Adhoc Networks and Transport Layer

Mobile Communications
Exercise: Adhoc Networks and
Transport Layer
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
1/34
Exercise: Adhoc Networks 1
a) 
b) 
N°1
Why is routing in multi-hop ad hoc networks complicated, what are
the special challenges? Why should existing routing protocols from
classical wired networks not be reused?
Recall the distance vector and link state routing algorithms for fixed
networks. Why are both difficult to use in multi-hop ad hoc
networks?
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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Solution: Adhoc Networks 1.a
a) 
• 
• 
• 
• 
• 
Why is routing in multi-hop ad hoc networks complicated, what are
the special challenges? Why should existing routing protocols from
classical wired networks not be reused?
Very frequent topology changes compared to wired networks
Varying propagation characteristics
Lack of a central instance
Reuse of routing protocols may technically be possible, but very
inefficient and error prone
Specialized protocols can greatly increase efficiency
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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Solution: Adhoc Networks 1.b
b) 
• 
• 
• 
• 
• 
Recall the distance vector and link state routing algorithms for fixed
networks. Why are both difficult to use in multi-hop ad hoc
networks?
Both algorithms assume a (more or less) stable networks (seldom
topology changes compared to routing information exchange)
BUT: topology of ad-hoc networks may change often
Both algorithms build routing tables independent of demand
High communication burden for each topology change
Routing information may be outdated, when communication happens
right after topology change
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
4/34
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Exercise: Adhoc Networks 2
a) 
b) 
N°2
Please outline the categories or classes into which MANET routing
protocols can be characterized. Please also give a brief explanation
of the differences of the classes and name at least one example
protocol per class.
What are the benefits of location information for routing in ad hoc
networks, which problems do arise?
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
5/34
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Exercise: Adhoc Networks 2.a
a) 
Please outline the categories or classes into which MANET routing
protocols can be characterized. Please also give a brief explanation
of the differences of the classes and name at least one example
protocol per class.
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
6/34
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Solution: Adhoc Networks 2.a
Table-driven / Proactive routing
q 
q 
Maintain routes to all other nodes permanently
Constant, high signalling overhead
Hybrid routing
On-demand-driven / Reactive routing
q 
q 
q 
Routes are discovered if needed
Delayed packet forwarding since route must be established first
Signalling overhead depends on traffic patterns
Cluster-based/Hierarchical
q 
q 
Nodes have different roles, cluster-heads determine routes
Dependant on scenario and application
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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Solution: Adhoc Networks 2.a
Unicast routing protocols
for MANETs
(topologie-based)
Table-driven/
pro-active
DistanceVector
LinkState
•  DSDV
•  ...
•  OLSR
•  TBRPF
•  FSR
•  STAR
•  ...
Hybrid
•  ZRP
•  ...
On-Demand
-driven/reactive
•  DSR
•  AODV
•  TORA
•  ...
Cluster-based/
hierarchical
•  LANMAR
•  CEDAR
•  ...
not covered: position-based routing protocols
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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Solution: Adhoc Networks 2.b
b) 
What are the benefits of location information for routing in ad hoc
networks, which problems do arise?
Benefits
•  Outgoing route can be selected based on direction of destination
Problems
•  Privacy
•  Local Minimum
E"
A"
Z"
B"
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
9/34
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Exercise: Adhoc Networks 3
a) 
b) 
N°3
What makes DSR different from Distance-Vector and Link-State
Routing?
Please outline the steps of route discovery between nodes A and Z
in the following network.
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
Sicherheit
10/34
Solution: Adhoc Networks 3.a
a) 
What makes DSR different from Distance-Vector and Link-State
Routing?
Distance-Vector / Link State
•  Routes are discovered pro-active
•  Routers keep routing tables to route packets
Dynamic Source Routing
•  Routes are discovered on-demand
•  Source node stores (and determines) route, no routing tables on
Routers
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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11/34
Solution: Adhoc Networks 3.b
b) 
Please outline the steps of route discovery between nodes A and Z
in the following network.
Step 1
[Z, A, 42]
A
E
B
G
D
C
F
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
Z
Sicherheit
12/34
Solution: Adhoc Networks 3.b
b) 
Please outline the steps of route discovery between nodes A and Z
in the following network.
Step 2
[Z, A/E, 42]
A
E
[Z, A/B, 42]
B
G
D
C
F
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
Z
Sicherheit
13/34
Solution: Adhoc Networks 3.b
b) 
Please outline the steps of route discovery between nodes A and Z
in the following network.
Step 3
A
E
[Z, A/E/G, 42]
G
B
D
C
[Z, A/B/D, 42]
[Z, A/E/D, 42]
F
Z
[Z, A/B/C, 42]
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
Sicherheit
14/34
Solution: Adhoc Networks 3.b
b) 
Please outline the steps of route discovery between nodes A and Z
in the following network.
Step 4
A
E
B
G
D
C
F
Z
Path: A E G Z
[Z, A/B/D/F, 42]
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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15/34
Exercise: Transport Layer 4
a) 
b) 
c) 
N°4
What is the reaction of standard TCP in case of packet loss? In what
situation does this reaction make sense and why is it quite often
problematic in the case of wireless networks and mobility?
Can the problems using TCP be solved by replacing TCP with UDP?
Where could this be useful and why may this be dangerous for
network stability?
Please name and very briefly describe the mechanisms discussed in
the lecture to mitigate problems of TCP in wireless networks.
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
Sicherheit
16/34
Solution: Transport Layer 4.a
a) 
What is the reaction of standard TCP in case of packet loss? In what
situation does this reaction make sense and why is it quite often
problematic in the case of wireless networks and mobility?
Problem Statement
•  TCP interprets packet loss as (temporary) overload situation
•  TCP reacts with the slow-start mechanism
In wired networks: Packet losses usually indicate overload.
In mobile networks, packet losses may occur due to
•  Transmission errors
•  Roaming due to mobility
•  Actual congestion in the network
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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17/34
Solution: Transport Layer 4.b
b) 
Can the problems using TCP be solved by replacing TCP with UDP?
Where could this be useful and why may this be dangerous for
network stability?
UDP
+  Higher throughput for (some) users
-  No congestion control
-  no reliable data transmission
⟶  Fast overload of networks
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
Sicherheit
18/34
Solution: Transport Layer 4.c
c) 
• 
Please name and very briefly describe the mechanisms discussed in
the lecture to mitigate problems of TCP in wireless networks.
Indirect TCP
• 
• 
Snooping TCP
• 
• 
After roaming, MN sends multiple ACKs, CN avoids slow-start
Transmission/time-out freezing
• 
• 
Set sender window to 0 upon network interruption, freeze transfer
Fast retransmit / Fast recovery
• 
• 
AP snoops into packet stream and retransmits packets locally
Mobile TCP
• 
• 
Split of TCP connection into 2: MN <-> AP, AP <-> CN
MN + CN freeze connection when wireless network becomes unavailable
Selective retransmission
• 
Receiver ACKs single packets not sequences, avoids retransmissions
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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19/34
Solution: Transport Layer 4.c
Approach
Mechanism
Advantages
Indirect TCP
splits TCP connection
into two connections
isolation of wireless
link, simple
Disadvantages
loss of TCP semantics,
higher latency at
handover
Snooping TCP
“snoops” data and
transparent for end-to- problematic with
acknowledgements, local end connection, MAC
encryption, bad isolation
retransmission
integration possible
of wireless link
M-TCP
splits TCP connection,
Maintains end-to-end
Bad isolation of wireless
chokes sender via
semantics, handles
link, processing
window size
long term and frequent overhead due to
disconnections
bandwidth management
Fast retransmit/ avoids slow-start after
simple and efficient
mixed layers, not
fast recovery
roaming
transparent
Transmission/
freezes TCP state at
independent of content changes in TCP
time-out freezing disconnect, resumes
or encryption, works for required, MAC
after reconnection
longer interrupts
dependent
Selective
retransmit only lost data very efficient
slightly more complex
retransmission
receiver software, more
buffer needed
Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de
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20/31

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