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.
["Section 3: Definition of the Connectionless-Mode Service"]
''ITU-T Recommendation X.213: Open Systems Interconnection – Network service definition'', International Telecommunication Union, October 2001
, Chapter 2, Section 1 in ''IS-IS Network Design Solutions'', Cisco Press, 7 February 2002, As such it is a "
best-effort
Best-effort delivery describes a network service in which the network does ''not'' provide any guarantee that data is delivered or that delivery meets any quality of service. In a best-effort network, all users obtain best-effort service. Under b ...
" 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 suite. The service is specified in ISO/IEC 8348, the OSI Network Service Definition (which also defines the connection-oriented service, CONS.)
Connectionless-mode Network Protocol
Connectionless-mode Network Protocol (CLNP) is 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
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diode ...
(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
Internet 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/IEC 10589 and RFC 1142).
* Interdomain Routing Protocol (IDRP), the OSI equivalent of BGP.
* (SCCP), as specified in ITU-T Recommendation Q.711''ITU-T Recommendation Q.711: Specifications of Signalling System No. 7 – Signalling connection control part (SCCP)'']
International Telecommunication Union, March 2001 is a
Signaling System 7
Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in 1975, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also perform ...
protocol.
See also
*
X.25 protocol suite
X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts a ...
, an OSI Connection Oriented Network Service (CONS)
References
{{Reflist
External links
What is CLNS?- a brief introduction by Ivan Pepelnjak
ITU-T recommendations
OSI protocols
Network layer protocols