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MTU

HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\ interface-name

Data type

Range

Default value

REG_DWORD

0x44– Dynamically determined MTU (in bytes) | 0xFFFFFFFF

0xFFFFFFFF

Description

Reduces the size of the maximum transmission unit (MTU) that TCP/IP uses for the network interface. The value of this entry takes precedence over the MTU that the network adapter detects dynamically.

The MTU is the size of the largest packet that can be transmitted over the underlying network, including the size of the transport header. The MTU is configured separately for each interface.

To prevent fragmentation, the MTU should be large enough to hold any IP datagram in a single frame. IP datagrams larger than the MTU are divided into fragments whose size is a multiple of eight octets. The fragments travel separately to the destination computer, where they are reassembled before the datagram is processed.

MTU detection is determined for all interfaces by the value of the EnablePMTUDiscovery entry. By default, the network adapter for each interface detects the largest MTU that the interface can transmit, and it uses that MTU for its transmissions. However, if MTU detection is disabled (that is, the value of EnablePMTUDiscovery is 0), the system uses a fixed MTU of 576 bytes. If you change the default value of the MTU entry, you override either setting as it pertains to the interface represented by this subkey.

Value

Meaning

0x44 (68 bytes) - dynamically determined MTU.

Specifies the MTU used for the network interface. This value overrides the MTU that the network adapter dynamically determines.

0xFFFFFFFF (or any value greater than the dynamically-determined MTU)

Use the dynamically-determined MTU.

Note Image Note

If you enter a value greater than the dynamically-determined MTU, the system uses the value of the dynamically-determined MTU instead. You can use this entry to reduce, but not to increase, the size of the MTU.

Windows 2000 does not add this entry to the registry. You can add it by editing the registry or by using a program that edits the registry.

Tip Image Tip

For more information, see the Microsoft Knowledge Base link on the Web Resources page. Search the Knowledge Base by using the keywords MTU or PMTU.

Caution Image Caution

In general, replacing a dynamically-determined value with a fixed value degrades the performance of the operating system. Do not change the value of this entry unless the detected MTU is not compatible with the network media.

Related Entries

Page Image

EnablePMTUDiscovery

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