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.
>>
>>  
>>
>>
>>
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>