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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: August 12, 2017 Cisco Systems, Inc. 6 B. Decraene 7 S. Litkowski 8 Orange 9 February 8, 2017 11 Segment Routing interworking with LDP 12 draft-ietf-spring-segment-routing-ldp-interop-06 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 August 12, 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 Thanks to the defined segment allocation rule and specifically the 133 notion of the Segment Routing Global Block (SRGB, as defined in 134 [I-D.ietf-spring-segment-routing]), SR can 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 attach the respective Node Segment Identifiers (Node- 180 SID's, as defined in [I-D.ietf-spring-segment-routing]): 202, 101, 181 102, 103 and 204 to the loopbacks of nodes PE2, A, B, C and PE4. 182 The Node-SID's are configured to request penultimate-hop-popping. 184 PE1, A, B, C and PE3 are LDP capable. 186 PE1 and PE3 are not SR capable. 188 PE3 sends an ODD VPN route to PE1 with next-hop 192.0.2.203 and VPN 189 label 10001. 191 From an LDP viewpoint: PE1 received an LDP label binding (1037) for 192 FEC 192.0.2.203/32 from its nhop A. A received an LDP label binding 193 (2048) for that FEC from its nhop B. B received an LDP label binding 194 (3059) for that FEC from its nhop C. C received implicit-null LDP 195 binding from its next-hop PE3. 197 As a result, PE1 sends its traffic to the ODD service route 198 advertised by PE3 to next-hop A with two labels: the top label is 199 1037 and the bottom label is 10001. A swaps 1037 with 2048 and 200 forwards to B. B swaps 2048 with 3059 and forwards to C. C pops 201 3059 and forwards to PE3. 203 PE4 sends an EVEN VPN route to PE2 with next-hop 192.0.2.204 and VPN 204 label 10002. 206 From an SR viewpoint: PE2 maps the IGP route 192.0.2.204/32 onto 207 Node-SID 204; A swaps 204 with 204 and forwards to B; B swaps 204 208 with 204 and forwards to C; C pops 204 and forwards to PE4. 210 As a result, PE2 sends its traffic to the VPN service route 211 advertised by PE4 to next-hop A with two labels: the top label is 204 212 and the bottom label is 10002. A swaps 204 with 204 and forwards to 213 B. B swaps 204 with 204 and forwards to C. C pops 204 and forwards 214 to PE4. 216 The two modes of MPLS tunneling co-exist. 218 The ODD service is tunneled from PE1 to PE3 through a continuous 219 LDP LSP traversing A, B and C. 221 The EVEN service is tunneled from PE2 to PE4 through a continuous 222 SR node segment traversing A, B and C. 224 2.1. MPLS2MPLS co-existence 226 We want to highlight that several MPLS2MPLS entries can be installed 227 in the data plane for the same prefix. 229 Let us examine A's MPLS forwarding table as an example: 231 Incoming label: 1037 233 - outgoing label: 2048 234 - outgoing nhop: B 235 Note: this entry is programmed by LDP for 192.0.2.203/32 237 Incoming label: 203 238 - outgoing label: 203 239 - outgoing nhop: B 240 Note: this entry is programmed by SR for 192.0.2.203/32 242 These two entries can co-exist because their incoming label is 243 unique. The uniqueness is guaranteed by the label manager allocation 244 rules. 246 The same applies for the MPLS2IP forwarding entries. 248 2.2. IP2MPLS co-existence 250 By default, if both LDP and SR propose an IP to MPLS entry (IP2MPLS) 251 for the same IP prefix, then the LDP route SHOULD be selected. 253 A local policy on a router MUST allow to prefer the SR-provided 254 IP2MPLS entry. 256 Note that this policy may be locally defined. There is no 257 requirement that all routers use the same policy. 259 3. Migration from LDP to SR 261 PE2 PE4 262 \ / 263 PE1----P5--P6--P7---PE3 265 Figure 2: Migration 267 Several migration techniques are possible. We describe one technique 268 inspired by the commonly used method to migrate from one IGP to 269 another. 271 At time T0, all the routers run LDP. Any service is tunneled from an 272 ingress PE to an egress PE over a continuous LDP LSP. 274 At time T1, all the routers are upgraded to SR. They are configured 275 with the SRGB range [100, 300]. PE1, PE2, PE3, PE4, P5, P6 and P7 276 are respectively configured with the node segments 101, 102, 103, 277 104, 105, 106 and 107 (attached to their service-recursing loopback). 279 At this time, the service traffic is still tunneled over LDP LSP. 280 For example, PE1 has an SR node segment to PE3 and an LDP LSP to 281 PE3 but by default, as seen earlier, the LDP IP2MPLS encapsulation 282 is preferred. However, it has to be noted that the SR 283 infrastructure is usable, e.g. for Fast Reroute (FRR) or IGP Loop 284 Free Convergence to protect existing IP and LDP traffic. FRR 285 mechanisms are described in 286 [I-D.francois-rtgwg-segment-routing-ti-lfa]. 288 At time T2, the operator enables the local policy at PE1 to prefer SR 289 IP2MPLS encapsulation over LDP IP2MPLS. 291 The service from PE1 to any other PE is now riding over SR. All 292 other service traffic is still transported over LDP LSP. 294 At time T3, gradually, the operator enables the preference for SR 295 IP2MPLS encapsulation across all the edge routers. 297 All the service traffic is now transported over SR. LDP is still 298 operational and services could be reverted to LDP. 300 However, any traffic switched through LDP entries will still 301 suffer from LDP-IGP synchronization. 303 At time T4, LDP is unconfigured from all routers. 305 4. SR and LDP Interworking 307 In this section, we analyze the case where SR is available in one 308 part of the network and LDP is available in another part. We 309 describe how a continuous MPLS tunnel can be built throughout the 310 network. 312 PE2 PE4 313 \ / 314 PE1----P5--P6--P7--P8---PE3 316 Figure 3: SR and LDP Interworking 318 Let us analyze the following example: 320 P6, P7, P8, PE4 and PE3 are LDP capable. 322 PE1, PE2, P5 and P6 are SR capable. PE1, PE2, P5 and P6 are 323 configured with SRGB (100, 200) and respectively with node 324 segments 101, 102, 105 and 106. 326 A service flow must be tunneled from PE1 to PE3 over a continuous 327 MPLS tunnel encapsulation. We need SR and LDP to interwork. 329 If the SR/LDP node operates in LDP ordered label distribution control 330 mode (as defined in [RFC5036]), then the SR/LDP node MUST consider SR 331 learned labels as if they were learned through an LDP neighbor and 332 create LDP bindings for each Prefix-SID and Node-SID learned in the 333 SR domain. 335 4.1. LDP to SR 337 In this section, we analyze a right-to-left traffic flow. 339 PE3 has learned a service route whose nhop is PE1. PE3 has an LDP 340 label binding from the nhop P8 for the FEC "PE1". Hence PE3 sends 341 its service packet to P8 as per classic LDP behavior. 343 P8 has an LDP label binding from its nhop P7 for the FEC "PE1" and 344 hence P8 forwards to P7 as per classic LDP behavior. 346 P7 has an LDP label binding from its nhop P6 for the FEC "PE1" and 347 hence P7 forwards to P6 as per classic LDP behavior. 349 P6 does not have an LDP binding from its nhop P5 for the FEC "PE1". 350 However P6 has an SR node segment to the IGP route "PE1". Hence, P6 351 forwards the packet to P5 and swaps its local LDP-label for FEC "PE1" 352 by the equivalent node segment (i.e. 101). 354 P5 pops 101 (assuming PE1 advertised its node segment 101 with the 355 penultimate-pop flag set) and forwards to PE1. 357 PE1 receives the tunneled packet and processes the service label. 359 The end-to-end MPLS tunnel is built from an LDP LSP from PE3 to P6 360 and the related node segment from P6 to PE1. 362 4.1.1. LDP to SR Behavior 364 It has to be noted that no additional signaling or state is required 365 in order to provide interworking in the direction LDP to SR. 367 A SR node having LDP neighbors MUST create LDP bindings for each 368 Prefix-SID and Node-SID learned in the SR domain and, for each FEC, 369 stitch the incoming LDP label to the outgoing SR label. This has to 370 be done in both LDP independent and ordered label distribution 371 control modes as defined in [RFC5036]. 373 4.2. SR to LDP 375 In this section, we analyze the left-to-right traffic flow. 377 We assume that the operator configures P5 to act as a Segment Routing 378 Mapping Server (SRMS) and advertises the following mappings: (P7, 379 107), (P8, 108), (PE3, 103) and (PE4, 104). 381 These mappings are advertised as Remote-Binding SID as described in 382 [I-D.ietf-isis-segment-routing-extensions]. 384 The mappings advertised by one or more SR mapping servers result from 385 local policy information configured by the operator. 387 If PE3 had been SR capable, the operator would have configured PE3 388 with node segment 103. Instead, as PE3 is not SR capable, the 389 operator configures that policy at the SRMS and it is the latter 390 which advertises the mapping. 392 The mapping server advertisements are only understood by the SR 393 capable routers. The SR capable routers install the related node 394 segments in the MPLS data plane exactly like if the node segments had 395 been advertised by the nodes themselves. 397 For example, PE1 installs the node segment 103 with nhop P5 exactly 398 as if PE3 had advertised node segment 103. 400 PE1 has a service route whose nhop is PE3. PE1 has a node segment 401 for that IGP route: 103 with nhop P5. Hence PE1 sends its service 402 packet to P5 with two labels: the bottom label is the service label 403 and the top label is 103. 405 P5 swaps 103 for 103 and forwards to P6. 407 P6's next-hop for the IGP route "PE3" is not SR capable (P7 does not 408 advertise the SR capability). However, P6 has an LDP label binding 409 from that next-hop for the same FEC (e.g. LDP label 1037). Hence, 410 P6 swaps 103 for 1037 and forwards to P7. 412 P7 swaps this label with the LDP-label received from P8 and forwards 413 to P8. 415 P8 pops the LDP label and forwards to PE3. 417 PE3 receives the tunneled packet and processes the service label. 419 The end-to-end MPLS tunnel is built from an SR node segment from PE1 420 to P6 and an LDP LSP from P6 to PE3. 422 Note: SR mappings advertisements cannot set Penultimate Hop Popping. 423 In the previous example, P6 requires the presence of the segment 103 424 such as to map it to the LDP label 1037. For that reason, the P flag 425 available in the Prefix-SID is not available in the Remote-Binding 426 SID. 428 4.2.1. SR to LDP Behavior 430 SR to LDP interworking requires a SRMS as defined in 431 [I-D.ietf-isis-segment-routing-extensions]. 433 The SRMS MUST be configured by the operator in order to advertise 434 Node-SIDs on behalf of non-SR nodes. 436 At least one SRMS MUST be present in the routing domain. Multiple 437 SRMSs SHOULD be present for redundancy. 439 Each SR capable router installs in the MPLS data plane Node-SIDs 440 learned from the SRMS exactly like if these SIDs had been advertised 441 by the nodes themselves. 443 A SR node having LDP neighbors MUST create LDP bindings for each 444 Prefix-SID and Node-SID learned in the SR domain and, for each FEC, 445 stitch the incoming SR label to the outgoing LDP label. This has to 446 be done in both LDP independent and ordered label distribution 447 control modes as defined in [RFC5036]. 449 The encodings of the SRMS advertisements are specific to the routing 450 protocol. See [I-D.ietf-isis-segment-routing-extensions], 451 [I-D.ietf-ospf-segment-routing-extensions] and 452 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] for details of SRMS 453 encodings. See also [I-D.ietf-spring-conflict-resolution] for the 454 specific rules on SRMS advertisements. 456 It has to be noted that the SR to LDP behavior does not propagate the 457 status of the LDP FEC which was signaled if LDP was configured to use 458 the ordered mode. 460 It has to be noted that in the case of SR to LDP, the label binding 461 is equivalent to the independent LDP Label Distribution Control Mode 462 ([RFC5036]) where a label in bound to a FEC independently from the 463 received binding for the same FEC. 465 5. SR/LDP Interworking Use Cases 467 SR can be deployed such as to enhance LDP transport. The SR 468 deployment can be limited to the network region where the SR benefits 469 are most desired. 471 5.1. SR Protection of LDP-based Traffic 473 In Figure 4, let us assume: 475 All link costs are 10 except FG which is 30. 477 All routers are LDP capable. 479 X, Y and Z are PE's participating to an important service S. 481 The operator requires 50msec link-based Fast Reroute (FRR) for 482 service S. 484 A, B, C, D, E, F and G are SR capable. 486 X, Y, Z are not SR capable, e.g. as part of a staged migration 487 from LDP to SR, the operator deploys SR first in a sub-part of the 488 network and then everywhere. 490 X 491 | 492 Y--A---B---E--Z 493 | | \ 494 D---C--F--G 495 30 497 Figure 4: SR/LDP interworking example 499 The operator would like to resolve the following issues: 501 To protect the link BA along the shortest-path of the important 502 flow XY, B requires a Remote LFA (RLFA, [RFC7490]) repair tunnel 503 to D and hence a targeted LDP session from B to D. Typically, 504 network operators prefer avoiding these dynamically established 505 multi-hop LDP sessions in order to reduce the number of protocols 506 running in the network and hence simplify network operations. 508 There is no LFA/RLFA solution to protect the link BE along the 509 shortest path of the important flow XZ. The operator wants a 510 guaranteed link-based FRR solution. 512 The operator can meet these objectives by deploying SR only on A, B, 513 C, D, E, F and G: 515 The operator configures A, B, C, D, E, F and G with SRGB (100, 516 200) and respective node segments 101, 102, 103, 104, 105, 106 and 517 107. 519 The operator configures D as an SR Mapping Server with the 520 following policy mapping: (X, 201), (Y, 202), (Z, 203). 522 Each SR node automatically advertises local adjacency segment for 523 its IGP adjacencies. Specifically, F advertises adjacency segment 524 9001 for its adjacency FG. 526 A, B, C, D, E, F and G keep their LDP capability and hence the flows 527 XY and XZ are transported over end-to-end LDP LSP's. 529 For example, LDP at B installs the following MPLS data plane entries: 531 Incoming label: local LDP label bound by B for FEC Y 532 Outgoing label: LDP label bound by A for FEC Y 533 Outgoing nhop: A 535 Incoming label: local LDP label bound by B for FEC Z 536 Outgoing label: LDP label bound by E for FEC Z 537 Outgoing nhop: E 539 The novelty comes from how the backup chains are computed for these 540 LDP-based entries. While LDP labels are used for the primary nhop 541 and outgoing labels, SR information is used for the FRR construction. 542 In steady state, the traffic is transported over LDP LSP. In 543 transient FRR state, the traffic is backup thanks to the SR enhanced 544 capabilities. 546 The RLFA paths are dynamically pre-computed as defined in [RFC7490]. 547 Typically, implementations allow to enable RLFA mechanism through a 548 simple configuration command that triggers both the pre-computation 549 and installation of the repair path. The details on how RLFA 550 mechanisms are implemented and configured is outside the scope of 551 this document and not relevant to the aspects of SR/LDP interwork 552 explained in this document. 554 This helps meet the requirements of the operator: 556 Eliminate targeted LDP session. 558 Guaranteed FRR coverage. 560 Keep the traffic over LDP LSP in steady state. 562 Partial SR deployment only where needed. 564 5.2. Eliminating Targeted LDP Session 566 B's MPLS entry to Y becomes: 568 - Incoming label: local LDP label bound by B for FEC Y 569 Outgoing label: LDP label bound by A for FEC Y 570 Backup outgoing label: SR node segment for Y {202} 571 Outgoing nhop: A 572 Backup nhop: repair tunnel: node segment to D {104} 573 with outgoing nhop: C 575 It has to be noted that D is selected as Remote Free Alternate 576 (R-LFA) as defined in [RFC7490]. 578 In steady-state, X sends its Y-destined traffic to B with a top label 579 which is the LDP label bound by B for FEC Y. B swaps that top label 580 for the LDP label bound by A for FEC Y and forwards to A. A pops the 581 LDP label and forwards to Y. 583 Upon failure of the link BA, B swaps the incoming top-label with the 584 node segment for Y (202) and sends the packet onto a repair tunnel to 585 D (node segment 104). Thus, B sends the packet to C with the label 586 stack {104, 202}. C pops the node segment 104 and forwards to D. D 587 swaps 202 for 202 and forwards to A. A's nhop to Y is not SR capable 588 and hence A swaps the incoming node segment 202 to the LDP label 589 announced by its next-hop (in this case, implicit null). 591 After IGP convergence, B's MPLS entry to Y will become: 593 - Incoming label: local LDP label bound by B for FEC Y 594 Outgoing label: LDP label bound by C for FEC Y 595 Outgoing nhop: C 597 And the traffic XY travels again over the LDP LSP. 599 Conclusion: the operator has eliminated the need for targeted LDP 600 sessions (no longer required) and the steady-state traffic is still 601 transported over LDP. The SR deployment is confined to the area 602 where these benefits are required. 604 Despite that in general, an implementation would not require a manual 605 configuration of LDP Targeted sessions however, it is always a gain 606 if the operator is able to reduce the set of protocol sessions 607 running on the network infrastructure. 609 5.3. Guaranteed FRR coverage 611 As mentioned in Section 5.1 above, in the example topology described 612 in Figure 4, there is no RLFA-based solution for protecting the 613 traffic flow YZ against the failure of link BE because there is no 614 intersection between the extended P-space and Q-space (see [RFC7490] 615 for details). However: 617 o G belongs to the Q space of Z. 619 o G can be reached from B via a "repair SR path" {106, 9001} that is 620 not affected by failure of link BE (The method by which G and the 621 repair tunnel to it from B are identified are out of scope of this 622 document.) 624 B's MPLS entry to Z becomes: 626 - Incoming label: local LDP label bound by B for FEC Z 627 Outgoing label: LDP label bound by E for FEC Z 628 Backup outgoing label: SR node segment for Z {203} 629 Outgoing nhop: E 630 Backup nhop: repair tunnel to G: {106, 9001} 632 G is reachable from B via the combination of a 633 node segment to F {106} and an adjacency segment 634 FG {9001} 636 Note that {106, 107} would have equally work. 637 Indeed, in many case, P's shortest path to Q is 638 over the link PQ. The adjacency segment from P to 639 Q is required only in very rare topologies where 640 the shortest-path from P to Q is not via the link 641 PQ. 643 In steady-state, X sends its Z-destined traffic to B with a top label 644 which is the LDP label bound by B for FEC Z. B swaps that top label 645 for the LDP label bound by E for FEC Z and forwards to E. E pops the 646 LDP label and forwards to Z. 648 Upon failure of the link BE, B swaps the incoming top-label with the 649 node segment for Z (203) and sends the packet onto a repair tunnel to 650 G (node segment 106 followed by adjacency segment 9001). Thus, B 651 sends the packet to C with the label stack {106, 9001, 203}. C pops 652 the node segment 106 and forwards to F. F pops the adjacency segment 653 9001 and forwards to G. G swaps 203 for 203 and forwards to E. E's 654 nhop to Z is not SR capable and hence E swaps the incoming node 655 segment 203 for the LDP label announced by its next-hop (in this 656 case, implicit null). 658 After IGP convergence, B's MPLS entry to Z will become: 660 - Incoming label: local LDP label bound by B for FEC Z 661 Outgoing label: LDP label bound by C for FEC Z 662 Outgoing nhop: C 664 And the traffic XZ travels again over the LDP LSP. 666 Conclusions: 668 o the operator has eliminated its second problem: guaranteed FRR 669 coverage is provided. The steady-state traffic is still 670 transported over LDP. The SR deployment is confined to the area 671 where these benefits are required. 673 o FRR coverage has been achieved without any signaling for setting 674 up the repair LSP and without setting up a targeted LDP session 675 between B and G. 677 5.4. Inter-AS Option C, Carrier's Carrier 679 In inter-AS Option C, two interconnected ASes sets up inter-AS MPLS 680 connectivity. SR may be independently deployed in each AS. 682 PE1---R1---B1---B2---R2---PE2 683 <-----------> <-----------> 684 AS1 AS2 686 Figure 5: Inter-AS Option C 688 In Inter-AS Option C [RFC4364], B2 advertises to B1 a BGP3107 route 689 for PE2 and B1 reflects it to its internal peers, such as PE1. PE1 690 learns from a service route reflector a service route whose nhop is 691 PE2. PE1 resolves that service route on the BGP3107 route to PE2. 692 That BGP3107 route to PE2 is itself resolved on the AS1 IGP route to 693 B1. 695 If AS1 operates SR, then the tunnel from PE1 to B1 is provided by the 696 node segment from PE1 to B1. 698 PE1 sends a service packet with three labels: the top one is the node 699 segment to B1, the next-one is the BGP3107 label provided by B1 for 700 the route "PE2" and the bottom one is the service label allocated by 701 PE2. 703 6. IANA Considerations 705 This document does not introduce any new codepoint. 707 7. Manageability Considerations 709 7.1. SR and LDP co-existence 711 As illustrated in Section 2.2, when both SR and LDP co-exist, the 712 following applies: 714 o If both SR and LDP propose an IP2MPLS entry for the same IP 715 prefix, then by default the LDP route MUST be selected. 717 o A local policy on a router MUST allow to prefer the SR-provided 718 IP2MPLS entry. 720 o Note that this policy may be locally defined. There is no 721 requirement that all routers use the same policy. 723 7.2. SRMS Management 725 In the case of SR/LDP interoperability through the use of a SRMS, 726 mappings are advertised by one or more SRMS. 728 SRMS function is implemented in the link-state protocol (such as IS- 729 IS and OSPF). Link-state protocols allow propagation of updates 730 across area boundaries and therefore SRMS advertisements are 731 propagated through the usual inter-area advertisement procedures in 732 link-state protocols. 734 Multiple SRMSs can be provisioned in a network for redundancy. 735 Moreover, a preference mechanism may also be used among SRMSs so to 736 deploy a primary/secondary SRMS scheme allowing controlled 737 modification or migration of SIDs. 739 The content of SRMS advertisement (i.e.: mappings) are a matter of 740 local policy determined by the operator. When multiple SRMSs are 741 active, it is necessary that the information (mappings) advertised by 742 the different SRMSs is aligned and consistent. 743 [I-D.ietf-spring-conflict-resolution] illustrates mechanisms through 744 which such consistency is achieved. 746 When the SRMS advertise mappings, an implementation SHOULD provide a 747 mechanism through which the operator determines which of the IP2MPLS 748 mappings are preferred among the one advertised by the SRMS and the 749 ones advertised by LDP. 751 7.3. Dataplane Verification 753 When Label switch paths (LSPs) are defined by stitching LDP LSPs with 754 SR LSPs, it is necessary to have mechanisms allowing the verification 755 of the LSP connectivity as well as validation of the path. These 756 mechanisms are described in [I-D.ietf-mpls-spring-lsp-ping]. 758 8. Security Considerations 760 This document does not introduce any change to the MPLS dataplane and 761 therefore no additional security of the MPLS dataplane is required. 763 This document introduces another form of label binding 764 advertisements. The security associated with these advertisement is 765 part of the security applied to routing protocols such as IS-IS and 766 OSPF which both make use of cryptographic authentication mechanisms. 768 9. Acknowledgements 770 We would like to thank Pierre Francois, Ruediger Geib and Alexander 771 Vainshtein for their contribution to the content of this document. 773 10. Contributors' Addresses 775 Edward Crabbe 776 Individual 777 Email: edward.crabbe@gmail.com 779 Igor Milojevic 780 Email: milojevicigor@gmail.com 782 Saku Ytti 783 TDC 784 Email: saku@ytti.fi 786 Rob Shakir 787 Individual 788 Email: rjs@rob.sh 790 Martin Horneffer 791 Deutsche Telekom 792 Email: Martin.Horneffer@telekom.de 794 Wim Henderickx 795 Alcatel-Lucent 796 Email: wim.henderickx@alcatel-lucent.com 798 Jeff Tantsura 799 Ericsson 800 Email: Jeff.Tantsura@ericsson.com 802 11. References 804 11.1. Normative References 806 [I-D.ietf-spring-conflict-resolution] 807 Ginsberg, L., Psenak, P., Previdi, S., and M. Pilka, 808 "Segment Routing Conflict Resolution", draft-ietf-spring- 809 conflict-resolution-02 (work in progress), October 2016. 811 [I-D.ietf-spring-segment-routing] 812 Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., 813 and R. Shakir, "Segment Routing Architecture", draft-ietf- 814 spring-segment-routing-10 (work in progress), November 815 2016. 817 [I-D.ietf-spring-segment-routing-mpls] 818 Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., 819 Litkowski, S., Horneffer, M., Shakir, R., 820 jefftant@gmail.com, j., and E. Crabbe, "Segment Routing 821 with MPLS data plane", draft-ietf-spring-segment-routing- 822 mpls-07 (work in progress), February 2017. 824 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 825 Requirement Levels", BCP 14, RFC 2119, 826 DOI 10.17487/RFC2119, March 1997, 827 . 829 11.2. Informative References 831 [I-D.francois-rtgwg-segment-routing-ti-lfa] 832 Francois, P., Bashandy, A., Filsfils, C., Decraene, B., 833 and S. Litkowski, "Abstract", draft-francois-rtgwg- 834 segment-routing-ti-lfa-04 (work in progress), December 835 2016. 837 [I-D.ietf-isis-segment-routing-extensions] 838 Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., 839 Litkowski, S., Decraene, B., and j. jefftant@gmail.com, 840 "IS-IS Extensions for Segment Routing", draft-ietf-isis- 841 segment-routing-extensions-09 (work in progress), October 842 2016. 844 [I-D.ietf-mpls-spring-lsp-ping] 845 Kumar, N., Swallow, G., Pignataro, C., Akiya, N., Kini, 846 S., Gredler, H., and M. Chen, "Label Switched Path (LSP) 847 Ping/Trace for Segment Routing Networks Using MPLS 848 Dataplane", draft-ietf-mpls-spring-lsp-ping-02 (work in 849 progress), December 2016. 851 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] 852 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 853 Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3 854 Extensions for Segment Routing", draft-ietf-ospf-ospfv3- 855 segment-routing-extensions-07 (work in progress), October 856 2016. 858 [I-D.ietf-ospf-segment-routing-extensions] 859 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 860 Shakir, R., Henderickx, W., and J. Tantsura, "OSPF 861 Extensions for Segment Routing", draft-ietf-ospf-segment- 862 routing-extensions-10 (work in progress), October 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 899 Ahmed Bashandy 900 Cisco Systems, Inc. 901 170, West Tasman Drive 902 San Jose, CA 95134 903 US 905 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