Our proxyIn recent years, proxies have become more prevalent.Generally, these systems are used to process data tlowing between two endpoints using an intermediary.More specifically, they can be used to filter or process traffic flowing to and from a network-limited host.Benefits include more efficient use of network resources, reduced cost, and increased security.In this paper, we describe the design and capabilities of such a system, how it has been applied to the mobile environment, and provide an evaluation of the work.The environment for which our work is intended is shown in figure 1.In this figure a pair of mobile hosts are communicating with a number of fixed hosts.To simplify the discussion, we assume that an application (or "client") runs on the mobile host and communicates with a server on a fixed host, but the proxy mechanism can work with any two communicating endpoints located anywhere, including two mobile hosts.In our model of the environment, we assume the "last link" to the mobile host is the likely problem area.Currently and in the foreseeable future, this link will typically be slower, more costly, less reliable, and perhaps less secure than typical WANs or LANs.Accordingly, a proxy is placed somewhere on the opposite side of this link from the mobile host, and the two work together to overcome or attenuate the link's limitations.The proxy need not be physically attached to this last link."lids work was performed at Columbia University.

We advocate a new architecture for distributed systems supporting mobility. In this architecture an intermediary is interposed between client and server. Its purpose is to filter or delay all but the most essential data that would travel over the slow link to the mobile host, as instructed by the client. The goal of our work is to provide a general framework to ease the installation and management of application specific filtering mechanisms.

The performance of a high-speed commercial spread-spectrum wireless LAN that uses the CSMA/CA multiple-access strategy was studied. Using synthetic workloads, packet capture success rather than signal propagation characteristics was measured. Specifically throughput, packet loss rates, range, and patterns of errors within packets were measured. It is concluded that CSMA/CA is quite successful in allocating bandwidth under stress, but that packet capture rate degrades very quickly once the LAN's effective range is exceeded. Hence, network maintainers should plan the layout of wireless networks at least as carefully as they plan wired networks.>

The work habits of many individuals yield file access patterns that are quite pronounced and can be regarded as defining working sets of files used for particular applications. A client management technique for detecting these patterns and then exploiting them to successfully prefetch files from servers is described. Trace-driven simulations show the technique substantially increases file cache hit rate in a single-user environment. Successful file prefetching carries three major advantages: applications run faster, there is less burst load placed on the network, and properly loaded client caches can better survive network outages. The technique requires little extra code, and-because it is simply an augmentation of the standard LRU client cache management algorithm-is easily incorporated into existing software.>

We consider the problem of providing network access to hosts whose physical location clianges with time.Such hosts cannot depend on traditional forms of network connectivity and routing because their location, and hence the route to reach them, cannot be deduced from their network address.In this paper, we explore the concept of providing continuous network access to mobile computers, and present a set of IP-based protocols that achieve that goal.They are primarily targeted at supporting a campus environment with mobile computers, but also extend gracefully to accommodate hosts moving between different networks.The key feature is the dependence on ancillary machines, the Mobile Support Stations (MSSS), to track the location of the Mobile Hosts.Using a combination of caching, forwarding pointers, and timeouts, a minimal amount of state is kept in each MSS.The state information is kept in a distributed fashion; the system scales well, reacts quickly to changing topologies, and does not place an overwhelming burden on the network.

There is currently much interest in incorporating transactions into both operating systems and general-purpose programming languages. This paper provides a detailed examination of the design and performance of the transaction manager of the Camelot system. Camelot is a transaction facility that provides a rich model of transactions intended to support a wide variety of general-purpose applications. The transaction manager's principal function is to execute the protocols that ensure atomicity.

Facilities that support distributed transactions on user-defined types can be implemented efficiently and can simplify the construction of reliable distributed programs.To demonstrate these points, this paper describes a prototype transaction facility, called TABS, that supports objects, transparent communication, synchronization, recovery, and transaction management.Various objects that use the facilities of TABS are exemplified and the performance of the system is discussed in detail.The paper concludes that the prototype provides useful facilities, and that it would be feasible to build a high performance implementation based on its ideas.

The TABS prototype is an experimental facility that provides operating system-level support for distributed transactions that operate on shared abstract types. The facility is designed to simplify the construction of highly available and reliable distributed applications. This paper describes the TABS system model, the TABS prototype's structure, and certain aspects of its operation. The paper concludes with a discussion of the status of the project and a preliminary evaluation.