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Lecture 4
Advance Topics in
Networking
McGraw-Hill Technology Education
Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.
Lecture Overview
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Presenting a Research Topic
Sample Thesis Topics
More Thesis Topics
Ad hoc Networking
Reviewers Guidelines
Paper Review Guidelines
First Papers: For this week
Presenting a Research Topic
Typical steps of thesis research
• accumulate background
– network track courses
– independent studies, research group meetings
• define problem, search literature, and develop
solution
• implement a prototype (in JAVA or C/C++)
• measure and analyze performance of prototype
• summarize results in one technical paper / thesis
Sample Thesis Topics
• Programming of CISCO Routers
– How to deploy new services without modifying IOS?
• Policy-based Networking
– How to efficiently detect conflicts among policy rules
specified
• Smart Routing and Rerouting Algorithms
– How to reduce call blocking probability and data loss rate?
• Study of Distributed Denial of Service attacks
– How to identify sources of attacks?
– How to filter out malicious traffic early?
More Thesis Topics
• Security Protocols for Wireless LANs
– How to strengthen WEP?
– How to detect intrusions?
• Extreme (Ad hoc) Networking
– How to mitigate effect of large propagation
delays?
– How to guarantee performance to selected
traffic?
More Thesis Topics
• Mobile Agents and Survivable
Networking
– How to make a service ubiquitous, i.e.,
available while moving around the network
and regenerating if necessary
• Software Architecture for
Dynamically Reconfigurable Systems
– How to reduce programming complexity of
these systems?
802.11a/b/g Networks
Some slides taken from UIUC Wireless Networking Group
802.11a/b/g
• Operates in 2 different modes:
– Infrastructure mode
• Associates with an access point
• All communication goes through the access
point
• Used for wireless access at a company or
campus
– Peer-to-Peer Ad Hoc Mode
• If two nodes are within range of each other they
can communicate directly with no access point
• A few users in a room could quickly exchange
files with no access point required
Infrastructure Access
• Access Points:
– Provide infrastructure access to mobile
users
– Cover a fixed area
– Wired into LAN
Peer to Peer Ad Hoc Mode
Infrastructure Access
Infrastructure Access
802.11a/b/g are multi-rate devices
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
MAC Layer Fairness Models
• Per Packet Fairness: If two adjacent senders
continuously are attempting to send packets,
they should each send the same number of
packets.
• Temporal Fairness: If two adjacent senders
are continuously attempting to send packets,
they should each be able to send for the same
amount of medium time.
• In single rate networks these are the SAME!
Temporal Fairness Example
Per Packet Fairness
11 Mbps
802.11
Packet
Fairness
OAR
Temporal
Fairness
11 Mbps
Link
0.896
3.533
1 Mbps Link
0.713
0.450
Total
Throughput
1 Mbps
Temporal Fairness
11 Mbps
1.609
3.983
1 Mbps
802.11b Channels
• 11 available channels (in US)
• Only 3 are non-overlapping!
802.11b Channel Usage
Channel 1
Channel 6
Channel 11
802.11b Channel Reuse
Problems
• Access Point placement depends on
wired network availability
• Obstructions make it difficult to provide
total coverage of an area
• Site surveys are performed to determine
coverage areas
• Security Concerns: rogue access points
in companies etc..
• Each Access Point has limited range
Peer to Peer Ad Hoc Mode
Peer to Peer Ad Hoc Mode
X
Problems
• Communication is only possible
between nodes which are directly in
range of each other
Problems for both Infrastructure and Ad hoc Mode
• If nodes move out of range of the
access point (Infrastructure Mode)
OR
• nodes are not in direct range of each
other (Ad Hoc Mode)
• Then communication is not possible!!
What if ??
Multi-hop Infrastructure Access
Multi-hop Ad Hoc Network
OR
Multi-hop Infrastructure Access
• Nodes might be out of range of the
access point, BUT in range of other
nodes.
• The nodes in range of the access point
could relay packets to allow out of range
nodes to communicate.
• NOT part of 802.11
Multi-hop Ad Hoc Network
• If communication is required between
two nodes which are out of range of
each other, intermediary nodes can
forward the packets.
• NOT part of 802.11
Source
Destination
How can this be done? -< ROUTING!!
– Wired Networks:
• Hierarchical Routing
– Network is divided into subnets
– Nodes look at network address and determine if the
address is directly reachable. If not, just forward to
the default gateway.
– Different protocols for different levels of the hierarchy
» RIP, OSPF, BGP
Wireless Routing
• Flat routing
– You can’t assume that since a node is in
your subnet that it is directly accessible
– Node must maintain or discover routes to
the destination
– All nodes are routers
Ad Hoc Networking
29
Initial Architectures
- Low power sensors networks
“surveillance” web
- small, relatively static, embedded ad hoc networks
`“bluetooth-type” networks
- Small-to-medium sized, mobile ad hoc networks
“802.11-style”
Terminlology
Mobile Ad Hoc Networking =
= Mobile, Multi hop, Wireless Networking
= Mobile Mesh Networking
= Mobile Packet Networking
Ad hoc network applicability
Scale
Network type
Commercial
Small scale
(few nodes)
home/office personal
industrial local
networks
Government specific Public Safety
Community/urban
communications
networks
“covert” networks
Large scale
(many nodes)
mobile cellular like
Large-scale military
network
local
Hybrid Communication Networks
Satellite overlay
MANET
No fixed infrastructure
High speed
backbone
network
Fixed/static infrastructure
IETF MANET standardization

MANET - established in 1997 chartered working group
within Internet Engineering Task Force (IETF)

Focussed on studying routing specification with the goal of
supporting network scaling up to hundreds of routers

Unicast routing protocol

Multicast routing protocol

Work on routing for large and small scale networks

Work relies on the existing IETF standards such as mobileIP and IP addressing

For large-scale MANET the lack of interest have put this
work in question

Flooding: work on requirements had started
Mobile Ad Hoc Networking (MANET)
 Dynamic topologies
 Bandwidth-constrained
 Asymmetric links with variable capacity
 Energy constrained
 Multiple technologies can be used simultaneously
Open issues
A
optimisation network layer and radio layers for
different systems (incl. 802.11, HiperLAN)
B
QoS support
C
secuirity
D
mobility
•
B, C, D issues could be orthogonal, joint optimization is very difficult (system design choice)
•
tradeoff between centralized and distributed
algorithms for B,C,D
Relevant ETSI activities
 MESA Project - ad hoc network on future Public
Safety communications
 BRAN - HiperLAN-2, other
Standardization challenges =>
There is need for standard-based approach at
the network layer.
Mobile Ad Hoc Networks
• Formed by wireless hosts which may be
mobile
• Without (necessarily) using a preexisting infrastructure
• Routes between nodes may potentially
contain multiple hops
Mobile Ad Hoc Networks
• May need to traverse multiple links to reach a
destination
Mobile Ad Hoc Networks (MANET)
• Mobility causes route changes
Why Ad Hoc Networks ?
• Ease of deployment
• Speed of deployment
• Decreased dependence on
infrastructure
Many Applications
• Personal area networking
– cell phone, laptop, ear phone, wrist watch
• Military environments
– soldiers, tanks, planes
• Civilian environments
– taxi cab network
– meeting rooms
– sports stadiums
– boats, small aircraft
• Emergency operations
– search-and-rescue
– policing and fire fighting
Challenges
• Limited wireless transmission range
• Broadcast nature of the wireless medium
• Packet losses due to transmission errors
• Mobility-induced route changes
• Mobility-induced packet losses
• Battery constraints
• Potentially frequent network partitions
• Ease of snooping on wireless transmissions (security
hazard)
Unicast Routing
in
Mobile Ad Hoc Networks
Why is Routing in MANET different ?
• Host mobility
– link failure/repair due to mobility may have
different characteristics than those due to other
causes
• Rate of link failure/repair may be high when nodes
move fast
• New performance criteria may be used
– route stability despite mobility
– energy consumption
Unicast Routing Protocols
• Many protocols have been proposed
• Some have been invented specifically for MANET
• Others are adapted from previously proposed
protocols for wired networks
• No single protocol works well in all environments
– some attempts made to develop adaptive protocols
Routing Protocols
• Proactive protocols
– Determine routes independent of traffic pattern
– Traditional link-state and distance-vector routing
protocols are proactive
• Reactive protocols
– Maintain routes only if needed
• Hybrid protocols
Trade-Offs
• Latency of route discovery
– Proactive protocols may have lower latency since routes are
maintained at all times
– Reactive protocols may have higher latency because a route
from X to Y will be found only when X attempts to send to Y
• Overhead of route discovery/maintenance
– Reactive protocols may have lower overhead since routes
are determined only if needed
– Proactive protocols can (but not necessarily) result in higher
overhead due to continuous route updating
• Which approach achieves a better trade-off depends on the
traffic and mobility patterns
Overview of Unicast Routing
Protocols
Flooding for Data Delivery
• Sender S broadcasts data packet P to all its
neighbors
• Each node receiving P forwards P to its neighbors
• Sequence numbers used to avoid the possibility of
forwarding the same packet more than once
• Packet P reaches destination D provided that D is
reachable from sender S
• Node D does not forward the packet
Flooding for Data Delivery
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
N
Represents a node that has received packet P
Represents that connected nodes are within each
other’s transmission range
Flooding for Data Delivery
Y
Broadcast transmission
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
Represents a node that receives packet P for
the first time
Represents transmission of packet P
N
Flooding for Data Delivery
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
• Node H receives packet P from two neighbors:
potential for collision
D
N
Flooding for Data Delivery
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
• Node C receives packet P from G and H, but does not forward
it again, because node C has already forwarded packet P once
Flooding for Data Delivery
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
• Nodes J and K both broadcast packet P to node D
• Since nodes J and K are hidden from each other, their
transmissions may collide
=> Packet P may not be delivered to node D at all,
despite the use of flooding
Flooding for Data Delivery
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
• Node D does not forward packet P, because node D
is the intended destination of packet P
N
Flooding for Data Delivery
Y
Z
S
E
F
B
C
M
J
A
L
G
H
• Flooding completed
K
I
D
N
• Nodes unreachable from S do not receive packet P (e.g., node Z)
• Nodes for which all paths from S go through the destination D
also do not receive packet P (example: node N)
Flooding for Data Delivery
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
D
I
• Flooding may deliver packets to too many nodes
(in the worst case, all nodes reachable from sender
may receive the packet)
N
Flooding for Data Delivery: Advantages
• Simplicity
• May be more efficient than other protocols when rate of
information transmission is low enough so that the overhead of
explicit route discovery/maintenance incurred by other
protocols is relatively higher
– this scenario may occur, for instance, when nodes transmit
small data packets relatively infrequently, and many
topology changes occur between consecutive packet
transmissions
• Potentially higher reliability of data delivery
– Because packets may be delivered to the destination on
multiple paths
Flooding for Data Delivery: Disadvantages
• Potentially, very high overhead
– Data packets may be delivered to too many nodes who do
not need to receive them
• Potentially lower reliability of data delivery
– Flooding uses broadcasting -- hard to implement reliable
broadcast delivery without significantly increasing overhead
– Broadcasting in IEEE 802.11 MAC is unreliable
– In our example, nodes J and K may transmit to node D
simultaneously, resulting in loss of the packet
– in this case, destination would not receive the
packet at all
Flooding of Control Packets
• Many protocols perform (potentially limited) flooding
of control packets, instead of data packets
• The control packets are used to discover routes
• Discovered routes are subsequently used to send
data packet(s)
• Overhead of control packet flooding is amortized over
data packets transmitted between consecutive control
packet floods
Broadcast Storm Problem [Ni99Mobicom]
• When node A broadcasts a route query, nodes B and
C both receive it
• B and C both forward to their neighbors
• B and C transmit at about the same time since they
are reacting to receipt of the same message from A
• This results in a high probability of collisions
D
B
C
A
62
Broadcast Storm Problem
• Redundancy: A given node may receive
the same route request from too many
nodes, when one copy would have
sufficed
• Node D may receive from nodes B and
C both
D
B
C
A
Solutions for Broadcast Storm
• Probabilistic scheme: On receiving a route
request for the first time, a node will rebroadcast (forward) the request with
probability p
• Also, re-broadcasts by different nodes should
be staggered by using a collision avoidance
technique (wait a random delay when channel
is idle)
– this would reduce the probability that nodes B and
C would forward a packet simultaneously in the
previous example
64
Solutions for Broadcast Storms
• Counter-Based Scheme: If node E hears
more than k neighbors broadcasting a given
route request, before it can itself forward it,
then node E will not forward the request
• Intuition: k neighbors together have probably
already forwarded the request to all of E’s
neighbors
D
E
B
C
F
A
Solutions for Broadcast Storms
• Distance-Based Scheme: If node E hears RREQ
broadcasted by some node Z within physical distance
d, then E will not re-broadcast the request
• Intuition: Z and E are too close, so transmission
areas covered by Z and E are not very different
–
if E re-broadcasts the request, not many nodes who have not already heard the
request from Z will hear the request
E
<d
Z
Summary: Broadcast Storm Problem
• Flooding is used in many protocols, such as Dynamic
Source Routing (DSR)
• Problems associated with flooding
– collisions
– redundancy
• Collisions may be reduced by “jittering” (waiting for a
random interval before propagating the flood)
• Redundancy may be reduced by selectively rebroadcasting packets from only a subset of the nodes
67
Generic On-demand Routing Protocol
• Routes are maintained only between nodes which
need to communicate
• Route Requests (RREQ) are flooded through the
network
• When a node re-broadcasts a Route Request, it sets
up a reverse path pointing towards the source
• When the intended destination receives a Route
Request, it replies by sending a Route Reply
• Route Reply travels along the reverse path set-up
when Route Request is forwarded
68
Route Requests Phase
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents a node that has received RREQ for D from S
Route Requests Phase
Y
Broadcast transmission
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
Represents transmission of RREQ
N
Route Requests Phase
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
Represents links on Reverse Path
N
Route Requests Phase
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
Route Requests Phase
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Route Requests Phase
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
• Node D does not forward RREQ, because node D
is the intended target of the RREQ
N
Route Reply Phase
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
Represents links on path taken by RREP
N
Data Delivery
Y
DATA
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Routing table entries are used to forward data packet.
Summary: Generic On-demand Routing Protocols
• Nodes maintain routing tables containing
entries only for routes that are in active use
• Next-hop per destination maintained at each
node
• Unused routes expire even if topology does
not change
The End
Questions?
McGraw-Hill Technology Education
Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.