DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Menon — Mon, 11 Aug 2025 13:43:03 GMT

hello,

The root trigger for the this issue is HCI/L2CAP fragmentation from the central. This is common with Bluetooth 4.x devices that lack Data Length Extension (DLE), they have no choice but to fragment large L2CAP packets into multiple ACL packets. We can not guarantee that a modern phone/device supporting v5.x data length extensions will not start using fragmentation (depending on its installed applications using Bluetooth/co-existence/BR-EDR audio in use, it can switched to fragmenting to better schedule/use its radio).

Kind Regards,

Abhijith

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Prashant Humbre — Fri, 08 Aug 2025 09:04:29 GMT

Hello Abhijith,

You mentioned that earlier in the thread that you don't think this is specific to the Bluetooth controller, it is more of an nRF Connect SDK issue related to how the Bluetooth Host and Controller manage ACL buffer and timing.

Our testing indicates the following.

1) When we update our product (OTA) over BLE using an iOS application we don't see this issue or the ACL error associated with this issue.

2) When we update our product (OTA) over BLE using a PC with a BLE USB dongle that has a new 5.0 software driver, we don't see this ACL Buffer failure, or the error associated with it.

3) When we update our product over BLE using a PC with the same BLE USB dongle that has a the original 4.0 driver that, we see an ACL Buffer failure at a frequency of 1 in 10 firmware update cycles, with the 2.8.0 SDK. With the same dongle and driver we didn't see this failure in the 2.7.0 SDK.

The PC Dongle is used for cycle testing our FW Update(OTA) process, and isn't part of the consumer operating process for our product. Our question is do you believe that this ACL Buffer error will occur using an iOS or Android device as the Central, or should we expect to only see this issue if the central is doing something unexpected with or over allocating the ACL Buffer mechanism.

Thanks & regards,

Prashant H

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Menon — Wed, 06 Aug 2025 13:45:34 GMT

Hello Prashant,

The workaround mentioned in the known issue may not be effective because the configuration option CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA was only introduced in NCS version 3.0.0. As a result, I recommend updating to NCS v3.0.0, since there’s no straightforward workaround available for earlier versions.

For reference, please see this issue: https://github.com/zephyrproject-rtos/zephyr/issues/86091 A related fix was added to the SDC driver here: https://github.com/nrfconnect/sdk-nrf/pull/19869 , this was included in NCS v2.9.0 and later.

Kind Regards,

Abhijith

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Prashant Humbre — Tue, 05 Aug 2025 12:14:22 GMT

Hello Abhijit,

I already uploaded the .config files for above test results already.
But in addition, below is the additional .config file from:

build\ipc_radio\zephyr\.config

devzone.nordicsemi.com/.../build_5F00_ipc_5F00_radio_5F00_zephyr.zip

lock\build\lock\zephyr\.config
devzone.nordicsemi.com/.../build_5F00_lock_5F00_zephyr.zip

Let me know if you need any more information.

Thanks & regards,
Prashant Humbre.

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Menon — Tue, 05 Aug 2025 11:34:49 GMT

Hello Prashant,

Could you please share the .config file you are using for the network core? Specifically, the one you use for the hci_ipc or ipc_radio firmware. Kindly share it here.

Kind regards,
Abhijith

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Menon — Tue, 05 Aug 2025 09:40:25 GMT

Hello Prashant,

I have reported this internally and am gathering more information on the issue. I will update you here as soon as I receive any important information.

Kind regards,
Abhijith

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Prashant Humbre — Tue, 05 Aug 2025 07:12:30 GMT

Hello, any update on this issue?

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Prashant Humbre — Wed, 30 Jul 2025 12:03:51 GMT

Hello Abhijith,

As per your suggestion, we have used six different configurations in our code as below and compiled each one individually. We have completed testing for all six configurations built, but unfortunately, no noticeable improvements were observed.

Test-1:
nRF5340 App Version: 17.37.01.03
CONFIG_BT_BUF_ACL_RX_COUNT = 6
CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA = 4

Test-2:
nRF5340 App Version: 17.37.02.03
CONFIG_BT_BUF_CMD_TX_COUNT = 13
CONFIG_BT_BUF_ACL_RX_COUNT = 6
CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA = 6

Test-3:
nRF5340 App Version: 17.37.03.03
CONFIG_BT_BUF_CMD_TX_COUNT = 19
CONFIG_BT_BUF_ACL_RX_COUNT = 6
CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA = 12

Test-4:
nRF5340 App Version: 17.37.04.03
CONFIG_BT_BUF_CMD_TX_COUNT = 25
CONFIG_BT_BUF_ACL_RX_COUNT = 6
CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA = 18

Test-5:
nRF5340 App Version: 17.37.05.03
CONFIG_BT_BUF_CMD_TX_COUNT = 15
CONFIG_BT_BUF_ACL_RX_COUNT = 10
CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA = 4

Test-6.
nRF5340 App Version: 17.37.06.03
CONFIG_BT_BUF_CMD_TX_COUNT = 19
CONFIG_BT_BUF_ACL_RX_COUNT = 10
CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA = 8

We have attached the nRF5340 Application debug logs for all test cases, along with the generated .config files for both AppCore and NetCore.

Kindly investigate the issue further and advise if there is any SDK cherry-pick that could potentially resolve this problem.

Attachment:

devzone.nordicsemi.com/.../TestResult_2600_Configuration.zip

Thanks & Regards,
Prashant Humbre

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Menon — Mon, 28 Jul 2025 13:20:04 GMT

Hello Prashant,

I don't think this is specific to the Bluetooth controller, it is more of an nRF Connect SDK issue related to how the Bluetooth Host and Controller manage ACL buffers and timing.
I looked into the generated configs, can you see if increasing both CONFIG_BT_BUF_ACL_RX_COUNT and CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA resolves the issue? If not, please share the complete log here.

Sorry for getting back to you on this late. It's summer vacation here in Norway, and many are on vacation, which created the delay in responding.

Kind regards,
Abhijith

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Prashant Humbre — Mon, 21 Jul 2025 13:12:32 GMT

I checked the workaround mentioned in the Know Issue mentioned by you, but its not working.
Instead of sharing the prj.conf file, this is the \build\lock\zephyr\.config file:
devzone.nordicsemi.com/.../1362_2E00_config

Let me know if it helps.

Thanks & regards,

Prashant H.

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Prashant Humbre — Mon, 21 Jul 2025 06:00:02 GMT

While reviewing similar DevZone tickets related to the "No available ACL buffers!" and "Unexpected first L2CAP frame" errors, I came across discussions suggesting these issues might originate from the central side, not the peripheral.

To test this, I changed the Bluetooth dongle on the central side from Bluetooth 4.0 to Bluetooth 5.0, and surprisingly, the error logs no longer appeared during the DFU process with MCUMgr.

This raises a concern:
What is the root cause of this incompatibility?
In real-world deployments, I cannot expect end users to always have Bluetooth 5.0-capable devices. Many may still use phones supporting only Bluetooth 4.0.

Could you please help clarify:

Is this issue specific to older Bluetooth controllers?
Are there known workarounds to improve compatibility with Bluetooth 4.0 centrals?

Thanks again for your support!

Best regards,
Prashant

RE: DFU over BLE using MCUMgr fails with “No available ACL buffers!” and “Unexpected first L2CAP frame” – SDK v2.8.0

Menon — Fri, 18 Jul 2025 14:45:40 GMT

Hello,

Could you share the prj.conf you are using for your application? See this known issue.

Kind Regards,

Abhijith