Sharad Jaiswal

Modeling peer-peer file sharing systems

Peer-peer networking has recently emerged as a new paradigm for building distributed networked applications. We develop simple mathematical models to explore and illustrate fundamental performance issues of peer-peer file sharing systems. The modeling framework introduced and the corresponding solution methods are flexible enough to accommodate different characteristics of such systems. Through the specification of model parameters, we apply our framework to three different peer-peer architectures: centralized indexing, distributed indexing with flooded queries, and distributed indexing with hashing directed queries. Using our model, we investigate the effects of system scaling, freeloaders, file popularity and availability on system performance. In particular, we observe that a system with distributed indexing and flooded queries cannot exploit the full capacity of peer-peer systems. We further show that peer-peer file sharing systems can tolerate a significant number of freeloaders without suffering much performance degradation. In many cases, freeloaders can benefit from the available spare capacity of peer-peer systems and increase overall system throughput. Our work shows that simple models coupled with efficient solution methods can be used to understand and answer questions related to the performance of peer-peer file sharing systems.

Inferring TCP connection characteristics through passive measurements

We propose a passive measurement methodology to infer and keep track of the values of two important variables associated with a TCP connection: the senders congestion window (cwnd) and the connection round trip time (RTT). Together, these variables provide a valuable diagnostic of end-user-perceived network performance. Our methodology is validated via both simulation and concurrent active measurements, and is shown to be able to handle various flavors of TCP. Given our passive approach and measurement points within a Tier-1 network provider, we are able to analyze more than 10 million connections, with senders located in more than 45% of the autonomous systems in todays Internet. Our results indicate that sender throughput is frequently limited by a lack of data to send, that the TCP congestion control flavor often has minimal impact on throughput, and that the vast majority of connections do not experience significant variations in RTT during their lifetime

Real-Time Video Multicast in WiMAX Networks

IEEE 802.16e WiMAX is a promising new technology for broadband access networks. Amongst the class of applications that can be supported is real time video services (such as IPTV, broadcast of live events etc.). These applications are bandwidth hungry and have stringent delay constraints. Thus, scalable support for such applications is a challenging problem. To address this challenge, we consider a combination of approaches using multicast, layer encoded video and adaptive modulation of transmissions. Using these, we develop algorithms to ensure efficient, fair and timely delivery of video in WiMAX networks. The corresponding resource allocation problem is challenging because scheduling decisions (within a WiMAX base station) are performed in real-time across two dimensions, time and frequency. Moreover, combining layered video with appropriate modulation calls for novel MAC algorithms. We model the multicast resource allocation problem in WiMAX and demonstrate this problem to be NP-hard. We present a fast greedy algorithm that is (i) provably within a constant approximation of the optimal solution (based on a metric that reflects video quality as perceived by the user), and (ii) performs within 87-95% of the optimal as demonstrated by realistic simulations. We also demonstrate that our algorithm offers a 25% improvement over a naive algorithm. Moreover, in terms of the average rate received by each user, our algorithm out-performs the naive algorithm by more than 50%.

On the hierarchical structure of the logical Internet graph

The study of the Internet topology has recently received much attention from the research community. In particular, the observation that the network graph has interesting properties, such as power laws, that might be explored in a myriad of ways. Most of the work in characterizing the Internet graph is based on the physical network graph, i.e., the connectivity graph. In this paper we investigate how logical relationships between nodes of the AS graph can be used to gain insight to its structure. We characterize the logical graph using various metrics and identify the presence of power laws in the number of customers that a provider has. Using these logical relationships we define a structural model of the AS graph. The model highlights the hierarchical nature of logical relationships and the preferential connection to larger providers. We also investigate the consistency of this model over time and observe interesting properties of the hierarchical structure.

Identifying 802.11 traffic from passive measurements using iterative Bayesian inference

In this paper, we propose a classification scheme that differentiates Ethernet and WLAN TCP flows based on measurements collected passively at the edge of a network. This scheme computes two quantities, the fraction of wireless TCP flows and the degree of belief that a TCP flow traverses a WLAN inside the network, using an iterative Bayesian inference algorithm that we developed. We prove that this iterative Bayesian inference algorithm converges to the unique maximum likelihood estimate (MLE) of these two quantities. Furthermore, it has the advantage that it can handle any general K-classification problem given the marginal distributions of these classes. Numerical and experimental evaluations demonstrate that our classification scheme obtains accurate results. We apply this scheme to two sets of traces collected from two campus networks: one set collected from UMass in mid 2005 and the other collected from UConn in late 2010. Our technique infers that 4%-7% and 52%-55% of incoming TCP flows traverse an IEEE 802.11 wireless link in these two networks, respectively.

Do not embarrass: re-examining user concerns for online tracking and advertising

Recent studies have highlighted user concerns with respect to third-party tracking and online behavioral advertising (OBA) and the need for better consumer choice mechanisms to address these phenomena. We re-investigate the question of perceptions of third-party tracking while situating it in the larger context of how online ads, in general, are perceived by users. Via in-depth interviews with 53 Web users in India, we find that although concerns for third-party tracking and OBA remain noticeable amongst this population, other aspects of online advertising---like the possibility of being shown ads with embarrassing and suggestive content---are voiced as greater concerns than the concern of being tracked. Current-day blocking tools are insufficient to redress the situation: users demand selective filtering of ad content (as opposed to blocking out all ads) and are not satisfied with mechanisms that only control tracking and OBA. We conclude with design recommendations for enduser tools to control online ad consumption keeping in mind the concerns brought forth by our study.

Comparing the structure of power-law graphs and the Internet AS graph

In this work we devise algorithmic techniques to compare the interconnection structure of the Internet AS graph with that of graphs produced by topology generators that match the power-law degree distribution of the AS graph. We are guided by the existing notion that nodes in the AS graph can be placed in tiers with the resulting graph having an hierarchical structure. Our techniques are based on identifying graph nodes at each tier, decomposing the graph by removing such nodes and their incident edges, and thus explicitly revealing the interconnection structure of the graph. We define quantitative metrics to analyze and compare the decomposition of synthetic power-law graphs with the Internet-AS graph. Through experiments, we observe qualitative similarities in the decomposition structure of the different families of power-law graphs and explain any quantitative differences based on their generative models. We believe our approach provides insight into the interconnection structure of the AS graph and finds continuing applications in evaluating the representativeness of synthetic topology generators.

A New Channel Assignment Mechanism for Rural Wireless Mesh Networks

In this paper we present a new channel allocation scheme for IEEE 802.11 based mesh networks with point-to- point links, designed for rural areas. Our channel allocation scheme allows continuous full-duplex data transfer on every link in the network. Moreover, we do not require any synchronization across the links as the channel assignment prevents cross link interference. Our approach is simple. We consider any link in the network as made up of two directed edges. To each directed edge at a node, we assign a non-interfering IEEE 802.11 channel so that the set of channels assigned to the outgoing edges is disjoint from channels assigned to the incoming edges. Evaluation of this scheme in a testbed demonstrate throughput gains of between 50 - 100%, and significantly less end-to-end delays, over existing link scheduling/channel allocation protocols (such as 2P [11]) designed for point-to-point mesh networks. Formally speaking, this channel allocation scheme is equivalent to an edge-coloring problem, that we call the directed edge coloring (DEC) problem. We establish a relationship between this coloring problem and the classical vertex coloring problem, and thus, show that this problem is NP-hard. More precisely, we give an algorithm that, given k vertex coloring of a graph can directed edge color it using xi(k) colors, where xi(k) is the smallest integer n such that (lfloorn/2rfloor/n ) ges k.

Minimum Cost Topology Construction for Rural Wireless Mesh Networks

IEEE 802.11 WiFi equipment based wireless mesh networks have recently been proposed as an inexpensive approach to connect far-flung rural areas. Such networks are built using high-gain directional antennas that can establish long-distance wireless point-to-point links. Some nodes in the network (called gateway nodes) are directly connected to the wired internet, and the remaining nodes connect to the gateway(s) using one or more hops. The dominant cost of constructing such a mesh network is the cost of constructing antenna towers at nodes. The cost of a tower depends on its height, which in turn depends on the length of its links and the physical obstructions along those links. We investigate the problem of selecting which links should be established such that all nodes are connected, while the cost of constructing the antenna towers required to establish the selected links is minimized. We show that this problem is NP-hard and that a better than O(log n) approximation cannot be expected, where n is the number of vertices in the graph. We then present the first algorithm in the literature, for this problem, with provable performance bounds. More precisely, we present a greedy algorithm that is an O(log n) approximation algorithm for this problem. Finally, through simulations, we compare our approximation algorithm with both the optimal solution, and a naive heuristic.

VillageNet: A low-cost, 802.11-based mesh network for rural regions

VillageNet is a wireless mesh network that aims to provide low-cost broadband Internet access for rural regions. The cost of building the network is kept low by using off-the-shelf IEEE 802.11 equipment and optimizing the network topology to minimize cost. In this paper we describe the over-all operation of VillageNet and discuss two fundamental problems in building such a network. Nodes in VillageNet communicate using long-distance point-to-point wireless links that are established using high-gain directional antenna. VillageNet uses the 2P MAC protocol [?], that is suited for the interference pattern within such a network. However, the 2P protocol requires the underlying mesh graph (for each 802.11 channel) to be bi-partite. Thus, if K channels are available, then an important consideration is how to select K bi-partite subgraphs to activate, such that the demands of the nodes are best met. We formally pose this problem and present some initial results. Second, we observe that the dominant cost of constructing such a mesh network is the cost of constructing antenna towers at nodes. The cost of a tower depends on its height, which in turn depends on the length of its links, and the physical obstructions along those links. Thus to minimize cost, we pose the problem of deciding which links should be established, such that all villages are connected and the cost of constructing antenna towers to establish the selected links is minimized.