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[Didier Barvaux <question100514@xxxxxxxxxxxxxxxxxxxxx>]

Question #100514 on rohc changed:
https://answers.launchpad.net/rohc/+question/100514

    Status: Open => Answered

Didier Barvaux proposed the following answer:
> My understanding is that these parameters should start
> the compressor and associated decompressor with a
> small BER and feedback enabled.

Yes, you're right.

> When streaming in one direction, it shows that packets are
> being dropped, but it does not reply with any feedback.
> Wouldn't the context become invalidated?

Not necessarily. The context becomes invalid only if the ROHC
decompressor detects a problem such as a CRC failure. In such
cases, the decompressor _may_ send a negative feedback if the
(decompression failures / decompression successes) ratio
increases too much.

> An example of this behavior is shown below in the output of
> the decompressor on the receiving node.

In your example, there is no decompression failure, so no problem
is detected, so no feedback is transmitted. It probably appends because
your UDP stream is very regular (all successive packets are very similar),
so the lost packets do not cause a desynchonization between the
compressor context and the decompressor context.

> Furthermore, no feedback is sent on initialization of the tunnel. This
> makes me think that the mode is possibly stuck in unidirectional. It
> never seems to switch into optimistic or reliable. Is there a way to
> force the mode into optimistic or reliable?

It should transit to O-mode upon first successful decompression of packet.
Search for string "change from mode 1 to mode 2" in compression traces
and search for string "send an ACK feedback" in decompression traces.
Do you find them ? If not, contact me again with the compression and
decompression traces of the first 10 packets.

Didier Barvaux
Viveris Technologies
http://www.tech.viveris.com/opensource/

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