SCTP Zero Checksum: Zero Checksum for the Stream Control Transmission Protocol

Internet Engineering Task Force (IETF)                          M. Tüxen
Request for Comments: 9653               Münster Univ. of Appl. Sciences
Category: Standards Track                                      V. Boivie
ISSN: 2070-1721                                              F. Castelli
                                                                  Google
                                                                R. Jesup
                                                                 Mozilla
                                                          September 2024


       Zero Checksum for the Stream Control Transmission Protocol

Abstract

   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 already provides the
   same or a higher level of data integrity, computing this checksum
   does not provide any additional protection but does consume computing
   resources.

   This document provides a simple extension allowing SCTP 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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9653.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions
   3.  Alternate Error Detection Methods
   4.  A New Chunk Parameter
   5.  Procedures
     5.1.  Declaration of Feature Support
     5.2.  Sender-Side Considerations
     5.3.  Receiver-Side Considerations
   6.  Error Detection via SCTP over DTLS
   7.  Socket API Considerations
     7.1.  Set Accepting a Zero Checksum (SCTP_ACCEPT_ZERO_CHECKSUM)
   8.  IANA Considerations
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   SCTP as specified in [RFC9260] uses a CRC32c checksum 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
   [RFC8261], using the CRC32c checksum does not provide any additional
   protection over that already provided by DTLS.  However, computing
   the CRC32c checksum at the sender and receiver sides does consume
   computational resources for no benefit.  This is particularly
   important for endpoints that are computationally limited and use SCTP
   over DTLS.

   The extension described in this document allows an SCTP endpoint 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 endpoints
   that support this extension to be interoperable with endpoints that
   don't.  To provide this backwards compatibility, endpoints using this
   extension still need to implement the CRC32c checksum algorithm.

2.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Alternate Error Detection Methods

   SCTP uses a CRC32c checksum to provide some level of data integrity.
   The CRC32c checksum is computed based on the SCTP common header and
   the chunks contained in the packet.  In particular, the computation
   of the CRC32c checksum does not involve a pseudo header for IPv4 or
   IPv6 like the computation of the TCP checksum, as specified in
   [RFC9293], or the UDP checksum, as specified in [RFC0768].

   Zero is a valid result of the CRC32c checksum algorithm.  For
   example, the following figure depicts an SCTP packet containing a
   minimal INIT chunk with a correct CRC32c checksum 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                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 1: SCTP Packet with a Correct CRC32c Checksum of Zero

   Using SCTP in combination with other mechanisms or protocol
   extensions might provide data integrity protection with an equal or
   lower probability of false negatives than the one provided by using
   the CRC32c checksum algorithm.  When using such alternate error
   detection methods, the SCTP common header containing the 32-bit
   checksum field might or might not be visible to middleboxes on the
   paths between the two endpoints.

   Alternate error detection methods have two requirements:

   1.  An alternate error detection method MUST provide an equal or
       lower probability of false negatives than the one provided by
       using the CRC32c checksum algorithm.  This MAY only apply to
       packets satisfying some method-specific constraints.

   2.  Using an alternate error detection method MUST NOT result in a
       path failure for more than two retransmission timeouts (RTOs) due
       to middleboxes on the path expecting correct CRC32c checksums.

   To fulfill the second requirement, alternate error detection methods
   could use a heuristic to detect the existence of such middleboxes and
   use correct CRC32c checksums on these affected paths.

   Using DTLS as the lower layer of SCTP as specified in [RFC8261] is
   one example that fulfills the first requirement.  Another example is
   using SCTP Authentication as specified in [RFC4895].  Of course, this
   only applies to each SCTP packet having an AUTH chunk as its first
   chunk.  However, using SCTP Authentication without any heuristic does
   not fulfill the second requirement.  Since using DTLS as the lower
   layer of SCTP as specified in [RFC8261] also fulfills the second
   requirement, it can be used as an alternate error detection method
   (see Section 6).

   If an alternate error detection method is used, the computation of
   the CRC32c checksum consumes computational resources without
   providing any benefit.  To avoid this, an SCTP endpoint 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 checksum algorithm, a
   receiver of an SCTP packet containing a checksum value of zero cannot
   determine whether the sender included an incorrect CRC32c checksum of
   zero to reduce the CPU cost or the result of the CRC32c checksum
   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
   checksum to protect incoming packets.

4.  A New Chunk Parameter

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

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

             Figure 2: Zero Checksum Acceptable Chunk Parameter

   Type: 16 bits (unsigned integer)
      This field holds the IANA-defined parameter type for the "Zero
      Checksum Acceptable" chunk parameter.  IANA has assigned 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 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 endpoint 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 [RFC9260]
   because the highest-order two bits of the Type are '10'.

5.  Procedures

5.1.  Declaration of Feature Support

   An endpoint 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
   before including the corresponding Zero Checksum Acceptable Chunk
   Parameter.

5.2.  Sender-Side Considerations

   An SCTP endpoint cannot just use an incorrect CRC32c checksum value
   of zero for all SCTP packets it sends.  The following restrictions
   apply:

   1.  If an endpoint 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 endpoint

       a.  sends a packet containing an INIT chunk, it MUST include a
           correct CRC32c checksum in the packet containing the INIT
           chunk.

       b.  responds to an "Out of the Blue" (OOTB) SCTP packet, it MUST
           include a correct CRC32c checksum in the response packet.

   2.  When an endpoint 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 endpoint supports the dynamic address reconfiguration
       specified in [RFC5061] and sends a packet containing an ASCONF
       chunk, it MUST include a correct CRC32c checksum in the packet
       containing the ASCONF chunk.

   4.  If an alternate error detection method has some method-specific
       constraints, the sender MUST include a correct CRC32c checksum in
       all packets that don't fulfill these method-specific constraints.

   The first restriction allows backwards compatibility.  The second and
   third restrictions allow a simpler implementation of the extension
   defined in this document, because looking up the association for SCTP
   packets containing a COOKIE ECHO chunk or an ASCONF chunk might be
   more complex than for other packets.  Finally, the last restriction
   covers constraints specific to the alternate error detection method.

   An SCTP endpoint MAY require that the upper layer allow the use of
   the alternate error detection method that was announced by the peer
   before sending packets with an incorrect checksum of zero.

   If none of the above restrictions apply, an endpoint SHOULD use zero
   as the checksum when sending an SCTP packet.

5.3.  Receiver-Side Considerations

   If an endpoint has sent the Zero Checksum Acceptable Chunk Parameter
   indicating the support of an alternate error detection method in an
   INIT or INIT ACK chunk, in addition to SCTP packets containing the
   correct CRC32c checksum value it MUST accept SCTP packets that have
   an incorrect checksum value of zero and that fulfill the requirements
   of the announced alternate error detection method used for this
   association.  Otherwise, the endpoint MUST drop all SCTP packets with
   an incorrect CRC32c checksum.

   In addition to processing OOTB packets with a correct CRC32c checksum
   as specified in [RFC9260], an SCTP implementation MAY also process
   OOTB packets having an incorrect zero checksum.  Doing so might
   result in faster SCTP association failure detection.

6.  Error Detection via SCTP over DTLS

   Using SCTP over DTLS as specified in [RFC8261] provides a stronger
   error detection method than using the CRC32c checksum algorithm.
   Since middleboxes will not observe the unencrypted SCTP packet, there
   is no risk in interfering with using zero as an incorrect checksum.
   There are no additional constraints (specific to the error detection
   method) on packets when using DTLS encapsulation.

7.  Socket API Considerations

   This section describes how the socket API defined in [RFC6458] needs
   to be extended to provide a way for the application to control the
   acceptance of a zero checksum.

   A 'Socket API Considerations' section is contained in all SCTP-
   related specifications published after [RFC6458] describing an
   extension for which implementations using the socket API as specified
   in [RFC6458] would require some extension of the socket API.  Please
   note that this section is informational only.

   A socket API implementation based on [RFC6458] is extended by
   supporting one new write-only IPPROTO_SCTP-level socket option.

7.1.  Set Accepting a Zero Checksum (SCTP_ACCEPT_ZERO_CHECKSUM)

   This IPPROTO_SCTP-level socket option with the 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 any alternate error detection
      method.  This means that all SCTP packets being received are only
      accepted if they have a correct CRC32c checksum value.

   SCTP_EDMID_LOWER_LAYER_DTLS:  Use the alternate error detection
      method described in Section 6.

   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.

8.  IANA Considerations

   A new chunk parameter type has been assigned by IANA in the "Chunk
   Parameter Types" registry for SCTP:

       +==========+===================================+===========+
       | ID Value | Chunk Parameter Type              | Reference |
       +==========+===================================+===========+
       | 32769    | Zero Checksum Acceptable (0x8001) | RFC 9653  |
       +----------+-----------------------------------+-----------+

          Table 1: New Entry in "Chunk Parameter Types" Registry

   Furthermore, IANA has established a new "Error Detection Method"
   registry for SCTP.  The assignment of new error detection methods is
   done through the Specification Required policy as defined in
   [RFC8126].  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 a specification describing:

       (a)  the alternate error detection method,

       (b)  why the alternate error detection method provides an equal
            or lower probability of false negatives than the one
            provided by using the CRC32c checksum,

       (c)  any constraints (specific to the alternate error detection
            method) that are referred to in the fourth exception in
            Section 5.2, and

       (d)  why using the alternate error detection method does not
            result in path failures due to middleboxes expecting correct
            CRC32c checksums for more than two RTOs.  In case the
            alternate error detection method uses a heuristic for
            detecting such middleboxes, this heuristic needs to be
            described.

   The initial contents of the registry are as follows:

          +================+========================+===========+
          | ID Value       | Error Detection Method | Reference |
          +================+========================+===========+
          | 0              | Reserved               | RFC 9653  |
          +----------------+------------------------+-----------+
          | 1              | SCTP over DTLS         | RFC 9653  |
          +----------------+------------------------+-----------+
          | 2 - 4294967295 | Unassigned             |           |
          +----------------+------------------------+-----------+

             Table 2: Initial Contents of the "Error Detection
                              Method" Registry

   A designated expert (DE) is expected to ascertain the existence of
   suitable documentation (a specification) as described in [RFC8126]
   and to verify that the document is permanently and publicly
   available.  Furthermore, the DE is expected to ensure that the above
   four points have been addressed appropriately.

9.  Security Considerations

   This document does not change the considerations given in [RFC9260].

   Due to the first requirement in Section 3, using an alternate error
   detection method provides an equal or better level of data integrity
   than the one provided by using the CRC32c checksum algorithm.  The
   second requirement in Section 3 ensures that the existence of
   middleboxes expecting correct CRC32c checksums does not result in
   permanent path failures.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              DOI 10.17487/RFC5061, September 2007,
              <https://www.rfc-editor.org/info/rfc5061>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8261]  Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
              "Datagram Transport Layer Security (DTLS) Encapsulation of
              SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
              2017, <https://www.rfc-editor.org/info/rfc8261>.

   [RFC9260]  Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
              Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
              June 2022, <https://www.rfc-editor.org/info/rfc9260>.

10.2.  Informative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
              2007, <https://www.rfc-editor.org/info/rfc4895>.

   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458,
              DOI 10.17487/RFC6458, December 2011,
              <https://www.rfc-editor.org/info/rfc6458>.

   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/info/rfc9293>.

Acknowledgments

   The authors wish to thank Bernard Aboba, Deb Cooley, Martin Duke,
   Gorry Fairhurst, Mike Heard, Peter Lei, Nils Ohlmeier, Claudio
   Porfiri, Greg Skinner, Timo Völker, Éric Vyncke, and Magnus
   Westerlund for their invaluable comments.

Authors' Addresses

   Michael Tüxen
   Münster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   Germany
   Email: tuexen@fh-muenster.de


   Victor Boivie
   Google
   Kungsbron 2
   SE-11122 Stockholm
   Sweden
   Email: boivie@google.com


   Florent Castelli
   Google
   Kungsbron 2
   SE-11122 Stockholm
   Sweden
   Email: orphis@google.com


   Randell Jesup
   Mozilla Corporation
   1835 Horse Shoe Trl
   Malvern, PA 19355
   United States of America
   Email: randell-ietf@jesup.org