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 nodes on its zone boundary
Inter-zone routing: When a packet is to be routed to someone outside the zone, the packet is sent to a zone-boundary node in the direction of the destination
The packet is forwarded in this manner until it reaches someone within the destination’s zone
This node then uses intra-zone routing to deliver the packet
Similar to the GEDIR protocol [Lin98]
Anchored Geodesic Routing [Blazevic00,Hubaux00wcnc]: Anchored Geodesic Routing [Blazevic00,Hubaux00wcnc]
Anchors can be used to go around connectivity holes
Anchors are physical locations/areas. The route may be specified as a series of intermediate physical areas to be traversed to reach the destination B A
Routing: Routing Protocols discussed so far find/maintain a route provided it exists
Some protocols attempt to ensure that a route exists by
Power Control [Ramanathan00Infocom]
Limiting movement of hosts or forcing them to take detours [Reuben98thesis]
Power Control: Power Control
Protocols discussed so far find a route, on a given network topology
Some researchers propose controlling network topology by transmission power control to yield network properties which may be desirable [Ramanathan00Infocom]
Such approaches can significantly impact performance at several layers of protocol stack
[Wattwnhofer00Infocom] provides a distributed mechanism for power control which allows for local decisions, but guarantees global connectivity
Each node uses a power level that ensures that the node has at least one neighbor in each cone with angle 2p/3
Other Routing Protocols: Other Routing Protocols
Plenty of other routing protocols
Discussion here is far from exhaustive
Many of the existing protocols could potentially be adapted for MANET (some have already been adapted as discussed earlier)
Some Variations: Some Variations
Power-Aware Routing [Singh98Mobicom,Chang00Infocom]: Power-Aware Routing [Singh98Mobicom,Chang00Infocom] Define optimization criteria as a function of energy
consumption. Examples:
Minimize energy consumed per packet
Minimize time to network partition due to energy depletion
Maximize duration before a node fails due to energy depletion
Power-Aware Routing [Singh98Mobicom]: Power-Aware Routing [Singh98Mobicom] Assign a weigh to each link
Weight of a link may be a function of energy consumed when transmitting a packet on that link, as well as the residual energy level
low residual energy level may correspond to a high cost
Prefer a route with the smallest aggregate weight
Power-Aware Routing: Power-Aware Routing Possible modification to DSR to make it power aware (for simplicity, assume no route caching):
Route Requests aggregate the weights of all traversed links
Destination responds with a Route Reply to a Route Request if
it is the first RREQ with a given ('current') sequence number, or
its weight is smaller than all other RREQs received with the current sequence number
Signal Stability Based Adaptive Routing (SSA) [Dube97]: Signal Stability Based Adaptive Routing (SSA) [Dube97] Similar to DSR
A node X re-broadcasts a Route Request received from Y only if the (X,Y) link is deemed to have a strong signal stability
Signal stability is evaluated as a moving average of the signal strength of packets received on the link in recent past
An alternative approach would be to assign a cost as a function of signal stability
Associativity-Based Routing (ABR)[Toh97]: Associativity-Based Routing (ABR) [Toh97] Only links that have been stable for some minimum duration are utilized
motivation: If a link has been stable beyond some minimum threshold, it is likely to be stable for a longer interval. If it has not been stable longer than the threshold, then it may soon break (could be a transient link)
Association stability determined for each link
measures duration for which the link has been stable
Prefer paths with high aggregate association stability
Geography Adaptive Fidelity [Xu01MobiCom]: Geography Adaptive Fidelity [Xu01MobiCom]
Each node associates itself with a square in a virtual grid
Node in each grid square coordinate to determine who will sleep and how long
Preemptive Routing [Goff01MobiCom]: Preemptive Routing [Goff01MobiCom]
Add some proactivity to reactive routing protocols such as DSR and AODV
Route discovery initiated when it appears that an active route will break in the near future
Initiating route discover before existing route breaks reduces discovery latency
QoS Routing: QoS Routing
Quality-of-Service: Quality-of-Service
Several proposals for reserving bandwidth for a flow in MANET
Due to lack of time, these are not being discussed in this tutorial
Performance of Unicast Routing in MANET: Performance of Unicast Routing in MANET
Several performance comparisons [Broch98Mobicom,Johansson99Mobicom,Das00Infocom,Das98ic3n]
We will discuss performance issue later in the tutorial
MulticastinginMobile Ad Hoc Networks: Multicasting in Mobile Ad Hoc Networks
Multicasting: Multicasting A multicast group is defined with a unique group identifier
Nodes may join or leave the multicast group anytime
In traditional networks, the physical network topology does not change often
In MANET, the physical topology can change often
Multicasting in MANET: Multicasting in MANET
Need to take topology change into account when designing a multicast protocol
Several new protocols have been proposed for multicasting in MANET
AODV Multicasting [Royer00Mobicom]: AODV Multicasting [Royer00Mobicom] Each multicast group has a group leader
Group leader is responsible for maintaining group sequence number (which is used to ensure freshness of routing information)
Similar to sequence numbers for AODV unicast
First node joining a group becomes group leader
A node on becoming a group leader, broadcasts a Group Hello message
AODV Group Sequence Number: AODV Group Sequence Number
In our illustrations, we will ignore the group sequence numbers
However, note that a node makes use of information received only with recent enough sequence number
AODV Multicast Tree: AODV Multicast Tree E L H J D C G A K N Group and multicast tree member Tree (but not group) member Group leader B Multicast tree links
Joining the Multicast Tree: AODV: Joining the Multicast Tree: AODV E L H J D C G A K N Group leader B N wishes to
join the group:
it floods RREQ Route Request (RREQ)
Joining the Multicast Tree: AODV: Joining the Multicast Tree: AODV E L H J D C G A K N Group leader B N wishes to
join the group Route Reply (RREP)
Joining the Multicast Tree: AODV: Joining the Multicast Tree: AODV E L H J D C G A K N Group leader B N wishes to
join the group Multicast Activation (MACT)
Joining the Multicast Tree: AODV: Joining the Multicast Tree: AODV E L H J D C G A K N Group leader B N has joined
the group Multicast tree links Group member Tree (but not group) member
Sending Data on the Multicast Tree: Sending Data on the Multicast Tree
Data is delivered along the tree edges maintained by the Multicast AODV algorithm
If a node which does not belong to the multicast group wishes to multicast a packet
It sends a non-join RREQ which is treated similar in many ways to RREQ for joining the group
As a result, the sender finds a route to a multicast group member
Once data is delivered to this group member, the data is delivered to remaining members along multicast tree edges
Leaving a Multicast Tree: AODV: Leaving a Multicast Tree: AODV E L H J D C G A Group leader B J wishes to
leave the group Multicast tree links K N
Leaving a Multicast Tree: AODV: Leaving a Multicast Tree: AODV E L H J D C G A Group leader B J has left
the group Since J is not a leaf
node, it must remain
a tree member K N
Leaving a Multicast Tree: AODV: Leaving a Multicast Tree: AODV E L H J D C G A Group leader B K N N wishes to leave
the multicast group MACT (prune)
Leaving a Multicast Tree: AODV: Leaving a Multicast Tree: AODV E L H J D C G A Group leader B K N MACT
(prune) Node N has removed itself from the multicast group.
Now node K has become a leaf, and K is not in the group.
So node K removes itself from the tree as well.
Leaving a Multicast Tree: AODV: Leaving a Multicast Tree: AODV E L H J D C G A Group leader B K N Nodes N and K are no more in the multicast tree.
Handling a Link Failure: AODV Multicasting: Handling a Link Failure: AODV Multicasting When a link (X,Y) on the multicast tree breaks, the node that is further away from the leader is responsible to reconstruct the tree, say node X
Node X, which is further downstream, transmits a Route Request (RREQ)
Only nodes which are closer to the leader than node X’s last known distance are allowed to send RREP in response to the RREQ, to prevent nodes that are further downstream from node X from responding
Handling Partitions: AODV: Handling Partitions: AODV When failure of link (X,Y) results in a partition, the downstream node, say X, initiates Route Request
If a Route Reply is not received in response, then node X assumes that it is partitioned from the group leader
A new group leader is chosen in the partition containing node X
If node X is a multicast group member, it becomes the group leader, else a group member downstream from X is chosen as the group leader
Merging Partitions: AODV: Merging Partitions: AODV
If the network is partitioned, then each partition has its own group leader
When two partitions merge, group leader from one of the two partitions is chosen as the leader for the merged network
The leader with the larger identifier remains group leader
Merging Partitions: AODV: Merging Partitions: AODV Each group leader periodically sends Group Hello
Assume that two partitions exist with nodes P and Q as group leaders, and let P andlt; Q
Assume that node A is in the same partition as node P, and that node B is in the same partition as node Q
Assume that a link forms between nodes A and B
A P Q B
Merging Partitions: AODV: Merging Partitions: AODV Assume that node A receives Group Hello originated by node Q through its new neighbor B
Node A asks exclusive permission from its leader P to merge the two trees using a special Route Request
Node A sends a special Route Request to node Q
Node Q then sends a Group Hello message (with a special flag)
All tree nodes receiving this Group Hello record Q as the leader
Merging Partitions: AODV: Merging Partitions: AODV A P Q B Hello (Q)
Merging Partitions: AODV: Merging Partitions: AODV A P Q B RREQ
(can I repair
partition) RREP (Yes)
Merging Partitions: AODV: Merging Partitions: AODV A P Q B RREQ (repair)
Merging Partitions: AODV: Merging Partitions: AODV A P Q B Group Hello
(update) Q becomes leader of the merged multicast tree
New group sequence number is larger than most
recent ones known to P and Q both
Summary: Multicast AODV: Summary: Multicast AODV
Similar to unicast AODV
Uses leaders to maintain group sequence numbers, and to help in tree maintenance
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP)
ODMRP requires cooperation of nodes wishing to send data to the multicast group
To construct the multicast mesh
A sender node wishing to send multicast packets periodically floods a Join Data packet throughput the network
Periodic transmissions are used to update the routes
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP) Each multicast group member on receiving a Join Data, broadcasts a Join Table to all its neighbors
Join Table contains (sender S, next node N) pairs
next node N denotes the next node on the path from the group member to the multicast sender S
When node N receives the above broadcast, N becomes member of the forwarding group
When node N becomes a forwarding group member, it transmits Join Table containing the entry (S,M) where M is the next hop towards node S
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP) Assume that S is a sender node S T N D Join Data Multicast group member M C A B
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP) S T N D Join Data Multicast group member M C A B Join Data Join Data
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP) S T N D Multicast group member M C A B Join Table (S,M) Join Table (S,C)
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP) S T N D F marks a forwarding group member M C A B Join Table (S,N) Join Table (S,N) F F
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP) S T N D Multicast group member M C A B Join Table (S,S) F F F
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP) S T N D Multicast group member M C A B F F F Join Data (T)
On-Demand Multicast Routing Protocol (ODMRP): On-Demand Multicast Routing Protocol (ODMRP) S T N D Multicast group member M C A B F F F Join Table (T,C) Join Table (T,C) Join Table (T,D) F Join Table (T,T)
ODMRP Multicast Delivery: ODMRP Multicast Delivery
A sender broadcasts data packets to all its neighbors
Members of the forwarding group forward the packets
Using ODMRP, multiple routes from a sender to a multicast receiver may exist due to the mesh structure created by the forwarding group members
ODMRP: ODMRP No explicit join or leave procedure
A sender wishing to stop multicasting data simply stops sending Join Data messages
A multicast group member wishing to leave the group stops sending Join Table messages
A forwarding node ceases its forwarding status unless refreshed by receipt of a Join Table message
Link failure/repair taken into account when updating routes in response to periodic Join Data floods from the senders
Other Multicasting Protocols: Other Multicasting Protocols
Several other multicasting proposals have been made
For a comparison study, see [Lee00Infocom]
GeocastinginMobile Ad Hoc Networks: Geocasting in Mobile Ad Hoc Networks
Multicasting and Geocasting: Multicasting and Geocasting Multicast members may join or leave a multicast group whenever they desire
Geocast group is defined as the set of nodes that reside in a specified geographical region
Membership of a node to a geocast group is a function of the node’s physical location
Unlike multicasting
Geocasts are useful to deliver location-dependent information
Geocasting [Navas97Mobicom]: Geocasting [Navas97Mobicom]
Navas et al. proposed the notion of geocasting in the traditional internet
Need new protocols for geocasting in mobile ad hoc networks
Geocast region: Region to which a geocast message is to be delivered
Geocasting in MANET: Geocasting in MANET
Flooding-based protocol [Ko99Wmcsa]
Graph-based protocol [Ko2000icnp,Ko2000tech]
Simple Flooding-Based Geocasting: Simple Flooding-Based Geocasting Use the basic flooding algorithm, where a packet sent by a geocast sender is flooded to all reachable nodes in the network
The geocast region is tagged onto the geocast message
When a node receives a geocast packet by the basic flooding protocol, the packet is delivered (to upper layers) only if the node’s location is within the geocast region
Simple Flooding-Based Geocasting: Simple Flooding-Based Geocasting
Advantages:
Simplicity
Disadvantages
High overhead
Packet reaches all nodes reachable from the source
Geocasting based onLocation-Aided Routing (LAR)[Ko99Wmcsa]: Geocasting based on Location-Aided Routing (LAR) [Ko99Wmcsa]
Similar to unicast LAR protocol
Expected zone in unicast LAR now replaced by the geocast region
Request zone determined as in unicast LAR
Only nodes in the request zone forward geocast packets
Geocast LAR: Geocast LAR X Y r S Request Zone Network Space B A Geocast region
Geocast LAR: Geocast LAR If all routes between a geocast member and the source may contain nodes that are outside the request zone, geocast will not be delivered to that member
Trade-off between accuracy and overhead
Larger request zone increases accuracy but may also increase overhead
Advantage of LAR for geocasting: No need to keep track of network topology
Good approach when geocasting is performed infrequently
GeoTORA [Ko2000icnp,Ko2000tech]: GeoTORA [Ko2000icnp,Ko2000tech]
Based on link reversal algorithm TORA for unicasting in MANET
TORA maintains a Directed Acyclic Graph (DAG) with only the destination node being a sink
Anycasting with Modified TORA [Ko2000tech]: Anycasting with Modified TORA [Ko2000tech] A packet is delivered to any one member of an anycast group
Maintain a DAG for each anycast group
Make each member of the anycast group a sink
By using the outgoing links, packets may be delivered to any one sink
Anycasting: Anycasting A F B C E G D Maintain an directed acyclic
graph (DAG) for each
anycast group, with each group
member being a sink
Link between two sinks is
not directed Links are bi-directional
But algorithm imposes
logical directions on them Anycast group
member
DAG for Anycasting: DAG for Anycasting
Since links between anycast group members are not given a direction, the graph is not exactly a 'directed' acyclic graph
So use of the term DAG here is imprecise
Ignoring links between anycast group members, rest of the graph is a DAG
Geocasting using Modified Anycasting: Geocasting using Modified Anycasting A F B C E G D All nodes within a
specified geocasting
region are made sinks
When a group member
receives a packet, it
floods it within the
geocast region Geocast group
member Geocast region
Geocasting using Modified Anycasting: Geocasting using Modified Anycasting A F B C E G D Links may have to be
updated when a node
leaves geocast region Geocast group
member Geocast region
Geocasting using Modified Anycasting: Geocasting using Modified Anycasting A F B C E G D Links may have to be
updated when a node
enters geocast region Geocast group
member Geocast region
Other Geocasting Schemes: Other Geocasting Schemes
[Macwan01thesis] divides space into a grid, and maintains a graph structure for each grid square.
Data transmitted using grid structures for the grid squares that intersect with the geocast region. d a b e f c
Other Geocasting Schemes: Other Geocasting Schemes
Mesh-based geocast routing [Boleng01]
Some Related Work: Some Related Work
Content-based Multicasting [Zhou00MobiHoc]
Recipients of a packet are determined by the contents of a packet
Example: A soldier may receive information on events within his 1-mile radius
Role-Based Multicast [Briesmeister00MobiHoc]
Characteristics such as direction of motion are used to determine relevance of data to a node
Application: Informing car drivers of road accidents, emergencies, etc.
Capacity of Ad Hoc Networks: Capacity of Ad Hoc Networks
Capacity of Fixed Ad Hoc Networks [Gupta00it]: Capacity of Fixed Ad Hoc Networks [Gupta00it]
n nodes in area A transmitting at W bits/sec using a fixed range (distance between a random pair of nodes is O(sqrt(n))
Bit-distance product that can be transported by the network per second is
Q ( W sqrt (A n) )
Throughput per node
Q ( W / sqrt (n) )
Capacity of Mobile Ad Hoc Networks [Grossglauser01Infocom]: Capacity of Mobile Ad Hoc Networks [Grossglauser01Infocom] Assume random motion
Any two nodes become neighbors once in a while
Each node assumed sender for one session, and destination for another session
Relay packets through at most one other node
Packet go from S to D directly, when S and D are neighbors, or from S to a relay and the the relay to D, when each pair becomes neighbor respectively
Throughput of each session is O(1)
Independent of n
Continues from last slide …: Continues from last slide …
Delay in packet delivery can be large if O(1) throughput is to be achieved
Delay incurred waiting for the destination to arrive close to a relay or the sender
Trade-off between delay and throughput
Measured Capacity [Li01MobiCom]: Measured Capacity [Li01MobiCom]
Confirms intuition
In fixed networks, capacity is higher if average distance between source-destination pairs is small
Measured Scaling Law [Gupta01]: Measured Scaling Law [Gupta01]
Measured in static networks
Throughput declines worse with n than theoretically predicted
Due to limitations of existing MAC protocols
Unable to exploit 'parallelism' in channel access
Capacity: Capacity
How to design MAC and routing protocols to approach theoretical capacity ?
Open problem
Medium Access Control Protocols: Medium Access Control Protocols
Medium Access Control: Medium Access Control
Wireless channel is a shared medium
Need access control mechanism to avoid interference
MAC protocol design has been an active area of research for many years [Chandra00survey]
MAC: A Simple Classification: MAC: A Simple Classification Wireless
MAC Centralized Distributed Guaranteed
or
controlled
access Random
access This
tutorial
This tutorial: This tutorial
Mostly focus on random access protocols
Not a comprehensive overview of MAC protocols
Provides discussion of some example protocols
Hidden Terminal Problem [Tobagi75]: Hidden Terminal Problem [Tobagi75]
Node B can communicate with A and C both
A and C cannot hear each other
When A transmits to B, C cannot detect the transmission using the carrier sense mechanism
If C transmits, collision will occur at node B
Busy Tone [Tobagi75,Haas98] : Busy Tone [Tobagi75,Haas98]
A receiver transmits busy tone when receiving data
All nodes hearing busy tone keep silent
Avoids interference from hidden terminals
Requires a separate channel for busy tone
MACA Solution for Hidden Terminal Problem [Karn90]: MACA Solution for Hidden Terminal Problem [Karn90] When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS) to A
On receiving RTS, node A responds by sending Clear-to-Send (CTS), provided node A is able to receive the packet
When a node (such as C) overhears a CTS, it keeps quiet for the duration of the transfer
Transfer duration is included in RTS and CTS both
Reliability: Reliability
Wireless links are prone to errors. High packet loss rate detrimental to transport-layer performance.
Mechanisms needed to reduce packet loss rate experienced by upper layers
A Simple Solution to Improve Reliability: A Simple Solution to Improve Reliability When node B receives a data packet from node A, node B sends an Acknowledgement (Ack). This approach adopted in many protocols [Bharghavan94,IEEE 802.11]
If node A fails to receive an Ack, it will retransmit the packet
IEEE 802.11 Wireless MAC: IEEE 802.11 Wireless MAC
Distributed and centralized MAC components
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
DCF suitable for multi-hop ad hoc networking
DCF is a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol
IEEE 802.11 DCF : IEEE 802.11 DCF Uses RTS-CTS exchange to avoid hidden terminal problem
Any node overhearing a CTS cannot transmit for the duration of the transfer
Uses ACK to achieve reliability
Any node receiving the RTS cannot transmit for the duration of the transfer
To prevent collision with ACK when it arrives at the sender
When B is sending data to C, node A will keep quite
Collision Avoidance: Collision Avoidance With half-duplex radios, collision detection is not possible
CSMA/CA: Wireless MAC protocols often use collision avoidance techniques, in conjunction with a (physical or virtual) carrier sense mechanism
Carrier sense: When a node wishes to transmit a packet, it first waits until the channel is idle.
Collision avoidance: Nodes hearing RTS or CTS stay silent for the duration of the corresponding transmission. Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit.
IEEE 802.11: C F A B E D RTS RTS = Request-to-Send IEEE 802.11 Pretending a circular range
IEEE 802.11: C F A B E D RTS RTS = Request-to-Send IEEE 802.11 NAV = 10 NAV = remaining duration to keep quiet
IEEE 802.11: C F A B E D CTS CTS = Clear-to-Send IEEE 802.11
IEEE 802.11: C F A B E D CTS CTS = Clear-to-Send IEEE 802.11 NAV = 8
IEEE 802.11: C F A B E D DATA DATA packet follows CTS. Successful data reception acknowledged using ACK. IEEE 802.11
IEEE 802.11: IEEE 802.11 C F A B E D ACK
IEEE 802.11: C F A B E D ACK IEEE 802.11 Reserved area
IEEE 802.11: IEEE 802.11 C F A B E D DATA
CSMA/CA: CSMA/CA
Physical carrier sense, and
Virtual carrier sense using Network Allocation Vector (NAV)
NAV is updated based on overheard RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission
Nodes stay silent when carrier sensed (physical/virtual)
Backoff intervals used to reduce collision probability
Backoff Interval : Backoff Interval
When transmitting a packet, choose a backoff interval in the range [0,cw]
cw is contention window
Count down the backoff interval when medium is idle
Count-down is suspended if medium becomes busy
When backoff interval reaches 0, transmit RTS
DCF Example: DCF Example B1 and B2 are backoff intervals
at nodes 1 and 2 cw = 31
Backoff Interval: Backoff Interval The time spent counting down backoff intervals is a part of MAC overhead
Choosing a large cw leads to large backoff intervals and can result in larger overhead
Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously)
Slide288:
Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed
IEEE 802.11 DCF: contention window cw is chosen dynamically depending on collision occurrence
Binary Exponential Backoff in DCF: Binary Exponential Backoff in DCF
When a node fails to receive CTS in response to its RTS, it increases the contention window
cw is doubled (up to an upper bound)
When a node successfully completes a data transfer, it restores cw to Cwmin
cw follows a sawtooth curve
MILD Algorithm in MACAW [Bharghavan94]: MILD Algorithm in MACAW [Bharghavan94]
When a node successfully completes a transfer, reduces cw by 1
In 802.11 cw is restored to cwmin
In 802.11, cw reduces much faster than it increases
MACAW: cw reduces slower than it increases
Exponential Increase Linear Decrease
MACAW can avoid wild oscillations of cw when large number of nodes contend for the channel
Alternative Contention Resolution Mechanism [Hiperlan]: Alternative Contention Resolution Mechanism [Hiperlan] Elimination phase
A node transmits a burst for a random number (geometrically distributed) of slots
If medium idle at the end of the burst, go to yield phase, else give up until next round
Yield phase
Stay silent for a random number (geometrical distributed) of slots
If medium still silent, transmit
Receive-Initiated Mechanism [Talucci97,Garcia99]: Receive-Initiated Mechanism [Talucci97,Garcia99] In most protocols, sender initiates a transfer
Alternatively, a receiver may send a
Ready-To-Receive (RTR) message to a sender requesting it to being a packet transfer
Sender node on receiving the RTR transmits data
How does a receiver determine when to poll a sender with RTR?
Based on history, and prediction of traffic from the sender
Fairness: Fairness
Fairness Issue: Fairness Issue
Many definitions of fairness plausible
Simplest definition: All nodes should receive equal bandwidth A B C D Two flows
Fairness Issue: Fairness Issue Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide
Nodes A and B then choose from range [0,63]
Node A chooses 4 slots and B choose 60 slots
After A transmits a packet, it next chooses from range [0,31]
It is possible that A may transmit several packets before B transmits its first packet A B C D Two flows
Fairness Issue: Fairness Issue
Unfairness occurs when one node has backed off much more than some other node A B C D Two flows
MACAW Solution for Fairness: MACAW Solution for Fairness When a node transmits a packet, it appends the cw value to the packet, all nodes hearing that cw value use it for their future transmission attempts
Since cw is an indication of the level of congestion in the vicinity of a specific receiver node, MACAW proposes maintaining cw independently for each receiver
Using per-receiver cw is particularly useful in multi-hop environments, since congestion level at different receivers can be very different
Another MACAW Proposal: Another MACAW Proposal For the scenario below, when node A sends an RTS to B, while node C is receiving from D, node B cannot reply with a CTS, since B knows that D is sending to C
When the transfer from C to D is complete, node B can send a Request-to-send-RTS to node A [Bharghavan94Sigcomm]
Node A may then immediately send RTS to node B A B C D
Slide299:
This approach, however, does not work in the scenario below
Node B may not receive the RTS from A at all, due to interference with transmission from C A B C D
Weighted Fair Queueing [Keshav97book]: Weighted Fair Queueing [Keshav97book]
Assign a weight to each node
Bandwidth used by each node should be proportional to the weight assigned to the node
Distributed Fair Scheduling (DFS) [Vaidya00Mobicom]: Distributed Fair Scheduling (DFS) [Vaidya00Mobicom]
A fully distributed algorithm for achieving weighted fair queueing
Chooses backoff intervals proportional to
(packet size / weight)
DFS attempts to mimic the centralized Self-Clocked Fair Queueing algorithm [Golestani]
Works well on a LAN
Distributed Fair Scheduling (DFS): Distributed Fair Scheduling (DFS) B1 = 15 (DFS actually picks a random value
with mean 15)
B2 = 5 (DFS picks a value with mean 5) Weight of node 1 = 1
Weight of node 2 = 3
Assume equal
packet size B1 = 5 B2 = 5 Collision !
Impact of Collisions: Impact of Collisions
After collision resolution, either node 1 or node 2 may transmit a packet
The two alternatives may have different fairness properties (since collision resolution can result in priority inversion)
Distributed Fair Scheduling (DFS): Distributed Fair Scheduling (DFS) data wait B1 = 5 B2 = 5 Collision resolution data wait data
Distributed Fair Scheduling: Distributed Fair Scheduling
DFS uses randomization to reduce collisions
Alleviates negative impact of synchronization
DFS also uses a shifted contention window for choosing initial backoff interval
Reduces priority inversion (which leads to short-term unfairness) 0 31 0 31 802.11 DFS
DFS: DFS
Due to large cw, DFS can potentially yield lower throughput than IEEE 802.11
trade-off between fairness and throughput
On multi-hop network, properties of DFS still need to be characterized
Fairness in multi-hop case affected by hidden terminals
May need use of a copying technique, analogous to window copying in MACAW, to share some protocol state
Fairness in Multi-Hop Networks: Fairness in Multi-Hop Networks
Several definitions of fairness [Ozugur98,Vaidya99MSR,Luo00Mobicom, Nandagopal00Mobicom]
Hidden terminals make it difficult to achieve a desired notion of fairness
Balanced MAC [Ozugur98]: Balanced MAC [Ozugur98]
Variation on p-persistent protocol
A link access probability p_ij is assigned to each link (i,j) from node i to node j
p_ij is a function of the 1-hop neighbors of node i and 1-hop neighbors of all neighbors of node i
Node i picks a back-off interval, and when it counts to 0, node i transmits with probability p_ij
Otherwise, it picks another backoff interval, and repeats
Balanced MAC: Balanced MAC
degree of node j
p_ij is typically = --------------------------------------------------
maximum degree of all neighbors of node i
With an exception for the node whose degree is highest among all neighbors of i
For this neighbor k, link access probability is set to
min (1,degree of i/degree of k)
Balanced MAC: Balanced MAC K E D B J L F C A H G 2/3 2/3 2/3 2/3 2/5 2/5 2/5 2/5 2/5 3/5 3/5 1/2 1/4 1/4 1/4 1/2 1/5 1/2 4/5 4/5 1 3/4 3/5 1/5 1/3 3/4 1 2 2 4 3 5
Balanced MAC: Balanced MAC
Results show that it can sometimes (not always) improve fairness
Fairness definition used here: max throughput / min throughout
Large fairness index indicates poor fairness
Balanced MAC does not seem to be based on a mathematical argument
Not clear what properties it satisfies (approximates) in general
Estimation-Based Fair MAC [Bansou00MobiHoc]: Estimation-Based Fair MAC [Bansou00MobiHoc] Attempts to equalize throughput/weight ratio for all nodes
Two parts of the algorithm
Fair share estimation
Window adjustment
Each node estimates how much bandwidth (W) it is able to use, and the amount of bandwidth used by each station in its vicinity
Estimation based on overheard RTS, CTS, DATA packets
Estimation-Based Fair MAC: Estimation-Based Fair MAC
Fair share estimation: Node estimates how much bandwidth (Wi) it is able to use, and the amount of bandwidth (Wo) used by by all other neighbors combined
Estimation based on overheard RTS, CTS, DATA packets
Estimation-Based Fair MAC: Estimation-Based Fair MAC
Define:
Ti = Wi / weight of i
To = Wo / weight assigned to the group of neighbors of i
Fairness index = Ti / To
Window adjustment:
If fairness index is too large, cw = cw * 2
Else if fairness index is too small, cw = cw / 2
Else no change to cw (contention window)
Proportional Fair Contention Resolution (PFCR) [Nandagopal00Mobicom]: Proportional Fair Contention Resolution (PFCR) [Nandagopal00Mobicom] Proportional fairness: Allocate bandwidth Ri to node i such that any other allocation Si has the following property
Si (Si-Ri) / Ri andlt; 0
Link access probability is dynamically changed depending on success/failure at transmitting a packet
On success: Link access probability is increased by an additive factor a
On failure: Link access probability is decreased by a multiplicative factor (1-b)
Proportional Fair Contention Resolution (PFCR): Proportional Fair Contention Resolution (PFCR) Comparison with Balanced MAC
Both dynamically choose link access probability, but balanced MAC chooses it based on connectivity, while PFCR bases it on link access success/failure
Balanced MAC does not attempt to achieve any particular formal definition of fairness, unlike PFCR
Comparison with Estimation-based MAC
Estimation-based MAC needs an estimate of bandwidth used by other nodes
Estimation-based MAC chooses contention window dynamically, while PFCR chooses link access probability
Sender-Initiated Protocols: Sender-Initiated Protocols
The protocols discussed so far are sender-initiated protocols
The sender initiates a packet transfer to a receiver
Receive-Initiated Collision Avoidance [Garcia99Mobicom]: Receive-Initiated Collision Avoidance [Garcia99Mobicom]
A receiver sends a message to a sender requesting it to being a packet transfer
Difficulty: The receiver must somehow know (or poll to find out) when a sender has a packet to send
Issue of fairness using receiver-based protocols has not been studied (to my knowledge)
No reason to believe that receiver-initiated approach can achieve better fairness than source-initiate approach
Using Receiver’s Help in a Sender-Initiated Protocol: Using Receiver’s Help in a Sender-Initiated Protocol For the scenario below, when node A sends an RTS to B, while node C is transmitting to D, node B cannot reply with a CTS, since B knows that D is sending to C
When the transfer from C to D is complete, node B can send a Request-to-send-RTS to node A [Bharghavan94Sigcomm]
Node A then immediately sends RTS to node B A B C D
Slide320:
This approach, however, does not work in the scenario below
Node B may not receive the RTS from A at all, due to interference with transmission from C A B C D
Capacity and MAC Protocols: Capacity and MAC Protocols The MAC protocols such as 802.11 are unable to achieve performance close to theoretical capacity
Recent work attempts to improve on this [Rozosvsky01]
Distributed a pseudo-random transmission schedule to one-hop and two-hop neighbors (pseudo-random schedule can be distributed by distributing a seed)
Transmit state, listen state specified for each slot
In each transmit slot, transmission probability is chosen as a function of number of nearby nodes in transmit state
Energy Conservation: Energy Conservation
Energy Conservation: Energy Conservation
Since many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest
Two approaches to reduce energy consumption
Power save: Turn off wireless interface when desirable
Power control: Reduce transmit power
Power Aware Multi-Access Protocol (PAMAS) [Singh98]: Power Aware Multi-Access Protocol (PAMAS) [Singh98]
A node powers off its radio while a neighbor is transmitting to someone else
Node A sending to B Node C stays powered off C B A
Power Aware Multi-Access Protocol (PAMAS): Power Aware Multi-Access Protocol (PAMAS)
What should node C do when it wakes up and finds that D is transmitting to someone else
C does not know how long the transfer will last
Node A sending to B C stays powered off C B A D E Node D sending to E C wakes up and
finds medium busy
PAMAS: PAMAS PAMAS uses a control channel separate from the data channel
Node C on waking up performs a binary probe to determine the length of the longest remaining transfer
C sends a probe packet with parameter L
All nodes which will finish transfer in interval [L/2,L] respond
Depending on whether node C see silence, collision, or a unique response it takes varying actions
Node C (using procedure above) determines the duration of time to go back to sleep
Disadvantages of PAMAS: Disadvantages of PAMAS
Use of a separate control channel
Nodes have to be able to receive on the control channel while they are transmitting on the data channel
And also transmit on data and control channels simultaneously
A node (such as C) should be able to determine when probe responses from multiple senders collide
Another Proposal in PAMAS: Another Proposal in PAMAS To avoid the probing, a node should switch off the interface for data channel, but not for the control channel (which carries RTS/CTS packets)
Advantage: Each sleeping node always know how long to sleep by watching the control channel
Disadvantage: This may not be useful when hardware is shared for the control and data channels
It may not be possible turn off much hardware due to the sharing
Power Save in IEEE 802.11 Ad Hoc Mode: Power Save in IEEE 802.11 Ad Hoc Mode Time is divided into beacon intervals
Each beacon interval begins with an ATIM window
ATIM = Beacon interval ATIM
window
Power Save in IEEE 802.11 Ad Hoc Mode: Power Save in IEEE 802.11 Ad Hoc Mode If host A has a packet to transmit to B, A must send an ATIM Request to B during an ATIM Window
On receipt of ATIM Request from A, B will reply by sending an ATIM Ack, and stay up during the rest of the beacon interval
If a host does not receive an ATIM Request during an ATIM window, and has no pending packets to transmit, it may sleep during rest of the beacon interval
Power Save in IEEE 802.11 Ad Hoc Mode: Power Save in IEEE 802.11 Ad Hoc Mode ATIM
Req ATIM
Ack Ack Data Sleep Node A Node C Node B
Power Save in IEEE 802.11 Ad Hoc Mode: Power Save in IEEE 802.11 Ad Hoc Mode Size of ATIM window and beacon interval affects performance [Woesner98]
If ATIM window is too large, reduction in energy consumption reduced
Energy consumed during ATIM window
If ATIM window is too small, not enough time to send ATIM request
Power Save in IEEE 802.11 Ad Hoc Mode: Power Save in IEEE 802.11 Ad Hoc Mode How to choose ATIM window dynamically?
Based on observed load [Jung02infocom]
How to synchronize hosts?
If two hosts’ ATIM windows do not overlap in time, they cannot exchange ATIM requests
Coordination requires that each host stay awake long enough (at least periodically) to discover out-of-sync neighbors [Tseng02infocom] ATIM ATIM
Impact on Upper Layers: Impact on Upper Layers If each node uses the 802.11 power-save mechanism, each hop will require one beacon interval
This delay could be intolerable
Allow upper layers to dictate whether a node should enter the power save mode or not [Chen01mobicom]
Energy Conservation: Energy Conservation
Power save
Power control
Power Control: Power Control
Power control has two potential benefit
Reduced interference andamp; increased spatial reuse
Energy saving
Power Control: Power Control When C transmits to D at a high power level, B cannot receive A’s transmission due to interference from C
B C D A
Power Control: Power Control If C reduces transmit power, it can still communicate with D
Reduces energy consumption at node C
Allows B to receive A’s transmission (spatial reuse)
B C D A
Power Control: Power Control Received power level is proportional to 1/d , a andgt;= 2
If power control is utilized, energy required to transmit to a host at distance d is proportional to
d + constant
Shorter hops typically preferred for energy consumption (depending on the constant) [Rodoplu99]
Transmit to C from A via B, instead of directly from A to C a a
Power Control with 802.11: Power Control with 802.11 Transmit RTS/CTS/DATA/ACK at least power level needed to communicate with the received
A/B do not receive RTS/CTS from C/D. Also do not sense D’s data transmission
B’s transmission to A at high power interferes with reception of ACK at C B C D A
A Plausible Solution: A Plausible Solution RTS/CTS at highest power, and DATA/ACK at smallest necessary power level
A cannot sense C’s data transmission, and may transmit DATA to some other host
This DATA will interfere at C
This situation unlikely if DATA transmitted at highest power level
Interference range ~ sensing range
B C D A RTS Data Interference range Ack Data sensed
Slide342: Transmitting RTS at the highest power level also reduces spatial reuse
Nodes receiving RTS/CTS have to defer transmissions
Modification to Avoid Interference: Modification to Avoid Interference Transmit RTS/CTS at highest power level, DATA/ACK at least required power level
Increase DATA power periodically so distant hosts can sense transmission [Jung02tech]
Need to be able to change power level rapidly Power
level
Caveat: Caveat
Energy saving by power control is limited to savings in transmit energy
Other energy costs may not change
For some 802.11 devices, the energy consumption of the wireless interface reduces only by a factor of 2 when transmit power reduced from max to min possible for the device
Power Controlled Multiple Access (PCMA) [Monks01infocom]: Power Controlled Multiple Access (PCMA) [Monks01infocom] If receiver node R can tolerate noise E, it sends a busy tone at power level C/E, where C is an appropriate constant
When some node X receives a busy-tone a power level Pr, it may transmit at power level Pt andlt;= C/Pr R S data X busy tone C/E Y Pt
Power Controlled Multiple Access (PCMA) [Monks01infocom]: Power Controlled Multiple Access (PCMA) [Monks01infocom] If receiver node R can tolerate noise E, it sends a busy tone at power level C/E, where C is an appropriate constant
When some node X receives a busy-tone a power level Pr, it may transmit at power level Pt andlt;= C/Pr
Explanation:
Gain of channel RX = gain of channel XR = g
Busy tone signal level at X = Pr = g * C / E
Node X may transmit at level = Pt = C/Pr = E/g
Interference received by R = Pt * g = E
PCMA: PCMA
Advantage
Allows higher spatial reuse, as well as power saving using power control
Disadvantages:
Need a separate channel for the busy tone
Since multiple nodes may transmit the busy tones simultaneously, spatial reuse is less than optimal
Small Addresses Save Energy [Schurgers01mobihoc]: Small Addresses Save Energy [Schurgers01mobihoc] In sensor networks, packet sizes are small, and MAC addresses may be a substantial fraction of the packet
Observation: MAC addresses need only be unique within two hops
Fewer addresses are sufficient: Address size can be smaller. [Schurgers00mobihoc] uses Huffman coding to assign variable size encoding to the addresses
Energy consumption reduced due to smaller addresses C0 D3 A2 E1 F2 B1 G0
Adaptive Modulation: Adaptive Modulation
Adaptive Modulation: Adaptive Modulation
Channel conditions are time-varying
Received signal-to-noise ratio changes with time
A B
Adaptive Modulation: Adaptive Modulation Multi-rate radios are capable of transmitting at several rates, using different modulation schemes
Choose modulation scheme as a function of channel conditions Distance Throughput Modulation schemes provide
a trade-off between
throughput and range
Adaptive Modulation: Adaptive Modulation If physical layer chooses the modulation scheme transparent to MAC
MAC cannot know the time duration required for the transfer
Must involve MAC protocol in deciding the modulation scheme
Some implementations use a sender-based scheme for this purpose [Kamerman97]
Receiver-based schemes can perform better
Sender-Based “Autorate Fallback” [Kamerman97]: Sender-Based 'Autorate Fallback' [Kamerman97]
Probing mechanisms
Sender decreases bit rate after X consecutive transmission attempts fail
Sender increases bit rate after Y consecutive transmission attempt succeed
Autorate Fallback: Autorate Fallback Advantage
Can be implemented at the sender, without making any changes to the 802.11 standard specification
Disadvantage
Probing mechanism does not accurately detect channel state
Channel state detected more accurately at the receiver
Performance can suffer
Since the sender will periodically try to send at a rate higher than optimal
Also, when channel conditions improve, the rate is not increased immediately
Receiver-Based Autorate MAC [Holland01mobicom]: Receiver-Based Autorate MAC [Holland01mobicom]
Sender sends RTS containing its best rate estimate
Receiver chooses best rate for the conditions and sends it in the CTS
Sender transmits DATA packet at new rate
Information in data packet header implicitly updates nodes that heard old rate
Receiver-Based Autorate MAC Protocol: Receiver-Based Autorate MAC Protocol D C B A RTS (2 Mbps)
Multiple Channels: Multiple Channels
Multiple Channels: Multiple Channels
Multiple channels in ad hoc networks: typically defined by a particular code (CDMA) or frequency band (FDMA)
TDMA requires time synchronization among hosts in ad hoc network
Difficult
Many MAC protocols have been proposed
Multi-Channel MAC: A simple approach: Multi-Channel MAC: A simple approach
Divide bandwidth into multiple channels
Choose any one of the idle channels
Use a single-channel protocol on the chosen channel
ALOHA
MACA
Multi-Channel MAC with Soft Reservation [Nasipuri00]: Multi-Channel MAC with Soft Reservation [Nasipuri00]
Similar to the simple scheme, channel used recently for a successful transmission preferred
Tends to 'reserve' channels
Another Protocol: Another Protocol Use one (control) channel for RTS/CTS and remaining (data) channels for DATA/ACK
Each host maintains NAV table, with one entry for each data channel
Sender sends RTS to destination, specifying the channels that are free per sender’s table
Receiver replies with CTS specifying a channel that it also thinks is free
A channel is used only if both sender and receiver conclude that it is free
Impact of Directional Antennason MAC and Routing: Impact of Directional Antennas on MAC and Routing
Impact of Antennas on MAC: Impact of Antennas on MAC Wireless hosts traditionally use single-mode antennas
Typically, the single-mode = omni-directional
Recently, antennas with multiple modes (often, but not necessarily, directional) have been develop
We will now focus on directional antennas with multiple modes
IEEE 802.11: IEEE 802.11
Implicitly assumes single mode antennas
Typically, omnidirectional antennas (though not necessarily)
IEEE 802.11: C F A B E D RTS IEEE 802.11 Reserved area CTS
Omni-Directional Antennas: C D X Y Omni-Directional Antennas Red nodes
Cannot
Communicate
presently
Directional Antennas: Directional Antennas C D X Y Not possible using Omni
Question: Question
How to exploit directional antennas in ad hoc networks ?
Medium access control
Routing
MAC Protocols forDirectional Antennas: MAC Protocols for Directional Antennas
Antenna Model: Antenna Model 2 Operation Modes: Omni and Directional A node may operate in any one mode at any given time
Antenna Model: Antenna Model In Omni Mode:
Nodes receive signals with gain Go
While idle a node stays in omni mode
In Directional Mode:
Capable of beamforming in specified direction
Directional Gain Gd (Gd andgt; Go)
Symmetry: Transmit gain = Receive gain
Antenna Model: Antenna Model
More recent work models sidelobes approximately
Directional Communication: Directional Communication Received Power
(Transmit power) *(Tx Gain) * (Rx Gain)
Directional gain is higher
Potential Benefits ofDirectional Antennas: Potential Benefits of Directional Antennas
Increase 'range', keeping transmit power constant
Reduce transmit power, keeping range comparable with omni mode
Reduces interference, potentially increasing spatial reuse
Neighbors: Neighbors
Notion of a 'neighbor' needs to be reconsidered
Similarly, the notion of a 'broadcast' must also be reconsidered
Directional Neighborhood:
B Directional Neighborhood
A When C transmits directionally
Node A sufficiently close to receive in omni mode
Node C and A are Directional-Omni (DO) neighbors
Nodes C and B are not DO neighbors C Transmit Beam Receive Beam
Directional Neighborhood: Directional Neighborhood
A B C When C transmits directionally
Node B receives packets from C only in directional mode
C and B are Directional-Directional (DD) neighbors
Transmit Beam Receive Beam
Potential Benefits ofDirectional Antennas: Potential Benefits of Directional Antennas Increase 'range', keeping transmit power constant
Reduce transmit power, keeping range comparable with omni mode
Several proposal focus on this benefit
Assume that range of omni-directional and directional transmission is equal
Directional transmissions at lower power
Caveats: Caveats
Only most important features of the protocols discussed here
Antenna characteristics assumed are often different in different papers
Simple Tone Sense (STS) Protocol[Yum1992IEEE Trans. Comm.]: Simple Tone Sense (STS) Protocol [Yum1992IEEE Trans. Comm.]
STS Protocol: STS Protocol Based on busy tone signaling:
Each host is assigned a tone (sinusoidal wave at a certain frequency)
Tone frequency unique in each host’s neighborhood
When a host detects a packet destined to itself, it transmit a tone
If a host receive a tone on directional antenna A,it assumes that some host in that direction is receiving a packet
Cannot transmit using antenna A presently
OK to transmit using other antennas
STS Protocol: STS Protocol Tone duration used to encode information
Duration t1 implies transmitting node is busy
Duration t2 implies the transmitting node successfully received a transmission from another node
Example: Example S R B C A DATA Tone t1 Node A cannot
Initiate a
transmission.
But B can send
to C
Because B does
not receive t1
STS Protocol: STS Protocol Issues:
Assigning tones to hosts
Assigning hosts to antennas: It is assumed that the directions/angles can be chosen
distribute neighbor hosts evenly among the antennas
choose antenna angles such that adjacent antennas have some minimum separation
D-MAC Protocol[Ko2000Infocom]: D-MAC Protocol [Ko2000Infocom]
IEEE 802.11: DATA DATA RTS RTS CTS CTS ACK ACK B C E D Reserved area A F IEEE 802.11
Directional MAC (D-MAC) : Directional MAC (D-MAC)
Directional antenna can limit transmission to a smaller region (e.g., 90 degrees).
Basic philosophy: MAC protocol similar to IEEE 802.11, but on a per-antenna basis
D-MAC: D-MAC
IEEE802.11: Node X is blocked if node X has received an RTS or CTS for on-going transfer between two other nodes
D-MAC: Antenna T at node X is blocked if antenna T received an RTS or CTS for an on-going transmission
Transfer allowed using unblocked antennas
If multiple transmissions are received on different antennas, they are assumed to interfere
D-MAC Protocols: D-MAC Protocols
Based on location information of the receiver, sender selects an appropriate directional antenna
Several variations are possible
D-MAC Scheme 1: D-MAC Scheme 1 Uses directional antenna for sending RTS, DATA and ACK in a particular direction, whereas CTS sent omni-directionally
Directional RTS (DRTS) and Omni-directional CTS (OCTS)
D-MAC Scheme 1: DRTS/OCTS: DATA DRTS(B) OCTS(B,C) OCTS(B,C) ACK A B C E D DRTS(D) DATA ACK OCTS(D,E) DRTS(B) - Directional RTS including
location information of node B OCTS(B,C) – Omni-directional CTS
including location information
of nodes B and C
D-MAC Scheme 1: DRTS/OCTS
Drawback of Scheme 1: DATA DRTS(B) OCTS(B,C) OCTS(B,C) ACK A B C D DRTS(A) ? DRTS(A) Drawback of Scheme 1 Collision-free ACK transmission not guaranteed
D-MAC Scheme 2: D-MAC Scheme 2 Scheme 2 is similar to Scheme 1, except for using two types of RTS
Directional RTS (DRTS) / Omni-directional RTS (ORTS) both used
If none of the sender’s directional antennas are blocked, send ORTS
Otherwise, send DRTS when the desired antenna is not blocked
D-MAC Scheme 2: D-MAC Scheme 2
Probability of ACK collision lower than scheme 1
Possibilities for simultaneous transmission by neighboring nodes reduced compared to scheme 1
Variations: Variations Paper discusses further
variations on the theme
Reducing ACK collisions
Reducing wasteful transmission of RTS to busy nodes
Performance Comparison: Performance Comparison Which scheme will perform better depends on
location of various hosts
traffic patterns
antenna characteristics
Performance Evaluation: Performance Evaluation Mesh topology
No mobility
Bulk TCP traffic
2 Mbps channel
Performance Measurement: Performance Measurement Reference throughput of single TCP connection using IEEE 802.11
1 hop (1383 Kbps)
2 hops (687 Kbps)
3 hops (412 Kbps)
4 hops (274 Kbps)
Performance Measurement: Performance Measurement
Scenario 1
1 2
Performance Measurement: Performance Measurement Scenario 2: Best case for scheme 1 3 4
Performance Measurement: Performance Measurement
Scenario 3
5 6
Performance Measurement: Performance Measurement Scenario 4
Limitations of D-MAC : Limitations of D-MAC
No guarantee of collision-free ACK
Some improvements suggested in paper
Inaccurate/outdated location information can degrade performance
Conclusion: Conclusion Benefit: Can allow more simultaneous transmissions by improving spatial reuse
Disadvantage: Can increase Ack collisions
Alternatives for determining location information should be considered
Location information does not always correlate well with direction
Busy Tone Directional MAC[Huang2002MILCOM]: Busy Tone Directional MAC [Huang2002MILCOM] Extends the busy tone (DBTMA) protocol originally proposed by omni-directional antennas [Deng98ICUPC]
Three channels
Data channel
Two Busy Tone channels
Receive tone (BTr)
Transmit tone (BTt)
DBTMA: DBTMA Sender:
Sense BTr. If sensed busy, defer transmission.
If BTr idle, transmit RTS to receiver
Receiver
On receiving RTS, sense BTt.
If BTt idle, reply with a CTS, and transmit BTr until DATA is completely received
Sender
On receiving CTS, transmit DATA and BTt both
DBTMA + Directional Antennas: DBTMA + Directional Antennas DBTMA reduces reduction in throughput caused by collisions by hidden terminals
Directional antennas can be used to transmit the busy tones directionally
RTS/CTS, DATA, busy tones all may be sent directionally
Trade-offs similar to directional versus omni-directional transmission of RTS/CTS
Another Directional MAC protocol[Roychoudhury02mobicom]: Another Directional MAC protocol [Roychoudhury02mobicom] Derived from IEEE 802.11 (similar to [Takai02mobihoc])
A node listens omni-directionally when idle
Sender transmits Directional-RTS (DRTS) towards receiver
RTS received in Omni mode (idle receiver in when idle)
Receiver sends Directional-CTS (DCTS)
DATA, ACK transmitted and received directionally
Directional MAC: C F A B E D RTS RTS = Request-to-Send Directional MAC Pretending a circular range for omni X
Directional MAC: C F A B E D CTS CTS = Clear-to-Send Directional MAC X
Directional MAC: C F A B E D DATA DATA packet follows CTS. Successful data reception acknowledged using ACK. Directional MAC X
Directional MAC: C F A B E D ACK Directional MAC X
Directional NAV (DNAV) [Roychoudhury02mobicom]: Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA)
X D Y C CTS Directional NAV (DNAV) [Roychoudhury02mobicom] Similar DNAV mechanism proposed in [Takai02mobihoc]
Directional NAV (DNAV): Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA)
X Y Directional NAV (DNAV) D C DNAV
Directional NAV (DNAV): Directional NAV (DNAV) A B C θ DNAV D New transmission initiated only if direction of transmission does not overlap with DNAV, i.e., if (θ andgt; 0) RTS
DMAC Example:
DMAC Example
B
C
A D E B and C communicate D and E cannot: D blocked with DNAV from C D and A communicate
Issues with DMAC: Issues with DMAC Two types of Hidden Terminal Problems
Due to asymmetry in gain
C
A
B A’s RTS may interfere with C’s reception of DATA A is unaware of communication between B and C
Issues with DMAC: Issues with DMAC Node A beamformed in direction of D
C
B D A Two types of Hidden Terminal Problems
Due to unheard RTS/CTS Node A does not hear RTS/CTS from B andamp; C
Issues with DMAC: Issues with DMAC Node A may now interfere at node C by transmitting in C’s direction
C
B D A Two types of Hidden Terminal Problems
Due to unheard RTS/CTS
Issues with DMAC: Issues with DMAC RTS RTS RTS X does not know node A is busy.
X keeps transmitting RTSs to node A A B Using omni antennas, X would be aware that A is busy, and defer its own transmission X Z Y Deafness DATA
Issues with DMAC: Issues with DMAC
Uses DO links, but not DD links
DMAC Tradeoffs: DMAC Tradeoffs Benefits
Better Network Connectivity
Spatial Reuse
Disadvantages
Hidden terminals
Deafness
No DD Links
Using Training Sequences[Bellofiore2002IEEETrans.Ant.Prop]: Using Training Sequences [Bellofiore2002IEEETrans.Ant.Prop] Training packets used for DoA determination, after RTS/CTS exchange omni-directionally RTS CTS RXTRN TXTRN DATA ACK Sender Receiver
Slide424: Performance depends on the TXTRN and RXTRN delays
If direction is known a priori, then these delays can potentially be avoided
But mobility can change direction over time
Another Variation[Nasipuri2000WCNC]: Another Variation [Nasipuri2000WCNC] Similar to 802.11, but adapted for directional antennas
Assumptions:
Antenna model: Several directional antennas which can all be used simultaneously
Omni-directional reception is possible (by using all directional antennas together)
Direction of arrival (DoA) can be determined when receiving omni-directionally
Range of directional and omni transmissions are identical
Protocol Description: Protocol Description Sender sends omni-directional RTS
Receiver sends omni-directional CTS
Receiver also records direction of sender by determining the antenna on which the RTS signal was received with highest power level
Similarly, the sender, on receiving CTS, records the direction of the receiver
All nodes overhearing RTS/CTS defer transmissions
Sender then sends DATA directionally to the receiver
Receiver sends directional ACK
Discussion: Discussion Protocol takes advantage of reduction in interference due to directional transmission/reception of DATA
All neighbors of sender/receiver defer transmission on receiving omni-directional RTS/CTS
spatial reuse benefit not realized
Enhancing DMAC: Enhancing DMAC Are improvements possible to make DMAC more effective ?
Possible improvements:
Make Use of DD Links
Overcome deafness [Roychoudhury03 – UIUC Tech report under preparation]
Using DD Links: Using DD Links Exploit larger range of Directional antennas A and C are DD neighbors, but cannot communicate using DMAC
Transmit Beam Receive Beam A C
Exploiting Larger Range of Directional Antennas [Roychoudhury02tech]: Exploiting Larger Range of Directional Antennas [Roychoudhury02tech]
When transmission needs to be scheduled, receiving node is in omni-receive mode smaller gain A B C D E F G Omni
neighbors Directional
neighbors
Multi Hop RTS (MMAC) – Basic Idea: Multi Hop RTS (MMAC) – Basic Idea A source-routes RTS to D through adjacent DO neighbors (i.e., A-B-C-D)
When D receives RTS, it beamforms towards A, forming a DD link
Exploiting Larger Range of Directional Antennas: Exploiting Larger Range of Directional Antennas
Cannot send RTS from A to D directly
Send RTS over multiple hops A-B-C-D
Send CTS directionally from D to A
Send DATA directionally from D to A (single hop)
Send ACK directionally from A to D A B C D E F G Omni
neighbors Directional
neighbors
Exploiting Larger Range of Directional Antennas: Exploiting Larger Range of Directional Antennas
Reduces number of hops traversed by data
Can improve delay and throughput A B C D E F G Omni
neighbors Directional
neighbors
Performance of DMAC and MMAC[Roychoudhury02mobicom]: Performance of DMAC and MMAC [Roychoudhury02mobicom]
Impact of Topology: Impact of Topology Nodes arranged in 'linear' configuration reduce spatial reuse 802.11 – 1.19 Mbps
DMAC – 2.7 Mbps 802.11 – 1.19 Mbps
DMAC – 1.42 Mbps Aggregate throughput Aggregate throughput A B C Power control may improve performance
Slide436: Aligned Routes in Grid
Slide437: Unaligned Routes in Grid
Slide438: 'Random' Topology
Slide439: 'Random' Topology: delay
MMAC - Concerns: MMAC - Concerns Neighbor discovery overheads may offset the advantages of MMAC Lower probability of RTS delivery
Multi-hop RTS may not reach DD neighbor due to
deafness or collision
TDMA with Directional Antennas[Bao2002MobiCom]: TDMA with Directional Antennas [Bao2002MobiCom] Each node uses multiple beams, and can participate in multiple transmissions simultaneously
Link activation schedule determined for each slot, by a priori coordination among the nodes
Protocol needs neighborhood information (obtained using periodic broadcasts on a common control channel)
Directional MAC: Summary: Directional MAC: Summary Directional MAC protocols show improvement in aggregate throughput and delay
But not always
Performance dependent on topology
Routing with Directional Antennas: Routing with Directional Antennas
Routing Protocols: Routing Protocols
Many routing protocols for ad hoc networks rely on broadcast messages
For instance, flood of route requests (RREQ)
Using omni antennas for broadcast will not discover DD links
Need to implement broadcast using directional transmissions
Dynamic Source Routing [Johnson]: Dynamic Source Routing [Johnson]
Sender floods RREQ through the network
Nodes forward RREQs after appending their names
Destination node receives RREQ and unicasts a RREP back to sender node, using the route in which RREQ traveled
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 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
N L [S,C,G,K] [S,E,F,J]
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
DSR over Directional Antennas [Roychoudhury03PWC, Roychoudhury02UIUCTechrep]: DSR over Directional Antennas [Roychoudhury03PWC, Roychoudhury02UIUCTechrep] RREQ broadcast by sweeping
To use DD links
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
N L [S,C,G,K] [S,E,F,J]
Directional Routing : Larger Tx Range Fewer Hop Routes
Few Hop Routes Low Data Latency
Small Beamwidth High Sweep Delay
More Sweeping High Overhead Directional Routing
Tradeoffs Broadcast by sweeping
Issues: Issues Sub-optimal routes may be chosen if destination node misses shortest request, while beamformed
Broadcast storm: Using broadcasts, nodes receive multiple copies of same packet D misses request from K
Optimize by having destination wait before replying RREP RREQ Use K antenna elements to forward broadcast packet
Performance: Performance
Preliminary results indicate that routing performance can be improved using directional antennas
Route discovery latency … Single flow, grid topology (200 m distance): Route discovery latency … Single flow, grid topology (200 m distance) DSR DDSR4 DDSR6
Observations: Observations Advantage of higher transmit range significant only at higher distance of separation.
Grid distance = 200 m --- thus no gain with higher tx range of DDSR4 (350 m) over 802.11 (250 m).
However, DDSR4 has sweeping delay. Thus route discovery delay higher
Throughput: Throughput Sub-optimal routes chosen by DSR because destination node misses the shortest RREQ, while beamformed. DDSR18 DDSR9 DSR
Route Discovery in DSR: Route Discovery in DSR F J D receives RREQ from J, and replies with RREP
D misses RREQ from K N J RREP RREQ D K
Delayed RREP Optimization: Delayed RREP Optimization Due to sweeping – earliest RREQ need not have traversed shortest hop path.
RREQ packets sent to different neighbors at different points of time
If destination replies to first arriving RREP, it might miss shorter-path RREQ
Optimize by having DSR destination wait before replying with RREP
Routing Overhead: Routing Overhead
Using omni broadcast, nodes receive multiple copies of same packet - Redundant !!!
Broadcast Storm Problem
Using directional Antennas – can do better ?
Routing Overhead: Use K antenna elements to forward broadcast packet. K = N/2 in simulations
Routing Overhead Footprint of Tx
Routing Overhead: Routing Overhead Control overhead reduces
Beamwidth of antenna element (degrees)
Mobility: Mobility
Link lifetime increases using directional antennas.
Higher transmission range - link failures are less frequent
Nodes moving out of beam coverage in order of packet-transmission-time
Low probability
Mobility:
Antenna handoff
If no response to RTS, MAC layer uses N adjacent antenna elements to transmit same packet
Route error avoided if communication re-established [RoyChoudhury02UIUC Techrep] Mobility
Aggregate throughput over random mobile scenarios: Aggregate throughput over random mobile scenarios
Performance: Performance Control overhead
Throughput Vs Mobility Control overhead higher using DDSR
Throughput of DDSR higher, even under mobility
Latency in packet delivery lower using DDSR
Observations: Observations Randomness in topology aids DDSR.
Voids in network topology bridged by higher transmission range (prevents partition)
Higher transmission range increases link lifetime – reduces frequency of link failure under mobility
Antenna handoff due to nodes crossing antenna elements – not too serious
Other Approaches to Routingwith Directional Antennas[Nasipuri2000ICCCN]: Other Approaches to Routing with Directional Antennas [Nasipuri2000ICCCN]
Modified version of DSR
Transmit Route Request in the last known direction of the receiver
If the source S perceives receiver R to have been in direction d, then all nodes forward the route request from S in direction d.
Example 1: Example 1 B A S E F H J D C G I K Z Y M N L
Example 1: Example 1 B A S E F H J D C G I K Z Y M N L Route Reply
Example 2: Example 2 B A S E F H J D C G I K Z Y M N L D does
not
receive
RREQ
Limited Forwarding: Limited Forwarding
Benefit: Limits the forwarding of the Route Request
Disadvantage: Effectively assumes that each node has a sense of orientation
Routing with Directional Antennas: Conclusion: Routing with Directional Antennas: Conclusion Directional antennas can improve routing performance
But suitable protocol adaptations necessary
Directional Antennas: Conclusion: Directional Antennas: Conclusion
Directional antennas can potentially benefit
But also create difficulties in MAC and routing protocol design
UDP onMobile Ad Hoc Networks: UDP on Mobile Ad Hoc Networks
User Datagram Protocol (UDP): User Datagram Protocol (UDP)
UDP provides unreliable delivery
Studies comparing different routing protocols for MANET typically measure UDP performance
Several performance metrics are often used
Routing overhead per data packet
Packet loss rate
Packet delivery delay
UDP Performance: UDP Performance Several relevant studies [Broch98Mobicom,Das9ic3n,Johansson99Mobicom,Das00Infocom,Jacquet00Inria]
Results comparing a specific pair of protocols do not always agree, but some general (and intuitive) conclusions can be drawn
Reactive protocols may yield lower routing overhead than proactive protocols when communication density is low
Reactive protocols tend to loose more packets (assuming than network layer drops packets if a route is not known)
Proactive protocols perform better with high mobility and dense communication graph
UDP Performance: UDP Performance Many variables affect performance
Traffic characteristics
one-to-many, many-to-one, many-to-many
small bursts, large file transfers, real-time, non-real-time
Mobility characteristics
low/high rate of movement
do nodes tend to move in groups
Node capabilities
transmission range (fixed, changeable)
battery constraints
Performance metrics
delay
throughput
latency
routing overhead
Static or dynamic system characteristics (listed above)
UDP Performance: UDP Performance
Difficult to identify a single scheme that will perform well in all environments
Holy grail: Routing protocol that dynamically adapts to all environments so as to optimize 'performance'
Performance metrics may differ in different environments
TCP onMobile Ad Hoc Networks: TCP on Mobile Ad Hoc Networks
Overview ofTransmission Control Protocol / Internet Protocol(TCP/IP): Overview of Transmission Control Protocol / Internet Protocol (TCP/IP)
Internet Protocol (IP): Internet Protocol (IP)
Packets may be delivered out-of-order
Packets may be lost
Packets may be duplicated
Transmission Control Protocol (TCP): Transmission Control Protocol (TCP)
Reliable ordered delivery
Implements congestion avoidance and control
Reliability achieved by means of retransmissions if necessary
End-to-end semantics
Acknowledgements sent to TCP sender confirm delivery of data received by TCP receiver
Ack for data sent only after data has reached receiver
TCP Basics: TCP Basics Cumulative acknowledgements
An acknowledgement ack’s all contiguously received data
TCP assigns byte sequence numbers
For simplicity, we will assign packet sequence numbers
Also, we use slightly different syntax for acks than normal TCP syntax
In our notation, ack i acknowledges receipt of packets through packet i
Cumulative Acknowledgements: 40 39 37 38 35 33 Cumulative Acknowledgements A new cumulative acknowledgement is generated only on receipt of a new in-sequence packet 41 40 38 39 35 37 36 34 36 34 i data ack i
Duplicate Acknowledgements: Duplicate Acknowledgements A dupack is generated whenever an
out-of-order segment arrives at the receiver
Window Based Flow Control: Window Based Flow Control Sliding window protocol
Window size minimum of
receiver’s advertised window - determined by available buffer space at the receiver
congestion window - determined by the sender, based on feedback from the network 2 3 4 5 6 7 8 9 10 11 13 1 12 Sender’s window Acks received Not transmitted
Window Based Flow Control: Window Based Flow Control 2 3 4 5 6 7 8 9 10 11 13 1 12 Sender’s window Ack 5
Window Based Flow Control: Window Based Flow Control Congestion window size bounds the amount of data that can be sent per round-trip time
Throughput andlt;= W / RTT
Ideal Window Size: Ideal Window Size Ideal size = delay * bandwidth
delay-bandwidth product
What if window size andlt; delay*bw ?
Inefficiency (wasted bandwidth)
What if andgt; delay*bw ?
Queuing at intermediate routers
increased RTT due to queuing delays
Potentially, packet loss
How does TCP detect a packet loss?: How does TCP detect a packet loss?
Retransmission timeout (RTO)
Duplicate acknowledgements
Detecting Packet Loss Using Retransmission Timeout (RTO): Detecting Packet Loss Using Retransmission Timeout (RTO)
At any time, TCP sender sets retransmission timer for only one packet
If acknowledgement for the timed packet is not received before timer goes off, the packet is assumed to be lost
RTO dynamically calculated
Retransmission Timeout (RTO) calculation: Retransmission Timeout (RTO) calculation
RTO = mean + 4 mean deviation
Standard deviation s : s = average of (sample – mean)
Mean deviation d = average of |sample – mean|
Mean deviation easier to calculate than standard deviation
Mean deviation is more conservative: d andgt;= s 2 2
Exponential Backoff: Exponential Backoff Double RTO on each timeout Packet
transmitted Time-out occurs
before ack received,
packet retransmitted Timeout interval doubled T1 T2 = 2 * T1
Fast Retransmission: Fast Retransmission
Timeouts can take too long
how to initiate retransmission sooner?
Fast retransmit
Detecting Packet Loss Using Dupacks:Fast Retransmit Mechanism : Detecting Packet Loss Using Dupacks: Fast Retransmit Mechanism Dupacks may be generated due to
packet loss, or
out-of-order packet delivery
TCP sender assumes that a packet loss has occurred if it receives three dupacks consecutively 12 11 7 8 9 10 Receipt of packets 9, 10 and 11 will each generate
a dupack from the receiver. The sender, on getting
these dupacks, will retransmit packet 8.
Congestion Avoidance and Control: Congestion Avoidance and Control
Slow Start: cwnd grows exponentially with time during slow start
When cwnd reaches slow-start threshold, congestion avoidance is performed
Congestion avoidance: cwnd increases linearly with time during congestion avoidance
Rate of increase could be lower if sender does not always have data to send
Slide500: Slow start Congestion
avoidance Slow start threshold Example assumes that acks are not delayed
Congestion Control: Congestion Control On detecting a packet loss, TCP sender assumes that network congestion has occurred
On detecting packet loss, TCP sender drastically reduces the congestion window
Reducing congestion window reduces amount of data that can be sent per RTT
Congestion Control -- Timeout: Congestion Control -- Timeout On a timeout, the congestion window is reduced to the initial value of 1 MSS
The slow start threshold is set to half the window size before packet loss
more precisely,
ssthresh = maximum of min(cwnd,receiver’s advertised window)/2 and 2 MSS
Slow start is initiated
Slide503: ssthresh = 8 ssthresh = 10 cwnd = 20 After timeout
Congestion Control - Fast retransmit: Congestion Control - Fast retransmit Fast retransmit occurs when multiple (andgt;= 3) dupacks come back
Fast recovery follows fast retransmit
Different from timeout : slow start follows timeout
timeout occurs when no more packets are getting across
fast retransmit occurs when a packet is lost, but latter packets get through
ack clock is still there when fast retransmit occurs
no need to slow start
Fast Recovery: Fast Recovery ssthresh =
min(cwnd, receiver’s advertised window)/2 (at least 2 MSS)
retransmit the missing segment (fast retransmit)
cwnd = ssthresh + number of dupacks
when a new ack comes: cwnd = ssthreh
enter congestion avoidance
Congestion window cut into half
Slide506: After fast retransmit and fast recovery window size is
reduced in half. Receiver’s advertised window After fast recovery
TCP Reno: TCP Reno Slow-start
Congestion avoidance
Fast retransmit
Fast recovery
TCP PerformanceinMobile Ad Hoc Networks: TCP Performance in Mobile Ad Hoc Networks
Performance of TCP: Performance of TCP Several factors affect TCP performance in MANET:
Wireless transmission errors
Multi-hop routes on shared wireless medium
For instance, adjacent hops typically cannot transmit simultaneously
Route failures due to mobility
Random Errors: Random Errors
If number of errors is small, they may be corrected by an error correcting code
Excessive bit errors result in a packet being discarded, possibly before it reaches the transport layer
Random Errors May Cause Fast Retransmit: Random Errors May Cause Fast Retransmit Example assumes delayed ack - every other packet ack’d
Random Errors May Cause Fast Retransmit: Random Errors May Cause Fast Retransmit 41 40 38 39 36 34 Example assumes delayed ack - every other packet ack’d
Random Errors May Cause Fast Retransmit: Random Errors May Cause Fast Retransmit 42 41 39 40 36 Duplicate acks are not delayed 36 dupack
Random Errors May Cause Fast Retransmit: Random Errors May Cause Fast Retransmit 40 36 36 36 Duplicate acks 41 43 42
Random Errors May Cause Fast Retransmit: Random Errors May Cause Fast Retransmit 41 36 36 3 duplicate acks trigger
fast retransmit at sender 42 44 43 36
Random Errors May Cause Fast Retransmit: Random Errors May Cause Fast Retransmit
Fast retransmit results in
retransmission of lost packet
reduction in congestion window
Reducing congestion window in response to errors is unnecessary
Reduction in congestion window reduces the throughput
Sometimes Congestion Response May be Appropriate in Response to Errors: Sometimes Congestion Response May be Appropriate in Response to Errors
On a CDMA channel, errors occur due to interference from other user, and due to noise [Karn99pilc]
Interference due to other users is an indication of congestion. If such interference causes transmission errors, it is appropriate to reduce congestion window
If noise causes errors, it is not appropriate to reduce window
When a channel is in a bad state for a long duration, it might be better to let TCP backoff, so that it does not unnecessarily attempt retransmissions while the channel remains in the bad state [Padmanabhan99pilc]
Impact of Random Errors [Vaidya99]: Impact of Random Errors [Vaidya99] Exponential error model
2 Mbps wireless full duplex link
No congestion losses
Burst Errors May Cause Timeouts: Burst Errors May Cause Timeouts If wireless link remains unavailable for extended duration, a window worth of data may be lost
driving through a tunnel
passing a truck
Timeout results in slow start
Slow start reduces congestion window to 1 MSS,
reducing throughput
Reduction in window in response to errors unnecessary
Random Errors May Also Cause Timeout: Random Errors May Also Cause Timeout
Multiple packet losses in a window can result in timeout when using TCP-Reno (and to a lesser extent when using SACK)
Impact of Transmission Errors: Impact of Transmission Errors
TCP cannot distinguish between packet losses due to congestion and transmission errors
Unnecessarily reduces congestion window
Throughput suffers
This Tutorial: This Tutorial This tutorial does not consider techniques to improve TCP performance in presence of transmission errors
Please refer to the Tutorial on TCP for Wireless and Mobile Hosts presented by Vaidya at MobiCom 1999, Seattle
The tutorial slides are presently available from http://www.cs.tamu.edu/faculty/vaidya/ (follow the link to Seminars)
[Montenegro00-RFC2757] discusses related issues
This Tutorial: This Tutorial
This tutorial considers impact of multi-hop routes and route failures due to mobility
Mobile Ad Hoc Networks: Mobile Ad Hoc Networks May need to traverse multiple links to reach a destination
Mobile Ad Hoc Networks: Mobile Ad Hoc Networks Mobility causes route changes
Throughput over Multi-Hop Wireless Paths [Gerla99]: Throughput over Multi-Hop Wireless Paths [Gerla99]
Connections over multiple hops are at a disadvantage compared to shorter connections, because they have to contend for wireless access at each hop
Impact of Multi-Hop Wireless Paths [Holland99]: Impact of Multi-Hop Wireless Paths [Holland99] TCP Throughput using 2 Mbps 802.11 MAC
Throughput Degradations withIncreasing Number of Hops: Throughput Degradations with Increasing Number of Hops Packet transmission can occur on at most one hop among three consecutive hops
Increasing the number of hops from 1 to 2, 3 results in increased delay, and decreased throughput
Increasing number of hops beyond 3 allows simultaneous transmissions on more than one link, however, degradation continues due to contention between TCP Data and Acks traveling in opposite directions
When number of hops is large enough, the throughput stabilizes due to effective pipelining
Ideal Throughput: Ideal Throughput f(i) = fraction of time for which shortest path length between sender and destination is I
T(i) = Throughput when path length is I
From previous figure
Ideal throughput = S f(i) * T(i)
Impact of MobilityTCP Throughput: Impact of Mobility TCP Throughput Ideal throughput (Kbps) Actual throughput 2 m/s 10 m/s
Impact of Mobility: Impact of Mobility Ideal throughput Actual throughput 20 m/s 30 m/s
Throughput generally degrades with increasing speed …: Throughput generally degrades with increasing speed … Speed (m/s) Average
Throughput
Over
50 runs Ideal Actual
But not always …: But not always … Mobility pattern # Actual
throughput 20 m/s 30 m/s
Why Does Throughput Degrade?: Why Does Throughput Degrade?
Why Does Throughput Degrade?: Why Does Throughput Degrade?
Why Does Throughput Improve?Low Speed Scenario: Why Does Throughput Improve? Low Speed Scenario C B D A C B D A C B D A 1.5 second route failure Route from A to D is broken for ~1.5 second.
When TCP sender times after 1 second, route still broken.
TCP times out after another 2 seconds, and only then resumes.
Why Does Throughput Improve?Higher (double) Speed Scenario: Why Does Throughput Improve? Higher (double) Speed Scenario C B D A C B D A C B D A 0.75 second route failure
Route from A to D is broken for ~ 0.75 second.
When TCP sender times after 1 second, route is repaired.
Why Does Throughput Improve?General Principle: Why Does Throughput Improve? General Principle The previous two slides show a plausible cause for improved throughput
TCP timeout interval somewhat (not entirely) independent of speed
Network state at higher speed, when timeout occurs, may be more favorable than at lower speed
Network state
Link/route status
Route caches
Congestion
How to Improve Throughput(Bring Closer to Ideal): How to Improve Throughput (Bring Closer to Ideal) Network feedback
Inform TCP of route failure by explicit message
Let TCP know when route is repaired
Probing
Explicit notification
Reduces repeated TCP timeouts and backoff
Performance Improvement: Performance Improvement Without network
feedback Ideal throughput
2 m/s speed With feedback Actual throughput
Performance Improvement: Performance Improvement Without network
feedback With feedback Ideal throughput
30 m/s speed Actual throughput
Performance with Explicit Notification[Holland99]: Performance with Explicit Notification [Holland99]
IssuesNetwork Feedback: Issues Network Feedback
Network knows best (why packets are lost)
+ Network feedback beneficial
Need to modify transport andamp; network layer to receive/send feedback
Need mechanisms for information exchange between layers
[Holland99] discusses alternatives for providing feedback (when routes break and repair)
[Chandran98] also presents a feedback scheme
Impact of Caching: Impact of Caching
Route caching has been suggested as a mechanism to reduce route discovery overhead [Broch98]
Each node may cache one or more routes to a given destination
When a route from S to D is detected as broken, node S may:
Use another cached route from local cache, or
Obtain a new route using cached route at another node
To Cache or Not to Cache: To Cache or Not to Cache Average speed (m/s) Actual throughput (as fraction of expected throughput)
Why Performance Degrades With Caching: Why Performance Degrades With Caching When a route is broken, route discovery returns a cached route from local cache or from a nearby node
After a time-out, TCP sender transmits a packet on the new route.
However, the cached route has also broken after it was cached
Another route discovery, and TCP time-out interval
Process repeats until a good route is found
IssuesTo Cache or Not to Cache: Issues To Cache or Not to Cache Caching can result in faster route 'repair'
Faster does not necessarily mean correct
If incorrect repairs occur often enough, caching performs poorly
Need mechanisms for determining when cached routes are stale
Caching and TCP performance: Caching and TCP performance Caching can reduce overhead of route discovery even if cache accuracy is not very high
But if cache accuracy is not high enough, gains in routing overhead may be offset by loss of TCP performance due to multiple time-outs
TCP Performance: TCP Performance Two factors result in degraded throughput in presence of mobility:
Loss of throughput that occurs while waiting for TCP sender to timeout (as seen earlier)
This factor can be mitigated by using explicit notifications and better route caching mechanisms
Poor choice of congestion window and RTO values after a new route has been found
How to choose cwnd and RTO after a route change?
Issues Window Size After Route Repair: Issues Window Size After Route Repair
Same as before route break: may be too optimistic
Same as startup: may be too conservative
Better be conservative than overly optimistic
Reset window to small value after route repair
Let TCP figure out the suitable window size
Impact low on paths with small delay-bw product
IssuesRTO After Route Repair: Issues RTO After Route Repair
Same as before route break
If new route long, this RTO may be too small, leading to timeouts
Same as TCP start-up (6 second)
May be too large
May result in slow response to next packet loss
Another plausible approach: new RTO = function of old RTO, old route length, and new route length
Example: new RTO = old RTO * new route length / old route length
Not evaluated yet
Pitfall: RTT is not just a function of route length
Out-of-Order Packet Delivery: Out-of-Order Packet Delivery Out-of-order (OOO) delivery may occur due to:
Route changes
Link layer retransmissions schemes that deliver OOO
Significantly OOO delivery confuses TCP, triggering fast retransmit
Potential solutions:
Deterministically prefer one route over others, even if multiple routes are known
Reduce OOO delivery by re-ordering received packets
can result in unnecessary delay in presence of packet loss
Turn off fast retransmit
can result in poor performance in presence of congestion
Impact of Acknowledgements: Impact of Acknowledgements
TCP Acks (and link layer acks) share the wireless bandwidth with TCP data packets
Data and Acks travel in opposite directions
In addition to bandwidth usage, acks require additional receive-send turnarounds, which also incur time penalty
To reduce frequency of send-receive turnaround and contention between acks and data
Impact of Acks: Mitigation [Balakrishnan97]: Impact of Acks: Mitigation [Balakrishnan97]
Piggybacking link layer acks with data
Sending fewer TCP acks - ack every d-th packet (d may be chosen dynamically)
but need to use rate control at sender to reduce burstiness (for large d)
Ack filtering - Gateway may drop an older ack in the queue, if a new ack arrives
reduces number of acks that need to be delivered to the sender
Security Issues: Security Issues
Caveat: Caveat
Much of security-related stuff is mostly beyond my expertise
So coverage of this topic is very limited
Security Issues in Mobile Ad Hoc Networks: Security Issues in Mobile Ad Hoc Networks
Not much work in this area as yet
Many of the security issues are same as those in traditional wired networks and cellular wireless
What’s new ?
What’s New ?: What’s New ? Wireless medium is easy to snoop on
Due to ad hoc connectivity and mobility, it is hard to guarantee access to any particular node (for instance, to obtain a secret key)
Easier for trouble-makers to insert themselves into a mobile ad hoc network (as compared to a wired network)
Resurrecting Duckling [Stajano99]: Resurrecting Duckling [Stajano99]
Battery exhaustion threat: A malicious node may interact with a mobile node often with the goal of draining the mobile node’s battery
Authenticity: Who can a node talk to safely?
Resurrecting duckling: Analogy based on a duckling and its mother. Apparently, a duckling assumes that the first object it hears is the mother
A mobile device will trust first device which sends a secret key
Secure Routing [Zhou99]: Secure Routing [Zhou99]
Attackers may inject erroneous routing information
By doing so, an attacker may be able to divert network traffic, or make routing inefficient
[Zhou] suggests use of digital signatures to protect routing information and data both
Such schemes need a Certification Authority to manage the private-public keys
Secure Routing: Secure Routing Establishing a Certification Authority (CA) difficult in a mobile ad hoc network, since the authority may not be reachable from all nodes at all times
[Zhou] suggests distributing the CA function over multiple nodes
MANET Authentication Architecture[Jacobs99ietf-id]: MANET Authentication Architecture [Jacobs99ietf-id]
Digital signatures to authenticate a message
Key distribution via certificates
Need access to a certification authority
[Jacobs99ietf-id] specifies message formats to be used to carry signature, etc.
Techniques for Intrusion-Resistant Ad Hoc Routing Algorithms (TIARA) [Ramanujan00Milcom]: Techniques for Intrusion-Resistant Ad Hoc Routing Algorithms (TIARA) [Ramanujan00Milcom]
Flow disruption attack: Intruder (or compromised) node T may delay/drop/corrupt all data passing through, but leave all routing traffic unmodified A C B D T intruder
Techniques for Intrusion-Resistant Ad Hoc Routing Algorithms (TIARA) [Ramanujan00Milcom]: Techniques for Intrusion-Resistant Ad Hoc Routing Algorithms (TIARA) [Ramanujan00Milcom]
Resource Depletion Attack: Intruders may send data with the objective of congesting a network or depleting batteries A C B D T intruder U intruder Bogus traffic
Intrusion Detection [Zhang00Mobicom]: Intrusion Detection [Zhang00Mobicom] Detection of abnormal routing table updates
Uses 'training' data to determine characteristics of normal routing table updates (such as rate of change of routing info)
Efficacy of this approach is not evaluated, and is debatable
Similar abnormal behavior may be detected at other protocol layers
For instance, at the MAC layer, normal behavior may be characterized for access patterns by various hosts
Abnormal behavior may indicate intrusion
Solutions proposed in [Zhang00Mobicom] are preliminary, not enough detail provided
Preventing Traffic Analysis [Jiang00iaas,Jiang00tech]: Preventing Traffic Analysis [Jiang00iaas,Jiang00tech] Even with encryption, an eavesdropper may be able to identify the traffic pattern in the network
Traffic patterns can give away information about the mode of operation
Attack versus retreat
Traffic analysis can be prevented by presenting 'constant' traffic pattern independent of the underlying operational mode
May need insertion of dummy traffic to achieve this
Packet Purse Model [Byttayn00MobiHoc]: Packet Purse Model [Byttayn00MobiHoc] Cost-based approach for motivating collaboration between mobile nodes
The packet purse model assigns a cost to each packet transfer
Link-level recipient of a packet pays the link-level sender for the service
Virtual money ('beans') used for this purpose
Security issues:
How to ensure that some node does not sale the same packet to too many people to make money ?
How to ensure that each receiver indeed has money to pay for service?
Implementation Issues: Implementation Issues
Existing Implementations: Existing Implementations
Several implementations apparently exist (see IETF MANET web site)
Only a few available publicly [Maltz99,Broch99]
Most implementations focus on unicast routing
CMU Implementation [Maltz99]: CMU Implementation [Maltz99] Physical
devices Kernel space Kernel space WaveLan-I CDPD User space IP TCP/UDP DSR option processing (RREQ, RREP,…) Route cache DSR Output dsr_xmit Send
buffer rexmit
buffer Route table
CMU Implementation: Lessons Learned: CMU Implementation: Lessons Learned Multi-level priority queues helpful: Give higher priority to routing control packets, and lower for data
If retransmission is implemented above the link layer, it must be adaptive to accommodate congestion
Since Wavelan-I MAC does not provide retransmissions, DSR performs retransmits itself
DSR per-hop ack needs to contend for wireless medium
Time to get the ack (RTT) is dependent on congestion
TCP-like RTT estimation and RTO used for triggering retransmits by DSR on each hop
This is not very relevant when using IEEE 802.11 where the ack is sent immediately after data reception
CMU Implementation: Lessons Learned: CMU Implementation: Lessons Learned
'Wireless propagation is not what you would expect' [Maltz99]
Straight flat areas with line-of-sight connectivity had worst error rates
'Bystanders will think you are nuts' [Maltz99]
If you are planning experimental studies in the streets, it may be useful to let police and security guards know in advance what you are up to
BBN Implementation [Ramanathan00Wcnc]: BBN Implementation [Ramanathan00Wcnc] Density and Asymmetric-Adaptive Wireless Network (DAWN)
Quote from [Ramanathan00Wcnc]: DAWN is a 'subnet' or 'link' level system from IP’s viewpoint and runs 'below' IP
DAWN Features: DAWN Features Topology control by transmit power control
To avoid topologies that are too sparse or too dense
To extend battery life
Scalable link state routing: Link state updates with small TTL (time-to-live) sent more often, than those with greater TTL
As a packet gets closer to the destination, more accurate info is used for next hop determination
Elastic Virtual Circuits (VC):
Label switching through the DAWN nodes (label = VC id)
Path repaired transparent to the endpoints when hosts along the path move away
Implementation Issues:Where to Implement Ad Hoc Routing: Implementation Issues: Where to Implement Ad Hoc Routing
Link layer
Network layer
Application layer
Implementation Issues:Address Assignment: Implementation Issues: Address Assignment
Restrict all nodes within a given ad hoc network to belong to the same subnet
Routing within the subnet using ad hoc routing protocol
Routing to/from outside the subnet using standard internet routing
Nodes may be given random addresses
Routing to/from outside world becomes difficult unless Mobile IP is used
Implementation Issues:Address Assignment: Implementation Issues: Address Assignment How to assign the addresses ?
Non-random address assignment:
DHCP for ad hoc network ?
Random assignment
What happens if two nodes get the same address ?
Duplicate address detection needed
One procedure for detecting duplicates within a connected component [Perkins00ietf-id]: When a node picks address A, it first performs a few route discoveries for destination A. If no route reply is received, then address A is assumed to be unique.
Duplicate Address Detection: Duplicate Address Detection
Duplicate address detection harder when partitioned networks merge
Problem can be solved by associating a unique identifier to each node (such as MAC address), and including the unique identifier with IP address when sending routing-related control packet [Patchipulusu01thesis]
Duplicate addresses detected when routing information for identical IP addresses is received with different unique identifiers
Implementation Issues:Security: Implementation Issues: Security
How can I trust you to forward my packets without tampering?
Need to be able to detect tampering
How do I know you are what you claim to be ?
Authentication issues
Hard to guarantee access to a certification authority
Implementation Issues: Implementation Issues
Can we make any guarantees on performance?
When using a non-licensed band, difficult to provide hard guarantees, since others may be using the same band
Must use an licensed channel to attempt to make any guarantees
Implementation Issues: Implementation Issues
Only some issues have been addresses in existing implementations
Security issues typically ignored
Address assignment issue also has not received sufficient attention
Integrating MANET with the Internet [Broch99]: Integrating MANET with the Internet [Broch99]
Mobile IP + MANET routing
At least one node in a MANET should act as a gateway to the rest of the world
Such nodes may be used as foreign agents for Mobile IP
IP packets would be delivered to the foreign agent of a MANET node using Mobile IP. Then, MANET routing will route the packet from the foreign agent to the mobile host.
Distributed Algorithms forMobile Ad Hoc Networks: Distributed Algorithms for Mobile Ad Hoc Networks
Distributed Algorithms: Distributed Algorithms
For traditional networks, there is a rich history of work on distributed algorithms for various problems including
clock synchronization
mutual exclusion
leader election
Byzantine agreement
….
Distributed Algorithms: Distributed Algorithms There is also a large body of work on distributed algorithms for dynamic networks wherein links may come up or down [Afek89]
Similarity: Work on dynamic networks is applicable to ad hoc networks, since both share the dynamic topology change property
Difference: In ad hoc networks, link failure and repair caused by the movement of a single node are likely to be in vicinity of each other, and hence correlated
In dynamic networks research, link events are usually assumed
to be independent
Distributed Algorithms: Research Opportunities: Distributed Algorithms: Research Opportunities
Evaluation of existing algorithms for dynamic networks when applied to MANET
Identify shortcomings, if any
Design improvements
New distributed algorithms designed for mobile ad hoc networks
Limited research on distributed algorithms designed for MANET. Some examples:
Mutual exclusion [Walter98DialM]
Leader election [Royer99Mobicom,Malpani00DialM]
…
Related Standards Activities: Related Standards Activities
Internet Engineering Task Force (IETF)Activities: Internet Engineering Task Force (IETF) Activities
IETF manet (Mobile Ad-hoc Networks) working group
http://www.ietf.org/html.charters/manet-charter.html
IETF mobileip (IP Routing for Wireless/Mobile Hosts) working group
http://www.ietf.org/html.charters/mobileip-charter.html
Internet Engineering Task Force (IETF)Activities: Internet Engineering Task Force (IETF) Activities
IETF pilc (Performance Implications of Link Characteristics) working group
http://www.ietf.org/html.charters/pilc-charter.html
http://pilc.grc.nasa.gov
Refer [RFC2757] for an overview of related work
Related Standards Activities: Related Standards Activities
BlueTooth
http://www.bluetooth.com
HomeRF [Lansford00ieee]
http://www.homerf.org
IEEE 802.11
http://grouper.ieee.org/groups/802/11/
Hiperlan/2
http://www.etsi.org/technicalactiv/hiperlan2.htm
Bluetooth[Haartsen98,Bhagawat00Tutorial]: Bluetooth [Haartsen98,Bhagawat00Tutorial]
Features: Cheaper, smaller, low power, ubiquitous, unlicensed frequency band
Spec version 1.0B released December 1999
(1000+ pages)
Promoter group consisting of 9
Ericsson, IBM, Intel, Nokia, Toshiba, 3Com, Lucent, Microsoft, Motorola
1800+ adopters
Bluetooth: Link Types: Bluetooth: Link Types Designed to support multimedia applications that mix voice and data
Synchronous Connection-Oriented (SCO) link
Symmetrical, circuit-switched, point-to-point connections
Suitable for voice
Two consecutive slots (forward and return slots) reserved at fixed intervals
Asynchronous Connectionless (ACL) link
Symmetrical or asymmetric, packet-switched, point-to-multipoint
Suitable for bursty data
Master units use a polling scheme to control ACL connections
Bluetooth: Piconet: Bluetooth: Piconet A channel is characterized by a frequency-hopping pattern
Two or more terminals sharing a channel form a piconet
1 Mbps per Piconet
One terminal in a piconet acts as a master and up to 7 slaves
Other terminals are slaves
Polling scheme: A slave may send in a slave-to-master slot when it has been addressed by its MAC address in the previous master-to-slave slot
Inter-Piconet Communication: Inter-Piconet Communication
A slave can belong to two different piconets, but not at the same time
A slave can leave its current piconet (after informing its current master the duration of the leave) and join another piconet
A maser of one piconet can also join another piconet temporarily as a slave
Bluetooth: Scatternet: Bluetooth: Scatternet Several piconets may exist in the same area (such that units in different piconets are in each other’s range)
Each piconet uses a different channel and gets 1 Mbps for the piconet
Since two independently chosen hopping patterns may select same hop simultaneously with non-zero probability, some collisions between piconets are possible, reducing effective throughput
A group of piconets is called a scatternet
Routing: Routing
Ad hoc routing protocols needed to route between multiple piconets
Existing protocols may need to be adapted for Bluetooth [Bhagwat99Momuc]
For instance, not all nodes within transmission range of node X will hear node X
Only nodes which belong to node X’s current piconet can hear the transmission from X
Flooding-based schemes need to take this limitation into account
Open IssuesinMobile Ad Hoc Networking: Open Issues in Mobile Ad Hoc Networking
Open Problems: Open Problems Issues other than routing have received much less attention so far
Other interesting problems:
Address assignment problem
MAC protocols
Improving interaction between protocol layers
Distributed algorithms for MANET
QoS issues
Applications for MANET
Related Research Areas: Related Research Areas
Algorithms for dynamic networks (e.g., [Afek89])
Sensor networks [DARPA-SensIT]
Ad hoc network of sensors
Addressing based on data (or function) instead of name
'send this packet to a temperature sensor'
References: References
Please see attached listing for the references cited in the tutorial
Thank you !!For more information, send e-mail toNitin Vaidya atnhv@uiuc.edu: Thank you !! For more information, send e-mail to Nitin Vaidya at nhv@uiuc.edu
© 2003 Nitin Vaidya