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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 TRAM WG T. Reddy, Ed. 3 Internet-Draft McAfee 4 Obsoletes: 5766,6156 (if approved) A. Johnston, Ed. 5 Intended status: Standards Track Rowan University 6 Expires: September 28, 2018 P. Matthews 7 Alcatel-Lucent 8 J. Rosenberg 9 jdrosen.net 10 March 27, 2018 12 Traversal Using Relays around NAT (TURN): Relay Extensions to Session 13 Traversal Utilities for NAT (STUN) 14 draft-ietf-tram-turnbis-16 16 Abstract 18 If a host is located behind a NAT, then in certain situations it can 19 be impossible for that host to communicate directly with other hosts 20 (peers). In these situations, it is necessary for the host to use 21 the services of an intermediate node that acts as a communication 22 relay. This specification defines a protocol, called TURN (Traversal 23 Using Relays around NAT), that allows the host to control the 24 operation of the relay and to exchange packets with its peers using 25 the relay. TURN differs from some other relay control protocols in 26 that it allows a client to communicate with multiple peers using a 27 single relay address. 29 The TURN protocol was designed to be used as part of the ICE 30 (Interactive Connectivity Establishment) approach to NAT traversal, 31 though it also can be used without ICE. 33 This document obsoletes RFC 5766 and RFC 6156. 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at https://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on September 28, 2018. 51 Copyright Notice 53 Copyright (c) 2018 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents 58 (https://trustee.ietf.org/license-info) in effect on the date of 59 publication of this document. Please review these documents 60 carefully, as they describe your rights and restrictions with respect 61 to this document. Code Components extracted from this document must 62 include Simplified BSD License text as described in Section 4.e of 63 the Trust Legal Provisions and are provided without warranty as 64 described in the Simplified BSD License. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 69 2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6 70 2.1. Transports . . . . . . . . . . . . . . . . . . . . . . . 8 71 2.2. Allocations . . . . . . . . . . . . . . . . . . . . . . . 9 72 2.3. Permissions . . . . . . . . . . . . . . . . . . . . . . . 11 73 2.4. Send Mechanism . . . . . . . . . . . . . . . . . . . . . 12 74 2.5. Channels . . . . . . . . . . . . . . . . . . . . . . . . 14 75 2.6. Unprivileged TURN Servers . . . . . . . . . . . . . . . . 16 76 2.7. Avoiding IP Fragmentation . . . . . . . . . . . . . . . . 16 77 2.8. RTP Support . . . . . . . . . . . . . . . . . . . . . . . 18 78 2.9. Happy Eyeballs for TURN . . . . . . . . . . . . . . . . . 18 79 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 19 80 4. Discovery of TURN server . . . . . . . . . . . . . . . . . . 21 81 4.1. TURN URI Scheme Semantics . . . . . . . . . . . . . . . . 21 82 5. General Behavior . . . . . . . . . . . . . . . . . . . . . . 21 83 6. Allocations . . . . . . . . . . . . . . . . . . . . . . . . . 24 84 7. Creating an Allocation . . . . . . . . . . . . . . . . . . . 25 85 7.1. Sending an Allocate Request . . . . . . . . . . . . . . . 25 86 7.2. Receiving an Allocate Request . . . . . . . . . . . . . . 27 87 7.3. Receiving an Allocate Success Response . . . . . . . . . 32 88 7.4. Receiving an Allocate Error Response . . . . . . . . . . 32 89 8. Refreshing an Allocation . . . . . . . . . . . . . . . . . . 34 90 8.1. Sending a Refresh Request . . . . . . . . . . . . . . . . 35 91 8.2. Receiving a Refresh Request . . . . . . . . . . . . . . . 35 92 8.3. Receiving a Refresh Response . . . . . . . . . . . . . . 36 93 9. Permissions . . . . . . . . . . . . . . . . . . . . . . . . . 36 94 10. CreatePermission . . . . . . . . . . . . . . . . . . . . . . 38 95 10.1. Forming a CreatePermission Request . . . . . . . . . . . 38 96 10.2. Receiving a CreatePermission Request . . . . . . . . . . 38 97 10.3. Receiving a CreatePermission Response . . . . . . . . . 39 98 11. Send and Data Methods . . . . . . . . . . . . . . . . . . . . 39 99 11.1. Forming a Send Indication . . . . . . . . . . . . . . . 39 100 11.2. Receiving a Send Indication . . . . . . . . . . . . . . 40 101 11.3. Receiving a UDP Datagram . . . . . . . . . . . . . . . . 40 102 11.4. Receiving a Data Indication . . . . . . . . . . . . . . 41 103 11.5. Receiving an ICMP Packet . . . . . . . . . . . . . . . . 41 104 11.6. Receiving a Data Indication with an ICMP attribute . . . 42 105 12. Channels . . . . . . . . . . . . . . . . . . . . . . . . . . 43 106 12.1. Sending a ChannelBind Request . . . . . . . . . . . . . 45 107 12.2. Receiving a ChannelBind Request . . . . . . . . . . . . 45 108 12.3. Receiving a ChannelBind Response . . . . . . . . . . . . 46 109 12.4. The ChannelData Message . . . . . . . . . . . . . . . . 46 110 12.5. Sending a ChannelData Message . . . . . . . . . . . . . 47 111 12.6. Receiving a ChannelData Message . . . . . . . . . . . . 48 112 12.7. Relaying Data from the Peer . . . . . . . . . . . . . . 49 113 13. Packet Translations . . . . . . . . . . . . . . . . . . . . . 49 114 13.1. IPv4-to-IPv6 Translations . . . . . . . . . . . . . . . 49 115 13.2. IPv6-to-IPv6 Translations . . . . . . . . . . . . . . . 50 116 13.3. IPv6-to-IPv4 Translations . . . . . . . . . . . . . . . 52 117 14. IP Header Fields . . . . . . . . . . . . . . . . . . . . . . 52 118 15. STUN Methods . . . . . . . . . . . . . . . . . . . . . . . . 54 119 16. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 54 120 16.1. CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . 55 121 16.2. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . 55 122 16.3. XOR-PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . 55 123 16.4. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . 55 124 16.5. XOR-RELAYED-ADDRESS . . . . . . . . . . . . . . . . . . 56 125 16.6. REQUESTED-ADDRESS-FAMILY . . . . . . . . . . . . . . . . 56 126 16.7. EVEN-PORT . . . . . . . . . . . . . . . . . . . . . . . 56 127 16.8. REQUESTED-TRANSPORT . . . . . . . . . . . . . . . . . . 57 128 16.9. DONT-FRAGMENT . . . . . . . . . . . . . . . . . . . . . 57 129 16.10. RESERVATION-TOKEN . . . . . . . . . . . . . . . . . . . 57 130 16.11. ADDITIONAL-ADDRESS-FAMILY . . . . . . . . . . . . . . . 57 131 16.12. ADDRESS-ERROR-CODE Attribute . . . . . . . . . . . . . . 58 132 16.13. ICMP Attribute . . . . . . . . . . . . . . . . . . . . . 58 133 17. STUN Error Response Codes . . . . . . . . . . . . . . . . . . 59 134 18. Detailed Example . . . . . . . . . . . . . . . . . . . . . . 60 135 19. Security Considerations . . . . . . . . . . . . . . . . . . . 68 136 19.1. Outsider Attacks . . . . . . . . . . . . . . . . . . . . 68 137 19.1.1. Obtaining Unauthorized Allocations . . . . . . . . . 68 138 19.1.2. Offline Dictionary Attacks . . . . . . . . . . . . . 68 139 19.1.3. Faked Refreshes and Permissions . . . . . . . . . . 69 140 19.1.4. Fake Data . . . . . . . . . . . . . . . . . . . . . 69 141 19.1.5. Impersonating a Server . . . . . . . . . . . . . . . 70 142 19.1.6. Eavesdropping Traffic . . . . . . . . . . . . . . . 70 143 19.1.7. TURN Loop Attack . . . . . . . . . . . . . . . . . . 71 144 19.2. Firewall Considerations . . . . . . . . . . . . . . . . 71 145 19.2.1. Faked Permissions . . . . . . . . . . . . . . . . . 72 146 19.2.2. Blacklisted IP Addresses . . . . . . . . . . . . . . 72 147 19.2.3. Running Servers on Well-Known Ports . . . . . . . . 73 148 19.3. Insider Attacks . . . . . . . . . . . . . . . . . . . . 73 149 19.3.1. DoS against TURN Server . . . . . . . . . . . . . . 73 150 19.3.2. Anonymous Relaying of Malicious Traffic . . . . . . 73 151 19.3.3. Manipulating Other Allocations . . . . . . . . . . . 74 152 19.4. Tunnel Amplification Attack . . . . . . . . . . . . . . 74 153 19.5. Other Considerations . . . . . . . . . . . . . . . . . . 75 154 20. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 75 155 21. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 76 156 22. Changes since RFC 5766 . . . . . . . . . . . . . . . . . . . 78 157 23. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 78 158 24. References . . . . . . . . . . . . . . . . . . . . . . . . . 78 159 24.1. Normative References . . . . . . . . . . . . . . . . . . 78 160 24.2. Informative References . . . . . . . . . . . . . . . . . 80 161 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 83 163 1. Introduction 165 A host behind a NAT may wish to exchange packets with other hosts, 166 some of which may also be behind NATs. To do this, the hosts 167 involved can use "hole punching" techniques (see [RFC5128]) in an 168 attempt discover a direct communication path; that is, a 169 communication path that goes from one host to another through 170 intervening NATs and routers, but does not traverse any relays. 172 As described in [RFC5128] and [RFC4787], hole punching techniques 173 will fail if both hosts are behind NATs that are not well behaved. 174 For example, if both hosts are behind NATs that have a mapping 175 behavior of "address-dependent mapping" or "address- and port- 176 dependent mapping", then hole punching techniques generally fail. 178 When a direct communication path cannot be found, it is necessary to 179 use the services of an intermediate host that acts as a relay for the 180 packets. This relay typically sits in the public Internet and relays 181 packets between two hosts that both sit behind NATs. 183 This specification defines a protocol, called TURN, that allows a 184 host behind a NAT (called the TURN client) to request that another 185 host (called the TURN server) act as a relay. The client can arrange 186 for the server to relay packets to and from certain other hosts 187 (called peers) and can control aspects of how the relaying is done. 188 The client does this by obtaining an IP address and port on the 189 server, called the relayed transport address. When a peer sends a 190 packet to the relayed transport address, the server relays the packet 191 to the client. When the client sends a data packet to the server, 192 the server relays it to the appropriate peer using the relayed 193 transport address as the source. 195 A client using TURN must have some way to communicate the relayed 196 transport address to its peers, and to learn each peer's IP address 197 and port (more precisely, each peer's server-reflexive transport 198 address, see Section 2). How this is done is out of the scope of the 199 TURN protocol. One way this might be done is for the client and 200 peers to exchange email messages. Another way is for the client and 201 its peers to use a special-purpose "introduction" or "rendezvous" 202 protocol (see [RFC5128] for more details). 204 If TURN is used with ICE [RFC5245], then the relayed transport 205 address and the IP addresses and ports of the peers are included in 206 the ICE candidate information that the rendezvous protocol must 207 carry. For example, if TURN and ICE are used as part of a multimedia 208 solution using SIP [RFC3261], then SIP serves the role of the 209 rendezvous protocol, carrying the ICE candidate information inside 210 the body of SIP messages. If TURN and ICE are used with some other 211 rendezvous protocol, then [I-D.rosenberg-mmusic-ice-nonsip] provides 212 guidance on the services the rendezvous protocol must perform. 214 Though the use of a TURN server to enable communication between two 215 hosts behind NATs is very likely to work, it comes at a high cost to 216 the provider of the TURN server, since the server typically needs a 217 high-bandwidth connection to the Internet. As a consequence, it is 218 best to use a TURN server only when a direct communication path 219 cannot be found. When the client and a peer use ICE to determine the 220 communication path, ICE will use hole punching techniques to search 221 for a direct path first and only use a TURN server when a direct path 222 cannot be found. 224 TURN was originally invented to support multimedia sessions signaled 225 using SIP. Since SIP supports forking, TURN supports multiple peers 226 per relayed transport address; a feature not supported by other 227 approaches (e.g., SOCKS [RFC1928]). However, care has been taken to 228 make sure that TURN is suitable for other types of applications. 230 TURN was designed as one piece in the larger ICE approach to NAT 231 traversal. Implementors of TURN are urged to investigate ICE and 232 seriously consider using it for their application. However, it is 233 possible to use TURN without ICE. 235 TURN is an extension to the STUN (Session Traversal Utilities for 236 NAT) protocol [I-D.ietf-tram-stunbis]. Most, though not all, TURN 237 messages are STUN-formatted messages. A reader of this document 238 should be familiar with STUN. 240 2. Overview of Operation 242 This section gives an overview of the operation of TURN. It is non- 243 normative. 245 In a typical configuration, a TURN client is connected to a private 246 network [RFC1918] and through one or more NATs to the public 247 Internet. On the public Internet is a TURN server. Elsewhere in the 248 Internet are one or more peers with which the TURN client wishes to 249 communicate. These peers may or may not be behind one or more NATs. 250 The client uses the server as a relay to send packets to these peers 251 and to receive packets from these peers. 253 Peer A 254 Server-Reflexive +---------+ 255 Transport Address | | 256 192.0.2.150:32102 | | 257 | /| | 258 TURN | / ^| Peer A | 259 Client's Server | / || | 260 Host Transport Transport | // || | 261 Address Address | // |+---------+ 262 198.51.100.2:49721 192.0.2.15:3478 |+-+ // Peer A 263 | | ||N| / Host Transport 264 | +-+ | ||A|/ Address 265 | | | | v|T| 203.0.113.2:49582 266 | | | | /+-+ 267 +---------+| | | |+---------+ / +---------+ 268 | || |N| || | // | | 269 | TURN |v | | v| TURN |/ | | 270 | Client |----|A|----------| Server |------------------| Peer B | 271 | | | |^ | |^ ^| | 272 | | |T|| | || || | 273 +---------+ | || +---------+| |+---------+ 274 | || | | 275 | || | | 276 +-+| | | 277 | | | 278 | | | 279 Client's | Peer B 280 Server-Reflexive Relayed Transport 281 Transport Address Transport Address Address 282 192.0.2.1:7000 192.0.2.15:50000 192.0.2.210:49191 284 Figure 1 286 Figure 1 shows a typical deployment. In this figure, the TURN client 287 and the TURN server are separated by a NAT, with the client on the 288 private side and the server on the public side of the NAT. This NAT 289 is assumed to be a "bad" NAT; for example, it might have a mapping 290 property of "address-and-port-dependent mapping" (see [RFC4787]). 292 The client talks to the server from a (IP address, port) combination 293 called the client's HOST TRANSPORT ADDRESS. (The combination of an 294 IP address and port is called a TRANSPORT ADDRESS.) 296 The client sends TURN messages from its host transport address to a 297 transport address on the TURN server that is known as the TURN SERVER 298 TRANSPORT ADDRESS. The client learns the TURN server transport 299 address through some unspecified means (e.g., configuration), and 300 this address is typically used by many clients simultaneously. 302 Since the client is behind a NAT, the server sees packets from the 303 client as coming from a transport address on the NAT itself. This 304 address is known as the client's SERVER-REFLEXIVE transport address; 305 packets sent by the server to the client's server-reflexive transport 306 address will be forwarded by the NAT to the client's host transport 307 address. 309 The client uses TURN commands to create and manipulate an ALLOCATION 310 on the server. An allocation is a data structure on the server. 311 This data structure contains, amongst other things, the RELAYED 312 TRANSPORT ADDRESS for the allocation. The relayed transport address 313 is the transport address on the server that peers can use to have the 314 server relay data to the client. An allocation is uniquely 315 identified by its relayed transport address. 317 Once an allocation is created, the client can send application data 318 to the server along with an indication of to which peer the data is 319 to be sent, and the server will relay this data to the appropriate 320 peer. The client sends the application data to the server inside a 321 TURN message; at the server, the data is extracted from the TURN 322 message and sent to the peer in a UDP datagram. In the reverse 323 direction, a peer can send application data in a UDP datagram to the 324 relayed transport address for the allocation; the server will then 325 encapsulate this data inside a TURN message and send it to the client 326 along with an indication of which peer sent the data. Since the TURN 327 message always contains an indication of which peer the client is 328 communicating with, the client can use a single allocation to 329 communicate with multiple peers. 331 When the peer is behind a NAT, then the client must identify the peer 332 using its server-reflexive transport address rather than its host 333 transport address. For example, to send application data to Peer A 334 in the example above, the client must specify 192.0.2.150:32102 (Peer 335 A's server-reflexive transport address) rather than 203.0.113.2:49582 336 (Peer A's host transport address). 338 Each allocation on the server belongs to a single client and has 339 exactly one relayed transport address that is used only by that 340 allocation. Thus, when a packet arrives at a relayed transport 341 address on the server, the server knows for which client the data is 342 intended. 344 The client may have multiple allocations on a server at the same 345 time. 347 2.1. Transports 349 TURN, as defined in this specification, always uses UDP between the 350 server and the peer. However, this specification allows the use of 351 any one of UDP, TCP, Transport Layer Security (TLS) over TCP or 352 Datagram Transport Layer Security (DTLS) over UDP to carry the TURN 353 messages between the client and the server. 355 +----------------------------+---------------------+ 356 | TURN client to TURN server | TURN server to peer | 357 +----------------------------+---------------------+ 358 | UDP | UDP | 359 | TCP | UDP | 360 | TLS-over-TCP | UDP | 361 | DTLS-over-UDP | UDP | 362 +----------------------------+---------------------+ 364 If TCP or TLS-over-TCP is used between the client and the server, 365 then the server will convert between these transports and UDP 366 transport when relaying data to/from the peer. 368 Since this version of TURN only supports UDP between the server and 369 the peer, it is expected that most clients will prefer to use UDP 370 between the client and the server as well. That being the case, some 371 readers may wonder: Why also support TCP and TLS-over-TCP? 373 TURN supports TCP transport between the client and the server because 374 some firewalls are configured to block UDP entirely. These firewalls 375 block UDP but not TCP, in part because TCP has properties that make 376 the intention of the nodes being protected by the firewall more 377 obvious to the firewall. For example, TCP has a three-way handshake 378 that makes in clearer that the protected node really wishes to have 379 that particular connection established, while for UDP the best the 380 firewall can do is guess which flows are desired by using filtering 381 rules. Also, TCP has explicit connection teardown; while for UDP, 382 the firewall has to use timers to guess when the flow is finished. 384 TURN supports TLS-over-TCP transport and DTLS-over-UDP transport 385 between the client and the server because (D)TLS provides additional 386 security properties not provided by TURN's default digest 387 authentication; properties that some clients may wish to take 388 advantage of. In particular, (D)TLS provides a way for the client to 389 ascertain that it is talking to the correct server, and provides for 390 confidentiality of TURN control messages. TURN does not require 391 (D)TLS because the overhead of using (D)TLS is higher than that of 392 digest authentication; for example, using (D)TLS likely means that 393 most application data will be doubly encrypted (once by (D)TLS and 394 once to ensure it is still encrypted in the UDP datagram). 396 There is an extension to TURN for TCP transport between the server 397 and the peers [RFC6062]. For this reason, allocations that use UDP 398 between the server and the peers are known as UDP allocations, while 399 allocations that use TCP between the server and the peers are known 400 as TCP allocations. This specification describes only UDP 401 allocations. 403 In some applications for TURN, the client may send and receive 404 packets other than TURN packets on the host transport address it uses 405 to communicate with the server. This can happen, for example, when 406 using TURN with ICE. In these cases, the client can distinguish TURN 407 packets from other packets by examining the source address of the 408 arriving packet: those arriving from the TURN server will be TURN 409 packets. The algorithm of demultiplexing packets received from 410 multiple protocols on the host transport address is discussed in 411 [RFC7983]. 413 2.2. Allocations 415 To create an allocation on the server, the client uses an Allocate 416 transaction. The client sends an Allocate request to the server, and 417 the server replies with an Allocate success response containing the 418 allocated relayed transport address. The client can include 419 attributes in the Allocate request that describe the type of 420 allocation it desires (e.g., the lifetime of the allocation). Since 421 relaying data has security implications, the server requires that the 422 client authenticate itself, typically using STUN's long-term 423 credential mechanism or the STUN Extension for Third-Party 424 Authorization [RFC7635], to show that it is authorized to use the 425 server. 427 Once a relayed transport address is allocated, a client must keep the 428 allocation alive. To do this, the client periodically sends a 429 Refresh request to the server. TURN deliberately uses a different 430 method (Refresh rather than Allocate) for refreshes to ensure that 431 the client is informed if the allocation vanishes for some reason. 433 The frequency of the Refresh transaction is determined by the 434 lifetime of the allocation. The default lifetime of an allocation is 435 10 minutes -- this value was chosen to be long enough so that 436 refreshing is not typically a burden on the client, while expiring 437 allocations where the client has unexpectedly quit in a timely 438 manner. However, the client can request a longer lifetime in the 439 Allocate request and may modify its request in a Refresh request, and 440 the server always indicates the actual lifetime in the response. The 441 client must issue a new Refresh transaction within "lifetime" seconds 442 of the previous Allocate or Refresh transaction. Once a client no 443 longer wishes to use an allocation, it should delete the allocation 444 using a Refresh request with a requested lifetime of 0. 446 Both the server and client keep track of a value known as the 447 5-TUPLE. At the client, the 5-tuple consists of the client's host 448 transport address, the server transport address, and the transport 449 protocol used by the client to communicate with the server. At the 450 server, the 5-tuple value is the same except that the client's host 451 transport address is replaced by the client's server-reflexive 452 address, since that is the client's address as seen by the server. 454 Both the client and the server remember the 5-tuple used in the 455 Allocate request. Subsequent messages between the client and the 456 server use the same 5-tuple. In this way, the client and server know 457 which allocation is being referred to. If the client wishes to 458 allocate a second relayed transport address, it must create a second 459 allocation using a different 5-tuple (e.g., by using a different 460 client host address or port). 462 NOTE: While the terminology used in this document refers to 463 5-tuples, the TURN server can store whatever identifier it likes 464 that yields identical results. Specifically, an implementation 465 may use a file-descriptor in place of a 5-tuple to represent a TCP 466 connection. 468 TURN TURN Peer Peer 469 client server A B 470 |-- Allocate request --------------->| | | 471 | | | | 472 |<--------------- Allocate failure --| | | 473 | (401 Unauthenticated) | | | 474 | | | | 475 |-- Allocate request --------------->| | | 476 | | | | 477 |<---------- Allocate success resp --| | | 478 | (192.0.2.15:50000) | | | 479 // // // // 480 | | | | 481 |-- Refresh request ---------------->| | | 482 | | | | 483 |<----------- Refresh success resp --| | | 484 | | | | 486 Figure 2 488 In Figure 2, the client sends an Allocate request to the server 489 without credentials. Since the server requires that all requests be 490 authenticated using STUN's long-term credential mechanism, the server 491 rejects the request with a 401 (Unauthorized) error code. The client 492 then tries again, this time including credentials (not shown). This 493 time, the server accepts the Allocate request and returns an Allocate 494 success response containing (amongst other things) the relayed 495 transport address assigned to the allocation. Sometime later, the 496 client decides to refresh the allocation and thus sends a Refresh 497 request to the server. The refresh is accepted and the server 498 replies with a Refresh success response. 500 2.3. Permissions 502 To ease concerns amongst enterprise IT administrators that TURN could 503 be used to bypass corporate firewall security, TURN includes the 504 notion of permissions. TURN permissions mimic the address-restricted 505 filtering mechanism of NATs that comply with [RFC4787]. 507 An allocation can have zero or more permissions. Each permission 508 consists of an IP address and a lifetime. When the server receives a 509 UDP datagram on the allocation's relayed transport address, it first 510 checks the list of permissions. If the source IP address of the 511 datagram matches a permission, the application data is relayed to the 512 client, otherwise the UDP datagram is silently discarded. 514 A permission expires after 5 minutes if it is not refreshed, and 515 there is no way to explicitly delete a permission. This behavior was 516 selected to match the behavior of a NAT that complies with [RFC4787]. 518 The client can install or refresh a permission using either a 519 CreatePermission request or a ChannelBind request. Using the 520 CreatePermission request, multiple permissions can be installed or 521 refreshed with a single request -- this is important for applications 522 that use ICE. For security reasons, permissions can only be 523 installed or refreshed by transactions that can be authenticated; 524 thus, Send indications and ChannelData messages (which are used to 525 send data to peers) do not install or refresh any permissions. 527 Note that permissions are within the context of an allocation, so 528 adding or expiring a permission in one allocation does not affect 529 other allocations. 531 2.4. Send Mechanism 533 There are two mechanisms for the client and peers to exchange 534 application data using the TURN server. The first mechanism uses the 535 Send and Data methods, the second mechanism uses channels. Common to 536 both mechanisms is the ability of the client to communicate with 537 multiple peers using a single allocated relayed transport address; 538 thus, both mechanisms include a means for the client to indicate to 539 the server which peer should receive the data, and for the server to 540 indicate to the client which peer sent the data. 542 The Send mechanism uses Send and Data indications. Send indications 543 are used to send application data from the client to the server, 544 while Data indications are used to send application data from the 545 server to the client. 547 When using the Send mechanism, the client sends a Send indication to 548 the TURN server containing (a) an XOR-PEER-ADDRESS attribute 549 specifying the (server-reflexive) transport address of the peer and 550 (b) a DATA attribute holding the application data. When the TURN 551 server receives the Send indication, it extracts the application data 552 from the DATA attribute and sends it in a UDP datagram to the peer, 553 using the allocated relay address as the source address. Note that 554 there is no need to specify the relayed transport address, since it 555 is implied by the 5-tuple used for the Send indication. 557 In the reverse direction, UDP datagrams arriving at the relayed 558 transport address on the TURN server are converted into Data 559 indications and sent to the client, with the server-reflexive 560 transport address of the peer included in an XOR-PEER-ADDRESS 561 attribute and the data itself in a DATA attribute. Since the relayed 562 transport address uniquely identified the allocation, the server 563 knows which client should receive the data. 565 Some ICMP (Internet Control Message Protocol) packets arriving at the 566 relayed transport address on the TURN server may be converted into 567 Data indications and sent to the client, with the transport address 568 of the peer included in an XOR-PEER-ADDRESS attribute and the ICMP 569 type and code in a ICMP attribute. ICMP attribute forwarding always 570 uses Data indications containing the XOR-PEER-ADDRESS and ICMP 571 attributes, even when using the channel mechanism to forward UDP 572 data. 574 Send and Data indications cannot be authenticated, since the long- 575 term credential mechanism of STUN does not support authenticating 576 indications. This is not as big an issue as it might first appear, 577 since the client-to-server leg is only half of the total path to the 578 peer. Applications that want proper security should encrypt the data 579 sent between the client and a peer. 581 Because Send indications are not authenticated, it is possible for an 582 attacker to send bogus Send indications to the server, which will 583 then relay these to a peer. To partly mitigate this attack, TURN 584 requires that the client install a permission towards a peer before 585 sending data to it using a Send indication. 587 TURN TURN Peer Peer 588 client server A B 589 | | | | 590 |-- CreatePermission req (Peer A) -->| | | 591 |<-- CreatePermission success resp --| | | 592 | | | | 593 |--- Send ind (Peer A)-------------->| | | 594 | |=== data ===>| | 595 | | | | 596 | |<== data ====| | 597 |<-------------- Data ind (Peer A) --| | | 598 | | | | 599 | | | | 600 |--- Send ind (Peer B)-------------->| | | 601 | | dropped | | 602 | | | | 603 | |<== data ==================| 604 | dropped | | | 605 | | | | 607 Figure 3 609 In Figure 3, the client has already created an allocation and now 610 wishes to send data to its peers. The client first creates a 611 permission by sending the server a CreatePermission request 612 specifying Peer A's (server-reflexive) IP address in the XOR-PEER- 613 ADDRESS attribute; if this was not done, the server would not relay 614 data between the client and the server. The client then sends data 615 to Peer A using a Send indication; at the server, the application 616 data is extracted and forwarded in a UDP datagram to Peer A, using 617 the relayed transport address as the source transport address. When 618 a UDP datagram from Peer A is received at the relayed transport 619 address, the contents are placed into a Data indication and forwarded 620 to the client. Later, the client attempts to exchange data with Peer 621 B; however, no permission has been installed for Peer B, so the Send 622 indication from the client and the UDP datagram from the peer are 623 both dropped by the server. 625 2.5. Channels 627 For some applications (e.g., Voice over IP), the 36 bytes of overhead 628 that a Send indication or Data indication adds to the application 629 data can substantially increase the bandwidth required between the 630 client and the server. To remedy this, TURN offers a second way for 631 the client and server to associate data with a specific peer. 633 This second way uses an alternate packet format known as the 634 ChannelData message. The ChannelData message does not use the STUN 635 header used by other TURN messages, but instead has a 4-byte header 636 that includes a number known as a channel number. Each channel 637 number in use is bound to a specific peer and thus serves as a 638 shorthand for the peer's host transport address. 640 To bind a channel to a peer, the client sends a ChannelBind request 641 to the server, and includes an unbound channel number and the 642 transport address of the peer. Once the channel is bound, the client 643 can use a ChannelData message to send the server data destined for 644 the peer. Similarly, the server can relay data from that peer 645 towards the client using a ChannelData message. 647 Channel bindings last for 10 minutes unless refreshed -- this 648 lifetime was chosen to be longer than the permission lifetime. 649 Channel bindings are refreshed by sending another ChannelBind request 650 rebinding the channel to the peer. Like permissions (but unlike 651 allocations), there is no way to explicitly delete a channel binding; 652 the client must simply wait for it to time out. 654 TURN TURN Peer Peer 655 client server A B 656 | | | | 657 |-- ChannelBind req ---------------->| | | 658 | (Peer A to 0x4001) | | | 659 | | | | 660 |<---------- ChannelBind succ resp --| | | 661 | | | | 662 |-- (0x4001) data ------------------>| | | 663 | |=== data ===>| | 664 | | | | 665 | |<== data ====| | 666 |<------------------ (0x4001) data --| | | 667 | | | | 668 |--- Send ind (Peer A)-------------->| | | 669 | |=== data ===>| | 670 | | | | 671 | |<== data ====| | 672 |<------------------ (0x4001) data --| | | 673 | | | | 675 Figure 4 677 Figure 4 shows the channel mechanism in use. The client has already 678 created an allocation and now wishes to bind a channel to Peer A. To 679 do this, the client sends a ChannelBind request to the server, 680 specifying the transport address of Peer A and a channel number 681 (0x4001). After that, the client can send application data 682 encapsulated inside ChannelData messages to Peer A: this is shown as 683 "(0x4001) data" where 0x4001 is the channel number. When the 684 ChannelData message arrives at the server, the server transfers the 685 data to a UDP datagram and sends it to Peer A (which is the peer 686 bound to channel number 0x4001). 688 In the reverse direction, when Peer A sends a UDP datagram to the 689 relayed transport address, this UDP datagram arrives at the server on 690 the relayed transport address assigned to the allocation. Since the 691 UDP datagram was received from Peer A, which has a channel number 692 assigned to it, the server encapsulates the data into a ChannelData 693 message when sending the data to the client. 695 Once a channel has been bound, the client is free to intermix 696 ChannelData messages and Send indications. In the figure, the client 697 later decides to use a Send indication rather than a ChannelData 698 message to send additional data to Peer A. The client might decide 699 to do this, for example, so it can use the DONT-FRAGMENT attribute 700 (see the next section). However, once a channel is bound, the server 701 will always use a ChannelData message, as shown in the call flow. 703 Note that ChannelData messages can only be used for peers to which 704 the client has bound a channel. In the example above, Peer A has 705 been bound to a channel, but Peer B has not, so application data to 706 and from Peer B would use the Send mechanism. 708 2.6. Unprivileged TURN Servers 710 This version of TURN is designed so that the server can be 711 implemented as an application that runs in user space under commonly 712 available operating systems without requiring special privileges. 713 This design decision was made to make it easy to deploy a TURN 714 server: for example, to allow a TURN server to be integrated into a 715 peer-to-peer application so that one peer can offer NAT traversal 716 services to another peer. 718 This design decision has the following implications for data relayed 719 by a TURN server: 721 o The value of the Diffserv field may not be preserved across the 722 server; 724 o The Time to Live (TTL) field may be reset, rather than 725 decremented, across the server; 727 o The Explicit Congestion Notification (ECN) field may be reset by 728 the server; 730 o There is no end-to-end fragmentation, since the packet is re- 731 assembled at the server. 733 Future work may specify alternate TURN semantics that address these 734 limitations. 736 2.7. Avoiding IP Fragmentation 738 For reasons described in [Frag-Harmful], applications, especially 739 those sending large volumes of data, should try hard to avoid having 740 their packets fragmented. Applications using TCP can more or less 741 ignore this issue because fragmentation avoidance is now a standard 742 part of TCP, but applications using UDP (and thus any application 743 using this version of TURN) must handle fragmentation avoidance 744 themselves. 746 The application running on the client and the peer can take one of 747 two approaches to avoid IP fragmentation. 749 The first approach is to avoid sending large amounts of application 750 data in the TURN messages/UDP datagrams exchanged between the client 751 and the peer. This is the approach taken by most VoIP (Voice-over- 752 IP) applications. In this approach, the application exploits the 753 fact that the IP specification [RFC0791] specifies that IP packets up 754 to 576 bytes should never need to be fragmented. 756 The exact amount of application data that can be included while 757 avoiding fragmentation depends on the details of the TURN session 758 between the client and the server: whether UDP, TCP, or (D)TLS 759 transport is used, whether ChannelData messages or Send/Data 760 indications are used, and whether any additional attributes (such as 761 the DONT-FRAGMENT attribute) are included. Another factor, which is 762 hard to determine, is whether the MTU is reduced somewhere along the 763 path for other reasons, such as the use of IP-in-IP tunneling. 765 As a guideline, sending a maximum of 500 bytes of application data in 766 a single TURN message (by the client on the client-to-server leg) or 767 a UDP datagram (by the peer on the peer-to-server leg) will generally 768 avoid IP fragmentation. To further reduce the chance of 769 fragmentation, it is recommended that the client use ChannelData 770 messages when transferring significant volumes of data, since the 771 overhead of the ChannelData message is less than Send and Data 772 indications. 774 The second approach the client and peer can take to avoid 775 fragmentation is to use a path MTU discovery algorithm to determine 776 the maximum amount of application data that can be sent without 777 fragmentation. The classic path MTU discovery algorithm defined in 778 [RFC1191] may not be able to discover the MTU of the transmission 779 path between the client and the peer since: 781 - a probe packet with DF bit set to test a path for a larger MTU 782 can be dropped by routers, or 784 - ICMP error messages can be dropped by middle boxes. 786 As a result, the client and server need to use a path MTU discovery 787 algorithm that does not require ICMP messages. The Packetized Path 788 MTU Discovery algorithm defined in [RFC4821] is one such algorithm. 790 [I-D.ietf-tram-stun-pmtud] is an implementation of [RFC4821] that 791 uses STUN to discover the path MTU, and so might be a suitable 792 approach to be used in conjunction with a TURN server that supports 793 the DONT-FRAGMENT attribute. When the client includes the DONT- 794 FRAGMENT attribute in a Send indication, this tells the server to set 795 the DF bit in the resulting UDP datagram that it sends to the peer. 796 Since some servers may be unable to set the DF bit, the client should 797 also include this attribute in the Allocate request -- any server 798 that does not support the DONT-FRAGMENT attribute will indicate this 799 by rejecting the Allocate request. 801 2.8. RTP Support 803 One of the envisioned uses of TURN is as a relay for clients and 804 peers wishing to exchange real-time data (e.g., voice or video) using 805 RTP. To facilitate the use of TURN for this purpose, TURN includes 806 some special support for older versions of RTP. 808 Old versions of RTP [RFC3550] required that the RTP stream be on an 809 even port number and the associated RTP Control Protocol (RTCP) 810 stream, if present, be on the next highest port. To allow clients to 811 work with peers that still require this, TURN allows the client to 812 request that the server allocate a relayed transport address with an 813 even port number, and to optionally request the server reserve the 814 next-highest port number for a subsequent allocation. 816 2.9. Happy Eyeballs for TURN 818 If an IPv4 path to reach a TURN server is found, but the TURN 819 server's IPv6 path is not working, a dual-stack TURN client can 820 experience a significant connection delay compared to an IPv4-only 821 TURN client. To overcome these connection setup problems, the TURN 822 client MUST query both A and AAAA records for the TURN server 823 specified using a domain name and try connecting to the TURN server 824 using both IPv6 and IPv4 addresses in a fashion similar to the Happy 825 Eyeballs mechanism defined in [RFC8305]. The TURN client performs 826 the following steps based on the transport protocol being used to 827 connect to the TURN server. 829 o For TCP or TLS-over-TCP, initiate TCP connection to both IP 830 address families as discussed in [RFC8305], and use the first TCP 831 connection that is established. If connections are established on 832 both IP address families then terminate the TCP connection using 833 the IP address family with lower precedence [RFC6724]. 835 o For clear text UDP, send TURN Allocate requests to both IP address 836 families as discussed in [RFC8305], without authentication 837 information. If the TURN server requires authentication, it will 838 send back a 401 unauthenticated response and the TURN client uses 839 the first UDP connection on which a 401 error response is 840 received. If a 401 error response is received from both IP 841 address families then the TURN client can silently abandon the UDP 842 connection on the IP address family with lower precedence. If the 843 TURN server does not require authentication (as described in 844 Section 9 of [RFC8155]), it is possible for both Allocate requests 845 to succeed. In this case, the TURN client sends a Refresh with 846 LIFETIME value of 0 on the allocation using the IP address family 847 with lower precedence to delete the allocation. 849 o For DTLS over UDP, initiate DTLS handshake to both IP address 850 families as discussed in [RFC8305] and use the first DTLS session 851 that is established. If the DTLS session is established on both 852 IP address families then the client sends DTLS close_notify alert 853 to terminate the DTLS session using the IP address family with 854 lower precedence. If TURN over DTLS server has been configured to 855 require a cookie exchange (Section 4.2 in [RFC6347]) and 856 HelloVerifyRequest is received from the TURN servers on both IP 857 address families then the client can silently abandon the 858 connection on the IP address family with lower precedence. 860 3. Terminology 862 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 863 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 864 document are to be interpreted as described in RFC 2119 [RFC2119]. 866 Readers are expected to be familiar with [I-D.ietf-tram-stunbis] and 867 the terms defined there. 869 The following terms are used in this document: 871 TURN: The protocol spoken between a TURN client and a TURN server. 872 It is an extension to the STUN protocol [I-D.ietf-tram-stunbis]. 873 The protocol allows a client to allocate and use a relayed 874 transport address. 876 TURN client: A STUN client that implements this specification. 878 TURN server: A STUN server that implements this specification. It 879 relays data between a TURN client and its peer(s). 881 Peer: A host with which the TURN client wishes to communicate. The 882 TURN server relays traffic between the TURN client and its 883 peer(s). The peer does not interact with the TURN server using 884 the protocol defined in this document; rather, the peer receives 885 data sent by the TURN server and the peer sends data towards the 886 TURN server. 888 Transport Address: The combination of an IP address and a port. 890 Host Transport Address: A transport address on a client or a peer. 892 Server-Reflexive Transport Address: A transport address on the 893 "public side" of a NAT. This address is allocated by the NAT to 894 correspond to a specific host transport address. 896 Relayed Transport Address: A transport address on the TURN server 897 that is used for relaying packets between the client and a peer. 898 A peer sends to this address on the TURN server, and the packet is 899 then relayed to the client. 901 TURN Server Transport Address: A transport address on the TURN 902 server that is used for sending TURN messages to the server. This 903 is the transport address that the client uses to communicate with 904 the server. 906 Peer Transport Address: The transport address of the peer as seen by 907 the server. When the peer is behind a NAT, this is the peer's 908 server-reflexive transport address. 910 Allocation: The relayed transport address granted to a client 911 through an Allocate request, along with related state, such as 912 permissions and expiration timers. 914 5-tuple: The combination (client IP address and port, server IP 915 address and port, and transport protocol (currently one of UDP, 916 TCP, or (D)TLS)) used to communicate between the client and the 917 server. The 5-tuple uniquely identifies this communication 918 stream. The 5-tuple also uniquely identifies the Allocation on 919 the server. 921 Channel: A channel number and associated peer transport address. 922 Once a channel number is bound to a peer's transport address, the 923 client and server can use the more bandwidth-efficient ChannelData 924 message to exchange data. 926 Permission: The IP address and transport protocol (but not the port) 927 of a peer that is permitted to send traffic to the TURN server and 928 have that traffic relayed to the TURN client. The TURN server 929 will only forward traffic to its client from peers that match an 930 existing permission. 932 Realm: A string used to describe the server or a context within the 933 server. The realm tells the client which username and password 934 combination to use to authenticate requests. 936 Nonce: A string chosen at random by the server and included in the 937 message-digest. To prevent replay attacks, the server should 938 change the nonce regularly. 940 (D)TLS: This term is used for statements that apply to both 941 Transport Layer Security [RFC5246] and Datagram Transport Layer 942 Security [RFC6347]. 944 4. Discovery of TURN server 946 Methods of TURN server discovery, including using anycast, are 947 described in [RFC8155]. The syntax of the "turn" and "turns" URIs 948 are defined in Section 3.1 of [RFC7065]. 950 4.1. TURN URI Scheme Semantics 952 The "turn" and "turns" URI schemes are used to designate a TURN 953 server (also known as a relay) on Internet hosts accessible using the 954 TURN protocol. The TURN protocol supports sending messages over UDP, 955 TCP, TLS-over-TCP or DTLS-over-UDP. The "turns" URI scheme MUST be 956 used when TURN is run over TLS-over-TCP or in DTLS-over-UDP, and the 957 "turn" scheme MUST be used otherwise. The required part of 958 the "turn" URI denotes the TURN server host. The part, if 959 present, denotes the port on which the TURN server is awaiting 960 connection requests. If it is absent, the default port is 3478 for 961 both UDP and TCP. The default port for TURN over TLS and TURN over 962 DTLS is 5349. 964 5. General Behavior 966 This section contains general TURN processing rules that apply to all 967 TURN messages. 969 TURN is an extension to STUN. All TURN messages, with the exception 970 of the ChannelData message, are STUN-formatted messages. All the 971 base processing rules described in [I-D.ietf-tram-stunbis] apply to 972 STUN-formatted messages. This means that all the message-forming and 973 message-processing descriptions in this document are implicitly 974 prefixed with the rules of [I-D.ietf-tram-stunbis]. 976 [I-D.ietf-tram-stunbis] specifies an authentication mechanism called 977 the long-term credential mechanism. TURN servers and clients MUST 978 implement this mechanism. The server MUST demand that all requests 979 from the client be authenticated using this mechanism, or that a 980 equally strong or stronger mechanism for client authentication is 981 used. 983 Note that the long-term credential mechanism applies only to requests 984 and cannot be used to authenticate indications; thus, indications in 985 TURN are never authenticated. If the server requires requests to be 986 authenticated, then the server's administrator MUST choose a realm 987 value that will uniquely identify the username and password 988 combination that the client must use, even if the client uses 989 multiple servers under different administrations. The server's 990 administrator MAY choose to allocate a unique username to each 991 client, or MAY choose to allocate the same username to more than one 992 client (for example, to all clients from the same department or 993 company). For each Allocate request, the server SHOULD generate a 994 new random nonce when the allocation is first attempted following the 995 randomness recommendations in [RFC4086] and SHOULD expire the nonce 996 at least once every hour during the lifetime of the allocation. 998 All requests after the initial Allocate must use the same username as 999 that used to create the allocation, to prevent attackers from 1000 hijacking the client's allocation. Specifically, if the server 1001 requires the use of the long-term credential mechanism, and if a non- 1002 Allocate request passes authentication under this mechanism, and if 1003 the 5-tuple identifies an existing allocation, but the request does 1004 not use the same username as used to create the allocation, then the 1005 request MUST be rejected with a 441 (Wrong Credentials) error. 1007 When a TURN message arrives at the server from the client, the server 1008 uses the 5-tuple in the message to identify the associated 1009 allocation. For all TURN messages (including ChannelData) EXCEPT an 1010 Allocate request, if the 5-tuple does not identify an existing 1011 allocation, then the message MUST either be rejected with a 437 1012 Allocation Mismatch error (if it is a request) or silently ignored 1013 (if it is an indication or a ChannelData message). A client 1014 receiving a 437 error response to a request other than Allocate MUST 1015 assume the allocation no longer exists. 1017 [I-D.ietf-tram-stunbis] defines a number of attributes, including the 1018 SOFTWARE and FINGERPRINT attributes. The client SHOULD include the 1019 SOFTWARE attribute in all Allocate and Refresh requests and MAY 1020 include it in any other requests or indications. The server SHOULD 1021 include the SOFTWARE attribute in all Allocate and Refresh responses 1022 (either success or failure) and MAY include it in other responses or 1023 indications. The client and the server MAY include the FINGERPRINT 1024 attribute in any STUN-formatted messages defined in this document. 1026 TURN does not use the backwards-compatibility mechanism described in 1027 [I-D.ietf-tram-stunbis]. 1029 TURN, as defined in this specification, supports both IPv4 and IPv6. 1030 IPv6 support in TURN includes IPv4-to-IPv6, IPv6-to-IPv6, and IPv6- 1031 to-IPv4 relaying. The REQUESTED-ADDRESS-FAMILY attribute allows a 1032 client to explicitly request the address type the TURN server will 1033 allocate (e.g., an IPv4-only node may request the TURN server to 1034 allocate an IPv6 address). The ADDITIONAL-ADDRESS-FAMILY attribute 1035 allows a client to request the server to allocate one IPv4 and one 1036 IPv6 relay address in a single Allocate request. This saves local 1037 ports on the client and reduces the number of messages sent between 1038 the client and the TURN server. 1040 By default, TURN runs on the same ports as STUN: 3478 for TURN over 1041 UDP and TCP, and 5349 for TURN over (D)TLS. However, TURN has its 1042 own set of Service Record (SRV) names: "turn" for UDP and TCP, and 1043 "turns" for (D)TLS. Either the DNS resolution procedures or the 1044 ALTERNATE-SERVER procedures, both described in Section 7, can be used 1045 to run TURN on a different port. 1047 To ensure interoperability, a TURN server MUST support the use of UDP 1048 transport between the client and the server, and SHOULD support the 1049 use of TCP, TLS-over-TCP and DTLS-over-UDP transports. 1051 When UDP or DTLS-over-UDP transport is used between the client and 1052 the server, the client will retransmit a request if it does not 1053 receive a response within a certain timeout period. Because of this, 1054 the server may receive two (or more) requests with the same 5-tuple 1055 and same transaction id. STUN requires that the server recognize 1056 this case and treat the request as idempotent (see 1057 [I-D.ietf-tram-stunbis]). Some implementations may choose to meet 1058 this requirement by remembering all received requests and the 1059 corresponding responses for 40 seconds. Other implementations may 1060 choose to reprocess the request and arrange that such reprocessing 1061 returns essentially the same response. To aid implementors who 1062 choose the latter approach (the so-called "stateless stack 1063 approach"), this specification includes some implementation notes on 1064 how this might be done. Implementations are free to choose either 1065 approach or choose some other approach that gives the same results. 1067 When TCP transport is used between the client and the server, it is 1068 possible that a bit error will cause a length field in a TURN packet 1069 to become corrupted, causing the receiver to lose synchronization 1070 with the incoming stream of TURN messages. A client or server that 1071 detects a long sequence of invalid TURN messages over TCP transport 1072 SHOULD close the corresponding TCP connection to help the other end 1073 detect this situation more rapidly. 1075 To mitigate either intentional or unintentional denial-of-service 1076 attacks against the server by clients with valid usernames and 1077 passwords, it is RECOMMENDED that the server impose limits on both 1078 the number of allocations active at one time for a given username and 1079 on the amount of bandwidth those allocations can use. The server 1080 should reject new allocations that would exceed the limit on the 1081 allowed number of allocations active at one time with a 486 1082 (Allocation Quota Exceeded) (see Section 7.2), and should discard 1083 application data traffic that exceeds the bandwidth quota. 1085 6. Allocations 1087 All TURN operations revolve around allocations, and all TURN messages 1088 are associated with either a single or dual allocation. An 1089 allocation conceptually consists of the following state data: 1091 o the relayed transport address or addresses; 1093 o the 5-tuple: (client's IP address, client's port, server IP 1094 address, server port, transport protocol); 1096 o the authentication information; 1098 o the time-to-expiry for each relayed transport address; 1100 o a list of permissions for each relayed transport address; 1102 o a list of channel to peer bindings for each relayed transport 1103 address. 1105 The relayed transport address is the transport address allocated by 1106 the server for communicating with peers, while the 5-tuple describes 1107 the communication path between the client and the server. On the 1108 client, the 5-tuple uses the client's host transport address; on the 1109 server, the 5-tuple uses the client's server-reflexive transport 1110 address. The relayed transport address MUST be unique across all 1111 allocations, so it can be used to uniquely identify the allocation. 1113 Both the relayed transport address and the 5-tuple MUST be unique 1114 across all allocations, so either one can be used to uniquely 1115 identify the allocation, and an allocation in this context can be 1116 either a single or dual allocation. 1118 The authentication information (e.g., username, password, realm, and 1119 nonce) is used to both verify subsequent requests and to compute the 1120 message integrity of responses. The username, realm, and nonce 1121 values are initially those used in the authenticated Allocate request 1122 that creates the allocation, though the server can change the nonce 1123 value during the lifetime of the allocation using a 438 (Stale Nonce) 1124 reply. Note that, rather than storing the password explicitly, for 1125 security reasons, it may be desirable for the server to store the key 1126 value, which is a secure hash over the username, realm, and password 1127 (see [I-D.ietf-tram-stunbis]). 1129 The time-to-expiry is the time in seconds left until the allocation 1130 expires. Each Allocate or Refresh transaction sets this timer, which 1131 then ticks down towards 0. By default, each Allocate or Refresh 1132 transaction resets this timer to the default lifetime value of 600 1133 seconds (10 minutes), but the client can request a different value in 1134 the Allocate and Refresh request. Allocations can only be refreshed 1135 using the Refresh request; sending data to a peer does not refresh an 1136 allocation. When an allocation expires, the state data associated 1137 with the allocation can be freed. 1139 The list of permissions is described in Section 9 and the list of 1140 channels is described in Section 12. 1142 7. Creating an Allocation 1144 An allocation on the server is created using an Allocate transaction. 1146 7.1. Sending an Allocate Request 1148 The client forms an Allocate request as follows. 1150 The client first picks a host transport address. It is RECOMMENDED 1151 that the client pick a currently unused transport address, typically 1152 by allowing the underlying OS to pick a currently unused port for a 1153 new socket. 1155 The client then picks a transport protocol to use between the client 1156 and the server. The transport protocol MUST be one of UDP, TCP, TLS- 1157 over-TCP or DTLS-over-UDP. Since this specification only allows UDP 1158 between the server and the peers, it is RECOMMENDED that the client 1159 pick UDP unless it has a reason to use a different transport. One 1160 reason to pick a different transport would be that the client 1161 believes, either through configuration or by experiment, that it is 1162 unable to contact any TURN server using UDP. See Section 2.1 for 1163 more discussion. 1165 The client also picks a server transport address, which SHOULD be 1166 done as follows. The client uses one or more procedures described in 1167 [RFC8155] to discover a TURN server and uses the TURN server 1168 resolution mechanism defined in [RFC5928] to get a list of server 1169 transport addresses that can be tried to create a TURN allocation. 1171 The client MUST include a REQUESTED-TRANSPORT attribute in the 1172 request. This attribute specifies the transport protocol between the 1173 server and the peers (note that this is NOT the transport protocol 1174 that appears in the 5-tuple). In this specification, the REQUESTED- 1175 TRANSPORT type is always UDP. This attribute is included to allow 1176 future extensions to specify other protocols. 1178 If the client wishes to obtain a relayed transport address of a 1179 specific address type then it includes a REQUESTED-ADDRESS-FAMILY 1180 attribute in the request. This attribute indicates the specific 1181 address type the client wishes the TURN server to allocate. Clients 1182 MUST NOT include more than one REQUESTED-ADDRESS-FAMILY attribute in 1183 an Allocate request. Clients MUST NOT include a REQUESTED-ADDRESS- 1184 FAMILY attribute in an Allocate request that contains a RESERVATION- 1185 TOKEN attribute, for the reasons outlined in [RFC6156]. 1187 If the client wishes to obtain one IPv6 and one IPv4 relayed 1188 transport address then it includes an ADDITIONAL-ADDRESS-FAMILY 1189 attribute in the request. This attribute specifies that the server 1190 must allocate both address types. The attribute value in the 1191 ADDITIONAL-ADDRESS-FAMILY MUST be set to 0x02 (IPv6 address family). 1192 Clients MUST NOT include REQUESTED-ADDRESS-FAMILY and ADDITIONAL- 1193 ADDRESS-FAMILY attributes in the same request. Clients MUST NOT 1194 include ADDITIONAL-ADDRESS-FAMILY attribute in a Allocate request 1195 that contains a RESERVATION-TOKEN attribute. Clients MUST NOT 1196 include ADDITIONAL-ADDRESS-FAMILY attribute in a Allocate request 1197 that contains an EVEN-PORT attribute with the R bit set to 1. The 1198 reason behind the restriction is if EVEN-PORT with R bit set to 1 is 1199 allowed with the ADDITIONAL-ADDRESS-FAMILY attribute, two tokens will 1200 have to be returned in success response and requires changes to the 1201 way RESERVATION-TOKEN is handled. 1203 If the client wishes the server to initialize the time-to-expiry 1204 field of the allocation to some value other than the default 1205 lifetime, then it MAY include a LIFETIME attribute specifying its 1206 desired value. This is just a hint, and the server may elect to use 1207 a different value. Note that the server will ignore requests to 1208 initialize the field to less than the default value. 1210 If the client wishes to later use the DONT-FRAGMENT attribute in one 1211 or more Send indications on this allocation, then the client SHOULD 1212 include the DONT-FRAGMENT attribute in the Allocate request. This 1213 allows the client to test whether this attribute is supported by the 1214 server. 1216 If the client requires the port number of the relayed transport 1217 address be even, the client includes the EVEN-PORT attribute. If 1218 this attribute is not included, then the port can be even or odd. By 1219 setting the R bit in the EVEN-PORT attribute to 1, the client can 1220 request that the server reserve the next highest port number (on the 1221 same IP address) for a subsequent allocation. If the R bit is 0, no 1222 such request is made. 1224 The client MAY also include a RESERVATION-TOKEN attribute in the 1225 request to ask the server to use a previously reserved port for the 1226 allocation. If the RESERVATION-TOKEN attribute is included, then the 1227 client MUST omit the EVEN-PORT attribute. 1229 Once constructed, the client sends the Allocate request on the 1230 5-tuple. 1232 7.2. Receiving an Allocate Request 1234 When the server receives an Allocate request, it performs the 1235 following checks: 1237 1. The server SHOULD require that the request be authenticated. 1238 The authentication of the request is optional to allow TURN 1239 servers provided by the local or access network to accept 1240 Allocation requests from new and/or guest users in the network 1241 who do not necessarily possess long term credentials for STUN 1242 authentication and its security implications are discussed in 1243 [RFC8155]. If the request is authenticated, the authentication 1244 MUST be done using the long-term credential mechanism of 1245 [I-D.ietf-tram-stunbis] unless the client and server agree to 1246 use another mechanism through some procedure outside the scope 1247 of this document. 1249 2. The server checks if the 5-tuple is currently in use by an 1250 existing allocation. If yes, the server rejects the request 1251 with a 437 (Allocation Mismatch) error. 1253 3. The server checks if the request contains a REQUESTED-TRANSPORT 1254 attribute. If the REQUESTED-TRANSPORT attribute is not included 1255 or is malformed, the server rejects the request with a 400 (Bad 1256 Request) error. Otherwise, if the attribute is included but 1257 specifies a protocol other that UDP, the server rejects the 1258 request with a 442 (Unsupported Transport Protocol) error. 1260 4. The request may contain a DONT-FRAGMENT attribute. If it does, 1261 but the server does not support sending UDP datagrams with the 1262 DF bit set to 1 (see Section 14), then the server treats the 1263 DONT-FRAGMENT attribute in the Allocate request as an unknown 1264 comprehension-required attribute. 1266 5. The server checks if the request contains a RESERVATION-TOKEN 1267 attribute. If yes, and the request also contains an EVEN-PORT 1268 or REQUESTED-ADDRESS-FAMILY or ADDITIONAL-ADDRESS-FAMILY 1269 attribute, the server rejects the request with a 400 (Bad 1270 Request) error. Otherwise, it checks to see if the token is 1271 valid (i.e., the token is in range and has not expired and the 1272 corresponding relayed transport address is still available). If 1273 the token is not valid for some reason, the server rejects the 1274 request with a 508 (Insufficient Capacity) error. 1276 6. The server checks if the request contains both REQUESTED- 1277 ADDRESS-FAMILY and ADDITIONAL-ADDRESS-FAMILY attributes. If 1278 yes, then the server rejects the request with a 400 (Bad 1279 Request) error. 1281 7. If the server does not support the address family requested by 1282 the client in REQUESTED-ADDRESS-FAMILY or is disabled by local 1283 policy, it MUST generate an Allocate error response, and it MUST 1284 include an ERROR-CODE attribute with the 440 (Address Family not 1285 Supported) response code. If the REQUESTED-ADDRESS-FAMILY 1286 attribute is absent and the server does not support IPv4 address 1287 family, the server MUST include an ERROR-CODE attribute with the 1288 440 (Address Family not Supported) response code. If the 1289 REQUESTED-ADDRESS-FAMILY attribute is absent and the server 1290 supports IPv4 address family, the server MUST allocate an IPv4 1291 relayed transport address for the TURN client. 1293 8. The server checks if the request contains an EVEN-PORT attribute 1294 with the R bit set to 1. If yes, and the request also contains 1295 an ADDITIONAL-ADDRESS-FAMILY attribute, the server rejects the 1296 request with a 400 (Bad Request) error. Otherwise, the server 1297 checks if it can satisfy the request (i.e., can allocate a 1298 relayed transport address as described below). If the server 1299 cannot satisfy the request, then the server rejects the request 1300 with a 508 (Insufficient Capacity) error. 1302 9. The server checks if the request contains an ADDITIONAL-ADDRESS- 1303 FAMILY attribute. If yes, and the attribute value is 0x01 (IPv4 1304 address family), then the server rejects the request with a 400 1305 (Bad Request) error. Otherwise, the server checks if it can 1306 allocate relayed transport addresses of both address types. If 1307 the server cannot satisfy the request, then the server rejects 1308 the request with a 508 (Insufficient Capacity) error. If the 1309 server can partially meet the request, i.e. if it can only 1310 allocate one relayed transport address of a specific address 1311 type, then it includes ADDRESS-ERROR-CODE attribute in the 1312 response to inform the client the reason for partial failure of 1313 the request. The error code value signaled in the ADDRESS- 1314 ERROR-CODE attribute could be 440 (Address Family not Supported) 1315 or 508 (Insufficient Capacity). If the server can fully meet 1316 the request, then the server allocates one IPv4 and one IPv6 1317 relay address, and returns an Allocate success response 1318 containing the relayed transport addresses assigned to the dual 1319 allocation in two XOR-RELAYED-ADDRESS attributes. 1321 10. At any point, the server MAY choose to reject the request with a 1322 486 (Allocation Quota Reached) error if it feels the client is 1323 trying to exceed some locally defined allocation quota. The 1324 server is free to define this allocation quota any way it 1325 wishes, but SHOULD define it based on the username used to 1326 authenticate the request, and not on the client's transport 1327 address. 1329 11. Also at any point, the server MAY choose to reject the request 1330 with a 300 (Try Alternate) error if it wishes to redirect the 1331 client to a different server. The use of this error code and 1332 attribute follow the specification in [I-D.ietf-tram-stunbis]. 1334 If all the checks pass, the server creates the allocation. The 1335 5-tuple is set to the 5-tuple from the Allocate request, while the 1336 list of permissions and the list of channels are initially empty. 1338 The server chooses a relayed transport address for the allocation as 1339 follows: 1341 o If the request contains a RESERVATION-TOKEN attribute, the server 1342 uses the previously reserved transport address corresponding to 1343 the included token (if it is still available). Note that the 1344 reservation is a server-wide reservation and is not specific to a 1345 particular allocation, since the Allocate request containing the 1346 RESERVATION-TOKEN uses a different 5-tuple than the Allocate 1347 request that made the reservation. The 5-tuple for the Allocate 1348 request containing the RESERVATION-TOKEN attribute can be any 1349 allowed 5-tuple; it can use a different client IP address and 1350 port, a different transport protocol, and even different server IP 1351 address and port (provided, of course, that the server IP address 1352 and port are ones on which the server is listening for TURN 1353 requests). 1355 o If the request contains an EVEN-PORT attribute with the R bit set 1356 to 0, then the server allocates a relayed transport address with 1357 an even port number. 1359 o If the request contains an EVEN-PORT attribute with the R bit set 1360 to 1, then the server looks for a pair of port numbers N and N+1 1361 on the same IP address, where N is even. Port N is used in the 1362 current allocation, while the relayed transport address with port 1363 N+1 is assigned a token and reserved for a future allocation. The 1364 server MUST hold this reservation for at least 30 seconds, and MAY 1365 choose to hold longer (e.g., until the allocation with port N 1366 expires). The server then includes the token in a RESERVATION- 1367 TOKEN attribute in the success response. 1369 o Otherwise, the server allocates any available relayed transport 1370 address. 1372 In all cases, the server SHOULD only allocate ports from the range 1373 49152 - 65535 (the Dynamic and/or Private Port range [Port-Numbers]), 1374 unless the TURN server application knows, through some means not 1375 specified here, that other applications running on the same host as 1376 the TURN server application will not be impacted by allocating ports 1377 outside this range. This condition can often be satisfied by running 1378 the TURN server application on a dedicated machine and/or by 1379 arranging that any other applications on the machine allocate ports 1380 before the TURN server application starts. In any case, the TURN 1381 server SHOULD NOT allocate ports in the range 0 - 1023 (the Well- 1382 Known Port range) to discourage clients from using TURN to run 1383 standard services. 1385 NOTE: The use of randomized port assignments to avoid certain 1386 types of attacks is described in [RFC6056]. It is RECOMMENDED 1387 that a TURN server implement a randomized port assignment 1388 algorithm from [RFC6056]. This is especially applicable to 1389 servers that choose to pre-allocate a number of ports from the 1390 underlying OS and then later assign them to allocations; for 1391 example, a server may choose this technique to implement the EVEN- 1392 PORT attribute. 1394 The server determines the initial value of the time-to-expiry field 1395 as follows. If the request contains a LIFETIME attribute, then the 1396 server computes the minimum of the client's proposed lifetime and the 1397 server's maximum allowed lifetime. If this computed value is greater 1398 than the default lifetime, then the server uses the computed lifetime 1399 as the initial value of the time-to-expiry field. Otherwise, the 1400 server uses the default lifetime. It is RECOMMENDED that the server 1401 use a maximum allowed lifetime value of no more than 3600 seconds (1 1402 hour). Servers that implement allocation quotas or charge users for 1403 allocations in some way may wish to use a smaller maximum allowed 1404 lifetime (perhaps as small as the default lifetime) to more quickly 1405 remove orphaned allocations (that is, allocations where the 1406 corresponding client has crashed or terminated or the client 1407 connection has been lost for some reason). Also, note that the time- 1408 to-expiry is recomputed with each successful Refresh request, and 1409 thus the value computed here applies only until the first refresh. 1411 Once the allocation is created, the server replies with a success 1412 response. The success response contains: 1414 o An XOR-RELAYED-ADDRESS attribute containing the relayed transport 1415 address. 1417 o A LIFETIME attribute containing the current value of the time-to- 1418 expiry timer. 1420 o A RESERVATION-TOKEN attribute (if a second relayed transport 1421 address was reserved). 1423 o An XOR-MAPPED-ADDRESS attribute containing the client's IP address 1424 and port (from the 5-tuple). 1426 NOTE: The XOR-MAPPED-ADDRESS attribute is included in the response 1427 as a convenience to the client. TURN itself does not make use of 1428 this value, but clients running ICE can often need this value and 1429 can thus avoid having to do an extra Binding transaction with some 1430 STUN server to learn it. 1432 The response (either success or error) is sent back to the client on 1433 the 5-tuple. 1435 NOTE: When the Allocate request is sent over UDP, 1436 [I-D.ietf-tram-stunbis] requires that the server handle the 1437 possible retransmissions of the request so that retransmissions do 1438 not cause multiple allocations to be created. Implementations may 1439 achieve this using the so-called "stateless stack approach" as 1440 follows. To detect retransmissions when the original request was 1441 successful in creating an allocation, the server can store the 1442 transaction id that created the request with the allocation data 1443 and compare it with incoming Allocate requests on the same 1444 5-tuple. Once such a request is detected, the server can stop 1445 parsing the request and immediately generate a success response. 1446 When building this response, the value of the LIFETIME attribute 1447 can be taken from the time-to-expiry field in the allocate state 1448 data, even though this value may differ slightly from the LIFETIME 1449 value originally returned. In addition, the server may need to 1450 store an indication of any reservation token returned in the 1451 original response, so that this may be returned in any 1452 retransmitted responses. 1454 For the case where the original request was unsuccessful in 1455 creating an allocation, the server may choose to do nothing 1456 special. Note, however, that there is a rare case where the 1457 server rejects the original request but accepts the retransmitted 1458 request (because conditions have changed in the brief intervening 1459 time period). If the client receives the first failure response, 1460 it will ignore the second (success) response and believe that an 1461 allocation was not created. An allocation created in this matter 1462 will eventually timeout, since the client will not refresh it. 1463 Furthermore, if the client later retries with the same 5-tuple but 1464 different transaction id, it will receive a 437 (Allocation 1465 Mismatch), which will cause it to retry with a different 5-tuple. 1466 The server may use a smaller maximum lifetime value to minimize 1467 the lifetime of allocations "orphaned" in this manner. 1469 7.3. Receiving an Allocate Success Response 1471 If the client receives an Allocate success response, then it MUST 1472 check that the mapped address and the relayed transport address or 1473 addresses are part of an address family or families that the client 1474 understands and is prepared to handle. If these addresses are not 1475 part of an address family or families which the client is prepared to 1476 handle, then the client MUST delete the allocation (Section 8) and 1477 MUST NOT attempt to create another allocation on that server until it 1478 believes the mismatch has been fixed. 1480 Otherwise, the client creates its own copy of the allocation data 1481 structure to track what is happening on the server. In particular, 1482 the client needs to remember the actual lifetime received back from 1483 the server, rather than the value sent to the server in the request. 1484 The client must also remember the 5-tuple used for the request and 1485 the username and password it used to authenticate the request to 1486 ensure that it reuses them for subsequent messages. The client also 1487 needs to track the channels and permissions it establishes on the 1488 server. 1490 If the client receives an Allocate success response but with ADDRESS- 1491 ERROR-CODE attribute in the response and the error code value 1492 signaled in the ADDRESS-ERROR-CODE attribute is 440 (Address Family 1493 not Supported), the client MUST NOT retry its request for the 1494 rejected address type. If the client receives an ADDRESS-ERROR-CODE 1495 attribute in the response and the error code value signaled in the 1496 ADDRESS-ERROR-CODE attribute is 508 (Insufficient Capacity), the 1497 client SHOULD wait at least 1 minute before trying to request any 1498 more allocations on this server for the rejected address type. 1500 The client will probably wish to send the relayed transport address 1501 to peers (using some method not specified here) so the peers can 1502 communicate with it. The client may also wish to use the server- 1503 reflexive address it receives in the XOR-MAPPED-ADDRESS attribute in 1504 its ICE processing. 1506 7.4. Receiving an Allocate Error Response 1508 If the client receives an Allocate error response, then the 1509 processing depends on the actual error code returned: 1511 o (Request timed out): There is either a problem with the server, or 1512 a problem reaching the server with the chosen transport. The 1513 client considers the current transaction as having failed but MAY 1514 choose to retry the Allocate request using a different transport 1515 (e.g., TCP instead of UDP). 1517 o 300 (Try Alternate): The server would like the client to use the 1518 server specified in the ALTERNATE-SERVER attribute instead. The 1519 client considers the current transaction as having failed, but 1520 SHOULD try the Allocate request with the alternate server before 1521 trying any other servers (e.g., other servers discovered using the 1522 DNS resolution procedures). When trying the Allocate request with 1523 the alternate server, the client follows the ALTERNATE-SERVER 1524 procedures specified in [I-D.ietf-tram-stunbis]. 1526 o 400 (Bad Request): The server believes the client's request is 1527 malformed for some reason. The client considers the current 1528 transaction as having failed. The client MAY notify the user or 1529 operator and SHOULD NOT retry the request with this server until 1530 it believes the problem has been fixed. 1532 o 401 (Unauthorized): If the client has followed the procedures of 1533 the long-term credential mechanism and still gets this error, then 1534 the server is not accepting the client's credentials. In this 1535 case, the client considers the current transaction as having 1536 failed and SHOULD notify the user or operator. The client SHOULD 1537 NOT send any further requests to this server until it believes the 1538 problem has been fixed. 1540 o 403 (Forbidden): The request is valid, but the server is refusing 1541 to perform it, likely due to administrative restrictions. The 1542 client considers the current transaction as having failed. The 1543 client MAY notify the user or operator and SHOULD NOT retry the 1544 same request with this server until it believes the problem has 1545 been fixed. 1547 o 420 (Unknown Attribute): If the client included a DONT-FRAGMENT 1548 attribute in the request and the server rejected the request with 1549 a 420 error code and listed the DONT-FRAGMENT attribute in the 1550 UNKNOWN-ATTRIBUTES attribute in the error response, then the 1551 client now knows that the server does not support the DONT- 1552 FRAGMENT attribute. The client considers the current transaction 1553 as having failed but MAY choose to retry the Allocate request 1554 without the DONT-FRAGMENT attribute. 1556 o 437 (Allocation Mismatch): This indicates that the client has 1557 picked a 5-tuple that the server sees as already in use. One way 1558 this could happen is if an intervening NAT assigned a mapped 1559 transport address that was used by another client that recently 1560 crashed. The client considers the current transaction as having 1561 failed. The client SHOULD pick another client transport address 1562 and retry the Allocate request (using a different transaction id). 1563 The client SHOULD try three different client transport addresses 1564 before giving up on this server. Once the client gives up on the 1565 server, it SHOULD NOT try to create another allocation on the 1566 server for 2 minutes. 1568 o 438 (Stale Nonce): See the procedures for the long-term credential 1569 mechanism [I-D.ietf-tram-stunbis]. 1571 o 440 (Address Family not Supported): The server does not support 1572 the address family requested by the client. If the client 1573 receives an Allocate error response with the 440 (Unsupported 1574 Address Family) error code, the client MUST NOT retry the request. 1576 o 441 (Wrong Credentials): The client should not receive this error 1577 in response to a Allocate request. The client MAY notify the user 1578 or operator and SHOULD NOT retry the same request with this server 1579 until it believes the problem has been fixed. 1581 o 442 (Unsupported Transport Address): The client should not receive 1582 this error in response to a request for a UDP allocation. The 1583 client MAY notify the user or operator and SHOULD NOT reattempt 1584 the request with this server until it believes the problem has 1585 been fixed. 1587 o 486 (Allocation Quota Reached): The server is currently unable to 1588 create any more allocations with this username. The client 1589 considers the current transaction as having failed. The client 1590 SHOULD wait at least 1 minute before trying to create any more 1591 allocations on the server. 1593 o 508 (Insufficient Capacity): The server has no more relayed 1594 transport addresses available, or has none with the requested 1595 properties, or the one that was reserved is no longer available. 1596 The client considers the current operation as having failed. If 1597 the client is using either the EVEN-PORT or the RESERVATION-TOKEN 1598 attribute, then the client MAY choose to remove or modify this 1599 attribute and try again immediately. Otherwise, the client SHOULD 1600 wait at least 1 minute before trying to create any more 1601 allocations on this server. 1603 An unknown error response MUST be handled as described in 1604 [I-D.ietf-tram-stunbis]. 1606 8. Refreshing an Allocation 1608 A Refresh transaction can be used to either (a) refresh an existing 1609 allocation and update its time-to-expiry or (b) delete an existing 1610 allocation. 1612 If a client wishes to continue using an allocation, then the client 1613 MUST refresh it before it expires. It is suggested that the client 1614 refresh the allocation roughly 1 minute before it expires. If a 1615 client no longer wishes to use an allocation, then it SHOULD 1616 explicitly delete the allocation. A client MAY refresh an allocation 1617 at any time for other reasons. 1619 8.1. Sending a Refresh Request 1621 If the client wishes to immediately delete an existing allocation, it 1622 includes a LIFETIME attribute with a value of 0. All other forms of 1623 the request refresh the allocation. 1625 When refreshing a dual allocation, the client includes REQUESTED- 1626 ADDRESS-FAMILY attribute indicating the address family type that 1627 should be refreshed. If no REQUESTED-ADDRESS-FAMILY is included then 1628 the request should be treated as applying to all current allocations. 1629 The client MUST only include family types it previously allocated and 1630 has not yet deleted. This process can also be used to delete an 1631 allocation of a specific address type, by setting the lifetime of 1632 that refresh request to 0. Deleting a single allocation destroys any 1633 permissions or channels associated with that particular allocation; 1634 it MUST NOT affect any permissions or channels associated with 1635 allocations for the other address family. 1637 The Refresh transaction updates the time-to-expiry timer of an 1638 allocation. If the client wishes the server to set the time-to- 1639 expiry timer to something other than the default lifetime, it 1640 includes a LIFETIME attribute with the requested value. The server 1641 then computes a new time-to-expiry value in the same way as it does 1642 for an Allocate transaction, with the exception that a requested 1643 lifetime of 0 causes the server to immediately delete the allocation. 1645 8.2. Receiving a Refresh Request 1647 When the server receives a Refresh request, it processes the request 1648 as per Section 5 plus the specific rules mentioned here. 1650 If the server receives a Refresh Request with a REQUESTED-ADDRESS- 1651 FAMILY attribute and the attribute value does not match the address 1652 family of the allocation, the server MUST reply with a 443 (Peer 1653 Address Family Mismatch) Refresh error response. 1655 The server computes a value called the "desired lifetime" as follows: 1656 if the request contains a LIFETIME attribute and the attribute value 1657 is 0, then the "desired lifetime" is 0. Otherwise, if the request 1658 contains a LIFETIME attribute, then the server computes the minimum 1659 of the client's requested lifetime and the server's maximum allowed 1660 lifetime. If this computed value is greater than the default 1661 lifetime, then the "desired lifetime" is the computed value. 1662 Otherwise, the "desired lifetime" is the default lifetime. 1664 Subsequent processing depends on the "desired lifetime" value: 1666 o If the "desired lifetime" is 0, then the request succeeds and the 1667 allocation is deleted. 1669 o If the "desired lifetime" is non-zero, then the request succeeds 1670 and the allocation's time-to-expiry is set to the "desired 1671 lifetime". 1673 If the request succeeds, then the server sends a success response 1674 containing: 1676 o A LIFETIME attribute containing the current value of the time-to- 1677 expiry timer. 1679 NOTE: A server need not do anything special to implement 1680 idempotency of Refresh requests over UDP using the "stateless 1681 stack approach". Retransmitted Refresh requests with a non-zero 1682 "desired lifetime" will simply refresh the allocation. A 1683 retransmitted Refresh request with a zero "desired lifetime" will 1684 cause a 437 (Allocation Mismatch) response if the allocation has 1685 already been deleted, but the client will treat this as equivalent 1686 to a success response (see below). 1688 8.3. Receiving a Refresh Response 1690 If the client receives a success response to its Refresh request with 1691 a non-zero lifetime, it updates its copy of the allocation data 1692 structure with the time-to-expiry value contained in the response. 1694 If the client receives a 437 (Allocation Mismatch) error response to 1695 a request to delete the allocation, then the allocation no longer 1696 exists and it should consider its request as having effectively 1697 succeeded. 1699 9. Permissions 1701 For each allocation, the server keeps a list of zero or more 1702 permissions. Each permission consists of an IP address and an 1703 associated time-to-expiry. While a permission exists, all peers 1704 using the IP address in the permission are allowed to send data to 1705 the client. The time-to-expiry is the number of seconds until the 1706 permission expires. Within the context of an allocation, a 1707 permission is uniquely identified by its associated IP address. 1709 By sending either CreatePermission requests or ChannelBind requests, 1710 the client can cause the server to install or refresh a permission 1711 for a given IP address. This causes one of two things to happen: 1713 o If no permission for that IP address exists, then a permission is 1714 created with the given IP address and a time-to-expiry equal to 1715 Permission Lifetime. 1717 o If a permission for that IP address already exists, then the time- 1718 to-expiry for that permission is reset to Permission Lifetime. 1720 The Permission Lifetime MUST be 300 seconds (= 5 minutes). 1722 Each permission's time-to-expiry decreases down once per second until 1723 it reaches 0; at which point, the permission expires and is deleted. 1725 CreatePermission and ChannelBind requests may be freely intermixed on 1726 a permission. A given permission may be initially installed and/or 1727 refreshed with a CreatePermission request, and then later refreshed 1728 with a ChannelBind request, or vice versa. 1730 When a UDP datagram arrives at the relayed transport address for the 1731 allocation, the server extracts the source IP address from the IP 1732 header. The server then compares this address with the IP address 1733 associated with each permission in the list of permissions for the 1734 allocation. If no match is found, relaying is not permitted, and the 1735 server silently discards the UDP datagram. If an exact match is 1736 found, then the permission check is considered to have succeeded and 1737 the server continues to process the UDP datagram as specified 1738 elsewhere (Section 11.3). Note that only addresses are compared and 1739 port numbers are not considered. 1741 The permissions for one allocation are totally unrelated to the 1742 permissions for a different allocation. If an allocation expires, 1743 all its permissions expire with it. 1745 NOTE: Though TURN permissions expire after 5 minutes, many NATs 1746 deployed at the time of publication expire their UDP bindings 1747 considerably faster. Thus, an application using TURN will 1748 probably wish to send some sort of keep-alive traffic at a much 1749 faster rate. Applications using ICE should follow the keep-alive 1750 guidelines of ICE [RFC5245], and applications not using ICE are 1751 advised to do something similar. 1753 10. CreatePermission 1755 TURN supports two ways for the client to install or refresh 1756 permissions on the server. This section describes one way: the 1757 CreatePermission request. 1759 A CreatePermission request may be used in conjunction with either the 1760 Send mechanism in Section 11 or the Channel mechanism in Section 12. 1762 10.1. Forming a CreatePermission Request 1764 The client who wishes to install or refresh one or more permissions 1765 can send a CreatePermission request to the server. 1767 When forming a CreatePermission request, the client MUST include at 1768 least one XOR-PEER-ADDRESS attribute, and MAY include more than one 1769 such attribute. The IP address portion of each XOR-PEER-ADDRESS 1770 attribute contains the IP address for which a permission should be 1771 installed or refreshed. The port portion of each XOR-PEER-ADDRESS 1772 attribute will be ignored and can be any arbitrary value. The 1773 various XOR-PEER-ADDRESS attributes MAY appear in any order. The 1774 client MUST only include XOR-PEER-ADDRESS attributes with addresses 1775 of the same address family as that of the relayed transport address 1776 for the allocation. For dual allocations obtained using the 1777 ADDITIONAL-ADDRESS-FAMILY attribute, the client MAY include XOR-PEER- 1778 ADDRESS attributes with addresses of IPv4 and IPv6 address families. 1780 10.2. Receiving a CreatePermission Request 1782 When the server receives the CreatePermission request, it processes 1783 as per Section 5 plus the specific rules mentioned here. 1785 The message is checked for validity. The CreatePermission request 1786 MUST contain at least one XOR-PEER-ADDRESS attribute and MAY contain 1787 multiple such attributes. If no such attribute exists, or if any of 1788 these attributes are invalid, then a 400 (Bad Request) error is 1789 returned. If the request is valid, but the server is unable to 1790 satisfy the request due to some capacity limit or similar, then a 508 1791 (Insufficient Capacity) error is returned. 1793 If an XOR-PEER-ADDRESS attribute contains an address of an address 1794 family that is not the same as that of a relayed transport address 1795 for the allocation, the server MUST generate an error response with 1796 the 443 (Peer Address Family Mismatch) response code. 1798 The server MAY impose restrictions on the IP address allowed in the 1799 XOR-PEER-ADDRESS attribute -- if a value is not allowed, the server 1800 rejects the request with a 403 (Forbidden) error. 1802 If the message is valid and the server is capable of carrying out the 1803 request, then the server installs or refreshes a permission for the 1804 IP address contained in each XOR-PEER-ADDRESS attribute as described 1805 in Section 9. The port portion of each attribute is ignored and may 1806 be any arbitrary value. 1808 The server then responds with a CreatePermission success response. 1809 There are no mandatory attributes in the success response. 1811 NOTE: A server need not do anything special to implement 1812 idempotency of CreatePermission requests over UDP using the 1813 "stateless stack approach". Retransmitted CreatePermission 1814 requests will simply refresh the permissions. 1816 10.3. Receiving a CreatePermission Response 1818 If the client receives a valid CreatePermission success response, 1819 then the client updates its data structures to indicate that the 1820 permissions have been installed or refreshed. 1822 11. Send and Data Methods 1824 TURN supports two mechanisms for sending and receiving data from 1825 peers. This section describes the use of the Send and Data 1826 mechanisms, while Section 12 describes the use of the Channel 1827 mechanism. 1829 11.1. Forming a Send Indication 1831 The client can use a Send indication to pass data to the server for 1832 relaying to a peer. A client may use a Send indication even if a 1833 channel is bound to that peer. However, the client MUST ensure that 1834 there is a permission installed for the IP address of the peer to 1835 which the Send indication is being sent; this prevents a third party 1836 from using a TURN server to send data to arbitrary destinations. 1838 When forming a Send indication, the client MUST include an XOR-PEER- 1839 ADDRESS attribute and a DATA attribute. The XOR-PEER-ADDRESS 1840 attribute contains the transport address of the peer to which the 1841 data is to be sent, and the DATA attribute contains the actual 1842 application data to be sent to the peer. 1844 The client MAY include a DONT-FRAGMENT attribute in the Send 1845 indication if it wishes the server to set the DF bit on the UDP 1846 datagram sent to the peer. 1848 11.2. Receiving a Send Indication 1850 When the server receives a Send indication, it processes as per 1851 Section 5 plus the specific rules mentioned here. 1853 The message is first checked for validity. The Send indication MUST 1854 contain both an XOR-PEER-ADDRESS attribute and a DATA attribute. If 1855 one of these attributes is missing or invalid, then the message is 1856 discarded. Note that the DATA attribute is allowed to contain zero 1857 bytes of data. 1859 The Send indication may also contain the DONT-FRAGMENT attribute. If 1860 the server is unable to set the DF bit on outgoing UDP datagrams when 1861 this attribute is present, then the server acts as if the DONT- 1862 FRAGMENT attribute is an unknown comprehension-required attribute 1863 (and thus the Send indication is discarded). 1865 The server also checks that there is a permission installed for the 1866 IP address contained in the XOR-PEER-ADDRESS attribute. If no such 1867 permission exists, the message is discarded. Note that a Send 1868 indication never causes the server to refresh the permission. 1870 The server MAY impose restrictions on the IP address and port values 1871 allowed in the XOR-PEER-ADDRESS attribute -- if a value is not 1872 allowed, the server silently discards the Send indication. 1874 If everything is OK, then the server forms a UDP datagram as follows: 1876 o the source transport address is the relayed transport address of 1877 the allocation, where the allocation is determined by the 5-tuple 1878 on which the Send indication arrived; 1880 o the destination transport address is taken from the XOR-PEER- 1881 ADDRESS attribute; 1883 o the data following the UDP header is the contents of the value 1884 field of the DATA attribute. 1886 The handling of the DONT-FRAGMENT attribute (if present), is 1887 described in Section 14. 1889 The resulting UDP datagram is then sent to the peer. 1891 11.3. Receiving a UDP Datagram 1893 When the server receives a UDP datagram at a currently allocated 1894 relayed transport address, the server looks up the allocation 1895 associated with the relayed transport address. The server then 1896 checks to see whether the set of permissions for the allocation allow 1897 the relaying of the UDP datagram as described in Section 9. 1899 If relaying is permitted, then the server checks if there is a 1900 channel bound to the peer that sent the UDP datagram (see 1901 Section 12). If a channel is bound, then processing proceeds as 1902 described in Section 12.7. 1904 If relaying is permitted but no channel is bound to the peer, then 1905 the server forms and sends a Data indication. The Data indication 1906 MUST contain both an XOR-PEER-ADDRESS and a DATA attribute. The DATA 1907 attribute is set to the value of the 'data octets' field from the 1908 datagram, and the XOR-PEER-ADDRESS attribute is set to the source 1909 transport address of the received UDP datagram. The Data indication 1910 is then sent on the 5-tuple associated with the allocation. 1912 11.4. Receiving a Data Indication 1914 When the client receives a Data indication, it checks that the Data 1915 indication contains an XOR-PEER-ADDRESS attribute, and discards the 1916 indication if it does not. The client SHOULD also check that the 1917 XOR-PEER-ADDRESS attribute value contains an IP address with which 1918 the client believes there is an active permission, and discard the 1919 Data indication otherwise. 1921 NOTE: The latter check protects the client against an attacker who 1922 somehow manages to trick the server into installing permissions 1923 not desired by the client. 1925 If the XOR-PEER-ADDRESS is present and valid, the client checks that 1926 the Data indication contains either a DATA attribute or an ICMP 1927 attribute and discards the indication if it does not. Note that a 1928 DATA attribute is allowed to contain zero bytes of data. Processing 1929 of Data indications with an ICMP attribute is described in 1930 Section 11.6. 1932 If the Data indication passes the above checks, the client delivers 1933 the data octets inside the DATA attribute to the application, along 1934 with an indication that they were received from the peer whose 1935 transport address is given by the XOR-PEER-ADDRESS attribute. 1937 11.5. Receiving an ICMP Packet 1939 When the server receives an ICMP packet, the server verifies that the 1940 type is either 3, 11 or 12 for an ICMPv4 [RFC0792] packet or either 1941 1, 2, or 3 for an ICMPv6 [RFC4443] packet. It also verifies that the 1942 IP packet in the ICMP packet payload contains a UDP header. If 1943 either of these conditions fail, then the ICMP packet is silently 1944 dropped. 1946 The server looks up the allocation whose relayed transport address 1947 corresponds to the encapsulated packet's source IP address and UDP 1948 port. If no such allocation exists, the packet is silently dropped. 1949 The server then checks to see whether the set of permissions for the 1950 allocation allows the relaying of the ICMP packet. For ICMP packets, 1951 the source IP address MUST NOT be checked against the permissions 1952 list as it would be for UDP packets. Instead, the server extracts 1953 the destination IP address from the encapsulated IP header. The 1954 server then compares this address with the IP address associated with 1955 each permission in the list of permissions for the allocation. If no 1956 match is found, relaying is not permitted, and the server silently 1957 discards the ICMP packet. Note that only addresses are compared and 1958 port numbers are not considered. 1960 If relaying is permitted then the server forms and sends a Data 1961 indication. The Data indication MUST contain both an XOR-PEER- 1962 ADDRESS and an ICMP attribute. The ICMP attribute is set to the 1963 value of the type and code fields from the ICMP packet. The IP 1964 address portion of XOR-PEER-ADDRESS attribute is set to the 1965 destination IP address in the encapsulated IP header. At the time of 1966 writing of this specification, Socket APIs on some operating systems 1967 do not deliver the destination port in the encapsulated UDP header to 1968 applications without superuser privileges. If destination port in 1969 the encapsulated UDP header is available to the server then the port 1970 portion of XOR-PEER-ADDRESS attribute is set to the destination port 1971 otherwise the port portion is set to 0. The Data indication is then 1972 sent on the 5-tuple associated with the allocation. 1974 11.6. Receiving a Data Indication with an ICMP attribute 1976 When the client receives a Data indication with an ICMP attribute, it 1977 checks that the Data indication contains an XOR-PEER-ADDRESS 1978 attribute, and discards the indication if it does not. The client 1979 SHOULD also check that the XOR-PEER-ADDRESS attribute value contains 1980 an IP address with an active permission, and discard the Data 1981 indication otherwise. 1983 If the Data indication passes the above checks, the client signals 1984 the application of the error condition, along with an indication that 1985 it was received from the peer whose transport address is given by the 1986 XOR-PEER-ADDRESS attribute. The application can make sense of the 1987 meaning of the type and code values in the ICMP attribute by using 1988 the family field in the XOR-PEER-ADDRESS attribute. 1990 12. Channels 1992 Channels provide a way for the client and server to send application 1993 data using ChannelData messages, which have less overhead than Send 1994 and Data indications. 1996 The ChannelData message (see Section 12.4) starts with a two-byte 1997 field that carries the channel number. The values of this field are 1998 allocated as follows: 2000 0x0000 through 0x3FFF: These values can never be used for channel 2001 numbers. 2003 0x4000 through 0x4FFF: These values are the allowed channel 2004 numbers (4096 possible values). 2006 0x5000-0xFFFF: Reserved (For DTLS-SRTP multiplexing collision 2007 avoidance, see [RFC7983]. 2009 According to [RFC7983], ChannelData messages can be distinguished 2010 from other multiplexed protocols by examining the first byte of the 2011 message: 2013 [0..3] STUN 2015 [16..19] ZRTP 2017 [20..63] DTLS 2019 [64..79] TURN Channel 2021 [128..191] RTP/RTCP 2023 others Reserved, MUST be dropped and an alert MAY be logged 2025 Reserved values may be used in the future by other protocols. When 2026 the client uses channel binding, it MUST comply with the 2027 demultiplexing scheme discussed above. 2029 Channel bindings are always initiated by the client. The client can 2030 bind a channel to a peer at any time during the lifetime of the 2031 allocation. The client may bind a channel to a peer before 2032 exchanging data with it, or after exchanging data with it (using Send 2033 and Data indications) for some time, or may choose never to bind a 2034 channel to it. The client can also bind channels to some peers while 2035 not binding channels to other peers. 2037 Channel bindings are specific to an allocation, so that the use of a 2038 channel number or peer transport address in a channel binding in one 2039 allocation has no impact on their use in a different allocation. If 2040 an allocation expires, all its channel bindings expire with it. 2042 A channel binding consists of: 2044 o a channel number; 2046 o a transport address (of the peer); and 2048 o A time-to-expiry timer. 2050 Within the context of an allocation, a channel binding is uniquely 2051 identified either by the channel number or by the peer's transport 2052 address. Thus, the same channel cannot be bound to two different 2053 transport addresses, nor can the same transport address be bound to 2054 two different channels. 2056 A channel binding lasts for 10 minutes unless refreshed. Refreshing 2057 the binding (by the server receiving a ChannelBind request rebinding 2058 the channel to the same peer) resets the time-to-expiry timer back to 2059 10 minutes. 2061 When the channel binding expires, the channel becomes unbound. Once 2062 unbound, the channel number can be bound to a different transport 2063 address, and the transport address can be bound to a different 2064 channel number. To prevent race conditions, the client MUST wait 5 2065 minutes after the channel binding expires before attempting to bind 2066 the channel number to a different transport address or the transport 2067 address to a different channel number. 2069 When binding a channel to a peer, the client SHOULD be prepared to 2070 receive ChannelData messages on the channel from the server as soon 2071 as it has sent the ChannelBind request. Over UDP, it is possible for 2072 the client to receive ChannelData messages from the server before it 2073 receives a ChannelBind success response. 2075 In the other direction, the client MAY elect to send ChannelData 2076 messages before receiving the ChannelBind success response. Doing 2077 so, however, runs the risk of having the ChannelData messages dropped 2078 by the server if the ChannelBind request does not succeed for some 2079 reason (e.g., packet lost if the request is sent over UDP, or the 2080 server being unable to fulfill the request). A client that wishes to 2081 be safe should either queue the data or use Send indications until 2082 the channel binding is confirmed. 2084 12.1. Sending a ChannelBind Request 2086 A channel binding is created or refreshed using a ChannelBind 2087 transaction. A ChannelBind transaction also creates or refreshes a 2088 permission towards the peer (see Section 9). 2090 To initiate the ChannelBind transaction, the client forms a 2091 ChannelBind request. The channel to be bound is specified in a 2092 CHANNEL-NUMBER attribute, and the peer's transport address is 2093 specified in an XOR-PEER-ADDRESS attribute. Section 12.2 describes 2094 the restrictions on these attributes. The client MUST only include 2095 an XOR-PEER-ADDRESS attribute with an address of the same address 2096 family as that of a relayed transport address for the allocation. 2098 Rebinding a channel to the same transport address that it is already 2099 bound to provides a way to refresh a channel binding and the 2100 corresponding permission without sending data to the peer. Note 2101 however, that permissions need to be refreshed more frequently than 2102 channels. 2104 12.2. Receiving a ChannelBind Request 2106 When the server receives a ChannelBind request, it processes as per 2107 Section 5 plus the specific rules mentioned here. 2109 The server checks the following: 2111 o The request contains both a CHANNEL-NUMBER and an XOR-PEER-ADDRESS 2112 attribute; 2114 o The channel number is in the range 0x4000 through 0x7FFE 2115 (inclusive); 2117 o The channel number is not currently bound to a different transport 2118 address (same transport address is OK); 2120 o The transport address is not currently bound to a different 2121 channel number. 2123 o If the XOR-PEER-ADDRESS attribute contains an address of an 2124 address family that is not the same as that of a relayed transport 2125 address for the allocation, the server MUST generate an error 2126 response with the 443 (Peer Address Family Mismatch) response 2127 code. 2129 If any of these tests fail, the server replies with a 400 (Bad 2130 Request) error. 2132 The server MAY impose restrictions on the IP address and port values 2133 allowed in the XOR-PEER-ADDRESS attribute -- if a value is not 2134 allowed, the server rejects the request with a 403 (Forbidden) error. 2136 If the request is valid, but the server is unable to fulfill the 2137 request due to some capacity limit or similar, the server replies 2138 with a 508 (Insufficient Capacity) error. 2140 Otherwise, the server replies with a ChannelBind success response. 2141 There are no required attributes in a successful ChannelBind 2142 response. 2144 If the server can satisfy the request, then the server creates or 2145 refreshes the channel binding using the channel number in the 2146 CHANNEL-NUMBER attribute and the transport address in the XOR-PEER- 2147 ADDRESS attribute. The server also installs or refreshes a 2148 permission for the IP address in the XOR-PEER-ADDRESS attribute as 2149 described in Section 9. 2151 NOTE: A server need not do anything special to implement 2152 idempotency of ChannelBind requests over UDP using the "stateless 2153 stack approach". Retransmitted ChannelBind requests will simply 2154 refresh the channel binding and the corresponding permission. 2155 Furthermore, the client must wait 5 minutes before binding a 2156 previously bound channel number or peer address to a different 2157 channel, eliminating the possibility that the transaction would 2158 initially fail but succeed on a retransmission. 2160 12.3. Receiving a ChannelBind Response 2162 When the client receives a ChannelBind success response, it updates 2163 its data structures to record that the channel binding is now active. 2164 It also updates its data structures to record that the corresponding 2165 permission has been installed or refreshed. 2167 If the client receives a ChannelBind failure response that indicates 2168 that the channel information is out-of-sync between the client and 2169 the server (e.g., an unexpected 400 "Bad Request" response), then it 2170 is RECOMMENDED that the client immediately delete the allocation and 2171 start afresh with a new allocation. 2173 12.4. The ChannelData Message 2175 The ChannelData message is used to carry application data between the 2176 client and the server. It has the following format: 2178 0 1 2 3 2179 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 2180 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2181 | Channel Number | Length | 2182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2183 | | 2184 / Application Data / 2185 / / 2186 | | 2187 | +-------------------------------+ 2188 | | 2189 +-------------------------------+ 2191 The Channel Number field specifies the number of the channel on which 2192 the data is traveling, and thus the address of the peer that is 2193 sending or is to receive the data. 2195 The Length field specifies the length in bytes of the application 2196 data field (i.e., it does not include the size of the ChannelData 2197 header). Note that 0 is a valid length. 2199 The Application Data field carries the data the client is trying to 2200 send to the peer, or that the peer is sending to the client. 2202 12.5. Sending a ChannelData Message 2204 Once a client has bound a channel to a peer, then when the client has 2205 data to send to that peer it may use either a ChannelData message or 2206 a Send indication; that is, the client is not obligated to use the 2207 channel when it exists and may freely intermix the two message types 2208 when sending data to the peer. The server, on the other hand, MUST 2209 use the ChannelData message if a channel has been bound to the peer. 2210 The server uses a Data indication to signal the XOR-PEER-ADDRESS and 2211 ICMP attributes to the client even if a channel has been bound to the 2212 peer. 2214 The fields of the ChannelData message are filled in as described in 2215 Section 12.4. 2217 Over TCP and TLS-over-TCP, the ChannelData message MUST be padded to 2218 a multiple of four bytes in order to ensure the alignment of 2219 subsequent messages. The padding is not reflected in the length 2220 field of the ChannelData message, so the actual size of a ChannelData 2221 message (including padding) is (4 + Length) rounded up to the nearest 2222 multiple of 4. Over UDP, the padding is not required but MAY be 2223 included. 2225 The ChannelData message is then sent on the 5-tuple associated with 2226 the allocation. 2228 12.6. Receiving a ChannelData Message 2230 The receiver of the ChannelData message uses the first two bits to 2231 distinguish it from STUN-formatted messages, as described above. If 2232 the message uses a value in the reserved range (0x8000 through 2233 0xFFFF), then the message is silently discarded. 2235 If the ChannelData message is received in a UDP datagram, and if the 2236 UDP datagram is too short to contain the claimed length of the 2237 ChannelData message (i.e., the UDP header length field value is less 2238 than the ChannelData header length field value + 4 + 8), then the 2239 message is silently discarded. 2241 If the ChannelData message is received over TCP or over TLS-over-TCP, 2242 then the actual length of the ChannelData message is as described in 2243 Section 12.5. 2245 If the ChannelData message is received on a channel that is not bound 2246 to any peer, then the message is silently discarded. 2248 On the client, it is RECOMMENDED that the client discard the 2249 ChannelData message if the client believes there is no active 2250 permission towards the peer. On the server, the receipt of a 2251 ChannelData message MUST NOT refresh either the channel binding or 2252 the permission towards the peer. 2254 On the server, if no errors are detected, the server relays the 2255 application data to the peer by forming a UDP datagram as follows: 2257 o the source transport address is the relayed transport address of 2258 the allocation, where the allocation is determined by the 5-tuple 2259 on which the ChannelData message arrived; 2261 o the destination transport address is the transport address to 2262 which the channel is bound; 2264 o the data following the UDP header is the contents of the data 2265 field of the ChannelData message. 2267 The resulting UDP datagram is then sent to the peer. Note that if 2268 the Length field in the ChannelData message is 0, then there will be 2269 no data in the UDP datagram, but the UDP datagram is still formed and 2270 sent. 2272 12.7. Relaying Data from the Peer 2274 When the server receives a UDP datagram on the relayed transport 2275 address associated with an allocation, the server processes it as 2276 described in Section 11.3. If that section indicates that a 2277 ChannelData message should be sent (because there is a channel bound 2278 to the peer that sent to the UDP datagram), then the server forms and 2279 sends a ChannelData message as described in Section 12.5. 2281 When the server receives an ICMP packet, the server processes it as 2282 described in Section 11.5. A Data indication MUST be sent regardless 2283 of whether there is a channel bound to the peer that was the 2284 destination of the UDP datagram that triggered the reception of the 2285 ICMP packet. 2287 13. Packet Translations 2289 This section addresses IPv4-to-IPv6, IPv6-to-IPv4, and IPv6-to-IPv6 2290 translations. Requirements for translation of the IP addresses and 2291 port numbers of the packets are described above. The following 2292 sections specify how to translate other header fields. 2294 As discussed in Section 2.6, translations in TURN are designed so 2295 that a TURN server can be implemented as an application that runs in 2296 userland under commonly available operating systems and that does not 2297 require special privileges. The translations specified in the 2298 following sections follow this principle. 2300 The descriptions below have two parts: a preferred behavior and an 2301 alternate behavior. The server SHOULD implement the preferred 2302 behavior. Otherwise, the server MUST implement the alternate 2303 behavior and MUST NOT do anything else for the reasons detailed in 2304 [RFC7915]. 2306 13.1. IPv4-to-IPv6 Translations 2308 Traffic Class 2310 Preferred behavior: As specified in Section 4 of [RFC7915]. 2312 Alternate behavior: The relay sets the Traffic Class to the 2313 default value for outgoing packets. 2315 Flow Label 2317 Preferred behavior: The relay sets the Flow label to 0. The relay 2318 can choose to set the Flow label to a different value if it 2319 supports the IPv6 Flow Label field [RFC6437]. 2321 Alternate behavior: The relay sets the Flow label to the default 2322 value for outgoing packets. 2324 Hop Limit 2326 Preferred behavior: As specified in Section 4 of [RFC7915]. 2328 Alternate behavior: The relay sets the Hop Limit to the default 2329 value for outgoing packets. 2331 Fragmentation 2333 Preferred behavior: As specified in Section 4 of [RFC7915]. 2335 Alternate behavior: The relay assembles incoming fragments. The 2336 relay follows its default behavior to send outgoing packets. 2338 For both preferred and alternate behavior, the DONT-FRAGMENT 2339 attribute MUST be ignored by the server. 2341 Extension Headers 2343 Preferred behavior: The relay sends outgoing packet without any 2344 IPv6 extension headers, with the exception of the Fragmentation 2345 header as described above. 2347 Alternate behavior: Same as preferred. 2349 13.2. IPv6-to-IPv6 Translations 2351 Flow Label 2353 The relay should consider that it is handling two different IPv6 2354 flows. Therefore, the Flow label [RFC6437] SHOULD NOT be copied as 2355 part of the translation. 2357 Preferred behavior: The relay sets the Flow label to 0. The relay 2358 can choose to set the Flow label to a different value if it 2359 supports the IPv6 Flow Label field [RFC6437]. 2361 Alternate behavior: The relay sets the Flow label to the default 2362 value for outgoing packets. 2364 Hop Limit 2366 Preferred behavior: The relay acts as a regular router with 2367 respect to decrementing the Hop Limit and generating an ICMPv6 2368 error if it reaches zero. 2370 Alternate behavior: The relay sets the Hop Limit to the default 2371 value for outgoing packets. 2373 Fragmentation 2375 Preferred behavior: If the incoming packet did not include a 2376 Fragment header and the outgoing packet size does not exceed the 2377 outgoing link's MTU, the relay sends the outgoing packet without a 2378 Fragment header. 2380 If the incoming packet did not include a Fragment header and the 2381 outgoing packet size exceeds the outgoing link's MTU, the relay 2382 drops the outgoing packet and send an ICMP message of type 2 code 2383 0 ("Packet too big") to the sender of the incoming packet. If 2384 the packet is being sent to the peer, the relay reduces the MTU 2385 reported in the ICMP message by 48 bytes to allow room for the 2386 overhead of a Data indication. 2388 If the incoming packet included a Fragment header and the outgoing 2389 packet size (with a Fragment header included) does not exceed the 2390 outgoing link's MTU, the relay sends the outgoing packet with a 2391 Fragment header. The relay sets the fields of the Fragment header 2392 as appropriate for a packet originating from the server. 2394 If the incoming packet included a Fragment header and the outgoing 2395 packet size exceeds the outgoing link's MTU, the relay MUST 2396 fragment the outgoing packet into fragments of no more than 1280 2397 bytes. The relay sets the fields of the Fragment header as 2398 appropriate for a packet originating from the server. 2400 Alternate behavior: The relay assembles incoming fragments. The 2401 relay follows its default behavior to send outgoing packets. 2403 For both preferred and alternate behavior, the DONT-FRAGMENT 2404 attribute MUST be ignored by the server. 2406 Extension Headers 2408 Preferred behavior: The relay sends outgoing packet without any 2409 IPv6 extension headers, with the exception of the Fragmentation 2410 header as described above. 2412 Alternate behavior: Same as preferred. 2414 13.3. IPv6-to-IPv4 Translations 2416 Type of Service and Precedence 2418 Preferred behavior: As specified in Section 5 of [RFC7915]. 2420 Alternate behavior: The relay sets the Type of Service and 2421 Precedence to the default value for outgoing packets. 2423 Time to Live 2425 Preferred behavior: As specified in Section 5 of [RFC7915]. 2427 Alternate behavior: The relay sets the Time to Live to the default 2428 value for outgoing packets. 2430 Fragmentation 2432 Preferred behavior: As specified in Section 5 of [RFC7915]. 2433 Additionally, when the outgoing packet's size exceeds the outgoing 2434 link's MTU, the relay needs to generate an ICMP error (ICMPv6 2435 Packet Too Big) reporting the MTU size. If the packet is being 2436 sent to the peer, the relay SHOULD reduce the MTU reported in the 2437 ICMP message by 48 bytes to allow room for the overhead of a Data 2438 indication. 2440 Alternate behavior: The relay assembles incoming fragments. The 2441 relay follows its default behavior to send outgoing packets. 2443 For both preferred and alternate behavior, the DONT-FRAGMENT 2444 attribute MUST be ignored by the server. 2446 14. IP Header Fields 2448 This section describes how the server sets various fields in the IP 2449 header when relaying between the client and the peer or vice versa. 2450 The descriptions in this section apply: (a) when the server sends a 2451 UDP datagram to the peer, or (b) when the server sends a Data 2452 indication or ChannelData message to the client over UDP transport. 2453 The descriptions in this section do not apply to TURN messages sent 2454 over TCP or TLS transport from the server to the client. 2456 The descriptions below have two parts: a preferred behavior and an 2457 alternate behavior. The server SHOULD implement the preferred 2458 behavior, but if that is not possible for a particular field, then it 2459 SHOULD implement the alternative behavior. 2461 Time to Live (TTL) field 2462 Preferred Behavior: If the incoming value is 0, then the drop the 2463 incoming packet. Otherwise, set the outgoing Time to Live/Hop 2464 Count to one less than the incoming value. 2466 Alternate Behavior: Set the outgoing value to the default for 2467 outgoing packets. 2469 Differentiated Services Code Point (DSCP) field [RFC2474] 2471 Preferred Behavior: Set the outgoing value to the incoming value, 2472 unless the server includes a differentiated services classifier 2473 and marker [RFC2474]. 2475 Alternate Behavior: Set the outgoing value to a fixed value, which 2476 by default is Best Effort unless configured otherwise. 2478 In both cases, if the server is immediately adjacent to a 2479 differentiated services classifier and marker, then DSCP MAY be 2480 set to any arbitrary value in the direction towards the 2481 classifier. 2483 Explicit Congestion Notification (ECN) field [RFC3168] 2485 Preferred Behavior: Set the outgoing value to the incoming value, 2486 UNLESS the server is doing Active Queue Management, the incoming 2487 ECN field is ECT(1) (=0b01) or ECT(0) (=0b10), and the server 2488 wishes to indicate that congestion has been experienced, in which 2489 case set the outgoing value to CE (=0b11). 2491 Alternate Behavior: Set the outgoing value to Not-ECT (=0b00). 2493 IPv4 Fragmentation fields 2495 Preferred Behavior: When the server sends a packet to a peer in 2496 response to a Send indication containing the DONT-FRAGMENT 2497 attribute, then set the DF bit in the outgoing IP header to 1. In 2498 all other cases when sending an outgoing packet containing 2499 application data (e.g., Data indication, ChannelData message, or 2500 DONT-FRAGMENT attribute not included in the Send indication), copy 2501 the DF bit from the DF bit of the incoming packet that contained 2502 the application data. 2504 Set the other fragmentation fields (Identification, More 2505 Fragments, Fragment Offset) as appropriate for a packet 2506 originating from the server. 2508 Alternate Behavior: As described in the Preferred Behavior, except 2509 always assume the incoming DF bit is 0. 2511 In both the Preferred and Alternate Behaviors, the resulting 2512 packet may be too large for the outgoing link. If this is the 2513 case, then the normal fragmentation rules apply [RFC1122]. 2515 IPv4 Options 2517 Preferred Behavior: The outgoing packet is sent without any IPv4 2518 options. 2520 Alternate Behavior: Same as preferred. 2522 15. STUN Methods 2524 This section lists the codepoints for the STUN methods defined in 2525 this specification. See elsewhere in this document for the semantics 2526 of these methods. 2528 0x003 : Allocate (only request/response semantics defined) 2529 0x004 : Refresh (only request/response semantics defined) 2530 0x006 : Send (only indication semantics defined) 2531 0x007 : Data (only indication semantics defined) 2532 0x008 : CreatePermission (only request/response semantics defined 2533 0x009 : ChannelBind (only request/response semantics defined) 2535 16. STUN Attributes 2537 This STUN extension defines the following attributes: 2539 0x000C: CHANNEL-NUMBER 2540 0x000D: LIFETIME 2541 0x0010: Reserved (was BANDWIDTH) 2542 0x0012: XOR-PEER-ADDRESS 2543 0x0013: DATA 2544 0x0016: XOR-RELAYED-ADDRESS 2545 0x0017: REQUESTED-ADDRESS-FAMILY 2546 0x0018: EVEN-PORT 2547 0x0019: REQUESTED-TRANSPORT 2548 0x001A: DONT-FRAGMENT 2549 0x0021: Reserved (was TIMER-VAL) 2550 0x0022: RESERVATION-TOKEN 2551 TBD-CA: ADDITIONAL-ADDRESS-FAMILY 2552 TBD-CA: ADDRESS-ERROR-CODE 2553 TBD-CA: ICMP 2555 Some of these attributes have lengths that are not multiples of 4. 2556 By the rules of STUN, any attribute whose length is not a multiple of 2557 4 bytes MUST be immediately followed by 1 to 3 padding bytes to 2558 ensure the next attribute (if any) would start on a 4-byte boundary 2559 (see [I-D.ietf-tram-stunbis]). 2561 16.1. CHANNEL-NUMBER 2563 The CHANNEL-NUMBER attribute contains the number of the channel. The 2564 value portion of this attribute is 4 bytes long and consists of a 2565 16-bit unsigned integer, followed by a two-octet RFFU (Reserved For 2566 Future Use) field, which MUST be set to 0 on transmission and MUST be 2567 ignored on reception. 2569 0 1 2 3 2570 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 2571 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2572 | Channel Number | RFFU = 0 | 2573 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2575 16.2. LIFETIME 2577 The LIFETIME attribute represents the duration for which the server 2578 will maintain an allocation in the absence of a refresh. The value 2579 portion of this attribute is 4-bytes long and consists of a 32-bit 2580 unsigned integral value representing the number of seconds remaining 2581 until expiration. 2583 16.3. XOR-PEER-ADDRESS 2585 The XOR-PEER-ADDRESS specifies the address and port of the peer as 2586 seen from the TURN server. (For example, the peer's server-reflexive 2587 transport address if the peer is behind a NAT.) It is encoded in the 2588 same way as XOR-MAPPED-ADDRESS [I-D.ietf-tram-stunbis]. 2590 16.4. DATA 2592 The DATA attribute is present in all Send and Data indications. The 2593 value portion of this attribute is variable length and consists of 2594 the application data (that is, the data that would immediately follow 2595 the UDP header if the data was been sent directly between the client 2596 and the peer). If the length of this attribute is not a multiple of 2597 4, then padding must be added after this attribute. 2599 16.5. XOR-RELAYED-ADDRESS 2601 The XOR-RELAYED-ADDRESS is present in Allocate responses. It 2602 specifies the address and port that the server allocated to the 2603 client. It is encoded in the same way as XOR-MAPPED-ADDRESS 2604 [I-D.ietf-tram-stunbis]. 2606 16.6. REQUESTED-ADDRESS-FAMILY 2608 This attribute is used in Allocate and Refresh requests to specify 2609 the address type requested by the client. The value of this 2610 attribute is 4 bytes with the following format: 2612 0 1 2 3 2613 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 2614 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2615 | Family | Reserved | 2616 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2618 Family: there are two values defined for this field and specified in 2619 [I-D.ietf-tram-stunbis], Section 14.1: 0x01 for IPv4 addresses and 2620 0x02 for IPv6 addresses. 2622 Reserved: at this point, the 24 bits in the Reserved field MUST be 2623 set to zero by the client and MUST be ignored by the server. 2625 16.7. EVEN-PORT 2627 This attribute allows the client to request that the port in the 2628 relayed transport address be even, and (optionally) that the server 2629 reserve the next-higher port number. The value portion of this 2630 attribute is 1 byte long. Its format is: 2632 0 2633 0 1 2 3 4 5 6 7 2634 +-+-+-+-+-+-+-+-+ 2635 |R| RFFU | 2636 +-+-+-+-+-+-+-+-+ 2638 The value contains a single 1-bit flag: 2640 R: If 1, the server is requested to reserve the next-higher port 2641 number (on the same IP address) for a subsequent allocation. If 2642 0, no such reservation is requested. 2644 The other 7 bits of the attribute's value must be set to zero on 2645 transmission and ignored on reception. 2647 Since the length of this attribute is not a multiple of 4, padding 2648 must immediately follow this attribute. 2650 16.8. REQUESTED-TRANSPORT 2652 This attribute is used by the client to request a specific transport 2653 protocol for the allocated transport address. The value of this 2654 attribute is 4 bytes with the following format: 2656 0 1 2 3 2657 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 2658 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2659 | Protocol | RFFU | 2660 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2662 The Protocol field specifies the desired protocol. The codepoints 2663 used in this field are taken from those allowed in the Protocol field 2664 in the IPv4 header and the NextHeader field in the IPv6 header 2665 [Protocol-Numbers]. This specification only allows the use of 2666 codepoint 17 (User Datagram Protocol). 2668 The RFFU field MUST be set to zero on transmission and MUST be 2669 ignored on reception. It is reserved for future uses. 2671 16.9. DONT-FRAGMENT 2673 This attribute is used by the client to request that the server set 2674 the DF (Don't Fragment) bit in the IP header when relaying the 2675 application data onward to the peer. This attribute has no value 2676 part and thus the attribute length field is 0. 2678 16.10. RESERVATION-TOKEN 2680 The RESERVATION-TOKEN attribute contains a token that uniquely 2681 identifies a relayed transport address being held in reserve by the 2682 server. The server includes this attribute in a success response to 2683 tell the client about the token, and the client includes this 2684 attribute in a subsequent Allocate request to request the server use 2685 that relayed transport address for the allocation. 2687 The attribute value is 8 bytes and contains the token value. 2689 16.11. ADDITIONAL-ADDRESS-FAMILY 2691 This attribute is used by clients to request the allocation of a IPv4 2692 and IPv6 address type from a server. It is encoded in the same way 2693 as REQUESTED-ADDRESS-FAMILY Section 16.6. The ADDITIONAL-ADDRESS- 2694 FAMILY attribute MAY be present in Allocate request. The attribute 2695 value of 0x02 (IPv6 address) is the only valid value in Allocate 2696 request. 2698 16.12. ADDRESS-ERROR-CODE Attribute 2700 This attribute is used by servers to signal the reason for not 2701 allocating the requested address family. The value portion of this 2702 attribute is variable length with the following format: 2704 0 1 2 3 2705 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 2706 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2707 | Family | Rsvd |Class| Number | 2708 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2709 | Reason Phrase (variable) .. 2710 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2712 Family: there are two values defined for this field and specified in 2713 [I-D.ietf-tram-stunbis], Section 14.1: 0x01 for IPv4 addresses and 2714 0x02 for IPv6 addresses. 2716 Reserved: at this point, the 13 bits in the Reserved field MUST be 2717 set to zero by the client and MUST be ignored by the server. 2719 Class: The Class represents the hundreds digit of the error code and 2720 is defined in section 14.8 of [I-D.ietf-tram-stunbis]. 2722 Number: this 8-bit field contains the reason server cannot allocate 2723 one of the requested address types. The error code values could 2724 be either 440 (unsupported address family) or 508 (insufficient 2725 capacity). The number representation is defined in section 14.8 2726 of [I-D.ietf-tram-stunbis]. 2728 Reason Phrase: The recommended reason phrases for error codes 440 2729 and 508 are explained in Section 17. 2731 16.13. ICMP Attribute 2733 This attribute is used by servers to signal the reason an UDP packet 2734 was dropped. The following is the format of the ICMP attribute. 2736 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 2737 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2738 | Reserved | ICMP Type | ICMP Code | 2739 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2741 Reserved: This field MUST be set to 0 when sent, and MUST be ignored 2742 when received. 2744 ICMP Type: The field contains the value in the ICMP type. Its 2745 interpretation depends whether the ICMP was received over IPv4 or 2746 IPv6. 2748 ICMP Code: The field contains the value in the ICMP code. Its 2749 interpretation depends whether the ICMP was received over IPv4 or 2750 IPv6. 2752 17. STUN Error Response Codes 2754 This document defines the following error response codes: 2756 403 (Forbidden): The request was valid but cannot be performed due 2757 to administrative or similar restrictions. 2759 437 (Allocation Mismatch): A request was received by the server that 2760 requires an allocation to be in place, but no allocation exists, 2761 or a request was received that requires no allocation, but an 2762 allocation exists. 2764 440 (Address Family not Supported): The server does not support the 2765 address family requested by the client. 2767 441 (Wrong Credentials): The credentials in the (non-Allocate) 2768 request do not match those used to create the allocation. 2770 442 (Unsupported Transport Protocol): The Allocate request asked the 2771 server to use a transport protocol between the server and the peer 2772 that the server does not support. NOTE: This does NOT refer to 2773 the transport protocol used in the 5-tuple. 2775 443 (Peer Address Family Mismatch). A peer address is part of a 2776 different address family than that of the relayed transport 2777 address of the allocation. 2779 486 (Allocation Quota Reached): No more allocations using this 2780 username can be created at the present time. 2782 508 (Insufficient Capacity): The server is unable to carry out the 2783 request due to some capacity limit being reached. In an Allocate 2784 response, this could be due to the server having no more relayed 2785 transport addresses available at that time, having none with the 2786 requested properties, or the one that corresponds to the specified 2787 reservation token is not available. 2789 18. Detailed Example 2791 This section gives an example of the use of TURN, showing in detail 2792 the contents of the messages exchanged. The example uses the network 2793 diagram shown in the Overview (Figure 1). 2795 For each message, the attributes included in the message and their 2796 values are shown. For convenience, values are shown in a human- 2797 readable format rather than showing the actual octets; for example, 2798 "XOR-RELAYED-ADDRESS=192.0.2.15:9000" shows that the XOR-RELAYED- 2799 ADDRESS attribute is included with an address of 192.0.2.15 and a 2800 port of 9000, here the address and port are shown before the xor-ing 2801 is done. For attributes with string-like values (e.g., 2802 SOFTWARE="Example client, version 1.03" and 2803 NONCE="obMatJos2AAABadl7W7PeDU4hKE72jda"), the value of the attribute 2804 is shown in quotes for readability, but these quotes do not appear in 2805 the actual value. 2807 TURN TURN Peer Peer 2808 client server A B 2809 | | | | 2810 |--- Allocate request -------------->| | | 2811 | Transaction-Id=0xA56250D3F17ABE679422DE85 | | 2812 | SOFTWARE="Example client, version 1.03" | | 2813 | LIFETIME=3600 (1 hour) | | | 2814 | REQUESTED-TRANSPORT=17 (UDP) | | | 2815 | DONT-FRAGMENT | | | 2816 | | | | 2817 |<-- Allocate error response --------| | | 2818 | Transaction-Id=0xA56250D3F17ABE679422DE85 | | 2819 | SOFTWARE="Example server, version 1.17" | | 2820 | ERROR-CODE=401 (Unauthorized) | | | 2821 | REALM="example.com" | | | 2822 | NONCE="obMatJos2AAABadl7W7PeDU4hKE72jda" | | 2823 | PASSWORD-ALGORITHMS=MD5 and SHA256 | | 2824 | | | | 2825 |--- Allocate request -------------->| | | 2826 | Transaction-Id=0xC271E932AD7446A32C234492 | | 2827 | SOFTWARE="Example client 1.03" | | | 2828 | LIFETIME=3600 (1 hour) | | | 2829 | REQUESTED-TRANSPORT=17 (UDP) | | | 2830 | DONT-FRAGMENT | | | 2831 | USERNAME="George" | | | 2832 | REALM="example.com" | | | 2833 | NONCE="obMatJos2AAABadl7W7PeDU4hKE72jda" | | 2834 | PASSWORD-ALGORITHMS=MD5 and SHA256 | | 2835 | PASSWORD-ALGORITHM=SHA256 | | | 2836 | MESSAGE-INTEGRITY=... | | | 2837 | MESSAGE-INTEGRITY-SHA256=... | | | 2838 | | | | 2839 |<-- Allocate success response ------| | | 2840 | Transaction-Id=0xC271E932AD7446A32C234492 | | 2841 | SOFTWARE="Example server, version 1.17" | | 2842 | LIFETIME=1200 (20 minutes) | | | 2843 | XOR-RELAYED-ADDRESS=192.0.2.15:50000 | | 2844 | XOR-MAPPED-ADDRESS=192.0.2.1:7000 | | 2845 | MESSAGE-INTEGRITY=... | | | 2847 The client begins by selecting a host transport address to use for 2848 the TURN session; in this example, the client has selected 2849 198.51.100.2:49721 as shown in Figure 1. The client then sends an 2850 Allocate request to the server at the server transport address. The 2851 client randomly selects a 96-bit transaction id of 2852 0xA56250D3F17ABE679422DE85 for this transaction; this is encoded in 2853 the transaction id field in the fixed header. The client includes a 2854 SOFTWARE attribute that gives information about the client's 2855 software; here the value is "Example client, version 1.03" to 2856 indicate that this is version 1.03 of something called the Example 2857 client. The client includes the LIFETIME attribute because it wishes 2858 the allocation to have a longer lifetime than the default of 10 2859 minutes; the value of this attribute is 3600 seconds, which 2860 corresponds to 1 hour. The client must always include a REQUESTED- 2861 TRANSPORT attribute in an Allocate request and the only value allowed 2862 by this specification is 17, which indicates UDP transport between 2863 the server and the peers. The client also includes the DONT-FRAGMENT 2864 attribute because it wishes to use the DONT-FRAGMENT attribute later 2865 in Send indications; this attribute consists of only an attribute 2866 header, there is no value part. We assume the client has not 2867 recently interacted with the server, thus the client does not include 2868 USERNAME, USERHASH, REALM, NONCE, PASSWORD-ALGORITHMS, PASSWORD- 2869 ALGORITHM, MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute. 2870 Finally, note that the order of attributes in a message is arbitrary 2871 (except for the MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256 and 2872 FINGERPRINT attributes) and the client could have used a different 2873 order. 2875 Servers require any request to be authenticated. Thus, when the 2876 server receives the initial Allocate request, it rejects the request 2877 because the request does not contain the authentication attributes. 2878 Following the procedures of the long-term credential mechanism of 2879 STUN [I-D.ietf-tram-stunbis], the server includes an ERROR-CODE 2880 attribute with a value of 401 (Unauthorized), a REALM attribute that 2881 specifies the authentication realm used by the server (in this case, 2882 the server's domain "example.com"), and a nonce value in a NONCE 2883 attribute. The NONCE attribute starts with the "nonce cookie" with 2884 the STUN Security Feature "Password algorithm" bit set to 1. The 2885 server includes a PASSWORD-ALGORITHMS attribute that specifies the 2886 list of algorithms that the server can use to derive the long-term 2887 password. If the server sets the STUN Security Feature "Username 2888 anonymity" bit to 1 then the client uses the USERHASH attribute 2889 instead of the USERNAME attribute in the Allocate request to 2890 anonymise the username. The server also includes a SOFTWARE 2891 attribute that gives information about the server's software. 2893 The client, upon receipt of the 401 error, re-attempts the Allocate 2894 request, this time including the authentication attributes. The 2895 client selects a new transaction id, and then populates the new 2896 Allocate request with the same attributes as before. The client 2897 includes a USERNAME attribute and uses the realm value received from 2898 the server to help it determine which value to use; here the client 2899 is configured to use the username "George" for the realm 2900 "example.com". The client includes the PASSWORD-ALGORITHM attribute 2901 indicating the algorithm that the server must use to derive the long- 2902 term password. The client also includes the REALM and NONCE 2903 attributes, which are just copied from the 401 error response. 2904 Finally, the client includes MESSAGE-INTEGRITY and MESSAGE-INTEGRITY- 2905 SHA256 attributes as the last attributes in the message, whose values 2906 are Hashed Message Authentication Code - Secure Hash Algorithm 1 2907 (HMAC-SHA1) hash and Hashed Message Authentication Code - Secure Hash 2908 Algorithm 2 (HMAC-SHA2) hash over the contents of the message (shown 2909 as just "..." above); this HMAC-SHA1 and HMAC-SHA2 computation 2910 includes a password value. Thus, an attacker cannot compute the 2911 message integrity value without somehow knowing the secret password. 2913 The server, upon receipt of the authenticated Allocate request, 2914 checks that everything is OK, then creates an allocation. The server 2915 replies with an Allocate success response. The server includes a 2916 LIFETIME attribute giving the lifetime of the allocation; here, the 2917 server has reduced the client's requested 1-hour lifetime to just 20 2918 minutes, because this particular server doesn't allow lifetimes 2919 longer than 20 minutes. The server includes an XOR-RELAYED-ADDRESS 2920 attribute whose value is the relayed transport address of the 2921 allocation. The server includes an XOR-MAPPED-ADDRESS attribute 2922 whose value is the server-reflexive address of the client; this value 2923 is not used otherwise in TURN but is returned as a convenience to the 2924 client. The server includes either a MESSAGE-INTEGRITY or MESSAGE- 2925 INTEGRITY-SHA256 attribute to authenticate the response and to ensure 2926 its integrity; note that the response does not contain the USERNAME, 2927 REALM, and NONCE attributes. The server also includes a SOFTWARE 2928 attribute. 2930 TURN TURN Peer Peer 2931 client server A B 2932 |--- CreatePermission request ------>| | | 2933 | Transaction-Id=0xE5913A8F460956CA277D3319 | | 2934 | XOR-PEER-ADDRESS=192.0.2.150:0 | | | 2935 | USERNAME="George" | | | 2936 | REALM="example.com" | | | 2937 | NONCE="obMatJos2AAABadl7W7PeDU4hKE72jda" | | 2938 | MESSAGE-INTEGRITY=... | | | 2939 | | | | 2940 |<-- CreatePermission success resp.--| | | 2941 | Transaction-Id=0xE5913A8F460956CA277D3319 | | 2942 | MESSAGE-INTEGRITY=... | | | 2944 The client then creates a permission towards Peer A in preparation 2945 for sending it some application data. This is done through a 2946 CreatePermission request. The XOR-PEER-ADDRESS attribute contains 2947 the IP address for which a permission is established (the IP address 2948 of peer A); note that the port number in the attribute is ignored 2949 when used in a CreatePermission request, and here it has been set to 2950 0; also, note how the client uses Peer A's server-reflexive IP 2951 address and not its (private) host address. The client uses the same 2952 username, realm, and nonce values as in the previous request on the 2953 allocation. Though it is allowed to do so, the client has chosen not 2954 to include a SOFTWARE attribute in this request. 2956 The server receives the CreatePermission request, creates the 2957 corresponding permission, and then replies with a CreatePermission 2958 success response. Like the client, the server chooses not to include 2959 the SOFTWARE attribute in its reply. Again, note how success 2960 responses contain a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 2961 attribute (assuming the server uses the long-term credential 2962 mechanism), but no USERNAME, REALM, and NONCE attributes. 2964 TURN TURN Peer Peer 2965 client server A B 2966 |--- Send indication --------------->| | | 2967 | Transaction-Id=0x1278E9ACA2711637EF7D3328 | | 2968 | XOR-PEER-ADDRESS=192.0.2.150:32102 | | 2969 | DONT-FRAGMENT | | | 2970 | DATA=... | | | 2971 | |-- UDP dgm ->| | 2972 | | data=... | | 2973 | | | | 2974 | |<- UDP dgm --| | 2975 | | data=... | | 2976 |<-- Data indication ----------------| | | 2977 | Transaction-Id=0x8231AE8F9242DA9FF287FEFF | | 2978 | XOR-PEER-ADDRESS=192.0.2.150:32102 | | 2979 | DATA=... | | | 2981 The client now sends application data to Peer A using a Send 2982 indication. Peer A's server-reflexive transport address is specified 2983 in the XOR-PEER-ADDRESS attribute, and the application data (shown 2984 here as just "...") is specified in the DATA attribute. The client 2985 is doing a form of path MTU discovery at the application layer and 2986 thus specifies (by including the DONT-FRAGMENT attribute) that the 2987 server should set the DF bit in the UDP datagram to send to the peer. 2988 Indications cannot be authenticated using the long-term credential 2989 mechanism of STUN, so no MESSAGE-INTEGRITY or MESSAGE-INTEGRITY- 2990 SHA256 attribute is included in the message. An application wishing 2991 to ensure that its data is not altered or forged must integrity- 2992 protect its data at the application level. 2994 Upon receipt of the Send indication, the server extracts the 2995 application data and sends it in a UDP datagram to Peer A, with the 2996 relayed transport address as the source transport address of the 2997 datagram, and with the DF bit set as requested. Note that, had the 2998 client not previously established a permission for Peer A's server- 2999 reflexive IP address, then the server would have silently discarded 3000 the Send indication instead. 3002 Peer A then replies with its own UDP datagram containing application 3003 data. The datagram is sent to the relayed transport address on the 3004 server. When this arrives, the server creates a Data indication 3005 containing the source of the UDP datagram in the XOR-PEER-ADDRESS 3006 attribute, and the data from the UDP datagram in the DATA attribute. 3007 The resulting Data indication is then sent to the client. 3009 TURN TURN Peer Peer 3010 client server A B 3011 |--- ChannelBind request ----------->| | | 3012 | Transaction-Id=0x6490D3BC175AFF3D84513212 | | 3013 | CHANNEL-NUMBER=0x4000 | | | 3014 | XOR-PEER-ADDRESS=192.0.2.210:49191 | | 3015 | USERNAME="George" | | | 3016 | REALM="example.com" | | | 3017 | NONCE="obMatJos2AAABadl7W7PeDU4hKE72jda" | | 3018 | MESSAGE-INTEGRITY=... | | | 3019 | | | | 3020 |<-- ChannelBind success response ---| | | 3021 | Transaction-Id=0x6490D3BC175AFF3D84513212 | | 3022 | MESSAGE-INTEGRITY=... | | | 3024 The client now binds a channel to Peer B, specifying a free channel 3025 number (0x4000) in the CHANNEL-NUMBER attribute, and Peer B's 3026 transport address in the XOR-PEER-ADDRESS attribute. As before, the 3027 client re-uses the username, realm, and nonce from its last request 3028 in the message. 3030 Upon receipt of the request, the server binds the channel number to 3031 the peer, installs a permission for Peer B's IP address, and then 3032 replies with ChannelBind success response. 3034 TURN TURN Peer Peer 3035 client server A B 3036 |--- ChannelData ------------------->| | | 3037 | Channel-number=0x4000 |--- UDP datagram --------->| 3038 | Data=... | Data=... | 3039 | | | | 3040 | |<-- UDP datagram ----------| 3041 | | Data=... | | 3042 |<-- ChannelData --------------------| | | 3043 | Channel-number=0x4000 | | | 3044 | Data=... | | | 3046 The client now sends a ChannelData message to the server with data 3047 destined for Peer B. The ChannelData message is not a STUN message, 3048 and thus has no transaction id. Instead, it has only three fields: a 3049 channel number, data, and data length; here the channel number field 3050 is 0x4000 (the channel the client just bound to Peer B). When the 3051 server receives the ChannelData message, it checks that the channel 3052 is currently bound (which it is) and then sends the data onward to 3053 Peer B in a UDP datagram, using the relayed transport address as the 3054 source transport address and 192.0.2.210:49191 (the value of the XOR- 3055 PEER-ADDRESS attribute in the ChannelBind request) as the destination 3056 transport address. 3058 Later, Peer B sends a UDP datagram back to the relayed transport 3059 address. This causes the server to send a ChannelData message to the 3060 client containing the data from the UDP datagram. The server knows 3061 to which client to send the ChannelData message because of the 3062 relayed transport address at which the UDP datagram arrived, and 3063 knows to use channel 0x4000 because this is the channel bound to 3064 192.0.2.210:49191. Note that if there had not been any channel 3065 number bound to that address, the server would have used a Data 3066 indication instead. 3068 TURN TURN Peer Peer 3069 client server A B 3070 |--- Refresh request --------------->| | | 3071 | Transaction-Id=0x0864B3C27ADE9354B4312414 | | 3072 | SOFTWARE="Example client 1.03" | | | 3073 | USERNAME="George" | | | 3074 | REALM="example.com" | | | 3075 | NONCE="obMatJos2AAABadl7W7PeDU4hKE72jda" | | 3076 | MESSAGE-INTEGRITY=... | | | 3077 | | | | 3078 |<-- Refresh error response ---------| | | 3079 | Transaction-Id=0x0864B3C27ADE9354B4312414 | | 3080 | SOFTWARE="Example server, version 1.17" | | 3081 | ERROR-CODE=438 (Stale Nonce) | | | 3082 | REALM="example.com" | | | 3083 | NONCE="obMatJos2AAABnpSw1Xw239bBwGYhjN" | | 3084 | PASSWORD-ALGORITHMS=MD5 and SHA256 | | 3085 | | | | 3086 |--- Refresh request --------------->| | | 3087 | Transaction-Id=0x427BD3E625A85FC731DC4191 | | 3088 | SOFTWARE="Example client 1.03" | | | 3089 | USERNAME="George" | | | 3090 | REALM="example.com" | | | 3091 | NONCE="obMatJos2AAABnpSw1Xw239bBwGYhjNj" | | 3092 | PASSWORD-ALGORITHMS=MD5 and SHA256 | | 3093 | PASSWORD-ALGORITHM=SHA256 | | | 3094 | MESSAGE-INTEGRITY=... | | | 3095 | | | | 3096 |<-- Refresh success response -------| | | 3097 | Transaction-Id=0x427BD3E625A85FC731DC4191 | | 3098 | SOFTWARE="Example server, version 1.17" | | 3099 | LIFETIME=600 (10 minutes) | | | 3101 Sometime before the 20 minute lifetime is up, the client refreshes 3102 the allocation. This is done using a Refresh request. As before, 3103 the client includes the latest username, realm, and nonce values in 3104 the request. The client also includes the SOFTWARE attribute, 3105 following the recommended practice of always including this attribute 3106 in Allocate and Refresh messages. When the server receives the 3107 Refresh request, it notices that the nonce value has expired, and so 3108 replies with 438 (Stale Nonce) error given a new nonce value. The 3109 client then reattempts the request, this time with the new nonce 3110 value. This second attempt is accepted, and the server replies with 3111 a success response. Note that the client did not include a LIFETIME 3112 attribute in the request, so the server refreshes the allocation for 3113 the default lifetime of 10 minutes (as can be seen by the LIFETIME 3114 attribute in the success response). 3116 19. Security Considerations 3118 This section considers attacks that are possible in a TURN 3119 deployment, and discusses how they are mitigated by mechanisms in the 3120 protocol or recommended practices in the implementation. 3122 Most of the attacks on TURN are mitigated by the server requiring 3123 requests be authenticated. Thus, this specification requires the use 3124 of authentication. The mandatory-to-implement mechanism is the long- 3125 term credential mechanism of STUN. Other authentication mechanisms 3126 of equal or stronger security properties may be used. However, it is 3127 important to ensure that they can be invoked in an inter-operable 3128 way. 3130 19.1. Outsider Attacks 3132 Outsider attacks are ones where the attacker has no credentials in 3133 the system, and is attempting to disrupt the service seen by the 3134 client or the server. 3136 19.1.1. Obtaining Unauthorized Allocations 3138 An attacker might wish to obtain allocations on a TURN server for any 3139 number of nefarious purposes. A TURN server provides a mechanism for 3140 sending and receiving packets while cloaking the actual IP address of 3141 the client. This makes TURN servers an attractive target for 3142 attackers who wish to use it to mask their true identity. 3144 An attacker might also wish to simply utilize the services of a TURN 3145 server without paying for them. Since TURN services require 3146 resources from the provider, it is anticipated that their usage will 3147 come with a cost. 3149 These attacks are prevented using the long-term credential mechanism, 3150 which allows the TURN server to determine the identity of the 3151 requestor and whether the requestor is allowed to obtain the 3152 allocation. 3154 19.1.2. Offline Dictionary Attacks 3156 The long-term credential mechanism used by TURN is subject to offline 3157 dictionary attacks. An attacker that is capable of eavesdropping on 3158 a message exchange between a client and server can determine the 3159 password by trying a number of candidate passwords and seeing if one 3160 of them is correct. This attack works when the passwords are low 3161 entropy, such as a word from the dictionary. This attack can be 3162 mitigated by using strong passwords with large entropy. In 3163 situations where even stronger mitigation is required, (D)TLS 3164 transport between the client and the server can be used. 3166 19.1.3. Faked Refreshes and Permissions 3168 An attacker might wish to attack an active allocation by sending it a 3169 Refresh request with an immediate expiration, in order to delete it 3170 and disrupt service to the client. This is prevented by 3171 authentication of refreshes. Similarly, an attacker wishing to send 3172 CreatePermission requests to create permissions to undesirable 3173 destinations is prevented from doing so through authentication. The 3174 motivations for such an attack are described in Section 19.2. 3176 19.1.4. Fake Data 3178 An attacker might wish to send data to the client or the peer, as if 3179 they came from the peer or client, respectively. To do that, the 3180 attacker can send the client a faked Data Indication or ChannelData 3181 message, or send the TURN server a faked Send Indication or 3182 ChannelData message. 3184 Since indications and ChannelData messages are not authenticated, 3185 this attack is not prevented by TURN. However, this attack is 3186 generally present in IP-based communications and is not substantially 3187 worsened by TURN. Consider a normal, non-TURN IP session between 3188 hosts A and B. An attacker can send packets to B as if they came 3189 from A by sending packets towards A with a spoofed IP address of B. 3190 This attack requires the attacker to know the IP addresses of A and 3191 B. With TURN, an attacker wishing to send packets towards a client 3192 using a Data indication needs to know its IP address (and port), the 3193 IP address and port of the TURN server, and the IP address and port 3194 of the peer (for inclusion in the XOR-PEER-ADDRESS attribute). To 3195 send a fake ChannelData message to a client, an attacker needs to 3196 know the IP address and port of the client, the IP address and port 3197 of the TURN server, and the channel number. This particular 3198 combination is mildly more guessable than in the non-TURN case. 3200 These attacks are more properly mitigated by application-layer 3201 authentication techniques. In the case of real-time traffic, usage 3202 of SRTP [RFC3711] prevents these attacks. 3204 In some situations, the TURN server may be situated in the network 3205 such that it is able to send to hosts to which the client cannot 3206 directly send. This can happen, for example, if the server is 3207 located behind a firewall that allows packets from outside the 3208 firewall to be delivered to the server, but not to other hosts behind 3209 the firewall. In these situations, an attacker could send the server 3210 a Send indication with an XOR-PEER-ADDRESS attribute containing the 3211 transport address of one of the other hosts behind the firewall. If 3212 the server was to allow relaying of traffic to arbitrary peers, then 3213 this would provide a way for the attacker to attack arbitrary hosts 3214 behind the firewall. 3216 To mitigate this attack, TURN requires that the client establish a 3217 permission to a host before sending it data. Thus, an attacker can 3218 only attack hosts with which the client is already communicating, 3219 unless the attacker is able to create authenticated requests. 3220 Furthermore, the server administrator may configure the server to 3221 restrict the range of IP addresses and ports to which it will relay 3222 data. To provide even greater security, the server administrator can 3223 require that the client use (D)TLS for all communication between the 3224 client and the server. 3226 19.1.5. Impersonating a Server 3228 When a client learns a relayed address from a TURN server, it uses 3229 that relayed address in application protocols to receive traffic. 3230 Therefore, an attacker wishing to intercept or redirect that traffic 3231 might try to impersonate a TURN server and provide the client with a 3232 faked relayed address. 3234 This attack is prevented through the long-term credential mechanism, 3235 which provides message integrity for responses in addition to 3236 verifying that they came from the server. Furthermore, an attacker 3237 cannot replay old server responses as the transaction id in the STUN 3238 header prevents this. Replay attacks are further thwarted through 3239 frequent changes to the nonce value. 3241 19.1.6. Eavesdropping Traffic 3243 TURN concerns itself primarily with authentication and message 3244 integrity. Confidentiality is only a secondary concern, as TURN 3245 control messages do not include information that is particularly 3246 sensitive. The primary protocol content of the messages is the IP 3247 address of the peer. If it is important to prevent an eavesdropper 3248 on a TURN connection from learning this, TURN can be run over (D)TLS. 3250 Confidentiality for the application data relayed by TURN is best 3251 provided by the application protocol itself, since running TURN over 3252 (D)TLS does not protect application data between the server and the 3253 peer. If confidentiality of application data is important, then the 3254 application should encrypt or otherwise protect its data. For 3255 example, for real-time media, confidentiality can be provided by 3256 using SRTP. 3258 19.1.7. TURN Loop Attack 3260 An attacker might attempt to cause data packets to loop indefinitely 3261 between two TURN servers. The attack goes as follows. First, the 3262 attacker sends an Allocate request to server A, using the source 3263 address of server B. Server A will send its response to server B, 3264 and for the attack to succeed, the attacker must have the ability to 3265 either view or guess the contents of this response, so that the 3266 attacker can learn the allocated relayed transport address. The 3267 attacker then sends an Allocate request to server B, using the source 3268 address of server A. Again, the attacker must be able to view or 3269 guess the contents of the response, so it can send learn the 3270 allocated relayed transport address. Using the same spoofed source 3271 address technique, the attacker then binds a channel number on server 3272 A to the relayed transport address on server B, and similarly binds 3273 the same channel number on server B to the relayed transport address 3274 on server A. Finally, the attacker sends a ChannelData message to 3275 server A. 3277 The result is a data packet that loops from the relayed transport 3278 address on server A to the relayed transport address on server B, 3279 then from server B's transport address to server A's transport 3280 address, and then around the loop again. 3282 This attack is mitigated as follows. By requiring all requests to be 3283 authenticated and/or by randomizing the port number allocated for the 3284 relayed transport address, the server forces the attacker to either 3285 intercept or view responses sent to a third party (in this case, the 3286 other server) so that the attacker can authenticate the requests and 3287 learn the relayed transport address. Without one of these two 3288 measures, an attacker can guess the contents of the responses without 3289 needing to see them, which makes the attack much easier to perform. 3290 Furthermore, by requiring authenticated requests, the server forces 3291 the attacker to have credentials acceptable to the server, which 3292 turns this from an outsider attack into an insider attack and allows 3293 the attack to be traced back to the client initiating it. 3295 The attack can be further mitigated by imposing a per-username limit 3296 on the bandwidth used to relay data by allocations owned by that 3297 username, to limit the impact of this attack on other allocations. 3298 More mitigation can be achieved by decrementing the TTL when relaying 3299 data packets (if the underlying OS allows this). 3301 19.2. Firewall Considerations 3303 A key security consideration of TURN is that TURN should not weaken 3304 the protections afforded by firewalls deployed between a client and a 3305 TURN server. It is anticipated that TURN servers will often be 3306 present on the public Internet, and clients may often be inside 3307 enterprise networks with corporate firewalls. If TURN servers 3308 provide a 'backdoor' for reaching into the enterprise, TURN will be 3309 blocked by these firewalls. 3311 TURN servers therefore emulate the behavior of NAT devices that 3312 implement address-dependent filtering [RFC4787], a property common in 3313 many firewalls as well. When a NAT or firewall implements this 3314 behavior, packets from an outside IP address are only allowed to be 3315 sent to an internal IP address and port if the internal IP address 3316 and port had recently sent a packet to that outside IP address. TURN 3317 servers introduce the concept of permissions, which provide exactly 3318 this same behavior on the TURN server. An attacker cannot send a 3319 packet to a TURN server and expect it to be relayed towards the 3320 client, unless the client has tried to contact the attacker first. 3322 It is important to note that some firewalls have policies that are 3323 even more restrictive than address-dependent filtering. Firewalls 3324 can also be configured with address- and port-dependent filtering, or 3325 can be configured to disallow inbound traffic entirely. In these 3326 cases, if a client is allowed to connect the TURN server, 3327 communications to the client will be less restrictive than what the 3328 firewall would normally allow. 3330 19.2.1. Faked Permissions 3332 In firewalls and NAT devices, permissions are granted implicitly 3333 through the traversal of a packet from the inside of the network 3334 towards the outside peer. Thus, a permission cannot, by definition, 3335 be created by any entity except one inside the firewall or NAT. With 3336 TURN, this restriction no longer holds. Since the TURN server sits 3337 outside the firewall, at attacker outside the firewall can now send a 3338 message to the TURN server and try to create a permission for itself. 3340 This attack is prevented because all messages that create permissions 3341 (i.e., ChannelBind and CreatePermission) are authenticated. 3343 19.2.2. Blacklisted IP Addresses 3345 Many firewalls can be configured with blacklists that prevent a 3346 client behind the firewall from sending packets to, or receiving 3347 packets from, ranges of blacklisted IP addresses. This is 3348 accomplished by inspecting the source and destination addresses of 3349 packets entering and exiting the firewall, respectively. 3351 This feature is also present in TURN, since TURN servers are allowed 3352 to arbitrarily restrict the range of addresses of peers that they 3353 will relay to. 3355 19.2.3. Running Servers on Well-Known Ports 3357 A malicious client behind a firewall might try to connect to a TURN 3358 server and obtain an allocation which it then uses to run a server. 3359 For example, a client might try to run a DNS server or FTP server. 3361 This is not possible in TURN. A TURN server will never accept 3362 traffic from a peer for which the client has not installed a 3363 permission. Thus, peers cannot just connect to the allocated port in 3364 order to obtain the service. 3366 19.3. Insider Attacks 3368 In insider attacks, a client has legitimate credentials but defies 3369 the trust relationship that goes with those credentials. These 3370 attacks cannot be prevented by cryptographic means but need to be 3371 considered in the design of the protocol. 3373 19.3.1. DoS against TURN Server 3375 A client wishing to disrupt service to other clients might obtain an 3376 allocation and then flood it with traffic, in an attempt to swamp the 3377 server and prevent it from servicing other legitimate clients. This 3378 is mitigated by the recommendation that the server limit the amount 3379 of bandwidth it will relay for a given username. This won't prevent 3380 a client from sending a large amount of traffic, but it allows the 3381 server to immediately discard traffic in excess. 3383 Since each allocation uses a port number on the IP address of the 3384 TURN server, the number of allocations on a server is finite. An 3385 attacker might attempt to consume all of them by requesting a large 3386 number of allocations. This is prevented by the recommendation that 3387 the server impose a limit of the number of allocations active at a 3388 time for a given username. 3390 19.3.2. Anonymous Relaying of Malicious Traffic 3392 TURN servers provide a degree of anonymization. A client can send 3393 data to peers without revealing its own IP address. TURN servers may 3394 therefore become attractive vehicles for attackers to launch attacks 3395 against targets without fear of detection. Indeed, it is possible 3396 for a client to chain together multiple TURN servers, such that any 3397 number of relays can be used before a target receives a packet. 3399 Administrators who are worried about this attack can maintain logs 3400 that capture the actual source IP and port of the client, and perhaps 3401 even every permission that client installs. This will allow for 3402 forensic tracing to determine the original source, should it be 3403 discovered that an attack is being relayed through a TURN server. 3405 19.3.3. Manipulating Other Allocations 3407 An attacker might attempt to disrupt service to other users of the 3408 TURN server by sending Refresh requests or CreatePermission requests 3409 that (through source address spoofing) appear to be coming from 3410 another user of the TURN server. TURN prevents this by requiring 3411 that the credentials used in CreatePermission, Refresh, and 3412 ChannelBind messages match those used to create the initial 3413 allocation. Thus, the fake requests from the attacker will be 3414 rejected. 3416 19.4. Tunnel Amplification Attack 3418 An attacker might attempt to cause data packets to loop numerous 3419 times between a TURN server and a tunnel between IPv4 and IPv6. The 3420 attack goes as follows. 3422 Suppose an attacker knows that a tunnel endpoint will forward 3423 encapsulated packets from a given IPv6 address (this doesn't 3424 necessarily need to be the tunnel endpoint's address). Suppose he 3425 then spoofs two packets from this address: 3427 1. An Allocate request asking for a v4 address, and 3429 2. A ChannelBind request establishing a channel to the IPv4 address 3430 of the tunnel endpoint 3432 Then he has set up an amplification attack: 3434 o The TURN relay will re-encapsulate IPv6 UDP data in v4 and send it 3435 to the tunnel endpoint 3437 o The tunnel endpoint will de-encapsulate packets from the v4 3438 interface and send them to v6 3440 So if the attacker sends a packet of the following form... 3442 IPv6: src=2001:DB8:1::1 dst=2001:DB8::2 3443 UDP: 3444 TURN: 3445 IPv6: src=2001:DB8:1::1 dst=2001:DB8::2 3446 UDP: 3447 TURN: 3448 IPv6: src=2001:DB8:1::1 dst=2001:DB8::2 3449 UDP: 3450 TURN: 3451 ... 3453 Then the TURN relay and the tunnel endpoint will send it back and 3454 forth until the last TURN header is consumed, at which point the TURN 3455 relay will send an empty packet, which the tunnel endpoint will drop. 3457 The amplification potential here is limited by the MTU, so it's not 3458 huge: IPv6+UDP+TURN takes 334 bytes, so a four-to-one amplification 3459 out of a 1500-byte packet is possible. But the attacker could still 3460 increase traffic volume by sending multiple packets or by 3461 establishing multiple channels spoofed from different addresses 3462 behind the same tunnel endpoint. 3464 The attack is mitigated as follows. It is RECOMMENDED that TURN 3465 relays not accept allocation or channel binding requests from 3466 addresses known to be tunneled, and that they not forward data to 3467 such addresses. In particular, a TURN relay MUST NOT accept Teredo 3468 or 6to4 addresses in these requests. 3470 19.5. Other Considerations 3472 Any relay addresses learned through an Allocate request will not 3473 operate properly with IPsec Authentication Header (AH) [RFC4302] in 3474 transport or tunnel mode. However, tunnel-mode IPsec Encapsulating 3475 Security Payload (ESP) [RFC4303] should still operate. 3477 20. IANA Considerations 3479 [Paragraphs in braces should be removed by the RFC Editor upon 3480 publication] 3482 The codepoints for the STUN methods defined in this specification are 3483 listed in Section 15. [IANA is requested to update the reference 3484 from [RFC5766] to RFC-to-be for the STUN methods listed in 3485 Section 15.] 3487 The codepoints for the STUN attributes defined in this specification 3488 are listed in Section 16. [IANA is requested to update the reference 3489 from [RFC5766] to RFC-to-be for the STUN attributes CHANNEL-NUMBER, 3490 LIFETIME, Reserved (was BANDWIDTH), XOR-PEER-ADDRESS, DATA, XOR- 3491 RELAYED-ADDRESS, REQUESTED-ADDRESS-FAMILY, EVEN-PORT, REQUESTED- 3492 TRANSPORT, DONT-FRAGMENT, Reserved (was TIMER-VAL) and RESERVATION- 3493 TOKEN listed in Section 16.] 3495 [The ADDITIONAL-ADDRESS-FAMILY, ADDRESS-ERROR-CODE and ICMP 3496 attributes requires that IANA allocate a value in the "STUN 3497 attributes Registry" from the comprehension-optional range 3498 (0x8000-0xFFFF), to be replaced for TBD-CA throughout this document] 3500 The codepoints for the STUN error codes defined in this specification 3501 are listed in Section 17. [IANA is requested to update the reference 3502 from [RFC5766] to RFC-to-be for the STUN error codes listed in 3503 Section 17.] 3505 IANA has allocated the SRV service name of "turn" for TURN over UDP 3506 or TCP, and the service name of "turns" for TURN over (D)TLS. 3508 IANA has created a registry for TURN channel numbers, initially 3509 populated as follows: 3511 o 0x0000 through 0x3FFF: Reserved and not available for use, since 3512 they conflict with the STUN header. 3514 o 0x4000 through 0x7FFF: A TURN implementation is free to use 3515 channel numbers in this range. 3517 o 0x8000 through 0xFFFF: Unassigned. 3519 Any change to this registry must be made through an IETF Standards 3520 Action. 3522 21. IAB Considerations 3524 The IAB has studied the problem of "Unilateral Self Address Fixing" 3525 (UNSAF), which is the general process by which a client attempts to 3526 determine its address in another realm on the other side of a NAT 3527 through a collaborative protocol-reflection mechanism [RFC3424]. The 3528 TURN extension is an example of a protocol that performs this type of 3529 function. The IAB has mandated that any protocols developed for this 3530 purpose document a specific set of considerations. These 3531 considerations and the responses for TURN are documented in this 3532 section. 3534 Consideration 1: Precise definition of a specific, limited-scope 3535 problem that is to be solved with the UNSAF proposal. A short-term 3536 fix should not be generalized to solve other problems. Such 3537 generalizations lead to the prolonged dependence on and usage of the 3538 supposed short-term fix -- meaning that it is no longer accurate to 3539 call it "short-term". 3541 Response: TURN is a protocol for communication between a relay (= 3542 TURN server) and its client. The protocol allows a client that is 3543 behind a NAT to obtain and use a public IP address on the relay. As 3544 a convenience to the client, TURN also allows the client to determine 3545 its server-reflexive transport address. 3547 Consideration 2: Description of an exit strategy/transition plan. 3548 The better short-term fixes are the ones that will naturally see less 3549 and less use as the appropriate technology is deployed. 3551 Response: TURN will no longer be needed once there are no longer any 3552 NATs. Unfortunately, as of the date of publication of this document, 3553 it no longer seems very likely that NATs will go away any time soon. 3554 However, the need for TURN will also decrease as the number of NATs 3555 with the mapping property of Endpoint-Independent Mapping [RFC4787] 3556 increases. 3558 Consideration 3: Discussion of specific issues that may render 3559 systems more "brittle". For example, approaches that involve using 3560 data at multiple network layers create more dependencies, increase 3561 debugging challenges, and make it harder to transition. 3563 Response: TURN is "brittle" in that it requires the NAT bindings 3564 between the client and the server to be maintained unchanged for the 3565 lifetime of the allocation. This is typically done using keep- 3566 alives. If this is not done, then the client will lose its 3567 allocation and can no longer exchange data with its peers. 3569 Consideration 4: Identify requirements for longer-term, sound 3570 technical solutions; contribute to the process of finding the right 3571 longer-term solution. 3573 Response: The need for TURN will be reduced once NATs implement the 3574 recommendations for NAT UDP behavior documented in [RFC4787]. 3575 Applications are also strongly urged to use ICE [RFC5245] to 3576 communicate with peers; though ICE uses TURN, it does so only as a 3577 last resort, and uses it in a controlled manner. 3579 Consideration 5: Discussion of the impact of the noted practical 3580 issues with existing deployed NATs and experience reports. 3582 Response: Some NATs deployed today exhibit a mapping behavior other 3583 than Endpoint-Independent mapping. These NATs are difficult to work 3584 with, as they make it difficult or impossible for protocols like ICE 3585 to use server-reflexive transport addresses on those NATs. A client 3586 behind such a NAT is often forced to use a relay protocol like TURN 3587 because "UDP hole punching" techniques [RFC5128] do not work. 3589 22. Changes since RFC 5766 3591 This section lists the major changes in the TURN protocol from the 3592 original [RFC5766] specification. 3594 o IPv6 support. 3596 o REQUESTED-ADDRESS-FAMILY, ADDITIONAL-ADDRESS-FAMILY, AND ADDRESS- 3597 ERR-CODE attributes. 3599 o 440 (Address Family not Supported) and 443 (Peer Address Family 3600 Mismatch) responses. 3602 o Description of the tunnel amplification attack. 3604 o DTLS support. 3606 o More details on packet translations. 3608 o Add support for receiving ICMP packets. 3610 o Updates PMTUD. 3612 23. Acknowledgements 3614 Most of the text in this note comes from the original TURN 3615 specification, [RFC5766]. The authors would like to thank Rohan Mahy 3616 co-author of original TURN specification and everyone who had 3617 contributed to that document. The authors would also like to 3618 acknowledge that this document inherits material from [RFC6156]. 3620 Thanks to Justin Uberti, Pal Martinsen, Oleg Moskalenko, Aijun Wang 3621 and Simon Perreault for their help on SSODA mechanism. Authors would 3622 like to thank Gonzalo Salgueiro, Simon Perreault, Jonathan Lennox, 3623 Brandon Williams, Karl Stahl, Noriyuki Torii and Oleg Moskalenko for 3624 comments and review. The authors would like to thank Marc for his 3625 contributions to the text. 3627 24. References 3629 24.1. Normative References 3631 [I-D.ietf-tram-stunbis] 3632 Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing, 3633 D., Mahy, R., and P. Matthews, "Session Traversal 3634 Utilities for NAT (STUN)", draft-ietf-tram-stunbis-16 3635 (work in progress), March 2018. 3637 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 3638 RFC 792, DOI 10.17487/RFC0792, September 1981, 3639 . 3641 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - 3642 Communication Layers", STD 3, RFC 1122, 3643 DOI 10.17487/RFC1122, October 1989, 3644 . 3646 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 3647 Requirement Levels", BCP 14, RFC 2119, 3648 DOI 10.17487/RFC2119, March 1997, 3649 . 3651 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 3652 "Definition of the Differentiated Services Field (DS 3653 Field) in the IPv4 and IPv6 Headers", RFC 2474, 3654 DOI 10.17487/RFC2474, December 1998, 3655 . 3657 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 3658 of Explicit Congestion Notification (ECN) to IP", 3659 RFC 3168, DOI 10.17487/RFC3168, September 2001, 3660 . 3662 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet 3663 Control Message Protocol (ICMPv6) for the Internet 3664 Protocol Version 6 (IPv6) Specification", STD 89, 3665 RFC 4443, DOI 10.17487/RFC4443, March 2006, 3666 . 3668 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 3669 (TLS) Protocol Version 1.2", RFC 5246, 3670 DOI 10.17487/RFC5246, August 2008, 3671 . 3673 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 3674 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 3675 January 2012, . 3677 [RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme, 3678 "IPv6 Flow Label Specification", RFC 6437, 3679 DOI 10.17487/RFC6437, November 2011, 3680 . 3682 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 3683 "Default Address Selection for Internet Protocol Version 6 3684 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 3685 . 3687 [RFC7065] Petit-Huguenin, M., Nandakumar, S., Salgueiro, G., and P. 3688 Jones, "Traversal Using Relays around NAT (TURN) Uniform 3689 Resource Identifiers", RFC 7065, DOI 10.17487/RFC7065, 3690 November 2013, . 3692 [RFC7915] Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont, 3693 "IP/ICMP Translation Algorithm", RFC 7915, 3694 DOI 10.17487/RFC7915, June 2016, 3695 . 3697 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: 3698 Better Connectivity Using Concurrency", RFC 8305, 3699 DOI 10.17487/RFC8305, December 2017, 3700 . 3702 24.2. Informative References 3704 [Frag-Harmful] 3705 "Fragmentation Considered Harmful", . 3708 [I-D.ietf-tram-stun-pmtud] 3709 Petit-Huguenin, M. and G. Salgueiro, "Path MTU Discovery 3710 Using Session Traversal Utilities for NAT (STUN)", draft- 3711 ietf-tram-stun-pmtud-07 (work in progress), March 2018. 3713 [I-D.rosenberg-mmusic-ice-nonsip] 3714 Rosenberg, J., "Guidelines for Usage of Interactive 3715 Connectivity Establishment (ICE) by non Session Initiation 3716 Protocol (SIP) Protocols", draft-rosenberg-mmusic-ice- 3717 nonsip-01 (work in progress), July 2008. 3719 [Port-Numbers] 3720 "IANA Port Numbers Registry", 2005, 3721 . 3723 [Protocol-Numbers] 3724 "IANA Protocol Numbers Registry", 2005, 3725 . 3727 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 3728 DOI 10.17487/RFC0791, September 1981, 3729 . 3731 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 3732 DOI 10.17487/RFC1191, November 1990, 3733 . 3735 [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., 3736 and E. Lear, "Address Allocation for Private Internets", 3737 BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996, 3738 . 3740 [RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and 3741 L. Jones, "SOCKS Protocol Version 5", RFC 1928, 3742 DOI 10.17487/RFC1928, March 1996, 3743 . 3745 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 3746 A., Peterson, J., Sparks, R., Handley, M., and E. 3747 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 3748 DOI 10.17487/RFC3261, June 2002, 3749 . 3751 [RFC3424] Daigle, L., Ed. and IAB, "IAB Considerations for 3752 UNilateral Self-Address Fixing (UNSAF) Across Network 3753 Address Translation", RFC 3424, DOI 10.17487/RFC3424, 3754 November 2002, . 3756 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 3757 Jacobson, "RTP: A Transport Protocol for Real-Time 3758 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 3759 July 2003, . 3761 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 3762 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 3763 RFC 3711, DOI 10.17487/RFC3711, March 2004, 3764 . 3766 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 3767 "Randomness Requirements for Security", BCP 106, RFC 4086, 3768 DOI 10.17487/RFC4086, June 2005, 3769 . 3771 [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, 3772 DOI 10.17487/RFC4302, December 2005, 3773 . 3775 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", 3776 RFC 4303, DOI 10.17487/RFC4303, December 2005, 3777 . 3779 [RFC4787] Audet, F., Ed. and C. Jennings, "Network Address 3780 Translation (NAT) Behavioral Requirements for Unicast 3781 UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 3782 2007, . 3784 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 3785 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, 3786 . 3788 [RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to- 3789 Peer (P2P) Communication across Network Address 3790 Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March 3791 2008, . 3793 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 3794 (ICE): A Protocol for Network Address Translator (NAT) 3795 Traversal for Offer/Answer Protocols", RFC 5245, 3796 DOI 10.17487/RFC5245, April 2010, 3797 . 3799 [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using 3800 Relays around NAT (TURN): Relay Extensions to Session 3801 Traversal Utilities for NAT (STUN)", RFC 5766, 3802 DOI 10.17487/RFC5766, April 2010, 3803 . 3805 [RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT 3806 (TURN) Resolution Mechanism", RFC 5928, 3807 DOI 10.17487/RFC5928, August 2010, 3808 . 3810 [RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport- 3811 Protocol Port Randomization", BCP 156, RFC 6056, 3812 DOI 10.17487/RFC6056, January 2011, 3813 . 3815 [RFC6062] Perreault, S., Ed. and J. Rosenberg, "Traversal Using 3816 Relays around NAT (TURN) Extensions for TCP Allocations", 3817 RFC 6062, DOI 10.17487/RFC6062, November 2010, 3818 . 3820 [RFC6156] Camarillo, G., Novo, O., and S. Perreault, Ed., "Traversal 3821 Using Relays around NAT (TURN) Extension for IPv6", 3822 RFC 6156, DOI 10.17487/RFC6156, April 2011, 3823 . 3825 [RFC7635] Reddy, T., Patil, P., Ravindranath, R., and J. Uberti, 3826 "Session Traversal Utilities for NAT (STUN) Extension for 3827 Third-Party Authorization", RFC 7635, 3828 DOI 10.17487/RFC7635, August 2015, 3829 . 3831 [RFC7983] Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme 3832 Updates for Secure Real-time Transport Protocol (SRTP) 3833 Extension for Datagram Transport Layer Security (DTLS)", 3834 RFC 7983, DOI 10.17487/RFC7983, September 2016, 3835 . 3837 [RFC8155] Patil, P., Reddy, T., and D. Wing, "Traversal Using Relays 3838 around NAT (TURN) Server Auto Discovery", RFC 8155, 3839 DOI 10.17487/RFC8155, April 2017, 3840 . 3842 Authors' Addresses 3844 Tirumaleswar Reddy (editor) 3845 McAfee, Inc. 3846 Embassy Golf Link Business Park 3847 Bangalore, Karnataka 560071 3848 India 3850 Email: kondtir@gmail.com 3852 Alan Johnston (editor) 3853 Rowan University 3854 Glassboro, NJ 3855 USA 3857 Email: alan.b.johnston@gmail.com 3859 Philip Matthews 3860 Alcatel-Lucent 3861 600 March Road 3862 Ottawa, Ontario 3863 Canada 3865 Email: philip_matthews@magma.ca 3866 Jonathan Rosenberg 3867 jdrosen.net 3868 Edison, NJ 3869 USA 3871 Email: jdrosen@jdrosen.net 3872 URI: http://www.jdrosen.net