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List:       bitcoin-dev
Subject:    [bitcoin-dev] Issolated Bitcoin Nodes
From:       achow101-lists () achow101 ! com (Andrew Chow)
Date:       2017-03-24 3:38:21
Message-ID: 2caa270f-9feb-4720-9b68-eff458cdc956 () achow101 ! com
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A correction to my previous email (because people are quoting me on
r/btc and what I wrote was wrong)

This statement is incorrect:

> Given that Testnet has a smaller number of nodes and less difficulty,
this could result in some miners using 0.13.0+ mining blocks which do
not propagate well and thus causing multiple chain splits and reorgs as
other miners find blocks for the same height before receiving a block
for that height.

Miners using 0.13.0+ will produce blocks that propagate well. This is
because 0.12.x- nodes will accept those blocks, and so will 0.13.0+.
Furthermore Core 0.13.0+ will use its outbound connections to connect to
segwit enabled peers so that it will be relaying segwit blocks to
someone. However Bitcoin Core 0.13.0+ will not request blocks from peers
that are not segwit enabled (because with segwit, they will be receiving
blocks without witnesses which are invalid to a segwit node), so they
will not receive blocks mined by a 0.12.x- node. This means that 0.12.x-
mined blocks propagate poorly, even though are valid. Because of the
poor propagation, a 0.13.0+ miner can find a block at the same height
which is more likely to get built upon. However, the poorly propagated
block can still reach other 0.12.x- miners who can build upon it due to
the low difficulty and difficulty resets, thus causing multiple chains
to exist, particularly among pockets of 0.12.x- nodes. The reorgs happen
when either the 0.12.x- nodes hear of the segwit blockchain that is
presumably longer because it has the majority hashrate, or when people
run bridges which allow 0.12.x- nodes relay blocks to 0.13.0+ nodes.

On 3/23/2017 7:14 PM, Andrew Chow wrote:
> 
> The issue is due to Segwit blocks since Testnet has already activated
> Segwit. 0.12.x- nodes will receive a Segwit block with all of the
> witnesses stripped. When they relay this block to a 0.13.0+ node, the
> block will be rejected because those have Segwit functionality and
> require the witnesses to be in the block. Given that Testnet has a
> smaller number of nodes and less difficulty, this could result in some
> miners using 0.13.0+ mining blocks which do not propagate well and
> thus causing multiple chain splits and reorgs as other miners find
> blocks for the same height before receiving a block for that height.
> 
> 
> On 3/23/2017 6:37 PM, Juan Garavaglia via bitcoin-dev wrote:
> > 
> > We notice some reorgs in Bitcoin testnet, while reorgs in testnet are
> > common and may be part of different tests and experiments, it seems
> > the forks are not created by a single user and multiple blocks were
> > mined by different users in each chain.  My first impression was that
> > the problem was related to network issues but some Bitcoin explorers
> > were following one chain while others follow the other one. 
> > Nonetheless, well established explorers like blocktrail.com or
> > blockr.io were following different chains at different heights which
> > led to me to believe that it was not a network issue. After some
> > time, a reorg occurs and it all comes to normal state as a single chain.
> > 
> > We started investigating more and we identified that the fork occurs
> > with nodes 0.12; in some situations, nodes 0.12 has longer/different
> > chains. The blocks in both chains are valid so something must be
> > occurring in the communication between nodes but not related with the
> > network itself.
> > 
> > Long story short, when nodes 0.13+ receive blocks from 0.13+ nodes
> > all is ok, and those blocks propagate to older nodes with no issues.
> > But when a block tries to be propagated from bitcoind 0.12.+ to newer
> > ones those blocks are NOT being propagated to the peers with newer
> > versions while these newer blocks are being propagated to peers with
> > older versions with no issues.
> > 
> > My conclusion is that we have a backward compatibility issue between
> > 0.13.X+ and older versions.
> > 
> > The issue is simple to replicate, first, get latest version of
> > bitcoind, complete the IBD after is at current height, then force it
> > to use exclusively one or more peers of versions 0.12.X and older,
> > and you will notice that the latest version node will never receive a
> > new block.
> > 
> > Probably some alternative bitcoin implementations act as bridges
> > between these two versions and facilitate the chain reorgs.
> > 
> > I have not yet found any way where/how it can be used in a malicious
> > way or be exploited by a miner but in theory Bitcoin 0.13.X+ should
> > remain compatible with older ones, but a 0.13+ node may become
> > isolated by 0.12 peers, and there is not notice for the node owner.
> > 
> > 
> > 
> > 
> > 
> > _______________________________________________
> > bitcoin-dev mailing list
> > bitcoin-dev at lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> 

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