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Thread: Weird TCP problems using a vnc connection(Cable /T1)

  1. #1
    Junior Member
    Join Date
    Jun 2007
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    2

    Weird TCP problems using a vnc connection(Cable /T1)

    Hello all,
    first time on this forum, I hope I can be of assistance to some of you guys too.

    My first post however, will be a problem of my own

    Situation:
    - wireless GPRS modem with a working Internet connection situated in Jordan.
    - connected to the GPRS modem: a WinXP PC with VNC server (UltraVNC) installed.
    - WinXP PC with Cable Internet Connection situated in The Netherlands.

    Goal:
    - to take over the WinXP PC using a VNC connection form the The Netherlands to control te WInXP PC situated in Jordan. We always connect FROM the Netherlands TO the GPRS modem in Jordan.

    The Problem:
    - establishing the VNC connection using a cable connection.

    The 'funny' thing is: VNC connection WORKS with a T1 fibre connection. With a CABLE connection however, the connection fails.
    We tried various connection types: ADSL, CABLE (we tried various ISP's and operating systems!) but only the Fibre connection works. These test were done using the same pc (and some others pc's but this made no difference).
    Speed should not be the issue here, because the GPRS modem clearly is the bottleneck.

    Below are 2 dumps made using WireShark.
    We deleted the Ip adresses from the dump and changed them to "Client" and "Server" for security reasons..
    The first one is made with the WORKING connection. (WinXP->100MbitLan->T1 fibre connection->...)

    The second dump is made using a CABLE connection.
    (WinXP->100MbitLan->Cable connection->...)
    As you can see, this connection is not established correctly...

    Code:
    Time       Source       Destination   Protocol   Info
    
    29.221180   Client         Server        TCP      4764 > 5900 [SYN] Seq=0 Len=0 MSS=1460
    31.071030   Server         Client        TCP      5900 > 4764 [SYN, ACK] Seq=0 Ack=1 Win=65535 Len=0 MSS=1460
    31.071094   Client         Server        TCP      4764 > 5900 [ACK] Seq=1 Ack=1 Win=65535 Len=0
    31.920577   Server         Client        VNC      Server protocol version: 003.006
    31.920750   Client         Server        VNC      Client protocol version: 003.003
    
    Time       Source       Destination   Protocol   Info
    
    5.434876   Client          Server        TCP      1469 > 5900 [SYN] Seq=0 Len=0 MSS=1460
    7.729111   Server          Client        TCP      5900 > 1469 [SYN, ACK] Seq=0 Ack=1 Win=8760 Len=0 MSS=1460
    7.729172   Client          Server        TCP      1469 > 5900 [ACK] Seq=1 Ack=1 Win=65535 Len=0
    10.524501  Server          Client        TCP      5900 > 1469 [SYN, ACK] Seq=0 Ack=1 Win=8760 Len=0 MSS=1460
    10.524550  Client          Server        TCP      [TCP Dup ACK 10#1] 1469 > 5900 [ACK] Seq=1 Ack=1 Win=65535 Len=0
    16.541817  Server          Client        TCP      5900 > 1469 [SYN, ACK] Seq=0 Ack=1 Win=8760 Len=0 MSS=1460
    16.541870  Client          Server        TCP      [TCP Dup ACK 10#2] 1469 > 5900 [ACK] Seq=1 Ack=1 Win=65535 Len=0
    28.576874  Server          Client        TCP      5900 > 1469 [SYN, ACK] Seq=0 Ack=1 Win=8760 Len=0 MSS=1460
    28.576922  Client          Server        TCP      [TCP Dup ACK 10#3] 1469 > 5900 [ACK] Seq=1 Ack=1 Win=65535 Len=0
    52.672502  Server          Client        TCP      5900 > 1469 [SYN, ACK] Seq=0 Ack=1 Win=8760 Len=0 MSS=1460
    52.672554  Client          Server        TCP      [TCP Dup ACK 10#4] 1469 > 5900 [ACK] Seq=1 Ack=1 Win=65535 Len=0
    82.548013  Server          Client        TCP      5900 > 1469 [RST] Seq=1 Len=0


    We did so much testing at this point, and we are out of ideas. I hope you guys can have a look and maybe give us some hints on where to look for the problem..

    Muchos Thanks in advance.

  2. #2
    Security Specialist greEd's Avatar
    Join Date
    May 2001
    Location
    Maryland
    Posts
    807
    The first thing that jumped out at me is the window size difference. With your T1 connection it's reporting a receive buffer of 65535 and on your cable connection its down to 8760.

    I think that may be causing your ACK's to be delayed and causing the connection to time out.
    Last edited by greEd; 06-14-07 at 12:07 PM.
    "I'm doing a (free) operating system (just a hobby, won't be big and professional...) for AT clones... It's not portable and it probably [won't ever] support anything other than AT hard disks, as thats all I have :-(." --Posted on Usenet August 1991 by Linus Trovalds
    http://www.computerglitch.net
    curiosity builds security | dd if=/dev/zero of=/dev/hda bs=512 count=100
    EOF

  3. #3
    Junior Member
    Join Date
    Jun 2007
    Posts
    2
    Thanks for your reply!

    We already tried to set the window size on the client side manually using the Windows Registry (TcpWindowSize). When the window sizes match, the result is the same: the conenction times out.
    But I'm not an expert on window sizes so if you have a better idea on how to set the window size, pelase tell

    Another thing we tried is setting the Rtt time to a higher value, so it wouldn't time out very quick. No result. (Though maybe that has to do something with WInXP ignoring the set value)

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