gtps2m0w | ACF/SNA Data Communications Reference |
TPF supports connections to packet switched data networks (PSDNs) through the use of the NCP Packet Switching Interface (NPSI).
SNA devices that attach to TPF through NPSI are not defined as NPSI device types; instead, they are defined using other supported TPF RSC device types.
Defining a network containing X.25 resources using NPSI is described in the NPSI publications for the single-domain case. This task requires understanding of VTAM and NCP as well as NPSI and X.25, and consists of many considerations not relevant to TPF X.25 support or even a TPF application using the X.25 network. Such topics, mostly concerning X.25 link and physical level protocols, or X.25 vendor options, are not covered here because they have no influence on the TPF X.25 support, or on the definition of NPSI LUs to TPF.
TPF provides connectivity support for the following NPSI facilities defined by Logical Link Control (LLC) type:
TPF uses the ROUTC interface for TPF applications to communicate with logical end points reached through a PSDN. This is achieved by extending TPF SNA support to the LU_T1 interface with the NPSI product in a local or remote NCP node. Additionally, user-written system modules, which collectively form the basis of a user Communication and Transmission Control Program (CTCP), provide the user with the capabilities offered by NPSI GATE support. Process selection vector (PSV) routines can be used for this purpose. See Process Selection Vector (PSV) for additional information.
The NPSI link, PU, and LU resources are owned by VTAM. TPF needs network awareness of only the NPSI LU resources. Therefore, all LU sessions with NPSI LUs, or in the case of LLC=3 with the LU residing across the PSDN, have cross-domain/network session characteristics.
Non-SNA devices are supported as follows:
Non-SNA devices appear to TPF as LU_T1 LU_LU sessions and use the following session characteristics:
SNA devices are supported using NPSI LLC=3.
The following functions are not supported:
An "X.25 message" is a sequence of one or more X.25 data packets ending with a packet with the "more data" bit in its packet header turned off (indicating no more data). During a RECEIVE, for example, NPSI combines related data packets in one X.25 message and places it in 1 inbound PIU for application processing. During a SEND, an outbound PIU contains 1 X.25 message that NPSI transforms into 1 or more X.25 data packets for network transmission.
TPF applications, therefore, send or receive X.25 messages only. Furthermore, TPFs message length support allows up to a 4K block maximum. Because TPF message blocks contain headers and control bytes, user applications must adhere to the following data length constraints:
Input messages will continue to be given to a TPF application in the smallest core block possible. However, a complete X.25 message must be placed in 1 core block. If an inbound message exceeds 1020 bytes of user data (size of text area in a 1055-byte core block), that message is placed in a 4K block for the application.
Each use of the ROUTC macro sends 1 and only 1 X.25 message. For resources accessed through NSPI, OMT creates a single SNA PIU with an RU maximum length of 4027 bytes. Messages longer than 4027 bytes are discarded and a system error is taken. An application may present outbound messages using the existing interfaces (127, 381 byte core blocks or file chained 381 pool records) or as a single 1055-byte or 4K block.
The NPSI session protocols that are used by TPF X.25 support are:
These options are given in the BIND image detailed in TPF ACF/SNA Network Generation.
Because different LLC types use different message formats, the TPF application must be aware of the LLC type used on each NPSI session. This information is not maintained by TPF X.25 support and must be supplied and tracked by the-user in user defined tables. (See the user exits information in TPF System Installation Support Reference.)
The SNA commands shown in the Table 1 are used for the TPF NPSI support:
Table 1. SNA Commands Supported for NPSI
Session Control | Data Flow Control |
---|---|
BIND CLEAR SDT UNBIND |
CHASE LU_STAT RSHUT RSHUTD SHUTC SHUTD SIG (LLC=5 Integrated PAD) |