Re: skype and NAT

Hallo,

alpe@xxxxxxxxxx wrote:

hallo.
Pag 44 of wireless networking in the developing world (wndv.net) says
that:

"some applications (such as voice over the ip -eg.skype-) can have
difficulties dealing with NAT"

I would be very thankfull if someone could explain to me in full
details axactly what these difficulties entail.

Thank you.

Aldo Pellizzi

If host A and host B are behind a NAT firewall, in is impossible to
open a connection from A to B or reverse without explicitly opening ports.
Point-to-Point VoIP protocols will fail in this case.
Also IPsec based VPN has troubles with NAT.
And if you open a port, only one host behind the NAT can be reached
from outside on this port.


Skype found a trick, in short: host A opens a UDP connection to the Skype
erver. The NAT firewall of A has to open a UDP port to receive the reply,
but the Skype server tells host B what this IP address and port number of A are, so B can open a connection to A.
In that way A and B can communicate with each other without intermediate server.

See "UDP hole punching" in Wikipedia.

Kind Regards,


.

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