[CentOS] Check my math please

Warren Young warren at etr-usa.com
Thu Nov 6 20:32:32 UTC 2008

Sean Carolan wrote:
> For a back-of-the napkin calculation can we not assume that data equal
> to the entire size of the file will be streamed to the client during
> playback?  

You can if you're using some of the fudge factors others have mentioned 
here.  The headers for IP + UDP + RTP take at least 3.5% of the 
bandwidth for a network using a PMTU of 1500 bytes.  If the smallest MTU 
between your system and the receiver is smaller, the percentage goes up. 
  And, the percentage goes up anyway because those numbers only talk 
about the minimum overhead.  RTP headers are flexible, as are most 
higher-level protocols, like TCP.

We're also ignoring any other traffic on the link.  With RTP, for 
instance, there are usually RTCP and RTSP channels as well.  Without 
these parallel data channels, the stream doesn't work.

You also have to account for retries and buffering.  If your network 
link is exactly as big as it has to be for the ideal case, you will 
still have problems because the time it takes to deal with lost or 
damaged packets comes out of your bit rate budget.  There's a limit to 
the effectiveness of prebuffering.

10% total overhead isn't unreasonable.

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