Johan Garcia

KauNet: improving reproducibility for wireless and mobile research

This paper presents the KauNet emulation system that provides pattern-based emulation. KauNet enables bit precise placement of bit-errors, exact and repeatable packet losses, delays and bandwidth variations. The design and performance of KauNet is discussed. An example is also provided of how it can be integrated in a specific emulation framework to enhance emulation for mobile and wireless systems.

Impact of TCP congestion control on bufferbloat in cellular networks

The existence of excessively large and too filled network buffers, known as bufferbloat, has recently gained attention as a major performance problem for delay-sensitive applications. One important network scenario where bufferbloat may occur is cellular networks. This paper investigates the interaction between TCP congestion control and buffering in cellular networks. Extensive measurements have been performed in commercial 3G, 3.5G and 4G cellular networks, with a mix of long and short TCP flows using the CUBIC, NewReno and Westwood+ congestion control algorithms. The results show that the completion times of short flows increase significantly when concurrent long flow traffic is introduced. This is caused by increased buffer occupancy from the long flows. In addition, for 3G and 3.5G the completion times are shown to depend significantly on the congestion control algorithms used for the background flows, with CUBIC leading to significantly larger completion times.

A taxonomy and survey of SCTP research

The Stream Control Transmission Protocol (SCTP) is a relatively recent general-purpose transport layer protocol for IP networks that has been introduced as a complement to the well-established TCP and UDP transport protocols. Although initially conceived for the transport of PSTN signaling messages over IP networks, the introduction of key features in SCTP, such as multihoming and multistreaming, has spurred considerable research interest surrounding SCTP and its applicability to different networking scenarios. This article aims to provide a detailed survey of one of these new features—multihoming—which, as it is shown, is the subject of evaluation in more than half of all published SCTP-related articles. To this end, the article first summarizes and organizes SCTP-related research conducted so far by developing a four-dimensional taxonomy reflecting the (1) protocol feature examined, (2) application area, (3) network environment, and (4) study approach. Over 430 SCTP-related publications have been analyzed and classified according to the proposed taxonomy. As a result, a clear perspective on this research area in the decade since the first protocol standardization in 2000 is given, covering both current and future research trends. On continuation, a detailed survey of the SCTP multihoming feature is provided, examining possible applications of multihoming, such as robustness, handover support, and loadsharing.

Classification of indecent videos by low complexity repetitive motion detection

This paper proposes a fast method for detection of indecent video content using repetitive motion analysis. Unlike skin detection, motion will provide invariant features irrespective of race and color. The video material to be evaluated is divided into short fixed-length sections. By filtering different combinations of B-frame motion vectors using adjacency in time and space, one dominant motion vector is constructed for each frame. The power spectral density estimate of this dominant motion vector is then computed using a periodogram with a Hamming window. The resulting power spectrum is then subjected to a Slepian selection window to restrict the spectrum to a limited frequency range typical of indecent movement, as empirically derived by us. A threshold detector is then applied to detect repetitive motion in video sections. However, there are instances where repetitive motion occurs in these shorter sections without the video as a whole being indecent. As a second step, an additional detector can be employed to determine if the sections over a longer period of time can be classified as containing indecent material. The proposed method is resource efficient and do not require the typical IDCT step of video decoding. Further, the computationally expensive spectral estimation calculations are done using only one value per frame. Evaluations performed using a restricted set of videos show promising results with high true positive probability (>85%) for a low false positive probability (<10%) for the repetitive motion detection.

Checksum-based loss differentiation

In a wireless environment the common transport protocol assumption that all packet losses are due to congestion no longer holds true. If the transport protocols react identically to both congestion and non-congestion related losses, performance will degrade. To avoid this performance problem, transport protocols must be able to differentiate between losses that are due to congestion and losses that are due to wireless link errors. Suitable congestion avoidance behavior can then be applied as appropriate. This paper overviews previous work on loss differentiation, and proposes a checksum-based scheme. The proposed scheme reuses the checksums already available at various layers in the receiver end-host protocol stack. It is an end-to-end solution for the case where the lossy link is the last hop. Header decoding, data integrity and other issues related to the implementation of our checksum-based scheme are also discussed.

A measurement based study of TCP protocol efficiency in cellular networks

This paper examines the efficiency of resource utilization with respect to short-lived TCP flows in various cellular networks. The examination is done from the vantage point of an end-user who would like to use as much as possible of the cellular transmission resources that are available at any given time, thus minimizing the delays associated with communication. Based on a comprehensive measurement campaign we first derive network characteristics with regards to base RTT, RTT under load, and average throughput. A protocol efficiency metric is introduced to capture how efficiently short TCP flows are in fact able to use the instantaneously available transmission resources in a cellular network. The measurements show that short TCP connections have low efficiency in 3.5G (HSPA+) and 4G (LTE) mobile broadband networks, and that the improved latency and throughput characteristics of 4G in relation to 3.5G nevertheless results in lower short-flow efficiency for 4G.

Taxonomy and survey of retransmission-based partially reliable transport protocols

The mismatch between the services offered by the two standard transport protocols in the Internet, TCP and UDP, and the services required by distributed multimedia applications has led to the development of a large number of partially reliable transport protocols: protocols that in terms of reliability place themselves between TCP and UDP. This paper presents taxonomy for retransmission-based, partially reliable transport protocols, i.e. the subclass of partially reliable transport protocols that makes error recovery through retransmissions. The taxonomy comprises two classification schemes: one that classifies protocols with respect to the reliability service they offer and one that classifies them with respect to their error control scheme. The objective of our taxonomy is fourfold: to introduce a unified terminology; to provide a framework in which this class of protocols can be examined, compared and contrasted; to make explicit the error control schemes used by these protocols; and, finally, to gain new insight into these protocols and thereby suggest avenues for future research. Based on our taxonomy, a survey is made of existing retransmission-based, partially reliable transport protocols. The survey shows how protocols are categorized according to our taxonomy and exemplifies the majority of reliability services and error control schemes detailed in our taxonomy.

Improved IP-Level Emulation for Mobile and Wireless Systems

More and more applications and protocols are now running on wireless networks. Testing such applications and protocols is a real challenge as the position of the mobile terminals and environmental effects strongly affect the overall performance. Network emulation is often perceived as a good trade-off between experiments on operational wireless networks and discrete-event simulations on Opnet or ns-2. However, ensuring repeatability and realism in network emulation while taking into account mobility in a wireless environment is very difficult. This paper proposes a network emulation architecture based on off-line computations preceding online pattern-based traffic shaping. The underlying concepts of repeatability, dynamicity and accuracy are defined in the emulation context. Three different simple case studies illustrate the validity of our approach with respect to these concepts.

Examining TCP Short Flow Performance in Cellular Networks Through Active and Passive Measurements

In this study we examine the conditions in a current cellular network by examining data passively collected in the core of a cellular operator during a 24-hour period. More than 2 billion traffic measurement data points from over 500,000 cellular users are analyzed. The analysis characterizes the Time-of-Day (ToD) variations for traffic intensity and session length and serves as a complement to the active measurements also performed. A comprehensive active measurement campaign was completed in the HSDPA+ and LTE networks of the four major Swedish operators. We collect around 50,000 data points from stationary cellular modems and analyze the ToD variation pattern for underlying network layer metrics such as delay and throughput. In conjunction with the time-varying session size distribution obtained from the passive measurements, we then analyze the ToD impact on TCP flows of varying sizes. The ToD effects are examined using time series analysis with Lomb-Scargle periodograms and differential Bayesian Information Criterion to allow comparison of the relative impact of the network ToD effects. The results show that ToD effects are predominantly impacting longer-running flows, and although short flows are also impacted they are mostly constrained by other issues such as protocol efficiency.

Fragment retention characteristics in slack space — Analysis and measurements

When files are deleted, their information is not removed from the storage media. This is a well known fact, and there exist numerous undelete utilities to recover newly deleted files. When deleted files have been partly overwritten, the data from the part of the file that remains in unallocated space can be readily extracted by file carving. Such carving is often performed in forensic investigations. Furthermore, as a consequence of file system implementation specifics, there additionally exist small remains of the previous files in the space at the end of new files. In this paper we focus on these small remains of previous files, or micro-fragments, that exist even after all the space allocated to the previous file has been reallocated to new files. We derive expressions for modeling the number of micro-fragments that can be expected to be found, and perform experiments to evaluate the analytical model. The obtained results indicate good correspondence between the analytical predictions and the measured results.