Possibly I used the term "retransmission" non-correctly for T.38 over
RTP. In the draft-ietf-avt-rtp-retransmission-11.txt, the term
"retransmission" is applied to packets retransmitted as a response onto
the request of packet receiver detected packet lost. Such retransmission
reduces a reliability of fax relay, because enlarges the round-trip
delay of fax commands and responses.
But I assume a simple packet repetition following an original packet
transmission to improve a reliability of delivering important T.30
commands or responses. This repetition shall be done without any request
on retransmission from remote side. In T.38 UDPTL protocol the packet
repetition is widely used by different gateway vendors.
Taking into account for T.38-over-RTP that
1. Fax relay is asynchronous communication having long periods of packet
absence;
2. The redundancy and repetition of important fax packets significantly
improves the fax session reliability;
3. Incrementing RTP sequence number for repeated T.38 packets cannot be
accepted;
4. Reliability of fax session has a much higher priority vs. a correct
RTCP statistics;
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
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