RFC 3261 Session Initiation Protocol

1
RFC 3261Session Initiation Protocol

• Aqeeluddin MohammedFarhan Ali Khan

2
Introduction

• The need for it?- Creating Managing a
  session.- Users may move between endpoints, may
  be addressable by different names, may
  communicate in several different media.
• What is SIP?- SIP enables internet endpoints to
  discover each other to agree on a
  characterization of the session they would like
  to share.- Independent of the underlying
  transport protocol without any dependency on
  the type of session being established.

3
SIP does not provide services
Important

• It provides primitives that can be used to
  implement different services.
• E.g. SIP can locate a user deliver an opaque
  object to his current location.
• Session Description Protocol (SDP) can be used to
  describe sessions.
• RTP can be used to carry real-time session
  multimedia data.
• SIP should be used with other protocols in order
  to provide complete services to the user

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Facets of Establishing/Terminating Multimedia
Communications

• User Locationdetermination of end-system to be
  used for communication
• User availabilitydetermination of the
  willingness of the called party to engage in
  communications
• User capabilitiesdetermination of the media
  media parameters to be used
• Session Setupringing, establishment of session
  parameters at both called calling party
• Session Managementmodifying session parameters,
  transfer/termination of session

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SIP Protocol Structure

• Structured as a layered protocol
• Each layer is independent
• Transport layer defines howclient server send
  receiverequests/responses
• A transaction is a request its responses.
• Transaction layer handles retransmissions,
  matching responses to requests
• Every SIP entity except stateless proxies is a
  Transaction User

Transaction User
Transaction
Transport
Syntax Encoding
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SIP Message Structure
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Message Structure (continued)

• SIP Addresses
• Format e.g. sipkhan_at_uta.edu
• must include host, may include username, port
  number.
• Request Line
• request method followed by SIP address version
• Response Line
• SIP version followed by response code a textual
  description of the response code

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Message Header Fields

• Via address at which requester is expecting to
  receive responses to this request
• Max-Forwards limits the number of hops a request
  can make on its way to destination
• To SIP URI towards which the request was made
• From SIP URI that indicates the originator of
  request
• Call-ID GUID for this call
• CSeq incremented for each new request within a
  dialog, traditional sequence number
• Contact SIP URI representing direct route to
  entity
• Content-Type contains a description of message
  body
• Content-Length contains octet count of message
  body

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Request Methods/Messages

• INVITE initiate sessions
• session description included in message body
• re-invites used to change session states
• ACK confirms session establishment
• can only be used with INVITE
• BYE terminates session
• CANCEL cancels a pending INVITE
• OPTIONS capability inquiry
• REGISTER binds a permanent address to current
  location

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Response Codes

• Borrowed from HTTPStructure xxx explanatory
  text
• Receivers only need to understand the numeric
  code (xxx)
• 1xx Informational- 100 Trying- 180 Ringing-
  181 Call is being forwarded
• 2xx Success- 200 OK

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Response Codes (continued)

• 3xx Redirection- 300 Multiple Choices- 301
  Moved Permanently- 302 Moved Temporarily
• 4xx Client Error- 400 Bad Request- 401
  Unauthorized- 486 Busy Here
• 5xx Server Failure- 500 Server Internal Error
• 6xx Global Failure- 600 Busy Everywhere

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SIP Entities

• User Agent (user application)- UA Client
  originates call- UA Server listens for incoming
  calls
• SIP Proxy Server- relays call signaling
• SIP Redirect Server- redirects calls to other
  servers
• SIP Registrar- accepts registrations from
  users- maintains users whereabouts at a location
  server

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Registering with a SIP Registrar

• 1 User issues request to register itself with
  SIP Registrar
• 2 SIP Registrar creates/updates user entry in
  location server
• 3 SIP Registrar responds back with OK message
  indicating successful registration

Image Source IPTel.org SIP Tutorial
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SIP Operation
Request Message
Response Message
BYE
INVITE
Media Session
Alice
Bob
100 Trying
180 Ringing
200 OK
200 OK
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SIP Operation using Proxy

• INVITE
• INVITE
• 100 Trying
• INVITE
• 100 Trying
• 180 Ringing
• 180 Ringing
• 180 Ringing
• 200 OK
• 200 OK
• 200 OK
• ACK

LocationServer
Request Message
Response Message
Get the address for Bob
2
AliceProxy
BobsProxy
5
7
10
1
6
9
3
4
8
11
12
Media Session
Alice
Bob
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Session teardown
LocationServer
Request Message
Response Message
AliceProxy
BobsProxy
BYE
Media Session
Alice
Bob
200 OK
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SIP Operation in Redirection

• INVITE
• INVITE
• 100 Trying
• 30x Moved
• INVITE
• INVITE
• 100 Trying
• 180 Ringing
• 180 Ringing
• 180 Ringing
• 200 OK
• 200 OK
• 200 OK
• ACK

BobsProxy1
Request Message
Response Message
LocationServer
2
4
Get the address for Bob
5
AliceProxy
BobsProxy2
7
9
12
1
6
8
11
3
10
13
14
Media Session
Alice
Bob
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Proxy vs. Redirect

• A SIP server may proxy or redirect a request.
• Which of the two applies, is a configuration
  issue.
• Redirection useful if a user moves or changes his
  provider.
• Proxy useful when relaying user request, forking.
• Serves as rendezvous point at which callees are
  globally reachable
• Determine next hop at which signal should be
  relayed (UA, Redirect, Proxy)
• Several destinations may be tried for a request
  either sequentially or in parallel before failing

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Stateful vs. Stateless Proxy

• Stateful Proxy
• they keep the state of the transaction during the
  request processing. This allows the division of a
  request message in several ones (forking), with
  the purpose to find in parallel the location of
  the called in order to obtain the best path.
• Stateless Proxy
• they do not keep the state of the transaction
  during the requests processing, they only resend
  messages.

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Summary

• TCP/UDP port 5060 used to connect to servers
• Provides mechanism to locate users
• Only used in setup, maintenance tearing down of
  sessions
• Similar to HTTP, human readable request-response
  structure
• Doesnt provide services, rather provides
  primitives that can be used to implement
  different services.
• Works independent of underlying transport protocol

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SIP being used in

• Most widely used in IP Telephony
• Vonage
• BroadVoice
• SJPhone
• by many other VoIP solution providers
• Instant Messaging Presence
• Gizmo

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References

• RFC 3261
• http//www.en.voipforo.com/SIP/SIP_architecture.ph
  p
• http//en.wikipedia.org/wiki/Session_Initiation_Pr
  otocol
• http//www.iptel.org/sip/siptutorial.pdf
• http//en.wikipedia.org/wiki/List_of_SIP_software

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Questions ?