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Multi media streaming with UDP and related protocols

Posted on 2007-11-18
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Hi,

I was reading about TCP vs UDP and how UDP is mostly used for real time applications when data loss can be compromised and compensated using algorithms to compensate the data loss. But i have also read about RTP (real time transport protocol) and SIP (Session Initiation protocol) and SDP (Session Description protocol) and it was mentioned that they work in conjuction with the UDP. I dont understand the part of how they work with UDP? Could someone explain it to me?

thanks alot
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Question by:jaxrpc
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by:grahamnonweiler
ID: 20307169
Some background first on UDP - which is typically used where packet loss is not detrimental to the overall transmission.

Essentially UDP continues to transmit packets one after the other to the receiver without checking whether the packet was correctly received - whereas with TCP each packet is verified and if needed it is resent.

In general terms this is why UDP is NOT used in data sensitive applications and is often referred to as a "real-time" protocol - where the primary concern is speed and not data integrity.

The similarity between UDP and RTP/SIP/SDP  is that applications that use these protocols can still function through packet loss - typically multimedia streaming (RTP) and VoIP (SIP). In these environments the loss/corruption of one packet does not affect the other packets and the "application" can continue to provide the functionality for which it was intended.

Be careful not to misinterpret the meaning of "real-time applications" in that these protocols should NEVER be used where data integrity is required. The algorithms you are talking about simply compensate for the lost packets by "skipping" over them. Obviously in a data sensitive application this would be completely unacceptable, but in the case of streaming (video or music for instance) the packet loss simply manifests itself as a gap in the stream that is skipped over (i.e. a jittery picture or silence)

So to your specific question - where RTP/SIP/SDP will be the final tranmission layer the initial interaction between the end user application and the server is performed over UDP channels - and subsequent "keep-alive" requests will be actioned in a similar manner.


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by:jaxrpc
ID: 20307496
hi thanks for the explanation.... what is meant by RTP running on top of UDP? does it means...it still uses UDP but adds in extra support like number sequencing?
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grahamnonweiler earned 500 total points
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"RTP running on top of UDP" is a way to provide some continuity to UDP connections.

What this means, following on from my earlier description, is  while UDP does not provide any form of protection in respect to packet loss - just continuing to send packets until stopped/closed - it does have the advantage of speed. Therefore to circumvent the packet loss issue a form of packet sequencing with send/receipt validation, that will in turn resend a packet if it is lost or damaged, would make UDP a more reliable (and useful protocol) in certain circumstances.

This is often referred to as "RTP running on top of UDP " and can be thought of as an "application" rather than a protocol in its own right. In that the code is written to use UDP but it is the code itself that is handling the verification of packets and therefore the code must be present at either end of the link.

Interestingly "RTP" is an abbreviation for "Reliable Transport Protocol" which strictly speaking it isn't!
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