Ticket Created
over 4 years ago

WERETECH-8478

messages from BTLE GATT server truncated at 20 bytes

All messages from a BTLE GATT server are truncated at 20 bytes.

The server can be tested against a different client and the full payload is visible (in my case 150 bytes).

This issue is reported by other users as well (see https://forums.garmin.com/developer/connect-iq/f/q-a/196823/bluetooth-low-energy-mtu-size-for-characteristics)

As 20 bytes is the default MTU size, I guess this bug might be MTU size related.

5458292
Parents
  • over 2 years ago

    I can confirm this issue and add the following information:

    Issue occurs on Edge1030 FW 11.10 & FW 13.40 as well as on Edge 1030plus FW 6.40

    Messages are truncated, if the BLE device (operating as GATT server to the Garmin) negotiates an MTU size > 23.

    For me, this looks like the Garmin BLE stack is not capable of spanning ATT packets over multiple Link Layer packets.  Thus, payloads which do not fit in a single ATT packet, are truncated. Very bad.

    If this shall be a feature, not a bug, for some reason, this wouldn't be BLE 4.0 specs compliant to my understanding.

Comment
  • over 2 years ago

    I can confirm this issue and add the following information:

    Issue occurs on Edge1030 FW 11.10 & FW 13.40 as well as on Edge 1030plus FW 6.40

    Messages are truncated, if the BLE device (operating as GATT server to the Garmin) negotiates an MTU size > 23.

    For me, this looks like the Garmin BLE stack is not capable of spanning ATT packets over multiple Link Layer packets.  Thus, payloads which do not fit in a single ATT packet, are truncated. Very bad.

    If this shall be a feature, not a bug, for some reason, this wouldn't be BLE 4.0 specs compliant to my understanding.

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