Network Working Group M. Tüxen Internet-Draft Münster Univ. of Appl. Sciences Intended status: Standards Track V. Boivie Expires: 11 November 2023 F. Castelli Google R. Jesup Mozilla 10 May 2023 Zero Checksum for the Stream Control Transmission Protocol draft-tuexen-tsvwg-sctp-zero-checksum-03 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. When the lower layer 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 the constant 0 as the checksum in a backwards compatible way. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 11 November 2023. Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. Tüxen, et al. Expires 11 November 2023 [Page 1] Internet-Draft Zero Checksum for SCTP May 2023 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. A New Chunk Parameter . . . . . . . . . . . . . . . . . . . . 3 4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 3 4.1. Declaration of Feature Support . . . . . . . . . . . . . 4 4.2. Sender Side Considerations . . . . . . . . . . . . . . . 4 4.3. Receiver Side Considerations . . . . . . . . . . . . . . 5 5. Socket API Considerations . . . . . . . . . . . . . . . . . . 5 5.1. Set Accepting a Zero Checksum (SCTP_ACCEPT_ZERO_CHECKSUM) . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . 6 8.2. Informative References . . . . . . . . . . . . . . . . . 7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction SCTP as specified in [RFC9260] 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 [RFC8261], 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 computationally 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. To provide this backwards compatibility, end points using this extension still need to implement the CRC32c checksum algorithm. Tüxen, et al. Expires 11 November 2023 [Page 2] Internet-Draft Zero Checksum for SCTP May 2023 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. 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 (suggested) | Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: 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 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 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 this parameter and continues to process further parameters in the chunk. This behavior is REQUIRED by [RFC9260] because the highest-order two bits of the Type are '10'. 4. Procedures Tüxen, et al. Expires 11 November 2023 [Page 3] Internet-Draft Zero Checksum for SCTP May 2023 4.1. Declaration of Feature Support If the lower layer of SCTP provides an equal or better level of data integrity protection than the one provided by using the CRC32c algorithm, the computation of the CRC32c checksum requires computational resources without providing any benefit. To avoid this, an SCTP end point MAY be willing to accept SCTP packets with an incorrect CRC32c checksum value of zero in addition to SCTP packets with correct CRC32c checksum values. An SCTP endpoint SHOULD NOT be willing to accept SCTP packets with an incorrect CRC32c checksum value of zero, if the lower layer does not provide at least the level of data integrity the CRC32c checksum algorithm provides. One example of such a lower layer is the use of SCTP over DTLS as described in [RFC8261] (as used in the WebRTC context). Counter examples include: * SCTP over IPv4 or IPv6 as specified in [RFC9260]. * SCTP over UDP as specified in [RFC6951]. * The use of SCTP Authentication as specified in [RFC4895]. Therefore using an incorrect zero checksum, in particular when using SCTP over DTLS, will not result in problems with the middle boxes expecting correct CRC32c checksums in SCTP packets. An SCTP implementation MAY also require the upper layer to indicate that it is fine to accept SCTP packets with an incorrect CRC32c value of zero. An end point willing to accept SCTP packets with a checksum of zero MUST include the Zero Checksum Acceptable Chunk Parameter in the INIT or INIT ACK chunk it sends. 4.2. Sender Side Considerations If an end point has received an INIT or INIT ACK chunk containing a Zero Checksum Acceptable Chunk Parameter from its peer during the association setup, it SHOULD use zero as the checksum for all packets sent in this association with the following three exceptions: * When an end point sends a packet containing an INIT chunk, it MUST include a correct CRC32c checksum in the packet containing the INIT chunk. Tüxen, et al. Expires 11 November 2023 [Page 4] Internet-Draft Zero Checksum for SCTP May 2023 * 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. * When an end point 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. The first exception allows backwards compatibility and the second and third exception allow a simpler implementation of the extension defined in this document. When an end point responds to an "Out of the Blue" (OOTB) SCTP packet, it MUST include a correct CRC32c checksum in the response packet. An SCTP end point MAY only send packets with an incorrect checksum of zero, if the upper layer allowed the reception of SCTP packets with an incorrect zero checksum. 4.3. Receiver Side Considerations Zero is a valid result of the CRC32c algorithm. Therefore, 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 has sent the Zero Checksum Acceptable Chunk Parameter during the handshake, this ambiguity is irrelevant, since the receiver is fine with not using the CRC32c to protect incoming packets. 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 an incorrect checksum value of zero in addition to SCTP packets containing the correct CRC32c checksum value for this association. An SCTP implementation MAY process OOTB SCTP packets having an incorrect zero checksum in addition to OOTB packets with a correct CRC32c checksum. 5. 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. Please note that this section is informational only. Tüxen, et al. Expires 11 November 2023 [Page 5] Internet-Draft Zero Checksum for SCTP May 2023 A socket API implementation based on [RFC6458] is extended by supporting one new write-only IPPROTO_SCTP-level socket option. 5.1. Set Accepting a Zero Checksum (SCTP_ACCEPT_ZERO_CHECKSUM) This socket option 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 integer boolean flag, where a non-zero value enables the option, and a zero value disables the option. An implementation might only send packets with an incorrect checksum of zero, if this option is enabled. This option is disabled by default. 6. 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: +===================+==========================+===========+ | ID Value | Chunk Parameter Type | Reference | +===================+==========================+===========+ | 32769 (suggested) | Zero Checksum Acceptable | [RFCXXXX] | | | (0x8001 (suggested)) | | +-------------------+--------------------------+-----------+ Table 1: New entry in "Chunk Parameter Types" registry 7. Security Considerations This document does not change the considerations given in [RFC9260]. 8. References 8.1. Normative References Tüxen, et al. Expires 11 November 2023 [Page 6] Internet-Draft Zero Checksum for SCTP May 2023 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . [RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260, June 2022, . 8.2. Informative References [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, . [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, . [RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication", RFC 6951, DOI 10.17487/RFC6951, May 2013, . Acknowledgments The authors wish to thank Gorry Fairhurst, Mike Heard, Nils Ohlmeier, Claudio Porfiri, and Magnus Westerlund for their invaluable comments. Tüxen, et al. Expires 11 November 2023 [Page 7] Internet-Draft Zero Checksum for SCTP May 2023 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 Tüxen, et al. Expires 11 November 2023 [Page 8]