I don't know exactly iPlanet's error messages, but "PR_SYS_DESC_TABLE_FULL_ERROR" sounds like your system might have hit the maximum number of opened files (or network sockets)- i.e. the kernel parameter nfiles.
When you run out of nfiles, it is a server-wide problem: each process in the server will receive an error whenever it tries to open a file or use a network socket. Many programs don't handle this type of error well. This may cause things like sshd/inetd/syslogd to die, which makes the recovery more complicated.
Web server processes usually start up a large number of processes and/or threads to cope with loads of simultaneous requests. You should check the iPlanet's configuration to see when it will start limiting the number of requests. The maximum number of processes/threads multiplied by the number of files each thread needs to access must always be a number that is less than the nfiles value. If not, you must either increase the nfiles value or decrease the web server's load limits.
A public Web server is an easy target for Denial-of-Service attacks, many of which are based on creating a huge number of requests. You should always set the various limits so that the web server will hit the application-specific limits first, so there will be some reserve against overrunning the system-wide limit. When the web server hits its own internal limit, it should start replying to requests with HTTP error 503 (Out of resources) and recover gracefully when the load gets lighter again.
MK
