Issue with UDP in Visual Basic 6 - TCP-IP

This is a discussion on Issue with UDP in Visual Basic 6 - TCP-IP ; I have a pair of programs which use UDP to transfer a test message. The transmitter has a test string and every second sends out the string to a user selected port and IP address, or to the localhost address, ...

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Thread: Issue with UDP in Visual Basic 6

  1. Issue with UDP in Visual Basic 6

    I have a pair of programs which use UDP to transfer a test message. The
    transmitter has a test string and every second sends out the string to a
    user selected port and IP address, or to the localhost address, or to a
    broadcast address. The receiver receives the message and displays it. The
    programs will be used to determine whether UDP can transit a switch/router
    architecture over which we have no control.

    Everything works fine on a number of Windows XP machines connected on a LAN
    on which this has been tried and on several Windows 2003 servers, both
    coupling via IP address or localhost on the same machine, or via IP address
    or broadcast address on separate machines.

    However, one pair of XP machines connected on a LAN in the same subnet
    exhibit a problem in that it takes 16 seconds for the SendData call to
    occur - putting a printout of time before and after the call show this is
    the case, as well as inspection of the received data on the receiver
    program. Using the UDPTestTools program shows this is not a network
    problem, as coupling the transmitter of the UDP TestTools program to my
    receiver does not exhibit the problem - which is only occuring on this
    system. Anyone any ideas as to the cause? Might there be issues with the
    builds of the various ocx files used?



  2. Re: Issue with UDP in Visual Basic 6

    On Feb 4, 1:06 pm, "peterfelgate"
    wrote:

    > However, one pair of XP machines connected on a LAN in the same subnet
    > exhibit a problem in that it takes 16 seconds for the SendData call to
    > occur - putting a printout of time before and after the call show this is
    > the case, as well as inspection of the received data on the receiver
    > program. Using the UDPTestTools program shows this is not a network
    > problem, as coupling the transmitter of the UDP TestTools program to my
    > receiver does not exhibit the problem - which is only occuring on this
    > system. Anyone any ideas as to the cause? Might there be issues with the
    > builds of the various ocx files used?


    This sounds like a DNS timeout or similar issue. Does the machine have
    some firewall or other security product that might be delaying the
    SendData call until it can finish some security checks?

    DS

  3. Re: Issue with UDP in Visual Basic 6

    peterfelgate wrote:
    >I have a pair of programs which use UDP to transfer a test message.


    You might like to try Jim Harrison's Winsock Tool as another way to
    send and receive messages (just to confirm that it's not your tool,
    but is something else). You install it on two Windows computers, then
    run one as a "server" to listen on TCP or UDP and whatever port you
    like. The other then attempts to connect. Then you can send messages
    in either direction.

    http://www.isatools.org/tools.asp?Context=ISA2000

    When I last tried it, there was an irritating error message during
    install which I believe it is safe to ignore.

    Assuming you still have your delay, I would next open a command prompt
    window on one machine and execute 'ping -t x.x.x.x' to continuously
    ping the IP of the other machine. Then do your testing again (while
    the pings are active). I'm wondering if some switch weirdness is
    causing it to delay for the first packet or two (is there a delay when
    traffic is already flowing?).

    John


  4. Re: Issue with UDP in Visual Basic 6

    On Mon, 04 Feb 2008 16:32:44 -0800, David Schwartz wrote:

    > On Feb 4, 1:06 pm, "peterfelgate" wrote:
    >
    >> However, one pair of XP machines connected on a LAN in the same subnet
    >> exhibit a problem in that it takes 16 seconds for the SendData call to
    >> occur - putting a printout of time before and after the call show this
    >> is the case, as well as inspection of the received data on the receiver
    >> program. Using the UDPTestTools program shows this is not a network
    >> problem, as coupling the transmitter of the UDP TestTools program to my
    >> receiver does not exhibit the problem - which is only occuring on this
    >> system. Anyone any ideas as to the cause? Might there be issues with
    >> the builds of the various ocx files used?

    >
    > This sounds like a DNS timeout or similar issue. Does the machine have
    > some firewall or other security product that might be delaying the
    > SendData call until it can finish some security checks?


    Or a PMTU blackhole, which Windows tries to work around.

    HTH,
    M4

  5. Re: Issue with UDP in Visual Basic 6


    "Martijn Lievaart" wrote in message
    newsan.2008.02.05.07.38.10@rtij.nl.invlalid...
    > On Mon, 04 Feb 2008 16:32:44 -0800, David Schwartz wrote:
    >
    >> On Feb 4, 1:06 pm, "peterfelgate" wrote:
    >>
    >>> However, one pair of XP machines connected on a LAN in the same subnet
    >>> exhibit a problem in that it takes 16 seconds for the SendData call to
    >>> occur - putting a printout of time before and after the call show this
    >>> is the case, as well as inspection of the received data on the receiver
    >>> program. Using the UDPTestTools program shows this is not a network
    >>> problem, as coupling the transmitter of the UDP TestTools program to my
    >>> receiver does not exhibit the problem - which is only occuring on this
    >>> system. Anyone any ideas as to the cause? Might there be issues with
    >>> the builds of the various ocx files used?

    >>
    >> This sounds like a DNS timeout or similar issue. Does the machine have
    >> some firewall or other security product that might be delaying the
    >> SendData call until it can finish some security checks?

    >
    > Or a PMTU blackhole, which Windows tries to work around.
    >
    > HTH,
    > M4


    I already have the UDPTestTool program - which incidentally seems to be
    written in VB as the error messages are VB ones and the install reports
    about updating the VB6 run ddll file - and that shows no issues when coupled
    to its own receiver, or to my receiver.

    Just to clarify - this 16 second delay occurs when using the SAME machine
    (i.e. using the localhost IP address 127.0.01 or the machine's valid IP
    address) for the TX and the destination. It is definitely a TX program
    problem since if I set up receivers on several machines then use the
    broadcast address, I get receipt of the sent packets but only at the 16
    second interval at which they are transmitted.

    The security issue is a possibility. I will investigate this, as there are
    several security features implemented.

    Sorry for my ignorance, but what is a PMTU backhole?

    PS the original purpose of knocking up this program was to see if UDP
    broadcasts worked in the target environment - and the UDPTestTools has shown
    they don't, and so we can progress on finding a solution to that issue!



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