me as the To header. It is recommended that the same Call - ID be used for all registrations by a user agent. A user agent sending a REGISTER request may receive a 3xx redirection or 4xx failure response containing a Contact header of the location to which registrations should be sent. A third-party registration occurs when the party sending the registration request is not the party that is being registered. In this case, the From header will contain the URI of the party submitting the registration on behalf of the party identified in the To header. Chapter 3 contains an example of a first-party SIP: Understanding the Session Initiation Protocol registration. An example third-party registration request for the user Euclid is shown below: REGISTER sip: :registrar.athens.gr SIP/2.0 Via: SIP/2.0/UDP 201.202.203.204:5060;branch=z9hG4bk313 Max-Forwards:70 To: sip:euclid@athens.gr From: <sip:secretary@academy.athens.gr>;tag=543131 Call-ID: 2000-July-07-23:59:59.1234@201.202.203.204 CSeq: 1 REGISTER Contact: sip:euclid@parthenon.athens.gr Contact: mailto:euclid@geometry.org Content-Length: 0 The mandatory headers in a REGISTER request are shown in Table 4.3. 4.1.3 BYE The BYE method is used to terminate an established media session. In teleph- ony, it is similar to a release message. A session is considered established if an INVITE has received a success class response (2xx) or an ACK has been sent. A BYE is sent only by user agents participating in the session, never by proxies or other third parties. It is an end-to-end method, SO responses are only generated by the other user agent. A user agent responds with a 481 Dialog/Trans- action Does Not Exist to a BYE for an unknown dialog. It is not recommended that a BYE be used to cancel pending INVITES because it will not be forked like an INVITE and may not reach the same set of user agents as the INVITE. An example BYE request looks like the following: BYE sip: info@hypotenuse.org SIP/2.0 Via: SIP/2.0/UDP port443.hotmail.com:5060;branch=z9hG4bk312bc Max-Forwards: 70 To: