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SCTP Headers
This will be a very brief introduction to the SCTP headers. SCTP has a lot of different types of packets, and hence I will try to follow the RFC's as close as possible and how they depict the different headers, starting with a general overview of the headers applicable to all SCTP packets.
SCTP Generic header format
This is a generic overview of how a SCTP packet is laid out. Basically, you have a common header first with information describing the whole packet, and the source and destination ports etc. See more below for information on the common header.
After the common header a variable number of chunks are sent, up to the maximum possible in the MTU. All chunks can be bundled except for INIT, INIT ACK and SHUTDOWN COMPLETE, which must not be bundled. DATA chunks may be broken down to fit inside the MTU of the packets.
SCTP Common and generic headers
Every SCTP packet contains the Common header as seen above. The header contains four different fields and is set for every SCTP packet.
Source port - bit 0-15. This field gives the source port of the packet, which port it was sent from. The same as for TCP and UDP source port.
Destination port - bit 16-31. This is the destination port of the packet, ie., the port that the packet is going to. It is the same as for the TCP and UDP destination port.
Verification Tag - bit 32-63. The verification tag is used to verify that the packet comes from the correct sender. It is always set to the same value as the value received by the other peer in the Initiate Tag during the association initialization, with a few exceptions:
• An SCTP packet containing an INIT chunk must have the Verification tag set to 0.
• A SHUTDOWN COMPLETE chunk with the T-bit set must have the verification tag copied from the verification tag of the SHUTDOWN-ACK chunk.
• Packets containing ABORT chunk may have the verification tag set to the same verification tag as the packet causing the ABORT.
Checksum - bit 64-95. A checksum calculated for the whole SCTP packet based on the Adler-32 algorithm. Read RFC 2960 - Stream Control Transmission Protocol, appendix B for more information about this algorithm.
All SCTP chunks has a special layout that they all adhere to as can be seen above. This isn't an actual header, but rather a formalized way of how they do look.
Type - bit 0-7. This field specifies the chunk type of the packet, for example is it an INIT or SHUTDOWN chunk or what? Each chunk type has a specific number, and is specified in the image below. Here is a complete list of Chunk types:
Table 2-1. SCTP Types