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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 PALS Working Group S. Bryant 3 Internet-Draft A. Malis 4 Updates: 4448 (if approved) Huawei 5 Intended status: Standards Track I. Bagdonas 6 Expires: August 31, 2018 Equinix 7 February 27, 2018 9 Use of Ethernet Control Word RECOMMENDED 10 draft-ietf-pals-ethernet-cw-01 12 Abstract 14 The pseudowire (PW) encapsulation of Ethernet, as defined in RFC4448, 15 specifies that the use of the control word (CW) is optional. In the 16 absence of the CW an Ethernet pseudowire packet can be misidentified 17 as an IP packet by a label switching router (LSR). This in turn may 18 lead to the selection of the wrong equal-cost-multi-path (ECMP) path 19 for the packet, leading in turn to the misordering of packets. This 20 problem has become more serious due to the deployment of equipment 21 with Ethernet MAC addresses that start with 0x4 or 0x6. The use of 22 the Ethernet PW CW addresses this problem. This document recommends 23 the use of the Ethernet pseudowire control word in all but 24 exceptional circumstances. 26 This document updates RFC4448. 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 August 31, 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 RFC4448, 80 specifies that the use of the control word (CW) is optional. It is 81 common for label switching routers (LSRs) to search past the end of 82 the label stack to determine whether the payload is an IP packet, and 83 if the payload is an IP packet, to select the next hop based of the 84 so called "five-tuple" (IP source address, IP destination address, 85 protocol/next-header, transport layer source port and transport layer 86 destination port). In the absence of a PW CW an Ethernet pseudowire 87 packet can be misidentified as an IP packet by a label switching 88 router (LSR) selecting the ECMP path based on the five-tuple. This 89 in turn may lead to the selection of the wrong equal-cost-multi-path 90 (ECMP) path for the packet, leading in turn to the misordering of 91 packets. Further discussion of this topic is published in [RFC4928]. 93 Flow misordering can also happen in a single path scenario when 94 traffic classification and differential forwarding treatment 95 mechanisms are in use. This occurs when a forwarder incorrectly 96 assumes that the packet is IP and applies forwarding policy based on 97 fields in the PW payload. 99 This problem has recently become more serious for a number of 100 reasons. Firstly, due to the deployment of equipment with Ethernet 101 MAC addresses that start with 0x4 or 0x6 assigned by the IEEE 102 Registration Authority Committee (RAC). Secondly, concerns over 103 privacy have led to the use of MAC address randomization which 104 assigns local MAC addresses randomly for privacy. Random assignment 105 produce addresses starting with one of the two values about 1/8 of 106 the time. 108 The use of the Ethernet PW CW addresses this problem. 110 This document recommends the use of the Ethernet pseudowire control 111 word in all but exceptional circumstances. 113 2. Specification of Requirements 115 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 116 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 117 document are to be interpreted as described in RFC 2119 [RFC2119]. 119 3. Background 121 Ethernet pseudowire encapsulation is specified in [RFC4448]. In 122 particular the reader is drawn to section 4.6, part of which is 123 quoted below for the convenience of the reader: 125 "The control word defined in this section is based on the Generic 126 PW MPLS Control Word as defined in [RFC4385]. It provides the 127 ability to sequence individual frames on the PW, avoidance of 128 equal-cost multiple-path load-balancing (ECMP) [RFC2992], and 129 Operations and Management (OAM) mechanisms including VCCV 130 [RFC5085]. 132 "[RFC4385] states, "If a PW is sensitive to packet misordering 133 and is being carried over an MPLS PSN that uses the contents 134 of the MPLS payload to select the ECMP path, it MUST employ a 135 mechanism which prevents packet misordering." This is necessary 136 because ECMP implementations may examine the first nibble after 137 the MPLS label stack to determine whether the labeled packet 138 is IP or not. Thus, if the source MAC address of an Ethernet 139 frame carried over the PW without a control word present begins 140 with 0x4 or 0x6, it could be mistaken for an IPv4 or IPv6 141 packet. This could, depending on the configuration and 142 topology of the MPLS network, lead to a situation where all 143 packets for a given PW do not follow the same path. This may 144 increase out-of-order frames on a given PW, or cause OAM packets 145 to follow a different path than actual traffic (see 146 Section 4.4.3, "Frame Ordering"). 148 "The features that the control word provides may not be needed 149 for a given Ethernet PW. For example, ECMP may not be present 150 or active on a given MPLS network, strict frame sequencing may 151 not be required, etc. If this is the case, the control word 152 provides little value and is therefore optional. Early Ethernet 153 PW implementations have been deployed that do not include a 154 control word or the ability to process one if present. To 155 aid in backwards compatibility, future implementations MUST 156 be able to send and receive frames without the control word 157 present." 159 At the time when pseudowires were first deployed, some equipment of 160 commercial significance was unable to process the Ethernet Control 161 Word. In addition, at that time it was considered that no Ethernet 162 MAC address had been issued by the IEEE Registration Authority 163 Committee (RAC) that starts with 0x4 or 0x6, and thus it was thought 164 to be safe to deploy Ethernet PWs without the CW. 166 Since that time the RAC has issued Ethernet MAC addresses start with 167 0x4 or 0x6 and thus the assumption that in practical networks there 168 would be no confusion between an Ethernet PW packet without the CW 169 and an IP packet is no longer correct. 171 Possibly through the use of unauthorized Ethernet MAC addresses, this 172 assumption has been unsafe for a while, leading some equipment 173 vendors to implement more complex, proprietary, methods to 174 discriminate between Ethernet PW packets and IP packets. Such 175 mechanisms rely on the heuristics of examining the transit packets in 176 trying to find out the exact payload type of the packet and cannot be 177 reliable due to the random nature of the payload carried within such 178 packets. 180 A recent posting on the Nanog email list has highlighted this 181 problem: 183 https://mailman.nanog.org/pipermail/nanog/2016-December/089395.html 185 RFC EDITOR Please delete this paragraph. 186 Kramdown does not include references when they are only found in 187 literal text so I include them here: [RFC4385] [RFC2992] [RFC5085] as 188 a fixup. 190 4. Recommendation 192 The ambiguity between an MPLS payload that is a Ethernet PW and one 193 that is an IP packet is resolved when the Ethernet PW control word is 194 used. This document updates RFC4448 [RFC4448] to state that where 195 both both the ingress PE and the egress PE support the Ethernet 196 pseudowire control word, then the CW MUST be used. 198 Where the application ECMP to an Ethernet PW traffic is required, 199 then where the both ingress and egress PEs support RFC6790 [RFC6790] 200 (ELI) or both ingress and egress PEs support RFC6391 [RFC6391] (FAT 201 PW), then either method may be used. The use of both methods on the 202 same PW is not normally necessary and should be avoided unless 203 circumstances require it. In the case of multi-segment PWs, if ELI 204 is used then it should be used on every segment of the PW. 206 Where the ingress PE supports RFC6790 [RFC6790] (ELI) and both 207 ingress and egress PEs support RFC6391 [RFC6391] (FAT PW) and ECMP of 208 Ethernet PW traffic is required, then only one of these methods 209 should be used. In the case of multi-segment PWs, if ELI is used 210 then it should be used on every segment of the PW. Since the reason 211 for this recommendation is to avoid DPI based ECMP in the network, 212 the Entropy Label value SHOULD be calculated at ingress PE where 213 there is more payload context, and copied from ingress to egress LSP 214 at an S-PE. 216 5. Equal Cost Multi-path (ECMP) 218 Where the volume of traffic on an Ethernet PW is such that ECMP is 219 required then one of two methods may be used: 221 o Flow-Aware Transport (FAT) of Pseudowires over an MPLS Packet 222 Switched Network specified in [RFC6391], or 224 o LSP entropy labels specified [RFC6790] 226 RFC6391 works by increasing the entropy of the bottom of stack label. 227 It requires that both the ingress and egress provider edge (PE)s 228 support this feature. It also requires that sufficient LSRs on the 229 LSP between the ingress and egress PE be able to select an 230 ECMP path on an MPLS packet with the resultant stack depth. 232 RFC6790 works by including an entropy value in the LSP part of the 233 label stack. This requires that the Ingress and Egress PEs support 234 the insertion and removal of the entropy label (EL) and the entropy 235 label indicator, and that sufficient LSRs on the LSP are able to 236 preform ECMP based on the EL. 238 In both cases there considerations in getting Operations, 239 Administration, and Maintenance (OAM) packets to follow the same path 240 as a data packet. This is described in detail section 7 of 241 [RFC6391], and section 6 of RFC6790. However in both cases the 242 situation is improved compared to the ECMP behavior in the case where 243 the Ethernet PW CW was not used, since there is currently no known 244 method of getting a PW OAM packet to follow the same path as a PW 245 data packet subjected to ECMP based on the five tuple of the IP 246 payload. 248 The PW label is pushed before the LSP label. As the EL/ELI labels 249 are part of the LSP layer rather than part of the PW layer, they are 250 pushed after the PW label has been pushed. 252 6. Mitigations 254 Where it is not possible to use the Ethernet PW CW, the effects of 255 ECMP can be disabled by carrying the PW over a traffic engineered 256 path that does not subject the payload to load balancing (for example 257 [RFC3209]. However such paths may be subjected to link bundle load 258 balancing and of course the single LSP has to carry the full PW load. 260 7. Operational Considerations 262 CW presence on the PW is controlled by the configuration and may be 263 subject to default operational mode of not being enabled. Care needs 264 to be taken to ensure that software that implements this 265 recommendation does not depend on existing configuration setting that 266 prevents the use of control word. It is recommended that platform 267 software emits a rate limited message indicating that CW can be used 268 but is disabled due to existing configuration. 270 Instead of including a payload type in the packet, MPLS relies on the 271 control plane to signal the payload type that follows the bottom of 272 the label stack. Some LSRs attempt to deduce the packet type by MPLS 273 payload inspection, in some cases looking past the PW CW. If the 274 payload appears to be IP or IP carried in an Ethernet header they 275 perform an ECMP calculation based on what they assume to be the five 276 tuple fields. However deduction of the payload type in this way is 277 not an exact science, and where a packet that is not IP is mistaken 278 for an IP packet the result can be packets delivered out of order. 279 Misordering of this type can be difficult for an operator to 280 diagnose. Operators therefore need to careful when enabling 281 capability that allows information gleaned from packet inspection 282 past the PW CW to be be used in any ECMP calculation. 284 8. Security Considerations 286 This document expresses a preference for one existing and widely 287 deployed Ethernet PW encapsulation over another. These methods have 288 identical security considerations, which are discussed in [RFC4448]. 289 This document introduces no additional security issues. 291 9. IANA Considerations 293 This document makes no IANA requests. 295 10. Acknowledgments 297 The authors thank Job Snijders for drawing attention to this problem. 298 The authors also thank Pat Thaler for clarifying the matter of local 299 MAC address assignment. We thank Sasha Vainshtein for his valuable 300 review comments. 302 11. References 304 11.1. Normative References 306 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 307 Requirement Levels", BCP 14, RFC 2119, 308 DOI 10.17487/RFC2119, March 1997, . 311 [RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, 312 "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for 313 Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385, 314 February 2006, . 316 [RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron, 317 "Encapsulation Methods for Transport of Ethernet over MPLS 318 Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006, 319 . 321 [RFC4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal 322 Cost Multipath Treatment in MPLS Networks", BCP 128, 323 RFC 4928, DOI 10.17487/RFC4928, June 2007, 324 . 326 [RFC6391] Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V., 327 Regan, J., and S. Amante, "Flow-Aware Transport of 328 Pseudowires over an MPLS Packet Switched Network", 329 RFC 6391, DOI 10.17487/RFC6391, November 2011, 330 . 332 [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and 333 L. Yong, "The Use of Entropy Labels in MPLS Forwarding", 334 RFC 6790, DOI 10.17487/RFC6790, November 2012, 335 . 337 11.2. Informative References 339 [RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path 340 Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000, 341 . 343 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 344 and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 345 Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001, 346 . 348 [RFC5085] Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual 349 Circuit Connectivity Verification (VCCV): A Control 350 Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085, 351 December 2007, . 353 Authors' Addresses 355 Stewart Bryant 356 Huawei 358 Email: stewart.bryant@gmail.com 359 Andrew G Malis 360 Huawei 362 Email: agmalis@gmail.com 364 Ignas Bagdonas 365 Equinix 367 Email: ibagdona.ietf@gmail.com>