## Overview - RPC Fundamentals

DCE RPC provides a call environment that behaves essentially like a local call environment. However, some special requirements are imposed on remote procedure calls by the remoteness of calling code to the called procedure. Therefore, a remote procedure call may not always behave exactly like a local procedure call.

This topic discusses the following:

· Universal unique identifiers

· Communications protocols

· Binding information

· Endpoints

· Execution semantics

· Communication failures

· Scaling applications

· RPC Objects

Distributed applications have the following implications:

· Client/server relationship - binding

Like a local procedure call, a remote procedure call depends on a static relationship between the calling code and the called procedure. In a local application, this relationship is established by linking the calling and called code. Linking gives the calling code access to the address of each procedure to be called. Enabling a remote procedure call to go to the right procedure requires a similar relationship (called a binding) between a client and a server. A binding is a temporary relationship that depends on a communications link between the client and server RPC runtimes. A client establishes a binding over a specific protocol sequence to a specific host system and endpoint.

· Independent address spaces

The calling code and called remote procedure reside in different address spaces, generally on separate systems. The calling and called code cannot share global variables or other global program state such as open files. All data shared between the caller and the called remote procedure must be specified as procedure parameters in the IDL specification. Unlike a local procedure call that commonly uses the call-by-reference passing mechanism for input/output parameters, remote procedure calls with input/output parameters have copy-in/copy-out semantics due to the differing address spaces of the calling and called code. These two passing mechanisms are only slightly different, and most procedure calls are not sensitive to the differences between call-by-reference and copy-in/copy-out semantics.

· Independent failure

Distributing a calling program and the called procedures to physically separate machines increases the complexity of procedure calls. Remoteness introduces issues such as a remote system crash, communications failures, naming and binding issues, security problems, and protocol incompatibilities. Such issues can require error handling that is unnecessary for local procedure calls. Also, as with local procedure calls, remote procedure calls are subject to execution errors that arise from the procedure call itself.

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