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Examples of Receive Segment Coalescing

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This section illustrates the coalescing algorithm by using examples of segments that are received in order and processed in a single deferred procedure call (DPC).

This page uses X and X’ for labeling successive segments. All other segment and single coalesced unit (SCU) fields are as described in Rules for Coalescing TCP/IP Segments.

Example 1: Data segments

Segment Description

10 successive segments belonging to the same TCP connection are processed. All of the following conditions are true for each:

  • X’.SEQ == X.NXT

  • X’SEQ > X.SEQ

  • X’.ACK == X.ACK

None of these segments generates an exception.

Result

A single SCU is formed out of the 10 segments. This is indicated as a single NET_BUFFER in a single NET_BUFFER_LIST.

Example 2: Data segments, followed by an exception, followed by data segments

Segment Description

5 successive segments belonging to the same TCP connection are processed. All of the following conditions are true for each:

  • X’.SEQ == X.NXT

  • X’SEQ > X.SEQ

  • X’.ACK == X.ACK

None of these segments generates an exception. The 6th segment is a duplicate ACK segment with a TCP SACK option and generates an exception based on rule number 3 in Rules for Coalescing TCP/IP Segments.

Note  In this case, the exception rule for handling a TCP option takes precedence and thus overrides the coalescing rule.

2 successive segments belonging to the same TCP connection are processed. All of the following conditions are true for each:

  • X’.SEQ == X.NXT

  • X’SEQ > X.SEQ

  • X’.ACK == X.ACK

None of these segments generates an exception.

Result

A single SCU is formed out of the first 5 segments. The 6th segment does not form an SCU.

The 7th and 8th segments form an SCU together.

A NET_BUFFER_LIST chain is indicated with three NET_BUFFER_LIST structures each having a single NET_BUFFER. The ordering of received segments is maintained.

Example 3: Data segments, followed by multiple window updates

Segment Description

5 successive segments belonging to the same TCP connection are processed. All of the following conditions are true for each:

  • X’.SEQ == X.NXT

  • X’SEQ > X.SEQ

  • X’.ACK == X.ACK

None of these segments generates an exception. The 6th segment is a pure ACK that is a window update with SEG.WND = 65535 as shown in the following flowchart.

Flowchart that shows rules for coalescing segments with TCP timestamp option.

The 7th segment is a pure ACK that is a window update with SEG.WND = 131070 as shown in the same flowchart.

Result

A single SCU is formed out of the 7 segments. This is indicated as a single NET_BUFFER in a single NET_BUFFER_LIST.

The SCU.WND = 131070, and the checksum is updated based on this value.

Example 4: Piggybacked ACKs mixed with data segments

Segment Description

3 successive segments belonging to the same TCP connection are processed. All of the following conditions are true for each:

  • X’.SEQ == X.NXT

  • X’SEQ > X.SEQ

  • X’.ACK == X.ACK

None of these segments generates an exception. 2 successive segments belonging to the same TCP connection are processed. All of the following conditions are true for each:

  • X’.SEQ == X.NXT

  • X’SEQ > X.SEQ

  • X’.ACK == X.ACK

None of these segments generates an exception.

Result

A single SCU is formed out of the 5 segments. This is indicated as a single NET_BUFFER in a single NET_BUFFER_LIST. The SCU.ACK is set to the last SEG.ACK.