Bufferbloat refers to the phenomenon where network buffers (queues) become so large that they cause significant delays, especially when an upload saturates the internet connection. This can lead to laggy games, stuttering calls, and slow web pages, even though speed tests show good performance.
Why Ping Spikes During Uploads
When someone starts a big upload (like cloud backups or streaming), the router's buffer fills up because it cannot send data as fast as it arrives. This delay affects all packets, including those for time-critical applications like gaming and video calls. The longer the queue, the higher the latency.
The Cause: Oversized Buffers
Routers have buffers to handle temporary overflows, but manufacturers often make these buffers much larger than necessary. In a home network with an asymmetric connection (fast download, slow upload), this buffer can easily fill up during heavy uploads. TCP exacerbates the problem by continuing to send data until it detects packet loss, which doesn't happen immediately due to the large buffer.
Symptoms
- Games lag, calls stutter.
- Web pages load slowly even though speed tests show good performance.
- Ping spikes from 15ms to 200-800ms during uploads.
How to Test for Bufferbloat
You can test for bufferbloat using tools like the Waveform bufferbloat test, Fast.com with latency under load, or simply pinging a stable server (like 1.1.1.1) while uploading a large file. A significant difference in ping between idle and loaded conditions indicates bufferbloat.
The Fix: SMART QUEUE MANAGEMENT (SQM)
To mitigate bufferbloat, use Smart Queue Management (SQM). Modern queue algorithms like fq_codel and CAKE keep queues short by sharing bandwidth fairly and signaling TCP early to prevent congestion. Here’s how to implement it:
Where to Get SQM
- OpenWrt: Use sqm-scripts.
- OPNsense/Pfsense: Configure limiters with fq_codel.
- Ubiquiti: Utilize smart queues.
- Some stock firmwares may have SQM under "QoS."
Configuration
- Set the SQM bandwidth to about 90-95% of your measured real up/down speeds, ensuring the router is the bottleneck.
- This will control the queue and keep latency low.
Cost vs. Benefit
Implementing SQM costs only a few percent in throughput but can significantly reduce loaded latency, keeping gaming and calls smooth during backups.
Why ISPs Don’t Fix It
ISPs often leave large buffers in their modems because they own that part of the network. DOCIS 3.1 low-latency modes exist but are not widely deployed yet. By using SQM on your router, you can route around these buffers by ensuring the queue never fills up.
By understanding bufferbloat and implementing SQM, you can significantly improve your home network's performance during heavy uploads.