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Cmpe 491 Special Project In
Computer Engineering
SIP User Agent In JAVA
Alp Eren YILMAZ
&
Serdar YALÇINKAYA
Our Agenda - I
Introduction to SIP – Alp Eren
 A Sample of SIP Call – Alp Eren
 Behaviors of SIP User Agents – Alp Eren
 SIP Programming – Alp Eren
 SIP Servers – Serdar
 SIP Message Structure - Serdar

Our Agenda – II

Evaluation of Tested User Agents - Serdar
 Proposed System - Serdar
– Overview of the System
– Development Steps
– Typical GUI of the Sytem

Questions
Introduction to SIP:
Session Initiation Protocol (SIP) is an
application – layer control (signaling) protocol
for creating, modifying and terminating
sessions with one or more participants.
These sessions includes Internet telephone
calls, multimedia distributions and multimedia
conferences.

Important Properties of SIP :

The ability of establishing and modifying
peer – to – peer multimedia sessions
independently of underlying transport
protocols and characteristic of session

SIP supports the personal mobility by
assigning a single SIP Uniform Resource
Identifier (URI)
SIP Communication Messages:
REGISTRATION – Locate the user
 INVITE – Initiate the session
 ACK – Acknowledge of the INVITE
 BYE – Termination of the session
 CANCEL – Cancel pending request
 OPTIONS – Capability check

Complementary IETF Protocols :
Resources Reservation Protocol – RSVP
 Real Time Protocol – RTP
 Real Time Streaming Protocol – RTSP
 Session Announcement Protocol – SAP
 Session Description Protocol – SDP

Protocol Layers:

Syntax and Encoding Layer
 Transportation Layer
 Transaction Layer
 User Transaction Layer
Typical SIP Call :

The caller agent Alp @boun.edu.tr
– PC user (Soft Phone)

The called party Serdar @ibm.com.tr
– SIP Phone user

Two Proxy Servers
Typical SIP Call :
Alp ‘s PC
@boun.edu..tr
Serdar’s
SIP Phone
Message Sequence:
SIP User Agent Behaviors:
Three common characterisctic :
• Generating the request
• Sending the request
• Processing the responses
SIP Programming:

SIP has texture encoding feature.[1]

SIP allows third parties or user to program

SIP follows HTTP programming model
Programming Mechanisms:


In order to develop services programmers needs APIs
Three mechanisms suggested in IETF:[2]
( SIP – CPL )
– SIP Common Gateway Interface ( SIP – CGI )
– SIP Servlet
– SIP Call Processing Language

Other efforts
– Parlay
– JAIN™
Call Processing Language (CPL) :

Designed by the IETF to support sophisticated telephony services
– May be used by both SIP or H.323.

XML based scripting language for describing controlling call services[3]
– Simple Syntax
– Extendible
– Easily edited by GUI tools

Scripts runs on network SIP signaling server to create end user services
– Lightweight CPL interpreter is need to parser & validate scripts.
CPL Example :
A simple script that blocks anonymous callers [4];
<?xml version="1.0" ?>
<!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">
<cpl>
<incoming>
<address-switch field="origin" subfield="user">
<address is="anonymous">
<reject status="reject"
reason="I don't accept anonymous calls" />
</address>
</address-switch>
</incoming>
</cpl>
Common Gateway Interface (CGI) :

Almost identical to HTTP CGI [5]

Language independent ( Perl, Tcl, C, C++, ... )
– Any binary may be executed as a separate program

Suitable for services that contains substantial web content

Passes message parameters through environmental variables to a
separate program.
– More flexible but more risky

Feb. 1, 2001: RFC 3050 (Common Gateway Interface for SIP)
published [6]
Java Servlets :





Similar to HTTP servlets
Instead of using a separate process, messages are passed to
a class
The class runs within a JVM (Java Virtual Machine) on
server
Security provided by Java
Portable between OSs & servers
JAIN™ SIP API:



Low level API that maps to IETF - RFC 2543
Interfaces for services across circuit switched and packet
networks
Three major objectives :
– Service Portability – Write Once Run Anywhere
– Network Converges – Any underlying network architecture
IP, ATM,Wireless,...
– Service Provider Access by Anyone
JAIN™ SIP API: ( Cont.)

Three SIP APIs under JAIN initiative;
– JAIN™ SIP API (JSR 32) :
 Low level API for almost any signaling protocol ( SIP, H.323, ... )
 Requires extensive knowledge of SIP.
 Avaliable at Final Release, http://jcp.org/jsr/stage/final.jsp
– JAIN™ SIP Lite :
 High Level API for rapid application development
– Especially User Agent development
 Under development , http://jcp.org/jsr/detail/125.jsp
– SIP Servlets : [7]
 API for SIP servlets
 Under development, http://jcp.org/jsr/detail/116.jsp
Conclusion:

The proposed user agent client will be
implemented by using the JAIN SIP API [9]
that is released in December 2001.

GUI is designed by using Java Swing
SIP Servers
Three types of servers:
–
–
–
Registrars
Proxy
Redirect
Registrars
Registrars keep track of users within their
assigned network domain
Registrars take register requests and stores
the information in the SIP message in a
location service
Registrars
Registrar first looks at the Request-URI
if it has access to a location service
responsible for the domain
if not, act like a proxy server and
forward the message
Registrar
When gets a register message, it should
authenticate the user agent client.
Registrar takes the address in the TO field
Checks this address in its location service
for the set of bindings
Compares the contact addresses
If exists overwrite, else create new binding
Registrar
 Checks expiration interval of binding
– if it is not refreshed then remove
 The registrar must send a 200 (OK) response back
if everything Ok
 If it fails then a 404 (not found) response is sent.
Proxy
Proxy servers are application-layer routers
that forward the SIP requests and responses to
the proper destinations.
Proxy
Proxies will make routing decisions and
modify messages
Responses will route the same proxies in
reverse order
Proxy
Two kinds of proxy:
– Stateless
– Stateful
Stateless Proxy
Acts as a simple forwarding element
Decides the route for the request &
forwards it
Then forget all information about the
message
Stateful Proxy
Remembers information (transaction state)
about each incoming and outgoing
(processed incoming request) request
This information is used to process future
messages related to that particular request
Proxy can decide to fork an incoming
request, which must be handled statefully
Stateful Proxy
Creates new server transactions for requests
Check validity of message
Decide route
Redirect
 Redirect servers are useful for reducing the load of
the proxy servers that are responsible for routing
requests by relying on redirection
 Servers push redirect information in the response
 Client got the redirection & will send a new
request based on the new routing information
SIP Messages
SIP messages are text based
generic message = start line
message header
CRLF
[message body]
SIP Messages
Two groups of messages:
– Requests
– Responses
Requests
Register
Invite
Ack
Cancel
Bye
Options
Requests-Register
Register provides a new binding between an
address and one or more contact addresses
Can remove previous bindings
Query which bindings are currently in place
Requests-Invite
Invite indicates that the user is being invited
to participate in a session
The message body contains a description of
the session to which the callee is being
invited
SDP is used generally
Requests-Ack
Indicates that the caller has received a final
response to the Invite
Ack may contain a body with the final
session description
Empty body indicates the session
description in the Invite will be used
Requests-Cancel & Bye
The Cancel request cancels a pending
request with given Call-ID, To, From and
CSeq
Bye indicates to the server that the call will
be ended
Requests-Options
Options allows a client to query a client or
server for their capabilities
Client discovers information about methods,
content types, extensions, codecs etc.
supported without actually ”ringing” the
other party
Responses
SIP Version – Status Code – Reason Phrase
3-digit Status Code
The first digit describes the class of the
response
Responses-Status Code
1xx: Informational
2xx: Success
3xx: Redirection
4xx: Client Error
5xx: Server Error
6xx: Global Failure
Responses-Examples










100 – Trying
180 – Ringing
200 – OK
300 – Multiple Choices
301 – Moved Temporarily
400 – Bad Request
401 – Unauthorized
500 – Internal Server Error
504 – Gateway Time-out
600 – Busy Everywhere
SDP
SDP is intended for describing multimedia
sessions for the purposes of session
announcement, session invitation, and other
forms of multimedia session initiation.
SDP
The purpose of SDP is to convey
information about media streams in
multimedia sessions to allow the
recipients of a session description to
participate in the session
SDP includes description of
Media to use (codec, sampling rate)
Media destination (IP address and port
number)
Session name and purpose
Times the session is active
Contact information
User Agents- EZ-Phone
User Agents- Ubiquity
User Agents- Ubiquity
Proposed User Agent
Implementing a JAVA based User Agent
providing functions:
 Registering
 Making, canceling, forwarding, ending a call
 Address book
 Debugging Window
 (Optional) Video Conferencing
Proposed User Agent
Proposed User Agent
Proposed User Agent