OPSAWG                                                      M. Boucadair
Internet-Draft                                                    Orange
Obsoletes: 7125 (if approved)                              26 March 2023
Intended status: Standards Track                                        
Expires: 27 September 2023


   An Update to the tcpControlBits IP Flow Information Export (IPFIX)
                          Information Element
                  draft-ietf-opsawg-rfc7125-update-02

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 Flags header field since RFC 793.
   However, that update is still problematic for interoperability
   because some flag values were deprecated since then.

   This document removes stale information from the IPFIX registry and
   avoiding 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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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on 27 September 2023.

Copyright Notice

   Copyright (c) 2023 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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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The tcpControlBits Information Element  . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Changes from RFC 7125  . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

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, 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



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   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.

   [RFC7125] revised the tcpControlBits IP Flow Information Export
   (IPFIX) Information Element (IE) that was originally defined in
   [RFC5102] to reflect changes to the TCP Flags header field 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 update 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].

   This document fixes that problem by removing stale information from
   the IPFIX registry and avoiding future conflicts with the
   authoritative TCP registry [IPFIX].

   Also, 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].  See Section 3 for more
   details.

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

   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; 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.

   Units:

   Range:

   References:  [RFC9293][This-Document]

   Revision:  2



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4.  IANA Considerations

   IANA is requested to update the "tcpControlBits" entry of the [IPFIX]
   as follows:

   *  Update the description of to reflect the change in Section 3.

   *  Add [TCP-FLAGS] to the Additional Information field.

   *  Set the revision to 2 and the revision date to the date of
      publication of this document.

   *  Add this document to the references.

5.  Security Considerations

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

6.  References

6.1.  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/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>.

6.2.  Informative References



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   [I-D.ietf-opsawg-ipfix-srv6-srh]
              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-07, 13 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-
              ipfix-srv6-srh-07>.

   [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>.

   [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>.

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.



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   *  Update the references of the tcpControlBits Information Element.

   *  Bump the revision of the tcpControlBits Information Element.

   *  Replace obsolete RFCs (e.g., RFC793).

Acknowledgments

   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.

   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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