The INVITE request message format describes which content an INVITE request message must have.
System Specific Information
The header described by the request message format contains the following elements:
Request-LineParent topic: SIP Overview
Contains Method SP (Single Space), Request-URI, SP SIP version, CRLF (Carriage Return + Line Feed).
Contains the local address of the calling user, that is, pc33.server1.com where it is expecting the responses to come.
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.
Contains the display name of the called user and a SIP or SIPS URI.
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 a SIP or SIPS URI that is a direct route to the calling user. It contains a username and a FQDN (Fully Qualified Domain Name). It may also have an IP address.
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.