Subject: Re: [bitcoin-dev] Alternative name for CHECKSEQUENCEVERIFY (BIP112)

From: "Eric Lombrozo" <[email-masked]>
Date: Wed, 25 Nov 2015 23:41:03 +0000

As I said in an earlier post, a systems developer and an application 
developer have very different perspectives on this. From the former's 
perspective, it is entirely sensible to name things based on basic 
features of the system's design (i.e. a field in the txin or tx that 
gets checked) - but from an app developer's perspective, what matters is 
how they will use a particular feature in an actual app.

I think that part of what systems developers should strive to do is to 
abstract out the inner minutiae of the system's guts and expose to app 
developers the clearest interface with which to develop apps. This is 
even more the case when the details of the inner workings are completely 
irrelevant to the application logic and there's no real gains to be had 
from attempting to optimize for the inner workings when designing an 
application.

>From an app developer's perspective, I think it is pretty blatantly 
clear that relative timelock is *the* critical exposed functionality 
intended here. Now, one could argue that the satoshi script is still a 
systems level component of the system...but with the advent of overlay 
protocols such as payment channels and the Lightning Network, it is 
clear that we now require a new abstraction layer for reasoning about 
the higher level logic of the system that doesn't burden the protocol 
designer with having to know the intimate and esoteric details of the 
lower system levels. Of course, many of those who work on these higher 
level protocols will also be experts in the underlying system design. 
However, it greatly increases the learning curve and can easily 
frustrate people looking to work on these ideas...and ultimately, 
knowing the inner details of how the nSequence field is structured and 
what the bits actually mean is irrelevant to someone trying to design 
scripts for such applications.

We've already deployed another opcode, CHECKLOCKTIMEVERIFY, which does 
refer to the field name. However, in this particular situation, the 
field name reflects *far* more closely what the app developer actually 
cares about than nSequence, which to the app developer might as well be 
called foo. As such, I stick with my original vote - we should call the 
opcode RCHECKLOCKTIMEVERIFY, which has the advantage of communicating 
fairly directly to developers and protocol designers the semantics they 
actually care about and also makes clear the relationship between 
absolute and relative timelock...that's to say, the ability for the 
script designer to lock specific coins until either a specific moment in 
time or until a certain delay has passed since the coin output was 
created (added to blockchain).

Let's face it - the entire motivation behind BIP68/BIP112 is relative 
timelock. Explicitly calling the opcode RCHECKLOCKTIMEVERIFY will make 
life easier for everyone and will help sell the idea and help it gain 
greater acceptance more quickly; while stubbornly adhering to an 
esoteric detail that is only there for historical reasons will only 
continue to delay the idea's acceptance and adoptance.

- Eric

------ Original Message ------
From: "Mark Friedenbach via bitcoin-dev" 
<[email-masked]>
To: "Btc Drak" <[email-masked]>
Cc: "Bitcoin Dev" <[email-masked]>
Sent: 11/25/2015 3:05:50 PM
Subject: Re: [bitcoin-dev] Alternative name for CHECKSEQUENCEVERIFY 
(BIP112)

>Looks like I'm the long dissenting voice here? As the originator of the 
>name CHECKSEQUENCEVERIFY, perhaps I can explain why the name was 
>appropriately chosen and why the proposed alternatives don't stand up.
>
>First, the names are purposefully chosen to illustrate what they do:
>
>What does CHECKLOCKTIMEVERIFY do? It verifies the range of 
>tx.nLockTime.
>What does CHECKSEQUENCEVERIFY do? It verifies the range of 
>txin.nSequence.
>
>Second, the semantics are not limited to relative lock-time / maturity 
>only. They both leave open ranges with possible, but currently 
>undefined future consensus-enforced behavior. We don't know what sort 
>of future behavior these values might trigger, but the associated 
>opcodes are generic enough to handle them:
>
>CHECKLOCKTIMEVERIFY will pass an nSequence between 1985 and 2009, even 
>though such constraints have no meaning in Bitcoin.
>CHECKSEQUENCEVERIFY is explicitly written to permit a 5-byte push 
>operand, while checking only 17 of the available 39 bits of both the 
>operand and the nSequence. Indeed the most recent semantic change of 
>CSV was justified in part because it relaxes all constraints over the 
>values of these bits freeing them for other purposes in transaction 
>validation and/or future extensions of the opcode semantics.
>
>Third, single-byte opcode space is limited. There are less than 10 such 
>opcodes left. Maybe space won't be so precious in a post-segwitness 
>world, but I don't want to presume that just yet.
>
>
>As for the alternatives, they capture only the initial use case of 
>nSequence. My objection would relax if nSequence were renamed, but I 
>think that would be too disruptive and unnecessary. In any case, the 
>imagined use cases for CHECKSEQUENCEVERIFY has to do with sequencing 
>execution pathways of script, so it's not a stretch in meaning. 
>Previously CHECKMATURITYVERIFY was a hypothicated opcode that directly 
>checked the minimum age of inputs of a transaction. The indirect naming 
>of CHECKSEQUENCEVERIFY on the other hand is due to its indirect 
>behavior. RELATIVELOCKTIMEVERIFY was also a hypothicated opcode that 
>would check a ficticious nRelativeLockTime field, which does not exist. 
>Again my objection would go away if we renamed nSequence, but I 
>actually think the nSequence name is better...
>
>On Tue, Nov 24, 2015 at 2:30 AM, Btc Drak via bitcoin-dev 
><[email-masked]> wrote:
>>BIP68 introduces relative lock-time semantics to part of the nSequence 
>>field leaving the majority of bits undefined for other future 
>>applications.
>>
>>BIP112 introduces opcode CHECKSEQUENCEVERIFY (OP_CSV) that is 
>>specifically limited to verifying transaction inputs according to 
>>BIP68's relative lock-time[1], yet the _name_ OP_CSV is much boarder 
>>than that. We spent months limiting the number of bits used in BIP68 
>>so they would be available for future use cases, thus we have 
>>acknowledged there will be completely different usecases that take 
>>advantage of unused nSequence bits.
>>
>>For this reason I believe the BIP112 should be renamed specifically 
>>for it's usecase, which is verifying the time/maturity of transaction 
>>inputs relative to their inclusion in a block.
>>
>>Suggestions:-
>>
>>CHECKMATURITYVERIFY
>>RELATIVELOCKTIMEVERIFY
>>RCHECKLOCKTIMEVERIFY
>>RCLTV
>>
>>We could of course softfork additional meaning into OP_CSV each time 
>>we add new sequence number usecases, but that would become obscure and 
>>confusing. We have already shown there is no shortage of opcodes so it 
>>makes no sense to cram everything into one generic opcode.
>>
>>TL;DR: let's give BIP112 opcode a name that reflects it's actual 
>>usecase rather than focusing on the bitcoin internals.
>>
>>[1] 
>>https://github.com/bitcoin/bitcoin/pull/6564/files#diff-be2905e2f5218ecdbe4e55637dac75f3R1223
>>
>>_______________________________________________
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>>[email-masked]
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>>
>