For example, a 1.0 client would remain compatible with a 1.X based server.
Servers can add optional parameters and new methods, but backwards compatibility is retained.
Once the server revs to 2.0, backwards compatibility is lost with 1.X clients. More importantly, this gives servers a way to move forward, wo breaking legacy clients. XML-RPC would likely need to be rev'd to support this. Clients would need to notify servers which interface version they intend to use. All rights reserved.