Terminal Server and the 2048 Open File Limitation

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.

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.

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.