Re: [avt] Use of redundancy in rfc2793bis text transmission - optimizing interoperability

Colin Perkins <csp@csperkins.org> Thu, 15 April 2004 10:24 UTC

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From: Colin Perkins <csp@csperkins.org>
Subject: Re: [avt] Use of redundancy in rfc2793bis text transmission - optimizing interoperability
Date: Thu, 15 Apr 2004 10:55:10 +0100
To: Gunnar Hellström <gunnar.hellstrom@omnitor.se>
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On 14 Apr 2004, at 21:59, Gunnar Hellström wrote:
> I often get comments on the real time interactive text conversation 
> transport
> RFC2793bis, that it must more clearly require the use of redundancy to 
> assure
> good success rate in text transmission even in bad network conditions.
>
> Next version of draft-ietf-avt-rfc2793bis is about to be published, 
> and I would like to have  agreeable wording on this issue.
>
> A traditional requirement for basic text conversation quality is that 
> no more
> than 1% characters may be lost in conditions where voice 
> communications is
> barely usable. Where characters are dropped marks for missing text 
> should be
> inserted in the received text. A higher quality level, called good 
> text quality requires no more than 0.2% characters to be dropped. This 
> is of course no exact scientific measure and many factors influence 
> both the loss and the perception of usability of voice conversation, 
> but it gives a fair design goal.
>
> With voice coding and transmission schemes prevailing today, voice 
> gets barely
> usable around 20% packet loss.
>
> Therefore, in order to assure proper interoperability with the 
> required quality level for text at 20% packet loss, we need to require 
> one original and two redundant transmissions according to RFC2198. 
> (lowering text loss to 0.8%)

Why do you think it acceptable to be sending at full rate when there is 
20% packet loss, rather than reducing the sending rate to reduce packet 
loss? Do you have some analysis to show that, with typical text 
conversation packet rates and network RTTs, transmission with 20% 
packet loss is TCP friendly (or otherwise)?

The requirements you list may be appropriate from a media quality 
viewpoint, however you also need to demonstrate that the payload format 
is safe to deploy. This is particularly important for a format which 
adds a large amount of FEC to allow it to operate in the presence of 
large amounts of congestion, but doesn't specify a congestion control 
response.

-- 
Colin Perkins
http://csperkins.org/

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