Reflections on ad-hoc and partially disconnected networks

1
Reflections on ad-hoc and partially disconnected
networks

• Henning Schulzrinne
• Suman Srinivasan
• Arezu Moghadam
• Andy Yuen
• Columbia University

2
Introduction

• Are ad-hoc and sensor networks the next active
  networks?
• What are the uses and users?
• What are missing pieces in the wireless puzzle?

3
Ad-hoc/sensor networks

• More research interest than application interest
• limited, mostly military applications
• always repeat the same handful of examples
• vineyards, glaciers,
• number of papers gtgt number of users
• cf. active networks
• brittle for regular users
• easily splits into disconnected sub-networks
• difficult to plan
• mesh networks early experiences dubious
• business model? (Singapore)
• reliability and availability
• frequency management in dense deployments

4
Whats missing?

• Lots of practical problems
• 802.11 configuration and debugging
• IETF experience 1500 engineers cant keep
  networks up and running
• manual channel assignment, no load balancing,
  gratuitous channel dropping ? application crash,
  long association delays
• no location information (cf. Skyhook)
• security mechanisms
• something other than typing in 16 hex digits
• opportunistic security and association (e.g., get
  token)
• 3G (IMS)
• configuration
• system complexity
• new applications?

5
A set of predictions

• WiMax for rural areas (water tower)
• 3G/4G ( 3G without the PSTN legacy) in
  (sub)urban areas and on major transportation
  corridors
• easier to deploy than mesh
• better power management
• but hard to deploy for non-carriers
• 2.5G in rural areas
• 802.11g/n indoors and as last-hop access
• cheap
• on every laptop
• reasonably fast
• easy to deploy

6
Motivation

• 802.11 currently hard to deploy across city or
  large area
• Problem How can mobile devices / gadgets get
  information?
• Peer-to-Peer data sharing Network
• Solution 7DS!

7
Wireless networks
access (802.11) cellular (3G) mesh ad-hoc sensor 7DS
speed 10 Mb/s 1 Mb/s 500 kb/s 500 kb/s? 100 kb/s 10 Mb/s
ubiquity islands (100) urban urban islands (500) islands (500) dispersed
density high high locally high low
dataflow sink sink mesh mesh sink sinks
power high medium high low -- medium low low
8
Illustration
In the absence of the Internet, nodes can
exchange information amongst themselves
Internet
9
7DS Overview

• Information Dissemination and Resource Sharing
• Disconnected
• No Global Network Connection
• Dynamically Changing Topology
• Reactive Routing
• Data-Centric
• Unattended Network
• Uses Multicast to propagate request

10
Network
11
System Architecture Proxy Server

• Proxy Server listens to the incoming HTTP
  Requests
• Peers user client uses localhost proxy server
  by default
• Query Multicast is sent through a Query Listener
  Scheduler
• SMTP Server listens to the incoming messages and
  dumps them up to the MTA

12
Search Engine

• Provides ability to query self for results
• Searches the cache index using Swish-e library
• Presents results in any of three formats HTML,
  XML and plain text
• Similar in concept to Google Desktop

13
Query Multicast Engine

• Used to actually exchange information among peers
• Requesting peer broadcasts a query to the network
• Responding peers reply if they have information
• Send encoded string with list of matching items
• Requesting peer retrieves suitable information

14
Email Delivery

• 7DS enables mobile nodes to discover each other
• and relay messages behaving as MTA.
• Each node calculates statistics and keeps track
  of
• each outgoing message using a database.

15
Node Discovery

• Zero-Configuration Network
• On-Demand Publishing and Discovering of Services
  
• Connection set up on-demand using zeroconf
  protocol
• Similar to AppleTalk, Microsoft NETBIOS, Novell
  IPX

Wireless Coverage
Wireless Coverage
Zero Configuration
Zero Configuration
AP
AP
16
Community Extensions (Proposal)
Users can generate and share content in the
spirit of Web 2.0
7DS Access Box at 116th Broadway
1. Users can contribute community information
2. Users can search for and read community
information
17
7DS in Cluster Networks

• Sparse scenario
• Heavily partitioned network opportunistic p2p
  data sharing
• Dense scenario
• full network connectivity multihop routing for
  communication
• Cluster network
• A cluster is an isolated island disconnected from
  the world
• Nodes within a cluster connected by multihop
  routes
• Network consists of multiple clusters
• Likely scenario since nodes are heterogeneous
  distributed
• Context Email delivery application
• Should we incorporate multihop forwarding to 7DS?

18
A Snapshot of a Cluster Network
AP
Route existsto connectto AP
No route toconnect toAP
19
Mean Cluster Size ECu(n)
2000 simulations n200 nodes uniformly
distributed

• ECu(n)A exp (B ?)
• ? denotes neighbors
• Least square fitting ? A0.9694 B0.9992
• Mean cluster size exponentially related to mean
  neighbors
• Percolation theory shows that many metrics are
  bounded by exponential function of node density
• We have identified bound is (almost) exact for
  ECu(n)

Small variance of sample meanof cluster size
Small variance of sample meanof cluster size
20
Email Delivery Application
Pc Prob. of connecting to AP nAP APs (nAP
n) n nodes (n200)

• If multihop route discovery fails to find AP,
  i.e. Pc lt1,
• it is likely ?lt4
• mean cluster size lt e?55
• If route discovery fails to find AP, it is likely
  cluster size is small
• Flooding cluster with replicas is justified
• Overhead for finding cluster boundary using MST
  is also small
• Always perform route discovery to find route to
  AP for immediate email delivery
• If no route is found, SRC node creates replicas
  according to message replication schemes

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3 Message Replication Schemes

• Boundary
• Nodes at cluster boundary are more likely to
  meet an AP
• Discover cluster boundary using MST or Dijkstra
  shortest path algorithm

• Gossiping
• Each node forwards amessage with some prob.
• No boundary discovery
• Most replicas are close to SRC, not boundary?
  inefficient

• Random Walk
• Source node creates m replicas
• Tx node deletes thereplica after successful
  transmission
• replicas independent of cluster size

22
Conclusions

• 7DS makes transparent data exchange, even in
  absence of Internet, possible
• Data Propagation through and out side of the
  local network
• By new nodes joining and others leaving 7DS
  Network.
• No user intervention unless absolutely necessary
• New step in practical, large-scale wireless
  networking with gadgets?
• Remains to be seen