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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: November 1, 2021 D. Voyer 6 Bell Canada 7 A. Bogdanov 8 Google, Inc. 9 P. Mattes 10 Microsoft 11 April 30, 2021 13 Segment Routing Policy Architecture 14 draft-ietf-spring-segment-routing-policy-11 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 November 1, 2021. 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 . . . . . . . . . . . . . . . . . . 26 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 . . . . . . . . . . . . . . . . 30 105 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 106 12.1. Guidance for Designated Experts . . . . . . . . . . . . 31 107 13. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 32 108 14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 32 109 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 110 15.1. Normative References . . . . . . . . . . . . . . . . . . 33 111 15.2. Informative References . . . . . . . . . . . . . . . . . 34 112 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37 114 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 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 with a specific intent for traffic steering (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 (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 below: 279 o ASN : represented as a 4-byte number. If 2-byte ASNs are in use, 280 the low-order 16 bits MUST be used, and the high-order bits MUST 281 be set to zero. 283 o Node Address : represented as a 128-bit value. IPv4 addresses 284 MUST be encoded in the lowest 32 bits, and the high-order bits 285 MUST be set to zero. 287 Its application in the candidate path selection is described in 288 Section 2.9. 290 When provisioning is via configuration, the ASN and node address MAY 291 be set to either the headend or the provisioning controller/node ASN 292 and address. The default value is 0 for both AS and node address. 294 When signaling is via PCEP, it is the IPv4 or IPv6 address of the PCE 295 and the AS number SHOULD be set to 0 by default when not available or 296 known. 298 When signaling is via BGP SR Policy, the ASN and Node Address are 299 provided by BGP (refer to [I-D.ietf-idr-segment-routing-te-policy]) 300 on the headend. 302 2.5. Discriminator of a Candidate Path 304 The Discriminator is a 32-bit value associated with a candidate path 305 that uniquely identifies it within the context of an SR Policy from a 306 specific Protocol-Origin as specified below: 308 When provisioning is via configuration, this is an implementation's 309 configuration model-specific unique identifier for a candidate path. 310 The default value is 0. 312 When signaling is via PCEP, the method to uniquely signal an 313 individual candidate path along with its discriminator is described 314 in [I-D.ietf-pce-segment-routing-policy-cp]. The default value is 0. 316 When signaling is via BGP SR Policy, the BGP process receiving the 317 route provides the distinguisher (refer to Section 2.1 of 318 [I-D.ietf-idr-segment-routing-te-policy]) as the discriminator. 320 Its application in the candidate path selection is described in 321 Section 2.9. 323 2.6. Identification of a Candidate Path 325 A candidate path is identified in the context of a single SR Policy. 327 A candidate path is not shared across SR Policies. 329 A candidate path is not identified by its Segment-List(s). 331 If CP1 is a candidate path of SR Policy Pol1 and CP2 is a 332 candidate path of SR Policy Pol2, then these two candidate paths 333 are independent, even if they happen to have the same Segment- 334 List. The Segment-List does not identify a candidate path. The 335 Segment-List is an attribute of a candidate path. 337 The identity of a candidate path MUST be uniquely established in the 338 context of an SR Policy to handle add, 339 delete or modify operations on them in an unambiguous manner 340 regardless of their source(s). 342 The tuple uniquely 343 identifies a candidate path. 345 Candidate paths MAY also be assigned or signaled with a symbolic name 346 comprising printable ASCII characters to serve as a user-friendly 347 attribute for debugging and troubleshooting purposes. Such symbolic 348 names MUST NOT be considered as identifiers for a candidate path. 350 2.7. Preference of a Candidate Path 352 The preference of the candidate path is used to select the best 353 candidate path for an SR Policy. It is a 32-bit value and the 354 default preference is 100. 356 It is RECOMMENDED that each candidate path of a given SR policy has a 357 different preference. 359 2.8. Validity of a Candidate Path 361 A candidate path is usable when it is valid. A common path validity 362 criterion is the validity of its constituent Segment-Lists. The 363 validation rules are specified in Section 5. 365 2.9. Active Candidate Path 367 A candidate path is selected when it is valid and it is determined to 368 be the best path of the SR Policy. The selected path is referred to 369 as the "active path" of the SR policy in this document. 371 Whenever a new path is learned or an active path is deleted, the 372 validity of an existing path changes or an existing path is changed, 373 the selection process MUST be re-executed. 375 The candidate path selection process operates on the candidate path 376 Preference. A candidate path is selected when it is valid and it has 377 the highest preference value among all the candidate paths of the SR 378 Policy. 380 In the case of multiple valid candidate paths of the same preference, 381 the tie-breaking rules are evaluated on the identification tuple in 382 the following order until only one valid best path is selected: 384 1. Higher value of Protocol-Origin is selected. 386 2. If specified by configuration, prefer the existing installed 387 path. 389 3. Lower value of originator is selected. 391 4. Finally, the higher value of discriminator is selected. 393 The rules are framed with multiple protocols and sources in mind and 394 hence may not follow the logic of a single protocol (e.g. BGP best 395 path selection). The motivation behind these rules are as follows: 397 o The Protocol-Origin allows an operator to set up a default 398 selection mechanism across protocol sources, e.g., to prefer 399 configured over paths signaled via BGP SR Policy or PCEP. 401 o The preference, being the first tiebreaker, allows an operator to 402 influence selection across paths thus allowing provisioning of 403 multiple path options, e.g., CP1 is preferred and if it becomes 404 invalid then fallback to CP2 and so on. Since preference works 405 across protocol sources, it also enables (where necessary) 406 selective override of the default Protocol-Origin preference, 407 e.g., to prefer a path signaled via BGP SR Policy over what is 408 configured. 410 o The originator allows an operator to have multiple redundant 411 controllers and still maintain a deterministic behavior over which 412 of them are preferred even if they are providing the same 413 candidate paths for the same SR policies to the headend. 415 o The discriminator performs the final tiebreaking step to ensure a 416 deterministic outcome of selection regardless of the order in 417 which candidate paths are signaled across multiple transport 418 channels or sessions. 420 Section 4 of [I-D.filsfils-spring-sr-policy-considerations] provides 421 a set of examples to illustrate the active candidate path selection 422 rules. 424 2.10. Validity of an SR Policy 426 An SR Policy is valid when it has at least one valid candidate path. 428 2.11. Instantiation of an SR Policy in the Forwarding Plane 430 A valid SR Policy is instantiated in the forwarding plane. 432 Only the active candidate path SHOULD be used for forwarding traffic 433 that is being steered onto that policy. 435 If a set of Segment-Lists is associated with the active path of the 436 policy, then the steering is per-flow and weighted-ECMP (W-ECMP) 437 based according to the relative weight of each Segment-List. 439 The fraction of the flows associated with a given Segment-List is w/ 440 Sw, where w is the weight of the Segment-List and Sw is the sum of 441 the weights of the Segment-Lists of the selected path of the SR 442 Policy. 444 When a composite candidate path is active, the fraction of flows 445 steered into each constituent SR Policy is equal to the relative 446 weight of each constituent SR Policy. Further load balancing of 447 flows steered into a constituent SR Policy is performed based on the 448 weights of the Segment-List of the active candidate path of that 449 constituent SR Policy. 451 The accuracy of the weighted load-balancing depends on the platform 452 implementation. 454 2.12. Priority of an SR Policy 456 Upon topological change, many policies could be recomputed or 457 revalidated. An implementation MAY provide a per-policy priority 458 configuration. The operator MAY set this field to indicate the order 459 in which the policies should be re-computed. Such a priority is 460 represented by an integer in the range (0, 255) where the lowest 461 value is the highest priority. The default value of priority is 128. 463 An SR Policy may comprise multiple Candidate Paths received from the 464 same or different sources. A candidate path MAY be signaled with a 465 priority value. When an SR Policy has multiple candidate paths with 466 distinct signaled non-default priority values, the SR Policy as a 467 whole takes the lowest value (i.e. the highest priority) amongst 468 these signaled priority values. 470 2.13. Summary 472 In summary, the information model is the following: 474 SR policy POL1 475 Candidate-path CP1 477 Preference 200 478 Weight W1, SID-List1 479 Weight W2, SID-List2 480 Candidate-path CP2 482 Preference 100 483 Weight W3, SID-List3 484 Weight W4, SID-List4 486 The SR Policy POL1 is identified by the tuple . It has two candidate paths CP1 and CP2. Each is 488 identified by a tuple . 489 CP1 is the active candidate path (it is valid and has the highest 490 preference). The two Segment-Lists of CP1 are installed as the 491 forwarding instantiation of SR policy POL1. Traffic steered on POL1 492 is flow-based hashed on Segment-List with a ratio 493 W1/(W1+W2). 495 The information model of SR Policy POL100 having a composite 496 candidate path is the following: 498 SR policy POL100 499 Candidate-path CP1 501 Preference 200 502 Weight W1, SR policy 503 Weight W2, SR policy 505 The constituent SR Policies POL1 and POL2 have an information model 506 as described at the start of this section. They are referenced only 507 by color in the composite candidate path since their headend and 508 endpoint are identical to the POL100. The valid Segment-Lists of the 509 active candidate path of POL1 and POL2 are installed in the 510 forwarding. Traffic steered on POL100 is flow-based hashed on POL1 511 with a ratio W1/(W1+W2). Within the POL1, the flow-based hashing 512 over its Segment-Lists are performed as described earlier in this 513 section. 515 3. Segment Routing Database 517 An SR Policy computation node (e.g. headend or controller) maintains 518 the Segment Routing Database (SR-DB). The SR-DB is a conceptual 519 database to illustrate the various pieces of information and their 520 sources that may help in SR Policy computation and validation. There 521 is no specific requirement for an implementation to create a new 522 database as such. 524 An SR headend leverages the SR-DB to validate explicit candidate 525 paths and compute dynamic candidate paths. 527 The information in the SR-DB MAY include: 529 o IGP information (topology, IGP metrics based on ISIS [RFC1195] and 530 OSPF [RFC2328] [RFC5340]) 531 o Segment Routing information (such as Segment Routing Global Block, 532 Segment Routing Local Block, Prefix-SIDs, Adj-SIDs, BGP Peering 533 SID, SRv6 SIDs) [RFC8402] [RFC8986] 534 o TE Link Attributes (such as TE metric, Shared Risk Link Groups, 535 attribute-flag, extended admin group) [RFC5305] [RFC3630]. 536 o Extended TE Link attributes (such as latency, loss) [RFC8570] 537 [RFC7471] 538 o Inter-AS Topology information 539 [I-D.ietf-idr-bgpls-segment-routing-epe]. 541 The attached domain topology MAY be learned via IGP, BGP-LS or 542 NETCONF. 544 A non-attached (remote) domain topology MAY be learned via BGP-LS or 545 NETCONF. 547 In some use-cases, the SR-DB may only contain the attached domain 548 topology while in others, the SR-DB may contain the topology of 549 multiple domains and in this case, it is multi-domain capable. 551 The SR-DB MAY also contain the SR Policies instantiated in the 552 network. This can be collected via BGP-LS 553 [I-D.ietf-idr-te-lsp-distribution] or PCEP [RFC8231], 554 [I-D.ietf-pce-segment-routing-policy-cp], and 555 [I-D.ietf-pce-binding-label-sid]. This information allows to build 556 an end-to-end policy on the basis of intermediate SR policies (see 557 Section 6 for further details). 559 The SR-DB MAY also contain the Maximum SID Depth (MSD) capability of 560 nodes in the topology. This can be collected via ISIS [RFC8491], 561 OSPF [RFC8476], BGP-LS [RFC8814] or PCEP [RFC8664]. 563 The use of the SR-DB for computation and validation of SR Policies is 564 outside the scope of this document. Some implementation aspects 565 related to this are covered in 566 [I-D.filsfils-spring-sr-policy-considerations]. 568 4. Segment Types 570 A Segment-List is an ordered set of segments represented as where S1 is the first segment. 573 Based on the desired dataplane, either the MPLS label stack or the 574 SRv6 Segment Routing Header [RFC8754] is built from the Segment-List. 575 However, the Segment-List itself can be specified using different 576 segment-descriptor types and the following are currently defined: 578 Type A: SR-MPLS Label: 579 An MPLS label corresponding to any of the segment types defined 580 for SR-MPLS (as defined in [RFC8402] or other SR-MPLS 581 specifications) can be used. Additionally, reserved labels 582 like explicit-null or in general any MPLS label may also be 583 used. E.g. this type can be used to specify a label 584 representation that maps to an optical transport path on a 585 packet transport node. This type does not require the headend 586 to perform SID resolution. 588 Type B: SRv6 SID: 589 An IPv6 address corresponding to any of the SID behaviors for 590 SRv6 (as defined in [RFC8986] or other SRv6 specifications) can 591 be used. This type does not require the headend to perform SID 592 resolution. Optionally, the SRv6 SID behavior (as defined in 593 [RFC8986] or other SRv6 specifications) and structure (as 594 defined in [RFC8986]) MAY also be provided for the headend to 595 perform validation of the SID when using it for building the 596 Segment List. 598 Type C: IPv4 Prefix with optional SR Algorithm: 599 The headend is required to resolve the specified IPv4 Prefix 600 Address to the SR-MPLS label corresponding to a Prefix SID 601 segment (as defined in [RFC8402]). The SR algorithm (refer to 602 Section 3.1.1 of [RFC8402]) to be used MAY also be provided. 604 Type D: IPv6 Global Prefix with optional SR Algorithm for SR-MPLS: 605 In this case, the headend is required to resolve the specified 606 IPv6 Global Prefix Address to the SR-MPLS label corresponding 607 to its Prefix SID segment (as defined in [RFC8402]). The SR 608 Algorithm (refer to Section 3.1.1 of [RFC8402]) to be used MAY 609 also be provided. 611 Type E: IPv4 Prefix with Local Interface ID: 612 This type allows identification of Adjacency SID or BGP Peer 613 Adjacency SID (as defined in [RFC8402]) label for point-to- 614 point links including IP unnumbered links. The headend is 615 required to resolve the specified IPv4 Prefix Address to the 616 Node originating it and then use the Local Interface ID to 617 identify the point-to-point link whose adjacency is being 618 referred to. The Local Interface ID link descriptor follows 619 semantics as specified in [RFC7752]. This type can also be 620 used to indicate indirection into a layer 2 interface (i.e. 621 without IP address) like a representation of an optical 622 transport path or a layer 2 Ethernet port or circuit at the 623 specified node. 625 Type F: IPv4 Addresses for link endpoints as Local, Remote pair: 626 This type allows identification of Adjacency SID or BGP Peer 627 Adjacency SID (as defined in [RFC8402]) label for links. The 628 headend is required to resolve the specified IPv4 Local Address 629 to the Node originating it and then use the IPv4 Remote Address 630 to identify the link adjacency being referred to. The Local 631 and Remote Address pair link descriptors follow semantics as 632 specified in [RFC7752]. 634 Type G: IPv6 Prefix and Interface ID for link endpoints as Local, 635 Remote pair for SR-MPLS: 636 This type allows identification of Adjacency SID or BGP Peer 637 Adjacency SID (as defined in [RFC8402]) label for links 638 including those with only Link-Local IPv6 addresses. The 639 headend is required to resolve the specified IPv6 Prefix 640 Address to the Node originating it and then use the Local 641 Interface ID to identify the point-to-point link whose 642 adjacency is being referred to. For other than point-to-point 643 links, additionally the specific adjacency over the link needs 644 to be resolved using the Remote Prefix and Interface ID. The 645 Local and Remote pair of Prefix and Interface ID link 646 descriptor follows semantics as specified in [RFC7752]. This 647 type can also be used to indicate indirection into a layer 2 648 interface (i.e. without IP address) like a representation of an 649 optical transport path or a layer 2 Ethernet port or circuit at 650 the specified node. 652 Type H: IPv6 Addresses for link endpoints as Local, Remote pair for 653 SR-MPLS: 654 This type allows identification of Adjacency SID or BGP Peer 655 Adjacency SID (as defined in [RFC8402]) label for links with 656 Global IPv6 addresses. The headend is required to resolve the 657 specified Local IPv6 Address to the Node originating it and 658 then use the Remote IPv6 Address to identify the link adjacency 659 being referred to. The Local and Remote Address pair link 660 descriptors follow semantics as specified in [RFC7752]. 662 Type I: IPv6 Global Prefix with optional SR Algorithm for SRv6: 663 The headend is required to resolve the specified IPv6 Global 664 Prefix Address to an SRv6 SID corresponding to a Prefix SID 665 segment (as defined in [RFC8402]), such as a SID associated 666 with the End behavior (as defined in [RFC8986]), of the node 667 which is originating the prefix. The SR Algorithm (refer to 668 Section 3.1.1 of [RFC8402]), the SRv6 SID behavior (as defined 669 in [RFC8986] or other SRv6 specifications) and structure (as 670 defined in [RFC8986]) MAY also be provided. 672 Type J: IPv6 Prefix and Interface ID for link endpoints as Local, 673 Remote pair for SRv6: 674 This type allows identification of an SRv6 SID corresponding to 675 an Adjacency SID or BGP Peer Adjacency SID (as defined in 676 [RFC8402]), such as a SID associated with the End.X behavior 677 (as defined in [RFC8986]), associated with link or adjacency 678 with only Link-Local IPv6 addresses. The headend is required 679 to resolve the specified IPv6 Prefix Address to the Node 680 originating it and then use the Local Interface ID to identify 681 the point-to-point link whose adjacency is being referred to. 682 For other than point-to-point links, additionally the specific 683 adjacency needs to be resolved using the Remote Prefix and 684 Interface ID. The Local and Remote pair of Prefix and 685 Interface ID link descriptor follows semantics as specified in 686 [RFC7752]. The SR Algorithm (refer to Section 3.1.1 of 687 [RFC8402]), the SRv6 SID behavior (as defined in [RFC8986] or 688 other SRv6 specifications) and structure (as defined in 689 [RFC8986]) MAY also be provided. 691 Type K: IPv6 Addresses for link endpoints as Local, Remote pair for 692 SRv6: 693 This type allows identification of an SRv6 SID corresponding to 694 an Adjacency SID or BGP Peer Adjacency SID (as defined in 695 [RFC8402]), such as a SID associated with the End.X behavior 696 (as defined in [RFC8986]), associated with link or adjacency 697 with Global IPv6 addresses. The headend is required to resolve 698 the specified Local IPv6 Address to the Node originating it and 699 then use the Remote IPv6 Address to identify the link adjacency 700 being referred to. The Local and Remote Address pair link 701 descriptors follow semantics as specified in [RFC7752]. The SR 702 Algorithm (refer to Section 3.1.1 of [RFC8402]), the SRv6 SID 703 behavior (as defined in [RFC8986] or other SRv6 specifications) 704 and structure (as defined in [RFC8986]) MAY also be provided. 706 When the algorithm is not specified for the SID types above which 707 optionally allow for it, the headend SHOULD use the Strict Shortest 708 Path algorithm if available; otherwise, it SHOULD use the default 709 Shortest Path algorithm. The specification of the algorithm enables 710 the use of the IGP Flex Algorithm [I-D.ietf-lsr-flex-algo] specific 711 SIDs in SR Policy. 713 For SID types C-through-K, a SID value may also be optionally 714 provided to the headend for verification purposes. Section 5.1. 715 describes the resolution and verification of the SIDs and Segment 716 Lists on the headend. 718 When building the MPLS label stack or the IPv6 Segment list from the 719 Segment List, the node instantiating the policy MUST interpret the 720 set of Segments as follows: 722 o The first Segment represents the topmost label or the first IPv6 723 segment. It identifies the active segment the traffic will be 724 directed toward along the explicit SR path. 725 o The last Segment represents the bottommost label or the last IPv6 726 segment the traffic will be directed toward along the explicit SR 727 path. 729 4.1. Explicit Null 731 A Type A SID may be any MPLS label, including reserved labels. 733 For example, assuming that the desired traffic-engineered path from a 734 headend 1 to an endpoint 4 can be expressed by the Segment-List 735 <16002, 16003, 16004> where 16002, 16003 and 16004 respectively refer 736 to the IPv4 Prefix SIDs bound to node 2, 3, and 4, then IPv6 traffic 737 can be traffic-engineered from nodes 1 to 4 via the previously 738 described path using an SR Policy with Segment-List <16002, 16003, 739 16004, 2> where the MPLS label value of 2 represents the "IPv6 740 Explicit NULL Label". 742 The penultimate node before node 4 will pop 16004 and will forward 743 the frame on its directly connected interface to node 4. 745 The endpoint receives the traffic with the top label "2" which 746 indicates that the payload is an IPv6 packet. 748 When steering unlabeled IPv6 BGP destination traffic using an SR 749 policy composed of Segment-List(s) based on IPv4 SIDs, the Explicit 750 Null Label Policy is processed as specified in 751 [I-D.ietf-idr-segment-routing-te-policy]) Section 2.4.4. When an 752 "IPv6 Explicit NULL label" is not present as the bottom label, the 753 headend SHOULD automatically impose one. Refer to Section 8 for more 754 details. 756 5. Validity of a Candidate Path 758 5.1. Explicit Candidate Path 760 An explicit candidate path is associated with a Segment-List or a set 761 of Segment-Lists. 763 An explicit candidate path is provisioned by the operator directly or 764 via a controller. 766 The computation/logic that leads to the choice of the Segment-List is 767 external to the SR Policy headend. The SR Policy headend does not 768 compute the Segment-List. The SR Policy headend only confirms its 769 validity. 771 An explicit candidate path MAY consist of a single explicit Segment- 772 List containing only an implicit-null label to indicate pop-and- 773 forward behavior. The Binding SID (BSID) is popped and the traffic 774 is forwarded based on the inner label or an IP lookup in the case of 775 unlabeled IP packets. Such an explicit path can serve as a fallback 776 or path of last resort for traffic being steered into an SR Policy 777 using its BSID (refer to Section 8.3). 779 A Segment-List of an explicit candidate path MUST be declared invalid 780 when: 782 o It is empty. 783 o Its weight is 0. 784 o It comprises of a mix of SR-MPLS and SRv6 segment types. 785 o The headend is unable to perform path resolution for the first SID 786 into one or more outgoing interface(s) and next-hop(s). 787 o The headend is unable to perform SID resolution for any non-first 788 SID of type C-through-K into an MPLS label or an SRv6 SID. 789 o The headend verification fails for any SID for which verification 790 has been explicitly requested. 792 "Unable to perform path resolution" means that the headend has no 793 path to the SID in its SR database. 795 SID verification is performed when the headend is explicitly 796 requested to verify SID(s) by the controller via the signaling 797 protocol used. Implementations MAY provide a local configuration 798 option to enable verification on a global or per policy or per 799 candidate path basis. 801 "Verification fails" for a SID means any of the following: 803 o The headend is unable to find the SID in its SR-DB 804 o The headend detects a mis-match between the SID value and its 805 context provided for SIDs of type C-through-L in its SR-DB. 806 o The headend is unable to perform SID resolution for any non-first 807 SID of type C-through-K into an MPLS label or an SRv6 SID. 809 In multi-domain deployments, it is expected that the headend be 810 unable to verify the reachability of the SIDs in remote domains. 811 Types A or B MUST be used for the SIDs for which the reachability 812 cannot be verified. Note that the first SID MUST always be reachable 813 regardless of its type. 815 Additionally, a Segment-List MAY be declared invalid when: 817 o Its last segment is not a Prefix SID (including BGP Peer Node-SID) 818 advertised by the node specified as the endpoint of the 819 corresponding SR policy. 820 o Its last segment is not an Adjacency SID (including BGP Peer 821 Adjacency SID) of any of the links present on neighbor nodes and 822 that terminate on the node specified as the endpoint of the 823 corresponding SR policy. 825 An explicit candidate path is invalid as soon as it has no valid 826 Segment-List. 828 Additionally, an explicit candidate path MAY be declared invalid when 829 its constituent segment lists (valid or invalid) are using segment 830 types of different SR dataplanes. 832 5.2. Dynamic Candidate Path 834 A dynamic candidate path is specified as an optimization objective 835 and constraints. 837 The headend of the policy leverages its SR database to compute a 838 Segment-List ("solution Segment-List") that solves this optimization 839 problem for either the SR-MPLS or the SRv6 data-plane as specfied. 841 The headend re-computes the solution Segment-List any time the inputs 842 to the problem change (e.g., topology changes). 844 When the local computation is not possible (e.g., a policy's tail-end 845 is outside the topology known to the headend) or not desired, the 846 headend MAY send path computation request to a PCE supporting PCEP 847 extension specified in [RFC8664]. 849 If no solution is found to the optimization objective and 850 constraints, then the dynamic candidate path MUST be declared 851 invalid. 853 Section 3 of [I-D.filsfils-spring-sr-policy-considerations] discusses 854 some of the optimization objectives and constraints that may be 855 considered by a dynamic candidate path. It illustrates some of the 856 desirable properties of the computation of the solution Segment-List. 858 5.3. Composite Candidate Path 860 A composite candidate path is specified as a group of its constituent 861 SR Policies. 863 A composite candidate path is valid when it has at least one valid 864 constituent SR Policy. 866 6. Binding SID 868 The Binding SID (BSID) is fundamental to Segment Routing [RFC8402]. 869 It provides scaling, network opacity, and service independence. 870 Section 6 of [I-D.filsfils-spring-sr-policy-considerations] 871 illustrates some of these benefits. This section describes the 872 association of BSID with an SR Policy. 874 6.1. BSID of a candidate path 876 Each candidate path MAY be defined with a BSID. 878 Candidate Paths of the same SR policy SHOULD have the same BSID. 880 Candidate Paths of different SR policies MUST NOT have the same BSID. 882 6.2. BSID of an SR Policy 884 The BSID of an SR Policy is the BSID of its active candidate path. 886 When the active candidate path has a specified BSID, the SR Policy 887 uses that BSID if this value (label in MPLS, IPv6 address in SRv6) is 888 available (i.e., not associated with any other usage: e.g. to another 889 MPLS client, to another SRv6 client, to another SID, to another SR 890 Policy, outside the range of SRv6 Locators). 892 In the case of SR-MPLS, SRv6 BSIDs (e.g. with the behavior End.BM 893 [RFC8986]) MAY be associated with the SR Policy in addition to the 894 MPLS BSID. In the case of SRv6, multiple SRv6 BSIDs (e.g. with 895 different behaviors like End.B6.Encap and End.B6.Encap.Red [RFC8986]) 896 MAY be associated with the SR Policy. 898 Optionally, instead of only checking that the BSID of the active path 899 is available, a headend MAY check that it is available within a given 900 SID range i.e., Segment Routing Local Block (SRLB) as specified in 901 [RFC8402]. 903 When the specified BSID is not available (optionally is not in the 904 SRLB), an alert message MUST be generated. 906 In the cases (as described above) where SR Policy does not have a 907 BSID available, then the SR Policy MAY dynamically bind a BSID to 908 itself. Dynamically bound BSID SHOULD use an available SID outside 909 the SRLB. 911 Assuming that at time t the BSID of the SR Policy is B1, if at time 912 t+dt a different candidate path becomes active and this new active 913 path does not have a specified BSID or its BSID is specified but is 914 not available (e.g. it is in use by something else), then the SR 915 Policy keeps the previous BSID B1. 917 The association of an SR Policy with a BSID thus MAY change over the 918 life of the SR Policy (e.g., upon active path change). Hence, the 919 BSID SHOULD NOT be used as an identification of an SR Policy. 921 6.2.1. Frequent use-case : unspecified BSID 923 All the candidate paths of the same SR Policy can have an unspecified 924 BSID. 926 In such a case, a BSID MAY be dynamically bound to the SR Policy as 927 soon as the first valid candidate path is received. That BSID is 928 kept through the life of the SR Policy and across changes of active 929 candidate path. 931 6.2.2. Frequent use-case: all specified to the same BSID 933 All the paths of the SR Policy can have the same specified BSID. 935 6.2.3. Specified-BSID-only 937 An implementation MAY support the configuration of the Specified- 938 BSID-only restrictive behavior on the headend for all SR Policies or 939 individual SR Policies. Further, this restrictive behavior MAY also 940 be signaled on a per SR Policy basis to the headend. 942 When this restrictive behavior is enabled, if the candidate path has 943 an unspecified BSID or if the specified BSID is not available when 944 the candidate path becomes active then no BSID is bound to it and it 945 is considered invalid. An alert MUST be triggered for this error. 947 Other candidate paths MUST then be evaluated for becoming the active 948 candidate path. 950 6.3. Forwarding Plane 952 A valid SR Policy installs a BSID-keyed entry in the forwarding plane 953 with the action of steering the packets matching this entry to the 954 selected path of the SR Policy. 956 If the Specified-BSID-only restrictive behavior is enabled and the 957 BSID of the active path is not available (optionally not in the 958 SRLB), then the SR Policy does not install any entry indexed by a 959 BSID in the forwarding plane. 961 6.4. Non-SR usage of Binding SID 963 An implementation MAY choose to associate a Binding SID with any type 964 of interface (e.g. a layer 3 termination of an Optical Circuit) or a 965 tunnel (e.g. IP tunnel, GRE tunnel, IP/UDP tunnel, MPLS RSVP-TE 966 tunnel, etc). This enables the use of other non-SR enabled 967 interfaces and tunnels as segments in an SR Policy Segment-List 968 without the need of forming routing protocol adjacencies over them. 970 The details of this kind of usage are beyond the scope of this 971 document. A specific packet-optical integration use case is 972 described in [I-D.anand-spring-poi-sr]. 974 7. SR Policy State 976 The SR Policy State is maintained on the headend to represent the 977 state of the policy and its candidate paths. This is to provide an 978 accurate representation of whether the SR Policy is being 979 instantiated in the forwarding plane and which of its candidate paths 980 and segment-list(s) are active. The SR Policy state MUST also 981 reflect the reason when a policy and/or its candidate path is not 982 active due to validation errors or not being preferred. 984 The SR Policy state can be reported by the headend node via BGP-LS 985 [I-D.ietf-idr-te-lsp-distribution] or PCEP [RFC8231] and 986 [I-D.ietf-pce-binding-label-sid]. 988 SR Policy state on the headend also includes traffic accounting 989 information for the flows being steered via the policies. The 990 details of the SR Policy accounting are beyond the scope of this 991 document. The aspects related to the SR traffic counters and their 992 usage in the broader context of traffic accounting in an SR network 993 are covered in [I-D.filsfils-spring-sr-traffic-counters] and 994 [I-D.ali-spring-sr-traffic-accounting] respectively. 996 Implementations MAY support an administrative state to control 997 locally provisioned policies via mechanisms like CLI or NETCONF. 999 8. Steering into an SR Policy 1001 A headend can steer a packet flow into a valid SR Policy in various 1002 ways: 1004 o Incoming packets have an active SID matching a local BSID at the 1005 headend. 1006 o Per-destination Steering: incoming packets match a BGP/Service 1007 route which recurses on an SR policy. 1008 o Per-flow Steering: incoming packets match or recurse on a 1009 forwarding array of where some of the entries are SR Policies. 1010 o Policy-based Steering: incoming packets match a routing policy 1011 that directs them on an SR policy. 1013 8.1. Validity of an SR Policy 1015 An SR Policy is invalid when all its candidate paths are invalid as 1016 described in Section 5 and Section 2.10. 1018 By default, upon transitioning to the invalid state, 1020 o an SR Policy and its BSID are removed from the forwarding plane. 1021 o any steering of a service (Pseudowire (PW)), destination (BGP- 1022 VPN), flow or packet on the related SR policy is disabled and the 1023 related service, destination, flow, or packet is routed per the 1024 classic forwarding table (e.g. longest-match to the destination or 1025 the recursing next-hop). 1027 8.2. Drop upon invalid SR Policy 1029 An SR Policy MAY be enabled for the Drop-Upon-Invalid behavior: 1031 o an invalid SR Policy and its BSID is kept in the forwarding plane 1032 with an action to drop. 1033 o any steering of a service (PW), destination (BGP-VPN), flow or 1034 packet on the related SR policy is maintained with the action to 1035 drop all of this traffic. 1037 The drop-upon-invalid behavior has been deployed in use-cases where 1038 the operator wants some PW to only be transported on a path with 1039 specific constraints. When these constraints are no longer met, the 1040 operator wants the PW traffic to be dropped. Specifically, the 1041 operator does not want the PW to be routed according to the IGP 1042 shortest path to the PW endpoint. 1044 8.3. Incoming Active SID is a BSID 1046 Let us assume that headend H has a valid SR Policy P of Segment-List 1047 and BSID B. 1049 When H receives a packet K with label stack , H pops B and 1050 pushes and forwards the resulting packet according to 1051 SID S1. 1053 "Forwarding the resulting packet according to S1" means: If S1 is 1054 an Adj SID or a PHP-enabled prefix SID advertised by a neighbor, H 1055 sends the resulting packet with label stack on 1056 the outgoing interface associated with S1; Else H sends the 1057 resulting packet with label stack along the 1058 path of S1. 1060 H has steered the packet into the SR policy P. 1062 H did not have to classify the packet. The classification was done 1063 by a node upstream of H (e.g., the source of the packet or an 1064 intermediate ingress edge node of the SR domain) and the result of 1065 this classification was efficiently encoded in the packet header as a 1066 BSID. 1068 This is another key benefit of the segment routing in general and the 1069 binding SID in particular: the ability to encode a classification and 1070 the resulting steering in the packet header to better scale and 1071 simplify intermediate aggregation nodes. 1073 If the SR Policy P is invalid, the BSID B is not in the forwarding 1074 plane and hence the packet K is dropped by H. 1076 8.4. Per-Destination Steering 1078 Let us assume that headend H: 1080 o learns a BGP route R/r via next-hop N, extended-color community C 1081 and VPN label V. 1082 o has a valid SR Policy P to (color = C, endpoint = N) of Segment- 1083 List and BSID B. 1084 o has a BGP policy that matches on the extended-color community C 1085 and allows its usage as SLA steering information. 1087 If all these conditions are met, H installs R/r in RIB/FIB with next- 1088 hop = SR Policy P of BSID B instead of via N. 1090 Indeed, H's local BGP policy and the received BGP route indicate that 1091 the headend should associate R/r with an SR Policy path to endpoint N 1092 with the SLA associated with color C. The headend, therefore, 1093 installs the BGP route on that policy. 1095 This can be implemented by using the BSID as a generalized next-hop 1096 and installing the BGP route on that generalized next-hop. 1098 When H receives a packet K with a destination matching R/r, H pushes 1099 the label stack and sends the resulting packet along 1100 the path to S1. 1102 Note that any SID associated with the BGP route is inserted after the 1103 Segment-List of the SR Policy (i.e., ). 1105 The same behavior applies to any type of service route: any AFI/SAFI 1106 of BGP [RFC4760] any AFI/SAFI of LISP [RFC6830]. 1108 In a BGP multi-path scenario, the BGP route may be resolved over a 1109 mix of paths that include those that are steered over SR Policies and 1110 others resolved via the normal BGP nexthop resolution. 1111 Implementations MAY provide options to prefer one type over the other 1112 or other forms of local policy to determine the paths that are 1113 selected. 1115 8.4.1. Multiple Colors 1117 When a BGP route has multiple extended-color communities each with a 1118 valid SR Policy, the BGP process installs the route on the SR Policy 1119 giving preference to the color with the highest numerical value. 1121 Let us assume that headend H: 1123 o learns a BGP route R/r via next-hop N, extended-color communities 1124 C1 and C2 and VPN label V. 1125 o has a valid SR Policy P1 to (color = C1, endpoint = N) of Segment- 1126 List and BSID B1. 1127 o has a valid SR Policy P2 to (color = C2, endpoint = N) of Segment- 1128 List and BSID B2. 1129 o has a BGP policy that matches the extended-color communities C1 1130 and C2 and allows their usage as SLA steering information 1132 If all these conditions are met, H installs R/r in RIB/FIB with next- 1133 hop = SR Policy P2 of BSID=B2 (instead of N) because C2 > C1. 1135 When the SR Policy with a specific color is not instantiated or in 1136 the down/inactive state, the SR Policy with the next highest 1137 numerical value of color is considered. 1139 8.5. Recursion on an on-demand dynamic BSID 1141 In the previous section, it was assumed that H had a pre-established 1142 "explicit" SR Policy (color C, endpoint N). 1144 In this section, independent of the a-priori existence of any 1145 explicit candidate path of the SR policy (C, N), it is to be noted 1146 that the BGP process at headend node H triggers the instantiation of 1147 a dynamic candidate path for the SR policy (C, N) as soon as: 1149 o the BGP process learns of a route R/r via N and with color C. 1150 o a local policy at node H authorizes the on-demand SR Policy path 1151 instantiation and maps the color to a dynamic SR Policy path 1152 optimization template. 1154 8.5.1. Multiple Colors 1156 When a BGP route R/r via N has multiple extended-color communities Ci 1157 (with i=1 ... n), an individual on-demand SR Policy dynamic path 1158 request (color Ci, endpoint N) is triggered for each color Ci. The 1159 SR Policy that is used for steering is then determined as described 1160 in Section 8.4.1. 1162 8.6. Per-Flow Steering 1164 Let us assume that headend H: 1166 o has a valid SR Policy P1 to (color = C1, endpoint = N) of Segment- 1167 List and BSID B1. 1168 o has a valid SR Policy P2 to (color = C2, endpoint = N) of Segment- 1169 List and BSID B2. 1170 o is configured to instantiate an array of paths to N where the 1171 entry 0 is the IGP path to N, color C1 is the first entry and 1172 Color C2 is the second entry. The index into the array is called 1173 a Forwarding Class (FC). The index can have values 0 to 7. 1174 o is configured to match flows in its ingress interfaces (upon any 1175 field such as Ethernet destination/source/VLAN/TOS or IP 1176 destination/source/DSCP or transport ports etc.) and color them 1177 with an internal per-packet forwarding-class variable (0, 1 or 2 1178 in this example). 1180 If all these conditions are met, H installs in RIB/FIB: 1182 o N via recursion on an array A (instead of the immediate outgoing 1183 link associated with the IGP shortest-path to N). 1184 o Entry A(0) set to the immediate outgoing link of the IGP shortest- 1185 path to N. 1186 o Entry A(1) set to SR Policy P1 of BSID=B1. 1188 o Entry A(2) set to SR Policy P2 of BSID=B2. 1190 H receives three packets K, K1, and K2 on its incoming interface. 1191 These three packets either longest-match on N or more likely on a 1192 BGP/service route which recurses on N. H colors these 3 packets 1193 respectively with forwarding-class 0, 1, and 2. As a result: 1195 o H forwards K along the shortest path to N (which in SR-MPLS 1196 results in the pushing of the prefix-SID of N). 1197 o H pushes on packet K1 and forwards the resulting 1198 frame along the shortest path to S1. 1199 o H pushes on packet K2 and forwards the resulting 1200 frame along the shortest path to S4. 1202 If the local configuration does not specify any explicit forwarding 1203 information for an entry of the array, then this entry is filled with 1204 the same information as entry 0 (i.e. the IGP shortest path). 1206 If the SR Policy mapped to an entry of the array becomes invalid, 1207 then this entry is filled with the same information as entry 0. When 1208 all the array entries have the same information as entry0, the 1209 forwarding entry for N is updated to bypass the array and point 1210 directly to its outgoing interface and next-hop. 1212 The array index values (e.g. 0, 1, and 2) and the notion of 1213 forwarding-class are implementation-specific and only meant to 1214 describe the desired behavior. The same can be realized by other 1215 mechanisms. 1217 This realizes per-flow steering: different flows bound to the same 1218 BGP endpoint are steered on different IGP or SR Policy paths. 1220 A headend MAY support options to apply per-flow steering only for 1221 traffic matching specific prefixes (e.g. specific IGP or BGP 1222 prefixes). 1224 8.7. Policy-based Routing 1226 Finally, headend H may be configured with a local routing policy 1227 which overrides any BGP/IGP path and steer a specified packet on an 1228 SR Policy. This includes the use of mechanisms like IGP Shortcut for 1229 automatic routing of IGP prefixes over SR Policies intended for such 1230 purpose. 1232 8.8. Optional Steering Modes for BGP Destinations 1234 8.8.1. Color-Only BGP Destination Steering 1236 In the previous section, it is seen that the steering on an SR Policy 1237 is governed by the matching of the BGP route's next-hop N and the 1238 authorized color C with an SR Policy defined by the tuple (N, C). 1240 This is the most likely form of BGP destination steering and the one 1241 recommended for most use-cases. 1243 This section defines an alternative steering mechanism based only on 1244 the color. 1246 This color-only steering variation is governed by two new "CO" bits 1247 defined in the color extended community in section 3 of 1248 [I-D.ietf-idr-segment-routing-te-policy]. 1250 The Color-Only flags "CO" are set to 00 by default. 1252 When 00, the BGP destination is steered as follows: 1254 IF there is a valid SR Policy (N, C) where N is the IPv4 or IPv6 1256 endpoint address and C is a color; 1257 Steer into SR Policy (N, C); 1258 ELSE; 1259 Steer on the IGP path to the next-hop N. 1261 This is the classic case described in this document previously and 1262 what is recommended in most scenarios. 1264 When 01, the BGP destination is steered as follows: 1266 IF there is a valid SR Policy (N, C) where N is the IPv4 or IPv6 1268 endpoint address and C is a color; 1269 Steer into SR Policy (N, C); 1270 ELSE IF there is a valid SR Policy (null endpoint, C) of the 1271 same address-family of N; 1272 Steer into SR Policy (null endpoint, C); 1273 ELSE IF there is any valid SR Policy 1274 (any address-family null endpoint, C); 1275 Steer into SR Policy (any null endpoint, C); 1276 ELSE; 1277 Steer on the IGP path to the next-hop N. 1279 When 10, the BGP destination is steered as follows: 1281 IF there is a valid SR Policy (N, C) where N is an 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) 1285 of the same address-family of N; 1286 Steer into SR Policy (null endpoint, C); 1287 ELSE IF there is any valid SR Policy 1288 (any address-family null endpoint, C); 1289 Steer into SR Policy (any null endpoint, C); 1290 ELSE IF there is any valid SR Policy (any endpoint, C) 1291 of the same address-family of N; 1292 Steer into SR Policy (any endpoint, C); 1293 ELSE IF there is any valid SR Policy 1294 (any address-family endpoint, C); 1295 Steer into SR Policy (any address-family endpoint, C); 1296 ELSE; 1297 Steer on the IGP path to the next-hop N. 1299 The null endpoint is 0.0.0.0 for IPv4 and ::0 for IPv6 (all bits set 1300 to the 0 value). 1302 The value 11 is reserved for future use and SHOULD NOT be used. Upon 1303 reception, an implementation MUST treat it like 00. 1305 8.8.2. Multiple Colors and CO flags 1307 The steering preference is first based on the highest color value and 1308 then CO-dependent for the color. Assuming a Prefix via (NH, 1309 C1(CO=01), C2(CO=01)); C1>C2 The steering preference order is: 1311 o SR policy (NH, C1). 1312 o SR policy (null, C1). 1313 o SR policy (NH, C2). 1314 o SR policy (null, C2). 1315 o IGP to NH. 1317 8.8.3. Drop upon Invalid 1319 This document defined earlier that when all the following conditions 1320 are met, H installs R/r in RIB/FIB with next-hop = SR Policy P of 1321 BSID B instead of via N. 1323 o H learns a BGP route R/r via next-hop N, extended-color community 1324 C and VPN label V. 1325 o H has a valid SR Policy P to (color = C, endpoint = N) of Segment- 1326 List and BSID B. 1327 o H has a BGP policy that matches the extended-color community C and 1328 allows its usage as SLA steering information. 1330 This behavior is extended by noting that the BGP policy may require 1331 the BGP steering to always stay on the SR policy whatever its 1332 validity. 1334 This is the "drop upon invalid" option described in Section 8.2 1335 applied to BGP-based steering. 1337 9. Protection 1339 9.1. Leveraging TI-LFA local protection of the constituent IGP segments 1341 In any topology, Topology-Independent Loop-Free Alternate (TI-LFA) 1342 [I-D.ietf-rtgwg-segment-routing-ti-lfa] provides a 50msec local 1343 protection technique for IGP SIDs. The backup path is computed on a 1344 per IGP SID basis along the post-convergence path. 1346 In a network that has deployed TI-LFA, an SR Policy built on the 1347 basis of TI-LFA protected IGP segments leverages the local protection 1348 of the constituent segments. Since TI-LFA protection is based on IGP 1349 computation, there are cases where the path used during the fast- 1350 reroute time window may not meet the exact constraints of the SR 1351 Policy. 1353 In a network that has deployed TI-LFA, an SR Policy instantiated only 1354 with non-protected Adj SIDs does not benefit from any local 1355 protection. 1357 9.2. Using an SR Policy to locally protect a link 1359 1----2-----6----7 1360 | | | | 1361 4----3-----9----8 1363 Figure 1: Local protection using SR Policy 1365 An SR Policy can be instantiated at node 2 to protect the link 2to6. 1366 A typical explicit Segment-List would be <3, 9, 6>. 1368 A typical use-case occurs for links outside an IGP domain: e.g. 1, 2, 1369 3, and 4 are part of IGP/SR sub-domain 1 while 6, 7, 8, and 9 are 1370 part of IGP/SR sub-domain 2. In such a case, links 2to6 and 3to9 1371 cannot benefit from TI-LFA automated local protection. The SR Policy 1372 with Segment-List <3, 9, 6> on node 2 can be locally configured to be 1373 a fast-reroute backup path for the link 2to6. 1375 9.3. Using a Candidate Path for Path Protection 1377 An SR Policy allows for multiple candidate paths, of which at any 1378 point in time there is a single active candidate path that is 1379 provisioned in the forwarding plane and used for traffic steering. 1380 However, another (lower preference) candidate path MAY be designated 1381 as the backup for a specific or all (active) candidate path(s). The 1382 following options are possible: 1384 o A pair of disjoint candidate paths are provisioned with one of 1385 them as primary and the other is identified as its backup. 1386 o A specific candidate path is provisioned as the backup for any 1387 (active) candidate path. 1388 o The headend picks the next (lower) preference valid candidate path 1389 as the backup for the active candidate path. 1391 The headend MAY compute a-priori and validate such backup candidate 1392 paths as well as provision them into the forwarding plane as a backup 1393 for the active path. The backup candidate path may be dynamically 1394 computed or explicitly provisioned in such a way that they provide 1395 the most appropriate alternative for the active candidate path. A 1396 fast re-route mechanism MAY then be used to trigger sub 50msec 1397 switchover from the active to the backup candidate path in the 1398 forwarding plane. Mechanisms like Bidirectional Forwarding Detection 1399 (BFD) MAY be used for fast detection of such failures. 1401 10. Security Considerations 1403 This document specifies in detail the SR Policy construct introduced 1404 in [RFC8402] and its instantiation on a router supporting SR along 1405 with descriptions of mechanisms for steering of traffic flows over 1406 it. Therefore, the security considerations of [RFC8402] apply. This 1407 document does not define any new protocol extensions and does not 1408 introduce any further security considerations. 1410 11. Manageability Considerations 1412 This document specifies in detail the SR Policy construct introduced 1413 in [RFC8402] and its instantiation on a router supporting SR along 1414 with descriptions of mechanisms for steering of traffic flows over 1415 it. Therefore, the manageability considerations of [RFC8402] apply. 1417 A YANG model for the configuration and operation of SR Policy has 1418 been defined in [I-D.ietf-spring-sr-policy-yang]. 1420 12. IANA Considerations 1422 The document requests IANA to create a new sub-registry called 1423 "Segment Types" under the top-level "Segment Routing" registry. This 1424 sub-registry maintains the alphabetic identifiers for the segment 1425 types (as specified in section 4) that may be used within a Segment 1426 List of an SR Policy. The alphabetical identifiers run from A to Z 1427 and may be extended on exhaustion with the identifiers AA to AZ, BA 1428 to BZ and so on through till ZZ. This sub-registry would follow the 1429 Specification Required allocation policy as specified in [RFC8126]. 1431 The initial registrations for this sub-registry are as follows: 1433 +-------+-----------------------------------------------+-----------+ 1434 | Value | Description | Reference | 1435 +-------+-----------------------------------------------+-----------+ 1436 | A | SR-MPLS Label | [This.ID] | 1437 | B | SRv6 SID | [This.ID] | 1438 | C | IPv4 Prefix with optional SR Algorithm | [This.ID] | 1439 | D | IPv6 Global Prefix with optional SR Algorithm | [This.ID] | 1440 | | for SR-MPLS | | 1441 | E | IPv4 Prefix with Local Interface ID | [This.ID] | 1442 | F | IPv4 Addresses for link endpoints as Local, | [This.ID] | 1443 | | Remote pair | | 1444 | G | IPv6 Prefix and Interface ID for link | [This.ID] | 1445 | | endpoints as Local, Remote pair for SR-MPLS | | 1446 | H | IPv6 Addresses for link endpoints as Local, | [This.ID] | 1447 | | Remote pair for SR-MPLS | | 1448 | I | IPv6 Global Prefix with optional SR Algorithm | [This.ID] | 1449 | | for SRv6 | | 1450 | J | IPv6 Prefix and Interface ID for link | [This.ID] | 1451 | | endpoints as Local, Remote pair for SRv6 | | 1452 | K | IPv6 Addresses for link endpoints as Local, | [This.ID] | 1453 | | Remote pair for SRv6 | | 1454 +-------+-----------------------------------------------+-----------+ 1456 Table 2: Initial IANA Registration 1458 12.1. Guidance for Designated Experts 1460 The Designated Expert (DE) is expected to ascertain the existence of 1461 suitable documentation (a specification) as described in [RFC8126] 1462 and to verify that the document is permanently and publicly 1463 available. The DE is also expected to check the clarity of purpose 1464 and use of the requested assignment. Additionally, the DE must 1465 verify that any request for one of these assignments has been made 1466 available for review and comment within the IETF: the DE will post 1467 the request to the SPRING Working Group mailing list (or a successor 1468 mailing list designated by the IESG). If the request comes from 1469 within the IETF, it should be documented in an Internet-Draft. 1470 Lastly, the DE must ensure that any other request for a code point 1471 does not conflict with work that is active or already published 1472 within the IETF. 1474 13. Acknowledgement 1476 The authors would like to thank Tarek Saad, Dhanendra Jain, Ruediger 1477 Geib, Rob Shakir, Cheng Li, Dhruv Dhody and Gyan Mishra for their 1478 review comments and suggestions. 1480 14. Contributors 1482 The following people have contributed to this document: 1484 Siva Sivabalan 1485 Cisco Systems 1486 Email: msiva@cisco.com 1488 Zafar Ali 1489 Cisco Systems 1490 Email: zali@cisco.com 1492 Jose Liste 1493 Cisco Systems 1494 Email: jliste@cisco.com 1496 Francois Clad 1497 Cisco Systems 1498 Email: fclad@cisco.com 1500 Kamran Raza 1501 Cisco Systems 1502 Email: skraza@cisco.com 1504 Mike Koldychev 1505 Cisco Systems 1506 Email: mkoldych@cisco.com 1508 Shraddha Hegde 1509 Juniper Networks 1510 Email: shraddha@juniper.net 1512 Steven Lin 1513 Google, Inc. 1514 Email: stevenlin@google.com 1515 Przemyslaw Krol 1516 Google, Inc. 1517 Email: pkrol@google.com 1519 Martin Horneffer 1520 Deutsche Telekom 1521 Email: martin.horneffer@telekom.de 1523 Dirk Steinberg 1524 Steinberg Consulting 1525 Email: dws@steinbergnet.net 1527 Bruno Decraene 1528 Orange Business Services 1529 Email: bruno.decraene@orange.com 1531 Stephane Litkowski 1532 Orange Business Services 1533 Email: stephane.litkowski@orange.com 1535 Luay Jalil 1536 Verizon 1537 Email: luay.jalil@verizon.com 1539 15. References 1541 15.1. Normative References 1543 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1544 Requirement Levels", BCP 14, RFC 2119, 1545 DOI 10.17487/RFC2119, March 1997, 1546 . 1548 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1549 Writing an IANA Considerations Section in RFCs", BCP 26, 1550 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1551 . 1553 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1554 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1555 May 2017, . 1557 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 1558 Decraene, B., Litkowski, S., and R. Shakir, "Segment 1559 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 1560 July 2018, . 1562 [RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., 1563 Decraene, B., Litkowski, S., and R. Shakir, "Segment 1564 Routing with the MPLS Data Plane", RFC 8660, 1565 DOI 10.17487/RFC8660, December 2019, 1566 . 1568 [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., 1569 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header 1570 (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, 1571 . 1573 [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, 1574 D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 1575 (SRv6) Network Programming", RFC 8986, 1576 DOI 10.17487/RFC8986, February 2021, 1577 . 1579 15.2. Informative References 1581 [I-D.ali-spring-sr-traffic-accounting] 1582 Filsfils, C., Talaulikar, K., Sivabalan, S., Horneffer, 1583 M., Raszuk, R., Litkowski, S., Voyer, D., and R. Morton, 1584 "Traffic Accounting in Segment Routing Networks", draft- 1585 ali-spring-sr-traffic-accounting-05 (work in progress), 1586 April 2021. 1588 [I-D.anand-spring-poi-sr] 1589 Anand, M., Bardhan, S., Subrahmaniam, R., Tantsura, J., 1590 Mukhopadhyaya, U., and C. Filsfils, "Packet-Optical 1591 Integration in Segment Routing", draft-anand-spring-poi- 1592 sr-08 (work in progress), July 2019. 1594 [I-D.filsfils-spring-sr-policy-considerations] 1595 Filsfils, C., Talaulikar, K., Krol, P., Horneffer, M., and 1596 P. Mattes, "SR Policy Implementation and Deployment 1597 Considerations", draft-filsfils-spring-sr-policy- 1598 considerations-07 (work in progress), April 2021. 1600 [I-D.filsfils-spring-sr-traffic-counters] 1601 Filsfils, C., Ali, Z., Horneffer, M., Voyer, D., Durrani, 1602 M., and R. Raszuk, "Segment Routing Traffic Accounting 1603 Counters", draft-filsfils-spring-sr-traffic-counters-01 1604 (work in progress), April 2021. 1606 [I-D.ietf-idr-bgpls-segment-routing-epe] 1607 Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray, 1608 S., and J. Dong, "BGP-LS extensions for Segment Routing 1609 BGP Egress Peer Engineering", draft-ietf-idr-bgpls- 1610 segment-routing-epe-19 (work in progress), May 2019. 1612 [I-D.ietf-idr-segment-routing-te-policy] 1613 Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., 1614 Rosen, E., Jain, D., and S. Lin, "Advertising Segment 1615 Routing Policies in BGP", draft-ietf-idr-segment-routing- 1616 te-policy-11 (work in progress), November 2020. 1618 [I-D.ietf-idr-te-lsp-distribution] 1619 Previdi, S., Talaulikar, K., Dong, J., Chen, M., Gredler, 1620 H., and J. Tantsura, "Distribution of Traffic Engineering 1621 (TE) Policies and State using BGP-LS", draft-ietf-idr-te- 1622 lsp-distribution-14 (work in progress), October 2020. 1624 [I-D.ietf-lsr-flex-algo] 1625 Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and 1626 A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex- 1627 algo-15 (work in progress), April 2021. 1629 [I-D.ietf-pce-binding-label-sid] 1630 Sivabalan, S., Filsfils, C., Tantsura, J., Previdi, S., 1631 and C. Li, "Carrying Binding Label/Segment Identifier in 1632 PCE-based Networks.", draft-ietf-pce-binding-label-sid-08 1633 (work in progress), April 2021. 1635 [I-D.ietf-pce-segment-routing-policy-cp] 1636 Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H. 1637 Bidgoli, "PCEP extension to support Segment Routing Policy 1638 Candidate Paths", draft-ietf-pce-segment-routing-policy- 1639 cp-04 (work in progress), March 2021. 1641 [I-D.ietf-rtgwg-segment-routing-ti-lfa] 1642 Litkowski, S., Bashandy, A., Filsfils, C., Francois, P., 1643 Decraene, B., and D. Voyer, "Topology Independent Fast 1644 Reroute using Segment Routing", draft-ietf-rtgwg-segment- 1645 routing-ti-lfa-06 (work in progress), February 2021. 1647 [I-D.ietf-spring-sr-policy-yang] 1648 Raza, K., Sawaya, R., Shunwan, Z., Voyer, D., Durrani, M., 1649 Matsushima, S., and V. Beeram, "YANG Data Model for 1650 Segment Routing Policy", draft-ietf-spring-sr-policy- 1651 yang-01 (work in progress), April 2021. 1653 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 1654 dual environments", RFC 1195, DOI 10.17487/RFC1195, 1655 December 1990, . 1657 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 1658 DOI 10.17487/RFC2328, April 1998, 1659 . 1661 [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 1662 (TE) Extensions to OSPF Version 2", RFC 3630, 1663 DOI 10.17487/RFC3630, September 2003, 1664 . 1666 [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, 1667 "Multiprotocol Extensions for BGP-4", RFC 4760, 1668 DOI 10.17487/RFC4760, January 2007, 1669 . 1671 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 1672 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 1673 2008, . 1675 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 1676 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 1677 . 1679 [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The 1680 Locator/ID Separation Protocol (LISP)", RFC 6830, 1681 DOI 10.17487/RFC6830, January 2013, 1682 . 1684 [RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S. 1685 Previdi, "OSPF Traffic Engineering (TE) Metric 1686 Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015, 1687 . 1689 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 1690 S. Ray, "North-Bound Distribution of Link-State and 1691 Traffic Engineering (TE) Information Using BGP", RFC 7752, 1692 DOI 10.17487/RFC7752, March 2016, 1693 . 1695 [RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path 1696 Computation Element Communication Protocol (PCEP) 1697 Extensions for Stateful PCE", RFC 8231, 1698 DOI 10.17487/RFC8231, September 2017, 1699 . 1701 [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 1702 "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, 1703 DOI 10.17487/RFC8476, December 2018, 1704 . 1706 [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 1707 "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, 1708 DOI 10.17487/RFC8491, November 2018, 1709 . 1711 [RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward, 1712 D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE) 1713 Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March 1714 2019, . 1716 [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 1717 and J. Hardwick, "Path Computation Element Communication 1718 Protocol (PCEP) Extensions for Segment Routing", RFC 8664, 1719 DOI 10.17487/RFC8664, December 2019, 1720 . 1722 [RFC8814] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G., 1723 and N. Triantafillis, "Signaling Maximum SID Depth (MSD) 1724 Using the Border Gateway Protocol - Link State", RFC 8814, 1725 DOI 10.17487/RFC8814, August 2020, 1726 . 1728 Authors' Addresses 1730 Clarence Filsfils 1731 Cisco Systems, Inc. 1732 Pegasus Parc 1733 De kleetlaan 6a, DIEGEM BRABANT 1831 1734 BELGIUM 1736 Email: cfilsfil@cisco.com 1738 Ketan Talaulikar (editor) 1739 Cisco Systems, Inc. 1740 India 1742 Email: ketant@cisco.com 1743 Daniel Voyer 1744 Bell Canada 1745 671 de la gauchetiere W 1746 Montreal, Quebec H3B 2M8 1747 Canada 1749 Email: daniel.voyer@bell.ca 1751 Alex Bogdanov 1752 Google, Inc. 1754 Email: bogdanov@google.com 1756 Paul Mattes 1757 Microsoft 1758 One Microsoft Way 1759 Redmond, WA 98052-6399 1760 USA 1762 Email: pamattes@microsoft.com