The Free On-line Dictionary of Computing (30 December 2018):

Remote Reference Layer
RRL

    (RRL) Part of Java's Remote Method Invocation
   protocol.  RRL exists in both the RMI client and server.
   It is used by the stub or skeleton protocol layer and
   uses the transport layer.  RRL is reponsible for
   transport-independent functioning of RMI, such as connection
   management or unicast/multicast object invocation.

   [Specification URL?]

   (1997-12-03)