[BitTorrent] UPnP, NATs, and P2P

Justin Cormack justin at street-vision.com
Wed Feb 16 06:17:40 EST 2005


> 
> 
> On Tue, 2005-02-15 at 21:56 +0000, Justin Cormack wrote:
> > 
> > 
> > http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/teredo.mspx
> > 
> > > 
> > > 
> > > On Tue, 15 Feb 2005 16:06:02 -0500, Yejun Yang <yejunx at gmail.com> wrote:
> > > 
> > > > Currently MS Torendo IPv6 server protocal can let 2 peers both behind
> > > > NAT to communicate with each other directly without server relay.
> > > 
> > > Can you post an URL or something? I did not find anything on
> > > this subject.
> > > 
> > > Kosta
> 
> This seems to only document IPv6 tunneling through NATs and IPv4
> routers, much the way we do it today with plain IPv4.  The goal seems to
> be to help aid the transition to IPv6.
> 
> I didn't seen any recommended solution to the problem of two peers, both
> behind NATs, that want to talk.  From my understanding of simple NATs, I
> don't see how it's possible without configuring the NATs to forward
> ports, or support UPnP.
> 
> Basically, if I'm behind a NAT, and so are you, and either we don't have
> the gateway passwords, or we're not computer savvy, we can't talk
> directly.
> 
> Bill
> 

See the section entitled

Initial communication between Teredo clients in different sites

It relies on the fact that many NATs (not what they call symmetric NATs
which includes Linux) set up dumb mappings so incoming packets for a port
that has a mapping already set up in the NAT get forwarded by that rule
whichever external host they come from. Because UDP doesnt have sequence
numbers that means anyone can send you packets if they know the external
IP address of the NAT box and the port that you are keeping as a natted
connection. You need a server (actually 2) available that you can send
packets to to keep the mapping alive and for other machines to find out
what your mapping is (from your ipv6 address) and to assign ipv6 addresses.

In some situations it relies on sending packets that it knows will be discarded
to set up NAT mappings, again relying on the server to store the mappings.

Its what STUN uses.

My VOIP phone tried to use STUN and it didnt work at all, but then I use
Linux to do my NAT.

> 
> 
>  
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> 
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