[H-GEN] TCP question

[Jason McDonald]

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> now i run into a problem which makes no sence to me. My test linux-PC
> doesn't even blink an eyelid...it just keeps on seeing the share like
> nothing has happened.  BUT. My NT4-PC disconnects and will not
> ping/map/anything the IP/share for several minutes. Eventualy it switches
> over but the time  it takes is toooo long for my needs.
> 
> so my question (now that you have the background) is, what is causing the
> TCP in NT to be so slow in responding to a change in possition on the
> network for an IP?   anyone?  and how can i fix this?

I suspect that the NT box is not updating it's ARP cache (the list of
mappings from IP addresses to Ethernet hardware addresses) very often.
>From memory the standard expiry time for an ARP entry is about 20
minutes.

It would be interesting to run tcpdump and see how long it takes for the
NT box to send out an ARP request (to get the new ethernet hardware
address that matches the target IP address) after the changeover to
confirm this theory.

Unfortunately, I'm not sure how to change the default ARP expiry time in
Windoze.  But something to try would be to have the machine that takes
over the share send out a broadcast ICMP request packet (i.e. a ping to
the network's broadcast address (possibly 10.0.0.255 or the like)) when it
takes control of the IP address.  

Some TCP stacks will use received ICMP request packets to update their ARP
cache (on the basis that they will probably have to send replies to the
machine sneding the request and the ICMP packet has the needed data so
that a separate ARP request would be waste of time).  

If you're lucky NT might be one of them.

Good luck,
J.


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