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Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 7752 (Obsoleted by RFC 9552) Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 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. Beeram 4 Intended status: Informational C. Barth 5 Expires: August 19, 2021 Juniper Networks, Inc. 6 S. Sivabalan 7 Ciena Corporation. 8 February 15, 2021 10 Segment-Routing over Forwarding Adjacency Links 11 draft-saad-sr-fa-link-03 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 August 19, 2021. 43 Copyright Notice 45 Copyright (c) 2021 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 50 (https://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 3. Forwarding Adjacency Links . . . . . . . . . . . . . . . . . 3 63 3.1. Creation and Management . . . . . . . . . . . . . . . . . 4 64 3.2. Link Flooding . . . . . . . . . . . . . . . . . . . . . . 4 65 3.3. Underlay LSP(s) . . . . . . . . . . . . . . . . . . . . . 5 66 3.4. State Changes . . . . . . . . . . . . . . . . . . . . . . 5 67 3.5. TE Parameters . . . . . . . . . . . . . . . . . . . . . . 5 68 3.6. Link Local and Remote Identifiers . . . . . . . . . . . . 6 69 4. Segment-Routing over FA Links . . . . . . . . . . . . . . . . 6 70 4.1. SR IGP Segments for FA . . . . . . . . . . . . . . . . . 7 71 4.1.1. Parallel Adjacencies . . . . . . . . . . . . . . . . 7 72 4.2. SR BGP Segments for FA . . . . . . . . . . . . . . . . . 7 73 4.3. Applicability to Interdomain . . . . . . . . . . . . . . 8 74 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 75 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 76 7. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 9 77 8. Normative References . . . . . . . . . . . . . . . . . . . . 9 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 80 1. Introduction 82 To improve scalability in Multi-Protocol Label Switching (MPLS) 83 networks, it may be useful to create a hierarchy of LSPs as 84 Forwarding Adjacencies (FA). The concept of FA link(s) and FA-LSP(s) 85 was introduced in [RFC4206]. 87 In Segment-Routing (SR), this is particularly useful for two main 88 reasons. 90 First, it allows the stitching of sub-path(s) so as to realize an 91 end-to-end SR path. Each sub-path can be represented by a FA link 92 that is supported by one or more underlying LSP(s). The underlying 93 LSP(s) that support an FA link can be setup using different 94 technologies- including RSVP-TE, LDP, and SR. The sub-path(s), or FA 95 link(s) in this case, can possibly interconnect multiple 96 administrative domains, allowing each FA link within a domain to use 97 a different technology to setup the underlying LSP(s). 99 Second, it allows shortening of a large SR Segment-List by 100 compressing one or more slice(s) of the list into a corresponding FA 101 TE link that each can be represented by a single segment- see 102 Section 4. Effectively, it reduces the number of segments that an 103 ingress router has to impose to realize an end-to-end path. 105 The FA links are treated as normal link(s) in the network and hence 106 it can leverage existing link state protocol extensions to advertise 107 properties associated with the FA link. For example, Traffic- 108 Engineering (TE) link parameters and Segment-Routing (SR) segments 109 parameters can be associated with the FA link and advertised 110 throughout the network. 112 Once advertised in the network using a suitable protocols that 113 support carrying link state information, such as OSPF, ISIS or BGP 114 Link State (LS)), other LSR(s) in the network can use the FA TE 115 link(s) as well as possibly other normal TE link(s) when performing 116 path computation and/or when specifying the desired explicit path. 118 Though the concepts discussed in this document are specific to MPLS 119 technology, these are also extensible to other dataplane technologies 120 - e.g. SRv6. 122 2. Terminology 124 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 125 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 126 "OPTIONAL" in this document are to be interpreted as described in BCP 127 14 [RFC2119] [RFC8174] when, and only when, they appear in all 128 capitals, as shown here. 130 3. Forwarding Adjacency Links 132 FA Link(s) can be created and supported by underlying FA LSPs. The 133 FA link is of type point-to-point. FA links may be represented as 134 either unnumbered or numbered. The nodes connected by an FA link do 135 not usually establish a routing adjacency over the FA link. When FA 136 links are numbered with IPv4 addresses, the local and remote IPv4 137 addresses can come out of a /31 that is allocated by the LSR that 138 originates the FA-LSP. For unnumbered FA link(s), other provisions 139 may exist to exchange link identifier(s) between the endpoints of the 140 FA. 142 3.1. Creation and Management 144 In general, the creation/termination of an FA link and its FA-LSP is 145 driven either via configuration on the LSR at the head-end of the 146 adjacency, or dynamically using suitable North Bound Interface (NBI) 147 protocol, e.g. Netconf, gRPC, PCEP, etc. 149 The following FA-LSP attributes may be configured, including: 150 bandwidth and resource colors, and other constraints. The path taken 151 by the FA-LSP may be either computed by the LSR at the head-end of 152 the FA-LSP, or externally by a PCE and furnished to the headend. 154 The attributes of the FA link can be inherited from the underlying 155 LSP(s) that induced its creation. In general, for dynamically 156 provisioned FAs, a policy-based mechanism may be needed to associate 157 link attributes to those of the FA-LSPs. 159 When the FA link is supported by bidirectional FA LSP(s), a pair of 160 FA link(s) are advertised from each endpoint of the FA. These are 161 usually referred to as symmetrical link(s). 163 3.2. Link Flooding 165 Multiple protocols exist that can exchange link state information in 166 the network. For example, when advertising TE link(s) and their 167 attribute(s) using OSPF and ISIS protocols, the respective extensions 168 are defined in [RFC3630] and [RFC5305]. Also, when exchanging such 169 information in BGP protocol, extensions for BGP link state are 170 defined in [RFC7752] and [RFC8571]. The same protocol encodings can 171 be used to advertise FA(s) as TE link(s). As a result, the FA TE 172 link(s) and other normal TE link(s) will appear in the TE link state 173 database of any LSR in the network, and can be used for computing 174 end-to-end TE path(s). 176 When IGP protocols are used to advertise link state information about 177 FA links, the FA link(s) can appear in both the TE topology, as well 178 as the IGP topology. The use of FA link in the IGP topology may 179 result in undesirable routing loops. A router SHOULD leverage 180 exisitng mechanisms to exclude the FA link from the IGP Shortest Path 181 First (SPF) computations, and to restrict its use within the TE 182 topology for traffic engineered paths computation. 184 For example, when using ISIS to carry FA link state information, 185 [RFC5305] section 3 describes a way to restrict the link to the TE 186 topology by setting the IGP link metric to maximum (2^24 - 1). 187 Alternatively, when using OSPF, the FA link(s) can be advertised 188 using TE Opaque LSA(s) only, and hence, strictly show up in the TE 189 topology as described in [RFC3630] . 191 3.3. Underlay LSP(s) 193 The LSR that hosts an FA link can setup the underlying LSP(s) using 194 different technologies - e.g. RSVP-TE, LDP, and SR. 196 The FA link can be supported by one or more underlay LSP(s) that 197 terminate on the same remote endpoint. The underlay path(s) can be 198 setup using different signaling technologies, e.g. using RSVP-TE, 199 LDP, SR, etc. When multiple LSP(s) support the same FA link, the 200 attributes of the FA link can be derived from the aggregate 201 properties of each of the underlying LSP(s). 203 3.4. State Changes 205 The state of an FA TE link reflects the state of the underlying LSP 206 path that supports it. The TE link is assumed operational and is 207 advertised as long as the underlying LSP path is valid. When all 208 underlying LSP paths are invalidated, the FA TE link advertisement is 209 withdrawn. 211 3.5. TE Parameters 213 The TE metrics and TE attributes are used by path computation 214 algorithms to select the TE link(s) that a TE path traverses. When 215 advertising an FA link in OSPF or ISIS, or BGP-LS, the following TE 216 parameters are defined: 218 TE Path metrics: the FA link advertisement can include information 219 about TE, IGP, and other performance metrics (e.g. delay, and 220 loss). The FA link TE metrics, in this case, can be derived from 221 the underlying path(s) that support the FA link by producing the 222 path accumulative metrics. When multiple LSP(s) support the same 223 FA link, then the higher accumulative metric amongst the LSP(s) is 224 inherited by the FA link. 226 Resource Class/Color: An FA link can be assigned (e.g. via 227 configuration) a specific set of admin-groups. Alternatively, in 228 some cases, this can be derived from the underlying path affinity 229 - for example, the underlying path strictly includes a specific 230 admin-group. 232 SRLGs: An FA advertisement could contain the information about the 233 Shared Risk Link Groups (SRLG) for the path taken by the FA LSP 234 associated with that FA. This information may be used for path 235 calculation by other LSRs. The information carried is the union 236 of the SRLGs of the underlying TE links that make up the FA LSP 237 path. It is possible that the underlying path information might 238 change over time, via configuration updates or dynamic route 239 modifications, resulting in the change of the union of SRLGs for 240 the FA link. If multiple LSP(s) support the same FA link, then it 241 is expected all LSP(s) have the same SRLG union - note, that the 242 exact paths need not be the same. 244 It is worth noting, that topology changes in the network may affect 245 the FA link underlying LSP path(s), and hence, can dynamically change 246 the TE metrics and TE attributes of the FA links. 248 3.6. Link Local and Remote Identifiers 250 It is possible for the FA link to be numbered or unnumbered. 251 [RFC4206] describes a procedure for identifying a numbered FA TE link 252 using IPv4 addresses. 254 For unnumbered FA link(s), the assignment and handling of the local 255 and remote link identifiers is specified in [RFC3477]. The LSR at 256 each end of the unnumbered FA link assigns an identifier to that 257 link. This identifier is a non-zero 32-bit number that is unique 258 within the scope of the LSR that assigns it. There is no a priori 259 relationship between the identifiers assigned to a link by the LSRs 260 at each end of that link. 262 The FA link is a unidirectional and point-to-point link. Hence, the 263 combination of link local identifier and advertising node can 264 uniquely identify the link in the TED. In some cases, however, it is 265 desirable to associate the forward and reverse FA links in the TED. 266 In this case, the combination of link local and remote identifier can 267 identify the pair of forward and reverse FA link(s). The LSRs at the 268 two end points of an unnumbered link can exchange with each other the 269 identifiers they assign to the link. Exchanging the identifiers may 270 be accomplished by configuration, or by means of protocol extensions. 271 For example, when the FA link is established over RSVP-TE FA LSP(s), 272 then RSVP extensions have been introduced to exchange the FA link 273 identifier in [RFC3477]. Other protocol extensions pertaining to 274 specific link state protocols, and LSP setup technologies will be 275 discussed in a separate document. 277 If the link remote identifier is unknown, the value advertised is set 278 to 0 [RFC5307]. 280 4. Segment-Routing over FA Links 282 The Segment Routing (SR) architecture [RFC4206] describes that an IGP 283 adjacency can be formed over a FA link - in which the remote node of 284 an IGP adjacency is a non-adjacent IGP neighbor. 286 In Segment-Routing (SR), the adjacency that is established over a 287 link can be assigned an SR Segment [RFC8402]. For example, the Adj- 288 SID allows to strictly steer traffic on to the specific adjacency 289 that is associated with the Adj-SID. 291 4.1. SR IGP Segments for FA 293 Extensions have been defined to ISIS [RFC8667] and OSPF [RFC8665] in 294 order to advertise the the Adjacency-SID associated with a specific 295 IGP adjacency. The same extensions apply to adjacencies over FA 296 link. A node can bind an Adj-SID to an FA data-link. The Adj-SID 297 dictates the forwarding of packets through the specific FA link or FA 298 link(s) identified by the Adj-SID, regardless of its IGP/SPF cost. 300 When the FA link Adj-SID is supported by a single underlying LSP that 301 is associated with a binding label or SID, the same binding label can 302 be used for the FA link Adj-SID. For example, if the FA link is 303 supported by an SR Policy that is assigned a Binding SID B, the Adj- 304 SID of the FA link can be assigned the same Binding SID B. 306 When the FA link Adj-SID is supported by multiple underlying LSP(s) 307 or SR Policies - each having its own Binding label or SID, an 308 independent FA link Adj-SID is allocated and bound to the multiple 309 underlying LSP(s). 311 4.1.1. Parallel Adjacencies 313 Adj-SIDs can also be used in order to represent a set of parallel FA 314 link(s) between two endpoints. 316 When parallel FA links are associated with the same Adj-SID, a 317 "weight" factor can be assigned to each link and advertised with the 318 Adj-SID advertised with each FA link. The weight informs the ingress 319 (or an SDN/orchestration system) about the load-balancing factor over 320 the parallel adjacencies. 322 4.2. SR BGP Segments for FA 324 BGP segments are allocated and distributed by BGP. The SR 325 architecture [RFC8402] defines three types of BGP segments for Egress 326 Peer Engineering (EPE): PeerNode SID, PeerAdj SID, and PeerSet SID. 328 The applicability of each of the three types to FA links is discussed 329 below: 331 o PeerNode SID: a BGP PeerNode segment/SID is a local segment. At 332 the BGP node advertising, the forwarding semantics are: 334 * SR operation: NEXT. 336 * Next-Hop: forward over any FA link associated with the segment 337 that terminates on remote endpoint. 339 o PeerAdj SID: a BGP PeerAdj segment/SID is a local segment. At the 340 BGP node advertising it, the forwarding semantics are: 342 * SR operation: NEXT. 344 * Next-Hop: forward over the specific FA link to the remote 345 endpoint to which the segment is related. 347 o PeerSet SID: a BGP PeerSet segment/SID is a local segment. At the 348 BGP node advertising it, the semantics are: 350 * SR operation: NEXT. 352 * Next-Hop: load-balance across any of the FA links to any remote 353 endpoint in the related set. The group definition is a policy 354 set by the operator. 356 4.3. Applicability to Interdomain 358 In order to determine the potential to establish a TE path through a 359 series of interconnected domains or multi-domain network, it is 360 necessary to have available a certain amount of TE information about 361 each network domain. This need not be the full set of TE information 362 available within each network but does need to express the potential 363 of providing such TE connectivity. 365 Topology abstraction is described in [RFC7926]. Abstraction allows 366 applying a policy to the available TE information within a domain so 367 to produce selective information that represents the potential 368 ability to connect across the domain. Thus, abstraction does not 369 necessarily offer all possible connectivity options, but presents a 370 general view of potential connectivity according to the policies that 371 determine how the domain's administrator wants to allow the domain 372 resources to be used. 374 Hence, the domain may be constructed as a mesh of border node to 375 border node TE FA links. When computing a path for an LSP that 376 crosses the domain, a computation point can see which domain entry 377 points can be connected to which others, and with what TE attributes. 379 5. IANA Considerations 381 This document has no IANA actions. 383 6. Security Considerations 385 TBD. 387 7. Acknowledgement 389 The authors would like to thank Peter Psenak for reviewing and 390 providing valuable feedback on this document. 392 8. Normative References 394 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 395 Requirement Levels", BCP 14, RFC 2119, 396 DOI 10.17487/RFC2119, March 1997, 397 . 399 [RFC3477] Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links 400 in Resource ReSerVation Protocol - Traffic Engineering 401 (RSVP-TE)", RFC 3477, DOI 10.17487/RFC3477, January 2003, 402 . 404 [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 405 (TE) Extensions to OSPF Version 2", RFC 3630, 406 DOI 10.17487/RFC3630, September 2003, 407 . 409 [RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP) 410 Hierarchy with Generalized Multi-Protocol Label Switching 411 (GMPLS) Traffic Engineering (TE)", RFC 4206, 412 DOI 10.17487/RFC4206, October 2005, 413 . 415 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 416 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 417 2008, . 419 [RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions 420 in Support of Generalized Multi-Protocol Label Switching 421 (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008, 422 . 424 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 425 S. Ray, "North-Bound Distribution of Link-State and 426 Traffic Engineering (TE) Information Using BGP", RFC 7752, 427 DOI 10.17487/RFC7752, March 2016, 428 . 430 [RFC7926] Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G., 431 Ceccarelli, D., and X. Zhang, "Problem Statement and 432 Architecture for Information Exchange between 433 Interconnected Traffic-Engineered Networks", BCP 206, 434 RFC 7926, DOI 10.17487/RFC7926, July 2016, 435 . 437 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 438 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 439 May 2017, . 441 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 442 Decraene, B., Litkowski, S., and R. Shakir, "Segment 443 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 444 July 2018, . 446 [RFC8571] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and 447 C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of 448 IGP Traffic Engineering Performance Metric Extensions", 449 RFC 8571, DOI 10.17487/RFC8571, March 2019, 450 . 452 [RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler, 453 H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF 454 Extensions for Segment Routing", RFC 8665, 455 DOI 10.17487/RFC8665, December 2019, 456 . 458 [RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., 459 Bashandy, A., Gredler, H., and B. Decraene, "IS-IS 460 Extensions for Segment Routing", RFC 8667, 461 DOI 10.17487/RFC8667, December 2019, 462 . 464 Authors' Addresses 466 Tarek Saad 467 Juniper Networks, Inc. 469 Email: tsaad@juniper.net 470 Vishnu Pavan Beeram 471 Juniper Networks, Inc. 473 Email: vbeeram@juniper.net 475 Colby Barth 476 Juniper Networks, Inc. 478 Email: cbarth@juniper.net 480 Siva Sivabalan 481 Ciena Corporation. 483 Email: ssivabal@ciena.com