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[Didier Barvaux <question266708@xxxxxxxxxxxxxxxxxxxxx>]

Question #266708 on rohc changed:
https://answers.launchpad.net/rohc/+question/266708

    Status: Open => Answered

Didier Barvaux proposed the following answer:
Hello,

> So whenever I see a packet comes late from the sn difference I can use
> ROHC_LSB_REF_MINUS_1 to determine the outer-ip-id for that late
> packet. So I don't get anymore CRC error packets for that scenario.
> I don't if that feasible or not. :)

I'm afraid that your solution is quite specific to your use case. You
avoid a CRC failure* if the SN is correctly decompressed but not the IP-
ID. This means that the SN was more protected (against packet loss or
reordering) than IP-ID. A better way to fix your problem is probably to
increase the protection of the IP-ID. This could be performed by
increasing the width of the W-LSB window. See the
rohc_comp_set_wlsb_window_width() function**. Default value is 4. So, by
default a packet reordering of more than 4 packets might lead to a
decompression failure (in fact it depends of how quickly the IP-ID value
changes). Change it for another power of 2 value such as 64 for example.

* CRC matches but is the decompressed packet correct? several different packets may have the same CRC, especially with packet types that are only protected with CRC-3. So maybe decompression fails but it is not detected by CRC.
** https://rohc-lib.org/support/documentation/API/rohc-doc-1.7.0/group__rohc__comp.html#ga2b68070dddbc038cd55490952a7b3fa4


>  Another confusion here for me is at rohc_decompress3() previously
> we use to send feedback_send data using piggy_back. It seems its been
> deprecated at new version. So how we can send the feedback_send data
> from now on ?

Yes, the function was deprecated in 1.7.x versions and removed for
future versions 2.0.x. The master branch from Git is close to the future
2.0.0 version.

The migration between library versions is explained on the wiki:
  https://rohc-lib.org/support/wiki/doku.php?id=library-migration

However, the migration from 1.6.x to 1.7.x version that you need is
incomplete at the moment. I forgot to update it when I released the
final 1.7.0 version. I'll update it asap. Stay tuned.

Regards,
Didier

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