Internet Engineering Task Force (IETF)                      M. Boucadair
Request for Comments: 9565                                        Orange
Obsoletes: 7125                                               March 2024
Category: Standards Track
ISSN: 2070-1721


  An Update to the tcpControlBits IP Flow Information Export (IPFIX)
                         Information Element

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 header control bits since RFC 793.
  However, that update is still problematic for interoperability
  because some flag values have subsequently been deprecated.

  This document removes stale information from the IANA "IPFIX
  Information Elements" registry and avoids future conflicts with the
  authoritative IANA "TCP Header Flags" registry.

  This document obsoletes RFC 7125.

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/rfc9565.

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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  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

  1.  Introduction
  2.  Terminology
  3.  Revised tcpControlBits Information Element
  4.  An Example
  5.  IANA Considerations
  6.  Security Considerations
  7.  References
    7.1.  Normative References
    7.2.  Informative References
  Appendix A.  Changes from RFC 7125
  Acknowledgments
    Acknowledgments from RFC 7125
  Contributors
  Author's Address

1.  Introduction

  TCP defines a set of control bits (also known as "flags") for
  managing connections (Section 3.1 of [RFC9293]).  The "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
  [TCP-FLAGS], adding bits that had previously been specified in
  [RFC0793], and removing the NS (Nonce Sum) bit 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] is thus settled as the
  authoritative reference for the assigned TCP control bits.

     Note: The bits in offsets 0 through 3 are not header flags, but
     the TCP segment Data Offset field.

  [RFC7125] revised the tcpControlBits IP Flow Information Export
  (IPFIX) Information Element that was originally defined in [RFC5102]
  to reflect changes to the TCP control bits 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 deviating from the authoritative "TCP Header Flags"
  registry [TCP-FLAGS].

  This document fixes that problem by removing stale information from
  the "IPFIX Information Elements" registry [IPFIX] and avoiding future
  conflicts with the authoritative "TCP Header Flags" registry
  [TCP-FLAGS].  The update in this document also enhances
  observability.  For example, network operators can identify packets
  that are 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].

  The main changes from [RFC7125] are listed in Appendix A.

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.  Revised tcpControlBits Information Element

  ElementID:  6

  Name:  tcpControlBits

  Abstract Data Type:  unsigned16

  Data Type Semantics:  flags

  Status:  current

  Description:  TCP control bits observed for the packets of this Flow.
     This information is encoded as a bit field; each TCP control bit
     has a corresponding bit in that field.  A bit is set to 1 if any
     observed packet of this Flow has the corresponding TCP control bit
     set to 1.  The bit is cleared to 0 otherwise.

     Per [RFC9293], the assignment of TCP control bits is managed by
     IANA via the "TCP Header Flags" registry [TCP-FLAGS].
     Implementers can retrieve the current TCP control bits from that
     registry, which is authoritative for them.

     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 reported by the Exporter 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.

     If exported as a single octet with reduced-size encoding
     (Section 6.2 of [RFC7011]), this Information Element covers the
     low-order octet of this field (i.e., bit offset positions 8 to 15)
     [TCP-FLAGS].  A Collector receiving this Information Element with
     reduced-size encoding must not assume anything about the content
     of the four bits with bit offset positions 4 to 7.

     Exporting Processes exporting this Information Element on behalf
     of a Metering Process that is not capable of observing any of the
     flags with bit offset positions 4 to 7 SHOULD use reduced-size
     encoding, and only export the least significant 8 bits of this
     Information Element.

     Note that previous revisions of this Information Element's
     definition specified that flags with bit offset positions 8 and 9
     must be exported as zero, even if observed.  Collectors should
     therefore not assume that a value of zero for these bits in this
     Information Element indicates the bits were never set in the
     observed traffic, especially if these bits are zero in every Flow
     Record sent by a given Exporter.

     Note also that the "TCP Header Flags" registry [TCP-FLAGS] indexes
     the bit offset from the most significant bit of octet 12 to the
     least significant bit of octet 13 in the TCP header, but the
     tcpControlBits is encoded as a regular unsigned 16-bit integer.

  Units:

  Range:

  Additional Information:  See the assigned TCP control bits in the
     "TCP Header Flags" registry [TCP-FLAGS].

  Reference:  [RFC9293], RFC 9565

  Revision:  2

4.  An Example

  Figure 1 shows an example of a tcpControlBits Information Element set
  to 0x92, where MSB indicates the most significant bit and LSB
  indicates the least significant bit.  This Information Element is
  used to report TCP control bits for a Flow that has CWR (Congestion
  Window Reduced), ACK, and SYN flag bits set (that is, bit offset
  positions 8, 11, and 14).

                    MSB                           LSB
                                         1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |0|0|0|0|0|0|0|0|1|0|0|1|0|0|1|0|
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 1: An Example of the tcpControlBits Information Element

5.  IANA Considerations

  IANA has updated the "tcpControlBits" entry of the "IPFIX Information
  Elements" registry [IPFIX] to echo the details provided in Section 3.

6.  Security Considerations

  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 requirements
  language compared to [RFC7125].

  This document does not add new security considerations to those
  already discussed for IPFIX in [RFC7011].

7.  References

7.1.  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>.

  [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,
             <https://www.rfc-editor.org/info/rfc7011>.

  [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>.

  [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>.

  [TCP-FLAGS]
             IANA, "TCP Header Flags",
             <https://www.iana.org/assignments/tcp-parameters/>.

7.2.  Informative References

  [IPFIX]    IANA, "IPFIX Information Elements",
             <https://www.iana.org/assignments/ipfix/>.

  [RFC0793]  Postel, J., "Transmission Control Protocol", RFC 793,
             DOI 10.17487/RFC0793, September 1981,
             <https://www.rfc-editor.org/info/rfc793>.

  [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,
             <https://www.rfc-editor.org/info/rfc3168>.

  [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,
             <https://www.rfc-editor.org/info/rfc5102>.

  [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, <https://www.rfc-editor.org/info/rfc7125>.

  [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
             Notification (ECN) Experimentation", RFC 8311,
             DOI 10.17487/RFC8311, January 2018,
             <https://www.rfc-editor.org/info/rfc8311>.

  [RFC9487]  Graf, T., Claise, B., and P. Francois, "Export of Segment
             Routing over IPv6 Information in IP Flow Information
             Export (IPFIX)", RFC 9487, DOI 10.17487/RFC9487, November
             2023, <https://www.rfc-editor.org/info/rfc9487>.

Appendix A.  Changes from RFC 7125

  *  Cleaned up the description of the tcpControlBits Information
     Element by removing mentions of stale flag bits, referring to the
     flag bits by their bit offset position, and relying upon the IANA
     "TCP Header Flags" registry.

  *  Removed the table of TCP flag bits from the description of the
     tcpControlBits Information Element.

  *  Added the reference [TCP-FLAGS] to the Additional Information
     field of the tcpControlBits Information Element.

  *  Used strong normative language for exporting observed flags.

  *  Updated the references of the tcpControlBits Information Element.

  *  Bumped the revision of the tcpControlBits Information Element.

  *  Replaced obsolete RFCs (e.g., [RFC0793]).

  *  Added an example section (Section 4).

Acknowledgments

  This document was triggered by a discussion in the opsawg working
  group between the author and the authors of [RFC9487].

  Thanks to Christian Jacquenet, Thomas Graf, and Benoît Claise for the
  review and comments.

  Thanks to Michael Scharf for the tsvart review, Ketan Talaulikar for
  the rtgdir review, and Elwyn Davies for the genart review.

  Thanks to Rob Wilton for the AD review.

  Thanks to Tim Bray for the artart review and Shawn Emery for the
  secdir review.

  Thanks to Éric Vyncke and Paul Wouters for the comments in the IESG
  review.

Acknowledgments from RFC 7125

  Thanks to Andrew Feren, Lothar Braun, Michael Scharf, and Simon
  Josefsson for comments on the revised definition.  This work is
  partially supported by the European Commission under grant agreement
  FP7-ICT-318627 mPlane; this does not imply endorsement by the
  Commission.

Contributors

  The authors of [RFC7125] are as follows:

  Brian Trammell


  Paul Aitken


Author's Address

  Mohamed Boucadair
  Orange
  35000 Rennes
  France
  Email: [email protected]

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