DOCUMENT:Q140325  03-APR-2002  [crossnet]
TITLE   :How to Determine Loss of Client/Server Connection
PRODUCT :Windows for Workgroups and Windows NT Networking Issues
PROD/VER::3.5,3.51,4.0
OPER/SYS:
KEYWORDS:kbnetwork

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0 
 - Microsoft Windows NT Server versions 3.5, 3.51, 4.0 
 - Microsoft Windows 95 
-------------------------------------------------------------------------------

SUMMARY
=======

This article explains how to use keepalive probes to determine loss of stream
connection.

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

In the context of this article, the "client" is the peer computer requesting a
connection, and the "server" is the peer computer accepting a connection. This
notation does not reflect Client/Server relationships as an architectural
principal.

Consider the following scenario (half-open connection):

The client has established stream connection with the server. If client crashes
or a network error occurs (for example, intermediate router goes down), the
server has no way of knowing this, unless it tries to use send(). In this case,
send fails with the WSAECONNRESET error. If the server does not try to send data
to a client, the connection is kept open on the server side for infinite amount
of time.

Even if WSAAsyncSelect() with the FD_CLOSE parameter is used to determine loss of
connection, event notification is not triggered. No events get posted because
virtual circuit never go to a FIN WAIT or CLOSE WAIT state (crashed client never
called shutdown or closesocket).

This situation can be avoided if keepalive probe packets are used. Note that
keepalive probe packets are not a required part of the TCP specification.
Keepalive packets are used to probe a connection that has been inactive for a
long time. The peer initiates a disconnect when the probes do not get through.
Keepalive packets for TCP connection can be turned on by using setsockopt() call
with SO_KEEPALIVE option.

The following registry entries control keepalive probe packet parameters on
computers running Windows NT or Windows 95 (note that not all of the entries can
be found in the registry by default). Changing registry parameters affects all
TCP/IP stream connections on the system.

KeepAliveInterval - This parameter determines the interval separating keep alive
retransmissions until a response is received.

KeepAliveTime - This parameter controls how often TCP attempts to verify that an
idle connection is still intact by sending a keep alive packet.

TcpMaxDataRetransmissions - This parameter controls the number of times TCP will
retransmit an individual data segment (non-connect segment) before aborting the
connection.

NOTE: The TcpMaxDataRetransmission parameter is NOT a valid setting under Windows
95. For additional information on valid TCP/IP registry parameters in Windows
95, please see the following article in the Microsoft Knowledge Base:

   Q158474 Windows 95 TCP/IP Registry Entries

The above entries are found in following registry locations:

Windows 95
----------

   \HKEY_LOCAL_MACHINE
    \System
     \CurrentControlSet
      \Services
       \VxD
        \MSTCP

NOTE: To make the appropriate changes to the value above in the registry under
Windows 95, go to the above location, click the Edit menu, point to New, and
click String Value.

Windows NT
----------

   \HKEY_LOCAL_MACHINE
    \System
     \CurrentControlSet
      \Services
       \TCPIP
        \Parameters

NOTE: To make the appropriate changes to the values above in the registry under
Windows NT, go to the above location, click Add Value on the Edit menu. The Data
Type should be REG_DWORD.

For information on how to configure above entries, please see the following
article in the Microsoft Knowledge Base:

   Q120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows
   NT

IMPORTANT: After you alter the registry, you must restart your computer.


Additional query words:

======================================================================
Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT351search kbWinNT350search kbWinNT400search kbWinNTW350 kbWinNTW350search kbWinNTW351search kbWinNTW351 kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbWinNTS351 kbWinNTS350 kbWinNTS351search kbWinNTS350search kbWin95search kbZNotKeyword3
Version           : :3.5,3.51,4.0
Issue type        : kbinfo

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 2002.