Alert-Info: <http://www.provider.com/tones/internal_caller.pcm> 6.1.2 Allow-Events TheAllow-Events header field [3] is used to list the support event packages that are supported. A UA that supports SIP Events then knows that it may send a SUBSCRIBE to that event package. The list of currently defined packages is in Table 4.8. The compact form is u. Examples are shown here: Allow-Events : dialog u: conference 6.1.3 Call-ID The Call - ID header field is mandatory in all SIP requests and responses. It is part of the dialog used to uniquely identify a call between two user agents. A SIP: Understanding the Session Initiation Protocol Call-ID must be unique across calls, except in the case of a Call-ID in reg- istration requests. All registrations for a user agent should use the same Call-ID A Call - ID is always created by a user agent and is never modified by a server. The Call-II is usually made up of a local-id, which should be a crypto- graphically random identifier, or a local-id, the @ symbol, and a host name or IP address. Because a user agent can ensure that its local-id is unique within its domain, the addition of the globally unique host name makes the Call-ID globally unique. Some security is provided by the randomness of the Call-ID, because this prevents a third party from guessing a Call-ID and presenting false requests. The compact form of the Call - ID header field is i. Examples of Call-ID are shown here: Call-ID: 34a5d553192cc35@15.34.3 Call-ID: 44fer23ei4291dekfer34231232 i: : 35866383092031257@port34.carrier.com 6.1.4 Contact The Contact header field is used to convey a URI that identifies the resource requested or the request originator, depending on whether it is present in a request or response. Once a Contact header field has been received, that URI can be cached and used for routing future requests within a dialog. For example, a Contact header field in a 200 OK response to an INVITE can allow the acknowledgment ACK message and all future requests during this call to bypass proxies and go dire