The Free On-line Dictionary of Computing (19 January 2023):

Estelle

   A Pascal extension for formal specification of computer
   network protocols.  Protocols are described by modules
   which are communicating NFAs.  Modules are arranged in a
   dynamic hierarchy and communicate at named interaction points.

   EstPC (ftp:osi.ncsl.nist.gov/pub/osikit/estpc) Compiles
   Estelle into C.  petdingo Translates Estelle into C++

   Adopted by ITU-T.  ISO 9074 (1989).

   ["The Formal Description Technique Estelle", M.  Diaz et al
   eds, N-H 1989].