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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@intel.com>
This commit is contained in:
Jesus Sanchez-Palencia 2017-11-10 14:34:36 -08:00 committed by Stephen Hemminger
parent 8595cc40e9
commit 1915af404f
1 changed files with 13 additions and 1 deletions
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14
man/man8/tc-cbs.8
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@ -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