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Most Important Terminal Server Registry Keys and Values

Posted by Benny Tritsch on December 10, 2003 – updated on May 31, 2005

[Introduction] [General] [System and User] [Default User] [Folders] [Printers] [Network]

TCP/IP Network Control

Network settings that are relevant to Terminal Servers are located under HKLM \SYSTEM \CurrentControlSet \Services \Tcpip \Parameters.

 

Value Names Data Types, Default Value Description
EnableDeadGWDetect DWORD: 0x1 Determines whether TCP performs dead gateway detection. Dead gateway detection is a TCP feature that identifies gateways that are not operating properly and that switches the computer to a new default gateway. When enabled, any given connection defines a gateway as non-operational (dead) when a packet sent to the gateway must be retransmitted more than half of the number of times specified in the value of the TcpMaxDataRetransmissions entry. The connection switches to the next gateway in the list in the DefaultGateway or DhcpDefaultGateway entries. The system defines a gateway as dead when more than 25 percent of its connections have switched to the next default gateway in the list.
EnablePMTUBHDetect DWORD: 0x0 Determines whether TCP tries to detect black hole routers during the Path MTU (maximum transmission unit) discovery process. Enabling black hole detection increases the maximum number of times TCP retransmits a given segment. If the value of this entry is 1, TCP recognizes when it has transmitted the same segment several times without receiving an acknowledgement. It reduces the maximum segment size (MSS) to 536 bytes, and it sets the Don't-Fragment bit. If, as a result, receipt of the segment is acknowledged, TCP continues this practice in all subsequent transmissions on the connection. This entry is used only when the Path MTU discovery process is performed, that is, when the value of the EnablePMTUDiscovery entry is 1.
EnablePMTUDiscovery DWORD: 0x1 Determines whether TCP uses a fixed, default maximum transmission unit (MTU) or attempts to detect the actual MTU. By discovering the Path MTU and limiting TCP segments to this size, TCP can eliminate fragmentation at routers connecting networks with different MTUs. Fragmentation reduces TCP throughput and increases network congestion. By default, this entry applies to all interfaces. However, the MTU can be reduced for any particular interface by changing the default value of the MTU entry in the subkey for that interface.

0: TCP uses an MTU of 576 bytes for all connections to computers outside the local subnet.

1: TCP attempts to discover the MTU of the path to a remote host.
TcpMaxDataRetransmissions DWORD: 0x5 Windows provides a mechanism to control the initial retransmit time, and then the retransmit time is self-tuning. The timer for a given segment is doubled after each retransmission of that segment. Using this algorithm, TCP tunes itself to the normal delay of a connection. TCP connections over high-delay links will take much longer to time out than those over low- delay links. By default, after the retransmission timer hits 240 seconds, it uses that value for retransmission of any segment that needs to be retransmitted. This can be a cause of long delays for a client to time out on a slow link.

TcpMaxDataRetransmissions controls the number of times TCP retransmits an individual data segment (non connect segment) before aborting the connection. The retransmission timeout is doubled with each successive retransmission on a connection. It is reset when responses resume. The base timeout value is dynamically determined by the measured round-trip time on the connection. The valid range of this value is 0 - 0xffffffff.
TCPInitialRtt DWORD: 0xBB8 This parameter (default value = 3 seconds) controls the initial retransmission timeout used by TCP on each new connection. It applies to the connection request (SYN) and to the first data segments sent on each connection. For example, the value data 5000 decimal sets the initial retransmit time to five seconds. You can only increase the value for the initial timeout. Decreasing the value is not supported, even if the valid range for this value is 0 - 65535 (decimal).