[PATCH net v2] TUN/TAP: Improving throughput and latency by avoiding SKB drops

[Simon Schippers]

Stephen Hemminger wrote:
> On Tue, 12 Aug 2025 00:03:48 +0200
> Simon Schippers <simon.schippers@xxxxxxxxxxxxxx> wrote:
>
>> This patch is the result of our paper with the title "The NODROP Patch:
>> Hardening Secure Networking for Real-time Teleoperation by Preventing
>> Packet Drops in the Linux TUN Driver" [1].
>> It deals with the tun_net_xmit function which drops SKB's with the reason
>> SKB_DROP_REASON_FULL_RING whenever the tx_ring (TUN queue) is full,
>> resulting in reduced TCP performance and packet loss for bursty video
>> streams when used over VPN's.
>>
>> The abstract reads as follows:
>> "Throughput-critical teleoperation requires robust and low-latency
>> communication to ensure safety and performance. Often, these kinds of
>> applications are implemented in Linux-based operating systems and transmit
>> over virtual private networks, which ensure encryption and ease of use by
>> providing a dedicated tunneling interface (TUN) to user space
>> applications. In this work, we identified a specific behavior in the Linux
>> TUN driver, which results in significant performance degradation due to
>> the sender stack silently dropping packets. This design issue drastically
>> impacts real-time video streaming, inducing up to 29 % packet loss with
>> noticeable video artifacts when the internal queue of the TUN driver is
>> reduced to 25 packets to minimize latency. Furthermore, a small queue
>> length also drastically reduces the throughput of TCP traffic due to many
>> retransmissions. Instead, with our open-source NODROP Patch, we propose
>> generating backpressure in case of burst traffic or network congestion.
>> The patch effectively addresses the packet-dropping behavior, hardening
>> real-time video streaming and improving TCP throughput by 36 % in high
>> latency scenarios."
>>
>> In addition to the mentioned performance and latency improvements for VPN
>> applications, this patch also allows the proper usage of qdisc's. For
>> example a fq_codel can not control the queuing delay when packets are
>> already dropped in the TUN driver. This issue is also described in [2].
>>
>> The performance evaluation of the paper (see Fig. 4) showed a 4%
>> performance hit for a single queue TUN with the default TUN queue size of
>> 500 packets. However it is important to notice that with the proposed
>> patch no packet drop ever occurred even with a TUN queue size of 1 packet.
>> The utilized validation pipeline is available under [3].
>>
>> As the reduction of the TUN queue to a size of down to 5 packets showed no
>> further performance hit in the paper, a reduction of the default TUN queue
>> size might be desirable accompanying this patch. A reduction would
>> obviously reduce buffer bloat and memory requirements.
>>
>> Implementation details:
>> - The netdev queue start/stop flow control is utilized.
>> - Compatible with multi-queue by only stopping/waking the specific
>> netdevice subqueue.
>> - No additional locking is used.
>>
>> In the tun_net_xmit function:
>> - Stopping the subqueue is done when the tx_ring gets full after inserting
>> the SKB into the tx_ring.
>> - In the unlikely case when the insertion with ptr_ring_produce fails, the
>> old dropping behavior is used for this SKB.
>>
>> In the tun_ring_recv function:
>> - Waking the subqueue is done after consuming a SKB from the tx_ring when
>> the tx_ring is empty. Waking the subqueue when the tx_ring has any
>> available space, so when it is not full, showed crashes in our testing. We
>> are open to suggestions.
>> - When the tx_ring is configured to be small (for example to hold 1 SKB),
>> queuing might be stopped in the tun_net_xmit function while at the same
>> time, ptr_ring_consume is not able to grab a SKB. This prevents
>> tun_net_xmit from being called again and causes tun_ring_recv to wait
>> indefinitely for a SKB in the blocking wait queue. Therefore, the netdev
>> queue is woken in the wait queue if it has stopped.
>> - Because the tun_struct is required to get the tx_queue into the new txq
>> pointer, the tun_struct is passed in tun_do_read aswell. This is likely
>> faster then trying to get it via the tun_file tfile because it utilizes a
>> rcu lock.
>>
>> We are open to suggestions regarding the implementation :)
>> Thank you for your work!
>>
>> [1] Link:
>> https://cni.etit.tu-dortmund.de/storages/cni-etit/r/Research/Publications/2025/Gebauer_2025_VTCFall/Gebauer_VTCFall2025_AuthorsVersion.pdf
>> [2] Link:
>> https://unix.stackexchange.com/questions/762935/traffic-shaping-ineffective-on-tun-device
>> [3] Link: https://github.com/tudo-cni/nodrop
>>
>> Co-developed-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
>> Signed-off-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
>> Signed-off-by: Simon Schippers <simon.schippers@xxxxxxxxxxxxxx>
>
> I wonder if it would be possible to implement BQL in TUN/TAP?
>
> https://lwn.net/Articles/454390/
>
> BQL provides a feedback mechanism to application when queue fills.

Thank you very much for your reply,
I also thought about BQL before and like the idea!

However I see the following challenges in the implementation:
- netdev_tx_sent_queue is no problem, it would just be called in
tun_net_xmit function.
- netdev_tx_completed_queue is challenging, because there is no completion
routine like in a "normal" network driver. tun_ring_recv reads one SKB at
a time and therefore I am not sure when and with what parameters to call
the function.
- What to do with the existing TUN queue packet limit (500 packets
default)? Use it as an upper limit?

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