Abstract not found

Caching in the World Wide Web is based on two critical assumptions: that a significant fraction of requests reaccess resources that have already been retrieved; and that those resources do not change between accesses. We tested the validity of these assumptions, and their dependence on characteristics of Web resources, including access rate, age at time of reference, content type, resource size, and Internet top-level domain. We also measured the rate at which resources change, and the prevalence of duplicate copies in the Web. We quantified the potential benefit of a shared proxycaching server in a large environment by using traces that were collected at the Internet connection points for two large corporations, representing significant numbers of references. Only 22% of the resources referenced in the traces we analyzed were accessed more than once, but about half of the references were to those multiplyreferenced resources. Of this half, 13% were to a resource that had been modifi...

Much work on the performance of Web proxy caching has focused on high-level metrics such as hit rates, but has ignored low-level details such as "cookies," aborted connections, and persistent connections between clients and proxies as well as between proxies and servers. These details have a strong impact on performance, particularly in heterogeneous bandwidth environments where network speeds between clients and proxies are significantly different than speeds between proxies and servers. We evaluate through detailed simulations the latency and bandwidth effects of Web proxy caching in such environments. We drive our simulations with packet traces from two scenarios: clients connected through slow dialup modems to a commercial ISP, and clients on a fast LAN in an industrial research lab. We present three main results. First, caching persistent connections at the proxy can improve latency much more than simply caching Web data. Second, aborted connections can waste more bandwidth than that saved by caching data. Third, cookies can dramatically reduce hit rates by making many documents effectively uncachable.

When a machine is connected to the Internet via a slow network, such as a 28.8 Kbps modem, the cumulative latency to communicate over the Internet to World Wide Web servers and then transfer documents over the slow network can be significant. We have built a system that optimistically transfers data that may be out of date, then sends either a subsequent confirmation that the data is current or a delta to change the older version to the current one. In addition, if both sides of the slow link already store the same older version, just the delta need be transferred to update it. Our mechanism is optimistic because it assumes that much of the time there will be sufficient idle time to transfer most or all of the older version before the newer version is available, and because it assumes that the changes between the two versions will be small relative to the actual document. Timings of retrievals of random URLs in the Internet support the former assumption, while experiments using a versi...

Much work in the analysis of proxy cachinghas focused on high-level metrics such as hit rates, and has approximated actual reference patterns by ignoring exceptional cases such as connection aborts. Several of these low-level details have a strong impact on performance, particularly in heterogeneous bandwidth environments such as modem pools connected to faster networks. Trace-driven simulation of the modem pool of a large ISP suggests that "cookies" dramatically affect the cachability of resources; wasted bandwidth due to aborted connections can more than offset the savings from cached documents; and using a proxy to keep from repeatedly opening new TCP connections can reduce latency more than simply caching data.

A number of techniques are available for reducing latency and bandwidth requirements for resources on the World Wide Web, including caching, compression, and delta-encoding [12]. These approaches are limited: much data on the Web is dynamic, for which traditional caching is of limited use, and delta-encoding requires both a common version base against which to apply a delta and the complete generation of the resource prior to encoding it. In contrast to these approaches, we take an application-specific view, in which we separate the static and dynamic portions of a resource. The static portions (called the template) can then be cached, with (presumably small) dynamic portions obtained on each access. Our HTML extension, which we refer to as HPP (for HTML Pre-Processing) supports resources that contain variable number of static and dynamic elements, such as query responses. Results with macro-encoding of query response resources from local CGI scripts and two popular search engines indicate that our approach promises a substan-Copyright to this work is retained by the authors. Permission is granted for the noncommercial reproduction of the complete work for educational or research purposes.

Despite traditional web caching techniques, redundant data is often transferred over HTTP links. These redundant transfers result from both resource modification and aliasing. Resource modification causes the data represented by a single URI to change; often, in transferring the new data, some old data is retransmitted. Aliasing, in contrast, occurs when the same data is named by multiple URIs, often in the context of dynamic or advertising content. Traditional web caching techniques index data by its name and thus often fail to recognize and take advantage of aliasing. In this work we present Value-Based Web Caching, a technique that eliminates redundant data transfers due to both resource modification and aliasing using the same algorithm. This algorithm caches data based on its value, rather than its name. It is designed for use between a parent and child proxy over a low bandwidth link, and in the common case it requires no additional message round trips. The parent proxy stores a small amount of soft-state per client that it uses to eliminate redundant transfers. The additional computational requirements on the parent proxy are small, and there are virtually no additional computational or storage requirements on the child proxy. Finally, our algorithm allows the parent proxy to serve simultaneously as a traditional web cache and is orthogonal to other bandwidth-saving measures such as data compression. In our experiments, this algorithm yields a significant reduction in both bandwidth usage and user-perceived time-to-display versus traditional web caching.

distributed computing services that essentially abstract the underlying network services to a monolithic “black box. ” In a mobile operating environment, the fundamental assumption of middleware abstracting a unified distributed service for all types of applications operating over a static network infrastructure is no longer valid. In particular, mobile applications are not able to leverage the benefits of adaptive computing to optimize its computation based on current contextual situations. In this paper, we introduce the Mobile Platform for Actively Deployable Service (MobiPADS) system. MobiPADS is designed to support context-aware processing by providing an executing platform to enable active service deployment and reconfiguration of the service composition in response to environments of varying contexts. Unlike most mobile middleware, MobiPADS supports dynamic adaptation at both the middleware and application layers to provide flexible configuration of resources to optimize the operations of mobile applications. Within the MobiPADS system, services (known as mobilets) are configured as chained service objects to provide augmented services to the underlying mobile applications so as to alleviate the adverse conditions of a wireless environment. Index Terms—Middleware, mobile applications, mobile computing support services, mobile environments. 1

The AT&T Internet Difference Engine (aide) is a system that finds and displays changes to pages on the World Wide Web. The system consists of several components, including a webcrawler that detects changes, an archive of past versions of pages, a tool called HtmlDiff to highlight changes between versions of a page, and a graphical interface to view the relationship between pages over time. This paper describes aide, with an emphasis on the evolution of the system and experiences with it. It also raises some sociological and legal issues.

Many objects, such as les, electronic messages, and web pages, contain overlapping content. Numerous past research projects have observed that one can compress one object relative to another one by computing the differences between the two, but these delta-encoding systems have almost invariably required knowledge of a specific relationship between them most commonly, two versions using the same name at different points in time. We consider cases in which this relationship is determined dynamically, by efficiently determining when a sufficient resemblance exists between two objects in a relatively large collection. We look at specific examples of this technique, namely web pages, email, and files in a file system, and evaluate the potential data reduction and the factors that influence this reduction. We find that delta-encoding using this resemblance detection technique can improve on simple compression by up to a factor of two, depending on workload, and that a small fraction of objects can potentially account for a large portion of these savings.