Internet Engineering Task Force (IETF) B. Trammell

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.

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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",
"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

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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
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.

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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
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.

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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: [email protected]

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

Phone: +44 131 561 3616
EMail: [email protected]

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