Pascal Hennen after successfully defending his thesis. Photo: MPI-INF/Bertram Somieski
On Monday, 09 February 2026, Pascal Hennen defended his thesis with the title: "An Empirical Evaluation of Messy BGP Data Sources". From January 2023 until December 2026 he was a PhD student in the department "Internet Architecture" supervised by its head Prof. Anja Feldmann. The doctoral degree is awarded by Saarland University.
Abstract of the thesis:
The Internet is the world's largest human-build system and as such evolved to – be rather complex. Operators use the Border Gateway Protocol (BGP) – the Internet's de-facto inter-AS routing protocol – to enable global connectivity. However, routing on the Internet is evolving. Although the specification of BGP has not changed since decades, its additions and usage patterns have. Thus, BGP has become an important topic to study for researchers. They use BGP data to, e.g., understand routing decisions, map the Internet's topology, and improve security. Each autonomous system (AS) uses BGP to realize its routing policies based on the business agreements that they have with its neighboring ASes. ASes typically do not share their business agreements publicly. Yet, ASes need to see the effects of a change in their BGP configuration. Route collector projects such as RouteViews and RIPE RIS collect BGP data from as many ASes as possible and make that data publicly available in BGP archives. In addition, data broker services provide interfaces to these BGP archives.
Whereas operators use this data to optimize their networks, researchers frequently use this data to study and understand the routing ecosystem. Until now the consistency and reliability of these data sources was usually assumed to be a given. However, it is not. In this dissertation, we fill this gap by investigating the temporal consistency (are routes recorded when they should be) and internal consistency (are routes recorded correctly).
Furthermore, we evaluate whether a popular BGP route collector data broker (BGPStream's broker) reliably returns all data files according to supplied search terms. As a policy-based protocol, BGP is implemented on the border routers of ASes. A border router maintains multiple BGP sessions and selects the best route for a prefix by evaluating all learned routes. This is done via BGP attributes. Adjusting these BGP attributes and/or filtering routes allows an AS to implement its routing policies and manage its relationships with other networks. It is commonly assumed that ASes use the same BGP policies for all sessions with the same neighbor AS, preferring the same next-hop AS for the same prefix.
In this dissertation, we show that this is often not the case – we refer to such ASes as being heterogeneous. We propose two inference methods to
(i) quantify the number of heterogeneous ASes as observed by the route collectors, and
(ii) identify ASes which explicitly diverge from the conventional BGP behavior. Route collectors yield a public view of the Internet – they do not show privately assigned BGP attributes. Thus, ASes collaborate with each other and operate publicly accessible Looking Glasses (LGs). LGs are websites that allow other operators to perform queries on a subset of routers within the ASes to gather routing information. In this dissertation, we collect a LG dataset that focuses on collecting BGP attributes from more than 149 LGs in 154 ASes from 931 routers via scraping LGs. Hereby, the difficulties relate to the non-uniformity of the LGs – most interfaces differ, the fluctuating accessibility of the LGs, as well as the different output formats. To overcome this, we combined manual configuration with an automated scraping process followed by careful post-processing and manual checks.