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'3') == Outdated reference: A later version (-11) exists of draft-ietf-rtgwg-remote-lfa-02 ** Downref: Normative reference to an Historic draft: draft-bryant-ipfrr-tunnels (ref. '5') Summary: 6 errors (**), 0 flaws (~~), 6 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Pierre Francois 3 Internet-Draft Institute IMDEA Networks 4 Intended status: Standards Track Clarence Filsfils 5 Expires: May 22, 2014 Ahmed Bashandy 6 Cisco Systems, Inc. 7 Bruno Decraene 8 Stephane Litkowski 9 Orange 10 November 18, 2013 12 Topology Independent Fast Reroute using Segment Routing 13 draft-francois-segment-routing-ti-lfa-00 15 Abstract 17 This document presents a Fast Reroute (FRR) approach aimed at 18 providing link and node protection of node and adjacency segments 19 within the Segment Routing (SR) framework. This FRR behavior builds 20 on proven IP-FRR concepts being LFAs, remote LFAs (RLFA), and remote 21 LFAs with directed forwarding (DLFA). It extends these concepts to 22 provide guaranteed coverage in any IGP network. We accommodate the 23 FRR discovery and selection approaches in order to establish 24 protection over post-convergence paths from the point of local 25 repair, dramatically reducing the operator's need to control the tie- 26 breaks among various FRR options. 28 Status of this Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on May 22, 2014. 45 Copyright Notice 47 Copyright (c) 2013 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 3. Intersecting P-Space and Q-Space with post-convergence 65 paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 66 3.1. P-Space property computation for a resource X . . . . . . . 5 67 3.2. Q-Space property computation for a link S-F, over 68 post-convergence paths . . . . . . . . . . . . . . . . . . 5 69 3.3. Q-Space property computation for a node F, over 70 post-convergence paths . . . . . . . . . . . . . . . . . . 6 71 4. EPC Repair Tunnel . . . . . . . . . . . . . . . . . . . . . . . 6 72 4.1. The repair node is a direct neighbor . . . . . . . . . . . 6 73 4.2. The repair node is a PQ node . . . . . . . . . . . . . . . 6 74 4.3. The repair is a Q node, neighbor of the last P node . . . . 7 75 4.4. Connecting distant P and Q nodes along 76 post-convergence paths . . . . . . . . . . . . . . . . . . 7 77 5. Protecting segments . . . . . . . . . . . . . . . . . . . . . . 7 78 5.1. The active segment is a node segment . . . . . . . . . . . 7 79 5.2. The active segment is an adjacency segment . . . . . . . . 7 80 5.2.1. Protecting [Adjacency, Adjacency] segment lists . . . . 8 81 5.2.2. Protecting [Adjacency, Node] segment lists . . . . . . 8 82 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 83 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 85 1. Introduction 87 Segment Routing aims at supporting services with tight SLA guarantees 88 [1]. This document provides local repair mechanisms using SR, 89 capable of restoring end-to-end connectivity in the case of a sudden 90 failure of a link or a node, with guaranteed coverage properties. 92 Using segment routing, there is no need to establish TLDP sessions 93 with remote nodes in order to take advantage of the applicability of 94 remote LFAs (RLFA) or remote LFAs with directed forwarding (DLFA) 95 [2]. As a result, preferring LFAs over RLFAs or DLFAs, as well as 96 minimizing the number of RLFA or DLFA repair nodes is not required. 97 Using SR, there is no need to create state in the network in order to 98 enforce an explicit FRR path. As a result, we can use optimized 99 detour paths for each specific destination and for each possible 100 failure in the network without creating additional forwarding state. 102 Building on such an easier forwarding environment, the FRR behavior 103 suggested in this document tailors the repair paths over the post- 104 convergence path from the PLR to the protected destination. 106 As the capacity of the post-convergence path is typically planned by 107 the operator to support the post-convergence routing of the traffic 108 for any expected failure, there is much less need for the operator to 109 tune the decision among which protection path to choose. The 110 protection path will automatically follow the natural backup path 111 that would be used after local convergence. This also helps to 112 reduce the amount of path changes and hence service transients: one 113 transition (pre-convergence to post-convergence) instead of two (pre- 114 convergence to FRR and then post-convergence). 116 We provide an EPC-FRR approach that achieves guaranteed coverage 117 against link or node failure, in any IGP network, relying on the 118 flexibility of SR. 120 L ____ 121 S----------F--{____}--D 122 _|_ ___________ / 123 {___}--Q--{___________} 125 Figure 1: EPC Protection 127 We use Figure 1 to illustrate the EPC-FRR approach. 129 The Point of Local Repair (PLR), S, needs to find a node Q (a repair 130 node) that is capable of safely forwarding the traffic to a 131 destination D affected by the failure of the protected link L, or 132 node F. The PLR also needs to find a way to reach Q without being 133 affected by the convergence state of the nodes over the paths it 134 wants to use to reach Q. 136 In Section 2 we define the main notations used in the document. They 137 are in line with [2]. 139 In Section 3, we suggest to compute the P-Space and Q-Space 140 properties defined in Section 2, for the specific case of nodes lying 141 over the post-convergence paths towards the protected destinations. 142 The failure of a link S-F as well as the failure of a neighbor F is 143 discussed. 145 Using the properties defined in Section 3, we describe how to compute 146 protection lists that encode a loopfree post-convergence towards the 147 destination, in Section 4. 149 Finally, we define the segment operations to be applied by the PLR to 150 ensure consistency with the forwarding state of the repair node, in 151 Section 5. 153 2. Terminology 155 We define the main notations used in this document as the following. 157 We refer to "old" and "new" topologies as the LSDB state before and 158 after the considered failure. 160 SPT_old(R) is the Shortest Path Tree rooted at node R in the initial 161 state of the network. 163 SPT_new(R, X) is the Shortest Path Tree rooted at node R in the state 164 of the network after the resource X has failed. 166 Dist_old(A,B) is the distance from node A to node B in SPT_old(A). 168 Dist_new(A,B, X) is the distance from node A to node B in SPT_new(A, 169 X). 171 The P-Space P(R,X) of a node R w.r.t. a resource X (e.g. a link S-F, 172 or a node F) is the set of nodes that are reachable from R without 173 passing through X. It is the set of nodes that are not downstream of 174 X in SPT_old(R). 176 The Extended P-Space P'(R,X) of a node R w.r.t. a resource X is the 177 set of nodes that are reachable from R or a neighbor of R, without 178 passing through X. 180 The Q-Space Q(D,X) of a destination node D w.r.t. a resource X is the 181 set of nodes which do not use X to reach D in the initial state of 182 the network. In other words, it is the set of nodes which have D in 183 their P-Space w.r.t. S-F (or F). 185 A symmetric network is a network such that the IGP metric of each 186 link is the same in both directions of the link. 188 3. Intersecting P-Space and Q-Space with post-convergence paths 190 In this section, we suggest to determine the P-Space and Q-Space 191 properties of the nodes along on the post-convergence paths from the 192 PLR to the protected destination and compute an SR-based explicit 193 path from P to Q when they are not adjacent. Such properties will be 194 used in Section 4 to compute the EPC-FRR repair list. 196 3.1. P-Space property computation for a resource X 198 A node N is in P(R, X) if it is not downstream of X in SPT_old(R). 200 A node N is in P'(R,X) if it is not downstream of X in SPT_old(N), 201 for at least one neighbor N of R. 203 3.2. Q-Space property computation for a link S-F, over post-convergence 204 paths 206 We want to determine which nodes on the post-convergence from the PLR 207 to the destination D are in the Q-Space of destination D w.r.t. link 208 S-F. 210 This can be found by intersecting the post-convergence path to D, 211 assuming the failure of S-F, with Q(D, S-F). 213 The post-convergence path to D requires to compute SPT_new(S, S-F). 215 A node N is in Q(D,S-F) if it is not downstream of S-F in 216 rSPT_old(D). 218 3.3. Q-Space property computation for a node F, over post-convergence 219 paths 221 We want to determine which nodes on the post-convergence from the PLR 222 to the destination D are in the Q-Space of destination D w.r.t. node 223 F. 225 This can be found by intersecting the post-convergence path to D, 226 assuming the failure of F with Q(D, F). 228 The post-convergence path to D requires to compute SPT_new(S, F). 230 A node N is in Q(D,F) if it is not downstream of F in rSPT_old(D). 232 4. EPC Repair Tunnel 234 The EPC repair tunnel consists of an outgoing interface and a list of 235 segments (repair list) to insert on the SR header. The repair list 236 encodes the explicit post-convergence path to the destination, which 237 avoids the protected resource X. 239 The EPC repair tunnel is found by intersecting P(S,X) and Q(D,X) with 240 the post-convergence path to D and computing the explicit SR-based 241 path EP(P, Q) from P to Q when these nodes are not adjacent along the 242 post convergence path. The EPC repair list is expressed generally as 243 (Node_SID(P), EP(P, Q)). 245 Most often, the EPC repair list has a simpler form, as described in 246 the following sections. 248 4.1. The repair node is a direct neighbor 250 When the repair node is a direct neighbor, the outgoing interface is 251 set to that neighbor and the repair segment list is empty. 253 This is comparable to an LFA FRR repair. 255 4.2. The repair node is a PQ node 257 When the repair node is in P(S,X), the repair list is made of a 258 single node segment to the repair node. 260 This is comparable to an RLFA repair tunnel. 262 4.3. The repair is a Q node, neighbor of the last P node 264 When the repair node is adjacent to P(S,X), the repair list is made 265 of two segments: A node segment to the adjacent P node, and an 266 adjacency segment from that node to the repair node. 268 This is comparable to a DLFA repair tunnel. 270 4.4. Connecting distant P and Q nodes along post-convergence paths 272 In some cases, there is no adjacent P and Q node along the post- 273 convergence path. However, the PLR can perform additional 274 computations to compute a list of segments that represent a loopfree 275 path from P to Q. 277 5. Protecting segments 279 In this section, we explain how a protecting router S processes the 280 active segment of a packet upon the failure of its primary outgoing 281 interface. 283 The behavior depends on the type of active segment to be protected. 285 5.1. The active segment is a node segment 287 The active segment is kept on the SR header, unchanged (1). The 288 repair list is inserted at the head of the list. The active segment 289 becomes the first segment of the inserted repair list. 291 A future version of the document will describe the FRR behavior when 292 the active segment is a node segment destined to F, and F has failed. 294 Note (1): If the SRGB at the repair node is different from the SRGB 295 at the PLR, then the active segment must be updated to fit the SRGB 296 of the repair node. 298 5.2. The active segment is an adjacency segment 300 We define hereafter the FRR behavior applied by S for any packet 301 received with an active adjacency segment S-F for which protection 302 was enabled. We distinguish the case where this active segment is 303 followed by another adjacency segment from the case where it is 304 followed by a node segment. 306 5.2.1. Protecting [Adjacency, Adjacency] segment lists 308 If the next segment in the list is an Adjacency segment, then the 309 packet has to be conveyed to F. 311 To do so, S applies a "NEXT" operation on Adj(S-F) and then two 312 consecutive "PUSH" operations: first it pushes a node segment for F, 313 and then it pushes a protection list allowing to reach F while 314 bypassing S-F. 316 Upon failure of S-F, a packet reaching S with a segment list matching 317 [adj(S-F),adj(M),...] will thus leave S with a segment list matching 318 [RT(F),node(F),adj(M)], where RT(F) is the repair tunnel for 319 destination F. 321 5.2.2. Protecting [Adjacency, Node] segment lists 323 If the next segment in the stack is a node segment, say for node T, 324 the packet segment list matches [adj(S-F),node(T),...]. 326 A first solution would consist in steering the packet back to F while 327 avoiding S-F, similarly to the previous case. To do so, S applies a 328 "NEXT" operation on Adj(S-F) and then two consecutive "PUSH" 329 operations: first it pushes a node segment for F, and then it pushes 330 a repair list allowing to reach F while bypassing S-F. 332 Upon failure of S-F, a packet reaching S with a segment list matching 333 [adj(S-F),node(T),...] will thus leave S with a segment list matching 334 [RT(F),node(F),node(T)]. 336 Another solution is to not steer the packet back via F but rather 337 follow the new shortest path to T. In this case, S just needs to 338 apply a "NEXT" operation on the Adjacency segment related to S-F, and 339 push a repair list redirecting the traffic to a node Q, whose path to 340 node segment T is not affected by the failure. 342 Upon failure of S-F, packets reaching S with a segment list matching 343 [adj(L), node(T), ...], would leave S with a segment list matching 344 [RT(Q),node(T), ...]. 346 6. References 348 [1] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., 349 Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, 350 S., Henderickx, W., Tantsura, J., and E. Crabbe, "Segment 351 Routing Architecture", draft-filsfils-rtgwg-segment-routing-00 352 (work in progress), June 2013. 354 [2] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, 355 January 2010. 357 [3] Filsfils, C., Francois, P., Shand, M., Decraene, B., Uttaro, J., 358 Leymann, N., and M. Horneffer, "Loop-Free Alternate (LFA) 359 Applicability in Service Provider (SP) Networks", RFC 6571, 360 June 2012. 362 [4] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S. Ning, 363 "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-02 (work in 364 progress), May 2013. 366 [5] Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP Fast 367 Reroute using tunnels", draft-bryant-ipfrr-tunnels-03 (work in 368 progress), November 2007. 370 Authors' Addresses 372 Pierre Francois 373 Institute IMDEA Networks 374 Leganes 375 ES 377 Email: pierre.francois@imdea.org 379 Clarence Filsfils 380 Cisco Systems, Inc. 381 Brussels 382 BE 384 Email: cfilsfil@cisco.com 386 Ahmed Bashandy 387 Cisco Systems, Inc. 388 San Jose 389 US 391 Email: bashandy@cisco.com 392 Bruno Decraene 393 Orange 394 Issy-les-Moulineaux 395 FR 397 Email: bruno.decraene@orange.com 399 Stephane Litkowski 400 Orange 401 FR 403 Email: bruno.decraene@orange.com