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Summary: 0 errors (**), 0 flaws (~~), 0 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SPRING Working Group T. Saad 3 Internet-Draft V.P. Beeram 4 Intended status: Informational C. Barth 5 Expires: 26 August 2022 Juniper Networks, Inc. 6 S. Sivabalan 7 Ciena Corporation. 8 22 February 2022 10 Segment-Routing over Forwarding Adjacency Links 11 draft-saad-spring-srfa-link-01 13 Abstract 15 Label Switched Paths (LSPs) set up in Multiprotocol Label Switching 16 (MPLS) networks can be used to form Forwarding Adjacency (FA) links 17 that carry traffic in those networks. An FA link can be assigned 18 Traffic Engineering (TE) parameters that allow other LSR(s) to 19 include it in their constrained path computation. FA link(s) can be 20 also assigned Segment-Routing (SR) segments that enable the steering 21 of traffic on to the associated FA link(s). The TE and SR attributes 22 of an FA link can be advertised using known protocols that carry link 23 state information. This document elaborates on the usage of FA 24 link(s) and their attributes in SR enabled networks. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at https://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on 26 August 2022. 43 Copyright Notice 45 Copyright (c) 2022 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 50 license-info) in effect on the date of publication of this document. 51 Please review these documents carefully, as they describe your rights 52 and restrictions with respect to this document. Code Components 53 extracted from this document must include Revised BSD License text as 54 described in Section 4.e of the Trust Legal Provisions and are 55 provided without warranty as described in the Revised BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 60 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 61 3. Forwarding Adjacency Links . . . . . . . . . . . . . . . . . 3 62 3.1. Creation and Management . . . . . . . . . . . . . . . . . 4 63 3.2. Link Flooding . . . . . . . . . . . . . . . . . . . . . . 4 64 3.3. Underlay LSP(s) . . . . . . . . . . . . . . . . . . . . . 5 65 3.4. State Changes . . . . . . . . . . . . . . . . . . . . . . 5 66 3.5. TE Parameters . . . . . . . . . . . . . . . . . . . . . . 5 67 3.6. Link Local and Remote Identifiers . . . . . . . . . . . . 6 68 4. Segment-Routing over FA Links . . . . . . . . . . . . . . . . 7 69 4.1. SR IGP Segments for FA . . . . . . . . . . . . . . . . . 7 70 4.1.1. Parallel Adjacencies . . . . . . . . . . . . . . . . 7 71 4.2. SR BGP Segments for FA . . . . . . . . . . . . . . . . . 7 72 4.3. Applicability to Interdomain . . . . . . . . . . . . . . 8 73 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 74 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 75 7. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 9 76 8. Normative References . . . . . . . . . . . . . . . . . . . . 9 77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 79 1. Introduction 81 To improve scalability in Multi-Protocol Label Switching (MPLS) 82 networks, it may be useful to create a hierarchy of LSPs as 83 Forwarding Adjacencies (FA). The concept of FA link(s) and FA-LSP(s) 84 was introduced in [RFC4206]. 86 In Segment-Routing (SR), this is particularly useful for two main 87 reasons. 89 First, it allows the stitching of sub-path(s) so as to realize an 90 end-to-end SR path. Each sub-path can be represented by a FA link 91 that is supported by one or more underlying LSP(s). The underlying 92 LSP(s) that support an FA link can be setup using different 93 technologies- including RSVP-TE, LDP, and SR. The sub-path(s), or FA 94 link(s) in this case, can possibly interconnect multiple 95 administrative domains, allowing each FA link within a domain to use 96 a different technology to setup the underlying LSP(s). 98 Second, it allows shortening of a large SR Segment-List by 99 compressing one or more slice(s) of the list into a corresponding FA 100 TE link that each can be represented by a single segment- see 101 Section 4. Effectively, it reduces the number of segments that an 102 ingress router has to impose to realize an end-to-end path. 104 The FA links are treated as normal link(s) in the network and hence 105 it can leverage existing link state protocol extensions to advertise 106 properties associated with the FA link. For example, Traffic- 107 Engineering (TE) link parameters and Segment-Routing (SR) segments 108 parameters can be associated with the FA link and advertised 109 throughout the network. 111 Once advertised in the network using a suitable protocols that 112 support carrying link state information, such as OSPF, ISIS or BGP 113 Link State (LS)), other LSR(s) in the network can use the FA TE 114 link(s) as well as possibly other normal TE link(s) when performing 115 path computation and/or when specifying the desired explicit path. 117 Though the concepts discussed in this document are specific to MPLS 118 technology, these are also extensible to other dataplane technologies 119 - e.g. SRv6. 121 2. Terminology 123 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 124 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 125 "OPTIONAL" in this document are to be interpreted as described in 126 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 127 capitals, as shown here. 129 3. Forwarding Adjacency Links 131 FA Link(s) can be created and supported by underlying FA LSPs. The 132 FA link is of type point-to-point. FA links may be represented as 133 either unnumbered or numbered. The nodes connected by an FA link do 134 not usually establish a routing adjacency over the FA link. When FA 135 links are numbered with IPv4 addresses, the local and remote IPv4 136 addresses can come out of a /31 that is allocated by the LSR that 137 originates the FA-LSP. For unnumbered FA link(s), other provisions 138 may exist to exchange link identifier(s) between the endpoints of the 139 FA. 141 3.1. Creation and Management 143 In general, the creation/termination of an FA link and its FA-LSP is 144 driven either via configuration on the LSR at the head-end of the 145 adjacency, or dynamically using suitable North Bound Interface (NBI) 146 protocol, e.g. Netconf, gRPC, PCEP, etc. 148 The following FA-LSP attributes may be configured, including: 149 bandwidth and resource colors, and other constraints. The path taken 150 by the FA-LSP may be either computed by the LSR at the head-end of 151 the FA-LSP, or externally by a PCE and furnished to the headend. 153 The attributes of the FA link can be inherited from the underlying 154 LSP(s) that induced its creation. In general, for dynamically 155 provisioned FAs, a policy-based mechanism may be needed to associate 156 link attributes to those of the FA-LSPs. 158 When the FA link is supported by bidirectional FA LSP(s), a pair of 159 FA link(s) are advertised from each endpoint of the FA. These are 160 usually referred to as symmetrical link(s). 162 3.2. Link Flooding 164 Multiple protocols exist that can exchange link state information in 165 the network. For example, when advertising TE link(s) and their 166 attribute(s) using OSPF and ISIS protocols, the respective extensions 167 are defined in [RFC3630] and [RFC5305]. Also, when exchanging such 168 information in BGP protocol, extensions for BGP link state are 169 defined in [RFC7752] and [RFC8571]. The same protocol encodings can 170 be used to advertise FA(s) as TE link(s). As a result, the FA TE 171 link(s) and other normal TE link(s) will appear in the TE link state 172 database of any LSR in the network, and can be used for computing 173 end-to-end TE path(s). 175 When IGP protocols are used to advertise link state information about 176 FA links, the FA link(s) can appear in both the TE topology, as well 177 as the IGP topology. The use of FA link in the IGP topology may 178 result in undesirable routing loops. A router SHOULD leverage 179 exisitng mechanisms to exclude the FA link from the IGP Shortest Path 180 First (SPF) computations, and to restrict its use within the TE 181 topology for traffic engineered paths computation. 183 For example, when using ISIS to carry FA link state information, 184 [RFC5305] section 3 describes a way to restrict the link to the TE 185 topology by setting the IGP link metric to maximum (2^24 - 1). 186 Alternatively, when using OSPF, the FA link(s) can be advertised 187 using TE Opaque LSA(s) only, and hence, strictly show up in the TE 188 topology as described in [RFC3630] . 190 3.3. Underlay LSP(s) 192 The LSR that hosts an FA link can setup the underlying LSP(s) using 193 different technologies - e.g. RSVP-TE, LDP, and SR. 195 The FA link can be supported by one or more underlay LSP(s) that 196 terminate on the same remote endpoint. The underlay path(s) can be 197 setup using different signaling technologies, e.g. using RSVP-TE, 198 LDP, SR, etc. When multiple LSP(s) support the same FA link, the 199 attributes of the FA link can be derived from the aggregate 200 properties of each of the underlying LSP(s). 202 3.4. State Changes 204 The state of an FA TE link reflects the state of the underlying LSP 205 path that supports it. The TE link is assumed operational and is 206 advertised as long as the underlying LSP path is valid. When all 207 underlying LSP paths are invalidated, the FA TE link advertisement is 208 withdrawn. 210 3.5. TE Parameters 212 The TE metrics and TE attributes are used by path computation 213 algorithms to select the TE link(s) that a TE path traverses. When 214 advertising an FA link in OSPF or ISIS, or BGP-LS, the following TE 215 parameters are defined: 217 TE Path metrics: the FA link advertisement can include information 218 about TE, IGP, and other performance metrics (e.g. delay, and 219 loss). The FA link TE metrics, in this case, can be derived from 220 the underlying path(s) that support the FA link by producing the 221 path accumulative metrics. When multiple LSP(s) support the same 222 FA link, then the higher accumulative metric amongst the LSP(s) is 223 inherited by the FA link. 225 Resource Class/Color: An FA link can be assigned (e.g. via 226 configuration) a specific set of admin-groups. Alternatively, in 227 some cases, this can be derived from the underlying path affinity 228 - for example, the underlying path strictly includes a specific 229 admin-group. 231 SRLGs: An FA advertisement could contain the information about the 232 Shared Risk Link Groups (SRLG) for the path taken by the FA LSP 233 associated with that FA. This information may be used for path 234 calculation by other LSRs. The information carried is the union 235 of the SRLGs of the underlying TE links that make up the FA LSP 236 path. It is possible that the underlying path information might 237 change over time, via configuration updates or dynamic route 238 modifications, resulting in the change of the union of SRLGs for 239 the FA link. If multiple LSP(s) support the same FA link, then it 240 is expected all LSP(s) have the same SRLG union - note, that the 241 exact paths need not be the same. 243 It is worth noting, that topology changes in the network may affect 244 the FA link underlying LSP path(s), and hence, can dynamically change 245 the TE metrics and TE attributes of the FA links. 247 3.6. Link Local and Remote Identifiers 249 It is possible for the FA link to be numbered or unnumbered. 250 [RFC4206] describes a procedure for identifying a numbered FA TE link 251 using IPv4 addresses. 253 For unnumbered FA link(s), the assignment and handling of the local 254 and remote link identifiers is specified in [RFC3477]. The LSR at 255 each end of the unnumbered FA link assigns an identifier to that 256 link. This identifier is a non-zero 32-bit number that is unique 257 within the scope of the LSR that assigns it. There is no a priori 258 relationship between the identifiers assigned to a link by the LSRs 259 at each end of that link. 261 The FA link is a unidirectional and point-to-point link. Hence, the 262 combination of link local identifier and advertising node can 263 uniquely identify the link in the TED. In some cases, however, it is 264 desirable to associate the forward and reverse FA links in the TED. 265 In this case, the combination of link local and remote identifier can 266 identify the pair of forward and reverse FA link(s). The LSRs at the 267 two end points of an unnumbered link can exchange with each other the 268 identifiers they assign to the link. Exchanging the identifiers may 269 be accomplished by configuration, or by means of protocol extensions. 270 For example, when the FA link is established over RSVP-TE FA LSP(s), 271 then RSVP extensions have been introduced to exchange the FA link 272 identifier in [RFC3477]. Other protocol extensions pertaining to 273 specific link state protocols, and LSP setup technologies will be 274 discussed in a separate document. 276 If the link remote identifier is unknown, the value advertised is set 277 to 0 [RFC5307]. 279 4. Segment-Routing over FA Links 281 The Segment Routing (SR) architecture [RFC4206] describes that an IGP 282 adjacency can be formed over a FA link - in which the remote node of 283 an IGP adjacency is a non-adjacent IGP neighbor. 285 In Segment-Routing (SR), the adjacency that is established over a 286 link can be assigned an SR Segment [RFC8402]. For example, the Adj- 287 SID allows to strictly steer traffic on to the specific adjacency 288 that is associated with the Adj-SID. 290 4.1. SR IGP Segments for FA 292 Extensions have been defined to ISIS [RFC8667] and OSPF [RFC8665] in 293 order to advertise the the Adjacency-SID associated with a specific 294 IGP adjacency. The same extensions apply to adjacencies over FA 295 link. A node can bind an Adj-SID to an FA data-link. The Adj-SID 296 dictates the forwarding of packets through the specific FA link or FA 297 link(s) identified by the Adj-SID, regardless of its IGP/SPF cost. 299 When the FA link Adj-SID is supported by a single underlying LSP that 300 is associated with a binding label or SID, the same binding label can 301 be used for the FA link Adj-SID. For example, if the FA link is 302 supported by an SR Policy that is assigned a Binding SID B, the Adj- 303 SID of the FA link can be assigned the same Binding SID B. 305 When the FA link Adj-SID is supported by multiple underlying LSP(s) 306 or SR Policies - each having its own Binding label or SID, an 307 independent FA link Adj-SID is allocated and bound to the multiple 308 underlying LSP(s). 310 4.1.1. Parallel Adjacencies 312 Adj-SIDs can also be used in order to represent a set of parallel FA 313 link(s) between two endpoints. 315 When parallel FA links are associated with the same Adj-SID, a 316 "weight" factor can be assigned to each link and advertised with the 317 Adj-SID advertised with each FA link. The weight informs the ingress 318 (or an SDN/orchestration system) about the load-balancing factor over 319 the parallel adjacencies. 321 4.2. SR BGP Segments for FA 323 BGP segments are allocated and distributed by BGP. The SR 324 architecture [RFC8402] defines three types of BGP segments for Egress 325 Peer Engineering (EPE): PeerNode SID, PeerAdj SID, and PeerSet SID. 327 The applicability of each of the three types to FA links is discussed 328 below: 330 o PeerNode SID: a BGP PeerNode segment/SID is a local segment. At 331 the BGP node advertising, the forwarding semantics are: 333 - SR operation: NEXT. 335 - Next-Hop: forward over any FA link associated with the segment 336 that terminates on remote endpoint. 338 o PeerAdj SID: a BGP PeerAdj segment/SID is a local segment. At the 339 BGP node advertising it, the forwarding semantics are: 341 - SR operation: NEXT. 343 - Next-Hop: forward over the specific FA link to the remote 344 endpoint to which the segment is related. 346 o PeerSet SID: a BGP PeerSet segment/SID is a local segment. At the 347 BGP node advertising it, the semantics are: 349 - SR operation: NEXT. 351 - Next-Hop: load-balance across any of the FA links to any remote 352 endpoint in the related set. The group definition is a policy 353 set by the operator. 355 4.3. Applicability to Interdomain 357 In order to determine the potential to establish a TE path through a 358 series of interconnected domains or multi-domain network, it is 359 necessary to have available a certain amount of TE information about 360 each network domain. This need not be the full set of TE information 361 available within each network but does need to express the potential 362 of providing such TE connectivity. 364 Topology abstraction is described in [RFC7926]. Abstraction allows 365 applying a policy to the available TE information within a domain so 366 to produce selective information that represents the potential 367 ability to connect across the domain. Thus, abstraction does not 368 necessarily offer all possible connectivity options, but presents a 369 general view of potential connectivity according to the policies that 370 determine how the domain's administrator wants to allow the domain 371 resources to be used. 373 Hence, the domain may be constructed as a mesh of border node to 374 border node TE FA links. When computing a path for an LSP that 375 crosses the domain, a computation point can see which domain entry 376 points can be connected to which others, and with what TE attributes. 378 5. IANA Considerations 380 This document has no IANA actions. 382 6. Security Considerations 384 TBD. 386 7. Acknowledgement 388 The authors would like to thank Peter Psenak for reviewing and 389 providing valuable feedback on this document. 391 8. Normative References 393 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 394 Requirement Levels", BCP 14, RFC 2119, 395 DOI 10.17487/RFC2119, March 1997, 396 . 398 [RFC3477] Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links 399 in Resource ReSerVation Protocol - Traffic Engineering 400 (RSVP-TE)", RFC 3477, DOI 10.17487/RFC3477, January 2003, 401 . 403 [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 404 (TE) Extensions to OSPF Version 2", RFC 3630, 405 DOI 10.17487/RFC3630, September 2003, 406 . 408 [RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP) 409 Hierarchy with Generalized Multi-Protocol Label Switching 410 (GMPLS) Traffic Engineering (TE)", RFC 4206, 411 DOI 10.17487/RFC4206, October 2005, 412 . 414 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 415 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 416 2008, . 418 [RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions 419 in Support of Generalized Multi-Protocol Label Switching 420 (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008, 421 . 423 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 424 S. Ray, "North-Bound Distribution of Link-State and 425 Traffic Engineering (TE) Information Using BGP", RFC 7752, 426 DOI 10.17487/RFC7752, March 2016, 427 . 429 [RFC7926] Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G., 430 Ceccarelli, D., and X. Zhang, "Problem Statement and 431 Architecture for Information Exchange between 432 Interconnected Traffic-Engineered Networks", BCP 206, 433 RFC 7926, DOI 10.17487/RFC7926, July 2016, 434 . 436 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 437 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 438 May 2017, . 440 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 441 Decraene, B., Litkowski, S., and R. Shakir, "Segment 442 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 443 July 2018, . 445 [RFC8571] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and 446 C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of 447 IGP Traffic Engineering Performance Metric Extensions", 448 RFC 8571, DOI 10.17487/RFC8571, March 2019, 449 . 451 [RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler, 452 H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF 453 Extensions for Segment Routing", RFC 8665, 454 DOI 10.17487/RFC8665, December 2019, 455 . 457 [RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., 458 Bashandy, A., Gredler, H., and B. Decraene, "IS-IS 459 Extensions for Segment Routing", RFC 8667, 460 DOI 10.17487/RFC8667, December 2019, 461 . 463 Authors' Addresses 464 Tarek Saad 465 Juniper Networks, Inc. 466 Email: tsaad@juniper.net 468 Vishnu Pavan Beeram 469 Juniper Networks, Inc. 470 Email: vbeeram@juniper.net 472 Colby Barth 473 Juniper Networks, Inc. 474 Email: cbarth@juniper.net 476 Siva Sivabalan 477 Ciena Corporation. 478 Email: ssivabal@ciena.com