Using SNA chaining to segment large messages
Systems Network Architecture (SNA) allows terminal messages to be chained, and lets large messages be split into smaller parts while still logically treating the multiple message as a single message. Chaining can be used in systems that use z/OS® Communications Server SNA LUs of types that tolerate chaining.
Chaining characteristics are specified with the SENDSIZE, BUILDCHAIN, and RECEIVESIZE attributes.
The hardware requirements of each terminal normally dictate the input chain size and characteristics. The BUILDCHAIN and RECEIVESIZE attributes have default values that depend on device attributes. The size of an output chain is specified by the SENDSIZE attribute.
Effects
Because the network control program (NCP) also segments messages into 256 byte blocks for normal LU Type 0, 1, 2, and 3 devices, a SENDSIZE value of zero eliminates the processing effects of output chaining. A value of 0 or 1536 is required for local devices of this type.
If you specify the SENDSIZE attribute for intersystem communication (ISC) sessions, this attribute must match the RECEIVESIZE attribute in the other system. The SENDSIZE attribute or TCT BUFFER operand controls the size of the SNA element that is to be sent, and the RECEIVESIZE must match so that there is a corresponding buffer of the same size able to receive the element.
If you specify BUILDCHAIN(YES), CICS® assembles a complete chain of elements before passing them to an application. If you do not specify BUILDCHAIN(YES), each individual RU is passed to an individual receive-any in the application. With SNA/3270, BMS does not work correctly if you do not specify BUILDCHAIN(YES).
If you are dealing with large inbound elements that exceed a maximum of 32 KB, you cannot use the BUILDCHAIN attribute or CHNASSY operand. You must use multiple individual RUs, which extends the transaction life in the system.
Limitations
If you specify a low SENDSIZE value, this setting causes additional processing. Real and virtual storage are used to break the single logical message into multiple parts.
Chaining might be required for some terminal devices. Output chaining can cause flickering on display screens, which users might find disruptive. Chaining also causes additional I/O processing effects between the z/OS Communications Server and the NCP by requiring additional z/OS Communications Server subtasks and STARTIO operations. These effects are eliminated with applicable ACF/SNA releases by using the large message performance enhancement option (LMPEO).
Suggestions
The RECEIVESIZE value for IBM® 3274-connected display terminals is 1024 and for IBM 3276-connected display terminals it is 2048. These values give good line characteristics while keeping processor usage to a minimum.
Monitoring
Use of chaining and chain size can be determined by examining a z/OS Communications Server trace. You can also use the CICS internal and auxiliary trace facilities, where the VIO ZCP trace shows the chain elements. Some network monitoring tools such as NetView Performance Monitor (NPM) give this data.