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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group A. Bashandy, Ed. 2 Internet Draft Arrcus 3 Intended status: Standards Track C. Filsfils, Ed. 4 Expires: May 2019 S. Previdi, 5 Cisco Systems, Inc. 6 B. Decraene 7 S. Litkowski 8 Orange 9 R. Shakir 10 Google 11 November 19, 2018 13 Segment Routing with MPLS data plane 14 draft-ietf-spring-segment-routing-mpls-16 16 Abstract 18 Segment Routing (SR) leverages the source routing paradigm. A node 19 steers a packet through a controlled set of instructions, called 20 segments, by prepending the packet with an SR header. In the MPLS 21 dataplane, the SR header is instantiated through a label stack. This 22 document specifies the forwarding behavior to allow instantiating SR 23 over the MPLS dataplane. 25 Status of this Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at http://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on May 19, 2019. 42 Copyright Notice 44 Copyright (c) 2018 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction...................................................3 60 1.1. Requirements Language.....................................4 61 2. MPLS Instantiation of Segment Routing..........................4 62 2.1. Multiple Forwarding Behaviors for the Same Prefix.........5 63 2.2. SID Representation in the MPLS Forwarding Plane...........5 64 2.3. Segment Routing Global Block and Local Block..............6 65 2.4. Mapping a SID Index to an MPLS label......................6 66 2.5. Incoming Label Collision..................................7 67 2.5.1. Tie-breaking Rules...................................9 68 2.5.2. Redistribution between Routing Protocol Instances...12 69 2.5.2.1. Illustration...................................13 70 2.5.2.2. Illustration 2.................................13 71 2.6. Effect of Incoming Label Collision on Outgoing Label 72 Programming...................................................14 73 2.7. PUSH, CONTINUE, and NEXT.................................14 74 2.7.1. PUSH................................................14 75 2.7.2. CONTINUE............................................15 76 2.7.3. NEXT................................................15 77 2.7.3.1. Mirror SID.....................................15 78 2.8. MPLS Label Downloaded to FIB for Global and Local SIDs...15 79 2.9. Active Segment...........................................16 80 2.10. Forwarding behavior for Global SIDs.....................16 81 2.10.1. Forwarding for PUSH and CONTINUE of Global SIDs....16 82 2.10.2. Forwarding for NEXT Operation for Global SIDs......17 83 2.11. Forwarding Behavior for Local SIDs......................18 84 2.11.1. Forwarding for PUSH Operation on Local SIDs........18 85 2.11.2. Forwarding for CONTINUE Operation for Local SIDs...18 86 2.11.3. Outgoing label for NEXT Operation for Local SIDs...19 87 3. IANA Considerations...........................................19 88 4. Manageability Considerations..................................19 89 5. Security Considerations.......................................19 90 6. Contributors..................................................19 91 7. Acknowledgements..............................................20 92 8. References....................................................20 93 8.1. Normative References.....................................20 94 8.2. Informative References...................................21 95 9. Authors' Addresses............................................24 96 Appendix A. Examples.............................................26 97 A.1. IGP Segments Example.....................................26 98 A.2. Incoming Label Collision Examples........................28 99 A.2.1. Example 1...........................................28 100 A.2.2. Example 2...........................................29 101 A.2.3. Example 3...........................................30 102 A.2.4. Example 4...........................................30 103 A.2.5. Example 5...........................................31 104 A.2.6. Example 6...........................................31 105 A.2.7. Example 7...........................................32 106 A.2.8. Example 8...........................................32 107 A.2.9. Example 9...........................................33 108 A.2.10. Example 10.........................................33 109 A.2.11. Example 11.........................................34 110 A.2.12. Example 12.........................................35 111 A.2.13. Example 13.........................................35 112 A.2.14. Example 14.........................................36 113 A.3. Examples for the Effect of Incoming Label Collision on 114 Outgoing Label................................................36 115 A.3.1. Example 1...........................................36 116 A.3.2. Example 2...........................................37 118 1. Introduction 120 The Segment Routing architecture RFC8402 can be directly applied to 121 the MPLS architecture with no change in the MPLS forwarding plane. 122 This document specifies the forwarding plane behavior to allow 123 Segment Routing to operate on top of the MPLS data plane. This 124 document does not address the control plane behavior. Control plane 125 behavior is specified in other documents such as [I-D.ietf-isis- 126 segment-routing-extensions], [I-D.ietf-ospf-segment-routing- 127 extensions], and [I-D.ietf-ospf-ospfv3-segment-routing-extensions]. 129 The Segment Routing problem statement is described in [RFC7855]. 131 Co-existence of SR over MPLS forwarding plane with LDP [RFC5036] is 132 specified in [I-D.ietf-spring-segment-routing-ldp-interop]. 134 Policy routing and traffic engineering using segment routing can be 135 found in [I-D.ietf-spring-segment-routing-policy] 137 1.1. Requirements Language 139 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 140 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 141 "OPTIONAL" in this document are to be interpreted as described in BCP 142 14 [RFC2119] [RFC8174] when, and only when, they appear in all 143 capitals, as shown here. 145 2. MPLS Instantiation of Segment Routing 147 MPLS instantiation of Segment Routing fits in the MPLS architecture 148 as defined in [RFC3031] both from a control plane and forwarding 149 plane perspective: 151 o From a control plane perspective, [RFC3031] does not mandate a 152 single signaling protocol. Segment Routing makes use of various 153 control plane protocols such as link state IGPs [I-D.ietf-isis- 154 segment-routing-extensions], [I-D.ietf-ospf-segment-routing- 155 extensions] and [I-D.ietf-ospf-ospfv3-segment-routing-extensions]. 156 The flooding mechanisms of link state IGPs fits very well with 157 label stacking on ingress. Future control layer protocol and/or 158 policy/configuration can be used to specify the label stack. 160 o From a forwarding plane perspective, Segment Routing does not 161 require any change to the forwarding plane because Segment IDs 162 (SIDs) are instantiated as MPLS labels and the Segment routing 163 header instantiated as a stack of MPLS labels. 165 We call "MPLS Control Plane Client (MCC)" any control plane entity 166 installing forwarding entries in the MPLS data plane. IGPs with SR 167 extensions [I-D.ietf-isis-segment-routing-extensions], [I-D.ietf- 168 ospf-segment-routing-extensions], [I-D.ietf-ospf-ospfv3-segment- 169 routing-extensions] and LDP [RFC5036] are examples of MCCs. Local 170 configuration and policies applied on a router are also examples of 171 MCCs. 173 In order to have a node segment to reach the node, a network operator 174 SHOULD configure at least one node segment per routing instance, 175 topology, algorithm. Otherwise, the node is not reachable within the 176 routing instance, topology or along the routing algorithm, which 177 restrict its ability to be used by a SR policy, including for TI-LFA. 178 An implementation MAY check that an IGP node-SID is not associated 179 with a prefix that is owned by more than one router within the same 180 routing domain. If so, it SHOULD NOT use this Node-SID, MAY use 181 another one if available and SHOULD log an error. 183 2.1. Multiple Forwarding Behaviors for the Same Prefix 185 The SR architecture does not prohibit having more than one SID for 186 the same prefix. In fact, by allowing multiple SIDs for the same 187 prefix, it is possible to have different forwarding behaviors (such 188 as different paths, different ECMP/UCMP behaviors,...,etc) for the 189 same destination. 191 Instantiating Segment routing over the MPLS forwarding plane fits 192 seamlessly with this principle. An operator may assign multiple MPLS 193 labels or indices to the same prefix and assign different forwarding 194 behaviors to each label/SID. The MCC in the network downloads 195 different MPLS labels/SIDs to the FIB for different forwarding 196 behaviors. The MCC at the entry of an SR domain or at any point in 197 the domain can choose to apply a particular forwarding behavior to a 198 particular packet by applying the PUSH action to that packet using 199 the corresponding SID. 201 2.2. SID Representation in the MPLS Forwarding Plane 203 When instantiating SR over the MPLS forwarding plane, a SID is 204 represented by an MPLS label or an index [RFC8402]. 206 A global segment MUST be a label, or an index which may be mapped to 207 an MPLS label within the Segment Routing Global Block (SRGB) of the 208 node installing the global segment in its FIB/receiving the labeled 209 packet. Section 2.4 specifies the procedure to map a global segment 210 represented by an index to an MPLS label within the SRGB. 212 The MCC MUST ensure that any label value corresponding to any SID it 213 installs in the forwarding plane follows the following rules: 215 o The label value MUST be unique within the router on which the MCC 216 is running. i.e. the label MUST only be used to represent the SID 217 and MUST NOT be used to represent more than one SID or for any 218 other forwarding purpose on the router. 220 o The label value MUST NOT come from the range of special purpose 221 labels [RFC7274]. 223 Labels allocated in this document are considered per platform down- 224 stream allocated labels [RFC3031]. 226 2.3. Segment Routing Global Block and Local Block 228 The concepts of Segment Routing Global Block (SRGB) and global SID 229 are explained in [RFC8402]. In general, the SRGB need not be a 230 contiguous range of labels. 232 For the rest of this document, the SRGB is specified by the list of 233 MPLS Label ranges [Ll(1),Lh(1)], [Ll(2),Lh(2)],..., [Ll(k),Lh(k)] 234 where Ll(i) =< Lh(i). 236 The following rules apply to the list of MPLS ranges representing the 237 SRGB 239 o The list of ranges comprising the SRGB MUST NOT overlap. 241 o Every range in the list of ranges specifying the SRGB MUST NOT 242 cover or overlap with a reserved label value or range [RFC7274], 243 respectively. 245 o If the SRGB of a node does not conform to the structure specified 246 in this section or to the previous two rules, then this SRGB MUST 247 be completely ignored by all routers in the routing domain and the 248 node MUST be treated as if it does not have an SRGB. 250 o The list of label ranges MUST only be used to instantiate global 251 SIDs into the MPLS forwarding plane 253 A Local segment MAY be allocated from the Segment Routing Local Block 254 (SRLB) [RFC8402] or from any unused label as long as it does not use 255 a special purpose label. The SRLB consists of the range of local 256 labels reserved by the node for certain local segments. In a 257 controller-driven network, some controllers or applications MAY use 258 the control plane to discover the available set of local SIDs on a 259 particular router [I-D.ietf-spring-segment-routing-policy]. The rules 260 applicable to the SRGB are also applicable to the SRLB, except rule 261 that says that the SRGB MUST only be used to instantiate global SIDs 262 into the MPLS forwarding plane. The recommended, minimum, or maximum 263 size of the SRGB and/or SRLB is a matter of future study 265 2.4. Mapping a SID Index to an MPLS label 267 This sub-section specifies how the MPLS label value is calculated 268 given the index of a SID. The value of the index is determined by an 269 MCC such as IS-IS [I-D.ietf-isis-segment-routing-extensions] or OSPF 270 [I-D.ietf-ospf-segment-routing-extensions]. This section only 271 specifies how to map the index to an MPLS label. The calculated MPLS 272 label is downloaded to the FIB, sent out with a forwarded packet, or 273 both. 275 Consider a SID represented by the index "I". Consider an SRGB as 276 specified in Section 2.3. The total size of the SRGB, represented by 277 the variable "Size", is calculated according to the formula: 279 size = Lh(1)- Ll(1) + 1 + Lh(2)- Ll(2) + 1 + ... + Lh(k)- Ll(k) + 1 281 The following rules MUST be applied by the MCC when calculating the 282 MPLS label value corresponding the SID index value "I". 284 o 0 =< I < size. If the index "I" does not satisfy the previous 285 inequality, then the label cannot be calculated. 287 o The label value corresponding to the SID index "I" is calculated 288 as follows 290 o j = 1 , temp = 0 292 o While temp + Lh(j)- Ll(j) < I 294 . temp = temp + Lh(j)- Ll(j) + 1 296 . j = j+1 298 o label = I - temp + Ll(j) 300 An example for how a router calculates labels and forwards traffic 301 based on the procedure described in this section can be found in 302 Appendix A.1. 304 2.5. Incoming Label Collision 306 MPLS Architecture [RFC3031] defines Forwarding Equivalence Class 307 (FEC) term as the set of packets with similar and / or identical 308 characteristics which are forwarded the same way and are bound to the 309 same MPLS incoming (local) label. In Segment-Routing MPLS, local 310 label serves as the SID for given FEC. 312 We define Segment Routing (SR) FEC as one of the following [RFC8402]: 314 o (Prefix, Routing Instance, Topology, Algorithm [RFC8402]), where a 315 topology identifies a set of links with metrics. For the purpose 316 of incoming label collision resolution, the same Topology 317 numerical value SHOULD be used on all routers to identify the same 318 set of links with metrics. For MCCs where the "Topology" and/or 319 "Algorithm" fields are not defined, the numerical value of zero 320 MUST be used for these two fields. For the purpose of incoming 321 label collision resolution, a routing instance is identified by a 322 single incoming label downloader to FIB. Two MCCs running on the 323 same router are considered different routing instances if the only 324 way the two instances can know about the other's incoming labels 325 is through redistribution. The numerical value used to identify a 326 routing instance MAY be derived from other configuration or MAY be 327 explicitly configured. If it is derived from other configuration, 328 then the same numerical value SHOULD be derived from the same 329 configuration as long as the configuration survives router reload. 330 If the derived numerical value varies for the same configuration, 331 then an implementation SHOULD make numerical value used to 332 identify a routing instance configurable. 334 o (next-hop, outgoing interface), where the outgoing interface is 335 physical or virtual. 337 o (number of adjacencies, list of next-hops, list of outgoing 338 interfaces IDs in ascending numerical order). This FEC represents 339 parallel adjacencies [RFC8402] 341 o (Endpoint, Color) representing an SR policy [RFC8042] 343 o (Mirrored SID) The Mirrored SID [RFC8042, Section 5.1] is the IP 344 address advertised by the advertising node to identify the mirror- 345 SID. The IP address is encoded as specified in Section 2.5.1. 347 This section covers the RECOMMENDED procedure to handle the scenario 348 where, because of an error/misconfiguration, more than one SR FEC as 349 defined in this section, map to the same incoming MPLS label. 350 Examples illustrating the behavior specified in this section can be 351 found in Appendix A.2. 353 An incoming label collision occurs if the SIDs of the set of FECs 354 {FEC1, FEC2,..., FECk} maps to the same incoming SR MPLS label "L1". 356 An implementation MUST NOT allow the MCCs belonging to the same 357 router to assign the same incoming label to more than one SR FEC. An 358 implementation that allows such behavior is considered a faulty 359 implementation and is not covered in this document. 361 The objective of the following steps is to deterministically install 362 in the MPLS Incoming Label Map, also known as label FIB, a single FEC 363 with the incoming label "L1". Remaining FECs may be installed in the 364 IP FIB without incoming label. 366 The procedure in this section relies completely on the local FEC and 367 label database within a given router. 369 The collision resolution procedure is as follows 371 1. Given the SIDs of the set of FECs, {FEC1, FEC2,..., FECk} map to 372 the same MPLS label "L1". 374 2. Within an MCC, apply tie-breaking rules to select one FEC only and 375 assign the label to it. The losing FECs are handled as if no 376 labels are attached to them. The losing FECs with a non-zero 377 algorithm are not installed in FIB. 379 a. If the same set of FECs are attached to the same label "L1", 380 then the tie-breaking rules MUST always select the same FEC 381 irrespective of the order in which the FECs and the label "L1" 382 are received. In other words, the tie-breaking rule MUST be 383 deterministic. For example, a first-come-first-serve tie- 384 breaking is not allowed. 386 3. If there is still collision between the FECs belonging to 387 different MCCs, then re-apply the tie-breaking rules to the 388 remaining FECs to select one FEC only and assign the label to that 389 FEC 391 4. Install into the IP FIB the selected FEC and its incoming label in 392 the label FIB. 394 5. The remaining FECs with the default algorithm (see the 395 specification of prefix-SID algorithm [RFC8402]) are installed in 396 the FIB natively, such as pure IP entries in case of Prefix FEC, 397 without any incoming labels corresponding to their SIDs. The 398 remaining FECs with a non-zero algorithm are not installed in the 399 FIB. 401 2.5.1. Tie-breaking Rules 403 The default tie-breaking rules SHOULD be as follows: 405 1. if FECi has the lowest FEC administrative distance among the 406 competing FECs as defined in this section below, filter away all 407 the competing FECs with higher administrative distance. 409 2. if more than one competing FEC remains after step 1, select the 410 smallest numerical FEC value 412 These rules deterministically select the FEC to install in the MPLS 413 forwarding plane for the given incoming label. 415 This document defines the default tie breaking rules that SHOULD be 416 implemented. An implementation MAY choose to implement additional 417 tie-breaking rules. All routers in a routing domain SHOULD use the 418 same tie-breaking rules to maximize forwarding consistency. 420 Each FEC is assigned an administrative distance. The FEC 421 administrative distance is encoded as an 8-bit value. The lower the 422 value, the better the administrative distance. 424 The default FEC administrative distance order starting from the 425 lowest value SHOULD be 427 o Explicit SID assignment to a FEC that maps to a label outside the 428 SRGB irrespective of the owner MCC. An explicit SID assignment is 429 a static assignment of a label to a FEC such that the assignment 430 survives router reboot. 432 o An example of explicit SID allocation is static assignment of 433 a specific label to an adj-SID. 435 o An implementation of explicit SID assignment MUST guarantee 436 collision freeness on the same router 438 o Dynamic SID assignment: 440 o For all FEC types except for SR policy, the FEC types are 441 ordered using the default administrative distance ordering 442 defined by the implementation. 444 o Binding SID [RFC8402] assigned to SR Policy always has a 445 higher default administrative distance than the default 446 administrative distance of any other FEC type 448 A user SHOULD ensure that the same administrative distance preference 449 is used on all routers to maximize forwarding consistency. 451 The numerical sort across FECs SHOULD be performed as follows: 453 o Each FEC is assigned a FEC type encoded in 8 bits. The following 454 are the type code point for each SR FEC defined at the beginning 455 of this Section: 457 o 120: (Prefix, Routing Instance, Topology, Algorithm) 459 o 130: (next-hop, outgoing interface) 461 o 140: Parallel Adjacency [RFC8402] 463 o 150: an SR policy [RFC8402]. 465 o 160: Mirror SID [RFC8402] 467 o The numerical values above are mentioned to guide 468 implementation. If other numerical values are used, then the 469 numerical values must maintain the same greater-than ordering 470 of the numbers mentioned here. 472 o The fields of each FEC are encoded as follows 474 o Routing Instance ID represented by 16 bits. For routing 475 instances that are identified by less than 16 bits, encode the 476 Instance ID in the least significant bits while the most 477 significant bits are set to zero 479 o Address Family represented by 8 bits, where IPv4 encoded as 480 100 and IPv6 is encoded as 110. These numerical values are 481 mentioned to guide implementations. If other numerical values 482 are used, then the numerical value of IPv4 MUST be less than 483 the numerical value for IPv6 485 o All addresses are represented in 128 bits as follows 487 . IPv6 address is encoded natively 489 . IPv4 address is encoded in the most significant bits and 490 the remaining bits are set to zero 492 o All prefixes are represented by (128 + 8) bits. 494 . A prefix is encoded in the most significant bits and the 495 remaining bits are set to zero. 497 . The prefix length is encoded before the prefix in a field 498 of size 8 bits. 500 o Topology ID is represented by 16 bits. For routing instances 501 that identify topologies using less than 16 bits, encode the 502 topology ID in the least significant bits while the most 503 significant bits are set to zero 505 o Algorithm is encoded in a 16 bits field. 507 o The Color ID is encoded using 32 bits 509 o Choose the set of FECs of the smallest FEC type code point 511 o Out of these FECs, choose the FECs with the smallest address 512 family code point 514 o Encode the remaining set of FECs as follows 516 o Prefix, Routing Instance, Topology, Algorithm: (Prefix Length, 517 Prefix, routing_instance_id, Topology, SR Algorithm,) 519 o (next-hop, outgoing interface): (next-hop, 520 outgoing_interface_id) 522 o (number of adjacencies, list of next-hops in ascending 523 numerical order, list of outgoing interface IDs in ascending 524 numerical order). This encoding is used to encode a parallel 525 adjacency [RFC8402] 527 o (Endpoint, Color): (Endpoint_address, Color_id) 529 o (IP address): This is the encoding for a mirror SID FEC. The IP 530 address is encoded as described above in this section 532 o Select the FEC with the smallest numerical value 534 The numerical values mentioned in this section are for guidance only. 535 If other numerical values are used then the other numerical values 536 MUST maintain the same numerical ordering among different 538 2.5.2. Redistribution between Routing Protocol Instances 540 The following rule SHOULD be applied when redistributing SIDs with 541 prefixes between routing protocol instances: 543 o If the receiving instance's SRGB is the same as the SRGB of origin 544 instance, then 546 o the index is redistributed with the route 548 o Else 549 o the index is not redistributed and if needed it is the duty of 550 the receiving instance to allocate a fresh index relative to 551 its own SRGB. Note that in that case, the receiving instance 552 MUST compute its local label according to section 2.4 and 553 install it in FIB. 555 It is outside the scope of this document to define local node 556 behaviors that would allow to map the original index into a new index 557 in the receiving instance via the addition of an offset or other 558 policy means. 560 2.5.2.1. Illustration 562 A----IS-IS----B---OSPF----C-192.0.2.1/32 (20001) 564 Consider the simple topology above. 566 o A and B are in the IS-IS domain with SRGB [16000-17000] 568 o B and C are in OSPF domain with SRGB [20000-21000] 570 o B redistributes 192.0.2.1/32 into IS-IS domain 572 o In that case A learns 192.0.2.1/32 as an IP leaf connected to B as 573 usual for IP prefix redistribution 575 o However, according to the redistribution rule above rule, B 576 decides not to advertise any index with 192.0.2.1/32 into IS-IS 577 because the SRGB is not the same. 579 2.5.2.2. Illustration 2 581 Consider the example in the illustration described in Section 582 2.5.2.1. 584 When router B redistributes the prefix 192.0.2.1/32, router B decides 585 to allocate and advertise the same index 1 with the prefix 586 192.0.2.1/32 588 Within the SRGB of the IS-IS domain, index 1 corresponds to the local 589 label 16001 591 o Hence according to the redistribution rule above, router B 592 programs the incoming label 16001 in its FIB to match traffic 593 arriving from the IS-IS domain destined to the prefix 594 192.0.2.1/32. 596 2.6. Effect of Incoming Label Collision on Outgoing Label Programming 598 For the determination of the outgoing label to use, the ingress node 599 pushing new segments, and hence a stack of MPLS labels, MUST use, for 600 a given FEC, the same label that has been selected by the node 601 receiving the packet with that label exposed as top label. So in case 602 of incoming label collision on this receiving node, the ingress node 603 MUST resolve this collision using this same "Incoming Label Collision 604 resolution procedure", using the data of the receiving node. 606 In the general case, the ingress node may not have exactly the same 607 data of the receiving node, so the result may be different. This is 608 under the responsibility of the network operator. But in typical 609 case, e.g. where a centralized node or a distributed link state IGP 610 is used, all nodes would have the same database. However to minimize 611 the chance of misforwarding, a FEC that loses its incoming label to 612 the tie-breaking rules specified in Section 2.5 MUST NOT be 613 installed in FIB with an outgoing segment routing label based on the 614 SID corresponding to the lost incoming label. 616 Examples for the behavior specified in this section can be found in 617 Appendix A.3. 619 2.7. PUSH, CONTINUE, and NEXT 621 PUSH, NEXT, and CONTINUE are operations applied by the forwarding 622 plane. The specifications of these operations can be found in 623 [RFC8402]. This sub-section specifies how to implement each of these 624 operations in the MPLS forwarding plane. 626 2.7.1. PUSH 628 PUSH corresponds to pushing one or more labels on top of an incoming 629 packet then sending it out of a particular physical interface or 630 virtual interface, such as UDP tunnel [RFC7510] or L2TPv3 tunnel 631 [RFC4817], towards a particular next-hop. When pushing labels onto a 632 packet's label stack, the Time-to-Live (TTL) field ([RFC3032], 633 [RFC3443]) and the Traffic Class (TC) field ([RFC3032], [RFC5462]) of 634 each label stack entry must, of course, be set. This document does 635 not specify any set of rules for setting these fields; that is a 636 matter of local policy. Sections 2.10 and 2.11 specify additional 637 details about forwarding behavior. 639 2.7.2. CONTINUE 641 In the MPLS forwarding plane, the CONTINUE operation corresponds to 642 swapping the incoming label with an outgoing label. The value of the 643 outgoing label is calculated as specified in Sections 2.10 and 2.11. 645 2.7.3. NEXT 647 In the MPLS forwarding plane, NEXT corresponds to popping the topmost 648 label. The action before and/or after the popping depends on the 649 instruction associated with the active SID on the received packet 650 prior to the popping. For example suppose the active SID in the 651 received packet was an Adj-SID [RFC8402], then on receiving the 652 packet, the node applies NEXT operation, which corresponds to popping 653 the top most label, and then sends the packet out of the physical or 654 virtual interface (e.g. UDP tunnel [RFC7510] or L2TPv3 tunnel 655 [RFC4817]) towards the next-hop corresponding to the adj-SID. 657 2.7.3.1. Mirror SID 659 If the active SID in the received packet was a Mirror SID [RFC8402, 660 Section 5.1] allocated by the receiving router, then the receiving 661 router applies NEXT operation, which corresponds to popping the top 662 most label, then performs a lookup using the contents of the packet 663 after popping the outer most label in the mirrored forwarding table. 664 The method by which the lookup is made, and/or the actions applied to 665 the packet after the lookup in the mirror table depends on the 666 contents of the packet and the mirror table. Note that the packet 667 exposed after popping the top most label may or may not be an MPLS 668 packet. A mirror SID can be viewed as a generalization of the context 669 label in [RFC5331] because a mirror SID does not make any 670 assumptions about the packet underneath the top label. 672 2.8. MPLS Label Downloaded to FIB for Global and Local SIDs 674 The label corresponding to the global SID "Si" represented by the 675 global index "I" downloaded to FIB is used to match packets whose 676 active segment (and hence topmost label) is "Si". The value of this 677 label is calculated as specified in Section 2.4. 679 For Local SIDs, the MCC is responsible for downloading the correct 680 label value to FIB. For example, an IGP with SR extensions [I-D.ietf- 681 isis-segment-routing-extensions, I-D.ietf-ospf-segment-routing- 682 extensions] allocates and downloads the MPLS label corresponding to 683 an Adj-SID [RFC8402]. 685 2.9. Active Segment 687 When instantiated in the MPLS domain, the active segment on a packet 688 corresponds to the topmost label on the packet that is calculated 689 according to the procedure specified in Sections 2.10 and 2.11. When 690 arriving at a node, the topmost label corresponding to the active SID 691 matches the MPLS label downloaded to FIB as specified in Section 2.4. 693 2.10. Forwarding behavior for Global SIDs 695 This section specifies forwarding behavior, including the calculation 696 of outgoing labels, that corresponds to a global SID when applying 697 PUSH, CONTINUE, and NEXT operations in the MPLS forwarding plane. 699 This document covers the calculation of the outgoing label for the 700 top label only. The case where the outgoing label is not the top 701 label and is part of a stack of labels that instantiates a routing 702 policy or a traffic engineering tunnel is outside the scope of this 703 document and may be covered in other documents such as [I-D.ietf- 704 spring-segment-routing-policy]. 706 2.10.1. Forwarding for PUSH and CONTINUE of Global SIDs 708 Suppose an MCC on a router "R0" determines that PUSH or CONTINUE 709 operation is to be applied to an incoming packet related to the 710 global SID "Si" represented by the global index "I" and owned by the 711 router Ri before sending the packet towards a neighbor "N" directly 712 connected to "R0" through a physical or virtual interface such as UDP 713 tunnel [RFC7510] or L2TPv3 tunnel [RFC4817]. 715 The method by which the MCC on router "R0" determines that PUSH or 716 CONTINUE operation must be applied using the SID "Si" is beyond the 717 scope of this document. An example of a method to determine the SID 718 "Si" for PUSH operation is the case where IS-IS [I-D.ietf-isis- 719 segment-routing-extensions] receives the prefix-SID "Si" sub-TLV 720 advertised with prefix "P/m" in TLV 135 and the destination address 721 of the incoming IPv4 packet is covered by the prefix "P/m". 723 For CONTINUE operation, an example of a method to determine the SID 724 "Si" is the case where IS-IS [I-D.ietf-isis-segment-routing- 725 extensions] receives the prefix-SID "Si" sub-TLV advertised with 726 prefix "P" in TLV 135 and the top label of the incoming packet 727 matches the MPLS label in FIB corresponding to the SID "Si" on the 728 router "R0". 730 The forwarding behavior for PUSH and CONTINUE corresponding to the 731 SID "Si" 732 o If the neighbor "N" does not support SR or advertises an invalid 733 SRGB or a SRGB that is too small for the SID "Si" 735 o If it is possible to send the packet towards the neighbor "N" 736 using standard MPLS forwarding behavior as specified in 737 [RFC3031] and [RFC3032], then forward the packet. The method 738 by which a router decides whether it is possible to send the 739 packet to "N" or not is beyond the scope of this document. For 740 example, the router "R0" can use the downstream label 741 determined by another MCC, such as LDP [RFC5036], to send the 742 packet. 744 o Else if there are other useable next-hops, then use other next- 745 hops to forward the incoming packet. The method by which the 746 router "R0" decides on the possibility of using other next- 747 hops is beyond the scope of this document. For example, the 748 MCC on "R0" may chose the send an IPv4 packet without pushing 749 any label to another next-hop. 751 o Otherwise drop the packet. 753 o Else 755 o Calculate the outgoing label as specified in Section 2.4 using 756 the SRGB of the neighbor "N" 758 o If the operation is PUSH 760 . Push the calculated label according the MPLS label 761 pushing rules specified in [RFC3032] 763 o Else 765 . swap the incoming label with the calculated label 766 according to the label swapping rules in [RFC3032] 768 o Send the packet towards the neighbor "N" 770 2.10.2. Forwarding for NEXT Operation for Global SIDs 772 As specified in Section 2.7.3 NEXT operation corresponds to popping 773 the top most label. The forwarding behavior is as follows 775 o Pop the topmost label 776 o Apply the instruction associated with the incoming label that has 777 been popped 779 The action on the packet after popping the topmost label depends on 780 the instruction associated with the incoming label as well as the 781 contents of the packet right underneath the top label that got 782 popped. Examples of NEXT operation are described in Appendix A.1. 784 2.11. Forwarding Behavior for Local SIDs 786 This section specifies the forwarding behavior for local SIDs when SR 787 is instantiated over the MPLS forwarding plane. 789 2.11.1. Forwarding for PUSH Operation on Local SIDs 791 Suppose an MCC on a router "R0" determines that PUSH operation is to 792 be applied to an incoming packet using the local SID "Si" before 793 sending the packet towards a neighbor "N" directly connected to R0 794 through a physical or virtual interface such as UDP tunnel [RFC7510] 795 or L2TPv3 tunnel [RFC4817]. 797 An example of such local SID is an Adj-SID allocated and advertised 798 by IS-IS [I-D.ietf-isis-segment-routing-extensions]. The method by 799 which the MCC on "R0" determines that PUSH operation is to be applied 800 to the incoming packet is beyond the scope of this document. An 801 example of such method is backup path used to protect against a 802 failure using TI-LFA [I-D.bashandy-rtgwg-segment-routing-ti-lfa]. 804 As mentioned in [RFC8402], a local SID is specified by an MPLS label. 805 Hence the PUSH operation for a local SID is identical to label push 806 operation [RFC3032] using any MPLS label. The forwarding action after 807 pushing the MPLS label corresponding to the local SID is also 808 determined by the MCC. For example, if the PUSH operation was done to 809 forward a packet over a backup path calculated using TI-LFA, then the 810 forwarding action may be sending the packet to a certain neighbor 811 that will in turn continue to forward the packet along the backup 812 path 814 2.11.2. Forwarding for CONTINUE Operation for Local SIDs 816 A local SID on a router "R0" corresponds to a local label. In such 817 scenario, the outgoing label towards a next-hop "N" is determined by 818 the MCC running on the router "R0"and the forwarding behavior for 819 CONTINUE operation is identical to swap operation [RFC3032] on an 820 MPLS label. 822 2.11.3. Outgoing label for NEXT Operation for Local SIDs 824 NEXT operation for Local SIDs is identical to NEXT operation for 825 global SIDs specified in Section 2.10.2. 827 3. IANA Considerations 829 This document does not make any request to IANA. 831 4. Manageability Considerations 833 This document describes the applicability of Segment Routing over the 834 MPLS data plane. Segment Routing does not introduce any change in 835 the MPLS data plane. Manageability considerations described in 836 [RFC8402] applies to the MPLS data plane when used with Segment 837 Routing. SR OAM use cases for the MPLS data plane are defined in 838 [RFC8403]. SR OAM procedures for the MPLS data plane are defined in 839 [RFC8287]. 841 5. Security Considerations 843 This document does not introduce additional security requirements and 844 mechanisms other than the ones described in [RFC8402]. 846 6. Contributors 848 The following contributors have substantially helped the definition 849 and editing of the content of this document: 851 Martin Horneffer 852 Deutsche Telekom 853 Email: Martin.Horneffer@telekom.de 855 Wim Henderickx 856 Nokia 857 Email: wim.henderickx@nokia.com 859 Jeff Tantsura 860 Email: jefftant@gmail.com 861 Edward Crabbe 862 Email: edward.crabbe@gmail.com 864 Igor Milojevic 865 Email: milojevicigor@gmail.com 867 Saku Ytti 868 Email: saku@ytti.fi 870 7. Acknowledgements 872 The authors would like to thank Les Ginsberg, Chris Bowers, Himanshu 873 Shah, Adrian Farrel, Alexander Vainshtein, Przemyslaw Krol, Darren 874 Dukes, and Zafar Ali for their valuable comments on this document. 876 This document was prepared using 2-Word-v2.0.template.dot. 878 8. References 880 8.1. Normative References 882 [RFC8402] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and 883 R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 884 10.17487/RFC8402 July 2018, . 887 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 888 Requirement Levels", BCP 14, RFC 2119, DOI 889 0.17487/RFC2119, March 1997, . 892 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 893 Label Switching Architecture", RFC 3031, DOI 894 10.17487/RFC3031, January 2001, . 897 [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., 898 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack 899 Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001, 900 . 902 [RFC3443] P. Agarwal, P. and Akyol, B. "Time To Live (TTL) Processing 903 in Multi-Protocol Label Switching (MPLS) Networks", RFC 904 3443, DOI 10.17487/RFC3443, January 2003, . 907 [RFC5462] Andersson, L., and Asati, R., " Multiprotocol Label 908 Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to 909 "Traffic Class" Field", RFC 5462, DOI 10.17487/RFC5462, 910 February 2009, . 912 [RFC7274] K. Kompella, L. Andersson, and A. Farrel, "Allocating and 913 Retiring Special-Purpose MPLS Labels", RFC7274 DOI 914 10.17487/RFC7274, May 2014 917 [RFC8174] B. Leiba, " Ambiguity of Uppercase vs Lowercase in RFC 2119 918 Key Words", RFC7274 DOI 10.17487/RFC8174, May 2017 919 921 8.2. Informative References 923 [I-D.ietf-isis-segment-routing-extensions] Previdi, S., Filsfils, C., 924 Bashandy, A., Gredler, H., Litkowski, S., Decraene, B., and 925 j. jefftant@gmail.com, "IS-IS Extensions for Segment 926 Routing", draft-ietf-isis-segment-routing-extensions-13 927 (work in progress), June 2017. 929 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] Psenak, P., 930 Previdi, S., Filsfils, C., Gredler, H., Shakir, R., 931 Henderickx, W., and J. Tantsura, "OSPFv3 Extensions for 932 Segment Routing", draft-ietf-ospf-ospfv3-segment-routing- 933 extensions-09 (work in progress), March 2017. 935 [I-D.ietf-ospf-segment-routing-extensions] Psenak, P., Previdi, S., 936 Filsfils, C., Gredler, H., Shakir, R., Henderickx, W., and 937 J. Tantsura, "OSPF Extensions for Segment Routing", draft- 938 ietf-ospf-segment-routing-extensions-16 (work in progress), 939 May 2017. 941 [I-D.ietf-spring-segment-routing-ldp-interop] Filsfils, C., Previdi, 942 S., Bashandy, A., Decraene, B., and S. Litkowski, "Segment 943 Routing interworking with LDP", draft-ietf-spring-segment- 944 routing-ldp-interop-08 (work in progress), June 2017. 946 [I-D.bashandy-rtgwg-segment-routing-ti-lfa], Bashandy, A., Filsfils, 947 C., Decraene, B., Litkowski, S., Francois, P., Voyer, P. 948 Clad, F., and Camarillo, P., "Topology Independent Fast 949 Reroute using Segment Routing", draft-bashandy-rtgwg- 950 segment-routing-ti-lfa-05 (work in progress), October 2018, 952 [RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B., 953 Litkowski, S., Horneffer, M., and R. Shakir, "Source Packet 954 Routing in Networking (SPRING) Problem Statement and 955 Requirements", RFC 7855, DOI 10.17487/RFC7855, May 2016, 956 . 958 [RFC5036] Andersson, L., Acreo, AB, Minei, I., Thomas, B., " LDP 959 Specification", RFC5036, DOI 10.17487/RFC5036, October 960 2007, 962 [RFC5331] Aggarwal, R., Rekhter, Y., Rosen, E., " MPLS Upstream Label 963 Assignment and Context-Specific Label Space", RFC5331 DOI 964 10.17487/RFC5331, August 2008, . 967 [RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black, 968 "Encapsulating MPLS in UDP", RFC 7510, DOI 969 10.17487/RFC7510, April 2015, . 972 [RFC4817] Townsley, M., Pignataro, C., Wainner, S., Seely, T., Young, 973 T., "Encapsulation of MPLS over Layer 2 Tunneling Protocol 974 Version 3", RFC4817, DOI 10.17487/RFC4817, March 2007, 975 977 [RFC8287] N. Kumar, C. Pignataro, G. Swallow, N. Akiya, S. Kini, and 978 M. Chen " Label Switched Path (LSP) Ping/Traceroute for 979 Segment Routing (SR) IGP-Prefix and IGP-Adjacency Segment 980 Identifiers (SIDs) with MPLS Data Planes" RFC8287, DOI 981 10.17487/RFC8287, December 2017, https://www.rfc- 982 editor.org/info/rfc8287 984 [RFC8403] R. Geib, C. Filsfils, C. Pignataro, N. Kumar, "A Scalable 985 and Topology-Aware MPLS Data-Plane Monitoring System", 986 RFC8403, DOI 10.17487/RFC8403, July 2018, 989 [I-D.ietf-spring-segment-routing-policy] Filsfils, C., Sivabalan, 990 S., Raza, K., Liste, J. , Clad, F., Voyer, D., Bogdanov, A., 991 Mattes, P., " Segment Routing Policy for Traffic Engineering", 992 draft-ietf-spring-segment-routing-policy-01 (work in progress), June 993 2018 995 9. Authors' Addresses 997 Ahmed Bashandy (editor) 998 Arrcus 1000 Email: abashandy.ietf@gmail.com 1002 Clarence Filsfils (editor) 1003 Cisco Systems, Inc. 1004 Brussels 1005 BE 1007 Email: cfilsfil@cisco.com 1009 Stefano Previdi 1010 Cisco Systems, Inc. 1011 Italy 1013 Email: stefano@previdi.net 1015 Bruno Decraene 1016 Orange 1017 FR 1019 Email: bruno.decraene@orange.com 1020 Stephane Litkowski 1021 Orange 1022 FR 1024 Email: stephane.litkowski@orange.com 1026 Rob Shakir 1027 Google 1028 US 1030 Email: robjs@google.com 1032 Appendix A. Examples 1034 A.1. IGP Segments Example 1036 Consider the network diagram of Figure 1 and the IP address and IGP 1037 Segment allocation of Figure 2. Assume that the network is running 1038 IS-IS with SR extensions [I-D.ietf-isis-segment-routing-extensions] 1039 and all links have the same metric. The following examples can be 1040 constructed. 1042 +--------+ 1043 / \ 1044 R0-----R1-----R2----------R3-----R8 1045 | \ / | 1046 | +--R4--+ | 1047 | | 1048 +-----R5-----+ 1050 Figure 1: IGP Segments - Illustration 1052 +-----------------------------------------------------------+ 1053 | IP address allocated by the operator: | 1054 | 192.0.2.1/32 as a loopback of R1 | 1055 | 192.0.2.2/32 as a loopback of R2 | 1056 | 192.0.2.3/32 as a loopback of R3 | 1057 | 192.0.2.4/32 as a loopback of R4 | 1058 | 192.0.2.5/32 as a loopback of R5 | 1059 | 192.0.2.8/32 as a loopback of R8 | 1060 | 198.51.100.9/32 as an anycast loopback of R4 | 1061 | 198.51.100.9/32 as an anycast loopback of R5 | 1062 | | 1063 | SRGB defined by the operator as 1000-5000 | 1064 | | 1065 | Global IGP SID indices allocated by the operator: | 1066 | 1 allocated to 192.0.2.1/32 | 1067 | 2 allocated to 192.0.2.2/32 | 1068 | 3 allocated to 192.0.2.3/32 | 1069 | 4 allocated to 192.0.2.4/32 | 1070 | 8 allocated to 192.0.2.8/32 | 1071 | 1009 allocated to 198.51.100.9/32 | 1072 | | 1073 | Local IGP SID allocated dynamically by R2 | 1074 | for its "north" adjacency to R3: 9001 | 1075 | for its "north" adjacency to R3: 9003 | 1076 | for its "south" adjacency to R3: 9002 | 1077 | for its "south" adjacency to R3: 9003 | 1078 +-----------------------------------------------------------+ 1080 Figure 2: IGP Address and Segment Allocation - Illustration 1082 Suppose R1 wants to send an IPv4 packet P1 to R8. In this case, R1 1083 needs to apply PUSH operation to the IPv4 packet. 1085 Remember that the SID index "8" is a global IGP segment attached to 1086 the IP prefix 192.0.2.8/32. Its semantic is global within the IGP 1087 domain: any router forwards a packet received with active segment 8 1088 to the next-hop along the ECMP-aware shortest-path to the related 1089 prefix. 1091 R2 is the next-hop along the shortest path towards R8. By applying 1092 the steps in Section 2.8 the outgoing label downloaded to R1's FIB 1093 corresponding to the global SID index 8 is 1008 because the SRGB of 1094 R2 is [1000,5000] as shown in Figure 2. 1096 Because the packet is IPv4, R1 applies the PUSH operation using the 1097 label value 1008 as specified in Section 2.10.1. The resulting MPLS 1098 header will have the "S" bit [RFC3032] set because it is followed 1099 directly by an IPv4 packet. 1101 The packet arrives at router R2. Because the top label 1008 1102 corresponds to the IGP SID "8", which is the prefix-SID attached to 1103 the prefix 192.0.2.8/32 owned by the node R8, then the instruction 1104 associated with the SID is "forward the packet using all ECMP/UCMP 1105 interfaces and all ECMP/UCMP next-hop(s) along the shortest/useable 1106 path(s) towards R8". Because R2 is not the penultimate hop, R2 1107 applies the CONTINUE operation to the packet and sends it to R3 using 1108 one of the two links connected to R3 with top label 1008 as specified 1109 in Section 2.10.1. 1111 R3 receives the packet with top label 1008. Because the top label 1112 1008 corresponds to the IGP SID "8", which is the prefix-SID attached 1113 to the prefix 192.0.2.8/32 owned by the node R8, then the instruction 1114 associated with the SID is "send the packet using all ECMP interfaces 1115 and all next-hop(s) along the shortest path towards R8". Because R3 1116 is the penultimate hop, we assume that R3 performs penumtimate hop 1117 popping, which corresponds to the NEXT operation, then sends the 1118 packet to R8. The NEXT operation results in popping the outer label 1119 and sending the packet as a pure IPv4 packet to R8. 1121 In conclusion, the path followed by P1 is R1-R2--R3-R8. The ECMP- 1122 awareness ensures that the traffic be load-shared between any ECMP 1123 path, in this case the two links between R2 and R3. 1125 A.2. Incoming Label Collision Examples 1127 This section describes few examples to illustrate the handling of 1128 label collision described in Section 2.5. 1130 For the examples in this section, we assume that Node A has the 1131 following: 1133 o OSPF default admin distance for implementation=50 1135 o ISIS default admin distance for implementation=60 1137 A.2.1. Example 1 1139 Illustration of incoming label collision resolution for the same FEC 1140 type using MCC administrative distance. 1142 FEC1: 1144 o OSPF prefix SID advertisement from node B for 198.51.100.5/32 with 1145 index=5 1147 o OSPF SRGB on node A = [1000,1999] 1149 o Incoming label=1005 1151 FEC2: 1152 o ISIS prefix SID advertisement from node C for 203.0.113.105/32 1153 with index=5 1155 o ISIS SRGB on node A = [1000,1999] 1157 o Incoming label=1005 1159 FEC1 and FEC2 both use dynamic SID assignment. Since neither ofthe 1160 FEC types is SR Policy, we use the default admin distances of 50 and 1161 60 to break the tie. So FEC1 wins. 1163 A.2.2. Example 2 1165 Illustration of incoming label collision resolution for different FEC 1166 types using the MCC administrative distance. 1168 FEC1: 1169 o Node A receives an OSPF prefix sid advertisement from node B for 1170 198.51.100.6/32 with index=6 1172 o OSPF SRGB on node A = [1000,1999] 1174 o Hence the incoming label on node A corresponding to 1175 198.51.100.6/32 is 1006 1177 FEC2: 1178 ISIS on node A assigns the label 1006 to the globally significant 1179 adj-SID (I.e. when advertised the "L" flag is clear in the adj-SID 1180 sub-TLV as described in [I-D.ietf-isis-segment-routing-extensions]) 1181 towards one of its neighbors. Hence the incoming label corresponding 1182 to this adj-SID 1006. Assume Node A allocates this adj-SID 1183 dynamically, and it may differ across router reboots. 1185 FEC1 and FEC2 both use dynamic SID assignment. Since neither of the 1186 FEC types is SR Policy, we use the default admin distances of 50 and 1187 60 to break the tie. So FEC1 wins. 1189 A.2.3. Example 3 1191 Illustration of incoming label collision resolution based on 1192 preferring static over dynamic SID assignment 1194 FEC1: 1195 OSPF on node A receives a prefix SID advertisement from node B for 1196 198.51.100.7/32 with index=7. Assuming that the OSPF SRGB on node A 1197 is [1000,1999], then incoming label corresponding to 198.51.100.7/32 1198 is 1007 1200 FEC2: 1201 The operator on node A configures ISIS on node A to assign the label 1202 1007 to the globally significant adj-SID (I.e. when advertised the 1203 "L" flag is clear in the adj-SID sub-TLV as described in [I-D.ietf- 1204 isis-segment-routing-extensions]) towards one of its neighbor 1205 advertisement from node A with label=1007 1207 Node A assigns this adj-SID explicitly via configuration, so the adj- 1208 SID survives router reboots. 1210 FEC1 uses dynamic SID assignment, while FEC2 uses explicit SID 1211 assignment. So FEC2 wins. 1213 A.2.4. Example 4 1215 Illustration of incoming label collision resolution using FEC type 1216 default administrative distance 1218 FEC1: 1219 OSPF on node A receives a prefix SID advertisement from node B for 1220 198.51.100.8/32 with index=8. Assuming that OSPF SRGB on node A = 1221 [1000,1999], the incoming label corresponding to 198.51.100.8/32 is 1222 1008. 1224 FEC2: 1225 Suppose the SR Policy advertisement from controller to node A for the 1226 policy identified by (Endpoint = 192.0.2.208, color = 100) and 1227 consisting of SID-List = assigns the globally significant 1228 Binding-SID label 1008 1230 From the point of view of node A, FEC1 and FEC2 both use dynamic SID 1231 assignment. Based on the default administrative distance outlined in 1232 Section 2.5.1, the binding SID has a higher administrative distance 1233 than the prefix-SID and hence FEC1 wins. 1235 A.2.5. Example 5 1237 Illustration of incoming label collision resolution based on FEC type 1238 preference 1240 FEC1: 1241 ISIS on node A receives a prefix SID advertisement from node B for 1242 203.0.113.110/32 with index=10. Assuming that the ISIS SRGB on node A 1243 is [1000,1999], then incoming label corresponding to 203.0.113.110/32 1244 is 1010. 1246 FEC2: 1247 ISIS on node A assigns the label 1010 to the globally significant 1248 adj-SID (I.e. when advertised the "L" flag is clear in the adj-SID 1249 sub-TLV as described in [I-D.ietf-isis-segment-routing-extensions]) 1250 towards one of its neighbors). 1252 Node A allocates this adj-SID dynamically, and it may differ across 1253 router reboots. Hence both FEC1 and FEC2 both use dynamic SID 1254 assignment. 1256 Since both FECs are from the same MCC, they have the same default 1257 admin distance. So we compare FEC type code-point. FEC1 has FEC type 1258 code-point=120, while FEC2 has FEC type code-point=130. Therefore, 1259 FEC1 wins. 1261 A.2.6. Example 6 1263 Illustration of incoming label collision resolution based on address 1264 family preference. 1266 FEC1: 1267 ISIS on node A receives prefix SID advertisement from node B for 1268 203.0.113.111/32 with index 11. Assuming that the ISIS SRGB on node A 1269 is [1000,1999], the incoming label on node A for 203.0.113.111/32 is 1270 1011. 1272 FEC2: 1273 ISIS on node A prefix SID advertisement from node C for 1274 2001:DB8:1000::11/128 with index=11. Assuming that the ISIS SRGB on 1275 node A is [1000,1999], the incoming label on node A for 1276 2001:DB8:1000::11/128 is 1011 1278 FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are 1279 from the same MCC, they have the same default admin distance. So we 1280 compare FEC type code-point. Both FECs have FEC type code-point=120. 1281 So we compare address family. Since IPv4 is preferred over IPv6, FEC1 1282 wins. 1284 A.2.7. Example 7 1286 Illustration incoming label collision resolution based on prefix 1287 length. 1289 FEC1: 1290 ISIS on node A receives a prefix SID advertisement from node B for 1291 203.0.113.112/32 with index 12. Assuming that ISIS SRGB on node A is 1292 [1000,1999], the incoming label for 203.0.113.112/32 on node A is 1293 1012. 1295 FEC2: 1296 ISIS on node A receives a prefix SID advertisement from node C for 1297 203.0.113.128/30 with index 12. Assuming that the ISIS SRGB on node A 1298 is [1000,1999], then incoming label for 203.0.113.128/30 on node A is 1299 1012 1301 FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are 1302 from the same MCC, they have the same default admin distance. So we 1303 compare FEC type code-point. Both FECs have FEC type code-point=120. 1304 So we compare address family. Both are IPv4 address family, so we 1305 compare prefix length. FEC1 has prefix length=32, and FEC2 has 1306 prefix length=30, so FEC2 wins. 1308 A.2.8. Example 8 1310 Illustration of incoming label collision resolution based on the 1311 numerical value of the FECs. 1313 FEC1: 1314 ISIS on node A receives a prefix SID advertisement from node B for 1315 203.0.113.113/32 with index 13. Assuming that ISIS SRGB on node A is 1317 [1000,1999], then the incoming label for 203.0.113.113/32 on node A 1318 is 1013 1320 FEC2: 1321 ISIS on node A receives a prefix SID advertisement from node C for 1322 203.0.113.213/32 with index 13. Assuming that ISIS SRGB on node A is 1323 [1000,1999], then the incoming label for 203.0.113.213/32 on node A 1324 is 1013 1326 FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are 1327 from the same MCC, they have the same default admin distance. So we 1328 compare FEC type code-point. Both FECs have FEC type code-point=120. 1329 So we compare address family. Both are IPv4 address family, so we 1330 compare prefix length. Prefix lengths are the same, so we compare 1331 prefix. FEC1 has the lower prefix, so FEC1 wins. 1333 A.2.9. Example 9 1335 Illustration of incoming label collision resolution based on routing 1336 instance ID. 1338 FEC1: 1339 ISIS on node A receives a prefix SID advertisement from node B for 1340 203.0.113.114/32 with index 14. Assume that this ISIS instance on 1341 node A has the Routing Instance ID 1000 and SRGB [1000,1999]. Hence 1342 the incoming label for 203.0.113.114/32 on node A is 1014 1344 FEC2: 1345 ISIS on node A receives a prefix SID advertisement from node C for 1346 203.0.113.114/32 with index=14. Assume that this is another instance 1347 of ISIS on node A with a different routing Instance ID 2000 but the 1348 same SRGB [1000,1999]. Hence incoming label for 203.0.113.114/32 on 1349 node A 1014 1351 These two FECs match all the way through the prefix length and 1352 prefix. So Routing Instance ID breaks the tie, with FEC1 winning. 1354 A.2.10. Example 10 1356 Illustration of incoming label collision resolution based on topology 1357 ID. 1359 FEC1: 1360 ISIS on node A receives a prefix SID advertisement from node B for 1361 203.0.113.115/32 with index=15. Assume that this ISIS instance on 1362 node A has Routing Instance ID 1000. Assume that the prefix 1363 advertisement of 203.0.113.115/32 was received in ISIS Multi-topology 1364 advertisement with ID = 50. If the ISIS SRGB for this routing 1365 instance on node A is [1000,1999], then incoming label of 1366 203.0.113.115/32 for topology 50 on node A is 1015 1368 FEC2: 1369 ISIS on node A receives a prefix SID advertisement from node C for 1370 203.0.113.115/32 with index 15. Assume that it is the same routing 1371 Instance ID = 1000 but 203.0.113.115/32 was advertised with a 1372 different ISIS Multi-topology ID = 40. If the ISIS SRGB on node A is 1373 [1000,1999], then incoming label of 203.0.113.115/32 for topology 40 1374 on node A is also 1015 1376 These two FECs match all the way through the prefix length, prefix, 1377 and Routing Instance ID. We compare ISIS Multi-topology ID, so FEC2 1378 wins. 1380 A.2.11. Example 11 1382 Illustration of incoming label collision for resolution based on 1383 algorithm ID. 1385 FEC1: 1386 ISIS on node A receives a prefix SID advertisement from node B for 1387 203.0.113.116/32 with index=16 Assume that ISIS on node A has Routing 1388 Instance ID = 1000. Assume that node B advertised 203.0.113.116/32 1389 with ISIS Multi-topology ID = 50 and SR algorithm = 0. Assume that 1390 the ISIS SRGB on node A = [1000,1999]. Hence the incoming label 1391 corresponding to this advertisement of 203.0.113.116/32 is 1016. 1393 FEC2: 1394 ISIS on node A receives a prefix SID advertisement from node C for 1395 203.0.113.116/32 with index=16. Assume that it is the same ISIS 1396 instance on node A with Routing Instance ID = 1000. Also assume that 1397 node C advertised 203.0.113.116/32 with ISIS Multi-topology ID = 50 1398 but with SR algorithm = 22. Since it is the same routing instance, 1399 the SRGB on node A = [1000,1999]. Hence the incoming label 1400 corresponding to this advertisement of 203.0.113.116/32 by node C is 1401 also 1016. 1403 These two FECs match all the way through the prefix length, prefix, 1404 and Routing Instance ID, and Multi-topology ID. We compare SR 1405 algorithm ID, so FEC1 wins. 1407 A.2.12. Example 12 1409 Illustration of incoming label collision resolution based on FEC 1410 numerical value and independent of how the SID assigned to the 1411 colliding FECs. 1413 FEC1: 1414 ISIS on node A receives a prefix SID advertisement from node B for 1415 203.0.113.117/32 with index 17. Assume that the ISIS SRGB on node A 1416 is [1000,1999], then the incoming label is 1017 1418 FEC2: 1419 Suppose there is an ISIS mapping server advertisement (SID/Label 1420 Binding TLV) from node D has Range 100 and Prefix = 203.0.113.1/32. 1421 Suppose this mapping server advertisement generates 100 mappings, one 1422 of which maps 203.0.113.17/32 to index 17. Assuming that it is the 1423 same ISIS instance, then the SRGB is [1000,1999] and hence the 1424 incoming label for 1017. 1426 The fact that FEC1 comes from a normal prefix SID advertisement and 1427 FEC2 is generated from a mapping server advertisement is not used as 1428 a tie-breaking parameter. Both FECs use dynamic SID assignment, are 1429 from the same MCC, have the same FEC type code-point=120. Their 1430 prefix lengths are the same as well. FEC2 wins based on lower 1431 numerical prefix value, since 203.0.113.17 is less than 1432 203.0.113.117. 1434 A.2.13. Example 13 1436 Illustration of incoming label collision resolution based on address 1437 family preference 1439 FEC1: 1440 SR Policy advertisement from controller to node A. Endpoint 1441 address=2001:DB8:3000::100, color=100, SID-List= and the 1442 Binding-SID label=1020 1444 FEC2: 1445 SR Policy advertisement from controller to node A. Endpoint 1446 address=192.0.2.60, color=100, SID-List= and the Binding-SID 1447 label=1020 1448 The FECs match through the tie-breaks up to and including having the 1449 same FEC type code-point=140. FEC2 wins based on IPv4 address family 1450 being preferred over IPv6. 1452 A.2.14. Example 14 1454 Illustration of incoming label resolution based on numerical value of 1455 the policy endpoint. 1457 FEC1: 1458 SR Policy advertisement from controller to node A. Endpoint 1459 address=192.0.2.70, color=100, SID-List= and Binding-SID 1460 label=1021 1462 FEC2: 1463 SR Policy advertisement from controller to node A Endpoint 1464 address=192.0.2.71, color=100, SID-List= and Binding-SID 1465 label=1021 1467 The FECs match through the tie-breaks up to and including having the 1468 same address family. FEC1 wins by having the lower numerical endpoint 1469 address value. 1471 A.3. Examples for the Effect of Incoming Label Collision on Outgoing 1472 Label 1474 This section presents examples to illustrate the effect of incoming 1475 label collision on the selection of the outgoing label described in 1476 Section 2.6. 1478 A.3.1. Example 1 1480 Illustration of the effect of incoming label resolution on the 1481 outgoing label 1483 FEC1: 1484 ISIS on node A receives a prefix SID advertisement from node B for 1485 203.0.113.122/32 with index 22. Assuming that the ISIS SRGB on node A 1486 is [1000,1999] the corresponding incoming label is 1022. 1488 FEC2: 1489 ISIS on node A receives a prefix SID advertisement from node C for 1490 203.0.113.222/32 with index=22 Assuming that the ISIS SRGB on node A 1491 is [1000,1999] the corresponding incoming label is 1022. 1493 FEC1 wins based on lowest numerical prefix value. This means that 1494 node A installs a transit MPLS forwarding entry to SWAP incoming 1495 label 1022, with outgoing label N and use outgoing interface I. N is 1496 determined by the index associated with FEC1 (index 22) and the SRGB 1497 advertised by the next-hop node on the shortest path to reach 1498 203.0.113.122/32. 1500 Node A will generally also install an imposition MPLS forwarding 1501 entry corresponding to FEC1 for incoming prefix=203.0.113.122/32 1502 pushing outgoing label N, and using outgoing interface I. 1504 The rule in Section 2.6 means node A MUST NOT install an ingress 1505 MPLS forwarding entry corresponding to FEC2 (the losing FEC, which 1506 would be for prefix 203.0.113.222/32). 1508 A.3.2. Example 2 1510 Illustration of the effect of incoming label collision resolution on 1511 outgoing label programming on node A 1513 FEC1: 1514 o SR Policy advertisement from controller to node A 1516 o Endpoint address=192.0.2.80, color=100, SID-List= 1518 o Binding-SID label=1023 1520 FEC2: 1521 o SR Policy advertisement from controller to node A 1523 o Endpoint address=192.0.2.81, color=100, SID-List= 1525 o Binding-SID label=1023 1527 FEC1 wins by having the lower numerical endpoint address value. This 1528 means that node A installs a transit MPLS forwarding entry to SWAP 1529 incoming label=1023, with outgoing labels and outgoing interface 1530 determined by the SID-List for FEC1. 1532 In this example, we assume that node A receives two BGP/VPN routes: 1534 o R1 with VPN label=V1, BGP next-hop = 192.0.2.80, and color=100, 1536 o R2 with VPN label=V2, BGP next-hop = 192.0.2.81, and color=100, 1537 We also assume that A has a BGP policy which matches on color=100 1538 that allows that its usage as SLA steering information. In this case, 1539 node A will install a VPN route with label stack = 1540 (corresponding to FEC1). 1542 The rule described in section 2.6 means that node A MUST NOT install 1543 a VPN route with label stack = (corresponding to FEC2.)