Comment # 58
on bug 12687
from Guy Harris
(In reply to Guy Harris from comment #57)
> (In reply to Oliver Hartkopp from comment #56)
> > > > > > 2. Up to now only the Packet type 1 (Broadcast) was displayed inside
> > > > > > Wireshark. With the use of PF_PACKET sockets we now see the type 1 and type
> > > > > > 4 (from me) packet types - so we see sent frames two times, which is not
> > > > > > usual.
>
> So are you saying that, if a CANbus device sends a broadcast packet, the
> sending hardware receives the packet it's sending and provides it as an
> input packet, so that software reading from a PF_PACKET socket will see two
> copies - a copy looped back in the networking stack, and a copy received by
> the hardware? That's not the case with regular LAN hardware, which doesn't
> see its own transmissions, so that's not an issue for Ethernet, 802.11, etc..
No, it looks as if the CAN networking stack loops back transmitted packets.
So does dev_queue_xmit_nit(), so we get *two* loopback packets.
If the CAN layer is *guaranteed* to loop back all transmitted packets, that
means that all PACKET_OUTGOING CAN packets can be discarded, unless there's a
way to detect which PACKET_BROADCAST CAN packets are looped back and which are
received, in which case we should discard the looped-back PACKET_BROADCAST
packets so that the packet type reflects whether we sent the packet or not.
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