a final response to the trig- gered request. However, a transfer target that does not wish to establish a sub- scription and provide a final result of the REFER may send an immediate NOTIFY indicating that the subscription has been terminated. Each REFER sent creates a separate subscription. If more than one REFER is sent within a dialog, the resulting notifications (and subscriptions) are identified by an parameter in the Event header field. The id parameter is optional in refer triggered NOTIFYS except when multiple REFERs have been accepted, in which case it is mandatory. The optional Referred-By header field can be included in a REFER request. Table 4.7 lists the mandatory header fields in a REFER request. 4.1.8 SUBSCRIBE The SUBSCRIBE method [5] is used by a user agent to establish a subscrip- tion for the purpose of receiving notifications (via the NOTIFY method) about a particular event. A successful subscription establishes a dialog between the UAC and the UAS. The subscription request contains an Expires (see Section 6.4.7) header field, which indicates the desired duration of the existence of the subscription. After this time period passes, the subscription is automati- cally terminated. The subscription can be refreshed by sending another SUBSCRIBE within the dialog before the expiration time. A server accepting a subscription returns a 200 OK response also containing an Expires header Table 4.7 Mandatory Header Fields for a REFER Call-ID Contact Max-Forwards Refer-To SIP Request Messages field. The expiration timer can be the same as the request, or the server may shorten the interval, but it may not lengthen the interval. There is no "UNSUBSCRIBE" method used in SIP-instead a SUBSCRIBE with Expires : 0 requests the termination of a subscription and hence the dialog. A terminated subscription (either due to timeout out or a termination request) will result in a final NOTIFY indicating that the subscription has been terminated (see Section 4.1.9 on NOTIFY). A 202 Accepted response to a SUBSCRIBE does n