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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: May 20, 2008 P. Matthews 7 Avaya 8 D. Wing 9 Cisco 10 November 17, 2007 12 Session Traversal Utilities for (NAT) (STUN) 13 draft-ietf-behave-rfc3489bis-13 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 May 20, 2008. 40 Copyright Notice 42 Copyright (C) The IETF Trust (2007). 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 . . . . . . . . . . . . . . . . 15 75 7.3.1. Processing a Request . . . . . . . . . . . . . . . . . 16 76 7.3.1.1. Forming a Success or Error Response . . . . . . . 17 77 7.3.1.2. Sending the Success or Error Response . . . . . . 17 78 7.3.2. Processing an Indication . . . . . . . . . . . . . . . 18 79 7.3.3. Processing a Success Response . . . . . . . . . . . . 18 80 7.3.4. Processing an Error Response . . . . . . . . . . . . . 18 81 8. FINGERPRINT Mechanism . . . . . . . . . . . . . . . . . . . . 19 82 9. DNS Discovery of a Server . . . . . . . . . . . . . . . . . . 20 83 10. Authentication and Message-Integrity Mechanisms . . . . . . . 21 84 10.1. Short-Term Credential Mechanism . . . . . . . . . . . . . 21 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 . . . . . . . . . . . . . 23 89 10.2.1. Forming a Request . . . . . . . . . . . . . . . . . . 24 90 10.2.1.1. First Request . . . . . . . . . . . . . . . . . . 24 91 10.2.1.2. Subsequent Requests . . . . . . . . . . . . . . . 24 92 10.2.2. Receiving a Request . . . . . . . . . . . . . . . . . 25 93 10.2.3. Receiving a Response . . . . . . . . . . . . . . . . . 26 94 11. ALTERNATE-SERVER Mechanism . . . . . . . . . . . . . . . . . . 26 95 12. Backwards Compatibility with RFC 3489 . . . . . . . . . . . . 27 96 12.1. Changes to Client Processing . . . . . . . . . . . . . . 27 97 12.2. Changes to Server Processing . . . . . . . . . . . . . . 28 98 13. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 28 99 14. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 29 100 14.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 30 101 14.2. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 31 102 14.3. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 32 103 14.4. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 32 104 14.5. FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . . 33 105 14.6. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 34 106 14.7. REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 35 107 14.8. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 35 108 14.9. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 36 109 14.10. SERVER . . . . . . . . . . . . . . . . . . . . . . . . . 36 110 14.11. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 36 111 15. Security Considerations . . . . . . . . . . . . . . . . . . . 37 112 15.1. Attacks against the Protocol . . . . . . . . . . . . . . 37 113 15.1.1. Outside Attacks . . . . . . . . . . . . . . . . . . . 37 114 15.1.2. Inside Attacks . . . . . . . . . . . . . . . . . . . . 37 115 15.2. Attacks Affecting the Usage . . . . . . . . . . . . . . . 38 116 15.2.1. Attack I: DDoS Against a Target . . . . . . . . . . . 38 117 15.2.2. Attack II: Silencing a Client . . . . . . . . . . . . 38 118 15.2.3. Attack III: Assuming the Identity of a Client . . . . 39 119 15.2.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 39 120 15.3. Hash Agility Plan . . . . . . . . . . . . . . . . . . . . 39 121 16. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 39 122 17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40 123 17.1. STUN Methods Registry . . . . . . . . . . . . . . . . . . 40 124 17.2. STUN Attribute Registry . . . . . . . . . . . . . . . . . 40 125 17.3. STUN Error Code Registry . . . . . . . . . . . . . . . . 41 126 17.4. STUN UDP and TCP Port Numbers . . . . . . . . . . . . . . 42 127 18. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 42 128 19. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 43 129 20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43 130 21. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44 131 21.1. Normative References . . . . . . . . . . . . . . . . . . 44 132 21.2. Informational References . . . . . . . . . . . . . . . . 45 133 Appendix A. C Snippet to Determine STUN Message Types . . . . . . 46 134 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46 135 Intellectual Property and Copyright Statements . . . . . . . . . . 48 137 1. Introduction 139 The protocol defined in this specification, Session Traversal 140 Utilities for NAT, provides a tool for dealing with NATs. It 141 provides a means for an endpoint to determine the IP address and port 142 allocated by a NAT that corresponds to its private IP address and 143 port. It also provides a way for an endpoint to keep a NAT binding 144 alive. With some extensions, the protocol can be used to do 145 connectivity checks between two endpoints [I-D.ietf-mmusic-ice], or 146 to relay packets between two endpoints [I-D.ietf-behave-turn]. 148 In keeping with its tool nature, this specification defines an 149 extensible packet format, defines operation over several transport 150 protocols, and provides for two forms of authentication. 152 STUN is intended to be used in context of one or more NAT traversal 153 solutions. These solutions are known as STUN usages. Each usage 154 describes how STUN is utilized to achieve the NAT traversal solution. 155 Typically, a usage indicates when STUN messages get sent, which 156 optional attributes to include, what server is used, and what 157 authentication mechanism is to be used. Interactive Connectivity 158 Establishment (ICE) [I-D.ietf-mmusic-ice] is one usage of STUN. SIP 159 Outbound [I-D.ietf-sip-outbound] is another usage of STUN. In some 160 cases, a usage will require extensions to STUN. A STUN extension can 161 be in the form of new methods, attributes, or error response codes. 162 More information on STUN usages can be found in Section 13. 164 2. Evolution from RFC 3489 166 STUN was originally defined in RFC 3489 [RFC3489]. That 167 specification, sometimes referred to as "classic STUN", represented 168 itself as a complete solution to the NAT traversal problem. In that 169 solution, a client would discover whether it was behind a NAT, 170 determine its NAT type, discover its IP address and port on the 171 public side of the outermost NAT, and then utilize that IP address 172 and port within the body of protocols, such as the Session Initiation 173 Protocol (SIP) [RFC3261]. However, experience since the publication 174 of RFC 3489 has found that classic STUN simply does not work 175 sufficiently well to be a deployable solution. The address and port 176 learned through classic STUN are sometimes usable for communications 177 with a peer, and sometimes not. Classic STUN provided no way to 178 discover whether it would, in fact, work or not, and it provided no 179 remedy in cases where it did not. Furthermore, classic STUN's 180 algorithm for classification of NAT types was found to be faulty, as 181 many NATs did not fit cleanly into the types defined there. Classic 182 STUN also had security vulnerabilities which required an extremely 183 complicated mechanism to address, and despite the complexity of the 184 mechanism, were not fully remedied. 186 For these reasons, this specification obsoletes RFC 3489, and instead 187 describes STUN as a tool that is utilized as part of a complete NAT 188 traversal solution. ICE [I-D.ietf-mmusic-ice] is a complete NAT 189 traversal solution for protocols based on the offer/answer [RFC3264] 190 methodology, such as SIP. SIP Outbound [I-D.ietf-sip-outbound] is a 191 complete solution for traversal of SIP signaling, and it uses STUN in 192 a very different way. Though it is possible that a protocol may be 193 able to use STUN by itself (classic STUN) as a traversal solution, 194 such usage is not described here and is strongly discouraged for the 195 reasons described above. 197 The on-the-wire protocol described here is changed only slightly from 198 classic STUN. The protocol now runs over TCP in addition to UDP. 199 Extensibility was added to the protocol in a more structured way. A 200 magic-cookie mechanism for demultiplexing STUN with application 201 protocols was added by stealing 32 bits from the 128 bit transaction 202 ID defined in RFC 3489, allowing the change to be backwards 203 compatible. Mapped addresses are encoded using a new exclusive-or 204 format. There are other, more minor changes. See Section 18 for a 205 more complete listing. 207 Due to the change in scope, STUN has also been renamed from "Simple 208 Traversal of UDP Through NAT" to "Session Traversal Utilities for 209 NAT". The acronym remains STUN, which is all anyone ever remembers 210 anyway. 212 3. Overview of Operation 214 This section is descriptive only. 216 /-----\ 217 // STUN \\ 218 | Server | 219 \\ // 220 \-----/ 222 +--------------+ Public Internet 223 ................| NAT 2 |....................... 224 +--------------+ 226 +--------------+ Private NET 2 227 ................| NAT 1 |....................... 228 +--------------+ 230 /-----\ 231 // STUN \\ 232 | Client | 233 \\ // Private NET 1 234 \-----/ 236 Figure 1: One possible STUN Configuration 238 One possible STUN configuration is shown in Figure 1. In this 239 configuration, there are two entities (called STUN agents) that 240 implement the STUN protocol. The lower agent in the figure is the 241 client, and is connected to private network 1. This network connects 242 to private network 2 through NAT 1. Private network 2 connects to 243 the public Internet through NAT 2. The upper agent in the figure is 244 the server, and resides on the public Internet. 246 STUN is a client-server protocol. It supports two types of 247 transactions. One is a request/response transaction in which a 248 client sends a request to a server, and the server returns a 249 response. The second is an indication transaction in which either 250 agent - client or server - sends an indication which generates no 251 response. Both types of transactions include a transaction ID, which 252 is a randomly selected 96-bit number. For request/response 253 transactions, this transaction ID allows the client to associate the 254 response with the request that generated it; for indications, this 255 simply serves as a debugging aid. 257 All STUN messages start with a fixed header that includes a method, a 258 class, and the transaction ID. The method indicates which of the 259 various requests or indications this is; this specification defines 260 just one method, Binding, but other methods are expected to be 261 defined in other documents. The class indicates whether this is a 262 request, a success response, an error response, or an indication. 263 Following the fixed header comes zero or more attributes, which are 264 type-length-value extensions that convey additional information for 265 the specific message. 267 This document defines a single method called Binding. The Binding 268 method can be used either in request/response transactions or in 269 indication transactions. When used in request/response transactions, 270 the Binding method can be used to determine the particular "binding" 271 a NAT has allocated to a STUN client. When used in either request/ 272 response or in indication transactions, the Binding method can also 273 be used to keep these "bindings" alive. 275 In the Binding request/response transaction, a Binding Request is 276 sent from a STUN client to a STUN server. When the Binding Request 277 arrives at the STUN server, it may have passed through one or more 278 NATs between the STUN client and the STUN server (in Figure 1, there 279 were two such NATs). As the Binding Request message passes through a 280 NAT, the NAT will modify the source transport address (that is, the 281 source IP address and the source port) of the packet. As a result, 282 the source transport address of the request received by the server 283 will be the public IP address and port created by the NAT closest to 284 the server. This is called a reflexive transport address. The STUN 285 server copies that source transport address into an XOR-MAPPED- 286 ADDRESS attribute in the STUN Binding Response and sends the Binding 287 Response back to the STUN client. As this packet passes back through 288 a NAT, the NAT will modify the destination transport address in the 289 IP header, but the transport address in the XOR-MAPPED-ADDRESS 290 attribute within the body of the STUN response will remain untouched. 291 In this way, the client can learn its reflexive transport address 292 allocated by the outermost NAT with respect to the STUN server. 294 In some usages, STUN must be multiplexed with other protocols (e.g., 295 [I-D.ietf-mmusic-ice], [I-D.ietf-sip-outbound]). In these usages, 296 there must be a way to inspect a packet and determine if it is a STUN 297 packet or not. STUN provides three fields in the STUN header with 298 fixed values that can be used for this purpose. If this is not 299 sufficient, then STUN packets can also contain a FINGERPRINT value 300 which can further be used to distinguish the packets. 302 STUN defines a set of optional procedures that a usage can decide to 303 use, called mechanisms. These mechanisms include DNS discovery, a 304 redirection technique to an alternate server, a fingerprint attribute 305 for demultiplexing, and two authentication and message integrity 306 exchanges. The authentication mechanisms revolve around the use of a 307 username, password, and message-integrity value. Two authentication 308 mechanisms, the long-term credential mechanism and the short-term 309 credential mechanism, are defined in this specification. Each usage 310 specifies the mechanisms allowed with that usage. 312 In the long-term credential mechanism, the client and server share a 313 pre-provisioned username and password and perform a digest challenge/ 314 response exchange inspired by (but differing in details) to the one 315 defined for HTTP [RFC2617]. In the short-term credential mechanism, 316 the client and the server exchange a username and password through 317 some out-of-band method prior to the STUN exchange. For example, in 318 the ICE usage [I-D.ietf-mmusic-ice] the two endpoints use out-of-band 319 signaling to exchange a username and password. These are used to 320 integrity protect and authenticate the request and response. There 321 is no challenge or nonce used. 323 4. Terminology 325 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 326 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 327 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 328 [RFC2119] and indicate requirement levels for compliant STUN 329 implementations. 331 5. Definitions 333 STUN Agent: An entity that implements the STUN protocol. The entity 334 can either be a STUN client or a STUN server. 336 STUN Client: A STUN client is an entity that sends STUN requests, 337 and receives STUN responses. STUN clients can also send 338 indications. In this specification, the terms STUN client and 339 client are synonymous. 341 STUN Server: A STUN server is an entity that receives STUN requests 342 and sends STUN responses. A STUN server can also send 343 indications. In this specification, the terms STUN server and 344 server are synonymous. 346 Transport Address: The combination of an IP address and port number 347 (such as a UDP or TCP port number). 349 Reflexive Transport Address: A transport address learned by a client 350 that identifies that client as seen by another host on an IP 351 network, typically a STUN server. When there is an intervening 352 NAT between the client and the other host, the reflexive transport 353 address represents the mapped address allocated to the client on 354 the public side of the NAT. Reflexive transport addresses are 355 learned from the mapped address attribute (MAPPED-ADDRESS or XOR- 356 MAPPED-ADDRESS) in STUN responses. 358 Mapped Address: Same meaning as Reflexive Address. This term is 359 retained only for for historic reasons and due to the naming of 360 the MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes. 362 Long Term Credential: A username and associated password that 363 represent a shared secret between client and server. Long term 364 credentials are generally granted to the client when a subscriber 365 enrolls in a service and persist until the subscriber leaves the 366 service or explicitly changes the credential. 368 Long Term Password: The password from a long term credential. 370 Short Term Credential: A temporary username and associated password 371 which represent a shared secret between client and server. Short 372 term credentials are obtained through some kind of protocol 373 mechanism between the client and server, preceding the STUN 374 exchange. A short term credential has an explicit temporal scope, 375 which may be based on a specific amount of time (such as 5 376 minutes) or on an event (such as termination of a SIP dialog). 377 The specific scope of a short term credential is defined by the 378 application usage. 380 Short Term Password: The password component of a short term 381 credential. 383 STUN Indication: A STUN message that does not receive a response 385 Attribute: The STUN term for a Type-Length-Value (TLV) object that 386 can be added to a STUN message. Attributes are divided into two 387 types: comprehension-required and comprehension-optional. STUN 388 agents can safely ignore comprehension-optional attributes they 389 don't understand, but cannot successfully process a message if it 390 contains comprehension-required attributes that are not 391 understood. 393 RTO: Retransmission TimeOut, which defines the initial period of 394 time between transmission of a request and the first retransmit of 395 that request. 397 6. STUN Message Structure 399 STUN messages are encoded in binary using network-oriented format 400 (most significant byte or octet first, also commonly known as big- 401 endian). The transmission order is described in detail in Appendix B 402 of RFC791 [RFC0791]. Unless otherwise noted, numeric constants are 403 in decimal (base 10). 405 All STUN messages MUST start with a 20-byte header followed by zero 406 or more Attributes. The STUN header contains a STUN message type, 407 magic cookie, transaction ID, and message length. 409 0 1 2 3 410 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 1 411 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 412 |0 0| STUN Message Type | Message Length | 413 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 414 | Magic Cookie | 415 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 416 | | 417 | Transaction ID (96 bits) | 418 | | 419 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 421 Figure 2: Format of STUN Message Header 423 The most significant two bits of every STUN message MUST be zeroes. 424 This can be used to differentiate STUN packets from other protocols 425 when STUN is multiplexed with other protocols on the same port. 427 The message type defines the message class (request, success 428 response, failure response, or indication) and the message method 429 (the primary function) of the STUN message. Although there are four 430 message classes, there are only two types of transactions in STUN: 431 request/response transactions (which consist of a request message and 432 a response message), and indication transactions (which consists of a 433 single indication message). Response classes are split into error 434 and success responses to aid in quickly processing the STUN message. 436 The message type field is decomposed further into the following 437 structure: 439 0 1 440 2 3 4 5 6 7 8 9 0 1 2 3 4 5 442 +--+--+-+-+-+-+-+-+-+-+-+-+-+-+ 443 |M |M |M|M|M|C|M|M|M|C|M|M|M|M| 444 |11|10|9|8|7|1|6|5|4|0|3|2|1|0| 445 +--+--+-+-+-+-+-+-+-+-+-+-+-+-+ 447 Figure 3: Format of STUN Message Type Field 449 Here the bits in the message type field are shown as most-significant 450 (M11) through least-significant (M0). M11 through M0 represent a 12- 451 bit encoding of the method. C1 and C0 represent a 2 bit encoding of 452 the class. A class of 0b00 is a Request, a class of 0b01 is an 453 indication, a class of 0b10 is a success response, and a class of 454 0b11 is an error response. This specification defines a single 455 method, Binding. The method and class are orthogonal, so that for 456 each method, a request, success response, error response and 457 indication are defined for that method. 459 For example, a Binding Request has class=0b00 (request) and 460 method=0b000000000001 (Binding), and is encoded into the first 16 461 bits as 0x0001. A Binding response has class=0b10 (success response) 462 and method=0b000000000001, and is encoded into the first 16 bits as 463 0x0101. 465 Note: This unfortunate encoding is due to assignment of values in 466 [RFC3489] which did not consider encoding Indications, Success, 467 and Errors using bit fields. 469 The magic cookie field MUST contain the fixed value 0x2112A442 in 470 network byte order. In RFC 3489 [RFC3489], this field was part of 471 the transaction ID; placing the magic cookie in this location allows 472 a server to detect if the client will understand certain attributes 473 that were added in this revised specification. In addition, it aids 474 in distinguishing STUN packets from packets of other protocols when 475 STUN is multiplexed with those other protocols on the same port. 477 The transaction ID is a 96 bit identifier, used to uniquely identify 478 STUN transactions. For request/response transactions, the 479 transaction ID is chosen by the STUN client for the request and 480 echoed by the server in the response. For indications, it is chosen 481 by the agent sending the indication. It primarily serves to 482 correlate requests with responses, though it also plays a small role 483 in helping to prevent certain types of attacks. As such, the 484 transaction ID MUST be uniformly and randomly chosen from the 485 interval 0 .. 2**96-1. Resends of the same request reuse the same 486 transaction ID, but the client MUST choose a new transaction ID for 487 new transactions unless the new request is bit-wise identical to the 488 previous request and sent from the same transport address to the same 489 IP address. Success and error responses MUST carry the same 490 transaction ID as their corresponding request. When an agent is 491 acting as a STUN server and STUN client on the same port, the 492 transaction IDs in requests sent by the agent have no relationship to 493 the transaction IDs in requests received by the agent. 495 The message length MUST contain the size, in bytes, of the message 496 not including the 20 byte STUN header. Since all STUN attributes are 497 padded to a multiple of four bytes, the last two bits of this field 498 are always zero. This provides another way to distinguish STUN 499 packets from packets of other protocols. 501 Following the STUN fixed portion of the header are zero or more 502 attributes. Each attribute is TLV (type-length-value) encoded. The 503 details of the encoding, and of the attributes themselves is given in 504 Section 14. 506 7. Base Protocol Procedures 508 This section defines the base procedures of the STUN protocol. It 509 describes how messages are formed, how they are sent, and how they 510 are processed when they are received. It also defines the detailed 511 processing of the Binding method. Other sections in this document 512 describe optional procedures that a usage may elect to use in certain 513 situations. Other documents may define other extensions to STUN, by 514 adding new methods, new attributes, or new error response codes. 516 7.1. Forming a Request or an Indication 518 When formulating a request or indication message, the agent MUST 519 follow the rules in Section 6 when creating the header. In addition, 520 the message class MUST be either "Request" or "Indication" (as 521 appropriate), and the method must be either Binding or some method 522 defined in another document. 524 The agent then adds any attributes specified by the method or the 525 usage. For example, some usages may specify that the agent use an 526 authentication method (Section 10) or the FINGERPRINT attribute 527 (Section 8). 529 For the Binding method with no authentication, no attributes are 530 required unless the usage specifies otherwise. 532 All STUN requests (and responses) sent over UDP MUST be less than the 533 path MTU, or 1500 bytes if the MTU is not known. STUN provides no 534 ability to handle the case where the request is under the MTU but the 535 response would be larger than the MTU. It is not envisioned that 536 this limitation will be an issue for STUN. 538 7.2. Sending the Request or Indication 540 The agent then sends the request or indication. This document 541 specifies how to send STUN messages over UDP, TCP, or TLS-over-TCP; 542 other transport protocols may be added in the future. The STUN usage 543 must specify which transport protocol is used, and how the agent 544 determines the IP address and port of the recipient. Section 9 545 describes a DNS-based method of determining the IP address and port 546 of a server which a usage may elect to use. STUN may be used with 547 anycast addresses, but only with UDP and in usages where 548 authentication is not used. 550 At any time, a client MAY have multiple outstanding STUN requests 551 with the same STUN server (that is, multiple transactions in 552 progress, with different transaction ids). 554 7.2.1. Sending over UDP 556 When running STUN over UDP it is possible that the STUN message might 557 be dropped by the network. Reliability of STUN request/response 558 transactions is accomplished through retransmissions of the request 559 message by the client application itself. STUN indications are not 560 retransmitted; thus indication transactions over UDP are not 561 reliable. 563 A client SHOULD retransmit a STUN request message starting with an 564 interval of RTO ("Retransmission TimeOut"), doubling after each 565 retransmission. The RTO is an estimate of the round-trip-time, and 566 is computed as described in RFC 2988 [RFC2988], with two exceptions. 567 First, the initial value for RTO SHOULD be configurable (rather than 568 the 3s recommended in RFC 2988) and SHOULD be greater than 100ms. In 569 fixed-line access links, a value of 100ms is RECOMMENDED. Secondly, 570 the value of RTO MUST NOT be rounded up to the nearest second. 571 Rather, a 1ms accuracy MUST be maintained. As with TCP, the usage of 572 Karn's algorithm is RECOMMENDED [KARN87]. When applied to STUN, it 573 means that RTT estimates SHOULD NOT be computed from STUN 574 transactions which result in the retransmission of a request. 576 The value for RTO SHOULD be cached by a client after the completion 577 of the transaction, and used as the starting value for RTO for the 578 next transaction to the same server (based on equality of IP 579 address). The value SHOULD be considered stale and discarded after 580 10 minutes. 582 Retransmissions continue until a response is received, or until a 583 total of 7 requests have been sent. If, after the last request, a 584 duration equal to 16 times the RTO has passed without a response 585 (providing ample time to get a response if only this final request 586 actually succeeds), the client SHOULD consider the transaction to 587 have failed. A STUN transaction over UDP is also considered failed 588 if there has been a transport failure of some sort, such as a fatal 589 ICMP error. For example, assuming an RTO of 100ms, requests would be 590 sent at times 0ms, 100ms, 300ms, 700ms, 1500ms, 3100ms, and 6300ms. 591 If the client has not received a response after 7900ms, the client 592 will consider the transaction to have timed out. 594 7.2.2. Sending over TCP or TLS-over-TCP 596 For TCP and TLS-over-TCP, the client opens a TCP connection to the 597 server. 599 In some usage of STUN, STUN is sent as the only protocol over the TCP 600 connection. In this case, it can be sent without the aid of any 601 additional framing or demultiplexing. In other usages, or with other 602 extensions, it may be multiplexed with other data over a TCP 603 connection. In that case, STUN MUST be run on top of some kind of 604 framing protocol, specified by the usage or extension, which allows 605 for the agent to extract complete STUN messages and complete 606 application layer messages. 608 For TLS-over-TCP, the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST 609 be supported at a minimum. Implementations MAY also support any 610 other ciphersuite. When it receives the TLS Certificate message, the 611 client SHOULD verify the certificate and inspect the site identified 612 by the certificate. If the certificate is invalid, revoked, or if it 613 does not identify the appropriate party, the client MUST NOT send the 614 STUN message or otherwise proceed with the STUN transaction. The 615 client MUST verify the identity of the server. To do that, it 616 follows the identification procedures defined in Section 3.1 of RFC 617 2818 [RFC2818]. Those procedures assume the client is dereferencing 618 a URI. For purposes of usage with this specification, the client 619 treats the domain name or IP address used in Section 8.1 as the host 620 portion of the URI that has been dereferenced. If DNS was not used, 621 the client MUST be configured with a set of authorized domains whose 622 certificates will be accepted. 624 Reliability of STUN over TCP and TLS-over-TCP is handled by TCP 625 itself, and there are no retransmissions at the STUN protocol level. 626 However, for a request/response transaction, if the client has not 627 received a response 7900ms after it sent the SYN to establish the 628 connection, it considers the transaction to have timed out. This 629 value has been chosen to equalize the TCP and UDP timeouts for the 630 default initial RTO. 632 In addition, if the client is unable to establish the TCP connection, 633 or the TCP connection is reset or fails before a response is 634 received, any request/response transaction in progress is considered 635 to have failed 637 The client MAY send multiple transactions over a single TCP (or TLS- 638 over-TCP) connection, and it MAY send another request before 639 receiving a response to the previous. The client SHOULD keep the 640 connection open until it 642 o has no further STUN requests or indications to send over that 643 connection, and; 645 o has no plans to use any resources (such as a mapped address 646 (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address 647 [I-D.ietf-behave-turn]) that were learned though STUN requests 648 sent over that connection, and; 650 o if multiplexing other application protocols over that port, has 651 finished using that other application, and; 653 o if using that learned port with a remote peer, has established 654 communications with that remote peer, as is required by some TCP 655 NAT traversal techniques (e.g., [I-D.ietf-mmusic-ice-tcp]). 657 At the server end, the server SHOULD keep the connection open, and 658 let the client close it. If a server becomes overloaded and needs to 659 close connections to free up resources, it SHOULD close an existing 660 connection rather than reject new connection requests. The server 661 SHOULD NOT close a connection if a request was received over that 662 connection for which a response was not sent. A server MUST NOT ever 663 open a connection back towards the client in order to send a 664 response. 666 7.3. Receiving a STUN Message 668 This section specifies the processing of a STUN message. The 669 processing specified here is for STUN messages as defined in this 670 specification; additional rules for backwards compatibility are 671 defined in in Section 12. Those additional procedures are optional, 672 and usages can elect to utilize them. First, a set of processing 673 operations are applied that are independent of the class. This is 674 followed by class-specific processing, described in the subsections 675 which follow. 677 When a STUN agent receives a STUN message, it first checks that the 678 message obeys the rules of Section 6. It checks that the first two 679 bits are 0, that the magic cookie field has the correct value, that 680 the message length is sensible, and that the method value is a 681 supported method. If the message-class is Success Response or Error 682 Response, the agent checks that the transaction ID matches a 683 transaction that is still in progress. If the FINGERPRINT extension 684 is being used, the agent checks that the FINGERPRINT attribute is 685 present and contains the correct value. If any errors are detected, 686 the message is silently discarded. In the case when STUN is being 687 multiplexed with another protocol, an error may indicate that this is 688 not really a STUN message; in this case, the agent should try to 689 parse the message as a different protocol. 691 The STUN agent then does any checks that are required by a 692 authentication mechanism that the usage has specified (see 693 Section 10. 695 Once the authentication checks are done, the STUN agent checks for 696 unknown attributes and known-but-unexpected attributes in the 697 message. Unknown comprehension-optional attributes MUST be ignored 698 by the agent. Known-but-unexpected attributes SHOULD be ignored by 699 the agent. Unknown comprehension-required attributes cause 700 processing that depends on the message-class and is described below. 702 At this point, further processing depends on the message class of the 703 request. 705 7.3.1. Processing a Request 707 If the request contains one or more unknown comprehension-required 708 attributes, the server replies with an error response with an error 709 code of 420 (Unknown Attribute), and includes an UNKNOWN-ATTRIBUTES 710 attribute in the response that lists the unknown comprehension- 711 required attributes. 713 The server then does any additional checking that the method or the 714 specific usage requires. If all the checks succeed, the server 715 formulates a success response as described below. 717 If the request uses UDP transport and is a retransmission of a 718 request for which the server has already generated a success response 719 within the last 10 seconds, the server MUST retransmit the same 720 success response. One way for a server to do this is to remember all 721 transaction IDs received over UDP and their corresponding responses 722 in the last 10 seconds. Another way is to reprocess the request and 723 recompute the response. The latter technique MUST only be applied to 724 requests which are idempotent (a request is considered idempotent 725 when the same request can be safely repeated without impacting the 726 overall state of the system) and result in the same success response 727 for the same request. The Binding method is considered to idempotent 728 in this way (even though certain rare network events could cause the 729 reflexive transport address value to change). Extensions to STUN 730 SHOULD state whether their request types have this property or not. 732 7.3.1.1. Forming a Success or Error Response 734 When forming the response (success or error), the server follows the 735 rules of section 6. The method of the response is the same as that 736 of the request, and the message class is either "Success Response" or 737 "Error Response". 739 For an error response, the server MUST add an ERROR-CODE attribute 740 containing the error code specified in the processing above. The 741 reason phrase is not fixed, but SHOULD be something suitable for the 742 error code. For certain errors, additional attributes are added to 743 the message. These attributes are spelled out in the description 744 where the error code is specified. For example, for an error code of 745 420 (Unknown Attribute), the server MUST include an UNKNOWN- 746 ATTRIBUTES attribute. Certain authentication errors also cause 747 attributes to be added (see Section 10). Extensions may define other 748 errors and/or additional attributes to add in error cases. 750 If the server authenticated the request using an authentication 751 mechanism, then the server SHOULD add the appropriate authentication 752 attributes to the response (see Section 10). 754 The server also adds any attributes required by the specific method 755 or usage. In addition, the server SHOULD add a SERVER attribute to 756 the message. 758 For the Binding method, no additional checking is required unless the 759 usage specifies otherwise. When forming the success response, the 760 server adds a XOR-MAPPED-ADDRESS attribute to the response, where the 761 contents of the attribute are the source transport address of the 762 request message. For UDP, this is the source IP address and source 763 UDP port of the request message. For TCP and TLS-over-TCP, this is 764 the source IP address and source TCP port of the TCP connection as 765 seen by the server. 767 7.3.1.2. Sending the Success or Error Response 769 The response (success or error) is sent over the same transport as 770 the request was received on. If the request was received over UDP, 771 the destination IP address and port of the response is the source IP 772 address and port of the received request message, and the source IP 773 address and port of the response is equal to the destination IP 774 address and port of the received request message. If the request was 775 received over TCP or TLS-over-TCP, the response is sent back on the 776 same TCP connection as the request was received on. 778 7.3.2. Processing an Indication 780 If the indication contains unknown comprehension-required attributes, 781 the indication is discarded and processing ceases. 783 The agent then does any additional checking that the method or the 784 specific usage requires. If all the checks succeed, the agent then 785 processes the indication. No response is generated for an 786 indication. 788 For the Binding method, no additional checking or processing is 789 required, unless the usage specifies otherwise. The mere receipt of 790 the message by the agent has refreshed the "bindings" in the 791 intervening NATs. 793 Since indications are not re-transmitted over UDP (unlike requests), 794 there is no need to handle re-transmissions of indications at the 795 sending agent. 797 7.3.3. Processing a Success Response 799 If the success response contains unknown comprehension-required 800 attributes, the response is discarded and the transaction is 801 considered to have failed. 803 The client then does any additional checking that the method or the 804 specific usage requires. If all the checks succeed, the client then 805 processes the success response. 807 For the Binding method, the client checks that the XOR-MAPPED-ADDRESS 808 attribute is present in the response. The client checks the address 809 family specified. If it is an unsupported address family, the 810 attribute SHOULD be ignored. If it is an unexpected but supported 811 address family (for example, the Binding transaction was sent over 812 IPv4, but the address family specified is IPv6), then the client MAY 813 accept and use the value. 815 7.3.4. Processing an Error Response 817 If the error response contains unknown comprehension-required 818 attributes, or if the error response does not contain an ERROR-CODE 819 attribute, then the transaction is simply considered to have failed. 821 The client then does any processing specified by the authentication 822 mechanism (see Section 10). This may result in a new transaction 823 attempt. 825 The processing at this point depends on the error-code, the method, 826 and the usage; the following are the default rules: 828 o If the error code is 300 through 399, the client SHOULD consider 829 the transaction as failed unless the ALTERNATE-SERVER extension is 830 being used. See Section 11. 832 o If the error code is 400 through 499, the client declares the 833 transaction failed; in the case of 420 (Unknown Attribute), the 834 response should contain a UNKNOWN-ATTRIBUTES attribute that gives 835 additional information. 837 o If the error code is 500 through 599, the client MAY resend the 838 request; clients that do so MUST limit the number of times they do 839 this. 841 Any other error code causes the client to consider the transaction 842 failed. 844 8. FINGERPRINT Mechanism 846 This section describes an optional mechanism for STUN that aids in 847 distinguishing STUN messages from packets of other protocols when the 848 two are multiplexed on the same transport address. This mechanism is 849 optional, and a STUN usage must describe if and when it is used. 851 In some usages, STUN messages are multiplexed on the same transport 852 address as other protocols, such as RTP. In order to apply the 853 processing described in Section 7, STUN messages must first be 854 separated from the application packets. Section 6 describes three 855 fixed fields in the STUN header that can be used for this purpose. 856 However, in some cases, these three fixed fields may not be 857 sufficient. 859 When the FINGERPRINT extension is used, an agent includes the 860 FINGERPRINT attribute in messages it sends to another agent. 861 Section 14.5 describes the placement and value of this attribute. 862 When the agent receives what it believes is a STUN message, then, in 863 addition to other basic checks, the agent also checks that the 864 message contains a FINGERPRINT attribute and that the attribute 865 contains the correct value. Section 7.3 describes when in the 866 overall processing of a STUN message the FINGERPRINT check is 867 performed. This additional check helps the agent detect messages of 868 other protocols that might otherwise seem to be STUN messages. 870 9. DNS Discovery of a Server 872 This section describes an optional procedure for STUN that allows a 873 client to use DNS to determine the IP address and port of a server. 874 A STUN usage must describe if and when this extension is used. To 875 use this procedure, the client must know a server's domain name and a 876 service name; the usage must also describe how the client obtains 877 these. Hard-coding the domain-name of the server into software is 878 NOT RECOMMENDED in case the domain name is lost or needs to change 879 for legal or other reasons. 881 When a client wishes to locate a STUN server in the public Internet 882 that accepts Binding Request/Response transactions, the SRV service 883 name is "stun". When it wishes to locate a STUN server which accepts 884 Binding Request/Response transactions over a TLS session, the SRV 885 service name is "stuns". STUN usages MAY define additional DNS SRV 886 service names. 888 The domain name is resolved to a transport address using the SRV 889 procedures specified in [RFC2782]. The DNS SRV service name is the 890 service name provided as input to this procedure. The protocol in 891 the SRV lookup is the transport protocol the client will run STUN 892 over: "udp" for UDP and "tcp" for TCP. Note that only "tcp" is 893 defined with "stuns" at this time. 895 The procedures of RFC 2782 are followed to determine the server to 896 contact. RFC 2782 spells out the details of how a set of SRV records 897 are sorted and then tried. However, RFC2782 only states that the 898 client should "try to connect to the (protocol, address, service)" 899 without giving any details on what happens in the event of failure. 900 When following these procedures, if the STUN transaction times out 901 without receipt of a response, the client SHOULD retry the request to 902 the next server in the ordered defined by RFC 2782. Such a retry is 903 only possible for request/response transmissions, since indication 904 transactions generate no response or timeout. 906 The default port for STUN requests is 3478, for both TCP and UDP. 907 Administrators of STUN servers SHOULD use this port in their SRV 908 records for UDP and TCP, but MAY use others. In all cases, the port 909 in DNS MUST reflect the one the server is listening on. There is no 910 default port for STUN over TLS, however a STUN server SHOULD use a 911 port number for TLS different from 3478 so that the server can 912 determine whether the first message it will receive after the TCP 913 connection is set up, is a STUN message or a TLS message. 915 If no SRV records were found, the client performs an A or AAAA record 916 lookup of the domain name. The result will be a list of IP 917 addresses, each of which can be contacted at the default port using 918 UDP or TCP, independent of the STUN usage. For usages that require 919 TLS, lack of SRV records is equivalent to a failure of the 920 transaction, since the request or indication MUST NOT be sent unless 921 SRV records provided a transport address specifically for TLS. 923 10. Authentication and Message-Integrity Mechanisms 925 This section defines two mechanisms for STUN that a client and server 926 can use to provide authentication and message-integrity; these two 927 mechanisms are known as the short-term credential mechanism and the 928 long-term credential mechanism. These two mechanisms are optional, 929 and each usage must specify if and when these mechanisms are used. 930 Consequently, both clients and servers will know which mechanism (if 931 any) to follow based on knowledge of which usage applies. For 932 example, a STUN server on the public Internet supporting ICE would 933 have no authentication, whereas the STUN server functionality in an 934 agent supporting connectivity checks would utilize short term 935 credentials. An overview of these two mechanisms is given in 936 Section 3. 938 Each mechanism specifies the additional processing required to use 939 that mechanism, extending the processing specified in Section 7. The 940 additional processing occurs in three different places: when forming 941 a message; when receiving a message immediately after the basic 942 checks have been performed; and when doing the detailed processing of 943 error responses. 945 10.1. Short-Term Credential Mechanism 947 The short-term credential mechanism assumes that, prior to the STUN 948 transaction, the client and server have used some other protocol to 949 exchange a credential in the form of a username and password. This 950 credential is time-limited. The time-limit is defined by the usage. 951 As an example, in the ICE usage [I-D.ietf-mmusic-ice], the two 952 endpoints use out-of-band signaling to agree on a username and 953 password, and this username and password is applicable for the 954 duration of the media session. 956 This credential is used to form a message integrity check in each 957 request and in many responses. There is no challenge and response as 958 in the long term mechanism; consequently, replay is prevented by 959 virtue of the time-limited nature of the credential. 961 10.1.1. Forming a Request or Indication 963 For a request or indication message, the agent MUST include the 964 USERNAME and MESSAGE-INTEGRITY attributes in the message. The HMAC 965 for the MESSAGE-INTEGRITY attribute is computed as described in 966 Section 14.4. Note that the password is never included in the 967 request or indication. 969 10.1.2. Receiving a Request or Indication 971 After the agent has done the basic processing of a message, the agent 972 performs the checks listed below in order specified: 974 o If the message does not contain both a MESSAGE-INTEGRITY and a 975 USERNAME attribute: 977 * If the message is a request, the server MUST reject the request 978 with an error response. This response MUST use an error code 979 of 400 (Bad Request). 981 * If the message is an indication, the agent MUST silently 982 discard the indication. 984 o If the USERNAME does not contain a username value currently valid 985 within the server: 987 * If the message is a request, the server MUST reject the request 988 with an error response. This response MUST use an error code 989 of 401 (Unauthorized). 991 * If the message is an indication, the agent MUST silently 992 discard the indication. 994 o Using the password associated with the username, compute the value 995 for the message-integrity as described in Section 14.4. If the 996 resulting value does not match the contents of the MESSAGE- 997 INTEGRITY attribute: 999 * If the message is a request, the server MUST reject the request 1000 with an error response. This response MUST use an error code 1001 of 401 (Unauthorized). 1003 * If the message is an indication, the agent MUST silently 1004 discard the indication. 1006 If these checks pass, the agent continues to process the request or 1007 indication. Any response generated by a server MUST include the 1008 MESSAGE-INTEGRITY attribute, computed using the password utilized to 1009 authenticate the request. The response MUST NOT contain the USERNAME 1010 attribute. 1012 If any of the checks fail, a server MUST NOT include a MESSAGE- 1013 INTEGRITY or USERNAME attribute in the error response. This is 1014 because, in these failure cases, the server cannot determine the 1015 shared secret necessary to compute MESSAGE-INTEGRITY. 1017 10.1.3. Receiving a Response 1019 The client looks for the MESSAGE-INTEGRITY attribute in the response. 1020 If present, the client computes the message integrity over the 1021 response as defined in Section 14.4, using the same password it 1022 utilized for the request. If the resulting value matches the 1023 contents of the MESSAGE-INTEGRITY attribute, the response is 1024 considered authenticated. If the value does not match, or if 1025 MESSAGE-INTEGRITY was absent, the response MUST be discarded, as if 1026 it was never received. This means that retransmits, if applicable, 1027 will continue. 1029 10.2. Long-term Credential Mechanism 1031 The long-term credential mechanism relies on a long term credential, 1032 in the form of a username and password, that are shared between 1033 client and server. The credential is considered long-term since it 1034 is assumed that it is provisioned for a user, and remains in effect 1035 until the user is no longer a subscriber of the system, or is 1036 changed. This is basically a traditional "log-in" username and 1037 password given to users. 1039 Because these usernames and passwords are expected to be valid for 1040 extended periods of time, replay prevention is provided in the form 1041 of a digest challenge. In this mechanism, the client initially sends 1042 a request, without offering any credentials or any integrity checks. 1043 The server rejects this request, providing the user a realm (used to 1044 guide the user or agent in selection of a username and password) and 1045 a nonce. The nonce provides the replay protection. It is a cookie, 1046 selected by the server, and encoded in such a way as to indicate a 1047 duration of validity or client identity from which it is valid. The 1048 client retries the request, this time including its username, the 1049 realm, and echoing the nonce provided by the server. The client also 1050 includes a message-integrity, which provides an HMAC over the entire 1051 request, including the nonce. The server validates the nonce, and 1052 checks the message-integrity. If they match, the request is 1053 authenticated. If the nonce is no longer valid, it is considered 1054 "stale", and the server rejects the request, providing a new nonce. 1056 In subsequent requests to the same server, the client reuses the 1057 nonce, username, realm and password it used previously. In this way, 1058 subsequent requests are not rejected until the nonce becomes invalid 1059 by the server, in which case the rejection provides a new nonce to 1060 the client. 1062 Note that the long-term credential mechanism cannot be used to 1063 protect indications, since indications cannot be challenged. Usages 1064 utilizing indications must either use a short-term credential, or 1065 omit authentication and message integrity for them. 1067 Since the long-term credential mechanism is susceptible to offline 1068 dictionary attacks, deployments SHOULD utilize strong passwords. 1070 10.2.1. Forming a Request 1072 There are two cases when forming a request. In the first case, this 1073 is the first request from the client to the server (as identified by 1074 its IP address and port). In the second case, the client is 1075 submitting a subsequent request once a previous request/response 1076 transaction has completed successfully. Forming a request as a 1077 consequence of a 401 or 438 error response is covered in 1078 Section 10.2.3 and is not considered a "subsequent request" and thus 1079 does not utilize the rules described in Section 10.2.1.2. 1081 10.2.1.1. First Request 1083 If the client has not completed a successful request/response 1084 transaction with the server (as identified by hostname, if the DNS 1085 procedures of Section 9 are used, else IP address if not), it SHOULD 1086 omit the USERNAME, MESSAGE-INTEGRITY, REALM, and NONCE attributes. 1087 In other words, the very first request is sent as if there were no 1088 authentication or message integrity applied. The exception to this 1089 rule are requests sent to another server as a consequence of the 1090 ALTERNATE-SERVER mechanism described in Section 11. Those requests 1091 do include the USERNAME, REALM and NONCE from the original request, 1092 along with a newly computed MESSAGE-INTEGRITY based on them. 1094 10.2.1.2. Subsequent Requests 1096 Once a request/response transaction has completed successfully, the 1097 client will have been been presented a realm and nonce by the server, 1098 and selected a username and password with which it authenticated. 1099 The client SHOULD cache the username, password, realm, and nonce for 1100 subsequent communications with the server. When the client sends a 1101 subsequent request, it SHOULD include the USERNAME, REALM, and NONCE 1102 attributes with these cached values. It SHOULD include a MESSAGE- 1103 INTEGRITY attribute, computed as described in Section 14.4 using the 1104 cached password. 1106 10.2.2. Receiving a Request 1108 After the server has done the basic processing of a request, it 1109 performs the checks listed below in the order specified: 1111 o If the message does not contain a MESSAGE-INTEGRITY attribute, the 1112 server MUST generate an error response with an error code of 401 1113 (Unauthorized). This response MUST include a REALM value. It is 1114 RECOMMENDED that the REALM value be the domain name of the 1115 provider of the STUN server. The response MUST include a NONCE, 1116 selected by the server. The response SHOULD NOT contain a 1117 USERNAME or MESSAGE-INTEGRITY attribute. 1119 o If the message contains a MESSAGE-INTEGRITY attribute, but is 1120 missing the USERNAME, REALM or NONCE attributes, the server MUST 1121 generate an error response with an error code of 400 (Bad 1122 Request). This response SHOULD NOT include a USERNAME, NONCE, 1123 REALM or MESSAGE-INTEGRITY attribute. 1125 o If the NONCE is no longer valid, the server MUST generate an error 1126 response with an error code of 438 (Stale Nonce). This response 1127 MUST include a NONCE and REALM attribute and SHOULD NOT incude the 1128 USERNAME or MESSAGE-INTEGRITY attribute. 1130 o If the username in the USERNAME attribute is not valid, the server 1131 MUST generate an error response with an error code of 401 1132 (Unauthorized). This response MUST include a REALM value. It is 1133 RECOMMENDED that the REALM value be the domain name of the 1134 provider of the STUN server. The response MUST include a NONCE, 1135 selected by the server. The response SHOULD NOT contain a 1136 USERNAME or MESSAGE-INTEGRITY attribute. 1138 o Using the password associated with the username in the USERNAME 1139 attribute, compute the value for the message-integrity as 1140 described in Section 14.4. If the resulting value does not match 1141 the contents of the MESSAGE-INTEGRITY attribute, the server MUST 1142 reject the request with an error response. This response MUST use 1143 an error code of 401 (Unauthorized). It MUST include a REALM and 1144 NONCE attribute and SHOULD NOT include the USERNAME or MESSAGE- 1145 INTEGRITY attribute. 1147 If these checks pass, the server continues to process the request. 1148 Any response generated by the server (excepting the cases described 1149 above) MUST include the MESSAGE-INTEGRITY attribute, computed using 1150 the username and password utilized to authenticate the request. The 1151 REALM, NONCE, and USERNAME attributes SHOULD NOT be included. 1153 10.2.3. Receiving a Response 1155 If the response is an error response, with an error code of 401 1156 (Unauthorized), the client SHOULD retry the request with a new 1157 transaction. This request MUST contain a USERNAME, determined by the 1158 client as the appropriate username for the REALM from the error 1159 response. The request MUST contain the REALM, copied from the error 1160 response. The request MUST contain the NONCE, copied from the error 1161 response. The request MUST contain the MESSAGE-INTEGRITY attribute, 1162 computed using the password associated with the username in the 1163 USERNAME attribute. The client MUST NOT perform this retry if it is 1164 not changing the USERNAME or REALM or its associated password, from 1165 the previous attempt. 1167 If the response is an error response with an error code of 438 (Stale 1168 Nonce), the client MUST retry the request, using the new NONCE 1169 supplied in the 438 (Stale Nonce) response. This retry MUST also 1170 include the USERNAME, REALM and MESSAGE-INTEGRITY. 1172 The client looks for the MESSAGE-INTEGRITY attribute in the response 1173 (either success or failure). If present, the client computes the 1174 message integrity over the response as defined in Section 14.4, using 1175 the same password it utilized for the request. If the resulting 1176 value matches the contents of the MESSAGE-INTEGRITY attribute, the 1177 response is considered authenticated. If the value does not match, 1178 or if MESSAGE-INTEGRITY was absent, the response MUST be discarded, 1179 as if it was never received. This means that retransmits, if 1180 applicable, will continue. 1182 11. ALTERNATE-SERVER Mechanism 1184 This section describes a mechanism in STUN that allows a server to 1185 redirect a client to another server. This extension is optional, and 1186 a usage must define if and when this extension is used. To prevent 1187 denial-of-service attacks, this extension MUST only be used in 1188 situations where the client and server are using an authentication 1189 and message-integrity mechanism. 1191 A server using this extension redirects a client to another server by 1192 replying to a request message with an error response message with an 1193 error code of 300 (Try Alternate). The server MUST include a 1194 ALTERNATE-SERVER attribute in the error response. The error response 1195 message MUST be authenticated, which in practice means the request 1196 message must have passed the authentication checks. 1198 A client using this extension handles a 300 (Try Alternate) error 1199 code as follows. If the error response has passed the authentication 1200 checks, then the client looks for a ALTERNATE-SERVER attribute in the 1201 error response. If one is found, then the client considers the 1202 current transaction as failed, and re-attempts the request with the 1203 server specified in the attribute. The client SHOULD reuse any 1204 authentication credentials from the old request in the new 1205 transaction. 1207 12. Backwards Compatibility with RFC 3489 1209 This section define procedures that allow a degree of backwards 1210 compatible with the original protocol defined in RFC 3489 [RFC3489]. 1211 This mechanism is optional, meant to be utilized only in cases where 1212 a new client can connect to an old server, or vice-a-versa. A usage 1213 must define if and when this procedure is used. 1215 Section 18 lists all the changes between this specification and RFC 1216 3489 [RFC3489]. However, not all of these differences are important, 1217 because "classic STUN" was only used in a few specific ways. For the 1218 purposes of this extension, the important changes are the following. 1219 In RFC 3489: 1221 o UDP was the only supported transport; 1223 o The field that is now the Magic Cookie field was a part of the 1224 transaction id field, and transaction ids were 128 bits long; 1226 o The XOR-MAPPED-ADDRESS attribute did not exist, and the Binding 1227 method used the MAPPED-ADDRESS attribute instead; 1229 o There were three comprehension-required attributes, RESPONSE- 1230 ADDRESS, CHANGE-REQUEST, and CHANGED-ADDRESS that have been 1231 removed from this specification; 1233 * These attributes are now part of the NAT Behavior Discovery 1234 usage. [I-D.ietf-behave-nat-behavior-discovery] 1236 12.1. Changes to Client Processing 1238 A client that wants to interoperate with a [RFC3489] server SHOULD 1239 send a request message that uses the Binding method, contains no 1240 attributes, and uses UDP as the transport protocol to the server. If 1241 successful, the success response received from the server will 1242 contain a MAPPED-ADDRESS attribute rather than an XOR-MAPPED-ADDRESS 1243 attribute; other than this change, the processing of the response is 1244 identical to the procedures described above. 1246 12.2. Changes to Server Processing 1248 A STUN server can detect when a given Binding Request message was 1249 sent from an RFC 3489 [RFC3489] client by the absence of the correct 1250 value in the Magic Cookie field. When the server detects an RFC 3489 1251 client, it SHOULD copy the value seen in the Magic Cookie field in 1252 the Binding Request to the Magic Cookie field in the Binding Response 1253 message, and insert a MAPPED-ADDRESS attribute instead of an XOR- 1254 MAPPED-ADDRESS attribute. 1256 The client might, in rare situations, include either the RESPONSE- 1257 ADDRESS or CHANGE-REQUEST attributes. In these situations, the 1258 server will view these as unknown comprehension-required attributes 1259 and reply with an error response. Since the mechanisms utilizing 1260 those attributes are no longer supported, this behavior is 1261 acceptable. 1263 The RFC 3489 version of STUN lacks both the Magic Cookie and the 1264 FINGERPRINT attribute that allows for a very high probablility of 1265 correctly identifying STUN messages when multiplexed with other 1266 protocols. Therefore, STUN implementations that are backwards 1267 compatible with RFC 3489 SHOULD NOT be used in cases where STUN will 1268 be multiplexed with another protocol. However, that should not be an 1269 issues as such multiplexing was not available in RFC 3489. 1271 13. STUN Usages 1273 STUN by itself is not a solution to the NAT traversal problem. 1274 Rather, STUN defines a tool that can be used inside a larger 1275 solution. The term "STUN Usage" is used for any solution that uses 1276 STUN as a component. 1278 At the time of writing, three STUN usages are defined: Interactive 1279 Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice], Client- 1280 initiated connections for SIP [I-D.ietf-sip-outbound], and NAT 1281 Behavior Discovery [I-D.ietf-behave-nat-behavior-discovery]. Other 1282 STUN usages may be defined in the future. 1284 A STUN usage defines how STUN is actually utilized - when to send 1285 requests, what to do with the responses, and which optional 1286 procedures defined here (or in an extension to STUN) are to be used. 1287 A usage would also define: 1289 o Which STUN methods are used; 1291 o What authentication and message integrity mechanisms are used; 1292 o What mechanisms are used to distinguish STUN messages from other 1293 messages. When STUN is run over TCP, a framing mechanism may be 1294 required; 1296 o How a STUN client determines the IP address and port of the STUN 1297 server; 1299 o Whether backwards compatibility to RFC 3489 is required; 1301 o What optional attributes defined here (such as FINGERPRINT and 1302 ALTERNATE-SERVER) or in other extensions are required. 1304 In addition, any STUN usage must consider the security implications 1305 of using STUN in that usage. A number of attacks against STUN are 1306 known (see the Security Considerations section in this document) and 1307 any usage must consider how these attacks can be thwarted or 1308 mitigated. 1310 Finally, a usage must consider whether its usage of STUN is an 1311 example of the Unilateral Self-Address Fixing approach to NAT 1312 traversal, and if so, address the questions raised in RFC 3424. 1313 [RFC3424] 1315 14. STUN Attributes 1317 After the STUN header are zero or more attributes. Each attribute 1318 MUST be TLV encoded, with a 16 bit type, 16 bit length, and value. 1319 Each STUN attribute MUST end on a 32 bit boundary. As mentioned 1320 above, all fields in an attribute are transmitted most significant 1321 bit first. 1323 0 1 2 3 1324 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 1325 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1326 | Type | Length | 1327 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1328 | Value (variable) .... 1329 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1331 Figure 4: Format of STUN Attributes 1333 The value in the Length field MUST contain the length of the Value 1334 part of the attribute, prior to padding, measured in bytes. Since 1335 STUN aligns attributes on 32 bit boundaries, attributes whose content 1336 is not a multiple of 4 bytes are padded with 1, 2 or 3 bytes of 1337 padding so that its value contains a multiple of 4 bytes. The 1338 padding bits are ignored, and may be any value. 1340 Any attribute type MAY appear more than once in a STUN message. 1341 Unless specified otherwise, the order of appearance is significant: 1342 only the first occurance needs to be processed by a receiver, and any 1343 duplicates MAY be ignored by a receiver. 1345 To allow future revisions of this specification to add new attributes 1346 if needed, the attribute space is divided into two ranges. 1347 Attributes with type values between 0x0000 and 0x7FFF are 1348 comprehension-required attributes, which means that the STUN agent 1349 cannot successfully process the message unless it understands the 1350 attribute. Attributes with type values between 0x8000 and 0xFFFF are 1351 comprehension-optional attributes, which means that those attributes 1352 can be ignored by the STUN agent if it does not understand them. 1354 The STUN Attribute types defined by this specification are: 1356 Comprehension-required range (0x0000-0x7FFF): 1357 0x0000: (Reserved) 1358 0x0001: MAPPED-ADDRESS 1359 0x0006: USERNAME 1360 0x0007: (Reserved; was PASSWORD) 1361 0x0008: MESSAGE-INTEGRITY 1362 0x0009: ERROR-CODE 1363 0x000A: UNKNOWN-ATTRIBUTES 1364 0x0014: REALM 1365 0x0015: NONCE 1366 0x0020: XOR-MAPPED-ADDRESS 1368 Comprehension-optional range (0x8000-0xFFFF) 1369 0x8022: SERVER 1370 0x8023: ALTERNATE-SERVER 1371 0x8028: FINGERPRINT 1373 The rest of this section describes the format of the various 1374 attributes defined in this specification. 1376 14.1. MAPPED-ADDRESS 1378 The MAPPED-ADDRESS attribute indicates a reflexive transport address 1379 of the client. It consists of an eight bit address family, and a 1380 sixteen bit port, followed by a fixed length value representing the 1381 IP address. If the address family is IPv4, the address MUST be 32 1382 bits. If the address family is IPv6, the address MUST be 128 bits. 1383 All fields must be in network byte order. 1385 The format of the MAPPED-ADDRESS attribute is: 1387 0 1 2 3 1388 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 1389 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1390 |0 0 0 0 0 0 0 0| Family | Port | 1391 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1392 | | 1393 | Address (32 bits or 128 bits) | 1394 | | 1395 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1397 Figure 6: Format of MAPPED-ADDRESS attribute 1399 The address family can take on the following values: 1401 0x01:IPv4 1402 0x02:IPv6 1404 The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be 1405 ignored by receivers. These bits are present for aligning parameters 1406 on natural 32 bit boundaries. 1408 This attribute is used only by servers for achieving backwards 1409 compatibility with RFC 3489 [RFC3489] clients. 1411 14.2. XOR-MAPPED-ADDRESS 1413 The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS 1414 attribute, except that the reflexive transport address is obfuscated 1415 through the XOR function. 1417 The format of the XOR-MAPPED-ADDRESS is: 1419 0 1 2 3 1420 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 1421 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1422 |x x x x x x x x| Family | X-Port | 1423 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1424 | X-Address (Variable) 1425 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1427 Figure 8: Format of XOR-MAPPED-ADDRESS Attribute 1429 The Family represents the IP address family, and is encoded 1430 identically to the Family in MAPPED-ADDRESS. 1432 X-Port is computed by taking the mapped port in host byte order, 1433 XOR'ing it with the most significant 16 bits of the magic cookie, and 1434 then the converting the result to network byte order. If the IP 1435 address family is IPv4, X-Address is computed by taking the mapped IP 1436 address in host byte order, XOR'ing it with the magic cookie, and 1437 converting the result to network byte order. If the IP address 1438 family is IPv6, X-Address is computed by taking the mapped IP address 1439 in host byte order, XOR'ing it with the concatenation of the magic 1440 cookie and the 96-bit transaction ID, and converting the result to 1441 network byte order. 1443 The rules for encoding and processing the first 8 bits of the 1444 attribute's value, the rules for handling multiple occurrences of the 1445 attribute, and the rules for processing addresses families are the 1446 same as for MAPPED-ADDRESS. 1448 NOTE: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their 1449 encoding of the transport address. The former encodes the transport 1450 address by exclusive-or'ing it with the magic cookie. The latter 1451 encodes it directly in binary. RFC 3489 originally specified only 1452 MAPPED-ADDRESS. However, deployment experience found that some NATs 1453 rewrite the 32-bit binary payloads containing the NAT's public IP 1454 address, such as STUN's MAPPED-ADDRESS attribute, in the well-meaning 1455 but misguided attempt at providing a generic ALG function. Such 1456 behavior interferes with the operation of STUN and also causes 1457 failure of STUN's message integrity checking. 1459 14.3. USERNAME 1461 The USERNAME attribute is used for message integrity. It identifies 1462 the username and password combination used in the message integrity 1463 check. 1465 The value of USERNAME is a variable length value. It MUST contain a 1466 UTF-8 [RFC3629] encoded sequence of less than 513 bytes. 1468 14.4. MESSAGE-INTEGRITY 1470 The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of 1471 the STUN message. The MESSAGE-INTEGRITY attribute can be present in 1472 any STUN message type. Since it uses the SHA1 hash, the HMAC will be 1473 20 bytes. The text used as input to HMAC is the STUN message, 1474 including the header, up to and including the attribute preceding the 1475 MESSAGE-INTEGRITY attribute. With the exception of the FINGERPRINT 1476 attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore 1477 all other attributes that follow MESSAGE-INTEGRITY. 1479 The key for the HMAC depends on whether long term or short term 1480 credentials are in use. For long term credentials: 1482 key = MD5(username ":" realm ":" password) 1484 For short term credentials: 1486 key = password 1488 Where MD5 is defined in RFC 1321 [RFC1321]. 1490 The structure of the key when used with long term credentials 1491 facilitates deployment in systems that also utilize SIP. Typically, 1492 SIP systems utilizing SIP's digest authentication mechanism do not 1493 actually store the password in the database. Rather, they store a 1494 value called H(A1), which is equal to the key defined above. 1496 Based on the rules above, the hash includes the length field from the 1497 STUN message header. This length indicates the length of the entire 1498 message, including the MESSAGE-INTEGRITY attribute itself. 1499 Consequently, the MESSAGE-INTEGRITY attribute MUST be inserted into 1500 the message (with dummy content) prior to the computation of the 1501 integrity check. Once the computation is performed, the value of the 1502 attribute can be filled in. This ensures the length has the correct 1503 value when the hash is performed. Similarly, when validating the 1504 MESSAGE-INTEGRITY, the length field should be adjusted to point to 1505 the end of the MESSAGE-INTEGRITY attribute prior to calculating the 1506 HMAC. Such adjustment is necessary when attributes, such as 1507 FINGERPRINT, appear after MESSAGE-INTEGRITY. 1509 14.5. FINGERPRINT 1511 The FINGERPRINT attribute MAY be present in all STUN messages. The 1512 value of the attribute is computed as the CRC-32 of the STUN message 1513 up to (but excluding) the FINGERPRINT attribute itself, xor-d with 1514 the 32 bit value 0x5354554e (the XOR helps in cases where an 1515 application packet is also using CRC-32 in it). The 32 bit CRC is 1516 the one defined in ITU V.42 [ITU.V42.2002], which has a generator 1517 polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1. 1518 When present, the FINGERPRINT attribute MUST be the last attribute in 1519 the message, and thus will appear after MESSAGE-INTEGRITY. 1521 The FINGERPRINT attribute can aid in distinguishing STUN packets from 1522 packets of other protocols. See Section 8. 1524 As with MESSAGE-INTEGRITY, the CRC used in the FINGERPRINT attribute 1525 covers the length field from the STUN message header. Therefore, 1526 this value must be correct, and include the CRC attribute as part of 1527 the message length, prior to computation of the CRC. When using the 1528 FINGERPRINT attribute in a message, the attribute is first placed 1529 into the message with a dummy value, then the CRC is computed, and 1530 then the value of the attribute is updated. If the MESSAGE-INTEGRITY 1531 attribute is also present, then it must be present with the correct 1532 message-integrity value before the CRC is computed, since the CRC is 1533 done over the value of the MESSAGE-INTEGRITY attribute as well. 1535 14.6. ERROR-CODE 1537 The ERROR-CODE attribute is used in Error Response messages. It 1538 contains a numeric error code value in the range of 300 to 699 plus a 1539 textual reason phrase encoded in UTF-8 [RFC3629], and is consistent 1540 in its code assignments and semantics with SIP [RFC3261] and HTTP 1541 [RFC2616]. The reason phrase is meant for user consumption, and can 1542 be anything appropriate for the error code. Recommended reason 1543 phrases for the defined error codes are presented below. The reason 1544 phrase MUST be a UTF-8 [RFC3629] encoded sequence of less than 128 1545 characters (which can be as long as 763 bytes). 1547 0 1 2 3 1548 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 1549 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1550 | Reserved, should be 0 |Class| Number | 1551 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1552 | Reason Phrase (variable) .. 1553 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1555 Figure 11: ERROR-CODE Attribute 1557 To facilitate processing, the class of the error code (the hundreds 1558 digit) is encoded separately from the rest of the code, as shown in 1559 Figure 11. 1561 The Reserved bits SHOULD be 0, and are for alignment on 32-bit 1562 boundaries. Receivers MUST ignore these bits. The Class represents 1563 the hundreds digit of the error code. The value MUST be between 3 1564 and 6. The number represents the error code modulo 100, and its 1565 value MUST be between 0 and 99. 1567 The following error codes, along with their recommended reason 1568 phrases are defined: 1570 300 Try Alternate: The client should contact an alternate server for 1571 this request. This error response MUST only be sent if the 1572 request included a USERNAME attribute and a valid MESSAGE- 1573 INTEGRITY attribute; otherwise it MUST NOT be sent and error 1574 code 400 (Bad Request) is suggested. This error response MUST 1575 be protected with the MESSAGE-INTEGRITY attribute, and receivers 1576 MUST validate the MESSAGE-INTEGRITY of this response before 1577 redirecting themselves to an alternate server. 1579 Note: failure to generate and validate message-integrity 1580 for a 300 response allows an on-path attacker to falsify a 1581 300 response thus causing subsequent STUN messages to be 1582 sent to a victim. 1584 400 Bad Request: The request was malformed. The client SHOULD NOT 1585 retry the request without modification from the previous 1586 attempt. The server may not be able to generate a valid 1587 MESSAGE-INTEGRITY for this error, so the client MUST NOT expect 1588 a valid MESSAGE-INTEGRITY attribute on this response. 1590 401 Unauthorized: The request did not contain the correct 1591 credentials to proceed. The client should retry the request 1592 with proper credentials. 1594 420 Unknown Attribute: The server received STUN packet containing a 1595 comprehension-required attribute which it did not understand. 1596 The server MUST put this unknown attribute in the UNKNOWN- 1597 ATTRIBUTE attribute of its error response. 1599 438 Stale Nonce: The NONCE used by the client was no longer valid. 1600 The client should retry, using the NONCE provided in the 1601 response. 1603 500 Server Error: The server has suffered a temporary error. The 1604 client should try again. 1606 14.7. REALM 1608 The REALM attribute may be present in requests and responses. It 1609 contains text which meets the grammar for "realm-value" as described 1610 in RFC 3261 [RFC3261] but without the double quotes and their 1611 surrounding whitespace. That is, it is an unquoted realm-value (and 1612 is therefore a sequence of qdtext or quoted-pair). It MUST be a 1613 UTF-8 [RFC3629] encoded sequence of less than 128 characters (which 1614 can be as long as 763 bytes). 1616 Presence of the REALM attribute in a request indicates that long-term 1617 credentials are being used for authentication. Presence in certain 1618 error responses indicates that the server wishes the client to use a 1619 long-term credential for authentication. 1621 14.8. NONCE 1623 The NONCE attribute may be present in requests and responses. It 1624 contains a sequence of qdtext or quoted-pair, which are defined in 1625 RFC 3261 [RFC3261]. Note that this means that the NONCE attribute 1626 will not contain actual quote characters. See RFC 2617 [RFC2617], 1627 Section 4.3, for guidance on selection of nonce values in a server. 1628 It MUST be less than 128 characters (which can be as long as 763 1629 bytes). 1631 14.9. UNKNOWN-ATTRIBUTES 1633 The UNKNOWN-ATTRIBUTES attribute is present only in an error response 1634 when the response code in the ERROR-CODE attribute is 420. 1636 The attribute contains a list of 16 bit values, each of which 1637 represents an attribute type that was not understood by the server. 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 | Attribute 1 Type | Attribute 2 Type | 1643 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1644 | Attribute 3 Type | Attribute 4 Type ... 1645 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1647 Figure 12: Format of UNKNOWN-ATTRIBUTES attribute 1649 Note: In [RFC3489], this field was padded to 32 by duplicating the 1650 last attribute. In this version of the specification, the normal 1651 padding rules for attributes are used instead. 1653 14.10. SERVER 1655 The server attribute contains a textual description of the software 1656 being used by the server, including manufacturer and version number. 1657 The attribute has no impact on operation of the protocol, and serves 1658 only as a tool for diagnostic and debugging purposes. The value of 1659 SERVER is variable length. It MUST be a UTF-8 [RFC3629] encoded 1660 sequence of less than 128 characters (which can be as long as 763 1661 bytes). 1663 14.11. ALTERNATE-SERVER 1665 The alternate server represents an alternate transport address 1666 identifying a different STUN server which the STUN client should try. 1668 It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a 1669 single server by IP address. The IP address family MUST be identical 1670 to that of the source IP address of the request. 1672 This attribute MUST only appear in an error response that contains a 1673 MESSAGE-INTEGRITY attribute. This prevents it from being used in 1674 denial-of-service attacks. 1676 15. Security Considerations 1678 15.1. Attacks against the Protocol 1680 15.1.1. Outside Attacks 1682 An attacker can try to modify STUN messages in transit, in order to 1683 cause a failure in STUN operation. These attacks are detected for 1684 both requests and responses through the message integrity mechanism, 1685 using either a short term or long term credential. Of course, once 1686 detected, the manipulated packets will be dropped, causing the STUN 1687 transaction to effectively fail. This attack is possible only by an 1688 on-path attacker. 1690 An attacker that can observe, but not modify STUN messages in-transit 1691 (for example, an attacker present on a shared access medium, such as 1692 Wi-Fi), can see a STUN request, and then immediately send a STUN 1693 response, typically an error response, in order to disrupt STUN 1694 processing. This attack is also prevented for messages that utilize 1695 MESSAGE-INTEGRITY. However, some error responses, those related to 1696 authentication in particular, cannot be protected by MESSAGE- 1697 INTEGRITY. When STUN itself is run over a secure transport protocol 1698 (e.g., TLS), these attacks are completely mitigated. 1700 15.1.2. Inside Attacks 1702 A rogue client may try to launch a DoS attack against a server by 1703 sending it a large number of STUN requests. Fortunately, STUN 1704 requests can be processed statelessly by a server, making such 1705 attacks hard to launch. 1707 A rogue client may use a STUN server as a reflector, sending it 1708 requests with a falsified source IP address and port. In such a 1709 case, the response would be delivered to that source IP and port. 1710 There is no amplification of the number of packets with this attack 1711 (the STUN server sends one packet for each packet sent by the 1712 client), though there is a small increase in the amount of data, 1713 since STUN responses are typically larger than requests. This attack 1714 is mitigated by ingress source address filtering. 1716 15.2. Attacks Affecting the Usage 1718 This section lists attacks that might be launched against a usage of 1719 STUN. Each STUN usage must consider whether these attacks are 1720 applicable to it, and if so, discuss counter-measures. 1722 Most of the attacks in this section revolve around an attacker 1723 modifying the reflexive address learned by a STUN client through a 1724 Binding Request/Binding Response transaction. Since the usage of the 1725 reflexive address is a function of the usage, the applicability and 1726 remediation of these attacks is usage-specific. In common 1727 situations, modification of the reflexive address by an on-path 1728 attacker is easy to do. Consider, for example, the common situation 1729 where STUN is run directly over UDP. In this case, an on-path 1730 attacker can modify the source IP address of the Binding Request 1731 before it arrives at the STUN server. The STUN server will then 1732 return this IP address in the XOR-MAPPED-ADDRESS attribute to the 1733 client, and send the response back to that (falsified) IP address and 1734 port. If the attacker can also intercept this response, it can 1735 direct it back towards the client. Protecting against this attack by 1736 using a message-integrity check is impossible, since a message- 1737 integrity value cannot cover the source IP address, since the 1738 intervening NAT must be able to modify this value. Instead, one 1739 solution to preventing the attacks listed below is for the client to 1740 verify the reflexive address learned, as is done in ICE 1741 [I-D.ietf-mmusic-ice]. Other usages may use other means to prevent 1742 these attacks. 1744 15.2.1. Attack I: DDoS Against a Target 1746 In this attack, the attacker provides one or more clients with the 1747 same faked reflexive address that points to the intended target. 1748 This will trick the STUN clients into thinking that their reflexive 1749 addresses are equal to that of the target. If the clients hand out 1750 that reflexive address in order to receive traffic on it (for 1751 example, in SIP messages), the traffic will instead be sent to the 1752 target. This attack can provide substantial amplification, 1753 especially when used with clients that are using STUN to enable 1754 multimedia applications. However, it can only be launched against 1755 targets for which packets from the STUN server to the target pass 1756 through the attacker, limiting the cases in which it is possible 1758 15.2.2. Attack II: Silencing a Client 1760 In this attack, the attacker provides a STUN client with a faked 1761 reflexive address. The reflexive address it provides is a transport 1762 address that routes to nowhere. As a result, the client won't 1763 receive any of the packets it expects to receive when it hands out 1764 the reflexive address. This exploitation is not very interesting for 1765 the attacker. It impacts a single client, which is frequently not 1766 the desired target. Moreover, any attacker that can mount the attack 1767 could also deny service to the client by other means, such as 1768 preventing the client from receiving any response from the STUN 1769 server, or even a DHCP server. As with the attack in Section 15.2.1, 1770 this attack is only possible when the attacker is on path for packets 1771 sent from the STUN server towards this unused IP address. 1773 15.2.3. Attack III: Assuming the Identity of a Client 1775 This attack is similar to attack II. However, the faked reflexive 1776 address points to the attacker itself. This allows the attacker to 1777 receive traffic which was destined for the client. 1779 15.2.4. Attack IV: Eavesdropping 1781 In this attack, the attacker forces the client to use a reflexive 1782 address that routes to itself. It then forwards any packets it 1783 receives to the client. This attack would allow the attacker to 1784 observe all packets sent to the client. However, in order to launch 1785 the attack, the attacker must have already been able to observe 1786 packets from the client to the STUN server. In most cases (such as 1787 when the attack is launched from an access network), this means that 1788 the attacker could already observe packets sent to the client. This 1789 attack is, as a result, only useful for observing traffic by 1790 attackers on the path from the client to the STUN server, but not 1791 generally on the path of packets being routed towards the client. 1793 15.3. Hash Agility Plan 1795 This specification uses HMAC-SHA-1 for computation of the message 1796 integrity. If, at a later time, HMAC-SHA-1 is found to be 1797 compromised, the following is the remedy that will be applied. 1799 We will define a STUN extension which introduces a new message 1800 integrity attribute, computed using a new hash. Clients would be 1801 required to include both the new and old message integrity attributes 1802 in their requests or indications. A new server will utilize the new 1803 message integrity attribute, and an old one, the old. After a 1804 transition period where mixed implementations are in deployment, the 1805 old message-integrity attribute will be deprecated by another 1806 specification, and clients will cease including it in requests. 1808 16. IAB Considerations 1810 The IAB has studied the problem of "Unilateral Self Address Fixing" 1811 (UNSAF), which is the general process by which a client attempts to 1812 determine its address in another realm on the other side of a NAT 1813 through a collaborative protocol reflection mechanism (RFC3424 1814 [RFC3424]). STUN can be used to perform this function using a 1815 Binding Request/Response transaction if one agent is behind a NAT and 1816 the other is on the public side of the NAT. 1818 The IAB has mandated that protocols developed for this purpose 1819 document a specific set of considerations. Because some STUN usages 1820 provide UNSAF functions (such as ICE [I-D.ietf-mmusic-ice] ), and 1821 others do not (such as SIP Outbound [I-D.ietf-sip-outbound]), answers 1822 to these considerations need to be addressed by the usages 1823 themselves. 1825 17. IANA Considerations 1827 IANA is hereby requested to create three new registries: a STUN 1828 methods registry, a STUN Attributes registry, and a STUN Error Codes 1829 registry. IANA is also requested to change the name of the assigned 1830 IANA port for STUN from "nat-stun-port" to "stun". 1832 17.1. STUN Methods Registry 1834 A STUN method is a hex number in the range 0x000 - 0x3FF. The 1835 encoding of STUN method into a STUN message is described in 1836 Section 6. 1838 The initial STUN methods are: 1840 0x000: (Reserved) 1841 0x001: Binding 1842 0x002: (Reserved; was SharedSecret) 1844 STUN methods in the range 0x000 - 0x1FF are assigned by IETF 1845 Consensus [RFC2434]. STUN methods in the range 0x200 - 0x3FF are 1846 assigned on a First Come First Served basis [RFC2434] 1848 17.2. STUN Attribute Registry 1850 A STUN Attribute type is a hex number in the range 0x0000 - 0xFFFF. 1851 STUN attribute types in the range 0x0000 - 0x7FFF are considered 1852 comprehension-required; STUN attribute types in the range 0x8000 - 1853 0xFFFF are considered comprehension-optional. A STUN agent handles 1854 unknown comprehension-required and comprehension-optional attributes 1855 differently. 1857 The initial STUN Attributes types are: 1859 Comprehension-required range (0x0000-0x7FFF): 1860 0x0000: (Reserved) 1861 0x0001: MAPPED-ADDRESS 1862 0x0002: (Reserved; was RESPONSE-ADDRESS) 1863 0x0006: USERNAME 1864 0x0007: (Reserved; was PASSWORD) 1865 0x0008: MESSAGE-INTEGRITY 1866 0x0009: ERROR-CODE 1867 0x000A: UNKNOWN-ATTRIBUTES 1868 0x0014: REALM 1869 0x0015: NONCE 1870 0x0020: XOR-MAPPED-ADDRESS 1872 Comprehension-optional range (0x8000-0xFFFF) 1873 0x8022: SERVER 1874 0x8023: ALTERNATE-SERVER 1875 0x8028: FINGERPRINT 1877 STUN Attribute types in the first half of the comprehension-required 1878 range (0x0000 - 0x3FFF) and in the first half of the comprehension- 1879 optional range (0x8000 - 0xBFFF) are assigned by IETF Consensus 1880 [RFC2434]. STUN Attribute types in the second half of the 1881 comprehension-required range (0x4000 - 0x7FFF) and in the second half 1882 of the comprehension-optional range (0xC000 - 0xFFFF) are assigned on 1883 a First Come First Served basis [RFC2434]. 1885 17.3. STUN Error Code Registry 1887 A STUN Error code is a number in the range 0 - 699. STUN error codes 1888 are accompanied by a textual reason phrase in UTF-8 [RFC3629] which 1889 is intended only for human consumption and can be anything 1890 appropriate; this document proposes only suggested values. 1892 STUN error codes are consistent in codepoint assignments and 1893 semantics with SIP [RFC3261] and HTTP [RFC2616]. 1895 The initial values in this registry are given in Section 14.6. 1897 New STUN error codes are assigned on a Specification-Required basis 1898 [RFC2434]. The specification must carefully consider how clients 1899 that do not understand this error code will process it before 1900 granting the request. See the rules in Section 7.3.4. 1902 17.4. STUN UDP and TCP Port Numbers 1904 IANA has previously assigned port 3478 for STUN. This port appears 1905 in the IANA registry under the moniker "nat-stun-port". In order to 1906 align the DNS SRV procedures with the registered protocol service, 1907 IANA is requested to change the name of protocol assigned to port 1908 3478 from "nat-stun-port" to "stun", and the textual name from 1909 "Simple Traversal of UDP Through NAT (STUN)" to "Session Traversal 1910 Utilities for NAT", so that the IANA port registry would read: 1912 stun 3478/tcp Session Traversal Utilities for NAT (STUN) port 1913 stun 3478/udp Session Traversal Utilities for NAT (STUN) port 1915 18. Changes Since RFC 3489 1917 This specification obsoletes RFC3489 [RFC3489]. This specification 1918 differs from RFC3489 in the following ways: 1920 o Removed the notion that STUN is a complete NAT traversal solution. 1921 STUN is now a tool that can be used to produce a NAT traversal 1922 solution. As a consequence, changed the name of the protocol to 1923 Session Traversal Utilities for NAT. 1925 o Introduced the concept of STUN usages, and described what a usage 1926 of STUN must document. 1928 o Removed the usage of STUN for NAT type detection and binding 1929 lifetime discovery. These techniques have proven overly brittle 1930 due to wider variations in the types of NAT devices than described 1931 in this document. Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS, 1932 CHANGE-REQUEST, SOURCE-ADDRESS, and REFLECTED-FROM attributes. 1934 o Added a fixed 32-bit magic cookie and reduced length of 1935 transaction ID by 32 bits. The magic cookie begins at the same 1936 offset as the original transaction ID. 1938 o Added the XOR-MAPPED-ADDRESS attribute, which is included in 1939 Binding Responses if the magic cookie is present in the request. 1940 Otherwise the RFC3489 behavior is retained (that is, Binding 1941 Response includes MAPPED-ADDRESS). See discussion in XOR-MAPPED- 1942 ADDRESS regarding this change. 1944 o Introduced formal structure into the Message Type header field, 1945 with an explicit pair of bits for indication of request, response, 1946 error response or indication. Consequently, the message type 1947 field is split into the class (one of the previous four) and 1948 method. 1950 o Explicitly point out that the most significant two bits of STUN 1951 are 0b00, allowing easy differentiation with RTP packets when used 1952 with ICE. 1954 o Added the FINGERPRINT attribute to provide a method of definitely 1955 detecting the difference between STUN and another protocol when 1956 the two protocols are multiplexed together. 1958 o Added support for IPv6. Made it clear that an IPv4 client could 1959 get a v6 mapped address, and vice-a-versa. 1961 o Added long-term credential-based authentication. 1963 o Added the SERVER, REALM, NONCE, and ALTERNATE-SERVER attributes. 1965 o Removed the SharedSecret method, and thus the PASSWORD attribute. 1966 This method was almost never implemented and is not needed with 1967 current usages. 1969 o Removed recommendation to continue listening for STUN Responses 1970 for 10 seconds in an attempt to recognize an attack. 1972 o Changed transaction timers to be more TCP friendly. 1974 o Removed the STUN example that centered around the separation of 1975 the control and media planes. Instead, provided more information 1976 on using STUN with protocols. 1978 o Defined a generic padding mechanism that changes the 1979 interpretation of the length attribute. This would, in theory, 1980 break backwards compatibility. However, the mechanism in RFC 3489 1981 never worked for the few attributes that weren't aligned naturally 1982 on 32 bit boundaries. 1984 o REALM, SERVER, reason phrases and NONCE limited to 127 characters. 1985 USERNAME to 513 bytes. 1987 19. Contributors 1989 Christian Huitema and Joel Weinberger were original co-authors of RFC 1990 3489. 1992 20. Acknowledgements 1994 The authors would like to thank Cedric Aoun, Pete Cordell, Cullen 1995 Jennings, Bob Penfield, Xavier Marjou, Magnus Westerlund, Miguel 1996 Garcia, Bruce Lowekamp and Chris Sullivan for their comments, and 1997 Baruch Sterman and Alan Hawrylyshen for initial implementations. 1998 Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning 1999 Schulzrinne for IESG and IAB input on this work. 2001 21. References 2003 21.1. Normative References 2005 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2006 Requirement Levels", BCP 14, RFC 2119, March 1997. 2008 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 2009 September 1981. 2011 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 2012 specifying the location of services (DNS SRV)", RFC 2782, 2013 February 2000. 2015 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 2017 [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 2018 Leach, P., Luotonen, A., and L. Stewart, "HTTP 2019 Authentication: Basic and Digest Access Authentication", 2020 RFC 2617, June 1999. 2022 [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission 2023 Timer", RFC 2988, November 2000. 2025 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 2026 Hashing for Message Authentication", RFC 2104, 2027 February 1997. 2029 [ITU.V42.2002] 2030 International Telecommunications Union, "Error-correcting 2031 Procedures for DCEs Using Asynchronous-to-Synchronous 2032 Conversion", ITU-T Recommendation V.42, March 2002. 2034 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 2035 10646", STD 63, RFC 3629, November 2003. 2037 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 2038 April 1992. 2040 21.2. Informational References 2042 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 2043 A., Peterson, J., Sparks, R., Handley, M., and E. 2044 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 2045 June 2002. 2047 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 2048 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 2049 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 2051 [I-D.ietf-mmusic-ice] 2052 Rosenberg, J., "Interactive Connectivity Establishment 2053 (ICE): A Protocol for Network Address Translator (NAT) 2054 Traversal for Offer/Answer Protocols", 2055 draft-ietf-mmusic-ice-19 (work in progress), October 2007. 2057 [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, 2058 "STUN - Simple Traversal of User Datagram Protocol (UDP) 2059 Through Network Address Translators (NATs)", RFC 3489, 2060 March 2003. 2062 [I-D.ietf-behave-turn] 2063 Rosenberg, J., "Traversal Using Relays around NAT (TURN): 2064 Relay Extensions to Session Traversal Utilities for NAT 2065 (STUN)", draft-ietf-behave-turn-04 (work in progress), 2066 July 2007. 2068 [I-D.ietf-sip-outbound] 2069 Jennings, C. and R. Mahy, "Managing Client Initiated 2070 Connections in the Session Initiation Protocol (SIP)", 2071 draft-ietf-sip-outbound-10 (work in progress), July 2007. 2073 [I-D.ietf-behave-nat-behavior-discovery] 2074 MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery 2075 Using STUN", draft-ietf-behave-nat-behavior-discovery-01 2076 (work in progress), July 2007. 2078 [I-D.ietf-mmusic-ice-tcp] 2079 Rosenberg, J., "TCP Candidates with Interactive 2080 Connectivity Establishment (ICE", 2081 draft-ietf-mmusic-ice-tcp-04 (work in progress), 2082 July 2007. 2084 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2085 with Session Description Protocol (SDP)", RFC 3264, 2086 June 2002. 2088 [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral 2089 Self-Address Fixing (UNSAF) Across Network Address 2090 Translation", RFC 3424, November 2002. 2092 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 2093 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 2094 October 1998. 2096 [KARN87] Karn, P. and C. Partridge, "Improving Round-Trip Time 2097 Estimates in Reliable Transport Protocols", SIGCOMM 1987, 2098 August 1987. 2100 Appendix A. C Snippet to Determine STUN Message Types 2102 Given an 16-bit STUN message type value in host byte order in 2103 msg_type parameter, below are C macros to determine the STUN message 2104 types: 2106 #define IS_REQUEST(msg_type) (((msg_type) & 0x0110) == 0x0000) 2107 #define IS_INDICATION(msg_type) (((msg_type) & 0x0110) == 0x0010) 2108 #define IS_SUCCESS_RESP(msg_type) (((msg_type) & 0x0110) == 0x0100) 2109 #define IS_ERR_RESP(msg_type) (((msg_type) & 0x0110) == 0x0110) 2111 Authors' Addresses 2113 Jonathan Rosenberg 2114 Cisco 2115 Edison, NJ 2116 US 2118 Email: jdrosen@cisco.com 2119 URI: http://www.jdrosen.net 2121 Rohan Mahy 2122 Plantronics 2123 345 Encinal Street 2124 Santa Cruz, CA 95060 2125 US 2127 Email: rohan@ekabal.com 2128 Philip Matthews 2129 Avaya 2130 1135 Innovation Drive 2131 Ottawa, Ontario K2K 3G7 2132 Canada 2134 Phone: +1 613 592 4343 x224 2135 Fax: 2136 Email: philip_matthews@magma.ca 2137 URI: 2139 Dan Wing 2140 Cisco 2141 771 Alder Drive 2142 San Jose, CA 95035 2143 US 2145 Email: dwing@cisco.com 2147 Full Copyright Statement 2149 Copyright (C) The IETF Trust (2007). 2151 This document is subject to the rights, licenses and restrictions 2152 contained in BCP 78, and except as set forth therein, the authors 2153 retain all their rights. 2155 This document and the information contained herein are provided on an 2156 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 2157 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 2158 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 2159 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 2160 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 2161 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 2163 Intellectual Property 2165 The IETF takes no position regarding the validity or scope of any 2166 Intellectual Property Rights or other rights that might be claimed to 2167 pertain to the implementation or use of the technology described in 2168 this document or the extent to which any license under such rights 2169 might or might not be available; nor does it represent that it has 2170 made any independent effort to identify any such rights. Information 2171 on the procedures with respect to rights in RFC documents can be 2172 found in BCP 78 and BCP 79. 2174 Copies of IPR disclosures made to the IETF Secretariat and any 2175 assurances of licenses to be made available, or the result of an 2176 attempt made to obtain a general license or permission for the use of 2177 such proprietary rights by implementers or users of this 2178 specification can be obtained from the IETF on-line IPR repository at 2179 http://www.ietf.org/ipr. 2181 The IETF invites any interested party to bring to its attention any 2182 copyrights, patents or patent applications, or other proprietary 2183 rights that may cover technology that may be required to implement 2184 this standard. Please address the information to the IETF at 2185 ietf-ipr@ietf.org. 2187 Acknowledgment 2189 Funding for the RFC Editor function is provided by the IETF 2190 Administrative Support Activity (IASA).