[Lightning-dev] [RFC] Simplified (but less optimal) HTLC Negotiation
rusty at rustcorp.com.au
Mon Oct 12 23:37:07 UTC 2020
Our HTLC state machine is optimal, but complex; the Lightning
Labs team recently did some excellent work finding another place the spec
is insufficient. 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
Otherwise, (arbitrarily) opener goes first again.
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.
1. If it's not your turn, it adds 1 RTT latency.
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.
 This is my fault; I was persuaded early on that optimality was more
important than simplicity in a classic nerd-snipe.
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