Duplicate SACK原理

SACK & D-SACK Interaction
No difference between SACK & D-SACK, except that first SACK block is used to report a duplicate PDU
in D-SACK.
No separate negotiation / options for D-SACK.
D-SACK is compatible with current implementations of SACK option in TCP.

D-SACK TCP & Retransmissions
D-SACK allows TCP to determine when retransmission is not necessary and thereby undo congestion
control measures.
D-SACK allows TCP to determine if the network is duplicating packets.
D-SACK does not allow a sender to determine if both the original and retransmitted PDU are received,
or the original is lost and the retransmitted PDU is duplicated by the network.

Extension to SACK
When D-SACK is used, the first block of the SACK option should specify the duplicate PDU.
D-SACK block is used to report duplicate contiguous sequence of data received by the receiver in the
most recent packet.
Each duplicate is reported at most once.
Allows the sender TCP to determine when a retransmission is not necessary.

D-SACK Rules
If D-SACK block reports duplicate PDU from (possibly larger) block of data in the receiver buffer above
the cumulative acknowledgement, the second SACK block (the first non D-SACK block) should specify
this block.
The first SACK block is considered as D-SACK block, if multiple sequences are duplicated, only the
first is contained in the D-SACK block.

Motivations
This could be used to make TCP more robust to reordered packets, ACK loss, packet replication,
and/or early retransmit timeouts.
As a result, TCP could be more robust in environments with：
link-level retransmissions
widely-varying round trip times
loss on the return (ACK) path
routing mechanisms that result in packet reordering

D-SACK处理流程
1）look at the first SACK block :
—If the first SACK block is covered by the Cumulative Acknowledgement field, then it is a D-SACK
block, and is reporting duplicate data.
—Else, if the first SACK block is covered by the second SACK block, then the first SACK block is a
D-SACK block, and is reporting duplicate data.

2）otherwise, interpret the SACK blocks using the normal SACK procedures.

D-SACK作用举例
1）Identify a retransmit timeout due to ACK loss

2）Identify an early retransmission timeout

3）Identify a false retransmit due to reordering

4）Identify data packet replication in the network

我们可以看到，一个算是比较小的改进，就能给SACK带来较大的提升。