The Internet provides no information on the fate of transmitted packets, and end systems cannot determine who is responsible for dropping or delaying their traffic. As a result, they cannot verify that their ISPs are honoring their service level agreements, nor can they react to adverse network conditions appropriately. While current probing tools provide some assistance in this regard, they only give feedback on probes, not actual traffic. Moreover, service providers could, at any time, render their network opaque to such tools. We propose an explicit accountability interface, through which ISPs can pro-actively supply feedback to traffic sources on loss and delay, at administrative-domain granularity. Notably, our interface is resistant to ISP lies and can be implemented with a modest NetFlow modification. On our Click-based prototype, playback of real traces from a Tier-1 ISP reveals less than 2% bandwidth overhead. Finally, our proposal benefits not only end systems, but also ISPs, who can now control the amount and quality of information revealed about their internals.