Services for Mobile Users

1
Services for Mobile Users

• Mobility was the requirement of the 90s, first
  in communications and then in computing.
• rapidly growing demand by users
• many interested players
• equipment manufacturers, infrastructure and
  service providers
• Current technology (devices, access) makes mobile
  computing feasible, but present support for it is
  limited.

2
Introduction to Mobile Computing

• 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.

3
What Mobile Users Want

• Seamless mobility
• connect from any location, at any time
• convenience 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.

4
The Mobile Computing Stack
Mobile User
5
Technical 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)

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Technical Challenges (continued)

• Other Challenges
• device design size and weight, usability
• energy conservation
• security, authentication, authorization
• application development
• . . .

7
Recent 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

8
Accommodating Mobile Host Relocations

• The problem
• IP routing is based on the network component of a
  hosts 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.

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Mobile IP An Emerging Standard

• 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.

10
Mobile 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 agents
• IP-in-IP encapsulation
• Mobile unit deregisters (explicitly or
  implicitly) upon leaving foreign network.

11
Datagram forwarding

• HA tells local nodes and routers to send MNs
  datagrams to it
• HA intercepts datagrams intended for MN, then
  encapsulates and forwards them to MNs care-of
  address
• FA receives encapsulated datagrams, then
  decapsulates them and delivers them to MN

12
Mobile IP Routing
13
Integrating 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.

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Wireless 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

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TCP in a Wireless Environment

• 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

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Sample Measurement Results

• Retransmission Time-out Behaviour

17
The Proxy Model
Sender
Receiver
Proxy
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Sample Simulation Results

• Impact of proxy on end-to-end throughput

Proxy ON
Proxy OFF
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Summary 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 TCPs response to it and thus
  improve end-to-end performance.

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File System Support for Mobile Clients

• 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.

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File Caching for Mobile Computing

• 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

22
File System Operation
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Strongly 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

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Disconnected Operation (CODA file system)

• no connection to home file server
• users 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

25
Weakly 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

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Project 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

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Sample Results
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Summary 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.

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The Future Wearable Computers
http//www.media.mit.edu/wearables/
A whole new meaning to the term smarty pants
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Concluding 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, theres still much work to be done to get us
  there.