idnits 2.17.1 draft-ietf-spring-segment-routing-ldp-interop-03.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- -- The document has examples using IPv4 documentation addresses according to RFC6890, but does not use any IPv6 documentation addresses. Maybe there should be IPv6 examples, too? Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (May 17, 2016) is 2899 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '100' on line 272 -- Looks like a reference, but probably isn't: '300' on line 272 == Outdated reference: A later version (-04) exists of draft-francois-rtgwg-segment-routing-ti-lfa-01 == Outdated reference: A later version (-25) exists of draft-ietf-isis-segment-routing-extensions-06 == Outdated reference: A later version (-13) exists of draft-ietf-mpls-spring-lsp-ping-00 == Outdated reference: A later version (-23) exists of draft-ietf-ospf-ospfv3-segment-routing-extensions-05 == Outdated reference: A later version (-27) exists of draft-ietf-ospf-segment-routing-extensions-08 == Outdated reference: A later version (-15) exists of draft-ietf-spring-segment-routing-08 == Outdated reference: A later version (-22) exists of draft-ietf-spring-segment-routing-mpls-04 Summary: 0 errors (**), 0 flaws (~~), 8 warnings (==), 4 comments (--). 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 18, 2016 Cisco Systems, Inc. 6 B. Decraene 7 S. Litkowski 8 Orange 9 May 17, 2016 11 Segment Routing interworking with LDP 12 draft-ietf-spring-segment-routing-ldp-interop-03 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 18, 2016. 51 Copyright Notice 53 Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . . . . . . 7 75 4.1.1. LDP to SR Behavior . . . . . . . . . . . . . . . . . 8 76 4.2. SR to LDP . . . . . . . . . . . . . . . . . . . . . . . . 8 77 4.2.1. SR to LDP Behavior . . . . . . . . . . . . . . . . . 9 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 . . . . . . . . . . . . . . . . . 17 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 2. SR/LDP Ship-in-the-night coexistence 120 We call "MPLS Control Plane Client (MCC)" any control plane protocol 121 installing forwarding entries in the MPLS data plane. SR, LDP, RSVP- 122 TE, BGP 3107, VPNv4, etc are examples of MCCs. 124 An MCC, operating at node N, must ensure that the incoming label it 125 installs in the MPLS data plane of Node N has been uniquely allocated 126 to himself. 128 Thanks to the defined segment allocation rule and specifically the 129 notion of the Segment Routing Global Block (SRGB, as defined in 130 [I-D.ietf-spring-segment-routing]), SR can co-exist with any other 131 MCC. 133 This is clearly the case for the adjacency segment: it is a local 134 label allocated by the label manager, as for any MCC. 136 This is clearly the case for the prefix segment: the label manager 137 allocates the SRGB set of labels to the SR MCC client and the 138 operator ensures the unique allocation of each global prefix segment/ 139 label within the allocated SRGB set. 141 Note that this static label allocation capability of the label 142 manager exists for many years across several vendors and hence is not 143 new. Furthermore, note that the label-manager ability to statically 144 allocate a range of labels to a specific application is not new 145 either. This is required for MPLS-TP operation. In this case, the 146 range is reserved by the label manager and it is the MPLS-TP 147 ([RFC5960]) NMS (acting as an MCC) that ensures the unique allocation 148 of any label within the allocated range and the creation of the 149 related MPLS forwarding entry. 151 Let us illustrate an example of ship-in-the-night (SIN) coexistence. 153 PE2 PE4 154 \ / 155 PE1----A----B---C---PE3 157 Figure 1: SIN coexistence 159 The EVEN VPN service is supported by PE2 and PE4 while the ODD VPN 160 service is supported by PE1 and PE3. The operator wants to tunnel 161 the ODD service via LDP and the EVEN service via SR. 163 This can be achieved in the following manner: 165 The operator configures PE1, PE2, PE3, PE4 with respective 166 loopbacks 192.0.2.201/32, 192.0.2.202/32, 192.0.2.203/32, 167 192.0.2.204/32. These PE's advertised their VPN routes with next- 168 hop set on their respective loopback address. 170 The operator configures A, B, C with respective loopbacks 171 192.0.2.1/32, 192.0.2.2/32, 192.0.2.3/32. 173 The operator configures PE2, A, B, C and PE4 with SRGB [100, 300]. 175 The operator attach the respective Node Segment Identifiers (Node- 176 SID's, as defined in [I-D.ietf-spring-segment-routing]): 202, 101, 177 102, 103 and 204 to the loopbacks of nodes PE2, A, B, C and PE4. 178 The Node-SID's are configured to request penultimate-hop-popping. 180 PE1, A, B, C and PE3 are LDP capable. 182 PE1 and PE3 are not SR capable. 184 PE3 sends an ODD VPN route to PE1 with next-hop 192.0.2.203 and VPN 185 label 10001. 187 From an LDP viewpoint: PE1 received an LDP label binding (1037) for 188 FEC 192.0.2.203/32 from its nhop A. A received an LDP label binding 189 (2048) for that FEC from its nhop B. B received an LDP label binding 190 (3059) for that FEC from its nhop C. C received implicit-null LDP 191 binding from its next-hop PE3. 193 As a result, PE1 sends its traffic to the ODD service route 194 advertised by PE3 to next-hop A with two labels: the top label is 195 1037 and the bottom label is 10001. A swaps 1037 with 2048 and 196 forwards to B. B swaps 2048 with 3059 and forwards to C. C pops 197 3059 and forwards to PE3. 199 PE4 sends an EVEN VPN route to PE2 with next-hop 192.0.2.204 and VPN 200 label 10002. 202 From an SR viewpoint: PE1 maps the IGP route 192.0.2.204/32 onto 203 Node-SID 204; A swaps 204 with 204 and forwards to B; B swaps 204 204 with 204 and forwards to C; C pops 204 and forwards to PE4. 206 As a result, PE2 sends its traffic to the VPN service route 207 advertised by PE4 to next-hop A with two labels: the top label is 204 208 and the bottom label is 10002. A swaps 204 with 204 and forwards to 209 B. B swaps 204 with 204 and forwards to C. C pops 204 and forwards 210 to PE4. 212 The two modes of MPLS tunneling co-exist. 214 The ODD service is tunneled from PE1 to PE3 through a continuous 215 LDP LSP traversing A, B and C. 217 The EVEN service is tunneled from PE2 to PE4 through a continuous 218 SR node segment traversing A, B and C. 220 2.1. MPLS2MPLS co-existence 222 We want to highlight that several MPLS2MPLS entries can be installed 223 in the data plane for the same prefix. 225 Let us examine A's MPLS forwarding table as an example: 227 Incoming label: 1037 229 - outgoing label: 2048 230 - outgoing nhop: B 231 Note: this entry is programmed by LDP for 192.0.2.203/32 233 Incoming label: 203 235 - outgoing label: 203 236 - outgoing nhop: B 237 Note: this entry is programmed by SR for 192.0.2.203/32 239 These two entries can co-exist because their incoming label is 240 unique. The uniqueness is guaranteed by the label manager allocation 241 rules. 243 The same applies for the MPLS2IP forwarding entries. 245 2.2. IP2MPLS co-existence 247 By default, if both LDP and SR propose an IP to MPLS entry (IP2MPLS) 248 for the same IP prefix, then the LDP route SHOULD be selected. 250 A local policy on a router MUST allow to prefer the SR-provided 251 IP2MPLS entry. 253 Note that this policy may be locally defined. There is no 254 requirement that all routers use the same policy. 256 3. Migration from LDP to SR 258 PE2 PE4 259 \ / 260 PE1----P5--P6--P7---PE3 262 Figure 2: Migration 264 Several migration techniques are possible. We describe one technique 265 inspired by the commonly used method to migrate from one IGP to 266 another. 268 At time T0, all the routers run LDP. Any service is tunneled from an 269 ingress PE to an egress PE over a continuous LDP LSP. 271 At time T1, all the routers are upgraded to SR. They are configured 272 with the SRGB range [100, 300]. PE1, PE2, PE3, PE4, P5, P6 and P7 273 are respectively configured with the node segments 101, 102, 103, 274 104, 105, 106 and 107 (attached to their service-recursing loopback). 276 At this time, the service traffic is still tunneled over LDP LSP. 277 For example, PE1 has an SR node segment to PE3 and an LDP LSP to 278 PE3 but by default, as seen earlier, the LDP IP2MPLS encapsulation 279 is preferred. However, it has to be noted that the SR 280 infrastructure is usable, e.g. for Fast Reroute (FRR) or IGP Loop 281 Free Convergence to protect existing IP and LDP traffic. FRR 282 mechanisms are described in 283 [I-D.francois-rtgwg-segment-routing-ti-lfa]. 285 At time T2, the operator enables the local policy at PE1 to prefer SR 286 IP2MPLS encapsulation over LDP IP2MPLS. 288 The service from PE1 to any other PE is now riding over SR. All 289 other service traffic is still transported over LDP LSP. 291 At time T3, gradually, the operator enables the preference for SR 292 IP2MPLS encapsulation across all the edge routers. 294 All the service traffic is now transported over SR. LDP is still 295 operational and services could be reverted to LDP. 297 However, any traffic switched through LDP entries will still 298 suffer from LDP-IGP synchronization. 300 At time T4, LDP is unconfigured from all routers. 302 4. SR and LDP Interworking 304 In this section, we analyze the case where SR is available in one 305 part of the network and LDP is available in another part. We 306 describe how a continuous MPLS tunnel can be built throughout the 307 network. 309 PE2 PE4 310 \ / 311 PE1----P5--P6--P7--P8---PE3 313 Figure 3: SR and LDP Interworking 315 Let us analyze the following example: 317 P6, P7, P8, PE4 and PE3 are LDP capable. 319 PE1, PE2, P5 and P6 are SR capable. PE1, PE2, P5 and P6 are 320 configured with SRGB (100, 200) and respectively with node 321 segments 101, 102, 105 and 106. 323 A service flow must be tunneled from PE1 to PE3 over a continuous 324 MPLS tunnel encapsulation. We need SR and LDP to interwork. 326 4.1. LDP to SR 328 In this section, we analyze a right-to-left traffic flow. 330 PE3 has learned a service route whose nhop is PE1. PE3 has an LDP 331 label binding from the nhop P8 for the FEC "PE1". Hence PE3 sends 332 its service packet to P8 as per classic LDP behavior. 334 P8 has an LDP label binding from its nhop P7 for the FEC "PE1" and 335 hence P8 forwards to P7 as per classic LDP behavior. 337 P7 has an LDP label binding from its nhop P6 for the FEC "PE1" and 338 hence P7 forwards to P6 as per classic LDP behavior. 340 P6 does not have an LDP binding from its nhop P5 for the FEC "PE1". 341 However P6 has an SR node segment to the IGP route "PE1". Hence, P6 342 forwards the packet to P5 and swaps its local LDP-label for FEC "PE1" 343 by the equivalent node segment (i.e. 101). 345 P5 pops 101 (assuming PE1 advertised its node segment 101 with the 346 penultimate-pop flag set) and forwards to PE1. 348 PE1 receives the tunneled packet and processes the service label. 350 The end-to-end MPLS tunnel is built from an LDP LSP from PE3 to P6 351 and the related node segment from P6 to PE1. 353 4.1.1. LDP to SR Behavior 355 It has to be noted that no additional signaling or state is required 356 in order to provide interworking in the direction LDP to SR. 358 A SR node having LDP neighbors MUST create LDP bindings for each 359 Prefix-SID and Node-SID learned in the SR domain and, for each FEC, 360 stitch the incoming LDP label to the outgoing SR label. 362 4.2. SR to LDP 364 In this section, we analyze the left-to-right traffic flow. 366 We assume that the operator configures P5 to act as a Segment Routing 367 Mapping Server (SRMS) and advertises the following mappings: (P7, 368 107), (P8, 108), (PE3, 103) and (PE4, 104). 370 These mappings are advertised as Remote-Binding SID as described in 371 [I-D.ietf-isis-segment-routing-extensions]. 373 The mappings advertised by one or more SR mapping servers result from 374 local policy information configured by the operator. 376 If PE3 had been SR capable, the operator would have configured PE3 377 with node segment 103. Instead, as PE3 is not SR capable, the 378 operator configures that policy at the SRMS and it is the latter 379 which advertises the mapping. 381 The mapping server advertisements are only understood by the SR 382 capable routers. The SR capable routers install the related node 383 segments in the MPLS data plane exactly like if the node segments had 384 been advertised by the nodes themselves. 386 For example, PE1 installs the node segment 103 with nhop P5 exactly 387 as if PE3 had advertised node segment 103. 389 PE1 has a service route whose nhop is PE3. PE1 has a node segment 390 for that IGP route: 103 with nhop P5. Hence PE1 sends its service 391 packet to P5 with two labels: the bottom label is the service label 392 and the top label is 103. 394 P5 swaps 103 for 103 and forwards to P6. 396 P6's next-hop for the IGP route "PE3" is not SR capable (P7 does not 397 advertise the SR capability). However, P6 has an LDP label binding 398 from that next-hop for the same FEC (e.g. LDP label 1037). Hence, 399 P6 swaps 103 for 1037 and forwards to P7. 401 P7 swaps this label with the LDP-label received from P8 and forwards 402 to P8. 404 P8 pops the LDP label and forwards to PE3. 406 PE3 receives the tunneled packet and processes the service label. 408 The end-to-end MPLS tunnel is built from an SR node segment from PE1 409 to P6 and an LDP LSP from P6 to PE3. 411 Note: SR mappings advertisements cannot set Penultimate Hop Popping. 412 In the previous example, P6 requires the presence of the segment 103 413 such as to map it to the LDP label 1037. For that reason, the P flag 414 available in the Prefix-SID is not available in the Remote-Binding 415 SID. 417 4.2.1. SR to LDP Behavior 419 SR to LDP interworking requires a SRMS as defined in 420 [I-D.ietf-isis-segment-routing-extensions]. 422 The SRMS MUST be configured by the operator in order to advertise 423 Node-SIDs on behalf of non-SR nodes. 425 At least one SRMS MUST be present in the routing domain. Multiple 426 SRMSs SHOULD be present for redundancy. 428 Each SR capable router installs in the MPLS data plane Node-SIDs 429 learned from the SRMS exactly like if these SIDs had been advertised 430 by the nodes themselves. 432 A SR node having LDP neighbors MUST create LDP bindings for each 433 Prefix-SID and Node-SID learned in the SR domain and, for each FEC, 434 stitch the incoming SR label to the outgoing LDP label. 436 The encodings of the SRMS advertisements are specific to the routing 437 protocol. See [I-D.ietf-isis-segment-routing-extensions], 438 [I-D.ietf-ospf-segment-routing-extensions] and 439 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] for details of SRMS 440 encodings. See also [I-D.ginsberg-spring-conflict-resolution] for 441 the specific rules on SRMS advertisements. 443 It has to be noted, The SR to LDP behavior does not propagate the 444 status of the LDP FEC which was signaled if LDP was configured to use 445 the ordered mode. 447 It has to be noted that in the case of SR to LDP, the label binding 448 is equivalent to the LDP Label Distribution Control Mode ([RFC5036]) 449 where a label in bound to a FEC independently from the received 450 binding for the same FEC. 452 5. SR/LDP Interworking Use Cases 454 SR can be deployed such as to enhance LDP transport. The SR 455 deployment can be limited to the network region where the SR benefits 456 are most desired. 458 5.1. SR Protection of LDP-based Traffic 460 In Figure 4, let us assume: 462 All link costs are 10 except FG which is 30. 464 All routers are LDP capable. 466 X, Y and Z are PE's participating to an important service S. 468 The operator requires 50msec link-based Fast Reroute (FRR) for 469 service S. 471 A, B, C, D, E, F and G are SR capable. 473 X, Y, Z are not SR capable, e.g. as part of a staged migration 474 from LDP to SR, the operator deploys SR first in a sub-part of the 475 network and then everywhere. 477 X 478 | 479 Y--A---B---E--Z 480 | | \ 481 D---C--F--G 482 30 484 Figure 4: SR/LDP interworking example 486 The operator would like to resolve the following issues: 488 To protect the link BA along the shortest-path of the important 489 flow XY, B requires a Remote LFA (RLFA, [RFC7490]) repair tunnel 490 to D and hence a targeted LDP session from B to D. Typically, 491 network operators prefer avoiding these dynamically established 492 multi-hop LDP sessions in order to reduce the number of protocols 493 running in the network and hence simplify network operations. 495 There is no LFA/RLFA solution to protect the link BE along the 496 shortest path of the important flow XZ. The operator wants a 497 guaranteed link-based FRR solution. 499 The operator can meet these objectives by deploying SR only on A, B, 500 C, D, E, F and G: 502 The operator configures A, B, C, D, E, F and G with SRGB (100, 503 200) and respective node segments 101, 102, 103, 104, 105, 106 and 504 107. 506 The operator configures D as an SR Mapping Server with the 507 following policy mapping: (X, 201), (Y, 202), (Z, 203). 509 Each SR node automatically advertises local adjacency segment for 510 its IGP adjacencies. Specifically, F advertises adjacency segment 511 9001 for its adjacency FG. 513 A, B, C, D, E, F and G keep their LDP capability and hence the flows 514 XY and XZ are transported over end-to-end LDP LSP's. 516 For example, LDP at B installs the following MPLS data plane entries: 518 Incoming label: local LDB label bound by B for FEC Y 519 Outgoing label: LDP label bound by A for FEC Y 520 Outgoing nhop: A 522 Incoming label: local LDB label bound by B for FEC Z 523 Outgoing label: LDP label bound by E for FEC Z 524 Outgoing nhop: E 526 The novelty comes from how the backup chains are computed for these 527 LDP-based entries. While LDP labels are used for the primary nhop 528 and outgoing labels, SR information is used for the FRR construction. 529 In steady state, the traffic is transported over LDP LSP. In 530 transient FRR state, the traffic is backup thanks to the SR enhanced 531 capabilities. 533 The RLFA paths are dynamically pre-computed as defined in [RFC7490]. 534 Typically, implementations allow to enable RLFA mechanism through a 535 simple configuration command that triggers both the pre-computation 536 and installation of the repair path. The details on how RLFA 537 mechanisms are implemented and configured is outside the scope of 538 this document and not relevant to the aspects of SR/LDP interwork 539 explained in this document. 541 This helps meet the requirements of the operator: 543 Eliminate targeted LDP session. 545 Guaranteed FRR coverage. 547 Keep the traffic over LDP LSP in steady state. 549 Partial SR deployment only where needed. 551 5.2. Eliminating Targeted LDP Session 553 B's MPLS entry to Y becomes: 555 - Incoming label: local LDB label bound by B for FEC Y 556 Outgoing label: LDP label bound by A for FEC Y 557 Backup outgoing label: SR node segment for Y {202} 558 Outgoing nhop: A 559 Backup nhop: repair tunnel: node segment to D {104} 560 with outgoing nhop: C 562 It has to be noted that D is selected as Remote Free Alternate 563 (R-LFA) as defined in [RFC7490]. 565 In steady-state, X sends its Y-destined traffic to B with a top label 566 which is the LDP label bound by B for FEC Y. B swaps that top label 567 for the LDP label bound by A for FEC Y and forwards to A. A pops the 568 LDP label and forwards to Y. 570 Upon failure of the link BA, B swaps the incoming top-label with the 571 node segment for Y (202) and sends the packet onto a repair tunnel to 572 D (node segment 104). Thus, B sends the packet to C with the label 573 stack {104, 202}. C pops the node segment 104 and forwards to D. D 574 swaps 202 for 202 and forwards to A. A's nhop to Y is not SR capable 575 and hence A swaps the incoming node segment 202 to the LDP label 576 announced by its next-hop (in this case, implicit null). 578 After IGP convergence, B's MPLS entry to Y will become: 580 - Incoming label: local LDB label bound by B for FEC Y 581 Outgoing label: LDP label bound by C for FEC Y 582 Outgoing nhop: C 584 And the traffic XY travels again over the LDP LSP. 586 Conclusion: the operator has eliminated the need for targeted LDP 587 sessions (no longer required) and the steady-state traffic is still 588 transported over LDP. The SR deployment is confined to the area 589 where these benefits are required. 591 Despite that in general, an implementation would not require a manual 592 configuration of LDP Targeted sessions however, it is always a gain 593 if the operator is able to reduce the set of protocol sessions 594 running on the network infrastructure. 596 5.3. Guaranteed FRR coverage 598 As mentioned in Section 5.1 above, in the example topology described 599 in Figure 4, there is no RLFA-based solution for protecting the 600 traffic flow YZ against the failure of link BE because there is no 601 intersection between the extended P-space and Q-space (see [RFC7490] 602 for details). However: 604 o G belongs to the Q space of Z. 606 o G can be reached from B via a "repair SR path" {106, 9001} that is 607 not affected by failure of link BE (The method by which G and the 608 repair tunnel to it from B are identified are out of scope of this 609 document.) 611 B's MPLS entry to Z becomes: 613 - Incoming label: local LDB label bound by B for FEC Z 614 Outgoing label: LDP label bound by E for FEC Z 615 Backup outgoing label: SR node segment for Z {203} 616 Outgoing nhop: E 617 Backup nhop: repair tunnel to G: {106, 9001} 619 G is reachable from B via the combination of a 620 node segment to F {106} and an adjacency segment 621 FG {9001} 623 Note that {106, 107} would have equally work. 624 Indeed, in many case, P's shortest path to Q is 625 over the link PQ. The adjacency segment from P to 626 Q is required only in very rare topologies where 627 the shortest-path from P to Q is not via the link 628 PQ. 630 In steady-state, X sends its Z-destined traffic to B with a top label 631 which is the LDP label bound by B for FEC Z. B swaps that top label 632 for the LDP label bound by E for FEC Z and forwards to E. E pops the 633 LDP label and forwards to Z. 635 Upon failure of the link BE, B swaps the incoming top-label with the 636 node segment for Z (203) and sends the packet onto a repair tunnel to 637 G (node segment 106 followed by adjacency segment 9001). Thus, B 638 sends the packet to C with the label stack {106, 9001, 203}. C pops 639 the node segment 106 and forwards to F. F pops the adjacency segment 640 9001 and forwards to G. G swaps 203 for 203 and forwards to E. E's 641 nhop to Z is not SR capable and hence E swaps the incoming node 642 segment 203 for the LDP label announced by its next-hop (in this 643 case, implicit null). 645 After IGP convergence, B's MPLS entry to Z will become: 647 - Incoming label: local LDB label bound by B for FEC Z 648 Outgoing label: LDP label bound by C for FEC Z 649 Outgoing nhop: C 651 And the traffic XZ travels again over the LDP LSP. 653 Conclusions: 655 o the operator has eliminated its second problem: guaranteed FRR 656 coverage is provided. The steady-state traffic is still 657 transported over LDP. The SR deployment is confined to the area 658 where these benefits are required. 660 o FRR coverage has been achieved without any signaling for setting 661 up the repair LSP and without setting up a targeted LDP session 662 between B and G. 664 5.4. Inter-AS Option C, Carrier's Carrier 666 In inter-AS Option C, two interconnected ASes sets up inter-AS MPLS 667 connectivity. SR may be independently deployed in each AS. 669 PE1---R1---B1---B2---R2---PE2 670 <-----------> <-----------> 671 AS1 AS2 673 Figure 5: Inter-AS Option C 675 In Inter-AS Option C [RFC4364], B2 advertises to B1 a BGP3107 route 676 for PE2 and B1 reflects it to its internal peers, such as PE1. PE1 677 learns from a service route reflector a service route whose nhop is 678 PE2. PE1 resolves that service route on the BGP3107 route to PE2. 679 That BGP3107 route to PE2 is itself resolved on the AS1 IGP route to 680 B1. 682 If AS1 operates SR, then the tunnel from PE1 to B1 is provided by the 683 node segment from PE1 to B1. 685 PE1 sends a service packet with three labels: the top one is the node 686 segment to B1, the next-one is the BGP3107 label provided by B1 for 687 the route "PE2" and the bottom one is the service label allocated by 688 PE2. 690 6. IANA Considerations 692 This document does not introduce any new codepoint. 694 7. Manageability Considerations 696 7.1. SR and LDP co-existence 698 As illustrated in Section 2.2, when both SR and LDP co-exist, the 699 following applies: 701 o If both SR and LDP propose an IP2MPLS entry for the same IP 702 prefix, then by default the LDP route MUST be selected. 704 o A local policy on a router MUST allow to prefer the SR-provided 705 IP2MPLS entry. 707 o Note that this policy may be locally defined. There is no 708 requirement that all routers use the same policy. 710 7.2. SRMS Management 712 In the case of SR/LDP interoperability through the use of a SRMS, 713 mappings are advertised by one or more SRMS. 715 SRMS function is implemented in the link-state protocol (such as IS- 716 IS and OSPF). Link-state protocols allow propagation of updates 717 across area boundaries and therefore SRMS advertisements are 718 propagated through the usual inter-area advertisement procedures in 719 link-state protocols. 721 Multiple SRMSs can be provisioned in a network for redundancy. 722 Moreover, a preference mechanism may also be used among SRMSs so to 723 deploy a primary/secondary SRMS scheme allowing controlled 724 modification or migration of SIDs. 726 The content of SRMS advertisement (i.e.: mappings) are a matter of 727 local policy determined by the operator. When multiple SRMSs are 728 active, it is necessary that the information (mappings) advertised by 729 the different SRMSs is aligned and consistent. 730 [I-D.ginsberg-spring-conflict-resolution] illustrates mechanisms 731 through which such consistency is achieved. 733 When the SRMS advertise mappings, an implementation SHOULD provide a 734 mechanism through which the operator determines which of the IP2MPLS 735 mappings are preferred among the one advertised by the SRMS and the 736 ones advertised by LDP. 738 7.3. Dataplane Verification 740 When Label switch paths (LSPs) are defined by stitching LDP LSPs with 741 SR LSPs, it is necessary to have mechanisms allowing the verification 742 of the LSP connectivity as well as validation of the path. These 743 mechanisms are described in [I-D.ietf-mpls-spring-lsp-ping]. 745 8. Security Considerations 747 This document does not introduce any change to the MPLS dataplane and 748 therefore no additional security of the MPLS dataplane is required. 750 This document introduces another form of label binding 751 advertisements. The security associated with these advertisement is 752 part of the security applied to routing protocols such as IS-IS and 753 OSPF which both make use of cryptographic authentication mechanisms. 755 9. Acknowledgements 757 We would like to thank Pierre Francois, Ruediger Geib and Alexander 758 Vainshtein for their contribution to the content of this document. 760 10. Contributors' Addresses 762 Edward Crabbe 763 Individual 764 Email: edward.crabbe@gmail.com 766 Igor Milojevic 767 Email: milojevicigor@gmail.com 769 Saku Ytti 770 TDC 771 Email: saku@ytti.fi 773 Rob Shakir 774 Individual 775 Email: rjs@rob.sh 777 Martin Horneffer 778 Deutsche Telekom 779 Email: Martin.Horneffer@telekom.de 781 Wim Henderickx 782 Alcatel-Lucent 783 Email: wim.henderickx@alcatel-lucent.com 785 Jeff Tantsura 786 Ericsson 787 Email: Jeff.Tantsura@ericsson.com 789 11. References 791 11.1. Normative References 793 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 794 Requirement Levels", BCP 14, RFC 2119, 795 DOI 10.17487/RFC2119, March 1997, 796 . 798 11.2. Informative References 800 [I-D.francois-rtgwg-segment-routing-ti-lfa] 801 Francois, P., Filsfils, C., Bashandy, A., and B. Decraene, 802 "Topology Independent Fast Reroute using Segment Routing", 803 draft-francois-rtgwg-segment-routing-ti-lfa-01 (work in 804 progress), May 2016. 806 [I-D.ginsberg-spring-conflict-resolution] 807 Ginsberg, L., Psenak, P., Previdi, S., and M. Pilka, 808 "Segment Routing Conflict Resolution", draft-ginsberg- 809 spring-conflict-resolution-01 (work in progress), April 810 2016. 812 [I-D.ietf-isis-segment-routing-extensions] 813 Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., 814 Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS 815 Extensions for Segment Routing", draft-ietf-isis-segment- 816 routing-extensions-06 (work in progress), December 2015. 818 [I-D.ietf-mpls-spring-lsp-ping] 819 Kumar, N., Swallow, G., Pignataro, C., Akiya, N., Kini, 820 S., Gredler, H., and M. Chen, "Label Switched Path (LSP) 821 Ping/Trace for Segment Routing Networks Using MPLS 822 Dataplane", draft-ietf-mpls-spring-lsp-ping-00 (work in 823 progress), May 2016. 825 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] 826 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 827 Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3 828 Extensions for Segment Routing", draft-ietf-ospf-ospfv3- 829 segment-routing-extensions-05 (work in progress), March 830 2016. 832 [I-D.ietf-ospf-segment-routing-extensions] 833 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 834 Shakir, R., Henderickx, W., and J. Tantsura, "OSPF 835 Extensions for Segment Routing", draft-ietf-ospf-segment- 836 routing-extensions-08 (work in progress), April 2016. 838 [I-D.ietf-spring-segment-routing] 839 Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., 840 and R. Shakir, "Segment Routing Architecture", draft-ietf- 841 spring-segment-routing-08 (work in progress), May 2016. 843 [I-D.ietf-spring-segment-routing-mpls] 844 Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., 845 Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J., 846 and E. Crabbe, "Segment Routing with MPLS data plane", 847 draft-ietf-spring-segment-routing-mpls-04 (work in 848 progress), March 2016. 850 [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private 851 Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February 852 2006, . 854 [RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed., 855 "LDP Specification", RFC 5036, DOI 10.17487/RFC5036, 856 October 2007, . 858 [RFC5960] Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS 859 Transport Profile Data Plane Architecture", RFC 5960, 860 DOI 10.17487/RFC5960, August 2010, 861 . 863 [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. 864 So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", 865 RFC 7490, DOI 10.17487/RFC7490, April 2015, 866 . 868 Authors' Addresses 870 Clarence Filsfils (editor) 871 Cisco Systems, Inc. 872 Brussels 873 BE 875 Email: cfilsfil@cisco.com 877 Stefano Previdi (editor) 878 Cisco Systems, Inc. 879 Via Del Serafico, 200 880 Rome 00142 881 Italy 883 Email: sprevidi@cisco.com 884 Ahmed Bashandy 885 Cisco Systems, Inc. 886 170, West Tasman Drive 887 San Jose, CA 95134 888 US 890 Email: bashandy@cisco.com 892 Bruno Decraene 893 Orange 894 FR 896 Email: bruno.decraene@orange.com 898 Stephane Litkowski 899 Orange 900 FR 902 Email: stephane.litkowski@orange.com