The INVITE is a SIP method that specifies the action that the requester (Calling Party) wants the server (Called Party) to take. The INVITE request contains a number of header fields. Header fields are named attributes that provide additional information about a message. The ones present in an INVITE include a unique identifier for the call, the destination address, Calling Party Address, and information about the type of session that the requester wishes to establish with the server.
INVITE sip:email@example.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
To: Bob <sip:firstname.lastname@example.org>
From: Alice <sip:email@example.com>;tag=1928301774
CSeq: 314159 INVITE
The first line of the text-encoded message contains the method name (INVITE). The lines that follow are a list of header fields. This example contains a minimum required set. The header fields are briefly described below:
Via- Contains the address (pc33.atlanta.com) at which Alice is expecting to receive responses to this request. It also contains a branch parameter that identifies this transaction.
To- Contains a display name (Bob) and a SIP or SIPS URI (sip:firstname.lastname@example.org) towards which the request was originally directed.
From- Also contains a display name (Alice) and a SIP or SIPS URI (sip:email@example.com) that indicate the originator of the request. This header field also has a tag parameter containing a random string (1928301774) that was added to the URI by the softphone. It is used for identification purposes.
Call-ID- Contains a globally unique identifier for this call, generated by the combination of a random string and the softphone's host name or IP address. The combination of the To tag, From tag, and Call-ID completely defines a peer-to-peer SIP relationship between Alice and Bob and is referred to as a dialog.
CSeq- or Command Sequence, contains an integer and a method name. The CSeq number is incremented for each new request within a dialog and is a traditional sequence number.
Contact- Contains a SIP or SIPS URI that represents a direct route to contact Alice, usually composed of a username at a fully qualified domain name (FQDN). While an FQDN is preferred, many end systems do not have registered domain names, so IP addresses are permitted. While the Via header field tells other elements where to send the response, the Contact header field tells other elements where to send future requests.
Max-Forwards- Serves to limit the number of hops a request can make on the way to its destination. It consists of an integer that is decremented by one at each hop.
Content-Type- Contains a description of the message body (not shown).
Content-Length- Contains an octet (byte) count of the message body.
Copyright (C) The Internet Society (2004). All Rights Reserved
The INVITE method is used to establish media sessions between User Agents. The RE-INVITE message permits the IMG to change parameters of an existing or pending call.