Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 9565 Orange
Obsoletes: 7125 March 2024
Category: Standards Track
ISSN: 2070-1721
An Update to the tcpControlBits IP Flow Information Export (IPFIX)
Information Element
Abstract
RFC 7125 revised the tcpControlBits IP Flow Information Export
(IPFIX) Information Element that was originally defined in RFC 5102
to reflect changes to the TCP header control bits since RFC 793.
However, that update is still problematic for interoperability
because some flag values have subsequently been deprecated.
This document removes stale information from the IANA "IPFIX
Information Elements" registry and avoids future conflicts with the
authoritative IANA "TCP Header Flags" registry.
This document obsoletes RFC 7125.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9565.
Copyright Notice
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Table of Contents
1. Introduction
2. Terminology
3. Revised tcpControlBits Information Element
4. An Example
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Changes from RFC 7125
Acknowledgments
Acknowledgments from RFC 7125
Contributors
Author's Address
1. Introduction
TCP defines a set of control bits (also known as "flags") for
managing connections (Section 3.1 of [RFC9293]). The "TCP Header
Flags" registry was initially set by [RFC3168], but it was populated
with only TCP control bits that were defined in [RFC3168]. [RFC9293]
fixed that by moving that registry to be listed as a subregistry
under the "Transmission Control Protocol (TCP) Parameters" registry
[TCP-FLAGS], adding bits that had previously been specified in
[RFC0793], and removing the NS (Nonce Sum) bit per [RFC8311]. Also,
Section 6 of [RFC9293] introduces "Bit Offset" to ease referencing
each header flag's offset within the 16-bit aligned view of the TCP
header (Figure 1 of [RFC9293]). [TCP-FLAGS] is thus settled as the
authoritative reference for the assigned TCP control bits.
Note: The bits in offsets 0 through 3 are not header flags, but
the TCP segment Data Offset field.
[RFC7125] revised the tcpControlBits IP Flow Information Export
(IPFIX) Information Element that was originally defined in [RFC5102]
to reflect changes to the TCP control bits since [RFC0793]. However,
that update is still problematic for interoperability because a value
was deprecated since then (Section 7 of [RFC8311]), and, therefore,
[RFC7125] risks deviating from the authoritative "TCP Header Flags"
registry [TCP-FLAGS].
This document fixes that problem by removing stale information from
the "IPFIX Information Elements" registry [IPFIX] and avoiding future
conflicts with the authoritative "TCP Header Flags" registry
[TCP-FLAGS]. The update in this document also enhances
observability. For example, network operators can identify packets
that are observed with unassigned TCP flags set and, therefore,
identify which applications in the network should be upgraded to
reflect the changes to TCP flags that were introduced, e.g., in
[RFC8311].
The main changes from [RFC7125] are listed in Appendix A.
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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the terms defined in Section 2 of [RFC7011].
3. Revised tcpControlBits Information Element
ElementID: 6
Name: tcpControlBits
Abstract Data Type: unsigned16
Data Type Semantics: flags
Status: current
Description: TCP control bits observed for the packets of this Flow.
This information is encoded as a bit field; each TCP control bit
has a corresponding bit in that field. A 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.
Per [RFC9293], the assignment of TCP control bits is managed by
IANA via the "TCP Header Flags" registry [TCP-FLAGS].
Implementers can retrieve the current TCP control bits from that
registry, which is authoritative for them.
As the most significant 4 bits of octets 12 and 13 (counting from
zero) of the TCP header [RFC9293] are used to encode the TCP data
offset (header length), the corresponding bits in this Information
Element MUST be reported by the Exporter with a value of zero and
MUST be ignored by the Collector. Use the tcpHeaderLength
Information Element to encode this value.
All TCP control bits (including those unassigned) MUST be exported
as observed in the TCP headers of the packets of this Flow.
If exported as a single octet with reduced-size encoding
(Section 6.2 of [RFC7011]), this Information Element covers the
low-order octet of this field (i.e., bit offset positions 8 to 15)
[TCP-FLAGS]. A Collector receiving this Information Element with
reduced-size encoding must not assume anything about the content
of the four bits with bit offset positions 4 to 7.
Exporting Processes exporting this Information Element on behalf
of a Metering Process that is not capable of observing any of the
flags with bit offset positions 4 to 7 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 flags with bit offset positions 8 and 9
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.
Note also that the "TCP Header Flags" registry [TCP-FLAGS] indexes
the bit offset from the most significant bit of octet 12 to the
least significant bit of octet 13 in the TCP header, but the
tcpControlBits is encoded as a regular unsigned 16-bit integer.
Units:
Range:
Additional Information: See the assigned TCP control bits in the
"TCP Header Flags" registry [TCP-FLAGS].
Reference: [RFC9293], RFC 9565
Revision: 2
4. An Example
Figure 1 shows an example of a tcpControlBits Information Element set
to 0x92, where MSB indicates the most significant bit and LSB
indicates the least significant bit. This Information Element is
used to report TCP control bits for a Flow that has CWR (Congestion
Window Reduced), ACK, and SYN flag bits set (that is, bit offset
positions 8, 11, and 14).
MSB LSB
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|0|0|0|0|1|0|0|1|0|0|1|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: An Example of the tcpControlBits Information Element
5. IANA Considerations
IANA has updated the "tcpControlBits" entry of the "IPFIX Information
Elements" registry [IPFIX] to echo the details provided in Section 3.
6. Security Considerations
Because the setting of TCP control bits may be misused in some Flows
(e.g., Distributed Denial-of-Service (DDoS) attacks), an Exporter has
to report all observed control bits even if no meaning is associated
with a given TCP flag. This document uses a stronger requirements
language compared to [RFC7125].
This document does not add new security considerations to those
already discussed for IPFIX in [RFC7011].
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
"Specification of the IP Flow Information Export (IPFIX)
Protocol for the Exchange of Flow Information", STD 77,
RFC 7011, DOI 10.17487/RFC7011, September 2013,
<https://www.rfc-editor.org/info/rfc7011>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
[TCP-FLAGS]
IANA, "TCP Header Flags",
<https://www.iana.org/assignments/tcp-parameters/>.
7.2. Informative References
[IPFIX] IANA, "IPFIX Information Elements",
<https://www.iana.org/assignments/ipfix/>.
[RFC0793] Postel, J., "Transmission Control Protocol", RFC 793,
DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC5102] Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
Meyer, "Information Model for IP Flow Information Export",
RFC 5102, DOI 10.17487/RFC5102, January 2008,
<https://www.rfc-editor.org/info/rfc5102>.
[RFC7125] Trammell, B. and P. Aitken, "Revision of the
tcpControlBits IP Flow Information Export (IPFIX)
Information Element", RFC 7125, DOI 10.17487/RFC7125,
February 2014, <https://www.rfc-editor.org/info/rfc7125>.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.
[RFC9487] Graf, T., Claise, B., and P. Francois, "Export of Segment
Routing over IPv6 Information in IP Flow Information
Export (IPFIX)", RFC 9487, DOI 10.17487/RFC9487, November
2023, <https://www.rfc-editor.org/info/rfc9487>.
Appendix A. Changes from RFC 7125
* Cleaned up the description of the tcpControlBits Information
Element by removing mentions of stale flag bits, referring to the
flag bits by their bit offset position, and relying upon the IANA
"TCP Header Flags" registry.
* Removed the table of TCP flag bits from the description of the
tcpControlBits Information Element.
* Added the reference [TCP-FLAGS] to the Additional Information
field of the tcpControlBits Information Element.
* Used strong normative language for exporting observed flags.
* Updated the references of the tcpControlBits Information Element.
* Bumped the revision of the tcpControlBits Information Element.
* Replaced obsolete RFCs (e.g., [RFC0793]).
* Added an example section (Section 4).
Acknowledgments
This document was triggered by a discussion in the opsawg working
group between the author and the authors of [RFC9487].
Thanks to Christian Jacquenet, Thomas Graf, and Benoît Claise for the
review and comments.
Thanks to Michael Scharf for the tsvart review, Ketan Talaulikar for
the rtgdir review, and Elwyn Davies for the genart review.
Thanks to Rob Wilton for the AD review.
Thanks to Tim Bray for the artart review and Shawn Emery for the
secdir review.
Thanks to Éric Vyncke and Paul Wouters for the comments in the IESG
review.
Acknowledgments from RFC 7125
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.
Contributors
The authors of [RFC7125] are as follows:
Brian Trammell
Paul Aitken
Author's Address