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Chen 9 Huawei 10 December 5, 2017 12 Bidirectional Forwarding Detection (BFD) Directed Return Path 13 draft-ietf-mpls-bfd-directed-08 15 Abstract 17 Bidirectional Forwarding Detection (BFD) is expected to be able to 18 monitor wide variety of encapsulations of paths between systems. 19 When a BFD session monitors an explicitly routed unidirectional path 20 there may be a need to direct egress BFD peer to use a specific path 21 for the reverse direction of the BFD session. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on June 8, 2018. 40 Copyright Notice 42 Copyright (c) 2017 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (https://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 1.1. Conventions used in this document . . . . . . . . . . . . 3 59 1.1.1. Requirements Language . . . . . . . . . . . . . . . . 3 60 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 61 3. Control of the Reverse BFD Path . . . . . . . . . . . . . . . 3 62 3.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 3 63 3.2. Static and RSVP-TE sub-TLVs . . . . . . . . . . . . . . . 5 64 3.3. Return Codes . . . . . . . . . . . . . . . . . . . . . . 5 65 4. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 5 66 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 67 5.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 6 68 5.2. Return Code . . . . . . . . . . . . . . . . . . . . . . . 6 69 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 70 7. Normative References . . . . . . . . . . . . . . . . . . . . 6 71 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 8 72 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 74 1. Introduction 76 [RFC5880], [RFC5881], and [RFC5883] established the BFD protocol for 77 IP networks. [RFC5884] and [RFC7726] set rules of using BFD 78 asynchronous mode over IP/MPLS LSPs. These standards implicitly 79 assume that the egress BFD peer will use the shortest path route 80 regardless of route being used to send BFD control packets towards 81 it. 83 For the case where a LSP is explicitly routed it is likely that the 84 shortest return path to the ingress BFD peer would not follow the 85 same path as the LSP in the forward direction. The fact that BFD 86 control packets are not guaranteed to follow the same links and nodes 87 in both forward and reverse directions is a significant factor in 88 producing false positive defect notifications, i.e. false alarms, if 89 used by the ingress BFD peer to deduce the state of the forward 90 direction. 92 This document defines the BFD Reverse Path TLV as an extension to LSP 93 Ping [RFC8029] and proposes that it is to be used to instruct the 94 egress BFD peer to use an explicit path for its BFD control packets 95 associated with a particular BFD session. The TLV will be allocated 96 from the TLV and sub-TLV registry defined in [RFC8029]. As a special 97 case, forward and reverse directions of the BFD session can form a 98 bi-directional co-routed associated channel. 100 1.1. Conventions used in this document 102 1.1.1. Requirements Language 104 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 105 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 106 "OPTIONAL" in this document are to be interpreted as described in BCP 107 14 [RFC2119] [RFC8174] when, and only when, they appear in all 108 capitals, as shown here. 110 2. Problem Statement 112 When BFD is used to monitor unidirectional explicitly routed path, 113 e.g. MPLS-TE LSP, BFD control packets in forward direction would be 114 in-band using the mechanism defined in [RFC5884] and [RFC5586]. But 115 the reverse direction of the BFD session would follow the shortest 116 path route and that might lead to the problem in detecting failures 117 on a unidirectional explicit path as described below: 119 o a failure detection by ingress node on the reverse path cannot be 120 interpreted as bi-directional failure unambiguously and thus 121 trigger, for example, protection switchover of the forward 122 direction without possibility of being a false positive. 124 To address this scenario the egress BFD peer would be instructed to 125 use a specific path for BFD control packets. 127 3. Control of the Reverse BFD Path 129 LSP ping, defined in [RFC8029], uses BFD Discriminator TLV [RFC5884] 130 to bootstrap a BFD session over an MPLS LSP. This document defines a 131 new TLV, BFD Reverse Path TLV, that MUST contain a single sub-TLV 132 that can be used to carry information about the reverse path for the 133 BFD session that is specified by value in BFD Discriminator TLV. 135 3.1. BFD Reverse Path TLV 137 The BFD Reverse Path TLV is an optional TLV within the LSP ping 138 [RFC8029]. However, if used, the BFD Discriminator TLV MUST be 139 included in an Echo Request message as well. If the BFD 140 Discriminator TLV is not present when the BFD Reverse Path TLV is 141 included, then it MUST be treated as malformed Echo Request, as 142 described in [RFC8029]. 144 The BFD Reverse Path TLV carries information about the path onto 145 which the egress BFD peer of the BFD session referenced by the BFD 146 Discriminator TLV MUST transmit BFD control packets. The format of 147 the BFD Reverse Path TLV is as presented in Figure 1. 149 0 1 2 3 150 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 1 151 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 152 | BFD Reverse Path TLV Type | Length | 153 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 154 | Reverse Path | 155 ~ ~ 156 | | 157 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 159 Figure 1: BFD Reverse Path TLV 161 BFD Reverse Path TLV Type is 2 octets in length and has a value of 162 TBD1 (to be assigned by IANA as requested in Section 5). 164 Length field is 2 octets long and defines the length in octets of the 165 Reverse Path field. 167 Reverse Path field contains a sub-TLV. Any non-multicast Target FEC 168 Stack sub-TLV (already defined, or to be defined in the future) for 169 TLV Types 1, 16, and 21 of MPLS LSP Ping Parameters registry MAY be 170 used in this field. Multicast Target FEC Stack sub-TLVs, i.e. p2mp 171 and mp2mp, SHOULD NOT be included into Reverse Path field. If the 172 egress LSR finds multicast Target Stack sub-TLV it MUST send echo 173 reply with the received Reverse Path TLV, BFD Discriminator TLV and 174 set the Return Code to "Inappropriate Target FEC Stack sub-TLV 175 present" Section 3.3. None, one or more sub-TLVs MAY be included in 176 the BFD Reverse Path TLV. If none sub-TLVs found in the BFD Reverse 177 Path TLV, the egress BFD peer MUST revert to using the local policy 178 based decision as described in Section 7 [RFC5884], i.e., routed over 179 IP network. 181 If the egress LSR cannot find the path specified in the Reverse Path 182 TLV it MUST send Echo Reply with the received BFD Discriminator TLV, 183 Reverse Path TLV and set the Return Code to "Failed to establish the 184 BFD session. The specified reverse path was not found" Section 3.3. 185 The egress BFD peer MAY establish the BFD session over IP network as 186 defined in [RFC5884]. 188 3.2. Static and RSVP-TE sub-TLVs 190 When an explicit path on an MPLS data plane is set either as Static 191 or RSVP-TE LSP respective sub-TLVs defined in [RFC7110] MAY be used 192 to identify the explicit reverse path for the BFD session. 194 3.3. Return Codes 196 This document defines the following Return Codes for MPLS LSP Echo 197 Reply: 199 o "Inappropriate Target FEC Stack sub-TLV present", (TBD3). When 200 multicast Target FEC Stack sub-TLV found in the received Echo 201 Request by the egress BFD peer, an Echo Reply with the return code 202 set to "Inappropriate Target FEC Stack sub-TLV present" MUST be 203 sent to the ingress BFD peer Section 3.1. 205 o "Failed to establish the BFD session. The specified reverse path 206 was not found", (TBD4). When a specified reverse path is not 207 available at the egress BFD peer, an Echo Reply with the return 208 code set to "Failed to establish the BFD session. The specified 209 reverse path was not found" MUST be sent back to the ingress BFD 210 peer Section 3.1. 212 4. Use Case Scenario 214 In the network presented in Figure 2 node A monitors two tunnels to 215 node H: A-B-C-D-G-H and A-B-E-F-G-H. To bootstrap a BFD session to 216 monitor the first tunnel, node A MUST include a BFD Discriminator TLV 217 with Discriminator value (e.g. foobar-1) and MAY include a BFD 218 Reverse Path TLV that references H-G-D-C-B-A tunnel. To bootstrap a 219 BFD session to monitor the second tunnel, node A MUST include a BFD 220 Discriminator TLV with a different Discriminator value (e.g. foobar- 221 2) [RFC7726] and MAY include a BFD Reverse Path TLV that references 222 H-G-F-E-B-A tunnel. 224 C---------D 225 | | 226 A-------B G-----H 227 | | 228 E---------F 230 Figure 2: Use Case for BFD Reverse Path TLV 232 If an operator needs node H to monitor a path to node A, e.g. 233 H-G-D-C-B-A tunnel, then by looking up list of known Reverse Paths it 234 MAY find and use the existing BFD session. 236 5. IANA Considerations 238 5.1. BFD Reverse Path TLV 240 The IANA is requested to assign a new value for BFD Reverse Path TLV 241 from the "Multiprotocol Label Switching Architecture (MPLS) Label 242 Switched Paths (LSPs) Ping Parameters - TLVs" registry, "TLVs and 243 sub-TLVs" sub-registry. 245 +--------+----------------------+---------------+ 246 | Value | Description | Reference | 247 +--------+----------------------+---------------+ 248 | (TBD1) | BFD Reverse Path TLV | This document | 249 +--------+----------------------+---------------+ 251 Table 1: New BFD Reverse Type TLV 253 5.2. Return Code 255 The IANA is requested to assign a new Return Code value from the 256 "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) 257 Ping Parameters" registry, "Return Codes" sub-registry, as follows 258 using a Standards Action value. 260 +--------+------------------------------------------+---------------+ 261 | Value | Description | Reference | 262 +--------+------------------------------------------+---------------+ 263 | (TBD3) | Inappropriate Target FEC Stack sub-TLV | This document | 264 | | present. | | 265 | (TBD4) | Failed to establish the BFD session. The | This document | 266 | | specified reverse path was not found. | | 267 +--------+------------------------------------------+---------------+ 269 Table 2: New Return Code 271 6. Security Considerations 273 Security considerations discussed in [RFC5880], [RFC5884], [RFC7726], 274 and [RFC8029], apply to this document. 276 7. Normative References 278 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 279 Requirement Levels", BCP 14, RFC 2119, 280 DOI 10.17487/RFC2119, March 1997, 281 . 283 [RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., 284 "MPLS Generic Associated Channel", RFC 5586, 285 DOI 10.17487/RFC5586, June 2009, 286 . 288 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 289 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 290 . 292 [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 293 (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, 294 DOI 10.17487/RFC5881, June 2010, 295 . 297 [RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 298 (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, 299 June 2010, . 301 [RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, 302 "Bidirectional Forwarding Detection (BFD) for MPLS Label 303 Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884, 304 June 2010, . 306 [RFC7110] Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord, 307 "Return Path Specified Label Switched Path (LSP) Ping", 308 RFC 7110, DOI 10.17487/RFC7110, January 2014, 309 . 311 [RFC7726] Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S. 312 Aldrin, "Clarifying Procedures for Establishing BFD 313 Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726, 314 DOI 10.17487/RFC7726, January 2016, 315 . 317 [RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N., 318 Aldrin, S., and M. Chen, "Detecting Multiprotocol Label 319 Switched (MPLS) Data-Plane Failures", RFC 8029, 320 DOI 10.17487/RFC8029, March 2017, 321 . 323 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 324 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 325 May 2017, . 327 Appendix A. Acknowledgments 329 Authors greatly appreciate thorough review and the most helpful 330 comments from Eric Gray and Carlos Pignataro. 332 Authors' Addresses 334 Greg Mirsky 335 ZTE 337 Email: gregimirsky@gmail.com 339 Jeff Tantsura 340 Individual 342 Email: jefftant.ietf@gmail.com 344 Ilya Varlashkin 345 Google 347 Email: Ilya@nobulus.com 349 Mach(Guoyi) Chen 350 Huawei 352 Email: mach.chen@huawei.com