RE: [PATCH iproute2/net-next 1/3] tc: Add support for the sample tc action

-----Original Message-----
From: Florian Fainelli [mailto:f.fainelli@gmail.com]
Sent: Sunday, February 05, 2017 10:55 PM
To: Yotam Gigi <redacted>; stephen@networkplumber.org;
netdev@vger.kernel.org; Jiri Pirko [off-list ref]; Elad Raz
[off-list ref]
Subject: Re: [PATCH iproute2/net-next 1/3] tc: Add support for the sample tc action

Le 02/05/17 à 12:22, Yotam Gigi a écrit :

quoted

quoted

-----Original Message-----
From: Florian Fainelli [mailto:f.fainelli@gmail.com]
Sent: Sunday, February 05, 2017 8:37 PM
To: Yotam Gigi <redacted>; stephen@networkplumber.org;
netdev@vger.kernel.org; Jiri Pirko [off-list ref]; Elad Raz
[off-list ref]
Subject: Re: [PATCH iproute2/net-next 1/3] tc: Add support for the sample tc

action

quoted

quoted

On 02/04/2017 11:58 PM, Yotam Gigi wrote:

quoted

The sample tc action allows sampling packets matching a classifier. It
peeks randomly packets, and samples them using the psample netlink
channel. The user can specify the psample group, which the packet will be
sampled to, the sampling rate and the packet truncation (to save
kernel-user traffic).

The sampled packets contain informative metadata, for example, the input
interface and the original packet length.

The action syntax:
tc filter add [...] \
	action sample rate <RATE> group <GROUP> [trunc <SIZE>]
	[...]

Where:
  RATE := The sampling rate which is the ratio of packets observed at the
	  data source to the samples generated
  GROUP := the psample module sampling group
  SIZE := optional truncation size

An example for a common usecase of the sample tc action: to sample ingress
traffic from interface eth1, one may use the commands:

tc qdisc add dev eth1 handle ffff: ingress

tc filter add dev eth1 parent ffff: \
       matchall action sample rate 12 group 4

Where the first command adds an ingress qdisc and the second starts
sampling randomly with an average of one sampled packet per 12 packets
on dev eth1 to psample group 4.

The group argument seems to be mandatory from looking at the code, but
what if just wanted to have a port mirroring between, say sw0p1 and
sw0p2 with the sample rate specified instead (without using the psample
netlink channel at all)? Could we make this group an optional argument
instead?

The kernel action currently don't support it, and I am not sure it should.

If I understand you correctly, you want to make the sample action identical
to mirred-mirror, only with random behavior. This can be done using the
matchall and mirred action, plus the 'random' gact keyword.

It sounds like we can indeed, with random determ and using the VAL
argument we should be able to configure the capture divider; thanks!

You're welcome. It took me some time to find that keyword too :)

quoted

The sample action attaches some metadata in addition to the original packet
data, and that cannot be achieved by mirroring the packets, thus making it
unusable for our usecase. In the former version we attached the metadata
using the IFE protocol, but we decided to use a dedicated netlink channel
instead.

Yeah I see that now, thanks for the explanation!
--
Florian