Can you explain why a fixed offset along the whole circuit is enough to ensure safely as opposed to an increased delta at each hop?
On Tue, May 1, 2018, 5:05 AM Christian Decker <decker.christ...@gmail.com> wrote: > Jim Posen <jim.po...@gmail.com> writes: > > If my understanding is correct though, this construction would > > significantly increase the safe CLTV delta requirements because HTLCs > > cannot be timed out immediately on the settlement transaction. Consider a > > case where node B receives an HTLC from A and forwards to C. If the HTLC > > offered to C times out and C does not fail the HTLC off-chain, Lightning > > currently guarantees that the CLTV delta is sufficient that I may close > the > > channel to C on-chain and claim the timed-out HTLC before my upstream > HTLC > > to A times out. If the CLTV delta is too small, I may fail the upstream > > HTLC as soon as it times out, and then C may still claim the downstream > > HTLC with the preimage on-chain. With eltoo, when B closes the downstream > > channel on-chain, it must wait the CSV timeout on the update transaction > > before locking in the timed-out HTLC. This effectively means the CLTV > delta > > has to be greater than the CSV timeout, plus some extra (whereas it is > > currently safe to make it significantly shorter). Is that true or am I > > missing something? > > That's a good point Jim. We need to make sure that the CLTVs are far > enough in the future for the CSV timeout to expire and to grab any > preimage downstream and insert it upstream. Overall this results in an > offset of all the CLTVs to (less than) the maximum CSV timeout along the > path. This would be a fixed offset for each channel and can be announced > using the gossip protocol, so senders can take it into consideration > when computing the routes. Notice that this is not really the CLTV > delta, which would accumulate along the path, but an offset on which the > CLTV deltas build on. > > In today's network we have many nodes that have a CLTV delta of 144 > blocks, which quickly results in HTLC funds unavailable for several days > depending on the route length, so I don't think that adding a fixed > offset is much worse. Once we have watch-towers we can reduce both the > offset as well as the CLTV deltas. Since eltoo makes watch-towers less > expensive, given the reduced storage costs, I'd argue that it's a net > positive for the Lightning network (but then again I'm biased) :-) >
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