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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Transport Area Working Group B. Briscoe 3 Internet-Draft Independent 4 Updates: 6040, 2661, 2784, 3931, 4380, March 9, 2020 5 7450 (if approved) 6 Intended status: Standards Track 7 Expires: September 10, 2020 9 Propagating Explicit Congestion Notification Across IP Tunnel Headers 10 Separated by a Shim 11 draft-ietf-tsvwg-rfc6040update-shim-10 13 Abstract 15 RFC 6040 on "Tunnelling of Explicit Congestion Notification" made the 16 rules for propagation of ECN consistent for all forms of IP in IP 17 tunnel. This specification updates RFC 6040 to clarify that its 18 scope includes tunnels where two IP headers are separated by at least 19 one shim header that is not sufficient on its own for wide area 20 packet forwarding. It surveys widely deployed IP tunnelling 21 protocols that use such shim header(s) and updates the specifications 22 of those that do not mention ECN propagation (L2TPv2, L2TPv3, GRE, 23 Teredo and AMT). This specification also updates RFC 6040 with 24 configuration requirements needed to make any legacy tunnel ingress 25 safe. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on September 10, 2020. 44 Copyright Notice 46 Copyright (c) 2020 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (https://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 3. Scope of RFC 6040 . . . . . . . . . . . . . . . . . . . . . . 3 64 3.1. Feasibility of ECN Propagation between Tunnel Headers . . 4 65 3.2. Desirability of ECN Propagation between Tunnel Headers . 5 66 4. Making a non-ECN Tunnel Ingress Safe by Configuration . . . . 5 67 5. ECN Propagation and Fragmentation/Reassembly . . . . . . . . 7 68 6. IP-in-IP Tunnels with Tightly Coupled Shim Headers . . . . . 8 69 6.1. Specific Updates to Protocols under IETF Change Control . 10 70 6.1.1. L2TP (v2 and v3) ECN Extension . . . . . . . . . . . 10 71 6.1.2. GRE . . . . . . . . . . . . . . . . . . . . . . . . . 13 72 6.1.3. Teredo . . . . . . . . . . . . . . . . . . . . . . . 14 73 6.1.4. AMT . . . . . . . . . . . . . . . . . . . . . . . . . 15 74 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 75 8. Security Considerations . . . . . . . . . . . . . . . . . . . 17 76 9. Comments Solicited . . . . . . . . . . . . . . . . . . . . . 17 77 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 78 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 79 11.1. Normative References . . . . . . . . . . . . . . . . . . 18 80 11.2. Informative References . . . . . . . . . . . . . . . . . 19 81 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22 83 1. Introduction 85 RFC 6040 on "Tunnelling of Explicit Congestion Notification" 86 [RFC6040] made the rules for propagation of Explicit Congestion 87 Notification (ECN [RFC3168]) consistent for all forms of IP in IP 88 tunnel. 90 A common pattern for many tunnelling protocols is to encapsulate an 91 inner IP header (v4 or v6) with shim header(s) then an outer IP 92 header (v4 or v6). Some of these shim headers are designed as 93 generic encapsulations, so they do not necessarily directly 94 encapsulate an inner IP header. Instead they can encapsulate headers 95 such as link-layer (L2) protocols that in turn often encapsulate IP. 97 To clear up confusion, this specification clarifies that the scope of 98 RFC 6040 includes any IP-in-IP tunnel, including those with shim 99 header(s) and other encapsulations between the IP headers. Where 100 necessary, it updates the specifications of the relevant 101 encapsulation protocols with the specific text necessary to comply 102 with RFC 6040. 104 This specification also updates RFC 6040 to state how operators ought 105 to configure a legacy tunnel ingress to avoid unsafe system 106 configurations. 108 2. Terminology 110 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 111 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 112 document are to be interpreted as described in RFC 2119 [RFC2119] 113 when, and only when, they appear in all capitals, as shown here. 115 This specification uses the terminology defined in RFC 6040 116 [RFC6040]. 118 3. Scope of RFC 6040 120 In section 1.1 of RFC 6040, its scope is defined as: 122 "...ECN field processing at encapsulation and decapsulation for 123 any IP-in-IP tunnelling, whether IPsec or non-IPsec tunnels. It 124 applies irrespective of whether IPv4 or IPv6 is used for either 125 the inner or outer headers. ..." 127 This was intended to include cases where shim header(s) sit between 128 the IP headers. Many tunnelling implementers have interpreted the 129 scope of RFC 6040 as it was intended, but it is ambiguous. 130 Therefore, this specification updates RFC 6040 by adding the 131 following scoping text after the sentences quoted above: 133 It applies in cases where an outer IP header encapsulates an inner 134 IP header either directly or indirectly by encapsulating other 135 headers that in turn encapsulate (or might encapsulate) an inner 136 IP header. 138 There is another problem with the scope of RFC 6040. Like many IETF 139 specifications, RFC 6040 is written as a specification that 140 implementations can choose to claim compliance with. This means it 141 does not cover two important cases: 143 1. those cases where it is infeasible for an implementation to 144 access an inner IP header when adding or removing an outer IP 145 header; 147 2. those implementations that choose not to propagate ECN between IP 148 headers. 150 However, the ECN field is a non-optional part of the IP header (v4 151 and v6). So any implementation that creates an outer IP header has 152 to give the ECN field some value. There is only one safe value a 153 tunnel ingress can use if it does not know whether the egress 154 supports propagation of the ECN field; it has to clear the ECN field 155 in any outer IP header to 0b00. 157 However, an RFC has no jurisdiction over implementations that choose 158 not to comply with it or cannot comply with it, including all those 159 implementations that pre-dated the RFC. Therefore it would have been 160 unreasonable to add such a requirement to RFC 6040. Nonetheless, to 161 ensure safe propagation of the ECN field over tunnels, it is 162 reasonable to add requirements on operators, to ensure they configure 163 their tunnels safely (where possible). Before stating these 164 configuration requirements in Section 4, the factors that determine 165 whether propagating ECN is feasible or desirable will be briefly 166 introduced. 168 3.1. Feasibility of ECN Propagation between Tunnel Headers 170 In many cases shim header(s) and an outer IP header are always added 171 to (or removed from) an inner IP packet as part of the same 172 procedure. We call this a tightly coupled shim header. Processing 173 the shim and outer together is often necessary because the shim(s) 174 are not sufficient for packet forwarding in their own right; not 175 unless complemented by an outer header. In these cases it will often 176 be feasible for an implementation to propagate the ECN field between 177 the IP headers. 179 In some cases a tunnel adds an outer IP header and a tightly coupled 180 shim header to an inner header that is not an IP header, but that in 181 turn encapsulates an IP header (or might encapsulate an IP header). 182 For instance an inner Ethernet (or other link layer) header might 183 encapsulate an inner IP header as its payload. We call this a 184 tightly coupled shim over an encapsulating header. 186 Digging to arbitrary depths to find an inner IP header within an 187 encapsulation is strictly a layering violation so it cannot be a 188 required behaviour. Nonetheless, some tunnel endpoints already look 189 within a L2 header for an IP header, for instance to map the Diffserv 190 codepoint between an encapsulated IP header and an outer IP header 192 [RFC2983]. In such cases at least, it should be feasible to also 193 (independently) propagate the ECN field between the same IP headers. 194 Thus, access to the ECN field within an encapsulating header can be a 195 useful and benign optimization. The guidelines in section 5 of 196 [I-D.ietf-tsvwg-ecn-encap-guidelines] give the conditions for this 197 layering violation to be benign. 199 3.2. Desirability of ECN Propagation between Tunnel Headers 201 Developers and network operators are encouraged to implement and 202 deploy tunnel endpoints compliant with RFC 6040 (as updated by the 203 present specification) in order to provide the benefits of wider ECN 204 deployment [RFC8087]. Nonetheless, propagation of ECN between IP 205 headers, whether separated by shim headers or not, has to be optional 206 to implement and to use, because: 208 o Legacy implementations of tunnels without any ECN support already 209 exist 211 o A network might be designed so that there is usually no bottleneck 212 within the tunnel 214 o If the tunnel endpoints would have to search within an L2 header 215 to find an encapsulated IP header, it might not be worth the 216 potential performance hit 218 4. Making a non-ECN Tunnel Ingress Safe by Configuration 220 Even when no specific attempt has been made to implement propagation 221 of the ECN field at a tunnel ingress, it ought to be possible for the 222 operator to render a tunnel ingress safe by configuration. The main 223 safety concern is to disable (clear to zero) the ECN capability in 224 the outer IP header at the ingress if the egress of the tunnel does 225 not implement ECN logic to propagate any ECN markings into the packet 226 forwarded beyond the tunnel. Otherwise the non-ECN egress could 227 discard any ECN marking introduced within the tunnel, which would 228 break all the ECN-based control loops that regulate the traffic load 229 over the tunnel. 231 Therefore this specification updates RFC 6040 by inserting the 232 following text at the end of section 4.3: 234 " 235 Whether or not an ingress implementation claims compliance with 236 RFC 6040, RFC 4301 or RFC3168, when the outer tunnel header is IP 237 (v4 or v6), if possible, the operator MUST configure the ingress 238 to zero the outer ECN field in any of the following cases: 240 * if it is known that the tunnel egress does not support any of 241 the RFCs that define propagation of the ECN field (RFC 6040, 242 RFC 4301 or the full functionality mode of RFC 3168) 244 * or if the behaviour of the egress is not known or an egress 245 with unknown behaviour might be dynamically paired with the 246 ingress. 248 * or if an IP header might be encapsulated within a non-IP header 249 that the tunnel ingress is encapsulating, but the ingress does 250 not inspect within the encapsulation. 252 For the avoidance of doubt, the above only concerns the outer IP 253 header. The ingress MUST NOT alter the ECN field of the arriving 254 IP header that will become the inner IP header. 256 In order that the network operator can comply with the above 257 safety rules, even if an implementation of a tunnel ingress does 258 not claim to support RFC 6040, RFC 4301 or the full functionality 259 mode of RFC 3168: 261 * it MUST NOT treat the former ToS octet (IPv4) or the former 262 Traffic Class octet (IPv6) as a single 8-bit field, as the 263 resulting linkage of ECN and Diffserv field propagation between 264 inner and outer is not consistent with the definition of the 265 6-bit Diffserv field in [RFC2474] and [RFC3260]; 267 * it SHOULD be able to be configured to zero the ECN field of the 268 outer header. 270 " 272 For instance, if a tunnel ingress with no ECN-specific logic had a 273 configuration capability to refer to the last 2 bits of the old ToS 274 Byte of the outer (e.g. with a 0x3 mask) and set them to zero, while 275 also being able to allow the DSCP to be re-mapped independently, that 276 would be sufficient to satisfy both the above implementation 277 requirements. 279 There might be concern that the above "MUST NOT" makes compliant 280 implementations non-compliant at a stroke. However, by definition it 281 solely applies to equipment that provides Diffserv configuration. 282 Any such Diffserv equipment that is configuring treatment of the 283 former ToS octet (IPv4) or the former Traffic Class octet (IPv6) as a 284 single 8-bit field must have always been non-compliant with the 285 definition of the 6-bit Diffserv field in [RFC2474] and [RFC3260]. 286 If a tunnel ingress does not have any ECN logic, copying the ECN 287 field as a side-effect of copying the DSCP is a seriously unsafe bug 288 that risks breaking the feedback loops that regulate load on a 289 tunnel. 291 Zeroing the outer ECN field of all packets in all circumstances would 292 be safe, but it would not be sufficient to claim compliance with RFC 293 6040 because it would not meet the aim of introducing ECN support to 294 tunnels (see Section 4.3 of [RFC6040]). 296 5. ECN Propagation and Fragmentation/Reassembly 298 The following requirements update RFC6040, which omitted handling of 299 the ECN field during fragmentation or reassembly. These changes 300 might alter how many ECN-marked packets are propagated by a tunnel 301 that fragments packets, but this would not raise any backward 302 compatibility issues: 304 If a tunnel ingress fragments a packet, it MUST set the outer ECN 305 field of all the fragments to the same value as it would have set if 306 it had not fragmented the packet. 308 During reassembly of outer fragments [I-D.ietf-intarea-tunnels], if 309 the ECN fields of the outer headers being reassembled into a single 310 packet consist of a mixture of Not-ECT and other ECN codepoints, the 311 packet MUST be discarded. 313 As a tunnel egress reassembles sets of outer fragments 314 [I-D.ietf-intarea-tunnels] into packets, as long as no fragment 315 carries the Not-ECT codepoint, it SHOULD propagate CE markings such 316 that the proportion of reassembled packets output with CE markings is 317 broadly the same as the proportion of fragments arriving with CE 318 markings. 320 The above statement describes the approximate desired outcome, not 321 the specific mechanism. A simple to achieve this outcome would be to 322 leave a CE-mark on a reassembled packet if the head fragment is CE- 323 marked, irrespective of the markings on the other fragments. 324 Nonetheless, "SHOULD" is used in the above requirement to allow 325 similar perhaps more efficient approaches that result in 326 approximately the same outcome. 328 In RFC 3168 the approach to propagating CE markings during fragment 329 reassembly required that a reassembled packet has to be be CE-marked 330 if any of its fragments is CE-marked. This "logical OR" approach to 331 CE marking during reassembly was intended to ensure that no 332 individual CE marking is ever lost. However, an unintended 333 consequence is that the proportion of packets with CE markings 334 increases. For instance, with the logical OR approach, once a 335 sequence of packets each consisting of 2 fragments, has been 336 reassembled, the fraction of packets that are CE-marked roughly 337 doubles (because the number of marks remains roughly the same, but 338 the number of packets halves). 340 This specification does not rule out the logical OR approach of RFC 341 3168. So a tunnel egress MAY CE-mark a reassembled packet if any of 342 the fragments are CE-marked (and none are Not-ECT). However, this 343 approach could result in reduced link utilization, or bias against 344 flows that are fragmented relative to those that are not. 346 6. IP-in-IP Tunnels with Tightly Coupled Shim Headers 348 There follows a list of specifications of encapsulations with tightly 349 coupled shim header(s), in rough chronological order. The list is 350 confined to standards track or widely deployed protocols. The list 351 is not necessarily exhaustive so, for the avoidance of doubt, the 352 scope of RFC 6040 is defined in Section 3 and is not limited to this 353 list. 355 o PPTP (Point-to-Point Tunneling Protocol) [RFC2637]; 357 o L2TP (Layer 2 Tunnelling Protocol), specifically L2TPv2 [RFC2661] 358 and L2TPv3 [RFC3931], which not only includes all the L2-specific 359 specializations of L2TP, but also derivatives such as the Keyed 360 IPv6 Tunnel [RFC8159]; 362 o GRE (Generic Routing Encapsulation) [RFC2784] and NVGRE (Network 363 Virtualization using GRE) [RFC7637]; 365 o GTP (GPRS Tunnelling Protocol), specifically GTPv1 [GTPv1], GTP v1 366 User Plane [GTPv1-U], GTP v2 Control Plane [GTPv2-C]; 368 o Teredo [RFC4380]; 370 o CAPWAP (Control And Provisioning of Wireless Access Points) 371 [RFC5415]; 373 o LISP (Locator/Identifier Separation Protocol) [RFC6830]; 375 o AMT (Automatic Multicast Tunneling) [RFC7450]; 377 o VXLAN (Virtual eXtensible Local Area Network) [RFC7348] and VXLAN- 378 GPE [I-D.ietf-nvo3-vxlan-gpe]; 380 o The Network Service Header (NSH [RFC8300]) for Service Function 381 Chaining (SFC); 383 o Geneve [I-D.ietf-nvo3-geneve]; 384 o GUE (Generic UDP Encapsulation) [I-D.ietf-intarea-gue]; 386 o Direct tunnelling of an IP packet within a UDP/IP datagram (see 387 Section 3.1.11 of [RFC8085]); 389 o TCP Encapsulation of IKE and IPsec Packets (see Section 12.5 of 390 [RFC8229]). 392 Some of the listed protocols enable encapsulation of a variety of 393 network layer protocols as inner and/or outer. This specification 394 applies in the cases where there is an inner and outer IP header as 395 described in Section 3. Otherwise 396 [I-D.ietf-tsvwg-ecn-encap-guidelines] gives guidance on how to design 397 propagation of ECN into other protocols that might encapsulate IP. 399 Where protocols in the above list need to be updated to specify ECN 400 propagation and they are under IETF change control, update text is 401 given in the following subsections. For those not under IETF 402 control, it is RECOMMENDED that implementations of encapsulation and 403 decapsulation comply with RFC 6040. It is also RECOMMENDED that 404 their specifications are updated to add a requirement to comply with 405 RFC 6040 (as updated by the present document). 407 PPTP is not under the change control of the IETF, but it has been 408 documented in an informational RFC [RFC2637]. However, there is no 409 need for the present specification to update PPTP because L2TP has 410 been developed as a standardized replacement. 412 NVGRE is not under the change control of the IETF, but it has been 413 documented in an informational RFC [RFC7637]. NVGRE is a specific 414 use-case of GRE (it re-purposes the key field from the initial 415 specification of GRE [RFC1701] as a Virtual Subnet ID). Therefore 416 the text that updates GRE in Section 6.1.2 below is also intended to 417 update NVGRE. 419 Although the definition of the various GTP shim headers is under the 420 control of the 3GPP, it is hard to determine whether the 3GPP or the 421 IETF controls standardization of the _process_ of adding both a GTP 422 and an IP header to an inner IP header. Nonetheless, the present 423 specification is provided so that the 3GPP can refer to it from any 424 of its own specifications of GTP and IP header processing. 426 The specification of CAPWAP already specifies RFC 3168 ECN 427 propagation and ECN capability negotiation. Without modification the 428 CAPWAP specification already interworks with the backward compatible 429 updates to RFC 3168 in RFC 6040. 431 LISP made the ECN propagation procedures in RFC 3168 mandatory from 432 the start. RFC 3168 has since been updated by RFC 6040, but the 433 changes are backwards compatible so there is still no need for LISP 434 tunnel endpoints to negotiate their ECN capabilities. 436 VXLAN is not under the change control of the IETF but it has been 437 documented in an informational RFC. In contrast, VXLAN-GPE (Generic 438 Protocol Extension) is being documented under IETF change control. 439 It is RECOMMENDED that VXLAN and VXLAN-GPE implementations comply 440 with RFC 6040 when the VXLAN header is inserted between (or removed 441 from between) IP headers. The authors of any future update to these 442 specifications are encouraged to add a requirement to comply with RFC 443 6040 as updated by the present specification. 445 The Network Service Header (NSH [RFC8300]) has been defined as a 446 shim-based encapsulation to identify the Service Function Path (SFP) 447 in the Service Function Chaining (SFC) architecture [RFC7665]. A 448 proposal has been made for the processing of ECN when handling 449 transport encapsulation [I-D.ietf-sfc-nsh-ecn-support]. 451 The specifications of Geneve and GUE already refer to RFC 6040 for 452 ECN encapsulation. 454 Section 3.1.11 of RFC 8085 already explains that a tunnel that 455 encapsulates an IP header within a UDP/IP datagram needs to follow 456 RFC 6040 when propagating the ECN field between inner and outer IP 457 headers. The requirements in Section 4 update RFC 6040, and hence 458 implicitly update the UDP usage guidelines in RFC 8085 to add the 459 important but previously unstated requirement that, if the UDP tunnel 460 egress does not, or might not, support ECN propagation, a UDP tunnel 461 ingress has to clear the outer IP ECN field to 0b00, e.g. by 462 configuration. 464 Section 12.5 of TCP Encapsulation of IKE and IPsec Packets [RFC8229] 465 already recommends the compatibility mode of RFC 6040 in this case, 466 because there is not a one-to-one mapping between inner and outer 467 packets. 469 6.1. Specific Updates to Protocols under IETF Change Control 471 6.1.1. L2TP (v2 and v3) ECN Extension 473 The L2TP terminology used here is defined in [RFC2661] and [RFC3931]. 475 L2TPv3 [RFC3931] is used as a shim header between any packet-switched 476 network (PSN) header (e.g. IPv4, IPv6, MPLS) and many types of layer 477 2 (L2) header. The L2TPv3 shim header encapsulates an L2-specific 478 sub-layer then an L2 header that is likely to contain an inner IP 479 header (v4 or v6). Then this whole stack of headers can be 480 encapsulated optionally within an outer UDP header then an outer PSN 481 header that is typically IP (v4 or v6). 483 L2TPv2 is used as a shim header between any PSN header and a PPP 484 header, which is in turn likely to encapsulate an IP header. 486 Even though these shims are rather fat (particularly in the case of 487 L2TPv3), they still fit the definition of a tightly coupled shim 488 header over an encapsulating header (Section 3.1), because all the 489 headers encapsulating the L2 header are added (or removed) together. 490 L2TPv2 and L2TPv3 are therefore within the scope of RFC 6040, as 491 updated by Section 3 above. 493 L2TP maintainers are RECOMMENDED to implement the ECN extension to 494 L2TPv2 and L2TPv3 defined in Section 6.1.1.2 below, in order to 495 provide the benefits of ECN [RFC8087], whenever a node within an L2TP 496 tunnel becomes the bottleneck for an end-to-end traffic flow. 498 6.1.1.1. Safe Configuration of a 'Non-ECN' Ingress LCCE 500 The following text is appended to both Section 5.3 of [RFC2661] and 501 Section 4.5 of [RFC3931] as an update to the base L2TPv2 and L2TPv3 502 specifications: 504 The operator of an LCCE that does not support the ECN Extension in 505 Section 6.1.1.2 of RFCXXXX MUST follow the configuration 506 requirements in Section 4 of RFCXXXX to ensure it clears the outer 507 IP ECN field to 0b00 when the outer PSN header is IP (v4 or v6). 508 {RFCXXXX refers to the present document so it will need to be 509 inserted by the RFC Editor} 511 In particular, for an LCCE implementation that does not support the 512 ECN Extension, this means that configuration of how it propagates the 513 ECN field between inner and outer IP headers MUST be independent of 514 any configuration of the Diffserv extension of L2TP [RFC3308]. 516 6.1.1.2. ECN Extension for L2TP (v2 or v3) 518 When the outer PSN header and the payload inside the L2 header are 519 both IP (v4 or v6), to comply with RFC 6040, an LCCE will follow the 520 rules for propagation of the ECN field at ingress and egress in 521 Section 4 of RFC 6040 [RFC6040]. 523 Before encapsulating any data packets, RFC 6040 requires an ingress 524 LCCE to check that the egress LCCE supports ECN propagation as 525 defined in RFC 6040 or one of its compatible predecessors ([RFC4301] 526 or the full functionality mode of [RFC3168]). If the egress supports 527 ECN propagation, the ingress LCCE can use the normal mode of 528 encapsulation (copying the ECN field from inner to outer). 529 Otherwise, the ingress LCCE has to use compatibility mode [RFC6040] 530 (clearing the outer IP ECN field to 0b00). 532 An LCCE can determine the remote LCCE's support for ECN either 533 statically (by configuration) or by dynamic discovery during setup of 534 each control connection between the LCCEs, using the Capability AVP 535 defined in Section 6.1.1.2.1 below. 537 Where the outer PSN header is some protocol other than IP that 538 supports ECN, the appropriate ECN propagation specification will need 539 to be followed, e.g. "Explicit Congestion Marking in MPLS" 540 [RFC5129]. Where no specification exists for ECN propagation by a 541 particular PSN, [I-D.ietf-tsvwg-ecn-encap-guidelines] gives general 542 guidance on how to design ECN propagation into a protocol that 543 encapsulates IP. 545 6.1.1.2.1. LCCE Capability AVP for ECN Capability Negotiation 547 The LCCE Capability Attribute-Value Pair (AVP) defined here has 548 Attribute Type ZZ. The Attribute Value field for this AVP is a bit- 549 mask with the following 16-bit format: 551 0 1 552 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 553 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 554 |X X X X X X X X X X X X X X X E| 555 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 557 Figure 1: Value Field for the LCCE Capability Attribute 559 This AVP MAY be present in the following message types: SCCRQ and 560 SCCRP (Start-Control-Connection-Request and Start-Control-Connection- 561 Reply). This AVP MAY be hidden (the H-bit set to 0 or 1) and is 562 optional (M-bit not set). The length (before hiding) of this AVP 563 MUST be 8 octets. The Vendor ID is the IETF Vendor ID of 0. 565 Bit 15 of the Value field of the LCCE Capability AVP is defined as 566 the ECN Capability flag (E). When the ECN Capability flag is set to 567 1, it indicates that the sender supports ECN propagation. When the 568 ECN Capability flag is cleared to zero, or when no LCCE Capabiliy AVP 569 is present, it indicates that the sender does not support ECN 570 propagation. All the other bits are reserved. They MUST be cleared 571 to zero when sent and ignored when received or forwarded. 573 An LCCE initiating a control connection will send a Start-Control- 574 Connection-Request (SCCRQ) containing an LCCE Capability AVP with the 575 ECN Capability flag set to 1. If the tunnel terminator supports ECN, 576 it will return a Start-Control-Connection-Reply (SCCRP) that also 577 includes an LCCE Capability AVP with the ECN Capability flag set to 578 1. Then, for any sessions created by that control connection, both 579 ends of the tunnel can use the normal mode of RFC 6040, i.e. it can 580 copy the IP ECN field from inner to outer when encapsulating data 581 packets. 583 If, on the other hand, the tunnel terminator does not support ECN it 584 will ignore the ECN flag in the LCCE Capability AVP and send an SCCRP 585 to the tunnel initiator without a Capability AVP (or with a 586 Capability AVP but with the ECN Capability flag cleared to zero). 587 The tunnel initiator interprets the absence of the ECN Capability 588 flag in the SCCRP as an indication that the tunnel terminator is 589 incapable of supporting ECN. When encapsulating data packets for any 590 sessions created by that control connection, the tunnel initiator 591 will then use the compatibility mode of RFC 6040 to clear the ECN 592 field of the outer IP header to 0b00. 594 If the tunnel terminator does not support this ECN extension, the 595 network operator is still expected to configure it to comply with the 596 safety provisions set out in Section 6.1.1.1 above, when it acts as 597 an ingress LCCE. 599 6.1.2. GRE 601 The GRE terminology used here is defined in [RFC2784]. GRE is often 602 used as a tightly coupled shim header between IP headers. Sometimes 603 the GRE shim header encapsulates an L2 header, which might in turn 604 encapsulate an IP header. Therefore GRE is within the scope of RFC 605 6040 as updated by Section 3 above. 607 GRE tunnel endpoint maintainers are RECOMMENDED to support [RFC6040] 608 as updated by the present specification, in order to provide the 609 benefits of ECN [RFC8087] whenever a node within a GRE tunnel becomes 610 the bottleneck for an end-to-end IP traffic flow tunnelled over GRE 611 using IP as the delivery protocol (outer header). 613 GRE itself does not support dynamic set-up and configuration of 614 tunnels. However, control plane protocols such as Mobile IPv4 (MIP4) 615 [RFC5944], Mobile IPv6 (MIP6) [RFC6275], Proxy Mobile IP (PMIP) 616 [RFC5845] and IKEv2 [RFC7296] are sometimes used to set up GRE 617 tunnels dynamically. 619 When these control protocols set up IP-in-IP or IPSec tunnels, it is 620 likely that they propagate the ECN field as defined in RFC 6040 or 621 one of its compatible predecessors (RFC 4301 or the full 622 functionality mode of RFC 3168). However, if they use a GRE 623 encapsulation, this presumption is less sound. 625 Therefore, If the outer delivery protocol is IP (v4 or v6) the 626 operator is obliged to follow the safe configuration requirements in 627 Section 4 above. Section 6.1.2.1 below updates the base GRE 628 specification with this requirement, to emphasize its importance. 630 Where the delivery protocol is some protocol other than IP that 631 supports ECN, the appropriate ECN propagation specification will need 632 to be followed, e.g Explicit Congestion Marking in MPLS [RFC5129]. 633 Where no specification exists for ECN propagation by a particular 634 PSN, [I-D.ietf-tsvwg-ecn-encap-guidelines] gives more general 635 guidance on how to propagate ECN to and from protocols that 636 encapsulate IP. 638 6.1.2.1. Safe Configuration of a 'Non-ECN' GRE Ingress 640 The following text is appended to Section 3 of [RFC2784] as an update 641 to the base GRE specification: 643 The operator of a GRE tunnel ingress MUST follow the configuration 644 requirements in Section 4 of RFCXXXX when the outer delivery 645 protocol is IP (v4 or v6). {RFCXXXX refers to the present document 646 so it will need to be inserted by the RFC Editor} 648 6.1.3. Teredo 650 Teredo [RFC4380] provides a way to tunnel IPv6 over an IPv4 network, 651 with a UDP-based shim header between the two. 653 For Teredo tunnel endpoints to provide the benefits of ECN, the 654 Teredo specification would have to be updated to include negotiation 655 of the ECN capability between Teredo tunnel endpoints. Otherwise it 656 would be unsafe for a Teredo tunnel ingress to copy the ECN field to 657 the IPv6 outer. 659 It is believed that current implementations do not support 660 propagation of ECN, but that they do safely zero the ECN field in the 661 outer IPv6 header. However the specification does not mention 662 anything about this. 664 To make existing Teredo deployments safe, it would be possible to add 665 ECN capability negotiation to those that are subject to remote OS 666 update. However, for those implementations not subject to remote OS 667 update, it will not be feasible to require them to be configured 668 correctly, because Teredo tunnel endpoints are generally deployed on 669 hosts. 671 Therefore, until ECN support is added to the specification of Teredo, 672 the only feasible further safety precaution available here is to 673 update the specification of Teredo implementations with the following 674 text, as a new section 5.1.3: 676 "5.1.3 Safe 'Non-ECN' Teredo Encapsulation 678 A Teredo tunnel ingress implementation that does not support ECN 679 propagation as defined in RFC 6040 or one of its compatible 680 predecessors (RFC 4301 or the full functionality mode of RFC 3168) 681 MUST zero the ECN field in the outer IPv6 header." 683 6.1.4. AMT 685 Automatic Multicast Tunneling (AMT [RFC7450]) is a tightly coupled 686 shim header that encapsulates an IP packet and is itself encapsulated 687 within a UDP/IP datagram. Therefore AMT is within the scope of RFC 688 6040 as updated by Section 3 above. 690 AMT tunnel endpoint maintainers are RECOMMENDED to support [RFC6040] 691 as updated by the present specification, in order to provide the 692 benefits of ECN [RFC8087] whenever a node within an AMT tunnel 693 becomes the bottleneck for an IP traffic flow tunnelled over AMT. 695 To comply with RFC 6040, an AMT relay and gateway will follow the 696 rules for propagation of the ECN field at ingress and egress 697 respectively, as described in Section 4 of RFC 6040 [RFC6040]. 699 Before encapsulating any data packets, RFC 6040 requires an ingress 700 AMT relay to check that the egress AMT gateway supports ECN 701 propagation as defined in RFC 6040 or one of its compatible 702 predecessors (RFC 4301 or the full functionality mode of RFC 3168). 703 If the egress gateway supports ECN, the ingress relay can use the 704 normal mode of encapsulation (copying the IP ECN field from inner to 705 outer). Otherwise, the ingress relay has to use compatibility mode, 706 which means it has to clear the outer ECN field to zero [RFC6040]. 708 An AMT tunnel is created dynamically (not manually), so the relay 709 will need to determine the remote gateway's support for ECN using the 710 ECN capability declaration defined in Section 6.1.4.2 below. 712 6.1.4.1. Safe Configuration of a 'Non-ECN' Ingress AMT Relay 714 The following text is appended to Section 4.2.2 of [RFC7450] as an 715 update to the AMT specification: 717 The operator of an AMT relay that does not support RFC 6040 or one 718 of its compatible predecessors (RFC 4301 or the full functionality 719 mode of RFC 3168) MUST follow the configuration requirements in 720 Section 4 of RFCXXXX to ensure it clears the outer IP ECN field to 721 zero. {RFCXXXX refers to the present document so it will need to 722 be inserted by the RFC Editor} 724 6.1.4.2. ECN Capability Declaration of an AMT Gateway 726 0 1 2 3 727 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 1 728 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 729 | V=0 |Type=3 | Reserved |E|P| Reserved | 730 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 731 | Request Nonce | 732 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 734 Figure 2: Updated AMT Request Message Format 736 Bit 14 of the AMT Request Message counting from 0 (or bit 7 of the 737 Reserved field counting from 1) is defined here as the AMT Gateway 738 ECN Capability flag (E), as shown in Figure 2. The definitions of 739 all other fields in the AMT Request Message are unchanged from RFC 740 7450. 742 When the E flag is set to 1, it indicates that the sender of the 743 message supports RFC 6040 ECN propagation. When it is cleared to 744 zero, it indicates the sender of the message does not support RFC 745 6040 ECN propagation. An AMT gateway "that supports RFC 6040 ECN 746 propagation" means one that propagates the ECN field to the forwarded 747 data packet based on the combination of arriving inner and outer ECN 748 fields, as defined in Section 4 of RFC 6040. 750 The other bits of the Reserved field remain reserved. They will 751 continue to be cleared to zero when sent and ignored when either 752 received or forwarded, as specified in Section 5.1.3.3. of RFC 7450. 754 An AMT gateway that does not support RFC 6040 MUST NOT set the E flag 755 of its Request Message to 1. 757 An AMT gateway that supports RFC 6040 ECN propagation MUST set the E 758 flag of its Relay Discovery Message to 1. 760 The action of the corresponding AMT relay that receives a Request 761 message with the E flag set to 1 depends on whether the relay itself 762 supports RFC 6040 ECN propagation: 764 o If the relay supports RFC 6040 ECN propagation, it will store the 765 ECN capability of the gateway along with its address. Then 766 whenever it tunnels datagrams towards this gateway, it MUST use 767 the normal mode of RFC 6040 to propagate the ECN field when 768 encapsulating datagrams (i.e. it copies the IP ECN field from 769 inner to outer). 771 o If the discovered AMT relay does not support RFC 6040 ECN 772 propagation, it will ignore the E flag in the Reserved field, as 773 per section 5.1.3.3. of RFC 7450. 775 If the AMT relay does not support RFC 6040 ECN propagation, the 776 network operator is still expected to configure it to comply with 777 the safety provisions set out in Section 6.1.4.1 above. 779 7. IANA Considerations 781 IANA is requested to assign the following L2TP Control Message 782 Attribute Value Pair: 784 +----------------+----------------+-----------+ 785 | Attribute Type | Description | Reference | 786 +----------------+----------------+-----------+ 787 | ZZ | ECN Capability | RFCXXXX | 788 +----------------+----------------+-----------+ 790 [TO BE REMOVED: This registration should take place at the following 791 location: https://www.iana.org/assignments/l2tp-parameters/l2tp- 792 parameters.xhtml ] 794 8. Security Considerations 796 The Security Considerations in [RFC6040] and 797 [I-D.ietf-tsvwg-ecn-encap-guidelines] apply equally to the scope 798 defined for the present specification. 800 9. Comments Solicited 802 Comments and questions are encouraged and very welcome. They can be 803 addressed to the IETF Transport Area working group mailing list 804 , and/or to the authors. 806 10. Acknowledgements 808 Thanks to Ing-jyh (Inton) Tsang for initial discussions on the need 809 for ECN propagation in L2TP and its applicability. Thanks also to 810 Carlos Pignataro, Tom Herbert, Ignacio Goyret, Alia Atlas, Praveen 811 Balasubramanian, Joe Touch, Mohamed Boucadair, David Black, Jake 812 Holland and Sri Gundavelli for helpful advice and comments. "A 813 Comparison of IPv6-over-IPv4 Tunnel Mechanisms" [RFC7059] helped to 814 identify a number of tunnelling protocols to include within the scope 815 of this document. 817 Bob Briscoe was part-funded by the Research Council of Norway through 818 the TimeIn project. The views expressed here are solely those of the 819 authors. 821 11. References 823 11.1. Normative References 825 [I-D.ietf-tsvwg-ecn-encap-guidelines] 826 Briscoe, B., Kaippallimalil, J., and P. Thaler, 827 "Guidelines for Adding Congestion Notification to 828 Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn- 829 encap-guidelines-13 (work in progress), May 2019. 831 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 832 Requirement Levels", BCP 14, RFC 2119, 833 DOI 10.17487/RFC2119, March 1997, 834 . 836 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 837 "Definition of the Differentiated Services Field (DS 838 Field) in the IPv4 and IPv6 Headers", RFC 2474, 839 DOI 10.17487/RFC2474, December 1998, 840 . 842 [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, 843 G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"", 844 RFC 2661, DOI 10.17487/RFC2661, August 1999, 845 . 847 [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. 848 Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, 849 DOI 10.17487/RFC2784, March 2000, 850 . 852 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 853 of Explicit Congestion Notification (ECN) to IP", 854 RFC 3168, DOI 10.17487/RFC3168, September 2001, 855 . 857 [RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., 858 "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", 859 RFC 3931, DOI 10.17487/RFC3931, March 2005, 860 . 862 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 863 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 864 December 2005, . 866 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through 867 Network Address Translations (NATs)", RFC 4380, 868 DOI 10.17487/RFC4380, February 2006, 869 . 871 [RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion 872 Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January 873 2008, . 875 [RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion 876 Notification", RFC 6040, DOI 10.17487/RFC6040, November 877 2010, . 879 11.2. Informative References 881 [GTPv1] 3GPP, "GPRS Tunnelling Protocol (GTP) across the Gn and Gp 882 interface", Technical Specification TS 29.060. 884 [GTPv1-U] 3GPP, "General Packet Radio System (GPRS) Tunnelling 885 Protocol User Plane (GTPv1-U)", Technical Specification TS 886 29.281. 888 [GTPv2-C] 3GPP, "Evolved General Packet Radio Service (GPRS) 889 Tunnelling Protocol for Control plane (GTPv2-C)", 890 Technical Specification TS 29.274. 892 [I-D.ietf-intarea-gue] 893 Herbert, T., Yong, L., and O. Zia, "Generic UDP 894 Encapsulation", draft-ietf-intarea-gue-09 (work in 895 progress), October 2019. 897 [I-D.ietf-intarea-tunnels] 898 Touch, J. and M. Townsley, "IP Tunnels in the Internet 899 Architecture", draft-ietf-intarea-tunnels-10 (work in 900 progress), September 2019. 902 [I-D.ietf-nvo3-geneve] 903 Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic 904 Network Virtualization Encapsulation", draft-ietf- 905 nvo3-geneve-16 (work in progress), March 2020. 907 [I-D.ietf-nvo3-vxlan-gpe] 908 Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol 909 Extension for VXLAN", draft-ietf-nvo3-vxlan-gpe-09 (work 910 in progress), December 2019. 912 [I-D.ietf-sfc-nsh-ecn-support] 913 Eastlake, D., Briscoe, B., and A. Malis, "Explicit 914 Congestion Notification (ECN) and Congestion Feedback 915 Using the Network Service Header (NSH)", draft-ietf-sfc- 916 nsh-ecn-support-02 (work in progress), January 2020. 918 [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic 919 Routing Encapsulation (GRE)", RFC 1701, 920 DOI 10.17487/RFC1701, October 1994, 921 . 923 [RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, 924 W., and G. Zorn, "Point-to-Point Tunneling Protocol 925 (PPTP)", RFC 2637, DOI 10.17487/RFC2637, July 1999, 926 . 928 [RFC2983] Black, D., "Differentiated Services and Tunnels", 929 RFC 2983, DOI 10.17487/RFC2983, October 2000, 930 . 932 [RFC3260] Grossman, D., "New Terminology and Clarifications for 933 Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002, 934 . 936 [RFC3308] Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer 937 Two Tunneling Protocol (L2TP) Differentiated Services 938 Extension", RFC 3308, DOI 10.17487/RFC3308, November 2002, 939 . 941 [RFC5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley, 942 Ed., "Control And Provisioning of Wireless Access Points 943 (CAPWAP) Protocol Specification", RFC 5415, 944 DOI 10.17487/RFC5415, March 2009, 945 . 947 [RFC5845] Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung, 948 "Generic Routing Encapsulation (GRE) Key Option for Proxy 949 Mobile IPv6", RFC 5845, DOI 10.17487/RFC5845, June 2010, 950 . 952 [RFC5944] Perkins, C., Ed., "IP Mobility Support for IPv4, Revised", 953 RFC 5944, DOI 10.17487/RFC5944, November 2010, 954 . 956 [RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility 957 Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July 958 2011, . 960 [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The 961 Locator/ID Separation Protocol (LISP)", RFC 6830, 962 DOI 10.17487/RFC6830, January 2013, 963 . 965 [RFC7059] Steffann, S., van Beijnum, I., and R. van Rein, "A 966 Comparison of IPv6-over-IPv4 Tunnel Mechanisms", RFC 7059, 967 DOI 10.17487/RFC7059, November 2013, 968 . 970 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 971 Kivinen, "Internet Key Exchange Protocol Version 2 972 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 973 2014, . 975 [RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger, 976 L., Sridhar, T., Bursell, M., and C. Wright, "Virtual 977 eXtensible Local Area Network (VXLAN): A Framework for 978 Overlaying Virtualized Layer 2 Networks over Layer 3 979 Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014, 980 . 982 [RFC7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450, 983 DOI 10.17487/RFC7450, February 2015, 984 . 986 [RFC7637] Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network 987 Virtualization Using Generic Routing Encapsulation", 988 RFC 7637, DOI 10.17487/RFC7637, September 2015, 989 . 991 [RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function 992 Chaining (SFC) Architecture", RFC 7665, 993 DOI 10.17487/RFC7665, October 2015, 994 . 996 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage 997 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, 998 March 2017, . 1000 [RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using 1001 Explicit Congestion Notification (ECN)", RFC 8087, 1002 DOI 10.17487/RFC8087, March 2017, 1003 . 1005 [RFC8159] Konstantynowicz, M., Ed., Heron, G., Ed., Schatzmayr, R., 1006 and W. Henderickx, "Keyed IPv6 Tunnel", RFC 8159, 1007 DOI 10.17487/RFC8159, May 2017, 1008 . 1010 [RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation 1011 of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229, 1012 August 2017, . 1014 [RFC8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed., 1015 "Network Service Header (NSH)", RFC 8300, 1016 DOI 10.17487/RFC8300, January 2018, 1017 . 1019 Author's Address 1021 Bob Briscoe 1022 Independent 1023 UK 1025 EMail: ietf@bobbriscoe.net 1026 URI: http://bobbriscoe.net/