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Terminal server and packet loss

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  • Terminal server and packet loss

    Hi there,
    Had a nightmare of today and had to abandon a cabling installation to look into a TS problem.
    The crux of the problem is that a 2003 terminal server is having network 'issues'. If you open up websites on it sometimes they don't work, sometimes they do. Similarly, users connecting in can't always connect. Sometimes they connect and it hangs randomly and other times it resets the connection completely.

    I have run Wireshark on both the TS and a test client machine and can easily see the problem traffic. It generally starts with a TCP window size ack that gets sent from the server to the client. This shows up in both captures.

    First point of call - we ruled out the Internet being the problem. They have 2 broadband lines, 2 routers and the problem is seen on both. So I went local. With the TS and client connected directly into a new switch we see the same problems - with the exception that it tends to recover from the problems quicker more often without a disconnect.

    I've updated all the NIC drivers, the server is a HP ML310 that has a network diagnostics program that runs a load of tests that all came back fine. The only thing I haven't been able to do is try an alternate NIC as the machine only has one adapter (built in broadcom gigabit - tried on both gigabit and 100Mb switches).

    I'm left with the inclination (and hope) that it is simply a faulty NIC as we can just pop a new one in. My worst fears are that it is something in the windows build.

    Does anyone have any suggestions that I can take with me when I go back out to the client tomorrow? I've about run out of options if this NIC doesn't work as having to rebuild the machine will *not* go down well..

  • #2
    Re: Terminal server and packet loss

    Are you running NLB on the TS? Is there any management software for the NIC installed and running? are you seeing any TCP Resets in Wireshark. do you see alot of broadcasting in the switch? How about errors or collisions?

    Comment


    • #3
      Re: Terminal server and packet loss

      No load balancing at all. The TS is on the same subnet as everything else, it just has a different default gateway for talking to the outside world but that is irrelevant as we have tested every combination we can think of of things on the same subnets/gateways and the problem did not go away.

      I see lots of things like this:

      Code:
      5817    60.129876    client.ip    ts.ip    TCP    [TCP Dup ACK 5816#1] 50085 > ms-wbt-server [ACK] Seq=5369 Ack=1651320 Win=32348 Len=0 SLE=1651690 SRE=1653337
      
      5818    60.133067    client.ip    ts.ip    TCP    [TCP Dup ACK 5816#2] 50085 > ms-wbt-server [ACK] Seq=5369 Ack=1651320 Win=32348 Len=0 SLE=1651690 SRE=1653519
      
      5819    60.133077    ts.ip    client.ip    TPKT    [TCP Fast Retransmission] Continuation
      If the connection recovers, that is it. If it doesn't recover then there will be loads more Dup ACKs.

      I don't think I've seen any resets, just the duplicate ACKs and I've seen "TCP Out-of-order" along with "TCP Previous segment lost". I've not seen any problems with the switch and we've even isolated the TS and client (well almost, there was still an uplink to the normal switch for connection to the main server but no problems showed up on the switch.)

      I wish I still had a working hub that I could run a sniff from to see if the missing packets (which I believe to all be coming from the terminal server, not the client) are making it out onto the wire.

      Comment


      • #4
        Re: Terminal server and packet loss

        Are you using a managed switch? Does the switch support SPAN (port mirroring)? Are there counters in the switch you can look at such as collisions, broadcasts, etc.?

        Comment


        • #5
          Re: Terminal server and packet loss

          Not in any of the switches I'm afraid. This customer has a Netgear Prosafe and the alternate test ones are smaller. I don't think we have any managed switches around as we don't tend to need them normally.

          Comment


          • #6
            Re: Terminal server and packet loss

            How about disconnecting this "test" switch from the rest of the network and running Wireshark to see if anything is different.

            Comment


            • #7
              Re: Terminal server and packet loss

              I'm not sure we're on the same lines here. The only reason we have used test switches is because we were having problems on the main switch. The fact that the problem also exists on the test switch (actually a 4 port built into a Draytek router..nothing fancy but enough for testing) proves that it isn't the main switch that is the problem.

              Comment


              • #8
                Re: Terminal server and packet loss

                You previously said that the "test" switch was linked to the production switch. Therefore both switches are in the same broadcast domain. Any link or network layer broadcasts occurring in the main switch will also show up in the test switch, that's why I asked if you could disconnect the "test" switch from the production switch. If you disconnect them and the problem goes away then you've isolated the problem to the production switch and it's connected hosts. If the problem remains then you've isolated it to the TS server. Unless and until you can definitively rule out one or the other you can't say where the problem is.

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                • #9
                  Re: Terminal server and packet loss

                  Also, the reason I'm being so adamant on this is that I also had a TS problem several years ago exhibiting very similar symptoms. the root cause was NLB broadcasting causing congestion in our switch. I'm thinking that even though you're not using NLB, any other broadcasts (link or network layer broadcast storms) may be causing your problem as well. Any broadcasts above 3% or so of the total traffic through the switch constitutes a broadcast storm and should be investigated and resolved.

                  Comment


                  • #10
                    Re: Terminal server and packet loss

                    Unfortunately Active directory is on the main server as are all the profiles thus it might be a tad difficult to log on with no link :/

                    Comment


                    • #11
                      Re: Terminal server and packet loss

                      Log on from the client to the server and then disconnect the switches.

                      Comment


                      • #12
                        Re: Terminal server and packet loss

                        That should work, as long as the connection is stable enough to actually log in!

                        Cheers, I had it in my mind that I had to keep the two connected but when you take a step back or a fresh mind comes at it things often become clearer.

                        There's not a huge amount on the network though, 15 odd pcs and a few printers as they moved a load of stuff to a different site (the one that is now kicking up a storm because they can't access the terminal server :/

                        Comment


                        • #13
                          Re: Terminal server and packet loss

                          Worst case scenario is you have to connect your DC to the "test" switch and conduct your test after hours. The only way you're going to narrow it down IMHO is to isolate the TS and client from the rest of the network and run Wireshark. At least this should give you a good idea of where the problem is originating. Good luck and keep us posted on your progress.

                          Comment


                          • #14
                            Re: Terminal server and packet loss

                            Its not looking good. With the TS completely isolated from the main network the same thing happens. I've not had a replacement PCI Express network card yet but I have tried a USB NIC and exactly the same thing happens. Its worth noting that the problems seem to happen at the same points so maybe its something about particular packets that isn't going through but I'm at a loss of what to do now except rebuild the machine.

                            Comment


                            • #15
                              Re: Terminal server and packet loss

                              Try turning off the SNP features as decribed in the following link. I always use method 3.

                              http://support.microsoft.com/kb/948496

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