netdev - [PATCH iproute2] man: Clarify idleslope calculation for tc-cbs

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Message-Id: <20171110223436.18221-1-jesus.sanchez-palencia@intel.com>
Date:   Fri, 10 Nov 2017 14:34:36 -0800
From:   Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
To:     netdev@...r.kernel.org
Cc:     jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        andre.guedes@...el.com, vinicius.gomes@...el.com,
        ivan.briano@...el.com,
        Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
Subject: [PATCH iproute2] man: Clarify idleslope calculation for tc-cbs

In order to calculate the idleSlope parameter of CBS correctly, users
must take into account the entire packet size, including the overhead
from all layers.

Add some more details to the man page to clarify that, giving one
simple example and pointing users to the correct 802.1Q section for
further clarifications if needed.

Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
---
 man/man8/tc-cbs.8 | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/man/man8/tc-cbs.8 b/man/man8/tc-cbs.8
index 97e00c84..32e1e0d4 100644
--- a/man/man8/tc-cbs.8
+++ b/man/man8/tc-cbs.8
@@ -43,7 +43,19 @@ second) when there is at least one packet waiting for transmission.
 Packets are transmitted when the current value of credits is equal or
 greater than zero. When there is no packet to be transmitted the
 amount of credits is set to zero. This is the main tunable of the CBS
-algorithm.
+algorithm and represents the bandwidth that will be consumed.
+Note that when calculating idleslope, the entire packet size must be
+considered, including headers from all layers (i.e. MAC framing and any
+overhead from the physical layer), as described by IEEE 802.1Q-2014
+section 34.4.
+
+As an example, for an ethernet frame carrying 284 bytes of payload,
+and with no VLAN tags, you must add 14 bytes for the Ethernet headers,
+4 bytes for the Frame check sequence (CRC), and 20 bytes for the L1
+overhead: 12 bytes of interpacket gap, 7 bytes of preamble and 1 byte
+of start of frame delimiter. That results in 322 bytes for the total
+packet size, which is then used for calculating the idleslope.
+
 .TP
 sendslope
 Sendslope is the rate of credits that is depleted (it should be a
-- 
2.14.1