[OLUG] collisions - tcpdump, ethereal, etc

[puzzled]

   If there are only three machines and only connected with one hub shutdown
the machine not involved in the test and make sure the others are set to half
duplex.


   What sort of rate are you getting collisions/frames sent? With a simple
environment like you describe this could be something exotic like a NIC that
is going deaf or dumb or maybe a bad cable. A failing repeater (hub) can raise
all kinds of hell too but you'd see that as garbage mac addresses via tcpdump.
Maybe you should get a crossover cable and hook the two stations directly
together then compare that behavior to when the hub is in the mix.


  Sorry for the lecture earlier - I didn't read the intial post.




brian at cbiowa.com wrote:

>         Thanks for all the info.  I was looking for the quick fix for what
> seemed like an easy problem.  When you have 3 computers connected
> together and the only thing all of them are doing is ftping a file from
> one machine to another, other than the normal NetBUI traffic, there
> shouldn't be any trouble.  I am going to give tcpdump a try while I am
> doing the ftp and hopefully figure out from that what packets are
> colliding.  I believe that is what you were suggesting below.  It sounds
> like the best road to go down.  Thanks again to all who helped.
>
> puzzled wrote:
> >
> >    Collisions are a normal part of ethernet operation. When connected to
> > a hub a workstation listens for a short period of time to be sure its
> > got a free wire, starts to transmit, and then if it bumps into another
> > machines transmission both of the machines back off for a random period
> > of time.
> >
> >     Divide the number of collisions by the number of frames sent, the
> > rule of thumb is it should be less than 15% on a hub and just about zero
> > on a switch port. Your mileage may vary considerably on the hub rule but
> > the switch should almost never give a report of a collision ...
> > theoretically it never happens with full duplex but in practice I've
> > seen continuous very low rates of that behavior on 3com switches and
> > various Cisco products.
> >
> >    Describe the topology you're using - if your workstation is on a
> > switch port and the target server is on a hub your frames could be dying
> > by remote collision and you have no visibility into that unless you're
> > examining network stats on the other end of the connection. Switch
> > forwarding mode also makes a difference ... do you have access to the
> > switch if there is such a device involved?
> >
> >    Fire up tcpdump and set it to watching the afflicated interface. Dump
> > all of the output to a file and write a simple perl script that will
> > parse the output and look for retransmission of tcp sequence numbers in
> > packets. If this greek to you there is an excellent english/greek
> > translation called "TCP/IP Illustrated Volume 1" by W Richard Stevens.
> >
> >    Calculate the overhead associated with the connection. 10 mbit/8
> > bits/byte = 1.25 mbytes/sec *but* you have to consider the ack packet
> > for every packet you receive, the effects of packet size vs MTU for the
> > media, buffer issues on the systems involved, etc, etc.
> >
> >   If tcpdump scares you take a look at ethereal instead - easy to
> > install, easy to get started using. You can find it on
> > www.freshmeat.net.
> >
> >   I can't recommend the TCP/IP Illustrated book highly enough but you
> > only need volume 1 unless you're going to code network utilities.
> >
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> --
>              Brian Weber
>          Computer Consultant
>              Cap Gemini
>       brian at mail.cbiowa.com
>
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