Hi Y'all, 

Thanks to luke-jr and jl2012 for publishing your analysis of the

xblocks proposal. I'd like to also present some analysis but instead focus

on the professed LN safety enhancing scheme in the proposal. It's a bit

underspecified, so I've taken the liberty of extrapolating a bit to fill

in the gaps to the point that I can analyze it.

TLDR; The xblock proposal includes a sub-proposal for LN which is

essentially a block-size decrease for each open channel within the network.

This decrease reserves space in blocks to allow honest parties guaranteed

space in the blocks to punish dishonest channel counter parties. As a result

the block size is permanently decreased for each channel open. Some may

consider this cost prohibitively high.

>> If the second highest transaction version bit (30th bit) is set to to `1`

>> within an extension block transaction, an extra 700-bytes is reserved on

>> the transaction space used up in the block.

> Why wouldn't users set this on all transactions?

As the proposal stands now, it seems that users _are_ able to unilaterally

use this for all their Bitcoin transactions, as there's no additional cost

to using the smart-contract safety feature outlined in the proposal. 

The new safety measures proposed near the end of this xblock proposal

could itself consume a dedicated document outlining the prior background,

context, and implications of this new safety feature. Throughout the rest

of this post, I'll be referring to the scheme as a Pre-Allocated

Smart-contract Dispute arena (PASDA, chosen because it sounds kinda like

"pasta", which brings me many keks). It's rather insufficiently described and

under specified as it stands in the proposal. As a result, if one doesn't

have the necessary prior context, it might've been skipped over entirely

as it's difficult to extract the sub-proposal from the greater proposal. I

think I possess the necessary prior context required to required to

properly analyze the sub-proposal. As a result, I would like to illuminate

the readers of the ML so y'all may also be able to evaluate this

sub-proposal independently.

## Background

First, some necessary background. Within LN as it exists today there is

one particularly nasty systematic risk related to blockchain availability

in the case of a channel dispute. This risk is clearly outlined in the

original white paper, and in my opinion a satisfactory solution to the

risks which safe guard the use of very high-value channels has yet to be

presented. 

### Chain Spam/Censorship Attack Vector

The attack vector mentioned in the original paper is a reoccurring attack

in systems of this nature: DoS attacks. As it stands today, if a channel

counterparty is able to (solely, or in collaboration with other attackers)

prevent one from committing a transaction to the chain, they're able to

steal money from the honest participant in the channel. The attack

proceeds something like this:

   * Mallory opens a very large channel with me.

   * We transfer money back and forth in the channel as normal. The nature

     of these transfers isn't very important. The commitment balances may

     be modified due to Mallory making multi-hop payments through my

     channel, or possibly from Mallory directly purchasing some goods I

     offer, paying via the channel.

   * Let's call the current commitment number state S_i. In the lifetime

     of the channel there may exist some state S_j (i < j) s.t Mallory's

     balance in S_i, is larger than S_j. 

   * At this point, depending on the value of the channel's time-based

     security parameter (T) it may be possible for Mallory to broadcast

     state S_i (which has been revoked), and prevent me being able to

     include by my punishment transaction (PTX) within the blockchain.

   * If Mallory is able to incapacitate me for a period of time T, or

     censor my transactions from the chain (either selectively or via a

     spam attack), then at time K (K > T + B, where B is the time the

     commitment transaction was stamped in the chain), then she'll be free

     to walk away with her settled balance at state S_i. For the sake of

     simplicity, we're ignoring HTLC's.

   * Mallory's gain is the difference between the balance at state S_i and

     S_j. Deepening on the gap between the states, my settled balance at

     state S_i and the her balance delta, she may be able to fully recoup

     the funds she initially place in the channel.

### The Role of Channel Reserves as Partial Mitigation

A minor mitigation to this attack that's purely commitment transaction

policy is to mandate that Mallory's balance in the channel never dips

below some reserve value R. Otherwise, if at state S_j, Mallory has a

settled balance of 0 within he channel (all the money if on my side), then

the attack outline above can under certain conditions be _costless_ from

her PoV. Replicate this simultaneously across the network in a synchronized

manner (possibly getting some help from your miner friends) and this

becomes a bit of a problem (to say the least).

Taking this a step further another mitigation that's been proposed is to

also use the channel reserve to implement a _ceiling_ on the maximum size

of _any_ in flight HTLC. Similar to the scheme above, this is meant to

eliminate the possibility of a "costless" attack, as if channel throughput

is artificially constrained, then the value of pending HTLC's isn't

enticing enough to launch a channel breach attack.

### Analysis of Attack Feasibility/Difficulty

The difficulty of the attack is dependant on the time-denominated security

parameter T, and the adversaries ability to collude with miners. Purely

spamming the chain given a very larger T value may be prohibitively

expensive for the attacker and their profit from launching the attack

would need to outweigh the cost in transaction fees and idle bitcoin

required to launch the attack. Considering the case of colluding with

miners, if mining is highly centralized (as it is now), then that may be a

more attractive attack avenue. In a world of highly decentralized mining

(let's say a lofty goal of no pool commanding > 5% of the hash power),

then the attack is much more difficult. 

(as an aside schemes that involve transactions committing to the inputs

they're spending and revealing them at a later date/block (committed

transactions) may address the miner censorship attack vector)

Depending one's target use of channels, the individuals they open channels

with, the applications that run on top of the channels, the amount of

coins within the channel, and the choice of the time parameter T, the

attack outline above may or may not be an issue from your PoV.  However,

in order to realize LN's maximum potential of being able to enter a

smart-contract with a complete stranger on the internet trustlessly,

without fearing conditions that may lead to monetary losses, the attack

vector should be mitigated if possible.

In the words of The Architect of the Matrix (and referenced by Tadge at

his "Level of LN" talk at Scaling Bitcoin Hong Kong: "There are levels of

survival we are prepared to accept". There exist levels of LN and usage of

channels, that may not consider this a dire issue.

OK, with the necessary background and context laid out, I'll now analyze

the solution proposed within the greater xblock proposal, making a brief

detour to briefly described another proposed solution.

### Timestop

A prior proposed solution to the failure scenario described above is

what's known as "time stop". This was proposed by gmaxwell and was briefly

touched upon in the original LN white paper. The mechanism of the

time-denominated security parameter T in today's channel construction is

enforced using OpCheckSequenceVerify. After broadcasting a commitment

transaction, all outputs paying to the broadcaster of the commitment are

encumbered with a relative time delay of T blocks, meaning they are unable

to claim the funds until time T has elapsed. This time margin gives the

honest party an opportunity to broadcast their punishment transaction

iff, the broadcaster has broadcast a prior revoked state. 

The idea of time stomp is to introduce a special sequence-locks block

height to the system. This block height would increase with each block

along with the regular block height _unless_ the block reaches a certain

sustained "high water mark". As an example, let's assume that when 3

blocks in row are above 75% capacity, then the sequence-lock clock stops

ticking.

The effect of this change is to morph the security risk into simply a

postponement of the judgment within the contract. With this, DoS attacks

simply delay the (seemingly) inevitable punishment of the dishonest party

within the contract. 

Aside from some informal discussions and the brief section within the

original white paper, many details of this proposal are left

underspecified. For example: how do miners signal to full nodes that the

sequence-lock clock has stopped? What's the high water mark threshold? Can

it go on indefinitely? Should this feature be opt-in? 

I think this proposal should be considered in tandem with the proposal

within the xblock proposal as both have a few unanswered questions that

need to be further explored.

## Pre-Allocated Smart-Contract Dispute Area (PASDA)

Aight, now to the LN enhancing proposal that's buried within the

greater xblock proposal. Introducing some new terminology, I've been

calling this a: Pre-Allocated Smart-contract Dispute Arena or (PASDA) for

short. In a nut shell, the key idea of the proposal is this: transactions

that mark the commencement of a smart contract who's security depends on

availability of block space for disputes are able to _pre allocate_ a

section of the block that will _always_ be _reserved_ for dispute

transactions. With this, contracts is  _guaranteed_ space in blocks to

handle disputes in the case that the contract breaks down. As an analogy:

when you enter in a contract with a contractor to build your dream

kitchen, you _also_ go to a court and reserve a 1-hour block in their

scheduled to handle a dispute _just in case_ one arises. In the event of a

peaceful resolution to the contract, the space is freed up.

The description in the paper is a bit light on the details, so I'll say up

front that I'm extrapolating w.r.t to some mechanisms of the construction.

However, I've been involved in some private conversations where the idea

was thrown around, so I think I have enough context to _maybe_ fill in

some of the gaps in the proposal.

I'll now restate the proposal. Smart contract transactions set a certain

bit in their version number. This bit indicates that they wish to

pre-allocate N bytes in _all_ further blocks _until_ the contract has been

reserved. In the specific context of payment channels, this means that

once a channel is open, until it has been closed, it _decreases_ the

available block size for all other transactions. As this is a very

aggressive proposal I think the authors took advantage of the new design

space within xblocks to include something that may not be readily accepted

as a modification to the rules of the main chain.

The concrete parameters chosen in the proposal are: each channel opening

transaction reserves 700-bytes within _each_ block in the chain until the

transaction has been closed. This pre-allocation has the following

constraint: a transaction can _only_ take advantage of this allocation iff

it's spending the _first_ output of a smart-contract transaction (has a

particular bit in the version set). This means that only dispute

resolution transactions can utilize this space. 

The proposal references two allocations, which I've squinted very hard at

for half a day in an attempt to parse the rules governing them, but so far

I've been unable to glean any further details. From my squinting, I

interpret that half of the allocation is reserved for spending the

self-output of a transaction in the last 2016 blocks (two weeks) and the

other half is dedicated to spending the first output of a commitment

transaction in the _same_ block.

I'm unsure as to why these allocations are separate, and why they aren't

just combined into a single allocation.

### Modification to LN Today

This change would require a slight modification to LN as it's currently

defined today. ATM, we use BIP 69 in order the inputs and outputs of a

transaction. This is helpful as it lets us just send of signatures for new

states as both sides already know the order of the inputs and outputs.

With PASDA, we'd now need to omit the to-self-output (the output in my

commitment transaction paying to myself my settled balance) from this

ordering and _always_ make it the first output (txid:0).

The second change is that this proposal puts a ceiling on on the CSV value

allowed by any channel. All CSV delays _must-weeks otherwise, they're

unable to take advantage of the arena.

### Modifications to Bitcoin

In order to implement this within Bitcoin, a third utxo set (regular

block, xblock) must be maintained by all full nodes. Alternatively, this

can just be a bit in the xblock utxo set. The implementation doesn't

really matter. Before attempting to pack transactions into a block, the

total allocation within the PASDA utxo-set must be summed up, and

subtracted from the block size cap. Only transactions which validly spend

from one of these UTXO's are able to take advantage of the new space in

the block.

## Analysis of PASDA

OK, now for some analysis. First, let's assume that transactions which

create PASDA UTXO's aren't subject to any additional constraints. If so,

then this means that _any_ transaction can freely create PASDA UTXO's and

_decrease_ the block size for _all_ transactions until the UTXO has been

spent. If my interpretation is correct, then this introduces a new attack

vector which allows _anyone_ to nearly permanently decrease the block size

for all-time with next to zero additional cost. If this is correct, then

it seems that miners have _zero_ incentive to _ever_ include a transaction

that creates a PASDA output in their blocks as it robs them of future

revenue and decreases the available capacity in the system, possibly

permanently proportionally to _each_ unspent PASDA output in the chain.

Alternatively, let's say the transactions which create PASDA outputs

_must_ pay a disproportionately high fee in order to pay up front for

their consumption of the size within all future blocks. If so, then a

question that arises is: How large a fee? If the fee is very large, then

the utilization of the smart-contract battling arena is only reserved to

very high valued channels who can afford very high fees. This may be

acceptable as if you have a $5 channel, then are you really at risk at

such a large scale attack on Bitcoin just to steal $5 from you? It's

important to note that many attacks on LN's contract resolution

capabilities are also a direct attack on Bitcoin. However, in a world of

dynamic fees, then it may be the case that the fee paid 6 months ago is

now a measly fee an no longer covers the costs to miners (and even the

entire system...).

Finally, here's something I thought of earlier today that possibly

mitigates the downside from the PoV of the miners (everyone else must

still accept the costs of a permanent block size decrease). Let's say that

in order to create a PASDA output fees are paid as normal. However, for

_each_ subsequent block, the participants of the contract _must_ pay a

tribute to miners to account for their loss in revenue due to the

reduction in block size. Essentially, all PASDA outputs must pay _rent_

for their pre-allocated space. If rent isn't paid sufficiently and on-time,

then the pre-allocate arena space is revoked by miners. There're a few

ways to construct this payment, but I'll leave that to follow up work as I

just want to shed some light on the PASDA and its implications.

## Conclusion

I've attempted to fill in some gaps for y'all w.r.t exactly what the

sub-proposal within the greater xblock proposal consists of and some

possible implications. I'd like to note that I've taken the liberty of

filling on some gaps within the sub-proposal as only a single section

within the greater proposal has been allocated to it. PASDA itself could

likely fill up an entirely distinct propsal by itself spanning several

pages. To the authors of the proposal: if my interpretation is inaccurate

please correct me as I'd also like to better understand the proposal. It's

possible that everything I've said in this (now rather long) email is

incorrect.

If you've made it this far, thank you for taking the time out of your day

to consider my thoughts. It's my hope that we can further analyze this

sub-proposal in detail and discuss its construction as well as its

implications on smart-contracts like payment channels on top of Bitcoin.

PASDA purports to address one half of the systematic risks in LN by

possibly eliminating the DoS vector attack against LN. However, the costs

of PASDA are very high, and possibly prohibitively so. In my opinion, the

second attack vector lies in the ability of miners to arbitrarily censor

transactions spending a particular output. Fungibility enhancing

techniques such as Committed Transactions may be a viable path forward to

patch this attack vector.

-- roasbeef

On Tue, Apr 4, 2017 at 8:35 PM Johnson Lau via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

I feel particularly disappointed that while this BIP is 80% similar to my proposal made 2 months ago ( https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-January/013490.html ), Matt Corallo was only the person replied me. Also, this BIP seems ignored the txid malleability of the resolution tx, as my major technical critique of xblock design.

But anyway, here I’m only making comments on the design. As I said in my earlier post, I consider this more as an academic topic than something really ready for production use.

> This specification defines a method of increasing bitcoin transaction throughput without altering any existing consensus rules.

Softforks by definition tighten consensus rules

> There has been great debate regarding other ways of increasing transaction throughput, with no proposed consensus-layer solutions that have proven themselves to be particularly safe.

so the authors don’t consider segwit as a consensus-layer solution to increase transaction throughput, or not think segwit is safe? But logically speaking if segwit is not safe, this BIP could only be worse. OTOH, segwit also obviously increases tx throughput, although it may not be as much as some people wish to have.

> This specification refines many of Lau's ideas, and offers a much simpler method of tackling the value transfer issue, which, in Lau's proposal, was solved with consensus-layer UTXO selection.

The 2013 one is outdated. As the authors are not quoting it, not sure if they read my January proposal

>  extension block activation entails BIP141 activation.

I think extension block in the proposed form actually breaks BIP141. It may say it activates segregated witness as a general idea, but not a specific proposal like BIP141

> The merkle root is to be calculated as a merkle tree with all extension block txids and wtxids as the leaves.

It needs to be more specific here. How are they exactly arranged? I suggest it uses a root of all txids, and a root of all wtxids, and combine them as the commitment. The reason is to allow people to prune the witness data, yet still able to serve the pruned tx to light wallets. If it makes txid and wtxid as pairs, after witness pruning it still needs to store all the wtxids or it can’t reconstruct the tree

> Outputs signal to exit the extension block if the contained script is either a minimally encoded P2PKH or P2SH script.

This hits the biggest question I asked in my January post: do you want to allow direct exit payment to legacy addresses? As a block reorg will almost guarantee changing txid of the resolution tx, that will permanently invalidate all the child txs based on the resolution tx. This is a significant change to the current tx model. To fix this, you need to make exit outputs unspendable for up to 100 blocks. Doing this, however, will make legacy wallet users very confused as they do not anticipate funding being locked up for a long period of time. So you can’t let the money sent back to a legacy address directly, but sent to a new format address that only recognized by new wallet, which understands the lock up requirement. This way, however, introduces friction and some fungibility issues, and I’d expect people using cross chain atomic swap to exchange bitcoin and xbitcoin

To summarise, my questions are:
1. Is it acceptable to have massive txid malleability and transaction chain invalidation for every natural happening reorg?  Yes: the current spec is ok; No: next question (I’d say no)
2. Is locking up exit outputs the best way to deal with the problem? (I tried really hard to find a better solution but failed)
3. How long the lock-up period should be? Answer could be anywhere from 1 to 100
4. With a lock-up period, should it allow direct exit to legacy address? (I think it’s ok if the lock-up is short, like 1-2 block. But is that safe enough?)
5. Due to the fungibility issues, it may need a new name for the tokens in the ext-block

> Verification of transactions within the extension block shall enforce all currently deployed softforks, along with an extra BIP141-like ruleset.

I suggest to only allow push-only and OP_RETURN scriptPubKey in xblock. Especially, you don’t want to replicate the sighash bug to xblock. Also, requires scriptSig to be always empty

> This leaves room for 7 future soft-fork upgrades to relax DoS limits.

Why 7? There are 16 unused witness program versions

> Witness script hash v0 shall be worth the number of accurately counted sigops in the redeem script, multiplied by a factor of 8.

There is a flaw here: witness script with no sigop will be counted as 0 and have a lot free space

> every 73 bytes in the serialized witness vector is worth 1 additional point.

so 72 bytes is 1 point or 0 point? Maybe it should just scale everything up by 64 or 128, and make 1 witness byte = 1 point . So it won’t provide any “free space” in the block.

> Currently defined witness programs (v0) are each worth 8 points. Unknown witness program outputs are worth 1 point. Any exiting output is always worth 8 points.

I’d suggest to have at least 16 points for each witness v0 output, so it will make it always more expensive to create than spend UTXO. It may even provide extra “discount” if a tx has more input than output. The overall objective is to limit the UTXO growth. The ext block should be mainly for making transactions, not store of value (I’ll explain later)

> Dust Threshold

In general I think it’s ok, but I’d suggest a higher threshold like 5000 satoshi. It may also combine the threshold with the output witness version, so unknown version may have a lower or no threshold. Alternatively, it may start with a high threshold and leave a backdoor softfork to reduce it.

> Deactivation

It is a double-edged sword. While it is good for us to be able to discard an unused chain, it may create really bad user experience and people may even lose money. For example, people may have opened Lightning channels and they will find it not possible to close the channel. So you need to make sure people are not making time-locked tx for years, and require people to refresh their channel regularly. And have big red warning when the deactivation SF is locked in. Generally, xblock with deactivation should never be used as long-term storage of value.

————
some general comments:

1. This BIP in current form is not compatible with BIP141. Since most nodes are already upgraded to BIP141, this BIP must not be activated unless BIP141 failed to activate. However, if the community really endorse the idea of ext block, I see no reason why we couldn’t activate BIP141 first (which could be done in 2 weeks), then work together to make ext block possible. Ext block is more complicated than segwit. If it took dozens of developers a whole year to release segwit, I don’t see how ext block could become ready for production with less time and efforts.

2. Another reason to make this BIP compatible with BIP141 is we also need malleability fix in the main chain. As the xblock has a deactivation mechanism, it can’t be used for longterm value storage.

3. I think the size and cost limit of the xblock should be lower at the beginning, and increases as we find it works smoothly. It could be a predefined growth curve like BIP103, or a backdoor softfork. With the current design, it leaves a massive space for miners to fill up with non-tx garbage. Also, I’d also like to see a complete SPV fraud-proof solution before the size grows bigger.


> On 5 Apr 2017, at 02:03, Luke Dashjr via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> Recently there has been some discussion of an apparent work-in-progress
> extension block proposal by Christopher Jeffrey, Joseph Poon, Fedor Indutny,
> and Steven Pair. Since this hasn't been formally posted on the ML yet, perhaps
> it is still in pre-draft stages and not quite ready for review, but in light
> of public interest, I think it is appropriate to open it to discussion, and
> toward this end, I have reviewed the current revision.
>
> For reference, the WIP proposal itself is here:
>    https://github.com/tothemoon-org/extension-blocks
>
> ==Overall analysis & comparison==
>
> This is a relatively complicated proposal, creating a lot of additional
> technical debt and complexity in comparison to both BIP 141 and hardforks. It
> offers no actual benefits beyond BIP 141 or hardforks, so seems irrational to
> consider at face value. In fact, it fits much better the inaccurate criticisms
> made by segwit detractors against BIP 141.
>
> That being said, this proposal is very interesting in construction and is for
> the most part technically sound. While ill-fit to merely making blocks larger,
> it may be an ideal fit for fundamentally different block designs such as
> Rootstock and MimbleWimble in absence of decentralised non-integrated
> sidechains (extension blocks are fundamentally sidechains tied into Bitcoin
> directly).
>
> ==Fundamental problem==
>
> Extension blocks are a risk of creating two classes of "full nodes": those
> which verify the full block (and are therefore truly full nodes), and those
> which only verify the "base" block. However, because the extension is
> consensus-critical, the latter are in fact not full nodes at all, and are left
> insecure like pseudo-SPV (not even real SPV) nodes. This technical nature is
> of course true of a softfork as well, but softforks are intentionally designed
> such that all nodes are capable of trivially upgrading, and there is no
> expectation for anyone to run with pre-softfork rules.
>
> In general, hardforks can provide the same benefits of an extension block, but
> without the false expectation and pointless complexity.
>
> ==Other problems & questions==
>
>> These outpoints may not be spent inside the mempool (they must be redeemed
> from the next resolution txid in reality).
>
> This breaks the ability to spend unconfirmed funds in the same block (as is
> required for CPFP).
>
> The extension block's transaction count is not cryptographically committed-to
> anywhere. (This is an outstanding bug in Bitcoin today, but impractical to
> exploit in practice; however, exploiting it in an extension block may not be
> as impractical, and it should be fixed given the opportunity.)
>
>> The merkle root is to be calculated as a merkle tree with all extension
> block txids and wtxids as the leaves.
>
> This needs to elaborate how the merkle tree is constructed. Are all the txids
> followed by all the wtxids (tx hashes)? Are they alternated? Are txid and
> wtxid trees built independently and merged at the tip?
>
>> Output script code aside from witness programs, p2pkh or p2sh is considered
> invalid in extension blocks.
>
> Why? This prevents extblock users from sending to bare multisig or other
> various possible destinations. (While static address forms do not exist for
> other types, they can all be used by the payment protocol.)
>
> Additionally, this forbids datacarrier (OP_RETURN), and forces spam to create
> unprovably-unspendable UTXOs. Is that intentional?
>
>> The maximum extension size should be intentionally high.
>
> This has the same "attacks can do more damage than ordinary benefit" issue as
> BIP141, but even more extreme since it is planned to be used for future size
> increases.
>
>> Witness key hash v0 shall be worth 1 point, multiplied by a factor of 8.
>
> What is a "point"? What does it mean multiplied by a factor of 8? Why not just
> say "8 points"?
>
>> Witness script hash v0 shall be worth the number of accurately counted
> sigops in the redeem script, multiplied by a factor of 8.
>
> Please define "accurately counted" here. Is this using BIP16 static counting,
> or accurately counting sigops during execution?
>
>> To reduce the chance of having redeem scripts which simply allow for garbage
> data in the witness vector, every 73 bytes in the serialized witness vector is
> worth 1 additional point.
>
> Is the size rounded up or down? If down, 72-byte scripts will carry 0
> points...)
>
> ==Trivial & process==
>
> BIPs must be in MediaWiki format, not Markdown. They should be submitted for
> discussion to the bitcoin-dev mailing list, not social media and news.
>
>> Layer: Consensus (soft-fork)
>
> Extension blocks are more of a hard-fork IMO.
>
>> License: Public Domain
>
> BIPs may not be "public domain" due to non-recognition in some jurisdictions.
> Can you agree on one or more of these?
> https://github.com/bitcoin/bips/blob/master/bip-0002.mediawiki#Recommended_licenses
>
>> ## Abstract
>>
>> This specification defines a method of increasing bitcoin transaction
> throughput without altering any existing consensus rules.
>
> This is inaccurate. Even softforks alter consensus rules.
>
>> ## Motivation
>>
>> Bitcoin retargetting ensures that the time in between mined blocks will be
> roughly 10 minutes. It is not possible to change this rule. There has been
> great debate regarding other ways of increasing transaction throughput, with
> no proposed consensus-layer solutions that have proven themselves to be
> particularly safe.
>
> Block time seems entirely unrelated to this spec. Motivation is unclear.
>
>> Extension blocks leverage several features of BIP141, BIP143, and BIP144 for
> transaction opt-in, serialization, verification, and network services, and as
> such, extension block activation entails BIP141 activation.
>
> As stated in the next paragraph, the rules in BIP 141 are fundamentally
> incompatible with this one, so saying BIP 141 is activated is confusingly
> incorrect.
>
>> This specification should be considered an extension and modification to
> these BIPs. Extension blocks are _not_ compatible with BIP141 in its current
> form, and will require a few minor additional rules.
>
> Extension blocks should be compatible with BIP 141, there doesn’t appear to be
> any justification for not making them compatible.
>
>> This specification prescribes a way of fooling non-upgraded nodes into
> believing the existing UTXO set is still behaving as they would expect.
>
> The UTXO set behaves fundamentally different to old nodes with this proposal,
> albeit in a mostly compatible manner.
>
>> Note that canonical blocks containing entering outputs MUST contain an
> extension block commitment (all zeroes if nothing is present in the extension
> block).
>
> Please explain why in Rationale.
>
>> Coinbase outputs MUST NOT contain witness programs, as they cannot be
> sweeped by the resolution transaction due to previously existing consensus
> rules.
>
> Seems like an annoying technical debt. I wonder if it can be avoided.
>
>> The genesis resolution transaction MAY also include a 1-100 byte pushdata in
> the first input script, allowing the miner of the genesis resolution to add a
> special message. The pushdata MUST be castable to a true boolean.
>
> Why? Unlike the coinbase, this seems to create additional technical debt with
> no apparent purpose. Better to just have a consensus rule every input must be
> null.
>
>> The resolution transaction's version MUST be set to the uint32 max (`2^32 -
> 1`).
>
> Transaction versions are signed, so I assume this is actually simply -1.
> (While signed transaction versions seemed silly to me, using it for special
> cases like this actually makes sense.)
>
>> ### Exiting the extension block
>
> Should specify that spending such an exit must use the resolution txid, not
> the extblock's txid.
>
>> On the policy layer, transaction fees may be calculated by transaction cost
> as well as additional size/legacy-sigops added to the canonical block due to
> entering or exiting outputs.
>
> BIPs should not specify policy at all. Perhaps prefix "For the avoidance of
> doubt:" to be clear that miners may perform any fee logic they like.
>
>> Transactions within the extended transaction vector MAY include a witness
> vector using BIP141 transaction serialization.
>
> Since extblock transactions are all required to be segwit, why wouldn't this
> be mandatory?
>
>> - BIP141's nested P2SH feature is no longer available, and no longer a
> consensus rule.
>
> Note this makes adoption slower: wallets cannot use the extblock until the
> economy has updated to support segwit-native addresses.
>
>> To reduce the chance of having redeem scripts which simply allow for garbage
> data in the witness vector, every 73 bytes in the serialized witness vector is
> worth 1 additional point.
>
> Please explain why 73 bytes in Rationale.
>
>> This leaves room for 7 future soft-fork upgrades to relax DoS limits.
>
> How so? Please explain.
>
>> A consensus dust threshold is now enforced within the extension block.
>
> Why?
>
>> If the second highest transaction version bit (30th bit) is set to to `1`
> within an extension block transaction, an extra 700-bytes is reserved on the
> transaction space used up in the block.
>
> Why wouldn't users set this on all transactions?
>
>> `default_witness_commitment` has been renamed to
> `default_extension_commitment` and includes the extension block commitment
> script.
>
> `default_witness_commitment` was never part of the GBT spec. At least describe
> what this new key is.
>
>> - Deployment name: `extblk` (appears as `!extblk` in GBT).
>
> Should be just `extblk` if backward compatibility is supported (and `!extblk`
> when not).
>
>> The "deactivation" deployment's start time...
>
> What about timeout? None? To continue the extension block, must it be
> deactivated and reactivated in parallel?
>
>
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