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Eckert 5 Expires: November 19, 2021 Futurewei Technologies, Inc. 6 May 18, 2021 8 Use of an MPLS LSE as an Ancillary Data Pointer 9 draft-bryant-mpls-aux-data-pointer-00 11 Abstract 13 The purpose of this memo is to describe how Label Stack Entries 14 (LSEs) can be used to point to ancillary or meta-data carried below 15 the MPLS label stack. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at https://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on November 19, 2021. 34 Copyright Notice 36 Copyright (c) 2021 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (https://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 2. Background Documents . . . . . . . . . . . . . . . . . . . . 2 53 3. Use of SPLs as Pointers . . . . . . . . . . . . . . . . . . . 3 54 4. Label Operations: Popping and Swapping . . . . . . . . . . . 5 55 5. Use of Multiple Pointers . . . . . . . . . . . . . . . . . . 6 56 6. Disposition of the Ancillary Data . . . . . . . . . . . . . . 9 57 7. Structure of Ancillary Data . . . . . . . . . . . . . . . . . 9 58 8. Structure of Pointer Label . . . . . . . . . . . . . . . . . 9 59 9. Backward Compatibility . . . . . . . . . . . . . . . . . . . 10 60 10. Security Considerations . . . . . . . . . . . . . . . . . . . 10 61 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 62 12. Appendix 1: CONTROVERSY ALERT - Use Of Ordinary Labels . . . 11 63 13. Appendix 2: Other Issues for Discussion . . . . . . . . . . . 12 64 14. Appendix 3: Ancillary vs Auxiliary vs Metadata . . . . . . . 12 65 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 66 15.1. Normative References . . . . . . . . . . . . . . . . . . 13 67 15.2. Informative References . . . . . . . . . . . . . . . . . 13 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 70 1. Introduction 72 There has been significant recent interest in developing the MPLS 73 data plane to address new needs, and in particular to carry ancillary 74 or meta-data below the stack. In this document we consider that this 75 ancillary data is further subdivided into a sequence of blocks. This 76 draft does not prescribe the information or its structure of the 77 ancillary data. For the sake of examples, it could range from a 78 single ancillary data unit to a structured set of ancillary data 79 blocks similar to an IPv6 extension header. There has also been 80 recent interest in carrying additional flags or other indicators to 81 qualify the forwarding operations. 83 This memo proposes the use of "spare" bits in a Special Purpose Label 84 (SPL) [I-D.kompella-mpls-mspl4fa] be used as a pointer to items of 85 ancillary data carried below the bottom of stack (BoS). Finally we 86 speculate that in certain network scopes we may usefully be able to 87 create pseudo-SPLs from the ordinary label pool. 89 2. Background Documents 91 [I-D.kompella-mpls-mspl4fa] notes that the forwarder does not need to 92 use the TC, or TTL fields in an LSE [RFC3032] that does not become 93 top of stack (ToS). It proposes to exploit these fields as 94 indicators of forwarding actions, by modifying the semantics of these 95 fields. 97 There are a number of key proposals in that draft: 99 o Using the "spare bits" as forwarding indicator flags to specify 100 actions or in some cases inactions 102 o Using the method to multi-purpose SPLs and thus expand the number 103 of single label SPLs available to the IETF. 105 o Reuse the Entropy Label (EL) fields to carry additional data 106 needed by the forwarder. This latter point could be adopted by 107 any eSPL. One use for this additional data that was proposed 108 (certainly in discussion but I cannot see it in the draft) was the 109 use of this facility to carry a network slice identifier. 111 This draft proposes that these "spare" bits in an SPL or pseudo-SPL 112 be used as a pointer to ancillary data below the stack. 114 This proposal can be used in conjunction with the other indicator 115 proposals, for example by using different SPLs for different options, 116 such as one SPL indicating the presence of a pointer vs one or more 117 other SPLs for the other proposals. 119 3. Use of SPLs as Pointers 121 Previously it had been proposed to use the "spare" bits in an SPL 122 that is not ToS as a bit field or as an enumerator of a slice. 123 However, it would appear to be an advantage to take things a bit 124 further and use them as a pointer to ancillary data below the BoS. 125 This ancillary data can then be accessed and processed as needed 126 whenever the SPL is being processed. 128 The advantages of doing this are: 130 o The ability to find the ancillary data without scanning the whole 131 stack. Speculatively scanning the label stack can be expensive in 132 Network Processor Unit (NPU) processing time, particularly if the 133 stack is deep. 135 o Ability to specify which ancillary data is applicable at the hop 136 being processed. 138 o The use of a pointer or set of pointers allows for a simple packet 139 parser. 141 o The approach is inherently general and extensible. 143 This concept is illustrated in Figure 1. 145 +-------------------------+ 146 L1 | Top of Stack | 147 +-------------------------+ 148 L2 | Pointer SPL |-----+ 149 +-------------------------+ | 150 | | | 151 . . | 152 . . | 153 | | | 154 +-------------------------+ | 155 | Bottom of Stack | | 156 =============================== | 157 | Ancillary Data | | 158 . . | 159 . .<----+ 160 | | 161 +-------------------------+ 163 Figure 1: Use of In-stack MPLS pointer 165 The ToS label (L1) and Pointer SPL (L2) form a tuple with the 166 semantic "process the action that the Forwarding Equivalence Class 167 (FEC) of the ToS label specifies with the assistance of the 168 information pointed to by the following SPL". Ideally L2 is SPL 169 requiring a single LSE rather than an Extended SPL (ESPL) requiring 170 two LSEs. Whilst the additional LSE required for an ESPL may not 171 initially seem significant, the authors imagine that there may be 172 cases where multiple pointer labels will be required. 174 Let us consider the case when the ToS is not an SPL of any kind. In 175 this case, the forwarder looks at the following LSE (i.e., the LSE 176 that immediately succeeds the ToS). If that LSE is not a pointer 177 SPL, forwarding is performed as normal. If, on the other hand, the 178 following label is a pointer SPL, the forwarder uses the information 179 pointed to by the pointer as assistance in the forwarding operation. 181 Note that whilst the pointer can be simply point to the end of the 182 stack, which aligns with the other MPLS proposals being made, the 183 ideas discussed here can actually point to a specific item within the 184 MPLS payload i.e. to a specific item of ancillary data. This in turn 185 also means that different LSEs can point to different ancillary data 186 components. This allows the MPLS application or packet designer to 187 express sophisticated behavior in which it is possibly to apply 188 different ancillary data to different LSEs, i.e. different network 189 segments. 191 4. Label Operations: Popping and Swapping 193 When the ToS is popped, consideration needs to be given to any 194 Pointer SPL immediately following it. 195 In the basic case, a Pointer SPL will simply be popped along with the 196 ToS. 198 There will be cases in in which the same Pointer SPL applies to 199 multiple labels. In those cases, requiring the forwarder to pop the 200 Pointer SPL along with the ToS results in the need to carry multiple 201 instances of the same Pointer SPL, one for each label to which it 202 applies. As an optimization, it will make sense to offer a second 203 behavioral option in which, upon popping the ToS, any subsequent 204 Pointer SPL will be swapped with the next FEC label. This case is 205 depicted in Figure 2 and Figure 3. 207 +-------------------------+ 208 L1 | Top of Stack | 209 +-------------------------+ 210 L2 | Pointer SPL |-----+ 211 +-------------------------+ | 212 L3 | Next LSE | | 213 +-------------------------+ | 214 L4 | Whatever | | 215 +-------------------------+ | 216 . . | 217 | | | 218 +-------------------------+ | 219 | Bottom of Stack | | 220 =============================== | 221 | Ancillary Data | | 222 . . | 223 . .<----+ 224 | | 225 +-------------------------+ 227 Figure 2: Before Pop - Swap operation: 229 +-------------------------+ 230 L3 | Next LSE (New ToS) | 231 +-------------------------+ 232 L2 | Pointer SPL |-----+ 233 +-------------------------+ | 234 L4 | Whatever | | 235 +-------------------------+ | 236 . . | 237 . . | 238 | | | 239 +-------------------------+ | 240 | Bottom of Stack | | 241 =============================== | 242 | Ancillary Data | | 243 . . | 244 . .<----+ 245 | | 246 +-------------------------+ 248 Figure 3: After Pop - Swap operation: 250 When this optimization is applied, there needs to be a distinction 251 that allows a forwarder to determine whether a Pointer SPL should be 252 popped along with its ToS, or whether it should be swapped with the 253 next FEC label below. 254 One possibility is to indicate this in the FEC of the LSE that 255 precedes the Pointer SPL, or perhaps it can be indicated by using one 256 of its bits as a corresponding flag. Alternatively a perhaps some 257 method can be found whereby a "TTL" can be associated with the 258 pointer label. How to best indicate this end-of-use distinction is 259 for further study. 261 5. Use of Multiple Pointers 263 One problem to be solved is how to support multiple independent (sets 264 of) ancillary data in the MPLS header in support of different 265 (forwarding, OAM etc) operations associated with the ancillary data. 266 In IP/IPv6, ancillary data is encoded in the packet header through a 267 sequence of Extension Headers (EH). For IPv6, [RFC8200] defines 268 several EH types, each of which implies a specific set of nodes that 269 have to process the EH and their order. This approach results in a 270 complex parsing requirement for IPv6 packets when multiple extension 271 headers are used and very rigid encoding and EH semantic difficult to 272 extend. 274 Instead of limiting processing to a single pointer, it is possible to 275 generalize the above concepts to allow for multiple pointers. This 276 increases flexibility by allowing the packet designer to include 277 pointers to multiple sets of ancillary data, each of which can be 278 potentially used for a different purpose. 279 Therefore, the use of multiple adjacent Pointer SPLs is allowed. 280 This means that ToS processing takes into considerations all Pointer 281 SPLs that immediately follow. 283 A pointer mechanism in the MPLS label stack provides a method of 284 using multiple pointers to express two (or more) sets of ancillary 285 data, for example a latency object and an iOAM object. An example is 286 shown in Figure 4. 288 +-------------------------+ 289 L1 | Top of Stack | 290 +-------------------------+ 291 L2 | Pointer SPL |-----+ 292 +-------------------------+ | 293 L3 | Pointer SPL |-----|---+ 294 +-------------------------+ | | 295 | | | | 296 . . | | 297 . . | | 298 | | | | 299 +-------------------------+ | | 300 | Bottom of Stack | | | 301 =============================== | | 302 | Ancillary Data | | | 303 . . | | 304 . .<----+ | 305 . . | 306 . .<--------+ 307 | | 308 +-------------------------+ 310 Figure 4: Use of Multiple In-stack MPLS Pointers 312 As in Figure 1 the top three labels are a tuple that in this case 313 have the semantics "process the action that the FEC of the ToS label 314 specifies with the assistance of the information pointed to by the 315 following SPLs", in this case the label set L1, L2, L3. The tuple 316 terminates when either an "ordinary" label, an SPL that is not a 317 pointer SPL, or a label with the S bit set is encountered. 319 The Figure 5 further illustrates this capability. Here in this 320 example two LSEs, Li and Lj, are each associated with two pointers. 321 The FEC of Li therefore indicates that execution of Li includes the 322 use of Ancillary Data 1 and 2, and execution of Lj includes the use 323 of ancillary data 2 and 3. 325 +-------------------------+ 326 L1 | Top of Stack | 327 +-------------------------+ 328 . . 329 +-------------------------+ 330 Li | FEC LSE | 331 | Pointer |---------+ 332 | Pointer |-----+ | 333 +-------------------------+ | | 334 | | | | 335 . . | | 336 +-------------------------+ | | 337 Lj | FEC LSE | | | 338 | Pointer |-----|---+ 339 | Pointer |---+ | | 340 +-------------------------+ | | | 341 . . | | | 342 | | | | | 343 +-------------------------+ | | | 344 | Bottom of Stack | | | | 345 =========================== | | | 346 | Ancillary Data 1 |<--|-+ | 347 . . | | 348 +-------------------------+ | | 349 | Ancillary Data 2 |<--|-----+ 350 . . | 351 +-------------------------+ | 352 | Ancillary Data 3 |<--+ 353 . . 354 ........................... 355 . ... . 356 +-------------------------+ 358 Figure 5: Further Example of Multiple In-stack MPLS Pointers 360 To support multiple Pointer SPLs, the following additional 361 considerations apply: 363 The pop operation for the ToS needs to be extended to apply to the 364 entire tuple of Pointer SPLs that are in its scope, i.e. that 365 immediately succeed it. The default pop behavior will be to pop the 366 entire tuple of Pointer SPLs along with the ToS. 368 As described earlier, as an optimization to reduce the size of the 369 label stack, Pointer SPLs can be designated to not be popped but 370 instead swapped with other LSEs in the stack. This will allow the 371 same Pointer SPL set to be applied to multiple LSEs. 373 For an in stack swap operation where multiple pointers form a pointer 374 set, the entire tuple, or group, would be swapped with the next FEC 375 label below. 377 In addition, mixed cases are conceivable in which some Pointer SPLs 378 are popped whereas others are swapped. Whether to pop or swap a 379 Pointer SPL needs to be specified as part of the associated LSE's 380 disposition behavior. 382 6. Disposition of the Ancillary Data 384 The ancillary data must be removed before the payload is passed out 385 of the MPLS domain. There are three methods whereby the egress PE 386 can know of the presence of the ancillary data: 388 o The FEC of the BoS LSE can indicate the need to do this in a 389 manner similar to pseudowires or MPLS VPN. 391 o The BoS LSE can be a special purpose label indicating the presence 392 of the ancillary data. 394 o The BOS LSE can point to an item of ancillary data that describes 395 the disposition of the ancillary data. 397 The removal of the ancillary data may be relatively complex depending 398 on its purpose, i.e. it may be more complex than removing some number 399 of bytes, for example, if it is carrying latency or iOAM information. 401 The structure and quantity of ancillary data including any methods 402 whereby the ancillary data points to other ancillary, or whether 403 there are pointer to the payload itself is out of scope for this 404 document. Such information will need to be included in the ancillary 405 data design so that it can be safely processed and/or removed. 407 7. Structure of Ancillary Data 409 The structure of the ancillary data is outside the scope of this 410 memo. 412 8. Structure of Pointer Label 414 A possible structure for a pointer LSE is shown in Figure 6. 416 0 1 2 3 417 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 418 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 419 | Label | Flg |S| Pointer | 420 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 422 Label : contains the label that triggers the pointer behavior 424 Flg (Flags): Contains a number of flags that clarify the pointer 425 Bit 20: Size of pointer units, Bit 20 = 0 units are octets, 426 Bit = 1 units are 16 bit quantities 427 S : BoS as per {{RFC3032}} 428 Pointer : Pointer to the start of the specific ancillary data 429 block. 431 Figure 6: MPLS Pointer LSE 433 The label is recognized by the forwarder as being the trigger for the 434 pointer behavior. The pointer is the offset from the pointer LSE to 435 the start of the auxiliary data that is to be used at this hop to 436 process the ToS label. The pointer may be in units of octets of 16 437 bit words (or 32 bit words TBD) as specified by the flag. The S bit 438 had its normal meaning in an MPLS LSE. 440 9. Backward Compatibility 442 If the LSP includes a legacy node that does not understand the 443 pointer SPL it will forward based on the FEC of the ToS alone 444 omitting the feature. If that feature absence results in a service 445 shortfall for traffic on the LSP or MPLS-SR path then obviously the 446 LSP or path has to be constructed to avoid any node that is unable to 447 execute the feature. The various methods of constructing LSPs via 448 LSRs with certain capabilities is well known routing technology and 449 will not be further discussed in this memo. 451 10. Security Considerations 453 This proposal does not change the security of the MPLS data plane. 454 Normal operational practice is to prohibit the ingress of an MPLS 455 packet from other than a trusted source. An attacker that breaches 456 the physical security of an MPLS domain has many methods of attack by 457 manipulating the label stack, and this mechanism does not 458 significantly increase that risk. 460 11. IANA Considerations 462 This document has no IANA requests. 464 12. Appendix 1: CONTROVERSY ALERT - Use Of Ordinary Labels 466 Given the restricted number of Base SPLs [RFC9017] it is interesting 467 to consider whether we might use an ordinary label for this purpose. 468 At the time of writing there are eight out of 16 base SPLS still 469 available. This is a dilemma since there is a protocol need for 470 single label SPLs to support MPLS stack efficiency, but those that we 471 have available must last the development lifetime of MPLS. On the 472 other hand using a non-SPL has potential run-time/hardware issues if 473 we need lots of them. However there probably exists a compromise 474 number where we can safe allocating Base SPLs but not significantly 475 impact the forwarder performance with this approach. 477 The label becomes a run-time constant that the forwarder needs to 478 check during the parsing of the label stack. This is a 20 bit 479 compare of a run-time constant. This is simple for a software or 480 microcoded forwarder but needs a programmable register in a fully 481 hardware based forwarder. Clearly from a protocol design perspective 482 it is necessary to check the restrictions on the deployed hardware, 483 but this certainly seems feasible. In deployment it will of course 484 be necessary to verify that the routers along the LSP can support 485 this feature before the LSP can be constructed. 487 If an ordinary label were assigned to this purpose from the 488 16-104857516 label set, there are two cases to consider: LSRs that 489 have the capability of associating a FEC with a label of this value 490 and LSRs that do not. 492 If an LSR has the capability to allocate a FEC with the chosen value 493 it is necessary to preallocate this label before any MPLS application 494 takes that value. This may impact a number of MPLS applications, but 495 it seems feasible. 497 An LSR that does not have the capability to allocate a FEC with this 498 value simply has the issue of adapting the forwarding behavior. 500 The matter of choosing a suitable value and distributing it is 501 outside the scope of this memo, but is something that is routinely 502 done in the routing system so is not a factor in assessing 503 feasibility. 505 Clearly the LSP needs to be constructed so as to avoid any LSR that 506 is unable to process a packet with one of these sequestered ordinary 507 labels, but that is no different to the case where an SPL is used. 509 The final consideration is what happens if the label every becomes 510 ToS. For this to happen the packet must have been incorrectly 511 processed, and that is no different from any other case of a 512 incorrectly processed MPLS packet. 514 13. Appendix 2: Other Issues for Discussion 516 This appendix briefly describes a number of issue that require 517 further consideration. 519 1) Pointer labels as described earlier in this document are defined 520 using an offset that is calculated from the pointer label. 521 An alternative approach, given that we may not get rid of scanning 522 for the BoS in MPLS header parsing, is to consider a design in which 523 offsets were relative to the BoS instead. We could use this as a 524 standard method, or we could specify this via a flag in the LSE 525 carrying the offset. 527 The relative to BoS relative approach can be more efficient in some 528 circumstances, e.g.: When the ancillary data is applicable to 529 multiple hops of a label stack that is indicating a steering path, 530 such as in SR-MPLS, the FEC of every steering hop label could 531 indicate to "reuse" the ancillary data for every hop. The MPLS 532 operation would then consist of a pop of the ToS label followed by a 533 swap of the two top labels with each other, so that the following 534 steering label effectively gets pulled up as ToS. 536 2) To support pointers being valid across multiple hops, the pointer 537 either needs to be indicated either as an offset relative to BoS, or 538 the value of the pointer in the pointer-SPL needs to be adjusted by a 539 swap operation. 541 3) The LSE pointer could include a lifetime indicating the number of 542 times it is to be propagated. This raises the following issue. 544 4) The pointer mechanism has to be able to deal with multiple 545 instances of ancillary data applicable to specific elements within 546 the LSP. So it is important to know when to stop propagating any 547 pointer if that approach is adopted (instead of, for example adopting 548 an approach of one pointer LSE per ToS label. 550 5) We need to decide of correct name for pointer SPL. 552 14. Appendix 3: Ancillary vs Auxiliary vs Metadata 554 From the Oxford English Dictionary: 556 o Ancillary: Designating activities and services that provide 557 essential support to the functioning of a central service or 558 industry; 560 o Auxiliary: Helpful, assistant, affording aid, rendering 561 assistance, giving support or succour(sic). 563 o Metadata(sic): data whose purpose is to describe and give 564 information about other data. 566 The two terms ancillary and auxiliary are similar but the additional 567 qualifier that ancillary is _essential_ support make it, in the 568 author's view, the preferred term. 570 Metadata, the term often used in the technical discussions does 571 appear to be sufficiently descriptive of the purpose of this 572 information that is included in the packet. 574 15. References 576 15.1. Normative References 578 [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., 579 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack 580 Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001, 581 . 583 [RFC9017] Andersson, L., Kompella, K., and A. Farrel, "Special- 584 Purpose Label Terminology", RFC 9017, 585 DOI 10.17487/RFC9017, April 2021, 586 . 588 15.2. Informative References 590 [I-D.kompella-mpls-mspl4fa] 591 Kompella, K., Beeram, V. P., Saad, T., and I. Meilik, 592 "Multi-purpose Special Purpose Label for Forwarding 593 Actions", draft-kompella-mpls-mspl4fa-00 (work in 594 progress), February 2021. 596 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 597 (IPv6) Specification", STD 86, RFC 8200, 598 DOI 10.17487/RFC8200, July 2017, 599 . 601 Authors' Addresses 603 Stewart Bryant 604 Futurewei Technologies, Inc. 606 Email: sb@stewartbryant.com 608 Alexander Clemm 609 Futurewei Technologies, Inc. 611 Email: ludwig@clemm.org 613 Toerless Eckert 614 Futurewei Technologies, Inc. 616 Email: tte@cs.fau.de