Mostafa H. Ammar

Algorithms for Assigning Substrate Network Resources to Virtual Network Components

Recent proposals for network virtualization provide a promising way to overcome the Internet ossification. The key idea of network virtualization is to build a diversified Internet to support a variety of network services and architectures through a shared substrate. A major challenge in network virtualization is the assigning of substrate resources to virtual networks (VN) efficiently and on-demand. This paper focuses on two versions of the VN assignment problem: VN assignment without reconfiguration (VNA-I) and VN assignment with reconfiguration (VNAII). For the VNA-I problem, we develop a basic scheme as a building block for all other advanced algorithms. Subdividing heuristics and adaptive optimization strategies are then presented to further improve the performance. For the VNA-II problem, we develop a selective VN reconfiguration scheme that prioritizes the reconfiguration of the most critical VNs. Extensive simulation experiments demonstrate that the proposed algorithms can achieve good performance under a wide range of network conditions.

A reputation system for peer-to-peer networks

We investigate the design of a reputation system for decentralized unstructured P2P networks like Gnutella. Having reliable reputation information about peers can form the basis of an incentive system and can guide peers in their decision making (e.g., who to download a file from). The reputation system uses objective criteria to track each peers contribution in the system and allows peers to store their reputations locally. Reputation are computed using either of the two schemes, debit-credit reputation computation (DCRC) and credit-only reputation computation (CORC). Using a reputation computation agent (RCA), we design a public key based mechanism that periodically updates the peer reputations in a secure, light-weight, and partially distributed manner. We evaluate using simulations the performance tradeoffs inherent in the design of our system.

Controlling the mobility of multiple data transport ferries in a delay-tolerant network

As technology rapidly progresses, more devices will combine both communication and mobility capabilities. With mobility in devices, we envision a new class of proactive networks that are able to adapt themselves, via physical movement, to meet the needs of applications. To fully realize these opportunities, effective control of device mobility and the interaction between devices is needed. In this paper, we consider the message ferrying (MF) scheme which exploits controlled mobility to transport data in delay-tolerant networks, where end-to-end paths may not exist between nodes. In the MF scheme, a set of special mobile nodes called message ferries are responsible for carrying data for nodes in the network. We study the use of multiple ferries in such networks, which may be necessary to address performance and robustness concerns. We focus on the design of ferry routes. With the possibilities of interaction between ferries, the route design problem is challenging. We present algorithms to calculate routes such that the traffic demand is met and the data delivery delay is minimized. We evaluate these algorithms under a variety of network conditions via simulations. Our goal is to guide the design of MF systems and understand the tradeoff between the incurred cost of multiple ferries and the improved performance. We show that the performance scales well with the number of ferries in terms of throughput, delay and resource requirements in both ferries and nodes.

The use of multicast delivery to provide a scalable and interactive video-on-demand service

In typical proposals for video-on-demand (VoD) systems, customers are serviced individually by allocating and dedicating a transmission channel and a set of server resources to each customer. This approach leads to an expensive to operate, nonscalable system. We consider a VoD system that uses multicast delivery to service multiple customers with a single set of resources. The use of multicast communication requires that part of the on-demand nature of the system be sacrificed to achieve scalability and cost-effectiveness. One drawback to using multicast communication is that it complicates the provision or interactive VCR-style functions. Interactivity can be provided by either increasing the complexity of the customer set-top box (STB) or by modifying the semantics of the interactive functions to make them easier to provide. We describe a framework and mechanisms by which such interactive functions can be incorporated into a multicast delivery VoD system. Through the use of simulation, we evaluate and compare the performance of a unicast VoD system and multicast VoD systems offering various levels of interactivity.

Message ferrying: proactive routing in highly-partitioned wireless ad hoc networks

An ad hoc network allows devices with wireless interfaces to communicate with each other without any pre-installed infrastructure. Due to node mobility, limited radio power, node failure and wide deployment area, ad hoc networks are often vulnerable to network partitioning. A number of examples are in battlefield, disaster recovery and wide area surveillance. Unfortunately, most existing ad hoc routing protocols will fail to deliver messages under these circumstances since no route to the destination exists. In this work we propose the Message Ferrying or MF scheme that provides efficient data delivery in disconnected ad hoc networks. In the MF scheme, nodes move proactively to send or receive messages. By introducing non-randomness in a nodes proactive movement and exploiting such nonrandomness to deliver messages, the MF scheme improves data delivery performance in a disconnected network. In this paper, we propose the basic design of the MF scheme and develop a general framework to classify variations of MF systems. We also study ferry route design problem in stationary node case which is shown to be NP-hard and provide an efficient algorithm to compute ferry route.

A novel server selection technique for improving the response time of a replicated service

Server replication is an approach often used to improve the ability of a service to handle a large number of clients. One of the important factors in the efficient utilization of replicated servers is the ability to direct client requests to the best server, according to some optimality criteria. In this paper we target an environment in which servers are distributed across the Internet, and clients identify servers using our application-layer any-casting service. Our goal is to allocate servers to clients in a way that minimizes a clients response time. To that end, we develop an approach for estimating the performance that a client would experience when accessing particular servers. Such information is maintained in a resolver that clients can query to obtain the identity of the server with the best response time. Our performance collection technique combines server push with client probes to estimate the expected response time. A set of experiments is used to demonstrate the properties of our performance determination approach and to show its advantages when used within the application-layer anycasting architecture.

PeopleRank: Social Opportunistic Forwarding

In opportunistic networks, end-to-end paths between two communicating nodes are rarely available. In such situations, the nodes might still copy and forward messages to nodes that are more likely to meet the destination. The question is which forwarding algorithm offers the best trade off between cost (number of message replicas) and rate of successful message delivery. We address this challenge by developing the PeopleRank approach in which nodes are ranked using a tunable weighted social information. Similar to the PageRank idea, PeopleRank gives higher weight to nodes if they are socially connected to important other nodes of the network. We develop centralized and distributed variants for the computation of PeopleRank. We present an evaluation using real mobility traces of nodes and their social interactions to show that PeopleRank manages to deliver messages with near optimal success rate (close to Epidemic Routing) while reducing the number of message retransmissions by 50% compared to Epidemic Routing.

Prefix-preserving IP address anonymization: measurement-based security evaluation and a new cryptography-based scheme

Real-world traffic traces are crucial for Internet research, but only a very small percentage of traces collected are made public. One major reason why traffic trace owners hesitate to make the traces publicly available is the concern that confidential and private information may be inferred from the trace. We focus on the problem of anonymizing IP addresses in a trace. More specifically, we are interested in prefix-preserving anonymization in which the prefix relationship among IP addresses is preserved in the anonymized trace, making such a trace usable in situations where prefix relationships are important. The goal of our work is two fold. First, we develop a cryptography-based, prefix-preserving anonymization technique that is provably as secure as the existing well-known TCPdpriv scheme, and unlike TCPdpriv, provides consistent prefix-preservation in large scale distributed setting. Second, we evaluate the security properties inherent in all prefix-preserving IP address anonymization schemes (including TCPdpriv). Through the analysis of Internet backbone traffic traces, we investigate the effect of some types of attacks on the security of any prefix-preserving anonymization algorithm. We also derive results for the optimum manner in which an attack should proceed, which provides a bound on the effectiveness of attacks in general.

Capacity Enhancement using Throwboxes in DTNs

Disruption tolerant networks (DTNs) are designed to overcome limitations in connectivity due to conditions such as mobility, poor infrastructure, and short range radios. DTNs rely on the inherent mobility in the network to deliver packets around frequent and extended network partitions using a store-carry-and-forward paradigm. However, missed contact opportunities decrease throughput and increase delay in the network. We propose the use of throwboxes in mobile DTNs to create a greater number of contact opportunities, consequently improving the performance of the network. Throwboxes are wireless nodes that act as relays, creating additional contact opportunities in the DTN. We propose algorithms to deploy stationary throwboxes in the network that simultaneously consider routing as well as placement. We also present placement algorithms that use more limited knowledge about the network structure. We perform an extensive evaluation of our algorithms by varying both the underlying routing and mobility models. Our results suggest several findings to guide the design and operation of throwbox-augmented DTNs

Dynamic Topology Configuration in Service Overlay Networks: A Study of Reconfiguration Policies

The routing infrastructure of the Internet has become resistant to fundamental changes and the use of overlay networks has been proposed to provide additional flexibility and control. One of the most prominent configurable components of an overlay network is its topology, which can be dynamically reconfigured to accommodate communication requirements that vary over time. In this paper, we study the problem of determining dynamic topology reconfiguration for service overlay networks with dynamic communication requirement, and the ideal goal is to find the optimal reconfiguration policies that can minimize the potential overall cost of using an overlay. We start by observing the properties of the optimal reconfiguration policies through studies on small systems and find structures in the optimal reconfiguration policies. Based on these observations, we propose heuristic methods for constructing different flavors of reconfiguration policies, i.e., never-change policy, always-change policy and cluster-based policies, to mimic and approximate the optimal ones. Our experiments show that our policy construction methods are applicable to large systems and generate policies with good performance. Our work does not only provide solutions to practical overlay topology design problems, but also provides theoretical evidence for the advantage of overlay network due to its configurability.