About ISA Server RPC filtering
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The ISA Server computer handles traffic for all remote procedure calls between clients outside your network and RPC servers located inside your network. Using ISA Server RPC Filter Configuration Add-in, you can define one or more UUID interfaces as an RPC protocol definition. This protocol definition is used in ISA Server publishing rules for the server so that external clients can access UUID interfaces on the internal RPC server.
For information about creating an RPC protocol definition, see Create an RPC protocol definition.
When Windows starts, the RPC endpoint mapper service (port mapper) automatically loads. When an RPC server starts on that computer, it registers its UUID interfaces on the port mapper. The RPC server listens on TCP port 135 for incoming RPC requests. At the same time, ISA Server listens for incoming RPC requests from external clients on its external port 135. When an RPC request from an external client is received, ISA Server emulates the role of the RPC server endpoint mapper, accepting the request and verifying that the requested UUID is allowed in a server publishing rule. ISA Server sends the request through to port 135 on the internal RPC server.
When the RPC server receives the request, it verifies that the requested UUID is registered and dynamically allocates a port for each UUID in the RPC protocol definition. After UUID verification, the allocated port number for the requested UUID is sent back to the ISA Server and the client request is allowed through to that port. This process is transparent to the client who communicates only with the ISA Server computer.