TCPM Working Group J. Touch
Internet Draft USC/ISI
Intended status: Proposed Standard May 30, 2012
Expires: November 2012
Shared Use of Experimental TCP Options
draft-ietf-tcpm-experimental-options-01.txt
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Abstract
This document describes how TCP option codepoints can support
concurrent experiments using a magic number field. This mechanism
avoids the need for a coordinated registry, and is backward-
compatible with currently known uses.
Table of Contents
1. Introduction...................................................2
2. Conventions used in this document..............................3
3. TCP Experimental Option Structure..............................3
3.1. Reducing the Impact of False Positives....................5
3.2. Migration to Assigned Options.............................6
4. Security Considerations........................................6
5. IANA Considerations............................................6
6. References.....................................................6
6.1. Normative References......................................6
6.2. Informative References....................................7
7. Acknowledgments................................................7
1. Introduction
TCP includes options to enable new protocol capabilities that can be
activated only where needed and supported [RFC793]. The space for
identifying such options is small - 256 values, of which 31 are
assigned at the time this document was published [IANA]. Two of
these codepoints are allocated to support experiments (253, 254)
[RFC4727]. These numbers are intended for testing purposes, and
implementations need to assume they can be used for other purposes,
but this is often not the case.
There is no mechanism to support shared use of the experimental
option codepoints. Experimental options 253 and 254 are deployed in
operational code to support an early version of TCP authentication.
Option 253 is also documented for the experimental TCP Cookie
Transaction option [RFC6013]. This shared use results in collisions
in which a single codepoint can appear multiple times in a single
TCP segment and each use is ambiguous.
Other codepoints have been used without assignment, notably 31-32
(TCP cookie transactions, as originally distributed and in its API
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doc) and 76-78 (tcpcrypt) [Bi11][Si11]. Commercial products
reportedly also use unassigned options 33 and 76-78 as well. Even
though these uses are inappropriate, they can impact legitimate
assignees.
There are a variety of proposed approaches to address this issue.
The first is to relax the requirements for assignment of TCP
options, allowing them to be assigned more readily for protocols
that have not been standardized through the IETF process [RFC5226].
A second would be to assign a larger pool to options, and to manage
their sharing through IANA coordination [Ed11].
This document proposes a solution that does not require additional
codepoints and also avoids IANA involvement. The solution involves
adding a field to the structure of the experimental TCP option. This
field is typically populated with a fixed "magic number" defined as
part of a specific option experiment. The magic number helps reduce
the probability of a collision of independent experimental uses of
the same option codepoint. This feature increases the number of
bytes used by experimental options, but the size can be reduced when
the experiment is converted to a standard protocol with a
conventional codepoint assignment.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
In this document, the characters ">>" preceding an indented line(s)
indicates a compliance requirement statement using the key words
listed above. This convention aids reviewers in quickly identifying
or finding the explicit compliance requirements of this RFC.
3. TCP Experimental Option Structure
TCP options have the current common structure, where the first byte
is the codepoint (Kind) and the second is the length of the option
in bytes (Length):
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+--------+--------+--------+--------+
| Kind | Length | ... |
+--------+--------+--------+--------+
| ...
+--------
Figure 1 TCP Option Structure [RFC793]
This document extends the option structure for experimental
codepoints (253, 254) with a magic number. The magic number is used
to differentiate different experiments, and is the first field after
the Kind and Length, as follows:
+--------+--------+--------+--------+
| Kind | Length | Magic Number |
+--------+--------+--------+--------+
| Magic Number | ...
+--------+--------+--------+---
Figure 2 TCP Experimental Option with a Magic Number
>> Protocols using the TCP experimental option codepoints (253, 254)
SHOULD use magic numbers as described in this document.
Magic numbers are used in other protocols, e.g., BOOTP [RFC951] and
DHCP [RFC2131]. Here they help ensure that concurrent experiments
that share the same TCP option codepoint do not interfere.
The magic number is selected by the protocol designer when an
experimental option is defined. The magic number is selected any of
a variety of ways, e.g., using the Unix time() command or bits
selected by an arbitrary function (such as a hash).
>> The magic number size and value SHOULD be selected to reduce the
probability of collision.
This document does not proscribe a minimum magic number size.
However, a reasonable suggested size is 32 bits, in network standard
byte order:
>> The magic number SHOULD be 32 bits, but MAY be either longer or
shorter.
The magic number is considered part of the TCP option, not the TCP
option header. The presence of the magic number increases the
effective option Length field by the size of the magic number. The
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presence of this magic number is thus transparent to implementations
that do not support TCP options where it is used.
During TCP processing, experimental options are matched against both
the experimental codepoints and the magic number value for each
implemented protocol.
>> Experimental options that have magic numbers that do not match
implemented protocols MUST be ignored.
The remainder of the option is specified by the particular
experimental protocol. This includes the possibility that the magic
number could appear in only a subset of instances of the option.
Because TCP option capabilities are negotiated during connection
establishment, the magic number might be omitted afterwards (e.g.,
in non-SYN segments).
>> Experimental option magic numbers, if used, MUST be present in
TCP SYN segments.
The specification of an experimental option needs to describe
whether the magic number appears in non-SYN segments. If the magic
number does not appear in all segments, the experimental option may
need to be rejected during connection negotiation because options
for different experiments in non-SYN segments may not be
distinguishable. As a result, this document recommends that:
>> Experimental option magic numbers, if used, SHOULD be used in all
TCP segments where the option is present.
Use of a magic number uses additional space in the TCP header and
requires additional protocol processing by experimental protocols.
Because these are experiments, neither consideration is a
substantial impediment; a finalized protocol can avoid both issues
with the assignment of a dedicated option codepoint later.
3.1. Reducing the Impact of False Positives
False positives are always possible, where a magic number matches
the value of a field in the legacy use of these options or a
protocol that does not implement the mechanism described in this
document.
>> Protocols that are not robust to magic number false positives
SHOULD implement other measures to ensure they process options for
their protocol only, such as checksums or digital signatures among
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cooperating parties of their protocol. Such measures SHOULD
supplement, rather than substitute for, the use of magic numbers.
Use of checksums or signatures may help an experiment use a shorter
magic number while reducing the corresponding increased potential
for false positives. However this document recommends magic numbers
are used together with such checksums/signatures, not as a
substitute thereof. Magic numbers are static and thus more easily
identify the experiment using the experimental option; they can also
be more efficiently interpreted at the TCP receiver.
3.2. Migration to Assigned Options
This document does not address a specific migration plan to avoid
the use of magic numbers once an experimental TCP option is
considered for operational deployment, e.g., if it transitions to
proposed standard. The expectation is that such options would be
assigned their own TCP codepoints and their specifications updated
to avoid the need to support the experimental codepoint.
4. Security Considerations
The mechanism described in this document is not intended to provide
security for TCP option processing.
5. IANA Considerations
This document has no IANA considerations. This section should be
removed prior to publication.
6. References
6.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, Sep. 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4727] Fenner, B., "Experimental Values in IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, Nov. 2006.
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6.2. Informative References
[Bi11] Bittau, A., D. Boneh, M. Hamburg, M. Handley, D. Mazieres,
Q. Slack, "Cryptographic protection of TCP Streams
(tcpcrypt)", work in progress, draft-bittau-tcp-crypt-02,
Feb. 20, 2012.
[Ed11] Eddy, W., "Additional TCP Experimental-Use Options", work
in progress, draft-eddy-tcpm-addl-exp-options-00, Aug. 16,
2011.
[IANA] IANA web pages, http://www.iana.org/
[RFC951] Croft, B., J. Gilmore, "BOOTSTRAP PROTOCOL (BOOTP)", RFC
951, Sept. 1985.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, Mar. 1997.
[RFC5226] Narten, T., H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May
2008.
[RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
Jan. 2011.
[Si11] Simpson, W., "TCP Cookie Transactions (TCPCT) Sockets
Application Program Interface (API)", work in progress,
draft-simpson-tcpct-api-04, Apr. 7, 2011.
7. Acknowledgments
This document was motivated by discussions on the IETF TCPM mailing
list and by Wes Eddy's proposal [Ed11]. Yoshifumi Nishida, Pasi
Sarolathi, and Michael Sharf provided detailed feedback.
This document was prepared using 2-Word-v2.0.template.dot.
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Authors' Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695 U.S.A.
Phone: +1 (310) 448-9151
Email: touch@isi.edu
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