1 cmpt 422.3 services for mobile users mobility was the requirement of the 90’s, first in...
Post on 18-Dec-2015
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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
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
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
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.