[P4-dev] tables ordering in pipeline using "conditionals", "next_tables", ...?

Hardik Soni hardik.soni at inria.fr
Tue Aug 15 11:59:26 EDT 2017


Hello, 

I have a confusion between two constructs. 

1) Why next tables are dependent on actions? 
I have attached a screenshot, it shows fragment of .p4 against .json side by side. 
For 3 actions, there are 3 values in "next_tables" tag from line 784 to 786 in .json snippet. 
In lines 1697 to 1713 of P4Objects.cpp, why MatchTableAbstract types have next_tables based on actions? 
These actions are not kind of "goto table" semantics. 
It is not really a pipeline if there are next_table options based on actions in one table, its a graph. 
This semantic(control invocation from table) is not specified in the spec of P4-16 dated May 22, 2017 version 1.0.0. 
For example, the idea of having different next nodes even for __HIT__ and __MISS__ are also not specified. 

2) T here is apply{...} in .p4 or conditionals in .json, so which order of table arrangement is finally used? 
To me it seems, conditionals have final say. 
I went through https://github.com/p4lang/behavioral-model/blob/master/docs/JSON_format.md link. 
Still, did not understand why two ways to define control flow even if it is a graph? 
What is the difference, except the scope at which control transition is programmed? 

Best Regards, 
Hardik Soni 

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