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

From: "Vladimir Ulybin" <[email-masked]>
Date: Mon, 4 Jul 2005 12:58:47 +0300

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

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From: Colin Perkins <[email-masked]>
Date: Mon, 4 Jul 2005 11:35:15 +0100

> SHOULD or MUST for sequence number advances (in contrast to RFC  

The RTP specification says "The sequence number increments by one for  
each RTP data packet sent".

> 3. Different RTP sequence numbers assigned to a fax state (or binary
> data) detected (or demodulated) at ONE moment of time cannot be
> considered as unique, because the one fax state will share some
> different packets. There is no mutual correspondence.

Using either RFC 2733, the RTP retransmission framework, or RTP over  
TCP will retain the sequence number. This is not a problem.

> gateways may not repeat fax packets.

As you say, gateways may not repeat the packets. This is because RTP  
requires all packets to have a unique sequence number - a gateway  
which suppresses the duplicates is behaving correctly.

> loose connection during fax relay.

The AVT working group has developed three alternatives to improve the  
quality of the media: RFC 2733, the RTP retransmission framework, or  
RTP over TCP. I believe one of those would be appropriate for your  
usage.

Colin

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From: "Paul E. Jones" <[email-masked]>
Date: Sat, 16 Jul 2005 18:53:28 -0400

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
>