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ISIS Protocol, The first CCIE Journey

Connectionless-mode Network Service (CLNS) ?

Connectionless-mode Network Service

Connectionless-mode Network Service (CLNS) or simply Connectionless Network Service is an OSI Network Layer datagram service that does not require a circuit to be established before data is transmitted and routes messages to their destinations independently of any other messages.[1][2] As such it is a “best-effort” rather than a “reliable” delivery service. CLNS is not an Internet service, but provides capabilities in an OSI network environment similar to those provided by the Internet Protocol (IP) and the User Datagram Protocol (UDP).

Connectionless-mode Network Protocol (CLNP)

In an OSI protocol deployment, CLNS is the service provided by the Connectionless-mode Network Protocol (CLNP). CLNP is widely used in many telecommunications networks around the world because IS-IS (an OSI routing protocol) is mandated by the ITU-T as the protocol for management of Synchronous Digital Hierarchy (SDH) elements. From August 1990 to April 1995 the NSFNET backbone supported CLNP in addition to TCP/IP.[3] However, CLNP usage remained low compared to TCP/IP.

Transport Protocol Class 4 (TP4) in conjunction with CLNS

CLNS is used by ISO Transport Protocol Class 4 (TP4), one of the five transport layer protocols in the OSI suite. TP4 offers error recovery, performs segmentation and reassembly, and supplies multiplexing and demultiplexing of data streams over a single virtual circuit. TP4 sequences PDUs and retransmits them or re-initiates the connection if an excessive number are unacknowledged. TP4 provides reliable transport service and functions with either connection-oriented or connectionless network service. TP4 is the most commonly used of all the OSI transport protocols and is similar to the Transmission Control Protocol (TCP) in the TCP/IP protocol suite.

Protocols providing CLNS

Several protocols provide the CLNS service:[2]





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