Chapter 4. Connection Management 4.1 TCP Connections

4.1.1 TCP Reliable Data Pipes

HTTP connections really are nothing more than TCP connections, plus a few rules about how to use them. TCP connections are the reliable connections of the Internet. To send data accurately and quickly, you need to know the basics of TCP.^[2]

TCP gives HTTP a reliable bit pipe. Bytes stuffed in one side of a TCP connection come out the other side correctly, and in the right order (see Figure 4-2).

Figure 4-2. TCP carries HTTP data in order, and without corruption

4.1.2 TCP Streams Are Segmented and Shipped by IP Packets

TCP sends its data in little chunks called IP packets (or IP datagrams). In this way, HTTP is the top layer in a "protocol stack" of "HTTP over TCP over IP," as depicted in Figure 4-3a. A secure variant, HTTPS, inserts a cryptographic encryption layer (called TLS or SSL) between HTTP and TCP (Figure 4-3b).

4.1.2 TCP Streams Are Segmented and Shipped by IP Packets

TCP sends its data in little chunks called IP packets (or IP datagrams). In this way, HTTP is the top layer in a "protocol stack" of "HTTP over TCP over IP," as depicted in Figure 4-3a. A secure variant, HTTPS, inserts a cryptographic encryption layer (called TLS or SSL) between HTTP and TCP (Figure 4-3b).

Figure 4-3. HTTP and HTTPS network protocol stacks

When HTTP wants to transmit a message, it streams the contents of the message data, in order, through an open TCP connection. TCP takes the stream of data, chops up the data stream into chunks called segments, and transports the segments across the Internet inside envelopes called IP packets (see Figure 4-4). This is all handled by the TCP/IP software; the HTTP programmer sees none of it.

Each TCP segment is carried by an IP packet from one IP address to another IP address. Each of these IP packets contains:

· An IP packet header (usually 20 bytes)

· A TCP segment header (usually 20 bytes)

· A chunk of TCP data (0 or more bytes)

The IP header contains the source and destination IP addresses, the size, and other flags. The TCP segment header contains TCP port numbers, TCP control flags, and numeric values used for data ordering and integrity checking.

Figure 4-4. IP packets carry TCP segments, which carry chunks of the TCP data stream

4.1.4 Programming with TCP Sockets

Operating systems provide different facilities for manipulating their TCP connections. Let's take a quick look at one TCP programming interface, to make things concrete. Table 4-2 shows some of the primary interfaces provided by the sockets API. This sockets API hides all the details of TCP and IP from the HTTP programmer. The sockets API was first developed for the Unix operating system, but variants are now available for almost every operating system and language.

Table 4-2. Common socket interface functions for programming TCP connections

Sockets API call  &  Description

s = socket(<parameters>)

Creates a new, unnamed, unattached socket.

bind(s, <local IP:port>)

Assigns a local port number and interface to the socket.

connect(s, <remote IP:port>)

Establishes a TCP connection to a local socket and a remote host and port.

listen(s,...)

Marks a local socket as legal to accept connections.

s2 = accept(s)

Waits for someone to establish a connection to a local port.

n = read(s,buffer,n)

Tries to read n bytes from the socket into the buffer.

n = write(s,buffer,n)

Tries to write n bytes from the buffer into the socket.

close(s)

Completely closes the TCP connection.

shutdown(s,<side>)

Closes just the input or the output of the TCP connection.

getsockopt(s, . . . )

Reads the value of an internal socket configuration option.

setsockopt(s, . . . )

Changes the value of an internal socket configuration option.

The sockets API lets you create TCP endpoint data structures, connect these endpoints to remote server TCP endpoints, and read and write data streams. The TCP API hides all the details of the underlying network protocol handshaking and the segmentation and reassembly of the TCP data stream to and from IP packets.

In Figure 4-1, we showed how a web browser could download the power-tools.html web page from Joe's Hardware store using HTTP. The pseudocode in Figure 4-6 sketches how we might use the sockets API to highlight the steps the client and server could perform to implement this HTTP transaction.