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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group S. Kini, Ed. 3 Internet-Draft Ericsson 4 Intended status: Informational K. Kompella 5 Expires: March 9, 2016 Juniper 6 S. Sivabalan 7 Cisco 8 S. Litkowski 9 Orange 10 R. Shakir 12 X. Xu 13 Huawei 14 W. Hendrickx 15 Alcatel-Lucent 16 J. Tantsura 17 Ericsson 18 September 6, 2015 20 Entropy labels for source routed stacked tunnels 21 draft-ietf-mpls-spring-entropy-label-01 23 Abstract 25 Source routed tunnel stacking is a technique that can be leveraged to 26 provide a method to steer a packet through a controlled set of 27 segments. This can be applied to the Multi Protocol Label Switching 28 (MPLS) data plane. Entropy label (EL) is a technique used in MPLS to 29 improve load balancing. This document examines and describes how ELs 30 are to be applied to source routed stacked tunnels. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at http://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on March 9, 2016. 49 Copyright Notice 51 Copyright (c) 2015 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (http://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 67 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 68 2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 3 69 3. Use-case requiring multipath load balancing in source stacked 70 tunnels . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 71 4. Recommended EL solution for SPRING . . . . . . . . . . . . . 5 72 5. Options considered . . . . . . . . . . . . . . . . . . . . . 6 73 5.1. Single EL at the bottom of the stack of tunnels . . . . . 6 74 5.2. An EL per tunnel in the stack . . . . . . . . . . . . . . 7 75 5.3. A re-usable EL for a stack of tunnels . . . . . . . . . . 7 76 5.3.1. EL at top of stack . . . . . . . . . . . . . . . . . 8 77 5.4. ELs at readable label stack depths . . . . . . . . . . . 8 78 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 79 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 80 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 81 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 82 9.1. Normative References . . . . . . . . . . . . . . . . . . 9 83 9.2. Informative References . . . . . . . . . . . . . . . . . 10 84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 86 1. Introduction 88 The source routed stacked tunnels paradigm is leveraged by techniques 89 such as Segment Routing (SR) [I-D.filsfils-spring-segment-routing] to 90 steer a packet through a set of segments. This can be directly 91 applied to the MPLS data plane, but it has implications on label 92 stack depth. 94 Clarifying statements on label stack depth have been provided in 95 [RFC7325] but they do not address the case of source routed stacked 96 MPLS tunnels as described in [I-D.filsfils-spring-segment-routing] 97 where deeper label stacks are more prevalent. 99 Entropy label (EL) [RFC6790] is a technique used in the MPLS data 100 plane to provide entropy for load balancing. When using LSP 101 hierarchies there are implications on how [RFC6790] should be 102 applied. One such issue is addressed by 103 [I-D.ravisingh-mpls-el-for-seamless-mpls] but that is when different 104 levels of the hierarchy are created at different LSRs. The current 105 document addresses the case where the hierarchy is created at a 106 single LSR as required by source stacked tunnels. 108 A use-case requiring load balancing with source stacked tunnels is 109 given in Section 3. A recommended solution is described in Section 4 110 keeping in consideration the limitations of implementations when 111 applying [RFC6790] to deeper label stacks. Options that were 112 considered to arrive at the recommended solution are documented for 113 historical purposes in Section 5. 115 1.1. Requirements Language 117 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 118 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 119 document are to be interpreted as described in RFC 2119 [RFC2119]. 121 Although this document is not a protocol specification, the use of 122 this language clarifies the instructions to protocol designers 123 producing solutions that satisfy the requirements set out in this 124 document. 126 2. Abbreviations and Terminology 128 EL - Entropy Label 130 ELI - Entropy Label Identifier 132 ELC - Entropy Label Capability 134 SR - Segment Routing 136 ECMP - Equal Cost Multi Paths 138 MPLS - Multiprotocol Label Switching 140 SID - Segment Identifier 142 RLD - Readable Label Depth 143 OAM - Operation, Administration and Maintenance 145 3. Use-case requiring multipath load balancing in source stacked 146 tunnels 148 +------+ 149 | | 150 +-------| P3 |-----+ 151 | +-----| |---+ | 152 L3| |L4 +------+ L1| |L2 +----+ 153 | | | | +--| P4 |--+ 154 +-----+ +-----+ +-----+ | +----+ | +-----+ 155 | S |-----| P1 |------------| P2 |--+ +--| D | 156 | | | | | |--+ +--| | 157 +-----+ +-----+ +-----+ | +----+ | +-----+ 158 +--| P5 |--+ 159 +----+ 161 S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs, 162 L1,L2,L3,L4=Links 164 Figure 1: Traffic engineering use-case 166 Traffic-engineering (TE) is one of the applications of MPLS and is 167 also a requirement for source stacked tunnels. Consider the topology 168 shown in Figure 1. Lets say the LSR P1 has a limitation that it can 169 only look four labels deep in the stack to do multipath decisions. 170 All other transit LSRs in the figure can read deep label stacks and 171 the LSR S can insert as many pairs as needed. The LSR S 172 requires data to be sent to LSR D along a traffic-engineered path 173 that goes over the link L1. Good load balancing is also required 174 across equal cost paths (including parallel links). To engineer 175 traffic along a path that takes link L1, the label stack that LSR S 176 creates consists of a label to the node SID of LSR P3, stacked over 177 the label for the adjacency SID of link L1 and that in turn is 178 stacked over the label to the node SID of LSR D. For simplicity lets 179 assume that all LSRs use the same label space for source stacked 180 tunnels. Lets L_N-P denote the label to be used to reach the node 181 SID of LSR P. Let L_A-Ln denote the label used for the adjacency SID 182 for link Ln. The LSR S must use the label stack for traffic-engineering. However to achieve good load 184 balancing over the equal cost paths P2-P4-D, P2-P5-D and the parallel 185 links L3, L4, a mechanism such as Entropy labels [RFC6790] should be 186 adapted for source stacked tunnels. Multiple ways to apply entropy 187 labels were considered and are documented in Section 5 along with 188 their tradeoffs. A recommended solution is described in Section 4. 190 4. Recommended EL solution for SPRING 192 The solution described in this section follows [RFC6790]. 194 An LSR may have a limitation in its ability to read and process the 195 label stack in order to do multipath load balancing. This limitation 196 expressed in terms of the number of label stack entries that the LSR 197 can read is henceforth referred to as the Readable Label Depth (RLD) 198 capability of that LSR. If an EL does not occur within the RLD of an 199 LSR in the label stack of the MPLS packet that it receives, then it 200 would lead to poor load balancing at that LSR. The RLD of an LSR is 201 a characteristic of the forwarding plane of that LSR's implementation 202 and determining it is outside the scope of this document. 204 In order for the EL to occur within the RLD of LSRs along the path 205 corresponding to a label stack, multiple pairs MAY be 206 inserted in the label stack as long as the tunnel's label below which 207 they are inserted are advertised with entropy label capability 208 enabled. The LSR that inserts pairs MAY have limitations 209 on the number of such pairs that it can insert and also the depth at 210 which it can insert them. If due to any limitation, the inserted ELs 211 are at positions such that an LSR along the path receives an MPLS 212 packet without an EL in the label stack within that LSR's RLD, then 213 the load balancing performed by that LSR would be poor. Special 214 attention should be paid when a forwarding adjacency LSP (FA-LSP) 215 [RFC4206] is used as a link along the path of a source stacked LSP, 216 since the labels of the FA-LSP would additionally count towards the 217 depth of the label stack when calculating the appropriate positions 218 to insert the ELs. The recommendations for inserting pairs 219 are: 221 o An LSR that is limited in the number of pairs that it 222 can insert SHOULD insert such pairs deeper in the stack. 224 o An LSR SHOULD try to insert pairs at positions so that 225 for the maximum number of transit LSRs, the EL occurs within the 226 RLD of the incoming packet to that LSR. 228 o An LSR SHOULD try to insert the minimum number of such pairs while 229 trying to satisfy the above criteria. 231 A sample algorithm to insert ELs is shown below. Implementations can 232 choose any algorithm as long as it follows the above recommendations. 234 Initialize the current EL insertion point to the 235 bottommost label in the stack that is EL-capable 236 while (local-node can push more pairs OR 237 insertion point is not above label stack) { 238 insert an pair below current insertion point 239 move new insertion point up from current insertion point until 240 ((last inserted EL is below the RLD) AND (RLD > 2) 241 AND 242 (new insertion point is EL-capable)) 243 set current insertion point to new insertion point 244 } 246 Figure 2: Algorithm to insert pairs in a label stack 248 When this algorithm is applied to the example described in Section 3 249 it will result in ELs being inserted in two positions, one below the 250 label L_N-D and another below L_N-P3. Thus the resulting label stack 251 would be 253 The RLD can be advertised via protocols and those extensions would be 254 described in separate documents [I-D.xu-isis-mpls-elc] and 255 [I-D.xu-ospf-mpls-elc]. 257 The recommendations above are not expected to bring any additional 258 OAM considerations beyond those described in section 6 of [RFC6790]. 259 However, the OAM requirements and solutions for source stacked 260 tunnels are still under discussion and future revisions of this 261 document will address those if needed. 263 5. Options considered 265 5.1. Single EL at the bottom of the stack of tunnels 267 In this option a single EL is used for the entire label stack. The 268 source LSR S encodes the entropy label (EL) below the labels of all 269 the stacked tunnels. In the example described in Section 3 it will 270 result in the label stack at LSR S to look like . Note that the notation in 272 [RFC6790] is used to describe the label stack. An issue with this 273 approach is that as the label stack grows due an increase in the 274 number of SIDs, the EL goes correspondingly deeper in the label 275 stack. Hence transit LSRs have to access a larger number of bytes in 276 the packet header when making forwarding decisions. In the example 277 described in Section 3 the LSR P1 would poorly load-balance traffic 278 on the parallel links L3, L4 since the EL is below the RLD of the 279 packet received by P1. A load balanced network design using this 280 approach must ensure that all intermediate LSRs have the capability 281 to traverse the maximum label stack depth as required for that 282 application that uses source routed stacking. 284 In the case where the hardware is capable of pushing a single pair at any depth, this option is the same as the recommended 286 solution in Section 4. 288 This option was discounted since there exist a number of hardware 289 implementations which have a low maximum readable label depth. 290 Choosing this option can lead to a loss of load-balancing using EL in 291 a significant part of the network but that is a critical requirement 292 in a service provider network. 294 5.2. An EL per tunnel in the stack 296 In this option each tunnel in the stack can be given its own EL. The 297 source LSR pushes an before pushing a tunnel label when 298 load balancing is required to direct traffic on that tunnel. In the 299 example described in Section 3, the source LSR S encoded label stack 300 would be where all the ELs 301 can be the same. Accessing the EL at an intermediate LSR is 302 independent of the depth of the label stack and hence independent of 303 the specific application that uses source stacking on that network. 304 A drawback is that the depth of the label stack grows significantly, 305 almost 3 times as the number of labels in the label stack. The 306 network design should ensure that source LSRs should have the 307 capability to push such a deep label stack. Also, the bandwidth 308 overhead and potential MTU issues of deep label stacks should be 309 accounted for in the network design. 311 In the case where the RLD is the minimum value (3) for all LSRs, all 312 LSRs are EL capable and the LSR that is inserting pairs has 313 no limit on how many it can insert then this option is the same as 314 the recommended solution in Section 4. 316 This option was discounted due to the existence of hardware 317 implementations that can push a limited number of labels on the label 318 stack. Choosing this option would result in a hardware requirement 319 to push two additional labels per tunnel label. Hence it would 320 restrict the number of tunnels that can form a LSP and constrain the 321 types of LSPs that can be created. This was considered unacceptable. 323 5.3. A re-usable EL for a stack of tunnels 325 In this option an LSR that terminates a tunnel re-uses the EL of the 326 terminated tunnel for the next inner tunnel. It does this by storing 327 the EL from the outer tunnel when that tunnel is terminated and re- 328 inserting it below the next inner tunnel label during the label swap 329 operation. The LSR that stacks tunnels SHOULD insert an EL below the 330 outermost tunnel. It SHOULD NOT insert ELs for any inner tunnels. 331 Also, the penultimate hop LSR of a segment MUST NOT pop the ELI and 332 EL even though they are exposed as the top labels since the 333 terminating LSR of that segment would re-use the EL for the next 334 segment. 336 In Section 3 above, the source LSR S encoded label stack would be 337 . At P1 the outgoing label stack 338 would be after it has load balanced 339 to one of the links L3 or L4. At P3 the outgoing label stack would 340 be . At P2 the outgoing label stack would be and it would load balance to one of the nexthop LSRs P4 or 342 P5. Accessing the EL at an intermediate LSR (e.g. P1) is 343 independent of the depth of the label stack and hence independent of 344 the specific use-case to which the stacked tunnels are applied. 346 This option was discounted due to the significant change in label 347 swap operations that would be required for existing hardware. 349 5.3.1. EL at top of stack 351 A slight variant of the re-usable EL option is to keep the EL at the 352 top of the stack rather than below the tunnel label. In this case 353 each LSR that is not terminating a segment should continue to keep 354 the received EL at the top of the stack when forwarding the packet 355 along the segment. An LSR that terminates a segment should use the 356 EL from the terminated segment at the top of the stack when 357 forwarding onto the next segment. 359 This option was discounted due to the significant change in label 360 swap operations that would be required for existing hardware. 362 5.4. ELs at readable label stack depths 364 In this option the source LSR inserts ELs for tunnels in the label 365 stack at depths such that each LSR along the path that must load 366 balance is able to access at least one EL. Note that the source LSR 367 may have to insert multiple ELs in the label stack at different 368 depths for this to work since intermediate LSRs may have differing 369 capabilities in accessing the depth of a label stack. The label 370 stack depth access value of intermediate LSRs must be known to create 371 such a label stack. How this value is determined is outside the 372 scope of this document. This value can be advertised using a 373 protocol such as an IGP. For the same Section 3 above, if LSR P1 374 needs to have the EL within a depth of 4, then the source LSR S 375 encoded label stack would be where all the ELs would typically have the same value. 378 In the case where the RLD has different values along the path and the 379 LSR that is inserting pairs has no limit on how many pairs 380 it can insert, and it knows the appropriate positions in the stack 381 where they should be inserted, then this option is the same as the 382 recommended solution in Section 4. 384 A variant of this solution was selected which balances the number of 385 labels that need to be pushed against the requirement for entropy. 387 6. Acknowledgements 389 The authors would like to thank John Drake, Loa Andersson, Curtis 390 Villamizar, Greg Mirsky, Markus Jork, Kamran Raza and Nobo Akiya for 391 their review comments and suggestions. 393 7. IANA Considerations 395 This memo includes no request to IANA. 397 8. Security Considerations 399 This document does not introduce any new security considerations 400 beyond those already listed in [RFC6790]. 402 9. References 404 9.1. Normative References 406 [I-D.filsfils-spring-segment-routing] 407 Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., 408 Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., 409 Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe, 410 "Segment Routing Architecture", draft-filsfils-spring- 411 segment-routing-04 (work in progress), July 2014. 413 [I-D.ravisingh-mpls-el-for-seamless-mpls] 414 Singh, R., Shen, Y., and J. Drake, "Entropy label for 415 seamless MPLS", draft-ravisingh-mpls-el-for-seamless- 416 mpls-04 (work in progress), October 2014. 418 [I-D.xu-isis-mpls-elc] 419 Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. 420 Litkowski, "Signaling Entropy Label Capability Using IS- 421 IS", draft-xu-isis-mpls-elc-02 (work in progress), April 422 2015. 424 [I-D.xu-ospf-mpls-elc] 425 Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. 426 Litkowski, "Signaling Entropy Label Capability Using 427 OSPF", draft-xu-ospf-mpls-elc-01 (work in progress), 428 October 2014. 430 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 431 Requirement Levels", BCP 14, RFC 2119, 432 DOI 10.17487/RFC2119, March 1997, 433 . 435 [RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP) 436 Hierarchy with Generalized Multi-Protocol Label Switching 437 (GMPLS) Traffic Engineering (TE)", RFC 4206, 438 DOI 10.17487/RFC4206, October 2005, 439 . 441 [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and 442 L. Yong, "The Use of Entropy Labels in MPLS Forwarding", 443 RFC 6790, DOI 10.17487/RFC6790, November 2012, 444 . 446 9.2. Informative References 448 [I-D.filsfils-spring-segment-routing-use-cases] 449 Filsfils, C., Francois, P., Previdi, S., Decraene, B., 450 Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., 451 Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E. 452 Crabbe, "Segment Routing Use Cases", draft-filsfils- 453 spring-segment-routing-use-cases-01 (work in progress), 454 October 2014. 456 [I-D.ietf-isis-segment-routing-extensions] 457 Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., 458 Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS 459 Extensions for Segment Routing", draft-ietf-isis-segment- 460 routing-extensions-05 (work in progress), June 2015. 462 [I-D.ietf-ospf-segment-routing-extensions] 463 Psenak, P., Previdi, S., Filsfils, C., Gredler, H., 464 Shakir, R., Henderickx, W., and J. Tantsura, "OSPF 465 Extensions for Segment Routing", draft-ietf-ospf-segment- 466 routing-extensions-05 (work in progress), June 2015. 468 [RFC7325] Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A., 469 and C. Pignataro, "MPLS Forwarding Compliance and 470 Performance Requirements", RFC 7325, DOI 10.17487/RFC7325, 471 August 2014, . 473 Authors' Addresses 475 Sriganesh Kini (editor) 476 Ericsson 478 Email: sriganesh.kini@ericsson.com 480 Kireeti Kompella 481 Juniper 483 Email: kireeti@juniper.net 485 Siva Sivabalan 486 Cisco 488 Email: msiva@cisco.com 490 Stephane Litkowski 491 Orange 493 Email: stephane.litkowski@orange.com 495 Rob Shakir 497 Email: rjs@rob.sh 499 Xiaohu Xu 500 Huawei 502 Email: xuxiaohu@huawei.com 504 Wim Hendrickx 505 Alcatel-Lucent 507 Email: wim.henderickx@alcatel-lucent.com 509 Jeff Tantsura 510 Ericsson 512 Email: jeff.tantsura@ericsson.com