Services for Mobile Users

1. 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.

2. Introduction to Mobile Computing • Various definitions of mobile computing: • not the same as wireless computing • nomadic (orlocation-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. Application System Services Network Services Transport Medium 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)

6. 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 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.

9. 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. IP header To: care-of address IP header To: mobile node datagram payload Datagram 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

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.

14. 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

15. 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

16. Sample Measurement Results • Retransmission Time-out Behaviour

17. The Proxy Model Sender Receiver Proxy

18. Sample Simulation Results • Impact of proxy on end-to-end throughput Proxy ON Proxy OFF

19. 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 TCP’s response to it and thus improve end-to-end performance.

20. 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.

21. 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

23. 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

24. Disconnected Operation(CODA file system) • no connection to home file server • users optimisticallyhoard 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

26. 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

27. Lotto NoWB, PWB Effect of write-back policy (10 MB cache) Resource tradeoff Sample Results

28. 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.

29. The Future: Wearable Computers http://www.media.mit.edu/wearables/ A whole new meaning to the term “smarty pants”

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