ihave/sendme
Usenet's ask-before-sending scheme: a neighbour received a list of article IDs and ordered only the articles it was missing.
Over UUCP, news moved in batches across dial-up modems, and long-distance calls were billed by the minute. Pushing every article to every neighbour meant the same text often crossed the line two or three times, because it had already reached the recipient by another route. B News therefore added two control messages: ihave, carrying a list of the Message-IDs the sender held, and sendme, in which the receiver answered with the ones it wanted.
The trade saved bandwidth and cost time, since each article now needed two extra round trips over a link that might be dialled once a night. The scheme was documented in RFC 1036 and survives as the name of NNTP's IHAVE command, where question and answer take milliseconds instead.