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From:   Emil Lenngren <emil.lenngren@gmail.com>
Date:   Wed, 4 Mar 2020 16:56:10 +0100
Message-ID: <CAO1O6sfwcEswTkuuntoBM_S5oFkQv7mCP1GH8Z0S0vgHk2Bp5A@mail.gmail.com>
Subject: Re: Get negotiated ATT MTU?
To:     Jamie Mccrae <Jamie.Mccrae@lairdconnect.com>
Cc:     Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        Bluez mailing list <linux-bluetooth@vger.kernel.org>
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Hi,

Den ons 4 mars 2020 kl 11:55 skrev Jamie Mccrae <Jamie.Mccrae@lairdconnect.=
com>:
>
> Hi,
> > It should be fine also if the remote end sends an Exchange MTU request
> > at the beginning of the connection since we can then immediately send
> > a response and assign the MTU property without waiting for the
> > Exchange MTU response (that corresponds to our request).
> >
> > Let me know if you think I've missed some edge case...
>
> In the core specification 5.2 volume 3 part A, there is a command, L2CAP_=
CREDIT_BASED_RECONFIGURE_REQ, which allows for the MTU to be changed after =
it has been established. This requires an enhanced ATT service however, but=
 it means that the initial MTU is subject to change.

I just read the L2CAP/ATT/GATT parts in the new spec. Is EATT
implemented yet for the dbus-api, and will it affect the API? Anyway,
for EATT it's a requirement that the MTU can only increase, never
decrease, which shouldn't cause issues for apps. But change my "ATT
MTU exchanged" property name proposal to "Initial ATT MTU exchanged"
then, if waiting for the ServicesResolved wouldn't be enough, and set
it true immediately if EATT is used and after an Exchange MTU
procedure for unenhanced ATT. Then update the ATT MTU property when
the MTU is increased.

/Emil