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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Filsfils, Ed. 3 Internet-Draft S. Previdi, Ed. 4 Intended status: Standards Track A. Bashandy 5 Expires: November 3, 2017 Cisco Systems, Inc. 6 B. Decraene 7 S. Litkowski 8 Orange 9 May 2, 2017 11 Segment Routing interworking with LDP 12 draft-ietf-spring-segment-routing-ldp-interop-07 14 Abstract 16 A Segment Routing (SR) node steers a packet through a controlled set 17 of instructions, called segments, by prepending the packet with an SR 18 header. A segment can represent any instruction, topological or 19 service-based. SR allows to enforce a flow through any topological 20 path and service chain while maintaining per-flow state only at the 21 ingress node to the SR domain. 23 The Segment Routing architecture can be directly applied to the MPLS 24 data plane with no change in the forwarding plane. This drafts 25 describes how Segment Routing operates in a network where LDP is 26 deployed and in the case where SR-capable and non-SR-capable nodes 27 coexist. 29 Requirements Language 31 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 32 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 33 document are to be interpreted as described in RFC 2119 [RFC2119]. 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at http://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on November 3, 2017. 51 Copyright Notice 53 Copyright (c) 2017 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents 58 (http://trustee.ietf.org/license-info) in effect on the date of 59 publication of this document. Please review these documents 60 carefully, as they describe your rights and restrictions with respect 61 to this document. Code Components extracted from this document must 62 include Simplified BSD License text as described in Section 4.e of 63 the Trust Legal Provisions and are provided without warranty as 64 described in the Simplified BSD License. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 69 2. SR/LDP Ship-in-the-night coexistence . . . . . . . . . . . . 3 70 2.1. MPLS2MPLS co-existence . . . . . . . . . . . . . . . . . 5 71 2.2. IP2MPLS co-existence . . . . . . . . . . . . . . . . . . 6 72 3. Migration from LDP to SR . . . . . . . . . . . . . . . . . . 6 73 4. SR and LDP Interworking . . . . . . . . . . . . . . . . . . . 7 74 4.1. LDP to SR . . . . . . . . . . . . . . . . . . . . . . . . 8 75 4.1.1. LDP to SR Behavior . . . . . . . . . . . . . . . . . 8 76 4.2. SR to LDP . . . . . . . . . . . . . . . . . . . . . . . . 8 77 4.2.1. SR to LDP Behavior . . . . . . . . . . . . . . . . . 10 78 5. SR/LDP Interworking Use Cases . . . . . . . . . . . . . . . . 10 79 5.1. SR Protection of LDP-based Traffic . . . . . . . . . . . 10 80 5.2. Eliminating Targeted LDP Session . . . . . . . . . . . . 12 81 5.3. Guaranteed FRR coverage . . . . . . . . . . . . . . . . . 13 82 5.4. Inter-AS Option C, Carrier's Carrier . . . . . . . . . . 15 83 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 84 7. Manageability Considerations . . . . . . . . . . . . . . . . 15 85 7.1. SR and LDP co-existence . . . . . . . . . . . . . . . . . 15 86 7.2. SRMS Management . . . . . . . . . . . . . . . . . . . . . 16 87 7.3. Dataplane Verification . . . . . . . . . . . . . . . . . 16 88 8. Security Considerations . . . . . . . . . . . . . . . . . . . 16 89 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 90 10. Contributors' Addresses . . . . . . . . . . . . . . . . . . . 17 91 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 92 11.1. Normative References . . . . . . . . . . . . . . . . . . 17 93 11.2. Informative References . . . . . . . . . . . . . . . . . 18 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 96 1. Introduction 98 Segment Routing, as described in [I-D.ietf-spring-segment-routing], 99 can be used on top of the MPLS data plane without any modification as 100 described in [I-D.ietf-spring-segment-routing-mpls]. 102 Segment Routing control plane can co-exist with current label 103 distribution protocols such as LDP ([RFC5036]). 105 This draft outlines the mechanisms through which SR interworks with 106 LDP in cases where a mix of SR-capable and non-SR-capable routers co- 107 exist within the same network and more precisely in the same routing 108 domain. 110 Section 2 describes the co-existence of SR with other MPLS Control 111 Plane. Section 3 documents a method to migrate from LDP to SR-based 112 MPLS tunneling. Section 4 documents the interworking between SR and 113 LDP in the case of non-homogeneous deployment. Section 5 describes 114 how a partial SR deployment can be used to provide SR benefits to 115 LDP-based traffic including a possible application of SR in the 116 context of inter-domain MPLS use-cases. 118 Typically, an implementation will allow an operator to select 119 (through configuration) which of the described modes of SR and LDP 120 co-existence to use. 122 2. SR/LDP Ship-in-the-night coexistence 124 We call "MPLS Control Plane Client (MCC)" any control plane protocol 125 installing forwarding entries in the MPLS data plane. SR, LDP, RSVP- 126 TE, BGP 3107, VPNv4, etc are examples of MCCs. 128 An MCC, operating at node N, MUST ensure that the incoming label it 129 installs in the MPLS data plane of Node N has been uniquely allocated 130 to himself. 132 Segment Routing makes use of the Segment Routing Global Block (SRGB, 133 as defined in [I-D.ietf-spring-segment-routing]) for the label 134 allocation. The use of the SRGB allows SR to co-exist with any other 135 MCC. 137 This is clearly the case for the adjacency segment: it is a local 138 label allocated by the label manager, as for any MCC. 140 This is clearly the case for the prefix segment: the label manager 141 allocates the SRGB set of labels to the SR MCC client and the 142 operator ensures the unique allocation of each global prefix segment/ 143 label within the allocated SRGB set. 145 Note that this static label allocation capability of the label 146 manager exists for many years across several vendors and hence is not 147 new. Furthermore, note that the label-manager ability to statically 148 allocate a range of labels to a specific application is not new 149 either. This is required for MPLS-TP operation. In this case, the 150 range is reserved by the label manager and it is the MPLS-TP 151 ([RFC5960]) NMS (acting as an MCC) that ensures the unique allocation 152 of any label within the allocated range and the creation of the 153 related MPLS forwarding entry. 155 Let us illustrate an example of ship-in-the-night (SIN) coexistence. 157 PE2 PE4 158 \ / 159 PE1----A----B---C---PE3 161 Figure 1: SIN coexistence 163 The EVEN VPN service is supported by PE2 and PE4 while the ODD VPN 164 service is supported by PE1 and PE3. The operator wants to tunnel 165 the ODD service via LDP and the EVEN service via SR. 167 This can be achieved in the following manner: 169 The operator configures PE1, PE2, PE3, PE4 with respective 170 loopbacks 192.0.2.201/32, 192.0.2.202/32, 192.0.2.203/32, 171 192.0.2.204/32. These PE's advertised their VPN routes with next- 172 hop set on their respective loopback address. 174 The operator configures A, B, C with respective loopbacks 175 192.0.2.1/32, 192.0.2.2/32, 192.0.2.3/32. 177 The operator configures PE2, A, B, C and PE4 with SRGB [100, 300]. 179 The operator attaches the respective Node Segment Identifiers 180 (Node-SID's, as defined in [I-D.ietf-spring-segment-routing]): 181 202, 101, 102, 103 and 204 to the loopbacks of nodes PE2, A, B, C 182 and PE4. The Node-SID's are configured to request penultimate- 183 hop-popping. 185 PE1, A, B, C and PE3 are LDP capable. 187 PE1 and PE3 are not SR capable. 189 PE3 sends an ODD VPN route to PE1 with next-hop 192.0.2.203 and VPN 190 label 10001. 192 From an LDP viewpoint: PE1 received an LDP label binding (1037) for 193 FEC 192.0.2.203/32 from its next-hop A. A received an LDP label 194 binding (2048) for that FEC from its next-hop B. B received an LDP 195 label binding (3059) for that FEC from its next-hop C. C received 196 implicit-null LDP binding from its next-hop PE3. 198 As a result, PE1 sends its traffic to the ODD service route 199 advertised by PE3 to next-hop A with two labels: the top label is 200 1037 and the bottom label is 10001. Node A swaps 1037 with 2048 and 201 forwards to B. B swaps 2048 with 3059 and forwards to C. C pops 202 3059 and forwards to PE3. 204 PE4 sends an EVEN VPN route to PE2 with next-hop 192.0.2.204 and VPN 205 label 10002. 207 From an SR viewpoint: PE2 maps the IGP route 192.0.2.204/32 onto 208 Node-SID 204; node A swaps 204 with 204 and forwards to B; B swaps 209 204 with 204 and forwards to C; C pops 204 and forwards to PE4. 211 As a result, PE2 sends its traffic to the VPN service route 212 advertised by PE4 to next-hop A with two labels: the top label is 204 213 and the bottom label is 10002. Node A swaps 204 with 204 and 214 forwards to B. B swaps 204 with 204 and forwards to C. C pops 204 215 and forwards to PE4. 217 The two modes of MPLS tunneling co-exist. 219 The ODD service is tunneled from PE1 to PE3 through a continuous 220 LDP LSP traversing A, B and C. 222 The EVEN service is tunneled from PE2 to PE4 through a continuous 223 SR node segment traversing A, B and C. 225 2.1. MPLS2MPLS co-existence 227 We want to highlight that several MPLS2MPLS entries can be installed 228 in the data plane for the same prefix. 230 Let us examine A's MPLS forwarding table as an example: 232 Incoming label: 1037 234 - outgoing label: 2048 235 - outgoing next-hop: B 236 Note: this entry is programmed by LDP for 192.0.2.203/32 238 Incoming label: 203 239 - outgoing label: 203 240 - outgoing next-hop: B 241 Note: this entry is programmed by SR for 192.0.2.203/32 243 These two entries can co-exist because their incoming label is 244 unique. The uniqueness is guaranteed by the label manager allocation 245 rules. 247 The same applies for the MPLS2IP forwarding entries. 249 2.2. IP2MPLS co-existence 251 By default, if both LDP and SR propose an IP to MPLS entry (IP2MPLS) 252 for the same IP prefix, then the LDP route SHOULD be selected. 254 A local policy on a router MUST allow to prefer the SR-provided 255 IP2MPLS entry. 257 Note that this policy MAY be locally defined. There is no 258 requirement that all routers use the same policy. 260 3. Migration from LDP to SR 262 PE2 PE4 263 \ / 264 PE1----P5--P6--P7---PE3 266 Figure 2: Migration 268 Several migration techniques are possible. We describe one technique 269 inspired by the commonly used method to migrate from one IGP to 270 another. 272 At time T0, all the routers run LDP. Any service is tunneled from an 273 ingress PE to an egress PE over a continuous LDP LSP. 275 At time T1, all the routers are upgraded to SR. They are configured 276 with the SRGB range [100, 300]. PE1, PE2, PE3, PE4, P5, P6 and P7 277 are respectively configured with the node segments 101, 102, 103, 278 104, 105, 106 and 107 (attached to their service-recursing loopback). 280 At this time, the service traffic is still tunneled over LDP LSP. 281 For example, PE1 has an SR node segment to PE3 and an LDP LSP to 282 PE3 but by default, as seen earlier, the LDP IP2MPLS encapsulation 283 is preferred. However, it has to be noted that the SR 284 infrastructure is usable, e.g. for Fast Reroute (FRR) or IGP Loop 285 Free Convergence to protect existing IP and LDP traffic. FRR 286 mechanisms are described in 287 [I-D.francois-rtgwg-segment-routing-ti-lfa]. 289 At time T2, the operator enables the local policy at PE1 to prefer SR 290 IP2MPLS encapsulation over LDP IP2MPLS. 292 The service from PE1 to any other PE is now riding over SR. All 293 other service traffic is still transported over LDP LSP. 295 At time T3, gradually, the operator enables the preference for SR 296 IP2MPLS encapsulation across all the edge routers. 298 All the service traffic is now transported over SR. LDP is still 299 operational and services could be reverted to LDP. 301 However, any traffic switched through LDP entries will still 302 suffer from LDP-IGP synchronization. 304 At time T4, LDP is unconfigured from all routers. 306 4. SR and LDP Interworking 308 In this section, we analyze the case where SR is available in one 309 part of the network and LDP is available in another part. We 310 describe how a continuous MPLS tunnel can be built throughout the 311 network. 313 PE2 PE4 314 \ / 315 PE1----P5--P6--P7--P8---PE3 317 Figure 3: SR and LDP Interworking 319 Let us analyze the following example: 321 P6, P7, P8, PE4 and PE3 are LDP capable. 323 PE1, PE2, P5 and P6 are SR capable. PE1, PE2, P5 and P6 are 324 configured with SRGB (100, 200) and respectively with node 325 segments 101, 102, 105 and 106. 327 A service flow must be tunneled from PE1 to PE3 over a continuous 328 MPLS tunnel encapsulation. We need SR and LDP to interwork. 330 If the SR/LDP node operates in LDP ordered label distribution control 331 mode (as defined in [RFC5036]), then the SR/LDP node MUST consider SR 332 learned labels as if they were learned through an LDP neighbor and 333 create LDP bindings for each Prefix-SID and Node-SID learned in the 334 SR domain. 336 4.1. LDP to SR 338 In this section, we analyze a right-to-left traffic flow. 340 PE3 has learned a service route whose next-hop is PE1. PE3 has an 341 LDP label binding from the next-hop P8 for the FEC "PE1". Hence PE3 342 sends its service packet to P8 as per classic LDP behavior. 344 P8 has an LDP label binding from its next-hop P7 for the FEC "PE1" 345 and hence P8 forwards to P7 as per classic LDP behavior. 347 P7 has an LDP label binding from its next-hop P6 for the FEC "PE1" 348 and hence P7 forwards to P6 as per classic LDP behavior. 350 P6 does not have an LDP binding from its next-hop P5 for the FEC 351 "PE1". However P6 has an SR node segment to the IGP route "PE1". 352 Hence, P6 forwards the packet to P5 and swaps its local LDP-label for 353 FEC "PE1" by the equivalent node segment (i.e. 101). 355 P5 pops 101 (assuming PE1 advertised its node segment 101 with the 356 penultimate-pop flag set) and forwards to PE1. 358 PE1 receives the tunneled packet and processes the service label. 360 The end-to-end MPLS tunnel is built from an LDP LSP from PE3 to P6 361 and the related node segment from P6 to PE1. 363 4.1.1. LDP to SR Behavior 365 It has to be noted that no additional signaling or state is required 366 in order to provide interworking in the direction LDP to SR. 368 A SR node having LDP neighbors MUST create LDP bindings for each 369 Prefix-SID and Node-SID learned in the SR domain and, for each FEC, 370 stitch the incoming LDP label to the outgoing SR label. This has to 371 be done in both LDP independent and ordered label distribution 372 control modes as defined in [RFC5036]. 374 4.2. SR to LDP 376 In this section, we analyze the left-to-right traffic flow. 378 We assume that the operator configures P5 to act as a Segment Routing 379 Mapping Server (SRMS) and advertises the following mappings: (P7, 380 107), (P8, 108), (PE3, 103) and (PE4, 104). 382 These mappings are advertised as Remote-Binding SID as described in 383 [I-D.ietf-isis-segment-routing-extensions]. 385 The mappings advertised by one or more SR mapping servers result from 386 local policy information configured by the operator. 388 If PE3 had been SR capable, the operator would have configured PE3 389 with node segment 103. Instead, as PE3 is not SR capable, the 390 operator configures that policy at the SRMS and it is the latter 391 which advertises the mapping. 393 The mapping server advertisements are only understood by the SR 394 capable routers. The SR capable routers install the related node 395 segments in the MPLS data plane exactly like if the node segments had 396 been advertised by the nodes themselves. 398 For example, PE1 installs the node segment 103 with next-hop P5 399 exactly as if PE3 had advertised node segment 103. 401 PE1 has a service route whose next-hop is PE3. PE1 has a node 402 segment for that IGP route: 103 with next-hop P5. Hence PE1 sends 403 its service packet to P5 with two labels: the bottom label is the 404 service label and the top label is 103. 406 P5 swaps 103 for 103 and forwards to P6. 408 P6's next-hop for the IGP route "PE3" is not SR capable (P7 does not 409 advertise the SR capability). However, P6 has an LDP label binding 410 from that next-hop for the same FEC (e.g. LDP label 1037). Hence, 411 P6 swaps 103 for 1037 and forwards to P7. 413 P7 swaps this label with the LDP-label received from P8 and forwards 414 to P8. 416 P8 pops the LDP label and forwards to PE3. 418 PE3 receives the tunneled packet and processes the service label. 420 The end-to-end MPLS tunnel is built from an SR node segment from PE1 421 to P6 and an LDP LSP from P6 to PE3. 423 Note: SR mappings advertisements cannot set Penultimate Hop Popping. 424 In the previous example, P6 requires the presence of the segment 103 425 such as to map it to the LDP label 1037. For that reason, the P flag 426 available in the Prefix-SID is not available in the Remote-Binding 427 SID. 429 4.2.1. SR to LDP Behavior 431 SR to LDP interworking requires a SRMS as defined in 432 [I-D.ietf-isis-segment-routing-extensions]. 434 The SRMS MUST be configured by the operator in order to advertise 435 Node-SIDs on behalf of non-SR nodes. 437 At least one SRMS MUST be present in the routing domain. Multiple 438 SRMSs SHOULD be present for redundancy. 440 Each SR capable router installs in the MPLS data plane Node-SIDs 441 learned from the SRMS exactly like if these SIDs had been advertised 442 by the nodes themselves. 444 A SR node having LDP neighbors MUST create LDP bindings for each 445 Prefix-SID and Node-SID learned in the SR domain and, for each FEC, 446 stitch the incoming SR label to the outgoing LDP label. This has to 447 be done in both LDP independent and ordered label distribution 448 control modes as defined in [RFC5036]. 450 The encodings of the SRMS advertisements are specific to the routing 451 protocol. See [I-D.ietf-isis-segment-routing-extensions], 452 [I-D.ietf-ospf-segment-routing-extensions] and 453 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] for details of SRMS 454 encodings. See also [I-D.ietf-spring-conflict-resolution] for the 455 specific rules on SRMS advertisements. 457 It has to be noted that the SR to LDP behavior does not propagate the 458 status of the LDP FEC which was signaled if LDP was configured to use 459 the ordered mode. 461 It has to be noted that in the case of SR to LDP, the label binding 462 is equivalent to the independent LDP Label Distribution Control Mode 463 ([RFC5036]) where a label in bound to a FEC independently from the 464 received binding for the same FEC. 466 5. SR/LDP Interworking Use Cases 468 SR can be deployed such as to enhance LDP transport. The SR 469 deployment can be limited to the network region where the SR benefits 470 are most desired. 472 5.1. SR Protection of LDP-based Traffic 474 In Figure 4, let us assume: 476 All link costs are 10 except FG which is 30. 478 All routers are LDP capable. 480 X, Y and Z are PE's participating to an important service S. 482 The operator requires 50msec link-based Fast Reroute (FRR) for 483 service S. 485 A, B, C, D, E, F and G are SR capable. 487 X, Y, Z are not SR capable, e.g. as part of a staged migration 488 from LDP to SR, the operator deploys SR first in a sub-part of the 489 network and then everywhere. 491 X 492 | 493 Y--A---B---E--Z 494 | | \ 495 D---C--F--G 496 30 498 Figure 4: SR/LDP interworking example 500 The operator would like to resolve the following issues: 502 To protect the link BA along the shortest-path of the important 503 flow XY, B requires a Remote LFA (RLFA, [RFC7490]) repair tunnel 504 to D and hence a targeted LDP session from B to D. Typically, 505 network operators prefer avoiding these dynamically established 506 multi-hop LDP sessions in order to reduce the number of protocols 507 running in the network and hence simplify network operations. 509 There is no LFA/RLFA solution to protect the link BE along the 510 shortest path of the important flow XZ. The operator wants a 511 guaranteed link-based FRR solution. 513 The operator can meet these objectives by deploying SR only on A, B, 514 C, D, E, F and G: 516 The operator configures A, B, C, D, E, F and G with SRGB (100, 517 200) and respective node segments 101, 102, 103, 104, 105, 106 and 518 107. 520 The operator configures D as an SR Mapping Server with the 521 following policy mapping: (X, 201), (Y, 202), (Z, 203). 523 Each SR node automatically advertises local adjacency segment for 524 its IGP adjacencies. Specifically, F advertises adjacency segment 525 9001 for its adjacency FG. 527 A, B, C, D, E, F and G keep their LDP capability and hence the flows 528 XY and XZ are transported over end-to-end LDP LSP's. 530 For example, LDP at B installs the following MPLS data plane entries: 532 Incoming label: local LDP label bound by B for FEC Y 533 Outgoing label: LDP label bound by A for FEC Y 534 Outgoing next-hop: A 536 Incoming label: local LDP label bound by B for FEC Z 537 Outgoing label: LDP label bound by E for FEC Z 538 Outgoing next-hop: E 540 The novelty comes from how the backup chains are computed for these 541 LDP-based entries. While LDP labels are used for the primary next- 542 hop and outgoing labels, SR information is used for the FRR 543 construction. In steady state, the traffic is transported over LDP 544 LSP. In transient FRR state, the traffic is backup thanks to the SR 545 enhanced capabilities. 547 The RLFA paths are dynamically pre-computed as defined in [RFC7490]. 548 Typically, implementations allow to enable RLFA mechanism through a 549 simple configuration command that triggers both the pre-computation 550 and installation of the repair path. The details on how RLFA 551 mechanisms are implemented and configured is outside the scope of 552 this document and not relevant to the aspects of SR/LDP interwork 553 explained in this document. 555 This helps meet the requirements of the operator: 557 Eliminate targeted LDP session. 559 Guaranteed FRR coverage. 561 Keep the traffic over LDP LSP in steady state. 563 Partial SR deployment only where needed. 565 5.2. Eliminating Targeted LDP Session 567 B's MPLS entry to Y becomes: 569 - Incoming label: local LDP label bound by B for FEC Y 570 Outgoing label: LDP label bound by A for FEC Y 571 Backup outgoing label: SR node segment for Y {202} 572 Outgoing next-hop: A 573 Backup next-hop: repair tunnel: node segment to D {104} 574 with outgoing next-hop: C 576 It has to be noted that D is selected as Remote Loop-Free Alternate 577 (R-LFA) as defined in [RFC7490]. 579 In steady-state, X sends its Y-destined traffic to B with a top label 580 which is the LDP label bound by B for FEC Y. B swaps that top label 581 for the LDP label bound by A for FEC Y and forwards to A. A pops the 582 LDP label and forwards to Y. 584 Upon failure of the link BA, B swaps the incoming top-label with the 585 node segment for Y (202) and sends the packet onto a repair tunnel to 586 D (node segment 104). Thus, B sends the packet to C with the label 587 stack {104, 202}. C pops the node segment 104 and forwards to D. D 588 swaps 202 for 202 and forwards to A. A's next-hop to Y is not SR 589 capable and hence node A swaps the incoming node segment 202 to the 590 LDP label announced by its next-hop (in this case, implicit null). 592 After IGP convergence, B's MPLS entry to Y will become: 594 - Incoming label: local LDP label bound by B for FEC Y 595 Outgoing label: LDP label bound by C for FEC Y 596 Outgoing next-hop: C 598 And the traffic XY travels again over the LDP LSP. 600 Conclusion: the operator has eliminated the need for targeted LDP 601 sessions (no longer required) and the steady-state traffic is still 602 transported over LDP. The SR deployment is confined to the area 603 where these benefits are required. 605 Despite that in general, an implementation would not require a manual 606 configuration of LDP Targeted sessions however, it is always a gain 607 if the operator is able to reduce the set of protocol sessions 608 running on the network infrastructure. 610 5.3. Guaranteed FRR coverage 612 As mentioned in Section 5.1 above, in the example topology described 613 in Figure 4, there is no RLFA-based solution for protecting the 614 traffic flow YZ against the failure of link BE because there is no 615 intersection between the extended P-space and Q-space (see [RFC7490] 616 for details). However: 618 o G belongs to the Q space of Z. 620 o G can be reached from B via a "repair SR path" {106, 9001} that is 621 not affected by failure of link BE (The method by which G and the 622 repair tunnel to it from B are identified are out of scope of this 623 document.) 625 B's MPLS entry to Z becomes: 627 - Incoming label: local LDP label bound by B for FEC Z 628 Outgoing label: LDP label bound by E for FEC Z 629 Backup outgoing label: SR node segment for Z {203} 630 Outgoing next-hop: E 631 Backup next-hop: repair tunnel to G: {106, 9001} 633 G is reachable from B via the combination of a 634 node segment to F {106} and an adjacency segment 635 FG {9001} 637 Note that {106, 107} would have equally work. 638 Indeed, in many case, P's shortest path to Q is 639 over the link PQ. The adjacency segment from P to 640 Q is required only in very rare topologies where 641 the shortest-path from P to Q is not via the link 642 PQ. 644 In steady-state, X sends its Z-destined traffic to B with a top label 645 which is the LDP label bound by B for FEC Z. B swaps that top label 646 for the LDP label bound by E for FEC Z and forwards to E. E pops the 647 LDP label and forwards to Z. 649 Upon failure of the link BE, B swaps the incoming top-label with the 650 node segment for Z (203) and sends the packet onto a repair tunnel to 651 G (node segment 106 followed by adjacency segment 9001). Thus, B 652 sends the packet to C with the label stack {106, 9001, 203}. C pops 653 the node segment 106 and forwards to F. F pops the adjacency segment 654 9001 and forwards to G. G swaps 203 for 203 and forwards to E. E's 655 next-hop to Z is not SR capable and hence E swaps the incoming node 656 segment 203 for the LDP label announced by its next-hop (in this 657 case, implicit null). 659 After IGP convergence, B's MPLS entry to Z will become: 661 - Incoming label: local LDP label bound by B for FEC Z 662 Outgoing label: LDP label bound by C for FEC Z 663 Outgoing next-hop: C 665 And the traffic XZ travels again over the LDP LSP. 667 Conclusions: 669 o the operator has eliminated its second problem: guaranteed FRR 670 coverage is provided. The steady-state traffic is still 671 transported over LDP. The SR deployment is confined to the area 672 where these benefits are required. 674 o FRR coverage has been achieved without any signaling for setting 675 up the repair LSP and without setting up a targeted LDP session 676 between B and G. 678 5.4. Inter-AS Option C, Carrier's Carrier 680 In inter-AS Option C, two interconnected ASes sets up inter-AS MPLS 681 connectivity. SR may be independently deployed in each AS. 683 PE1---R1---B1---B2---R2---PE2 684 <-----------> <-----------> 685 AS1 AS2 687 Figure 5: Inter-AS Option C 689 In Inter-AS Option C [RFC4364], B2 advertises to B1 a BGP3107 route 690 for PE2 and B1 reflects it to its internal peers, such as PE1. PE1 691 learns from a service route reflector a service route whose next-hop 692 is PE2. PE1 resolves that service route on the BGP3107 route to PE2. 693 That BGP3107 route to PE2 is itself resolved on the AS1 IGP route to 694 B1. 696 If AS1 operates SR, then the tunnel from PE1 to B1 is provided by the 697 node segment from PE1 to B1. 699 PE1 sends a service packet with three labels: the top one is the node 700 segment to B1, the next-one is the BGP3107 label provided by B1 for 701 the route "PE2" and the bottom one is the service label allocated by 702 PE2. 704 6. IANA Considerations 706 This document does not introduce any new codepoint. 708 7. Manageability Considerations 710 7.1. SR and LDP co-existence 712 As illustrated in Section 2.2, when both SR and LDP co-exist, the 713 following applies: 715 o If both SR and LDP propose an IP2MPLS entry for the same IP 716 prefix, then by default the LDP route MUST be selected. 718 o A local policy on a router MUST allow to prefer the SR-provided 719 IP2MPLS entry. 721 o Note that this policy MAY be locally defined. There is no 722 requirement that all routers use the same policy. 724 7.2. SRMS Management 726 In the case of SR/LDP interoperability through the use of a SRMS, 727 mappings are advertised by one or more SRMS. 729 SRMS function is implemented in the link-state protocol (such as IS- 730 IS and OSPF). Link-state protocols allow propagation of updates 731 across area boundaries and therefore SRMS advertisements are 732 propagated through the usual inter-area advertisement procedures in 733 link-state protocols. 735 Multiple SRMSs can be provisioned in a network for redundancy. 736 Moreover, a preference mechanism may also be used among SRMSs so to 737 deploy a primary/secondary SRMS scheme allowing controlled 738 modification or migration of SIDs. 740 The content of SRMS advertisement (i.e.: mappings) are a matter of 741 local policy determined by the operator. When multiple SRMSs are 742 active, it is necessary that the information (mappings) advertised by 743 the different SRMSs is aligned and consistent. 744 [I-D.ietf-spring-conflict-resolution] illustrates mechanisms through 745 which such consistency is achieved. 747 When the SRMS advertise mappings, an implementation SHOULD provide a 748 mechanism through which the operator determines which of the IP2MPLS 749 mappings are preferred among the one advertised by the SRMS and the 750 ones advertised by LDP. 752 7.3. Dataplane Verification 754 When Label switch paths (LSPs) are defined by stitching LDP LSPs with 755 SR LSPs, it is necessary to have mechanisms allowing the verification 756 of the LSP connectivity as well as validation of the path. These 757 mechanisms are described in [I-D.ietf-mpls-spring-lsp-ping]. 759 8. Security Considerations 761 This document does not introduce any change to the MPLS dataplane and 762 therefore no additional security of the MPLS dataplane is required. 764 This document introduces another form of label binding 765 advertisements. The security associated with these advertisement is 766 part of the security applied to routing protocols such as IS-IS and 767 OSPF which both make use of cryptographic authentication mechanisms. 769 9. Acknowledgements 771 We would like to thank Pierre Francois, Ruediger Geib and Alexander 772 Vainshtein for their contribution to the content of this document. 774 10. Contributors' Addresses 776 Edward Crabbe 777 Individual 778 Email: edward.crabbe@gmail.com 780 Igor Milojevic 781 Email: milojevicigor@gmail.com 783 Saku Ytti 784 TDC 785 Email: saku@ytti.fi 787 Rob Shakir 788 Google 789 Email: robjs@google.com 791 Martin Horneffer 792 Deutsche Telekom 793 Email: Martin.Horneffer@telekom.de 795 Wim Henderickx 796 Nokia 797 Email: wim.henderickx@nokia.com 799 Jeff Tantsura 800 Individual 801 Email: jefftant@gmail.com 803 11. References 805 11.1. Normative References 807 [I-D.ietf-isis-segment-routing-extensions] 808 Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., 809 Litkowski, S., Decraene, B., and j. jefftant@gmail.com, 810 "IS-IS Extensions for Segment Routing", draft-ietf-isis- 811 segment-routing-extensions-12 (work in progress), April 812 2017. 814 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] 815 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 816 Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3 817 Extensions for Segment Routing", draft-ietf-ospf-ospfv3- 818 segment-routing-extensions-09 (work in progress), March 819 2017. 821 [I-D.ietf-ospf-segment-routing-extensions] 822 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 823 Shakir, R., Henderickx, W., and J. Tantsura, "OSPF 824 Extensions for Segment Routing", draft-ietf-ospf-segment- 825 routing-extensions-12 (work in progress), March 2017. 827 [I-D.ietf-spring-conflict-resolution] 828 Ginsberg, L., Psenak, P., Previdi, S., and M. Pilka, 829 "Segment Routing Conflict Resolution", draft-ietf-spring- 830 conflict-resolution-03 (work in progress), April 2017. 832 [I-D.ietf-spring-segment-routing] 833 Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., 834 and R. Shakir, "Segment Routing Architecture", draft-ietf- 835 spring-segment-routing-11 (work in progress), February 836 2017. 838 [I-D.ietf-spring-segment-routing-mpls] 839 Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., 840 Litkowski, S., and R. Shakir, "Segment Routing with MPLS 841 data plane", draft-ietf-spring-segment-routing-mpls-08 842 (work in progress), March 2017. 844 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 845 Requirement Levels", BCP 14, RFC 2119, 846 DOI 10.17487/RFC2119, March 1997, 847 . 849 11.2. Informative References 851 [I-D.francois-rtgwg-segment-routing-ti-lfa] 852 Francois, P., Bashandy, A., Filsfils, C., Decraene, B., 853 and S. Litkowski, "Abstract", draft-francois-rtgwg- 854 segment-routing-ti-lfa-04 (work in progress), December 855 2016. 857 [I-D.ietf-mpls-spring-lsp-ping] 858 Kumar, N., Swallow, G., Pignataro, C., Akiya, N., Kini, 859 S., Gredler, H., and M. Chen, "Label Switched Path (LSP) 860 Ping/Trace for Segment Routing Networks Using MPLS 861 Dataplane", draft-ietf-mpls-spring-lsp-ping-02 (work in 862 progress), December 2016. 864 [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private 865 Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February 866 2006, . 868 [RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed., 869 "LDP Specification", RFC 5036, DOI 10.17487/RFC5036, 870 October 2007, . 872 [RFC5960] Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS 873 Transport Profile Data Plane Architecture", RFC 5960, 874 DOI 10.17487/RFC5960, August 2010, 875 . 877 [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. 878 So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", 879 RFC 7490, DOI 10.17487/RFC7490, April 2015, 880 . 882 Authors' Addresses 884 Clarence Filsfils (editor) 885 Cisco Systems, Inc. 886 Brussels 887 BE 889 Email: cfilsfil@cisco.com 891 Stefano Previdi (editor) 892 Cisco Systems, Inc. 893 Via Del Serafico, 200 894 Rome 00142 895 Italy 897 Email: sprevidi@cisco.com 898 Ahmed Bashandy 899 Cisco Systems, Inc. 900 170, West Tasman Drive 901 San Jose, CA 95134 902 US 904 Email: bashandy@cisco.com 906 Bruno Decraene 907 Orange 908 FR 910 Email: bruno.decraene@orange.com 912 Stephane Litkowski 913 Orange 914 FR 916 Email: stephane.litkowski@orange.com