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(The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The document date (April 11, 2018) is 2199 days in the past. Is this intentional? 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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-03) exists of draft-chen-bfd-unsolicited-02 == Outdated reference: A later version (-12) exists of draft-ietf-idr-bgp-bestpath-selection-criteria-08 Summary: 0 errors (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group R. Bush 3 Internet-Draft Internet Initiative Japan 4 Intended status: Standards Track J. Haas 5 Expires: October 13, 2018 J. Scudder 6 Juniper Networks, Inc. 7 A. Nipper 8 C. Dietzel 9 DE-CIX 10 April 11, 2018 12 Making Route Servers Aware of Data Link Failures at IXPs 13 draft-ietf-idr-rs-bfd-05 15 Abstract 17 When BGP route servers are used, the data plane is not congruent with 18 the control plane. Therefore, peers at an Internet exchange can lose 19 data connectivity without the control plane being aware of it, and 20 packets are lost. This document proposes the use of a newly defined 21 BGP Subsequent Address Family Identifier (SAFI) both to allow the 22 route server to request its clients use BFD to track data plane 23 connectivity to their peers' addresses, and for the clients to signal 24 that connectivity state back to the route server. 26 Requirements Language 28 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 29 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to 30 be interpreted as described in [RFC2119] only when they appear in all 31 upper case. They may also appear in lower or mixed case as English 32 words, without normative meaning. 34 Status of This Memo 36 This Internet-Draft is submitted in full conformance with the 37 provisions of BCP 78 and BCP 79. 39 Internet-Drafts are working documents of the Internet Engineering 40 Task Force (IETF). Note that other groups may also distribute 41 working documents as Internet-Drafts. The list of current Internet- 42 Drafts is at https://datatracker.ietf.org/drafts/current/. 44 Internet-Drafts are draft documents valid for a maximum of six months 45 and may be updated, replaced, or obsoleted by other documents at any 46 time. It is inappropriate to use Internet-Drafts as reference 47 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on October 13, 2018. 50 Copyright Notice 52 Copyright (c) 2018 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (https://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 68 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 69 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4 70 4. Next Hop Validation . . . . . . . . . . . . . . . . . . . . . 5 71 4.1. ReachAsk . . . . . . . . . . . . . . . . . . . . . . . . 6 72 4.2. LocReach . . . . . . . . . . . . . . . . . . . . . . . . 6 73 4.3. ReachTell . . . . . . . . . . . . . . . . . . . . . . . . 7 74 4.4. NHIB . . . . . . . . . . . . . . . . . . . . . . . . . . 7 75 5. Advertising NH-Reach state in BGP . . . . . . . . . . . . . . 7 76 6. Client Procedures for NH-Reach Changes . . . . . . . . . . . 9 77 7. Recommendations for Using BFD . . . . . . . . . . . . . . . . 9 78 8. Other Considerations . . . . . . . . . . . . . . . . . . . . 10 79 9. Acknolwedgments . . . . . . . . . . . . . . . . . . . . . . . 10 80 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 81 11. Security Considerations . . . . . . . . . . . . . . . . . . . 10 82 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 83 12.1. Normative References . . . . . . . . . . . . . . . . . . 11 84 12.2. Informative References . . . . . . . . . . . . . . . . . 12 85 Appendix A. Summary of Document Changes . . . . . . . . . . . . 12 86 Appendix B. Other Forms of Connectity Checks . . . . . . . . . . 12 87 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 89 1. Introduction 91 In configurations (typically Internet Exchange Points (IXPs)) where 92 EBGP routing information is exchanged between client routers through 93 the agency of a route server (RS) [RFC7947], but traffic is exchanged 94 directly, operational issues can arise when partial data plane 95 connectivity exists among the route server client routers. Since the 96 data plane is not congruent with the control plane, the client 97 routers on the IXP can lose data connectivity without the control 98 plane - the route server - being aware of it, resulting in 99 significant data loss. 101 To remedy this, two basic problems need to be solved: 103 1. Client routers must have a means of verifying connectivity 104 amongst themselves, and 105 2. Client routers must have a means of communicating the knowledge 106 of the failure (and restoration) back to the route server. 108 The first can be solved by application of Bidirectional Forwarding 109 Detection [RFC5880]. The second can be solved by exchanging BGP 110 routes which use the NH-Reach Subsequent Address Family Identifier 111 (SAFI) defined in this document. 113 Throughout this document, we generally assume that the route server 114 being discussed is able to represent different RIBs towards different 115 clients, as discussed in section 2.3.2.1 of [RFC7947]. If this is 116 not the case, the procedures described here to allow BFD to be 117 automatically provisioned between clients still have value; however, 118 the procedures for signaling reachability back to the route server 119 may not. 121 Throughout this document, we refer to the "route server", "RS" or 122 just "server" and the "client" to describe the two BGP routers 123 engaging in the exchange of information. We observe that there could 124 be other applications for this extension. Our use of terminology is 125 intended for clarity of description, and not to limit the future 126 applicability of the proposal. 128 [I-D.ietf-idr-bgp-bestpath-selection-criteria] discusses enhancement 129 of the route resolvability condition of section 9.1.2.1 of [RFC4271] 130 to include next hop reachability and path availability checks. This 131 specification represents in part an instance of such, implemented 132 using BFD as the OAM mechanism. 134 2. Definitions 136 o Indirect peer: If a route server is configured such that routes 137 from a given client might be sent to some other client, or vice- 138 versa, those two clients are considered to be indirect peers. 139 o Indirect Peer's Address, IPA, next hop: We refer frequently to a 140 next hop. It should generally be clear from context what is 141 intended, almost always an address associated with an indirect 142 peer (the exception, when an indirect peer sends a third party 143 next hop, is discussed in Section 3). In Section 5 we discuss the 144 MP-BGP [RFC4760] Next Hop field; this is distinguished by its 145 capitalization and should also be clear from context. Later in 146 that section we define the Indirect Peer's Address field of the 147 NLRI, also called "IPA". It will be clear to the reader that this 148 refers to the "next hops" discussed elsewhere in the document, but 149 we don't use the name "next hop" for this field to avoid confusion 150 with the pre-existing next hop path attribute of [RFC4271] and 151 attribute field of [RFC4760]. 152 o RS: Route Server. See [RFC7947]. 154 3. Overview 156 As with the base BGP protocol, we model the function of this 157 extension as the interaction between a conceptual set of databases: 159 o ReachAsk: The reachability request database. A database of next 160 hops (host addresses) for which data plane reachability is being 161 queried. 162 o ReachAsk-Out: A set of queries sent to the client. 163 o ReachAsk-In: A set of queries received from the route server. 164 o ReachTell: The reachability response database. A database of 165 responses to ReachAsk queries, indicating what is known about data 166 plane reachability. 167 o ReachTell-Out: The responses being sent to the route server. 168 o ReachTell-In: The response received from the client. 169 o LocReach: The local reachability database. 170 o NHIB: Next Hop Information Base. Stores what is known about the 171 client's reachability to its next hops. 173 +--------------------------------------------------------+ 174 | +------------+ +------------+ +------------+ | 175 | | Per- | | Configured | | Per- | | 176 | | Client | | indirect | | Client | | 177 | | NHIB | | peers | | RIB | | 178 | +-----^------+ +------------+ +-----+------+ | 179 | | \ | | 180 | +-----+------+ `-->-----v------+ | 181 | |ReachTell-In| |ReachAsk-Out| | 182 | +------^-----+ Route Server +-----+------+ | 183 +----------|----------------------------------|----------+ 184 | | 185 | | 186 | | 187 | | 188 +----------|----------------------------------|----------+ 189 | +------+------+ RS Client +-----v-----+ | 190 | |ReachTell-Out| |ReachAsk-In| | 191 | +------^------+ +-----+-----+ | 192 | | +------------+ | | 193 | | | | | | 194 | `----------+ LocReach <----------' | 195 | | | | 196 | +------------+ | 197 +--------------------------------------------------------+ 199 Route Server, RS Client, and Reachability Ask and Tell databases with 200 In/Out Queues 202 In outline, the route server requests its client to track 203 connectivity for all the potential next hops the RS might send to the 204 client, by sending these next hops as ReachAsk "routes". The client 205 tracks connectivity using BFD and reports its connectivity status to 206 the RS using ReachTell "routes". Connectivity status may be that the 207 next hop is reachable, unreachable, or unknown. Once the RS has been 208 informed by the client of its connectivity, it uses this information 209 to influence the route selection the RS performs on behalf of the 210 client. Details are elaborated in the following sections. 212 4. Next Hop Validation 214 Below, we detail procedures where a route server tells its client 215 router about other client next hops by sending it ReachAsk routes and 216 the client router verifies connectivity to those other client routers 217 and communicates its findings back to the RS using ReachTell routes. 218 The RS uses the received ReachTell routes as input to the NHIB and 219 hence the route selection process it performs on behalf of the 220 client. 222 4.1. ReachAsk 224 The route server maintains a ReachAsk database for each client that 225 supports this proposal, that is, for each client that has advertised 226 support (Section 5) for the NH-Reach SAFI. This database is the 227 union of: 229 o The set of next hops found in the associated per-client Loc-RIB 230 (see section 2.3.2.1 of [RFC7947]). 231 o The set of addresses of this client's indirect peers (Section 2). 232 o The RS MAY also add other entries, for example under configuration 233 control. 235 We note that under most circumstances, the first (Loc-RIB next hops) 236 set will be a subset of the second (indirect peers) set. For this 237 not to be the case, a client would have to have sent a "third party" 238 next hop [RFC4271] to the server. To cover such a case, an 239 implementation MAY note any such next hops, and include them in its 240 list of indirect peers. (This implies that if a third party next hop 241 for client C is conveyed to client A, not only will C be placed in 242 A's ReachAsk database, but A will be placed in C's ReachAsk 243 database.) 245 The contents of the ReachAsk database are communicated to the client 246 using the NLRI format and procedures described in Section 5. 248 4.2. LocReach 250 The client MUST attempt to track data plane connectivity to each host 251 address depicted in the ReachAsk database. It MAY also track 252 connectivity to other addresses. The use of BFD for this purpose is 253 detailed in Section 6. 255 For each address being tracked, its state is maintained by the client 256 in a LocReach entry. The state can be: 258 o Unknown. Connectivity status is unknown. This may be due to a 259 temporary or permanent lack of feasible OAM mechanism to determine 260 the status. 261 o Up. The address has been determined to be reachable. 262 o Down. The address has been determined to be unreachable. 264 The LocReach database is used as input for the ReachTell database; it 265 MAY also be used as input to the client's route resolvability 266 condition (section 9.1.2.1 of [RFC4271]). 268 4.3. ReachTell 270 The ReachTell database contains an entry for every entry in the 271 LocReach database. 273 The contents of the ReachTell database are communicated to the server 274 using the NLRI format and procedures described in Section 5. 276 4.4. NHIB 278 The route server maintains a per-client Next Hop Information Base, or 279 NHIB. This contains the information about next hop status received 280 from ReachTell. 282 In computing its per-client Loc-RIB, the RS uses the content of the 283 related per-client NHIB as input to the route resolvability condition 284 (section 9.1.2.1 of [RFC4271]). The next hop being resolved is 285 looked up in the NHIB and its state determined: 287 o Up next hops are considered resolvable. 288 o Unknown next hops MAY be considered resolvable. They MAY be less 289 preferred for selection. 290 o Down next hops MUST NOT be considered resolvable. 291 o If a given next hop is not present in the NHIB, but is present in 292 ReachAsk-Out, either the client has not responded yet (a transient 293 condition) or an error exists. Similar to Unknown next hops, such 294 routes MAY be considered resolvable; they MAY be less preferred. 296 5. Advertising NH-Reach state in BGP 298 A new BGP SAFI, the NH-Reach SAFI, is defined in this document. It 299 has been assigned value TBD. A route server or a route server client 300 using the procedures in this document MUST advertise support for this 301 SAFI, for the IPv4 and/or IPv6 Address Family Identifier (AFI). The 302 use of this SAFI with any other AFI is not defined by this document. 304 NH-Reach NLRI "routes" have a Length of Next Hop Network Address 305 value of 0, therefore they have an empty Network Address of Next Hop 306 field (section 3 of [RFC4760]). 308 Since as specified here, ReachTell "routes" from different clients 309 populate distinct databases on the RS, there will generally be only a 310 single path per "route"; this implies that route selection need not 311 be performed (or equivalently, that it's trivial to perform). 313 In the other direction, a client might peer with multiple route 314 servers and receive differing sets of ReachAsk routes from them. An 315 implementation MAY handle this situation by implementing a distinct 316 ReachAsk and ReachTell per server, but it MAY also handle it by 317 placing all servers' ReachAsk "routes" into a single ReachAsk, and 318 sending the results to all servers from a single ReachTell. This 319 would imply some route server(s) might get ReachTell results they had 320 not asked for, but this is permissible in any case. Again, since the 321 contents of ReachAsk are simply a set of host routes to be tested, 322 route selection over a combined ReachAsk MAY be omitted. 324 ReachAsk and ReachTell entries are exchanged using the NH-Reach NLRI 325 encoding: 327 0 1 2 3 328 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 1 329 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 330 |T|Reserved |Sta| Indirect Peer's Address (4 or 16 octets) | 331 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 332 . ... Indirect Peer's Address (4 or 16 octets) ... . 333 . . 334 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 336 NH-Reach NLRI Format 338 o T: Type is a one-bit field that can take the value 0, meaning the 339 NLRI is a ReachAsk entry, or 1, meaning it is a ReachTell entry. 340 o Reserved: These five bits are reserved. They MUST be sent as zero 341 and MUST be disregarded on receipt. 342 o Sta: State is a two-bit field used to signal the LocReach 343 (Section 4.2) state: 345 * 0 or 3: Unknown. 346 * 1: Up. 347 * 2: Down. 349 Although either 0 or 3 is to be interpreted as "Unknown", the 350 value 0 MUST be used on transmission. The value 3 MUST be 351 accepted as an alias for 0 on receipt. 352 o The Indirect Peer's Address ("IPA") field is an IPv4 or IPv6 host 353 route, depending on whether the AFI is IPv4 or IPv6. 355 ReachAsk and ReachTell entries MUST NOT be propagated from one BGP 356 peering session to another; the routes are not transitive. 358 The IPA field is the key for the NH-Reach NLRI type; the information 359 encoded in the top octet is non-key information. It is possible in 360 principle (although unlikely) for two NLRI to be validly present in 361 an UPDATE message with identical IPA fields but different types. 362 However, two NLRI with the same IPA field and different State fields 363 MUST NOT be encoded in the same UPDATE message. If such is 364 encountered, the receiver MUST behave as though the state "Unknown" 365 was received for the IPA in question. 367 6. Client Procedures for NH-Reach Changes 369 When an entry is added to a route server client's ReachAsk-In for a 370 route server peering session, the client will then attempt to verify 371 connectivity to the host depicted by that entry. The procedure 372 described in this specification utilizes BFD. 374 If no existing BFD session exists to this next hop, a BFD session is 375 provisioned to that IP address and the LocReach reachability state 376 (Section 4.2) is set to Unknown. 378 If the client cannot establish a BFD session with an entry in its 379 ReachAsk-In, the next hop remains in LocReach with its Reachable 380 state Unknown. 382 Once the BFD session moves to the Up state, the LocReach reachability 383 state is set to Up. 385 When the BFD session transitions out of the Up state to the Down 386 state, the LocReach reachability state is set to Down. 388 If the BFD session transitions out of the Up state to the AdminDown 389 state, the LocReach reachability state is set to Unknown. 391 When entries are removed from the route server client's ReachAsk-In 392 for a route server peering session, the client MAY delay de- 393 provisioning the BFD peering session. If the client delays de- 394 provisioning the session, it should remove it if the BFD session 395 transitions to the Down or AdminDown states. 397 7. Recommendations for Using BFD 399 The RECOMMENDED way a client router can confirm the data plane 400 connectivity to its next hops is available, is the use of BFD in 401 asynchronous mode. Echo mode MAY be used if both client routers 402 running a BFD session support this. The use of authentication in BFD 403 is OPTIONAL as there is a certain level of trust between the 404 operators of the client routers at a particular IXP. If trust cannot 405 be assumed, it is recommended to use pair-wise keys (how this can be 406 achieved is outside the scope of this document). The ttl/hop limit 407 values as described in section 5 [RFC5881] MUST be obeyed in order to 408 shield BFD sessions against packets coming from outside the IXP. 410 The following values of the BFD configuration of client routers (see 411 section 6.8.1 [RFC5880]) are RECOMMENDED: 413 o DesiredMinTxInterval: 1,000,000 (microseconds) 414 o RequiredMinRxInterval: 1,000,000 (microseconds) 415 o DetectMult: 3 417 A client router administrator MAY select more appropriate values to 418 meet the special needs of a particular deployment. 420 8. Other Considerations 422 For purposes of routing stability, implementations may wish to apply 423 hysteresis ("holddown") to next hops that have transitioned from 424 reachable to unreachable and back. 426 Implementations MAY restrict the range of addresses with which they 427 will attempt to form BFD relationships. For example, an 428 implementation might by default only allow BFD relationships with 429 peers that share a subnetwork with the route server. An 430 implementation MAY apply such restrictions by default. 432 In a route-server environment, use of this feature SHOULD be 433 restricted to consider only routes that are advertised from within 434 the IXP network. This might include checks on AS_PATH length. 436 9. Acknolwedgments 438 The authors would like to thanks Thomas King for his contributions 439 toward this work. 441 10. IANA Considerations 443 IANA is requested to allocate a value from the Subsequent Address 444 Family Identifiers (SAFI) Parameters registry for this proposal. Its 445 Description in that registry shall be NH-Reach with a Reference of 446 this RFC. 448 11. Security Considerations 450 The mechanism in this document permits a route server client to 451 influence the contents of the route server's Adj-Ribs-Out through its 452 reports of next hop reachability state using the NH-Reach SAFI. 453 Since this state is per-client, if a route server client is able to 454 inject NH-Reach routes for another route server's BGP session to a 455 client, it can cause the route server to select different forwarding 456 than otherwise expected. This issue may be mitigated using transport 457 security on the BGP sessions between the route server and its 458 clients. See [RFC4272]. 460 The NH-Reach SAFI enables the server to trigger creation of a BFD 461 session on its client. A malicious or misbehaving server could 462 trigger an unreasonable number of sessions, a potential resource 463 exhaustion attack. The sedate default timers proposed in Section 7 464 mitigate this; they also mitigate concerns about use of the client as 465 a source of packets in a flooding attack. An implementation MAY also 466 impose limits on the number of BFD sessions it will create at the 467 request of the server. 469 The reachability tests between route server clients themselves may be 470 a target for attack. Such attacks may include forcing a BFD session 471 Down through injecting false BFD state. A less likely attack 472 includes forcing a BFD session to stay Up when its real state is 473 Down. These attacks may be mitigated using the BFD security 474 mechanisms defined in [RFC5880]. 476 12. References 478 12.1. Normative References 480 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 481 Requirement Levels", BCP 14, RFC 2119, 482 DOI 10.17487/RFC2119, March 1997, 483 . 485 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 486 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 487 DOI 10.17487/RFC4271, January 2006, 488 . 490 [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, 491 "Multiprotocol Extensions for BGP-4", RFC 4760, 492 DOI 10.17487/RFC4760, January 2007, 493 . 495 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 496 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 497 . 499 [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 500 (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, 501 DOI 10.17487/RFC5881, June 2010, 502 . 504 [RFC7947] Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker, 505 "Internet Exchange BGP Route Server", RFC 7947, 506 DOI 10.17487/RFC7947, September 2016, 507 . 509 12.2. Informative References 511 [I-D.chen-bfd-unsolicited] 512 Chen, E., Shen, N., and R. Raszuk, "Unsolicited BFD for 513 Sessionless Applications", draft-chen-bfd-unsolicited-02 514 (work in progress), January 2018. 516 [I-D.ietf-idr-bgp-bestpath-selection-criteria] 517 Asati, R., "BGP Bestpath Selection Criteria Enhancement", 518 draft-ietf-idr-bgp-bestpath-selection-criteria-08 (work in 519 progress), October 2017. 521 [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", 522 RFC 4272, DOI 10.17487/RFC4272, January 2006, 523 . 525 [RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S. 526 Pallagatti, "Seamless Bidirectional Forwarding Detection 527 (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016, 528 . 530 Appendix A. Summary of Document Changes 532 idr-04 to idr-05: Added reference to "BGP Bestpath Selection 533 Criteria Enhancement" draft. Rename "next hop" field of NLRI to 534 "Indirect Peer's Address". Add suggestion about AS_PATH length 535 checks. 536 idr-03 to idr-04: Note other forms of connectivity checks. 537 idr-02 to idr-03: Substantial rewrite. Introduce NLRI format that 538 embeds state. 539 idr-01 to idr-02: Move from BGP-LS to NH-Reach SAFI. Lots of 540 editorial changes. 541 idr-00 to idr-01: Add BGP Capability. Move from NH-Cost to BGP-LS. 542 ymbk-01 to idr-00: No technical changes; adopted by IDR. 543 ymbk-00 to ymbk-01: Clarifications to BFD procedures. Use BFD state 544 as an input to BGP route selection. 546 Appendix B. Other Forms of Connectity Checks 548 RFC 5880/5881 BFD is a well-deployed feature. For this reason, it 549 was chosen as the connectivity check utilized for nexthop 550 reachability by this document. As other forms of BFD become more 551 widely deployed, they may also be utilized to provide the 552 connectivity check functionality. 554 Examples of other such BFD mechanisms include: 556 o Seamless BFD [RFC7880] 557 o Unsolicited BFD for Sessionless Applications 558 [I-D.chen-bfd-unsolicited] 560 Implementations MUST support RFC 5880/5881 BFD to be compliant with 561 this specification. Implementations MAY support other forms of 562 connectivity check, including those mechanisms listed above, so long 563 as they provide the ability to fall-back to RFC 5880/5881 BFD. 565 Authors' Addresses 567 Randy Bush 568 Internet Initiative Japan 569 5147 Crystal Springs 570 Bainbridge Island, Washington 98110 571 US 573 Email: randy@psg.com 575 Jeffrey Haas 576 Juniper Networks, Inc. 577 1133 Innovation Way 578 Sunnyvale, CA 94089 579 US 581 Email: jhaas@juniper.net 583 John G. Scudder 584 Juniper Networks, Inc. 585 1133 Innovation Way 586 Sunnyvale, CA 94089 587 US 589 Email: jgs@juniper.net 591 Arnold Nipper 592 DE-CIX Management GmbH 593 Lichtstrasse 43i 594 Cologne 50825 595 Germany 597 Email: arnold.nipper@de-cix.net 598 Christoph Dietzel 599 DE-CIX Management GmbH 600 Lichtstrasse 43i 601 Cologne 50825 602 Germany 604 Email: christoph.dietzel@de-cix.net