Re: [exim] GMail and TCP Fast Open

Hello.

On Fri, Dec 03, 2021 at 12:00:33PM +0000, Andrew C Aitchison via Exim-users wrote:
> > We currently have about 1,000 emails queued for google accounts and
> > these are failing with a timeout, eg from exim:
> >
> > 2021-12-02 11:48:05 1msize-005feQ-DI H=gmail-smtp-in.l.google.com
> > [142.250.13.26] TLS error on connection (send): The TLS connection was
> > non-properly terminated.
> >
> > 2021-12-02 11:48:05 1msize-005feQ-DI H=gmail-smtp-in.l.google.com
> > [142.250.13.26]: SMTP timeout after sending data block (525043 bytes
> > written): Connection timed out

Looks as broken Path MTU Discovery.

> > Based on past experience, eventually, the emails may will through.
> > This is also intermittent, as sending to my own gmail address tends to
> > get through eventually, though they do still time out once or twice
> > before eventually getting through.

If some mails could be passed, this is a hint that path is changing
over time, so effect is not stable. With MTR utility path can be
monitored in real time (like traceroute, but much better).

> > This report is rather vague, but we do have further examples, logs,
> > pcaps.

Use pcaps, study the moments when connections are timed out.

> > After trying a few things, disabling TCP Fast Open seems to have worked...

TFO affects connection open stage, but this problem is definitely due to
timeout in the middle of data transfer.

Many internet sites (may be millions) communicate with Google, so this
problem is very likely local for one site and its internet connection.
--
Eugene Berdnikov