[e2e] Internet Draft and survey on P2P in the presence of NAT

[Christian Huitema]

> Midcom/UPnP doesn't really solve the incoming connection
> problem, even for firewalls.  There's an implicit assumption
> that you've got something listening at an accessible port/
> address, and that thing communicates with the firewall or
> NAT to request pinholes.  That model makes a lot of sense
> for applications like telephony but doesn't work as well for
> peer-to-peer applications.

That is definitely not the architecture used in the UPNP IGD. The thing
inside the NAT (usually a PC) is expected to request a mapping, and the
NAT is expected to return the external address and a TCP or UDP port
number. The application can then listen for incoming calls or UDP
packets bound to that port.

In fact, UPNP IGD are normally programmed to ignore any UPNP command
that originate from an external host, for obvious security reasons.

-- Christian Huitema