[bitcoin-dev] Segregated Witness in the context of Scaling Bitcoin

Jeff Garzik jgarzik at gmail.com
Wed Dec 16 20:38:30 UTC 2015


1. Summary

Segregated Witness (SegWitness, SW) is being presented in the context of
Scaling Bitcoin.  It has useful attributes, notably addressing a major
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 limited 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.  Not
changed by SW.


Extended transaction - Newer, upgraded version of transaction data format.

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 increase.
This email focuses directly on that.

Useful features outside block size context, such as anti-malleability or
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 and
extended blocks.  Similarly, there exists transactions and extended
transactions.

This view is rendered to clients depending on compatibility level.  Newer
clients see extended blocks and extended transactions.  Older clients see
blocks (limit 1M), and do not see extended blocks.  Older clients see
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 transaction.


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 today.


4.3.  Observations on new block economic model

SW complicates block economics by creating two separate, supply limited
resources.

The core block economic resource is heavily contended.  Older clients use
core blocks exclusively.  Newer clients use core blocks more
conservatively, storing as much data as possible in extended blocks.

The extended block economic resource is less heavily contended, though that
of course grows over time as clients upgrade.

Because core blocks are more heavily contended, it is presumed that older
clients will pay a higher fee than newer clients (subject to elasticity
etc.).


5.1.  Problem:  Pace of roll-out will be slow - Whole Ecosystem must be
considered.

The current apparent proposal is to roll out Segregated Witness as a soft
fork, and keep block size at 1M.

The roll-out pace cannot simply be judged by soft fork speed - which is
months at best.  Analysis must the layers above:  Updating bitcoin-core
(JS) and bitcoinj (Java), and then the timelines to roll out those updates
to apps, and then the timeline to update those apps to create extended
transactions.

Overall, wallet software and programmer libraries must be upgraded to make
use of this new format, adding many more months (12+ in some stacks) to the
roll out timeline.  In the meantime, clients continue to contend entirely
for core block space.


5.2.  Problem:   Hard fork to bigger block size Just Works(tm) with most
software, unlike SW.

A simple hard fork such as BIP 102 is automatically compatible with the
vast range of today's ecosystem software.

SW requires merchants to upgrade almost immediately, requires wallet and
other peripheral software upgrades to make use of.  Other updates are
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 the
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 not
address the risk of a Fee Event and associated Economic Change in the
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 - that
is the current mining policy of (1) fee/KB followed by (2) priority/age.

Valuable or not - e.g. incentivizing older clients to upgrade - the fact
remains that SW creates a more-complex bidding structure by creating a
second economic resource.

*This is clearly a change to a new economic policy* with standard risks
associated with that.  Will that induce an Economic Change Event (see def
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 some naive
assumptions about the fee market across an entire extended block.  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 risk
factors:  splitting the signatures away from transactions, and creating a
new apparently-unsigned version of the transaction opens the possibility of
some network attacks which cause some clients to degrade down from 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 but 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 and
ecosystem risks that SW does not.

Bump + SW should proceed in parallel, independent tracks, as orthogonal
issues.


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 a huge
amount of trust on miners to validate transaction signatures, versus the
rest of the network, as the network slowly upgrades to newer clients.

An SW hard fork could also add several zero-filled placeholders in a merkle
tree for future use.
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