Connectionless Network Protocol (CLNP) is an ISO network layer datagram protocol by the layers defined in the Reference Model for Open Systems Interconnection. CLNP provides fundamentally the same underlying service to a transport layer as IP in the TCP/IP environment and therefore also named ISO-IP. Another ISO protocol in the network layer is CONP (Connection-Oriented Network Protocol), which provides connection oriented services at the network layer.

CLNP (Connectionless Network Protocol) may be used between network-entities in end systems or in Network Layer relay systems (or both). CLNP provides the Connectionless-mode Network Service. 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 is defined to accommodate variability where Subnetwork Dependent Convergence Protocols and/or Subnetwork Access Protocols do not provide all of the functions necessary to support the Connectionless-mode Network Service over all or part of the path from one NSAP to another. CLNP may also be used to fulfill other roles and may therefore be used in the context of other approaches to subnetwork interconnection.

CLNP uses NSAP addresses and titles to identify network devices. The Source Address and Destination Address parameters are OSI Network Service Access Point Addresses (NSAP address). A network-entity title is an identifier for a network-entity in an end-system or intermediate-system. Network-entity titles are allocated from the same name space as NSAP addresses, and the determination of whether an address is an NSAP address or a network-entity title depends on the context in which the address is interpreted.

CLNP (Connectionless Network Protocol) provides the same maximum datagram size as IP, 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 (Connectionless Network Protocol) 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.

Protocol Structure

CLNP has the following PDU structure:

CLNP PDU header:

• 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, Protocol for Providing the Connectionless- mode Network Service. 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 - 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
• Seg. 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.

Address Part - it contains information of destination and source addresses, which are defined in OSI 8348/AD2 with variable length.

Segmentation Part - If the Segmentation Permitted Flag in the Fixed Part of the PDU Header (Octet 4, Bit 8) is set to one, the segmentation part of the header, illustrated in Figure 6, must be present: If the Segmentation Permitted flag is set to zero, the non-segmenting protocol subset is in use.

Option Part - The options part is used to convey optional parameters.

Data Part - The Data part of the PDU is structured as an ordered multiple of octets.

Related protocols: CLNP, CONP, IP, ISO Transport protocol, ISO Session protocol

Sponsor Source: CLNP is defined in ISO (http://www.iso.org) document 8473 and ITU (www.itu.org) X.213, X.233

Reference:
http://www.javvin.com/rfc994.pdf : Final Text of DIS 8473, Protocol for Providing the Connectionless-mode Network Service
http://www.doc.ua.pt/arch/itu/rec/product/X.htm:
X.213: Information technology: Open Systems Interconnection ?Network service definition
X.233: Information technology - Protocol for providing the connectionless-mode network service: Protocol specification