Setting MTU Back to Blank/Optional doesn't seem to work

[franklin]

I own both a SlateAX and a Beryl, however I am currently playing with the AX (FW 4.0.3) .

I have a bunch of WG configs for surfshark. On one of the configs, I played with the MTU setting trying to make it work with a cruise connection. I don’t seem to be able to remove the “optional” field anymore.

I have tried the following.

1. VPN Dashboard → Options → MTU → [blank] → Apply. I get a sucess, however nothing seems to have changed.
2. Wireguard client → Surfshark → specific config → edit → deleting the MTU line → apply.
   This gives me success, but does not stick.
3. Wireguard client → Surfshark → specific config → edit → item mode → deleting the MTU field → apply.
   This gives me success, but does not stick.

Is this really an optional field? If so, shouldn’t i be able to remove this field and have it figure itself out? Or does it make sense to just set it to 1420 and let it be.

[LupusE]

As the MTU is the Maximum Transfer Unit (aka How big is my Packet before it needs to be split), it makes no sense to delete the value.

In IPv6 this value should be negotiated. Bit as you played around with the MTU of the tunnel, I think there’s is simply a default.
In worst case the MTU of the physical adapter is used. This means it will work, but lead to a huge amount of fragments with only the size of the overhead in the second frame.

What is your goal by deleting this value?

[wcs2228]

@franklin

I played with the MTU setting trying to make it work with a cruise connection

Are you referring to Internet on a cruise ship? Internet on cruise ships tend to be flaky and have high latency, although some cruise lines offer premium plans for extra $$. Lowering the MTU for WireGuard may help:

Slate GL-AR750S-Ext - Slow Wireguard Upload Performance

Hello, I have been getting poor performance with my Slate GL-AR750S (client) connected to my Flint GL-AX1800 (server) router at home. I sometimes get decent speeds (atleast 6mb upload), but often 0.6mb upload. My computer is connect via ethernet to my Slate. Its not consistent and I don’t know why. On my iPhone connecting to my Flint server at home I get consistent 24mb upload. This is using the same WiFi connection. Any help would be greatly appreciated! Thanks

I do not work for and I am not directly associated with GL.iNet

[alzhao]

The MAX mtu is 1500, which is also the default one.

As vpn has headers, so it should have a lower MTU.

For Wireguard you may need 1380 etc.

[franklin]

Ok, so it sounds like when the field is blank, it defaults to a 1500 byte MTU.

Is there a MTU discovery process when using IPv4?

If there were some discovery process, then having a blank option of “do MTU discovery” vs a hard coded value could have different effects.

However, if there is no MTU discovery process, typing in 1500 would be equivalent of just deleting the MTU field?

[wcs2228]

You should see the MTU for all interfaces by running the ifconfig command in SSH.

[alzhao]

@franklin

Is there a MTU discovery process when using IPv4?

If there were some discovery process, then having a blank option of “do MTU discovery” vs a hard coded value could have different effects.

There is MTU discovery but it is not reliable. It is better just to set the correct mtu.

The MTU discovery is done by applications, not the router. So a lot of problems.