The Real-time Transport Protocol (RTP) is widely used in telephony, video conferencing, and telepresence applications. Such applications are often run on best-effort UDP/IP networks. If congestion control is not implemented in the applications, then network congestion will deteriorate the user's multimedia experience. This document does not propose a congestion control algorithm; instead, it defines a minimal set of RTP "circuit-breakers". Circuit-breakers are conditions under which an RTP sender needs to stop transmitting media data in order to protect the network from excessive congestion. It is expected that, in the absence of severe congestion, all RTP applications running on best-effort IP networks will be able to run without triggering these circuit breakers. Any future RTP congestion control specification will be expected to operate within the constraints defined by these circuit breakers.