Reflections on ad-hoc and partially disconnected networks Henning Schulzrinne Suman Srinivasan Arezu Moghadam Andy Yuen Columbia University

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?

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 >> 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

What’s missing? Lots of practical problems configuration and debugging –IETF experience: 1500 engineers can’t 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?

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 g/n indoors and as last-hop access –cheap –on every laptop –reasonably fast –easy to deploy

Motivation  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!

Wireless networks access (802.11) cellular (3G) meshad-hocsensor7DS speed10 Mb/s1 Mb/s500 kb/s500 kb/s?100 kb/s10 Mb/s ubiquityislands (100’) urban islands (500’) islands (500’) dispersed densityhigh locally high low dataflowsink mesh sinksinks powerhighmediumhighlow -- medium low

Illustration In the absence of the Internet, nodes can exchange information amongst themselves Internet

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

Network Alice Bob 7DS Network Discovers 7DS Multicast Query Retrieved Object Internet ProxyMulticast SMTP ServerCache

System Architecture & Proxy Server  Proxy Server listens to the incoming HTTP Requests  Peer’s 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 Proxy Server Multicast Server Web Server (Mini HTTP) Search Engine SMTP Server Cache Manager MTA Relay To Next Client

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

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

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.

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 AP Zero Configuration Wireless Coverage AP Zero Configuration Wireless Coverage

Community Extensions (Proposal) 7DS Access Box at 116 th & Broadway 1. Users can contribute community information 2. Users can search for and read community information Users can generate and share content in the spirit of Web 2.0

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: delivery application  Should we incorporate multihop forwarding to 7DS?

A Snapshot of a Cluster Network AP Route exists to connect to AP No route to connect to AP

Mean Cluster Size E[C u (n)] E[C u (n)]=A exp (B  )  denotes # neighbors Least square fitting  A=0.9694; B= 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 E[C u (n)] Small variance of sample mean of cluster size 2000 simulations n=200 nodes uniformly distributed Small variance of sample mean of cluster size

Delivery Application  If multihop route discovery fails to find AP, i.e. P c <1,  it is likely  <4  mean cluster size < 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 delivery  If no route is found, SRC node creates replicas according to message replication schemes P c : Prob. of connecting to AP n AP : #APs (n AP « n) n: # nodes (n=200)

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 a message 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 the replica after successful transmission  # replicas independent of cluster size

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