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An Update to the tcpControlBits IP Flow Information Export (IPFIX) Information Element
RFC 9565

Document Type RFC - Proposed Standard (March 2024)
Obsoletes RFC 7125
Author Mohamed Boucadair
Last updated 2024-03-11
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Robert Wilton
Send notices to (None)
RFC 9565

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


   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

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
   Provisions Relating to IETF Documents
   ( 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.  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 from RFC 7125
   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

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

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.



   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
                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

       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,

   [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,

   [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,

              IANA, "TCP Header Flags",

7.2.  Informative References

   [IPFIX]    IANA, "IPFIX Information Elements",

   [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,

   [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,

   [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, <>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,

   [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, <>.

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).


   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

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


   The authors of [RFC7125] are as follows:

   Brian Trammell

   Paul Aitken

Author's Address

   Mohamed Boucadair
   35000 Rennes