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Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Looks like a reference, but probably isn't: '1000' on line 369 -- Looks like a reference, but probably isn't: '1999' on line 369 == Outdated reference: draft-ietf-spring-segment-routing has been published as RFC 8402 == Outdated reference: draft-ietf-spring-segment-routing-mpls has been published as RFC 8660 == Outdated reference: draft-ietf-isis-mpls-elc has been published as RFC 9088 == Outdated reference: draft-ietf-ospf-mpls-elc has been published as RFC 9089 == Outdated reference: draft-ietf-isis-l2bundles has been published as RFC 8668 Summary: 0 errors (**), 0 flaws (~~), 6 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group S. Kini 3 Internet-Draft 4 Intended status: Informational K. Kompella 5 Expires: August 3, 2018 Juniper 6 S. Sivabalan 7 Cisco 8 S. Litkowski 9 Orange 10 R. Shakir 11 Google 12 J. Tantsura 13 January 30, 2018 15 Entropy label for SPRING tunnels 16 draft-ietf-mpls-spring-entropy-label-08 18 Abstract 20 Segment Routing (SR) leverages the source routing paradigm. A node 21 steers a packet through an ordered list of instructions, called 22 segments. Segment Routing can be applied to the Multi Protocol Label 23 Switching (MPLS) data plane. Entropy label (EL) is a technique used 24 in MPLS to improve load-balancing. This document examines and 25 describes how ELs are to be applied to Segment Routing when applied 26 to the MPLS dataplane. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on August 3, 2018. 45 Copyright Notice 47 Copyright (c) 2018 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (https://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 63 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 64 2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 4 65 3. Use-case requiring multipath load-balancing . . . . . . . . . 4 66 4. Entropy Readable Label Depth . . . . . . . . . . . . . . . . 5 67 5. Maximum SID Depth . . . . . . . . . . . . . . . . . . . . . . 7 68 6. LSP stitching using the binding SID . . . . . . . . . . . . . 8 69 7. Insertion of entropy labels for SPRING path . . . . . . . . . 10 70 7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10 71 7.1.1. Example 1 where the ingress node has a sufficient MSD 11 72 7.1.2. Example 2 where the ingress node has not a sufficient 73 MSD . . . . . . . . . . . . . . . . . . . . . . . . . 12 74 7.2. Considerations for the placement of entropy labels . . . 12 75 7.2.1. ERLD value . . . . . . . . . . . . . . . . . . . . . 13 76 7.2.2. Segment type . . . . . . . . . . . . . . . . . . . . 14 77 7.2.2.1. Node-SID . . . . . . . . . . . . . . . . . . . . 14 78 7.2.2.2. Adjacency-set SID . . . . . . . . . . . . . . . . 15 79 7.2.2.3. Adjacency-SID representing a single IP link . . . 15 80 7.2.2.4. Adjacency-SID representing a single link within a 81 L2 bundle . . . . . . . . . . . . . . . . . . . . 15 82 7.2.2.5. Adjacency-SID representing a L2 bundle . . . . . 15 83 7.2.3. Maximizing number of LSRs that will load-balance . . 15 84 7.2.4. Preference for a part of the path . . . . . . . . . . 16 85 7.2.5. Combining criteria . . . . . . . . . . . . . . . . . 16 86 8. A simple example algorithm . . . . . . . . . . . . . . . . . 16 87 9. Deployment Considerations . . . . . . . . . . . . . . . . . . 17 88 10. Options considered . . . . . . . . . . . . . . . . . . . . . 18 89 10.1. Single EL at the bottom of the stack . . . . . . . . . . 18 90 10.2. An EL per segment in the stack . . . . . . . . . . . . . 18 91 10.3. A re-usable EL for a stack of tunnels . . . . . . . . . 19 92 10.4. EL at top of stack . . . . . . . . . . . . . . . . . . . 19 93 10.5. ELs at readable label stack depths . . . . . . . . . . . 20 94 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 95 12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20 96 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 97 14. Security Considerations . . . . . . . . . . . . . . . . . . . 21 98 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 99 15.1. Normative References . . . . . . . . . . . . . . . . . . 21 100 15.2. Informative References . . . . . . . . . . . . . . . . . 22 101 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 103 1. Introduction 105 Segment Routing [I-D.ietf-spring-segment-routing] is based on source 106 routed tunnels to steer a packet along a particular path. This path 107 is encoded as an ordered list of segments. When applied to the MPLS 108 dataplane [I-D.ietf-spring-segment-routing-mpls], each segment is an 109 LSP with an associated MPLS label value. Hence, label stacking is 110 used to represent the ordered list of segments and the label stack 111 associated with an SR tunnel can be seen as nested LSPs (LSP 112 hierarchy) in the MPLS architecture. 114 Using label stacking to encode the list of segment has implications 115 on the label stack depth. 117 Entropy label (EL) [RFC6790] is a technique used in the MPLS data 118 plane to provide entropy for load-balancing. When using LSP 119 hierarchies, there are implications on how [RFC6790] should be 120 applied. The current document addresses the case where a hierarchy 121 is created at a single LSR as required by Segment Routing. 123 A use-case requiring load-balancing with SR is given in Section 3. A 124 recommended solution is described in Section 7 keeping in 125 consideration the limitations of implementations when applying 126 [RFC6790] to deeper label stacks. Options that were considered to 127 arrive at the recommended solution are documented for historical 128 purposes in Section 10. 130 1.1. Requirements Language 132 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 133 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 134 "OPTIONAL" in this document are to be interpreted as described in BCP 135 14 [RFC2119] [RFC8174] when, and only when, they appear in all 136 capitals, as shown here. 138 2. Abbreviations and Terminology 140 EL - Entropy Label 142 ELI - Entropy Label Identifier 144 ELC - Entropy Label Capability 146 ERLD - Entropy Readable Label Depth 148 SR - Segment Routing 150 ECMP - Equal Cost Multi Path 152 LSR - Label Switch Router 154 MPLS - Multiprotocol Label Switching 156 MSD - Maximum SID Depth 158 SID - Segment Identifier 160 RLD - Readable Label Depth 162 OAM - Operation, Administration and Maintenance 164 3. Use-case requiring multipath load-balancing 166 +------+ 167 | | 168 +-------| P3 |-----+ 169 | +-----| |---+ | 170 L3| |L4 +------+ L1| |L2 +----+ 171 | | | | +--| P4 |--+ 172 +-----+ +-----+ +-----+ | +----+ | +-----+ 173 | S |-----| P1 |------------| P2 |--+ +--| D | 174 | | | | | |--+ +--| | 175 +-----+ +-----+ +-----+ | +----+ | +-----+ 176 +--| P5 |--+ 177 +----+ 178 S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs, 179 L1,L2,L3,L4=Links 181 Figure 1: Traffic engineering use-case 183 Traffic-engineering is one of the applications of MPLS and is also a 184 requirement for source routed tunnels with label stacks [RFC7855]. 186 Consider the topology shown in Figure 1. The LSR S requires data to 187 be sent to LSR D along a traffic-engineered path that goes over the 188 link L1. Good load-balancing is also required across equal cost 189 paths (including parallel links). To engineer traffic along a path 190 that takes link L1, the label stack that LSR S creates consists of a 191 label to the node SID of LSR P3, stacked over the label for the 192 adjacency SID of link L1 and that in turn is stacked over the label 193 to the node SID of LSR D. For simplicity lets assume that all LSRs 194 use the same label space (SRGB) for source routed label stacks. Let 195 L_N-Px denote the label to be used to reach the node SID of LSR Px. 196 Let L_A-Ln denote the label used for the adjacency SID for link Ln. 197 The LSR S must use the label stack for 198 traffic-engineering. However to achieve good load-balancing over the 199 equal cost paths P2-P4-D, P2-P5-D and the parallel links L3, L4, a 200 mechanism such as Entropy labels [RFC6790] should be adapted for 201 source routed label stacks. Indeed, the SPRING architecture with the 202 MPLS dataplane ([I-D.ietf-spring-segment-routing-mpls]) uses nested 203 MPLS LSPs composing the source routed label stacks. As each MPLS 204 node may have limitations in the number of labels it can push when it 205 is ingress or inspect when doing load-balancing, an entropy label 206 insertion strategy becomes important to keep the benefit of the load- 207 balancing. Multiple ways to apply entropy labels were considered and 208 are documented in Section 10 along with their trade-offs. A 209 recommended solution is described in Section 7. 211 4. Entropy Readable Label Depth 213 The Entropy Readable Label Depth (ERLD) is defined as the number of 214 labels a router can both: 216 a. Read in an MPLS packet received on its incoming interface(s) 217 (starting from the top of the stack). 219 b. Use in its load-balancing function. 221 The ERLD means that the router will perform load-balancing using the 222 EL label if the EL is placed within the ERLD first labels. 224 A router capable of reading N labels but not using an EL located 225 within those N labels MUST consider its ERLD to be 0. In a 226 distributed switching architecture, each linecard may have a 227 different capability in terms of ERLD. For simplicity, an 228 implementation MAY use the minimum ERLD between each linecard as the 229 ERLD value for the system. 231 Examples: 233 | Payload | 234 +----------+ 235 | Payload | | EL | P7 236 +----------+ +----------+ 237 | Payload | | EL | | ELI | 238 +----------+ +----------+ +----------+ 239 | Payload | | EL | | ELI | | Label 50 | 240 +----------+ +----------+ +----------+ +----------+ 241 | Payload | | EL | | ELI | | Label 40 | | Label 40 | 242 +----------+ +----------+ +----------+ +----------+ +----------+ 243 | EL | | ELI | | Label 30 | | Label 30 | | Label 30 | 244 +----------+ +----------+ +----------+ +----------+ +----------+ 245 | ELI | | Label 20 | | Label 20 | | Label 20 | | Label 20 | 246 +----------+ +----------+ +----------+ +----------+ +----------+ 247 | Label 16 | | Label 16 | | Label 16 | | Label 16 | | Label 16 | P1 248 +----------+ +----------+ +----------+ +----------+ +----------+ 249 Packet 1 Packet 2 Packet 3 Packet 4 Packet 5 251 Figure 2: Label stacks with ELI/EL 253 In the figure 2, we consider the displayed packets received on a 254 router interface. We consider also a single ERLD value for the 255 router. 257 o If the router has an ERLD of 3, it will be able to load-balance 258 Packet 1 displayed in Figure 2 using the EL as part of the load- 259 balancing keys. The ERLD value of 3 means that the router can 260 read and take into account the entropy label for load-balancing if 261 it is placed between position 1 (top) and position 3. 263 o If the router has an ERLD of 5, it will be able to load-balance 264 Packets 1 to 3 in Figure 2 using the EL as part of the load- 265 balancing keys. Packets 4 and 5 have the EL placed at a position 266 greater than 5, so the router is not able to read it and use as 267 part of the load-balancing keys. 269 o If the router has an ERLD of 10, it will be able to load-balance 270 all the packets displayed in Figure 2 using the EL as part of the 271 load-balancing keys. 273 To allow an efficient load-balancing based on entropy labels, a 274 router running SPRING SHOULD advertise its ERLD (or ERLDs), so all 275 the other SPRING routers in the network are aware of its capability. 276 How this advertisement is done is outside the scope of this document. 278 To advertise an ERLD value, a SPRING router: 280 o MUST be entropy label capable and, as a consequence, MUST apply 281 the dataplane procedures defined in [RFC6790]. 283 o MUST be able to read an ELI/EL which is located within its ERLD 284 value. 286 o MUST take into account this EL in its load-balancing function. 288 5. Maximum SID Depth 290 The Maximum SID Depth defines the maximum number of labels that a 291 particular node can impose on a packet. This includes any kind of 292 labels (service, entropy, transport...). In an MPLS network, the MSD 293 is a limit of the Ingress LSR (I-LSR) or any stitching node that 294 would perform an imposition of additional labels on an existing label 295 stack. 297 Depending of the number of MPLS operations (POP, SWAP...) to be 298 performed before the PUSH, the MSD may vary due to the hardware or 299 software limitations. As for the ERLD, there may also be different 300 MSD limits based on the linecard type used in a distributed switching 301 system. 303 When an external controller is used to program a label stack on a 304 particular node, this node MAY advertise its MSD value or a subset of 305 its MSD value to the controller. How this advertisement is done is 306 outside the scope of this document. As the controller does not have 307 the knowledge of the entire label stack to be pushed by the node, the 308 node may advertise an MSD value which is lower than its actual limit. 309 This gives the ability for the controller to program a label stack up 310 to the advertised MSD value while leaving room for the local node to 311 add more labels (e.g., service, entropy, transport...) without 312 reaching the hardware/software limit. 314 P7 ---- P8 ---- P9 315 / \ 316 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2 317 | \ | 318 ----> P10 \ | 319 IP Pkt | \ | 320 P11 --- P12 --- P13 321 100 10000 323 Figure 3 325 In the figure 3, an IP packet comes in the MPLS network at PE1. All 326 metrics are considered equal to 1 except P12-P13 which is 10000 and 327 P11-P12 which is 100. PE1 wants to steer the traffic using a SPRING 328 path to PE2 along 329 PE1->P1->P7->P8->P9->P4->P5->P10->P11->P12->P13->PE2. By using 330 adjacency SIDs only, PE1 (acting as an I-LSR) will be required to 331 push 10 labels on the IP packet received and thus requires an MSD of 332 10. If the IP packet should be carried over an MPLS service like a 333 regular layer 3 VPN, an additional service label should be imposed, 334 requiring an MSD of 11 for PE1. In addition, if PE1 wants to insert 335 an ELI/EL for load-balancing purpose, PE1 will need to push 13 labels 336 on the IP packet requiring an MSD of 13. 338 In the SPRING architecture, Node SIDs or Binding SIDs can be used to 339 reduce the label stack size. As an example, to steer the traffic on 340 the same path as before, PE1 may be able to use the following label 341 stack: . In this example we 342 consider a combination of Node SIDs and a Binding SID advertised by 343 P5 that will stitch the traffic along the path P10->P11->P12->P13. 344 The instruction associated with the binding SID at P5 is thus to swap 345 Binding_P5 to Adj_P12-P13 and then push . P5 346 acts as a stitching node that pushes additional labels on an existing 347 label stack, P5's MSD needs also to be taken into account and may 348 limit the number of labels that could be imposed. 350 6. LSP stitching using the binding SID 352 The binding SID allows binding a segment identifier to an existing 353 LSP. As examples, the binding SID can represent an RSVP-TE tunnel, 354 an LDP path (through the mapping server advertisement), or a SPRING 355 path. Each LSP associated with a binding SID has its own entropy 356 label capability. 358 In the figure 3, we consider that: 360 o P6, PE2, P10, P11, P12, P13 are pure LDP routers. 362 o PE1, P1, P2, P3, P4, P7, P8, P9 are pure SPRING routers. 364 o P5 is running SPRING and LDP. 366 o P5 acts as a mapping server and advertises Prefix SIDs for the LDP 367 FECs: an index value of 20 is used for PE2. 369 o All SPRING routers use an SRGB of [1000, 1999]. 371 o P6 advertises label 20 for the PE2 FEC. 373 o Traffic from PE1 to PE2 uses the shortest path. 375 PE1 ----- P1 -- P2 -- P3 -- P4 ---- P5 --- P6 --- PE2 377 --> +----+ +----+ +----+ +----+ 378 IP Pkt | IP | | IP | | IP | | IP | 379 +----+ +----+ +----+ +----+ 380 |1020| |1020| | 20 | 381 +----+ +----+ +----+ 382 SPRING LDP 384 In term of packet forwarding, by learning the mapping-server 385 advertisement from PE5, PE1 imposes a label 1020 to an IP packet 386 destinated to PE2. SPRING routers along the shortest path to PE2 387 will switch the traffic until it reaches P5 which will perform the 388 LSP stitching. P5 will swap the SPRING label 1020 to the LDP label 389 20 advertised by the nexthop P6. P6 will then forward the packet 390 using the LDP label towards PE2. 392 PE1 cannot push an ELI/EL for the binding SID without knowing that 393 the tail-end of the LSP associated with the binding (PE2) is entropy 394 label capable. 396 To accomodate the mix of signalling protocols involved during the 397 stitching, the entropy label capability SHOULD be propagated between 398 the signalling protocols. Each binding SID SHOULD have its own 399 entropy label capability that MUST be inherited from the entropy 400 label capability of the associated LSP. If the router advertising 401 the binding SID does not know the ELC state of the target FEC, it 402 MUST NOT set the ELC for the binding SID. An ingress node MUST NOT 403 push an ELI/EL associated with a binding SID unless this binding SID 404 has the entropy label capability. How the entropy label capability 405 is advertised for a binding SID is outside the scope of this 406 document. 408 In our example, if PE2 is LDP entropy label capable, it will add the 409 entropy label capability in its LDP advertisement. When P5 receives 410 the FEC/label binding for PE2, it learns about the ELC and can set 411 the ELC in the mapping server advertisement. Thus PE1 learns about 412 the ELC of PE2 and may push an ELI/EL associated with the binding 413 SID. 415 The proposed solution only works if the SPRING router advertising the 416 binding SID is also performing the dataplane LSP stitching. In our 417 example, if the mapping server function is hosted on P8 instead of 418 P5, P8 does not know about the ELC state of PE2's LDP FEC. As a 419 consequence, it does not set the ELC for the associated binding SID. 421 7. Insertion of entropy labels for SPRING path 423 7.1. Overview 425 The solution described in this section follows the dataplane 426 processing defined in [RFC6790]. Within a SPRING path, a node may be 427 ingress, egress, transit (regarding the entropy label processing 428 described in [RFC6790]), or it can be any combination of those. For 429 example: 431 o The ingress node of a SPRING domain may be an ingress node from an 432 entropy label perspective. 434 o Any LSR terminating a segment of the SPRING path is an egress node 435 (because it terminates the segment) but may also be a transit node 436 if the SPRING path is not terminated because there is a subsequent 437 SPRING MPLS label in the stack. 439 o Any LSR processing a binding SID may be a transit node and an 440 ingress node (because it may push additional labels when 441 processing the binding SID). 443 As described earlier, an LSR may have a limitation, ERLD, on the 444 depth of the label stack that it can read and process in order to do 445 multipath load-balancing based on entropy labels. 447 If an EL does not occur within the ERLD of an LSR in the label stack 448 of an MPLS packet that it receives, then it would lead to poor load- 449 balancing at that LSR. Hence an ELI/EL pair must be within the ERLD 450 of the LSR in order for the LSR to use the EL during load-balancing. 452 Adding a single ELI/EL pair for the entire SPRING path may lead also 453 to poor load-balancing as well because the EL/ELI may not occur 454 within the ERLD of some LSR on the path (if too deep) or may not be 455 present in the stack when it reaches some LSRs if it is too shallow. 457 In order for the EL to occur within the ERLD of LSRs along the path 458 corresponding to a SPRING label stack, multiple pairs MAY 459 be inserted in this label stack. 461 The insertion of the ELI/EL SHOULD occur only with a SPRING label 462 advertised by an LSR that advertised an ERLD (the LSR is entropy 463 label capable) or with a SPRING label associated with a binding SID 464 that has the ELC set. 466 The ELs among multiple pairs inserted in the stack MAY be 467 the same or different. The LSR that inserts pairs MAY have 468 limitations on the number of such pairs that it can insert and also 469 the depth at which it can insert them. If, due to limitations, the 470 inserted ELs are at positions such that an LSR along the path 471 receives an MPLS packet without an EL in the label stack within that 472 LSR's ERLD, then the load-balancing performed by that LSR would be 473 poor. An implementation MAY consider multiple criteria when 474 inserting pairs. 476 7.1.1. Example 1 where the ingress node has a sufficient MSD 478 ECMP LAG LAG 479 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2 481 Figure 4 483 In the figure 4, PE1 wants to forward some MPLS VPN traffic over an 484 explicit path to PE2 resulting in the following label stack to be 485 pushed onto the received IP header: . PE1 is limited 487 to push a maximum of 11 labels (MSD=11). P2, P3 and P6 have an ERLD 488 of 3 while others have an ERLD of 10. 490 PE1 can only add two ELI/EL pairs in the label stack due to its MSD 491 limitation. It should insert them strategically to benefit load- 492 balancing along the longest part of the path. 494 PE1 may take into account multiple parameters when inserting ELs, as 495 examples: 497 o The ERLD value advertised by transit nodes. 499 o The requirement of load-balancing for a particular label value. 501 o Any service provider preference: favor beginning of the path or 502 end of the path. 504 In the figure 4, a good strategy may be to use the following stack 505 . The original stack requests P2 to forward 507 based on a L3 adjacency set that will require load-balancing. 508 Therefore it is important to ensure that P2 can load-balance 509 correctly. As P2 has a limited ERLD of 3, ELI/EL must be inserted 510 just next to the label that P2 will use to forward. On the path to 511 PE2, P3 has also a limited ERLD, but P3 will forward based on a basic 512 adjacency segment that may require no load-balancing. Therefore it 513 does not seem important to ensure that P3 can do load-balancing 514 despite of its limited ERLD. The next nodes along the forwarding 515 path have a high ERLD that does not cause any issue, except P6, 516 moreover P6 is using some LAGs to PE2 and so is expected to load- 517 balance. It becomes important to insert a new ELI/EL just next to P6 518 forwarding label. 520 In the case above, the ingress node had enough label push capacity to 521 ensure end-to-end load-balancing taking into the path attributes. 522 There might be some cases, where the ingress node may not have the 523 necessary label imposition capacity. 525 7.1.2. Example 2 where the ingress node has not a sufficient MSD 527 ECMP LAG ECMP ECMP 528 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- P7 --- P8 --- PE2 530 Figure 5 532 In the figure 5, PE1 wants to forward MPLS VPN traffic over an 533 explicit path to PE2 resulting in the following label stack to be 534 pushed onto the IP header: . PE1 is limited to push a maximum of 11 labels, P2, P3 537 and P6 have an ERLD of 3 while others have an ERLD of 15. 539 Using a similar strategy as the previous case may lead to a dilemma, 540 as PE1 can only push a single ELI/EL while we may need a minimum of 541 three to load-balance the end-to-end path. An optimized stack that 542 would enable end-to-end load-balancing may be: . 546 A decision needs to be taken to favor some part of the path for load- 547 balancing considering that load-balancing may not work on the other 548 part. A service provider may decide to place the ELI/EL after the P6 549 forwarding label as it will allow P4 and P6 to load-balance. Placing 550 the ELI/EL at bottom of the stack is also a possibility enabling 551 load-balancing for P4 and P8. 553 7.2. Considerations for the placement of entropy labels 555 The sample cases described in the previous section showed that 556 placing the ELI/EL when the maximum number of labels to be pushed is 557 limited is not an easy decision and multiple criteria may be taken 558 into account. 560 This section describes some considerations that could be taken into 561 account when placing ELI/ELs. This list of criteria is not 562 considered as exhaustive and an implementation MAY take into account 563 additional criteria or tie-breakers that are not documented here. 565 An implementation SHOULD try to maximize the load-balancing where 566 multiple ECMP paths are available and minimize the number of EL/ELIs 567 that need to be inserted. In case of a trade-off, an implementation 568 MAY provide flexibility to the operator to select the criteria to be 569 considered when placing EL/ELIs or the sub-objective for which to 570 optimize. 572 2 2 573 PE1 -- P1 -- P2 --P3 --- P4 --- P5 -- ... -- P8 -- P9 -- PE2 574 | | 575 P3'--- P4'--- P5' 577 Figure 6 579 The figure above will be used as reference in the following 580 subsections. All metrics are equal to 1, except P3-P4 and P4-P5 581 which have a metric 2. 583 7.2.1. ERLD value 585 As mentioned in Section 7.1, the ERLD value is an important parameter 586 to consider when inserting ELI/EL. If an ELI/EL does not fall within 587 the ERLD of a node on the path, the node will not be able to load- 588 balance the traffic efficiently. 590 The ERLD value can be advertised via protocols and those extensions 591 are described in separate documents [I-D.ietf-isis-mpls-elc] and 592 [I-D.ietf-ospf-mpls-elc]. 594 Let's consider a path from PE1 to PE2 using the following stack 595 pushed by PE1: . 597 Using the ERLD as an input parameter may help to minimize the number 598 of required ELI/EL pairs to be inserted. An ERLD value must be 599 retrieved for each SPRING label in the label stack. 601 For a label bound to an adjacency segment, the ERLD is the ERLD of 602 the node that advertised the adjacency segment. In the example 603 above, the ERLD associated with Adj_P1P2 would be the ERLD of router 604 P1 as P1 will perform the forwarding based on the Adj_P1P2 label. 606 For a label bound to a node segment, multiple strategies MAY be 607 implemented. An implementation may try to evaluate the minimum ERLD 608 value along the node segment path. If an implementation cannot find 609 the minimum ERLD along the path of the segment, it can use the ERLD 610 of the starting node instead. In the example above, if the 611 implementation supports computation of minimum ERLD along the path, 612 the ERLD associated with label Node_P9 would be the minimum ERLD 613 between nodes {P2,P3,P4 ..., P8}. If an implementation does not 614 support the computation of minimum ERLD, it should consider the ERLD 615 of P2 (starting node that will forward based on the Node_P9 label). 617 For a label bound to a binding segment, if the binding segment 618 describes a path, an implementation may also try to evaluate the 619 minimum ERLD along this path. If the implementation cannot find the 620 minimum ERLD along the path of the segment, it can use the ERLD of 621 the starting node instead. 623 7.2.2. Segment type 625 Depending of the type of segment a particular label is bound to, an 626 implementation may deduce that this particular label will be subject 627 to load-balancing on the path. 629 7.2.2.1. Node-SID 631 An MPLS label bound to a Node-SID represents a path that may cross 632 multiple hops. Load-balancing may be needed on the node starting 633 this path but also on any node along the path. 635 In the figure 6, let's consider a path from PE1 to PE2 using the 636 following stack pushed by PE1: . 639 If, for example, PE1 is limited to push 6 labels, it can add a single 640 ELI/EL within the label stack. An operator may want to favor a 641 placement that would allow load-balancing along the Node-SID path. 642 In the figure above, P3 which is along the Node-SID path requires 643 load-balancing on two equal-cost paths. 645 An implementation may try to evaluate if load-balancing is really 646 required within a node segment path. This could be done by running 647 an additional SPT computation and analysis of the node segment path 648 to prevent a node segment that does not really require load-balancing 649 from being preferred when placing EL/ELIs. Such inspection may be 650 time consuming for implementations and without a 100% guarantee, as a 651 node segment path may use LAG that could be invisible from the IP 652 topology. A simpler approach would be to consider that a label bound 653 to a Node-SID will be subject to load-balancing and requires an EL/ 654 ELI. 656 7.2.2.2. Adjacency-set SID 658 An adjacency-set is an adjacency SID that refers to a set of 659 adjacencies. When an adjacency-set segment is used within a label 660 stack, an implementation can deduce that load-balancing is expected 661 at the node that advertised this adjacency segment. An 662 implementation could then favor this particular label value when 663 placing ELI/ELs. 665 7.2.2.3. Adjacency-SID representing a single IP link 667 When an adjacency segment representing a single IP link is used 668 within a label stack, an implementation can deduce that load- 669 balancing may not be expected at the node that advertised this 670 adjacency segment. 672 The implementation could then decide to place ELI/ELs to favor other 673 LSRs than the one advertising this adjacency segment. 675 Readers should note that an adjacency segment representing a single 676 IP link may require load-balancing. This is the case when a LAG (L2 677 bundle) is implemented between two IP nodes and the L2 bundle SR 678 extensions [I-D.ietf-isis-l2bundles] are not implemented. In such a 679 case, it may be useful to insert an EL/ELI in a readable position for 680 the LSR advertising the label associated with the adjacency segment. 682 7.2.2.4. Adjacency-SID representing a single link within a L2 bundle 684 When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used, 685 adjacency segments may be advertised for each member of the bundle. 686 In this case, an implementation can deduce that load-balancing is not 687 expected on the LSR advertising this segment and could then decide to 688 place ELI/ELs to favor other LSRs than the one advertising this 689 adjacency segment. 691 7.2.2.5. Adjacency-SID representing a L2 bundle 693 When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used, an 694 adjacency segment may be advertised to represent the bundle. In this 695 case, an implementation can deduce that load-balancing is expected on 696 the LSR advertising this segment and could then decide to place ELI/ 697 ELs to favor this LSR. 699 7.2.3. Maximizing number of LSRs that will load-balance 701 When placing ELI/ELs, an implementation may try to maximize the 702 number of LSRs that both need to load-balance (i.e., have ECMP paths) 703 and that will be able to perform load-balancing (i.e., the EL label 704 is within their ERLD). 706 Let's consider a path from PE1 to PE2 using the following stack 707 pushed by PE1: . All 708 routers have an ERLD of 10, expect P1 and P2 which have an ERLD of 4. 709 PE1 is able to push 6 labels, so only a single ELI/EL can be added. 711 In the example above, adding ELI/EL next to Adj_P1P2 will only allow 712 load-balancing at P1 while inserting it next to Adj_PE2P9, will allow 713 load-balancing at P2,P3 ... P9 and maximizing the number of LSRs that 714 could perform load-balancing. 716 7.2.4. Preference for a part of the path 718 An implementation may propose to favor a part of the end-to-end path 719 when the number of EL/ELI that can be pushed is not enough to cover 720 the entire path. As example, a service provider may want to favor 721 load-balancing at the beginning of the path or at the end of path, so 722 the implementation should prefer putting the ELI/ELs near the top or 723 near of the bottom of the stack. 725 7.2.5. Combining criteria 727 An implementation can combine multiple criteria to determine the best 728 EL/ELIs placement. However, combining too many criteria may lead to 729 implementation complexity and high resource consumption. Each time 730 the network topology changes, a new evaluation of the EL/ELI 731 placement will be necessary for each impacted LSPs. 733 8. A simple example algorithm 735 A simple implementation might take into account ERLD when placing 736 ELI/EL while trying to minimize the number of EL/ELIs inserted and 737 trying to maximize the number of LSRs that can load-balance. 739 The example algorithm is based on the following considerations: 741 o An LSR that is limited in the number of pairs that it 742 can insert SHOULD insert such pairs deeper in the stack. 744 o An LSR should try to insert pairs at positions so that 745 for the maximum number of transit LSRs, the EL occurs within the 746 ERLD of those LSRs. 748 o An LSR should try to insert the minimum number of such pairs while 749 trying to satisfy the above criteria. 751 The pseudocode of the example algorithm is shown below. 753 Initialize the current EL insertion point to the 754 bottommost label in the stack that is EL-capable 755 while (local-node can push more pairs OR 756 insertion point is not above label stack) { 757 insert an pair below current insertion point 758 move new insertion point up from current insertion point until 759 ((last inserted EL is below the ERLD) AND (ERLD > 2) 760 AND 761 (new insertion point is EL-capable)) 762 set current insertion point to new insertion point 763 } 765 Figure 7: Example algorithm to insert pairs in a label 766 stack 768 When this algorithm is applied to the example described in Section 3, 769 it will result in ELs being inserted in two positions, one below the 770 label L_N-D and another below L_N-P3. Thus the resulting label stack 771 would be 773 9. Deployment Considerations 775 As long as LSR node dataplane capabilities are limited (number of 776 labels that can be pushed, or number of labels that can be 777 inspected), hop-by-hop load-balancing of SPRING encapsulated flows 778 will require trade-offs. 780 Entropy label is still a good and usable solution as it allows load- 781 balancing without having to perform a deep packet inspection on each 782 LSR: it does not seem reasonable to have an LSR inspecting UDP ports 783 within a GRE tunnel carried over a 15 label SPRING tunnel. 785 Due to the limited capacity of reading a deep stack of MPLS labels, 786 multiple EL/ELIs may be required within the stack which directly 787 impacts the capacity of the head-end to push a deep stack: each EL/ 788 ELI inserted requires two additional labels to be pushed. 790 Placement strategies of EL/ELIs are required to find the best trade- 791 off. Multiple criteria may be taken into account and some level of 792 customization (by the user) may be required to accommodate the 793 different deployments. Analyzing the path of each destination to 794 determine the best EL/ELI placement may be time consuming for the 795 control plane, we encourage implementations to find the best trade- 796 off between simplicity, resource consumption, and load-balancing 797 efficiency. 799 In future, hardware and software capacity may increase dataplane 800 capabilities and may be remove some of these limitations, increasing 801 load-balancing capability using entropy labels. 803 10. Options considered 805 Different options that were considered to arrive at the recommended 806 solution are documented in this section. 808 These options are detailed here only for historical purposes. 810 10.1. Single EL at the bottom of the stack 812 In this option, a single EL is used for the entire label stack. The 813 source LSR S encodes the entropy label at the bottom of the label 814 stack. In the example described in Section 3, it will result in the 815 label stack at LSR S to look like 816 . Note that the notation in [RFC6790] is 817 used to describe the label stack. An issue with this approach is 818 that as the label stack grows due an increase in the number of SIDs, 819 the EL goes correspondingly deeper in the label stack. Hence, 820 transit LSRs have to access a larger number of bytes in the packet 821 header when making forwarding decisions. In the example described in 822 Section 3, if we consider that the LSR P1 has an ERLD of 3, P1 would 823 load-balance traffic poorly on the parallel links L3 and L4 since the 824 EL is below the ERLD of P1. A load-balanced network design using 825 this approach must ensure that all intermediate LSRs have the 826 capability to read the maximum label stack depth as required for the 827 application that uses source routed stacking. 829 This option was rejected since there exist a number of hardware 830 implementations which have a low maximum readable label depth. 831 Choosing this option can lead to a loss of load-balancing using EL in 832 a significant part of the network when that is a critical requirement 833 in a service-provider network. 835 10.2. An EL per segment in the stack 837 In this option, each segment/label in the stack can be given its own 838 EL. When load-balancing is required to direct traffic on a segment, 839 the source LSR pushes an before pushing the label 840 associated to this segment . In the example described in Section 3, 841 the source LSR S encoded label stack would be where all the ELs can be the same. Accessing the 843 EL at an intermediate LSR is independent of the depth of the label 844 stack and hence independent of the specific application that uses 845 source routed tunnels with label stacking. A drawback is that the 846 depth of the label stack grows significantly, almost 3 times as the 847 number of labels in the label stack. The network design should 848 ensure that source LSRs have the capability to push such a deep label 849 stack. Also, the bandwidth overhead and potential MTU issues of deep 850 label stacks should be considered in the network design. 852 This option was rejected due to the existence of hardware 853 implementations that can push a limited number of labels on the label 854 stack. Choosing this option would result in a hardware requirement 855 to push two additional labels per tunnel label. Hence it would 856 restrict the number of tunnels that can be stacked in a LSP and hence 857 constrain the types of LSPs that can be created. This was considered 858 unacceptable. 860 10.3. A re-usable EL for a stack of tunnels 862 In this option an LSR that terminates a tunnel re-uses the EL of the 863 terminated tunnel for the next inner tunnel. It does this by storing 864 the EL from the outer tunnel when that tunnel is terminated and re- 865 inserting it below the next inner tunnel label during the label swap 866 operation. The LSR that stacks tunnels should insert an EL below the 867 outermost tunnel. It should not insert ELs for any inner tunnels. 868 Also, the penultimate hop LSR of a segment must not pop the ELI and 869 EL even though they are exposed as the top labels since the 870 terminating LSR of that segment would re-use the EL for the next 871 segment. 873 In Section 3 above, the source LSR S encoded label stack would be 874 . At P1, the outgoing label stack 875 would be after it has load-balanced 876 to one of the links L3 or L4. At P3 the outgoing label stack would 877 be . At P2, the outgoing label stack would be and it would load-balance to one of the nexthop LSRs P4 879 or P5. Accessing the EL at an intermediate LSR (e.g., P1) is 880 independent of the depth of the label stack and hence independent of 881 the specific use-case to which the label stack is applied. 883 This option was rejected due to the significant change in label swap 884 operations that would be required for existing hardware. 886 10.4. EL at top of stack 888 A slight variant of the re-usable EL option is to keep the EL at the 889 top of the stack rather than below the tunnel label. In this case, 890 each LSR that is not terminating a segment should continue to keep 891 the received EL at the top of the stack when forwarding the packet 892 along the segment. An LSR that terminates a segment should use the 893 EL from the terminated segment at the top of the stack when 894 forwarding onto the next segment. 896 This option was rejected due to the significant change in label swap 897 operations that would be required for existing hardware. 899 10.5. ELs at readable label stack depths 901 In this option the source LSR inserts ELs for tunnels in the label 902 stack at depths such that each LSR along the path that must load 903 balance is able to access at least one EL. Note that the source LSR 904 may have to insert multiple ELs in the label stack at different 905 depths for this to work since intermediate LSRs may have differing 906 capabilities in accessing the depth of a label stack. The label 907 stack depth access value of intermediate LSRs must be known to create 908 such a label stack. How this value is determined is outside the 909 scope of this document. This value can be advertised using a 910 protocol such as an IGP. 912 Applying this method to the example in Section 3 above, if LSR P1 913 needs to have the EL within a depth of 4, then the source LSR S 914 encoded label stack would be where all the ELs would typically have the same value. 917 In the case where the ERLD has different values along the path and 918 the LSR that is inserting pairs has no limit on how many 919 pairs it can insert, and it knows the appropriate positions in the 920 stack where they should be inserted, this option is the same as the 921 recommended solution in Section 7. 923 Note that a refinement of this solution which balances the number of 924 pushed labels against the desired entropy is the solution described 925 in Section 7. 927 11. Acknowledgements 929 The authors would like to thank John Drake, Loa Andersson, Curtis 930 Villamizar, Greg Mirsky, Markus Jork, Kamran Raza, Carlos Pignataro, 931 Bruno Decraene, Chris Bowers and Nobo Akiya for their review comments 932 and suggestions. 934 12. Contributors 935 Xiaohu Xu 936 Huawei 938 Email: xuxiaohu@huawei.com 940 Wim Hendrickx 941 Nokia 943 Email: wim.henderickx@nokia.com 945 Gunter Van De Velde 946 Nokia 948 Email: gunter.van_de_velde@nokia.com 950 Acee Lindem 951 Cisco 953 Email: acee@cisco.com 955 13. IANA Considerations 957 This memo includes no request to IANA. Note to RFC Editor: Remove 958 this section before publication. 960 14. Security Considerations 962 This document does not introduce any new security considerations 963 beyond those already listed in [RFC6790]. 965 15. References 967 15.1. Normative References 969 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 970 Requirement Levels", BCP 14, RFC 2119, 971 DOI 10.17487/RFC2119, March 1997, 972 . 974 [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and 975 L. Yong, "The Use of Entropy Labels in MPLS Forwarding", 976 RFC 6790, DOI 10.17487/RFC6790, November 2012, 977 . 979 [RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B., 980 Litkowski, S., Horneffer, M., and R. Shakir, "Source 981 Packet Routing in Networking (SPRING) Problem Statement 982 and Requirements", RFC 7855, DOI 10.17487/RFC7855, May 983 2016, . 985 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 986 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 987 May 2017, . 989 [I-D.ietf-spring-segment-routing] 990 Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B., 991 Litkowski, S., and R. Shakir, "Segment Routing 992 Architecture", draft-ietf-spring-segment-routing-15 (work 993 in progress), January 2018. 995 [I-D.ietf-spring-segment-routing-mpls] 996 Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., 997 Litkowski, S., and R. Shakir, "Segment Routing with MPLS 998 data plane", draft-ietf-spring-segment-routing-mpls-11 999 (work in progress), October 2017. 1001 15.2. Informative References 1003 [I-D.ietf-isis-mpls-elc] 1004 Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. 1005 Litkowski, "Signaling Entropy Label Capability and 1006 Readable Label-stack Depth Using IS-IS", draft-ietf-isis- 1007 mpls-elc-03 (work in progress), January 2018. 1009 [I-D.ietf-ospf-mpls-elc] 1010 Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. 1011 Litkowski, "Signaling Entropy Label Capability and 1012 Readable Label-stack Depth Using OSPF", draft-ietf-ospf- 1013 mpls-elc-05 (work in progress), January 2018. 1015 [I-D.ietf-isis-l2bundles] 1016 Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and 1017 E. Aries, "Advertising L2 Bundle Member Link Attributes in 1018 IS-IS", draft-ietf-isis-l2bundles-07 (work in progress), 1019 May 2017. 1021 Authors' Addresses 1023 Sriganesh Kini 1025 EMail: sriganeshkini@gmail.com 1026 Kireeti Kompella 1027 Juniper 1029 EMail: kireeti@juniper.net 1031 Siva Sivabalan 1032 Cisco 1034 EMail: msiva@cisco.com 1036 Stephane Litkowski 1037 Orange 1039 EMail: stephane.litkowski@orange.com 1041 Rob Shakir 1042 Google 1044 EMail: rjs@rob.sh 1046 Jeff Tantsura 1048 EMail: jefftant@gmail.com