The invite method can also be used for establishing a multiparty call. Once it has received a final response to an INVITE, the client that initi ated the INVITE sends an ACK. The ACK method is used as a confirmation that the final response has been received. For example, if the response to an INVITE indicates that the called user is busy and the call cannot be com pleted at that time, then the calling client will send an ACK. On the other hand, if the response to the INVITE indicates that the called user is being alerted or that the call is being forwarded, then the client does not send ACK, because such responses are not considered final. The BYE method terminates a session. The method can be issued by either the calling or called party and is used when the party in question hangs up. The OPTIONS method queries a server as to its capabilities. This method could be used, for example, to determine whether a called user agent can support a particular type of media or how a called user agent would respond if sent an INVITE. In such a case, the response might indi cate that the user can support certain types of media or perhaps that the user is currently unavailable. The CANCEL method is used to terminate a pending request. For exam ple, CANCEL could be used to terminate a session where an INVITE has been sent, but a final response has not yet been received. The REGISTER method is used by a useragent client to log in and reg ister its address with a SIP server, thereby letting the registrar know the address at which the user can be reached. The useragent client might reg ister with a local SIP server at startup, with a known registrar server whose address is configured within the user agent, or by multicasting to the allSIPservers multicast address. A client can register with multiple servers and a given client can have multiple registrations at a single server. Such a situation would occur in the event that a user has logged in at several terminals or devices. If a user has multiple active registrations, then calls to the user may be sent to all regis tered destinations. This capability can enable a onenumber service where a user publishes just a single number, but when that number is called, the users office phone, home office phone, and wireless phone all ring. Fig ure 57 shows an example of how this would work and also shows the usage of the CANCEL message in such a scenario.