Comment # 33
on bug 12687
from Michael Mann
(In reply to Oliver Hartkopp from comment #32)
> With CAN FD we now have two options:
>
> 1. Create a second link type DLT_CANFD_SOCKETCAN and provide a separate data
> source to Wireshark. Additionally libpcap could be moved to use PF_PACKET
> too.
>
> 2. Enhance the PDU that contain the current CAN frames to be able to
> transport CAN FD frames too. This kind of tunnelling from libpcap to
> Wireshark is currently done with the CANFD_USE bit - and that's why this bit
> must not show up in the Wiresharks proto bits.
>
> As you can see from our patches option 2 is obviously easy to do.
> But is it the right way to go?
Now we're on the same page as I was trying to ask similar questions earlier. I
don't have a strong opinion on the route taken, but I do believe Wireshark
should show ALL "frame data" which includes CANFD_USE bit if you are going to
piggyback on the DLT_CAN_SOCKETCAN format.
My patch (and current dissection functionality) has the fields/flags in order
as they appear on the frame:
32-bit value for CAN ID + "original" flags (ERR, RTR, EFF/SFF)
8-bit length field
8-bit flag field for CAN_FD (so only visible if frame is CAN_FD)
16-bit "pad" (which probably should be added because people don't like seeing
gaps in dissection - consider 24-bit pad for "Classic" CAN)
0-x bytes - packet data.
If a DLT_CANFD_SOCKETCAN "frame format" can better consolidate its respective
flags, then Wireshark would dissect them as such (but still in order). It
would then probably lend itself to a separate dissection function (like I had
in the first version of my patch as I thought this was going to be the
direction).
CANFD dissection will not be included in the 2.2 release, so we have a good
amount of time to finalize a solution. We can go the route of piggybacking on
CAN for now until a DLT_CANFD_SOCKETCAN value can be secured (email
tcpdump-workers to work that out).
I don't have a problem submitting my patch once the minor bugs/comments have
been worked through as it does enable the dissection of CANFD and you can use
Wireshark moving forward (nightly builds) in your efforts with it.
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