SCTP Packet Structure - Chunks

Chunks

Each SCTP packet consists, in addition to the common header, of chunks. Each chunk has a common format, but the contents can vary. One chunk appears in the diagram to the right with the green background.

Chunk type

An 8-bit value predefined by the IETF to identify the contents of the chunk value field.

Chunk flags

Eight flag-bits whose definition varies with the chunk type. The default value is zero.

Chunk length

A 16-bit unsigned value specifying the total length of the chunk in bytes (excludes any padding) that includes chunk type, flags, length, and value fields.

Chunk data

General purpose data field whose definition varies with the chunk type.

If the chunk length does not equate to a multiple of 4 bytes then the protocol implicitly pads the chunk with trailing zeros.

Additionally, each chunk type may define a set of parameters which it includes inside the chunk value field (and, consequently, their length in the chunk length).

Two types of parameter exist:

• fixed parameters — they must appear and in the order specified
• variable-length or optional parameters — they appear after the fixed parameters and may appear in any order and in any number.

For optional/variable-length parameters, the parameter type, parameter length, and parameter value fields all behave just like their chunk counterparts. The minimum size of parameter is 4 bytes and this occurs when the parameter value field is empty and the parameter consists only of the type & length fields.