[Cake] A few puzzling Cake results

Jonas Mårtensson martensson.jonas at gmail.com
Tue Apr 17 09:16:04 EDT 2018

On Tue, Apr 17, 2018 at 2:22 PM, Toke Høiland-Jørgensen <toke at toke.dk>

> Y via Cake <cake at lists.bufferbloat.net> writes:
> > From: Y <intruder_tkyf at yahoo.fr>
> > Subject: Re: [Cake] A few puzzling Cake results
> > To: cake at lists.bufferbloat.net
> > Date: Tue, 17 Apr 2018 21:05:12 +0900
> >
> > Hi.
> >
> > Any certain fomula of fq_codel flow number?
> Well, given N active bulk flows with packet size L, and assuming the
> quantum Q=L (which is the default for FQ-CoDel at full-size 1500-byte
> packets), the maximum rate for a sparse flow, R_s, is bounded by
> R_s < R / ((L/L_s)(N+1))
> Where R is the link rate and L_s is the packet size of the sparse flow.
> This assumes that the sparse flow has constant spacing between its
> packets, which is often the case for a VoIP flow...

For 10-Mbit/s link rate and 32 bulk flows with 1500-byte packets this
formula gives roughly 25 pps (packets per second) as maximum for a sparse
flow. A VoIP flow is typically 50 pps (20 ms voice payload).

Does this mean that cake sets the quantum to less than 750 bytes for a
10-Mbit/s link?

Do you see any benefit with cake diffserv if you increase the number of

Does the adjusted quantum also explain the "*way* higher" TCP RTT for cake?

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