The INVITE response message format describes which content a response to an INVITE request message must have.
System Specific Information
The header described by the Response Message Format contains the following elements:
Status LineParent topic: SIP Overview
Contains SIP version, SP status code, SP reason phrase and CRLF.
Contains the local address of the calling user, that is, pc33.server1.com where it is expecting the responses to come. There can be more than one Via fields, because each element through which the INVITE request was being passed has added its identity in the Via field.
Used to limit the number of hops that this request may take before reaching the recipient. It is decreased by one at each hop. It is necessary to prevent the request from traveling forever in a loop.
It contains the display name of the called user, a SIP or SIPS (Secure SIP) URI and a tag. The tag is used to represent the called user in a dialog.
Contains the display name of the calling user and a SIP or SIPS URI. It also contains a tag which is a pseudo-random sequence inserted by the SIP application. It works as an identifier of the caller in the dialog.
Globally unique identifier of the call generated as the combination of a pseudo-random string and the SIP phone’s IP address. The Call ID is unique for a call. A call may contain several dialogs. Each dialog is uniquely identified by a combination of From, To and Call ID.
Contains an integer and a method name. When a transaction starts, the first message is given a random CSeq. After that it is incremented by one with each new message. It is used to detect non-delivery of a message or out-of-order delivery of messages.
Contains the exact address of the called user. So the calling user doesn't need to use the proxy servers to find the called user in the future.
Used to send the response to the request. Contact field is used to send future requests. When a user generates a BYE request (a new request and not a response to INVITE), it goes directly to the other user bypassing the proxies.
Contains a description of the message body.
An octet (byte) count of the message body.