Bitcoin miners want their newly-found blocks to propagate across the network as quickly as possible, because every millisecond of delay increases the chances that another block, found at about the same time, wins the "block race."
Luzius Meisser luziusmeisser
See https://en.bitcoin.it/wiki/BIP_0070 for the latest version of this document; I'll keep this document so the process of discussion/revision isn't lost.
This document proposes protocol buffer-based formats for a simple payment protocol between a customer's bitcoin client software and a merchant.