@cft btw: a goog p2p protocol needs to be robust to partial failures - we must assume that there are always some connections that fail, and some peers that cannot talk to some other peers, so we can treat upgrades as partial failures. Some peers can eagerly upgrade, some peers can upgrade with compatibilty, and some are stuck on the legacy mode - but the compatiblity peers provide a bridge between old and new formats. At least, this makes the transport protocol easy to upgrade (we already have done this with EBT), message encoding formats are still hard, but it's still an improvement over the web. However, I think that getting things to upgrade is much easier now days. I remember installing my first web browser (netscape) from a CD our ISP gave us, that makes shipping updates quite difficult! But I havn't owned a computer with a CD drive in years!