Münster University of Applied Sciences

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Mozilla Corporation

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The Stream Control Transmission Protocol (SCTP) uses a 32-bit checksum in the common header of each packet to provide some level of data integrity. If another method used by SCTP provides already the same or a higher level of data integrity, computing this checksum does not provide any additional protection, but does require computing resources. This document provides a simple extension to SCTP allowing to save these computing resources by using zero as the checksum in a backwards compatible way. It also defines how this feature can be used when SCTP packets are encapsulated in Datagram Transport Layer Security (DTLS) packets.

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 consume computational resources for no benefit. This is particularly important for end points that are computational-limited and use 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 when using a specific alternate error detection method. 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. To provide this backwards compatibility, end points using this extension still need to implement the CRC32c checksum algorithm.

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.

Alternate Error Detection Methods SCTP uses a CRC32c to provide some level of data integrity. The CRC32c is computed based on the SCTP common header and the chunks contained in the packet. In particular, the computation of the CRC32c does not involve a pseudo header for IPv4 or IPv6 like the computation of the TCP or UDP checksum. Zero is a valid result of the CRC32c algorithm. For example, the following figure depicts an SCTP packet containing a minimal INIT chunk with a correct CRC32c or zero.

SCTP Packet with a correct CRC32c of zero 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Port Number = 5001 |Destination Port Number = 5001 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Verification Tag = 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum = 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 1 |Chunk Flags = 0| Chunk Length = 20 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initiate Tag = 0xFCB75CCA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertised Receiver Window Credit (a_rwnd) = 1500 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Number of Outbound Streams = 1 | Number of Inbound Streams = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initial TSN = 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Using SCTP in combination with other mechanisms or protocol extensions might provide an equal or better level of data integrity protection than the one provided by using the CRC32c algorithm. Such alternate error detection methods have two requirements:

1. An alternate error detection method MUST provide an equal or better level of data integrity than the one provided by using the CRC32c algorithm. This MAY only apply to packets satisfying some method specific constraints.
2. An alternate error detection method MUST NOT interfere with middle boxes expecting correct CRC32c checksums.

One example fulfilling the first requirement is using DTLS as the lower layer of SCTP as specified in . Another example is using SCTP Authentication as specified in . Of course, this only applies to all SCTP packets having an AUTH chunk as its first chunk. The use of SCTP over UDP as specified in is not fulfilling the first requirement. As using DTLS as the lower layer of SCTP as specified in also fulfills the second requirement, it can be used as an alternate error detection method (see ). If an alternate error detection method is available, the computation of the CRC32c checksum consumes computational resources without providing any benefit. To avoid this, an SCTP end point could be willing to accept SCTP packets with an incorrect CRC32c checksum value of zero in addition to SCTP packets with correct CRC32c checksum values. Because zero is a valid result of the CRC32c algorithm, a receiver of an SCTP packet containing a checksum value of zero cannot determine whether the sender included an incorrect CRC32c of zero to reduce the CPU cost or the result of the CRC32c computation was actually zero. However, if the receiver is willing to use an alternate error detection method, this ambiguity is irrelevant, since the receiver is fine with not using the CRC32c to protect incoming packets.

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 (suggested) | Length = 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Detection Method Identifier (EDMID) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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) (suggested) for this parameter type.

Length: 16 bits (unsigned integer)

This field holds the length in bytes of the chunk parameter; the value MUST be 8.

Error Detection Method Identifier (EDMID): 32 bits (unsigned integer)

An IANA registered value specifying the alternate error detection method the sender of this parameter is willing to use for received packets.

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 and MUST NOT appear in any other chunk. The Parameter MUST NOT appear more than once in any chunk. If an end point not supporting the extension described in this document receives this parameter in an INIT or INIT ACK chunk, it is REQUIRED to skip this parameter and continue to process further parameters in the chunk. This behavior is specified by because the highest-order two bits of the Type are '10'.

Procedures

Declaration of Feature Support An end point willing to accept SCTP packets with an incorrect checksum of zero MUST include the Zero Checksum Acceptable Chunk Parameter indicating the alternate error detection method it is willing to use in the INIT or INIT ACK chunk it sends. An SCTP implementation MAY also require the upper layer to indicate that it is fine to use a specific alternate error detection method for accepting SCTP packets with an incorrect CRC32c value of zero.

Sender Side Considerations An SCTP end point cannot just use an incorrect CRC32c checksum value of zero for all SCTP packets it sends. Therefore, the following restrictions apply:

1. If an end point has not received an INIT or INIT ACK chunk containing a Zero Checksum Acceptable Chunk Parameter indicating an alternate error detection method it supports from its peer during the association setup, it MUST use a correct CRC32c checksum. In particular, when an end point
   1. sends a packet containing an INIT chunk, it MUST include a correct CRC32c checksum in the packet containing the INIT chunk.
   2. responds to an "Out of the Blue" (OOTB) SCTP packet, it MUST include a correct CRC32c checksum in the response packet.
2. When an end point sends a packet containing a COOKIE ECHO chunk, it MUST include a correct CRC32c checksum in the packet containing the COOKIE ECHO chunk.
3. When an end point supports the dynamic address reconfiguration specified in and sends a packet containing an ASCONF chunk, it MUST include a correct CRC32c checksum in the packet containing the ASCONF chunk.
4. Alternate error detection methods might have some additional conditions requiring that the sender MUST include a correct CRC32c checksum in the packet.

The first restriction allows backwards compatibility and the second and third restriction allow a simpler implementation of the extension defined in this document. The last restriction covers alternate error detection method specific constraints. An SCTP end point MAY only send packets with an incorrect checksum of zero, if the upper layer allowed the use of the alternate error detection method that was announced by the peer. If none of the above restrictions apply, an end point SHOULD use zero as the checksum when sending an SCTP packet.

Receiver Side Considerations If an end point has sent the Zero Checksum Acceptable Chunk Parameter indicating the support of an alternate error detection method in an INIT or INIT ACK chunk, it MUST accept SCTP packets fulfilling the requirements of the announced alternate error detection method using an incorrect checksum value of zero in addition to SCTP packets containing the correct CRC32c checksum value for this association. In addition to processing OOTB packets with a correct CRC32c checksum as specified in , an SCTP implementation MAY also process OOTB packets having an incorrect zero checksum. Doing so might result in faster SCTP association failure detection.

Error Detection via SCTP over DTLS Using SCTP over DTLS as specified in provides a stronger error detection method than using the CRC32c. Since middle boxes will not observe the unencrypted SCTP packet, there is no risk in interferring with using zero as an incorrect checksum. There are no additional error detection specific constraints on packets when using DTLS encapsulation. IANA is requested to assign the Error Detection Method Identifier of 1 for this method.

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 acceptance of a zero checksum. Please note that this section is informational only. A socket API implementation based on is extended by supporting one new write-only IPPROTO_SCTP-level socket option.

Set Accepting a Zero Checksum (SCTP_ACCEPT_ZERO_CHECKSUM) This IPPROTO_SCTP-level socket option with name SCTP_ACCEPT_ZERO_CHECKSUM can be used to control the acceptance of a zero checksum. It is a write-only socket option and applies only to future SCTP associations on the socket. This option expects an unsigned integer. Possible values include:

SCTP_EDMID_NONE:

Disable the use of alternate error detection method. This means that all SCTP packets being sent have a correct CRC32c.

SCTP_EDMID_LOWER_LAYER_DTLS:

Use the alternate error detection method described in .

An implementation might only send packets with an incorrect checksum of zero, if the alternate error detection method announced by the peer is also enabled locally via this socket option. The default for this socket option is that the use of alternate error detection methods is disabled.

IANA Considerations [NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number you assign this document.] [NOTE to RFC-Editor: The suggested 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 (suggested)	Zero Checksum Acceptable (0x8001 (suggested))	[RFCXXXX]

Furthermore, IANA is requested to establish a new "Error Detection Method" registry for SCTP. The assignment of new error detection methods is done through the RFC Required policy as defined in . Documentation for a new error detection method MUST contain the following information:

1. A name of an alternate error detection method.
2. A reference to an RFC describing:
   1. the alternate error detection method,
   2. any method specific constraints referred to in the fourth exception in , and
   3. why using the alternate error detection method does not interfere with middle boxes expecting correct CRC32c checksums.

IANA is requested to use the following as the initial contents of the registry: Initial Contents of the "Error Detection Method" registry

ID Value	Error Detection Method	Reference
0	Reserved	[RFCXXXX]
1	SCTP over DTLS	[RFCXXXX]
2 - 4294967295	Unassigned

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

References Normative References Informative References

Acknowledgments The authors wish to thank , , , , , and for their invaluable comments.