MTU - VNC dies and requires reconnection every few minutes (W in NT - AT T Extranet - Cable Modem)

mpatient@att.com mpatient "at" att.com
Thu, 24 Jan 2002 15:38:12 +0000


Chris and Mark,

Regarding MTUs. I'm not working at that knowledge level, so I asked our
technical support people...

They said we are not using TCP/IP for transport. Instead, we are using
"IPSEC" and the data is both encrypted and encapsulated for transport. 

Thanks for the "shot in the dark". I'm still looking for the light switch...

Mike Patient
Solutions Engineer
> *	Email :	mpatient "at" ems.att.com
> *	Phone:		407.805.1558

-----Original Message-----
From: Mark.Reeder "at" reuters.com [mailto:Mark.Reeder "at" reuters.com]
Sent: Thursday, January 24, 2002 8:00 AM
To: vnc-list "at" uk.research.att.com
Subject: Re: MTU - VNC dies and requires reconnection every few minutes
(Win NT - AT T Extranet - Cable Modem)


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\Paramet
ers\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\Enable
PMTUBHDetect
>
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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