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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SPRING Working Group C. Filsfils 3 Internet-Draft K. Talaulikar, Ed. 4 Intended status: Standards Track Cisco Systems, Inc. 5 Expires: April 28, 2022 D. Voyer 6 Bell Canada 7 A. Bogdanov 8 British Telecom 9 P. Mattes 10 Microsoft 11 October 25, 2021 13 Segment Routing Policy Architecture 14 draft-ietf-spring-segment-routing-policy-14 16 Abstract 18 Segment Routing (SR) allows a headend node to steer a packet flow 19 along any path. Intermediate per-path states are eliminated thanks 20 to source routing. The headend node steers a flow into an SR Policy. 21 The packets steered into an SR Policy carry an ordered list of 22 segments associated with that SR Policy. This document details the 23 concepts of SR Policy and steering into an SR Policy. 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 https://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 April 28, 2022. 42 Copyright Notice 44 Copyright (c) 2021 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 (https://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 . . . . . . . . . . . . . . . . . . 3 61 2. SR Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 4 62 2.1. Identification of an SR Policy . . . . . . . . . . . . . 4 63 2.2. Candidate Path and Segment List . . . . . . . . . . . . . 5 64 2.3. Protocol-Origin of a Candidate Path . . . . . . . . . . . 6 65 2.4. Originator of a Candidate Path . . . . . . . . . . . . . 6 66 2.5. Discriminator of a Candidate Path . . . . . . . . . . . . 7 67 2.6. Identification of a Candidate Path . . . . . . . . . . . 7 68 2.7. Preference of a Candidate Path . . . . . . . . . . . . . 8 69 2.8. Validity of a Candidate Path . . . . . . . . . . . . . . 8 70 2.9. Active Candidate Path . . . . . . . . . . . . . . . . . . 8 71 2.10. Validity of an SR Policy . . . . . . . . . . . . . . . . 10 72 2.11. Instantiation of an SR Policy in the Forwarding Plane . . 10 73 2.12. Priority of an SR Policy . . . . . . . . . . . . . . . . 10 74 2.13. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 11 75 3. Segment Routing Database . . . . . . . . . . . . . . . . . . 12 76 4. Segment Types . . . . . . . . . . . . . . . . . . . . . . . . 13 77 4.1. Explicit Null . . . . . . . . . . . . . . . . . . . . . . 16 78 5. Validity of a Candidate Path . . . . . . . . . . . . . . . . 17 79 5.1. Explicit Candidate Path . . . . . . . . . . . . . . . . . 17 80 5.2. Dynamic Candidate Path . . . . . . . . . . . . . . . . . 18 81 5.3. Composite Candidate Path . . . . . . . . . . . . . . . . 19 82 6. Binding SID . . . . . . . . . . . . . . . . . . . . . . . . . 19 83 6.1. BSID of a candidate path . . . . . . . . . . . . . . . . 19 84 6.2. BSID of an SR Policy . . . . . . . . . . . . . . . . . . 19 85 6.3. Forwarding Plane . . . . . . . . . . . . . . . . . . . . 21 86 6.4. Non-SR usage of Binding SID . . . . . . . . . . . . . . . 21 87 7. SR Policy State . . . . . . . . . . . . . . . . . . . . . . . 21 88 8. Steering into an SR Policy . . . . . . . . . . . . . . . . . 22 89 8.1. Validity of an SR Policy . . . . . . . . . . . . . . . . 22 90 8.2. Drop upon invalid SR Policy . . . . . . . . . . . . . . . 22 91 8.3. Incoming Active SID is a BSID . . . . . . . . . . . . . . 23 92 8.4. Per-Destination Steering . . . . . . . . . . . . . . . . 23 93 8.5. Recursion on an on-demand dynamic BSID . . . . . . . . . 25 94 8.6. Per-Flow Steering . . . . . . . . . . . . . . . . . . . . 25 95 8.7. Policy-based Routing . . . . . . . . . . . . . . . . . . 27 96 8.8. Optional Steering Modes for BGP Destinations . . . . . . 27 98 9. Protection . . . . . . . . . . . . . . . . . . . . . . . . . 29 99 9.1. Leveraging TI-LFA local protection of the constituent IGP 100 segments . . . . . . . . . . . . . . . . . . . . . . . . 29 101 9.2. Using an SR Policy to locally protect a link . . . . . . 29 102 9.3. Using a Candidate Path for Path Protection . . . . . . . 30 103 10. Security Considerations . . . . . . . . . . . . . . . . . . . 30 104 11. Manageability Considerations . . . . . . . . . . . . . . . . 31 105 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 106 12.1. Guidance for Designated Experts . . . . . . . . . . . . 32 107 13. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 32 108 14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 33 109 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 110 15.1. Normative References . . . . . . . . . . . . . . . . . . 34 111 15.2. Informative References . . . . . . . . . . . . . . . . . 35 112 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38 114 1. Introduction 116 Segment Routing (SR) [RFC8402] allows a headend node to steer a 117 packet flow along any path. Intermediate per-path states are 118 eliminated thanks to source routing. 120 The headend node is said to steer a flow into a Segment Routing 121 Policy (SR Policy). [RFC8402] introduces the SR Policy construct and 122 provides an overview of how it is leveraged for Segment Routing use- 123 cases. 125 The packets steered into an SR Policy carry an ordered list of 126 segments associated with that SR Policy. [RFC8660] describes the 127 representation and processing of these ordered list of segments as an 128 MPLS label stack for SR-MPLS. While [RFC8754] and [RFC8986] describe 129 the same for Segment Routing over IPv6 (SRv6) with the use of the 130 Segment Routing Header (SRH). 132 This document details the concepts of SR Policy and steering packets 133 into an SR Policy. These apply equally to the SR-MPLS and SRv6 134 instantiations of segment routing. 136 1.1. Requirements Language 138 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 139 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 140 "OPTIONAL" in this document are to be interpreted as described in BCP 141 14 [RFC2119] [RFC8174] when, and only when, they appear in all 142 capitals, as shown here. 144 2. SR Policy 146 An SR Policy is a framework that enables the instantiation of an 147 ordered list of segments on a node for implementing a source routing 148 policy for the steering of traffic for a specific purpose (e.g. for a 149 specific SLA) from that node. 151 The Segment Routing architecture [RFC8402] specifies that any 152 instruction can be bound to a segment. Thus, an SR Policy can be 153 built using any type of Segment Identifier (SID) including those 154 associated with topological or service instructions. 156 This section defines the key aspects and constituents of an SR 157 Policy. 159 2.1. Identification of an SR Policy 161 An SR Policy is identified through the tuple . In the context of a specific headend, one may identify an 163 SR policy by the tuple. 165 The headend is the node where the policy is instantiated/implemented. 166 The headend is specified as an IPv4 or IPv6 address and is expected 167 to be unique in the domain. 169 The endpoint indicates the destination of the policy. The endpoint 170 is specified as an IPv4 or IPv6 address and is expected to be unique 171 in the domain. In a specific case (refer to Section 8.8.1), the 172 endpoint can be the null address (0.0.0.0 for IPv4, ::0 for IPv6). 174 The color is a 32-bit numerical value that associates the SR Policy 175 with an intent or objective (e.g. low-latency). 177 The endpoint and the color are used to automate the steering of 178 service or transport routes on SR Policies (refer to Section 8). 180 An implementation MAY allow assignment of a symbolic name comprising 181 of printable ASCII characters to an SR Policy to serve as a user- 182 friendly attribute for debugging and troubleshooting purposes. Such 183 symbolic names may identify an SR Policy when the naming scheme 184 ensures uniqueness. The SR Policy name MAY also be signaled along 185 with a candidate path of the SR Policy (refer to Section 2.2). An SR 186 Policy may have multiple names associated with it in the scenario 187 where the headend receives different SR Policy names along with 188 candidate paths for the same SR Policy. 190 2.2. Candidate Path and Segment List 192 An SR Policy is associated with one or more candidate paths. A 193 candidate path is the unit for signaling of an SR Policy to a headend 194 via protocol extensions like Path Computation Element (PCE) 195 Communication Protocol (PCEP) [RFC8664] 196 [I-D.ietf-pce-segment-routing-policy-cp] or BGP SR Policy 197 [I-D.ietf-idr-segment-routing-te-policy]. 199 A Segment-List represents a specific source-routed path to send 200 traffic from the headend to the endpoint of the corresponding SR 201 policy. 203 A candidate path is either dynamic, explicit, or composite. 205 An explicit candidate path is expressed as a Segment-List or a set of 206 Segment-Lists. 208 A dynamic candidate path expresses an optimization objective and a 209 set of constraints. The headend (potentially with the help of a PCE) 210 computes the solution Segment-List (or set of Segment-Lists) that 211 solves the optimization problem. 213 If a candidate path is associated with a set of Segment-Lists, each 214 Segment-List is associated with weight for weighted load balancing 215 (refer to Section 2.11 for details). The default weight is 1. 217 A composite candidate path acts as a container for grouping SR 218 Policies. The composite candidate path construct enables the 219 combination of SR Policies, each with explicit candidate paths and/or 220 dynamic candidate paths with potentially different optimization 221 objectives and constraints, for load-balanced steering of packet 222 flows over its constituent SR Policies. The following criteria apply 223 for inclusion of constituent SR Policies using a composite candidate 224 path under a parent SR Policy: 226 o the endpoints of the constituent SR Policies and the parent SR 227 Policy MUST be identical 229 o The colors of each of the constituent SR Policies and the parent 230 SR Policy MUST be different 232 o the constituent SR Policies MUST NOT use composite candidate paths 234 Each constituent SR Policy of a composite candidate path is 235 associated with weight for load-balancing purposes (refer to 236 Section 2.11 for details). The default weight is 1. 238 2.3. Protocol-Origin of a Candidate Path 240 A headend may be informed about a candidate path for an SR Policy 241 by various means including: via configuration, PCEP 242 [RFC8664] [I-D.ietf-pce-segment-routing-policy-cp] or BGP 243 [I-D.ietf-idr-segment-routing-te-policy]. 245 Protocol-Origin of a candidate path is an 8-bit value associated with 246 the mechanism or protocol used for signaling/provisioning the SR 247 Policy. It helps identify the protocol/mechanism that provides or 248 signals the candidate path and indicates its preference relative to 249 other protocols/mechanisms. 251 The head-end assigns different Protocol-Origin values to each source 252 of SR Policy information. The Protocol-Origin value is used as a 253 tie-breaker between candidate-paths of equal preference, as described 254 in Section 2.9. The table below specifies the RECOMMENDED default 255 values of Protocol-Origin: 257 +-----------------+-------------------+ 258 | Protocol-Origin | Description | 259 +-----------------+-------------------+ 260 | 10 | PCEP | 261 | 20 | BGP SR Policy | 262 | 30 | Via Configuration | 263 +-----------------+-------------------+ 265 Table 1: Protocol-Origin default values 267 Implementations MAY allow modifications of these default values 268 assigned to protocols on the headend along similar lines as a routing 269 administrative distance. Its application in the candidate path 270 selection is described in Section 2.9. 272 2.4. Originator of a Candidate Path 274 Originator identifies the node which provisioned or signaled the 275 candidate path on the headend. The originator is expressed in the 276 form of a 160-bit numerical value formed by the concatenation of the 277 fields of the tuple as 278 below: 280 o Autonomous System Number (ASN) : represented as a 4-byte number. 281 If 2-byte ASNs are in use, the low-order 16 bits MUST be used, and 282 the high-order bits MUST be set to zero. 284 o Node Address : represented as a 128-bit value. IPv4 addresses 285 MUST be encoded in the lowest 32 bits, and the high-order bits 286 MUST be set to zero. 288 Its application in the candidate path selection is described in 289 Section 2.9. 291 When provisioning is via configuration, the ASN and node address MAY 292 be set to either the headend or the provisioning controller/node ASN 293 and address. The default value is 0 for both AS and node address. 295 When signaling is via PCEP, it is the IPv4 or IPv6 address of the PCE 296 and the AS number SHOULD be set to 0 by default when not available or 297 known. 299 When signaling is via BGP SR Policy, the ASN and Node Address are 300 provided by BGP (refer to [I-D.ietf-idr-segment-routing-te-policy]) 301 on the headend. 303 2.5. Discriminator of a Candidate Path 305 The Discriminator is a 32-bit value associated with a candidate path 306 that uniquely identifies it within the context of an SR Policy from a 307 specific Protocol-Origin as specified below: 309 When provisioning is via configuration, this is an implementation's 310 configuration model-specific unique identifier for a candidate path. 311 The default value is 0. 313 When signaling is via PCEP, the method to uniquely signal an 314 individual candidate path along with its discriminator is described 315 in [I-D.ietf-pce-segment-routing-policy-cp]. The default value is 0. 317 When signaling is via BGP SR Policy, the BGP process receiving the 318 route provides the distinguisher (refer to Section 2.1 of 319 [I-D.ietf-idr-segment-routing-te-policy]) as the discriminator. 321 Its application in the candidate path selection is described in 322 Section 2.9. 324 2.6. Identification of a Candidate Path 326 A candidate path is identified in the context of a single SR Policy. 328 A candidate path is not shared across SR Policies. 330 A candidate path is not identified by its Segment-List(s). 332 If CP1 is a candidate path of SR Policy Pol1 and CP2 is a 333 candidate path of SR Policy Pol2, then these two candidate paths 334 are independent, even if they happen to have the same Segment- 335 List. The Segment-List does not identify a candidate path. The 336 Segment-List is an attribute of a candidate path. 338 The identity of a candidate path MUST be uniquely established in the 339 context of an SR Policy to handle add, 340 delete or modify operations on them in an unambiguous manner 341 regardless of their source(s). 343 The tuple uniquely 344 identifies a candidate path. 346 Candidate paths MAY also be assigned or signaled with a symbolic name 347 comprising printable ASCII characters to serve as a user-friendly 348 attribute for debugging and troubleshooting purposes. Such symbolic 349 names MUST NOT be considered as identifiers for a candidate path. 351 2.7. Preference of a Candidate Path 353 The preference of the candidate path is used to select the best 354 candidate path for an SR Policy. It is a 32-bit value and the 355 default preference is 100. 357 It is RECOMMENDED that each candidate path of a given SR policy has a 358 different preference. 360 2.8. Validity of a Candidate Path 362 A candidate path is usable when it is valid. A common path validity 363 criterion is the validity of any of its constituent Segment-Lists. 364 The validation rules are specified in Section 5. 366 2.9. Active Candidate Path 368 A candidate path is selected when it is valid and it is determined to 369 be the best path of the SR Policy. The selected path is referred to 370 as the "active path" of the SR policy in this document. 372 Whenever a new path is learned or an active path is deleted, the 373 validity of an existing path changes or an existing path is changed, 374 the selection process MUST be re-executed. 376 The candidate path selection process operates on the candidate path 377 Preference. A candidate path is selected when it is valid and it has 378 the highest preference value among all the candidate paths of the SR 379 Policy. 381 In the case of multiple valid candidate paths of the same preference, 382 the tie-breaking rules are evaluated on the identification tuple in 383 the following order until only one valid best path is selected: 385 1. Higher value of Protocol-Origin is selected. 387 2. If specified by configuration, prefer the existing installed 388 path. 390 3. Lower value of originator is selected. 392 4. Finally, the higher value of discriminator is selected. 394 The rules are framed with multiple protocols and sources in mind and 395 hence may not follow the logic of a single protocol (e.g. BGP best 396 path selection). The motivation behind these rules are as follows: 398 o The Protocol-Origin allows an operator to set up a default 399 selection mechanism across protocol sources, e.g., to prefer 400 configured over paths signaled via BGP SR Policy or PCEP. 402 o The preference, being the first tiebreaker, allows an operator to 403 influence selection across paths thus allowing provisioning of 404 multiple path options, e.g., CP1 is preferred and if it becomes 405 invalid then fallback to CP2 and so on. Since preference works 406 across protocol sources, it also enables (where necessary) 407 selective override of the default Protocol-Origin preference, 408 e.g., to prefer a path signaled via BGP SR Policy over what is 409 configured. 411 o The originator allows an operator to have multiple redundant 412 controllers and still maintain a deterministic behavior over which 413 of them are preferred even if they are providing the same 414 candidate paths for the same SR policies to the headend. 416 o The discriminator performs the final tiebreaking step to ensure a 417 deterministic outcome of selection regardless of the order in 418 which candidate paths are signaled across multiple transport 419 channels or sessions. 421 Section 4 of [I-D.filsfils-spring-sr-policy-considerations] provides 422 a set of examples to illustrate the active candidate path selection 423 rules. 425 2.10. Validity of an SR Policy 427 An SR Policy is valid when it has at least one valid candidate path. 429 2.11. Instantiation of an SR Policy in the Forwarding Plane 431 A valid SR Policy is instantiated in the forwarding plane. 433 Only the active candidate path SHOULD be used for forwarding traffic 434 that is being steered onto that policy. 436 If a set of Segment-Lists is associated with the active path of the 437 policy, then the steering is per-flow and weighted-ECMP (W-ECMP) 438 based according to the relative weight of each Segment-List. 440 The fraction of the flows associated with a given Segment-List is w/ 441 Sw, where w is the weight of the Segment-List and Sw is the sum of 442 the weights of the Segment-Lists of the selected path of the SR 443 Policy. 445 When a composite candidate path is active, the fraction of flows 446 steered into each constituent SR Policy is equal to the relative 447 weight of each constituent SR Policy. Further load balancing of 448 flows steered into a constituent SR Policy is performed based on the 449 weights of the Segment-List of the active candidate path of that 450 constituent SR Policy. 452 The accuracy of the weighted load-balancing depends on the platform 453 implementation. 455 2.12. Priority of an SR Policy 457 Upon topological change, many policies could be recomputed or 458 revalidated. An implementation MAY provide a per-policy priority 459 configuration. The operator may set this field to indicate the order 460 in which the policies should be re-computed. Such a priority is 461 represented by an integer in the range (0, 255) where the lowest 462 value is the highest priority. The default value of priority is 128. 464 An SR Policy may comprise multiple Candidate Paths received from the 465 same or different sources. A candidate path MAY be signaled with a 466 priority value. When an SR Policy has multiple candidate paths with 467 distinct signaled non-default priority values and the SR Policy 468 itself does not have a priority value configured, the SR Policy as a 469 whole takes the lowest value (i.e. the highest priority) amongst 470 these signaled priority values. 472 2.13. Summary 474 In summary, the information model is the following: 476 SR policy POL1 477 Candidate-path CP1 479 Preference 200 480 Priority 10 481 Weight W1, SID-List1 482 Weight W2, SID-List2 483 Candidate-path CP2 485 Preference 100 486 Priority 10 487 Weight W3, SID-List3 488 Weight W4, SID-List4 490 The SR Policy POL1 is identified by the tuple . It has two candidate paths CP1 and CP2. Each is 492 identified by a tuple . 493 CP1 is the active candidate path (it is valid and has the highest 494 preference). The two Segment-Lists of CP1 are installed as the 495 forwarding instantiation of SR policy POL1. Traffic steered on POL1 496 is flow-based hashed on Segment-List with a ratio 497 W1/(W1+W2). 499 The information model of SR Policy POL100 having a composite 500 candidate path is the following: 502 SR policy POL100 503 Candidate-path CP1 505 Preference 200 506 Weight W1, SR policy 507 Weight W2, SR policy 509 The constituent SR Policies POL1 and POL2 have an information model 510 as described at the start of this section. They are referenced only 511 by color in the composite candidate path since their headend and 512 endpoint are identical to the POL100. The valid Segment-Lists of the 513 active candidate path of POL1 and POL2 are installed in the 514 forwarding. Traffic steered on POL100 is flow-based hashed on POL1 515 with a ratio W1/(W1+W2). Within the POL1, the flow-based hashing 516 over its Segment-Lists are performed as described earlier in this 517 section. 519 3. Segment Routing Database 521 An SR Policy computation node (e.g. headend or controller) maintains 522 the Segment Routing Database (SR-DB). The SR-DB is a conceptual 523 database to illustrate the various pieces of information and their 524 sources that may help in SR Policy computation and validation. There 525 is no specific requirement for an implementation to create a new 526 database as such. 528 An SR headend leverages the SR-DB to validate explicit candidate 529 paths and compute dynamic candidate paths. 531 The information in the SR-DB may include: 533 o IGP information (topology, IGP metrics based on ISIS [RFC1195] and 534 OSPF [RFC2328] [RFC5340]) 535 o Segment Routing information (such as Segment Routing Global Block, 536 Segment Routing Local Block, Prefix-SIDs, Adj-SIDs, BGP Peering 537 SID, SRv6 SIDs) [RFC8402] [RFC8986] 538 o TE Link Attributes (such as TE metric, Shared Risk Link Groups, 539 attribute-flag, extended admin group) [RFC5305] [RFC3630]. 540 o Extended TE Link attributes (such as latency, loss) [RFC8570] 541 [RFC7471] 542 o Inter-AS Topology information [RFC9086]. 544 The attached domain topology may be learned via IGP, BGP-LS or 545 NETCONF. 547 A non-attached (remote) domain topology may be learned via BGP-LS or 548 NETCONF. 550 In some use-cases, the SR-DB may only contain the attached domain 551 topology while in others, the SR-DB may contain the topology of 552 multiple domains and in this case, it is multi-domain capable. 554 The SR-DB may also contain the SR Policies instantiated in the 555 network. This can be collected via BGP-LS 556 [I-D.ietf-idr-te-lsp-distribution] or PCEP [RFC8231], 557 [I-D.ietf-pce-segment-routing-policy-cp], and 558 [I-D.ietf-pce-binding-label-sid]. This information allows to build 559 an end-to-end policy on the basis of intermediate SR policies (see 560 Section 6 for further details). 562 The SR-DB may also contain the Maximum SID Depth (MSD) capability of 563 nodes in the topology. This can be collected via ISIS [RFC8491], 564 OSPF [RFC8476], BGP-LS [RFC8814] or PCEP [RFC8664]. 566 The use of the SR-DB for computation and validation of SR Policies is 567 outside the scope of this document. Some implementation aspects 568 related to this are covered in 569 [I-D.filsfils-spring-sr-policy-considerations]. 571 4. Segment Types 573 A Segment-List is an ordered set of segments represented as where S1 is the first segment. 576 Based on the desired dataplane, either the MPLS label stack or the 577 SRv6 Segment Routing Header [RFC8754] is built from the Segment-List. 578 However, the Segment-List itself can be specified using different 579 segment-descriptor types and the following are currently defined: 581 Type A: SR-MPLS Label: 582 An MPLS label corresponding to any of the segment types defined 583 for SR-MPLS (as defined in [RFC8402] or other SR-MPLS 584 specifications) can be used. Additionally, reserved labels 585 like explicit-null or in general any MPLS label may also be 586 used. E.g. this type can be used to specify a label 587 representation that maps to an optical transport path on a 588 packet transport node. This type does not require the headend 589 to perform SID resolution. 591 Type B: SRv6 SID: 592 An IPv6 address corresponding to any of the SID behaviors for 593 SRv6 (as defined in [RFC8986] or other SRv6 specifications) can 594 be used. This type does not require the headend to perform SID 595 resolution. Optionally, the SRv6 SID behavior (as defined in 596 [RFC8986] or other SRv6 specifications) and structure (as 597 defined in [RFC8986]) MAY also be provided for the headend to 598 perform validation of the SID when using it for building the 599 Segment List. 601 Type C: IPv4 Prefix with optional SR Algorithm: 602 The headend is required to resolve the specified IPv4 Prefix 603 Address to the SR-MPLS label corresponding to a Prefix SID 604 segment (as defined in [RFC8402]). The SR algorithm (refer to 605 Section 3.1.1 of [RFC8402]) to be used MAY also be provided. 607 Type D: IPv6 Global Prefix with optional SR Algorithm for SR-MPLS: 608 In this case, the headend is required to resolve the specified 609 IPv6 Global Prefix Address to the SR-MPLS label corresponding 610 to its Prefix SID segment (as defined in [RFC8402]). The SR 611 Algorithm (refer to Section 3.1.1 of [RFC8402]) to be used MAY 612 also be provided. 614 Type E: IPv4 Prefix with Local Interface ID: 615 This type allows identification of Adjacency SID or BGP Peer 616 Adjacency SID (as defined in [RFC8402]) label for point-to- 617 point links including IP unnumbered links. The headend is 618 required to resolve the specified IPv4 Prefix Address to the 619 Node originating it and then use the Local Interface ID to 620 identify the point-to-point link whose adjacency is being 621 referred to. The Local Interface ID link descriptor follows 622 semantics as specified in [RFC7752]. This type can also be 623 used to indicate indirection into a layer 2 interface (i.e. 624 without IP address) like a representation of an optical 625 transport path or a layer 2 Ethernet port or circuit at the 626 specified node. 628 Type F: IPv4 Addresses for link endpoints as Local, Remote pair: 629 This type allows identification of Adjacency SID or BGP Peer 630 Adjacency SID (as defined in [RFC8402]) label for links. The 631 headend is required to resolve the specified IPv4 Local Address 632 to the Node originating it and then use the IPv4 Remote Address 633 to identify the link adjacency being referred to. The Local 634 and Remote Address pair link descriptors follow semantics as 635 specified in [RFC7752]. 637 Type G: IPv6 Prefix and Interface ID for link endpoints as Local, 638 Remote pair for SR-MPLS: 639 This type allows identification of Adjacency SID or BGP Peer 640 Adjacency SID (as defined in [RFC8402]) label for links 641 including those with only Link-Local IPv6 addresses. The 642 headend is required to resolve the specified IPv6 Prefix 643 Address to the Node originating it and then use the Local 644 Interface ID to identify the point-to-point link whose 645 adjacency is being referred to. For other than point-to-point 646 links, additionally the specific adjacency over the link needs 647 to be resolved using the Remote Prefix and Interface ID. The 648 Local and Remote pair of Prefix and Interface ID link 649 descriptor follows semantics as specified in [RFC7752]. This 650 type can also be used to indicate indirection into a layer 2 651 interface (i.e. without IP address) like a representation of an 652 optical transport path or a layer 2 Ethernet port or circuit at 653 the specified node. 655 Type H: IPv6 Addresses for link endpoints as Local, Remote pair for 656 SR-MPLS: 657 This type allows identification of Adjacency SID or BGP Peer 658 Adjacency SID (as defined in [RFC8402]) label for links with 659 Global IPv6 addresses. The headend is required to resolve the 660 specified Local IPv6 Address to the Node originating it and 661 then use the Remote IPv6 Address to identify the link adjacency 662 being referred to. The Local and Remote Address pair link 663 descriptors follow semantics as specified in [RFC7752]. 665 Type I: IPv6 Global Prefix with optional SR Algorithm for SRv6: 666 The headend is required to resolve the specified IPv6 Global 667 Prefix Address to an SRv6 SID corresponding to a Prefix SID 668 segment (as defined in [RFC8402]), such as a SID associated 669 with the End behavior (as defined in [RFC8986]), of the node 670 which is originating the prefix. The SR Algorithm (refer to 671 Section 3.1.1 of [RFC8402]), the SRv6 SID behavior (as defined 672 in [RFC8986] or other SRv6 specifications) and structure (as 673 defined in [RFC8986]) MAY also be provided. 675 Type J: IPv6 Prefix and Interface ID for link endpoints as Local, 676 Remote pair for SRv6: 677 This type allows identification of an SRv6 SID corresponding to 678 an Adjacency SID or BGP Peer Adjacency SID (as defined in 679 [RFC8402]), such as a SID associated with the End.X behavior 680 (as defined in [RFC8986]), associated with link or adjacency 681 with only Link-Local IPv6 addresses. The headend is required 682 to resolve the specified IPv6 Prefix Address to the Node 683 originating it and then use the Local Interface ID to identify 684 the point-to-point link whose adjacency is being referred to. 685 For other than point-to-point links, additionally the specific 686 adjacency needs to be resolved using the Remote Prefix and 687 Interface ID. The Local and Remote pair of Prefix and 688 Interface ID link descriptor follows semantics as specified in 689 [RFC7752]. The SR Algorithm (refer to Section 3.1.1 of 690 [RFC8402]), the SRv6 SID behavior (as defined in [RFC8986] or 691 other SRv6 specifications) and structure (as defined in 692 [RFC8986]) MAY also be provided. 694 Type K: IPv6 Addresses for link endpoints as Local, Remote pair for 695 SRv6: 696 This type allows identification of an SRv6 SID corresponding to 697 an Adjacency SID or BGP Peer Adjacency SID (as defined in 698 [RFC8402]), such as a SID associated with the End.X behavior 699 (as defined in [RFC8986]), associated with link or adjacency 700 with Global IPv6 addresses. The headend is required to resolve 701 the specified Local IPv6 Address to the Node originating it and 702 then use the Remote IPv6 Address to identify the link adjacency 703 being referred to. The Local and Remote Address pair link 704 descriptors follow semantics as specified in [RFC7752]. The SR 705 Algorithm (refer to Section 3.1.1 of [RFC8402]), the SRv6 SID 706 behavior (as defined in [RFC8986] or other SRv6 specifications) 707 and structure (as defined in [RFC8986]) MAY also be provided. 709 When the algorithm is not specified for the SID types above which 710 optionally allow for it, the headend SHOULD use the Strict Shortest 711 Path algorithm if available; otherwise, it SHOULD use the default 712 Shortest Path algorithm. The specification of the algorithm enables 713 the use of the IGP Flex Algorithm [I-D.ietf-lsr-flex-algo] specific 714 SIDs in SR Policy. 716 For SID types C-through-K, a SID value may also be optionally 717 provided to the headend for verification purposes. Section 5.1. 718 describes the resolution and verification of the SIDs and Segment 719 Lists on the headend. 721 When building the MPLS label stack or the IPv6 Segment list from the 722 Segment List, the node instantiating the policy MUST interpret the 723 set of Segments as follows: 725 o The first Segment represents the topmost label or the first IPv6 726 segment. It identifies the active segment the traffic will be 727 directed toward along the explicit SR path. 728 o The last Segment represents the bottommost label or the last IPv6 729 segment the traffic will be directed toward along the explicit SR 730 path. 732 4.1. Explicit Null 734 A Type A SID may be any MPLS label, including reserved labels. 736 For example, assuming that the desired traffic-engineered path from a 737 headend 1 to an endpoint 4 can be expressed by the Segment-List 738 <16002, 16003, 16004> where 16002, 16003 and 16004 respectively refer 739 to the IPv4 Prefix SIDs bound to node 2, 3, and 4, then IPv6 traffic 740 can be traffic-engineered from nodes 1 to 4 via the previously 741 described path using an SR Policy with Segment-List <16002, 16003, 742 16004, 2> where the MPLS label value of 2 represents the "IPv6 743 Explicit NULL Label". 745 The penultimate node before node 4 will pop 16004 and will forward 746 the frame on its directly connected interface to node 4. 748 The endpoint receives the traffic with the top label "2" which 749 indicates that the payload is an IPv6 packet. 751 When steering unlabeled IPv6 BGP destination traffic using an SR 752 policy composed of Segment-List(s) based on IPv4 SIDs, the Explicit 753 Null Label Policy is processed as specified in 754 [I-D.ietf-idr-segment-routing-te-policy]) Section 2.4.4. When an 755 "IPv6 Explicit NULL label" is not present as the bottom label, the 756 headend SHOULD automatically impose one. Refer to Section 8 for more 757 details. 759 5. Validity of a Candidate Path 761 5.1. Explicit Candidate Path 763 An explicit candidate path is associated with a Segment-List or a set 764 of Segment-Lists. 766 An explicit candidate path is provisioned by the operator directly or 767 via a controller. 769 The computation/logic that leads to the choice of the Segment-List is 770 external to the SR Policy headend. The SR Policy headend does not 771 compute the Segment-List. The SR Policy headend only confirms its 772 validity. 774 An explicit candidate path MAY consist of a single explicit Segment- 775 List containing only an implicit-null label to indicate pop-and- 776 forward behavior. The Binding SID (BSID) is popped and the traffic 777 is forwarded based on the inner label or an IP lookup in the case of 778 unlabeled IP packets. Such an explicit path can serve as a fallback 779 or path of last resort for traffic being steered into an SR Policy 780 using its BSID (refer to Section 8.3). 782 A Segment-List of an explicit candidate path MUST be declared invalid 783 when: 785 o It is empty. 786 o Its weight is 0. 787 o It comprises of a mix of SR-MPLS and SRv6 segment types. 788 o The headend is unable to perform path resolution for the first SID 789 into one or more outgoing interface(s) and next-hop(s). 790 o The headend is unable to perform SID resolution for any non-first 791 SID of type C-through-K into an MPLS label or an SRv6 SID. 792 o The headend verification fails for any SID for which verification 793 has been explicitly requested. 795 "Unable to perform path resolution" means that the headend has no 796 path to the SID in its SR database. 798 SID verification is performed when the headend is explicitly 799 requested to verify SID(s) by the controller via the signaling 800 protocol used. Implementations MAY provide a local configuration 801 option to enable verification on a global or per policy or per 802 candidate path basis. 804 "Verification fails" for a SID means any of the following: 806 o The headend is unable to find the SID in its SR-DB 807 o The headend detects a mis-match between the SID value and its 808 context provided for SIDs of type C-through-K in its SR-DB. 809 o The headend is unable to perform SID resolution for any non-first 810 SID of type C-through-K into an MPLS label or an SRv6 SID. 812 In multi-domain deployments, it is expected that the headend be 813 unable to verify the reachability of the SIDs in remote domains. 814 Types A or B MUST be used for the SIDs for which the reachability 815 cannot be verified. Note that the first SID MUST always be reachable 816 regardless of its type. 818 Additionally, a Segment-List MAY be declared invalid when: 820 o Its last segment is not a Prefix SID (including BGP Peer Node-SID) 821 advertised by the node specified as the endpoint of the 822 corresponding SR policy. 823 o Its last segment is not an Adjacency SID (including BGP Peer 824 Adjacency SID) of any of the links present on neighbor nodes and 825 that terminate on the node specified as the endpoint of the 826 corresponding SR policy. 828 An explicit candidate path is invalid as soon as it has no valid 829 Segment-List. 831 Additionally, an explicit candidate path MAY be declared invalid when 832 its constituent segment lists (valid or invalid) are using segment 833 types of different SR dataplanes. 835 5.2. Dynamic Candidate Path 837 A dynamic candidate path is specified as an optimization objective 838 and constraints. 840 The headend of the policy leverages its SR database to compute a 841 Segment-List ("solution Segment-List") that solves this optimization 842 problem for either the SR-MPLS or the SRv6 data-plane as specified. 844 The headend re-computes the solution Segment-List any time the inputs 845 to the problem change (e.g., topology changes). 847 When the local computation is not possible (e.g., a policy's tail-end 848 is outside the topology known to the headend) or not desired, the 849 headend MAY send path computation request to a PCE supporting PCEP 850 extension specified in [RFC8664]. 852 If no solution is found to the optimization objective and 853 constraints, then the dynamic candidate path MUST be declared 854 invalid. 856 Section 3 of [I-D.filsfils-spring-sr-policy-considerations] discusses 857 some of the optimization objectives and constraints that may be 858 considered by a dynamic candidate path. It illustrates some of the 859 desirable properties of the computation of the solution Segment-List. 861 5.3. Composite Candidate Path 863 A composite candidate path is specified as a group of its constituent 864 SR Policies. 866 A composite candidate path is valid when it has at least one valid 867 constituent SR Policy. 869 6. Binding SID 871 The Binding SID (BSID) is fundamental to Segment Routing [RFC8402]. 872 It provides scaling, network opacity, and service independence. 873 Section 6 of [I-D.filsfils-spring-sr-policy-considerations] 874 illustrates some of these benefits. This section describes the 875 association of BSID with an SR Policy. 877 6.1. BSID of a candidate path 879 Each candidate path MAY be defined with a BSID. 881 Candidate Paths of the same SR policy SHOULD have the same BSID. 883 Candidate Paths of different SR policies MUST NOT have the same BSID. 885 6.2. BSID of an SR Policy 887 The BSID of an SR Policy is the BSID of its active candidate path. 889 When the active candidate path has a specified BSID, the SR Policy 890 uses that BSID if this value (label in MPLS, IPv6 address in SRv6) is 891 available (i.e., not associated with any other usage: e.g. to another 892 MPLS client, to another SRv6 client, to another SID, to another SR 893 Policy, outside the range of SRv6 Locators). 895 In the case of SR-MPLS, SRv6 BSIDs (e.g. with the behavior End.BM 896 [RFC8986]) MAY be associated with the SR Policy in addition to the 897 MPLS BSID. In the case of SRv6, multiple SRv6 BSIDs (e.g. with 898 different behaviors like End.B6.Encap and End.B6.Encap.Red [RFC8986]) 899 MAY be associated with the SR Policy. 901 Optionally, instead of only checking that the BSID of the active path 902 is available, a headend MAY check that it is available within a given 903 SID range i.e., Segment Routing Local Block (SRLB) as specified in 904 [RFC8402]. 906 When the specified BSID is not available (optionally is not in the 907 SRLB), an alert message MUST be generated. 909 In the cases (as described above) where SR Policy does not have a 910 BSID available, then the SR Policy MAY dynamically bind a BSID to 911 itself. Dynamically bound BSID SHOULD use an available SID outside 912 the SRLB. 914 Assuming that at time t the BSID of the SR Policy is B1, if at time 915 t+dt a different candidate path becomes active and this new active 916 path does not have a specified BSID or its BSID is specified but is 917 not available (e.g. it is in use by something else), then the SR 918 Policy MAY keep the previous BSID B1. 920 The association of an SR Policy with a BSID thus MAY change over the 921 life of the SR Policy (e.g., upon active path change). Hence, the 922 BSID SHOULD NOT be used as an identification of an SR Policy. 924 6.2.1. Frequent use-case : unspecified BSID 926 All the candidate paths of the same SR Policy can have an unspecified 927 BSID. 929 In such a case, a BSID MAY be dynamically bound to the SR Policy as 930 soon as the first valid candidate path is received. That BSID is 931 kept through the life of the SR Policy and across changes of active 932 candidate path. 934 6.2.2. Frequent use-case: all specified to the same BSID 936 All the paths of the SR Policy can have the same specified BSID. 938 6.2.3. Specified-BSID-only 940 An implementation MAY support the configuration of the Specified- 941 BSID-only restrictive behavior on the headend for all SR Policies or 942 individual SR Policies. Further, this restrictive behavior MAY also 943 be signaled on a per SR Policy basis to the headend. 945 When this restrictive behavior is enabled, if the candidate path has 946 an unspecified BSID or if the specified BSID is not available when 947 the candidate path becomes active then no BSID is bound to it and the 948 candidate path is considered invalid. An alert MUST be triggered for 949 this error. Other candidate paths MUST then be evaluated for 950 becoming the active candidate path. 952 6.3. Forwarding Plane 954 A valid SR Policy installs a BSID-keyed entry in the forwarding plane 955 with the action of steering the packets matching this entry to the 956 selected path of the SR Policy. 958 If the Specified-BSID-only restrictive behavior is enabled and the 959 BSID of the active path is not available (optionally not in the 960 SRLB), then the SR Policy does not install any entry indexed by a 961 BSID in the forwarding plane. 963 6.4. Non-SR usage of Binding SID 965 An implementation MAY choose to associate a Binding SID with any type 966 of interface (e.g. a layer 3 termination of an Optical Circuit) or a 967 tunnel (e.g. IP tunnel, GRE tunnel, IP/UDP tunnel, MPLS RSVP-TE 968 tunnel, etc). This enables the use of other non-SR enabled 969 interfaces and tunnels as segments in an SR Policy Segment-List 970 without the need of forming routing protocol adjacencies over them. 972 The details of this kind of usage are beyond the scope of this 973 document. A specific packet-optical integration use case is 974 described in [I-D.anand-spring-poi-sr]. 976 7. SR Policy State 978 The SR Policy State is maintained on the headend to represent the 979 state of the policy and its candidate paths. This is to provide an 980 accurate representation of whether the SR Policy is being 981 instantiated in the forwarding plane and which of its candidate paths 982 and segment-list(s) are active. The SR Policy state MUST also 983 reflect the reason when a policy and/or its candidate path is not 984 active due to validation errors or not being preferred. 986 The SR Policy state can be reported by the headend node via BGP-LS 987 [I-D.ietf-idr-te-lsp-distribution] or PCEP [RFC8231] and 988 [I-D.ietf-pce-binding-label-sid]. 990 SR Policy state on the headend also includes traffic accounting 991 information for the flows being steered via the policies. The 992 details of the SR Policy accounting are beyond the scope of this 993 document. The aspects related to the SR traffic counters and their 994 usage in the broader context of traffic accounting in an SR network 995 are covered in [I-D.filsfils-spring-sr-traffic-counters] and 996 [I-D.ali-spring-sr-traffic-accounting] respectively. 998 Implementations MAY support an administrative state to control 999 locally provisioned policies via mechanisms like CLI or NETCONF. 1001 8. Steering into an SR Policy 1003 A headend can steer a packet flow into a valid SR Policy in various 1004 ways: 1006 o Incoming packets have an active SID matching a local BSID at the 1007 headend. 1008 o Per-destination Steering: incoming packets match a BGP/Service 1009 route which recurses on an SR policy. 1010 o Per-flow Steering: incoming packets match or recurse on a 1011 forwarding array of where some of the entries are SR Policies. 1012 o Policy-based Steering: incoming packets match a routing policy 1013 that directs them on an SR policy. 1015 8.1. Validity of an SR Policy 1017 An SR Policy is invalid when all its candidate paths are invalid as 1018 described in Section 5 and Section 2.10. 1020 By default, upon transitioning to the invalid state, 1022 o an SR Policy and its BSID are removed from the forwarding plane. 1023 o any steering of a service (Pseudowire (PW)), destination (BGP- 1024 VPN), flow or packet on the related SR policy is disabled and the 1025 related service, destination, flow, or packet is routed per the 1026 classic forwarding table (e.g. longest-match to the destination or 1027 the recursing next-hop). 1029 8.2. Drop upon invalid SR Policy 1031 An SR Policy MAY be enabled for the Drop-Upon-Invalid behavior: 1033 o an invalid SR Policy and its BSID is kept in the forwarding plane 1034 with an action to drop. 1035 o any steering of a service (PW), destination (BGP-VPN), flow or 1036 packet on the related SR policy is maintained with the action to 1037 drop all of this traffic. 1039 The drop-upon-invalid behavior has been deployed in use-cases where 1040 the operator wants some PW to only be transported on a path with 1041 specific constraints. When these constraints are no longer met, the 1042 operator wants the PW traffic to be dropped. Specifically, the 1043 operator does not want the PW to be routed according to the IGP 1044 shortest path to the PW endpoint. 1046 8.3. Incoming Active SID is a BSID 1048 Let us assume that headend H has a valid SR Policy P of Segment-List 1049 and BSID B. 1051 In the case of SR-MPLS, when H receives a packet K with label stack 1052 , H pops B and pushes and forwards the 1053 resulting packet according to SID S1. 1055 "Forwarding the resulting packet according to S1" means: If S1 is 1056 an Adj SID or a PHP-enabled prefix SID advertised by a neighbor, H 1057 sends the resulting packet with label stack on 1058 the outgoing interface associated with S1; Else H sends the 1059 resulting packet with label stack along the 1060 path of S1. 1062 In the case of SRv6, the processing is similar and follows the SR 1063 Policy headend behaviors as specified in section 5 of [RFC8986]. 1065 H has steered the packet into the SR policy P. 1067 H did not have to classify the packet. The classification was done 1068 by a node upstream of H (e.g., the source of the packet or an 1069 intermediate ingress edge node of the SR domain) and the result of 1070 this classification was efficiently encoded in the packet header as a 1071 BSID. 1073 This is another key benefit of the segment routing in general and the 1074 binding SID in particular: the ability to encode a classification and 1075 the resulting steering in the packet header to better scale and 1076 simplify intermediate aggregation nodes. 1078 If the SR Policy P is invalid, the BSID B is not in the forwarding 1079 plane and hence the packet K is dropped by H. 1081 8.4. Per-Destination Steering 1083 In the case of SR-MPLS, let us assume that headend H: 1085 o learns a BGP route R/r via next-hop N, Color Extended community C 1086 and VPN label V. 1087 o has a valid SR Policy P to (color = C, endpoint = N) of Segment- 1088 List and BSID B. 1089 o has a BGP policy that matches on the Color Extended community C 1090 and allows its usage as SLA steering information. 1092 If all these conditions are met, H installs R/r in RIB/FIB with next- 1093 hop = SR Policy P of BSID B instead of via N. 1095 Indeed, H's local BGP policy and the received BGP route indicate that 1096 the headend should associate R/r with an SR Policy path to endpoint N 1097 with the SLA associated with color C. The headend, therefore, 1098 installs the BGP route on that policy. 1100 This can be implemented by using the BSID as a generalized next-hop 1101 and installing the BGP route on that generalized next-hop. 1103 When H receives a packet K with a destination matching R/r, H pushes 1104 the label stack and sends the resulting packet along 1105 the path to S1. 1107 Note that any SID associated with the BGP route is inserted after the 1108 Segment-List of the SR Policy (i.e., ). 1110 In the case of SRv6, the processing is similar and follows the SR 1111 Policy headend behaviors as specified in section 5 of [RFC8986]. 1113 The same behavior applies to any type of service route: any AFI/SAFI 1114 of BGP [RFC4760] any AFI/SAFI of LISP [RFC6830]. 1116 In a BGP multi-path scenario, the BGP route may be resolved over a 1117 mix of paths that include those that are steered over SR Policies and 1118 others resolved via the normal BGP nexthop resolution. 1119 Implementations MAY provide options to prefer one type over the other 1120 or other forms of local policy to determine the paths that are 1121 selected. 1123 8.4.1. Multiple Colors 1125 When a BGP route has multiple Color Extended communities each with a 1126 valid SR Policy, the BGP process installs the route on the SR Policy 1127 giving preference to the color with the highest numerical value. 1129 Let us assume that headend H: 1131 o learns a BGP route R/r via next-hop N, Color Extended communities 1132 C1 and C2. 1133 o has a valid SR Policy P1 to (color = C1, endpoint = N) of Segment- 1134 List and BSID B1. 1135 o has a valid SR Policy P2 to (color = C2, endpoint = N) of Segment- 1136 List and BSID B2. 1137 o has a BGP policy that matches the Color Extended communities C1 1138 and C2 and allows their usage as SLA steering information 1140 If all these conditions are met, H installs R/r in RIB/FIB with next- 1141 hop = SR Policy P2 of BSID=B2 (instead of N) because C2 > C1. 1143 When the SR Policy with a specific color is not instantiated or in 1144 the down/inactive state, the SR Policy with the next highest 1145 numerical value of color is considered. 1147 8.5. Recursion on an on-demand dynamic BSID 1149 In the previous section, it was assumed that H had a pre-established 1150 "explicit" SR Policy (color C, endpoint N). 1152 In this section, independent of the a-priori existence of any 1153 explicit candidate path of the SR policy (C, N), it is to be noted 1154 that the BGP process at headend node H triggers the instantiation of 1155 a dynamic candidate path for the SR policy (C, N) as soon as: 1157 o the BGP process learns of a route R/r via N and with color C. 1158 o a local policy at node H authorizes the on-demand SR Policy path 1159 instantiation and maps the color to a dynamic SR Policy path 1160 optimization template. 1162 8.5.1. Multiple Colors 1164 When a BGP route R/r via N has multiple Color Extended communities Ci 1165 (with i=1 ... n), an individual on-demand SR Policy dynamic path 1166 request (color Ci, endpoint N) is triggered for each color Ci. The 1167 SR Policy that is used for steering is then determined as described 1168 in Section 8.4.1. 1170 8.6. Per-Flow Steering 1172 Let us assume that headend H: 1174 o has a valid SR Policy P1 to (color = C1, endpoint = N) of Segment- 1175 List and BSID B1. 1176 o has a valid SR Policy P2 to (color = C2, endpoint = N) of Segment- 1177 List and BSID B2. 1178 o is configured to instantiate an array of paths to N where the 1179 entry 0 is the IGP path to N, color C1 is the first entry and 1180 Color C2 is the second entry. The index into the array is called 1181 a Forwarding Class (FC). The index can have values 0 to 7. 1182 o is configured to match flows in its ingress interfaces (upon any 1183 field such as Ethernet destination/source/VLAN/TOS or IP 1184 destination/source/DSCP or transport ports etc.) and color them 1185 with an internal per-packet forwarding-class variable (0, 1 or 2 1186 in this example). 1188 If all these conditions are met, H installs in RIB/FIB: 1190 o N via recursion on an array A (instead of the immediate outgoing 1191 link associated with the IGP shortest-path to N). 1192 o Entry A(0) set to the immediate outgoing link of the IGP shortest- 1193 path to N. 1194 o Entry A(1) set to SR Policy P1 of BSID=B1. 1195 o Entry A(2) set to SR Policy P2 of BSID=B2. 1197 H receives three packets K, K1, and K2 on its incoming interface. 1198 These three packets either longest-match on N or more likely on a 1199 BGP/service route which recurses on N. H colors these 3 packets 1200 respectively with forwarding-class 0, 1, and 2. 1202 As a result, for SR-MPLS: 1204 o H forwards K along the shortest path to N (i.e., pushes the 1205 prefix-SID of N). 1206 o H pushes on packet K1 and forwards the resulting 1207 frame along the shortest path to S1. 1208 o H pushes on packet K2 and forwards the resulting 1209 frame along the shortest path to S4. 1211 For SRv6, the processing is similar and the segment lists of the 1212 individual SR Policies P1 and P2 are enforced for packet K1 and K2 1213 using the SR Policy headend behaviors as specified in section 5 of 1214 [RFC8986]. 1216 If the local configuration does not specify any explicit forwarding 1217 information for an entry of the array, then this entry is filled with 1218 the same information as entry 0 (i.e. the IGP shortest path). 1220 If the SR Policy mapped to an entry of the array becomes invalid, 1221 then this entry is filled with the same information as entry 0. When 1222 all the array entries have the same information as entry0, the 1223 forwarding entry for N is updated to bypass the array and point 1224 directly to its outgoing interface and next-hop. 1226 The array index values (e.g. 0, 1, and 2) and the notion of 1227 forwarding-class are implementation-specific and only meant to 1228 describe the desired behavior. The same can be realized by other 1229 mechanisms. 1231 This realizes per-flow steering: different flows bound to the same 1232 BGP endpoint are steered on different IGP or SR Policy paths. 1234 A headend MAY support options to apply per-flow steering only for 1235 traffic matching specific prefixes (e.g. specific IGP or BGP 1236 prefixes). 1238 8.7. Policy-based Routing 1240 Finally, headend H may be configured with a local routing policy 1241 which overrides any BGP/IGP path and steer a specified packet on an 1242 SR Policy. This includes the use of mechanisms like IGP Shortcut for 1243 automatic routing of IGP prefixes over SR Policies intended for such 1244 purpose. 1246 8.8. Optional Steering Modes for BGP Destinations 1248 8.8.1. Color-Only BGP Destination Steering 1250 In the previous section, it is seen that the steering on an SR Policy 1251 is governed by the matching of the BGP route's next-hop N and the 1252 authorized color C with an SR Policy defined by the tuple (N, C). 1254 This is the most likely form of BGP destination steering and the one 1255 recommended for most use-cases. 1257 This section defines an alternative steering mechanism based only on 1258 the color. 1260 This color-only steering variation is governed by two new "CO" bits 1261 defined in the Color Extended community in section 3 of 1262 [I-D.ietf-idr-segment-routing-te-policy]. 1264 The Color-Only flags "CO" are set to 00 by default. 1266 When 00, the BGP destination is steered as follows: 1268 IF there is a valid SR Policy (N, C) where N is the IPv4 or IPv6 1270 endpoint address and C is a color; 1271 Steer into SR Policy (N, C); 1272 ELSE; 1273 Steer on the IGP path to the next-hop N. 1275 This is the classic case described in this document previously and 1276 what is recommended in most scenarios. 1278 When 01, the BGP destination is steered as follows: 1280 IF there is a valid SR Policy (N, C) where N is the IPv4 or IPv6 1282 endpoint address and C is a color; 1283 Steer into SR Policy (N, C); 1284 ELSE IF there is a valid SR Policy (null endpoint, C) of the 1285 same address-family of N; 1287 Steer into SR Policy (null endpoint, C); 1288 ELSE IF there is any valid SR Policy 1289 (any address-family null endpoint, C); 1290 Steer into SR Policy (any null endpoint, C); 1291 ELSE; 1292 Steer on the IGP path to the next-hop N. 1294 When 10, the BGP destination is steered as follows: 1296 IF there is a valid SR Policy (N, C) where N is an IPv4 or IPv6 1297 endpoint address and C is a color; 1298 Steer into SR Policy (N, C); 1299 ELSE IF there is a valid SR Policy (null endpoint, C) 1300 of the same address-family of N; 1301 Steer into SR Policy (null endpoint, C); 1302 ELSE IF there is any valid SR Policy 1303 (any address-family null endpoint, C); 1304 Steer into SR Policy (any null endpoint, C); 1305 ELSE IF there is any valid SR Policy (any endpoint, C) 1306 of the same address-family of N; 1307 Steer into SR Policy (any endpoint, C); 1308 ELSE IF there is any valid SR Policy 1309 (any address-family endpoint, C); 1310 Steer into SR Policy (any address-family endpoint, C); 1311 ELSE; 1312 Steer on the IGP path to the next-hop N. 1314 The null endpoint is 0.0.0.0 for IPv4 and ::0 for IPv6 (all bits set 1315 to the 0 value). 1317 The value 11 is reserved for future use and SHOULD NOT be used. Upon 1318 reception, an implementation MUST treat it like 00. 1320 8.8.2. Multiple Colors and CO flags 1322 The steering preference is first based on the highest color value and 1323 then CO-dependent for the color. Assuming a Prefix via (NH, 1324 C1(CO=01), C2(CO=01)); C1>C2 The steering preference order is: 1326 o SR policy (NH, C1). 1327 o SR policy (null, C1). 1328 o SR policy (NH, C2). 1329 o SR policy (null, C2). 1330 o IGP to NH. 1332 8.8.3. Drop upon Invalid 1334 This document defined earlier that when all the following conditions 1335 are met, H installs R/r in RIB/FIB with next-hop = SR Policy P of 1336 BSID B instead of via N. 1338 o H learns a BGP route R/r via next-hop N, Color Extended community 1339 C. 1340 o H has a valid SR Policy P to (color = C, endpoint = N) of Segment- 1341 List and BSID B. 1342 o H has a BGP policy that matches the Color Extended community C and 1343 allows its usage as SLA steering information. 1345 This behavior is extended by noting that the BGP policy may require 1346 the BGP steering to always stay on the SR policy whatever its 1347 validity. 1349 This is the "drop upon invalid" option described in Section 8.2 1350 applied to BGP-based steering. 1352 9. Protection 1354 9.1. Leveraging TI-LFA local protection of the constituent IGP segments 1356 In any topology, Topology-Independent Loop-Free Alternate (TI-LFA) 1357 [I-D.ietf-rtgwg-segment-routing-ti-lfa] provides a 50msec local 1358 protection technique for IGP SIDs. The backup path is computed on a 1359 per IGP SID basis along the post-convergence path. 1361 In a network that has deployed TI-LFA, an SR Policy built on the 1362 basis of TI-LFA protected IGP segments leverages the local protection 1363 of the constituent segments. Since TI-LFA protection is based on IGP 1364 computation, there are cases where the path used during the fast- 1365 reroute time window may not meet the exact constraints of the SR 1366 Policy. 1368 In a network that has deployed TI-LFA, an SR Policy instantiated only 1369 with non-protected Adj SIDs does not benefit from any local 1370 protection. 1372 9.2. Using an SR Policy to locally protect a link 1373 1----2-----6----7 1374 | | | | 1375 4----3-----9----8 1377 Figure 1: Local protection using SR Policy 1379 An SR Policy can be instantiated at node 2 to protect the link 2to6. 1380 A typical explicit Segment-List would be <3, 9, 6>. 1382 A typical use-case occurs for links outside an IGP domain: e.g. 1, 2, 1383 3, and 4 are part of IGP/SR sub-domain 1 while 6, 7, 8, and 9 are 1384 part of IGP/SR sub-domain 2. In such a case, links 2to6 and 3to9 1385 cannot benefit from TI-LFA automated local protection. The SR Policy 1386 with Segment-List <3, 9, 6> on node 2 can be locally configured to be 1387 a fast-reroute backup path for the link 2to6. 1389 9.3. Using a Candidate Path for Path Protection 1391 An SR Policy allows for multiple candidate paths, of which at any 1392 point in time there is a single active candidate path that is 1393 provisioned in the forwarding plane and used for traffic steering. 1394 However, another (lower preference) candidate path MAY be designated 1395 as the backup for a specific or all (active) candidate path(s). The 1396 following options are possible: 1398 o A pair of disjoint candidate paths are provisioned with one of 1399 them as primary and the other is identified as its backup. 1400 o A specific candidate path is provisioned as the backup for any 1401 (active) candidate path. 1402 o The headend picks the next (lower) preference valid candidate path 1403 as the backup for the active candidate path. 1405 The headend MAY compute a-priori and validate such backup candidate 1406 paths as well as provision them into the forwarding plane as a backup 1407 for the active path. The backup candidate path may be dynamically 1408 computed or explicitly provisioned in such a way that they provide 1409 the most appropriate alternative for the active candidate path. A 1410 fast re-route mechanism MAY then be used to trigger sub 50msec 1411 switchover from the active to the backup candidate path in the 1412 forwarding plane. Mechanisms like Bidirectional Forwarding Detection 1413 (BFD) MAY be used for fast detection of such failures. 1415 10. Security Considerations 1417 This document specifies in detail the SR Policy construct introduced 1418 in [RFC8402] and its instantiation on a router supporting SR along 1419 with descriptions of mechanisms for steering of traffic flows over 1420 it. Therefore, the security considerations of [RFC8402] apply. This 1421 document does not define any new protocol extensions and does not 1422 introduce any further security considerations. 1424 11. Manageability Considerations 1426 This document specifies in detail the SR Policy construct introduced 1427 in [RFC8402] and its instantiation on a router supporting SR along 1428 with descriptions of mechanisms for steering of traffic flows over 1429 it. Therefore, the manageability considerations of [RFC8402] apply. 1431 A YANG model for the configuration and operation of SR Policy has 1432 been defined in [I-D.ietf-spring-sr-policy-yang]. 1434 12. IANA Considerations 1436 The document requests IANA to create a new sub-registry called 1437 "Segment Types" under the top-level "Segment Routing" registry that 1438 was created by [RFC8986]. This sub-registry maintains the alphabetic 1439 identifiers for the segment types (as specified in section 4) that 1440 may be used within a Segment List of an SR Policy. The alphabetical 1441 identifiers run from A to Z and may be extended on exhaustion with 1442 the identifiers AA to AZ, BA to BZ and so on through till ZZ. This 1443 sub-registry would follow the Specification Required allocation 1444 policy as specified in [RFC8126]. 1446 The initial registrations for this sub-registry are as follows: 1448 +-------+-----------------------------------------------+-----------+ 1449 | Value | Description | Reference | 1450 +-------+-----------------------------------------------+-----------+ 1451 | A | SR-MPLS Label | [This.ID] | 1452 | B | SRv6 SID | [This.ID] | 1453 | C | IPv4 Prefix with optional SR Algorithm | [This.ID] | 1454 | D | IPv6 Global Prefix with optional SR Algorithm | [This.ID] | 1455 | | for SR-MPLS | | 1456 | E | IPv4 Prefix with Local Interface ID | [This.ID] | 1457 | F | IPv4 Addresses for link endpoints as Local, | [This.ID] | 1458 | | Remote pair | | 1459 | G | IPv6 Prefix and Interface ID for link | [This.ID] | 1460 | | endpoints as Local, Remote pair for SR-MPLS | | 1461 | H | IPv6 Addresses for link endpoints as Local, | [This.ID] | 1462 | | Remote pair for SR-MPLS | | 1463 | I | IPv6 Global Prefix with optional SR Algorithm | [This.ID] | 1464 | | for SRv6 | | 1465 | J | IPv6 Prefix and Interface ID for link | [This.ID] | 1466 | | endpoints as Local, Remote pair for SRv6 | | 1467 | K | IPv6 Addresses for link endpoints as Local, | [This.ID] | 1468 | | Remote pair for SRv6 | | 1469 +-------+-----------------------------------------------+-----------+ 1471 Table 2: Initial IANA Registration 1473 12.1. Guidance for Designated Experts 1475 The Designated Expert (DE) is expected to ascertain the existence of 1476 suitable documentation (a specification) as described in [RFC8126] 1477 and to verify that the document is permanently and publicly 1478 available. The DE is also expected to check the clarity of purpose 1479 and use of the requested assignment. Additionally, the DE must 1480 verify that any request for one of these assignments has been made 1481 available for review and comment within the IETF: the DE will post 1482 the request to the SPRING Working Group mailing list (or a successor 1483 mailing list designated by the IESG). If the request comes from 1484 within the IETF, it should be documented in an Internet-Draft. 1485 Lastly, the DE must ensure that any other request for a code point 1486 does not conflict with work that is active or already published 1487 within the IETF. 1489 13. Acknowledgement 1491 The authors would like to thank Tarek Saad, Dhanendra Jain, Ruediger 1492 Geib, Rob Shakir, Cheng Li, Dhruv Dhody and Gyan Mishra for their 1493 review comments and suggestions. 1495 14. Contributors 1497 The following people have contributed to this document: 1499 Siva Sivabalan 1500 Cisco Systems 1501 Email: msiva@cisco.com 1503 Zafar Ali 1504 Cisco Systems 1505 Email: zali@cisco.com 1507 Jose Liste 1508 Cisco Systems 1509 Email: jliste@cisco.com 1511 Francois Clad 1512 Cisco Systems 1513 Email: fclad@cisco.com 1515 Kamran Raza 1516 Cisco Systems 1517 Email: skraza@cisco.com 1519 Mike Koldychev 1520 Cisco Systems 1521 Email: mkoldych@cisco.com 1523 Shraddha Hegde 1524 Juniper Networks 1525 Email: shraddha@juniper.net 1527 Steven Lin 1528 Google, Inc. 1529 Email: stevenlin@google.com 1531 Przemyslaw Krol 1532 Google, Inc. 1533 Email: pkrol@google.com 1535 Martin Horneffer 1536 Deutsche Telekom 1537 Email: martin.horneffer@telekom.de 1539 Dirk Steinberg 1540 Steinberg Consulting 1541 Email: dws@steinbergnet.net 1542 Bruno Decraene 1543 Orange Business Services 1544 Email: bruno.decraene@orange.com 1546 Stephane Litkowski 1547 Orange Business Services 1548 Email: stephane.litkowski@orange.com 1550 Luay Jalil 1551 Verizon 1552 Email: luay.jalil@verizon.com 1554 15. References 1556 15.1. Normative References 1558 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1559 Requirement Levels", BCP 14, RFC 2119, 1560 DOI 10.17487/RFC2119, March 1997, 1561 . 1563 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1564 Writing an IANA Considerations Section in RFCs", BCP 26, 1565 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1566 . 1568 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1569 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1570 May 2017, . 1572 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 1573 Decraene, B., Litkowski, S., and R. Shakir, "Segment 1574 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 1575 July 2018, . 1577 [RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., 1578 Decraene, B., Litkowski, S., and R. Shakir, "Segment 1579 Routing with the MPLS Data Plane", RFC 8660, 1580 DOI 10.17487/RFC8660, December 2019, 1581 . 1583 [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., 1584 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header 1585 (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, 1586 . 1588 [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, 1589 D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 1590 (SRv6) Network Programming", RFC 8986, 1591 DOI 10.17487/RFC8986, February 2021, 1592 . 1594 15.2. Informative References 1596 [I-D.ali-spring-sr-traffic-accounting] 1597 Filsfils, C., Talaulikar, K., Sivabalan, S., Horneffer, 1598 M., Raszuk, R., Litkowski, S., Voyer, D., and R. Morton, 1599 "Traffic Accounting in Segment Routing Networks", draft- 1600 ali-spring-sr-traffic-accounting-05 (work in progress), 1601 April 2021. 1603 [I-D.anand-spring-poi-sr] 1604 Anand, M., Bardhan, S., Subrahmaniam, R., Tantsura, J., 1605 Mukhopadhyaya, U., and C. Filsfils, "Packet-Optical 1606 Integration in Segment Routing", draft-anand-spring-poi- 1607 sr-08 (work in progress), July 2019. 1609 [I-D.filsfils-spring-sr-policy-considerations] 1610 Filsfils, C., Talaulikar, K., Krol, P., Horneffer, M., and 1611 P. Mattes, "SR Policy Implementation and Deployment 1612 Considerations", draft-filsfils-spring-sr-policy- 1613 considerations-08 (work in progress), October 2021. 1615 [I-D.filsfils-spring-sr-traffic-counters] 1616 Filsfils, C., Ali, Z., Horneffer, M., Voyer, D., Durrani, 1617 M., and R. Raszuk, "Segment Routing Traffic Accounting 1618 Counters", draft-filsfils-spring-sr-traffic-counters-02 1619 (work in progress), October 2021. 1621 [I-D.ietf-idr-segment-routing-te-policy] 1622 Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., 1623 Rosen, E., Jain, D., and S. Lin, "Advertising Segment 1624 Routing Policies in BGP", draft-ietf-idr-segment-routing- 1625 te-policy-13 (work in progress), June 2021. 1627 [I-D.ietf-idr-te-lsp-distribution] 1628 Previdi, S., Talaulikar, K., Dong, J., Chen, M., Gredler, 1629 H., and J. Tantsura, "Distribution of Traffic Engineering 1630 (TE) Policies and State using BGP-LS", draft-ietf-idr-te- 1631 lsp-distribution-16 (work in progress), October 2021. 1633 [I-D.ietf-lsr-flex-algo] 1634 Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and 1635 A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex- 1636 algo-17 (work in progress), July 2021. 1638 [I-D.ietf-pce-binding-label-sid] 1639 Sivabalan, S., Filsfils, C., Tantsura, J., Previdi, S., 1640 and C. L. (editor), "Carrying Binding Label/Segment 1641 Identifier in PCE-based Networks.", draft-ietf-pce- 1642 binding-label-sid-11 (work in progress), October 2021. 1644 [I-D.ietf-pce-segment-routing-policy-cp] 1645 Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H. 1646 Bidgoli, "PCEP extension to support Segment Routing Policy 1647 Candidate Paths", draft-ietf-pce-segment-routing-policy- 1648 cp-06 (work in progress), October 2021. 1650 [I-D.ietf-rtgwg-segment-routing-ti-lfa] 1651 Litkowski, S., Bashandy, A., Filsfils, C., Francois, P., 1652 Decraene, B., and D. Voyer, "Topology Independent Fast 1653 Reroute using Segment Routing", draft-ietf-rtgwg-segment- 1654 routing-ti-lfa-07 (work in progress), June 2021. 1656 [I-D.ietf-spring-sr-policy-yang] 1657 Raza, K., Sawaya, R., Shunwan, Z., Voyer, D., Durrani, M., 1658 Matsushima, S., and V. P. Beeram, "YANG Data Model for 1659 Segment Routing Policy", draft-ietf-spring-sr-policy- 1660 yang-01 (work in progress), April 2021. 1662 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 1663 dual environments", RFC 1195, DOI 10.17487/RFC1195, 1664 December 1990, . 1666 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 1667 DOI 10.17487/RFC2328, April 1998, 1668 . 1670 [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 1671 (TE) Extensions to OSPF Version 2", RFC 3630, 1672 DOI 10.17487/RFC3630, September 2003, 1673 . 1675 [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, 1676 "Multiprotocol Extensions for BGP-4", RFC 4760, 1677 DOI 10.17487/RFC4760, January 2007, 1678 . 1680 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 1681 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 1682 2008, . 1684 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 1685 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 1686 . 1688 [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The 1689 Locator/ID Separation Protocol (LISP)", RFC 6830, 1690 DOI 10.17487/RFC6830, January 2013, 1691 . 1693 [RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S. 1694 Previdi, "OSPF Traffic Engineering (TE) Metric 1695 Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015, 1696 . 1698 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 1699 S. Ray, "North-Bound Distribution of Link-State and 1700 Traffic Engineering (TE) Information Using BGP", RFC 7752, 1701 DOI 10.17487/RFC7752, March 2016, 1702 . 1704 [RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path 1705 Computation Element Communication Protocol (PCEP) 1706 Extensions for Stateful PCE", RFC 8231, 1707 DOI 10.17487/RFC8231, September 2017, 1708 . 1710 [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 1711 "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, 1712 DOI 10.17487/RFC8476, December 2018, 1713 . 1715 [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 1716 "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, 1717 DOI 10.17487/RFC8491, November 2018, 1718 . 1720 [RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward, 1721 D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE) 1722 Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March 1723 2019, . 1725 [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 1726 and J. Hardwick, "Path Computation Element Communication 1727 Protocol (PCEP) Extensions for Segment Routing", RFC 8664, 1728 DOI 10.17487/RFC8664, December 2019, 1729 . 1731 [RFC8814] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G., 1732 and N. Triantafillis, "Signaling Maximum SID Depth (MSD) 1733 Using the Border Gateway Protocol - Link State", RFC 8814, 1734 DOI 10.17487/RFC8814, August 2020, 1735 . 1737 [RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K., 1738 Ray, S., and J. Dong, "Border Gateway Protocol - Link 1739 State (BGP-LS) Extensions for Segment Routing BGP Egress 1740 Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August 1741 2021, . 1743 Authors' Addresses 1745 Clarence Filsfils 1746 Cisco Systems, Inc. 1747 Pegasus Parc 1748 De kleetlaan 6a, DIEGEM BRABANT 1831 1749 BELGIUM 1751 Email: cfilsfil@cisco.com 1753 Ketan Talaulikar (editor) 1754 Cisco Systems, Inc. 1755 India 1757 Email: ketant.ietf@gmail.com 1759 Daniel Voyer 1760 Bell Canada 1761 671 de la gauchetiere W 1762 Montreal, Quebec H3B 2M8 1763 Canada 1765 Email: daniel.voyer@bell.ca 1766 Alex Bogdanov 1767 British Telecom 1769 Email: alex.bogdanov@bt.com 1771 Paul Mattes 1772 Microsoft 1773 One Microsoft Way 1774 Redmond, WA 98052-6399 1775 USA 1777 Email: pamattes@microsoft.com