Type - bit 0-15. Always set to 7 for all State Cookie parameters.
Length - bit 16-31. The size of the whole parameter, including the type, length and State Cookie field in bytes.
State Cookie - bit 31-n. This parameter contains a cookie of variable length. For a description on how this cookie is created, see the RFC 2960 - Stream Control Transmission Protocol document.
SCTP SACK chunk
The SACK chunk is used to tell the sender of DATA chunks which chunks has been received and where there has been a gap in the stream, based on the received TSN's. Basically, the SACK chunk acknowledges that it has received data up to a certain point (the Cumulative TSN Ack parameter), and then adds Gap Ack Blocks for all of the data that it has received after the Cumulative TSN Ack point. A SACK chunk must not be sent more than once for every DATA chunk that is received.
Type - bit 0-7. This header is always set to 3 for SACK chunks.
Chunk flags - bit 8-15. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.
Chunk Length - bit 16-31. The chunk length is the length of the whole chunk, including everything in the headers and all the parameters.
Cumulative TSN Ack - bit 32-63. This is the Cumulative TSN Ack parameter, which is used to acknowledge data. The DATA chunk receiver will use this field to tell the sending host that it has received all data up to this point of the association. After this point, all data that has not been specifically acknowledged by the Gap Ack Blocks will, basically, be considered unaccounted for.
Advertised Receiver Window Credit (a_rwnd) - bit 64-95. The a_rwnd field is basically the same as the a_rwnd in the INIT and INIT ACK chunks, but can be used to raise or lower the a_rwnd value. Please read more in the RFC 2960 - Stream Control Transmission Protocol document about this.
Number of Gap Ack Blocks - bit 96-111. The number of Gap Ack Blocks listed in this chunk. Each Gap Ack Block takes up 32 bits in the chunk.
Number of Duplicate TSNs - bit 112-127. The number of DATA chunks that has been duplicated. Each duplicated TSN is listed after the Gap Ack Blocks in the chunk, and each TSN takes 32 bits to send.
Gap Ack Block #1 Start - bit 128-143. This is the first Gap Ack Block in the SACK chunk. If there are no gaps in the received DATA chunk TSN numbers, there will be no Gap Ack Blocks at all. However, if DATA chunks are received out of order or some DATA chunks where lost during transit to the host, there will be gaps. The gaps that has been seen will be reported with Gap Ack Blocks. The Gap Ack Block start point is calculated by adding the Gap Ack Block Start parameter to the Cumulative TSN value. The calculated value is the start of the block.
Gap Ack Block #1 End - bit 144-159. This value reports the end of the first Gap Ack Block in the stream. All the DATA chunks with the TSN between the Gap Ack Block Start and the Gap Ack Block End has been received. The Gap Ack Block End value is added to the Cumulative TSN, just as the Start parameter, to get the actual last TSN of the block chunks to be Acknowledged.
Gap Ack Block #N Start - bits variable. For every Gap Ack Block counted in the Number of Gap Ack Blocks parameter, one Gap Ack Block is added, until the final N block. Ie, if Number of Gap Ack Blocks = 2, then there will be two Gap Ack Blocks in the SACK chunk. This is the last one simply, and contains the same type of value as the Gap Ack Block #1 Start.
Gap Ack Block #N End - bits variable. Same as for the Gap Ack Block #N End, but for the end of the gap.
Duplicate TSN #1 - bits variable. These fields report a duplicate TSN, in which case we have already received a specific chunk, but receive the same TSN several times more. This can either be router glitches (retransmitting already sent data) or a case of retransmission from the sending endpoint, or a score of other possibilities. Each instance of a duplicate TSN should be reported once. For example, if 2 duplicate TSN's has been received after acknowledging the first one, each of these duplicate TSN's should be sent sent in the next SACK message that is being sent. If even more duplicate TSN's should appear after this second SACK is sent, the new duplicates should be added in the next SACK, and so on.
Duplicate TSN #X - bits variable. This is the last duplicate TSN parameter, containing the same type of information as the first parameter.
SCTP SHUTDOWN chunk
The SHUTDOWN chunk is issued when one of the endpoints of a connection wants to close the current association. The sending party must empty all of its sending buffers before sending the SHUTDOWN chunk, and must not send any more DATA chunks afterwards. The receiver must also empty its sending buffers and must then send the responding SHUTDOWN ACK chunk.
Type - bit 0-7. This header is always set to 7 for SHUTDOWN chunks.
Chunk flags - bit 8-15. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.
Chunk Length - bit 16-31. The chunk length is the length of the whole packet, including the Cumulative TSN Ack parameter. The length of the SHUTDOWN chunk should always be 8.
Cumulative TSN Ack - bit 32-63. This is a Cumulative TSN Ack field, just the same as in the SACK chunk. The Cumulative TSN Ack acknowledges the last TSN received in sequence from the opposite endpoint. This parameter does not, nor can the rest of the SHUTDOWN chunk either, acknowledge Gap Ack Blocks. The lack of a Gap Ack Block in the SHUTDOWN chunk that was acknowledged before should not be interpreted as if the previously acknowledged block was lost again.
SCTP SHUTDOWN ACK chunk
The SHUTDOWN ACK chunk is used to acknowledge a SHUTDOWN chunk that has been received. Before the SHUTDOWN ACK chunk is sent, all data in the sending buffers should be sent, but the buffers must not accept any new data from the application. SCTP does not support half-open connections as TCP does.
Type - bit 0-7. This header is always set to 8 for SHUTDOWN ACK chunks.
Chunk flags - bit 8-15. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.
Chunk Length - bit 16-31. The chunk length is the length of the whole chunk. The length of the SHUTDOWN ACK chunk should always be 4.
SCTP SHUTDOWN COMPLETE chunk
The SHUTDOWN COMPLETE chunk is sent, by the originating host of the SHUTDOWN, in response to the SHUTDOWN ACK chunk. It is sent to acknowledge that the association is finally closed.
Type - bit 0-7. Always set to 14 for SHUTDOWN COMPLETE chunks.
Reserved - bit 8-14. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.
T-bit - bit 15. The T-bit is not set to signal that the sending host had a Transmission Control Block (TCB) associated with this connection and that it destroyed. If the T-bit was set, it had no TCB to destroy.
Length - bit 16-31. This is always set to 4 for SHUTDOWN COMPLETE chunks, since the chunk should never be any larger, as long as no updates to the standards are made.