ITU-T H.248.82 PDF

Explicit congestion notification is a mechanism to provide indications of incipient congestion affecting an RTP stream to a receiver and, usually, to a sender. ECN, when used with an RTP stream over UDP, uses the RTP control protocol to provide feedback of ECN congestion markings to an RTP sender. Depending on previous signalling the sender and/or receiver can take action to minimize congestion in reception of the ECN congestion markings.

The session description protocol (SDP) is utilized to indicate support of ECN and what parameters relate to its use. As a media gateway (MG) sends/receives RTP based media (and RTCP), these parameters must be agreed and coordinated between the media gateway controller (MGC) and the MG. In ITU-T H.248 controlled gateways, this information must be provided via ITU-T H.248.

This Recommendation defines the "ECN for RTP-over-UDP Support Package" in order to establish ECN support on media gateways.