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SIP Instant Messaging

Instant Messaging (IM) is an application that allows exchanging messages in near real-time between two users or more. These messages can be sent within the context of a call (in-dialog) or outside a call (out-of-dialog). There can be different applications like chat box app, even control signals can be exchanged among users. Session Initiation Protocol (SIP) already developed the fundamental framework for real-time communication that IM requires, RFC 3428 extends SIP with the MESSAGE method that allows the transfer of IMs. Since, the MESSAGE request is just an extension, it has all the built in routing and security features of SIP. It carries the content in the form of MIME body parts. Usually the MESSAGE method does not create a dialog, each IM stands alone like a pager message. However, different limitations of SIP MESSAGE method have been resolved in RFC 4975 known as Message Session Relay Protocol (MSRP). MSRP is especially useful if the contents are multimedia in nature.
SIP IM Call flow
In the example message flow (RFC 3428), shown in the figure, an IM is sent from user agent 1 (UA1) to user agent 2 (UA 2) through s single proxy.

1.    MESSAGE UA1 -> PROXY
MESSAGE sip:ua2@vocal.com SIP/2.0
Via: SIP/2.0/TCP ua1.vocal.com;branch=z9hG4bK123abcabc
Max-Forwards: 70
From: <sip:ua1@vocal.com>;tag=12345
To: <sip:ua2@vocal.com>
Call-ID: 418447bfc2f658aa@1.2.3.4
CSeq: 1 MESSAGE
Content-Type: text/plain
Content-Length: 13
Hello, there.

2.    MESSAGE PROXY -> UA2
MESSAGE sip:ua2@vocal.com SIP/2.0
Via: SIP/2.0/TCP proxy.vocal.com;branch=z9hG4bK123fsdfdvc
Via: SIP/2.0/TCP ua1.vocal.com;branch=z9hG4bK123abcabc;received=1.2.3.4
Max-Forwards: 69
From: <sip:ua1@vocal.com>;tag=12394
To: <sip:ua2@vocal.com>
Call-ID: 418447bfc2f658aa@1.2.3.4
CSeq: 1 MESSAGE
Content-Type: text/plain
Content-Length: 13
Hello, there.

This message when received by UA2, displayed and a response is generated and sent to the proxy.

3.    MESSAGE UA2 -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/TCP proxy.vocal.com; branch= z9hG4bK123fsdfdvc;received=192.0.2.1
Via: SIP/2.0/TCP ua1.vocal.com; branch= z9hG4bK123abcabc;received=1.2.3.4
From: <sip:ua1@vocal.com>;tag=12394
To: sip:ua2@vocal.com;tag=abc56abc
Call-ID: 418447bfc2f658aa@1.2.3.4
CSeq: 1 MESSAGE

4.    MESSAGE PROXY -> UA1
SIP/2.0 200 OK
Via: SIP/2.0/TCP ua1.vocal.com;branch=z9hG4bK123abcabc;received=1.2.3.4
From: <sip:ua1@vocal.com>;tag=12345
To: sip:ua2@vocal.com;tag=abc56abc
Call-ID: 418447bfc2f658aa@1.2.3.4
CSeq: 1 MESSAGE

More Information

 

VOCAL Technologies has been in business for over 30 years and is an engineering design house that can provide a custom solution that meets your unique communication requirements.

Please contact us to discuss your communication application requirements.

VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Amherst New York 14228
Phone: +1-716-688-4675
Fax: +1-716-639-0713
Email: sales@vocal.com