On Thu, Aug 16, 2012 at 05:16:53PM +0900, Isaku Yamahata wrote:
> Hi. Right now OVS tells a controller that 256 tables available
> as SWITCH_FEATURES response.
> (i.e. ofp_switch_features->n_tables = 256 = N_TABLES)
It should say 255, are you sure about 256?
> But the table whose id is TBL_INTERNAL = N_TABLES - 1 is used for ovs
> internal use so can't be available to a controller.
The controller can't modify it, but it is allowed to view it.
> Addition to the above, the document, DESIGN, says that
> > Tables 128 and above are reserved for use by the switch itself.
> > Controllers should use only tables 0 through 127.
>
> So what should be returned as n_tables?
> - 256 (current value)
> With OF1.0, there is no way to know that table id 255 is unusable.
OF1.0 uses table id 255 as a special value in many places, but what
you write is true of table 254.
I don't think that it is meaningful to say that table 254 is not
usable in OF1.0, because in OF1.0 there is no way to place a flow in a
particular table. Open vSwitch will always place flows in table 0, if
the controller uses OF1.0 without extensions. So, I can't see how the
controller can be bothered by such a reserved table.
> With OF1.1+, allowed operations can be described. So a controller can know
> that which tables are actually usable.
I hadn't noticed that but it is a useful fact.
> - 255
> This reflects the reality, but contradicts to the document, DESIGN.
DESIGN says:
The Open vSwitch software switch implementation offers 255 flow
tables.
> - 128
> Actually 255 tables are usable.
I don't see how there is a practical problem here.
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