On 03/07/2013 10:44 PM, Ben Pfaff wrote:
On Thu, Mar 07, 2013 at 09:00:49PM -0600, Saul St. John wrote:
1) Is there a reason for not allowing this field as the target of
mangling actions?
I don't know a good one. I guess that no one has implemented it, and
that is probably because there has been little demand for it. (I don't
remember previous requests.)
Am I correct in thinking that the only change needed to support this
would be to change the 'writable' boolean in the mf_field for
NXM_OF_ARP_OP from false to true in lib/meta-flow.c?
3) I don't have a OF1.2-speaking controller at-hand to test this, but it
looks like the OFPAT_SET_FIELD action calls the same implementation as
the NXM actions. Does OVS enforce the same restrictions on this action?
Yes.
If so, is that correct behavior? (I can't tell from the spec if a switch
implementation supporting set_field needs to support setting all fields
it supports matching (except those excluded in A.2.5).)
I doubt that a switch implementation is required to support set_field
for all the fields it supports. It does not make much sense to change
some fields; for example, changing the Ethertype is ordinarily going to,
at most, cause confusion.
I'm not sure I take your point. Any switch that implements the MPLS or
VLAN push/pop actions is going to need to support ethertype rewriting--
so preventing set_field on ethertype would seem to be an artificial
restriction unmotivated by underlying capabilities. I understand that
there's usually little utility in directly manipulating that field, and
that doing so could cause confusion, but I'd suggest that the simplest
way to prevent that confusion would just be to not set ethertype by
flow-rule action in the first place.
From a practical perspective, I'm left wondering if there's any
practical method for a controller to interrogate a switch's
field-setting capabilities. (I apologize if that's an o/t subject for
this list.)
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