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
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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
5
Many Applications

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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
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


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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




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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
20

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



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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

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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
33
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
35
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 ?
40
Network Layer
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Reactive versus Proactive Routing
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Reactive protocols
Maintain routes between nodes that need to communicate

Proactive protocols
Maintain routes between all node-pairs
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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 ?
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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
52
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
63
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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