The Cognitive Packet Network: A Survey

The Computer Journal Advance Access published June 5, 2009
© The Author 2009. Published by Oxford University Press on behalf of The British Computer Society. All rights reserved.
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doi:10.1093/comjnl/bxp053
The Cognitive Packet Network:
A Survey
Georgia Sakellari1,∗
1 Intelligent
Systems and Networks Group, Department of Electrical and Electronic Engineering,
Imperial College London, London SW7 2BT UK
∗Corresponding author: g.sakellari@imperial.ac.uk
Current and future multimedia networks require connections under specific quality of service (QoS)
constraints which can no longer be provided by the best-effort Internet. Therefore, ‘smarter’networks
have been proposed in order to cover this need. The cognitive packet network (CPN) is a routing
protocol that provides QoS-driven routing and performs self-improvement in a distributed manner,
by learning from the experience of special packets, which gather on-line QoS measurements and
discover new routes. The CPN was first introduced in 1999 and has been used in several applications
since then. Here we provide a comprehensive survey of its variations, applications and experimental
performance evaluations.
Keywords: cognitive packet networks; random neural network; quality of service; routing protocol;
self-aware networks
Received 10 October 2008; revised 7 May 2009
Handling editor: Taskin Kocak
1.
INTRODUCTION
The need to provide quality of service (QoS) to the users of
current and future multimedia networks has led to the growth of
‘smarter’ networks that use QoS-driven architectures to provide
a more stable and more reliable environment and guarantee
packet delivery under specific and user-defined QoS constraints.
Such an architecture is the cognitive packet network (CPN), a
packet routing protocol designed to perform self-improvement
and address QoS by using adaptive techniques based on on-line
measurements, first introduced in [1] and inspired by an earlier
approach on learning agents [2].
Contrary to conventional routing protocol mechanisms, in
CPN it is the users rather than the nodes that are given control
of the routing. Each user can specify the QoS criteria based
on which data will be routed in the network, since the routing
algorithm is directly related to the QoS desired by the end-user.
Different users can have different QoS goals depending on the
application and the service they want to use. This is done at the
software level, and therefore CPN reduces the complexity of
the routes, which in turn significantly reduces the cost of the
network. The CPN is a distributed protocol, where the routing
decisions are made at each node of the network and are based on
adaptive learning techniques such as random neural networks
(RNNs) with reinforcement learning (RL).
The remainder of this paper is organized as follows. Section 2
describes the functionality of the CPN in detail, while Section 3
describes the learning algorithms used to optimize userspecified QoS goals and to make routing decisions. Section 4
explains how the CPN collaborates with IP applications and
other protocols and Section 5 reviews the research undertaken
in its various aspects and describes some of its applications and
enhancements. In Section 6 we review experimental evaluation
results, and we conclude in Section 7 with a discussion on the
role that CPN can play in the intelligent networks of the near
future.
2. THE OPERATION OF CPN
CPN is an adaptive packet routing protocol with enhanced
monitoring and self-improvement capabilities that address QoS
by using adaptive techniques based on on-line measurements
[3–7]. It is a distributed protocol with which users, or the
network itself, declare their QoS requirements (QoS goals) such
as minimum delay, maximum bandwidth, minimum cost etc. It
is designed to perform self-improvement by learning from the
experience of smart packets (SPs).
It makes use of three types of packets:
• SPs (also known as cognitive packets), for discovery;
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G. Sakellari
• source routed dumb packets (DPs) to carry the payload;
and
• acknowledgement (ACK) packets to bring back information that has been discovered by SPs, and is used in nodes
to train neural networks.
SPs are generated by a user that requests to create a path
to some CPN node or to discover parts of the network state,
including location of certain fixed or mobile nodes, power levels
at nodes, topology, paths and their QoS metrics. Their role is
to explore the network and discover the ‘best QoS routes’ for
each source–destination pair in the network according to goals
assigned from the users. At each hop SPs are routed according
to the experiences of previous packets with the same goals and
the same destination. The decisions of the SPs are based on a
learning algorithm. In order to explore all possible routes, at
some hops, each SP makes a random routing decision, with
a small probability (usually 5%). To avoid overburdening the
system with unsuccessful requests or packets which are in effect
lost, all packets have a life-time constraint based on the number
of nodes they have visited. The term ‘goal’ is used instead of
‘QoS specifications’ to emphasize the fact that there are no QoS
guarantees and that the CPN provides a best effort service [8].
The goal is a QoS criterion that the SPs try to achieve such as
minimum delay, maximum bandwidth, minimum packet loss,
minimum variance of the packet delay, maximum security level
in a path, minimum power consumption in a wireless node or a
weighted combination of these.
In the CPN, routers have restricted functionality. The tasks of
the nodes include receiving packets from several ports, storing
them into an input buffer and transmitting packets to other nodes
via output buffers based on their priority discipline. In addition,
routers store information they receive from SPs in special shortterm memory stores, referred to as mailboxes (MBs) [3,4]. In a
node, there may be more than one MB, since each one may be
used by a certain class of SPs that have common characteristics,
such as QoS goals or destination. SPs read their MBs and use
the information contained in them to execute their code via the
node, update their MBs in the node and decide their next hop
movement. Routers are also responsible for discarding packets
that have exceeded the allowed ‘time-out’ value.
Each SP or DP in a CPN contains all the usual fields one
would find in usual TCP/IP packets, plus some fields that
contain the code needed to interact with the nodes they visit
[4,7]. Examples include reading the MB, computing the goal,
running the adaptive learning algorithm, making the next hop
decision. The learning algorithms used by the SPs are analysed
in Section 3. An SP stores the route it follows and also keeps
‘timestamp’ information regarding the local time at which it
visited a node.
When it arrives at its destination, an ACK packet is generated
and the routing and measurement information collected by the
SP are transferred to the ACK. The ACK packet will follow
the reverse path back to the source, of the one followed by
the corresponding SP (using a loop-removal algorithm to avoid
circuits). The reverse path is not necessary the shortest one or the
one resulting in the best goal satisfaction. As the ACK follows
its route, it deposits QoS information in the MBs of the nodes it
visits. At the source, the route carried by an ACK, along with its
QoS data is cached in a table, the dumb packet route repository
(DPRR).
The last path inserted in the DPRR is the one that the DPs
will follow. DPs are the packets that carry the payload of
a particular connection. Since the path discovery process is
continuous, DPs select the most recently discovered best route
from the source to the destination, which is a result of the SPs
of the same QoS class previously sent in the network. DPs
can also collect time information during their trip through the
network, which can be brought back to a source node by a dumb
acknowledgement (DACK) packet and can update the MBs of
the nodes traversed.
3.
LEARNING AND DECISION ALGORITHMS
Several algorithms have been used in the CPN as learning
and decision techniques for SPs to find satisfactory routes
from source to destination based on the desired goals. The
simplest algorithm used is the bang-bang algorithm while other
more sophisticated algorithms based on RNN have also been
used, such as the learning feedforward algorithm and the RL
algorithm. Finally, genetic algorithms (GAs) have recently been
tested with CPN.
3.1.
Bang-bang algorithm
This algorithm makes use of the most recently available data
stored in the MBs of the present node and makes the decision
that requires the lowest cost or highest gain. To illustrate
how bang-bang algorithm works we assume that our goal is
a weighted combination of packet loss (L) and delay (W). This
means that the QoS Goal will be calculated as: G = aW + bL.
Each time a decision has to be made the SPs read the MB of the
node and estimate a running average of the delay to destination
D of the form: Wd = αWd + (1 − α)Vd , where Vd is the
most recent value of the delay for the particular destination.
Similarly, information is also collected regarding the loss and a
running average for it, ld , is computed as well [1]. In order for
the SP to take a decision, it makes an evaluation of the average
delay and loss it would require to the destination Dd and Ld
respectively. Note that Dd and Ld are a priori information since
their evaluation requires the knowledge of the grid structure by
the SPs [2]. If Ld < ld or Dd < Wd , then the SPs select at
random an output link since they assessed that better paths to
the destination exist while if the aforementioned conditions are
not valid, then the SP takes the direction followed by an SP
with exactly the same characteristics (e.g. same destination D
and QoS class). A significant disadvantage of the bang-bang
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The Cognitive Packet Network: A Survey
algorithm is the fact that it uses a priori information. Later
algorithms addressed this issue.
3.2.
RNN-based algorithms
The RNN is a biologically inspired neural network model,
which is characterized by the existence of positive (excitation)
and negative (inhibition) signals in the form of spikes of
unit amplitude that circulate among nodes and alter the
potential of the neurons. Each neuron can be connected to
another neuron and each connection is characterized by an
excitatory or inhibitory weight [2]. It was originally introduced
in [9] and extended in [10–15]. Although the RNN model
was initially inspired by biophysical neural networks, it has
been successfully applied in many areas such as associative
memory [16–18], image processing [19–21], texture generation
[22,23], video QoS and compression [19,24–29], as well as task
assignment [30] and resource allocation [31], and has inspired
the use of negative customers in queuing networks, which has
led to G-networks [32–36].
Apart from the CPN, RNN-based routing is employed in
[37], where RNN is used to learn the characteristics of the
minefield by fusing data from multiple sensors and navigating
autonomous vehicles, robots or agents. The predecessor of CPN
was the routing of agents described in [2]. Here agents are routed
according to predefined goals, such as traversing a dangerous
metropolitan grid safely and rapidly, using the RNN to learn
from their own observations and from the experience of other
agents with whom they exchange information. The RNN is also
used to control the movement of agents within a realistic live
scene of a simulator [38].
In a RNN, the state qi of the ith neuron, which represents the
probability that the ith neuron is excited, satisfies the following
system of nonlinear equations:
qi =
λ+ (i)
,
r(i) + λ− (i)
(1)
where
λ+ (i) =
qj wj+i + i ,
(2)
qj wj+i + λi ,
(3)
j
λ− (i) =
j
r(i) =
wij+ + wij− ,
(4)
j
in which wj+i is the rate at which neuron j sends ‘excitation
spikes’ to neuron i when j is excited, wj−i is the rate at which
neuron j sends ‘inhibition spikes’ to neuron i when j is excited
and r(i) is the total firing rate from the neuron i. i and λi
are the constant rates of the external positive and negative
signal arrivals, respectively, which follow stationary Poisson
distributions. For an N neuron network, the network parameters
3
are these N by N ‘weight matrices’ W + = {w+ (i, j )} and
W − = {w− (i, j )}, which need to be ‘learned’ from input data.
The weights wij+ and wj−i as well as i and λi are assumed to
be known for every i,j .
Several learning techniques have been proposed for the RNN.
Hebbian learning was tested at the early stages of the CPN
development and was shown to be inefficient and slow [8]. Other
algorithms include feedforward learning RNN with the use of a
gradient descent quadratic error function, as well as RL. These
two algorithms are briefly described next.
3.2.1. Learning feedforward random neural networks
These networks update their weights W + and W − based on
a gradient descent quadratic error function, which is used
to minimize the error observed when a new input-output
pair of training data is introduced. In CPN, recent samples
of the goals (e.g. packet loss and transit delay) are used
to train the learning feedforward random neural networks.
When an update of the weights has taken place, the CPN
provides its own estimate of the cost or reward incurred in
each output direction and a decision is made based on the
direction associated with the highest reward [2]. This algorithm
is computationally less efficient than the bang-bang and the RL
algorithm, because it requires computation at every step and also
because mathematical analysis of the model leads to a ‘backpropagation type algorithm’ that requires the solution of a linear
and a nonlinear system of N equations each time [10].
3.2.2. Reinforcement learning random neural networks
This is the algorithm that eventually prevailed in the
implementations of the CPN. The RL algorithm was initially
based on an RL algorithm with internal expectation for the
RNN that was developed for use in maze navigation [39]. The
arrival of an SP triggers the execution of the RL algorithm. More
specifically, each router stores a specific RNN for each QoS
class and for each active source–destination pair. Each RNN
node, which represents the decision to choose a given output
link for a SP, has as many neurons as the possible outgoing links
[5]. Decisions are taken by selecting the output link j for which
the corresponding neuron is the most excited, i.e. qi ≤ qj for
all i = 1, . . . , N, where N is the number of neurons (possible
outgoing links). The state qi of the ith neuron in the network
represents the probability that the ith neuron is excited and thus
the probability that the ith outgoing link will be selected for the
SP’s routing.
In CPN, the RL process changes the weights to reward or
punish a neuron according to the level of goal satisfaction
measured on the corresponding output, and thus according to
whether the SP succeeded or failed to achieve its QoS goal.
Each QoS class, for each source–destination pair, has a QoS
Goal G, which expresses a function to be minimized. The level
of goal satisfaction is expressed by a reward. Given some goal
G that a packet has to minimize, the reward R is formulated
simply as R = 1/G. The RNN weights are updated based on
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G. Sakellari
the following threshold T :
Tk = αTk−1 + (1 − α)Rk ,
(5)
where Rk , k = 1, 2, . . ., are successive measured values of
reward R and α is some constant (0 < α < 1) that is used to
tune the responsiveness of the algorithm: for instance, α = 0.8
means that on the average five past values of R are being taken
into account. Neurons are rewarded or punished based on the
difference between the current reward Rk and the last threshold
Tk−1 . Hence, if the most recent value of the reward Rk , which
corresponds to neuron j , is larger than the previous value of
the threshold Tk−1 , then the excitatory weights going into that
neuron are significantly increased (in order to reward it for
its new success), and the inhibitory weights leading to other
neurons are slightly increased. If the new reward is not greater
than the previous threshold, then all excitatory weights leading
to all neurons are moderately increased, except for the previous
winner, and the inhibitory weights leading to the previous
winning neuron are significantly increased, in order to punish
it for not being very successful this time. Thus, if ri is the firing
rate before the update takes place for every neuron i it is given by
ri =
n
[w + (i, m) + w − (i, m)].
(6)
m=1
First Tk−1 is computed and then the network weights are
updated for all neurons i = j as follows:
• If Tk−1 ≤ Rk
• w + (i, j ) ← w+ (i, j ) + Rk ,
Rk
• w − (i, l) ← w− (i, l) +
, if l = j .
n−2
• Else
Rk
• w+ (i, l) ← w+ (i, l) +
, if l = j ,
n−2
• w− (i, j ) ← w− (i, j ) + Rk .
To avoid large weights, which would lead to numerical difficulties when running the algorithms, and since the relative
size of the weights of the RNN, rather than the actual values,
determine the state of the neural network, the weights can be
re-normalized by carrying out the following operations. First,
for each i ri∗ is computed by
ri∗ =
n
[w + (i, m) + w − (i, m)],
(7)
m=1
and then the weights are re-normalized with:
ri
w+ (i, j ) ← w+ (i, j ) ∗ ∗ ,
ri
ri
−
−
w (i, j ) ← w (i, j ) ∗ ∗ .
ri
(8)
(9)
Having computed the new values of the weights the nonlinear
system of equations (1–4) can be solved in order to obtain the
qi and choose the most excited, which will provide the link
that the SP will follow. This procedure is repeated for each SP,
QoS class and source–destination pair.
3.3.
Genetic algorithms
Recently, GAs have been applied in the CPN for adaptive route
discovery. The authors of [6,40] use GAs to modify, filter and
combine the paths already found by the SPs in order to generate
new undiscovered but valid source–destination paths and select
the most advantageous ones. In the CPN, the populations of
individuals (or genotypes), which in GAs are the possible
solution to the problem being investigated, are the routes from a
source to a destination, represented as a list of the nodes’ names
in a particular route. Accordingly, the fitness function, which
estimates the usefulness of each individual to the solution of
the problem, is the measured or evaluated QoS or goal value of
a given path. Therefore, the CPN protocol is interested in paths
that have the best fitness and can use the crossover operator of
the GA to combine paths already discovered and create new
valid undiscovered paths.
When SPs discover valid routes, these are returned back to the
source via ACK packets to become individuals. The individuals’
population is stored at the source in a repository of finite length
called stack, where individuals are placed in order of ascending
goal value. The fittest individual is deposited first in the stack.
New valid paths are generated from the GA using the crossover
operator. Note that paths can only be combined if they contain
the same source and destination as well as same intermediate
nodes. Since we are only interested in the fittest paths, only paths
with small goal values are combined while their resulting goal
value of a new path is evaluated. Each time a DP needs to be
forwarded to the destination the fittest path is taken from the top
of the stack and is used as the source route of the particular DP.
4.
INTEGRATION WITH IP APPLICATIONS AND
OTHER PROTOCOLS
In CPN, both SPs and DPs contain all the usual fields one would
find in usual TCP/IP packets plus some fields containing the
code needed to interact with the nodes they visit [7]. When
this additional information is discarded, the smart and DPs can
be viewed as ordinary TCP/IP packets. However, ACKs are
specific to the CPN framework, even though they too are derived
from TCP/IP packets. Since the routing decisions are based
on the code carried by the CPN packets, the nodes in a CPN
network do not need the routing tables or routing algorithms,
which are typically stored in TCP/IP routers, and therefore can
be implemented with much simpler and less costly hardware
and software than conventional TCP/IP routers. The CPN is
a replacement for the IP layer, since it introduces a different
manner to handle routing.
The Computer Journal, 2009
The Cognitive Packet Network: A Survey
Integrating with IP applications and end-hosts is an important
characteristic of any networking protocol. Since most network
environments use IP as standard, it is important that CPN is
compatible with it [41]. On operating systems such as Linux,
which is used in most routers, a great majority of applications
have their source code available. When this is the case, adding
CPN support can amount to a few very simple changes to the
code that creates and connects a socket [41]. Since the size of the
address in the CPN is kept at 32 bits, the CPN and IP addresses
are completely compatible, making it very simple to convert an
IP application to a CPN one. Even for applications for which
the source code is not available, the CPN can still be used. By
intercepting the various socket functions and redirecting them to
their CPN-specific counterparts, an application which has been
written to use IP can use the CPN without any requirements
from the application’s side.
Another approach to bridging IP with CPN, presented also
in [41], is tunnelling. This involves modifying the IP routing
table such that before an IP packet is sent out on the wire, it
is first encapsulated into a CPN packet. This process requires
the designation of a tunnel source and a tunnel edge by an
administrator. This way conventional IP packets may tunnel
through the CPN to seamlessly operate mixed IP and CPN
networks.
By using an overlay network, the benefits of the CPN routing
can be introduced into existing networks with low overhead
and without modifying their underlying routing mechanisms
[41]. Thus, the CPN protocol is fully compatible at its edges
with the IP protocol, while internally it offers dynamic routing
based on on-line sensing and monitoring. Furthermore, the CPN
technology could be used to build private user networks, which
are implemented on top of existing TCP/IP, Asynchronous
Transfer Mode or other forms of network communication.
5.
The use of broadcasts allows SPs to reach all neighbouring
nodes without requiring the explicit selection of one. It also
avoids the need of an explicit neighbour discovery process
(e.g. by a periodic broadcast of hello packets) that may
unnecessarily use network resources. AHCPN assumes that
there is an insufficient knowledge about the neighbourhood if
the number of known neighbours is less than two when the node
is the source of the traffic, or three otherwise [43].
Neighbours receiving the packet repeat this decision process,
and thus the SP may use a broadcast again or select a given
neighbour to continue network exploration. It is interesting to
note that, in general, unicast decisions are preferred by SPs
[44]. Unicasts have the advantage of being more reliable and
are able to restrict the number of nodes involved in the route
discovery. Broadcasts, on the other hand, are prone to collide,
although a small random delay is introduced before transmitting
a broadcast to reduce the probability of collision. AHCPN
defines specific goals for wireless environments, which take
into account link reliability and energy consumption.
5.2. Traffic engineering with CPN
The use of CPN to achieve traffic engineering (TE) goals is
described in [45]. TE allows the internet service providers (ISPs)
to optimize resource utilization and network performance. With
the CPN technology an ISP is capable of providing value-added
IP services, such as QoS, to its customers and perform TE on the
network. For the end-user CPN provides QoS at the application
layer, based on the delay constraint, and at the service level
it redistributes the traffic in a flow-basis over the edge nodes
(similar to IntServ and RSVP). Thus, the network minimizes
congestion at the link level and minimizes the delay experienced
by the packets. The authors of [45] also provide experimental
results of how the ISPs can fulfil TE requirements through the
use of CPN.
ENHANCEMENTS AND APPLICATIONS
The CPN has been shown to be effective for a variety of uses,
including traffic balancing, power-based routing in mobile ad
hoc networks, denial of service (DoS) protection and admission
control (AC). This section describes some of the applications
that make use of the CPN protocol.
5.1.
5
CPN in wireless networks (mobile AD HOC CPN)
The CPN architecture has also been implemented in mobile
networks [42]. However, in mobile ad hoc environments, node
neighbours may change unexpectedly because of the link
breaks caused by changes in the position of the nodes or the
environment. To deal with such situations, the SPs of the ad
hoc CPN (AHCPN) allow the use of broadcasts in addition to
unicast decisions to continue exploring the network whenever
they reach a node with no information, or stale information
about its neighbourhood [42].
5.3.
Hardware implementation of CPN
The use of CPN with the routers currently used in Internet-based
network architectures may impose limitations due to higher and
higher traffic rates introduced to the networks, while at the same
time the processing demand of the packets could significantly
increase. Therefore, a hardware implementation of the CPN
protocol (a ‘CPN router’) would be low cost and provide a
more efficient processing of the packets. The main problem in
the hardware implementation of the CPN is the complexity and
the memory requirements of the RNN with RL.
A hardware implementation of the RNN-based routing
engine of a CPN network processor chip, called the smart
packet processor (SPP), is presented in [46]. The SPP is
a dual port device that stores, modifies and interprets the
defining characteristics of multiple RNN models. The authors
of [46] suggest hardware design improvements over the
software implementation, such as the dual access memory,
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G. Sakellari
output calculation step and reduced output calculation module,
and introduce a modification over the reinforcement learning
random neural networks such that the number of weight terms
are reduced from 2n2 to 2n, in order to not only save memory, but
also simplify the calculations for the steady-state probabilities.
Simulations have been conducted in [46] for the functionality
of the isolated SPPs design as well as for the multiple SPPs in
a networked environment.
Alternative algorithms to the RNN were also proposed in
[47,48] in order to keep the benefits of the RNN while being
less complex and less resource demanding, and therefore, more
suitable for implementation in hardware. The approximated
RNN (aRNN) [48] is not a neural network model, but attempts
to approximate properties of the RNN while using only simple
arithmetic operators such as addition and subtraction, and
bitwise shifts. A sensible routing (SR) policy (also called 1-SR),
introduced in [49], bases its routing decisions on the expected
QoS of a possible outcome and selects a path with a probability
that is inversely proportional to the expected QoS metric of
that path. In an m-SR policy the inverted expectation of the
QoS metric is raised to the power of m in order to increase the
probability of selecting the better performing routes. Therefore
m-SR shares the RNN’s problem of costly divisions, both from
the inversions and the probability calculations. However, if
m → ∞ the probability of selecting the best route equals 1,
which makes the implementation simpler. Thus, by avoiding
divisions, the ∞-SR and the approximated RNN offer smaller
and faster implementations than the RNN. They also require
less memory, and scale better with respect to the number of
neighbours. This is beneficial as it makes the CPN suitable for
implementation in dedicated hardware for high-speed routers,
and also enables it to be used in software-driven devices that
lack powerful processors and large memories, such as PDAs
or wireless sensor network devices. The authors in [47,48] also
propose an architecture for a field programmable gate arraybased hardware CPN router. This design demonstrates that
the traditional architectural approaches used in high-speed IP
routers are applicable to CPN routers, and thus demonstrates
the possibility for hybrid IP/CPN routers in hardware.
5.4.
CPN in sensor networks
Wireless sensor networks consist of many devices of limited
energy, memory and processing capabilities. The author of [50]
proposed a simplified CPN-based routing protocol for sensor
motes. This scheme uses smart routing in order to minimize the
transmission power use and also enables on-line aggregation
of data based on the locality of motes. The tinyCPN protocol
tracks the success of messages in reaching the sink, the length
of the route they discover and the power required.
Similar to the wired version of CPN, in tinyCPN smart
messages attempt to find a reliable low-power route to the
destination. They are forwarded to the neighbour that has the
lowest recorded energy requirements to the sink. When the smart
message reaches the sink, a smart acknowledgement message is
sent along the reverse route to the source. It updates the routing
tables and route power requirements for the sink at each node
in the route.
In contrast to the CPN, apart from the end-to-end
acknowledgements that are sent when smart messages reach
their destinations to update the routing and QoS tables, hopby-hop acknowledgements are also used every time a node
has received a message. Due to the fact that the probability
of a message being lost or corrupted in transit is higher in
sensor networks, when a node receives a message, it will
reply to the previous hop with a hop-by-hop acknowledgement.
If no acknowledgement is received, the message can be resent. The number of re-sent messages is recorded and is
used as a multiplier when determining the hop-by-hop power
requirements of the route for smart messages. Whenever a data
carrying dumb message is sent, there is a small probability of
a smart message also being sent. This ensures that power and
route data remain current.
5.5.
Defence mechanisms against DoS attacks in CPN
Recognizing the fact that the networks of the near future
will feature self-awareness and on-line interaction with users,
the authors of [51] investigated the application of defence
techniques on the resilience of the CPN against DoS attacks.
They introduced a generic framework of DoS protection based
on the dropping of probable illegitimate traffic, and presented a
mathematical model with which one can measure the impact
that both attack and defence have on the performance of a
network. More analytically, the CPN-based distributed DoS
defence technique exploits the ability of the CPN to trace traffic
going both downstream and upstream, owing to SPs and ACK
packets. When a node detects an attack, it uses the ACKs to ask
all intermediate nodes upstream to drop the packets of the attack
flow. Each node is allowed to select the maximum bandwidth
that it will accept from any flow that terminates at the node
and the maximum bandwidth that it allocates to a flow that
traverses the node. These parameters may vary dynamically as
a result of other conditions, and they can also be selected based
on the identity and the QoS needs of the flows. When a node
receives an SP or DP from a flow that it has not previously
encountered (e.g. with a new source–destination pair, or a new
QoS class), it sends a Flow-ACK packet back to the source
along the reverse path and informs the source of its bandwidth
allocation. The node monitors the flows that traverse it and
drops packets of any flow that exceeds the allocation; it may
also inform upstream nodes that packets of this flow should be
dropped. Other possible actions include diverting the flow into a
‘honeypot’ or to a special network. The mathematical results of
[51] were validated with simulation results and experimental
measurements in a CPN environment. This generic defence
was further improved by using prioritization and rate-limiting
instead of simple dropping [52,53]. To further improve the
The Computer Journal, 2009
The Cognitive Packet Network: A Survey
resilience against DoS attacks, the same authors have also
introduced a DoS detection mechanism that makes use of
on-line statistics collected by the CPN protocol’s monitoring
system and fused them with a RNN [54]. More analytically, the
scheme uses input features to capture both the instantaneous
behaviour and the longer-term statistical properties of the
traffic. In an off-line information gathering step, it obtains
the probability density function estimates, and evaluates the
likelihood ratios for the input features. During the real-time
decision step it measures and calculates the features of the
incoming traffic, finds the likelihood ratios corresponding to
those values and aggregates these likelihood values using an
RNN. The overall architecture outputs a numerical value that is
a measure of having an on-going attack in the network, which
is consequently used in the prioritization and rate-limiting
mechanisms previously mentioned [55,56].
5.6.
CPN with sorting
In the original CPN implementation, the route brought back
by an SP’s ACK is considered as the best route and would
be immediately used by the data traffic. However, this could
lead to poor data packet performance since it does not take into
consideration the random component of SP exploration, where
each router will, with a small probability, sometimes choose
a random next hop for an SP rather than the one decided by
the RNN. Therefore, the authors of [57] propose to compare the
quality of routes brought back by an ACK packet against the
current route and switch routes only when the reward of the new
route is greater than that of the current route. They named this
version ‘CPN with Sorting’ and the experiments show that it
performs better than the original CPN algorithm.
5.7.
Recursive CPN
A QoS-based recursive routing algorithm, which can break a
large-scale routing problem into some smaller routing problems,
has been proposed in [58]. Partial routes are cached in the
intermediate nodes, which can be used to provide a fast estimate
of the “best” QoS-based route that is needed by an arriving
packet. The experiments conducted on a 46-node network testbed showed that when recursive routing is applied, the average
time for a SP to discover a valid route is reduced dramatically,
the QoS that the users experience is also slightly improved and
more routes are discovered by the SPs. They also show that
selecting the paths based on QoS rather than on the ‘freshness’
of data does not reduce the adaptivity of the CPN, and that it
improves the QoS experienced by the users by recommending
only the best routes.
5.8. AC with CPN
CPN offers QoS-based best-effort routing and therefore, high
demand and network congestion, can prevent multimedia
7
applications and users from obtaining the network service
they require for a successful operation. A way to control
traffic congestion and satisfy all users’ QoS requirements,
without overprovisioning, is AC. The authors in [59,60]
propose a centralized, measurement-based, multiple criteria,AC
algorithm that is based on measurements of the QoS metrics
on each link of the network. This does not require any special
mechanism since CPN already collects QoS information on all
links and paths that the SPs have explored and on all paths that
any user is using in the network. A novelty of the proposed AC
algorithm is that the users can be the ones that specify the QoS
constraints they need in order to obtain the network service they
require for a successful connection, so that each user can have
different QoS criteria and QoS values.
The scheme decides whether a new call should be allowed to
enter the network based on measurements of the QoS metrics
on each link of the network before and after the transmission
of probe packets. Contrary to existing methods, the scheme
estimates the new flow’s impact by probing at a small rate, so
that probe packets will not contribute noticeably to the network’s
congestion. Finally, the decision on whether to accept a new
flow or not is based on an algebra of QoS metrics, inspired by
Warshall’s algorithm [61], which searches whether there is a
feasible path to accommodate the new flow without affecting
the existing users. Experimental results showing the efficiency
of the algorithm, under highly congested circumstances, in a
46-node real test-bed were presented in [62].
5.9. Autonomic auctions and CPN
The role which CPN can play in the future network-based
markets, where autonomic auctions will be part of the
web-based economy, is described in [63,64]. The auctions
considered, involve network auctions in which any bidder,
except for the one who has made the most recent bid and is
waiting for a response from the seller, is allowed to move at any
time from one auction to another one. A mathematical analysis
of such automated networked auctions is given in [65,66], where
the author studies different auction types, both in terms of the
price a good will fetch and the income per unit time provided
by the auction, and provides mathematical models of automated
bidders and sellers who interact through a network.
A number of CPN nodes, geographically distributed, could
constitute a network market. This network can provide the users
with information such as where and when to buy or sell an item.
The network could consider application-level requirements,
for instance price-related or history-related factors, along with
network-level ones like routing or communication factors
[64,67,68]. Decisions could be based on ‘sensible decision’
algorithms, studied in [49], where the estimations are based
on probabilities that mix auction-specific factors, such as the
current price of the item under auction, the average selling price
in the past or the average wait-time for acceptance of the bid, and
network or routing factors, such as the current average delay in
The Computer Journal, 2009
8
G. Sakellari
the path towards the destination and the average loss probability
in the path. Therefore, with the CPN protocol, each user (seller
or buyer) of a networked auction would self-adapt their own
behaviour according to heterogeneous factors either auction or
network based.
5.10.
CPN and next-generation battlespace information
services
The CPN protocol is used for decision making in the communication layer of an agent-based architecture implemented as
part of the HYPERION project, funded by the UK Ministry
of Defence, which aims to provide an automated and adaptive information management capability embedded in defence
networks [69]. The overall system architecture is designed to
improve the situational awareness of field commanders by providing the ability to fuse and compose information services in
real time. The key technologies adopted to enable this include:
autonomous software agents, self-organizing middleware, a
smart data filtering system and a 3D battlespace simulation
environment. The role of CPN in this project is to provide
an adaptive network architecture with enhanced functionality
and resilience for networks for battlespace communication and
information services. Since network security is of primary concern for such communication networks, the DoS defence mechanism described in a previous section is a vital aspect of this
scheme.
6.
EXPERIMENTAL PERFORMANCE EVALUATION
Apart from the individual experiments conducted for each
enhancement and application presented in the previous sections,
the CPN has been thoroughly investigated for a variety of
performance metrics. All performance evaluation work has been
carried out in a real networking test-bed, running the CPN as a
module of the Linux kernel.
6.1. Adaptability
The ability of CPN to adapt to changing network conditions,
such as changes in traffic load, link failures or buffer overflows
has been experimentally evaluated in [70]. The experiments
showed that the CPN managed to find new routes in order to
avoid obstructing traffic introduced in some of the links used by
the data traffic and also avoided links that were under failures.
Another issue studied experimentally in [70] was the impact of
the ratio of SPs to total packets, on the overall performance of
CPN. The experiments concluded that in order to achieve the
best performance for the DPs the percentage of SPs that should
be sent for discovery is 10% to 20% of the data packets’ rate.
Going beyond these values does not significantly improve the
QoS values for DPs. This was further investigated in [71], where
experimental data also show that a relatively small fraction of
SPs and ACKs, compared to total user traffic, is needed to serve
the users’ QoS goals. One must bear in mind that in CPN, SPs
and ACKs are not full sized Ethernet packets, but are actually
10% of the DPs’ size. If 20% of SP traffic is added, this will
result in 14% traffic overhead, when ACKs are generated by
both DPs and SPs, and only 4% of traffic overhead when ACKs
are only generated in response to SPs. Additionally it was shown
that a small number of SPs would suffice to initially establish
a connection. In addition to the experiments on the test-bed in
[71], simulations were conducted for a 1000-node network with
results similar to the real experiments.
6.2.
QoS goals
The choice of a ‘goal’ and ‘reward’ function for packetized
voice applications is discussed in [5] and experiments conducted
for ‘voice over CPN’ are presented. The performance of
CPN is detailed via several measurements, and the resulting
QoS is compared with that of the IP routing protocol under
identical conditions showing the gain resulting from the use
of CPN.
Measurements indicating how the CPN protocol can respond
to different QoS goals are also presented in [6,8]. Composite
goal functions for taking into account both delay and packet
loss are proposed. In [8], the measurements suggest that CPN
networks effectively adapt routing behaviour to the QoS goal
that is specified; [6] also provides experimental results when a
GA is used to create new routes from the information discovered
by the SPs. The results showed that the GA daemon significantly
improves QoS under light network traffic but not under high
traffic conditions. An explanation given by the authors is that
the GA tends to delay decision making, since it stores more
information and makes recommendations based on longer-term
trends.
The experiments in [72] show that the CPN can implement
distributed adaptive shortest-path routing and approximately
find the shortest paths. Extensive experiments compare the
shortest-path CPN, where the QoS goal is the minimum hop
count, with a CPN routing using minimum delay and a version
where routing is based on a combination of hop count and
forward delay. The experiments where conducted under low,
medium and high background traffic and show that the use of
criteria more complex than the shortest number of hops can
provide better overall QoS.
The use of delay implies a collection of timestamps along
packets’ paths, which add overhead to the packets that may
become significant in long routes. Therefore the authors of
[73] implemented a composite QoS goal metric that consists
of path length and buffer occupancy of nodes to achieve
traffic balancing and to identify low-delay paths in a network.
Experimental results in a wired test-bed and wireless ad hoc
simulations show that a routing goal that combines path length
and buffer occupancy in nodes offers the advantage of producing
approximately the same performance as that of using delay but
with a smaller packet overhead.
The Computer Journal, 2009
The Cognitive Packet Network: A Survey
6.3.
Realistic environments
A set of experiments, which demonstrate how the CPN performs
in a realistic environment of a 46-node test-bed, have been
presented in [57]. The performance of CPN was compared to
that of an industry standard routing protocol, the open shortest
path first (OSPF) routing protocol, the current industry standard
and widely used in IP networks. A 46-node test-bed was used,
the topology of which represents a real-world topology, the
Swiss Education and Research Network (SWITCHlan), which
is used by universities and some education sites in Switzerland.
The administrators of this network provided the authors of
[57] with details on their 46-router backbone, complete with
bandwidth, OSPF costs and link-level delays. Because the cost
of each link is proportional to its delay, OSPF routing converges
to the minimal delay path, giving a baseline for comparison. The
experiments show that the routes computed by CPN are as good
as those computed a priori using administrator-defined costs.
Furthermore, the paper gives experimental results showing that
an RNN with RL can autonomously learn the best route in
the network simply through exploration in a very short timeframe and demonstrates that the CPN protocol is able to adapt
to changes in the network environment quickly, by switching to
a new optimal route in the network. The experiments also show
that the original algorithm of the CPN can be improved when
data traffic is re-routed only when a better route is found (CPN
with sorting).
6.4.
Intermittent node failures
In [74], the authors compare the performance of CPN in the
existence of intermittent node failures, caused by the spread
of a network worm, with that of the OSPF routing protocol.
The experimental results showed that the CPN performs much
better than the OSPF routing, by adapting more quickly to the
network changes and avoiding both the failed nodes and the
congestion created by the failures, while keeping the average
delay that the users experience at smaller levels. The authors of
[74] also describe a failure detection element used to enhance
the resilience of the CPN during node failures.
6.5.
Routing oscillations
Although oscillations are generally considered a weakness
of a network, performance evaluations indicate that routing
oscillations do not severely degrade performance as would be
expected, and high performance can still be obtained even in
the presence of oscillations [75,76]. The way oscillations can
be controlled was studied, and two different parameters that
largely impact the rate at which oscillations are observed were
tested: the use of probabilistic path switching, which can be
used both to make path switching more asynchronous and to
vary the rate at which switching decisions are made, and the
introduction of a decision threshold, which will only allow
9
path switching if the gain expected from switching exceeds a
certain minimal value. Both of these control schemes are easy
to implement and provide an effective way to limit oscillations
and their negative consequences, although [75] brings into
question whether there are actually such negative consequences
in routing protocols that are based on self-monitoring and
adaptation such as the CPN protocol.
7.
CPN AND FUTURE INTELLIGENT NETWORKS
Scalability issues barricade the broad use of QoS mechanisms
in the Internet, since it is impossible for each Internet router to
deal with the QoS needs of each individual connection of the
network. The scalability issue can be tackled with the use of
intelligent network routers (INRs) that would be responsible
only for the local users and services [77]. Source routing
removes the burden of routing decisions from all but the local
INR, which reduces overhead and removes the need of ‘per
flow’ information handling apart from the INRs where the
flows are initiated. Thus, the CPN algorithm, which runs at
the packet transport level, can be abstracted to a higher level
where it searches for services. Users and services can specify
their requests in terms of the services that they seek and QoS
criteria that they need, and also the price that they are willing
to pay. In turn the services and the network would dynamically
try to find the best-suited service for the users, satisfy them as
best as they could and inform them of the level at which their
requests are being satisfied and at what cost. Thus an intelligent
network of the future, according to [77], can be viewed as an
overlay network composed of INRs (CPN routers) that offer
a flexible and self-organizing communication environment for
users and services and can be used for finding services and
users, for routing through the network and for self-observation
and network monitoring in order to obtain the best QoS and
performance.
8.
CONCLUSIONS
We have described the CPN routing protocol, the underlying
mathematical principles and the learning and decision
algorithms that it uses.
In CPN, the routing decisions are made at each node of the
network. Thanks to this distributed nature and the fact that it
uses real-time QoS measurements, it is more resilient to link or
node failures and more adaptive to network changes, such as
network congestion.
Also, since it is the users rather than the nodes that control
the routing, the traffic of different users can be routed under
different QoS criteria, depending on the application and the
required service. This is a unique contribution that CPN brings
in computer network research.
Finally, its numerous applications and extensive experimental
implementations in wired, wireless communication and sensor
The Computer Journal, 2009
10
G. Sakellari
networks, indicate that it is a highly adaptable protocol that is
easy to integrate with existing networking systems.
FUNDING
The Hyperion cluster project funded by the UK MoD’s Data
and Information Fusion Defence Technology Centre (DIF DTC)
research program and British Telecommunications plc (BT).
The CASCADAS project (EU FP6-027807). The ALADDIN
(Autonomous Learning Agents for Decentralised Data and
Information Networks) project which is jointly funded by a
BAE (British Aerospace) Systems and EPSRC (Engineering
and Physical Sciences Research Council) strategic partnership
(EP/C548051/1).
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