29.3 XDR: External Data Representation

XDR, External Data Representation, is the standard used to encode the values in the RPC call and reply messages ” the RPC header fields (XID, program number, accept status, etc.), the procedure parameters, and the procedure results. Having a standard way of encoding all these values is what lets a client on one system call a procedure on a system with a different architecture. XDR is defined in RFC 1014 [Sun Microsystems 1987].

XDR defines numerous data types and exactly how they are transmitted in an RPC message (bit order, byte order, etc.). The sender must build an RPC message in XDR format, then the receiver converts the XDR format into its native representation. We see, for example, in Figures 29.1 and 29.2, that all the integer values we show (XID, call, program number, etc.) are 4-byte integers. Indeed, all integers occupy 4 bytes in XDR. Other data types supported by XDR include unsigned integers, booleans, floating point numbers , fixed-length arrays, variable-length arrays, and structures.