CLNS is an abbreviation of Connectionless Network Service.
It is an OSI Network Layer service that (unlike CONS) does not require a circuit to be established before data is transmitted. CLNS routes messages to their destinations independently of any other messages.
In an OSI protocol deployment, CLNS would be the service provided by CLNP (Connectionless Network Protocol) and used by TP4 (Transport Protocol Class 4). However CLNP is not used on the Internet, instead its function is provided by IP. CLNP is still widely used today in many telecommunications networks around the world. This is because IS-IS (an OSI layer 3 protocol) is mandated by the ITU-T as the protocol for management of the SDH (Synchronous Digital Hierarchy) elements.
CLNP uses IS-IS (developed by DEC) for routing. CLNP type supports two values; 1 and 28. 1 is used to indicate Error Report. All the other packets use the value 28 (Hex 0x1c) (TP4, TARP). CLNP does not have a Protocol field like IP. The NSEL field in the NSAP is used for forwarding to the appropriate protocol handler. In IP it is always assumed that the end detecting and handling errors in the packet will send an error packet (via ICMP) back to the source. CLNP PDUs (PDUs are the OSI equivalent of packets) will only elicit error reports if the Error Report Requested flag is set. The CLNP header does not require the messages to be padded to the nearest 32 bit boundary. The length of a CLNP header can go up to 254 octets.
CLNP has separate fields for indicating total length and segment length which helps in reassembly.
CLNP is standardized in ISO 8473. TP4 is standardized in ISO 8073.
Transport Protocol Class 4 (TP4), one of the five transport layer protocols existing in the OSI suite, 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, which is similar to the Transmission Control Protocol (TCP) in the TCP/IP suite.
Some protocols that provide the CLNS service: