1CMPT 422.3

Services for Mobile Services for Mobile UsersUsersServices for Mobile Services for Mobile UsersUsers

Mobility was the requirement of the 90’s, first in communications and then in computing.rapidly growing demand by usersmany interested players:

– equipment manufacturers, infrastructure and service providers

Current technology (devices, access) makes mobile computing feasible, but present support for it is limited.

2CMPT 422.3

Introduction to Mobile Introduction to Mobile ComputingComputing

Various definitions of mobile computing: not the same as wireless computing nomadic (or location-independent) computing

Our interest is in supporting users who work from multiple locations, and whose means of “connection” to their home system may take different forms at different times.

Emphasis to date has been on functionality, with little attention to performance.

3CMPT 422.3

What Mobile Users What Mobile Users WantWantWhat Mobile Users What Mobile Users WantWant

Seamless mobility“connect” from any location, at any timeconvenience of use (no extra setup, “plug and

play”)same computing environment, same services,

consistent interfaces, regardless of location Mobile users may be willing to sacrifice some

performance for mobility, but only some.

4CMPT 422.3

The Mobile Computing StackThe Mobile Computing Stack

Application

System Services

Network Services

Transport Medium

Mobile User

5CMPT 422.3

Technical ChallengesTechnical ChallengesTechnical ChallengesTechnical Challenges

Networking Challenges communications issues: protocols (old and new),

technologies (old and new) accommodating host relocations network services to mobile users (e.g., mobile multicast)

Operating System Challenges OS support for mobility oriented devices (e.g. intermittently

powered hard drives, limited resources) OS services for mobile clients (e.g., to ensure data

availability, data integrity)

6CMPT 422.3

Technical Challenges Technical Challenges (continued)(continued)Technical Challenges Technical Challenges (continued)(continued)

Other Challengesdevice design: size and weight, usability

energy conservationsecurity, authentication, authorizationapplication development. . .

7CMPT 422.3

Recent Research ProjectsRecent Research ProjectsRecent Research ProjectsRecent Research Projects

Accommodating mobile host relocations with Carey Williamson, Vineet Chikarmane, Wayne Mackrell

Multicast support for mobile hosts with Carey Williamson , Tim Harrison, Wayne Mackrell

TCP over wireless links with Venkat Josyula

File system support for mobile users with Kevin Froese

8CMPT 422.3

Accommodating Mobile Host Accommodating Mobile Host RelocationsRelocationsAccommodating Mobile Host Accommodating Mobile Host RelocationsRelocations

The problem: IP routing is based on the network component of a host’s IP

address, which is bound inextricably with its location. Moving to a new location means acquiring a new IP address

and then informing all “correspondents”. Roaming must be handled on an ad hoc case-by-case basis

(by individual users, system administrators, or both).

Mobile IP aims to provide for seamless relocation by providing services to mobile users as if they were at their home network.

9CMPT 422.3

Mobile IP: An Emerging Mobile IP: An Emerging StandardStandardMobile IP: An Emerging Mobile IP: An Emerging StandardStandard

Features of Mobile IP: Separates “location” from “address”. No new IP addresses or address formats required. Only “mobile aware” routers and mobile units need new

software. Other routers and hosts use current IP.

Impact of Mobile IP on users: Can take any computer to any location; routing of

communications from correspondents is done “automatically”.

Services provided as if at home network.

10CMPT 422.3

Mobile IP: How It WorksMobile IP: How It WorksMobile IP: How It WorksMobile IP: How It Works

Mobile unit registers with the foreign network upon arrival.

Home Agent and Foreign Agent cooperate to deliver IP datagrams to the mobile unit.forwarding caches at both agentsIP-in-IP encapsulation

Mobile unit deregisters (explicitly or implicitly) upon leaving foreign network.

11CMPT 422.3

Datagram forwardingDatagram forwarding

HA tells local nodes and routers to send MN’s datagrams to it HA intercepts datagrams intended for MN, then encapsulates

and forwards them to MN’s care-of address

FA receives encapsulated datagrams, then decapsulates them and delivers them to MN datagram payload

IP headerTo: mobile node

IP headerTo: care-of address

12CMPT 422.3

Mobile IP: RoutingMobile IP: RoutingMobile IP: RoutingMobile IP: Routing

13CMPT 422.3

Integrating Wireless AccessIntegrating Wireless Access

What are the implications of integrating wireless connections into the internetworking fabric?

Our focus was TCP, with emphasis on short range connections: tests of functionality and performance by

experiment and simulation.

14CMPT 422.3

Wireless ComputingWireless Computing

Existing wireless technologies (such as infrared, radio or cellular) can be employed for signal propagation

Can provide for tetherless computing Wireless links are characterized by

higher error rates, more lost packets, longer delays For wireless links to integrate seamlessly into the

internet, TCP must work well over wireless connections since TCP/IP is the basis for many current network applications

15CMPT 422.3

TCP in a Wireless TCP in a Wireless EnvironmentEnvironment Problems with TCP in a wireless environment:

TCP congestion management: uses loss as congestion indicator TCP timers: use delays for timeouts and retransmissions

Proposed solution: sender manages end-to-end packet transmission a (transparent) proxy looks after loss on the wireless link

– caches packets from sender for transmission over wireless link– performs retransmissions of dropped packets– ACKS from receiver flow through to sender

sender retransmissions reduced TCP semantics preserved

16CMPT 422.3

Sample Measurement Sample Measurement ResultsResultsSample Measurement Sample Measurement ResultsResults

Retransmission Time-out Behaviour

17CMPT 422.3

The Proxy ModelThe Proxy Model

Sender Receiver

Proxy

18CMPT 422.3

Sample Simulation ResultsSample Simulation Results

Impact of proxy on end-to-end throughput

Proxy OFF

Proxy ON

19CMPT 422.3

Summary of FindingsSummary of Findings

Design decisions within TCP present problems when applications run over wireless (lossy) links. These problems have a profound impact on end-to-end performance of the application.

While proxy solutions cannot affect the loss, they can control TCP’s response to it and thus improve end-to-end performance.

20CMPT 422.3

Location-independent computing characterised by disconnection, movement to a new working location,

reconnection type and quality of connection (to home file server) varies

Mobile users want access to remotely stored files, regardless of current type of connection. this research is focused on maintaining acceptable file access

performance across different forms of connection.

The ultimate distributed file system File caching at the client is fundamental to any

solution.

File System Support for Mobile File System Support for Mobile ClientsClientsFile System Support for Mobile File System Support for Mobile ClientsClients

21CMPT 422.3

File Caching for Mobile File Caching for Mobile ComputingComputing

Goal is to provide effective file system service to mobile clients.

Optimistic caching of file replicas at the client is a key to availability.

Project considered impact on performance of configuration issues, at the client and on the

network cache management strategies demand characteristics

22CMPT 422.3

File System OperationFile System OperationFile System OperationFile System Operation

23CMPT 422.3

Strongly Connected OperationStrongly Connected OperationStrongly Connected OperationStrongly Connected Operation

a high-bandwidth connection is available, over which read and write operations are serviced

file caching can improve performance (by reducing latency)

the conventional distributed file system

24CMPT 422.3

Disconnected OperationDisconnected Operation (CODA file (CODA file system)system)Disconnected OperationDisconnected Operation (CODA file (CODA file system)system)

no connection to home file serverusers optimistically hoard replicas of desired files

prior to disconnection all file operations processed in the cache

read misses are fatal updates to file system are logged at

the client upon reconnection, replay of logged

events reintegrates changes with home file system

25CMPT 422.3

Weakly Connected OperationWeakly Connected OperationWeakly Connected OperationWeakly Connected Operation

a low-bandwidth connection is available

read misses are no longer fatal asynchronous write backs provide for

reintegration of logged changes with home file system, but must share the bandwidth available with reads

reads should have priority

26CMPT 422.3

Project ObjectivesProject ObjectivesProject ObjectivesProject Objectives

To investigate performance issues relating to mobility-aware file caching using trace-driven simulations.configuration parameters:

– cache unit, cache size, bandwidth available

policy parameters: – what to write, when to write, read/write scheduling

performance measures

27CMPT 422.3

Sample ResultsSample Results

Resource tradeoff Effect of write-back policy (10 MB cache)

Lotto

NoWB, PWB

28CMPT 422.3

Summary of FindingsSummary of FindingsSummary of FindingsSummary of Findings

It is possible to provide quite acceptable remote file service to weakly connected mobile clients, even when very little bandwidth is available. Reads can be serviced in a timely manner. Even very simple write-back policies can provide

timely reintegration. Requires only reasonably sized caches at the

mobile client.

29CMPT 422.3

The Future: The Future: Wearable Wearable ComputersComputersThe Future: The Future: Wearable Wearable ComputersComputers

http://www.media.mit.edu/wearables/

A whole new meaning to the term “smarty

pants”

30CMPT 422.3

Concluding RemarksConcluding Remarks Mobile functionality is available now, but

performance remains an issue. What the future holds:

Better devices for mobile users. Seamless and transparent mobility. Better mobility infrastructure.

– Mobile IP everywhere: foreign agent capabilities at conference sites, hotels, airports, ...

– widespread support for wireless access: base stations on many networks

But, there’s still much work to be done to get us there.