Wireless & Mobile Networking CS 752/852 - Spring 2011

Wireless & Mobile Networking

CS 752/852 - Spring 2011

Tamer NadeemDept. of Computer Science

Ad Hoc Routing Layer - I

Page 2 Spring 2011 CS 752/852 - Wireless and Mobile Networking

The OSI Communication Model


The OSI Communication Model


MANET – Mobile Ad Hoc Networks

Collection of mobile nodes connected by wireless links, forming an autonomous network

Nodes run on batteries – power consumption is an issue

Thus routing is multi-hop each node also acts as a router

Scalability is an issue

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MANET

• No infrastructure, lack of centralized control

• Frequent topology changes due to node mobility

• All communications over wireless medium

Cellular Communications

Fixed, pre-located base stations

Static backbone network topology

Reliable backbone links; only last link is wireless

MANET vs Cellular


Applications

• Civilian environments

• meeting rooms

• smart homes

• multimedia classrooms

• Emergency operations

• disaster relief

• search and rescue

• law enforcement

• Military applications

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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)


Network connectivity defined by node proximity and wireless medium characteristics

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Topology Challenges

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Due to mobility the network topology undergoes rapid changes


MAC Challenges

• Hidden terminal problem

• Unreliable physical layer

• multi-path fading

• high Bit Error Rate (BER)

• other impairments

• Mobility


Routing Schemes

Broadcast

Multicast

Unicast

Anycast

Geocast


Unicast Routing inMobile Ad Hoc Networks


Routing in MANET

• Why is routing in MANET challenging?

• Without (necessarily) using a pre-existing infrastructure

• Main culprit: mobility

• network topology is changing rapidly (link failure/repair)

• Rate of link failure/repair may be high when nodes move fast

• Main challenge: maintain and distribute up-to-date routing information without saturating the network


Routing in MANET

• Numerous protocols have been proposed

• Flavors:

• adapted from protocols previously proposed for wired networks

• invented specifically for MANET

• Major approaches

• Proactive: routing information is maintained proactively regardless of communication requests

• e.g. traditional link-state and distance-vector routing protocols are proactive

• Reactive: a route to a destination is found and maintained only when the route is needed


Proactive vs Reactive

• 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 is better depends on traffic and mobility patterns


Sample Routing Protocols

• Pure flooding

• DSR

• AODV

• LAR


Flooding (1/5)

• Sender S broadcasts data packet P to all its neighbors

• Each node receiving P forwards P to its neighbors

• Sequence numbers are used to avoid forwarding the same packet more than once

• The destination D does not forward the packet


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Represents transmission of packet P

Represents a node that receives packet P forthe first time

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YBroadcast transmission

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Flooding (2/5)


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Node H receives packet P from two neighbors: potential for collision

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Flooding (3/5)


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Node C receives packet P from G and H, but does not forward it again, because node C has already forwarded packet P

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Flooding (4/5)


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• 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)

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Flooding (5/5)


• Advantage: simplicity

• 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 MAC broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead

• Broadcasting in IEEE 802.11 MAC is unreliable

• Need end-to-end reliability

Flooding: Summary


Flooding Control Packets

• Many protocols perform limited flooding of control packets, instead of data packets

• Control packets are used to discover routes or to send link-state updates

• 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


Dynamic Source Routing (DSR) [Johnson96]

• When source S wants to send a packet to node D, but does not know a route to D, it initiates a route discovery

• Source node S floods a Route Request (RREQ)

• Each node appends own identifier when forwarding the RREQ


DSR: Route Discovery (1/3)

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Represents transmission of RREQ

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[S]

[X,Y] Represents list of identifiers appended to RREQ


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[S,E]

[S,C]



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Nodes J and K both broadcast RREQ to node DSince nodes J and K are hidden from each other, their transmissions may collide

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[S,C,G,K]

[S,E,F,J]



• Destination D upon receiving the first RREQ, sends back to S a Route Reply (RREP)

• RREP is sent on the route obtained by reversing the route appended to the received RREQ

• RREP includes the route from S to D on which RREQ was received by node D

DSR: Route Reply (1/2)


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RREP [S,E,F,J,D]

Represents RREP control message

DSR: Route Reply (2/2)


• S upon receiving RREP, caches the route included in the RREP

• When S sends a data packet to D, the entire route is included in the packet header

• hence the name source routing

• Intermediate nodes use the source route included in a packet to determine to whom the packet should be forwarded

DSR: Data Delivery (1/2)


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DATA [S,E,F,J,D]

Packet header size grows with route length

DSR: Data Delivery (2/2)


DSR Optimization: Route Caching

• Each node caches a new route it learns by some means

• When S finds route [S,E,F,J,D] to node D,

S also learns route [S,E,F] to node F

• When node K receives the Route Request [S,C,G],

K learns route [K,G,C,S] to S

• When node F forwards Route Reply RREP [S,E,F,J,D],

F learns route [F,J,D] to D

• When node E forwards Data [S,E,F,J,D],

it learns route [E,F,J,D] to node D

• A node may also learn a route when it overhears data packets


• Advantages:

• can speed up route discovery

• can reduce propagation of route requests (RREQ)

DSR: Route Caching (1/3)


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[S,E,F,J,D][E,F,J,D]

[C,S]

[G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

RREQ

Route Reply (RREP) from node K limits flooding of RREQIn general, the reduction may be less dramatic

[K,G,C,S]RREP



• Stale caches can adversely affect performance

• With passage of time and host mobility,

cached routes may become invalid

• In some cases, a sender may try several stale routes (obtained from its local cache, or from cache of some other node), before finding a good route



DSR: Route Maintenance - Route Error (RERR)

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RERR [J-D]

When J’s attempt to forward a data packet (with route S-E-F-J-D) on link (J-D) fails, J sends a route error (RERR) to S along route J-F-E-SNodes hearing RERR update their route cache to remove link (J-D)When node S receives the RERR, it initiates a new RREQ


Summary of DSR

• Routes maintained only between nodes who need to communicate

• Route caching can further reduce route discovery overhead

• A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches

• Complicated Cache Management mechanisms

• Packet header size grows with route length due to source routing


Ad-hoc On-Demand Distance Vector Routing (AODV)

• Route Requests (RREQ) are forwarded in a manner similar to DSR

• 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


AODV: Route Request (1/5)

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Represents transmission of RREQ

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Represents links on Reverse Path

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F sets up a reverse path entry in its routing table:

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Destination Next Hop

S Ereverse path entry

Represents links on Reverse Path



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D does not forward RREQ, because it is the intended target of the RREQ

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Represents links on path taken by RREP

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AODV: Route Reply (1/2)


• An intermediate node may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender S

• To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used

• Forward links are set up when RREP travels along the reverse path

AODV: Route Reply (1/2)


AODV: Forward Path Setup

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F adds a forward path entry in its routing table:

Destination Next Hop

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forward path entry


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• Routing table entries used to forward data packet• Route is not included in packet header

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Represents a link on the forward path

AODV: Data Delivery


AODV: Route Maintenance - Route Error (RERR)

• When node X is unable to forward packet P (from node S to node D) on link (X,Y), it generates a RERR message

• Node X increments the destination sequence number for D cached at node X

• The incremented sequence number N is included in the RERR

• When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N


Information “Freshness” Assured

• Each originating node maintains a monotonically increasing sequence number.

• Used by other nodes to determine the freshness of the information.

• Every nodes routing table contains the latest information available about the sequence number for the IP address of the destination node for which the routing information is maintained.

• Updated whenever a node receives new information about the sequence number from RREQ, RREP, or RERR messages received related to that destination.


Information “Freshness” Assured

• AODV depends on each node in the network to own and maintain its destination sequence number.

• A destination node increments its own sequence number immediately before it originates a route discovery

• A destination node increments its own sequence number immediately before it originates a RREP in response to a RREQ

• The node treats its sequence number as an unsigned number when incrementing accomplishing sequence number rollover.

• Destination information is assured by comparing the sequence number of the incoming AODV message with its sequence number for that destination.


ADOV: Summary

• Nodes maintain routing tables containing entries only for routes that are in active use

• Routes are not included in packet headers

• At most one next-hop per destination is maintained at each node

• DSR may maintain several routes for a single destination


Flooding of Control Packets

• How to reduce the scope of the route request flood ?

• LAR [Ko98Mobicom]

• Query localization [Castaneda99Mobicom]

• How to reduce redundant broadcasts ?

• The Broadcast Storm Problem [Ni99Mobicom]


Location-Aided Routing (LAR) [Ko98Mobicom]

• Exploits location information to limit scope of route request flood

• Location information may be obtained using GPS

• Expected Zone is determined as a region that is expected to hold the current location of the destination

• Expected region determined based on potentially old location information, and knowledge of the destination’s speed

• Route requests limited to a Request Zone that contains the Expected Zone and location of the sender node


Expected Zone in LAR

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X = last known location of node D, at time t0

Y = location of node D at current time t1, unknown to node S

r = (t1 - t0) * estimate of D’s speed

Expected Zone


Request Zone in LAR

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Request Zone

Network Space

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LAR

• Only nodes within the request zone forward route requests

• Node A does not forward RREQ, but node B does (see previous slide)

• Request zone explicitly specified in the route request

• Each node must know its physical location to determine whether it is within the request zone

• If route discovery using the smaller request zone fails to find a route, the sender initiates another route discovery (after a timeout) using a larger request zone

• the larger request zone may be the entire network


LAR Variations: Adaptive Request Zone

• Each node may modify the request zone included in the forwarded request

• Modified request zone may be determined using more recent/accurate information, and may be smaller than the original request zone

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Request zone adapted by B

Request zone defined by sender S

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Location Aided Routing (LAR)

• Advantages

• reduces the scope of route request flood

• reduces overhead of route discovery

• Disadvantages

• Nodes need to know their physical locations

• Does not take into account possible existence of obstructions for radio transmissions


Other Routing Protocols

• Plenty of other routing protocols

• Geographic Distance Routing (GEDIR)

• Energy-aware routing

• QoS routing

• Routing with Guaranteed Delivery


MANET Routing: Issues

• How to compare these protocols fairly?

• Message overhead

• Routing delays

• Throughput

• Energy Consumption

• Comparisons are based mainly on empirical results

• Lack of analytical results

• Interaction with MAC protocols

• Scalability:

• >200 nodes is already a problem

• A possible solution for scalability:

• hierarchical organization of ad hoc networks


• Fully Symmetric Environment

• all nodes have identical capabilities and responsibilities

• Asymmetric Capabilities

• transmission ranges and radios may differ

• battery life at different nodes may differ

• processing capacity may be different at different nodes

• speed of movement

• Asymmetric Responsibilities

• only some nodes may route packets

• some nodes may act as leaders of nearby nodes (e.g., cluster head)

MANET Routing: Many Variations


• Traffic characteristics may differ in different ad hoc networks

• bit rate

• timeliness constraints

• reliability requirements

• unicast / multicast / geocast

• host-based addressing / content-based addressing / capability-based addressing

• May co-exist (and co-operate) with an infrastructure-based network



• Mobility patterns may be different

• people sitting at an airport lounge

• New York taxi cabs

• kids playing

• military movements

• personal area network

• Mobility characteristics

• speed

• predictability

• direction of movement

• pattern of movement

• uniformity (or lack thereof) of mobility characteristics among different nodes



Research on Mobile Ad Hoc Networks

Variations in capabilities & responsibilities

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Variations in traffic characteristics, mobility models, etc.

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Performance criteria (e.g., optimize throughput, reduce energy consumption)

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Significant research activity


Questions


More on IEEE 802.11 DCF

• Collision Avoidance Mechanism

• CSMA + RTS/CTS (physical/virtual carrier sensing)

• Collision Resolution Mechanism

• Binary Exponential Backoff (BEB)

• Does IEEE802.11 DCF work well in multi-hop networks?

• Answer: No! (has known fairness problems)


What is Fairness?

• What does fairness mean in a multi-hop environment?

• Max-min fairness

• an allocation is max-min fair if there is no way to give more bandwidth to a flow without decreasing the allocation of a flow of lesser or equal bandwidth

• Practical approach

• ensure that each flow gets a minimum share of the bandwidth

• allow those flows that are not interfering with the current transmitting flows to send simultaneously


Throughput of each flow:

f0: 1298

f1: 684

f2: 47

f3: 1173

f4: 1913

f0 f1 f2 f3 f4

A B C D E F

Unfairness in IEEE 802.11 DCF


f0 f1 f2 f3 f4

A B C D E F

f0 f2 f4

f1 f3

The Flow Contention Graph

Nodes represent flows

Two flows connected by an edge means they conflict with each other


f0 f2 f4

f1 f3

Flow Contention Graph

Scheduling:Slot 1: f0 + f3/f4Slot 2: f1 + f4Slot 3: f2Slot 4: f3 + f0Slot 5: f4 +f0/f1……Slot 6: f0 + f3/f4Slot 7: f1 + f4Slot 8: f2Slot 9: f3 + f0Slot 10: f4 +f0/f1……Each flow is guaranteed to be scheduled once in every 5 slots!

An example of scheduling


f0 f2 f4

f1 f3

Improved schedule:Slot 1: f0 + f3Slot 2: f1 + f4Slot 3: f2

Slot 4: f0+f3Slot 5: f1+f4Slot 6: f2……Each flow is guaranteed to be scheduled once in every 3 slots!

Flow Contention Graph

One can do better…


Challenges

• Which scheduling is better?

• Trade-off between fairness and overall throughput

• How to implement a fair scheduler in a fully distributed way?

Documents

Wireless & Mobile Networking CS 752/852 - Spring 2011