Tracing Internet Path Transparency

Investigating Internet Path Transparency means measuring if a network path between two endhosts is impaired by in-network functions on the path. A path is considered transparent if it provides connectivity and the same performance independent of the protocol or protocol stack that is used for the transmission. Unfortunately this is not always the case. Simple firewalls that block e.g. UDP, are an example. Of course such in-network functions are often valuable, like firewalls. However, these middleboxes also, sometimes unintentionally, make assumptions about the traffic passing through them that restricts innovation in the Internet on the higher layers, e.g. the deployment of new UDP-based protocols such as QUIC, to stick with the previous example. PATHspider is an active measurement tool to test for Path Transparency. In this paper we present a new feature of PATH-spider that integrates tracebox-based functionality and analysis to not only detect in-transparency but also further locate the origin of the impairment observed. As an example study we show updated and extended measurements on ECN support and connectivity. By using our enhanced ECN PATHspider plugin to test network support of the ECN IP codepoint and additional path tracing that is correlated with DSCP testing, we show that most in-network ECN IP codepoint zeroing is due to use of the deprecated definition of the IP ToS field for domain-internal service differentiation, while pure resetting of the ECN IP field is more likely an active inference in border networks.