This document discusses the nature of signals seen by on-path
elements examining transport protocols, contrasting implicit and
explicit signals. For example, TCP's state machine uses a series of
well-known messages that are exchanged in the clear. Because these
are visible to network elements on the path between the two nodes
setting up the transport connection, they are often used as signals
by those network elements. In transports that do not exchange these
messages in the clear, on-path network elements lack those signals.
Often, the removal of those signals is intended by those moving the
messages to confidential channels. Where the endpoints desire that
network elements along the path receive these signals, this document
recommends explicit signals be used.
