Just a little correctionThis feature already exists in Bitcoin, since version 0.12.0 I believe (i.e. nearly 18 months ago).
Oct 3, 2015Default time limit is 1 week.
2 weeks, still not changed.
Transactions do not have a timestamp to it neither do the nodes have a synchronized time with each other. Since every node sees the transaction at a different time, they will not drop the transaction at the same time. In addition, if the transaction gets rebroadcast, the nodes wouldn't drop it at all. Its not up to you, anyone can do it.
but it works locally (dropping on local mempool) and other nodes may (re)broadcast it again (read ranochigo post)
thus the unconfirmed transaction goes back into (local) mempool again as new unconf.tx
it would create never ending loop which only breakable by RBF or successful double spend transaction
but by setting certain synchronized time based on block number similar to lock_time,
the unconf.tx will be dropped completely by all nodes and never be re-broadcasted again past specified block number
or I misunderstood the way that protocol works?