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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-20) exists of draft-ietf-babel-rfc6126bis-06 == Outdated reference: A later version (-07) exists of draft-ietf-rtgwg-dst-src-routing-06 -- Obsolete informational reference (is this intentional?): RFC 6126 (Obsoleted by RFC 8966) Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Boutier 3 Internet-Draft J. Chroboczek 4 Intended status: Standards Track IRIF, University of Paris-Diderot 5 Expires: April 26, 2019 October 23, 2018 7 Source-Specific Routing in Babel 8 draft-ietf-babel-source-specific-04 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 April 26, 2019. 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 . . . . . . . . . . . . . . . . . . . . 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 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", "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 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 is 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 of its extensions). More precisely, if non-source-specific 305 routers and source-specific routers are mixed in a single routing 306 domain, Babel's loop-avoidance properties are preserved, and, in 307 particular, no persistent routing loops will occur. 309 However, this extension is encoded using mandatory sub-TLVs, 310 introduced in [BABEL], and therefore is not compatible with the older 311 version of the Babel Routing Protocol [RFC6126] which does not 312 support such sub-TLVs. Consequently, this extension MUST NOT be used 313 with routers implementing RFC 6126, otherwise persistent routing 314 loops may occur. 316 6.1. Loop-avoidance 318 The extension defined in this protocol uses a new Mandatory sub-TLV 319 to carry the source prefix information. As discussed in Section 4.4 320 of [BABEL], this encoding ensures that non-source-specific routers 321 will silently ignore the whole TLV, which is necessary to avoid 322 persistent routing loops in hybrid networks. 324 Consider two nodes A and B, with A source-specific announcing a route 325 to (D, S). Suppose that B (non-source-specific) merely ignores the 326 source prefix information when it receives the update rather than 327 ignoring the whole TLV, and re-announces the route as D. This re- 328 announcement reaches A, which treats it as (D, ::/0). Packets 329 destined to D but not sourced in S will be forwarded by A to B, and 330 by B to A, causing a persistent routing loop: 332 (D,S) (D) 333 <-- <-- 334 ------ A ----------------- B 335 --> 336 (D,::/0) 338 6.2. Starvation and Blackholes 340 In general, discarding source-specific routes by non-source-specific 341 routers will cause route starvation. Intuitively, unless there are 342 enough non-source-specific routes in the network, non-source-specific 343 routers will suffer starvation, and discard packets for destinations 344 that are only announced by source-specific routers. 346 A simple yet sufficient condition for avoiding starvation is to build 347 a connected source-specific backbone that includes all of the edge 348 routers, and announce a (non-source-specific) default route towards 349 the backbone. 351 7. Protocol Encoding 353 This extension defines a new sub-TLV used to carry a source prefix: 354 the Source Prefix sub-TLV. It can be used within an Update, a Route 355 Request or a Seqno Request TLV to match a source-specific entry of 356 the Route Table, in conjunction with the destination prefix natively 357 carried by these TLVs. 359 Since a source-specific routing entry is characterized by a single 360 destination prefix and a single source prefix, a source-specific 361 message contains exactly one Source Prefix sub-TLV. A node MUST NOT 362 send more that one Source Prefix sub-TLV in a TLV, and a node 363 receiving more than one Source Prefix sub-TLV in a single TLV SHOULD 364 ignore this TLV. It MAY ignore the whole packet. 366 7.1. Source Prefix sub-TLV 368 0 1 2 3 369 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 1 370 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 371 | Type = 128 | Length | Source Plen | Source Prefix... 372 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 374 Fields: 376 Type Set to 128 to indicate a Source Prefix sub-TLV. 378 Length The length of the body, exclusive of the Type and Length 379 fields. 381 Source Plen The length of the advertised source prefix. This MUST 382 NOT be 0. 384 Source Prefix The source prefix being advertised. This field's size 385 is (Source Plen)/8 rounded upwards. 387 The contents of the source prefix sub-TLV are interpreted according 388 to the AE of the enclosing TLV. 390 Note that this sub-TLV is a mandatory sub-TLV. Therefore, as 391 described in Section 4.4 of [BABEL], the whole TLV MUST be ignored if 392 that sub-TLV is not understood (or malformed). Otherwise, routing 393 loops may occur (see Section 6.1). 395 7.2. Source-specific Update 397 The source-specific Update is an Update TLV with a Source Prefix sub- 398 TLV. It advertises or retracts source-specific routes in the same 399 manner than routes with non-source-specific Updates (see [BABEL]). A 400 wildcard retraction (Update with AE equal to 0) MUST NOT carry a 401 Source Prefix sub-TLV. 403 Babel uses a stateful compression scheme to reduce the size taken by 404 destination prefixes in update TLVs (see Section 4.5 of [BABEL]). 405 The source prefix defined by this extension is not compressed. On 406 the other hand, compression is allowed for the destination prefixes 407 carried by source-specific updates. As described in Section 4.5 of 408 [BABEL], unextended implementations will correctly update their 409 parser state while otherwise ignoring the whole TLV. 411 7.3. Source-specific (Route) Request 413 A source-specific Route Request is a Route Request TLV with a Source 414 Prefix sub-TLV. It prompts the receiver to send an update for a 415 given pair of destination and source prefixes, as described in 416 Section 3.8.1.1 of [BABEL]. A wildcard request (Route Request with 417 AE equals to 0) MUST NOT carry a Source Prefix sub-TLV. 419 7.4. Source-Specific Seqno Request 421 A source-specific Seqno Request is a Seqno Request TLV with a Source 422 Prefix sub-TLV. It requests the receiving node to perform the 423 procedure described in Section 3.8.1.2 of [BABEL], but applied to a 424 pair of a destination and source prefix. 426 8. IANA Considerations 428 IANA has allocated sub-TLV number 128 for the Source Prefix sub-TLV 429 in the Babel sub-TLV types registry. 431 9. Security considerations 433 The extension defined in this document adds a new sub-TLV to three 434 TLVs already present in the original Babel protocol, and does not in 435 itself change the security properties of the protocol. However, 436 source-specific routing gives more control over routing to the 437 sending hosts, which might have security implications (see Section 8 438 of [DSR]). 440 10. Acknowledgments 442 The authors are grateful to Joel Halpern for his help with this 443 document. 445 11. References 447 11.1. Normative References 449 [BABEL] Chroboczek, J., "The Babel Routing Protocol", Internet 450 Draft draft-ietf-babel-rfc6126bis-06, October 2018. 452 [BCP84] Baker, F. and P. Savola, "Ingress Filtering for Multihomed 453 Networks", BCP 84, RFC 3704, March 2004. 455 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 456 Requirement Levels", BCP 14, RFC 2119, 457 DOI 10.17487/RFC2119, March 1997. 459 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 460 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 461 May 2017. 463 11.2. Informative References 465 [DSR] Lamparter, D. and A. Smirnov, "Destination/Source 466 Routing", Internet Draft draft-ietf-rtgwg-dst-src-routing- 467 06, May 2018. 469 [RFC6126] Chroboczek, J., "The Babel Routing Protocol", RFC 6126, 470 DOI 10.17487/RFC6126, April 2011, 471 . 473 [SS-ROUTING] 474 Boutier, M. and J. Chroboczek, "Source-Specific Routing", 475 August 2014. 477 In Proc. IFIP Networking 2015. A slightly earlier 478 version is available online from http://arxiv.org/ 479 pdf/1403.0445. 481 Authors' Addresses 483 Matthieu Boutier 484 IRIF, University of Paris-Diderot 485 Case 7014 486 75205 Paris Cedex 13 487 France 489 Email: boutier@irif.fr 491 Juliusz Chroboczek 492 IRIF, University of Paris-Diderot 493 Case 7014 494 75205 Paris Cedex 13 495 France 497 Email: jch@irif.fr