CLNP (Connectionless Network Protocol)

CLNP is a datagram network protocol. It provides fundamentally the same underlying service to a transport layer as IP. CLNP provides essentially the same maximum datagram size, and for those circumstances where datagrams may need to traverse a network whose maximum packet size is smaller than the size of the datagram, CLNP provides mechanisms for fragmentation (data unit identification, fragment/total length and offset). Like IP, a checksum computed on the CLNP header provides a verification that the information used in processing the CLNP datagram has been transmitted correctly, and a lifetime control mechanism ("Time to Live") imposes a limit on the amount of time a datagram is allowed to remain in the internet system.

CLNP (Connectionless Network Protocol) may be used between network-entities in end systems or in Network Layer relay systems (or both). CLNP is intended for use in the Subnetwork Independent Convergence Protocol (SNICP) role, which operates to construct the OSI Network Service over a defined set of underlying services, performing functions necessary to support the uniform appearance of the OSI Connectionless-mode Network Service over a homogeneous or heterogeneous set of interconnected subnetworks.

CLNP Protocol Structure

- NLP ID - Network Layer Protocol Identifier : The value of this field is set to binary 1000 0001 to identify this Network Layer protocol as ISO 8473. The value of this field is set to binary 0000 0000 to identify the Inactive Network Layer protocol subset.
- Length ID : Length Indicator is the length in octets of the header.
- Version : Version/Protocol Id Extension identifies the standard Version of ISO 8473.
- Lifetime : PDU Lifetime representing the remaining lifetime of the PDU, in units of 500 milliseconds.
- Flags : There are three flags : segmentation permitted, more segments, error report.
- Type : The Type code field identifies the type of the protocol data unit, which could be data PDU or Error Report PDU.
- Segment Length : The Segment Length field specifies the entire length, in octets, of the Derived PDU, including both header and data (if present).
- Checksum : The checksum is computed on the entire PDU header.