Editing Connectionless-mode Network Service

{{Short description|OSI network layer datagram service}}
{{OSIModel}}
{{Main |OSI protocols}}
{{Technical|date=September 2010}}

'''Connectionless-mode Network Service''' ('''CLNS''') or simply '''Connectionless Network Service''' is an [[OSI_protocols#Layer_3:_network_layer|OSI network layer]] [[datagram|datagram service]] that does not require a circuit to be established before data is transmitted, and routes messages to their [[NSAP address|destinations]] independently of any other messages.<ref name=ITU-T-X.213>[http://www.itu.int/rec/T-REC-X.213/en/ "Section 3: Definition of the Connectionless-Mode Service"], ''ITU-T Recommendation X.213: Open Systems Interconnection – Network service definition'', International Telecommunication Union, October 2001</ref><ref name=CISCOIS-IS>[http://fengnet.com/book/IS-ISDesign/ch02lev1sec1.html "ISO Connectionless Network Service"] {{webarchive|url=https://web.archive.org/web/20080621062139/http://fengnet.com/book/IS-ISDesign/ch02lev1sec1.html |date=2008-06-21 }}, Chapter 2, Section 1 in ''IS-IS Network Design Solutions'', Cisco Press, 7 February 2002, {{ISBN|1-57870-220-8}}</ref> As such it is a "[[Best-effort delivery|best-effort]]" rather than a "[[Reliability (computer networking)|reliable]]" delivery service. CLNS is not an [[Internet]] service, but provides capabilities in an [[OSI model|OSI network environment]] similar to those provided by the [[Internet protocol suite]].  The service is specified in ISO/IEC 8348,<ref>{{cite web |title=ISO/IEC 8348: OSI Network Service Definition |url=https://www.iso.org/obp/ui/#iso:std:iso-iec:8348:ed-3:v1:en}}</ref> 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). From August 1990 to April 1995 the [[NSFNET]] backbone supported CLNP in addition to [[TCP/IP]].<ref>[http://babel.hathitrust.org/cgi/pt?id=mdp.39015035356347;page=root;view=image;size=100;seq=1;num=1 ''Link Letter''], Volume 4, No. 3 (Sept/Oct 1991), p. 1, NSFNET Information Services, Merit Network, Inc., Ann Arbor</ref> 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|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:<ref name=CISCOIS-IS/>

* Connectionless-mode Network Protocol (CLNP), as specified in ITU-T Recommendation X.233.<ref>[http://www.itu.int/rec/T-REC-X.233/en/ ''ITU-T Recommendation X.233: Protocol for providing the connectionless-mode network service: Protocol specification''], International Telecommunication Union, August 1997</ref>
* 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|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),<ref>{{cite web |title=ISO 10747 |url=https://www.iso.org/standard/21417.html}}</ref> the OSI equivalent of BGP.
* [[Signalling Connection Control Part]] (SCCP), as specified in ITU-T Recommendation Q.711<ref>[http://www.itu.int/rec/T-REC-Q.711/en/ ''ITU-T Recommendation Q.711: Specifications of Signalling System No. 7 – Signalling connection control part (SCCP)''], International Telecommunication Union, March 2001</ref> is a [[Signaling System 7]] protocol.

==See also==
* [[X.25 protocol suite]], an OSI Connection Oriented Network Service (CONS)

==References==
{{Reflist}}

==External links ==
* [https://blog.ipspace.net/2008/04/what-is-clns.html What is CLNS?] - a brief introduction by Ivan Pepelnjak

[[Category:ITU-T recommendations]]
[[Category:OSI protocols]]
[[Category:Network layer protocols]]