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Draves 2 Internet Draft Microsoft Research 3 Document: draft-draves-ipngwg-simple-srcaddr-01.txt June 25, 1999 4 Category: Standards Track 6 Simple Source Address Selection for IPv6 8 Status of this Memo 10 This document is an Internet-Draft and is in full conformance with 11 all provisions of Section 10 of RFC 2026 [1]. 13 Internet-Drafts are working documents of the Internet Engineering 14 Task Force (IETF), its areas, and its working groups. Note that 15 other groups may also distribute working documents as Internet- 16 Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six 19 months and may be updated, replaced, or obsoleted by other documents 20 at any time. It is inappropriate to use Internet-Drafts as reference 21 material or to cite them other than as "work in progress." 23 The list of current Internet-Drafts can be accessed at 24 http://www.ietf.org/ietf/1id-abstracts.txt. 26 The list of Internet-Draft Shadow Directories can be accessed at 27 http://www.ietf.org/shadow.html. 29 1. Abstract 31 This document describes a simple algorithm by which IPv6 32 implementations can choose an appropriate source address to use for 33 communication with a specified destination address. 35 2. Conventions used in this document 37 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 38 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 39 this document are to be interpreted as described in RFC-2119 [2]. 41 3. Introduction 43 The IPv6 addressing architecture [3] allows multiple unicast 44 addresses to be assigned to interfaces. These addresses may have 45 different reachability scopes (link-local, site-local, or global). 46 Furthermore, these addresses may be "preferred" or "deprecated" [4. 48 Draves Standards Track - Expires January 2000 1 50 Simple Source Address Selection for IPv6 April 28, 1999 52 On occasion, an IPv6 implementation must choose from a set of 53 available addresses an appropriate source address to use for a given 54 destination address. This document specifies a simple set of rules 55 for choosing a source address of appropriate scope and configuration 56 status (preferred or deprecated). Furthermore, this document 57 suggests a preferred method, longest matching prefix, for choosing 58 among otherwise equivalent source addresses in the absence of better 59 information. 61 This document does not address the more general problem of choosing 62 the "best" destination address / source address pair for 63 communication with another node, given a set of possible destination 64 addresses and a set of possible source addresses. 66 The rules specified in this document MUST NOT be construed to 67 override an application or upper-layer's explicit choice of source 68 address. 70 4. Source Address Selection 72 This document specifies source address selection in two steps. 73 First, it specifies a set of candidate source addresses for a given 74 destination address. Second, it specifies a pair-wise source address 75 selection algorithm. Given a destination address and a pair of 76 possible source addresses SA and SB (SA not equal to SB), it chooses 77 a source address. Obviously, the pair-wise algorithm may be extended 78 to select an address from the set of candidate source addresses. 80 4.1 Multicast Scopes 82 Multicast destination addresses have a 4-bit scope field that 83 controls the propagation of the multicast packet. The IPv6 84 addressing architecture defines scope field values for node-local 85 (0x1), link-local (0x2), site-local (0x5), organization-local (0x8), 86 and global (0xE) scopes. 88 Application of the source address selection algorithm to a multicast 89 destination address requires the comparison of a unicast source 90 address scope with a multicast destination address scope. We map 91 unicast link-local to multicast link-local, unicast site-local to 92 multicast site-local, and unicast global scope to multicast global 93 scope. For example, unicast site-local is equal to multicast site- 94 local, which is smaller than multicast organization-local, which is 95 smaller than unicast global, which is equal to multicast global. 97 This mapping implicitly conflates unicast site boundaries and 98 multicast site boundaries. 100 4.2 Candidate Source Addresses 102 Draves Standards Track - Expires January 2000 2 104 Simple Source Address Selection for IPv6 April 28, 1999 106 It is RECOMMENDED that the candidate source addresses be the set of 107 unicast addresses assigned to the interface that will be used to 108 send to the destination. (The "outgoing" interface.) 110 For multicast and link-local destination addresses, the set of 111 candidate source addresses MUST only include addresses assigned to 112 the outgoing interface. 114 For site-local destination addresses, the set of candidate source 115 addresses MUST only include addresses assigned to interfaces 116 belonging to the same site as the outgoing interface. 118 In any case, anycast and multicast addresses MUST NOT be included in 119 the candidate set. 121 4.3 Pair-Wise Source Address Selection 123 The algorithm consists of four rules, which MUST be applied in 124 order. If a rule chooses a source address, then the remaining rules 125 are not relevant and MUST be ignored. Subsequent rules act as tie- 126 breakers for earlier rules. If the four rules fail to choose a 127 source address, some unspecified tie-breaker MUST be used. 129 Rule 1: If one of the source addresses is equal to the destination 130 address, an implementation MUST choose that source address. 132 Rule 2: If the source addresses SA and SB have different scope, then 133 an implementation MUST choose the source address as follows. Without 134 loss of generality, assume that SA has smaller scope than SB. If 135 SA's scope is smaller than the destination address scope, then 136 choose SB. Otherwise, if one of the source addresses is "preferred" 137 and one of them is "deprecated", then choose the "preferred" 138 address. Otherwise, choose SA. 140 Rule 3: The two source addresses have the same scope. If one of the 141 source addresses is "preferred" and one of them is "deprecated", an 142 implementation MUST choose the one that is preferred. 144 Rule 4: The two source addresses have the same scope and the same 145 configuration status (both preferred or both deprecated). If one of 146 the source addresses has a longer prefix matching the destination 147 address, an implementation SHOULD choose the source address with the 148 longer matching prefix. 150 The fourth rule MAY be superceded if the implementation has other 151 means of choosing among source addresses. For example, if the 152 implementation somehow knows which source address will result in the 153 "best" communications performance. 155 5. IPv4-Compatible Addresses and Other Format Prefixes 157 Draves Standards Track - Expires January 2000 3 159 Simple Source Address Selection for IPv6 April 28, 1999 161 For the purposes of this document, IPv4-compatible addresses have 162 global scope and "preferred" configuration status. 164 Similarly, NSAP addresses, IPX addresses, or addresses with as-yet- 165 undefined format prefixes should be treated as having global scope 166 and "preferred" configuration status. Later standards may supercede 167 this treatment. 169 The loopback address should be treated as having node-local scope 170 and "preferred" configuration status. 172 6. Security Considerations 174 This document has no direct impact on Internet infrastructure 175 security. 177 7. References 179 1 S. Bradner, "The Internet Standards Process -- Revision 3", BCP 180 9, RFC 2026, October 1996. 182 2 S. Bradner, "Key words for use in RFCs to Indicate Requirement 183 Levels", BCP 14, RFC 2119, March 1997. 185 3 R. Hinden, S. Deering, "IP Version 6 Addressing Architecture", 186 RFC 2373, July 1998. 188 4 S. Thompson, T. Narten, "IPv6 Stateless Address 189 Autoconfiguration", RFC 2462 , December 1998. 191 8. Acknowledgments 193 The author would like to acknowledge the contributions of the IPng 194 Working Group. 196 9. Author's Address 198 Richard Draves 199 Microsoft Research 200 One Microsoft Way 201 Redmond, WA 98052 202 Email: richdr@microsoft.com 204 10. Changes from 00 to 01 206 Minor wording changes because DHCPv6 also supports "preferred" and 207 "deprecated" addresses. 209 Draves Standards Track - Expires January 2000 4 211 Simple Source Address Selection for IPv6 April 28, 1999 213 Specified treatment of other format prefixes; now they are 214 considered global scope, "preferred" addresses. 216 Reiterated that anycast and multicast addresses are not allowed as 217 source addresses. 219 Recommended that source addresses be taken from the outgoing 220 interface. Required this for multicast destinations. Added analogous 221 requirements for link-local and site-local destinations. 223 Specified treatment of the loopback address. 225 Changed the second selection rule so that if both candidate source 226 addresses have scope greater or equal than the destination address 227 and only of them is preferred, the preferred address is chosen. 229 Draves Standards Track - Expires January 2000 5 231 Simple Source Address Selection for IPv6 April 28, 1999 233 Full Copyright Statement 235 Copyright (C) The Internet Society (1999). All Rights Reserved. 236 This document and translations of it may be copied and furnished to 237 others, and derivative works that comment on or otherwise explain it 238 or assist in its implementation may be prepared, copied, published 239 and distributed, in whole or in part, without restriction of any 240 kind, provided that the above copyright notice and this paragraph 241 are included on all such copies and derivative works. However, this 242 document itself may not be modified in any way, such as by removing 243 the copyright notice or references to the Internet Society or other 244 Internet organizations, except as needed for the purpose of 245 developing Internet standards in which case the procedures for 246 copyrights defined in the Internet Standards process must be 247 followed, or as required to translate it into languages other than 248 English. 250 The limited permissions granted above are perpetual and will not be 251 revoked by the Internet Society or its successors or assigns. 253 This document and the information contained herein is provided on an 254 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 255 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 256 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 257 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 258 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 260 Draves Standards Track - Expires January 2000 6