Provides the description of a means to permit a protocol to be identified; a record of the structure and allowable ranges of protocol identifiers which can be assigned by ISO and other authorities, a record of the values of protocol identifiers used by OSI network layer protocols and non-OSI protocols occupying a similar position, in particular, only protocols with protocol control information commencing in octet 1 of the protocol data unit are covered. This Standard is identical with and has been reproduced from ISO/IEC TR 9577:1990.