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Bagdonas 6 Expires: November 26, 2018 Equinix 7 May 25, 2018 9 Use of Ethernet Control Word RECOMMENDED 10 draft-ietf-pals-ethernet-cw-06 12 Abstract 14 The pseudowire (PW) encapsulation of Ethernet, as defined in RFC 15 4448, specifies that the use of the control word (CW) is optional. 16 In the absence of the CW an Ethernet pseudowire packet can be 17 misidentified as an IP packet by a label switching router (LSR). 18 This in turn may lead to the selection of the wrong equal-cost-multi- 19 path (ECMP) path for the packet, leading in turn to the misordering 20 of packets. This problem has become more serious due to the 21 deployment of equipment with Ethernet MAC addresses that start with 22 0x4 or 0x6. The use of the Ethernet PW CW addresses this problem. 23 This document recommends the use of the Ethernet pseudowire control 24 word in all but exceptional circumstances. 26 This document updates RFC 4448. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on November 26, 2018. 45 Copyright Notice 47 Copyright (c) 2018 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 63 2. Specification of Requirements . . . . . . . . . . . . . . . . 3 64 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 65 4. Recommendation . . . . . . . . . . . . . . . . . . . . . . . 5 66 5. Equal Cost Multi-path (ECMP) . . . . . . . . . . . . . . . . 5 67 6. Mitigations . . . . . . . . . . . . . . . . . . . . . . . . . 6 68 7. Operational Considerations . . . . . . . . . . . . . . . . . 6 69 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 70 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 71 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 72 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 73 11.1. Normative References . . . . . . . . . . . . . . . . . . 7 74 11.2. Informative References . . . . . . . . . . . . . . . . . 8 75 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 77 1. Introduction 79 The pseudowire(PW) encapsulation of Ethernet, as defined in 80 [RFC4448], specifies that the use of the control word (CW) is 81 optional. It is common for label switching routers (LSRs) to search 82 past the end of the label stack to determine whether the payload is 83 an IP packet, and if the payload is an IP packet, to select the next 84 hop based on the so called "five-tuple" (IP source address, IP 85 destination address, protocol/next-header, transport layer source 86 port and transport layer destination port). In the absence of a PW 87 CW an Ethernet pseudowire packet can be misidentified as an IP packet 88 by a label switching router (LSR) selecting the equal-cost-multi-path 89 (ECMP) path based on the five-tuple. This in turn may lead to the 90 selection of the wrong ECMP path for the packet, leading in turn to 91 the misordering of packets. Further discussion of this topic is 92 published in [RFC4928]. 94 Flow misordering can also happen in a single path scenario when 95 traffic classification and differential forwarding treatment 96 mechanisms are in use. This occurs when a forwarder incorrectly 97 assumes that the packet is IP and applies forwarding policy based on 98 fields in the PW payload. 100 This problem has recently become more serious for a number of 101 reasons. Firstly, due to the deployment of equipment with Ethernet 102 MAC addresses that start with 0x4 or 0x6 assigned by the IEEE 103 Registration Authority Committee (RAC). Secondly, concerns over 104 privacy have led to the use of MAC address randomization which 105 assigns local MAC addresses randomly for privacy. Random assignment 106 results in addresses starting with one of these two values one time 107 in eight. 109 The use of the Ethernet PW CW addresses this problem. 111 This document recommends the use of the Ethernet pseudowire control 112 word in all but exceptional circumstances. 114 2. Specification of Requirements 116 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 117 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 118 "OPTIONAL" in this document are to be interpreted as described in BCP 119 14 [RFC2119] [RFC8174] when, and only when, they appear in all 120 capitals, as shown here. 122 3. Background 124 Ethernet pseudowire encapsulation is specified in [RFC4448]. In 125 particular the reader is drawn to section 4.6, part of which is 126 quoted below for the convenience of the reader: 128 "The control word defined in this section is based on the Generic 129 PW MPLS Control Word as defined in [RFC4385]. It provides the 130 ability to sequence individual frames on the PW, avoidance of 131 equal-cost multiple-path load-balancing (ECMP) [RFC2992], and 132 Operations and Management (OAM) mechanisms including VCCV 133 [RFC5085]. 135 "[RFC4385] states, "If a PW is sensitive to packet misordering 136 and is being carried over an MPLS PSN that uses the contents 137 of the MPLS payload to select the ECMP path, it MUST employ a 138 mechanism which prevents packet misordering." This is necessary 139 because ECMP implementations may examine the first nibble after 140 the MPLS label stack to determine whether the labeled packet 141 is IP or not. Thus, if the source MAC address of an Ethernet 142 frame carried over the PW without a control word present begins 143 with 0x4 or 0x6, it could be mistaken for an IPv4 or IPv6 144 packet. This could, depending on the configuration and 145 topology of the MPLS network, lead to a situation where all 146 packets for a given PW do not follow the same path. This may 147 increase out-of-order frames on a given PW, or cause OAM packets 148 to follow a different path than actual traffic (see 149 Section 4.4.3, "Frame Ordering"). 151 "The features that the control word provides may not be needed 152 for a given Ethernet PW. For example, ECMP may not be present 153 or active on a given MPLS network, strict frame sequencing may 154 not be required, etc. If this is the case, the control word 155 provides little value and is therefore optional. Early Ethernet 156 PW implementations have been deployed that do not include a 157 control word or the ability to process one if present. To 158 aid in backwards compatibility, future implementations MUST 159 be able to send and receive frames without the control word 160 present." 162 At the time when pseudowires were first deployed, some equipment of 163 commercial significance was unable to process the Ethernet Control 164 Word. In addition, at that time it was considered that no Ethernet 165 MAC address had been issued by the IEEE Registration Authority 166 Committee (RAC) that starts with 0x4 or 0x6, and thus it was thought 167 to be safe to deploy Ethernet PWs without the CW. 169 Since that time the RAC has issued Ethernet MAC addresses start with 170 0x4 or 0x6 and thus the assumption that in practical networks there 171 would be no confusion between an Ethernet PW packet without the CW 172 and an IP packet is no longer correct. 174 Possibly through the use of unauthorized Ethernet MAC addresses, this 175 assumption has been unsafe for a while, leading some equipment 176 vendors to implement more complex, proprietary, methods to 177 discriminate between Ethernet PW packets and IP packets. Such 178 mechanisms rely on the heuristics of examining the transit packets in 179 trying to find out the exact payload type of the packet and cannot be 180 reliable due to the random nature of the payload carried within such 181 packets. 183 A recent posting on the Nanog email list has highlighted this 184 problem: 186 https://mailman.nanog.org/pipermail/nanog/2016-December/089395.html 188 RFC EDITOR Please delete this paragraph. 189 Kramdown does not include references when they are only found in 190 literal text so I include them here: [RFC4385] [RFC2992] [RFC5085] as 191 a fixup. 193 4. Recommendation 195 The ambiguity between an MPLS payload that is an Ethernet PW and one 196 that is an IP packet is resolved when the Ethernet PW control word is 197 used. This document updates [RFC4448] to state that where both the 198 ingress PE and the egress PE support the Ethernet pseudowire control 199 word, then the CW MUST be used. 201 Where the application of ECMP to an Ethernet PW traffic is required, 202 and where both the ingress and the egress PEs support [RFC6790] 203 (Entropy Label Indicator/Entropy Label (ELI/EL)) or both the ingress 204 and the egress PEs support [RFC6391] (FAT PW), then either method may 205 be used. The use of both methods on the same PW is not normally 206 necessary and should be avoided unless circumstances require it. In 207 the case of multi-segment PWs, if ELI/EL is used then it SHOULD be 208 used on every segment of the PW. The method by which usage of ELI/EL 209 on every segment is guaranteed is out of scope of this document. 211 5. Equal Cost Multi-path (ECMP) 213 Where the volume of traffic on an Ethernet PW is such that ECMP is 214 required then one of two methods may be used: 216 o Flow-Aware Transport (FAT) of Pseudowires over an MPLS Packet 217 Switched Network specified in [RFC6391], or 219 o LSP entropy labels specified in [RFC6790] 221 RFC6391 works by increasing the entropy of the bottom of stack label. 222 It requires that both the ingress and egress provider edge (PE)s 223 support this feature. It also requires that sufficient LSRs on the 224 LSP between the ingress and egress PE be able to select an 225 ECMP path on an MPLS packet with the resultant stack depth. 227 RFC6790 works by including an entropy value in the LSP part of the 228 label stack. This requires that the Ingress and Egress PEs support 229 the insertion and removal of the EL and the entropy label indicator, 230 and that sufficient LSRs on the LSP are able to preform ECMP based on 231 the EL. 233 In both cases there are considerations in getting Operations, 234 Administration, and Maintenance (OAM) packets to follow the same path 235 as a data packet. This is described in detail section 7 of 236 [RFC6391], and section 6 of RFC6790. However in both cases the 237 situation is improved compared to the ECMP behavior in the case where 238 the Ethernet PW CW was not used, since there is currently no known 239 method of getting a PW OAM packet to follow the same path as a PW 240 data packet subjected to ECMP based on the five tuple of the IP 241 payload. 243 The PW label is pushed before the LSP label. As the EL/ELI labels 244 are part of the LSP layer rather than part of the PW layer, they are 245 pushed after the PW label has been pushed. 247 6. Mitigations 249 Where it is not possible to use the Ethernet PW CW, the effects of 250 ECMP can be disabled by carrying the PW over a traffic engineered 251 path that does not subject the payload to load balancing (for example 252 [RFC3209]). However such paths may be subjected to link bundle load 253 balancing and of course the single LSP has to carry the full PW load. 255 7. Operational Considerations 257 CW presence on the PW is controlled by the configuration and may be 258 subject to default operational mode of not being enabled. Care needs 259 to be taken to ensure that software that implements this 260 recommendation does not depend on existing configuration setting that 261 prevents the use of control word. It is recommended that platform 262 software emits a rate limited message indicating that CW can be used 263 but is disabled due to existing configuration. 265 Instead of including a payload type in the packet, MPLS relies on the 266 control plane to signal the payload type that follows the bottom of 267 the label stack. Some LSRs attempt to deduce the packet type by MPLS 268 payload inspection, in some cases looking past the PW CW. If the 269 payload appears to be IP or IP carried in an Ethernet header they 270 perform an ECMP calculation based on what they assume to be the five 271 tuple fields. However deduction of the payload type in this way is 272 not an exact science, and where a packet that is not IP is mistaken 273 for an IP packet the result can be packets delivered out of order. 274 Misordering of this type can be difficult for an operator to 275 diagnose. 276 Operators should be aware when enabling capability that allows 277 information gleaned from packet inspection past the PW CW to be used 278 in any ECMP calculation, that this may cause Ethernet frames to be 279 delivered out of order despite the presence of the CW. 281 8. Security Considerations 283 This document expresses a preference for one existing and widely 284 deployed Ethernet PW encapsulation over another. These methods have 285 identical security considerations, which are discussed in [RFC4448]. 286 This document introduces no additional security issues. 288 9. IANA Considerations 290 This document makes no IANA requests. 292 10. Acknowledgments 294 The authors thank Job Snijders for drawing attention to this problem. 295 The authors also thank Pat Thaler for clarifying the matter of local 296 MAC address assignment. We thank Sasha Vainshtein for his valuable 297 review comments. 299 11. References 301 11.1. Normative References 303 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 304 Requirement Levels", BCP 14, RFC 2119, 305 DOI 10.17487/RFC2119, March 1997, . 308 [RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, 309 "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for 310 Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385, 311 February 2006, . 313 [RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron, 314 "Encapsulation Methods for Transport of Ethernet over MPLS 315 Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006, 316 . 318 [RFC4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal 319 Cost Multipath Treatment in MPLS Networks", BCP 128, 320 RFC 4928, DOI 10.17487/RFC4928, June 2007, 321 . 323 [RFC6391] Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V., 324 Regan, J., and S. Amante, "Flow-Aware Transport of 325 Pseudowires over an MPLS Packet Switched Network", 326 RFC 6391, DOI 10.17487/RFC6391, November 2011, 327 . 329 [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and 330 L. Yong, "The Use of Entropy Labels in MPLS Forwarding", 331 RFC 6790, DOI 10.17487/RFC6790, November 2012, 332 . 334 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 335 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 336 May 2017, . 338 11.2. Informative References 340 [RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path 341 Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000, 342 . 344 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 345 and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 346 Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001, 347 . 349 [RFC5085] Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual 350 Circuit Connectivity Verification (VCCV): A Control 351 Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085, 352 December 2007, . 354 Authors' Addresses 356 Stewart Bryant 357 Huawei 359 Email: stewart.bryant@gmail.com 361 Andrew G Malis 362 Huawei 364 Email: agmalis@gmail.com 365 Ignas Bagdonas 366 Equinix 368 Email: ibagdona.ietf@gmail.com>