[Winpcap-users] winpcap sends packets double (or libpcap receives 'm double)

Bryan Kadzban bryan at kadzban.is-a-geek.net
Thu Aug 31 17:18:10 GMT 2006

On Thu, Aug 31, 2006 at 02:02:26PM +0200, Tiemen Schut wrote:
> What I can't quite imagine is packets being magically duplicated on the
> physical layer. Note that I'm not talking high level packet, just plain raw
> ethernet (mac addresses & data). What possible cause is there for a packet
> to get duplicated on the wire? I believe none.

The fact that there is no cause does not imply that the 802.3 standard
requires implementations to be reliable, though.  ;-)  And if the
standard doesn't require cards to be reliable, you really can't rely
on never getting duplicates.

But there are still reasons for packets to be duplicated -- non-switched
networks see collisions fairly regularly, which cause the sending device
to back off and retransmit.  If the collision (for some reason) did not
really affect the receiver, then the receiver will see the same frame at
least twice.  I'm not sure if this is possible in practice (especially
on a switched network), but in theory it is.

> Thanks anyway. Oh, and I know TCP uses error correction and stuff, but as
> the final product I'm developing will receive it's data from specialistic
> hardware,

Oh, never mind then.  Sounds like you'll have to discard duplicates in
your user-mode code then.  If the packets have unique identifiers, this
shouldn't be too difficult.

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