Transcript 422 Lecture Notes

Services for Mobile Users

Mobility was the requirement of the 90’s, 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.
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Introduction to Mobile Computing
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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.
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What Mobile Users Want
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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.
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The Mobile Computing Stack
Mobile User
Application
System Services
Network Services
Transport Medium
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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)
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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
 Other
(continued)
Challenges
device
design: size and weight, usability
energy conservation
security, authentication, authorization
application development
. . .
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Recent Research Projects
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Accommodating mobile host relocations
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Multicast support for mobile hosts
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with Carey Williamson , Tim Harrison, Wayne Mackrell
TCP over wireless links
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with Carey Williamson, Vineet Chikarmane, Wayne Mackrell
with Venkat Josyula
File system support for mobile users
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with Kevin Froese
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Accommodating Mobile Host Relocations
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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).

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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
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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.

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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.

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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.
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Datagram forwarding
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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
IP header
To: care-of address
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IP header
To: mobile node
datagram payload
FA receives encapsulated datagrams, then decapsulates
them and delivers them to MN
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Mobile IP: Routing
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Integrating Wireless Access
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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
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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
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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
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Problems with TCP in a wireless environment:
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TCP congestion management: uses loss as congestion indicator
TCP timers: use delays for timeouts and retransmissions
Proposed solution:
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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
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sender retransmissions reduced
TCP semantics preserved
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Sample Measurement Results
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Retransmission Time-out Behaviour
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The Proxy Model
Sender
Receiver
Proxy
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Sample Simulation Results
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Impact of proxy on end-to-end throughput
Proxy ON
Proxy OFF
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Summary of Findings
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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.
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File System Support for Mobile Clients
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Location-independent computing characterised by
disconnection, movement to a new working location, reconnection
 type and quality of connection (to home file server) varies
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Mobile users want access to remotely stored
files, regardless of current type of connection.
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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
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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
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configuration issues, at the client and on the network
cache management strategies
demand characteristics
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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
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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
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Sample Results
Lotto
NoWB, PWB
Resource tradeoff
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Effect of write-back policy
(10 MB cache)
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Summary of Findings
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It is possible to provide quite acceptable
remote file service to weakly connected
mobile clients, even when very little
bandwidth is available.
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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
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Mobile functionality is available now, but
performance remains an issue.
What the future holds:
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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
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But, there’s still much work to be done to get us
there.
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