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Chen 9 Huawei 10 June 13, 2017 12 Bidirectional Forwarding Detection (BFD) Directed Return Path 13 draft-ietf-mpls-bfd-directed-07 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 http://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 December 15, 2017. 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 (http://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 . . . . . . . . . . . . . . . 4 64 3.3. Return Codes . . . . . . . . . . . . . . . . . . . . . . 5 65 4. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 5 66 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 67 5.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 5 68 5.2. Return Code . . . . . . . . . . . . . . . . . . . . . . . 6 69 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 70 7. Normative References . . . . . . . . . . . . . . . . . . . . 6 71 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 7 72 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 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 Target FEC sub-TLV 168 (already defined, or to be defined in the future) for TLV Types 1, 169 16, and 21 of MPLS LSP Ping Parameters registry MAY be used in this 170 field. None, one or more sub-TLVs MAY be included in the BFD Reverse 171 Path TLV. If none sub-TLVs found in the BFD Reverse Path TLV, the 172 egress BFD peer MUST revert to using the default, i.e., over IP 173 network, reverse path. 175 If the egress LSR cannot find the path specified in the Reverse Path 176 TLV it MUST send Echo Reply with the received Reverse Path TLV and 177 set the Return Code to "Failed to establish the BFD session. The 178 specified reverse path was not found" Section 3.3. The egress BFD 179 peer MAY establish the BFD session over IP network as defined in 180 [RFC5884]. 182 3.2. Static and RSVP-TE sub-TLVs 184 When an explicit path on an MPLS data plane is set either as Static 185 or RSVP-TE LSP respective sub-TLVs defined in [RFC7110] MAY be used 186 to identify the explicit reverse path for the BFD session. 188 3.3. Return Codes 190 This document defines the following Return Codes for MPLS LSP Echo 191 Reply: 193 o "Failed to establish the BFD session. The specified reverse path 194 was not found", (TBD4). When a specified reverse path is not 195 available at the egress BFD peer, an Echo Reply with the return 196 code set to "Failed to establish the BFD session. The specified 197 reverse path was not found" MUST be sent back to the ingress BFD 198 peer Section 3.1. 200 4. Use Case Scenario 202 In the network presented in Figure 2 node A monitors two tunnels to 203 node H: A-B-C-D-G-H and A-B-E-F-G-H. To bootstrap a BFD session to 204 monitor the first tunnel, node A MUST include a BFD Discriminator TLV 205 with Discriminator value (e.g. foobar-1) and MAY include a BFD 206 Reverse Path TLV that references H-G-D-C-B-A tunnel. To bootstrap a 207 BFD session to monitor the second tunnel, node A MUST include a BFD 208 Discriminator TLV with a different Discriminator value (e.g. foobar- 209 2) [RFC7726] and MAY include a BFD Reverse Path TLV that references 210 H-G-F-E-B-A tunnel. 212 C---------D 213 | | 214 A-------B G-----H 215 | | 216 E---------F 218 Figure 2: Use Case for BFD Reverse Path TLV 220 If an operator needs node H to monitor a path to node A, e.g. 221 H-G-D-C-B-A tunnel, then by looking up list of known Reverse Paths it 222 MAY find and use the existing BFD session. 224 5. IANA Considerations 226 5.1. BFD Reverse Path TLV 228 The IANA is requested to assign a new value for BFD Reverse Path TLV 229 from the "Multiprotocol Label Switching Architecture (MPLS) Label 230 Switched Paths (LSPs) Ping Parameters - TLVs" registry, "TLVs and 231 sub-TLVs" sub-registry. 233 +----------+----------------------+---------------+ 234 | Value | Description | Reference | 235 +----------+----------------------+---------------+ 236 | X (TBD1) | BFD Reverse Path TLV | This document | 237 +----------+----------------------+---------------+ 239 Table 1: New BFD Reverse Type TLV 241 5.2. Return Code 243 The IANA is requested to assign a new Return Code value from the 244 "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) 245 Ping Parameters" registry, "Return Codes" sub-registry, as follows 246 using a Standards Action value. 248 +----------+----------------------------------------+---------------+ 249 | Value | Description | Reference | 250 +----------+----------------------------------------+---------------+ 251 | X (TBD4) | Failed to establish the BFD session. | This document | 252 | | The specified reverse path was not | | 253 | | found. | | 254 +----------+----------------------------------------+---------------+ 256 Table 2: New Return Code 258 6. Security Considerations 260 Security considerations discussed in [RFC5880], [RFC5884], [RFC7726], 261 and [RFC8029], apply to this document. 263 7. Normative References 265 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 266 Requirement Levels", BCP 14, RFC 2119, 267 DOI 10.17487/RFC2119, March 1997, 268 . 270 [RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., 271 "MPLS Generic Associated Channel", RFC 5586, 272 DOI 10.17487/RFC5586, June 2009, 273 . 275 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 276 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 277 . 279 [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 280 (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, 281 DOI 10.17487/RFC5881, June 2010, 282 . 284 [RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 285 (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, 286 June 2010, . 288 [RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, 289 "Bidirectional Forwarding Detection (BFD) for MPLS Label 290 Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884, 291 June 2010, . 293 [RFC7110] Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord, 294 "Return Path Specified Label Switched Path (LSP) Ping", 295 RFC 7110, DOI 10.17487/RFC7110, January 2014, 296 . 298 [RFC7726] Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S. 299 Aldrin, "Clarifying Procedures for Establishing BFD 300 Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726, 301 DOI 10.17487/RFC7726, January 2016, 302 . 304 [RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N., 305 Aldrin, S., and M. Chen, "Detecting Multiprotocol Label 306 Switched (MPLS) Data-Plane Failures", RFC 8029, 307 DOI 10.17487/RFC8029, March 2017, 308 . 310 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 311 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 312 May 2017, . 314 Appendix A. Acknowledgments 316 Authors greatly appreciate thorough review and the most helpful 317 comments from Eric Gray and Carlos Pignataro. 319 Authors' Addresses 321 Greg Mirsky 322 ZTE 324 Email: gregimirsky@gmail.com 325 Jeff Tantsura 326 Individual 328 Email: jefftant.ietf@gmail.com 330 Ilya Varlashkin 331 Google 333 Email: Ilya@nobulus.com 335 Mach(Guoyi) Chen 336 Huawei 338 Email: mach.chen@huawei.com