Communicating with VM/CMS Virtual Machines › Server Code: OPVMSV › Data Message Formats

Data Message Formats

The server deals with several kinds of messages:

• Client text from the server's message queue
• Commands which also originate from the message queue but are intended for local server control and might contain private protocol exchanges between server and client

The general syntax for any item stored into the global variable message queue is 'header: text'. The header: is identified by a trailing colon “:”, and is separated from the text by at least one space. Text can be any character string, with imbedded spaces, which will get uppercased and sent to the client VM system.

The header can contain the following:

• CMD:

  This identifies the message as a local command to the server, and is not intended for any client. The associated text must contain a valid command:

  • QUIT (or any of the candidate termination commands)

    Closes all active client connections, and terminates server processing

  • DISPLAY

    Writes a list of active connections to the log. Each line item contains the socket number, client port number, client IP address, VM user ID, VM node, connection status, and a count of messages sent to that client.

  • SEVER n

    Explicitly terminates the connection with the client identified by the socket ID n. The socket ID is displayed using the DISPLAY command. The socket assignments are dynamic, and a client's socket ID should not be considered a constant unless only one client is expected to connect.

• Client_ID:

  Due to the multiple client possibility, each client message contains a destination identifier as the header. The possible formats are:

  • nnn.nnn.nnn.nnn,port:

    The destination is a fully qualified IP and port combination. Use when several clients are on one system.

  • nnn.nnn.nnn.nnn:

    The destination is an IP address only. Normally this would be sufficient.

  • VMuserid@Vmnode:

    The destination is specified as a user ID and node combination.

  • N:

    The destination is a socket ID.

  • *:

    The destination is a broadcast to all connected clients.

  The following are several examples:

  • This sends the text 'Hi, VM' to the client at IP address 11.22.3.44, port 12345:

    '11.22.3.44,12345: Hi, VM'
    

  • This presumes only one active client at IP address 11.22.3.44 and sends the same text to the client at that address:

    '11.22.3.44: Hi, VM'
    

  • This sends 'Hi, VM' to VM user ID CLIENTA on VM node SYSTEMC:

    'CLIENTA@SYSTEMC: Hi, VM'
    

  • This sends the same text to whomever is connected on socket ID 3:

    '3: Hi, VM'
    

  • Every connected client receives the message:

    '*: Hi, VM'
    

• REPLY:

  This is used internally, but may be visible when tracing is activated. Messages from the client are delivered to the appropriate service as defined by the disposition option, and the resulting return code is sent back to the client. This is a one-way message from server to client.

• PING:

  Server code monitors connection status. If there is no activity for one minute, a health-check is done by sending a probe message to the quiet client. If there is no response within another minute, the server will report the condition as a possible network or system outage. If a series of DISPLAY commands are issued during that two-minute period, connection status will be seen transitioning from Active to Probe to NoResp. As soon as any kind of input arrives from that client, the status returns to its normal active status. The low frequency of the probe message and its small size should not contribute significantly to network traffic.