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Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Looks like a reference, but probably isn't: '1000' on line 395 -- Looks like a reference, but probably isn't: '1999' on line 395 == Outdated reference: A later version (-22) exists of draft-ietf-spring-segment-routing-mpls-13 == Outdated reference: A later version (-13) exists of draft-ietf-isis-mpls-elc-03 == Outdated reference: A later version (-15) exists of draft-ietf-ospf-mpls-elc-05 Summary: 0 errors (**), 0 flaws (~~), 4 warnings (==), 3 comments (--).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: October 28, 2018 Juniper 6 S. Sivabalan 7 Cisco 8 S. Litkowski 9 Orange 10 R. Shakir 11 Google 12 J. Tantsura 13 April 26, 2018 15 Entropy label for SPRING tunnels 16 draft-ietf-mpls-spring-entropy-label-09 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 MPLS. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on October 28, 2018. 44 Copyright Notice 46 Copyright (c) 2018 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (https://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 62 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 63 2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 4 64 3. Use-case requiring multipath load-balancing . . . . . . . . . 4 65 4. Entropy Readable Label Depth . . . . . . . . . . . . . . . . 6 66 5. Maximum SID Depth . . . . . . . . . . . . . . . . . . . . . . 7 67 6. LSP stitching using the binding SID . . . . . . . . . . . . . 9 68 7. Insertion of entropy labels for SPRING path . . . . . . . . . 10 69 7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10 70 7.1.1. Example 1 where the ingress node has a sufficient MSD 11 71 7.1.2. Example 2 where the ingress node has not a sufficient 72 MSD . . . . . . . . . . . . . . . . . . . . . . . . . 12 73 7.2. Considerations for the placement of entropy labels . . . 13 74 7.2.1. ERLD value . . . . . . . . . . . . . . . . . . . . . 14 75 7.2.2. Segment type . . . . . . . . . . . . . . . . . . . . 14 76 7.2.2.1. Node-SID . . . . . . . . . . . . . . . . . . . . 15 77 7.2.2.2. Adjacency-set SID . . . . . . . . . . . . . . . . 15 78 7.2.2.3. Adjacency-SID representing a single IP link . . . 15 79 7.2.2.4. Adjacency-SID representing a single link within a 80 L2 bundle . . . . . . . . . . . . . . . . . . . . 16 81 7.2.2.5. Adjacency-SID representing a L2 bundle . . . . . 16 82 7.2.3. Maximizing number of LSRs that will load-balance . . 16 83 7.2.4. Preference for a part of the path . . . . . . . . . . 16 84 7.2.5. Combining criteria . . . . . . . . . . . . . . . . . 17 85 8. A simple example algorithm . . . . . . . . . . . . . . . . . 17 86 9. Deployment Considerations . . . . . . . . . . . . . . . . . . 18 87 10. Options considered . . . . . . . . . . . . . . . . . . . . . 18 88 10.1. Single EL at the bottom of the stack . . . . . . . . . . 18 89 10.2. An EL per segment in the stack . . . . . . . . . . . . . 19 90 10.3. A re-usable EL for a stack of tunnels . . . . . . . . . 19 91 10.4. EL at top of stack . . . . . . . . . . . . . . . . . . . 20 92 10.5. ELs at readable label stack depths . . . . . . . . . . . 20 93 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21 94 12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21 95 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 96 14. Security Considerations . . . . . . . . . . . . . . . . . . . 22 97 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 98 15.1. Normative References . . . . . . . . . . . . . . . . . . 22 99 15.2. Informative References . . . . . . . . . . . . . . . . . 22 100 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 102 1. Introduction 104 Segment Routing [I-D.ietf-spring-segment-routing] is based on source 105 routed tunnels to steer a packet along a particular path. This path 106 is encoded as an ordered list of segments. When applied to the MPLS 107 dataplane [I-D.ietf-spring-segment-routing-mpls], each segment is an 108 LSP with an associated MPLS label value. Hence, label stacking is 109 used to represent the ordered list of segments and the label stack 110 associated with an SR tunnel can be seen as nested LSPs (LSP 111 hierarchy) in the MPLS architecture. 113 Using label stacking to encode the list of segment has implications 114 on the label stack depth. 116 Traffic load-balancing over ECMP (Equal Cost MultiPath) or LAGs (Link 117 Aggregation Groups) is usually based on a hashing function. The 118 local node who performs the load-balancing is required to read some 119 header fields in the incoming packets and then computes a hash based 120 on these fields. The result of the hash is finally mapped to a list 121 of outgoing nexthops. The hashing technique is required to perfom a 122 per-flow load-balancing and thus prevent packet disordering. For IP 123 traffic, the usual fields that are looked up are the source address, 124 the destination address, the protocol type, and, if the upper layer 125 is TCP or UDP, the source port and destination port can be added as 126 well in the hash. 128 The MPLS architecture brings some challenges on the load-balancing as 129 an LSR (Label Switch Router) should be able to look at header fields 130 that are beyond the MPLS label stack. An LSR must perform a deeper 131 inspection compared to an ingress router which could be challenging 132 for some hardware. Entropy label (EL) [RFC6790] is a technique used 133 in the MPLS data plane to provide entropy for load-balancing. The 134 idea behind entropy label is that the ingress router computes a hash 135 based on several fields from a given packet and place the result in 136 an additional label, named "entropy label". When using entropy 137 label, an LSR is only required to hash on the MPLS label stack or 138 solely on the entropy label to get a full benefit of load-balancing; 139 deep hashing is not required anymore. 141 When using LSP hierarchies, there are implications on how [RFC6790] 142 should be applied. The current document addresses the case where a 143 hierarchy is created at a single LSR as required by Segment Routing. 145 A use-case requiring load-balancing with SR is given in Section 3. A 146 recommended solution is described in Section 7 keeping in 147 consideration the limitations of implementations when applying 148 [RFC6790] to deeper label stacks. Options that were considered to 149 arrive at the recommended solution are documented for historical 150 purposes in Section 10. 152 1.1. Requirements Language 154 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 155 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 156 "OPTIONAL" in this document are to be interpreted as described in BCP 157 14 [RFC2119] [RFC8174] when, and only when, they appear in all 158 capitals, as shown here. 160 2. Abbreviations and Terminology 162 EL - Entropy Label 164 ELI - Entropy Label Identifier 166 ELC - Entropy Label Capability 168 ERLD - Entropy Readable Label Depth 170 SR - Segment Routing 172 ECMP - Equal Cost Multi Path 174 LSR - Label Switch Router 176 MPLS - Multiprotocol Label Switching 178 MSD - Maximum SID Depth 180 SID - Segment Identifier 182 RLD - Readable Label Depth 184 OAM - Operation, Administration and Maintenance 186 3. Use-case requiring multipath load-balancing 187 +------+ 188 | | 189 +-------| P3 |-----+ 190 | +-----| |---+ | 191 L3| |L4 +------+ L1| |L2 +----+ 192 | | | | +--| P4 |--+ 193 +-----+ +-----+ +-----+ | +----+ | +-----+ 194 | S |-----| P1 |------------| P2 |--+ +--| D | 195 | | | | | |--+ +--| | 196 +-----+ +-----+ +-----+ | +----+ | +-----+ 197 +--| P5 |--+ 198 +----+ 199 S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs, 200 L1,L2,L3,L4=Links 202 Figure 1: Traffic engineering use-case 204 Traffic-engineering is one of the applications of MPLS and is also a 205 requirement for source routed tunnels with label stacks [RFC7855]. 206 Consider the topology shown in Figure 1. The LSR S requires data to 207 be sent to LSR D along a traffic-engineered path that goes over the 208 link L1. Good load-balancing is also required across equal cost 209 paths (including parallel links). To engineer traffic along a path 210 that takes link L1, the label stack that LSR S creates consists of a 211 label to the node SID of LSR P3, stacked over the label for the 212 adjacency SID of link L1 and that in turn is stacked over the label 213 to the node SID of LSR D. For simplicity lets assume that all LSRs 214 use the same label space (SRGB) for source routed label stacks. Let 215 L_N-Px denote the label to be used to reach the node SID of LSR Px. 216 Let L_A-Ln denote the label used for the adjacency SID for link Ln. 217 The LSR S must use the label stack for 218 traffic-engineering. However to achieve good load-balancing over the 219 equal cost paths P2-P4-D, P2-P5-D and the parallel links L3, L4, a 220 mechanism such as Entropy labels [RFC6790] should be adapted for 221 source routed label stacks. Indeed, the SPRING architecture with the 222 MPLS dataplane ([I-D.ietf-spring-segment-routing-mpls]) uses nested 223 MPLS LSPs composing the source routed label stacks. 225 An MPLS node may have limitations in the number of labels it can 226 push. It may also have a limitation in the number of labels it can 227 inspect when looking for hash keys during load-balancing. While 228 entropy label is normally inserted at the bottom of the transport 229 tunnel, this may prevent an LSR to take into account the EL in its 230 load-balancing function if the EL is too deep in the stack. In a 231 segment routing environment, it is important to define the 232 considerations that needs to be taken into account when inserting EL. 233 Multiple ways to apply entropy labels were considered and are 234 documented in Section 10 along with their trade-offs. A recommended 235 solution is described in Section 7. 237 4. Entropy Readable Label Depth 239 The Entropy Readable Label Depth (ERLD) is defined as the number of 240 labels a router can both: 242 a. Read in an MPLS packet received on its incoming interface(s) 243 (starting from the top of the stack). 245 b. Use in its load-balancing function. 247 The ERLD means that the router will perform load-balancing using the 248 EL label if the EL is placed within the ERLD first labels. 250 A router capable of reading N labels but not using an EL located 251 within those N labels MUST consider its ERLD to be 0. In a 252 distributed switching architecture, each linecard may have a 253 different capability in terms of ERLD. For simplicity, an 254 implementation MAY use the minimum ERLD between each linecard as the 255 ERLD value for the system. 257 Examples: 259 | Payload | 260 +----------+ 261 | Payload | | EL | P7 262 +----------+ +----------+ 263 | Payload | | EL | | ELI | 264 +----------+ +----------+ +----------+ 265 | Payload | | EL | | ELI | | Label 50 | 266 +----------+ +----------+ +----------+ +----------+ 267 | Payload | | EL | | ELI | | Label 40 | | Label 40 | 268 +----------+ +----------+ +----------+ +----------+ +----------+ 269 | EL | | ELI | | Label 30 | | Label 30 | | Label 30 | 270 +----------+ +----------+ +----------+ +----------+ +----------+ 271 | ELI | | Label 20 | | Label 20 | | Label 20 | | Label 20 | 272 +----------+ +----------+ +----------+ +----------+ +----------+ 273 | Label 16 | | Label 16 | | Label 16 | | Label 16 | | Label 16 | P1 274 +----------+ +----------+ +----------+ +----------+ +----------+ 275 Packet 1 Packet 2 Packet 3 Packet 4 Packet 5 277 Figure 2: Label stacks with ELI/EL 279 In the figure 2, we consider the displayed packets received on a 280 router interface. We consider also a single ERLD value for the 281 router. 283 o If the router has an ERLD of 3, it will be able to load-balance 284 Packet 1 displayed in Figure 2 using the EL as part of the load- 285 balancing keys. The ERLD value of 3 means that the router can 286 read and take into account the entropy label for load-balancing if 287 it is placed between position 1 (top) and position 3. 289 o If the router has an ERLD of 5, it will be able to load-balance 290 Packets 1 to 3 in Figure 2 using the EL as part of the load- 291 balancing keys. Packets 4 and 5 have the EL placed at a position 292 greater than 5, so the router is not able to read it and use as 293 part of the load-balancing keys. 295 o If the router has an ERLD of 10, it will be able to load-balance 296 all the packets displayed in Figure 2 using the EL as part of the 297 load-balancing keys. 299 To allow an efficient load-balancing based on entropy labels, a 300 router running SPRING SHOULD advertise its ERLD (or ERLDs), so all 301 the other SPRING routers in the network are aware of its capability. 302 How this advertisement is done is outside the scope of this document. 304 To advertise an ERLD value, a SPRING router: 306 o MUST be entropy label capable and, as a consequence, MUST apply 307 the dataplane procedures defined in [RFC6790]. 309 o MUST be able to read an ELI/EL which is located within its ERLD 310 value. 312 o MUST take into account this EL in its load-balancing function. 314 5. Maximum SID Depth 316 The Maximum SID Depth defines the maximum number of labels that a 317 particular node can impose on a packet. This includes any kind of 318 labels (service, entropy, transport...). In an MPLS network, the MSD 319 is a limit of the Ingress LSR (I-LSR) or any stitching node that 320 would perform an imposition of additional labels on an existing label 321 stack. 323 Depending of the number of MPLS operations (POP, SWAP...) to be 324 performed before the PUSH, the MSD may vary due to the hardware or 325 software limitations. As for the ERLD, there may also be different 326 MSD limits based on the linecard type used in a distributed switching 327 system. 329 When an external controller is used to program a label stack on a 330 particular node, this node MAY advertise its MSD value or a subset of 331 its MSD value to the controller. How this advertisement is done is 332 outside the scope of this document. As the controller does not have 333 the knowledge of the entire label stack to be pushed by the node, the 334 node may advertise an MSD value which is lower than its actual limit. 335 This gives the ability for the controller to program a label stack up 336 to the advertised MSD value while leaving room for the local node to 337 add more labels (e.g., service, entropy, transport...) without 338 reaching the hardware/software limit. 340 P7 ---- P8 ---- P9 341 / \ 342 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2 343 | \ | 344 ----> P10 \ | 345 IP Pkt | \ | 346 P11 --- P12 --- P13 347 100 10000 349 Figure 3 351 In the figure 3, an IP packet comes in the MPLS network at PE1. All 352 metrics are considered equal to 1 except P12-P13 which is 10000 and 353 P11-P12 which is 100. PE1 wants to steer the traffic using a SPRING 354 path to PE2 along 355 PE1->P1->P7->P8->P9->P4->P5->P10->P11->P12->P13->PE2. By using 356 adjacency SIDs only, PE1 (acting as an I-LSR) will be required to 357 push 10 labels on the IP packet received and thus requires an MSD of 358 10. If the IP packet should be carried over an MPLS service like a 359 regular layer 3 VPN, an additional service label should be imposed, 360 requiring an MSD of 11 for PE1. In addition, if PE1 wants to insert 361 an ELI/EL for load-balancing purpose, PE1 will need to push 13 labels 362 on the IP packet requiring an MSD of 13. 364 In the SPRING architecture, Node SIDs or Binding SIDs can be used to 365 reduce the label stack size. As an example, to steer the traffic on 366 the same path as before, PE1 may be able to use the following label 367 stack: . In this example we 368 consider a combination of Node SIDs and a Binding SID advertised by 369 P5 that will stitch the traffic along the path P10->P11->P12->P13. 370 The instruction associated with the binding SID at P5 is thus to swap 371 Binding_P5 to Adj_P12-P13 and then push . P5 372 acts as a stitching node that pushes additional labels on an existing 373 label stack, P5's MSD needs also to be taken into account and may 374 limit the number of labels that could be imposed. 376 6. LSP stitching using the binding SID 378 The binding SID allows binding a segment identifier to an existing 379 LSP. As examples, the binding SID can represent an RSVP-TE tunnel, 380 an LDP path (through the mapping server advertisement), or a SPRING 381 path. Each LSP associated with a binding SID has its own entropy 382 label capability. 384 In the figure 3, we consider that: 386 o P6, PE2, P10, P11, P12, P13 are pure LDP routers. 388 o PE1, P1, P2, P3, P4, P7, P8, P9 are pure SPRING routers. 390 o P5 is running SPRING and LDP. 392 o P5 acts as a mapping server and advertises Prefix SIDs for the LDP 393 FECs: an index value of 20 is used for PE2. 395 o All SPRING routers use an SRGB of [1000, 1999]. 397 o P6 advertises label 20 for the PE2 FEC. 399 o Traffic from PE1 to PE2 uses the shortest path. 401 PE1 ----- P1 -- P2 -- P3 -- P4 ---- P5 --- P6 --- PE2 403 --> +----+ +----+ +----+ +----+ 404 IP Pkt | IP | | IP | | IP | | IP | 405 +----+ +----+ +----+ +----+ 406 |1020| |1020| | 20 | 407 +----+ +----+ +----+ 408 SPRING LDP 410 In term of packet forwarding, by learning the mapping-server 411 advertisement from P5, PE1 imposes a label 1020 to an IP packet 412 destinated to PE2. SPRING routers along the shortest path to PE2 413 will switch the traffic until it reaches P5 which will perform the 414 LSP stitching. P5 will swap the SPRING label 1020 to the LDP label 415 20 advertised by the nexthop P6. P6 will then forward the packet 416 using the LDP label towards PE2. 418 PE1 cannot push an ELI/EL for the binding SID without knowing that 419 the tail-end of the LSP associated with the binding (PE2) is entropy 420 label capable. 422 To accomodate the mix of signalling protocols involved during the 423 stitching, the entropy label capability SHOULD be propagated between 424 the signalling protocols. Each binding SID SHOULD have its own 425 entropy label capability that MUST be inherited from the entropy 426 label capability of the associated LSP. If the router advertising 427 the binding SID does not know the ELC state of the target FEC, it 428 MUST NOT set the ELC for the binding SID. An ingress node MUST NOT 429 push an ELI/EL associated with a binding SID unless this binding SID 430 has the entropy label capability. How the entropy label capability 431 is advertised for a binding SID is outside the scope of this 432 document. 434 In our example, if PE2 is LDP entropy label capable, it will add the 435 entropy label capability in its LDP advertisement. When P5 receives 436 the FEC/label binding for PE2, it learns about the ELC and can set 437 the ELC in the mapping server advertisement. Thus PE1 learns about 438 the ELC of PE2 and may push an ELI/EL associated with the binding 439 SID. 441 The proposed solution only works if the SPRING router advertising the 442 binding SID is also performing the dataplane LSP stitching. In our 443 example, if the mapping server function is hosted on P8 instead of 444 P5, P8 does not know about the ELC state of PE2's LDP FEC. As a 445 consequence, it does not set the ELC for the associated binding SID. 447 7. Insertion of entropy labels for SPRING path 449 7.1. Overview 451 The solution described in this section follows the dataplane 452 processing defined in [RFC6790]. Within a SPRING path, a node may be 453 ingress, egress, transit (regarding the entropy label processing 454 described in [RFC6790]), or it can be any combination of those. For 455 example: 457 o The ingress node of a SPRING domain may be an ingress node from an 458 entropy label perspective. 460 o Any LSR terminating a segment of the SPRING path is an egress node 461 (because it terminates the segment) but may also be a transit node 462 if the SPRING path is not terminated because there is a subsequent 463 SPRING MPLS label in the stack. 465 o Any LSR processing a binding SID may be a transit node and an 466 ingress node (because it may push additional labels when 467 processing the binding SID). 469 As described earlier, an LSR may have a limitation, ERLD, on the 470 depth of the label stack that it can read and process in order to do 471 multipath load-balancing based on entropy labels. 473 If an EL does not occur within the ERLD of an LSR in the label stack 474 of an MPLS packet that it receives, then it would lead to poor load- 475 balancing at that LSR. Hence an ELI/EL pair must be within the ERLD 476 of the LSR in order for the LSR to use the EL during load-balancing. 478 Adding a single ELI/EL pair for the entire SPRING path may lead also 479 to poor load-balancing as well because the EL/ELI may not occur 480 within the ERLD of some LSR on the path (if too deep) or may not be 481 present in the stack when it reaches some LSRs if it is too shallow. 483 In order for the EL to occur within the ERLD of LSRs along the path 484 corresponding to a SPRING label stack, multiple pairs MAY 485 be inserted in this label stack. 487 The insertion of the ELI/EL SHOULD occur only with a SPRING label 488 advertised by an LSR that advertised an ERLD (the LSR is entropy 489 label capable) or with a SPRING label associated with a binding SID 490 that has the ELC set. 492 The ELs among multiple pairs inserted in the stack MAY be 493 the same or different. The LSR that inserts pairs MAY have 494 limitations on the number of such pairs that it can insert and also 495 the depth at which it can insert them. If, due to limitations, the 496 inserted ELs are at positions such that an LSR along the path 497 receives an MPLS packet without an EL in the label stack within that 498 LSR's ERLD, then the load-balancing performed by that LSR would be 499 poor. An implementation MAY consider multiple criteria when 500 inserting pairs. 502 7.1.1. Example 1 where the ingress node has a sufficient MSD 504 ECMP LAG LAG 505 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2 507 Figure 4 509 In the figure 4, PE1 wants to forward some MPLS VPN traffic over an 510 explicit path to PE2 resulting in the following label stack to be 511 pushed onto the received IP header: . PE1 is limited 513 to push a maximum of 11 labels (MSD=11). P2, P3 and P6 have an ERLD 514 of 3 while others have an ERLD of 10. 516 PE1 can only add two ELI/EL pairs in the label stack due to its MSD 517 limitation. It should insert them strategically to benefit load- 518 balancing along the longest part of the path. 520 PE1 may take into account multiple parameters when inserting ELs, as 521 examples: 523 o The ERLD value advertised by transit nodes. 525 o The requirement of load-balancing for a particular label value. 527 o Any service provider preference: favor beginning of the path or 528 end of the path. 530 In the figure 4, a good strategy may be to use the following stack 531 . The original stack requests P2 to forward 533 based on a L3 adjacency set that will require load-balancing. 534 Therefore it is important to ensure that P2 can load-balance 535 correctly. As P2 has a limited ERLD of 3, ELI/EL must be inserted 536 just next to the label that P2 will use to forward. On the path to 537 PE2, P3 has also a limited ERLD, but P3 will forward based on a basic 538 adjacency segment that may require no load-balancing. Therefore it 539 does not seem important to ensure that P3 can do load-balancing 540 despite of its limited ERLD. The next nodes along the forwarding 541 path have a high ERLD that does not cause any issue, except P6, 542 moreover P6 is using some LAGs to PE2 and so is expected to load- 543 balance. It becomes important to insert a new ELI/EL just next to P6 544 forwarding label. 546 In the case above, the ingress node had enough label push capacity to 547 ensure end-to-end load-balancing taking into the path attributes. 548 There might be some cases, where the ingress node may not have the 549 necessary label imposition capacity. 551 7.1.2. Example 2 where the ingress node has not a sufficient MSD 553 ECMP LAG ECMP ECMP 554 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- P7 --- P8 --- PE2 556 Figure 5 558 In the figure 5, PE1 wants to forward MPLS VPN traffic over an 559 explicit path to PE2 resulting in the following label stack to be 560 pushed onto the IP header: . PE1 is limited to push a maximum of 11 labels, P2, P3 563 and P6 have an ERLD of 3 while others have an ERLD of 15. 565 Using a similar strategy as the previous case may lead to a dilemma, 566 as PE1 can only push a single ELI/EL while we may need a minimum of 567 three to load-balance the end-to-end path. An optimized stack that 568 would enable end-to-end load-balancing may be: . 572 A decision needs to be taken to favor some part of the path for load- 573 balancing considering that load-balancing may not work on the other 574 part. A service provider may decide to place the ELI/EL after the P6 575 forwarding label as it will allow P4 and P6 to load-balance. Placing 576 the ELI/EL at bottom of the stack is also a possibility enabling 577 load-balancing for P4 and P8. 579 7.2. Considerations for the placement of entropy labels 581 The sample cases described in the previous section showed that 582 placing the ELI/EL when the maximum number of labels to be pushed is 583 limited is not an easy decision and multiple criteria may be taken 584 into account. 586 This section describes some considerations that could be taken into 587 account when placing ELI/ELs. This list of criteria is not 588 considered as exhaustive and an implementation MAY take into account 589 additional criteria or tie-breakers that are not documented here. 591 An implementation SHOULD try to maximize the load-balancing where 592 multiple ECMP paths are available and minimize the number of EL/ELIs 593 that need to be inserted. In case of a trade-off, an implementation 594 MAY provide flexibility to the operator to select the criteria to be 595 considered when placing EL/ELIs or the sub-objective for which to 596 optimize. 598 2 2 599 PE1 -- P1 -- P2 --P3 --- P4 --- P5 -- ... -- P8 -- P9 -- PE2 600 | | 601 P3'--- P4'--- P5' 603 Figure 6 605 The figure above will be used as reference in the following 606 subsections. All metrics are equal to 1, except P3-P4 and P4-P5 607 which have a metric 2. 609 7.2.1. ERLD value 611 As mentioned in Section 7.1, the ERLD value is an important parameter 612 to consider when inserting ELI/EL. If an ELI/EL does not fall within 613 the ERLD of a node on the path, the node will not be able to load- 614 balance the traffic efficiently. 616 The ERLD value can be advertised via protocols and those extensions 617 are described in separate documents [I-D.ietf-isis-mpls-elc] and 618 [I-D.ietf-ospf-mpls-elc]. 620 Let's consider a path from PE1 to PE2 using the following stack 621 pushed by PE1: . 623 Using the ERLD as an input parameter may help to minimize the number 624 of required ELI/EL pairs to be inserted. An ERLD value must be 625 retrieved for each SPRING label in the label stack. 627 For a label bound to an adjacency segment, the ERLD is the ERLD of 628 the node that advertised the adjacency segment. In the example 629 above, the ERLD associated with Adj_P1P2 would be the ERLD of router 630 P1 as P1 will perform the forwarding based on the Adj_P1P2 label. 632 For a label bound to a node segment, multiple strategies MAY be 633 implemented. An implementation may try to evaluate the minimum ERLD 634 value along the node segment path. If an implementation cannot find 635 the minimum ERLD along the path of the segment, it can use the ERLD 636 of the starting node instead. In the example above, if the 637 implementation supports computation of minimum ERLD along the path, 638 the ERLD associated with label Node_P9 would be the minimum ERLD 639 between nodes {P2,P3,P4 ..., P8}. If an implementation does not 640 support the computation of minimum ERLD, it should consider the ERLD 641 of P2 (starting node that will forward based on the Node_P9 label). 643 For a label bound to a binding segment, if the binding segment 644 describes a path, an implementation may also try to evaluate the 645 minimum ERLD along this path. If the implementation cannot find the 646 minimum ERLD along the path of the segment, it can use the ERLD of 647 the starting node instead. 649 7.2.2. Segment type 651 Depending of the type of segment a particular label is bound to, an 652 implementation may deduce that this particular label will be subject 653 to load-balancing on the path. 655 7.2.2.1. Node-SID 657 An MPLS label bound to a Node-SID represents a path that may cross 658 multiple hops. Load-balancing may be needed on the node starting 659 this path but also on any node along the path. 661 In the figure 6, let's consider a path from PE1 to PE2 using the 662 following stack pushed by PE1: . 665 If, for example, PE1 is limited to push 6 labels, it can add a single 666 ELI/EL within the label stack. An operator may want to favor a 667 placement that would allow load-balancing along the Node-SID path. 668 In the figure above, P3 which is along the Node-SID path requires 669 load-balancing on two equal-cost paths. 671 An implementation may try to evaluate if load-balancing is really 672 required within a node segment path. This could be done by running 673 an additional SPT computation and analysis of the node segment path 674 to prevent a node segment that does not really require load-balancing 675 from being preferred when placing EL/ELIs. Such inspection may be 676 time consuming for implementations and without a 100% guarantee, as a 677 node segment path may use LAG that could be invisible from the IP 678 topology. A simpler approach would be to consider that a label bound 679 to a Node-SID will be subject to load-balancing and requires an EL/ 680 ELI. 682 7.2.2.2. Adjacency-set SID 684 An adjacency-set is an adjacency SID that refers to a set of 685 adjacencies. When an adjacency-set segment is used within a label 686 stack, an implementation can deduce that load-balancing is expected 687 at the node that advertised this adjacency segment. An 688 implementation could then favor this particular label value when 689 placing ELI/ELs. 691 7.2.2.3. Adjacency-SID representing a single IP link 693 When an adjacency segment representing a single IP link is used 694 within a label stack, an implementation can deduce that load- 695 balancing may not be expected at the node that advertised this 696 adjacency segment. 698 The implementation could then decide to place ELI/ELs to favor other 699 LSRs than the one advertising this adjacency segment. 701 Readers should note that an adjacency segment representing a single 702 IP link may require load-balancing. This is the case when a LAG (L2 703 bundle) is implemented between two IP nodes and the L2 bundle SR 704 extensions [I-D.ietf-isis-l2bundles] are not implemented. In such a 705 case, it may be useful to insert an EL/ELI in a readable position for 706 the LSR advertising the label associated with the adjacency segment. 708 7.2.2.4. Adjacency-SID representing a single link within a L2 bundle 710 When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used, 711 adjacency segments may be advertised for each member of the bundle. 712 In this case, an implementation can deduce that load-balancing is not 713 expected on the LSR advertising this segment and could then decide to 714 place ELI/ELs to favor other LSRs than the one advertising this 715 adjacency segment. 717 7.2.2.5. Adjacency-SID representing a L2 bundle 719 When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used, an 720 adjacency segment may be advertised to represent the bundle. In this 721 case, an implementation can deduce that load-balancing is expected on 722 the LSR advertising this segment and could then decide to place ELI/ 723 ELs to favor this LSR. 725 7.2.3. Maximizing number of LSRs that will load-balance 727 When placing ELI/ELs, an implementation may try to maximize the 728 number of LSRs that both need to load-balance (i.e., have ECMP paths) 729 and that will be able to perform load-balancing (i.e., the EL label 730 is within their ERLD). 732 Let's consider a path from PE1 to PE2 using the following stack 733 pushed by PE1: . All 734 routers have an ERLD of 10, expect P1 and P2 which have an ERLD of 4. 735 PE1 is able to push 6 labels, so only a single ELI/EL can be added. 737 In the example above, adding ELI/EL next to Adj_P1P2 will only allow 738 load-balancing at P1 while inserting it next to Adj_PE2P9, will allow 739 load-balancing at P2,P3 ... P9 and maximizing the number of LSRs that 740 could perform load-balancing. 742 7.2.4. Preference for a part of the path 744 An implementation may propose to favor a part of the end-to-end path 745 when the number of EL/ELI that can be pushed is not enough to cover 746 the entire path. As example, a service provider may want to favor 747 load-balancing at the beginning of the path or at the end of path, so 748 the implementation should prefer putting the ELI/ELs near the top or 749 near of the bottom of the stack. 751 7.2.5. Combining criteria 753 An implementation can combine multiple criteria to determine the best 754 EL/ELIs placement. However, combining too many criteria may lead to 755 implementation complexity and high resource consumption. Each time 756 the network topology changes, a new evaluation of the EL/ELI 757 placement will be necessary for each impacted LSPs. 759 8. A simple example algorithm 761 A simple implementation might take into account ERLD when placing 762 ELI/EL while trying to minimize the number of EL/ELIs inserted and 763 trying to maximize the number of LSRs that can load-balance. 765 The example algorithm is based on the following considerations: 767 o An LSR that is limited in the number of pairs that it 768 can insert SHOULD insert such pairs deeper in the stack. 770 o An LSR should try to insert pairs at positions so that 771 for the maximum number of transit LSRs, the EL occurs within the 772 ERLD of those LSRs. 774 o An LSR should try to insert the minimum number of such pairs while 775 trying to satisfy the above criteria. 777 The pseudocode of the example algorithm is shown below. 779 Initialize the current EL insertion point to the 780 bottommost label in the stack that is EL-capable 781 while (local-node can push more pairs OR 782 insertion point is not above label stack) { 783 insert an pair below current insertion point 784 move new insertion point up from current insertion point until 785 ((last inserted EL is below the ERLD) AND (ERLD > 2) 786 AND 787 (new insertion point is EL-capable)) 788 set current insertion point to new insertion point 789 } 791 Figure 7: Example algorithm to insert pairs in a label 792 stack 794 When this algorithm is applied to the example described in Section 3, 795 it will result in ELs being inserted in two positions, one below the 796 label L_N-D and another below L_N-P3. Thus the resulting label stack 797 would be 799 9. Deployment Considerations 801 As long as LSR node dataplane capabilities are limited (number of 802 labels that can be pushed, or number of labels that can be 803 inspected), hop-by-hop load-balancing of SPRING encapsulated flows 804 will require trade-offs. 806 Entropy label is still a good and usable solution as it allows load- 807 balancing without having to perform a deep packet inspection on each 808 LSR: it does not seem reasonable to have an LSR inspecting UDP ports 809 within a GRE tunnel carried over a 15 label SPRING tunnel. 811 Due to the limited capacity of reading a deep stack of MPLS labels, 812 multiple EL/ELIs may be required within the stack which directly 813 impacts the capacity of the head-end to push a deep stack: each EL/ 814 ELI inserted requires two additional labels to be pushed. 816 Placement strategies of EL/ELIs are required to find the best trade- 817 off. Multiple criteria may be taken into account and some level of 818 customization (by the user) may be required to accommodate the 819 different deployments. Analyzing the path of each destination to 820 determine the best EL/ELI placement may be time consuming for the 821 control plane, we encourage implementations to find the best trade- 822 off between simplicity, resource consumption, and load-balancing 823 efficiency. 825 In future, hardware and software capacity may increase dataplane 826 capabilities and may be remove some of these limitations, increasing 827 load-balancing capability using entropy labels. 829 10. Options considered 831 Different options that were considered to arrive at the recommended 832 solution are documented in this section. 834 These options are detailed here only for historical purposes. 836 10.1. Single EL at the bottom of the stack 838 In this option, a single EL is used for the entire label stack. The 839 source LSR S encodes the entropy label at the bottom of the label 840 stack. In the example described in Section 3, it will result in the 841 label stack at LSR S to look like 842 . Note that the notation in [RFC6790] is 843 used to describe the label stack. An issue with this approach is 844 that as the label stack grows due an increase in the number of SIDs, 845 the EL goes correspondingly deeper in the label stack. Hence, 846 transit LSRs have to access a larger number of bytes in the packet 847 header when making forwarding decisions. In the example described in 848 Section 3, if we consider that the LSR P1 has an ERLD of 3, P1 would 849 load-balance traffic poorly on the parallel links L3 and L4 since the 850 EL is below the ERLD of P1. A load-balanced network design using 851 this approach must ensure that all intermediate LSRs have the 852 capability to read the maximum label stack depth as required for the 853 application that uses source routed stacking. 855 This option was rejected since there exist a number of hardware 856 implementations which have a low maximum readable label depth. 857 Choosing this option can lead to a loss of load-balancing using EL in 858 a significant part of the network when that is a critical requirement 859 in a service-provider network. 861 10.2. An EL per segment in the stack 863 In this option, each segment/label in the stack can be given its own 864 EL. When load-balancing is required to direct traffic on a segment, 865 the source LSR pushes an before pushing the label 866 associated to this segment . In the example described in Section 3, 867 the source LSR S encoded label stack would be where all the ELs can be the same. Accessing the 869 EL at an intermediate LSR is independent of the depth of the label 870 stack and hence independent of the specific application that uses 871 source routed tunnels with label stacking. A drawback is that the 872 depth of the label stack grows significantly, almost 3 times as the 873 number of labels in the label stack. The network design should 874 ensure that source LSRs have the capability to push such a deep label 875 stack. Also, the bandwidth overhead and potential MTU issues of deep 876 label stacks should be considered in the network design. 878 This option was rejected due to the existence of hardware 879 implementations that can push a limited number of labels on the label 880 stack. Choosing this option would result in a hardware requirement 881 to push two additional labels per tunnel label. Hence it would 882 restrict the number of tunnels that can be stacked in a LSP and hence 883 constrain the types of LSPs that can be created. This was considered 884 unacceptable. 886 10.3. A re-usable EL for a stack of tunnels 888 In this option an LSR that terminates a tunnel re-uses the EL of the 889 terminated tunnel for the next inner tunnel. It does this by storing 890 the EL from the outer tunnel when that tunnel is terminated and re- 891 inserting it below the next inner tunnel label during the label swap 892 operation. The LSR that stacks tunnels should insert an EL below the 893 outermost tunnel. It should not insert ELs for any inner tunnels. 894 Also, the penultimate hop LSR of a segment must not pop the ELI and 895 EL even though they are exposed as the top labels since the 896 terminating LSR of that segment would re-use the EL for the next 897 segment. 899 In Section 3 above, the source LSR S encoded label stack would be 900 . At P1, the outgoing label stack 901 would be after it has load-balanced 902 to one of the links L3 or L4. At P3 the outgoing label stack would 903 be . At P2, the outgoing label stack would be and it would load-balance to one of the nexthop LSRs P4 905 or P5. Accessing the EL at an intermediate LSR (e.g., P1) is 906 independent of the depth of the label stack and hence independent of 907 the specific use-case to which the label stack is applied. 909 This option was rejected due to the significant change in label swap 910 operations that would be required for existing hardware. 912 10.4. EL at top of stack 914 A slight variant of the re-usable EL option is to keep the EL at the 915 top of the stack rather than below the tunnel label. In this case, 916 each LSR that is not terminating a segment should continue to keep 917 the received EL at the top of the stack when forwarding the packet 918 along the segment. An LSR that terminates a segment should use the 919 EL from the terminated segment at the top of the stack when 920 forwarding onto the next segment. 922 This option was rejected due to the significant change in label swap 923 operations that would be required for existing hardware. 925 10.5. ELs at readable label stack depths 927 In this option the source LSR inserts ELs for tunnels in the label 928 stack at depths such that each LSR along the path that must load 929 balance is able to access at least one EL. Note that the source LSR 930 may have to insert multiple ELs in the label stack at different 931 depths for this to work since intermediate LSRs may have differing 932 capabilities in accessing the depth of a label stack. The label 933 stack depth access value of intermediate LSRs must be known to create 934 such a label stack. How this value is determined is outside the 935 scope of this document. This value can be advertised using a 936 protocol such as an IGP. 938 Applying this method to the example in Section 3 above, if LSR P1 939 needs to have the EL within a depth of 4, then the source LSR S 940 encoded label stack would be where all the ELs would typically have the same value. 943 In the case where the ERLD has different values along the path and 944 the LSR that is inserting pairs has no limit on how many 945 pairs it can insert, and it knows the appropriate positions in the 946 stack where they should be inserted, this option is the same as the 947 recommended solution in Section 7. 949 Note that a refinement of this solution which balances the number of 950 pushed labels against the desired entropy is the solution described 951 in Section 7. 953 11. Acknowledgements 955 The authors would like to thank John Drake, Loa Andersson, Curtis 956 Villamizar, Greg Mirsky, Markus Jork, Kamran Raza, Carlos Pignataro, 957 Bruno Decraene, Chris Bowers and Nobo Akiya for their review comments 958 and suggestions. 960 12. Contributors 962 Xiaohu Xu 963 Huawei 965 Email: xuxiaohu@huawei.com 967 Wim Hendrickx 968 Nokia 970 Email: wim.henderickx@nokia.com 972 Gunter Van De Velde 973 Nokia 975 Email: gunter.van_de_velde@nokia.com 977 Acee Lindem 978 Cisco 980 Email: acee@cisco.com 982 13. IANA Considerations 984 This memo includes no request to IANA. Note to RFC Editor: Remove 985 this section before publication. 987 14. Security Considerations 989 This document does not introduce any new security considerations 990 beyond those already listed in [RFC6790]. 992 15. References 994 15.1. Normative References 996 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 997 Requirement Levels", BCP 14, RFC 2119, 998 DOI 10.17487/RFC2119, March 1997, 999 . 1001 [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and 1002 L. Yong, "The Use of Entropy Labels in MPLS Forwarding", 1003 RFC 6790, DOI 10.17487/RFC6790, November 2012, 1004 . 1006 [RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B., 1007 Litkowski, S., Horneffer, M., and R. Shakir, "Source 1008 Packet Routing in Networking (SPRING) Problem Statement 1009 and Requirements", RFC 7855, DOI 10.17487/RFC7855, May 1010 2016, . 1012 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1013 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1014 May 2017, . 1016 [I-D.ietf-spring-segment-routing] 1017 Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B., 1018 Litkowski, S., and R. Shakir, "Segment Routing 1019 Architecture", draft-ietf-spring-segment-routing-15 (work 1020 in progress), January 2018. 1022 [I-D.ietf-spring-segment-routing-mpls] 1023 Bashandy, A., Filsfils, C., Previdi, S., Decraene, B., 1024 Litkowski, S., and R. Shakir, "Segment Routing with MPLS 1025 data plane", draft-ietf-spring-segment-routing-mpls-13 1026 (work in progress), April 2018. 1028 15.2. Informative References 1030 [I-D.ietf-isis-mpls-elc] 1031 Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. 1032 Litkowski, "Signaling Entropy Label Capability and 1033 Readable Label-stack Depth Using IS-IS", draft-ietf-isis- 1034 mpls-elc-03 (work in progress), January 2018. 1036 [I-D.ietf-ospf-mpls-elc] 1037 Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. 1038 Litkowski, "Signaling Entropy Label Capability and 1039 Readable Label-stack Depth Using OSPF", draft-ietf-ospf- 1040 mpls-elc-05 (work in progress), January 2018. 1042 [I-D.ietf-isis-l2bundles] 1043 Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and 1044 E. Aries, "Advertising L2 Bundle Member Link Attributes in 1045 IS-IS", draft-ietf-isis-l2bundles-07 (work in progress), 1046 May 2017. 1048 Authors' Addresses 1050 Sriganesh Kini 1052 EMail: sriganeshkini@gmail.com 1054 Kireeti Kompella 1055 Juniper 1057 EMail: kireeti@juniper.net 1059 Siva Sivabalan 1060 Cisco 1062 EMail: msiva@cisco.com 1064 Stephane Litkowski 1065 Orange 1067 EMail: stephane.litkowski@orange.com 1069 Rob Shakir 1070 Google 1072 EMail: rjs@rob.sh 1074 Jeff Tantsura 1076 EMail: jefftant@gmail.com