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Connectionless-mode Network Service

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Main article: OSI protocols

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.[2][3] 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)

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 (SDH) elements. From August 1990 to April 1995 the NSFNET backbone supported CLNP in addition to TCP/IP.[4] However, CLNP usage remained low compared to TCP/IP.

Protocols providing CLNS

Several protocols provide the CLNS service:[3]

  • Connectionless-mode Network Protocol (CLNP), as specified in ITU-T Recommendation X.233.[5]
  • 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[6] is a Signaling System 7 protocol.

• OSPF supports only IP, IS-IS supports both IP and CLNS. • IS-IS does not require IP connectivity between routers to share routing information. Updates are sent via CLNS instead of IP. • In OSPF, interfaces belong to areas. In IS-IS, the entire router belongs to an area. • An IS-IS router belongs to only one Level-2 area, which results in less LSP traffic. IS-IS is thus more efficient and scalable than OSPF, and supports more routers per area. • There is no Area 0 backbone area for IS-IS. The IS-IS backbone is a contiguous group of Level 1-2 and Level 2 routers. • IS-IS does not elect a backup DIS. Additionally, DIS election is preemptive. • On broadcast networks, even with an elected DIS, IS-IS routers still form adjacencies with all other routers. In OSPF, routers will only form adjacencies with the DR and BDR on broadcast links. • IS-IS uses an arbitrary cost metric. OSPF’s cost metric is based on the bandwidth of the link. • IS-IS provides far more granular control of link-state and SPF timers than OSPF

See also

References

  1. ^ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on 1 February 2021. Retrieved 10 March 2023.
  2. ^ "Section 3: Definition of the Connectionless-Mode Service", ITU-T Recommendation X.213: Open Systems Interconnection – Network service definition, International Telecommunications Union, October 2001
  3. ^ a b "ISO Connectionless Network Service", Chapter 2, Section 1 in IS-IS Network Design Solutions, Cisco Press, 7 February 2002, ISBN 1-57870-220-8
  4. ^ Link Letter, Volume 4, No. 3 (Sept/Oct 1991), p. 1, NSFNET Information Services, Merit Network, Inc., Ann Arbor
  5. ^ ITU-T Recommendation X.233: Protocol for providing the connectionless-mode network service: Protocol specification, International Telecommunications Union, August 1997
  6. ^ ITU-T Recommendation Q.711: Specifications of Signalling System No. 7 – Signalling connection control part (SCCP), International Telecommunications Union, March 2001
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