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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for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-13) exists of draft-song-mpls-extension-header-04 Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--).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: November 6, 2021 T. Zhou 6 Huawei 7 L. Andersson 8 Bronze Dragon Consulting 9 May 5, 2021 11 Options for MPLS Extension Header Indicator 12 draft-song-mpls-eh-indicator-02 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 November 6, 2021. 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. Configured FEC Labels . . . . . . . . . . . . . . . . . . . . 6 73 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 74 6. Considerations of EHI . . . . . . . . . . . . . . . . . . . . 7 75 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 76 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 77 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8 78 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 79 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 80 11.1. Normative References . . . . . . . . . . . . . . . . . . 8 81 11.2. Informative References . . . . . . . . . . . . . . . . . 9 82 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 84 1. Introduction 86 The document [I-D.song-mpls-extension-header] presents the 87 motivation, specification, and use cases of MPLS Extension Header 88 (EH). An indicator is needed in the MPLS label stack to indicate the 89 presence of the extension header(s). Multiple options are possible 90 for this purpose. As the discussion progresses, more options could 91 emerge. 93 In this document, we propound three categories of methods which can 94 be further partitioned into five unique schemes. Four of them use 95 explicit data plane encoding to indicate the EH and the last one 96 implies the EH through control plane configuration. This document 97 details and compares these schemes, in order to foster further 98 discussions until a final decision is made. 100 2. Dedicated Extension Header Label 102 A straightforward method is to directly encode an Extension Header 103 Label (EHL) in the MPLS label stack. Two derived schemes are as 104 follows. 106 2.1. Special Purpose Label 108 A new special purpose label, EHL, can be used to indicate EHs. As 109 specified in [RFC7274], so far eight special purpose label values are 110 still left unsigned by IANA (which are 4 to 6 and 8 to 12). This 111 single label scheme is elegant but arguably demands a scarce 112 resource. We cannot rule out the possibility of requiring more than 113 one label value to differentiate EH classes (e.g., Hop-by-Hop, End- 114 to-End, or both). If this happens, it can only aggravate the 115 situation. 117 Another benefit of this scheme is that an EHL can potentially be 118 located anywhere in an MPLS label stack. It is easier and quicker 119 for a router to figure out the existence of extension header(s) if 120 the EHL is close to or at the top of the label stack. However, if 121 there are legacy devices which can reach the EHL but do not recognize 122 it in a network, then for backward compatibility, the EHL must be 123 located at the bottom of the stack (i.e., only the MPLS tunnel ends 124 and EHL-aware nodes will look up and process it). 126 The format of an EHL is the same as an MPLS label. The first 20-bit 127 label value will be assigned by IANA. The BoS bit is used to 128 indicate the location of the label. The other fields, CoS and TTL, 129 currently have no use in the context of EHL. However, these two 130 fields can potentially be used to encode other information. If such 131 code points are open for other purpose, it will make the single EHL 132 idea more compelling. E.g., the EH category and/or other 133 information, if needed, can be encoded in these fields, so that only 134 one special label value is needed. 136 The following figure shows a potential scheme in which one bit from 137 the CoS field ('H') is used to indicate the presence of HbH EHs in 138 the packet. If 'H' bit is 0, it means no HbH EH follows so a 139 P-router will not need to check the EH. 141 0 1 2 3 142 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 1 143 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 144 | EHL (TBD) |H| |S| TTL | 145 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 147 Figure 1: Special EHL with EH Category Encoding 149 2.2. Extension Label plus an Extended Special Purpose Label 151 [RFC7274] specifies the Extension Label (XL) with the value of 15. 152 An extended special purpose label (ESPL) following XL can be used as 153 EHL. A large number of ESPL values are available for allocation. 154 The XL+EHL scheme eases the concern on the reserved label space at 155 the cost of one more label in the label stack. 157 Except for the fact that one more label is needed, The XL+EHL scheme 158 shares the same property as the single special purpose EHL scheme. 160 3. Generic Associated Channel Extension 162 The similar "header extension" requirement for MPLS has led to some 163 proposals before. A special Generic Associated Channel Label (GAL) 164 [RFC5586] with the value of 13 has been assigned to support the 165 identification of an Associated Channel Header (ACH). We can extend 166 this existing mechanism to encode the MPLS EH indicator. 168 3.1. GAL and Associated Channel Header 170 The ACH is located below the bottom label. It has a 16-bit Channel 171 Type field which provides abundant space to encode the MPLS EH 172 indicator. This scheme has the same header overhead as the XL+EHL 173 scheme. The format is depicted in Figure 2. 175 0 1 2 3 176 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 1 177 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 178 | GAL (13) | EXP |1| TTL | 179 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 180 |0 0 0 1|Version| Reserved | Extension Header Indicator | 181 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 182 | | 183 | HEH and EH(s) | 184 | | 185 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 187 Figure 2: Associated Channel Header as Extension Header Indicator 189 GAL has several applications already yet its heritage also has 190 several limitations. The GAL must be located at the bottom of a 191 label stack for its chief use cases such as MPLS-TP. So a router 192 needs to search the entire label stack for the BoS bit and check if 193 the corresponding label is GAL. This can impact the performance when 194 the label stack is deep. A more serious concern is that [RFC5586] 195 states that GAL+ACH MUST NOT be used to transport user traffic and an 196 ACH is supposed to be followed by a non-service payload. 198 None of these is insurmountable but it does require an overhaul of 199 the existing RFC in order to extend the usage of GAL. 201 3.2. GAL and a Different Nibble Value 203 To avoid changing the established semantics of ACH, a variation can 204 be used. ACH starts with a nibble value "0001". A different nibble 205 value may be used to redefine the remaining part of the word. The 206 idea has been exploited by [I-D.guichard-sfc-mpls-metadata] to define 207 a Metadata Channel Header (MCH) with the leading nibble value "0000". 208 Similarly, we can use another nibble value (e.g., "0010") to define a 209 new header, namely the MPLS Extension Header Indicator (EHI). 211 The format of the GAL and EHI is depicted in Figure 3. 213 0 1 2 3 214 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 1 215 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 216 | GAL (13) | EXP |1| TTL | 217 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 218 |0 0 1 0|Version| Reserved | Extension Header Class |<-EHI 219 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 220 | | 221 | HEH and EH(s) | 222 | | 223 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 225 Figure 3: Extension Header Indicator Format 227 The Extension Header Class field in EHI is used to differentiate the 228 extension headers. Potentially there are three classes: Hop-by-Hop 229 (HbH), End-to-End (E2E), or both. If finally we decide to not 230 differentiate the extension headers, we have the opportunity to merge 231 the HEH (see [I-D.song-mpls-extension-header] for details) into EHI, 232 so we can reduce the header overhead by four bytes. The header 233 format is depicted in Figure 4. 235 0 1 2 3 236 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 1 237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 238 | GAL (13) | EXP |1| TTL | 239 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 240 |0 0 1 0| EHCNT | EHTLEN | NH |<-HEH 241 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 242 | | 243 | EH(s) | 244 | | 245 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 247 Figure 4: Merge HEH to EHI 249 4. Configured FEC Labels 251 It is also possible to use FEC labels to indicate the presence of 252 extension headers. An FEC label has the same forwarding semantics as 253 the original label, but it also means that one or more extension 254 headers exist below the label stack. 256 Although this approach avoids the need of new header encoding 257 standards, it introduces a good deal of complexity into the control 258 plane. Since every label needs an FEC label to indicate EH, this 259 scheme also significantly reduces the available label space. Another 260 issue is that this solution may not work for incremental deployment 261 where some legacy routers cannot understand and apply the FEC labels 262 for EH. Moreover, this configuration-based solution certainly makes 263 the cross-domain interoperability more difficult. Hence, this is the 264 least preferred option. We only include it here for the completeness 265 of the discussion. 267 5. Summary 269 Evidenced by the existing and emerging use cases, MPLS networks need 270 a standard way to support extension headers. In Figure 5, we 271 summarize the potential schemes that allow MPLS packets to carry 272 extension headers and list the main pros and cons for each scheme. 274 +---+---------------------------+---------------------------------+ 275 |No.| Description | Pros and Cons | 276 +---+---------------------------+---------------------------------+ 277 | 1 | Special purpose EHL |+ Single label | 278 | | |+ Location freedom | 279 | | |- Need standard extension | 280 | | |- Use scarce resource | 281 +---+---------------------------+---------------------------------+ 282 | 2 | XL(15) + EHL |+ Location freedom | 283 | | |+ Established mechanism | 284 | | |+ Abundant resource | 285 | | |- One extra label than Optiona 1 | 286 | | |- Need standard extension | 287 +---+---------------------------+---------------------------------+ 288 | 3 | GAL + ACH with channel |+ Reuse existing mechanism | 289 | | type extension |+ Abundant resource | 290 | | |- Label location limitation | 291 | | |- One more word than Option 1 | 292 | | |- Not for user traffic | 293 | | |- Need standard extension/update | 294 +---+---------------------------+---------------------------------+ 295 | 4 | GAL + another nibble value|+ No change to ACH semantics | 296 | | to indicate EHs (e.g., |+ Potential overhead saving | 297 | | "0010") |- Label location limitation | 298 | | |- Hack scarce resource (nibble) | 299 | | |- Need standard extension | 300 +---+---------------------------+---------------------------------+ 301 | 5 | FEC label as EH indicator |+ No need for header standard | 302 | | |- Complex control plane | 303 | | |- Cross-domain interoperability | 304 | | |- Label space issue | 305 | | |- Not for incremental deployment | 306 +---+---------------------------+---------------------------------+ 308 Figure 5: Potential Schemes for MPLS Extension Headers 310 Through comprehensive considerations on the pros and cons of each 311 scheme, we expect a working group consensus can be reached to pick 312 the final winner. 314 6. Considerations of EHI 316 The existence of Extension Headers will make the ECMP based on inner 317 IP packet header impossible or harder. If legacy routers need to 318 conduct this kind of ECMP, the process either fails or generates 319 unexpected results. EH-aware routers can do this kind of ECMP but 320 they need to skip all the EHs in order to access the inner packet 321 header which may not be efficient (we make provision in HEH to help 322 accelerate this process). In this case, the Entropy Label (EL) is 323 preferred for ECMP. The Entropy Label Indicator (ELI) and EL should 324 be put in front of the EHI to avoid confusing the legacy routers. 326 7. Security Considerations 328 TBD 330 8. IANA Considerations 332 If the EHL approach is adopted to indicate the presence of MPLS 333 extension header(s), this document requests IANA to assign one or 334 more new Special-Purpose MPLS Label Values from the Special-Purpose 335 Multiprotocol Label Switching (MPLS) Label Values Registry of 336 "Extension Header Label (EHL)". 338 9. Contributors 340 The other contributors of this document are listed as follows. 342 o James Guichard 344 o Stewart Bryant 346 10. Acknowledgments 348 TBD. 350 11. References 352 11.1. Normative References 354 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 355 Requirement Levels", BCP 14, RFC 2119, 356 DOI 10.17487/RFC2119, March 1997, 357 . 359 [RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., 360 "MPLS Generic Associated Channel", RFC 5586, 361 DOI 10.17487/RFC5586, June 2009, 362 . 364 [RFC7274] Kompella, K., Andersson, L., and A. Farrel, "Allocating 365 and Retiring Special-Purpose MPLS Labels", RFC 7274, 366 DOI 10.17487/RFC7274, June 2014, 367 . 369 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 370 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 371 May 2017, . 373 11.2. Informative References 375 [I-D.guichard-sfc-mpls-metadata] 376 Guichard, J., Pignataro, C., Spraggs, S., and S. Bryant, 377 "Carrying Metadata in MPLS Networks", draft-guichard-sfc- 378 mpls-metadata-00 (work in progress), September 2013. 380 [I-D.song-mpls-extension-header] 381 Song, H., Li, Z., Zhou, T., and L. Andersson, "MPLS 382 Extension Header", draft-song-mpls-extension-header-04 383 (work in progress), April 2021. 385 Authors' Addresses 387 Haoyu Song (editor) 388 Futurewei Technologies 389 2330 Central Expressway 390 Santa Clara 391 USA 393 Email: haoyu.song@futurewei.com 395 Zhenbin Li 396 Huawei 397 156 Beiqing Road 398 Beijing, 100095 399 P.R. China 401 Email: lizhenbin@huawei.com 403 Tianran Zhou 404 Huawei 405 156 Beiqing Road 406 Beijing, 100095 407 P.R. China 409 Email: zhoutianran@huawei.com 410 Loa Andersson 411 Bronze Dragon Consulting 413 Stockholm 414 Sweden 416 Email: loa@pi.nu