This document describes an abstract mechanism by which senders inform
the network about the congestion recently encountered by packets in
the same flow. Today, network elements at any layer may signal
congestion to the receiver by dropping packets or by Explicit
Congestion Notification (ECN) markings, and the receiver passes this
information back to the sender in transport-layer feedback. The
mechanism described here enables the sender to also relay this
congestion information back into the network in-band at the IP layer,
such that the total amount of congestion from all elements on the
path is revealed to all IP elements along the path, where it could,
for example, be used to provide input to traffic management. This
mechanism is called Congestion Exposure, or ConEx. The companion
document, "Congestion Exposure (ConEx) Concepts and Use Cases"
(RFC 6789), provides the entry point to the set of ConEx documentation.