BIP68 and BIP112 collectively define the CHECKSEQUENCEVERIFY semantics,
which can be summarized conceptually as a relative CHECKLOCKTIMEVERIFY.
However, CSV does define behavior for the previously undefined nSequence
field, which is the only "free-form" field we currently have in the
transaction serialization format that can be used for future upgrades -
we should justify this new behavior carefully as it limits our options
in the future. Adding new fields to the serialization format is very
difficult, due to the very broad system-wide impact of the hard-fork
required to do so.

So we need to make the case for two main things:

1) We have applications that need a relative (instead of absolute CLTV)

2) Additionally to RCLTV, we need to implement this via nSequence

To show we need RCLTV BIP112 provides the example "Escrow with Timeout",
which is a need that was brought up by GreenAddress, among others; I
don't think we have an issue there, though getting more examples would
be a good thing. (the CLTV BIP describes seven use cases, and one
additional future use-case)

However I don't think we've done a good job showing why we need to
implement this feature via nSequence. BIP68 describes the new nSequence
semantics, and gives the rational for them as being a
"Consensus-enforced tx replacement" mechanism, with a bidirectional
payment channel as an example of this in action. However, the
bidirectional payment channel concept itself can be easily implemented
with CLTV alone. There is a small drawback in that the initial
transaction could be delayed, reducing the overall time the channel
exists, but the protocol already assumes that transactions can be
reliably confirmed within a day - significantly less than the proposed
30 days duration of the channel. That example alone I don't think
justifies a fairly complex soft-fork that limits future upgrades; we
need more justification.

So, what else can the community come up with? nSequence itself exists
because of a failed feature that turned out to not work as intended;
it'd be a shame to make that kind of mistake again, so let's get our
semantics and use-cases in the BIPs and documented before we deploy.

-- 
'peter'[:-1]@petertodd.org
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