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

Thanks.

Magnus Westerlund wrote:
> The first one is to use FEC to provide the packet repetition... 
> The second is actually to use that Retransmission RTP payload format
for 
> repetitions...

But,
1. Not all gateways support FEC.
2. The RTP payload retransmission is under consideration (draft
document) and is not supported by all gateways. Also, the using it may
give additional interoperability conflicts, when one gateway supports
NACK but the other ignores it. 

Regards,
Vladimir Ulybin

-----Original Message-----
From: Magnus Westerlund [mailto:magnus.westerlund@ericsson.com] 
Sent: Monday, July 04, 2005 10:21 AM
To: Vladimir Ulybin
Cc: avt@ietf.org
Subject: Re: [AVT] T.38 over RTP: RTP Sequence Number

Vladimir Ulybin wrote:
> 
> 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.
> 

There exist at least two possible ways of performing this without 
screwing up the RTCP statistics and fulfilling your other requirements:

The first one is to use FEC to provide the packet repetition. I would 
suggest that you use RFC 2733 to provide repetition in a smarter way. 
This way one FEC packet can cover a few RTP packets. If this is done 
using RFC 2198 packetization then you don't even need to run two RTP 
sessions.

The second is actually to use that Retransmission RTP payload format for

repetitions. The payload format is really for repetitions and the spec 
provides with necessary information on bind and sync. Simple skip the 
NACK process.

Cheers

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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