Hi Emil, Olivier, On Mon, Jul 17, 2017 at 5:59 PM, Emil Lenngren <emil.lenngren@xxxxxxxxx> wrote: > 2017-07-17 16:07 GMT+02:00 Olivier MARTIN <olivier@xxxxxxxxxxxx>: > >> When I started to do the same exercise last week, I exposed two different MTUs: GATT Server MTU (in GATT Manager DBUS API) and GATT Client MTU (in Device DBUS API) as I had the impression they were different. >> Instead of only exposing the negociated MTU I also exposed the current MTU (ie: the value before being negociated). The idea was to send a DBUS Signal when a new MTU was negociated to tell DBus-based applications a new MTU has been negociated. > > > As you can read in the standard (Core 5.0) at Vol 3, Part F, Chapter 3.4.2.2, > "If a device is both a client and a server, the following rules shall apply: > 1. A device's Exchange MTU Request shall contain the same MTU as the > device's Exchange MTU Response (i.e. the MTU shall be symmetric). > 3. It is permitted, (but not necessary - see 2.) to exchange MTU in both > directions, but the MTUs shall be the same in each direction (see 1.)" > > So for a given link, the MTU is same for both being a client and server. As long as it is and LE link you are correct, but ATT over BR/EDR uses L2CAP signaling to negotiate the MTU which does not impose the same rules as above, so ultimately the ATT can be different, luckily the AcquireWrite and AcquireRead don't need changing since we can have different MTU for each direction. > Also make sure there are no race conditions in the way that an app > sends some value with the wrong MTU since it is just being updated. > (There is a large list of "gotchas" in 3.4.2.2 as well as in figure > 3.1). And this gotcha of the MTU may not be symmetric over BR/EDR is not even there, no wonder most OSes get it wrong, this maybe the reason with some Androids, perhaps most, ATT/GATT don't work over BR/EDR since they might assume MTU of 23 bytes. > -- > To unsubscribe from this list: send the line "unsubscribe linux-bluetooth" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Luiz Augusto von Dentz -- To unsubscribe from this list: send the line "unsubscribe linux-bluetooth" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
