SCTP Packet Structure - Chunk Types - SACK Chunk

SACK Chunk

+ Bits 0 - 7 8 - 15 16 - 31
0 Chunk type = 3 Chunk flags Chunk length
32 Cumulative TSN ACK
64 Advertised receiver window credit
96 Number of gap ACK blocks = N Number of duplicate TSNs = X
128 Gap ACK block #1 start Gap ACK block #1 end
... ... ...
96+N*32 Gap ACK block #N start Gap ACK block #N end
128+N*32 Duplicate TSN #1
... ...
96+N*32+X*32 Duplicate TSN #X
Chunk type
always three for selective acknowledgment (SACK).
Chunk flags
There are currently no flags used.
Chunk length
This is the chunk length which has a minimum value of 16 when no gaps or duplicates are sent.
Fixed parameters:
Cumulative TSN ACK
Acknowledges all sequence numbers received in sequence so is the sequence number of the last received byte. The immediate value after this one has not been received yet.
Advertised receiver window credit
Amount of dedicated buffer space for this association that should never be reduced.
Number of gap ACK blocks
Indicates the number of gap ACK block start and gap ACK block end pairs included.
Number of duplicate TSNs
Indicates the number of duplicate TSNs the endpoint has received.
Optional parameters appear with alternating background colors of green and blue:
Gap ACK block #n start
Indicates the TSN (added to the cumulative TSN ACK) of a block of TSNs to acknowledge.
Gap ACK block #n end
Indicates the TSN (added to the cumulative TSN ACK) of a block of TSNs to acknowledge.
Duplicate TSN #x
A TSN that was received more than once. A TSN will appear in this list for each time it is received after the first time.

Read more about this topic:  SCTP Packet Structure, Chunk Types

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