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

["Paul E. Jones" <paulej at packetizer.com>]

Vladimir,

Forget RFC 2198 for a moment.  You're going to sample the audio, you will
extract T.30 information from that audio, you will form an RTP packet using
the time from the clock that is necessarily running.

As Packets arrive, you can do what you want with the timestamps, but they
can certainly be used for ordering packets if you want or they can be
largely ignored.

Now, if you add RFC 2198 to the picture, you are simply re-transmitting the
media from a previous packet in the current packet.  The timestamp was in
the previous packet and the current packet simply has an offset to it.  When
it arrives, you can determine the order of packets.  Isn't that all you
need?

Honestly, I do not see why this is causing such a problem.  I will not
disagree that an alternative (e.g., UDPTL) might be more fitting as it does
not have timestamps, but as Colin pointing out: it's an RTP tax.  But, you
get benefits with that tax, including security, RTCP, RTCP-XR, etc.

Paul


> -----Original Message-----
> From: Vladimir Ulybin [mailto:Vladimir at audiocodes.com]
> Sent: Wednesday, July 20, 2005 5:51 AM
> To: Magnus Westerlund
> Cc: Colin Perkins; Paul E. Jones; avt at ietf.org
> Subject: RE: [AVT] T.38 over RTP: RTP Sequence Number
> 
> Hi,
> 
> You are right. The play out concept of RFC 2198 is problematic for fax
> relay. Because, T.38 gateways do not play the buffers, but transmit
> according to T.30 standard. Generally, the fax rates may be different at
> two sides of communication, for example, 2400bps at calling fax side and
> 14400bps at answer fax side. Also T.30 control signals may have no
> synchronization between gateways, but should be synchronized with near
> fax machine.
> 
> The problem of timestamps is not only our (AudioCodes) problem but is
> general for different vendors.
> 
> The packet buffer of a gateway receiving T.38 packets has no any time
> mapping. The sequence number is the only ID of T.38 packet. Combining
> sequence numbers with timestamps in packet recovery module is highly
> problematic for interoperability (and from my point of view is wrong for
> fax transfer).
> 
> Unfortunately, typical gateways do not support a complex protocol RFC
> 2733. So, we cannot use it as a basic for our implementation.
> 
> Thanks,
> Vladimir Ulybin
> 
> 
> -----Original Message-----
> From: Magnus Westerlund [mailto:magnus.westerlund at ericsson.com]
> Sent: Wednesday, July 20, 2005 10:49 AM
> To: Vladimir Ulybin
> Cc: Colin Perkins; Paul E. Jones; avt at ietf.org
> Subject: Re: [AVT] T.38 over RTP: RTP Sequence Number
> 
> Hi,
> 
> I think a lot of the problems you are having is based on the fact you
> are using RFC 2198 for something it wasn't designed for. It was designed
> 
> and works as intended for audio payloads that relies on a single data
> block (ADU) per timestamp. The formats that fulfill this can for
> synchronization and detecting duplicates rely solely on timestamp. It
> uses the RTP timestamp primarily to detect when discontinuous
> transmissions occur. RFC 2198 doesn't have full sequence number recovery
> 
> due to the fact that it wasn't needed for all the audio payloads one was
> 
> considering to use. Also the solution RFC 2198 employs aren't suitable
> at all when the payloads become larger than half of the MTU.
> 
> If you want sequence number recovery, less hassle with timestamps and
> so: Use RFC 2733 FEC resolves these issue. Or rather the updated version
> 
> as RFC 2733 has some issues. Unfortunately we haven't finished this
> update yet, but we are getting close.
> 
> Vladimir Ulybin wrote:
> > Let consider an option to update the
> "draft-jones-avt-audio-t38-05.txt"
> > or write a new draft for T.38 over RTP.
> 
> This is an ITU defined RTP payload format. I agree that it has issues
> and some of them could have been avoided if ITU had involved AVT in the
> loop earlier when it was under proposal. However it is ITU that has
> change control of it.
> 
> >
> > I think the problems opened in our discussions
> > - repetition of T.38 packets and
> 
> If you are using RTP you have certain rules to follow. These involve the
> 
> fact that packets can't be repeated using the same RTP sequence number.
> This requires solution like the RTP Retransmission format or the use of
> 2733 FEC.
> 
> > - excessive complexity of T.38 over RTP caused by timestamps
> > (non-required by T.38)
> 
> As Colin says RTP timestamps must be set in RTP. However one can make
> them simple to only indicate time of transmission. This would be simpler
> 
> if people hadn't insisted on making it go over the same RTP session as
> audio. FAX isn't audio and it shouldn't be handled the same way as audio
> 
> packets. Thus it should go in its own RTP session where it can be given
> somewhat different treatment. Yes, audio/t38 should in fact be image/t38
> 
> or possibly application/t38.
> 
> Then as I said, stop using RFC 2198 and your timestamp issues mostly
> goes away.
> 
> 
> Cheers
> 
> Magnus Westerlund
> 
> Multimedia Technologies, Ericsson Research EAB/TVA/A
> ----------------------------------------------------------------------
> Ericsson AB                | Phone +46 8 4048287
> Torshamsgatan 23           | Fax   +46 8 7575550
> S-164 80 Stockholm, Sweden | mailto: magnus.westerlund at ericsson.com


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