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Network Working Group                                           M. Menth
Internet-Draft                                               M. Hartmann
Intended status: Experimental                                    D. Hock
Expires: January 6, 2011                         University of Wuerzburg
                                                            July 5, 2010


               Routing Optimization with IP Fast Reroute
               draft-menth-ipfrr-routing-optimization-00

Abstract

   This draft gives a summary of findings about routing optimization of
   not-via addresses [I-D.ietf-rtgwg-ipfrr-notvia-addresses] and LFAs
   [RFC5286] in IP networks.  The optimization is done only by tuning IP
   link costs.  Optimization goals for both IP FRR techniques is the
   maximum link utilization in the failure-free scenario and for a set
   of failure scenarios such as all single link (and node) failures as
   well as the failure coverage by LFAs.  For LFAs, the failure coverage
   is another optimization goal.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 6, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.


Table of Contents

   1.  Unique Shortest Paths for IP Networks with Not-Via
       Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Routing Optimization of Loop-Free Alternates (LFAs):
       Minimizing Maximum Link Utilization and Maximizing Failure
       Coverage  . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5































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1.  Unique Shortest Paths for IP Networks with Not-Via Addresses

   Routes in IP networks are determined by administrative IP link costs,
   i.e., traffic is forwarded on the least-cost paths from source to
   destination.  When the equal-cost paths option is enabled, traffic is
   equally distributed over all interfaces leading to a least-cost path
   towards the destination.  If the equal-cost path option is not
   enabled, only a single interface towards a least-cost path is chosen
   to forward traffic to a specific destination even though multiple
   least-cost paths may exist.  However, it is not clear which interface
   is chosen in that case.  As a consequence, traffic loads and link
   utilization are difficult to predict in such networks.  As paths
   cannot be predicted, routing optimization is difficult.

   We studied this problem in [HoHa10].  We optimized routing in example
   networks for the failure-free scenario and for a set of failure
   scenarios, i.e., we chose link costs that minimize the maximum link
   utilization.  We assumed some tie-breaker function to choose next-
   hops in case of equal-cost paths and showed how much more traffic
   than expected can occur on some links when routers choose different
   next-hops than predicted.  We proposed to fix this problem by
   choosing link costs such that equal-cost paths are avoided for the
   failure-free case and for the set of considered failure scenarios.
   We call this type of paths "unique shortest paths (USPs)".  It is
   possible to find USPs and to minimize the maximum link utilization
   without any performance loss compared to unconstraint routing
   optimization.

   In IP networks, equal-cost paths is another option to avoid the
   presented problem.  However, with not-via addresses
   [I-D.ietf-rtgwg-ipfrr-notvia-addresses], traffic is forwarded around
   non-reachable nodes only on a single interface as the traffic
   tunnelled by the point of local repair to the next-next-hop has the
   same source and destination so that it cannot be distributed equally
   over multiple equal-cost tunnels.  Hence, the load on not-via paths
   in failure cases cannot be predicted in spite of ECMP when multiple
   equal-cost paths exist.  USP is a solution to make traffic
   distribution in not-via networks predictable and allows to optimize
   routing.


2.  Routing Optimization of Loop-Free Alternates (LFAs): Minimizing
    Maximum Link Utilization and Maximizing Failure Coverage

   We used link cost optimization to maximize the coverage of LFAs
   [RFC5286] in several example networks.  We defined three different
   requirements.  Destinations need to be protected at least by




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   a.  link-protecting LFAs

   b.  node-protecting LFAs (do not cause loops in case of node
       failures)

   c.  downstream LFAs (do not cause loops in case of multiple failures)

   ad a) In most networks, 100% of all destinations could be protected
   after optimization.  In some networks with a special structure, only
   a slightly lower coverage could be achieved.

   ad b) Even after optimization, less than 100% coverage could be
   achieved in any network.

   ad c) After optimization, it was not possible to achieve 100%
   coverage.  This is obvious since the closest node to a specific
   destination does not have a downstream LFA for that destination.

   When only the coverage is optimized, the maximum link utilization in
   failure cases can become extremely high.

   Optimizing the coverage as primary goal and the link utilization as
   secondary goal does not lead to low link utilizations.  However,
   changing the order of the optimization goals leads to low utilization
   and relatively high coverage.

   When optimizing for coverage and link utilization, the maximum link
   utilization in failure cases is quite high if only the maximum link
   utilization in the failure-free scenario is optimized.

   Optimizing the maximum link utilization for a set of failure
   scenarios and optimizing the coverage leads to low maximum link
   utilization and to high failure coverage.


3.  IANA Considerations

   This document makes no request to IANA.


4.  Security Considerations

   This document makes no security considerations.


5.  Acknowledgements

   The authors thank Christian Schwartz, Tim Neubert, and Michael



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   Hoefling for their contributions and efforts.


6.  References

6.1.  Normative References

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

6.2.  Informative References

   [HoHa10]   Hock, D., Hartmann, M., Menth, M., and C. Schwartz,
              "Optimizing Unique Shortest Paths for Resilient Routing
              and Fast Reroute in IP-Based Networks", in Proceedings of
              the 12th IEEE/IFIP Network Operations and Management
              Symposium (NOMS), Osaka, Japan, preprint available at:
              http://www.menth.net/Publications/papers/Menth10g.pdf,
              April 2010.

   [I-D.ietf-rtgwg-ipfrr-notvia-addresses]
              Shand, M., Bryant, S., and S. Previdi, "IP Fast Reroute
              Using Not-via Addresses",
              draft-ietf-rtgwg-ipfrr-notvia-addresses-05 (work in
              progress), March 2010.


Authors' Addresses

   Michael Menth
   University of Wuerzburg
   room B206, Institute of Computer Science
   Am Hubland
   Wuerzburg  97074
   Germany

   Phone: +49 931 31 86644
   Email: menth@informatik.uni-wuerzburg.de













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   Matthias Hartmann
   University of Wuerzburg
   room A208, Institute of Computer Science
   Am Hubland
   Wuerzburg  97074
   Germany

   Phone: +49 931 31 83381
   Email: hartmann@informatik.uni-wuerzburg.de


   David Hock
   University of Wuerzburg
   room A208, Institute of Computer Science
   Am Hubland
   Wuerzburg  97074
   Germany

   Phone: +49 931 31 88523
   Email: david.hock@informatik.uni-wuerzburg.de































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