Subject: [mpls] MPLS-TP ring protection: draft-weingarten & draft-umansky

From: Zhi-Wei Lin <[email-masked]>
Date: Thu, 25 Mar 2010 16:13:10 -0400

Hi,

I wanted to summarize my issues with both ring protection drafts. Hopefully
the authors can integrate the two and come up with one document that helps
to address this:

   - Neither draft-weingarten or draft-umansky covers dual-ring
   interconnection. draft-weingarten discusses it, but the mechanism included
   does not seem to prevent against single point of failure (or I don't
   understand it well enough)
   - draft-weingarten seems to re-use linear-protection + path segment
   tunnel to keep protocol simple and single-phase. However, per presentation
   this may need to change to include a multi-phase messaging. Although it does
   seem optimized and simple, it seems to lock the protocol to support only
   current mechanism. It seems that any extensions or addition of protection
   switching requires extensive changes to message format (e.g., PT codes are
   all used up). How would this affect future interoperability and upgrade of
   equipment functionality? Hopefully it won't...
   - draft-umansky's mechanism is multi-phase and seems to borrow heavily on
   existing SONET/SDH messaging processes. Is this necessary, or could the
   protocol be simplified in some way? Too many message exchanges means longer
   protection time (maybe not too bad)
   - draft-weingarten's use of PST seems to prevent future extra traffic
   usage (intermediate ring nodes have no access to the PST tunnel?), while
   draft-umansky's mechanism (while doesn't mention extra traffic) seems to be
   more flexible. Am I interpreting correctly?
   - draft-weingarten describes a wrapping mechanism using PST. I'm not sure
   how this generalizes to protecting any links and nodes in between the
   ingress/egress of LSP. For example, if my traffic actually goes from LSR_A
   to LSR_D (LSR_B & LSR_C, and LSR_E & LSR_F are intermediate nodes), does the
   wrapping approach mean that I have to create the following in order to wrap
   (how is this done anyway?):
      - PST from A-to-B (to protect A-B link),
      - PST from A-to-C (to protect node B),
      - PST from B-to-C (to protect B-C link),
      - PST from B-to-D (to protect node C), etc.?
      - If that's the case, how are these PSTs associated with the original
      traffic protected by the ring?
      - If not the case, how is this wrapping as compared to steering?
      - draft-umansky seems to have a more logical definition of the state
   machines, with specific rules for going from state to state. This should be
   adopted in draft-weingarten as well (in the form of updated
   draft-ietf-linear-protection).

Also, another point: It has been mentioned to me that most service providers
did not make heavy use of extra traffic in TDM networks and does not see a
need for this in MPLS-TP (as evidenced in RFC 5654, note after rqt #67). I
think from my perspective, since MPLS-TP is a packet-based network and most
likely used to carry bursty data traffic, not having a mechanism to re-use
all that "stranded" capacity just sitting there waiting for a failure so it
can be used, seems wasteful - especially given that in packet world, much
traffic is best-effort and a perfect fit for extra traffic...I think adding
the (1:1)^n and oversubscription would alleviate this problem, but having
the option of extra traffic would be nice also.

However, I do note that in this morning meeting, the "control plane" version
of MPLS-TP does seems to support extra traffic while the management-plane
version of MPLS-TP seems to leave this out...

Thanks
Zhi