SixXS::Sunset 2017-06-06
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FAQ : Connectivity (Tunnels and Subnets) : What is the MTU of a tunnel?

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What is the MTU of a tunnel?

The MTU (Maximum Transmission Unit) of a tunnel defaults to 1280. This is also the value that is recommended by us for normal usage.

The MTU defines how large the packets are that can be transmitted over a link.

A pMTU (Path MTU) is the MTU used over a complete path. As a tunnel crosses over possibly several IPv4 network links one should check the IPv4 pMTU between your host and the PoP. The IPv4 pMTU to the PoP becomes then the IPv6 MTU for your tunnel, though minus the overhead for the tunneling protocol used, see Tunneling Overhead.

Standard Ethernet uses an MTU of 1500, thus in the most ideal case one will have a pMTU of 1500. In most cases the MTU will be lower though and thus to avoid fragmentation and misbehaviour per default the MTU of a tunnel is 1280, which is the minimum IPv6 packet size.

A larger MTU can improve performance of the tunnel when sending/receiving large streams as less packets are required to send/receive the same amount of actual data. When the MTU is too large for the underlying infrastructure performance will drop and in cases where there is a disagreement about which size the MTU is, packets may also be lost.

Changing the MTU of the tunnel

In case you want to change the MTU to something higher, one can use the webinterface to change it. The MTU options are available in the Tunnel Detail page, log in and then select the link of the tunnel you want to modify in the tunnel overview table. There you can change the MTU and various other properties of the tunnel, next to seeing the performance statistics.

The allowed MTU size is between 1280, minimum IPv6 packetsize, and 1480, which comes forth from the tunneling overhead and the fact that the PoPs are connected to Ethernet networks, which use a MTU of 1500.

Don't forget to update the client (your) side of the tunnel too by checking the FAQ for the configuration needed. In cases where AICCU is used, a restart is needed to refetch the new TIC parameters.

Warning One should only change the MTU if one knows what one is doing. The MTU should match on both sides of the tunnel. One should check, eg using tracepath if the IPv4 path between you and the PoP really can handle the MTU.

Tunneling Overhead

The following table shows the overhead that the various protocols have when they are tunneled over the Internet. Note that 1500 is the ideal MTU for Ethernet, you might have a slightly smaller MTU and especially on the path between you and the PoP the MTU might be quite a bit lower.

ProtocolOverheadTotal OverheadPossible MTU
Proto-41IPv420 = 201500 - 20= 1480
AYIYAIPv4 UDP AYIYA20 + 8 + 44 = 721500 - 72= 1428

Note that proto-41 is used for both static and heartbeat tunnels.

Statistics detailing what MTUs are in common use can be found on the MTU Statistics page.

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