Resolvable Private Address Generation Cycle

Question

The Central device is using Resolvable Private Addresses. 
 Even though private_addr_cycle_s is set to BLE_GAP_DEFAULT_PRIVATE_ADDR_CYCLE_INTERVAL_S (15 minutes), 
 When observed with a BLE sniffer, the Resolvable Private Address changes with each BLE connection. 
 I thought the address would change according to the set cycle regardless of BLE connections. Is that correct? 
 We're using a fixed IRK without bonding. Could that be causing this?

We are developing in the following environment: 
 
 SDK: nRF5 SDK 17.1.0 
 SoftDevice: S140 v7.2.0

Amanda Hsieh · Accepted Answer

Hi, 
 I thought the address would change according to the set cycle regardless of BLE connections. Is that correct? 
 Correct. See this post . 
 Regards, Amanda H.

Genyu · Answer

I understand that the recommended update cycle for RPA is 15 minutes. Why is the RPA being updated more frequently than the 15-minute cycle in my code? 
 ble_gap_privacy_params_t privacy_params;
memset(&privacy_params, 0, sizeof(privacy_params));
privacy_params.privacy_mode = BLE_GAP_PRIVACY_MODE_DEVICE_PRIVACY;
privacy_params.private_addr_cycle_s = BLE_GAP_DEFAULT_PRIVATE_ADDR_CYCLE_INTERVAL_S;
privacy_params.private_addr_type = BLE_GAP_ADDR_TYPE_RANDOM_PRIVATE_RESOLVABLE;
privacy_params.p_device_irk = &m_irk;

err_code = sd_ble_gap_privacy_set(&privacy_params);
APP_ERROR_CHECK(err_code);

Amanda Hsieh · Answer

Hi, 
 See the private_addr_cycle_s : 
 Private address cycle interval in seconds. Providing an address cycle value of 0 will use the default value defined by BLE_GAP_DEFAULT_PRIVATE_ADDR_CYCLE_INTERVAL_S . 
 Could you try to set privacy_params.private_addr_cycle_s =0? 
 -Amanda H.

Genyu · Answer

Hi Amanda, 
 > Could you try to set privacy_params.private_addr_cycle_s =0? 
 I tried it, but it doesn't seem to work. 
 PRA changes for each CONNECT_IND.

Amanda Hsieh · Answer

Genyu said: Would it be appropriate to consider this behavior to correspond to the following description in the Bluetooth SIG Specification? 
 From the developer: Resolvable Private Address (RPA) is more used to protect an advertising device from being tracked by malicious peers. In the Bluetooth SIG Specification, "If the Link Layer is reset, a new resolvable private address shall be generated and the timer started with any value in the allowed range." Here "Link layer reset" means HCI reset, softdevice regenerates a new address not only on HCI reset and "when the timer expired", but also when starting a new advertising/scanning. 
 However, the spec does not prevent us from generating a new address more often. Therefore, the softdevice follows the privacy requirement in the spec. 
 -Amanda H.

Resolvable Private Address Generation Cycle

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