Call-Back Header
Topic Last Modified: 2006-06-13
This topic applies to Exchange 2000 Server SP2 or later.
The WebDAV Call-Back header is used to specify a User Datagram Protocol (UDP) callback address so that notifications may be actively pushed to the client from the server using the NOTIFY Method.
Call-back := "Call-Back:" URI
URI := "httpu://" machine_name ":" port "/" path
A UDP callback URL is of the form: "httpu://machine_name:port/path", where the path may be defined by the client so that multiple clients can subscribe to the same event without conflict. The port number must be specified.
If the Call-Back header is present in the SUBSCRIBE Method and includes a callback address, then the client has chosen the delivery model using the NOTIFY Method.
If the Call-Back header is not present in the SUBSCRIBE Method (or it is empty), then the client has chosen the delivery model using the POLL Method.
Example
The following subscription instructs the server to use the NOTIFY Method to call back the UDP server www.fourthcoffee.com using port 8080.
SUBSCRIBE /public/subtest HTTP/1.1
Host: www.contoso.com
Notification-type: Update
Call-Back: httpu://www.fourthcoffee.com:8080/510