Networking Working Group O. Gnawali
Internet-Draft P. Levis
Intended status: Standards Track Stanford University
Expires: April 14, 2011 October 11, 2010
The Minimum Rank Objective Function with Hysteresis
draft-ietf-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 the Minimum Rank Objective Function with
Hysteresis (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
roots of a Directed Acyclic Graph (DAG).
Status of this Memo
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Copyright Notice
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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 cost . . . . . . . . . . . . . . . . . . 4
3.2. Parent Selection . . . . . . . . . . . . . . . . . . . . . 5
3.3. Computing Rank . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Advertising the path cost . . . . . . . . . . . . . . . . . 6
4. MRHOF Variables and Parameters . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
An objective function allows RPL [I-D.ietf-roll-rpl] to select paths
that are best in terms of a given routing metric or select paths that
meet certain constraints in terms of the routing metric. 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.
The nodes running RPL might use a number of metrics to describe a
link or a node [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 cost 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 terms:
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 cost: Path cost quantifies a property of an end-to-end path.
Path cost is composed using the selected metric of the links
along the path. Path cost 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 path cost while
preventing excessive churn in the network. It does so by switching
to the minimum cost path only if the path cost of the current path is
larger than the path cost of the minimum cost 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 cost
Nodes compute the path cost for each candidate neighbor reachable on
all the interfaces. The Path cost 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_cost
to MIN_PATH_COST.
A non-root node computes the path cost 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 value of the selected metric in the metric container in the
DIO sent by that neighbor.
A node SHOULD compute the path cost for the path through each
candidate neighbor reachable through all the interfaces. If a node
cannot compute the path cost for the path through a candidate
neighbor, the node MUST NOT select the candidate neighbor as its
preferred parent with one exception. If the node does not have
metrics to compute the path cost through any of the candidate
neighbors, it SHOULD join one of the candidate neighbors as a leaf
node.
If the selected metric of the link to a neighbor is not available,
the path cost for the path through that neighbor SHOULD be set to
MAX_PATH_COST. This cost value will prevent this path from being
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considered for path selection.
The path cost corresponding to a neighbor SHOULD be re-computed each
time:
1. The selected metric of the link to the candidate neighbor is
updated.
2. A node receives a new 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 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 cost 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 cost corresponding to that neighbor is smaller than the path
cost corresponding to the rest of the neighbors, except as indicated
below:
1. If the smallest path cost for paths through the candidate
neighbors is smaller than cur_min_path_cost 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 cost and that
the smallest path cost is smaller than cur_min_path_cost 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 minimum cost.
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.
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5. If cur_min_path_cost is greater than MAX_PATH_COST, 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
parent, and MUST set cur_min_path_cost to MAX_PATH_COST.
3.3. Computing Rank
Once a node selects its preferred parent, it can use the following
table to covert its path cost to the DAG root through its preferred
parent (written as Cost in the table) to its rank:
+--------------------+------------+
| Node/link Metric | Rank |
+--------------------+------------+
| Node Energy | 255 - Cost |
| Hop-Count | Cost |
| Latency | Cost/65536 |
| Link Quality Level | Cost |
| ETX | Cost |
+--------------------+------------+
Table 1: Conversion of metric to rank.
Node rank is undefined for these node/link metrics: Node state and
attributes, node fanout ratio, throughput, and link color.
3.4. Advertising the path cost
Once the preferred parent is selected, the node sets its
cur_min_path_cost variable to the path cost corresponding to the
preferred parent. Thus, cur_min_path_cost is the cost of the minimum
cost path from the node to the root. The value of the
cur_min_path_cost is carried in the metric container whenever DIO
messages are sent.
4. MRHOF Variables and Parameters
MRHOF uses the following variable:
cur_min_path_cost: The cost of the path from a node through its
preferred parent to the root computed at the last parent
selection.
MRHOF uses the following parameters:
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MAX_LINK_METRIC: Maximum allowed value for the selected link
metric for each link on the path.
MAX_PATH_COST: Maximum allowed value for the path metric of a
selected path.
MIN_PATH_COST: 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_COST: 100. Disallow paths with greater than 100 expected
transmission count.
MIN_PATH_COST: 0. At root, the expected transmission count is 0.
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
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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.
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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