Subject: Re: [AVT] RE: [dccp] Re: DCCP Media Guide

From: [email-masked]
Date: Wed, 26 Nov 2003 17:30:54 +0100

Henning, TFRC experts,

>> It would be interesting to look at the voice response to changing
>> packetization period.

..

>
>We have looked at this systematically, varying the packetization
>threshold. See the papers by Wenyu Jiang and myself at
>http://netbib.info and http://www1.cs.columbia.edu/~wenyu/
>

Does anyone have a clue of how changing the packetization
impacts the behavior of TFRC in presence of TCP traffic ?

For example if an application increases its packet rate
(same media rate, higher transmit rate due to packet headers contribution increasing)
is that added "pressure" against competing TCP traffic ?

regards,

Philippe Gentric
Chief Architect
Philips Software
[email-masked]
http://www.software.philips.com

_______________________________________________
dccp IETF mailing list: [email-masked]
list info:  https://www1.ietf.org/mailman/listinfo/dccp
wg charter: http://www.ietf.org/html.charters/dccp-charter.html

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

---

From: Henning Schulzrinne <[email-masked]>
Date: Wed, 26 Nov 2003 08:59:23 -0500

> I fully agree this is the fundamental reason why another protocol than TCP
> is a good idea: media does not need 100% reliability.

The only reason not to get 100% reliability is that relaxing reliability 
constraints reduces delay, as even modest loss rates, if unfortuitously 
placed, can wreak havoc with user experience. For the higher-latency 
streaming application contemplated here, it is unclear that removing 
reliability adds any significant value, particularly given that you end 
up having to add some protection (FEC) to deal with losses, thus 
increasing the overall average bit rate or reducing the quality. 
(Obviously, FEC doesn't deal well with loss spurts, so quality will be 
rather poor if losses are indeed bursty, even with aggressive FEC.) 
Typical FEC overhead for useful error resiliency is usually in excess of 
20%, so the bandwidth penalty is substantial.

Thus, before accepting that DCCP is the right thing for streaming media, 
a bit more human-factors and measurement evidence seems called for.