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Q190162: Terminal Server and the 2048 Open File Limitation

Article: Q190162
Product(s): Microsoft Windows NT
Version(s): 4.0
Operating System(s): 
Keyword(s): 
Last Modified: 11-JUN-2002

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The information in this article applies to:

- Microsoft Windows NT Server version 4.0, Terminal Server Edition 
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IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:

  Q256986 Description of the Microsoft Windows Registry

SYMPTOMS
========

To maintain compatibility with existing Server Message Block (SMB)-based
products (for example, Microsoft Windows NT 3.x and 4.0, Microsoft Windows 95),
Terminal Server's use of SMB has not been modified from Microsoft Windows NT
Server 4.0. This can cause a problem if many Terminal Server users connect to a
single network share, either on the Terminal Server or elsewhere on the network.
The potential problem is an SMB limitation of 2048 open file handles.

RESOLUTION
==========

WARNING: If you use Registry Editor incorrectly, you may cause serious problems
that may require you to reinstall your operating system. Microsoft cannot
guarantee that you can solve problems that result from using Registry Editor
incorrectly. Use Registry Editor at your own risk.

This behavior has been modified in SP4 to enable each Terminal Server client to
maintain a separate virtual circuit. The new functionality is not enabled by
default.

To enable the new functionality, follow these steps:

1. Ensure that the server is at or above SP4.

2. From the server, run Registry Editor (Regedt32.exe).

3. From the HKEY_LOCAL_MACHINE subtree, go to the following key:

  \SYSTEM\CurrentControlSet\Services\Rdr\Parameters

4. On the Edit menu, click Add Value.

5. Add the following:

  Value Name: MultipleUsersOnConnection
  Data Type: REG_DWORD
  Data: 0

6. After you make this registry change, quit Registry Editor and restart the
  server.

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

Windows NT normally multiplexes all file requests sent to a single server over
one virtual circuit. A locally unique identifier (LUID) and SessionID on
Terminal Server sort user account credentials. These identifiers are used to
receive credentials from the server for that user. Each request to access a
specific file supplies this server credential handle. This enables multiple
users on the workstation (really security contexts since Windows NT can have
services access the network with different accounts) to share the virtual
circuit and have their own security reflected on the server. This is known as
User Level Security in SMB terminology.

The SMB data structures reserve 11 bits for the file handle value. This reserve
results in the 2048 files per connection situation. Terminal Server aggravates
this problem, as many users may be simultaneously connecting from Terminal
Server to the same share. It is therefore easier in Terminal Server to reach the
file open limit than in Windows NT.

Since the limitation is 2,048 open files per connection and all users use the
same connection, a file-intensive program may be unsuccessful when the limit is
reached. The Server Manager on the server that is hosting the share can display
that the files are opened by the Terminal Server's computer name, since files
are opened with a null user name even though security is checked through user
name and credentials.

Theoretically, a Terminal Server with 200 users simultaneously using the share
would be limited to 10 open files per user.

Regarding Windows 2000:
-----------------------

The MultipleUsersOnConnection registry value does not exist in Windows 2000
because this is the behavior by default. The limit in Windows 2000 is 8192
simultaneously open file handles per Terminal Server session.

Regarding TSE4:
---------------

A 2048 simultaneously open file handles per Terminal Server; and with the TSE4
MUX switch enabled, 2048 simultaneously open file handles per Terminal Server
session.

The preceding numbers only apply to the situation where a single Terminal Server
has all users on that terminal server establishing SMB connections to a single
file server. On Terminal Server, SMB is "smart" in the sense that if two users
establish an SMB connection to the same file server, the SMB simply shares that
connection. If another user establishes another SMB connection to a different
file server, a new SMB connection is established to that file server.

Additional query words:

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Keywords          :  
Technology        : kbWinNTsearch kbWinNT400search kbWinNTSsearch kbWinNTS400search kbNTTermServ400 kbNTTermServSearch
Version           : :4.0
Issue type        : kbprb

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