Debugging SQM to Eliminate Bufferbloat

Every now and again, someone posts a question expressing trouble setting up their SQM/QoS in LEDE. They're hoping to eliminate the lag/latency caused by Bufferbloat in their router.

If you're in this situation, follow the SQM Howto at https://openwrt.org/docs/guide-user/network/traffic-shaping/sqm If you don't have success (or you get paradoxical results), we're happy to help, but are going to want to know the answers to a few questions:

  • What brand/model router do you have?
  • What version of OpenWrt/LEDE are you using?
  • How do you connect to the internet? Cable? DSL, other?
  • What's your nominal/expected/advertised download speed? Upload speed?
  • If you turn off all QoS/management, what are your measured download/upload speeds? (We like to use www.dslreports.com/speedtest - it's robust, accurate, and doesn't rely on Flash...)
  • What is the WAN "interface name" in the Network -> Interfaces page? (its name is in the red box below "WAN" on that page)
  • What parameters do you see in the Network -> SQM-QoS values? The most important are:
    • Enable this SQM instance (should be checked)
    • Interface name (should be the same as the "WAN interface name" from above)
    • The number entered for Download Speed (in kbps)
    • The number entered for Upload Speed (in kbps)
    • Queue discipline (default is "piece of cake" if it's a new build, or fq_codel)
    • Set "Which Link Layer to account for:"
      • If you're on cable, should be "none"
      • If you're using DSL/ADSL, should be "ATM", and Per-Packet Overhead (in the Link Adaptation tab) should be 44
      • If you're using VDSL2, should be Ethernet with overhead and Per-Packet Overhead should be 8

Copy the questions above and paste them into a new topic in this forum. Then try www.dslreports.com/speedtest again and look at the "Results and Share" to see the latency values, and add a link to that page to share your results.

Update: Fixed LEDE/OpenWrt name and URL to the SQM Howto

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