Unless I've misunderstood the news months gone by, the UK has always requested an extension which has then been voted on by the other EU member states.
However I've also read that the UK can, at any time, revoke article 50 - without requiring the agreement or cooperation of the EU, the ball is firmly in our court, the power of revelation is the UKs.
With the UK able to cancel and simply reissue the article 50 notification as it sees fit, it seems needless to agree extensions. It also seems unusual that there is a default timeline given altogether (why say 2 years from submitting another article 50 notification to leaving the EU, if the leaving state can do as I've suggested). I understand that this may be a politically bad move (essentially the leaving state annoying the EU) but as negotiations continue it seems like there is little love lost anyway.