What is SUBSCRIBE Request Extension Method in SIP?

What is SUBSCRIBE Request Extension Method?

SUBSCRIBE is used by user agents to begin a subscription for the purpose of getting notification about a special event.

1. SUBSCRIBE request has a time period in the Expires header field that shows the desired duration of existence of a subscription.

2. After the specified time period passes, the subscription is automatically ended.

3. A successful subscription begins a basic call flow between the user agents.

4. A subscription can be recharged by sending another SUBSCRIBE within the dialog before the expiration time.

5. The server accepting a subscription returns a 200 OK.

6. Clients can unsubscribe by sending another SUBSCRIBE method with Expires value 0 (zero).