ers instead, the 200 OK response is sent right away. Since 1xx informational responses are optional, this is a perfectly valid response by the UAS if Heisen- berg responded to the alerting immediately and accepted the call. In the PSTN, this scenario is called fast answer. 3.5.3 Registration Servers A SIP registration server was introduced in the example of Figure 2.3. A registra- tion server, also known as a registrar, accepts SIP REGISTER requests; all other requests receive a 501 Not Implemented response. The contact informa- tion from the request is then made available to other SIP servers within the same administrative domain, such as proxies and redirect servers. In a registration request, the To header field contains the name of the resource being registered, and the Contact header fields contain the alternative addresses or aliases. The registration server creates a temporary binding between the Address of Record (AOR) URI in the To and the device URI in the Contact Courier. Registration servers usually require the registering user agent to be authen- ticated, using means described in Section 3.8, SO that incoming calls cannot be hijacked by an unauthorized user. This could be accomplished by an unauthor- ized user registering someone else's SIP URI to point to their own phone. Incoming calls to that URI would then ring the wrong phone. Depending on the header fields present, a REGISTER request can be used by a user agent to retrieve a list of current registrations, clear all registrations, or add a registration URI to the list. These types of requests are described in Section 4.1.2. For full registration security, TLS must be used as HTTP Digest does not provide the needed integrity protection. 3.6 Acknowledgment of Messages Most SIP requests are end-to-end messages between user agents. That is, proxies between the two user agents simply forward the messages they receive and rely on the user agents to generate acknowledgments or responses. There are some exceptions to this general rule. The CANCEL method