ICMP is a key protocol to exchange control and error messages over the Internet. An appropriate ICMP's processing throughout a path is therefore a key requirement both for troubleshooting operations (e.g. debugging routing problems) and for several functionalities (e.g. Path Maximum Transmission Unit Discovery, PMTUD). Unfortunately it is common to see ICMP malfunctions, thereby causing various levels of problems. The contributions of this paper are threefold. The authors first introduce a taxonomy of the way routers process ICMP, which is of great help to understand for instance certain trace-route outputs. Secondly, they introduce IB-Track, a tool that any user can use to automatically characterize ICMP issues within the Internet, without requiring any additional in-network assistance (e.g. there is no vantage point).