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.
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