Network Working Group J. Rosenberg
`Request for Comments: 3261 dynamicsoft
`Obsoletes: 2543 H. Schulzrinne
`Category: Standards Track Columbia U.
` G. Camarillo
` Ericsson
` A. Johnston
` WorldCom
` J. Peterson
` Neustar
` R. Sparks
` dynamicsoft
` M. Handley
` ICIR
` E. Schooler
` AT&T
` June 2002
`
` SIP: Session Initiation Protocol
`
`Status of this Memo
`
` This document specifies an Internet standards track protocol for the
` Internet community, and requests discussion and suggestions for
` improvements. Please refer to the current edition of the "Internet
` Official Protocol Standards" (STD 1) for the standardization state
` and status of this protocol. Distribution of this memo is unlimited.
`
`Copyright Notice
`
` Copyright (C) The Internet Society (2002). All Rights Reserved.
`
`Abstract
`
` This document describes Session Initiation Protocol (SIP), an
` application-layer control (signaling) protocol for creating,
` modifying, and terminating sessions with one or more participants.
` These sessions include Internet telephone calls, multimedia
` distribution, and multimedia conferences.
`
` SIP invitations used to create sessions carry session descriptions
` that allow participants to agree on a set of compatible media types.
` SIP makes use of elements called proxy servers to help route requests
` to the user’s current location, authenticate and authorize users for
` services, implement provider call-routing policies, and provide
` features to users. SIP also provides a registration function that
` allows users to upload their current locations for use by proxy
` servers. SIP runs on top of several different transport protocols.
`
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`Table of Contents
`
` 1 Introduction ........................................ 8
` 2 Overview of SIP Functionality ....................... 9
` 3 Terminology ......................................... 10
` 4 Overview of Operation ............................... 10
` 5 Structure of the Protocol ........................... 18
` 6 Definitions ......................................... 20
` 7 SIP Messages ........................................ 26
` 7.1 Requests ............................................ 27
` 7.2 Responses ........................................... 28
` 7.3 Header Fields ....................................... 29
` 7.3.1 Header Field Format ................................. 30
` 7.3.2 Header Field Classification ......................... 32
` 7.3.3 Compact Form ........................................ 32
` 7.4 Bodies .............................................. 33
` 7.4.1 Message Body Type ................................... 33
` 7.4.2 Message Body Length ................................. 33
` 7.5 Framing SIP Messages ................................ 34
` 8 General User Agent Behavior ......................... 34
` 8.1 UAC Behavior ........................................ 35
` 8.1.1 Generating the Request .............................. 35
` 8.1.1.1 Request-URI ......................................... 35
` 8.1.1.2 To .................................................. 36
` 8.1.1.3 From ................................................ 37
` 8.1.1.4 Call-ID ............................................. 37
` 8.1.1.5 CSeq ................................................ 38
` 8.1.1.6 Max-Forwards ........................................ 38
` 8.1.1.7 Via ................................................. 39
` 8.1.1.8 Contact ............................................. 40
` 8.1.1.9 Supported and Require ............................... 40
` 8.1.1.10 Additional Message Components ....................... 41
` 8.1.2 Sending the Request ................................. 41
` 8.1.3 Processing Responses ................................ 42
` 8.1.3.1 Transaction Layer Errors ............................ 42
` 8.1.3.2 Unrecognized Responses .............................. 42
` 8.1.3.3 Vias ................................................ 43
` 8.1.3.4 Processing 3xx Responses ............................ 43
` 8.1.3.5 Processing 4xx Responses ............................ 45
` 8.2 UAS Behavior ........................................ 46
` 8.2.1 Method Inspection ................................... 46
` 8.2.2 Header Inspection ................................... 46
` 8.2.2.1 To and Request-URI .................................. 46
` 8.2.2.2 Merged Requests ..................................... 47
` 8.2.2.3 Require ............................................. 47
` 8.2.3 Content Processing .................................. 48
` 8.2.4 Applying Extensions ................................. 49
` 8.2.5 Processing the Request .............................. 49
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` 8.2.6 Generating the Response ............................. 49
` 8.2.6.1 Sending a Provisional Response ...................... 49
` 8.2.6.2 Headers and Tags .................................... 50
` 8.2.7 Stateless UAS Behavior .............................. 50
` 8.3 Redirect Servers .................................... 51
` 9 Canceling a Request ................................. 53
` 9.1 Client Behavior ..................................... 53
` 9.2 Server Behavior ..................................... 55
` 10 Registrations ....................................... 56
` 10.1 Overview ............................................ 56
` 10.2 Constructing the REGISTER Request ................... 57
` 10.2.1 Adding Bindings ..................................... 59
` 10.2.1.1 Setting the Expiration Interval of Contact Addresses 60
` 10.2.1.2 Preferences among Contact Addresses ................. 61
` 10.2.2 Removing Bindings ................................... 61
` 10.2.3 Fetching Bindings ................................... 61
` 10.2.4 Refreshing Bindings ................................. 61
` 10.2.5 Setting the Internal Clock .......................... 62
` 10.2.6 Discovering a Registrar ............................. 62
` 10.2.7 Transmitting a Request .............................. 62
` 10.2.8 Error Responses ..................................... 63
` 10.3 Processing REGISTER Requests ........................ 63
` 11 Querying for Capabilities ........................... 66
` 11.1 Construction of OPTIONS Request ..................... 67
` 11.2 Processing of OPTIONS Request ....................... 68
` 12 Dialogs ............................................. 69
` 12.1 Creation of a Dialog ................................ 70
` 12.1.1 UAS behavior ........................................ 70
` 12.1.2 UAC Behavior ........................................ 71
` 12.2 Requests within a Dialog ............................ 72
` 12.2.1 UAC Behavior ........................................ 73
` 12.2.1.1 Generating the Request .............................. 73
` 12.2.1.2 Processing the Responses ............................ 75
` 12.2.2 UAS Behavior ........................................ 76
` 12.3 Termination of a Dialog ............................. 77
` 13 Initiating a Session ................................ 77
` 13.1 Overview ............................................ 77
` 13.2 UAC Processing ...................................... 78
` 13.2.1 Creating the Initial INVITE ......................... 78
` 13.2.2 Processing INVITE Responses ......................... 81
` 13.2.2.1 1xx Responses ....................................... 81
` 13.2.2.2 3xx Responses ....................................... 81
` 13.2.2.3 4xx, 5xx and 6xx Responses .......................... 81
` 13.2.2.4 2xx Responses ....................................... 82
` 13.3 UAS Processing ...................................... 83
` 13.3.1 Processing of the INVITE ............................ 83
` 13.3.1.1 Progress ............................................ 84
` 13.3.1.2 The INVITE is Redirected ............................ 84
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` 13.3.1.3 The INVITE is Rejected .............................. 85
` 13.3.1.4 The INVITE is Accepted .............................. 85
` 14 Modifying an Existing Session ....................... 86
` 14.1 UAC Behavior ........................................ 86
` 14.2 UAS Behavior ........................................ 88
` 15 Terminating a Session ............................... 89
` 15.1 Terminating a Session with a BYE Request ............ 90
` 15.1.1 UAC Behavior ........................................ 90
` 15.1.2 UAS Behavior ........................................ 91
` 16 Proxy Behavior ...................................... 91
` 16.1 Overview ............................................ 91
` 16.2 Stateful Proxy ...................................... 92
` 16.3 Request Validation .................................. 94
` 16.4 Route Information Preprocessing ..................... 96
` 16.5 Determining Request Targets ......................... 97
` 16.6 Request Forwarding .................................. 99
` 16.7 Response Processing ................................. 107
` 16.8 Processing Timer C .................................. 114
` 16.9 Handling Transport Errors ........................... 115
` 16.10 CANCEL Processing ................................... 115
` 16.11 Stateless Proxy ..................................... 116
` 16.12 Summary of Proxy Route Processing ................... 118
` 16.12.1 Examples ............................................ 118
` 16.12.1.1 Basic SIP Trapezoid ................................. 118
` 16.12.1.2 Traversing a Strict-Routing Proxy ................... 120
` 16.12.1.3 Rewriting Record-Route Header Field Values .......... 121
` 17 Transactions ........................................ 122
` 17.1 Client Transaction .................................. 124
` 17.1.1 INVITE Client Transaction ........................... 125
` 17.1.1.1 Overview of INVITE Transaction ...................... 125
` 17.1.1.2 Formal Description .................................. 125
` 17.1.1.3 Construction of the ACK Request ..................... 129
` 17.1.2 Non-INVITE Client Transaction ....................... 130
` 17.1.2.1 Overview of the non-INVITE Transaction .............. 130
` 17.1.2.2 Formal Description .................................. 131
` 17.1.3 Matching Responses to Client Transactions ........... 132
` 17.1.4 Handling Transport Errors ........................... 133
` 17.2 Server Transaction .................................. 134
` 17.2.1 INVITE Server Transaction ........................... 134
` 17.2.2 Non-INVITE Server Transaction ....................... 137
` 17.2.3 Matching Requests to Server Transactions ............ 138
` 17.2.4 Handling Transport Errors ........................... 141
` 18 Transport ........................................... 141
` 18.1 Clients ............................................. 142
` 18.1.1 Sending Requests .................................... 142
` 18.1.2 Receiving Responses ................................. 144
` 18.2 Servers ............................................. 145
` 18.2.1 Receiving Requests .................................. 145
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` 18.2.2 Sending Responses ................................... 146
` 18.3 Framing ............................................. 147
` 18.4 Error Handling ...................................... 147
` 19 Common Message Components ........................... 147
` 19.1 SIP and SIPS Uniform Resource Indicators ............ 148
` 19.1.1 SIP and SIPS URI Components ......................... 148
` 19.1.2 Character Escaping Requirements ..................... 152
` 19.1.3 Example SIP and SIPS URIs ........................... 153
` 19.1.4 URI Comparison ...................................... 153
` 19.1.5 Forming Requests from a URI ......................... 156
` 19.1.6 Relating SIP URIs and tel URLs ...................... 157
` 19.2 Option Tags ......................................... 158
` 19.3 Tags ................................................ 159
` 20 Header Fields ....................................... 159
` 20.1 Accept .............................................. 161
` 20.2 Accept-Encoding ..................................... 163
` 20.3 Accept-Language ..................................... 164
` 20.4 Alert-Info .......................................... 164
` 20.5 Allow ............................................... 165
` 20.6 Authentication-Info ................................. 165
` 20.7 Authorization ....................................... 165
` 20.8 Call-ID ............................................. 166
` 20.9 Call-Info ........................................... 166
` 20.10 Contact ............................................. 167
` 20.11 Content-Disposition ................................. 168
` 20.12 Content-Encoding .................................... 169
` 20.13 Content-Language .................................... 169
` 20.14 Content-Length ...................................... 169
` 20.15 Content-Type ........................................ 170
` 20.16 CSeq ................................................ 170
` 20.17 Date ................................................ 170
` 20.18 Error-Info .......................................... 171
` 20.19 Expires ............................................. 171
` 20.20 From ................................................ 172
` 20.21 In-Reply-To ......................................... 172
` 20.22 Max-Forwards ........................................ 173
` 20.23 Min-Expires ......................................... 173
` 20.24 MIME-Version ........................................ 173
` 20.25 Organization ........................................ 174
` 20.26 Priority ............................................ 174
` 20.27 Proxy-Authenticate .................................. 174
` 20.28 Proxy-Authorization ................................. 175
` 20.29 Proxy-Require ....................................... 175
` 20.30 Record-Route ........................................ 175
` 20.31 Reply-To ............................................ 176
` 20.32 Require ............................................. 176
` 20.33 Retry-After ......................................... 176
` 20.34 Route ............................................... 177
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` 20.35 Server .............................................. 177
` 20.36 Subject ............................................. 177
` 20.37 Supported ........................................... 178
` 20.38 Timestamp ........................................... 178
` 20.39 To .................................................. 178
` 20.40 Unsupported ......................................... 179
` 20.41 User-Agent .......................................... 179
` 20.42 Via ................................................. 179
` 20.43 Warning ............................................. 180
` 20.44 WWW-Authenticate .................................... 182
` 21 Response Codes ...................................... 182
` 21.1 Provisional 1xx ..................................... 182
` 21.1.1 100 Trying .......................................... 183
` 21.1.2 180 Ringing ......................................... 183
` 21.1.3 181 Call Is Being Forwarded ......................... 183
` 21.1.4 182 Queued .......................................... 183
` 21.1.5 183 Session Progress ................................ 183
` 21.2 Successful 2xx ...................................... 183
` 21.2.1 200 OK .............................................. 183
` 21.3 Redirection 3xx ..................................... 184
` 21.3.1 300 Multiple Choices ................................ 184
` 21.3.2 301 Moved Permanently ............................... 184
` 21.3.3 302 Moved Temporarily ............................... 184
` 21.3.4 305 Use Proxy ....................................... 185
` 21.3.5 380 Alternative Service ............................. 185
` 21.4 Request Failure 4xx ................................. 185
` 21.4.1 400 Bad Request ..................................... 185
` 21.4.2 401 Unauthorized .................................... 185
` 21.4.3 402 Payment Required ................................ 186
` 21.4.4 403 Forbidden ....................................... 186
` 21.4.5 404 Not Found ....................................... 186
` 21.4.6 405 Method Not Allowed .............................. 186
` 21.4.7 406 Not Acceptable .................................. 186
` 21.4.8 407 Proxy Authentication Required ................... 186
` 21.4.9 408 Request Timeout ................................. 186
` 21.4.10 410 Gone ............................................ 187
` 21.4.11 413 Request Entity Too Large ........................ 187
` 21.4.12 414 Request-URI Too Long ............................ 187
` 21.4.13 415 Unsupported Media Type .......................... 187
` 21.4.14 416 Unsupported URI Scheme .......................... 187
` 21.4.15 420 Bad Extension ................................... 187
` 21.4.16 421 Extension Required .............................. 188
` 21.4.17 423 Interval Too Brief .............................. 188
` 21.4.18 480 Temporarily Unavailable ......................... 188
` 21.4.19 481 Call/Transaction Does Not Exist ................. 188
` 21.4.20 482 Loop Detected ................................... 188
` 21.4.21 483 Too Many Hops ................................... 189
` 21.4.22 484 Address Incomplete .............................. 189
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` 21.4.23 485 Ambiguous ....................................... 189
` 21.4.24 486 Busy Here ....................................... 189
` 21.4.25 487 Request Terminated .............................. 190
` 21.4.26 488 Not Acceptable Here ............................. 190
` 21.4.27 491 Request Pending ................................. 190
` 21.4.28 493 Undecipherable .................................. 190
` 21.5 Server Failure 5xx .................................. 190
` 21.5.1 500 Server Internal Error ........................... 190
` 21.5.2 501 Not Implemented ................................. 191
` 21.5.3 502 Bad Gateway ..................................... 191
` 21.5.4 503 Service Unavailable ............................. 191
` 21.5.5 504 Server Time-out ................................. 191
` 21.5.6 505 Version Not Supported ........................... 192
` 21.5.7 513 Message Too Large ............................... 192
` 21.6 Global Failures 6xx ................................. 192
` 21.6.1 600 Busy Everywhere ................................. 192
` 21.6.2 603 Decline ......................................... 192
` 21.6.3 604 Does Not Exist Anywhere ......................... 192
` 21.6.4 606 Not Acceptable .................................. 192
` 22 Usage of HTTP Authentication ........................ 193
` 22.1 Framework ........................................... 193
` 22.2 User-to-User Authentication ......................... 195
` 22.3 Proxy-to-User Authentication ........................ 197
` 22.4 The Digest Authentication Scheme .................... 199
` 23 S/MIME .............................................. 201
` 23.1 S/MIME Certificates ................................. 201
` 23.2 S/MIME Key Exchange ................................. 202
` 23.3 Securing MIME bodies ................................ 205
` 23.4 SIP Header Privacy and Integrity using S/MIME:
` Tunneling SIP ....................................... 207
` 23.4.1 Integrity and Confidentiality Properties of SIP
` Headers ............................................. 207
` 23.4.1.1 Integrity ........................................... 207
` 23.4.1.2 Confidentiality ..................................... 208
` 23.4.2 Tunneling Integrity and Authentication .............. 209
` 23.4.3 Tunneling Encryption ................................ 211
` 24 Examples ............................................ 213
` 24.1 Registration ........................................ 213
` 24.2 Session Setup ....................................... 214
` 25 Augmented BNF for the SIP Protocol .................. 219
` 25.1 Basic Rules ......................................... 219
` 26 Security Considerations: Threat Model and Security
` Usage Recommendations ............................... 232
` 26.1 Attacks and Threat Models ........................... 233
` 26.1.1 Registration Hijacking .............................. 233
` 26.1.2 Impersonating a Server .............................. 234
` 26.1.3 Tampering with Message Bodies ....................... 235
` 26.1.4 Tearing Down Sessions ............................... 235
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` 26.1.5 Denial of Service and Amplification ................. 236
` 26.2 Security Mechanisms ................................. 237
` 26.2.1 Transport and Network Layer Security ................ 238
` 26.2.2 SIPS URI Scheme ..................................... 239
` 26.2.3 HTTP Authentication ................................. 240
` 26.2.4 S/MIME .............................................. 240
` 26.3 Implementing Security Mechanisms .................... 241
` 26.3.1 Requirements for Implementers of SIP ................ 241
` 26.3.2 Security Solutions .................................. 242
` 26.3.2.1 Registration ........................................ 242
` 26.3.2.2 Interdomain Requests ................................ 243
` 26.3.2.3 Peer-to-Peer Requests ............................... 245
` 26.3.2.4 DoS Protection ...................................... 246
` 26.4 Limitations ......................................... 247
` 26.4.1 HTTP Digest ......................................... 247
` 26.4.2 S/MIME .............................................. 248
` 26.4.3 TLS ................................................. 249
` 26.4.4 SIPS URIs ........................................... 249
` 26.5 Privacy ............................................. 251
` 27 IANA Considerations ................................. 252
` 27.1 Option Tags ......................................... 252
` 27.2 Warn-Codes .......................................... 252
` 27.3 Header Field Names .................................. 253
` 27.4 Method and Response Codes ........................... 253
` 27.5 The "message/sip" MIME type. ....................... 254
` 27.6 New Content-Disposition Parameter Registrations ..... 255
` 28 Changes From RFC 2543 ............................... 255
` 28.1 Major Functional Changes ............................ 255
` 28.2 Minor Functional Changes ............................ 260
` 29 Normative References ................................ 261
` 30 Informative References .............................. 262
` A Table of Timer Values ............................... 265
` Acknowledgments ................................................ 266
` Authors’ Addresses ............................................. 267
` Full Copyright Statement ....................................... 269
`
`1 Introduction
`
` There are many applications of the Internet that require the creation
` and management of a session, where a session is considered an
` exchange of data between an association of participants. The
` implementation of these applications is complicated by the practices
` of participants: users may move between endpoints, they may be
` addressable by multiple names, and they may communicate in several
` different media - sometimes simultaneously. Numerous protocols have
` been authored that carry various forms of real-time multimedia
` session data such as voice, video, or text messages. The Session
` Initiation Protocol (SIP) works in concert with these protocols by
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` enabling Internet endpoints (called user agents) to discover one
` another and to agree on a characterization of a session they would
` like to share. For locating prospective session participants, and
` for other functions, SIP enables the creation of an infrastructure of
` network hosts (called proxy servers) to which user agents can send
` registrations, invitations to sessions, and other requests. SIP is
` an agile, general-purpose tool for creating, modifying, and
` terminating sessions that works independently of underlying transport
` protocols and without dependency on the type of session that is being
` established.
`
`2 Overview of SIP Functionality
`
` SIP is an application-layer control protocol that can establish,
` modify, and terminate multimedia sessions (conferences) such as
` Internet telephony calls. SIP can also invite participants to
` already existing sessions, such as multicast conferences. Media can
` be added to (and removed from) an existing session. SIP
` transparently supports name mapping and redirection services, which
` supports personal mobility [27] - users can maintain a single
` externally visible identifier regardless of their network location.
`
` SIP supports five facets of establishing and terminating multimedia
` communications:
`
` User location: determination of the end system to be used for
` communication;
`
` User availability: determination of the willingness of the called
` party to engage in communications;
`
` User capabilities: determination of the media and media parameters
` to be used;
`
` Session setup: "ringing", establishment of session parameters at
` both called and calling party;
`
` Session management: including transfer and termination of
` sessions, modifying session parameters, and invoking
` services.
`
` SIP is not a vertically integrated communications system. SIP is
` rather a component that can be used with other IETF protocols to
` build a complete multimedia architecture. Typically, these
` architectures will include protocols such as the Real-time Transport
` Protocol (RTP) (RFC 1889 [28]) for transporting real-time data and
` providing QoS feedback, the Real-Time streaming protocol (RTSP) (RFC
` 2326 [29]) for controlling delivery of streaming media, the Media
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` Gateway Control Protocol (MEGACO) (RFC 3015 [30]) for controlling
` gateways to the Public Switched Telephone Network (PSTN), and the
` Session Description Protocol (SDP) (RFC 2327 [1]) for describing
` multimedia sessions. Therefore, SIP should be used in conjunction
` with other protocols in order to provide complete services to the
` users. However, the basic functionality and operation of SIP does
` not depend on any of these protocols.
`
` SIP does not provide services. Rather, SIP provides primitives that
` can be used to implement different services. For example, SIP can
` locate a user and deliver an opaque object to his current location.
` If this primitive is used to deliver a session description written in
` SDP, for instance, the endpoints can agree on the parameters of a
` session. If the same primitive is used to deliver a photo of the
` caller as well as the session description, a "caller ID" service can
` be easily implemented. As this example shows, a single primitive is
` typically used to provide several different services.
`
` SIP does not offer conference control services such as floor control
` or voting and does not prescribe how a conference is to be managed.
` SIP can be used to initiate a session that uses some other conference
` control protocol. Since SIP messages and the sessions they establish
` can pass through entirely different networks, SIP cannot, and does
` not, provide any kind of network resource reservation capabilities.
`
` The nature of the services provided make security particularly
` important. To that end, SIP provides a suite of security services,
` which include denial-of-service prevention, authentication (both user
`