There may be many machanisms to tell the PEP agent to invoke an extension.The extension map is likely to be the most common. It maintians an associative list between URLs and extensions. The PEP model extension map is implemented as a WildCardVector and is managed by the PEPAgent.
The server policy is established by adding (URL, extension) pairs to it's own extension map. The server checks requested URLs against this list and inokes the proper extension(s).
The client may start out completely ignorant of a server's policies or it may have it's extension map populated by some resource (optional hardcoding, in the model Client's case).
Extension maps are just one of the
mechanisms that may trigger a
PEP extension.