TCP for Mobile and Wireless Hosts
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Transcript TCP for Mobile and Wireless Hosts
“Open” Problems in Mobile Ad Hoc Networking
Nitin Vaidya
University of Illinois at Urbana-Champaign
[email protected]
www.crhc.uiuc.edu/~nhv
Keynote talk presented at the Workshop on Wireless Local Networks
(in conjunction with 26th Conference on Local Computer Networks),
Tampa, Florida, November 14, 2001
© 2001 Nitin Vaidya
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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
2
Mobile Ad Hoc Networks
May need to traverse multiple links to reach a
destination
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Mobile Ad Hoc Networks (MANET)
Mobility causes route changes
4
Why Ad Hoc Networks ?
Potential ease of deployment
Decreased dependence on infrastructure
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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
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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)
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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 infrastructurebased network
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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
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Challenges
Limited wireless transmission range
Broadcast nature of the wireless medium
– Hidden terminal problem
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)
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Research on Mobile Ad Hoc Networks
Variations in capabilities & 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
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Hidden Terminals
&
RTS/CTS Handshake
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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
A
B
C
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RTS/CTS Handshake
Sender sends Ready-to-Send (RTS)
Receiver responds with Clear-to-Send (CTS)
RTS and CTS announce the duration of the transfer
Nodes overhearing RTS/CTS keep quiet for that duration
RTS/CTS used in IEEE 802.11
C
10
RTS (10)
A
B
CTS (10)
D
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Problems
in
Ad Hoc Networking
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Problem Space
Practical considerations
Consumer demand or lack thereof
Standardization
Government regulations
Technical issues
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Problem Space
Upper layers
Transport
Network
Link
Physical
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Physical Layer
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Physical Layer
Traditionally, not much interaction between physical
layer and upper layers
Many physical layer mechanisms not beneficial
without help from upper layers
Example: Adaptive modulation
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Adaptive Modulation
Channel conditions are time-varying
A
B
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Choose modulation scheme as a function of channel
conditions
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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 802.11-compliant implementations use a senderbased scheme for this purpose
Receiver-based schemes can perform better
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Sender-Based “Autorate Fallback” MAC Protocol
Sender decreases rate after N consecutive ACKS are not received
Sender increases rate after Y consecutive ACKS are received
C
DATA2Mbps
A
B
D
2Mbps
1Mbps
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Performance of Sender-Based
“Autorate Fallback”
BPSK (1Mbps)
QPSK (2Mbps)
CCK (5.5Mbps)
CCK (11Mbps)
Expected
ARF
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Receiver-Based Autorate MAC Protocol
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
C
1
RTS (2)
A
B
CTS (1)
D
2
2Mbps
1Mbps
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Physical Layer
Several other physical layer capabilities call for
changes to upper layers of protocol stack
Example: Power control
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Power Control
Transmit power determines
“Range” of a transmission
Interference caused at other nodes
A
B
C
D
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Power Control
Transmit power determines
“Range” of a transmission
Interference caused at other nodes
A
B
C
D
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Benefits of Power Control
Transmit a packet with least transmit power
necessary to deliver to the receiver
Save energy: Important benefit to battery-powered hosts
Reduce interference
Can allow greater spatial reuse
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Power Control
A
Power control introduces asymmetry
B
C
D
D transmits to C at low power, but B uses high transmit power to
transmit to A
B may not about D-to-C transmission, but can interfere with it
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Power Control
Proposals for medium access control and routing with
power control exist
Do not solve the problem satisfactorily
Ideal solution will
Reduce energy consumption, and
Maximize spatial reuse
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Directional / Smart Antennas
Various capabilities
Sectored antennas (fixed beam positions)
Beam steering
Tracking a transmitter
MAC and routing protocols for ad hoc networks using
such antennas
How to take into account antenna capabilities?
• Network may be heterogeneous
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Physical Layer
Are ad hoc networks benefiting from the progress
made at physical layer ?
Other interesting areas
Efficient coding schemes
Various diversity techniques
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Physical Layer: Simulation Models
Insufficient accuracy in commonly used physical layer
models
Physical link state is not binary as often assumed
Reliable packet reception does not depend just on
distance
Transmit power
Modulation scheme
Interference level
Coding
Fading
Need to use realistic models
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Link Layer
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Interesting Link Layer Issues
Medium access control
Retransmission mechanisms
Transmission scheduling
Which pending packet should a node attempt to transmit?
Adaptive parameter selection
Frame size
Retransmission limit
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QoS in Medium Access Control
Many proposals for achieving fairness
Fair scheduling schemes attempt to provide equitable
sharing of channel
Unpredictable nature of transmission errors makes it
difficult to make hard guarantees
Need to develop a probabilistic framework
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QoS in MAC
Easier in a centralized protocol (such as 802.11 point
coordination function), than in a distributed protocol
Distributed MAC appears more suitable for ad hoc
networks, however
Perhaps a hybrid protocol will be best
How to design such a protocol ?
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Transmission Scheduling
When multiple packets pending transmission, which
packet to transmit next?
Choice should depend on
Receiver status (blocked by some other transmission?)
Congestion at receivers
Noise level at receivers
Tolerable delay for pending packets
– Need interaction between upper layers and MAC
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MAC for Multiple Channels
How to split bandwidth into channels?
How to use the multiple channels ?
• Dedicated channel for control ?
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Network Layer
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Reactive versus Proactive Routing
Reactive protocols
Maintain routes between nodes that need to communicate
Proactive protocols
Maintain routes between all node-pairs
Lot of activity on routing protocol design
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Routing
Reactive and proactive protocols are quite
well-understood
Designing reactive protocols: “Solved” problem
Designing proactive protocols: “Solved” problem
At least, when using common assumptions about the
network
Interesting problems exist when other issues are considered
(such as QoS or physical layer properties)
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Reactive versus Proactive
Choice of protocol depends on
Mobility characteristics of the nodes
Traffic characteristics
How to design adaptive protocols ?
Existing proposals use a straightforward combination
of reactive and proactive
Proactive within “radius” K
Reactive outside K
Choose K somehow
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Reactive versus Proactive
Need a more flexible way to manage protocol
behavior
Assign proactive/reactive tag to each route (A,B) ?
How to determine when proactive behavior is better
than reactive ?
45
Address Assignment
How to assign addresses to nodes in an ad hoc
network ?
Static assignment
Easier to guarantee unique address
Dynamic assignment
How to guarantee unique addresses when partitions merge?
Do we need to guarantee unique addresses ?
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Transport Protocols
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TCP
TCP performance degrades in presence of route
failures
TCP cannot distinguish between packet losses due to
route change and due to congestion
Reduces congestion window in response
• Unnecessary degradation in throughput
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TCP
Several solutions have been proposed to fix this
These techniques somehow inform TCP sender that
the packet losses are due to route failure
TCP does not decrease congestion window in
response
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TCP
New route may differ significantly from old route
Proposals for TCP-over-ad-hoc tend to use old
timeout and congestion window after a route change
Does not seem like a good idea
How to choose appropriate timeout and congestion
window after detecting a route change ?
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Other Issues
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Algorithms
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Distributed Algorithms
Rich body of work on distributed algorithms in
traditional distributed environments
Shared memory
Message ordering
Clock synchronization
Leader election
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Distributed Algorithms
Existing algorithms can usually be used on ad hoc
networks without affecting correctness
Performance on ad hoc networks may not be good
Existing algorithm treat link repairs/failures as
random events
With mobility, link failure/repairs are correlated with
host movement
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Distributed Algorithms
How to design distributed algorithm exploiting the
correlation between mobility and link failure/repair ?
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Distributed Algorithms
Traditionally, complexity is measured as a function of
problem “size”
Number of nodes
Number of failures
How to analyze algorithm complexity as a function of
mobility ?
What measure of mobility is amenable to such an
analysis ?
Need to capture the correlation without making the measure
too complex
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Security Issues
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What’s New ?
Wireless medium easy to snoop on
With ad hoc networking, hard to guarantee
connectivity
Easier for intruders to insert themselves into network
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Authentication
How to authenticate a node ?
May not have access to a certification authority
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Resource Depletion Attack
Intruders may send data with the objective of
congesting a network or depleting batteries
U
B
intruder
C
A
D
T
Bogus traffic
intruder
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Routing Attacks
Intruders may mis-route the data
not delivering it to the destination at all, or
delaying it significantly
How to detect such attacks ?
How to tolerate such attacks ?
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Traffic Analysis
Despite encryption, an eavesdropper can identify
traffic patterns
Traffic patterns can divulge information about the
operation mode
Traffic analysis can be prevented by presenting
“constant” traffic pattern
– Insert dummy traffic
How to make this cheaper ?
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Other Issues
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Incentives for Ad Hoc Routing
Why should I forward packets for some other nodes ?
Need some incentive mechanism
Policies to determine reward for performing each
operation
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Applications
New applications for ad hoc networks ?
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Hybrid Environments
Use infrastructure when convenient
Use ad hoc connectivity when necessary or superior
infrastructure
BS1
BS2
E
A
Z
Ad hoc connectivity
X
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Summary
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Summary
Plenty of interesting research problems
Research community disproportionately obsessed
with routing protocols
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Summary
Interesting problems elsewhere at the two ends of the
protocol stack
Upper layers
How to design
algorithms and applications ?
How to exploit physical
layer techniques ?
• Increase interaction
between physical layer
and upper layers
Transport
Network
Link
Physical
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Summary
Hybrid environments require revisiting protocol
design decisions
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Tutorials
Visit http://www.crhc.uiuc.edu/~nhv for my tutorials
on
Mobile ad hoc networking
TCP over Wireless
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Thank you !!
Comments/questions to
[email protected]
© 2001 Nitin Vaidya
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