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

From: "Vladimir Ulybin" <[email-masked]>
Date: Sun, 17 Jul 2005 09:06:05 +0300

Paul,

Finally, for resending of T.38 packets over RTP, Colin suggested us to
use FEC per RFC 2733 or new retransmission protocol. Both ways present
optional features which may not be supported by gateways.

Basic configuration for T.38 over RTP is a basic RFC 3550 plus
redundancy per RFC 2198.

This configuration has limited capabilities vs. T.38 UDPTL to improve
reliability of fax transport, if gateways do not resend (==duplicate)
fax packets, refer to explanation that I done in previous e-mails.

The other open issue for T.38 over RTP is the time stamps for redundant
fax packets. The T.38 implementations are not dependant on time stamps.
So, the time offset fields required per RFC 2198 for redundant fax
payloads are absolutely usefulness. There is no problem to ignore these
fields on receiving, but transmitting accurate offsets requires
additional resources in gateways.

Regards,
Vladimir Ulybin

-----Original Message-----
From: Paul E. Jones [mailto:[email-masked]] 
Sent: Sunday, July 17, 2005 1:53 AM
To: Vladimir Ulybin; 'Colin Perkins'
Cc: [email-masked]
Subject: RE: [AVT] T.38 over RTP: RTP Sequence Number

Vladimir,

I apologize that I missed much of this discussion while I was traveling.

The bottom line on this, though, is that T.38 over RTP should use
whatever
mechanisms are defined in the IETF for redundancy or FEC.  This might
include RFC 2198, RFC 2733, or other tools that come along.  It's also
expected that this will be the means by which security is provided for
fax
(SRTP).

Is there a problem you see that is specific to fax?  Timing is certainly
an
issue, but the delay should be no worse than UDPTL.  However, if there
is a
flaw somewhere, we should fix it sooner than later.

Paul

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]] On Behalf Of
> Vladimir Ulybin
> Sent: Monday, July 04, 2005 5:59 AM
> To: Colin Perkins
> Cc: [email-masked]
> Subject: RE: [AVT] T.38 over RTP: RTP Sequence Number
> 
> On my suggestion from 3 Jul 2005, at 15:44:
> >> 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.
> 
> Colin Perkins wrote:
> > Not if you wish to be compatible with RTP: RTP requires the sequence
> > numbers to be unique.
> 
> 1. The current T.38 Rec. defining T.38 over RTP refers to RFC 3550 as
a
> basic RTP protocol to be used for encapsulation.
> 2. The RFC 3550 does not define the packet repetition, also does not
use
> SHOULD or MUST for sequence number advances (in contrast to RFC
2833bis
> were the MUST is used).
> 3. Different RTP sequence numbers assigned to the same fax signal
state
> or the same binary data cannot be considered as unique.
> 4. I try to find a more reliable transport for T.38 over RTP. The
blind
> assignment of new sequence numbers for ALL packets is full compatible
> with RFC 3550, but highly reduces the reliability of fax relay,
because
> gateways may not repeat fax packets.
> 5. As I understand from draft-ietf-avt-rtp-retransmission-11.txt the
> only problem of packet repetition with the same SN is a distorted RTCP
> statistics. In absence of other ways this violation is better than to
> loose connection during fax relay.
> 
> Regards,
> Vladimir Ulybin
> 
> _______________________________________________
> Audio/Video Transport Working Group
> [email-masked]
> https://www1.ietf.org/mailman/listinfo/avt

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: "Paul E. Jones" <[email-masked]>
Date: Sun, 17 Jul 2005 23:17:08 -0400

Vladimir,

T.38 (2004) addresses the transport of T.38 over RTP and does not say that
RFC 2198 is the basic configuration.  Both RFC 2198 and RFC 2733 are
suggested as possible choices for protecting the media stream.

While T.38 data may not have the same kind of timing as, say, an audio
codec, the fact that it is placed into a media stream means it needs a
clock.  The draft I wrote suggests the use of an 8000hz clock or one that
matches the primary audio codec.  In any case, the timing information would
be meaningful.  In theory, one would not play out packets so quickly that a
redundant piece of information received one or two packets later would be
too old to be useful.

In any case, you are certainly free to use RFC 2198 or RFC 2733.  Most
people I've talked to prefer RFC 2198, simply because of the computational
complexity introduced by RFC 2733.  With that said, I've certainly not
talked to every GW maker in the world ;-)

Paul

> From: Vladimir Ulybin [mailto:[email-masked]]
> Sent: Sunday, July 17, 2005 2:06 AM
> To: Paul E. Jones; Colin Perkins
> Cc: [email-masked]