Fahad Hilal after after successfully defending his thesis. Photo: MPI-INF/ Iris Wagner
On Wednesday, 30 September 2026 Fahad Hilal defended his thesis with the title: “From Ossification to Off-Nets: Detecting Middleboxes and Revealing Hypergiant Server Deployment Strategies”. Since December 2020 until January 2022 he was a students helper and Master student, following until May 2026 PhD student in Computer Science at Saarland University in Saarbrücken and the Max Planck Institute for Informatics under the supervision of Prof. Anja Feldmann, head of department Internet Architecture. The doctoral degree is awarded by Saarland University.
Abstract of the thesis:
The Internet has grown from a small research project into one of the most complex inventions, supporting communication, commerce, and content delivery on a global scale. Its early design embraced design philosophies that emphasized simplicity, yet the demands of billions of users and diverse applications have driven it far beyond these original blueprints. This dissertation investigates how the Internet has grown in complexity, focusing on two central themes: the proliferation of path-impairing middleboxes that challenge protocol evolution, and the large-scale restructuring of content delivery through hypergiant off-net deployments.
The first part of this work examines the feasibility of detecting path-impairing middle-boxes at Internet scale, with a focus on unexpected alterations to network- and transport layer headers, particularly those affecting TCP. Such in-transit modifications can interfere with protocol operation and complicate the deployment of extensions. To enable large-scale detection, this dissertation develops Yarrpbox, a stateless and high-speed measurement tool designed to uncover header changes along Internet paths. By making Internet-scale measurement of these specific impairments practical, this work provides a basis for assessing the extent of protocol ossification and offers insights relevant to the evolution of transport protocols.
The second part of this work builds on the measurement capabilities of Yarrpbox to assess the behavioral characteristics, topological position, and the persistence of path-impairing middleboxes. Through geographically distributed measurements, it studies the extent to which middleboxes affect the IPv6 and the IPv4 Internet, with the main focus on their impact on TCP traffic. The work also seeks to understand their underlying motivations behind impairments, drawing not only on measurement data but also on engagement with the network operator community to place these practices in their operational context.
The third part of this work turns to the theme of shifting content delivery paradigms, focusing on the growing trend of off-net server deployments by hypergiants such as Google, Meta, and Netflix. By embedding infrastructure directly within access and ser-vice provider networks, these actors reshape traffic flows and challenge the Internets traditional hierarchical structure. This study investigates the presence of such deployments over both IPv4 and IPv6, asking to what extent strategies converge across the two address families or diverge in terms of network type, size, and regional footprint. Beyond mapping deployment patterns, the work also considers their broader implications by comparing the performance and security posture of these infrastructures.
By engaging with these research themes, this dissertation sheds light on the ways in which deviations from traditional design principles both constrain and transform the Internet. It contributes new tools, measurement methodologies, and Internet-scale data along both address families for assessing how middleboxes affect protocol evolution and network transparency, and how hypergiants content delivery strategies alter Internet structure while driving improvements in performance. Together, these contributions provide fresh perspectives on the Internets evolving architecture.