Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)

This document defines a padding chunk and a padding parameter and describes the required receiver side procedures. The padding chunk is used to pad an SCTP packet to an arbitrary size. The padding parameter is used to pad an SCTP INIT chunk to an arbitrary size. The usage of the PAD chunk for path MTU discovery is described in PMTU. The inappropriate usage of the PAD parameter or PAD chunk can result in wasted bandwidth.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL", when they appear in this document, are to be interpreted as described in RFC 2119.

This chunk is used to pad an SCTP packet. A PAD chunk can be used to enlarge the packet by 4 to 65536 bytes in steps of 4 bytes. An SCTP packet MAY contain multiple PAD chunks.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x84 | Flags=0 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | \ Padding Data / / \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This value MUST be set to 0x84 for all PAD chunks. This value SHOULD be set to zero on transmit and MUST be ignored on receipt. This value holds the length of the Padding Data plus 4. This holds the Padding Data. The Padding Data MUST be ignored by the receiver. The receiver of the PAD chunk MUST discard this chunk and continue processing the rest of the chunks in the packet. Please note that this is also the required processing behavior for any unknown chunk having the same highest-order two bits of the type as the PAD chunk.

This parameter is used to pad an INIT chunk. A PAD parameter can be used to enlarge the INIT chunk by 4 bytes as the minimum to the maximum size of the INIT chunk in steps of 4 bytes. An INIT chunk MAY contain multiple PAD parameters.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Parameter Type = 0x8005 | Parameter Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / \ Padding Data \ / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This value MUST be set to 0x8005. This value holds the length of the Padding Data plus 4. The PAD parameter MAY be included only in the INIT chunk. It MUST NOT be included in any other chunk. The receiver of the PAD parameter MUST silently discard this parameter and continue processing the rest of the INIT chunk. This means that the size of the generated COOKIE parameter in the INIT-ACK MUST NOT depend on the existence of the PAD parameter in the INIT chunk. A receiver of a PAD parameter MUST NOT include the PAD parameter within any State Cookie parameter it generates.

This document is the reference for all registrations described in this section. All registrations have been listed in the document available at sctp-parameters. The changes are described below.

A chunk type for the PAD chunk has been assigned by IANA. The value has been assigned as described in . The following has been added to the "CHUNK TYPES" table of sctp-parameters:

CHUNK TYPES ID Value Chunk Type Reference -------- ---------- --------- 132(0x84) Padding Chunk (PAD) [RFC4820]

A parameter type has been assigned for the PAD parameter by IANA. The value has been assigned as described in . The following has been added to the "CHUNK PARAMETER TYPES" table in sctp-parameters: INIT Chunk Parameter Types

Chunk Parameter Type Value -------------------- ----- Padding 32773(0x8005)

This document does not add any additional security considerations to the ones given in RFC 2960.

The authors wish to thank Matthew J. Zekauskas and Lars Eggert for their invaluable comments.