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A Review of Current Routing Protocols
for Ad Hoc Mobile Wireless Networks
 IEEE Personal Communications,
 April 1999, pp. 46-55
 E. Royer and C.-K. Toh
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Introduction
 Two types of wireless networks:
 infrastructured network:
 base stations are the bridges
 a mobile host will communicate with the nearest base station
 handoff is taken when a host roams from one base to another
 ad hoc network:
 infrastructureless: no fixed base stations
 without the assistance of base stations for communication
 Due to transmission range constraint,
 two MHs need multi-hop routing for communication
 quickly and unpredictably changing topology
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MANET
 MANET = Mobile Ad Hoc Networks
 a set of mobile hosts, each with a transceiver
 no base stations; no fixed network infrastructure
 multi-hop communication
 needs a routing protocol which can handle changing
topology
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Applications of MANET
 battlefields
 nature disaster areas
 fleet in oceans
 historical cites
 festival ground
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Related Research
 IEEE 802.11 for Wireless LANs
 MAC
 PHY
 IETF manet group
 to stimulate research and discuss possible standards in this
area
 Routing Protocols:
 unicast – AODV, DSR, ZRP, TORA, CBRP, CEDAR
 multicast – MAODV, AMRoute, ODMRP, AMRIS
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Resources and Applications
 NS-2:
 AODV, DSR, DSDV, TORA
 Telcordia: Intelligent Transportation System
 AODV
 MAODV: to distributed emergency information
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Challenge of Ad Hoc Networks
 No centralized entity
 Mobile host is no longer just an end system
 Acting as an intermediate system
 Changing network topology over time
 Every node can be mobile
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Routing in MANET
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Can Existing Internet Routing Protocols
Be Used for MANET?
 Link-state Routing
 Distance-vector Routing
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Link State Routing: Dijkstra’s Algorithm
 Each node keeps its link state to its neighbors.
 From each node, we gradually expand a spanning tree,
until all nodes are scanned.
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Link State Routing: Dijkstra’s Algorithm
Initial State: each host only knows its direct neighbors
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Evolution of States in C
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Evolution of States in C (cont.)
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Evolution of States in C (cont.)
 Comments: This is a centralized algorithm, not appropriate.
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Overview of Current Routing Protocols
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On-demand vs. Table-driven
 Table-Driven Routing Protocol:
 proactive!!
 continuously evaluate the routes
 attempt to maintain consistent, up-to-date routing
information
 when a route is needed, one may be ready immediately
 when the network topology changes
 the protocol responds by propagating updates throughout the
network to maintain a consistent view
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 Source-Initiated On-Demand Routing Protocol:
 reactive!!
 on-demand style: create routes only when it is desired by the
source node
 route discovery: invoke a route-determination procedure
 the procedure is terminated when
 a route has been found
 no route is found after all route permutations are examined
 longer delay: sometimes a route may not be ready for use
immediately when data packets come
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Table-Driven Routing Protocols
 Protocol 1:
 DSDV: Destination Sequence Distance Vector
 Protocol 2:
 CGSR: Clustered Gateway Switch Routing
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Protocol 1: DSDV (Destination Sequence
Distance Vector)
 “Highly Dynamic Destination-Sequence Distance-Vector
Routing (DSDV) for Mobile Computers”
 Charles E. Perkins & Pravin Bhagwat
 Dated: 1994
 Computer Communications Review, ‘94
 pp. 234-244
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DSDV Outline
 Each node keeps a routing table to all other nodes.
 based on next-hop routing
 Once its routing table changes, a node broadcasts its table
to other nodes.
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DSDV(cont.)
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DSDV(cont.)
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Protocol 2: CGSR (Clusterhead Gateway
Switch Routing)
 “Routing in Clustered Multihop, Mobile Wireless
Networks with Fading Channel”, C.-C. Chiang, 1996, Proc.
IEEE SICON ’97, pp. 197-211.
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CGSR: Cluster Head and Gateway
 The arrangement of cluster head is similar to dominating
set in graph theory.
 Definition: each node is either in the dominating set or is
neighboring to a node in the dominating set.
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CGSR(cont.)
(5 hops)
(3 hops)
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CGSR (cont.)
 A routing table among cluster heads are maintained.
 also based on the DSDV manner
 Data forwarding steps:
 from cluster head to cluster head
 in a hierarchical manner
 then from cluster head to cluster members
 between two cluster heads, gateways are used to forward the
packets
 Adv: less routing information to be kept
 Disadv: longer route
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Source-Initiated On-Demand
Routing Protocols






AODV
DSR
TORA
ABR
SSR
ZRP
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Protocol 1:AODV
 AODV: Ad hoc On-demand Distance Vector routing
protocol
 On track to become an IETF Experimental RFC
 References
 C. E. Perkins, E. M. Belding-Royer, and S. R. Das, “Ad hoc
On-Demand Distance Vector (AODV) Routing,” IETF
Internet Draft, draft-ietf-manet-aodv-13.txt, Feb. 17, 2003
(work in progress).
 C. E. Perkins and E. M. Royer, “Ad hoc On-Demand
Distance Vector Routing,” Proceedings 2nd IEEE Workshop
on Mobile Computing Systems and Applications, February
1999, pp. 90-100.
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AODV Concepts (1)
 Pure on-demand routing protocol
 A node does not perform route discovery or maintenance
until it needs a route to another node or it offers its services
as an intermediate node
 Nodes that are not on active paths do not maintain routing
information and do not participate in routing table exchanges
 Uses a broadcast route discovery mechanism
 Uses hop-by-hop routing
 Routes are based on dynamic table entries maintained at
intermediate nodes
 Comparison: Dynamic Source Routing (DSR) uses source
routing
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AODV Concepts (2)
 Local HELLO messages are used to determine local
connectivity
 Can reduce response time to routing requests
 Can trigger updates when necessary
 Sequence numbers are assigned to routes and routing table
entries
 to supersede stale cached routing entries
 Every node maintains two counters
 Node sequence number
 Broadcast ID
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AODV Route Request (1)
 Initiated when a node wants to communicate with another
node, but does not have a route to that node
 Source node broadcasts a route request (RREQ) packet to
its neighbors
type
flags resvd hopcnt
broadcast_id
dest_addr
dest_sequence_#
source_addr
source_sequence_#
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AODV Route Request (2)
 Sequence numbers
 Source sequence indicates “freshness” of reverse route to the
source
 Destination sequence number indicates freshness of route to
the destination
 Every neighbor receives the RREQ and either …
 Returns a route reply (RREP) packet, or
 Forwards the RREQ to its neighbors
 (source_addr, broadcast_id) uniquely identifies the RREQ
 broadcast_id is incremented for every RREQ packet sent
 Receivers can identify and discard duplicate RREQ packets
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AODV Route Request (3)
 If a node cannot respond to the RREQ
 The node increments the hop count
 The node saves the following information to set up a reverse
path (AODV assumes symmetrical links)
 Neighbor that sent this RREQ packet
 Destination IP address
 Source IP address
 Broadcast ID
 Source node’s sequence number
 Expiration time for reverse path entry (to enable garbage
collection)
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AODV Example (1)
4
6
1
3
5
7
2
 Node 1 needs to send a data packet to Node 7
 Assume Node 6 knows a current route to Node 7
 Assume that no other route information exists in the
network (related to Node 7)
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AODV Example (2)
4
6
1
3
5
7
2
 Node 1 sends a RREQ packet to its neighbors
 source_addr = 1
 dest_addr = 7
 broadcast_id = broadcast_id + 1
 source_sequence_# = source_sequence_# + 1
 dest_sequence_# = last dest_sequence_# for Node 7
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AODV Example (3)
4
6
1
3
5
7
2
 Nodes 2 and 4 verify that this is a new RREQ and that the
source_sequence_# is not stale with respect to the reverse
route to Node 1
 Nodes 2 and 4 forward the RREQ
 Update source_sequence_# for Node 1
 Increment hop_cnt in the RREQ packet
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AODV Example (4)
4
6
1
3
5
7
2
 RREQ reaches Node 6, which knows a route to 7
 Node 6 must verify that the destination sequence number is
less than or equal to the destination sequence number it has
recorded for Node 7
 Nodes 3 and 5 will forward the RREQ packet, but the
receivers recognize the packets as duplicates
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AODV Route Reply (1)
 If a node receives an RREQ packet and it has a current
route to the target destination, then it unicasts a route reply
packet (RREP) to the neighbor that sent the RREQ packet
type
flags rsvd prsz hopcnt
dest_addr
dest_sequence_#
source_addr
lifetime
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AODV Route Reply (2)
 Intermediate nodes propagate the first RREP for the source
towards the source using cached reverse route entries
 Other RREP packets are discarded unless…
 dest_sequence_# number is higher than the previous, or
 destination_sequence_# is the same, but hop_cnt is smaller
(i.e., there’s a better path)
 RREP eventually makes it to the source, which can use the
neighbor sending the RREP as its next hop for sending to
the destination
 Cached reverse routes will timeout in nodes not seeing a
RREP packet
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AODV Example (5)
4
6
1
3
5
7
2
 Node 6 knows a route to Node 7 and sends an RREP to
Node 4
 source_addr = 1
 dest_addr = 7
 dest_sequence_# = maximum(own sequence number,
dest_sequence_# in RREQ)
 hop_cnt = 1
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AODV Example (6)
4
6
1
3
5
7
2
 Node 4 verifies that this is a new route reply (the case here)
or one that has a lower hop count and, if so, propagates the
RREP packet to Node 1
 Increments hop_cnt in the RREP packet
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AODV Example (7)
Dest Next Hops
7
4
3
4
6
1
3
5
7
2
 Node 1 now has a route to Node 7 in three hops and can
use it immediately to send data packets
 Note that the first data packet that prompted path discovery
has been delayed until the first RREP was returned
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AODV Route Maintenance
 Route changes can be detected by…
 Failure of periodic HELLO packets
 Failure or disconnect indication from the link level
 Failure of transmission of a packet to the next hop (can
detect by listening for the retransmission if it is not the final
destination)
 The upstream (toward the source) node detecting a failure
propagates an route error (RERR) packet to the source
node with a new destination sequence number and a hop
count of infinity (unreachable)
 The source (or another node on the path) can rebuild a path
by sending a RREQ packet
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AODV Example (8)
4
6
1
3
2
7
5
7
 Assume that Node 7 moves and link 6-7 breaks
 Node 6 issues an RERR packet indicating the broken path
 The RERR propagates back to Node 1
 Node 1 can discover a new route
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Protocol 2: DSR (Dynamic Source Routing)
 “Dynamic Source Routing in Ad-Hoc Wireless Networks”,
D. B. Johnson and D. A. Maltz, Mobile Computing, 1996,
pp. 153-181.
 on-demand
 Each host maintains a route cache which contains all routes
it has learnt.
 Source Routing:
 routes are denoted with complete information (each hop is
registered)
 Two major parts:
 route discovery
 route maintenance
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Route Discovery
Route Reply
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Route Discovery of DSR
 When a host has a packet to send, it first consults its route
cache.
 If there is an unexpired route, then it will use it.
 Otherwise, a route discovery will be performed.
 Route Discovery:
 There is a “route record” field in the packet.
 The source node will add its address to the record.
 On receipt of the packet, a host will add its address to the
“route record” and rebroadcast the packet.
 To limit the number of ROUTE_REQUEST packets:
 Each node only rebroadcasts the packet at most once.
 Each node will consult its route cache to see if a route is
already known.
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ROUTE_REPLY of DSR
 A ROUTE_REPLY packet is generated when
 the route request packet reaches the destination
 an intermediate host has an unexpired route to the
destination
 The ROUTE_REPLY packet will contain a route generated
in two manner:
 from destination:
 the route that was traversed by the ROUTE_REQUEST packet
 from intermediate host:
 the route that was traversed by the ROUTE_REQUEST packet
concatenated with the route in the intermediate host’s route
cache
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Stale Route Cache Problem
 Definition:
 A route may become broken (i.e., stale), but is unaware by a
host X.
 With route cache, host X may keep on distributing erroneous
information to other hosts.
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Route Maintenance of DSR
 When the data link layer encounters a link breakage, a
ROUTE_ERROR packet will be initiated.
 The packet will traverse in the backward direction to the
source.
 The source will then initiate another ROUTE_REQUEST.
 Example: (next page)
 Maintenance of route cache:
 All routes which contain the breakage hop have to be
removed from the route cache.
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Route_Error
x
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Packet Type: Route Request (RREQ)
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Packet Type: Route Reply (RREP)
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Packet Type: Data Packet
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Packet Type: Route_Error
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Route Concentration Problem
 With route cache, hosts are likely to share the same links
(routes).
(1)
(2)
(3)
(4)
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Protocol 3: TORA
(Temporally Ordered Routing Algorithm)
 “A Highly Adaptive Distributed Routing Algorithm for
Mobile Wireless Networks”, University of Maryland, V. D.
Park and M. S. Corson, 1996, Proc. INFOCOM ’97.
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TORA Outline
 source-initiated protocol
 provide multiple paths for
any source-destination
pair
high level
 Like water flowing, it
goes from upstream to
downstream.
 for highly dynamic mobile
networks
low level
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Main Idea
 Regard the network as a directed graph.
 For each destination, a DAG (directed acyclic graph) will
be maintained.
 Note: There are n copies of DAG’s, each associated with one
destination, where n is the number of hosts.
 In the following discussion, we only discuss one DAG
associated with a destination.
 The DAG is accomplished by assigning each node i a
height metric hi.
 A link from i to j means hi > hj.
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Full Reversal Method
 A node will update its height to adapt to the change of
network topology.
 Height hi = (valuei, IDi)
 a node will change its value to change the direction of a link
 Relation: hi > hj if the following is true:
 valuei > valuej
 (valuei = valuej) and (Di > Dj)
 Ex: (5, 4) > (4, 6)
 Ex: (5, 4) > (5, 2)
 Property: Given any to heights, there must exist a “>”
relation between them.
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 Rule:
 Each node other than the destination that has no outgoing
links reverses the direction of ALL its incoming links.
 This means that the node’s height is a local minimum.
 This is done by getting a larger height such that the node
becomes a local maximum.
 MAX{all neighbors’ heights} + 1
 Example: a graph with a random direction for each link
a, 5
b, 6
e, 3
d, 4
c, 3
dest, 8
f, 1
g, 2
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a, 7
b, 6
original network
e, 6
a, 5
d, 9
c, 9
b, 6
f, 7
e, 3
dest, 8
d, 4
c, 3
dest, 8
g, 5
f, 1
a, 5
g, 2
b, 6
e, 6
d, 4
c, 9
a, 5
f, 4
b, 6
e, 3
d, 4
c, 9
dest, 8
dest, 8
f, 4
g, 2
g, 5
a, 7
b, 10
e, 10
d, 9
c, 9
dest, 8
f, 7
g, 10
a, 11
b, 10
e, 10
d, 9
c, 9
dest, 8
f, 11
g, 10
Eventually, the DAG will stablize.
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TORA Details
 Three basic functions:
 route creation
 route maintenance
 route erasure
 Three control packets:
 query (QRY)
 update (UPD)
 clear (CLR)
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Data Structure
 Each node keep a 5-tuple (τ, oid, r, δ, i)
 τ: time of the link failure.
 oid (originator ID):
 Unique identifier of the node that defined a new reference level
 Most likely, the node who detects link breakage.
 r: reflection indicator bit.
 initially set to 0.
 δ: propagation ordering parameter (i.e., height)
 i: node ID
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Creating Routes
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Maintaining Routes
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Maintaining Routes (with Reaction)
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Maintaining Routes (cont.)
The reflection bit (r)
is used here to mean
“no exit”.
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Protocol 4: ABR
(Associativity-Based Routing)
 ABR considers the stability of a link.
 The metric is called degree of association stability.
 Basic Idea:
 Each node periodically generates a beacon to signify its
existence.
 On receipt of the beacon, a neighboring node will increase
the “tick” of the sender by 1.
 A higher degree of association stability (i.e., ticks) may
indicate a low mobility of that node.
 A low degree of association stability may indicate a high
mobility of that node.
 When a link becomes broken, the node will set the tick of
the other node to 0.
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ABR Outline
 Route Discovery:
 (similar to DSR)
 On needing a route, a host will broadcast a
ROUTE_REQUEST packet.
 Each receiving host will append its address to the packet.
 The association stability (represented by “ticks”) is also
appended in the ROUTE_REQUEST packet.
 The destination node will select the best route (in terms of
association stability), and then respond a packet to the source.
7
5
8
source
10
4
destination
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 Route Reconstruction:
 On route error, a node will perform a local search in hope of
rebuild the path.
 If the local search fails, a ROUTE_ERROR will be reported
to the source.
source
local
searched zone
destination
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Protocol 5: SSA
(Signal Stability-Based Adaptive Routing)
 “Signal Stability-Based Adaptive Routing (SSA) for Ad
Hoc Wireless Networks”
 University of Maryland
 R. Dube, C. D. Rais, K.-Y. Wang & S. K. Tripathi
 IEEE Personal Communications, ‘97
 pp. 36-45
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Basic Idea of SSA
 Observation:
 The ABR only considers the connectivity to nodes.
 Two more metrics:
 signal stability:
 the strength of a signal
 provided by link layer
 location stability
 how fast a host moves
 could be measure by:
 the change of signal strength over a period of time
 location devices (such as GPS)
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SSA
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SSA(cont.)
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Protocol 6: ZRP
(Zone Routing Protocol)
 The Zone Routing Protocol (ZRP) for Ad Hoc Networks
 Cornell University
 Z.J. Haas and M.R. Pearlman
 draft-ietf-manet-zone-zrp-01.txt, 1998
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ZRP Outline
 Hybrid of table-driven and on-demand!!
 From each node, there is a concept of “zone”.
 Within each zone, the routing is performed in a table-driven
manner (proactive).
 However, a node does not try to keep global routing
information.
 For inter-zone routing, on-demand routing is used.
 This is similar to DSR.
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ZRP Example
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Route Discovery
 By an operation called “boardercast”:
 sending the route-request to boarder nodes
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Comparison of Table-Driven and
On-Demand Protocols
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Research Highlight: Resource Allocation by
Pricing
 Ref: Y. Xue, et al., “Optimal resource allocation in wireless
ad hoc networks: a price-based approach”, IEEE Trans. on
Mobile Computing, 2006.
 Goal: A mobile might be selfish by asking others to relay
its data, but avoiding relaying data of others.
 Approaches:
 using clique to represent interference relations:
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cont.
 price of a clique:
 going up when the demand is higher than requested
 going down when the demand is lower than requested
 sources of flows:
 adding more traffic when path price is going down
 reducing traffic when path price is going up
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