Understanding & Predicting User Lifetime with Machine Learning in an Anonymous Location-Based Social Network · WWW LocWeb 2021

International Workshop on Location and the Web at WWW 2021

Jens Helge Reelfs, Max Bergmann, Oliver Hohlfeld, Niklas Henckell

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In this work, we predict the user lifetime within the anonymous and location-based social network Jodel in the Kingdom of Saudi Arabia. Jodel’s location-based nature yields to the establishment of disjoint communities country-wide and enables for the first time the study of user lifetime in the case of a large set of disjoint communities. A user’s lifetime is an important measurement for evaluating and steering customer bases as it can be leveraged to predict churn and possibly apply suitable methods to circumvent potential user losses. We train and test off the shelf machine learning techniques with 5-fold crossvalidation to predict user lifetime as a regression and classification problem; identifying the Random Forest to provide very strong results. Discussing model complexity and quality tradeoffs, we also dive deep into a time-dependent feature subset analysis, which does not work very well; Easing up the classification problem into a binary decision (lifetime longer than timespan 𝑥) enables a practical lifetime predictor with very good performance.We identify implicit similarities across community models according to strong correlations in feature importance. A single countrywide model generalizes the problem and works equally well for any tested community; the overall model internally works similar to others also indicated by its feature importances.

Word-Emoji embeddings from large scale messaging data reflect real world semantic associations of expressive icons · ICWSM Emoji 2020

ICWSM Workshop on Emoji Understanding and Applications in Social Media 2020

Jens Helge Reelfs, Oliver Hohlfeld, Markus Strohmaier, Niklas Henckell

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We train word-emoji embeddings on large scale messaging data obtained from the Jodel online social network. Our data set contains more than 40 million sentences, of which 11 million sentences are annotated with a subset of the Unicode 13.0 standard Emoji list. We explore semantic emoji associations contained in this embedding by analyzing associations between emojis, between emojis and text, and between text and emojis. Our investigations demonstrate anecdotally that word-emoji embeddings trained on large scale messaging data can reflect real-world semantic associations. To enable further research we release the Jodel Emoji Embedding Dataset (JEED1488) containing 1488 emojis and their embeddings along 300 dimensions.

Corona-Warn-App: Tracing the Start of the Official COVID-19 Exposure Notification App for Germany · SIGCOMM Poster 2020

ACM Special Interest Group on Data Communication - Poster 2020

Jens Helge Reelfs, Oliver Hohlfeld, Ingmar Poese

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On June 16, 2020, Germany launched an open-source smartphone contact tracing app (“Corona-Warn-App”) to help tracing SARSCoV- 2 (coronavirus) infection chains. It uses a decentralized, privacy preserving design based on the Exposure Notification APIs in which a centralized server is only used to distribute a list of keys of SARSCoV-2 infected users that is fetched by the app once per day. Its success, however, depends on its adoption. In this poster, we characterize the early adoption of the app using Netflow traces captured directly at its hosting infrastructure. We show that the app generated traffic from allover Germany—already on the first day. We further observe that local COVID-19 outbreaks do not result in noticeable traffic increases.

Application-Agnostic Offloading of Packet Processing · arXiv 2019


Oliver Hohlfeld, Jens Helge Reelfs, Jan Rüth, Florian Schmidt, Torsten Zimmermann, Jens Hiller, Klaus Wehrle

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[ More Hot Science · BMC: Accelerating Memcached using Safe In-kernel Caching and Pre-stack Processing, Ghigoff et al. USENIX Symposium on Networked Systems Design and Implementation NSDI'21 PDF ]

As network speed increases, servers struggle to serve all requests directed at them. This challenge is rooted in a partitioned data path where the split between the kernel space networking stack and user space applications induces overheads. To address this challenge, we propose Santa, a new architecture to optimize the data path by enabling server applications to partially offload packet processing to a generic rule processor. We exemplify Santa by showing how it can drastically accelerate kernel-based packet processing - a currently neglected domain. Our evaluation of a broad class of applications, namely DNS, Memcached, and HTTP, highlights that Santa can substantially improve the server performance by a factor of 5.5, 2.1, and 2.5, respectively.

Determining Response-generating Contexts on Microblogging Platforms · KONVENS 2019

Conference on Natural Language Processing (KONVENS) 2019

Jennifer Fest, Arndt Heilmann, Oliver Hohlfeld, Stella Neumann, Jens Helge Reelfs, Marco Schmitt, Alina Vogelgesang

[ Venue in Liguistics ]

In recent years the study of social media communities has come into the focus of research. One open but central question is which properties stimulate user interaction within communities and thus contribute to community building. In this paper, we provide a first step towards answering this question by identifying features in the Jodel microblogging app that trigger user responses as one form of attention. Jodel is a geographically restricted app that allows users to post threads and comments anonymously. The absence of displayed user information on Jodel makes the posted content the only trigger for user interaction, making the language the one and only means for users to gather contextual implications about their discourse partners. This enhanced function of language promises a revealing baseline investigation into linguistic behavior on social media.

To approach this issue, we conducted a sequence of lexico-grammatical analyses and subjected the quantitative results to various statistical tests. While a Principal Component Analysis did not show a significant difference between the grammatical structure of original posts with and without answers, a negative binomial regression model focusing on the interpersonal meta-function yielded significant results. A further analysis of these features correlated to shorter or longer response times showed significant results for the interrogative mood. Additionally, keyword analyses revealed significant differences between posts with answers and without answers. Our study provides a promising first step towards understanding textual features triggering user interaction and thereby community building – an unresolved problem of practical relevance to social network operation.

Demystifying the Performance of XDP BPF · Netsoft 2019

IEEE Conference on Network Softwarization 2019

Oliver Hohlfeld, Johannes Krude, Jens Helge Reelfs, Jan Rüth, Klaus Wehrle


High packet rates at ≥ 10 GBit/s challenge the packet processing performance of network stacks. A common solution is to offload (parts of) the user-space packet processing to other execution environments, e.g., into the device driver (kernel-space), the NIC or even from virtual machines into the host operating system (OS), or any combination of those. While common wisdom states that offloading optimizes performance, neither benefits nor negative effects are comprehensively studied. In this paper, we aim to shed light on the benefits and shortcomings of eBPF/XDP-based offloading from the user-space to i) the kernel-space or ii) a smart NIC-including VM virtualization. We show that offloading can indeed optimize packet processing, but only if the task is small and optimized for the target environment. Otherwise, offloading can even lead to detrimental performance.

Hashtag Usage in a Geographically-Local Microblogging App · WWW LocWeb 2019

International Workshop on Location and the Web at WWW 2019

Jens Helge Reelfs, Timon Mohaupt, Oliver Hohlfeld, Niklas Henckell

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This paper studies for the first time the usage and propagation of hashtags in a new and fundamentally different type of social media that is i) without profiles and ii) location-based to only show nearby posted content. Our study is based on analyzing the mobile-only Jodel microblogging app, which has an established user base in several European countries and Saudi Arabia. All posts are user to user anonymous (i.e., no displayed user handles) and are only displayed in the proximity of the user’s location (up to 20 km). It thereby forms local communities and opens the question of how information propagates within and between these communities. We tackle this question by applying established metrics for Twitter hashtags to a ground-truth data set of Jodel posts within Germany that spans three years. We find the usage of hashtags in Jodel to differ from Twitter; despite embracing local communication in its design, Jodel hashtags are mostly used country-wide.

SPLIT: Smart Protocol Loading for the IoT · EWSN 2018

Proceedings of the 2018 International Conference on Embedded Wireless Systems and Networks 2018

Torsten Zimmermann, Jens Hiller, Jens Helge Reelfs, Pascal Hein, Klaus Wehrle


The Internet of Things (IoT) permeates our everyday life, e.g., in the area of health monitoring, wearables, industry, and home automation. It comprises devices that provide only limited resources, operate in challenging network conditions, and are often battery-powered. To embed these devices into the Internet, they are envisioned to operate standard protocols. Yet, these protocols occupy the majority of limited program memory resources. Thus, devices can neither add application logic nor apply security updates or adopt optimizations for efficiency. This problem will further exacerbate in the future as the further ongoing permeation of smart devices in our environment demands for more and more functionality.

To overcome limited functionality due to resource constraints, we show that not all functionality is required in parallel, and thus can be SPLIT in a feasible manner. This enables on-demand loading of functionality outsourced as (multiple) modules to the significantly lesser constrained flash storage of devices. We exemplify efficient modularization of DTLS and show that SPLIT enables operation of large protocol stacks while it incurs reasonable, tunable performance trade-offs. Our use case specific results show an initial runtime overhead of 23.34 % and 4.9 % for subsequent protocol executions.

Application-Agnostic Offloading of Datagram Processing · ITC 2018

International Teletraffic Congress 2018

Oliver Hohlfeld, Jens Helge Reelfs, Jan Rüth, Florian Schmidt, Torsten Zimmermann, Jens Hiller, Klaus Wehrle

PDF slides
[ Related · see our full tech report ]

As network speed increases, servers struggle to serve all requests directed at them. This challenge is rooted in a partitioned data path where the split between the kernel space networking stack and user space applications induces overheads. To address this challenge, we propose Santa, an architecture to optimize the data path by enabling server applications to (partially) offload packet processing to a generic rule processor. We exemplify Santa by showing how it can drastically accelerate UDP packet processing in the Linux kernel—a currently neglected domain. Our evaluation focuses on accelerating DNS traffic for which we find a performance increase by a factor of 5.5 on realworld request pattern.

How the construction of community-like social orders can stop the proliferation of polarizing communication in social media -- The case of Jodel · 2017


Marco Schmitt, Markus Baum, Daniel Kerpen, Jens Helge Reelfs, Oliver Hohlfeld

The Rise of an Anonymous Social Network · IMC Poster 2017

Internet Measurement Conference 2017

Jens Helge Reelfs, Oliver Hohlfeld


Social networks have become a popular Internet service and exist in various flavors. These flavors can be categorized by i) infrastructure (e.g., centralized vs. decentralized / P2P), ii) user profile (e.g., real names, pseudonyms, or anonymity), iii) geographic coverage (e.g., global vs. regional), or iv) functionality (e.g., microblogging on Twitter or questions on Stackoverflow). Most prominent networks are centralized, provide global coverage and usually do not permit anonymous use (e.g., Facebook). A large body of research has focused on understanding these networks and user behavior by analyzing underlying friendship/connection graphs, content, or usage pattern. Besides these well-known and well-studied networks, there exists a rising demand for both i) location-based/regional (e.g., Nextdoor) and ii) anonymous (e.g., Whisper) networks. Anonymous networks and location-based network have yet, however, not received much attention in scientific literature.

In this poster, we present initial results obtained by crawling an emerging anonymous and location-based social network for more than one year. This network is predominant in Germany and Scandinavia and is currently expanding across Europe and other continents. It enables users to post text content or images and to comment on posts within threads. It differs from other social networks by two aspects: 1) It is location based and only displays posts sent within close (e.g., 10km) geographic proximity; 2) All communication is anonymous by not displaying user handles. Users are only enumerated by posting order within a single discussion thread, to enable users referencing to each other within a discussion.

To prevent abuse, the network employs a community filtering and moderation system. This community moderation follows a simple filtering scheme which prefers mainstream content: posts may be voted up or down resulting in a cumulative score. When the per-post or answer score exceeds a negative score of -5, it is not being shown anymore. To increase user engagement and voting, the network applies gamification by awarding “Karma” points that users collect by either voting or posting content that is upvoted by others. Further, harmful or non-policy compliant content can be flagged for moderation. Such reported content is reviewed by volunteering community moderators (selected by properties like activity or Karma scores) and is kept or removed according to the moderators' majority vote. For an anonymous social network, community moderation is a key success parameter to prevent harmful or abusive content. The recent downfall of the YikYak anonymous network highlighted that unsuccessfully preventing adverse content can seriously hurt the network.

To study the network, we retrieve posts by performing continuous crawls from about 200 cities for a period of more than one year summing up to about NNN posts. Based on this data, we present preliminary results on the network in terms of activity within different cities and posted content. We further present preliminary results from studying the networks community moderation system. By discussing our first results with the IMC community, we aim to shed light on an emerging and less studied type of social network.

Dynamic algorithm selection for the logic of tasks in IoT stream processing systems · CNSM 2017

IEEE Conference on Network and Service Management 2017

Ehsan Poormohammady, Jens Helge Reelfs, Mirko Stoffers, Klaus Wehrle, Apostolos Papageorgiou


Various Internet of Things and Industry 4.0 use cases such as city-wide monitoring, Smart Grid control, or machine control, require low-latency distributed processing of continuous data streams. This fact has boosted research on making Stream Processing Frameworks (SPFs) IoT-ready, meaning that their cloud and IoT service management mechanisms (e.g., task placement, load balancing, algorithm selection) need to consider new requirements derived from IoT-specific characteristics, i.e., ultra low latency due to physical interactions. Although various extensions have appeared to optimize such SPF-provided mechanisms, they still lack the modules, data models, and algorithms to properly handle algorithm selection in IoT deployments. The algorithm selection problem refers to selecting dynamically which internal logic a deployed streaming task should use in case of various alternatives. To the best of our knowledge, this work is the first solution that adds this capability to SPFs. Our solution is based on i) architectural extensions of typical SPF middleware, ii) a new schema for characterizing algorithmic performance in the targeted context, and iii) a streaming-specific optimization problem formulation. We implemented our solution as an extension to Apache Storm and demonstrate how it can reduce stream processing latency by up to a factor of 2.9 in the tested scenarios.

It's time to combine Network Advances and Databases · CoNEXT Poster 2016

ACM International Conference on emerging Networking EXperiments and Technologies - Student Workshop 2016

Jens Helge Reelfs, Oliver Hohlfeld, Klaus Wehrle

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[ More Hot Science · BMC: Accelerating Memcached using Safe In-kernel Caching and Pre-stack Processing, Ghigoff et al. USENIX Symposium on Networked Systems Design and Implementation NSDI'21 PDF ]

Database architectures have fundamentally advanced (in-memory, parallelization, distribution) to support faster query execution and to manage higher workloads. Due to these advances, networking overheads have now become a new performance challenge, which is currently only tackled by Remote Direct Memory Access (RDMA). Fortunately, networking has recently made considerable latency and throughput improvements via kernel optimizations or by employing new architectures, e.g., kernel bypassing or specialized hardware.

We show that improved networking architectures indeed offer substantial—and currently unexplored—potential for database performance improvements regarding throughput, CPU-load, end-to-end and tail latency. To prove this potential, we used Santa, a packet processing offloading engine exemplarily implemented in the Linux kernel. Our results clearly remark reduced end-to-end latencies and throughput increases for both Memcached and MySQL by a factor of up to 1.5 and 3.3, respectively.

Attitude Independent Movement Determination using MEMS for Screen-less Pedestrian Navigation · MS Thesis 2013

RWTH Aachen - Master Thesis 2013

Jens Helge Reelfs

PDF slides

While indoor navigation systems attract more and more attention, smartphone-based dead reckoning systems heavily depend on the user holding the device in aspecific way. As soon as a user puts the device in her trouser pocket or jacket, they fail. In this thesis, we enable smartphones to accurately determine steps and the current bearing of a user even in these conditions.

We build a general model of the walking motion with specific instances for different device locations (trousers pocket, jacket pocket). Slicing the measured data within a step to only extract the segment which provides the most information on the user’s walking direction, our approach has a median absolute error of only 12° (mean: 22°, q75: 25°) on a total of 15 participants completing a total of 49 test runs. Therefore, together with proposed feedback generation methods, this thesis lies the foundation of true hands-free indoor navigation without the need for any infrastructure.

Implementation of a OneAPI-REST to SIP interface for integrating web services in an IMS-based telecommunication network · BS Thesis 2011

RWTH Aachen - Bachelor Thesis 2011

Jens Helge Reelfs

PDF slides

Latest development shows that telecommunication providers merge their networksinto all-IP-based networks. For this pupose, IMS (IP-Multimedia Subsystem) has been introduced. The architecture allows to deploy application servers providing several telecommunication services ontop of the IMS.

Motivated by the rapidly growing popularity of Web 2.0 applications and the emerging market with value added services on mobile devices, there have been introduced standards to combine the telecommunication and internet domain. The most recent standard is OneAPI which defines some key services in a RESTful design. RESTful Web Services are kept small by directly using the HTTP’s protocol mechanisms which makes them very efficient.

This thesis shows a prototype implementation of a generic REST to SIP interface, and a gateway module translating between specific OneAPI REST messages on a JAIN SLEE Application Server being an event driven platform to SIP. To perfect an example of the complete vertical communication through this AS, we also present an SMS service implementation.

A brief overview of a performance evaluation is given and the results are discussed.



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