ATT_MTU change for connectable Eddystone service

[harshg]

I am trying to change the default ATT_MTU size from 23 to 64. 

my code uses Ble_nus service and Eddystone_Service. 

Eddystone service is in connectable mode and it's where the advertisement starts.

Question: Can I change the ATT_MTU for Eddystone service using

 nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 64);

OR is there any restriction for Eddystone service

[Hung Bui]

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.

[harshg]

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?

 

[Hung Bui]

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.