[P4-dev] DIfference between on_miss and default

Anandakumar ananth.padfoot at gmail.com
Thu Oct 5 13:57:34 EDT 2017

Just to confirm that my understanding is right, the keys for which the
`on_miss` action is programmed in the table, the control
`process_outer_multicast` is executed. This is inline with the guidelines
shared in the p4 trainings where a table-apply is not to be performed in an
action but instead invoked in this imperative style.

And when no such matching key is seen in the table, instead of a nop that
is implicitly done by the compiler, here the `default` process_ipv4_vtep or
process_ipv6_vtep control flow takes place. Right?

Also in this case, the on_miss action is a nop. So I understand that the
`process_outer_multicast` will be performed. But what if the action has
some modify_field operations? Is it ok to do it? If so what is the expected
behavior? Which will be done first  - the action or the imperatively
defined control?


On 5 October 2017 at 11:21, Anandakumar <ananth.padfoot at gmail.com> wrote:

> Thank you very much for the clarification!
> On 05-Oct-2017 7:25 AM, "Vladimir Gurevich" <vladimir.gurevich@
> barefootnetworks.com> wrote:
>> Hello Anand,
>> The keyword "default" used in the context of the apply() statement with
>> the case clause does not mean "the default" action. Instead it means "any
>> action, other that those that were explicitly listed".  In that sense it is
>> very much the "catch all" keyword, similar to "default" as used in C switch
>> statements.
>> So, the statement in question reads like this:
>> If the chosen action (after the outer_rmac table has been applied) is
>> "on_miss", then execute "process_outer_multicast" control. In all other
>> cases, if ipv4 header is valid, then call process_ipv4_vtep() control
>> otherwise, if ipv6 header is valid, then call (apply) process_ip6_vtep()
>> control.
>> Normally, you would expect the "on_miss" action to be the default action
>> in the table outer_rmac, but individual entries can have it as well.
>> I hope this answers the rest of your questions as well. Again, "default"
>> does not mean "default action".
>> Happy Hacking,
>> Vladimir
>> On Wed, Oct 4, 2017 at 6:01 PM, Anandakumar <ananth.padfoot at gmail.com>
>> wrote:
>>> Hi Everyone,
>>> I’m fairly new to P4 and was looking at this snippet in process_tunnel
>>> and was wondering how the on_miss and default are different?
>>> Can someone please help me in demystifying the following?
>>>    1. How the on_miss and default are different?
>>>    2. Is the on_miss taken only if there’s an on_miss action programmed
>>>    in the table against a specific key?
>>>    3. What’s the behavior expected here, if on_miss is programmed as
>>>    the default_action for the table in the program itself or from the
>>>    control plane via the PD API?
>>>    Would it make missed take the path of process_outer_multicast or the
>>>    default?
>>>    4. What’s the behavior expected here, if say an example action like
>>>    another_action is programmed as the default_action for the table?
>>>    Would it still execute the default block or just the action and
>>>    nothing in this snippet?
>>> What’s the suggested way to easily search the whole mailing list archive
>>> so that I don’t repeat the questions that are already probably answered! :)
>>> Thank you very much.
>>>         /* outer RMAC lookup for tunnel termination */
>>>           apply(outer_rmac) {
>>>               on_miss {
>>>                   process_outer_multicast();
>>>               }
>>>               default {
>>>                   if (valid(ipv4)) {
>>>                       process_ipv4_vtep();
>>>                   } else {
>>>                       if (valid(ipv6)) {
>>>                           process_ipv6_vtep();
>>>                       }
>>>                   }
>>>               }
>>>           }
>>> Thank you very much.
>>> -Anand
>>> _______________________________________________
>>> P4-dev mailing list
>>> P4-dev at lists.p4.org
>>> http://lists.p4.org/mailman/listinfo/p4-dev_lists.p4.org
>> --
>> *Vladimir Gurevich*
>> *Barefoot Networks*
>> *Technical Lead, Customer Engineering*
>> Email: vag at barefootnetworks.com
>> Phone: (408) 833-4505
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