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rfc:rfc9565



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
 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.  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: mohamed.boucadair@orange.com
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