Chen-Mou Cheng

Use of spectral analysis in defense against DoS attacks

We propose using spectral analysis to identify normal TCP traffic so that it will not be dropped or rate-limited in defense against denial of service (DoS) attacks. The approach can reduce false positives of attacker identification schemes and thus decrease the associated unnecessary slowdown or stoppage of legitimate traffic. For the spectral analysis, we use the number of packet arrivals of a flow in fixed-length time intervals as the signal. We then estimate the power spectral density of the signal, in which information of periodicity, or lack thereof, in the signal reveals itself. A normal TCP flow should exhibit strong periodicity around its round-trip time in both flow directions, whereas an attack flow usually does not. We validate the effectiveness of the approach with simulation and trace analysis. We argue that the approach complements existing DoS defense mechanisms that focus on identifying attack traffic.

WSN07-1: Adjacent Channel Interference in Dual-radio 802.11a Nodes and Its Impact on Multi-hop Networking

We evaluate the performance impact of adjacent channel interference (ACI) in multi-hop wireless networks based on dual-radio 802.11a nodes. Although these nodes use chipsets that satisfy the transmit-mask requirements set by the IEEE 802.11 standard, the multi-hop performance is still significantly affected by ACI. That is, a nodes transmitter can interfere with its own receiver on a different channel; as a result, multi-hop throughput is severely degraded. This degradation is especially pronounced for 802.11a. We use a spectrum analyzer with a signal combiner to quantify ACI under various conditions and propose solutions to mitigate the effect of such interference on multi-hop forwarding. Field experiments with multi-hop relay have validated these findings as well as the effectiveness of our solutions.

Performance Measurement of 802.11a Wireless Links from UAV to Ground Nodes with Various Antenna Orientations

We report measured performance of 802.11a wireless links from an unmanned aerial vehicle (UAV) to ground stations. In a set of field experiments, we record the received signal strength indicator (RSSI) and measure the raw link-layer throughput for various antenna orientations, communication distances and ground-station elevations. By comparing the performance of 32 simultaneous pairs of UAV and ground station configurations, we are able to conclude that, in order to achieve the highest throughput under a typical flyover UAV flight path, both the UAV and the ground station should use omni-directional dipole (as opposed to high-gain, narrow- beam) antennas positioned horizontally, with their respective antenna null pointing to a direction perpendicular to the UAVs flight path. In addition, a moderate amount of elevation of the ground stations can also improve performance significantly.

Maximizing Throughput of UAV-Relaying Networks with the Load-Carry-and-Deliver Paradigm

We consider the task of using one or more unmanned aerial vehicles (UAVs) to relay messages between two distant ground nodes. For delay-tolerant applications like latency-insensitive bulk data transfer, we seek to maximize throughput by having a UAV load from a source ground node, carry the data while flying to the destination, and finally deliver the data to a destination ground node. We term this the load-carry-and-deliver (LCAD) paradigm and compare it against the conventional multi-hop, store-and-forward paradigm. We identify and analyze several of the most important factors in constructing a throughput-maximizing framework subject to constraints on both application allowable delay and UAV maneuverability. We report performance measurement results for IEEE 802.11g devices in three flight tests, based on which we derive a statistical model for predicting throughput performance for LCAD. Due to the nature of commercial off-the-shelf systems, this methodology is of essential importance for allowing better flight-path design to achieve high throughput.

Parallel Use of Multiple Channels in Multi-hop 802.11 Wireless Networks

We consider parallel use of multiple channels in a multi-radio, multi-hop 802.11 wireless network, with the goal of maximizing the total multi-hop throughput. We first quantify several fundamental forms of radio interference that cause performance degradation when the number of hops increases and that prevent total throughput from scaling up with number of radio interfaces at each node. We then evaluate three different methods of parallel channel use: ad-hoc, frequency-division multiplexing (FDM), and time-division multiplexing (TDM). We measure their performance on a linearly connected multi-hop network of dual-radio nodes. Although theoretically these three methods should have comparable performance, their actual measured performances are quite different. We find that TDM has the best performance, followed by ad-hoc and then FDM. The performance differences are due to these methods capabilities of combating interference. We conclude that interference, especially adjacent channel interference, has significant effect on the achievable performance of a multi-radio, multi-hop network and hence should be carefully taken into account in the design and deployment of such a network.

Path probing relay routing for achieving high end-to-end performance

We present an overlay network routing scheme, called path probing relay routing (PPRR), which is capable of promptly switching to alternative paths when the direct paths provided by the underlying IP networks suffer from serious performance degradation or outage. PPRR uses a randomized search algorithm to discover available alternative paths and employs an end-to-end, on-demand probing technique to determine their quality. To assess the effectiveness of PPRR, we conduct performance simulations using four sets of real-world traces, collected by various research groups at different times and places. Our simulation results show that the performance of PPRR is comparable to that of a typical link state relay routing algorithm. Compared with the latter, PPRR has lower probing overhead in the sense that the overhead remains constant as network size grows. In particular, PPRR avoids the need to flood the overlay network with link state updates.

Rainbow: A wireless medium access control using network coding for multi-hop content distribution

We consider the problem of multi-hop content distribution over a wireless ad-hoc network. Such mechanisms are relevant to a broad spectrum of applications, but are particularly important to data broadcast in wireless distributed computing where speedy I/O is critical to overall performance. In this paper, we present Rainbow, a content distribution protocol for multi-hop wireless ad-hoc networks. The protocol uses a content-directed medium access control (MAC), through which transmission priority is given to those nodes most capable of delivering useful content to their neighbors. We describe an efficient implementation of Rainbow based on network coding. Specifically, Rainbow uses a MAC priority scheme, where the priority of packet transmission from a node depends on the rank of the coefficient matrix associated with the coded content the node holds. We demonstrate that Rainbow achieves a 1.3-to 1.9-fold improvement in content distribution time over other flooding protocols, as measured on a testbed of 29 wireless nodes. We attribute this performance gain in part to Rainbowpsilas ability to address a MAC-level bottleneck in multi-hop wireless networks, which we refer to as the ldquobridge lock-out problemrdquo.

Transmit Antenna Selection Based on Link-layer Channel Probing

In this paper, we propose transmit antenna selection based on receiver feedback of channel information obtained via link-layer probing. Furthermore, we report the performance gain of the proposed antenna selection scheme in an experimental multi-antenna 802.11 network. We built a low-altitude Unmanned Aerial Vehicle (UAV) testbed using commodity dual-antenna 802.11 hardware and performed field experiments to collect traces of link performance using antennas of various types and orientations. Based on the collected traces, we demonstrate that transmit antenna selection can achieve a significant amount of gain using a link-layer channel probing protocol at a relatively low probing rate. The largest improvement we observed with joint transmit/receive antenna selection in 2x2 systems was 32%, about twice as much as that of receive-only antenna selection in 1x2 systems, which achieved 17%. Moreover, a similar improvement is obtained with probing intervals up to about 200 milliseconds, which is infrequently enough to consume only a small fraction of the available 802.11 channel capacity. Since these results require only a low implementation and operational cost, we conclude that transmit antenna selection is a worthwhile technique to use with the kind of multi-antenna mobile ad-hoc networks we examined.

Design and analysis of an IP-Layer anonymizing infrastructure

This paper describes an IP-layer anonymizing infrastructure, called ANON which allows server addresses to be hidden from clients and vice versa. In providing address anonymity, ANON uses a network resident set of IP-layer anonymizing forwarders that can forward IP packets with nested encryption and decryption applied to their source and destination addresses. To prevent adversaries from compromising the anonymity by learning the forwarding path, ANON incorporates a suite of countermeasures, including non-malleable, semantically secure link encryption and link padding. To lower the bandwidth cost of padding traffic, two novel algorithms are suggested: on demand link padding and probabilistic link padding. To prevent inband denial of service (DoS) attacks through the anonymizing infrastructure itself ANON uses rate limiting. Finally, ANON makes use of fault-tolerant transport networks to enhance its resilience against failures and out-band attacks.

Use of relays in extending network lifetime

We describe a simple physical-layer relay scheme for wireless communication in which a set of low-noise linear amplify-and-forward relays are placed between transmitter and receiver to assist radio communication. By exploiting the geometric gain resulting from the location of properly deployed relays, part of the radio transmission power of the information source can be off-loaded to these relays. We argue that such a simple relay scheme can thus provide a transparent and management-free alleviation to the problem of limited battery time in application scenarios like sensor networks, where radio transmission can consume a significant portion of the battery-provided energy. We use simulation to quantify the bit error rate (BER) performance of the relay scheme in a low signal-to-noise ratio (SNR) regime when used in conjunction with the maximal ratio combining (MRC) scheme and Alamouti space-time codes. We show that, under a reasonably precise indoor radio propagation model derived from measurements, the proposed relay scheme is capable of reaping the geometric gain attained from a set of relays deployed midway between the transmitter and the receiver to extend effectively the battery lifetime of the network.