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<pre>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
<span class="h1">Revision of the tcpControlBits</span>
<span class="h1">IP Flow Information Export (IPFIX) Information Element</span>
Abstract
This document revises the tcpControlBits IP Flow Information Export
(IPFIX) Information Element as originally defined in <a href="./rfc5102">RFC 5102</a> to
reflect changes to the TCP Flags header field since <a href="./rfc793">RFC 793</a>.
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 <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc7125">http://www.rfc-editor.org/info/rfc7125</a>.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.
<span class="grey">Trammell & Aitken Informational [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc7125">RFC 7125</a> IPFIX tcpControlBits February 2014</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
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 [<a href="./rfc0793" title=""Transmission Control Protocol"">RFC0793</a>] as
URG, ACK, PSH, RST, SYN, and FIN for TCP control. Subsequently,
[<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>] defined the CWR and ECE flags for Explicit Congestion
Notification (ECN) negotiation and signaling; [<a href="./rfc3540" title=""Robust Explicit Congestion Notification (ECN) Signaling with Nonces"">RFC3540</a>] additionally
defined the NS flag for the ECN Nonce Sum.
As defined in the IANA IPFIX Information Element Registry
[<a href="#ref-IANA-IPFIX">IANA-IPFIX</a>], taken from [<a href="./rfc5102" title=""Information Model for IP Flow Information Export"">RFC5102</a>], the tcpControlBits Information
Element for IPFIX [<a href="./rfc7011" title=""Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of Flow Information"">RFC7011</a>] only covers the original 6 bits from
[<a href="./rfc0793" title=""Transmission Control Protocol"">RFC0793</a>]. 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 <a href="#section-3">Section 3</a> was
developed and approved through the IE-DOCTORS process [<a href="./rfc7013" title=""Guidelines for Authors and Reviewers of IP Flow Information Export (IPFIX) Information Elements"">RFC7013</a>] in
August 2013. <a href="./rfc7013#section-5.1">Section 5.1 of [RFC7013]</a> 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
[<a href="./rfc5102" title=""Information Model for IP Flow Information Export"">RFC5102</a>], an IETF Proposed Standard, any revision of this
Information Element definition requires IETF Consensus. The
publication of this document fulfills that requirement.
<a href="#section-3">Section 3</a> defines the revised tcpControlBits Information Element as
in <a href="./rfc7013#section-9.1">Section 9.1 of [RFC7013]</a>.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Terminology</span>
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
[<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. The tcpControlBits Information Element</span>
ElementId: 6
Data Type: unsigned16
Data Type Semantics: flags
<span class="grey">Trammell & Aitken Informational [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc7125">RFC 7125</a> IPFIX tcpControlBits February 2014</span>
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 [<a href="./rfc0793" title=""Transmission Control Protocol"">RFC0793</a>] [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>] [<a href="./rfc3540" title=""Robust Explicit Congestion Notification (ECN) Signaling with Nonces"">RFC3540</a>]:
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 [<a href="./rfc0793" title=""Transmission Control Protocol"">RFC0793</a>] 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 [<a href="./rfc0793" title=""Transmission Control Protocol"">RFC0793</a>], SHOULD be exported as observed in the
TCP headers of the packets of this Flow.
<span class="grey">Trammell & Aitken Informational [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc7125">RFC 7125</a> IPFIX tcpControlBits February 2014</span>
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: [<a href="./rfc0793" title=""Transmission Control Protocol"">RFC0793</a>] [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>] [<a href="./rfc3540" title=""Robust Explicit Congestion Notification (ECN) Signaling with Nonces"">RFC3540</a>]
Revision: 1
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. IANA Considerations</span>
IANA has updated the definition of the tcpControlBits Information
Element in the IANA IPFIX Information Element Registry [<a href="#ref-IANA-IPFIX">IANA-IPFIX</a>]
to reflect the changes in <a href="#section-3">Section 3</a> above, setting the revision to 1
as noted, and the revision date to the date of publication of this
document.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Security and Privacy Considerations</span>
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 [<a href="./rfc7011" title=""Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of Flow Information"">RFC7011</a>] apply.
<span class="grey">Trammell & Aitken Informational [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc7125">RFC 7125</a> IPFIX tcpControlBits February 2014</span>
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Acknowledgments</span>
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.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. References</span>
<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>. Normative References</span>
[<a id="ref-RFC0793">RFC0793</a>] Postel, J., "Transmission Control Protocol", STD 7, <a href="./rfc793">RFC</a>
<a href="./rfc793">793</a>, September 1981.
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC3168">RFC3168</a>] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", <a href="./rfc3168">RFC</a>
<a href="./rfc3168">3168</a>, September 2001.
[<a id="ref-RFC3540">RFC3540</a>] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces", <a href="./rfc3540">RFC</a>
<a href="./rfc3540">3540</a>, June 2003.
[<a id="ref-RFC7011">RFC7011</a>] Claise, B., Trammell, B., and P. Aitken, "Specification of
the IP Flow Information Export (IPFIX) Protocol for the
Exchange of Flow Information", STD 77, <a href="./rfc7011">RFC 7011</a>, September
2013.
[<a id="ref-RFC7013">RFC7013</a>] Trammell, B. and B. Claise, "Guidelines for Authors and
Reviewers of IP Flow Information Export (IPFIX)
Information Elements", <a href="https://www.rfc-editor.org/bcp/bcp184">BCP 184</a>, <a href="./rfc7013">RFC 7013</a>, September 2013.
<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>. Informative References</span>
[<a id="ref-IANA-IPFIX">IANA-IPFIX</a>]
IANA, "IP Flow Information Export (IPFIX) Entities",
<<a href="http://www.iana.org/assignments/ipfix">http://www.iana.org/assignments/ipfix</a>>.
[<a id="ref-RFC5102">RFC5102</a>] Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
Meyer, "Information Model for IP Flow Information Export",
<a href="./rfc5102">RFC 5102</a>, January 2008.
<span class="grey">Trammell & Aitken Informational [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc7125">RFC 7125</a> IPFIX tcpControlBits February 2014</span>
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]
</pre>
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