Subject: Micro-loop prevention DT recommendation

From: Alex Zinin <[email-masked]>
Date: Tue, 29 Mar 2005 15:38:08 -0800

Folks-

The microloop prevention DT has concluded its discussion and is ready to provide
the recommendation on the method to supplement the Basic IP FRR mechanism.

The rough consensus within the DT team indicates that the recommended micro-loop
prevention method for the Basic IP FRR should be the PLSN (path locking via safe
neighbors) mechanism outlined in [1].

As discussed in a previous message [2], the DT previously eliminated two
methods: incremental cost changes, and synchronized FIB installation. See [2]
for more details.

After further discussion where advantages and disadvantages of different methods
were acknowledged (see [2]), PLSN appeared to be a good match to the Basic IP
FRR method from the perspective of coverage, required additional
forwarding/routing mechanisms, involved complexity, as well as operational
considerations. This is explained by the fact that both Basic FRR and PLSN
mechanisms use similar constructs--loopfreeness conditions of a specific
neighbor wrt a specific destination. Consequently, even though PLSN does not
provide 100% coverage, the coverage it provides is very similar (topology-wise)
to the coverage provided by the Basic FRR mechanism. This suggests that
modifications of network topology to improve FRR coverage should improve PSLN
coverage as well. Another positive aspect of the PLSN method is that it does not
preclude later extensions to improve coverage and prevent loops for destinations
left out by PLSN by employing other mechanisms for them (e.g. tunnel-based or
FIB ordering).

On top of the general recommendation, the DT also provided a list of related
topics that would benefit a discussion in the WG:

 1. Given small incremental coverage provided by type-B routers, it's worth
    considering whether retaining type-Bs in the spec is advisable.

 2. Guidelines for topology tuning to improve basic FRR and PLSN coverage
    would be useful

 3. How should cascading failures be dealt with?

 4. How will Basic FRR and PLSN interplay with fast convergence?

From this point, discussion on micro-loop prevention is moved to main WG mailing
list. The design team, therefore, has concluded its tasks and is disbanded.

The WG chairs would like to thank the DT members: Alia Atlas, Olivier
Bonaventure, Chris Martin, Don Fedyk, Joel M. Halpern, Loa Andersson, Mike
Shand, Pierre Francois, and Stewart Bryant.

[1] A. Zinin, "Analysis and Minimization of Microloops in Link-state Routing
    Protocols", Internet Draft, draft-zinin-microloop-analysis-00.txt

[2] A. Zinin, "Micro-loop prevention DT report", e-mail message to RTGWG mailing list,
    22 Feb 2005, http://www.ietf.org/mail-archive/web/rtgwg/current/msg00470.html

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

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