A Multi-perspective Analysis of Carrier-Grade NAT Deployment

As ISPs face IPv4 address scarcity they increasingly turn to network address translation (NAT) to accommodate the address needs of their customers. Recently, ISPs have moved beyond employing NATs only directly at individual customers and instead begun deploying Carrier-Grade NATs (CGNs) to apply address translation to many independent and disparate endpoints spanning physical locations, a phenomenon that so far has received little in the way of empirical assessment. In this work we present a broad and systematic study of the deployment and behavior of these middleboxes. We develop a methodology to detect the existence of hosts behind CGNs by extracting non-routable IP addresses from peer lists we obtain by crawling the BitTorrent DHT. We complement this approach with improvements to our Netalyzr troubleshooting service, enabling us to determine a range of indicators of CGN presence as well as detailed insights into key properties of CGNs. Combining the two data sources we illustrate the scope of CGN deployment on today's Internet, and report on characteristics of commonly deployed CGNs and their effect on end users

Data Cleaning Methods for Client and Proxy Logs

In this paper we present our experiences with the cleaning of Web client and proxy usage logs, based on a long-term browsing study with 25 participants. A detailed clickstream log, recorded using a Web intermediary, was combined with a second log of user interface actions, which was captured by a modified Firefox browser for a subset of the participants. The consolidated data from both records revealed many page requests that were not directly related to user actions. For participants who had no ad-filtering system installed, these artifacts made up one third of all transferred Web pages. Three major reasons could be identified: HTML Frames and iFrames, advertisements, and automatic page reloads. The experiences made during the data cleaning process might help other researchers to choose adequate filtering methods for their data

A Configurable Transport Layer for CAF

The message-driven nature of actors lays a foundation for developing scalable and distributed software. While the actor itself has been thoroughly modeled, the message passing layer lacks a common definition. Properties and guarantees of message exchange often shift with implementations and contexts. This adds complexity to the development process, limits portability, and removes transparency from distributed actor systems. In this work, we examine actor communication, focusing on the implementation and runtime costs of reliable and ordered delivery. Both guarantees are often based on TCP for remote messaging, which mixes network transport with the semantics of messaging. However, the choice of transport may follow different constraints and is often governed by deployment. As a first step towards re-architecting actor-to-actor communication, we decouple the messaging guarantees from the transport protocol. We validate our approach by redesigning the network stack of the C++ Actor Framework (CAF) so that it allows to combine an arbitrary transport protocol with additional functions for remote messaging. An evaluation quantifies the cost of composability and the impact of individual layers on the entire stack

Is Our Model for Contention Resolution Wrong?

Randomized binary exponential backoff (BEB) is a popular algorithm for coordinating access to a shared channel. With an operational history exceeding four decades, BEB is currently an important component of several wireless standards. Despite this track record, prior theoretical results indicate that under bursty traffic (1) BEB yields poor makespan and (2) superior algorithms are possible. To date, the degree to which these findings manifest in practice has not been resolved. To address this issue, we examine one of the strongest cases against BEB: $n$ packets that simultaneously begin contending for the wireless channel. Using Network Simulator 3, we compare against more recent algorithms that are inspired by BEB, but whose makespan guarantees are superior. Surprisingly, we discover that these newer algorithms significantly underperform. Through further investigation, we identify as the culprit a flawed but common abstraction regarding the cost of collisions. Our experimental results are complemented by analytical arguments that the number of collisions -- and not solely makespan -- is an important metric to optimize. We believe that these findings have implications for the design of contention-resolution algorithms.Comment: Accepted to the 29th ACM Symposium on Parallelism in Algorithms and Architectures (SPAA 2017

Consistency mechanisms for a distributed lookup service supporting mobile applications

This paper presents a general-purpose distributed lookup service, denoted Passive Distributed Indexing (PDI). PDI stores entries in form of (key, value) pairs in index caches located in each mobile device. Index caches are filled by epidemic dissemination of popular index entries. By exploiting node mobility, PDI can resolve most queries locally without sending messages outside the radio coverage of the inquiring node. Thus, PDI reduces network traffic for the resolution of keys to values. For keeping index caches coherent, configurable value timeouts implementing implicit invalidation and lazy invalidation caches implementing explicit invalidation are introduced. Inconsistency in index caches due to weak connectivity or node failure is handled by value timeouts. Lazy invalidation caches reduce the fraction of stale index entries due to modified data at the origin node. Similar to index caches, invalidation caches are filled by epidemic distributions of invalidation messages. Simulation results show that with the suitable integration of both invalidation mechanisms, more than 95% of results delivered by PDI index caches are up-to-date for the considered scenario

Scoring dynamics across professional team sports: tempo, balance and predictability

Despite growing interest in quantifying and modeling the scoring dynamics within professional sports games, relative little is known about what patterns or principles, if any, cut across different sports. Using a comprehensive data set of scoring events in nearly a dozen consecutive seasons of college and professional (American) football, professional hockey, and professional basketball, we identify several common patterns in scoring dynamics. Across these sports, scoring tempo---when scoring events occur---closely follows a common Poisson process, with a sport-specific rate. Similarly, scoring balance---how often a team wins an event---follows a common Bernoulli process, with a parameter that effectively varies with the size of the lead. Combining these processes within a generative model of gameplay, we find they both reproduce the observed dynamics in all four sports and accurately predict game outcomes. These results demonstrate common dynamical patterns underlying within-game scoring dynamics across professional team sports, and suggest specific mechanisms for driving them. We close with a brief discussion of the implications of our results for several popular hypotheses about sports dynamics.Comment: 18 pages, 8 figures, 4 tables, 2 appendice

Exploring user and system requirements of linked data visualization through a visual dashboard approach

One of the open problems in SemanticWeb research is which tools should be provided to users to explore linked data. This is even more urgent now that massive amount of linked data is being released by governments worldwide. The development of single dedicated visualization applications is increasing, but the problem of exploring unknown linked data to gain a good understanding of what is contained is still open. An effective generic solution must take into account the user’s point of view, their tasks and interaction, as well as the system’s capabilities and the technical constraints the technology imposes. This paper is a first step in understanding the implications of both, user and system by evaluating our dashboard-based approach. Though we observe a high user acceptance of the dashboard approach, our paper also highlights technical challenges arising out of complexities involving current infrastructure that need to be addressed while visualising linked data. In light of the findings, guidelines for the development of linked data visualization (and manipulation) are provided

Exploiting epidemic data dissemination for consistent lookup operations in mobile applications

This paper presents a general-purpose distributed lookup service, denoted Passive Distributed Indexing (PDI). PDI stores entries in form of (key, value) pairs in index caches located at mobile devices. Index caches are filled by epidemic dissemination of popular index entries. By exploiting node mobility, PDI can resolve most queries locally without sending messages outside the radio coverage of the inquiring node. For keeping index caches coherent, configurable value timeouts implementing implicit invalidation and lazy invalidation caches implementing explicit invalidation are introduced. Inconsistency in index caches due to weak connectivity or node failure is handled by value timeouts. Lazy invalidation caches reduce the fraction of stale index entries due to modified data at the origin node. Similar to index caches, invalidation caches are filled by epidemic distributions of invalidation messages. We evaluate the performance of PDI for a mobile P2P file sharing a mobile instant messaging application. Simulation results show that with the suitable integration of both invalidation mechanisms, up to 80% of the lookup operations return correct results and more than 90% of results delivered by PDI index caches are up-to-date