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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 793 (Obsoleted by RFC 9293) == Outdated reference: A later version (-04) exists of draft-bittau-tcp-crypt-02 -- Obsolete informational reference (is this intentional?): RFC 5226 (Obsoleted by RFC 8126) -- Obsolete informational reference (is this intentional?): RFC 6013 (Obsoleted by RFC 7805) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 TCPM Working Group J. Touch 2 Internet Draft USC/ISI 3 Intended status: Proposed Standard May 30, 2012 4 Expires: November 2012 6 Shared Use of Experimental TCP Options 7 draft-ietf-tcpm-experimental-options-01.txt 9 Status of this Memo 11 This Internet-Draft is submitted in full conformance with the 12 provisions of BCP 78 and BCP 79. 14 Internet-Drafts are working documents of the Internet Engineering 15 Task Force (IETF), its areas, and its working groups. Note that 16 other groups may also distribute working documents as Internet- 17 Drafts. 19 Internet-Drafts are draft documents valid for a maximum of six 20 months and may be updated, replaced, or obsoleted by other documents 21 at any time. It is inappropriate to use Internet-Drafts as 22 reference material or to cite them other than as "work in progress." 24 The list of current Internet-Drafts can be accessed at 25 http://www.ietf.org/ietf/1id-abstracts.txt 27 The list of Internet-Draft Shadow Directories can be accessed at 28 http://www.ietf.org/shadow.html 30 This Internet-Draft will expire on November 30, 2012. 32 Copyright Notice 34 Copyright (c) 2012 IETF Trust and the persons identified as the 35 document authors. All rights reserved. 37 This document is subject to BCP 78 and the IETF Trust's Legal 38 Provisions Relating to IETF Documents 39 (http://trustee.ietf.org/license-info) in effect on the date of 40 publication of this document. Please review these documents 41 carefully, as they describe your rights and restrictions with 42 respect to this document. Code Components extracted from this 43 document must include Simplified BSD License text as described in 44 Section 4.e of the Trust Legal Provisions and are provided without 45 warranty as described in the Simplified BSD License. 47 Abstract 49 This document describes how TCP option codepoints can support 50 concurrent experiments using a magic number field. This mechanism 51 avoids the need for a coordinated registry, and is backward- 52 compatible with currently known uses. 54 Table of Contents 56 1. Introduction...................................................2 57 2. Conventions used in this document..............................3 58 3. TCP Experimental Option Structure..............................3 59 3.1. Reducing the Impact of False Positives....................5 60 3.2. Migration to Assigned Options.............................6 61 4. Security Considerations........................................6 62 5. IANA Considerations............................................6 63 6. References.....................................................6 64 6.1. Normative References......................................6 65 6.2. Informative References....................................7 66 7. Acknowledgments................................................7 68 1. Introduction 70 TCP includes options to enable new protocol capabilities that can be 71 activated only where needed and supported [RFC793]. The space for 72 identifying such options is small - 256 values, of which 31 are 73 assigned at the time this document was published [IANA]. Two of 74 these codepoints are allocated to support experiments (253, 254) 75 [RFC4727]. These numbers are intended for testing purposes, and 76 implementations need to assume they can be used for other purposes, 77 but this is often not the case. 79 There is no mechanism to support shared use of the experimental 80 option codepoints. Experimental options 253 and 254 are deployed in 81 operational code to support an early version of TCP authentication. 82 Option 253 is also documented for the experimental TCP Cookie 83 Transaction option [RFC6013]. This shared use results in collisions 84 in which a single codepoint can appear multiple times in a single 85 TCP segment and each use is ambiguous. 87 Other codepoints have been used without assignment, notably 31-32 88 (TCP cookie transactions, as originally distributed and in its API 89 doc) and 76-78 (tcpcrypt) [Bi11][Si11]. Commercial products 90 reportedly also use unassigned options 33 and 76-78 as well. Even 91 though these uses are inappropriate, they can impact legitimate 92 assignees. 94 There are a variety of proposed approaches to address this issue. 95 The first is to relax the requirements for assignment of TCP 96 options, allowing them to be assigned more readily for protocols 97 that have not been standardized through the IETF process [RFC5226]. 98 A second would be to assign a larger pool to options, and to manage 99 their sharing through IANA coordination [Ed11]. 101 This document proposes a solution that does not require additional 102 codepoints and also avoids IANA involvement. The solution involves 103 adding a field to the structure of the experimental TCP option. This 104 field is typically populated with a fixed "magic number" defined as 105 part of a specific option experiment. The magic number helps reduce 106 the probability of a collision of independent experimental uses of 107 the same option codepoint. This feature increases the number of 108 bytes used by experimental options, but the size can be reduced when 109 the experiment is converted to a standard protocol with a 110 conventional codepoint assignment. 112 2. Conventions used in this document 114 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 115 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 116 document are to be interpreted as described in RFC-2119 [RFC2119]. 118 In this document, these words will appear with that interpretation 119 only when in ALL CAPS. Lower case uses of these words are not to be 120 interpreted as carrying RFC-2119 significance. 122 In this document, the characters ">>" preceding an indented line(s) 123 indicates a compliance requirement statement using the key words 124 listed above. This convention aids reviewers in quickly identifying 125 or finding the explicit compliance requirements of this RFC. 127 3. TCP Experimental Option Structure 129 TCP options have the current common structure, where the first byte 130 is the codepoint (Kind) and the second is the length of the option 131 in bytes (Length): 133 +--------+--------+--------+--------+ 134 | Kind | Length | ... | 135 +--------+--------+--------+--------+ 136 | ... 137 +-------- 139 Figure 1 TCP Option Structure [RFC793] 141 This document extends the option structure for experimental 142 codepoints (253, 254) with a magic number. The magic number is used 143 to differentiate different experiments, and is the first field after 144 the Kind and Length, as follows: 146 +--------+--------+--------+--------+ 147 | Kind | Length | Magic Number | 148 +--------+--------+--------+--------+ 149 | Magic Number | ... 150 +--------+--------+--------+--- 152 Figure 2 TCP Experimental Option with a Magic Number 154 >> Protocols using the TCP experimental option codepoints (253, 254) 155 SHOULD use magic numbers as described in this document. 157 Magic numbers are used in other protocols, e.g., BOOTP [RFC951] and 158 DHCP [RFC2131]. Here they help ensure that concurrent experiments 159 that share the same TCP option codepoint do not interfere. 161 The magic number is selected by the protocol designer when an 162 experimental option is defined. The magic number is selected any of 163 a variety of ways, e.g., using the Unix time() command or bits 164 selected by an arbitrary function (such as a hash). 166 >> The magic number size and value SHOULD be selected to reduce the 167 probability of collision. 169 This document does not proscribe a minimum magic number size. 170 However, a reasonable suggested size is 32 bits, in network standard 171 byte order: 173 >> The magic number SHOULD be 32 bits, but MAY be either longer or 174 shorter. 176 The magic number is considered part of the TCP option, not the TCP 177 option header. The presence of the magic number increases the 178 effective option Length field by the size of the magic number. The 179 presence of this magic number is thus transparent to implementations 180 that do not support TCP options where it is used. 182 During TCP processing, experimental options are matched against both 183 the experimental codepoints and the magic number value for each 184 implemented protocol. 186 >> Experimental options that have magic numbers that do not match 187 implemented protocols MUST be ignored. 189 The remainder of the option is specified by the particular 190 experimental protocol. This includes the possibility that the magic 191 number could appear in only a subset of instances of the option. 192 Because TCP option capabilities are negotiated during connection 193 establishment, the magic number might be omitted afterwards (e.g., 194 in non-SYN segments). 196 >> Experimental option magic numbers, if used, MUST be present in 197 TCP SYN segments. 199 The specification of an experimental option needs to describe 200 whether the magic number appears in non-SYN segments. If the magic 201 number does not appear in all segments, the experimental option may 202 need to be rejected during connection negotiation because options 203 for different experiments in non-SYN segments may not be 204 distinguishable. As a result, this document recommends that: 206 >> Experimental option magic numbers, if used, SHOULD be used in all 207 TCP segments where the option is present. 209 Use of a magic number uses additional space in the TCP header and 210 requires additional protocol processing by experimental protocols. 211 Because these are experiments, neither consideration is a 212 substantial impediment; a finalized protocol can avoid both issues 213 with the assignment of a dedicated option codepoint later. 215 3.1. Reducing the Impact of False Positives 217 False positives are always possible, where a magic number matches 218 the value of a field in the legacy use of these options or a 219 protocol that does not implement the mechanism described in this 220 document. 222 >> Protocols that are not robust to magic number false positives 223 SHOULD implement other measures to ensure they process options for 224 their protocol only, such as checksums or digital signatures among 225 cooperating parties of their protocol. Such measures SHOULD 226 supplement, rather than substitute for, the use of magic numbers. 228 Use of checksums or signatures may help an experiment use a shorter 229 magic number while reducing the corresponding increased potential 230 for false positives. However this document recommends magic numbers 231 are used together with such checksums/signatures, not as a 232 substitute thereof. Magic numbers are static and thus more easily 233 identify the experiment using the experimental option; they can also 234 be more efficiently interpreted at the TCP receiver. 236 3.2. Migration to Assigned Options 238 This document does not address a specific migration plan to avoid 239 the use of magic numbers once an experimental TCP option is 240 considered for operational deployment, e.g., if it transitions to 241 proposed standard. The expectation is that such options would be 242 assigned their own TCP codepoints and their specifications updated 243 to avoid the need to support the experimental codepoint. 245 4. Security Considerations 247 The mechanism described in this document is not intended to provide 248 security for TCP option processing. 250 5. IANA Considerations 252 This document has no IANA considerations. This section should be 253 removed prior to publication. 255 6. References 257 6.1. Normative References 259 [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 260 793, Sep. 1981. 262 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 263 Requirement Levels", BCP 14, RFC 2119, March 1997. 265 [RFC4727] Fenner, B., "Experimental Values in IPv4, IPv6, ICMPv4, 266 ICMPv6, UDP, and TCP Headers", RFC 4727, Nov. 2006. 268 6.2. Informative References 270 [Bi11] Bittau, A., D. Boneh, M. Hamburg, M. Handley, D. Mazieres, 271 Q. Slack, "Cryptographic protection of TCP Streams 272 (tcpcrypt)", work in progress, draft-bittau-tcp-crypt-02, 273 Feb. 20, 2012. 275 [Ed11] Eddy, W., "Additional TCP Experimental-Use Options", work 276 in progress, draft-eddy-tcpm-addl-exp-options-00, Aug. 16, 277 2011. 279 [IANA] IANA web pages, http://www.iana.org/ 281 [RFC951] Croft, B., J. Gilmore, "BOOTSTRAP PROTOCOL (BOOTP)", RFC 282 951, Sept. 1985. 284 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 285 2131, Mar. 1997. 287 [RFC5226] Narten, T., H. Alvestrand, "Guidelines for Writing an IANA 288 Considerations Section in RFCs", BCP 26, RFC 5226, May 289 2008. 291 [RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013, 292 Jan. 2011. 294 [Si11] Simpson, W., "TCP Cookie Transactions (TCPCT) Sockets 295 Application Program Interface (API)", work in progress, 296 draft-simpson-tcpct-api-04, Apr. 7, 2011. 298 7. Acknowledgments 300 This document was motivated by discussions on the IETF TCPM mailing 301 list and by Wes Eddy's proposal [Ed11]. Yoshifumi Nishida, Pasi 302 Sarolathi, and Michael Sharf provided detailed feedback. 304 This document was prepared using 2-Word-v2.0.template.dot. 306 Authors' Addresses 308 Joe Touch 309 USC/ISI 310 4676 Admiralty Way 311 Marina del Rey, CA 90292-6695 U.S.A. 313 Phone: +1 (310) 448-9151 314 Email: touch@isi.edu