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Pignataro 7 Cisco Systems 8 May 3, 2019 10 Bidirectional Forwarding Detection (BFD) for Segment Routing Policies 11 for Traffic Engineering 12 draft-ali-spring-bfd-sr-policy-03 14 Abstract 16 Segment Routing (SR) allows a headend node to steer a packet flow 17 along any path using a segment list which is referred to as a SR 18 Policy. Intermediate per-flow states are eliminated thanks to source 19 routing. The header of a packet steered in an SR Policy is augmented 20 with the ordered list of segments associated with that SR Policy. 21 Bidirectional Forwarding Detection (BFD) is used to monitor different 22 kinds of paths between node. BFD mechanisms can be also used to 23 monitor the availability of the path indicated by a SR Policy and to 24 detect any failures. Seamless BFD (S-BFD) extensions provide a 25 simplified mechanism which is suitable for monitoring of paths that 26 are setup dynamically and on a large scale. 28 This document describes the use of Seamless BFD (S-BFD) mechanism to 29 monitor the SR Policies that are used for Traffic Engineering (TE) in 30 SR deployments. 32 Requirements Language 34 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 35 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 36 document are to be interpreted as described in RFC 2119 [RFC2119]. 38 Status of This Memo 40 This Internet-Draft is submitted in full conformance with the 41 provisions of BCP 78 and BCP 79. 43 Internet-Drafts are working documents of the Internet Engineering 44 Task Force (IETF). Note that other groups may also distribute 45 working documents as Internet-Drafts. The list of current Internet- 46 Drafts is at https://datatracker.ietf.org/drafts/current/. 48 Internet-Drafts are draft documents valid for a maximum of six months 49 and may be updated, replaced, or obsoleted by other documents at any 50 time. It is inappropriate to use Internet-Drafts as reference 51 material or to cite them other than as "work in progress." 53 This Internet-Draft will expire on November 4, 2019. 55 Copyright Notice 57 Copyright (c) 2019 IETF Trust and the persons identified as the 58 document authors. All rights reserved. 60 This document is subject to BCP 78 and the IETF Trust's Legal 61 Provisions Relating to IETF Documents 62 (https://trustee.ietf.org/license-info) in effect on the date of 63 publication of this document. Please review these documents 64 carefully, as they describe your rights and restrictions with respect 65 to this document. Code Components extracted from this document must 66 include Simplified BSD License text as described in Section 4.e of 67 the Trust Legal Provisions and are provided without warranty as 68 described in the Simplified BSD License. 70 Table of Contents 72 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 73 2. Choice of S-BFD over BFD . . . . . . . . . . . . . . . . . . 4 74 3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 4 75 3.1. S-BFD Discriminator . . . . . . . . . . . . . . . . . . . 5 76 3.2. S-BFD session Initiation by SBFDInitiator . . . . . . . . 5 77 3.3. Controlled Return Path . . . . . . . . . . . . . . . . . 6 78 3.4. S-BFD Echo Recommendation . . . . . . . . . . . . . . . . 7 79 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 80 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 81 6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8 82 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 83 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 84 8.1. Normative References . . . . . . . . . . . . . . . . . . 8 85 8.2. Informative References . . . . . . . . . . . . . . . . . 9 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 88 1. Introduction 90 Segment Routing (SR) ([RFC8402]) allows a headend node to steer a 91 packet flow along any path for specific objectives like Traffic 92 Engineering (TE) and to provide it treatment according to the 93 specific established service level agreement (SLA) for it. 94 Intermediate per-flow states are eliminated thanks to source routing. 95 The headend node steers a flow into an SR Policy. The header of a 96 packet steered in an SR Policy is augmented with the ordered list of 97 segments associated with that SR Policy. SR Policy 98 [I-D.ietf-spring-segment-routing-policy] specifies the concepts of SR 99 Policy and steering into an SR Policy. 101 SR Policy state is instantiated only on the head-end node and any 102 intermediate node or the endpoint node does not require any state to 103 be maintained or instantiated for it. SR Policies are not signaled 104 through the network nodes except the signaling required to 105 instantiate them on the head-end in the case of a controller based 106 deployment. This enables SR Policies to scale far better than 107 previous TE mechanisms. This also enables SR Policies to be 108 instantiated dynamically and on demand basis for steering specific 109 traffic flows corresponding to service routes as they are signaled. 110 These automatic steering and signaling mechanisms for SR Policies are 111 described in SR Policy [I-D.ietf-spring-segment-routing-policy]. 113 There is a requirement to continuously monitor the availability of 114 the path corresponding to the SR Policy along the nodes in the 115 network to rapidly detect any failures in the forwarding path so that 116 it could take corrective action to restore service. The corrective 117 actions may be either to invalidate the candidate path that has 118 experienced failure and to switch to another candidate path within 119 the same SR Policy OR to activate another backup SR Policy or 120 candidate path for end-to-end path protection. These mechanisms are 121 beyond the scope of this document. 123 Bidirectional Forwarding Detection (BFD) mechanisms have been 124 specified for use for monitoring of unidirectional MPLS LSPs via BFD 125 MPLS [RFC5884]. Seamless BFD [RFC7880] defines a simplified 126 mechanism for using BFD by eliminating the negotiation aspect and the 127 need to maintain per session state entries on the tail end of the 128 policy, thus providing benefits such as quick provisioning, as well 129 as improved control and flexibility for network nodes initiating path 130 monitoring. When BFD or S-BFD is used for verification of such 131 unidirectional LSP paths, the reverse path is via the shortest path 132 from the tail-end router back to the head-end router as determined by 133 routing. 135 The SR Policy is essentially a unidirectional path through the 136 network. This document describes the use of BFD and more 137 specifically S-BFD for monitoring of SR Policy paths through the 138 network. SR can be instantiated using both MPLS and IPv6 dataplanes. 139 The mechanism described in this document applies to both these 140 instantiations of SR Policy. 142 2. Choice of S-BFD over BFD 144 BFD MPLS [RFC5884] describes a mechanism where LSP Ping [RFC8029] is 145 used to bootstrap the BFD session over an MPLS TE LSP path. The LSP 146 Ping mechanism was extended to support SR LSPs via SR LSP Ping 147 [RFC8287] and a similar mechanism could have been considered for BFD 148 monitoring of SR Policies on MPLS data-plane. However, this document 149 proposes instead to use S-BFD mechanism as it is more suitable for SR 150 Policies. 152 Some of the key aspects of SR Policies that are considered in 153 arriving at this decision are as follows: 155 o SR Policies do not require any signaling to be performed through 156 the network nodes in order to be setup. They are simply 157 instantiated on the head-end node via provisioning or even 158 dynamically by a controller via BGP SR-TE 159 [I-D.ietf-idr-segment-routing-te-policy] or using PCEP (PCEP SR 160 [I-D.ietf-pce-segment-routing], PCE Initiated [RFC8281], PCEP 161 Stateful [RFC8231]). 163 o SR Policies result in state being instantiated only on the head- 164 end node and no other node in the network. 166 o In many deployments, SR Policies are instantiated dynamically and 167 on-demand or in the case of automated steering for BGP routes, 168 when routes are learnt with specific color communities (refer SR 169 Policy [I-D.ietf-spring-segment-routing-policy] for details). 171 o SR Policies are expected to be deployed in much higher scale. 173 o SR Policies can be instantiated both for MPLS and IPv6 data-planes 174 and hence a monitoring mechanism which works for both is 175 desirable. 177 In view of the above, the BFD mechanism to be used for monitoring 178 them needs to be simple, lightweight, one that does not result in 179 instantiation of per SR Policy state anywhere but the head-end and 180 which can be setup and deleted dynamically and on-demand. The S-BFD 181 extensions provide this support as described in Seamless BFD 182 [RFC7880]. Furthermore, S-BFD Use-Cases [RFC7882] clarifies the 183 applicability in the Centralized TE and SR scenarios. 185 3. Procedures 187 The general procedures and mechanisms for S-BFD operations are 188 specified in Seamless BFD [RFC7880]. This section describes the 189 specifics related to S-BFD use for SR Policies. 191 SR Policies are represented on a head-end router as tuple. The SRTE process on the head-end determines the 193 tail-end node of a SR Policy on the basis of the endpoint IP address. 194 In the cases where the SR Policy endpoint is outside the domain of 195 the head-end node, this information is available with the centralized 196 controller that computed the multi-domain SR Policy path for the 197 head-end. 199 3.1. S-BFD Discriminator 201 In order to enable S-BFD monitoring for a given SR Policy, the S-BFD 202 Discriminator for the tail-end node (i.e. one with the endpoint IP 203 address) which is going to be the S-BFD Reflector is required. ISIS 204 S-BFD [RFC7883] and OSPF S-BFD [RFC7884] describe the extensions to 205 the ISIS and OSPF link state routing protocols that allow all nodes 206 to advertise their S-BFD Discriminators across the network. BGP-LS 207 S-BFD [I-D.li-idr-bgp-ls-sbfd-extensions] describes extensions for 208 advertising the S-BFD discriminators via BGP-LS across domains and to 209 a controller. Thus, either the SRTE head-end node or the controller, 210 as the case may be, have the S-BFD Discriminator of the tail-end node 211 of the SR Policy available. 213 When the end point IP address configured in the SR policy is IPv4, an 214 implementation may support the use of end point address as the S-BFD 215 Discriminator if SBFDReflector is enabled to associate the end point 216 address as Discriminator for the target identifier. 218 The selection of S-BFD Discriminator from IGP or end point address is 219 a local implementation matter and can be controlled by configuration 220 knob. 222 3.2. S-BFD session Initiation by SBFDInitiator 224 The SRTE Process can straightaway instantiate the S-BFD mechanism on 225 the SR Policy as soon as it is provisioned in the forwarding to start 226 verification of the path to the endpoint. No signaling or 227 provisioning is required for the tail-end node on a per SR Policy 228 basis and it just performs its role as a stateless S-BFD Reflector. 229 The return path used by S-BFD is via the normal IP routing back to 230 the head-end node. Once the specific SR Policy path is verified via 231 S-BFD, then it is considered as active and may be used for traffic 232 steering. 234 The S-BFD monitoring continues for the SR Policy and any failure is 235 notified to the SRTE process. In response to the failure of a 236 specific candidate path, the SRTE process may trigger any of the 237 following based on local policy or implementation specific aspects 238 which are outside the scope of this document: 240 o Trigger path-protection for the SR Policy 242 o Declare the specific candidate path as invalid and switch to using 243 the next valid candidate path based on preference 245 o If no alternate candidate path is available, then handle the 246 steering over that SR Policy based on its invalidation policy 247 (e.g. drop or switch to best effort routing). 249 3.3. Controlled Return Path 251 S-BFD response from SBFDResponder is IP routed and so the procedure 252 defined in the above sections will receive the response through 253 uncontrolled return path. S-BFD echo packets with relevant stack of 254 segment ID can be used to control the return path. 256 +-----B-------C-----+ 257 / \ 258 A-----------E-----------D 259 \ / 260 +-----F-------G-----+ 262 Forward Paths: A-B-C-D 263 IP Return Paths: D-E-A 265 Figure 1: S-BFD Echo Example 267 Node A sending S-BFD control packets with segment stack {B, C, D} 268 will cause S-BFD control packets to traverse the paths A-B-C-D in the 269 forward direction. The response S-BFD control packets from node D 270 back to node A will be IP routed and will traverse the paths D-E-A. 271 The SBFDInitiator sending such packets can also send S-BFD echo 272 packets with segment stack {B, C, D, C, A}. S-BFD echo packets will 273 u-turn on node D and traverse the paths D-C-B-A. If required, the 274 SBFDInitiator can possess multiple types of S-BFD echo packets, with 275 each having varying return paths. In this particular example, the 276 SBFDInitiator can be sending two types of S-BFD echo packets in 277 addition to S-BFD control packets. 279 o S-BFD Control Packets 281 * Segment Stack: {B, C, D} 283 * Return Path: D->E->A 285 o S-BFD Echo packets #1 286 * Segment Stack: {B, C, D, C, A} 288 * Return Path: D->C->B->A 290 o S-BFD Echo packets #2 292 * Segment Stack: {B, C, D, G, A} 294 * Return Path: D->G->F->A 296 The SBFDInitiator can correlate the result of each packet type to 297 determine the nature of the failure. One such example of failure 298 correlation is described in the figure below. 300 +---+-----------------------------------------------------------+ 301 | | S-BFD Echo Pkt | 302 | +------------------------------------+----------------------+ 303 | | Success | Failure | 304 +-+-+------------------------------------+----------------------+ 305 | |S| | | 306 |S|u| | | 307 |||c| |Forward SID stack good| 308 |B|c| All is well |Return SID stack bad | 309 |F|e| |Return IP path good | 310 |D|s| | | 311 | |s| | | 312 |C+-+----------------------+-------------+----------------------+ 313 |t|F|Forward SID stack good| | | 314 |r|a|Return SID stack good |Send Alert | | 315 |l|i|Return IP path bad |Discrim S-BFD| | 316 | |l+--------- OR ---------+w/ Forward |Forward SID stack bad | 317 |P|u|Forward SID stack is |SID stack to | | 318 |k|r|terminating on wrong |differentiate| | 319 |t|e|node | | | 320 +-+-+----------------------+-------------+----------------------+ 322 Figure 2: SBFDInitiator Failure Correlation Example 324 3.4. S-BFD Echo Recommendation 326 o It is RECOMMENDED to compute and use smallest number of segment 327 stack to describe the return path of S-BFD echo packets to prevent 328 the segment stack being too large. How SBFDInitiator determines 329 when to use S-BFD echo packets and how to identify corresponding 330 segment stack for the return paths are outside the scope of this 331 document. 333 o It is RECOMMENDED that SBFDInitiator does not send only S-BFD echo 334 packets. S-BFD echo packets are crafted to traverse the network 335 and to come back to self, thus there is no guarantee that S-BFD 336 echo are u-turning on the intended remote target. On the other 337 hand, S-BFD control packets can verify that segment stack of the 338 forward direction reaches the intended remote target. Therefore, 339 an SBFDInitiator SHOULD send S-BFD control packets when sending 340 S-BFD echo packets. 342 4. IANA Considerations 344 None 346 5. Security Considerations 348 Procedures described in this document do not affect the BFD or 349 Segment Routing security model. See the 'Security Considerations' 350 section of [RFC7880] for a discussion of S-BFD security and to 351 [RFC8402] for analysis of security in SR deployments. 353 6. Contributors 355 Mallik Mudigonda 356 Cisco Systems Inc. 358 Email: mmudigon@cisco.com 360 7. Acknowledgements 362 8. References 364 8.1. Normative References 366 [I-D.ietf-spring-segment-routing-policy] 367 Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d., 368 bogdanov@google.com, b., and P. Mattes, "Segment Routing 369 Policy Architecture", draft-ietf-spring-segment-routing- 370 policy-02 (work in progress), October 2018. 372 [I-D.li-idr-bgp-ls-sbfd-extensions] 373 Li, Z., Aldrin, S., Tantsura, J., Mirsky, G., Zhuang, S., 374 and K. Talaulikar, "BGP Link-State Extensions for Seamless 375 BFD", draft-li-idr-bgp-ls-sbfd-extensions-03 (work in 376 progress), February 2019. 378 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 379 Requirement Levels", BCP 14, RFC 2119, 380 DOI 10.17487/RFC2119, March 1997, 381 . 383 [RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S. 384 Pallagatti, "Seamless Bidirectional Forwarding Detection 385 (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016, 386 . 388 [RFC7882] Aldrin, S., Pignataro, C., Mirsky, G., and N. Kumar, 389 "Seamless Bidirectional Forwarding Detection (S-BFD) Use 390 Cases", RFC 7882, DOI 10.17487/RFC7882, July 2016, 391 . 393 [RFC7883] Ginsberg, L., Akiya, N., and M. Chen, "Advertising 394 Seamless Bidirectional Forwarding Detection (S-BFD) 395 Discriminators in IS-IS", RFC 7883, DOI 10.17487/RFC7883, 396 July 2016, . 398 [RFC7884] Pignataro, C., Bhatia, M., Aldrin, S., and T. Ranganath, 399 "OSPF Extensions to Advertise Seamless Bidirectional 400 Forwarding Detection (S-BFD) Target Discriminators", 401 RFC 7884, DOI 10.17487/RFC7884, July 2016, 402 . 404 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 405 Decraene, B., Litkowski, S., and R. Shakir, "Segment 406 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 407 July 2018, . 409 8.2. Informative References 411 [I-D.ietf-idr-segment-routing-te-policy] 412 Previdi, S., Filsfils, C., Jain, D., Mattes, P., Rosen, 413 E., and S. Lin, "Advertising Segment Routing Policies in 414 BGP", draft-ietf-idr-segment-routing-te-policy-05 (work in 415 progress), November 2018. 417 [I-D.ietf-pce-segment-routing] 418 Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 419 and J. Hardwick, "PCEP Extensions for Segment Routing", 420 draft-ietf-pce-segment-routing-16 (work in progress), 421 March 2019. 423 [RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, 424 "Bidirectional Forwarding Detection (BFD) for MPLS Label 425 Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884, 426 June 2010, . 428 [RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N., 429 Aldrin, S., and M. Chen, "Detecting Multiprotocol Label 430 Switched (MPLS) Data-Plane Failures", RFC 8029, 431 DOI 10.17487/RFC8029, March 2017, 432 . 434 [RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path 435 Computation Element Communication Protocol (PCEP) 436 Extensions for Stateful PCE", RFC 8231, 437 DOI 10.17487/RFC8231, September 2017, 438 . 440 [RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path 441 Computation Element Communication Protocol (PCEP) 442 Extensions for PCE-Initiated LSP Setup in a Stateful PCE 443 Model", RFC 8281, DOI 10.17487/RFC8281, December 2017, 444 . 446 [RFC8287] Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya, 447 N., Kini, S., and M. Chen, "Label Switched Path (LSP) 448 Ping/Traceroute for Segment Routing (SR) IGP-Prefix and 449 IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data 450 Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017, 451 . 453 Authors' Addresses 455 Zafar Ali 456 Cisco Systems 458 Email: zali@cisco.com 460 Ketan Talaulikar 461 Cisco Systems 463 Email: ketant@cisco.com 465 Clarence Filsfils 466 Cisco Systems 468 Email: cfilsfil@cisco.com 469 Nagendra Kumar Nainar 470 Cisco Systems 472 Email: naikumar@cisco.com 474 Carlos Pignataro 475 Cisco Systems 477 Email: cpignata@cisco.com