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iosbluetoothcore-bluetooth

Why is the MTU different for With Response vs Without Response?

On iOS when I request the MTU I get different values for .withoutResponse vs .withResponse. I realize it could be different since the function takes that parameter, but why is it different?

Here's on device (BLE 4.2):

(lldb) po peripheral.maximumWriteValueLength(for: .withoutResponse)
182

(lldb) po peripheral.maximumWriteValueLength(for: .withResponse)
512

On another device (BLE 5) I get:

(lldb) po peripheral.maximumWriteValueLength(for: .withoutResponse)
509

(lldb) po peripheral.maximumWriteValueLength(for: .withResponse)
512
Solution

  • Write Without Response is directly related to the MTU. (MTU - 3).

    When you use writeWithResponse, iOS can either perform a Write With Response (max value length is MTU - 3), or a "Write Long Characteristic Values", which uses several ATT transactions to write the value. In the latter case the only restricting factor is the maximum length of a characteristic value, which is set to 512 bytes by the standard.