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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 S. Sivabalan, Ed. 4 Intended status: Standards Track Cisco Systems, Inc. 5 Expires: June 16, 2020 D. Voyer 6 Bell Canada 7 A. Bogdanov 8 Google, Inc. 9 P. Mattes 10 Microsoft 11 December 14, 2019 13 Segment Routing Policy Architecture 14 draft-ietf-spring-segment-routing-policy-06.txt 16 Abstract 18 Segment Routing (SR) allows a headend node to steer a packet flow 19 along any path. Intermediate per-flow states are eliminated thanks 20 to source routing. The headend node steers a flow into an SR Policy. 21 The header of a packet steered in an SR Policy is augmented with an 22 ordered list of segments associated with that SR Policy. This 23 document details the concepts of SR Policy and steering into an SR 24 Policy. 26 Requirements Language 28 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 29 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 30 document are to be interpreted as described in [RFC2119]. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at https://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on June 16, 2020. 49 Copyright Notice 51 Copyright (c) 2019 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (https://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 67 2. SR Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 3 68 2.1. Identification of an SR Policy . . . . . . . . . . . . . 4 69 2.2. Candidate Path and Segment List . . . . . . . . . . . . . 4 70 2.3. Protocol-Origin of a Candidate Path . . . . . . . . . . . 5 71 2.4. Originator of a Candidate Path . . . . . . . . . . . . . 5 72 2.5. Discriminator of a Candidate Path . . . . . . . . . . . . 6 73 2.6. Identification of a Candidate Path . . . . . . . . . . . 7 74 2.7. Preference of a Candidate Path . . . . . . . . . . . . . 7 75 2.8. Validity of a Candidate Path . . . . . . . . . . . . . . 7 76 2.9. Active Candidate Path . . . . . . . . . . . . . . . . . . 7 77 2.10. Validity of an SR Policy . . . . . . . . . . . . . . . . 9 78 2.11. Instantiation of an SR Policy in the Forwarding Plane . . 9 79 2.12. Priority of an SR Policy . . . . . . . . . . . . . . . . 9 80 2.13. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 9 81 3. Segment Routing Database . . . . . . . . . . . . . . . . . . 10 82 4. Segment Types . . . . . . . . . . . . . . . . . . . . . . . . 11 83 4.1. Explicit Null . . . . . . . . . . . . . . . . . . . . . . 14 84 5. Validity of a Candidate Path . . . . . . . . . . . . . . . . 15 85 5.1. Explicit Candidate Path . . . . . . . . . . . . . . . . . 15 86 5.2. Dynamic Candidate Path . . . . . . . . . . . . . . . . . 16 87 6. Binding SID . . . . . . . . . . . . . . . . . . . . . . . . . 17 88 6.1. BSID of a candidate path . . . . . . . . . . . . . . . . 17 89 6.2. BSID of an SR Policy . . . . . . . . . . . . . . . . . . 17 90 6.3. Forwarding Plane . . . . . . . . . . . . . . . . . . . . 18 91 6.4. Non-SR usage of Binding SID . . . . . . . . . . . . . . . 19 92 7. SR Policy State . . . . . . . . . . . . . . . . . . . . . . . 19 93 8. Steering into an SR Policy . . . . . . . . . . . . . . . . . 19 94 8.1. Validity of an SR Policy . . . . . . . . . . . . . . . . 20 95 8.2. Drop upon invalid SR Policy . . . . . . . . . . . . . . . 20 96 8.3. Incoming Active SID is a BSID . . . . . . . . . . . . . . 20 97 8.4. Per-Destination Steering . . . . . . . . . . . . . . . . 21 98 8.5. Recursion on an on-demand dynamic BSID . . . . . . . . . 22 99 8.6. Per-Flow Steering . . . . . . . . . . . . . . . . . . . . 23 100 8.7. Policy-based Routing . . . . . . . . . . . . . . . . . . 24 101 8.8. Optional Steering Modes for BGP Destinations . . . . . . 24 102 9. Protection . . . . . . . . . . . . . . . . . . . . . . . . . 26 103 9.1. Leveraging TI-LFA local protection of the constituent IGP 104 segments . . . . . . . . . . . . . . . . . . . . . . . . 26 105 9.2. Using an SR Policy to locally protect a link . . . . . . 27 106 9.3. Using a Candidate Path for Path Protection . . . . . . . 27 107 10. Security Considerations . . . . . . . . . . . . . . . . . . . 27 108 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 109 11.1. Guidance for Designated Experts . . . . . . . . . . . . 28 110 12. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 29 111 13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29 112 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 113 14.1. Normative References . . . . . . . . . . . . . . . . . . 30 114 14.2. Informative References . . . . . . . . . . . . . . . . . 31 115 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 117 1. Introduction 119 Segment Routing (SR) allows a headend node to steer a packet flow 120 along any path. Intermediate per-flow states are eliminated thanks 121 to source routing [RFC8402]. 123 The headend node is said to steer a flow into an Segment Routing 124 Policy (SR Policy). 126 The header of a packet steered into an SR Policy is augmented with an 127 ordered list of segments associated with that SR Policy. 129 This document details the concepts of SR Policy and steering packets 130 into an SR Policy. These apply equally to the MPLS and SRv6 131 instantiations of segment routing. 133 For reading simplicity, the illustrations are provided for the MPLS 134 instantiations. 136 2. SR Policy 138 An SR Policy is a framework that enables instantiation of an ordered 139 list of segments on a node for implementing a source routing policy 140 with a specific intent for traffic steering from that node. 142 The Segment Routing architecture [RFC8402] specifies that any 143 instruction can be bound to a segment. Thus, an SR Policy can be 144 built using any type of Segment Identifier (SID) including those 145 associated with topological or service instructions. 147 This section defines the key aspects and constituents of an SR 148 Policy. 150 2.1. Identification of an SR Policy 152 An SR Policy is identified through the tuple . In the context of a specific headend, one may identify an 154 SR policy by the tuple. 156 The headend is the node where the policy is instantiated/implemented. 157 The headend is specified as an IPv4 or IPv6 address and is expected 158 to be unique in the domain. 160 The endpoint indicates the destination of the policy. The endpoint 161 is specified as an IPv4 or IPv6 address and is expected to be unique 162 in the domain. In a specific case (refer to Section 8.8.1), the 163 endpoint can be the null address (0.0.0.0 for IPv4, ::0 for IPv6). 165 The color is a 32-bit numerical value that associates the SR Policy 166 with an intent (e.g. low-latency). 168 The endpoint and the color are used to automate the steering of 169 service or transport routes on SR Policies (refer to Section 8). 171 An implementation MAY allow assignment of a symbolic name comprising 172 of printable ASCII characters to an SR Policy to serve as a user- 173 friendly attribute for debug and troubleshooting purposes. Such 174 symbolic names may identify an SR Policy when the naming scheme 175 ensures uniqueness. 177 2.2. Candidate Path and Segment List 179 An SR Policy is associated with one or more candidate paths. A 180 candidate path is the unit for signaling of an SR Policy to a headend 181 via protocols like Path Computation Element (PCE) Communication 182 Protocol (PCEP) [RFC8281] or BGP SR Policy 183 [I-D.ietf-idr-segment-routing-te-policy]. 185 A Segment-List represents a specific source-routed path to send 186 traffic from the headend to the endpoint of the corresponding SR 187 policy. 189 A candidate path is either dynamic or explicit. 191 An explicit candidate path is expressed as a Segment-List or a set of 192 Segment-Lists. 194 A dynamic candidate path expresses an optimization objective and a 195 set of constraints. The headend (potentially with the help of a PCE) 196 computes the solution Segment-List (or set of Segment-Lists) that 197 solves the optimization problem. 199 If a candidate path is associated with a set of Segment-Lists, each 200 Segment-List is associated with a weight for weighted load balancing 201 (refer Section 2.11 for details). The default weight is 1. 203 2.3. Protocol-Origin of a Candidate Path 205 A headend may be informed about a candidate path for an SR Policy 206 by various means including: via configuration, PCEP 207 [RFC8281] or BGP [I-D.ietf-idr-segment-routing-te-policy]. 209 Protocol-Origin of a candidate path is an 8-bit value which 210 identifies the component or protocol that originates or signals the 211 candidate path. 213 The table below specifies the RECOMMENDED default values: 215 +-------+-------------------+ 216 | Value | Protocol-Origin | 217 +-------+-------------------+ 218 | 10 | PCEP | 219 | 20 | BGP SR Policy | 220 | 30 | Via Configuration | 221 +-------+-------------------+ 223 Table 1: Protocol-origin Identifier 225 Implementations MAY allow modifications of these default values 226 assigned to protocols on the headend along similar lines as a routing 227 administrative distance. Its application in the candidate path 228 selection is described in Section 2.9. 230 2.4. Originator of a Candidate Path 232 Originator identifies the node which provisioned or signalled the 233 candidate path on the headend. The originator is expressed in the 234 form of a 160 bit numerical value formed by the concatenation of the 235 fields of the tuple as below: 237 o ASN : represented as a 4 byte number. 239 o Node Address : represented as a 128 bit value. IPv4 addresses are 240 encoded in the lowest 32 bits. 242 Its application in the candidate path selection is described in 243 Section 2.9. 245 When Protocol-Origin is Via Configuration, the ASN and node address 246 MAY be set to either the headend or the provisioning controller/node 247 ASN and address. Default value is 0 for both AS and node address. 249 When Protocol-Origin is PCEP, it is the IPv4 or IPv6 address of the 250 PCE and the AS number SHOULD be set to 0 by default when not 251 available or known. 253 Protocol-Origin is BGP SR Policy, it is provided by the BGP component 254 on the headend and is: 256 o the BGP Router ID and ASN of the node/controller signalling the 257 candidate path when it has a BGP session to the headend, OR 259 o the BGP Router ID of the eBGP peer signalling the candidate path 260 along with ASN of origin when the signalling is done via one or 261 more intermediate eBGP routers, OR 263 o the BGP Originator ID [RFC4456] and the ASN of the node/controller 264 when the signalling is done via one or more route-reflectors over 265 iBGP session. 267 2.5. Discriminator of a Candidate Path 269 The Discriminator is a 32 bit value associated with a candidate path 270 that uniquely identifies it within the context of an SR Policy from a 271 specific Protocol-Origin as specified below: 273 When Protocol-Origin is Via Configuration, this is an 274 implementation's configuration model specific unique identifier for a 275 candidate path. Default value is 0. 277 When PCEP is the Protocol-Origin, the method to uniquely identify 278 signalled path will be specified in a future PCEP document. Default 279 value is 0. 281 When BGP SR Policy is the Protocol-Origin, it is the distinguisher 282 specified in Section 2.1 of [I-D.ietf-idr-segment-routing-te-policy]. 284 Its application in the candidate path selection is described in 285 Section 2.9. 287 2.6. Identification of a Candidate Path 289 A candidate path is identified in the context of a single SR Policy. 291 A candidate path is not shared across SR Policies. 293 A candidate path is not identified by its Segment-List(s). 295 If CP1 is a candidate path of SR Policy Pol1 and CP2 is a 296 candidate path of SR Policy Pol2, then these two candidate paths 297 are independent, even if they happen to have the same Segment- 298 List. The Segment-List does not identify a candidate path. The 299 Segment-List is an attribute of a candidate path. 301 The identity of a candidate path MUST be uniquely established in the 302 context of an SR Policy in order to handle 303 add, delete or modify operations on them in an unambiguous manner 304 regardless of their source(s). 306 The tuple uniquely 307 identifies a candidate path. 309 Candidate paths MAY also be assigned or signaled with a symbolic name 310 comprising printable ASCII characters to serve as a user-friendly 311 attribute for debug and troubleshooting purposes. Such symbolic 312 names MUST NOT be considered as identifiers for a candidate path. 314 2.7. Preference of a Candidate Path 316 The preference of the candidate path is used to select the best 317 candidate path for an SR Policy. The default preference is 100. 319 It is RECOMMENDED that each candidate path of a given SR policy has a 320 different preference. 322 2.8. Validity of a Candidate Path 324 A candidate path is usable when it valid. A common path validity 325 criterion is the reachability of its constituent SIDs. The 326 validation rules are specified in Section 5. 328 2.9. Active Candidate Path 330 A candidate path is selected when it is valid and it is determined to 331 be the best path of the SR Policy. The selected path is referred to 332 as the "active path" of the SR policy in this document. 334 Whenever a new path is learned or an active path is deleted, the 335 validity of an existing path changes or an existing path is changed, 336 the selection process MUST be re-executed. 338 The candidate path selection process operates on the candidate path 339 Preference. A candidate path is selected when it is valid and it has 340 the highest preference value among all the candidate paths of the SR 341 Policy. 343 In the case of multiple valid candidate paths of the same preference, 344 the tie-breaking rules are evaluated on the identification tuple in 345 the following order until only one valid best path is selected: 347 1. Higher value of Protocol-Origin is selected. 349 2. If specified by configuration, prefer the existing installed 350 path. 352 3. Lower value of originator is selected. 354 4. Finally, the higher value of discriminator is selected. 356 The rules are framed with multiple protocols and sources in mind and 357 hence may not follow the logic of a single protocol (e.g. BGP best 358 path selection). The motivation behind these rules are as follows: 360 o The Protocol-Origin allows an operator to setup a default 361 selection mechanism across protocol sources, e.g., to prefer 362 configured over paths signalled via BGP SR Policy or PCEP. 364 o The preference, being the first tiebreaker, allows an operator to 365 influence selection across paths thus allowing provisioning of 366 multiple path options, e.g., CP1 is preferred and if it becomes 367 invalid then fall-back to CP2 and so on. Since preference works 368 across protocol sources it also enables (where necessary) 369 selective override of the default protocol-origin preference, 370 e.g., to prefer a path signalled via BGP SR Policy over what is 371 configured. 373 o The originator allows an operator to have multiple redundant 374 controllers and still maintain a deterministic behaviour over 375 which of them are preferred even if they are providing the same 376 candidate paths for the same SR policies to the headend. 378 o The discriminator performs the final tiebreaking step to ensure a 379 deterministic outcome of selection regardless of the order in 380 which candidate paths are signalled across multiple transport 381 channels or sessions. 383 [I-D.filsfils-spring-sr-policy-considerations] provides a set of 384 examples to illustrate the active candidate path selection rules. 386 2.10. Validity of an SR Policy 388 An SR Policy is valid when it has at least one valid candidate path. 390 2.11. Instantiation of an SR Policy in the Forwarding Plane 392 A valid SR Policy is instantiated in the forwarding plane. 394 Only the active candidate path SHOULD be used for forwarding traffic 395 that is being steered onto that policy. 397 If a set of Segment-Lists is associated with the active path of the 398 policy, then the steering is per flow and W-ECMP based according to 399 the relative weight of each Segment-List. 401 The fraction of the flows associated with a given Segment-List is w/ 402 Sw where w is the weight of the Segment-List and Sw is the sum of the 403 weights of the Segment-Lists of the selected path of the SR Policy. 405 The accuracy of the weighted load-balancing depends on the platform 406 implementation. 408 2.12. Priority of an SR Policy 410 Upon topological change, many policies could be recomputed or 411 revalidated. An implementation MAY provide a per-policy priority 412 configuration. The operator MAY set this field to indicate order in 413 which the policies should be re-computed. Such a priority is 414 represented by an integer in the range (0, 255) where the lowest 415 value is the highest priority. The default value of priority is 128. 417 An SR Policy may comprise multiple Candidate Paths received from the 418 same or different sources. A candidate path MAY be signaled with a 419 priority value. When an SR Policy has multiple candidate paths with 420 distinct signaled non-default priority values, the SR Policy as a 421 whole takes the lowest value (i.e. the highest priority) amongst 422 these signaled priority values. 424 2.13. Summary 426 In summary, the information model is the following: 428 SR policy POL1 429 Candidate-path CP1 431 Preference 200 432 Weight W1, SID-List1 433 Weight W2, SID-List2 434 Candidate-path CP2 436 Preference 100 437 Weight W3, SID-List3 438 Weight W4, SID-List4 440 The SR Policy POL1 is identified by the tuple . It has two candidate paths CP1 and CP2. Each is 442 identified by a tuple . 443 CP1 is the active candidate path (it is valid and it has the highest 444 preference). The two Segment-Lists of CP1 are installed as the 445 forwarding instantiation of SR policy Pol1. Traffic steered on Pol1 446 is flow-based hashed on Segment-List with a ratio 447 W1/(W1+W2). 449 3. Segment Routing Database 451 An SR headend maintains the Segment Routing Database (SR-DB). The 452 SR-DB is a conceptual database to illustrate the various pieces of 453 information and their sources that may help in SR Policy computation 454 and validation. There is no specific requirement for an 455 implementation to create a new database as such. 457 An SR headend leverages the SR-DB to validate explicit candidate 458 paths and compute dynamic candidate paths. 460 The information in the SR-DB MAY include: 462 o IGP information (topology, IGP metrics based on ISIS [RFC1195] and 463 OSPF [RFC2328] [RFC5340]) 464 o Segment Routing information (such as SRGB, SRLB, Prefix-SIDs, Adj- 465 SIDs, BGP Peering SID, SRv6 SIDs) [RFC8402] 466 [I-D.ietf-idr-bgpls-segment-routing-epe] 467 [I-D.filsfils-spring-srv6-network-programming] 468 o TE Link Attributes (such as TE metric, SRLG, attribute-flag, 469 extended admin group) [RFC5305] [RFC3630]. 470 o Extended TE Link attributes (such as latency, loss) [RFC7810] 471 [RFC7471] 472 o Inter-AS Topology information 473 [I-D.ietf-idr-bgpls-segment-routing-epe]. 475 The attached domain topology MAY be learned via IGP, BGP-LS or 476 NETCONF. 478 A non-attached (remote) domain topology MAY be learned via BGP-LS or 479 NETCONF. 481 In some use-cases, the SR-DB may only contain the attached domain 482 topology while in others, the SR-DB may contain the topology of 483 multiple domains and in this case it is multi-domain capable. 485 The SR-DB MAY also contain the SR Policies instantiated in the 486 network. This can be collected via BGP-LS 487 [I-D.ietf-idr-te-lsp-distribution] or PCEP [RFC8231] and 488 [I-D.sivabalan-pce-binding-label-sid]. This information allows to 489 build an end-to-end policy on the basis of intermediate SR policies 490 (see Section 6 for further details). 492 The SR-DB MAY also contain the Maximum SID Depth (MSD) capability of 493 nodes in the topology. This can be collected via ISIS 494 [I-D.ietf-isis-segment-routing-msd], OSPF 495 [I-D.ietf-ospf-segment-routing-msd], BGP-LS 496 [I-D.ietf-idr-bgp-ls-segment-routing-msd] or PCEP 497 [I-D.ietf-pce-segment-routing]. 499 The use of the SR-DB for computation and validation of SR Policies is 500 outside the scope of this document. Some implementation aspects 501 related to this are covered in 502 [I-D.filsfils-spring-sr-policy-considerations]. 504 4. Segment Types 506 A Segment-List is an ordered set of segments represented as where S1 is the first segment. 509 Based on the desired dataplane, either the MPLS label stack or the 510 SRv6 SRH is built from the Segment-List. However, the Segment-List 511 itself can be specified using different segment-descriptor types and 512 the following are currently defined: 514 Type A: SR-MPLS Label: 515 A MPLS label corresponding to any of the segment types defined 516 for SR-MPLS (as defined in [RFC8402] or other SR-MPLS 517 specifications) can be used. Additionally, reserved labels 518 like explicit-null or in general any MPLS label may also be 519 used. E.g. this type can be used to specify a label 520 representation which maps to an optical transport path on a 521 packet transport node. This type does not require the headend 522 to perform SID resolution. 524 Type B: SRv6 SID: 526 An IPv6 address corresponding to any of the segment types 527 defined for SRv6 (as defined in 528 [I-D.filsfils-spring-srv6-network-programming] or other SRv6 529 specifications) can be used. This type does not require the 530 headend to perform SID resolution. 532 Type C: IPv4 Prefix with optional SR Algorithm: 533 The headend is required to resolve the specified IPv4 Prefix 534 Address to the SR-MPLS label corresponding to a Prefix SID 535 segment (as defined in [RFC8402]). The SR algorithm (refer to 536 Section 3.1.1 of [RFC8402]) to be used MAY also be provided. 538 Type D: IPv6 Global Prefix with optional SR Algorithm for SR-MPLS: 539 In this case the headend is required to resolve the specified 540 IPv6 Global Prefix Address to the SR-MPLS label corresponding 541 to its Prefix SID segment (as defined in [RFC8402]). The SR 542 Algorithm (refer to Section 3.1.1 of [RFC8402]) to be used MAY 543 also be provided. 545 Type E: IPv4 Prefix with Local Interface ID: 546 This type allows identification of Adjacency SID (as defined in 547 [RFC8402]) or BGP EPE Peering SID (as defined in 548 [I-D.ietf-idr-bgpls-segment-routing-epe]) label for point-to- 549 point links including IP unnumbered links. The headend is 550 required to resolve the specified IPv4 Prefix Address to the 551 Node originating it and then use the Local Interface ID to 552 identify the point-to-point link whose adjacency is being 553 referred to. The Local Interface ID link descriptor follows 554 semantics as specified in [RFC7752]. This type can also be 555 used to indicate indirection into a layer 2 interface (i.e. 556 without IP address) like a representation of an optical 557 transport path or a layer 2 Ethernet port or circuit at the 558 specified node. 560 Type F: IPv4 Addresses for link endpoints as Local, Remote pair: 561 This type allows identification of Adjacency SID (as defined in 562 [RFC8402]) or BGP EPE Peering SID (as defined in 563 [I-D.ietf-idr-bgpls-segment-routing-epe]) label for links. The 564 headend is required to resolve the specified IPv4 Local Address 565 to the Node originating it and then use the IPv4 Remote Address 566 to identify the link adjacency being referred to. The Local 567 and Remote Address pair link descriptors follows semantics as 568 specified in [RFC7752]. 570 Type G: IPv6 Prefix and Interface ID for link endpoints as Local, 571 Remote pair for SR-MPLS: 572 This type allows identification of Adjacency SID (as defined in 573 [RFC8402]) or BGP EPE Peering SID (as defined in 575 [I-D.ietf-idr-bgpls-segment-routing-epe]) label for links 576 including those with only Link Local IPv6 addresses. The 577 headend is required to resolve the specified IPv6 Prefix 578 Address to the Node originating it and then use the Local 579 Interface ID to identify the point-to-point link whose 580 adjacency is being referred to. For other than point-to-point 581 links, additionally the specific adjacency over the link needs 582 to be resolved using the Remote Prefix and Interface ID. The 583 Local and Remote pair of Prefix and Interface ID link 584 descriptor follows semantics as specified in [RFC7752]. This 585 type can also be used to indicate indirection into a layer 2 586 interface (i.e. without IP address) like a representation of an 587 optical transport path or a layer 2 Ethernet port or circuit at 588 the specified node. 590 Type H: IPv6 Addresses for link endpoints as Local, Remote pair for 591 SR-MPLS: 592 This type allows identification of Adjacency SID (as defined in 593 [RFC8402]) or BGP EPE Peering SID (as defined in 594 [I-D.ietf-idr-bgpls-segment-routing-epe]) label for links with 595 Global IPv6 addresses. The headend is required to resolve the 596 specified Local IPv6 Address to the Node originating it and 597 then use the Remote IPv6 Address to identify the link adjacency 598 being referred to. The Local and Remote Address pair link 599 descriptors follows semantics as specified in [RFC7752]. 601 Type I: IPv6 Global Prefix with optional SR Algorithm for SRv6: 602 The headend is required to resolve the specified IPv6 Global 603 Prefix Address to the SRv6 END function SID (as defined in 604 [I-D.filsfils-spring-srv6-network-programming]) corresponding 605 to the node which is originating the prefix. The SR Algorithm 606 (refer to Section 3.1.1 of [RFC8402]) to be used MAY also be 607 provided. 609 Type J: IPv6 Prefix and Interface ID for link endpoints as Local, 610 Remote pair for SRv6: 611 This type allows identification of SRv6 END.X SID (as defined 612 in [I-D.filsfils-spring-srv6-network-programming]) for links 613 with only Link Local IPv6 addresses. The headend is required 614 to resolve the specified IPv6 Prefix Address to the Node 615 originating it and then use the Local Interface ID to identify 616 the point-to-point link whose adjacency is being referred to. 617 For other than point-to-point links, additionally the specific 618 adjacency needs to be resolved using the Remote Prefix and 619 Interface ID. The Local and Remote pair of Prefix and 620 Interface ID link descriptor follows semantics as specified in 621 [RFC7752]. 623 Type K: IPv6 Addresses for link endpoints as Local, Remote pair for 624 SRv6: 625 This type allows identification of SRv6 END.X SID (as defined 626 in [I-D.filsfils-spring-srv6-network-programming]) for links 627 with Global IPv6 addresses. The headend is required to resolve 628 the specified Local IPv6 Address to the Node originating it and 629 then use the Remote IPv6 Address to identify the link adjacency 630 being referred to. The Local and Remote Address pair link 631 descriptors follows semantics as specified in [RFC7752]. 633 When the algorithm is not specified for the SID types above which 634 optionally allow for it, the headend SHOULD use the Strict Shortest 635 Path algorithm if available; otherwise it SHOULD use the default 636 Shortest Path algorithm. The specification of algorithm enables the 637 use of IGP Flex Algorithm [I-D.ietf-lsr-flex-algo] specific SIDs in 638 SR Policy. 640 For SID types C-through-K, a SID value may also be optionally 641 provided to the headend for verification purposes. Section 5.1. 642 describes the resolution and verification of the SIDs and Segment 643 Lists on the headend. 645 When building the MPLS label stack or the IPv6 Segment list from the 646 Segment List, the node instantiating the policy MUST interpret the 647 set of Segments as follows: 649 o The first Segment represents the topmost label or the first IPv6 650 segment. It identifies the active segment the traffic will be 651 directed toward along the explicit SR path. 652 o The last Segment represents the bottommost label or the last IPv6 653 segment the traffic will be directed toward along the explicit SR 654 path. 656 4.1. Explicit Null 658 A Type A SID may be any MPLS label, including reserved labels. 660 For example, assuming that the desired traffic-engineered path from a 661 headend 1 to an endpoint 4 can be expressed by the Segment-List 662 <16002, 16003, 16004> where 16002, 16003 and 16004 respectively refer 663 to the IPv4 Prefix SIDs bound to node 2, 3 and 4, then IPv6 traffic 664 can be traffic-engineered from nodes 1 to 4 via the previously 665 described path using an SR Policy with Segment-List <16002, 16003, 666 16004, 2> where mpls label value of 2 represents the "IPv6 Explicit 667 NULL Label". 669 The penultimate node before node 4 will pop 16004 and will forward 670 the frame on its directly connected interface to node 4. 672 The endpoint receives the traffic with top label "2" which indicates 673 that the payload is an IPv6 packet. 675 When steering unlabeled IPv6 BGP destination traffic using an SR 676 policy composed of Segment-List(s) based on IPv4 SIDs, the Explicit 677 Null Label Policy is processed as specified in 678 [I-D.ietf-idr-segment-routing-te-policy]) Section 2.4.4. When an 679 "IPv6 Explicit NULL label" is not present as the bottom label, the 680 headend SHOULD automatically impose one. Refer to Section 8 for more 681 details. 683 5. Validity of a Candidate Path 685 5.1. Explicit Candidate Path 687 An explicit candidate path is associated with a Segment-List or a set 688 of Segment-Lists. 690 An explicit candidate path is provisioned by the operator directly or 691 via a controller. 693 The computation/logic that leads to the choice of the Segment-List is 694 external to the SR Policy headend. The SR Policy headend does not 695 compute the Segment-List. The SR Policy headend only confirms its 696 validity. 698 A Segment-List of an explicit candidate path MUST be declared invalid 699 when: 701 o It is empty. 702 o Its weight is 0. 703 o The headend is unable to perform path resolution for the first SID 704 into one or more outgoing interface(s) and next-hop(s). 705 o The headend is unable to perform SID resolution for any non-first 706 SID of type C-through-K into an MPLS label or an SRv6 SID. 707 o The headend verification fails for any SID for which verification 708 has been explicitly requested. 710 "Unable to perform path resolution" means that the headend has no 711 path to the SID in its SR database. 713 SID verification is performed when the headend is explicitly 714 requested to verify SID(s) by the controller via the signaling 715 protocol used. Implementations MAY provide a local configuration 716 option to enable verification on a global or per policy or per 717 candidate path basis. 719 "Verification fails" for a SID means any of the following: 721 o The headend is unable to find the SID in its SR DB 722 o The headend detects mis-match between the SID value and its 723 context provided for SIDs of type C-through-K in its SR DB. 724 o The headend is unable to perform SID resolution for any non-first 725 SID of type C-through-K into an MPLS label or an SRv6 SID. 727 In multi-domain deployments, it is expected that the headend be 728 unable to verify the reachability of the SIDs in remote domains. 729 Types A or B MUST be used for the SIDs for which the reachability 730 cannot be verified. Note that the first SID MUST always be reachable 731 regardless of its type. 733 In addition, a Segment-List MAY be declared invalid when: 735 o Its last segment is not a Prefix SID (including BGP Peer Node-SID) 736 advertised by the node specified as the endpoint of the 737 corresponding SR policy. 738 o Its last segment is not an Adjacency SID (including BGP Peer 739 Adjacency SID) of any of the links present on neighbor nodes and 740 that terminate on the node specified as the endpoint of the 741 corresponding SR policy. 743 An explicit candidate path is invalid as soon as it has no valid 744 Segment-List. 746 5.2. Dynamic Candidate Path 748 A dynamic candidate path is specified as an optimization objective 749 and constraints. 751 The headend of the policy leverages its SR database to compute a 752 Segment-List ("solution Segment-List") that solves this optimization 753 problem. 755 The headend re-computes the solution Segment-List any time the inputs 756 to the problem change (e.g., topology changes). 758 When local computation is not possible (e.g., a policy's tailend is 759 outside the topology known to the headend) or not desired, the 760 headend MAY send path computation request to a PCE supporting PCEP 761 extension specified in [I-D.ietf-pce-segment-routing]. 763 If no solution is found to the optimization objective and 764 constraints, then the dynamic candidate path MUST be declared 765 invalid. 767 [I-D.filsfils-spring-sr-policy-considerations] discusses some of the 768 optimization objectives and constraints that may be considered by a 769 dynamic candidate path. It illustrates some of the desirable 770 properties of the computation of the solution Segment-List. 772 6. Binding SID 774 The Binding SID (BSID) is fundamental to Segment Routing [RFC8402]. 775 It provides scaling, network opacity and service independence. 776 [I-D.filsfils-spring-sr-policy-considerations] illustrates some of 777 these benefits. This section describes the association of BSID with 778 an SR Policy. 780 6.1. BSID of a candidate path 782 Each candidate path MAY be defined with a BSID. 784 Candidate Paths of the same SR policy SHOULD have the same BSID. 786 Candidate Paths of different SR policies MUST NOT have the same BSID. 788 6.2. BSID of an SR Policy 790 The BSID of an SR Policy is the BSID of its active candidate path. 792 When the active candidate path has a specified BSID, the SR Policy 793 uses that BSID if this value (label in MPLS, IPv6 address in SRv6) is 794 available (i.e., not associated with any other usage: e.g. to another 795 MPLS client, to another SID, to another SR Policy). 797 Optionally, instead of only checking that the BSID of the active path 798 is available, a headend MAY check that it is available within a given 799 SID range i.e., Segment Routing Local Block (SRLB) as specified in 800 [RFC8402]. 802 When the specified BSID is not available (optionally is not in the 803 SRLB), an alert message MUST be generated. 805 In the cases (as described above) where SR Policy does not have a 806 BSID available, then the SR Policy MAY dynamically bind a BSID to 807 itself. Dynamically bound BSID SHOULD use an available SID outside 808 the SRLB. 810 Assuming that at time t the BSID of the SR Policy is B1, if at time 811 t+dt a different candidate path becomes active and this new active 812 path does not have a specified BSID or its BSID is specified but is 813 not available (e.g. it is in use by something else), then the SR 814 Policy keeps the previous BSID B1. 816 The association of an SR Policy with a BSID thus MAY change over the 817 life of the SR Policy (e.g., upon active path change). Hence, the 818 BSID SHOULD NOT be used as an identification of an SR Policy. 820 6.2.1. Frequent use-case : unspecified BSID 822 All the candidate paths of the same SR Policy can have an unspecified 823 BSID. 825 In such a case, a BSID MAY be dynamically bound to the SR Policy as 826 soon as the first valid candidate path is received. That BSID is 827 kept along all the life of the SR Policy and across changes of active 828 candidate path. 830 6.2.2. Frequent use-case: all specified to the same BSID 832 All the paths of the SR Policy can have the same specified BSID. 834 6.2.3. Specified-BSID-only 836 An implementation MAY support the configuration of the Specified- 837 BSID-only restrictive behavior on the headend for all SR Policies or 838 individual SR Policies. Further, this restrictive behavior MAY also 839 be signaled on a per SR Policy basis to the headend. 841 When this restrictive behavior is enabled, if the candidate path has 842 an unspecified BSID or if the specified BSID is not available when 843 the candidate path becomes active then no BSID is bound to it and it 844 is considered invalid. An alert MUST be triggered for this error. 845 Other candidate paths MUST then be evaluated for becoming the active 846 candidate path. 848 6.3. Forwarding Plane 850 A valid SR Policy installs a BSID-keyed entry in the forwarding plane 851 with the action of steering the packets matching this entry to the 852 selected path of the SR Policy. 854 If the Specified-BSID-only restrictive behavior is enabled and the 855 BSID of the active path is not available (optionally not in the 856 SRLB), then the SR Policy does not install any entry indexed by a 857 BSID in the forwarding plane. 859 6.4. Non-SR usage of Binding SID 861 An implementation MAY choose to associate a Binding SID with any type 862 of interface (e.g. a layer 3 termination of an Optical Circuit) or a 863 tunnel (e.g. IP tunnel, GRE tunnel, IP/UDP tunnel, MPLS RSVP-TE 864 tunnel, etc). This enables the use of other non-SR enabled 865 interfaces and tunnels as segments in an SR Policy Segment-List 866 without the need of forming routing protocol adjacencies over them. 868 The details of this kind of usage are beyond the scope of this 869 document. A specific packet optical integration use case is 870 described in [I-D.anand-spring-poi-sr] 872 7. SR Policy State 874 The SR Policy State is maintained on the headend to represent the 875 state of the policy and its candidate paths. This is to provide an 876 accurate representation of whether the SR Policy is being 877 instantiated in the forwarding plane and which of its candidate paths 878 and segment-list(s) are active. The SR Policy state MUST also 879 reflect the reason when a policy and/or its candidate path is not 880 active due to validation errors or not being preferred. 882 The SR Policy state can be reported by the headend node via BGP-LS 883 [I-D.ietf-idr-te-lsp-distribution] or PCEP [RFC8231] and 884 [I-D.sivabalan-pce-binding-label-sid]. 886 SR Policy state on the headend also includes traffic accounting 887 information for the flows being steered via the policies. The 888 details of the SR Policy accounting are beyond the scope of this 889 document. The aspects related to the SR traffic counters and their 890 usage in the broader context of traffic accounting in a SR network 891 are covered in [I-D.filsfils-spring-sr-traffic-counters] and 892 [I-D.ali-spring-sr-traffic-accounting] respectively. 894 Implementations MAY support an administrative state to control 895 locally provisioned policies via mechanisms like CLI or NETCONF. 897 8. Steering into an SR Policy 899 A headend can steer a packet flow into a valid SR Policy in various 900 ways: 902 o Incoming packets have an active SID matching a local BSID at the 903 headend. 904 o Per-destination Steering: incoming packets match a BGP/Service 905 route which recurses on an SR policy. 907 o Per-flow Steering: incoming packets match or recurse on a 908 forwarding array of where some of the entries are SR Policies. 909 o Policy-based Steering: incoming packets match a routing policy 910 which directs them on an SR policy. 912 For simplicity of illustration, this document uses the SR-MPLS 913 example. 915 8.1. Validity of an SR Policy 917 An SR Policy is invalid when all its candidate paths are invalid as 918 described in Section 5 and Section 2.10. 920 By default, upon transitioning to the invalid state, 922 o an SR Policy and its BSID are removed from the forwarding plane. 923 o any steering of a service (PW), destination (BGP-VPN), flow or 924 packet on the related SR policy is disabled and the related 925 service, destination, flow or packet is routed per the classic 926 forwarding table (e.g. longest-match to the destination or the 927 recursing next-hop). 929 8.2. Drop upon invalid SR Policy 931 An SR Policy MAY be enabled for the Drop-Upon-Invalid behavior: 933 o an invalid SR Policy and its BSID is kept in the forwarding plane 934 with an action to drop. 935 o any steering of a service (PW), destination (BGP-VPN), flow or 936 packet on the related SR policy is maintained with the action to 937 drop all of this traffic. 939 The drop-upon-invalid behavior has been deployed in use-cases where 940 the operator wants some PW to only be transported on a path with 941 specific constraints. When these constraints are no longer met, the 942 operator wants the PW traffic to be dropped. Specifically, the 943 operator does not want the PW to be routed according to the IGP 944 shortest-path to the PW endpoint. 946 8.3. Incoming Active SID is a BSID 948 Let us assume that headend H has a valid SR Policy P of Segment-List 949 and BSID B. 951 When H receives a packet K with label stack , H pops B and 952 pushes and forwards the resulting packet according to 953 SID S1. 955 "Forwarding the resulting packet according to S1" means: If S1 is 956 an Adj SID or a PHP-enabled prefix SID advertised by a neighbor, H 957 sends the resulting packet with label stack on 958 the outgoing interface associated with S1; Else H sends the 959 resulting packet with label stack along the 960 path of S1. 962 H has steered the packet into the SR policy P. 964 H did not have to classify the packet. The classification was done 965 by a node upstream of H (e.g., the source of the packet or an 966 intermediate ingress edge node of the SR domain) and the result of 967 this classification was efficiently encoded in the packet header as a 968 BSID. 970 This is another key benefit of the segment routing in general and the 971 binding SID in particular: the ability to encode a classification and 972 the resulting steering in the packet header to better scale and 973 simplify intermediate aggregation nodes. 975 If the SR Policy P is invalid, the BSID B is not in the forwarding 976 plane and hence the packet K is dropped by H. 978 8.4. Per-Destination Steering 980 Let us assume that headend H: 982 o learns a BGP route R/r via next-hop N, extended-color community C 983 and VPN label V. 984 o has a valid SR Policy P to (color = C, endpoint = N) of Segment- 985 List and BSID B. 986 o has a BGP policy which matches on the extended-color community C 987 and allows its usage as SLA steering information. 989 If all these conditions are met, H installs R/r in RIB/FIB with next- 990 hop = SR Policy P of BSID B instead of via N. 992 Indeed, H's local BGP policy and the received BGP route indicate that 993 the headend should associate R/r with an SR Policy path to endpoint N 994 with the SLA associated with color C. The headend therefore installs 995 the BGP route on that policy. 997 This can be implemented by using the BSID as a generalized next-hop 998 and installing the BGP route on that generalized next-hop. 1000 When H receives a packet K with a destination matching R/r, H pushes 1001 the label stack and sends the resulting packet along 1002 the path to S1. 1004 Note that any SID associated with the BGP route is inserted after the 1005 Segment-List of the SR Policy (i.e., ). 1007 The same behavior is applicable to any type of service route: any 1008 AFI/SAFI of BGP [RFC4760] any AFI/SAFI of LISP [RFC6830]. 1010 8.4.1. Multiple Colors 1012 When a BGP route has multiple extended-color communities each with a 1013 valid SR Policy NLRI, the BGP process installs the route on the SR 1014 policy whose color is of highest numerical value. 1016 Let us assume that headend H: 1018 o learns a BGP route R/r via next-hop N, extended-color communities 1019 C1 and C2 and VPN label V. 1020 o has a valid SR Policy P1 to (color = C1, endpoint = N) of Segment- 1021 List and BSID B1. 1022 o has a valid SR Policy P2 to (color = C2, endpoint = N) of Segment- 1023 List and BSID B2. 1024 o has a BGP policy which matches on the extended-color communities 1025 C1 and C2 and allows their usage as SLA steering information 1027 If all these conditions are met, H installs R/r in RIB/FIB with next- 1028 hop = SR Policy P2 of BSID=B2 (instead of N) because C2 > C1. 1030 8.5. Recursion on an on-demand dynamic BSID 1032 In the previous section, it was assumed that H had a pre-established 1033 "explicit" SR Policy (color C, endpoint N). 1035 In this section, independently to the a-priori existence of any 1036 explicit candidate path of the SR policy (C, N), it is to be noted 1037 that the BGP process at headend node H triggers the instantiation of 1038 a dynamic candidate path for the SR policy (C, N) as soon as: 1040 o the BGP process learns of a route R/r via N and with color C. 1041 o a local policy at node H authorizes the on-demand SR Policy path 1042 instantiation and maps the color to a dynamic SR Policy path 1043 optimization template. 1045 8.5.1. Multiple Colors 1047 When a BGP route R/r via N has multiple extended-color communities Ci 1048 (with i=1 ... n), an individual on-demand SR Policy dynamic path 1049 request (color Ci, endpoint N) is triggered for each color Ci. 1051 8.6. Per-Flow Steering 1053 Let us assume that headend H: 1055 o has a valid SR Policy P1 to (color = C1, endpoint = N) of Segment- 1056 List and BSID B1. 1057 o has a valid SR Policy P2 to (color = C2, endpoint = N) of Segment- 1058 List and BSID B2. 1059 o is configured to instantiate an array of paths to N where the 1060 entry 0 is the IGP path to N, color C1 is the first entry and 1061 Color C2 is the second entry. The index into the array is called 1062 a Forwarding Class (FC). The index can have values 0 to 7. 1063 o is configured to match flows in its ingress interfaces (upon any 1064 field such as Ethernet destination/source/vlan/tos or IP 1065 destination/source/DSCP or transport ports etc.) and color them 1066 with an internal per-packet forwarding-class variable (0, 1 or 2 1067 in this example). 1069 If all these conditions are met, H installs in RIB/FIB: 1071 o N via a recursion on an array A (instead of the immediate outgoing 1072 link associated with the IGP shortest-path to N). 1073 o Entry A(0) set to the immediate outgoing link of the IGP shortest- 1074 path to N. 1075 o Entry A(1) set to SR Policy P1 of BSID=B1. 1076 o Entry A(2) set to SR Policy P2 of BSID=B2. 1078 H receives three packets K, K1 and K2 on its incoming interface. 1079 These three packets either longest-match on N or more likely on a 1080 BGP/service route which recurses on N. H colors these 3 packets 1081 respectively with forwarding-class 0, 1 and 2. As a result: 1083 o H forwards K along the shortest-path to N (which in SR-MPLS 1084 results in the pushing of the prefix-SID of N). 1085 o H pushes on packet K1 and forwards the resulting 1086 frame along the shortest-path to S1. 1087 o H pushes on packet K2 and forwards the resulting 1088 frame along the shortest-path to S4. 1090 If the local configuration does not specify any explicit forwarding 1091 information for an entry of the array, then this entry is filled with 1092 the same information as entry 0 (i.e. the IGP shortest-path). 1094 If the SR Policy mapped to an entry of the array becomes invalid, 1095 then this entry is filled with the same information as entry 0. When 1096 all the array entries have the same information as entry0, the 1097 forwarding entry for N is updated to bypass the array and point 1098 directly to its outgoing interface and next-hop. 1100 The array index values (e.g. 0, 1 and 2) and the notion of 1101 forwarding-class are implementation specific and only meant to 1102 describe the desired behavior. The same can be realized by other 1103 mechanisms. 1105 This realizes per-flow steering: different flows bound to the same 1106 BGP endpoint are steered on different IGP or SR Policy paths. 1108 A headend MAY support options to apply per-flow steering only for 1109 traffic matching specific prefixes (e.g. specific IGP or BGP 1110 prefixes). 1112 8.7. Policy-based Routing 1114 Finally, headend H may be configured with a local routing policy 1115 which overrides any BGP/IGP path and steer a specified packet on an 1116 SR Policy. This includes the use of mechanisms like IGP Shortcut for 1117 automatic routing of IGP prefixes over SR Policies intended for such 1118 purpose. 1120 8.8. Optional Steering Modes for BGP Destinations 1122 8.8.1. Color-Only BGP Destination Steering 1124 In the previous section, it is seen that the steering on an SR Policy 1125 is governed by the matching of the BGP route's next-hop N and the 1126 authorized color C with an SR Policy defined by the tuple (N, C). 1128 This is the most likely form of BGP destination steering and the one 1129 recommended for most use-cases. 1131 This section defines an alternative steering mechanism based only on 1132 the color. 1134 This color-only steering variation is governed by two new flags "C" 1135 and "O" defined in the color extended community [ref draft-ietf-idr- 1136 segment-routing-te-policy section 3]. 1138 The Color-Only flags "CO" are set to 00 by default. 1140 When 00, the BGP destination is steered as follows: 1142 IF there is a valid SR Policy (N, C) where N is the IPv4 or IPv6 1144 endpoint address and C is a color; 1145 Steer into SR Policy (N, C); 1146 ELSE; 1147 Steer on the IGP path to the next-hop N. 1149 This is the classic case described in this document previously and 1150 what is recommended in most scenarios. 1152 When 01, the BGP destination is steered as follows: 1154 IF there is a valid SR Policy (N, C) where N is the IPv4 or IPv6 1156 endpoint address and C is a color; 1157 Steer into SR Policy (N, C); 1158 ELSE IF there is a valid SR Policy (null endpoint, C) of the 1159 same address-family of N; 1160 Steer into SR Policy (null endpoint, C); 1161 ELSE IF there is any valid SR Policy 1162 (any address-family null endpoint, C); 1163 Steer into SR Policy (any null endpoint, C); 1164 ELSE; 1165 Steer on the IGP path to the next-hop N. 1167 When 10, the BGP destination is steered as follows: 1169 IF there is a valid SR Policy (N, C) where N is an IPv4 or IPv6 1170 endpoint address and C is a color; 1171 Steer into SR Policy (N, C); 1172 ELSE IF there is a valid SR Policy (null endpoint, C) 1173 of the same address-family of N; 1174 Steer into SR Policy (null endpoint, C); 1175 ELSE IF there is any valid SR Policy 1176 (any address-family null endpoint, C); 1177 Steer into SR Policy (any null endpoint, C); 1178 ELSE IF there is any valid SR Policy (any endpoint, C) 1179 of the same address-family of N; 1180 Steer into SR Policy (any endpoint, C); 1181 ELSE IF there is any valid SR Policy 1182 (any address-family endpoint, C); 1183 Steer into SR Policy (any address-family endpoint, C); 1184 ELSE; 1185 Steer on the IGP path to the next-hop N. 1187 The null endpoint is 0.0.0.0 for IPv4 and ::0 for IPv6 (all bits set 1188 to the 0 value). 1190 The value 11 is reserved for future use and SHOULD NOT be used. Upon 1191 reception, an implementations MUST treat it like 00. 1193 8.8.2. Multiple Colors and CO flags 1195 The steering preference is first based on highest color value and 1196 then CO-dependent for the color. Assuming a Prefix via (NH, 1197 C1(CO=01), C2(CO=01)); C1>C2 The steering preference order is: 1199 o SR policy (NH, C1). 1200 o SR policy (null, C1). 1201 o SR policy (NH, C2). 1202 o SR policy (null, C2). 1203 o IGP to NH. 1205 8.8.3. Drop upon Invalid 1207 This document defined earlier that when all the following conditions 1208 are met, H installs R/r in RIB/FIB with next-hop = SR Policy P of 1209 BSID B instead of via N. 1211 o H learns a BGP route R/r via next-hop N, extended-color community 1212 C and VPN label V. 1213 o H has a valid SR Policy P to (color = C, endpoint = N) of Segment- 1214 List and BSID B. 1215 o H has a BGP policy which matches on the extended-color community C 1216 and allows its usage as SLA steering information. 1218 This behavior is extended by noting that the BGP policy may require 1219 the BGP steering to always stay on the SR policy whatever its 1220 validity. 1222 This is the "drop upon invalid" option described in Section 8.2 1223 applied to BGP-based steering. 1225 9. Protection 1227 9.1. Leveraging TI-LFA local protection of the constituent IGP segments 1229 In any topology, Topology-Independent Loop Free Alternate (TI-LFA) 1230 [I-D.bashandy-rtgwg-segment-routing-ti-lfa] provides a 50msec local 1231 protection technique for IGP SIDs. The backup path is computed on a 1232 per IGP SID basis along the post-convergence path. 1234 In a network that has deployed TI-LFA, an SR Policy built on the 1235 basis of TI-LFA protected IGP segments leverages the local protection 1236 of the constituent segments. 1238 In a network that has deployed TI-LFA, an SR Policy instantiated only 1239 with non-protected Adj SIDs does not benefit from any local 1240 protection. 1242 9.2. Using an SR Policy to locally protect a link 1244 1----2-----6----7 1245 | | | | 1246 4----3-----9----8 1248 Figure 1: Local protection using SR Policy 1250 An SR Policy can be instantiated at node 2 to protect the link 2to6. 1251 A typical explicit Segment-List would be <3, 9, 6>. 1253 A typical use-case occurs for links outside an IGP domain: e.g. 1, 2, 1254 3 and 4 are part of IGP/SR sub-domain 1 while 6, 7, 8 and 9 are part 1255 of IGP/SR sub-domain 2. In such a case, links 2to6 and 3to9 cannot 1256 benefit from TI-LFA automated local protection. The SR Policy with 1257 Segment-List <3, 9, 6> on node 2 can be locally configured to be a 1258 fast-reroute backup path for the link 2to6. 1260 9.3. Using a Candidate Path for Path Protection 1262 An SR Policy allows for multiple candidate paths, of which at any 1263 point in time there is a single active candidate path that is 1264 provisioned in the forwarding plane and used for traffic steering. 1265 However, another (lower preference) candidate path MAY be designated 1266 as the backup for a specific or all (active) candidate path(s). The 1267 following options are possible: 1269 o A pair of disjoint candidate paths are provisioned with one of 1270 them as primary and the other is identified as its backup. 1271 o A specific candidate path is provisioned as the backup for any 1272 (active) candidate path. 1273 o The headend picks the next (lower) preference valid candidate path 1274 as the backup for the active candidate path. 1276 The headend MAY compute a-priori and validate such backup candidate 1277 paths as well as provision them into forwarding plane as backup for 1278 the active path. A fast re-route mechanism MAY then be used to 1279 trigger sub 50msec switchover from the active to the backup candidate 1280 path in the forwarding plane. Mechanisms like BFD MAY be used for 1281 fast detection of such failures. 1283 10. Security Considerations 1285 This document does not define any new protocol extensions and does 1286 not impose any additional security challenges. 1288 11. IANA Considerations 1290 This document requests IANA to create a new top-level registry called 1291 "Segment Routing Parameters". This registry is being defined to 1292 serve as a top-level registry for keeping all other Segment Routing 1293 sub-registries. 1295 The document also requests creation of a new sub-registry called 1296 "Segment Types" to be defined under the top-level "Segment Routing 1297 Parameters" registry. This sub-registry maintains the alphabetic 1298 identifiers for the segment types (as specified in section 4) that 1299 may be used within a Segment List of an SR Policy. This sub-registry 1300 would follow the Specification Required allocation policy as 1301 specified in [RFC8126]. 1303 The initial registrations for this sub-registry are as follows: 1305 +-------+-----------------------------------------------+-----------+ 1306 | Value | Description | Reference | 1307 +-------+-----------------------------------------------+-----------+ 1308 | A | SR-MPLS Label | [This.ID] | 1309 | B | SRv6 SID | [This.ID] | 1310 | C | IPv4 Prefix with optional SR Algorithm | [This.ID] | 1311 | D | IPv6 Global Prefix with optional SR Algorithm | [This.ID] | 1312 | | for SR-MPLS | | 1313 | E | IPv4 Prefix with Local Interface ID | [This.ID] | 1314 | F | IPv4 Addresses for link endpoints as Local, | [This.ID] | 1315 | | Remote pair | | 1316 | G | IPv6 Prefix and Interface ID for link | [This.ID] | 1317 | | endpoints as Local, | | 1318 | | Remote pair for SR-MPLS | | 1319 | H | IPv6 Addresses for link endpoints as Local, | [This.ID] | 1320 | | Remote pair for SR-MPLS | | 1321 | I | IPv6 Global Prefix with optional SR Algorithm | [This.ID] | 1322 | | for SRv6 | | 1323 | J | IPv6 Prefix and Interface ID for link | [This.ID] | 1324 | | endpoints as Local, Remote pair for SRv6 | | 1325 | K | IPv6 Addresses for link endpoints as Local, | [This.ID] | 1326 | | Remote pair for SRv6 | | 1327 +-------+-----------------------------------------------+-----------+ 1329 Table 2: Initial IANA Registration 1331 11.1. Guidance for Designated Experts 1333 The Designated Expert (DE) is expected to ascertain the existence of 1334 suitable documentation (a specification) as described in [RFC8126] 1335 and to verify that the document is permanently and publicly 1336 available. The DE is also expected to check the clarity of purpose 1337 and use of the requested assignment. Additionally, the DE must 1338 verify that any request for one of these assignments has been made 1339 available for review and comment within the IETF: the DE will post 1340 the request to the SPRING Working Group mailing list (or a successor 1341 mailing list designated by the IESG). If the request comes from 1342 within the IETF, it should be documented in an Internet-Draft. 1343 Lastly, the DE must ensure that any other request for a code point 1344 does not conflict with work that is active or already published 1345 within the IETF. 1347 12. Acknowledgement 1349 The authors would like to thank Tarek Saad, Dhanendra Jain, Ruediger 1350 Geib and Rob Shakir for their valuable comments and suggestions. 1352 13. Contributors 1354 The following people have contributed to this document: 1356 Ketan Talaulikar 1357 Cisco Systems 1358 Email: ketant@cisco.com 1360 Zafar Ali 1361 Cisco Systems 1362 Email: zali@cisco.com 1364 Jose Liste 1365 Cisco Systems 1366 Email: jliste@cisco.com 1368 Francois Clad 1369 Cisco Systems 1370 Email: fclad@cisco.com 1372 Kamran Raza 1373 Cisco Systems 1374 Email: skraza@cisco.com 1376 Shraddha Hegde 1377 Juniper Networks 1378 Email: shraddha@juniper.net 1380 Steven Lin 1381 Google, Inc. 1382 Email: stevenlin@google.com 1383 Przemyslaw Krol 1384 Google, Inc. 1385 Email: pkrol@google.com 1387 Martin Horneffer 1388 Deutsche Telekom 1389 Email: martin.horneffer@telekom.de 1391 Dirk Steinberg 1392 Steinberg Consulting 1393 Email: dws@steinbergnet.net 1395 Bruno Decraene 1396 Orange Business Services 1397 Email: bruno.decraene@orange.com 1399 Stephane Litkowski 1400 Orange Business Services 1401 Email: stephane.litkowski@orange.com 1403 Luay Jalil 1404 Verizon 1405 Email: luay.jalil@verizon.com 1407 14. References 1409 14.1. Normative References 1411 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1412 Requirement Levels", BCP 14, RFC 2119, 1413 DOI 10.17487/RFC2119, March 1997, 1414 . 1416 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1417 IANA Considerations Section in RFCs", RFC 5226, 1418 DOI 10.17487/RFC5226, May 2008, 1419 . 1421 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1422 Writing an IANA Considerations Section in RFCs", BCP 26, 1423 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1424 . 1426 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 1427 Decraene, B., Litkowski, S., and R. Shakir, "Segment 1428 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 1429 July 2018, . 1431 14.2. Informative References 1433 [I-D.ali-spring-sr-traffic-accounting] 1434 Ali, Z., Filsfils, C., Talaulikar, K., Sivabalan, S., 1435 Horneffer, M., Raszuk, R., Litkowski, S., and D. Voyer, 1436 "Traffic Accounting in Segment Routing Networks", draft- 1437 ali-spring-sr-traffic-accounting-03 (work in progress), 1438 August 2019. 1440 [I-D.anand-spring-poi-sr] 1441 Anand, M., Bardhan, S., Subrahmaniam, R., Tantsura, J., 1442 Mukhopadhyaya, U., and C. Filsfils, "Packet-Optical 1443 Integration in Segment Routing", draft-anand-spring-poi- 1444 sr-08 (work in progress), July 2019. 1446 [I-D.bashandy-rtgwg-segment-routing-ti-lfa] 1447 Bashandy, A., Filsfils, C., Decraene, B., Litkowski, S., 1448 Francois, P., daniel.voyer@bell.ca, d., Clad, F., and P. 1449 Camarillo, "Topology Independent Fast Reroute using 1450 Segment Routing", draft-bashandy-rtgwg-segment-routing-ti- 1451 lfa-05 (work in progress), October 2018. 1453 [I-D.filsfils-spring-sr-policy-considerations] 1454 Filsfils, C., Talaulikar, K., Krol, P., Horneffer, M., and 1455 P. Mattes, "SR Policy Implementation and Deployment 1456 Considerations", draft-filsfils-spring-sr-policy- 1457 considerations-04 (work in progress), October 2019. 1459 [I-D.filsfils-spring-sr-traffic-counters] 1460 Filsfils, C., Ali, Z., Horneffer, M., 1461 daniel.voyer@bell.ca, d., Durrani, M., and R. Raszuk, 1462 "Segment Routing Traffic Accounting Counters", draft- 1463 filsfils-spring-sr-traffic-counters-00 (work in progress), 1464 June 2018. 1466 [I-D.filsfils-spring-srv6-network-programming] 1467 Filsfils, C., Camarillo, P., Leddy, J., 1468 daniel.voyer@bell.ca, d., Matsushima, S., and Z. Li, "SRv6 1469 Network Programming", draft-filsfils-spring-srv6-network- 1470 programming-07 (work in progress), February 2019. 1472 [I-D.ietf-idr-bgp-ls-segment-routing-msd] 1473 Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G., 1474 and N. Triantafillis, "Signaling MSD (Maximum SID Depth) 1475 using Border Gateway Protocol Link-State", draft-ietf-idr- 1476 bgp-ls-segment-routing-msd-09 (work in progress), October 1477 2019. 1479 [I-D.ietf-idr-bgpls-segment-routing-epe] 1480 Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray, 1481 S., and J. Dong, "BGP-LS extensions for Segment Routing 1482 BGP Egress Peer Engineering", draft-ietf-idr-bgpls- 1483 segment-routing-epe-19 (work in progress), May 2019. 1485 [I-D.ietf-idr-segment-routing-te-policy] 1486 Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., 1487 Rosen, E., Jain, D., and S. Lin, "Advertising Segment 1488 Routing Policies in BGP", draft-ietf-idr-segment-routing- 1489 te-policy-08 (work in progress), November 2019. 1491 [I-D.ietf-idr-te-lsp-distribution] 1492 Previdi, S., Talaulikar, K., Dong, J., Chen, M., Gredler, 1493 H., and J. Tantsura, "Distribution of Traffic Engineering 1494 (TE) Policies and State using BGP-LS", draft-ietf-idr-te- 1495 lsp-distribution-12 (work in progress), October 2019. 1497 [I-D.ietf-isis-segment-routing-msd] 1498 Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 1499 "Signaling MSD (Maximum SID Depth) using IS-IS", draft- 1500 ietf-isis-segment-routing-msd-19 (work in progress), 1501 October 2018. 1503 [I-D.ietf-lsr-flex-algo] 1504 Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and 1505 A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex- 1506 algo-05 (work in progress), November 2019. 1508 [I-D.ietf-ospf-segment-routing-msd] 1509 Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 1510 "Signaling MSD (Maximum SID Depth) using OSPF", draft- 1511 ietf-ospf-segment-routing-msd-25 (work in progress), 1512 October 2018. 1514 [I-D.ietf-pce-segment-routing] 1515 Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 1516 and J. Hardwick, "PCEP Extensions for Segment Routing", 1517 draft-ietf-pce-segment-routing-16 (work in progress), 1518 March 2019. 1520 [I-D.sivabalan-pce-binding-label-sid] 1521 Sivabalan, S., Filsfils, C., Tantsura, J., Hardwick, J., 1522 Previdi, S., and C. Li, "Carrying Binding Label/Segment-ID 1523 in PCE-based Networks.", draft-sivabalan-pce-binding- 1524 label-sid-07 (work in progress), July 2019. 1526 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 1527 dual environments", RFC 1195, DOI 10.17487/RFC1195, 1528 December 1990, . 1530 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 1531 DOI 10.17487/RFC2328, April 1998, 1532 . 1534 [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 1535 (TE) Extensions to OSPF Version 2", RFC 3630, 1536 DOI 10.17487/RFC3630, September 2003, 1537 . 1539 [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route 1540 Reflection: An Alternative to Full Mesh Internal BGP 1541 (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006, 1542 . 1544 [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, 1545 "Multiprotocol Extensions for BGP-4", RFC 4760, 1546 DOI 10.17487/RFC4760, January 2007, 1547 . 1549 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 1550 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 1551 2008, . 1553 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 1554 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 1555 . 1557 [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The 1558 Locator/ID Separation Protocol (LISP)", RFC 6830, 1559 DOI 10.17487/RFC6830, January 2013, 1560 . 1562 [RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S. 1563 Previdi, "OSPF Traffic Engineering (TE) Metric 1564 Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015, 1565 . 1567 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 1568 S. Ray, "North-Bound Distribution of Link-State and 1569 Traffic Engineering (TE) Information Using BGP", RFC 7752, 1570 DOI 10.17487/RFC7752, March 2016, 1571 . 1573 [RFC7810] Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and 1574 Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions", 1575 RFC 7810, DOI 10.17487/RFC7810, May 2016, 1576 . 1578 [RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path 1579 Computation Element Communication Protocol (PCEP) 1580 Extensions for Stateful PCE", RFC 8231, 1581 DOI 10.17487/RFC8231, September 2017, 1582 . 1584 [RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path 1585 Computation Element Communication Protocol (PCEP) 1586 Extensions for PCE-Initiated LSP Setup in a Stateful PCE 1587 Model", RFC 8281, DOI 10.17487/RFC8281, December 2017, 1588 . 1590 Authors' Addresses 1592 Clarence Filsfils 1593 Cisco Systems, Inc. 1594 Pegasus Parc 1595 De kleetlaan 6a, DIEGEM BRABANT 1831 1596 BELGIUM 1598 Email: cfilsfil@cisco.com 1600 Siva Sivabalan (editor) 1601 Cisco Systems, Inc. 1602 2000 Innovation Drive 1603 Kanata, Ontario K2K 3E8 1604 Canada 1606 Email: msiva@cisco.com 1608 Daniel Voyer 1609 Bell Canada 1610 671 de la gauchetiere W 1611 Montreal, Quebec H3B 2M8 1612 Canada 1614 Email: daniel.voyer@bell.ca 1615 Alex Bogdanov 1616 Google, Inc. 1618 Email: bogdanov@google.com 1620 Paul Mattes 1621 Microsoft 1622 One Microsoft Way 1623 Redmond, WA 98052-6399 1624 USA 1626 Email: pamattes@microsoft.com