Hi Alex, I didn't think that you were - I was just trying to advise you on the best course of action to see a resolution to the VPN/MTU problem, it being a pain to have to configure these things manually!
It sounds from what you say as though there is a manual-configuration component to the VPN setup that is the source of the connection hang problem. I think you are correct in asserting that this results from over-large do-not-fragment packets, which result from the server system's TCP stack using the underlying network MTU rather than the VPN's MTU. (The VPN's MTU is obviously slightly smaller due to VPN headers.) TCP has end-to-end Maximum Segment Size handling which is supposed to take care of this sort of thing, but it sounds like it's not in your case, or in SB's. Retransmission won't help (in fact, it can make things worse), because TCP will keep trying to retransmit with the same MSS/MTU and so the retransmissions will be dropped in the same way as the original packets. Thanks for the recommendation, much appreciated! :) Cheers, Wez @ RealVNC Ltd. > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Alex Pelts > Sent: 26 April 2006 18:44 > To: James Weatherall > Cc: 'S B'; [email protected] > Subject: Re: VNC freezing on Win XP Media Center Edition SP2 > > 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:[EMAIL PROTECTED] > >> Sent: 26 April 2006 16:50 > >> To: James Weatherall > >> Cc: 'S B'; [email protected] > >> 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: [EMAIL PROTECTED] > >>>> [mailto:[EMAIL PROTECTED] On Behalf Of Alex Pelts > >>>> Sent: 25 April 2006 19:14 > >>>> To: James Weatherall > >>>> Cc: 'S B'; [email protected] > >>>> 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: [EMAIL PROTECTED] > >>>>>> [mailto:[EMAIL PROTECTED] On Behalf Of Alex Pelts > >>>>>> Sent: 25 April 2006 07:07 > >>>>>> To: S B > >>>>>> Cc: [email protected] > >>>>>> 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 > >>>>>>> _______________________________________________ > >>>>>>> VNC-List mailing list > >>>>>>> [email protected] > >>>>>>> To remove yourself from the list visit: > >>>>>>> http://www.realvnc.com/mailman/listinfo/vnc-list > >>>>>> _______________________________________________ > >>>>>> VNC-List mailing list > >>>>>> [email protected] > >>>>>> To remove yourself from the list visit: > >>>>>> http://www.realvnc.com/mailman/listinfo/vnc-list > >>>>> _______________________________________________ > >>>>> VNC-List mailing list > >>>>> [email protected] > >>>>> To remove yourself from the list visit: > >>>>> http://www.realvnc.com/mailman/listinfo/vnc-list > >>>> _______________________________________________ > >>>> VNC-List mailing list > >>>> [email protected] > >>>> To remove yourself from the list visit: > >>>> http://www.realvnc.com/mailman/listinfo/vnc-list > >>> _______________________________________________ > >>> VNC-List mailing list > >>> [email protected] > >>> To remove yourself from the list visit: > >>> http://www.realvnc.com/mailman/listinfo/vnc-list > _______________________________________________ > VNC-List mailing list > [email protected] > To remove yourself from the list visit: > http://www.realvnc.com/mailman/listinfo/vnc-list _______________________________________________ VNC-List mailing list [email protected] To remove yourself from the list visit: http://www.realvnc.com/mailman/listinfo/vnc-list
