Networking Working Group O. Gnawali
Internet-Draft P. Levis
Intended status: Standards Track Stanford University
Expires: September 7, 2012 March 6, 2012
The Minimum Rank with Hysteresis Objective Function
draft-ietf-roll-minrank-hysteresis-of-06
Abstract
The Routing Protocol for Low Power and Lossy Networks (RPL) uses
objective functions to construct routes that optimize or constrain
the routes it selects and uses. This specification describes the
Minimum Rank Objective Function with Hysteresis (MRHOF), an objective
function that selects routes that minimize a metric, while using
hysteresis to reduce churn in response to small metric changes.
MRHOF works with metrics that are additive along a route, and the
metric it uses is determined by the metrics RPL Destination
Information Object (DIO) messages advertise.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on September 7, 2012.
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
3.2.1. When Parent Selection Runs . . . . . . . . . . . . . . 6
3.2.2. Parent Selection Algorithm . . . . . . . . . . . . . . 6
3.3. Computing Rank . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Advertising the Path Cost . . . . . . . . . . . . . . . . 8
3.5. Working Without Metric Containers . . . . . . . . . . . . 8
4. Using MRHOF for Metric Maximization . . . . . . . . . . . . . 8
5. MRHOF Variables and Parameters . . . . . . . . . . . . . . . . 9
6. Manageability . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Device Configuration . . . . . . . . . . . . . . . . . . . 10
6.2. Device Monitoring . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
An objective function specifies how RPL [I-D.ietf-roll-rpl] selects
paths. For example, if an RPL instance uses an objective function
that minimizes hop-count, RPL will select paths with minimum hop
count. RPL requires that all nodes in a network use a common OF;
relaxing this requirement may be a subject of future study.
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 these metrics
available for route selection. RPL advertises metrics in RPL
Destination Information Object (DIO) messages with a Metric Container
suboption. An objective function can use these metrics to choose
routes.
To decouple the details of an individual metric or objective function
from forwarding and routing, RPL describes routes through a value
called Rank. Rank, roughly speaking, corresponds to the distance
associated with a route. RPL defines how nodes decide on paths based
on Rank and advertise their Rank. An objective function defines how
nodes calculate Rank, based on the Rank of its potential parents,
metrics, and other network properties.
This specification describes MRHOF, an objective function for RPL.
MRHOF uses hysteresis while selecting the path with the smallest
metric value. The metric that MRHOF uses is determined by the
metrics in the DIO Metric Container. 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. In the absence of a
metric in the DIO Metric Container or the lack of a DIO Metric
Container, MRHOF defaults to using ETX to compute Rank, as described
in Section 3.5.
Because MRHOF seeks to minimize path costs as described by metrics,
it can only be used with additive metrics. MRHOF ignores metrics t
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],
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[I-D.ietf-roll-rpl], and [I-D.ietf-roll-routing-metrics].
This document introduces three 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].
Path cost: Path cost quantifies a property of an end-to-end path.
Path cost is obtained by summing up the selected metric of the
links or nodes along the path. Path cost can be used by RPL to
compare different paths.
Worst parent: The node in the parent set with the largest path cost.
3. The Minimum Rank Objective Function with Hysteresis
The Minimum Rank with Hysteresis Objective Function, MRHOF, is
designed to find the paths with the smallest path cost while
preventing excessive churn in the network. It does so by finding the
minimum cost path and switching to that path only if it is shorter
(in terms of path cost) than the current path by at least 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. Nodes MUST support at least one
of these metrics: node energy, hop count, latency, link quality
level, and ETX. Nodes SHOULD support the ETX metric. MRHOF does not
support non-additive metrics.
3.1. Computing the Path cost
Root nodes (Grounded or Floating) set the variable cur_min_path_cost
to MIN_PATH_COST.
If a non-root node does not have metrics to compute the path cost
through any of the candidate neighbors, it MUST join one of the
candidate neighbors as a RPL Leaf.
Otherwise, nodes compute the path cost for each candidate neighbor
reachable on an interface. The path cost of a neighbor represents
the cost of the path, in terms of the selected metric, from a node to
the root of the DODAG through that neighbor. A non-root node
computes a neighbor's path cost by adding two components:
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1. If the selected metric is a link metric, the selected metric for
the link to a candidate neighbor. If the selected metric is a
node metric, the selected metric for the node.
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 an interface. 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,
except if it cannot compute the path cost through any neighbor, in
which case it may join as a leaf as described above.
If the selected metric is a link metric and the 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 considered for path selection.
If the selected metric is a node metric, and the metric is not
available, the path cost through all the neighbors SHOULD be set to
MAX_PATH_COST.
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. If the selected metric is a node metric and the metric is
updated.
3. A node receives a new metric advertisement from the candidate
neighbor.
This computation MAY also be performed periodically. Too much delay
in updating the path cost after the metric is updated or a new metric
advertisement is received can lead to stale information.
3.2. Parent Selection
After computing the path cost for all the candidate neighbors
reachable through an interface for the current DODAG iteration, a
node selects the preferred parent. This process is called parent
selection. To allow hysteresis, parent selection maintains a
variable, cur_min_path_cost, which is the path cost of the current
preferred parent.
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3.2.1. When Parent Selection Runs
A MRHOF implementation SHOULD perform Parent Selection each time:
1. The path cost for an existing candidate neighbor, including the
preferred parent, changes. This condition can be checked
immediately after the path cost is computed.
2. A new candidate neighbor is inserted into the neighbor table.
The parent selection MAY be deferred until a later time. Deferring
the parent selection can delay the use of better paths available in
the network.
3.2.2. Parent Selection Algorithm
If the selected metric for a link is greater than MAX_LINK_METRIC,
the node SHOULD exclude that link from consideration during parent
selection.
A node MUST select the candidate neighbor with the lowest path cost
as its preferred parent, except as indicated below:
1. A node MAY declare itself as a Floating root, and hence have no
preferred parent, depending on system configuration.
2. If cur_min_path_cost is greater than MAX_PATH_COST, the node MAY
declare itself as a Floating root.
3. 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. This is the hysteresis component of MRHOF.
4. If ALLOW_FLOATING_ROOT is 0 and no neighbors are discovered, the
node does not have a preferred parent, and MUST set
cur_min_path_cost to MAX_PATH_COST.
If there are multiple neighbors which share the smallest path cost, a
node MAY use a different objective function to select which of these
neighbors should be considered to have the lowest cost.
A node MAY include up to PARENT_SET_SIZE-1 additional candidate
neighbors in its parent set. The cost of path through the nodes in
the parent set is smaller than or equal to the cost of the paths
through any of the nodes that are not in the parent set. If the cost
of the path through the preferred parent and the worst parent is too
large, a node MAY keep a smaller parent set than PARENT_SET_SIZE.
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Once the preferred parent is selected, the node sets its
cur_min_path_cost variable to the path cost corresponding to the
preferred parent. The value of the cur_min_path_cost is carried in
the metric container corresponding to the selected metric when DIO
messages are sent.
3.3. Computing Rank
DAG roots set their Rank to MinHopRankIncrease.
Once a non-root node selects its parent set, it can use the following
table to covert the path cost of a parent (written as Cost in the
table) to a Rank value:
+--------------------+------------+
| 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.
Rank is undefined for these node/link metrics: Node state and
attributes, throughput, and link color. If the rank is undefined,
the node must join one of the neighbors as a RPL Leaf node according
to [I-D.ietf-roll-rpl].
MRHOF uses this Rank value to compute the Rank it associates with the
path through each member of the parent set. The Rank associated with
a path through a member of the parent set is the maximum of the
corresponding calculated Rank value from above and that node's
advertised Rank plus MinHopRankIncrease.
A node sets its Rank to the maximum of three values:
1. The Rank associated with the path through the preferred parent
2. The Rank of the member of the parent set with the highest
advertised Rank plus one
3. The Rank of the path through the member of the parent set with
the highest path cost, minus MaxRankIncrease
The first case is the Rank associated with the path through the
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preferred parent. The second case covers requirement 4 of Rank
advertisements in Section 8.2.1 of [I-D.ietf-roll-rpl]. The third
case ensures that a node does not advertise a Rank which then
precludes it from using members of its parent set.
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 its
preferred parent. It then calculates the metric it will advertise in
its metric container. This value is the path cost of the member of
the parent set with the highest path cost. Thus, while
cur_min_path_cost is the cost through the preferred parent, a node
advertises the highest cost path from the node to the root through a
member of the parent set. The value of the highest cost path is
carried in the metric container corresponding to the selected metric
when DIO messages are sent.
If ETX is the selected metric, cur_min_path_cost is directly used as
Rank and never advertised in a metric container.
DODAG Roots advertise a metric value which computes to a cost of
MIN_PATH_COST.
3.5. Working Without Metric Containers
In the absence of metric container, MRHOF uses ETX as its metric. It
locally computes the ETX of links to its neighbors and adds this
value to their advertised Rank to compute the associated Rank of
routes. Once parent selection and rank computation is performed
using the ETX metric, the node advertises a Rank equal to the ETX
cost and MUST NOT include a metric container in its DIO messages.
4. Using MRHOF for Metric Maximization
MRHOF cannot be directly used for parent selection using metrics
which require finding paths with maximum value of the selected
metric, such as path reliability. It is possible to convert such a
metric maximization problem to a metric minimization problem for some
metrics and use MRHOF provided:
There is a fixed and well-known maximum metric value corresponding
to the best path. This is the path cost for the DAG root. For
example, the logarithm of the best link reliability has a value of
0.
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The metrics in the maximization problem are all negative. The
logarithm of the link reliability is always negative.
For metrics meeting the above conditions, the problem of maximizing
the metric value is equivalent to minimizing the modified metric
value, e.g., logarithm of link reliability. MRHOF is not required to
work with these metrics.
5. 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:
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 the cost of the
path through the preferred parent and the minimum cost path in
order to trigger the selection of a new preferred parent.
PARENT_SET_SIZE: The number of candidate parents, including the
preferred parent, in the parent set.
ALLOW_FLOATING_ROOT: If set to 1, allows a node to become a
floating root.
The parameter values are assigned depending on the selected metric.
The best values for these parameters should be experimentally
determined. The working group has long experience routing with the
ETX metric. Based on those experiences, these values are
RECOMMENDED:
MAX_LINK_METRIC: 10. Disallow links with greater than 10 expected
transmission count on the selected path.
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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: 172. Switch to a new path only if it is
expected to require at least 1.5 fewer transmissions than the
current path. As ETX is encoded as number of transmissions times
128, this is a threshold of 172.
PARENT_SET_SIZE: 3. If the preferred parent is not available, two
candidate parents are still available without triggering a new
round of route discovery.
ALLOW_FLOATING_ROOT: 0. Do not allow a node to become a floating
root.
6. Manageability
Section 18 of [I-D.ietf-roll-rpl] depicts the management of RPL.
This specification inherits from that section and its subsections,
with the exception that metrics as specified in
[I-D.ietf-roll-routing-metrics] are not used and do not require
management.
6.1. Device Configuration
An implementation SHOULD allow the following parameters to be
configured at installation time: MAX_LINK_METRIC, MAX_PATH_COST,
MIN_PATH_COST, PARENT_SWITCH_THRESHOLD, PARENT_SET_SIZE, and
ALLOW_FLOATING_ROOT. An implementation MAY allow these parameters to
be configured dynamically at run time once a network has been
deployed.
A MRHOF implementation SHOULD support the DODAG Configuration option
as described in [I-D.ietf-roll-rpl] and apply the parameters it
specifies. Care should be taken in the relationship between the
MRHOF PARENT_SWITCH_THRESHOLD parameter and the RPL MaxRankIncrease
parameter. For example, if MaxRankIncrease is smaller than
PARENT_SWITCH_THRESHOLD, a RPL node using MRHOF could enter a
situation where its current preferred parent causes the nodes Rank to
increase more than MaxRankIncrease but MRHOF does not change
preferred parents: this could cause the node to leave the routing
topology even though there may be other members of the parent set
which would allow the node's Rank to remain within MaxRankIncrease.
Unless configured otherwise, a MRHOF implementation SHOULD use the
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default parameters as specified in Section 5.
Because of the partially-coupled relationship between Rank and metric
values, networks using MRHOF require care in setting
MinHopRankIncrease. A large MinHopRankIncrease will cause MRHOF to
be unable to select paths with different hop counts but similar
metric values. If MinHopRankIncrease is large enough that its
increment is greater than that caused by link cost, then metrics will
be used to select a preferred parent but the advertised Rank will be
a simple hopcount. This behavior might be desirable, but it also
might be unintended: care is recommended.
6.2. Device Monitoring
A MRHOF implementation should provide an interface for monitoring its
operation. At a minimum, the information provided should include:
DAG information as specified in Section 6.3.1 of
[I-D.ietf-roll-rpl], and including the DODAGID, the RPLInstanceID,
the Mode of Operation, the Rank of this node, the current Version
Number, and the value of the Grounded flag.
A list of neighbors indicating the preferred parent. The list
should indicate, for each neighbor, the Rank, the current Version
Number, the value of the Grounded flag, and associated metrics.
7. Acknowledgements
Thanks to Antonio Grilo, Nicolas Tsiftes, Matteo Paris, JP Vasseur,
and Phoebus Chen for their comments.
8. IANA Considerations
This specification requires a value allocated from the "Objective
Code Point (OCP)" sub-registry of the "Routing Protocol for Low Power
and Lossy Networks (RPL)" registry. A value of 1 is suggested.
9. Security Considerations
This specification makes simple extensions to RPL and so is
vulnerable to and benefits from the security issues and mechanisms
described in [I-D.ietf-roll-rpl] and
[I-D.ietf-roll-security-framework]. This document does not introduce
new flows or new messages, thus requires no specific mitigation for
new threats.
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MRHOF depends on information exchanged in a number RPL protocol
elements. If those elements were compromised, then an implementation
of MRHOF might generate the wrong path for a packet, resulting in it
being misrouted. Therefore, deployments are RECOMMENDED to use RPL
security mechanisms if there is a risk that routing information might
be modified or spoofed.
10. References
10.1. Normative References
[I-D.ietf-roll-routing-metrics]
Barthel, D., Vasseur, J., Pister, K., Kim, M., and N.
Dejean, "Routing Metrics used for Path Calculation in Low
Power and Lossy Networks",
draft-ietf-roll-routing-metrics-19 (work in progress),
March 2011.
[I-D.ietf-roll-rpl]
Brandt, A., Vasseur, J., Hui, J., Pister, K., Thubert, P.,
Levis, P., Struik, R., Kelsey, R., Clausen, T., and T.
Winter, "RPL: IPv6 Routing Protocol for Low power and
Lossy Networks", draft-ietf-roll-rpl-19 (work in
progress), March 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[I-D.ietf-roll-security-framework]
Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
Lozano, "A Security Framework for Routing over Low Power
and Lossy Networks", draft-ietf-roll-security-framework-07
(work in progress), January 2012.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-05 (work in
progress), March 2011.
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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
412 Gates Hall, Stanford University
Stanford, CA 94305
USA
Email: pal@cs.stanford.edu
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