Network Layer

Unlike the ARPA protocol suite that only has a single network protocol (IP), ISO has defined two network layer services: CONS (Connection-Oriented Network Service) and CLNS (Connectionless Network Service). OTS supports CONS over the X.25 protocol, and CLNS over the X.25 and 802.3/ FDDI protocols. The OTS programmatic interfaces, such as XTI, are designed to allow applications to communicate over either network service. The choice of which service (CONS or CLNS) to use is made in one of two ways. The first way is by OTS automatically examining the destination address and determining over which network service this address may best be reached. The second way to use the HP-UX bind command to specifically identify one service or the other.

The OSI protocol suite communicates with other systems over physical subnetworks. Examples of physical subnetwork types are X.25 and 802.3. An address as defined above is used by the OSI protocol suite to route information up from, and down to, the network layer. It is the job of the network layer to route information to destination NSAPs over the proper subnetwork. This is called network routing.

Network routing is accomplished by associating a destination NSAP with its point of attachment on a physical subnetwork, that is, the point on a physical subnetwork where that NSAP may be reached. An NSAP's point of attachment onto a subnetwork is identified by a subnetwork address.

On an X.25 subnetwork, a point of attachment is identified by an X.121 address. An X.121 address is one to 15 digits in length and has two parts: the switch address portion, and the subaddress portion. The switch address tells the X.25 protocol which switch on the subnetwork is the destination. The subaddress portion tells the switch which entity above X.25 is to receive the sent information.

In this example, two X.121 addresses have been defined: the subnetwork address for the CONS entity is 21223401, and the subnetwork address for the CLNS entity is 21223402.

	NOTE: Although the use of subaddresses is the recommended method to identify different X.25 users, some switches and X.25 networks do not support its use. If this is the case for your installation, see the Installing and Administering OSI Transport Services manual for more information.

On an 802.3 subnetwork, a point of attachment is identified by an IEEE MAC address. Instead of configured subaddress portions, the 802.3 protocol uses embedded, well known LSAP (Link Service Access Point) values for the CLNS and ARPA IP entities.

The network layer obtains its routing information in one of two ways: automatically using the ISO End System to Intermediate System Routing Protocol (ES-IS), or from statically configured tables entered by using the OTS Destination System and Routes configuration screens in osiadmin.