Re: [bitcoin-dev] The BIP148 chain split may be inevitable

[Jacob Eliosoff <jacob.eliosoff@gmail.com>]

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Just a quick follow-up on BIP91's prospects of avoiding a BIP148 chain
split, because I may have left an overly pessimistic impression -

In short: the timing isn't as dire as I suggested, BUT unless concrete
progress on a plan starts taking shape, esp miner support, *the split is
indeed coming.*

THE GOOD NEWS: several refinements have been noted which could get BIP91
(or splitprotection, Segwit2x, etc) deployed faster:
- The lock-in window could be shortened, eg to splitprotection's 504 blocks
(3.5 days)
- Of course the 80% threshold could just be reduced, eg to
splitprotection's 65%
- BIP91 nodes could start signaling on bit 1 the moment bit 4 reaches
lock-in, rather than waiting another period until it  "activates".
 (Orphaning of non-bit-1-signaling blocks would probably also have to start
at or shortly after the same time [1].)

Combining these approaches, *July 26* is an approximate hard deadline for
>50% of miners to be running BIP91 in order to prevent the split.  So,
significantly less tight than my previous June 30 (or my original,
erroneous "a few days ago").

THE BAD NEWS: no one should underestimate the steps that would need to be
completed by that deadline:
1. Coordinate on a solution (BIP91, splitprotection, Segwit2x, BIP148
itself, ...)
2. Implement and test it
3. Convince >50% of miners to run it [2]
4. Miners upgrade to the new software and begin signaling

In particular, #3: afaict a lot of convincing is still needed before miner
support for any of these reaches anything like 50%.  (With the exception of
Segwit2x, but it has the additional handicap that it probably needs to
include deployable hard fork code, obviously ambitious in 1.5 months.)


[1] See Saicere's comment:
https://github.com/btc1/bitcoin/pull/11#discussion_r121086886, and related
discussion at https://github.com/btc1/bitcoin/pull/11#issuecomment-307330011
.

[2] Note that >50% need to run the *solution*, eg BIP91; old BIP141 nodes
signaling segwit support do *not* count, since they won't orphan non-bit-1
blocks.  The impending split isn't between nodes that support segwit vs
don't, but between those that reject non-segwit-supporting blocks vs don't.


On Fri, Jun 9, 2017 at 1:23 AM, Jacob Eliosoff <jacob.eliosoff@gmail.com>
wrote:

> Ah, two corrections:
> 1. I meant to include an option c): of course >50% of hashpower running
> BIP148 by Aug 1 avoids a split.
> 2. More seriously, I misrepresented BIP148's logic: it doesn't require
> segwit *activation*, just orphans non-segwit-*signaling* (bit 1) blocks
> from Aug 1.
>
> I believe that means 80% of hashrate would need to be running BIP91
> (signaling bit 4) by ~June 30 (so BIP91 locks in ~July 13, activates ~July
> 27), not "a few days ago" as I claimed.  So, tight timing, but not
> impossible.
>
> Sorry about the errors.
>
>
> On Fri, Jun 9, 2017 at 12:40 AM, Jacob Eliosoff <jacob.eliosoff@gmail.com>
> wrote:
>
>> I've been trying to work out the expected interaction between James
>> Hilliard's BIP91 [1] (or splitprotection [2], or Segwit2x [3], which both
>> use variants of BIP91 activation) and the BIP148 UASF [4].  Some of this is
>> subtle so CORRECTIONS WELCOME, but my conclusions are:
>> 1. It's extremely unlikely BIP91-type logic can activate segwit in time
>> to avoid a BIP148 chain split.
>> 2. So, in practice all we can do is ensure the BIP148 split is as
>> painless as possible.
>>
>> REASONING:  First, some dates.  BIP148, whose deadline is already
>> deployed and thus unlikely to be postponed, starts orphaning non-segwit
>> blocks on midnight (GMT) the morning of August 1.  Meanwhile, here are
>> Bitcoin's rough expected next four difficulty adjustment dates (they could
>> vary by ~1-3 days depending on block times, but it's unlikely to matter
>> here):
>> 1. June 17
>> 2. June 30
>> 3. July 13
>> 4. July 27
>>
>> If Segwit activates on adj date #5 or later (August), it will be too late
>> to avoid BIP148's split, which will have occurred the moment August began.
>> So, working backwards, and assuming we want compatibility with old BIP141
>> nodes:
>>
>> - Segwit MUST activate by adj #4 (~July 27)
>> - Therefore segwit MUST be locked in by adj #3 (~July 13: this is
>> inflexible, since this logic is in already-deployed BIP141 nodes)
>> - Therefore, I *think* >50% of hashpower needs to be BIP91 miners,
>> signaling bit 1 and orphaning non-BIP91 (ie, BIP91's bit 4 must activate),
>> by adj #2 (June 30)?
>> - Therefore, as currently designed, BIP91 bit 4 must be locked in by adj
>> #1 (June 17)
>> - Therefore, >=80% of hashrate must start signaling BIP91's bit 4 by a
>> few days ago...
>>
>> There are ways parts of this could be sped up, eg, James' "rolling
>> 100-block lock-in periods" [5], to get BIP91 signaling bit 1 sooner.  But
>> to be compatible with old BIP141 nodes, >50% of hashrate must be activated
>> BIP91 miners by ~June 30: there's no fudging that.
>>
>> So, it seems to me that to avoid the BIP148 split, one of two things
>> would have to happen:
>> a) 95% of hashrate start signaling bit 1 by ~June 30.  Given current stat
>> is 32%, this would basically require magic.
>> b) BIP91 is deployed and >50% (80% or whatever) of hashrate is
>> *activated* BIP91 miners by ~June 30, ~3 weeks from now.  Again, much too
>> soon.
>>
>> So, I think the BIP148 split is inevitable.  I actually expect that few
>> parts of the ecosystem will join the fork, so disruption will be bearable.
>> But anyway let me know any flaws in the reasoning above.
>>
>> [1] https://github.com/bitcoin/bips/blob/master/bip-0091.mediawiki
>> [2] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017
>> -June/014508.html
>> [3] https://github.com/btc1/bitcoin/pull/11
>> [4] https://github.com/bitcoin/bips/blob/master/bip-0148.mediawiki
>> [5] https://github.com/btc1/bitcoin/pull/6#issuecomment-305917729
>>
>
>

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