[bitcoin-dev] Segregated Witness in the context of Scaling Bitcoin
lf-lists at mattcorallo.com
Wed Dec 16 22:29:39 UTC 2015
We should probably start by defining "economically important". To me, it's pretty clear that every, or at least around 99% of, " economically important" node have upgraded by the time the fork kicks in, with way more than sufficient time given to everyone to upgrade (minding that this is not an emergency situation and that people have lives and many Bitcoin services are hobby projects and upgrading isn't always as easy as just restarting your node). I'd define "economically important" as any node that is used for anything more than simply "being a node" (ie people who started a node to provide resources to the network, and only using their node for that). Note, of course, that we should avoid breaking all such "non-economically important" nodes, but breaking many of them is not a big deal. Note that my proposal includes nodes such as the one doing transaction selection for the relay network. Though it is not used for payments, if it is not upgraded, things will break.
With this definition in mind, I think a year is an aggressive timeline.
On December 16, 2015 1:51:47 PM PST, Jameson Lopp <jameson.lopp at gmail.com> wrote:
>On Wed, Dec 16, 2015 at 12:50 PM, Matt Corallo via bitcoin-dev <
>bitcoin-dev at lists.linuxfoundation.org> wrote:
>> A large part of your argument is that SW will take longer to deploy
>> hard fork, but I completely disagree. Though I do not agree with some
>> people claiming we can deploy SW significantly faster than a hard
>> once the code is ready (probably a six month affair) we can get it
>> very quickly. It's true the ecosystem may take some time to upgrade,
>> see that as a feature, not a bug - we can build up some fee pressure
>> an immediate release valve available for people to use if they want
>> fewer fees.
>> On the other hand, a hard fork, while simpler for the ecosystem to
>> to, is a 1-2 year affair (after the code is shipped, so at least
>> from today if we all put off heads down and work). One thing that has
>> concerned me greatly through this whole debate is how quickly people
>> to think we can roll out a hard fork. Go look at the distribution of
>> versions on the network today and work backwards to get nearly every
>> upgraded... Even with a year between fork-version-release and
>> fork-activation, we'd still kill a bunch of nodes and instead of
>> their security model, lead them to be outright robbed.
>Over a year seems to be an extraordinarily long time frame is for
>a hard fork. It looks like <https://bitnodes.21.co/dashboard/?days=365>
>of reachable nodes have upgraded in the past 6 months while as much as
>may not have been upgraded in over a year. However, viewing historical
>stats of version upgrades doesn't seem to be an appropriate comparison
>because node operators have never been faced with the same incentive to
>upgrade. We can point to unintentional forks in the past that have been
>resolved fairly quickly by reaching out to miners, but it's also a poor
>comparison. Unfortunately, we have no way of knowing what percentage of
>nodes are economically important - a great deal of them may be running
>not even be used by the operators.
>Perhaps it would be better if we were to formalize the expectations for
>full node operators, but it seems to me that node operators have a
>responsibility to keep themselves informed and decide when it is
>appropriate to update their software. I'm not so sure that it's the
>the ecosystem's responsibility to wait around for laggards.
>On December 16, 2015 12:38:30 PM PST, Jeff Garzik via bitcoin-dev <
>> bitcoin-dev at lists.linuxfoundation.org> wrote:
>>> 1. Summary
>>> Segregated Witness (SegWitness, SW) is being presented in the
>>> Scaling Bitcoin. It has useful attributes, notably addressing a
>>> malleability vector, but is not a short term scaling solution.
>>> 2. Definitions
>>> Import Fee Event, ECE, TFM, FFM from previous email.
>>> Older clients - Any software not upgraded to SW
>>> Newer clients - Upgraded, SW aware software
>>> Block size - refers to the core block economic resource limit ed by
>>> MAX_BLOCK_SIZE. Witness data (or extension block data) is excluded.
>>> Requires a hard fork to change.
>>> Core block - Current bitcoin block, with upper bound MAX_BLOCK_SIZE.
>>> changed by SW.
>>> Extended transaction - Newer, upgraded version of transaction data
>>> Extended block - Newer, upgraded version of block data format.
>>> EBS - Extended block size. Block size seen by newer clients.
>>> 3. Context of analysis
>>> One proposal presents SW *in lieu of* a hard fork block size
>>> This email focuses directly on that.
>>> Useful features outside block size context, such as
>>> fraud proof features, are not covered in depth.
>>> 4.1. Observations on data structure formats and views
>>> SW creates two *views* of each transaction and block. SW has blocks
>>> extended blocks. Similarly, there exists transactions and extended
>>> This view is rendered to clients depending on compatibility level.
>>> clients see extended blocks and extended transactions. Older
>>> blocks (limit 1M), and do not see extended blocks. Older clients
>>> upgraded transactions as unsigned, anyone-can-pay transactions.
>>> Each extended transaction exists in two states, one unsigned and one
>>> signed, each of which passes validation as a valid bitcoin
>>> 4.2. Observations on behavior of older transaction creation
>>> Transactions created by older clients will not use the extended
>>> transaction format. All data is stored the standard 1M block as
>>> 4.3. Observations on new block economic model
>>> SW complicates block economics by creating two separate, supply
>>> The core block economic resource is heavily contended. Older
>>> core blocks exclusively. Newer clients use core block s more
>>> conservatively, storing as much data as possible in extended blocks.
>>> The extended block economic resource is less heavily contended,
>>> that of course grows over time as clients upgrade.
>>> Because core blocks are more heavily contended, it is presumed that
>>> clients will pay a higher fee than newer clients (subject to
>>> 5.1. Problem: Pace of roll-out will be slow - Whole Ecosystem must
>>> The current apparent proposal is to roll out Segregated Witness as a
>>> fork, and keep block size at 1M.
>>> The roll-out pace cannot simply be judged by soft fork speed - which
>>> months at best. Analysis must the layers above: Updating
>>> (JS) and bitcoinj (Java), and then the timelines to roll out those
>>> to apps, and then the timeline to update those apps to create
>>> Overall, wallet software and programmer libraries must be upgraded
>>> make use of this new format, adding many more months (12+ in some
>>> to the roll out timeline. In the meantime, clients continue to
>>> entirely for core block space.
>>> 5.2. Problem: Hard fork to bigger block size Just Works(tm) with
>>> software, unlike SW.
>>> A simple hard fork such as BIP 102 is automatically compatible with
>>> vast range of today's ecosystem software.
>>> SW requires merchants to upgrade almost immediately, requires wallet
>>> other peripheral software upgrades to make use of. Other updates
>>> opt-in and occur more slowly. BIP 70 processors need some updates.
>>> The number of LOC that must change for BIP 102 is very small, and
>>> problem domain well known, versus SW.
>>> 5.3. Problem: Due to pace, Fee Event not forestalled.
>>> Even presuming SW is merged into Bitcoin Core tomorrow, this does
>>> address the risk of a Fee Event and associated Economic Change in
>>> coming months.
>>> 5.4. Problem: More complex economic policy, new game theory, new
>>> bidding structure risks.
>>> Splitting blocks into two pieces, each with separate and distinct
>>> behaviors and resource values, creates *two fee markets.*
>>> Having two pricing strata within each block has certainly feasible -
>>> is the current mining policy of (1) fee/KB followed by (2)
>>> Valuable or not - e.g. incentivizing older clients to upgrade - the
>>> remains that SW creates a more-complex bidding structure by creating
>>> second economic resource.
>>> *This is clearly a change to a new economic policy* with standard
>>> associated with that. Will that induce an Economic C hange Event
>>> last email)? *Unlikely*, due to slow rollout pace.
>>> 5.5. Problem: Current SW mining algorithm needs improvement
>>> Current SW block template maker does a reasonable job, but makes
>>> naive assumptions about the fee market across an entire extended
>>> This is a mismatch with the economic reality (just described).
>>> 5.6. Problem: New, under-analyzed attack surfaces
>>> Less significant and fundamental but still worth noting.
>>> This is not a fundamental SW problem, but simply standard complexity
>>> factors: splitting the signatures away from transactions, and
>>> new apparently-unsigned version of the transaction opens t he
>>> of some network attacks which cause some clients to degrade down
>>> extended block to core block mode temporarily.
>>> There is a chance of a failure mode that fools older clients into
>>> thinking fraudulent data is valid (judgement: unlikely vis hashpower
>>> not impossible)
>>> 6. Conclusions and recommendations
>>> It seems unlikely that SW provides scaling in the short term, and SW
>>> introduces new economics complexities.
>>> A "short term bump" hard fork block size increase addresses economic
>>> ecosystem risks that SW does not.
>>> Bump + SW should proce ed in parallel, independent tracks, as
>>> 7. Appendix - Other SW comments
>>> Hard forks provide much stronger validation, and ensure the network
>>> operates at a fully trustless level.
>>> SW hard fork is preferred, versus soft fork. Soft forking SW places
>>> huge amount of trust on miners to validate transaction signatures,
>>> the rest of the network, as the network slowly upgrades to newer
>>> An SW hard fork could also add several zero-filled placeholders in a
>>> merkle tree for future use.
>>> bitcoin-dev mailing list
>>> bitcoin-dev at lists.linuxfoundation.org
>> bitcoin-dev mailing list
>> bitcoin-dev at lists.linuxfoundation.org
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