On Wed, Mar 06, 2013 at 07:46:16AM -0800, Jesse Gross wrote:
> On Tue, Mar 5, 2013 at 11:08 PM, Simon Horman <ho...@verge.net.au> wrote:
> > [ Cc Pravin B Shelar ]
> >
> > On Tue, Mar 05, 2013 at 06:11:27PM -0800, Ben Pfaff wrote:
> >> On Wed, Mar 06, 2013 at 10:42:02AM +0900, Simon Horman wrote:
> >> > On Tue, Mar 05, 2013 at 10:14:44AM -0800, Ben Pfaff wrote:
> >> > > On Thu, Feb 28, 2013 at 06:15:07PM +0900, Simon Horman wrote:
> >> > > > This adds support for the OpenFlow 1.1+ dec_mpls_ttl action.
> >> > > > And also adds an NX dec_mpls_ttl action.
> >> > > >
> >> > > > The handling of the TTL modification is entirely handled in
> >> > > > userspace.
> >> > > >
> >> > > > Reviewed-by: Isaku Yamahata <yamah...@valinux.co.jp>
> >> > > > Signed-off-by: Simon Horman <ho...@verge.net.au>
> >> > >
> >> > > The only issue I see with this is that it seems uncertain about what is
> >> > > an invalid MPLS TTL. The code says that, pre-decrement, values 0 and 1
> >> > > are invalid:
> >> > >
> >> > > + if (ttl > 1) {
> >> > > + ttl--;
> >> > > + set_mpls_lse_ttl(&ctx->flow.mpls_lse, ttl);
> >> > > + return false;
> >> > > + } else {
> >> > > + execute_controller_action(ctx, UINT16_MAX, OFPR_INVALID_TTL,
> >> > > 0);
> >> > >
> >> > > The documentation says that, pre-decrement, value 0 is invalid:
> >> > >
> >> > > +.IP \fBdec_mpls_ttl\fR
> >> > > +Decrement TTL of the outer MPLS label stack entry of a packet. If
> >> > > the TTL
> >> > > +is initially zero, no decrement occurs. Instead, a ``packet-in''
> >> > > message
> >> > >
> >> > > I don't know MPLS, so I don't know which definition is correct.
> >> >
> >> > I am not sure and to be honest I had just followed dec_ttl implementation
> >> > when adding the code. Reading up I think that section 2.3 of RFC3443
> >> > implies this is the desired behaviour.
> >> >
> >> > I'm not sure why the documentation conflicts with the code, most likely
> >> > it documents a previous incantation of the code. In any case I propose
> >> > updating it rather than the code.
> >>
> >> OK, that makes sense, thank you.
> >
> > It seems to me that dec_ttl has the same problem.
> >
> > But it also seems to me that it is the code rather than the documentation
> > that should be updated.
> >
> > The commit log for the addition of the TTL decrement action states:
> >
> > commit f0fd1a1772665ea57662281d9cccadb0f0146196
> > Author: Pravin B Shelar <pshe...@nicira.com>
> > Date: Fri Jan 13 17:54:04 2012 -0800
> >
> > ofproto: New action TTL decrement.
> >
> > Following patch implements dec_ttl as vendor action with similar
> > semantics as OpenFlow 1.2. If TTL reaches zero while procession
> > actions in current table, the remaining actions in previous tables
> > are processed. A configuration parameter is added to make TTL
> > decrement to zero generate packet in.
> >
> > Feature #8758
> > Signed-off-by: Pravin B Shelar <pshe...@nicira.com>
> >
> > But the code features "if (ttl > 1)".
> >
> > That now seems like a logic bug to me (though perhaps the afternoon
> > air is affecting my brain adversely).
> >
> >
> > I have revised the dec_mpls_ttl patch to use "(ttl > 0)".
> > Should I provide a patch to update dec_ttl?
>
> I think you were right the first time - if a decrement causes the TTL
> to hit zero then it should go to the exception case (in theory we
> should never receive a packet with TTL zero here, so it's always an
> exception). I believe this applies equally to IP and MPLS.
My understanding is that, in IP, a host accepts packets with TTL=0, but
a router discards them. If that is correct, then decrementing a TTL to
0 should not discard the packet, only decrementing a TTL that is
initially zero.
I think that this isn't a very important question for the purpose of
accepting these patches. I'm going to do my best to adjust them to do
what I think is correct. If there is a bug, then we can fix it.
Thanks,
Ben.
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