netdev - tc-cake(8) needs to explain a common mistake

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Message-ID: <CAN_LGv1h8Ut4bGm7ZgYaGV_Tbdy3ABW+epb_p6jeX=TxnAvH1g@mail.gmail.com>
Date:   Fri, 3 Apr 2020 23:44:39 +0500
From:   "Alexander E. Patrakov" <patrakov@...il.com>
To:     netdev@...r.kernel.org
Subject: tc-cake(8) needs to explain a common mistake

Hello,

there is a recurring cargo cult pattern in many forums (e.g. OpenWRT):
people keep suggesting various overhead compensation parameters to
tc-cake without checking what's the bottleneck. They just assume that
it is always related to the link-layer technology of the connection.

This assumption is mostly incorrect, and this needs to be explained in
the manual page to stop the cargo cult. E.g., here in Russia, in the
past year, I had a 1Gbit/s link (1000BASE-X) but they shaped my
connection down to 500 Mbit/s because that's the bandwidth that I paid
for. I.e. the link from my router to the ISP equipment was not the
bottleneck, it was the ISP's shaper.

How about the following addition to the tc-cake(8) manual page, just
before "Manual Overhead Specification"? Feel free to edit.

General considerations
-------------------------------

Do not blindly set the overhead compensation parameters to match the
internet connection link type and protocols running on it. Doing so
makes sense only if that link (and not something further in the path,
like the ISP's shaper) is indeed the bottleneck.

Example 1: the ADSL modem connects at 18 Mbit/s, but the ISP further
throttles the speed to 15 Mbit/s because that's what the user pays
for, and does so with a shaper that has bufferbloat. Then, the "adsl"
keyword is likely not appropriate, because the ISP's shaper operates
on the IP level. The bandwidth needs to be set slightly below 15
Mbit/s.

Example 2: the ADSL modem connects at 18 Mbit/s, and the user pays for
"as fast as the modem can get" connection. Then, the "adsl" keyword is
relevant, and the bandwidth needs to be set to 18 Mbit/s.

Example 3: the user has a 100BASE-TX Ethernet connection, and pays for
the full 100 Mbit/s bandwidth (i.e. there is no shaper further up).
Then, the "ethernet" keyword is relevant, and the bandwidth needs to
be set to 100 Mbit/s.

-- 
Alexander E. Patrakov
CV: http://pc.cd/PLz7