Today's user community demands a level of mobility not previously anticipated by designers of computer systems and networks. The need to support and manage the computing activities of mobile users, whose point and form of attachment to an internetwork will vary over time, gives rise to many research and commercial challenges. Effective technical and management support for mobility at the system and network level will provide the basis for new classes of applications and, potentially, new classes of users.

A mobile (or nomadic) user is one who, on occasion, disconnects from his/her home network to change location, and then reconnects, possibly using a different access technology. A necessary feature of mobility management is the ability to continue to provide system and network services to mobile users seamlessly, regardless of their location and the form of their connection.

Our research addresses issues of functionality and performance. As a mobile user wanders around an internetwork, attaching to it at different locations at different times (perhaps in different ways), it is the responsibility of the network and system software to provide mobility support in a transparent and seamless fashion, but also in ways which mobile users do not incur unacceptable performance penalties. The projects can be grouped in 4 categories: routing (Mobile IP), file system support, TCP issues, and adaptivity.

Mobile IP: In a thesis completed in 1995, Vineet Chikarmane first addressed the problem of providing network layer routing support for mobile hosts. We selected IETF Mobile IP as the basis for further investigation and proceeded with a prototype implementation, both to gain first-hand practical experience with Mobile IP and to provide a platform for future projects in mobility. A paper entitled Implementing Mobile IP Routing in 4.3 BSD Unix: A Case Study describes this implementation and the issues that had to be addressed. Wayne Mackrell conducted a series of performance experiments with Mobile IP to determine how effectively it is able to meet the needs of higher level network traffic and its impact on networtks and routers.

In collaboration with Carey Williamson, Tim Harrison, and Wayne Mackrell, we also considered the provision of multicast services for mobile hosts by extending Mobile IP unicast through a protocol we called MoM. This work is described in a pair of papers entitled Multicast for Mobile Hosts using Mobile IP: Design Issues and Proposed Architecture and Mobile Multicast (MoM) Protocol: Multicast Support for Mobile Hosts

File System Support: We are also interested in issues relating to supporting the file service needs of mobile users, whose point and quality of connection to the network may be highly variable. A necessary feature of mobility management is the ability to continue to provide system and network services to mobile users, regardless of their location and the form of their connection. Access to data is a fundamental computing requirement. This project concerns the need to provide remote file service to mobile users in low bandwidth (weakly connected) and disconnected environments. Accommodation for periods of disconnection is typically made through caching or hoarding of files so that some degree of operability can be maintained. Write-backs of changed data are delayed until reconnection (introducing possible concerns for consistency) but cache misses cannot be processed (and may thus result in termination of the session). Weakly connected users are those connected through some low bandwidth medium (such as a modem or some wireless device). The small amount of bandwidth that is available (perhaps only intermittently) must be managed wisely. It can be used to process cache misses (thus maintaining operability) and/or it can be used to perform update processing (thus addressing the data consistency problem).

Our focus in this area was on file caching at the mobile client, in particular the design of appropriate cache semantics for weakly connected and disconnected operation, and the impact on performance of various caching parameters in these settings (eg., cache size, cache unit, and policies for replacement, prefetch, hoarding, and update of cached file data). A paper entitled Cache Management for Mobile File Service describes some of this work. Jiangmei Mei extended this research by investigating the feasibility of file cache management approaches that can adapt dynamically to changes in the state of connection, and Lixin Wang considered the provision of serverless file system support for nodes in an ad hoc network.

TCP: We also investigated issues arising from the introduction of wireless access technologies into the present networking environment. The emergence of wireless access technologies has sparked considerable research and commercial interest in their application to computing, as wireless capabilities offer enhanced opportunities for users to access network services from different places at different times. Effective technical and management support will be required at both the system and network level if these services are to be made available in a seamless and transparent manner.

Because the vast majority of network applications use TCP as the transport layer protocol for reliable delivery of packets across a network connection, the seamless integration of wireless links into the internet requires that TCP perform well over wireless connections. Venkat Josyula measured the performance of TCP over wireless connections under a range of induced circumstances, both to to identify problems that might exist in the specification of a protocol designed for wired connections and to evaluate proposed solutions.

Adaptivity: As circumstances change, both system support and applications must adapt if performance is to be maintained. For example, a web browser could elect not to download images when bandwidth is low or a file cache manager could elect to hold back updates. Jiangmei Mei studied a number of issues relating to adaptivity at the system support level and Xiaozhen Cao looked at adaptation at the application level.

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