TCPM Working Group J. Touch Internet Draft USC/ISI Intended status: Proposed Standard October 5, 2012 Expires: April 2013 Shared Use of Experimental TCP Options draft-ietf-tcpm-experimental-options-02.txt Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on April 5, 2013. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 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 Touch Expires April 5, 2013 [Page 1] Internet-Draft Shared Use of Experimental TCP Options October 2012 Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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. It is recommended for all new experimental RFCs that require TCP option codepoints. Table of Contents 1. Introduction...................................................2 2. Conventions used in this document..............................3 3. TCP Experimental Option Structure..............................4 3.1. Reducing the Impact of False Positives....................6 3.2. Migration to Assigned Options.............................6 4. Security Considerations........................................7 5. IANA Considerations............................................7 6. References.....................................................7 6.1. Normative References......................................7 6.2. Informative References....................................7 7. Acknowledgments................................................8 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 30 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. Touch, (TBD) Expires April 5, 2013 [Page 2] Internet-Draft Shared Use of Experimental TCP Options October 2012 Other codepoints have been used without assignment, notably 31-32 (TCP cookie transactions, as originally distributed and in its API doc) and 76-78 (tcpcrypt) [Bi11][Si11]. Commercial products reportedly also use unassigned options 33, 69-70, and 76-78 as well. Even though these uses are unauthorized, 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. The solution proposed in this document is recommended for all new experimental protocols that require TCP option codpoints. This document also contains suggestions for the transition from this proposed mechanism to conventionally assigned codepoints, e.g., upon transition of an experimental protocol to more standards-track use. 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. Touch, (TBD) Expires April 5, 2013 [Page 3] Internet-Draft Shared Use of Experimental TCP Options October 2012 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): +--------+--------+--------+--------+ | 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 defined in experimental RFCs or their precursor Internet Drafts (expecting experimental RFC publication) requiring new TCP option codepoints SHOULD use the existing TCP experimental option codepoints (253, 254) with magic numbers as described in this document. >> All 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. Touch, (TBD) Expires April 5, 2013 [Page 4] Internet-Draft Shared Use of Experimental TCP Options October 2012 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 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). >> TCP experimental option magic numbers, if used in any TCP segment of a connection, MUST be present in TCP SYN segments of that connection. 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: >> TCP experimental option magic numbers, if used in any TCP segment of a connection, SHOULD be used in all TCP segments of that connection in which any experimental 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. Touch, (TBD) Expires April 5, 2013 [Page 5] Internet-Draft Shared Use of Experimental TCP Options October 2012 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 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 require a specific migration plan to avoid the use of magic numbers once a protocol using a experimental TCP option codepoint 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. Use of a magic number represents unnecessary overhead in an assigned TCP codepoint. As a result: >> Once a specific TCP option codepoint is assigned to a protocol, that protocol SHOULD NOT continue to use a magic number as part of that assigned codepoint. >> The updated protocol specification SHOULD recommend that implementations intended to be backward-compatible with experimental deployments MUST support both the experimental codepoint/magic number and assigned codepoint variants of the option. Discontinuing support for the experimental codepoint/magic number variant saves only a small amount of code. Touch, (TBD) Expires April 5, 2013 [Page 6] Internet-Draft Shared Use of Experimental TCP Options October 2012 >> Support for the experimental codepoint/magic number variant SHOULD be discontinued for implementations where the protocol has been revised in a non-backward-compatible way. 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. 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-03, Sep. 3, 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. Touch, (TBD) Expires April 5, 2013 [Page 7] Internet-Draft Shared Use of Experimental TCP Options October 2012 [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 Scharf provided detailed feedback. This document was prepared using 2-Word-v2.0.template.dot. 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 Touch, (TBD) Expires April 5, 2013 [Page 8]