Subject: Re: [bitcoin-dev] "Compressed" headers stream

From: Tier Nolan <[email-masked]>
Date: Mon, 11 Dec 2017 22:41:50 +0000

On Mon, Dec 11, 2017 at 9:56 PM, Jim Posen via bitcoin-dev <
[email-masked]> wrote:

> Omitting nBits entirely seems reasonable, I wrote up a possible
> implementation here
> <https://github.com/bitcoin/bitcoin/compare/master...jimpo:compact-headers-difficulty>.
> The downside is that it is more complex because it leaks into the
> validation code. The extra 4 byte savings is certainly nice though.
>

A compromise would be to have 1 byte indicating the difference since the
last header.

Since the exponent doesn't use the full range you could steal bits from
there to indicate mode.

- no change
- mantissa offset (for small changes)
- full difficulty

This would support any nBits rule and you say 3 of the 4 bytes.

> Can you elaborate on how parallel header fetching might work? getheaders
> requests could probably already be pipelined, where the node requests the
> next 2,000 headers before processing the current batch (though would make
> sense to check that they are all above min difficulty first).
>

I suggest adding a message where you can ask for the lowest N hashes
between 2 heights on the main chain.

The reply is an array of {height, header} pairs for the N headers with the
lowest hash in the specified range.

All peers should agree on which headers are in the array.  If there is
disagreement, then you can at least narrow down on which segment there is
disagreement.

It works kind of like a cut and choose.  You pick one segment of the ones
he gave you recursively.

You can ask a peer for proof for a segment between 2 headers of the form.

- first header + coinbase with merkle branch
- all headers in the segment

This proves the segment has the correct height and that all the headers
link up.

There is a method called "high hash highway" that allows compact proofs of
total POW.