Introduction to SIP

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Introduction to SIP

• Speaker Min-Hua Yang
• Advisor Ho-Ting Wu
• Date2005/3/29

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Outline

• SIP Overview
• SIP Architecture and Components
• User Agent , Servers
• SIP Messages
• Request, Response and Headers
• Reliability Mechanism
• Summary

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Outline

• SIP Overview
• SIP Architecture and Components
• SIP Messages
• Reliability Mechanism
• Summary

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SIP Overview(1) - History

• SIP ( Session initiation protocol)
• Work began in 1995
• Developed in SIP Working Group in IETF
• Proposed standard RFC2543, February 1999
• New Version of SIP - RFC 3261, June 2002
• Split from MMUSIC (Multiparty Multimedia Session
  Control), Sep. 1999

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SIP Overview(2) - Main Functions

• SIP is an application-layer control (signaling)
  protocol for
• initiating sessions
• Find the users current location
• Carry session descriptions
• Modifying sessions
• Terminating sessions

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SIP Overview(3) - What is a session?

• Refers to an active connection between two
  computers for the purpose of communicating and
  transferring information
• Internet telephone calls
• multimedia conferences
• Instant Messaging
• However its not limited to the above

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SIP Overview(4)

• SIP is similar to HTTP
• both use a request/response model
• both are text-based
• much of SIPs message and header field syntax
  is identical to HTTP/1.1.
• Works independent of the underlying network
• transmission protocol and indifferent to media

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SIP Overview(5)

• Scalability
• Functionality such as proxying, redirection,
  location, or registration can reside in
  different physical servers.
• Distributed functionality allows new processes
  to be added without affecting other components.

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SIP Overview(6)

• Interoperability
• An open standard
• Can implement to communicate with other SIP
  based products

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SIP Overview(7)

• Mobility
• The user can be using a PC at work, PC at home
  ,wireless phone, IP phone, or regular phone.
• Users must register/Update their current
  location.
• Supports user mobility by proxying or redirecting
  requests to a users current location.
• Proxy servers will forward calls to the users
  current location.

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Integration with IETF Protocols

• SIP forms only part of an overall IP telephony
  system
• Other IETF protocol standards are used to build a
  fully functioning VoIP system.
• example
• RSVP - to reserve network resources.
• RTP (Real Time Transport Protocol) - to
  transport real
• time data
• RTSP (Real Time Streaming Protocol) - for
  controlling
• delivery of streaming media.
• RTCP (Real Time Transport Control Protocol)
  provides
• information about the quality of the
  communication.

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Related Protocols
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SIP Capabilities(1/2)

• Determine location of target points Support
  address
• resolution, name mapping, call redirection
• Determine media capabilities SIP uses Session
• Description Protocol (SDP) for this
• Determine availability returns a message why
  the
• remote party cannot be contacted
• Establish a session between end points also
  support mid call changes, changes of media
  characteristics or codec

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SIP Capabilities(2/2)

• Permits interaction between devices via
• signaling messages
• These messages can
• Register a user with a system
• Invite a user to join an interactive session
• Establish a media stream between 2 or more end
• points
• Terminate a session

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Outline

• SIP Overview
• SIP Architecture and Components
• SIP Messages
• Reliability Mechanism
• Summary

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SIP Network Architecture
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SIP Component User Agent

• User Agent Client (UAC)- Initiate SIP Request
• User Agent Server (UAS)-Accepts or rejects call
• Phones acts as UAC or UAS
• Implemented in Hardware or Software
• Components
• Includes softphones, sip ip phones

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User Agents
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SIP Component Proxy Server

• Heart of SIP network that contains all service
  logic.
• Receives SIP messages, forwards to next SIP UAS.
• Interprets, rewrites or translates a request
  message before forwarding it.

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Scenario with Proxy Server
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SIP Component Redirect Server

• Return routing information to the originating
  endpoint.
• Does not accept or terminate calls
• Does not initiate its own SIP request

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Scenario with Redirect Server
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SIP Component Registrar

• Accept registration requests from users
• Maintains users whereabouts at a
• Location Server
• Typically co-located with a proxy server
• or a redirect server and may offer
• location services

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Scenario with Registrar
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Invitation for SIP Proxy Server
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Invitation for Redirect Server
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SIP Addressing

• The SIP address is identified by a SIP URL,in the
  format user_at_host.
• Email-like identifier of the form
• Examples of SIP URLs
• siphostname_at_ntut.edu.tw
• siphostname_at_192.168.10.1
• sip14083831088_at_vovida.org

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Outline

• SIP Overview
• SIP Architecture and Components
• SIP Messages
• Reliability Mechanism
• Summary

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SIP Messages

• The message syntax is identical to
  HTTP/1.1(RFC2616).
• Two kinds of SIP messages
• SIP-message Request Response

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SIP Messages - Requests

• INVITE Initiates a call by inviting user to
  participate in session.
• ACK - Confirms that the client has received a
  final response to an INVITE request.
• BYE - Indicates termination of the call.
• CANCEL - Cancels a pending request.
• REGISTER Registers the user agent.
• OPTIONS Used to query the capabilities of a
  server.

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SIP Messages - Responses

• 1xx - Informational Responses
• 180 ringing
• 2xx - Successful Responses
• 200 OK
• 3xx - Redirection Responses
• 302 Moved Temporarily
• 4xx - Request Failure Responses.
• 404 Not Found
• 5xx - Server Failure Responses.
• 503 Service Unavailable
• 6xx - Global Failures Responses.
• 600 Busy Everywhere
• All responses, except for 1XX, are considered
  final responses

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Message Headers

• Provide further information about the message
• E.g.,
• To header in an INVITE
• The called party
• From header
• The calling party
• Four main categories
• General, Request, Response, and Entity headers

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Header Fields

• General Header
• Be applied to both request and response
  messages
• Entity Header
• Define information about the message body
• Request Header
• Allow the client to pass additional information
  about
• the request
• Response Header
• Allow the server to pass additional information
• about the response

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SIP Headers
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SIP Headers From

• A field required in all requests and response
  messages
• Provides identity of requests initiator
• Example
• From Laura Brown ltsipLaura.Brown_at_university.comgt
  

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SIP Headers To

• Provides identity of the intended
• recipient of the request
• Example
• To Bob Johnson ltsipBob.Johnson_at_company.comgt

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SIP Headers Call-ID(1/2)

• Provides a globally unique identifier to
• distinguish specific invitations
• Typically uses a 32-bit cryptographically
• random numbers
• Example
• Call-ID ges456fcdw21lkfgte12ax_at_workstation1234.un
  iversity.com

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SIP Headers Call-ID(2/2)
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SIP Headers Contact(1/2)

• A Contact header provides a URL where the user
  can be reached directly.
• This feature is important because it offloads SIP
  servers that do not need to be in the signalling
  path after routing the first INVITE.
• Example
• Contact Bob JohnsonltsipBob_at_140.124.181.227gt

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SIP Headers Contact(2/2)
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SIP Header - Via

• History of messages path through network(s)
• Helps to prevent looping and ensures
• replies route back to originator
• Indicates the used transport protocol, ip
• address and port of sender
• Via SIP/2.0/UDP 192.168.6.215060

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SIP Header Content-Type

• Provides information about media type of message
  body
• Content-Type application/sdp

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SIP Header CSeq(1/2)

• CSeq or command sequence
• Needed in both request messages as well
• as response messages
• Need to increment this when a user with the
• same Call-ID wants to send different SIP
• methods or content ( Except ACK and Cancel)
• When sending responses to requests, CSeq
• should be the same
• The numerical part of the Cseq is used to order
  different requests within the same session.
• CSeq 1 INVITE

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SIP Header CSeq(2/2)
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An example of SIP Request
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An example of SIP Response
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Outline

• SIP Overview
• SIP Architecture and Components
• SIP Messages
• Reliability Mechanism
• Summary

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Reliable Transmission of Responses(1/2)

• Final responses are transmitted reliably between
  server and client, using retransmissions or a
  reliable transport protocol to ensure delivery.
• Provisional responses may either be received by
  the client or be lost in the network.

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Reliable Transmission of Responses(2/2)
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Reliable Transmission of Requests(1/2)
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Reliable Transmission of Requests(2/2)
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Outline

• SIP Overview
• SIP Architecture and Components
• SIP Messages
• Reliability Mechanism
• Summary

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Summary

• SIP is gaining acceptance in the industry
• Open Source projects are taking the lead
• in SIP implementations
• New generation of services are already
• being offered
• Staying with single a vender solution is
  currently workable
• Interoperability between vender products is still
• in early stages

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Reference

• RFC 2543(SIP/1.0)
• RFC 3261(SIP/2.0)
• RFC 2327(SDP)
• Related Material provided by