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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RFC 3261 SIP: Session Initiation Protocol June 2002
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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RFC 3261 SIP: Session Initiation Protocol June 2002
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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RFC 3261 SIP: Session Initiation Protocol June 2002
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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RFC 3261 SIP: Session Initiation Protocol June 2002
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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RFC 3261 SIP: Session Initiation Protocol June 2002
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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RFC 3261 SIP: Session Initiation Protocol June 2002
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
to user and proxy to user), integrity protection, and encryption and
privacy services.
SIP works with both IPv4 and IPv6.
3 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [2] and indicate requirement levels for
compliant SIP implementations.
4 Overview of Operation
This section introduces the basic operations of SIP using simple
examples. This section is tutorial in nature and does not contain
any normative statements.
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RFC 3261 SIP: Session Initiation Protocol June 2002
The first example shows the basic functions of SIP: location of an
end point, signal of a desire to communicate, negotiation of session
parameters to establish the session, and teardown of the session once
established.
Figure 1 shows a typical example of a SIP message exchange between
two users, Alice and Bob. (Each message is labeled with the letter
"F" and a number for reference by the text.) In this example, Alice
uses a SIP application on her PC (referred to as a softphone) to call
Bob on his SIP phone over the Internet. Also shown are two SIP proxy
servers that act on behalf of Alice and Bob to facilitate the session
establishment. This typical arrangement is often referred to as the
"SIP trapezoid" as shown by the geometric shape of the dotted lines
in Figure 1.
Alice "calls" Bob using his SIP identity, a type of Uniform Resource
Identifier (URI) called a SIP URI. SIP URIs are defined in Section
19.1. It has a similar form to an email address, typically
containing a username and a host name. In this case, it is
sip:[email protected], where biloxi.com is the domain of Bob's SIP
service provider. Alice has a SIP URI of sip:[email protected].
Alice might have typed in Bob's URI or perhaps clicked on a hyperlink
or an entry in an address book. SIP also provides a secure URI,
called a SIPS URI. An example would be sips:[email protected]. A call
made to a SIPS URI guarantees that secure, encrypted transport
(namely TLS) is used to carry all SIP messages from the caller to the
domain of the callee. From there, the request is sent securely to
the callee, but with security mechanisms that depend on the policy of
the domain of the callee.
SIP is based on an HTTP-like request/response transaction model.
Each transaction consists of a request that invokes a particular
method, or function, on the server and at least one response. In
this example, the transaction begins with Alice's softphone sending
an INVITE request addressed to Bob's SIP URI. INVITE is an example
of a SIP method that specifies the action that the requestor (Alice)
wants the server (Bob) to take. The INVITE request contains a number
of header fields. Header fields are named attributes that provide
additional information about a message. The ones present in an
INVITE include a unique identifier for the call, the destination
address, Alice's address, and information about the type of session
that Alice wishes to establish with Bob. The INVITE (message F1 in
Figure 1) might look like this:
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RFC 3261 SIP: Session Initiation Protocol June 2002
atlanta.com . . . biloxi.com
. proxy proxy .
. .
Alice's . . . . . . . . . . . . . . . . . . . . Bob's
softphone SIP Phone
| | | |
| INVITE F1 | | |
|--------------->| INVITE F2 | |
| 100 Trying F3 |--------------->| INVITE F4 |
|<---------------| 100 Trying F5 |--------------->|
| |<-------------- | 180 Ringing F6 |
| | 180 Ringing F7 |<---------------|
| 180 Ringing F8 |<---------------| 200 OK F9 |
|<---------------| 200 OK F10 |<---------------|
| 200 OK F11 |<---------------| |
|<---------------| | |
| ACK F12 |
|------------------------------------------------->|
| Media Session |
|<================================================>|
| BYE F13 |
|<-------------------------------------------------|
| 200 OK F14 |
|------------------------------------------------->|
| |
Figure 1: SIP session setup example with SIP trapezoid
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
Max-Forwards: 70
To: Bob
From: Alice ;tag=1928301774
Call-ID: [email protected]
CSeq: 314159 INVITE
Contact:
Content-Type: application/sdp
Content-Length: 142
(Alice's SDP not shown)
The first line of the text-encoded message contains the method name
(INVITE). The lines that follow are a list of header fields. This
example contains a minimum required set. The header fields are
briefly described below:
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RFC 3261 SIP: Session Initiation Protocol June 2002
Via contains the address (pc33.atlanta.com) at which Alice is
expecting to receive responses to this request. It also contains a
branch parameter that identifies this transaction.
To contains a display name (Bob) and a SIP or SIPS URI
(sip:[email protected]) towards which the request was originally
directed. Display names are described in RFC 2822 [3].
From also contains a display name (Alice) and a SIP or SIPS URI
(sip:[email protected]) that indicate the originator of the request.
This header field also has a tag parameter containing a random string
(1928301774) that was added to the URI by the softphone. It is used
for identification purposes.
Call-ID contains a globally unique identifier for this call,
generated by the combination of a random string and the softphone's
host name or IP address. The combination of the To tag, From tag,
and Call-ID completely defines a peer-to-peer SIP relationship
between Alice and Bob and is referred to as a dialog.
CSeq or Command Sequence contains an integer and a method name. The
CSeq number is incremented for each new request within a dialog and
is a traditional sequence number.
Contact contains a SIP or SIPS URI that represents a direct route to
contact Alice, usually composed of a username at a fully qualified
domain name (FQDN). While an FQDN is preferred, many end systems do
not have registered domain names, so IP addresses are permitted.
While the Via header field tells other elements where to send the
response, the Contact header field tells other elements where to send
future requests.
Max-Forwards serves to limit the number of hops a request can make on
the way to its destination. It consists of an integer that is
decremented by one at each hop.
Content-Type contains a description of the message body (not shown).
Content-Length contains an octet (byte) count of the message body.
The complete set of SIP header fields is defined in Section 20.
The details of the session, such as the type of media, codec, or
sampling rate, are not described using SIP. Rather, the body of a
SIP message contains a description of the session, encoded in some
other protocol format. One such format is the Session Description
Protocol (SDP) (RFC 2327 [1]). This SDP message (not shown in the
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example) is carried by the SIP message in a way that is analogous to
a document attachment being carried by an email message, or a web
page being carried in an HTTP message.
Since the softphone does not know the location of Bob or the SIP
server in the biloxi.com domain, the softphone sends the INVITE to
the SIP server that serves Alice's domain, atlanta.com. The address
of the atlanta.com SIP server could have been configured in Alice's
softphone, or it could have been discovered by DHCP, for example.
The atlanta.com SIP server is a type of SIP server known as a proxy
server. A proxy server receives SIP requests and forwards them on
behalf of the requestor. In this example, the proxy server receives
the INVITE request and sends a 100 (Trying) response back to Alice's
softphone. The 100 (Trying) response indicates that the INVITE has
been received and that the proxy is working on her behalf to route
the INVITE to the destination. Responses in SIP use a three-digit
code followed by a descriptive phrase. This response contains the
same To, From, Call-ID, CSeq and branch parameter in the Via as the
INVITE, which allows Alice's softphone to correlate this response to
the sent INVITE. The atlanta.com proxy server locates the proxy
server at biloxi.com, possibly by performing a particular type of DNS
(Domain Name Service) lookup to find the SIP server that serves the
biloxi.com domain. This is described in [4]. As a result, it
obtains the IP address of the biloxi.com proxy server and forwards,
or proxies, the INVITE request there. Before forwarding the request,
the atlanta.com proxy server adds an additional Via header field
value that contains its own address (the INVITE already contains
Alice's address in the first Via). The biloxi.com proxy server
receives the INVITE and responds with a 100 (Trying) response back to
the atlanta.com proxy server to indicate that it has received the
INVITE and is processing the request. The proxy server consults a
database, generically called a location service, that contains the
current IP address of Bob. (We shall see in the next section how
this database can be populated.) The biloxi.com proxy server adds
another Via header field value with its own address to the INVITE and
proxies it to Bob's SIP phone.
Bob's SIP phone receives the INVITE and alerts Bob to the incoming
call from Alice so that Bob can decide whether to answer the call,
that is, Bob's phone rings. Bob's SIP phone indicates this in a 180
(Ringing) response, which is routed back through the two proxies in
the reverse direction. Each proxy uses the Via header field to
determine where to send the response and removes its own address from
the top. As a result, although DNS and location service lookups were
required to route the initial INVITE, the 180 (Ringing) response can
be returned to the caller without lookups or without state being
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maintained in the proxies. This also has the desirable property that
each proxy that sees the INVITE will also see all responses to the
INVITE.
When Alice's softphone receives the 180 (Ringing) response, it passes
this information to Alice, perhaps using an audio ringback tone or by
displaying a message on Alice's screen.
In this example, Bob decides to answer the call. When he picks up
the handset, his SIP phone sends a 200 (OK) response to indicate that
the call has been answered. The 200 (OK) contains a message body
with the SDP media description of the type of session that Bob is
willing to establish with Alice. As a result, there is a two-phase
exchange of SDP messages: Alice sent one to Bob, and Bob sent one
back to Alice. This two-phase exchange provides basic negotiation
capabilities and is based on a simple offer/answer model of SDP
exchange. If Bob did not wish to answer the call or was busy on
another call, an error response would have been sent instead of the
200 (OK), which would have resulted in no media session being
established. The complete list of SIP response codes is in Section
21. The 200 (OK) (message F9 in Figure 1) might look like this as
Bob sends it out:
SIP/2.0 200 OK
Via: SIP/2.0/UDP server10.biloxi.com
;branch=z9hG4bKnashds8;received=192.0.2.3
Via: SIP/2.0/UDP bigbox3.site3.atlanta.com
;branch=z9hG4bK77ef4c2312983.1;received=192.0.2.2
Via: SIP/2.0/UDP pc33.atlanta.com
;branch=z9hG4bK776asdhds ;received=192.0.2.1
To: Bob ;tag=a6c85cf
From: Alice ;tag=1928301774
Call-ID: [email protected]
CSeq: 314159 INVITE
Contact:
Content-Type: application/sdp
Content-Length: 131
(Bob's SDP not shown)
The first line of the response contains the response code (200) and
the reason phrase (OK). The remaining lines contain header fields.
The Via, To, From, Call-ID, and CSeq header fields are copied from
the INVITE request. (There are three Via header field values - one
added by Alice's SIP phone, one added by the atlanta.com proxy, and
one added by the biloxi.com proxy.) Bob's SIP phone has added a tag
parameter to the To header field. This tag will be incorporated by
both endpoints into the dialog and will be included in all future
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requests and responses in this call. The Contact header field
contains a URI at which Bob can be directly reached at his SIP phone.
The Content-Type and Content-Length refer to the message body (not
shown) that contains Bob's SDP media information.
In addition to DNS and location service lookups shown in this
example, proxy servers can make flexible "routing decisions" to
decide where to send a request. For example, if Bob's SIP phone
returned a 486 (Busy Here) response, the biloxi.com proxy server
could proxy the INVITE to Bob's voicemail server. A proxy server can
also send an INVITE to a number of locations at the same time. This
type of parallel search is known as forking.
In this case, the 200 (OK) is routed back through the two proxies and
is received by Alice's softphone, which then stops the ringback tone
and indicates that the call has been answered. Finally, Alice's
softphone sends an acknowledgement message, ACK, to Bob's SIP phone
to confirm the reception of the final response (200 (OK)). In this
example, the ACK is sent directly from Alice's softphone to Bob's SIP
phone, bypassing the two proxies. This occurs because the endpoints
have learned each other's address from the Contact header fields
through the INVITE/200 (OK) exchange, which was not known when the
initial INVITE was sent. The lookups performed by the two proxies
are no longer needed, so the proxies drop out of the call flow. This
completes the INVITE/200/ACK three-way handshake used to establish
SIP sessions. Full details on session setup are in Section 13.
Alice and Bob's media session has now begun, and they send media
packets using the format to which they agreed in the exchange of SDP.
In general, the end-to-end media packets take a different path from
the SIP signaling messages.
During the session, either Alice or Bob may decide to change the
characteristics of the media session. This is accomplished by
sending a re-INVITE containing a new media description. This re-
INVITE references the existing dialog so that the other party knows
that it is to modify an existing session instead of establishing a
new session. The other party sends a 200 (OK) to accept the change.
The requestor responds to the 200 (OK) with an ACK. If the other
party does not accept the change, he sends an error response such as
488 (Not Acceptable Here), which also receives an ACK. However, the
failure of the re-INVITE does not cause the existing call to fail -
the session continues using the previously negotiated
characteristics. Full details on session modification are in Section
14.
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At the end of the call, Bob disconnects (hangs up) first and
generates a BYE message. This BYE is routed directly to Alice's
softphone, again bypassing the proxies. Alice confirms receipt of
the BYE with a 200 (OK) response, which terminates the session and
the BYE transaction. No ACK is sent - an ACK is only sent in
response to a response to an INVITE request. The reasons for this
special handling for INVITE will be discussed later, but relate to
the reliability mechanisms in SIP, the length of time it can take for
a ringing phone to be answered, and forking. For this reason,
request handling in SIP is often classified as either INVITE or non-
INVITE, referring to all other methods besides INVITE. Full details
on session termination are in Section 15.
Section 24.2 describes the messages shown in Figure 1 in full.
In some cases, it may be useful for proxies in the SIP signaling path
to see all the messaging between the endpoints for the duration of
the session. For example, if the biloxi.com proxy server wished to
remain in the SIP messaging path beyond the initial INVITE, it would
add to the INVITE a required routing header field known as Record-
Route that contained a URI resolving to the hostname or IP address of
the proxy. This information would be received by both Bob's SIP
phone and (due to the Record-Route header field being passed back in
the 200 (OK)) Alice's softphone and stored for the duration of the
dialog. The biloxi.com proxy server would then receive and proxy the
ACK, BYE, and 200 (OK) to the BYE. Each proxy can independently
decide to receive subsequent messages, and those messages will pass
through all proxies that elect to receive it. This capability is
frequently used for proxies that are providing mid-call features.
Registration is another common operation in SIP. Registration is one
way that the biloxi.com server can learn the current location of Bob.
Upon initialization, and at periodic intervals, Bob's SIP phone sends
REGISTER messages to a server in the biloxi.com domain known as a SIP
registrar. The REGISTER messages associate Bob's SIP or SIPS URI
(sip:[email protected]) with the machine into which he is currently
logged (conveyed as a SIP or SIPS URI in the Contact header field).
The registrar writes this association, also called a binding, to a
database, called the location service, where it can be used by the
proxy in the biloxi.com domain. Often, a registrar server for a
domain is co-located with the proxy for that domain. It is an
important concept that the distinction between types of SIP servers
is logical, not physical.
Bob is not limited to registering from a single device. For example,
both his SIP phone at home and the one in the office could send
registrations. This information is stored together in the location
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service and allows a proxy to perform various types of searches to
locate Bob. Similarly, more than one user can be registered on a
single device at the same time.
The location service is just an abstract concept. It generally
contains information that allows a proxy to input a URI and receive a
set of zero or more URIs that tell the proxy where to send the
request. Registrations are one way to create this information, but
not the only way. Arbitrary mapping functions can be configured at
the discretion of the administrator.
Finally, it is important to note that in SIP, registration is used
for routing incoming SIP requests and has no role in authorizing
outgoing requests. Authorization and authentication are handled in
SIP either on a request-by-request basis with a challenge/response
mechanism, or by using a lower layer scheme as discussed in Section
26.
The complete set of SIP message details for this registration example
is in Section 24.1.
Additional operations in SIP, such as querying for the capabilities
of a SIP server or client using OPTIONS, or canceling a pending
request using CANCEL, will be introduced in later sections.
5 Structure of the Protocol
SIP is structured as a layered protocol, which means that its
behavior is described in terms of a set of fairly independent
processing stages with only a loose coupling between each stage. The
protocol behavior is described as layers for the purpose of
presentation, allowing the description of functions common across
elements in a single section. It does not dictate an implementation
in any way. When we say that an element "contains" a layer, we mean
it is compliant to the set of rules defined by that layer.
Not every element specified by the protocol contains every layer.
Furthermore, the elements specified by SIP are logical elements, not
physical ones. A physical realization can choose to act as different
logical elements, perhaps even on a transaction-by-transaction basis.
The lowest layer of SIP is its syntax and encoding. Its encoding is
specified using an augmented Backus-Naur Form grammar (BNF). The
complete BNF is specified in Section 25; an overview of a SIP
message's structure can be found in Section 7.
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The second layer is the transport layer. It defines how a client
sends requests and receives responses and how a server receives
requests and sends responses over the network. All SIP elements
contain a transport layer. The transport layer is described in
Section 18.
The third layer is the transaction layer. Transactions are a
fundamental component of SIP. A transaction is a request sent by a
client transaction (using the transport layer) to a server
transaction, along with all responses to that request sent from the
server transaction back to the client. The transaction layer handles
application-layer retransmissions, matching of responses to requests,
and application-layer timeouts. Any task that a user agent client
(UAC) accomplishes takes place using a series of transactions.
Discussion of transactions can be found in Section 17. User agents
contain a transaction layer, as do stateful proxies. Stateless
proxies do not contain a transaction layer. The transaction layer
has a client component (referred to as a client transaction) and a
server component (referred to as a server transaction), each of which
are represented by a finite state machine that is constructed to
process a particular request.
The layer above the transaction layer is called the transaction user
(TU). Each of the SIP entities, except the stateless proxy, is a
transaction user. When a TU wishes to send a request, it creates a
client transaction instance and passes it the request along with the
destination IP address, port, and transport to which to send the
request. A TU that creates a client transaction can also cancel it.
When a client cancels a transaction, it requests that the server stop
further processing, revert to the state that existed before the
transaction was initiated, and generate a specific error response to
that transaction. This is done with a CANCEL request, which
constitutes its own transaction, but references the transaction to be
cancelled (Section 9).
The SIP elements, that is, user agent clients and servers, stateless
and stateful proxies and registrars, contain a core that
distinguishes them from each other. Cores, except for the stateless
proxy, are transaction users. While the behavior of the UAC and UAS
cores depends on the method, there are some common rules for all
methods (Section 8). For a UAC, these rules govern the construction
of a request; for a UAS, they govern the processing of a request and
generating a response. Since registrations play an important role in
SIP, a UAS that handles a REGISTER is given the special name
registrar. Section 10 describes UAC and UAS core behavior for the
REGISTER method. Section 11 describes UAC and UAS core behavior for
the OPTIONS method, used for determining the capabilities of a UA.
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Certain other requests are sent within a dialog. A dialog is a
peer-to-peer SIP relationship between two user agents that persists
for some time. The dialog facilitates sequencing of messages and
proper routing of requests between the user agents. The INVITE
method is the only way defined in this specification to establish a
dialog. When a UAC sends a request that is within the context of a
dialog, it follows the common UAC rules as discussed in Section 8 but
also the rules for mid-dialog requests. Section 12 discusses dialogs
and presents the procedures for their construction and maintenance,
in addition to construction of requests within a dialog.
The most important method in SIP is the INVITE method, which is used
to establish a session between participants. A session is a
collection of participants, and streams of media between them, for
the purposes of communication. Section 13 discusses how sessions are
initiated, resulting in one or more SIP dialogs. Section 14
discusses how characteristics of that session are modified through
the use of an INVITE request within a dialog. Finally, section 15
discusses how a session is terminated.
The procedures of Sections 8, 10, 11, 12, 13, 14, and 15 deal
entirely with the UA core (Section 9 describes cancellation, which
applies to both UA core and proxy core). Section 16 discusses the
proxy element, which facilitates routing of messages between user
agents.
6 Definitions
The following terms have special significance for SIP.
Address-of-Record: An address-of-record (AOR) is a SIP or SIPS URI
that points to a domain with a location service that can map
the URI to another URI where the user might be available.
Typically, the location service is populated through
registrations. An AOR is frequently thought of as the "public
address" of the user.
Back-to-Back User Agent: A back-to-back user agent (B2BUA) is a
logical entity that receives a request and processes it as a
user agent server (UAS). In order to determine how the request
should be answered, it acts as a user agent client (UAC) and
generates requests. Unlike a proxy server, it maintains dialog
state and must participate in all requests sent on the dialogs
it has established. Since it is a concatenation of a UAC and
UAS, no explicit definitions are needed for its behavior.
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Call: A call is an informal term that refers to some communication
between peers, generally set up for the purposes of a
multimedia conversation.
Call Leg: Another name for a dialog [31]; no longer used in this
specification.
Call Stateful: A proxy is call stateful if it retains state for a
dialog from the initiating INVITE to the terminating BYE
request. A call stateful proxy is always transaction stateful,
but the converse is not necessarily true.
Client: A client is any network element that sends SIP requests
and receives SIP responses. Clients may or may not interact
directly with a human user. User agent clients and proxies are
clients.
Conference: A multimedia session (see below) that contains
multiple participants.
Core: Core designates the functions specific to a particular type
of SIP entity, i.e., specific to either a stateful or stateless
proxy, a user agent or registrar. All cores, except those for
the stateless proxy, are transaction users.
Dialog: A dialog is a peer-to-peer SIP relationship between two
UAs that persists for some time. A dialog is established by
SIP messages, such as a 2xx response to an INVITE request. A
dialog is identified by a call identifier, local tag, and a
remote tag. A dialog was formerly known as a call leg in RFC
2543.
Downstream: A direction of message forwarding within a transaction
that refers to the direction that requests flow from the user
agent client to user agent server.
Final Response: A response that terminates a SIP transaction, as
opposed to a provisional response that does not. All 2xx, 3xx,
4xx, 5xx and 6xx responses are final.
Header: A header is a component of a SIP message that conveys
information about the message. It is structured as a sequence
of header fields.
Header Field: A header field is a component of the SIP message
header. A header field can appear as one or more header field
rows. Header field rows consist of a header field name and zero
or more header field values. Multiple header field values on a
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RFC 3261 SIP: Session Initiation Protocol June 2002
given header field row are separated by commas. Some header
fields can only have a single header field value, and as a
result, always appear as a single header field row.
Header Field Value: A header field value is a single value; a
header field consists of zero or more header field values.
Home Domain: The domain providing service to a SIP user.
Typically, this is the domain present in the URI in the
address-of-record of a registration.
Informational Response: Same as a provisional response.
Initiator, Calling Party, Caller: The party initiating a session
(and dialog) with an INVITE request. A caller retains this
role from the time it sends the initial INVITE that established
a dialog until the termination of that dialog.
Invitation: An INVITE request.
Invitee, Invited User, Called Party, Callee: The party that
r
