The poor performance of TCP in wireless networks is a well-known problem, and a large amount of research effort has been devoted to it. However, our own experiments show that existing solutions including split TCP and recently developed congestion control algorithms still suffer significant performance degradation in lossy environments, leaving considerable room for improvement. Rather than developing a more sophisticated congestion control algorithm, we explore a radically different approach: we instead propose to completely remove congestion control between the Wi-Fi access points and the wireless receivers. We introduce TCP Fixed, a TCP used in split TCP environments that eliminates congestion control for the last lossy hop, allowing TCP to send data as fast as the channel allows. Extensive evaluations in both lab and real-world settings demonstrate that our approach significantly improves TCP performance in the presence of packet losses. In addition, our results indicate that existing 802.11 rate adaptation schemes which strive to minimize frame loss unnecessarily decrease the data rate in the presence of TCP Fixed. These results offer new opportunities for future link-layer rate adaptation designs.

The Dynamic Host Configuration Protocol (DHCP) was introduced nearly 20 years ago as a mechanism for hosts to automatically acquire IP addresses. While the protocol remains the same, its usage has evolved, especially in the last decade with the introduction of mobile devices and wireless local area networks. In this paper we investigate the impact that new types of wireless devices, such as smartphones, have on DHCP. We use two one-month long traces, collected at a corporate and an educational network, and we compare side-by-side DHCP usage patterns. We develop a novel passive fingerprinting technique based on DHCP messages to determine the device type and operating system. We show that DHCP implementations vary among device types and have an effect on DHCP lease durations. To improve network address utilization, without introducing any protocol changes, we propose a new leasing strategy which takes into account device types. This strategy, compared to current approaches, improves the address utilization without considerably increasing DHCP overhead.

In this work we present the differences and similarities of the web browsing behavior in most common mobile platforms. We devise a novel Operating System (OS) fingerprinting methodology to distinguish different types of wireless devices (smartphone vs laptops) as well as operating system instances (iOS, Android, BlackBerry etc.). We showcase that most of the multimedia content in smartphone devices is delivered via Range-Requests, and a large portion of the video transfers are aborted. We also show that laptop devices have more intelligent browser caching capabilities. We investigate the impact of an additional browser cache, and demonstrate that a 10MB browser cache that is able to handle partial downloads in smartphones would be enough to handle the majority of the savings. Finally, we showcase that caching policies need to be amended to attain the maximum possible savings in proxy caches. Based on those optimizations the emulated proxy cache provides 10%-20% in bandwidth savings.

The scalability of modern data centers has become a practical concern and has attracted significant attention in recent years. In contrast to existing solutions that require changes in the network architecture and the routing protocols, this paper proposes using traffic-aware virtual machine (VM) placement to improve the network scalability. By optimizing the placement of VMs on host machines, traffic patterns among VMs can be better aligned with the communication distance between them, e.g. VMs with large mutual bandwidth usage are assigned to host machines in close proximity. We formulate the VM placement as an optimization problem and prove its hardness. We design a two-tier approximate algorithm that efficiently solves the VM placement problem for very large problem sizes. Given the significant difference in the traffic patterns seen in current data centers and the structural differences of the recently proposed data center architectures, we further conduct a comparative analysis on the impact of the traffic patterns and the network architectures on the potential performance gain of traffic-aware VM placement. We use traffic traces collected from production data centers to evaluate our proposed VM placement algorithm, and we show a significant performance improvement compared to existing general methods that do not take advantage of traffic patterns and data center network characteristics.

During the past twenty years the Domain Name System (DNS) has sustained phenomenal growth while maintaining satisfactory user-level performance. However, the original design focused mainly on system robustness against physical failures, and neglected the impact of operational errors such as mis-configurations. Our measurement efforts have revealed a number of mis-configurations in DNS today: delegation inconsistency, lame delegation, diminished server redundancy, and cyclic zone dependency. Zones with configuration errors suffer from reduced availability and increased query delays up to an order of magnitude. The original DNS design assumed that redundant DNS servers fail independently, but our measurements show that operational choices create dependencies between servers. We found that, left unchecked, DNS configuration errors are widespread. Specifically, lame delegation affects 15% of the measured DNS zones, delegation inconsistency appears in 21% of the zones, diminished server redundancy is even more prevalent, and cyclic dependency appears in 2% of the zones. We also noted that the degrees of mis-configuration vary from zone to zone, with the most popular zones having the lowest percentage of errors. Our results indicate that DNS, as well as any other truly robust large-scale system, must include systematic checking mechanisms to cope with operational errors.

In this paper, we propose a novel duty cycling algorithm for a large-scale dense wireless sensor networks. The proposed algorithm is based on a social behavior of nodes in the sense that individual node's sleep/wakeup decision is influenced by the state of its neighbors. We analyze the behavior of the proposed duty cycling algorithm using a stochastic spatial process. In particular, we consider a geometric form of neighborhood dependence and a reversible Markov chain, and apply this model to analyze the behavior of the duty cycling network. We then identify a set of parameters for the reversible spatial process model, and study the steady state of the network with respect to these parameters. We report that our algorithm is scalable to a large network, and can effectively control the active node density while achieving a small variance. We also report that the social behavior of nodes has interesting and non-obvious impacts on the performance of duty cycling. Finally, we present how to set the parameters of the algorithm to obtain a desirable duty cycling behavior.

Basic properties of wireless ad-hoc networks, such as connectivity, have traditionally been analyzed and evaluated under the assumption that all nodes have homogeneous communication capabilities. While this assumption usually holds for small networks, it is unlikely to be true in most large scale networks. This paper studies how heterogeneous communication capabilities affect basic properties of ad-hoc networks. More specifically, we consider one type of heterogeneity in which nodes are equipped with two different types of antenna technologies, omni-directional and directional, and evaluate them against four properties: network connectivity, energy consumption, interference tolerance and resilience to failures. Our main results show that even under a very simple network model that does not assume any global knowledge of the network, an ad-hoc network where only 10% to 20% of the nodes are equipped with directional antennas, can outperform a comparable homogeneous network (where all nodes have only omni-directional antennas) in all four metrics.

The Domain Name System (DNS) is a critical Internet infrastructure that provides name to address mapping services. In the past few years, distributed denial of service (DDoS) attacks have targeted the DNS infrastructure and threaten to disrupt this critical service. In this paper we show that the existing DNS can gain significant resilience against DDoS attacks through a simple change to the current DNS operations, by setting longer time-to-live values for a special class of DNS resource records, the infrastructure records. These records are used to navigate the DNS hierarchy and change infrequently. Furthermore, in combination with a set of simple and incrementally deployable record renewal policies, the DNS service availability can be improved by one order of magnitude. Our approach requires neither additional physical resources nor any change to the existing DNS design. We evaluate the effectiveness of our proposed enhancement by using DNS traces collected from multiple locations.

Overlay networks are widely used as a way to introduce novel Internet services and to improve the reliability and performance of today's Internet. Traditionally, overlay networks perform routing by mimicking the underlying IP routing protocols, which does not scale well with the system size. In this paper, we propose a fundamentally new approach to overlay routing that is based on network coordinates. Our evaluation shows that this approach is scalable to a large number of overlay nodes with a message complexity of O(N) while providing routing decisions that are close to optimal in terms of error resilience and path delay.

In this paper, we measure the performance impact of anycast on DNS. We study four top-level DNS servers to evaluate how anycast improves DNS service and compare different anycast configurations. Increased availability is one of the supposed advantages of anycast and we found that indeed the number of observed outages was smaller for anycast, suggesting that it provides a mostly stable service. On the other hand, outages can last up to multiple minutes, mainly due to slow BGP convergence. We also found that anycast indeed reduces query latency. Furthermore, depending on the anycast configuration used, 37% to 80% of the queries are directed to the closest anycast instance. Our measurements revealed an inherent trade-off between increasing the percentage of queries answered by the closest server and the stability of the DNS zone, measured by the number of query failures and server switches. We believe that these findings will help network providers to deploy anycast more effectively in the future.

The current Domain Name System (DNS) follows a hierarchical tree structure. Several recent efforts proposed to re-implement DNS as a peer-to-peer network with a flat structure that uses Distributed Hash Tables (DHT) to improve the system availability. In this paper we compare the performance and availability of these two designs, enabled by caching and redundancy in both cases. We show that the caching and redundancy mechanisms in each design are closely bound to its system structure. We further demonstrate that each of the two system structures provides unique advantages over the other, while each has its own shortcomings. Using analysis and tracedriven simulations, we show that hierarchical structure enables high performance caching and that DHT structures provide high degree of robustness against targeted attacks. We further show that the current DNS design offers engineering flexibilities which have been utilized to optimize system performance under typical Internet failures and traffic loads, and which can be further extended to overcome DNS weaknesses against the aforementioned attacks.

We present the initial results from our evaluation study on the performance implications of anycast in DNS, using four anycast servers deployed at top-level DNS zones. Our results show that 15% to 55% of the queries sent to an anycast group, are answered by the topologically closest server and at least 10% of the queries experience an additional delay in the order of 100ms. While increased availability is one of the supposed advantages of anycast, we found that outages can last up to multiple minutes, mainly due to slow BGP convergence. On the other hand, the number of outages observed was fairly small, suggesting that anycast provides a generally stable service.

Overlay networks represent an emerging technology for rapid deployment of novel network services and applications. However, since public overlay networks are built out of loosely coupled end-hosts, individual nodes are less trustworthy than Internet routers in carrying out the data forwarding function. Here we describe a set of mechanisms designed to detect and repair errors in the data stream. Utilizing the highly redundant connectivity in overlay networks, our design splits each data stream to multiple sub-streams which are delivered over disjoint paths. Each sub-stream carries additional information that enables receivers to detect damaged or lost packets. Furthermore, each node can verify the validity of data by periodically exchanging Bloom filters, the digests of recently received packets, with other nodes in the overlay. We have evaluated our design through both simulations and experiments over a network testbed. The results show that most nodes can effectively detect corrupted data streams even in the presence of multiple tampering nodes.

During the past twenty years the Domain Name System (DNS) has sustained phenomenal growth while maintaining satisfactory performance. However, the original design focused mainly on system robustness against physical failures, and neglected the impact of operational errors such as misconfigurations. Our recent measurement effort revealed three specific types of misconfigurations in DNS today: lame delegation, diminished server redundancy, and cyclic zone dependency. Zones with configuration errors suffer from reduced availability and increased query delays up to an order of magnitude. Furthermore, while the original DNS design assumed that redundant DNS servers fail independently, our measurements show that operational choices made at individual zones can severely affect the availability of other zones. We found that, left unchecked, DNS configuration errors are widespread, with lame delegation affecting 15% of the DNS zones, diminished server redundancy being even more prevalent, and cyclic dependency appearing in 2% of the zones. We also noted that the degrees of misconfiguration vary from zone to zone, with most popular zones having the lowest percentage of errors. Our results indicate that DNS, as well as any other truly robust large-scale system, must include systematic checking mechanisms to cope with operational errors.

The Internet distance map service (IDMaps) [P. Francis, S. Jamin, C. Jin, D. Raz, Y. Shavitt, and L. Zhang, 2001] provides distance estimates between any pair of hosts connected to the Internet. The IDMaps system comprises two component types: tracers that measure distance between IP address prefixes, and servers that collect measurement results and answer distance queries. The distance information protocol (DIP) is used for tracers to report measured distance data to servers. The dynamics on the Internet topology, the distributed nature of autonomous tracers and servers, and the vast size of the data set require that DIP provide highly adaptive and scalable data dissemination from tracers to servers. DIP is a soft-state announce/listen protocol and scales independently from the total amount of measurement data by all tracers. DIP achieves its scalability through combination of staged timers, positive feedback, and feedback suppression techniques, which enable DIP to disseminate only the most useful measurement data to servers in a dynamic way. Simulations verified DIP's scalability and adaptability under various network conditions.