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Chroboczek 4 Intended status: Standards Track IRIF, University of Paris-Diderot 5 Expires: August 4, 2018 January 31, 2018 7 Source-Specific Routing in Babel 8 draft-ietf-babel-source-specific-03 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 August 4, 2018. 35 Copyright Notice 37 Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . . . . 3 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 . . . . . . . . . . . . . . . . . . . . . 9 71 9. Security considerations . . . . . . . . . . . . . . . . . . . 9 72 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 73 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 74 11.1. Normative References . . . . . . . . . . . . . . . . . . 10 75 11.2. Informative References . . . . . . . . . . . . . . . . . 10 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 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 case of equality, the one with the most specific 103 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", "MAY", and "OPTIONAL" in this 130 document are to be interpreted as described in [RFC2119]. 132 3. Data Structures 134 A number of the conceptual data structures described in Section 3.2 135 of [BABEL] contain a destination prefix. This specification extends 136 these data structures with a source prefix. Data from the original 137 protocol, which do not specify a source prefix, are stored with a 138 zero length source prefix, which matches exactly the same set of 139 packets as the original, non-source-specific data. 141 3.1. The Source Table 143 Every Babel node maintains a source table, as described in [BABEL] 144 Section 3.2.5. A source-specific Babel node extends this table with 145 the following field: 147 o The source prefix specifying the source address of packets to 148 which this entry applies. 150 The source table is now indexed by triples of the form (prefix, 151 source prefix, router-id). 153 Note that the route entry contains a source which itself contains a 154 source prefix. These are two very different concepts that should not 155 be confused. 157 3.2. The Route Table 159 Every Babel node maintains a route table, as described in [BABEL] 160 Section 3.2.6. Each route table entry contains, among other data, a 161 source, which this specification extends with a source prefix as 162 described above. The route table is now indexed by triples of the 163 form (prefix, source prefix, neighbour), where the prefix and source 164 prefix are obtained from the source. 166 3.3. The Table of Pending Seqno Requests 168 Every Babel node maintains a table of pending seqno requests, as 169 described in [BABEL], Section 3.2.7. A source-specific Babel node 170 extends this table with the following entry: 172 o The source prefix being requested. 174 The table of pending seqno requests is now indexed by triples of the 175 form (prefix, source prefix, router-id). 177 4. Data Forwarding 179 In next-hop routing, if two routing table entries overlap, then one 180 is necessarily more specific than the other; the "longest prefix 181 rule" specifies that the most specific applicable routing table entry 182 is chosen. 184 With source-specific routing, there might no longer be a most 185 specific applicable entry: two routing table entries might match a 186 given packet without one necessarily being more specific than the 187 other. Consider for example the following routing table: 189 destination source next-hop 190 2001:DB8:0:1::/64 ::/0 A 191 ::/0 2001:DB8:0:2::/64 B 193 This specifies that all packets with destination in 2001:DB8:0:1::/64 194 are to be routed through A, while all packets with source in 195 2001:DB8:0:2::/64 are to be routed through B. A packet with source 196 2001:DB8:0:2::42 and destination 2001:DB8:0:1::57 matches both rules, 197 although neither is more specific than the other. A choice is 198 necessary, and unless the choice being made is the same on all 199 routers in a routing domain, persistent routing loops may occur. 200 More details are given in [SS-ROUTING] Section IV.C. 202 A Babel implementation MUST choose routing table entries by using the 203 so-called destination-first ordering, where a routing table entry R1 204 is preferred to a routing table entry R2 when either R1's destination 205 prefix is more specific than R2's, or the destination prefixes are 206 equal and R1's source prefix is more specific than R2's. (In more 207 formal terms, routing table entries are compared using the 208 lexicographic product of the destination prefix ordering by the 209 source prefix ordering.) This is consistent with the behaviour 210 described in Section 3.3 of [DSR]. 212 In practice, this means that a source-specific Babel implementation 213 must take care that any lower layer that performs packet forwarding 214 obey this semantics. In particular: 216 o If the lower layers implement the destination-first ordering, then 217 the Babel implementation MAY use them directly; 219 o If the lower layers can hold source-specific routes, but not with 220 the right semantics, then the Babel implementation MUST 221 disambiguate the routing table by using a suitable disambiguation 222 algorithm (see [SS-ROUTING] Section V.B for such an algorithm); 224 o If the lower layers cannot hold source-specific routes, then a 225 Babel implementation MUST silently ignore (drop) any source- 226 specific routes. 228 5. Protocol Operation 230 This extension does not fundamentally change the operation of the 231 Babel protocol, and we therefore only describe differences between 232 the original protocol and the extended protocol. 234 In the original protocol, three TLVs carry a destination prefix: 235 Updates, Route Requests and Seqno Requests. This specification 236 extends these messages to optionally carry a source prefix sub-TLV, 237 as described in Section 7 below. The sub-TLV is marked as mandatory, 238 so that an unextended implementation will silently ignore the whole 239 enclising TLV. A node obeying this specification MUST NOT send a TLV 240 with a zero-length source prefix: instead, it sends a TLV with no 241 source prefix sub-TLV. Conversely, an extended implementation MUST 242 interpret an unextended TLV as carrying a source prefix of zero 243 length. Taken together, these properties ensure interoperability 244 between the original and extended protocols (see Section 6 below). 246 5.1. Protocol Messages 248 This extension allows three TLVs of the original Babel protocol to 249 carry a source prefix: Update TLVs, Route Request TLVs and Seqno 250 Request TLVs. 252 In order to advertise a route with a non-zero-length source prefix, a 253 node sends a source-specific Update, i.e., an Update with a source 254 prefix sub-TLV. When a node receives a source-specific Update 255 (prefix, source prefix, router-id, seqno, metric) from a neighbour 256 neigh, it behaves as described in [BABEL] Section 3.5.4, except that 257 the entry under consideration is indexed by (prefix, source prefix, 258 neigh) rather than just (prefix, neigh). 260 Similarly, when a node needs to send a Request of either kind that 261 applies to a route with a non-zero length source prefix, it sends a 262 source-specific Request, i.e., a Request with a source prefix sub- 263 TLV. When a node receives a source-specific Request, it behaves as 264 described in Section 3.8 of [BABEL], except that the request applies 265 to the Route Table entry carrying the source prefix indicated by the 266 sub-TLV. 268 5.2. Wildcard Messages 270 In the original protocol, the Address Encoding value 0 is used for 271 wildcard messages: messages that apply to all routes, of any address 272 family and with any destination prefix. Wildcard messages are 273 allowed in two places in the protocol: wildcard retractions are used 274 to retract all of the routes previously advertised by a node on a 275 given interface, and wildcard Route Requests are used to request a 276 full dump of the Route Table from a given node. Wildcard messages 277 are intended to apply to all routes, including routes decorated with 278 additional data and AE values to be defined by future extensions, and 279 hence this specification extends wildcard operations to apply to all 280 routes, whatever the value of the source prefix. 282 More precisely, a node receiving an Update with the AE field set to 0 283 and the Metric field set to infinity (a wildcard retraction) MUST 284 apply the route acquisition procedure described in Section 3.5.4 of 285 [BABEL] to all of the routes that is has learned from the sending 286 node, whatever the value of the source prefix. A node MUST NOT send 287 a wildcard retraction with an attached source prefix, and a node that 288 receives a wildcard retraction with a source prefix MUST silently 289 ignore it. 291 Similarly, a node that receives a route request with the AE field set 292 to 0 (a wildcard route request) SHOULD send a full routing table 293 dump, including routes with a non-zero-length source prefix. A node 294 MUST NOT send a wildcard request that carries a source prefix, and a 295 node receiving a wildcard request with a with a source prefix MUST 296 silently ignore it. 298 6. Compatibility with the base protocol 300 The protocol extension defined in this document is, to a great 301 extent, interoperable with the base protocol defined in [BABEL] (and 302 all of its extensions). More precisely, if non-source-specific 303 routers and source-specific routers are mixed in a single routing 304 domain, Babel's loop-avoidance properties are preserved, and, in 305 particular, no persistent routing loops will occur. 307 However, this extension is encoded using mandatory sub-TLVs, 308 introduced in [BABEL], and therefore is not compatible with the older 309 version of the Babel Routing Protocol [RFC6126]. Consequently, this 310 extension MUST NOT be used with routers implementing RFC 6126, 311 otherwise persistent routing loops may occur. 313 6.1. Loop-avoidance 315 The extension defined in this protocol uses a new Mandatory sub-TLV 316 to carry the source prefix information. As discussed in Section 4.4 317 of [BABEL], this encoding ensures that non-source-specific routers 318 will silently ignore the whole TLV, which is necessary to avoid 319 persistent routing loops in hybrid networks. 321 Consider two nodes A and B, with A source-specific announcing a route 322 to (D, S). Suppose that B (non source-specific) merely ignores the 323 source prefix information when it receives the update rather than 324 ignoring the whole TLV, and re-announces the route as D. This re- 325 announcement reaches A, which treats it as (D, ::/0). Packets 326 destined to D but not sourced in S will be forwarded by A to B, and 327 by B to A, causing a persistent routing loop: 329 (D,S) (D) 330 <-- <-- 331 ------ A ----------------- B 332 --> 333 (D,::/0) 335 6.2. Starvation and Blackholes 337 In general, discarding source-specific routes by non-source-specific 338 routers will cause route starvation. Intuitively, unless there are 339 enough non-source-specific routes in the network, non-source-specific 340 routers will suffer starvation, and discard packets for destinations 341 that are only announced by source-specific routers. 343 A simple yet sufficient condition for avoiding starvation is to build 344 a connected source-specific backbone that includes all of the edge 345 routers, and announce a (non-source-specific) default route towards 346 the backbone. 348 7. Protocol Encoding 350 This extension defines a new sub-TLV used to carry a source prefix: 351 the Source Prefix sub-TLV. It can be used within an Update, a Route 352 Request or a Seqno Request TLV to match a source-specific entry of 353 the Route Table, in conjunction with the destination prefix natively 354 carried by these TLVs. 356 Since a source-specific routing entry is characterized by a single 357 destination prefix and a single source prefix, a source-specific 358 message contains exactly one Source Prefix sub-TLV. A node MUST NOT 359 send more that one Source Prefix sub-TLV in a TLV, and a node 360 receiving more than one Source Prefix sub-TLV in a single TLV SHOULD 361 ignore this TLV. It MAY ignore the whole packet. 363 7.1. Source Prefix sub-TLV 365 0 1 2 3 366 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 1 367 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 368 | Type = 128 | Length | Source Plen | Source Prefix... 369 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 371 Fields: 373 Type Set to 128 to indicate a Source Prefix sub-TLV. 375 Length The length of the body, exclusive of the Type and Length 376 fields. 378 Source Plen The length of the advertised source prefix. This MUST 379 NOT be 0. 381 Source Prefix The source prefix being advertised. This field's size 382 is (Source Plen)/8 rounded upwards. 384 The contents of the source prefix sub-TLV are interpreted according 385 to the AE of the enclosing TLV. 387 Note that this sub-TLV is a mandatory sub-TLV. Threfore, as 388 described in Section 4.4 of [BABEL], the whole TLV MUST be ignored if 389 that sub-TLV is not understood (or malformed). Otherwise, routing 390 loops may occur (see Section 6.1). 392 7.2. Source-specific Update 394 The source-specific Update is an Update TLV with a Source Prefix sub- 395 TLV. It advertises or retracts source-specific routes in the same 396 manner than routes with non-source-specific Updates (see [BABEL]). A 397 wildcard retraction (Update with AE equals to 0) MUST NOT carry a 398 Source Prefix sub-TLV. 400 Contrary to the destination prefix, this extension does not compress 401 the source prefix attached to Updates. However, compression is 402 allowed for the destination prefix of source-specific routes. As 403 described in Section 4.5 of [BABEL], unextended implementations will 404 correctly update their parser state while otherwise ignoring the 405 whole TLV. 407 7.3. Source-specific (Route) Request 409 A source-specific Route Request is a Route Request TLV with a Source 410 Prefix sub-TLV. It prompts the receiver to send an update for a 411 given pair of destination and source prefixes, as described in 412 Section 3.8.1.1 of [BABEL]. A wildcard request (Route Request with 413 AE equals to 0) MUST NOT carry a Source Prefix sub-TLV. 415 7.4. Source-Specific Seqno Request 417 A source-specific Seqno Request is a Seqno Request TLV with a Source 418 Prefix sub-TLV. It requests the receiving node to perform the 419 procedure described in Section 3.8.1.2 of [BABEL], but applied to a 420 pair of a destination and source prefix. 422 8. IANA Considerations 424 IANA has allocated sub-TLV number 128 for the Source Prefix sub-TLV 425 in the Babel Sub-TLV Numbers registry. 427 9. Security considerations 429 The extension defined in this document adds a new sub-TLV to three 430 TLVs already present in the original Babel protocol, and does not in 431 itself change the security properties of the protocol. However, 432 source-specific routing gives more control over routing to the 433 sending hosts, which might have security implications (see Section 8 434 of [DSR]). 436 10. Acknowledgments 438 The authors are grateful to Joel Halpern for his help with this 439 document. 441 11. References 443 11.1. Normative References 445 [BABEL] Chroboczek, J., "The Babel Routing Protocol", Internet 446 Draft draft-ietf-babel-rfc6126bis-04, May 2017. 448 [BCP84] Baker, F. and P. Savola, "Ingress Filtering for Multihomed 449 Networks", BCP 84, RFC 3704, March 2004. 451 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 452 Requirement Levels", BCP 14, RFC 2119, 453 DOI 10.17487/RFC2119, March 1997. 455 11.2. Informative References 457 [DSR] Lamparter, D. and A. Smirnov, "Destination/Source 458 Routing", Internet Draft draft-ietf-rtgwg-dst-src-routing- 459 06, May 2018. 461 [RFC6126] Chroboczek, J., "The Babel Routing Protocol 462 (Experimental)", RFC 6126, February 2011. 464 [SS-ROUTING] 465 Boutier, M. and J. Chroboczek, "Source-Specific Routing", 466 August 2014. 468 In Proc. IFIP Networking 2015. A slightly earlier 469 version is available online from http://arxiv.org/ 470 pdf/1403.0445. 472 Authors' Addresses 473 Matthieu Boutier 474 IRIF, University of Paris-Diderot 475 Case 7014 476 75205 Paris Cedex 13 477 France 479 Email: boutier@irif.fr 481 Juliusz Chroboczek 482 IRIF, University of Paris-Diderot 483 Case 7014 484 75205 Paris Cedex 13 485 France 487 Email: jch@irif.fr