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docs:guide-user:network:traffic-shaping:sqm-details [2022/11/17 19:58] – [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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   * **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). 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.+  * **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:** All shaping below the physical gross-rate of a link requires correct per-packet overhead accounting to be precise, so **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).   * **None:** All shaping below the physical gross-rate of a link requires correct per-packet overhead accounting to be precise, so **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).
  
  • Last modified: 2024/06/14 10:22
  • by moeller0