Session Initiation Protocol (SIP)
Introduction
A powerful alternative to H.323
More flexible, simpler
Easier to implement
Advanced features
Better suited to the support of intelligent user devices
A part of IETF multimedia data and control architecture
SDP, RTSP (Real-Time Streaming Protocol), SAP
(Session Announcement Protocol)
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The Popularity of SIP
Originally Developed in the MMUSIC (Multiparty Multimedia Session Control)
A separate SIP working group
RFC 2543
Many developers
The latest version: RFC 3261 (June 2002 )
SIP + MGCP/MEGACO
The VoIP signaling in the future
“bake-offs” or SIP Interoperability Tests
The development of SIP and its implementation by system developers has involved a number of events.
Various vendors come together and test their products against each other
to ensure that they have implemented the specification correctly
to ensure compatibility with other implementations
SIP Architecture
A signaling protocol
The setup, modification, and tear-down of multimedia sessions
SIP + SDP
Describe the session characteristics to potential session participants
Separate signaling and media streams
Signaling may pass via one or more proxy or redirect servers
Media stream takes a more direct path.
SIP Signaling
IP Network
RTP Media Stream
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SIP Network Entities [1/4]
Clients
User agent clients
Application programs sending SIP requests
Servers
Responds to clients’ requests
Clients and servers may be in the same platform.
Proxy acts as both clients and servers
SIP Network Entities [2/4]
Four types of servers
Proxy servers
Act in a similar way to a proxy server used for web access
Handle requests or forward requests to other servers after some translation
Can be used for call forwarding, time-of-day routing, or follow-me services
SIP Proxy
Caller@work.com Collins@home.net
1.Request
Collins@work.com 2.Request
Collins@home.net
4.Response 3.Response
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SIP Network Entities [3/4]
Redirect servers
Accept SIP requests
Map the destination address to zero or more new addresses
Return the new address(es) to the originator of the request
Redirect Server 1.Request
Collins@work.com
Caller@work.com
Collins@home.net 2.Moved temporarily
Contact: Collins@home.net 3.ACK
4.Request
Collins@home.net
5.Response
SIP Network Entities [4/4]
A user agent server
Accepts SIP requests and contacts the user
The user responds → an SIP response
A SIP device
E.g., a SIP-enabled telephone
A registrar (location server)
Accepts SIP REGISTER requests
Indicating that the user is at a particular address
Personal mobility
Typically combined with a proxy or redirect server
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SIP Call Establishment
A SIP call establishment is simple.
A number of interim responses may be made to the INVITE prior to the called party accepting the call.
a b c d e
f g
INVITE
Ringing OK ACK
Conversation
BYE
OK
SIP Advantages
Attempt to keep the signaling as simple as possible
Offer a great deal of flexibility
Does not care what type of media is to be exchanged during a session or the type of transport to be used for the media
Various pieces of information can be included within the messages
Including non-standard information
Text-based encoding
Enable the users to make intelligent decisions
The control of the intelligent features is placed in the hands of the customer, not the network operator.
E.g., SUBJECT header
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Call Completion to Busy Subscriber Service
a b c d e f g
Conversation
BYE
OK ACK
OK Ringing INVITE
Busy (Try at 4pm) INVITE
ACK
j i h
Overview of SIP Messaging Syntax
Text-based
Similar to HTTP
Disadvantage – more bandwidth consumption
SIP messages
message = start-line
*message-header CRLF [message-body]
start-line = request-line | status-line
Request-line specifies the type of request
The response line indicates the success or
failure of a given request.
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Message headers
Additional information of the request or response
E.g.,
The originator and recipient
Retry-after header
Subject header
Message body
Describe the type of session
The most common structure for the message body is SDP (Session Description Protocol).
Could include an ISDN User Part message
Examined only at the two ends
SIP Requests [1/2]
Method SP Request-URI SP SIP-version CRLF
Request-URI
The SIP address of the destination
Methods
INVITE, ACK, OPTIONS, BYE, CANCLE, REGISTER
INVITE
Initiate a session
Information of the calling and called parties
The type of media
～IAM (initial address message) of ISUP
ACK only when receiving the final response
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SIP Requests [2/2]
BYE
Terminate a session
Can be issued by either the calling or called party
OPTIONS
Query a server as to its capabilities
To support a particular type of media
CANCEL
Terminate a pending request
Pending Request: an INVITE did not receive a final response
REGISTER
Log in and register the address with a SIP server
“all SIP servers” – multicast address (224.0.1.175)
Can register with multiple servers
Can have several registrations with one server
“One Number” Service
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SIP INFO Method
Specified in RFC 2976
For transferring information during an ongoing session
The transfer of DTMF digits
The transfer of account balance information
Pre-paid service
The transfer of mid-call signaling information
SIP Responses
SIP Version SP Status Code SP Reason-Phrase CRLF
Reason-Phrase
A textual description of the outcome
Could be presented to the user
Status code
A three-digit number
1XX Informational
2XX Success (only 200 is defined: the request has been understood and has been performed)
3XX Redirection (302: the called party is not available at the address used in the request, and the request should be reissued to a new address
included with the response)
4XX Request Failure (401: the client is not authorized to make the request)
5XX Server Failure (505: the server does not support the SIP version specified in the request)
6XX Global Failure (600: busy)
All responses, except for 1XX, are considered final
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SIP Addressing
SIP URIs (Uniform Resource Indicators)
user@host
sip:collins@home.net
sip:3344556789@telco.net
Message Headers
Provide further information about the message
E.g.,
To:header in an INVITE
The called party
From:header
The calling party
M (mandatory), M* (the header field should be present in the request, but a receiver should be prepared to process the
request even if the header is absent), O (optional), T (the header should be included in the request if a stream-based transport is used), C (the presence of the header depends on the context of the message), N/A (the header should not be sent in the request)
Four main categories
General, Request, Response, and Entity headers
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General Headers
Used in both requests and responses
Basic information
E.g., To:, From:, Call-ID: (uniquely identifies a specific invitation to a session), …
Contact:
Provides a URL for use in future communication regarding a particular session
Examples 1: In a SIP INVITE, the Contact header might be different from the From header.
An third-party administrator initiates a multiparty session.
Example 2: Used in response, it is useful for directing further requests directly to the called user.
Example 3: It is used to indicate a more appropriate
address if an INVITE issued to a given URI failed to reach the user.
Request Headers
Apply only to SIP requests
Addition information about the request or the client
E.g.,
Subject:
Priority: urgency of the request (emergency, urgent, normal, or non-urgent)
Response Headers
Further information about the response that cannot be included in the status line
E.g.,
Unsupported
Retry-After
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Entity Headers
Indicate the type and format of information included in the message body
Content-Length: the length of the message body
Content-Type: the media type of the message body
E.g., application/sdp
Content-Encoding: for message compression
Content Disposition: how a message part should be interpreted
