Hello Clark Moody,
Thank you for your review of our proposal.
For reference please see the pull request: https://github.com/bitcoin/bips/pull/555 and the proposed specification: https://github.com/veleslavs/bips/blob/Bech32_Encoded_TxRef/bip-XXXX-Bech32_Encoded_Transaction_Position_References.mediawiki. (I will inform the mailing if a BIP number is assigned and these become obsolete).
On variable length encodings:
We considered in-depth various variable length encodings; and found that for such short data-lengths they all came with a too-high overheard: especially when you design them to work correctly with 5-bit chunks. We will update the rational section of the BIP to include this comment.
Hence, we have proposed the concept of "Display Formats". Display Formats are individually tailored to a particular purpose and thus can be optimised to a much greater extent.
In the case there is a Hard-Fork that extends Bitcoin's Block Transaction Capacity over the 8191 of the provided display format, we will simply define a "Bitcoin Display Format 1". Considering the considerable disruption a Hard Fork causes; the creation of a secondary Display Format should not be of significant concern.
In the case there is a Drive-Chain style extension or other indirect extension to Bitcoin's transactional capacity; it makes no sense to try and define for an undefined format now. We will simply use a new Magic Value and create a tailored Display Format for the new system.
In the case that it is nearing Year 2048, I think that we will be in a far-better position to define a new format that suits the needs of the Bitcoin users than now.
Many thanks for your careful review, it is much appreciated,
С наилучшими пожеланиями,
Велеслав
-------- Original Message --------
Subject: Re: [bitcoin-dev] A BIP proposal for conveniently referring to confirmed transactions
Local Time: July 15, 2017 1:43 AM
UTC Time: July 14, 2017 6:43 PM
From: bitcoin-dev@lists.linuxfoundation.org
To: bitcoin-dev@lists.linuxfoundation.org
(copying from GitHub per jonasschnelli"s request)
I can understand the desire to keep all reference strings to the nice
14-character version by keeping the data payload to 40 bits, but it
seems to place artificial limitations on the format (year 2048 & 8191
transactions). I also understand that this might be addressed with
Version 1 encoding. But current blocks are not that far from having
8191 transactions.
You could go with a variable-length encoding similar to Bitcoin"s
variable ints and gain the benefit of having a format that will work
for very large blocks and the very far future.
Also, the Bech32 reference libraries allow encoding from byte arrays
into the base-5 arrays native to Bech32. It seems like bit-packing to
these 40 bits might be overkill. As an alternative you could have one
bit-packed byte to start:
# First two bits are the protocol version, supporting values 0-3
V = ((protocol version) & 0x03) << 6
# Next two bits are magic for the blockchain
# 0x00 = Bitcoin
# 0x01 = Testnet3
# 0x02 = Byte1 is another coin"s magic code (gives 256 options)
# 0x03 = Byte1-2 is treated as the coin magic code (gives 65280 more options)
M = (magic & 0x03) << 4
# Next two bits are the byte length of the block reference
B = ((byte length of block reference) & 0x03) << 2
# Final two bits are the byte length of the transaction index
T = ((byte length of transaction index) & 0x03)
# Assemble into the first byte
Byte0 = V | M | B | T
This gives you up to 3 bytes for each block and transaction reference,
which is 16.7 M blocks, or year 2336, and 16.7 M transaction slots.
Data part: [Byte0][optional magic bytes 1-2][block reference bytes][tx
reference bytes]
So the shortest data part would have 3 bytes in it, with the reference
version 0 genesis coinbase transaction having data part 0x050000.
I know this is a departure from your vision, but it would be much more
flexible for the long term.
Clark
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