When someone in the household uploads a video or downloads a large file, hundreds of ms worth of packets pile up in the router’s queue, and game packets wait behind them.
Why The connection fills up with a family member’s video upload or cloud backup, your own live stream, or a large download → Effect The router or modem holds the overflow of packets in a large queue → On screen Game packets wait at the back of the queue too, and ping shoots up to hundreds of ms
Primary owner External (External) · Also Game team (Client development)
Game team action items
Show network status on screen when ping suddenly jumps to hundreds of ms (mention a possible large transfer on the same connection).
External action items
Tell players to use a router with SQM (fq_codel, CAKE) or QoS, set the SQM speed to 90–95% of the connection speed (so the queue forms inside the router and SQM can take effect), and cap upload speeds.
Ballpark numbers
On a connection with 10 Mbps upload, a 1 MB buffer lets the queue grow to 800 ms.
On the graph
Rises with load · RTT, upload and download usage on the connection
Where to look
With ping running, saturate the connection with a speed test, or use a web test that measures latency under load (see Bufferbloat.net). Check alongside the upload and download usage shown on the router’s admin page
Confirmed if
Ping climbs to hundreds of ms while an upload or download saturates the connection and recovers when the transfer ends (suspect it when latency under load exceeds 50 ms). Gone with SQM on
Ruled out if
Ping spikes even when the connection is idle: “Wi-Fi interference and weak signal” or “Poor line quality”
Check with
The player’s own environment
Learn more
The upload side clogs especially easily, because cable and mobile connections often have far less upload bandwidth than download. In homes with plenty of fiber bandwidth, the Wi-Fi link becomes the bottleneck, and the same thing happens in the router’s wireless queue. Game packets are small and use almost no bandwidth, but they still have to wait in the queue. If only the upload direction is clogged, only your own input is late, while other players’ movement looks fine. On phones, photo backups and app updates on the same phone fill the queues in the phone’s modem and at the cell tower, with the same result.
Sources
Setting up SQM for CeroWrt 3.10Bufferbloat.net Set SQM to 95% of the measured speed (85% if based on the advertised speed) to move the bottleneck from the ISP’s equipment into the router; that’s what makes it work
SQM (Smart Queue Management)OpenWrt Enter 90% of the measured download and upload speeds; cake is the recommended queue discipline (fq_codel if the CPU is weak)
Tests for BufferbloatBufferbloat.net If ping rises while a speed test saturates the connection with ping running, it’s bufferbloat; a fix is recommended when latency under load exceeds 50 ms (or the grade is below B)