- Source
- RFC 9110, sections 10.1.1 and 15.2.1
A 100 is a server saying "so far, so good, carry on". It's an interim response, so a final one is still on its way. A client that's about to send something large can send only its headers along with "Expect: 100-continue", and wait to hear whether it's worth the trip.
The headers alone can be enough for the server to say no, which is what happened to the tribbles. The spec suggests this for huge uploads, or when you're not sure your credentials are still good. The Content-Length is made up, but the tribble count is Spock's own sum: one tribble, a litter of ten, and a new generation every 12 hours, which makes 1,771,561 after three days.