Mobile Ad Hoc Networks:Routing, MAC and Transport Issues : Mobile Ad Hoc Networks: Routing, MAC and Transport Issues
Nitin H. Vaidya
University of Illinois at Urbana-Champaign
nhv@uiuc.edu
http://www.crhc.uiuc.edu/~nhv
© 2003 Nitin Vaidya
Note : Note Versions of this tutorial have been presented at several conferences
These slides for the most part consist of a compilation of the slides used in prior tutorials by Nitin Vaidya at MobiCom 2001 (Rome) and 2000 (Boston), MobiHoc 2003 (Annapolis) and 2002 (Lausanne), Hot Interconnects 2002 (Palo Alto) and VTC 2000 (Boston)
Notes : Notes
Names in brackets, as in [Xyz00], refer to a document in the list of references
The handout may not be as readable as the original slides, since the slides contain colored text and figures
Note that different colors in the colored slides may look identically black in the black-and-white handout
Statutory Warnings : Statutory Warnings Only most important features of various schemes are typically discussed, i.e, features I consider as being important
Others may disagree
Most schemes include many more details, and optimizations
Not possible to cover all details in this tutorial
Be aware that some protocol specs have changed several times, and the slides may not reflect the most current specifications
Jargon used to discuss a scheme may occasionally differ from that used by the proposers
Coverage : Coverage
Not intended to be exhaustive
Many interesting papers not covered in the tutorial due to lack of time
No judgement on those papers is implied
Tutorial Outline : Tutorial Outline Introduction
Unicast routing
Multicast routing
Geocast routing
Medium Access Control
Performance of UDP and TCP
Security Issues
Implementation Issues
Distributed Algorithms
Standards activities
Open problems
Mobile Ad Hoc Networks (MANET)Introduction and Generalities : Mobile Ad Hoc Networks (MANET) Introduction and Generalities
Mobile Ad Hoc Networks : Mobile Ad Hoc Networks
Formed by wireless hosts which may be mobile
Without (necessarily) using a pre-existing infrastructure
Routes between nodes may potentially contain multiple hops
Mobile Ad Hoc Networks : Mobile Ad Hoc Networks May need to traverse multiple links to reach a destination
Mobile Ad Hoc Networks (MANET) : Mobile Ad Hoc Networks (MANET) Mobility causes route changes
Why Ad Hoc Networks ? : Why Ad Hoc Networks ?
Ease of deployment
Speed of deployment
Decreased dependence on infrastructure
Many Applications : Many Applications Personal area networking
cell phone, laptop, ear phone, wrist watch
Military environments
soldiers, tanks, planes
Civilian environments
taxi cab network
meeting rooms
sports stadiums
boats, small aircraft
Emergency operations
search-and-rescue
policing and fire fighting
Many Variations : Many Variations 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)
Many Variations : 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
Many Variations : Many Variations 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
Challenges : Challenges
Limited wireless transmission range
Broadcast nature of the wireless medium
Hidden terminal problem (see next slide)
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)
Hidden Terminal Problem : Hidden Terminal Problem Nodes A and C cannot hear each other
Transmissions by nodes A and C can collide at node B
Nodes A and C are hidden from each other
Research on Mobile Ad Hoc Networks : Research on Mobile Ad Hoc Networks
Variations in capabilities andamp; responsibilities
X
Variations in traffic characteristics, mobility models, etc.
X
Performance criteria (e.g., optimize throughput, reduce energy consumption)
+
Increased research funding
=
Significant research activity
The Holy Grail : The Holy Grail
A one-size-fits-all solution
Perhaps using an adaptive/hybrid approach that can adapt to situation at hand
Difficult problem
Many solutions proposed trying to address a
sub-space of the problem domain
Assumption : Assumption
Unless stated otherwise, fully symmetric environment is assumed implicitly
all nodes have identical capabilities and responsibilities
Unicast RoutinginMobile Ad Hoc Networks : Unicast Routing in Mobile Ad Hoc Networks
Why is Routing in MANET different ? : Why is Routing in MANET different ? Host mobility
link failure/repair due to mobility may have different characteristics than those due to other causes
Rate of link failure/repair may be high when nodes move fast
New performance criteria may be used
route stability despite mobility
energy consumption
Unicast Routing Protocols : Unicast Routing Protocols
Many protocols have been proposed
Some have been invented specifically for MANET
Others are adapted from previously proposed protocols for wired networks
No single protocol works well in all environments
some attempts made to develop adaptive protocols
Routing Protocols : Routing Protocols Proactive protocols
Determine routes independent of traffic pattern
Traditional link-state and distance-vector routing protocols are proactive
Reactive protocols
Maintain routes only if needed
Hybrid protocols
Trade-Off : Trade-Off 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 achieves a better trade-off depends on the traffic and mobility patterns
Overview of Unicast Routing Protocols : Overview of Unicast Routing Protocols
Flooding for Data Delivery : Flooding for Data Delivery Sender S broadcasts data packet P to all its neighbors
Each node receiving P forwards P to its neighbors
Sequence numbers used to avoid the possibility of forwarding the same packet more than once
Packet P reaches destination D provided that D is reachable from sender S
Node D does not forward the packet
Flooding for Data Delivery : Flooding for Data Delivery B A S E F H J D C G I K Represents that connected nodes are within each
other’s transmission range Z Y Represents a node that has received packet P M N L
Flooding for Data Delivery : Flooding for Data Delivery B A S E F H J D C G I K Represents transmission of packet P Represents a node that receives packet P for
the first time Z Y Broadcast transmission M N L
Flooding for Data Delivery : Flooding for Data Delivery B A S E F H J D C G I K Node H receives packet P from two neighbors:
potential for collision Z Y M N L
Flooding for Data Delivery : Flooding for Data Delivery B A S E F H J D C G I K Node C receives packet P from G and H, but does not forward
it again, because node C has already forwarded packet P once Z Y M N L
Flooding for Data Delivery : Flooding for Data Delivery B A S E F H J D C G I K Z Y M Nodes J and K both broadcast packet P to node D
Since nodes J and K are hidden from each other, their
transmissions may collide
=andgt; Packet P may not be delivered to node D at all,
despite the use of flooding N L
Flooding for Data Delivery : Flooding for Data Delivery B A S E F H J D C G I K Z Y Node D does not forward packet P, because node D
is the intended destination of packet P M N L
Flooding for Data Delivery : Flooding for Data Delivery B A S E F H J D C G I K 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) Z Y M N L
Flooding for Data Delivery : Flooding for Data Delivery B A S E F H J D C G I K Flooding may deliver packets to too many nodes
(in the worst case, all nodes reachable from sender
may receive the packet) Z Y M N L
Flooding for Data Delivery: Advantages : Flooding for Data Delivery: Advantages Simplicity
May be more efficient than other protocols when rate of information transmission is low enough that the overhead of explicit route discovery/maintenance incurred by other protocols is relatively higher
this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissions
Potentially higher reliability of data delivery
Because packets may be delivered to the destination on multiple paths
Flooding for Data Delivery: Disadvantages : Flooding for Data Delivery: Disadvantages
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 broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead
Broadcasting in IEEE 802.11 MAC is unreliable
In our example, nodes J and K may transmit to node D simultaneously, resulting in loss of the packet
in this case, destination would not receive the packet at all
Flooding of Control Packets : Flooding of Control Packets Many protocols perform (potentially limited) flooding of control packets, instead of data packets
The control packets are used to discover routes
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] : Dynamic Source Routing (DSR) [Johnson96]
When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery
Source node S floods Route Request (RREQ)
Each node appends own identifier when forwarding RREQ
Route Discovery in DSR : Route Discovery in DSR B A S E F H J D C G I K Z Y Represents a node that has received RREQ for D from S M N L
Route Discovery in DSR : Route Discovery in DSR B A S E F H J D C G I K Represents transmission of RREQ Z Y Broadcast transmission M N L [S] [X,Y] Represents list of identifiers appended to RREQ
Route Discovery in DSR : Route Discovery in DSR B A S E F H J D C G I K Node H receives packet RREQ from two neighbors:
potential for collision Z Y M N L [S,E] [S,C]
Route Discovery in DSR : Route Discovery in DSR B A S E F H J D C G I K Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once Z Y M N L [S,C,G] [S,E,F]
Route Discovery in DSR : Route Discovery in DSR B A S E F H J D C G I K Z Y M Nodes J and K both broadcast RREQ to node D
Since nodes J and K are hidden from each other, their
transmissions may collide N L [S,C,G,K] [S,E,F,J]
Route Discovery in DSR : Route Discovery in DSR B A S E F H J D C G I K Z Y Node D does not forward RREQ, because node D
is the intended target of the route discovery M N L [S,E,F,J,M]
Route Discovery in DSR : Route Discovery in DSR
Destination D on receiving the first RREQ, sends a Route Reply (RREP)
RREP is sent on a route obtained by reversing the route appended to received RREQ
RREP includes the route from S to D on which RREQ was received by node D
Route Reply in DSR : Route Reply in DSR B A S E F H J D C G I K Z Y M N L RREP [S,E,F,J,D] Represents RREP control message
Route Reply in DSR : Route Reply in DSR Route Reply can be sent by reversing the route in Route Request (RREQ) only if links are guaranteed to be bi-directional
To ensure this, RREQ should be forwarded only if it received on a link that is known to be bi-directional
If unidirectional (asymmetric) links are allowed, then RREP may need a route discovery for S from node D
Unless node D already knows a route to node S
If a route discovery is initiated by D for a route to S, then the Route Reply is piggybacked on the Route Request from D.
If IEEE 802.11 MAC is used to send data, then links have to be bi-directional (since Ack is used)
Dynamic Source Routing (DSR) : Dynamic Source Routing (DSR)
Node S on receiving RREP, caches the route included in the RREP
When node 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 a packet should be forwarded
Data Delivery in DSR : Data Delivery in DSR B A S E F H J D C G I K Z Y M N L DATA [S,E,F,J,D] Packet header size grows with route length
When to Perform a Route Discovery : When to Perform a Route Discovery
When node S wants to send data to node D, but does not know a valid route node D
DSR Optimization: Route Caching : DSR Optimization: Route Caching Each node caches a new route it learns by any means
When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F
When node K receives Route Request [S,C,G] destined for node, node K learns route [K,G,C,S] to node S
When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route [F,J,D] to node 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
Use of Route Caching : Use of Route Caching
When node S learns that a route to node D is broken, it uses another route from its local cache, if such a route to D exists in its cache. Otherwise, node S initiates route discovery by sending a route request
Node X on receiving a Route Request for some node D can send a Route Reply if node X knows a route to node D
Use of route cache
can speed up route discovery
can reduce propagation of route requests
Use of Route Caching : Use of Route Caching B A S E F H J D C G I K [P,Q,R] Represents cached route at a node
(DSR maintains the cached routes in a tree format) M N L [S,E,F,J,D] [E,F,J,D] [C,S] [G,C,S] [F,J,D],[F,E,S] [J,F,E,S] Z
Use of Route Caching:Can Speed up Route Discovery : Use of Route Caching: Can Speed up Route Discovery B A S E F H J D C G I K Z M N L [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 When node Z sends a route request
for node C, node K sends back a route
reply [Z,K,G,C] to node Z using a locally
cached route [K,G,C,S] RREP
Use of Route Caching:Can Reduce Propagation of Route Requests : Use of Route Caching: Can Reduce Propagation of Route Requests B A S E F H J D C G I K Z Y M N L [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 Assume that there is no link between D and Z.
Route Reply (RREP) from node K limits flooding of RREQ.
In general, the reduction may be less dramatic. [K,G,C,S] RREP
Route Error (RERR) : Route Error (RERR) B A S E F H J D C G I K Z Y M N L RERR [J-D] J sends a route error to S along route J-F-E-S when its attempt to forward the data packet S (with route SEFJD) on J-D fails
Nodes hearing RERR update their route cache to remove link J-D
Route Caching: Beware! : Route Caching: Beware! Stale caches can adversely affect performance
With passage of time and host mobility, cached routes may become invalid
A sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good route
An illustration of the adverse impact on TCP will be discussed later in the tutorial [Holland99]
Dynamic Source Routing: Advantages : Dynamic Source Routing: Advantages
Routes maintained only between nodes who need to communicate
reduces overhead of route maintenance
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
Dynamic Source Routing: Disadvantages : Dynamic Source Routing: Disadvantages Packet header size grows with route length due to source routing
Flood of route requests may potentially reach all nodes in the network
Care must be taken to avoid collisions between route requests propagated by neighboring nodes
insertion of random delays before forwarding RREQ
Increased contention if too many route replies come back due to nodes replying using their local cache
Route Reply Storm problem
Reply storm may be eased by preventing a node from sending RREP if it hears another RREP with a shorter route
Dynamic Source Routing: Disadvantages : Dynamic Source Routing: Disadvantages
An intermediate node may send Route Reply using a stale cached route, thus polluting other caches
This problem can be eased if some mechanism to purge (potentially) invalid cached routes is incorporated.
For some proposals for cache invalidation, see [Hu00Mobicom]
Static timeouts
Adaptive timeouts based on link stability
Flooding of Control Packets : 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] : 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 : Expected Zone in LAR X Y r 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 : Request Zone in LAR X Y r S Request Zone Network Space B A
LAR : 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
LAR : LAR Only nodes within the request zone forward route requests
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
Rest of route discovery protocol similar to DSR
LAR Variations: Adaptive Request Zone : 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 S B Request zone adapted by B Request zone defined by sender S
LAR Variations: Implicit Request Zone : LAR Variations: Implicit Request Zone
In the previous scheme, a route request explicitly specified a request zone
Alternative approach: A node X forwards a route request received from Y if node X is deemed to be closer to the expected zone as compared to Y
The motivation is to attempt to bring the route request physically closer to the destination node after each forwarding
Location-Aided Routing : Location-Aided Routing The basic proposal assumes that, initially, location information for node X becomes known to Y only during a route discovery
This location information is used for a future route discovery
Each route discovery yields more updated information which is used for the next discovery
Variations
Location information can also be piggybacked on any message from Y to X
Y may also proactively distribute its location information
Similar to other protocols discussed later (e.g., DREAM, GLS)
Location Aided Routing (LAR) : 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
Detour : Detour Routing Using Location Information
Distance Routing Effect Algorithm for Mobility (DREAM) [Basagni98Mobicom] : Distance Routing Effect Algorithm for Mobility (DREAM) [Basagni98Mobicom]
Uses location and speed information (like LAR)
DREAM uses flooding of data packets as the routing mechanism (unlike LAR)
DREAM uses location information to limit the flood of data packets to a small region
Distance Routing Effect Algorithm for Mobility (DREAM) : Distance Routing Effect Algorithm for Mobility (DREAM) S D Expected zone
(in the LAR jargon) A Node A, on receiving the
data packet, forwards it to
its neighbors within the
cone rooted at node A S sends data packet to all
neighbors in the cone rooted
at node S
Distance Routing Effect Algorithm for Mobility (DREAM) : Distance Routing Effect Algorithm for Mobility (DREAM) Nodes periodically broadcast their physical location
Nearby nodes are updated more frequently, far away nodes less frequently
Distance effect: Far away nodes seem to move at a lower angular speed as compared to nearby nodes
Location update’s time-to-live field used to control how far the information is propagated
Relative Distance Micro-Discovery Routing (RDMAR) [Aggelou99Wowmom] : Relative Distance Micro-Discovery Routing (RDMAR) [Aggelou99Wowmom]
Estimates distance between source and intended destination in number of hops
Sender node sends route request with time-to-live (TTL) equal to the above estimate
Hop distance estimate based on the physical distance that the nodes may have traveled since the previous route discovery, and transmission range
Geographic Distance Routing (GEDIR) [Lin98] : Geographic Distance Routing (GEDIR) [Lin98] Location of the destination node is assumed known
Each node knows location of its neighbors
Each node forwards a packet to its neighbor closest to the destination
Route taken from S to D shown below S A B D C F E obstruction H G
Geographic Distance Routing (GEDIR) [Stojmenovic99] : Geographic Distance Routing (GEDIR) [Stojmenovic99] The algorithm terminates when same edge traversed twice consecutively
Algorithm fails to route from S to E
Node G is the neighbor of C who is closest from destination E, but C does not have a route to E S A B D C F E obstruction H G
Routing with Guaranteed Delivery [Bose99Dialm] : Routing with Guaranteed Delivery [Bose99Dialm]
Improves on GEDIR [Lin98]
Guarantees delivery (using location information) provided that a path exists from source to destination
Routes around obstacles if necessary
A similar idea also appears in [Karp00Mobicom]
Grid Location Service (GLS) [Li00Mobicom] : Grid Location Service (GLS) [Li00Mobicom] A cryptic discussion of this scheme due to lack of time:
Each node maintains its location information at other nodes in the network
Density of nodes who know location of node X decreases as distance from X increases
Each node updates its location periodically -- nearby nodes receive the updates more often than far away nodes
A hierarchical grid structure used to define near and far
End of Detour : Back to
Reducing Scope of
the Route Request Flood End of Detour
Query Localization [Castaneda99Mobicom] : Query Localization [Castaneda99Mobicom]
Limits route request flood without using physical information
Route requests are propagated only along paths that are close to the previously known route
The closeness property is defined without using physical location information
Query Localization : Query Localization
Path locality heuristic: Look for a new path that contains at most k nodes that were not present in the previously known route
Old route is piggybacked on a Route Request
Route Request is forwarded only if the accumulated route in the Route Request contains at most k new nodes that were absent in the old route
this limits propagation of the route request
Query Localization: Example : Query Localization: Example B E A S D C G F Initial route
from S to D B E A S D C G F Permitted routes
with k = 2 Node F does not forward the route
request since it is not on any route
from S to D that contains at most
2 new nodes Node D moved
Query Localization : Query Localization
Advantages:
Reduces overhead of route discovery without using physical location information
Can perform better in presence of obstructions by searching for new routes in the vicinity of old routes
Disadvantage:
May yield routes longer than LAR
(Shortest route may contain more than k new nodes)
Broadcast Storm Problem [Ni99Mobicom] : B D C A Broadcast Storm Problem [Ni99Mobicom] When node A broadcasts a route query, nodes B and C both receive it
B and C both forward to their neighbors
B and C transmit at about the same time since they are reacting to receipt of the same message from A
This results in a high probability of collisions
Broadcast Storm Problem : Broadcast Storm Problem Redundancy: A given node may receive the same route request from too many nodes, when one copy would have sufficed
Node D may receive from nodes B and C both B D C A
Solutions for Broadcast Storm : Solutions for Broadcast Storm Probabilistic scheme: On receiving a route request for the first time, a node will re-broadcast (forward) the request with probability p
Also, re-broadcasts by different nodes should be staggered by using a collision avoidance technique (wait a random delay when channel is idle)
this would reduce the probability that nodes B and C would forward a packet simultaneously in the previous example
Solutions for Broadcast Storms : B D C A F E Solutions for Broadcast Storms Counter-Based Scheme: If node E hears more than k neighbors broadcasting a given route request, before it can itself forward it, then node E will not forward the request
Intuition: k neighbors together have probably already forwarded the request to all of E’s neighbors
Solutions for Broadcast Storms : E Z andlt;d Solutions for Broadcast Storms Distance-Based Scheme: If node E hears RREQ broadcasted by some node Z within physical distance d, then E will not re-broadcast the request
Intuition: Z and E are too close, so transmission areas covered by Z and E are not very different
if E re-broadcasts the request, not many nodes who have not already heard the request from Z will hear the request
Summary: Broadcast Storm Problem : Summary: Broadcast Storm Problem Flooding is used in many protocols, such as Dynamic Source Routing (DSR)
Problems associated with flooding
collisions
redundancy
Collisions may be reduced by 'jittering' (waiting for a random interval before propagating the flood)
Redundancy may be reduced by selectively re-broadcasting packets from only a subset of the nodes
Ad Hoc On-Demand Distance Vector Routing (AODV) [Perkins99Wmcsa] : Ad Hoc On-Demand Distance Vector Routing (AODV) [Perkins99Wmcsa] DSR includes source routes in packet headers
Resulting large headers can sometimes degrade performance
particularly when data contents of a packet are small
AODV attempts to improve on DSR by maintaining routing tables at the nodes, so that data packets do not have to contain routes
AODV retains the desirable feature of DSR that routes are maintained only between nodes which need to communicate
AODV : 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
AODV assumes symmetric (bi-directional) links
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
Route Requests in AODV : Route Requests in AODV B A S E F H J D C G I K Z Y Represents a node that has received RREQ for D from S M N L
Route Requests in AODV : Route Requests in AODV B A S E F H J D C G I K Represents transmission of RREQ Z Y Broadcast transmission M N L
Route Requests in AODV : Route Requests in AODV B A S E F H J D C G I K Represents links on Reverse Path Z Y M N L
Reverse Path Setup in AODV : Reverse Path Setup in AODV B A S E F H J D C G I K Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once Z Y M N L
Reverse Path Setup in AODV : Reverse Path Setup in AODV B A S E F H J D C G I K Z Y M N L
Reverse Path Setup in AODV : Reverse Path Setup in AODV B A S E F H J D C G I K Z Y Node D does not forward RREQ, because node D
is the intended target of the RREQ M N L
Route Reply in AODV : Route Reply in AODV B A S E F H J D C G I K Z Y Represents links on path taken by RREP M N L
Route Reply in AODV : Route Reply in AODV An intermediate node (not the destination) 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
The likelihood that an intermediate node will send a Route Reply when using AODV not as high as DSR
A new Route Request by node S for a destination is assigned a higher destination sequence number. An intermediate node which knows a route, but with a smaller sequence number, cannot send Route Reply
Forward Path Setup in AODV : Forward Path Setup in AODV B A S E F H J D C G I K Z Y M N L Forward links are setup when RREP travels along
the reverse path
Represents a link on the forward path
Data Delivery in AODV : Data Delivery in AODV B A S E F H J D C G I K Z Y M N L Routing table entries used to forward data packet.
Route is not included in packet header. DATA
Timeouts : Timeouts
A routing table entry maintaining a reverse path is purged after a timeout interval
timeout should be long enough to allow RREP to come back
A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval
if no is data being sent using a particular routing table entry, that entry will be deleted from the routing table (even if the route may actually still be valid)
Link Failure Reporting : Link Failure Reporting
A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry
When the next hop link in a routing table entry breaks, all active neighbors are informed
Link failures are propagated by means of Route Error messages, which also update destination sequence numbers
Route Error : Route Error 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
Destination Sequence Number : Destination Sequence Number
Continuing from the previous slide …
When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N
Link Failure Detection : Link Failure Detection Hello messages: Neighboring nodes periodically exchange hello message
Absence of hello message is used as an indication of link failure
Alternatively, failure to receive several MAC-level acknowledgement may be used as an indication of link failure
Why Sequence Numbers in AODV : Why Sequence Numbers in AODV To avoid using old/broken routes
To determine which route is newer
To prevent formation of loops
Assume that A does not know about failure of link C-D because RERR sent by C is lost
Now C performs a route discovery for D. Node A receives the RREQ (say, via path C-E-A)
Node A will reply since A knows a route to D via node B
Results in a loop (for instance, C-E-A-B-C ) A B C D E
Why Sequence Numbers in AODV : Why Sequence Numbers in AODV
Loop C-E-A-B-C
A B C D E
Optimization: Expanding Ring Search : Optimization: Expanding Ring Search
Route Requests are initially sent with small Time-to-Live (TTL) field, to limit their propagation
DSR also includes a similar optimization
If no Route Reply is received, then larger TTL tried
Summary: AODV : Summary: AODV
Routes need not be included in packet headers
Nodes maintain routing tables containing entries only for routes that are in active use
At most one next-hop per destination maintained at each node
DSR may maintain several routes for a single destination
Unused routes expire even if topology does not change
So far ... : So far ...
All protocols discussed so far perform some form of flooding
Now we will consider protocols which try to reduce/avoid such behavior
Link Reversal Algorithm [Gafni81] : Link Reversal Algorithm [Gafni81] A F B C E G D
Link Reversal Algorithm : Link Reversal Algorithm A F B C E G D Maintain a directed acyclic
graph (DAG) for each
destination, with the destination
being the only sink
This DAG is for destination
node D Links are bi-directional
But algorithm imposes
logical directions on them
Link Reversal Algorithm : Link Reversal Algorithm Link (G,D) broke A F B C E G D Any node, other than the destination, that has no outgoing links
reverses all its incoming links.
Node G has no outgoing links
Link Reversal Algorithm : Link Reversal Algorithm A F B C E G D Now nodes E and F have no outgoing links Represents a
link that was
reversed recently
Link Reversal Algorithm : Link Reversal Algorithm A F B C E G D Now nodes B and G have no outgoing links Represents a
link that was
reversed recently
Link Reversal Algorithm : Link Reversal Algorithm A F B C E G D Now nodes A and F have no outgoing links Represents a
link that was
reversed recently
Link Reversal Algorithm : Link Reversal Algorithm A F B C E G D Now all nodes (other than destination D) have an outgoing link Represents a
link that was
reversed recently
Link Reversal Algorithm : Link Reversal Algorithm A F B C E G D DAG has been restored with only the destination as a sink
Link Reversal Algorithm : Link Reversal Algorithm
Attempts to keep link reversals local to where the failure occurred
But this is not guaranteed
When the first packet is sent to a destination, the destination oriented DAG is constructed
The initial construction does result in flooding of control packets
Link Reversal Algorithm : Link Reversal Algorithm The previous algorithm is called a full reversal method since when a node reverses links, it reverses all its incoming links
Partial reversal method [Gafni81]: A node reverses incoming links from only those neighbors who have not themselves reversed links 'previously'
If all neighbors have reversed links, then the node reverses all its incoming links
'Previously' at node X means since the last link reversal done by node X
Partial Reversal Method : Partial Reversal Method Link (G,D) broke A F B C E G D Node G has no outgoing links
Partial Reversal Method : Partial Reversal Method A F B C E G D Now nodes E and F have no outgoing links Represents a
link that was
reversed recently Represents a
node that has
reversed links
Partial Reversal Method : Partial Reversal Method A F B C E G D Nodes E and F do not reverse links from node G
Now node B has no outgoing links Represents a
link that was
reversed recently
Partial Reversal Method : Partial Reversal Method A F B C E G D Now node A has no outgoing links Represents a
link that was
reversed recently
Partial Reversal Method : Partial Reversal Method A F B C E G D Now all nodes (except destination D) have outgoing links Represents a
link that was
reversed recently
Partial Reversal Method : Partial Reversal Method A F B C E G D DAG has been restored with only the destination as a sink
Link Reversal Methods: Advantages : Link Reversal Methods: Advantages
Link reversal methods attempt to limit updates to routing tables at nodes in the vicinity of a broken link
Partial reversal method tends to be better than full reversal method
Each node may potentially have multiple routes to a destination
Link Reversal Methods: Disadvantage : Link Reversal Methods: Disadvantage
Need a mechanism to detect link failure
hello messages may be used
but hello messages can add to contention
If network is partitioned, link reversals continue indefinitely
Link Reversal in a Partitioned Network : Link Reversal in a Partitioned Network A F B C E G D This DAG is for destination node D
Full Reversal in a Partitioned Network : Full Reversal in a Partitioned Network A F B C E G D A and G do not have outgoing links
Full Reversal in a Partitioned Network : Full Reversal in a Partitioned Network A F B C E G D E and F do not have outgoing links
Full Reversal in a Partitioned Network : Full Reversal in a Partitioned Network A F B C E G D B and G do not have outgoing links
Full Reversal in a Partitioned Network : Full Reversal in a Partitioned Network A F B C E G D E and F do not have outgoing links
Full Reversal in a Partitioned Network : Full Reversal in a Partitioned Network A F B C E G D In the partition
disconnected from
destination D, link
reversals continue, until
the partitions merge
Need a mechanism to
minimize this wasteful
activity
Similar scenario can
occur with partial
reversal method too
Temporally-Ordered Routing Algorithm(TORA) [Park97Infocom] : Temporally-Ordered Routing Algorithm (TORA) [Park97Infocom]
TORA modifies the partial link reversal method to be able to detect partitions
When a partition is detected, all nodes in the partition are informed, and link reversals in that partition cease
Partition Detection in TORA : Partition Detection in TORA A B E D F C DAG for
destination D
Partition Detection in TORA : Partition Detection in TORA A B E D F C TORA uses a
modified partial
reversal method Node A has no outgoing links
Partition Detection in TORA : Partition Detection in TORA A B E D F C TORA uses a
modified partial
reversal method Node B has no outgoing links
Partition Detection in TORA : Partition Detection in TORA A B E D F C Node B has no outgoing links
Partition Detection in TORA : Partition Detection in TORA A B E D F C Node C has no outgoing links -- all its neighbor have
reversed links previously.
Partition Detection in TORA : Partition Detection in TORA A B E D F C Nodes A and B receive the reflection from node C
Node B now has no outgoing link
Partition Detection in TORA : Partition Detection in TORA A B E D F C Node A has received the reflection from all its neighbors.
Node A determines that it is partitioned from destination D. Node B propagates
the reflection to node A
Partition Detection in TORA : Partition Detection in TORA A B E D F C On detecting a partition,
node A sends a clear (CLR)
message that purges all
directed links in that
partition
TORA : TORA
Improves on the partial link reversal method in [Gafni81] by detecting partitions and stopping non-productive link reversals
Paths may not be shortest
The DAG provides many hosts the ability to send packets to a given destination
Beneficial when many hosts want to communicate with a single destination
TORA Design Decision : TORA Design Decision
TORA performs link reversals as dictated by [Gafni81]
However, when a link breaks, it looses its direction
When a link is repaired, it may not be assigned a direction, unless some node has performed a route discovery after the link broke
if no one wants to send packets to D anymore, eventually, the DAG for destination D may disappear
TORA makes effort to maintain the DAG for D only if someone needs route to D
Reactive behavior
TORA Design Decision : TORA Design Decision
One proposal for modifying TORA optionally allowed a more proactive behavior, such that a DAG would be maintained even if no node is attempting to transmit to the destination
Moral of the story: The link reversal algorithm in [Gafni81] does not dictate a proactive or reactive response to link failure/repair
Decision on reactive/proactive behavior should be made based on environment under consideration
So far ... : So far ...
All nodes had identical responsibilities
Some schemes propose giving special responsibilities to a subset of nodes
Even if all nodes are physically identical
Core-based schemes are examples of such schemes
Asymmetric Responsibilities : Asymmetric Responsibilities
Core-Extraction Distributed Ad Hoc Routing (CEDAR) [Sivakumar99] : Core-Extraction Distributed Ad Hoc Routing (CEDAR) [Sivakumar99] A subset of nodes in the network is identified as the core
Each node in the network must be adjacent to at least one node in the core
Each node picks one core node as its dominator (or leader)
Core is determined by periodic message exchanges between each node and its neighbors
attempt made to keep the number of nodes in the core small
Each core node determines paths to nearby core nodes by means of a localized broadcast
Each core node guaranteed to have a core node at andlt;=3 hops
CEDAR: Core Nodes : CEDAR: Core Nodes B A C E J S K D F H G A core node Node E is the dominator
for nodes D, F and K
Link State Propagation in CEDAR : Link State Propagation in CEDAR The distance to which the state of a link is propagated in the network is a function of
whether the link is stable -- state of unstable links is not propagated very far
whether the link bandwidth is high or low -- only state of links with high bandwidth is propagated far
Link state propagation occurs among core nodes
Link state information includes dominators of link end-points
Each core node knows the state of local links and stable high bandwidth links far away
Route Discovery in CEDAR : Route Discovery in CEDAR When a node S wants to send packets to destination D
Node S informs its dominator core node B
Node B finds a route in the core network to the core node E which is the dominator for destination D
This is done by means of a DSR-like route discovery (but somewhat optimized) process among the core nodes
Core nodes on the above route then build a route from S to D using locally available link state information
Route from S to D may or may not include core nodes
CEDAR: Core Maintenance : CEDAR: Core Maintenance B A C E J S K D F H G A core node
Link State at Core Nodes : Link State at Core Nodes B A C E J S K D F H G Links that node B is aware of
CEDAR Route Discovery : CEDAR Route Discovery B A C E J S K D F H G Partial route constructed by B Links that node C is aware of
CEDAR Route Discovery : CEDAR Route Discovery B A C E J S K D F H G Complete route -- last two hops determined by node C
CEDAR : CEDAR Advantages
Route discovery/maintenance duties limited to a small number of core nodes
Link state propagation a function of link stability/quality
Disadvantages
Core nodes have to handle additional traffic, associated with route discovery and maintenance
Asymmetric Responsibilities:Cluster-Based Schemes : Asymmetric Responsibilities: Cluster-Based Schemes Some cluster-based schemes have also been proposed [Gerla95,Krishna97,Amis00]
In some cluster-based schemes, a leader is elected for each cluster of node
The leader often has some special responsibilities
Different schemes may differ in
how clusters are determined
the way cluster head (leader) is chosen
duties assigned to the cluster head
Proactive Protocols : Proactive Protocols
Most of the schemes discussed so far are reactive
Proactive schemes based on distance-vector and link-state mechanisms have also been proposed
Link State Routing [Huitema95] : Link State Routing [Huitema95]
Each node periodically floods status of its links
Each node re-broadcasts link state information received from its neighbor
Each node keeps track of link state information received from other nodes
Each node uses above information to determine next hop to each destination
Optimized Link State Routing (OLSR) [Jacquet00ietf,Jacquet99Inria] : Optimized Link State Routing (OLSR) [Jacquet00ietf,Jacquet99Inria] The overhead of flooding link state information is reduced by requiring fewer nodes to forward the information
A broadcast from node X is only forwarded by its multipoint relays
Multipoint relays of node X are its neighbors such that each two-hop neighbor of X is a one-hop neighbor of at least one multipoint relay of X
Each node transmits its neighbor list in periodic beacons, so that all nodes can know their 2-hop neighbors, in order to choose the multipoint relays
Optimized Link State Routing (OLSR) : Optimized Link State Routing (OLSR)
Nodes C and E are multipoint relays of node A
A B F C D E H G K J Node that has broadcast state information from A
Optimized Link State Routing (OLSR) : Optimized Link State Routing (OLSR)
Nodes C and E forward information received from A A B F C D E H G K J Node that has broadcast state information from A
Optimized Link State Routing (OLSR) : Optimized Link State Routing (OLSR) Nodes E and K are multipoint relays for node H
Node K forwards information received from H
E has already forwarded the same information once A B F C D E H G K J Node that has broadcast state information from A
OLSR : OLSR
OLSR floods information through the multipoint relays
The flooded itself is fir links connecting nodes to respective multipoint relays
Routes used by OLSR only include multipoint relays as intermediate nodes
Destination-Sequenced Distance-Vector (DSDV) [Perkins94Sigcomm] : Destination-Sequenced Distance-Vector (DSDV) [Perkins94Sigcomm] Each node maintains a routing table which stores
next hop towards each destination
a cost metric for the path to each destination
a destination sequence number that is created by the destination itself
Sequence numbers used to avoid formation of loops
Each node periodically forwards the routing table to its neighbors
Each node increments and appends its sequence number when sending its local routing table
This sequence number will be attached to route entries created for this node
Destination-Sequenced Distance-Vector (DSDV) : Destination-Sequenced Distance-Vector (DSDV) Assume that node X receives routing information from Y about a route to node Z
Let S(X) and S(Y) denote the destination sequence number for node Z as stored at node X, and as sent by node Y with its routing table to node X, respectively X Y Z
Destination-Sequenced Distance-Vector (DSDV) : Destination-Sequenced Distance-Vector (DSDV) Node X takes the following steps:
If S(X) andgt; S(Y), then X ignores the routing information received from Y
If S(X) = S(Y), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z
If S(X) andlt; S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y) X Y Z
Hybrid Protocols : Hybrid Protocols
Zone Routing Protocol (ZRP) [Haas98] : Zone Routing Protocol (ZRP) [Haas98] Zone routing protocol combines
Proactive protocol: which pro-actively updates network state and maintains route regardless of whether any data traffic exists or not
Reactive protocol: which only determines route to a destination if there is some data to be sent to the destination
ZRP : ZRP
All nodes within hop distance at most d from a node X are said to be in the routing zone of node X
All nodes at hop distance exactly d are said to be peripheral nodes of node X’s routing zone
ZRP : ZRP
Intra-zone routing: Pro-actively maintain state information for links within a short distance from any given node
Routes to nodes within short distance are thus maintained proactively (using, say, link state or distance vector protocol)
Inter-zone routing: Use a route discovery protocol for determining routes to far away nodes. Route discovery is similar to DSR with the exception that route requests are propagated via peripheral nodes.
ZRP: Example withZone Radius = d = 2 : ZRP: Example with Zone Radius = d = 2 S F D S performs route
discovery for D Denotes route request
ZRP: Example with d = 2 : ZRP: Example with d = 2 S F D S performs route
discovery for D Denotes route reply E knows route from E to D,
so route request need not be
forwarded to D from E
ZRP: Example with d = 2 : ZRP: Example with d = 2 S F D S performs route
discovery for D Denotes route taken by Data
Landmark Routing (LANMAR) for MANET with Group Mobility [Pei00Mobihoc] : Landmark Routing (LANMAR) for MANET with Group Mobility [Pei00Mobihoc] A landmark node is elected for a group of nodes that are likely to move together
A scope is defined such that each node would typically be within the scope of its landmark node
Each node propagates link state information corresponding only to nodes within it scope and distance-vector information for all landmark nodes
Combination of link-state and distance-vector
Distance-vector used for landmark nodes outside the scope
No state information for non-landmark nodes outside scope maintained
LANMAR Routing to Nodes Within Scope : LANMAR Routing to Nodes Within Scope Assume that node C is within scope of node A
Routing from A to C: Node A can determine next hop to node C using the available link state information A B C F H G E D
LANMAR Routing to Nodes Outside Scope : LANMAR Routing to Nodes Outside Scope Routing from node A to F which is outside A’s scope
Let H be the landmark node for node F
Node A somehow knows that H is the landmark for C
Node A can determine next hop to node H using the available distance vector information A B C F H G E D
LANMAR Routing to Nodes Outside Scope : LANMAR Routing to Nodes Outside Scope Node D is within scope of node F
Node D can determine next hop to node F using link state information
The packet for F may never reach the landmark node H, even though initially node A sends it towards H A B C F H G E D
Slide183 : LANMAR scheme uses node identifiers as landmarks
Anchored Geodesic Scheme [LeBoudec00] uses geographical regions as landmarks
Geodesic Routing Without Anchors [Blazevic00,Hubaux00wcnc] : Geodesic Routing Without Anchors [Blazevic00,Hubaux00wcnc] Each node somehow keeps track of routes to nodes within its zone (intra-zone routing)
Each node also records physical locations of