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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-22) exists of draft-ietf-spring-segment-routing-policy-13 == Outdated reference: A later version (-17) exists of draft-ietf-idr-segment-routing-te-policy-13 == Outdated reference: A later version (-17) exists of draft-ietf-idr-te-lsp-distribution-15 == Outdated reference: A later version (-20) exists of draft-ietf-lsr-flex-algo-17 == Outdated reference: A later version (-15) exists of draft-ietf-pce-binding-label-sid-11 Summary: 0 errors (**), 0 flaws (~~), 6 warnings (==), 1 comment (--). 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: Informational Cisco Systems, Inc. 5 Expires: April 25, 2022 P. Krol 6 Google, Inc. 7 M. Horneffer 8 Deutsche Telekom 9 P. Mattes 10 Microsoft 11 October 22, 2021 13 SR Policy Implementation and Deployment Considerations 14 draft-filsfils-spring-sr-policy-considerations-08 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. SR Policy framework enables the instantiation and 21 the management of necessary state on the headend node for flows along 22 a source routed paths using an ordered list of segments associated 23 with their specific SR Policies. This document describes some of the 24 implementation and deployment aspects that are useful for 25 operationalizing the SR Policy architecture. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on April 25, 2022. 44 Copyright Notice 46 Copyright (c) 2021 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (https://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 2. SR Policy Headend Architecture . . . . . . . . . . . . . . . 3 63 3. Dynamic Path Computation . . . . . . . . . . . . . . . . . . 4 64 3.1. Optimization Objective . . . . . . . . . . . . . . . . . 4 65 3.2. Constraints . . . . . . . . . . . . . . . . . . . . . . . 5 66 3.3. SR Native Algorithm . . . . . . . . . . . . . . . . . . . 6 67 3.4. Path to SID . . . . . . . . . . . . . . . . . . . . . . . 7 68 4. Candidate Path Selection . . . . . . . . . . . . . . . . . . 7 69 5. Distributed and/or Centralized Control Plane . . . . . . . . 11 70 5.1. Distributed Control Plane within a single Link-State IGP 71 area . . . . . . . . . . . . . . . . . . . . . . . . . . 11 72 5.2. Distributed Control Plane across several Link-State IGP 73 areas . . . . . . . . . . . . . . . . . . . . . . . . . . 11 74 5.3. Centralized Control Plane . . . . . . . . . . . . . . . . 12 75 5.4. Distributed and Centralized Control Plane . . . . . . . . 12 76 6. Binding SID Aspects . . . . . . . . . . . . . . . . . . . . . 13 77 6.1. Benefits of Binding SID . . . . . . . . . . . . . . . . . 13 78 6.2. Centralized Discovery of available BSID . . . . . . . . . 14 79 7. Flex-Algorithm Based SR Policies . . . . . . . . . . . . . . 16 80 8. Layer 2 and Optical Transport . . . . . . . . . . . . . . . . 17 81 9. Security Considerations . . . . . . . . . . . . . . . . . . . 18 82 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 83 11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 18 84 12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18 85 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 86 13.1. Normative References . . . . . . . . . . . . . . . . . . 20 87 13.2. Informative References . . . . . . . . . . . . . . . . . 20 88 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 90 1. Introduction 92 Segment Routing (SR) allows a headend node to steer a packet flow 93 along any path. Intermediate per-flow states are eliminated with 94 source routing [RFC8402]. 96 The headend node steers a flow into a Segment Routing Policy (SR 97 Policy) by augmenting packet headers with the ordered list of 98 segments associated with that SR Policy. 99 [I-D.ietf-spring-segment-routing-policy] defines the SR Policy 100 architecture and details the concepts of SR Policy and steering into 101 an SR Policy. 103 This document describes some of the implementation aspects for SR 104 Policy framework which should be considered as suggestions. The same 105 behavior, as defined in [I-D.ietf-spring-segment-routing-policy], may 106 in fact be realized with other alternate approaches. The deployment 107 aspects described in this document are also meant to only serve as 108 guidelines. This document describes these aspects and other 109 considerations related to SR Policy concepts as they are important to 110 facilitate multi-vendor interoperable deployments for various SR 111 Policy use-cases. 113 These apply equally to the MPLS [RFC8660] and SRv6 [RFC8986] 114 instantiations of segment routing. 116 For reading simplicity, the illustrations are provided for the MPLS 117 instantiation. 119 2. SR Policy Headend Architecture 121 This section provides a conceptual overview of components (or 122 functions) that interact to implement SR Policy on a headend 124 +--------+ +--------+ 125 | BGP | | PCEP | 126 +--------+ +--------+ 127 \ / 128 +--------+ +----------+ +--------+ 129 | | | SR | | | 130 | CLI |--| Policy |--| NETCONF| 131 | | | | | | 132 +--------+ +----------+ +--------+ 133 | 134 +--------+ 135 | FIB | 136 +--------+ 138 Figure 1: SR Policy Architecture at a Headend 140 The SR Policy functionality at a headend can be implemented in an SR 141 Policy (SRP) process as illustrated in Figure 1 . 143 The SRP process interacts with other processes to learn candidate 144 paths. 146 The SRP process selects the active path of an SR Policy. 148 The SRP process interacts with the RIB/FIB process to install an 149 active SR Policy in the dataplane. 151 In order to validate explicit candidate paths and compute dynamic 152 candidate paths, the SRP process maintains an SR Database (SR-DB) as 153 specified in [I-D.ietf-spring-segment-routing-policy]. The SRP 154 process interacts with other processes as shown in Figure 2 to 155 collect the SR-DB information. 157 +--------+ +--------+ +--------+ 158 | BGP SR | | BGP-LS | | IGP | 159 | Policy | +--------+ +--------+ 160 +--------+ \ | / 161 +--------+ +-----------+ +--------+ 162 | PCEP |---| SRP |--| NETCONF| 163 +--------+ +-----------+ +--------+ 165 Figure 2: Topology/link-state database architecture 167 The SR Policy architecture supports both centralized and distributed 168 control-plane. 170 3. Dynamic Path Computation 172 A dynamic candidate path for SR Policy is specified as an 173 optimization objective and constraints and needs to be computed by 174 either the headend or a Path Computation Element (PCE). The 175 distributed or centralized computation aspect is described further in 176 Section 5. This section describes the computation aspects of a 177 dynamic path. 179 3.1. Optimization Objective 181 This document describes two optimization objectives: 183 o Min-Metric - requests computation of a solution Segment-List 184 optimized for a selected metric. 186 o Min-Metric with margin and maximum number of SIDs - Min-Metric 187 with two changes: a margin of by which two paths with similar 188 metrics would be considered equal, a constraint on the max number 189 of SIDs in the Segment-List. 191 The "Min-Metric" optimization objective requests to compute a 192 solution Segment-List such that packets flowing through the solution 193 Segment-List use ECMP-aware paths optimized for the selected metric. 194 The "Min-Metric" objective can be instantiated for the IGP metric 195 ([RFC1195] [RFC2328] [RFC5340]) xor the TE metric ([RFC5305] 196 [RFC3630]) xor the latency extended TE metric ([RFC8570] [RFC7471]). 197 This metric is called the O metric (the optimized metric) to 198 distinguish it from the IGP metric. The solution Segment-List must 199 be computed to minimize the number of SIDs and the number of Segment- 200 Lists. 202 If the selected O metric is the IGP metric and the headend and 203 tailend are in the same IGP domain, then the solution Segment-List is 204 made of the single prefix-SID of the tailend. 206 When the selected O metric is not the IGP metric, then the solution 207 Segment-List is made of prefix SIDs of intermediate nodes, Adjacency 208 SIDs along intermediate links and potentially Binding SIDs (BSIDs) of 209 intermediate policies. 211 In many deployments there are insignificant metric differences 212 between mostly equal path (e.g. a difference of 100 usec of latency 213 between two paths from NYC to SFO would not matter in most cases). 214 The "Min-Metric with margin" objective supports such requirement. 216 The "Min-Metric with margin and maximum number of SIDs" optimization 217 objective requests to compute a solution Segment-List such that 218 packets flowing through the solution Segment-List do not use a path 219 whose cumulative O metric is larger than the shortest-path O metric + 220 margin. 222 If this is not possible because of the number of SIDs constraint, 223 then one option is that the solution Segment-List minimizes the O 224 metric while meeting the maximum number of SID constraints (i.e. path 225 with the least value of O metric while using <= the number of SIDs 226 specified). The other default option is to not come up with a 227 solution unless the desired SLA is guaranteed. 229 Section 7 describes another approach for computing a solution 230 Segment-List consisting of a single segment when the O metric is not 231 the IGP metric by using the Flex Algorithm Prefix-SID of the tailend. 233 3.2. Constraints 235 The following constraints can be described: 237 o Inclusion and/or exclusion of TE affinity. 239 o Inclusion and/or exclusion of IP address. 241 o Inclusion and/or exclusion of SRLG. 243 o Inclusion and/or exclusion of admin-tag. 245 o Maximum accumulated metric (IGP, TE and latency). 247 o Maximum number of SIDs in the solution Segment-List. 249 o Maximum number of weighted Segment-Lists in the solution set. 251 o Diversity to another service instance (e.g., link, node, or SRLG 252 disjoint paths originating from different head-ends). 254 3.3. SR Native Algorithm 256 1----------------2----------------3 257 |\ / 258 | \ / 259 | 4-------------5-------------7 260 | \ /| 261 | +-----------6-----------+ | 262 8------------------------------9 264 Figure 3: Illustration used to describe SR native algorithm 266 Let us assume that all the links have the same IGP metric of 10 and 267 let us consider the dynamic path defined as: Min-Metric(from 1, to 3, 268 IGP metric, margin 0) with constraint "avoid link 2-to-3". 270 A classical circuit implementation would do: prune the graph, compute 271 the shortest-path, pick a single non-ECMP branch of the ECMP-aware 272 shortest-path and encode it as a Segment-List. The solution Segment- 273 List would be <4, 5, 7, 3>. 275 An SR-native algorithm would find a Segment-List that minimizes the 276 number of SIDs and maximize the use of all the ECMP branches along 277 the ECMP shortest path. In this illustration, the solution Segment- 278 List would be <7, 3>. 280 In the vast majority of SR use-cases, SR-native algorithms should be 281 preferred: they preserve the native ECMP of IP and they minimize the 282 dataplane header overhead. 284 In some specific use-case (e.g. TDM migration over IP where the 285 circuit notion prevails), one may prefer a classic circuit 286 computation followed by an encoding into SIDs (potentially only using 287 non-protected Adj SIDs that pin the path to specific links and avoid 288 ECMP to reflect the TDM paradigm). 290 SR-native algorithms are a local node behavior and are thus outside 291 the scope of this document. 293 3.4. Path to SID 295 Let us assume the below diagram where all the links have an IGP 296 metric of 10 and a TE metric of 10 except the link AB which has an 297 IGP metric of 20 and the link AD which has a TE metric of 100. Let 298 us consider the min-metric(from A, to D, TE metric, margin 0). 300 B---C 301 | | 302 A---D 304 Figure 4: Illustration used to describe path to SID conversion 306 The solution path to this problem is ABCD. 308 This path can be expressed in SIDs as where B and D are the 309 IGP prefix SIDs respectively associated with nodes B and D in the 310 diagram. 312 Indeed, from A, the IGP path to B is AB (IGP metric 20 better than 313 ADCB of IGP metric 30). From B, the IGP path to D is BCD (IGP metric 314 20 better than BAD of IGP metric 30). 316 While the details of the algorithm remain a local node behavior, a 317 high-level description follows: start at the headend and find an IGP 318 prefix SID that leads as far down the desired path as 319 possible(without using any link not included in the desired path). 320 If no prefix SID exists, use the Adj SID to the first neighbor along 321 the path. Restart from the node that was reached. 323 4. Candidate Path Selection 325 An SR Policy may have multiple candidate paths that are provisioned 326 or signaled [I-D.ietf-idr-segment-routing-te-policy] [RFC8664] from 327 one of more sources. The tie-breaker rules defined in 328 [I-D.ietf-spring-segment-routing-policy] result in determination of a 329 single "active path" in a formal definition. 331 This section describe some examples for the candidate path selection 332 based on the same rules. 334 Example 1: 336 Consider headend H where two candidate paths of the same SR Policy 337 are signaled via BGP 338 [I-D.ietf-idr-segment-routing-te-policy] and whose respective NLRIs 339 have the same route distinguishers: 341 NLRI A with distinguisher = RD1, color = C, endpoint = N, preference 342 P1. 344 NLRI B with distinguisher = RD1, color = C, endpoint = N, preference 345 P2. 347 o Because the NLRIs are identical (same distinguisher), BGP will 348 perform bestpath selection. Note that there are no changes to BGP 349 best path selection algorithm. 351 o H installs one advertisement as bestpath into the BGP table. 353 o A single advertisement is passed to the SR Policy instantiation 354 process. 356 o The SRP process does not perform any path selection. 358 Note that the candidate path's preference value does not have any 359 effect on the BGP bestpath selection process. 361 Example 2: 363 Consider headend H where two candidate paths of the same SR Policy 364 are signaled via BGP and whose respective NLRIs 365 have different route distinguishers: 367 NLRI A with distinguisher = RD1, color = C, endpoint = N, preference 368 P1. 370 NLRI B with distinguisher = RD2, color = C, endpoint = N, preference 371 P2. 373 o Because the NLRIs are different (different distinguisher), BGP 374 will not perform bestpath selection. 376 o H installs both advertisements into the BGP table. 378 o Both advertisements are passed to the SR Policy instantiation 379 process. 381 o SRP process at H selects the candidate path advertised by NLRI B 382 as the active path for the SR policy since P2 is greater than P1. 384 Note that the recommended approach is to use NLRIs with different 385 distinguishers when several candidate paths for the same SR Policy 386 (color, endpoint) are signaled via BGP to a headend. 388 Example 3: 390 Consider that a headend H learns two candidate paths of the same SR 391 Policy one signaled via BGP and another via Local 392 configuration. 394 NLRI A with distinguisher = RD1, color = C, endpoint = N, preference 395 P1. 397 Local "foo" with color = C, endpoint = N, preference P2. 399 o H installs NLRI A into the BGP table. 401 o NLRI A and "foo" are both passed to the SRP process. 403 o SRP process at H selects the candidate path indicated by "foo" as 404 the active path for the SR policy since P2 is greater than P1. 406 Now, let us consider cases, when an SR Policy has multiple valid 407 candidate paths with the same best preference, the SRP process at a 408 headend uses the rules described in 409 [I-D.ietf-spring-segment-routing-policy] section 2.9 to select the 410 active path. This is explained in the following examples: 412 Example 4: 414 Consider headend H with two candidate paths of the same SR Policy 415 and the same preference value received from the 416 same controller R and where RD2 is higher than RD1. 418 o NLRI A with distinguisher RD1, color C, endpoint N, preference 419 P1(selected as active path at time t0). 421 o NLRI B with distinguisher RD2 (RD2 is greater than RD1), color C, 422 endpoint N, preference P1 (passed to SR Policy instantiation 423 process at time t1 > t0). 425 After t1, SRP process at H selects candidate path associated with 426 NLRI B as active path of the SR policy since RD2 is higher than RD1. 427 Here the time when the headend receives the candidate path via BGP is 428 not a factor in the selection. 430 Note that, in such a scenario where there are redundant sessions to 431 the same controller, the recommended approach is to use the same RD 432 value for conveying the same candidate paths and let the BGP best 433 path algorithm pick the best path. 435 Example 5: 437 Consider headend H with two candidate paths of the same SR Policy 438 and the same preference value both received from 439 the same controller R and where RD2 is higher than RD1. 441 Consider also that headend H is configured to override the 442 discriminator tiebreaker specified in 443 [I-D.ietf-spring-segment-routing-policy] section 2.9 445 o NLRI A with distinguisher RD1, color C, endpoint N, preference P1 446 (selected as active path at time t0). 448 o NLRI B with distinguisher RD2, color C, endpoint N, preference P1 449 (passed to SR Policy instantiation process at time t1). 451 Even after t1, SRP process at H retains candidate path associated 452 with NLRI A as active path of the SR policy since the discriminator 453 tiebreaker is disabled at H. 455 Example 6: 457 Consider headend H with two candidate paths of the same SR Policy 458 and the same preference value. 460 o Local "foo" with color C, endpoint N, preference P1 (selected as 461 active path at time t0). 463 o NLRI A with distinguisher RD1, color C, endpoint N, preference P1 464 (passed to SRP process at time t1). 466 Even after t1, SRP process at H retains candidate path associated 467 with local candidate path "foo" as active path of the SR policy since 468 the Local protocol is preferred over BGP by default based on its 469 higher protocol identifier value. 471 Example 7: 473 Consider headend H with two candidate paths of the same SR Policy 474 and the same preference value but received via 475 NETCONF from two controllers R and S (where S > R) 477 o Path A from R with distinguisher D1, color C, endpoint N, 478 preference P1 (selected as active path at time t0). 480 o Path B from S with distinguisher D2, color C, endpoint N, 481 preference P1 (passed to SRP process at time t1). 483 Note that the NETCONF process sends both paths to the SRP process 484 since it does not have any tiebreaker logic. After t1, SRP process 485 at H selects candidate path associated with Path B as active path of 486 the SR policy. 488 5. Distributed and/or Centralized Control Plane 490 5.1. Distributed Control Plane within a single Link-State IGP area 492 Consider a single-area IGP with per-link latency measurement and 493 advertisement of the measured latency in the extended-TE IGP TLV. 495 A head-end H is configured with a single dynamic candidate path for 496 SR policy P with a low-latency optimization objective and endpoint E. 498 Clearly the SRP process at H learns the topology (and extended TE 499 latency information) from the IGP and computes the solution Segment- 500 List providing the low-latency path to E. 502 No centralized controller is involved in such a deployment. 504 The SR-DB at H only uses the Link-State Database (LSDB) provided by 505 the IGP. 507 5.2. Distributed Control Plane across several Link-State IGP areas 509 Consider a domain D composed of two link-state IGP single-area 510 instances (I1 and I2) where each sub-domain benefits from per-link 511 latency measurement and advertisement of the measured latency in the 512 related IGP. The link-state information of each IGP is advertised 513 via BGP-LS [RFC7752] towards a set of BGP-LS route reflectors (RR). 515 H is a headend in IGP I1 sub-domain and E is an endpoint in IGP I2 516 sub-domain. 518 Using a BGP-LS session to any BGP-LS RR, H's SRP process may learn 519 the link-state information of the remote domain I2. H can thus 520 compute the low-latency path from H to E as a solution Segment-List 521 that spans the two domains I1 and I2. 523 The SR-DB at H collects the LSDB from both sub-domains (I1 and I2). 525 No centralized controller is required. 527 5.3. Centralized Control Plane 529 Considering the same domain D as in the previous section, let us now 530 assume that H does not have a BGP-LS session to the BGP-LS RR's. 531 Instead, let us assume a controller "C" has at least one BGP-LS 532 session to the BGP-LS RR's. 534 The controller C learns the topology and extended latency information 535 from both sub-domains via BGP-LS. It computes a low-latency path 536 from H to E as a Segment-List and programs H with the 537 related explicit candidate path. 539 The headend H does not compute the solution Segment-List (it cannot). 540 The headend only validates the received explicit candidate path. 541 Most probably, the controller encodes the SID's of the Segment-List 542 with Type-1. In that case, The headend's validation simply consists 543 in resolving the first SID on an outgoing interface and next-hop. 545 The SR-DB at H only includes the LSDB provided by the IGP I1. 547 The SR-DB of the controller collects the LSDB from both sub- 548 domains(I1 and I2). 550 5.4. Distributed and Centralized Control Plane 552 Consider the same domain D as in the previous section. 554 H's SRP process is configured to associate color C1 with a low- 555 latency optimization objective. 557 H's BGP process is configured to steer a Route R/r of extended-color 558 community C1 and of next-hop N via an SR policy (N, C1). 560 Upon receiving a first BGP route of color C1 and of next-hop N, H 561 recognizes the need for an SR Policy (N, C1) with a low-latency 562 objective to N. As N is outside the SRTE DB of H, H requests a 563 controller to compute such Segment-List (e.g., PCEP [RFC8664]). 565 This is an example of hybrid control-plane: the BGP distributed 566 control plane signals the routes and their TE requirements. Upon 567 receiving these BGP routes, a local headend either computes the 568 solution Segment-List (entirely distributed when the endpoint is in 569 the SR-DB of the headend) else delegates the computation to a 570 controller (hybrid distributed/centralized control-plane). 572 The SR-DB at H only includes the LSDB provided by the IGP. 574 The SR-DB of the controller collects the LSDB from both sub-domains. 576 6. Binding SID Aspects 578 The Binding SID (BSID) is fundamental to Segment Routing. It 579 provides scaling, network opacity and service independence. 581 This section describes implementation and operational aspects related 582 to the Binding SID. 584 6.1. Benefits of Binding SID 586 A simplified illustration is provided on the basis of Figure 5 where 587 it is assumed that S, A, B, Data Center Interconnect DCI1 and DCI2 588 share the same IGP-SR instance in the data-center 1 (DC1). DCI1, 589 DCI2, C, D, E, F, G, DCI3 and DCI4 share the same IGP-SR domain in 590 the core. DCI3, DCI4, H, K and Z share the same IGP-SR domain in the 591 data-center 2 (DC2). 593 A---DCI1----C----D----E----DCI3---H 594 / | | \ 595 S | | Z 596 \ | | / 597 B---DCI2----F---------G----DCI4---K 598 <==DC1==><=========Core========><==DC2==> 600 Figure 5: A Simple Datacenter Topology 602 In this example, it is assumed no redistribution between the IGP's 603 and no presence of BGP-LU. The inter-domain communication is only 604 provided by SR through SR Policies. 606 The latency from S to DCI1 equals to DCI2. The latency from Z to 607 DCI3 equals to DCI4. All the intra-DC links have the same IGP metric 608 10. 610 The path DCI1, C, D, E, DCI3 has a lower latency and lower capacity 611 than the path DCI2, F, G, DCI4. 613 The IGP metrics of all the core links are set to 10 except the links 614 D-E which is set to 100. 616 A low-latency multi-domain policy from S to Z may be expressed as 617 where: 619 o DCI1 is the prefix SID of DCI1. 621 o BSID is the Binding SID bound to an SR policy 622 instantiated at DCI1. 624 o Z is the prefix SID of Z. 626 Without the use of an intermediate core SR Policy (efficiently 627 summarized by a single BSID), S would need to steer its low-latency 628 flow into the policy . 630 The use of a BSID (and the intermediate bound SR Policy) decreases 631 the number of segments imposed by the source. 633 A BSID acts as a stable anchor point which isolates one domain from 634 the churn of another domain. Upon topology changes within the core 635 of the network, the low-latency path from DCI1 to DCI3 may change. 636 While the path of an intermediate policy changes, its BSID does not 637 change. Hence the policy used by the source does not change, hence 638 the source is shielded from the churn in another domain. 640 A BSID provides opacity and independence between domains. The 641 administrative authority of the core domain may not want to share 642 information about its topology. The use of a BSID allows keeping the 643 service opaque. S is not aware of the details of how the low-latency 644 service is provided by the core domain. S is not aware of the need 645 of the core authority to temporarily change the intermediate path. 647 6.2. Centralized Discovery of available BSID 649 This section explains how controllers can discover the local SIDs 650 available at a node N so as to pick an explicit BSID for a SR Policy 651 to be instantiated at headend N. 653 Any controller can discover the following properties of a node N 654 (e.g., via BGP-LS , NETCONF etc.): 656 o its local topology [RFC7752]. 658 o its topology-related SIDs (Prefix SIDs, Adj SID and EPE SID 659 [RFC9085] [RFC9086]). 661 o its Segment Routing Label Block (SRLB). 663 o its SR Policies and their BSID ([RFC8664] 664 [I-D.ietf-pce-binding-label-sid] 665 [I-D.ietf-idr-te-lsp-distribution]). 667 Any controller can thus infer the available SIDs in the SRLB of any 668 node with the assumption that all SIDs allocated from the SRLB on 669 that node are being advertised by it via some protocols or mechanisms 670 to the controller. 672 As an example, a controller discovers the following characteristics 673 of N: SRLB (4000, 8000), 3 Adj SIDs (4001, 4002, 4003), 2 EPE SIDs 674 (4004, 4005) and 3 SRTE policies (whose BSIDs are respectively 4006, 675 4007 and 4008). This controller can deduce that the SRLB sub-range 676 (4009, 8000) is free for allocation. 678 A controller is not restricted to use the next numerically available 679 SID in the available SRLB sub-range. It can pick any label in the 680 subset of available labels. This random pick make the chance for a 681 collision unlikely. 683 An operator could also sub-allocate the SRLB between different 684 controllers (e.g. (4000-4499) to controller 1 and (4500-5000) to 685 controller 2). 687 Inter-controller state-synchronization may be used to avoid/detect 688 collision in BSID. 690 All these techniques make the likelihood of a collision between 691 different controllers very unlikely. 693 In the unlikely case of a collision, the controllers will detect it 694 through system alerts, BGP-LS reporting using 695 [I-D.ietf-idr-te-lsp-distribution] or PCEP notification [RFC8231]. 696 They then have the choice to continue the operation of their SR 697 Policy with the dynamically allocated BSID or re-try with another 698 explicit pick. 700 Note: in deployments where PCE Protocol (PCEP) is used between head- 701 end and controller (PCE), a head-end can report BSID as well as 702 policy attributes (e.g., type of disjointness) and operational and 703 administrative states to controller. Similarly, a controller can 704 also assign/update the BSID of a policy via PCEP when instantiating 705 or updating SR Policy. 707 7. Flex-Algorithm Based SR Policies 709 SR allows for association of algorithms to Prefix SIDs [RFC8402]. 710 [I-D.ietf-lsr-flex-algo] defines the IGP based Flex-Algorithm 711 solution which allows IGPs themselves to compute constraint based 712 paths over the network. Prefix SIDs for the specific flex-algorithm 713 and associated with a node are used in the forwarding plane to steer 714 along the specific constraint path to that node. 716 As specified in [RFC8402] these IGP Flex Algo Prefix SIDs can be used 717 as segments within SR Policies thereby leveraging the underlying IGP 718 Flex Algo solution. 720 1--RED--2-------6 721 | | | 722 4-------3--RED--9 724 Figure 6: Illustration for Flex-Alg SID 726 Now let us assume that 728 o 1, 2, 3 and 4 are part of IGP 1. 730 o 2, 6, 9 and 3 are part of IGP 2. 732 o All the IGP link costs are 10. 734 o Links 1to2 and 3to9 are colored with IGP Link Affinity Red. 736 o Flex-Alg1 is defined in both IGPs as: avoid red, minimize IGP 737 metric. 739 o All nodes of each IGP domain are enabled for FlexAlg1 741 o SID(k, 0) represents the Prefix SID of node k according to Alg=0. 743 o SID(k, FlexAlg1) represents the Prefix SID of node k according to 744 Flex-Alg1. 746 A controller can steer a flow from 1 to 9 through an end-to-end path 747 that avoids the RED links of both IGP domains thanks to the explicit 748 SR Policy . 750 8. Layer 2 and Optical Transport 752 1----2----3----4----5 753 I2(lambda L241)\ / I4(lambda L241) 754 Optical 756 Figure 7: SR Policy with integrated DWDM 758 An explicit candidate path can express a path through a transport 759 layer beneath IP (ATM, FR, DWDM). The transport layer could be ATM, 760 FR, DWDM, back-to-back Ethernet etc. The transport path is modelled 761 as a link between two IP nodes with the specific assumption that no 762 distributed IP routing protocol runs over the link. The link may 763 have IP address or be IP unnumbered. Depending on the transport 764 protocol case, the link can be a physical DWDM interface and a lambda 765 (integrated solution), an Ethernet interface and a VLAN, an ATM 766 interface with a VPI/VCI, a FR interface with a DLCI etc. 768 Using the DWDM integrated use-case of Figure 7 as an illustration, 769 let us assume 771 o nodes 1, 2, 3, 4 and 5 are IP routers running an SR-enable IGP on 772 the links 1-2, 2-3, 3-4 and 4-5. 774 o The SRGB is homogeneous (16000, 24000). 776 o Node K's prefix SID is 16000+K. 778 o node 2 has an integrated DWDM interface I2 with Lambda L1. 780 o node 4 has an integrated DWDM interface I4 with Lambda L2. 782 o the optical network is provisioned with a circuit from 2 to 4 with 783 continuous lambda L241 (details outside the scope of this 784 document). 786 o Node 2 is provisioned with an SR policy with Segment-List 787 and Binding SID B where I2(L241) is of type 5 (IPv4) or 788 type 7 (IPv6), see section 4 of 789 [I-D.ietf-spring-segment-routing-policy] . 791 o node 1 steers a packet P1 towards the prefix SID of node 5 792 (16005). 794 o node 1 steers a packet P2 on the SR policy <16002, B, 16005>. 796 In such a case, the journey of P1 will be 1-2-3-4-5 while the journey 797 of P2 will be 1-2-lambda(L241)-4-5. P2 skips the IP hop 3 and 798 leverages the DWDM circuit from node 2 to node 4. P1 follows the 799 shortest-path computed by the distributed routing protocol. The path 800 of P1 is unaltered by the addition, modification or deletion of 801 optical bypass circuits. 803 The salient point of this example is that the SR Policy architecture 804 seamlessly support explicit candidate paths through any transport 805 sub-layer. 807 BGP-LS Extensions to describe the sub-IP-layer characteristics of the 808 SR Policy are out of scope of this document (e.g. in Figure 7, the 809 DWDM characteristics of the SR Policy at node 2 in terms of latency, 810 loss, security, domain/country traversed by the circuit etc.). 812 Further details of the SR Policy use-case for Packet Optical networks 813 are specified in [I-D.anand-spring-poi-sr] . 815 9. Security Considerations 817 The security considerations related to Segment Routing architecture 818 are described in [RFC8402] and for SR Policy architecture are 819 described in [I-D.ietf-spring-segment-routing-policy] and they apply 820 to this document as well. 822 10. IANA Considerations 824 This document has no actions for IANA. 826 11. Acknowledgement 828 The authors like to thank Tarek Saad, Dhanendra Jain, Muhammad 829 Durrani and Rob Shakir for their valuable comments and suggestions. 831 12. Contributors 833 The following people have contributed to this document: 835 Siva Sivabalan 836 Cisco Systems 837 Email: msiva@cisco.com 839 Zafar Ali 840 Cisco Systems 841 Email: zali@cisco.com 842 Jose Liste 843 Cisco Systems 844 Email: jliste@cisco.com 846 Francois Clad 847 Cisco Systems 848 Email: fclad@cisco.com 850 Kamran Raza 851 Cisco Systems 852 Email: skraza@cisco.com 854 Shraddha Hegde 855 Juniper Networks 856 Email: shraddha@juniper.net 858 Steven Lin 859 Google, Inc. 860 Email: stevenlin@google.com 862 Alex Bogdanov 863 Google, Inc. 864 Email: bogdanov@google.com 866 Daniel Voyer 867 Bell Canada 868 Email: daniel.voyer@bell.ca 870 Dirk Steinberg 871 Steinberg Consulting 872 Email: dws@steinbergnet.net 874 Bruno Decraene 875 Orange Business Services 876 Email: bruno.decraene@orange.com 878 Stephane Litkowski 879 Orange Business Services 880 Email: stephane.litkowski@orange.com 882 Luay Jalil 883 Verizon 884 Email: luay.jalil@verizon.com 886 13. References 888 13.1. Normative References 890 [I-D.ietf-spring-segment-routing-policy] 891 Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and 892 P. Mattes, "Segment Routing Policy Architecture", draft- 893 ietf-spring-segment-routing-policy-13 (work in progress), 894 May 2021. 896 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 897 Decraene, B., Litkowski, S., and R. Shakir, "Segment 898 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 899 July 2018, . 901 13.2. Informative References 903 [I-D.anand-spring-poi-sr] 904 Anand, M., Bardhan, S., Subrahmaniam, R., Tantsura, J., 905 Mukhopadhyaya, U., and C. Filsfils, "Packet-Optical 906 Integration in Segment Routing", draft-anand-spring-poi- 907 sr-08 (work in progress), July 2019. 909 [I-D.ietf-idr-segment-routing-te-policy] 910 Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., 911 Rosen, E., Jain, D., and S. Lin, "Advertising Segment 912 Routing Policies in BGP", draft-ietf-idr-segment-routing- 913 te-policy-13 (work in progress), June 2021. 915 [I-D.ietf-idr-te-lsp-distribution] 916 Previdi, S., Talaulikar, K., Dong, J., Chen, M., Gredler, 917 H., and J. Tantsura, "Distribution of Traffic Engineering 918 (TE) Policies and State using BGP-LS", draft-ietf-idr-te- 919 lsp-distribution-15 (work in progress), May 2021. 921 [I-D.ietf-lsr-flex-algo] 922 Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and 923 A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex- 924 algo-17 (work in progress), July 2021. 926 [I-D.ietf-pce-binding-label-sid] 927 Sivabalan, S., Filsfils, C., Tantsura, J., Previdi, S., 928 and C. L. (editor), "Carrying Binding Label/Segment 929 Identifier in PCE-based Networks.", draft-ietf-pce- 930 binding-label-sid-11 (work in progress), October 2021. 932 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 933 dual environments", RFC 1195, DOI 10.17487/RFC1195, 934 December 1990, . 936 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 937 DOI 10.17487/RFC2328, April 1998, 938 . 940 [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 941 (TE) Extensions to OSPF Version 2", RFC 3630, 942 DOI 10.17487/RFC3630, September 2003, 943 . 945 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 946 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 947 2008, . 949 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 950 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 951 . 953 [RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S. 954 Previdi, "OSPF Traffic Engineering (TE) Metric 955 Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015, 956 . 958 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 959 S. Ray, "North-Bound Distribution of Link-State and 960 Traffic Engineering (TE) Information Using BGP", RFC 7752, 961 DOI 10.17487/RFC7752, March 2016, 962 . 964 [RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path 965 Computation Element Communication Protocol (PCEP) 966 Extensions for Stateful PCE", RFC 8231, 967 DOI 10.17487/RFC8231, September 2017, 968 . 970 [RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward, 971 D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE) 972 Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March 973 2019, . 975 [RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., 976 Decraene, B., Litkowski, S., and R. Shakir, "Segment 977 Routing with the MPLS Data Plane", RFC 8660, 978 DOI 10.17487/RFC8660, December 2019, 979 . 981 [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 982 and J. Hardwick, "Path Computation Element Communication 983 Protocol (PCEP) Extensions for Segment Routing", RFC 8664, 984 DOI 10.17487/RFC8664, December 2019, 985 . 987 [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, 988 D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 989 (SRv6) Network Programming", RFC 8986, 990 DOI 10.17487/RFC8986, February 2021, 991 . 993 [RFC9085] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler, 994 H., and M. Chen, "Border Gateway Protocol - Link State 995 (BGP-LS) Extensions for Segment Routing", RFC 9085, 996 DOI 10.17487/RFC9085, August 2021, 997 . 999 [RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K., 1000 Ray, S., and J. Dong, "Border Gateway Protocol - Link 1001 State (BGP-LS) Extensions for Segment Routing BGP Egress 1002 Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August 1003 2021, . 1005 Authors' Addresses 1007 Clarence Filsfils 1008 Cisco Systems, Inc. 1009 Pegasus Parc 1010 De kleetlaan 6a, DIEGEM BRABANT 1831 1011 BELGIUM 1013 Email: cfilsfil@cisco.com 1015 Ketan Talaulikar (editor) 1016 Cisco Systems, Inc. 1018 Email: ketant.ietf@gmail.com 1020 Przemyslaw Krol 1021 Google, Inc. 1023 Email: pkrol@google.com 1024 Martin Horneffer 1025 Deutsche Telekom 1027 Email: martin.horneffer@telekom.de 1029 Paul Mattes 1030 Microsoft 1031 One Microsoft Way 1032 Redmond, WA 98052-6399 1033 USA 1035 Email: pamattes@microsoft.com