Connectionless-mode Network Service

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.

Protocols providing CLNS

Some protocols that provide the CLNS service:

• CLNP, as specified in ITU-T Recommendation X.233 is a Public Data Network protocol that provides the connectionless mode network service as described in ITU-T Recommendation X.213.

• Signalling Connection Control Part (SCCP), as specified in ITU-T Recommendation Q.711 is a Signaling System 7 protocol that provides the connectionless mode network service as described in ITU-T Recommendation X.213.

• Internet Protocol and User Datagram Protocol essentially provide the connectionless mode network service as described in ITU-T Recommendation X.213.