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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-13) exists of draft-song-mpls-extension-header-04 Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MPLS H. Song, Ed. 3 Internet-Draft Futurewei Technologies 4 Intended status: Informational Z. Li 5 Expires: January 3, 2022 T. Zhou 6 Huawei 7 L. Andersson 8 Bronze Dragon Consulting 9 July 2, 2021 11 Options for MPLS Extension Header Indicator 12 draft-song-mpls-eh-indicator-03 14 Abstract 16 The intention of this document is to enumerate and describe the 17 candidate schemes that can be used to indicate the presence of the 18 MPLS extension header(s) following the MPLS label stack. After a 19 careful evaluation of these options by comparing their pros and cons, 20 it is expected that one should be chosen as the final standard scheme 21 for MPLS extension header indicator. 23 Requirements Language 25 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 26 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 27 "OPTIONAL" in this document are to be interpreted as described in BCP 28 14 [RFC2119][RFC8174] when, and only when, they appear in all 29 capitals, as shown here. 31 Status of This Memo 33 This Internet-Draft is submitted in full conformance with the 34 provisions of BCP 78 and BCP 79. 36 Internet-Drafts are working documents of the Internet Engineering 37 Task Force (IETF). Note that other groups may also distribute 38 working documents as Internet-Drafts. The list of current Internet- 39 Drafts is at https://datatracker.ietf.org/drafts/current/. 41 Internet-Drafts are draft documents valid for a maximum of six months 42 and may be updated, replaced, or obsoleted by other documents at any 43 time. It is inappropriate to use Internet-Drafts as reference 44 material or to cite them other than as "work in progress." 46 This Internet-Draft will expire on January 3, 2022. 48 Copyright Notice 50 Copyright (c) 2021 IETF Trust and the persons identified as the 51 document authors. All rights reserved. 53 This document is subject to BCP 78 and the IETF Trust's Legal 54 Provisions Relating to IETF Documents 55 (https://trustee.ietf.org/license-info) in effect on the date of 56 publication of this document. Please review these documents 57 carefully, as they describe your rights and restrictions with respect 58 to this document. Code Components extracted from this document must 59 include Simplified BSD License text as described in Section 4.e of 60 the Trust Legal Provisions and are provided without warranty as 61 described in the Simplified BSD License. 63 Table of Contents 65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 66 2. Dedicated Extension Header Label . . . . . . . . . . . . . . 3 67 2.1. Special Purpose Label . . . . . . . . . . . . . . . . . . 3 68 2.2. Extension Label plus an Extended Special Purpose Label . 4 69 3. Generic Associated Channel Extension . . . . . . . . . . . . 4 70 3.1. GAL and Associated Channel Header . . . . . . . . . . . . 4 71 3.2. GAL and a Different Nibble Value . . . . . . . . . . . . 5 72 4. Extend MPLS Entropy Label . . . . . . . . . . . . . . . . . . 6 73 5. Configured FEC Labels . . . . . . . . . . . . . . . . . . . . 7 74 6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 75 7. Considerations of EHI . . . . . . . . . . . . . . . . . . . . 9 76 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 77 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 78 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9 79 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 80 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 81 12.1. Normative References . . . . . . . . . . . . . . . . . . 9 82 12.2. Informative References . . . . . . . . . . . . . . . . . 10 83 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 85 1. Introduction 87 The document [I-D.song-mpls-extension-header] presents the 88 motivation, specification, and use cases of MPLS Extension Header 89 (EH). An indicator is needed in the MPLS label stack to indicate the 90 presence of the extension header(s). Multiple options are possible 91 for this purpose. As the discussion progresses, more options could 92 emerge. 94 In this document, we propound three categories of methods which can 95 be further partitioned into five unique schemes. Four of them use 96 explicit data plane encoding to indicate the EH and the last one 97 implies the EH through control plane configuration. This document 98 details and compares these schemes, in order to foster further 99 discussions until a final decision is made. 101 2. Dedicated Extension Header Label 103 A straightforward method is to directly encode an Extension Header 104 Label (EHL) in the MPLS label stack. Two derived schemes are as 105 follows. 107 2.1. Special Purpose Label 109 A new special purpose label, EHL, can be used to indicate EHs. As 110 specified in [RFC7274], so far eight special purpose label values are 111 still left unsigned by IANA (which are 4 to 6 and 8 to 12). This 112 single label scheme is elegant but arguably demands a scarce 113 resource. We cannot rule out the possibility of requiring more than 114 one label value to differentiate EH classes (e.g., Hop-by-Hop, End- 115 to-End, or both). If this happens, it can only aggravate the 116 situation. 118 Another benefit of this scheme is that an EHL can potentially be 119 located anywhere in an MPLS label stack. It is easier and quicker 120 for a router to figure out the existence of extension header(s) if 121 the EHL is close to or at the top of the label stack. However, if 122 there are legacy devices which can reach the EHL but do not recognize 123 it in a network, then for backward compatibility, the EHL must be 124 located at the bottom of the stack (i.e., only the MPLS tunnel ends 125 and EHL-aware nodes will look up and process it). 127 The format of an EHL is the same as an MPLS label. The first 20-bit 128 label value will be assigned by IANA. The BoS bit is used to 129 indicate the location of the label. The other fields, CoS and TTL, 130 currently have no use in the context of EHL. However, these two 131 fields can potentially be used to encode other information. If such 132 code points are open for other purpose, it will make the single EHL 133 idea more compelling. E.g., the EH category and/or other 134 information, if needed, can be encoded in these fields, so that only 135 one special label value is needed. 137 The following figure shows a potential scheme in which one bit from 138 the CoS field ('H') is used to indicate the presence of HbH EHs in 139 the packet. If 'H' bit is 0, it means no HbH EH follows so a 140 P-router will not need to check the EH. The last 8 bits is used to 141 find the location of the extension headers (i.e., the first byte 142 after the MPLS label stack). This information can help to avoid the 143 scan of the label stack in case the extension headers need to be 144 accessed. 146 0 1 2 3 147 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 1 148 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 149 | EHL (TBD) |H| |S| Offset | 150 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 152 Figure 1: Special EHL with EH Category Encoding 154 Note that the Cos/TTL fields can be encoded to include more 155 information. For example, in addition to indicate the EH, it can 156 also indicate the presence of some other label-based services (e.g., 157 EL). If we want to explore such possibilities, we have 11 bits in 158 total at our disposal. 160 2.2. Extension Label plus an Extended Special Purpose Label 162 [RFC7274] specifies the Extension Label (XL) with the value of 15. 163 An extended special purpose label (ESPL) following XL can be used as 164 EHL. A large number of ESPL values are available for allocation. 165 The XL+EHL scheme eases the concern on the reserved label space at 166 the cost of one more label in the label stack. 168 Except for the fact that one more label is needed, The XL+EHL scheme 169 shares the same property as the single special purpose EHL scheme. 171 3. Generic Associated Channel Extension 173 The similar "header extension" requirement for MPLS has led to some 174 proposals before. A special Generic Associated Channel Label (GAL) 175 [RFC5586] with the value of 13 has been assigned to support the 176 identification of an Associated Channel Header (ACH). We can extend 177 this existing mechanism to encode the MPLS EH indicator. 179 3.1. GAL and Associated Channel Header 181 The ACH is located below the bottom label. It has a 16-bit Channel 182 Type field which provides abundant space to encode the MPLS EH 183 indicator. This scheme has the same header overhead as the XL+EHL 184 scheme. The format is depicted in Figure 2. 186 0 1 2 3 187 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 1 188 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 189 | GAL (13) | EXP |1| TTL | 190 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 191 |0 0 0 1|Version| Reserved | Extension Header Indicator | 192 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 193 | | 194 | HEH and EH(s) | 195 | | 196 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 198 Figure 2: Associated Channel Header as Extension Header Indicator 200 GAL has several applications already yet its heritage also has 201 several limitations. The GAL must be located at the bottom of a 202 label stack for its chief use cases such as MPLS-TP. So a router 203 needs to search the entire label stack for the BoS bit and check if 204 the corresponding label is GAL. This can impact the performance when 205 the label stack is deep. A more serious concern is that [RFC5586] 206 states that GAL+ACH MUST NOT be used to transport user traffic and an 207 ACH is supposed to be followed by a non-service payload. 209 None of these is insurmountable but it does require an overhaul of 210 the existing RFC in order to extend the usage of GAL. 212 3.2. GAL and a Different Nibble Value 214 To avoid changing the established semantics of ACH, a variation can 215 be used. ACH starts with a nibble value "0001". A different nibble 216 value may be used to redefine the remaining part of the word. The 217 idea has been exploited by [I-D.guichard-sfc-mpls-metadata] to define 218 a Metadata Channel Header (MCH) with the leading nibble value "0000". 219 Similarly, we can use another nibble value (e.g., "0010") to define a 220 new header, namely the MPLS Extension Header Indicator (EHI). 222 The format of the GAL and EHI is depicted in Figure 3. 224 0 1 2 3 225 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 1 226 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 227 | GAL (13) | EXP |1| TTL | 228 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 229 |0 0 1 0|Version| Reserved | Extension Header Class |<-EHI 230 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 231 | | 232 | HEH and EH(s) | 233 | | 234 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 236 Figure 3: Extension Header Indicator Format 238 The Extension Header Class field in EHI is used to differentiate the 239 extension headers. Potentially there are three classes: Hop-by-Hop 240 (HbH), End-to-End (E2E), or both. If finally we decide to not 241 differentiate the extension headers, we have the opportunity to merge 242 the HEH (see [I-D.song-mpls-extension-header] for details) into EHI, 243 so we can reduce the header overhead by four bytes. The header 244 format is depicted in Figure 4. 246 0 1 2 3 247 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 1 248 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 249 | GAL (13) | EXP |1| TTL | 250 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 251 |0 0 1 0| EHCNT | EHTLEN | NH |<-HEH 252 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 253 | | 254 | EH(s) | 255 | | 256 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 258 Figure 4: Merge HEH to EHI 260 4. Extend MPLS Entropy Label 262 Instead of introducing a new SPL as the EH indicator, we can 263 piggyback the indicator in some existing SPL to avoid claiming extra 264 SPL resource and save a label overhead. The best candidate is the 265 entropy label (EL) [RFC6790]. If we can make EL default for every 266 MPLS packet, we can encode the EH indicator in the unused ELI/EL 267 label fields such as CoS and TTL. 269 In Figure 5 we show a possible encoding method, in which the first 270 bit of the CoS field in ELI is used to indicate the presence of EH 271 and teh TTL field in ELI is used to indicate the location of the EH. 273 Note that the CoS field of the EL can also be used to encode other 274 information, if necessary. 276 0 1 2 3 277 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 1 278 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 279 | ELI (7) |I| |S| Offset | 280 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 281 | EL | |S| 0 | 282 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 284 Figure 5: Special EHL with EH Category Encoding 286 5. Configured FEC Labels 288 It is also possible to use FEC labels to indicate the presence of 289 extension headers. An FEC label has the same forwarding semantics as 290 the original label, but it also means that one or more extension 291 headers exist below the label stack. 293 Although this approach avoids the need of new header encoding 294 standards, it introduces a good deal of complexity into the control 295 plane. Since every label needs an FEC label to indicate EH, this 296 scheme also significantly reduces the available label space. Another 297 issue is that this solution may not work for incremental deployment 298 where some legacy routers cannot understand and apply the FEC labels 299 for EH. Moreover, this configuration-based solution certainly makes 300 the cross-domain interoperability more difficult. Hence, this is the 301 least preferred option. We only include it here for the completeness 302 of the discussion. 304 6. Summary 306 Evidenced by the existing and emerging use cases, MPLS networks need 307 a standard way to support extension headers. In Figure 6, we 308 summarize the potential schemes that allow MPLS packets to carry 309 extension headers and list the main pros and cons for each scheme. 311 +---+---------------------------+---------------------------------+ 312 |No.| Description | Pros and Cons | 313 +---+---------------------------+---------------------------------+ 314 | 1 | Special purpose EHL |+ Single label | 315 | | |+ Location freedom | 316 | | |- Need standard extension | 317 | | |- Use scarce resource | 318 +---+---------------------------+---------------------------------+ 319 | 2 | XL(15) + EHL |+ Location freedom | 320 | | |+ Established mechanism | 321 | | |+ Abundant resource | 322 | | |- One extra label than Option 1 | 323 | | |- Need standard extension | 324 +---+---------------------------+---------------------------------+ 325 | 3 | GAL + ACH with channel |+ Reuse existing mechanism | 326 | | type extension |+ Abundant resource | 327 | | |- Label location limitation | 328 | | |- One more word than Option 1 | 329 | | |- Not for user traffic | 330 | | |- Need standard extension/update | 331 +---+---------------------------+---------------------------------+ 332 | 4 | GAL + another nibble value|+ No change to ACH semantics | 333 | | to indicate EHs (e.g., |+ Potential overhead saving | 334 | | "0010") |- Label location limitation | 335 | | |- Hack scarce resource (nibble) | 336 | | |- Need standard extension | 337 +---+---------------------------+---------------------------------+ 338 | 5 | Extend ELI/EL |+ No need for new label | 339 | | |- Need standard update | 340 | | |- Need to make EL mandatory | 341 | | |- One extra label than Option 1 | 342 +---+---------------------------+---------------------------------+ 343 | 6 | FEC label as EH indicator |+ No need for header standard | 344 | | |- Complex control plane | 345 | | |- Cross-domain interoperability | 346 | | |- Label space issue | 347 | | |- Not for incremental deployment | 348 +---+---------------------------+---------------------------------+ 350 Figure 6: Potential Schemes for MPLS Extension Headers 352 Basically we have three groups of solutions. The scheme 1 and 2 353 introduce new labels, the scheme 3, 4, and 5 extend the existing 354 solutions, and the scheme 6 relies on the control plane. Through 355 comprehensive considerations on the pros and cons of each scheme, we 356 expect a working group consensus can be reached to pick the final 357 winner. 359 7. Considerations of EHI 361 The existence of Extension Headers will make the ECMP based on inner 362 IP packet header impossible or harder. If legacy routers need to 363 conduct this kind of ECMP, the process either fails or generates 364 unexpected results. EH-aware routers can do this kind of ECMP but 365 they need to skip all the EHs in order to access the inner packet 366 header which may not be efficient (we make provision in HEH to help 367 accelerate this process). In this case, the Entropy Label (EL) is 368 preferred for ECMP. The Entropy Label Indicator (ELI) and EL should 369 be put in front of the EHI to avoid confusing the legacy routers. 371 8. Security Considerations 373 TBD 375 9. IANA Considerations 377 If the EHL approach is adopted to indicate the presence of MPLS 378 extension header(s), this document requests IANA to assign one or 379 more new Special-Purpose MPLS Label Values from the Special-Purpose 380 Multiprotocol Label Switching (MPLS) Label Values Registry of 381 "Extension Header Label (EHL)". 383 10. Contributors 385 The other contributors of this document are listed as follows. 387 o James Guichard 389 o Stewart Bryant 391 o Bruno Decraene 393 11. Acknowledgments 395 TBD. 397 12. References 399 12.1. Normative References 401 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 402 Requirement Levels", BCP 14, RFC 2119, 403 DOI 10.17487/RFC2119, March 1997, 404 . 406 [RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., 407 "MPLS Generic Associated Channel", RFC 5586, 408 DOI 10.17487/RFC5586, June 2009, 409 . 411 [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and 412 L. Yong, "The Use of Entropy Labels in MPLS Forwarding", 413 RFC 6790, DOI 10.17487/RFC6790, November 2012, 414 . 416 [RFC7274] Kompella, K., Andersson, L., and A. Farrel, "Allocating 417 and Retiring Special-Purpose MPLS Labels", RFC 7274, 418 DOI 10.17487/RFC7274, June 2014, 419 . 421 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 422 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 423 May 2017, . 425 12.2. Informative References 427 [I-D.guichard-sfc-mpls-metadata] 428 Guichard, J., Pignataro, C., Spraggs, S., and S. Bryant, 429 "Carrying Metadata in MPLS Networks", draft-guichard-sfc- 430 mpls-metadata-00 (work in progress), September 2013. 432 [I-D.song-mpls-extension-header] 433 Song, H., Li, Z., Zhou, T., and L. Andersson, "MPLS 434 Extension Header", draft-song-mpls-extension-header-04 435 (work in progress), April 2021. 437 Authors' Addresses 439 Haoyu Song (editor) 440 Futurewei Technologies 441 2330 Central Expressway 442 Santa Clara 443 USA 445 Email: haoyu.song@futurewei.com 446 Zhenbin Li 447 Huawei 448 156 Beiqing Road 449 Beijing, 100095 450 P.R. China 452 Email: lizhenbin@huawei.com 454 Tianran Zhou 455 Huawei 456 156 Beiqing Road 457 Beijing, 100095 458 P.R. China 460 Email: zhoutianran@huawei.com 462 Loa Andersson 463 Bronze Dragon Consulting 465 Stockholm 466 Sweden 468 Email: loa@pi.nu