Ahoy: A Proximity-Based Discovery Protocol Robbert Haarman Contents - - PowerPoint PPT Presentation

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Ahoy: A Proximity-Based Discovery Protocol Robbert Haarman Contents - - PowerPoint PPT Presentation

Nov 01, 2022 511 likes •1.24k views

Ahoy: A Proximity-Based Discovery Protocol Robbert Haarman Contents 1. Introduction to Ahoy 2. Protocol Overview 3. Message Types 4. Attenuated Bloom Filters 5. My Contributions 6. Summary 7. Questions Part 1 Introduction to Ahoy What is

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Ahoy: A Proximity-Based Discovery Protocol

Robbert Haarman

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Contents

  • 1. Introduction to Ahoy
  • 2. Protocol Overview
  • 3. Message Types
  • 4. Attenuated Bloom Filters
  • 5. My Contributions
  • 6. Summary
  • 7. Questions
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Part 1 Introduction to Ahoy

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What is Ahoy?

  • 1. Ahoy is a service discovery protocol.
  • 2. Ahoy is designed for mobile ad-hoc

networks.

  • 3. Ahoy is decentralized.
  • 4. Ahoy is efficient.
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What is Ahoy?

  • 1. Ahoy is a service discovery protocol.
  • 2. Ahoy is designed for mobile ad-hoc

networks.

  • 3. Ahoy is decentralized.
  • 4. Ahoy is efficient.
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Service Discovery

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What is Ahoy?

  • 1. Ahoy is a service discovery protocol.
  • 2. Ahoy is designed for mobile ad-hoc

networks.

  • 3. Ahoy is decentralized.
  • 4. Ahoy is efficient.
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Traditional Infrastructure Network

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Mobile Ad-Hoc Network

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Challenges

  • Services may be located multiple hops

away

  • Connectivity may change
  • Limited resources
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What is Ahoy?

  • 1. Ahoy is a service discovery protocol.
  • 2. Ahoy is designed for mobile ad-hoc

networks.

  • 3. Ahoy is decentralized.
  • 4. Ahoy is efficient.
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Decentralized

  • No reliance on a central authority
  • Helps deal with connectivity changes
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What is Ahoy?

  • 1. Ahoy is a service discovery protocol.
  • 2. Ahoy is designed for mobile ad-hoc

networks.

  • 3. Ahoy is decentralized.
  • 4. Ahoy is efficient.
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Efficiency

  • Ahoy sends few messages
  • Ahoy sends small messages
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How is Efficiency Accomplished?

  • Do not send all information to everyone
  • Do not send all information
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Part 2 Protocol Overview

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Protocol Overview

  • Tell where information can be found
  • When information is needed, find it
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Ahoy

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Ahoy

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Ahoy

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Ahoy

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Part 3 Message Types

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Five Message Types

  • Announcements
  • Queries
  • Responses
  • Keep-Alive Messages
  • Update Requests
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Five Message Types

  • Announcements
  • Queries
  • Responses
  • Keep-Alive Messages
  • Update Requests
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Announcements

  • Tell where to find service information
  • Multiple layers

– First layer: services of sending node – Second layer: services of sender's neighbors – And so on, up to configurable limit

  • Sent to all direct neighbors
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Distribution of Announcements

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Distribution of Announcements

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Distribution of Announcements

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Five Message Types

  • Announcements
  • Queries
  • Responses
  • Keep-Alive Messages
  • Update Requests
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Queries

  • Request service details
  • Contain service name
  • Are sent from neighbor to neighbor
  • Only to neighbors who know about the

service

  • Only to neighbors who are close to the

service

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Distribution of Queries

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Distribution of Queries

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Distribution of Queries

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Five Message Types

  • Announcements
  • Queries
  • Responses
  • Keep-Alive Messages
  • Update Requests
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Responses

  • Tell service details

– Specifically: IP address and port number

  • Sent from node offering service to node

sending query

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Response Distribution

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Response Distribution

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Response Distribution

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Five Message Types

  • Announcements
  • Queries
  • Responses
  • Keep-Alive Messages
  • Update Requests
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Keep-Alive Messages

  • “I'm still here!”
  • Sent to direct neighbors
  • Detect changes in topology
  • Detect missed announcements

– Through announcement id

  • No keep-alive received for a while:

– Information from neighbor is discarded

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Five Message Types

  • Announcements
  • Queries
  • Responses
  • Keep-Alive Messages
  • Update Requests
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Update Requests

  • “Could you repeat that?”
  • Sent when an announcement has been

missed

  • Causes announcement to be sent again
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Part 4 Attenuated Bloom Filters

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How To Tell Where Information Is

  • Announcements do not contain service

names

  • But they do tell where information can be

found

  • How?
  • Answer: Attenuated Bloom Filters
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Bloom Filters

  • Two operations:

– Adding an item – Testing if an item is present

  • Compact representation
  • Small chance of false positives
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Bloom Filter Implementation

  • Array of bits (initially all 0)
  • Set of hash functions
  • Each hash function maps a service name

to a bit in the array

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Bloom Filters: Adding Items

  • Apply each hash function to item
  • Set corresponding bits to 1
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Adding Service “printer”

  • Two hash functions

– First returns 1 – Second returns 5

  • Resulting Bloom filter:
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Bloom Filters: Item Present?

  • Apply each hash function to item
  • Test if corresponding bits are 1
  • If not all are 1, item is absent
  • If all are 1, item is probably present
  • Bits might be 1 because of other items
  • This is called a false positive
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Bloom Filters: Item Present

  • Bloom filter:
  • Service name “printer”
  • Two hash functions

– First returns 1 – Second returns 5

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Bloom Filters: Item Not Present

  • Bloom filter:
  • Service name “thermometer”
  • Two hash functions

– First returns 2 – Second returns 7

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Bloom Filters: False Positive

  • Bloom filter:
  • Service name “mail server”
  • Two hash functions

– First returns 5 – Second returns 1

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False Positives

  • False positives are bad
  • They cause queries to be sent
  • But no information will be found
  • Thus, resources are wasted
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Attenuated Bloom Filters

  • Multiple layers of Bloom filters
  • One layer for services of the sender
  • One layer for services of its neighbors
  • One layer for services of their neighbors
  • Etc.
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Combining Filters

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Combining Filters

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Combining Filters

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Combining Filters

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Attenuated Bloom Filters

  • Are small
  • Do not contain service names
  • But do tell which neighbor is likely to

know about a service

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Part 5 My Contributions

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My Contributions

  • Original idea from Geert, Fei, and Patrick
  • I implemented a prototype
  • I decided protocol details
  • I contributed some protocol

enhancements

  • I came up with the name
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The Prototype

  • Shows that Ahoy works
  • Forced details to be decided
  • Serves as a platform for further

experimentation

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Protocol Details

  • What message types exist?
  • What exactly do we put in them?
  • When do we send messages?
  • What technology do we build on top of?
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Protocol Enhancements

  • Original protocol sent announcements

periodically

  • I added keep-alive messages and update

requests

  • This saves resources:

– Keep-alive messages are 5 bytes – Announcements can be 100s or 1000s

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The Name

  • Ahoy is a pun on “Bonjour”, the service

discovery mechanism used by Apple

  • It also alludes to my fondness of sailing
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Part 6 Summary

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What is Ahoy?

  • Ahoy is an efficient, decentralized

service discovery protocol for mobile ad- hoc networks

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What Have I Contributed?

  • Prototype
  • Protocol Details
  • Protocol Enhancements
  • The Name
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What Have We Gained?

  • We know Ahoy works
  • We have an implementation
  • Design alternatives have been

documented

  • Other alternatives can be tried
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Part 7 Questions

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Thanks

  • Geert, Patrick, Fei, and Hartmut for their

ideas and feedback

  • Everyone for attending