1. A method of triggering content substitution, comprising: receiving an ordered stream of packets containing content marked by a first packet identifier (PID) and substitute content marked by a secondary PID, and where the number and placement of packets marked by secondary PIDs ahead of packets marked by the first PID in the stream is retained during transmission of the ordered stream of packets; determining if a number of intervening packets having secondary PIDs that reside between the packets having the first PID and the packets having a selected secondary PID is an odd number or an even number; if the number is a first of either an odd number or an even number of intervening packets having secondary PIDs that reside between the packets having the first PID and the packets having the selected secondary PID is received, interpreting receipt of said number of intervening packets as an instruction that triggers ignoring subsequent packets marked with a primary PID, and mapping the portions of the content having the secondary PIDs to the primary PID and placing the mapped content into the data stream as a substitute for the first portion of the content; if the number is the other of either an even number or an odd number of intervening packets having secondary PIDs that reside between the packets having the first PID and the packets having the selected secondary PID is received, interpreting receipt of said number of intervening packets as an instruction that triggers processing subsequent packets marked with the primary PID.