[bitcoin-dev] Issolated Bitcoin Nodes

[Juan Garavaglia via bitcoin-dev]

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.

[Andrew Chow via bitcoin-dev]

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.

Post by Juan Garavaglia via bitcoin-dev
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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[Andrew Chow via bitcoin-dev]

A correction to my previous email (because people are quoting me on
r/btc and what I wrote was wrong)

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.

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.

Post by Juan Garavaglia via bitcoin-dev
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
https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev

[Matt Corallo via bitcoin-dev]

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.

Post by Juan Garavaglia via bitcoin-dev
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.

[Pieter Wuille via bitcoin-dev]

On Thu, Mar 23, 2017 at 3:37 PM, Juan Garavaglia via bitcoin-dev

Post by Juan Garavaglia via bitcoin-dev
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.

[James Hilliard via bitcoin-dev]

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

Post by Pieter Wuille via bitcoin-dev
On Thu, Mar 23, 2017 at 3:37 PM, Juan Garavaglia via bitcoin-dev

Post by Juan Garavaglia via bitcoin-dev
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
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[Eric Voskuil via bitcoin-dev]

Post by Pieter Wuille via bitcoin-dev
On Thu, Mar 23, 2017 at 3:37 PM, Juan Garavaglia via bitcoin-dev

Post by Juan Garavaglia via bitcoin-dev
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