Networking Working Group O. Gnawali
Internet-Draft P. Levis
Intended status: Standards Track Stanford University
Expires: December 16, 2010 June 14, 2010
The Minimum Rank Objective Function with Hysteresis
draft-gnawali-roll-minrank-hysteresis-of-00
Abstract
Hysteresis delays the effect of changes in link metric on parent
selection. Such delay makes the topology stable despite jitters in
link metrics. The Routing Protocol for Low Power and Lossy Networks
(RPL) allows the use of objective functions to construct routes that
optimize or constrain a routing metric on the paths. This
specification describes MRHOF, an objective function that minimizes
the node rank in terms of a given metric, while using hysteresis to
prevent excessive rank churn. The use of MRHOF with RPL results in
nodes selecting stable paths that minimize the given routing metric
to the DAG roots.
Status of this Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Minimum Rank Objective Function with Hysteresis . . . . . . 4
3.1. Computing the Path metric . . . . . . . . . . . . . . . . . 4
3.2. Parent Selection . . . . . . . . . . . . . . . . . . . . . 5
3.3. Advertising the path metric . . . . . . . . . . . . . . . . 6
4. MRHOF Variables and Parameters . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
An objective function allows RPL [I-D.ietf-roll-rpl] to optimize or
constrain the routing metric of a path. RPL achieves this goal by
selecting the parent among the alternate parents as dictated by that
objective function. For example, if an RPL instance uses an
objective function that minimizes hop-count, RPL will select paths
with minimum hop count. Different objective functions optimize or
constrain metrics differently.
The nodes running RPL might use a number of metrics to describe a
link [I-D.ietf-roll-routing-metrics] and make it available for route
selection. A metric can be used by different objective functions to
optimize or constrain the metric in different ways.
This specification describes MRHOF, an objective function for RPL.
MRHOF uses hysteresis while selecting the path with the smallest
metric value. The path with the minimum metric value has different
property depending on the metric used for path selection. For
example, the use of MRHOF with the latency metric allows RPL to find
stable minimum-latency paths from the nodes to a root in the DAG
instance. The use of MRHOF with the ETX metric allows RPL to find
the stable minimum-ETX paths from the nodes to a root in the DAG
instance.
MRHOF can be used with additive metric that must be minimized on the
paths selected for routing. Although MRHOF can be used with a number
of metrics, this draft is based on experiences with the ETX metric.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This terminology used in this document is consistent with the
terminologies described in [I-D.ietf-roll-terminology],
[I-D.ietf-roll-rpl], and [I-D.ietf-roll-routing-metrics].
This document introduces two term:
Selected metric: The metric chosen by the network operator to use
for path selection. This metric can be any additive metric
listed in [I-D.ietf-roll-routing-metrics]
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Path metric: Path metric quantifies a property of an end-to-end
path. Path metric is composed using the selected metric of the
links along the path. Path metrics can be used by RPL to
compare different paths.
3. The Minimum Rank Objective Function with Hysteresis
The Minimum Rank Objective Function with Hysteresis, MRHOF, is
designed to find the paths with the smallest metric values while
preventing excessive churn in the network. It does so by switching
to the minimum metric path only if the path metric of the current
path is larger than the path metric of the minimum metric path by a
given threshold. MRHOF may be used with any additive metric listed
in [I-D.ietf-roll-routing-metrics] as long the routing objective is
to minimize the given routing metric. MRHOF cannot be used if the
routing objective is to maximize the metric.
3.1. Computing the Path metric
Nodes compute the path metric for each candidate neighbor reachable
on all the interfaces. The Path metric represents the cost of the
path, in terms of the selected metric, from a node to the root of the
DODAG through the neighbor.
Root nodes (Grounded or Floating) set the variable
cur_min_path_metric to MIN_PATH_METRIC.
A non-root node computes the path metric for a path to the root
through each candidate neighbor by adding these two components:
1. The selected metric for the link to a candidate neighbor.
2. The cur_min_path_metric advertised by that neighbor.
A node SHOULD compute the path metric for the path through each
candidate neighbor reachable through all interfaces. If a node
cannot compute the path metric for the path through a candidate
neighbor, the node MUST NOT make that candidate neighbor its
preferred parent.
If the selected metric of the link to a neighbor is not available,
the path metric for the path through that neighbor SHOULD be set to
MAX_PATH_METRIC. This metric value will prevent this path from being
considered for path selection.
The path metric corresponding to a neighbor MUST be re-computed each
time:
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1. The selected metric of the link to the candidate neighbor is
updated.
2. A node receives a new cur_min_path_metric advertisement from the
candidate neighbor.
This computation MAY also be performed periodically. However, long
intervals between periodic computation or deferring the computation
for too long after new cur_min_path_metric advertisements or updates
to the selected link metric prevents results in node making parent
selection based on stale link and path information.
3.2. Parent Selection
After computing the path metric for all the candidate neighbors
reachable through all the interfaces for the current DODAG iteration,
a node selects the preferred parent. This process is called parent
selection.
A node MUST select a candidate neighbor as its preferred parent if
the path metric corresponding to that neighbor is smaller than the
path metric corresponding to the rest of the neighbors, except as
indicated below:
1. If the smallest path metric for paths through the candidate
neighbors is smaller than cur_min_path_metric by less than
PARENT_SWITCH_THRESHOLD, the node MAY continue to use the current
preferred parent.
2. If there are multiple paths with the smallest path metric and
that smallest path metric is smaller than cur_min_path_metric by
at least PARENT_SWITCH_THRESHOLD, a node MAY use a different
objective function to select the preferred parent among the
candidates which are first hop on the path with the smallest path
metric.
3. A node MAY declare itself as a Floating root, and hence no
preferred parent, depending on the configuration.
4. If the selected metric for a link is greater than
MAX_LINK_METRIC, the node SHOULD exclude that link from
consideration for parent selection.
5. If cur_min_path_metric is greater than MAX_PATH_METRIC, the node
MAY declare itself as a Floating root.
6. If the configuration disallows a node to be a Floating root and
no neighbors are discovered, the node does not have a preferred
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parent, and MUST set cur_min_path_metric to MAX_PATH_METRIC.
3.3. Advertising the path metric
Once the preferred parent is selected, the node sets its
cur_min_path_metric variable to the path metric corresponding to the
preferred parent. Thus, cur_min_path_metric is the cost of the path
with the smallest path metric from the node to the root. The value
of the cur_min_path_metric is carried in the metric container
whenever DIO messages are sent.
4. MRHOF Variables and Parameters
MRHOF uses the following variable:
cur_min_path_metric: The path metric of the path from a node
through its preferred parent to the root computed at the last
parent selection.
MRHOF uses the following parameters:
MAX_LINK_METRIC: Maximum allowed value for the selected link
metric for each link on the path.
MAX_PATH_METRIC: Maximum allowed value for the path metric of a
selected path.
MIN_PATH_METRIC: The minimum allowed value for the path metric of
the selected path.
PARENT_SWITCH_THRESHOLD: The difference between metric of the path
through the preferred parent and the minimum-metric path to
trigger new preferred parent selection.
The parameter values are assigned depending on the selected metric.
Here is an example parameter assignment for the ETX metric:
MAX_LINK_METRIC: 10. Disallow links with greater than 10 expected
transmission count on the selected path.
MAX_PATH_METRIC: 100. Disallow paths with greater than 100
expected transmission count.
MIN_PATH_METRIC: 0. At root, the expected transmission count is
0.
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PARENT_SWITCH_THRESHOLD: 1.0. Switch to a new path only if it is
requires at least one fewer transmission than the current path.
5. Acknowledgements
6. IANA Considerations
This specification requires an allocated OCP. A value of 1 is
requested.
7. Security Considerations
Security considerations to be developed in accordance to the output
of the WG.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[I-D.ietf-roll-routing-metrics]
Vasseur, J. and D. Networks, "Routing Metrics used for
Path Calculation in Low Power and Lossy Networks",
draft-ietf-roll-routing-metrics-01 (work in progress),
October 2009.
[I-D.ietf-roll-rpl]
Winter, T., Thubert, P., and R. Team, "RPL: IPv6 Routing
Protocol for Low power and Lossy Networks",
draft-ietf-roll-rpl-05 (work in progress), December 2009.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-01 (work in
progress), May 2009.
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Authors' Addresses
Omprakash Gnawali
Stanford University
S255 Clark Center, 318 Campus Drive
Stanford, CA 94305
USA
Phone: +1 650 725 6086
Email: gnawali@cs.stanford.edu
Philip Levis
Stanford University
358 Gates Hall, Stanford University
Stanford, CA 94305
USA
Email: pal@cs.stanford.edu
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