Unicast Routing in

Mobile Ad-Hoc Networks

Overview

• Introduction• MaNet Protocols• Proactive & Hybrid Protocols

• WRP/GSR/FSR/LAR• ZRP

• Reactive Protocols• DSR/DSDV/AODV

• Challenges in Ad-Hoc Networks• References

Mobile Ad-Hoc Network

• Collection of mobile nodes forming a temporary network

• No centralized administration or standard support services

• Each Host is an independent router

• Hosts use wireless RF transceivers as network interface

• Conferences/Meetings

• Search and Rescue

• Disaster Recovery

• Automated Battlefields

MaNet Issues

• Lack of a centralized entity• Network topology changes frequently and unpredictably• Channel access/Bandwidth availability• Hidden/Exposed station problem• Lack of symmetrical links• Power limitation

• Multipath Fading• Doppler Effect

MaNet Protocols

• Proactive Protocols• Table driven• Continuously evaluate

routes• No latency in route

discovery• Large network capacity

to keep info. current• Most routing info. may

never be used!

• Reactive Protocols• On Demand• Route discovery by some

global search• Bottleneck due to

latency of route discovery

• May not be appropriate for real time commn.

Conventional Routing Protocols ???

• Distributed Bellman-Ford (DBF) shows a degradation in performance

• Slow convergence due to “Count to Infinity” Problem• Creates loops during node failure, network partition or

congestion• Protocols that use flooding techniques create excessive

traffic and control overhead

MaNet Protocol Considerations

• Simple, Reliable and Efficient• Distributed but lightweight in nature• Quickly adapt to changes in topology and traffic pattern• Protocol reaction to topology changes should result in

minimal control overhead • Bandwidth efficient• Mobility Management involving user location management

and Hand-off management

Wireless Routing Protocol (WRP)

• A path-finding algorithm• Utilizes information regarding the the length and the

predecessor-to-dest in the shortest path to each destination

• Eliminates the “Count to Infinity” Problem and converges faster

• An Update message is sent after processing updates from neighbors or a change in link to a neighbor is detected

• Each route update from neighbor k causes route entries of other neighbors that use k to be re-computed

The Algorithm

• Each node i maintains a Distance table (iDjk), Routing table (Destination Identifier, Distance iDj, Predecessor Pj, the successor Sj and a marker tag), link cost table (Cost, Update Period) , message retransmission list (Seq. No., Counter, Acknowledgement flag, Update List)

• Listen for updates/ACKs which include i in the response list. Acknowledge each update

• Process the Distance table entries. Compute Djb = Dkb + kDj . Update predecessor as reported by k

• Update own Distance and predecessor information

• Choose neighbor p such that the path from p to j does not include i and iDjp <= iDjx and iDyp <= iDyx

• Broadcast new update message. Delete stale entries from MRL for new updates. Decrement counter for all entries in list.

• Retransmit MRL entry when counter hits zero, setting response list of update message to those neighbors who have not yet acknowledged

• Lack of NULL updates for given HelloInterval indicate change in link to a neighbor.

WRP Example

Performance Measurements

Comparison against DBF and ILS

• For a single resource failure, WRP outperforms DBF in terms of number of steps and control overhead

• Performance of WRP better for resource recoveries in terms of number of control messages

• Average number of control messages least for WRP with node mobility

• Average message length comparable to DBF and ILS (Ideal Link State)in most cases

Global State Routing (GSR)

• Combination of Distance Vector and Link State • Global Network Topology stored in a Table• Topology Table broadcast to immediate neighbors only• Link State/Changes updates are time triggered

The Algorithm

• Ad-Hoc Network modeled as an undirected graph G(V,E)• L(i,j) formed when D(i,j) less than or equal to RTr

• A List and Three Tables are maintained at each Node i: Neighbor list Ai, Topology Table TTi [TTi.LS(j), TTi.SEQ(j)], Next Hop Table Nexti and Distance Table Di

• Initialize Ai, TTi. Use pkt.SEQ(j) to update TTi.SEQ(j) • Compute shortest path tree rooted at i. Rebuild the Routing

Table• Broadcast information to neighbors periodically

Performance Measurements

• Routing Inaccuracy :• Link State reacts fastest to topology changes; GSR still

outperforms DBF• Control Overhead :

• Link State has maximum overhead since it is event triggered

• Overhead is constant for GSR & DBF • Mobility Impact :

• No impact on control overhead for GSR & DBF• Routing inaccuracy increases for all schemes, but least

for ILS

Performance Measurements cont.

• Update Interval :• Routing inaccuracy may be improved or degraded with

change in update interval• Radio Range :

• Routing inaccuracy decreases for larger transmission ranges

Advantages/Disadvantages of GSR

• Advantages :• Avoids Flooding for disconnects/reconnects• Updates are time triggered than event triggered• Greatly reduces control overhead

• Disadvantages :• Hogs bandwidth since entire topology table is

broadcast with each update• Link state latency depends on update interval

• Can GSR be modified to rectify it’s drawbacks ?

Fisheye State Routing

• The network is logically divided into “Fisheye” circles with respect to each node. The scope of the circle may be defined in terms of number of hops

• Reduction in update message achieved by using different exchange periods for different entries in the table

• Creates larger latencies for stations that are far• Routes become progressively accurate as the packet gets

closer to the destination• FSR scales well to large networks, by keeping the LS

exchange overhead low

FSR cont.

Zone Routing Protocol

• A Hybrid Routing Protocol• A Zone is defined for each node• Proactive maintenance of topology within a zone (IARP)

Distance Vector or Link State• Reactive query/reply mechanism between zones (IERP)

With Route Caching : Reactive Distance VectorW/O Route Caching : Source Routing

• Uses ‘Bordercast’ instead of neighbor broadcast• Neighbor Discovery/Maintenance (NMD) and Border

Resolution Protocol (BRP) used for query control, route accumulation etc.

ZRP Example

A

B

C

DE

F

G H

1 Hop

2 Hops

Multi Hops

Zone Routing Protocol cont.

• Routing Zone and IntrAzone Routing Protocol• Zone Radius may be based on hop count• Identity and distance of each Node within the Zone is

proactively maintained• The Interzone Routing Protocol

• Check if destination is within the routing zone• Bordercast a route query to all peripheral nodes• Peripheral nodes execute the same algorithm

Zone Routing Protocol cont.

• Route Accumulation :• Provide reverse path from discovery node to source

node• May employ global caching to reduce query packet

length• Query Detection/Control :

• Terminate Query thread in previously queried regions• Intermediate nodes update a Detected Queries

Table[Query Source, ID]• Route Maintenance may be reactive or proactive

Location Aided Routing (LAR)

• A Modified Flooding Algorithm• Utilizes location information of mobile hosts using a GPS for

route discovery• Flooding is restricted to a “request zone”, defined by an

“expected zone”• A node forwards a route request only if it belongs to the

“request zone”• Tradeoff between latency of route determination and

message overhead• Resorts to flooding when prior information of destination is

not available

LAR Schemes

N

S (xs,ys)

I

J

D(xd,yd)

R = v(t-t0)

N

S (xs,ys)

I

J

D(xd,yd)

Scheme 1 Scheme 2

LAR cont.

• Scheme 1.• Source calculates the “expected zone”, defines a “request

zone in the request packet and initiates route discovery”• Node I receiving the route request forwards the request if

it falls inside the “request zone”, otherwise discards it• When destination receives the request, replies with a

route reply including current location, time and average speed

• Size of request zone is large at low and high node speeds

LAR cont.

• Scheme 2.• Source calculates the distance Dists to destination (xd, yd)

and initiates route discovery with both parameters• Node I calculates it’s distance Disti from (xd, yd) and

forwards the request only if Disti<= Dists + δ, otherwise discards the request

• Node I replaces Dists with Disti before forwarding the request

• None zero δ increases probability of route discovery

Performance Comparison

• Scheme 2 performs better than Scheme 1 and Flooding• For low transmission ranges and degree of connectivity, LAR

schemes are comparable to Flooding• Routing overhead increases with increase in location error• Impact of location error more on Scheme 2 than Scheme 1• “Local Search” may be employed by intermediate nodes to

reconstruct routes, thus reducing “request zone” area and thereby reducing routing overhead

Destination-Sequenced Distance Vector Routing

• Protocol Overview• Route Advertisements• Routing Table Entry Structure

Protocol Overview

• Each Routing Table List all destinations and number of hops to each

• Each Route is tagged with a sequence number originated by destination

• Updates are transmitted periodically and when there is any significant topology change

• Routing information is transmitted by broadcast

Route Table Entry Structure

• Destination’s Address• Number of hops required to reach the destination• Destination Sequence Number

Responding to Topology Changes

• Broken links indicated by ∝• Any route through a hop with a broken link is also assigned ∝

• ∝ routes are immediately broadcast• Sequence number of Destination is incremented and

information is broadcast• Nodes with same or higher sequence number broadcast

their metric information• Limit broadcast by “full dump” and “incremental”

Route Selection Criteria

• Route broadcast are asynchronous events• Fluctuations are caused due to possibility of

receiving routes with worse metric first• Solution is maintain routing tables, one for routing

and one for broadcast

Example of DSDV

Example of DSDV

Example of DSDV

Example of DSDV

Example of DSDV

Ad-Hoc On-Demand Distance Vector Routing

• Protocol overview and objectives• Path Discovery• Reverse Path Setup• Forward Path Setup• Route Table Management• Path Maintenance• Local Connectivity Management

Protocol Overview and Objectives

• Pure on-demand protocol• Node does not need to maintain knowledge of

another node unless it communicates with it• Broadcast discovery packets only when necessary • Distinguish between local connectivity and general

topology maintenance• To disseminate Information about changes in local

connectivity to those neighboring nodes that are likely to need it

Path Discovery

• Initiated whenever nodes want to communicate• Route Request packets are broadcast• RREQ format

< source_addr, source_sequence-# , broadcast_id, dest_addr, dest_sequence_#, hop_cnt >

• RREQ uniquely identified by <source_addr , broadcast_id>

Path Discovery Cont.

• Broadcast id incremented with every RREQ• Neighboring nodes satisfy RREQ by sending RREP

or broadcast RREQ after incrementing hop_cnt• Each intermediate node keeps following

information• Destination Address• Source Address• Broadcast_id

Path discovery Cont.

• Expiration time for reverse path entry• Source node’s sequence number

Reverse Path Setup

• Source sequence number is used to maintain freshness about reverse route to source

• Destination number specified for freshness of route before accepted by source

• Reverse path setup by having link to neighbor

Forward Path Setup

• Node processing on reception of RREQ• checks if the route is current by comparing dest_seq#• if route is current node unicasts RREP back to neighbor from which it received

the RREQ

• RREP contains <source_addr, dest_addr, dest_sequence_#, hop_cnt,lifetime>

• Node propagates RREP back to source• Updates information on reception of subsequent

RREPs

Forward Path Setup

Route Table Management

• Route Request Expiration Timer for purging reverse paths which do not lie on source-destination route

• Route Caching Timeout for time after which the route is considered invalid

• Active_timeout Period used to determine if neighboring node is active

Route Table Management

• Route Table entry • Destination• Next Hop• Number of hops (metric)• Sequence numbers of Destination• Active Neighbors for this route• Expiration time for the route table entry

Path Maintenance

• When dest node or intermediate node moves a special RREP is sent

• When next hop become unreachable the upstream node sends propagates RREP with fresh sequence number and hop cnt ∝

• restart route discovery process from source on receipt of RREP

Local Connectivity Management

• Node learn of their neighbors in following way• on receipt of broadcast message• on receipt of hello message

Dynamic Source Routing

• Overview• Constructs a source route in packet header listing source route• Each host maintains a route cache• Route discovery used for routes not in cache

Route Discovery

• Route Request Packet • Route Reply Packet• Route Record • Unique Request id

Route Discovery

• Possible scenarios on receipt of route request packet

• discard packet if <initiator address, request id> already seen• if host already listed then discard packet• if target address is same as host address then obtain reverse route from

route record and send route reply• add host address in case of none of the above

Route Maintenance

• Route error packet sent on detection of break containing addresses on both sides of error, the host that detected the error and the host to which it was trying to send the packet

• All upstream node then deletes routes with that particular hop

Optimizations

• Full Use of Route Cache• Piggy Backing on Route Discoveries• Reflecting Shorter Routes• Improved Handling of errors

Full Use of Route Cache

• Each forwarding host can add route information to cache

• Nodes can operate in promiscuous mode and add information to cache from any packets that they hear

• Each intermediate node having a route can send a route reply packet

Piggy Backing on Route Discoveries

• Data is piggybacked on Route Request Packet and Route Reply Packet (like SYN for TCP)

• While piggybacking on Route Request and intermediate node send Route Reply propagate Route Request with data

Reflecting Shorter Routes

• D on receipt of packet from B can modify cache and send route reply

Improved Handling of errors

• In scenario when network becomes partitioned buffer packets and use exponential backoff

• Use promiscuous mode to remove entries

Performance Comparison of AODV and DSR

• DSR has access to significantly greater amount of routing information than AODV by virtue of source routing and promiscuous listening

• DSR replies to all requests reaching a destination from a single request cycle whereas AODV only replies once thereby learning only one route

• In DSR no particular mechanism to delete stale routes unlike AODV

• In AODV the route deletion causes all the nodes using that link to delete it, but in DSR only the nodes on that particular part are deleted

Performance Comparison of AODV and DSR

• Both protocols do not perform any load balancing• Routing load in DSR is less than AODV but in terms

of bits the difference is less• In AODV the path discovery is mostly due to

RREQ( but in RREP & RERR’s but in terms of network load both of them are nearly the same)

• Both protocols use hop-wise determination of routes

Challenges in Ad-hoc Design

• Protocols still in Nascent Stage, analysis for which protocol does well in which scenario

• QOS issues in Ad-hoc• TCP performance over Ad-hoc• Integration of Ad-Hoc Networks in Internet• Multicasting in Ad-hoc Networks

References

• S. Murthy and J.J Garcia Luna Aceves, “A Routing Protocol for Packet Radio Networks”, Proc. IEEE Mobicom, Nov. 1995

• A. Iwata, C.C. Chiang, G. Pei, M. Gerla and T. W. Chen, “Scalable Routing Strategies for Ad-Hoc Wireless Networks”, IEEE journal on Selected Areas in Communcations, Aug. 1999

• M. Gerla and T. W. Chen, “Global State Routing : A new routing scheme for Ad-Hoc wireless networks”, Proc. IEEE ICC ‘98

• Z. J. Haas, M. R. Pearlman, “The Zone Routing Protocol for Ad-Hoc networks”, Internet Draft, Aug. 1998

• Y. B. Ko, N. H. Vaidya, “Location Aided Routing in Ad-Hoc networks”, Proceedings of ACM/IEEE Mobicom’98, Dallas, TX, Oct. 1998

References

• Charles Perkin's Home Page• http://www.srvloc.org/charliep/charliep.html• Routing in Ad Hoc Networks of Mobile Hosts

http://phantom.me.uvic.ca/clesiuk/thesis/reports/adhoc/adhoc.html#E18E3

• Wireless Adaptive Mobility LAB, UCLAhttp://www.cs.ucla.edu/NRL/wireless/

• Monarch Project Research Papershttp://www.monarch.cs.cmu.edu/papers.html

• Papers on Mobile Computinghttp://db.nthu.edu.tw/paper/Mobile/

• Simulation Environment for an Ad-Hoc Wireless Network Running the AODV Routing Algorithm

References

• http://www.ctr.columbia.edu/~angin/e6950/sameh/aodv_final.html• ADHOC Networks Presentation

http://www.ittc.ukans.edu/~eshwar/eecs845/intro.html• Mobile Ad hoc Networking

http://www.varium.com/~pso/AdhocNetworking.html• Paper Collection on Ad-hoc Networks

http://147.46.59.102/~imhyo/papers/papers.html• Ad-Hoc Networking Resource Page

http://147.46.59.102/~imhyo/papers/papers.html• Papers on Mobile Computing http://www-

sor.inria.fr/~aline/mobile/biblio.html• Papers on Ad-hoc Multihop Wireless Networks

http://www.ics.uci.edu/~atm/adhoc/paper-collection/papers.html