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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Filsfils, Ed. 3 Internet-Draft S. Previdi, Ed. 4 Intended status: Standards Track Cisco Systems, Inc. 5 Expires: April 16, 2016 B. Decraene 6 S. Litkowski 7 Orange 8 R. Shakir 9 Individual 10 October 14, 2015 12 Segment Routing Architecture 13 draft-ietf-spring-segment-routing-06 15 Abstract 17 Segment Routing (SR) leverages the source routing paradigm. A node 18 steers a packet through an ordered list of instructions, called 19 segments. A segment can represent any instruction, topological or 20 service-based. A segment can have a local semantic to an SR node or 21 global within an SR domain. SR allows to enforce a flow through any 22 topological path and service chain while maintaining per-flow state 23 only at the ingress node to the SR domain. 25 Segment Routing can be directly applied to the MPLS architecture with 26 no change on the forwarding plane. A segment is encoded as an MPLS 27 label. An ordered list of segments is encoded as a stack of labels. 28 The segment to process is on the top of the stack. Upon completion 29 of a segment, the related label is popped from the stack. 31 Segment Routing can be applied to the IPv6 architecture, with a new 32 type of routing header. A segment is encoded as an IPv6 address. An 33 ordered list of segments is encoded as an ordered list of IPv6 34 addresses in the routing header. The active segment is indicated by 35 the Destination Address of the packet. The next active segment is 36 indicated by a pointer in the new routing header. 38 Requirements Language 40 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 41 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 42 document are to be interpreted as described in RFC 2119 [RFC2119]. 44 Status of This Memo 46 This Internet-Draft is submitted in full conformance with the 47 provisions of BCP 78 and BCP 79. 49 Internet-Drafts are working documents of the Internet Engineering 50 Task Force (IETF). Note that other groups may also distribute 51 working documents as Internet-Drafts. The list of current Internet- 52 Drafts is at http://datatracker.ietf.org/drafts/current/. 54 Internet-Drafts are draft documents valid for a maximum of six months 55 and may be updated, replaced, or obsoleted by other documents at any 56 time. It is inappropriate to use Internet-Drafts as reference 57 material or to cite them other than as "work in progress." 59 This Internet-Draft will expire on April 16, 2016. 61 Copyright Notice 63 Copyright (c) 2015 IETF Trust and the persons identified as the 64 document authors. All rights reserved. 66 This document is subject to BCP 78 and the IETF Trust's Legal 67 Provisions Relating to IETF Documents 68 (http://trustee.ietf.org/license-info) in effect on the date of 69 publication of this document. Please review these documents 70 carefully, as they describe your rights and restrictions with respect 71 to this document. Code Components extracted from this document must 72 include Simplified BSD License text as described in Section 4.e of 73 the Trust Legal Provisions and are provided without warranty as 74 described in the Simplified BSD License. 76 Table of Contents 78 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 79 1.1. Companion Documents . . . . . . . . . . . . . . . . . . . 4 80 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 81 3. Link-State IGP Segments . . . . . . . . . . . . . . . . . . . 7 82 3.1. IGP Segment, IGP SID . . . . . . . . . . . . . . . . . . 7 83 3.2. IGP-Prefix Segment, Prefix-SID . . . . . . . . . . . . . 7 84 3.2.1. Prefix-SID Algorithm . . . . . . . . . . . . . . . . 7 85 3.2.2. MPLS Dataplane . . . . . . . . . . . . . . . . . . . 8 86 3.2.3. IPv6 Dataplane . . . . . . . . . . . . . . . . . . . 9 87 3.3. IGP-Node Segment, Node-SID . . . . . . . . . . . . . . . 10 88 3.4. IGP-Anycast Segment, Anycast SID . . . . . . . . . . . . 10 89 3.5. IGP-Adjacency Segment, Adj-SID . . . . . . . . . . . . . 13 90 3.5.1. Parallel Adjacencies . . . . . . . . . . . . . . . . 14 91 3.5.2. LAN Adjacency Segments . . . . . . . . . . . . . . . 15 92 3.6. Binding Segment . . . . . . . . . . . . . . . . . . . . . 16 93 3.6.1. Mapping Server . . . . . . . . . . . . . . . . . . . 16 94 3.6.2. Tunnel Headend . . . . . . . . . . . . . . . . . . . 16 95 3.7. Inter-Area Considerations . . . . . . . . . . . . . . . . 16 96 4. BGP Peering Segments . . . . . . . . . . . . . . . . . . . . 17 97 5. IGP Mirroring Context Segment . . . . . . . . . . . . . . . 18 98 6. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 18 99 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 100 8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 101 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 19 102 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 103 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 104 11.1. Normative References . . . . . . . . . . . . . . . . . . 20 105 11.2. Informative References . . . . . . . . . . . . . . . . . 20 106 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 108 1. Introduction 110 With Segment Routing (SR), a node steers a packet through an ordered 111 list of instructions, called segments. A segment can represent any 112 instruction, topological or service-based. A segment can have a 113 local semantic to an SR node or global within an SR domain. SR 114 allows to enforce a flow through any path and service chain while 115 maintaining per-flow state only at the ingress node of the SR domain. 117 Segment Routing can be directly applied to the MPLS architecture 118 ([RFC3031]) with no change on the forwarding plane. A segment is 119 encoded as an MPLS label. An ordered list of segments is encoded as 120 a stack of labels. The active segment is on the top of the stack. A 121 completed segment is popped off the stack. The addition of a segment 122 is performed with a push. 124 In the Segment Routing MPLS instantiation, a segment could be of 125 several types: 127 o an IGP segment, 129 o a BGP Peering segments, 131 o an LDP LSP segment, 133 o an RSVP-TE LSP segment, 135 o a BGP LSP segment. 137 The first two (IGP and BGP Peering segments) types of segments are 138 defined in this document. The use of the last three types of 139 segments is illustrated in [I-D.ietf-spring-segment-routing-mpls]. 141 Segment Routing can be applied to the IPv6 architecture ([RFC2460]), 142 with a new type of routing header. A segment is encoded as an IPv6 143 address. An ordered list of segments is encoded as an ordered list 144 of IPv6 addresses in the routing header. The active segment is 145 indicated by the Destination Address of the packet. Upon completion 146 of a segment, a pointer in the new routing header is incremented and 147 indicates the next segment. 149 Numerous use-cases illustrate the benefits of source routing either 150 for FRR, OAM or Traffic Engineering reasons. 152 This document defines a set of instructions (called segments) that 153 are required to fulfill the described use-cases. These segments can 154 either be used in isolation (one single segment defines the source 155 route of the packet) or in combination (these segments are part of an 156 ordered list of segments that define the source route of the packet). 158 1.1. Companion Documents 160 This document defines the SR architecture, its routing model, the 161 IGP-based segments, the BGP-based segments and the service segments. 163 Use cases are described in [I-D.ietf-spring-problem-statement], 164 [I-D.filsfils-spring-segment-routing-central-epe], 165 [I-D.filsfils-spring-segment-routing-msdc], 166 [I-D.filsfils-spring-large-scale-interconnect], 167 [I-D.ietf-spring-ipv6-use-cases], 168 [I-D.ietf-spring-resiliency-use-cases], [I-D.geib-spring-oam-usecase] 169 and [I-D.ietf-spring-sr-oam-requirement]. 171 Segment Routing for MPLS dataplane is documented in 172 [I-D.ietf-spring-segment-routing-mpls]. 174 Segment Routing for IPv6 dataplane is documented in 175 [I-D.previdi-6man-segment-routing-header]. 177 IGP protocol extensions for Segment Routing are described in 178 [I-D.ietf-isis-segment-routing-extensions], 179 [I-D.ietf-ospf-segment-routing-extensions] and 180 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] referred in this 181 document as "IGP SR extensions documents". 183 The FRR solution for SR is documented in 184 [I-D.francois-rtgwg-segment-routing-ti-lfa]. 186 The PCEP protocol extensions for Segment Routing are defined in 187 [I-D.ietf-pce-segment-routing]. 189 The interaction between SR/MPLS with other MPLS Signaling planes is 190 documented in [I-D.filsfils-spring-segment-routing-ldp-interop]. 192 2. Terminology 194 Segment: an instruction a node executes on the incoming packet (e.g.: 195 forward packet according to shortest path to destination, or, forward 196 packet through a specific interface, or, deliver the packet to a 197 given application/service instance). 199 SID: a Segment Identifier. Examples of SIDs are: a MPLS label, an 200 index value in a MPLS label space, an IPv6 address. Other types of 201 SIDs can be defined in the future. 203 Segment List: ordered list of SID's encoding the topological and 204 service source route of the packet. It is a stack of labels in the 205 MPLS architecture. It is an ordered list of IPv6 addresses in the 206 IPv6 architecture. 208 Active segment: the segment that MUST be used by the receiving router 209 to process the packet. In the MPLS dataplane is the top label. In 210 the IPv6 dataplane is the destination address of a packet having the 211 Segment Routing Header as defined in 212 [I-D.previdi-6man-segment-routing-header]. 214 PUSH: the insertion of a segment at the head of the Segment list. 216 NEXT: the active segment is completed, the next segment becomes 217 active. 219 CONTINUE: the active segment is not completed and hence remains 220 active. The CONTINUE instruction is implemented as the SWAP 221 instruction in the MPLS dataplane. In IPv6, this is the plain IPv6 222 forwarding action of a regular IPv6 packet according to its 223 Destination Address. 225 SR Global Block (SRGB): local property of an SR node. In the MPLS 226 architecture, SRGB is the set of local labels reserved for global 227 segments. Using the same SRGB on all nodes within the SR domain ease 228 operations and troubleshooting and is expected to be a deployment 229 guideline. In the IPv6 architecture, the equivalent of the SRGB is 230 in fact the set of addresses used as global segments. Since there 231 are no restrictions on which IPv6 address can be used, the concept of 232 the SRGB includes all IPv6 global address space used within the SR 233 domain. 235 Global Segment: the related instruction is supported by all the SR- 236 capable nodes in the domain. In the MPLS architecture, a Global 237 Segment has a globally-unique index. The related local label at a 238 given node N is found by adding the globally-unique index to the SRGB 239 of node N. In the IPv6 architecture, a global segment is a globally- 240 unique IPv6 address. 242 Local Segment: the related instruction is supported only by the node 243 originating it. In the MPLS architecture, this is a local label 244 outside the SRGB. In the IPv6 architecture, this can be any IPv6 245 address whose reachability is not advertised in any routing protocol 246 (hence, the segment is known only by the local node). 248 IGP Segment: the generic name for a segment attached to a piece of 249 information advertised by a link-state IGP, e.g. an IGP prefix or an 250 IGP adjacency. 252 IGP-prefix Segment, Prefix-SID: an IGP-Prefix Segment is an IGP 253 segment attached to an IGP prefix. An IGP-Prefix Segment is global 254 (unless explicitly advertised otherwise) within the SR IGP instance/ 255 topology and identifies an instruction to forward the packet along 256 the path computed using the algorithm field, in the topology and the 257 IGP instance where it is advertised. The Prefix-SID is the SID of 258 the IGP-Prefix Segment. 260 IGP-Anycast: an IGP-Anycast Segment is an IGP-prefix segment which 261 does not identify a specific router, but a set of routers. The terms 262 "Anycast Segment" or "Anycast-SID" are often used as an abbreviation. 264 IGP-Adjacency: an IGP-Adjacency Segment is an IGP segment attached to 265 an unidirectional adjacency or a set of unidirectional adjacencies. 266 By default, an IGP-Adjacency Segment is local (unless explicitly 267 advertised otherwise) to the node that advertises it. 269 IGP-Node: an IGP-Node Segment is an IGP-Prefix Segment which 270 identifies a specific router (e.g. a loopback). The terms "Node 271 Segment" or Node-SID" are often used as an abbreviation. 273 SR Tunnel: a list of segments to be pushed on the packets directed on 274 the tunnel. The list of segments can be specified explicitly or 275 implicitly via a set of abstract constraints (latency, affinity, 276 SRLG, ...). In the latter case, a constraint-based path computation 277 is used to determine the list of segments associated with the tunnel. 278 The computation can be local or delegated to a PCE server. An SR 279 tunnel can be configured by the operator, provisioned via netconf or 280 provisioned via PCEP. An SR tunnel can be used for traffic- 281 engineering, OAM or FRR reasons. 283 Segment List Depth: the number of segments of an SR tunnel. The 284 entity instantiating an SR Tunnel at a node N should be able to 285 discover the depth insertion capability of the node N. The PCEP 286 discovery capability is described in [I-D.ietf-pce-segment-routing]. 288 3. Link-State IGP Segments 290 Within a link-state IGP domain, an SR-capable IGP node advertises 291 segments for its attached prefixes and adjacencies. These segments 292 are called IGP segments or IGP SIDs. They play a key role in Segment 293 Routing and use-cases as they enable the expression of any 294 topological path throughout the IGP domain. Such a topological path 295 is either expressed as a single IGP segment or a list of multiple IGP 296 segments. 298 3.1. IGP Segment, IGP SID 300 The terms "IGP Segment" and "IGP SID" are the generic names for a 301 segment attached to a piece of information advertised by a link-state 302 IGP, e.g. an IGP prefix or an IGP adjacency. 304 3.2. IGP-Prefix Segment, Prefix-SID 306 An IGP-Prefix Segment is an IGP segment attached to an IGP prefix. 307 An IGP-Prefix Segment is global (unless explicitly advertised 308 otherwise) within the SR/IGP domain. 310 The required IGP protocol extensions are defined in IGP SR extensions 311 documents. 313 3.2.1. Prefix-SID Algorithm 315 The IGP protocol extensions for Segment Routing define the Prefix-SID 316 advertisement which includes a set of flags and the algorithm field. 317 The algorithm field has the purpose of associating a given Prefix-SID 318 to a routing algorithm. 320 In the context of an instance and a topology, multiple Prefix-SID's 321 MAY be allocated to the same IGP Prefix as long as the algorithm 322 value is different in each one. 324 Multiple instances and topologies are defined in IS-IS and OSPF in: 325 [RFC5120], [RFC6822], [RFC6549] and [RFC4915]. 327 Initially, two "algorithms" have been defined: 329 o "Shortest Path": this algorithm is the default behavior. The 330 packet is forwarded along the well known ECMP-aware SPF algorithm 331 however it is explicitly allowed for a midpoint to implement 332 another forwarding based on local policy.. The "Shortest Path" 333 algorithm is in fact the default and current behavior of most of 334 the networks where local policies may override the SPF decision. 336 o "Strict Shortest Path": This algorithm mandates that the packet is 337 forwarded according to ECMP-aware SPF algorithm and instruct any 338 router in the path to ignore any possible local policy overriding 339 SPF decision. The SID advertised with "Strict Shortest Path" 340 algorithm ensures that the path the packet is going to take is the 341 expected, and not altered, SPF path. 343 An IGP-Prefix Segment identifies the path, to the related prefix, 344 along the path computed as per the algorithm field. 346 A packet injected anywhere within the SR/IGP domain with an active 347 Prefix-SID will be forwarded along path computed by the algorithm 348 expressed in the algorithm field. 350 The ingress node of an SR domain validates that the path to a prefix, 351 advertised with a given algorithm, includes nodes all supporting the 352 advertised algorithm. In other words, when computing paths for a 353 given algorithm, the transit nodes MUST compute the algorithm X on 354 the IGP topology, regardless of the support of the algorithm X by the 355 nodes in that topology. As a consequence, if a node on the path does 356 not support algorithm X, the IGP-Prefix segment will be interrupted 357 and will drop packet on that node. It's the responsibility of the 358 ingress node using a segment to check that all downstream nodes 359 support the algorithm of the segment. 361 Details of the two defined algorithms are defined in 362 [I-D.ietf-isis-segment-routing-extensions], 363 [I-D.ietf-ospf-segment-routing-extensions] and 364 [I-D.ietf-ospf-ospfv3-segment-routing-extensions]. 366 3.2.2. MPLS Dataplane 368 When SR is used over the MPLS dataplane: 370 o the IGP signaling extension for IGP-Prefix segment includes the 371 P-Flag. A Node N advertising a Prefix-SID SID-R for its attached 372 prefix R resets the P-Flag to allow its connected neighbors to 373 perform the NEXT operation while processing SID-R. This behavior 374 is equivalent to Penultimate Hop Popping in MPLS. When set, the 375 neighbors of N must perform the CONTINUE operation while 376 processing SID-R. 378 o A Prefix-SID is allocated in the form of an index in the SRGB (or 379 as a local MPLS label) according to a process similar to IP 380 address allocation. Typically the Prefix-SID is allocated by 381 policy by the operator (or NMS) and the SID very rarely changes. 383 o While SR allows to attach a local segment to an IGP prefix (using 384 the L-Flag), we specifically assume that when the terms "IGP- 385 Prefix Segment" and "Prefix-SID" are used, the segment is global 386 (the SID is allocated from the SRGB or as an index). This is 387 consistent with all the described use-cases that require global 388 segments attached to IGP prefixes. 390 o The allocation process MUST NOT allocate the same Prefix-SID to 391 different IP prefixes. 393 o If a node learns a Prefix-SID having a value that falls outside 394 the locally configured SRGB range, then the node MUST NOT use the 395 Prefix-SID and SHOULD issue an error log warning for 396 misconfiguration. 398 o A node N attaching a Prefix-SID SID-R to its attached prefix R 399 MUST maintain the following FIB entry: 401 Incoming Active Segment: SID-R 402 Ingress Operation: NEXT 403 Egress interface: NULL 405 o A remote node M MUST maintain the following FIB entry for any 406 learned Prefix-SID SID-R attached to IP prefix R: 408 Incoming Active Segment: SID-R 409 Ingress Operation: 410 If the next-hop of R is the originator of R 411 and instructed to remove the active segment: NEXT 412 Else: CONTINUE 413 Egress interface: the interface towards the next-hop along the 414 path computed using the algorithm advertised with 415 the SID toward prefix R. 417 3.2.3. IPv6 Dataplane 419 When SR is used over the IPv6 dataplane: 421 o The Prefix-SID is the prefix itself. No additional identifier is 422 needed for Segment Routing over IPv6. 424 o Any address belonging to any of the node's prefixes can be used as 425 Prefix-SIDs. 427 o An operator may want to explicitly indicate which of the node's 428 prefixes can be used as Prefix-SIDs through the setting of a flag 429 (e.g.: using the IGP prefix attribute defined in 431 [I-D.ietf-isis-prefix-attributes]) in the routing protocol used 432 for advertising the prefix. 434 o A global SID is instantiated through any globally advertised IPv6 435 address. 437 o A local SID is instantiated through a local IPv6 prefix not being 438 advertised and therefore known only by the local node. 440 A node N advertising an IPv6 address R usable as a segment identifier 441 MUST maintain the following FIB entry: 443 Incoming Active Segment: R 444 Ingress Operation: NEXT 445 Egress interface: NULL 447 Regardless Segment Routing, any remote IPv6 node will maintain a 448 plain IPv6 FIB entry for any prefix, no matter if they represent a 449 segment or not. 451 3.3. IGP-Node Segment, Node-SID 453 An IGP Node Segment is a an IGP Prefix Segment which identifies a 454 specific router (e.g. a loopback). The terms "Node Segment" or 455 "Node-SID" are often used as an abbreviation. The IGP SR extensions 456 define a flag that identifies Node-SIDs. 458 A "Node Segment" or "Node-SID" is fundamental to SR. From anywhere 459 in the network, it enforces the ECMP-aware shortest-path forwarding 460 of the packet towards the related node. 462 An IGP Node-SID MUST NOT be associated with a prefix that is owned by 463 more than one router within the same routing domain. 465 3.4. IGP-Anycast Segment, Anycast SID 467 An IGP-Anycast Segment is an IGP-prefix segment which does not 468 identify a specific router, but a set of routers. The terms "Anycast 469 Segment" or "Anycast-SID" are often used as an abbreviation. 471 An "Anycast Segment" or "Anycast SID" enforces the ECMP-aware 472 shortest-path forwarding towards the closest node of the anycast set. 473 This is useful to express macro-engineering policies or protection 474 mechanisms. 476 An IGP-Anycast Segment MUST NOT reference a particular node. 478 Within an anycast group, all routers MUST advertise the same prefix 479 with the same SID value. 481 +--------------+ 482 | Group A | 483 |192.0.2.10/32 | 484 | SID:100 | 485 | | 486 +-----------A1---A3----------+ 487 | | | \ / | | | 488 SID:10 | | | / | | | SID:30 489 203.0.113.1/32 | | | / \ | | | 203.0.113.3/32 490 PE1------R1----------A2---A4---------R3------PE3 491 \ /| | | |\ / 492 \ / | +--------------+ | \ / 493 \ / | | \ / 494 / | | / 495 / \ | | / \ 496 / \ | +--------------+ | / \ 497 / \| | | |/ \ 498 PE2------R2----------B1---B3----+----R4------PE4 499 203.0.113.2/32 | | | \ / | | | 203.0.113.4/32 500 SID:20 | | | / | | | SID:40 501 | | | / \ | | | 502 +-----+-----B2---B4----+-----+ 503 | | 504 | Group B | 505 | 192.0.2.1/32 | 506 | SID:200 | 507 +--------------+ 509 Transit device groups 511 The figure above describes a network example with two groups of 512 transit devices. Group A consists of devices {A1, A2, A3 and A4}. 513 They are all provisioned with the anycast address 192.0.2.10/32 and 514 the anycast SID 100. 516 Similarly, group B consists of devices {B1, B2, B3 and B4} and are 517 all provisioned with the anycast address 192.0.2.1/32, anycast SID 518 200. In the above network topology, each PE device is connected to 519 two routers in each of the groups A and B. 521 PE1 can choose a particular transit device group when sending traffic 522 to PE3 or PE4. This will be done by pushing the anycast SID of the 523 group in the stack. 525 Processing the anycast, and subsequent segments, requires special 526 care. 528 Obviously, the value of the SID following the anycast SID MUST be 529 understood by all nodes advertising the same anycast segment. 531 +-------------------------+ 532 | Group A | 533 | 192.0.2.10/32 | 534 | SID:100 | 535 |-------------------------| 536 | | 537 | SRGB: SRGB: | 538 SID:10 |(1000-2000) (3000-4000)| SID:30 539 PE1---+ +-------A1-------------A3-------+ +---PE3 540 \ / | | \ / | | \ / 541 \ / | | +-----+ / | | \ / 542 SRGB: \ / | | \ / | | \ / SRGB: 543 (7000-8000) R1 | | \ | | R3 (6000-7000) 544 / \ | | / \ | | / \ 545 / \ | | +-----+ \ | | / \ 546 / \ | | / \ | | / \ 547 PE2---+ +-------A2-------------A4-------+ +---PE4 548 SID:20 | SRGB: SRGB: | SID:40 549 |(2000-3000) (4000-5000)| 550 | | 551 +-------------------------+ 553 Transit paths via anycast group A 555 Considering a MPLS deployment, in the above topology, if device PE1 556 (or PE2) requires to send a packet to the device PE3 (or PE4) it 557 needs to encapsulate the packet in a MPLS payload with the following 558 stack of labels. 560 o Label allocated by R1 for anycast SID 100 (outer label). 562 o Label allocated by the nearest router in group A for SID 30 (for 563 destination PE3). 565 While the first label is easy to compute, in this case since there 566 are more than one topologically nearest devices (A1 and A2), unless 567 A1 and A2 allocated the same label value to the same prefix, 568 determining the second label is impossible. Devices A1 and A2 may be 569 devices from different hardware vendors. If both don't allocate the 570 same label value for SID 30, it is impossible to use the anycast 571 group "A" as a transit anycast group towards PE3. Hence, PE1 (or 572 PE2) cannot compute an appropriate label stack to steer the packet 573 exclusively through the group A devices. Same holds true for devices 574 PE3 and PE4 when trying to send a packet to PE1 or PE2. 576 To ease the use of anycast segment in a short term, it is recommended 577 to configure the same SRGB on all nodes of a particular anycast 578 group. Using this method, as mentioned above, computation of the 579 label following the anycast segment is straightforward. 581 Using anycast segment without configuring the same SRGB on nodes 582 belonging to the same device group may lead to misrouting (in a MPLS 583 VPN deployment, some traffic may leak between VPNs). 585 3.5. IGP-Adjacency Segment, Adj-SID 587 An IGP-Adjacency Segment is an IGP segment attached to a 588 unidirectional adjacency or a set of unidirectional adjacencies. By 589 default, an IGP-Adjacency Segment is local to the node which 590 advertises it. However, an Adjacency Segment can be global if 591 advertised by the IGP as such. The SID of the IGP-Adjacency Segment 592 is called the Adj-SID. 594 The adjacency is formed by the local node (i.e., the node advertising 595 the adjacency in the IGP) and the remote node (i.e., the other end of 596 the adjacency). The local node MUST be an IGP node. The remote node 597 MAY be an adjacent IGP neighbor or a non-adjacent neighbor (e.g.: a 598 Forwarding Adjacency, [RFC4206]). 600 A packet injected anywhere within the SR domain with a segment list 601 {SN, SNL}, where SN is the Node-SID of node N and SNL is an Adj-SID 602 attached by node N to its adjacency over link L, will be forwarded 603 along the shortest-path to N and then be switched by N, without any 604 IP shortest-path consideration, towards link L. If the Adj-SID 605 identifies a set of adjacencies, then the node N load- balances the 606 traffic among the various members of the set. 608 Similarly, when using a global Adj-SID, a packet injected anywhere 609 within the SR domain with a segment list {SNL}, where SNL is a global 610 Adj-SID attached by node N to its adjacency over link L, will be 611 forwarded along the shortest-path to N and then be switched by N, 612 without any IP shortest-path consideration, towards link L. If the 613 Adj-SID identifies a set of adjacencies, then the node N load- 614 balances the traffic among the various members of the set. The use 615 of global Adj-SID allows to reduce the size of the segment list when 616 expressing a path at the cost of additional state (i.e.: the global 617 Adj-SID will be inserted by all routers within the area in their 618 forwarding table). 620 An "IGP Adjacency Segment" or "Adj-SID" enforces the switching of the 621 packet from a node towards a defined interface or set of interfaces. 622 This is key to theoretically prove that any path can be expressed as 623 a list of segments. 625 The encodings of the Adj-SID include the B-flag. When set, the Adj- 626 SID refers to an adjacency that is eligible for protection (e.g.: 627 using IPFRR or MPLS-FRR). 629 The encodings of the Adj-SID include the L-flag. When set, the Adj- 630 SID has local significance. By default the L-flag is set. 632 A node SHOULD allocate one Adj-SIDs for each of its adjacencies. 634 A node MAY allocate multiple Adj-SIDs to the same adjacency. An 635 example is where the adjacency is established over a bundle 636 interface. Each bundle member MAY have its own Adj-SID. 638 A node MAY allocate the same Adj-SID to multiple adjacencies. 640 Adjacency suppression MUST NOT be performed by the IGP. 642 A node MUST install a FIB entry for any Adj-SID of value V attached 643 to data-link L: 645 Incoming Active Segment: V 646 Operation: NEXT 647 Egress Interface: L 649 The Adj-SID implies, from the router advertising it, the forwarding 650 of the packet through the adjacency identified by the Adj-SID, 651 regardless its IGP/SPF cost. In other words, the use of Adjacency 652 Segments overrides the routing decision made by SPF algorithm. 654 3.5.1. Parallel Adjacencies 656 Adj-SIDs can be used in order to represent a set of parallel 657 interfaces between two adjacent routers. 659 A node MUST install a FIB entry for any locally originated Adjacency 660 Segment (Adj-SID) of value W attached to a set of link B with: 662 Incoming Active Segment: W 663 Ingress Operation: NEXT 664 Egress interface: loadbalance between any data-link within set B 666 When parallel adjacencies are used and associated to the same Adj- 667 SID, and in order to optimize the load balancing function, a "weight" 668 factor can be associated to the Adj-SID advertised with each 669 adjacency. The weight tells the ingress (or a SDN/orchestration 670 system) about the loadbalancing factor over the parallel adjacencies. 671 As shown in Figure 1, A and B are connected through two parallel 672 adjacencies 674 link-1 675 +--------+ 676 | | 677 S---A B---C 678 | | 679 +--------+ 680 link-2 682 Figure 1: Parallel Links and Adj-SIDs 684 Node A advertises following Adj-SIDs and weights: 686 o Link-1: Adj-SID 1000, weight: 1 688 o Link-2: Adj-SID 1000, weight: 2 690 Node S receives the advertisements of the parallel adjacencies and 691 understands that by using Adj-SID 1000 node A will loadbalance the 692 traffic across the parallel links (link-1 and link-2) according to a 693 1:2 ratio. 695 The weight value is advertised with the Adj-SID as defined in IGP SR 696 extensions documents. 698 3.5.2. LAN Adjacency Segments 700 In LAN subnetworks, link-state protocols define the concept of 701 Designated Router (DR, in OSPF) or Designated Intermediate System 702 (DIS, in IS-IS) that conduct flooding in broadcast subnetworks and 703 that describe the LAN topology in a special routing update (OSPF 704 Type2 LSA or IS-IS Pseudonode LSP). 706 The difficulty with LANs is that each router only advertises its 707 connectivity to the DR/DIS and not to each other individual nodes in 708 the LAN. Therefore, additional protocol mechanisms (IS-IS and OSPF) 709 are necessary in order for each router in the LAN to advertise an 710 Adj-SID associated to each neighbor in the LAN. These extensions are 711 defined in IGP SR extensions documents. 713 3.6. Binding Segment 715 3.6.1. Mapping Server 717 A Remote-Binding SID S advertised by the mapping server M for remote 718 prefix R attached to non-SR-capable node N signals the same 719 information as if N had advertised S as a Prefix-SID. Further 720 details are described in the SR/LDP interworking procedures 721 ([I-D.filsfils-spring-segment-routing-ldp-interop]. 723 The segment allocation and SRGB Maintenance rules are the same as 724 those defined for Prefix-SID. 726 3.6.2. Tunnel Headend 728 The segment allocation and SRGB Maintenance rules are the same as 729 those defined for Adj-SID. A tunnel attached to a head-end H acts as 730 an adjacency attached to H. 732 Note: an alternative consists of representing tunnels as forwarding- 733 adjacencies ( [RFC4206]). In such case, the tunnel is presented to 734 the routing area as a routing adjacency and is considered as such by 735 all area routers. The Remote-Binding SID is preferred as it allows 736 to advertise the presence of a tunnel without influencing the LSDB 737 and the SPF computation. 739 3.7. Inter-Area Considerations 741 In the following example diagram we assume an IGP deployed using 742 areas and where SR has been deployed. 744 ! ! 745 ! ! 746 B------C-----F----G-----K 747 / | | | 748 S---A/ | | | 749 \ | | | 750 \D------I----------J-----L----Z (192.0.2.1/32, Node-SID: 150) 751 ! ! 752 Area-1 ! Backbone ! Area 2 753 ! area ! 755 Figure 2: Inter-Area Topology Example 757 In area 2, node Z allocates Node-SID 150 to his local prefix 758 192.0.2.1/32. ABRs G and J will propagate the prefix into the 759 backbone area by creating a new instance of the prefix according to 760 normal inter-area/level IGP propagation rules. 762 Nodes C and I will apply the same behavior when leaking prefixes from 763 the backbone area down to area 1. Therefore, node S will see prefix 764 192.0.2.1/32 with Prefix-SID 150 and advertised by nodes C and I. 766 It therefore results that a Prefix-SID remains attached to its 767 related IGP Prefix through the inter-area process. 769 When node S sends traffic to 192.0.2.1/32, it pushes Node-SID(150) as 770 active segment and forward it to A. 772 When packet arrives at ABR I (or C), the ABR forwards the packet 773 according to the active segment (Node-SID(150)). Forwarding 774 continues across area borders, using the same Node-SID(150), until 775 the packet reaches its destination. 777 When an ABR propagates a prefix from one area to another it MUST set 778 the R-Flag. 780 4. BGP Peering Segments 782 In the context of BGP Egress Peer Engineering (EPE), as described in 783 [I-D.filsfils-spring-segment-routing-central-epe], an EPE enabled 784 Egress PE node MAY advertise segments corresponding to its attached 785 peers. These segments are called BGP peering segments or BGP Peering 786 SIDs. They enable the expression of source-routed inter-domain 787 paths. 789 An ingress border router of an AS may compose a list of segments to 790 steer a flow along a selected path within the AS, towards a selected 791 egress border router C of the AS and through a specific peer. At 792 minimum, a BGP Peering Engineering policy applied at an ingress PE 793 involves two segments: the Node SID of the chosen egress PE and then 794 the BGP Peering Segment for the chosen egress PE peer or peering 795 interface. 797 Hereafter, we will define three types of BGP peering segments/SID's: 798 PeerNodeSID, PeerAdjSID and PeerSetSID. 800 o PeerNode SID. A BGP PeerNode segment/SID is a local segment. At 801 the BGP node advertising it, its semantics is: 803 * SR header operation: NEXT. 805 * Next-Hop: the connected peering node to which the segment is 806 related. 808 o PeerAdj SID: A BGP PeerAdj segment/SID is a local segment. At the 809 BGP node advertising it, its semantics is: 811 * SR header operation: NEXT. 813 * Next-Hop: the peer connected through the interface to which the 814 segment is related. 816 o PeerSet SID. A BGP PeerSet segment/SID is a local segment. At 817 the BGP node advertising it, its semantics is: 819 * SR header operation: NEXT. 821 * Next-Hop: loadbalance across any connected interface to any 822 peer in the related group. 824 A peer set could be all the connected peers from the same AS or a 825 subset of these. A group could also span across AS. The group 826 definition is a policy set by the operator. 828 The BGP extensions necessary in order to signal these BGP peering 829 segments will be defined in a separate document. 831 5. IGP Mirroring Context Segment 833 It is beneficial for an IGP node to be able to advertise its ability 834 to process traffic originally destined to another IGP node, called 835 the Mirrored node and identified by an IP address or a Node-SID, 836 provided that a "Mirroring Context" segment be inserted in the 837 segment list prior to any service segment local to the mirrored node. 839 When a given node B wants to provide egress node A protection, it 840 advertises a segment identifying node's A context. Such segment is 841 called "Mirror Context Segment" and identified by the Mirror SID. 843 The Mirror SID is advertised using the Binding Segment defined in SR 844 IGP protocol extensions ( [I-D.ietf-isis-segment-routing-extensions], 845 [I-D.ietf-ospf-segment-routing-extensions] and 846 [I-D.ietf-ospf-ospfv3-segment-routing-extensions]). 848 In the event of a failure, a point of local repair (PLR) diverting 849 traffic from A to B does a PUSH of the Mirror SID on the protected 850 traffic. B, when receiving the traffic with the Mirror SID as the 851 active segment, uses that segment and process underlying segments in 852 the context of A. 854 6. Multicast 856 Segment Routing is defined for unicast. The application of the 857 source-route concept to Multicast is not in the scope of this 858 document. 860 7. IANA Considerations 862 This document does not require any action from IANA. 864 8. Security Considerations 866 This document doesn't introduce new security considerations when 867 applied to the MPLS dataplane. 869 There are a number of security concerns with source routing at the 870 IPv6 dataplane [RFC5095]. The new IPv6-based segment routing header 871 defined in [I-D.previdi-6man-segment-routing-header] and its 872 associated security measures address these concerns. The IPv6 873 Segment Routing Header is defined in a way that blind attacks are 874 never possible, i.e., attackers will be unable to send source routed 875 packets that get successfully processed, without being part of the 876 negations for setting up the source routes or being able to eavesdrop 877 legitimate source routed packets. In some networks this base level 878 security may be complemented with other mechanisms, such as packet 879 filtering, cryptographic security, etc. 881 9. Contributors 883 The following people have substantially contributed to the definition 884 of the Segment Routing architecture and to the editing of this 885 document: 887 Ahmed Bashandy 888 Cisco Systems, Inc. 889 Email: bashandy@cisco.com 891 Martin Horneffer 892 Deutsche Telekom 893 Email: Martin.Horneffer@telekom.de 895 Wim Henderickx 896 Alcatel-Lucent 897 Email: wim.henderickx@alcatel-lucent.com 899 Jeff Tantsura 900 Ericsson 901 Email: Jeff.Tantsura@ericsson.com 903 Edward Crabbe 904 Individual 905 Email: edward.crabbe@gmail.com 906 Igor Milojevic 907 Email: milojevicigor@gmail.com 909 Saku Ytti 910 TDC 911 Email: saku@ytti.fi 913 10. Acknowledgements 915 We would like to thank Dave Ward, Dan Frost, Stewart Bryant, Pierre 916 Francois, Thomas Telkamp, Les Ginsberg, Ruediger Geib, Hannes 917 Gredler, Pushpasis Sarkar, Eric Rosen and Chris Bowers for their 918 comments and review of this document. 920 11. References 922 11.1. Normative References 924 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 925 Requirement Levels", BCP 14, RFC 2119, 926 DOI 10.17487/RFC2119, March 1997, 927 . 929 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 930 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, 931 December 1998, . 933 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 934 Label Switching Architecture", RFC 3031, 935 DOI 10.17487/RFC3031, January 2001, 936 . 938 [RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP) 939 Hierarchy with Generalized Multi-Protocol Label Switching 940 (GMPLS) Traffic Engineering (TE)", RFC 4206, 941 DOI 10.17487/RFC4206, October 2005, 942 . 944 11.2. Informative References 946 [I-D.filsfils-spring-large-scale-interconnect] 947 Filsfils, C., Cai, D., Previdi, S., Henderickx, W., 948 Shakir, R., Cooper, D., Ferguson, F., Laberge, T., Lin, 949 S., Decraene, B., and L. Jalil, "Interconnecting Millions 950 Of Endpoints With Segment Routing", draft-filsfils-spring- 951 large-scale-interconnect-00 (work in progress), July 2015. 953 [I-D.filsfils-spring-segment-routing-central-epe] 954 Filsfils, C., Previdi, S., Patel, K., Shaw, S., Ginsburg, 955 D., and D. Afanasiev, "Segment Routing Centralized Egress 956 Peer Engineering", draft-filsfils-spring-segment-routing- 957 central-epe-05 (work in progress), August 2015. 959 [I-D.filsfils-spring-segment-routing-ldp-interop] 960 Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., 961 Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., 962 Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe, 963 "Segment Routing interoperability with LDP", draft- 964 filsfils-spring-segment-routing-ldp-interop-03 (work in 965 progress), March 2015. 967 [I-D.filsfils-spring-segment-routing-msdc] 968 Filsfils, C., Previdi, S., Mitchell, J., Lapukhov, P., 969 Gaya, G., Afanasiev, D., Laberge, T., Nkposong, E., 970 Nanduri, M., Uttaro, J., and S. Ray, "BGP-Prefix Segment 971 in large-scale data centers", draft-filsfils-spring- 972 segment-routing-msdc-03 (work in progress), July 2015. 974 [I-D.francois-rtgwg-segment-routing-ti-lfa] 975 Francois, P., Filsfils, C., Bashandy, A., and B. Decraene, 976 "Topology Independent Fast Reroute using Segment Routing", 977 draft-francois-rtgwg-segment-routing-ti-lfa-00 (work in 978 progress), August 2015. 980 [I-D.geib-spring-oam-usecase] 981 Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "Use 982 case for a scalable and topology aware MPLS data plane 983 monitoring system", draft-geib-spring-oam-usecase-06 (work 984 in progress), July 2015. 986 [I-D.ietf-isis-prefix-attributes] 987 Ginsberg, L., Decraene, B., Filsfils, C., Litkowski, S., 988 Previdi, S., Xu, X., and U. Chunduri, "IS-IS Prefix 989 Attributes for Extended IP and IPv6 Reachability", draft- 990 ietf-isis-prefix-attributes-01 (work in progress), June 991 2015. 993 [I-D.ietf-isis-segment-routing-extensions] 994 Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., 995 Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS 996 Extensions for Segment Routing", draft-ietf-isis-segment- 997 routing-extensions-05 (work in progress), June 2015. 999 [I-D.ietf-ospf-ospfv3-segment-routing-extensions] 1000 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 1001 Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3 1002 Extensions for Segment Routing", draft-ietf-ospf-ospfv3- 1003 segment-routing-extensions-03 (work in progress), June 1004 2015. 1006 [I-D.ietf-ospf-segment-routing-extensions] 1007 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 1008 Shakir, R., Henderickx, W., and J. Tantsura, "OSPF 1009 Extensions for Segment Routing", draft-ietf-ospf-segment- 1010 routing-extensions-05 (work in progress), June 2015. 1012 [I-D.ietf-pce-segment-routing] 1013 Sivabalan, S., Medved, J., Filsfils, C., Crabbe, E., 1014 Lopez, V., Tantsura, J., Henderickx, W., and J. Hardwick, 1015 "PCEP Extensions for Segment Routing", draft-ietf-pce- 1016 segment-routing-06 (work in progress), August 2015. 1018 [I-D.ietf-spring-ipv6-use-cases] 1019 Brzozowski, J., Leddy, J., Leung, I., Previdi, S., 1020 Townsley, W., Martin, C., Filsfils, C., and R. Maglione, 1021 "IPv6 SPRING Use Cases", draft-ietf-spring-ipv6-use- 1022 cases-05 (work in progress), September 2015. 1024 [I-D.ietf-spring-problem-statement] 1025 Previdi, S., Filsfils, C., Decraene, B., Litkowski, S., 1026 Horneffer, M., and R. Shakir, "SPRING Problem Statement 1027 and Requirements", draft-ietf-spring-problem-statement-04 1028 (work in progress), April 2015. 1030 [I-D.ietf-spring-resiliency-use-cases] 1031 Francois, P., Filsfils, C., Decraene, B., and R. Shakir, 1032 "Use-cases for Resiliency in SPRING", draft-ietf-spring- 1033 resiliency-use-cases-01 (work in progress), March 2015. 1035 [I-D.ietf-spring-segment-routing-mpls] 1036 Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., 1037 Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J., 1038 and E. Crabbe, "Segment Routing with MPLS data plane", 1039 draft-ietf-spring-segment-routing-mpls-01 (work in 1040 progress), May 2015. 1042 [I-D.ietf-spring-sr-oam-requirement] 1043 Kumar, N., Pignataro, C., Akiya, N., Geib, R., Mirsky, G., 1044 and S. Litkowski, "OAM Requirements for Segment Routing 1045 Network", draft-ietf-spring-sr-oam-requirement-00 (work in 1046 progress), June 2015. 1048 [I-D.previdi-6man-segment-routing-header] 1049 Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova, 1050 J., Kosugi, T., Vyncke, E., and D. Lebrun, "IPv6 Segment 1051 Routing Header (SRH)", draft-previdi-6man-segment-routing- 1052 header-08 (work in progress), October 2015. 1054 [RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. 1055 Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", 1056 RFC 4915, DOI 10.17487/RFC4915, June 2007, 1057 . 1059 [RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation 1060 of Type 0 Routing Headers in IPv6", RFC 5095, 1061 DOI 10.17487/RFC5095, December 2007, 1062 . 1064 [RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi 1065 Topology (MT) Routing in Intermediate System to 1066 Intermediate Systems (IS-ISs)", RFC 5120, 1067 DOI 10.17487/RFC5120, February 2008, 1068 . 1070 [RFC6549] Lindem, A., Roy, A., and S. Mirtorabi, "OSPFv2 Multi- 1071 Instance Extensions", RFC 6549, DOI 10.17487/RFC6549, 1072 March 2012, . 1074 [RFC6822] Previdi, S., Ed., Ginsberg, L., Shand, M., Roy, A., and D. 1075 Ward, "IS-IS Multi-Instance", RFC 6822, 1076 DOI 10.17487/RFC6822, December 2012, 1077 . 1079 Authors' Addresses 1081 Clarence Filsfils (editor) 1082 Cisco Systems, Inc. 1083 Brussels 1084 BE 1086 Email: cfilsfil@cisco.com 1088 Stefano Previdi (editor) 1089 Cisco Systems, Inc. 1090 Via Del Serafico, 200 1091 Rome 00142 1092 Italy 1094 Email: sprevidi@cisco.com 1095 Bruno Decraene 1096 Orange 1097 FR 1099 Email: bruno.decraene@orange.com 1101 Stephane Litkowski 1102 Orange 1103 FR 1105 Email: stephane.litkowski@orange.com 1107 Rob Shakir 1108 Individual 1110 Email: rjs@rob.sh