[Lightning-dev] [RFC] Simplified (but less optimal) HTLC Negotiation

[Rusty Russell]

Hi all,

        Our HTLC state machine is optimal, but complex[1]; the Lightning
Labs team recently did some excellent work finding another place the spec
is insufficient[2].  Also, the suggestion for more dynamic changes makes it
more difficult, usually requiring forced quiescence.

The following protocol returns to my earlier thoughts, with cost of
latency in some cases.

1. The protocol is half-duplex, with each side taking turns; opener first.
2. It's still the same form, but it's always one-direction so both sides
   stay in sync.
        update+-> commitsig-> <-revocation <-commitsig revocation->
3. A new message pair "turn_request" and "turn_reply" let you request
   when it's not your turn.
4. If you get an update in reply to your turn_request, you lost the race
   and have to defer your own updates until after peer is finished.
5. On reconnect, you send two flags: send-in-progress (if you have
   sent the initial commitsig but not the final revocation) and
   receive-in-progress (if you have received the initial commitsig
   not not received the final revocation).  If either is set,
   the sender (as indicated by the flags) retransmits the entire
   sequence.
   Otherwise, (arbitrarily) opener goes first again.

Pros:
1. Way simpler.  There is only ever one pair of commitment txs for any
   given commitment index.
2. Fee changes are now deterministic.  No worrying about the case where
   the peer's changes are also in flight.
3. Dynamic changes can probably happen more simply, since we always
   negotiate both sides at once.

Cons:
1. If it's not your turn, it adds 1 RTT latency.

Unchanged:
1. Database accesses are unchanged; you need to commit when you send or
   receive a commitsig.
2. You can use the same state machine as before, but one day (when
   this would be compulsory) you'll be able signficantly simplify;
   you'll need to record the index at which HTLCs were changed
   (added/removed) in case peer wants you to rexmit though.

Cheers,
Rusty.

[1] This is my fault; I was persuaded early on that optimality was more
    important than simplicity in a classic nerd-snipe.
[2] https://github.com/lightningnetwork/lightning-rfc/issues/794