Distributed systems are notorious for harboring subtle bugs. Verification can, in principle, eliminate these bugs a priori, but verification has historically been difficult to apply at full-program scale, much less distributed-system scale.

Using measurements from VanLAN, a modest-size testbed that we have deployed, we analyze the fundamental characteristics of WiFi-based connectivity between basestations and vehicles in urban settings. Our results uncover a more complex picture than previous work which was conducted in more benign settings. The interval between a vehicle coming into and going out of range of a basestation is often marred by intermittent periods of very poor connectivity. These "gray periods" are hard to reliably predict because their arrival is not signaled by metrics such as signal strength, loss rate, speed or distance from the basestation. At the same time, they also do not consistently occur at the same spot. Our analysis suggests that gray periods are not caused by the motion of the vehicle per se but by the variability in the urban radio environment combined with the vehicle traversing locations that are poorly covered by the basestation. We also find that knowledge of past connectivity can be used to identify regions where gray periods are more likely to occur as well as regions where the vehicle is likely to experience good connectivit.

We present a framework for monitoring enterprise wireless networks using desktop infrastructure. The framework is called DAIR, which is short for Dense Array of Inexpensive Radios. We demonstrate that the DAIR framework is useful for detecting rogue wireless devices (e.g., access points) attached to corporate networks, as well as for detecting Denial of Service attacks on Wi-Fi networks.Prior proposals in this area include monitoring the network via a combination of access points (APs), mobile clients, and dedicated sensor nodes. We show that a dense deployment of sensors is necessary to effectively monitor Wi-Fi networks for certain types of threats, and one can not accomplish this using access points alone. An ordinary, single-radio AP can not monitor multiple channels effectively, without adversely impacting the associated clients. Moreover, we show that a typical deployment of access points is not sufficiently dense to detect the presence of rogue wireless devices. Due to power constraints, mobile devices can provide only limited assistance in monitoring wireless networks. Deploying a dense array of dedicated sensor nodes is an expensive proposition.Our solution is based on two simple observations. First, in most enterprise environments, one finds plenty of desktop machines with good wired connectivity, and spare CPU and disk resources. Second, inexpensive USB-based wireless adapters are commonly available. By attaching these adapters to desktop machines, and dedicating the adapters to the task of monitoring the wireless network, we create a low cost management infrastructure.

We present a new metric for routing in multi-radio, multi-hop wireless networks. We focus on wireless networks with stationary nodes, such as community wireless networks.The goal of the metric is to choose a high-throughput path between a source and a destination. Our metric assigns weights to individual links based on the Expected Transmission Time (ETT) of a packet over the link. The ETT is a function of the loss rate and the bandwidth of the link. The individual link weights are combined into a path metric called Weighted Cumulative ETT (WCETT) that explicitly accounts for the interference among links that use the same channel. The WCETT metric is incorporated into a routing protocol that we call Multi-Radio Link-Quality Source Routing.We studied the performance of our metric by implementing it in a wireless testbed consisting of 23 nodes, each equipped with two 802.11 wireless cards. We find that in a multi-radio environment, our metric significantly outperforms previously-proposed routing metrics by making judicious use of the second radio.

Routing protocols for wireless ad hoc networks have traditionally focused on finding paths with minimum hop count. However, such paths can include slow or lossy links, leading to poor throughput. A routing algorithm can select better paths by explicitly taking the quality of the wireless links into account. In this paper, we conduct a detailed, empirical evaluation of the performance of three link-quality metrics---ETX, per-hop RTT, and per-hop packet pair---and compare them against minimum hop count. We study these metrics using a DSR-based routing protocol running in a wireless testbed. We find that the ETX metric has the best performance when all nodes are stationary. We also find that the per-hop RTT and per-hop packet-pair metrics perform poorly due to self-interference. Interestingly, the hop-count metric outperforms all of the link-quality metrics in a scenario where the sender is mobile.

The Border Gateway Protocol (BGP) populates Internet backbone routers with routes or prefixes. We present an algorithm to locally compute (without any modification to BGP) equivalent forwarding tables that provably contain the minimal number of prefixes. For large backbone routers, the Optimal Routing Table Constructor (ORTC) algorithm that we present produces routing tables with roughly 60% of the original number of prefixes. The publicly available MaeEast database with 41315 prefixes reduces to 23007 prefixes when ORTC is applied. We present performance measurements on four publicly available databases and a formal proof that ORTC does produce the optimal set of routes.

Data copying and checksumming are the most expensive operations when doing high-bandwidth network IO over a high-speed network. Under some conditions, outboard buffering and checksumming can eliminate accesses to the data, thus making communication less expensive and faster. One of the scenarios in which outboard buffering pays off is the common case of applications accessing the network using the Berkeley sockets interface and the Internet protocol stack. In this paper we describe the changes that were made to a BSD protocol stack to make use of a network adaptor that supports outboard buffering and checksumming. Our goal is not only to achieve "single copy" communication for application that use sockets, but to also have efficient communication for in-kernel applications and for applications using other networks. Performance measurements show that for large reads and writes the single-copy path through the stack is significantly more efficient than the original implementation.

Achieving high-speed network I/O on distributed-memory systems is difficult because their architecture is in general ill-suited for communication processing. Some of the common problems are: inability to do protocol processing, inefficient handling of data distribution, and poor management of the I/O. The authors present an I/O architecture that addresses these problems and supports high-speed network I/O on distributed-memory systems. The key to good performance is to partition the work appropriately between the system and the network interface. The authors perform some communication tasks on the distributed-memory parallel system since it is more powerful, and less likely to become a bottleneck than the network interface. Tasks that do not parallelize well are performed on the network interface and hardware support is provided for the most time-critical operations. They emphasize the use of simple I/O mechanisms that can be used by programming tools that map applications on the distributed-memory system to implement efficient I/O for the class of applications they support. This architecture has been implemented for the iWarp distributed-memory system. The authors describe this implementation and present performance results.>

Multicomputersbuiltaround a general network are now a viable alternative to multicomputersbased on a system-speci~c interconnect because of architectural improvements in two areas.First, the host-network interface overhead can be minimized by reducing copy operations and host interrupts.Second, the network can provide high bandwidth and low latency by using high-speed crossbar switches and efficient protocol implementations.While still enjoying thejexibility of general networks, the resulting network-based multicomputers achieve high performance for typical multicomputer applications that use system-specijic interconnects.We have developed a network-based mtdticomputer called Nectar that supports these claims.

We have built a high-speed local-area network called Nectar that uses programmable communication processors as host interfaces. In contrast to most protocol engines, our communication processors have a flexible runtime system that supports multiple transport protocols as well as application-specific activities. In particular, we have implemented the TCP/IP protocol suite and Nectar-specific communication protocols on the communication processor. The Nectar network currently has 25 hosts and has been in use for over a year.