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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) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Link State Routing K. Talaulikar 3 Internet-Draft P. Psenak 4 Intended status: Standards Track Cisco Systems, Inc. 5 Expires: January 1, 2021 A. Fu 6 Bloomberg 7 M. Rajesh 8 Juniper Networks 9 June 30, 2020 11 OSPF Strict-Mode for BFD 12 draft-ietf-lsr-ospf-bfd-strict-mode-01 14 Abstract 16 This document specifies the extensions to OSPF that enable an OSPF 17 router to signal the requirement for a Bidirectional Forwarding 18 Detection (BFD) session prior to adjacency formation. Link-Local 19 Signaling (LLS) is used to advertise this requirement of "strict- 20 mode" of BFD session establishment for OSPF adjacency. If both OSPF 21 neighbors advertise the "strict-mode" of BFD, adjacency formation 22 will be blocked until a BFD session has been successfully 23 established. 25 Status of This Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at https://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on January 1, 2021. 42 Copyright Notice 44 Copyright (c) 2020 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (https://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 60 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 61 2. LLS B-bit Flag . . . . . . . . . . . . . . . . . . . . . . . 3 62 3. Local Interface IPv4 Address TLV . . . . . . . . . . . . . . 3 63 4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 4.1. OSPFv3 IPv4 Address-Family Specifics . . . . . . . . . . 6 65 4.2. Graceful Restart Considerations . . . . . . . . . . . . . 6 66 5. Operations & Management Considerations . . . . . . . . . . . 6 67 6. Backward Compatibility . . . . . . . . . . . . . . . . . . . 7 68 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 69 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 70 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 71 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 72 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 73 10.2. Informative References . . . . . . . . . . . . . . . . . 9 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 76 1. Introduction 78 Bidirectional Forwarding Detection (BFD) [RFC5880] enables routers to 79 monitor dataplane connectivity and to detect faults in the 80 bidirectional path between them. BFD is leveraged by routing 81 protocols like OSPFv2[RFC2328] and OSPFv3 [RFC5340] to detect 82 connectivity failures for established adjacencies and trigger the 83 rerouting of traffic around the failure more quickly than with OSPF 84 hello packet monitoring. 86 The use of BFD for monitoring routing protocols adjacencies is 87 described in [RFC5882]. When BFD monitoring is enabled for OSPF 88 adjacencies, the BFD session is bootstrapped based on the neighbor 89 address information discovered by the exchange of OSPF hello packets. 90 Faults in the bidirectional forwarding detected via BFD then result 91 in the OSPF adjacency being brought down. Note that it is possible 92 in some failure scenarios for the network to be in a state such that 93 an OSPF adjacency can be established but a BFD session cannot be 94 established and maintained. In certain other scenarios, a degraded 95 or poor quality link may result in OSPF adjacency formation to 96 succeed only to result in BFD session establishment not being 97 successful or flapping of the BFD session. 99 To avoid the routing churn associated with these scenarios, it would 100 be beneficial to not allow OSPF to establish an adjacency until a BFD 101 session is successfully established and has stabilized. However, 102 this would preclude the OSPF operation in an environment in which not 103 all OSPF routers support BFD and are enabled for BFD on the link. A 104 solution is to block OSPF adjacency establishment until a BFD session 105 is established as long as both neighbors advertise such a 106 requirement. Such a mode of OSPF BFD usage is referred to as 107 "strict-mode". 109 This document specifies the OSPF protocol extensions using link-local 110 signaling (LLS) [RFC5613] for a router to indicate to its neighbor 111 the willingness to establish a BFD session in the "strict-mode". It 112 also introduces an extension for OSPFv3 link-local signaling of 113 interface IPv4 address when used for IPv4 address-family (AF) 114 instance to enable discovery of the IPv4 addresses for BFD session 115 setup. 117 A similar functionality for IS-IS is specified [RFC6213]. 119 1.1. Requirements Language 121 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 122 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 123 "OPTIONAL" in this document are to be interpreted as described in BCP 124 14 [RFC2119] [RFC8174] when, and only when, they appear in all 125 capitals, as shown here. 127 2. LLS B-bit Flag 129 This document defines the B-bit in the LLS Type 1 Extended Options 130 and Flags field. This bit is defined for the LLS block included in 131 Hello packets and indicates that BFD is enabled on the link and that 132 the router requests BFD strict-mode. Section 7 describes the 133 position of the B-bit. 135 A router MUST include the LLS block with the LLS Type 1 Extended 136 Options and Flags TLV with the B-bit set its Hello messages when BFD 137 is enabled on the link. 139 3. Local Interface IPv4 Address TLV 141 The Local Interface IPv4 Address TLV is an LLS TLV meant for OSPFv3 142 protocol operations for IPv4 AF instances [RFC5838]. It has 143 following format: 145 0 1 2 3 146 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 1 147 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 148 | Type | Length | 149 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 150 | Local Interface IPv4 Address | 151 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 153 where: 155 Type: TBD, suggested value 21 157 Length: 4 octet 159 Local Interface IPv4 Address: The primary IPv4 address of the 160 local interface. 162 4. Procedures 164 A router supporting BFD strict-mode advertises this capability 165 through its hello messages as described in Section 2. When a router 166 supporting BFD strict-mode discovers a new neighbor router that also 167 supports BFD strict-mode, then it will establish a BFD session first 168 with that neighbor before bringing up the OSPF adjacency as described 169 further in this section. 171 This document updates the OSPF neighbor state machine as described in 172 [RFC2328]. Specifically, the operations related to the Init state as 173 below when BFD strict-mode is used: 175 Init (without BFD strict-mode) 177 In this state, a Hello packet has recently been received from the 178 neighbor. However, bidirectional communication has not yet been 179 established with the neighbor (i.e., the router itself did not 180 appear in the neighbor's Hello packet). All neighbors in this 181 state (or higher) are listed in the Hello packets sent from the 182 associated interface. 184 Init (with BFD strict-mode) 186 In this state, an Hello packet has recently been received from the 187 neighbor. However, bidirectional communication has not yet been 188 established with the neighbor (i.e., the router itself did not 189 appear in the neighbor's Hello packet). A BFD session 190 establishment to the neighbor is requested, if not already done 191 (e.g. in the event of transition from 2-way state). Neighbors in 192 Init state or higher will be listed in the Hello packets 193 associated with the interface if they either have a corresponding 194 BFD session established or have not advertised "strict-mode" BFD 195 in the Hello packet LLS Extended Options and Flags. 197 Whenever the neighbor state transitions to Down state, the removal of 198 the BFD session associated with that neighbor SHOULD be requested by 199 OSPF and subsequent BFD session establishement SHOULD similarly be 200 requested by OSPF upon transitioning into Init state. This may 201 result in the deletion and creation of the BFD session respectively 202 when OSPF is the only client interested in the BFD session to the 203 neighbor address. 205 An implementation MUST NOT wait for BFD session establishment in Init 206 state unless BFD strict-mode is enabled on the router and the 207 specific neighbor indicates BFD strict-mode capability via its Hello 208 LLS options. When BFD is enabled, but the strict-mode of operation 209 has not be signaled by both neighbors, then an implementation SHOULD 210 start the BFD session establishment only in 2-Way state or higher 211 state. This makes it possible for an OSPF router to operate a mix of 212 BFD operation in strict-mode or normal mode across different 213 interfaces or even different neighbors on the same multi-access LAN 214 interface. 216 Once the OSPF state machine has moved beyond the Init state, any 217 change in the B-bit advertised in subsequent Hello messages MUST NOT 218 result in any trigger in either the OSPF adjacency or the BFD session 219 management (i.e., the B-bit is considered only when in the Init 220 state). Disabling BFD (or BFD strict-mode) on an OSPF router would 221 result in it not setting the B-bit in its subsequent Hello LLS 222 options. Disabling BFD strict-mode has no effect on the BFD 223 operations and would not result in bringing down of any established 224 BFD session. Disabling BFD would result in the BFD session brought 225 down due to Admin reason and hence would not bring down the OSPF 226 adjacency. 228 When BFD is enabled on an interface over which we already have an 229 existing OSPF adjacency, it would result in the router setting the 230 B-bit in its subsequent Hello messages. If the adjacency is already 231 up (i.e., in its terminal state of Full or 2-way with non-DR routers 232 on a LAN) with a neighbor that also supports BFD strict-mode, then an 233 implemantion SHOULD NOT bring this adjacency down but instead use the 234 BFD strict-mode of operation after the next transition into Init 235 state. However, if the adjacency is not up, then an implementation 236 MAY bring such an adjacency down so it can use the BFD strict-mode 237 for its bring up. 239 4.1. OSPFv3 IPv4 Address-Family Specifics 241 Multiple AF support in OSPFv3 [RFC5838] requires the use of an IPv6 242 link-local address as the source address for hello packets even when 243 forming adjacencies for IPv4 AF instances. In most deployments of 244 OSPFv3 IPv4 AF, it is required that BFD is used to monitor and verify 245 the IPv4 data plane connectivity between the routers on the link and, 246 hence, the BFD session is setup using IPv4 neighbor addresses. The 247 IPv4 neighbor address on the interface is learnt only later in the 248 adjacency formation process when the neighbor's Link-LSA is received. 249 This results in the setup of the BFD session either after the 250 adjacency is established or later in the adjacency formation 251 sequence. 253 To enable BFD operation in strict-mode, it is necessary for an OSPF 254 router to learn it's neighbor's IPv4 link address during the Init 255 state of adjacency formation (ideally when it receives the first 256 hello). The use of the Local Interface IPv4 Address TLV (as defined 257 in Section 3) in the LLS block of the OSPFv3 Hello messages for IPv4 258 AF instances makes this possible. Implementations that support 259 strict-mode of BFD operation for OSPFv3 IPv4 AF instances MUST 260 include the Local Interface IPv4 Address TLV in the LLS block of 261 their hello messages whenever the B-bit is also set in the LLS 262 Options and Flags field. A receiver MUST ignore the B-bit (i.e., not 263 operate in BFD strict mode) when the Local Interface IPv4 Address TLV 264 is not present in OSPFv3 Hello message for IPv4 AF OSPFv3 instances. 266 4.2. Graceful Restart Considerations 268 An implementation needs to handle scenarios where both graceful 269 restart (GR) and the strict-mode of BFD operation are deployed 270 together. The GR aspects discussed in [RFC5882] also apply with 271 strict-mode of BFD operation. Additionally, in strict-mode of BFD 272 operation, since the OSPF adjacency formation is delayed until the 273 BFD session establishment, the resultant delay in adajcency formation 274 may affect or break the GR-based recovery. In such cases, it is 275 RECOMMENDED that the GR timers are set such that they provide 276 sufficient time to allow for normal BFD session establishment delays. 278 5. Operations & Management Considerations 280 An implementation SHOULD report the BFD session status along with the 281 OSPF Init adjacency state when operating in BFD strict-mode and 282 perform logging operations on state transitions to include the BFD 283 events. This allows an operator to detect scenarios where an OSPF 284 adjacency may be stuck waiting for BFD session establishment. 286 In network deployments with noisy links or those with packet loss, 287 BFD sessions may flap frequently. In such scenarions, OSPF strict- 288 mode for BFD may be deployed in conjunction with a BFD dampening or 289 hold-down mechanism to help avoid frequent adjacency flaps that cause 290 routing churn. 292 6. Backward Compatibility 294 An implementation MUST support OSPF adjacency formation and 295 operations with a neighbor router that does not advertise the BFD 296 strict-mode capability - both when that neighbor router does not 297 support BFD and when it does support BFD but not in the strict-mode 298 of operation as described in this document. Implementations MAY 299 provide an option to specifically enable BFD operations only in the 300 strict-mode. In this case, an OSPF adjacency with a neighbor that 301 does not support BFD strict-mode would not be established 302 successfully. Implementations MAY provide an option to disable BFD 303 strict-mode which results in the router not advertising the B-bit and 304 BFD operations being performed in the same way as prior to this 305 specification. 307 The signaling specified in this document happens at a link-local 308 level between routers on that link. A router that does not support 309 this specification would ignore the B-bit in the LLS block of hello 310 messages from its neighbors and continue to establish BFD sessions, 311 if enabled, without delaying the OSPF adjacency formation. Since the 312 router that does not support this specification would not have set 313 the B-bit in the LLS block of its own hello messages, its neighbor 314 routers that support this specification would not use BFD strict-mode 315 with such OSPF routers. As a result, the behavior would be the same 316 as before this specification. Therefore, there are no backward 317 compatibility issues or implementations considerations beyond what is 318 specified herein. 320 7. IANA Considerations 322 This specification updates Link Local Signaling TLV Identifiers 323 registry. 325 Following values are requested for allocation: 327 o B-bit from "LLS Type 1 Extended Options and Flags" registry at bit 328 position 0x00000010. 330 o TBD (Suggested value 21) - Local Interface IPv4 Address TLV 332 8. Security Considerations 334 The security considerations for "OSPF Link-Local Signaling" [RFC5613] 335 also apply to the extension described in this document. 336 Inappropriate use of the B-bit in the LLS block of an OSPF hello 337 message could prevent an OSPF adjacency from forming or lead to 338 failure to detect bidirectional forwarding failures. If 339 authentication is being used in the OSPF routing domain 340 [RFC5709][RFC7474], then the Cryptographic Authentication TLV 341 [RFC5613] SHOULD also be used to protect the contents of the LLS 342 block. 344 9. Acknowledgements 346 The authors would like to acknowledge the review and inputs from Acee 347 Lindem, Manish Gupta and Balaji Ganesh. 349 The authors would like to acknowledge Dylan van Oudheusden for 350 highlighting the problems in using strict-mode for BFD session for 351 IPv4 AF instance with OSPFv3 and Baalajee S for his suggestions on 352 the approach to address it. 354 10. References 356 10.1. Normative References 358 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 359 Requirement Levels", BCP 14, RFC 2119, 360 DOI 10.17487/RFC2119, March 1997, 361 . 363 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 364 DOI 10.17487/RFC2328, April 1998, 365 . 367 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 368 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 369 . 371 [RFC5613] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D. 372 Yeung, "OSPF Link-Local Signaling", RFC 5613, 373 DOI 10.17487/RFC5613, August 2009, 374 . 376 [RFC5838] Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and 377 R. Aggarwal, "Support of Address Families in OSPFv3", 378 RFC 5838, DOI 10.17487/RFC5838, April 2010, 379 . 381 [RFC5882] Katz, D. and D. Ward, "Generic Application of 382 Bidirectional Forwarding Detection (BFD)", RFC 5882, 383 DOI 10.17487/RFC5882, June 2010, 384 . 386 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 387 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 388 May 2017, . 390 10.2. Informative References 392 [RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M., 393 Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic 394 Authentication", RFC 5709, DOI 10.17487/RFC5709, October 395 2009, . 397 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 398 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 399 . 401 [RFC6213] Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV", 402 RFC 6213, DOI 10.17487/RFC6213, April 2011, 403 . 405 [RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed., 406 "Security Extension for OSPFv2 When Using Manual Key 407 Management", RFC 7474, DOI 10.17487/RFC7474, April 2015, 408 . 410 Authors' Addresses 412 Ketan Talaulikar 413 Cisco Systems, Inc. 414 India 416 Email: ketant@cisco.com 418 Peter Psenak 419 Cisco Systems, Inc. 420 Apollo Business Center 421 Mlynske nivy 43 422 Bratislava 821 09 423 Slovakia 425 Email: ppsenak@cisco.com 426 Albert Fu 427 Bloomberg 428 USA 430 Email: afu14@bloomberg.net 432 Rajesh M 433 Juniper Networks 434 India 436 Email: mrajesh@juniper.net