[Ecn-sane] quick question

[Sebastian Moeller]

Dear ECN-experts,

I have a quick question, when running downloads, say x flows (with MTU ~1500), over a Y Mbps link with Zms RTT, what kind of CE rate do I need to expect?

I ask, because the kids started to play some games on steam, and when these do their multi-GB updates I accumulate quite a lot of CE marks for the respective cake tin:

Before:
qdisc cake 810a: dev ifb4pppoe-wan root refcnt 2 bandwidth 105Mbit diffserv3 dual-dsthost nat nowash ingress no-ack-filter split-gso rtt 100ms noatm overhead 34 mpu 88 memlimit 32Mb 
 Sent 21471648426 bytes 16712159 pkt (dropped 9179, overlimits 27961170 requeues 0) 
 backlog 2984b 2p requeues 0
 memory used: 864Kb of 32Mb
 capacity estimate: 105Mbit
 min/max network layer size:           28 /    1492
 min/max overhead-adjusted size:       88 /    1526
 average network hdr offset:            0

                   Bulk  Best Effort        Voice
  thresh       6562Kbit      105Mbit    26250Kbit
  target            5ms          5ms          5ms
  interval        100ms        100ms        100ms
  pk_delay        116us       2.68ms         88us
  av_delay         31us       1.29ms         18us
  sp_delay          6us         57us          6us
  backlog            0b        2984b           0b
  pkts           170434     16500879        50027
  bytes        73422115  21408109703      3480229
  way_inds           56      1320752         2152
  way_miss         3879       101928          341
  way_cols            0            0            0
  drops              12         9167            0
  marks              34          559            0
  ack_drop            0            0            0
  sp_flows            1            5            1
  bk_flows            0            1            0
  un_flows            0            0            0
  max_len          1492         1492          192
  quantum           300         1514          801


After:

qdisc cake 810a: dev ifb4pppoe-wan root refcnt 2 bandwidth 105Mbit diffserv3 dual-dsthost nat nowash ingress no-ack-filter split-gso rtt 100ms noatm overhead 34 mpu 88 memlimit 32Mb 
 Sent 26301367224 bytes 19973069 pkt (dropped 9381, overlimits 34351737 requeues 0) 
 backlog 0b 0p requeues 0
 memory used: 1025408b of 32Mb
 capacity estimate: 105Mbit
 min/max network layer size:           28 /    1492
 min/max overhead-adjusted size:       88 /    1526
 average network hdr offset:            0

                   Bulk  Best Effort        Voice
  thresh       6562Kbit      105Mbit    26250Kbit
  target            5ms          5ms          5ms
  interval        100ms        100ms        100ms
  pk_delay        336us        238us        180us
  av_delay         62us         96us         36us
  sp_delay          2us          2us          8us
  backlog            0b           0b           0b
  pkts           171622     19760656        50172
  bytes        73494415  26237802841      3493005
  way_inds           56      1321363         2237
  way_miss         3904       102830          345
  way_cols            0            0            0
  drops              12         9369            0
  marks              34      2346888            0
  ack_drop            0            0            0
  sp_flows            1           19            1
  bk_flows            0            1            0
  un_flows            0            0            0
  max_len          1492         1492          192
  quantum           300         1514          801

# note there was other traffic ongoing as well, so some drops/marks are unrelated, but the bulk of marks was correlated with the steam download
delta packets: (19760656 - 16500879) = 3259777
delta bytes: (26237802841 - 21408109703) / 1000^3 = 4.829693138 GB
average packet size: (26237802841 - 21408109703) / (19760656 - 16500879) = 1481.60231145
delta drops: (9369 - 9167) = 202
delta marks: (2346888 - 559) = 2346329
fraction of packets marked: 2346329 / 3259777 = 0.719782058711
percentage of packets marked: 100 * (2346329 / 3259777) = 72%

This seems like too high a marking rate to me. I would naively expect that a flow on getting a mark scale back by its cwin by 20-50% and then slowly increaer it again, so I expect the actual marking rate to be considerably below 50% per flow...
My gut feeling is that these steam flows do not obey RFC3168 ECN (or something wipes the CE marks my router sends upstream along the path)... but without a good model what marking rate I should expect this is very hand-wavy, so if anybody could help me out with an easy derivation of the expected average marking rate I would be grateful.

Regards
	Sebastian