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

["Vladimir Ulybin" <Vladimir at audiocodes.com>]

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

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