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SIP – Functionality
and Structure of the Protocol
By
Omar A. Abouabdalla
Network Research Group
(USM)
What is SIP ?!
SIP or Session Initiation Protocol is an application-level control
protocol for setting up, changing and terminating multimedia
sessions between participants on IP data networks.
SIP is a text-based protocol, similar to HTTP and SMTP, for
initiating interactive communication sessions between users.
Such sessions include voice, video, chat, interactive games, and
virtual reality.
When it Starts ?!
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The protocol's roots extend back to 1996.
SIP was first developed within the Multiparty
Multimedia Session Control (MMUSIC) working
group at Internet Engineering Task Force (IETF).
In 1999 SIP was approved as an official standard.
In 1999 RFC2543 was published .
Its real value has only recently been uncovered.
How it Works ?!
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SIP requires the use of user data gram protocol
(UDP), and TCP is optional.
 SIP contains its own retransmission mechanisms for
the purpose of establishing sessions.
 Session Description Protocol (SDP), which is the
encoded body of the SIP message contains
information about what media types the parties can
and will use.
 Destinations in SIP are represented with Uniform
Resource Indicators (URI), which have the same
format as e-mail addresses.
SIP Advantages ?!
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It is short, simple and flexible protocol.
It needs only a small set of messages and responses.
Can run over most fixed and wireless networks.
It uses the internet model and maps it onto telecom
world.
It uses email-like addresses to identify users.
SIP address to identify users rather than the devices
they happen to be using.
How it Functions ?!
SIP supports five facets of establishing and terminating
multimedia communications:
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User Location: determination of the end system to be used for
communication.
User Availability: determination of the willingness of the called
party to engage in communications.
User Capabilities: determination of the media and media
parameters to be used.
Session Setup: "ringing", establishment of session parameters at
both called and calling party.
Session Management: including transfer and termination of
sessions, modifying session parameters.
How it Functions ?! (cont’d..)
SIP is a component that can be used with other IETF
protocols to build a complete multimedia architecture.
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Real-time Transport Protocol (RTP) for transporting real time
data.
Real-time Streaming Protocol (RTSP) for controlling delivery of
streaming media.
The Media Gateway Control Protocol (MEGACO) for controlling
gateways to Public Switched Telephone Network (PSTN).
Session Description Protocol (SDP) for describing multimedia
sessions.
Tan’s
SIP Client
penang.com
proxy
Bob’s
SIP Client
tokyo.com
proxy
INVITE F1
INVITE F2
100 Trying F3
INVITE F4
100 Trying F5
180 Ringing F6
180 Ringing F7
180 Ringing F8
200 OK F9
200 OK F10
200 OK F11
ACK F12
Media Session
BYE F13
200 OK F14
SIP Methods
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Invite - Indicates that the user is invited to a
session.
Ack - Used to confirm a session establishment.
Bye - Terminates the sessions.
Cancel - Used to cancel a pending Invite.
Options - Used to query the server for its
capabilities.
Register - Used to bind a permanent address to
the current location of the user.
SIP Responds Codes
 1## Informational Responses
100 Trying
180 Ringing
181 Call is being forwarded
 2## Success
200 OK
 3## Redirection
300 Multiple Choices
301 Moved Permanently
302 Moved Temporarily
SIP Responds Codes (cont’d..)
 4## Client Error
400 Bad Request
401 Unauthorized
482 Loop Detected
486 Busy Here
 5## Server Failure
500 Server Internal Error
 6## Global Failure
600 Busy Everywhere
SIP Structure
Transaction User (TU)
Transaction Layer
Client
Component
Server
Component
Transport Layer
Syntax Layer
Conclusion
SIP is short, simple and flexible signaling
protocol for creating, modifying, and terminating
sessions with one or more participants.
Future work will be to bridge between SIP and
other multimedia control protocols like RSW
control protocol.
Thank You