Introduction to SIP

SIP stands for Session Initiation Protocol a signaling protocol defined by IETF used for controlling communication sessions. SIP is used to create, manage and terminate sessions in an IP based network. SIP is widely used in VOIP (Voice over IP) .

 

The two main entities in a SIP session are

User Agent Client

User Agent Server

There are several types of server

 

Proxy Server

Redirect Server

Registrar

Location Server

 

SIP is similar to HTTP protocol. To understand the differences lets first learn how HTTP Protocol works.

Here is a sample HTTP request

GET /index.html HTTP/1.1

Host: www.w3.org

User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:20.0) Gecko/20100101 Firefox/20.0

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

Accept-Language: en-US,en;q=0.5

The first line is the request line which has the request method (GET), the URI that is being requested (index.html) and the protocol version (HTTP/1.1)

Following the request line is one or more request headers like Host, User-Agent etc.

An empty line break follows the request headers which is followed an options message body.

Once you understand the protocol you can mimic a browser and send a request to web server using telnet or a simple java program that uses plain socket.

TBD

Now lets take a look at SIP protocol and find out the differences between HTTP and SIP

Here is a sample SIP request

REGISTER sip:ss2.biloxi.example.com SIP/2.0
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Bob <sips:bob@biloxi.example.com>;tag=a73kszlfl
To: Bob <sips:bob@biloxi.example.com>
Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
CSeq: 1 REGISTER
Contact: <sips:bob@client.biloxi.example.com>
Content-Length: 0

The first line is the request line which has the request method (REGISTER), the URI that is being requested (index.html) and the protocol version (HTTP/1.1)

Following the request line is one or more request headers like Host, User-Agent etc.

An empty line break follows the request headers which is followed an options message body.