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.
>
>  
>
>
>
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