A NOTIFY method without a Subscription-ID Header is an unsolicited notification. This can be used if the server wants to notify all clients, for example, that the store was shut down and that notifications will not be working properly for some period of time. Call back responses may not occur in real time because the store may automatically coalesce events.
Because UDP packets sent by the NOTIFY method may be lost, the server will keep track of past events that have subscriptions. The server will maintain subscriptions for which events occurred until the client sends an acknowledgement (using the Subscription-ID Header in any method, typically the POLL Method) that it is refreshing the subscription.
The following is an example UDP Request from a server. The Uniform Resource Identifier (URI) that is used was originally specified in the Call-Back Header of the SUBSCRIBE Method when the subscription was created. The Subscription-ID Header is the identifier of the subscription for the event being fired.
NOTIFY httpu://www.fourthcoffee.com:8080/510 HTTP/1.1 Subscription-id: 15