netdev - Re: [PATCH v2.51 1/5] ofp-actions: Allow Consistency checking of OF1.3+ VLAN actions after mpls

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Date:	Thu, 5 Dec 2013 08:58:49 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Ben Pfaff <blp@...ira.com>
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 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.
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