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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, November 15, 2020 5 7450 (if approved) 6 Intended status: Standards Track 7 Expires: May 19, 2021 9 Propagating Explicit Congestion Notification Across IP Tunnel Headers 10 Separated by a Shim 11 draft-ietf-tsvwg-rfc6040update-shim-12 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 May 19, 2021. 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 . . . . . 7 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 . . . . . . . . . . . . . . . . . . . . . . . . . 14 74 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 75 8. Security Considerations . . . . . . . . . . . . . . . . . . . 17 76 9. Comments Solicited . . . . . . . . . . . . . . . . . . . . . 17 77 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 78 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 79 11.1. Normative References . . . . . . . . . . . . . . . . . . 17 80 11.2. Informative References . . . . . . . . . . . . . . . . . 18 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 Section 5.3 of [RFC3168] defines the process that a tunnel egress 314 follows to reassemble sets of outer fragments 315 [I-D.ietf-intarea-tunnels] into packets. 317 6. IP-in-IP Tunnels with Tightly Coupled Shim Headers 319 There follows a list of specifications of encapsulations with tightly 320 coupled shim header(s), in rough chronological order. The list is 321 confined to standards track or widely deployed protocols. The list 322 is not necessarily exhaustive so, for the avoidance of doubt, the 323 scope of RFC 6040 is defined in Section 3 and is not limited to this 324 list. 326 o PPTP (Point-to-Point Tunneling Protocol) [RFC2637]; 328 o L2TP (Layer 2 Tunnelling Protocol), specifically L2TPv2 [RFC2661] 329 and L2TPv3 [RFC3931], which not only includes all the L2-specific 330 specializations of L2TP, but also derivatives such as the Keyed 331 IPv6 Tunnel [RFC8159]; 333 o GRE (Generic Routing Encapsulation) [RFC2784] and NVGRE (Network 334 Virtualization using GRE) [RFC7637]; 336 o GTP (GPRS Tunnelling Protocol), specifically GTPv1 [GTPv1], GTP v1 337 User Plane [GTPv1-U], GTP v2 Control Plane [GTPv2-C]; 339 o Teredo [RFC4380]; 341 o CAPWAP (Control And Provisioning of Wireless Access Points) 342 [RFC5415]; 344 o LISP (Locator/Identifier Separation Protocol) [RFC6830]; 346 o AMT (Automatic Multicast Tunneling) [RFC7450]; 348 o VXLAN (Virtual eXtensible Local Area Network) [RFC7348] and VXLAN- 349 GPE [I-D.ietf-nvo3-vxlan-gpe]; 351 o The Network Service Header (NSH [RFC8300]) for Service Function 352 Chaining (SFC); 354 o Geneve [I-D.ietf-nvo3-geneve]; 356 o GUE (Generic UDP Encapsulation) [I-D.ietf-intarea-gue]; 358 o Direct tunnelling of an IP packet within a UDP/IP datagram (see 359 Section 3.1.11 of [RFC8085]); 361 o TCP Encapsulation of IKE and IPsec Packets (see Section 12.5 of 362 [RFC8229]). 364 Some of the listed protocols enable encapsulation of a variety of 365 network layer protocols as inner and/or outer. This specification 366 applies in the cases where there is an inner and outer IP header as 367 described in Section 3. Otherwise 368 [I-D.ietf-tsvwg-ecn-encap-guidelines] gives guidance on how to design 369 propagation of ECN into other protocols that might encapsulate IP. 371 Where protocols in the above list need to be updated to specify ECN 372 propagation and they are under IETF change control, update text is 373 given in the following subsections. For those not under IETF 374 control, it is RECOMMENDED that implementations of encapsulation and 375 decapsulation comply with RFC 6040. It is also RECOMMENDED that 376 their specifications are updated to add a requirement to comply with 377 RFC 6040 (as updated by the present document). 379 PPTP is not under the change control of the IETF, but it has been 380 documented in an informational RFC [RFC2637]. However, there is no 381 need for the present specification to update PPTP because L2TP has 382 been developed as a standardized replacement. 384 NVGRE is not under the change control of the IETF, but it has been 385 documented in an informational RFC [RFC7637]. NVGRE is a specific 386 use-case of GRE (it re-purposes the key field from the initial 387 specification of GRE [RFC1701] as a Virtual Subnet ID). Therefore 388 the text that updates GRE in Section 6.1.2 below is also intended to 389 update NVGRE. 391 Although the definition of the various GTP shim headers is under the 392 control of the 3GPP, it is hard to determine whether the 3GPP or the 393 IETF controls standardization of the _process_ of adding both a GTP 394 and an IP header to an inner IP header. Nonetheless, the present 395 specification is provided so that the 3GPP can refer to it from any 396 of its own specifications of GTP and IP header processing. 398 The specification of CAPWAP already specifies RFC 3168 ECN 399 propagation and ECN capability negotiation. Without modification the 400 CAPWAP specification already interworks with the backward compatible 401 updates to RFC 3168 in RFC 6040. 403 LISP made the ECN propagation procedures in RFC 3168 mandatory from 404 the start. RFC 3168 has since been updated by RFC 6040, but the 405 changes are backwards compatible so there is still no need for LISP 406 tunnel endpoints to negotiate their ECN capabilities. 408 VXLAN is not under the change control of the IETF but it has been 409 documented in an informational RFC. In contrast, VXLAN-GPE (Generic 410 Protocol Extension) is being documented under IETF change control. 411 It is RECOMMENDED that VXLAN and VXLAN-GPE implementations comply 412 with RFC 6040 when the VXLAN header is inserted between (or removed 413 from between) IP headers. The authors of any future update to these 414 specifications are encouraged to add a requirement to comply with RFC 415 6040 as updated by the present specification. 417 The Network Service Header (NSH [RFC8300]) has been defined as a 418 shim-based encapsulation to identify the Service Function Path (SFP) 419 in the Service Function Chaining (SFC) architecture [RFC7665]. A 420 proposal has been made for the processing of ECN when handling 421 transport encapsulation [I-D.ietf-sfc-nsh-ecn-support]. 423 The specifications of Geneve and GUE already refer to RFC 6040 for 424 ECN encapsulation. 426 Section 3.1.11 of RFC 8085 already explains that a tunnel that 427 encapsulates an IP header within a UDP/IP datagram needs to follow 428 RFC 6040 when propagating the ECN field between inner and outer IP 429 headers. The requirements in Section 4 update RFC 6040, and hence 430 implicitly update the UDP usage guidelines in RFC 8085 to add the 431 important but previously unstated requirement that, if the UDP tunnel 432 egress does not, or might not, support ECN propagation, a UDP tunnel 433 ingress has to clear the outer IP ECN field to 0b00, e.g. by 434 configuration. 436 Section 12.5 of TCP Encapsulation of IKE and IPsec Packets [RFC8229] 437 already recommends the compatibility mode of RFC 6040 in this case, 438 because there is not a one-to-one mapping between inner and outer 439 packets. 441 6.1. Specific Updates to Protocols under IETF Change Control 443 6.1.1. L2TP (v2 and v3) ECN Extension 445 The L2TP terminology used here is defined in [RFC2661] and [RFC3931]. 447 L2TPv3 [RFC3931] is used as a shim header between any packet-switched 448 network (PSN) header (e.g. IPv4, IPv6, MPLS) and many types of layer 449 2 (L2) header. The L2TPv3 shim header encapsulates an L2-specific 450 sub-layer then an L2 header that is likely to contain an inner IP 451 header (v4 or v6). Then this whole stack of headers can be 452 encapsulated optionally within an outer UDP header then an outer PSN 453 header that is typically IP (v4 or v6). 455 L2TPv2 is used as a shim header between any PSN header and a PPP 456 header, which is in turn likely to encapsulate an IP header. 458 Even though these shims are rather fat (particularly in the case of 459 L2TPv3), they still fit the definition of a tightly coupled shim 460 header over an encapsulating header (Section 3.1), because all the 461 headers encapsulating the L2 header are added (or removed) together. 462 L2TPv2 and L2TPv3 are therefore within the scope of RFC 6040, as 463 updated by Section 3 above. 465 L2TP maintainers are RECOMMENDED to implement the ECN extension to 466 L2TPv2 and L2TPv3 defined in Section 6.1.1.2 below, in order to 467 provide the benefits of ECN [RFC8087], whenever a node within an L2TP 468 tunnel becomes the bottleneck for an end-to-end traffic flow. 470 6.1.1.1. Safe Configuration of a 'Non-ECN' Ingress LCCE 472 The following text is appended to both Section 5.3 of [RFC2661] and 473 Section 4.5 of [RFC3931] as an update to the base L2TPv2 and L2TPv3 474 specifications: 476 The operator of an LCCE that does not support the ECN Extension in 477 Section 6.1.1.2 of RFCXXXX MUST follow the configuration 478 requirements in Section 4 of RFCXXXX to ensure it clears the outer 479 IP ECN field to 0b00 when the outer PSN header is IP (v4 or v6). 481 {RFCXXXX refers to the present document so it will need to be 482 inserted by the RFC Editor} 484 In particular, for an LCCE implementation that does not support the 485 ECN Extension, this means that configuration of how it propagates the 486 ECN field between inner and outer IP headers MUST be independent of 487 any configuration of the Diffserv extension of L2TP [RFC3308]. 489 6.1.1.2. ECN Extension for L2TP (v2 or v3) 491 When the outer PSN header and the payload inside the L2 header are 492 both IP (v4 or v6), to comply with RFC 6040, an LCCE will follow the 493 rules for propagation of the ECN field at ingress and egress in 494 Section 4 of RFC 6040 [RFC6040]. 496 Before encapsulating any data packets, RFC 6040 requires an ingress 497 LCCE to check that the egress LCCE supports ECN propagation as 498 defined in RFC 6040 or one of its compatible predecessors ([RFC4301] 499 or the full functionality mode of [RFC3168]). If the egress supports 500 ECN propagation, the ingress LCCE can use the normal mode of 501 encapsulation (copying the ECN field from inner to outer). 502 Otherwise, the ingress LCCE has to use compatibility mode [RFC6040] 503 (clearing the outer IP ECN field to 0b00). 505 An LCCE can determine the remote LCCE's support for ECN either 506 statically (by configuration) or by dynamic discovery during setup of 507 each control connection between the LCCEs, using the Capability AVP 508 defined in Section 6.1.1.2.1 below. 510 Where the outer PSN header is some protocol other than IP that 511 supports ECN, the appropriate ECN propagation specification will need 512 to be followed, e.g. "Explicit Congestion Marking in MPLS" 513 [RFC5129]. Where no specification exists for ECN propagation by a 514 particular PSN, [I-D.ietf-tsvwg-ecn-encap-guidelines] gives general 515 guidance on how to design ECN propagation into a protocol that 516 encapsulates IP. 518 6.1.1.2.1. LCCE Capability AVP for ECN Capability Negotiation 520 The LCCE Capability Attribute-Value Pair (AVP) defined here has 521 Attribute Type ZZ. The Attribute Value field for this AVP is a bit- 522 mask with the following 16-bit format: 524 0 1 525 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 526 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 527 |X X X X X X X X X X X X X X X E| 528 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 530 Figure 1: Value Field for the LCCE Capability Attribute 532 This AVP MAY be present in the following message types: SCCRQ and 533 SCCRP (Start-Control-Connection-Request and Start-Control-Connection- 534 Reply). This AVP MAY be hidden (the H-bit set to 0 or 1) and is 535 optional (M-bit not set). The length (before hiding) of this AVP 536 MUST be 8 octets. The Vendor ID is the IETF Vendor ID of 0. 538 Bit 15 of the Value field of the LCCE Capability AVP is defined as 539 the ECN Capability flag (E). When the ECN Capability flag is set to 540 1, it indicates that the sender supports ECN propagation. When the 541 ECN Capability flag is cleared to zero, or when no LCCE Capabiliy AVP 542 is present, it indicates that the sender does not support ECN 543 propagation. All the other bits are reserved. They MUST be cleared 544 to zero when sent and ignored when received or forwarded. 546 An LCCE initiating a control connection will send a Start-Control- 547 Connection-Request (SCCRQ) containing an LCCE Capability AVP with the 548 ECN Capability flag set to 1. If the tunnel terminator supports ECN, 549 it will return a Start-Control-Connection-Reply (SCCRP) that also 550 includes an LCCE Capability AVP with the ECN Capability flag set to 551 1. Then, for any sessions created by that control connection, both 552 ends of the tunnel can use the normal mode of RFC 6040, i.e. it can 553 copy the IP ECN field from inner to outer when encapsulating data 554 packets. 556 If, on the other hand, the tunnel terminator does not support ECN it 557 will ignore the ECN flag in the LCCE Capability AVP and send an SCCRP 558 to the tunnel initiator without a Capability AVP (or with a 559 Capability AVP but with the ECN Capability flag cleared to zero). 560 The tunnel initiator interprets the absence of the ECN Capability 561 flag in the SCCRP as an indication that the tunnel terminator is 562 incapable of supporting ECN. When encapsulating data packets for any 563 sessions created by that control connection, the tunnel initiator 564 will then use the compatibility mode of RFC 6040 to clear the ECN 565 field of the outer IP header to 0b00. 567 If the tunnel terminator does not support this ECN extension, the 568 network operator is still expected to configure it to comply with the 569 safety provisions set out in Section 6.1.1.1 above, when it acts as 570 an ingress LCCE. 572 6.1.2. GRE 574 The GRE terminology used here is defined in [RFC2784]. GRE is often 575 used as a tightly coupled shim header between IP headers. Sometimes 576 the GRE shim header encapsulates an L2 header, which might in turn 577 encapsulate an IP header. Therefore GRE is within the scope of RFC 578 6040 as updated by Section 3 above. 580 GRE tunnel endpoint maintainers are RECOMMENDED to support [RFC6040] 581 as updated by the present specification, in order to provide the 582 benefits of ECN [RFC8087] whenever a node within a GRE tunnel becomes 583 the bottleneck for an end-to-end IP traffic flow tunnelled over GRE 584 using IP as the delivery protocol (outer header). 586 GRE itself does not support dynamic set-up and configuration of 587 tunnels. However, control plane protocols such as Mobile IPv4 (MIP4) 588 [RFC5944], Mobile IPv6 (MIP6) [RFC6275], Proxy Mobile IP (PMIP) 589 [RFC5845] and IKEv2 [RFC7296] are sometimes used to set up GRE 590 tunnels dynamically. 592 When these control protocols set up IP-in-IP or IPSec tunnels, it is 593 likely that they propagate the ECN field as defined in RFC 6040 or 594 one of its compatible predecessors (RFC 4301 or the full 595 functionality mode of RFC 3168). However, if they use a GRE 596 encapsulation, this presumption is less sound. 598 Therefore, If the outer delivery protocol is IP (v4 or v6) the 599 operator is obliged to follow the safe configuration requirements in 600 Section 4 above. Section 6.1.2.1 below updates the base GRE 601 specification with this requirement, to emphasize its importance. 603 Where the delivery protocol is some protocol other than IP that 604 supports ECN, the appropriate ECN propagation specification will need 605 to be followed, e.g Explicit Congestion Marking in MPLS [RFC5129]. 606 Where no specification exists for ECN propagation by a particular 607 PSN, [I-D.ietf-tsvwg-ecn-encap-guidelines] gives more general 608 guidance on how to propagate ECN to and from protocols that 609 encapsulate IP. 611 6.1.2.1. Safe Configuration of a 'Non-ECN' GRE Ingress 613 The following text is appended to Section 3 of [RFC2784] as an update 614 to the base GRE specification: 616 The operator of a GRE tunnel ingress MUST follow the configuration 617 requirements in Section 4 of RFCXXXX when the outer delivery 618 protocol is IP (v4 or v6). {RFCXXXX refers to the present document 619 so it will need to be inserted by the RFC Editor} 621 6.1.3. Teredo 623 Teredo [RFC4380] provides a way to tunnel IPv6 over an IPv4 network, 624 with a UDP-based shim header between the two. 626 For Teredo tunnel endpoints to provide the benefits of ECN, the 627 Teredo specification would have to be updated to include negotiation 628 of the ECN capability between Teredo tunnel endpoints. Otherwise it 629 would be unsafe for a Teredo tunnel ingress to copy the ECN field to 630 the IPv6 outer. 632 It is believed that current implementations do not support 633 propagation of ECN, but that they do safely zero the ECN field in the 634 outer IPv6 header. However the specification does not mention 635 anything about this. 637 To make existing Teredo deployments safe, it would be possible to add 638 ECN capability negotiation to those that are subject to remote OS 639 update. However, for those implementations not subject to remote OS 640 update, it will not be feasible to require them to be configured 641 correctly, because Teredo tunnel endpoints are generally deployed on 642 hosts. 644 Therefore, until ECN support is added to the specification of Teredo, 645 the only feasible further safety precaution available here is to 646 update the specification of Teredo implementations with the following 647 text, as a new section 5.1.3: 649 "5.1.3 Safe 'Non-ECN' Teredo Encapsulation 651 A Teredo tunnel ingress implementation that does not support ECN 652 propagation as defined in RFC 6040 or one of its compatible 653 predecessors (RFC 4301 or the full functionality mode of RFC 3168) 654 MUST zero the ECN field in the outer IPv6 header." 656 6.1.4. AMT 658 Automatic Multicast Tunneling (AMT [RFC7450]) is a tightly coupled 659 shim header that encapsulates an IP packet and is itself encapsulated 660 within a UDP/IP datagram. Therefore AMT is within the scope of RFC 661 6040 as updated by Section 3 above. 663 AMT tunnel endpoint maintainers are RECOMMENDED to support [RFC6040] 664 as updated by the present specification, in order to provide the 665 benefits of ECN [RFC8087] whenever a node within an AMT tunnel 666 becomes the bottleneck for an IP traffic flow tunnelled over AMT. 668 To comply with RFC 6040, an AMT relay and gateway will follow the 669 rules for propagation of the ECN field at ingress and egress 670 respectively, as described in Section 4 of RFC 6040 [RFC6040]. 672 Before encapsulating any data packets, RFC 6040 requires an ingress 673 AMT relay to check that the egress AMT gateway supports ECN 674 propagation as defined in RFC 6040 or one of its compatible 675 predecessors (RFC 4301 or the full functionality mode of RFC 3168). 676 If the egress gateway supports ECN, the ingress relay can use the 677 normal mode of encapsulation (copying the IP ECN field from inner to 678 outer). Otherwise, the ingress relay has to use compatibility mode, 679 which means it has to clear the outer ECN field to zero [RFC6040]. 681 An AMT tunnel is created dynamically (not manually), so the relay 682 will need to determine the remote gateway's support for ECN using the 683 ECN capability declaration defined in Section 6.1.4.2 below. 685 6.1.4.1. Safe Configuration of a 'Non-ECN' Ingress AMT Relay 687 The following text is appended to Section 4.2.2 of [RFC7450] as an 688 update to the AMT specification: 690 The operator of an AMT relay that does not support RFC 6040 or one 691 of its compatible predecessors (RFC 4301 or the full functionality 692 mode of RFC 3168) MUST follow the configuration requirements in 693 Section 4 of RFCXXXX to ensure it clears the outer IP ECN field to 694 zero. {RFCXXXX refers to the present document so it will need to 695 be inserted by the RFC Editor} 697 6.1.4.2. ECN Capability Declaration of an AMT Gateway 699 0 1 2 3 700 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 1 701 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 702 | V=0 |Type=3 | Reserved |E|P| Reserved | 703 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 704 | Request Nonce | 705 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 707 Figure 2: Updated AMT Request Message Format 709 Bit 14 of the AMT Request Message counting from 0 (or bit 7 of the 710 Reserved field counting from 1) is defined here as the AMT Gateway 711 ECN Capability flag (E), as shown in Figure 2. The definitions of 712 all other fields in the AMT Request Message are unchanged from RFC 713 7450. 715 When the E flag is set to 1, it indicates that the sender of the 716 message supports RFC 6040 ECN propagation. When it is cleared to 717 zero, it indicates the sender of the message does not support RFC 718 6040 ECN propagation. An AMT gateway "that supports RFC 6040 ECN 719 propagation" means one that propagates the ECN field to the forwarded 720 data packet based on the combination of arriving inner and outer ECN 721 fields, as defined in Section 4 of RFC 6040. 723 The other bits of the Reserved field remain reserved. They will 724 continue to be cleared to zero when sent and ignored when either 725 received or forwarded, as specified in Section 5.1.3.3. of RFC 7450. 727 An AMT gateway that does not support RFC 6040 MUST NOT set the E flag 728 of its Request Message to 1. 730 An AMT gateway that supports RFC 6040 ECN propagation MUST set the E 731 flag of its Relay Discovery Message to 1. 733 The action of the corresponding AMT relay that receives a Request 734 message with the E flag set to 1 depends on whether the relay itself 735 supports RFC 6040 ECN propagation: 737 o If the relay supports RFC 6040 ECN propagation, it will store the 738 ECN capability of the gateway along with its address. Then 739 whenever it tunnels datagrams towards this gateway, it MUST use 740 the normal mode of RFC 6040 to propagate the ECN field when 741 encapsulating datagrams (i.e. it copies the IP ECN field from 742 inner to outer). 744 o If the discovered AMT relay does not support RFC 6040 ECN 745 propagation, it will ignore the E flag in the Reserved field, as 746 per section 5.1.3.3. of RFC 7450. 748 If the AMT relay does not support RFC 6040 ECN propagation, the 749 network operator is still expected to configure it to comply with 750 the safety provisions set out in Section 6.1.4.1 above. 752 7. IANA Considerations 754 IANA is requested to assign the following L2TP Control Message 755 Attribute Value Pair: 757 +----------------+----------------+-----------+ 758 | Attribute Type | Description | Reference | 759 +----------------+----------------+-----------+ 760 | ZZ | ECN Capability | RFCXXXX | 761 +----------------+----------------+-----------+ 763 [TO BE REMOVED: This registration should take place at the following 764 location: https://www.iana.org/assignments/l2tp-parameters/l2tp- 765 parameters.xhtml ] 767 8. Security Considerations 769 The Security Considerations in [RFC6040] and 770 [I-D.ietf-tsvwg-ecn-encap-guidelines] apply equally to the scope 771 defined for the present specification. 773 9. Comments Solicited 775 Comments and questions are encouraged and very welcome. They can be 776 addressed to the IETF Transport Area working group mailing list 777 , and/or to the authors. 779 10. Acknowledgements 781 Thanks to Ing-jyh (Inton) Tsang for initial discussions on the need 782 for ECN propagation in L2TP and its applicability. Thanks also to 783 Carlos Pignataro, Tom Herbert, Ignacio Goyret, Alia Atlas, Praveen 784 Balasubramanian, Joe Touch, Mohamed Boucadair, David Black, Jake 785 Holland and Sri Gundavelli for helpful advice and comments. "A 786 Comparison of IPv6-over-IPv4 Tunnel Mechanisms" [RFC7059] helped to 787 identify a number of tunnelling protocols to include within the scope 788 of this document. 790 Bob Briscoe was part-funded by the Research Council of Norway through 791 the TimeIn project. The views expressed here are solely those of the 792 authors. 794 11. References 796 11.1. Normative References 798 [I-D.ietf-tsvwg-ecn-encap-guidelines] 799 Briscoe, B., Kaippallimalil, J., and P. Thaler, 800 "Guidelines for Adding Congestion Notification to 801 Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn- 802 encap-guidelines-13 (work in progress), May 2019. 804 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 805 Requirement Levels", BCP 14, RFC 2119, 806 DOI 10.17487/RFC2119, March 1997, 807 . 809 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 810 "Definition of the Differentiated Services Field (DS 811 Field) in the IPv4 and IPv6 Headers", RFC 2474, 812 DOI 10.17487/RFC2474, December 1998, 813 . 815 [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, 816 G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"", 817 RFC 2661, DOI 10.17487/RFC2661, August 1999, 818 . 820 [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. 821 Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, 822 DOI 10.17487/RFC2784, March 2000, 823 . 825 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 826 of Explicit Congestion Notification (ECN) to IP", 827 RFC 3168, DOI 10.17487/RFC3168, September 2001, 828 . 830 [RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., 831 "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", 832 RFC 3931, DOI 10.17487/RFC3931, March 2005, 833 . 835 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 836 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 837 December 2005, . 839 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through 840 Network Address Translations (NATs)", RFC 4380, 841 DOI 10.17487/RFC4380, February 2006, 842 . 844 [RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion 845 Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January 846 2008, . 848 [RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion 849 Notification", RFC 6040, DOI 10.17487/RFC6040, November 850 2010, . 852 11.2. Informative References 854 [GTPv1] 3GPP, "GPRS Tunnelling Protocol (GTP) across the Gn and Gp 855 interface", Technical Specification TS 29.060. 857 [GTPv1-U] 3GPP, "General Packet Radio System (GPRS) Tunnelling 858 Protocol User Plane (GTPv1-U)", Technical Specification TS 859 29.281. 861 [GTPv2-C] 3GPP, "Evolved General Packet Radio Service (GPRS) 862 Tunnelling Protocol for Control plane (GTPv2-C)", 863 Technical Specification TS 29.274. 865 [I-D.ietf-intarea-gue] 866 Herbert, T., Yong, L., and O. Zia, "Generic UDP 867 Encapsulation", draft-ietf-intarea-gue-09 (work in 868 progress), October 2019. 870 [I-D.ietf-intarea-tunnels] 871 Touch, J. and M. Townsley, "IP Tunnels in the Internet 872 Architecture", draft-ietf-intarea-tunnels-10 (work in 873 progress), September 2019. 875 [I-D.ietf-nvo3-geneve] 876 Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic 877 Network Virtualization Encapsulation", draft-ietf- 878 nvo3-geneve-16 (work in progress), March 2020. 880 [I-D.ietf-nvo3-vxlan-gpe] 881 Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol 882 Extension for VXLAN (VXLAN-GPE)", draft-ietf-nvo3-vxlan- 883 gpe-10 (work in progress), July 2020. 885 [I-D.ietf-sfc-nsh-ecn-support] 886 Eastlake, D., Briscoe, B., and A. Malis, "Explicit 887 Congestion Notification (ECN) and Congestion Feedback 888 Using the Network Service Header (NSH)", draft-ietf-sfc- 889 nsh-ecn-support-03 (work in progress), July 2020. 891 [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic 892 Routing Encapsulation (GRE)", RFC 1701, 893 DOI 10.17487/RFC1701, October 1994, 894 . 896 [RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, 897 W., and G. Zorn, "Point-to-Point Tunneling Protocol 898 (PPTP)", RFC 2637, DOI 10.17487/RFC2637, July 1999, 899 . 901 [RFC2983] Black, D., "Differentiated Services and Tunnels", 902 RFC 2983, DOI 10.17487/RFC2983, October 2000, 903 . 905 [RFC3260] Grossman, D., "New Terminology and Clarifications for 906 Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002, 907 . 909 [RFC3308] Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer 910 Two Tunneling Protocol (L2TP) Differentiated Services 911 Extension", RFC 3308, DOI 10.17487/RFC3308, November 2002, 912 . 914 [RFC5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley, 915 Ed., "Control And Provisioning of Wireless Access Points 916 (CAPWAP) Protocol Specification", RFC 5415, 917 DOI 10.17487/RFC5415, March 2009, 918 . 920 [RFC5845] Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung, 921 "Generic Routing Encapsulation (GRE) Key Option for Proxy 922 Mobile IPv6", RFC 5845, DOI 10.17487/RFC5845, June 2010, 923 . 925 [RFC5944] Perkins, C., Ed., "IP Mobility Support for IPv4, Revised", 926 RFC 5944, DOI 10.17487/RFC5944, November 2010, 927 . 929 [RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility 930 Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July 931 2011, . 933 [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The 934 Locator/ID Separation Protocol (LISP)", RFC 6830, 935 DOI 10.17487/RFC6830, January 2013, 936 . 938 [RFC7059] Steffann, S., van Beijnum, I., and R. van Rein, "A 939 Comparison of IPv6-over-IPv4 Tunnel Mechanisms", RFC 7059, 940 DOI 10.17487/RFC7059, November 2013, 941 . 943 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 944 Kivinen, "Internet Key Exchange Protocol Version 2 945 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 946 2014, . 948 [RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger, 949 L., Sridhar, T., Bursell, M., and C. Wright, "Virtual 950 eXtensible Local Area Network (VXLAN): A Framework for 951 Overlaying Virtualized Layer 2 Networks over Layer 3 952 Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014, 953 . 955 [RFC7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450, 956 DOI 10.17487/RFC7450, February 2015, 957 . 959 [RFC7637] Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network 960 Virtualization Using Generic Routing Encapsulation", 961 RFC 7637, DOI 10.17487/RFC7637, September 2015, 962 . 964 [RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function 965 Chaining (SFC) Architecture", RFC 7665, 966 DOI 10.17487/RFC7665, October 2015, 967 . 969 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage 970 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, 971 March 2017, . 973 [RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using 974 Explicit Congestion Notification (ECN)", RFC 8087, 975 DOI 10.17487/RFC8087, March 2017, 976 . 978 [RFC8159] Konstantynowicz, M., Ed., Heron, G., Ed., Schatzmayr, R., 979 and W. Henderickx, "Keyed IPv6 Tunnel", RFC 8159, 980 DOI 10.17487/RFC8159, May 2017, 981 . 983 [RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation 984 of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229, 985 August 2017, . 987 [RFC8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed., 988 "Network Service Header (NSH)", RFC 8300, 989 DOI 10.17487/RFC8300, January 2018, 990 . 992 Author's Address 994 Bob Briscoe 995 Independent 996 UK 998 EMail: ietf@bobbriscoe.net 999 URI: http://bobbriscoe.net/