IS-IS: OSI Network Layer

The OSI reference model and related protocols were developed independently of TCP/IP and TCP/IP creators never intended to follow the OSI reference model.  As a result this makes certain concepts completely foreign to those used to working with TCP/IP predominantly.

ISO OSI terminology refers to hosts as End Systems (ES) and refers to routers as Intermediate Systems (IS).  the term System describes a network node, and the term Circuit stands for a specific interface.  The term Domain stands for Autonomous System.

The network layer specification of the OSI reference model is concerned with end to end communications between two ES entities, this calls for connectionless-mode and connection-mode network layer communication.

Connection-less mode is identical to IP as a pure data-gram service without any prior establishments.  In OSI networks the layer 3 network protocol provides a connectionless communication is called Connection-less mode Network Protocol (CLNP)

CLNP is to OSI networks as IPv4 is to TCP/IP networks. The services provided by CLNP are referred to as connection-less Network Services or CLNS.

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