Re: [mpls] MPLS-RT review of draft-cheng-mpls-tp-shared-ring-protection-05

Yimin Shen <> Fri, 10 July 2015 01:37 UTC

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Thread-Topic: MPLS-RT review of draft-cheng-mpls-tp-shared-ring-protection-05
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Subject: Re: [mpls] MPLS-RT review of draft-cheng-mpls-tp-shared-ring-protection-05
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Dear authors, chairs, and WG

I've been asked by the WG chairs to review draft-cheng-mpls-tp-shared-ring-protection-05 for WG adoption. Overall, I think the draft is trying to address a valid MPLS-TP scenario by using a promising solution. However, I have the following comments and questions  for sections 1 to 4, which I would like to request the authors to answer and clarify.

[1]     Section 2.1 - Recovery of multiple failures
-       This is misleading to me. The draft seems to only be able to handle a single failure per ring, rather than multiple failures per ring. This may need some clarification.

[2]     I'd like to suggest to change term "exit node" to "egress node".

[3]     Section 4.1 - The ring tunnel layer.

-       Given a ring of N nodes, this layer would consist of 4xN ring tunnels. The draft is a little light on the provisioning and management aspects of this layer, while pushing a lot of complexity to this layer.
-       Each node is traversed by many tunnels. How does it know whether a tunnel is working or protecting,  and whether a tunnel is CW or AW ? How does it associate a working tunnel with a protecting tunnel (to the same egress node, but opposite in direction) ?
-       When an ingress node needs a working tunnel to a particular egress node, how does choose between the CW  tunnel and the AW tunnel, especially when one is significantly longer  (i.e. of higher cost) than the other ?

[4]     The draft describes 4 tunnels (CW, AW, CP, AP) per egress node ? Is it possible to use only 2 tunnels (i.e. CW and AW) and have them protect each other ?

[5]     Section 4.3.2 -- Short wrapping vs. "traditional wrapping"

-       Does "traditional wrapping" refer to the scenario in section 4.3.1 ?
-       The 4.3.2 mode seems superior to the 4.3.1 mode in all aspects. If so, would there be any value to keep 4.3.1 mode.
-       In 4.3.2 mode, CW tunnel ==CP tunnel and AW tunnel == AP tunnel in path topology, correct ? If so, you could reduce the number of tunnels per egress node from 4 to 2 (as comment [4] above), correct ?

[6]     Section 4.3.3

-       This is equivalent to "global repair" in MPLS. However, why does the ingress node switch traffic from CW tunnel to AP tunnel instead of AW tunnel ? A working tunnel should be "better" than a protecting tunnel to permanently carry traffic.

[7]     Section 4.4

-       I'm not sure how label swap would work on an interconnection node, as the draft describes it. In Fig-12, in normal state, F receives a packet from ring-1; The top label X is a tunnel to the node group; The next label Y is the service LSP. So, F pops label X, and " the label used for working ring tunnel R2cW_I will be pushed based on the inner label" (i.e. label Y). How could F have the knowledge about the service LSP, its label Y, and its egress node, etc ?
-       I may have more questions to follow after the above...


Yimin Shen
Juniper Networks