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Garrido 6 Cisco 7 December 17, 2019 9 Packetization Layer Path MTU Discovery (PLMTUD) For UDP Transports Using 10 Session Traversal Utilities for NAT (STUN) 11 draft-ietf-tram-stun-pmtud-15 13 Abstract 15 The datagram exchanged between two Internet endpoints have to go 16 through a series of physical and virtual links that may have 17 different limits on the upper size of the datagram they can transmit 18 without fragmentation. Because fragmentation is considered harmful, 19 most transports and protocols are designed with a mechanism that 20 permits dynamic measurement of the maximum size of a datagram. This 21 mechanism is called Packetization Layer Path MTU Discovery (PLPMTUD). 22 But the UDP transport and some of the protocols that use UDP were 23 designed without that feature. The Session Traversal Utilities for 24 NAT (STUN) Usage described in this document permits retrofitting an 25 existing UDP-based protocol with such a feature. Similarly, a new 26 UDP-based protocol could simply reuse the mechanism described in this 27 document. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at https://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on June 19, 2020. 46 Copyright Notice 48 Copyright (c) 2019 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (https://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 64 2. Overview of Operations . . . . . . . . . . . . . . . . . . . 4 65 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 66 4. Probing Mechanisms . . . . . . . . . . . . . . . . . . . . . 6 67 4.1. Simple Probing Mechanism . . . . . . . . . . . . . . . . 7 68 4.1.1. Sending a Probe Request . . . . . . . . . . . . . . . 7 69 4.1.2. Receiving a Probe Request . . . . . . . . . . . . . . 8 70 4.1.3. Receiving a Probe Response . . . . . . . . . . . . . 8 71 4.2. Complete Probing Mechanism . . . . . . . . . . . . . . . 8 72 4.2.1. Sending a Probe Indications and Report Request . . . 9 73 4.2.2. Receiving an ICMP Packet . . . . . . . . . . . . . . 9 74 4.2.3. Receiving a Probe Indication and Report Request . . . 9 75 4.2.4. Receiving a Report Response . . . . . . . . . . . . . 10 76 4.2.5. Using Checksums as Packet Identifiers . . . . . . . . 10 77 4.2.6. Using Sequence Numbers as Packet Identifiers . . . . 11 78 5. Probe Support Signaling Mechanisms . . . . . . . . . . . . . 12 79 5.1. Explicit Probe Support Signaling Mechanism . . . . . . . 12 80 5.2. Implicit Probe Support Signaling Mechanism . . . . . . . 13 81 6. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 13 82 6.1. IDENTIFIERS . . . . . . . . . . . . . . . . . . . . . . . 13 83 6.2. PMTUD-SUPPORTED . . . . . . . . . . . . . . . . . . . . . 13 84 6.3. PADDING . . . . . . . . . . . . . . . . . . . . . . . . . 13 85 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 86 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 87 8.1. New STUN Methods . . . . . . . . . . . . . . . . . . . . 14 88 8.2. New STUN Attributes . . . . . . . . . . . . . . . . . . . 14 89 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 90 9.1. Normative References . . . . . . . . . . . . . . . . . . 15 91 9.2. Informative References . . . . . . . . . . . . . . . . . 16 92 Appendix A. Release Notes . . . . . . . . . . . . . . . . . . . 16 93 A.1. Modifications between draft-ietf-tram-stun-pmtud-15 and 94 draft-ietf-tram-stun-pmtud-14 . . . . . . . . . . . . . . 16 95 A.2. Modifications between draft-ietf-tram-stun-pmtud-14 and 96 draft-ietf-tram-stun-pmtud-13 . . . . . . . . . . . . . . 16 97 A.3. Modifications between draft-ietf-tram-stun-pmtud-13 and 98 draft-ietf-tram-stun-pmtud-12 . . . . . . . . . . . . . . 17 99 A.4. Modifications between draft-ietf-tram-stun-pmtud-12 and 100 draft-ietf-tram-stun-pmtud-11 . . . . . . . . . . . . . . 17 101 A.5. Modifications between draft-ietf-tram-stun-pmtud-11 and 102 draft-ietf-tram-stun-pmtud-10 . . . . . . . . . . . . . . 17 103 A.6. Modifications between draft-ietf-tram-stun-pmtud-10 and 104 draft-ietf-tram-stun-pmtud-09 . . . . . . . . . . . . . . 17 105 A.7. Modifications between draft-ietf-tram-stun-pmtud-09 and 106 draft-ietf-tram-stun-pmtud-08 . . . . . . . . . . . . . . 17 107 A.8. Modifications between draft-ietf-tram-stun-pmtud-08 and 108 draft-ietf-tram-stun-pmtud-07 . . . . . . . . . . . . . . 17 109 A.9. Modifications between draft-ietf-tram-stun-pmtud-07 and 110 draft-ietf-tram-stun-pmtud-06 . . . . . . . . . . . . . . 17 111 A.10. Modifications between draft-ietf-tram-stun-pmtud-06 and 112 draft-ietf-tram-stun-pmtud-05 . . . . . . . . . . . . . . 17 113 A.11. Modifications between draft-ietf-tram-stun-pmtud-05 and 114 draft-ietf-tram-stun-pmtud-04 . . . . . . . . . . . . . . 18 115 A.12. Modifications between draft-ietf-tram-stun-pmtud-04 and 116 draft-ietf-tram-stun-pmtud-03 . . . . . . . . . . . . . . 18 117 A.13. Modifications between draft-ietf-tram-stun-pmtud-03 and 118 draft-ietf-tram-stun-pmtud-02 . . . . . . . . . . . . . . 18 119 A.14. Modifications between draft-ietf-tram-stun-pmtud-02 and 120 draft-ietf-tram-stun-pmtud-01 . . . . . . . . . . . . . . 19 121 A.15. Modifications between draft-ietf-tram-stun-pmtud-01 and 122 draft-ietf-tram-stun-pmtud-00 . . . . . . . . . . . . . . 19 123 A.16. Modifications between draft-ietf-tram-stun-pmtud-00 and 124 draft-petithuguenin-tram-stun-pmtud-01 . . . . . . . . . 19 125 A.17. Modifications between draft-petithuguenin-tram-stun- 126 pmtud-01 and draft-petithuguenin-tram-stun-pmtud-00 . . . 19 127 A.18. Modifications between draft-petithuguenin-tram-stun- 128 pmtud-00 and draft-petithuguenin-behave-stun-pmtud-03 . . 19 129 A.19. Modifications between draft-petithuguenin-behave-stun- 130 pmtud-03 and draft-petithuguenin-behave-stun-pmtud-02 . . 19 131 A.20. Modifications between draft-petithuguenin-behave-stun- 132 pmtud-02 and draft-petithuguenin-behave-stun-pmtud-01 . . 20 133 A.21. Modifications between draft-petithuguenin-behave-stun- 134 pmtud-01 and draft-petithuguenin-behave-stun-pmtud-00 . . 20 135 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20 136 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 138 1. Introduction 140 The Packetization Layer Path MTU Discovery (PMTUD) specification 141 [RFC4821] describes a method to discover the Path MTU, but does not 142 describe a practical protocol to do so with UDP. Many application 143 layer protocols based on the transport layer protocol UDP do not 144 implement the Path MTU discovery mechanism described in [RFC4821]. 145 These application layer protocols can make use of the probing 146 mechanisms described in this document instead of designing their own 147 adhoc extension. These probing mechanisms are implemented with 148 Session Traversal Utilities for NAT (STUN), but their usage is not 149 limited to STUN-based protocols. 151 The STUN usage defined in this document for Packetization Layer Path 152 MTU Discovery (PLPMTUD) between a client and a server permits proper 153 measurement of the Path MTU for application layer protocols based on 154 the transport layer protocol UDP in the network. It also simplifies 155 troubleshooting and has multiple other applications across a wide 156 variety of technologies. 158 Complementary techniques can be used to discover additional network 159 characteristics, such as the network path (using the STUN Traceroute 160 mechanism described in [I-D.martinsen-tram-stuntrace]) and bandwidth 161 availability (using the mechanism described in 162 [I-D.martinsen-tram-turnbandwidthprobe]). In addition, 163 [I-D.ietf-tsvwg-datagram-plpmtud] provides a robust method for Path 164 MTU Discovery for a broader range of protocols and applications. 166 2. Overview of Operations 168 This section is meant to be informative only and is not intended as a 169 substitute for [RFC4821]. 171 A UDP endpoint that uses this specification to discover the Path MTU 172 over UDP and knows that the endpoint it is communicating with also 173 supports this specification can choose to use either the Simple 174 Probing mechanism (as described in Section 4.1) or the Complete 175 Probing mechanism (as described in Section 4.2). The selection of 176 which Probing Mechanism to use is dependent on performance and 177 security and complexity trade-offs. 179 If the Simple Probing mechanism is chosen, then the client initiates 180 Probe transactions, as shown in Figure 1, which decrease in size 181 until transactions succeed, indicating that the Path MTU has been 182 discovered. It then uses that information to update the Path MTU. 184 Client Server 185 | | 186 | Probe Request | 187 |---------------->| 188 | | 189 | Probe Response | 190 |<----------------| 191 | | 193 Figure 1: Simple Probing Example 195 If the Complete Probing mechanism (as described in Section 4.2) is 196 chosen, then the client sends Probe Indications of various sizes (as 197 specified in [RFC4821]) interleaved with UDP packets sent by the UDP 198 protocol. The client then sends a Report Request for the ordered 199 list of identifiers for the UDP packets and Probe Indications 200 received by the server. The client then compares the list returned 201 in the Report Response with its own list of identifiers for the UDP 202 packets and Probe Indications it sent. The client examines the 203 received reports to determine which probes were successful. When a 204 probe succeeds with a larger size than the current PMTU, the PMTU is 205 increased. When the probes indicate the current PMTU is not 206 supported the size is decreased. 208 Because of the possibility of amplification attack, the Complete 209 Probing mechanism must be authenticated as specified in Section 5.1. 210 Particular care must be taken to prevent amplification when an 211 external mechanism is used to trigger the Complete Probing mechanism. 213 Client Server 214 | UDP Packet | 215 |------------------>| 216 | | 217 | UDP Packet | 218 |------------------>| 219 | | 220 | Probe Indication | 221 |------------------>| 222 | | 223 | UDP Packet | 224 |------------------>| 225 | | 226 | Probe Indication | 227 |------------------>| 228 | | 229 | Report Request | 230 |------------------>| 231 | Report Response | 232 |<------------------| 233 | | 235 Figure 2: Complete Probing Example 237 3. Terminology 239 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 240 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 241 "OPTIONAL" in this document are to be interpreted as described in BCP 242 14 [RFC2119][RFC8174] when, and only when, they appear in all 243 capitals, as shown here. 245 4. Probing Mechanisms 247 The Probing mechanism is used to discover the Path MTU in one 248 direction only: from the client to the server. Both endpoints MAY 249 behave as a client and a server to achieve bi-directional path 250 discovery. 252 Two Probing mechanisms are described: a Simple Probing mechanism and 253 a more complete mechanism that can converge more quickly and find an 254 appropriate Path MTU in the presence of congestion. Additionally, 255 the Simple Probing mechanism does not require authentication except 256 where used as an implicit signaling mechanism, whereas the complete 257 mechanism does. 259 Implementations supporting this specification MUST implement the 260 server side of both the Simple Probing mechanism (Section 4.1) and 261 the Complete Probing mechanism (Section 4.2). 263 Implementations supporting this specification MUST implement the 264 client side of the Complete Probing mechanism. They MAY implement 265 the client side of the Simple Probing mechanism. 267 4.1. Simple Probing Mechanism 269 The Simple Probing mechanism is implemented by sending a Probe 270 Request with a PADDING attribute over UDP with the DF bit set in the 271 IP header for IPv4 packets and IPv6 packets without the Fragment 272 Header included. 274 Note: Routers can be configured to clear the DF bit or ignore the DF 275 bit which can be difficult or impossible to detect if reassembly 276 occurs prior to receiving the packet, rendering PLPMTUD inaccurate. 278 4.1.1. Sending a Probe Request 280 A client forms a Probe Request by using the Probe Method and 281 following the rules in Section 6.1 of [I-D.ietf-tram-stunbis]. 283 The Probe transaction MUST be authenticated if the Simple Probing 284 mechanism is used in conjunction with the Implicit Probing Support 285 mechanism described in Section 5.2. If not, the Probe transaction 286 MAY be authenticated. 288 The client adds a PADDING attribute with a length that, when added to 289 the IP and UDP headers and the other STUN components, is equal to the 290 Selected Probe Size, as defined in [RFC4821] Section 7.3. The 291 PADDING bits SHOULD be set to zero. The client MUST add the 292 FINGERPRINT attribute so the STUN messages are disambiguated from the 293 other protocol packets as specified in Section 7 of 294 [I-D.ietf-tram-stunbis]. 296 Then the client sends the Probe Request to the server over UDP with 297 the DF bit set for IPv4 packets and IPv6 packets without the Fragment 298 Header included. For the purpose of this transaction, the Rc 299 parameter is set to 3 and the initial value for RTO stays at 500 ms 300 as specified in Section 6.2.1 of [I-D.ietf-tram-stunbis] 302 A client MUST NOT send a probe if it does not have knowledge that the 303 server supports this specification. This is done either by external 304 signalling or by a mechanism specific to the UDP protocol to which 305 PMTUD capabilities are added or by one of the mechanisms specified in 306 Section 5. 308 4.1.2. Receiving a Probe Request 310 A server receiving a Probe Request MUST process it as specified in 311 [I-D.ietf-tram-stunbis]. 313 The server then creates a Probe Response. The server MUST add the 314 FINGERPRINT attribute so the STUN messages are disambiguated from the 315 other protocol packets as specified in Section 7 of 316 [I-D.ietf-tram-stunbis]. The server then sends the response to the 317 client. 319 4.1.3. Receiving a Probe Response 321 A client receiving a Probe Response MUST process it as specified in 322 section 6.3.1 of [I-D.ietf-tram-stunbis] and MUST ignore the PADDING 323 attribute. If a response is received this is interpreted as a Probe 324 Success, as defined in [RFC4821] Section 7.6.1. If an ICMP packet 325 "Fragmentation needed" or "Packet Too Big" is received then this is 326 interpreted as a Probe Failure, as defined in [RFC4821] 327 Section 7.6.2. If the Probe transaction times out, then this is 328 interpreted as a Probe Inconclusive, as defined in [RFC4821] 329 Section 7.6.4. Validation SHOULD be performed on the ICMP packet as 330 specified in [RFC8085]. 332 4.2. Complete Probing Mechanism 334 The Complete Probing mechanism is implemented by sending one or more 335 Probe Indications with a PADDING attribute over UDP with the DF bit 336 set in the IP header for IPv4 packets and IPv6 packets without the 337 Fragment Header included followed by a Report Request to the same 338 server. A router on the path to the server can reject this 339 Indication with an ICMP message or drop it. The server keeps a 340 chronologically ordered list of identifiers for all packets received 341 (including retransmitted packets) and sends this list back to the 342 client in the Report Response. The client analyzes this list to find 343 which packets were not received. Because UDP packets do not contain 344 an identifier, the Complete Probing mechanism needs a way to identify 345 each packet received. 347 Some application layer protocols may already have a way of 348 identifying each individual UDP packet, in which case these 349 identifiers SHOULD be used in the IDENTIFIERS attribute of the Report 350 Response. While there are other possible packet identification 351 schemes, this document describes two different ways to identify a 352 specific packet when no application layer protocol-specific 353 identification mechanism is available. 355 In the first packet identification mechanism, the server computes a 356 checksum over each packet received and sends back to the sender the 357 list of checksums ordered chronologically. The client compares this 358 list to its own list of checksums. 360 In the second packet identification mechanism, the client prepends 361 the UDP data with a header that provides a sequence number. The 362 server sends back the chronologically ordered list of sequence 363 numbers received that the client then compares with its own list. 365 4.2.1. Sending a Probe Indications and Report Request 367 A client forms a Probe Indication by using the Probe Method and 368 following the rules in [I-D.ietf-tram-stunbis] Section 6.1. The 369 client adds to a Probe Indication a PADDING attribute with a size 370 that, when added to the IP and UDP headers and the other STUN 371 components, is equal to the Selected Probe Size, as defined in 372 [RFC4821] Section 7.3. The PADDING bits SHOULD be set to zero. If 373 the authentication mechanism permits it, then the Indication MUST be 374 authenticated. The client MUST add the FINGERPRINT attribute so the 375 STUN messages are disambiguated from the other protocol packets. 377 Then the client sends a Probe Indication to the server over UDP with 378 the DF bit set for IPv4 packets and IPv6 packets without the Fragment 379 Header included. 381 Then the client forms a Report Request by following the rules in 382 [I-D.ietf-tram-stunbis] Section 6.1. The Report transaction MUST be 383 authenticated to prevent amplification attacks. The client MUST add 384 the FINGERPRINT attribute so the STUN messages are disambiguated from 385 the other protocol packets. 387 Then the client waits half the RTO after sending the last Probe 388 Indication and then sends the Report Request to the server over UDP. 390 4.2.2. Receiving an ICMP Packet 392 If an ICMP packet "Fragmentation needed" or "Packet Too Big" is 393 received then this is interpreted as a Probe Failure, as defined in 394 [RFC4821] Section 7.5. Validation SHOULD be performed on the ICMP 395 packet as specified in [RFC8085]. 397 4.2.3. Receiving a Probe Indication and Report Request 399 A server supporting this specification will keep the identifiers of 400 all packets received in a chronologically ordered list. The packets 401 that are to be associated to a given flow's identifier are selected 402 according to Section 5.2 of [RFC4821]. The same identifier can 403 appear multiple times in the list because of retransmissions. The 404 maximum size of this list is calculated such that when the list is 405 added to the Report Response, the total size of the packet does not 406 exceed the unknown Path MTU, as defined in [I-D.ietf-tram-stunbis] 407 Section 6.1. Older identifiers are removed when new identifiers are 408 added to a list that is already full. 410 A server receiving a Report Request MUST process it as specified in 411 [I-D.ietf-tram-stunbis] and MUST ignore the PADDING attribute. 413 The server creates a Report Response and adds an IDENTIFIERS 414 attribute that contains the chronologically ordered list of all 415 identifiers received so far. The server MUST add the FINGERPRINT 416 attribute. The server then sends the response to the client. 418 The exact content of the IDENTIFIERS attribute depends on what type 419 of identifiers have been chosen for the protocol. Each protocol 420 adding PMTUD capabilities as specified by this specification MUST 421 describe the format of the contents of the IDENTIFIERS attribute, 422 unless it is using one of the formats described in this 423 specification. See Section 6.1 for details about the IDENTIFIERS 424 attribute. 426 4.2.4. Receiving a Report Response 428 A client receiving a Report Response processes it as specified in 429 [I-D.ietf-tram-stunbis]. If the response IDENTIFIERS attribute 430 contains the identifier of a Probe Indication, then this is 431 interpreted as a Probe Success for this probe, as defined in 432 [RFC4821] Section 7.5. If a Probe Indication identifier cannot be 433 found in the Report Response, this is interpreted as a Probe Failure, 434 as defined in [RFC4821] Section 7.5. If a Probe Indication 435 identifier cannot be found in the Report Response but identifiers for 436 other packets sent before or after the Probe Indication can all be 437 found, this is interpreted as a Probe Failure as defined in [RFC4821] 438 Section 7.5. If the Report Transaction times out, this is 439 interpreted as a Full-Stop Timeout, as defined in [RFC4821] 440 Section 3. 442 4.2.5. Using Checksums as Packet Identifiers 444 When using a checksum as a packet identifier, the client keeps a 445 chronologically ordered list of the packets it transmits, along with 446 an associated checksum value. For STUN Probe Indication or Request 447 packets, the associated checksum value is the FINGERPRINT value from 448 the packet; for other packets a checksum value is computed using a 449 similar algorithm to the FINGERPRINT calculation. (i.e., the CRC-32 450 calculated per the algorithm defined in [ITU.V42.2002], such as 451 subsequently been XOR'ed with 32-bit value 0x5354554e). 453 For each STUN Probe Indication or Request, the server retrieves the 454 STUN FINGERPRINT value. For all other packets, the server calculates 455 the checksum as described above. It puts these FINGERPRINT and 456 checksum values in a chronologically ordered list that is sent back 457 in the Report Response. 459 The contents of the IDENTIFIERS attribute is a list of 4 byte 460 numbers, each using the same encoding that is used for the contents 461 of the FINGERPRINT attribute. 463 It could have been possible to use the checksum generated in the UDP 464 checksum for this, but this value is generally not accessible to 465 applications. Also, sometimes the checksum is not calculated or is 466 off-loaded to network hardware. 468 4.2.6. Using Sequence Numbers as Packet Identifiers 470 When using sequence numbers, a small header similar to the TURN 471 ChannelData header, as defined in Section 11.4 of [RFC5766], is added 472 in front of all packets that are not a STUN Probe Indication or 473 Request. The initial sequence number MUST be randomized and is 474 monotonically incremented by one for each packet sent. The most 475 significant bit of the sequence number is always 0. The server 476 collects the sequence number of the packets sent, or the 4 first 477 bytes of the transaction ID if a STUN Probe Indication or Request is 478 sent. In that case, the most significant bit of the 4 first bytes is 479 set to 1. 481 0 1 2 3 482 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 1 483 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 484 | Channel Number | Length | 485 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 486 |0| Sequence number | 487 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 488 | | 489 / Application Data / 490 / / 491 | | 492 | +-------------------------------+ 493 | | 494 +-------------------------------+ 495 The Channel Number is always 0xFFFF. The Length field specifies the 496 length in bytes of the sequence number and application data fields. 497 The header values are encoded using network order. 499 The contents of the IDENTIFIERS attribute is a chronologically 500 ordered list of 4 byte numbers, each containing either a sequence 501 number, if the packet was not a STUN Probe Indication or Request, or 502 the 4 first bytes of the transaction ID, with the most significant 503 bit forced to 1, if the packet is a STUN Probe Indication or Request. 505 5. Probe Support Signaling Mechanisms 507 The PMTUD mechanism described in this document is intended to be used 508 by any UDP-based protocols that do not have built-in PMTUD 509 capabilities, irrespective of whether those UDP-based protocols are 510 STUN-based or not. So the manner in which a specific protocol 511 discovers that it is safe to send PMTUD probes is largely dependent 512 on the details of that specific protocol, with the exception of the 513 Implicit Mechanism described below, which applies to any protocol. 515 5.1. Explicit Probe Support Signaling Mechanism 517 Some of these mechanisms can use a separate signalling mechanism (for 518 instance, an SDP attribute in an Offer/Answer exchange [RFC3264]), or 519 an optional flag that can be set in the protocol that is augmented 520 with PMTUD capabilities. STUN Usages that can benefit from PMTUD 521 capabilities can signal in-band that they support probing by 522 inserting a PMTUD-SUPPORTED attribute in some STUN methods. The 523 decision of which methods support this attribute is left to each 524 specific STUN Usage. 526 UDP-based protocols that want to use any of these mechanisms, 527 including the PMTUD-SUPPORTED attribute, to signal PMTUD capabilities 528 MUST ensure that it cannot be used to launch an amplification attack. 530 An amplification attack can be prevented using techniques such as: 532 o Authentication, where the source of the packet and the destination 533 share a secret. 535 o 3 way handshake with some form of unpredictable cookie. 537 o Make sure that the total size of the traffic potentially generated 538 is lower than the size of the request that generated it. 540 5.2. Implicit Probe Support Signaling Mechanism 542 As a result of the fact that all endpoints implementing this 543 specification are both clients and servers, a Probe Request or 544 Indication received by an endpoint acting as a server implicitly 545 signals that this server can now act as a client and MAY send a Probe 546 Request or Indication to probe the Path MTU in the reverse direction 547 toward the former client, that will now be acting as a server. 549 The Probe Request or Indication that are used to implicitly signal 550 probing support in the reverse direction MUST be authenticated to 551 prevent amplification attacks. 553 6. STUN Attributes 555 6.1. IDENTIFIERS 557 The IDENTIFIERS attribute carries a chronologically ordered list of 558 UDP packet identifiers. 560 While Section 4.2.5 and Section 4.2.6 describe two possible methods 561 for acquiring and formatting the identifiers used for this purpose, 562 ultimately each protocol has to define how these identifiers are 563 acquired and formatted. Therefore, the contents of the IDENTIFIERS 564 attribute is opaque. 566 6.2. PMTUD-SUPPORTED 568 The PMTUD-SUPPORTED attribute indicates that its sender supports this 569 mechanism, as incorporated into the STUN usage or protocol being 570 used. This attribute has no value part and thus the attribute length 571 field is 0. 573 6.3. PADDING 575 The PADDING attribute allows for the entire message to be padded to 576 force the STUN message to be divided into IP fragments. PADDING 577 consists entirely of a free-form string, the value of which does not 578 matter. PADDING can be used in either Binding Requests or Binding 579 Responses. 581 PADDING MUST NOT be longer than the length that brings the total IP 582 datagram size to 64K. It SHOULD be equal in length to the MTU of the 583 outgoing interface, rounded up to an even multiple of four bytes and 584 SHOULD ensure a probe does not result in a packet larger than the MTU 585 fo the outgoing interface. STUN messages sent with PADDING are 586 intended to test the behavior of UDP fragments, therefore they are an 587 exception to the usual rule that STUN messages need to be less than 588 the PMTU for the path. 590 7. Security Considerations 592 The PMTUD mechanism described in this document, when used without the 593 signalling mechanism described in Section 5.1, does not introduce any 594 specific security considerations beyond those described in [RFC4821] 595 and [I-D.ietf-tsvwg-datagram-plpmtud]. 597 The attacks described in Section 11 of [RFC4821] apply equally to the 598 mechanism described in this document. 600 The amplification attacks introduced by the signalling mechanism 601 described in Section 5.1 can be prevented by using one of the 602 techniques described in that section. 604 The Simple Probing mechanism may be used without authentication 605 because this usage by itself cannot trigger an amplification attack 606 as the Probe Response is smaller than the Probe Request except when 607 used in conjunction with the Implicit Probing Support Signaling 608 mechanism. 610 8. IANA Considerations 612 This specification defines two new STUN methods and two new STUN 613 attributes. 615 8.1. New STUN Methods 617 IANA is requested to add the following methods to the STUN Method 618 Registry: 620 0xXXX : Probe 622 0xXXX : Report 624 See Sections Section 4.1 and Section 4.2 for the semantics of these 625 new methods. 627 8.2. New STUN Attributes 629 IANA is requested to add the following attributes to the STUN Method 630 Registry: 632 Comprehension-required range (0x0000-0x7FFF): 633 0xXXXX: IDENTIFIERS 635 Comprehension-optional range (0x8000-0xFFFF) 636 0xXXXX: PMTUD-SUPPORTED 638 0x0026: PADDING 640 The IDENTIFIERS STUN attribute is defined in Section 6.1, the PMTUD- 641 SUPPORTED STUN attribute is defined in Section 6.2; the PADDING STUN 642 attribute is defined in Section 6.3. 644 NOTE: TO BE DELETED BEFORE PUBLICATION. PLEASE NOTE THAT THE PADDING 645 ATTRIBUTE ENTRY IS REPLACING THE ENTRY MADE BY RFC5780 646 (EXPERIMENTAL). THE SAME VALUE AND NAME ARE USED BUT THE REFERENCE 647 SHOULD BE CHANGED TO THIS STANDARDS TRACK DOCUMENT. 649 9. References 651 9.1. Normative References 653 [I-D.ietf-tram-stunbis] 654 Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing, 655 D., Mahy, R., and P. Matthews, "Session Traversal 656 Utilities for NAT (STUN)", draft-ietf-tram-stunbis-21 657 (work in progress), March 2019. 659 [ITU.V42.2002] 660 International Telecommunications Union, "Error-correcting 661 Procedures for DCEs Using Asynchronous-to-Synchronous 662 Conversion", ITU-T Recommendation V.42, 2002. 664 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 665 Requirement Levels", BCP 14, RFC 2119, 666 DOI 10.17487/RFC2119, March 1997, 667 . 669 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 670 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, 671 . 673 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage 674 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, 675 March 2017, . 677 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 678 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 679 May 2017, . 681 9.2. Informative References 683 [I-D.ietf-tsvwg-datagram-plpmtud] 684 Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and 685 T. Voelker, "Packetization Layer Path MTU Discovery for 686 Datagram Transports", draft-ietf-tsvwg-datagram-plpmtud-12 687 (work in progress), December 2019. 689 [I-D.martinsen-tram-stuntrace] 690 Martinsen, P. and D. Wing, "STUN Traceroute", draft- 691 martinsen-tram-stuntrace-01 (work in progress), June 2015. 693 [I-D.martinsen-tram-turnbandwidthprobe] 694 Martinsen, P., Andersen, T., Salgueiro, G., and M. Petit- 695 Huguenin, "Traversal Using Relays around NAT (TURN) 696 Bandwidth Probe", draft-martinsen-tram- 697 turnbandwidthprobe-00 (work in progress), May 2015. 699 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 700 with Session Description Protocol (SDP)", RFC 3264, 701 DOI 10.17487/RFC3264, June 2002, 702 . 704 [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using 705 Relays around NAT (TURN): Relay Extensions to Session 706 Traversal Utilities for NAT (STUN)", RFC 5766, 707 DOI 10.17487/RFC5766, April 2010, 708 . 710 Appendix A. Release Notes 712 This section must be removed before publication as an RFC. 714 A.1. Modifications between draft-ietf-tram-stun-pmtud-15 and draft- 715 ietf-tram-stun-pmtud-14 717 o Modifications to address DISCUSS and COMMENT from IESG review 719 A.2. Modifications between draft-ietf-tram-stun-pmtud-14 and draft- 720 ietf-tram-stun-pmtud-13 722 o Modifications to address COMMENTS from IESG review 724 A.3. Modifications between draft-ietf-tram-stun-pmtud-13 and draft- 725 ietf-tram-stun-pmtud-12 727 o Modifications to address nits 729 A.4. Modifications between draft-ietf-tram-stun-pmtud-12 and draft- 730 ietf-tram-stun-pmtud-11 732 o Modifications following IESG review. Incorporated RFC5780 PADDING 733 attribute (Adam's Discuss) and added IPv6 language (Suresh's 734 Discuss). 736 A.5. Modifications between draft-ietf-tram-stun-pmtud-11 and draft- 737 ietf-tram-stun-pmtud-10 739 o Modifications following IESG review. 741 A.6. Modifications between draft-ietf-tram-stun-pmtud-10 and draft- 742 ietf-tram-stun-pmtud-09 744 o Modifications following reviews for gen-art (Roni Even) and secdir 745 (Carl Wallace). 747 A.7. Modifications between draft-ietf-tram-stun-pmtud-09 and draft- 748 ietf-tram-stun-pmtud-08 750 o Add 3 ways of preventing amplification attacks. 752 A.8. Modifications between draft-ietf-tram-stun-pmtud-08 and draft- 753 ietf-tram-stun-pmtud-07 755 o Updates following Spencer's review. 757 A.9. Modifications between draft-ietf-tram-stun-pmtud-07 and draft- 758 ietf-tram-stun-pmtud-06 760 o Updates following Shepherd review. 762 A.10. Modifications between draft-ietf-tram-stun-pmtud-06 and draft- 763 ietf-tram-stun-pmtud-05 765 o Nits. 767 o Restore missing changelog for previous version. 769 A.11. Modifications between draft-ietf-tram-stun-pmtud-05 and draft- 770 ietf-tram-stun-pmtud-04 772 o Modifications following Brandon Williams review. 774 A.12. Modifications between draft-ietf-tram-stun-pmtud-04 and draft- 775 ietf-tram-stun-pmtud-03 777 o Modifications following Simon Perreault and Brandon Williams 778 reviews. 780 A.13. Modifications between draft-ietf-tram-stun-pmtud-03 and draft- 781 ietf-tram-stun-pmtud-02 783 o Add new Overview of Operations section with ladder diagrams. 785 o Authentication is mandatory for the Complete Probing mechanism, 786 optional for the Simple Probing mechanism. 788 o All the ICE specific text moves to a separate draft to be 789 discussed in the ICE WG. 791 o The TURN usage is removed because probing between a TURN server 792 and TURN client is not useful. 794 o Any usage of PMTUD-SUPPORTED or other signaling mechanisms 795 (formerly knows as discovery mechanisms) must now be 796 authenticated. 798 o Both probing mechanisms are MTI in the server, the complete 799 probing mechanism is MTI in the client. 801 o Make clear that stopping after 3 retransmission is done by 802 changing the STUN parameter. 804 o Define the format of the attributes. 806 o Make clear that the specification is for any UDP protocol that 807 does not already have PMTUD capabilities, not just STUN based 808 protocols. 810 o Change the default delay to send the Report Request to 250 ms 811 after the last Indication if the RTO is unknown. 813 o Each usage of this specification must the format of the 814 IDENTIFIERS attribute contents. 816 o Better define the implicit signaling mechanism. 818 o Extend the Security Consideration section. 820 o Tons of nits. 822 A.14. Modifications between draft-ietf-tram-stun-pmtud-02 and draft- 823 ietf-tram-stun-pmtud-01 825 o Cleaned up references. 827 A.15. Modifications between draft-ietf-tram-stun-pmtud-01 and draft- 828 ietf-tram-stun-pmtud-00 830 o Added Security Considerations Section. 832 o Added IANA Considerations Section. 834 A.16. Modifications between draft-ietf-tram-stun-pmtud-00 and draft- 835 petithuguenin-tram-stun-pmtud-01 837 o Adopted by WG - Text unchanged. 839 A.17. Modifications between draft-petithuguenin-tram-stun-pmtud-01 and 840 draft-petithuguenin-tram-stun-pmtud-00 842 o Moved some Introduction text to the Probing Mechanism section. 844 o Added cross-reference to the other two STUN troubleshooting 845 mechanism drafts. 847 o Updated references. 849 o Added Gonzalo Salgueiro as co-author. 851 A.18. Modifications between draft-petithuguenin-tram-stun-pmtud-00 and 852 draft-petithuguenin-behave-stun-pmtud-03 854 o General refresh for republication. 856 A.19. Modifications between draft-petithuguenin-behave-stun-pmtud-03 857 and draft-petithuguenin-behave-stun-pmtud-02 859 o Changed author address. 861 o Changed the IPR to trust200902. 863 A.20. Modifications between draft-petithuguenin-behave-stun-pmtud-02 864 and draft-petithuguenin-behave-stun-pmtud-01 866 o Defined checksum and sequential numbers as possible packet 867 identifiers. 869 o Updated the reference to RFC 5389 871 o The FINGERPRINT attribute is now mandatory. 873 o Changed the delay between Probe indication and Report request to 874 be RTO/2 or 50 milliseconds. 876 o Added ICMP packet processing. 878 o Added Full-Stop Timeout detection. 880 o Stated that Binding request with PMTUD-SUPPORTED does not start 881 the PMTUD process if already started. 883 A.21. Modifications between draft-petithuguenin-behave-stun-pmtud-01 884 and draft-petithuguenin-behave-stun-pmtud-00 886 o Removed the use of modified STUN transaction but shorten the 887 retransmission for the simple probing mechanism. 889 o Added a complete probing mechanism. 891 o Removed the PADDING-RECEIVED attribute. 893 o Added release notes. 895 Acknowledgements 897 Thanks to Eilon Yardeni, Geir Sandbakken, Paal-Erik Martinsen, 898 Tirumaleswar Reddy, Ram Mohan R, Simon Perreault, Brandon Williams, 899 Tolga Asveren, Spencer Dawkins, Carl Wallace, and Roni Even for their 900 review comments, suggestions and questions that helped to improve 901 this document. 903 Special thanks to Dan Wing, who supported this document since its 904 first publication back in 2008. 906 Authors' Addresses 907 Marc Petit-Huguenin 908 Impedance Mismatch 910 Email: marc@petit-huguenin.org 912 Gonzalo Salgueiro 913 Cisco Systems, Inc. 914 7200-12 Kit Creek Road 915 Research Triangle Park, NC 27709 916 United States 918 Email: gsalguei@cisco.com 920 Felipe Garrido 921 Cisco Systems, Inc. 922 7200-12 Kit Creek Road 923 Research Triangle Park, NC 27709 924 United States 926 Email: fegarrid@cisco.com