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

On 3 Jul 2005, at 15:44, Vladimir Ulybin wrote:
> Possibly I used the term "retransmission" non-correctly for T.38 over
> RTP. In the draft-ietf-avt-rtp-retransmission-11.txt, the term
> "retransmission" is applied to packets retransmitted as a response  
> onto
> the request of packet receiver detected packet lost. Such  
> retransmission
> reduces a reliability of fax relay, because enlarges the round-trip
> delay of fax commands and responses.
>
> But I assume a simple packet repetition following an original packet
> transmission to improve a reliability of delivering important T.30
> commands or responses. This repetition shall be done without any  
> request
> on retransmission from remote side. In T.38 UDPTL protocol the packet
> repetition is widely used by different gateway vendors.

As Magnus says, the RFC 2733 FEC format is appropriate, or the  
retransmission draft I suggested.

> Taking into account for T.38-over-RTP that
>
> 1. Fax relay is asynchronous communication having long periods of  
> packet
> absence;
> 2. The redundancy and repetition of important fax packets  
> significantly
> improves the fax session reliability;
> 3. Incrementing RTP sequence number for repeated T.38 packets  
> cannot be
> accepted;
> 4. Reliability of fax session has a much higher priority vs. a correct
> RTCP statistics;
>
> I think we may use for RTP sequence number the same rules of
> incrementing as for T.38 UDPTL SN, i.e. increment SN per every new
> primary IFP packet and do not increment SN if a fax packet is  
> repeated.

Not if you wish to be compatible with RTP: RTP requires the sequence  
numbers to be unique.

Colin

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