An Architecture for Tetherless Computing

S. KeshavUniversity of Waterloo

October 21, 2004

Joint work with Aaditeshwar Seth and Patrick Darragh

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What if…

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Applications

Healthcaretell a doctor about latest test results

Environmental monitoringfly a plane over a site to collect data

Rural developmentbus on a rural route becomes a data mule

Personal productivityAlmost always in-sync iPod

Image sharing for migrant workers

Tetherless computing

Smart mobile devices that opportunistically communicate with resource-rich data centers over heterogeneously administered wireless and wireline networks

Internet cloud

Server

Data center

1. Computing costs are plummeting

From www.icknowledge.com

Processor costs have come down by six orders of magnitude in three decades

CMOS allows on-chip logic, memory, imaging and RF components

Devices will merge computing, audio, and video

•Cell phone•Still camera•Video camera•MP3 player•PDA

2. Batteries are lasting longer

3. Wireless networks are proliferating

© Intel

4. Data Centers aggregate resources

Requirements

Must build on existing Internet architectureMobility transparency

Address changes as a mobile host moves from one subnet to anotherHow to locate a mobile?

Disconnection resilienceTCP cannot sustain long duration disconnectionsShould not require both ends of a connection to be simultaneously present

Identity managementMobile should have the same identity no matter where it goesMobile and infrastructure should mutually authenticate

Access sensingIs a mobile in a hot spot in the first place?Important to make the right decision

Need low control overheadTo maximize use of opportunistic communication

Existing solutions have limitations

Network layer solutions: Mobile IP/HMIPv6Provide mobility transparencyBut do not provide disconnection transparency

Transport layer solutions: TCP Migrate/Rocks and RacksProvide both mobility and limited disconnection transparencyBoth endpoints need to be simultaneously present Only support short disconnectionsOnly work with TCP

Session layer solutions: DHARMA, PCMPProvide both mobility and disconnection transparencyBoth endpoints need to be simultaneously present Only work with TCP

Application layer solutions: Message ferrying, GnutellaApplication specificDo not deal with control overheads

Tetherless Computing Architecture

Disconnection tolerance using DTN

Sender

Receiver

DTN OverlayDTN router

DTN is just like email but…

Built-in support for disconnectionReactive fragmentationBoth ends can move

Better routingDynamic, instead of MX recordsCan be based on opportunistic or scheduled links

General purpose APISend()/Recv() like sockets

Support for multiple classes of service Better identity management

We hope… Avoid spam

Mobility transparency using a DHT

Sender

Internet Region

DHT Overlay

Receiver at R-1

Receiver at R-2

Mobile AP

Overview

Bundles sent to DTN addresses with GUIDsLocation management

DHT stores translations from GUID to DTN regionWithin a region store translation from a GUID to custodian

Late binding from GUID to destination region

Internet Region (R) Custodian (C) Mobile (I)(I, R) (I, C) TCP

Local DTN router (M) TCP

HLR VLRL LR

Fig. 1: Three tiered hierarchy of lookups in TCA

Details

When a bus drives past a mobile, it uses GUID to deliver bundles to the mobile, and picks up late-bound bundles from the mobileDTN routers with an interface on the Internet resolve late-bound addresses to a destination region gateway using DHTDestination region gateway maps from GUID to the custodian and forwards bundles to the custodianMobile picks up bundles from the custodianAfter a move, tables have to be carefully updated

‘Make then break’

Features

Sender can inject all its data into the network, whenever it gets a chanceReceiver can extract all waiting data from the network, whenever it gets a chanceFar notifications not needed in near movementsShort latency stretch

Innovations

Unbound destination address allows disconnected endpoints

Lookup in the forwarding path

GUID is MD5 hash of email addressProvides security without PKI or DNS

DHT for HLRMakes lookup scaleable and robust

Compatible with DTN routingNo extra overhead on headers or routing tables

Ongoing research

Solving underlying point problemsBuilding a prototypeBuilding real-world tetherless applications

1. Fundamental underlying problems

How to manage identities of mobile disconnected users?How do you know you’re in a hotspot?How to boost priority of communicating processes?

Identity management (Seth)

Use Hierarchical Identity Based EncryptionPublic Key = Hash (identity)Each user must get Private Key from a PK serverAllows disconnected operationCan limit need for revocation using timed keysWorking on a secure architecture for tetherless computing

Hot spot detection (Lifchits and Liang)

Monitor signal/noise strength of incoming beacons

sent out once every 100 ms

Hard to make the right choiceWe are still collecting raw data

sensitivity to positionsensitivity to orientationsensitivity to motion

Is signal strength the right indicator?

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Priority boosting (Liang)

Simple ideaBoost priority of communicating processes when in a hotspot

Complex implementation!We have implemented in Linux 2.4, Linux 2.6 and FreeBSDEach OS has its own quirks and needs a lot of careful kernel hackingCan show some improvement for each OS by using a combination of priority boosting, quantum size boosting, and memory locking

2. Building a prototype

Have successfully installed DTN on a variety of platforms (Darragh and Thomas)Have successfully integrated DTN with Internet Indirection Infrastructure (Darragh)First app is a ‘mobile blog’ (Fung and Darragh)Working on rewriting DTN bundle daemon (Ahmed)

Hardware platforms

Soekris single board computersx86 compatible, running Pebble Linuxpowered by lead acid cellruns DTN bundle daemon and I3 client

Sensor motesintegrated with DTN mote proxy

Sharp Zaurus 5500 PDAsSomewhat flaky hardware, but works OK most of the time

IPAQ 4150nice platform, but Linux support is missinginvestigating CELib

3. Applications

Medical applications (Hilliker)working with Guelph General Hospitalearly stages -- still learning context

Plant operations (Hilliker)simplifying workflow

UW Formula SAE racing team (Hilliker and Salmon)

working with on-board DAS to send information to the pit

Course projects

Integration with JabberFormula SAEMobilityVMPod

Conclusions

Tetherless vision will take years to realizeAttacking on several frontsProgress is encouragingMore at http://mindstream.watsmore.net