Communications Server for z/OS V1R7 TCP/IP Implementation, Volume 1: Base Functions, Connectivity, and Routing

6 Communications Server for z/OS V1R7 TCP/IP Implementation, Volume 1 - Base Functions, Connectivity, and Routing

VTAM and TCP/IP tasks are typical examples. The CSM facility also manages storage as it

automates the addition and subtraction of the different types and sizes of storage requests.

1.3.3 Protocols and devices

As illustrated in Figure 1-1 on page 5, the Data Link Control (DLC) is a protocol layer that

manages and provides communication between the file I/O subsystem and the I/O device

driver of the particular device. It also shows two categories of DLCs: TCP/IP exclusive DLCs

and shared DLCs.

Exclusive DLCs

TCP/IP exclusive DLCs are those only available for TCP/IP usage and are not shared with

ACF/VTAM. Examples of TCP/IP exclusive DLCs supported by CS for z/OS IP are as follows.

LCS

LAN Channel Station (LCS) protocol is used by OSA-Express, some routers, and the

3746-9x0 MAE.

SAMEHOST

SAMEHOST is another TCP/IP exclusive DLC protocol that exists, although it does not make

use of System z9 or zSeries channels. In the past, this communication was provided by

IUCV. Currently, three such servers (SNALINK LU0, SNALINK LU6.2, X.25) exploit the

SAMEHOST DLC.

Shared DLCs

Shared DLCs are those that can be simultaneously used by TCP/IP and ACF/VTAM. The

shared DLCs are indicated in Figure 1-1 on page 5; however, we only focus on the most

commonly used DLCs, such as the following.

Multipath Channel+ (MPC+)

MPC+ is an enhanced version of VTAM’s Multipath Channel (MPC) protocol. The Multipath

Channel I/O process (MPC) defines the implementation of the MPC protocols. It allows for the

efficient use of multiple read and write channels. MPC handles protocol headers and data

separately and executes multiple I/O dispatchable units of work. This, when used in

conjunction with Communication Storage Management, creates efficient I/O throughput. High

Performance Data Transfer uses MPC+ together with Communication Storage Manager

(CSM) to decrease the number of data copies required to transmit data. This type of

connection may be used in two ways:

򐂰 MPCPTP allows a CS for z/OS IP environment to connect to a peer IP stack in a

point-to-point configuration. With MPCPTP a CS for z/OS IP stack may be connected to:

– Another CS for z/OS IP stack

– An IP router with corresponding support

– A non-z/OS server

– 3746-9x0 MAE

PTP Samehost (MPCPTP), sometimes referred to IUTSAMEH: This connection type is

used to connect two or more CS for z/OS IP stacks running on the same z/OS LPAR. In

addition, it can be used to connect these CS for z/OS IP stacks to z/OS VTAM for the use

of Enterprise Extender.

򐂰 MPCIPA allows an Open Systems Adapter-Express (OSA-Express) port to act as an

extension of the CS for z/OS TCP/IP stack and not as a peer TCP/IP stack, as with

MPCPTP.