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.

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.

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

Chunk Flags - bit 8-15. The chunk flags are generally not used but are set up for future usage if nothing else. They are chunk specific flags or bits of information that might be needed for the other peer. According to specifications, flags are only used in DATA, ABORT and SHUTDOWN COMPLETE packets at this moment. This may change however.

Important! A lot of times when you read an RFC, you might run into some old proven problems. The RFC 2960 - Stream Control Transmission Protocol document is one example of this, where they specifically specify that the Chunk flags should always be set to 0 and ignored unless used for something. This is written all over the place, and it begs for problems in the future. If you do firewalling or routing, watch out very carefully for this, since specifications for fields like this may change in the future and hence break at your firewall without any legit reason. This happened before with the implementation of ECN in the IP headers for example. See more in the IP headers section of this chapter.

Chunk Length - bit 16-31. This is the chunk length calculated in bytes. It includes all headers, including the chunk type, chunk flags, chunk length and chunk value. If there is no chunk value, the chunk length will be set to 4 (bytes).

Chunk Value - bit 32-n. This is specific to each chunk and may contain more flags and data pertaining to the chunk type. Sometimes it might be empty, in which case the chunk length will be set to 4.

The ABORT chunk is used to abort an association as previously described in the Shutdown and abort section of this chapter. ABORT is issued upon unrecoverable errors in the association such as bad headers or data.

Type - bit 0-7. Always set to 6 for this chunk type.

Reserved - bit 8-14. Reserved for future chunk flags but not used as of writing this. See the SCTP Common and generic headers for more information about the chunk flags field.

T-bit - bit 15. If this bit is set to 0, the sender had a TCB associated with this packet that it has destroyed. If the sender had no TCB the T-bit should be set to 1.

Length - bit 16-31. Sets the length of the chunk in bytes including error causes.