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Versions: 00 01 02 03 04                                                
INTERNET DRAFT     Synch of Loop Free Timer Values           Jul 2005




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

                                                               A. Atlas
                                                             Google Inc

                                                              July 2005

              Synchronisation of Loop Free Timer Values
             <draft-atlas-bryant-shand-lf-timers-00.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
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   Internet-Drafts are working documents of the Internet Engineering
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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
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   Information on the procedures with respect to rights in RFC
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   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 (2005). 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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