[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

On 19 Jul 2005, at 07:40, Vladimir Ulybin wrote:
1. Zero is a simplest solution. The RFC 3550 allow the same time stamp
for several consecutive packets. So, when these packets will be
transmitted as redundant one relative to the other, the time offsets per
RFC 2198 will be zero. It means that receiving gateway should handle
correctly zero time offsets in RFC 2198 headers.

No, since it will treat the data as being a redundant copy of the packet in which it is sent. If you use an offset of zero, there's no way to correctly place the data at the right time instant in the playout buffer.

2. The computation of redundant packet time offset relative to primary
packet can be applied for synchronous operation like audio or
t4-non-ECM-data having a fixed time interval between packets. But, for
asynchronous packet stream, which is typical for T.30 control and ECM
fax, such approach cannot be used.

Correct - this is why RFC 2198 has an explicit timestamp for the redundancy offset.

Why I opened this issue?

Assume a T.38 module of DSP/gateway which delivers the T.38 UDPTL
packets ready for transmission to IP. To enable new feature of T.38 over
RTP, it is easy to convert T.38 UDPTL into RFC 2198 at output of
DSP/gateway. The obstacle is in time stamp offsets of redundant packets.
We all (fax engineers) know that these offsets are not required at T.38
packet receivers. But accurate transmission of those per RFC 2198
requires deep re-writing of working and verified module(s) plus
additional allocation of RedundancyLevel*NofChannels timestamps.

Correct - this is required for robust operation over an unreliable packet network that may disrupt media timing.

There are two cases:

1) There is a constant mapping from sequence number to timestamp. In this case, you can trivially calculate the timestamp and need no extra storage. The media timing can be reconstructed using either the sequence number or timestamp, and transporting both is essentially a waste of bandwidth for this application. In this case, you treat the extra bits on the wire as an "RTP tax" needed for interoperability, but don't need to store the extra data in the gateway.

2) There is not a constant mapping from sequence number to timestamp. This case requires you to store the extra timestamps for inclusion in RFC 2198 redundancy headers, however that extra information is required by the receiver to manage a playout buffer, since a playout buffer managed by sequence number only is not sufficient to reconstruct media timing.

Colin

_______________________________________________
Audio/Video Transport Working Group
avt at ietf.org
https://www1.ietf.org/mailman/listinfo/avt