Re: [mpls] Doubts on draft-helvoort-mpls-tp-ring-protection-switching-03

[Naveen T <[e-mail]>]

Dear Huub,

Thank you very much for clearing up my doubts. Your answers addressed my concerns.

I have two more questions. 

This is with respect to the link failure example in the Wrapping solution as described in section 3.1.2.1. As proposed by the solution, when a failure occurs on the link between B and C, the traffic flow and the label usage are as below:
A[W1]->B[P6]->A[P1]->F[P2]->E[P3]->D[P4]->C[W3]->D

Consider the case where the packet has traversed the ring on the protection path (...D[P4]->C) and is now at node C. At this point in time, if the link between B and C comes back up again, i.e., it is operational again, then;

1) Will the node C swap the label it receives (P4) with the protection label P3 and forward the packet to B? If so, there is a possibility that the packet continues to traverse the ring via the protection path in the loop, or;


2) Will the node C always swap the label it receives (P4) with the working label W3 and forward the packet to D irrespective of whether there is a failure on the link between C and B?

Thanks and Regards,
Naveen T

________________________________
From: Huub helvoort <[e-mail]>
To: Naveen T <[e-mail]>; MRPS Helvoort <[e-mail]> 
Sent: Monday, 4 February 2013 6:50 PM
Subject: RE: [mpls] Doubts on draft-helvoort-mpls-tp-ring-protection-switching-03



Dear Naveen,

Thank you very much for your patience.

The last two weeks I have been traveling, and email access was very limited.

Please find below the answers to your questions [Huub].

> The proposed solution recommends the usage of section layer OAM to find signal failures on a link. Since the two nodes will be able to detect failures at either end of a link using the link OAM, why would it be a necessity to send the APS payload to the adjacent node in the direction opposite to the failure?

[Huub] The nodes at each side of a section (e.g. Nodes A and B) can detect failures. However the failure may have occurred only in one direction, e.g node A --> B, the APS message s used to informA that B has detected a failure and will switch to protection, A will have to switch to protection too. 
[Huub] also if a node fails e.g. node B located between nodes A and C then both A and C will detect the failure of
the section connecting them to B, and they have to inform the other side C resp. A that they will perform a protectiuoin switch based on the detected failure.
[Huub] there may be more than a single section or node failure in the ring, all the nodes in the ring have to be aware of the failure and its location to take proper protection switch consequent actions.

> If the switching commands Lockout, Manual Switch and Forced Switch are taken out of the equation, and assuming that a signal failure will be the only reason considered for a switch-over, will it still be necessary to use the APS protocol at all? 

[Huub] also in this case the APS messages sent colockwise and counter-clockwise will be necessary to inform all the nodes in the ring of the fault location and the switch action. There may be point-to-multipoint connections on the ring, and each node has to decide wich signal should be used. 

I hope this addresses your concerns. 

Best regards, Huub.