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Versions: 00 01 02 03 04                                                
INTERNET DRAFT     Synch of Loop Free Timer Values           Mar 2006




Network Working Group                                        S. Bryant
Internet Draft                                                M. Shand
Expiration Date: September 2006                          Cisco Systems

                                                              A. Atlas
                                                            Google Inc

                                                            March 2005

              Synchronisation of Loop Free Timer Values
             <draft-atlas-bryant-shand-lf-timers-01.txt>


Status of this Memo

  By submitting this Internet-Draft, each author represents that any
  applicable patent or other IPR claims of which he or she is aware
  have been or will be disclosed, and any of which he or she becomes
  aware will be disclosed, in accordance with Section 6 of BCP 79.

  Internet-Drafts are working documents of the Internet Engineering
  Task Force (IETF), its areas, and its working groups.  Note that
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  Internet-Drafts.

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  http://www.ietf.org/1id-abstracts.html

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Abstract
  This draft describes a mechanism that enables routers to agree on a
  common convergence delay time for use in loop-free convergence.

Conventions used in this document

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
  this document are to be interpreted as described in RFC 2119
  [RFC2119].



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1.     Introduction

  Most of the loop-free convergence mechanisms [LFFWK] require one or
  more convergence delay timers that MUST have a duration that is
  consistent throughout the routing domain. This time is the worst
  case time that any router will take to calculate the new topology,
  and to make the necessary changes to the FIB. The timer is used by
  the routers to know when it is safe to transition between the loop-
  free convergence states.

  The time taken by a router to complete each phase of the loop-free
  transition will be dependent on the size of the network and the
  design and implementation of the router. It can therefore be
  expected that the optimum delay will need to be tuned from time to
  time as the network evolves.

  Manual configuration of the timer is fraught for two reasons,
  firstly it is always difficult to ensure that the correct value is
  installed in all of the routers, and secondly, if any change is
  introduced into the network that results in a need to change the
  timer, for example due to a change in hardware or software version,
  then all of the routers need to be reconfigured to use the new
  timer value.

  It is therefore desirable that a means be provided by which the
  convergence delay timer can be automatically synchronized
  throughout the network.



2.    Required Properties

  The timer synchronization mechanism MUST have the following
  properties:

     o The convergence delay time must be consistent amongst all
       routers that are converging on the new topology.

     o The convergence delay time must be the highest delay required
       by any router in the new topology.

     o The mechanism must increase the delay when a new router in
       introduced to the network that requires a higher delay than is
       currently in use.

     o When the router that had the longest delay requirements is
       removed from the topology, the convergence delay timer value
       must, within some reasonable time, be reduced to the longest
       delay required by the remaining routers.



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     o It must be possible for a router to change the convergence
       delay timer value that it requires.

     o A router which is in multiple routing areas, or is running
       multiple routing protocols may signal a different loop-free
       convergence delay for each area, and for each protocol.

  How a router determines the time that it needs to execute each
  convergence phase is an implementation issue, and outside the scope
  of this specification. However a router that dynamically determines
  its proposed timer value must do so in such a way that it does not
  cause the synchronized value to continually fluctuate.



3.    Mechanism

  The following mechanism is proposed.

  A new information element is introduced into the routing protocol
  that specifies the maximum time (in milliseconds) that the router
  will take to calculate the new topology and to update its FIB as a
  result of any topology change.

  When a topology change occurs, the largest convergence delay time
  required by any router in the new topology is used by the loop-free
  convergence mechanism.

  If a routing protocol message is issued that changes the
  convergence delay timer value, but does not change the topology,
  the new timer value MUST be taken into consideration during the
  next loop-free transition, but MUST NOT instigate a loop-free
  transition.

  If a routing protocol message is issued that changes the
  convergence timer value and changes the topology, a loop-free
  transition is instigated and the new timer value is taken into
  consideration.

  The loop-free convergence mechanism should specify the action to be
  taken if a timer change (only) message and a topology change
  message are independently generated during the hold-off time. A
  suitable action would be to take the same action that would be
  taken if two uncorrelated topology changes occurred in the network.

  All routers that support loop-free convergence MUST advertise a
  loop-free convergence delay time. The loop-free convergence
  mechanism MUST specify the action to be taken if a router does not
  advertise a convergence delay time.





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4.    Protocol Details

  This section describes the protocol changes needed to implement the
  timer synchronization function.


4.1.     ISIS

  The controlled convergence timer value will be carried in a new
  Sub-TLV of the capability TLV as defined in [ISIS-CAP].

  This draft defines one such SUB-TLV where the type is for the
  worst-case FIB compute/install time, the value is 16 bits and is
  specified in milliseconds; this gives a max value of about 65s.

  The format of the Sub-TLV is as shown below.

      Sub-TLV FIB-Convergence Timer

      TYPE: <TBD>

      Length: 2 octets

      Value: <16-bit timer value expressed in milliseconds>

  This MUST be carried in a capability TLV with the S-bit set to zero
  (indicating that it MUST NOT be leaked between levels).


4.2.     OSPF

  A new type-10 opaque LSA (the controlled convergence LSA) will be
  defined as part of OSPF changed needed to define the loop-free
  convergence mechanism. This will consist of one or more TLVs.  This
  draft defines one such TLV where the type is for the worst-case FIB
  compute/install time, the value is 16 bits and is specified in
  milliseconds; this gives a max value of about 65s.



5.    IANA considerations

  There will be IANA considerations that arise as a result of this
  draft, but they are not yet determined.



6.    Security Considerations

  If an abnormally large timer value is proposed by a router, the
  there is a danger that the loop-free convergence process will take
  an excessive time. If during that time the routing protocol signals
  the need for another transition, the loop-free transition will be

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  abandoned and the default best case (traditional) convergence
  mechanism used.

  It is still undesirable that the routers select a convergence delay
  time that has an excessive value. The maximum value that can be
  specified in the LSP/LSA is limited through the use of a 16 bit
  field to about 65 seconds. When sufficient implementation
  experience is gained, an architectural constant will be specified
  which sets the upper limit of the convergence delay timer.



7.    Intellectual Property Statement


  The IETF takes no position regarding the validity or scope of any
  Intellectual Property Rights or other rights that might be claimed
  to pertain to the implementation or use of the technology described
  in this document or the extent to which any license under such
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  it has made any independent effort to identify any such rights.
  Information on the procedures with respect to rights in RFC
  documents can be found in BCP 78 and BCP 79.

  Copies of IPR disclosures made to the IETF Secretariat and any
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  specification can be obtained from the IETF on-line IPR repository
  at http://www.ietf.org/ipr.

  The IETF invites any interested party to bring to its attention any
  copyrights, patents or patent applications, or other proprietary
  rights that may cover technology that may be required to implement
  this standard.  Please address the information to the IETF at
  ietf-ipr@ietf.org.




8.     Full copyright statement

  Copyright (C) The Internet Society (2006). This document is subject
  to the rights, licenses and restrictions contained in BCP 78, and
  except as set forth therein, the authors retain all their rights.

  This document and the information contained herein are provided on
  an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
  REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
  THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
  THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR


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  ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
  PARTICULAR PURPOSE.



9.    Normative References

  Internet-drafts are works in progress available from
  <http://www.ietf.org/internet-drafts/>

   [ISIS-CAP]     Vasseur JP. et al,  "IS-IS extensions for
                  advertising router information",  draft-ietf-
                  isis-caps-03.txt , Work in Progress.





10.     Informative References

  Internet-drafts are works in progress available from
  <http://www.ietf.org/internet-drafts/>

  [LFFWK]       Bryant, S., Shand, M., A Framework for Loop-
                 free Convergence <draft-bryant-shand-lf-conv-
                 frmwk-01.txt>, (work on progress)




11.    Acknowledgements

  Our thanks to Stefano Previdi for his useful coments.





















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12.    Authors' Addresses

  Alia K. Atlas
  1600 Amphitheatre Parkway
  Mountain View CA 94043    Email: akatlas@gmail.com

  Stewart Bryant
  Cisco Systems,
  250, Longwater,
  Green Park,
  Reading, RG2 6GB,
  United Kingdom.            Email: stbryant@cisco.com

  Mike Shand
  Cisco Systems,
  250, Longwater,
  Green Park,
  Reading, RG2 6GB,
  United Kingdom.            Email: mshand@cisco.com

































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