What is the maximum number of bonded i.e. 'Paired Devices' for Bluetooth LE devices in Android?

6
votes

The information being sought with this question is I think similar (but not the same) as the information sought here...

"How many devices we can pair via Bluetooth of BLE to Android?"

...and so let's re-ask it a different way:

Let's say we have a use case where someone with a mobile phone may want to connect securely (using industry standard security mechanisms) to a Bluetooth LE device using Pairing. When Pairing occurs, bonding information is stored in the mobile phones - - > Settings - - > Wireless - -> Bluetooth - - > Paired Devices window (dialog).

These 'Paired Devices' will remain present in this list regardless of whether or not active connections are maintained, correct?

And if this answer is correct, how many hundreds or thousands of previously Paired/bonded devices can be accrued in this list before the Android OS (or iPhone's iOS) begins to start to incur performance issues?

My understanding is... neither Android OS or iPhone OS offer programmatic ability to 'clean out' the list of old paired / bonded devices. And mobile phone users who neglect to remove those devices manually could have a challenging task at hand if they've allowed the list of Paired/Bonded devices to get out-of-hand.

Thoughts? Thanks much in advance, --DJ Regan

2018-November-12 Update... New news today: In profiling many BLE paired devices with Android - - > a team mate of mine discovered their Bluetooth radio had become unavailable (as in un-useable) after attempting to pair to the 102nd device+ today. Additional profiling is on-going. So... perhaps this is a use case that neither iOS or Android should be trusted to support? --DJ Regan

3
androidiosbluetooth
I doubt that the list would ever grow large enough to cause performance issues. – Paulw11
Hi Paulw11, You may be right. :) But, if folks are going to roll-out a solution that grows at a rate of hundreds of thousands of new devices per year... they need to have 'beyond a doubt' confidence in solid performance. Does anyone already have an example use case where this confidence is known with 'beyond a doubt' certainty? I realize this is an 'expensive' use case to profile. – DJ Regan
You are going to require a user to personally bond their device with 100,000 different devices per year. 273 per day, every day? I suspect you have a bigger issue than the paired device table. There would be a negative impact on the useability of the Bluetooth settings screen. This is probably reached at about 100 devices – Paulw11
Here is a practical real use case... Let's say a mobile phone app is created that enables authorized users (utility service persons, in this case) to read customer utility meters. In this case... the user with mobile phone would go door-to-door, and would pair with the BLE enabled sensor securely. This secure pairing effort will cause bonding information in the "Paired Devices" to grow indefinitely. Who has experience with this use case? – DJ Regan
I don't think anyone is going to have experience with this use case, because I think pairing/bonding is impractical in this case; what PIN is the meter reader going to enter? If it is a "well known" PIN that each meter has in common, the pairing is pointless. If the reader needs to obtain a PIN, say from the meter itself, then the reading process will be pretty slow, and again is probably pointless. Are utility meter readings a big secret? – Paulw11

3 Answers

4
votes

From sources I guess 100 is the right number pointing to the max devices you can bond with. Also proved in practise.

 /* The number of security records for peer devices. */
    #ifndef BTM_SEC_MAX_DEVICE_RECORDS
    #define BTM_SEC_MAX_DEVICE_RECORDS  100
    #endif

... 

/************************************************************************************
    **  Local type definitions
    ************************************************************************************/
    typedef struct
    {
        uint32_t num_devices;
        bt_bdaddr_t devices[BTM_SEC_MAX_DEVICE_RECORDS];
    } btif_bonded_devices_t;
3
votes

According to the bluetooth implementation, if there are more than 100 device records the allocation will fail.

The responsible BTM_SEC_MAX_DEVICE_RECORDS is defined in bt_target.h:

/* The number of security records for peer devices. */
#ifndef BTM_SEC_MAX_DEVICE_RECORDS
#define BTM_SEC_MAX_DEVICE_RECORDS  100
#endif

This is then used inside btm_dev.c, specifically in BTM_SecAddDevice which returns false after 100 pairings:

   /* There is no device record, allocate one.
    * If we can not find an empty spot for this one, let it fail. */
    for (i = 0; i < BTM_SEC_MAX_DEVICE_RECORDS; i++)
0
votes

You can connect maximum 8 devices via bluetooth.