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Chroboczek 4 Intended status: Standards Track IRIF, University of Paris-Diderot 5 Expires: October 13, 2019 April 11, 2019 7 Source-Specific Routing in Babel 8 draft-ietf-babel-source-specific-05 10 Abstract 12 Source-specific routing (also known as Source-Address Dependent 13 Routing, SADR) is an extension to traditional next-hop routing where 14 packets are forwarded according to both their destination and their 15 source address. This document describes an extension for source- 16 specific routing to the Babel routing protocol. 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 October 13, 2019. 35 Copyright Notice 37 Copyright (c) 2019 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 42 (https://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction and background . . . . . . . . . . . . . . . . . 2 53 2. Specification of Requirements . . . . . . . . . . . . . . . . 3 54 3. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 3 55 3.1. The Source Table . . . . . . . . . . . . . . . . . . . . 4 56 3.2. The Route Table . . . . . . . . . . . . . . . . . . . . . 4 57 3.3. The Table of Pending Seqno Requests . . . . . . . . . . . 4 58 4. Data Forwarding . . . . . . . . . . . . . . . . . . . . . . . 4 59 5. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 5 60 5.1. Protocol Messages . . . . . . . . . . . . . . . . . . . . 6 61 5.2. Wildcard Messages . . . . . . . . . . . . . . . . . . . . 6 62 6. Compatibility with the base protocol . . . . . . . . . . . . 7 63 6.1. Loop-avoidance . . . . . . . . . . . . . . . . . . . . . 7 64 6.2. Starvation and Blackholes . . . . . . . . . . . . . . . . 8 65 7. Protocol Encoding . . . . . . . . . . . . . . . . . . . . . . 8 66 7.1. Source Prefix sub-TLV . . . . . . . . . . . . . . . . . . 8 67 7.2. Source-specific Update . . . . . . . . . . . . . . . . . 9 68 7.3. Source-specific (Route) Request . . . . . . . . . . . . . 9 69 7.4. Source-Specific Seqno Request . . . . . . . . . . . . . . 9 70 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 71 9. Security considerations . . . . . . . . . . . . . . . . . . . 10 72 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 73 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 74 11.1. Normative References . . . . . . . . . . . . . . . . . . 10 75 11.2. Informative References . . . . . . . . . . . . . . . . . 10 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 78 1. Introduction and background 80 The Babel routing protocol [BABEL] is a distance vector routing 81 protocol for next-hop routing. In next-hop routing, each node 82 maintains a forwarding table which maps destination prefixes to next 83 hops. The forwarding decision is a per-packet operation which 84 depends on the destination address of the packets and on the entries 85 of the forwarding table. When a packet is about to be routed, its 86 destination address is compared to the prefixes of the routing table: 87 the entry with the most specific prefix containing the destination 88 address of the packet is chosen, and the packet is forwarded to the 89 associated next-hop. Next-hop routing is a simple, well understood 90 paradigm that works satisfactorily in a large number of cases. 92 Source-specific routing [SS-ROUTING], or Source Address Dependent 93 Routing (SADR) [DSR], is a modest extension to next-hop routing where 94 the forwarding decision depends not only on the destination address 95 but also on the source address of the packet being routed, which 96 makes it possible for two packets with the same destination but 97 different source addresses to be routed following different paths. 99 The forwarding tables are extended to map pairs of prefixes 100 (destination, source) to next hops. When multiple entries match a 101 given packet, the one with the most specific destination prefix is 102 chosen, and, in the case of equally specific destination prefixes, 103 the one with the most specific source prefix. 105 The main application of source-specific routing is a form of 106 multihoming known as multihoming with multiple addresses. When using 107 this technique in a network connected to multiple providers, every 108 host is assigned multiple addresses, one per provider. When a host 109 sources a packet, it picks one of its addresses as the source 110 address, and source-specific routing is used to route the packet to 111 an edge router that is connected to the corresponding provider, which 112 is compatible with [BCP84]. Unlike classical multihoming, this 113 technique is applicable to small networks, as it does not require the 114 use of provider-independent addresses, or cause excessive growth of 115 the global routing table. More details are given in [SS-ROUTING] and 116 [DSR]. 118 This document describes a source-specific routing extension for the 119 Babel routing protocol [BABEL]. This involves minor changes to the 120 data structures, which must include a source prefix in addition to 121 the destination prefix already present, and some changes to the 122 Update, Route Request and Seqno Request TLVs, which are extended with 123 a source prefix. The source prefix is encoded using a mandatory sub- 124 TLV ([BABEL] Section 4.4). 126 2. Specification of Requirements 128 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 129 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 130 "OPTIONAL" in this document are to be interpreted as described in 131 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 132 capitals, as shown here. 134 3. Data Structures 136 A number of the conceptual data structures described in Section 3.2 137 of [BABEL] contain a destination prefix. This specification extends 138 these data structures with a source prefix. Data from the original 139 protocol, which do not specify a source prefix, are stored with a 140 zero length source prefix, which matches exactly the same set of 141 packets as the original, non-source-specific data. 143 3.1. The Source Table 145 Every Babel node maintains a source table, as described in [BABEL] 146 Section 3.2.5. A source-specific Babel node extends this table with 147 the following field: 149 o The source prefix specifying the source address of packets to 150 which this entry applies. 152 The source table is now indexed by triples of the form (prefix, 153 source prefix, router-id). 155 Note that the route entry contains a source (see sections 2 and 3.2.5 156 of [BABEL]) which itself contains a source prefix. These are two 157 very different concepts that should not be confused. 159 3.2. The Route Table 161 Every Babel node maintains a route table, as described in [BABEL] 162 Section 3.2.6. Each route table entry contains, among other data, a 163 source, which this specification extends with a source prefix as 164 described above. The route table is now indexed by triples of the 165 form (prefix, source prefix, neighbour), where the prefix and source 166 prefix are obtained from the source. 168 3.3. The Table of Pending Seqno Requests 170 Every Babel node maintains a table of pending seqno requests, as 171 described in [BABEL], Section 3.2.7. A source-specific Babel node 172 extends this table with the following entry: 174 o The source prefix being requested. 176 The table of pending seqno requests is now indexed by triples of the 177 form (prefix, source prefix, router-id). 179 4. Data Forwarding 181 In next-hop routing, if two routing table entries overlap, then one 182 is necessarily more specific than the other; the "longest prefix 183 rule" specifies that the most specific applicable routing table entry 184 is chosen. 186 With source-specific routing, there might no longer be a most 187 specific applicable entry: two routing table entries might match a 188 given packet without one necessarily being more specific than the 189 other. Consider for example the following routing table: 191 destination source next-hop 192 2001:DB8:0:1::/64 ::/0 A 193 ::/0 2001:DB8:0:2::/64 B 195 This specifies that all packets with destination in 2001:DB8:0:1::/64 196 are to be routed through A, while all packets with source in 197 2001:DB8:0:2::/64 are to be routed through B. A packet with source 198 2001:DB8:0:2::42 and destination 2001:DB8:0:1::57 matches both rules, 199 although neither is more specific than the other. A choice is 200 necessary, and unless the choice being made is the same on all 201 routers in a routing domain, persistent routing loops may occur. 202 More details are given in Section IV.C of [SS-ROUTING]. 204 A Babel implementation MUST choose routing table entries by using the 205 so-called destination-first ordering, where a routing table entry R1 206 is preferred to a routing table entry R2 when either R1's destination 207 prefix is more specific than R2's, or the destination prefixes are 208 equal and R1's source prefix is more specific than R2's. (In more 209 formal terms, routing table entries are compared using the 210 lexicographic product of the destination prefix ordering by the 211 source prefix ordering.) This is consistent with the behaviour 212 described in Section 3.3 of [DSR]. 214 In practice, this means that a source-specific Babel implementation 215 must take care that any lower layer that performs packet forwarding 216 obey this semantics. More precisely: 218 o If the lower layers implement the destination-first ordering, then 219 the Babel implementation MAY use them directly; 221 o If the lower layers can hold source-specific routes, but not with 222 the right semantics, then the Babel implementation MUST 223 disambiguate the routing table by using a suitable disambiguation 224 algorithm (see Section V.B of [SS-ROUTING] for such an algorithm); 226 o If the lower layers cannot hold source-specific routes, then a 227 Babel implementation MUST silently ignore (drop) any source- 228 specific routes. 230 5. Protocol Operation 232 This extension does not fundamentally change the operation of the 233 Babel protocol, and we therefore only describe differences between 234 the original protocol and the extended protocol. 236 In the original protocol, three TLVs carry a destination prefix: 237 Updates, Route Requests and Seqno Requests. This specification 238 extends these messages to optionally carry a Source Prefix sub-TLV, 239 as described in Section 7 below. The sub-TLV is marked as mandatory, 240 so that an unextended implementation will silently ignore the whole 241 enclosing TLV. A node obeying this specification MUST NOT send a TLV 242 with a zero-length source prefix: instead, it sends a TLV with no 243 Source Prefix sub-TLV. Conversely, an extended implementation MUST 244 interpret an unextended TLV as carrying a source prefix of zero 245 length. Taken together, these properties ensure interoperability 246 between the original and extended protocols (see Section 6 below). 248 5.1. Protocol Messages 250 This extension allows three TLVs of the original Babel protocol to 251 carry a source prefix: Update TLVs, Route Request TLVs and Seqno 252 Request TLVs. 254 In order to advertise a route with a non-zero length source prefix, a 255 node sends a source-specific Update, i.e., an Update with a Source 256 Prefix sub-TLV. When a node receives a source-specific Update 257 (prefix, source prefix, router-id, seqno, metric) from a neighbour 258 neigh, it behaves as described in [BABEL] Section 3.5.4, except that 259 the entry under consideration is indexed by (prefix, source prefix, 260 neigh) rather than just (prefix, neigh). 262 Similarly, when a node needs to send a Request of either kind that 263 applies to a route with a non-zero length source prefix, it sends a 264 source-specific Request, i.e., a Request with a Source Prefix sub- 265 TLV. When a node receives a source-specific Request, it behaves as 266 described in Section 3.8 of [BABEL], except that the request applies 267 to the Route Table entry carrying the source prefix indicated by the 268 Source Prefix sub-TLV. 270 5.2. Wildcard Messages 272 In the original protocol, the Address Encoding value 0 is used for 273 wildcard messages: messages that apply to all routes, of any address 274 family and with any destination prefix. Wildcard messages are 275 allowed in two places in the protocol: wildcard retractions are used 276 to retract all of the routes previously advertised by a node on a 277 given interface, and wildcard Route Requests are used to request a 278 full dump of the Route Table from a given node. Wildcard messages 279 are intended to apply to all routes, including routes decorated with 280 additional data and AE values to be defined by future extensions, and 281 hence this specification extends wildcard operations to apply to all 282 routes, whatever the value of the source prefix. 284 More precisely, a node receiving an Update with the AE field set to 0 285 and the Metric field set to infinity (a wildcard retraction) MUST 286 apply the route acquisition procedure described in Section 3.5.4 of 288 [BABEL] to all of the routes that it has learned from the sending 289 node, whatever the value of the source prefix. A node MUST NOT send 290 a wildcard retraction with an attached source prefix, and a node that 291 receives a wildcard retraction with a source prefix MUST ignore it. 293 Similarly, a node that receives a route request with the AE field set 294 to 0 (a wildcard route request) SHOULD send a full routing table 295 dump, including routes with a non-zero length source prefix. A node 296 MUST NOT send a wildcard request that carries a source prefix, and a 297 node receiving a wildcard request with a source prefix MUST ignore 298 it. 300 6. Compatibility with the base protocol 302 The protocol extension defined in this document is, to a great 303 extent, interoperable with the base protocol defined in [BABEL] (and 304 all previously standardised extensions). More precisely, if non- 305 source-specific routers and source-specific routers are mixed in a 306 single routing domain, Babel's loop-avoidance properties are 307 preserved, and, in particular, no persistent routing loops will 308 occur. 310 However, this extension is encoded using mandatory sub-TLVs, 311 introduced in [BABEL], and therefore is not compatible with the older 312 version of the Babel Routing Protocol [RFC6126] which does not 313 support such sub-TLVs. Consequently, this extension MUST NOT be used 314 with routers implementing RFC 6126, otherwise persistent routing 315 loops may occur. 317 6.1. Loop-avoidance 319 The extension defined in this protocol uses a new Mandatory sub-TLV 320 to carry the source prefix information. As discussed in Section 4.4 321 of [BABEL], this encoding ensures that non-source-specific routers 322 will silently ignore the whole TLV, which is necessary to avoid 323 persistent routing loops in hybrid networks. 325 Consider two nodes A and B, with A source-specific announcing a route 326 to (D, S). Suppose that B (non-source-specific) merely ignores the 327 source prefix information when it receives the update rather than 328 ignoring the whole TLV, and re-announces the route as D. This re- 329 announcement reaches A, which treats it as (D, ::/0). Packets 330 destined to D but not sourced in S will be forwarded by A to B, and 331 by B to A, causing a persistent routing loop: 333 (D,S) (D) 334 <-- <-- 335 ------ A ----------------- B 336 --> 337 (D,::/0) 339 6.2. Starvation and Blackholes 341 In general, the discarding of source-specific routes by non-source- 342 specific routers will cause route starvation. Intuitively, unless 343 there are enough non-source-specific routes in the network, non- 344 source-specific routers will suffer starvation, and discard packets 345 for destinations that are only announced by source-specific routers. 347 A simple yet sufficient condition for avoiding starvation is to build 348 a connected source-specific backbone that includes all of the edge 349 routers, and announce a (non-source-specific) default route towards 350 the backbone. 352 7. Protocol Encoding 354 This extension defines a new sub-TLV used to carry a source prefix: 355 the Source Prefix sub-TLV. It can be used within an Update, a Route 356 Request or a Seqno Request TLV to match a source-specific entry of 357 the Route Table, in conjunction with the destination prefix natively 358 carried by these TLVs. 360 Since a source-specific routing entry is characterized by a single 361 destination prefix and a single source prefix, a source-specific 362 message contains exactly one Source Prefix sub-TLV. A node MUST NOT 363 send more that one Source Prefix sub-TLV in a TLV, and a node 364 receiving more than one Source Prefix sub-TLV in a single TLV SHOULD 365 ignore this TLV. It MAY ignore the whole packet. 367 7.1. Source Prefix sub-TLV 369 0 1 2 3 370 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 371 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 372 | Type = 128 | Length | Source Plen | Source Prefix... 373 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 375 Fields: 377 Type Set to 128 to indicate a Source Prefix sub-TLV. 379 Length The length of the body, exclusive of the Type and Length 380 fields. 382 Source Plen The length of the advertised source prefix. This MUST 383 NOT be 0. 385 Source Prefix The source prefix being advertised. This field's size 386 is (Source Plen)/8 rounded upwards. 388 The contents of the Source Prefix sub-TLV are interpreted according 389 to the AE of the enclosing TLV. If a TLV with AE equal to 0 contains 390 a Source Prefix sub-TLV, then the whole TLV MUST be ignored. 391 Similarly, if a TLV contains multiple Source Prefix sub-TLVs, then 392 the whole TLV MUST be ignored. 394 Note that this sub-TLV is a mandatory sub-TLV. Therefore, as 395 described in Section 4.4 of [BABEL], the whole TLV MUST be ignored if 396 that sub-TLV is not understood (or malformed). Otherwise, routing 397 loops may occur (see Section 6.1). 399 7.2. Source-specific Update 401 The source-specific Update is an Update TLV with a Source Prefix sub- 402 TLV. It advertises or retracts source-specific routes in the same 403 manner as routes with non-source-specific Updates (see [BABEL]). A 404 wildcard retraction (Update with AE equal to 0) MUST NOT carry a 405 Source Prefix sub-TLV. 407 Babel uses a stateful compression scheme to reduce the size taken by 408 destination prefixes in update TLVs (see Section 4.5 of [BABEL]). 409 The source prefix defined by this extension is not compressed. On 410 the other hand, compression is allowed for the destination prefixes 411 carried by source-specific updates. As described in Section 4.5 of 412 [BABEL], unextended implementations will correctly update their 413 parser state while otherwise ignoring the whole TLV. 415 7.3. Source-specific (Route) Request 417 A source-specific Route Request is a Route Request TLV with a Source 418 Prefix sub-TLV. It prompts the receiver to send an update for a 419 given pair of destination and source prefixes, as described in 420 Section 3.8.1.1 of [BABEL]. A wildcard request (Route Request with 421 AE equals to 0) MUST NOT carry a Source Prefix sub-TLV; if a wildcard 422 request with a Source Prefix sub-TLV is received, then the request 423 MUST be ignored. 425 7.4. Source-Specific Seqno Request 427 A source-specific Seqno Request is a Seqno Request TLV with a Source 428 Prefix sub-TLV. It requests the receiving node to perform the 429 procedure described in Section 3.8.1.2 of [BABEL], but applied to a 430 pair of a destination and source prefix. 432 8. IANA Considerations 434 IANA has allocated sub-TLV number 128 for the Source Prefix sub-TLV 435 in the Babel sub-TLV types registry. 437 9. Security considerations 439 The extension defined in this document adds a new sub-TLV to three 440 TLVs already present in the original Babel protocol, and does not in 441 itself change the security properties of the protocol. However, 442 source-specific routing gives more control over routing to the 443 sending hosts, which might have security implications (see Section 8 444 of [DSR]). 446 10. Acknowledgments 448 The authors are grateful to Donald Eastlake and Joel Halpern for 449 their help with this document. 451 11. References 453 11.1. Normative References 455 [BABEL] Chroboczek, J., "The Babel Routing Protocol", Internet 456 Draft draft-ietf-babel-rfc6126bis-06, October 2018. 458 [BCP84] Baker, F. and P. Savola, "Ingress Filtering for Multihomed 459 Networks", BCP 84, RFC 3704, March 2004. 461 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 462 Requirement Levels", BCP 14, RFC 2119, 463 DOI 10.17487/RFC2119, March 1997. 465 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 466 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 467 May 2017. 469 11.2. Informative References 471 [DSR] Lamparter, D. and A. Smirnov, "Destination/Source 472 Routing", Internet Draft draft-ietf-rtgwg-dst-src-routing- 473 06, May 2018. 475 [RFC6126] Chroboczek, J., "The Babel Routing Protocol", RFC 6126, 476 DOI 10.17487/RFC6126, April 2011, 477 . 479 [SS-ROUTING] 480 Boutier, M. and J. Chroboczek, "Source-Specific Routing", 481 August 2014. 483 In Proc. IFIP Networking 2015. A slightly earlier 484 version is available online from http://arxiv.org/ 485 pdf/1403.0445. 487 Authors' Addresses 489 Matthieu Boutier 490 IRIF, University of Paris-Diderot 491 Case 7014 492 75205 Paris Cedex 13 493 France 495 Email: boutier@irif.fr 497 Juliusz Chroboczek 498 IRIF, University of Paris-Diderot 499 Case 7014 500 75205 Paris Cedex 13 501 France 503 Email: jch@irif.fr