EZcheats

The following provides an overview of the data provided in a SSDP
   system.

   Services are identified by a unique pairing of a service type URI
   and a Unique Service Name (USN) URI.

   Service types identify a type of service, such as a refrigerator,
   clock/radio, what have you. The exact meaning of a service type is
   outside the scope of this specification. For the purposes of this
   specification, a service type is an opaque identifier that
   identifies a particular type of service.

   A USN is a URI that uniquely identifies a particular instance of a
   service. USNs are used to differentiate between two services with
   the same service type.

   In addition to providing both a service type and a USN, discovery
   results and presence announcements also provide expiration and
   location information.

   Location information identifies how one should contact a particular
   service. One or more location URIs may be included in a discovery
   response or a presence announcement.

   Expiration information identifies how long a SSDP client should keep
   information about the service in its cache. Once the entry has
   expired it is to be removed from the SSDP client's cache.

   Thus a SSDP client service cache might look like:

   USN URI          | Service Type URI | Expiration | Location
   -----------------|------------------|------------|------------------
   upnp:uuid:k91... | upnp:clockradio  | 3 days     | http://foo.com/cr
   -----------------|------------------|------------|------------------
   uuid:x7z...      | ms:wince         | 1 week     | http://msce/win
   -----------------|------------------|------------|------------------

   In the previous example both USN URIs are actually UUIDs such as
   upnp:uuid:k91d4fae-7dec-11d0-a765-00a0c91c6bf6.