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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 CableLabs 4 Updates: 6040, 2661, 2784, 3931, 4380, November 12, 2017 5 7450 (if approved) 6 Intended status: Standards Track 7 Expires: May 16, 2018 9 Propagating Explicit Congestion Notification Across IP Tunnel Headers 10 Separated by a Shim 11 draft-ietf-tsvwg-rfc6040update-shim-05 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 separated by such shim header(s) and updates the 22 specifications of those that do not mention ECN propagation (L2TPv2, 23 L2TPv3, GRE, Teredo and AMT). This specification also updates RFC 24 6040 with configuration requirements needed to make any legacy tunnel 25 ingress 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 16, 2018. 44 Copyright Notice 46 Copyright (c) 2017 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. IP-in-IP Tunnels with Tightly Coupled Shim Headers . . . . . 6 68 5.1. Specific Updates to Protocols under IETF Change Control . 9 69 5.1.1. L2TP (v2 and v3) ECN Extension . . . . . . . . . . . 9 70 5.1.2. GRE . . . . . . . . . . . . . . . . . . . . . . . . . 12 71 5.1.3. Teredo . . . . . . . . . . . . . . . . . . . . . . . 13 72 5.1.4. AMT . . . . . . . . . . . . . . . . . . . . . . . . . 14 73 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 74 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 75 8. Comments Solicited . . . . . . . . . . . . . . . . . . . . . 16 76 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 77 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 78 10.1. Normative References . . . . . . . . . . . . . . . . . . 17 79 10.2. Informative References . . . . . . . . . . . . . . . . . 18 80 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 82 1. Introduction 84 RFC 6040 on "Tunnelling of Explicit Congestion Notification" 85 [RFC6040] made the rules for propagation of Explicit Congestion 86 Notification (ECN [RFC3168]) consistent for all forms of IP in IP 87 tunnel. 89 A common pattern for many tunnelling protocols is to encapsulate an 90 inner IP header (v4 or v6) with shim header(s) then an outer IP 91 header (v4 or v6). Some of these shim headers are designed as 92 generic encapsulations, so they do not necessarily directly 93 encapsulate an inner IP header. Instead they can encapsulate headers 94 such as link-layer (L2) protocols that in turn often encapsulate IP. 96 To clear up confusion, this specification clarifies that the scope of 97 RFC 6040 includes any IP-in-IP tunnel, including those with shim 98 header(s) and other encapsulations between the IP headers. Where 99 necessary, it updates the specifications of the relevant 100 encapsulation protocols with the specific text necessary to comply 101 with RFC 6040. 103 This specification also updates RFC 6040 to state how operators ought 104 to configure a legacy tunnel ingress to avoid unsafe system 105 configurations. 107 2. Terminology 109 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 110 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 111 document are to be interpreted as described in RFC 2119 [RFC2119] 112 when, and only when, they appear in all capitals, as shown here. 114 This specification uses the terminology defined in RFC 6040 115 [RFC6040]. 117 3. Scope of RFC 6040 119 In section 1.1 of RFC 6040, its scope is defined as: 121 "...ECN field processing at encapsulation and decapsulation for 122 any IP-in-IP tunnelling, whether IPsec or non-IPsec tunnels. It 123 applies irrespective of whether IPv4 or IPv6 is used for either 124 the inner or outer headers. ..." 126 This was intended to include cases where shim header(s) sit between 127 the IP headers. Many tunnelling implementers have interpreted the 128 scope of RFC 6040 as it was intended, but it is ambiguous. 129 Therefore, this specification updates RFC 6040 by adding the 130 following scoping text after the sentences quoted above: 132 It applies in cases where an outer IP header encapsulates an inner 133 IP header either directly or indirectly by encapsulating other 134 headers that in turn encapsulate (or might encapsulate) an inner 135 IP header. 137 There is another problem with the scope of RFC 6040. Like many IETF 138 specifications, RFC 6040 is written as a specification that 139 implementations can choose to claim compliance with. This means it 140 does not cover two important cases: 142 1. those cases where it is infeasible for an implementation to 143 access an inner IP header when adding or removing an outer IP 144 header; 146 2. those implementations that choose not to propagate ECN between IP 147 headers. 149 However, the ECN field is a non-optional part of the IP header (v4 150 and v6). So any implementation that creates an outer IP header has 151 to give the ECN field some value. There is only one safe value a 152 tunnel ingress can use if it does not know whether the egress 153 supports propagation of the ECN field; it has to clear the ECN field 154 in any outer IP header to 0b00. 156 However, an RFC has no jurisdiction over implementations that choose 157 not to comply with it or cannot comply with it, including all those 158 implementations that pre-dated the RFC. Therefore it would have been 159 unreasonable to add such a requirement to RFC 6040. Nonetheless, to 160 ensure safe propagation of the ECN field over tunnels, it is 161 reasonable to add requirements on operators, to ensure they configure 162 their tunnels safely (where possible). Before stating these 163 configuration requirements in Section 4, the factors that determine 164 whether propagating ECN is feasible or desirable will be briefly 165 introduced. 167 3.1. Feasibility of ECN Propagation between Tunnel Headers 169 In many cases shim header(s) and an outer IP header are always added 170 to (or removed from) an inner IP packet as part of the same 171 procedure. We call this a tightly coupled shim header. Processing 172 the shim and outer together is often necessary because the shim(s) 173 are not sufficient for packet forwarding in their own right; not 174 unless complemented by an outer header. In these cases it will often 175 be feasible for an implementation to propagate the ECN field between 176 the IP headers. 178 In some cases a tunnel adds an outer IP header and a tightly coupled 179 shim header to an inner header that is not an IP header, but that in 180 turn encapsulates an IP header (or might encapsulate an IP header). 181 For instance an inner Ethernet (or other link layer) header might 182 encapsulate an inner IP header as its payload. We call this a 183 tightly coupled shim over an encapsulating header. 185 Digging to arbitrary depths to find an inner IP header within an 186 encapsulation is strictly a layering violation so it cannot be a 187 required behaviour. Nonetheless, some tunnel endpoints already look 188 within a L2 header for an IP header, for instance to map the Diffserv 189 codepoint between an encapsulated IP header and an outer IP header 191 [RFC2983]. In such cases at least, it should be feasible to also 192 (independently) propagate the ECN field between the same IP headers. 193 Thus, access to the ECN field within an encapsulating header can be a 194 useful and benign optimization. The guidelines in section 6 of 195 [I-D.ietf-tsvwg-ecn-encap-guidelines] give the conditions for this 196 layering violation to be benign. 198 3.2. Desirability of ECN Propagation between Tunnel Headers 200 Developers and network operators are encouraged to implement and 201 deploy tunnel endpoints compliant with RFC 6040 (as updated by the 202 present specification) in order to provide the benefits of wider ECN 203 deployment [RFC8087]. Nonetheless, propagation of ECN between IP 204 headers, whether separated by shim headers or not, has to be optional 205 to implement and to use, because: 207 o Legacy implementations of tunnels without any ECN support already 208 exist 210 o A network might be designed so that there is usually no bottleneck 211 within the tunnel 213 o If the tunnel endpoints would have to search within an L2 header 214 to find an encapsulated IP header, it might not be worth the 215 potential performance hit 217 4. Making a non-ECN Tunnel Ingress Safe by Configuration 219 Even when no specific attempt has been made to implement propagation 220 of the ECN field at a tunnel ingress, it ought to be possible for the 221 operator to render a tunnel ingress safe by configuration. The main 222 safety concern is to disable (clear to zero) the ECN capability in 223 the outer IP header at the ingress if the egress of the tunnel does 224 not implement ECN logic to propagate any ECN markings into the packet 225 forwarded beyond the tunnel. Otherwise the non-ECN egress could 226 discard any ECN marking introduced within the tunnel, which would 227 break all the ECN-based control loops that regulate the traffic load 228 over the tunnel. 230 Therefore this specification updates RFC 6040 by inserting the 231 following text just before the last paragraph of section 4.3: 233 When the outer tunnel header is IP (v4 or v6), if possible, the 234 operator MUST configure the ingress to zero the outer ECN field in 235 any of the following cases: 237 * if it is known that the tunnel egress does not support 238 propagation of the ECN field (RFC 6040, RFC 4301 or the full 239 functionality mode of RFC 3168) 241 * or if the behaviour of the egress is not known or an egress 242 with unknown behaviour might be dynamically paired with the 243 ingress. 245 * or if an IP header might be encapsulated within a non-IP header 246 that the tunnel ingress is encapsulating, but the ingress does 247 not inspect within the encapsulation. 249 In order that the network operator can comply with the above safety 250 rules, even if a tunnel ingress does not claim to support RFC 6040, 251 RFC 4301 or the full functionality mode of RFC 3168, the 252 implementation of the tunnel ingress: 254 o MUST make propagation of the ECN field between inner and outer IP 255 headers independent of any configuration of Diffserv codepoint 256 propagation; 258 o SHOULD zero the outer ECN field in its default configuration. 260 There might be concern that the above "MUST" makes compliant 261 equipment non-compliant at a stroke. However, any equipment that is 262 still treating the ToS octet (IPv4) or the Traffic Class octet (IPv6) 263 as a single 8-bit field is already non-compliant, and has been since 264 1998 when the upper 6 bits were separated off for the Diffserv 265 codepoint (DSCP) [RFC2474]. For instance, copying the ECN field as a 266 side-effect of copying the DSCP is a seriously unsafe bug that risks 267 breaking the feedback loops that regulate load on a tunnel. 269 Permanently zeroing the outer ECN field is safe, but it is not 270 sufficient to claim compliance with RFC 6040 because it does not meet 271 the aim of introducing ECN support to tunnels (see Section 4.3 of 272 [RFC6040]). 274 5. IP-in-IP Tunnels with Tightly Coupled Shim Headers 276 There follows a list of specifications of encapsulations with tightly 277 coupled shim header(s), in rough chronological order. The list is 278 confined to standards track or widely deployed protocols. The list 279 is not necessarily exhaustive so, for the avoidance of doubt, the 280 scope of RFC 6040 is defined in Section 3 and is not limited to this 281 list. 283 o PPTP (Point-to-Point Tunneling Protocol) [RFC2637]; 284 o L2TP (Layer 2 Tunnelling Protocol), specifically L2TPv2 [RFC2661] 285 and L2TPv3 [RFC3931], which not only includes all the L2-specific 286 specializations of L2TP, but also derivatives such as the Keyed 287 IPv6 Tunnel [RFC8159]; 289 o GRE (Generic Routing Encapsulation) [RFC2784] and NVGRE (Network 290 Virtualization using GRE) [RFC7637]; 292 o GTP (GPRS Tunnelling Protocol), specifically GTPv1 [GTPv1], GTP v1 293 User Plane [GTPv1-U], GTP v2 Control Plane [GTPv2-C]; 295 o Teredo [RFC4380]; 297 o CAPWAP (Control And Provisioning of Wireless Access Points) 298 [RFC5415]; 300 o LISP (Locator/Identifier Separation Protocol) [RFC6830]; 302 o AMT (Automatic Multicast Tunneling) [RFC7450]; 304 o VXLAN (Virtual eXtensible Local Area Network) [RFC7348] and VXLAN- 305 GPE [I-D.ietf-nvo3-vxlan-gpe]; 307 o SFC (Service Function Chaining) [RFC7665]; 309 o Geneve [I-D.ietf-nvo3-geneve]; 311 o GUE (Generic UDP Encapsulation) [I-D.ietf-intarea-gue]; 313 o Direct tunnelling of an IP packet within a UDP/IP datagram (see 314 Section 3.1.11 of [RFC8085]); 316 o TCP Encapsulation of IKE and IPsec Packets (see Section 12.5 of 317 [RFC8229]). 319 Some of the listed protocols enable encapsulation of a variety of 320 network layer protocols as inner and/or outer. This specification 321 applies in the cases where there is an inner and outer IP header as 322 described in Section 3. Otherwise 323 [I-D.ietf-tsvwg-ecn-encap-guidelines] gives guidance on how to design 324 propagation of ECN into other protocols that might encapsulate IP. 326 Where protocols in the above list need to be updated to specify ECN 327 propagation and they are under IETF change control, update text is 328 given in the following subsections. For those not under IETF 329 control, it is RECOMMENDED that implementations of encapsulation and 330 decapsulation comply with RFC 6040. It is also RECOMMENDED that 331 their specifications are updated to add a requirement to comply with 332 RFC 6040 (as updated by the present document). 334 PPTP is not under the change control of the IETF, but it has been 335 documented in an informational RFC [RFC2637]. However, there is no 336 need for the present specification to update PPTP because L2TP has 337 been developed as a standardized replacement. 339 NVGRE is not under the change control of the IETF, but it has been 340 documented in an informational RFC [RFC7637]. NVGRE is a specific 341 use-case of GRE (it re-purposes the key field from the initial 342 specification of GRE [RFC1701] as a Virtual Subnet ID). Therefore 343 the text that updates GRE in Section 5.1.2 below is also intended to 344 update NVGRE. 346 Although the definition of the various GTP shim headers is under the 347 control of the 3GPP, it is hard to determine whether the 3GPP or the 348 IETF controls standardization of the _process_ of adding both a GTP 349 and an IP header to an inner IP header. Nonetheless, the present 350 specification is provided so that the 3GPP can refer to it from any 351 of its own specifications of GTP and IP header processing. 353 The specification of CAPWAP already specifies RFC 3168 ECN 354 propagation and ECN capability negotiation. Without modification the 355 CAPWAP specification already interworks with the backward compatible 356 updates to RFC 3168 in RFC 6040. 358 LISP made the ECN propagation procedures in RFC 3168 mandatory from 359 the start. RFC 3168 has since been updated by RFC 6040, but the 360 changes are backwards compatible so there is still no need for LISP 361 tunnel endpoints to negotiate their ECN capabilities. 363 VXLAN is not under the change control of the IETF but it has been 364 documented in an informational RFC. In contrast, VXLAN-GPE (Generic 365 Protocol Extension) is being documented under IETF change control. 366 It is RECOMMENDED that VXLAN and VXLAN-GPE implementations comply 367 with RFC 6040 when the VXLAN header is inserted between (or removed 368 from between) IP headers. The authors of any future update to these 369 specifications are encouraged to add a requirement to comply with RFC 370 6040 as updated by the present specification. 372 Although the Service Function Chaining (SFC) architecture [RFC7665] 373 depends on a shim-based encapsulation to identify the service 374 function path (SFP), it does not specify the processing of ECN when 375 handling transport encapsulation. 377 The specifications of Geneve and GUE already refer to RFC 6040 for 378 ECN encapsulation. 380 Section 3.1.11 of the UDP usage guidelines [RFC8085] already explains 381 that a tunnel that encapsulates an IP header directly within a UDP/IP 382 datagram needs to follow RFC 6040 when propagating the ECN field 383 between inner and outer IP headers. The requirements in Section 4 384 update RFC 6040 so, by reference, they automatically update RFC 8085 385 to add the important but previously unstated requirement that, if the 386 UDP tunnel egress does not, or might not, support ECN propagation, a 387 legacy UDP tunnel ingress has to clear the outer IP ECN field to 388 0b00, e.g. by configuration. 390 Section 12.5 of TCP Encapsulation of IKE and IPsec Packets [RFC8229] 391 already recommends the compatibility mode of RFC 6040 in this case, 392 because there is not a one-to-one mapping between inner and outer 393 packets. 395 5.1. Specific Updates to Protocols under IETF Change Control 397 5.1.1. L2TP (v2 and v3) ECN Extension 399 The L2TP terminology used here is defined in [RFC2661] and [RFC3931]. 401 L2TPv3 [RFC3931] is used as a shim header between any packet-switched 402 network (PSN) header (e.g. IPv4, IPv6, MPLS) and many types of layer 403 2 (L2) header. The L2TPv3 shim header encapsulates an L2-specific 404 sub-layer then an L2 header that is likely to contain an inner IP 405 header (v4 or v6). Then this whole stack of headers can be 406 encapsulated optionally within an outer UDP header then an outer PSN 407 header that is typically IP (v4 or v6). 409 L2TPv2 is used as a shim header between any PSN header and a PPP 410 header, which is in turn likely to encapsulate an IP header. 412 Even though these shims are rather fat (particularly in the case of 413 L2TPv3), they still fit the definition of a tightly coupled shim 414 header over an encapsulating header (Section 3.1), because all the 415 headers encapsulating the L2 header are added (or removed) together. 416 L2TPv2 and L2TPv3 are therefore within the scope of RFC 6040, as 417 updated by Section 3 above. 419 L2TP maintainers are RECOMMENDED to implement the ECN extension to 420 L2TPv2 and L2TPv3 defined in Section 5.1.1.2 below, in order to 421 provide the benefits of ECN [RFC8087], whenever a node within an L2TP 422 tunnel becomes the bottleneck for an end-to-end traffic flow. 424 5.1.1.1. Safe Configuration of a 'Non-ECN' Ingress LCCE 426 The following text is appended to both Section 5.3 of [RFC2661] and 427 Section 4.5 of [RFC3931] as an update to the base L2TPv2 and L2TPv3 428 specifications: 430 The operator of an LCCE that does not support the ECN Extension in 431 Section 5.1.1.2 of RFCXXXX MUST follow the configuration 432 requirements in Section 4 of RFCXXXX to ensure it clears the outer 433 IP ECN field to 0b00 when the outer PSN header is IP (v4 or v6). 434 {RFCXXXX refers to the present document so it will need to be 435 inserted by the RFC Editor} 437 In particular, for an LCCE implementation that does not support the 438 ECN Extension, this means that configuration of how it propagates the 439 ECN field between inner and outer IP headers MUST be independent of 440 any configuration of the Diffserv extension of L2TP [RFC3308]. 442 5.1.1.2. ECN Extension for L2TP (v2 or v3) 444 When the outer PSN header and the payload inside the L2 header are 445 both IP (v4 or v6), to comply with RFC 6040, an LCCE will follow the 446 rules for propagation of the ECN field at ingress and egress in 447 Section 4 of RFC 6040 [RFC6040]. 449 Before encapsulating any data packets, RFC 6040 requires an ingress 450 LCCE to check that the egress LCCE supports ECN propagation as 451 defined in RFC 6040 or one of its compatible predecessors ([RFC4301] 452 or the full functionality mode of [RFC3168]). If the egress supports 453 ECN propagation, the ingress LCCE can use the normal mode of 454 encapsulation (copying the ECN field from inner to outer). 455 Otherwise, the ingress LCCE has to use compatibility mode [RFC6040] 456 (clearing the outer IP ECN field to 0b00). 458 An LCCE can determine the remote LCCE's support for ECN either 459 statically (by configuration) or by dynamic discovery during setup of 460 each control connection between the LCCEs, using the Capability AVP 461 defined in Section 5.1.1.2.1 below. 463 Where the outer PSN header is some protocol other than IP that 464 supports ECN, the appropriate ECN propagation specification will need 465 to be followed, e.g. "Explicit Congestion Marking in MPLS" 466 [RFC5129]. Where no specification exists for ECN propagation by a 467 particular PSN, [I-D.ietf-tsvwg-ecn-encap-guidelines] gives general 468 guidance on how to design ECN propagation into a protocol that 469 encapsulates IP. 471 5.1.1.2.1. LCCE Capability AVP for ECN Capability Negotiation 473 The LCCE Capability Attribute-Value Pair (AVP) defined here has 474 Attribute Type ZZ. The Attribute Value field for this AVP is a bit- 475 mask with the following 16-bit format: 477 0 1 478 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 479 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 480 |X X X X X X X X X X X X X X X E| 481 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 483 Figure 1: Value Field for the LCCE Capability Attribute 485 This AVP MAY be present in the following message types: SCCRQ and 486 SCCRP (Start-Control-Connection-Request and Start-Control-Connection- 487 Reply). This AVP MAY be hidden (the H-bit set to 0 or 1) and is 488 optional (M-bit not set). The length (before hiding) of this AVP 489 MUST be 8 octets. The Vendor ID is the IETF Vendor ID of 0. 491 Bit 15 of the Value field of the LCCE Capability AVP is defined as 492 the ECN Capability flag (E). When the ECN Capability flag is set to 493 1, it indicates that the sender supports ECN propagation. When the 494 ECN Capability flag is cleared to zero, or when no LCCE Capabiliy AVP 495 is present, it indicates that the sender does not support ECN 496 propagation. All the other bits are reserved. They MUST be cleared 497 to zero when sent and ignored when received or forwarded. 499 An LCCE initiating a control connection will send a Start-Control- 500 Connection-Request (SCCRQ) containing an LCCE Capability AVP with the 501 ECN Capability flag set to 1. If the tunnel terminator supports ECN, 502 it will return a Start-Control-Connection-Reply (SCCRP) that also 503 includes an LCCE Capability AVP with the ECN Capability flag set to 504 1. Then, for any sessions created by that control connection, both 505 ends of the tunnel can use the normal mode of RFC 6040, i.e. it can 506 copy the IP ECN field from inner to outer when encapsulating data 507 packets. 509 If, on the other hand, the tunnel terminator does not support ECN it 510 will ignore the ECN flag in the LCCE Capability AVP and send an SCCRP 511 to the tunnel initiator without a Capability AVP (or with a 512 Capability AVP but with the ECN Capability flag cleared to zero). 513 The tunnel initiator interprets the absence of the ECN Capability 514 flag in the SCCRP as an indication that the tunnel terminator is 515 incapable of supporting ECN. When encapsulating data packets for any 516 sessions created by that control connection, the tunnel initiator 517 will then use the compatibility mode of RFC 6040 to clear the ECN 518 field of the outer IP header to 0b00. 520 If the tunnel terminator does not support this ECN extension, the 521 network operator is still expected to configure it to comply with the 522 safety provisions set out in Section 5.1.1.1 above, when it acts as 523 an ingress LCCE. 525 5.1.2. GRE 527 The GRE terminology used here is defined in [RFC2784]. GRE is often 528 used as a tightly coupled shim header between IP headers. Sometimes 529 the GRE shim header encapsulates an L2 header, which might in turn 530 encapsulate an IP header. Therefore GRE is within the scope of RFC 531 6040 as updated by Section 3 above. 533 GRE tunnel endpoint maintainers are RECOMMENDED to support [RFC6040] 534 as updated by the present specification, in order to provide the 535 benefits of ECN [RFC8087] whenever a node within a GRE tunnel becomes 536 the bottleneck for an end-to-end IP traffic flow tunnelled over GRE 537 using IP as the delivery protocol (outer header). 539 GRE itself does not support dynamic set-up and configuration of 540 tunnels. However, control plane protocols such as Mobile IPv4 (MIP4) 541 [RFC5944], Mobile IPv6 (MIP6) [RFC6275], Proxy Mobile IP (PMIP) 542 [RFC5845] and IKEv2 [RFC5996] are sometimes used to set up GRE 543 tunnels dynamically. 545 When these control protocols set up IP-in-IP or IPSec tunnels, it is 546 likely that they propagate the ECN field as defined in RFC 6040 or 547 one of its compatible predecessors (RFC 4301 or the full 548 functionality mode of RFC 3168). However, if they use a GRE 549 encapsulation, this presumption is less sound. 551 Therefore, If the outer delivery protocol is IP (v4 or v6) the 552 operator is obliged to follow the safe configuration requirements in 553 Section 4 above. Section 5.1.2.1 below updates the base GRE 554 specification with this requirement, to emphasize its importance. 556 Where the delivery protocol is some protocol other than IP that 557 supports ECN, the appropriate ECN propagation specification will need 558 to be followed, e.g Explicit Congestion Marking in MPLS [RFC5129]. 559 Where no specification exists for ECN propagation by a particular 560 PSN, [I-D.ietf-tsvwg-ecn-encap-guidelines] gives more general 561 guidance on how to propagate ECN to and from protocols that 562 encapsulate IP. 564 5.1.2.1. Safe Configuration of a 'Non-ECN' GRE Ingress 566 The following text is appended to Section 3 of [RFC2784] as an update 567 to the base GRE specification: 569 The operator of a GRE tunnel ingress MUST follow the configuration 570 requirements in Section 4 of RFCXXXX when the outer delivery 571 protocol is IP (v4 or v6). {RFCXXXX refers to the present 572 document so it will need to be inserted by the RFC Editor} 574 5.1.3. Teredo 576 Teredo [RFC4380] provides a way to tunnel IPv6 over an IPv4 network, 577 with a UDP-based shim header between the two. 579 For Teredo tunnel endpoints to provide the benefits of ECN, the 580 Teredo specification would have to be updated to include negotiation 581 of the ECN capability between Teredo tunnel endpoints. Otherwise it 582 would be unsafe for a Teredo tunnel ingress to copy the ECN field to 583 the IPv6 outer. 585 It is believed that current implementations do not support 586 propagation of ECN, but that they do safely zero the ECN field in the 587 outer IPv6 header. However the specification does not mention 588 anything about this. 590 To make existing Teredo deployments safe, it would be possible to add 591 ECN capability negotiation to those that are subject to remote OS 592 update. However, for those implementations not subject to remote OS 593 update, it will not be feasible to require them to be configured 594 correctly, because Teredo tunnel endpoints are generally deployed on 595 hosts. 597 Therefore, until ECN support is added to the specification of Teredo, 598 the only feasible further safety precaution available here is to 599 update the specification of Teredo implementations with the following 600 text, as a new section 5.1.3: 602 "5.1.3 Safe 'Non-ECN' Teredo Encapsulation 604 A Teredo tunnel ingress implementation that does not support ECN 605 propagation as defined in RFC 6040 or one of its compatible 606 predecessors (RFC 4301 or the full functionality mode of RFC 3168) 607 MUST zero the ECN field in the outer IPv6 header." 609 5.1.4. AMT 611 Automatic Multicast Tunneling (AMT [RFC7450]) is a tightly coupled 612 shim header that encapsulates an IP packet and is itself encapsulated 613 within a UDP/IP datagram. Therefore AMT is within the scope of RFC 614 6040 as updated by Section 3 above. 616 AMT tunnel endpoint maintainers are RECOMMENDED to support [RFC6040] 617 as updated by the present specification, in order to provide the 618 benefits of ECN [RFC8087] whenever a node within an AMT tunnel 619 becomes the bottleneck for an IP traffic flow tunnelled over AMT. 621 To comply with RFC 6040, an AMT relay and gateway will follow the 622 rules for propagation of the ECN field at ingress and egress 623 respectively, as described in Section 4 of RFC 6040 [RFC6040]. 625 Before encapsulating any data packets, RFC 6040 requires an ingress 626 AMT relay to check that the egress AMT gateway supports ECN 627 propagation as defined in RFC 6040 or one of its compatible 628 predecessors (RFC 4301 or the full functionality mode of RFC 3168). 629 If the egress gateway supports ECN, the ingress relay can use the 630 normal mode of encapsulation (copying the IP ECN field from inner to 631 outer). Otherwise, the ingress relay has to use compatibility mode, 632 which means it has to clear the outer ECN field to zero [RFC6040]. 634 An AMT tunnel is created dynamically (not manually), so the relay 635 will need to determine the remote gateway's support for ECN using the 636 ECN capability declaration defined in Section 5.1.4.2 below. 638 5.1.4.1. Safe Configuration of a 'Non-ECN' Ingress AMT Relay 640 The following text is appended to Section 4.2.2 of [RFC7450] as an 641 update to the AMT specification: 643 The operator of an AMT relay that does not support RFC 6040 or one 644 of its compatible predecessors (RFC 4301 or the full functionality 645 mode of RFC 3168) MUST follow the configuration requirements in 646 Section 4 of RFCXXXX to ensure it clears the outer IP ECN field to 647 zero. {RFCXXXX refers to the present document so it will need to 648 be inserted by the RFC Editor} 650 5.1.4.2. ECN Capability Declaration of an AMT Gateway 651 0 1 2 3 652 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 1 653 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 654 | V=0 |Type=3 | Reserved |E|P| Reserved | 655 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 656 | Request Nonce | 657 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 659 Figure 2: Updated AMT Request Message Format 661 Bit 14 of the AMT Request Message counting from 0 (or bit 7 of the 662 Reserved field counting from 1) is defined here as the AMT Gateway 663 ECN Capability flag (E), as shown in Figure 2. The definitions of 664 all other fields in the AMT Request Message are unchanged from RFC 665 7450. 667 When the E flag is set to 1, it indicates that the sender of the 668 message supports RFC 6040 ECN propagation. When it is cleared to 669 zero, it indicates the sender of the message does not support RFC 670 6040 ECN propagation. An AMT gateway "that supports RFC 6040 ECN 671 propagation" means one that propagates the ECN field to the forwarded 672 data packet based on the combination of arriving inner and outer ECN 673 fields, as defined in Section 4 of RFC 6040. 675 The other bits of the Reserved field remain reserved. They will 676 continue to be cleared to zero when sent and ignored when either 677 received or forwarded, as specified in Section 5.1.3.3. of RFC 7450. 679 An AMT gateway that does not support RFC 6040 MUST NOT set the E flag 680 of its Request Message to 1. 682 An AMT gateway that supports RFC 6040 ECN propagation MUST set the E 683 flag of its Relay Discovery Message to 1. 685 The action of the corresponding AMT relay that receives a Request 686 message with the E flag set to 1 depends on whether the relay itself 687 supports RFC 6040 ECN propagation: 689 o If the relay supports RFC 6040 ECN propagation, it will store the 690 ECN capability of the gateway along with its address. Then 691 whenever it tunnels datagrams towards this gateway, it MUST use 692 the normal mode of RFC 6040 to propagate the ECN field when 693 encapsulating datagrams (i.e. it copies the IP ECN field from 694 inner to outer). 696 o If the discovered AMT relay does not support RFC 6040 ECN 697 propagation, it will ignore the E flag in the Reserved field, as 698 per section 5.1.3.3. of RFC 7450. 700 If the AMT relay does not support RFC 6040 ECN propagation, the 701 network operator is still expected to configure it to comply with 702 the safety provisions set out in Section 5.1.4.1 above. 704 6. IANA Considerations 706 IANA is requested to assign the following L2TP Control Message 707 Attribute Value Pair: 709 +----------------+----------------+-----------+ 710 | Attribute Type | Description | Reference | 711 +----------------+----------------+-----------+ 712 | ZZ | ECN Capability | RFCXXXX | 713 +----------------+----------------+-----------+ 715 [TO BE REMOVED: This registration should take place at the following 716 location: https://www.iana.org/assignments/l2tp-parameters/l2tp- 717 parameters.xhtml ] 719 7. Security Considerations 721 The Security Considerations in [RFC6040] and 722 [I-D.ietf-tsvwg-ecn-encap-guidelines] apply equally to the scope 723 defined for the present specification. 725 8. Comments Solicited 727 Comments and questions are encouraged and very welcome. They can be 728 addressed to the IETF Transport Area working group mailing list 729 , and/or to the authors. 731 9. Acknowledgements 733 Thanks to Ing-jyh (Inton) Tsang for initial discussions on the need 734 for ECN propagation in L2TP and its applicability. Thanks also to 735 Carlos Pignataro, Tom Herbert, Ignacio Goyret, Alia Atlas, Praveen 736 Balasubramanian, Joe Touch, Mohamed Boucadair, David Black, Jake 737 Holland and Sri Gundavelli for helpful advice and comments. "A 738 Comparison of IPv6-over-IPv4 Tunnel Mechanisms" [RFC7059] helped to 739 identify a number of tunnelling protocols to include within the scope 740 of this document. 742 Bob Briscoe was part-funded by the Research Council of Norway through 743 the TimeIn project. The views expressed here are solely those of the 744 authors. 746 10. References 748 10.1. Normative References 750 [I-D.ietf-tsvwg-ecn-encap-guidelines] 751 Briscoe, B., Kaippallimalil, J., and P. Thaler, 752 "Guidelines for Adding Congestion Notification to 753 Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn- 754 encap-guidelines-09 (work in progress), July 2017. 756 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 757 Requirement Levels", BCP 14, RFC 2119, 758 DOI 10.17487/RFC2119, March 1997, 759 . 761 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 762 "Definition of the Differentiated Services Field (DS 763 Field) in the IPv4 and IPv6 Headers", RFC 2474, 764 DOI 10.17487/RFC2474, December 1998, 765 . 767 [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, 768 G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"", 769 RFC 2661, DOI 10.17487/RFC2661, August 1999, 770 . 772 [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. 773 Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, 774 DOI 10.17487/RFC2784, March 2000, 775 . 777 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 778 of Explicit Congestion Notification (ECN) to IP", 779 RFC 3168, DOI 10.17487/RFC3168, September 2001, 780 . 782 [RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., 783 "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", 784 RFC 3931, DOI 10.17487/RFC3931, March 2005, 785 . 787 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 788 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 789 December 2005, . 791 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through 792 Network Address Translations (NATs)", RFC 4380, 793 DOI 10.17487/RFC4380, February 2006, 794 . 796 [RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion 797 Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January 798 2008, . 800 [RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion 801 Notification", RFC 6040, DOI 10.17487/RFC6040, November 802 2010, . 804 10.2. Informative References 806 [GTPv1] 3GPP, "GPRS Tunnelling Protocol (GTP) across the Gn and Gp 807 interface", Technical Specification TS 29.060. 809 [GTPv1-U] 3GPP, "General Packet Radio System (GPRS) Tunnelling 810 Protocol User Plane (GTPv1-U)", Technical Specification TS 811 29.281. 813 [GTPv2-C] 3GPP, "Evolved General Packet Radio Service (GPRS) 814 Tunnelling Protocol for Control plane (GTPv2-C)", 815 Technical Specification TS 29.274. 817 [I-D.ietf-intarea-gue] 818 Herbert, T., Yong, L., and O. Zia, "Generic UDP 819 Encapsulation", draft-ietf-intarea-gue-04 (work in 820 progress), May 2017. 822 [I-D.ietf-nvo3-geneve] 823 Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic 824 Network Virtualization Encapsulation", draft-ietf- 825 nvo3-geneve-05 (work in progress), September 2017. 827 [I-D.ietf-nvo3-vxlan-gpe] 828 Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol 829 Extension for VXLAN", draft-ietf-nvo3-vxlan-gpe-05 (work 830 in progress), October 2017. 832 [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic 833 Routing Encapsulation (GRE)", RFC 1701, 834 DOI 10.17487/RFC1701, October 1994, 835 . 837 [RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, 838 W., and G. Zorn, "Point-to-Point Tunneling Protocol 839 (PPTP)", RFC 2637, DOI 10.17487/RFC2637, July 1999, 840 . 842 [RFC2983] Black, D., "Differentiated Services and Tunnels", 843 RFC 2983, DOI 10.17487/RFC2983, October 2000, 844 . 846 [RFC3308] Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer 847 Two Tunneling Protocol (L2TP) Differentiated Services 848 Extension", RFC 3308, DOI 10.17487/RFC3308, November 2002, 849 . 851 [RFC5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley, 852 Ed., "Control And Provisioning of Wireless Access Points 853 (CAPWAP) Protocol Specification", RFC 5415, 854 DOI 10.17487/RFC5415, March 2009, 855 . 857 [RFC5845] Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung, 858 "Generic Routing Encapsulation (GRE) Key Option for Proxy 859 Mobile IPv6", RFC 5845, DOI 10.17487/RFC5845, June 2010, 860 . 862 [RFC5944] Perkins, C., Ed., "IP Mobility Support for IPv4, Revised", 863 RFC 5944, DOI 10.17487/RFC5944, November 2010, 864 . 866 [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, 867 "Internet Key Exchange Protocol Version 2 (IKEv2)", 868 RFC 5996, DOI 10.17487/RFC5996, September 2010, 869 . 871 [RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility 872 Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July 873 2011, . 875 [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The 876 Locator/ID Separation Protocol (LISP)", RFC 6830, 877 DOI 10.17487/RFC6830, January 2013, 878 . 880 [RFC7059] Steffann, S., van Beijnum, I., and R. van Rein, "A 881 Comparison of IPv6-over-IPv4 Tunnel Mechanisms", RFC 7059, 882 DOI 10.17487/RFC7059, November 2013, 883 . 885 [RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger, 886 L., Sridhar, T., Bursell, M., and C. Wright, "Virtual 887 eXtensible Local Area Network (VXLAN): A Framework for 888 Overlaying Virtualized Layer 2 Networks over Layer 3 889 Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014, 890 . 892 [RFC7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450, 893 DOI 10.17487/RFC7450, February 2015, 894 . 896 [RFC7637] Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network 897 Virtualization Using Generic Routing Encapsulation", 898 RFC 7637, DOI 10.17487/RFC7637, September 2015, 899 . 901 [RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function 902 Chaining (SFC) Architecture", RFC 7665, 903 DOI 10.17487/RFC7665, October 2015, 904 . 906 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage 907 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, 908 March 2017, . 910 [RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using 911 Explicit Congestion Notification (ECN)", RFC 8087, 912 DOI 10.17487/RFC8087, March 2017, 913 . 915 [RFC8159] Konstantynowicz, M., Ed., Heron, G., Ed., Schatzmayr, R., 916 and W. Henderickx, "Keyed IPv6 Tunnel", RFC 8159, 917 DOI 10.17487/RFC8159, May 2017, 918 . 920 [RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation 921 of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229, 922 August 2017, . 924 Author's Address 926 Bob Briscoe 927 CableLabs 928 UK 930 EMail: ietf@bobbriscoe.net 931 URI: http://bobbriscoe.net/