Question: In a Multimedia Broadcast/Multicast Service (MBMS) using FLUTE, what is the primary purpose of the 'max-stale' caching directive for a specific file or set of files?
Option 1: To indicate that the file should be cached for an indefinite period without an expiry date.
Option 2: To specify a short caching duration for highly dynamic content.
Option 3: To instruct the receiver to delete the cached file after a single use.
Option 4: To set a default caching validity of one week for all files.
Explanation: The 'max-stale' directive is utilized in MBMS's FLUTE protocol to signal to the receiver that a particular file (or set of files) should be retained in the cache indefinitely if possible, effectively meaning it has no specified expiry date. This approach is beneficial for content that does not frequently change or where updates are not time-sensitive, optimizing storage and reducing unnecessary downloads.