Congratulations to Florian Holzbauer on Successfully Passing His Rigorosum
On February 19, Florian Holzbauer successfully completed his Rigorosum, receiving excellent evaluations.
He successfully defended his dissertation, “Turning Failure into Knowledge: Extending the Observable Internet by Leveraging Delivery Failures,” before the committee and Florian’s PhD thesis supervisor Johanna Ullrich.
© Florian Holzbauer
Abstract
Delivery failures occur both in the postal system and on the Internet when packets cannot be forwarded to their destination. Active Internet measurements probe Internet-connected systems by sending packets and observing their responses. In practice, not all probes reach their target. When delivery fails, an error message may be returned indicating the cause of the failure, such as a nonexistent address or an unreachable system. Despite their diagnostic value, Internet measurements traditionally focus on successful delivery. As the Internet continues to evolve, including the transition to IPv6 and the deployment of new protocols, new measurement methods are required to observe previously unmeasured parts of the Internet.
This thesis shows how delivery failures can be systematically leveraged to expand the observable Internet. The thesis introduces three measurement approaches, email-security-scans.org, BValue Steps, and CountryMonitor, which use delivery failures to observe infrastructure and security properties that go beyond the capabilities of current state-of-the-art measurement techniques. These approaches extend measurement coverage in three domains: email delivery, IPv6 deployment, and Internet outage detection.
The thesis distinguishes between intentional and unintentional delivery failures. Intentional delivery failures are deliberately triggered by the measurement methodology to elicit protocol-defined responses, whereas unintentional delivery failures arise from real-world conditions such as host unavailability or network outages and are observed through missing responses. Email-security-scans.org and BValue Steps rely on intentional delivery failures, while CountryMonitor is based on unintentional delivery failures. In the domain of email measurements, delivery failures are used to extend observability beyond inbound email infrastructure. By intentionally triggering validation and rejection mechanisms, email-security-scans.org measures the configuration and adoption of security-relevant standards of outbound email servers at scale, including authentication and validation mechanisms that are otherwise difficult to observe. This enables a sender-side perspective on global email security practices. For IPv6 measurements, the large address space makes random probing of active hosts infeasible.
The thesis shows that delivery failures in the form of ICMPv6 error messages can be used to infer the presence of active networks and network boundaries without requiring successful packet delivery. By analyzing error message types and timing behavior, this approach extends the enumeration of active networks beyond previously observable scopes. Finally, delivery failures that manifest as missing responses are leveraged to monitor Internet disruptions. CountryMonitor uses loss of responsiveness to detect and track Internet outages over time in Ukraine, providing a longitudinal view of connectivity disruptions affecting numerous smaller Internet operators during an ongoing large-scale geopolitical conflict.