Internet DRAFT - draft-tripathi-roll-rpl-simulation
draft-tripathi-roll-rpl-simulation
Networking Working Group J. Tripathi, Ed.
Internet-Draft J. de Oliveira, Ed.
Intended status: Informational Drexel University
Expires: October 5, 2012 JP. Vasseur, Ed.
Cisco Systems, Inc.
April 3, 2012
Performance Evaluation of Routing Protocol for Low Power and Lossy
Networks (RPL)
draft-tripathi-roll-rpl-simulation-08
Abstract
This document presents a performance evaluation of the Routing
Protocol for Low power and Lossy Networks (RPL) for a small outdoor
deployment of sensor nodes and for a large scale smart meter network.
Detailed simulations are carried out to produce several routing
performance metrics using these real-life deployment scenarios.
Please refer to the pdf version of this document, which includes
several plots for the performance metrics not shown in the txt
version.
note
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Methodology and Simulation Setup . . . . . . . . . . . . . . . 5
4. Performance Metrics . . . . . . . . . . . . . . . . . . . . . 7
4.1. Common Assumptions . . . . . . . . . . . . . . . . . . . . 7
4.2. Path Quality . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Routing Table Size . . . . . . . . . . . . . . . . . . . . 10
4.4. Delay Bound for P2P Routing . . . . . . . . . . . . . . . 10
4.5. Control Packet Overhead . . . . . . . . . . . . . . . . . 11
4.6. Loss of Connectivity . . . . . . . . . . . . . . . . . . . 13
5. RPL in a Building Automation Routing Scenario . . . . . . . . 16
5.1. Path Quality . . . . . . . . . . . . . . . . . . . . . . . 17
5.2. Delay . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6. RPL in a Large Scale Network . . . . . . . . . . . . . . . . . 17
6.1. Path Quality . . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Delay . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.3. Control Packet Overhead . . . . . . . . . . . . . . . . . 19
7. Scaling Property and Routing Stability . . . . . . . . . . . . 20
8. Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Terminology
Please refer to the following document for terminology:
[I-D.ietf-roll-terminology]. In addition, the following terms are
specified:
PDR: Packet Delivery Ratio.
CDF: Cumulative Distribution Function.
Expected Transmission Count (ETX Metric): The expected number of
transmissions to reach the next hop is determined as the inverse
of the link PDR. Consequently, in every hop, if the link quality
(PDR) is high, the expected number of transmission to reach the
next hop may be as low as 1. However, if the PDR for the
particular link is low, multiple transmissions may be needed.
ETX Path Cost: The ETX path cost metric is determined as the
summation of the ETX value for each link on the route a packet
takes towards the destination.
ETX Path Cost Stretch: The ETX path stretch is defined as the
difference between the number of expected transmissions (ETX
Metric) taken by a packet traveling from source to destination,
following a route determined by RPL and a route determined by a
hypothetical ideal shortest path routing protocol (using link ETX
as the metric).
ETX Fractional Stretch (Fractional Stretch Factor of link ETX
Metric Against Ideal Shortest Path): The fractional path stretch
is the ratio of ETX path stretch to ETX path cost for the shortest
path route for the source-destination pair.
Hop Distance Stretch (Stretch Factor for Node Hop Distance Against
Ideal Shortest Path): The hop stretch is defined as the difference
between the number of hops taken by a packet traveling from source
to destination, following a route determined by RPL and by a
hypothetical ideal shortest path algorithm, both using ETX as the
link cost. The Fractional Hop Distance Stretch is computed as the
ratio of path stretch to count value between a source-destination
pair for the hypothetical shortest path route optimizing ETX path
cost.
2. Introduction
Designing a routing protocol for Low power and Lossy link Networks
(LLNs) imposes great challenges, mainly due to low data rates, high
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probability of packet delivery failure, and strict energy constraint
in the nodes. The IETF ROLL Working Group took on this task and
specified the Routing Protocol for Low power and Lossy Networks (RPL)
in [RFC6550].
RPL is designed to meet the core requirements specified in [RFC5826],
[RFC5867], [RFC5673] and [RFC5548].
This document's contribution is to provide a performance evaluation
of RPL with respect to several metrics of interest. This is
accomplished using real data and topologies in a discrete event
simulator developed to reproduce the protocol behavior.
The following metrics are evaluated:
o Path quality metrics, such as ETX path cost, ETX path stretch, ETX
fractional stretch, hop distance stretch, as defined in Section 1
(Terminology);
o Control plane overhead;
o End-to-end delay between nodes;
o Ability to cope with unstable situations (link churns, node
dying);
o Required resource constraints on nodes (routing table size).
Some of these metrics are mentioned in the aforementioned RFCs,
whereas others have been introduced considering the challenges and
unique requirements of LLNs, as discussed in [RFC6550]. For example,
routing in a home automation deployment has strict time bounds on
protocol convergence after any change in topology as mentioned in
section 3.4 of [RFC5826]. [RFC5673] requires bounded and guaranteed
end-to-end delay for routing in an industrial deployment and
[RFC5548] requires comparatively loose bound on latency for end-to-
end communication. [RFC5548] mandates scalability in terms of
protocol performance for a network of size ranging from 10^2 to 10^4
nodes.
Although simulation cannot prove formally that a protocol operates
properly in all situations, it can give a good level of confidence in
protocol behavior in highly stressful conditions, if and only if real
life data are used. Simulation is particularly useful when
theoretical model assumptions may not be applicable to such networks
and scenarios. In this document, real deployed network data traces
have been used to model link behaviors and network topologies.
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3. Methodology and Simulation Setup
In the context of this document, RPL has been simulated using OMNET++
[OMNETpp], a well-known discrete event based simulator written in C++
and NED. Castalia-2.2 [Castalia-2.2] has been used as Wireless
Sensor Network Simulator framework within OMNET++. The output and
events in the simulating are visualized with the help of the Network
AniMator or NAM, which is distributed with NS (Network Simulator)
[NS-2].
Note that NS or any of its versions are not used in this simulation
study. Only the visualization tool was borrowed for verification
purposes.
In contrast with theoretical models, which may have assumptions not
applicable to lossy links, real-life data was used for two aspects of
the simulations:
* Link Failure Model: Derived from time varying real network traces
containing packet delivery probability for each link, over all
channels, for both indoor network deployment and outdoor network
deployment.
* Topology: Gathered from real-life deployment (traces mentioned
above) as opposed to random topology simulations.
A 45 node topology, deployed as an outdoor network, shown in Figure
1, and a 2442 node topology, gathered from a smart meter network
deployment, were used in the simulations. In Figure 1, links between
a most preferred parent and child nodes are shown in red. Links
which are shown in black are also part of the topology, but are not
between a preferred parent and child node.
Figure 1
Figure 1: Outdoor network topology with 45 nodes.
Note that this is just a start to validate the simulation before
using large scale networks.
A set of time varying link quality data was gathered from real
network deployment to form a database used for the simulations. Each
link in the topology randomly 'picks up' a link model (trace) from
the database. Each link has a Packet Delivery Ratio (PDR) that
varies with time (in the simulation, a new PDR is read from the
database every 10 minutes) according to the gathered data. Packets
are dropped randomly from that link with probability (1 - PDR). Each
time a packet is about to be sent, the module generates a random
number using the Mersenne Twister Random number generation method.
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The random number is compared to the PDR to determine whether the
packet should be dropped. Note that each link uses a different
random number generator to maintain true randomness in the simulator,
and to avoid correlation between links. Also, the packet drop
applies to all kinds of data and control packets (RPL) such as the
DIO, DAO, DIS packets defined in [RFC6550]. Figure 2 shows a typical
temporal characteristic of links from the indoor network traces used
in the simulations. The figure shows several links with perfect
connectivity, some links with PDR as low as 10% and several for which
the PDR may vary from 30% to 80%, sharply changing back and forth
between high value (strong connectivity) and low value (weak
connectivity).
Figure 2
Figure 2: Example of link characteristics.
In the RPL simulator, the LBR (LLN Border Router) or the DAG root
first initiates sending out DIO messages, and the DAG is gradually
constructed. RPL makes use of trickle timers: the protocol sets a
minimum time period, with which the nodes start re-issuing DAOs, and
this minimum period is denoted by the parameter I_min. RPL also sets
an upper limit on how many times this time period can be doubled, and
is denoted by the parameter I_doubling, as defined in [RFC6550]. For
the simulation, I_min is initially set to 1 second and I_doubling is
equal to 16, and therefore the maximum time between two consecutive
DIO emissions by a node (under a steady network condition) is 18.2
hours. The trickle time interval for emitting DIO message assumes
the initial value of 1 second, and then changes over simulation time
as mentioned in [RFC6206].
Another objective of this study is to give insight to the network
administrator on how to tweak the trickle values. These
recommendations could then be used in applicability statement
documents.
Each node in the network, other than the LBR or DAG root, also emits
DAO messages as specified in [RFC6550], to initially populate the
routing tables with the prefixes received from children via the DAO
messages to support Point-to-Point (P2P) and Point-to-Multipoint
traffic (P2MP) in the "down" direction. During these simulations, it
is assumed that each node is capable of storing route information for
other nodes in the network (storing mode of RPL).
For nodes implementing RPL, as expected, the routing table memory
requirement varies according to the position in the DODAG
(Destination Oriented Directed Acyclic Graph). The (worst-case)
assumption is made that there is no route summarization (aggregation)
in the network. Thus a node closer to the DAG will have to store
more entries in its routing table. It is also assumed that all nodes
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have equal memory capacity to store the routing states.
For simulations of the indoor network, each node sends traffic
according to a Constant Bit Rate (CBR) to all other nodes in the
network, over the simulation period. Each node generates a new data
packet every 10 seconds. Each data packet has a size of 127 bytes
including 802.15.4 PHY/MAC headers and RPL packet headers. All
control packets are also encapsulated with 802.15.4 PHY/MAC headers.
To simulate a more realistic scenario, 80% of the generated packets
by each node are destined to the root, and the remaining 20% of the
packets are uniformly assigned as destined to nodes other than the
root. Therefore the root receives a considerably larger amount of
data than other nodes. These values may be revised when studying P2P
traffic so as to have a majority of traffic going to all nodes as
opposed to the root. In the later part of the simulation, a typical
home/building routing scenario is also simulated and different path
quality metrics are computed for that traffic pattern.
The packets are routed through the DODAG built by RPL according to
the mechanisms specified in [RFC6550].
A number of RPL parameters are varied (such as the packet rate from
each source, time period for emitting new DAG sequence number) to
observe their effect on the performance metric of interest.
4. Performance Metrics
4.1. Common Assumptions
As the DAO messages are used to feed the routing tables in the
network, they grow with time and size of the network. Nevertheless,
no constraint was imposed on the size of the routing table nor on how
much information the node can store. The routing table size is not
expressed in terms of Kbyte of memory usage but measured in terms of
number of entries for each node. Each entry has the next hop node
and path cost associated with the destination node.
The link ETX (Expected Transmission Count) metric is used to build
the DODAG and is specified in [RFC6551].
4.2. Path Quality
Hop Count: For each source-destination pair, the number of hops for
both RPL and shortest path routing is computed. Shortest path
routing refers to a hypothetical ideal routing protocol that would
always provide the shortest path in terms of ETX path cost (or
whichever metric is used) in the network.
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The Cumulative Distribution Function (CDF) of the hop count for all
paths (n*(n-1) in an n-node network) in the network with respect to
the hop count is plotted in Figure 3 for both RPL and shortest path
routing. One can observe that the CDF corresponding to 4 hops is
around 80% for RPL and 90% for shortest path routing. In other
words, for the given topology, 90% of paths have a path length of 4
hops or less with an ideal shortest path routing methodology, whereas
in RPL Point-to-Point (P2P) routing, 90% of the paths will have a
length of no more than 5 hops. This result indicates that despite
having a non-optimized P2P routing scheme, the path quality of RPL is
close to an optimized P2P routing mechanism for the topology in
consideration. Another reason for this may relate to the fact that
the DAG root is at the center of the network, thus routing through
the DAG root is often close to an optimal (shortest path) routing.
This result may be different in a topology where the DAG root is
located at one end of the network.
Figure 3
Figure 3: CDF of hop count versus hop count.
ETX Path Cost: In the simulation, the total ETX path cost (defined in
the Terminology section) from source to destination for each packet
is computed.
Figure 4 shows the CDF of the total ETX path cost, both with RPL and
shortest path routing. Here also one can observe that the ETX path
cost from all source to all destinations is close to that of a
shortest path routing for the network.
Figure 4
Figure 4: CDF of total ETX path cost along path versus ETX path cost.
Path Stretch: The path stretch metric encompasses stretch factor for
both hop distance and ETX path cost (as defined in the Terminology
section). The hop distance stretch, which is determined as the
difference between the number of hops taken by a packet while
following a route built via RPL and the number of hops taken by
shortest path routing (using link ETX as the metric) is computed.
The ETX path cost stretch is also provided.
The CDF of the both path stretch metrics are plotted against the
value of the corresponding path stretch over all packets in Figures 5
and 6, for hop distance stretch and ETX path stretch, respectively.
It can be observed that, for a few packets, the path built via RPL
has fewer hops than the ideal shortest path where path ETX is
minimized along the DAG. This is because there are a few source-
destination pairs where the total ETX path cost is equal to or less
than that of the ideal shortest path when the packet takes a longer
hop count. As the RPL implementation ignores 20% change in total ETX
path cost before switching to a new parent or emitting new DIO, it
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does not necessarily provide the shortest path in terms of total ETX
path cost. Thus, this implementation yields a few paths with smaller
hop count but larger (or equal) total ETX path cost.
Figure 5
Figure 5: CDF of hop distance stretch versus hop distance stretch
value.
Figure 6
Figure 6: CDF of ETX path stretch versus ETX path stretch value.
The data for the CDF of hop count and ETX path cost for the ideal
shortest path (SP) and a path built via RPL, along with the CDF of
the routing table size is given below in Table 1. Figures 3 to 7
relate to the data in this table.
+---------+--------+---------+-----------+------------+-------------+
| CDF | Hop | Hop | ETX Cost | ETX Cost | Routing |
| (%age) | (SP) | (RPL) | (SP) | (RPL) | Table Size |
+---------+--------+---------+-----------+------------+-------------+
| 0 | 1.0 | 1.0 | 1 | 1.0 | 0 |
| 5 | 1.0 | 1.03 | 1 | 1.242 | 1 |
| 10 | 2.0 | 2.0 | 2 | 2.048 | 2 |
| 15 | 2.0 | 2.01 | 2 | 2.171 | 2 |
| 20 | 2.0 | 2.06 | 2 | 2.400 | 2 |
| 25 | 2.0 | 2.11 | 2 | 2.662 | 3 |
| 30 | 2.0 | 2.42 | 2 | 2.925 | 3 |
| 35 | 2.0 | 2.90 | 3 | 3.082 | 3 |
| 40 | 3.0 | 3.06 | 3 | 3.194 | 4 |
| 45 | 3.0 | 3.1 | 3 | 3.41 | 4 |
| 50 | 3.0 | 3.15 | 3 | 3.626 | 4 |
| 55 | 3.0 | 3.31 | 3 | 3.823 | 5 |
| 60 | 3.0 | 3.50 | 3 | 4.032 | 6 |
| 65 | 3.0 | 3.66 | 3 | 4.208 | 7 |
| 70 | 3.0 | 3.92 | 4 | 4.474 | 7 |
| 75 | 4.0 | 4.16 | 4 | 4.694 | 7 |
| 80 | 4.0 | 4.55 | 4 | 4.868 | 8 |
| 85 | 4.0 | 4.70 | 4 | 5.091 | 9 |
| 90 | 4.0 | 4.89 | 4 | 5.488 | 10 |
| 95 | 4.0 | 5.65 | 5 | 5.923 | 12 |
| 100 | 5.0 | 7.19 | 9 | 10.125 | 44 |
+---------+--------+---------+-----------+------------+-------------+
Table 1: Path Quality CDFs.
Overall, the path quality metrics give us important information about
the protocol's performance when minimizing the ETX path cost is the
objective to form the DAG. The protocol, as explained, does not
always provide an optimum path, especially for peer-to-peer
communication. However, it does end-up reducing the control overhead
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cost, reducing unnecessary parent selection and DIO message
forwarding events, by choosing a non-optimized path. Despite this
specific implementation technique, around 30% of the packets travel
the same number of hops as an ideal shortest path routing mechanism,
and 20% of packets experience the same number of attempted
transmissions to reach the destination. On average, this
implementation costs only a few extra transmission attempts and saves
a large number of control packet transmissions.
4.3. Routing Table Size
The objective of this metric is to observe the distribution of the
number of entries per node. Figure 7 shows the CDF of the number of
routing table entries for all nodes. Note that 90% of the nodes need
to store less than 10 entries in their routing table for the topology
under study. The LBR does not have the same power or memory
constraints as regular nodes do, and hence it can accommodate entries
for all the nodes in the network. The requirement of accommodating
devices with low storage capacity has been mandated in [RFC5673],
[RFC5826] and [RFC5867]. However, in storing mode of implementation,
some nodes closer to the LBR or DAG Root will require more memory to
store bigger routing tables.
Figure 7
Figure 7: CDF of routing table size with respect to number of nodes.
4.4. Delay Bound for P2P Routing
For delay sensitive applications, such as home and building
automation, it is critical to optimize the end-to-end delay. Figure
8 shows the upper bound and distributions of delay for paths between
any two given nodes for different hop counts between source and
destination. Here, the hop count refers to the number of hops a
packet travels to reach the destination when using RPL paths. This
hop distance does not correspond to shortest path distance between
two nodes. Note that, each packet has a length of 127 bytes, with a
240 kbps radio, which makes the transmission delay approximately 4
ms.
Figure 8
Figure 8: Comparison of packet latency, for different path lengths,
expressed in hop count.
RFCs 5673 [RFC5673] and 5548 [RFC5548] mention a requirement for the
end-to-end delivery delay to remain within a bounded latency. For
instance, according to the industrial routing requirement, non-
critical closed-loop applications may have a latency requirement that
can be as low as 100 milliseconds (ms), whereas monitoring services
may tolerate a delay in the order of seconds. The results show that
about 99% of the end-to-end communication (where maximum hop-count is
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7 hops) are bounded within the 100 ms requirement, for the topology
under study. It should be noted that due to poor link condition,
there may be packet drops triggering retransmission, which may cause
larger end-to-end delivery delays. Nodes in the proximity of the LBR
may become congested at high traffic loads, which can also lead to
higher end-to-end delay.
4.5. Control Packet Overhead
The control plane overhead is an important routing characteristic in
LLNs. It is imperative to bound the control plane overhead. One of
the distinctive characteristics of RPL is that it makes use of
trickle timers so as to reduce the number of control plane packets by
eliminating redundant messages. The aim of this performance metric
is thus to analyze the control plane overhead both in stable
conditions (no network element failure overhead) and in the presence
of failures.
Data and control plane traffic comparison for each node: Figure 9
shows the comparison between the amount of data packets transmitted
(including forwarded) and control packets (DIO and DAO messages)
transmitted for all individual nodes when link ETX is used to
optimize the DAG. As mentioned earlier, each node generates a new
data packet every 10 seconds. Here one can observe that a
considerable amount of traffic is routed through the DAG root itself.
The x axis indicates the node ID in the network. Also, as expected,
the nodes closer to DAG root and that act as routers (as opposed to
leaves) handle much more data traffic than other nodes. Nodes 12,
36, 38 are example of nodes next to the DAG root, taking part in
routing most of the data packets, hence having much more data packet
transmissions than other nodes, as observed in Figure 9. We can also
observe that the proportion of control traffic is negligible for
those nodes. This result also reinforces the fact that the amount of
control plane traffic generated by RPL is negligible on these
topologies. Leaf nodes have comparable amount of data and control
packet transmission (they do not take part in routing the data).
Figure 9
Figure 9: Amount of data and control packets transmitted against node
ID using link ETX as routing metric.
Data and Control Packet Transmission with Respect to Time: In Figures
10, 11 and 12, the amount of data and control packets transmitted for
node 12 (low rank in DAG, closer to the root), node 43 (in the
middle) and node 31 (leaf node) are shown, respectively. These
values stand for number of data and control packets transmitted for
each 10 minute intervals for the particular node, to help understand
what is the ratio between data and control packets exchanged in the
network. One can observe that nodes closer to the DAG root have a
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higher proportion of data packets (as expected), and the proportion
of control traffic is negligible in comparison with the data traffic.
Also, the amount of data traffic handled by a node within given
interval varies largely over time for a node closer to DAG root,
because in each interval the destination of the packets from same
source changes, while 20% of the packets are destined to the DAG
root. As a result, pattern of the traffic handled changes widely in
each interval for the nodes closer to the DAG root. For the nodes
that are farther away from the DAG root, the ratio of data and
control traffic is smaller since the amount of data traffic is
greatly reduced.
The control traffic load exhibits a wave-like pattern. The amount of
control packets for each node drops quickly as the DODAG stabilizes
due to the effect of trickle timers. However, when a new DODAG
sequence is advertised (global repair of the DODAG), the trickle
timers are reset and the nodes start emitting DIOs frequently again
to rebuild the DODAG. For a node closer to the DAG root, the amount
of data packets is much larger than that of control packets, and
somewhat oscillatory around a mean value. The amount of control
packets exhibits a 'saw-tooth' behavior. As the ETX link metric was
used, when the PDR changes the ETX link metric for a node to its
child changes, which may lead to choosing a new parent, and changing
the DAG rank of the child. This event resets the trickle timer and
triggers the emission of a new DIO. Also, issue of a new DODAG
sequence number triggers DODAG re-computation and resets the trickle
timers. Therefore, one can observe that the number of control
packets attains a high value for one interval, and comes down to
lower values for subsequent intervals. The interval with high value
of control packets denotes the interval where the timers to emit new
DIO are reset more frequently. As the network stabilizes, the
control packets are less dense in volume. For leaf nodes, the amount
of control packets is comparable to that of data packets, as leaf
nodes are more prone to face changes in their DODAG rank as opposed
to nodes closer to DAG root when the link ETX value in the topology
changes dynamically.
Figure 10
Figure 10: Amount of data and control packets transmitted for node
12.
Figure 11
Figure 11: Amount of data and control packets transmitted for node
43.
Figure 12
Figure 12: Amount of data and control packets transmitted for node
31.
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4.6. Loss of Connectivity
Upon link failures, a node may lose its parents: preferred and backup
(if any) thus leading to a loss of connectivity (no path to the DODAG
root). RPL specifies two mechanisms for DODAG repairs, referred to
as the global repair and local repair. In this document, simulation
results are presented to evaluate the amount of time data packets are
dropped due to a loss of connectivity for the following two cases: a)
when only using global repair (i.e., the DODAG is rebuilt thanks to
the emission of new DODAG sequence numbers by the DODAG root), and b)
when using local repair (poisoning the sub-DAG in case of loss of
connectivity) in addition to global repair. The idea is to tune the
frequency at which new DODAG sequence numbers are generated by the
DODAG root, and also to observe the effect of varying the frequency
for global repair and the concurrent use of global and local repair.
It is expected that more frequent increments of DODAG sequence number
will lead to shorter duration of connectivity loss at a price of a
higher rate of control packet in the network. For the use of both
global and local repair, the simulation results show the trade-off in
amount of time that a node may remain without service and total
number of control packets for extra bit of signaling.
Figure 13 shows the CDF of time spent by any node without service,
when the data packet rate is one packet every 10 seconds, and new
DODAG sequence number is generated every 10 minutes. This plot
reflects the property of global repair without any local repair
scheme. When all the parents are temporarily unreachable from a
node, the time before it hears a DIO from another node is recorded,
which gives the time without service. We define DAG repair timer to
be the interval at which the LBR increments the DAG sequence number,
thus triggering a global re-optimization. In some cases this value
might go up to the DAG repair timer value, because until a DIO is
heard, the node does not have a parent, and hence no route to the LBR
or other nodes not in its own sub-DAG. Clearly, this situation
indicates a lack of connectivity and loss of service for the node.
Figure 13
Figure 13: CDF: Loss of connectivity with global repair.
The effect of the DAG repair timer on time without service is plotted
in Figure 14, where the source rate is 20 seconds/packet and in
Figure 15, where the source sends a packet every 10 seconds.
Figure 14
Figure 14: CDF: Loss of connectivity for different global repair
period, packet rate 20/s.
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Figure 15
Figure 15: CDF: Loss of connectivity for different global repair
period, packet rate 10/s.
The data for Figures 13 and 15 can be found in Table 2. The table
shows how the CDF of time without connectivity to LBR increases while
we increase the time period to emit new DAG sequence number, when the
nodes generate a packet every 10 seconds.
+---------+------------------+------------------+-------------------+
| CDF | Repair Period 10 | Repair Period 30 | Repair Period 60 |
| (%age) | Minutes | Minutes | Minutes |
+---------+------------------+------------------+-------------------+
| 0 | 0.464 | 0.045 | 0.027 |
| 5 | 0.609 | 0.424 | 0.396 |
| 10 | 1.040 | 1.451 | 0.396 |
| 15 | 1.406 | 3.035 | 0.714 |
| 20 | 1.934 | 3.521 | 0.714 |
| 25 | 2.113 | 5.461 | 1.856 |
| 30 | 3.152 | 5.555 | 1.856 |
| 35 | 3.363 | 7.756 | 6.173 |
| 40 | 4.9078 | 8.604 | 6.173 |
| 45 | 8.575 | 9.181 | 14.751 |
| 50 | 9.788 | 21.974 | 14.751 |
| 55 | 13.230 | 30.017 | 14.751 |
| 60 | 17.681 | 31.749 | 16.166 |
| 65 | 29.356 | 68.709 | 16.166 |
| 70 | 34.019 | 92.974 | 302.459 |
| 75 | 49.444 | 117.869 | 302.459 |
| 80 | 75.737 | 133.653 | 488.602 |
| 85 | 150.089 | 167.828 | 488.602 |
| 90 | 180.505 | 271.884 | 488.602 |
| 95 | 242.247 | 464.047 | 488.602 |
| 100 | 273.808 | 464.047 | 488.602 |
+---------+------------------+------------------+-------------------+
Table 2: Loss of Connectivity Time. Data Rate : 1 Packet / 10
Seconds.
The data for Figure 14 can be found in Table 3. The table shows how
the CDF of time without connectivity to LBR increases while we
increase the time period to emit new DAG sequence number, when the
nodes generate a packet every 20 seconds.
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+---------+------------------+------------------+-------------------+
| CDF | Repair Period 10 | Repair Period 30 | Repair Period 60 |
| (%age) | Minutes | Minutes | Minutes |
+---------+------------------+------------------+-------------------+
| 0 | 0.071 | 0.955 | 0.167 |
| 5 | 0.126 | 2.280 | 1.377 |
| 10 | 0.403 | 2.926 | 1.409 |
| 15 | 0.902 | 3.269 | 1.409 |
| 20 | 1.281 | 16.623 | 3.054 |
| 25 | 2.322 | 21.438 | 5.175 |
| 30 | 2.860 | 48.479 | 5.175 |
| 35 | 3.316 | 49.495 | 10.30 |
| 40 | 3.420 | 93.700 | 25.406 |
| 45 | 6.363 | 117.594 | 25.406 |
| 50 | 11.500 | 243.429 | 34.379 |
| 55 | 19.703 | 277.039 | 102.141 |
| 60 | 22.216 | 284.660 | 102.141 |
| 65 | 39.211 | 285.101 | 328.293 |
| 70 | 63.197 | 376.549 | 556.296 |
| 75 | 88.986 | 443.450 | 556.296 |
| 80 | 147.509 | 452.883 | 1701.52 |
| 85 | 154.26 | 653.420 | 2076.41 |
| 90 | 244.241 | 720.032 | 2076.41 |
| 95 | 518.835 | 1760.47 | 2076.41 |
| 100 | 555.57 | 1760.47 | 2076.41 |
+---------+------------------+------------------+-------------------+
Table 3: Loss of Connectivity Time. Data Rate: 1 Packet / 20 Seconds.
Figure 16 shows the effect of DAG global repair timer period on
control traffic. As expected, as the frequency at which new DAG
sequence numbers are generated increases, the amount of control
traffic decreases because DIO messages are sent less frequently to
rebuild the DODAG. However reducing the control traffic comes at a
price of increased loss of connectivity when only global repair is
used.
Figure 16
Figure 16: Amount of control traffic for different global repair
periods.
From the above results, it is clear that the time the protocol takes
to re-establish routes and to converge, after an unexpected link or
device failure happens, is fairly long. [RFC5826] mandates that "the
routing protocol MUST converge within 0.5 seconds if no nodes have
moved". Clearly, implementation of a repair mechanism based on new
DAG sequence number alone would not meet the requirements. Hence a
local repair mechanism, in form of poisoning the sub-DAG and issuing
DIS, has been adopted.
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The effect of the DAG repair timer on time without service when local
repair is activated is now observed and plotted in Figure 17, where
the source rate is 20 seconds/packet. A comparison of the CDF of
loss of connectivity for global repair mechanism and global + local
repair mechanism is shown in Figures 18 and 19 (semi-log plots, x
axis in logarithmic and y axis in linear scale), where the source
generates a packet every 10 seconds and 20 seconds, respectively.
For these plots, the x axis shows time in log scale, and y axis
denotes the corresponding CDF in linear scale. One can observe that
using local repair (with poisoning of the sub-DAG) greatly reduces
loss of connectivity.
Figure 17
Figure 17: CDF: Loss of connectivity for different DAG repair timer
values for global+local repair, packet rate 20/s.
Figure 18
Figure 18: CDF: Loss of connectivity for global repair and global+
local repair, packet rate 10/s.
Figure 19
Figure 19: CDF: Loss of connectivity for global repair and global+
local repair, packet rate 20/s.
A comparison between the amount of control plane overhead used for
global repair only and global plus local repair mechanism is shown in
Figure 20, which highlights the improved performance of RPL in terms
of convergence time at very little extra overhead. From Figure 19,
in 85% of the cases the protocol finds connectivity to the LBR for
the concerned nodes within fraction of seconds when local repair is
employed. Using only global repair leads to 150 - 154 seconds as
observed in Figures 13 and 14.
Figure 20
Figure 20: Number of control packets for different DAG sequence
number period, for both global repair and global+local repair.
5. RPL in a Building Automation Routing Scenario
Unlike the previous traffic pattern, where a majority of the total
traffic generated by any node is destined to the root, this section
considers a different traffic pattern, which is more prominent in
home or building routing scenario. In the simulations shown below,
the nodes send 60% of their total generated traffic to the physically
1-hop distant node, 20% of traffic to a 2-hop distant node and the
other 20% of traffic is distributed among other nodes in the network.
The CDF of path quality metrics such as hop count, ETX path cost,
average hop distance stretch, ETX path stretch, and delay for P2P
routing for all pair of nodes is calculated. Maintaining low delay
bound for P2P traffic is of high importance, as applications in home
and building routing typically have low delay tolerance.
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5.1. Path Quality
Figure 21 shows the CDF of hop count for both RPL and ideal shortest
path routing for the traffic pattern described above. Figure 22
shows the CDF of the expected number of transmission (ETX) for each
packet to reach its destination. Figures 23 and 24 show the CDF of
the stretch factor for these two metrics. To illustrate the stretch
factor, an example from Figure 24 will be given next. For all paths
built by RPL, 85% of the time the path cost is less than the path
cost for the ideal shortest path plus one.
Figure 21
Figure 21: CDF of end-to-end hop count for RPL and ideal shortest
path in home routing.
Figure 22
Figure 22: CDF of ETX path cost metric for RPL and ideal shortest
path in home routing.
Figure 23
Figure 23: CDF of hop distance stretch from ideal shortest path.
Figure 24
Figure 24: CDF of ETX metric stretch from ideal shortest path.
5.2. Delay
To get an idea of maximum observable delay in the mentioned traffic
pattern, the delay for different number of hops to the destination
for RPL is considered. Figure 25 shows how the end-to-end packet
latency is distributed for different packets with different hop
counts in the network.
Figure 25
Figure 25: Packet latency for different hop count in RPL.
For this deployment scenario, 60% of the traffic has been restricted
to 1-hop neighborhood. Hence, intuitively, the protocol is expected
to yield path qualities which are close to that of ideal shortest
path routing for most of the paths. From the CDF of hop count and
ETX path cost, it is clear that peer-to-peer paths are more often
closer to an ideal shortest path. The end-to-end delay for distances
within 2 hops are less than 60 ms for 99% of the delivered packets,
while packets traversing 5 hops and more are delivered within 100 ms
for 99% of the time. These results demonstrate that, for a normal
routing scenario of an LLN deployment in a building, RPL performs
fairly well without incurring much control plane overhead, and it can
be applied for delay critical applications as well.
6. RPL in a Large Scale Network
In this section we focus on simulating RPL in a large network and
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study its scalability by focusing on a few performance metrics: the
latency and path cost stretch, and the amount of control packets.
The 2442 nodes smart meter network with its corresponding link traces
were used in this scalability study. To simulate a more realistic
scenario for a smart meter network, 100% of the generated packets by
each node are destined to the root. Therefore, no traffic is
destined to nodes other than the root.
6.1. Path Quality
To investigate RPL's scalability, the CDF of ETX path cost in the
large scale smart meter network is compared to a hypothetical ideal
shortest path routing protocol which minimizes the total ETX path
cost (Figure 26). In this simulation, the path stretch is also
calculated for each packet that traverses the network. The path
stretch is determined as the difference between the path cost taken
by a packet while following a route built via RPL and a path computed
using an ideal shortest path routing protocol. The CDF of ETX
fractional stretch, which is determined as the ETX metric stretch
value over the ETX path cost of an ideal shortest path, is plotted in
Figure 27. The fractional hop distance stretch value, as defined in
the Terminology section, is shown in Figure 28.
Looking at the path quality plots, it is obvious that RPL works in a
non-optimal fashion in this deployment scenario as well. However, on
average, for each source-destination pair, the ETX fractional stretch
is limited to 30% of the ideal shortest path cost. This fraction is
higher for paths with shorter distance, and lower for paths where
source-destination are far apart. The negative stretch factor for
hop count is an interesting feature of this deployment and is due to
RPL's decision of not switching to another parent where the
improvement in path quality is not significant. As mentioned, in
this implementation, a node will only switch to a new parent if the
advertised ETX path cost to the LBR through the new candidate parent
is 20% better than the old one. The nodes tend to hear DIOs from a
smaller hop count first, and later do not always shift to a larger
hop count and smaller ETX path cost. As the traffic is mostly to the
DAG root, some P2P paths built via RPL do yield a smaller hop count
from source to destination, albeit at a larger ETX path cost.
As observed in Figure 26, 90% of the packets transmitted during the
simulation have a (shortest) ETX path cost to destination less than
or equal to 12. However, via RPL, 90% of the packets will follow
paths that have a total ETX path cost of up to 14. Though all
packets are destined to the LBR, it is to be noted that this
implementation ignores a change of up to 20% in total ETX path cost.
Figures 27 and 28 indicate all paths have a very low ETX fractional
stretch factor as total ETX path cost is concerned, and some of the
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paths have lesser hop counts to LBR or DAG root as well when compared
to the hop count of ideal shortest path.
Figure 26
Figure 26: CDF of total ETX path cost Vs. ETX path cost.
Figure 27
Figure 27: CDF of ETX fractional stretch Vs. ETX fractional stretch
value.
Figure 28
Figure 28: CDF of fractional hop count stretch.
6.2. Delay
Figure 29 shows how the end-to-end packet latency is distributed for
different hop counts in the network. According to [RFC5826], U-LLNs
are delay tolerant, and the information, except for critical alarms,
should arrive within a fraction of the reporting interval (within a
few seconds). The packet generation for this deployment has been set
higher than usual to incur high traffic volume, and nodes generate
data once every 30 seconds. However, the end-to-end latency for most
of the packets is condensed between 500 ms to 1s, where the upper
limit corresponds to packets traversing longer (larger than or equal
to 6 hops) paths.
Figure 29
Figure 29: End-to-end packet delivery latency for different hop
counts.
6.3. Control Packet Overhead
Figure 30 shows the comparison between data packets (originated and
forwarded) and control packets (DIO and DAO messages) transmitted by
each node (link ETX is used as the routing metric). Here one can
observe that in spite of the large scale of the network, the amount
of control traffic in the protocol is negligible in comparison to
data packet transmission. The smaller node id for this network
actually indicates closer proximity to the DAG root and nodes with
high ID are actually farther away from the DAG root. Also, as
expected, we can observe on Figures 31, 32 and 33 that the (non-leaf)
nodes closer to the DAG root have much more data packet transmissions
than other nodes. The leaf nodes have comparable amount of data and
control packet transmissions, as they do not take part in routing the
data. As seen before, the data traffic for a child node has much
less variation than the nodes which are closer to the DAG root. This
variation decreases with increase in DAG depth. In this topology,
Nodes 1, 2, and 3, etc., are direct children of the LBR.
Figure 30
Figure 30: Data and control packet comparison.
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Figure 31
Figure 31: Data and control packet over time for Node 1.
Figure 32
Figure 32: Data and control packet over time for Node 78.
Figure 33
Figure 33: Data and control packet over time for Node 300.
In Figure 34, the effect of global repair period timer on control
packet overhead is shown.
Figure 34
Figure 34: Amount of control packet for different global repair timer
period.
7. Scaling Property and Routing Stability
An important metric of interest is the maximum load experienced by
any node (CPU usage) in terms of the number of control packets
transmitted by the node. Also, to get an idea of scaling properties
of RPL in large scale networks, it is also key to analyze the number
of packets handled by the RPL nodes for different sizes of the
network.
In these simulations, at any given interval, the node with maximum
control overhead load is identified. The amount of maximum control
overhead processed by that node is plotted against time for three
different networks under study. The first one is Network 'A', which
has 45 nodes and is shown in Figure 1 (Section 3); Network 'B', which
is another deployed outdoor network with 86 nodes; and finally,
Network 'C', which is the large deployed smart meter network with
2442 nodes being considered in this document.
In Figure 35, the comparison of maximum control load is shown for
different network sizes. For the network with 45 nodes, the maximum
number of control packets in the network stays within a limit of 50
packets (per 1 minute interval), where for the networks with 86 and
2442 nodes, this limit stretches to 100 and 2 * 10^3 packets per 1
minute interval, respectively.
Figure 35
Figure 35: Scaling property of maximum control packets processed by
any node over time.
For a network built with low power devices interconnected by lossy
links, it is of the utmost importance to ensure that routing packets
are not flooded in the entire network, and that the routing topology
stays as stable as possible. Any change in routing information,
specially parent-child relationship, would reset the timer leading to
emitting new DIOs, and hence, change the node's path metric to reach
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the root. This change will trigger a series of control plane
messages (RPL packets) in the DODAG. Therefore, it is important to
carefully control the triggering of DIO control packets via the use
of thresholds.
In this study, the effect of the tolerance value which is considered
before emitting a DIO reflecting a new path cost is analyzed. Four
cases are considered:
o No change in DAG depth of a node is ignored;
o The implementation ignores 10% of change in the ETX path cost to
the DAG root. That is, if the change in total path cost to root/
LBR, due to a DIO reception from most preferred parent or due to
shifting to another parent, is less than 10%, the node will not
advertise the new metric to the root;
o The implementation ignores 20% change in ETX path cost to the DAG
root for any node before deciding to advertise a new depth;
o The implementation ignores 30% change in the total ETX path cost
to DAG root of a node before deciding to advertise a new depth.
This decision does affect the optimum path quality to the DAG root.
As observed in Figure 36, for 0% tolerance, 95% of paths used have an
ETX fractional stretch factor less than 10%. Similarly, for 10% and
20% tolerance level, 95% of paths will have a 15% and 20% ETX
fractional path stretch. However, the increased routing stability
and decreased control overhead is the profit gained from the 10%
extra increase in path length or ETX path cost, whichever is used as
the metric to optimize the DAG.
Figure 36
Figure 36: ETX fractional stretch factor for different tolerance
levels.
As the above mentioned threshold also affects the path taken by a
packet, this study also demonstrates the effect of the threshold on
routing stability (number of times P2P paths change between a source
and a destination). For Network 'A' shown in Figure 1 and the large
smart meter network 'C', the CDF of path change is plotted against
fraction of path change for different thresholds triggering the
emission of a new DIO upon path cost change.
In Figures 37 and 38, it is shown that the CDF of fraction of times a
path has changed (for each source-destination pair). If X packets
are transferred from source A to destination B, and out of X times, Y
times the path between this source-destination pair is changed, then
we compute the fraction of path change as Y/X * 100% . This metric
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is computed over all source-destination pairs, and the CDF is plotted
in the y axis.
Figure 37
Figure 37: Distribution of fraction of path change for network A.
Figure 38
Figure 38: Distribution of fraction of path change for large network
C.
This document also compares the CDF of fraction of path change for
three different networks, A, B and C. Figure 39 shows how the three
networks exhibit change of P2P path when 30% change in metric cost to
the root is ignored before shifting to a new parent.
Figure 39
Figure 39: Comparison of distribution of fraction of path change.
8. Comments
All the simulation results presented in this document corroborate the
expected protocol behavior for the topologies and traffic model used
in the study. For the particular discussed scenarios, the protocol
is shown to meet the desired delay and convergency requirements and
to exhibit self-healing properties without external intervention,
incurring negligible control overhead (only a small fraction of data
traffic). RPL provided near optimum path quality for most of the
packets in the scenarios considered and is able to trade-off control
overhead for path quality as per the application and device
requirement through configurable parameters (such as decision on when
to switch to new parent), and thus can trade-off routing stability
for control overhead as well. Finally, as per the requirement of
urban LLN deployments, the protocol is shown to scale to larger
topologies (few thousand nodes), for the topologies considered in
this implementation.
9. Acknowledgements
The authors would like to acknowledge Jerald P. Martocci, Mukul
Goyal, Emmanuel Monnerie, Philip Levis, Omprakash Gnawali and Craig
Partridge for their valuable and helpful suggestions over metrics to
include and overall feedback.
10. References
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10.1. Normative References
10.2. Informative References
[Castalia-2.2]
Boulis, A., "Castalia: Revealing pitfalls in designing
distributed algorithms in WSN, in Proceedings of the 5th
international conference on Embedded networked sensor
systems (SenSys'07)", 2007.
[I-D.ietf-roll-terminology]
JP Vasseur, "Terminology in Low power And Lossy Networks,
draft-ietf-roll-terminology-06 (work in progress)'",
September 2011.
[NS-2] "The Network Simulator-2, http://www.isi.edu/nsnam/ns/".
[OMNETpp] Varga, A., "The OMNeT++ Discrete Event Simulation System,
in Proceedings of the European Simulation Multiconference
(ESM'2001)", June 2001.
[RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
"Routing Requirements for Urban Low-Power and Lossy
Networks", RFC 5548, May 2009.
[RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low-Power and Lossy
Networks", RFC 5673, October 2009.
[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks",
RFC 5826, April 2010.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, June 2010.
[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, March 2011.
[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.
[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics Used for Path Calculation in
Low-Power and Lossy Networks", RFC 6551, March 2012.
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[draft-iphc]
J. Jurski, "Limited IP Header Compression over PPP,
draft-jurski-pppext-iphc-02.txt (work in progress)", March
2007.
Authors' Addresses
Joydeep Tripathi (editor)
Drexel University
3141 Chestnut Street 7-313
Philadelphia, PA 19104
USA
Email: jt369@drexel.edu
Jaudelice C. de Oliveira (editor)
Drexel University
3141 Chestnut Street 7-313
Philadelphia, PA 19104
USA
Email: jau@coe.drexel.edu
JP Vasseur (editor)
Cisco Systems, Inc.
11, Rue Camille Desmoulins
Issy Les Moulineaux, 92782
France
Email: jpv@cisco.com
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