-b  Cause Tethereal to run in "multiple files" mode.  In "multiple
           files" mode, Tethereal will write to several capture files. When
           the first capture file fills up, Tethereal will switch writing to
           the next file and so on.

           The created filenames are based on the filename given with the -w
           flag, the number of the file and on the creation date and time,
           e.g. savefile_00001_20050604120117.pcap, save-
           file_00001_20050604120523.pcap, ...

           With the files option it’s also possible to form a "ring buffer".
           This will fill up new files until the number of files specified, at
           which point Tethereal will discard the data in the first file and
           start writing to that file and so on. If the files option is not
           set, new files filled up until one of the capture stop conditions
           match (or until the disk if full).

           The criterion is of the form key:value, where key is one of:

           duration:value switch to the next file after value seconds have
           elapsed, even if the current file is not completely filled up.

           filesize:value switch to the next file after it reaches a size of
           value kilobytes (where a kilobyte is 1024 bytes).

           files:value begin again with the first file after value number of
           files were written (form a ring buffer).