Re: dealing with collisions, dropped packets

[Guy Harris <guy () alum mit edu>]

On Nov 3, 2004, at 11:34 PM, sthaug () nethelp no wrote:

> Um, I'm still not sure if you understand. A normal collision is 
> detected
> during the first 512 bits of the packet. There is no retransmission of
> the whole packet in case of collision (and thus no second copy of the
> packet). One packet is sent, one packet is received.

Or, to give some more details, the *sender* detects the collision (by 
listening to what they transmit), and stops transmitting as soon as 
they detect the collision, so the packet isn't fully transmitted.  In 
the case of a collision, another attempt will probably be made to 
transmit the packeet (after a backoff), unless enough unsuccessful 
attempts have been made - but those aren't "retransmissions" in the 
sense that the entire packet has been transmitted more than once; only 
on a *successful* transmission will the entire packet be transmitted, 
and transmission attempts that get a normal collision are unsuccessful 
attempts and the entire packet isn't transmitted.

So neither tcpdump nor any other software on the host will see a packet 
if the attempt to transmit that packet gets a normal collision - any 
network adapter that sees the beginning of that packet being 
transmitted will also see that the transmission stops before the end of 
the packet, and will discard whatever data it received and not supply 
it to the host.

-
This is the tcpdump-workers list.
Visit https://lists.sandelman.ca/ to unsubscribe.