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Chroboczek 3 Internet-Draft IRIF, University of Paris 4 Updates: 8966 (if approved) 9 April 2021 5 Intended status: Experimental 6 Expires: 11 October 2021 8 IPv4 routes with an IPv6 next-hop in the Babel routing protocol 9 draft-ietf-babel-v4viav6-01 11 Abstract 13 This document defines an extension to the Babel routing protocol that 14 allows annoncing routes to an IPv4 prefix with an IPv6 next-hop, 15 which makes it possible for IPv4 traffic to flow through interfaces 16 that have not been assigned an IPv4 address. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at https://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on 11 October 2021. 35 Copyright Notice 37 Copyright (c) 2021 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 42 license-info) in effect on the date of publication of this document. 43 Please review these documents carefully, as they describe your rights 44 and restrictions with respect to this document. Code Components 45 extracted from this document must include Simplified BSD License text 46 as described in Section 4.e of the Trust Legal Provisions and are 47 provided without warranty as described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 1.1. Specification of Requirements . . . . . . . . . . . . . . 3 53 2. Protocol operation . . . . . . . . . . . . . . . . . . . . . 3 54 2.1. Announcing v4-via-v6 routes . . . . . . . . . . . . . . . 3 55 2.2. Receiving v4-via-v6 routes . . . . . . . . . . . . . . . 4 56 2.3. Prefix and seqno requests . . . . . . . . . . . . . . . . 4 57 2.4. Other TLVs . . . . . . . . . . . . . . . . . . . . . . . 5 58 3. ICMPv4 and PMTU discovery . . . . . . . . . . . . . . . . . . 5 59 4. Backwards compatibility . . . . . . . . . . . . . . . . . . . 6 60 5. Protocol encoding . . . . . . . . . . . . . . . . . . . . . . 6 61 5.1. Prefix encoding . . . . . . . . . . . . . . . . . . . . . 6 62 5.2. Changes for existing TLVs . . . . . . . . . . . . . . . . 7 63 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 64 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 65 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 66 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 67 9.1. Normative References . . . . . . . . . . . . . . . . . . 8 68 9.2. Informative References . . . . . . . . . . . . . . . . . 9 69 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 71 1. Introduction 73 Traditionally, a routing table maps a network prefix of a given 74 address family to a next-hop address in the same address family. The 75 sole purpose of this next-hop address is to serve as an input to a 76 protocol that will map it to a link-layer address, Neighbour 77 Discovery (ND) [RFC4861] in the case of IPv6, Address Resolution 78 (ARP) [RFC0826] in the case of IPv4. Therefore, there is no reason 79 why the address family of the next hop address should match that of 80 the prefix being announced: an IPv6 next-hop yields a link-layer 81 address that is suitable for forwarding both IPv6 or IPv4 traffic. 83 We call a route towards an IPv4 prefix that uses an IPv6 next hop a 84 "v4-via-v6" route. Since an IPv6 next-hop can use a link-local 85 address that is autonomously configured, the use of v4-via-v6 routes 86 enables a mode of operation where the network core has no statically 87 assigned IP addresses of either family, thus significantly reducing 88 the amount of manual configuration. 90 This document describes an extension that allows the Babel routing 91 protocol [RFC8966] to announce routes towards IPv6 prefixes with IPv4 92 next hops. The extension is inspired by a previously defined 93 extension to the BGP protocol [RFC5549]. 95 1.1. Specification of Requirements 97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 99 "OPTIONAL" in this document are to be interpreted as described in BCP 100 14 [RFC2119] [RFC8174] when, and only when, they appear in all 101 capitals, as shown here. 103 2. Protocol operation 105 The Babel protocol fully supports double-stack operation: all data 106 that represent a neighbour address or a network prefix are tagged by 107 an Address Encoding (AE), a small integer that identifies the address 108 family (IPv4 or IPv6) of the address of prefix, and describes how it 109 is encoded. This extension defines a new AE, called v4-via-v6, which 110 has the same format as the existing AE for IPv4 addresses. This new 111 AE is only allowed in TLVs that carry network prefixes: TLVs that 112 carry a neighbour address use the normal encodings for IPv6 113 addresses. 115 2.1. Announcing v4-via-v6 routes 117 A Babel node that needs to announce an IPv4 route over an interface 118 that has no assigned IPv4 address MAY make a v4-via-v6 announcement. 119 In order to do so, it first establishes an IPv6 next-hop address in 120 the usual manner (either by sending the Babel packet over IPv6, or by 121 including a Next Hop TLV containing an IPv6 address); it then sends 122 an Update with AE equal to TBD containing the IPv4 prefix being 123 announced. 125 If the outgoing interface has been assigned an IPv4 address, then, in 126 the interest of maximising compatibility with existing routers, the 127 sender SHOULD prefer an ordinary IPv4 announcement; even in that 128 case, however, it MAY use a v4-via-v6 announcement. A node SHOULD 129 NOT send both ordinary IPv4 and v4-via-v6 annoucements for the same 130 prefix over a single interface (if the update is sent to a multicast 131 address) or to a single neighbour (if sent to a unicast address), 132 since doing that doubles the amount of routing traffic while 133 providing no benefit. 135 2.2. Receiving v4-via-v6 routes 137 Upon reception of an Update TLV with a v4-via-v6 AE and finite 138 metric, a Babel node computes the IPv6 next-hop, as described in 139 Section 4.6.9 of [RFC8966]. If no IPv6 next-hop exists, then the 140 Update MUST be silently ignored. If an IPv6 next-hop exists, then 141 the node MAY acquire the route being announced, as described in 142 Section 3.5.3 of [RFC8966]; the parameters of the route are as 143 follows: 145 * the prefix, plen, router-id, seqno, metric MUST be computed as for 146 an IPv4 route, as described in Section 4.6.9 of [RFC8966]; 148 * the next-hop MUST be computed as for an IPv6 route, as described 149 in Section 4.6.9 of [RFC8966]: it is taken from the last preceding 150 Next-Hop TLV with an AE field equal to 2 or 3; if no such entry 151 exists, and if the Update TLV has been sent in a Babel packet 152 carried over IPv6, then the next-hop is the network-layer source 153 address of the packet. 155 An Update TLV with a v4-via-v6 AE and metric equal to infinity is a 156 retraction: it announces that a previously available route is being 157 retracted. In that case, no next-hop is necessary, and the 158 retraction is treated as described in Section 4.6.9 of [RFC8966]. 160 As usual, a node MAY ignore the update, e.g., due to filtering 161 (Appendix C of [RFC8966]). If a node cannot install v4-via-v6 162 routes, eg., due to hardware or software limitations, then routes to 163 an IPv4 prefix with an IPv6 next-hop MUST NOT be selected, as 164 described in Section 3.5.3 of [RFC8966]. 166 2.3. Prefix and seqno requests 168 Prefix and seqno requests are used to request an update for a given 169 prefix. Since they are not related to a specific Next-Hop, there is 170 no semantic difference between IPv4 and v4-via-v6 requests. 171 Therefore, a node SHOULD NOT send requests of either kind with the AE 172 field being set to TBD (v4-via-v6); instead, it SHOULD request IPv4 173 updates using requests with the AE field being set to 1 (IPv4). 175 When receiving requests, AEs 1 (IPv4) and TBD (v4-via-v6) MUST be 176 treated in the same manner: the receiver processes the request as 177 described in Section 3.8 of [RFC8966]. If an Update is sent, then it 178 MAY be sent with AE 1 or TBD, as described in Section 2.1 above, 179 irrespective of which AE was used in the request. 181 When receiving a request with AE 0 (wildcard), the receiver SHOULD 182 send a full route dump, as described in Section 3.8.1.1 of [RFC8966]. 183 Any IPv4 routes contained in the route dump MAY use either AE 1 or AE 184 TBD, as described in Section 2.1 above. 186 2.4. Other TLVs 188 The only other TLVs defined by [RFC8966] that carry an AE field are 189 Next-Hop and TLV. Next-Hop and IHU TLVs MUST NOT carry the AE TBD 190 (v4-via-v6). 192 3. ICMPv4 and PMTU discovery 194 The Internet Control Message Protocol (ICMPv4, or simply ICMP) 195 [RFC792] is a protocol related to IPv4 that carries diagnostic and 196 debugging information. ICMPv4 packets may be originated by end hosts 197 (e.g., the "destination unreachable, port unreachable" ICMPv4 198 packet), but they may also be originated by intermediate routers 199 (e.g., most other kinds of "destination unreachable" packets). 201 Path MTU Discovery (PMTUd) [RFC1191] is an algorithm executed by end 202 hosts to discover the maximum packet size that a route is able to 203 carry. While there exist variants of PMTUd that are purely end-to- 204 end [RFC4821], the variant most commonly deployed in the Internet has 205 a hard dependency on ICMPv4 packets originated by intermediate 206 routers: if intermediate routers are unable to send ICMPv4 packets, 207 PMTUd may lead to persistent blackholing of IPv4 traffic. 209 For that reason, every Babel router that is able to forward IPv4 210 traffic MUST be able originate ICMPv4 traffic. Since the extension 211 described in this document enables routers to forward IPv4 traffic 212 even when they have not been assigned an IPv4 address, a router 213 implementing this extension MUST be able to originate ICMPv4 packets 214 even when it has not been assigned an IPv4 address. 216 There are various ways to meet this requirement, and choosing between 217 them is left to the implementation. For example, if a router has an 218 interface that has been assigned an IPv4 address, or if it has an 219 IPv4 address that has been assigned to the router itself (to the 220 "loopback interface"), then that IPv4 address may be "borrowed" to 221 serve as the source of originated ICMPv4 packets. If no IPv4 address 222 is available, a router may use a dummy IPv4 address as the source of 223 outgoing ICMPv4 packets, for example an address taken from a private 224 address range [RFC1918] that is known to not be used in the local 225 routing domain. Note however that using the same address on multiple 226 routers may hamper debugging and fault isolation. 228 4. Backwards compatibility 230 This protocol extension adds no new TLVs or sub-TLVs. 232 This protocol extension uses a new AE. As discussed in Appendix D of 233 [RFC8966] and specified in the same document, implementations that do 234 not understand the present extension will silently ignore the various 235 TLVs that use this new AE. As a result, incompatible versions will 236 ignore v4-via-v6 routes. They will also ignore requests with AE TBD, 237 which, as stated in Section 2.3, are NOT RECOMMENDED. 239 Using a new AE introduces a new compression state, used to parse the 240 network prefixes. As this compression state is separate from other 241 AEs' states, it will not interfere with the compression state of 242 unextended nodes. 244 This extension reuses the next-hop state from AEs 2 and 3 (IPv6), but 245 makes no changes to the way it is updated, and therefore causes no 246 compatibility issues. 248 As mentioned in Section 2.1, ordinary IPv4 announcements are 249 preferred to v4-via-v6 announcements when the outgoing interface has 250 an assigned IPv4 address; doing otherwise would prevent routers that 251 do not implement this extension from learning the route being 252 announced. 254 5. Protocol encoding 256 This extension defines the v4-via-v6 AE, whose value is TBD. This AE 257 is solely used to tag network prefixes, and MUST NOT be used to tag 258 peers' addresses, eg. in Next-Hop or IHU TLVs. 260 This extension defines no new TLVs or sub-TLVs. 262 5.1. Prefix encoding 264 Network prefixes tagged with AE TBD MUST be encoded and decoded as 265 prefixes tagged with AE 1 (IPv4), as described in Section 4.3.1 of 266 [RFC8966]. 268 A new compression state for AE TBD (v4-via-v6) distinct from that of 269 AE 1 (IPv4) is introduced, and MUST be used for address compression 270 of prefixes tagged with AE TBD, as described in Section 4.6.9 of 271 [RFC8966] 273 5.2. Changes for existing TLVs 275 The following TLVs MAY be tagged with AE TBD: 277 * Update (Type = 8) 279 * Route Request (Type = 9) 281 * Seqno Request (Type = 10) 283 As AE TBD is suitable only to tag network prefixes, IHU (Type = 5) 284 and Next-Hop (Type = 7) TLVs MUST NOT be tagged with AE TBD. Such 285 (incorrect) TLVs MUST be silently ignored upon reception. 287 5.2.1. Update 289 An Update (Type = 8) TLV with AE = TBD is constructed as described in 290 Section 4.6.9 of [RFC8966] for AE 1 (IPv4), with the following 291 specificities: 293 * Prefix. The Prefix field is constructed according to the 294 Section 5.1 above. 296 * Next hop. The next hop is determined as described in Section 2.2 297 above. 299 5.2.2. Other valid TLVs tagged with AE = TBD 301 Any other valid TLV tagged with AE = TBD MUST be constructed and 302 decoded as described in Section 4.6 of [RFC8966]. Network prefixes 303 within MUST be constructed and decoded as described in Section 5.1 304 above. 306 6. IANA Considerations 308 IANA is requested to allocate a value (4 suggested) in the "Babel 309 Address Encodings" registry as follows: 311 +=====+===========+=================+ 312 | AE | Name | Reference | 313 +=====+===========+=================+ 314 | TBD | v4-via-v6 | (this document) | 315 +-----+-----------+-----------------+ 317 Table 1 319 7. Security Considerations 321 The extension defined in this document does not fundamentally change 322 the security properties of the Babel protocol. However, by allowing 323 IPv4 routes to be propagated across routers that have not been 324 assigned IPv4 addresses, it might invalidate the assumptions made by 325 some network administatoris, which could conceivably lead to security 326 issues. 328 For example, if an island of IPv4-only hosts is separated from the 329 IPv4 Internet by an area of routers that have not been assigned IPv4 330 addresses, a network administrator might reasonably assume that the 331 IPv4-only hosts are unreachable from the IPv4 Internet. This 332 assumption is broken if the intermediary routers implement the 333 extension described in this document, which might expose the 334 IPv4-only hosts to traffic from the IPv4 Internet. If this is 335 undesirable, the flow of IPv4 traffic must be restricted by the use 336 of suitable filtering rules (Appendix C of [RFC8966]) together with 337 matching packet filters in the data plane. 339 8. Acknowledgments 341 This protocol extension was originally designed, described and 342 implemented in collaboration with Theophile Bastian. The author is 343 also indebted to Margaret Cullen, who pointed out the issues with 344 ICMP and helped coin the expression "V4-via-V6". 346 9. References 348 9.1. Normative References 350 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 351 Requirement Levels", BCP 14, RFC 2119, 352 DOI 10.17487/RFC2119, March 1997, 353 . 355 [RFC792] Postel, J., "Internet Control Message Protocol", STD 5, 356 RFC 792, DOI 10.17487/RFC0792, September 1981, 357 . 359 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 360 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 361 May 2017, . 363 [RFC8966] Chroboczek, J. and D. Schinazi, "The Babel Routing 364 Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021, 365 . 367 9.2. Informative References 369 [RFC0826] Plummer, D., "An Ethernet Address Resolution Protocol: Or 370 Converting Network Protocol Addresses to 48.bit Ethernet 371 Address for Transmission on Ethernet Hardware", STD 37, 372 RFC 826, DOI 10.17487/RFC0826, November 1982, 373 . 375 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 376 DOI 10.17487/RFC1191, November 1990, 377 . 379 [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. 380 J., and E. Lear, "Address Allocation for Private 381 Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, 382 February 1996, . 384 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 385 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, 386 . 388 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 389 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 390 DOI 10.17487/RFC4861, September 2007, 391 . 393 [RFC5549] Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network 394 Layer Reachability Information with an IPv6 Next Hop", 395 RFC 5549, DOI 10.17487/RFC5549, May 2009, 396 . 398 Author's Address 400 Juliusz Chroboczek 401 IRIF, University of Paris 402 Case 7014 403 75205 Paris Cedex 13 404 France 406 Email: jch@irif.fr