Re: [AVT] T.38 over RTP: RTP Sequence Number

[Colin Perkins <csp at csperkins.org>]

On 18 Jul 2005, at 07:38, Vladimir Ulybin wrote:
> The term "basic configuration" for RFC 3550 plus redundancy per RFC  
> 2198
> should be replaced on "typical configuration". I assumed the same as
> you, that most gateways support this.
>
> As for the time stamps for redundant fax packets, the theory case  
> given
> is not relevant for T.38.
> 1. The ITU-T T.38 itself does not uses any time stamps both for
> redundant and primary packets. T.38 gateways transmitting fax signals
> should care about compatibility with T.30 but not with time stamps
> received from a remote gateway. The only important things are the
> content of fax packets and the real-time of receiving.
> 2. We say about new feature of T.38 over RTP which is to be added to
> existing (==working) T.38 UDPTL.
>
> My suggestion is to reserve (zero) time stamps for redundant packets
> when sending T.38 over RTP.

Which will break any conformant RTP implementation...  The timestamps  
may well be redundant for this particular media format, but they are  
trivial to calculate and required by RTP.

We well understand that, in many cases, it's possible to develop a  
more optimised format for a particular media type or application  
scenario. The value of the RTP standard comes not because it's always  
the optimal solution, but because it's an acceptable solution which  
everyone can implement, to gain economies of scale and increased  
interoperability across a range of problem domains. Adjustments of  
the type you suggest are often locally beneficial, but frequently  
harmful to overall interoperability and implementation complexity.

Colin

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