Using XML as an Application-Level Protocol
http://softwaredev.earthweb.com/xml/article/0,,10697_1380861,00.html

Saleem, Usman
Developer.com
2002

Abstract:

In one of my past articles, A Pattern/Framework for Client/Server Programming in Java, I discussed a pattern for client/server development using java. That article does not answer exactly how the two parties, client and server, communicate with each other. We require an application-level protocol to do the talking between two entities. It sets up rules about how the two applications/entities communicate and understand each other over a network. If you happen to know the TCP/IP networking model or the OSI networking model, you will observe that network-based communication is implemented in layers, with the application layer at the top and the physical layer at the bottom. This article discusses issues you must face when implementing an application-level protocol and how XML proves to be an excellent choice to represent and implement the application-level protocol.