What Is MTU?
MTU stands for Maximum Transmission Unit. The concept is used in multiple areas in networking, but in most cases you will find it refers to the maximum number of bytes that a single layer 3 packet can be. Any packet larger than the MTU must be fragmented. Critically since the layer 3 headers are part of the packet they are counted towards the MTU as well as the size of the payload. MTU is related to the lower layers (2 and 1) so its value is related to their properties.
Why is MTU Important?
Some applications and/or protocols such as VPNs are particularly sensitive to the size of the MTU. The reason VPNs are sensitive is that they usually encapsulate layer 3 packets inside other layer 3 packets. Now two sets of headers need to be accounted for when generating packets otherwise fragmentation may occur! Some types of VPNs do not react well to fragmentation. Even if fragmentation does not cause problems with a VPN it may cause performance issues as the remote side has to de-fragment the packets upon receipt.
What Is My MTU?
This is a complicated question since MTU is a consequence of the link layer the MTU can vary depending on the path traffic is taking and what links it is transiting. Some applications or protocols are not affected by MTU, some will perform Path MTU Discovery (PMTUD) and others need a static value configured. Most applications and/or protocols will default to 1500 bytes if it's important to them as this is the default MTU for an Ethernet link. In fact 1500 bytes is the de-facto standard for almost all links across the Internet.
If you just want to know what your MTU is already you can see our article on determining your MTU.
How Does this Relate to Me?
If your connection has an MTU other than 1500 bytes some VPNs do not work correctly or do not perform adequately. For example DSL connections that utilize PPPoE normally have an MTU of 1492 bytes as the PPPoE header that must be included with every packet is 8 bytes. SDWAN connections utilize VPNs and as such they also reduce the MTU with their VPN headers. The amount SDWAN reduces the MTU varies depending on the backing connection. If you have SDWAN service from iTel you may either contact us or follow our article on determining your MTU to find out what the MTU is for your SDWAN connection.
How Do I Fix My VPN?
If you suspect your VPN is not working due to the MTU of your connection we suggest contacting the manufacturer of the device or the vendor you purchased the device from. Unfortunately we are not able to provide support for all manufacturers devices and/or services.