Subject: [lp-wan] L2 size and fragment retransmission problem

From: Shoichi Sakane <[email-masked]>
Date: Sun, 19 Nov 2017 14:06:42 +0900

Hi,

LoRaWAN L2 payload is likely to be variable.  In window mode, if a sender needs to retransmit a fragment, but the L2 size is smaller the one that the sender used to send the fragment first time, how does the sender fix the fragment into the smaller size of the L2 payload ?

Thanks,
Shoichi

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From: "Carles Gomez Montenegro" <[email-masked]>
Date: Sun, 19 Nov 2017 09:42:28 +0100

Hi Shoichi,

While designing SCHC fragmentation, we considered the problem you mention.
However, we found that the problem is relatively complex, and would
require use of further header and ACK/bitmap bits to support a solution.
(Something like a second-level bitmap and second-level FCNs should be
supported for refragmenting a fragment in that case.)

Therefore SCHC fragmentation, as of today, does not provide a solution to
this problem, and possibly the sender may only transmit an Abort message
when it happens.

Does the WG think there needs to be a solution to this problem (i.e. to
allow retransmitting a fragment that needs to be "refragmented", and
supporting ACK/bitmap feedback at the "subfragment" level)?

Cheers,

Carles

> Hi,
>
> LoRaWAN L2 payload is likely to be variable.  In window mode, if a sender
> needs to retransmit a fragment, but the L2 size is smaller the one that
> the sender used to send the fragment first time, how does the sender fix
> the fragment into the smaller size of the L2 payload ?
>
> Thanks,
> Shoichi