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3 Time Constraint Given the half-day duration of this DSN 2005 tutorial, some of the slides in this set of 300+ slides will not be actually discussed during the presentation The slides are included in the handout as a reference for the attendees
Citation preview
1
Mobile Ad Hoc Networks: Protocols and Security Issues
Nitin H. VaidyaUniversity of Illinois at Urbana-Champaign
nhv@uiuc.eduhttp://www.crhc.uiuc.edu/~nhv
© 2005 Nitin Vaidya
2
Notes Coverage not exhaustive. Only a few example schemes discussed
Only selected features of various schemes are typically discussed. Not possible to cover all details in this tutorial
Some protocol specs have changed over time, and the slides may not reflect the most current specifications
Jargon used to discuss a scheme may occasionally differ from that used in the original papers
Names in brackets, as in [Xyz00], refer to a document in the list of references
Abbreviation MAC used to mean either Medium Access Control or Message Authentication Code – implied meaning should be clear from context
3
Time Constraint
Given the half-day duration of this DSN 2005 tutorial, some of the slides in this set of 300+ slides will not be actually discussed during the presentation The slides are included in the handout as a reference for the
attendees
4
Outline
Introduction to ad hoc networks Selected routing protocols Selected MAC protocol mechanisms Security and misbehavior
Key management in wireless ad hoc networks Secure communication in ad hoc networks MAC layer issues Network layer issues
Related activities References
5
Mobile Ad Hoc Networks (MANET)
6
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
7
Mobile Ad Hoc Networks
May need to traverse multiple links to reach a destination
AB
C
D
8
Mobile Ad Hoc Networks (MANET)
Mobility causes route changes
AB
C D
9
Why Ad Hoc Networks ?
Ease of deployment
Speed of deployment
Decreased dependence on infrastructure
10
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
11
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)
12
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
13
Many Variations
Mobility pattern/characteristics may be different Application domain
– people sitting at an airport lounge– New York taxi cabs– Kids playing– Military movements– personal area network
speed predictability
– direction of movement– pattern of movement
uniformity (or lack thereof) of mobility characteristics among different nodes
14
Challenges
Limited wireless transmission range Broadcast nature of the wireless medium Packet losses due to transmission errors Mobility-induced route changes Mobility-induced packet losses Battery constraints Potentially frequent network partitions Ease of snooping on wireless transmissions (security
hazard)
15
Research on Mobile Ad Hoc Networks
Variations in capabilities & responsibilities XVariations in traffic characteristics, mobility models, etc. XPerformance criteria (e.g., throughput, energy, security)
=Significant research activity
16
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 asub-space of the problem domain
17
Outline
Introduction to ad hoc networks Selected routing and MAC protocols Key management in wireless ad hoc networks Secure communication in ad hoc networks Misbehavior at the MAC layer Misbehavior at the network layer Anomaly detection
18
Unicast Routingin
Mobile Ad Hoc Networks
19
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
20
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
21
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
22
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 may 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
23
Reactive Routing Protocols
24
Routing Protocols
Proactive protocols for ad hoc networks are often derived from link state or distance vector routing protocols
But with some optimizations
We will not discuss proactive protocols in detail
Before discussing an example reactive protocol, let us consider “flooding” as a routing protocol
25
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
26
Flooding for Data Delivery
B
A
S EF
H
J
D
C
G
IK
Represents that connected nodes are within each other’s transmission range
Z
Y
Represents a node that has received packet P
M
N
L
27
Flooding for Data Delivery
B
A
S EF
H
J
D
C
G
IK
Represents transmission of packet P
Represents a node that receives packet P forthe first time
Z
YBroadcast transmission
M
N
L
28
Flooding for Data Delivery
B
A
S EF
H
J
D
C
G
IK
• Node H receives packet P from two neighbors: potential for collision
Z
Y
M
N
L
29
Flooding for Data Delivery
B
A
S EF
H
J
D
C
G
IK
• 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
30
Flooding for Data Delivery
B
A
S EF
H
J
D
C
G
IK
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 Packet P may not be delivered to node D at all, despite the use of flooding
N
L
31
Flooding for Data Delivery
B
A
S EF
H
J
D
C
G
IK
Z
Y
• Node D does not forward packet P, because node D is the intended destination of packet P
M
N
L
32
Flooding for Data Delivery
B
A
S EF
H
J
D
C
G
IK
• 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
33
Flooding for Data Delivery
B
A
S EF
H
J
D
C
G
IK
• 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
34
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
35
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
Several protocols based on this (Examples: DSR, AODV)
36
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
37
Route Discovery in DSR
B
A
S EF
H
J
D
C
G
IK
Z
Y
Represents a node that has received RREQ for D from S
M
N
L
38
Route Discovery in DSR
B
A
S EF
H
J
D
C
G
IK
Represents transmission of RREQ
Z
YBroadcast transmission
M
N
L
[S]
[X,Y] Represents list of identifiers appended to RREQ
39
Route Discovery in DSR
B
A
S EF
H
J
D
C
G
IK
• Node H receives packet RREQ from two neighbors: potential for collision
Z
Y
M
N
L
[S,E]
[S,C]
40
Route Discovery in DSR
B
A
S EF
H
J
D
C
G
IK
• 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]
41
Route Discovery in DSR
B
A
S EF
H
J
D
C
G
IK
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]
42
Route Discovery in DSR
B
A
S EF
H
J
D
C
G
IK
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]
43
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
44
Route Reply in DSR
B
A
S EF
H
J
D
C
G
IK
Z
Y
M
N
L
RREP [S,E,F,J,D]
Represents RREP control message
45
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)
46
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
47
Data Delivery in DSR
B
A
S EF
H
J
D
C
G
IK
Z
Y
M
N
L
DATA [S,E,F,J,D]
Packet header size grows with route length
48
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
49
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
50
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
51
Use of Route Caching
B
A
S EF
H
J
D
C
G
IK
[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
52
Use of Route Caching:Can Speed up Route Discovery
B
A
S EF
H
J
D
C
G
IK
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 requestfor node C, node K sends back a routereply [Z,K,G,C] to node Z using a locallycached route
[K,G,C,S] RREP
53
Use of Route Caching:Can Reduce Propagation of Route Requests
B
A
S EF
H
J
D
C
G
IK
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
54
Route Error (RERR)
B
A
S EF
H
J
D
C
G
IK
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
55
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]
56
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
57
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
58
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
59
Reducing Route Discovery Overhead:Expanding Ring Search
Route Requests are initially sent with smallTime-to-Live (TTL) field, to limit their propagation
If no Route Reply is received, then larger TTL tried
60
Reducing Route Discovery Overhead: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
61
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
62
Request Zone in LAR
X
Y
r
S
Request Zone
Network Space
BA
63
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
64
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
65
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
66
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
67
Route Requests in AODV
B
A
S EF
H
J
D
C
G
IK
Z
Y
Represents a node that has received RREQ for D from S
M
N
L
68
Route Requests in AODV
B
A
S EF
H
J
D
C
G
IK
Represents transmission of RREQ
Z
YBroadcast transmission
M
N
L
69
Route Requests in AODV
B
A
S EF
H
J
D
C
G
IK
Represents links on Reverse Path
Z
Y
M
N
L
70
Reverse Path Setup in AODV
B
A
S EF
H
J
D
C
G
IK
• 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
71
Reverse Path Setup in AODV
B
A
S EF
H
J
D
C
G
IK
Z
Y
M
N
L
72
Reverse Path Setup in AODV
B
A
S EF
H
J
D
C
G
IK
Z
Y
• Node D does not forward RREQ, because node D is the intended target of the RREQ
M
N
L
73
Route Reply in AODV
B
A
S EF
H
J
D
C
G
IK
Z
Y
Represents links on path taken by RREP
M
N
L
74
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
75
Forward Path Setup in AODV
B
A
S EF
H
J
D
C
G
IK
Z
Y
M
N
L
Forward links are setup when RREP travels alongthe reverse path
Represents a link on the forward path
76
Data Delivery in AODV
B
A
S EF
H
J
D
C
G
IK
Z
Y
M
N
L
Routing table entries used to forward data packet.
Route is not included in packet header.
DATA
77
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
78
Proactive Protocols
79
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
80
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
81
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
82
Optimized Link State Routing (OLSR)
Nodes C and E are multipoint relays of node A
A
B F
C
D
E H
GK
J
Node that has broadcast state information from A
83
Optimized Link State Routing (OLSR)
Nodes C and E forward information received from A
A
B F
C
D
E H
GK
J
Node that has broadcast state information from A
84
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
GK
J
Node that has broadcast state information from A
85
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
86
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
87
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
88
Destination-Sequenced Distance-Vector (DSDV)
Node X takes the following steps:
If S(X) > 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) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y)
X Y Z
89
Unicast Routing Protocols
MANY other protocols have been proposed
Some use other metrics such as energy efficiency, load balancing, when choosing routes
Hybrid protocols combine reactive and proactive features
90
Outline
Introduction to ad hoc networks Selected routing protocols Selected MAC protocol mechanisms Security and misbehavior
Key management in wireless ad hoc networks Secure communication in ad hoc networks MAC layer issues Network layer issues
Related activities References
91
Medium Access Control Protocols
92
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 [Chandra00]
93
MAC: A Simple Classification
WirelessMAC
Centralized Distributed
Guaranteedor
controlledaccess
RandomaccessIEEE 802.11
94
A B C
Hidden Terminal Problem
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
95
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
A B C
96
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
97
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
A B C
98
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
99
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
A B C
100
Collision Avoidance
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/CTS stay silent for specified duration. Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit.
101
C FA B EDRTS
RTS = Request-to-Send
IEEE 802.11
Pretending a circular range
102
C FA B EDRTS
RTS = Request-to-Send
IEEE 802.11
NAV = 10
NAV = remaining duration to keep quiet
103
C FA B EDCTS
CTS = Clear-to-Send
IEEE 802.11
104
C FA B EDCTS
CTS = Clear-to-Send
IEEE 802.11
NAV = 8
105
C FA B EDDATA
•DATA packet follows CTS. Successful data reception acknowledged using ACK.
IEEE 802.11
106
IEEE 802.11
C FA B EDACK
107
C FA B EDACK
IEEE 802.11
Reserved area(not necessarilycircular inpractice)
108
Backoff Interval
Backoff intervals used to reduce collision probability
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
109
IEEE 802.11 DCF Example
data
waitB1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervalsat nodes 1 and 2cw = 31
B2 = 10
110
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)
111
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
112
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
113
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
ATIMwindow
114
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
115
Power Save in IEEE 802.11 Ad Hoc Mode
ATIMReq
ATIMAck
AckData
Sleep
Node A
Node C
Node B
116
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
117
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
118
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]
119
Adaptive Modulation
120
Adaptive Modulation
Channel conditions are time-varying
Received signal-to-noise ratio changes with time
A B
121
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 providea trade-off betweenthroughput and range
122
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
123
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
124
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
125
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
126
Receiver-Based Autorate MAC Protocol
D
C
BACTS (1 Mbps)
RTS (2 Mbps)
Data (1 Mbps)
NAV updated using rate
specified in the data packet
127
TCP Performancein
Mobile Ad Hoc Networks
128
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
129
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.crhc.uiuc.edu/wireless/ (follow the link to Tutorials)
[Montenegro00-RFC2757] discusses related issues
130
This Tutorial
This tutorial considers impact of multi-hop routes and route failures due to mobility
131
Mobile Ad Hoc Networks
May need to traverse multiple links to reach a destination
132
Mobile Ad Hoc Networks
Mobility causes route changes
133
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
134
Impact of Multi-Hop Wireless Paths [Holland99]
0200400600800
1000120014001600
1 2 3 4 5 6 7 8 9 10
Number of hops
TCPThroughtput(Kbps)
TCP Throughput using 2 Mbps 802.11 MAC
135
Throughput Degradations withIncreasing 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
136
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 = f(i) * T(i)
137
Impact of MobilityTCP Throughput
Ideal throughput (Kbps)
Act
ual t
hrou
ghpu
t
2 m/s 10 m/s
138
Impact of Mobility
Ideal throughput
Act
ual t
hrou
ghpu
t
20 m/s 30 m/s
139
Throughput generally degrades with increasing speed …
Speed (m/s)
AverageThroughputOver 50 runs
Ideal
Actual
140
But not always …
Mobility pattern #
Actualthroughput
20 m/s
30 m/s
141
mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
Why Does Throughput Degrade?
TCP sender times out.Starts sending packets again
Route isrepaired
No throughput
No throughputdespite route repair
142
mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
Why Does Throughput Degrade?
TCP sendertimes out.Backs off timer.
Route isrepaired
TCP sendertimes out.Resumessending
Larger route repair delaysespecially harmful
No throughputNo throughput
despite route repair
143
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.
144
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.
145
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
146
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
147
Performance Improvement
Without networkfeedback
Ideal throughput 2 m/s speed
With feedback
Act
ual t
hrou
ghpu
t
148
Performance Improvement
Without networkfeedback
With feedback
Ideal throughput 30 m/s speed
Act
ual t
hrou
ghpu
t
149
Performance with Explicit Notification[Holland99]
0
0.2
0.4
0.6
0.8
1
2 10 20 30
mean speed (m/s)
thro
ughp
ut a
s a
frac
tion
of
idea
l Base TCP
With explicitnotification
150
IssuesNetwork Feedback
Network knows best (why packets are lost)
+ Network feedback beneficial- Need to modify transport & 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
151
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
152
To Cache or Not to Cache
Average speed (m/s)Ac t
ual t
hro u
g hp u
t (as
frac
t ion
o f e
xpec
t ed
thro
u gh p
u t)
153
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
timeout dueto route failure
timeout, cachedroute is broken
timeout, second cachedroute also broken
154
IssuesTo 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
155
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
156
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?
157
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
158
IssuesRTO 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
159
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
160
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
161
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
162
Outline
Introduction to ad hoc networks Selected routing protocols Selected MAC protocol mechanisms Security and misbehavior
Key management in wireless ad hoc networks Secure communication in ad hoc networks MAC layer issues Network layer issues
Related activities References
163
Security and Misbehavior
164
Issues
Hosts may be misbehave or try to compromise security at all layers of the protocol stack
165
Transport Layer(End-to-End Communication)
How to secure end-to-end communication?
Need to know keys to be used for secure communication
May want to anonymize the communication
166
Network Layer
Misbehaving hosts may create many hazards
May disrupt route discovery and maintenance:Force use of poor routes (e.g., long routes)
Delay, drop, corrupt, misroute packets
May degrade performance by making good routeslook bad
167
MAC Layer
Disobey protocol specifications for selfish gains
Denial-of-service attacks
168
Scope of this Tutorial
Overview of selected issues at various protocol layers
Not an exhaustive survey of all relevant problems or solutions
169
Outline
Introduction to ad hoc networks Selected routing and MAC protocols Key management in wireless ad hoc networks Secure communication in ad hoc networks Misbehavior at the MAC layer Misbehavior at the network layer Anomaly detection
170
Key Management
171
Key Management
In “pure” ad hoc networks, access to infrastructure cannot be assumed
Network may also become partitioned
In “hybrid” networks, however, if access to infrastructure is typically available, traditional solutions can be extended with relative ease
172
Certification Authority
Certification Authority (CA) has a public/private key pair, with public key known to all
CA signs certificate binding public keys to other nodes
A single CA may not be enough – unavailability of the CA (due to partitioning, failure or compromise) will make it difficult for nodes to obtain public keys of other hosts
A compromised CA may sign erroneous certificates
173
Distributed Certification Authority [Zhou99]
Use threshold cryptography to implement CA functionality jointly at n nodes. The n CA servers collectively have a public/private key pair
Each CA only knows a part of the private key Can tolerate t compromised servers
Threshold cryptography: (n,t+1) threshold cryptography scheme allows n parties to share the ability to perform a cryptographic operation (e.g., creating a digital signature)
Any (t+1) parties can perform the operation jointly No t or fewer parties can perform the operation
174
Distributed Certification Authority [Zhou99]
Each server knows public key of other servers, so that the servers can communicate with each other securely
To sign a certificate, each server generates a partial signature for the certificate, and submits to a combiner
To protect against a compromised combiner, use t+1 combiners
175
Self-Organized Public Key Management [Capkun03]
Does not rely on availability of CA
Nodes form a “Certificate Graph” each vertex represents a public key an edge from Ku to Kw exists if there is a certificate signed by
the private key of node u that binds Kw to the identity of some node w.
Ku Kw
(w,Kw)Pr Ku
176
Self-Organized Public Key Management [Capkun03]
Four steps of the management scheme
Step 1: Each node creates its own private/public keys.Each node acts independently
177
Self-Organized Public Key Management
Step 2: When a node u believes that key Kw belongs to node w, node u issues a public-key certificate in which Kw is bound to w by the signature of u
u may believe this because u and w may have talked on a dedicated channel previously
Each node also issues a self-signed certificate for its own key
Step 3: Nodes periodically exchange certificates with other nodes they encounter Mobility allows faster dissemination of certificates through the
network
178
Self-Organized Public Key Management
Step 4: Each node forms a certificate graph using the certificates known to that node
Authentication: When a node u wants to verify the authenticity of the public key Kv of node v, u tries to find a directed graph from Ku to Kv in the certificate graph. If such a path is found, the key is authentic.
179
Self-Organized Public Key Management
Misbehaving hosts may issue incorrect certificates
If there are mismatching certificates, indicates presence of a misbehaving host (unless one of the mismatching certificate has expired) Mismatching certificates may bind same public key for two
different nodes, or same node to two different keys
To resolve the mismatch, a “confidence” level may be calculated for each certificate chain that verifies each of the mismatching certificates Choose the certificate that can be verified with high
confidence – else ignore both certificates
180
TESLA Broadcast Authentication [Perrig]
How to verify authenticity of broadcast packets? Use Message Authentication Code (MAC) for each
message, using a shared secret key But with broadcast, all receivers need to know the shared
key, and any of them can then impersonate the sender
Use digital signature with asymmetric cryptography Computationally expensive
Use asymmetric cryptography to bootstrap symmetric cryptography solution TESLA
181
TESLA
Uses one-way hash chains: Starting with initial value s0, use one-way function F to general a sequence of values s1 = F(s0), s2 = F(s1), … , sn = F(sn-1).
Knowing an earlier value in the chain, a latter value can be determined, but not vice-versa
Use the values in reverse order, starting from sn-1 Order of use opposite the order of generation
Distribute sn to all nodes with verifiable authenticity Use digital signature (this is the “bootstrap” step) Nodes need to know the source’s public key
182
TESLA
Messages sent during period i include Message Authentication Code (MAC) computed using another one-way function of si
The key si is revealed after a key disclosure delay of d intervals
On receiving a message in interval i, a node X waits for d-1 additional intervals for the key to be revealed)
When si is revealed, node X can verify that si+1 = F(si) to determine authenticity of si
183
TESLA
Authenticity of si can be determined so long as node X knows some sk with k>i Allows for loss of revealed keys during broadcast operation
Once a key is revealed, anyone can try to impersonate the sender using that key
To avoid this, TESLA assumes loose time synchronization Each receiver can place an upper bound on the sender’s clock The error needs to be small compared to key disclosure delay
184
TESLA
If impersonator I receives key si from source S first, and sends a packet to R impersonating S, R will find the packet valid only if The packet timestamp is smaller than the upper bound R
places on the time at S, and Now, the upper bound when S sends key si will be at least i+d
(since the key is not released until interval i+d) So if R only accepts packets sent with timestamp i but
received when the upper bound on S’s clock < i+d, there is no way an impersonator can pass above conditions (provided clock error small compared to d)
SR
I
185
TESLA
Advantage: Use of asymmetric cryptography required only initially (to distribute initial key using signatures)
Further communication uses MAC
Disadvantage: Messages can only be authenticated after delay d
186
Outline
Introduction to ad hoc networks Selected routing and MAC protocols Key management in wireless ad hoc networks Secure communication in ad hoc networks Misbehavior at the MAC layer Misbehavior at the network layer Anomaly detection
187
Secure Communication
188
Secure Communication
With the previously discussed mechanisms for key distribution, it is possible to authenticate the assignment of a public key to a node
This key can then be used for secure communication The public key can be used to set up a symmetric key
between a given node pair as well TESLA provides a mechanism for broadcast authentication
when a single source must broadcast packets to multiple receivers
189
Secure Communication
Sometimes security requirement may include anonymity
Availability of an authentic key is not enough to prevent traffic analysis
We may want to hide the source or the destination of a packet, or simply the amount of traffic between a given pair of nodes
190
Traffic Analysis
Traditional approaches for anonymous communication, for instance, based on MIX nodes or dummy traffic insertion, can be used in wireless ad hoc networks as well
However, it is possible to develop new approaches considering the broadcast nature of the wireless channel
191
Mix Nodes [Chaum]
Mix nodes can reorder packets from different flows, insert dummy packets, or delay packets, to reduce correlation between packets in and packets out
M1 B M2 E
A
M3C
DG
F
192
Mix Nodes
Node A wants to send message M to node G. Node A chooses 2 Mix nodes (in general n mix nodes), say, M1 and M2
M1 B M2 E
A
M3C
DG
F
193
Mix Nodes
Node A transmits to M1message K1(R1, K2(R2, M)) where Ki() denotes encryption using public key Ki of Mix i, and Ri is a random number
M1 B M2 E
A
M3C
DG
F
194
Mix Nodes
M1 recovers K2(R2,M) and send to M2
M1 B M2 E
A
M3C
DG
F
195
Mix Nodes
M2 recovers M and sends to G
M1 B M2 E
A
M3C
DG
F
196
Mix Nodes
If M is encrypted by a secret key, no one other than G or A can know M
Since M1 and M2 “mix” traffic, observers cannot determine the source-destination pair without compromising M1 and M2 both
197
Alternative Mix Nodes Suppose A uses M2 and M3 (not M1 and M2)
Need to take fewer hops
Choice of mix nodes affects overhead
M1 B M2 E
A
M3C
DG
F
198
Mix Node Selection
Intelligent selection of mix nodes can reduce overhead [Jiang04]
With mobility, the choice of mix nodes may have to be modified to reduce cost
However, change of mix selection has the potential for divulging more information
199
Traffic Mode Detection
Consider a node pair A and D. Depending on the “mode” of operation, the traffic rate from A to D is either R1 or R2.
To avoid detection of the mode, node A may always send at rate max (R1, R2) inserting dummy traffic if necessary [Venkatraman93]
This is an end-to-end approach, since it can be implemented entirely at source & destination of a flow
200
Traffic Mode Detection
Now consider two flow A-D and E-F Mode 1: A-D rate R1 E-F rate R2
Mode 2: A-D rate R2 E-F rate R1 End-to-end cover: A-D and E-F both at rate max (R1,R2) Link BC carries traffic 2*max (R1,R2)
A B C D
E
F
Max(R1,R2)
Max(R1,R2) 2 * Max(R1,R2)
201
Traffic Mode Detection
If we can encrypt link layer traffic in ad hoc networks, then a “link” cover mode can be used, such that each link carries fixed traffic independent of traffic mode
Reduces resource usage
A B C D
E
F
Max(R1,R2) on each link except BCR1+ R2 on link BC
202
Traffic Mode Detection
Insertion of dummy traffic on a per-link basis “cheaper” than end-to-end [Radosavljevic92,Jiang01]
But need to take into account rates of different flows to determine suitable level of padding
Also, need link layer encryption to disallow differentiation of different flows at the link layer
203
Traffic Mode Detection
Mode 1: A-D rate R1 E-F rate R2Mode 2: A-D rate R2 E-F rate R1
Need Max(R1,R2) on all links, since the two flows do not share links
Node B transmits 2 * Max(R1,R2) traffic
A B D
E
F
204
Traffic Mode Detection
Node-level dummy packet insertion cheaper, if we can hide link-level receiver of the packets
Without the dummy traffic, node B forwards traffic R1+R2 independent of the mode
Node-level insertion: Maintain rates Max(R1,R2) at nodes A and E, and rate R1+R2 at node B
A B D
E
F
205
Traffic Mode Detection
Node B needs to be able to remove dummy packets
Recipient of traffic from node B needs to be hidden
Additional mechanisms can be designed for this [Jiang05]
206
Outline
Introduction to ad hoc networks Selected routing protocols Selected MAC protocol mechanisms Security and misbehavior
Key management in wireless ad hoc networks Secure communication in ad hoc networks MAC layer issues Network layer issues
Related activities References
207
Misbehavior at the MAC Layer
208
MAC Layer Misbehavior
Wireless
channel
Access Point
A B
Nodes are required to follow Medium Access Control (MAC) rules
Misbehaving nodes may violate MAC rules
Wireless
channel
Access Point
C D
209
Example
We will illustrate MAC layer misbehavior with example misbehaviors that can occur with IEEE 802.11 DCF protocol
For ease of discussion, we sometimes refer to nodes communicating with an “access point”, but the discussion applies equally to nodes transmitting to any node in an ad hoc network acting as their receiver
210
Some Possible Misbehaviors
Causing collisions with other hosts’ RTS or CTS [Raya]
Those hosts will exponentially backoff on packet loss, giving free channel to the misbehaving host
211
Possible Misbehaviors:“Impatient” Transmitters
Smaller backoff intervals [Kyasanur]
Shorter Interframe Spacings [Raya]
212
“Impatient” Transmitters
Backoff from biased distribution
Example: Always select a small backoff value
Transmit
wait
B1 = 1
B2 = 20
Transmit
wait
B2 = 19
B1 = 1Misbehaving node
Well-behaved node
213
Impatient Transmitters
We will discuss the case of hosts that choose “too small” backoff intervals
But other cases of hosts waiting too little before talking can be handled analogously
214
Goals [Kyasanur03]
Diagnose node misbehavior Catch misbehaving nodes
Discourage misbehavior Punish misbehaving nodes
215
Potential Approaches
Watch idle times on the channel to detect when hosts wait too little
Design protocols that improve the ability to detect misbehavior
Protocols that discourage misbehavior [Konorski]• Certain game-theoretic approaches
216
Passive Observation [Kyasanur03](Conceptually Simplest Solution)
802.11 dictates that each host must be idle for a certain duration between transmissions
The duration can be expressed as(K + v) where K is a constant, and v is chosen probabilistically from a certain distribution
K due to inter-frame spacing
v due to randomly chosen backoff intervals
217
Passive Observation
The observer can measure the idle time on the channel and determine whether the idle time is drawn from the above distribution
If the observed idle time is smaller than expected, then misbehavior can be detected [Kyasanur03]
[Cagalj05] presents an implementation based on this approach
218
Passive Observation
With this approach, a receiver can try to diagnose behavior of nodes trying to send packets to the receiver
Wireless channel
Access Point
A
219
Issues
Wireless channel introduces uncertainties
Not all hosts see channel idle at the same time
AP1 sees channel busy, but A sees it as idle
Wireless channel
AP 1
A
Wireless channel
AP 2
B
220
Issues
Spatial channel variations bound the efficacy of misbehavior detection mechanisms
Many existing proposals ignore channel variation when performing evaluations, making the evaluations less reliable
221
Issues
Receiver does not know exact backoff value chosen by sender Sender chooses random backoff
Hard to distinguish between maliciously chosen small values and a legitimate value
222
Potential Solution:Use long-term statistics [Kyasanur]
Observe backoffs chosen by sender over multiple packets
Selecting right observation interval difficult
223
An Alternative Approach
Remove the non-determinism
224
An Alternative Approach
Receiver provides backoff values to sender Receiver specifies backoff for next packet in ACK for current
packet
Modification does not significantly change 802.11 behavior Backoffs of different nodes still independent
Uncertainty of sender’s backoff eliminated
225
Modifications to 802.11
• R provides backoff B to S in ACK B selected from [0,CWmin]
DATA
Sender S
Receiver R
CTS
ACK(
B)RTS
• S uses B for backoff
RTS
B
226
Protocol steps
Step 1: For each transmission: Detect deviations: Decide if sender backed off for less than
required number of slots Penalize deviations: Penalty is added, if the sender appears to
have deviated
Goal: Identify and penalize suspected misbehavior Reacting to individual transmission makes it harder for the
cheater to adapt to the protocol
227
Protocol steps
Step 2: Based on last W transmissions: Diagnose misbehavior: Identify misbehaving nodes
Goal: Identify misbehaving nodes with high probability Reduce impact of channel uncertainties Filter out misbehaving nodes from well-behaved nodes
228
Detecting deviations
Receiver counts number of idle slots Bobsr
Condition for detecting deviations: Bobsr < B (0 < <= 1)
Sender S
Receiver R
ACK(
B) RTS
Backoff
Bobsr
229
Penalizing Misbehavior
When Bobsr < B, penalty P added P proportional to B– Bobsr
ACK(
B+P
)
CTS DATA
Total backoff assigned = B + P
Bobsr
Sender SReceiver R
ACK(
B) RTS
Actual backoff < B
230
Penalty Scheme issues
Misbehaving sender has two options Ignore assigned penalty Easier to detect Follow assigned penalty No throughput gain
With penalty, sender has to misbehave more for same throughput gain
231
Diagnosing Misbehavior
Total deviation for last W packets used Deviation per packet is B – Bobsr
If total deviation > THRESH then sender is designated as misbehaving
Higher layers / administrator can be informed of misbehavior
232
Summary of Performance Results
Persistent misbehavior detected with high accuracy• Accuracy increases with misbehavior
Accuracy depends on channel conditions
Accuracy not 100% due to channel variations
233
Variations – Multiple Observers
In an ad hoc networks, a node can only diagnose, on its own, misbehavior by senders in its vicinity
Potential for error due to channel variations
Different hosts can cooperate to improve accuracy
Open problem: How to cooperate? How to “merge” information to arrive at a diagnosis?
234
Other Approaches
Game theory
Incentive-based mechanisms
235
MAC Selfishness: Game-Theoretic Approach
[MacKenzie] addresses selfish misbehavior in Aloha networks Nodes can choose arbitrary access probabilities Assign cost c for a transmission attempt
• Utility of a successful transmission = 1-c
• Utility of an unsuccessful transmission = -c
• Utility of no attempt = 0
MacKenzie’s contribution is to show that there exists a Nash equilibrium strategy
236
MAC: Selfishness
Others have also attempted game-theoretic solutions [Konorski,Cagalj05]
Limitation: Game-theoretic solutions (so far) assume that all hosts see identical channel state Not realistic Limits usefulness of solutions
237
Use payment schemes, charging per packet
Misbehaving hosts can get more throughput, but at a higher cost
• This solution does not ensure fairness• Also, misbehaving node can achieve lower delay at no extra
cost
• This suggests that per-packet payment is not enough• Need to factor delay as well (harder)
Incentive-Based Mechanisms [Zhong02]
238
Some Other MAC Layer Issues
239
MAC Layer Anonymous Broadcast
How to broadcast anonymously at the MAC layer? To maintain anonymity from “external” attackers
One possible solution: Encrypt the source address using secret key (attacker cannot determine the packet’s contents)
Source may be encrypted, but the signal energy will be highest closest to the transmitter
This may give away the identity of the source
240
MAC Layer Anonymous Broadcast
Alternate (expensive) solution: Require all hosts in a “broadcast domain” to periodically broadcast packets
Hosts may transmit dummy packets when no real packets need to be transmitted
Observer cannot determine which hosts are sending real packets (due to encryption)
Source cannot be determined uniquely, but overhead high
241
Link Layer Encryption
Link layer encryption provides protection for wireless transmissions on a per-hop basis.
Need mechanisms for agreeing on suitable keys for this purpose
IEEE 802.11 specifies one such approach
242
Outline
Introduction to ad hoc networks Selected routing protocols Selected MAC protocol mechanisms Security and misbehavior
Key management in wireless ad hoc networks Secure communication in ad hoc networks MAC layer issues Network layer issues
Related activities References
243
Network Layer Misbehavior
244
Network Layer Misbehavior
Many potential misbehaviors have been identified in various papers
We will discuss selected misbehaviors, and plausible solutions
245
Drop/Corrupt/Misroute
A node “agrees” to join a route(for instance, by forwarding route request in DSR)
but fails to forward packets correctly
A node may do so to conserve energy, or to launch a denial-of-service attack, due to failure of some sort, or because of overload
246
Watchdog Approach [Marti]
Verify whether a node has forwarded a packet or not
B DC EA
B sends packet to C
247
Watchdog Approach [Marti]
Verify whether a node has forwarded a packet or not B can learn whether C has forwarded packet or not B can also know whether packet is tampered with if no
per-link encryption
B DC EA
C forwards packet to D
B overhears CForwarding the packet
248
Watchdog Approach:Buffering & Failure Detection
Forwarding by C may not be immediate: B must buffer packets for some time, and compare them with overheard packets
• Buffered packet can be removed on a match
If packet stays in buffer at B too long, a “failure tally” for node C is incremented
If the failure rate is above a threshold, C is determined as misbehaving, and source node informed
249
Impact of Collisions
If A transmits while C is forwarding to D, A will not know
Failure tally at C is not reliable. Include a margin for such errors (which may be exploited by misbehaving hosts)
B DC EA
C forwards packet to D
250
Reliability of Reception Not Known
Even if B sees the transmission from C, it cannot always tell whether D received the packet reliably
Misbehaving C may reduce power such that B can receive from C, but D does not (provided path loss to D is higher)
B DC EA
C forwards packet to D
251
Channel Variations May Cause False Detection
If channel quality between B and C changes often, B may not overhear packets forwarded by C
This will increase C’s failure tally at B May cause false misbehavior accusation
B DC EA
252
Malicious Reporting
Host D may be a good node, but C may report that D is misbehaving
Source cannot tell whether this report is accurate
If the destination sends acknowledgement to source for the received packets, and if the forward-reverse routes are disjoint, this misbehavior (by C) may be caught
253
Collusion
If C forwards packets to D, but fails to report when D does not forward packets, the source node cannot determine who is misbehaving
B DC EA
Collusion hard to detect in many other schemes as well
254
Misdirection of Packets
C forwards packets, but to the wrong node! With DSR, B knows the next hop after C, so this
misbehavior may be detected
With other hop-by-hop forwarding protocols, B cannot detect this
B DC EA
F
255
Directional Transmissions
Directional transmissions make it difficult to use Watchdog
Power control for improved capacity or energy efficiency can create difficulties as well
B DC EA
B cannot hearC’s transmission to D
256
Watchdog + Pathrater [Marti]
“Pathrater” is run by each node. Each node assigns a rating to each known node Previously unknown nodes assigned “neutral” rating of 0.5 Rating assigned to nodes suspected of misbehaving are set
to large negative value Other nodes have positive ratings (between 0 and 0.8)
Ratings of well-behaved nodes increase over time up to a maximum So a temporary misbehavior can be overcome by sustained
good behavior
Routes with larger cumulative node ratings preferred
257
Watchdog: Summary
Can detect misbehaving hosts, although not always; false detection possible as well
Misbehaving hosts not punished
Effectively rewarded, by not sending any more traffic through them
Potential modification: Punishment could be to not forward any traffic from the misbehaving hosts
258
Hosts Bearing Grudges:CONFIDANT Protocol [Buchegger]
Motivated by “The Selfish Gene” by Dawkins (1976)
Consider three types of birds “Suckers” – Birds that always groom parasites off other
birds’ heads “Cheats” – Birds that never help other birds “Grudgers” – Birds that do not help known cheaters
If bird population starts out with only suckers and cheats, both categories become extinct over time
If bird population contains grudgers, eventually they dominate the population, and others become extinct
259
Hosts Bearing Grudges
Applying the “grudgers” concept to ad hoc networks
Each node determines whether its neighbor is misbehaving
• Similar to the previous scheme
A node ALARMs its “friends” when a misbehaving hosts is detected
Each node maintains reputation ratings for other nodes that are reduced on receipt of ALARMs
Ratings improve with time – a cheater can rehabilitate itself
260
Hosts Bearing Grudges: Issues
How to decide on friends?
What if “friends” cheat?
261
Hosts Bearing Grudges: Summary
Reputation-based scheme
Nodes prefer to route through & for nodes with higher reputation
Interesting concept, but cannot circumvent the difficulties in diagnosing misbehavior accurately
262
Exploiting Path Redundancy [Xue04]
Design routing algorithms that can deliver data despite misbehaving nodes
“Tolerate” misbehavior by using disjoint routes
Prefer routes that deliver packets at a higher “delivery ratio”
263
Exploiting Path Redundancy
Alternate routes: AFGE, ABCDE, ABFGE, ABCGE
B D
GE
A
F
C
264
Exploiting Path Redundancy
Misbehaving host F drops packets Delivery ratio poor on routes AFGE, ABFGE,
better on ABCDE, ABCGE
B D
GE
A
F
C
265
Best-Effort Fault Tolerant Routing (BFTR)– Modified DSR [Xue04]
The target of a route discovery is required to send multiple route replies (RREP)
The source can discover multiple routes (all are deemed feasible initially)
(1) The source chooses a feasible route based on the “shortest path” metric
(2) The source uses this route until its delivery ratio falls below a threshold (making the route infeasible)
(3) If existing route is deemed infeasible, go to (1)
266
BFTR: Issues
A route may look infeasible due to temporary overload on that route
The source may settle on a poorer (but feasible) route
No direct mechanism to differentiate misbehavior from lower capacity routes
This is both an advantage, and a potential shortcoming
267
Information Dispersal [Rabin89]
Map the N bit information F to n pieces, each N/m in size, such that any m pieces suffice to reconstruct original information
• Total size = n/m * N
Divide information F into N/m sequences of length m
S1 = (b1, …, bm)
S2 = (bm+1, …, b2m)
…
268
Information Dispersal
Choose n vectors ai = (ai1, …, aim)
Such that any set of m different vectors arelinearly independent
Let Fi = (ci1, ci2, …, ciN/m) 1<= i <= n
where cik = ai . Sk
Example: ci1 = ai.b1 + ai2.b2 + … + aim . bm
269
Information Dispersal [Rabin89]
Given m pieces, say, F1, …, Fm, we can reconstruct F as follows
Let A = (aij) 1<=i,j<= m
A . Sk’ = (c11, c21, …, cm1)’ ’ denotes transpose
Thus, knowing A and Fi= (ci1, ci2, …, ciN/m),we can recover S
270
Information Dispersal to Tolerate Misbehavior [Papadimitratos03]
Choose n node-disjoint paths to send the n pieces of information
Use a route rating scheme (based on delivery ratios) to select the routes
Acknowledgements for received pieces are sent
The missing pieces retransmitted on other routes
Need to be able to detect whether packets are tampered with
271
Route Tampering Attack
A node may make a route appear too long or too short by tampering with RREQ in DSR
By making a route appear too long, the node may avoid the route from being used This would happen if the destination replies to multiple
RREQ in DSR
By making a route appear too short, the node may make the source use that route, and then drop data packets (denial of service)
272
Node Insertion
B
A
S EF
H
J
D
C
G
IK
Z
Y
M
N
L[S,E,P,Q,F]
[S,E]
273
Node Deletion
B
A
S EF
H
J
D
C
G
IK
Z
Y
M
N
L
[S,G,K][S,C,G]
274
Route Tampering Attack
Useful to allow detection of route tampering
Solution:
Protect route accumulated in RREQ from tampering
Removal or insertion of nodes should both be detected
275
Ariadne [Hu]: Detecting Route Tampering
Source-Destination S-D pairs share secret keys Ksd and Kds for each direction of communication
One-way hash function H available
MAC = Message Authentication Code (MAC) computed using MAC keys
276
Ariadne [Hu]: Detecting Route Tampering
Let RREQ’ denote the RREQ that would have been sent in unmodified DSR
Source S broadcasts RREQ = RREQ’,h0,[]where h0 = HMACKsd(RREQ’)
When a node X receives anRREQ = (RREQ’, hi, [m list])
it broadcasts RREQ, mi+1
where RREQ = (RREQ’, hi+1, [m list]), mi+1
where hi+1 = H(X, hi) and mi+1=HMACKx(RREQ)
277
Ariadne
If D receives an RREQ that came via route S, A, B, C, then D should have receivedh = H(C, H(B, H(A, HMACKsd(initial RREQ’))))
Knowing H and Ksd, and the node identifiers appended in the RREQ, D can verify accuracy of received h Relies on the inability to invert function H A mismatch indicates tampering with h or node list A match indicates that the h value corresponds to the node-list
Not enough to know whether the node-list is accurate
If no tampering detected in h, send RREP including node-list and m-list, and HMAC for this information
278
Ariadne Node D sends the RREP to node C (first node on reverse route)
Node C forwards to the next node towards the source, but also appends its key Kc to the message One key used per route discovery (TESLA mechanism).
S can verify authenticity of this key Alternate mechanisms: Use pair-wise shared secret keys, or
signatures using authentic public keys
Node S receives all the keys, and also the m-list in RREP
S can verify that all m values in the m-list are accurate, in addition to the HMAC computed by D
If all check out, then no tampering, else discard RREP
279
Ariadne
If HMAC checks, then no one tampered with the node-list and m-list in the RREP
If m-list checks, then the m values were computed by legitimate nodes when RREQ forwarded
If all OK, accept RREP
Use of m-list ensures that a host cannot tamper with the RREP Route in RREP is the route taken by RREQ and RREP
280
Ariadne: Issues
Ensuring that RREQ and RREP follow the known route does not ensure that the nodes on the route will deliver packets correctly
So this is not a sufficient solution (and some might argue, not necessary!)
281
Wormhole Attack [Hu]
In this attack, the attacker makes a wireless “link” appear in the network when there isn’t one
The attacker may achieve this by using an out-of-band channel, or a channel that cannot be detected by other hosts
Not necessarily detrimental, since the additional link can improve performance
But the attacker may cause the network to funnel traffic through this link, giving the attacker control on the fate of the traffic
282
Wormhole Attack [Hu]
Host X can forward packets from F and E unaltered Hosts F and E will seem “adjacent” to each other
B D
XE
A
F
C
283
Wormhole Attack [Hu]
With DSR, RREQ via AFXE will likely arrive at E soonest The RREQ will contain route AFE
When RREP from E reaches A, it will start using AFE The fact that AFE really is AFXE will not be detected
B D
XE
A
F
C
284
Wormhole Attack [Hu]
With DSR, RREQ via AFXE will likely arrive at E soonest The RREQ will contain route AFE
When RREP from E reaches A, it will start using AFE The fact that AFE really is AFXE will not be detected
B D
XE
A
F
C
285
Wormhole Attack [Hu]
Subsequently when A sends data along AFE, node X will not forward the data to E
B D
XE
A
F
C
286
Wormhole Attack: Issues
Not that simple to launch an undetected wormhole attack
If node F can “see” someone else sending packets with F specified as sender, the attack is detected Transmissions from X must be invisible to F
B D
XE
A
F
C
287
Wormhole Attack: Issues
Transmissions from X must be invisible to F Use directional transmissions at X to forward packets Difficult for X to guarantee that F will not see its
transmissions (depends on beamforms, multipath)
B D
XE
A
F
C
288
Wormhole Attack: Issues
Transmissions from X must be invisible to F Out-of-band collusion between two attackers X and Y Difficult for Y to guarantee that F will not see its
transmissions
B D
XE
A
F
C
Y
289
Wormhole Attack: Issues
Timing: F may expect an “immediate ACK” In the absence of authentication, X can ACK packets
to F without having delivered them to E With authentication, this is difficult
B D
XE
A
F
C
290
Timing Issue
Alternatively, the attacker must be able to forward bits as soon as it starts receiving them from F X transmits to E while receiving from F on the same channel
If no delays introduced, E and F may not detect the attack
B D
XE
A
F
C
291
Detected Attack
If timing issue cannot be resolved by the attacker ….
If X cannot deliver a timely ACK, the link E F will appear broken to E (because no ACK when expected)
Thus, even though E appears to receive RREQ from F, it cannot deliver packets to F
The attack will make the link F-E seem unidirectional (unreliable broadcast from F to E works, but not reliable unicast from E to F).
Mechanisms to handle unidirectional links (“blacklist”) can potentially suffice
292
Other Detection Mechanisms:Geographical Leashes
Geographical Leashes: Each transmission from a host should be allowed to propagate over a limited distance
If E and F are too far, F should reject packets that seem to be transmitted by E, even if received reliably
Need an estimate of distance between E and F (GPS locations + mobility during packet transmission)
293
Geographical Leashes [Hu]
Difficulty: Packets may travel along non line-of-sight paths Hard to predict the actual “distance” traveled by the
transmissions
Difficulty: A related problem is that physically close hosts may not be able to communicate directly (because of obstacles) The attacker may still introduce a tunnel (wormhole)
between these hosts However, the attacker needs the information that the two
hosts cannot see each other – difficult to get this information
294
Temporal Leashes
Assume tight clock synchronization (e.g., GPS)
Sender timestamps the packet, and receiver determines the delay since the packet was sent
If delay too large, reject the packet
The timestamps must be protected by some authentication mechanism or signature
295
Wormhole Attack: Summary
Not clear that this attack is easy to launch undetected• The attacker needs knowledge of propagation to be sure
of avoiding detection
Solutions dealing with unidirectional links may suffice in some cases
296
Anomaly Detection
297
Anomaly Detection
Anomaly detection: Detect deviation from “normal” behavior Need to characterize “normal” Normal behavior hard to characterize accurately Need to be able to determine when observed behavior
departs significantly from the norm Avoid false positives
The MAC layer approach for detecting deviation from “normal” distribution of contention window parameters can be considered an “anomaly detection” scheme
298
Anomaly Detection in Ad Hoc Networks [Zhang00]
Anomaly detection may also be useful at other layers, particularly, network layer
How to characterize “normal” routing protocol behavior?
Some of the routing mechanisms we discussed earlier do detect specific forms of abnormal behavior, but a more generic approach is desired
Can we design a protocol-independent anomaly detection mechanism? Not clear
299
Anomaly Detection
We limit our discussion here
Wireless harder than wired networks due to spatial and temporal variations
300
Attacks on Sensor Networks
Compromised sensors may provide erroneous sensor readings
Need to protect from spurious data, by exploiting redundancy offered by dense sensor deployment
Take “vote” among nearby sensors to determine appropriate value
Nearby sensors (even if all good) may not yield identical readings
The “vote” needs to account for this
301
Attacks on Sensor Networks
Intruder may gain access to sensor data transmitted over wireless channel Use encryption
How to set up keys at various sensors? Static assignment
• Example:– Each sensor pre-loaded with a private key
Dynamic assignment• Example:
– Each sensor pre-loaded with a set of public-private key pairs
– Adjacent sensors use a key that both are aware of
302
Outline
Introduction to ad hoc networks Selected routing protocols Selected MAC protocol mechanisms Security and misbehavior
Key management in wireless ad hoc networks Secure communication in ad hoc networks MAC layer issues Network layer issues
Conclusion & Related activities References
303
Conclusions
304
Conclusion
Security an important consideration for widespread deployment of wireless ad hoc networks
We discussed a sampling of topics in security and misbehavior in ad hoc networks
Some issues are similar to those in wired networks
The differences from wired network arise due to Shared nature of the wireless channel with variations over
space/time Inability to rely on access to “infrastructure” Ease of intrusion (relative to wired networks)
305
Conclusion
A lot of interesting research ongoing
One concern is that not all attacks are equally likely Attackers will typically go after the weakest feature
Nevertheless an important area of research with potential for future applications
306
Related Standards Activities
IETF MANET Working group IEEE 802.11 IEEE 802.16
307
Some Relevant Conferences/Workshops
ACM Wireless Security Workshop (WiSe) – held at ACM MobiCom last few years
Traditional security conferences (Security and Privacy, DSN, etc.)
Networking conferences: ACM MobiCom, ACM MobiHoc, IEEE INFOCOM, etc.
308
Thanks!
www.crhc.uiuc.edu/wirelessnhv@uiuc.edu
309
References
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[Buchegger] S. Buchegger and J. Le Boudec, Nodes Bearing Grudges: Towards Routing, Security, Fairness, and Robustness in Mobile Ad Hoc Networks,' in Proceedings of the Tenth Euromicro Workshop on Parallel, Distributed and Network-based Processing, IEEE Computer Society, January 2002.
[Cagalj05] M. Cagalj, S. Ganeriwal, I. Aad, and J. P. Hubaux : On Selfish Behavior in CSMA/CA Ad Hoc Networks, to appear at Infocom 20
[Capkun93] S. Capkun, L. Buttyan, and J. P. Hubaux, "Self-Organized Public-Key Management for Mobile Ad Hoc Networks“ IEEE Transactions on Mobile Computing, Vol. 2, Nr. 1 (January - March 2003)
[Chandra00] A. Chandra, V. Gummalla, and J. O. Limb, "Wireless Medium Access Control Protocols," IEEE Commun. Surveys [online], available at: http://www.comsoc.org/pubs/surveys, 2nd Quarter 2000.
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[Chaum] D. Chaum, Untraceable Electronic Mail, Return Addresses, and Digital Pseudonyms", Communications of the ACM, 1981.
[IEEE 802.11] IEEE 802.11 Specification, IEEE
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References
[Hu02] Y. Hu, A. Perrig, and D. Johnson, ``Ariadne: A secure on-demand routing protocol for ad hoc networks,'' in The 8th ACM International Conference on Mobile Computing and Networking, MobiCom 2002, pp.~12--23, September 2002.
[Hu03] Y.-C. Hu, A. Perrig, and D. B. Johnson, ``Packet leashes: A defense against wormhole attacks in wireless networks,'' in Proceedings of IEEE INFOCOM'03, (San Francisco, CA), April 2003.
[Jiang04] S. Jiang, N. H. Vaidya and W. Zhao, A Mix Route Algorithm for Mix-Net in Wireless Ad Hoc Networks, IEEE International Conference on Mobile Ad-hoc and Sensor Systems (MASS), October 2004.
[Jiang01] S. Jiang, N. H. Vaidya, W. Zhao, Preventing traffic analysis in packet radio networks, DISCEX 2001.
[Jiang05] S. Jiang, N. H. Vaidya, W. Zhao, in preparation, 2005 [Johnson] David B. Johnson and David A. Maltz. Protocols for Adaptive Wireless
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[Karn90] MACA - A New Channel Access Method for Packet Radio. Appeared in the proceedings of the 9th ARRL Computer Networking Conference, London, Ontario, Canada, 1990
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[Kyasanur], Pradeep Kyasanur and N. H. Vaidya, Selfish MAC Layer Misbehavior in Wireless Networks, to appear in the IEEE Transactions on Mobile Computing.
[Kyasanur03] P. Kyasanur and N. H. Vaidya, Detection and Handling of MAC Layer Misbehavior in Wireless Networks, Dependable Computing and Communications Symposium (DCC) at the International Conference on Dependable Systems and Networks (DSN) , June 2003.
[Papadimitratos03] Papadimitratos and Haas, Secure message transmission in mobile ad hoc networks, Ad Hoc Networks journal, 2003.
[Perrig] A. Perrig, TESLA Project, http://www.ece.cmu.edu/~adrian/tesla.html. [Rabin89] M. O. Rabin, Efficient dispersal of information for security, load
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[Xue04] Yuan Xue and Klara Nahrstedt, "Providing Fault-Tolerant Ad-hoc Routing Service in Adversarial Environments," in Wireless Personal Communications, Special Issue on Security for Next Generation Communications, Kluwer Academic Publishers, vol 29, no 3-4, pp 367-388, 2004
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