What is the XPT protocol?

[brtzsnr]

First, hi all! I recently joined this forum and I'm interested in crytocurrencies more from a technical point of view rather than hopping to get rich.

I'm currently writing my own primecoin poolminer and I'd like to know what is the XPT (x.pushthrough) protocol that is used by some mining pools (e.g. ypool.net, primecoin). I have googled but could not find any reference to the details of XPT. What is the protocol to be able to write a basic client?