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Internet Engineering Task Force (IETF)                       B. TrammellRequest for Comments: 7125                                    ETH ZurichCategory: Informational                                        P. AitkenISSN: 2070-1721                                       Cisco Systems, Inc                                                           February 2014Revision of the tcpControlBitsIP Flow Information Export (IPFIX) Information ElementAbstract   This document revises the tcpControlBits IP Flow Information Export   (IPFIX) Information Element as originally defined inRFC 5102 to   reflect changes to the TCP Flags header field sinceRFC 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 20141.  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 inSection 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   inSection 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:  flagsTrammell & 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:  14.  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 inSection 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 20146.  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.  References7.1.  Normative References   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,RFC793, 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",RFC3168, September 2001.   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit              Congestion Notification (ECN) Signaling with Nonces",RFC3540, 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 2014Authors' 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.comTrammell & Aitken             Informational                     [Page 6]