idnits 2.17.1 draft-ietf-lsr-flex-algo-17.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (July 6, 2021) is 1022 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-19) exists of draft-ietf-lsr-isis-srv6-extensions-14 -- Possible downref: Non-RFC (?) normative reference: ref. 'ISO10589' ** Obsolete normative reference: RFC 8919 (Obsoleted by RFC 9479) ** Obsolete normative reference: RFC 8920 (Obsoleted by RFC 9492) == Outdated reference: A later version (-13) exists of draft-ietf-rtgwg-segment-routing-ti-lfa-06 Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Psenak, Ed. 3 Internet-Draft Cisco Systems 4 Intended status: Standards Track S. Hegde 5 Expires: January 7, 2022 Juniper Networks, Inc. 6 C. Filsfils 7 K. Talaulikar 8 Cisco Systems, Inc. 9 A. Gulko 10 Edward Jones 11 July 6, 2021 13 IGP Flexible Algorithm 14 draft-ietf-lsr-flex-algo-17 16 Abstract 18 IGP protocols traditionally compute best paths over the network based 19 on the IGP metric assigned to the links. Many network deployments 20 use RSVP-TE based or Segment Routing based Traffic Engineering to 21 steer traffic over a path that is computed using different metrics or 22 constraints than the shortest IGP path. This document proposes a 23 solution that allows IGPs themselves to compute constraint-based 24 paths over the network. This document also specifies a way of using 25 Segment Routing (SR) Prefix-SIDs and SRv6 locators to steer packets 26 along the constraint-based paths. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on January 7, 2022. 45 Copyright Notice 47 Copyright (c) 2021 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (https://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4 64 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 65 4. Flexible Algorithm . . . . . . . . . . . . . . . . . . . . . 5 66 5. Flexible Algorithm Definition Advertisement . . . . . . . . . 6 67 5.1. IS-IS Flexible Algorithm Definition Sub-TLV . . . . . . . 6 68 5.2. OSPF Flexible Algorithm Definition TLV . . . . . . . . . 8 69 5.3. Common Handling of Flexible Algorithm Definition TLV . . 9 70 6. Sub-TLVs of IS-IS FAD Sub-TLV . . . . . . . . . . . . . . . . 10 71 6.1. IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV . . 10 72 6.2. IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV 11 73 6.3. IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV 12 74 6.4. IS-IS Flexible Algorithm Definition Flags Sub-TLV . . . . 12 75 6.5. IS-IS Flexible Algorithm Exclude SRLG Sub-TLV . . . . . . 13 76 7. Sub-TLVs of OSPF FAD TLV . . . . . . . . . . . . . . . . . . 14 77 7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV . . . 14 78 7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV . 15 79 7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV . 15 80 7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV . . . . 15 81 7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV . . . . . . 17 82 8. IS-IS Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . 17 83 9. OSPF Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . . 18 84 10. OSPF Flexible Algorithm ASBR Reachability Advertisement . . . 20 85 10.1. OSPFv2 Extended Inter-Area ASBR LSA . . . . . . . . . . 20 86 10.1.1. OSPFv2 Extended Inter-Area ASBR TLV . . . . . . . . 22 87 10.2. OSPF Flexible Algorithm ASBR Metric Sub-TLV . . . . . . 22 88 11. Advertisement of Node Participation in a Flex-Algorithm . . . 24 89 11.1. Advertisement of Node Participation for Segment Routing 25 90 11.2. Advertisement of Node Participation for Other 91 Applications . . . . . . . . . . . . . . . . . . . . . . 25 92 12. Advertisement of Link Attributes for Flex-Algorithm . . . . . 25 93 13. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 26 94 13.1. Multi-area and Multi-domain Considerations . . . . . . . 28 95 14. Flex-Algorithm and Forwarding Plane . . . . . . . . . . . . . 30 96 14.1. Segment Routing MPLS Forwarding for Flex-Algorithm . . . 31 97 14.2. SRv6 Forwarding for Flex-Algorithm . . . . . . . . . . . 31 98 14.3. Other Applications' Forwarding for Flex-Algorithm . . . 32 99 15. Operational Considerations . . . . . . . . . . . . . . . . . 32 100 15.1. Inter-area Considerations . . . . . . . . . . . . . . . 32 101 15.2. Usage of SRLG Exclude Rule with Flex-Algorithm . . . . . 33 102 15.3. Max-metric consideration . . . . . . . . . . . . . . . . 34 103 16. Backward Compatibility . . . . . . . . . . . . . . . . . . . 34 104 17. Security Considerations . . . . . . . . . . . . . . . . . . . 34 105 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 106 18.1. IGP IANA Considerations . . . . . . . . . . . . . . . . 35 107 18.1.1. IGP Algorithm Types Registry . . . . . . . . . . . . 35 108 18.1.2. IGP Metric-Type Registry . . . . . . . . . . . . . . 35 109 18.2. Flexible Algorithm Definition Flags Registry . . . . . . 36 110 18.3. IS-IS IANA Considerations . . . . . . . . . . . . . . . 36 111 18.3.1. Sub TLVs for Type 242 . . . . . . . . . . . . . . . 36 112 18.3.2. Sub TLVs for for TLVs 135, 235, 236, and 237 . . . . 36 113 18.3.3. Sub-Sub-TLVs for Flexible Algorithm Definition Sub- 114 TLV . . . . . . . . . . . . . . . . . . . . . . . . 36 115 18.4. OSPF IANA Considerations . . . . . . . . . . . . . . . . 37 116 18.4.1. OSPF Router Information (RI) TLVs Registry . . . . . 37 117 18.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs . . . . . . . . 38 118 18.4.3. OSPFv3 Extended-LSA Sub-TLVs . . . . . . . . . . . . 38 119 18.4.4. OSPF Flex-Algorithm Prefix Metric Bits . . . . . . . 38 120 18.4.5. OSPF Opaque LSA Option Types . . . . . . . . . . . . 38 121 18.4.6. OSPFv2 Externded Inter-Area ASBR TLVs . . . . . . . 39 122 18.4.7. OSPFv2 Inter-Area ASBR Sub-TLVs . . . . . . . . . . 39 123 18.4.8. OSPF Flexible Algorithm Definition TLV Sub-TLV 124 Registry . . . . . . . . . . . . . . . . . . . . . . 39 125 18.4.9. Link Attribute Applications Registry . . . . . . . . 41 126 19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 41 127 20. References . . . . . . . . . . . . . . . . . . . . . . . . . 41 128 20.1. Normative References . . . . . . . . . . . . . . . . . . 41 129 20.2. Informative References . . . . . . . . . . . . . . . . . 43 130 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45 132 1. Introduction 134 An IGP-computed path based on the shortest IGP metric is often be 135 replaced by a traffic-engineered path due to the traffic requirements 136 which are not reflected by the IGP metric. Some networks engineer 137 the IGP metric assignments in a way that the IGP metric reflects the 138 link bandwidth or delay. If, for example, the IGP metric is 139 reflecting the bandwidth on the link and the application traffic is 140 delay sensitive, the best IGP path may not reflect the best path from 141 such an application's perspective. 143 To overcome this limitation, various sorts of traffic engineering 144 have been deployed, including RSVP-TE and SR-TE, in which case the TE 145 component is responsible for computing paths based on additional 146 metrics and/or constraints. Such paths need to be installed in the 147 forwarding tables in addition to, or as a replacement for, the 148 original paths computed by IGPs. Tunnels are often used to represent 149 the engineered paths and mechanisms like one described in [RFC3906] 150 are used to replace the native IGP paths with such tunnel paths. 152 This document specifies a set of extensions to IS-IS, OSPFv2, and 153 OSPFv3 that enable a router to advertise TLVs that (a) identify 154 calculation-type, (b) specify a metric-type, and (c) describe a set 155 of constraints on the topology, that are to be used to compute the 156 best paths along the constrained topology. A given combination of 157 calculation-type, metric-type, and constraints is known as a 158 "Flexible Algorithm Definition". A router that sends such a set of 159 TLVs also assigns a Flex-Algorithm value to the specified combination 160 of calculation-type, metric-type, and constraints. 162 This document also specifies a way for a router to use IGPs to 163 associate one or more SR Prefix-SIDs or SRv6 locators with a 164 particular Flex-Algorithm. Each such Prefix-SID or SRv6 locator then 165 represents a path that is computed according to the identified Flex- 166 Algorithm. 168 2. Requirements Language 170 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 171 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 172 "OPTIONAL" in this document are to be interpreted as described in BCP 173 14 [RFC2119] [RFC8174] when, and only when, they appear in all 174 capitals, as shown here. 176 3. Terminology 178 This section defines terms that are often used in this document. 180 Flexible Algorithm Definition (FAD) - the set consisting of (a) 181 calculation-type, (b) metric-type, and (c) a set of constraints. 183 Flexible Algorithm - a numeric identifier in the range 128-255 that 184 is associated via configuration with the Flexible-Algorithm 185 Definition. 187 Local Flexible Algorithm Definition - Flexible Algorithm Definition 188 defined locally on the node. 190 Remote Flexible Algorithm Definition - Flexible Algorithm Definition 191 received from other nodes via IGP flooding. 193 Flexible Algorithm Participation - per application configuration 194 state that expresses whether the node is participating in a 195 particular Flexible Algorithm. 197 IGP Algorithm - value from the the "IGP Algorithm Types" registry 198 defined under "Interior Gateway Protocol (IGP) Parameters" IANA 199 registries. IGP Algorithms represents the triplet (Calculation Type, 200 Metric, Constraints), where the second and third elements of the 201 triple MAY be unspecified. 203 ABR - Area Border Router. In IS-IS terminology it is also known as 204 L1/L2 router. 206 ASBR - Autonomous System Border Router. 208 4. Flexible Algorithm 210 Many possible constraints may be used to compute a path over a 211 network. Some networks are deployed as multiple planes. A simple 212 form of constraint may be to use a particular plane. A more 213 sophisticated form of constraint can include some extended metric as 214 described in [RFC8570]. Constraints which restrict paths to links 215 with specific affinities or avoid links with specific affinities are 216 also possible. Combinations of these are also possible. 218 To provide maximum flexibility, we want to provide a mechanism that 219 allows a router to (a) identify a particular calculation-type, (b) 220 metric-type, (c) describe a particular set of constraints, and (d) 221 assign a numeric identifier, referred to as Flex-Algorithm, to the 222 combination of that calculation-type, metric-type, and those 223 constraints. We want the mapping between the Flex-Algorithm and its 224 meaning to be flexible and defined by the user. As long as all 225 routers in the domain have a common understanding as to what a 226 particular Flex-Algorithm represents, the resulting routing 227 computation is consistent and traffic is not subject to any looping. 229 The set consisting of (a) calculation-type, (b) metric-type, and (c) 230 a set of constraints is referred to as a Flexible-Algorithm 231 Definition. 233 Flexible-Algorithm is a numeric identifier in the range 128-255 that 234 is associated via configuratin with the Flexible-Algorithm 235 Definition. 237 IANA "IGP Algorithm Types" registry defines the set of values for IGP 238 Algorithms. We propose to allocate the following values for Flex- 239 Algorithms from this registry: 241 128-255 - Flex-Algorithms 243 5. Flexible Algorithm Definition Advertisement 245 To guarantee the loop-free forwarding for paths computed for a 246 particular Flex-Algorithm, all routers that (a) are configured to 247 participate in a particular Flex-Algorithm, and (b) are in the same 248 Flex-Algorithm definition advertisement scope MUST agree on the 249 definition of the Flex-Algorithm. 251 5.1. IS-IS Flexible Algorithm Definition Sub-TLV 253 The IS-IS Flexible Algorithm Definition Sub-TLV (FAD Sub-TLV) is used 254 to advertise the definition of the Flex-Algorithm. 256 The IS-IS FAD Sub-TLV is advertised as a Sub-TLV of the IS-IS Router 257 Capability TLV-242 that is defined in [RFC7981]. 259 IS-IS FAD Sub-TLV has the following format: 261 0 1 2 3 262 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 263 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 264 | Type | Length |Flex-Algorithm | Metric-Type | 265 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 266 | Calc-Type | Priority | 267 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 268 | Sub-TLVs | 269 + + 270 | ... | 272 | | 273 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 275 where: 277 Type: 26 279 Length: variable, dependent on the included Sub-TLVs 280 Flex-Algorithm: Single octet value between 128 and 255 inclusive. 282 Metric-Type: Type of metric to be used during the calculation. 283 Following values are defined: 285 0: IGP Metric 287 1: Min Unidirectional Link Delay as defined in [RFC8570], 288 section 4.2, encoded as application specific link attribute as 289 specified in [RFC8919] and Section 12 of this document. 291 2: Traffic Engineering Default Metric as defined in [RFC5305], 292 section 3.7, encoded as application specific link attribute as 293 specified in [RFC8919] and Section 12 of this document. 295 Calc-Type: value from 0 to 127 inclusive from the "IGP Algorithm 296 Types" registry defined under "Interior Gateway Protocol (IGP) 297 Parameters" IANA registries. IGP algorithms in the range of 0-127 298 have a defined triplet (Calculation Type, Metric, Constraints). 299 When used to specify the Calc-Type in the FAD Sub-TLV, only the 300 Calculation Type defined for the specified IGP Algorithm is used. 301 The Metric/Constraints MUST NOT be inherited. If the required 302 calculation type is Shortest Path First, the value 0 SHOULD appear 303 in this field. 305 Priority: Value between 0 and 255 inclusive that specifies the 306 priority of the advertisement. 308 Sub-TLVs - optional sub-TLVs. 310 The IS-IS FAD Sub-TLV MAY be advertised in an LSP of any number, but 311 a router MUST NOT advertise more than one IS-IS FAD Sub-TLV for a 312 given Flexible-Algorithm. A router receiving multiple IS-IS FAD Sub- 313 TLVs for a given Flexible-Algorithm from the same originator MUST 314 select the first advertisement in the lowest numbered LSP. 316 The IS-IS FAD Sub-TLV has an area scope. The Router Capability TLV 317 in which the FAD Sub-TLV is present MUST have the S-bit clear. 319 IS-IS L1/L2 router MAY be configured to re-generate the winning FAD 320 from level 2, without any modification to it, to level 1 area. The 321 re-generation of the FAD Sub-TLV from level 2 to level 1 is 322 determined by the L1/L2 router, not by the originator of the FAD 323 advertisement in the level 2. In such case, the re-generated FAD 324 Sub-TLV will be advertised in the level 1 Router Capability TLV 325 originated by the L1/L2 router. 327 L1/L2 router MUST NOT re-generate any FAD Sub-TLV from level 1 to 328 level 2. 330 5.2. OSPF Flexible Algorithm Definition TLV 332 OSPF FAD TLV is advertised as a top-level TLV of the RI LSA that is 333 defined in [RFC7770]. 335 OSPF FAD TLV has the following format: 337 0 1 2 3 338 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 339 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 340 | Type | Length | 341 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 342 |Flex-Algorithm | Metric-Type | Calc-Type | Priority | 343 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 344 | Sub-TLVs | 345 + + 346 | ... | 348 | | 349 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 351 where: 353 Type: 16 355 Length: variable, dependent on the included Sub-TLVs 357 Flex-Algorithm:: Flex-Algorithm number. Value between 128 and 255 358 inclusive. 360 Metric-Type: Type of metric to be used during the calculation. 361 Following values are defined: 363 0: IGP Metric 365 1: Min Unidirectional Link Delay as defined in [RFC7471], 366 section 4.2, encoded as application specific link attribute as 367 specified in [RFC8920] and Section 12 of this document. 369 2: Traffic Engineering metric as defined in [RFC3630], section 370 2.5.5, encoded as application specific link attribute as 371 specified in [RFC8920] and Section 12 of this document. 373 Calc-Type: as described in Section 5.1 374 Priority: as described in Section 5.1 376 Sub-TLVs - optional sub-TLVs. 378 When multiple OSPF FAD TLVs, for the same Flexible-Algorithm, are 379 received from a given router, the receiver MUST use the first 380 occurrence of the TLV in the Router Information LSA. If the OSPF FAD 381 TLV, for the same Flex-Algorithm, appears in multiple Router 382 Information LSAs that have different flooding scopes, the OSPF FAD 383 TLV in the Router Information LSA with the area-scoped flooding scope 384 MUST be used. If the OSPF FAD TLV, for the same algorithm, appears 385 in multiple Router Information LSAs that have the same flooding 386 scope, the OSPF FAD TLV in the Router Information (RI) LSA with the 387 numerically smallest Instance ID MUST be used and subsequent 388 instances of the OSPF FAD TLV MUST be ignored. 390 The RI LSA can be advertised at any of the defined opaque flooding 391 scopes (link, area, or Autonomous System (AS)). For the purpose of 392 OSPF FAD TLV advertisement, area-scoped flooding is REQUIRED. The 393 Autonomous System flooding scope SHOULD NOT be used by default unless 394 local configuration policy on the originating router indicates domain 395 wide flooding. 397 5.3. Common Handling of Flexible Algorithm Definition TLV 399 This section describes the protocol-independent handling of the FAD 400 TLV (OSPF) or FAD Sub-TLV (IS-IS). We will refer to it as FAD TLV in 401 this section, even though in the case of IS-IS it is a Sub-TLV. 403 The value of the Flex-Algorithm MUST be between 128 and 255 404 inclusive. If it is not, the FAD TLV MUST be ignored. 406 Only a subset of the routers participating in the particular Flex- 407 Algorithm need to advertise the definition of the Flex-Algorithm. 409 Every router, that is configured to participate in a particular Flex- 410 Algorithm, MUST select the Flex-Algorithm definition based on the 411 following ordered rules. This allows for the consistent Flex- 412 Algorithm definition selection in cases where different routers 413 advertise different definitions for a given Flex-Algorithm: 415 1. From the advertisements of the FAD in the area (including both 416 locally generated advertisements and received advertisements) 417 select the one(s) with the highest priority value. 419 2. If there are multiple advertisements of the FAD with the same 420 highest priority, select the one that is originated from the 421 router with the highest System-ID, in the case of IS-IS, or Router 422 ID, in the case of OSPFv2 and OSPFv3. For IS-IS, the System-ID is 423 described in [ISO10589]. For OSPFv2 and OSPFv3, standard Router 424 ID is described in [RFC2328] and [RFC5340] respectively. 426 A router that is not configured to participate in a particular Flex- 427 Algorithm MUST ignore FAD Sub-TLVs advertisements for such Flex- 428 Algorithm. 430 A router that is not participating in a particular Flex-Algorithm is 431 allowed to advertise FAD for such Flex-Algorithm. Receiving routers 432 MUST consider FAD advertisement regardless of the Flex-Algorithm 433 participation of the FAD originator. 435 Any change in the Flex-Algorithm definition may result in temporary 436 disruption of traffic that is forwarded based on such Flex-Algorithm 437 paths. The impact is similar to any other event that requires 438 network-wide convergence. 440 If a node is configured to participate in a particular Flexible- 441 Algorithm, but the selected Flex-Algorithm definition includes 442 calculation-type, metric-type, constraint, flag, or Sub-TLV that is 443 not supported by the node, it MUST stop participating in such 444 Flexible-Algorithm. That implies that it MUST NOT announce 445 participation for such Flexible-Algorithm as specified in Section 11 446 and it MUST remove any forwarding state associated with it. 448 Flex-Algorithm definition is topology independent. It applies to all 449 topologies that a router participates in. 451 6. Sub-TLVs of IS-IS FAD Sub-TLV 453 6.1. IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV 455 The Flexible Algorithm definition can specify 'colors' that are used 456 by the operator to exclude links during the Flex-Algorithm path 457 computation. 459 The IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV is used to 460 advertise the exclude rule that is used during the Flex-Algorithm 461 path calculation as specified in Section 13. 463 The IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub- 464 TLV) is a Sub-TLV of the IS-IS FAD Sub-TLV. It has the following 465 format: 467 0 1 2 3 468 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 469 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 470 | Type | Length | 471 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 472 | Extended Admin Group | 473 +- -+ 474 | ... | 475 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 476 where: 478 Type: 1 480 Length: variable, dependent on the size of the Extended Admin 481 Group. MUST be a multiple of 4 octets. 483 Extended Administrative Group: Extended Administrative Group as 484 defined in [RFC7308]. 486 The IS-IS FAEAG Sub-TLV MUST NOT appear more than once in an IS-IS 487 FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV 488 MUST be ignored by the receiver. 490 6.2. IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV 492 The Flexible Algorithm definition can specify 'colors' that are used 493 by the operator to include links during the Flex-Algorithm path 494 computation. 496 The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV is used 497 to advertise include-any rule that is used during the Flex-Algorithm 498 path calculation as specified in Section 13. 500 The format of the IS-IS Flexible Algorithm Include-Any Admin Group 501 Sub-TLV is identical to the format of the FAEAG Sub-TLV in 502 Section 6.1. 504 The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV Type is 505 2. 507 The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT 508 appear more than once in an IS-IS FAD Sub-TLV. If it appears more 509 than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver. 511 6.3. IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV 513 The Flexible Algorithm definition can specify 'colors' that are used 514 by the operator to include link during the Flex-Algorithm path 515 computation. 517 The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV is used 518 to advertise include-all rule that is used during the Flex-Algorithm 519 path calculation as specified in Section 13. 521 The format of the IS-IS Flexible Algorithm Include-All Admin Group 522 Sub-TLV is identical to the format of the FAEAG Sub-TLV in 523 Section 6.1. 525 The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV Type is 526 3. 528 The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT 529 appear more than once in an IS-IS FAD Sub-TLV. If it appears more 530 than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver. 532 6.4. IS-IS Flexible Algorithm Definition Flags Sub-TLV 534 The IS-IS Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV) 535 is a Sub-TLV of the IS-IS FAD Sub-TLV. It has the following format: 537 0 1 2 3 538 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 539 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 540 | Type | Length | 541 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 542 | Flags | 543 +- -+ 544 | ... | 545 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 546 where: 548 Type: 4 550 Length: variable, non-zero number of octets of the Flags field 552 Flags: 554 0 1 2 3 4 5 6 7... 555 +-+-+-+-+-+-+-+-+... 556 |M| | | ... 557 +-+-+-+-+-+-+-+-+... 559 M-flag: when set, the Flex-Algorithm specific prefix metric 560 MUST be used for inter-area and external prefix calculation. 561 This flag is not applicable to prefixes advertised as SRv6 562 locators. 564 Bits are defined/sent starting with Bit 0 defined above. Additional 565 bit definitions that may be defined in the future SHOULD be assigned 566 in ascending bit order so as to minimize the number of bits that will 567 need to be transmitted. 569 Undefined bits MUST be transmitted as 0. 571 Bits that are NOT transmitted MUST be treated as if they are set to 0 572 on receipt. 574 The IS-IS FADF Sub-TLV MUST NOT appear more than once in an IS-IS FAD 575 Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be 576 ignored by the receiver. 578 If the IS-IS FADF Sub-TLV is not present inside the IS-IS FAD Sub- 579 TLV, all the bits are assumed to be set to 0. 581 If a node is configured to participate in a particular Flexible- 582 Algorithm, but the selected Flex-Algorithm definition includes a bit 583 in the IS-IS FADF Sub-TLV that is not supported by the node, it MUST 584 stop participating in such Flexible-Algorithm. 586 6.5. IS-IS Flexible Algorithm Exclude SRLG Sub-TLV 588 The Flexible Algorithm definition can specify Shared Risk Link Groups 589 (SRLGs) that the operator wants to exclude during the Flex-Algorithm 590 path computation. 592 The IS-IS Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG) is used 593 to advertise the exclude rule that is used during the Flex-Algorithm 594 path calculation as specified in Section 13. 596 The IS-IS FAESRLG Sub-TLV is a Sub-TLV of the IS-IS FAD Sub-TLV. It 597 has the following format: 599 0 1 2 3 600 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 601 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 602 | Type | Length | 603 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 604 | Shared Risk Link Group Value | 605 +- -+ 606 | ... | 607 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 608 where: 610 Type: 5 612 Length: variable, dependent on number of SRLG values. MUST be a 613 multiple of 4 octets. 615 Shared Risk Link Group Value: SRLG value as defined in [RFC5307]. 617 The IS-IS FAESRLG Sub-TLV MUST NOT appear more than once in an IS-IS 618 FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV 619 MUST be ignored by the receiver. 621 7. Sub-TLVs of OSPF FAD TLV 623 7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV 625 The Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is 626 a Sub-TLV of the OSPF FAD TLV. It's usage is described in 627 Section 6.1. It has the following format: 629 0 1 2 3 630 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 631 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 632 | Type | Length | 633 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 634 | Extended Admin Group | 635 +- -+ 636 | ... | 637 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 638 where: 640 Type: 1 642 Length: variable, dependent on the size of the Extended Admin 643 Group. MUST be a multiple of 4 octets. 645 Extended Administrative Group: Extended Administrative Group as 646 defined in [RFC7308]. 648 The OSPF FAEAG Sub-TLV MUST NOT appear more than once in an OSPF FAD 649 TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored 650 by the receiver. 652 7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV 654 The usage of this Sub-TLVs is described in Section 6.2. 656 The format of the OSPF Flexible Algorithm Include-Any Admin Group 657 Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in 658 Section 7.1. 660 The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV Type is 661 2. 663 The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT 664 appear more than once in an OSPF FAD TLV. If it appears more than 665 once, the OSPF FAD TLV MUST be ignored by the receiver. 667 7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV 669 The usage of this Sub-TLVs is described in Section 6.3. 671 The format of the OSPF Flexible Algorithm Include-Any Admin Group 672 Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in 673 Section 7.1. 675 The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV Type is 676 3. 678 The OSPF Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT 679 appear more than once in an OSPF FAD TLV. If it appears more than 680 once, the OSPF FAD TLV MUST be ignored by the receiver. 682 7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV 684 The OSPF Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV) 685 is a Sub-TLV of the OSPF FAD TLV. It has the following format: 687 0 1 2 3 688 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 689 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 690 | Type | Length | 691 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 692 | Flags | 693 +- -+ 694 | ... | 695 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 696 where: 698 Type: 4 700 Length: variable, dependent on the size of the Flags field. MUST 701 be a multiple of 4 octets. 703 Flags: 705 0 1 2 3 4 5 6 7... 706 +-+-+-+-+-+-+-+-+... 707 |M| | | ... 708 +-+-+-+-+-+-+-+-+... 710 M-flag: when set, the Flex-Algorithm specific prefix and ASBR 711 metric MUST be used for inter-area and external prefix 712 calculation. This flag is not applicable to prefixes 713 advertised as SRv6 locators. 715 Bits are defined/sent starting with Bit 0 defined above. Additional 716 bit definitions that may be defined in the future SHOULD be assigned 717 in ascending bit order so as to minimize the number of bits that will 718 need to be transmitted. 720 Undefined bits MUST be transmitted as 0. 722 Bits that are NOT transmitted MUST be treated as if they are set to 0 723 on receipt. 725 The OSPF FADF Sub-TLV MUST NOT appear more than once in an OSPF FAD 726 TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored 727 by the receiver. 729 If the OSPF FADF Sub-TLV is not present inside the OSPF FAD TLV, all 730 the bits are assumed to be set to 0. 732 If a node is configured to participate in a particular Flexible- 733 Algorithm, but the selected Flex-Algorithm definition includes a bit 734 in the OSPF FADF Sub-TLV that is not supported by the node, it MUST 735 stop participating in such Flexible-Algorithm. 737 7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV 739 The OSPF Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG Sub-TLV) is 740 a Sub-TLV of the OSPF FAD TLV. Its usage is described in 741 Section 6.5. It has the following format: 743 0 1 2 3 744 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 745 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 746 | Type | Length | 747 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 748 | Shared Risk Link Group Value | 749 +- -+ 750 | ... | 751 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 752 where: 754 Type: 5 756 Length: variable, dependent on the number of SRLGs. MUST be a 757 multiple of 4 octets. 759 Shared Risk Link Group Value: SRLG value as defined in [RFC4203]. 761 The OSPF FAESRLG Sub-TLV MUST NOT appear more than once in an OSPF 762 FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be 763 ignored by the receiver. 765 8. IS-IS Flexible Algorithm Prefix Metric Sub-TLV 767 The IS-IS Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports 768 the advertisement of a Flex-Algorithm specific prefix metric 769 associated with a given prefix advertisement. 771 The IS-IS FAPM Sub-TLV is a sub-TLV of TLVs 135, 235, 236, and 237 772 and has the following format: 774 0 1 2 3 775 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 776 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 777 | Type | Length |Flex-Algorithm | 778 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 779 | Metric | 780 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 781 where: 783 Type: 6 785 Length: 5 octets 787 Flex-Algorithm: Single octet value between 128 and 255 inclusive. 789 Metric: 4 octets of metric information 791 The IS-IS FAPM Sub-TLV MAY appear multiple times in its parent TLV. 792 If it appears more than once with the same Flex-Algorithm value, the 793 first instance MUST be used and any subsequent instances MUST be 794 ignored. 796 If a prefix is advertised with a Flex-Algorithm prefix metric larger 797 then MAX_PATH_METRIC as defined in [RFC5305] this prefix MUST NOT be 798 considered during the Flexible-Algorithm computation. 800 The usage of the Flex-Algorithm prefix metric is described in 801 Section 13. 803 The IS-IS FAPM Sub-TLV MUST NOT be advertised as a sub-TLV of the IS- 804 IS SRv6 Locator TLV [I-D.ietf-lsr-isis-srv6-extensions]. The IS-IS 805 SRv6 Locator TLV includes the Algorithm and Metric fields which MUST 806 be used instead. If the FAPM Sub-TLV is present as a sub-TLV of the 807 IS-IS SRv6 Locator TLV in the received LSP, such FAPM Sub-TLV MUST be 808 ignored. 810 9. OSPF Flexible Algorithm Prefix Metric Sub-TLV 812 The OSPF Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the 813 advertisement of a Flex-Algorithm specific prefix metric associated 814 with a given prefix advertisement. 816 The OSPF Flex-Algorithm Prefix Metric (FAPM) Sub-TLV is a Sub-TLV of 817 the: 819 - OSPFv2 Extended Prefix TLV [RFC7684] 821 - Following OSPFv3 TLVs as defined in [RFC8362]: 823 Inter-Area Prefix TLV 825 External Prefix TLV 827 OSPF FAPM Sub-TLV has the following format: 829 0 1 2 3 830 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 831 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 832 | Type | Length | 833 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 834 |Flex-Algorithm | Flags | Reserved | 835 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 836 | Metric | 837 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 839 where: 841 Type: 3 for OSPFv2, 26 for OSPFv3 843 Length: 8 octets 845 Flex-Algorithm: Single octet value between 128 and 255 inclusive. 847 Flags: single octet value 849 0 1 2 3 4 5 6 7 850 +-+-+-+-+-+-+-+-+ 851 |E| | 852 +-+-+-+-+-+-+-+-+ 854 E bit : position 0: The type of external metric. If bit is 855 set, the metric specified is a Type 2 external metric. This 856 bit is applicable only to OSPF External and NSSA external 857 prefixes. This is semantically the same as E bit in section 858 A.4.5 of [RFC2328] and section A.4.7 of [RFC5340] for OSPFv2 859 and OSPFv3 respectively. 861 Bits 1 through 7: MUST be cleared by sender and ignored by 862 receiver. 864 Reserved: Must be set to 0, ignored at reception. 866 Metric: 4 octets of metric information 868 The OSPF FAPM Sub-TLV MAY appear multiple times in its parent TLV. 869 If it appears more than once with the same Flex-Algorithm value, the 870 first instance MUST be used and any subsequent instances MUST be 871 ignored. 873 The usage of the Flex-Algorithm prefix metric is described in 874 Section 13. 876 10. OSPF Flexible Algorithm ASBR Reachability Advertisement 878 An OSPF ABR advertises the reachability of ASBRs in its attached 879 areas to enable routers within those areas to perform route 880 calculations for external prefixes advertised by the ASBRs. OSPF 881 extensions for advertisement of Flex-Algorithm specific reachability 882 and metric for ASBRs is similarly required for Flex-Algorithm 883 external prefix computations as described further in Section 13.1. 885 10.1. OSPFv2 Extended Inter-Area ASBR LSA 887 The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) LSA is an OSPF Opaque 888 LSA [RFC5250] that is used to advertise additional attributes related 889 to the reachability of the OSPFv2 ASBR that is external to the area 890 yet internal to the OSPF domain. Semantically, the OSPFv2 EIA-ASBR 891 LSA is equivalent to the fixed format Type 4 Summary LSA [RFC2328]. 892 Unlike the Type 4 Summary LSA, the LSID of the EIA-ASBR LSA does not 893 carry the ASBR Router-ID - the ASBR Router-ID is carried in the body 894 of the LSA. OSPFv2 EIA-ASBR LSA is advertised by an OSPFv2 ABR and 895 its flooding is defined to be area-scoped only. 897 An OSPFv2 ABR generates the EIA-ASBR LSA for an ASBR when it is 898 advertising the Type-4 Summary LSA for it and has the need for 899 advertising additional attributes for that ASBR beyond what is 900 conveyed in the fixed format Type-4 Summary LSA. An OSPFv2 ABR MUST 901 NOT advertise the EIA-ASBR LSA for an ASBR for which it is not 902 advertising the Type 4 Summary LSA. This ensures that the ABR does 903 not generate the EIA-ASBR LSA for an ASBR to which it does not have 904 reachability in the base OSPFv2 topology calculation. The OSPFv2 ABR 905 SHOULD NOT advertise the EIA-ASBR LSA for an ASBR when it does not 906 have additional attributes to advertise for that ASBR. 908 The OSPFv2 EIA-ASBR LSA has the following format: 910 0 1 2 3 911 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 912 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 913 | LS age | Options | LS Type | 914 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 915 | Opaque Type | Opaque ID | 916 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 917 | Advertising Router | 918 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 919 | LS sequence number | 920 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 921 | LS checksum | Length | 922 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 923 | | 924 +- TLVs -+ 925 | ... | 927 The Opaque Type used by the OSPFv2 EIA-ASBR LSA is TBD (suggested 928 value 11). The Opaque Type is used to differentiate the various 929 types of OSPFv2 Opaque LSAs and is described in Section 3 of 930 [RFC5250]. The LS Type MUST be 10, indicating that the Opaque LSA 931 flooding scope is area-local [RFC5250]. The LSA Length field 932 [RFC2328] represents the total length (in octets) of the Opaque LSA, 933 including the LSA header and all TLVs (including padding). 935 The Opaque ID field is an arbitrary value used to maintain multiple 936 OSPFv2 EIA-ASBR LSAs. For OSPFv2 EIA-ASBR LSAs, the Opaque ID has no 937 semantic significance other than to differentiate OSPFv2 EIA-ASBR 938 LSAs originated by the same OSPFv2 ABR. If multiple OSPFv2 EIA-ASBR 939 LSAs specify the same ASBR, the attributes from the Opaque LSA with 940 the lowest Opaque ID SHOULD be used. 942 The format of the TLVs within the body of the OSPFv2 EIA-ASBR LSA is 943 the same as the format used by the Traffic Engineering Extensions to 944 OSPFv2 [RFC3630]. The variable TLV section consists of one or more 945 nested TLV tuples. Nested TLVs are also referred to as sub- TLVs. 946 The Length field defines the length of the value portion in octets 947 (thus, a TLV with no value portion would have a length of 0). The 948 TLV is padded to 4-octet alignment; padding is not included in the 949 Length field (so a 3-octet value would have a length of 3, but the 950 total size of the TLV would be 8 octets). Nested TLVs are also 951 32-bit aligned. For example, a 1-byte value would have the Length 952 field set to 1, and 3 octets of padding would be added to the end of 953 the value portion of the TLV. The padding is composed of zeros. 955 10.1.1. OSPFv2 Extended Inter-Area ASBR TLV 957 The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) TLV is a top-level TLV 958 of the OSPFv2 EIA-ASBR LSA and is used to advertise additional 959 attributes associated with the reachability of an ASBR. 961 The OSPFv2 EIA-ASBR TLV has the following format: 963 0 1 2 3 964 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 965 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 966 | Type | Length | 967 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 968 | ASBR Router ID | 969 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 970 . . 971 . Sub-TLVs . 972 . . 973 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 975 where: 977 Type: 1 979 Length: variable 981 ASBR Router ID: four octets carrying the OSPF Router ID of the 982 ASBR whose information is being carried. 984 Sub-TLVs : variable 986 Only a single OSPFv2 EIA-ASBR TLV MUST be advertised in each OSPFv2 987 EIA-ASBR LSA and the receiver MUST ignore all instances of this TLV 988 other than the first one in an LSA. 990 OSPFv2 EIA-ASBR TLV MUST be present inside an OSPFv2 EIA-ASBR LSA 991 with at least a single sub-TLV included, otherwise the OSPFv2 EIA- 992 ASBR LSA MUST be ignored by the receiver. 994 10.2. OSPF Flexible Algorithm ASBR Metric Sub-TLV 996 The OSPF Flexible Algorithm ASBR Metric (FAAM) Sub-TLV supports the 997 advertisement of a Flex-Algorithm specific metric associated with a 998 given ASBR reachability advertisement by an ABR. 1000 The OSPF Flex-Algorithm ASBR Metric (FAAM) Sub-TLV is a Sub-TLV of 1001 the: 1003 - OSPFv2 Extended Inter-Area ASBR TLV as defined in Section 10.1.1 1005 - OSPFv3 Inter-Area-Router TLV defined in [RFC8362] 1007 OSPF FAAM Sub-TLV has the following format: 1009 0 1 2 3 1010 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1011 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1012 | Type | Length | 1013 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1014 |Flex-Algorithm | Reserved | 1015 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1016 | Metric | 1017 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1019 where: 1021 Type: 1 for OSPFv2, TBD (suggested value 30) for OSPFv3 1023 Length: 8 octets 1025 Flex-Algorithm: Single octet value between 128 and 255 inclusive. 1027 Reserved: Must be set to 0, ignored at reception. 1029 Metric: 4 octets of metric information 1031 The OSPF FAAM Sub-TLV MAY appear multiple times in its parent TLV. 1032 If it appears more than once with the same Flex-Algorithm value, the 1033 first instance MUST be used and any subsequent instances MUST be 1034 ignored. 1036 The advertisement of the ASBR reachability using the OSPF FAAM Sub- 1037 TLV inside the OSPFv2 EIA-ASBR LSA follows the section 12.4.3 of 1038 [RFC2328] and inside the OSPFv3 E-Inter-Area-Router LSA follows the 1039 section 4.8.5 of [RFC5340]. The reachability of the ASBR is 1040 evaluated in the context of the specific Flex-Algorithm. 1042 The FAAM computed by the ABR will be equal to the metric to reach the 1043 ASBR for a given Flex-Algorithm in a source area or the cumulative 1044 metric via other ABR(s) when the ASBR is in a remote area. This is 1045 similar in nature to how the metric is set when the ASBR reachability 1046 metric is computed in the default algorithm for the metric in the 1047 OSPFv2 Type 4 ASBR Summary LSA and the OSPFv3 Inter-Area-Router LSA. 1049 An OSPF ABR MUST NOT include the OSPF FAAM Sub-TLV with a specific 1050 Flex-Algorithm in its reachability advertisement for an ASBR between 1051 areas unless that ASBR is reachable for it in the context of that 1052 specific Flex-Algorithm. 1054 An OSPF ABR MUST include the OSPF FAAM Sub-TLVs as part of the ASBR 1055 reachability advertisement between areas for the Flex-Algorithm for 1056 which the winning FAD includes the M-flag and the ASBR is reachable 1057 in the context of that specific Flex-Algorithm. 1059 OSPF routers MUST use the OSPF FAAM Sub-TLV to calculate the 1060 reachability of the ASBRs if the winning FAD for the specific Flex- 1061 Algorithm includes the M-flag. OSPF routers MUST NOT use the OSPF 1062 FAAM Sub-TLV to calculate the reachability of the ASBRs for the 1063 specific Flex-Algorithm if the winning FAD for such Flex-Algorithm 1064 does not include the M-flag. Instead, the OSPFv2 Type 4 Summary LSAs 1065 or the OSPFv3 Inter-Area-Router-LSAs MUST be used instead as 1066 specified in section 16.2 of [RFC2328] and section 4.8.5 of [RFC5340] 1067 for OSPFv2 and OSPFv3 respectively. 1069 The processing of the new or changed OSPF FAAM Sub-TLV triggers the 1070 processing of the External routes similar to what is described in 1071 section 16.5 of the [RFC2328] for OSPFv2 and section 4.8.5 of 1072 [RFC5340] for OSPFv3 for the specific Flex-Algorithm. The External 1073 and NSSA External route calculation should be limited to Flex- 1074 Algorithm(s) for which the winning FAD(s) includes the M-flag. 1076 Processing of the OSPF FAAM Sub-TLV does not require the existence of 1077 the equivalent OSPFv2 Type 4 Summary LSA or the OSPFv3 Inter-Area- 1078 Router-LSA that is advertised by the same ABR inside the area. When 1079 the OSPFv2 EIA-ASBR LSA or the OSPFv3 E-Inter-Area-Router-LSA are 1080 advertised along with the OSPF FAAM Sub-TLV by the ABR for a specific 1081 ASBR, it is expected that the same ABR would advertise the 1082 reachability of the same ASBR in the equivalent base LSAs - i.e., the 1083 OSPFv2 Type 4 Summary LSA or the OSPFv3 Inter-Area-Router-LSA. The 1084 presence of the base LSA is not mandatory for the usage of the 1085 extended LSA with the OSPF FAAM Sub-TLV. This means that the order 1086 in which these LSAs are received is not significant. 1088 11. Advertisement of Node Participation in a Flex-Algorithm 1090 When a router is configured to support a particular Flex-Algorithm, 1091 we say it is participating in that Flex-Algorithm. 1093 Paths computed for a specific Flex-Algorithm MAY be used by various 1094 applications, each potentially using its own specific data plane for 1095 forwarding traffic over such paths. To guarantee the presence of the 1096 application specific forwarding state associated with a particular 1097 Flex-Algorithm, a router MUST advertise its participation for a 1098 particular Flex-Algorithm for each application specifically. 1100 11.1. Advertisement of Node Participation for Segment Routing 1102 [RFC8667], [RFC8665], and [RFC8666] (IGP Segment Routing extensions) 1103 describe how the SR-Algorithm is used to compute the IGP best path. 1105 Routers advertise the support for the SR-Algorithm as a node 1106 capability as described in the above mentioned IGP Segment Routing 1107 extensions. To advertise participation for a particular Flex- 1108 Algorithm for Segment Routing, including both SR MPLS and SRv6, the 1109 Flex-Algorithm value MUST be advertised in the SR-Algorithm TLV 1110 (OSPF) or sub-TLV (IS-IS). 1112 Segment Routing Flex-Algorithm participation advertisement is 1113 topology independent. When a router advertises participation in an 1114 SR-Algorithm, the participation applies to all topologies in which 1115 the advertising node participates. 1117 11.2. Advertisement of Node Participation for Other Applications 1119 This section describes considerations related to how other 1120 applications can advertise their participation in a specific Flex- 1121 Algorithm. 1123 Application-specific Flex-Algorithm participation advertisements MAY 1124 be topology specific or MAY be topology independent, depending on the 1125 application itself. 1127 Application-specific advertisement for Flex-Algorithm participation 1128 MUST be defined for each application and is outside of the scope of 1129 this document. 1131 12. Advertisement of Link Attributes for Flex-Algorithm 1133 Various link attributes may be used during the Flex-Algorithm path 1134 calculation. For example, include or exclude rules based on link 1135 affinities can be part of the Flex-Algorithm definition as defined in 1136 Section 6 and Section 7. 1138 Link attribute advertisements that are to be used during Flex- 1139 Algorithm calculation MUST use the Application-Specific Link 1140 Attribute (ASLA) advertisements defined in [RFC8919] or [RFC8920], 1141 unless, in the case of IS-IS, the L-Flag is set in the ASLA 1142 advertisement. If the L-Flag is set, as defined in [RFC8919] 1143 Section 4.2 subject to the constraints discussed in Section 6 of the 1144 [[RFC8919], then legacy advertisements are to be used instead. 1146 The mandatory use of ASLA advertisements applies to link attributes 1147 specifically mentioned in this document (Min Unidirectional Link 1148 Delay, TE Default Metric, Administrative Group, Extended 1149 Administrative Group and Shared Risk Link Group) and any other link 1150 attributes that may be used in support of Flex-Algorithm in the 1151 future. 1153 A new Application Identifier Bit is defined to indicate that the ASLA 1154 advertisement is associated with the Flex-Algorithm application. 1155 This bit is set in the Standard Application Bit Mask (SABM) defined 1156 in [RFC8919] or [RFC8920]: 1158 Bit-3: Flexible Algorithm (X-bit) 1160 ASLA Admin Group Advertisements to be used by the Flexible Algorithm 1161 Application MAY use either the Administrative Group or Extended 1162 Administrative Group encodings. If the Administrative Group encoding 1163 is used, then the first 32 bits of the corresponding FAD sub-TLVs are 1164 mapped to the link attribute advertisements as specified in RFC 7308. 1166 A receiver supporting this specification MUST accept both ASLA 1167 Administrative Group and Extended Administrative Group TLVs as 1168 defined in [RFC8919] or [RFC8920]. In the case of ISIS, if the 1169 L-Flag is set in ASLA advertisement, as defined in [RFC8919] 1170 Section 4.2, then the receiver MUST be able to accept both 1171 Administrative Group TLV as defined in [RFC5305] and Extended 1172 Administrative Group TLV as defined in [RFC7308]. 1174 13. Calculation of Flexible Algorithm Paths 1176 A router MUST be configured to participate in a given Flex-Algorithm 1177 K and MUST select the FAD based on the rules defined in Section 5.3 1178 before it can compute any path for that Flex-Algorithm. 1180 As described in Section 11, participation for any particular Flex- 1181 Algorithm MUST be advertised on a per-application basis. Calculation 1182 of the paths for any particular Flex-Algorithm MUST be application 1183 specific. 1185 The way applications handle nodes that do not participate in 1186 Flexible-Algorithm is application specific. If the application only 1187 wants to consider participating nodes during the Flex-Algorithm 1188 calculation, then when computing paths for a given Flex-Algorithm, 1189 all nodes that do not advertise participation for that Flex-Algorithm 1190 in their application-specific advertisements MUST be pruned from the 1191 topology. Segment Routing, including both SR MPLS and SRv6, are 1192 applications that MUST use such pruning when computing Flex-Algorithm 1193 paths. 1195 When computing the path for a given Flex-Algorithm, the metric-type 1196 that is part of the Flex-Algorithm definition (Section 5) MUST be 1197 used. 1199 When computing the path for a given Flex-Algorithm, the calculation- 1200 type that is part of the Flex-Algorithm definition (Section 5) MUST 1201 be used. 1203 Various link include or exclude rules can be part of the Flex- 1204 Algorithm definition. To refer to a particular bit within an AG or 1205 EAG we use the term 'color'. 1207 Rules, in the order as specified below, MUST be used to prune links 1208 from the topology during the Flex-Algorithm computation. 1210 For all links in the topology: 1212 1. Check if any exclude AG rule is part of the Flex-Algorithm 1213 definition. If such exclude rule exists, check if any color that 1214 is part of the exclude rule is also set on the link. If such a 1215 color is set, the link MUST be pruned from the computation. 1217 2. Check if any exclude SRLG rule is part of the Flex-Algorithm 1218 definition. If such exclude rule exists, check if the link is 1219 part of any SRLG that is also part of the SRLG exclude rule. If 1220 the link is part of such SRLG, the link MUST be pruned from the 1221 computation. 1223 3. Check if any include-any AG rule is part of the Flex-Algorithm 1224 definition. If such include-any rule exists, check if any color 1225 that is part of the include-any rule is also set on the link. If 1226 no such color is set, the link MUST be pruned from the 1227 computation. 1229 4. Check if any include-all AG rule is part of the Flex-Algorithm 1230 definition. If such include-all rule exists, check if all colors 1231 that are part of the include-all rule are also set on the link. 1232 If all such colors are not set on the link, the link MUST be 1233 pruned from the computation. 1235 5. If the Flex-Algorithm definition uses other than IGP metric 1236 (Section 5), and such metric is not advertised for the particular 1237 link in a topology for which the computation is done, such link 1238 MUST be pruned from the computation. A metric of value 0 MUST NOT 1239 be assumed in such case. 1241 13.1. Multi-area and Multi-domain Considerations 1243 Any IGP Shortest Path Tree calculation is limited to a single area. 1244 This applies to Flex-Algorithm calculations as well. Given that the 1245 computing router does not have visibility of the topology of the next 1246 areas or domain, the Flex-Algorithm specific path to an inter-area or 1247 inter-domain prefix will be computed for the local area only. The 1248 egress L1/L2 router (ABR in OSPF), or ASBR for inter-domain case, 1249 will be selected based on the best path for the given Flex-Algorithm 1250 in the local area and such egress ABR or ASBR router will be 1251 responsible to compute the best Flex-Algorithm specific path over the 1252 next area or domain. This may produce an end-to-end path, which is 1253 sub-optimal based on Flex-Algorithm constraints. In cases where the 1254 ABR or ASBR has no reachability to a prefix for a given Flex- 1255 Algorithm in the next area or domain, the traffic may be dropped by 1256 the ABR/ASBR. 1258 To allow the optimal end-to-end path for an inter-area or inter- 1259 domain prefix for any Flex-Algorithm to be computed, the FAPM has 1260 been defined in Section 8 and Section 9. For external route 1261 calculation for prefixes originated by ASBRs in remote areas in OSPF, 1262 the FAAM has been defined in Section 10.2 for the ABR to indicate its 1263 ASBR reachability along with the metric for the specific Flex- 1264 Algorithm. 1266 If the FAD selected based on the rules defined in Section 5.3 1267 includes the M-flag, an ABR or ASBR MUST include the FAPM (Section 8, 1268 Section 9) when advertising the prefix, that is reachable in a given 1269 Flex-Algorithm, between areas or domains. Such metric will be equal 1270 to the metric to reach the prefix for that Flex-Algorithm in its 1271 source area or domain. This is similar in nature to how the metric 1272 is set when prefixes are advertised between areas or domains for the 1273 default algorithm. When a prefix is unreachable in its source area 1274 or domain in a specific Flex-Algorithm, then an ABR or ASBR MUST NOT 1275 include the FAPM for that Flex-Algorithm when advertising the prefix 1276 between areas or domains. 1278 If the FAD selected based on the rules defined in Section 5.3 1279 includes the M-flag, the FAPM MUST be used during the calculation of 1280 prefix reachability for the inter-area and external prefixes. If the 1281 FAPM for the Flex-Algorithm is not advertised with the inter-area or 1282 external prefix reachability advertisement, the prefix MUST be 1283 considered as unreachable for that Flex-Algorithm. Similarly in the 1284 case of OSPF, for ASBRs in remote areas, if the FAAM is not 1285 advertised by the local ABR(s), the ASBR MUST be considered as 1286 unreachable for that Flex-Algorithm and the external prefix 1287 advertisements from such an ASBR are not considered for that Flex- 1288 Algorithm. 1290 Flex-Algorithm prefix metrics and the OSPF Flex-Algorithm ASBR 1291 metrics MUST NOT be used during the Flex-Algorithm computation unless 1292 the FAD selected based on the rules defined in Section 5.3 includes 1293 the M-Flag, as described in (Section 6.4 or Section 7.4). 1295 In the case of OSPF, when calculating external routes in a Flex- 1296 Algorithm (with FAD selected includes the M-Flag) where the 1297 advertising ASBR is in a remote area, the metric will be the sum of 1298 the following: 1300 o the FAPM for that Flex-Algorithm advertised with the external 1301 route by the ASBR 1303 o the metric to reach the ASBR for that Flex-Algorithm from the 1304 local ABR i.e., the FAAM for that Flex-Algorithm advertised by the 1305 ABR in the local area for that ASBR 1307 o the Flex-Algorithm specific metric to reach the local ABR 1309 This is similar in nature to how the metric is calculated for routes 1310 learned from remote ASBRs in the default algorithm using the OSPFv2 1311 Type 4 ASBR Summary LSA and the OSPFv3 Inter-Area-Router LSA. 1313 If the FAD selected based on the rules defined in Section 5.3 does 1314 not includes the M-flag, then the IGP metrics associated with the 1315 prefix reachability advertisements used by the base IS-IS and OSPF 1316 protocol MUST be used for the Flex-Algorithm route computation. 1317 Similarly, in the case of external route calculations in OSPF, the 1318 ASBR reachability is determined based on the base OSPFv2 Type 4 1319 Summary LSA and the OSFPv3 Inter-Area-Router LSA. 1321 It is NOT RECOMMENDED to use the Flex-Algorithm for inter-area or 1322 inter-domain prefix reachability without the M-flag set. The reason 1323 is that without the explicit Flex-Algorithm Prefix Metric 1324 advertisement (and the Flex-Algorithm ASBR metric advertisement in 1325 the case of OSPF external route calculation), it is not possible to 1326 conclude whether the ABR or ASBR has reachability to the inter-area 1327 or inter-domain prefix for a given Flex-Algorithm in the next area or 1328 domain. Sending the Flex-Algoritm traffic for such prefix towards 1329 the ABR or ASBR may result in traffic looping or black-holing. 1331 During the route computation, it is possible for the Flex-Algorithm 1332 specific metric to exceed the maximum value that can be stored in an 1333 unsigned 32-bit variable. In such scenarios, the value MUST be 1334 considered to be of value 4,294,967,295 during the computation and 1335 advertised as such. 1337 The FAPM MUST NOT be advertised with IS-IS L1 or L2 intra-area, 1338 OSPFv2 intra-area, or OSPFv3 intra-area routes. If the FAPM is 1339 advertised for these route-types, it MUST be ignored during the 1340 prefix reachability calculation. 1342 The M-flag in FAD is not applicable to prefixes advertised as SRv6 1343 locators. The IS-IS SRv6 Locator TLV 1344 [I-D.ietf-lsr-isis-srv6-extensions] includes the Algorithm and Metric 1345 fields. When the SRv6 Locator is advertised between areas or 1346 domains, the metric field in the Locator TLV of IS-IS MUST be used 1347 irrespective of the M-flag in the FAD advertisement. 1349 OSPF external and NSSA external prefix advertisements MAY include a 1350 non-zero forwarding address in the prefix advertisements in the base 1351 protocol. In such a scenario, the Flex-Algorithm specific 1352 reachability of the external prefix is determined by Flex-Algorithm 1353 specific reachability of the forwarding address. 1355 In OSPF, the procedures for translation of NSSA external prefix 1356 advertisements into external prefix advertisements performed by an 1357 NSSA ABR [RFC3101] remain unchanged for Flex-Algorithm. An NSSA 1358 translator MUST include the OSPF FAPM Sub-TLVs for all Flex- 1359 Algorithms that are in the original NSSA external prefix 1360 advertisement from the NSSA ASBR in the translated external prefix 1361 advertisement generated by it regardless of its participation in 1362 those Flex-Algorithms or its having reachability to the NSSA ASBR in 1363 those Flex-Algorithms. 1365 An area could become partitioned from the perspective of the Flex- 1366 Algorithm due to the constraints and/or metric being used for it, 1367 while maintaining the continuity in the algorithm 0. When that 1368 happens, some destinations inside that area could become unreachable 1369 in that Flex-Algorithm. These destinations will not be able to use 1370 an inter-area path. This is the consequence of the fact that the 1371 inter-area prefix reachability advertisement would not be available 1372 for these intra-area destinations within the area. It is RECOMMENDED 1373 to avoid such partitioning by providing enough redundancy inside the 1374 area for each Flex-Algorithm being used. 1376 14. Flex-Algorithm and Forwarding Plane 1378 This section describes how Flex-Algorithm paths are used in 1379 forwarding. 1381 14.1. Segment Routing MPLS Forwarding for Flex-Algorithm 1383 This section describes how Flex-Algorithm paths are used with SR MPLS 1384 forwarding. 1386 Prefix SID advertisements include an SR-Algorithm value and, as such, 1387 are associated with the specified SR-Algorithm. Prefix-SIDs are also 1388 associated with a specific topology which is inherited from the 1389 associated prefix reachability advertisement. When the algorithm 1390 value advertised is a Flex-Algorithm value, the Prefix SID is 1391 associated with paths calculated using that Flex-Algorithm in the 1392 associated topology. 1394 A Flex-Algorithm path MUST be installed in the MPLS forwarding plane 1395 using the MPLS label that corresponds to the Prefix-SID that was 1396 advertised for that Flex-algorithm. If the Prefix SID for a given 1397 Flex-algorithm is not known, the Flex-Algorithm specific path cannot 1398 be installed in the MPLS forwarding plane. 1400 Traffic that is supposed to be routed via Flex-Algorithm specific 1401 paths, MUST be dropped when there are no such paths available. 1403 Loop Free Alternate (LFA) paths for a given Flex-Algorithm MUST be 1404 computed using the same constraints as the calculation of the primary 1405 paths for that Flex-Algorithm. LFA paths MUST only use Prefix-SIDs 1406 advertised specifically for the given algorithm. LFA paths MUST NOT 1407 use an Adjacency-SID that belongs to a link that has been pruned from 1408 the Flex-Algorithm computation. 1410 If LFA protection is being used to protect a given Flex-Algorithm 1411 paths, all routers in the area participating in the given Flex- 1412 Algorithm SHOULD advertise at least one Flex-Algorithm specific Node- 1413 SID. These Node-SIDs are used to steer traffic over the LFA computed 1414 backup path. 1416 14.2. SRv6 Forwarding for Flex-Algorithm 1418 This section describes how Flex-Algorithm paths are used with SRv6 1419 forwarding. 1421 In SRv6 a node is provisioned with topology/algorithm specific 1422 locators for each of the topology/algorithm pairs supported by that 1423 node. Each locator is an aggregate prefix for all SIDs provisioned 1424 on that node which have the matching topology/algorithm. 1426 The SRv6 locator advertisement in IS-IS 1427 [I-D.ietf-lsr-isis-srv6-extensions] includes the MTID value that 1428 associates the locator with a specific topology. SRv6 locator 1429 advertisements also includes an Algorithm value that explicitly 1430 associates the locator with a specific algorithm. When the algorithm 1431 value advertised with a locator represents a Flex-Algorithm, the 1432 paths to the locator prefix MUST be calculated using the specified 1433 Flex-Algorithm in the associated topology. 1435 Forwarding entries for the locator prefixes advertised in IS-IS MUST 1436 be installed in the forwarding plane of the receiving SRv6 capable 1437 routers when the associated topology/algorithm is participating in 1438 them. Forwarding entries for locators associated with Flex- 1439 Algorithms in which the node is not participating MUST NOT be 1440 installed in the forwarding plane. 1442 When the locator is associated with a Flex-Algorithm, LFA paths to 1443 the locator prefix MUST be calculated using such Flex-Algorithm in 1444 the associated topology, to guarantee that they follow the same 1445 constraints as the calculation of the primary paths. LFA paths MUST 1446 only use SRv6 SIDs advertised specifically for the given Flex- 1447 Algorithm. 1449 If LFA protection is being used to protect locators associated with a 1450 given Flex-Algorithm, all routers in the area participating in the 1451 given Flex-Algorithm SHOULD advertise at least one Flex-Algorithm 1452 specific locator and END SID per node and one END.X SID for every 1453 link that has not been pruned from such Flex-Algorithm computation. 1454 These locators and SIDs are used to steer traffic over the LFA- 1455 computed backup path. 1457 14.3. Other Applications' Forwarding for Flex-Algorithm 1459 Any application that wants to use Flex-Algorithm specific forwarding 1460 needs to install some form of Flex-Algorithm specific forwarding 1461 entries. 1463 Application-specific forwarding for Flex-Algorithm MUST be defined 1464 for each application and is outside of the scope of this document. 1466 15. Operational Considerations 1468 15.1. Inter-area Considerations 1470 The scope of the Flex-Algorithm computation is an area, so is the 1471 scope of the FAD. In IS-IS, the Router Capability TLV in which the 1472 FAD Sub-TLV is advertised MUST have the S-bit clear, which prevents 1473 it to be flooded outside of the level in which it was originated. 1474 Even though in OSPF the FAD Sub-TLV can be flooded in an RI LSA that 1475 has AS flooding scope, the FAD selection is performed for each 1476 individual area in which it is being used. 1478 There is no requirement for the FAD for a particular Flex-Algorithm 1479 to be identical in all areas in the network. For example, traffic 1480 for the same Flex-Algorithm may be optimized for minimal delay (e.g., 1481 using delay metric) in one area or level, while being optimized for 1482 available bandwidth (e.g., using IGP metric) in another area or 1483 level. 1485 As described in Section 5.1, IS-IS allows the re-generation of the 1486 winning FAD from level 2, without any modification to it, into a 1487 level 1 area. This allows the operator to configure the FAD in one 1488 or multiple routers in the level 2, without the need to repeat the 1489 same task in each level 1 area, if the intent is to have the same FAD 1490 for the particular Flex-Algorithm across all levels. This can 1491 similarly be achieved in OSPF by using the AS flooding scope of the 1492 RI LSA in which the FAD Sub-TLV for the particular Flex-Algoritm is 1493 advertised. 1495 Re-generation of FAD from a level 1 area to the level 2 area is not 1496 supported in IS-IS, so if the intent is to regenerate the FAD between 1497 IS-IS levels, the FAD MUST be defined on router(s) that are in level 1498 2. In OSPF, the FAD definition can be done in any area and be 1499 propagated to all routers in the OSPF routing domain by using the AS 1500 flooding scope of the RI LSA. 1502 15.2. Usage of SRLG Exclude Rule with Flex-Algorithm 1504 There are two different ways in which SRLG information can be used 1505 with Flex-Algorithm: 1507 In a context of a single Flex-Algorithm, it can be used for 1508 computation of backup paths, as described in 1509 [I-D.ietf-rtgwg-segment-routing-ti-lfa]. This usage does not 1510 require association of any specific SRLG constraint with the given 1511 Flex-Algorithm definition. 1513 In the context of multiple Flex-Algorithms, it can be used for 1514 creating disjoint sets of paths by pruning the links belonging to 1515 a specific SRLG from the topology on which a specific Flex- 1516 Algorithm computes its paths. This usage: 1518 Facilitates the usage of already deployed SRLG configurations 1519 for setup of disjoint paths between two or more Flex- 1520 Algorithms. 1522 Requires explicit association of a given Flex-Algorithm with a 1523 specific set of SRLG constraints as defined in Section 6.5 and 1524 Section 7.5. 1526 The two usages mentioned above are orthogonal. 1528 15.3. Max-metric consideration 1530 Both IS-IS and OSPF have a mechanism to set the IGP metric on a link 1531 to a value that would make the link either non-reachable or to serve 1532 as the link of last resort. Similar functionality would be needed 1533 for the Min Unidirectional Link Delay and TE metric, as these can be 1534 used to compute Flex-Algorithm paths. 1536 The link can be made un-reachable for all Flex-Algorithms that use 1537 Min Unidirectional Link Delay as metric, as described in Section 5.1, 1538 by removing the Flex-Algorithm ASLA Min Unidirectional Link Delay 1539 advertisement for the link. The link can be made the link of last 1540 resort by setting the delay value in the Flex-Algorithm ASLA delay 1541 advertisement for the link to the value of 16,777,215 (2^24 - 1). 1543 The link can be made un-reachable for all Flex-Algorithms that use TE 1544 metric, as described in Section 5.1, by removing the Flex-Algorithm 1545 ASLA TE metric advertisement for the link. The link can be made the 1546 link of last resort by setting the TE metric value in the Flex- 1547 Algorithm ASLA delay advertisement for the link to the value of (2^24 1548 - 1) in IS-IS and (2^32 - 1) in OSPF. 1550 16. Backward Compatibility 1552 This extension brings no new backward compatibility issues. IS-IS, 1553 OSPFv2 and OSPFv3 all have well defined handling of unrecognized TLVs 1554 and sub-TLVs that allows the introduction of the new extensions, 1555 similar to those defined here, without introducing any 1556 interoperability issues. 1558 17. Security Considerations 1560 This draft adds two new ways to disrupt IGP networks: 1562 An attacker can hijack a particular Flex-Algorithm by advertising 1563 a FAD with a priority of 255 (or any priority higher than that of 1564 the legitimate nodes). 1566 An attacker could make it look like a router supports a particular 1567 Flex-Algorithm when it actually doesn't, or vice versa. 1569 Both of these attacks can be addressed by the existing security 1570 extensions as described in [RFC5304] and [RFC5310] for IS-IS, in 1571 [RFC2328] and [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552] 1572 for OSPFv3. 1574 18. IANA Considerations 1576 18.1. IGP IANA Considerations 1578 18.1.1. IGP Algorithm Types Registry 1580 This document makes the following registrations in the "IGP Algorithm 1581 Types" registry: 1583 Type: 128-255. 1585 Description: Flexible Algorithms. 1587 Reference: This document (Section 4). 1589 18.1.2. IGP Metric-Type Registry 1591 IANA is requested to set up a registry called "IGP Metric-Type 1592 Registry" under an "Interior Gateway Protocol (IGP) Parameters" IANA 1593 registries. The registration policy for this registry is "Standards 1594 Action" ([RFC8126] and [RFC7120]). 1596 Values in this registry come from the range 0-255. 1598 This document registers following values in the "IGP Metric-Type 1599 Registry": 1601 Type: 0 1603 Description: IGP metric 1605 Reference: This document (Section 5.1) 1607 Type: 1 1609 Description: Min Unidirectional Link Delay as defined in 1610 [RFC8570], section 4.2, and [RFC7471], section 4.2. 1612 Reference: This document (Section 5.1) 1614 Type: 2 1616 Description: Traffic Engineering Default Metric as defined in 1617 [RFC5305], section 3.7, and Traffic engineering metric as defined 1618 in [RFC3630], section 2.5.5 1620 Reference: This document (Section 5.1) 1622 18.2. Flexible Algorithm Definition Flags Registry 1624 IANA is requested to set up a registry called "IS-IS Flexible 1625 Algorithm Definition Flags Registry" under an "Interior Gateway 1626 Protocol (IGP) Parameters" IANA registries. The registration policy 1627 for this registry is "Standards Action" ([RFC8126] and [RFC7120]). 1629 This document defines the following single bit in Flexible Algorithm 1630 Definition Flags registry: 1632 Bit # Name 1633 ----- ------------------------------ 1634 0 Prefix Metric Flag (M-flag) 1636 Reference: This document (Section 6.4, Section 7.4). 1638 18.3. IS-IS IANA Considerations 1640 18.3.1. Sub TLVs for Type 242 1642 This document makes the following registrations in the "sub-TLVs for 1643 TLV 242" registry. 1645 Type: 26. 1647 Description: Flexible Algorithm Definition. 1649 Reference: This document (Section 5.1). 1651 18.3.2. Sub TLVs for for TLVs 135, 235, 236, and 237 1653 This document makes the following registrations in the "Sub-TLVs for 1654 for TLVs 135, 235, 236, and 237" registry. 1656 Type: 6 1658 Description: Flexible Algorithm Prefix Metric. 1660 Reference: This document (Section 8). 1662 18.3.3. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV 1664 This document creates the following Sub-Sub-TLV Registry: 1666 Registry: Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV 1668 Registration Procedure: Expert review 1669 Reference: This document (Section 5.1) 1671 This document defines the following Sub-Sub-TLVs in the "Sub-Sub-TLVs 1672 for Flexible Algorithm Definition Sub-TLV" registry: 1674 Type: 1 1676 Description: Flexible Algorithm Exclude Admin Group 1678 Reference: This document (Section 6.1). 1680 Type: 2 1682 Description: Flexible Algorithm Include-Any Admin Group 1684 Reference: This document (Section 6.2). 1686 Type: 3 1688 Description: Flexible Algorithm Include-All Admin Group 1690 Reference: This document (Section 6.3). 1692 Type: 4 1694 Description: Flexible Algorithm Definition Flags 1696 Reference: This document (Section 6.4). 1698 Type: 5 1700 Description: Flexible Algorithm Exclude SRLG 1702 Reference: This document (Section 6.5). 1704 18.4. OSPF IANA Considerations 1706 18.4.1. OSPF Router Information (RI) TLVs Registry 1708 This specification updates the OSPF Router Information (RI) TLVs 1709 Registry. 1711 Type: 16 1713 Description: Flexible Algorithm Definition TLV. 1715 Reference: This document (Section 5.2). 1717 18.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs 1719 This document makes the following registrations in the "OSPFv2 1720 Extended Prefix TLV Sub-TLVs" registry. 1722 Type: 3 1724 Description: Flexible Algorithm Prefix Metric. 1726 Reference: This document (Section 9). 1728 18.4.3. OSPFv3 Extended-LSA Sub-TLVs 1730 This document makes the following registrations in the "OSPFv3 1731 Extended-LSA Sub-TLVs" registry. 1733 Type: 26 1735 Description: Flexible Algorithm Prefix Metric. 1737 Reference: This document (Section 9). 1739 Type: TBD (suggested value 30) 1741 Description: OSPF Flexible Algorithm ASBR Metric Sub-TLV 1743 Reference: This document (Section 10.2). 1745 18.4.4. OSPF Flex-Algorithm Prefix Metric Bits 1747 This specification requests creation of "OSPF Flex-Algorithm Prefix 1748 Metric Bits" registry under the OSPF Parameters Registry with the 1749 following initial values. 1751 Bit Number: 0 1753 Description: E bit - External Type 1755 Reference: this document. 1757 The bits 1-7 are unassigned and the registration procedure to be 1758 followed for this registry is IETF Review. 1760 18.4.5. OSPF Opaque LSA Option Types 1762 This document makes the following registrations in the "OSPF Opaque 1763 LSA Option Types" registry. 1765 Value: TBD (suggested value 11) 1767 Description: OSPFv2 Extended Inter-Area ASBR LSA 1769 Reference: This document (Section 10.1). 1771 18.4.6. OSPFv2 Externded Inter-Area ASBR TLVs 1773 This specification requests creation of "OSPFv2 Extended Inter-Area 1774 ASBR TLVs" registry under the OSPFv2 Parameters Registry with the 1775 following initial values. 1777 Value: 1 1779 Description : Extended Inter-Area ASBR TLV 1781 Reference: this document 1783 The values 2 to 32767 are unassigned, values 32768 to 33023 are 1784 reserved for experimental use while the values 0 and 33024 to 65535 1785 are reserved. The registration procedure to be followed for this 1786 registry is IETF Review or IESG Approval. 1788 18.4.7. OSPFv2 Inter-Area ASBR Sub-TLVs 1790 This specification requests creation of "OSPFv2 Extended Inter-Area 1791 ASBR Sub-TLVs" registry under the OSPFv2 Parameters Registry with the 1792 following initial values. 1794 Value: 1 1796 Description : OSPF Flexible Algorithm ASBR Metric Sub-TLV 1798 Reference: this document 1800 The values 2 to 32767 are unassigned, values 32768 to 33023 are 1801 reserved for experimental use while the values 0 and 33024 to 65535 1802 are reserved. The registration procedure to be followed for this 1803 registry is IETF Review or IESG Approval. 1805 18.4.8. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry 1807 This document creates the following registry: 1809 Registry: OSPF Flexible Algorithm Definition TLV sub-TLV 1811 Registration Procedure: Expert review 1812 Reference: This document (Section 5.2) 1814 The "OSPF Flexible Algorithm Definition TLV sub-TLV" registry will 1815 define sub-TLVs at any level of nesting for the Flexible Algorithm 1816 TLV and should be added to the "Open Shortest Path First (OSPF) 1817 Parameters" registries group. New values can be allocated via IETF 1818 Review or IESG Approval. 1820 This document registers following Sub-TLVs in the "TLVs for Flexible 1821 Algorithm Definition TLV" registry: 1823 Type: 1 1825 Description: Flexible Algorithm Exclude Admin Group 1827 Reference: This document (Section 7.1). 1829 Type: 2 1831 Description: Flexible Algorithm Include-Any Admin Group 1833 Reference: This document (Section 7.2). 1835 Type: 3 1837 Description: Flexible Algorithm Include-All Admin Group 1839 Reference: This document (Section 7.3). 1841 Type: 4 1843 Description: Flexible Algorithm Definition Flags 1845 Reference: This document (Section 7.4). 1847 Type: 5 1849 Description: Flexible Algorithm Exclude SRLG 1851 Reference: This document (Section 7.5). 1853 Types in the range 32768-33023 are for experimental use; these will 1854 not be registered with IANA, and MUST NOT be mentioned by RFCs. 1856 Types in the range 33024-65535 are not to be assigned at this time. 1857 Before any assignments can be made in the 33024-65535 range, there 1858 MUST be an IETF specification that specifies IANA Considerations that 1859 covers the range being assigned. 1861 18.4.9. Link Attribute Applications Registry 1863 This document registers following bit in the Link Attribute 1864 Applications Registry: 1866 Bit-3 1868 Description: Flexible Algorithm (X-bit) 1870 Reference: This document (Section 12). 1872 19. Acknowledgements 1874 This draft, among other things, is also addressing the problem that 1875 the [I-D.gulkohegde-routing-planes-using-sr] was trying to solve. 1876 All authors of that draft agreed to join this draft. 1878 Thanks to Eric Rosen, Tony Przygienda, William Britto A J, Gunter Van 1879 De Velde, Dirk Goethals, Manju Sivaji and, Baalajee S for their 1880 detailed review and excellent comments. 1882 Thanks to Cengiz Halit for his review and feedback during initial 1883 phase of the solution definition. 1885 Thanks to Kenji Kumaki for his comments. 1887 Thanks to Acee Lindem for editorial comments. 1889 20. References 1891 20.1. Normative References 1893 [I-D.ietf-lsr-isis-srv6-extensions] 1894 Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and 1895 Z. Hu, "IS-IS Extension to Support Segment Routing over 1896 IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-14 1897 (work in progress), April 2021. 1899 [ISO10589] 1900 International Organization for Standardization, 1901 "Intermediate system to Intermediate system intra-domain 1902 routeing information exchange protocol for use in 1903 conjunction with the protocol for providing the 1904 connectionless-mode Network Service (ISO 8473)", ISO/ 1905 IEC 10589:2002, Second Edition, Nov 2002. 1907 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1908 Requirement Levels", BCP 14, RFC 2119, 1909 DOI 10.17487/RFC2119, March 1997, 1910 . 1912 [RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in 1913 Support of Generalized Multi-Protocol Label Switching 1914 (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005, 1915 . 1917 [RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The 1918 OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250, 1919 July 2008, . 1921 [RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions 1922 in Support of Generalized Multi-Protocol Label Switching 1923 (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008, 1924 . 1926 [RFC7308] Osborne, E., "Extended Administrative Groups in MPLS 1927 Traffic Engineering (MPLS-TE)", RFC 7308, 1928 DOI 10.17487/RFC7308, July 2014, 1929 . 1931 [RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W., 1932 Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute 1933 Advertisement", RFC 7684, DOI 10.17487/RFC7684, November 1934 2015, . 1936 [RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and 1937 S. Shaffer, "Extensions to OSPF for Advertising Optional 1938 Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, 1939 February 2016, . 1941 [RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions 1942 for Advertising Router Information", RFC 7981, 1943 DOI 10.17487/RFC7981, October 2016, 1944 . 1946 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1947 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1948 May 2017, . 1950 [RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and 1951 F. Baker, "OSPFv3 Link State Advertisement (LSA) 1952 Extensibility", RFC 8362, DOI 10.17487/RFC8362, April 1953 2018, . 1955 [RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler, 1956 H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF 1957 Extensions for Segment Routing", RFC 8665, 1958 DOI 10.17487/RFC8665, December 2019, 1959 . 1961 [RFC8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions 1962 for Segment Routing", RFC 8666, DOI 10.17487/RFC8666, 1963 December 2019, . 1965 [RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., 1966 Bashandy, A., Gredler, H., and B. Decraene, "IS-IS 1967 Extensions for Segment Routing", RFC 8667, 1968 DOI 10.17487/RFC8667, December 2019, 1969 . 1971 [RFC8919] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and 1972 J. Drake, "IS-IS Application-Specific Link Attributes", 1973 RFC 8919, DOI 10.17487/RFC8919, October 2020, 1974 . 1976 [RFC8920] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura, 1977 J., and J. Drake, "OSPF Application-Specific Link 1978 Attributes", RFC 8920, DOI 10.17487/RFC8920, October 2020, 1979 . 1981 20.2. Informative References 1983 [I-D.gulkohegde-routing-planes-using-sr] 1984 Hegde, S. and A. Gulko, "Separating Routing Planes using 1985 Segment Routing", draft-gulkohegde-routing-planes-using- 1986 sr-00 (work in progress), March 2017. 1988 [I-D.ietf-rtgwg-segment-routing-ti-lfa] 1989 Litkowski, S., Bashandy, A., Filsfils, C., Francois, P., 1990 Decraene, B., and D. Voyer, "Topology Independent Fast 1991 Reroute using Segment Routing", draft-ietf-rtgwg-segment- 1992 routing-ti-lfa-06 (work in progress), February 2021. 1994 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 1995 DOI 10.17487/RFC2328, April 1998, 1996 . 1998 [RFC3101] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option", 1999 RFC 3101, DOI 10.17487/RFC3101, January 2003, 2000 . 2002 [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 2003 (TE) Extensions to OSPF Version 2", RFC 3630, 2004 DOI 10.17487/RFC3630, September 2003, 2005 . 2007 [RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway 2008 Protocol (IGP) Routes Over Traffic Engineering Tunnels", 2009 RFC 3906, DOI 10.17487/RFC3906, October 2004, 2010 . 2012 [RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality 2013 for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006, 2014 . 2016 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic 2017 Authentication", RFC 5304, DOI 10.17487/RFC5304, October 2018 2008, . 2020 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 2021 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2022 2008, . 2024 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 2025 and M. Fanto, "IS-IS Generic Cryptographic 2026 Authentication", RFC 5310, DOI 10.17487/RFC5310, February 2027 2009, . 2029 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 2030 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 2031 . 2033 [RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code 2034 Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January 2035 2014, . 2037 [RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S. 2038 Previdi, "OSPF Traffic Engineering (TE) Metric 2039 Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015, 2040 . 2042 [RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed., 2043 "Security Extension for OSPFv2 When Using Manual Key 2044 Management", RFC 7474, DOI 10.17487/RFC7474, April 2015, 2045 . 2047 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2048 Writing an IANA Considerations Section in RFCs", BCP 26, 2049 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2050 . 2052 [RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward, 2053 D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE) 2054 Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March 2055 2019, . 2057 Authors' Addresses 2059 Peter Psenak (editor) 2060 Cisco Systems 2061 Apollo Business Center 2062 Mlynske nivy 43 2063 Bratislava, 82109 2064 Slovakia 2066 Email: ppsenak@cisco.com 2068 Shraddha Hegde 2069 Juniper Networks, Inc. 2070 Embassy Business Park 2071 Bangalore, KA, 560093 2072 India 2074 Email: shraddha@juniper.net 2076 Clarence Filsfils 2077 Cisco Systems, Inc. 2078 Brussels 2079 Belgium 2081 Email: cfilsfil@cisco.com 2083 Ketan Talaulikar 2084 Cisco Systems, Inc. 2085 S.No. 154/6, Phase I, Hinjawadi 2086 PUNE, MAHARASHTRA 411 057 2087 India 2089 Email: ketant@cisco.com 2090 Arkadiy Gulko 2091 Edward Jones 2093 Email: arkadiy.gulko@edwardjones.com