[OLUG] collisions - tcpdump, ethereal, etc

[puzzled]

   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.


-------------------------------------------------------------------------
Sent by OLUG Mailing list Manager, run by ezmlm.  http://olug.bstc.net/ 
To unsubscribe: `echo unsubsribe | mail olug-unsubscribe at bstc.net`