SNAX/APN Application Programming Manual

The SNALU Interface

SNAX/APN Application Programming Manual—420111-001

4-19

Flow Control

A set of SNA requests is used to control the flow of RUs within an LU-LU session. The

requests are discussed in sections describing data traffic protocols and DFC

request/response in the IBM Systems Network Architecture: Format and Protocol

Reference: Architectural Logic manual.

Chaining

Chaining provides a means of sending (and receiving) a sequence of requests as a single

logical entity. This SNA concept is described in the IBM Systems Network Architecture:

Format and Protocol Reference: Architectural Logic manual.

Within the SNALU programming environment, the application on the Tandem system

uses the BCI and ECI bits of the RH in the SNAX header for specifying chain

positioning on outgoing RUs and for interpreting chain positioning on incoming RUs.

If the use of chaining is agreed upon at session initiation, the BCI and ECI bits in the

RH (bits 6 and 7 respectively of byte 6 in the SNAX header) are used as shown in

Table 4-4.

Request/Response Modes

There are four possible request and response modes:

•

Immediate request mode—the sender must wait (after transmitting a request) for a

definite response before transmitting the next request.

•

Delayed request mode—the sender can transmit successive requests without waiting

for responses.

•

Immediate response mode—the receiver must respond to incoming RUs in the order

in which they were received.

•

Delayed response mode—the receiver can respond to incoming RUs in any order.

The choice of which modes are to be used is determined by the particular FM profile

specified in the BIND request at session initiation. The DR1I, DR2I, and ERI bits in the

RH (bits 0, 2, and 3 respectively of byte 7 in the SNAX header) are used in request units

to specify the desired response type, and in response units to specify the sent response

type:

Table 4-4. BCI and ECI Bit Settings

BCI ECI

First RU in chain 1 0

Middle RUs in chain 0 0

Last RU in chain 0 1

Only RU in chain 1 1