SIP is an agile, general-purpose tool for creating, modifying, and terminating sessions that works independently of underlying transport protocols and without dependency on the type of session that is being established.

SIP works with both IPv4 and IPv6.

The modern version of SIP is defined in RFC document 3261.

Terminology

·         User Agents (UA): Client (originates calls) and Server (listens for incoming calls) UA.

·         SIP Proxy Server: A SIP server that can receive SIP requests and handles or forward them to the intended recipient.

·         SIP Redirect Server: accepts a SIP request and conveys to the originating client the way to route the call.

·         SIP Registrar Server: accepts registration requests and maps client’s address to user’s sign-in name or SIP URI.

·         Gateways: Provide call control and translation functions from one format to another.

Introduction

·         It’s an application level protocol for managing sessions and is based on HTTP like request/response model. It allows to enable multi-user sessions regardless of media content.

·         Various kinds of SIP Servers exist: Proxy, Redirect and Registrar.

·         SIP clients send SIP request to SIP Servers which reply with at least one response.

·         Office Communicator 2007 is an example of SIP client.

·         Office Communications Server 2007 is an example of SIP Server.

·         SIP provides for the following basic requirements in communications:

1.    User location services

2.    Session establishment

3.    Session participant management

4.    Endpoint capabilities establishment using Session Description Protocol (SDP).

·         SIP can work with other protocols like LDAP, RTP, RADIUS, etc.

 

Users in a SIP network are identified by unique SIP addresses. A SIP address is similar to an e-mail address and is in the format of sip:userID@domain.com. Users register with a registrar server using their assigned SIP addresses. The registrar server provides this information to the location server upon request.

 

Over time, a SIP end user might move between end systems. The location of the end user can be dynamically registered with the SIP server. The location server can use one or more protocols (including finger, rwhois, and LDAP) to locate the end user. Because the end user can be logged in at more than one station and because the location server can sometimes have inaccurate information, it might return more than one address for the end user. If the request is coming through a SIP proxy server, the proxy server will try each of the returned addresses until it locates the end user. If the request is coming through a SIP redirect server, the redirect server forwards all the addresses to the caller in the Contact header field of the invitation response.

Session Initiation Commands

 

 

Method Name	Description
INVITE	Initiates a session. This method includes information about the calling and called users and the type of media that is to be exchanged.
ACK	Sent by the client who sends the INVITE. ACK is sent to confirm that the session is established. Media can then be exchanged.
BYE	Terminates a session. This method can be sent by either user.
CANCEL	Terminates a pending request, such as an outstanding INVITE. After a session is established, a BYE method needs to be used to terminate the session.
OPTIONS	Queries the capabilities of the server or other devices. It can be used to check media capabilities before issuing an INVITE.
REGISTER	Used by client to login and register its address with a SIP registrar server.

SIP Architecture

 

 

1.    User A calls User B

2.    SIP Proxy Server gets location of User B

3.    Call gets proxied to User B

4.    Response from User B

5.    Response gets proxied to User A

6.    Media Channel get established between User A & User B.

SIP Response Codes

SIP response contains following:

Status Code: Three digit number

Reason Phrase: text description of response

Status codes defined in SIP have values between 100 and 699. The first digit of status code indicates response class.

1xx       -           Provisional

2xx       -           Success

3xx       -           Redirection

4xx       -           Client Error

5xx       -           Server Error

6xx       -           Global Failure

SIP Messages

 

SIP messages have three kinds of headers:

General Headers: These are used in both requests & responses and contain basic information like TO and FROM fields.

Request Header: Part of Request messages and contains fields like SUBJECT and PRIORITY.

Response Header: Part of Response Messages and contains fields like UNSUPPORTED and RETRY-AFTER fields.

SAMPLE REQUEST MESSAGE

INVITE sip:userA@domain.com SIP/2.0
Via: SIP/2.0/UDP pc1.domain.com

Max-Forward: 70
To: “userB” <sip:userB@ domain.com >
From: “userA” <sip:userA@domain.com >;tag=123
Call-ID: 1234567890@171.1.1.1
CSeq: 1 INVITE
Contact: <sip:userB@ pc1 domain.com >
Content-Type: application/sdp
Content-Length: 100

---- Message Body (SDP) Not Shown ----

 

SAMPLE RESPONSE MESSAGE

SIP/2.0 200 OK
Via: SIP/2.0/UDP pc1.domain.com

To: “userB” sip:userB@ domain.com ; tag=999
From: “userA” <sip:userA@domain.com >;tag=123
Call-ID: 1234567890@171.1.1.1
CSeq: 1 INVITE
Contact: <sip:userB@ pc1 domain.com >
Content-Type: application/sdp
Content-Length: 100

---- Message Body (SDP) Not Shown ----