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docs:guide-user:network:traffic-shaping:sqm-details [2022/01/15 20:42] – [SQM: Link Layer Adaptation Tab] moeller0docs:guide-user:network:traffic-shaping:sqm-details [2022/11/17 20:00] – [SQM: Link Layer Adaptation Tab] moeller0
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 Smart Queue Management (SQM) is our name for an intelligent combination of better packet scheduling (flow queueing) techniques along with with active queue length management (AQM). Smart Queue Management (SQM) is our name for an intelligent combination of better packet scheduling (flow queueing) techniques along with with active queue length management (AQM).
  
-OpenWrt/LEDE has full capability of tuning the network traffic control parameters. If you want to do the work, you can read the full description at the [[doc/howto/traffic.control|Traffic Control HOWTO.]] You may still find it useful to get into all the details of classifying and prioritizing certain kinds of traffic, but the SQM algorithms and scripts (fq_codel, cake, and sqm-scripts) require a few minutes to set up, and work as well or better than most hand-tuned classification schemes.+OpenWrt/LEDE has full capability of tuning the network traffic control parameters. If you want to do the work, you can read the full description at the [[:docs:guide-user:network:traffic-shaping:packet.scheduler|QoS HOWTO]]You may still find it useful to get into all the details of classifying and prioritizing certain kinds of traffic, but the SQM algorithms and scripts (fq_codel, cake, and sqm-scripts) require a few minutes to set up, and work as well or better than most hand-tuned classification schemes.
  
 Current versions of OpenWrt/LEDE have SQM, fq_codel, and cake built in. These algorithms were developed as part of the [[http://www.bufferbloat.net/projects/cerowrt/wiki|CeroWrt]] project. They have been tested and refined over the last four years, and have been accepted back into OpenWrt, the Linux Kernel, and in dozens of commercial offerings.  Current versions of OpenWrt/LEDE have SQM, fq_codel, and cake built in. These algorithms were developed as part of the [[http://www.bufferbloat.net/projects/cerowrt/wiki|CeroWrt]] project. They have been tested and refined over the last four years, and have been accepted back into OpenWrt, the Linux Kernel, and in dozens of commercial offerings. 
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   * Choose **ATM: select for e.g. ADSL1, ADSL2, ADSL2+** and set the Per-packet Overhead to 44 bytes if you use any kind of DSL/ADSL connection to the Internet other than a modern VDSL high speed connection (20+Mbps). In other words if you have your internet service through a copper telephone line at around 1 or 2Mbps.   * Choose **ATM: select for e.g. ADSL1, ADSL2, ADSL2+** and set the Per-packet Overhead to 44 bytes if you use any kind of DSL/ADSL connection to the Internet other than a modern VDSL high speed connection (20+Mbps). In other words if you have your internet service through a copper telephone line at around 1 or 2Mbps.
   * Choose **Ethernet with overhead: select for e.g. VDSL2** and set the Per-packet Overhead to 34 if you know you have a VDSL2 connection (this is sometimes called Fiber to the Cabinet, for example in the UK). VDSL connections operate at 20-100Mbps over higher quality copper lines. If you are sure that PPPoE is not in use, you can reduce this to 26.   * Choose **Ethernet with overhead: select for e.g. VDSL2** and set the Per-packet Overhead to 34 if you know you have a VDSL2 connection (this is sometimes called Fiber to the Cabinet, for example in the UK). VDSL connections operate at 20-100Mbps over higher quality copper lines. If you are sure that PPPoE is not in use, you can reduce this to 26.
-  * If you have a cable modem, with a coaxial cable connector, you can try 22 bytes, or see the **Ethernet with Overhead** details below.+  * If you have a cable modem, with a coaxial cable connector, you can try 22 bytes, or see the **Ethernet with Overhead** details below. If your contracted rate is greater than 760 Mbps set overhead 42 (mpu 84) as the ethernet link to the modem now affects worst case per-packet-overhead.
   * Choose **Ethernet with overhead** if you have an actual Fiber to the Premises or metro-Ethernet connection and set the Per-Packet Overhead to 44 bytes. This can be reduced somewhat for example if you know you are not using VLAN tags, but will usually work well.   * Choose **Ethernet with overhead** if you have an actual Fiber to the Premises or metro-Ethernet connection and set the Per-Packet Overhead to 44 bytes. This can be reduced somewhat for example if you know you are not using VLAN tags, but will usually work well.
   * Choose **none (default)** if you have some reason to not include overhead. All the other parameters will be ignored.   * Choose **none (default)** if you have some reason to not include overhead. All the other parameters will be ignored.
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   * **ATM:** It is especially important to set the Link Layer Adaptation on links that use ATM framing (almost all DSL/ADSL links do), because ATM adds five additional bytes of overhead to a 48-byte frame. Unless the SQM algorithm knows to account for the ATM framing bytes, short packets will appear to take longer to send than expected, and will be penalized. For true ATM links, one often can measure the real per-packet overhead empirically, see https://github.com/moeller0/ATM_overhead_detector for further information how to do that. Getting the mpu right is tricky since ATM/AAL5 can either include the FCS or not, but setting mu to 96 should be save (that results in 2 ATM cells).   * **ATM:** It is especially important to set the Link Layer Adaptation on links that use ATM framing (almost all DSL/ADSL links do), because ATM adds five additional bytes of overhead to a 48-byte frame. Unless the SQM algorithm knows to account for the ATM framing bytes, short packets will appear to take longer to send than expected, and will be penalized. For true ATM links, one often can measure the real per-packet overhead empirically, see https://github.com/moeller0/ATM_overhead_detector for further information how to do that. Getting the mpu right is tricky since ATM/AAL5 can either include the FCS or not, but setting mu to 96 should be save (that results in 2 ATM cells).
-  * **Ethernet with Overhead:** SQM can also account for the overhead imposed by VDSL2 links - add 22 bytes of overhead (mpu 68). Cable Modems (DOCSIS) set both up- and downstream overhead to 18 bytes (6 bytes source MAC, 6 bytes destination MAC, 2 bytes ether-type, 4 bytes FCS), to allow for a possible 4 byte VLAN tag it is recommended to set the overhead to 18 + 4 = 22 (mpu 64). +  * **Ethernet with Overhead:** SQM can also account for the overhead imposed by //VDSL2// links - add 22 bytes of overhead (mpu 68). Cable Modems (//DOCSIS//) set both up- and downstream overhead to 18 bytes (6 bytes source MAC, 6 bytes destination MAC, 2 bytes ether-type, 4 bytes FCS), to allow for a possible 4 byte VLAN tag it is recommended to set the overhead to 18 + 4 = 22 (mpu 64); if you want to set shaper rates greater than 760 Mbps set overhead 42 (mpu 84) as now the worst case per-packet-overhead is on the ethernet link to the modem. For //FTTH// the answer is less clear cut, since different underlaying technologies have different relevant per-packet-overheads; however underestimating the per-packet-overhead is considerably worse for responsiveness than (gently) overestimating it, so for //FTTH// set the overhead to 44 (mpu 84) unless there is more detailed information about the true overhead on a link available
-  * **None:** Fiber, and direct Ethernet connections generally do not need any kind of link layer adaptation. Well, I am kidding, all shaping below the physical gross-rate requires correct per-packet overhead accounting, but for fiber and ethernet it is much harder to figure out the exact overhead to specify... (the question is typically how is the ISP's upstream traffic shaper configured). For true ethernet shaping without VLANs specify 38 bytes (mpu 84).+  * **None:** All shaping below the physical gross-rate of a link requires correct per-packet overhead accounting to be preciseso **None** is only useful if approximate shaping is sufficient, say if you want to clamp a guest network to at best ~50% of the available capacity or similar tasks, but even then configuring an approximate correct per-packet-overhead is recommended (overhead 44 (mpu 84) is a decent default to pick).
  
  
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 Also to make things even more complicated, CPU power/frequency scaling (to save power) can interfere negatively with SQM. Probably due to SQM's bursty nature it might not be recognised by the power governor and the CPU (that at 100% is capable of shaping to the desired sqm rate) gets too slow to service SQM in time and throughput suffers, and due to the burstyness the governor might never realise it should scale frequency back up. This can be remedied by trying to optimise the transition rules for up-scaling frequency/power or by switching to a non-scaling governor. The former requires a bit of trial and error but maintains power saving, while the latter probably is easier to achieve and hence might be a good way to figure out whether power saving might be an issue in the first place. @experts, please feel free to elaborate on which power save settings are worth exploring. Also to make things even more complicated, CPU power/frequency scaling (to save power) can interfere negatively with SQM. Probably due to SQM's bursty nature it might not be recognised by the power governor and the CPU (that at 100% is capable of shaping to the desired sqm rate) gets too slow to service SQM in time and throughput suffers, and due to the burstyness the governor might never realise it should scale frequency back up. This can be remedied by trying to optimise the transition rules for up-scaling frequency/power or by switching to a non-scaling governor. The former requires a bit of trial and error but maintains power saving, while the latter probably is easier to achieve and hence might be a good way to figure out whether power saving might be an issue in the first place. @experts, please feel free to elaborate on which power save settings are worth exploring.
  
 +** How do I get cake to consider IPv6 traffic in a 6in4 tunnel as separate flows?**
  
 +See [[:docs:guide-user:network:ipv6:ipv6_henet#in4_with_cake_sqm|6in4 with cake config]]
  
 ===== Troubleshooting SQM ===== ===== Troubleshooting SQM =====
  • Last modified: 2024/06/14 10:22
  • by moeller0