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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'CERTREQ' is mentioned on line 148, but not defined Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group V. Smyslov 3 Internet-Draft ELVIS-PLUS 4 Intended status: Standards Track September 10, 2020 5 Expires: March 14, 2021 7 Intermediate Exchange in the IKEv2 Protocol 8 draft-ietf-ipsecme-ikev2-intermediate-05 10 Abstract 12 This documents defines a new exchange, called Intermediate Exchange, 13 for the Internet Key Exchange protocol Version 2 (IKEv2). This 14 exchange can be used for transferring large amount of data in the 15 process of IKEv2 Security Association (SA) establishment. 16 Introducing Intermediate Exchange allows re-using existing IKE 17 fragmentation mechanism, that helps to avoid IP fragmentation of 18 large IKE messages, but cannot be used in the initial IKEv2 exchange. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at https://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on March 14, 2021. 37 Copyright Notice 39 Copyright (c) 2020 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (https://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3 56 3. Intermediate Exchange Details . . . . . . . . . . . . . . . . 3 57 3.1. Support for Intermediate Exchange Negotiation . . . . . . 3 58 3.2. Using Intermediate Exchange . . . . . . . . . . . . . . . 4 59 3.3. The IKE_INTERMEDIATE Exchange Protection and 60 Authentication . . . . . . . . . . . . . . . . . . . . . 5 61 3.3.1. Protection of the IKE_INTERMEDIATE Messages . . . . . 5 62 3.3.2. Authentication of the IKE_INTERMEDIATE Exchanges . . 5 63 3.4. Error Handling in the IKE_INTERMEDIATE Exchange . . . . . 8 64 4. Interaction with other IKEv2 Extensions . . . . . . . . . . . 8 65 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 66 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 67 7. Implementation Status . . . . . . . . . . . . . . . . . . . . 10 68 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 69 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 70 9.1. Normative References . . . . . . . . . . . . . . . . . . 10 71 9.2. Informative References . . . . . . . . . . . . . . . . . 11 72 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11 74 1. Introduction 76 The Internet Key Exchange protocol version 2 (IKEv2) defined in 77 [RFC7296] uses UDP as a transport for its messages. If size of a 78 message is large enough, IP fragmentation takes place, that may 79 interfere badly with some network devices. The problem is described 80 in more detail in [RFC7383], which also defines an extension to the 81 IKEv2 called IKE fragmentation. This extension allows IKE messages 82 to be fragmented at IKE level, eliminating possible issues caused by 83 IP fragmentation. However, the IKE fragmentation cannot be used in 84 the initial IKEv2 exchange (IKE_SA_INIT). This limitation in most 85 cases is not a problem, since the IKE_SA_INIT messages used to be 86 small enough not to cause IP fragmentation. 88 However, the situation has been changing recently. One example of 89 the need to transfer large amount of data before IKE SA is created is 90 using Quantum Computer resistant key exchange methods in IKEv2. 91 Recent progress in Quantum Computing has brought a concern that 92 classical Diffie-Hellman key exchange methods will become insecure in 93 a relatively near future and should be replaced with Quantum Computer 94 (QC) resistant ones. Currently most of QC-resistant key exchange 95 methods have large public keys. If these keys are exchanged in the 96 IKE_SA_INIT, then most probably IP fragmentation will take place, 97 therefore all the problems caused by it will become inevitable. 99 A possible solution to the problem would be to use TCP as a transport 100 for IKEv2, as defined in [RFC8229]. However this approach has 101 significant drawbacks and is intended to be a "last resort" when UDP 102 transport is completely blocked by intermediate network devices. 104 This specification describes a way to transfer large amount of data 105 in IKEv2 using UDP transport. For this purpose the document defines 106 a new exchange for the IKEv2 protocol, called Intermediate Exchange 107 or IKE_INTERMEDIATE. One or more these exchanges may take place 108 right after the IKE_SA_INIT exchange and prior to the IKE_AUTH 109 exchange. The IKE_INTERMEDIATE exchange messages can be fragmented 110 using IKE fragmentation mechanism, so these exchanges may be used to 111 transfer large amounts of data which don't fit into the IKE_SA_INIT 112 exchange without causing IP fragmentation. 114 The Intermediate Exchange can be used to transfer large public keys 115 of QC-resistant key exchange methods, but its application is not 116 limited to this use case. This exchange can also be used whenever 117 some data need to be transferred before the IKE_AUTH exchange and for 118 some reason the IKE_SA_INIT exchange is not suited for this purpose. 119 This document defines the IKE_INTERMEDIATE exchange without tying it 120 to any specific use case. It is expected that separate 121 specifications will define for which purposes and how the 122 IKE_INTERMEDIATE exchange is used in the IKEv2. 124 2. Terminology and Notation 126 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 127 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 128 "OPTIONAL" in this document are to be interpreted as described in BCP 129 14 [RFC2119] [RFC8174] when, and only when, they appear in all 130 capitals, as shown here. 132 It is expected that readers are familiar with the terms used in the 133 IKEv2 specification [RFC7296]. 135 3. Intermediate Exchange Details 137 3.1. Support for Intermediate Exchange Negotiation 139 The initiator indicates its support for Intermediate Exchange by 140 including a notification of type INTERMEDIATE_EXCHANGE_SUPPORTED in 141 the IKE_SA_INIT request message. If the responder also supports this 142 exchange, it includes this notification in the response message. 144 Initiator Responder 145 ----------- ----------- 146 HDR, SAi1, KEi, Ni, 147 [N(INTERMEDIATE_EXCHANGE_SUPPORTED)] --> 148 <-- HDR, SAr1, KEr, Nr, [CERTREQ], 149 [N(INTERMEDIATE_EXCHANGE_SUPPORTED)] 151 The INTERMEDIATE_EXCHANGE_SUPPORTED is a Status Type IKEv2 152 notification. Its Notify Message Type is 16438, Protocol ID and SPI 153 Size are both set to 0. This specification doesn't define any data 154 this notification may contain, so the Notification Data is left 155 empty. However, future enhancements of this specification may 156 override this. Implementations MUST ignore the non-empty 157 Notification Data if they don't understand its purpose. 159 3.2. Using Intermediate Exchange 161 If both peers indicated their support for the Intermediate Exchange, 162 the initiator may use one or more these exchanges to transfer 163 additional data. Using the Intermediate Exchange is optional, the 164 initiator may find it unnecessary even when support for this 165 exchanged has been already negotiated. 167 The Intermediate Exchange is denoted as IKE_INTERMEDIATE, its 168 Exchange Type is 43. 170 Initiator Responder 171 ----------- ----------- 172 HDR, ..., SK {...} --> 173 <-- HDR, ..., SK {...} 175 The initiator may use several IKE_INTERMEDIATE exchanges if 176 necessary. Since window size is initially set to one for both peers 177 (Section 2.3 of [RFC7296]), these exchanges MUST follow each other 178 and MUST all be completed before the IKE_AUTH exchange is initiated. 179 The IKE SA MUST NOT be considered as established until the IKE_AUTH 180 exchange is successfully completed. 182 The Message IDs for IKE_INTERMEDIATE exchanges MUST be chosen 183 according to the standard IKEv2 rule, described in the Section 2.2. 184 of [RFC7296], i.e. it is set to 1 for the first IKE_INTERMEDIATE 185 exchange, 2 for the next (if any) and so on. The Message ID for the 186 first pair of the IKE_AUTH messages is one more than the value used 187 in the last IKE_INTERMEDIATE exchange. 189 If the presence of NAT is detected in the IKE_SA_INIT exchange via 190 NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP 191 notifications, then the peers MUST switch to port 4500 and send all 192 IKE_INTERMEDIATE exchanges using port 4500. 194 The content of the IKE_INTERMEDIATE exchange messages depends on the 195 data being transferred and will be defined by specifications 196 utilizing this exchange. However, since the main motivation for the 197 IKE_INTERMEDIATE exchange is to avoid IP fragmentation when large 198 amount of data need to be transferred prior to IKE_AUTH, the 199 Encrypted payload MUST be present in the IKE_INTERMEDIATE exchange 200 messages and payloads containing large data MUST be placed inside it. 201 This will allow IKE fragmentation [RFC7383] to take place, provided 202 it is supported by the peers and negotiated in the initial exchange. 204 3.3. The IKE_INTERMEDIATE Exchange Protection and Authentication 206 3.3.1. Protection of the IKE_INTERMEDIATE Messages 208 The keys SK_e[i/r] and SK_a[i/r] for the IKE_INTERMEDIATE exchanges 209 protection are computed in a standard fashion, as defined in the 210 Section 2.14 of [RFC7296]. 212 Every subsequent IKE_INTERMEDIATE exchange uses the most recently 213 calculated IKE SA keys before this exchange is started. So, the 214 first IKE_INTERMEDIATE exchange always uses SK_e[i/r] and SK_a[i/r] 215 keys that were computed as a result of the IKE_SA_INIT exchange. If 216 additional key exchange is performed in the first IKE_INTERMEDIATE 217 exchange resulting in the update of SK_e[i/r] and SK_a[i/r], then 218 these updated keys are used for protection of the second 219 IKE_INTERMEDIATE exchange, otherwise the original SK_e[i/r] and 220 SK_a[i/r] keys are used again, and so on. 222 3.3.2. Authentication of the IKE_INTERMEDIATE Exchanges 224 The IKE_INTERMEDIATE messages must be authenticated in the IKE_AUTH 225 exchange, which is performed by adding their content into the AUTH 226 payload calculation. It is anticipated that in many use cases 227 IKE_INTERMEDIATE messages will be fragmented using IKE fragmentation 228 [RFC7383] mechanism. According to [RFC7383], when IKE fragmentation 229 is negotiated, initiator may first send request message in 230 unfragmented form, but later turn IKE fragmentation on and re-send it 231 fragmented if no response is received after few retransmissions. In 232 addition, peers may re-send fragmented message using different 233 fragment sizes to perform simple PMTU discovery. 235 The requirement to support this behavior makes authentication 236 challenging: it is not appropriate to add on-the-wire content of the 237 IKE_INTERMEDIATE messages into the AUTH payload calculation, because 238 peers generally are unaware in which form other side has received 239 them. Instead, a more complex scheme is used - authentication is 240 performed by adding content of these messages before their encryption 241 and possible fragmentation, so that data to be authenticated doesn't 242 depend on the form the messages are delivered in. 244 If any IKE_INTERMEDIATE exchange took place, the definition of the 245 blob to be signed (or MAC'ed) from the Section 2.15 of [RFC7296] is 246 modified as follows: 248 InitiatorSignedOctets = RealMsg1 | NonceRData | MACedIDForI | IntAuth 249 ResponderSignedOctets = RealMsg2 | NonceIData | MACedIDForR | IntAuth 251 IntAuth = IntAuth_1 [| IntAuth_2 [| IntAuth_3 ... ]] 253 IntAuth_1 = IntAuth_1_I | IntAuth_1_R 254 IntAuth_2 = IntAuth_2_I | IntAuth_2_R 255 IntAuth_3 = IntAuth_3_I | IntAuth_3_R 256 ... 258 IntAuth_1_I = prf(SK_pi_1, IntAuth_1_I_A [| IntAuth_1_I_P]) 259 IntAuth_2_I = prf(SK_pi_2, IntAuth_2_I_A [| IntAuth_2_I_P]) 260 IntAuth_3_I = prf(SK_pi_3, IntAuth_3_I_A [| IntAuth_3_I_P]) 261 ... 263 IntAuth_1_R = prf(SK_pr_1, IntAuth_1_R_A [| IntAuth_1_R_P]) 264 IntAuth_2_R = prf(SK_pr_2, IntAuth_2_R_A [| IntAuth_2_R_P]) 265 IntAuth_3_R = prf(SK_pr_3, IntAuth_3_R_A [| IntAuth_3_R_P]) 266 ... 268 IntAuth_1_I/IntAuth_1_R, IntAuth_2_I/IntAuth_2_R, IntAuth_3_I/ 269 IntAuth_3_R, etc. represent the results of applying the negotiated 270 prf to the content of the IKE_INTERMEDIATE messages sent by the 271 initiator (IntAuth_*_I) and by the responder (IntAuth_*_R) in an 272 order of increasing their Message IDs (i.e. in an order the 273 IKE_INTERMEDIATE exchanges took place). The prf is applied to the 274 the concatenation of two chunks of data: mandatory IntAuth_*_[I/R]_A 275 optionally followed by IntAuth_*_[I/R]_P. The IntAuth_*_[I/R]_A 276 chunk lasts from the first octet of the IKE Header (not including 277 prepended four octets of zeros, if port 4500 is used) to the last 278 octet of the Encrypted payload header. The IntAuth_*_[I/R]_P chunk 279 is present if the Encrypted payload is not empty. It consists of the 280 content of the Encrypted payload that is fully formed, but not yet 281 encrypted. The Initialization Vector, the Padding, the Pad Length 282 and the Integrity Checksum Data fields (see Section 3.14 of 283 [RFC7296]) are not included into the calculation. In other words, 284 the IntAuth_*_[I/R]_P chunk is the inner payloads of the Encrypted 285 payload in plaintext form. 287 1 2 3 288 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 1 289 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ ^ 290 | IKE SA Initiator's SPI | | | 291 | | | | 292 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I | 293 | IKE SA Responder's SPI | K | 294 | | E | 295 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 296 | Next Payload | MjVer | MnVer | Exchange Type | Flags | H | 297 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ d | 298 | Message ID | r A 299 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | 300 | Adjusted Length | | | 301 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v | 302 | | | 303 ~ Unencrypted payloads (if any) ~ | 304 | | | 305 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ | 306 | Next Payload |C| RESERVED | Adjusted Payload Length | | | 307 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ E v 308 | Initialization Vector | n 309 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ c ^ 310 | | r | 311 ~ Inner payloads (not yet encrypted) ~ P 312 | | P | 313 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ l v 314 | Padding (0-255 octets) | Pad Length | d 315 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 316 ~ Integrity Checksum Data ~ | 317 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 319 Figure 1: Data to Authenticate in the IKE_INTERMEDIATE Exchange 320 Messages 322 Figure 1 illustrates the layout of the IntAuth_*_[I/R]_P (denoted as 323 P) and the IntAuth_*_[I/R]_A (denoted as A) chunks in case the 324 Encrypted payload is not empty. 326 For the purpose of prf calculation the Length field in the IKE header 327 and the Payload Length field in the Encrypted payload header are 328 adjusted so that they don't count the lengths of Initialization 329 Vector, Integrity Checksum Data, Padding and Pad Length fields. In 330 other words, the Length field in the IKE header (denoted as Adjusted 331 Length in Figure 1) is set to the sum of the lengths of IntAuth_*_[I/ 332 R]_A and IntAuth_*_[I/R]_P, and the Payload Length field in the 333 Encrypted payload header (denoted as Adjusted Payload Length in 334 Figure 1) is set to the length of IntAuth_*_[I/R]_P plus the size of 335 the Encrypted payload header (four octets). 337 The prf calculations MUST be applied to whole messages only, before 338 possible IKE fragmentation. This ensures that the IntAuth will be 339 the same regardless of whether IKE fragmentation takes place or not. 340 If the message was received in fragmented form, it MUST be 341 reconstructed before calculating prf as if it were received 342 unfragmented. While reconstructing, the RESERVED field in the 343 reconstructed Encrypted payload header MUST be set to the value of 344 the RESERVED field in the Encrypted Fragment payload header from the 345 first fragment (with Fragment Number field set to 1). 347 Note that it is possible to avoid actual reconstruction of the 348 message by incrementally calculating prf on decrypted (or ready to be 349 encrypted) fragments. However care must be taken to properly replace 350 the content of the Next Header and the Length fields so that the 351 result of computing prf is the same as if it were computed on 352 reconstructed message. 354 Each calculation of IntAuth_*_[I/R] uses its own keys SK_p[i/r]_*, 355 which are the most recently updated SK_p[i/r] keys available before 356 the corresponded IKE_INTERMEDIATE exchange is started. The first 357 IKE_INTERMEDIATE exchange always uses SK_p[i/r] keys that were 358 computed in the IKE_SA_INIT as SK_p[i/r]_1. If the first 359 IKE_INTERMEDIATE exchange performs additional key exchange resulting 360 in SK_p[i/r] update, then this updated SK_p[i/r] are used as SK_p[i/ 361 r]_2, otherwise the original SK_p[i/r] are used, and so on. Note, 362 that if keys are updated then for any given IKE_INTERMEDIATE exchange 363 the keys SK_e[i/r] and SK_a[i/r] used for its messages protection 364 (see Section 3.3.1) and the keys SK_p[i/r] for its authentication are 365 always from the same generation. 367 3.4. Error Handling in the IKE_INTERMEDIATE Exchange 369 Since messages of the IKE_INTERMEDIATE exchange are not authenticated 370 until the IKE_AUTH exchange successfully completes, possible errors 371 need to be handled with care. There is a trade-off between providing 372 a better diagnostics of the problem and a risk to become a part of 373 DoS attack. See Section 2.21.1 and 2.21.2 of [RFC7296] describe how 374 errors are handled in initial IKEv2 exchanges, these considerations 375 are also applied to the IKE_INTERMEDIATE exchange. 377 4. Interaction with other IKEv2 Extensions 379 The IKE_INTERMEDIATE exchanges MAY be used during the IKEv2 Session 380 Resumption [RFC5723] between the IKE_SESSION_RESUME and the IKE_AUTH 381 exchanges. To be able to use it peers MUST negotiate support for 382 intermediate exchange by including INTERMEDIATE_EXCHANGE_SUPPORTED 383 notifications in the IKE_SESSION_RESUME messages. Note, that a flag 384 whether peers supported the IKE_INTERMEDIATE exchange is not stored 385 in the resumption ticket and is determined each time from the 386 IKE_SESSION_RESUME exchange. 388 5. Security Considerations 390 The data that is transferred by means of the IKE_INTERMEDIATE 391 exchanges is not authenticated until the subsequent IKE_AUTH exchange 392 is completed. However, if the data is placed inside the Encrypted 393 payload, then it is protected from passive eavesdroppers. In 394 addition the peers can be certain that they receives messages from 395 the party they performed the IKE_SA_INIT with if they can 396 successfully verify the Integrity Checksum Data of the Encrypted 397 payload. 399 The main application for Intermediate Exchange is to transfer large 400 amount of data before IKE SA is set up without causing IP 401 fragmentation. For that reason it is expected that in most cases IKE 402 fragmentation will be employed in the IKE_INTERMEDIATE exchanges. 403 Section 5 of [RFC7383] contains security considerations for IKE 404 fragmentation. 406 Note, that if an attacker was able to break key exchange in real time 407 (e.g. by means of Quantum Computer), then the security of the 408 IKE_INTERMEDIATE exchange would degrade. In particular, such an 409 attacker would be able both to read data contained in the Encrypted 410 payload and to forge it. The forgery would become evident in the 411 IKE_AUTH exchange (provided the attacker cannot break employed 412 authentication mechanism), but the ability to inject forged the 413 IKE_INTERMEDIATE exchange messages with valid ICV would allow the 414 attacker to mount Denial-of-Service attack. Moreover, if in this 415 situation the negotiated prf was not secure against preimage attack 416 with known key, then the attacker could forge the IKE_INTERMEDIATE 417 exchange messages without later being detected in the IKE_AUTH 418 exchange. To do this the attacker should find the same 419 IntAuth_*_[I|R] value for the forged message as for original. 421 6. IANA Considerations 423 This document defines a new Exchange Type in the "IKEv2 Exchange 424 Types" registry: 426 43 IKE_INTERMEDIATE 428 This document also defines a new Notify Message Type in the "Notify 429 Message Types - Status Types" registry: 431 16438 INTERMEDIATE_EXCHANGE_SUPPORTED 433 7. Implementation Status 435 [Note to RFC Editor: please, remove this section before publishing 436 RFC.] 438 At the time of writing the -05 version of the draft there were at 439 least three independent interoperable implementations of this 440 specifications from the following vendors: 442 o ELVIS-PLUS 444 o strongSwan 446 o libreswan (only one IKE_INTERMEDIATE exchange is supported) 448 8. Acknowledgements 450 The idea to use an intermediate exchange between IKE_SA_INIT and 451 IKE_AUTH was first suggested by Tero Kivinen. Scott Fluhrer and 452 Daniel Van Geest identified a possible problem with authentication of 453 the IKE_INTERMEDIATE exchange and helped to resolve it. Author is 454 also grateful to Tobias Brunner for raising good points concerning 455 authentication of the IKE_INTERMEDIATE exchange and to Paul Wouters 456 who suggested text improvements for the document. 458 9. References 460 9.1. Normative References 462 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 463 Requirement Levels", BCP 14, RFC 2119, 464 DOI 10.17487/RFC2119, March 1997, 465 . 467 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 468 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 469 May 2017, . 471 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 472 Kivinen, "Internet Key Exchange Protocol Version 2 473 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 474 2014, . 476 [RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2 477 (IKEv2) Message Fragmentation", RFC 7383, 478 DOI 10.17487/RFC7383, November 2014, 479 . 481 9.2. Informative References 483 [RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation 484 of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229, 485 August 2017, . 487 [RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange 488 Protocol Version 2 (IKEv2) Session Resumption", RFC 5723, 489 DOI 10.17487/RFC5723, January 2010, 490 . 492 Author's Address 494 Valery Smyslov 495 ELVIS-PLUS 496 PO Box 81 497 Moscow (Zelenograd) 124460 498 RU 500 Phone: +7 495 276 0211 501 Email: svan@elvis.ru