[SR-Users] Sync nonce between various servers

Hi, There are lots of parameters controlling the creation of nonce values on a server, and I'm curious if there is a way to kind of "sync" them between servers. The use case would be to have a UA send for example its registration to Proxy1. Proxy1 would challenge it, UA will send the registration again, this time with credentials. Proxy1 would look up the user based on $au/$ar in the subscriber table, and if it's not found, will look up the responsible proxy from another table (with key being $au@$ar), forward it to Proxy2, which then would be able authenticate the user. The reason for this is that the auth credentials are unique across all servers and reliably identify a user, whereas for example From could be something else (e.g. in case of an IP-PBX sending a CLI in the From-userpart). Challenging the user on the second proxy again would theoretically be possible, but if the UA gets a 401 twice (once from Proxy1, once from Proxy2), it'll most likely pop up a password form for soft-clients, so I want to avoid that. Any ideas how to accomplish that? Andreas