Applications and dynamic VIPAs

While most applications support multiple instances in a sysplex, very few applications expect IP addresses to move around under them. TCP applications use TCP connections to form a relationship between particular client and server instances to exchange data over an extended period. They rely on notification of TCP connection termination to initiate recovery and to reestablish a new relationship (possibly from a client to a different server). Conversely, most UDP applications do the equivalent function at the application layer. Movement of an IP address to a different server could be confusing to the client, unless the new server also is aware of the state of the client work.

UDP applications whose interactions consist of atomic interactions (a single request followed by one or more responses, with no state information maintained at the server between requests) can use dynamic VIPAs in the multiple application-instance scenario. However, if the server application maintains state information between interactions (for example, NFS), then moving a dynamic VIPA to another server might not work unless the client/server application protocol can detect the discontinuity. In that case, the unique application-instance scenario might apply, which would require the restart of the server instance on another TCP/IP.