Re: Question regarding how 2 hosts transfer files if they are both behind a fire

Thanks for the explanation. I've found an easy-to-understand
explanation of UDP hole punching, outlined step-by-step (for Skype,
but it applies to p2p as well):

http://www.heise-online.co.uk/security/How-Skype-Co-get-round-firewalls--/features/82481




--- In [email protected], "Philippe Verdy" <verdy_p@...> wrote:
>
>  
> 
> > -----Message d'origine-----
> > De : [email protected] [mailto:[email protected]] 
> > De la part de Raphael_Manfredi@...
> > Envoyé : mardi 1 juillet 2008 10:07
> > À : [email protected]
> > Objet : [the_gdf] Re: Question regarding how 2 hosts transfer 
> > files if they are both behind a firewallDate: Tue, 1 Jul 2008 
> > 08:06:33 +0000 (UTC)
> > 
> > 
> > Quoting david_chen100 <david_chen100@...> from 
> > ml.gnutella.dev-forum:
> > :The documentation says that if the servent that contains the 
> > file to :be transferred is behind a firewall, it can PUSH the 
> > file to its :counterparty (who is receiving the file) via GIV in HTTP.
> > :
> > :However, what happens when both hosts are behind firewalls? 
> > How does :the file transfer happen in this case?  
> > 
> > If both hosts are behind firewalls, then it is not possible 
> > for them to exchange files through TCP/IP.
> 
> Not exactly: it's not possible through TCP but possible through UDP that
> does not require the allocation of a port number to the incoming
connection.
> For this to work the firewall must just allow a technic called "port
> punching" for UDP: the recipient that wants to accept traffic from a UDP
> port just has to push some UDP datagram throuh it, in orer to have the
> firewall accept and direct incoming datagram coming to this same port to
> reach the intended recipient.
> 
> That's the basic of how the FW2FW sub-protocol in LimeWire works: it
> reimplements TCP over UDP, using a single port number, to allow incoming
> Gnutella-formatted traffic encapsulating a TCP emulation.
> 
> This does not solve all firewall issues, but at least, if you're able to
> send traffic to the network via UDP and recieve just one reply from
it, it
> will work.
> 
> The FW2FW subprotocol allows transporting anything that a TCP connection
> would transport, so it can be used as well to maintain sessions,
including
> file transfers and chat. You may think that the extra layer of
encapsulation
> slows down the traffic: it's true in theory, nut not in practice.
> 
> Note that some parts of the TCP protocol, that is used to autodetect
the IP
> MTU and avoid or autocorrect data fragmentation through negociation
may not
> work with FW2FW with just a basic emulation of it in an
encapsulation. This
> must be handled by the encapsulator itself detecting it internally
from the
> incoming datagrams, and inorming the upper layer of the TCP
emulation about
> the corrected MTU size. That's the complex issue to solve (otherwise you
> will suffer from poor performance.
>