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rfc:rfc7125

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

Abstract

 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
 http://www.rfc-editor.org/info/rfc7125.

Copyright Notice

 Copyright (c) 2014 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
 (http://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 Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Trammell & Aitken Informational [Page 1] 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].

3. The tcpControlBits Information Element

 ElementId:  6
 Data Type:  unsigned16
 Data Type Semantics:  flags

Trammell & Aitken Informational [Page 2] 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.

Trammell & Aitken Informational [Page 3] 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
    Element.
    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.
 Units:
 Range:
 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
 document.

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.

Trammell & Aitken Informational [Page 4] 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
 Commission.

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
            2013.
 [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-IPFIX]
            IANA, "IP Flow Information Export (IPFIX) Entities",
            <http://www.iana.org/assignments/ipfix>.
 [RFC5102]  Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
            Meyer, "Information Model for IP Flow Information Export",
            RFC 5102, January 2008.

Trammell & Aitken Informational [Page 5] RFC 7125 IPFIX tcpControlBits February 2014

Authors' Addresses

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

Trammell & Aitken Informational [Page 6]

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