BT BT & UCL University of Wuerzburg Pre-congestion notification (PCN) provides information to support admission control and flow termination in order to protect the Quality of Service of inelastic flows. It does this by marking packets when traffic load on a link is approaching or has exceeded a threshold below the physical link rate. This document specifies how such marks are to be encoded into the IP header. The baseline encoding described here provides for only two PCN encoding states. Other documents describe extended encoding schemes that allow for three encoding states. Status This memo is posted as an Internet-Draft with an intent to eventually progress to standards track.