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

From: "Vladimir Ulybin" <[email-masked]>
Date: Thu, 30 Jun 2005 11:31:53 +0300

Hi,

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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