Muxi Yan

Algorithms for weakly secure data exchange

We consider the problem of direct data exchange among a group of wireless clients in the presence of an eavesdropper. An instance of this problem includes a set of packets X and a set of clients, such that each client has an access to a subset of packets and requests the rest of the packets in X. The clients use a lossless broadcast channel to transmit packets to other clients in the group. All transmitted packets can be observed by an eavesdropper that might have prior side information about the packets exchanged by the clients. Our goal is to construct a coding scheme that prevents the eavesdropper from decoding any individual packet in X (beyond those it already knowns). In weakly secure schemes, the more packets the eavesdropper has, the harder it is to construct a scheme that prevents the eavesdropper from obtaining additional packets. Thus, it is natural to measure the degree of secrecy by the maximum number of packets that can be held by the eavesdropper. Accordingly, our goal is to construct a coding scheme that can handle an eavesdropper that has as many packets as possible. We also present randomized and deterministic solutions for this problem and analyze their performance. We also establish a tight upper bound on the maximum degree of secrecy that a weakly secure scheme can provide for each instance of the problem.

Weakly secure data exchange with Generalized Reed Solomon codes

We focus on secure data exchange among a group of wireless clients. The clients exchange data by broadcasting linear combinations of packets over a lossless channel. The data exchange is performed in the presence of an eavesdropper who has access to the channel and can obtain all transmitted data. Our goal is to develop a weakly secure coding scheme that prevents the eavesdropper from being able to decode any of the original packets held by the clients. We present a randomized algorithm based on Generalized Reed-Solomon (GRS) codes. The algorithm has two key advantages over the previous solutions: it operates over a small (polynomial-size) finite field and provides a way to verify that constructed code is feasible. In contrast, the previous approaches require exponential field size and do not provide an efficient (polynomial-time) algorithm to verify the secrecy properties of the constructed code. We formulate an algebraic-geometric conjecture that implies the correctness of our algorithm and prove its validity for special cases. Our simulation results indicate that the algorithm is efficient in practical settings.

Weakly Secure Network Coding for Wireless Cooperative Data Exchange

We consider the problem of secure cooperative data exchange in wireless networks. The cooperative data exchange problem includes a set of wireless clients that exchange information over a lossless broadcast channel. The clients are missing some packets but collectively know all packets. Each client can transmit packets it currently has or a combination thereof. The problem asks for a scheme that allows all clients to recover all packets with the minimum number of transmissions. We focus on secure information exchange in the presence of an eavesdropper that can observe all packets transmitted over the broadcast channel. Our goal is to construct a \emph{weakly secure} solution to the cooperative data exchange problem, i.e., a solution that does not reveal information about any single packet. This is in contrast to \emph{strongly secure} solutions that do not reveal any data about the entire set of packets. Weakly secure solutions can be implemented more efficiently in practical settings and do not require sharing secret keys. Our paper makes the following contributions. First, we establish necessary and sufficient conditions for the feasibility of weakly secure data exchange for any given instance of the weakly secure data exchange problem. Second, we show that if it is possible to obtain a weakly secure solution for an instance of the cooperative data exchange problem, then weak security can be achieved with no penalty in terms of the total number of transmissions. Finally, we present an algorithm that finds an optimal weakly secure solution to the cooperative data exchange problem.

ÆtherFlow: Principled Wireless Support in SDN

Software Defined Networking (SDN) drastically changes the meaning and process of designing, building, testing, and operating networks. The current support for wireless networking in SDN technologies has lagged behind its development and deployment for wired networks. The purpose of this work is to bring principled support for wireless access networks so that they can receive the same level of programmability as wireline interfaces. Specifically we aim to integrate wireless protocols into the general SDN framework by proposing a new set of abstractions in wireless devices and the interfaces to manipulate them. We validate our approach by implementing our design as an extension of an existing OpenFlow data plane and deploying it in an IEEE 802.11 access point. We demonstrate the viability of software-defined wireless access networks by developing and testing a wireless handoff application. The results of the experiment show that our framework is capable of providing new capabilities in an efficient manner.

Cooperative data exchange with priority classes

This paper considers the problem of cooperative data exchange with different client priority classes. In this problem, each client initially knows a subset of packets in the ground set X of size K, and all clients wish to learn all packets in X. The clients exchange packets by broadcasting coded combinations of their packets. The primary objective is to satisfy all high-priority clients in the first round of transmissions with minimum sum-rate, and the secondary objective is to satisfy low-priority clients in the second round of transmissions with minimum sum-rate, subject to minimizing the sum-rate in the first round. For any arbitrary problem instance, we provide a linear programming-based approach to find the minimum sum-rate in each round. Moreover, for the case in which the packets are randomly distributed among clients, we derive a closed-form expression for the minimum sum-rate in each round, which holds with probability approaching 1 as K tends to infinity.

On error correcting algorithms for the cooperative data exchange problem

In the cooperative data exchange problem, a group of wireless clients use a shared broadcast channel to exchange packets from a given set X. Each of the clients has a subset of packets in X available to it as a side information. The clients transmit linear combinations of the packets one after the other, until all clients are able to recover all packets in X. This problem arises naturally in many practical settings, in particular in battlefield networks and peer-to-peer wireless networks. In this paper, we focus on the Error Correcting Data Exchange (ECDE) problem in which some of the clients participating in data exchange can be faulty. Faulty clients may transmit packets with arbitrary errors that can occur due to corruption or malicious behavior. Our goal is to identify a coding strategy that allows every non-faulty client to obtain the correct copies of all original packets in X. The paper makes the following contributions. First, we establish necessary and sufficient conditions for the existence of a feasible solution to Problem ECDE. Next, we focus on a special case in which at most one client is faulty. We prove that the problem is NP-hard even for this special case and present an approximation algorithm with provable performance guarantees. Our simulation results indicate that the algorithm performs well in practical settings.

CrossFlow: A cross-layer architecture for SDR using SDN principles

Software defined radios (SDR) have revolutionized communications by enabling flexible implementation and deployment of physical layer and medium access control protocols through software, without the need for specialized hardware. Although SDRs have been around for many years now, there is still tremendous interest to make them work in scenarios that were not previously thought possible. One such scenario is to build a network of SDRs that can be handled in a programmatic manner. The challenge for building such an architecture is that there is a plethora of radio protocols that need to be supported, each having its own nuances. To address this, we need to build fundamental abstractions that provide enough visibility so that a programmer can implement protocols, while at the same time being rigid enough not to expose excessive details that will complicate the application development process. The purpose of this work is to introduce a principled approach to building a cross-layer architecture using the basic building blocks of SDR and the principles of Software Defined Networking (SDN). SDN provides a great level of programmability into devices through the separation of the control and data planes. This paradigm enforces development of abstractions necessary to support SDRs. We validate this design through a proof-of-concept implementation.

Supporting Voice over LTE: Solutions, Architectures, and Protocols

Modern cellular networks are expected to support both voice and a growing volume of data traffic. The rapid growth in data traffic has promoted network operators to move to Long Term Evolution (LTE), a 4th generation of wireless network infrastructure. However, LTE architecture does not support native circuit switching services and relies on the IP Multimedia Subsystem (IMS) for supporting voice and Short Messaging Service (SMS). Unfortunately, the uptake of IMS has not been as rapid as expected and deployments of IMS cores have been limited. This poses a major issue for operators who wish to deploy LTE in the near future. In particular, voice and SMS drive a majority of service provider revenue, who are concerned with voice quality, call continuity, and reliable SMS delivery in deployed LTE networks. In this paper we analyze several contending approaches to delivering voice services over LTE networks. Each approach will be illustrated with sequence diagrams to explain how voice and SMS services are rendered. We compare the proposed solutions in terms of complexity, cost, features, and interoperability.

Verifying interoperability and application performance of PDCs in synchrophasor system solution

This paper presents a new test methodology for verifying the interoperability and application performance of synchrophasor system solution containing PMUs, PDCs, and communication network. Two types of tests are defined to evaluate the performance of synchrophasors system components from two different aspects: design and application tests. The interoperability between PMUs and PDCs can be verified by a design test. A fault location algorithm using two-end synchronized measurement is used to evaluate the application performance. We have developed a special device, referred to as “Imparator.” that can emulate different network conditions by introducing impairments such as packet losses and delays. In addition, the imperator can measure performance characteristics of PDCs, such as data processing time. Nine commercial PMUs, three PDCs, and reference GPS receiver are selected to perform the tests. The test results at the end show that the PDCs meet the most of the functional requirements, and interoperability issues exist between the PMUs and PDCs of different vendors.

Approximation algorithms for erasure correcting Data Exchange

In the Cooperative Data Exchange (CDE) problem, a group of wireless clients need to exchange data over a shared broadcast channel, such that at the end of the transfer all clients have access to all packets held by other clients. In this paper, we focus on the robust version of the problem in which the clients must be able to complete the transfer even if one of the clients fails before the transfer is complete. The event of a client failure is referred to as an erasure. We show that, while the original CDE problem can be solved in polynomial time, the robust version of the problem is NP-hard, even in the special case when robustness to a single failure is required. Focusing on the practically important special case of a single failure, we establish an approximation algorithm for this problem that can find a solution within a constant factor of the optimum. Our simulation studies show that the algorithm is able to find solutions that are very close to the optimum.