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Micro-loop prevention DT report

Subject: Micro-loop prevention DT report
From: Alex Zinin
Date: Tue, 22 Feb 2005 15:52:44 -0800
Folks-

The Micro-loop Prevention DT has been considering the existing proposals and
exploring additional ones. Please refer to the following reading material:

  - http://psg.com/~zinin/ietf/rtgwg/IETF61-rtgwg-uloop-prev.ppt
  - draft-bryant-shand-lf-conv-frmwk-00.txt,
  - draft-zinin-microloop-analysis-00.txt
  - http://psg.com/~zinin/ietf/rtgwg/neverloop/ordered-spf.txt
  - http://www.info.ucl.ac.be/people/OBO/papers/pfr-infocom05.pdf

We have eliminated incremental cost changes, and synchronized FIB installation.

   Incremental cost changes has been eliminated due to the excessively long time
   (hours) for network convergence.

   Synchronized FIB installation has been eliminated due to the strong
   implementation constraints, dependency on NTP, and the fact that traffic loss
   due to loops would be minimized and not eliminated.

Proposals still under consideration are:

  - path-locking via safe neighbors (PLSN)
  - ordered FIB installation (OFIB)
  - tunnel-based
  - combination of PLSN with OFIB and tunnels

A brief description of each mechanism is given below:

Path Locking via Safe Neighbors (PLSN)

  Path locking via safe neighbors has a fixed convergence time regardless of
  network size. This method also works regardless of the topology change(s) that
  have occurred; this allows SRLG protection. The mechanism is easy to
  understand. Analysis of existing networks shows that about 90% of the
  potential 2-hop micro-loops are eliminated. The set of source/destination
  pairs that are not protected is similar to that not protected via loop-free
  alternates.

  The disadvantages of path locking via safe neighbors are as follows. First,
  the coverage is not 100% on existing networks. As with the Basic IP FRR
  mechanism, increasing redundancy in network topology can be used to increase
  micro-loop coverage. However, without 100% coverage, even traffic that is
  micro-loop free may suffer congestion or loss due to the traffic across a link
  that is not micro-loop free. Second, prevention of multi-hop micro-loops in
  networks with asymmetric link costs requires a stricter neighbor safety
  condition (downstream path before topology change), which reduces the coverage
  below the anticipated 90%.

  Path locking can be augmented in a number of different ways (requiring
  different forwarding mechanism) to provide 100% coverage. These methods
  include tunneling, marking traffic, or U-turn traffic redirection.

Ordered FIB (OFIB)

  Ordered FIB installation has the advantage that it provides 100% coverage for
  all micro-loops - including ones due to asymmetric link cost. This method also
  works with SRLGs. This however, may require different delays for different
  sets of prefixes.

  The disadvantage of Ordered FIB installation is that the worst-case network
  convergence time depends on the diameter of the network - and this can be
  quite long. When used with a repair mechanism that does not provide 100%
  coverage some traffic will suffer a significantly longer disruption that it
  would have experienced through conventional convergence. The length of the
  network convergence time also raises the risk that another topology event will
  occur before the network has converged; this could happen if there were
  another failure or even a maintenance event during the interval.

  The DT is also discussing the possibility of improving convergence time with
  the OFIB proposal through use of explicit FIB-update-completion signaling.

Tunnel-based approach

  The DT is in the process of discussing this method at the moment. Attractive
  properties of this method include being able to cover 100% of cases, and
  independence of transition time from the size of the network. On the other
  hand, this method requires modifications to the routing protocol to support
  "covert" topology change announcement, using tunneling introduces different
  operational and security considerations, and the distributed version of the
  method involves relatively higher level of complexity.

Combinations of PLSN with OFIB or Tunnels

  Such a combination would make it possible to use the simpler PLSN method to
  compliment the Basic FRR (LFA) mechanism, whose coverage would then be
  extended with OFIB or Tunnels to reach 100% for the advanced FRR methods.

Summary

  At this point the DT is still discussing different proposals. As the next
  step, the design team is expected to come up with the recommendation of a
  mechanism to compliment the Basic IP FRR specification.
  
The DT will give will provide a comprehensive report at the IETF-62 meeting in
Minneapolis.

Comments and feedback are appreciated.

-- 
Alex
http://www.psg.com/~zinin


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