In this section, we will present the Listen protocol, a data plane verification technique that detects reachability problems in the data plane. Reachability problems can occur due to a variety of reasons ranging from routing problems to misconfigurations to link failures. Listen primarily signals the existence of such problems as opposed to identifying the source or type of a problem.
Data plane verification mechanisms are necessary in two contexts: (a) connectivity problems due to stale routes or forwarding problems are detectable only by data plane solutions like Listen. (b) Blackhole attacks by malicious adversaries already present along a path to a destination. However, proactive malicious nodes can defeat any data plane solution by impersonating the behavior of a genuine end-hosts. The attractive features of Listen are: (a) passive (b) a standalone solution that can be incrementally deployed without any modifications to BGP; (c) quick detection of reachability problems for popular prefixes; (d) low overhead.
The basic form of the protocol described in this section is vulnerable to port scanners generating many incomplete connections. In Section 6.2, we use propose defensive measures against port scanners and motivate them using real world measurements.