Your Internet mail or news reader may stop responding (hang) or time out in Windows 95.

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This problem can occur if requests are routed through a "black hole" router.

A black hole router does not return an Internet Control Message Protocol (ICMP) "destination unreachable" message when it needs to fragment a packet that has its Don't Fragment bit enabled.

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NOTE: For information about how to edit the registry, view the Changing Keys And Values online Help topic in Registry Editor (Regedit.exe). Note that you should make a backup copy of the registry files (System.dat and User.dat) before you edit the registry.

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall Windows 95. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

To correct this problem, add the string value

   PMTUBlackHoleDetect
				

to the following registry key:

   Hkey_Local_Machine\System\CurrentControlSet\Services\VxD\MSTCP
				

Add a data value of 1 for the new PMTUBlackHoleDetect string value.

This setting specifies whether the stack attempts to detect Maximum Transmission Unit (MTU) routers that do not send ICMP fragmentation- needed messages. Setting this parameter when it is not needed can cause performance degradation. The default is 0.

If you continue to experience the problem described in this article, add the string value

   PMTUDiscovery
				

to the following registry key:

   Hkey_Local_Machine\System\CurrentControlSet\Services\VxD\MSTCP
				

Add a data value of 1 for the new PMTUDiscovery string value.

This setting specifies whether Microsoft TCP/IP attempts to perform path MTU discovery as specified in RFC 1191. The default is 1.

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Eudora versions 2.0.3 and 2.1.0 are examples of programs known to exhibit the problem described in this article. Eudora may pause when you try to send a message with more than 10 lines. After a few minutes, Eudora reports a time-out error.

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