MTU

Mark.Reeder@reuters.com Mark.Reeder "at" reuters.com
Thu, 24 Jan 2002 13:06:14 +0000


Chris,
can you clarify your reply  please.

On 24/01/2002 11:35:20 Chris Jaecker wrote:

> To test for this try sending out pings with 1500 mtu and df set (client to server or vice versa).

#ping -?

Usage: ping [-t] [-a] [-n count] [-l size] [-f] [-i TTL] [-v TOS]
            [-r count] [-s count] [[-j host-list] | [-k host-list]]
            [-w timeout] destination-list

Options:
    -t             Ping the specifed host until interrupted.
    -a             Resolve addresses to hostnames.
    -n count       Number of echo requests to send.
    -l size        Send buffer size.
    -f             Set Don't Fragment flag in packet.
    -i TTL         Time To Live.
    -v TOS         Type Of Service.
    -r count       Record route for count hops.
    -s count       Timestamp for count hops.
    -j host-list   Loose source route along host-list.
    -k host-list   Strict source route along host-list.
    -w timeout     Timeout in milliseconds to wait for each reply.



...so that would be
                                            ping -l 1500 -f  remote_host
      -  correct ?

>If it is the problem, try this:
>
> The solution was to reduce the MTU of the VNC host. This can be done in the registry permanently with:
>
> HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\<adapter>\Tcpip\Parameters\MTU
>
> which should be set to 1400

[Under NT4 system] I couldn't find any Tcpip\Parameter names under Services and couldn't really match <adapter> to my actual adapter
can you advise how to create an appropriate "<adapter>" name please.

> The NT server can also be forced to discover the MTU of a path by setting:
>
> HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters\EnablePMTUBHDetect
>
On my system, this did not exist, so presumably requires creation.
a) what is the behaviour when entry does not exist?
b) can you confirm that "discover" means "discover and use"
c) As a normal "end-user", when would you NOT want "discover and use" ?
 I assume it is useful when investigating performance and setting up a network.

> which is a REG_DWORD value. Setting it to 1 forces the NT server to
> discover the MTU of the path to the target system (in this case the VNC
> viewer). If you set the value to 0, it forces the MTU down to 576, and
> doesn't attempt to discover the MTU at all.

thanks for your advice, Mark



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