INTERNET DRAFT Synch of Loop Free Timer Values Oct 2006
Network Working Group S. Bryant
Internet Draft M. Shand
Expiration Date: May 2007 Cisco Systems
A. Atlas
Google Inc
October 2006
Synchronisation of Loop Free Timer Values
<draft-atlas-bryant-shand-lf-timers-02.txt>
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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
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the need for another transition, the loop-free transition will be
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
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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-06.txt , Work in Progress.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC
2119, March 1997.
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-03.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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