I guess this problem caused by the firewall in my friend side, but ,how can i bypass the FW? any solution? thanks
since you can both connect to Stratus, that suggests it's not a firewall issue: you can make UDP connections in general. most likely it is an incompatible combination of your NAT and your friend's NAT. P2P communications aren't always possible between all kinds of NATs. Matthew Kaufman wrote a detailed explanation of the various NAT issues that can hinder P2P communications with RTMFP here:
all of the other chat applications that you listed are client-server-client TCP-based systems. VideoPhoneLabs, using RTMFP, is a direct client-client system, so it will be affected by NAT and firewall issues in ways the others wouldn't be.
Thanks for your reply,, i am also wondering that the NAT type of my friend, Port-Restricted cone NAT or Restricted Cone NAT?? do you have any website can tell me the cone NAT type?? such as the http://cc.rtmfp.net/ can tell me he is using a cone NAT.