XML-RPC

KISS: Keep it simple, stupid

A fixed vocabulary XML-RPC has a very simple set of elements, no attributes, and a limited number of combinations. XML-RPC represents pretty much a lowest-common denominator of structures for exchange between programs.
Simple request reponse XML-RPC uses the HTTP request-response mechanism, with no built-in tools for things like maintaining state. The client sends information to the server, which then responds. That's all there is.
Structs and arrays as the only options XML-RPC offers simple numeric and string types, plus unordered structs and ordered arrays. There are no objects in XML-RPC, no notions of more complex structures.
Uses HTTP, few interoperability issues XML-RPC is bonded tightly to HTTP in the specification, though some implementations have used other transport mechanisms (notably Jabber). XML-RPC's simplicity has led to 62 implementations in a wide variety of environments, with interoperability working quite smoothly.