This Recommendation | International Standard specifies a protocol that is used to provide the connectionless-mode Network service described in ITU-T Rec. X.213 | ISO/IEC 8348 and to perform certain Network layer management functions. The protocol relies upon the provision of an underlying connectionless-mode service by real subnetworks and/or data links. The underlying connectionless-mode service assumed by the protocol may be obtained either directly, from a connectionless-mode real subnetwork, or indirectly, through the operation of an appropriate Subnetwork Dependent Convergence Function (SNDCF) or Protocol (SNDCP) over a connection-mode real subnetwork, as described in ISO/IEC 8648. This Recommendation | International Standard specifies the operation of the protocol with respect to a uniform, abstract "underlying subnetwork service". Other Recommendations | International Standards specify the way in which this "underlying subnetwork service" is obtained from real subnetworks, such as those which conform to ISO/IEC 8802 or ISO/IEC 8208. The "underlying subnetwork service" may be obtained from real subnetworks other than those that are specifically covered by the other Recommendations | International Standards. This Recommendation | International Standard specifies: a) procedures for the connectionless transmission of data and control information from one Network entity to one or more peer Network entities; b) the encoding of the Protocol Data Units (PDUs) used for the transmission of data and control information, comprising a variable-length protocol header format; c) procedures for the correct interpretation of protocol control information; and d) the functional requirements for implementations claiming conformance to this Recommendation | International Standard. The procedures are defined in terms of: a) the interactions among peer Network entities through the exchange of protocol data units; b) the interactions between a Network entity and a Network service user through the exchange of Network service primitives; and c) the interactions between a Network entity and an abstract underlying service provider through the exchange of service primitives. This Recommendation | International Standard also provides the PICS proforma for this protocol, in compliance with the relevant requirements, and in accordance with the relevant guidance, given in ITU-T Rec. X.290 | ISO/IEC 9646-1