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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 networks1
Unreliable communication

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

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

today s topics
Today’s Topics
  • Introduction
  • Modeling
  • Emulating a virtual node:
    • Convergent History Agreement
  • Emulating virtual infrastructure
wireless networks
Wireless Networks
  • Mobile nodes
    • Crash failures
    • Location updates
  • Unreliable wireless broadcast
    • Synchronous
    • Collision detection
    • Contention manager
wireless networks1
Wireless Networks
  • Broadcast: within some radius R.
  • Interference: up to some radius R’.

R’

R

wireless networks2
Wireless Networks
  • Eventually, if only one nearby node broadcasts in a round, then its message will be delivered.
wireless networks3
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
  • 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.
slide12

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
  • Mobile nodes
    • Crash failures
    • Location updates
  • Unreliable wireless broadcast
    • Synchronous
    • Collision detection
    • Contention manager
today s topics1
Today’s Topics
  • Introduction
  • Modeling
  • Emulating a virtual node:
    • Basic Idea
    • Convergent History Agreement (CHA)
    • CHA Protocol
  • Emulating virtual infrastructure
building virtual infrastructure2
Building Virtual Infrastructure

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
  • 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

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
  • 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
  • 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
  • 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
  • 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

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

Veto 1

Veto 2

Result

green

yellow

orange

red

Update State

Accept

Reject

update state1

Ballot

Veto 1

Veto 2

Result

green

yellow

orange

red

Update State

Accept

Tentative Accept

Tentative Reject

Reject

update state2

Ballot

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
  • Reconstructing the history

Example:

ballot history1
Ballot history
  • Reconstructing the history

Example:

ballot history2
Ballot history
  • Reconstructing the history

Example:

ballot history3
Ballot history
  • Reconstructing the history

Example:

ballot history4
Ballot history
  • Reconstructing the history

Example:

History:

1. propose(P)

2. propose(O)

3. propose(D)

4. propose(C)

=> PODC

chap output
CHAP Output
  • 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)
  • 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)
  • 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
  • Introduction
  • Modeling
  • Emulating a virtual node:
    • Convergent History Agreement
  • Emulating virtual infrastructure
virtual infrastructure1
Virtual Infrastructure

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
  • 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

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
  • Bounded round emulation
    • 10 + (size of schedule)
  • Constant overhead
    • After stabilization, constant message overhead.
    • Can be optimized further.
ongoing and future work1
Ongoing and Future Work
  • Optimization
    • Smaller messages
    • Less redundant communication
    • Shorter virtual rounds
  • Implementation aspects
    • Collision detectors
    • Contention managers
    • Synchronization
ongoing and future work2
Ongoing and Future Work
  • 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
  • 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
  • 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

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
ad