DOCUMENT:Q156438  28-JUL-2001  [win95x]
TITLE   :Internet Mail or News Reader Software Hangs or Times Out
PRODUCT :Microsoft Windows 95.x Retail Product
PROD/VER:WINDOWS:95
OPER/SYS:
KEYWORDS:win95 appscomp kbAppCompatibility

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The information in this article applies to:

 - Microsoft Windows 95 
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SYMPTOMS
========

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

CAUSE
=====

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.

RESOLUTION
==========

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.

MORE INFORMATION
================

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.

Additional query words:

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Keywords          : win95 appscomp kbAppCompatibility 
Technology        : kbWin95search kbZNotKeyword3
Version           : WINDOWS:95

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