t address of the tar- get agent. A registration server accepts a REGISTER message described below and makes contact information available as described below. The following are SIP messages to use during call establishment in SIP. The others are omitted. REGISTER message: The REGISTER message informs a SIP server of the current contact URIs of a user agent. The SIP server transfers a SIP request to a contact URI of the user agent. Multiple contact URIs can be specified to redirect and fork a SIP message SO that a SIP message is routed to the specified contact URI. In a contact header, each contact URI is separated by a comma. In the following case, a user registers a SIP address, a mail address, and a phone number as its contact address. An expires field specifies the duration during which the registered user agent is valid. If a value expires field is zero, the registered device is unregistered from the home network. REGISTER sip:userA@ai.u-hyogo.ac.jp SIP/2.0 To: :userA<sip:userA@ai.u-hyogo.ac.jp user=phone> From:userA<sip:userA@ai.u-hyogo.ac.jpx Expires 7200 Call-ID: 12-34-56-78@ai.u-hyogo.ac.jp CSeq: 1 REGISTER Contact: <sip:userA@ai.u-hyogo.ac.jp>, <mailto:userA@ai.u-hyogo.ac.jp> tel:+81-12-xxx-yyyy@phone.com; user=phone> Content-Type: application/sdp Content-Length: 0 INVITE message: The INVITE message establishes multimedia sessions between a UAC and a UAS. An example of the INVITE message is as follows: A UAC issues an INVITE message to establish a session with a UAS. The From and To header in the INVITE message contain an address of a UAC and a UAS in a URI form, respectively. Note that the SIP message header can be extended and the extended messages are sent to the To: address with- out processing the headers. The following example shows a calling request from user A to user B with an audio device. Information about a communica- tion media is described in the content of the INVITE message using Session Description Protocol (SDP). INVITE sip:123@ai.u-hyogo.ac.jp user=phone SIP/2.0 Max=Forward: To: 123@ai.