Why does Peripheral show MTU of 498 while iOS Controller shows a MTU of 495?

[lcj]

I am sending data from iOS to a nRF5340DK. I have some code showing the TX/RX MTU Size:

void mtu_updated(struct bt_conn *conn, uint16_t tx, uint16_t rx)
{
    mtu_rx = rx;
    mtu_tx = tx;
    LOG_DGB("Updated MTU: TX: %d RX: %d bytes", mtu_tx, mtu_rx);
}

Which shows:
Updated MTU: TX: 527 RX: 498 bytes

But on the iOS/Controller side I have code:

        let mtu = maximumWriteValueLength(for: .withResponse)
        print("mtu: \(mtu)")

Which shows:  mtu: 495

I also wanted to confirm the write from the controller is the RX for the peripheral, right?

[Vidar Berg]

Hello,

iOS is showing the maximum write length, which is the ATT MTU size minus the 3-byte ATT header. 

Best regards,

Vidar