draft-ietf-roll-minrank-hysteresis-of-07.txt   draft-ietf-roll-minrank-hysteresis-of-08.txt 
Networking Working Group O. Gnawali Networking Working Group O. Gnawali
Internet-Draft P. Levis Internet-Draft University of Houston
Intended status: Standards Track Stanford University Intended status: Standards Track P. Levis
Expires: September 10, 2012 March 9, 2012 Expires: October 22, 2012 Stanford University
April 20, 2012
The Minimum Rank with Hysteresis Objective Function The Minimum Rank with Hysteresis Objective Function
draft-ietf-roll-minrank-hysteresis-of-07 draft-ietf-roll-minrank-hysteresis-of-08
Abstract Abstract
The Routing Protocol for Low Power and Lossy Networks (RPL) uses The Routing Protocol for Low Power and Lossy Networks (RPL) uses
objective functions to construct routes that optimize or constrain objective functions to construct routes that optimize or constrain
the routes it selects and uses. This specification describes the the routes it selects and uses. This specification describes the
Minimum Rank Objective Function with Hysteresis (MRHOF), an objective Minimum Rank with Hysteresis Objective Function (MRHOF), an objective
function that selects routes that minimize a metric, while using function that selects routes that minimize a metric, while using
hysteresis to reduce churn in response to small metric changes. hysteresis to reduce churn in response to small metric changes.
MRHOF works with metrics that are additive along a route, and the MRHOF works with metrics that are additive along a route, and the
metric it uses is determined by the metrics RPL Destination metric it uses is determined by the metrics RPL Destination
Information Object (DIO) messages advertise. Information Object (DIO) messages advertise.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 10, 2012. This Internet-Draft will expire on October 22, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Minimum Rank Objective Function with Hysteresis . . . . . 4 3. The Minimum Rank with Hysteresis Objective Function . . . . . 4
3.1. Computing the Path cost . . . . . . . . . . . . . . . . . 4 3.1. Computing the Path cost . . . . . . . . . . . . . . . . . 4
3.2. Parent Selection . . . . . . . . . . . . . . . . . . . . . 5 3.2. Parent Selection . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. When Parent Selection Runs . . . . . . . . . . . . . . 6 3.2.1. When Parent Selection Runs . . . . . . . . . . . . . . 6
3.2.2. Parent Selection Algorithm . . . . . . . . . . . . . . 6 3.2.2. Parent Selection Algorithm . . . . . . . . . . . . . . 6
3.3. Computing Rank . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Computing Rank . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Advertising the Path Cost . . . . . . . . . . . . . . . . 8 3.4. Advertising the Path Cost . . . . . . . . . . . . . . . . 8
3.5. Working Without Metric Containers . . . . . . . . . . . . 8 3.5. Working Without Metric Containers . . . . . . . . . . . . 9
4. Using MRHOF for Metric Maximization . . . . . . . . . . . . . 9 4. Using MRHOF for Metric Maximization . . . . . . . . . . . . . 9
5. MRHOF Variables and Parameters . . . . . . . . . . . . . . . . 9 5. MRHOF Variables and Parameters . . . . . . . . . . . . . . . . 9
6. Manageability . . . . . . . . . . . . . . . . . . . . . . . . 10 6. Manageability . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Device Configuration . . . . . . . . . . . . . . . . . . . 10 6.1. Device Configuration . . . . . . . . . . . . . . . . . . . 11
6.2. Device Monitoring . . . . . . . . . . . . . . . . . . . . 11 6.2. Device Monitoring . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12 10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12 10.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
An objective function specifies how RPL [I-D.ietf-roll-rpl] selects An objective function specifies how RPL [RFC6550] selects paths. For
paths. For example, if an RPL instance uses an objective function example, if an RPL instance uses an objective function that minimizes
that minimizes hop-count, RPL will select paths with minimum hop hop-count, RPL will select paths with minimum hop count. RPL
count. RPL requires that all nodes in a network use a common OF; requires that all nodes in a network use a common OF; relaxing this
relaxing this requirement may be a subject of future study. requirement may be a subject of future study.
The nodes running RPL might use a number of metrics to describe a 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 link or a node [I-D.ietf-roll-routing-metrics] and make these metrics
available for route selection. RPL advertises metrics in RPL available for route selection. RPL advertises metrics in RPL
Destination Information Object (DIO) messages with a Metric Container Destination Information Object (DIO) messages with a Metric Container
suboption. An objective function can use these metrics to choose suboption. An objective function can use these metrics to choose
routes. routes.
To decouple the details of an individual metric or objective function To decouple the details of an individual metric or objective function
from forwarding and routing, RPL describes routes through a value from forwarding and routing, RPL describes routes through a value
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metrics in the DIO Metric Container. For example, the use of MRHOF metrics in the DIO Metric Container. For example, the use of MRHOF
with the latency metric allows RPL to find stable minimum-latency 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 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 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 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 metric in the DIO Metric Container or the lack of a DIO Metric
Container, MRHOF defaults to using ETX to compute Rank, as described Container, MRHOF defaults to using ETX to compute Rank, as described
in Section 3.5. in Section 3.5.
Because MRHOF seeks to minimize path costs as described by metrics, Because MRHOF seeks to minimize path costs as described by metrics,
it can only be used with additive metrics. MRHOF ignores metrics t it can only be used with additive metrics. MRHOF ignores metrics
that are not additive.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC "OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119]. 2119 [RFC2119].
This terminology used in this document is consistent with the The terminologies used in this document are consistent with the
terminologies described in [I-D.ietf-roll-terminology], terminologies described in [I-D.ietf-roll-terminology] and
[I-D.ietf-roll-routing-metrics].
[I-D.ietf-roll-rpl], and [I-D.ietf-roll-routing-metrics]. The terminologies used in this document are also consistent with the
terminologies described in RFC 6550 [RFC6550], except the term Rank.
In this document, Rank refers to the value of the Rank field, not
DAGRank as in RFC 6550 [RFC6550].
This document introduces three terms: This document introduces three terms:
Selected metric: The metric chosen by the network operator to use Selected metric: The metric chosen by the network operator to use
for path selection. This metric can be any additive metric for path selection. This metric can be any additive metric
listed in [I-D.ietf-roll-routing-metrics]. listed in [I-D.ietf-roll-routing-metrics].
Path cost: Path cost quantifies a property of an end-to-end path. 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 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 links or nodes along the path. Path cost can be used by RPL to
compare different paths. compare different paths.
Worst parent: The node in the parent set with the largest path cost. Worst parent: The node in the parent set with the largest path cost.
3. The Minimum Rank Objective Function with Hysteresis 3. The Minimum Rank with Hysteresis Objective Function
The Minimum Rank with Hysteresis Objective Function, MRHOF, is The Minimum Rank with Hysteresis Objective Function, MRHOF, is
designed to find the paths with the smallest path cost while designed to find the paths with the smallest path cost while
preventing excessive churn in the network. It does so by finding the 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 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 (in terms of path cost) than the current path by at least a given
threshold. threshold.
MRHOF may be used with any additive metric listed in MRHOF may be used with any additive metric listed in
[I-D.ietf-roll-routing-metrics] as long the routing objective is to [I-D.ietf-roll-routing-metrics] as long the routing objective is to
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| Latency | Cost/65536 | | Latency | Cost/65536 |
| Link Quality Level | Cost | | Link Quality Level | Cost |
| ETX | Cost | | ETX | Cost |
+--------------------+------------+ +--------------------+------------+
Table 1: Conversion of metric to rank. Table 1: Conversion of metric to rank.
Rank is undefined for these node/link metrics: node state and Rank is undefined for these node/link metrics: node state and
attributes, throughput, and link color. If the rank is undefined, attributes, throughput, and link color. If the rank is undefined,
the node must join one of the neighbors as a RPL Leaf node according the node must join one of the neighbors as a RPL Leaf node according
to [I-D.ietf-roll-rpl]. to [RFC6550].
MRHOF uses this Rank value to compute the Rank it associates with the MRHOF uses this Rank value to compute the Rank it associates with the
path through each meqber of the parent set. The Rank associated with 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 two a path through a member of the parent set is the maximum of two
values. The first is corresponding Rank value calculated with the values. The first is corresponding Rank value calculated with the
table above, the second is the that node's advertised Rank plus table above, the second is that node's advertised Rank plus
MinHopRankIncrease. MinHopRankIncrease.
A node sets its Rank to the maximum of three values: A node sets its Rank to the maximum of three values:
1. The Rank calculated for the path through the preferred parent 1. The Rank calculated for the path through the preferred parent
2. The Rank of the member of the parent set with the highest 2. The Rank of the member of the parent set with the highest
advertised Rank plus one advertised Rank, rounded to the next higher integral Rank, ie, to
MinHopRankIncrease * (1 + floor(Rank/MinHopRankIncrease)).
3. The largest calculated Rank among paths through the parent set, 3. The largest calculated Rank among paths through the parent set,
minus MaxRankIncrease minus MaxRankIncrease
The first case is the Rank associated with the path through the The first case is the Rank associated with the path through the
preferred parent. The second case covers requirement 4 of Rank preferred parent. The second case covers requirement 5 of Rank
advertisements in Section 8.2.1 of [I-D.ietf-roll-rpl]. The third advertisements in Section 8.2.1 of [RFC6550]. The third case ensures
case ensures that a node does not advertise a Rank which then that a node does not advertise a Rank which then precludes it from
precludes it from using members of its parent set. using members of its parent set.
Note that the third case means that a node advertises a conservative Note that the third case means that a node advertises a conservative
Rank value based on members of its parent set, which might be Rank value based on members of its parent set. This conservative
significantly higher than the Rank calculated for the path through value might be significantly higher than the Rank calculated for the
the preferred parent. Accordingly, picking a parent set whose paths path through the preferred parent. Accordingly, picking a parent set
have a large range of Ranks will likely result in sub-optimal whose paths have a large range of Ranks will likely result in sub-
routing: nodes will not choose good paths because they are advertised optimal routing: nodes might not choose good paths because they are
as much worse than they actually are. The exact selection of a advertised as much worse than they actually are. The exact selection
parent set is an implementation decision. of a parent set is an implementation decision.
3.4. Advertising the Path Cost 3.4. Advertising the Path Cost
Once the preferred parent is selected, the node sets its Once the preferred parent is selected, the node sets its
cur_min_path_cost variable to the path cost corresponding to its cur_min_path_cost variable to the path cost corresponding to its
preferred parent. It then calculates the metric it will advertise in preferred parent. It then calculates the metric it will advertise in
its metric container. This value is the path cost of the member of its metric container. This value is the path cost of the member of
the parent set with the highest path cost. Thus, while the parent set with the highest path cost. Thus, while
cur_min_path_cost is the cost through the preferred parent, a node 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 advertises the highest cost path from the node to the root through a
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PARENT_SET_SIZE: 3. If the preferred parent is not available, two PARENT_SET_SIZE: 3. If the preferred parent is not available, two
candidate parents are still available without triggering a new candidate parents are still available without triggering a new
round of route discovery. round of route discovery.
ALLOW_FLOATING_ROOT: 0. Do not allow a node to become a floating ALLOW_FLOATING_ROOT: 0. Do not allow a node to become a floating
root. root.
6. Manageability 6. Manageability
Section 18 of [I-D.ietf-roll-rpl] depicts the management of RPL. Section 18 of [RFC6550] depicts the management of RPL. This
This specification inherits from that section and its subsections, specification inherits from that section and its subsections, with
with the exception that metrics as specified in the exception that metrics as specified in
[I-D.ietf-roll-routing-metrics] are not used and do not require [I-D.ietf-roll-routing-metrics] are not used and do not require
management. management.
6.1. Device Configuration 6.1. Device Configuration
An implementation SHOULD allow the following parameters to be An implementation SHOULD allow the following parameters to be
configured at installation time: MAX_LINK_METRIC, MAX_PATH_COST, configured at installation time: MAX_LINK_METRIC, MAX_PATH_COST,
MIN_PATH_COST, PARENT_SWITCH_THRESHOLD, PARENT_SET_SIZE, and MIN_PATH_COST, PARENT_SWITCH_THRESHOLD, PARENT_SET_SIZE, and
ALLOW_FLOATING_ROOT. An implementation MAY allow these parameters to ALLOW_FLOATING_ROOT. An implementation MAY allow these parameters to
be configured dynamically at run time once a network has been be configured dynamically at run time once a network has been
deployed. deployed.
A MRHOF implementation SHOULD support the DODAG Configuration option A MRHOF implementation SHOULD support the DODAG Configuration option
as described in [I-D.ietf-roll-rpl] and apply the parameters it as described in [RFC6550] and apply the parameters it specifies.
specifies. Care should be taken in the relationship between the Care should be taken in the relationship between the MRHOF
MRHOF PARENT_SWITCH_THRESHOLD parameter and the RPL MaxRankIncrease PARENT_SWITCH_THRESHOLD parameter and the RPL MaxRankIncrease
parameter. For example, if MaxRankIncrease is smaller than parameter. For example, if MaxRankIncrease is smaller than
PARENT_SWITCH_THRESHOLD, a RPL node using MRHOF could enter a PARENT_SWITCH_THRESHOLD, a RPL node using MRHOF could enter a
situation where its current preferred parent causes the nodes Rank to situation where its current preferred parent causes the nodes Rank to
increase more than MaxRankIncrease but MRHOF does not change increase more than MaxRankIncrease but MRHOF does not change
preferred parents: this could cause the node to leave the routing preferred parents: this could cause the node to leave the routing
topology even though there may be other members of the parent set topology even though there may be other members of the parent set
which would allow the node's Rank to remain within MaxRankIncrease. which would allow the node's Rank to remain within MaxRankIncrease.
Unless configured otherwise, a MRHOF implementation SHOULD use the Unless configured otherwise, a MRHOF implementation SHOULD use the
default parameters as specified in Section 5. default parameters as specified in Section 5.
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increment is greater than that caused by link cost, then metrics will 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 be used to select a preferred parent but the advertised Rank will be
a simple hopcount. This behavior might be desirable, but it also a simple hopcount. This behavior might be desirable, but it also
might be unintended: care is recommended. might be unintended: care is recommended.
6.2. Device Monitoring 6.2. Device Monitoring
A MRHOF implementation should provide an interface for monitoring its A MRHOF implementation should provide an interface for monitoring its
operation. At a minimum, the information provided should include: operation. At a minimum, the information provided should include:
DAG information as specified in Section 6.3.1 of DAG information as specified in Section 6.3.1 of [RFC6550], and
[I-D.ietf-roll-rpl], and including the DODAGID, the RPLInstanceID, including the DODAGID, the RPLInstanceID, the Mode of Operation,
the Mode of Operation, the Rank of this node, the current Version the Rank of this node, the current Version Number, and the value
Number, and the value of the Grounded flag. of the Grounded flag.
A list of neighbors indicating the preferred parent. The list A list of neighbors indicating the preferred parent. The list
should indicate, for each neighbor, the Rank, the current Version should indicate, for each neighbor, the Rank, the current Version
Number, the value of the Grounded flag, and associated metrics. Number, the value of the Grounded flag, and associated metrics.
7. Acknowledgements 7. Acknowledgements
Thanks to Antonio Grilo, Nicolas Tsiftes, Matteo Paris, JP Vasseur, Thanks to Antonio Grilo, Nicolas Tsiftes, Matteo Paris, JP Vasseur,
and Phoebus Chen for their comments. and Phoebus Chen for their comments.
8. IANA Considerations 8. IANA Considerations
This specification requires a value allocated from the "Objective This specification requires a value allocated from the "Objective
Code Point (OCP)" sub-registry of the "Routing Protocol for Low Power Code Point (OCP)" sub-registry of the "Routing Protocol for Low Power
and Lossy Networks (RPL)" registry. A value of 1 is suggested. and Lossy Networks (RPL)" registry. A value of 1 is suggested.
9. Security Considerations 9. Security Considerations
This specification makes simple extensions to RPL and so is This specification makes simple extensions to RPL and so is
vulnerable to and benefits from the security issues and mechanisms vulnerable to and benefits from the security issues and mechanisms
described in [I-D.ietf-roll-rpl] and described in [RFC6550] and [I-D.ietf-roll-security-framework]. This
[I-D.ietf-roll-security-framework]. This document does not introduce document does not introduce new flows or new messages, thus requires
new flows or new messages, thus requires no specific mitigation for no specific mitigation for new threats.
new threats.
MRHOF depends on information exchanged in a number RPL protocol MRHOF depends on information exchanged in a number RPL protocol
elements. If those elements were compromised, then an implementation elements. If those elements were compromised, then an implementation
of MRHOF might generate the wrong path for a packet, resulting in it of MRHOF might generate the wrong path for a packet, resulting in it
being misrouted. Therefore, deployments are RECOMMENDED to use RPL being misrouted. Therefore, deployments are RECOMMENDED to use RPL
security mechanisms if there is a risk that routing information might security mechanisms if there is a risk that routing information might
be modified or spoofed. be modified or spoofed.
10. References 10. References
10.1. Normative References 10.1. Normative References
[I-D.ietf-roll-routing-metrics] [I-D.ietf-roll-routing-metrics]
Barthel, D., Vasseur, J., Pister, K., Kim, M., and N. Barthel, D., Vasseur, J., Pister, K., Kim, M., and N.
Dejean, "Routing Metrics used for Path Calculation in Low Dejean, "Routing Metrics used for Path Calculation in Low
Power and Lossy Networks", Power and Lossy Networks",
draft-ietf-roll-routing-metrics-19 (work in progress), draft-ietf-roll-routing-metrics-19 (work in progress),
March 2011. 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 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
10.2. Informative References 10.2. Informative References
[I-D.ietf-roll-security-framework] [I-D.ietf-roll-security-framework]
Tsao, T., Alexander, R., Dohler, M., Daza, V., and A. Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
Lozano, "A Security Framework for Routing over Low Power Lozano, "A Security Framework for Routing over Low Power
and Lossy Networks", draft-ietf-roll-security-framework-07 and Lossy Networks", draft-ietf-roll-security-framework-07
(work in progress), January 2012. (work in progress), January 2012.
[I-D.ietf-roll-terminology] [I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-05 (work in Networks", draft-ietf-roll-terminology-05 (work in
progress), March 2011. progress), March 2011.
Authors' Addresses Authors' Addresses
Omprakash Gnawali Omprakash Gnawali
Stanford University University of Houston
S255 Clark Center, 318 Campus Drive PGH 577, University of Houston
Stanford, CA 94305 Houston, TX 77204
USA USA
Phone: +1 650 725 6086 Phone: +1 713 743 3356
Email: gnawali@cs.stanford.edu Email: gnawali@cs.uh.edu
Philip Levis Philip Levis
Stanford University Stanford University
412 Gates Hall, Stanford University 412 Gates Hall, Stanford University
Stanford, CA 94305 Stanford, CA 94305
USA USA
Email: pal@cs.stanford.edu Email: pal@cs.stanford.edu
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