Re: Serious mssfix compatibility issue with OpenVPN Connect

On Wed, 4 Sep 2024 09:22:08 +0200, Antonio Quartulli wrote
...
> That practically means that QUIC is equally broken everytime macOS is 
> used behind a router with a smaller MTU (like the PPPoE case mentioned 
> by Gert).
> 
> Are we truly sure about these statement?

In fact, all UDP applications are extremely conservative about maximal packet sizes: e.g. QUIC never uses packets larger than 1385 bytes, MPEG-TS over RTP/UDP uses max. 1328 bytes etc.

Thus PPPoE is no problem, as it provides 1492 byte MTU. Also iPhone's/iPad's safety-belt default MTU 1450 in 5G/LTE networks still works.

However mssfix=1400 in OpenVPN might result in max. usable packet of e.g. 1376 bytes, which is 9 bytes less than needed for QUIC.

Combined with UDP packet blackholing, that's a serious problem - and much better approach would be to lower tun-mtu, if you want to completely avoid fragmentation for all protocols.

With kind regards,
MD