ATT_MTU change for connectable Eddystone service

RE: ATT_MTU change for connectable Eddystone service

Hung Bui — Fri, 06 Mar 2020 14:01:56 GMT

Hi Harsheen,

We need to verify that the new ATT_MTU is actually used. Please try capture a sniffer trace.

How many bytes do you see coming out of the RX notification ?

I'm not sure I understand what you meant by "it's unlock requirement is 16 bytes of data for AES_128 ECB Encryption."

I don't see any reason larger ATT_MTU size change the communication at GATT level.

RE: ATT_MTU change for connectable Eddystone service

harshg — Thu, 05 Mar 2020 18:05:54 GMT

Thanks Hung for your reply.

I tried changing the GATT ATT_MTU size to 64 or my defined size in ble_uart example and was able to successfully do it.

But when I try to change in my program, no change is taking place. Ideally, I want 32 bytes coming over ble_nus RX characteristic.. The big diff is I have eddystone service with ble_nus service.

Although the spec for Eddystone GATT service has nothing mentioned about the ATT_MTU size,

it's unlock requirement is 16 bytes of data for AES_128 ECB Encryption. Do you think this is limiting the ATT_MTU size?

RE: ATT_MTU change for connectable Eddystone service

Hung Bui — Thu, 05 Mar 2020 12:16:57 GMT

Hi Harsheen,

The ATT_MTU is for the whole GATT communication so if you change the ATT_MTU for one service, the other service will also use the new ATT_MTU. But if the actual data, for example of the Eddystone GATT service is smaller than the ATT_MTU then there wouldn't be any change in the communication.

The spec for Eddystone GATT service is defined here: https://github.com/google/eddystone/tree/master/configuration-service . I don't see any requirement for the ATT_MTU.