RE: Banking/Hospital Problems and MTU
There's a somewhat simpler reg hack on Windoze. Set HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\EnablePMTUBHDetect to 1 This enables automatic 'Black Hole' router detection and triggers automatic MTU scaling. BTW, the 'Don't Fragment' Bit is on by default with Windows. Rob
How incredibly serendipitous. I just ran into this problem at work the yesterday. The service techs. for our digital radiology system came in and applied the latest service update. All of a sudden users weren't able to log in via their browser loaded Java client. Funny thing is, these guys spent 2 days trying to figure out the problem before resorting to asking "the network guy"... one of those pride things I guess.
Some quality time (15 minutes) with tcpdump (well, windump) showed the DF bit being set and repeated acknowledgment requests during the "hang". After a modest amount of investigation and a few phone calls the following was found:
1. a remote firewall blocking all ICMP (the admin won't change it) 2. a router somewhere in the path with a lower MTU requirement 3. a software upgrade forcing the DF bit to be set (developer won't change it)
Lowering the MTU on the client end fixed the problem; however, I need to get the registry hack distributed across ~500 desktop and all potentially have different interface descriptions in the HKLM/system/current control set/service/ ...what a PITA.
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On Mon, 29 Jul 2002, rob.edkins(a)axon.co.nz wrote:
There's a somewhat simpler reg hack on Windoze.
Set HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\EnablePMTUBHDetect to 1
This enables automatic 'Black Hole' router detection and triggers automatic MTU scaling.
BTW, the 'Don't Fragment' Bit is on by default with Windows.
http://www.microsoft.com/windows2000/techinfo/reskit/en/cnet/cnbc_imp_yvjd.h... has more details. The ResKit says the following: Description Determines whether TCP tries to detect black hole routers during the Path MTU (maximum transmission unit) discovery process. Enabling black hole detection increases the maximum number of times TCP retransmits a given segment. If the value of this entry is 1, TCP recognizes when it has transmitted the same segment several times without receiving an acknowledgement. It reduces the maximum segment size (MSS) to 536 bytes, and it sets the Don't-Fragment bit. If, as a result, receipt of the segment is acknowledged, TCP continues this practice in all subsequent transmissions on the connection. -- Juha Saarinen - To unsubscribe from nznog, send email to majordomo(a)list.waikato.ac.nz where the body of your message reads: unsubscribe nznog
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Juha Saarinen -
rob.edkins＠axon.co.nz