29.8 Summary

RPC is a way to build a client-server application so that it appears that the client just calls server procedures. All the networking details are hidden in the client and server stubs, which are generated for an application by the RPC package, and in the RPC library routines. We showed the format of the RPC call and reply messages, and mentioned that XDR is used to encode the values, allowing RPC clients and servers to run on machines with different architectures.

One of the most widely used RPC applications is Sun's NFS, a heterogeneous file access protocol that is widely implemented on hosts of all sizes. We looked at NFS and the way that it uses UDP and TCP. Fifteen procedures define the NFS Version 2 protocol.

A client's access to an NFS server starts with the mount protocol, returning a file handle to the client. The client can then access files on the server's filesystem using that file handle. Filenames are looked up on the server one element at a time, returning a new file handle for each element. The end result is a file handle for the file being referenced, which is used in subsequent reads and writes .

NFS tries to make all its procedures idempotent, so that the client can just reissue a request if the response gets lost. We saw an example of this with a client reading a file while the server crashed and rebooted.