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Yes its a good link you must follow http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html

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Posted by HamidAliKhan

http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html

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Posted by Hash007

The following are the different parts of a request Messages:
First Line: Contains the method name, such as INVITE.
Via: Indicates the transport used for transaction and contains the address at which the caller is expecting responses to the request.
From: Contains a display name, and a SIP or secure SIP (SIPS) uniform resource identifier (URI) that indicates the originator of the request.
To: Contains the display name (TPU) and a SIP or a SIPS URI towards which the request was originally directed.
Subject: Contains the subject line for the call
Call ID: Contains a globally unique identifier for this call generated by the combination of a random string and the end point’s host name or IP address.
Content-Type: Contains a description of the message body.
Command sequence (CSeq): Contains an integer and a method name.
Contact: Contains a SIP or SIPS URI that represents a direct route to contact the end point. Contact usually contains a username that is a fully qualified domain name.
Content-Length: Contains an octet (byte) count of the message body.

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Posted by sagitraz

Start Line

Method Name: A command that states what you want to do. (Full list later)
• Request-URI: URI address of involved party. This address can be modified by SIP Servers. Addresses in other headers should not be modified. In the first message that initiates a call, the Request URI will be the UAC’s address
• Version: Currently always SIP 2.0
• Carriage Return and Line Feed: (Press “Enter” on your keyboard). Must be there.
Headers - Choose from a variety of headers. Only use the ones needed for the specific Message. (More later)
Message Body – Information destined for the UAC only . Not looked at by SIP Servers.

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Posted by nishanth