Connection-Less Network Protocol (CLNP)
OSI CLNP is a standard protocol by ISO which provides the ability to
send a datagram unit over to a destination system. Semantically it is
similar to IP, in fact, it originally was derived from IP. One of the
proposed uses for CLNP is in conjunction with IP in the network layer
proposed by TUBA.
Each datagram is an independent unit of transmission with no
relationship to other datagram units that are sent over the same network
service provider. In particular ,the order of delivery may not be
CLNP operates by exchanging Network Protocol Data Units
(NPDU). Since data units can exist in different sizes going from one
subnetwork to another, one of the main functions CLNP provides is segmentation.
The sending end system forms its NPDU up to the maximum size of its
originating network. Subsequent intermediate systems may need to divide
the original up into a number of smaller segments. The reassembly of the
segments takes place only at the destination system.
The fields in the NPDU can be classified as the following:
- Lifetime control
- This function requires that the NPDU be discarded by an intermediate network
entity, if it has been circulating in the network for an overly long time.
Options provide services such as source routing, route recording, error
reporting ,priority and some other QOS functions.
- Segmentation fields
- These tell the receiving end, the ordering, completeness and correctness of
the NPDUs, so that the exact PDU can be reassembled at the destination system.
- This includes both the source and destination NSAP
addresses and the length of each address.
Finally the actual user data from the data request primitive.
Work is progressing at this time on extensions to CLNP, including
network layer multicast capabilities, arbitrary packet coloring for
policy routing purposes and more flexible type of service selection.