The CICS Listener a Concurrent TCP/IP Server

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1 The CICS Listener a Concurrent TCP/IP Server Author Heinz Hagedorn VSE Development, IBM Böblingen Laboratory hagedorn@de.ibm.com Communications between TCP/IP socket applications are built on the client/server model. That means a TCP/IP client is communicating with and requesting services from a TCP/IP server. The TCP/IP server may be either an iterative or a concurrent server. The iterative server handles both the client's connection request and its service requests. As long as one of these connections exists, the iterative server is not available for other clients. A concurrent server accepts the client's connection request and then starts a child server to handle the client's service requests. While the child server is working on the client's service requests, the concurrent server is free for other clients' connection requests. VSE/ESA 2.5 provides a concurrent server for TCP/IP clients that can interact with TCP/IP applications running under CICS TS. This concurrent server is named the CICS Listener. In CICS terms, the CICS Listener is a long-running CICS transaction which acts like a "switchboard". With incoming requests from a client, the CICS Listener starts a CICS server transaction (as specified by the client). A TCP/IP communication path between the client and the CICS server transaction also is established. See Figure 44 on page 62. Articles Related to VSE/ESA 2.4 and

2 Figure 44. The CICS Listener in VSE/ESA 2.5 VSE/ESA 2.5 not only supplies the CICS Listener transaction (to act as a concurrent server). It also provides environmental support for this IBM-supplied listener. This environmental support can be used by user-written listeners as well, as long as they adhere to the requirements as described in TCP/IP for VSE/ESA IBM Program Setup and Supplementary Information. The following topics in this article describe: Environmental support for CICS Listeners, The IBM-supplied CICS Listener (page 63), and Cache support for GETHOSTBYNAME calls to Domain Name Servers (page 65). Environmental Support for CICS Listeners VSE/ESA 2.5 provides a set of programs and transactions that allow you to: Configure the CICS Listener environment and each listener that will be using this environment. Start and stop the CICS Listener environment, as well as selected CICS listeners. The configuration definitions are in the VSAM file, EZACONF. In particular, there is a: Macro EZACICD to define entries for the configuration file EZACONF. Transaction (EZAC) to maintain these configuration file entries. 62 VSE/ESA Software Newsletter

3 Program (EZACIC20) to dynamically start and stop the CICS Listener environment and the individual CICS listeners, either via a CICS PLT entry or via a dynamic link from another CICS program. Transaction (EZAO) to interactively start and stop the CICS Listener environment and the individual CICS listeners. Transient data queue (EZAM) for error messages. The VSE/ESA 2.5 system setup has all definitions for these programs, transactions, and files incorporated. These definitions have been made for both the CICSICCF and the PRODCICS partition. The only task left to the user is to fill the CICS Listener configuration file. For this, a skeleton (SKCICSLI) in VSE/ICCF library 59 is provided. This skeleton includes definitions for the CICS Listener environment and the IBM-supplied CICS Listener. What are the definitions to be made for the CICS Listener environment and for individual CICS listeners? A CICS Listener environment can be configured for each CICS TS partition in the system. Configuration for the environment includes: The CICS APPLID, The name of the transient data queue for error messages, and Refresh time values for a Domain Name Server Cache. For an individual CICS listener, the following can be configured: The listener's transaction ID, The TCP/IP port on which the listener is listening, Time-out values for socket calls like ACCEPT, READ and GIVESOCKET, The name of a security exit module, and The request to start the listener, along with its environment. Messages generated by the CICS Listener environment all start with prefix EZY. They are documented in TCP/IP for VSE/ESA IBM Program Setup and Supplementary Information. IBM-supplied CICS Listener The IBM-supplied CICS Listener is provided via program EZACIC02, driven by CICS transaction EZAL. After activation, EZAL waits on connection requests from TCP/IP clients. When a TCP/IP client has successfully CONNECTed to EZAL, it needs to submit the following data string on its first SEND request to EZAL: Articles Related to VSE/ESA 2.4 and

4 Table 2. Data String for First SEND Request to EZAL Field Length Required? Description Transaction ID 1-4 bytes mandatory CICS transaction ID that the listener is going to start Client-in-data 1-35 bytes optional First data to be passed to the child server transaction Startup type 2 bytes optional Type of startup of server transaction (IC=Interval Control or TD=Transient Data) Interval time 6 bytes optional Interval time (hhmmss) for IC startup type Fields must be separated by commas. EZAL analyzes this character string; and after having checked that the requested transaction is available, it calls the "security exit" module which is defined in the configuration file. This security exit module is provided with all the data passed from the client, including the client's IP address and port number. The security exit then has the option to permit or to reject startup of the requested transaction. Note: There is no IBM-supplied security exit module for the CICS Listener. If you want to use this feature, please refer to the TCP/IP for VSE/ESA IBM Program Setup and Supplementary Information for details on writing your own security exit module. When the security exit module does not reject the startup request, the socket communication is made available to the server transaction via a GIVESOCKET socket call; and the server transaction is started. The following data (without delimiters in between) is passed to the server transaction with the FROM parameter on the EXEC CICS START statement: Table 3. Data String Passed with FROM Parameter Field Format Description Socket descriptor Listener Client ID 4 bytes Socket descriptor to be used with TAKESOCKET request 40 bytes Listener's client ID to be used with TAKESOCKET request Client-in-data 36 bytes First data sent from client Socket Address 16 bytes Socket address of own host the 2-byte Addressing Family (AFINEY=2), the 2-byte port and the 4-byte IP address of the host, followed by 8 reserved bytes Once the server transaction has received this data, it has to pick up the socket communication via a TAKESOCKET function call. With the successful TAKESOCKET function, the CICS Listener is posted to close its socket communication with the client. From that point, direct communication exists between the client and the server transaction. EZAL is then again waiting on connection requests from other clients. 64 VSE/ESA Software Newsletter

5 Messages that are generated by the IBM-supplied CICS Listener all start with prefix EZY. They are documented in TCP/IP for VSE/ESA IBM Program Setup and Supplementary Information. Domain Name Server Cache Support One of the tasks a TCP/IP application may have to do is to translate a host name into its related IP address. The appropriate TCP/IP socket call for this task is the GETHOSTBYNAME call. This call first searches the local TCP/IP names table (created by DEFINE NAME) and then, if the requested entry is not found, calls an external Domain Name Server to perform the name resolution. To improve performance in a highly active CICS TCP/IP application environment, it may be helpful to use the Domain Name Server (DNS) cache support provided with the CICS Listener. This function caches the results of successful GETHOSTBYNAME calls in a VSAM KSDS file. Instead of doing the GETHOSTBYNAME call by itself, the CICS application may invoke program EZACIC25 by an EXEC CICS LINK command. EZACIC25 either retrieves the requested information from the local cache file or via a GETHOSTBYNAME call. Information between the CICS application program and EZACIC25 is passed via the CICS COMMAREA. This includes: As input from the application program, the host name and an indication whether the cache should be searched first or a GETHOSTBYNAME call should be done right away. As output from EZACIC25, the address of the HOSTENT structure that includes the host's IP address and an indication how the IP address was found (in cache or via a GETHOSTBYNAME call). When EZACIC25 is called from an application and the application has requested to use the cache for host name resolution, it first attempts to retrieve the entry from the cache. If not successful, it will issue a GETHOSTBYNAME call (unless prohibited by the application). If retrieval is successful, it will check the "age" of this entry. If its age adheres to certain refresh time values (defined in the CICS Listener configuration file), this entry will be passed back to the application. If not, a GETHOSTBYNAME call will be issued; and the returned HOSTENT structure is passed back to the application. In any case, after a successful GETHOSTBYNAME call, the cache is updated with the new/changed information. More Information Details for CICS Listener support are in TCP/IP for VSE/ESA IBM Program Setup and Supplementary Information, SC Articles Related to VSE/ESA 2.4 and

Expedite/CICS Messages

GXS EDI Services Expedite/CICS Messages Version 4 Release 5 GC34-2331-04 Fifth Edition (November 2005) This edition replaces document number GC34-2331-03. Copyright GXS, Inc. 1998, 2005. All rights reserved.