I was going over my notes for the IETF MMUSIC session last week, and came across an item which may be an issue for H.323. Thanks to the presence of Network Address Translation (NAT) in the net, an endpoint doesn't necessarily know its true network address. (NAT boxes translate between local addresses and public ones invisibly to the hosts concerned.) Thus the address an H.323 endpoint sends out during registration with the Gatekeeper may be bogus.