[ovs-dev] [PATCH v2.56] datapath: Add basic MPLS support to kernel

[Jesse Gross]

On Fri, Apr 25, 2014 at 1:06 AM, YAMAMOTO Takashi
<yamamoto at valinux.co.jp> wrote:
>> On Thu, Apr 24, 2014 at 05:57:29PM +0900, YAMAMOTO Takashi wrote:
>>> hi,
>>>
>>> > + * Due to the sample action there may be multiple possible eth types.
>>> > + * In order to correctly validate actions all possible types are tracked
>>> > + * and verified. This is done using struct eth_types.
>>>
>>> is there any real-world use cases of these actions inside a sample?
>>> otherwise, how about just rejecting such combinations?
>>> it doesn't seem to worth the code complexity to me.
>>> (sorry if it has been already discussed.  it's the first time for me
>>> to seriously read this long-lived patch.)
>>
>> Good point, the code is rather complex.
>>
>> My understanding is that it comes into effect in the case
>> of sflow or ipfix being configured on the bridge. I tend
>> to think these are real-world use-cases, though that thinking
>> is by no means fixed.
>>
>> My reading of the code is that in the case of sflow and ipfix a single
>> sample rule appears at the beginning of the flow. And that it may be
>> possible to replace the code that you are referring to with something
>> simpler to handle these cases.
>
> it seems that they put only a userland action inside a sample.
> it's what i expected from its name "sample".

Yes, that's the only current use case. In theory, this could be used
more generally although nothing has come up yet.

In retrospect, I regret the design of the sample action - not the part
that allows it to handle different actions but the fact that the
results can affect things outside of the sample action list. I think
that if we had made it more like a subroutine then that would have
retained all of the functionality but none of the complexity here.
Perhaps if we can find a clean way to restructure it without breaking
compatibility then that would simplify the validation here.

>>
>> My understanding is that the code you are referring to also comes into
>> effect when a sample action (a Nicira extension) is used directly in a
>> rule.  I am less sure that this is a real-world case but the complex logic
>> you are referring to should to handle this use-case.
>
> probably nicira folks can clarify?

It's the same set of use cases, just extending it to OpenFlow to
enable building sampling into different situations.