Theoretical vs experimental results for Telstra EuroDOCSIS 3.0 (with "Ultimate Speed Boost" add-on) upstream rates at various packet sizes
Telstra's supplied modem (CG3100D-2BPAUS) appears to be shaping (through a ~94KiB FIFO) based on ethernet frames (inc. FCS, excl. preamble etc). The 6-byte DOCSIS MAC header and 4-byte "Upstream Privacy EH version 2 Element" are not a factor.
Naive:
(12 / (12 + 8 + 20 + 14 + 4))*2500 = 517
Accounting for minimum datagram size of 64 bytes:
(12 / (12 + 8 + 20 + 14 + 4 + 6))*2500 = 468
Actual ~= 470kbps
(18 / (18 + 8 + 20 + 14 + 4))*2500 = 703
Actual ~= 703kbps
(22 / (22 + 8 + 20 + 14 + 4))*2500 = 808
Actual ~= 810kbps
(24 / (24 + 8 + 20 + 14 + 4))*2500 = 857
Actual ~= 860kbps
(1472 / (1472 + 8 + 20 + 14 + 4))*2500 = 2424
Actual ~= 2424kbps
The advertised "2Mbps" service appears to be configured for precisely 2.5Mbps
line rate (inc. ether + fcs overhead) = 2500kbps
desired TCP ACK (12 byte udp payload equiv) throughput = 468kbps
desired full size (1472 byte udp payload equiv) throughput = 2424kbps
(12 / (12 + 8 + 20 + 14 + x))*y = 468
(1472 / (1472 + 8 + 20 + 14 + x))*y = 2424
solve for x (spoofed overhead in bytes) and y (spoofed line rate in kbps)
x = 10.3649 (round up for safety) y = 2510.23 (round down for safety)
previous variables based on trial and error x = 10 y = 2500
The use of this linear optimisation may result in link saturation for very small packets (smaller than TCP ACK, rare), and slightly reduced link utilisation for mid-size packets.
Example cake configuration on LEDE
#!/bin/sh
[ "$ACTION" = ifup ] && [ "$DEVICE" = "eth0.2" ] && {
logger -t Cake Device: $DEVICE / Action: $ACTION
tc qdisc del dev eth0.2 root
tc qdisc add dev eth0.2 root cake bandwidth 2510Kbit triple-isolate overhead 11
}Ideally, a shaper with a parameter for the minimum packet size should be used. I have prepared a patched verion of cake with a simple "eth" framing compensation option (with a static minimum frame size of 64 bytes), in the same vein as the atm and ptm options. A more generic option to define the minimum frame size may be more appropriate, as well as being less similarly named to the existing "ether-" overhead macros.
The configuration of my patched cake is as follows. This allows for 99.96% upstream link utilisation across all possible packet sizes.
#!/bin/sh
[ "$ACTION" = ifup ] && [ "$DEVICE" = "eth0.2" ] && {
logger -t Cake Device: $DEVICE / Action: $ACTION
tc qdisc del dev eth0.2 root
tc qdisc add dev eth0.2 root cake bandwidth 2499Kbit triple-isolate overhead 4 eth
}