Re: [PWE3] [mpls] Seeking feedback on I-D "MPLS-TPLinearProtectionApplicability to MS-PW"

["Daniel Cohn" <[e-mail]>]

Hi Sasha, thanks again and see inline with [DC].

 

From: Alexander Vainshtein [mailto:[e-mail]] 
Sent: Monday, June 13, 2011 6:11 PM
To: Daniel Cohn
Cc: [e-mail]; [e-mail]; Sprecher, Nurit (NSN - IL/Hod
HaSharon); [e-mail]
Subject: RE: [mpls] [PWE3] Seeking feedback on I-D
"MPLS-TPLinearProtectionApplicability to MS-PW"

 

Daniel and all,

Please see some comments inline below.

 

Regards,

     Sasha

 

From: Daniel Cohn [mailto:[e-mail]] 
Sent: Monday, June 13, 2011 5:55 PM
To: Sprecher, Nurit (NSN - IL/Hod HaSharon); Alexander Vainshtein;
[e-mail]
Cc: [e-mail]; [e-mail]
Subject: RE: [mpls] [PWE3] Seeking feedback on I-D
"MPLS-TPLinearProtection Applicability to MS-PW"

 

Hi,

 

Thanks all for the feedback. I believe we all agree that PW protection
is only required in the event of S-PE failure at an MS-PW - this  is
clearly stated in the draft. Now, both Sasha and Nurit mention that the
PW redundancy mechanism can meet the MPLS-TP PW protection requirements.

 

First of all a word on scalability. Please note that in an MPLS-TP
environment without a control plane, the PW redundancy mechanism must
also rely on proactive connectivity check for fast failure detection to
meet the sub-50 ms requirement. [[[Sasha]]] IMO there is nothing
specific to MPLS-TP here. Detecting S-PE failures based on the control
plane would  not fast enough.

[DC] Agreed - I just pointed our that MPLS-TP has a requirement for
sub-50ms protection.

Therefore any scalability considerations that apply to the PW protection
draft apply to the PW redundancy draft as well.[[[Sasha]]] I have not
raised the scalability issue. My issue was limited applicability scope
when compared to PW redundancy. 

[DC] Correct - scalability issue was a response to Nurit's comment.

E.g., the PW redundancy mechanisms take care of dual-homed CEs in SS-
and MS-PWs - something that liner protection of PWs cannot do.

[DC] I'm afraid I don't follow you here. Where does PW redundancy take
care of dual-homed CE? Actually PW redundancy draft explicitly states:
"The method for dual-homing of CE1 to PE1 and to PE3 nodes, and the
protocols used, are outside the scope of this document". 

Also wrt scalability, there is no such thing as "scales" or "doesn't
scale" - scalability is not a binary concept. You can say that PW
protection scales worse than LSP protection, just like you can say that
LSP protection scales worse than interface protection. Which didn't stop
IETF from defining LSP protection for scenarios where interface
protection doesn't do the job.

 

Now let's turn our attention back to whether the PW redundancy draaft
can be used to meet MPLS-TP PW protection requirements. I can identify
the following reasons why in its current form it doesn't:

 

-          It explicitly ("outside the scope") does not define
protection triggers and how to handle coexisting triggers, as requested
in RFC 5654 (MPLS-TP Requirements), reqs #75, #76 and #79

[[[Sasha]]] So what? Definition of triggers is orthogonal to how
coordinated protection switching happens.

[DC] It is. But still it needs to be defined by the recovery framework,
e.g. what should the endpoint do when one PW is in SF and the other is
in SD, or when both are in SD. Operators expect well-defined behavior in
these and other scenarios, and PW redundancy does not define them
(because it was not in their scope). 

-          It does not support the ability to distinguish between
different types of triggers (i.e. one end doesn't know why the other end
triggered switch), as requested in RFC 5654 (MPLS-TP Requirements), req
#77

-          It does not define revertive/nonrevertive behavior, as
requested in RFC 5654 (MPLS-TP Requirements), req #64

-          It does not define holdoff support, which is especially
important to avoid race conditions with LSP protection when it exists

-          It doesn't support 1+1 mode, as requested in RFC 5654
(MPLS-TP Requirements), req #65 

[[[Sasha]]] All these claims are correct - and  this should not be a
surprise, because MPLS-TP requirements have been defined much later than
the PW redundancy mechanism. 

But I do not think that this justifies co-existence of two different
mechanisms.

[DC] So how do you propose to meet the PW protection requirement? 

-          It's a two-phase protocol, with the consequent impact on
timing[[[Sasha]]] Could you please elaborate? [DC] In
draft-ietf-mpls-tp-linear-protection, each endpoint will immediately
switch traffic to the other path upon identifying an SF condition in a
path, without waiting for the far end to acknowledge the switch
(1-phase). In PW redundancy, an endpoint detecting an SF condition in a
path will not switch until the far end has acknowledged the switch
(2-phase). Needless to say, recovery is slower in a 2-phase protocol.

 

-          It doesn't define retransmission of protection coordination
messages, so loss of a single PDU can result in switchover not taking
place, thus not supporting sub-50 ms recovery in this case

[[[Sasha]]] The PW redundancy protocol runs either on top of LDP (which
benefits from TCP retransmissions) or on top of static PW status
messages (where retransmission is defined). 

[DC] True - but static PW status defines slow retransmission ("will be
transmitted twice at an initial interval of one second") while
draft-ietf-mpls-tp-linear-protection specifies fast retransmission when
required for fast recovery (section 3.1.4). 

 

In summary, PW redundancy was not designed with TP requirements in mind,
and as such does not meet the TP requirements. Of course modifications
may be introduced, but why reinvent the wheel when there is a protocol
(draft-ietf-mpls-tp-linear-protection-06) in the standards track that
supports all the above requirements and can be applied to MS-PW
protection with minor modifications?

[[[Sasha]]] As I said, because the applicability scope is by far too
narrow to justify a dedicated protocol.

[DC] If we were discussing designing a new protocol, I might agree with
you. But from a practical standpoint, the PW protection draft is not a
dedicated protocol - it's an applicability statement to an existing
protocol. Both from the implementation and the operational point of view
it defines a new use case for an existing protocol and concept.

 

 

Regards,

 

Daniel

 

 

From: [e-mail] [mailto:[e-mail]] On Behalf Of
Sprecher, Nurit (NSN - IL/Hod HaSharon)
Sent: Monday, June 13, 2011 4:02 PM
To: ext Alexander Vainshtein; [e-mail]
Cc: [e-mail]; [e-mail]
Subject: Re: [mpls] [PWE3] Seeking feedback on I-D
"MPLS-TPLinearProtection Applicability to MS-PW"

 

Hi,

I would like to second Sasha.

End-to-end PW protection (with diverse paths) does not scale, and put
hard restrictions on the utilization of the resources.  

MPLS-TP PWs are carried across the network inside MPLS-TP LSPs.
Therefore, an obvious way to provide protection for a PW is to protect
the LSP that carries it.  

If the PW is a multi-segment PW, then LSP recovery can only protect the
PW in individual segments.  This means that a single LSP recovery action
cannot protect against a failure of a PW switching point (an S-PE).

When protecting against an AC or T/S-PE failure by dual connectivity, PW
redundancy mechanisms provide means for the PEs to coordinate over which
LSP the traffic of the PW is carried. 

I also doubt why there is a need for additional mechanism. 

Best regards,

Nurit

 

From: [e-mail] [mailto:[e-mail]] On Behalf Of
ext Alexander Vainshtein
Sent: Monday, June 13, 2011 3:43 PM
To: [e-mail]
Cc: [e-mail]; [e-mail]
Subject: Re: [PWE3] [mpls] Seeking feedback on I-D "MPLS-TP
LinearProtection Applicability to MS-PW"

 

Dear Ma and all,

Adding the PWE3 WG to my response.

 

The PW redundancy mechanism supports linear protection of MS-PWs as one
of many additional application use cases:

Appendix A of the PW redundancy Bit draft
<http://datatracker.ietf.org/doc/draft-ietf-pwe3-redundancy-bit/?include
_text=1>  describes 5 application uses cases in addition to MS-PW with
single-homed CEs (which is listed there as use case 5).

And it is equally applicable to IP/MPLS and MPLS - with the help of  the
Static PW Status Messages draft
<http://datatracker.ietf.org/doc/draft-ietf-pwe3-static-pw-status/?inclu
de_text=1> ( if, for whatever reason, you do not want  to, or cannot,
use RFC 4447 <http://datatracker.ietf.org/doc/rfc4447/?include_text=1>
).

 

Hence I doubt the need for yet another PW redundancy  mechanism with
narrow scope of applicability.

 

Regards,

     Sasha

 

From: [e-mail] [mailto:[e-mail]] On Behalf Of
[e-mail]
Sent: Monday, June 13, 2011 3:25 PM
To: [e-mail]
Subject: Re: [mpls] Seeking feedback on I-D "MPLS-TP Linear Protection
Applicability to MS-PW"

 

Hi all, 

The linear protection mechanism for LSP and PW(including MS-PW) should
be the same and it is valuable to describe it clearly. 

BTW, there is a typo, it is "T-PE Z" instead of "T-PE B". 

 " 
  Figure 1 illustrates such a scenario, where two MS-PWs are 
  established between T-PE A and T-PE B, over S-PEs 1-2 and 3-4 
  respectively. Each PW segment is established over an LSP (e.g. PW- 
  s12 over LSP12). 
 " 

-----Original Message-----
From: Daniel Cohn 
Sent: Tuesday, May 17, 2011 4:14 PM
To: mpls
Subject: Seeking feedback on I-D "MPLS-TP Linear Protection
Applicability to MS-PW"
Importance: High

Hi MPLSers,

I uploaded "MPLS-TP Linear Protection Applicability to MS-PW" I-D
(http://tools.ietf.org/html/draft-cohn-mpls-tp-pw-protection-00)

The abstract goes:

One of the requirements of the MPLS transport profile [RFC 5654] is
to provide linear protection for transport paths, which include both
LSPs and PWs. The functional architecture described in [SurvivFwk]
is applicable to both LSP and PWs, however [LinearProt] does not
explicitly describe mechanisms for PW protection in MPLS-TP.

This document extends the applicability of the linear protection
mechanism described in [LinearProt] to MPLS-TP segmented PWs 
(MS-PWs) as defined in [RFC 6073].

Could you please review it and send feedback to the mailing list or
directly to the author? 

Looking forward to your feedback, 

Daniel 

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