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.christian@gmail.com>
wrote:

> Jim Posen <jim.posen@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) :-)
>