[bitcoin-dev] Issolated Bitcoin Nodes

[bitcoin-dev] Issolated Bitcoin Nodes

[Juan Garavaglia]

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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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Re: [bitcoin-dev] Issolated Bitcoin Nodes

[Matt Corallo]

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I haven't investigated, but you may be seeing segwit-invalid blocks...0.13.0+ nodes will enforce segwit as it activated some time ago on testnet, 0.12.X nodes will not.

On March 23, 2017 3:37:34 PM PDT, Juan Garavaglia via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> 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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Re: [bitcoin-dev] Issolated Bitcoin Nodes

[Andrew Chow]

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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@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev


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Re: [bitcoin-dev] Issolated Bitcoin Nodes

[Pieter Wuille]

On Thu, Mar 23, 2017 at 3:37 PM, Juan Garavaglia via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> 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.

Hello Juan,

this is expected behaviour. Nodes with segwit active only download
blocks from other segwit peers, as old peers cannot provide the
witness data they need to verify the blocks.

-- 
Pieter

Re: [bitcoin-dev] Issolated Bitcoin Nodes

[James Hilliard]

There were bridge nodes being run on testnet at one point to prevent
forks https://github.com/jl2012/bitcoin/commit/9717d856e72baa939d4b273f0a56e6009978e11b

On Thu, Mar 23, 2017 at 7:20 PM, Pieter Wuille via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> On Thu, Mar 23, 2017 at 3:37 PM, Juan Garavaglia via bitcoin-dev
> <bitcoin-dev@lists.linuxfoundation.org> wrote:
>> 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.
>
> Hello Juan,
>
> this is expected behaviour. Nodes with segwit active only download
> blocks from other segwit peers, as old peers cannot provide the
> witness data they need to verify the blocks.
>
> --
> Pieter
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev

Re: [bitcoin-dev] Issolated Bitcoin Nodes

[Eric Voskuil]

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Hash: SHA256

On 03/23/2017 05:20 PM, Pieter Wuille via bitcoin-dev wrote:
> On Thu, Mar 23, 2017 at 3:37 PM, Juan Garavaglia via bitcoin-dev 
> <bitcoin-dev@lists.linuxfoundation.org> wrote:
>> 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.
> 
> Hello Juan,
> 
> this is expected behaviour. Nodes with segwit active only download 
> blocks from other segwit peers, as old peers cannot provide the 
> witness data they need to verify the blocks.

Juan's statement pertains to incompatibility, not mysterious causation.

Clearly it's a material consideration. Is it an oversight that this is
not documented as an incompatibility in any of the segwit BIPs?

I don't recall any discussion on the importance of segwit bridge
nodes. Is there a plan for bridging mainnet?

e
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Re: [bitcoin-dev] Issolated Bitcoin Nodes

[Andrew Chow]

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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@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>


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