Appendix C. Avoiding VTAM network addressing problems 405
Understanding VTAM network addressing
Network addresses are the architected means for identifying specific resources
in an SNA network, referred to as Network Addressable Units or NAUs. Network
addresses are assigned to SNA resources in a variety of ways (like pre-definition
or dynamic assignment), and are also used in a variety of ways. In general, most
network addresses are assigned to SNA resources that participate in LU-LU
sessions, such as SNA applications and devices. But other types of devices, like
PUs and LINEs, must also be assigned network addresses which are used for
SSCP-PU sessions or as a general means of identification.
SNA sessions
Every LU-LU session that is established must be represented by a unique pair of
network addresses: one network address that is assigned to the primary LU
(PLU) of the session and one that is assigned to the secondary LU (SLU).
Because many SNA application programs support “parallel sessions” (two or
more concurrently active sessions between the same two session partners),
assigning only a single network address to each session-capable SNA resource
is not always sufficient to guarantee network address pair uniqueness for all
sessions. Therefore, it is often necessary to assign more than one network
address to some SNA resources.
Depending on where a resource resides in the network (or attaches to the
network), network addresses may be assigned from any subarea node (typically
NCP or VTAM). However, it is the SSCP component of VTAM that manages the
assignment of network addresses during LU-LU session establishment, even if
one or both of the network addresses being assigned comes from an NCP
network address pool. To optimize the use (and reuse) of network addresses for
LU-LU sessions and to simplify the algorithms used to assign network addresses
to SNA resources, VTAM follows these general guidelines:
򐂰 Each session-capable SNA resource that is capable of being the SLU for a
session is assigned one network address that is used for every session in
which this resource acts as the SLU. For resources that support only one
active session at a time (like dependent LUs) and applications that do not
support parallel sessions, the SLU network address may also be used for
sessions in which the resource acts as the PLU, since network address pair
uniqueness is still guaranteed.
򐂰 Each session-capable SNA resource that is capable of acting as the PLU for a
session may also be assigned one or more network addresses (as needed)
that is only used for sessions in which this resource is acting as the PLU. For
SNA resources that establish parallel sessions, a new PLU network address
is assigned for each parallel session, since the SLU typically uses the same
network address for each parallel session. To conserve network addresses,

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