on_data_length_update_request_evt

[joe.ker]

Hi,

upon connection I get these messages via RTT:

nrf_ble_gatt:DEBUG:Peer on connection 0x0 requested an ATT MTU of 185 bytes. nrf_ble_gatt:DEBUG:Updating ATT MTU to 23 bytes (desired: 23) on connection 0x0.

Does anybody know what they mean?

Many thanks, Josef

[endnode]

This probably means that one peer performed Exchange MTU Request on ATTribute Protocol layer and the other peer responded with Exchange MTU Response which resulted into automatic ATT_MTU alignment to minimum of numbers indicated by both sides. One said it can go up to 185 bytes but second indicated it can handle only 23 bytes (default size mandated by BT SIG spec for ATT layer on LE) so ATT_MTU settled on default size 23.

[FormerMember]

@joe.ker: As you can see in this message sequence chart, the ATT_MTU request can be sent at any point after a connection has been established.

From the Bluetooth Core Specification v. 5.0, vol 3, part F, chapter 3.4.2.1:

This request [Exchange MTU Request] shall only be sent once during a connection by the client. The Client Rx MTU parameter shall be set to the maximum size of the attribute protocol PDU that the client can receive.