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 (mainly the transmission area) around the world. Still the new systems delivered utilize this.
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 an error in handling the packet (OSI speak for packet is PDU) will send an error packet (ICMP) back to the source. In CLNP, error reports are only sent back 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.
Some protocols that provide the CLNS service:
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