Internet-Draft tcpControlBits IPFIX October 2023
Boucadair Expires 14 April 2024 [Page]
7125 (if approved)
Intended Status:
Standards Track
M. Boucadair

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 IPFIX registry and avoids future conflicts with the authoritative TCP Header Flags registry.

This document obsoletes RFC 7125.

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 (, which is archived at

Source for this draft and an issue tracker can be found at

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

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 14 April 2024.

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 [TCP-FLAGS], 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] is thus settled as the authoritative reference for the assigned TCP control bits.

  • 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 (IE) 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 to deviate from the authoritative TCP registry [TCP-FLAGS].

This document fixes that problem by removing stale information from the IPFIX registry [IPFIX] and avoiding future conflicts with the authoritative TCP registry [TCP-FLAGS]. The update in this document 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].

The main changes to [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. The tcpControlBits Information Element



Data Type:


Data Type Semantics:



TCP control bits observed for the packets of this Flow. This information is encoded as a bit field; for each TCP control bit, there is a bit in this set. The 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.

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.

If exported as a single octet with reduced-size encoding, 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 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 [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.

For example, a tcpControlBits Information Element set to 0x90 is used to report TCP control bits for a segment that has CWR (Congestion Window Reduced) and ACK flag bits set (that is, bit offset positions 8 and 11).

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





Additional Information:

See the assigned TCP control bits in [TCP-FLAGS].



4. IANA Considerations

IANA is requested to update the "tcpControlBits" entry of the [IPFIX] to echo the details provided in Section 3.

5. 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 requirement language compared to [RFC7125].

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

6. References

6.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Eddy, W., Ed., "Transmission Control Protocol (TCP)", STD 7, RFC 9293, DOI 10.17487/RFC9293, , <>.
IANA, "TCP Header Flags", n.d., <>.

6.2. Informative References

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-14, , <>.
IANA, "IP Flow Information Export (IPFIX) Entities", n.d., <<>.
Postel, J., "Transmission Control Protocol", RFC 793, DOI 10.17487/RFC0793, , <>.
Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, , <>.
Quittek, J., Bryant, S., Claise, B., Aitken, P., and J. Meyer, "Information Model for IP Flow Information Export", RFC 5102, DOI 10.17487/RFC5102, , <>.
Trammell, B. and P. Aitken, "Revision of the tcpControlBits IP Flow Information Export (IPFIX) Information Element", RFC 7125, DOI 10.17487/RFC7125, , <>.
Black, D., "Relaxing Restrictions on Explicit Congestion Notification (ECN) Experimentation", RFC 8311, DOI 10.17487/RFC8311, , <>.

Appendix A. Changes from RFC 7125

  • Clean-up the description by removing mentions of stale flag bits, referring to the flag bits by their bit offset position, and relying upon the IANA TCP registry.

  • Remove the table of TCP flag bits from the description of the tcpControlBits Information Element.

  • Add [TCP-FLAGS] to the Additional Information field of the tcpControlBits Information Element.

  • Use strong normative language for exporting observed flags.

  • Update the references of the tcpControlBits Information Element.

  • Bump the revision of the tcpControlBits Information Element.

  • Replace obsolete RFCs (e.g., [RFC0793]).


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 Benoît Claise for the review and comments.

Thanks to Michael Scharf for the tsvart review and Ketan Talaulikar for the rtgdir review.

Thanks to Rob Wilton for the AD review.

Acknowledgments from [RFC7125]:

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.


The authors of [RFC7125] are as follows:

  • Brian Trammell

  • Paul Aitken

Author's Address

Mohamed Boucadair
35000 Rennes