TCPM Working Group J. Touch Internet Draft USC/ISI Intended status: Proposed Standard January 17, 2012 Expires: July 2012 Shared Use of Experimental TCP Options draft-ietf-tcpm-experimental-options-00.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 July 17, 2012. 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, (TBD) Expires July 17, 2012 [Page 1] Internet-Draft Shared Use of Experimental TCP Options January 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. The suggested 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 4. Security Considerations........................................5 5. IANA Considerations............................................5 6. References.....................................................5 6.1. Normative References......................................5 6.2. Informative References....................................6 7. Acknowledgments................................................6 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 245 and 255 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 options 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 and 76-78 as well. Touch, (TBD) Expires July 17, 2012 [Page 2] Internet-Draft Shared Use of Experimental TCP Options January 2012 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 participation. A short magic number is added to the structure of the experimental TCP option structure. The magic number helps reduce the probability of collision of independent experimental uses of the same option codepoint. This feature increases the size of 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): +--------+--------+--------+--------+ | 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 Touch, (TBD) Expires July 17, 2012 [Page 3] Internet-Draft Shared Use of Experimental TCP Options January 2012 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 value SHOULD be selected to reduce the probability of collision. The length of the magic number is a 32 bit value in network standard byte order. It can be shorter if desired (e.g., 16 bits), with a corresponding increased probability of collision and thus false positives. >> The magic number SHOULD be 32 bits long; it MAY be as few as 16 bits if desired. 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. Touch, (TBD) Expires July 17, 2012 [Page 4] Internet-Draft Shared Use of Experimental TCP Options January 2012 The remainder of the option is specified by the particular experimental protocol. 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. 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. 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 such 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. 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. Touch, (TBD) Expires July 17, 2012 [Page 5] Internet-Draft Shared Use of Experimental TCP Options January 2012 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-01, Aug. 29, 2011. [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]. 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 July 17, 2012 [Page 6]