We consider the problem of providing vehicular Internet access using roadside 802.11 access points. We build on previous work in this area [18, 8, 5, 11] with an extensive experimental analysis of protocol operation at a level of detail not previously explored. We report on data gathered with four capture devices from nearly 50 experimental runs conducted with vehicles on a rural highway. Our three primary contributions are: (1) We experimentally demonstrate that, on average, current protocols only achieve 50% of the overall throughput possible in this scenario. In particular, even with a streamlined connection setup procedure that does not use DHCP, high packet losses early in a vehicular connection are responsible for the loss of nearly 25% of overall throughput, 15% of the time. (2) We quantify the effects of ten problems caused by the mechanics of existing protocols that are responsible for this throughput loss; and (3) We recommend best practices for using vehicular opportunistic connections. Moreover, we show that overall throughput could be significantly improved if environmental information was made available to the 802.11 MAC and to TCP. The central message in this paper is that wireless conditions in the vicinity of a roadside access point are predictable, and by exploiting this information, vehicular opportunistic access can be greatly improved.
A multiprocessor system is unlikely to have access to information about the execution characteristics of the jobs it is to schedule. In this work, we are interested in scheduling algorithms for batch jobs that require no such knowledge (such algorithms are called non-
This paper introduces Time-lined TCP (TLTCP). TLTCP is a protocol designed to provide TCP-friendly delivery of time-sensitive data to applications that are loss-tolerant, such as streaming media players. Previous work on unicast delivev of streaming media over the Internet proposes using UDP and performs congestion control at the user level by regulating the application's sending rate (attempting to mimic the behavior of TCP in order to be TCP-friendly). TLTCP, on the other hand, is intended to be implemented at the transport level, and is based on TCP with modifications to support time-lines. Instead of treating all data as a byte stream TLTCP allows the application to associate data with deadlines. TLTCP sends data in a similar fashion to TCP until the deadline for a section of data has elapsed; at which point the now obsolete data is discarded in favor of new data. As a result, TLTCP supports TCP-friendly delivery of streaming media by retaining much of TCP 's congestion control functionality. We describe an API f o r TLTCP that involves augmenting the recvmsg and sendmsg socket calls. We also describe how streaming media applications that use various encoding schemes like MPEG-1 can associate data with deadlines and use TLTCP's API. We use simulations to examine the behavior of TLTCP under a wide range of networks and workloads. We find that it indeed performs time-lined data delivery and under most circunzstunces bandwidth is shared equally among conipeting TLTCP and TCP Jlows. Moreover, those scenarios under which TLTCP appears to be unfriendly are those under which TCPJEows competing only with other TCPJlows do not share bandwidth equitably,
SUMMARYThe rapid proliferation of the World-Wide Web has been due to the seamless access it provides to information that is distributed both within organizations and around the world. In this paper, we describe the design and implementation of a system, called Ajents, which provides the software infrastructure necessary to support a similar level of seamless access to organization-wide or world-wide heterogeneous computing resources.Ajents introduces class libraries which are written entirely in Java and that run on any standard compliant Java virtual machine. These class libraries implement and combine several important features that are essential to supporting distributed and parallel computing using Java. These features include: the ability to easily create objects on remote hosts, to interact with those objects through either synchronous or asynchronous remote method invocations, and to freely migrate objects to heterogeneous hosts. While some of these features have been implemented in other systems, Ajents provides support for the combination of all of these features using techniques that permit them to operate together in a fashion that is more transparent and/or and less restrictive than existing systems.Our experimental results show that in our test environment: we are able to achieve good speedup on a sample parallel application; the overheads introduced by our implementation do not adversely affect remote method invocation times; and (somewhat surprisingly) the cost of migration does not greatly impact the execution time of an example application.
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