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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-13) exists of draft-song-mpls-extension-header-02 Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 1 comment (--).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 Z. Li 4 Intended status: Informational T. Zhou 5 Expires: August 19, 2019 Huawei 6 L. Andersson 7 Bronze Dragon Consulting 8 February 15, 2019 10 Options for MPLS Extension Header Indicator 11 draft-song-mpls-eh-indicator-00 13 Abstract 15 This document describes the schemes that indicates the presence of 16 the MPLS extension header(s) following the MPLS label stack. After a 17 thorough evaluation of these options by comparing their pros and 18 cons, one should be chosen as the final scheme for MPLS extension 19 header indicator. 21 Requirements Language 23 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 24 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 25 "OPTIONAL" in this document are to be interpreted as described in BCP 26 14 [RFC2119][RFC8174] when, and only when, they appear in all 27 capitals, as shown here. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at https://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on August 19, 2019. 46 Copyright Notice 48 Copyright (c) 2019 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (https://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 64 2. Dedicated Extension Header Label . . . . . . . . . . . . . . 3 65 2.1. Special Purpose Label . . . . . . . . . . . . . . . . . . 3 66 2.2. Extension Label plus an Extended Special Purpose Label . 3 67 3. Generic Associated Channel Extension . . . . . . . . . . . . 4 68 3.1. GAL and Associated Channel Header . . . . . . . . . . . . 4 69 3.2. GAL and a Different Nibble Value . . . . . . . . . . . . 4 70 4. Configured FEC Labels . . . . . . . . . . . . . . . . . . . . 5 71 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 72 6. Considerations of EHI . . . . . . . . . . . . . . . . . . . . 7 73 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 74 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 75 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8 76 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 77 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 78 11.1. Normative References . . . . . . . . . . . . . . . . . . 8 79 11.2. Informative References . . . . . . . . . . . . . . . . . 9 80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 82 1. Introduction 84 The document [I-D.song-mpls-extension-header] presents the 85 motivation, specification, and use cases of MPLS Extension Header 86 (EH). However, multiple options are possible to indicate the 87 presence of the extension header(s). 89 We propound three categories of methods which can be further 90 partitioned into five unique schemes. Four of them use explicit data 91 plane encoding to indicate the EH and the last one implies the EH 92 through control plane configuration. This document details and 93 compares these schemes, in order to foster further discussions until 94 a final decision is made. 96 2. Dedicated Extension Header Label 98 A straightforward method is to directly encode an Extension Header 99 Label (EHL) in the MPLS label stack. Two derived schemes are as 100 follows. 102 2.1. Special Purpose Label 104 A new special purpose label, EHL, can be used to indicate EHs. As 105 specified in [RFC7274], so far eight special purpose label values are 106 still left unsigned by IANA (which are 4 to 6 and 8 to 12). This 107 single label scheme is elegant but arguably demands a scarce 108 resource. We cannot rule out the possibility of requiring more than 109 one label value to differentiate EH classes (e.g., Hop-by-Hop, End- 110 to-End, or both). If this happens, it can only aggravate the 111 situation. 113 Another benefit of this scheme is that an EHL can potentially be 114 located anywhere in an MPLS label stack. It is easier and quicker 115 for a router to figure out the existence of extension header(s) if 116 the EHL is close to or at the top of the label stack. However, if 117 there are legacy devices which can reach the EHL but do not recognize 118 it in a network, then for backward compatibility, the EHL must be 119 located at the bottom of the stack (i.e., only the MPLS tunnel ends 120 and EHL-aware nodes will look up and process it). 122 The format of an EHL is the same as an MPLS label. The first 20-bit 123 label value will be assigned by IANA. The BoS bit is used to 124 indicate the location of the label. The other fields, CoS and TTL, 125 currently have no use in the context of EHL. However, these two 126 fields can potentially be used to encode other information, which is 127 beyond the scope of this document. 129 2.2. Extension Label plus an Extended Special Purpose Label 131 [RFC7274] specifies the Extension Label (XL) with the value of 15. 132 An extended special purpose label (ESPL) following XL can be used as 133 EHL. A large number of ESPL values are available for allocation. 134 The XL+EHL scheme eases the concern on the reserved label space at 135 the cost of one more label in the label stack. 137 Except for the fact that one more label is needed, The XL+EHL scheme 138 shares the same property as the single special purpose EHL scheme. 140 3. Generic Associated Channel Extension 142 The similar "header extension" requirement for MPLS has led to some 143 proposals before. A special Generic Associated Channel Label (GAL) 144 [RFC5586] with the value of 13 has been assigned to support the 145 identification of an Associated Channel Header (ACH). We can extend 146 this existing mechanism to encode the MPLS EH indicator. 148 3.1. GAL and Associated Channel Header 150 The ACH is located below the bottom label. It has a 16-bit Channel 151 Type field which provides abundant space to encode the MPLS EH 152 indicator. This scheme has the same header overhead as the XL+EHL 153 scheme. The format is depicted in Figure 1. 155 0 1 2 3 156 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 1 157 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 158 | GAL (13) | EXP |1| TTL | 159 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 160 |0 0 0 1|Version| Reserved | Extension Header Indicator | 161 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 162 | | 163 | HEH and EH(s) | 164 | | 165 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 167 Figure 1: Associated Channel Header as Extension Header Indicator 169 GAL has several applications already yet its heritage also has 170 several limitations. The GAL must be located at the bottom of a 171 label stack for its chief use cases such as MPLS-TP. So a router 172 needs to search the entire label stack for the BoS bit and check if 173 the corresponding label is GAL. This can impact the performance when 174 the label stack is deep. A more serious concern is that [RFC5586] 175 states that GAL+ACH MUST NOT be used to transport user traffic and an 176 ACH is supposed to be followed by a non-service payload. 178 None of these is insurmountable but it does require an overhaul of 179 the existing RFC in order to extend the usage of GAL. 181 3.2. GAL and a Different Nibble Value 183 To avoid changing the established semantics of ACH, a variation can 184 be used. ACH starts with a nibble value "0001". A different nibble 185 value may be used to redefine the remaining part of the word. The 186 idea has been exploited by [I-D.guichard-sfc-mpls-metadata] to define 187 a Metadata Channel Header (MCH) with the leading nibble value "0000". 189 Similarly, we can use another nibble value (e.g., "0010") to define a 190 new header, namely the MPLS Extension Header Indicator (EHI). 192 The format of the GAL and EHI is depicted in Figure 2. 194 0 1 2 3 195 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 1 196 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 197 | GAL (13) | EXP |1| TTL | 198 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 199 |0 0 1 0|Version| Reserved | Extension Header Class |<-EHI 200 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 201 | | 202 | HEH and EH(s) | 203 | | 204 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 206 Figure 2: Extension Header Indicator Format 208 The Extension Header Class field in EHI is used to differentiate the 209 extension headers. Potentially there are three classes: Hop-by-Hop 210 (HbH), End-to-End (E2E), or both. If finally we decide to not 211 differentiate the extension headers, we have the opportunity to merge 212 the HEH (see [I-D.song-mpls-extension-header] for details) into EHI, 213 so we can reduce the header overhead by four bytes. The header 214 format is depicted in Figure 3. 216 0 1 2 3 217 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 1 218 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 219 | GAL (13) | EXP |1| TTL | 220 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 221 |0 0 1 0| EHCNT | EHTLEN | NH |<-HEH 222 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 223 | | 224 | EH(s) | 225 | | 226 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 228 Figure 3: Merge HEH to EHI 230 4. Configured FEC Labels 232 It is also possible to use FEC labels to indicate the presence of 233 extension headers. An FEC label has the same forwarding semantics as 234 the original label, but it also means that one or more extension 235 headers exist below the label stack. 237 Although this approach avoids the need of new header encoding 238 standards, it introduces a good deal of complexity into the control 239 plane. Since every label needs an FEC label to indicate EH, this 240 scheme also significantly reduces the available label space. Another 241 issue is that this solution may not work for incremental deployment 242 where some legacy routers cannot understand and apply the FEC labels 243 for EH. Moreover, this configuration-based solution certainly makes 244 the cross-domain interoperability more difficult. Hence, this is the 245 least preferred option. We only include it here for the completeness 246 of the discussion. 248 5. Summary 250 Evidenced by the existing and emerging use cases, MPLS networks need 251 a standard way to support extension headers. In Figure 4, we 252 summarize the potential schemes that allow MPLS packets to carry 253 extension headers and list the main pros and cons for each scheme. 255 +---+---------------------------+---------------------------------+ 256 |No.| Description | Pros and Cons | 257 +---+---------------------------+---------------------------------+ 258 | 1 | Special purpose EHL |+ Single label | 259 | | |+ Location freedom | 260 | | |- Need standard extension | 261 | | |- Use scarce resource | 262 +---+---------------------------+---------------------------------+ 263 | 2 | XL(15) + EHL |+ Location freedom | 264 | | |+ Established mechanism | 265 | | |+ Abundant resource | 266 | | |- One extra label than Optiona 1 | 267 | | |- Need standard extension | 268 +---+---------------------------+---------------------------------+ 269 | 3 | GAL + ACH with channel |+ Reuse existing mechanism | 270 | | type extension |+ Abundant resource | 271 | | |- Label location limitation | 272 | | |- One more word than Option 1 | 273 | | |- Not for user traffic | 274 | | |- Need standard extension/update | 275 +---+---------------------------+---------------------------------+ 276 | 4 | GAL + another nibble value|+ No change to ACH semantics | 277 | | to indicate EHs (e.g., |+ Potential overhead saving | 278 | | "0010") |- Label location limitation | 279 | | |- Hack scarce resource (nibble) | 280 | | |- Need standard extension | 281 +---+---------------------------+---------------------------------+ 282 | 5 | FEC label as EH indicator |+ No need for header standard | 283 | | |- Complex control plane | 284 | | |- Cross-domain interoperability | 285 | | |- Label space issue | 286 | | |- Not for incremental deployment | 287 +---+---------------------------+---------------------------------+ 289 Figure 4: Potential Schemes for MPLS Extension Headers 291 Through comprehensive considerations on the pros and cons of each 292 scheme, we expect a working group consensus can be reached to pick 293 the final winner. 295 6. Considerations of EHI 297 The existence of Extension Headers will make the ECMP based on inner 298 IP packet header impossible or harder. If legacy routers need to 299 conduct this kind of ECMP, the process either fails or generates 300 unexpected results. EH-aware routers can do this kind of ECMP but 301 they need to skip all the EHs in order to access the inner packet 302 header which may not be efficient. In this case, the Entropy Label 303 (EL) is preferred for ECMP. The Entropy Label Indicator (ELI) and EL 304 should be put in front of the EHI to avoid confusing the legacy 305 routers. 307 7. Security Considerations 309 TBD 311 8. IANA Considerations 313 If the EHL approach is adopted to indicate the presence of MPLS 314 extension header(s), this document requests IANA to assign one or 315 more new Special-Purpose MPLS Label Values from the Special-Purpose 316 Multiprotocol Label Switching (MPLS) Label Values Registry of 317 "Extension Header Label (EHL)". 319 9. Contributors 321 The other contributors of this document are listed as follows. 323 o James Guichard 325 o Stewart Bryant 327 10. Acknowledgments 329 TBD. 331 11. References 333 11.1. Normative References 335 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 336 Requirement Levels", BCP 14, RFC 2119, 337 DOI 10.17487/RFC2119, March 1997, 338 . 340 [RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., 341 "MPLS Generic Associated Channel", RFC 5586, 342 DOI 10.17487/RFC5586, June 2009, 343 . 345 [RFC7274] Kompella, K., Andersson, L., and A. Farrel, "Allocating 346 and Retiring Special-Purpose MPLS Labels", RFC 7274, 347 DOI 10.17487/RFC7274, June 2014, 348 . 350 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 351 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 352 May 2017, . 354 11.2. Informative References 356 [I-D.guichard-sfc-mpls-metadata] 357 Guichard, J., Pignataro, C., Spraggs, S., and S. Bryant, 358 "Carrying Metadata in MPLS Networks", draft-guichard-sfc- 359 mpls-metadata-00 (work in progress), September 2013. 361 [I-D.song-mpls-extension-header] 362 Song, H., Li, Z., Zhou, T., and L. Andersson, "MPLS 363 Extension Header", draft-song-mpls-extension-header-02 364 (work in progress), February 2019. 366 Authors' Addresses 368 Haoyu Song (editor) 369 Huawei 370 2330 Central Expressway 371 Santa Clara 372 USA 374 Email: haoyu.song@huawei.com 376 Zhenbin Li 377 Huawei 378 156 Beiqing Road 379 Beijing, 100095 380 P.R. China 382 Email: lizhenbin@huawei.com 384 Tianran Zhou 385 Huawei 386 156 Beiqing Road 387 Beijing, 100095 388 P.R. China 390 Email: zhoutianran@huawei.com 391 Loa Andersson 392 Bronze Dragon Consulting 393 Stockholm 394 Sweden 396 Email: loa@pi.nu