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2-16 TN3270 Design and Implementation Guide

The DLUS is a product feature (APPN option set 1066) of a T5 (VTAM) network node supporting session

services extensions. The DLUS function enables VTAM to provide SSCP services for dependent LUs in remote

APPN ENs or NNs. The DLUS provides SSCP services through standard SSCP-to-PU and SSCP-to-LU session

ﬂows that are encapsulated and sent over LU 6.2 sessions.

The DLUR is a function (APPN option set 1067) of an APPN EN or NN that owns dependent LUs but obtains

services from a DLUS. The DLUR function is conﬁgured on the CIP or CPA under the TN3270 Server. The PUs

are deﬁnedunder the DLURstatement. The hierarchical structureis: TN3270 Server, DLUR, andPU. The DLUR

function provides a remote boundary function for dependent LUs; that is, it removes the requirement that a node

supporting dependent LUs must be adjacent to a subarea boundary node.

DLUR/DLUS removes these restrictions by providing the following functions:

• The session between the dependent LU (or PU) and its SSCP is encapsulated in an LU 6.2 pipe.

This CP-to-server pipe consists of a pair of sessions between the CPs in the DLUR and DLUS nodes. These

sessions are called CPSVRMGR sessions.The pipe cancarry a number ofSSCP-to-PU and SSCP-to-LUsessions

and does not need to be between adjacent CPs. The pipe can cross network boundaries.

• LU-to-LU session routing is performed by the APPN function, not the subarea function.

When a primary LU requests a search for a dependent LU, it normally receives a positive response from the

DLUS, not the DLUR. The response identiﬁes the DLUS as the server for the dependent LU and includes the

correctCP name(theDLUR). Theroute canthenbe calculateddirectly totheDLUR, typicallyby theNN server

of the primary LU (which is never the dependent LU). In cases where the DLUR supports cross-network

CPSVRMGR sessions, the DLUR may respond to a search, but it still indicates that it is the owning CP.

Because the DLUS presents itself as the NN server for the dependent LUs, it must always be an NN.

Using the DLUR/DLUS, or CP-to-server pipe, SSCP-to-PU and SSCP-to-LU control messages can be exchanged

between VTAM, the PU, and the local dependent LUs. SSCP-to-PU and SSCP-to-LU control messages are

required for PU and LU activation and to initiate LU-to-LU session establishment. LU-LU sessions can, for

example, be initiated by a LOGON request from a terminal operator. The LOGON request is forwarded to

VTAM, which locatesthe destinationLU using existingsubarea (SSCP-to-SSCP)or APPN (CP-to-CP)data ﬂows.

Following the activation ﬂows, the primary LU sends a BIND to establish the session. The BIND, and successive

LU-to-LU session data, will be using the best route between the two session partners for the desired Class of

Service (COS).

When you plan to use DLUR/DLUS, remember the following guidelines:

• To participate in an APPN network, a VTAM APPN EN must have an extended session services-capable NN

server if it is to do any type of session initiation other than LU 6.2. Currently this function is only provided in

VTAM, so all VTAM ENs must have VTAM NNs as their NN servers.

• The DLUS feature in VTAM is offered only on NNs. A VTAM EN cannot be a DLUS.

• The APPN EN TN3270 DLUR function allows you to route TN3270 LUs to multiple VTAM hosts from the

CIP or CPA rather than on the VTAM hosts. This feature isparticularly useful with the introduction of the new

multi-CPU Complementary Metal Oxide Semiconductor (CMOS) mainframe, which is composed of up to 16

CPUs that appear as separate VTAMs.

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