Installing and Administering OSI Transport Services

44 Chapter 1
HP OTS/9000 Resources
Network Layer
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.