[P4-dev] Match + Actions on a cloned packet

David Hancock dhancock at cs.utah.edu
Wed Feb 3 13:10:27 EST 2016

Ramana, I haven't looked at copy_to_cpu, but I'm pretty sure receive.py 
is simply using SCAPY to receive, and in your case, send, packets.  
There is nothing special about the packets from the point of view of the 
switch.  So I'm a little surprised by your question. It sounds like you 
are wondering whether packets ever skip certain components.

Perhaps you are confused about data plane traffic vs. control plane 
traffic?  I'm not sure what the underlying confusion is but think things 
through and describe what *you* think should happen.


On 02/03/2016 10:25 AM, Ramana Reddy wrote:
> Thanks David. I will write some test code and try it out.
> In my second question, inject the packet means, assume that the packet 
> is cloned and received by an application which is
> listing on a particular port (ex: receive.py in copy_to_cpu), and if 
> the receive.py modified the packet and inject the modified
> packet back to switch, in that case, where exactly it injects into the 
> pipeline ( ingress or buffering mechanism or egress).
> Thanks & Regards,
> Ramana
> On Wed, Feb 3, 2016 at 10:03 PM, David Hancock <dhancock at cs.utah.edu 
> <mailto:dhancock at cs.utah.edu>> wrote:
>     Ramana, if you have reason to doubt the spec, why don't you write
>     some test code and try it out?  Then, if the results are not what
>     you expected, tell us about the difference between what you
>     expected and what you observed.  I have done this numerous times. 
>     In some cases, I uncovered a bug in the behavioral model.  In
>     other cases, I learned that a feature was not implemented in the
>     behavioral model, or that it was implemented a little bit
>     differently than what the spec said.  In yet other cases, I
>     learned that I did not understand the spec correctly.  I think you
>     will best serve yourself and the entire P4 community by taking
>     this approach.
>     That is my answer to your first question.  I don't understand your
>     second question - what do you mean, exactly, by "inject the
>     packet"?  Since this is under the topic of cloning packets, I
>     assume you are talking about cloning somehow.  I will tell you now
>     that you cannot do a clone_*_pkt_to_ingress - it has not been
>     implemented in bmv2, and per my discussion with Antonin, it may
>     never be and is likely to be removed from the spec.  If you are
>     talking about a resubmit, no clone is produced but the packet is
>     sent back to the parser.  If you are talking about
>     clone_*_pkt_to_egress, this sounds like your first question.
>     One additional hint - neither the cloning primitives nor the
>     resubmit primitive, nor the recirculate primitive take effect
>     immediately - the packet continues until the end of the
>     appropriate pipeline, at which point cloning / resubmission /
>     recirculation happens.  That fact caused me a bit of confusion
>     since I did not read the spec as carefully as I should have when I
>     started working with these primitives, so I hope this helps you
>     avoid the same confusion.
>     --David
>     On 02/03/2016 06:22 AM, Ramana Reddy wrote:
>>     Hi All,
>>     I have few doubts on cloned packets:
>>     1. When the packet is cloned using clone_ingress_to_egress
>>     primitive, //
>>     is the match+actions which are mentioned in ingress pipeline only
>>     applicable to original packet or to the cloned packet as well?
>>     In the diagram given in the spec (section 14.1.2), the cloned
>>     packet is directly going to the buffering mechanism.
>>     2. When we inject the packet through the application which
>>     is listing an a particular port, what is the control flow of that
>>     packet. Is it go again ingress pipeline match+actions tables.
>>     Thanks & Regards,
>>     Ramana
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