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Virtual Infrastructure for Collision-Prone Wireless Networks. Seth Gilbert Gregory Chockler Nancy Lynch. Wireless Ad Hoc Networks. Unreliable communication Contention Collisions Noise Lost messages. Unknown Availability Fault-prone devices

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Virtual infrastructure for collision prone wireless networks

Virtual Infrastructure for Collision-Prone Wireless Networks

Seth Gilbert

Gregory Chockler Nancy Lynch


Wireless ad hoc networks
Wireless Ad Hoc Networks Wireless Networks


Wireless ad hoc networks1

Unreliable communication Wireless Networks

Contention

Collisions

Noise

Lost messages

Unknown Availability

Fault-prone devices

Ad hoc deployments

Unknown topology

Unknown participants

Mobility

Dynamic joining/leaving

Wireless Ad Hoc Networks


Fixed infrastructure
Fixed Infrastructure Wireless Networks

For Example:

Simplifies wireless networks:

Communication

  • Avoids contention and collisions

    Unknown availability

  • Reliable overlay

  • Known topology

    Mobility

  • Fixed and predictable

cell towers

base stations

servers


Virtual infrastructure
Virtual Infrastructure Wireless Networks


Today s topics
Today’s Topics Wireless Networks

  • Introduction

  • Modeling

  • Emulating a virtual node:

    • Convergent History Agreement

  • Emulating virtual infrastructure


Wireless networks
Wireless Networks Wireless Networks

  • Mobile nodes

    • Crash failures

    • Location updates

  • Unreliable wireless broadcast

    • Synchronous

    • Collision detection

    • Contention manager


Wireless networks1
Wireless Networks Wireless Networks

  • Broadcast: within some radius R.

  • Interference: up to some radius R’.

R’

R


Wireless networks2
Wireless Networks Wireless Networks

  • Eventually, if only one nearby node broadcasts in a round, then its message will be delivered.


Wireless networks3
Wireless Networks Wireless Networks

Bob either:

  • Gets both messages

    or

  • Detects a collision

Bob

Collin

Alice

CD

A-C: complete, eventually accurate:

  • Detect if any message is lost.

  • Eventually no false positives (after stabilization).


Collision prone network
Collision-Prone Network Wireless Networks

  • Eventually reduces contention/collisions

    Example: backoff protocol

  • Advice only: nodes can ignore.

  • Eventually, if only 1 node broadcasts, then no collisions (after “stabilization”).

Each node contends when it wants to broadcast.

Each contention manager outputs advice:

  • active: it is safe for the mobile node to broadcast

  • passive: the mobile node should not broadcast.


Wireless Networks Wireless Networks

Eventually non-interfering:

  • Eventually, nearby nodes are not advised to be active.

    Eventually regionally fair:

  • Eventually, if:

    • Only nearby nodes contend.

    • Any non-failed node contends for sufficiently long.

  • Then:

    • Somenon-failed, contending node is advised to be active for sufficiently long.


Wireless networks4
Wireless Networks Wireless Networks

  • Mobile nodes

    • Crash failures

    • Location updates

  • Unreliable wireless broadcast

    • Synchronous

    • Collision detection

    • Contention manager


Today s topics1
Today’s Topics Wireless Networks

  • Introduction

  • Modeling

  • Emulating a virtual node:

    • Basic Idea

    • Convergent History Agreement (CHA)

    • CHA Protocol

  • Emulating virtual infrastructure




Building virtual infrastructure2
Building Virtual Infrastructure Wireless Networks

Leader / backup

Leader sends & receives messages for the virtual node

Each participant is a replica.

Replicas execute a consistency protocol


Consistency first attempt
Consistency: First Attempt Wireless Networks

  • For each virtual round:

    • Run consensus.

    • Replicas agree on:

      • Messages for virtual node to receive.

      • Message for virtual node to send.

      • State update.

    • Update state.

    • Leader sends message for virtual node.


First attempt
First Attempt Wireless Networks

Problems:

  • Consensus does not terminate until collisions stop.

    • Virtual rounds are non-constant sized

    • Rounds are unsynchronized!

  • Virtual nodes do not detect collisions

    • Delay until collisions stop.


Goals
Goals Wireless Networks

  • Can communicate with both real and virtual nodes.

  • Detect collisions when a message is lost.

  • Emulation of virtual nodes is transparent.

  • Rounds are synchronized

  • Rounds are constant length

  • In each round, replicas have a consistent view

  • Constant-sized messages


Convergent history agreement
Convergent History Agreement Wireless Networks

  • Sequence of instances:

  • For each instance k:

    • Each node proposes an input.

    • Each node receives a history output h, or .

CHA

k4

h1

propose(x)

h2

propose(y)

propose(z)

CHA

k7

CHA

k5

CHA

k3

CHA

k2

CHA

k6

CHA

k1

CHA

k4


Convergent history agreement1
Convergent History Agreement Wireless Networks

  • A history is a sequence of proposals, one per instance, e.g.:

    Validity: Each history contains only real proposals.

    Agreement: Every pair of histories shares a common prefix of proposals/non-proposals.

    Liveness: Eventually, every instance outputs histories, and every element in the history is a proposal.

CHA

k7

CHA

k5

CHA

k3

CHA

k2

CHA

k6

CHA

k1

CHA

k4


Emulating a virtual round
Emulating a Virtual Round Wireless Networks

  • Determine proposals.

    • Mobile nodes broadcast their messages for the virtual round.

    • Calculate virtual node’s broadcast message, using most recent history, extended by collisions.

  • Execute convergent history agreement (CHA).

  • If CHA outputs a history, then:

    • Simulate receiving virtual node’s broadcast.

  • Otherwise:

    • Report a collisions.


Cha protocol
CHA Protocol Wireless Networks

Each client begins with a proposal for the current instance.

Round 1: [Ballot]

  • Ifadvice=activethenbroadcast(ballot)

    Round 2: [Veto-1]

  • If“collision”thenbroadcast(veto)

    Round 3: [Veto-2]

  • If“collision”thenbroadcast(veto)

  • Update state


Update state

Ballot Wireless Networks

Veto 1

Veto 2

Result

green

yellow

orange

red

Update State

Accept

Reject


Update state1

Ballot Wireless Networks

Veto 1

Veto 2

Result

green

yellow

orange

red

Update State

Accept

Tentative Accept

Tentative Reject

Reject


Update state2

Ballot Wireless Networks

Veto 1

Veto 2

Result

green

yellow

orange

red

Update State

ballot={proposal, last green/yellow round}

Accept

Tentative Accept

Tentative Reject

Reject


Ballot history
Ballot history Wireless Networks

  • Reconstructing the history

    Example:


Ballot history1
Ballot history Wireless Networks

  • Reconstructing the history

    Example:


Ballot history2
Ballot history Wireless Networks

  • Reconstructing the history

    Example:


Ballot history3
Ballot history Wireless Networks

  • Reconstructing the history

    Example:


Ballot history4
Ballot history Wireless Networks

  • Reconstructing the history

    Example:

History:

1. propose(P)

2. propose(O)

3. propose(D)

4. propose(C)

=> PODC


Chap output
CHAP Output Wireless Networks

  • If round is green:

    • Output history.

    • Include proposal where indicated, otherwise.

  • Else if round is not green:

    • Output .


Analysis key idea 1
Analysis: Key Idea (1) Wireless Networks

  • If some instance is green, then all accept that instance’s ballot.

    • All histories agree on the same execution.

    • If a history is output for some instance, then all replicas receive the proposal for that instance.

  • If a ballot is red, then all the replicas reject it.

    • If a proposal is not received, then no history outputs it.

The color at any two replicas differs by at most one shade.


Analysis key idea 2
Analysis: Key Idea (2) Wireless Networks

  • When the underlying network is well-behaved, then there are no vetos.

  • Used to prove that the virtual node satisfies:

    • Eventual collision freedom.

    • Eventual accuracy of the collision detector.

    • Contention manager eventually well-behaved.

After the network, collision detector, and contention manager stabilize:

Eventually, every round is green.


Today s topics2
Today’s Topics Wireless Networks

  • Introduction

  • Modeling

  • Emulating a virtual node:

    • Convergent History Agreement

  • Emulating virtual infrastructure


Virtual infrastructure1
Virtual Infrastructure Wireless Networks

Problems:

  • Virtual nodes communicate with each other.

    • Sender adds message to send to proposal.

    • Receiver adds messages to receive to proposal.

    • Both run agreement.

    • If sender-agreement fails, then receiver cannot receive the message.

    • Too late!!

  • Virtual node emulators may interfere with each other.

    • Contention managers are insufficient.


Virtual infrastructure2
Virtual Infrastructure Wireless Networks

  • Choose a schedule for all the virtual nodes:

    • Non-conflicting: no neighboring virtual nodes scheduled at the same time.

    • Fair: every virtual node is scheduled.

  • Use schedule to avoid contention in the emulation.

    ** Not sufficient to emulate each virtual node according to the schedule, since virtual nodes communicate.


Virtual infrastructure3
Virtual Infrastructure Wireless Networks

Four part protocol:

  • Data phases:

    • Client phase: clients broadcast messages to vns, each other.

    • VN phase: representative emulator sends msg for vn.

  • Scheduled Agreement Instance

    • Scheduled virtual nodes run the 3 phase agreement protocol, all at once.

  • Unscheduled Agreement Instance

    • Unscheduled virtual nodes run the 3 phase agreement protocol.

    • Lasts for s +2 rounds, where s is the size of the schedule.

  • Join + Reset phases.


Virtual infrastructure4
Virtual Infrastructure Wireless Networks

  • Bounded round emulation

    • 10 + (size of schedule)

  • Constant overhead

    • After stabilization, constant message overhead.

    • Can be optimized further.


Ongoing and future work
Ongoing and Future Work Wireless Networks


Ongoing and future work1
Ongoing and Future Work Wireless Networks

  • Optimization

    • Smaller messages

    • Less redundant communication

    • Shorter virtual rounds

  • Implementation aspects

    • Collision detectors

    • Contention managers

    • Synchronization


Ongoing and future work2
Ongoing and Future Work Wireless Networks

  • Coordination Problems

    • Traffic Coordination

    • Air traffic control

    • Rescue worker / military scenarios

  • Control Problems

    • Actuated sensors

    • Real-time applications

  • Mobile Sensors

    • Floating / submergible devices

    • Zebranet-like scenarios


Ongoing and future work3
Ongoing and Future Work Wireless Networks

  • Malicious Devices

    • Can we emulate virtual infrastructure in the presence of non-cooperative mobile nodes?

    • Can we keep the state of the system secret from the participants?

    • Cryptography

    • Frequency hopping


Ongoing and future work4
Ongoing and Future Work Wireless Networks

  • Energy Efficiency

    • How efficiently can we implement virtual infrastructure?

    • Mobile nodes share the work of implementing the virtual nodes.

    • More efficient replication via coding theory techniques?


Summary
Summary Wireless Networks

Virtual Infrastructure

  • Simple abstraction for dependable ad hoc networks

  • Convergent History Agreement

  • CHA Protocol / VI Emulation

    • Collision detectors, contention managers

    • Efficient: constant overhead, constant time


The end
The End Wireless Networks


The end1
The End Wireless Networks


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