netdev - Re: [PATCH v2.34] datapath: Add basic MPLS support to kernel

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Message-ID: <20130703013102.GA21323@verge.net.au>
Date:	Wed, 3 Jul 2013 10:31:04 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Jesse Gross <jesse@...ira.com>
Cc:	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	netdev <netdev@...r.kernel.org>, Ravi K <rkerur@...il.com>,
	Isaku Yamahata <yamahata@...inux.co.jp>,
	Pravin B Shelar <pshelar@...ira.com>,
	Jarno Rajahalme <jarno.rajahalme@....com>,
	Joe Stringer <joe@...d.net.nz>
Subject: Re: [PATCH v2.34] datapath: Add basic MPLS support to kernel

On Tue, Jul 02, 2013 at 02:59:51PM -0700, Jesse Gross wrote:
> On Mon, Jul 1, 2013 at 9:16 PM, Simon Horman <horms@...ge.net.au> wrote:
> > On Mon, Jul 01, 2013 at 04:42:46PM -0700, Jesse Gross wrote:
> >> On Wed, Jun 26, 2013 at 1:18 AM, Simon Horman <horms@...ge.net.au> wrote:
> >> > Allow datapath to recognize and extract MPLS labels into flow keys
> >> > and execute actions which push, pop, and set labels on packets.
> >> >
> >> > Based heavily on work by Leo Alterman, Ravi K, Isaku Yamahata and Joe Stringer.
> >> >
> >> > Cc: Ravi K <rkerur@...il.com>
> >> > Cc: Leo Alterman <lalterman@...ira.com>
> >> > Cc: Isaku Yamahata <yamahata@...inux.co.jp>
> >> > Cc: Joe Stringer <joe@...d.net.nz>
> >> > Signed-off-by: Simon Horman <horms@...ge.net.au>
> >>
> >> Simon, have you thought any more about the header ordering issues? I
> >> don't think we've reached a conclusion at this point.
> >
> > I believe that you then raised the issue of QinQ, which somehow
> > I failed to respond to, I apologise for that.
> >
> > In particular, my understanding is that you are concerned the code will
> > miss-calculate the end of L2 headers in the presence of multiple VLAN tags.
> > Thus resulting in an MPLS push action inserting an MPLS LSE after the first
> > rather than the last VLAN tag. And that would likely change if QinQ support
> > was added to Open vSwtich.
> >
> > I wonder if a good way forwards is to enhance the calculation
> > of the end of L2 headers (mac_len) and the beginning of L3 headers
> > (network_header) in ovs_flow_extract() such that it takes into
> > account the presence of more than one VLAN tag.
> 
> The problem is that this requires being able to calculate the length
> of all possible headers that we might want to support in the future.
> In the case of QinQ, this would mean the various EtherTypes. You could
> also imagine other things like MAC-in-MAC that are farther afield from
> what we currently support.

That is true.

I think that the key problem is it that it is hard for us
to correctly determine where the end of the L2 header is,
or more specifically where the MPLS tag should go, for all cases.
Particularly cases which are yet to be defined.

Having spoken with Joe about this we see two main options:

1. The status quo as of this patch. Which is that MPLS actions
   may be invalid for some cases.

   While it should be be possible to solve individual cases,
   this doesn't solve the general case.

2. Only allow MPLS actions on ether types where the implementation
   is known to work.

   This could act as a white list of sorts. It could start with
   the obvious candidates: IPv4, IPv6, ARP, 802.1Q,...
   And support for more protocols could be added in the future.

   This would seem to reflect the somewhat special nature of MPLS.
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