Session Initiation Protocol (SIP)
Introduction
n
A powerful alternative to H.323
n
More flexible, simpler
n
Easier to implement
n
Advanced features
n
Better suited to the support of intelligent user devices
n
A part of IETF multimedia data and control architecture
n
SDP, RTSP (Real-Time Streaming Protocol), SAP
(Session Announcement Protocol)
The Popularity of SIP
n
Originally Developed in the MMUSIC (Multiparty Multimedia Session Control)
n A separate SIP working group
n RFC 2543
n Many developers
n The latest version: RFC 3261 (June 2002 )
n
SIP + MGCP/MEGACO
n The VoIP signaling in the future
n
“bake-offs” or SIP Interoperability Tests
n The development of SIP and its implementation by system developers has involved a number of events.
n Various vendors come together and test their products against each other
n to ensure that they have implemented the specification correctly
SIP Architecture
n
A signaling protocol
n The setup, modification, and tear-down of multimedia sessions
n
SIP + SDP
n Describe the session characteristics to potential session participants
n
Separate signaling and media streams
n Signaling may pass via one or more proxy or redirect servers
n Media stream takes a more direct path.
SIP Signaling
IP Network
SIP Network Entities [1/4]
n
Clients
n
User agent clients
n
Application programs sending SIP requests
n
Servers
n
Responds to clients’ requests
n
Clients and servers may be in the same platform.
n
Proxy acts as both clients and servers
SIP Network Entities [2/4]
n
Four types of servers
n
Proxy servers
n Act in a similar way to a proxy server used for web access
n Handle requests or forward requests to other servers after some translation
n Can be used for call forwarding, time-of-day routing, or follow-me services
ProxySIP 1.Request
Collins@work.com 2.Request
Collins@home.net
4.Response 3.Response
SIP Network Entities [3/4]
n
Redirect servers
n Accept SIP requests
n Map the destination address to zero or more new addresses
n Return the new address(es) to the originator of the request
Redirect Server 1.Request
Collins@work.com
Caller@work.com
2.Moved temporarily
Contact: Collins@home.net 3.ACK
4.Request
SIP Network Entities [4/4]
n
A user agent server
n Accepts SIP requests and contacts the user
n The user responds → an SIP response
n A SIP device
n E.g., a SIP-enabled telephone
n
A registrar (location server)
n Accepts SIP REGISTER requests
n Indicating that the user is at a particular address
n Personal mobility
n Typically combined with a proxy or redirect server
SIP Call Establishment
n
A SIP call establishment is simple.
n
A number of interim responses may be made to the
INVITE prior to the called party accepting the call.
SIP Advantages
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Attempt to keep the signaling as simple as possible
n
Offer a great deal of flexibility
n 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
n
Various pieces of information can be included within the messages
n Including non-standard information
n Text-based encoding
n Enable the users to make intelligent decisions
n The control of the intelligent features is placed in the hands of the customer, not the network operator.
n E.g., SUBJECT header
Call Completion to Busy Subscriber Service
Overview of SIP Messaging Syntax
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Text-based
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Similar to HTTP
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Disadvantage – more bandwidth consumption
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SIP messages
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message = start-line
*message-header CRLF [message-body]
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start-line = request-line | status-line
n
Request-line specifies the type of request
n
The response line indicates the success or
failure of a given request.
n
Message headers
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Additional information of the request or response
n
E.g.,
n The originator and recipient
n Retry-after header
n Subject header
n
Message body
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Describe the type of session
n
The most common structure for the message body is SDP (Session Description Protocol).
n
Could include an ISDN User Part message
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Examined only at the two ends
SIP Requests [1/2]
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Method SP Request-URI SP SIP-version CRLF
n
Request-URI
n
The SIP address of the destination
n
Methods
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INVITE, ACK, OPTIONS, BYE, CANCLE, REGISTER
n
INVITE
n Initiate a session
n Information of the calling and called parties
n The type of media
n ∼IAM (initial address message) of ISUP
n ACK only when receiving the final response
SIP Requests [2/2]
n BYE
n Terminate a session
n Can be issued by either the calling or called party
n OPTIONS
n Query a server as to its capabilities
n To support a particular type of media
n CANCEL
n Terminate a pending request
n Pending Request: an INVITE did not receive a final response
n REGISTER
n Log in and register the address with a SIP server
n “all SIP servers” – multicast address (224.0.1.175)
n Can register with multiple servers
Can have several registrations with one server
“One Number” Service
SIP INFO Method
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Specified in RFC 2976
n
For transferring information during an ongoing session
n
The transfer of DTMF digits
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The transfer of account balance information
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Pre-paid service
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The transfer of mid-call signaling information
SIP Responses
n SIP Version SP Status Code SP Reason-Phrase CRLF
n
Reason-Phrase
n A textual description of the outcome
n Could be presented to the user
n
Status code
n A three-digit number
n 1XX Informational
n 2XX Success (only code 200 is defined)
n 3XX Redirection
n 4XX Request Failure
n 5XX Server Failure
n 6XX Global Failure
SIP Addressing
n
SIP URLs (Uniform Resource Locators)
n
user@host
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sip:collins@home.net
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sip:3344556789@telco.net
Message Headers
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Provide further information about the message
n
E.g.,
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To:header in an INVITE
n The called party
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From:header
n The calling party
n
Four main categories
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General, Request, Response, and Entity headers
General Headers
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Used in both requests and responses
n
Basic information
n E.g., To:, From:, Call-ID: (uniquely identifies a specific invitation to a session), …
n
Contact:
n Provides a URL for use in future communication regarding a particular session
n Examples 1: In a SIP INVITE, the Contact header might be different from the From header.
n An third-party administrator initiates a multiparty session.
n Example 2: Used in response, it is useful for directing further requests directly to the called user.
n Example 3: It is used to indicate a more appropriate
address if an INVITE issued to a given URI failed to reach
n
Request Headers
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Apply only to SIP requests
n
Addition information about the request or the client
n
E.g.,
n Subject:
n Priority: urgency of the request (emergency, urgent, normal, or non-urgent)
n
Response Headers
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Further information about the response that cannot be included in the status line
n
E.g.,
n Unsupported Retry-After
Entity Headers
n
Indicate the type and format of information included in the message body
n
Content-Length: the length of the message body
n
Content-Type: the media type of the message body
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E.g., application/sdp
n
Content-Encoding: for message compression
n
Content Disposition: how a message part should be interpreted
n
session, alert, render …
Examples of SIP Message Sequences
n
Via:
n
From: and To:
n
Call-ID:
n host-specific
n
Contact: (for future SIP message transmission)
n *
n
Content-Length:
n Zero, no msg body
n
CSeq:
n A response to any request must use the same value of CSeq as used in the request.
Expires:
Invitation
n
A two-party call
n Subject:
n optional
n Content-Type:
n application/sdp
n A dialog ID
n To identify a peer-to-peer relationship between two user agents
n Tag in From
n Tag in To
n Call-ID
Termination of a Call
n
CSeq has changed.
Redirect Servers
n
An alternative address
n 302, Moved temporarily
n
Another INVITE
n Same Call-ID
n CSeq ++
Proxy Servers [1/2]
n
Sits between a user-agent client and the far-end user- agent server
n
Numerous proxies can reside in a chain between the caller and callee.
n The most common scenario will have at least two proxies: one at the caller and one at the callee end.
n It is likely that only the last proxy in the chain changes the Request-URI.
n The other proxies in the chain would simply use the domain part of the received Request-URI as input to a location
function (e.g., DNS) to determine the next hop.
Proxy Servers [2/2]
n
Via:
n The path taken by a request
n Loop detected, 482 (status code)
n For a response
n The 1st Via: header is checked and removed.
n The second Via: header is checked.
n If it exists, perform forwarding.
n If not, the response is destined to the proxy itself.
n The response finds its way back to the originator of the request.
n Branch: used to distinguish between multiple responses to the same request
n Forking Proxy: Issue a single request to multiple destinations
Proxy State [1/2]
n
Can be either stateless or stateful
n
If stateless, the proxy takes an incoming request, performs whatever translation and forwards the corresponding outgoing request and forgets
anything.
n Retransmission takes the same path (no change on retransmission).
n
If stateful, the proxy remembers incoming requests and corresponding outgoing request.
n The proxy is able to act more intelligently on subsequent requests and responses related to the same session.
Proxy State [2/2]
n
Record-Route: and Route: Headers
n The subsequent requests may not pass through the same path as the initial request/response.
n E.g., use Contact:
n A Proxy might require that it remains in the signaling path for all subsequent requests to provide some advanced
service.
n In particular for a stateful proxy
n Insert its address into the Record-Route: header
n The response includes the Record-Route: header
n The information contained in the Record-Route: header is used in the subsequent requests related to the same call.
n The Route: header is used to record the path that the request is enforced to pass.
lr (loose routing) vs. sr (strict routing)
Forking Proxy
n
A proxy can “fork” requests
n
A user is registered at several locations
n ;branch=xxx
n
