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Chen 9 Huawei 10 April 24, 2017 12 Bidirectional Forwarding Detection (BFD) Directed Return Path 13 draft-ietf-mpls-bfd-directed-06 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 October 26, 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. Direct Reverse BFD Path . . . . . . . . . . . . . . . . . . . 3 62 3.1. Case of MPLS Data Plane . . . . . . . . . . . . . . . . . 3 63 3.1.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . 3 64 3.1.2. Static and RSVP-TE sub-TLVs . . . . . . . . . . . . . 4 65 3.2. Return Codes . . . . . . . . . . . . . . . . . . . . . . 5 66 4. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 5 67 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 68 5.1. TLV . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 69 5.2. Return Codes . . . . . . . . . . . . . . . . . . . . . . 6 70 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 71 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 72 8. Normative References . . . . . . . . . . . . . . . . . . . . 6 73 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 75 1. Introduction 77 RFC 5880 [RFC5880], RFC 5881 [RFC5881], and RFC 5883 [RFC5883] 78 established the BFD protocol for IP networks. RFC 5884 [RFC5884] and 79 RFC 7726 [RFC7726] set rules of using BFD asynchronous mode over IP/ 80 MPLS LSPs. These standards implicitly assume that the egress BFD 81 peer will use the shortest path route regardless of route being used 82 to send BFD control packets towards it. 84 For the case where a LSP is explicitly routed it is likely that the 85 shortest return path to the ingress BFD peer would not follow the 86 same path as the LSP in the forward direction. The fact that BFD 87 control packets are not guaranteed to follow the same links and nodes 88 in both forward and reverse directions is a significant factor in 89 producing false positive defect notifications, i.e. false alarms, if 90 used by the ingress BFD peer to deduce the state of the forward 91 direction. 93 This document defines the BFD Reverse Path TLV as an extension to LSP 94 Ping [RFC8029] and proposes that it is to be used to instruct the 95 egress BFD peer to use an explicit path for its BFD control packets 96 associated with a particular BFD session. The TLV will be allocated 97 from the TLV and sub-TLV registry defined by RFC 8029 [RFC8029]. As 98 a special case, forward and reverse directions of the BFD session can 99 form a bi-directional co-routed associated channel. 101 1.1. Conventions used in this document 103 1.1.1. Requirements Language 105 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 106 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 107 "OPTIONAL" in this document are to be interpreted as described in 108 [RFC2119]. 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. Direct Reverse BFD Path 129 3.1. Case of MPLS Data Plane 131 LSP ping, defined in [RFC8029], uses BFD Discriminator TLV [RFC5884] 132 to bootstrap a BFD session over an MPLS LSP. This document defines a 133 new TLV, BFD Reverse Path TLV, that MUST contain a single sub-TLV 134 that can be used to carry information about the reverse path for the 135 BFD session that is specified by value in BFD Discriminator TLV. 137 3.1.1. BFD Reverse Path TLV 139 The BFD Reverse Path TLV is an optional TLV within the LSP ping 140 [RFC8029]. However, if used, the BFD Discriminator TLV MUST be 141 included in an Echo Request message as well. If the BFD 142 Discriminator TLV is not present when the BFD Reverse Path TLV is 143 included, then it MUST be treated as malformed Echo Request, as 144 described in [RFC8029]. 146 The BFD Reverse Path TLV carries information about the path onto 147 which the egress BFD peer of the BFD session referenced by the BFD 148 Discriminator TLV MUST transmit BFD control packets. The format of 149 the BFD Reverse Path TLV is as presented in Figure 1. 151 0 1 2 3 152 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 1 153 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 154 | BFD Reverse Path TLV Type | Length | 155 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 156 | Reverse Path | 157 ~ ~ 158 | | 159 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 161 Figure 1: BFD Reverse Path TLV 163 BFD Reverse Path TLV Type is 2 octets in length and has a value of 164 TBD1 (to be assigned by IANA as requested in Section 5). 166 Length field is 2 octets long and defines the length in octets of the 167 Reverse Path field. 169 Reverse Path field contains a sub-TLV. Any Target FEC sub-TLV 170 (already defined, or to be defined in the future) for TLV Types 1, 171 16, and 21 of MPLS LSP Ping Parameters registry MAY be used in this 172 field. Exactly one sub-TLV MUST be included in the Reverse Path TLV. 173 If more than one sub-TLV is present in the Reverse Path TLV, then, in 174 order to avoid ambiguity of which of TLVs to use, the egress BFD peer 175 MUST send Echo Reply with the received Reverse Path TLVs and set the 176 Return Code to "Too Many TLVs Detected" Section 3.2. 178 If the egress LSR cannot find the path specified in the Reverse Path 179 TLV it MUST send Echo Reply with the received Reverse Path TLV and 180 set the Return Code to "Failed to establish the BFD session. The 181 specified reverse path was not found" Section 3.2. The egress BFD 182 peer MAY establish the BFD session over IP network as defined in 183 [RFC5884]. 185 3.1.2. Static and RSVP-TE sub-TLVs 187 When an explicit path on an MPLS data plane is set either as Static 188 or RSVP-TE LSP respective sub-TLVs defined in [RFC7110] MAY be used 189 to identify the explicit reverse path for the BFD session. 191 3.2. Return Codes 193 This document defines the following Return Codes for MPLS LSP Echo 194 Reply: 196 o "Too Many TLVs Detected", (TBD3). When more than one Reverse Path 197 TLV found in the received Echo Request by the egress BFD peer, an 198 Echo Reply with the return code set to "Too Many TLVs Detected" 199 MUST be sent to the ingress BFD peer Section 3.1.1. 201 o "Failed to establish the BFD session. The specified reverse path 202 was not found", (TBD4). When a specified reverse path is not 203 available at the egress BFD peer, an Echo Reply with the return 204 code set to "Failed to establish the BFD session. The specified 205 reverse path was not found" MUST be sent back to the ingress BFD 206 peer Section 3.1.1. 208 4. Use Case Scenario 210 In the network presented in Figure 2 node A monitors two tunnels to 211 node H: A-B-C-D-G-H and A-B-E-F-G-H. To bootstrap a BFD session to 212 monitor the first tunnel, node A MUST include a BFD Discriminator TLV 213 with Discriminator value (e.g. foobar-1) and MAY include a BFD 214 Reverse Path TLV that references H-G-D-C-B-A tunnel. To bootstrap a 215 BFD session to monitor the second tunnel, node A MUST include a BFD 216 Discriminator TLV with a different Discriminator value (e.g. foobar- 217 2) [RFC7726] and MAY include a BFD Reverse Path TLV that references 218 H-G-F-E-B-A tunnel. 220 C---------D 221 | | 222 A-------B G-----H 223 | | 224 E---------F 226 Figure 2: Use Case for BFD Reverse Path TLV 228 If an operator needs node H to monitor a path to node A, e.g. 229 H-G-D-C-B-A tunnel, then by looking up list of known Reverse Paths it 230 MAY find and use the existing BFD session. 232 5. IANA Considerations 234 5.1. TLV 236 The IANA is requested to assign a new value for BFD Reverse Path TLV 237 from the "Multiprotocol Label Switching Architecture (MPLS) Label 238 Switched Paths (LSPs) Ping Parameters - TLVs" registry, "TLVs and 239 sub-TLVs" sub-registry. 241 +----------+----------------------+---------------+ 242 | Value | Description | Reference | 243 +----------+----------------------+---------------+ 244 | X (TBD1) | BFD Reverse Path TLV | This document | 245 +----------+----------------------+---------------+ 247 Table 1: New BFD Reverse Type TLV 249 5.2. Return Codes 251 The IANA is requested to assign a new Return Code value from the 252 "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) 253 Ping Parameters" registry, "Return Codes" sub-registry, as follows 254 using a Standards Action value. 256 +----------+----------------------------------------+---------------+ 257 | Value | Description | Reference | 258 +----------+----------------------------------------+---------------+ 259 | X (TBD3) | Too Many TLVs Detected. | This document | 260 | X (TBD4) | Failed to establish the BFD session. | This document | 261 | | The specified reverse path was not | | 262 | | found. | | 263 +----------+----------------------------------------+---------------+ 265 Table 2: New Return Code 267 6. Security Considerations 269 Security considerations discussed in [RFC5880], [RFC5884], [RFC7726], 270 and [RFC8029], apply to this document. 272 7. Acknowledgments 274 Authors greatly appreciate thorough review and the most helpful 275 comments from Eric Gray and Carlos Pignataro. 277 8. Normative References 279 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 280 Requirement Levels", BCP 14, RFC 2119, 281 DOI 10.17487/RFC2119, March 1997, 282 . 284 [RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., 285 "MPLS Generic Associated Channel", RFC 5586, 286 DOI 10.17487/RFC5586, June 2009, 287 . 289 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 290 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 291 . 293 [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 294 (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, 295 DOI 10.17487/RFC5881, June 2010, 296 . 298 [RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 299 (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, 300 June 2010, . 302 [RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, 303 "Bidirectional Forwarding Detection (BFD) for MPLS Label 304 Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884, 305 June 2010, . 307 [RFC7110] Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord, 308 "Return Path Specified Label Switched Path (LSP) Ping", 309 RFC 7110, DOI 10.17487/RFC7110, January 2014, 310 . 312 [RFC7726] Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S. 313 Aldrin, "Clarifying Procedures for Establishing BFD 314 Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726, 315 DOI 10.17487/RFC7726, January 2016, 316 . 318 [RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N., 319 Aldrin, S., and M. Chen, "Detecting Multiprotocol Label 320 Switched (MPLS) Data-Plane Failures", RFC 8029, 321 DOI 10.17487/RFC8029, March 2017, 322 . 324 Authors' Addresses 326 Greg Mirsky 327 ZTE 329 Email: gregimirsky@gmail.com 330 Jeff Tantsura 331 Individual 333 Email: jefftant.ietf@gmail.com 335 Ilya Varlashkin 336 Google 338 Email: Ilya@nobulus.com 340 Mach(Guoyi) Chen 341 Huawei 343 Email: mach.chen@huawei.com