> eDonkey servers do this - they connect to a user's listening port
 > and if it's unreachable, it puts them in their equivalent of
 > passive mode (lowID).  eDonkey servers also host a lot more users
 > than the biggest DC hubs as of yet do (the biggest I saw listed was
 > 205,628 users - though the median size seems closer to 20-30k).

Really? I would have thought that it would take to much hub resources
to do that. Do you really think that it's feasible?

I think it is possible for only one reason: the user list. ed2k does not have one but DC has and sending a big user list probably uses more bandwidth than checking a user port.

 > >If a user detects anything weird about another user (not being
 > >able to connect to him,
 >
 >  From my experience on the DC++ forums, if you implement a "warning
 > system" that can be triggered on timeouts by the user, you will get
 > a lot of misconfigured users using it.  (Many behind routers don't
 > realize they have to put in their external IP to become active.)

I have another problem with IP usage. Let's say a user has a computer with 2 network connection (like me for example :) ). How can I use the 2 connections (=2 IP) ? Opening 2 clients is a waste of bandwidth. I think a client should be able to have more than one address. I also think a client should send its IP(s) to the hub and the hub should check if commands containing IP and sent by a client has an IP matching the previously sent one (but this is not really a protocol problem, it is a hub problem).

About transfer port verification performed by the hub, I think it will be useless because some clients uses more than one port. Client verification seems to be more reliable.

Eric