idnits 2.17.1 draft-ietf-spring-segment-routing-mpls-19.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 : ---------------------------------------------------------------------------- == There are 1 instance of lines with non-RFC6890-compliant IPv4 addresses in the document. If these are example addresses, they should be changed. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (March 28, 2019) is 1856 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) == Missing Reference: '16000-17000' is mentioned on line 573, but not defined == Missing Reference: '20000-21000' is mentioned on line 575, but not defined -- Looks like a reference, but probably isn't: '1000' on line 1501 -- Looks like a reference, but probably isn't: '5000' on line 1104 -- Looks like a reference, but probably isn't: '1999' on line 1501 == Outdated reference: A later version (-25) exists of draft-ietf-isis-segment-routing-extensions-13 == Outdated reference: A later version (-23) exists of draft-ietf-ospf-ospfv3-segment-routing-extensions-09 == Outdated reference: A later version (-27) exists of draft-ietf-ospf-segment-routing-extensions-16 == Outdated reference: A later version (-15) exists of draft-ietf-spring-segment-routing-ldp-interop-08 == Outdated reference: A later version (-22) exists of draft-ietf-spring-segment-routing-policy-01 Summary: 0 errors (**), 0 flaws (~~), 9 warnings (==), 4 comments (--). 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: September 2019 S. Previdi, 5 Cisco Systems, Inc. 6 B. Decraene 7 S. Litkowski 8 Orange 9 R. Shakir 10 Google 11 March 28, 2019 13 Segment Routing with MPLS data plane 14 draft-ietf-spring-segment-routing-mpls-19 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 September 28, 2019. 42 Copyright Notice 44 Copyright (c) 2019 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..................................10 68 2.5.2. Redistribution between Routing Protocol Instances...13 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......18 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...19 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 SHOULD 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 [RFC8402] 343 o (Mirrored SID) The Mirrored SID [RFC8402, 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} map to the same incoming SR MPLS label "L1". 356 Suppose an anycast prefix is advertised with a prefix-SID by some, 357 but not all, of the nodes that advertise that prefix. If the prefix- 358 SID subTLVs result in mapping that anycast prefix to the same 359 incoming label, then the advertisement of the prefix-SID by some, but 360 not all, of advertising nodes SHOULD NOT be treated as a label 361 collision. 363 An implementation MUST NOT allow the MCCs belonging to the same 364 router to assign the same incoming label to more than one SR FEC. 366 The objective of the following steps is to deterministically install 367 in the MPLS Incoming Label Map, also known as label FIB, a single FEC 368 with the incoming label "L1". Remaining FECs may be installed in the 369 IP FIB without incoming label. 371 The procedure in this section relies completely on the local FEC and 372 label database within a given router. 374 The collision resolution procedure is as follows 376 1. Given the SIDs of the set of FECs, {FEC1, FEC2,..., FECk} map to 377 the same MPLS label "L1". 379 2. Within an MCC, apply tie-breaking rules to select one FEC only and 380 assign the label to it. The losing FECs are handled as if no 381 labels are attached to them. The losing FECs with a non-zero 382 algorithm are not installed in FIB. 384 a. If the same set of FECs are attached to the same label "L1", 385 then the tie-breaking rules MUST always select the same FEC 386 irrespective of the order in which the FECs and the label "L1" 387 are received. In other words, the tie-breaking rule MUST be 388 deterministic. For example, a first-come-first-serve tie- 389 breaking is not allowed. 391 3. If there is still collision between the FECs belonging to 392 different MCCs, then re-apply the tie-breaking rules to the 393 remaining FECs to select one FEC only and assign the label to that 394 FEC 396 4. Install into the IP FIB the selected FEC and its incoming label in 397 the label FIB. 399 5. The remaining FECs with the default algorithm (see the 400 specification of prefix-SID algorithm [RFC8402]) are installed in 401 the FIB natively, such as pure IP entries in case of Prefix FEC, 402 without any incoming labels corresponding to their SIDs. The 403 remaining FECs with a non-zero algorithm are not installed in the 404 FIB. 406 2.5.1. Tie-breaking Rules 408 The default tie-breaking rules are specified as follows: 410 1. if FECi has the lowest FEC administrative distance among the 411 competing FECs as defined in this section below, filter away all 412 the competing FECs with higher administrative distance. 414 2. if more than one competing FEC remains after step 1, select the 415 smallest numerical FEC value 417 These rules deterministically select the FEC to install in the MPLS 418 forwarding plane for the given incoming label. 420 This document defines the default tie breaking rules that SHOULD be 421 implemented. An implementation MAY choose to support different tie- 422 breaking rules and MAY use one of the these instead of the default 423 tie-breaking rules. All routers in a routing domain SHOULD use the 424 same tie-breaking rules to maximize forwarding consistency. 426 Each FEC is assigned an administrative distance. The FEC 427 administrative distance is encoded as an 8-bit value. The lower the 428 value, the better the administrative distance. 430 The default FEC administrative distance order starting from the 431 lowest value SHOULD be 433 o Explicit SID assignment to a FEC that maps to a label outside the 434 SRGB irrespective of the owner MCC. An explicit SID assignment is 435 a static assignment of a label to a FEC such that the assignment 436 survives router reboot. 438 o An example of explicit SID allocation is static assignment of 439 a specific label to an adj-SID. 441 o An implementation of explicit SID assignment MUST guarantee 442 collision freeness on the same router 444 o Dynamic SID assignment: 446 o For all FEC types except for SR policy, the FEC types are 447 ordered using the default administrative distance ordering 448 defined by the implementation. 450 o Binding SID [RFC8402] assigned to SR Policy always has a 451 higher default administrative distance than the default 452 administrative distance of any other FEC type 454 A user SHOULD ensure that the same administrative distance preference 455 is used on all routers to maximize forwarding consistency. 457 The numerical sort across FECs SHOULD be performed as follows: 459 o Each FEC is assigned a FEC type encoded in 8 bits. The following 460 are the type code point for each SR FEC defined at the beginning 461 of this Section: 463 o 120: (Prefix, Routing Instance, Topology, Algorithm) 465 o 130: (next-hop, outgoing interface) 467 o 140: Parallel Adjacency [RFC8402] 469 o 150: an SR policy [RFC8402]. 471 o 160: Mirror SID [RFC8402] 473 o The numerical values above are mentioned to guide 474 implementation. If other numerical values are used, then the 475 numerical values must maintain the same greater-than ordering 476 of the numbers mentioned here. 478 o The fields of each FEC are encoded as follows 480 o Routing Instance ID represented by 16 bits. For routing 481 instances that are identified by less than 16 bits, encode the 482 Instance ID in the least significant bits while the most 483 significant bits are set to zero 485 o Address Family represented by 8 bits, where IPv4 encoded as 486 100 and IPv6 is encoded as 110. These numerical values are 487 mentioned to guide implementations. If other numerical values 488 are used, then the numerical value of IPv4 MUST be less than 489 the numerical value for IPv6 491 o All addresses are represented in 128 bits as follows 493 . IPv6 address is encoded natively 495 . IPv4 address is encoded in the most significant bits and 496 the remaining bits are set to zero 498 o All prefixes are represented by (128 + 8) bits. 500 . A prefix is encoded in the most significant bits and the 501 remaining bits are set to zero. 503 . The prefix length is encoded before the prefix in a field 504 of size 8 bits. 506 o Topology ID is represented by 16 bits. For routing instances 507 that identify topologies using less than 16 bits, encode the 508 topology ID in the least significant bits while the most 509 significant bits are set to zero 511 o Algorithm is encoded in a 16 bits field. 513 o The Color ID is encoded using 32 bits 515 o Choose the set of FECs of the smallest FEC type code point 517 o Out of these FECs, choose the FECs with the smallest address 518 family code point 520 o Encode the remaining set of FECs as follows 522 o Prefix, Routing Instance, Topology, Algorithm: (Prefix Length, 523 Prefix, routing_instance_id, Topology, SR Algorithm,) 525 o (next-hop, outgoing interface): (next-hop, 526 outgoing_interface_id) 528 o (number of adjacencies, list of next-hops in ascending 529 numerical order, list of outgoing interface IDs in ascending 530 numerical order). This encoding is used to encode a parallel 531 adjacency [RFC8402] 533 o (Endpoint, Color): (Endpoint_address, Color_id) 535 o (IP address): This is the encoding for a mirror SID FEC. The IP 536 address is encoded as described above in this section 538 o Select the FEC with the smallest numerical value 540 The numerical values mentioned in this section are for guidance only. 541 If other numerical values are used then the other numerical values 542 MUST maintain the same numerical ordering among different SR FECs. 544 2.5.2. Redistribution between Routing Protocol Instances 546 The following rule SHOULD be applied when redistributing SIDs with 547 prefixes between routing protocol instances: 549 o If the receiving instance's SRGB is the same as the SRGB of origin 550 instance, then 552 o the index is redistributed with the route 554 o Else 556 o the index is not redistributed and if needed it is the duty of 557 the receiving instance to allocate a fresh index relative to 558 its own SRGB. Note that in that case, the receiving instance 559 MUST compute its local label according to section 2.4 and 560 install it in FIB. 562 It is outside the scope of this document to define local node 563 behaviors that would allow to map the original index into a new index 564 in the receiving instance via the addition of an offset or other 565 policy means. 567 2.5.2.1. Illustration 569 A----IS-IS----B---OSPF----C-192.0.2.1/32 (20001) 571 Consider the simple topology above. 573 o A and B are in the IS-IS domain with SRGB [16000-17000] 575 o B and C are in OSPF domain with SRGB [20000-21000] 577 o B redistributes 192.0.2.1/32 into IS-IS domain 579 o In that case A learns 192.0.2.1/32 as an IP leaf connected to B as 580 usual for IP prefix redistribution 582 o However, according to the redistribution rule above rule, B 583 decides not to advertise any index with 192.0.2.1/32 into IS-IS 584 because the SRGB is not the same. 586 2.5.2.2. Illustration 2 588 Consider the example in the illustration described in Section 589 2.5.2.1. 591 When router B redistributes the prefix 192.0.2.1/32, router B decides 592 to allocate and advertise the same index 1 with the prefix 593 192.0.2.1/32 595 Within the SRGB of the IS-IS domain, index 1 corresponds to the local 596 label 16001 598 o Hence according to the redistribution rule above, router B 599 programs the incoming label 16001 in its FIB to match traffic 600 arriving from the IS-IS domain destined to the prefix 601 192.0.2.1/32. 603 2.6. Effect of Incoming Label Collision on Outgoing Label Programming 605 For the determination of the outgoing label to use, the ingress node 606 pushing new segments, and hence a stack of MPLS labels, MUST use, for 607 a given FEC, the same label that has been selected by the node 608 receiving the packet with that label exposed as top label. So in case 609 of incoming label collision on this receiving node, the ingress node 610 MUST resolve this collision using this same "Incoming Label Collision 611 resolution procedure", using the data of the receiving node. 613 In the general case, the ingress node may not have exactly the same 614 data of the receiving node, so the result may be different. This is 615 under the responsibility of the network operator. But in typical 616 case, e.g. where a centralized node or a distributed link state IGP 617 is used, all nodes would have the same database. However to minimize 618 the chance of misforwarding, a FEC that loses its incoming label to 619 the tie-breaking rules specified in Section 2.5 MUST NOT be 620 installed in FIB with an outgoing segment routing label based on the 621 SID corresponding to the lost incoming label. 623 Examples for the behavior specified in this section can be found in 624 Appendix A.3. 626 2.7. PUSH, CONTINUE, and NEXT 628 PUSH, NEXT, and CONTINUE are operations applied by the forwarding 629 plane. The specifications of these operations can be found in 630 [RFC8402]. This sub-section specifies how to implement each of these 631 operations in the MPLS forwarding plane. 633 2.7.1. PUSH 635 PUSH corresponds to pushing one or more labels on top of an incoming 636 packet then sending it out of a particular physical interface or 637 virtual interface, such as UDP tunnel [RFC7510] or L2TPv3 tunnel 638 [RFC4817], towards a particular next-hop. When pushing labels onto a 639 packet's label stack, the Time-to-Live (TTL) field ([RFC3032], 640 [RFC3443]) and the Traffic Class (TC) field ([RFC3032], [RFC5462]) of 641 each label stack entry must, of course, be set. This document does 642 not specify any set of rules for setting these fields; that is a 643 matter of local policy. Sections 2.10 and 2.11 specify additional 644 details about forwarding behavior. 646 2.7.2. CONTINUE 648 In the MPLS forwarding plane, the CONTINUE operation corresponds to 649 swapping the incoming label with an outgoing label. The value of the 650 outgoing label is calculated as specified in Sections 2.10 and 2.11. 652 2.7.3. NEXT 654 In the MPLS forwarding plane, NEXT corresponds to popping the topmost 655 label. The action before and/or after the popping depends on the 656 instruction associated with the active SID on the received packet 657 prior to the popping. For example suppose the active SID in the 658 received packet was an Adj-SID [RFC8402], then on receiving the 659 packet, the node applies NEXT operation, which corresponds to popping 660 the top most label, and then sends the packet out of the physical or 661 virtual interface (e.g. UDP tunnel [RFC7510] or L2TPv3 tunnel 662 [RFC4817]) towards the next-hop corresponding to the adj-SID. 664 2.7.3.1. Mirror SID 666 If the active SID in the received packet was a Mirror SID [RFC8402, 667 Section 5.1] allocated by the receiving router, then the receiving 668 router applies NEXT operation, which corresponds to popping the top 669 most label, then performs a lookup using the contents of the packet 670 after popping the outer most label in the mirrored forwarding table. 671 The method by which the lookup is made, and/or the actions applied to 672 the packet after the lookup in the mirror table depends on the 673 contents of the packet and the mirror table. Note that the packet 674 exposed after popping the top most label may or may not be an MPLS 675 packet. A mirror SID can be viewed as a generalization of the context 676 label in [RFC5331] because a mirror SID does not make any 677 assumptions about the packet underneath the top label. 679 2.8. MPLS Label Downloaded to FIB for Global and Local SIDs 681 The label corresponding to the global SID "Si" represented by the 682 global index "I" downloaded to FIB is used to match packets whose 683 active segment (and hence topmost label) is "Si". The value of this 684 label is calculated as specified in Section 2.4. 686 For Local SIDs, the MCC is responsible for downloading the correct 687 label value to FIB. For example, an IGP with SR extensions [I-D.ietf- 688 isis-segment-routing-extensions, I-D.ietf-ospf-segment-routing- 689 extensions] downloads the MPLS label corresponding to an Adj-SID 690 [RFC8402]. 692 2.9. Active Segment 694 When instantiated in the MPLS domain, the active segment on a packet 695 corresponds to the topmost label on the packet that is calculated 696 according to the procedure specified in Sections 2.10 and 2.11. When 697 arriving at a node, the topmost label corresponding to the active SID 698 matches the MPLS label downloaded to FIB as specified in Section 2.4. 700 2.10. Forwarding behavior for Global SIDs 702 This section specifies forwarding behavior, including the calculation 703 of outgoing labels, that corresponds to a global SID when applying 704 PUSH, CONTINUE, and NEXT operations in the MPLS forwarding plane. 706 This document covers the calculation of the outgoing label for the 707 top label only. The case where the outgoing label is not the top 708 label and is part of a stack of labels that instantiates a routing 709 policy or a traffic engineering tunnel is outside the scope of this 710 document and may be covered in other documents such as [I-D.ietf- 711 spring-segment-routing-policy]. 713 2.10.1. Forwarding for PUSH and CONTINUE of Global SIDs 715 Suppose an MCC on a router "R0" determines that PUSH or CONTINUE 716 operation is to be applied to an incoming packet related to the 717 global SID "Si" represented by the global index "I" and owned by the 718 router Ri before sending the packet towards a neighbor "N" directly 719 connected to "R0" through a physical or virtual interface such as UDP 720 tunnel [RFC7510] or L2TPv3 tunnel [RFC4817]. 722 The method by which the MCC on router "R0" determines that PUSH or 723 CONTINUE operation must be applied using the SID "Si" is beyond the 724 scope of this document. An example of a method to determine the SID 725 "Si" for PUSH operation is the case where IS-IS [I-D.ietf-isis- 726 segment-routing-extensions] receives the prefix-SID "Si" sub-TLV 727 advertised with prefix "P/m" in TLV 135 and the destination address 728 of the incoming IPv4 packet is covered by the prefix "P/m". 730 For CONTINUE operation, an example of a method to determine the SID 731 "Si" is the case where IS-IS [I-D.ietf-isis-segment-routing- 732 extensions] receives the prefix-SID "Si" sub-TLV advertised with 733 prefix "P" in TLV 135 and the top label of the incoming packet 734 matches the MPLS label in FIB corresponding to the SID "Si" on the 735 router "R0". 737 The forwarding behavior for PUSH and CONTINUE corresponding to the 738 SID "Si" 740 o If the neighbor "N" does not support SR or advertises an invalid 741 SRGB or a SRGB that is too small for the SID "Si" 743 o If it is possible to send the packet towards the neighbor "N" 744 using standard MPLS forwarding behavior as specified in 745 [RFC3031] and [RFC3032], then forward the packet. The method 746 by which a router decides whether it is possible to send the 747 packet to "N" or not is beyond the scope of this document. For 748 example, the router "R0" can use the downstream label 749 determined by another MCC, such as LDP [RFC5036], to send the 750 packet. 752 o Else if there are other useable next-hops, then use other next- 753 hops to forward the incoming packet. The method by which the 754 router "R0" decides on the possibility of using other next- 755 hops is beyond the scope of this document. For example, the 756 MCC on "R0" may chose the send an IPv4 packet without pushing 757 any label to another next-hop. 759 o Otherwise drop the packet. 761 o Else 763 o Calculate the outgoing label as specified in Section 2.4 using 764 the SRGB of the neighbor "N" 766 o If the operation is PUSH 768 . Push the calculated label according to the MPLS label 769 pushing rules specified in [RFC3032] 771 o Else 773 . swap the incoming label with the calculated label 774 according to the label swapping rules in [RFC3032] 776 o Send the packet towards the neighbor "N" 778 2.10.2. Forwarding for NEXT Operation for Global SIDs 780 As specified in Section 2.7.3 NEXT operation corresponds to popping 781 the top most label. The forwarding behavior is as follows 783 o Pop the topmost label 785 o Apply the instruction associated with the incoming label that has 786 been popped 788 The action on the packet after popping the topmost label depends on 789 the instruction associated with the incoming label as well as the 790 contents of the packet right underneath the top label that got 791 popped. Examples of NEXT operation are described in Appendix A.1. 793 2.11. Forwarding Behavior for Local SIDs 795 This section specifies the forwarding behavior for local SIDs when SR 796 is instantiated over the MPLS forwarding plane. 798 2.11.1. Forwarding for PUSH Operation on Local SIDs 800 Suppose an MCC on a router "R0" determines that PUSH operation is to 801 be applied to an incoming packet using the local SID "Si" before 802 sending the packet towards a neighbor "N" directly connected to R0 803 through a physical or virtual interface such as UDP tunnel [RFC7510] 804 or L2TPv3 tunnel [RFC4817]. 806 An example of such local SID is an Adj-SID allocated and advertised 807 by IS-IS [I-D.ietf-isis-segment-routing-extensions]. The method by 808 which the MCC on "R0" determines that PUSH operation is to be applied 809 to the incoming packet is beyond the scope of this document. An 810 example of such method is backup path used to protect against a 811 failure using TI-LFA [I-D.bashandy-rtgwg-segment-routing-ti-lfa]. 813 As mentioned in [RFC8402], a local SID is specified by an MPLS label. 814 Hence the PUSH operation for a local SID is identical to label push 815 operation [RFC3032] using any MPLS label. The forwarding action after 816 pushing the MPLS label corresponding to the local SID is also 817 determined by the MCC. For example, if the PUSH operation was done to 818 forward a packet over a backup path calculated using TI-LFA, then the 819 forwarding action may be sending the packet to a certain neighbor 820 that will in turn continue to forward the packet along the backup 821 path 823 2.11.2. Forwarding for CONTINUE Operation for Local SIDs 825 A local SID on a router "R0" corresponds to a local label. In such 826 scenario, the outgoing label towards a next-hop "N" is determined by 827 the MCC running on the router "R0"and the forwarding behavior for 828 CONTINUE operation is identical to swap operation [RFC3032] on an 829 MPLS label. 831 2.11.3. Outgoing label for NEXT Operation for Local SIDs 833 NEXT operation for Local SIDs is identical to NEXT operation for 834 global SIDs specified in Section 2.10.2. 836 3. IANA Considerations 838 This document does not make any request to IANA. 840 4. Manageability Considerations 842 This document describes the applicability of Segment Routing over the 843 MPLS data plane. Segment Routing does not introduce any change in 844 the MPLS data plane. Manageability considerations described in 845 [RFC8402] applies to the MPLS data plane when used with Segment 846 Routing. SR OAM use cases for the MPLS data plane are defined in 847 [RFC8403]. SR OAM procedures for the MPLS data plane are defined in 848 [RFC8287]. 850 5. Security Considerations 852 This document does not introduce additional security requirements and 853 mechanisms other than the ones described in [RFC8402]. 855 6. Contributors 857 The following contributors have substantially helped the definition 858 and editing of the content of this document: 860 Martin Horneffer 861 Deutsche Telekom 862 Email: Martin.Horneffer@telekom.de 864 Wim Henderickx 865 Nokia 866 Email: wim.henderickx@nokia.com 868 Jeff Tantsura 869 Email: jefftant@gmail.com 870 Edward Crabbe 871 Email: edward.crabbe@gmail.com 873 Igor Milojevic 874 Email: milojevicigor@gmail.com 876 Saku Ytti 877 Email: saku@ytti.fi 879 7. Acknowledgements 881 The authors would like to thank Les Ginsberg, Chris Bowers, Himanshu 882 Shah, Adrian Farrel, Alexander Vainshtein, Przemyslaw Krol, Darren 883 Dukes, Zafar Ali, and Martin Vigoureax for their valuable comments on 884 this document. 886 This document was prepared using 2-Word-v2.0.template.dot. 888 8. References 890 8.1. Normative References 892 [RFC8402] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and 893 R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 894 10.17487/RFC8402 July 2018, . 897 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 898 Requirement Levels", BCP 14, RFC 2119, DOI 899 0.17487/RFC2119, March 1997, . 902 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 903 Label Switching Architecture", RFC 3031, DOI 904 10.17487/RFC3031, January 2001, . 907 [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., 908 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack 909 Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001, 910 . 912 [RFC3443] P. Agarwal, P. and Akyol, B. "Time To Live (TTL) Processing 913 in Multi-Protocol Label Switching (MPLS) Networks", RFC 914 3443, DOI 10.17487/RFC3443, January 2003, . 917 [RFC5462] Andersson, L., and Asati, R., " Multiprotocol Label 918 Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to 919 "Traffic Class" Field", RFC 5462, DOI 10.17487/RFC5462, 920 February 2009, . 922 [RFC7274] K. Kompella, L. Andersson, and A. Farrel, "Allocating and 923 Retiring Special-Purpose MPLS Labels", RFC7274 DOI 924 10.17487/RFC7274, May 2014 927 [RFC8174] B. Leiba, " Ambiguity of Uppercase vs Lowercase in RFC 2119 928 Key Words", RFC8174 DOI 10.17487/RFC8174, May 2017 929 931 8.2. Informative References 933 [I-D.ietf-isis-segment-routing-extensions] Previdi, S., Filsfils, C., 934 Bashandy, A., Gredler, H., Litkowski, S., Decraene, B., and 935 j. jefftant@gmail.com, "IS-IS Extensions for Segment 936 Routing", draft-ietf-isis-segment-routing-extensions-13 937 (work in progress), June 2017. 939 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] Psenak, P., 940 Previdi, S., Filsfils, C., Gredler, H., Shakir, R., 941 Henderickx, W., and J. Tantsura, "OSPFv3 Extensions for 942 Segment Routing", draft-ietf-ospf-ospfv3-segment-routing- 943 extensions-09 (work in progress), March 2017. 945 [I-D.ietf-ospf-segment-routing-extensions] Psenak, P., Previdi, S., 946 Filsfils, C., Gredler, H., Shakir, R., Henderickx, W., and 947 J. Tantsura, "OSPF Extensions for Segment Routing", draft- 948 ietf-ospf-segment-routing-extensions-16 (work in progress), 949 May 2017. 951 [I-D.ietf-spring-segment-routing-ldp-interop] Filsfils, C., Previdi, 952 S., Bashandy, A., Decraene, B., and S. Litkowski, "Segment 953 Routing interworking with LDP", draft-ietf-spring-segment- 954 routing-ldp-interop-08 (work in progress), June 2017. 956 [I-D.bashandy-rtgwg-segment-routing-ti-lfa], Bashandy, A., Filsfils, 957 C., Decraene, B., Litkowski, S., Francois, P., Voyer, P. 958 Clad, F., and Camarillo, P., "Topology Independent Fast 959 Reroute using Segment Routing", draft-bashandy-rtgwg- 960 segment-routing-ti-lfa-05 (work in progress), October 2018, 962 [RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B., 963 Litkowski, S., Horneffer, M., and R. Shakir, "Source Packet 964 Routing in Networking (SPRING) Problem Statement and 965 Requirements", RFC 7855, DOI 10.17487/RFC7855, May 2016, 966 . 968 [RFC5036] Andersson, L., Acreo, AB, Minei, I., Thomas, B., " LDP 969 Specification", RFC5036, DOI 10.17487/RFC5036, October 970 2007, 972 [RFC5331] Aggarwal, R., Rekhter, Y., Rosen, E., " MPLS Upstream Label 973 Assignment and Context-Specific Label Space", RFC5331 DOI 974 10.17487/RFC5331, August 2008, . 977 [RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black, 978 "Encapsulating MPLS in UDP", RFC 7510, DOI 979 10.17487/RFC7510, April 2015, . 982 [RFC4817] Townsley, M., Pignataro, C., Wainner, S., Seely, T., Young, 983 T., "Encapsulation of MPLS over Layer 2 Tunneling Protocol 984 Version 3", RFC4817, DOI 10.17487/RFC4817, March 2007, 985 987 [RFC8287] N. Kumar, C. Pignataro, G. Swallow, N. Akiya, S. Kini, and 988 M. Chen " Label Switched Path (LSP) Ping/Traceroute for 989 Segment Routing (SR) IGP-Prefix and IGP-Adjacency Segment 990 Identifiers (SIDs) with MPLS Data Planes" RFC8287, DOI 991 10.17487/RFC8287, December 2017, https://www.rfc- 992 editor.org/info/rfc8287 994 [RFC8403] R. Geib, C. Filsfils, C. Pignataro, N. Kumar, "A Scalable 995 and Topology-Aware MPLS Data-Plane Monitoring System", 996 RFC8403, DOI 10.17487/RFC8403, July 2018, 999 [I-D.ietf-spring-segment-routing-policy] Filsfils, C., Sivabalan, 1000 S., Raza, K., Liste, J. , Clad, F., Voyer, D., Bogdanov, A., 1001 Mattes, P., " Segment Routing Policy for Traffic Engineering", 1002 draft-ietf-spring-segment-routing-policy-01 (work in progress), June 1003 2018 1005 9. Authors' Addresses 1007 Ahmed Bashandy (editor) 1008 Arrcus 1010 Email: abashandy.ietf@gmail.com 1012 Clarence Filsfils (editor) 1013 Cisco Systems, Inc. 1014 Brussels 1015 BE 1017 Email: cfilsfil@cisco.com 1019 Stefano Previdi 1020 Cisco Systems, Inc. 1021 Italy 1023 Email: stefano@previdi.net 1025 Bruno Decraene 1026 Orange 1027 FR 1029 Email: bruno.decraene@orange.com 1030 Stephane Litkowski 1031 Orange 1032 FR 1034 Email: stephane.litkowski@orange.com 1036 Rob Shakir 1037 Google 1038 US 1040 Email: robjs@google.com 1042 Appendix A. Examples 1044 A.1. IGP Segments Example 1046 Consider the network diagram of Figure 1 and the IP address and IGP 1047 Segment allocation of Figure 2. Assume that the network is running 1048 IS-IS with SR extensions [I-D.ietf-isis-segment-routing-extensions] 1049 and all links have the same metric. The following examples can be 1050 constructed. 1052 +--------+ 1053 / \ 1054 R0-----R1-----R2----------R3-----R8 1055 | \ / | 1056 | +--R4--+ | 1057 | | 1058 +-----R5-----+ 1060 Figure 1: IGP Segments - Illustration 1062 +-----------------------------------------------------------+ 1063 | IP address allocated by the operator: | 1064 | 192.0.2.1/32 as a loopback of R1 | 1065 | 192.0.2.2/32 as a loopback of R2 | 1066 | 192.0.2.3/32 as a loopback of R3 | 1067 | 192.0.2.4/32 as a loopback of R4 | 1068 | 192.0.2.5/32 as a loopback of R5 | 1069 | 192.0.2.8/32 as a loopback of R8 | 1070 | 198.51.100.9/32 as an anycast loopback of R4 | 1071 | 198.51.100.9/32 as an anycast loopback of R5 | 1072 | | 1073 | SRGB defined by the operator as 1000-5000 | 1074 | | 1075 | Global IGP SID indices allocated by the operator: | 1076 | 1 allocated to 192.0.2.1/32 | 1077 | 2 allocated to 192.0.2.2/32 | 1078 | 3 allocated to 192.0.2.3/32 | 1079 | 4 allocated to 192.0.2.4/32 | 1080 | 8 allocated to 192.0.2.8/32 | 1081 | 1009 allocated to 198.51.100.9/32 | 1082 | | 1083 | Local IGP SID allocated dynamically by R2 | 1084 | for its "north" adjacency to R3: 9001 | 1085 | for its "north" adjacency to R3: 9003 | 1086 | for its "south" adjacency to R3: 9002 | 1087 | for its "south" adjacency to R3: 9003 | 1088 +-----------------------------------------------------------+ 1090 Figure 2: IGP Address and Segment Allocation - Illustration 1092 Suppose R1 wants to send an IPv4 packet P1 to R8. In this case, R1 1093 needs to apply PUSH operation to the IPv4 packet. 1095 Remember that the SID index "8" is a global IGP segment attached to 1096 the IP prefix 192.0.2.8/32. Its semantic is global within the IGP 1097 domain: any router forwards a packet received with active segment 8 1098 to the next-hop along the ECMP-aware shortest-path to the related 1099 prefix. 1101 R2 is the next-hop along the shortest path towards R8. By applying 1102 the steps in Section 2.8 the outgoing label downloaded to R1's FIB 1103 corresponding to the global SID index 8 is 1008 because the SRGB of 1104 R2 is [1000,5000] as shown in Figure 2. 1106 Because the packet is IPv4, R1 applies the PUSH operation using the 1107 label value 1008 as specified in Section 2.10.1. The resulting MPLS 1108 header will have the "S" bit [RFC3032] set because it is followed 1109 directly by an IPv4 packet. 1111 The packet arrives at router R2. Because the top label 1008 1112 corresponds to the IGP SID "8", which is the prefix-SID attached to 1113 the prefix 192.0.2.8/32 owned by the node R8, then the instruction 1114 associated with the SID is "forward the packet using all ECMP/UCMP 1115 interfaces and all ECMP/UCMP next-hop(s) along the shortest/useable 1116 path(s) towards R8". Because R2 is not the penultimate hop, R2 1117 applies the CONTINUE operation to the packet and sends it to R3 using 1118 one of the two links connected to R3 with top label 1008 as specified 1119 in Section 2.10.1. 1121 R3 receives the packet with top label 1008. Because the top label 1122 1008 corresponds to the IGP SID "8", which is the prefix-SID attached 1123 to the prefix 192.0.2.8/32 owned by the node R8, then the instruction 1124 associated with the SID is "send the packet using all ECMP interfaces 1125 and all next-hop(s) along the shortest path towards R8". Because R3 1126 is the penultimate hop, we assume that R3 performs penumtimate hop 1127 popping, which corresponds to the NEXT operation, then sends the 1128 packet to R8. The NEXT operation results in popping the outer label 1129 and sending the packet as a pure IPv4 packet to R8. 1131 In conclusion, the path followed by P1 is R1-R2--R3-R8. The ECMP- 1132 awareness ensures that the traffic be load-shared between any ECMP 1133 path, in this case the two links between R2 and R3. 1135 A.2. Incoming Label Collision Examples 1137 This section describes few examples to illustrate the handling of 1138 label collision described in Section 2.5. 1140 For the examples in this section, we assume that Node A has the 1141 following: 1143 o OSPF default admin distance for implementation=50 1145 o ISIS default admin distance for implementation=60 1147 A.2.1. Example 1 1149 Illustration of incoming label collision resolution for the same FEC 1150 type using MCC administrative distance. 1152 FEC1: 1154 o OSPF prefix SID advertisement from node B for 198.51.100.5/32 with 1155 index=5 1157 o OSPF SRGB on node A = [1000,1999] 1159 o Incoming label=1005 1161 FEC2: 1162 o ISIS prefix SID advertisement from node C for 203.0.113.105/32 1163 with index=5 1165 o ISIS SRGB on node A = [1000,1999] 1167 o Incoming label=1005 1169 FEC1 and FEC2 both use dynamic SID assignment. Since neither ofthe 1170 FEC types is SR Policy, we use the default admin distances of 50 and 1171 60 to break the tie. So FEC1 wins. 1173 A.2.2. Example 2 1175 Illustration of incoming label collision resolution for different FEC 1176 types using the MCC administrative distance. 1178 FEC1: 1179 o Node A receives an OSPF prefix sid advertisement from node B for 1180 198.51.100.6/32 with index=6 1182 o OSPF SRGB on node A = [1000,1999] 1184 o Hence the incoming label on node A corresponding to 1185 198.51.100.6/32 is 1006 1187 FEC2: 1188 ISIS on node A assigns the label 1006 to the globally significant 1189 adj-SID (I.e. when advertised the "L" flag is clear in the adj-SID 1190 sub-TLV as described in [I-D.ietf-isis-segment-routing-extensions]) 1191 towards one of its neighbors. Hence the incoming label corresponding 1192 to this adj-SID 1006. Assume Node A allocates this adj-SID 1193 dynamically, and it may differ across router reboots. 1195 FEC1 and FEC2 both use dynamic SID assignment. Since neither of the 1196 FEC types is SR Policy, we use the default admin distances of 50 and 1197 60 to break the tie. So FEC1 wins. 1199 A.2.3. Example 3 1201 Illustration of incoming label collision resolution based on 1202 preferring static over dynamic SID assignment 1204 FEC1: 1205 OSPF on node A receives a prefix SID advertisement from node B for 1206 198.51.100.7/32 with index=7. Assuming that the OSPF SRGB on node A 1207 is [1000,1999], then incoming label corresponding to 198.51.100.7/32 1208 is 1007 1210 FEC2: 1211 The operator on node A configures ISIS on node A to assign the label 1212 1007 to the globally significant adj-SID (I.e. when advertised the 1213 "L" flag is clear in the adj-SID sub-TLV as described in [I-D.ietf- 1214 isis-segment-routing-extensions]) towards one of its neighbor 1215 advertisement from node A with label=1007 1217 Node A assigns this adj-SID explicitly via configuration, so the adj- 1218 SID survives router reboots. 1220 FEC1 uses dynamic SID assignment, while FEC2 uses explicit SID 1221 assignment. So FEC2 wins. 1223 A.2.4. Example 4 1225 Illustration of incoming label collision resolution using FEC type 1226 default administrative distance 1228 FEC1: 1229 OSPF on node A receives a prefix SID advertisement from node B for 1230 198.51.100.8/32 with index=8. Assuming that OSPF SRGB on node A = 1231 [1000,1999], the incoming label corresponding to 198.51.100.8/32 is 1232 1008. 1234 FEC2: 1235 Suppose the SR Policy advertisement from controller to node A for the 1236 policy identified by (Endpoint = 192.0.2.208, color = 100) and 1237 consisting of SID-List = assigns the globally significant 1238 Binding-SID label 1008 1240 From the point of view of node A, FEC1 and FEC2 both use dynamic SID 1241 assignment. Based on the default administrative distance outlined in 1242 Section 2.5.1, the binding SID has a higher administrative distance 1243 than the prefix-SID and hence FEC1 wins. 1245 A.2.5. Example 5 1247 Illustration of incoming label collision resolution based on FEC type 1248 preference 1250 FEC1: 1251 ISIS on node A receives a prefix SID advertisement from node B for 1252 203.0.113.110/32 with index=10. Assuming that the ISIS SRGB on node A 1253 is [1000,1999], then incoming label corresponding to 203.0.113.110/32 1254 is 1010. 1256 FEC2: 1257 ISIS on node A assigns the label 1010 to the globally significant 1258 adj-SID (I.e. when advertised the "L" flag is clear in the adj-SID 1259 sub-TLV as described in [I-D.ietf-isis-segment-routing-extensions]) 1260 towards one of its neighbors). 1262 Node A allocates this adj-SID dynamically, and it may differ across 1263 router reboots. Hence both FEC1 and FEC2 both use dynamic SID 1264 assignment. 1266 Since both FECs are from the same MCC, they have the same default 1267 admin distance. So we compare FEC type code-point. FEC1 has FEC type 1268 code-point=120, while FEC2 has FEC type code-point=130. Therefore, 1269 FEC1 wins. 1271 A.2.6. Example 6 1273 Illustration of incoming label collision resolution based on address 1274 family preference. 1276 FEC1: 1277 ISIS on node A receives prefix SID advertisement from node B for 1278 203.0.113.111/32 with index 11. Assuming that the ISIS SRGB on node A 1279 is [1000,1999], the incoming label on node A for 203.0.113.111/32 is 1280 1011. 1282 FEC2: 1283 ISIS on node A prefix SID advertisement from node C for 1284 2001:DB8:1000::11/128 with index=11. Assuming that the ISIS SRGB on 1285 node A is [1000,1999], the incoming label on node A for 1286 2001:DB8:1000::11/128 is 1011 1288 FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are 1289 from the same MCC, they have the same default admin distance. So we 1290 compare FEC type code-point. Both FECs have FEC type code-point=120. 1291 So we compare address family. Since IPv4 is preferred over IPv6, FEC1 1292 wins. 1294 A.2.7. Example 7 1296 Illustration incoming label collision resolution based on prefix 1297 length. 1299 FEC1: 1300 ISIS on node A receives a prefix SID advertisement from node B for 1301 203.0.113.112/32 with index 12. Assuming that ISIS SRGB on node A is 1302 [1000,1999], the incoming label for 203.0.113.112/32 on node A is 1303 1012. 1305 FEC2: 1306 ISIS on node A receives a prefix SID advertisement from node C for 1307 203.0.113.128/30 with index 12. Assuming that the ISIS SRGB on node A 1308 is [1000,1999], then incoming label for 203.0.113.128/30 on node A is 1309 1012 1311 FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are 1312 from the same MCC, they have the same default admin distance. So we 1313 compare FEC type code-point. Both FECs have FEC type code-point=120. 1314 So we compare address family. Both are IPv4 address family, so we 1315 compare prefix length. FEC1 has prefix length=32, and FEC2 has 1316 prefix length=30, so FEC2 wins. 1318 A.2.8. Example 8 1320 Illustration of incoming label collision resolution based on the 1321 numerical value of the FECs. 1323 FEC1: 1324 ISIS on node A receives a prefix SID advertisement from node B for 1325 203.0.113.113/32 with index 13. Assuming that ISIS SRGB on node A is 1327 [1000,1999], then the incoming label for 203.0.113.113/32 on node A 1328 is 1013 1330 FEC2: 1331 ISIS on node A receives a prefix SID advertisement from node C for 1332 203.0.113.213/32 with index 13. Assuming that ISIS SRGB on node A is 1333 [1000,1999], then the incoming label for 203.0.113.213/32 on node A 1334 is 1013 1336 FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are 1337 from the same MCC, they have the same default admin distance. So we 1338 compare FEC type code-point. Both FECs have FEC type code-point=120. 1339 So we compare address family. Both are IPv4 address family, so we 1340 compare prefix length. Prefix lengths are the same, so we compare 1341 prefix. FEC1 has the lower prefix, so FEC1 wins. 1343 A.2.9. Example 9 1345 Illustration of incoming label collision resolution based on routing 1346 instance ID. 1348 FEC1: 1349 ISIS on node A receives a prefix SID advertisement from node B for 1350 203.0.113.114/32 with index 14. Assume that this ISIS instance on 1351 node A has the Routing Instance ID 1000 and SRGB [1000,1999]. Hence 1352 the incoming label for 203.0.113.114/32 on node A is 1014 1354 FEC2: 1355 ISIS on node A receives a prefix SID advertisement from node C for 1356 203.0.113.114/32 with index=14. Assume that this is another instance 1357 of ISIS on node A with a different routing Instance ID 2000 but the 1358 same SRGB [1000,1999]. Hence incoming label for 203.0.113.114/32 on 1359 node A 1014 1361 These two FECs match all the way through the prefix length and 1362 prefix. So Routing Instance ID breaks the tie, with FEC1 winning. 1364 A.2.10. Example 10 1366 Illustration of incoming label collision resolution based on topology 1367 ID. 1369 FEC1: 1370 ISIS on node A receives a prefix SID advertisement from node B for 1371 203.0.113.115/32 with index=15. Assume that this ISIS instance on 1372 node A has Routing Instance ID 1000. Assume that the prefix 1373 advertisement of 203.0.113.115/32 was received in ISIS Multi-topology 1374 advertisement with ID = 50. If the ISIS SRGB for this routing 1375 instance on node A is [1000,1999], then incoming label of 1376 203.0.113.115/32 for topology 50 on node A is 1015 1378 FEC2: 1379 ISIS on node A receives a prefix SID advertisement from node C for 1380 203.0.113.115/32 with index 15. Assume that it is the same routing 1381 Instance ID = 1000 but 203.0.113.115/32 was advertised with a 1382 different ISIS Multi-topology ID = 40. If the ISIS SRGB on node A is 1383 [1000,1999], then incoming label of 203.0.113.115/32 for topology 40 1384 on node A is also 1015 1386 These two FECs match all the way through the prefix length, prefix, 1387 and Routing Instance ID. We compare ISIS Multi-topology ID, so FEC2 1388 wins. 1390 A.2.11. Example 11 1392 Illustration of incoming label collision for resolution based on 1393 algorithm ID. 1395 FEC1: 1396 ISIS on node A receives a prefix SID advertisement from node B for 1397 203.0.113.116/32 with index=16 Assume that ISIS on node A has Routing 1398 Instance ID = 1000. Assume that node B advertised 203.0.113.116/32 1399 with ISIS Multi-topology ID = 50 and SR algorithm = 0. Assume that 1400 the ISIS SRGB on node A = [1000,1999]. Hence the incoming label 1401 corresponding to this advertisement of 203.0.113.116/32 is 1016. 1403 FEC2: 1404 ISIS on node A receives a prefix SID advertisement from node C for 1405 203.0.113.116/32 with index=16. Assume that it is the same ISIS 1406 instance on node A with Routing Instance ID = 1000. Also assume that 1407 node C advertised 203.0.113.116/32 with ISIS Multi-topology ID = 50 1408 but with SR algorithm = 22. Since it is the same routing instance, 1409 the SRGB on node A = [1000,1999]. Hence the incoming label 1410 corresponding to this advertisement of 203.0.113.116/32 by node C is 1411 also 1016. 1413 These two FECs match all the way through the prefix length, prefix, 1414 and Routing Instance ID, and Multi-topology ID. We compare SR 1415 algorithm ID, so FEC1 wins. 1417 A.2.12. Example 12 1419 Illustration of incoming label collision resolution based on FEC 1420 numerical value and independent of how the SID assigned to the 1421 colliding FECs. 1423 FEC1: 1424 ISIS on node A receives a prefix SID advertisement from node B for 1425 203.0.113.117/32 with index 17. Assume that the ISIS SRGB on node A 1426 is [1000,1999], then the incoming label is 1017 1428 FEC2: 1429 Suppose there is an ISIS mapping server advertisement (SID/Label 1430 Binding TLV) from node D has Range 100 and Prefix = 203.0.113.1/32. 1431 Suppose this mapping server advertisement generates 100 mappings, one 1432 of which maps 203.0.113.17/32 to index 17. Assuming that it is the 1433 same ISIS instance, then the SRGB is [1000,1999] and hence the 1434 incoming label for 1017. 1436 The fact that FEC1 comes from a normal prefix SID advertisement and 1437 FEC2 is generated from a mapping server advertisement is not used as 1438 a tie-breaking parameter. Both FECs use dynamic SID assignment, are 1439 from the same MCC, have the same FEC type code-point=120. Their 1440 prefix lengths are the same as well. FEC2 wins based on lower 1441 numerical prefix value, since 203.0.113.17 is less than 1442 203.0.113.117. 1444 A.2.13. Example 13 1446 Illustration of incoming label collision resolution based on address 1447 family preference 1449 FEC1: 1450 SR Policy advertisement from controller to node A. Endpoint 1451 address=2001:DB8:3000::100, color=100, SID-List= and the 1452 Binding-SID label=1020 1454 FEC2: 1455 SR Policy advertisement from controller to node A. Endpoint 1456 address=192.0.2.60, color=100, SID-List= and the Binding-SID 1457 label=1020 1458 The FECs match through the tie-breaks up to and including having the 1459 same FEC type code-point=140. FEC2 wins based on IPv4 address family 1460 being preferred over IPv6. 1462 A.2.14. Example 14 1464 Illustration of incoming label resolution based on numerical value of 1465 the policy endpoint. 1467 FEC1: 1468 SR Policy advertisement from controller to node A. Endpoint 1469 address=192.0.2.70, color=100, SID-List= and Binding-SID 1470 label=1021 1472 FEC2: 1473 SR Policy advertisement from controller to node A Endpoint 1474 address=192.0.2.71, color=100, SID-List= and Binding-SID 1475 label=1021 1477 The FECs match through the tie-breaks up to and including having the 1478 same address family. FEC1 wins by having the lower numerical endpoint 1479 address value. 1481 A.3. Examples for the Effect of Incoming Label Collision on Outgoing 1482 Label 1484 This section presents examples to illustrate the effect of incoming 1485 label collision on the selection of the outgoing label described in 1486 Section 2.6. 1488 A.3.1. Example 1 1490 Illustration of the effect of incoming label resolution on the 1491 outgoing label 1493 FEC1: 1494 ISIS on node A receives a prefix SID advertisement from node B for 1495 203.0.113.122/32 with index 22. Assuming that the ISIS SRGB on node A 1496 is [1000,1999] the corresponding incoming label is 1022. 1498 FEC2: 1499 ISIS on node A receives a prefix SID advertisement from node C for 1500 203.0.113.222/32 with index=22 Assuming that the ISIS SRGB on node A 1501 is [1000,1999] the corresponding incoming label is 1022. 1503 FEC1 wins based on lowest numerical prefix value. This means that 1504 node A installs a transit MPLS forwarding entry to SWAP incoming 1505 label 1022, with outgoing label N and use outgoing interface I. N is 1506 determined by the index associated with FEC1 (index 22) and the SRGB 1507 advertised by the next-hop node on the shortest path to reach 1508 203.0.113.122/32. 1510 Node A will generally also install an imposition MPLS forwarding 1511 entry corresponding to FEC1 for incoming prefix=203.0.113.122/32 1512 pushing outgoing label N, and using outgoing interface I. 1514 The rule in Section 2.6 means node A MUST NOT install an ingress 1515 MPLS forwarding entry corresponding to FEC2 (the losing FEC, which 1516 would be for prefix 203.0.113.222/32). 1518 A.3.2. Example 2 1520 Illustration of the effect of incoming label collision resolution on 1521 outgoing label programming on node A 1523 FEC1: 1524 o SR Policy advertisement from controller to node A 1526 o Endpoint address=192.0.2.80, color=100, SID-List= 1528 o Binding-SID label=1023 1530 FEC2: 1531 o SR Policy advertisement from controller to node A 1533 o Endpoint address=192.0.2.81, color=100, SID-List= 1535 o Binding-SID label=1023 1537 FEC1 wins by having the lower numerical endpoint address value. This 1538 means that node A installs a transit MPLS forwarding entry to SWAP 1539 incoming label=1023, with outgoing labels and outgoing interface 1540 determined by the SID-List for FEC1. 1542 In this example, we assume that node A receives two BGP/VPN routes: 1544 o R1 with VPN label=V1, BGP next-hop = 192.0.2.80, and color=100, 1546 o R2 with VPN label=V2, BGP next-hop = 192.0.2.81, and color=100, 1547 We also assume that A has a BGP policy which matches on color=100 1548 that allows that its usage as SLA steering information. In this case, 1549 node A will install a VPN route with label stack = 1550 (corresponding to FEC1). 1552 The rule described in section 2.6 means that node A MUST NOT install 1553 a VPN route with label stack = (corresponding to FEC2.)