Data decoding, description in xml

I would like to decode a certain packet of data. For example, let's say this packet carries two fields, the first and the last person's name. I want to decouple the app implementation from the particular format of the packet, so that if the packet changes only the packet description must be rewritten. I'm thinking about some technology which would allow me to describe the packet format outside the app, in XML for example. And that is actually the question, which technologies might help to accomplish that?