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I am evaluating the feasibility of this project. I'd like to be able to establish an encrypted VoIP call between two asterisk PBXs.

I plan to establish a traditional IAX call over an encrypted VPN tunnel between the two asterisk servers. The servers are linked via a traditional analog phone line and are able to dial each other and establish a PPP connection. I got acceptable results when placing the call over the plain PPP link.

However I've been having problems calling over an OpenVPN tunnel. The call goes through but the RTP audio packets are dropped, i keep getting the UDP4 no buffer space available. I've already tweaked multiple parameters and settings on the kernel and openvpn but still does not work.

Does anyone know any other way that this can be accomplished? It should be able to create an encrypted tunnel over two linux servers linked by 2 modems. TCP tunnels are not acceptable as VoIP calls are UDP.

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3 Answers 3

Encryption DOES increase bandwidth. It can be significant. People who say it doesn't haven't done any real world testing. Voip also isn't very efficient - quite often decent signal quality consumes considerably MORE bandwidth than the original 56k allocated for analog channels doing it the old way. Open vpn is nice, but real encryption sucks extra bytes out of the stream. If you don't mind trivial-to-crack pseudo security that still counts as encryption, try PPTP as your link.

btw, contrary to common belief, P=NP over some pretty significant problem domains. If you're trying to keep the men in black away from your idle chatter, you're wasting your time.

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Encryption should not increase the size of the encrypted objects, very much. The processors at each end may not be able to keep up with the encrypt/decrypt load. Also, is the VPN or something else you added giving priority to TCP packets over UDP? Have you tried running the VPN-VOIP over a faster link? If it works, try slowing the link down until it just begins to fail. Then you will know how far from your goal you are. You might also have some clues about how to fix it.

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Your main issue is going to be bandwidth, I'm afraid. Under best conditions, you're going to be using more than half of your dialup bandwidth for plain VoIP. Add encryption overhead to that, and you're going to be very close to your maximum. This is going to make it very susceptible to line noise and other interruptions.

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