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README-bluetooth-analysis.md

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Bluetooth analysis (see issue)

Tools used:

  • Logcat traces
  • HCI snoop debug traces on the phones
  • Bluetooth sniffer device (TI CC1352R1 dev kit)
  • Nordic nRF Connect app

The devices analyzed are:

  • Google Pixel4 (Bluetooth 5.0), LineageOS 17.1, SwissCovid app bundled with microG EN libraries (v0.2.13.203915)
  • Sony Z3 (Bluetooth 4.0), LineageOS 17.1, SwissCovid app bundled with microG EN libraries (v0.2.13.203915)

Advertisement: sequence of HCI commands

Every 10 minutes, aligned on a 10-minute boundary:

Pixel4

  • LE Rand
  • LE Set Extended Advertising Parameters handle 0, legacy PDUs, ADV_NONCONN_IND, interval 250-281.25 msec, channels 37,38,39, random device address
  • LE Set Advertising Set Random Address handle 0
  • LE Set Extended Advertising Data handle 0
  • LE Set Extended Scan Response Data handle 0
  • LE Set Extended Advertising Enable handle 0, on [10min]
  • LE Set Extended Advertising Enable handle 0, off
  • LE Remove Advertising Set handle 0

Z3

  • LE Rand
  • LE Set Advertising Parameters ADV_NONCONN_IND, interval 250-281.25 msec, channels 37,38,39, random device address
  • LE Set Random Address
  • LE Set Advertising Data
  • LE Set Scan Response Data
  • LE Set Advertise Enable on [10min]
  • LE Set Advertise Enable off

Scanning: sequence of HCI commands

Every 3 minutes (not aligned):

Pixel4

  • Vendor command 0x0157 (4x)
  • LE Set Extended Scan Parameters active, random device address, accept all adv (except direct not addressed here)
  • LE Set Extended Scan Enable on
  • LE Set Extended Scan Enable off
  • LE Set Extended Scan Parameters active, random device address, accept all adv (except direct not addressed here)
  • LE Set Extended Scan Parameters active, random device address, accept all adv (except direct not addressed here)
  • LE Set Extended Scan Enable on [20s]
  • LE Set Extended Scan Enable off

Z3

  • LE Set Scan Parameters active, random device address, accept all adv (except direct not addressed here)
  • LE Set Scan Enable on
  • LE Set Scan Enable off
  • LE Set Scan Parameters active, random device address, accept all adv (except direct not addressed here)
  • LE Set Scan Parameters active, random device address, accept all adv (except direct not addressed here)
  • LE Set Scan Enable on [20s]
  • LE Set Scan Enable off

Scanning events received

Pixel4

LE Meta -- LE Extended Advertising Report legacy, ADV_NONCONN_IND, random device address,

Z3

LE Meta -- LE Advertising Report ADV_NONCONN_IND, random device address,

Advertisements recorded from the sniffer, channel 37

Interval distribution over an RPI value (10 min):

Pixel4

mean: 287.21ms, median: 286,25ms

Z3

mean: 287.80ms, median: 286.26ms

Other

Every 15 minutes (not aligned):

Pixel4

  • LE Set Random Address

Z3

  • LE Set Random Address

NOTE: On the Z3, this has the effect of also changing the advertising random address in the middle of a 10-minute RPI cycle, without changing the RPI. This is probably due to it having an older Bluetooth HW and using legacy commands, which do not distinguish between setting the random address for advertising and for scanning.

RPI / BD Address overlap

  • The same BD address used with different advertised RPIs was never observed.
  • The same RPI value advertised with two different BD addresses was observed on the Z3, as indicated in the previous note.

In other words, whenever the RPI changes, the BD address changes as well, on both devices. On the Z3 however, sometimes the BD address will change while broadcasting a given RPI.

This behaviour is expected from GAEN as well.