In T.38 UDPTL protocol, every primary fax packet has a unique
sequence number.
To immune a fax session of packet loss in the network, gateways use
redundancy and packet retransmissions.
When a T.38 UDPTL packet is retransmitted, the sequence number is
unchanged.
In T.38 over RTP protocol, the RTP sequence number must be used.
The RFC 3550 defines the sequence number as monotonically
incremented by one for every packet sent to the network.
If RTP sequence number is incremented per every retransmission,
then the retransmission of fax packets containing T.38 DATA packets
in primary or redundant RTP payloads is problematic because the T.
30 data may be re-modulated incorrectly.
The problem exists also for retransmission of T.38 INDICATOR
packets: If a primary T.38 INDICATOR is lost, the retransmitted T.
38 INDICATOR arrived with a gap in RTP sequence number may be
delayed in packet recovery module, so the start of corresponding
fax signal may be delayed when it should not.
The obvious solution is to increment RTP sequence number
conditionally by the same way as in T.38 UDPTL:
The RTP sequence number is to be incremented per every new primary
fax packet, but remains unchanged if RTP packet transfers a
retransmitted primary fax packet.
The questions:
Does this conflict with some RTP protocol?
What kind of interoperability problems (excluding RTCP statistics
of packet loss) may exist?
Thanks,
Vladimir Ulybin
AudioCodes Ltd.
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Audio/Video Transport Working Group
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