10. Issues Raised by Monitoring and Policing Devices

10. Issues Raised by Monitoring and Policing Devices

One possibility is to install monitoring and policing devices in the network (or more informally called "penalty boxes") to monitor whether best-effort flows respond appropriately to congestion, and to preferentially drop packets from flows that are determined not to be using adequate end-to-end congestion control procedures.

We recommend that any "penalty box" that detects a flow or flow aggregate not responding to end-to-end congestion control should first change from marking to dropping packets from that flow before taking any additional action to limit the bandwidth available to that flow. Thus, initially, the router could drop packets that would otherwise have the CE codepoint set. This could include dropping those packets in that flow that have already arrived as ECN-capable and already have the CE codepoint set. In this way, any congestion indication that router sees for that flow will be guaranteed to also be seen by the end nodes, even if there are malicious or corrupted routers elsewhere on the path. If we assume that the first action that any "penalty box" takes on an ECN-capable flow will be to drop packets instead of marking them, then malicious or corrupted entities within the protocol are unlikely to be able to subvert end-to-end congestion control beyond routers dropping packets.