Determining the current MTU

The MTU or Maximum Transmission Unit of a given connection varies depending on different factors of the connection.

The important thing to remember is that the MTU is different from the MSS (Maximum Segment Size) of a frame that is sent across a particular connection.

The MTU is the complete Frame including all Headers (TCP/IP)

The MSS is the Maximum amount of Data excluding all Headers:

• MTU is 1500 - 20 bytes for TCP - 20 bytes for IP = MSS 1460

10/100/1000 Ethernet default to 1500, yet this can be modified via registry.

(1000 Ethernet has a feature called Jumbo Frames that can increase frame sizes to 64k, yet their MTU is still ‘defined’ at 1500)

Other forms of connections can be less or more:

• PPP Connections will be less.
• PPTP/L2TP/IPSec/Wireless connections can have 1500 byte MTU’s yet their MSS is smaller due to the number of headers and the size thereof.

To determine the current MTU of the existing connection, the following tests can be performed:

• Use the PING Command with the -f switch to localhost:
  • ping -f -l 1493 localhost
  • ping -f -l 1492 localhost

When Pinging LocalHost, 8 Bytes of Data are ICMP data, leaving us with an MTU of 1500 Bytes.

Testing MTU Issues with Ping.exe

MTU’s are a frequent issue when dealing with cross segment communication. If a Device sends a frame of a particular size, and the Intermediate Hardware does not pass frames larger than a set amount, communication can appear unstable.

A common test that can be used is to try and ‘ping’ remote devices using specific command line switches to verify whether the devices in between will allow the Frames of a particular size to go through.

Using ping with the following switches accommodates this:

• PING -f -l 1472 <Remote Host>

The -f Switch tells any device the receives the Frame not to Fragment the Frame into separate pieces for transmission.

The -l 1472 Switch tells Ping to use 1472 Bytes of data

(20 bytes for IP - 8 bytes for ICMP)

If ping returns:

• Packet needs to be fragmented but DF set.

This implies that a device in between recognized that the frame was too large and requested it to be Fragmented.

If Ping returns:

• Request Timed Out

This implies that the frame was unable to get to its destination, or the response was unable to return.

If this occurs try a regular ping:

• PING <Remote Host>

And if this works and returns:

• Reply from XXX

Then you are inevitably dealing with an MTU Problem.

If ping ALWAYS returns:

• Request Timed Out

Then ICMP may be blocked as a protocol, and is therefore not able to be valued as a valid testing tool.

Changing MTU w/out Reboot

If PING does not work successfully as a testing tool to confirm that communication works with higher MTU values. A method exists to modify the MTU of the specific Network Adapter and then reproduce the behavior that is causing the issue.

Perform the Following Steps to modify the MTU:

• Open Regedit.exe.
• Open the following Registry key:
  • HKLM\System\CCS\Services\TCPIP\Parameters\Interfaces
• Start looking through the list of GUID Keys under Interfaces to find the Adapter of interest. (Use IPAddress, to find match)
• Once identified, add a REG_DWORD value to this Key called “MTU”.
• Set the value to 576 Decimal (0×240 Hex)
• Close Regedit.
• Open Control Panel
• Open Network Connections
• Find the Connectoid that matches the Network Adapter of interest.
• Highlight the Connectoid with the Mouse and Click your right Mouse Button.
• Choose Disable from the Menu.
• After a brief moment, the Connectoid will display the word “Disabled” next to it.
• Highlight the Connectoid with the Mouse and Click your right Mouse Button.
• Choose Enable from the Menu.
• After a brief moment, the Connectoid will display the word “Enabled” next to it.
• By disabling and re-enabling the adapter, the MTU Defined in the registry should be implemented on the Adapter.
• Run through the set of steps to reproduce the problem and see if this issue still exists.
• If not, you may have identified cause.
• This step set the MTU to a much lower value than may be recommended for some environments. Discuss the issue with the customer to determine if the MTU can be more optimally configured.

MTU settings NetworkMTU settings Network

Filed under: Network