Web 开发之 HTTP/2 & SPDY & HTTP 1.1 & HTTP 对比分析详解！
1
https://zh.wikipedia.org/wiki/HTTP/2
HTTP/2
HTTP/2（超文本传输协议第2版，最初命名为HTTP 2.0），是HTTP协议的的第二个主要版本，使用于万维网。HTTP/2是HTTP协议自1999年HTTP 1.1发布后的首个更新，主要基于SPDY协议。它由互联网工程任务组（IETF）的Hypertext Transfer Protocol Bis（httpbis）工作小组进行开发。[1]该组织于2014年12月将HTTP/2标准提议递交至IESG进行讨论[2]，于2015年2月17日被批准。[3] HTTP/2标准于2015年5月以RFC 7540正式发表。[4]
https://github.com/xgqfrms/http2-spec
https://tools.ietf.org/html/rfc7540
https://tools.ietf.org/pdf/rfc7540.pdf
HTTP/2
This is the home page for HTTP/2, a major revision of the Web's protocol. It is maintained by the IETF HTTP Working Group.
See also HTTP/2 JP, maintained by the Japanese HTTP/2 community.
What is HTTP/2?
HTTP/2 is a replacement for how HTTP is expressed “on the wire.” It is not a ground-up rewrite of the protocol; HTTP methods, status codes and semantics are the same, and it should be possible to use the same APIs as HTTP/1.x (possibly with some small additions) to represent the protocol.
The focus of the protocol is on performance; specifically, end-user perceived latency, network and server resource usage. One major goal is to allow the use of a single connection from browsers to a Web site.
The basis of the work was SPDY, but HTTP/2 has evolved to take the community’s input into account, incorporating several improvements in the process.
See our charter for more details of the scope of the work, as well as our Frequently Asked Questions.
Specifications
HTTP/2 is comprised of two specifications:
Implementations
We track known implementations of HTTP/2 on our wiki.
1
https://httpwg.github.io/specs/rfc7541.html
| Internet Engineering Task Force (IETF) | R. Peon |
| Request for Comments: 7541 | Google, Inc |
| Category: Standards Track | H. Ruellan |
| ISSN: 2070-1721 | Canon CRF |
| May 2015 |
HPACK: Header Compression for HTTP/2
Abstract
This specification defines HPACK, a compression format for efficiently representing HTTP header fields, to be used in HTTP/2.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at <https://lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information can be found at <http://tools.ietf.org/wg/httpbis/>; that specific to HTTP/2 are at <http://http2.github.io/>.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7541.
Copyright Notice
Copyright © 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
1. Introduction
In HTTP/1.1 (see [RFC7230]), header fields are not compressed. As web pages have grown to require dozens to hundreds of requests, the redundant header fields in these requests unnecessarily consume bandwidth, measurably increasing latency.
SPDY [SPDY] initially addressed this redundancy by compressing header fields using the DEFLATE [DEFLATE] format, which proved very effective at efficiently representing the redundant header fields. However, that approach exposed a security risk as demonstrated by the CRIME (Compression Ratio Info-leak Made Easy) attack (see [CRIME]).
This specification defines HPACK, a new compressor that eliminates redundant header fields, limits vulnerability to known security attacks, and has a bounded memory requirement for use in constrained environments. Potential security concerns for HPACK are described in Section 7.
The HPACK format is intentionally simple and inflexible. Both characteristics reduce the risk of interoperability or security issues due to implementation error. No extensibility mechanisms are defined; changes to the format are only possible by defining a complete replacement.
1.1 Overview
The format defined in this specification treats a list of header fields as an ordered collection of name-value pairs that can include duplicate pairs. Names and values are considered to be opaque sequences of octets, and the order of header fields is preserved after being compressed and decompressed.
Encoding is informed by header field tables that map header fields to indexed values. These header field tables can be incrementally updated as new header fields are encoded or decoded.
In the encoded form, a header field is represented either literally or as a reference to a header field in one of the header field tables. Therefore, a list of header fields can be encoded using a mixture of references and literal values.
Literal values are either encoded directly or use a static Huffman code.
The encoder is responsible for deciding which header fields to insert as new entries in the header field tables. The decoder executes the modifications to the header field tables prescribed by the encoder, reconstructing the list of header fields in the process. This enables decoders to remain simple and interoperate with a wide variety of encoders.
Examples illustrating the use of these different mechanisms to represent header fields are available in Appendix C.
1.2 Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
All numeric values are in network byte order. Values are unsigned unless otherwise indicated. Literal values are provided in decimal or hexadecimal as appropriate.
1.3 Terminology
This specification uses the following terms:
- Header Field:
- A name-value pair. Both the name and value are treated as opaque sequences of octets.
- Dynamic Table:
- The dynamic table (see Section 2.3.2) is a table that associates stored header fields with index values. This table is dynamic and specific to an encoding or decoding context.
- Static Table:
- The static table (see Section 2.3.1) is a table that statically associates header fields that occur frequently with index values. This table is ordered, read-only, always accessible, and it may be shared amongst all encoding or decoding contexts.
- Header List:
- A header list is an ordered collection of header fields that are encoded jointly and can contain duplicate header fields. A complete list of header fields contained in an HTTP/2 header block is a header list.
- Header Field Representation:
- A header field can be represented in encoded form either as a literal or as an index (see Section 2.4).
- Header Block:
- An ordered list of header field representations, which, when decoded, yields a complete header list.
2. Compression Process Overview
This specification does not describe a specific algorithm for an encoder. Instead, it defines precisely how a decoder is expected to operate, allowing encoders to produce any encoding that this definition permits.
2.1 Header List Ordering
HPACK preserves the ordering of header fields inside the header list. An encoder MUST order header field representations in the header block according to their ordering in the original header list. A decoder MUST order header fields in the decoded header list according to their ordering in the header block.
2.2 Encoding and Decoding Contexts
To decompress header blocks, a decoder only needs to maintain a dynamic table (see Section 2.3.2) as a decoding context. No other dynamic state is needed.
When used for bidirectional communication, such as in HTTP, the encoding and decoding dynamic tables maintained by an endpoint are completely independent, i.e., the request and response dynamic tables are separate.
2.3 Indexing Tables
HPACK uses two tables for associating header fields to indexes. The static table (see Section 2.3.1) is predefined and contains common header fields (most of them with an empty value). The dynamic table (see Section 2.3.2) is dynamic and can be used by the encoder to index header fields repeated in the encoded header lists.
These two tables are combined into a single address space for defining index values (see Section 2.3.3).
2.3.1 Static Table
The static table consists of a predefined static list of header fields. Its entries are defined in Appendix A.
2.3.2 Dynamic Table
The dynamic table consists of a list of header fields maintained in first-in, first-out order. The first and newest entry in a dynamic table is at the lowest index, and the oldest entry of a dynamic table is at the highest index.
The dynamic table is initially empty. Entries are added as each header block is decompressed.
The dynamic table can contain duplicate entries (i.e., entries with the same name and same value). Therefore, duplicate entries MUST NOT be treated as an error by a decoder.
The encoder decides how to update the dynamic table and as such can control how much memory is used by the dynamic table. To limit the memory requirements of the decoder, the dynamic table size is strictly bounded (see Section 4.2).
The decoder updates the dynamic table during the processing of a list of header field representations (see Section 3.2).
2.3.3 Index Address Space
The static table and the dynamic table are combined into a single index address space.
Indices between 1 and the length of the static table (inclusive) refer to elements in the static table (see Section 2.3.1).
Indices strictly greater than the length of the static table refer to elements in the dynamic table (see Section 2.3.2). The length of the static table is subtracted to find the index into the dynamic table.
Indices strictly greater than the sum of the lengths of both tables MUST be treated as a decoding error.
For a static table size of s and a dynamic table size of k, the following diagram shows the entire valid index address space.
<---------- Index Address Space ---------->
<-- Static Table --> <-- Dynamic Table -->
+---+-----------+---+ +---+-----------+---+
| 1 | ... | s | |s+1| ... |s+k|
+---+-----------+---+ +---+-----------+---+
^ |
| V
Insertion Point Dropping Point
Figure 1: Index Address Space
2.4 Header Field Representation
An encoded header field can be represented either as an index or as a literal.
An indexed representation defines a header field as a reference to an entry in either the static table or the dynamic table (see Section 6.1).
A literal representation defines a header field by specifying its name and value. The header field name can be represented literally or as a reference to an entry in either the static table or the dynamic table. The header field value is represented literally.
Three different literal representations are defined:
- A literal representation that adds the header field as a new entry at the beginning of the dynamic table (see Section 6.2.1).
- A literal representation that does not add the header field to the dynamic table (see Section 6.2.2).
- A literal representation that does not add the header field to the dynamic table, with the additional stipulation that this header field always use a literal representation, in particular when re-encoded by an intermediary (see Section 6.2.3). This representation is intended for protecting header field values that are not to be put at risk by compressing them (see Section 7.1.3 for more details).
The selection of one of these literal representations can be guided by security considerations, in order to protect sensitive header field values (see Section 7.1).
The literal representation of a header field name or of a header field value can encode the sequence of octets either directly or using a static Huffman code (see Section 5.2).
3. Header Block Decoding
3.1 Header Block Processing
A decoder processes a header block sequentially to reconstruct the original header list.
A header block is the concatenation of header field representations. The different possible header field representations are described in Section 6.
Once a header field is decoded and added to the reconstructed header list, the header field cannot be removed. A header field added to the header list can be safely passed to the application.
By passing the resulting header fields to the application, a decoder can be implemented with minimal transitory memory commitment in addition to the memory required for the dynamic table.
3.2 Header Field Representation Processing
The processing of a header block to obtain a header list is defined in this section. To ensure that the decoding will successfully produce a header list, a decoder MUST obey the following rules.
All the header field representations contained in a header block are processed in the order in which they appear, as specified below. Details on the formatting of the various header field representations and some additional processing instructions are found in Section 6.
An indexed representation entails the following actions:
- The header field corresponding to the referenced entry in either the static table or dynamic table is appended to the decoded header list.
A literal representation that is not added to the dynamic table entails the following action:
- The header field is appended to the decoded header list.
A literal representation that is added to the dynamic table entails the following actions:
- The header field is appended to the decoded header list.
- The header field is inserted at the beginning of the dynamic table. This insertion could result in the eviction of previous entries in the dynamic table (see Section 4.4).
4. Dynamic Table Management
To limit the memory requirements on the decoder side, the dynamic table is constrained in size.
4.1 Calculating Table Size
The size of the dynamic table is the sum of the size of its entries.
The size of an entry is the sum of its name's length in octets (as defined in Section 5.2), its value's length in octets, and 32.
The size of an entry is calculated using the length of its name and value without any Huffman encoding applied.
Note: The additional 32 octets account for an estimated overhead associated with an entry. For example, an entry structure using two 64-bit pointers to reference the name and the value of the entry and two 64-bit integers for counting the number of references to the name and value would have 32 octets of overhead.
4.2 Maximum Table Size
Protocols that use HPACK determine the maximum size that the encoder is permitted to use for the dynamic table. In HTTP/2, this value is determined by the SETTINGS_HEADER_TABLE_SIZE setting (see Section 6.5.2 of [HTTP2]).
An encoder can choose to use less capacity than this maximum size (see Section 6.3), but the chosen size MUST stay lower than or equal to the maximum set by the protocol.
A change in the maximum size of the dynamic table is signaled via a dynamic table size update (see Section 6.3). This dynamic table size update MUST occur at the beginning of the first header block following the change to the dynamic table size. In HTTP/2, this follows a settings acknowledgment (see Section 6.5.3 of [HTTP2]).
Multiple updates to the maximum table size can occur between the transmission of two header blocks. In the case that this size is changed more than once in this interval, the smallest maximum table size that occurs in that interval MUST be signaled in a dynamic table size update. The final maximum size is always signaled, resulting in at most two dynamic table size updates. This ensures that the decoder is able to perform eviction based on reductions in dynamic table size (see Section 4.3).
This mechanism can be used to completely clear entries from the dynamic table by setting a maximum size of 0, which can subsequently be restored.
4.3 Entry Eviction When Dynamic Table Size Changes
Whenever the maximum size for the dynamic table is reduced, entries are evicted from the end of the dynamic table until the size of the dynamic table is less than or equal to the maximum size.
4.4 Entry Eviction When Adding New Entries
Before a new entry is added to the dynamic table, entries are evicted from the end of the dynamic table until the size of the dynamic table is less than or equal to (maximum size - new entry size) or until the table is empty.
If the size of the new entry is less than or equal to the maximum size, that entry is added to the table. It is not an error to attempt to add an entry that is larger than the maximum size; an attempt to add an entry larger than the maximum size causes the table to be emptied of all existing entries and results in an empty table.
A new entry can reference the name of an entry in the dynamic table that will be evicted when adding this new entry into the dynamic table. Implementations are cautioned to avoid deleting the referenced name if the referenced entry is evicted from the dynamic table prior to inserting the new entry.
5. Primitive Type Representations
HPACK encoding uses two primitive types: unsigned variable-length integers and strings of octets.
5.1 Integer Representation
Integers are used to represent name indexes, header field indexes, or string lengths. An integer representation can start anywhere within an octet. To allow for optimized processing, an integer representation always finishes at the end of an octet.
An integer is represented in two parts: a prefix that fills the current octet and an optional list of octets that are used if the integer value does not fit within the prefix. The number of bits of the prefix (called N) is a parameter of the integer representation.
If the integer value is small enough, i.e., strictly less than 2N-1, it is encoded within the N-bit prefix.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | ? | ? | ? | Value | +---+---+---+-------------------+
Figure 2: Integer Value Encoded within the Prefix (Shown for N = 5)
Otherwise, all the bits of the prefix are set to 1, and the value, decreased by 2N-1, is encoded using a list of one or more octets. The most significant bit of each octet is used as a continuation flag: its value is set to 1 except for the last octet in the list. The remaining bits of the octets are used to encode the decreased value.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ? | ? | ? | 1 1 1 1 1 |
+---+---+---+-------------------+
| 1 | Value-(2^N-1) LSB |
+---+---------------------------+
...
+---+---------------------------+
| 0 | Value-(2^N-1) MSB |
+---+---------------------------+
Figure 3: Integer Value Encoded after the Prefix (Shown for N = 5)
Decoding the integer value from the list of octets starts by reversing the order of the octets in the list. Then, for each octet, its most significant bit is removed. The remaining bits of the octets are concatenated, and the resulting value is increased by 2N-1 to obtain the integer value.
The prefix size, N, is always between 1 and 8 bits. An integer starting at an octet boundary will have an 8-bit prefix.
Pseudocode to represent an integer I is as follows:
if I < 2^N - 1, encode I on N bits
else
encode (2^N - 1) on N bits
I = I - (2^N - 1)
while I >= 128
encode (I % 128 + 128) on 8 bits
I = I / 128
encode I on 8 bits
Pseudocode to decode an integer I is as follows:
decode I from the next N bits
if I < 2^N - 1, return I
else
M = 0
repeat
B = next octet
I = I + (B & 127) * 2^M
M = M + 7
while B & 128 == 128
return I
Examples illustrating the encoding of integers are available in Appendix C.1.
This integer representation allows for values of indefinite size. It is also possible for an encoder to send a large number of zero values, which can waste octets and could be used to overflow integer values. Integer encodings that exceed implementation limits — in value or octet length — MUST be treated as decoding errors. Different limits can be set for each of the different uses of integers, based on implementation constraints.
5.2 String Literal Representation
Header field names and header field values can be represented as string literals. A string literal is encoded as a sequence of octets, either by directly encoding the string literal's octets or by using a Huffman code (see [HUFFMAN]).
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | H | String Length (7+) | +---+---------------------------+ | String Data (Length octets) | +-------------------------------+
Figure 4: String Literal Representation
A string literal representation contains the following fields:
- H:
- A one-bit flag, H, indicating whether or not the octets of the string are Huffman encoded.
- String Length:
- The number of octets used to encode the string literal, encoded as an integer with a 7-bit prefix (see Section 5.1).
- String Data:
- The encoded data of the string literal. If H is '0', then the encoded data is the raw octets of the string literal. If H is '1', then the encoded data is the Huffman encoding of the string literal.
String literals that use Huffman encoding are encoded with the Huffman code defined in Appendix B (see examples for requests in Appendix C.4 and for responses in Appendix C.6). The encoded data is the bitwise concatenation of the codes corresponding to each octet of the string literal.
As the Huffman-encoded data doesn't always end at an octet boundary, some padding is inserted after it, up to the next octet boundary. To prevent this padding from being misinterpreted as part of the string literal, the most significant bits of the code corresponding to the EOS (end-of-string) symbol are used.
Upon decoding, an incomplete code at the end of the encoded data is to be considered as padding and discarded. A padding strictly longer than 7 bits MUST be treated as a decoding error. A padding not corresponding to the most significant bits of the code for the EOS symbol MUST be treated as a decoding error. A Huffman-encoded string literal containing the EOS symbol MUST be treated as a decoding error.
6. Binary Format
This section describes the detailed format of each of the different header field representations and the dynamic table size update instruction.
6.1 Indexed Header Field Representation
An indexed header field representation identifies an entry in either the static table or the dynamic table (see Section 2.3).
An indexed header field representation causes a header field to be added to the decoded header list, as described in Section 3.2.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 | Index (7+) | +---+---------------------------+
Figure 5: Indexed Header Field
An indexed header field starts with the '1' 1-bit pattern, followed by the index of the matching header field, represented as an integer with a 7-bit prefix (see Section 5.1).
The index value of 0 is not used. It MUST be treated as a decoding error if found in an indexed header field representation.
6.2 Literal Header Field Representation
A literal header field representation contains a literal header field value. Header field names are provided either as a literal or by reference to an existing table entry, either from the static table or the dynamic table (see Section 2.3).
This specification defines three forms of literal header field representations: with indexing, without indexing, and never indexed.
6.2.1 Literal Header Field with Incremental Indexing
A literal header field with incremental indexing representation results in appending a header field to the decoded header list and inserting it as a new entry into the dynamic table.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | Index (6+) | +---+---+-----------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Figure 6: Literal Header Field with Incremental Indexing — Indexed Name
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | 0 | +---+---+-----------------------+ | H | Name Length (7+) | +---+---------------------------+ | Name String (Length octets) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Figure 7: Literal Header Field with Incremental Indexing — New Name
A literal header field with incremental indexing representation starts with the '01' 2-bit pattern.
If the header field name matches the header field name of an entry stored in the static table or the dynamic table, the header field name can be represented using the index of that entry. In this case, the index of the entry is represented as an integer with a 6-bit prefix (see Section 5.1). This value is always non-zero.
Otherwise, the header field name is represented as a string literal (see Section 5.2). A value 0 is used in place of the 6-bit index, followed by the header field name.
Either form of header field name representation is followed by the header field value represented as a string literal (see Section 5.2).
6.2.2 Literal Header Field without Indexing
A literal header field without indexing representation results in appending a header field to the decoded header list without altering the dynamic table.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 | Index (4+) | +---+---+-----------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Figure 8: Literal Header Field without Indexing — Indexed Name
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 | 0 | +---+---+-----------------------+ | H | Name Length (7+) | +---+---------------------------+ | Name String (Length octets) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Figure 9: Literal Header Field without Indexing — New Name
A literal header field without indexing representation starts with the '0000' 4-bit pattern.
If the header field name matches the header field name of an entry stored in the static table or the dynamic table, the header field name can be represented using the index of that entry. In this case, the index of the entry is represented as an integer with a 4-bit prefix (see Section 5.1). This value is always non-zero.
Otherwise, the header field name is represented as a string literal (see Section 5.2). A value 0 is used in place of the 4-bit index, followed by the header field name.
Either form of header field name representation is followed by the header field value represented as a string literal (see Section 5.2).
6.2.3 Literal Header Field Never Indexed
A literal header field never-indexed representation results in appending a header field to the decoded header list without altering the dynamic table. Intermediaries MUST use the same representation for encoding this header field.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 1 | Index (4+) | +---+---+-----------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Figure 10: Literal Header Field Never Indexed — Indexed Name
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 1 | 0 | +---+---+-----------------------+ | H | Name Length (7+) | +---+---------------------------+ | Name String (Length octets) | +---+---------------------------+ | H | Value Length (7+) | +---+---------------------------+ | Value String (Length octets) | +-------------------------------+
Figure 11: Literal Header Field Never Indexed — New Name
A literal header field never-indexed representation starts with the '0001' 4-bit pattern.
When a header field is represented as a literal header field never indexed, it MUST always be encoded with this specific literal representation. In particular, when a peer sends a header field that it received represented as a literal header field never indexed, it MUST use the same representation to forward this header field.
This representation is intended for protecting header field values that are not to be put at risk by compressing them (see Section 7.1 for more details).
The encoding of the representation is identical to the literal header field without indexing (see Section 6.2.2).
6.3 Dynamic Table Size Update
A dynamic table size update signals a change to the size of the dynamic table.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | Max size (5+) | +---+---------------------------+
Figure 12: Maximum Dynamic Table Size Change
A dynamic table size update starts with the '001' 3-bit pattern, followed by the new maximum size, represented as an integer with a 5-bit prefix (see Section 5.1).
The new maximum size MUST be lower than or equal to the limit determined by the protocol using HPACK. A value that exceeds this limit MUST be treated as a decoding error. In HTTP/2, this limit is the last value of the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 6.5.2 of [HTTP2]) received from the decoder and acknowledged by the encoder (see Section 6.5.3 of [HTTP2]).
Reducing the maximum size of the dynamic table can cause entries to be evicted (see Section 4.3).
7. Security Considerations
This section describes potential areas of security concern with HPACK:
- Use of compression as a length-based oracle for verifying guesses about secrets that are compressed into a shared compression context.
- Denial of service resulting from exhausting processing or memory capacity at a decoder.
7.1 Probing Dynamic Table State
HPACK reduces the length of header field encodings by exploiting the redundancy inherent in protocols like HTTP. The ultimate goal of this is to reduce the amount of data that is required to send HTTP requests or responses.
The compression context used to encode header fields can be probed by an attacker who can both define header fields to be encoded and transmitted and observe the length of those fields once they are encoded. When an attacker can do both, they can adaptively modify requests in order to confirm guesses about the dynamic table state. If a guess is compressed into a shorter length, the attacker can observe the encoded length and infer that the guess was correct.
This is possible even over the Transport Layer Security (TLS) protocol (see [TLS12]), because while TLS provides confidentiality protection for content, it only provides a limited amount of protection for the length of that content.
Note: Padding schemes only provide limited protection against an attacker with these capabilities, potentially only forcing an increased number of guesses to learn the length associated with a given guess. Padding schemes also work directly against compression by increasing the number of bits that are transmitted.
Attacks like CRIME [CRIME] demonstrated the existence of these general attacker capabilities. The specific attack exploited the fact that DEFLATE [DEFLATE] removes redundancy based on prefix matching. This permitted the attacker to confirm guesses a character at a time, reducing an exponential-time attack into a linear-time attack.
7.1.1 Applicability to HPACK and HTTP
HPACK mitigates but does not completely prevent attacks modeled on CRIME [CRIME] by forcing a guess to match an entire header field value rather than individual characters. Attackers can only learn whether a guess is correct or not, so they are reduced to brute-force guesses for the header field values.
The viability of recovering specific header field values therefore depends on the entropy of values. As a result, values with high entropy are unlikely to be recovered successfully. However, values with low entropy remain vulnerable.
Attacks of this nature are possible any time that two mutually distrustful entities control requests or responses that are placed onto a single HTTP/2 connection. If the shared HPACK compressor permits one entity to add entries to the dynamic table and the other to access those entries, then the state of the table can be learned.
Having requests or responses from mutually distrustful entities occurs when an intermediary either:
- sends requests from multiple clients on a single connection toward an origin server, or
- takes responses from multiple origin servers and places them on a shared connection toward a client.
Web browsers also need to assume that requests made on the same connection by different web origins [ORIGIN] are made by mutually distrustful entities.
7.1.2 Mitigation
Users of HTTP that require confidentiality for header fields can use values with entropy sufficient to make guessing infeasible. However, this is impractical as a general solution because it forces all users of HTTP to take steps to mitigate attacks. It would impose new constraints on how HTTP is used.
Rather than impose constraints on users of HTTP, an implementation of HPACK can instead constrain how compression is applied in order to limit the potential for dynamic table probing.
An ideal solution segregates access to the dynamic table based on the entity that is constructing header fields. Header field values that are added to the table are attributed to an entity, and only the entity that created a particular value can extract that value.
To improve compression performance of this option, certain entries might be tagged as being public. For example, a web browser might make the values of the Accept-Encoding header field available in all requests.
An encoder without good knowledge of the provenance of header fields might instead introduce a penalty for a header field with many different values, such that a large number of attempts to guess a header field value results in the header field no longer being compared to the dynamic table entries in future messages, effectively preventing further guesses.
Note: Simply removing entries corresponding to the header field from the dynamic table can be ineffectual if the attacker has a reliable way of causing values to be reinstalled. For example, a request to load an image in a web browser typically includes the Cookie header field (a potentially highly valued target for this sort of attack), and web sites can easily force an image to be loaded, thereby refreshing the entry in the dynamic table.
This response might be made inversely proportional to the length of the header field value. Marking a header field as not using the dynamic table anymore might occur for shorter values more quickly or with higher probability than for longer values.
7.1.3 Never-Indexed Literals
Implementations can also choose to protect sensitive header fields by not compressing them and instead encoding their value as literals.
Refusing to generate an indexed representation for a header field is only effective if compression is avoided on all hops. The never-indexed literal (see Section 6.2.3) can be used to signal to intermediaries that a particular value was intentionally sent as a literal.
An intermediary MUST NOT re-encode a value that uses the never-indexed literal representation with another representation that would index it. If HPACK is used for re-encoding, the never-indexed literal representation MUST be used.
The choice to use a never-indexed literal representation for a header field depends on several factors. Since HPACK doesn't protect against guessing an entire header field value, short or low-entropy values are more readily recovered by an adversary. Therefore, an encoder might choose not to index values with low entropy.
An encoder might also choose not to index values for header fields that are considered to be highly valuable or sensitive to recovery, such as the Cookie or Authorization header fields.
On the contrary, an encoder might prefer indexing values for header fields that have little or no value if they were exposed. For instance, a User-Agent header field does not commonly vary between requests and is sent to any server. In that case, confirmation that a particular User-Agent value has been used provides little value.
Note that these criteria for deciding to use a never-indexed literal representation will evolve over time as new attacks are discovered.
7.2 Static Huffman Encoding
There is no currently known attack against a static Huffman encoding. A study has shown that using a static Huffman encoding table created an information leakage; however, this same study concluded that an attacker could not take advantage of this information leakage to recover any meaningful amount of information (see [PETAL]).
7.3 Memory Consumption
An attacker can try to cause an endpoint to exhaust its memory. HPACK is designed to limit both the peak and state amounts of memory allocated by an endpoint.
The amount of memory used by the compressor is limited by the protocol using HPACK through the definition of the maximum size of the dynamic table. In HTTP/2, this value is controlled by the decoder through the setting parameter SETTINGS_HEADER_TABLE_SIZE (see Section 6.5.2 of [HTTP2]). This limit takes into account both the size of the data stored in the dynamic table, plus a small allowance for overhead.
A decoder can limit the amount of state memory used by setting an appropriate value for the maximum size of the dynamic table. In HTTP/2, this is realized by setting an appropriate value for the SETTINGS_HEADER_TABLE_SIZE parameter. An encoder can limit the amount of state memory it uses by signaling a lower dynamic table size than the decoder allows (see Section 6.3).
The amount of temporary memory consumed by an encoder or decoder can be limited by processing header fields sequentially. An implementation does not need to retain a complete list of header fields. Note, however, that it might be necessary for an application to retain a complete header list for other reasons; even though HPACK does not force this to occur, application constraints might make this necessary.
7.4 Implementation Limits
An implementation of HPACK needs to ensure that large values for integers, long encoding for integers, or long string literals do not create security weaknesses.
An implementation has to set a limit for the values it accepts for integers, as well as for the encoded length (see Section 5.1). In the same way, it has to set a limit to the length it accepts for string literals (see Section 5.2).
8. References
8.1 Normative References
- [HTTP2]
- Belshe, M., Peon, R., and M. Thomson, Ed., “Hypertext Transfer Protocol Version 2 (HTTP/2)”, RFC 7540, DOI 10.17487/RFC7540, May 2015, <http://www.rfc-editor.org/info/rfc7540>.
- [RFC2119]
- Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
- [RFC7230]
- Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>.
8.2 Informative References
- [CANONICAL]
- Schwartz, E. and B. Kallick, “Generating a canonical prefix encoding”, Communications of the ACM, Volume 7 Issue 3, pp. 166-169, March 1964, <https://dl.acm.org/citation.cfm?id=363991>.
- [CRIME]
- Wikipedia, “CRIME”, May 2015, <http://en.wikipedia.org/w/index.php?title=CRIME&oldid=660948120>.
- [DEFLATE]
- Deutsch, P., “DEFLATE Compressed Data Format Specification version 1.3”, RFC 1951, DOI 10.17487/RFC1951, May 1996, <http://www.rfc-editor.org/info/rfc1951>.
- [HUFFMAN]
- Huffman, D., “A Method for the Construction of Minimum-Redundancy Codes”, Proceedings of the Institute of Radio Engineers, Volume 40, Number 9, pp. 1098-1101, September 1952, <http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4051119>.
- [ORIGIN]
- Barth, A., “The Web Origin Concept”, RFC 6454, DOI 10.17487/RFC6454, December 2011, <http://www.rfc-editor.org/info/rfc6454>.
- [PETAL]
- Tan, J. and J. Nahata, “PETAL: Preset Encoding Table Information Leakage”, April 2013, <http://www.pdl.cmu.edu/PDL-FTP/associated/CMU-PDL-13-106.pdf>.
- [SPDY]
- Belshe, M. and R. Peon, “SPDY Protocol”, Work in Progress, draft-mbelshe-httpbis-spdy-00, February 2012.
- [TLS12]
- Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2”, RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>.
A. Static Table Definition
The static table (see Section 2.3.1) consists in a predefined and unchangeable list of header fields.
The static table was created from the most frequent header fields used by popular web sites, with the addition of HTTP/2-specific pseudo-header fields (see Section 8.1.2.1 of [HTTP2]). For header fields with a few frequent values, an entry was added for each of these frequent values. For other header fields, an entry was added with an empty value.
Table 1 lists the predefined header fields that make up the static table and gives the index of each entry.
| Index | Header Name | Header Value |
|---|---|---|
| 1 | :authority | |
| 2 | :method | GET |
| 3 | :method | POST |
| 4 | :path | / |
| 5 | :path | /index.html |
| 6 | :scheme | http |
| 7 | :scheme | https |
| 8 | :status | 200 |
| 9 | :status | 204 |
| 10 | :status | 206 |
| 11 | :status | 304 |
| 12 | :status | 400 |
| 13 | :status | 404 |
| 14 | :status | 500 |
| 15 | accept-charset | |
| 16 | accept-encoding | gzip, deflate |
| 17 | accept-language | |
| 18 | accept-ranges | |
| 19 | accept | |
| 20 | access-control-allow-origin | |
| 21 | age | |
| 22 | allow | |
| 23 | authorization | |
| 24 | cache-control | |
| 25 | content-disposition | |
| 26 | content-encoding | |
| 27 | content-language | |
| 28 | content-length | |
| 29 | content-location | |
| 30 | content-range | |
| 31 | content-type | |
| 32 | cookie | |
| 33 | date | |
| 34 | etag | |
| 35 | expect | |
| 36 | expires | |
| 37 | from | |
| 38 | host | |
| 39 | if-match | |
| 40 | if-modified-since | |
| 41 | if-none-match | |
| 42 | if-range | |
| 43 | if-unmodified-since | |
| 44 | last-modified | |
| 45 | link | |
| 46 | location | |
| 47 | max-forwards | |
| 48 | proxy-authenticate | |
| 49 | proxy-authorization | |
| 50 | range | |
| 51 | referer | |
| 52 | refresh | |
| 53 | retry-after | |
| 54 | server | |
| 55 | set-cookie | |
| 56 | strict-transport-security | |
| 57 | transfer-encoding | |
| 58 | user-agent | |
| 59 | vary | |
| 60 | via | |
| 61 | www-authenticate |
B. Huffman Code
The following Huffman code is used when encoding string literals with a Huffman coding (see Section 5.2).
This Huffman code was generated from statistics obtained on a large sample of HTTP headers. It is a canonical Huffman code (see [CANONICAL]) with some tweaking to ensure that no symbol has a unique code length.
Each row in the table defines the code used to represent a symbol:
- sym:
- The symbol to be represented. It is the decimal value of an octet, possibly prepended with its ASCII representation. A specific symbol, "EOS", is used to indicate the end of a string literal.
- code as bits:
- The Huffman code for the symbol represented as a base-2 integer, aligned on the most significant bit (MSB).
- code as hex:
- The Huffman code for the symbol, represented as a hexadecimal integer, aligned on the least significant bit (LSB).
- len:
- The number of bits for the code representing the symbol.
As an example, the code for the symbol 47 (corresponding to the ASCII character "/") consists in the 6 bits "0", "1", "1", "0", "0", "0". This corresponds to the value 0x18 (in hexadecimal) encoded in 6 bits.
code
code as bits as hex len
sym aligned to MSB aligned in
to LSB bits
( 0) |11111111|11000 1ff8 [13]
( 1) |11111111|11111111|1011000 7fffd8 [23]
( 2) |11111111|11111111|11111110|0010 fffffe2 [28]
( 3) |11111111|11111111|11111110|0011 fffffe3 [28]
( 4) |11111111|11111111|11111110|0100 fffffe4 [28]
( 5) |11111111|11111111|11111110|0101 fffffe5 [28]
( 6) |11111111|11111111|11111110|0110 fffffe6 [28]
( 7) |11111111|11111111|11111110|0111 fffffe7 [28]
( 8) |11111111|11111111|11111110|1000 fffffe8 [28]
( 9) |11111111|11111111|11101010 ffffea [24]
( 10) |11111111|11111111|11111111|111100 3ffffffc [30]
( 11) |11111111|11111111|11111110|1001 fffffe9 [28]
( 12) |11111111|11111111|11111110|1010 fffffea [28]
( 13) |11111111|11111111|11111111|111101 3ffffffd [30]
( 14) |11111111|11111111|11111110|1011 fffffeb [28]
( 15) |11111111|11111111|11111110|1100 fffffec [28]
( 16) |11111111|11111111|11111110|1101 fffffed [28]
( 17) |11111111|11111111|11111110|1110 fffffee [28]
( 18) |11111111|11111111|11111110|1111 fffffef [28]
( 19) |11111111|11111111|11111111|0000 ffffff0 [28]
( 20) |11111111|11111111|11111111|0001 ffffff1 [28]
( 21) |11111111|11111111|11111111|0010 ffffff2 [28]
( 22) |11111111|11111111|11111111|111110 3ffffffe [30]
( 23) |11111111|11111111|11111111|0011 ffffff3 [28]
( 24) |11111111|11111111|11111111|0100 ffffff4 [28]
( 25) |11111111|11111111|11111111|0101 ffffff5 [28]
( 26) |11111111|11111111|11111111|0110 ffffff6 [28]
( 27) |11111111|11111111|11111111|0111 ffffff7 [28]
( 28) |11111111|11111111|11111111|1000 ffffff8 [28]
( 29) |11111111|11111111|11111111|1001 ffffff9 [28]
( 30) |11111111|11111111|11111111|1010 ffffffa [28]
( 31) |11111111|11111111|11111111|1011 ffffffb [28]
' ' ( 32) |010100 14 [ 6]
'!' ( 33) |11111110|00 3f8 [10]
'"' ( 34) |11111110|01 3f9 [10]
'#' ( 35) |11111111|1010 ffa [12]
'$' ( 36) |11111111|11001 1ff9 [13]
'%' ( 37) |010101 15 [ 6]
'&' ( 38) |11111000 f8 [ 8]
''' ( 39) |11111111|010 7fa [11]
'(' ( 40) |11111110|10 3fa [10]
')' ( 41) |11111110|11 3fb [10]
'*' ( 42) |11111001 f9 [ 8]
'+' ( 43) |11111111|011 7fb [11]
',' ( 44) |11111010 fa [ 8]
'-' ( 45) |010110 16 [ 6]
'.' ( 46) |010111 17 [ 6]
'/' ( 47) |011000 18 [ 6]
'0' ( 48) |00000 0 [ 5]
'1' ( 49) |00001 1 [ 5]
'2' ( 50) |00010 2 [ 5]
'3' ( 51) |011001 19 [ 6]
'4' ( 52) |011010 1a [ 6]
'5' ( 53) |011011 1b [ 6]
'6' ( 54) |011100 1c [ 6]
'7' ( 55) |011101 1d [ 6]
'8' ( 56) |011110 1e [ 6]
'9' ( 57) |011111 1f [ 6]
':' ( 58) |1011100 5c [ 7]
';' ( 59) |11111011 fb [ 8]
'<' ( 60) |11111111|1111100 7ffc [15]
'=' ( 61) |100000 20 [ 6]
'>' ( 62) |11111111|1011 ffb [12]
'?' ( 63) |11111111|00 3fc [10]
'@' ( 64) |11111111|11010 1ffa [13]
'A' ( 65) |100001 21 [ 6]
'B' ( 66) |1011101 5d [ 7]
'C' ( 67) |1011110 5e [ 7]
'D' ( 68) |1011111 5f [ 7]
'E' ( 69) |1100000 60 [ 7]
'F' ( 70) |1100001 61 [ 7]
'G' ( 71) |1100010 62 [ 7]
'H' ( 72) |1100011 63 [ 7]
'I' ( 73) |1100100 64 [ 7]
'J' ( 74) |1100101 65 [ 7]
'K' ( 75) |1100110 66 [ 7]
'L' ( 76) |1100111 67 [ 7]
'M' ( 77) |1101000 68 [ 7]
'N' ( 78) |1101001 69 [ 7]
'O' ( 79) |1101010 6a [ 7]
'P' ( 80) |1101011 6b [ 7]
'Q' ( 81) |1101100 6c [ 7]
'R' ( 82) |1101101 6d [ 7]
'S' ( 83) |1101110 6e [ 7]
'T' ( 84) |1101111 6f [ 7]
'U' ( 85) |1110000 70 [ 7]
'V' ( 86) |1110001 71 [ 7]
'W' ( 87) |1110010 72 [ 7]
'X' ( 88) |11111100 fc [ 8]
'Y' ( 89) |1110011 73 [ 7]
'Z' ( 90) |11111101 fd [ 8]
'[' ( 91) |11111111|11011 1ffb [13]
'\' ( 92) |11111111|11111110|000 7fff0 [19]
']' ( 93) |11111111|11100 1ffc [13]
'^' ( 94) |11111111|111100 3ffc [14]
'_' ( 95) |100010 22 [ 6]
'`' ( 96) |11111111|1111101 7ffd [15]
'a' ( 97) |00011 3 [ 5]
'b' ( 98) |100011 23 [ 6]
'c' ( 99) |00100 4 [ 5]
'd' (100) |100100 24 [ 6]
'e' (101) |00101 5 [ 5]
'f' (102) |100101 25 [ 6]
'g' (103) |100110 26 [ 6]
'h' (104) |100111 27 [ 6]
'i' (105) |00110 6 [ 5]
'j' (106) |1110100 74 [ 7]
'k' (107) |1110101 75 [ 7]
'l' (108) |101000 28 [ 6]
'm' (109) |101001 29 [ 6]
'n' (110) |101010 2a [ 6]
'o' (111) |00111 7 [ 5]
'p' (112) |101011 2b [ 6]
'q' (113) |1110110 76 [ 7]
'r' (114) |101100 2c [ 6]
's' (115) |01000 8 [ 5]
't' (116) |01001 9 [ 5]
'u' (117) |101101 2d [ 6]
'v' (118) |1110111 77 [ 7]
'w' (119) |1111000 78 [ 7]
'x' (120) |1111001 79 [ 7]
'y' (121) |1111010 7a [ 7]
'z' (122) |1111011 7b [ 7]
'{' (123) |11111111|1111110 7ffe [15]
'|' (124) |11111111|100 7fc [11]
'}' (125) |11111111|111101 3ffd [14]
'~' (126) |11111111|11101 1ffd [13]
(127) |11111111|11111111|11111111|1100 ffffffc [28]
(128) |11111111|11111110|0110 fffe6 [20]
(129) |11111111|11111111|010010 3fffd2 [22]
(130) |11111111|11111110|0111 fffe7 [20]
(131) |11111111|11111110|1000 fffe8 [20]
(132) |11111111|11111111|010011 3fffd3 [22]
(133) |11111111|11111111|010100 3fffd4 [22]
(134) |11111111|11111111|010101 3fffd5 [22]
(135) |11111111|11111111|1011001 7fffd9 [23]
(136) |11111111|11111111|010110 3fffd6 [22]
(137) |11111111|11111111|1011010 7fffda [23]
(138) |11111111|11111111|1011011 7fffdb [23]
(139) |11111111|11111111|1011100 7fffdc [23]
(140) |11111111|11111111|1011101 7fffdd [23]
(141) |11111111|11111111|1011110 7fffde [23]
(142) |11111111|11111111|11101011 ffffeb [24]
(143) |11111111|11111111|1011111 7fffdf [23]
(144) |11111111|11111111|11101100 ffffec [24]
(145) |11111111|11111111|11101101 ffffed [24]
(146) |11111111|11111111|010111 3fffd7 [22]
(147) |11111111|11111111|1100000 7fffe0 [23]
(148) |11111111|11111111|11101110 ffffee [24]
(149) |11111111|11111111|1100001 7fffe1 [23]
(150) |11111111|11111111|1100010 7fffe2 [23]
(151) |11111111|11111111|1100011 7fffe3 [23]
(152) |11111111|11111111|1100100 7fffe4 [23]
(153) |11111111|11111110|11100 1fffdc [21]
(154) |11111111|11111111|011000 3fffd8 [22]
(155) |11111111|11111111|1100101 7fffe5 [23]
(156) |11111111|11111111|011001 3fffd9 [22]
(157) |11111111|11111111|1100110 7fffe6 [23]
(158) |11111111|11111111|1100111 7fffe7 [23]
(159) |11111111|11111111|11101111 ffffef [24]
(160) |11111111|11111111|011010 3fffda [22]
(161) |11111111|11111110|11101 1fffdd [21]
(162) |11111111|11111110|1001 fffe9 [20]
(163) |11111111|11111111|011011 3fffdb [22]
(164) |11111111|11111111|011100 3fffdc [22]
(165) |11111111|11111111|1101000 7fffe8 [23]
(166) |11111111|11111111|1101001 7fffe9 [23]
(167) |11111111|11111110|11110 1fffde [21]
(168) |11111111|11111111|1101010 7fffea [23]
(169) |11111111|11111111|011101 3fffdd [22]
(170) |11111111|11111111|011110 3fffde [22]
(171) |11111111|11111111|11110000 fffff0 [24]
(172) |11111111|11111110|11111 1fffdf [21]
(173) |11111111|11111111|011111 3fffdf [22]
(174) |11111111|11111111|1101011 7fffeb [23]
(175) |11111111|11111111|1101100 7fffec [23]
(176) |11111111|11111111|00000 1fffe0 [21]
(177) |11111111|11111111|00001 1fffe1 [21]
(178) |11111111|11111111|100000 3fffe0 [22]
(179) |11111111|11111111|00010 1fffe2 [21]
(180) |11111111|11111111|1101101 7fffed [23]
(181) |11111111|11111111|100001 3fffe1 [22]
(182) |11111111|11111111|1101110 7fffee [23]
(183) |11111111|11111111|1101111 7fffef [23]
(184) |11111111|11111110|1010 fffea [20]
(185) |11111111|11111111|100010 3fffe2 [22]
(186) |11111111|11111111|100011 3fffe3 [22]
(187) |11111111|11111111|100100 3fffe4 [22]
(188) |11111111|11111111|1110000 7ffff0 [23]
(189) |11111111|11111111|100101 3fffe5 [22]
(190) |11111111|11111111|100110 3fffe6 [22]
(191) |11111111|11111111|1110001 7ffff1 [23]
(192) |11111111|11111111|11111000|00 3ffffe0 [26]
(193) |11111111|11111111|11111000|01 3ffffe1 [26]
(194) |11111111|11111110|1011 fffeb [20]
(195) |11111111|11111110|001 7fff1 [19]
(196) |11111111|11111111|100111 3fffe7 [22]
(197) |11111111|11111111|1110010 7ffff2 [23]
(198) |11111111|11111111|101000 3fffe8 [22]
(199) |11111111|11111111|11110110|0 1ffffec [25]
(200) |11111111|11111111|11111000|10 3ffffe2 [26]
(201) |11111111|11111111|11111000|11 3ffffe3 [26]
(202) |11111111|11111111|11111001|00 3ffffe4 [26]
(203) |11111111|11111111|11111011|110 7ffffde [27]
(204) |11111111|11111111|11111011|111 7ffffdf [27]
(205) |11111111|11111111|11111001|01 3ffffe5 [26]
(206) |11111111|11111111|11110001 fffff1 [24]
(207) |11111111|11111111|11110110|1 1ffffed [25]
(208) |11111111|11111110|010 7fff2 [19]
(209) |11111111|11111111|00011 1fffe3 [21]
(210) |11111111|11111111|11111001|10 3ffffe6 [26]
(211) |11111111|11111111|11111100|000 7ffffe0 [27]
(212) |11111111|11111111|11111100|001 7ffffe1 [27]
(213) |11111111|11111111|11111001|11 3ffffe7 [26]
(214) |11111111|11111111|11111100|010 7ffffe2 [27]
(215) |11111111|11111111|11110010 fffff2 [24]
(216) |11111111|11111111|00100 1fffe4 [21]
(217) |11111111|11111111|00101 1fffe5 [21]
(218) |11111111|11111111|11111010|00 3ffffe8 [26]
(219) |11111111|11111111|11111010|01 3ffffe9 [26]
(220) |11111111|11111111|11111111|1101 ffffffd [28]
(221) |11111111|11111111|11111100|011 7ffffe3 [27]
(222) |11111111|11111111|11111100|100 7ffffe4 [27]
(223) |11111111|11111111|11111100|101 7ffffe5 [27]
(224) |11111111|11111110|1100 fffec [20]
(225) |11111111|11111111|11110011 fffff3 [24]
(226) |11111111|11111110|1101 fffed [20]
(227) |11111111|11111111|00110 1fffe6 [21]
(228) |11111111|11111111|101001 3fffe9 [22]
(229) |11111111|11111111|00111 1fffe7 [21]
(230) |11111111|11111111|01000 1fffe8 [21]
(231) |11111111|11111111|1110011 7ffff3 [23]
(232) |11111111|11111111|101010 3fffea [22]
(233) |11111111|11111111|101011 3fffeb [22]
(234) |11111111|11111111|11110111|0 1ffffee [25]
(235) |11111111|11111111|11110111|1 1ffffef [25]
(236) |11111111|11111111|11110100 fffff4 [24]
(237) |11111111|11111111|11110101 fffff5 [24]
(238) |11111111|11111111|11111010|10 3ffffea [26]
(239) |11111111|11111111|1110100 7ffff4 [23]
(240) |11111111|11111111|11111010|11 3ffffeb [26]
(241) |11111111|11111111|11111100|110 7ffffe6 [27]
(242) |11111111|11111111|11111011|00 3ffffec [26]
(243) |11111111|11111111|11111011|01 3ffffed [26]
(244) |11111111|11111111|11111100|111 7ffffe7 [27]
(245) |11111111|11111111|11111101|000 7ffffe8 [27]
(246) |11111111|11111111|11111101|001 7ffffe9 [27]
(247) |11111111|11111111|11111101|010 7ffffea [27]
(248) |11111111|11111111|11111101|011 7ffffeb [27]
(249) |11111111|11111111|11111111|1110 ffffffe [28]
(250) |11111111|11111111|11111101|100 7ffffec [27]
(251) |11111111|11111111|11111101|101 7ffffed [27]
(252) |11111111|11111111|11111101|110 7ffffee [27]
(253) |11111111|11111111|11111101|111 7ffffef [27]
(254) |11111111|11111111|11111110|000 7fffff0 [27]
(255) |11111111|11111111|11111011|10 3ffffee [26]
EOS (256) |11111111|11111111|11111111|111111 3fffffff [30]
C. Examples
This appendix contains examples covering integer encoding, header field representation, and the encoding of whole lists of header fields for both requests and responses, with and without Huffman coding.
C.1 Integer Representation Examples
This section shows the representation of integer values in detail (see Section 5.1).
C.1.1 Example 1: Encoding 10 Using a 5-Bit Prefix
The value 10 is to be encoded with a 5-bit prefix.
- 10 is less than 31 (25 - 1) and is represented using the 5-bit prefix.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | X | X | X | 0 | 1 | 0 | 1 | 0 | 10 stored on 5 bits +---+---+---+---+---+---+---+---+
C.1.2 Example 2: Encoding 1337 Using a 5-Bit Prefix
The value I=1337 is to be encoded with a 5-bit prefix.
- 1337 is greater than 31 (25 - 1).
-
- The 5-bit prefix is filled with its max value (31).
- I = 1337 - (25 - 1) = 1306.
-
- I (1306) is greater than or equal to 128, so the while loop body executes:
-
- I % 128 == 26
- 26 + 128 == 154
- 154 is encoded in 8 bits as: 10011010
- I is set to 10 (1306 / 128 == 10)
- I is no longer greater than or equal to 128, so the while loop terminates.
- I, now 10, is encoded in 8 bits as: 00001010.
- The process ends.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | X | X | X | 1 | 1 | 1 | 1 | 1 | Prefix = 31, I = 1306 | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1306>=128, encode(154), I=1306/128 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 10<128, encode(10), done +---+---+---+---+---+---+---+---+
C.1.3 Example 3: Encoding 42 Starting at an Octet Boundary
The value 42 is to be encoded starting at an octet boundary. This implies that a 8-bit prefix is used.
- 42 is less than 255 (28 - 1) and is represented using the 8-bit prefix.
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 42 stored on 8 bits +---+---+---+---+---+---+---+---+
C.2 Header Field Representation Examples
This section shows several independent representation examples.
C.2.1 Literal Header Field with Indexing
The header field representation uses a literal name and a literal value. The header field is added to the dynamic table.
Header list to encode:
custom-key: custom-header
Hex dump of encoded data:
400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | @.custom-key.cus 746f 6d2d 6865 6164 6572 | tom-header
Decoding process:
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0d | Literal value (len = 13)
6375 7374 6f6d 2d68 6561 6465 72 | custom-header
| -> custom-key:
| custom-header
Dynamic Table (after decoding):
[ 1] (s = 55) custom-key: custom-header
Table size: 55
Decoded header list:
custom-key: custom-header
C.2.2 Literal Header Field without Indexing
The header field representation uses an indexed name and a literal value. The header field is not added to the dynamic table.
Header list to encode:
:path: /sample/path
Hex dump of encoded data:
040c 2f73 616d 706c 652f 7061 7468 | ../sample/path
Decoding process:
04 | == Literal not indexed ==
| Indexed name (idx = 4)
| :path
0c | Literal value (len = 12)
2f73 616d 706c 652f 7061 7468 | /sample/path
| -> :path: /sample/path
Dynamic table (after decoding): empty.
Decoded header list:
:path: /sample/path
C.2.3 Literal Header Field Never Indexed
The header field representation uses a literal name and a literal value. The header field is not added to the dynamic table and must use the same representation if re-encoded by an intermediary.
Header list to encode:
password: secret
Hex dump of encoded data:
1008 7061 7373 776f 7264 0673 6563 7265 | ..password.secre 74 | t
Decoding process:
10 | == Literal never indexed ==
08 | Literal name (len = 8)
7061 7373 776f 7264 | password
06 | Literal value (len = 6)
7365 6372 6574 | secret
| -> password: secret
Dynamic table (after decoding): empty.
Decoded header list:
password: secret
C.2.4 Indexed Header Field
The header field representation uses an indexed header field from the static table.
Header list to encode:
:method: GET
Hex dump of encoded data:
82 | .
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
Dynamic table (after decoding): empty.
Decoded header list:
:method: GET
C.3 Request Examples without Huffman Coding
This section shows several consecutive header lists, corresponding to HTTP requests, on the same connection.
C.3.1 First Request
Header list to encode:
:method: GET :scheme: http :path: / :authority: www.example.com
Hex dump of encoded data:
8286 8441 0f77 7777 2e65 7861 6d70 6c65 | ...A.www.example 2e63 6f6d | .com
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
41 | == Literal indexed ==
| Indexed name (idx = 1)
| :authority
0f | Literal value (len = 15)
7777 772e 6578 616d 706c 652e 636f 6d | www.example.com
| -> :authority:
| www.example.com
Dynamic Table (after decoding):
[ 1] (s = 57) :authority: www.example.com
Table size: 57
Decoded header list:
:method: GET :scheme: http :path: / :authority: www.example.com
C.3.2 Second Request
Header list to encode:
:method: GET :scheme: http :path: / :authority: www.example.com cache-control: no-cache
Hex dump of encoded data:
8286 84be 5808 6e6f 2d63 6163 6865 | ....X.no-cache
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
be | == Indexed - Add ==
| idx = 62
| -> :authority:
| www.example.com
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
08 | Literal value (len = 8)
6e6f 2d63 6163 6865 | no-cache
| -> cache-control: no-cache
Dynamic Table (after decoding):
[ 1] (s = 53) cache-control: no-cache
[ 2] (s = 57) :authority: www.example.com
Table size: 110
Decoded header list:
:method: GET :scheme: http :path: / :authority: www.example.com cache-control: no-cache
C.3.3 Third Request
Header list to encode:
:method: GET :scheme: https :path: /index.html :authority: www.example.com custom-key: custom-value
Hex dump of encoded data:
8287 85bf 400a 6375 7374 6f6d 2d6b 6579 | ....@.custom-key 0c63 7573 746f 6d2d 7661 6c75 65 | .custom-value
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: https
85 | == Indexed - Add ==
| idx = 5
| -> :path: /index.html
bf | == Indexed - Add ==
| idx = 63
| -> :authority:
| www.example.com
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0c | Literal value (len = 12)
6375 7374 6f6d 2d76 616c 7565 | custom-value
| -> custom-key:
| custom-value
Dynamic Table (after decoding):
[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 53) cache-control: no-cache
[ 3] (s = 57) :authority: www.example.com
Table size: 164
Decoded header list:
:method: GET :scheme: https :path: /index.html :authority: www.example.com custom-key: custom-value
C.4 Request Examples with Huffman Coding
This section shows the same examples as the previous section but uses Huffman encoding for the literal values.
C.4.1 First Request
Header list to encode:
:method: GET :scheme: http :path: / :authority: www.example.com
Hex dump of encoded data:
8286 8441 8cf1 e3c2 e5f2 3a6b a0ab 90f4 | ...A......:k.... ff | .
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
41 | == Literal indexed ==
| Indexed name (idx = 1)
| :authority
8c | Literal value (len = 12)
| Huffman encoded:
f1e3 c2e5 f23a 6ba0 ab90 f4ff | .....:k.....
| Decoded:
| www.example.com
| -> :authority:
| www.example.com
Dynamic Table (after decoding):
[ 1] (s = 57) :authority: www.example.com
Table size: 57
Decoded header list:
:method: GET :scheme: http :path: / :authority: www.example.com
C.4.2 Second Request
Header list to encode:
:method: GET :scheme: http :path: / :authority: www.example.com cache-control: no-cache
Hex dump of encoded data:
8286 84be 5886 a8eb 1064 9cbf | ....X....d..
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
86 | == Indexed - Add ==
| idx = 6
| -> :scheme: http
84 | == Indexed - Add ==
| idx = 4
| -> :path: /
be | == Indexed - Add ==
| idx = 62
| -> :authority:
| www.example.com
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
86 | Literal value (len = 6)
| Huffman encoded:
a8eb 1064 9cbf | ...d..
| Decoded:
| no-cache
| -> cache-control: no-cache
Dynamic Table (after decoding):
[ 1] (s = 53) cache-control: no-cache
[ 2] (s = 57) :authority: www.example.com
Table size: 110
Decoded header list:
:method: GET :scheme: http :path: / :authority: www.example.com cache-control: no-cache
C.4.3 Third Request
Header list to encode:
:method: GET :scheme: https :path: /index.html :authority: www.example.com custom-key: custom-value
Hex dump of encoded data:
8287 85bf 4088 25a8 49e9 5ba9 7d7f 8925 | ....@.%.I.[.}..% a849 e95b b8e8 b4bf | .I.[....
Decoding process:
82 | == Indexed - Add ==
| idx = 2
| -> :method: GET
87 | == Indexed - Add ==
| idx = 7
| -> :scheme: https
85 | == Indexed - Add ==
| idx = 5
| -> :path: /index.html
bf | == Indexed - Add ==
| idx = 63
| -> :authority:
| www.example.com
40 | == Literal indexed ==
88 | Literal name (len = 8)
| Huffman encoded:
25a8 49e9 5ba9 7d7f | %.I.[.}.
| Decoded:
| custom-key
89 | Literal value (len = 9)
| Huffman encoded:
25a8 49e9 5bb8 e8b4 bf | %.I.[....
| Decoded:
| custom-value
| -> custom-key:
| custom-value
Dynamic Table (after decoding):
[ 1] (s = 54) custom-key: custom-value
[ 2] (s = 53) cache-control: no-cache
[ 3] (s = 57) :authority: www.example.com
Table size: 164
Decoded header list:
:method: GET :scheme: https :path: /index.html :authority: www.example.com custom-key: custom-value
C.5 Response Examples without Huffman Coding
This section shows several consecutive header lists, corresponding to HTTP responses, on the same connection. The HTTP/2 setting parameter SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing some evictions to occur.
C.5.1 First Response
Header list to encode:
:status: 302 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
Hex dump of encoded data:
4803 3330 3258 0770 7269 7661 7465 611d | H.302X.privatea. 4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013 2032 303a 3133 3a32 3120 474d 546e 1768 | 20:13:21 GMTn.h 7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp 6c65 2e63 6f6d | le.com
Decoding process:
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 32 | 302
| -> :status: 302
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
07 | Literal value (len = 7)
7072 6976 6174 65 | private
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3120 474d 54 | 20:13:21 GMT
| -> date: Mon, 21 Oct 2013
| 20:13:21 GMT
6e | == Literal indexed ==
| Indexed name (idx = 46)
| location
17 | Literal value (len = 23)
6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
706c 652e 636f 6d | ple.com
| -> location:
| https://www.example.com
Dynamic Table (after decoding):
[ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302
Table size: 222
Decoded header list:
:status: 302 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
C.5.2 Second Response
The (":status", "302") header field is evicted from the dynamic table to free space to allow adding the (":status", "307") header field.
Header list to encode:
:status: 307 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
Hex dump of encoded data:
4803 3330 37c1 c0bf | H.307...
Decoding process:
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
03 | Literal value (len = 3)
3330 37 | 307
| - evict: :status: 302
| -> :status: 307
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
c0 | == Indexed - Add ==
| idx = 64
| -> date: Mon, 21 Oct 2013
| 20:13:21 GMT
bf | == Indexed - Add ==
| idx = 63
| -> location:
| https://www.example.com
Dynamic Table (after decoding):
[ 1] (s = 42) :status: 307
[ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private
Table size: 222
Decoded header list:
:status: 307 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
C.5.3 Third Response
Several header fields are evicted from the dynamic table during the processing of this header list.
Header list to encode:
:status: 200 cache-control: private date: Mon, 21 Oct 2013 20:13:22 GMT location: https://www.example.com content-encoding: gzip set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
Hex dump of encoded data:
88c1 611d 4d6f 6e2c 2032 3120 4f63 7420 | ..a.Mon, 21 Oct 3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM 54c0 5a04 677a 6970 7738 666f 6f3d 4153 | T.Z.gzipw8foo=AS 444a 4b48 514b 425a 584f 5157 454f 5049 | DJKHQKBZXOQWEOPI 5541 5851 5745 4f49 553b 206d 6178 2d61 | UAXQWEOIU; max-a 6765 3d33 3630 303b 2076 6572 7369 6f6e | ge=3600; version 3d31 | =1
Decoding process:
88 | == Indexed - Add ==
| idx = 8
| -> :status: 200
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
1d | Literal value (len = 29)
4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
2032 303a 3133 3a32 3220 474d 54 | 20:13:22 GMT
| - evict: cache-control:
| private
| -> date: Mon, 21 Oct 2013
| 20:13:22 GMT
c0 | == Indexed - Add ==
| idx = 64
| -> location:
| https://www.example.com
5a | == Literal indexed ==
| Indexed name (idx = 26)
| content-encoding
04 | Literal value (len = 4)
677a 6970 | gzip
| - evict: date: Mon, 21 Oct
| 2013 20:13:21 GMT
| -> content-encoding: gzip
77 | == Literal indexed ==
| Indexed name (idx = 55)
| set-cookie
38 | Literal value (len = 56)
666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
206d 6178 2d61 6765 3d33 3630 303b 2076 | max-age=3600; v
6572 7369 6f6e 3d31 | ersion=1
| - evict: location:
| https://www.example.com
| - evict: :status: 307
| -> set-cookie: foo=ASDJKHQ
| KBZXOQWEOPIUAXQWEOIU; ma
| x-age=3600; version=1
Dynamic Table (after decoding):
[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU;
max-age=3600; version=1
[ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215
Decoded header list:
:status: 200 cache-control: private date: Mon, 21 Oct 2013 20:13:22 GMT location: https://www.example.com content-encoding: gzip set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
C.6 Response Examples with Huffman Coding
This section shows the same examples as the previous section but uses Huffman encoding for the literal values. The HTTP/2 setting parameter SETTINGS_HEADER_TABLE_SIZE is set to the value of 256 octets, causing some evictions to occur. The eviction mechanism uses the length of the decoded literal values, so the same evictions occur as in the previous section.
C.6.1 First Response
Header list to encode:
:status: 302 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
Hex dump of encoded data:
4882 6402 5885 aec3 771a 4b61 96d0 7abe | H.d.X...w.Ka..z. 9410 54d4 44a8 2005 9504 0b81 66e0 82a6 | ..T.D. .....f... 2d1b ff6e 919d 29ad 1718 63c7 8f0b 97c8 | -..n..)...c..... e9ae 82ae 43d3 | ....C.
Decoding process:
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
82 | Literal value (len = 2)
| Huffman encoded:
6402 | d.
| Decoded:
| 302
| -> :status: 302
58 | == Literal indexed ==
| Indexed name (idx = 24)
| cache-control
85 | Literal value (len = 5)
| Huffman encoded:
aec3 771a 4b | ..w.K
| Decoded:
| private
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
96 | Literal value (len = 22)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e082 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:21
| GMT
| -> date: Mon, 21 Oct 2013
| 20:13:21 GMT
6e | == Literal indexed ==
| Indexed name (idx = 46)
| location
91 | Literal value (len = 17)
| Huffman encoded:
9d29 ad17 1863 c78f 0b97 c8e9 ae82 ae43 | .)...c.........C
d3 | .
| Decoded:
| https://www.example.com
| -> location:
| https://www.example.com
Dynamic Table (after decoding):
[ 1] (s = 63) location: https://www.example.com
[ 2] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 3] (s = 52) cache-control: private
[ 4] (s = 42) :status: 302
Table size: 222
Decoded header list:
:status: 302 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
C.6.2 Second Response
The (":status", "302") header field is evicted from the dynamic table to free space to allow adding the (":status", "307") header field.
Header list to encode:
:status: 307 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
Hex dump of encoded data:
4883 640e ffc1 c0bf | H.d.....
Decoding process:
48 | == Literal indexed ==
| Indexed name (idx = 8)
| :status
83 | Literal value (len = 3)
| Huffman encoded:
640e ff | d..
| Decoded:
| 307
| - evict: :status: 302
| -> :status: 307
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
c0 | == Indexed - Add ==
| idx = 64
| -> date: Mon, 21 Oct 2013
| 20:13:21 GMT
bf | == Indexed - Add ==
| idx = 63
| -> location:
| https://www.example.com
Dynamic Table (after decoding):
[ 1] (s = 42) :status: 307
[ 2] (s = 63) location: https://www.example.com
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:21 GMT
[ 4] (s = 52) cache-control: private
Table size: 222
Decoded header list:
:status: 307 cache-control: private date: Mon, 21 Oct 2013 20:13:21 GMT location: https://www.example.com
C.6.3 Third Response
Several header fields are evicted from the dynamic table during the processing of this header list.
Header list to encode:
:status: 200 cache-control: private date: Mon, 21 Oct 2013 20:13:22 GMT location: https://www.example.com content-encoding: gzip set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
Hex dump of encoded data:
88c1 6196 d07a be94 1054 d444 a820 0595 | ..a..z...T.D. .. 040b 8166 e084 a62d 1bff c05a 839b d9ab | ...f...-...Z.... 77ad 94e7 821d d7f2 e6c7 b335 dfdf cd5b | w..........5...[ 3960 d5af 2708 7f36 72c1 ab27 0fb5 291f | 9`..'..6r..'..). 9587 3160 65c0 03ed 4ee5 b106 3d50 07 | ..1`e...N...=P.
Decoding process:
88 | == Indexed - Add ==
| idx = 8
| -> :status: 200
c1 | == Indexed - Add ==
| idx = 65
| -> cache-control: private
61 | == Literal indexed ==
| Indexed name (idx = 33)
| date
96 | Literal value (len = 22)
| Huffman encoded:
d07a be94 1054 d444 a820 0595 040b 8166 | .z...T.D. .....f
e084 a62d 1bff | ...-..
| Decoded:
| Mon, 21 Oct 2013 20:13:22
| GMT
| - evict: cache-control:
| private
| -> date: Mon, 21 Oct 2013
| 20:13:22 GMT
c0 | == Indexed - Add ==
| idx = 64
| -> location:
| https://www.example.com
5a | == Literal indexed ==
| Indexed name (idx = 26)
| content-encoding
83 | Literal value (len = 3)
| Huffman encoded:
9bd9 ab | ...
| Decoded:
| gzip
| - evict: date: Mon, 21 Oct
| 2013 20:13:21 GMT
| -> content-encoding: gzip
77 | == Literal indexed ==
| Indexed name (idx = 55)
| set-cookie
ad | Literal value (len = 45)
| Huffman encoded:
94e7 821d d7f2 e6c7 b335 dfdf cd5b 3960 | .........5...[9`
d5af 2708 7f36 72c1 ab27 0fb5 291f 9587 | ..'..6r..'..)...
3160 65c0 03ed 4ee5 b106 3d50 07 | 1`e...N...=P.
| Decoded:
| foo=ASDJKHQKBZXOQWEOPIUAXQ
| WEOIU; max-age=3600; versi
| on=1
| - evict: location:
| https://www.example.com
| - evict: :status: 307
| -> set-cookie: foo=ASDJKHQ
| KBZXOQWEOPIUAXQWEOIU; ma
| x-age=3600; version=1
Dynamic Table (after decoding):
[ 1] (s = 98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU;
max-age=3600; version=1
[ 2] (s = 52) content-encoding: gzip
[ 3] (s = 65) date: Mon, 21 Oct 2013 20:13:22 GMT
Table size: 215
Decoded header list:
:status: 200 cache-control: private date: Mon, 21 Oct 2013 20:13:22 GMT location: https://www.example.com content-encoding: gzip set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
Acknowledgments
This specification includes substantial input from the following individuals:
- Mike Bishop, Jeff Pinner, Julian Reschke, and Martin Thomson (substantial editorial contributions).
- Johnny Graettinger (Huffman code statistics).
Authors' Addresses
Roberto Peon
Google, Inc
Email: fenix@google.com
Hervé Ruellan
Canon CRF
Email: herve.ruellan@crf.canon.fr
1
https://httpwg.github.io/specs/rfc7540.html
| Internet Engineering Task Force (IETF) | M. Belshe |
| Request for Comments: 7540 | BitGo |
| Category: Standards Track | R. Peon |
| ISSN: 2070-1721 | Google, Inc |
| M. Thomson, Editor | |
| Mozilla | |
| May 2015 |
Hypertext Transfer Protocol Version 2 (HTTP/2)
Abstract
This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients.
This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged.
This document has errata.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7540.
Copyright Notice
Copyright © 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
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1. Introduction
The Hypertext Transfer Protocol (HTTP) is a wildly successful protocol. However, the way HTTP/1.1 uses the underlying transport ([RFC7230], Section 6) has several characteristics that have a negative overall effect on application performance today.
In particular, HTTP/1.0 allowed only one request to be outstanding at a time on a given TCP connection. HTTP/1.1 added request pipelining, but this only partially addressed request concurrency and still suffers from head-of-line blocking. Therefore, HTTP/1.0 and HTTP/1.1 clients that need to make many requests use multiple connections to a server in order to achieve concurrency and thereby reduce latency.
Furthermore, HTTP header fields are often repetitive and verbose, causing unnecessary network traffic as well as causing the initial TCP [TCP] congestion window to quickly fill. This can result in excessive latency when multiple requests are made on a new TCP connection.
HTTP/2 addresses these issues by defining an optimized mapping of HTTP's semantics to an underlying connection. Specifically, it allows interleaving of request and response messages on the same connection and uses an efficient coding for HTTP header fields. It also allows prioritization of requests, letting more important requests complete more quickly, further improving performance.
The resulting protocol is more friendly to the network because fewer TCP connections can be used in comparison to HTTP/1.x. This means less competition with other flows and longer-lived connections, which in turn lead to better utilization of available network capacity.
Finally, HTTP/2 also enables more efficient processing of messages through use of binary message framing.
2. HTTP/2 Protocol Overview
HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2 supports all of the core features of HTTP/1.1 but aims to be more efficient in several ways.
The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each frame type serves a different purpose. For example, HEADERS and DATA frames form the basis of HTTP requests and responses (Section 8.1); other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are used in support of other HTTP/2 features.
Multiplexing of requests is achieved by having each HTTP request/response exchange associated with its own stream (Section 5). Streams are largely independent of each other, so a blocked or stalled request or response does not prevent progress on other streams.
Flow control and prioritization ensure that it is possible to efficiently use multiplexed streams. Flow control (Section 5.2) helps to ensure that only data that can be used by a receiver is transmitted. Prioritization (Section 5.3) ensures that limited resources can be directed to the most important streams first.
HTTP/2 adds a new interaction mode whereby a server can push responses to a client (Section 8.2). Server push allows a server to speculatively send data to a client that the server anticipates the client will need, trading off some network usage against a potential latency gain. The server does this by synthesizing a request, which it sends as a PUSH_PROMISE frame. The server is then able to send a response to the synthetic request on a separate stream.
Because HTTP header fields used in a connection can contain large amounts of redundant data, frames that contain them are compressed (Section 4.3). This has especially advantageous impact upon request sizes in the common case, allowing many requests to be compressed into one packet.
2.1 Document Organization
The HTTP/2 specification is split into four parts:
- Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is initiated.
- The frame (Section 4) and stream (Section 5) layers describe the way HTTP/2 frames are structured and formed into multiplexed streams.
- Frame (Section 6) and error (Section 7) definitions include details of the frame and error types used in HTTP/2.
- HTTP mappings (Section 8) and additional requirements (Section 9) describe how HTTP semantics are expressed using frames and streams.
While some of the frame and stream layer concepts are isolated from HTTP, this specification does not define a completely generic frame layer. The frame and stream layers are tailored to the needs of the HTTP protocol and server push.
2.2 Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
All numeric values are in network byte order. Values are unsigned unless otherwise indicated. Literal values are provided in decimal or hexadecimal as appropriate. Hexadecimal literals are prefixed with 0x to distinguish them from decimal literals.
The following terms are used:
- client:
- The endpoint that initiates an HTTP/2 connection. Clients send HTTP requests and receive HTTP responses.
- connection:
- A transport-layer connection between two endpoints.
- connection error:
- An error that affects the entire HTTP/2 connection.
- endpoint:
- Either the client or server of the connection.
- frame:
- The smallest unit of communication within an HTTP/2 connection, consisting of a header and a variable-length sequence of octets structured according to the frame type.
- peer:
- An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint that is remote to the primary subject of discussion.
- receiver:
- An endpoint that is receiving frames.
- sender:
- An endpoint that is transmitting frames.
- server:
- The endpoint that accepts an HTTP/2 connection. Servers receive HTTP requests and send HTTP responses.
- stream:
- A bidirectional flow of frames within the HTTP/2 connection.
- stream error:
- An error on the individual HTTP/2 stream.
Finally, the terms "gateway", "intermediary", "proxy", and "tunnel" are defined in Section 2.3 of [RFC7230]. Intermediaries act as both client and server at different times.
The term "payload body" is defined in Section 3.3 of [RFC7230].
3. Starting HTTP/2
An HTTP/2 connection is an application-layer protocol running on top of a TCP connection ([TCP]). The client is the TCP connection initiator.
HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1. HTTP/2 shares the same default port numbers: 80 for "http" URIs and 443 for "https" URIs. As a result, implementations processing requests for target resource URIs like http://example.org/foo or https://example.com/bar are required to first discover whether the upstream server (the immediate peer to which the client wishes to establish a connection) supports HTTP/2.
The means by which support for HTTP/2 is determined is different for "http" and "https" URIs. Discovery for "http" URIs is described in Section 3.2. Discovery for "https" URIs is described in Section 3.3.
3.1 HTTP/2 Version Identification
The protocol defined in this document has two identifiers.
-
The string "h2" identifies the protocol where HTTP/2 uses Transport Layer Security (TLS) [TLS12]. This identifier is used in the TLS application-layer protocol negotiation (ALPN) extension [TLS-ALPN] field and in any place where HTTP/2 over TLS is identified.
The "h2" string is serialized into an ALPN protocol identifier as the two-octet sequence: 0x68, 0x32.
-
The string "h2c" identifies the protocol where HTTP/2 is run over cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade header field and in any place where HTTP/2 over TCP is identified.
The "h2c" string is reserved from the ALPN identifier space but describes a protocol that does not use TLS.
Negotiating "h2" or "h2c" implies the use of the transport, security, framing, and message semantics described in this document.
3.2 Starting HTTP/2 for "http" URIs
A client that makes a request for an "http" URI without prior knowledge about support for HTTP/2 on the next hop uses the HTTP Upgrade mechanism (Section 6.7 of [RFC7230]). The client does so by making an HTTP/1.1 request that includes an Upgrade header field with the "h2c" token. Such an HTTP/1.1 request MUST include exactly one HTTP2-Settings (Section 3.2.1) header field.
For example:
GET / HTTP/1.1 Host: server.example.com Connection: Upgrade, HTTP2-Settings Upgrade: h2c HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
Requests that contain a payload body MUST be sent in their entirety before the client can send HTTP/2 frames. This means that a large request can block the use of the connection until it is completely sent.
If concurrency of an initial request with subsequent requests is important, an OPTIONS request can be used to perform the upgrade to HTTP/2, at the cost of an additional round trip.
A server that does not support HTTP/2 can respond to the request as though the Upgrade header field were absent:
HTTP/1.1 200 OK Content-Length: 243 Content-Type: text/html ...
A server MUST ignore an "h2" token in an Upgrade header field. Presence of a token with "h2" implies HTTP/2 over TLS, which is instead negotiated as described in Section 3.3.
A server that supports HTTP/2 accepts the upgrade with a 101 (Switching Protocols) response. After the empty line that terminates the 101 response, the server can begin sending HTTP/2 frames. These frames MUST include a response to the request that initiated the upgrade.
For example:
HTTP/1.1 101 Switching Protocols Connection: Upgrade Upgrade: h2c [ HTTP/2 connection ...
The first HTTP/2 frame sent by the server MUST be a server connection preface (Section 3.5) consisting of a SETTINGS frame (Section 6.5). Upon receiving the 101 response, the client MUST send a connection preface (Section 3.5), which includes a SETTINGS frame.
The HTTP/1.1 request that is sent prior to upgrade is assigned a stream identifier of 1 (see Section 5.1.1) with default priority values (Section 5.3.5). Stream 1 is implicitly "half-closed" from the client toward the server (see Section 5.1), since the request is completed as an HTTP/1.1 request. After commencing the HTTP/2 connection, stream 1 is used for the response.
3.2.1 HTTP2-Settings Header Field
A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one HTTP2-Settings header field. The HTTP2-Settings header field is a connection-specific header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.
HTTP2-Settings = token68
A server MUST NOT upgrade the connection to HTTP/2 if this header field is not present or if more than one is present. A server MUST NOT send this header field.
The content of the HTTP2-Settings header field is the payload of a SETTINGS frame (Section 6.5), encoded as a base64url string (that is, the URL- and filename-safe Base64 encoding described in Section 5 of [RFC4648], with any trailing '=' characters omitted). The ABNF [RFC5234] production for token68 is defined in Section 2.1 of [RFC7235].
Since the upgrade is only intended to apply to the immediate connection, a client sending the HTTP2-Settings header field MUST also send HTTP2-Settings as a connection option in the Connection header field to prevent it from being forwarded (see Section 6.1 of [RFC7230]).
A server decodes and interprets these values as it would any other SETTINGS frame. Explicit acknowledgement of these settings (Section 6.5.3) is not necessary, since a 101 response serves as implicit acknowledgement. Providing these values in the upgrade request gives a client an opportunity to provide parameters prior to receiving any frames from the server.
3.3 Starting HTTP/2 for "https" URIs
A client that makes a request to an "https" URI uses TLS [TLS12] with the application-layer protocol negotiation (ALPN) extension [TLS-ALPN].
HTTP/2 over TLS uses the "h2" protocol identifier. The "h2c" protocol identifier MUST NOT be sent by a client or selected by a server; the "h2c" protocol identifier describes a protocol that does not use TLS.
Once TLS negotiation is complete, both the client and the server MUST send a connection preface (Section 3.5).
3.4 Starting HTTP/2 with Prior Knowledge
A client can learn that a particular server supports HTTP/2 by other means. For example, [ALT-SVC] describes a mechanism for advertising this capability.
A client MUST send the connection preface (Section 3.5) and then MAY immediately send HTTP/2 frames to such a server; servers can identify these connections by the presence of the connection preface. This only affects the establishment of HTTP/2 connections over cleartext TCP; implementations that support HTTP/2 over TLS MUST use protocol negotiation in TLS [TLS-ALPN].
Likewise, the server MUST send a connection preface (Section 3.5).
Without additional information, prior support for HTTP/2 is not a strong signal that a given server will support HTTP/2 for future connections. For example, it is possible for server configurations to change, for configurations to differ between instances in clustered servers, or for network conditions to change.
3.5 HTTP/2 Connection Preface
In HTTP/2, each endpoint is required to send a connection preface as a final confirmation of the protocol in use and to establish the initial settings for the HTTP/2 connection. The client and server each send a different connection preface.
The client connection preface starts with a sequence of 24 octets, which in hex notation is:
0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
That is, the connection preface starts with the string PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n). This sequence MUST be followed by a SETTINGS frame (Section 6.5), which MAY be empty. The client sends the client connection preface immediately upon receipt of a 101 (Switching Protocols) response (indicating a successful upgrade) or as the first application data octets of a TLS connection. If starting an HTTP/2 connection with prior knowledge of server support for the protocol, the client connection preface is sent upon connection establishment.
Note: The client connection preface is selected so that a large proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do not attempt to process further frames. Note that this does not address the concerns raised in [TALKING].
The server connection preface consists of a potentially empty SETTINGS frame (Section 6.5) that MUST be the first frame the server sends in the HTTP/2 connection.
The SETTINGS frames received from a peer as part of the connection preface MUST be acknowledged (see Section 6.5.3) after sending the connection preface.
To avoid unnecessary latency, clients are permitted to send additional frames to the server immediately after sending the client connection preface, without waiting to receive the server connection preface. It is important to note, however, that the server connection preface SETTINGS frame might include parameters that necessarily alter how a client is expected to communicate with the server. Upon receiving the SETTINGS frame, the client is expected to honor any parameters established. In some configurations, it is possible for the server to transmit SETTINGS before the client sends additional frames, providing an opportunity to avoid this issue.
Clients and servers MUST treat an invalid connection preface as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. A GOAWAY frame (Section 6.8) MAY be omitted in this case, since an invalid preface indicates that the peer is not using HTTP/2.
4. HTTP Frames
Once the HTTP/2 connection is established, endpoints can begin exchanging frames.
4.1 Frame Format
All frames begin with a fixed 9-octet header followed by a variable-length payload.
+-----------------------------------------------+ | Length (24) | +---------------+---------------+---------------+ | Type (8) | Flags (8) | +-+-------------+---------------+-------------------------------+ |R| Stream Identifier (31) | +=+=============================================================+ | Frame Payload (0...) ... +---------------------------------------------------------------+
Figure 1: Frame Layout
The fields of the frame header are defined as:
- Length:
-
The length of the frame payload expressed as an unsigned 24-bit integer. Values greater than 214 (16,384) MUST NOT be sent unless the receiver has set a larger value for SETTINGS_MAX_FRAME_SIZE.
The 9 octets of the frame header are not included in this value.
- Type:
-
The 8-bit type of the frame. The frame type determines the format and semantics of the frame. Implementations MUST ignore and discard any frame that has a type that is unknown.
- Flags:
-
An 8-bit field reserved for boolean flags specific to the frame type.
Flags are assigned semantics specific to the indicated frame type. Flags that have no defined semantics for a particular frame type MUST be ignored and MUST be left unset (0x0) when sending.
- R:
-
A reserved 1-bit field. The semantics of this bit are undefined, and the bit MUST remain unset (0x0) when sending and MUST be ignored when receiving.
- Stream Identifier:
-
A stream identifier (see Section 5.1.1) expressed as an unsigned 31-bit integer. The value 0x0 is reserved for frames that are associated with the connection as a whole as opposed to an individual stream.
The structure and content of the frame payload is dependent entirely on the frame type.
4.2 Frame Size
The size of a frame payload is limited by the maximum size that a receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This setting can have any value between 214 (16,384) and 224-1 (16,777,215) octets, inclusive.
All implementations MUST be capable of receiving and minimally processing frames up to 214 octets in length, plus the 9-octet frame header (Section 4.1). The size of the frame header is not included when describing frame sizes.
Note: Certain frame types, such as PING (Section 6.7), impose additional limits on the amount of payload data allowed.
An endpoint MUST send an error code of FRAME_SIZE_ERROR if a frame exceeds the size defined in SETTINGS_MAX_FRAME_SIZE, exceeds any limit defined for the frame type, or is too small to contain mandatory frame data. A frame size error in a frame that could alter the state of the entire connection MUST be treated as a connection error (Section 5.4.1); this includes any frame carrying a header block (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and CONTINUATION), SETTINGS, and any frame with a stream identifier of 0.
Endpoints are not obligated to use all available space in a frame. Responsiveness can be improved by using frames that are smaller than the permitted maximum size. Sending large frames can result in delays in sending time-sensitive frames (such as RST_STREAM, WINDOW_UPDATE, or PRIORITY), which, if blocked by the transmission of a large frame, could affect performance.
4.3 Header Compression and Decompression
Just as in HTTP/1, a header field in HTTP/2 is a name with one or more associated values. Header fields are used within HTTP request and response messages as well as in server push operations (see Section 8.2).
Header lists are collections of zero or more header fields. When transmitted over a connection, a header list is serialized into a header block using HTTP header compression [COMPRESSION]. The serialized header block is then divided into one or more octet sequences, called header block fragments, and transmitted within the payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6), or CONTINUATION (Section 6.10) frames.
The Cookie header field [COOKIE] is treated specially by the HTTP mapping (see Section 8.1.2.5).
A receiving endpoint reassembles the header block by concatenating its fragments and then decompresses the block to reconstruct the header list.
A complete header block consists of either:
- a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag set, or
- a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared and one or more CONTINUATION frames, where the last CONTINUATION frame has the END_HEADERS flag set.
Header compression is stateful. One compression context and one decompression context are used for the entire connection. A decoding error in a header block MUST be treated as a connection error (Section 5.4.1) of type COMPRESSION_ERROR.
Each header block is processed as a discrete unit. Header blocks MUST be transmitted as a contiguous sequence of frames, with no interleaved frames of any other type or from any other stream. The last frame in a sequence of HEADERS or CONTINUATION frames has the END_HEADERS flag set. The last frame in a sequence of PUSH_PROMISE or CONTINUATION frames has the END_HEADERS flag set. This allows a header block to be logically equivalent to a single frame.
Header block fragments can only be sent as the payload of HEADERS, PUSH_PROMISE, or CONTINUATION frames because these frames carry data that can modify the compression context maintained by a receiver. An endpoint receiving HEADERS, PUSH_PROMISE, or CONTINUATION frames needs to reassemble header blocks and perform decompression even if the frames are to be discarded. A receiver MUST terminate the connection with a connection error (Section 5.4.1) of type COMPRESSION_ERROR if it does not decompress a header block.
5. Streams and Multiplexing
A "stream" is an independent, bidirectional sequence of frames exchanged between the client and server within an HTTP/2 connection. Streams have several important characteristics:
- A single HTTP/2 connection can contain multiple concurrently open streams, with either endpoint interleaving frames from multiple streams.
- Streams can be established and used unilaterally or shared by either the client or server.
- Streams can be closed by either endpoint.
- The order in which frames are sent on a stream is significant. Recipients process frames in the order they are received. In particular, the order of HEADERS and DATA frames is semantically significant.
- Streams are identified by an integer. Stream identifiers are assigned to streams by the endpoint initiating the stream.
5.1 Stream States
The lifecycle of a stream is shown in Figure 2.
+--------+
send PP | | recv PP
,--------| idle |--------.
/ | | \
v +--------+ v
+----------+ | +----------+
| | | send H / | |
,------| reserved | | recv H | reserved |------.
| | (local) | | | (remote) | |
| +----------+ v +----------+ |
| | +--------+ | |
| | recv ES | | send ES | |
| send H | ,-------| open |-------. | recv H |
| | / | | \ | |
| v v +--------+ v v |
| +----------+ | +----------+ |
| | half | | | half | |
| | closed | | send R / | closed | |
| | (remote) | | recv R | (local) | |
| +----------+ | +----------+ |
| | | | |
| | send ES / | recv ES / | |
| | send R / v send R / | |
| | recv R +--------+ recv R | |
| send R / `----------->| |<-----------' send R / |
| recv R | closed | recv R |
`----------------------->| |<----------------------'
+--------+
send: endpoint sends this frame
recv: endpoint receives this frame
H: HEADERS frame (with implied CONTINUATIONs)
PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
ES: END_STREAM flag
R: RST_STREAM frame
Figure 2: Stream States
Note that this diagram shows stream state transitions and the frames and flags that affect those transitions only. In this regard, CONTINUATION frames do not result in state transitions; they are effectively part of the HEADERS or PUSH_PROMISE that they follow. For the purpose of state transitions, the END_STREAM flag is processed as a separate event to the frame that bears it; a HEADERS frame with the END_STREAM flag set can cause two state transitions.
Both endpoints have a subjective view of the state of a stream that could be different when frames are in transit. Endpoints do not coordinate the creation of streams; they are created unilaterally by either endpoint. The negative consequences of a mismatch in states are limited to the "closed" state after sending RST_STREAM, where frames might be received for some time after closing.
Streams have the following states:
- idle:
-
All streams start in the "idle" state.The following transitions are valid from this state:
- Sending or receiving a HEADERS frame causes the stream to become "open". The stream identifier is selected as described in Section 5.1.1. The same HEADERS frame can also cause a stream to immediately become "half-closed".
- Sending a PUSH_PROMISE frame on another stream reserves the idle stream that is identified for later use. The stream state for the reserved stream transitions to "reserved (local)".
- Receiving a PUSH_PROMISE frame on another stream reserves an idle stream that is identified for later use. The stream state for the reserved stream transitions to "reserved (remote)".
- Note that the PUSH_PROMISE frame is not sent on the idle stream but references the newly reserved stream in the Promised Stream ID field.
Receiving any frame other than HEADERS or PRIORITY on a stream in this state MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
- reserved (local):
-
A stream in the "reserved (local)" state is one that has been promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame reserves an idle stream by associating the stream with an open stream that was initiated by the remote peer (see Section 8.2).In this state, only the following transitions are possible:
- The endpoint can send a HEADERS frame. This causes the stream to open in a "half-closed (remote)" state.
- Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.
An endpoint MUST NOT send any type of frame other than HEADERS, RST_STREAM, or PRIORITY in this state.
A PRIORITY or WINDOW_UPDATE frame MAY be received in this state. Receiving any type of frame other than RST_STREAM, PRIORITY, or WINDOW_UPDATE on a stream in this state MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
- reserved (remote):
-
A stream in the "reserved (remote)" state has been reserved by a remote peer.In this state, only the following transitions are possible:
- Receiving a HEADERS frame causes the stream to transition to "half-closed (local)".
- Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.
An endpoint MAY send a PRIORITY frame in this state to reprioritize the reserved stream. An endpoint MUST NOT send any type of frame other than RST_STREAM, WINDOW_UPDATE, or PRIORITY in this state.
Receiving any type of frame other than HEADERS, RST_STREAM, or PRIORITY on a stream in this state MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
- open:
-
A stream in the "open" state may be used by both peers to send frames of any type. In this state, sending peers observe advertised stream-level flow-control limits (Section 5.2).From this state, either endpoint can send a frame with an END_STREAM flag set, which causes the stream to transition into one of the "half-closed" states. An endpoint sending an END_STREAM flag causes the stream state to become "half-closed (local)"; an endpoint receiving an END_STREAM flag causes the stream state to become "half-closed (remote)".
Either endpoint can send a RST_STREAM frame from this state, causing it to transition immediately to "closed".
- half-closed (local):
-
A stream that is in the "half-closed (local)" state cannot be used for sending frames other than WINDOW_UPDATE, PRIORITY, and RST_STREAM.A stream transitions from this state to "closed" when a frame that contains an END_STREAM flag is received or when either peer sends a RST_STREAM frame.
An endpoint can receive any type of frame in this state. Providing flow-control credit using WINDOW_UPDATE frames is necessary to continue receiving flow-controlled frames. In this state, a receiver can ignore WINDOW_UPDATE frames, which might arrive for a short period after a frame bearing the END_STREAM flag is sent.
PRIORITY frames received in this state are used to reprioritize streams that depend on the identified stream.
- half-closed (remote):
-
A stream that is "half-closed (remote)" is no longer being used by the peer to send frames. In this state, an endpoint is no longer obligated to maintain a receiver flow-control window.If an endpoint receives additional frames, other than WINDOW_UPDATE, PRIORITY, or RST_STREAM, for a stream that is in this state, it MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.
A stream that is "half-closed (remote)" can be used by the endpoint to send frames of any type. In this state, the endpoint continues to observe advertised stream-level flow-control limits (Section 5.2).
A stream can transition from this state to "closed" by sending a frame that contains an END_STREAM flag or when either peer sends a RST_STREAM frame.
- closed:
-
The "closed" state is the terminal state.An endpoint MUST NOT send frames other than PRIORITY on a closed stream. An endpoint that receives any frame other than PRIORITY after receiving a RST_STREAM MUST treat that as a stream error (Section 5.4.2) of type STREAM_CLOSED. Similarly, an endpoint that receives any frames after receiving a frame with the END_STREAM flag set MUST treat that as a connection error (Section 5.4.1) of type STREAM_CLOSED, unless the frame is permitted as described below.
WINDOW_UPDATE or RST_STREAM frames can be received in this state for a short period after a DATA or HEADERS frame containing an END_STREAM flag is sent. Until the remote peer receives and processes RST_STREAM or the frame bearing the END_STREAM flag, it might send frames of these types. Endpoints MUST ignore WINDOW_UPDATE or RST_STREAM frames received in this state, though endpoints MAY choose to treat frames that arrive a significant time after sending END_STREAM as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
PRIORITY frames can be sent on closed streams to prioritize streams that are dependent on the closed stream. Endpoints SHOULD process PRIORITY frames, though they can be ignored if the stream has been removed from the dependency tree (see Section 5.3.4).
If this state is reached as a result of sending a RST_STREAM frame, the peer that receives the RST_STREAM might have already sent — or enqueued for sending — frames on the stream that cannot be withdrawn. An endpoint MUST ignore frames that it receives on closed streams after it has sent a RST_STREAM frame. An endpoint MAY choose to limit the period over which it ignores frames and treat frames that arrive after this time as being in error.
Flow-controlled frames (i.e., DATA) received after sending RST_STREAM are counted toward the connection flow-control window. Even though these frames might be ignored, because they are sent before the sender receives the RST_STREAM, the sender will consider the frames to count against the flow-control window.
An endpoint might receive a PUSH_PROMISE frame after it sends RST_STREAM. PUSH_PROMISE causes a stream to become "reserved" even if the associated stream has been reset. Therefore, a RST_STREAM is needed to close an unwanted promised stream.
In the absence of more specific guidance elsewhere in this document, implementations SHOULD treat the receipt of a frame that is not expressly permitted in the description of a state as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that PRIORITY can be sent and received in any stream state. Frames of unknown types are ignored.
An example of the state transitions for an HTTP request/response exchange can be found in Section 8.1. An example of the state transitions for server push can be found in Sections 8.2.1 and 8.2.2.
5.1.1 Stream Identifiers
Streams are identified with an unsigned 31-bit integer. Streams initiated by a client MUST use odd-numbered stream identifiers; those initiated by the server MUST use even-numbered stream identifiers. A stream identifier of zero (0x0) is used for connection control messages; the stream identifier of zero cannot be used to establish a new stream.
HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are responded to with a stream identifier of one (0x1). After the upgrade completes, stream 0x1 is "half-closed (local)" to the client. Therefore, stream 0x1 cannot be selected as a new stream identifier by a client that upgrades from HTTP/1.1.
The identifier of a newly established stream MUST be numerically greater than all streams that the initiating endpoint has opened or reserved. This governs streams that are opened using a HEADERS frame and streams that are reserved using PUSH_PROMISE. An endpoint that receives an unexpected stream identifier MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The first use of a new stream identifier implicitly closes all streams in the "idle" state that might have been initiated by that peer with a lower-valued stream identifier. For example, if a client sends a HEADERS frame on stream 7 without ever sending a frame on stream 5, then stream 5 transitions to the "closed" state when the first frame for stream 7 is sent or received.
Stream identifiers cannot be reused. Long-lived connections can result in an endpoint exhausting the available range of stream identifiers. A client that is unable to establish a new stream identifier can establish a new connection for new streams. A server that is unable to establish a new stream identifier can send a GOAWAY frame so that the client is forced to open a new connection for new streams.
5.1.2 Stream Concurrency
A peer can limit the number of concurrently active streams using the SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within a SETTINGS frame. The maximum concurrent streams setting is specific to each endpoint and applies only to the peer that receives the setting. That is, clients specify the maximum number of concurrent streams the server can initiate, and servers specify the maximum number of concurrent streams the client can initiate.
Streams that are in the "open" state or in either of the "half-closed" states count toward the maximum number of streams that an endpoint is permitted to open. Streams in any of these three states count toward the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the "reserved" states do not count toward the stream limit.
Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a HEADERS frame that causes its advertised concurrent stream limit to be exceeded MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR or REFUSED_STREAM. The choice of error code determines whether the endpoint wishes to enable automatic retry (see Section 8.1.4) for details).
An endpoint that wishes to reduce the value of SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current number of open streams can either close streams that exceed the new value or allow streams to complete.
5.2 Flow Control
Using streams for multiplexing introduces contention over use of the TCP connection, resulting in blocked streams. A flow-control scheme ensures that streams on the same connection do not destructively interfere with each other. Flow control is used for both individual streams and for the connection as a whole.
HTTP/2 provides for flow control through use of the WINDOW_UPDATE frame (Section 6.9).
5.2.1 Flow-Control Principles
HTTP/2 stream flow control aims to allow a variety of flow-control algorithms to be used without requiring protocol changes. Flow control in HTTP/2 has the following characteristics:
- Flow control is specific to a connection. Both types of flow control are between the endpoints of a single hop and not over the entire end-to-end path.
- Flow control is based on WINDOW_UPDATE frames. Receivers advertise how many octets they are prepared to receive on a stream and for the entire connection. This is a credit-based scheme.
- Flow control is directional with overall control provided by the receiver. A receiver MAY choose to set any window size that it desires for each stream and for the entire connection. A sender MUST respect flow-control limits imposed by a receiver. Clients, servers, and intermediaries all independently advertise their flow-control window as a receiver and abide by the flow-control limits set by their peer when sending.
- The initial value for the flow-control window is 65,535 octets for both new streams and the overall connection.
- The frame type determines whether flow control applies to a frame. Of the frames specified in this document, only DATA frames are subject to flow control; all other frame types do not consume space in the advertised flow-control window. This ensures that important control frames are not blocked by flow control.
- Flow control cannot be disabled.
- HTTP/2 defines only the format and semantics of the WINDOW_UPDATE frame (Section 6.9). This document does not stipulate how a receiver decides when to send this frame or the value that it sends, nor does it specify how a sender chooses to send packets. Implementations are able to select any algorithm that suits their needs.
Implementations are also responsible for managing how requests and responses are sent based on priority, choosing how to avoid head-of-line blocking for requests, and managing the creation of new streams. Algorithm choices for these could interact with any flow-control algorithm.
5.2.2 Appropriate Use of Flow Control
Flow control is defined to protect endpoints that are operating under resource constraints. For example, a proxy needs to share memory between many connections and also might have a slow upstream connection and a fast downstream one. Flow-control addresses cases where the receiver is unable to process data on one stream yet wants to continue to process other streams in the same connection.
Deployments that do not require this capability can advertise a flow-control window of the maximum size (231-1) and can maintain this window by sending a WINDOW_UPDATE frame when any data is received. This effectively disables flow control for that receiver. Conversely, a sender is always subject to the flow-control window advertised by the receiver.
Deployments with constrained resources (for example, memory) can employ flow control to limit the amount of memory a peer can consume. Note, however, that this can lead to suboptimal use of available network resources if flow control is enabled without knowledge of the bandwidth-delay product (see [RFC7323]).
Even with full awareness of the current bandwidth-delay product, implementation of flow control can be difficult. When using flow control, the receiver MUST read from the TCP receive buffer in a timely fashion. Failure to do so could lead to a deadlock when critical frames, such as WINDOW_UPDATE, are not read and acted upon.
5.3 Stream Priority
A client can assign a priority for a new stream by including prioritization information in the HEADERS frame (Section 6.2) that opens the stream. At any other time, the PRIORITY frame (Section 6.3) can be used to change the priority of a stream.
The purpose of prioritization is to allow an endpoint to express how it would prefer its peer to allocate resources when managing concurrent streams. Most importantly, priority can be used to select streams for transmitting frames when there is limited capacity for sending.
Streams can be prioritized by marking them as dependent on the completion of other streams (Section 5.3.1). Each dependency is assigned a relative weight, a number that is used to determine the relative proportion of available resources that are assigned to streams dependent on the same stream.
Explicitly setting the priority for a stream is input to a prioritization process. It does not guarantee any particular processing or transmission order for the stream relative to any other stream. An endpoint cannot force a peer to process concurrent streams in a particular order using priority. Expressing priority is therefore only a suggestion.
Prioritization information can be omitted from messages. Defaults are used prior to any explicit values being provided (Section 5.3.5).
5.3.1 Stream Dependencies
Each stream can be given an explicit dependency on another stream. Including a dependency expresses a preference to allocate resources to the identified stream rather than to the dependent stream.
A stream that is not dependent on any other stream is given a stream dependency of 0x0. In other words, the non-existent stream 0 forms the root of the tree.
A stream that depends on another stream is a dependent stream. The stream upon which a stream is dependent is a parent stream. A dependency on a stream that is not currently in the tree — such as a stream in the "idle" state — results in that stream being given a default priority (Section 5.3.5).
When assigning a dependency on another stream, the stream is added as a new dependency of the parent stream. Dependent streams that share the same parent are not ordered with respect to each other. For example, if streams B and C are dependent on stream A, and if stream D is created with a dependency on stream A, this results in a dependency order of A followed by B, C, and D in any order.
A A / \ ==> /|\ B C B D C
Figure 3: Example of Default Dependency Creation
An exclusive flag allows for the insertion of a new level of dependencies. The exclusive flag causes the stream to become the sole dependency of its parent stream, causing other dependencies to become dependent on the exclusive stream. In the previous example, if stream D is created with an exclusive dependency on stream A, this results in D becoming the dependency parent of B and C.
A
A |
/ \ ==> D
B C / \
B C
Figure 4: Example of Exclusive Dependency Creation
Inside the dependency tree, a dependent stream SHOULD only be allocated resources if either all of the streams that it depends on (the chain of parent streams up to 0x0) are closed or it is not possible to make progress on them.
A stream cannot depend on itself. An endpoint MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.
5.3.2 Dependency Weighting
All dependent streams are allocated an integer weight between 1 and 256 (inclusive).
Streams with the same parent SHOULD be allocated resources proportionally based on their weight. Thus, if stream B depends on stream A with weight 4, stream C depends on stream A with weight 12, and no progress can be made on stream A, stream B ideally receives one-third of the resources allocated to stream C.
5.3.3 Reprioritization
Stream priorities are changed using the PRIORITY frame. Setting a dependency causes a stream to become dependent on the identified parent stream.
Dependent streams move with their parent stream if the parent is reprioritized. Setting a dependency with the exclusive flag for a reprioritized stream causes all the dependencies of the new parent stream to become dependent on the reprioritized stream.
If a stream is made dependent on one of its own dependencies, the formerly dependent stream is first moved to be dependent on the reprioritized stream's previous parent. The moved dependency retains its weight.
For example, consider an original dependency tree where B and C depend on A, D and E depend on C, and F depends on D. If A is made dependent on D, then D takes the place of A. All other dependency relationships stay the same, except for F, which becomes dependent on A if the reprioritization is exclusive.
x x x x
| / \ | |
A D A D D
/ \ / / \ / \ |
B C ==> F B C ==> F A OR A
/ \ | / \ /|\
D E E B C B C F
| | |
F E E
(intermediate) (non-exclusive) (exclusive)
Figure 5: Example of Dependency Reordering
5.3.4 Prioritization State Management
When a stream is removed from the dependency tree, its dependencies can be moved to become dependent on the parent of the closed stream. The weights of new dependencies are recalculated by distributing the weight of the dependency of the closed stream proportionally based on the weights of its dependencies.
Streams that are removed from the dependency tree cause some prioritization information to be lost. Resources are shared between streams with the same parent stream, which means that if a stream in that set closes or becomes blocked, any spare capacity allocated to a stream is distributed to the immediate neighbors of the stream. However, if the common dependency is removed from the tree, those streams share resources with streams at the next highest level.
For example, assume streams A and B share a parent, and streams C and D both depend on stream A. Prior to the removal of stream A, if streams A and D are unable to proceed, then stream C receives all the resources dedicated to stream A. If stream A is removed from the tree, the weight of stream A is divided between streams C and D. If stream D is still unable to proceed, this results in stream C receiving a reduced proportion of resources. For equal starting weights, C receives one third, rather than one half, of available resources.
It is possible for a stream to become closed while prioritization information that creates a dependency on that stream is in transit. If a stream identified in a dependency has no associated priority information, then the dependent stream is instead assigned a default priority (Section 5.3.5). This potentially creates suboptimal prioritization, since the stream could be given a priority that is different from what is intended.
To avoid these problems, an endpoint SHOULD retain stream prioritization state for a period after streams become closed. The longer state is retained, the lower the chance that streams are assigned incorrect or default priority values.
Similarly, streams that are in the "idle" state can be assigned priority or become a parent of other streams. This allows for the creation of a grouping node in the dependency tree, which enables more flexible expressions of priority. Idle streams begin with a default priority (Section 5.3.5).
The retention of priority information for streams that are not counted toward the limit set by SETTINGS_MAX_CONCURRENT_STREAMS could create a large state burden for an endpoint. Therefore, the amount of prioritization state that is retained MAY be limited.
The amount of additional state an endpoint maintains for prioritization could be dependent on load; under high load, prioritization state can be discarded to limit resource commitments. In extreme cases, an endpoint could even discard prioritization state for active or reserved streams. If a limit is applied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS. Implementations SHOULD also attempt to retain state for streams that are in active use in the priority tree.
If it has retained enough state to do so, an endpoint receiving a PRIORITY frame that changes the priority of a closed stream SHOULD alter the dependencies of the streams that depend on it.
5.3.5 Default Priorities
All streams are initially assigned a non-exclusive dependency on stream 0x0. Pushed streams (Section 8.2) initially depend on their associated stream. In both cases, streams are assigned a default weight of 16.
5.4 Error Handling
HTTP/2 framing permits two classes of error:
- An error condition that renders the entire connection unusable is a connection error.
- An error in an individual stream is a stream error.
A list of error codes is included in Section 7.
5.4.1 Connection Error Handling
A connection error is any error that prevents further processing of the frame layer or corrupts any connection state.
An endpoint that encounters a connection error SHOULD first send a GOAWAY frame (Section 6.8) with the stream identifier of the last stream that it successfully received from its peer. The GOAWAY frame includes an error code that indicates why the connection is terminating. After sending the GOAWAY frame for an error condition, the endpoint MUST close the TCP connection.
It is possible that the GOAWAY will not be reliably received by the receiving endpoint ([RFC7230], Section 6.6 describes how an immediate connection close can result in data loss). In the event of a connection error, GOAWAY only provides a best-effort attempt to communicate with the peer about why the connection is being terminated.
An endpoint can end a connection at any time. In particular, an endpoint MAY choose to treat a stream error as a connection error. Endpoints SHOULD send a GOAWAY frame when ending a connection, providing that circumstances permit it.
5.4.2 Stream Error Handling
A stream error is an error related to a specific stream that does not affect processing of other streams.
An endpoint that detects a stream error sends a RST_STREAM frame (Section 6.4) that contains the stream identifier of the stream where the error occurred. The RST_STREAM frame includes an error code that indicates the type of error.
A RST_STREAM is the last frame that an endpoint can send on a stream. The peer that sends the RST_STREAM frame MUST be prepared to receive any frames that were sent or enqueued for sending by the remote peer. These frames can be ignored, except where they modify connection state (such as the state maintained for header compression (Section 4.3) or flow control).
Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. However, an endpoint MAY send additional RST_STREAM frames if it receives frames on a closed stream after more than a round-trip time. This behavior is permitted to deal with misbehaving implementations.
To avoid looping, an endpoint MUST NOT send a RST_STREAM in response to a RST_STREAM frame.
5.4.3 Connection Termination
If the TCP connection is closed or reset while streams remain in "open" or "half-closed" state, then the affected streams cannot be automatically retried (see Section 8.1.4 for details).
5.5 Extending HTTP/2
HTTP/2 permits extension of the protocol. Within the limitations described in this section, protocol extensions can be used to provide additional services or alter any aspect of the protocol. Extensions are effective only within the scope of a single HTTP/2 connection.
This applies to the protocol elements defined in this document. This does not affect the existing options for extending HTTP, such as defining new methods, status codes, or header fields.
Extensions are permitted to use new frame types (Section 4.1), new settings (Section 6.5.2), or new error codes (Section 7). Registries are established for managing these extension points: frame types (Section 11.2), settings (Section 11.3), and error codes (Section 11.4).
Implementations MUST ignore unknown or unsupported values in all extensible protocol elements. Implementations MUST discard frames that have unknown or unsupported types. This means that any of these extension points can be safely used by extensions without prior arrangement or negotiation. However, extension frames that appear in the middle of a header block (Section 4.3) are not permitted; these MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when the revised layout comes into effect. Note that treating any frames other than DATA frames as flow controlled is such a change in semantics and can only be done through negotiation.
This document doesn't mandate a specific method for negotiating the use of an extension but notes that a setting (Section 6.5.2) could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the initial value MUST be defined in such a fashion that the extension is initially disabled.
6. Frame Definitions
This specification defines a number of frame types, each identified by a unique 8-bit type code. Each frame type serves a distinct purpose in the establishment and management either of the connection as a whole or of individual streams.
The transmission of specific frame types can alter the state of a connection. If endpoints fail to maintain a synchronized view of the connection state, successful communication within the connection will no longer be possible. Therefore, it is important that endpoints have a shared comprehension of how the state is affected by the use any given frame.
6.1 DATA
DATA frames (type=0x0) convey arbitrary, variable-length sequences of octets associated with a stream. One or more DATA frames are used, for instance, to carry HTTP request or response payloads.
DATA frames MAY also contain padding. Padding can be added to DATA frames to obscure the size of messages. Padding is a security feature; see Section 10.7.
+---------------+ |Pad Length? (8)| +---------------+-----------------------------------------------+ | Data (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+
Figure 6: DATA Frame Payload
The DATA frame contains the following fields:
- Pad Length:
- An 8-bit field containing the length of the frame padding in units of octets. This field is conditional (as signified by a "?" in the diagram) and is only present if the PADDED flag is set.
- Data:
- Application data. The amount of data is the remainder of the frame payload after subtracting the length of the other fields that are present.
- Padding:
- Padding octets that contain no application semantic value. Padding octets MUST be set to zero when sending. A receiver is not obligated to verify padding but MAY treat non-zero padding as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The DATA frame defines the following flags:
- END_STREAM (0x1):
- When set, bit 0 indicates that this frame is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of the "half-closed" states or the "closed" state (Section 5.1).
- PADDED (0x8):
- When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.
DATA frames MUST be associated with a stream. If a DATA frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
DATA frames are subject to flow control and can only be sent when a stream is in the "open" or "half-closed (remote)" state. The entire DATA frame payload is included in flow control, including the Pad Length and Padding fields if present. If a DATA frame is received whose stream is not in "open" or "half-closed (local)" state, the recipient MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.
The total number of padding octets is determined by the value of the Pad Length field. If the length of the padding is the length of the frame payload or greater, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Note: A frame can be increased in size by one octet by including a Pad Length field with a value of zero.
6.2 HEADERS
The HEADERS frame (type=0x1) is used to open a stream (Section 5.1), and additionally carries a header block fragment. HEADERS frames can be sent on a stream in the "idle", "reserved (local)", "open", or "half-closed (remote)" state.
+---------------+ |Pad Length? (8)| +-+-------------+-----------------------------------------------+ |E| Stream Dependency? (31) | +-+-------------+-----------------------------------------------+ | Weight? (8) | +-+-------------+-----------------------------------------------+ | Header Block Fragment (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+
Figure 7: HEADERS Frame Payload
The HEADERS frame payload has the following fields:
- Pad Length:
- An 8-bit field containing the length of the frame padding in units of octets. This field is only present if the PADDED flag is set.
- E:
- A single-bit flag indicating that the stream dependency is exclusive (see Section 5.3). This field is only present if the PRIORITY flag is set.
- Stream Dependency:
- A 31-bit stream identifier for the stream that this stream depends on (see Section 5.3). This field is only present if the PRIORITY flag is set.
- Weight:
- An unsigned 8-bit integer representing a priority weight for the stream (see Section 5.3). Add one to the value to obtain a weight between 1 and 256. This field is only present if the PRIORITY flag is set.
- Header Block Fragment:
- A header block fragment (Section 4.3).
- Padding:
- Padding octets.
The HEADERS frame defines the following flags:
- END_STREAM (0x1):
-
When set, bit 0 indicates that the header block (Section 4.3) is the last that the endpoint will send for the identified stream.
A HEADERS frame carries the END_STREAM flag that signals the end of a stream. However, a HEADERS frame with the END_STREAM flag set can be followed by CONTINUATION frames on the same stream. Logically, the CONTINUATION frames are part of the HEADERS frame.
- END_HEADERS (0x4):
-
When set, bit 2 indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames.
A HEADERS frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
- PADDED (0x8):
-
When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.
- PRIORITY (0x20):
-
When set, bit 5 indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present; see Section 5.3.
The payload of a HEADERS frame contains a header block fragment (Section 4.3). A header block that does not fit within a HEADERS frame is continued in a CONTINUATION frame (Section 6.10).
HEADERS frames MUST be associated with a stream. If a HEADERS frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The HEADERS frame changes the connection state as described in Section 4.3.
The HEADERS frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1). Padding that exceeds the size remaining for the header block fragment MUST be treated as a PROTOCOL_ERROR.
Prioritization information in a HEADERS frame is logically equivalent to a separate PRIORITY frame, but inclusion in HEADERS avoids the potential for churn in stream prioritization when new streams are created. Prioritization fields in HEADERS frames subsequent to the first on a stream reprioritize the stream (Section 5.3.3).
6.3 PRIORITY
The PRIORITY frame (type=0x2) specifies the sender-advised priority of a stream (Section 5.3). It can be sent in any stream state, including idle or closed streams.
+-+-------------------------------------------------------------+ |E| Stream Dependency (31) | +-+-------------+-----------------------------------------------+ | Weight (8) | +-+-------------+
Figure 8: PRIORITY Frame Payload
The payload of a PRIORITY frame contains the following fields:
- E:
- A single-bit flag indicating that the stream dependency is exclusive (see Section 5.3).
- Stream Dependency:
- A 31-bit stream identifier for the stream that this stream depends on (see Section 5.3).
- Weight:
- An unsigned 8-bit integer representing a priority weight for the stream (see Section 5.3). Add one to the value to obtain a weight between 1 and 256.
The PRIORITY frame does not define any flags.
The PRIORITY frame always identifies a stream. If a PRIORITY frame is received with a stream identifier of 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The PRIORITY frame can be sent on a stream in any state, though it cannot be sent between consecutive frames that comprise a single header block (Section 4.3). Note that this frame could arrive after processing or frame sending has completed, which would cause it to have no effect on the identified stream. For a stream that is in the "half-closed (remote)" or "closed" state, this frame can only affect processing of the identified stream and its dependent streams; it does not affect frame transmission on that stream.
The PRIORITY frame can be sent for a stream in the "idle" or "closed" state. This allows for the reprioritization of a group of dependent streams by altering the priority of an unused or closed parent stream.
A PRIORITY frame with a length other than 5 octets MUST be treated as a stream error (Section 5.4.2) of type FRAME_SIZE_ERROR.
6.4 RST_STREAM
The RST_STREAM frame (type=0x3) allows for immediate termination of a stream. RST_STREAM is sent to request cancellation of a stream or to indicate that an error condition has occurred.
+---------------------------------------------------------------+ | Error Code (32) | +---------------------------------------------------------------+
Figure 9: RST_STREAM Frame Payload
The RST_STREAM frame contains a single unsigned, 32-bit integer identifying the error code (Section 7). The error code indicates why the stream is being terminated.
The RST_STREAM frame does not define any flags.
The RST_STREAM frame fully terminates the referenced stream and causes it to enter the "closed" state. After receiving a RST_STREAM on a stream, the receiver MUST NOT send additional frames for that stream, with the exception of PRIORITY. However, after sending the RST_STREAM, the sending endpoint MUST be prepared to receive and process additional frames sent on the stream that might have been sent by the peer prior to the arrival of the RST_STREAM.
RST_STREAM frames MUST be associated with a stream. If a RST_STREAM frame is received with a stream identifier of 0x0, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
RST_STREAM frames MUST NOT be sent for a stream in the "idle" state. If a RST_STREAM frame identifying an idle stream is received, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
A RST_STREAM frame with a length other than 4 octets MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.
6.5 SETTINGS
The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior. The SETTINGS frame is also used to acknowledge the receipt of those parameters. Individually, a SETTINGS parameter can also be referred to as a "setting".
SETTINGS parameters are not negotiated; they describe characteristics of the sending peer, which are used by the receiving peer. Different values for the same parameter can be advertised by each peer. For example, a client might set a high initial flow-control window, whereas a server might set a lower value to conserve resources.
A SETTINGS frame MUST be sent by both endpoints at the start of a connection and MAY be sent at any other time by either endpoint over the lifetime of the connection. Implementations MUST support all of the parameters defined by this specification.
Each parameter in a SETTINGS frame replaces any existing value for that parameter. Parameters are processed in the order in which they appear, and a receiver of a SETTINGS frame does not need to maintain any state other than the current value of its parameters. Therefore, the value of a SETTINGS parameter is the last value that is seen by a receiver.
SETTINGS parameters are acknowledged by the receiving peer. To enable this, the SETTINGS frame defines the following flag:
- ACK (0x1):
- When set, bit 0 indicates that this frame acknowledges receipt and application of the peer's SETTINGS frame. When this bit is set, the payload of the SETTINGS frame MUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a length field value other than 0 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. For more information, see Section 6.5.3 ("Settings Synchronization").
SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a SETTINGS frame MUST be zero (0x0). If an endpoint receives a SETTINGS frame whose stream identifier field is anything other than 0x0, the endpoint MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The SETTINGS frame affects connection state. A badly formed or incomplete SETTINGS frame MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
A SETTINGS frame with a length other than a multiple of 6 octets MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.
6.5.1 SETTINGS Format
The payload of a SETTINGS frame consists of zero or more parameters, each consisting of an unsigned 16-bit setting identifier and an unsigned 32-bit value.
+-------------------------------+ | Identifier (16) | +-------------------------------+-------------------------------+ | Value (32) | +---------------------------------------------------------------+
Figure 10: Setting Format
6.5.2 Defined SETTINGS Parameters
The following parameters are defined:
- SETTINGS_HEADER_TABLE_SIZE (0x1):
-
Allows the sender to inform the remote endpoint of the maximum size of the header compression table used to decode header blocks, in octets. The encoder can select any size equal to or less than this value by using signaling specific to the header compression format inside a header block (see [COMPRESSION]). The initial value is 4,096 octets.
- SETTINGS_ENABLE_PUSH (0x2):
-
This setting can be used to disable server push (Section 8.2). An endpoint MUST NOT send a PUSH_PROMISE frame if it receives this parameter set to a value of 0. An endpoint that has both set this parameter to 0 and had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The initial value is 1, which indicates that server push is permitted. Any value other than 0 or 1 MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
- SETTINGS_MAX_CONCURRENT_STREAMS (0x3):
-
Indicates the maximum number of concurrent streams that the sender will allow. This limit is directional: it applies to the number of streams that the sender permits the receiver to create. Initially, there is no limit to this value. It is recommended that this value be no smaller than 100, so as to not unnecessarily limit parallelism.
A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be treated as special by endpoints. A zero value does prevent the creation of new streams; however, this can also happen for any limit that is exhausted with active streams. Servers SHOULD only set a zero value for short durations; if a server does not wish to accept requests, closing the connection is more appropriate.
- SETTINGS_INITIAL_WINDOW_SIZE (0x4):
-
Indicates the sender's initial window size (in octets) for stream-level flow control. The initial value is 216-1 (65,535) octets.
This setting affects the window size of all streams (see Section 6.9.2).
Values above the maximum flow-control window size of 231-1 MUST be treated as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.
- SETTINGS_MAX_FRAME_SIZE (0x5):
-
Indicates the size of the largest frame payload that the sender is willing to receive, in octets.
The initial value is 214 (16,384) octets. The value advertised by an endpoint MUST be between this initial value and the maximum allowed frame size (224-1 or 16,777,215 octets), inclusive. Values outside this range MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
- SETTINGS_MAX_HEADER_LIST_SIZE (0x6):
-
This advisory setting informs a peer of the maximum size of header list that the sender is prepared to accept, in octets. The value is based on the uncompressed size of header fields, including the length of the name and value in octets plus an overhead of 32 octets for each header field.
For any given request, a lower limit than what is advertised MAY be enforced. The initial value of this setting is unlimited.
An endpoint that receives a SETTINGS frame with any unknown or unsupported identifier MUST ignore that setting.
6.5.3 Settings Synchronization
Most values in SETTINGS benefit from or require an understanding of when the peer has received and applied the changed parameter values. In order to provide such synchronization timepoints, the recipient of a SETTINGS frame in which the ACK flag is not set MUST apply the updated parameters as soon as possible upon receipt.
The values in the SETTINGS frame MUST be processed in the order they appear, with no other frame processing between values. Unsupported parameters MUST be ignored. Once all values have been processed, the recipient MUST immediately emit a SETTINGS frame with the ACK flag set. Upon receiving a SETTINGS frame with the ACK flag set, the sender of the altered parameters can rely on the setting having been applied.
If the sender of a SETTINGS frame does not receive an acknowledgement within a reasonable amount of time, it MAY issue a connection error (Section 5.4.1) of type SETTINGS_TIMEOUT.
6.6 PUSH_PROMISE
The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in advance of streams the sender intends to initiate. The PUSH_PROMISE frame includes the unsigned 31-bit identifier of the stream the endpoint plans to create along with a set of headers that provide additional context for the stream. Section 8.2 contains a thorough description of the use of PUSH_PROMISE frames.
+---------------+ |Pad Length? (8)| +-+-------------+-----------------------------------------------+ |R| Promised Stream ID (31) | +-+-----------------------------+-------------------------------+ | Header Block Fragment (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+
Figure 11: PUSH_PROMISE Payload Format
The PUSH_PROMISE frame payload has the following fields:
- Pad Length:
- An 8-bit field containing the length of the frame padding in units of octets. This field is only present if the PADDED flag is set.
- R:
- A single reserved bit.
- Promised Stream ID:
- An unsigned 31-bit integer that identifies the stream that is reserved by the PUSH_PROMISE. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see "new stream identifier" in Section 5.1.1).
- Header Block Fragment:
- A header block fragment (Section 4.3) containing request header fields.
- Padding:
- Padding octets.
The PUSH_PROMISE frame defines the following flags:
- END_HEADERS (0x4):
-
When set, bit 2 indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames.
A PUSH_PROMISE frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
- PADDED (0x8):
-
When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.
PUSH_PROMISE frames MUST only be sent on a peer-initiated stream that is in either the "open" or "half-closed (remote)" state. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Promised streams are not required to be used in the order they are promised. The PUSH_PROMISE only reserves stream identifiers for later use.
PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of the peer endpoint is set to 0. An endpoint that has set this setting and has received acknowledgement MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Recipients of PUSH_PROMISE frames can choose to reject promised streams by returning a RST_STREAM referencing the promised stream identifier back to the sender of the PUSH_PROMISE.
A PUSH_PROMISE frame modifies the connection state in two ways. First, the inclusion of a header block (Section 4.3) potentially modifies the state maintained for header compression. Second, PUSH_PROMISE also reserves a stream for later use, causing the promised stream to enter the "reserved" state. A sender MUST NOT send a PUSH_PROMISE on a stream unless that stream is either "open" or "half-closed (remote)"; the sender MUST ensure that the promised stream is a valid choice for a new stream identifier (Section 5.1.1) (that is, the promised stream MUST be in the "idle" state).
Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame causes the stream state to become indeterminate. A receiver MUST treat the receipt of a PUSH_PROMISE on a stream that is neither "open" nor "half-closed (local)" as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. However, an endpoint that has sent RST_STREAM on the associated stream MUST handle PUSH_PROMISE frames that might have been created before the RST_STREAM frame is received and processed.
A receiver MUST treat the receipt of a PUSH_PROMISE that promises an illegal stream identifier (Section 5.1.1) as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that an illegal stream identifier is an identifier for a stream that is not currently in the "idle" state.
The PUSH_PROMISE frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).
6.7 PING
The PING frame (type=0x6) is a mechanism for measuring a minimal round-trip time from the sender, as well as determining whether an idle connection is still functional. PING frames can be sent from any endpoint.
+---------------------------------------------------------------+ | | | Opaque Data (64) | | | +---------------------------------------------------------------+
Figure 12: PING Payload Format
In addition to the frame header, PING frames MUST contain 8 octets of opaque data in the payload. A sender can include any value it chooses and use those octets in any fashion.
Receivers of a PING frame that does not include an ACK flag MUST send a PING frame with the ACK flag set in response, with an identical payload. PING responses SHOULD be given higher priority than any other frame.
The PING frame defines the following flags:
- ACK (0x1):
- When set, bit 0 indicates that this PING frame is a PING response. An endpoint MUST set this flag in PING responses. An endpoint MUST NOT respond to PING frames containing this flag.
PING frames are not associated with any individual stream. If a PING frame is received with a stream identifier field value other than 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Receipt of a PING frame with a length field value other than 8 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.
6.8 GOAWAY
The GOAWAY frame (type=0x7) is used to initiate shutdown of a connection or to signal serious error conditions. GOAWAY allows an endpoint to gracefully stop accepting new streams while still finishing processing of previously established streams. This enables administrative actions, like server maintenance.
There is an inherent race condition between an endpoint starting new streams and the remote sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last peer-initiated stream that was or might be processed on the sending endpoint in this connection. For instance, if the server sends a GOAWAY frame, the identified stream is the highest-numbered stream initiated by the client.
Once sent, the sender will ignore frames sent on streams initiated by the receiver if the stream has an identifier higher than the included last stream identifier. Receivers of a GOAWAY frame MUST NOT open additional streams on the connection, although a new connection can be established for new streams.
If the receiver of the GOAWAY has sent data on streams with a higher stream identifier than what is indicated in the GOAWAY frame, those streams are not or will not be processed. The receiver of the GOAWAY frame can treat the streams as though they had never been created at all, thereby allowing those streams to be retried later on a new connection.
Endpoints SHOULD always send a GOAWAY frame before closing a connection so that the remote peer can know whether a stream has been partially processed or not. For example, if an HTTP client sends a POST at the same time that a server closes a connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate what streams it might have acted on.
An endpoint might choose to close a connection without sending a GOAWAY for misbehaving peers.
A GOAWAY frame might not immediately precede closing of the connection; a receiver of a GOAWAY that has no more use for the connection SHOULD still send a GOAWAY frame before terminating the connection.
+-+-------------------------------------------------------------+ |R| Last-Stream-ID (31) | +-+-------------------------------------------------------------+ | Error Code (32) | +---------------------------------------------------------------+ | Additional Debug Data (*) | +---------------------------------------------------------------+
Figure 13: GOAWAY Payload Format
The GOAWAY frame does not define any flags.
The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST treat a GOAWAY frame with a stream identifier other than 0x0 as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The last stream identifier in the GOAWAY frame contains the highest-numbered stream identifier for which the sender of the GOAWAY frame might have taken some action on or might yet take action on. All streams up to and including the identified stream might have been processed in some way. The last stream identifier can be set to 0 if no streams were processed.
Note: In this context, "processed" means that some data from the stream was passed to some higher layer of software that might have taken some action as a result.
If a connection terminates without a GOAWAY frame, the last stream identifier is effectively the highest possible stream identifier.
On streams with lower- or equal-numbered identifiers that were not closed completely prior to the connection being closed, reattempting requests, transactions, or any protocol activity is not possible, with the exception of idempotent actions like HTTP GET, PUT, or DELETE. Any protocol activity that uses higher-numbered streams can be safely retried using a new connection.
Activity on streams numbered lower or equal to the last stream identifier might still complete successfully. The sender of a GOAWAY frame might gracefully shut down a connection by sending a GOAWAY frame, maintaining the connection in an "open" state until all in-progress streams complete.
An endpoint MAY send multiple GOAWAY frames if circumstances change. For instance, an endpoint that sends GOAWAY with NO_ERROR during graceful shutdown could subsequently encounter a condition that requires immediate termination of the connection. The last stream identifier from the last GOAWAY frame received indicates which streams could have been acted upon. Endpoints MUST NOT increase the value they send in the last stream identifier, since the peers might already have retried unprocessed requests on another connection.
A client that is unable to retry requests loses all requests that are in flight when the server closes the connection. This is especially true for intermediaries that might not be serving clients using HTTP/2. A server that is attempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the last stream identifier set to 231-1 and a NO_ERROR code. This signals to the client that a shutdown is imminent and that initiating further requests is prohibited. After allowing time for any in-flight stream creation (at least one round-trip time), the server can send another GOAWAY frame with an updated last stream identifier. This ensures that a connection can be cleanly shut down without losing requests.
After sending a GOAWAY frame, the sender can discard frames for streams initiated by the receiver with identifiers higher than the identified last stream. However, any frames that alter connection state cannot be completely ignored. For instance, HEADERS, PUSH_PROMISE, and CONTINUATION frames MUST be minimally processed to ensure the state maintained for header compression is consistent (see Section 4.3); similarly, DATA frames MUST be counted toward the connection flow-control window. Failure to process these frames can cause flow control or header compression state to become unsynchronized.
The GOAWAY frame also contains a 32-bit error code (Section 7) that contains the reason for closing the connection.
Endpoints MAY append opaque data to the payload of any GOAWAY frame. Additional debug data is intended for diagnostic purposes only and carries no semantic value. Debug information could contain security- or privacy-sensitive data. Logged or otherwise persistently stored debug data MUST have adequate safeguards to prevent unauthorized access.
6.9 WINDOW_UPDATE
The WINDOW_UPDATE frame (type=0x8) is used to implement flow control; see Section 5.2 for an overview.
Flow control operates at two levels: on each individual stream and on the entire connection.
Both types of flow control are hop by hop, that is, only between the two endpoints. Intermediaries do not forward WINDOW_UPDATE frames between dependent connections. However, throttling of data transfer by any receiver can indirectly cause the propagation of flow-control information toward the original sender.
Flow control only applies to frames that are identified as being subject to flow control. Of the frame types defined in this document, this includes only DATA frames. Frames that are exempt from flow control MUST be accepted and processed, unless the receiver is unable to assign resources to handling the frame. A receiver MAY respond with a stream error (Section 5.4.2) or connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept a frame.
+-+-------------------------------------------------------------+ |R| Window Size Increment (31) | +-+-------------------------------------------------------------+
Figure 14: WINDOW_UPDATE Payload Format
The payload of a WINDOW_UPDATE frame is one reserved bit plus an unsigned 31-bit integer indicating the number of octets that the sender can transmit in addition to the existing flow-control window. The legal range for the increment to the flow-control window is 1 to 231-1 (2,147,483,647) octets.
The WINDOW_UPDATE frame does not define any flags.
The WINDOW_UPDATE frame can be specific to a stream or to the entire connection. In the former case, the frame's stream identifier indicates the affected stream; in the latter, the value "0" indicates that the entire connection is the subject of the frame.
A receiver MUST treat the receipt of a WINDOW_UPDATE frame with an flow-control window increment of 0 as a stream error (Section 5.4.2) of type PROTOCOL_ERROR; errors on the connection flow-control window MUST be treated as a connection error (Section 5.4.1).
WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the END_STREAM flag. This means that a receiver could receive a WINDOW_UPDATE frame on a "half-closed (remote)" or "closed" stream. A receiver MUST NOT treat this as an error (see Section 5.1).
A receiver that receives a flow-controlled frame MUST always account for its contribution against the connection flow-control window, unless the receiver treats this as a connection error (Section 5.4.1). This is necessary even if the frame is in error. The sender counts the frame toward the flow-control window, but if the receiver does not, the flow-control window at the sender and receiver can become different.
A WINDOW_UPDATE frame with a length other than 4 octets MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.
6.9.1 The Flow-Control Window
Flow control in HTTP/2 is implemented using a window kept by each sender on every stream. The flow-control window is a simple integer value that indicates how many octets of data the sender is permitted to transmit; as such, its size is a measure of the buffering capacity of the receiver.
Two flow-control windows are applicable: the stream flow-control window and the connection flow-control window. The sender MUST NOT send a flow-controlled frame with a length that exceeds the space available in either of the flow-control windows advertised by the receiver. Frames with zero length with the END_STREAM flag set (that is, an empty DATA frame) MAY be sent if there is no available space in either flow-control window.
For flow-control calculations, the 9-octet frame header is not counted.
After sending a flow-controlled frame, the sender reduces the space available in both windows by the length of the transmitted frame.
The receiver of a frame sends a WINDOW_UPDATE frame as it consumes data and frees up space in flow-control windows. Separate WINDOW_UPDATE frames are sent for the stream- and connection-level flow-control windows.
A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amount specified in the frame.
A sender MUST NOT allow a flow-control window to exceed 231-1 octets. If a sender receives a WINDOW_UPDATE that causes a flow-control window to exceed this maximum, it MUST terminate either the stream or the connection, as appropriate. For streams, the sender sends a RST_STREAM with an error code of FLOW_CONTROL_ERROR; for the connection, a GOAWAY frame with an error code of FLOW_CONTROL_ERROR is sent.
Flow-controlled frames from the sender and WINDOW_UPDATE frames from the receiver are completely asynchronous with respect to each other. This property allows a receiver to aggressively update the window size kept by the sender to prevent streams from stalling.
6.9.2 Initial Flow-Control Window Size
When an HTTP/2 connection is first established, new streams are created with an initial flow-control window size of 65,535 octets. The connection flow-control window is also 65,535 octets. Both endpoints can adjust the initial window size for new streams by including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms part of the connection preface. The connection flow-control window can only be changed using WINDOW_UPDATE frames.
Prior to receiving a SETTINGS frame that sets a value for SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default initial window size when sending flow-controlled frames. Similarly, the connection flow-control window is set to the default initial window size until a WINDOW_UPDATE frame is received.
In addition to changing the flow-control window for streams that are not yet active, a SETTINGS frame can alter the initial flow-control window size for streams with active flow-control windows (that is, streams in the "open" or "half-closed (remote)" state). When the value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust the size of all stream flow-control windows that it maintains by the difference between the new value and the old value.
A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available space in a flow-control window to become negative. A sender MUST track the negative flow-control window and MUST NOT send new flow-controlled frames until it receives WINDOW_UPDATE frames that cause the flow-control window to become positive.
For example, if the client sends 60 KB immediately on connection establishment and the server sets the initial window size to be 16 KB, the client will recalculate the available flow-control window to be -44 KB on receipt of the SETTINGS frame. The client retains a negative flow-control window until WINDOW_UPDATE frames restore the window to being positive, after which the client can resume sending.
A SETTINGS frame cannot alter the connection flow-control window.
An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that causes any flow-control window to exceed the maximum size as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.
6.9.3 Reducing the Stream Window Size
A receiver that wishes to use a smaller flow-control window than the current size can send a new SETTINGS frame. However, the receiver MUST be prepared to receive data that exceeds this window size, since the sender might send data that exceeds the lower limit prior to processing the SETTINGS frame.
After sending a SETTINGS frame that reduces the initial flow-control window size, a receiver MAY continue to process streams that exceed flow-control limits. Allowing streams to continue does not allow the receiver to immediately reduce the space it reserves for flow-control windows. Progress on these streams can also stall, since WINDOW_UPDATE frames are needed to allow the sender to resume sending. The receiver MAY instead send a RST_STREAM with an error code of FLOW_CONTROL_ERROR for the affected streams.
6.10 CONTINUATION
The CONTINUATION frame (type=0x9) is used to continue a sequence of header block fragments (Section 4.3). Any number of CONTINUATION frames can be sent, as long as the preceding frame is on the same stream and is a HEADERS, PUSH_PROMISE, or CONTINUATION frame without the END_HEADERS flag set.
+---------------------------------------------------------------+ | Header Block Fragment (*) ... +---------------------------------------------------------------+
Figure 15: CONTINUATION Frame Payload
The CONTINUATION frame payload contains a header block fragment (Section 4.3).
The CONTINUATION frame defines the following flag:
- END_HEADERS (0x4):
-
When set, bit 2 indicates that this frame ends a header block (Section 4.3).
If the END_HEADERS bit is not set, this frame MUST be followed by another CONTINUATION frame. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The CONTINUATION frame changes the connection state as defined in Section 4.3.
CONTINUATION frames MUST be associated with a stream. If a CONTINUATION frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or CONTINUATION frame without the END_HEADERS flag set. A recipient that observes violation of this rule MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
7. Error Codes
Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey the reasons for the stream or connection error.
Error codes share a common code space. Some error codes apply only to either streams or the entire connection and have no defined semantics in the other context.
The following error codes are defined:
- NO_ERROR (0x0):
- The associated condition is not a result of an error. For example, a GOAWAY might include this code to indicate graceful shutdown of a connection.
- PROTOCOL_ERROR (0x1):
- The endpoint detected an unspecific protocol error. This error is for use when a more specific error code is not available.
- INTERNAL_ERROR (0x2):
- The endpoint encountered an unexpected internal error.
- FLOW_CONTROL_ERROR (0x3):
- The endpoint detected that its peer violated the flow-control protocol.
- SETTINGS_TIMEOUT (0x4):
- The endpoint sent a SETTINGS frame but did not receive a response in a timely manner. See Section 6.5.3 ("Settings Synchronization").
- STREAM_CLOSED (0x5):
- The endpoint received a frame after a stream was half-closed.
- FRAME_SIZE_ERROR (0x6):
- The endpoint received a frame with an invalid size.
- REFUSED_STREAM (0x7):
- The endpoint refused the stream prior to performing any application processing (see Section 8.1.4 for details).
- CANCEL (0x8):
- Used by the endpoint to indicate that the stream is no longer needed.
- COMPRESSION_ERROR (0x9):
- The endpoint is unable to maintain the header compression context for the connection.
- CONNECT_ERROR (0xa):
- The connection established in response to a CONNECT request (Section 8.3) was reset or abnormally closed.
- ENHANCE_YOUR_CALM (0xb):
- The endpoint detected that its peer is exhibiting a behavior that might be generating excessive load.
- INADEQUATE_SECURITY (0xc):
- The underlying transport has properties that do not meet minimum security requirements (see Section 9.2).
- HTTP_1_1_REQUIRED (0xd):
- The endpoint requires that HTTP/1.1 be used instead of HTTP/2.
Unknown or unsupported error codes MUST NOT trigger any special behavior. These MAY be treated by an implementation as being equivalent to INTERNAL_ERROR.
8. HTTP Message Exchanges
HTTP/2 is intended to be as compatible as possible with current uses of HTTP. This means that, from the application perspective, the features of the protocol are largely unchanged. To achieve this, all request and response semantics are preserved, although the syntax of conveying those semantics has changed.
Thus, the specification and requirements of HTTP/1.1 Semantics and Content [RFC7231], Conditional Requests [RFC7232], Range Requests [RFC7233], Caching [RFC7234], and Authentication [RFC7235] are applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax and Routing [RFC7230], such as the HTTP and HTTPS URI schemes, are also applicable in HTTP/2, but the expression of those semantics for this protocol are defined in the sections below.
8.1 HTTP Request/Response Exchange
A client sends an HTTP request on a new stream, using a previously unused stream identifier (Section 5.1.1). A server sends an HTTP response on the same stream as the request.
An HTTP message (request or response) consists of:
- for a response only, zero or more HEADERS frames (each followed by zero or more CONTINUATION frames) containing the message headers of informational (1xx) HTTP responses (see [RFC7230], Section 3.2 and [RFC7231], Section 6.2),
- one HEADERS frame (followed by zero or more CONTINUATION frames) containing the message headers (see [RFC7230], Section 3.2),
- zero or more DATA frames containing the payload body (see [RFC7230], Section 3.3), and
- optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing the trailer-part, if present (see [RFC7230], Section 4.1.2).
The last frame in the sequence bears an END_STREAM flag, noting that a HEADERS frame bearing the END_STREAM flag can be followed by CONTINUATION frames that carry any remaining portions of the header block.
Other frames (from any stream) MUST NOT occur between the HEADERS frame and any CONTINUATION frames that might follow.
HTTP/2 uses DATA frames to carry message payloads. The chunked transfer encoding defined in Section 4.1 of [RFC7230] MUST NOT be used in HTTP/2.
Trailing header fields are carried in a header block that also terminates the stream. Such a header block is a sequence starting with a HEADERS frame, followed by zero or more CONTINUATION frames, where the HEADERS frame bears an END_STREAM flag. Header blocks after the first that do not terminate the stream are not part of an HTTP request or response.
A HEADERS frame (and associated CONTINUATION frames) can only appear at the start or end of a stream. An endpoint that receives a HEADERS frame without the END_STREAM flag set after receiving a final (non-informational) status code MUST treat the corresponding request or response as malformed (Section 8.1.2.6).
An HTTP request/response exchange fully consumes a single stream. A request starts with the HEADERS frame that puts the stream into an "open" state. The request ends with a frame bearing END_STREAM, which causes the stream to become "half-closed (local)" for the client and "half-closed (remote)" for the server. A response starts with a HEADERS frame and ends with a frame bearing END_STREAM, which places the stream in the "closed" state.
An HTTP response is complete after the server sends — or the client receives — a frame with the END_STREAM flag set (including any CONTINUATION frames needed to complete a header block). A server can send a complete response prior to the client sending an entire request if the response does not depend on any portion of the request that has not been sent and received. When this is true, a server MAY request that the client abort transmission of a request without error by sending a RST_STREAM with an error code of NO_ERROR after sending a complete response (i.e., a frame with the END_STREAM flag). Clients MUST NOT discard responses as a result of receiving such a RST_STREAM, though clients can always discard responses at their discretion for other reasons.
8.1.1 Upgrading from HTTP/2
HTTP/2 removes support for the 101 (Switching Protocols) informational status code ([RFC7231], Section 6.2.2).
The semantics of 101 (Switching Protocols) aren't applicable to a multiplexed protocol. Alternative protocols are able to use the same mechanisms that HTTP/2 uses to negotiate their use (see Section 3).
8.1.2 HTTP Header Fields
HTTP header fields carry information as a series of key-value pairs. For a listing of registered HTTP headers, see the "Message Header Field" registry maintained at <https://www.iana.org/assignments/message-headers>.
Just as in HTTP/1.x, header field names are strings of ASCII characters that are compared in a case-insensitive fashion. However, header field names MUST be converted to lowercase prior to their encoding in HTTP/2. A request or response containing uppercase header field names MUST be treated as malformed (Section 8.1.2.6).
8.1.2.1 Pseudo-Header Fields
While HTTP/1.x used the message start-line (see [RFC7230], Section 3.1) to convey the target URI, the method of the request, and the status code for the response, HTTP/2 uses special pseudo-header fields beginning with ':' character (ASCII 0x3a) for this purpose.
Pseudo-header fields are not HTTP header fields. Endpoints MUST NOT generate pseudo-header fields other than those defined in this document.
Pseudo-header fields are only valid in the context in which they are defined. Pseudo-header fields defined for requests MUST NOT appear in responses; pseudo-header fields defined for responses MUST NOT appear in requests. Pseudo-header fields MUST NOT appear in trailers. Endpoints MUST treat a request or response that contains undefined or invalid pseudo-header fields as malformed (Section 8.1.2.6).
All pseudo-header fields MUST appear in the header block before regular header fields. Any request or response that contains a pseudo-header field that appears in a header block after a regular header field MUST be treated as malformed (Section 8.1.2.6).
8.1.2.2 Connection-Specific Header Fields
HTTP/2 does not use the Connection header field to indicate connection-specific header fields; in this protocol, connection-specific metadata is conveyed by other means. An endpoint MUST NOT generate an HTTP/2 message containing connection-specific header fields; any message containing connection-specific header fields MUST be treated as malformed (Section 8.1.2.6).
The only exception to this is the TE header field, which MAY be present in an HTTP/2 request; when it is, it MUST NOT contain any value other than "trailers".
This means that an intermediary transforming an HTTP/1.x message to HTTP/2 will need to remove any header fields nominated by the Connection header field, along with the Connection header field itself. Such intermediaries SHOULD also remove other connection-specific header fields, such as Keep-Alive, Proxy-Connection, Transfer-Encoding, and Upgrade, even if they are not nominated by the Connection header field.
Note: HTTP/2 purposefully does not support upgrade to another protocol. The handshake methods described in Section 3 are believed sufficient to negotiate the use of alternative protocols.
8.1.2.3 Request Pseudo-Header Fields
The following pseudo-header fields are defined for HTTP/2 requests:
-
The :method pseudo-header field includes the HTTP method ([RFC7231], Section 4).
-
The :scheme pseudo-header field includes the scheme portion of the target URI ([RFC3986], Section 3.1).
:scheme is not restricted to http and https schemed URIs. A proxy or gateway can translate requests for non-HTTP schemes, enabling the use of HTTP to interact with non-HTTP services.
-
The :authority pseudo-header field includes the authority portion of the target URI ([RFC3986], Section 3.2). The authority MUST NOT include the deprecated userinfo subcomponent for http or https schemed URIs.
To ensure that the HTTP/1.1 request line can be reproduced accurately, this pseudo-header field MUST be omitted when translating from an HTTP/1.1 request that has a request target in origin or asterisk form (see [RFC7230], Section 5.3). Clients that generate HTTP/2 requests directly SHOULD use the :authority pseudo-header field instead of the Host header field. An intermediary that converts an HTTP/2 request to HTTP/1.1 MUST create a Host header field if one is not present in a request by copying the value of the :authority pseudo-header field.
-
The :path pseudo-header field includes the path and query parts of the target URI (the path-absolute production and optionally a '?' character followed by the query production (see Sections 3.3 and 3.4 of [RFC3986]). A request in asterisk form includes the value '*' for the :path pseudo-header field.
This pseudo-header field MUST NOT be empty for http or https URIs; http or https URIs that do not contain a path component MUST include a value of '/'. The exception to this rule is an OPTIONS request for an http or https URI that does not include a path component; these MUST include a :path pseudo-header field with a value of '*' (see [RFC7230], Section 5.3.4).
All HTTP/2 requests MUST include exactly one valid value for the :method, :scheme, and :path pseudo-header fields, unless it is a CONNECT request (Section 8.3). An HTTP request that omits mandatory pseudo-header fields is malformed (Section 8.1.2.6).
HTTP/2 does not define a way to carry the version identifier that is included in the HTTP/1.1 request line.
8.1.2.4 Response Pseudo-Header Fields
For HTTP/2 responses, a single :status pseudo-header field is defined that carries the HTTP status code field (see [RFC7231], Section 6). This pseudo-header field MUST be included in all responses; otherwise, the response is malformed (Section 8.1.2.6).
HTTP/2 does not define a way to carry the version or reason phrase that is included in an HTTP/1.1 status line.
8.1.2.5 Compressing the Cookie Header Field
The Cookie header field [COOKIE] uses a semi-colon (";") to delimit cookie-pairs (or "crumbs"). This header field doesn't follow the list construction rules in HTTP (see [RFC7230], Section 3.2.2), which prevents cookie-pairs from being separated into different name-value pairs. This can significantly reduce compression efficiency as individual cookie-pairs are updated.
To allow for better compression efficiency, the Cookie header field MAY be split into separate header fields, each with one or more cookie-pairs. If there are multiple Cookie header fields after decompression, these MUST be concatenated into a single octet string using the two-octet delimiter of 0x3B, 0x20 (the ASCII string "; ") before being passed into a non-HTTP/2 context, such as an HTTP/1.1 connection, or a generic HTTP server application.
Therefore, the following two lists of Cookie header fields are semantically equivalent.
cookie: a=b; c=d; e=f cookie: a=b cookie: c=d cookie: e=f
8.1.2.6 Malformed Requests and Responses
A malformed request or response is one that is an otherwise valid sequence of HTTP/2 frames but is invalid due to the presence of extraneous frames, prohibited header fields, the absence of mandatory header fields, or the inclusion of uppercase header field names.
A request or response that includes a payload body can include a content-length header field. A request or response is also malformed if the value of a content-length header field does not equal the sum of the DATA frame payload lengths that form the body. A response that is defined to have no payload, as described in [RFC7230], Section 3.3.2, can have a non-zero content-length header field, even though no content is included in DATA frames.
Intermediaries that process HTTP requests or responses (i.e., any intermediary not acting as a tunnel) MUST NOT forward a malformed request or response. Malformed requests or responses that are detected MUST be treated as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.
For malformed requests, a server MAY send an HTTP response prior to closing or resetting the stream. Clients MUST NOT accept a malformed response. Note that these requirements are intended to protect against several types of common attacks against HTTP; they are deliberately strict because being permissive can expose implementations to these vulnerabilities.
8.1.3 Examples
This section shows HTTP/1.1 requests and responses, with illustrations of equivalent HTTP/2 requests and responses.
An HTTP GET request includes request header fields and no payload body and is therefore transmitted as a single HEADERS frame, followed by zero or more CONTINUATION frames containing the serialized block of request header fields. The HEADERS frame in the following has both the END_HEADERS and END_STREAM flags set; no CONTINUATION frames are sent.
GET /resource HTTP/1.1 HEADERS
Host: example.org ==> + END_STREAM
Accept: image/jpeg + END_HEADERS
:method = GET
:scheme = https
:path = /resource
host = example.org
accept = image/jpeg
Similarly, a response that includes only response header fields is transmitted as a HEADERS frame (again, followed by zero or more CONTINUATION frames) containing the serialized block of response header fields.
HTTP/1.1 304 Not Modified HEADERS
ETag: "xyzzy" ==> + END_STREAM
Expires: Thu, 23 Jan ... + END_HEADERS
:status = 304
etag = "xyzzy"
expires = Thu, 23 Jan ...
An HTTP POST request that includes request header fields and payload data is transmitted as one HEADERS frame, followed by zero or more CONTINUATION frames containing the request header fields, followed by one or more DATA frames, with the last CONTINUATION (or HEADERS) frame having the END_HEADERS flag set and the final DATA frame having the END_STREAM flag set:
POST /resource HTTP/1.1 HEADERS
Host: example.org ==> - END_STREAM
Content-Type: image/jpeg - END_HEADERS
Content-Length: 123 :method = POST
:path = /resource
{binary data} :scheme = https
CONTINUATION
+ END_HEADERS
content-type = image/jpeg
host = example.org
content-length = 123
DATA
+ END_STREAM
{binary data}
Note that data contributing to any given header field could be spread between header block fragments. The allocation of header fields to frames in this example is illustrative only.
A response that includes header fields and payload data is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames, followed by one or more DATA frames, with the last DATA frame in the sequence having the END_STREAM flag set:
HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> - END_STREAM
Content-Length: 123 + END_HEADERS
:status = 200
{binary data} content-type = image/jpeg
content-length = 123
DATA
+ END_STREAM
{binary data}
An informational response using a 1xx status code other than 101 is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames.
Trailing header fields are sent as a header block after both the request or response header block and all the DATA frames have been sent. The HEADERS frame starting the trailers header block has the END_STREAM flag set.
The following example includes both a 100 (Continue) status code, which is sent in response to a request containing a "100-continue" token in the Expect header field, and trailing header fields:
HTTP/1.1 100 Continue HEADERS
Extension-Field: bar ==> - END_STREAM
+ END_HEADERS
:status = 100
extension-field = bar
HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> - END_STREAM
Transfer-Encoding: chunked + END_HEADERS
Trailer: Foo :status = 200
content-length = 123
123 content-type = image/jpeg
{binary data} trailer = Foo
0
Foo: bar DATA
- END_STREAM
{binary data}
HEADERS
+ END_STREAM
+ END_HEADERS
foo = bar
8.1.4 Request Reliability Mechanisms in HTTP/2
In HTTP/1.1, an HTTP client is unable to retry a non-idempotent request when an error occurs because there is no means to determine the nature of the error. It is possible that some server processing occurred prior to the error, which could result in undesirable effects if the request were reattempted.
HTTP/2 provides two mechanisms for providing a guarantee to a client that a request has not been processed:
- The GOAWAY frame indicates the highest stream number that might have been processed. Requests on streams with higher numbers are therefore guaranteed to be safe to retry.
- The REFUSED_STREAM error code can be included in a RST_STREAM frame to indicate that the stream is being closed prior to any processing having occurred. Any request that was sent on the reset stream can be safely retried.
Requests that have not been processed have not failed; clients MAY automatically retry them, even those with non-idempotent methods.
A server MUST NOT indicate that a stream has not been processed unless it can guarantee that fact. If frames that are on a stream are passed to the application layer for any stream, then REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame MUST include a stream identifier that is greater than or equal to the given stream identifier.
In addition to these mechanisms, the PING frame provides a way for a client to easily test a connection. Connections that remain idle can become broken as some middleboxes (for instance, network address translators or load balancers) silently discard connection bindings. The PING frame allows a client to safely test whether a connection is still active without sending a request.
8.2 Server Push
HTTP/2 allows a server to pre-emptively send (or "push") responses (along with corresponding "promised" requests) to a client in association with a previous client-initiated request. This can be useful when the server knows the client will need to have those responses available in order to fully process the response to the original request.
A client can request that server push be disabled, though this is negotiated for each hop independently. The SETTINGS_ENABLE_PUSH setting can be set to 0 to indicate that server push is disabled.
Promised requests MUST be cacheable (see [RFC7231], Section 4.2.3), MUST be safe (see [RFC7231], Section 4.2.1), and MUST NOT include a request body. Clients that receive a promised request that is not cacheable, that is not known to be safe, or that indicates the presence of a request body MUST reset the promised stream with a stream error (Section 5.4.2) of type PROTOCOL_ERROR. Note this could result in the promised stream being reset if the client does not recognize a newly defined method as being safe.
Pushed responses that are cacheable (see [RFC7234], Section 3) can be stored by the client, if it implements an HTTP cache. Pushed responses are considered successfully validated on the origin server (e.g., if the "no-cache" cache response directive is present ([RFC7234], Section 5.2.2)) while the stream identified by the promised stream ID is still open.
Pushed responses that are not cacheable MUST NOT be stored by any HTTP cache. They MAY be made available to the application separately.
The server MUST include a value in the :authority pseudo-header field for which the server is authoritative (see Section 10.1). A client MUST treat a PUSH_PROMISE for which the server is not authoritative as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.
An intermediary can receive pushes from the server and choose not to forward them on to the client. In other words, how to make use of the pushed information is up to that intermediary. Equally, the intermediary might choose to make additional pushes to the client, without any action taken by the server.
A client cannot push. Thus, servers MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Clients MUST reject any attempt to change the SETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
8.2.1 Push Requests
Server push is semantically equivalent to a server responding to a request; however, in this case, that request is also sent by the server, as a PUSH_PROMISE frame.
The PUSH_PROMISE frame includes a header block that contains a complete set of request header fields that the server attributes to the request. It is not possible to push a response to a request that includes a request body.
Pushed responses are always associated with an explicit request from the client. The PUSH_PROMISE frames sent by the server are sent on that explicit request's stream. The PUSH_PROMISE frame also includes a promised stream identifier, chosen from the stream identifiers available to the server (see Section 5.1.1).
The header fields in PUSH_PROMISE and any subsequent CONTINUATION frames MUST be a valid and complete set of request header fields (Section 8.1.2.3). The server MUST include a method in the :method pseudo-header field that is safe and cacheable. If a client receives a PUSH_PROMISE that does not include a complete and valid set of header fields or the :method pseudo-header field identifies a method that is not safe, it MUST respond with a stream error (Section 5.4.2) of type PROTOCOL_ERROR.
The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to sending any frames that reference the promised responses. This avoids a race where clients issue requests prior to receiving any PUSH_PROMISE frames.
For example, if the server receives a request for a document containing embedded links to multiple image files and the server chooses to push those additional images to the client, sending PUSH_PROMISE frames before the DATA frames that contain the image links ensures that the client is able to see that a resource will be pushed before discovering embedded links. Similarly, if the server pushes responses referenced by the header block (for instance, in Link header fields), sending a PUSH_PROMISE before sending the header block ensures that clients do not request those resources.
PUSH_PROMISE frames MUST NOT be sent by the client.
PUSH_PROMISE frames can be sent by the server in response to any client-initiated stream, but the stream MUST be in either the "open" or "half-closed (remote)" state with respect to the server. PUSH_PROMISE frames are interspersed with the frames that comprise a response, though they cannot be interspersed with HEADERS and CONTINUATION frames that comprise a single header block.
Sending a PUSH_PROMISE frame creates a new stream and puts the stream into the “reserved (local)” state for the server and the “reserved (remote)” state for the client.
8.2.2 Push Responses
After sending the PUSH_PROMISE frame, the server can begin delivering the pushed response as a response (Section 8.1.2.4) on a server-initiated stream that uses the promised stream identifier. The server uses this stream to transmit an HTTP response, using the same sequence of frames as defined in Section 8.1. This stream becomes "half-closed" to the client (Section 5.1) after the initial HEADERS frame is sent.
Once a client receives a PUSH_PROMISE frame and chooses to accept the pushed response, the client SHOULD NOT issue any requests for the promised response until after the promised stream has closed.
If the client determines, for any reason, that it does not wish to receive the pushed response from the server or if the server takes too long to begin sending the promised response, the client can send a RST_STREAM frame, using either the CANCEL or REFUSED_STREAM code and referencing the pushed stream's identifier.
A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit the number of responses that can be concurrently pushed by a server. Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables server push by preventing the server from creating the necessary streams. This does not prohibit a server from sending PUSH_PROMISE frames; clients need to reset any promised streams that are not wanted.
Clients receiving a pushed response MUST validate that either the server is authoritative (see Section 10.1) or the proxy that provided the pushed response is configured for the corresponding request. For example, a server that offers a certificate for only the example.com DNS-ID or Common Name is not permitted to push a response for https://www.example.org/doc.
The response for a PUSH_PROMISE stream begins with a HEADERS frame, which immediately puts the stream into the "half-closed (remote)" state for the server and "half-closed (local)" state for the client, and ends with a frame bearing END_STREAM, which places the stream in the "closed" state.
Note: The client never sends a frame with the END_STREAM flag for a server push.
8.3 The CONNECT Method
In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) is used to convert an HTTP connection into a tunnel to a remote host. CONNECT is primarily used with HTTP proxies to establish a TLS session with an origin server for the purposes of interacting with https resources.
In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream to a remote host for similar purposes. The HTTP header field mapping works as defined in Section 8.1.2.3 ("Request Pseudo-Header Fields"), with a few differences. Specifically:
- The :method pseudo-header field is set to CONNECT.
- The :scheme and :path pseudo-header fields MUST be omitted.
- The :authority pseudo-header field contains the host and port to connect to (equivalent to the authority-form of the request-target of CONNECT requests (see [RFC7230], Section 5.3)).
A CONNECT request that does not conform to these restrictions is malformed (Section 8.1.2.6).
A proxy that supports CONNECT establishes a TCP connection [TCP] to the server identified in the :authority pseudo-header field. Once this connection is successfully established, the proxy sends a HEADERS frame containing a 2xx series status code to the client, as defined in [RFC7231], Section 4.3.6.
After the initial HEADERS frame sent by each peer, all subsequent DATA frames correspond to data sent on the TCP connection. The payload of any DATA frames sent by the client is transmitted by the proxy to the TCP server; data received from the TCP server is assembled into DATA frames by the proxy. Frame types other than DATA or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY) MUST NOT be sent on a connected stream and MUST be treated as a stream error (Section 5.4.2) if received.
The TCP connection can be closed by either peer. The END_STREAM flag on a DATA frame is treated as being equivalent to the TCP FIN bit. A client is expected to send a DATA frame with the END_STREAM flag set after receiving a frame bearing the END_STREAM flag. A proxy that receives a DATA frame with the END_STREAM flag set sends the attached data with the FIN bit set on the last TCP segment. A proxy that receives a TCP segment with the FIN bit set sends a DATA frame with the END_STREAM flag set. Note that the final TCP segment or DATA frame could be empty.
A TCP connection error is signaled with RST_STREAM. A proxy treats any error in the TCP connection, which includes receiving a TCP segment with the RST bit set, as a stream error (Section 5.4.2) of type CONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detects an error with the stream or the HTTP/2 connection.
9. Additional HTTP Requirements/Considerations
This section outlines attributes of the HTTP protocol that improve interoperability, reduce exposure to known security vulnerabilities, or reduce the potential for implementation variation.
9.1 Connection Management
HTTP/2 connections are persistent. For best performance, it is expected that clients will not close connections until it is determined that no further communication with a server is necessary (for example, when a user navigates away from a particular web page) or until the server closes the connection.
Clients SHOULD NOT open more than one HTTP/2 connection to a given host and port pair, where the host is derived from a URI, a selected alternative service [ALT-SVC], or a configured proxy.
A client can create additional connections as replacements, either to replace connections that are near to exhausting the available stream identifier space (Section 5.1.1), to refresh the keying material for a TLS connection, or to replace connections that have encountered errors (Section 5.4.1).
A client MAY open multiple connections to the same IP address and TCP port using different Server Name Indication [TLS-EXT] values or to provide different TLS client certificates but SHOULD avoid creating multiple connections with the same configuration.
Servers are encouraged to maintain open connections for as long as possible but are permitted to terminate idle connections if necessary. When either endpoint chooses to close the transport-layer TCP connection, the terminating endpoint SHOULD first send a GOAWAY (Section 6.8) frame so that both endpoints can reliably determine whether previously sent frames have been processed and gracefully complete or terminate any necessary remaining tasks.
9.1.1 Connection Reuse
Connections that are made to an origin server, either directly or through a tunnel created using the CONNECT method (Section 8.3), MAY be reused for requests with multiple different URI authority components. A connection can be reused as long as the origin server is authoritative (Section 10.1). For TCP connections without TLS, this depends on the host having resolved to the same IP address.
For https resources, connection reuse additionally depends on having a certificate that is valid for the host in the URI. The certificate presented by the server MUST satisfy any checks that the client would perform when forming a new TLS connection for the host in the URI.
An origin server might offer a certificate with multiple subjectAltName attributes or names with wildcards, one of which is valid for the authority in the URI. For example, a certificate with a subjectAltName of *.example.com might permit the use of the same connection for requests to URIs starting with https://a.example.com/ and https://b.example.com/.
In some deployments, reusing a connection for multiple origins can result in requests being directed to the wrong origin server. For example, TLS termination might be performed by a middlebox that uses the TLS Server Name Indication (SNI) [TLS-EXT] extension to select an origin server. This means that it is possible for clients to send confidential information to servers that might not be the intended target for the request, even though the server is otherwise authoritative.
A server that does not wish clients to reuse connections can indicate that it is not authoritative for a request by sending a 421 (Misdirected Request) status code in response to the request (see Section 9.1.2).
A client that is configured to use a proxy over HTTP/2 directs requests to that proxy through a single connection. That is, all requests sent via a proxy reuse the connection to the proxy.
9.1.2 The 421 (Misdirected Request) Status Code
The 421 (Misdirected Request) status code indicates that the request was directed at a server that is not able to produce a response. This can be sent by a server that is not configured to produce responses for the combination of scheme and authority that are included in the request URI.
Clients receiving a 421 (Misdirected Request) response from a server MAY retry the request — whether the request method is idempotent or not — over a different connection. This is possible if a connection is reused (Section 9.1.1) or if an alternative service is selected [ALT-SVC].
This status code MUST NOT be generated by proxies.
A 421 response is cacheable by default, i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [RFC7234]).
9.2 Use of TLS Features
Implementations of HTTP/2 MUST use TLS version 1.2 [TLS12] or higher for HTTP/2 over TLS. The general TLS usage guidance in [TLSBCP] SHOULD be followed, with some additional restrictions that are specific to HTTP/2.
The TLS implementation MUST support the Server Name Indication (SNI) [TLS-EXT] extension to TLS. HTTP/2 clients MUST indicate the target domain name when negotiating TLS.
Deployments of HTTP/2 that negotiate TLS 1.3 or higher need only support and use the SNI extension; deployments of TLS 1.2 are subject to the requirements in the following sections. Implementations are encouraged to provide defaults that comply, but it is recognized that deployments are ultimately responsible for compliance.
9.2.1 TLS 1.2 Features
This section describes restrictions on the TLS 1.2 feature set that can be used with HTTP/2. Due to deployment limitations, it might not be possible to fail TLS negotiation when these restrictions are not met. An endpoint MAY immediately terminate an HTTP/2 connection that does not meet these TLS requirements with a connection error (Section 5.4.1) of type INADEQUATE_SECURITY.
A deployment of HTTP/2 over TLS 1.2 MUST disable compression. TLS compression can lead to the exposure of information that would not otherwise be revealed [RFC3749]. Generic compression is unnecessary since HTTP/2 provides compression features that are more aware of context and therefore likely to be more appropriate for use for performance, security, or other reasons.
A deployment of HTTP/2 over TLS 1.2 MUST disable renegotiation. An endpoint MUST treat a TLS renegotiation as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that disabling renegotiation can result in long-lived connections becoming unusable due to limits on the number of messages the underlying cipher suite can encipher.
An endpoint MAY use renegotiation to provide confidentiality protection for client credentials offered in the handshake, but any renegotiation MUST occur prior to sending the connection preface. A server SHOULD request a client certificate if it sees a renegotiation request immediately after establishing a connection.
This effectively prevents the use of renegotiation in response to a request for a specific protected resource. A future specification might provide a way to support this use case. Alternatively, a server might use an error (Section 5.4) of type HTTP_1_1_REQUIRED to request the client use a protocol that supports renegotiation.
Implementations MUST support ephemeral key exchange sizes of at least 2048 bits for cipher suites that use ephemeral finite field Diffie-Hellman (DHE) [TLS12] and 224 bits for cipher suites that use ephemeral elliptic curve Diffie-Hellman (ECDHE) [RFC4492]. Clients MUST accept DHE sizes of up to 4096 bits. Endpoints MAY treat negotiation of key sizes smaller than the lower limits as a connection error (Section 5.4.1) of type INADEQUATE_SECURITY.
9.2.2 TLS 1.2 Cipher Suites
A deployment of HTTP/2 over TLS 1.2 SHOULD NOT use any of the cipher suites that are listed in the cipher suite black list (Appendix A).
Endpoints MAY choose to generate a connection error (Section 5.4.1) of type INADEQUATE_SECURITY if one of the cipher suites from the black list is negotiated. A deployment that chooses to use a black-listed cipher suite risks triggering a connection error unless the set of potential peers is known to accept that cipher suite.
Implementations MUST NOT generate this error in reaction to the negotiation of a cipher suite that is not on the black list. Consequently, when clients offer a cipher suite that is not on the black list, they have to be prepared to use that cipher suite with HTTP/2.
The black list includes the cipher suite that TLS 1.2 makes mandatory, which means that TLS 1.2 deployments could have non-intersecting sets of permitted cipher suites. To avoid this problem causing TLS handshake failures, deployments of HTTP/2 that use TLS 1.2 MUST support TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [TLS-ECDHE] with the P-256 elliptic curve [FIPS186].
Note that clients might advertise support of cipher suites that are on the black list in order to allow for connection to servers that do not support HTTP/2. This allows servers to select HTTP/1.1 with a cipher suite that is on the HTTP/2 black list. However, this can result in HTTP/2 being negotiated with a black-listed cipher suite if the application protocol and cipher suite are independently selected.
10. Security Considerations
10.2 Cross-Protocol Attacks
In a cross-protocol attack, an attacker causes a client to initiate a transaction in one protocol toward a server that understands a different protocol. An attacker might be able to cause the transaction to appear as a valid transaction in the second protocol. In combination with the capabilities of the web context, this can be used to interact with poorly protected servers in private networks.
Completing a TLS handshake with an ALPN identifier for HTTP/2 can be considered sufficient protection against cross-protocol attacks. ALPN provides a positive indication that a server is willing to proceed with HTTP/2, which prevents attacks on other TLS-based protocols.
The encryption in TLS makes it difficult for attackers to control the data that could be used in a cross-protocol attack on a cleartext protocol.
The cleartext version of HTTP/2 has minimal protection against cross-protocol attacks. The connection preface (Section 3.5) contains a string that is designed to confuse HTTP/1.1 servers, but no special protection is offered for other protocols. A server that is willing to ignore parts of an HTTP/1.1 request containing an Upgrade header field in addition to the client connection preface could be exposed to a cross-protocol attack.
10.3 Intermediary Encapsulation Attacks
The HTTP/2 header field encoding allows the expression of names that are not valid field names in the Internet Message Syntax used by HTTP/1.1. Requests or responses containing invalid header field names MUST be treated as malformed (Section 8.1.2.6). An intermediary therefore cannot translate an HTTP/2 request or response containing an invalid field name into an HTTP/1.1 message.
Similarly, HTTP/2 allows header field values that are not valid. While most of the values that can be encoded will not alter header field parsing, carriage return (CR, ASCII 0xd), line feed (LF, ASCII 0xa), and the zero character (NUL, ASCII 0x0) might be exploited by an attacker if they are translated verbatim. Any request or response that contains a character not permitted in a header field value MUST be treated as malformed (Section 8.1.2.6). Valid characters are defined by the field-content ABNF rule in Section 3.2 of [RFC7230].
10.4 Cacheability of Pushed Responses
Pushed responses do not have an explicit request from the client; the request is provided by the server in the PUSH_PROMISE frame.
Caching responses that are pushed is possible based on the guidance provided by the origin server in the Cache-Control header field. However, this can cause issues if a single server hosts more than one tenant. For example, a server might offer multiple users each a small portion of its URI space.
Where multiple tenants share space on the same server, that server MUST ensure that tenants are not able to push representations of resources that they do not have authority over. Failure to enforce this would allow a tenant to provide a representation that would be served out of cache, overriding the actual representation that the authoritative tenant provides.
Pushed responses for which an origin server is not authoritative (see Section 10.1) MUST NOT be used or cached.
10.5 Denial-of-Service Considerations
An HTTP/2 connection can demand a greater commitment of resources to operate than an HTTP/1.1 connection. The use of header compression and flow control depend on a commitment of resources for storing a greater amount of state. Settings for these features ensure that memory commitments for these features are strictly bounded.
The number of PUSH_PROMISE frames is not constrained in the same fashion. A client that accepts server push SHOULD limit the number of streams it allows to be in the "reserved (remote)" state. An excessive number of server push streams can be treated as a stream error (Section 5.4.2) of type ENHANCE_YOUR_CALM.
Processing capacity cannot be guarded as effectively as state capacity.
The SETTINGS frame can be abused to cause a peer to expend additional processing time. This might be done by pointlessly changing SETTINGS parameters, setting multiple undefined parameters, or changing the same setting multiple times in the same frame. WINDOW_UPDATE or PRIORITY frames can be abused to cause an unnecessary waste of resources.
Large numbers of small or empty frames can be abused to cause a peer to expend time processing frame headers. Note, however, that some uses are entirely legitimate, such as the sending of an empty DATA or CONTINUATION frame at the end of a stream.
Header compression also offers some opportunities to waste processing resources; see Section 7 of [COMPRESSION] for more details on potential abuses.
Limits in SETTINGS parameters cannot be reduced instantaneously, which leaves an endpoint exposed to behavior from a peer that could exceed the new limits. In particular, immediately after establishing a connection, limits set by a server are not known to clients and could be exceeded without being an obvious protocol violation.
All these features — i.e., SETTINGS changes, small frames, header compression — have legitimate uses. These features become a burden only when they are used unnecessarily or to excess.
An endpoint that doesn't monitor this behavior exposes itself to a risk of denial-of-service attack. Implementations SHOULD track the use of these features and set limits on their use. An endpoint MAY treat activity that is suspicious as a connection error (Section 5.4.1) of type ENHANCE_YOUR_CALM.
10.5.1 Limits on Header Block Size
A large header block (Section 4.3) can cause an implementation to commit a large amount of state. Header fields that are critical for routing can appear toward the end of a header block, which prevents streaming of header fields to their ultimate destination. This ordering and other reasons, such as ensuring cache correctness, mean that an endpoint might need to buffer the entire header block. Since there is no hard limit to the size of a header block, some endpoints could be forced to commit a large amount of available memory for header fields.
An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to advise peers of limits that might apply on the size of header blocks. This setting is only advisory, so endpoints MAY choose to send header blocks that exceed this limit and risk having the request or response being treated as malformed. This setting is specific to a connection, so any request or response could encounter a hop with a lower, unknown limit. An intermediary can attempt to avoid this problem by passing on values presented by different peers, but they are not obligated to do so.
A server that receives a larger header block than it is willing to handle can send an HTTP 431 (Request Header Fields Too Large) status code [RFC6585]. A client can discard responses that it cannot process. The header block MUST be processed to ensure a consistent connection state, unless the connection is closed.
10.5.2 CONNECT Issues
The CONNECT method can be used to create disproportionate load on an proxy, since stream creation is relatively inexpensive when compared to the creation and maintenance of a TCP connection. A proxy might also maintain some resources for a TCP connection beyond the closing of the stream that carries the CONNECT request, since the outgoing TCP connection remains in the TIME_WAIT state. Therefore, a proxy cannot rely on SETTINGS_MAX_CONCURRENT_STREAMS alone to limit the resources consumed by CONNECT requests.
10.6 Use of Compression
Compression can allow an attacker to recover secret data when it is compressed in the same context as data under attacker control. HTTP/2 enables compression of header fields (Section 4.3); the following concerns also apply to the use of HTTP compressed content-codings ([RFC7231], Section 3.1.2.1).
There are demonstrable attacks on compression that exploit the characteristics of the web (e.g., [BREACH]). The attacker induces multiple requests containing varying plaintext, observing the length of the resulting ciphertext in each, which reveals a shorter length when a guess about the secret is correct.
Implementations communicating on a secure channel MUST NOT compress content that includes both confidential and attacker-controlled data unless separate compression dictionaries are used for each source of data. Compression MUST NOT be used if the source of data cannot be reliably determined. Generic stream compression, such as that provided by TLS, MUST NOT be used with HTTP/2 (see Section 9.2).
Further considerations regarding the compression of header fields are described in [COMPRESSION].
10.7 Use of Padding
Padding within HTTP/2 is not intended as a replacement for general purpose padding, such as might be provided by TLS [TLS12]. Redundant padding could even be counterproductive. Correct application can depend on having specific knowledge of the data that is being padded.
To mitigate attacks that rely on compression, disabling or limiting compression might be preferable to padding as a countermeasure.
Padding can be used to obscure the exact size of frame content and is provided to mitigate specific attacks within HTTP, for example, attacks where compressed content includes both attacker-controlled plaintext and secret data (e.g., [BREACH]).
Use of padding can result in less protection than might seem immediately obvious. At best, padding only makes it more difficult for an attacker to infer length information by increasing the number of frames an attacker has to observe. Incorrectly implemented padding schemes can be easily defeated. In particular, randomized padding with a predictable distribution provides very little protection; similarly, padding payloads to a fixed size exposes information as payload sizes cross the fixed-sized boundary, which could be possible if an attacker can control plaintext.
Intermediaries SHOULD retain padding for DATA frames but MAY drop padding for HEADERS and PUSH_PROMISE frames. A valid reason for an intermediary to change the amount of padding of frames is to improve the protections that padding provides.
10.8 Privacy Considerations
Several characteristics of HTTP/2 provide an observer an opportunity to correlate actions of a single client or server over time. These include the value of settings, the manner in which flow-control windows are managed, the way priorities are allocated to streams, the timing of reactions to stimulus, and the handling of any features that are controlled by settings.
As far as these create observable differences in behavior, they could be used as a basis for fingerprinting a specific client, as defined in Section 1.8 of [HTML5].
HTTP/2's preference for using a single TCP connection allows correlation of a user's activity on a site. Reusing connections for different origins allows tracking across those origins.
Because the PING and SETTINGS frames solicit immediate responses, they can be used by an endpoint to measure latency to their peer. This might have privacy implications in certain scenarios.
11. IANA Considerations
A string for identifying HTTP/2 is entered into the "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLS-ALPN].
This document establishes a registry for frame types, settings, and error codes. These new registries appear in the new "Hypertext Transfer Protocol version 2 (HTTP/2) Parameters" section.
This document registers the HTTP2-Settings header field for use in HTTP; it also registers the 421 (Misdirected Request) status code.
This document registers the PRI method for use in HTTP to avoid collisions with the connection preface (Section 3.5).
11.1 Registration of HTTP/2 Identification Strings
This document creates two registrations for the identification of HTTP/2 (see Section 3.3) in the "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLS-ALPN].
The "h2" string identifies HTTP/2 when used over TLS:
- Protocol:
- HTTP/2 over TLS
- Identification Sequence:
- 0x68 0x32 ("h2")
- Specification:
- This document
The "h2c" string identifies HTTP/2 when used over cleartext TCP:
- Protocol:
- HTTP/2 over TCP
- Identification Sequence:
- 0x68 0x32 0x63 ("h2c")
- Specification:
- This document
11.2 Frame Type Registry
This document establishes a registry for HTTP/2 frame type codes. The "HTTP/2 Frame Type" registry manages an 8-bit space. The "HTTP/2 Frame Type" registry operates under either of the "IETF Review" or "IESG Approval" policies [RFC5226] for values between 0x00 and 0xef, with values between 0xf0 and 0xff being reserved for Experimental Use.
New entries in this registry require the following information:
- Frame Type:
- A name or label for the frame type.
- Code:
- The 8-bit code assigned to the frame type.
- Specification:
- A reference to a specification that includes a description of the frame layout, its semantics, and flags that the frame type uses, including any parts of the frame that are conditionally present based on the value of flags.
The entries in the following table are registered by this document.
| Frame Type | Code | Section |
|---|---|---|
| DATA | 0x0 | Section 6.1 |
| HEADERS | 0x1 | Section 6.2 |
| PRIORITY | 0x2 | Section 6.3 |
| RST_STREAM | 0x3 | Section 6.4 |
| SETTINGS | 0x4 | Section 6.5 |
| PUSH_PROMISE | 0x5 | Section 6.6 |
| PING | 0x6 | Section 6.7 |
| GOAWAY | 0x7 | Section 6.8 |
| WINDOW_UPDATE | 0x8 | Section 6.9 |
| CONTINUATION | 0x9 | Section 6.10 |
11.3 Settings Registry
This document establishes a registry for HTTP/2 settings. The "HTTP/2 Settings" registry manages a 16-bit space. The "HTTP/2 Settings" registry operates under the "Expert Review" policy [RFC5226] for values in the range from 0x0000 to 0xefff, with values between and 0xf000 and 0xffff being reserved for Experimental Use.
New registrations are advised to provide the following information:
- Name:
- A symbolic name for the setting. Specifying a setting name is optional.
- Code:
- The 16-bit code assigned to the setting.
- Initial Value:
- An initial value for the setting.
- Specification:
- An optional reference to a specification that describes the use of the setting.
The entries in the following table are registered by this document.
| Name | Code | Initial Value | Specification |
|---|---|---|---|
| HEADER_TABLE_SIZE | 0x1 | 4096 | Section 6.5.2 |
| ENABLE_PUSH | 0x2 | 1 | Section 6.5.2 |
| MAX_CONCURRENT_STREAMS | 0x3 | (infinite) | Section 6.5.2 |
| INITIAL_WINDOW_SIZE | 0x4 | 65535 | Section 6.5.2 |
| MAX_FRAME_SIZE | 0x5 | 16384 | Section 6.5.2 |
| MAX_HEADER_LIST_SIZE | 0x6 | (infinite) | Section 6.5.2 |
11.4 Error Code Registry
This document establishes a registry for HTTP/2 error codes. The "HTTP/2 Error Code" registry manages a 32-bit space. The "HTTP/2 Error Code" registry operates under the "Expert Review" policy [RFC5226].
Registrations for error codes are required to include a description of the error code. An expert reviewer is advised to examine new registrations for possible duplication with existing error codes. Use of existing registrations is to be encouraged, but not mandated.
New registrations are advised to provide the following information:
- Name:
- A name for the error code. Specifying an error code name is optional.
- Code:
- The 32-bit error code value.
- Description:
- A brief description of the error code semantics, longer if no detailed specification is provided.
- Specification:
- An optional reference for a specification that defines the error code.
The entries in the following table are registered by this document.
| Name | Code | Description | Specification |
|---|---|---|---|
| NO_ERROR | 0x0 | Graceful shutdown | Section 7 |
| PROTOCOL_ERROR | 0x1 | Protocol error detected | Section 7 |
| INTERNAL_ERROR | 0x2 | Implementation fault | Section 7 |
| FLOW_CONTROL_ERROR | 0x3 | Flow-control limits exceeded | Section 7 |
| SETTINGS_TIMEOUT | 0x4 | Settings not acknowledged | Section 7 |
| STREAM_CLOSED | 0x5 | Frame received for closed stream | Section 7 |
| FRAME_SIZE_ERROR | 0x6 | Frame size incorrect | Section 7 |
| REFUSED_STREAM | 0x7 | Stream not processed | Section 7 |
| CANCEL | 0x8 | Stream cancelled | Section 7 |
| COMPRESSION_ERROR | 0x9 | Compression state not updated | Section 7 |
| CONNECT_ERROR | 0xa | TCP connection error for CONNECT method | Section 7 |
| ENHANCE_YOUR_CALM | 0xb | Processing capacity exceeded | Section 7 |
| INADEQUATE_SECURITY | 0xc | Negotiated TLS parameters not acceptable | Section 7 |
| HTTP_1_1_REQUIRED | 0xd | Use HTTP/1.1 for the request | Section 7 |
11.5 HTTP2-Settings Header Field Registration
This section registers the HTTP2-Settings header field in the "Permanent Message Header Field Names" registry [BCP90].
- Header field name:
- HTTP2-Settings
- Applicable protocol:
- http
- Status:
- standard
- Author/Change controller:
- IETF
- Specification document(s):
- Section 3.2.1 of this document
- Related information:
- This header field is only used by an HTTP/2 client for Upgrade-based negotiation.
11.6 PRI Method Registration
This section registers the PRI method in the "HTTP Method Registry" ([RFC7231], Section 8.1).
- Method Name:
- PRI
- Safe:
- Yes
- Idempotent:
- Yes
- Specification document(s):
- Section 3.5 of this document
- Related information:
- This method is never used by an actual client. This method will appear to be used when an HTTP/1.1 server or intermediary attempts to parse an HTTP/2 connection preface.
11.7 The 421 (Misdirected Request) HTTP Status Code
This document registers the 421 (Misdirected Request) HTTP status code in the "HTTP Status Codes" registry ([RFC7231], Section 8.2).
- Status Code:
- 421
- Short Description:
- Misdirected Request
- Specification:
- Section 9.1.2 of this document
11.8 The h2c Upgrade Token
This document registers the "h2c" upgrade token in the "HTTP Upgrade Tokens" registry ([RFC7230], Section 8.6).
- Value:
- h2c
- Description:
- Hypertext Transfer Protocol version 2 (HTTP/2)
- Expected Version Tokens:
- None
- Reference:
- Section 3.2 of this document
12. References
12.1 Normative References
- [COMPRESSION]
- Peon, R. and H. Ruellan, “HPACK: Header Compression for HTTP/2”, RFC 7541, DOI 10.17487/RFC7541, May 2015, <http://www.rfc-editor.org/info/rfc7541>.
- [COOKIE]
- Barth, A., “HTTP State Management Mechanism”, RFC 6265, DOI 10.17487/RFC6265, April 2011, <http://www.rfc-editor.org/info/rfc6265>.
- [FIPS186]
- NIST, “Digital Signature Standard (DSS)”, FIPS PUB 186-4, July 2013, <http://dx.doi.org/10.6028/NIST.FIPS.186-4>.
- [RFC2119]
- Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
- [RFC2818]
- Rescorla, E., “HTTP Over TLS”, RFC 2818, DOI 10.17487/RFC2818, May 2000, <http://www.rfc-editor.org/info/rfc2818>.
- [RFC3986]
- Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <http://www.rfc-editor.org/info/rfc3986>.
- [RFC4648]
- Josefsson, S., “The Base16, Base32, and Base64 Data Encodings”, RFC 4648, DOI 10.17487/RFC4648, October 2006, <http://www.rfc-editor.org/info/rfc4648>.
- [RFC5226]
- Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>.
- [RFC5234]
- Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <http://www.rfc-editor.org/info/rfc5234>.
- [RFC7230]
- Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>.
- [RFC7231]
- Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, RFC 7231, DOI 10.17487/RFC7231, June 2014, <http://www.rfc-editor.org/info/rfc7231>.
- [RFC7232]
- Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests”, RFC 7232, DOI 10.17487/RFC7232, June 2014, <http://www.rfc-editor.org/info/rfc7232>.
- [RFC7233]
- Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Range Requests”, RFC 7233, DOI 10.17487/RFC7233, June 2014, <http://www.rfc-editor.org/info/rfc7233>.
- [RFC7234]
- Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”, RFC 7234, DOI 10.17487/RFC7234, June 2014, <http://www.rfc-editor.org/info/rfc7234>.
- [RFC7235]
- Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Authentication”, RFC 7235, DOI 10.17487/RFC7235, June 2014, <http://www.rfc-editor.org/info/rfc7235>.
- [TCP]
- Postel, J., “Transmission Control Protocol”, STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, <http://www.rfc-editor.org/info/rfc793>.
- [TLS-ALPN]
- Friedl, S., Popov, A., Langley, A., and E. Stephan, “Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension”, RFC 7301, DOI 10.17487/RFC7301, July 2014, <http://www.rfc-editor.org/info/rfc7301>.
- [TLS-ECDHE]
- Rescorla, E., “TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)”, RFC 5289, DOI 10.17487/RFC5289, August 2008, <http://www.rfc-editor.org/info/rfc5289>.
- [TLS-EXT]
- Eastlake 3rd, D., “Transport Layer Security (TLS) Extensions: Extension Definitions”, RFC 6066, DOI 10.17487/RFC6066, January 2011, <http://www.rfc-editor.org/info/rfc6066>.
- [TLS12]
- Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2”, RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>.
12.2 Informative References
- [ALT-SVC]
- Nottingham, M., McManus, P., and J. Reschke, “HTTP Alternative Services”, Work in Progress, draft-ietf-httpbis-alt-svc-06, February 2015.
- [BCP90]
- Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004, <http://www.rfc-editor.org/info/bcp90>.
- [BREACH]
- Gluck, Y., Harris, N., and A. Prado, “BREACH: Reviving the CRIME Attack”, July 2013, <http://breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.
- [HTML5]
- Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Doyle Navara, E., O'Connor, E., and S. Pfeiffer, “HTML5”, W3C Recommendation REC-html5-20141028, October 2014, <http://www.w3.org/TR/2014/REC-html5-20141028/>.
- [RFC3749]
- Hollenbeck, S., “Transport Layer Security Protocol Compression Methods”, RFC 3749, DOI 10.17487/RFC3749, May 2004, <http://www.rfc-editor.org/info/rfc3749>.
- [RFC4492]
- Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, “Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)”, RFC 4492, DOI 10.17487/RFC4492, May 2006, <http://www.rfc-editor.org/info/rfc4492>.
- [RFC6585]
- Nottingham, M. and R. Fielding, “Additional HTTP Status Codes”, RFC 6585, DOI 10.17487/RFC6585, April 2012, <http://www.rfc-editor.org/info/rfc6585>.
- [RFC7323]
- Borman, D., Braden, B., Jacobson, V., and R. Scheffenegger, Ed., “TCP Extensions for High Performance”, RFC 7323, DOI 10.17487/RFC7323, September 2014, <http://www.rfc-editor.org/info/rfc7323>.
- [TALKING]
- Huang, L., Chen, E., Barth, A., Rescorla, E., and C. Jackson, “Talking to Yourself for Fun and Profit”, 2011, <http://w2spconf.com/2011/papers/websocket.pdf>.
- [TLSBCP]
- Sheffer, Y., Holz, R., and P. Saint-Andre, “Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)”, BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, <http://www.rfc-editor.org/info/rfc7525>.
A. TLS 1.2 Cipher Suite Black List
An HTTP/2 implementation MAY treat the negotiation of any of the following cipher suites with TLS 1.2 as a connection error (Section 5.4.1) of type INADEQUATE_SECURITY:
- TLS_NULL_WITH_NULL_NULL
- TLS_RSA_WITH_NULL_MD5
- TLS_RSA_WITH_NULL_SHA
- TLS_RSA_EXPORT_WITH_RC4_40_MD5
- TLS_RSA_WITH_RC4_128_MD5
- TLS_RSA_WITH_RC4_128_SHA
- TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
- TLS_RSA_WITH_IDEA_CBC_SHA
- TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
- TLS_RSA_WITH_DES_CBC_SHA
- TLS_RSA_WITH_3DES_EDE_CBC_SHA
- TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
- TLS_DH_DSS_WITH_DES_CBC_SHA
- TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA
- TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
- TLS_DH_RSA_WITH_DES_CBC_SHA
- TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA
- TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
- TLS_DHE_DSS_WITH_DES_CBC_SHA
- TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
- TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
- TLS_DHE_RSA_WITH_DES_CBC_SHA
- TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA
- TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
- TLS_DH_anon_WITH_RC4_128_MD5
- TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
- TLS_DH_anon_WITH_DES_CBC_SHA
- TLS_DH_anon_WITH_3DES_EDE_CBC_SHA
- TLS_KRB5_WITH_DES_CBC_SHA
- TLS_KRB5_WITH_3DES_EDE_CBC_SHA
- TLS_KRB5_WITH_RC4_128_SHA
- TLS_KRB5_WITH_IDEA_CBC_SHA
- TLS_KRB5_WITH_DES_CBC_MD5
- TLS_KRB5_WITH_3DES_EDE_CBC_MD5
- TLS_KRB5_WITH_RC4_128_MD5
- TLS_KRB5_WITH_IDEA_CBC_MD5
- TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA
- TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA
- TLS_KRB5_EXPORT_WITH_RC4_40_SHA
- TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5
- TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5
- TLS_KRB5_EXPORT_WITH_RC4_40_MD5
- TLS_PSK_WITH_NULL_SHA
- TLS_DHE_PSK_WITH_NULL_SHA
- TLS_RSA_PSK_WITH_NULL_SHA
- TLS_RSA_WITH_AES_128_CBC_SHA
- TLS_DH_DSS_WITH_AES_128_CBC_SHA
- TLS_DH_RSA_WITH_AES_128_CBC_SHA
- TLS_DHE_DSS_WITH_AES_128_CBC_SHA
- TLS_DHE_RSA_WITH_AES_128_CBC_SHA
- TLS_DH_anon_WITH_AES_128_CBC_SHA
- TLS_RSA_WITH_AES_256_CBC_SHA
- TLS_DH_DSS_WITH_AES_256_CBC_SHA
- TLS_DH_RSA_WITH_AES_256_CBC_SHA
- TLS_DHE_DSS_WITH_AES_256_CBC_SHA
- TLS_DHE_RSA_WITH_AES_256_CBC_SHA
- TLS_DH_anon_WITH_AES_256_CBC_SHA
- TLS_RSA_WITH_NULL_SHA256
- TLS_RSA_WITH_AES_128_CBC_SHA256
- TLS_RSA_WITH_AES_256_CBC_SHA256
- TLS_DH_DSS_WITH_AES_128_CBC_SHA256
- TLS_DH_RSA_WITH_AES_128_CBC_SHA256
- TLS_DHE_DSS_WITH_AES_128_CBC_SHA256
- TLS_RSA_WITH_CAMELLIA_128_CBC_SHA
- TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA
- TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA
- TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA
- TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA
- TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA
- TLS_DHE_RSA_WITH_AES_128_CBC_SHA256
- TLS_DH_DSS_WITH_AES_256_CBC_SHA256
- TLS_DH_RSA_WITH_AES_256_CBC_SHA256
- TLS_DHE_DSS_WITH_AES_256_CBC_SHA256
- TLS_DHE_RSA_WITH_AES_256_CBC_SHA256
- TLS_DH_anon_WITH_AES_128_CBC_SHA256
- TLS_DH_anon_WITH_AES_256_CBC_SHA256
- TLS_RSA_WITH_CAMELLIA_256_CBC_SHA
- TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA
- TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA
- TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA
- TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA
- TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA
- TLS_PSK_WITH_RC4_128_SHA
- TLS_PSK_WITH_3DES_EDE_CBC_SHA
- TLS_PSK_WITH_AES_128_CBC_SHA
- TLS_PSK_WITH_AES_256_CBC_SHA
- TLS_DHE_PSK_WITH_RC4_128_SHA
- TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA
- TLS_DHE_PSK_WITH_AES_128_CBC_SHA
- TLS_DHE_PSK_WITH_AES_256_CBC_SHA
- TLS_RSA_PSK_WITH_RC4_128_SHA
- TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA
- TLS_RSA_PSK_WITH_AES_128_CBC_SHA
- TLS_RSA_PSK_WITH_AES_256_CBC_SHA
- TLS_RSA_WITH_SEED_CBC_SHA
- TLS_DH_DSS_WITH_SEED_CBC_SHA
- TLS_DH_RSA_WITH_SEED_CBC_SHA
- TLS_DHE_DSS_WITH_SEED_CBC_SHA
- TLS_DHE_RSA_WITH_SEED_CBC_SHA
- TLS_DH_anon_WITH_SEED_CBC_SHA
- TLS_RSA_WITH_AES_128_GCM_SHA256
- TLS_RSA_WITH_AES_256_GCM_SHA384
- TLS_DH_RSA_WITH_AES_128_GCM_SHA256
- TLS_DH_RSA_WITH_AES_256_GCM_SHA384
- TLS_DH_DSS_WITH_AES_128_GCM_SHA256
- TLS_DH_DSS_WITH_AES_256_GCM_SHA384
- TLS_DH_anon_WITH_AES_128_GCM_SHA256
- TLS_DH_anon_WITH_AES_256_GCM_SHA384
- TLS_PSK_WITH_AES_128_GCM_SHA256
- TLS_PSK_WITH_AES_256_GCM_SHA384
- TLS_RSA_PSK_WITH_AES_128_GCM_SHA256
- TLS_RSA_PSK_WITH_AES_256_GCM_SHA384
- TLS_PSK_WITH_AES_128_CBC_SHA256
- TLS_PSK_WITH_AES_256_CBC_SHA384
- TLS_PSK_WITH_NULL_SHA256
- TLS_PSK_WITH_NULL_SHA384
- TLS_DHE_PSK_WITH_AES_128_CBC_SHA256
- TLS_DHE_PSK_WITH_AES_256_CBC_SHA384
- TLS_DHE_PSK_WITH_NULL_SHA256
- TLS_DHE_PSK_WITH_NULL_SHA384
- TLS_RSA_PSK_WITH_AES_128_CBC_SHA256
- TLS_RSA_PSK_WITH_AES_256_CBC_SHA384
- TLS_RSA_PSK_WITH_NULL_SHA256
- TLS_RSA_PSK_WITH_NULL_SHA384
- TLS_RSA_WITH_CAMELLIA_128_CBC_SHA256
- TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA256
- TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA256
- TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA256
- TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA256
- TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA256
- TLS_RSA_WITH_CAMELLIA_256_CBC_SHA256
- TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA256
- TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA256
- TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA256
- TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA256
- TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA256
- TLS_EMPTY_RENEGOTIATION_INFO_SCSV
- TLS_ECDH_ECDSA_WITH_NULL_SHA
- TLS_ECDH_ECDSA_WITH_RC4_128_SHA
- TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA
- TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA
- TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA
- TLS_ECDHE_ECDSA_WITH_NULL_SHA
- TLS_ECDHE_ECDSA_WITH_RC4_128_SHA
- TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA
- TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
- TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
- TLS_ECDH_RSA_WITH_NULL_SHA
- TLS_ECDH_RSA_WITH_RC4_128_SHA
- TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA
- TLS_ECDH_RSA_WITH_AES_128_CBC_SHA
- TLS_ECDH_RSA_WITH_AES_256_CBC_SHA
- TLS_ECDHE_RSA_WITH_NULL_SHA
- TLS_ECDHE_RSA_WITH_RC4_128_SHA
- TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
- TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
- TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA
- TLS_ECDH_anon_WITH_NULL_SHA
- TLS_ECDH_anon_WITH_RC4_128_SHA
- TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA
- TLS_ECDH_anon_WITH_AES_128_CBC_SHA
- TLS_ECDH_anon_WITH_AES_256_CBC_SHA
- TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA
- TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA
- TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA
- TLS_SRP_SHA_WITH_AES_128_CBC_SHA
- TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA
- TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA
- TLS_SRP_SHA_WITH_AES_256_CBC_SHA
- TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA
- TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA
- TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
- TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384
- TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256
- TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384
- TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
- TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
- TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256
- TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384
- TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256
- TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384
- TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256
- TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384
- TLS_ECDHE_PSK_WITH_RC4_128_SHA
- TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA
- TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA
- TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA
- TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256
- TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384
- TLS_ECDHE_PSK_WITH_NULL_SHA
- TLS_ECDHE_PSK_WITH_NULL_SHA256
- TLS_ECDHE_PSK_WITH_NULL_SHA384
- TLS_RSA_WITH_ARIA_128_CBC_SHA256
- TLS_RSA_WITH_ARIA_256_CBC_SHA384
- TLS_DH_DSS_WITH_ARIA_128_CBC_SHA256
- TLS_DH_DSS_WITH_ARIA_256_CBC_SHA384
- TLS_DH_RSA_WITH_ARIA_128_CBC_SHA256
- TLS_DH_RSA_WITH_ARIA_256_CBC_SHA384
- TLS_DHE_DSS_WITH_ARIA_128_CBC_SHA256
- TLS_DHE_DSS_WITH_ARIA_256_CBC_SHA384
- TLS_DHE_RSA_WITH_ARIA_128_CBC_SHA256
- TLS_DHE_RSA_WITH_ARIA_256_CBC_SHA384
- TLS_DH_anon_WITH_ARIA_128_CBC_SHA256
- TLS_DH_anon_WITH_ARIA_256_CBC_SHA384
- TLS_ECDHE_ECDSA_WITH_ARIA_128_CBC_SHA256
- TLS_ECDHE_ECDSA_WITH_ARIA_256_CBC_SHA384
- TLS_ECDH_ECDSA_WITH_ARIA_128_CBC_SHA256
- TLS_ECDH_ECDSA_WITH_ARIA_256_CBC_SHA384
- TLS_ECDHE_RSA_WITH_ARIA_128_CBC_SHA256
- TLS_ECDHE_RSA_WITH_ARIA_256_CBC_SHA384
- TLS_ECDH_RSA_WITH_ARIA_128_CBC_SHA256
- TLS_ECDH_RSA_WITH_ARIA_256_CBC_SHA384
- TLS_RSA_WITH_ARIA_128_GCM_SHA256
- TLS_RSA_WITH_ARIA_256_GCM_SHA384
- TLS_DH_RSA_WITH_ARIA_128_GCM_SHA256
- TLS_DH_RSA_WITH_ARIA_256_GCM_SHA384
- TLS_DH_DSS_WITH_ARIA_128_GCM_SHA256
- TLS_DH_DSS_WITH_ARIA_256_GCM_SHA384
- TLS_DH_anon_WITH_ARIA_128_GCM_SHA256
- TLS_DH_anon_WITH_ARIA_256_GCM_SHA384
- TLS_ECDH_ECDSA_WITH_ARIA_128_GCM_SHA256
- TLS_ECDH_ECDSA_WITH_ARIA_256_GCM_SHA384
- TLS_ECDH_RSA_WITH_ARIA_128_GCM_SHA256
- TLS_ECDH_RSA_WITH_ARIA_256_GCM_SHA384
- TLS_PSK_WITH_ARIA_128_CBC_SHA256
- TLS_PSK_WITH_ARIA_256_CBC_SHA384
- TLS_DHE_PSK_WITH_ARIA_128_CBC_SHA256
- TLS_DHE_PSK_WITH_ARIA_256_CBC_SHA384
- TLS_RSA_PSK_WITH_ARIA_128_CBC_SHA256
- TLS_RSA_PSK_WITH_ARIA_256_CBC_SHA384
- TLS_PSK_WITH_ARIA_128_GCM_SHA256
- TLS_PSK_WITH_ARIA_256_GCM_SHA384
- TLS_RSA_PSK_WITH_ARIA_128_GCM_SHA256
- TLS_RSA_PSK_WITH_ARIA_256_GCM_SHA384
- TLS_ECDHE_PSK_WITH_ARIA_128_CBC_SHA256
- TLS_ECDHE_PSK_WITH_ARIA_256_CBC_SHA384
- TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_CBC_SHA256
- TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_CBC_SHA384
- TLS_ECDH_ECDSA_WITH_CAMELLIA_128_CBC_SHA256
- TLS_ECDH_ECDSA_WITH_CAMELLIA_256_CBC_SHA384
- TLS_ECDHE_RSA_WITH_CAMELLIA_128_CBC_SHA256
- TLS_ECDHE_RSA_WITH_CAMELLIA_256_CBC_SHA384
- TLS_ECDH_RSA_WITH_CAMELLIA_128_CBC_SHA256
- TLS_ECDH_RSA_WITH_CAMELLIA_256_CBC_SHA384
- TLS_RSA_WITH_CAMELLIA_128_GCM_SHA256
- TLS_RSA_WITH_CAMELLIA_256_GCM_SHA384
- TLS_DH_RSA_WITH_CAMELLIA_128_GCM_SHA256
- TLS_DH_RSA_WITH_CAMELLIA_256_GCM_SHA384
- TLS_DH_DSS_WITH_CAMELLIA_128_GCM_SHA256
- TLS_DH_DSS_WITH_CAMELLIA_256_GCM_SHA384
- TLS_DH_anon_WITH_CAMELLIA_128_GCM_SHA256
- TLS_DH_anon_WITH_CAMELLIA_256_GCM_SHA384
- TLS_ECDH_ECDSA_WITH_CAMELLIA_128_GCM_SHA256
- TLS_ECDH_ECDSA_WITH_CAMELLIA_256_GCM_SHA384
- TLS_ECDH_RSA_WITH_CAMELLIA_128_GCM_SHA256
- TLS_ECDH_RSA_WITH_CAMELLIA_256_GCM_SHA384
- TLS_PSK_WITH_CAMELLIA_128_GCM_SHA256
- TLS_PSK_WITH_CAMELLIA_256_GCM_SHA384
- TLS_RSA_PSK_WITH_CAMELLIA_128_GCM_SHA256
- TLS_RSA_PSK_WITH_CAMELLIA_256_GCM_SHA384
- TLS_PSK_WITH_CAMELLIA_128_CBC_SHA256
- TLS_PSK_WITH_CAMELLIA_256_CBC_SHA384
- TLS_DHE_PSK_WITH_CAMELLIA_128_CBC_SHA256
- TLS_DHE_PSK_WITH_CAMELLIA_256_CBC_SHA384
- TLS_RSA_PSK_WITH_CAMELLIA_128_CBC_SHA256
- TLS_RSA_PSK_WITH_CAMELLIA_256_CBC_SHA384
- TLS_ECDHE_PSK_WITH_CAMELLIA_128_CBC_SHA256
- TLS_ECDHE_PSK_WITH_CAMELLIA_256_CBC_SHA384
- TLS_RSA_WITH_AES_128_CCM
- TLS_RSA_WITH_AES_256_CCM
- TLS_RSA_WITH_AES_128_CCM_8
- TLS_RSA_WITH_AES_256_CCM_8
- TLS_PSK_WITH_AES_128_CCM
- TLS_PSK_WITH_AES_256_CCM
- TLS_PSK_WITH_AES_128_CCM_8
- TLS_PSK_WITH_AES_256_CCM_8
Note: This list was assembled from the set of registered TLS cipher suites at the time of writing. This list includes those cipher suites that do not offer an ephemeral key exchange and those that are based on the TLS null, stream, or block cipher type (as defined in Section 6.2.3 of [TLS12]). Additional cipher suites with these properties could be defined; these would not be explicitly prohibited.
Acknowledgements
This document includes substantial input from the following individuals:
- Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay, Paul Amer, Fan Yang, and Jonathan Leighton (SPDY contributors).
- Gabriel Montenegro and Willy Tarreau (Upgrade mechanism).
- William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto Peon, and Rob Trace (Flow control).
- Mike Bishop (Extensibility).
- Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike Bishop, and Herve Ruellan (Substantial editorial contributions).
- Kari Hurtta, Tatsuhiro Tsujikawa, Greg Wilkins, Poul-Henning Kamp, and Jonathan Thackray.
- Alexey Melnikov, who was an editor of this document in 2013.
A substantial proportion of Martin's contribution was supported by Microsoft during his employment there.
The Japanese HTTP/2 community provided invaluable contributions, including a number of implementations as well as numerous technical and editorial contributions.
Authors' Addresses
Mike Belshe
BitGo
Email: mike@belshe.com
Roberto Peon
Google, Inc
Email: fenix@google.com
Martin Thomson
(editor)
Mozilla
331 E Evelyn Street
Mountain View, CA 94041
United States
Email: martin.thomson@gmail.com
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本文首发于博客园，作者：xgqfrms，原文链接：https://www.cnblogs.com/xgqfrms/p/5046861.html
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