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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 BEHAVE Working Group J. Rosenberg 3 Internet-Draft Cisco 4 Obsoletes: 3489 (if approved) R. Mahy 5 Intended status: Standards Track Plantronics 6 Expires: August 26, 2008 P. Matthews 7 Avaya 8 D. Wing 9 Cisco 10 February 23, 2008 12 Session Traversal Utilities for (NAT) (STUN) 13 draft-ietf-behave-rfc3489bis-15 15 Status of this Memo 17 By submitting this Internet-Draft, each author represents that any 18 applicable patent or other IPR claims of which he or she is aware 19 have been or will be disclosed, and any of which he or she becomes 20 aware will be disclosed, in accordance with Section 6 of BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF), its areas, and its working groups. Note that 24 other groups may also distribute working documents as Internet- 25 Drafts. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 The list of current Internet-Drafts can be accessed at 33 http://www.ietf.org/ietf/1id-abstracts.txt. 35 The list of Internet-Draft Shadow Directories can be accessed at 36 http://www.ietf.org/shadow.html. 38 This Internet-Draft will expire on August 26, 2008. 40 Copyright Notice 42 Copyright (C) The IETF Trust (2008). 44 Abstract 46 Session Traversal Utilities for NAT (STUN) is a protocol that serves 47 as a tool for other protocols in dealing with NAT traversal. It can 48 be used by an endpoint to determine the IP address and port allocated 49 to it by a NAT. It can also be used to check connectivity between 50 two endpoints, and as a keep-alive protocol to maintain NAT bindings. 51 STUN works with many existing NATs, and does not require any special 52 behavior from them. 54 STUN is not a NAT traversal solution by itself. Rather, it is a tool 55 to be used in the context of a NAT traversal solution. This is an 56 important change from the previous version of this specification (RFC 57 3489), which presented STUN as a complete solution. 59 This document obsoletes RFC 3489. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 2. Evolution from RFC 3489 . . . . . . . . . . . . . . . . . . . 4 65 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5 66 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8 67 5. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 8 68 6. STUN Message Structure . . . . . . . . . . . . . . . . . . . . 10 69 7. Base Protocol Procedures . . . . . . . . . . . . . . . . . . . 12 70 7.1. Forming a Request or an Indication . . . . . . . . . . . 12 71 7.2. Sending the Request or Indication . . . . . . . . . . . . 13 72 7.2.1. Sending over UDP . . . . . . . . . . . . . . . . . . . 13 73 7.2.2. Sending over TCP or TLS-over-TCP . . . . . . . . . . . 14 74 7.3. Receiving a STUN Message . . . . . . . . . . . . . . . . 16 75 7.3.1. Processing a Request . . . . . . . . . . . . . . . . . 17 76 7.3.1.1. Forming a Success or Error Response . . . . . . . 17 77 7.3.1.2. Sending the Success or Error Response . . . . . . 18 78 7.3.2. Processing an Indication . . . . . . . . . . . . . . . 18 79 7.3.3. Processing a Success Response . . . . . . . . . . . . 19 80 7.3.4. Processing an Error Response . . . . . . . . . . . . . 19 81 8. FINGERPRINT Mechanism . . . . . . . . . . . . . . . . . . . . 20 82 9. DNS Discovery of a Server . . . . . . . . . . . . . . . . . . 20 83 10. Authentication and Message-Integrity Mechanisms . . . . . . . 21 84 10.1. Short-Term Credential Mechanism . . . . . . . . . . . . . 22 85 10.1.1. Forming a Request or Indication . . . . . . . . . . . 22 86 10.1.2. Receiving a Request or Indication . . . . . . . . . . 22 87 10.1.3. Receiving a Response . . . . . . . . . . . . . . . . . 23 88 10.2. Long-term Credential Mechanism . . . . . . . . . . . . . 24 89 10.2.1. Forming a Request . . . . . . . . . . . . . . . . . . 25 90 10.2.1.1. First Request . . . . . . . . . . . . . . . . . . 25 91 10.2.1.2. Subsequent Requests . . . . . . . . . . . . . . . 25 92 10.2.2. Receiving a Request . . . . . . . . . . . . . . . . . 25 93 10.2.3. Receiving a Response . . . . . . . . . . . . . . . . . 26 94 11. ALTERNATE-SERVER Mechanism . . . . . . . . . . . . . . . . . . 27 95 12. Backwards Compatibility with RFC 3489 . . . . . . . . . . . . 28 96 12.1. Changes to Client Processing . . . . . . . . . . . . . . 28 97 12.2. Changes to Server Processing . . . . . . . . . . . . . . 28 98 13. Basic Server Behavior . . . . . . . . . . . . . . . . . . . . 29 99 14. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 29 100 15. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 31 101 15.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 32 102 15.2. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 33 103 15.3. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 34 104 15.4. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 34 105 15.5. FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . . 35 106 15.6. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 36 107 15.7. REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 37 108 15.8. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 38 109 15.9. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 38 110 15.10. SERVER . . . . . . . . . . . . . . . . . . . . . . . . . 38 111 15.11. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 38 112 16. Security Considerations . . . . . . . . . . . . . . . . . . . 39 113 16.1. Attacks against the Protocol . . . . . . . . . . . . . . 39 114 16.1.1. Outside Attacks . . . . . . . . . . . . . . . . . . . 39 115 16.1.2. Inside Attacks . . . . . . . . . . . . . . . . . . . . 40 116 16.2. Attacks Affecting the Usage . . . . . . . . . . . . . . . 40 117 16.2.1. Attack I: DDoS Against a Target . . . . . . . . . . . 41 118 16.2.2. Attack II: Silencing a Client . . . . . . . . . . . . 41 119 16.2.3. Attack III: Assuming the Identity of a Client . . . . 41 120 16.2.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 41 121 16.3. Hash Agility Plan . . . . . . . . . . . . . . . . . . . . 42 122 17. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 42 123 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 124 18.1. STUN Methods Registry . . . . . . . . . . . . . . . . . . 42 125 18.2. STUN Attribute Registry . . . . . . . . . . . . . . . . . 43 126 18.3. STUN Error Code Registry . . . . . . . . . . . . . . . . 44 127 18.4. STUN UDP and TCP Port Numbers . . . . . . . . . . . . . . 44 128 19. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 44 129 20. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 46 130 21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46 131 22. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46 132 22.1. Normative References . . . . . . . . . . . . . . . . . . 46 133 22.2. Informational References . . . . . . . . . . . . . . . . 47 134 Appendix A. C Snippet to Determine STUN Message Types . . . . . . 49 135 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49 136 Intellectual Property and Copyright Statements . . . . . . . . . . 51 138 1. Introduction 140 The protocol defined in this specification, Session Traversal 141 Utilities for NAT, provides a tool for dealing with NATs. It 142 provides a means for an endpoint to determine the IP address and port 143 allocated by a NAT that corresponds to its private IP address and 144 port. It also provides a way for an endpoint to keep a NAT binding 145 alive. With some extensions, the protocol can be used to do 146 connectivity checks between two endpoints [I-D.ietf-mmusic-ice], or 147 to relay packets between two endpoints [I-D.ietf-behave-turn]. 149 In keeping with its tool nature, this specification defines an 150 extensible packet format, defines operation over several transport 151 protocols, and provides for two forms of authentication. 153 STUN is intended to be used in context of one or more NAT traversal 154 solutions. These solutions are known as STUN usages. Each usage 155 describes how STUN is utilized to achieve the NAT traversal solution. 156 Typically, a usage indicates when STUN messages get sent, which 157 optional attributes to include, what server is used, and what 158 authentication mechanism is to be used. Interactive Connectivity 159 Establishment (ICE) [I-D.ietf-mmusic-ice] is one usage of STUN. SIP 160 Outbound [I-D.ietf-sip-outbound] is another usage of STUN. In some 161 cases, a usage will require extensions to STUN. A STUN extension can 162 be in the form of new methods, attributes, or error response codes. 163 More information on STUN usages can be found in Section 14. 165 2. Evolution from RFC 3489 167 STUN was originally defined in RFC 3489 [RFC3489]. That 168 specification, sometimes referred to as "classic STUN", represented 169 itself as a complete solution to the NAT traversal problem. In that 170 solution, a client would discover whether it was behind a NAT, 171 determine its NAT type, discover its IP address and port on the 172 public side of the outermost NAT, and then utilize that IP address 173 and port within the body of protocols, such as the Session Initiation 174 Protocol (SIP) [RFC3261]. However, experience since the publication 175 of RFC 3489 has found that classic STUN simply does not work 176 sufficiently well to be a deployable solution. The address and port 177 learned through classic STUN are sometimes usable for communications 178 with a peer, and sometimes not. Classic STUN provided no way to 179 discover whether it would, in fact, work or not, and it provided no 180 remedy in cases where it did not. Furthermore, classic STUN's 181 algorithm for classification of NAT types was found to be faulty, as 182 many NATs did not fit cleanly into the types defined there. 184 Classic STUN also had a security vulnerability - attackers could 185 provide the client with incorrect mapped addresses under certain 186 topologies and constraints, and this was fundamentally not solvable 187 through any cryptographic means. Though this problem remains with 188 this specification, those attacks are now mitigated through the use 189 of more complete solutions that make use of STUN. 191 For these reasons, this specification obsoletes RFC 3489, and instead 192 describes STUN as a tool that is utilized as part of a complete NAT 193 traversal solution. ICE [I-D.ietf-mmusic-ice] is a complete NAT 194 traversal solution for protocols based on the offer/answer [RFC3264] 195 methodology, such as SIP. SIP Outbound [I-D.ietf-sip-outbound] is a 196 complete solution for traversal of SIP signaling, and it uses STUN in 197 a very different way. Though it is possible that a protocol may be 198 able to use STUN by itself (classic STUN) as a traversal solution, 199 such usage is not described here and is strongly discouraged for the 200 reasons described above. 202 The on-the-wire protocol described here is changed only slightly from 203 classic STUN. The protocol now runs over TCP in addition to UDP. 204 Extensibility was added to the protocol in a more structured way. A 205 magic-cookie mechanism for demultiplexing STUN with application 206 protocols was added by stealing 32 bits from the 128 bit transaction 207 ID defined in RFC 3489, allowing the change to be backwards 208 compatible. Mapped addresses are encoded using a new exclusive-or 209 format. There are other, more minor changes. See Section 19 for a 210 more complete listing. 212 Due to the change in scope, STUN has also been renamed from "Simple 213 Traversal of UDP Through NAT" to "Session Traversal Utilities for 214 NAT". The acronym remains STUN, which is all anyone ever remembers 215 anyway. 217 3. Overview of Operation 219 This section is descriptive only. 221 /-----\ 222 // STUN \\ 223 | Server | 224 \\ // 225 \-----/ 227 +--------------+ Public Internet 228 ................| NAT 2 |....................... 229 +--------------+ 231 +--------------+ Private NET 2 232 ................| NAT 1 |....................... 233 +--------------+ 235 /-----\ 236 // STUN \\ 237 | Client | 238 \\ // Private NET 1 239 \-----/ 241 Figure 1: One possible STUN Configuration 243 One possible STUN configuration is shown in Figure 1. In this 244 configuration, there are two entities (called STUN agents) that 245 implement the STUN protocol. The lower agent in the figure is the 246 client, and is connected to private network 1. This network connects 247 to private network 2 through NAT 1. Private network 2 connects to 248 the public Internet through NAT 2. The upper agent in the figure is 249 the server, and resides on the public Internet. 251 STUN is a client-server protocol. It supports two types of 252 transactions. One is a request/response transaction in which a 253 client sends a request to a server, and the server returns a 254 response. The second is an indication transaction in which either 255 agent - client or server - sends an indication which generates no 256 response. Both types of transactions include a transaction ID, which 257 is a randomly selected 96-bit number. For request/response 258 transactions, this transaction ID allows the client to associate the 259 response with the request that generated it; for indications, this 260 simply serves as a debugging aid. 262 All STUN messages start with a fixed header that includes a method, a 263 class, and the transaction ID. The method indicates which of the 264 various requests or indications this is; this specification defines 265 just one method, Binding, but other methods are expected to be 266 defined in other documents. The class indicates whether this is a 267 request, a success response, an error response, or an indication. 268 Following the fixed header comes zero or more attributes, which are 269 type-length-value extensions that convey additional information for 270 the specific message. 272 This document defines a single method called Binding. The Binding 273 method can be used either in request/response transactions or in 274 indication transactions. When used in request/response transactions, 275 the Binding method can be used to determine the particular "binding" 276 a NAT has allocated to a STUN client. When used in either request/ 277 response or in indication transactions, the Binding method can also 278 be used to keep these "bindings" alive. 280 In the Binding request/response transaction, a Binding Request is 281 sent from a STUN client to a STUN server. When the Binding Request 282 arrives at the STUN server, it may have passed through one or more 283 NATs between the STUN client and the STUN server (in Figure 1, there 284 were two such NATs). As the Binding Request message passes through a 285 NAT, the NAT will modify the source transport address (that is, the 286 source IP address and the source port) of the packet. As a result, 287 the source transport address of the request received by the server 288 will be the public IP address and port created by the NAT closest to 289 the server. This is called a reflexive transport address. The STUN 290 server copies that source transport address into an XOR-MAPPED- 291 ADDRESS attribute in the STUN Binding Response and sends the Binding 292 Response back to the STUN client. As this packet passes back through 293 a NAT, the NAT will modify the destination transport address in the 294 IP header, but the transport address in the XOR-MAPPED-ADDRESS 295 attribute within the body of the STUN response will remain untouched. 296 In this way, the client can learn its reflexive transport address 297 allocated by the outermost NAT with respect to the STUN server. 299 In some usages, STUN must be multiplexed with other protocols (e.g., 300 [I-D.ietf-mmusic-ice], [I-D.ietf-sip-outbound]). In these usages, 301 there must be a way to inspect a packet and determine if it is a STUN 302 packet or not. STUN provides three fields in the STUN header with 303 fixed values that can be used for this purpose. If this is not 304 sufficient, then STUN packets can also contain a FINGERPRINT value 305 which can further be used to distinguish the packets. 307 STUN defines a set of optional procedures that a usage can decide to 308 use, called mechanisms. These mechanisms include DNS discovery, a 309 redirection technique to an alternate server, a fingerprint attribute 310 for demultiplexing, and two authentication and message integrity 311 exchanges. The authentication mechanisms revolve around the use of a 312 username, password, and message-integrity value. Two authentication 313 mechanisms, the long-term credential mechanism and the short-term 314 credential mechanism, are defined in this specification. Each usage 315 specifies the mechanisms allowed with that usage. 317 In the long-term credential mechanism, the client and server share a 318 pre-provisioned username and password and perform a digest challenge/ 319 response exchange inspired by (but differing in details) to the one 320 defined for HTTP [RFC2617]. In the short-term credential mechanism, 321 the client and the server exchange a username and password through 322 some out-of-band method prior to the STUN exchange. For example, in 323 the ICE usage [I-D.ietf-mmusic-ice] the two endpoints use out-of-band 324 signaling to exchange a username and password. These are used to 325 integrity protect and authenticate the request and response. There 326 is no challenge or nonce used. 328 4. Terminology 330 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 331 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 332 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 333 [RFC2119] and indicate requirement levels for compliant STUN 334 implementations. 336 5. Definitions 338 STUN Agent: An entity that implements the STUN protocol. The entity 339 can either be a STUN client or a STUN server. 341 STUN Client: A STUN client is an entity that sends STUN requests, 342 and receives STUN responses. STUN clients can also send 343 indications. In this specification, the terms STUN client and 344 client are synonymous. 346 STUN Server: A STUN server is an entity that receives STUN requests 347 and sends STUN responses. A STUN server can also send 348 indications. In this specification, the terms STUN server and 349 server are synonymous. 351 Transport Address: The combination of an IP address and port number 352 (such as a UDP or TCP port number). 354 Reflexive Transport Address: A transport address learned by a client 355 that identifies that client as seen by another host on an IP 356 network, typically a STUN server. When there is an intervening 357 NAT between the client and the other host, the reflexive transport 358 address represents the mapped address allocated to the client on 359 the public side of the NAT. Reflexive transport addresses are 360 learned from the mapped address attribute (MAPPED-ADDRESS or XOR- 361 MAPPED-ADDRESS) in STUN responses. 363 Mapped Address: Same meaning as Reflexive Address. This term is 364 retained only for for historic reasons and due to the naming of 365 the MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes. 367 Long Term Credential: A username and associated password that 368 represent a shared secret between client and server. Long term 369 credentials are generally granted to the client when a subscriber 370 enrolls in a service and persist until the subscriber leaves the 371 service or explicitly changes the credential. 373 Long Term Password: The password from a long term credential. 375 Short Term Credential: A temporary username and associated password 376 which represent a shared secret between client and server. Short 377 term credentials are obtained through some kind of protocol 378 mechanism between the client and server, preceding the STUN 379 exchange. A short term credential has an explicit temporal scope, 380 which may be based on a specific amount of time (such as 5 381 minutes) or on an event (such as termination of a SIP dialog). 382 The specific scope of a short term credential is defined by the 383 application usage. 385 Short Term Password: The password component of a short term 386 credential. 388 STUN Indication: A STUN message that does not receive a response 390 Attribute: The STUN term for a Type-Length-Value (TLV) object that 391 can be added to a STUN message. Attributes are divided into two 392 types: comprehension-required and comprehension-optional. STUN 393 agents can safely ignore comprehension-optional attributes they 394 don't understand, but cannot successfully process a message if it 395 contains comprehension-required attributes that are not 396 understood. 398 RTO: Retransmission TimeOut, which defines the initial period of 399 time between transmission of a request and the first retransmit of 400 that request. 402 6. STUN Message Structure 404 STUN messages are encoded in binary using network-oriented format 405 (most significant byte or octet first, also commonly known as big- 406 endian). The transmission order is described in detail in Appendix B 407 of RFC791 [RFC0791]. Unless otherwise noted, numeric constants are 408 in decimal (base 10). 410 All STUN messages MUST start with a 20-byte header followed by zero 411 or more Attributes. The STUN header contains a STUN message type, 412 magic cookie, transaction ID, and message length. 414 0 1 2 3 415 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 1 416 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 417 |0 0| STUN Message Type | Message Length | 418 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 419 | Magic Cookie | 420 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 421 | | 422 | Transaction ID (96 bits) | 423 | | 424 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 426 Figure 2: Format of STUN Message Header 428 The most significant two bits of every STUN message MUST be zeroes. 429 This can be used to differentiate STUN packets from other protocols 430 when STUN is multiplexed with other protocols on the same port. 432 The message type defines the message class (request, success 433 response, failure response, or indication) and the message method 434 (the primary function) of the STUN message. Although there are four 435 message classes, there are only two types of transactions in STUN: 436 request/response transactions (which consist of a request message and 437 a response message), and indication transactions (which consists of a 438 single indication message). Response classes are split into error 439 and success responses to aid in quickly processing the STUN message. 441 The message type field is decomposed further into the following 442 structure: 444 0 1 445 2 3 4 5 6 7 8 9 0 1 2 3 4 5 447 +--+--+-+-+-+-+-+-+-+-+-+-+-+-+ 448 |M |M |M|M|M|C|M|M|M|C|M|M|M|M| 449 |11|10|9|8|7|1|6|5|4|0|3|2|1|0| 450 +--+--+-+-+-+-+-+-+-+-+-+-+-+-+ 452 Figure 3: Format of STUN Message Type Field 454 Here the bits in the message type field are shown as most-significant 455 (M11) through least-significant (M0). M11 through M0 represent a 12- 456 bit encoding of the method. C1 and C0 represent a 2 bit encoding of 457 the class. A class of 0b00 is a Request, a class of 0b01 is an 458 indication, a class of 0b10 is a success response, and a class of 459 0b11 is an error response. This specification defines a single 460 method, Binding. The method and class are orthogonal, so that for 461 each method, a request, success response, error response and 462 indication are defined for that method. 464 For example, a Binding Request has class=0b00 (request) and 465 method=0b000000000001 (Binding), and is encoded into the first 16 466 bits as 0x0001. A Binding response has class=0b10 (success response) 467 and method=0b000000000001, and is encoded into the first 16 bits as 468 0x0101. 470 Note: This unfortunate encoding is due to assignment of values in 471 [RFC3489] which did not consider encoding Indications, Success, 472 and Errors using bit fields. 474 The magic cookie field MUST contain the fixed value 0x2112A442 in 475 network byte order. In RFC 3489 [RFC3489], this field was part of 476 the transaction ID; placing the magic cookie in this location allows 477 a server to detect if the client will understand certain attributes 478 that were added in this revised specification. In addition, it aids 479 in distinguishing STUN packets from packets of other protocols when 480 STUN is multiplexed with those other protocols on the same port. 482 The transaction ID is a 96 bit identifier, used to uniquely identify 483 STUN transactions. For request/response transactions, the 484 transaction ID is chosen by the STUN client for the request and 485 echoed by the server in the response. For indications, it is chosen 486 by the agent sending the indication. It primarily serves to 487 correlate requests with responses, though it also plays a small role 488 in helping to prevent certain types of attacks. As such, the 489 transaction ID MUST be uniformly and randomly chosen from the 490 interval 0 .. 2**96-1. Resends of the same request reuse the same 491 transaction ID, but the client MUST choose a new transaction ID for 492 new transactions unless the new request is bit-wise identical to the 493 previous request and sent from the same transport address to the same 494 IP address. Success and error responses MUST carry the same 495 transaction ID as their corresponding request. When an agent is 496 acting as a STUN server and STUN client on the same port, the 497 transaction IDs in requests sent by the agent have no relationship to 498 the transaction IDs in requests received by the agent. 500 The message length MUST contain the size, in bytes, of the message 501 not including the 20 byte STUN header. Since all STUN attributes are 502 padded to a multiple of four bytes, the last two bits of this field 503 are always zero. This provides another way to distinguish STUN 504 packets from packets of other protocols. 506 Following the STUN fixed portion of the header are zero or more 507 attributes. Each attribute is TLV (type-length-value) encoded. The 508 details of the encoding, and of the attributes themselves is given in 509 Section 15. 511 7. Base Protocol Procedures 513 This section defines the base procedures of the STUN protocol. It 514 describes how messages are formed, how they are sent, and how they 515 are processed when they are received. It also defines the detailed 516 processing of the Binding method. Other sections in this document 517 describe optional procedures that a usage may elect to use in certain 518 situations. Other documents may define other extensions to STUN, by 519 adding new methods, new attributes, or new error response codes. 521 7.1. Forming a Request or an Indication 523 When formulating a request or indication message, the agent MUST 524 follow the rules in Section 6 when creating the header. In addition, 525 the message class MUST be either "Request" or "Indication" (as 526 appropriate), and the method must be either Binding or some method 527 defined in another document. 529 The agent then adds any attributes specified by the method or the 530 usage. For example, some usages may specify that the agent use an 531 authentication method (Section 10) or the FINGERPRINT attribute 532 (Section 8). 534 For the Binding method with no authentication, no attributes are 535 required unless the usage specifies otherwise. 537 All STUN requests and responses sent over UDP SHOULD be less than the 538 path MTU, if known. If the path MTU is unknown, requests and 539 responses SHOULD be the smaller of 576 bytes and the first-hop MTU 540 for IPv4 [RFC1122] and 1280 bytes for IPv6 [RFC2460]. This value 541 corresponds to the overall size of the IP packet. Consequently, for 542 IPv4, the actual STUN message would need to be less than 548 bytes 543 (576 minus 20 bytes IP header, minus 8 byte UDP header, assuming no 544 IP options are used). STUN provides no ability to handle the case 545 where the request is under the MTU but the response would be larger 546 than the MTU. It is not envisioned that this limitation will be an 547 issue for STUN. The MTU limitation is a SHOULD, and not a MUST, to 548 account for cases where STUN itself is being used to probe for MTU 549 characteristics [I-D.ietf-behave-nat-behavior-discovery]. Outside of 550 this or similar applications, the MTU constraint MUST be followed. 552 7.2. Sending the Request or Indication 554 The agent then sends the request or indication. This document 555 specifies how to send STUN messages over UDP, TCP, or TLS-over-TCP; 556 other transport protocols may be added in the future. The STUN usage 557 must specify which transport protocol is used, and how the agent 558 determines the IP address and port of the recipient. Section 9 559 describes a DNS-based method of determining the IP address and port 560 of a server which a usage may elect to use. STUN may be used with 561 anycast addresses, but only with UDP and in usages where 562 authentication is not used. 564 At any time, a client MAY have multiple outstanding STUN requests 565 with the same STUN server (that is, multiple transactions in 566 progress, with different transaction ids). Absent other limits to 567 the rate of new transactions (such as those specified by ICE for 568 connectivity checks), a client SHOULD space new transactions to a 569 server by RTO and SHOULD limit itself to ten outstanding transactions 570 to the same sevrer. 572 7.2.1. Sending over UDP 574 When running STUN over UDP it is possible that the STUN message might 575 be dropped by the network. Reliability of STUN request/response 576 transactions is accomplished through retransmissions of the request 577 message by the client application itself. STUN indications are not 578 retransmitted; thus indication transactions over UDP are not 579 reliable. 581 A client SHOULD retransmit a STUN request message starting with an 582 interval of RTO ("Retransmission TimeOut"), doubling after each 583 retransmission. The RTO is an estimate of the round-trip-time, and 584 is computed as described in RFC 2988 [RFC2988], with two exceptions. 586 First, the initial value for RTO SHOULD be configurable (rather than 587 the 3s recommended in RFC 2988) and SHOULD be greater than 500ms. 588 The exception cases for this SHOULD are when other mechanisms are 589 used to derive congestion thresholds (such as the ones defined in ICE 590 for fixed rate streams), or when STUN is used in non-Internet 591 environments with known network capacities. In fixed-line access 592 links, a value of 500ms is RECOMMENDED. Secondly, the value of RTO 593 MUST NOT be rounded up to the nearest second. Rather, a 1ms accuracy 594 MUST be maintained. As with TCP, the usage of Karn's algorithm is 595 RECOMMENDED [KARN87]. When applied to STUN, it means that RTT 596 estimates SHOULD NOT be computed from STUN transactions which result 597 in the retransmission of a request. 599 The value for RTO SHOULD be cached by a client after the completion 600 of the transaction, and used as the starting value for RTO for the 601 next transaction to the same server (based on equality of IP 602 address). The value SHOULD be considered stale and discarded after 603 10 minutes. 605 Retransmissions continue until a response is received, or until a 606 total of Rc requests have been sent. Rc SHOULD be configurable and 607 SHOULD have a default of 7. If, after the last request, a duration 608 equal to 16 times the RTO has passed without a response (providing 609 ample time to get a response if only this final request actually 610 succeeds), the client SHOULD consider the transaction to have failed. 611 A STUN transaction over UDP is also considered failed if there has 612 been a hard ICMP error [RFC1122]. For example, assuming an RTO of 613 500ms, requests would be sent at times 0ms, 500ms, 1500ms, 3500ms, 614 7500ms, 15500ms, and 31500ms. If the client has not received a 615 response after 39500ms, the client will consider the transaction to 616 have timed out. 618 7.2.2. Sending over TCP or TLS-over-TCP 620 For TCP and TLS-over-TCP, the client opens a TCP connection to the 621 server. 623 In some usages of STUN, STUN is sent as the only protocol over the 624 TCP connection. In this case, it can be sent without the aid of any 625 additional framing or demultiplexing. In other usages, or with other 626 extensions, it may be multiplexed with other data over a TCP 627 connection. In that case, STUN MUST be run on top of some kind of 628 framing protocol, specified by the usage or extension, which allows 629 for the agent to extract complete STUN messages and complete 630 application layer messages. The STUN service running on the well 631 known port or ports discovered through the the DNS procedures in 632 Section 9 is for STUN alone, and not for STUN multiplexed with other 633 data. Consequently, no framing protocols are used in connections to 634 those servers. When additional framing is utilized, the usage will 635 specify how the client knows to apply it and what port to connect to. 636 For example, in the case of ICE connectivity checks, this information 637 is learned through out-of-band negotiation between client and server. 639 When STUN is run by itself over TLS-over-TCP, the 640 TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST be supported at a 641 minimum. Implementations MAY also support any other ciphersuite. 642 When it receives the TLS Certificate message, the client SHOULD 643 verify the certificate and inspect the site identified by the 644 certificate. If the certificate is invalid, revoked, or if it does 645 not identify the appropriate party, the client MUST NOT send the STUN 646 message or otherwise proceed with the STUN transaction. The client 647 MUST verify the identity of the server. To do that, it follows the 648 identification procedures defined in Section 3.1 of RFC 2818 649 [RFC2818]. Those procedures assume the client is dereferencing a 650 URI. For purposes of usage with this specification, the client 651 treats the domain name or IP address used in Section 8.1 as the host 652 portion of the URI that has been dereferenced. If DNS was not used, 653 the client MUST be configured with a set of authorized domains whose 654 certificates will be accepted. 656 When STUN is run multiplexed with other protocols over a TLS-over-TCP 657 connection, the mandatory ciphersuites and TLS handling procedures 658 operate as defined by those protocols. 660 Reliability of STUN over TCP and TLS-over-TCP is handled by TCP 661 itself, and there are no retransmissions at the STUN protocol level. 662 However, for a request/response transaction, if the client has not 663 received a response 39500ms after it sent the SYN to establish the 664 connection, it considers the transaction to have timed out. This 665 value has been chosen to equalize the TCP and UDP timeouts for the 666 default initial RTO. 668 In addition, if the client is unable to establish the TCP connection, 669 or the TCP connection is reset or fails before a response is 670 received, any request/response transaction in progress is considered 671 to have failed 673 The client MAY send multiple transactions over a single TCP (or TLS- 674 over-TCP) connection, and it MAY send another request before 675 receiving a response to the previous. The client SHOULD keep the 676 connection open until it 678 o has no further STUN requests or indications to send over that 679 connection, and; 681 o has no plans to use any resources (such as a mapped address 682 (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address 683 [I-D.ietf-behave-turn]) that were learned though STUN requests 684 sent over that connection, and; 686 o if multiplexing other application protocols over that port, has 687 finished using that other application, and; 689 o if using that learned port with a remote peer, has established 690 communications with that remote peer, as is required by some TCP 691 NAT traversal techniques (e.g., [I-D.ietf-mmusic-ice-tcp]). 693 At the server end, the server SHOULD keep the connection open, and 694 let the client close it. Bindings learned by the client will remain 695 valid in intervening NATs only while the connection remains open. 696 Only the client knows how long it needs the binding. The server 697 SHOULD NOT close a connection if a request was received over that 698 connection for which a response was not sent. A server MUST NOT ever 699 open a connection back towards the client in order to send a 700 response. Servers SHOULD follow best practices regarding connection 701 management in cases of overload. 703 7.3. Receiving a STUN Message 705 This section specifies the processing of a STUN message. The 706 processing specified here is for STUN messages as defined in this 707 specification; additional rules for backwards compatibility are 708 defined in in Section 12. Those additional procedures are optional, 709 and usages can elect to utilize them. First, a set of processing 710 operations are applied that are independent of the class. This is 711 followed by class-specific processing, described in the subsections 712 which follow. 714 When a STUN agent receives a STUN message, it first checks that the 715 message obeys the rules of Section 6. It checks that the first two 716 bits are 0, that the magic cookie field has the correct value, that 717 the message length is sensible, and that the method value is a 718 supported method. If the message-class is Success Response or Error 719 Response, the agent checks that the transaction ID matches a 720 transaction that is still in progress. If the FINGERPRINT extension 721 is being used, the agent checks that the FINGERPRINT attribute is 722 present and contains the correct value. If any errors are detected, 723 the message is silently discarded. In the case when STUN is being 724 multiplexed with another protocol, an error may indicate that this is 725 not really a STUN message; in this case, the agent should try to 726 parse the message as a different protocol. 728 The STUN agent then does any checks that are required by a 729 authentication mechanism that the usage has specified (see 730 Section 10. 732 Once the authentication checks are done, the STUN agent checks for 733 unknown attributes and known-but-unexpected attributes in the 734 message. Unknown comprehension-optional attributes MUST be ignored 735 by the agent. Known-but-unexpected attributes SHOULD be ignored by 736 the agent. Unknown comprehension-required attributes cause 737 processing that depends on the message-class and is described below. 739 At this point, further processing depends on the message class of the 740 request. 742 7.3.1. Processing a Request 744 If the request contains one or more unknown comprehension-required 745 attributes, the server replies with an error response with an error 746 code of 420 (Unknown Attribute), and includes an UNKNOWN-ATTRIBUTES 747 attribute in the response that lists the unknown comprehension- 748 required attributes. 750 The server then does any additional checking that the method or the 751 specific usage requires. If all the checks succeed, the server 752 formulates a success response as described below. 754 If the request uses UDP transport and is a retransmission of a 755 request for which the server has already generated a success response 756 within the last 40 seconds, the server MUST retransmit the same 757 success response. One way for a server to do this is to remember all 758 transaction IDs received over UDP and their corresponding responses 759 in the last 10 seconds. Another way is to reprocess the request and 760 recompute the response. The latter technique MUST only be applied to 761 requests which are idempotent (a request is considered idempotent 762 when the same request can be safely repeated without impacting the 763 overall state of the system) and result in the same success response 764 for the same request. The Binding method is considered to idempotent 765 in this way (even though certain rare network events could cause the 766 reflexive transport address value to change). Extensions to STUN 767 SHOULD state whether their request types have this property or not. 769 7.3.1.1. Forming a Success or Error Response 771 When forming the response (success or error), the server follows the 772 rules of section 6. The method of the response is the same as that 773 of the request, and the message class is either "Success Response" or 774 "Error Response". 776 For an error response, the server MUST add an ERROR-CODE attribute 777 containing the error code specified in the processing above. The 778 reason phrase is not fixed, but SHOULD be something suitable for the 779 error code. For certain errors, additional attributes are added to 780 the message. These attributes are spelled out in the description 781 where the error code is specified. For example, for an error code of 782 420 (Unknown Attribute), the server MUST include an UNKNOWN- 783 ATTRIBUTES attribute. Certain authentication errors also cause 784 attributes to be added (see Section 10). Extensions may define other 785 errors and/or additional attributes to add in error cases. 787 If the server authenticated the request using an authentication 788 mechanism, then the server SHOULD add the appropriate authentication 789 attributes to the response (see Section 10). 791 The server also adds any attributes required by the specific method 792 or usage. In addition, the server SHOULD add a SERVER attribute to 793 the message. 795 For the Binding method, no additional checking is required unless the 796 usage specifies otherwise. When forming the success response, the 797 server adds a XOR-MAPPED-ADDRESS attribute to the response, where the 798 contents of the attribute are the source transport address of the 799 request message. For UDP, this is the source IP address and source 800 UDP port of the request message. For TCP and TLS-over-TCP, this is 801 the source IP address and source TCP port of the TCP connection as 802 seen by the server. 804 7.3.1.2. Sending the Success or Error Response 806 The response (success or error) is sent over the same transport as 807 the request was received on. If the request was received over UDP, 808 the destination IP address and port of the response is the source IP 809 address and port of the received request message, and the source IP 810 address and port of the response is equal to the destination IP 811 address and port of the received request message. If the request was 812 received over TCP or TLS-over-TCP, the response is sent back on the 813 same TCP connection as the request was received on. 815 7.3.2. Processing an Indication 817 If the indication contains unknown comprehension-required attributes, 818 the indication is discarded and processing ceases. 820 The agent then does any additional checking that the method or the 821 specific usage requires. If all the checks succeed, the agent then 822 processes the indication. No response is generated for an 823 indication. 825 For the Binding method, no additional checking or processing is 826 required, unless the usage specifies otherwise. The mere receipt of 827 the message by the agent has refreshed the "bindings" in the 828 intervening NATs. 830 Since indications are not re-transmitted over UDP (unlike requests), 831 there is no need to handle re-transmissions of indications at the 832 sending agent. 834 7.3.3. Processing a Success Response 836 If the success response contains unknown comprehension-required 837 attributes, the response is discarded and the transaction is 838 considered to have failed. 840 The client then does any additional checking that the method or the 841 specific usage requires. If all the checks succeed, the client then 842 processes the success response. 844 For the Binding method, the client checks that the XOR-MAPPED-ADDRESS 845 attribute is present in the response. The client checks the address 846 family specified. If it is an unsupported address family, the 847 attribute SHOULD be ignored. If it is an unexpected but supported 848 address family (for example, the Binding transaction was sent over 849 IPv4, but the address family specified is IPv6), then the client MAY 850 accept and use the value. 852 7.3.4. Processing an Error Response 854 If the error response contains unknown comprehension-required 855 attributes, or if the error response does not contain an ERROR-CODE 856 attribute, then the transaction is simply considered to have failed. 858 The client then does any processing specified by the authentication 859 mechanism (see Section 10). This may result in a new transaction 860 attempt. 862 The processing at this point depends on the error-code, the method, 863 and the usage; the following are the default rules: 865 o If the error code is 300 through 399, the client SHOULD consider 866 the transaction as failed unless the ALTERNATE-SERVER extension is 867 being used. See Section 11. 869 o If the error code is 400 through 499, the client declares the 870 transaction failed; in the case of 420 (Unknown Attribute), the 871 response should contain a UNKNOWN-ATTRIBUTES attribute that gives 872 additional information. 874 o If the error code is 500 through 599, the client MAY resend the 875 request; clients that do so MUST limit the number of times they do 876 this. 878 Any other error code causes the client to consider the transaction 879 failed. 881 8. FINGERPRINT Mechanism 883 This section describes an optional mechanism for STUN that aids in 884 distinguishing STUN messages from packets of other protocols when the 885 two are multiplexed on the same transport address. This mechanism is 886 optional, and a STUN usage must describe if and when it is used. The 887 FINGERPRINT mechanism is not backwards compatible with RFC3489, and 888 cannot be used in environments where such compatibility is required. 890 In some usages, STUN messages are multiplexed on the same transport 891 address as other protocols, such as RTP. In order to apply the 892 processing described in Section 7, STUN messages must first be 893 separated from the application packets. Section 6 describes three 894 fixed fields in the STUN header that can be used for this purpose. 895 However, in some cases, these three fixed fields may not be 896 sufficient. 898 When the FINGERPRINT extension is used, an agent includes the 899 FINGERPRINT attribute in messages it sends to another agent. 900 Section 15.5 describes the placement and value of this attribute. 901 When the agent receives what it believes is a STUN message, then, in 902 addition to other basic checks, the agent also checks that the 903 message contains a FINGERPRINT attribute and that the attribute 904 contains the correct value. Section 7.3 describes when in the 905 overall processing of a STUN message the FINGERPRINT check is 906 performed. This additional check helps the agent detect messages of 907 other protocols that might otherwise seem to be STUN messages. 909 9. DNS Discovery of a Server 911 This section describes an optional procedure for STUN that allows a 912 client to use DNS to determine the IP address and port of a server. 913 A STUN usage must describe if and when this extension is used. To 914 use this procedure, the client must know a server's domain name and a 915 service name; the usage must also describe how the client obtains 916 these. Hard-coding the domain-name of the server into software is 917 NOT RECOMMENDED in case the domain name is lost or needs to change 918 for legal or other reasons. 920 When a client wishes to locate a STUN server in the public Internet 921 that accepts Binding Request/Response transactions, the SRV service 922 name is "stun". When it wishes to locate a STUN server which accepts 923 Binding Request/Response transactions over a TLS session, the SRV 924 service name is "stuns". STUN usages MAY define additional DNS SRV 925 service names. 927 The domain name is resolved to a transport address using the SRV 928 procedures specified in [RFC2782]. The DNS SRV service name is the 929 service name provided as input to this procedure. The protocol in 930 the SRV lookup is the transport protocol the client will run STUN 931 over: "udp" for UDP and "tcp" for TCP. Note that only "tcp" is 932 defined with "stuns" at this time. 934 The procedures of RFC 2782 are followed to determine the server to 935 contact. RFC 2782 spells out the details of how a set of SRV records 936 are sorted and then tried. However, RFC2782 only states that the 937 client should "try to connect to the (protocol, address, service)" 938 without giving any details on what happens in the event of failure. 939 When following these procedures, if the STUN transaction times out 940 without receipt of a response, the client SHOULD retry the request to 941 the next server in the ordered defined by RFC 2782. Such a retry is 942 only possible for request/response transmissions, since indication 943 transactions generate no response or timeout. 945 The default port for STUN requests is 3478, for both TCP and UDP. 946 Administrators of STUN servers SHOULD use this port in their SRV 947 records for UDP and TCP. In all cases, the port in DNS MUST reflect 948 the one the server is listening on. The default port for STUN over 949 TLS is XXXX [[NOTE TO RFC EDITOR: Replace with IANA registered port 950 number for stuns]]. Servers can run STUN over TLS on the same port 951 as STUN over TCP if the server software supports determining whether 952 the initial message is a TLS or STUN message. 954 If no SRV records were found, the client performs an A or AAAA record 955 lookup of the domain name. The result will be a list of IP 956 addresses, each of which can be contacted at the default port using 957 UDP or TCP, independent of the STUN usage. For usages that require 958 TLS, lack of SRV records is equivalent to a failure of the 959 transaction, since the request or indication MUST NOT be sent unless 960 SRV records provided a transport address specifically for TLS. 962 10. Authentication and Message-Integrity Mechanisms 964 This section defines two mechanisms for STUN that a client and server 965 can use to provide authentication and message-integrity; these two 966 mechanisms are known as the short-term credential mechanism and the 967 long-term credential mechanism. These two mechanisms are optional, 968 and each usage must specify if and when these mechanisms are used. 969 Consequently, both clients and servers will know which mechanism (if 970 any) to follow based on knowledge of which usage applies. For 971 example, a STUN server on the public Internet supporting ICE would 972 have no authentication, whereas the STUN server functionality in an 973 agent supporting connectivity checks would utilize short term 974 credentials. An overview of these two mechanisms is given in 975 Section 3. 977 Each mechanism specifies the additional processing required to use 978 that mechanism, extending the processing specified in Section 7. The 979 additional processing occurs in three different places: when forming 980 a message; when receiving a message immediately after the basic 981 checks have been performed; and when doing the detailed processing of 982 error responses. 984 10.1. Short-Term Credential Mechanism 986 The short-term credential mechanism assumes that, prior to the STUN 987 transaction, the client and server have used some other protocol to 988 exchange a credential in the form of a username and password. This 989 credential is time-limited. The time-limit is defined by the usage. 990 As an example, in the ICE usage [I-D.ietf-mmusic-ice], the two 991 endpoints use out-of-band signaling to agree on a username and 992 password, and this username and password is applicable for the 993 duration of the media session. 995 This credential is used to form a message integrity check in each 996 request and in many responses. There is no challenge and response as 997 in the long term mechanism; consequently, replay is prevented by 998 virtue of the time-limited nature of the credential. 1000 10.1.1. Forming a Request or Indication 1002 For a request or indication message, the agent MUST include the 1003 USERNAME and MESSAGE-INTEGRITY attributes in the message. The HMAC 1004 for the MESSAGE-INTEGRITY attribute is computed as described in 1005 Section 15.4. Note that the password is never included in the 1006 request or indication. 1008 10.1.2. Receiving a Request or Indication 1010 After the agent has done the basic processing of a message, the agent 1011 performs the checks listed below in order specified: 1013 o If the message does not contain both a MESSAGE-INTEGRITY and a 1014 USERNAME attribute: 1016 * If the message is a request, the server MUST reject the request 1017 with an error response. This response MUST use an error code 1018 of 400 (Bad Request). 1020 * If the message is an indication, the agent MUST silently 1021 discard the indication. 1023 o If the USERNAME does not contain a username value currently valid 1024 within the server: 1026 * If the message is a request, the server MUST reject the request 1027 with an error response. This response MUST use an error code 1028 of 401 (Unauthorized). 1030 * If the message is an indication, the agent MUST silently 1031 discard the indication. 1033 o Using the password associated with the username, compute the value 1034 for the message-integrity as described in Section 15.4. If the 1035 resulting value does not match the contents of the MESSAGE- 1036 INTEGRITY attribute: 1038 * If the message is a request, the server MUST reject the request 1039 with an error response. This response MUST use an error code 1040 of 401 (Unauthorized). 1042 * If the message is an indication, the agent MUST silently 1043 discard the indication. 1045 If these checks pass, the agent continues to process the request or 1046 indication. Any response generated by a server MUST include the 1047 MESSAGE-INTEGRITY attribute, computed using the password utilized to 1048 authenticate the request. The response MUST NOT contain the USERNAME 1049 attribute. 1051 If any of the checks fail, a server MUST NOT include a MESSAGE- 1052 INTEGRITY or USERNAME attribute in the error response. This is 1053 because, in these failure cases, the server cannot determine the 1054 shared secret necessary to compute MESSAGE-INTEGRITY. 1056 10.1.3. Receiving a Response 1058 The client looks for the MESSAGE-INTEGRITY attribute in the response. 1059 If present, the client computes the message integrity over the 1060 response as defined in Section 15.4, using the same password it 1061 utilized for the request. If the resulting value matches the 1062 contents of the MESSAGE-INTEGRITY attribute, the response is 1063 considered authenticated. If the value does not match, or if 1064 MESSAGE-INTEGRITY was absent, the response MUST be discarded, as if 1065 it was never received. This means that retransmits, if applicable, 1066 will continue. 1068 10.2. Long-term Credential Mechanism 1070 The long-term credential mechanism relies on a long term credential, 1071 in the form of a username and password, that are shared between 1072 client and server. The credential is considered long-term since it 1073 is assumed that it is provisioned for a user, and remains in effect 1074 until the user is no longer a subscriber of the system, or is 1075 changed. This is basically a traditional "log-in" username and 1076 password given to users. 1078 Because these usernames and passwords are expected to be valid for 1079 extended periods of time, replay prevention is provided in the form 1080 of a digest challenge. In this mechanism, the client initially sends 1081 a request, without offering any credentials or any integrity checks. 1082 The server rejects this request, providing the user a realm (used to 1083 guide the user or agent in selection of a username and password) and 1084 a nonce. The nonce provides the replay protection. It is a cookie, 1085 selected by the server, and encoded in such a way as to indicate a 1086 duration of validity or client identity from which it is valid. The 1087 client retries the request, this time including its username, the 1088 realm, and echoing the nonce provided by the server. The client also 1089 includes a message-integrity, which provides an HMAC over the entire 1090 request, including the nonce. The server validates the nonce, and 1091 checks the message-integrity. If they match, the request is 1092 authenticated. If the nonce is no longer valid, it is considered 1093 "stale", and the server rejects the request, providing a new nonce. 1095 In subsequent requests to the same server, the client reuses the 1096 nonce, username, realm and password it used previously. In this way, 1097 subsequent requests are not rejected until the nonce becomes invalid 1098 by the server, in which case the rejection provides a new nonce to 1099 the client. 1101 Note that the long-term credential mechanism cannot be used to 1102 protect indications, since indications cannot be challenged. Usages 1103 utilizing indications must either use a short-term credential, or 1104 omit authentication and message integrity for them. 1106 Since the long-term credential mechanism is susceptible to offline 1107 dictionary attacks, deployments SHOULD utilize strong passwords. 1109 10.2.1. Forming a Request 1111 There are two cases when forming a request. In the first case, this 1112 is the first request from the client to the server (as identified by 1113 its IP address and port). In the second case, the client is 1114 submitting a subsequent request once a previous request/response 1115 transaction has completed successfully. Forming a request as a 1116 consequence of a 401 or 438 error response is covered in 1117 Section 10.2.3 and is not considered a "subsequent request" and thus 1118 does not utilize the rules described in Section 10.2.1.2. 1120 10.2.1.1. First Request 1122 If the client has not completed a successful request/response 1123 transaction with the server (as identified by hostname, if the DNS 1124 procedures of Section 9 are used, else IP address if not), it SHOULD 1125 omit the USERNAME, MESSAGE-INTEGRITY, REALM, and NONCE attributes. 1126 In other words, the very first request is sent as if there were no 1127 authentication or message integrity applied. The exception to this 1128 rule are requests sent to another server as a consequence of the 1129 ALTERNATE-SERVER mechanism described in Section 11. Those requests 1130 do include the USERNAME, REALM and NONCE from the original request, 1131 along with a newly computed MESSAGE-INTEGRITY based on them. 1133 10.2.1.2. Subsequent Requests 1135 Once a request/response transaction has completed successfully, the 1136 client will have been been presented a realm and nonce by the server, 1137 and selected a username and password with which it authenticated. 1138 The client SHOULD cache the username, password, realm, and nonce for 1139 subsequent communications with the server. When the client sends a 1140 subsequent request, it SHOULD include the USERNAME, REALM, and NONCE 1141 attributes with these cached values. It SHOULD include a MESSAGE- 1142 INTEGRITY attribute, computed as described in Section 15.4 using the 1143 cached password. 1145 10.2.2. Receiving a Request 1147 After the server has done the basic processing of a request, it 1148 performs the checks listed below in the order specified: 1150 o If the message does not contain a MESSAGE-INTEGRITY attribute, the 1151 server MUST generate an error response with an error code of 401 1152 (Unauthorized). This response MUST include a REALM value. It is 1153 RECOMMENDED that the REALM value be the domain name of the 1154 provider of the STUN server. The response MUST include a NONCE, 1155 selected by the server. The response SHOULD NOT contain a 1156 USERNAME or MESSAGE-INTEGRITY attribute. 1158 o If the message contains a MESSAGE-INTEGRITY attribute, but is 1159 missing the USERNAME, REALM or NONCE attributes, the server MUST 1160 generate an error response with an error code of 400 (Bad 1161 Request). This response SHOULD NOT include a USERNAME, NONCE, 1162 REALM or MESSAGE-INTEGRITY attribute. 1164 o If the NONCE is no longer valid, the server MUST generate an error 1165 response with an error code of 438 (Stale Nonce). This response 1166 MUST include a NONCE and REALM attribute and SHOULD NOT incude the 1167 USERNAME or MESSAGE-INTEGRITY attribute. Servers can invalidate 1168 nonces in order to provide additional security. See Section 4.3 1169 of [RFC2617] for guidelines. 1171 o If the username in the USERNAME attribute is not valid, the server 1172 MUST generate an error response with an error code of 401 1173 (Unauthorized). This response MUST include a REALM value. It is 1174 RECOMMENDED that the REALM value be the domain name of the 1175 provider of the STUN server. The response MUST include a NONCE, 1176 selected by the server. The response SHOULD NOT contain a 1177 USERNAME or MESSAGE-INTEGRITY attribute. 1179 o Using the password associated with the username in the USERNAME 1180 attribute, compute the value for the message-integrity as 1181 described in Section 15.4. If the resulting value does not match 1182 the contents of the MESSAGE-INTEGRITY attribute, the server MUST 1183 reject the request with an error response. This response MUST use 1184 an error code of 401 (Unauthorized). It MUST include a REALM and 1185 NONCE attribute and SHOULD NOT include the USERNAME or MESSAGE- 1186 INTEGRITY attribute. 1188 If these checks pass, the server continues to process the request. 1189 Any response generated by the server (excepting the cases described 1190 above) MUST include the MESSAGE-INTEGRITY attribute, computed using 1191 the username and password utilized to authenticate the request. The 1192 REALM, NONCE, and USERNAME attributes SHOULD NOT be included. 1194 10.2.3. Receiving a Response 1196 If the response is an error response, with an error code of 401 1197 (Unauthorized), the client SHOULD retry the request with a new 1198 transaction. This request MUST contain a USERNAME, determined by the 1199 client as the appropriate username for the REALM from the error 1200 response. The request MUST contain the REALM, copied from the error 1201 response. The request MUST contain the NONCE, copied from the error 1202 response. The request MUST contain the MESSAGE-INTEGRITY attribute, 1203 computed using the password associated with the username in the 1204 USERNAME attribute. The client MUST NOT perform this retry if it is 1205 not changing the USERNAME or REALM or its associated password, from 1206 the previous attempt. 1208 If the response is an error response with an error code of 438 (Stale 1209 Nonce), the client MUST retry the request, using the new NONCE 1210 supplied in the 438 (Stale Nonce) response. This retry MUST also 1211 include the USERNAME, REALM and MESSAGE-INTEGRITY. 1213 The client looks for the MESSAGE-INTEGRITY attribute in the response 1214 (either success or failure). If present, the client computes the 1215 message integrity over the response as defined in Section 15.4, using 1216 the same password it utilized for the request. If the resulting 1217 value matches the contents of the MESSAGE-INTEGRITY attribute, the 1218 response is considered authenticated. If the value does not match, 1219 or if MESSAGE-INTEGRITY was absent, the response MUST be discarded, 1220 as if it was never received. This means that retransmits, if 1221 applicable, will continue. 1223 11. ALTERNATE-SERVER Mechanism 1225 This section describes a mechanism in STUN that allows a server to 1226 redirect a client to another server. This extension is optional, and 1227 a usage must define if and when this extension is used. To prevent 1228 denial-of-service attacks, this extension MUST only be used in 1229 situations where the client and server are using an authentication 1230 and message-integrity mechanism. 1232 A server using this extension redirects a client to another server by 1233 replying to a request message with an error response message with an 1234 error code of 300 (Try Alternate). The server MUST include a 1235 ALTERNATE-SERVER attribute in the error response. The error response 1236 message MUST be authenticated, which in practice means the request 1237 message must have passed the authentication checks. 1239 A client using this extension handles a 300 (Try Alternate) error 1240 code as follows. If the error response has passed the authentication 1241 checks, then the client looks for a ALTERNATE-SERVER attribute in the 1242 error response. If one is found, then the client considers the 1243 current transaction as failed, and re-attempts the request with the 1244 server specified in the attribute, using the same transport protocol 1245 used for the previous request. The client SHOULD reuse any 1246 authentication credentials from the old request in the new 1247 transaction. If the server has been redirected to a server on which 1248 it has already tried this request within the last five minutes, it 1249 MUST ignore the redirection and consider the transaction to have 1250 failed. This prevents infinite ping-ponging between servers in case 1251 of redirection loops. 1253 12. Backwards Compatibility with RFC 3489 1255 This section define procedures that allow a degree of backwards 1256 compatible with the original protocol defined in RFC 3489 [RFC3489]. 1257 This mechanism is optional, meant to be utilized only in cases where 1258 a new client can connect to an old server, or vice-a-versa. A usage 1259 must define if and when this procedure is used. 1261 Section 19 lists all the changes between this specification and RFC 1262 3489 [RFC3489]. However, not all of these differences are important, 1263 because "classic STUN" was only used in a few specific ways. For the 1264 purposes of this extension, the important changes are the following. 1265 In RFC 3489: 1267 o UDP was the only supported transport; 1269 o The field that is now the Magic Cookie field was a part of the 1270 transaction id field, and transaction ids were 128 bits long; 1272 o The XOR-MAPPED-ADDRESS attribute did not exist, and the Binding 1273 method used the MAPPED-ADDRESS attribute instead; 1275 o There were three comprehension-required attributes, RESPONSE- 1276 ADDRESS, CHANGE-REQUEST, and CHANGED-ADDRESS that have been 1277 removed from this specification; 1279 * These attributes are now part of the NAT Behavior Discovery 1280 usage. [I-D.ietf-behave-nat-behavior-discovery] 1282 12.1. Changes to Client Processing 1284 A client that wants to interoperate with a [RFC3489] server SHOULD 1285 send a request message that uses the Binding method, contains no 1286 attributes, and uses UDP as the transport protocol to the server. If 1287 successful, the success response received from the server will 1288 contain a MAPPED-ADDRESS attribute rather than an XOR-MAPPED-ADDRESS 1289 attribute; other than this change, the processing of the response is 1290 identical to the procedures described above. 1292 12.2. Changes to Server Processing 1294 A STUN server can detect when a given Binding Request message was 1295 sent from an RFC 3489 [RFC3489] client by the absence of the correct 1296 value in the Magic Cookie field. When the server detects an RFC 3489 1297 client, it SHOULD copy the value seen in the Magic Cookie field in 1298 the Binding Request to the Magic Cookie field in the Binding Response 1299 message, and insert a MAPPED-ADDRESS attribute instead of an XOR- 1300 MAPPED-ADDRESS attribute. 1302 The client might, in rare situations, include either the RESPONSE- 1303 ADDRESS or CHANGE-REQUEST attributes. In these situations, the 1304 server will view these as unknown comprehension-required attributes 1305 and reply with an error response. Since the mechanisms utilizing 1306 those attributes are no longer supported, this behavior is 1307 acceptable. 1309 The RFC 3489 version of STUN lacks both the Magic Cookie and the 1310 FINGERPRINT attribute that allows for a very high probablility of 1311 correctly identifying STUN messages when multiplexed with other 1312 protocols. Therefore, STUN implementations that are backwards 1313 compatible with RFC 3489 SHOULD NOT be used in cases where STUN will 1314 be multiplexed with another protocol. However, that should not be an 1315 issues as such multiplexing was not available in RFC 3489. 1317 13. Basic Server Behavior 1319 This section defines the behavior of a basic, standalone STUN server. 1320 A basic STUN server provides clients with server reflexive transport 1321 addresses by receiving and replying to STUN Binding Requests. 1323 The STUN server MUST support the Binding method. It SHOULD NOT 1324 utilize the short term or long term credential mechanism. This is 1325 because the work involved in authenticating the request is more than 1326 the work in simply processing it. It SHOULD NOT utilize the 1327 ALTERNATE-SERVER mechanism for the same reason. It MUST support UDP 1328 and TCP. It MAY support STUN over TCP/TLS, however TLS provides 1329 minimal security benefits in this basic mode of operation. It MAY 1330 utilize the FINGERPRINT mechanism but MUST NOT require it. Since the 1331 standalove server only runs STUN, FINGERPRINT provides no benefit. 1332 Requiring it would break compatibility with RFC 3489, and such 1333 compatibility is desirable in a standalone server. Standalone STUN 1334 servers SHOULD support backwards compatibility with [RFC3489] 1335 clients, as described in Section 12. 1337 It is RECOMMENDED that administrators of STUN servers provide DNS 1338 entries for those servers as described in Section 9. 1340 A basic STUN server is not a solution for NAT traversal by itself. 1341 However, it can be utilized as part of a solution through STUN 1342 usages. This is discussed further in Section 14. 1344 14. STUN Usages 1346 STUN by itself is not a solution to the NAT traversal problem. 1347 Rather, STUN defines a tool that can be used inside a larger 1348 solution. The term "STUN Usage" is used for any solution that uses 1349 STUN as a component. 1351 At the time of writing, three STUN usages are defined: Interactive 1352 Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice], Client- 1353 initiated connections for SIP [I-D.ietf-sip-outbound], and NAT 1354 Behavior Discovery [I-D.ietf-behave-nat-behavior-discovery]. Other 1355 STUN usages may be defined in the future. 1357 A STUN usage defines how STUN is actually utilized - when to send 1358 requests, what to do with the responses, and which optional 1359 procedures defined here (or in an extension to STUN) are to be used. 1360 A usage would also define: 1362 o Which STUN methods are used; 1364 o What authentication and message integrity mechanisms are used; 1366 o The considerations around manual vs. automatic key derivation for 1367 the integrity mechanism, as discussed in [RFC4107]; 1369 o What mechanisms are used to distinguish STUN messages from other 1370 messages. When STUN is run over TCP, a framing mechanism may be 1371 required; 1373 o How a STUN client determines the IP address and port of the STUN 1374 server; 1376 o Whether backwards compatibility to RFC 3489 is required; 1378 o What optional attributes defined here (such as FINGERPRINT and 1379 ALTERNATE-SERVER) or in other extensions are required. 1381 In addition, any STUN usage must consider the security implications 1382 of using STUN in that usage. A number of attacks against STUN are 1383 known (see the Security Considerations section in this document) and 1384 any usage must consider how these attacks can be thwarted or 1385 mitigated. 1387 Finally, a usage must consider whether its usage of STUN is an 1388 example of the Unilateral Self-Address Fixing approach to NAT 1389 traversal, and if so, address the questions raised in RFC 3424. 1390 [RFC3424] 1392 15. STUN Attributes 1394 After the STUN header are zero or more attributes. Each attribute 1395 MUST be TLV encoded, with a 16 bit type, 16 bit length, and value. 1396 Each STUN attribute MUST end on a 32 bit boundary. As mentioned 1397 above, all fields in an attribute are transmitted most significant 1398 bit first. 1400 0 1 2 3 1401 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 1402 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1403 | Type | Length | 1404 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1405 | Value (variable) .... 1406 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1408 Figure 4: Format of STUN Attributes 1410 The value in the Length field MUST contain the length of the Value 1411 part of the attribute, prior to padding, measured in bytes. Since 1412 STUN aligns attributes on 32 bit boundaries, attributes whose content 1413 is not a multiple of 4 bytes are padded with 1, 2 or 3 bytes of 1414 padding so that its value contains a multiple of 4 bytes. The 1415 padding bits are ignored, and may be any value. 1417 Any attribute type MAY appear more than once in a STUN message. 1418 Unless specified otherwise, the order of appearance is significant: 1419 only the first occurance needs to be processed by a receiver, and any 1420 duplicates MAY be ignored by a receiver. 1422 To allow future revisions of this specification to add new attributes 1423 if needed, the attribute space is divided into two ranges. 1424 Attributes with type values between 0x0000 and 0x7FFF are 1425 comprehension-required attributes, which means that the STUN agent 1426 cannot successfully process the message unless it understands the 1427 attribute. Attributes with type values between 0x8000 and 0xFFFF are 1428 comprehension-optional attributes, which means that those attributes 1429 can be ignored by the STUN agent if it does not understand them. 1431 The STUN Attribute types defined by this specification are: 1433 Comprehension-required range (0x0000-0x7FFF): 1434 0x0000: (Reserved) 1435 0x0001: MAPPED-ADDRESS 1436 0x0006: USERNAME 1437 0x0007: (Reserved; was PASSWORD) 1438 0x0008: MESSAGE-INTEGRITY 1439 0x0009: ERROR-CODE 1440 0x000A: UNKNOWN-ATTRIBUTES 1441 0x0014: REALM 1442 0x0015: NONCE 1443 0x0020: XOR-MAPPED-ADDRESS 1445 Comprehension-optional range (0x8000-0xFFFF) 1446 0x8022: SERVER 1447 0x8023: ALTERNATE-SERVER 1448 0x8028: FINGERPRINT 1450 The rest of this section describes the format of the various 1451 attributes defined in this specification. 1453 15.1. MAPPED-ADDRESS 1455 The MAPPED-ADDRESS attribute indicates a reflexive transport address 1456 of the client. It consists of an eight bit address family, and a 1457 sixteen bit port, followed by a fixed length value representing the 1458 IP address. If the address family is IPv4, the address MUST be 32 1459 bits. If the address family is IPv6, the address MUST be 128 bits. 1460 All fields must be in network byte order. 1462 The format of the MAPPED-ADDRESS attribute is: 1464 0 1 2 3 1465 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 1466 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1467 |0 0 0 0 0 0 0 0| Family | Port | 1468 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1469 | | 1470 | Address (32 bits or 128 bits) | 1471 | | 1472 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1474 Figure 6: Format of MAPPED-ADDRESS attribute 1476 The address family can take on the following values: 1478 0x01:IPv4 1479 0x02:IPv6 1481 The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be 1482 ignored by receivers. These bits are present for aligning parameters 1483 on natural 32 bit boundaries. 1485 This attribute is used only by servers for achieving backwards 1486 compatibility with RFC 3489 [RFC3489] clients. 1488 15.2. XOR-MAPPED-ADDRESS 1490 The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS 1491 attribute, except that the reflexive transport address is obfuscated 1492 through the XOR function. 1494 The format of the XOR-MAPPED-ADDRESS is: 1496 0 1 2 3 1497 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 1498 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1499 |x x x x x x x x| Family | X-Port | 1500 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1501 | X-Address (Variable) 1502 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1504 Figure 8: Format of XOR-MAPPED-ADDRESS Attribute 1506 The Family represents the IP address family, and is encoded 1507 identically to the Family in MAPPED-ADDRESS. 1509 X-Port is computed by taking the mapped port in host byte order, 1510 XOR'ing it with the most significant 16 bits of the magic cookie, and 1511 then the converting the result to network byte order. If the IP 1512 address family is IPv4, X-Address is computed by taking the mapped IP 1513 address in host byte order, XOR'ing it with the magic cookie, and 1514 converting the result to network byte order. If the IP address 1515 family is IPv6, X-Address is computed by taking the mapped IP address 1516 in host byte order, XOR'ing it with the concatenation of the magic 1517 cookie and the 96-bit transaction ID, and converting the result to 1518 network byte order. 1520 The rules for encoding and processing the first 8 bits of the 1521 attribute's value, the rules for handling multiple occurrences of the 1522 attribute, and the rules for processing addresses families are the 1523 same as for MAPPED-ADDRESS. 1525 NOTE: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their 1526 encoding of the transport address. The former encodes the transport 1527 address by exclusive-or'ing it with the magic cookie. The latter 1528 encodes it directly in binary. RFC 3489 originally specified only 1529 MAPPED-ADDRESS. However, deployment experience found that some NATs 1530 rewrite the 32-bit binary payloads containing the NAT's public IP 1531 address, such as STUN's MAPPED-ADDRESS attribute, in the well-meaning 1532 but misguided attempt at providing a generic ALG function. Such 1533 behavior interferes with the operation of STUN and also causes 1534 failure of STUN's message integrity checking. 1536 15.3. USERNAME 1538 The USERNAME attribute is used for message integrity. It identifies 1539 the username and password combination used in the message integrity 1540 check. 1542 The value of USERNAME is a variable length value. It MUST contain a 1543 UTF-8 [RFC3629] encoded sequence of less than 513 bytes, and MUST 1544 have been processed using SASLPrep [RFC4013]. 1546 15.4. MESSAGE-INTEGRITY 1548 The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of 1549 the STUN message. The MESSAGE-INTEGRITY attribute can be present in 1550 any STUN message type. Since it uses the SHA1 hash, the HMAC will be 1551 20 bytes. The text used as input to HMAC is the STUN message, 1552 including the header, up to and including the attribute preceding the 1553 MESSAGE-INTEGRITY attribute. With the exception of the FINGERPRINT 1554 attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore 1555 all other attributes that follow MESSAGE-INTEGRITY. 1557 The key for the HMAC depends on whether long term or short term 1558 credentials are in use. For long term credentials, the key is 16 1559 bytes: 1561 key = MD5(username ":" realm ":" SASLPrep(password)) 1563 That is, the 16 byte key is formed by taking the MD5 hash of the 1564 result of concatenating the following five fields: (1) The username, 1565 with any quotes and trailing nulls removed, (2) A single colon, (3) 1566 The realm, with any quotes and trailing nulls removed, (4) A single 1567 colon, and (5) the password, with any trailing nulls removed and 1568 after processing using SASLPrep. For example, if the username was 1569 'user', the realm was 'realm', and the password was 'pass', then the 1570 16-byte HMAC key would be the result of performing an MD5 hash on the 1571 string 'user:realm:pass', the resulting hash being 1572 0x8493fbc53ba582fb4c044c456bdc40eb. 1574 For short term credentials: 1576 key = SASLPrep(password) 1578 Where MD5 is defined in RFC 1321 [RFC1321] and SASLPrep() is defined 1579 in [RFC4013]. 1581 The structure of the key when used with long term credentials 1582 facilitates deployment in systems that also utilize SIP. Typically, 1583 SIP systems utilizing SIP's digest authentication mechanism do not 1584 actually store the password in the database. Rather, they store a 1585 value called H(A1), which is equal to the key defined above. 1587 Based on the rules above, the hash includes the length field from the 1588 STUN message header. Prior to performing the hash, the MESSAGE- 1589 INTEGRITY attribute MUST be inserted into the message (with dummy 1590 content). The length MUST then be set to point to the length of the 1591 message up to, and including, the MESSAGE-INTEGRITY attribute itself, 1592 but excluding any attributes after it. Once the computation is 1593 performed, the value of the MESSAGE-INTEGRITY attribute can be filled 1594 in, and the value of the length in the STUN header can be set to its 1595 correct value - the length of the entire message. Similarly, when 1596 validating the MESSAGE-INTEGRITY, the length field should be adjusted 1597 to point to the end of the MESSAGE-INTEGRITY attribute prior to 1598 calculating the HMAC. Such adjustment is necessary when attributes, 1599 such as FINGERPRINT, appear after MESSAGE-INTEGRITY. 1601 15.5. FINGERPRINT 1603 The FINGERPRINT attribute MAY be present in all STUN messages. The 1604 value of the attribute is computed as the CRC-32 of the STUN message 1605 up to (but excluding) the FINGERPRINT attribute itself, xor-d with 1606 the 32 bit value 0x5354554e (the XOR helps in cases where an 1607 application packet is also using CRC-32 in it). The 32 bit CRC is 1608 the one defined in ITU V.42 [ITU.V42.2002], which has a generator 1609 polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1. 1610 When present, the FINGERPRINT attribute MUST be the last attribute in 1611 the message, and thus will appear after MESSAGE-INTEGRITY. 1613 The FINGERPRINT attribute can aid in distinguishing STUN packets from 1614 packets of other protocols. See Section 8. 1616 As with MESSAGE-INTEGRITY, the CRC used in the FINGERPRINT attribute 1617 covers the length field from the STUN message header. Therefore, 1618 this value must be correct, and include the CRC attribute as part of 1619 the message length, prior to computation of the CRC. When using the 1620 FINGERPRINT attribute in a message, the attribute is first placed 1621 into the message with a dummy value, then the CRC is computed, and 1622 then the value of the attribute is updated. If the MESSAGE-INTEGRITY 1623 attribute is also present, then it must be present with the correct 1624 message-integrity value before the CRC is computed, since the CRC is 1625 done over the value of the MESSAGE-INTEGRITY attribute as well. 1627 15.6. ERROR-CODE 1629 The ERROR-CODE attribute is used in Error Response messages. It 1630 contains a numeric error code value in the range of 300 to 699 plus a 1631 textual reason phrase encoded in UTF-8 [RFC3629], and is consistent 1632 in its code assignments and semantics with SIP [RFC3261] and HTTP 1633 [RFC2616]. The reason phrase is meant for user consumption, and can 1634 be anything appropriate for the error code. Recommended reason 1635 phrases for the defined error codes are presented below. The reason 1636 phrase MUST be a UTF-8 [RFC3629] encoded sequence of less than 128 1637 characters (which can be as long as 763 bytes). 1639 0 1 2 3 1640 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 1641 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1642 | Reserved, should be 0 |Class| Number | 1643 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1644 | Reason Phrase (variable) .. 1645 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1647 Figure 11: ERROR-CODE Attribute 1649 To facilitate processing, the class of the error code (the hundreds 1650 digit) is encoded separately from the rest of the code, as shown in 1651 Figure 11. 1653 The Reserved bits SHOULD be 0, and are for alignment on 32-bit 1654 boundaries. Receivers MUST ignore these bits. The Class represents 1655 the hundreds digit of the error code. The value MUST be between 3 1656 and 6. The number represents the error code modulo 100, and its 1657 value MUST be between 0 and 99. 1659 The following error codes, along with their recommended reason 1660 phrases are defined: 1662 300 Try Alternate: The client should contact an alternate server for 1663 this request. This error response MUST only be sent if the 1664 request included a USERNAME attribute and a valid MESSAGE- 1665 INTEGRITY attribute; otherwise it MUST NOT be sent and error 1666 code 400 (Bad Request) is suggested. This error response MUST 1667 be protected with the MESSAGE-INTEGRITY attribute, and receivers 1668 MUST validate the MESSAGE-INTEGRITY of this response before 1669 redirecting themselves to an alternate server. 1671 Note: failure to generate and validate message-integrity 1672 for a 300 response allows an on-path attacker to falsify a 1673 300 response thus causing subsequent STUN messages to be 1674 sent to a victim. 1676 400 Bad Request: The request was malformed. The client SHOULD NOT 1677 retry the request without modification from the previous 1678 attempt. The server may not be able to generate a valid 1679 MESSAGE-INTEGRITY for this error, so the client MUST NOT expect 1680 a valid MESSAGE-INTEGRITY attribute on this response. 1682 401 Unauthorized: The request did not contain the correct 1683 credentials to proceed. The client should retry the request 1684 with proper credentials. 1686 420 Unknown Attribute: The server received STUN packet containing a 1687 comprehension-required attribute which it did not understand. 1688 The server MUST put this unknown attribute in the UNKNOWN- 1689 ATTRIBUTE attribute of its error response. 1691 438 Stale Nonce: The NONCE used by the client was no longer valid. 1692 The client should retry, using the NONCE provided in the 1693 response. 1695 500 Server Error: The server has suffered a temporary error. The 1696 client should try again. 1698 15.7. REALM 1700 The REALM attribute may be present in requests and responses. It 1701 contains text which meets the grammar for "realm-value" as described 1702 in RFC 3261 [RFC3261] but without the double quotes and their 1703 surrounding whitespace. That is, it is an unquoted realm-value (and 1704 is therefore a sequence of qdtext or quoted-pair). It MUST be a 1705 UTF-8 [RFC3629] encoded sequence of less than 128 characters (which 1706 can be as long as 763 bytes), and MUST have been processed using 1707 SASLPrep [RFC4013]. 1709 Presence of the REALM attribute in a request indicates that long-term 1710 credentials are being used for authentication. Presence in certain 1711 error responses indicates that the server wishes the client to use a 1712 long-term credential for authentication. 1714 15.8. NONCE 1716 The NONCE attribute may be present in requests and responses. It 1717 contains a sequence of qdtext or quoted-pair, which are defined in 1718 RFC 3261 [RFC3261]. Note that this means that the NONCE attribute 1719 will not contain actual quote characters. See RFC 2617 [RFC2617], 1720 Section 4.3, for guidance on selection of nonce values in a server. 1721 It MUST be less than 128 characters (which can be as long as 763 1722 bytes). 1724 15.9. UNKNOWN-ATTRIBUTES 1726 The UNKNOWN-ATTRIBUTES attribute is present only in an error response 1727 when the response code in the ERROR-CODE attribute is 420. 1729 The attribute contains a list of 16 bit values, each of which 1730 represents an attribute type that was not understood by the server. 1732 0 1 2 3 1733 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 1734 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1735 | Attribute 1 Type | Attribute 2 Type | 1736 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1737 | Attribute 3 Type | Attribute 4 Type ... 1738 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1740 Figure 12: Format of UNKNOWN-ATTRIBUTES attribute 1742 Note: In [RFC3489], this field was padded to 32 by duplicating the 1743 last attribute. In this version of the specification, the normal 1744 padding rules for attributes are used instead. 1746 15.10. SERVER 1748 The server attribute contains a textual description of the software 1749 being used by the server, including manufacturer and version number. 1750 The attribute has no impact on operation of the protocol, and serves 1751 only as a tool for diagnostic and debugging purposes. The value of 1752 SERVER is variable length. It MUST be a UTF-8 [RFC3629] encoded 1753 sequence of less than 128 characters (which can be as long as 763 1754 bytes). 1756 15.11. ALTERNATE-SERVER 1758 The alternate server represents an alternate transport address 1759 identifying a different STUN server which the STUN client should try. 1761 It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a 1762 single server by IP address. The IP address family MUST be identical 1763 to that of the source IP address of the request. 1765 This attribute MUST only appear in an error response that contains a 1766 MESSAGE-INTEGRITY attribute. This prevents it from being used in 1767 denial-of-service attacks. 1769 16. Security Considerations 1771 16.1. Attacks against the Protocol 1773 16.1.1. Outside Attacks 1775 An attacker can try to modify STUN messages in transit, in order to 1776 cause a failure in STUN operation. These attacks are detected for 1777 both requests and responses through the message integrity mechanism, 1778 using either a short term or long term credential. Of course, once 1779 detected, the manipulated packets will be dropped, causing the STUN 1780 transaction to effectively fail. This attack is possible only by an 1781 on-path attacker. 1783 An attacker that can observe, but not modify STUN messages in-transit 1784 (for example, an attacker present on a shared access medium, such as 1785 Wi-Fi), can see a STUN request, and then immediately send a STUN 1786 response, typically an error response, in order to disrupt STUN 1787 processing. This attack is also prevented for messages that utilize 1788 MESSAGE-INTEGRITY. However, some error responses, those related to 1789 authentication in particular, cannot be protected by MESSAGE- 1790 INTEGRITY. When STUN itself is run over a secure transport protocol 1791 (e.g., TLS), these attacks are completely mitigated. 1793 Depending on the STUN usage, these attacks may be of minimal 1794 consequence and thus do not require message integrity to mitigate. 1795 For example, when STUN is used to a basic STUN server to discover a 1796 server reflexive candidate for usage with ICE, authentication and 1797 message integrity are not required since these attacks are detected 1798 during the connectivity check phase. The connectivity checks 1799 themselves, however, require protection for proper operation of ICE 1800 overall. As described in Section 14, STUN usages describe when 1801 authentication and message integrity are needed. 1803 Since STUN uses the HMAC of a shared secret for authentication and 1804 integrity protection, it is subject to offline dictionary attacks. 1805 When authentication is utilized, it SHOULD be with a strong password 1806 that is not readily subject to offline dictionary attacks. 1807 Protection of the channel itself, using TLS, mitigates these attacks. 1809 However, STUN is most often run over UDP and in those cases, strong 1810 passwords are the only way to protect against these attacks. 1812 16.1.2. Inside Attacks 1814 A rogue client may try to launch a DoS attack against a server by 1815 sending it a large number of STUN requests. Fortunately, STUN 1816 requests can be processed statelessly by a server, making such 1817 attacks hard to launch. 1819 A rogue client may use a STUN server as a reflector, sending it 1820 requests with a falsified source IP address and port. In such a 1821 case, the response would be delivered to that source IP and port. 1822 There is no amplification of the number of packets with this attack 1823 (the STUN server sends one packet for each packet sent by the 1824 client), though there is a small increase in the amount of data, 1825 since STUN responses are typically larger than requests. This attack 1826 is mitigated by ingress source address filtering. 1828 16.2. Attacks Affecting the Usage 1830 This section lists attacks that might be launched against a usage of 1831 STUN. Each STUN usage must consider whether these attacks are 1832 applicable to it, and if so, discuss counter-measures. 1834 Most of the attacks in this section revolve around an attacker 1835 modifying the reflexive address learned by a STUN client through a 1836 Binding Request/Binding Response transaction. Since the usage of the 1837 reflexive address is a function of the usage, the applicability and 1838 remediation of these attacks is usage-specific. In common 1839 situations, modification of the reflexive address by an on-path 1840 attacker is easy to do. Consider, for example, the common situation 1841 where STUN is run directly over UDP. In this case, an on-path 1842 attacker can modify the source IP address of the Binding Request 1843 before it arrives at the STUN server. The STUN server will then 1844 return this IP address in the XOR-MAPPED-ADDRESS attribute to the 1845 client, and send the response back to that (falsified) IP address and 1846 port. If the attacker can also intercept this response, it can 1847 direct it back towards the client. Protecting against this attack by 1848 using a message-integrity check is impossible, since a message- 1849 integrity value cannot cover the source IP address, since the 1850 intervening NAT must be able to modify this value. Instead, one 1851 solution to preventing the attacks listed below is for the client to 1852 verify the reflexive address learned, as is done in ICE 1853 [I-D.ietf-mmusic-ice]. Other usages may use other means to prevent 1854 these attacks. 1856 16.2.1. Attack I: DDoS Against a Target 1858 In this attack, the attacker provides one or more clients with the 1859 same faked reflexive address that points to the intended target. 1860 This will trick the STUN clients into thinking that their reflexive 1861 addresses are equal to that of the target. If the clients hand out 1862 that reflexive address in order to receive traffic on it (for 1863 example, in SIP messages), the traffic will instead be sent to the 1864 target. This attack can provide substantial amplification, 1865 especially when used with clients that are using STUN to enable 1866 multimedia applications. However, it can only be launched against 1867 targets for which packets from the STUN server to the target pass 1868 through the attacker, limiting the cases in which it is possible 1870 16.2.2. Attack II: Silencing a Client 1872 In this attack, the attacker provides a STUN client with a faked 1873 reflexive address. The reflexive address it provides is a transport 1874 address that routes to nowhere. As a result, the client won't 1875 receive any of the packets it expects to receive when it hands out 1876 the reflexive address. This exploitation is not very interesting for 1877 the attacker. It impacts a single client, which is frequently not 1878 the desired target. Moreover, any attacker that can mount the attack 1879 could also deny service to the client by other means, such as 1880 preventing the client from receiving any response from the STUN 1881 server, or even a DHCP server. As with the attack in Section 16.2.1, 1882 this attack is only possible when the attacker is on path for packets 1883 sent from the STUN server towards this unused IP address. 1885 16.2.3. Attack III: Assuming the Identity of a Client 1887 This attack is similar to attack II. However, the faked reflexive 1888 address points to the attacker itself. This allows the attacker to 1889 receive traffic which was destined for the client. 1891 16.2.4. Attack IV: Eavesdropping 1893 In this attack, the attacker forces the client to use a reflexive 1894 address that routes to itself. It then forwards any packets it 1895 receives to the client. This attack would allow the attacker to 1896 observe all packets sent to the client. However, in order to launch 1897 the attack, the attacker must have already been able to observe 1898 packets from the client to the STUN server. In most cases (such as 1899 when the attack is launched from an access network), this means that 1900 the attacker could already observe packets sent to the client. This 1901 attack is, as a result, only useful for observing traffic by 1902 attackers on the path from the client to the STUN server, but not 1903 generally on the path of packets being routed towards the client. 1905 16.3. Hash Agility Plan 1907 This specification uses HMAC-SHA-1 for computation of the message 1908 integrity. If, at a later time, HMAC-SHA-1 is found to be 1909 compromised, the following is the remedy that will be applied. 1911 We will define a STUN extension which introduces a new message 1912 integrity attribute, computed using a new hash. Clients would be 1913 required to include both the new and old message integrity attributes 1914 in their requests or indications. A new server will utilize the new 1915 message integrity attribute, and an old one, the old. After a 1916 transition period where mixed implementations are in deployment, the 1917 old message-integrity attribute will be deprecated by another 1918 specification, and clients will cease including it in requests. 1920 17. IAB Considerations 1922 The IAB has studied the problem of "Unilateral Self Address Fixing" 1923 (UNSAF), which is the general process by which a client attempts to 1924 determine its address in another realm on the other side of a NAT 1925 through a collaborative protocol reflection mechanism (RFC3424 1926 [RFC3424]). STUN can be used to perform this function using a 1927 Binding Request/Response transaction if one agent is behind a NAT and 1928 the other is on the public side of the NAT. 1930 The IAB has mandated that protocols developed for this purpose 1931 document a specific set of considerations. Because some STUN usages 1932 provide UNSAF functions (such as ICE [I-D.ietf-mmusic-ice] ), and 1933 others do not (such as SIP Outbound [I-D.ietf-sip-outbound]), answers 1934 to these considerations need to be addressed by the usages 1935 themselves. 1937 18. IANA Considerations 1939 IANA is hereby requested to create three new registries: a STUN 1940 methods registry, a STUN Attributes registry, and a STUN Error Codes 1941 registry. IANA is also requested to change the name of the assigned 1942 IANA port for STUN from "nat-stun-port" to "stun". 1944 18.1. STUN Methods Registry 1946 A STUN method is a hex number in the range 0x000 - 0x3FF. The 1947 encoding of STUN method into a STUN message is described in 1948 Section 6. 1950 The initial STUN methods are: 1952 0x000: (Reserved) 1953 0x001: Binding 1954 0x002: (Reserved; was SharedSecret) 1956 STUN methods in the range 0x000 - 0x1FF are assigned by IETF 1957 Consensus [RFC2434]. STUN methods in the range 0x200 - 0x3FF are 1958 assigned on a First Come First Served basis [RFC2434] 1960 18.2. STUN Attribute Registry 1962 A STUN Attribute type is a hex number in the range 0x0000 - 0xFFFF. 1963 STUN attribute types in the range 0x0000 - 0x7FFF are considered 1964 comprehension-required; STUN attribute types in the range 0x8000 - 1965 0xFFFF are considered comprehension-optional. A STUN agent handles 1966 unknown comprehension-required and comprehension-optional attributes 1967 differently. 1969 The initial STUN Attributes types are: 1971 Comprehension-required range (0x0000-0x7FFF): 1972 0x0000: (Reserved) 1973 0x0001: MAPPED-ADDRESS 1974 0x0002: (Reserved; was RESPONSE-ADDRESS) 1975 0x0006: USERNAME 1976 0x0007: (Reserved; was PASSWORD) 1977 0x0008: MESSAGE-INTEGRITY 1978 0x0009: ERROR-CODE 1979 0x000A: UNKNOWN-ATTRIBUTES 1980 0x0014: REALM 1981 0x0015: NONCE 1982 0x0020: XOR-MAPPED-ADDRESS 1984 Comprehension-optional range (0x8000-0xFFFF) 1985 0x8022: SERVER 1986 0x8023: ALTERNATE-SERVER 1987 0x8028: FINGERPRINT 1989 STUN Attribute types in the first half of the comprehension-required 1990 range (0x0000 - 0x3FFF) and in the first half of the comprehension- 1991 optional range (0x8000 - 0xBFFF) are assigned by IETF Consensus 1992 [RFC2434]. STUN Attribute types in the second half of the 1993 comprehension-required range (0x4000 - 0x7FFF) and in the second half 1994 of the comprehension-optional range (0xC000 - 0xFFFF) are assigned on 1995 a First Come First Served basis [RFC2434]. 1997 18.3. STUN Error Code Registry 1999 A STUN Error code is a number in the range 0 - 699. STUN error codes 2000 are accompanied by a textual reason phrase in UTF-8 [RFC3629] which 2001 is intended only for human consumption and can be anything 2002 appropriate; this document proposes only suggested values. 2004 STUN error codes are consistent in codepoint assignments and 2005 semantics with SIP [RFC3261] and HTTP [RFC2616]. 2007 The initial values in this registry are given in Section 15.6. 2009 New STUN error codes are assigned on an IETF Consensus basis 2010 [RFC2434]. The specification must carefully consider how clients 2011 that do not understand this error code will process it before 2012 granting the request. See the rules in Section 7.3.4. 2014 18.4. STUN UDP and TCP Port Numbers 2016 IANA has previously assigned port 3478 for STUN. This port appears 2017 in the IANA registry under the moniker "nat-stun-port". In order to 2018 align the DNS SRV procedures with the registered protocol service, 2019 IANA is requested to change the name of protocol assigned to port 2020 3478 from "nat-stun-port" to "stun", and the textual name from 2021 "Simple Traversal of UDP Through NAT (STUN)" to "Session Traversal 2022 Utilities for NAT", so that the IANA port registry would read: 2024 stun 3478/tcp Session Traversal Utilities for NAT (STUN) port 2025 stun 3478/udp Session Traversal Utilities for NAT (STUN) port 2027 In addition, IANA is requested to assign port numbers for the "stuns" 2028 service, defined over TCP and UDP. The UDP port is not currently 2029 defined however is reserved for future use. 2031 19. Changes Since RFC 3489 2033 This specification obsoletes RFC3489 [RFC3489]. This specification 2034 differs from RFC3489 in the following ways: 2036 o Removed the notion that STUN is a complete NAT traversal solution. 2037 STUN is now a tool that can be used to produce a NAT traversal 2038 solution. As a consequence, changed the name of the protocol to 2039 Session Traversal Utilities for NAT. 2041 o Introduced the concept of STUN usages, and described what a usage 2042 of STUN must document. 2044 o Removed the usage of STUN for NAT type detection and binding 2045 lifetime discovery. These techniques have proven overly brittle 2046 due to wider variations in the types of NAT devices than described 2047 in this document. Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS, 2048 CHANGE-REQUEST, SOURCE-ADDRESS, and REFLECTED-FROM attributes. 2050 o Added a fixed 32-bit magic cookie and reduced length of 2051 transaction ID by 32 bits. The magic cookie begins at the same 2052 offset as the original transaction ID. 2054 o Added the XOR-MAPPED-ADDRESS attribute, which is included in 2055 Binding Responses if the magic cookie is present in the request. 2056 Otherwise the RFC3489 behavior is retained (that is, Binding 2057 Response includes MAPPED-ADDRESS). See discussion in XOR-MAPPED- 2058 ADDRESS regarding this change. 2060 o Introduced formal structure into the Message Type header field, 2061 with an explicit pair of bits for indication of request, response, 2062 error response or indication. Consequently, the message type 2063 field is split into the class (one of the previous four) and 2064 method. 2066 o Explicitly point out that the most significant two bits of STUN 2067 are 0b00, allowing easy differentiation with RTP packets when used 2068 with ICE. 2070 o Added the FINGERPRINT attribute to provide a method of definitely 2071 detecting the difference between STUN and another protocol when 2072 the two protocols are multiplexed together. 2074 o Added support for IPv6. Made it clear that an IPv4 client could 2075 get a v6 mapped address, and vice-a-versa. 2077 o Added long-term credential-based authentication. 2079 o Added the SERVER, REALM, NONCE, and ALTERNATE-SERVER attributes. 2081 o Removed the SharedSecret method, and thus the PASSWORD attribute. 2082 This method was almost never implemented and is not needed with 2083 current usages. 2085 o Removed recommendation to continue listening for STUN Responses 2086 for 10 seconds in an attempt to recognize an attack. 2088 o Changed transaction timers to be more TCP friendly. 2090 o Removed the STUN example that centered around the separation of 2091 the control and media planes. Instead, provided more information 2092 on using STUN with protocols. 2094 o Defined a generic padding mechanism that changes the 2095 interpretation of the length attribute. This would, in theory, 2096 break backwards compatibility. However, the mechanism in RFC 3489 2097 never worked for the few attributes that weren't aligned naturally 2098 on 32 bit boundaries. 2100 o REALM, SERVER, reason phrases and NONCE limited to 127 characters. 2101 USERNAME to 513 bytes. 2103 o Changed the DNS SRV procedures for TCP and TLS. UDP remains the 2104 same as before. 2106 20. Contributors 2108 Christian Huitema and Joel Weinberger were original co-authors of RFC 2109 3489. 2111 21. Acknowledgements 2113 The authors would like to thank Cedric Aoun, Pete Cordell, Cullen 2114 Jennings, Bob Penfield, Xavier Marjou, Magnus Westerlund, Miguel 2115 Garcia, Bruce Lowekamp and Chris Sullivan for their comments, and 2116 Baruch Sterman and Alan Hawrylyshen for initial implementations. 2117 Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning 2118 Schulzrinne for IESG and IAB input on this work. 2120 22. References 2122 22.1. Normative References 2124 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2125 Requirement Levels", BCP 14, RFC 2119, March 1997. 2127 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 2128 September 1981. 2130 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 2131 specifying the location of services (DNS SRV)", RFC 2782, 2132 February 2000. 2134 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 2136 [RFC1122] Braden, R., "Requirements for Internet Hosts - 2137 Communication Layers", STD 3, RFC 1122, October 1989. 2139 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 2140 (IPv6) Specification", RFC 2460, December 1998. 2142 [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 2143 Leach, P., Luotonen, A., and L. Stewart, "HTTP 2144 Authentication: Basic and Digest Access Authentication", 2145 RFC 2617, June 1999. 2147 [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission 2148 Timer", RFC 2988, November 2000. 2150 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 2151 Hashing for Message Authentication", RFC 2104, 2152 February 1997. 2154 [ITU.V42.2002] 2155 International Telecommunications Union, "Error-correcting 2156 Procedures for DCEs Using Asynchronous-to-Synchronous 2157 Conversion", ITU-T Recommendation V.42, March 2002. 2159 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 2160 10646", STD 63, RFC 3629, November 2003. 2162 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 2163 April 1992. 2165 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 2166 and Passwords", RFC 4013, February 2005. 2168 22.2. Informational References 2170 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 2171 A., Peterson, J., Sparks, R., Handley, M., and E. 2172 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 2173 June 2002. 2175 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 2176 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 2177 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 2179 [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic 2180 Key Management", BCP 107, RFC 4107, June 2005. 2182 [I-D.ietf-mmusic-ice] 2183 Rosenberg, J., "Interactive Connectivity Establishment 2184 (ICE): A Protocol for Network Address Translator (NAT) 2185 Traversal for Offer/Answer Protocols", 2186 draft-ietf-mmusic-ice-19 (work in progress), October 2007. 2188 [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, 2189 "STUN - Simple Traversal of User Datagram Protocol (UDP) 2190 Through Network Address Translators (NATs)", RFC 3489, 2191 March 2003. 2193 [I-D.ietf-behave-turn] 2194 Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using 2195 Relays around NAT (TURN): Relay Extensions to Session 2196 Traversal Utilities for NAT (STUN)", 2197 draft-ietf-behave-turn-06 (work in progress), 2198 January 2008. 2200 [I-D.ietf-sip-outbound] 2201 Jennings, C. and R. Mahy, "Managing Client Initiated 2202 Connections in the Session Initiation Protocol (SIP)", 2203 draft-ietf-sip-outbound-11 (work in progress), 2204 November 2007. 2206 [I-D.ietf-behave-nat-behavior-discovery] 2207 MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery 2208 Using STUN", draft-ietf-behave-nat-behavior-discovery-02 2209 (work in progress), November 2007. 2211 [I-D.ietf-mmusic-ice-tcp] 2212 Rosenberg, J., "TCP Candidates with Interactive 2213 Connectivity Establishment (ICE)", 2214 draft-ietf-mmusic-ice-tcp-05 (work in progress), 2215 November 2007. 2217 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2218 with Session Description Protocol (SDP)", RFC 3264, 2219 June 2002. 2221 [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral 2222 Self-Address Fixing (UNSAF) Across Network Address 2223 Translation", RFC 3424, November 2002. 2225 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 2226 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 2227 October 1998. 2229 [KARN87] Karn, P. and C. Partridge, "Improving Round-Trip Time 2230 Estimates in Reliable Transport Protocols", SIGCOMM 1987, 2231 August 1987. 2233 Appendix A. C Snippet to Determine STUN Message Types 2235 Given an 16-bit STUN message type value in host byte order in 2236 msg_type parameter, below are C macros to determine the STUN message 2237 types: 2239 #define IS_REQUEST(msg_type) (((msg_type) & 0x0110) == 0x0000) 2240 #define IS_INDICATION(msg_type) (((msg_type) & 0x0110) == 0x0010) 2241 #define IS_SUCCESS_RESP(msg_type) (((msg_type) & 0x0110) == 0x0100) 2242 #define IS_ERR_RESP(msg_type) (((msg_type) & 0x0110) == 0x0110) 2244 Authors' Addresses 2246 Jonathan Rosenberg 2247 Cisco 2248 Edison, NJ 2249 US 2251 Email: jdrosen@cisco.com 2252 URI: http://www.jdrosen.net 2254 Rohan Mahy 2255 Plantronics 2256 345 Encinal Street 2257 Santa Cruz, CA 95060 2258 US 2260 Email: rohan@ekabal.com 2262 Philip Matthews 2263 Avaya 2264 1135 Innovation Drive 2265 Ottawa, Ontario K2K 3G7 2266 Canada 2268 Phone: +1 613 592 4343 x224 2269 Fax: 2270 Email: philip_matthews@magma.ca 2271 URI: 2273 Dan Wing 2274 Cisco 2275 771 Alder Drive 2276 San Jose, CA 95035 2277 US 2279 Email: dwing@cisco.com 2281 Full Copyright Statement 2283 Copyright (C) The IETF Trust (2008). 2285 This document is subject to the rights, licenses and restrictions 2286 contained in BCP 78, and except as set forth therein, the authors 2287 retain all their rights. 2289 This document and the information contained herein are provided on an 2290 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 2291 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 2292 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 2293 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 2294 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 2295 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 2297 Intellectual Property 2299 The IETF takes no position regarding the validity or scope of any 2300 Intellectual Property Rights or other rights that might be claimed to 2301 pertain to the implementation or use of the technology described in 2302 this document or the extent to which any license under such rights 2303 might or might not be available; nor does it represent that it has 2304 made any independent effort to identify any such rights. Information 2305 on the procedures with respect to rights in RFC documents can be 2306 found in BCP 78 and BCP 79. 2308 Copies of IPR disclosures made to the IETF Secretariat and any 2309 assurances of licenses to be made available, or the result of an 2310 attempt made to obtain a general license or permission for the use of 2311 such proprietary rights by implementers or users of this 2312 specification can be obtained from the IETF on-line IPR repository at 2313 http://www.ietf.org/ipr. 2315 The IETF invites any interested party to bring to its attention any 2316 copyrights, patents or patent applications, or other proprietary 2317 rights that may cover technology that may be required to implement 2318 this standard. Please address the information to the IETF at 2319 ietf-ipr@ietf.org. 2321 Acknowledgment 2323 Funding for the RFC Editor function is provided by the IETF 2324 Administrative Support Activity (IASA).