RFC 7125

Internet Engineering Task Force (IETF)                       B. Trammell
Request for Comments: 7125                                    ETH Zurich
Category: Informational                                        P. Aitken
ISSN: 2070-1721                                       Cisco Systems, Inc
                                                           February 2014

                     Revision of the tcpControlBits
         IP Flow Information Export (IPFIX) Information Element


   This document revises the tcpControlBits IP Flow Information Export
   (IPFIX) Information Element as originally defined in RFC 5102 to
   reflect changes to the TCP Flags header field since RFC 793.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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RFC 7125                  IPFIX tcpControlBits             February 2014

1.  Introduction

   Octets 12 and 13 (counting from zero) of the TCP header encode the
   data offset (header length) in 4 bits, as well as 12 bits of flags.
   The least significant 6 bits of these were defined in [RFC0793] as
   URG, ACK, PSH, RST, SYN, and FIN for TCP control.  Subsequently,
   [RFC3168] defined the CWR and ECE flags for Explicit Congestion
   Notification (ECN) negotiation and signaling; [RFC3540] additionally
   defined the NS flag for the ECN Nonce Sum.

   As defined in the IANA IPFIX Information Element Registry
   [IANA-IPFIX], taken from [RFC5102], the tcpControlBits Information
   Element for IPFIX [RFC7011] only covers the original 6 bits from
   [RFC0793].  To allow IPFIX to be used to measure the use of ECN, and
   to bring the IPFIX Information Element definition in line with the
   current definition of the TCP Flags header field, it is necessary to
   revise this definition.

   The revised definition of the Information Element in Section 3 was
   developed and approved through the IE-DOCTORS process [RFC7013] in
   August 2013.  Section 5.1 of [RFC7013] states "This process should
   not in any way be construed as allowing the IE-DOCTORS to overrule
   IETF consensus.  Specifically, Information Elements in the IANA
   Information Element registry that were added with IETF consensus
   require IETF consensus for revision or deprecation".  Since the
   tcpControlBits Information Element was originally defined in
   [RFC5102], an IETF Proposed Standard, any revision of this
   Information Element definition requires IETF Consensus.  The
   publication of this document fulfills that requirement.

   Section 3 defines the revised tcpControlBits Information Element as
   in Section 9.1 of [RFC7013].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

3.  The tcpControlBits Information Element

   ElementId:  6

   Data Type:  unsigned16

   Data Type Semantics:  flags

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RFC 7125                  IPFIX tcpControlBits             February 2014

   Description:  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.

      The values of each bit are shown below, per the definition of the
      bits in the TCP header [RFC0793] [RFC3168] [RFC3540]:

       MSb                                                         LSb
        0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
      |               |           | N | C | E | U | A | P | R | S | F |
      |     Zero      |   Future  | S | W | C | R | C | S | S | Y | I |
      | (Data Offset) |    Use    |   | R | E | G | K | H | T | N | N |

      bit    flag
      value  name  description
      0x8000       Zero (see tcpHeaderLength)
      0x4000       Zero (see tcpHeaderLength)
      0x2000       Zero (see tcpHeaderLength)
      0x1000       Zero (see tcpHeaderLength)
      0x0800       Future Use
      0x0400       Future Use
      0x0200       Future Use
      0x0100   NS  ECN Nonce Sum
      0x0080  CWR  Congestion Window Reduced
      0x0040  ECE  ECN Echo
      0x0020  URG  Urgent Pointer field significant
      0x0010  ACK  Acknowledgment field significant
      0x0008  PSH  Push Function
      0x0004  RST  Reset the connection
      0x0002  SYN  Synchronize sequence numbers
      0x0001  FIN  No more data from sender

      As the most significant 4 bits of octets 12 and 13 (counting from
      zero) of the TCP header [RFC0793] are used to encode the TCP data
      offset (header length), the corresponding bits in this Information
      Element MUST be exported as zero and MUST be ignored by the
      collector.  Use the tcpHeaderLength Information Element to encode
      this value.

      Each of the 3 bits (0x800, 0x400, and 0x200), which are reserved
      for future use in [RFC0793], SHOULD be exported as observed in the
      TCP headers of the packets of this Flow.

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RFC 7125                  IPFIX tcpControlBits             February 2014

      If exported as a single octet with reduced-size encoding, this
      Information Element covers the low-order octet of this field (i.e,
      bits 0x80 to 0x01), omitting the ECN Nonce Sum and the three
      Future Use bits.  A collector receiving this Information Element
      with reduced-size encoding must not assume anything about the
      content of these four bits.

      Exporting Processes exporting this Information Element on behalf
      of a Metering Process that is not capable of observing any of the
      ECN Nonce Sum or Future Use bits SHOULD use reduced-size encoding,
      and only export the least significant 8 bits of this Information

      Note that previous revisions of this Information Element's
      definition specified that the CWR and ECE bits 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.



   References:  [RFC0793] [RFC3168] [RFC3540]

   Revision:  1

4.  IANA Considerations

   IANA has updated the definition of the tcpControlBits Information
   Element in the IANA IPFIX Information Element Registry [IANA-IPFIX]
   to reflect the changes in Section 3 above, setting the revision to 1
   as noted, and the revision date to the date of publication of this

5.  Security and Privacy Considerations

   This document changes the data type (and therefore the native size)
   of the tcpControlBits Information Element, from unsigned8 (1 octet)
   to unsigned16 (2 octets).  As Exporting and Collecting Processes use
   the Information Element Length field in Templates, Options Templates,
   and specifications for reduced-size encoding where appropriate, as
   opposed to abstract data type sizes, for encoding and decoding Data
   Records, it is not expected that this will have any impact on buffer
   sizing during encoding and decoding.  Otherwise, note that the
   security considerations for IPFIX [RFC7011] apply.

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RFC 7125                  IPFIX tcpControlBits             February 2014

6.  Acknowledgments

   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

7.  References

7.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP", RFC
              3168, September 2001.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces", RFC
              3540, June 2003.

   [RFC7011]  Claise, B., Trammell, B., and P. Aitken, "Specification of
              the IP Flow Information Export (IPFIX) Protocol for the
              Exchange of Flow Information", STD 77, RFC 7011, September

   [RFC7013]  Trammell, B. and B. Claise, "Guidelines for Authors and
              Reviewers of IP Flow Information Export (IPFIX)
              Information Elements", BCP 184, RFC 7013, September 2013.

7.2.  Informative References

              IANA, "IP Flow Information Export (IPFIX) Entities",

   [RFC5102]  Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
              Meyer, "Information Model for IP Flow Information Export",
              RFC 5102, January 2008.

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RFC 7125                  IPFIX tcpControlBits             February 2014

Authors' Addresses

   Brian Trammell
   Swiss Federal Institute of Technology Zurich
   Gloriastrasse 35
   8092 Zurich

   Phone: +41 44 632 70 13
   EMail: trammell@tik.ee.ethz.ch

   Paul Aitken
   Cisco Systems, Inc.
   96 Commercial Quay
   Commercial Street, Edinburgh EH6 6LX
   United Kingdom

   Phone: +44 131 561 3616
   EMail: paitken@cisco.com

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