[Nice] What types of NATs can be traversed when one endpoint is symmetric?

[John Selbie]

Assume two hosts using SIP, TURN, and ICE to rendezvous and setup UDP based
P2P sessions with each other. Assume connectivity to the SIP and TURN
service is 100% reliable for these hosts.

User A  is a a host is behind a "symmetric" NAT - most likely a 3G mobile
client - or any other NAT that doesn't have a consistent port mapping
scheme.

User B - The other host is behind a port-restricted NAT.

Now A and B want to do a P2P session over UDP with each other.

My understanding is that the ICE negotiation will most likely fall back to
using the TURN relay port in this scenario. Is that correct? Or does the
connectivity check phase of ICE have some way of  connecting the hosts
directly?

I'm also certain that if User B were to be behind a Cone Nat, then ICE has
a much higher chance of succeeding with a direct connection.

Is my understanding correct so far?

Is there a table or data available showing what NAT-pairings are expected
to "go direct" vs fallback to TURN relay in an ICE session?

Also, does anyone have data or an "off the cuff" idea of how prevalent the
different NAT types are (cone, port-restricted, symmetric) in the
consumer/home case. I would guess most off-the-shelf routers these days are
port-restricted.  But that is only a guess based on the fact that mine is.

jrs
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