40 research outputs found
Skew detection and compensation for Internet audio applications
Long lived audio streams, such as music broadcasts, and small differences in clock rates lead to buffer underflow or overflow events in receiving applications that manifest themselves as audible interruptions. We present a low complexity algorithm for detecting clock skew in network audio applications that function with local clocks and in the absence of a synchronization mechanism. A companion algorithm to perform skew compensation is also presented. The compensation algorithm utilises the temporal redundancy inherent in audio streams to make inaudible playout adjustments. Both algorithms have been implemented in a simulator and in a network audio application. They perform effectively over the range of observed clock rate differences and beyond
Fully-distributed joint clock synchronization and ranging in wireless sensor networks under exponential delays
In this paper, we study the global clock synchro- nization and ranging problem for wireless sensor networks in the presence of unknown exponential delays using the two- way message exchange mechanism. Based on the Alternating Direction Method of Multipliers (ADMM), we propose a fully- distributed synchronization and ranging algorithm which has low communication overhead and computation cost. Simulation results show that the proposed algorithm achieves better accuracy than consensus algorithm, and can always converge to the centralized optimal solution.published_or_final_versio
Cloaking the Clock: Emulating Clock Skew in Controller Area Networks
Automobiles are equipped with Electronic Control Units (ECU) that communicate
via in-vehicle network protocol standards such as Controller Area Network
(CAN). These protocols are designed under the assumption that separating
in-vehicle communications from external networks is sufficient for protection
against cyber attacks. This assumption, however, has been shown to be invalid
by recent attacks in which adversaries were able to infiltrate the in-vehicle
network. Motivated by these attacks, intrusion detection systems (IDSs) have
been proposed for in-vehicle networks that attempt to detect attacks by making
use of device fingerprinting using properties such as clock skew of an ECU. In
this paper, we propose the cloaking attack, an intelligent masquerade attack in
which an adversary modifies the timing of transmitted messages in order to
match the clock skew of a targeted ECU. The attack leverages the fact that,
while the clock skew is a physical property of each ECU that cannot be changed
by the adversary, the estimation of the clock skew by other ECUs is based on
network traffic, which, being a cyber component only, can be modified by an
adversary. We implement the proposed cloaking attack and test it on two IDSs,
namely, the current state-of-the-art IDS and a new IDS that we develop based on
the widely-used Network Time Protocol (NTP). We implement the cloaking attack
on two hardware testbeds, a prototype and a real connected vehicle, and show
that it can always deceive both IDSs. We also introduce a new metric called the
Maximum Slackness Index to quantify the effectiveness of the cloaking attack
even when the adversary is unable to precisely match the clock skew of the
targeted ECU.Comment: 11 pages, 13 figures, This work has been accepted to the 9th ACM/IEEE
International Conference on Cyber-Physical Systems (ICCPS
TimeTrader: Exploiting Latency Tail to Save Datacenter Energy for On-line Data-Intensive Applications
Datacenters running on-line, data-intensive applications (OLDIs) consume
significant amounts of energy. However, reducing their energy is challenging
due to their tight response time requirements. A key aspect of OLDIs is that
each user query goes to all or many of the nodes in the cluster, so that the
overall time budget is dictated by the tail of the replies' latency
distribution; replies see latency variations both in the network and compute.
Previous work proposes to achieve load-proportional energy by slowing down the
computation at lower datacenter loads based directly on response times (i.e.,
at lower loads, the proposal exploits the average slack in the time budget
provisioned for the peak load). In contrast, we propose TimeTrader to reduce
energy by exploiting the latency slack in the sub- critical replies which
arrive before the deadline (e.g., 80% of replies are 3-4x faster than the
tail). This slack is present at all loads and subsumes the previous work's
load-related slack. While the previous work shifts the leaves' response time
distribution to consume the slack at lower loads, TimeTrader reshapes the
distribution at all loads by slowing down individual sub-critical nodes without
increasing missed deadlines. TimeTrader exploits slack in both the network and
compute budgets. Further, TimeTrader leverages Earliest Deadline First
scheduling to largely decouple critical requests from the queuing delays of
sub- critical requests which can then be slowed down without hurting critical
requests. A combination of real-system measurements and at-scale simulations
shows that without adding to missed deadlines, TimeTrader saves 15-19% and
41-49% energy at 90% and 30% loading, respectively, in a datacenter with 512
nodes, whereas previous work saves 0% and 31-37%.Comment: 13 page
Measuring the dynamical state of the Internet: Large-scale network tomography via the ETOMIC infrastructure
In this paper we show how to go beyond the study of the
topological properties of the Internet, by measuring its
dynamical state using special active probing techniques and the
methods of network tomography. We demonstrate this approach by
measuring the key state parameters of Internet paths, the
characteristics of queuing delay, in a part of the European
Internet. In the paper we describe in detail the ETOMIC
measurement platform that was used to conduct the experiments,
and the applied method of queuing delay tomography. The main
results of the paper are maps showing various spatial structure
in the characteristics of queuing delay corresponding to the
resolved part of the European Internet. These maps reveal that
the average queuing delay of network segments spans more than
two orders of magnitude, and that the distribution of this
quantity is very well fitted by the log-normal distribution.
Copyright © 2006 S. Karger AG
Coexisting Parallelogram Method to Handle Jump Point on Hough Transform-based Clock Skew Measurement
In this paper, we improve the robustness of the Hough transform-based clock skew measurement on the occurrence of a jump point. The current Hough transform-based skew method uses angle (θ), thickness (ω), and region (β), to create a parallelogram that covers the densest part of an offset-set. However, the assumption that all offsets are considered to line up roughly in only one direction restricts the ability of the current method when handling an offset-set in which its densest part is located separately, the jump point condition. By acquiring the parallelogram from coexisting angle-region tuples at the beginning and the ending parts of the offset-set, we completed the ability of the Hough transform-based method to handle the jump point. When handling the jump point problem, the proposed coexisting parallelogram method could reach 0.35 ppm accuracy compared with tens ppm by the current methods
Clock Estimation for Long-Term Synchronization in Wireless Sensor Networks with Exponential Delays
Although the existing time synchronization protocols in wireless sensor networks (WSNs) are efficient for short periods, many applications require long-term synchronization among the nodes, for example, coordinated sleep and wakeup modes, and synchronized sampling. In such applications, experiments have shown that even clock skew correction cannot maintain long-term clock synchronization and a quadratic model of clock variations can better capture the dynamics of the actual clock model involved, hence increasing the resynchronization period and conserving significant energy. This paper derives the maximum likelihood (ML) estimator for all the clock parameters in a two-way timing exchange model with exponential delays. The same estimation procedure can be applied to one-way timing exchange models with little modification
Computer Identification Based on Packet Timestamps
Tato práce pojednává o technice identifikace počítače na základě časových značek paketů. V práci je dále popsán algoritmus pro výpočet zkreslení hodin počítače, na kterém se celá technika identifikace počítače zakládá. Ukážeme využití této techniky k identifikaci několika počítačů v reálné počítačové síti, k identifikaci zařízení, na kterém běží překlad adres NAT a k identifikaci počítačů nacházejících se za NATem. Nakonec ukážeme, jak lze určit počet počítačů nacházejících se za NATem. This work describes perspective identification technique called computer identification based on packet timestamps. This work also describes an algorithm to calculate skew of computer clock, on which the entire computer identification technique is based on. We will use this technique to identify a number of computers in the real computer network, the device that is running NAT and computers located behind NAT. Finally we show, how to determine the number of computers located behind NAT.