[Bitcoin-development] Proposed alternatives to the 20MB step function

Rusty Russell rusty at rustcorp.com.au
Sat May 16 00:22:14 UTC 2015

Tier Nolan <tier.nolan at gmail.com> writes:
> On Sat, May 9, 2015 at 4:36 AM, Gregory Maxwell <gmaxwell at gmail.com> wrote:
>> An example would
>> be tx_size = MAX( real_size >> 1,  real_size + 4*utxo_created_size -
>> 3*utxo_consumed_size).
> This could be implemented as a soft fork too.
> * 1MB hard size limit
> * 900kB soft limit

I like this too.

Some tweaks:

1) Nomenclature: call tx_size "tx_cost" and real_size "tx_bytes"?

2) If we have a reasonable hard *byte* limit, I don't think that we need
   the MAX().  In fact, it's probably OK to go negative.

3) ... or maybe not, if any consumed UTXO was generated before the soft
   fork (reducing Tier's perverse incentive).

4) How do we measure UTXO size?  There are some constant-ish things in
   there (eg. txid as key, height, outnum, amount).  Maybe just add 32
   to scriptlen?

5) Add a CHECKSIG cost.  Naively, since we allow 20,000 CHECKSIGs and
   1MB blocks, that implies a cost of 50 bytes per CHECKSIG (but counted
   correctly, unlike now).   

This last one implies that the initial cost limit would be 2M, but in
practice probably somewhere in the middle.

  tx_cost = 50*num-CHECKSIG
                + tx_bytes
                + 4*utxo_created_size
                - 3*utxo_consumed_size

> A 250 byte transaction with 2 inputs and 2 outputs would have an adjusted
> size of 252 bytes.

Now cost == 352.


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