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Chen 9 Huawei 10 February 16, 2021 12 Bidirectional Forwarding Detection (BFD) Directed Return Path for MPLS 13 Label Switched Paths (LSPs) 14 draft-ietf-mpls-bfd-directed-17 16 Abstract 18 Bidirectional Forwarding Detection (BFD) is expected to be able to 19 monitor a wide variety of encapsulations of paths between systems. 20 When a BFD session monitors an explicitly routed unidirectional path 21 there may be a need to direct egress BFD peer to use a specific path 22 for the reverse direction of the BFD session. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at https://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on August 20, 2021. 41 Copyright Notice 43 Copyright (c) 2021 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (https://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 1.1. Conventions used in this document . . . . . . . . . . . . 3 60 1.1.1. Requirements Language . . . . . . . . . . . . . . . . 3 61 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 62 3. Control of the Reverse BFD Path . . . . . . . . . . . . . . . 3 63 3.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 3 64 3.2. Return Codes . . . . . . . . . . . . . . . . . . . . . . 5 65 4. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 5 66 5. Operational Considerations . . . . . . . . . . . . . . . . . 6 67 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 68 6.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 6 69 6.2. Return Code . . . . . . . . . . . . . . . . . . . . . . . 7 70 7. Implementation Status . . . . . . . . . . . . . . . . . . . . 7 71 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 72 9. Normative References . . . . . . . . . . . . . . . . . . . . 8 73 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 9 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 76 1. Introduction 78 [RFC5880], [RFC5881], and [RFC5883] established the BFD protocol for 79 IP networks. [RFC5884] and [RFC7726] set rules for using BFD 80 asynchronous mode over IP/MPLS LSPs, while not defining means to 81 control the path an egress BFD system uses to send BFD control 82 packets towards the ingress BFD system. 84 For the case when BFD is used to detect defects of the traffic 85 engineered LSP the path the BFD control packets transmitted by the 86 egress BFD system toward the ingress may be disjoint from the LSP in 87 the forward direction. The fact that BFD control packets are not 88 guaranteed to follow the same links and nodes in both forward and 89 reverse directions may be one of the factors contributing to 90 producing false positive defect notifications, i.e., false alarms, at 91 the ingress BFD peer. Ensuring that both directions of the BFD 92 session use co-routed paths may, in some environments, improve the 93 determinism of the failure detection and localization. 95 This document defines the BFD Reverse Path TLV as an extension to LSP 96 Ping [RFC8029] and proposes that it is to be used to instruct the 97 egress BFD system to use an explicit path for its BFD control packets 98 associated with a particular BFD session. The TLV will be allocated 99 from the TLV and sub-TLV registry defined in [RFC8029]. As a special 100 case, forward and reverse directions of the BFD session can form a 101 bi-directional co-routed associated channel. 103 1.1. Conventions used in this document 105 1.1.1. Requirements Language 107 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 108 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 109 "OPTIONAL" in this document are to be interpreted as described in BCP 110 14 [RFC2119] [RFC8174] when, and only when, they appear in all 111 capitals, as shown here. 113 2. Problem Statement 115 When BFD is used to monitor explicitly routed unidirectional path, 116 e.g., MPLS-TE LSP, BFD control packets in forward direction would be 117 in-band using the mechanism defined in [RFC5884]. But the reverse 118 direction of the BFD session would follow the shortest path route and 119 that might lead to the problem in detecting failures on an explicit 120 unidirectional path, as described below: 122 o detection by an ingress node of a failure on the reverse path may 123 not be unambiguously interpreted as the failure of the path in the 124 forward direction. 126 To address this scenario, the egress BFD peer would be instructed to 127 use a specific path for BFD control packets. 129 3. Control of the Reverse BFD Path 131 To bootstrap a BFD session over an MPLS LSP, LSP ping, defined in 132 [RFC8029], MUST be used with BFD Discriminator TLV [RFC5884]. This 133 document defines a new TLV, BFD Reverse Path TLV, that MAY contain 134 none, one or more sub-TLVs that can be used to carry information 135 about the reverse path for the BFD session that is specified by the 136 value in BFD Discriminator TLV. 138 3.1. BFD Reverse Path TLV 140 The BFD Reverse Path TLV is an optional TLV within the LSP ping 141 [RFC8029]. However, if used, the BFD Discriminator TLV MUST be 142 included in an Echo Request message as well. If the BFD 143 Discriminator TLV is not present when the BFD Reverse Path TLV is 144 included; then it MUST be treated as malformed Echo Request, as 145 described in [RFC8029]. 147 The BFD Reverse Path TLV carries information about the path onto 148 which the egress BFD peer of the BFD session referenced by the BFD 149 Discriminator TLV MUST transmit BFD control packets. The format of 150 the BFD Reverse Path TLV is as presented in Figure 1. 152 0 1 2 3 153 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 1 154 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 155 | BFD Reverse Path TLV Type | Length | 156 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 157 | Reverse Path | 158 ~ ~ 159 | | 160 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 162 Figure 1: BFD Reverse Path TLV 164 BFD Reverse Path TLV Type is two octets in length and has a value of 165 TBD1 (to be assigned by IANA as requested in Section 6). 167 Length field is two octets long and defines the length in octets of 168 the Reverse Path field. 170 Reverse Path field contains none, one or more sub-TLVs. Any non- 171 multicast Target FEC Stack sub-TLV (already defined, or to be defined 172 in the future) for TLV Types 1, 16, and 21 of MPLS LSP Ping 173 Parameters registry MAY be used in this field. Multicast Target FEC 174 Stack sub-TLVs, i.e., p2mp and mp2mp, SHOULD NOT be included in 175 Reverse Path field. If the egress LSR finds multicast Target Stack 176 sub-TLV, it MUST send echo reply with the received Reverse Path TLV, 177 BFD Discriminator TLV and set the Return Code to "Inappropriate 178 Target FEC Stack sub-TLV present" Section 3.2. None, one or more 179 sub-TLVs MAY be included in the BFD Reverse Path TLV. If no sub-TLVs 180 are found in the BFD Reverse Path TLV, the egress BFD peer MUST 181 revert to using the local policy-based decision as described in 182 Section 7 [RFC5884], i.e., routed over IP network. 184 If the egress LSR cannot find the path specified in the Reverse Path 185 TLV it MUST send Echo Reply with the received BFD Discriminator TLV, 186 Reverse Path TLV and set the Return Code to "Failed to establish the 187 BFD session. The specified reverse path was not found" Section 3.2. 188 An implementation MAY provide configuration options to define action 189 at the egress BFD peer. For example, if the egress LSR cannot find 190 the path specified in the Reverse Path TLV, it MAY establish the BFD 191 session over an IP network, as defined in [RFC5884]. 193 The BFD Reverse Path TLV MAY be used in the bootstrapping of a BFD 194 session process described in Section 6 [RFC5884]. A system that 195 supports this specification MUST support using the BFD Reverse Path 196 TLV after the BFD session has been established. If a system that 197 supports this specification receives an LSP Ping with the BFD 198 Discriminator TLV and no BFD Reverse Path TLV even though the reverse 199 path for the specified BFD session has been established according to 200 the previously received BFD Reverse Path TLV, the egress LSR MUST 201 transition to transmitting periodic BFD Control messages as defined 202 in Section 7 [RFC5884]. 204 3.2. Return Codes 206 This document defines the following Return Codes for MPLS LSP Echo 207 Reply: 209 o "Inappropriate Target FEC Stack sub-TLV present", (TBD3). When 210 multicast Target FEC Stack sub-TLV found in the received Echo 211 Request by the egress BFD peer, an Echo Reply with the return code 212 set to "Inappropriate Target FEC Stack sub-TLV present" MUST be 213 sent to the ingress BFD peer Section 3.1. 215 o "Failed to establish the BFD session. The specified reverse path 216 was not found", (TBD4). When a specified reverse path is not 217 available at the egress BFD peer, an Echo Reply with the return 218 code set to "Failed to establish the BFD session. The specified 219 reverse path was not found" MUST be sent back to the ingress BFD 220 peer Section 3.1. 222 4. Use Case Scenario 224 In the network presented in Figure 2 node A monitors two tunnels to 225 node H: A-B-C-D-G-H and A-B-E-F-G-H. To bootstrap a BFD session to 226 monitor the first tunnel, node A MUST include a BFD Discriminator TLV 227 with Discriminator value (e.g., foobar-1) and MAY include a BFD 228 Reverse Path TLV that references H-G-D-C-B-A tunnel. To bootstrap a 229 BFD session to monitor the second tunnel, node A MUST include a BFD 230 Discriminator TLV with a different Discriminator value (e.g., foobar- 231 2) [RFC7726] and MAY include a BFD Reverse Path TLV that references 232 H-G-F-E-B-A tunnel. 234 C---------D 235 | | 236 A-------B G-----H 237 | | 238 E---------F 240 Figure 2: Use Case for BFD Reverse Path TLV 242 If an operator needs node H to monitor a path to node A, e.g. 243 H-G-D-C-B-A tunnel, then by looking up the list of known Reverse 244 Paths it MAY find and use the existing BFD session. 246 5. Operational Considerations 248 When an explicit path is set either as Static or RSVP-TE LSP, 249 corresponding sub-TLVs, defined in [RFC7110], MAY be used to identify 250 the explicit reverse path for the BFD session. If any of defined in 251 [RFC7110] sub-TLVs used in BFD Reverse Path TLV, then the periodic 252 verification of the control plane against the data plane, as 253 recommended in Section 4 [RFC5884], MUST use the Return Path TLV, as 254 per [RFC7110], with that sub-TLV. By using the LSP Ping with Return 255 Path TLV, an operator monitors whether at the egress BFD node the 256 reverse LSP is mapped to the same FEC as the BFD session. Selection 257 and control of the rate of LSP Ping with Return Path TLV follows the 258 recommendation of [RFC5884]: "The rate of generation of these LSP 259 Ping Echo request messages SHOULD be significantly less than the rate 260 of generation of the BFD Control packets. An implementation MAY 261 provide configuration options to control the rate of generation of 262 the periodic LSP Ping Echo request messages." 264 Suppose an operator planned network maintenance activity that 265 possibly affects FEC used in the BFD Reverse Path TLV. In that case, 266 the operator MUST avoid the unnecessary disruption using the LSP Ping 267 with a new FEC in the BFD Reverse Path TLV. But in some scenarios, 268 proactive measures cannot be taken. Because the frequency of LSP 269 Ping messages will be lower than the defect detection time provided 270 by the BFD session. As a result, a change in the reverse-path FEC 271 will first be detected as the BFD session's failure. In such a case, 272 the ingress BFD node SHOULD immediately transmit the LSP Ping Echo 273 request with Return Path TLV to verify whether the FEC is still 274 valid. If the failure was caused by the change in the FEC used for 275 the reverse direction of the BFD session, the ingress BFD node SHOULD 276 bootstrap a new BFD session using another FEC in BFD Reverse Path 277 TLV. 279 6. IANA Considerations 281 6.1. BFD Reverse Path TLV 283 The IANA is requested to assign a new value for BFD Reverse Path TLV 284 from the "Multiprotocol Label Switching Architecture (MPLS) Label 285 Switched Paths (LSPs) Ping Parameters - TLVs" registry, "TLVs and 286 sub-TLVs" sub-registry. 288 +--------+----------------------+---------------+ 289 | Value | Description | Reference | 290 +--------+----------------------+---------------+ 291 | (TBD1) | BFD Reverse Path TLV | This document | 292 +--------+----------------------+---------------+ 294 Table 1: New BFD Reverse Type TLV 296 6.2. Return Code 298 The IANA is requested to assign a new Return Code value from the 299 "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) 300 Ping Parameters" registry, "Return Codes" sub-registry, as follows 301 using a Standards Action value. 303 +--------+----------------------------------------------+-----------+ 304 | Value | Description | Reference | 305 +--------+----------------------------------------------+-----------+ 306 | (TBD3) | Inappropriate Target FEC Stack sub-TLV | This docu | 307 | | present. | ment | 308 | (TBD4) | Failed to establish the BFD session. The | This docu | 309 | | specified reverse path was not found. | ment | 310 +--------+----------------------------------------------+-----------+ 312 Table 2: New Return Code 314 7. Implementation Status 316 - The organization responsible for the implementation: ZTE 317 Corporation. 319 - The implementation's name ROSng empowers traditional routers, e.g., 320 ZXCTN 6000. 322 - A brief general description: A Return Path can be specified for a 323 BFD session over RSVP tunnel or LSP. The same can be specified for a 324 backup RSVP tunnel/LSP. 326 The implementation's level of maturity: production. 328 - Coverage: RSVP LSP (no support for Static LSP) 330 - Version compatibility: draft-ietf-mpls-bfd-directed-10. 332 - Licensing: proprietary. 334 - Implementation experience: simple once you support RFC 7110. 336 - Contact information: Qian Xin qian.xin2@zte.com.cn 338 - The date when information about this particular implementation was 339 last updated: 12/16/2019 341 Note to RFC Editor: This section MUST be removed before publication 342 of the document. 344 8. Security Considerations 346 Security considerations discussed in [RFC5880], [RFC5884], [RFC7726], 347 [RFC8029], and [RFC7110] apply to this document. 349 9. Normative References 351 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 352 Requirement Levels", BCP 14, RFC 2119, 353 DOI 10.17487/RFC2119, March 1997, 354 . 356 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 357 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 358 . 360 [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 361 (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, 362 DOI 10.17487/RFC5881, June 2010, 363 . 365 [RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 366 (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, 367 June 2010, . 369 [RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, 370 "Bidirectional Forwarding Detection (BFD) for MPLS Label 371 Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884, 372 June 2010, . 374 [RFC7110] Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord, 375 "Return Path Specified Label Switched Path (LSP) Ping", 376 RFC 7110, DOI 10.17487/RFC7110, January 2014, 377 . 379 [RFC7726] Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S. 380 Aldrin, "Clarifying Procedures for Establishing BFD 381 Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726, 382 DOI 10.17487/RFC7726, January 2016, 383 . 385 [RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N., 386 Aldrin, S., and M. Chen, "Detecting Multiprotocol Label 387 Switched (MPLS) Data-Plane Failures", RFC 8029, 388 DOI 10.17487/RFC8029, March 2017, 389 . 391 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 392 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 393 May 2017, . 395 Appendix A. Acknowledgments 397 The authors greatly appreciate a thorough review and the most helpful 398 comments from Eric Gray and Carlos Pignataro. The authors much 399 appreciate the help of Qian Xin, who provided information about the 400 implementation of this specification. 402 Authors' Addresses 404 Greg Mirsky 405 ZTE 407 Email: gregimirsky@gmail.com 409 Jeff Tantsura 410 Juniper Networks 412 Email: jefftant.ietf@gmail.com 414 Ilya Varlashkin 415 Google 417 Email: Ilya@nobulus.com 419 Mach(Guoyi) Chen 420 Huawei 422 Email: mach.chen@huawei.com