On Wed, Mar 6, 2013 at 8:21 AM, Ben Pfaff <b...@nicira.com> wrote:
> 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.
>From the IPv6 RFC (since the meaning has somewhat changed over time):
Hop Limit 8-bit unsigned integer. Decremented by 1 by
each node that forwards the packet. The packet
is discarded if Hop Limit is decremented to
zero.
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