Date:	Thu, 5 Dec 2013 20:21:31 -0800
From:	Ben Pfaff <blp@...ira.com>
To:	Simon Horman <horms@...ge.net.au>
Cc:	dev@...nvswitch.org, netdev@...r.kernel.org,
	Jesse Gross <jesse@...ira.com>,
	Pravin B Shelar <pshelar@...ira.com>,
	Ravi K <rkerur@...il.com>, Joe Stringer <joe@...d.net.nz>
Subject: Re: [PATCH v2.51 1/5] ofp-actions: Allow Consistency checking of
 OF1.3+ VLAN actions after mpls_push

On Fri, Dec 06, 2013 at 12:29:24PM +0900, Simon Horman wrote:
> On Thu, Dec 05, 2013 at 09:56:25AM -0800, Ben Pfaff wrote:
> > On Thu, Dec 05, 2013 at 11:26:06AM +0900, Simon Horman wrote:
> > > On Wed, Dec 04, 2013 at 05:01:11PM -0800, Ben Pfaff wrote:
> > > > On Thu, Dec 05, 2013 at 09:51:39AM +0900, Simon Horman wrote:
> > > > > On Wed, Dec 04, 2013 at 04:44:17PM -0800, Ben Pfaff wrote:
> > > > > > On Thu, Dec 05, 2013 at 08:58:49AM +0900, Simon Horman wrote:
> > > > > > > On Wed, Dec 04, 2013 at 01:24:29PM -0800, Ben Pfaff wrote:
> > > > > > > > On Thu, Nov 21, 2013 at 12:46:42PM +0900, Simon Horman wrote:
> > > > > > > > > The aim of this patch is to support provide infrastructure for verification
> > > > > > > > > of VLAN actions after an mpls_push action for OpenFlow1.3. This supplements
> > > > > > > > > existing support for verifying these actions for pre-OpenFlow1.3.
> > > > > > > > > 
> > > > > > > > > In OpenFlow1.1 and 1.2 MPLS tags are pushed after any VLAN tags that
> > > > > > > > > immediately follow the ethernet header. This is pre-OpenFlow1.3 tag
> > > > > > > > > ordering. Open vSwitch also uses this ordering when supporting MPLS
> > > > > > > > > actions via Nicira extensions to OpenFlow1.0.
> > > > > > > > > 
> > > > > > > > > When using pre-OpenFlow1.3 tag ordering an MPLS push action does not
> > > > > > > > > affect the VLANs of a packet. If VLAN tags are present immediately after
> > > > > > > > > the ethernet header then they remain present there.
> > > > > > > > > 
> > > > > > > > > In of OpenFlow1.3+ MPLS LSEs are pushed before any VLAN tags that
> > > > > > > > > immediately follow the ethernet header. This is OpenFlow1.3+ tag
> > > > > > > > > ordering.
> > > > > > > > > 
> > > > > > > > > When using OpenFlow1.3+ tag ordering an MPLS push action affects the
> > > > > > > > > VLANs of a packet as any VLAN tags previously present after the ethernet
> > > > > > > > > header are moved to be immediately after the newly pushed MPLS LSE. Thus
> > > > > > > > > for the purpose of action consistency checking a packet may be changed
> > > > > > > > > from a VLAN packet to a non-VLAN packet.
> > > > > > > > > 
> > > > > > > > > In this way the effective value of the VLAN TCI of a packet may differ
> > > > > > > > > after an MPLS push depending on the OpenFlow version in use.
> > > > > > > > > 
> > > > > > > > > This patch does not enable the logic described above.
> > > > > > > > > Rather it is disabled in ofpacts_check__(). It should
> > > > > > > > > be enabled when support for OpenFlow1.3+ tag order is added
> > > > > > > > > and enabled.
> > > > > > > > > 
> > > > > > > > > Signed-off-by: Simon Horman <horms@...ge.net.au>
> > > > > > > > 
> > > > > > > > As far as I can tell this doesn't make sense, because where the MPLS
> > > > > > > > tag goes is a property of the action that we know at the time we parse
> > > > > > > > the push_mpls action.  So why isn't this patch just the following?
> > > > > > > > 
> > > > > > > > diff --git a/lib/ofp-actions.c b/lib/ofp-actions.c
> > > > > > > > index a02f842..f444374 100644
> > > > > > > > --- a/lib/ofp-actions.c
> > > > > > > > +++ b/lib/ofp-actions.c
> > > > > > > > @@ -2071,6 +2071,9 @@ ofpact_check__(enum ofputil_protocol *usable_protocols, struct ofpact *a,
> > > > > > > >           * Thus nothing can be assumed about the network protocol.
> > > > > > > >           * Temporarily mark that we have no nw_proto. */
> > > > > > > >          flow->nw_proto = 0;
> > > > > > > > +        if (ofpact_get_PUSH_MPLS(a)->position == OFPACT_MPLS_BEFORE_VLAN) {
> > > > > > > > +            flow->vlan_tci = 0;
> > > > > > > > +        }
> > > > > > > >          return 0;
> > > > > > > 
> > > > > > > That was more or less what I originally tried.  However I believe that it
> > > > > > > doesn't work because ofpact_get_PUSH_MPLS(a)->position may not have been
> > > > > > > set at the time that ofpact_check__ is called.  In particular this occurs
> > > > > > > when it is called indirectly from parse_ofp_str__.
> > > > > > > 
> > > > > > > Moreover, when ofpact_check__ is called indirectly from parse_ofp_str__ it
> > > > > > > is used to check actions when a one of number of protocols may be used,
> > > > > > > that is multiple bits of *usable_protocols.  If we could rely on
> > > > > > > ofpact_get_PUSH_MPLS(a)->position then I believe that implies that if it is
> > > > > > > set to OFPACT_MPLS_BEFORE_VLAN all pre-OpenFlow1.3 bits of
> > > > > > > *usable_protocols need to be cleared.  Otherwise all OpenFlow1.3+ bits
> > > > > > > would need to be cleared.
> > > > > > 
> > > > > > I think this might be a mistake in how we define the syntax that
> > > > > > parse_ofp_str__() parses.  If I write "actions=push_mpls" on an
> > > > > > ovs-ofctl command line, then I want that to have some specific
> > > > > > meaning.  I don't want it to mean "do one thing if you happen to
> > > > > > negotiate OpenFlow 1.2 or some other thing if you happen to negotiate
> > > > > > OpenFlow 1.3", because that's totally unusable and broken from a user
> > > > > > perspective.
> > > > > 
> > > > > To clarify, that is exactly what this series was trying to do.
> > > > > 
> > > > > I think there is some precedence in the handling of actions
> > > > > that set_vlans. Some OF versions implicitly push a tag, some don't.
> > > > 
> > > > Maybe that is worth fixing too.
> > > > 
> > > > > But I do agree that the behaviour you describe above would
> > > > > be very confusing for users.
> > > > > 
> > > > > > So if that the issue then I think we should change the
> > > > > > syntax.  One way would be to have "push_mpls" default to the 1.3
> > > > > > behavior (which seems generally saner) and allow the user to specify
> > > > > > an option to get the 1.2 behavior.
> > > > > 
> > > > > Sure, I think I am happy with that idea.
> > > > > 
> > > > > By an option do you mean a different action name, for example append_mpls,
> > > > > or push_mpls_after_vlan?
> > > > 
> > > > I was thinking of something like a push_mpls version of the
> > > > keyword-based fin_timeout syntax.  One option would be eth_type,
> > > > defaulting to ETH_TYPE_MPLS.  Another option would be position, with
> > > > after_vlan or before_vlan as allowed values, and probably after_vlan
> > > > as the default.
> > > > 
> > > > With this approach, any flow with a push_mpls could be used only with
> > > > pre-OF1.3 or OF1.3+, depending on the "position" value.  One wrinkle
> > > > that might be nice, if it isn't too nasty to implement, would be to
> > > > have a third value "no_vlan" as the default.  With no_vlan, we reject
> > > > the flow at check time if a VLAN is present; if no VLAN is present,
> > > > then push_mpls has the same behavior regardless of OpenFlow version.
> > > 
> > > From an ovs-ofctl point of view I think that makes a lot of sense and I
> > > think it should be clean enough to implement. My initial reaction is that
> > > using a position argument would be good and I don't see any particular
> > > problem with a no_vlan option. But I'll give some thought to an eth_type
> > > argument.
> > 
> > push_mpls currently takes a required eth_type argument.  There are only
> > two values and my understanding is that 0x8847 is more common, hence my
> > suggestion that it be the default.
> 
> I'm not sure that we can use the eth_type argument of an mpls_push action
> in order to differentiate between pushing the LSE before or after
> any VLAN tags that may be present. There are two acceptable values for the
> eth_type argument, 0x8847 and 0x8848. But I believe they are both
> equally acceptable regardless of where the LSE is to be pushed
> in relation to existing VLAN tags.

The eth_type is irrelevant to the change we're making, but there has to
be some way to specify it after the change, just as there is before the
change, so I'm suggesting a way.
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