1. Advance Topics in Networking
Lecture 4
Advance Topics in Networking

2. Lecture Overview Presenting a Research Topic Sample Thesis Topics
More Thesis Topics
Ad hoc Networking
Reviewers Guidelines
Paper Review Guidelines
First Papers: For this week

3. 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

4. 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?

5. 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?

6. 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?

7. 802.11a/b/g Networks
Some slides taken from UIUC Wireless Networking Group

8. 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

9. Infrastructure Access
Access Points:
Provide infrastructure access to mobile users
Cover a fixed area
Wired into LAN

10. Peer to Peer Ad Hoc Mode

11. Infrastructure Access

12. Infrastructure Access

13. 802.11a/b/g are multi-rate devices
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps

14. 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!

15. 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
1.609
3.983
1 Mbps
Temporal Fairness
OAR – Rice Networks Group – Mobicom 2002
11 Mbps
1 Mbps

16. 802.11b Channels 11 available channels (in US)
Only 3 are non-overlapping!

17. 802.11b Channel Usage
Channel 1
Channel 6
Channel 11

18. 802.11b Channel Reuse

19. 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

20. Peer to Peer Ad Hoc Mode

21. Peer to Peer Ad Hoc Mode X X X

22. Problems
Communication is only possible between nodes which are directly in range of each other

23. 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!!

24. What if ??
Multi-hop Infrastructure Access
Multi-hop Ad Hoc Network OR

25. 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

26. 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
Source
Destination

27. 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

28. 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

29. Ad Hoc Networking   29

30. 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
“ style”

31. Terminlology Mobile Ad Hoc Networking =
= Mobile, Multi hop, Wireless Networking
= Mobile Mesh Networking
= Mobile Packet Networking

32. Ad hoc network applicability
Scale
Small scale Large scale
Network type (few nodes) (many nodes)
Commercial home/office personal mobile cellular like
industrial local
networks
Government specific Public Safety Large-scale military
network
Community/urban “covert” networks local communications

33. Hybrid Communication Networks
Satellite overlay
High speed
backbone
network
MANET
No fixed infrastructure Fixed/static infrastructure

34. 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 mobile- IP and IP addressing
For large-scale MANET the lack of interest have put this work in question
Flooding: work on requirements had started

35. Mobile Ad Hoc Networking (MANET)
Dynamic topologies
Bandwidth-constrained
Asymmetric links with variable capacity
Energy constrained
Multiple technologies can be used simultaneously

36. Open issues
A optimisation network layer and radio layers for different systems (incl , 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

37. 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.

38. Mobile Ad Hoc Networks Formed by wireless hosts which may be mobile
Without (necessarily) using a pre- existing infrastructure
Routes between nodes may potentially contain multiple hops

39. Mobile Ad Hoc Networks
May need to traverse multiple links to reach a destination

40. Mobile Ad Hoc Networks (MANET)
Mobility causes route changes

41. Why Ad Hoc Networks ? Ease of deployment Speed of deployment
Decreased dependence on infrastructure

42. 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

43. 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)

44. Unicast Routing in Mobile Ad Hoc Networks

45. 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

46. 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

47. 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

48. 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

49. Overview of Unicast Routing Protocols

50. 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

51. Flooding for Data DeliveryZ S E F B C M L J A G H D K I N
Represents a node that has received packet P
Represents that connected nodes are within each
other’s transmission range

52. Flooding for Data Delivery
Broadcast transmission Z S E F B C M L J A G H D K I N
Represents a node that receives packet P for
the first time
Represents transmission of packet P

53. Flooding for Data DeliveryZ S E F B C M L J A G H D K I N
Node H receives packet P from two neighbors:
potential for collision

54. Flooding for Data DeliveryZ S E F B C M L J A G H D K I 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

55. Flooding for Data DeliveryZ S E F B C M L J A G H D K I 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

56. Flooding for Data DeliveryZ S E F B C M L J A G H D K I N
Node D does not forward packet P, because node D
is the intended destination of packet P

57. Flooding for Data DeliveryZ S E F B C M L J A G H D K I N
Flooding completed
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)

58. Flooding for Data DeliveryZ S E F B C M L J A G H D K I N
Flooding may deliver packets to too many nodes
(in the worst case, all nodes reachable from sender
may receive the packet)

59. 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

60. 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 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

61. 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

62. 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

63. 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

64. Solutions for Broadcast Storm
Probabilistic scheme: On receiving a route request for the first time, a node will re- broadcast (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

65. 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

66. 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 Z
<d

67. 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 re- broadcasting packets from only a subset of the nodes

68. 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

69. 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

70. Route Requests Phase Y Broadcast transmission Z S E F B C M L J A G HK I N
Represents transmission of RREQ

71. Route Requests Phase Y Z S E F B C M L J A G H D K I N
Represents links on Reverse Path

72. 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

73. Route Requests Phase Y Z S E F B C M L J A G H D K I N

74. Route Requests Phase Y Z S E F B C M L J A G H D K I N
Node D does not forward RREQ, because node D
is the intended target of the RREQ

75. Route Reply Phase Y Z S E F B C M L J A G H D K I N
Represents links on path taken by RREP

76. 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.

77. 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

78. The End
Questions?