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yufeiying1 · Guest

Dear friends:

I have a question with Indy. I want to create a communication app that
can send message from peer to peer. But the problem is that maybe one or
more machines that is in the local area network and borth maybe under
the firewall. Somebody tells me that I can use the UDP NAT to make pole.
But I don't know what is it and how to implement it with Indy.

Any suggestion? Or some code?

--- posted by geoForum on http://delphi.newswhat.com

Jamie Dale · Guest

If I remember correctly this can be done with the use of a 3rd server and
some TCP aswell as UDP.

Basically it works on the principle of a UDP broadcast OUTWARDS at both ends
to the other peer.

A Transmits data to B which has a firewall blocking it. The data never
reaches B BUT A's firewall expects a response.

Meanwhile B transmits data to A which DOES expect a response and so the data
gets through A's firewall. Meanwhile B's firewall realises that a reponse
might be needed and opens up to allow further incoming responses.

This of course only works if you have a 3rd party (EG switchboard script on
http server) being used to tell B that A has transmitted and that B should
give it a go. That means both peers clients must contact the switchboard
periodically to check for incoming peer requests.

Furthermore, it also depends on the firewall in use at both ends. If it's
automatic and recognises data that needs a response automatically it will
work. It it needs an administrator to open ports etc your stuffed.

Hope that gives you soem insight - If you don't understand it, try googling
skype.

JD

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