VNC freezing on Win XP Media Center Edition SP2
Alex Pelts
alexp "at" broadcom.com
Wed Apr 26 18:45:01 2006
James,
I am not trying to ask help on the forum. I am just telling what I
observed and how I fixed it. My VPN software made by cisco and I think
it is one of the better pieces of vpn out there or at least comparable
to any other vpn.
Note that I am not blaming VNC for my problems at all in fact I am very
happy with it and recommend anyone to upgrade to EE or personal edition
if they are running VNC on windows.
I think that default MTU in some vpn setups is too high and it is
causing the problems I described. Unfortunately I don't have the time or
desire to investigate the cause of this problem. It baffles me as well,
because I know that TCP is connection based protocol and packets are
retransmitted. But I do observe the problem and that was how I solved
it. I will be happy to provide more details if you want them.
Regards,
Alex
James Weatherall wrote:
> Hi Alex,
>
> Yes, TCP sends packets, but VNC doesn't - VNC sends a stream of bytes and
> the TCP stack is then responsible for splitting that stream into a sequence
> of packets of an appropriate size for the underlying network.
>
> The problems you're seeing are most likely caused by faulty VPN software not
> indicating the MTU correctly to the two ends of the connection, resulting in
> TCP splitting the data stream into chunks that are too large for the VPN.
> Your best bet is therefore to contact your VPN vendor for support for their
> product.
>
> Cheers,
>
> Wez @ RealVNC Ltd.
>
>
>
>> -----Original Message-----
>> From: Alex Pelts [mailto:alexp "at" broadcom.com]
>> Sent: 26 April 2006 16:50
>> To: James Weatherall
>> Cc: 'S B'; vnc-list "at" realvnc.com
>> Subject: Re: VNC freezing on Win XP Media Center Edition SP2
>>
>> Well tcp stack sends packets. It all has to do with the single write
>> length that most likely goes out as a packet. If the write is
>> large it
>> will fill one or more full vpn packets that can't be
>> fragmented and will
>> be dropped. You can try it yourself. To tell you the truth I am not a
>> TCP expert so I cant tell you the exact reason. I am sure that if you
>> run ethereal you can find some clues rather quickly.
>>
>> All I know that lowering MTU on my VPN interface fixed these problems.
>>
>> Regards,
>> Alex
>>
>>
>> James Weatherall wrote:
>>> Hi Alex,
>>>
>>> VNC doesn't send packets at all - it runs over TCP.
>>>
>>> Cheers,
>>>
>>> Wez @ RealVNC Ltd.
>>>
>>>
>>>> -----Original Message-----
>>>> From: vnc-list-admin "at" realvnc.com
>>>> [mailto:vnc-list-admin "at" realvnc.com] On Behalf Of Alex Pelts
>>>> Sent: 25 April 2006 19:14
>>>> To: James Weatherall
>>>> Cc: 'S B'; vnc-list "at" realvnc.com
>>>> Subject: Re: VNC freezing on Win XP Media Center Edition SP2
>>>>
>>>> The VPN has MTU larger than MTU of physical network. Lots of
>>>> packets are
>>>> dropped as somehow VNC sends rather large packets. I had
>> this problem
>>>> myself until I lowered MTU to below of the physical network MTU.
>>>>
>>>> The symptoms were:
>>>> 1. screen would render half way and then stop.
>>>> 2. screen updates were extremely slow
>>>> 3. Connection would go for some short time and then would
>> be dropped.
>>>> I am not sure why MTU is causing that effect. I think that
>>>> VPN packets
>>>> can not be fragmented so they are dropped and there is a
>> large packet
>>>> loss. I am not sure how VNC reacts to large packet loss.
>>>>
>>>> Regards,
>>>> Alex
>>>>
>>>>
>>>> James Weatherall wrote:
>>>>> Hi Alex,
>>>>>
>>>>> Can you provide any more information on why that causes the
>>>> effect SB is
>>>>> seeing?
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Wez @ RealVNC Ltd.
>>>>>
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: vnc-list-admin "at" realvnc.com
>>>>>> [mailto:vnc-list-admin "at" realvnc.com] On Behalf Of Alex Pelts
>>>>>> Sent: 25 April 2006 07:07
>>>>>> To: S B
>>>>>> Cc: vnc-list "at" realvnc.com
>>>>>> Subject: Re: VNC freezing on Win XP Media Center Edition SP2
>>>>>>
>>>>>> MTU is too large on your vpn connection.
>>>>>>
>>>>>> Regards,
>>>>>> Alex
>>>>>>
>>>>>>
>>>>>> S B wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> I just bought a new computer from Dell (Intel dual core
>>>>>> 2.8GHz, 1GB RAM
>>>>>>> running Win XP Media Center w/ SP2), and downloaded VNC 4.4
>>>>>> viewer on it.
>>>>>>> When I VNC to my work machine, VNC viewer freezes
>>>>>> intermittently. Some times
>>>>>>> immediately after logging in and some times after 10-15
>>>>>> minutes of work. Did
>>>>>>> anyone here faced a similar problem? How do I fix this problem.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> - SB
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