Subject: [mpls] 1:n protection for MPLS-TP open question (the 1000 Kg question?)

From: "Daniel Cohn" <[email-masked]>
Date: Mon, 25 Jul 2011 20:44:35 +0300

Hi Eric,

Wrt the "Two-phase without lock" option which you presented today - if
the side detecting the failure does not wait for acknowledgment before
switching traffic to the protection path, in which sense is this still
"two phase"?

Regards,

Daniel

PS: I also don't see a scenario where the "lock" is required

---

From: "Eric Osborne (eosborne)" <[email-masked]>
Date: Mon, 25 Jul 2011 13:34:08 -0500

Hi Daniel-

  That is indeed a good question.  To me, it's two-phase in that there
are two phases to the message (protection notification, and the ACK in
reply).  All we're doing with my proposed behavior is sending the
traffic that we would have converged on anyways, without waiting
unnecessarily.

  As I noted in my slides, we need this so that after all the dust
settles, both ends of the protection domain can agree on what they're
protecting.  This is really only necessary in the case of an oddball
unidirectional failure.  

  You could argue that all unidir failures should be caught by CC/Cv and
that we should not have to worry about anything in PSC other than
bidirectional failures.  And if you did argue that I'd be in vehement
agreement that it'd be nice to ignore unidir failures.  But we can't,
and so we need to be able to handle them in PSC, which makes several
things trickier than they would be otherwise.

eric

> -----Original Message-----
> From: Daniel Cohn [mailto:[email-masked]]
> Sent: Monday, July 25, 2011 1:45 PM
> To: Eric Osborne (eosborne)
> Cc: [email-masked]
> Subject: 1:n protection for MPLS-TP open question (the 1000 Kg
question?)
> Wrt the "Two-phase without lock" option which you presented today - if
the
> side detecting the failure does not wait for acknowledgment before

---

From: "Daniel Cohn" <[email-masked]>
Date: Mon, 25 Jul 2011 22:19:25 +0300

Eric,

"Classic" PSC or APS, which is explicitly 1-phase, also has an implicit
ACK when the far end reports which signal is being bridge to the
protection path in the PSC/APS messages. And this doesn't make it
two-phase. So I believe that your suggested behavior should be called
1-phase.
I'm not suggesting we ignore unidirectional failures, as I believe the
PSC mechanisms should work regardless of the underlying specific SF/SD
detection mechanism. I'm just splitting hairs on terminology ;-)

DC

-----Original Message-----
From: Eric Osborne (eosborne) [mailto:[email-masked]] 
Sent: Monday, July 25, 2011 2:34 PM
To: Daniel Cohn
Cc: [email-masked]
Subject: RE: 1:n protection for MPLS-TP open question (the 1000 Kg
question?)

Hi Daniel-

eric

question?)
the

---

From: "Eric Osborne (eosborne)" <[email-masked]>
Date: Tue, 26 Jul 2011 09:30:59 -0500

Hi Daniel-

  1:1 PSC is 1-phase, no argument there.  I'm not sure I follow your
point that APS is also explicitly 1-phase.  I'm looking at GR-253 issue
5.  Table 5-8 lists what I'd call a two-phase procedure:

- C detects failure, requests bridge
- A bridges, sends reverse request
- C selects and bridges

True, it's not exactly the same as PSC, but the idea as I read it is
that C needs an ACK (RR) from A in order to complete the protection.  If
that's one-phase then yeah, I think our terminologies are at odds with
each other. Right?

eric

> Sent: Monday, July 25, 2011 3:19 PM
> protection path in the PSC/APS messages. And this doesn't make it two-
> phase. So I believe that your suggested behavior should be called
1-phase.
> I'm not suggesting we ignore unidirectional failures, as I believe the
PSC
> mechanisms should work regardless of the underlying specific SF/SD
are
> two phases to the message (protection notification, and the ACK in
reply).
> All we're doing with my proposed behavior is sending the traffic that
we
> would have converged on anyways, without waiting unnecessarily.
> 
and
> so we need to be able to handle them in PSC, which makes several
things
> trickier than they would be otherwise.
> 
> 
> 
> 
> eric
> 
>

---

From: "Daniel Cohn" <[email-masked]>
Date: Tue, 26 Jul 2011 17:54:05 +0300

Right, GR-253 APS is 2-phase. G.8031 APS for example is 1-phase.
Regardless of examples, my point is that if you bridge the signal to the
protection path before waiting for the ACK, then this is a 1-phase
protocol, despite the ACK being eventually received.

DC

-----Original Message-----
From: Eric Osborne (eosborne) [mailto:[email-masked]] 
Sent: Tuesday, July 26, 2011 10:31 AM
To: Daniel Cohn
Cc: [email-masked]
Subject: RE: 1:n protection for MPLS-TP open question (the 1000 Kg
question?)

Hi Daniel-

eric

1-phase.
PSC
are
reply).
we
and
things

---

From: Pablo Frank <[email-masked]>
Date: Tue, 26 Jul 2011 22:04:04 -0400

I think this is true.  The only thing that makes it "2-phase-ish" is the way
the sender goes into a Wait-for-ack (Wfa) state while it waits for the other
side to acknowledge the state change.

However, the more I think about it, the more I'm unsure why we even need the
Wfa state, and thus why the PSC protocol has to be modified to be
2-phase-ish in the first place.  It seems like the only reason the Wfa state
exists is so that it can raise an alarm if the other side never acknowledges
the change.  But is that really necessary?  The current PSC will send 3 PSC
messages in quick succession following a state change and then will continue
to send periodic PSC messages indefinitely until the next change.  It seems
to me that as long as the senders continue to report on what they think the
highest priority failed path is, both sides will converge to protecting the
same path.  Am I missing something here?

regards,
Pablo

p.s. given the spelling, it's actually the 1016.04691 Kg question.  : )

On Tue, Jul 26, 2011 at 10:54 AM, Daniel Cohn <[email-masked]> wrote:

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