Zero Checksum for the Stream Control Transmission Protocol

Introduction SCTP as specified in uses a CRC32c to provide some level of data integrity. When using, for example, Datagram Transport Layer Security (DTLS) as the lower layer for SCTP as specified in , using the CRC32c does not provide any additional protection over the one already provided by DTLS. However, computing the CRC32c at the sender and receiver side does require computational resources for no benefit. This is in particular important for computational limited end points using SCTP encapsulated in DTLS. The extension described in this document allows an SCTP end point to declare that it accepts SCTP packets with a checksum of zero. This declaration happens during the setup of the SCTP association and allows end points supporting this extension to be interoperable with end points not supporting the extension described in this document. End points using this extension still need to implement the CRC32c checksum algorithms.

Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

A New Chunk Parameter The Zero Checksum Acceptable Chunk Parameter is defined by the following figure.

Zero Checksum Acceptable Chunk Parameter 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 = 0x8001 | Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: 16 bits (unsigned integer): This field holds the IANA defined parameter type for the "Zero Checksum Acceptable" chunk parameter. IANA is requested to assign the value 32769 (0x8001) for this parameter type.
Length: 16 bits (unsigned integer): This field holds the length in bytes of the chunk parameter; the value MUST be 4.

All transported integer numbers are in "network byte order" a.k.a., Big Endian. The Zero Checksum Acceptable Chunk Parameter MAY appear in INIT and INIT ACK chunks. It MUST NOT appear in any other chunk. If an end point not supporting the extension described in this document receives this parameter in an INIT or INIT ACK chunk, it skips it and continues to process further parameters in the chunk. This behaviour is REQUIRED by because the highest-order 2 bits of the Type are 10.

Procedures An end point willing to accept SCTP packets with a checksum of zero, MAY include the Zero Checksum Acceptable Chunk Parameter in the INIT or INIT ACK chunk it sends. An end point MUST do this consistently. If an end point has sent the Zero Checksum Acceptable Chunk Parameter in an INIT or INIT ACK chunk, it MUST accept SCTP packets using a checksum value of zero for this association. It MUST also accept SCTP packets containing the correct CRC32c checksum field. When an end point sends a packet containing an INIT chunk, it MUST include a CRC32c checksum in the packet containing the INIT chunk. When an end point receives an "Out of the Blue" (OOTB) SCTP packet and needs to send a response, it MUST include a CRC32c checksum in the response. If an end point has received an INIT or INIT ACK chunk from its peer during the association setup, it MAY use zero as the checksum for all packets sent in this association.

Socket API Considerations This section describes how the socket API defined in needs to be extended to provide a way for the application to control the UDP encapsulation. Please note that this section is informational only. A socket API implementation based on is extended by supporting one new read/write IPPROTO_SCTP level socket option.

Get or Set Accepting a Zero Checksum (SCTP_ACCEPT_ZERO_CHECKSUM) This socket option can be used to control the acceptance of a zero checksum. It applies only to future SCTP associations on the socket. This option expects an integer boolean flag, where a non-zero value turns on the option, and a zero value turns off the option. This option is off by default.

IANA Considerations [NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number you assign this document.] [NOTE to RFC-Editor: The requested value for the parameter type is tentative and to be confirmed by IANA.] This document (RFCXXXX) is the reference for the registration described in this section. A new chunk parameter type has to be assigned by IANA. This requires an additional line in the "Chunk Parameter Types" registry for SCTP: New entry in "Chunk Parameter Types" registry

ID Value	Chunk Parameter Type	Reference
32769	Zero Checksum Acceptable (0x8001)	[RFCXXXX]

Security Considerations This document does not change the considerations given in .