ree which causes the transferee to generate a new INVITE (called a "trig- gered" INVITE) to the target. The successful INVITE "replaces" the existing session between the transferor and the transfer target. When the transferee receives notification that the transfer was successful, the session between the transferor and the transferee is terminated with a BYE. This application uses "escaped" header fields in the Refer-To URI. That is, certain SIP header fields are specified and prepopulated in the URI, which are then copied into the SIP Request Messages Transferor Transferee Transfer target Media session INVITE 200 OK Media session REFER 202 Accepted INVITE (Replaces) NOTIFY 200 OK 200 OK NOTIFY 200 OK Media session 200 OK 200 OK Figure 4.7 Use of REFER and Replaces to perform attended transfer feature. triggered INVITE. In this case, the transferor generates the Replaces (see Section 6.2.23) header field necessary in the triggered INVITE to make the transfer succeed. The transferee copies the escaped Replaces header and places it in the INVITE. The acceptance of a REFER with a 202 Accepted response creates an implicit subscription (a subscription without sending a SUBSCRIBE request; see Section 4.1.8). After sending the 202 Accepted, the target must send an immediate NOTIFY with the status 100 Trying and Subscription - State: active; expires=60, which indicates that the subscription will expire in 60 seconds (the expiration value is chosen by the notifier). The Subscription-State header contains the expiration time of the SIP: Understanding the Session Initiation Protocol subscription. If that time period expires before the triggered request has com- pleted, both sides terminate the subscription, with the notifier sending a final notification as discussed in the next section. The subscription is terminated when the transfer target (the party that accepted the REFER) sends a final notification (a NOTIFY with Subscription-State terminated; reason=noresource). Usually, this is after the transfer target has received 