PTC is a TCP-based but extremely simplified transport protocol. It was (and still is!) developed in the context of Teoría de las Comunicaciones, an undergraduate course on networking of the MSc in Computer Science at the University of Buenos Aires. It is mainly aimed at providing a simple enough framework for delving into several transport-layer concepts (such as the implementation of "real" sliding window algorithms, connection establishment and maintenance, state transitions, and so forth) in a pragmatic way.
For further details, please see the wiki.