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(IJACSA) International Journal of Advanced Computer Science and Applications,
Vol. 2, No. 3, March 2011
54 | P a g e
http://ijacsa.thesai.org/
Multicasting over Overlay Networks – A Critical
Review
M.F.M Firdhous
Faculty of Information Technology,
University of Moratuwa,
Moratuwa,
Sri Lanka.
Mohamed.Firdhous@uom.lk
Abstract – Multicasting technology uses the minimum network
resources to serve multiple clients by duplicating the data packets
at the closest possible point to the clients. This way at most only
one data packets travels down a network link at any one time
irrespective of how many clients receive this packet. Traditionally
multicasting has been implemented over a specialized network
built using multicast routers. This kind of network has the
drawback of requiring the deployment of special routers that are
more expensive than ordinary routers. Recently there is new
interest in delivering multicast traffic over application layer
overlay networks. Application layer overlay networks though
built on top of the physical network, behave like an independent
virtual network made up of only logical links between the nodes.
Several authors have proposed systems, mechanisms and
protocols for the implementation of multicast media streaming
over overlay networks. In this paper, the author takes a critical
look at these systems and mechanism with special reference to
their strengths and weaknesses.
Keywords – Multicasting; overlay networks; streaming media;
I. INTRODUCTION
Media multicasting is one of the most attractive
applications that can exploit the network resources least while
delivering the most to the clients. Multicast is a very efficient
technology that can be used to deliver the same content to
multiple clients simultaneously with minimum bandwidth and
server loading. The use of this technology is many and web
TV, IP TV, web radio, online delivery of teaching are few of
them [1-4]. In all these applications, the server continues to
deliver the content while clients can join and leave the
network any time to receive the content, but they would
receive it only from where they joined the stream.
Traditionally multicasting was tied to the underlying
network with special multicast routers making the necessary
backbone. Multicast routers are special type of routers with the
capability of duplicating and delivering the same data packet
to many outgoing links depending on which links clients
reside downstream. Traditional IP based routers are unicast
that receive data packets on one link and either forward that
packet only to one outgoing link or drop it depending on the
routing table entries. Broadcasting is totally disabled in the
internet due to the unnecessary congestion caused by
broadcast traffic that may bring the entire internet down in a
short time.
Using the network layer multicast routers to create the
backbone of the internet is not that attractive as these routers
are more expensive compared to unicast routers. Also, the
absence of a multicast router at any point in the internet would
defeat the objective of multicasting throughout the internet
downstream from that point. Chu et al., have proposed to
replace the multicast routers with peer to peer clients for
duplicating and forwarding the packets to downstream clients
[5]. In this arrangement, the duplicating and forwarding
operation that makes multicasting attractive compared to
unicast and broadcast would be carried out at the application
layer. The multimedia application installed in end nodes
would carry out the duplicating and forwarding operation in
addition to the display of the content to the downstream nodes
that request the stream from an upstream node.
A peer to peer network is formed by nodes of equal
capability and act both as client and server at the same time
depending on the function performed. Peer to peer networks
and applications have advantages over traditional client server,
such as the elimination of single point of failure, balance the
network load uniformly and to provide alternate path routing
easily in case of link failures [6]. A peer to peer network forms
an overlay network on top of the existing network
infrastructure. The formation of this overlay network by the
peer to peer nodes make the resulting network resilient to
changes in the underlying network such as router and link
failures and congestion.
An overlay network is a computer network which is built
on top of another network. Nodes in the overlay can be
thought of as being connected by virtual or logical links, each
of which corresponds to a path, perhaps through many
physical links, in the underlying network. Usually overlay
network run at the application layer of the TCP/IP stack
making use of the underlying layer and independent of them
[7]. Figure 1 shows the basic architecture of an overlay
network. The nodes in an overlay network will form a network
architecture of their own that may be totally different and
independent of the underlying network.
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Figure 1: An Overlay Network
II. MULTICASTING OVER OVERLAY NETWORKS
Several authors have proposed protocols and services how
to implement overlay based multicast over the internet. In this
section, an in depth analysis would be carried out on the some
of the most recent protocols and application in terms of their
architecture, advantages and disadvantages.
Chen et al., have proposed ACOM Any-source Capacity-
constrained Overlay Multicast system. This system is made up
of three multicast algorithms namely Random Walk, Limited
Flooding, and Probabilistic Flooding on top of a non-DHT
(Distributed Hash table) overlay network with simple
structures [8]. ACOM divides the receiving nodes into
multicast groups based on the upload bandwidth of a node as
the upload bandwidth is determining factor as a node may be
required to transfer multiple copies of the same packet over
the uplink. The number of nodes in a multicast group hence
depends on the capacity of the uplink making the nodes with
higher capacity to support large number of neighbors. An
overlay network is established for each multicast group that
transforms the multicast stream to a broadcast stream with the
scope of the overlay.
The overlay network is made up of two components,
namely an unrestricted ring that is fundamentally different
from the location specific DHT based ring and the random
graph among the nodes. The ring maintenance is carried out
by requiring the nodes to the next node as a successor and a
few other nodes in order to avoid the problem of ring breakage
due to node leaving the network.
Packet delivery with the overlay network is carried out
using a two phase process. In Phase 1, packets are forwarded
to a random number of neighbors within a specific number (k
– a system parameter) of hops. This follows a tree structure of
delivery in Phase 1. For the purpose of delivery in Phase 2, the
ring is partitioned into segments and each Phase 1 node is
made responsible for a segment and required to deliver the
packet to its successor which in turn forwards to its successor
until the packet reaches a node that has already received the
packet.
In practice what ACOM does is to forward the packet in
Phase 1 using a tree structure to a number of random nodes
and then in Phase 2 the packet is forwarded in a unicast
fashion down the network until all the nodes within the
multicast subgroup receives the packet.
Even though ACOM presents several advantages such as
maintenance of virtual tree in place of multicast tree bound to
the physical networks, it has certain disadvantages too. The
main disadvantage is the total disregard of the physical
distances when setting up of the virtual tree. ACOM also has
certain other disadvantages in the formation and maintenance
of the ring network.
Liu and Ma have proposed a framework called Hierarchy
Overlay Multicast Network based on Transport Layer Multi-
homing (HOMN-SCTP) [9]. This framework uses transport
layer multi-homing techniques. HOMN-SCTP can be shared
by a variety of applications, and provide scalable, efficient,
and practical multicast support for a variety of group
communication applications. The main component of the
framework is the Service Broker (SvB) that is made up of the
cores of of HOMN-SCTP and Bandwidth- Satisfied Overlay
Multicast (BSOM). The BSOM searches for multicast paths
to form overlay networks for upper layer QoS-sensitive
applications, and balance overlay traffic load on SvBs and
overlay links.
HOMN-SCTP is capable of supporting multiple
applications on it and helps these applications meet the
required QoS requirements. Since it is built on top of TCP,
error control is totally delegated to the underlying layer.
Nevertheless HOMN-SCTP is not suitable for applications
such as streaming media that run on top of UDP.
Fair Load Sharing Scheme proposed by Makishi et al.,
concentrates on improving throughput instead of reducing the
delay on multisource multimedia delivery such as video
conferencing using Application Layer Multicasting (ALM)
[10]. This scheme is based on tree structure and tries to find
the tree with the highest possible throughput. The main
objective of the protocol is to improve the receiving bit rate of
the node that is having lowest bit rate out of all receivers. The
proposed scheme is an autonomous distributed protocol that
constructs the ALM where each node refines its own subtree.
The end result of this refinement of subtrees is the automatic
refinement of the entire tree structure.
This scheme assumes that there is backbone ring network
that has unlimited bandwidth and all the receiving nodes can
connect to this ring. The links from the backbone to the end
nodes make the tree network which this protocol tries to
improve for the receiver bandwidth. The end nodes initially
make a basic (ad-hoc) tree network that would be improved
iteratively over time until the all the nodes receive an optimum
bandwidth in terms of bit rate. Once the basic tree structure
has been established using unicast links, the nodes
communicate with each other exchanging estimation queries
with communication quality in terms of bandwidth to be
allocated and the delay. When a better path than that is
currently being used is found, the tree readjusts itself moving
to the node with the better quality. This process is continued
until the optimum link allocation is achieved.
Peng and Zhang have discussed the problem associated
with the intranet based multimedia education platform [4].
They mainly analyze the principles of this education platform,
but in the process they introduce Computer Assisted
Instruction (CAI) multicasting network as the backbone of the
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network. The CAI multicasting network is based on the IPv4
special class D multicasting (224.0.0.0 ~ 239.255.255.255)
address block. The multicasting network has been
implemented using the Winsock2 mechanism. Once the server
node (teacher) has initiated the stream the user (students) can
join the multicast network and receive the multicast stream on
their computers. This system is suitable only for in class
teaching or within limited distance where high networking
resources are available.
Reduced delay and delay jitter are very important to
viewing quality of any video from the viewers’ point. In
traditional IP multicasting RSVP and DiffServ algorithms
were used to reserve resources prior to starting the streaming.
Each router on the path from the server to the client uses a
dynamic scheduling algorithm to deliver the packet based on
the QoS requirements. Szymanski and Gilbert have proposed a
Guaranteed Rate (GR) scheduling algorithm for computing the
transmission schedules for each IQ packet-switched IP router
in a multicast tree. The simulation results have shown that this
algorithm manages on average two packets in a queue
resulting in very low delay and jitter which can practically
ignored as zero jitter [11].
Even though this algorithm virtually eliminates delay jitter,
it is bound to the underlying layers as it needs to be
implemented on routers.
Lua et al., have proposed a Network Aware Geometric
Overlay Multicast Streaming Network. This network exploits
the locality of the nodes in the underlay for the purpose of
node placement, routing and multicasting. This protocol
divides the nodes into two groups called SuperPeers and Peers.
SuperPeers form the low latency, high bandwidth backbone
and the Peer connect to the nearest SuperPeer to receive the
content [12].
SuperPeers have been elected based on two criteria,
namely: the SuperPeer should have sufficient resources to
serve other SuperPeers and Peers and they must be reliable in
terms of stability not join and leave the network very
frequently. Network embedding algorithm computes node
coordinates and geometric distances between nodes to
estimate the performance metrics of the underlying network
such as latency. Peers joining the overlay network calculate
the total Round Trip Time (RTT) to the SuperPeers and join
the SuperPeer that has the lowest RTT. A SuperPeer joining
the network calculates the RTT to all the existing SuperPeers
and joins the ones with the lowest RTT and then creates
connection with other six SuperPeers around it.
When a SuperPeer leaves the network, the other
SuperPeers would detect this by the loss of heartbeat signal
from the node that has left and will reorganize themselves by
sending discovery broadcast messages to all the SuperPeers.
The Peers who are affected by the leaving of a SuperPeer need
to reconnect to the overlay network by selecting the nearest
SuperPeer.
The main strengths of this scheme can be summarized as it
has good performance in terms of latency and efficient
transmission of packets via the high speed backbone network
whereas the weaknesses include the heavy dependence on the
geographical locality, election of SuperPeers.
Pompili et al., have presented two algorithms called
DIfferentiated service Multicast algorithm for Internet
Resource Optimization (DIMRO) and DIfferentiated service
Multicast algorithm for Internet Resource Optimization in
Groupshared applications (DIMRO-GS) to build virtual
multicast trees on an overlay network [13].
DIMRO constructs virtual source-rooted multicast trees for
source-specific applications taking the virtual link available
bandwidth into account. This avoids traffic congestion and
fluctuation on the underlay network. Traffic congestion in the
underlay would cause low performance. This keeps the
average link utilization low by distributing data flows among
the least loaded links. (DIMRO-GS) builds a virtual shared
tree for group-shared applications by connecting each member
node to all the other member nodes with a source-rooted tree
computed using DIMRO.
Both these algorithms support service differentiation
without the support of the underlying layers. Applications with
less stringent QoS requirements reuse resources already
exploited by members with more stringent requirements.
Better utilization of network bandwidth and improved QoS are
achieved due to this service differentiation.
System built using these algorithms would result in better
performance due to differentiation of applications based on
QoS requirements, but node dynamic may bring the quality of
the system down.
Wang et al., have proposed an adapted routing scheme that
minimizes delay and bandwidth consumption [14]. This
routing algorithm creates an optimum balanced tree where the
classical Dijkstra's algorithm is used to compute the shortest
path between two nodes. In this scheme the Optimal Balance
of Delay and Bandwidth consumption (OBDB) is formulated
as:
OBDB = (1−α )D +αB
Where D is the minimal delay criteria and B is the minimal
bandwidth consumption criteria.
From the above formula, it can be seen that one routing
method may lead to another bad performance. The value of α
is calculated based on the nature of the application. As the
delay and bandwidth can be tuned to suit application
requirements, applications performance can be controlled. But
these algorithms will have performance issues in the face of
node dynamics.
Kaafar et al., have proposed an overlay multicast tree
construction scheme called LCC: Locate, Cluster and
Conquer. The objective of this algorithm is to address
scalability and efficiency issues [15]. The scheme is made up
of two phases. One is a selective locating phase and the other
one is the overlay construction phase. The selective locating
phase algorithm locates the closest existing set of nodes
(cluster) in the overlay for a newcomer. The algorithm does
not need full knowledge of the network to carry out this
operation, partial knowledge of location-information of
participating nodes is sufficient for this operation. It then
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allows avoiding initially randomly-connected structures with
neither virtual coordinates system embedding nor fixed
landmarks measurements. Then, on the basis of this locating
process, the overlay construction phase consists in building
and managing a topology-aware clustered hierarchical overlay.
This algorithm builds an efficient initial tree architecture
with partial knowledge of the network but node dynamics may
result in poor performance with time.
Walters et al., have studied the effect of the attack by
adversaries after they become members of the overlay network
[16]. Most of the overlay protocol can handle benign node
failures and recover from those failures with relative ease, but
they all fail when adversaries in the network start attacking the
nodes. In this study, they have identified, demonstrated and
mitigated insider attacks against measurement-based
adaptation mechanisms in unstructured multicast overlay
networks.
Attacks usually target the overlay network construction,
maintenance, and availability and allow malicious nodes to
control significant traffic in the network, facilitating selective
forwarding, traffic analysis, and overlay partitioning. The
techniques proposed in this work decrease the number of
incorrect or unnecessary adaptations by using outlier
detection. The proposed solution is based on the performance
of spatial and temporal outlier analysis on measured and
probed metrics to allow an honest node to make better use of
available information before making an adaptation decision.
This algorithm creates a resilient overlay network in the
both structured and unstructured overlay network in the
presence of malicious attacks. But the strict nature of the
algorithm may delay the adaptation of the network in the event
of node dynamics disrupting the flow of information.
Alipour et al., have proposed an overlay protocol known as
Multicast Tree Protocol (OMTP) [17]. This protocol can be
used to build an overlay tree that reduces the latency between
any two pair of nodes. The delay between the nodes has been
reduced by adding a shortcut link by calculating the utility link
between two groups.
The main advantage of this algorithm is the efficient data
transfer but the efficiency may be affected by node dynamics
in the overlay network.
Bista has proposed a protocol where the nodes informs the
other nodes its leaving time when it joins the network [18].
Using this leaving time information new nodes are joined at
the tree in such a manner early leaving nodes would make the
leaf nodes down the line and the nodes that would stay longer
would be at the higher levels. It has also been proposed a
proactive recovery mechanism so that even if an upstream
node leaves the tree, the downstream nodes can rejoin at
predetermined nodes immediately, so that the recovery time of
the disrupted nodes is the minimum.
These algorithms have several drawbacks including, the
prior notice of the duration of stay in the network,
arrangement of nodes based on the time of stay and central
control to manage the node information.
Gao et al., have proposed a hybrid network combining the
IP multicast network and the mesh overlay network [19]. The
main objectives of the design were to build a network with
high performance (low end-to-end transmission latency, high
bandwidth mesh overlay links), low end-to-end hop count and
high reliability.
This algorithm results in a good structure combining
multicast and overlay, but the resulting network is still
dependant on the physical network and managing the mesh
network is expensive in terms of network resources.
Wang et al., have proposed hybrid overlay network
combining a tree and mesh networks called mTreebone [20].
In the mTreebone, the tree forms the backbone and local nodes
make a mesh to share content. The backbone tree network has
been constructed by identifying the stable nodes as the churn
of the backbone would be more expensive in terms of service
disruption than the leaf node churn. On top of the tree based
overlay network a mesh network has been created in order to
handle the effect of node dynamics. Since the mesh network
has been updated regularly using keep alive packets, any
change in the mesh network immediately notified to all the
other mesh nodes. In this design heuristics have been used to
predict the stability of the nodes assuming the age of the in the
network to be directly proportional to the probability of it
staying longer in the network. That is longer a node stays in
the network, larger the probability it would stay even further
and more stable.
This algorithm results in a resilient overlay network but the
network may carry duplicate packets in some part of the
network. Also, the maintenance of the mesh network requires
large network resources.
Guo and Jha have shown that the main problem in overlay
based multicast networks is to optimize routing among CDN
servers in the multicast overlay backbone in such a manner
that it reduces the maximal end-to-end latency from the origin
server to all end hosts is minimized [21]. They have identified
this as the Host-Aware Routing Problem (HARP) in the
multicast overlay backbone. The main reason for HARP is the
last mile latency between end hosts and their corresponding
proxy servers. The author of this paper have framed HARP as
a constrained spanning tree problem and shown that it is NP-
hard. As a solution they have presented a distributed algorithm
for HARP. They also have provided a genetic algorithm (GA)
to validate the quality of the distributed algorithm.
This structure results in a low latency routing path
improving QoS but the non-consideration of node dynamics
will affect the overall quality of the network.
Table I summarize the systems, algorithms and mechanism
discussed above with special reference to their advantages and
disadvantages. hows the comparison of these works with
special reference to their advantages and disadvantages.
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TABLE I: COMPARISON OF MULTICASTING OVERLAY NETWORKS
Work
System/Protocol Proposed
Advantages
Disadvantages
1.
[4]
Computer Assisted
Instruction (CAI)
Multicasting Network
This system is suitable for in class
teaching or teaching within a limited
area using the high technology to larger
classes.
This system uses the existing technologies to build a
platform and hence no new technology has been introduced
in terms of multicasting.
This technique may work well on an intranet but cannot
be ported to the internet as the internet lacks support for
class D multicast addresses.
2.
[8]
Any-source Capacity-
constrained Overlay
Multicast System
Only maintenance of virtual tree is
necessary, no strict multicast trees are
maintained.
Resilient to node dynamics as node
maintain multiple neighbors in its
neighbor table.
Simple maintenance of unrestricted
ring.
The random nature of the tree formation in Phase 1 totally
ignores the physical distance from the source to that node.
The researchers have considered this as an advantage, but
this will make unnecessary delay in delivering a packet to a
node that may be physically closer to the source but not
selected as a Phase 1 node.
Even though, the unrestricted ring is better than location
specific DHT ring in terms of node creation and
maintenance, the unrestricted node is inefficient in
forwarding packets and the logical neighbors may not
always be neighbors physically.
Phase 2 forwarding is essentially unicast and cascaded in
ACOM which may create a lot of delay in the ultimate
delivery to the last node. The problem will be more severe in
case of a high capacity Phase 1 node as it will have a large
number of neighbors.
Converting any portion of the multicast network to a
broadcast network is inherently in efficient as broadcast is an
inefficient protocol in terms of network utilization.
3.
[9]
Hierarchy Overlay
Multicast Network
This framework builds an
infrastructure on which multiple
applications can be built.
The framework helps application to
meet the required QoS.
The framework makes use of the
facilities of the underlying layer for the
error control as it is built on top of
TCP.
The framework has been built on top of TCP and hence is
not suitable for best effort services running on UDP
especially streaming media applications.
TCP is a high overhead protocol compared to UDP and
hence, this protocol would inherently have high overhead.
This framework will not meet the QoS requirement in
terms of delay and delay jitter on high loss links as TCP
would create delay and delay jitter on high loss networks and
hence not suitable for real time applications like media
streaming.
4.
[10]
Fair Load Sharing Scheme
This scheme works purely on the
application layer without directly
depending on the lower layers.
The protocol results in the optimum
receiving tree structure for a given
situation.
This protocol is only suitable for network with stable
receiving nodes.
Dynamic nature of the nodes joining and leaving the
network will make the tree fail as there is no mechanism in
the protocol to handle node dynamics.
The dynamic nature of the internet would make the tree
structure oscillating as the communication quality heavily
depends on the dynamics of the underlying network.
Tree management is costly in terms of information
exchanged between the nodes as the continuous information
exchange is needed.
5.
[11]
Guaranteed Rate (GR)
Scheduling Algorithm
This algorithm virtually eliminates
delay jitter in received video stream
resulting in high quality reception.
The algorithm is simple enough to
implement on any IP router.
This algorithm needs to be implemented on IP routers and
hence bound to the underlay network.
This does not provide a solution to the existing problem of
how to use the existing network as it is to transport
multimedia streaming without depending on the underlay
network.
6.
[12]
Network Aware Geometric
Overlay Multicast
Streaming Network
This algorithm has good performance
in terms of latency as the physical
location of the node has been computed
and used as a parameter in forming the
overlay network.
Breaking the network into two layers
result in efficient transmission of
Since the algorithm heavily depends on the geographical
location, an efficient geographical location computing
algorithm is vital in addition to managing the overlay
network.
Since the election of SuperPeers depends on the condition
that the SuperPeer should have high dependability in terms
of churn frequency, past historical data may be necessary to
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packets as the SuperPeers are connected
to each other via a high speed backbone
network.
determine the reliability of a node accurately.
SuperPeers should also have sufficient resources to
support other SuperPeers and Peers in the network, if the
resource availability information is obtained from the node
itself, this would be an invitation to rogue nodes to highjack
the entire backbone network using false information.
The Peers that are affected by a SuperPeer leaving the
network do not have automatic transfer to another
SuperPeer, this drastically affect the reliability of the entire
multicast operation.
7.
[13]
DIfferentiated service
Multicast algorithm for
Internet Resource
Optimization (DIMRO)
and
DIfferentiated service
Multicast algorithm for
Internet Resource
Optimization in
Groupshared applications
(DIMRO-GS)
These algorithms result in good
performance for both QoS stringent
applications and non QoS stringent
applications due to service
differentiation.
The node dynamics has not been considered when
designing these protocols, hence node dynamics would
drastically bring the quality of the network down.
8.
[14]
Adapted Routing Scheme
These algorithms results in better
performance for any kind of application
as delay and bandwidth can be tuned to
meet the application requirements.
The node dynamics has not been considered in this
routing scheme and hence node dynamics would drastically
bring the quality of the network down.
9.
[15]
LCC : Locate, Cluster and
Conquer multicast tree
This algorithm creates the initial tree
architecture very efficiently with partial
knowledge of the overlay network.
The node dynamics has not been considered in this
routing scheme and hence node dynamics would drastically
bring the quality of the network down.
10.
[16]
This algorithm creates a resilient
overlay network in the face of attacks
by malicious nodes.
The algorithm works on unstructured
overlays and hence can be easily
adapted to structured overlay networks
too.
The strict nature of the algorithm delays the adaptation of
the network to genuine node dynamics.
The delay in network adaptation may result in
unnecessary disruptions to the streaming and affect the
quality of service of the application.
11.
[17]
Multicast Tree Protocol
This algorithm results in efficient
data transfer between the source to
destination in terms of reduced delay.
The node dynamics has not been considered in
constructing the tree and hence node churn would result in
broken trees affecting the downstream nodes.
12.
[18]
Proactive Fault Resilient
Overlay Multicast Network
A proactive mechanism has been
proposed that keeps the downstream
nodes informed when to expect an
upstream node leaving for the purpose
of reconnection.
The setup shown in this algorithm is very artificial as the
nodes need to inform the other nodes their duration of stay at
the beginning itself. This against the basic spirit of the
overlay network where nodes can join and leave the network
at their choice.
Arranging the nodes in the in the reverse order of the
duration of the stay is very impractical as it may result in an
inefficient structure if the longest staying node is very far
away from the source.
Node dynamics has not been properly considered in this
design as the affected nodes need to rejoin the network
themselves.
A central control would be needed to keep the information
about the nodes joining time and duration of stay for proper
operation of these protocols.
13.
[19]
Hybrid IP multicast mesh
overlay network
Results in a good structure
combining both IP multicast and mesh
overlay.
This network is not totally independent of the underlying
network as it depends on the IP multicast protocol.
Establishing a full mesh network is not efficient as it
would need a large memory maintain information about each
and every node. This problem would become more acute as
the network size grows to very large. This would result in
scalability problems.
Maintaining the full mesh information also has the
problem of updating the mesh information table as all the
nodes need to update the information table every time a node
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joins or leaves the network. This would result in instability
in the network as the network grows.
14.
[20]
mTreebone
This structure show better resilience
to node dynamics compared to pure tree
structured overlay networks.
Also this structure would have lesser
load on the backbone tree as it would
carry only one data packet at any one
time.
This structure has the shortcoming of data duplicates of
content and unnecessary congestion in the local network in
managing the mesh network at the local level.
The maintenance of the mesh network is also more
expensive as it needs constant updates about the node
structure and availability and requires large memory to
maintain the mesh information on each and every mesh
node.
15.
[21]
Distributed Algorithm for
HARP
This structure optimizes the routing
between the source node and the
receiving nodes in such a manner that
the total latency is reduced. This results
in the better QoS in terms of reducing
the maximum delay between the source
and the clients.
The node dynamics has not been considered in
constructing the tree and hence node churn would result in
broken trees affecting the downstream nodes.
III. CONCLUSION
In this work, the author has taken a critical look at the
literature on multicasting over overlay networks. Multicasting
is one of the most promising applications over the Internet as it
requires relatively lower overhead to serve a large number of
clients compared to the traditional unicast or broadcast
applications. Under the most optimum conditions, multicast
systems will have only one data packet in any part of the
network irrespective of how many clients are served
downstream. Also, the multicast server will transmit only one
data packet irrespective of how many clients receive the copies
of such packets. In traditional multicast networks, multicast
routers placed at strategic locations duplicate the packets
depending on the requests they receive. Deploying multicast
routers throughout the Internet is not practical due to the
amount of investment required for such an operation. Hence
researchers have recently focused their attention on using
overlay networks to realize the goal of implementing multicast
applications in the Internet. By implementing multicasting over
application layer overlay networks, the function of duplicating
packets has been moved from the network layer (Internet
Protocol layer) to application layer.
One of the most challenging tasks in overlay networks in
the management of node dynamics. In overlay networks, nodes
can join and leave the network at their will. In traditional
multicast networks deployed using multicast routers, node
dynamics is not a major concern as the infrastructure is
considered to be stable and only the leaf or end nodes join and
leave the network. The churn of end nodes will not affect any
other client node as they are not dependant on each other.
Using overlay networks for multicasting presents a new
challenge as the end nodes are required to play a dual role of
clients as well as forwarding agents to other client nodes
downstream. Node dynamics will have different effects on the
end user applications depending on the type of application.
Real time applications will be more affected by node dynamics
than non-real time applications due to the disruptions resulting
from such dynamics.
In this work, media streaming has been selected as the
application to be run over the overlay network for the purpose
of testing the quality of the overlay networks. Media streaming
has been selected as the application due to its popularity and
the stringent QoS requirements that are required to be met for
successful deployment of such applications in the Internet.
Media streaming requires non disrupted flow of packets from
the source to destination. Hence managing node dynamics is
very important for successful implementation of these
applications on the overlay networks.
In this paper, the author presents a critical review of
systems, algorithms and mechanism proposed in the recent
literature. Special attention has been paid to the advantages and
disadvantages of these proposed systems with respect to
managing node dynamics. The paper looks at each proposal
critically on the mechanism proposed, their strengths and
weaknesses. Finally, the results of the analysis have been
presented in a table for easy reference.
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AUTHOR’S PROFILE
Mohamed Fazil Mohamed Firdhous is a senior lecturer
attached to the Faculty of Information Technology of the
University of Moratuwa, Sri Lanka. He received his BSc
Eng., MSc and MBA degrees from the University of
Moratuwa, Sri Lanka, Nanyang Technological University,
Singapore and University of Colombo Sri Lanka respectively.
In addition to his academic qualifications, he is a Chartered
Engineer and a Corporate Member of the Institution of Engineers, Sri Lanka,
the Institution of Engineering and Technology, United Kingdom and the
International Association of Engineers. Mohamed Firdhous has several years of
industry, academic and research experience in Sri Lanka, Singapore and the
United States of America.
