ISO 8473 Network Layer Standards

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4.5.4. ISO 8473

ISO 8473 is a standardization and simplification of IP. Despite its many similarities to IP, the two protocols remain functionally incompatible. ISO 8473 is also referred to as the ISO Internet Protocol IP and the Connectionless Network Protocol CLNP. The CLNP network service is embodied by a single service primitive, N- UNITDATA, of which there are two types see Figure 4.55. CLNP makes no assumptions about the underlying subnetwork architecture, except that it should be able to support point-to-point transfer of data units of at least 512 octets each. For packets larger than supported by the subnet, CLNP provides the necessary segmentation and re-assembly functions. Routing can be guided by the transmitter by including a routing option in the packet, but is otherwise determined by the internetworking nodes. Figure 4.55 CLNP service primitive. Primitive Types Parameters Purpose N-UNITDATA request indicate source address, destination address, quality of service, user data Used for data transfer between two NSAPs. Each packet is fully addressed and routed independently. The general structure of CLNP IPDUs is shown in Figure 4.56. These are used to represent user Network Service Data Units NSDUs. It follows that, depending on its size, an NSDU may be represented by a single IPDU or segmented into multiple IPDUs. Figure 4.56 ISO 8473 IPDU structure. Field Description Protocol ID Denotes the IPDU protocol i.e., ISO 8473. Header Length Length of the header fields in octets. Version Number Version of the ISO 8473 protocol. Lifetime IPDU lifetime. Flags For segmentation and error reporting. Type Denotes the IPDU type. Segment Length Total length of the segment in octets. Checksum Checksum for the IPDU header. Addresses Source and destination NSAP addresses. Data Unit ID Denotes the first segment of a segmented NSDU. Segment Offset Denotes the relative position of IPDU within NSDU. Total Length Total length of the NSDU. Options For source routing, route recording, QOS, etc. 72 Communication Networks Copyright © 2005 PragSoft Data Actual user data. The Protocol ID field uniquely identifies the IPDU from other network packets. Segmentation is facilitated by the Segment Length and Segment Offset fields. The former is set to the length of the segment for this IPDU; the latter denotes the relative position of this segment from the beginning of the original NSDU. The Total Length field denotes the total length of the original NSDU. Corrupt and expired IPDUs are discarded immediately. This causes an error report being generated by the respective station, which is then returned back to the originating station. ISO 8473 can be used in a variety of arrangements. It can be used on top of X.25, or even directly on top of LAP-B.

4.6. Further Reading