OPSAWG M. Boucadair Internet-Draft Orange Updates: 7125 (if approved) 24 February 2023 Intended status: Standards Track Expires: 28 August 2023 An Update to the tcpControlBits IP Flow Information Export (IPFIX) Information Element draft-ietf-opsawg-rfc7125-update-01 Abstract RFC 7125 revised the tcpControlBits IP Flow Information Export (IPFIX) Information Element that was originally defined in RFC 5102 to reflect changes to the TCP Flags header field since RFC 793. However, that update is still problematic for interoperability because some flag values were deprecated since then. This document updates RFC 7125 by removing stale information from the IPFIX registry and avoiding future conflicts with the authoritative TCP Header Flags registry. Discussion Venues This note is to be removed before publishing as an RFC. Discussion of this document takes place on the Operations and Management Area Working Group Working Group mailing list (opsawg@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/opsawg/. Source for this draft and an issue tracker can be found at https://github.com/boucadair/-ipfix-rfc7125-update. 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/. Boucadair Expires 28 August 2023 [Page 1] Internet-Draft tcpControlBits IPFIX February 2023 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 28 August 2023. Copyright Notice Copyright (c) 2023 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 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. An Update to tcpControlBits IP Flow Information Export (IPFIX) Information Element . . . . . . . . . . . . . . . . . . . 3 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 6.2. Informative References . . . . . . . . . . . . . . . . . 6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction TCP defines a set of control bits (also known as "flags") for managing connections (Section 3.1 of [RFC9293]). The "Transmission Control Protocol (TCP) Header Flags" registry was initially set by [RFC3168], but it was populated with only TCP control bits that were defined in [RFC3168]. [RFC9293] fixed that by moving that registry to be listed as a subregistry under the "Transmission Control Protocol (TCP) Parameters" registry, adding bits that had previously been specified in [RFC0793], and removing the NS (Nonce Sum) bit as per [RFC8311]. Also, Section 6 of [RFC9293] introduces "Bit Offset" to ease referencing each header flag's offset within the 16-bit aligned view of the TCP header (Figure 1 of [RFC9293]). [TCP-FLAGS] Boucadair Expires 28 August 2023 [Page 2] Internet-Draft tcpControlBits IPFIX February 2023 is thus settled as the authoritative reference for the assigned TCP control bits. [RFC7125] revised the tcpControlBits IP Flow Information Export (IPFIX) Information Element (IE) that was originally defined in [RFC5102] to reflect changes to the TCP Flags header field since [RFC0793]. However, that update is still problematic for interoperability because a value was deprecated since then (Section 7 of [RFC8311]) and, therefore, [RFC7125] risks to deviate from the authoritative TCP registry [TCP-FLAGS]. This update is also useful to enhance observability. For example, network operators can identify when packets are being observed with unassigned TCP flags set and, therefore, identify which applications in the network should be upgraded to reflect the changes to TCP flags that were introduced, e.g., in [RFC8311]. This document fixes that problem by removing stale information from the IPFIX registry and avoiding future conflicts with the authoritative TCP registry [IPFIX]. Also, because the setting of TCP control bits may be misused in some flows (e.g., Distributed Denial-of-Service (DDoS) attacks), an exporter has to report all observed control bits even if no meaning is associated with a given TCP flag. This document uses a stronger requirement language compared to [RFC7125]. See Section 3 for more details. 2. Terminology 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. This document uses the terms defined in Section 2 of [RFC7011]. 3. An Update to tcpControlBits IP Flow Information Export (IPFIX) Information Element This document updates Section 3 of [RFC7125] as follows: Boucadair Expires 28 August 2023 [Page 3] Internet-Draft tcpControlBits IPFIX February 2023 OLD: The values of each bit are shown below, per the definition of the bits in the TCP header [RFC0793][RFC3168][RFC3540]: MSb LSb 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | | N | C | E | U | A | P | R | S | F | | Zero | Future | S | W | C | R | C | S | S | Y | I | | (Data Offset) | Use | | R | E | G | K | H | T | N | N | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ bit flag value name description ------+-----+------------------------------------- 0x8000 Zero (see tcpHeaderLength) 0x4000 Zero (see tcpHeaderLength) 0x2000 Zero (see tcpHeaderLength) 0x1000 Zero (see tcpHeaderLength) 0x0800 Future Use 0x0400 Future Use 0x0200 Future Use 0x0100 NS ECN Nonce Sum 0x0080 CWR Congestion Window Reduced 0x0040 ECE ECN Echo 0x0020 URG Urgent Pointer field significant 0x0010 ACK Acknowledgment field significant 0x0008 PSH Push Function 0x0004 RST Reset the connection 0x0002 SYN Synchronize sequence numbers 0x0001 FIN No more data from sender As the most significant 4 bits of octets 12 and 13 (counting from zero) of the TCP header [RFC0793] are used to encode the TCP data offset (header length), the corresponding bits in this Information Element MUST be exported as zero and MUST be ignored by the collector. Use the tcpHeaderLength Information Element to encode this value. Each of the 3 bits (0x800, 0x400, and 0x200), which are reserved for future use in [RFC0793], SHOULD be exported as observed in the TCP headers of the packets of this Flow. Boucadair Expires 28 August 2023 [Page 4] Internet-Draft tcpControlBits IPFIX February 2023 NEW: As per [RFC9293], the assignment of the TCP control bits is managed by IANA from the "TCP Header Flags" registry [TCP-FLAGS]. That registry is authoritative to retrieve the most recent TCP control bits. As the most significant 4 bits of octets 12 and 13 (counting from zero) of the TCP header [RFC9293] are used to encode the TCP data offset (header length), the corresponding bits in this Information Element MUST be exported with a value of zero and MUST be ignored by the collector. Use the tcpHeaderLength Information Element to encode this value. All TCP control bits (including those unassigned) MUST be exported as observed in the TCP headers of the packets of this Flow. 4. IANA Considerations IANA is requested to update the "tcpControlBits" entry of the [IPFIX] as follows: * Update the description of to reflect the change in Section 3. * Add [TCP-FLAGS] to the Additional Information field. * Add this document to the references. 5. Security Considerations This document does not add any new security considerations to those already discussed in Section 5 of [RFC7125]. 6. References 6.1. 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, . [RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken, "Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of Flow Information", STD 77, RFC 7011, DOI 10.17487/RFC7011, September 2013, . Boucadair Expires 28 August 2023 [Page 5] Internet-Draft tcpControlBits IPFIX February 2023 [RFC7125] Trammell, B. and P. Aitken, "Revision of the tcpControlBits IP Flow Information Export (IPFIX) Information Element", RFC 7125, DOI 10.17487/RFC7125, February 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)", STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022, . [TCP-FLAGS] IANA, "TCP Header Flags", n.d., . 6.2. Informative References [I-D.ietf-opsawg-ipfix-srv6-srh] Graf, T., Claise, B., and P. Francois, "Export of Segment Routing over IPv6 Information in IP Flow Information Export (IPFIX)", Work in Progress, Internet-Draft, draft- ietf-opsawg-ipfix-srv6-srh-06, 5 January 2023, . [IPFIX] IANA, "IP Flow Information Export (IPFIX) Entities", n.d., <. [RFC0793] Postel, J., "Transmission Control Protocol", RFC 793, DOI 10.17487/RFC0793, September 1981, . [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001, . [RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, DOI 10.17487/RFC3540, June 2003, . Boucadair Expires 28 August 2023 [Page 6] Internet-Draft tcpControlBits IPFIX February 2023 [RFC5102] Quittek, J., Bryant, S., Claise, B., Aitken, P., and J. Meyer, "Information Model for IP Flow Information Export", RFC 5102, DOI 10.17487/RFC5102, January 2008, . [RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion Notification (ECN) Experimentation", RFC 8311, DOI 10.17487/RFC8311, January 2018, . Acknowledgments This document was triggered by a discussion of the author in opsawg with the authors of [I-D.ietf-opsawg-ipfix-srv6-srh]. Thanks to Christian Jacquenet, Thomas Graf, and BenoƮt Claise for the review and comments. Author's Address Mohamed Boucadair Orange 35000 Rennes France Email: mohamed.boucadair@orange.com Boucadair Expires 28 August 2023 [Page 7]