asterisk users - May 2005 - About Voip Technology : RTP over TCP

hello All

I am reading information about VoIP technology
For that i am concentrating on SIP (Session Initiation Protocol) and 
RTP (Real Time Transport Protocol).

I am interested in implementing RTP over TCP
I found that there are some disadvantages of TCP, some are

1) TCP doesn't support multicasting.

2) Through TCP is reliable, it heavily depends on retransmission of     
   lost or corrupted packets. That increases Delay. Our aim is real     
   time delivery of packets not reliability. 

3) TCP congestion control mechanism decreases the congestion window    
   when packet losses are detected.

Apart from these flaws i am interested in implementing RTP over TCP

Is it possible ? 
What is feasibility of implementing RTP over TCP ?

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

Need your valuable suggestion.

Thank You !

Atul Thosar

> 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

On 5/13/05, Atul Thosar <atul.mailinglist@gmail.com>
wrote:
> hello All
> 
> I am reading information about VoIP technology
> For that i am concentrating on SIP (Session Initiation Protocol) and
> RTP (Real Time Transport Protocol).
> 
> I am interested in implementing RTP over TCP
> I found that there are some disadvantages of TCP, some are
> 
> 1) TCP doesn't support multicasting.
TCP stacks don't always support the standard either......
> 
> 2) Through TCP is reliable, it heavily depends on retransmission of
>    lost or corrupted packets. That increases Delay. Our aim is real
>    time delivery of packets not reliability.
quite the opisite. reliably cheaper calls.......
> 
> 3) TCP congestion control mechanism decreases the congestion window
>    when packet losses are detected.
yes and your point....... 
> 
> Apart from these flaws i am interested in implementing RTP over TCP
read about beowulfs and the use of interconnects over Ethernet......
> 
> Is it possible ?
yes, been done
> What is feasibility of implementing RTP over TCP ?
see above
> 
> One more point is What is feasibility of implementing RTP over TCP in
> case of NAT (Network Address Translation) is there ?
what port? DNAT? SNAT? loaded question.....
> 
> Need your valuable suggestion.
> 
> Thank You !
> 
> Atul Thosar
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