An Update to the tcpControlBits IP Flow Information Export (IPFIX) Information Element
draft-ietf-opsawg-rfc7125-update-07

OPSAWG                                                      M. Boucadair
Internet-Draft                                                    Orange
Obsoletes: 7125 (if approved)                           29 November 2023
Intended status: Standards Track                                        
Expires: 1 June 2024

   An Update to the tcpControlBits IP Flow Information Export (IPFIX)
                          Information Element
                  draft-ietf-opsawg-rfc7125-update-07

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 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
   (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/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Revised tcpControlBits Information Element  . . . . . . . . .   3
   4.  An Example  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Changes from RFC 7125  . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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

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   "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 (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 deviating 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.  Revised tcpControlBits Information Element

   ElementId:  6

   Data Type:  unsigned16

   Data Type Semantics:  flags

   Description:  TCP control bits observed for the packets of this Flow.

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      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.

      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 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 [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:

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   Range:

   References:  [RFC9293][This-Document]

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

   Revision:  2

4.  An Example

   Figure 1 shows an example of a tcpControlBits Information Element set
   to 146.  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 tcpControlBits Information Element

5.  IANA Considerations

   IANA is requested to update the "tcpControlBits" entry of the [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

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   [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/rfc/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/rfc/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/rfc/rfc8174>.

   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9293>.

   [TCP-FLAGS]
              IANA, "TCP Header Flags", n.d.,
              <https://www.iana.org/assignments/tcp-parameters/tcp-
              parameters.xhtml#tcp-header-flags>.

7.2.  Informative References

   [IPFIX]    IANA, "IP Flow Information Export (IPFIX) Entities", n.d.,
              <https://www.iana.org/assignments/ipfix/ipfix.xhtml>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", RFC 793,
              DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/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/rfc/rfc7125>.

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   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc9487>.

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]).

   *  Add an Example Section

Acknowledgments

   This document was triggered by a discussion of the author in opsawg
   with 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 for Tim Bray for the artart review and Shawn Emery for the
   secdir review.

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

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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