idnits 2.17.1 draft-ietf-ipsecme-ikev2-fragmentation-05.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (November 3, 2013) is 3826 days in the past. Is this intentional? 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 171, but not defined ** Obsolete normative reference: RFC 5996 (Obsoleted by RFC 7296) -- Obsolete informational reference (is this intentional?): RFC 2460 (Obsoleted by RFC 8200) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). 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 November 3, 2013 5 Expires: May 7, 2014 7 IKEv2 Fragmentation 8 draft-ietf-ipsecme-ikev2-fragmentation-05 10 Abstract 12 This document describes the way to avoid IP fragmentation of large 13 IKEv2 messages. This allows IKEv2 messages to traverse network 14 devices that don't allow IP fragments to pass through. 16 Status of this Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on May 5, 2014. 33 Copyright Notice 35 Copyright (c) 2013 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 1.1. Conventions Used in This Document . . . . . . . . . . . . 3 52 2. Protocol details . . . . . . . . . . . . . . . . . . . . . . . 4 53 2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4 54 2.2. Limitations . . . . . . . . . . . . . . . . . . . . . . . 4 55 2.3. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 4 56 2.4. Using IKE Fragmentation . . . . . . . . . . . . . . . . . 5 57 2.5. Fragmenting Message . . . . . . . . . . . . . . . . . . . 6 58 2.5.1. Selecting Fragment Size . . . . . . . . . . . . . . . 8 59 2.5.2. PMTU Discovery . . . . . . . . . . . . . . . . . . . . 8 60 2.5.3. Fragmenting Messages containing unencrypted 61 Payloads . . . . . . . . . . . . . . . . . . . . . . . 10 62 2.6. Receiving IKE Fragment Message . . . . . . . . . . . . . . 10 63 2.6.1. Changes in Replay Protection Logic . . . . . . . . . . 12 64 3. Interaction with other IKE extensions . . . . . . . . . . . . 13 65 4. Transport Considerations . . . . . . . . . . . . . . . . . . . 14 66 5. Security Considerations . . . . . . . . . . . . . . . . . . . 15 67 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 68 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 69 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 70 8.1. Normative References . . . . . . . . . . . . . . . . . . . 18 71 8.2. Informative References . . . . . . . . . . . . . . . . . . 18 72 Appendix A. Design rationale . . . . . . . . . . . . . . . . . . 20 73 Appendix B. Correlation between IP Datagram size and 74 Encrypted Payload content size . . . . . . . . . . . 21 75 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22 77 1. Introduction 79 The Internet Key Exchange Protocol version 2 (IKEv2), specified in 80 [RFC5996], uses UDP as a transport for its messages. Most IKEv2 81 messages are relatively small, usually below several hundred bytes. 82 Noticeable exception is IKE_AUTH exchange, which requires fairly 83 large messages, up to several kbytes, especially when certificates 84 are transferred. When IKE message size exceeds path MTU, it gets 85 fragmented by IP level. The problem is that some network devices, 86 specifically some NAT boxes, don't allow IP fragments to pass 87 through. This apparently blocks IKE communication and, therefore, 88 prevents peers from establishing IPsec SA. 90 Widespread deployment of Carrier-Grade NATs (CGN) introduces new 91 challenges. RFC6888 [RFC6888] describes requirements for CGNs. It 92 states, that CGNs must comply with Section 11 of RFC4787 [RFC4787], 93 which requires NAT to support receiving IP fragments (REQ-14). In 94 real life fulfillment of this requirement creates an additional 95 burden in terms of memory, especially for high-capacity devices, used 96 in CGNs. It was found by people deploying IKE, that some ISPs have 97 begun to drop IP fragments, violating that requirement. 99 The solution to the problem described in this document is to perform 100 fragmentation of large messages by IKE itself, replacing them by 101 series of smaller messages. In this case the resulting IP Datagrams 102 will be small enough so that no fragmentation on IP level will take 103 place. 105 Avoiding IP fragmentation is beneficial for IKEv2 in general. 106 Security Considerations Section of [RFC5996] mentions exhausting of 107 the IP reassembly buffers as one of possible attacks on the protocol. 108 In the paper [DOSUDPPROT] several aspects of attacks on IKE using IP 109 fragmentation are discussed, and one of defenses it proposes is to 110 perform IKE-level fragmentation, similar to the solution, described 111 in this document. 113 1.1. Conventions Used in This Document 115 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 116 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 117 document are to be interpreted as described in [RFC2119]. 119 2. Protocol details 121 2.1. Overview 123 The idea of the protocol is to split large IKE message into a set of 124 smaller ones, called IKE Fragment Messages. Fragmentation takes 125 place before the original message is encrypted and authenticated, so 126 that each IKE Fragment Message receives individual protection. On 127 the receiving side IKE Fragment Messages are collected, verified, 128 decrypted and merged together to get the original message before 129 encryption. For design rationale see Appendix A. 131 2.2. Limitations 133 As IKE Fragment Messages are cryptographically protected, SK_a and 134 SK_e must already be calculated. In general, it means that original 135 message can be fragmented if and only if it contains Encrypted 136 Payload. 138 This implies that messages of the IKE_SA_INIT Exchange cannot be 139 fragmented. In most cases this is not a problem, since IKE_SA_INIT 140 messages are usually small enough to avoid IP fragmentation. But in 141 some cases (advertising a badly structured long list of algorithms, 142 using large MODP Groups, etc.) these messages may become fairly large 143 and get fragmented by IP level. In this case the described solution 144 won't help. 146 Among existing IKEv2 extensions, messages of IKE_SESSION_RESUME 147 Exchange, defined in [RFC5723], cannot be fragmented either. See 148 Section 3 for details. 150 Another limitation is that the minimal size of IP Datagram bearing 151 IKE Fragment Message is about 100 bytes depending on the algorithms 152 employed. According to [RFC0791] the minimum IP Datagram size that 153 is guaranteed not to be further fragmented is 68 bytes. So, even the 154 smallest IKE Fragment Messages could be fragmented by IP level in 155 some circumstances. But such extremely small PMTU sizes are very 156 rare in real life. 158 2.3. Negotiation 160 Initiator MAY indicate its support for IKE Fragmentation and 161 willingness to use it by including Notification Payload of type 162 IKE_FRAGMENTATION_SUPPORTED in IKE_SA_INIT request message. If 163 Responder also supports this extension and is willing to use it, it 164 includes this notification in response message. 166 Initiator Responder 167 ----------- ----------- 168 HDR, SAi1, KEi, Ni, 169 [N(IKE_FRAGMENTATION_SUPPORTED)] --> 171 <-- HDR, SAr1, KEr, Nr, [CERTREQ], 172 [N(IKE_FRAGMENTATION_SUPPORTED)] 174 The Notify payload is formatted as follows: 176 1 2 3 177 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 1 178 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 179 | Next Payload |C| RESERVED | Payload Length | 180 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 181 |Protocol ID(=0)| SPI Size (=0) | Notify Message Type | 182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 184 o Protocol ID (1 octet) MUST be 0. 186 o SPI Size (1 octet) MUST be 0, meaning no SPI is present. 188 o Notify Message Type (2 octets) - MUST be xxxxx, the value assigned 189 for IKE_FRAGMENTATION_SUPPORTED by IANA. 191 This Notification contains no data. 193 2.4. Using IKE Fragmentation 195 IKE Fragmentation MUST NOT be used unless both peers indicated their 196 support for it. After IKE Fragmentation is negotiated, it is up to 197 Initiator of each Exchange, whether to use it or not. In most cases 198 IKE Fragmentation will be used in IKE_AUTH Exchange, especially if 199 certificates are employed. Initiator may first try to send 200 unfragmented message and resend it fragmented only if it didn't 201 receive response after several retransmissions, or it may always send 202 messages fragmented (but see Section 3), or it may fragment only 203 large messages and messages causing large responses. 205 In general the following guidelines are applicable for initiator: 207 o Initiator MAY fragment outgoing message if it has some knowledge 208 (possibly from lower layer or from configuration) or suspicions 209 that either request or response message will be fragmented by IP 210 level. 212 o Initiator SHOULD fragment outgoing message if it has some 213 knowledge (possibly from lower layer or from configuration) or 214 suspicions that either request or response message will be 215 fragmented by IP level and IKE Fragmentation was already used in 216 one of previous Exchanges in the context of the current IKE SA. 218 o Initiator SHOULD NOT fragment outgoing message if both request and 219 response messages of the Exchange are small enough not to cause 220 fragmentation on IP level (for example, there is no point in 221 fragmenting Liveness Check messages). 223 In general the following guidelines are applicable for responder: 225 o Responder SHOULD send response message in the same form 226 (fragmented or not) as corresponded request message. If it 227 received unfragmented request message, responded with unfragmented 228 response message and then receives fragmented retransmission of 229 the same request, it SHOULD resend its response back to Initiator 230 fragmented. 232 o Responder MAY respond to unfragmented message with fragmented 233 response if it has some knowledge (possibly from lower layer or 234 from configuration) or suspicions that response message will be 235 fragmented by IP level. 237 o Responder MAY respond to fragmented message with unfragmented 238 response if the size of the response message is less than the 239 smallest fragmentation threshold, supported by Responder (for 240 example, there is no point in fragmenting Liveness Check 241 messages). 243 2.5. Fragmenting Message 245 Message to be fragmented MUST contain Encrypted Payload. For the 246 purpose of IKE Fragment Messages construction original (unencrypted) 247 content of Encrypted Payload is split into chunks. The content is 248 treated as a binary blob and is split regardless of inner Payloads 249 boundaries. Each of resulting chunks is treated as an original 250 content of Encrypted Fragment Payload and is then encrypted and 251 authenticated. Thus, the Encrypted Fragment Payload contains a chunk 252 of the original content of Encrypted Payload in encrypted form. The 253 cryptographic processing of Encrypted Fragment Payload is identical 254 to Section 3.14 of [RFC5996], as well as documents updating it for 255 particular algorithms or modes, such as [RFC5282]. 257 The Encrypted Fragment Payload, similarly to the Encrypted Payload, 258 if present in a message, MUST be the last payload in the message. 260 The Encrypted Fragment Payload is denoted SKF{...} and its payload 261 type is XXX (TBA by IANA). 263 1 2 3 264 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 1 265 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 266 | Next Payload |C| RESERVED | Payload Length | 267 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 268 | Fragment Number | Total Fragments | 269 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 270 | Initialization Vector | 271 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 272 ~ Encrypted content ~ 273 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 274 | | Padding (0-255 octets) | 275 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ 276 | | Pad Length | 277 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 278 ~ Integrity Checksum Data ~ 279 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 281 Encrypted Fragment Payload 283 o Next Payload (1 octet) - in the very first fragment MUST be set to 284 Payload Type of the first inner Payload (similarly to the 285 Encrypted Payload). In the rest fragments MUST be set to zero. 287 o Fragment Number (2 octets) - current fragment number starting from 288 1. This field MUST be less than or equal to the next field, Total 289 Fragments. This field MUST NOT be zero. 291 o Total Fragments (2 octets) - number of fragments original message 292 was divided into. With PMTU discovery this field plays additional 293 role. See Section 2.5.2 for details. This field MUST NOT be 294 zero. 296 The other fields are identical to those specified in Section 3.14 of 297 [RFC5996]. 299 When prepending IKE Header, Length field MUST be adjusted to reflect 300 the length of constructed message and Next Payload field MUST reflect 301 payload type of the first Payload in the constructed message (that in 302 most cases will be Encrypted Fragment Payload). All newly 303 constructed messages MUST retain the same Message ID as original 304 message. After prepending IKE Header and possibly any of Payloads 305 that precedes Encrypted Payload in original message (see 306 Section 2.5.3), the resulting messages are sent to the peer. 308 Below is an example of fragmenting a message. 310 HDR(MID=n), SK(NextPld=PLD1) {PLD1 ... PLDN} 311 Original Message 313 HDR(MID=n), SKF(NextPld=PLD1, Frag#=1, TotalFrags=m) {...}, 314 HDR(MID=n), SKF(NextPld=0, Frag#=2, TotalFrags=m) {...}, 315 ... 316 HDR(MID=n), SKF(NextPld=0, Frag#=m, TotalFrags=m) {...} 318 IKE Fragment Messages 320 2.5.1. Selecting Fragment Size 322 When splitting content of Encrypted into chunks sender SHOULD chose 323 size of those chunks so, that resulting IP Datagram size not exceed 324 some fragmentation threshold - be small enough to avoid IP 325 fragmentation. 327 If sender has some knowledge about PMTU size it MAY use it. If 328 sender is a Responder in the Exchange and it has received fragmented 329 request, it MAY use maximum size of received IKE Fragment Message IP 330 Datagrams as threshold when constructing fragmented response. 332 Otherwise for messages to be sent over IPv6 it is RECOMMENDED to use 333 value 1280 bytes as a maximum IP Datagram size ([RFC2460]). For 334 messages to be sent over IPv4 it is RECOMMENDED to use value 576 335 bytes as a maximum IP Datagram size. Presence of tunnels on the path 336 may reduce these values. 338 According to [RFC0791] the minimum IPv4 datagram size that is 339 guaranteed not to be further fragmented is 68 bytes, but it is 340 generally impossible to use such small value for solution, described 341 in this document. Using 576 bytes is a compromise - the value is 342 large enough for the presented solution and small enough to avoid IP 343 fragmentation in most situations. Several other UDP-based protocol 344 assume the value 576 bytes as a safe low limit for IP datagrams size 345 (Syslog, DNS, etc.). Sender MAY use other values if they are 346 appropriate. 348 See Appendix B for correlation between IP Datagram size and Encrypted 349 Payload content size. 351 2.5.2. PMTU Discovery 353 Initiator MAY try to discover path MTU by probing several values of 354 fragmentation threshold. While doing probes, node MUST start from 355 larger values and refragment message with next smaller value if it 356 doesn't receive response in a reasonable time after several 357 retransmissions. This time is supposed to be relatively short, so 358 that node could make all desired probes before exchange times out. 359 When starting new probe (with smaller threshold) node MUST reset its 360 retransmission timers so, that if it employs exponential back-off, 361 the timers start over. After reaching the smallest allowed value for 362 fragmentation threshold implementation MUST continue probing using it 363 untill either exchange completes ot times out. 365 PMTU discovery in IKE is supposed to be coarse-grained, i.e. it is 366 expected, that node will try only few fragmentation thresholds, in 367 order to minimize possible IKE SA establishment delay. In a corner 368 case, when host will use only one value, PMTU discovery will 369 effectively be disabled. In most cases PMTU discovery will not be 370 needed, as using values, recommended in section Section 2.5.1, should 371 suffice. It is expected, that PMTU discovery may be useful in 372 environments where PMTU size are smaller, than those listed in 373 Section 2.5.1, for example due to the presence of intermediate 374 tunnels. 376 PMTU discovery in IKE follows recommendations, given in Section 10.4 377 of RFC4821 [RFC4821] with some differences, induced by the 378 specialities of IKE. In particular: 380 o Unlike classical PMTUD [RFC1191] and PLMTUD [RFC4821] the goal of 381 Path MTU discovery in IKE is not to find the largest size of IP 382 packet, that will not be fragmented en route, but to find any 383 reasonal size, probably far from optimal. 385 o There is no goal to completely disallow IP fragmentation until its 386 presence leads to inability IKE to communicate (e.g. when IP 387 fragments are dropped) 389 o IKE usually sends large messages only in IKE_AUTH exchange, i.e. 390 once per IKE SA. Most of other messages will have size below 391 several hundred bytes. Performing full PMTUD for sending exactly 392 one large message is inefficient. 394 In case of PMTU discovery Total Fragments field is used to 395 distinguish between different sets of fragments, i.e. the sets that 396 were obtained by fragmenting original message using different 397 fragmentation thresholds. As sender will start from larger fragments 398 and then make them smaller, the value in Total Fragments field will 399 increase with each new try. When selecting next smaller value of 400 fragmentation threshold, sender MUST ensure that the value in Total 401 Fragments field is really increased. This requirement should not 402 become a problem for the sender, as PMTU discovery in IKE is supposed 403 to be coarse-grained, so difference between previous and next 404 fragmentation thresholds will be significant anyway. The necessity 405 to distinguish between the sets is vital for receiver as receiving 406 any valid fragment from newer set will mean that it have to start 407 reassembling over and not to mix fragments from different sets. 409 2.5.3. Fragmenting Messages containing unencrypted Payloads 411 Currently no one of IKEv2 Exchanges defines messages, containing both 412 unencrypted payloads and payloads, protected by Encrypted Payload. 413 But IKEv2 doesn't forbid such messages. If some future IKEv2 414 extension defines such a message and it needs to be fragmented, all 415 unprotected payloads MUST be in the first fragment, along with 416 Encrypted Fragment Payload, which MUST be present in any IKE Fragment 417 Message. 419 Below is an example of fragmenting message, containing both encrypted 420 and unencrypted Payloads. 422 HDR(MID=n), PLD0, SK(NextPld=PLD1) {PLD1 ... PLDN} 424 Original Message 426 HDR(MID=n), PLD0, SKF(NextPld=PLD1, Frag#=1, TotalFrags=m) {...}, 427 HDR(MID=n), SKF(NextPld=0, Frag#=2, TotalFrags=m) {...}, 428 ... 429 HDR(MID=n), SKF(NextPld=0, Frag#=m, TotalFrags=m) {...} 431 IKE Fragment Messages 433 Note, that the size of each IP Datagram bearing IKE Fragment Messages 434 SHOULD NOT exceed fragmentation threshold, including the very first, 435 which contains unprotected Payloads. This will reduce the size of 436 Encrypted Fragment Payload content in the first IKE Fragment Message 437 to accommodate unprotected Payloads. In extreme cases Encrypted 438 Fragment Payload will contain no data, but it is still MUST be 439 present in the message, because only its presence allows receiver to 440 distinguish IKE Fragment Message from regular IKE message. 442 2.6. Receiving IKE Fragment Message 444 Receiver identifies IKE Fragment Message by the presence of Encrypted 445 Fragment Payload in it. Note, that it is possible for this payload 446 to be not the first (and the only) payload in the message (see 447 Section 2.5.3). But for all currently defined IKEv2 exchanges this 448 payload will be the first and the only payload in the message. 450 Upon receiving IKE Fragment Message the following actions are 451 performed: 453 o Check message validity - in particular, check whether values of 454 Fragment Number and Total Fragments in Encrypted Fragment Payload 455 are valid. The following tests need to be performed. 457 * check that Fragment Number and Total Fragments fields are non- 458 zero 460 * check that Fragment Number field is less than or equal to Total 461 Fragments field 463 * if reassembling has already started, check that Total Fragments 464 field is equal to or greater than Total Fragments field in 465 fragments, that have already received 467 If any of this tests fails message MUST be silently discarded. 469 o Check, that this IKE Fragment Message is new for the receiver and 470 not a replay. If IKE Fragment message with the same Message ID, 471 same Fragment Number and same Total Fragments fields was already 472 received and successfully processed, this message is considered a 473 replay and MUST be silently discarded. 475 o Verify IKE Fragment Message authenticity by checking ICV in 476 Encrypted Fragment Payload. If ICV check fails message MUST be 477 silently discarded. 479 o If reassembling isn't finished yet and Total Fragments field in 480 received IKE Fragment Message is greater than this field in 481 previously received fragments, receiver MUST discard all received 482 fragments and start reassembling over with just received IKE 483 Fragment Message. 485 o Store message in the list waiting for the rest of fragments to 486 arrive. 488 When all IKE Fragment Messages (as indicated in the Total Fragments 489 field) are received, content of their already decrypted Encrypted 490 Fragment Payloads is merged together to form content of original 491 Encrypted Payload, and, therefore, along with IKE Header and 492 unencrypted Payloads (if any), original message. Then it is 493 processed as if it was received, verified and decrypted as regular 494 unfragmented message. 496 If receiver does't get all IKE Fragment Messages needed to reassemble 497 original Message for some Exchange within a timeout interval, it acts 498 according with Section 2.1 of [RFC5996], i.e. retransmits the 499 fragmented request Message (in case of Initiator) or deems Exchange 500 to have failed. If Exchange is abandoned, all received so far IKE 501 Fragment Messages for that Exchange MUST be discarded. 503 2.6.1. Changes in Replay Protection Logic 505 According to [RFC5996] IKEv2 MUST reject message with the same 506 Message ID as it has seen before (taking into consideration Response 507 bit). This logic has already been updated by [RFC6311], which 508 deliberately allows any number of messages with zero Message ID. 509 This document also updates this logic: if message contains Encrypted 510 Fragment Payload, the values of Fragment Number and Total Fragments 511 fields from this payload MUST be used along with Message ID to detect 512 retransmissions and replays. 514 If Responder receives IKE Fragment Message after it received, 515 successfully verified and processed regular message with the same 516 Message ID, it means that response message didn't reach Initiator and 517 it activated IKE Fragmentation. If Fragment Number in Encrypted 518 Fragment Payload in this message is equal to 1, Responder MUST 519 fragment its response and retransmit it back to Initiator in 520 fragmented form. 522 If Responder receives a replay IKE Fragment Message for already 523 reassembled, verified and processed fragmented message, it MUST 524 retransmit response back to Initiator, but only if Fragment Number 525 field in Encrypted Fragment Payload is equal to 1 and MUST silently 526 discard received message otherwise. If Total Fragments field in 527 received IKE Fragment Message is greater than in IKE Fragment 528 Messages that already processed fragmented message was reassembled 529 from, Responder MAY refragment its response message using smaller 530 fragmentation threshold before resending it back to Initiator. In 531 this case Total Fragments field in new IKE Fragment Messages MUST be 532 greater than in previously sent IKE Fragment Messages. 534 If Initiator doesn't receive any of response IKE Fragment Messages 535 withing a timeout interval, it MAY refragment request Message using 536 smaller fragmentation threshold before retransmitting it (see 537 Section 2.5.1). In this case Total Fragments field in new IKE 538 Fragment Messages MUST be greater than in previously sent IKE 539 Fragment Messages. Alternatively, if Initiator does receive some 540 (but not all) of response IKE Fragment Messages, it MAY retransmit 541 only the first of request IKE Fragment Messages, where Fragment 542 Number field is equal to 1. 544 3. Interaction with other IKE extensions 546 IKE Fragmentation is compatible with most of defined IKE extensions, 547 like IKE Session Resumption [RFC5723], Quick Crash Detection Method 548 [RFC6290] and so on. It neither affect their operation, nor is 549 affected by them. It is believed that IKE Fragmentation will also be 550 compatible with most future IKE extensions, if they follow general 551 principles of formatting, sending and receiving IKE messages, 552 described in [RFC5996]. 554 When IKE Fragmentation is used with IKE Session Resumption [RFC5723], 555 messages of IKE_SESSION_RESUME Exchange cannot be fragmented as they 556 don't contain Encrypted Payload. These messages may be large due to 557 ticket size. If this is the case the described solution won't help. 558 To avoid IP Fragmentation in this situation it is recommended to use 559 smaller tickets, e.g. by utilizing "ticket by reference" approach 560 instead of "ticket by value". 562 One exception that requires a special care is [RFC6311] - Protocol 563 Support for High Availability of IKEv2. As it deliberately allows 564 any number of synchronization Exchanges to have the same Message ID - 565 zero, standard replay detection logic, based on checking Message ID 566 is not applicable for such messages, and receiver has to check 567 message content to detect replays. When implementing IKE 568 Fragmentation along with [RFC6311], IKE Message ID Synchronization 569 messages MUST NOT be sent fragmented to simplify receiver's task of 570 detecting replays. Fortunately, these messages are small and there 571 is no point in fragmenting them anyway. 573 4. Transport Considerations 575 With IKE Fragmentation if any single IKE Fragment Message get lost, 576 receiver becomes unable to reassemble original Message. So, in 577 general, using IKE Fragmentation implies higher probability for the 578 Message not to be delivered to the peer. Although in most network 579 environments the difference will be insignificant, on some lossy 580 networks it may become noticeable. When using IKE Fragmentation 581 implementations MAY use longer timeouts and do more retransmits 582 before considering peer dead. 584 Note that Fragment Messages are not individually acknowledged. The 585 response Fragment Messages are sent back all together only when all 586 fragments of request are received, the original request Message is 587 reassembled and successfully processed. 589 5. Security Considerations 591 Most of the security considerations for IKE Fragmentation are the 592 same as those for base IKEv2 protocol described in [RFC5996]. This 593 extension introduces Encrypted Fragment Payload to protect content of 594 IKE Message Fragment. This allows receiver to individually check 595 authenticity of fragments, thus protecting peers from DoS attack. 597 Security Considerations Section of [RFC5996] mentions possible attack 598 on IKE by exhausting of the IP reassembly buffers. The mechanism, 599 described in this document, allows IKE to avoid IP-fragmentation and 600 therefore increases its robustness to DoS attacks. 602 6. IANA Considerations 604 This document defines new Payload in the "IKEv2 Payload Types" 605 registry: 607 Encrypted Fragment Payload SKF 609 This document also defines new Notify Message Types in the "Notify 610 Messages Types - Status Types" registry: 612 IKE_FRAGMENTATION_SUPPORTED 614 7. Acknowledgements 616 The author would like to thank Tero Kivinen, Yoav Nir, Paul Wouters, 617 Yaron Sheffer and others for their reviews and valueable comments. 619 8. References 621 8.1. Normative References 623 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 624 Requirement Levels", BCP 14, RFC 2119, March 1997. 626 [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, 627 "Internet Key Exchange Protocol Version 2 (IKEv2)", 628 RFC 5996, September 2010. 630 [RFC6311] Singh, R., Kalyani, G., Nir, Y., Sheffer, Y., and D. 631 Zhang, "Protocol Support for High Availability of IKEv2/ 632 IPsec", RFC 6311, July 2011. 634 8.2. Informative References 636 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 637 September 1981. 639 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 640 November 1990. 642 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 643 (IPv6) Specification", RFC 2460, December 1998. 645 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 646 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 647 RFC 4787, January 2007. 649 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 650 Discovery", RFC 4821, March 2007. 652 [RFC5282] Black, D. and D. McGrew, "Using Authenticated Encryption 653 Algorithms with the Encrypted Payload of the Internet Key 654 Exchange version 2 (IKEv2) Protocol", RFC 5282, 655 August 2008. 657 [RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange 658 Protocol Version 2 (IKEv2) Session Resumption", RFC 5723, 659 January 2010. 661 [RFC6290] Nir, Y., Wierbowski, D., Detienne, F., and P. Sethi, "A 662 Quick Crash Detection Method for the Internet Key Exchange 663 Protocol (IKE)", RFC 6290, June 2011. 665 [RFC6888] Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A., 666 and H. Ashida, "Common Requirements for Carrier-Grade NATs 667 (CGNs)", BCP 127, RFC 6888, April 2013. 669 [DOSUDPPROT] 670 Kaufman, C., Perlman, R., and B. Sommerfeld, "DoS 671 protection for UDP-based protocols", ACM Conference on 672 Computer and Communications Security, October 2003. 674 Appendix A. Design rationale 676 The simplest approach to the IKE fragmentation would have been to 677 fragment message that is fully formed and ready to be sent. But if 678 message got fragmented after being encrypted and authenticated, this 679 could open a possibility for a simple Denial of Service attack. The 680 attacker could infrequently emit forged but looking valid fragments 681 into the network, and some of these fragments would be fetched by 682 receiver into the reassempling queue. Receiver could not distinguish 683 forged fragments from valid ones and could only determine that some 684 of received fragments were forged when the whole message got 685 reassembled and check for its authenticity failed. 687 To prevent this kind of attack and also to reduce vulnerability to 688 some other kinds of DoS attacks it was decided to make fragmentation 689 before applying cryptographic protection to the message. In this 690 case each Fragment Message becomes individually encrypted and 691 authenticated, that allows receiver to determine forgeg fragments and 692 not to fetch them into the reassempling queue. 694 Appendix B. Correlation between IP Datagram size and Encrypted Payload 695 content size 697 For IPv4 Encrypted Payload content size is less than IP Datagram size 698 by the sum of the following values: 700 o IPv4 header size (typically 20 bytes, up to 60 if IP options are 701 present) 703 o UDP header size (8 bytes) 705 o non-ESP marker size (4 bytes if present) 707 o IKE Header size (28 bytes) 709 o Encrypted Payload header size (4 bytes) 711 o IV size (varying) 713 o padding and its size (at least 1 byte) 715 o ICV size (varying) 717 The sum may be estimated as 61..105 bytes + IV + ICV + padding. 719 For IPv6 this estimation is difficult as there may be varying IPv6 720 Extension headers included. 722 Author's Address 724 Valery Smyslov 725 ELVIS-PLUS 726 PO Box 81 727 Moscow (Zelenograd) 124460 728 RU 730 Phone: +7 495 276 0211 731 Email: svan@elvis.ru