mobile communications exercise: adhoc networks and...

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

Sicherheit Wolf-Bastian Pöttner, IBR, TU Braunschweig, www.ibr.cs.tu-bs.de 2/34

N°1 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?

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?

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

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tables independent of demand •  High communication burden for each topology change •  Routing information may be outdated, when communication happens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



N°2 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.

b)  What are the benefits of location information for routing in ad hoc networks, which problems do arise?

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.



Table-driven / Proactive routing q  Maintain routes to all other nodes permanently q  Constant, high signalling overhead

Hybrid routing On-demand-driven / Reactive routing

q  Routes are discovered if needed q  Delayed packet forwarding since route must be established first q  Signalling overhead depends on traffic patterns

Cluster-based/Hierarchical q  Nodes have different roles, cluster-heads determine routes q  Dependant on scenario and application



Unicast routing protocols for MANETs

(topologie-based)

Table-driven/ pro-active

On-Demand -driven/reactive Hybrid

•  DSDV •  ...

•  ZRP •  ...

•  DSR •  AODV •  TORA •  ...

Distance- Vector

Link- State

•  OLSR •  TBRPF •  FSR •  STAR •  ...

Cluster-based/ hierarchical

•  LANMAR •  CEDAR •  ...

not covered: position-based routing protocols



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

A"

B"

E"

Z"



N°3 a)  What makes DSR different from Distance-Vector and Link-State

Routing? b)  Please outline the steps of route discovery between nodes A and Z

in the following network.



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



b)  Please outline the steps of route discovery between nodes A and Z in the following network.

Step 1

A

B

E

C

D

F

G

Z

[Z, A, 42]




Step 2

A

B

E

C

D

F

G

Z

[Z, A/E, 42]

[Z, A/B, 42]




Step 3

A

B

E

C

D

F

G

Z

[Z, A/E/G, 42]

[Z, A/B/C, 42]

[Z, A/B/D, 42] [Z, A/E/D, 42]




Step 4

A

B

E

C

D

F

G

Z

[Z, A/B/D/F, 42]

Path: A E G Z

Exercise: Transport Layer 4


N°4 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?

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?

c)  Please name and very briefly describe the mechanisms discussed in the lecture to mitigate problems of TCP in wireless networks.

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

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

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

•  Split of TCP connection into 2: MN <-> AP, AP <-> CN •  Snooping TCP

•  AP snoops into packet stream and retransmits packets locally •  Mobile TCP

•  Set sender window to 0 upon network interruption, freeze transfer •  Fast retransmit / Fast recovery

•  After roaming, MN sends multiple ACKs, CN avoids slow-start •  Transmission/time-out freezing

•  MN + CN freeze connection when wireless network becomes unavailable •  Selective retransmission

•  Receiver ACKs single packets not sequences, avoids retransmissions

Solution: Transport Layer 4.c


Approach Mechanism Advantages Disadvantages Indirect TCP splits TCP connection

into two connections isolation of wireless link, simple

loss of TCP semantics, higher latency at handover

Snooping TCP “snoops” data and acknowledgements, local retransmission

transparent for end-to-end connection, MAC integration possible

problematic with encryption, bad isolation of wireless link

M-TCP splits TCP connection, chokes sender via window size

Maintains end-to-end semantics, handles long term and frequent disconnections

Bad isolation of wireless link, processing overhead due to bandwidth management

Fast retransmit/ fast recovery

avoids slow-start after roaming

simple and efficient mixed layers, not transparent

Transmission/ time-out freezing

freezes TCP state at disconnect, resumes after reconnection

independent of content or encryption, works for longer interrupts

changes in TCP required, MAC dependent

Selective retransmission

retransmit only lost data very efficient slightly more complex receiver software, more buffer needed

mobile communications exercise: adhoc networks and...

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