VPN Speed ​​problems due to wrong “MTU size”

In order to understand the MTU size or its effects on the transmission speeds in networks via TCP / IP, one has to learn something from the background of this technology. MTU size always only describes the maximum value, i.e. smaller sizes do not change the stability, values ​​that are too large lead to a transmission stop.

The MTU size determines the maximum data transport

What about the speed of using one VPN-ServiceI also explain it in this article.

Technical description:

The maximum transmission unit (MTU) describes the maximum packet size of a protocol of the network layer (layer 3) of the OSI model, measured in octets, which without fragmentation into the frame of a network of the data link layer ("frame"). Layer 2) can be transmitted.

This may sound a bit cryptic at first, but is very easy to explain in another form and also has an influence on whether you can use the maximum capacity of data transmission (speed) or not. Also in interaction with a VPN Service can have an impact, depending on which devices you use and how the size was set there.

MTU statement - example letter

The concept of MTU transferred to sending a letter is more understandable. An MTU 50 g means that you can max. 50 g can wrap content in the letter. The letter in general can be heavier than 50 g itself, as normally an envelope z. B. 4 g and a stamp 0,3 g added. Paid and sent the whole letter of 54,3 g mass.

  • But if the letter is too heavy, then this is simply no longer promoted and remains.
  • If the letter is filled with too little content, it will indeed be transmitted, but in total, then simply less data transported, which of course counteracts the speed of data transmission negative.
    (Example: 100 letters in one minute with 50gr of data each time or 100 letters in one minute with 30gr data each time) In the second case you could have transferred less data in one minute.)

The maximum MTU size is important to the performance and stability of a connection.

MTU size - example wagons
MTU size - example of wagons

A typical value for a maximum size is “1493”, but various Internet providers have also made individual settings in their networks, which is why this value cannot be used everywhere. Typically, however, the size is additionally determined by your own router and also on the device you are using. Why in the vast majority of cases with a direct internet connection this data can be used close to the maximum.

MTU has also joined VPN decide influence

The situation is different if you now have a network connection via VPN builds. because here too, a value for the maximum transmission size between the VPNServer and the VPNClient be possible. Because here, too, the data is broken down into individual packets, but if these data packets are now larger than the transmission channel allows, there are “loading problems”, which means that data is sent repeatedly and this can lead to the connection being completely broken off. If the data here is too small, the transport containers are not filled in and this also slows down the overall transmission performance.

To avoid these problems use VPN Providers often have very small MTU sizes, such as (1330), which is why the compatibility is of course better, but there is also an unnecessary loss of speed. Many above all VPNs with their own technology, often offer the option of manually adjusting the MTU size or even self-optimizing solutions.

Example MTU optimization with VyprVPN:

Bei VyprVPN Size tests are integrated in the apps and can determine the maximum possible value and thus enable more stable and faster connections. Link: VyprVPN Review

VyprVPN MTU sizes optimization integrated in the application
VyprVPN MTU sizes optimization integrated in the application

Determine the optimal “MTU size”

Depending on the Internet provider, it may happen that the MTU size (Maximum Transmission Unit) is different from the default value (1493 bytes), which causes the packets to be fragmented and therefore the existing connection is not optimally utilized.

To determine the maximum MTU size, it is important that we understand that our IPv4 protocol requires 28 bytes (20 bytes = IPv4 headers and 8 bytes = ICMP), and for an MTU of 1492 bytes for our data have only 1464 bytes per packet. If we now determine the optimal size If we want to make sure that we send packets that are not fragmented, this is best tested via a ping.

In our example, we ping Google.de with a ping packet size of 1464 bytes:

The Windows Ping (command line tool “CMD” in Windows) uses the parameter -l for the package size and to ensure that the packages are not fragmented the parameter -f is set.

ping google.de -f -l 1464

for MacOS:

ping -D -s 1464 google.com


MTU size 1464
Get MTU size - package lost: Ping google.de -f -l 1464

As you can see here, the ping package is too big and can not be shipped.

Now to determine the correct value, we now reduce our ping package until we can send it:

ping google.de -f -l 1456

for Mac OS:

ping -D -s 1456 google.com


MTU size 1456
Determine MTU size - Sent successfully: Ping google.de -f -l 1456

In this example, the ping package could be shipped with a size of 1456.

Now that we know the amount of data per packet, we can calculate the optimal MTU size. For this we take the determined Amount of data per packet (1456 bytes) and add to this the IPv4 protocol (28 bytes) what in our example one MTU size of 1484 bytes results.

The maximum value can now be between 1456 and 1464. You can approach this value by trying it out.


Created on:03/07/2018

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