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

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Public packet network, reliable delivery, realtime control.

Choose two.

If 1 and 2 is your choice, use TCP. If 1 and 3 is it, use RTP. If 2 and 3 is it, go shop for a different network offering.

Implementing RTP over TCP correctly is a non-starter. TCP "uses up" your realtime performance to acheive stream integrity; and use back-off plus retransmits to acheive it.

There is one reason though. That is firewall penetration. But for that you cannot implement TCP, just something that is indistiguishable from TCP as seen from the outside. TCP that just keeps sending at RTP paces into a huge window, and get acks back; and do retransmits to keep the intermediate stuff happy.

It could be done well enough for a firewall not to see much difference unless precise timin analysis were performed. But it would not be TCP.

And you would get better results in the current Internet by faking DNS packets.

-- mrr

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Morten Reistad

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