Re: [bitcoin-dev] CHECKSEQUENCEVERIFY - We need more usecases to motivate the change

From: jl2012@xbt•hk
To: Peter Todd <pete@petertodd•org>
Cc: bitcoin-dev@lists•linuxfoundation.org
Subject: Re: [bitcoin-dev] CHECKSEQUENCEVERIFY - We need more usecases to motivate the change
Date: Sat, 03 Oct 2015 14:49:20 -0400	[thread overview]
Message-ID: <e7b394187fd96bd77a1c49f7c9b7a9b2@xbt.hk> (raw)
In-Reply-To: <20151003143056.GA27942@muck>

BIP68 allows per-input locktime, though I don't know how this could be 
useful.

BIP68 and BIP112 are mostly ready. If we try to reimplement 
relative-locktime without using nSequence, we may need to wait for 
another year for deployment.

A compromise is to make BIP68 optional, indicated by a bit in tx 
nVersion, as I suggested earlier (1). This will allow deploying 
relative-locktime without further delay while not permanently limiting 
future upgrades.

(1) 
http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-August/010043.html

Peter Todd via bitcoin-dev 於 2015-10-03 10:30 寫到:
> 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.
> 
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