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SIP

Author
Marius Peter
Date
<2022-07-12 Tue>
Table of Contents

# -*- mode: org; -*-

#+SUBTITLE: The Session Initiation Protocol. #+OPTIONS: toc:2

#+DESCRIPTION: The Session Initiation Protocol.

#+begin_abstract SIP is a communication protocol used for VOIP. #+end_abstract

Introduction

What SIP does /not/ do: provide the media services.

Establishing and terminating multimedia sessions

User location
What end system will be used to communicate?
User availability
Is the called party willing to engage in communication?
User capabilities
What media and media parameters will be used?
Session setup
How are session parameters established between calling/called parties? (``Ringing'')
Session management
How are sessions transferred and terminated? How are session parameters modified? How are services invoked?

User Agents

# Section 2

SIP endpoints are called User Agents. At the very least, two types of UAs needed to setup a call: User-Agent Client and User-Agent Servers.

UAC

#+BEGIN_QUOTE A user agent client is a logical entity that creates a new request, and then uses the client transaction state machinery to send it. The role of UAC lasts only for the duration of the transaction. In other words, if a piece of software initiates a request, it acts as a UAC for the duration of the transaction. If it receives a request later, it assumes the role of a UAS for the processing of that transaction.

From RFC 3261 #+END_QUOTE

UAS

#+BEGIN_QUOTE A user agent server is a logical entity that generates a response to a SIP request. The response accepts, rejects or redirects the request. This role lasts only for the duration of the transaction. In other words, if a piece of software responds to a request, it acts as a UAS for the duration of the transaction. If it generates a request later, it assumes the role of a UAC for the processing of that transaction.

From RFC 3261 #+END_QUOTE

B2BUA

#+BEGIN_QUOTE A back-to-back user agent is a logical entity that receives a request and processes it as a UAS. In order to determine how the request should be answered, it acts as a 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 the concatenation of a UAC and a UAS, no explicit definitions are needed for its behavior.

From RFC 3261 #+END_QUOTE

Proxy Server

#+BEGIN_QUOTE A proxy server is an intermediary entity that acts as both a server and a client for the purpose of making requests on behalf of other clients. A proxy server primarily serves the role of routing, which means its job is to ensure that a request is sent to another entity ``closer'' to the targeted user. Proxies are also useful for enforcing policy (for example, making sure that a user is allowed to make a call). A proxy interprets and, if necessary, rewrites specific parts of a request message before forwarding it.

From RFC 3261 #+END_QUOTE

A proxy does not need to be involved for requests after a dialog has been created.

SIP messages

# Section 3

A SIP message is either a request or a response. When a UAC sends a request to a UAS, the UAS replies with a response, to inform the UAC that the request was received.

Message structure

: Start line (request line for request/status line for response) : Message headers : Empty line : Message body

SIP request methods

=REGISTER=
Register contact information
=INVITE=
Setting up a session
=ACK=
Acknowledge
=CANCEL=
Cancel session before other party answers
=BYE=
Hang up (tear down) the session
=OPTIONS=
Query UAS to determine capabilities, can be used as a ``ping'' method

RFC 3261 defines six types of REQUEST methods, other RFCs define additional methods.

SIP response codes

=1XX= provisional response

=100 TRYING=
Request has been received by the next-hop server, some unspecified action (e.g. database query) is being performed. 100 TRYING is never forwarded upstream by a stateful proxy.
=180 RINGING=
UAS has received the INVITE request, the UAC should initiate local ringback while further call processing takes place.
=181 CALL IS BEING FORWARDED=
A UAS may use this status code to indicate that the call is being forwarded to a different set of destinations.
=182 QUEUED=
Called party is temporarily unavailable, but UAS has decided to queue the call rather than reject it. When callee becomes available, it will return appropriate final status response. Reason phrase may give further details about call status, e.g. ``5 calls queued, expected waiting time is 15 minutes''. UAS may issue many 182 QUEUED responses to update caller about queued call status.
=183 SESSION IN PROGRESS=
Convey information about call progress. Typically used for ``early media session'', implying that audio or video is being sent before call is answered.

=200 OK=

=3XX= redirections

=300 MULTIPLE CHOICES=
Address in the request resolves to more than one choice, each with a specific location. UA can select preferred communication endpoint and redirect communication to that location. Response body may contain a list of resource characteristics and locations.
=301 MOVED PERMANENTLY=
User cannot be found at the address in the request URI, requesting client should retry at new address given by the CONTACT header. Requesting client should update any local directories, address books, and user location caches with this new value, and redirect future requests to this new address.
=302 MOVED TEMPORARILY=
Requesting client should retry at the new address. Duration of validity of the new address can be specified in the EXPIRES header. If no explicit expiration time, address is only valid once, and must not be cached for future transactions.
=380 ALTERNATIVE SERVICE=
Call was unsuccessful, but alternative services are available. These alternative services are described in the response body.

=4XX= request failures

=400 BAD REQUEST=
UAS cannot understand the request due to malformed syntax. Reason phrase should identify the syntax error so UAC can fix it. Associated with /fuzzing/, i.e. purposefully sending malformed data to the UAS for a DoS attack or to compromise a security weakness.
=401 UNAUTHORIZED= and =407 PROXY REQUIRES AUTHENTICATION=
UAS wants UAC to authenticate. Usually seen when UAC is attempting to REGISTER, can also be seen for other methods.
=403 FORBIDDEN=
Many systems issue this response if UAC sends an INVITE, but is not registered.
=404 NOT FOUND=
Request was received by UAS, but user does not exist at the specified domain. Can also be sent if domain does not match. Expected if you call the wrong number.
=405 METHOD NOT ALLOWED=
Request method is understood, but not allowed for the address specified in the URI. Response must include an ALLOW header that contains a list of allowed methods for the address.
=406 NOT ACCEPTABLE=
The UAS cannot generate a response with a content type that satisfies the ACCEPT header in the request.
=408 REQUEST TIMEOUT=
The UAS cannot produce a response within a suitable amount of time. UAC may repeat the request at any later time.
=410 GONE=
The requested resource is no longer available, and no forwarding address is known. This condition is expected to be permanent. If UAS does not/cannot know whether the condition is permanent, 404 NOT FOUND should be used instead.
=413 REQUEST ENTITY TOO LARGE=
Request body is larger than the UAS is willing or able to process. UAS may close the connection. If condition is temporary, UAS should include a RETRY-AFTER header to indicate the delay after which the UAC can try again.
=414 REQUEST URI TOO LONG=
Request URI is longer than the UAS is willing to interpret.
=415 UNSUPPORTED MEDIA TYPE= Request body
format is unsupported by UAS for the specified method. UAS must return a list of acceptable formats using the ACCEPT, ACCEPT-ENCODING, or ACCEPT-LANGUAGE headers depending on the specific problem.
=416 UNSUPPORTED URI SCHEME=
The request URI scheme is unknown to the server.
=420 BAD EXTENSION=
The UAS did not understand a protocol extension specified in the PROXY-REQUIRE or REQUIRE header fields. UAS must return a list of unsupported extensions in an UNSUPPORTED header.
=421 EXTENSION REQUIRED=
UAS needs a particular extension to process the request which is not listed in the SUPPORTED header. Response must contain a REQUIRE header listing the required extensions. UAS should only return this response if no service whatsoever can be provided to the UAC.
=423 INTERVAL TOO BRIEF=
Expiration time of the resource to be refreshed is too short.
=480 TEMPORARY UNAVAILABLE=
UAS was contacted successfully, but callee is unavailable, e.g. not logged in, ``do not disturb'' feature activated. Response may include better time to call in the RETRY-AFTER header. User might be available elsewhere. Reason phrase should include more precise reason why callee is unavailable, and should be settable by the UA. 486 BUSY HERE may be used for more details on why the call failed.
=481 TRANSACTION DOES NOT EXIST=
UAS received a request that does not match any existing transaction.
=482 LOOP DETECTED=
Observed frequently with packet loss, i.e. the same request is sent again, without packet sequence number being incremented.
=483 TOO MANY HOPS=
UAS has received a request in which MAX-FORWARDS header is equal to zero.
=484 ADDRESS INCOMPLETE=
Request URI is incomplete. Additional reason should be provided in reason phrase.
=486 BUSY HERE=
Callee's end system contacted successfully, but callee is unavailable at this end system. Response may include a better time to call in the RETRY-AFTER header. Callee might be available elsewhere, e.g. via a voicemail system.
=487 REQUEST TERMINATED=
Request terminated by a BYE or CANCEL request.
=488 NOT ACCEPTABLE HERE=
Request is not acceptable for the specific resource identified in the request URI. Request may succeed somewhere else. Description of media capabilities may be included in response body.
=491 REQUEST PENDING=
UAS received the request but had a pending request in the same dialog. This is a ``glare condition''.

=5XX= server failures

The UAS has experienced an error.

=501 NOT IMPLEMENTED=
UAS does not support the functionality required to fulfill the request. If UAS recognizes the request method, but it is not allowed or supported, use 405 METHOD NOT ALLOWED instead.
=502 BAD GATEWAY=
The UAS, whilst acting as a gateway or proxy, received an invalid response from the downstream server it accessed when attempting to fulfill the request.
=503 SERVER NOT AVAILABLE=
UAS is temporarily unable to fulfill the request due to temporary overload or server maintenance. UAS may include a RETRY-AFTER header; if it does not, UAC must act as if it has received a 500 SERVER INTERNAL ERROR response. UAC should attempt to forward the request to an alternate server. UAS may refuse the connection or drop the request instead of responding with 503 SERVER NOT AVAILABLE.
=504 SERVER TIMEOUT=
UAS did not receive a response from an external server in time. 408 REQUEST TIMEOUT should be used if there was no response within the time period specified in the EXPIRES header from the upstream server.
=505 VERSION NOT SUPPORTED=
The UAS does not or refuses to support the SIP protocol version used in the request.
=513 MESSAGE TOO LARGE=
Message length exceeds UAS capabilities.

=6XX= global failures

These responses indicate that the UAS has information about a particular user (not just the instance specified in request URI).

=600 BUSY EVERYWHERE=
Callee's end system was contacted successfully, but callee does not wish to take the call at this time. Response may indicate a better call back time in the RETRY-AFTER header. This response is returned if client know that callee is not available anywhere else (otherwise return 486 BUSY HERE).
=603 DECLINE=
Callee's end system was contacted successfully, but callee explicitly does not wish to or cannot participate. Response may indicate a better call back time in the RETRY-AFTER header. This response is returned if client know that callee is not available anywhere else (otherwise return 486 BUSY HERE).
=604 DOES NOT EXIST ANYWHERE=
Server is certain that the resource pointed to in request URI does not exist anywhere.
=605 NOT ACCEPTABLE=
The UAS was contacted successfully, but cannot properly support the described session. Response may contain a list of reasons in a WARNING header. This response is only returned if UAS knows that no other endpoint will answer the request.

SIP URI structure

: sip:user:password@host:port:parameters or : sip:user@host:port;parameters

| sip | bob | example.com | 5060 | ;userphone= | | scheme | user | host | port | parameters |

SIP headers

Mandatory headers

=CONTACT=
Contains a URI used to contact UA for subsequent requests.
=ALERT-INFO=
In an INVITE request, specify an alternative ringtone to the UAS. In a 180 RINGING response, specify an alternative ringback tone to the UAC. Can also be used for ``auto answering'' calls by certain endpoints.
=ALLOW=
Specify the type of methods supported by the UA generating the message.
=AUTHORIZATION=
Contains UA auth credentials.
=CALL-INFO=
Contains additional information about request originator or responder.
=CONTENT-DISPOSITION=
Describes how the body should be interpreted by UAC/UAS.
=CONTENT-LENGTH=
Size of the message body, not including CRLF separating header from body.
=CONTENT-TYPE=
Type of media sent in body.
=MIN-EXPIRES=
Minimum refresh interval of soft-state elements managed by the UA.
=RECORD-ROUTE=
Inserted by a proxy in a request to force future requests to route through the proxy.
=REQUIRE=
Used by a UAC to inform a UAS about options that are expected to be supported by the UAS.
=ROUTE=
Force the routing of a request through the listed set of proxies.
=SUPPORTED=
List all extensions supported by the UAC/UAS.
=USER-AGENT=
Information about the UA, e.g. manufacturer name, SIP UA version.
=WWW-AUTHENTICATE=
Authentication challenge.

Session Description Protocol

SIP and user mobility

SIP authentication

# Section 4

SIP routing and responses

# Section 5

SIP call routing

COMMENT SIP trapezoid model

This name results from the shape observed when plotting a typical SIP message flow on a sequence diagram.

COMMENT SIP registration

# Section 6

Media and RTP

# Section 7