Connectionless-mode Network Service
|The OSI model|
|7 Application layer|
|6 Presentation layer|
|5 Session layer|
|4 Transport layer|
|3 Network layer|
|2 Data link layer|
|1 Physical layer|
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. 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 layer 2 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. 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:
- Connectionless-mode Network Protocol (CLNP), as specified in ITU-T Recommendation X.233.
- End System-to-Intermediate System (ES-IS), a routing exchange protocol for use in conjunction with the protocol for providing the CLNS (ISO 9542).
- Intermediate System-to-Intermediate System (IS-IS), an intradomain routing exchange protocol used in both the OSI and Internet environments (ISO 10589 and RFC 1142).
- Signalling Connection Control Part (SCCP), as specified in ITU-T Recommendation Q.711 is a Signaling System 7 protocol.
See also 
- "Section 3: Definition of the Connectionless-Mode Service", ITU-T Recommendation X.213: Open Systems Interconnection – Network service definition, International Telecommunications Union, October 2001
- "ISO Connectionless Network Service", Chapter 2, Section 1 in IS-IS Network Design Solutions, Cisco Press, 7 February 2002, ISBN: 1-57870-220-8
- Link Letter, Volume 4, No. 3 (Sept/Oct 1991), p. 1, NSFNET Information Services, Merit Network, Inc., Ann Arbor
- ITU-T Recommendation X.233: Protocol for providing the connectionless-mode network service: Protocol specification, International Telecommunications Union, August 1997
- ITU-T Recommendation Q.711: Specifications of Signalling System No. 7 – Signalling connection control part (SCCP), International Telecommunications Union, March 2001
What is CLNS? - a brief introduction by Ivan Pepelnjak