[Asterisk-Users] About Voip Technology : RTP over TCP

Stewart Nelson sn at scgroup.com
Fri May 13 08:28:18 MST 2005


> I am interested in implementing RTP over TCP

Why?  If you want to permit operation through a firewall
that blocks UDP, there are packages that provide VPN
tunnels over TCP or even HTTP.  You could then run
any VoIP system over that VPN.  As you said, delay
performance would sometimes be awful.

Skype will automatically fall back to TCP if a UDP
connection attempt fails.

Most of the commercial instant messaging packages
that support voice or video can work over TCP.

If your purpose is to improve performance on networks
with high packet loss rates, IMHO you would get better
results from a UDP-based system that permits forward
error correction, by transmitting each voice frame
in two or more packets.  If you can't afford the
increased bandwidth, a system of retransmission such
as used by popular streaming protocols would still be
better than TCP.

> One more point is What is feasibility of implementing
> RTP over TCP in  case of NAT (Network Address
> Translation) is there ?

Any of the above systems can work through NAT.  If
both endpoints are behind NATs, and you can't
set up port forwarding on either, then of course you
must connect via an intermediate server.  Skype
and the IM services do that automatically.

If your desire for TCP is not related to firewalls
or packet loss, I'd be interested in hearing about
your application.

--Stewart




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