Subject: [AVT] RTP payload length

From: "Trevor" <[email-masked]>
Date: Wed, 24 Sep 2003 11:49:21 GMT

Hi experts,
 
What is the maximum payload length normally set for RTP? I
set it to 1460 to avoid exceeding MTU(1500 or ?) of lower
layers. But in my case, my video bitrate is at 1Mbps and
framerate at 25fps. i.e. In average each frame is of about
5KBps. That means that I have to segment all video frames
into 3 to 4. I feel the overhead is too much. Would anybody
here like to share me your settings for maximum payload
length? Thanks.
 
Regards,
Trevor

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

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From: Magnus Westerlund <[email-masked]>
Date: Wed, 24 Sep 2003 13:55:40 +0200

Hi Trevor,

What you should do in your application is running MTU discovery between 
transmitter and receiver. Based on the available MTU you can create 
packets of appropriate size. In your case near the upper limit of the 
MTU. That way you can avoid unnecessary fragmentation.

It seems common that many uses around 1400-1460 bytes as maximum size, 
allowing for 10 mbit ethernet MTU. The lower size allow for IPv6 or 
tunnelling. However using a standard size is not a good approach. This 
prevents one from taking advantage of larger MTU when available and may 
also if run over some older equipment be to large.

There are currently work ongoing in the IETF to update the ways of doing 
MTU discovery, see the PMTUD WG. It would probably be beneficial if we 
in AVT also takes the time to look at this and discuss on how the 
proposed methods are best applied for RTP traffic.

Cheers

Magnus

-- 

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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