Re-ECN: Adding Accountability for Causing Congestion to TCP/IP

This document introduces a new protocol for explicit congestion notification (ECN), termed re-ECN, which can be deployed incrementally around unmodified routers. The protocol arranges an extended ECN field in each packet so that, as it crosses any interface in an internetwork, it will carry a truthful prediction of congestion on the remainder of its path. Then the upstream party at any trust boundary in the internetwork can be held responsible for the congestion they cause, or allow to be caused. So, networks can introduce straightforward accountability and policing mechanisms for incoming traffic from end-customers or from neighbouring network domains. The purpose of this document is to specify the re-ECN protocol at the IP layer and to give guidelines on any consequent changes required to transport protocols. It includes the changes required to TCP both as an example and as a specification. It also gives examples of mechanisms that can use the protocol to ensure data sources respond correctly to congestion. And it describes example mechanisms that ensure the dominant selfish strategy of both network domains and end- points will be to set the extended ECN field honestly.