Re: [AVT] Re: draft-jones-avt-audio-t38-04.txt

[Colin Perkins <csp at csperkins.org>]

On Wednesday 26 May 2004 12:42, Magnus Westerlund wrote:
> Hi,
>
> As I understand it, the below SDP attributes are in fact only used when
> the usage is with m= lines that looks like the following (using udptl or
> tcp as protocol):
>
>  From section D.2.4.1 of T.38
>
> m=image 49170 udptl t38
> a=T38FaxRateManagement:transferredTCF
> a=T38FaxUdpEC:t38UDPFEC
> m=image 49172 tcp t38
> a=T38FaxRateManagement:localTCF

Agreed.

> While what we are now defining is the parameter that belongs to the T.38
> RTP payload format. This new payload format should really use the
> standard procedures for any m= line with RTP in it. Therefore you need
> to follow the methods in RFC 3555 for this.

I agree in principle, but I'm not sure it's that simple in practise. If we 
use the RFC 3555 approach, and define MIME parameters with a mapping to 
"a=fmtp:", there is a difference between the signalling for T38-over-RTP 
and for T38-over-udptl. This makes implementations more complex, since 
they need to understand both sets of attributes if they support both RTP 
and udptl transport, and makes the specification more complex since it has 
to define the interaction.

Using "a=fmtp:" for the parameters to the RTP payload format is probably 
the right approach, but we should be aware of the issues.

Colin

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