Hi all,

I've also been spending a few months coding upon the change's Pieter has been making with the headersfirst8 pull request.

My code updates are also ready to test, and are available on github at https://github.com/rebroad/bitcoin/ and the branch is "sipa-headersfirst8-patches".

I've made a number of improvement. Namely that it tracks the block as it downloads and won't disconnect if the block is downloading at a reasonable speed. The current stall logic of Pieter's is broken in that it will continue to disconnect a peer that is providing a block - particularly the next block needed to advance the current tip. I've raised this issue, but so far haven't been able to communicate the problem in a way that's been understood.

I've also added logic to avoid the node stalling due to many blocks being added to the ActiveTip (which would cause timeouts both from our node, and nodes we are connected to). It will also log and determine bandwidth per node, and the next changes I will be adding will be to make it prefer to download from the faster nodes (coming shortly).

I have also added code ready to adapt the window size for the download. Currently the start setting for blocks in flight is 3 per node, but for early on on the block chain this will be too small, so once it realises this after a few downloads and determines the average block size and speed, it will automatically adjust the number of blocks to request per node and revise this each minute.

Please do take a look at my code, and feel free to test it. It also improves some of the debug.log output to make it easier to read and provide useful information about concurrent downloads, etc.

Edmund

On Sun, Oct 12, 2014 at 7:34 AM, Pieter Wuille <pieter.wuille@gmail.com> wrote:

Hi all,

I believe that a large change that I've been working on for Bitcoin
Core is ready for review and testing: headers-first synchronization.
In short, it changes the way the best chain is discovered, downloaded
and verified, with several advantages:
* Parallel block downloading (much faster sync on typical network connections).
* No more stalled downloads.
* Much more robust against unresponsive or slow peers.
* Removes a class of DoS attacks related to peers feeding you
low-difficulty valid large blocks on a side branch.
* Reduces the need for checkpoints in the code.
* No orphan blocks stored in memory anymore (reducing memory usage during sync).
* A major step step towards an SPV mode using the reference codebase.

Historically, this mode of operation has been known for years (Greg
Maxwell wrote up a description of a very similar method in
https://en.bitcoin.it/wiki/User:Gmaxwell/Reverse_header-fetching_sync
in early 2012, but it was known before that), but it took a long time
to refactor these code enough to support it.

Technically, it works by replacing the single-peer blocks download by
a single-peer headers download (which typically takes seconds/minutes)
and verification, and simultaneously fetching blocks along the best
known headers chain from all peers that are known to have the relevant
blocks. Downloading is constrained to a moving window to avoid
unbounded unordering of blocks on disk (which would interfere with
pruning later).

At the protocol level, it increases the minimally supported version
for peers to 31800 (corresponding to bitcoin v3.18, released in
december 2010), as earlier versions did not support the getheaders P2P
message.

So, the code is available as a github pull request
(https://github.com/bitcoin/bitcoin/pull/4468), or packaged on
http://bitcoin.sipa.be/builds/headersfirst, where you can also find
binaries to test with.

Known issues:
* At the very start of the sync, especially before all headers are
processed, downloading is very slow due to a limited number of blocks
that are requested per peer simultaneously. The policies around this
will need some experimentation can certainly be improved.
* Blocks will be stored on disk out of order (in the order they are
received, really), which makes it incompatible with some tools or
other programs. Reindexing using earlier versions will also not work
anymore as a result of this.
* The block index database will now hold headers for which no block is
stored on disk, which earlier versions won't support. If you are fully
synced, it may still be possible to go back to an earlier version.

Unknown issues:
* Who knows, maybe it will replace your familiy pictures with Nyan
Cat? Use at your own risk.

TL;DR: Review/test https://github.com/bitcoin/bitcoin/pull/4468 or
http://bitcoin.sipa.be/builds/headersfirst.

--
Pieter

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