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

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 10 Nov 2017 14:34:36 -0800
Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com> wrote:

> 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>
> ---
>  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

Applied to net-next