The objective of the Pre-Congestion Notification (PCN) architecture is to protect the quality of service (QoS) of inelastic flows within a Diffserv domain. It achieves this by marking packets belonging to PCN-flows when the rate of traffic exceeds certain configured thresholds on links in the domain. These marks can then be evaluated to determine how close the domain is to being congested. This document specifies how such marks are encoded into the IP header by redefining the Explicit Congestion Notification (ECN) codepoints within such domains. The baseline encoding described here provides only two PCN encoding states: Not-marked and PCN-marked. Future extensions to this encoding may be needed in order to provide more than one level of marking severity.