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Block Structure
===============
The tendermint consensus engine records all agreements by a
supermajority of nodes into a blockchain, which is replicated among all
nodes. This blockchain is accessible via various rpc endpoints, mainly
``/block?height=`` to get the full block, as well as
``/blockchain?minHeight=_&maxHeight=_`` to get a list of headers. But
what exactly is stored in these blocks?
Block
~~~~~
A
`Block <https://godoc.org/github.com/tendermint/tendermint/types#Block>`__
contains:
- a `Header <#header>`__ contains merkle hashes for various chain
states
- the
`Data <https://godoc.org/github.com/tendermint/tendermint/types#Data>`__
is all transactions which are to be processed
- the `LastCommit <#commit>`__ > 2/3 signatures for the last block
The signatures returned along with block ``H`` are those validating
block ``H-1``. This can be a little confusing, but we must also consider
that the ``Header`` also contains the ``LastCommitHash``. It would be
impossible for a Header to include the commits that sign it, as it would
cause an infinite loop here. But when we get block ``H``, we find
``Header.LastCommitHash``, which must match the hash of ``LastCommit``.
Header
~~~~~~
The
`Header <https://godoc.org/github.com/tendermint/tendermint/types#Header>`__
contains lots of information (follow link for up-to-date info). Notably,
it maintains the ``Height``, the ``LastBlockID`` (to make it a chain),
and hashes of the data, the app state, and the validator set. This is
important as the only item that is signed by the validators is the
``Header``, and all other data must be validated against one of the
merkle hashes in the ``Header``.
The ``DataHash`` can provide a nice check on the
`Data <https://godoc.org/github.com/tendermint/tendermint/types#Data>`__
returned in this same block. If you are subscribed to new blocks, via
tendermint RPC, in order to display or process the new transactions you
should at least validate that the ``DataHash`` is valid. If it is
important to verify autheniticity, you must wait for the ``LastCommit``
from the next block to make sure the block header (including
``DataHash``) was properly signed.
The ``ValidatorHash`` contains a hash of the current
`Validators <https://godoc.org/github.com/tendermint/tendermint/types#Validator>`__.
Tracking all changes in the validator set is complex, but a client can
quickly compare this hash with the `hash of the currently known
validators <https://godoc.org/github.com/tendermint/tendermint/types#ValidatorSet.Hash>`__
to see if there have been changes.
The ``AppHash`` serves as the basis for validating any merkle proofs
that come from the `ABCI
application <https://github.com/tendermint/abci>`__. It represents the
state of the actual application, rather that the state of the blockchain
itself. This means it's necessary in order to perform any business
logic, such as verifying and account balance.
**Note** After the transactions are committed to a block, they still
need to be processed in a separate step, which happens between the
blocks. If you find a given transaction in the block at height ``H``,
the effects of running that transaction will be first visible in the
``AppHash`` from the block header at height ``H+1``.
Like the ``LastCommit`` issue, this is a requirement of the immutability
of the block chain, as the application only applies transactions *after*
they are commited to the chain.
Commit
~~~~~~
The
`Commit <https://godoc.org/github.com/tendermint/tendermint/types#Commit>`__
contains a set of
`Votes <https://godoc.org/github.com/tendermint/tendermint/types#Vote>`__
that were made by the validator set to reach consensus on this block.
This is the key to the security in any PoS system, and actually no data
that cannot be traced back to a block header with a valid set of Votes
can be trusted. Thus, getting the Commit data and verifying the votes is
extremely important.
As mentioned above, in order to find the ``precommit votes`` for block
header ``H``, we need to query block ``H+1``. Then we need to check the
votes, make sure they really are for that block, and properly formatted.
Much of this code is implemented in Go in the
`light-client <https://github.com/tendermint/light-client>`__ package.
If you look at the code, you will notice that we need to provide the
``chainID`` of the blockchain in order to properly calculate the votes.
This is to protect anyone from swapping votes between chains to fake (or
frame) a validator. Also note that this ``chainID`` is in the
``genesis.json`` from *Tendermint*, not the ``genesis.json`` from the
basecoin app (`that is a different
chainID... <https://github.com/tendermint/basecoin/issues/32>`__).
Once we have those votes, and we calculated the proper `sign
bytes <https://godoc.org/github.com/tendermint/tendermint/types#Vote.WriteSignBytes>`__
using the chainID and a `nice helper
function <https://godoc.org/github.com/tendermint/tendermint/types#SignBytes>`__,
we can verify them. The light client is responsible for maintaining a
set of validators that we trust. Each vote only stores the validators
``Address``, as well as the ``Signature``. Assuming we have a local copy
of the trusted validator set, we can look up the ``Public Key`` of the
validator given its ``Address``, then verify that the ``Signature``
matches the ``SignBytes`` and ``Public Key``. Then we sum up the total
voting power of all validators, whose votes fulfilled all these
stringent requirements. If the total number of voting power for a single
block is greater than 2/3 of all voting power, then we can finally trust
the block header, the AppHash, and the proof we got from the ABCI
application.
Vote Sign Bytes
^^^^^^^^^^^^^^^
The ``sign-bytes`` of a vote is produced by taking a
`stable-json <https://github.com/substack/json-stable-stringify>`__-like
deterministic JSON `wire <./wire-protocol.html>`__ encoding of
the vote (excluding the ``Signature`` field), and wrapping it with
``{"chain_id":"my_chain","vote":...}``.
For example, a precommit vote might have the following ``sign-bytes``:
.. code:: json
{"chain_id":"my_chain","vote":{"block_hash":"611801F57B4CE378DF1A3FFF1216656E89209A99","block_parts_header":{"hash":"B46697379DBE0774CC2C3B656083F07CA7E0F9CE","total":123},"height":1234,"round":1,"type":2}}
Block Hash
~~~~~~~~~~
The `block
hash <https://godoc.org/github.com/tendermint/tendermint/types#Block.Hash>`__
is the `Simple Tree hash <Merkle-Trees#simple-tree-with-dictionaries>`__
of the fields of the block ``Header`` encoded as a list of
``KVPair``\ s.
Transaction
~~~~~~~~~~~
A transaction is any sequence of bytes. It is up to your
`ABCI <https://github.com/tendermint/abci>`__ application to accept or
reject transactions.
BlockID
~~~~~~~
Many of these data structures refer to the
`BlockID <https://godoc.org/github.com/tendermint/tendermint/types#BlockID>`__,
which is the ``BlockHash`` (hash of the block header, also referred to
by the next block) along with the ``PartSetHeader``. The
``PartSetHeader`` is explained below and is used internally to
orchestrate the p2p propogation. For clients, it is basically opaque
bytes, but they must match for all votes.
PartSetHeader
~~~~~~~~~~~~~
The
`PartSetHeader <https://godoc.org/github.com/tendermint/tendermint/types#PartSetHeader>`__
contains the total number of pieces in a
`PartSet <https://godoc.org/github.com/tendermint/tendermint/types#PartSet>`__,
and the Merkle root hash of those pieces.
PartSet
~~~~~~~
PartSet is used to split a byteslice of data into parts (pieces) for
transmission. By splitting data into smaller parts and computing a
Merkle root hash on the list, you can verify that a part is legitimately
part of the complete data, and the part can be forwarded to other peers
before all the parts are known. In short, it's a fast way to securely
propagate a large chunk of data (like a block) over a gossip network.
PartSet was inspired by the LibSwift project.
Usage:
.. code:: go
data := RandBytes(2 << 20) // Something large
partSet := NewPartSetFromData(data)
partSet.Total() // Total number of 4KB parts
partSet.Count() // Equal to the Total, since we already have all the parts
partSet.Hash() // The Merkle root hash
partSet.BitArray() // A BitArray of partSet.Total() 1's
header := partSet.Header() // Send this to the peer
header.Total // Total number of parts
header.Hash // The merkle root hash
// Now we'll reconstruct the data from the parts
partSet2 := NewPartSetFromHeader(header)
partSet2.Total() // Same total as partSet.Total()
partSet2.Count() // Zero, since this PartSet doesn't have any parts yet.
partSet2.Hash() // Same hash as in partSet.Hash()
partSet2.BitArray() // A BitArray of partSet.Total() 0's
// In a gossip network the parts would arrive in arbitrary order, perhaps
// in response to explicit requests for parts, or optimistically in response
// to the receiving peer's partSet.BitArray().
for !partSet2.IsComplete() {
part := receivePartFromGossipNetwork()
added, err := partSet2.AddPart(part)
if err != nil {
// A wrong part,
// the merkle trail does not hash to partSet2.Hash()
} else if !added {
// A duplicate part already received
}
}
data2, _ := ioutil.ReadAll(partSet2.GetReader())
bytes.Equal(data, data2) // true

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Byzantine Consensus Algorithm
=============================
Terms
-----
- The network is composed of optionally connected *nodes*. Nodes
directly connected to a particular node are called *peers*.
- The consensus process in deciding the next block (at some *height*
``H``) is composed of one or many *rounds*.
- ``NewHeight``, ``Propose``, ``Prevote``, ``Precommit``, and
``Commit`` represent state machine states of a round. (aka
``RoundStep`` or just "step").
- A node is said to be *at* a given height, round, and step, or at
``(H,R,S)``, or at ``(H,R)`` in short to omit the step.
- To *prevote* or *precommit* something means to broadcast a `prevote
vote <https://godoc.org/github.com/tendermint/tendermint/types#Vote>`__
or `first precommit
vote <https://godoc.org/github.com/tendermint/tendermint/types#FirstPrecommit>`__
for something.
- A vote *at* ``(H,R)`` is a vote signed with the bytes for ``H`` and
``R`` included in its
`sign-bytes <block-structure.html#vote-sign-bytes>`__.
- *+2/3* is short for "more than 2/3"
- *1/3+* is short for "1/3 or more"
- A set of +2/3 of prevotes for a particular block or ``<nil>`` at
``(H,R)`` is called a *proof-of-lock-change* or *PoLC* for short.
State Machine Overview
----------------------
At each height of the blockchain a round-based protocol is run to
determine the next block. Each round is composed of three *steps*
(``Propose``, ``Prevote``, and ``Precommit``), along with two special
steps ``Commit`` and ``NewHeight``.
In the optimal scenario, the order of steps is:
::
NewHeight -> (Propose -> Prevote -> Precommit)+ -> Commit -> NewHeight ->...
The sequence ``(Propose -> Prevote -> Precommit)`` is called a *round*.
There may be more than one round required to commit a block at a given
height. Examples for why more rounds may be required include:
- The designated proposer was not online.
- The block proposed by the designated proposer was not valid.
- The block proposed by the designated proposer did not propagate in
time.
- The block proposed was valid, but +2/3 of prevotes for the proposed
block were not received in time for enough validator nodes by the
time they reached the ``Precommit`` step. Even though +2/3 of
prevotes are necessary to progress to the next step, at least one
validator may have voted ``<nil>`` or maliciously voted for something
else.
- The block proposed was valid, and +2/3 of prevotes were received for
enough nodes, but +2/3 of precommits for the proposed block were not
received for enough validator nodes.
Some of these problems are resolved by moving onto the next round &
proposer. Others are resolved by increasing certain round timeout
parameters over each successive round.
State Machine Diagram
---------------------
::
+-------------------------------------+
v |(Wait til `CommmitTime+timeoutCommit`)
+-----------+ +-----+-----+
+----------> | Propose +--------------+ | NewHeight |
| +-----------+ | +-----------+
| | ^
|(Else, after timeoutPrecommit) v |
+-----+-----+ +-----------+ |
| Precommit | <------------------------+ Prevote | |
+-----+-----+ +-----------+ |
|(When +2/3 Precommits for block found) |
v |
+--------------------------------------------------------------------+
| Commit |
| |
| * Set CommitTime = now; |
| * Wait for block, then stage/save/commit block; |
+--------------------------------------------------------------------+
Background Gossip
-----------------
A node may not have a corresponding validator private key, but it
nevertheless plays an active role in the consensus process by relaying
relevant meta-data, proposals, blocks, and votes to its peers. A node
that has the private keys of an active validator and is engaged in
signing votes is called a *validator-node*. All nodes (not just
validator-nodes) have an associated state (the current height, round,
and step) and work to make progress.
Between two nodes there exists a ``Connection``, and multiplexed on top
of this connection are fairly throttled ``Channel``\ s of information.
An epidemic gossip protocol is implemented among some of these channels
to bring peers up to speed on the most recent state of consensus. For
example,
- Nodes gossip ``PartSet`` parts of the current round's proposer's
proposed block. A LibSwift inspired algorithm is used to quickly
broadcast blocks across the gossip network.
- Nodes gossip prevote/precommit votes. A node NODE\_A that is ahead of
NODE\_B can send NODE\_B prevotes or precommits for NODE\_B's current
(or future) round to enable it to progress forward.
- Nodes gossip prevotes for the proposed PoLC (proof-of-lock-change)
round if one is proposed.
- Nodes gossip to nodes lagging in blockchain height with block
`commits <https://godoc.org/github.com/tendermint/tendermint/types#Commit>`__
for older blocks.
- Nodes opportunistically gossip ``HasVote`` messages to hint peers
what votes it already has.
- Nodes broadcast their current state to all neighboring peers. (but is
not gossiped further)
There's more, but let's not get ahead of ourselves here.
Proposals
---------
A proposal is signed and published by the designated proposer at each
round. The proposer is chosen by a deterministic and non-choking round
robin selection algorithm that selects proposers in proportion to their
voting power. (see
`implementation <https://github.com/tendermint/tendermint/blob/develop/types/validator_set.go>`__)
A proposal at ``(H,R)`` is composed of a block and an optional latest
``PoLC-Round < R`` which is included iff the proposer knows of one. This
hints the network to allow nodes to unlock (when safe) to ensure the
liveness property.
State Machine Spec
------------------
Propose Step (height:H,round:R)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Upon entering ``Propose``: - The designated proposer proposes a block at
``(H,R)``.
The ``Propose`` step ends: - After ``timeoutProposeR`` after entering
``Propose``. --> goto ``Prevote(H,R)`` - After receiving proposal block
and all prevotes at ``PoLC-Round``. --> goto ``Prevote(H,R)`` - After
`common exit conditions <#common-exit-conditions>`__
Prevote Step (height:H,round:R)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Upon entering ``Prevote``, each validator broadcasts its prevote vote.
- First, if the validator is locked on a block since ``LastLockRound``
but now has a PoLC for something else at round ``PoLC-Round`` where
``LastLockRound < PoLC-Round < R``, then it unlocks.
- If the validator is still locked on a block, it prevotes that.
- Else, if the proposed block from ``Propose(H,R)`` is good, it
prevotes that.
- Else, if the proposal is invalid or wasn't received on time, it
prevotes ``<nil>``.
The ``Prevote`` step ends: - After +2/3 prevotes for a particular block
or ``<nil>``. --> goto ``Precommit(H,R)`` - After ``timeoutPrevote``
after receiving any +2/3 prevotes. --> goto ``Precommit(H,R)`` - After
`common exit conditions <#common-exit-conditions>`__
Precommit Step (height:H,round:R)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Upon entering ``Precommit``, each validator broadcasts its precommit
vote. - If the validator has a PoLC at ``(H,R)`` for a particular block
``B``, it (re)locks (or changes lock to) and precommits ``B`` and sets
``LastLockRound = R``. - Else, if the validator has a PoLC at ``(H,R)``
for ``<nil>``, it unlocks and precommits ``<nil>``. - Else, it keeps the
lock unchanged and precommits ``<nil>``.
A precommit for ``<nil>`` means "I didn’t see a PoLC for this round, but
I did get +2/3 prevotes and waited a bit".
The Precommit step ends: - After +2/3 precommits for ``<nil>``. --> goto
``Propose(H,R+1)`` - After ``timeoutPrecommit`` after receiving any +2/3
precommits. --> goto ``Propose(H,R+1)`` - After `common exit
conditions <#common-exit-conditions>`__
common exit conditions
^^^^^^^^^^^^^^^^^^^^^^
- After +2/3 precommits for a particular block. --> goto ``Commit(H)``
- After any +2/3 prevotes received at ``(H,R+x)``. --> goto
``Prevote(H,R+x)``
- After any +2/3 precommits received at ``(H,R+x)``. --> goto
``Precommit(H,R+x)``
Commit Step (height:H)
~~~~~~~~~~~~~~~~~~~~~~
- Set ``CommitTime = now()``
- Wait until block is received. --> goto ``NewHeight(H+1)``
NewHeight Step (height:H)
~~~~~~~~~~~~~~~~~~~~~~~~~
- Move ``Precommits`` to ``LastCommit`` and increment height.
- Set ``StartTime = CommitTime+timeoutCommit``
- Wait until ``StartTime`` to receive straggler commits. --> goto
``Propose(H,0)``
Proofs
------
Proof of Safety
~~~~~~~~~~~~~~~
Assume that at most -1/3 of the voting power of validators is byzantine.
If a validator commits block ``B`` at round ``R``, it's because it saw
+2/3 of precommits at round ``R``. This implies that 1/3+ of honest
nodes are still locked at round ``R' > R``. These locked validators will
remain locked until they see a PoLC at ``R' > R``, but this won't happen
because 1/3+ are locked and honest, so at most -2/3 are available to
vote for anything other than ``B``.
Proof of Liveness
~~~~~~~~~~~~~~~~~
If 1/3+ honest validators are locked on two different blocks from
different rounds, a proposers' ``PoLC-Round`` will eventually cause
nodes locked from the earlier round to unlock. Eventually, the
designated proposer will be one that is aware of a PoLC at the later
round. Also, ``timeoutProposalR`` increments with round ``R``, while the
size of a proposal are capped, so eventually the network is able to
"fully gossip" the whole proposal (e.g. the block & PoLC).
Proof of Fork Accountability
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Define the JSet (justification-vote-set) at height ``H`` of a validator
``V1`` to be all the votes signed by the validator at ``H`` along with
justification PoLC prevotes for each lock change. For example, if ``V1``
signed the following precommits: ``Precommit(B1 @ round 0)``,
``Precommit(<nil> @ round 1)``, ``Precommit(B2 @ round 4)`` (note that
no precommits were signed for rounds 2 and 3, and that's ok),
``Precommit(B1 @ round 0)`` must be justified by a PoLC at round 0, and
``Precommit(B2 @ round 4)`` must be justified by a PoLC at round 4; but
the precommit for ``<nil>`` at round 1 is not a lock-change by
definition so the JSet for ``V1`` need not include any prevotes at round
1, 2, or 3 (unless ``V1`` happened to have prevoted for those rounds).
Further, define the JSet at height ``H`` of a set of validators ``VSet``
to be the union of the JSets for each validator in ``VSet``. For a given
commit by honest validators at round ``R`` for block ``B`` we can
construct a JSet to justify the commit for ``B`` at ``R``. We say that a
JSet *justifies* a commit at ``(H,R)`` if all the committers (validators
in the commit-set) are each justified in the JSet with no duplicitous
vote signatures (by the committers).
- **Lemma**: When a fork is detected by the existence of two
conflicting `commits <./validators.html#commiting-a-block>`__,
the union of the JSets for both commits (if they can be compiled)
must include double-signing by at least 1/3+ of the validator set.
**Proof**: The commit cannot be at the same round, because that would
immediately imply double-signing by 1/3+. Take the union of the JSets
of both commits. If there is no double-signing by at least 1/3+ of
the validator set in the union, then no honest validator could have
precommitted any different block after the first commit. Yet, +2/3
did. Reductio ad absurdum.
As a corollary, when there is a fork, an external process can determine
the blame by requiring each validator to justify all of its round votes.
Either we will find 1/3+ who cannot justify at least one of their votes,
and/or, we will find 1/3+ who had double-signed.
Alternative algorithm
~~~~~~~~~~~~~~~~~~~~~
Alternatively, we can take the JSet of a commit to be the "full commit".
That is, if light clients and validators do not consider a block to be
committed unless the JSet of the commit is also known, then we get the
desirable property that if there ever is a fork (e.g. there are two
conflicting "full commits"), then 1/3+ of the validators are immediately
punishable for double-signing.
There are many ways to ensure that the gossip network efficiently share
the JSet of a commit. One solution is to add a new message type that
tells peers that this node has (or does not have) a +2/3 majority for B
(or ) at (H,R), and a bitarray of which votes contributed towards that
majority. Peers can react by responding with appropriate votes.
We will implement such an algorithm for the next iteration of the
Tendermint consensus protocol.
Other potential improvements include adding more data in votes such as
the last known PoLC round that caused a lock change, and the last voted
round/step (or, we may require that validators not skip any votes). This
may make JSet verification/gossip logic easier to implement.
Censorship Attacks
~~~~~~~~~~~~~~~~~~
Due to the definition of a block
`commit <validators.html#commiting-a-block>`__, any 1/3+
coalition of validators can halt the blockchain by not broadcasting
their votes. Such a coalition can also censor particular transactions by
rejecting blocks that include these transactions, though this would
result in a significant proportion of block proposals to be rejected,
which would slow down the rate of block commits of the blockchain,
reducing its utility and value. The malicious coalition might also
broadcast votes in a trickle so as to grind blockchain block commits to
a near halt, or engage in any combination of these attacks.
If a global active adversary were also involved, it can partition the
network in such a way that it may appear that the wrong subset of
validators were responsible for the slowdown. This is not just a
limitation of Tendermint, but rather a limitation of all consensus
protocols whose network is potentially controlled by an active
adversary.
Overcoming Forks and Censorship Attacks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For these types of attacks, a subset of the validators through external
means should coordinate to sign a reorg-proposal that chooses a fork
(and any evidence thereof) and the initial subset of validators with
their signatures. Validators who sign such a reorg-proposal forego its
collateral on all other forks. Clients should verify the signatures on
the reorg-proposal, verify any evidence, and make a judgement or prompt
the end-user for a decision. For example, a phone wallet app may prompt
the user with a security warning, while a refrigerator may accept any
reorg-proposal signed by +½ of the original validators.
No non-synchronous Byzantine fault-tolerant algorithm can come to
consensus when ⅓+ of validators are dishonest, yet a fork assumes that
⅓+ of validators have already been dishonest by double-signing or
lock-changing without justification. So, signing the reorg-proposal is a
coordination problem that cannot be solved by any non-synchronous
protocol (i.e. automatically, and without making assumptions about the
reliability of the underlying network). It must be provided by means
external to the weakly-synchronous Tendermint consensus algorithm. For
now, we leave the problem of reorg-proposal coordination to human
coordination via internet media. Validators must take care to ensure
that there are no significant network partitions, to avoid situations
where two conflicting reorg-proposals are signed.
Assuming that the external coordination medium and protocol is robust,
it follows that forks are less of a concern than `censorship
attacks <#censorship-attacks>`__.

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Configuration
=============
TendermintCore can be configured via a TOML file in
``$TMHOME/config.toml``. Some of these parameters can be overridden by
command-line flags.
Config parameters
~~~~~~~~~~~~~~~~~
The main config parameters are defined
`here <https://github.com/tendermint/tendermint/blob/master/config/config.go>`__.
- ``abci``: ABCI transport (socket \| grpc). *Default*: ``socket``
- ``db_backend``: Database backend for the blockchain and
TendermintCore state. ``leveldb`` or ``memdb``. *Default*:
``"leveldb"``
- ``db_dir``: Database dir. *Default*: ``"$TMHOME/data"``
- ``fast_sync``: Whether to sync faster from the block pool. *Default*:
``true``
- ``genesis_file``: The location of the genesis file. *Default*:
``"$TMHOME/genesis.json"``
- ``log_level``: *Default*: ``"state:info,*:error"``
- ``moniker``: Name of this node. *Default*: ``"anonymous"``
- ``priv_validator_file``: Validator private key file. *Default*:
``"$TMHOME/priv_validator.json"``
- ``prof_laddr``: Profile listen address. *Default*: ``""``
- ``proxy_app``: The ABCI app endpoint. *Default*:
``"tcp://127.0.0.1:46658"``
- ``consensus.max_block_size_txs``: Maximum number of block txs.
*Default*: ``10000``
- ``consensus.timeout_*``: Various consensus timeout parameters
**TODO**
- ``consensus.wal_file``: Consensus state WAL. *Default*:
``"$TMHOME/data/cswal"``
- ``consensus.wal_light``: Whether to use light-mode for Consensus
state WAL. *Default*: ``false``
- ``mempool.*``: Various mempool parameters **TODO**
- ``p2p.addr_book_file``: Peer address book. *Default*:
``"$TMHOME/addrbook.json"``. **NOT USED**
- ``p2p.laddr``: Node listen address. (0.0.0.0:0 means any interface,
any port). *Default*: ``"0.0.0.0:46656"``
- ``p2p.pex``: Enable Peer-Exchange (dev feature). *Default*: ``false``
- ``p2p.seeds``: Comma delimited host:port seed nodes. *Default*:
``""``
- ``p2p.skip_upnp``: Skip UPNP detection. *Default*: ``false``
- ``rpc.grpc_laddr``: GRPC listen address (BroadcastTx only). Port
required. *Default*: ``""``
- ``rpc.laddr``: RPC listen address. Port required. *Default*:
``"0.0.0.0:46657"``
- ``rpc.unsafe``: Enabled unsafe rpc methods. *Default*: ``true``

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Fast Sync
=========
Background
----------
In a proof of work blockchain, syncing with the chain is the same
process as staying up-to-date with the consensus: download blocks, and
look for the one with the most total work. In proof-of-stake, the
consensus process is more complex, as it involves rounds of
communication between the nodes to determine what block should be
committed next. Using this process to sync up with the blockchain from
scratch can take a very long time. It's much faster to just download
blocks and check the merkle tree of validators than to run the real-time
consensus gossip protocol.
Fast Sync
---------
To support faster syncing, tendermint offers a ``fast-sync`` mode, which
is enabled by default, and can be toggled in the ``config.toml`` or via
``--fast_sync=false``.
In this mode, the tendermint daemon will sync hundreds of times faster
than if it used the real-time consensus process. Once caught up, the
daemon will switch out of fast sync and into the normal consensus mode.
After running for some time, the node is considered ``caught up`` if it
has at least one peer and it's height is at least as high as the max
reported peer height. See `the IsCaughtUp
method <https://github.com/tendermint/tendermint/blob/b467515719e686e4678e6da4e102f32a491b85a0/blockchain/pool.go#L128>`__.
If we're lagging sufficiently, we should go back to fast syncing, but
this is an open issue:
https://github.com/tendermint/tendermint/issues/129

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Genesis
=======
The genesis.json file in ``$TMROOT`` defines the initial TendermintCore
state upon genesis of the blockchain (`see
definition <https://github.com/tendermint/tendermint/blob/master/types/genesis.go>`__).
NOTE: This does not (yet) specify the application state (e.g. initial
distribution of tokens). Currently we leave it up to the application to
load the initial application genesis state. In the future, we may
include genesis SetOption messages that get passed from TendermintCore
to the app upon genesis.
Fields
~~~~~~
- ``genesis_time``: Official time of blockchain start.
- ``chain_id``: ID of the blockchain. This must be unique for every
blockchain. If your testnet blockchains do not have unique chain IDs,
you will have a bad time.
- ``validators``:
- ``pub_key``: The first element specifies the pub\_key type. 1 ==
Ed25519. The second element are the pubkey bytes.
- ``amount``: The validator's voting power.
- ``name``: Name of the validator (optional).
- ``app_hash``: The expected application hash (as returned by the
``Commit`` ABCI message) upon genesis. If the app's hash does not
match, a warning message is printed.
Sample genesis.json
~~~~~~~~~~~~~~~~~~~
.. code:: json
{
"genesis_time": "2016-02-05T06:02:31.526Z",
"chain_id": "chain-tTH4mi",
"validators": [
{
"pub_key": [
1,
"9BC5112CB9614D91CE423FA8744885126CD9D08D9FC9D1F42E552D662BAA411E"
],
"amount": 1,
"name": "mach1"
},
{
"pub_key": [
1,
"F46A5543D51F31660D9F59653B4F96061A740FF7433E0DC1ECBC30BE8494DE06"
],
"amount": 1,
"name": "mach2"
},
{
"pub_key": [
1,
"0E7B423C1635FD07C0FC3603B736D5D27953C1C6CA865BB9392CD79DE1A682BB"
],
"amount": 1,
"name": "mach3"
},
{
"pub_key": [
1,
"4F49237B9A32EB50682EDD83C48CE9CDB1D02A7CFDADCFF6EC8C1FAADB358879"
],
"amount": 1,
"name": "mach4"
}
],
"app_hash": "15005165891224E721CB664D15CB972240F5703F"
}

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Light Client Protocol
=====================
Light clients are an important part of the complete blockchain system
for most applications. Tendermint provides unique speed and security
properties for light client applications.
See our developing `light-client
repository <https://github.com/tendermint/light-client>`__.
Overview
--------
The objective of the light client protocol is to get a
`commit <./validators.html#committing-a-block>`__ for a recent
`block hash <./block-structure.html#block-hash>`__ where the commit
includes a majority of signatures from the last known validator set.
From there, all the application state is verifiable with `merkle
proofs <./merkle.html#iavl-tree>`__.
Properties
----------
- You get the full collateralized security benefits of Tendermint; No
need to wait for confirmations.
- You get the full speed benefits of Tendermint; transactions commit
instantly.
- You can get the most recent version of the application state
non-interactively (without committing anything to the blockchain).
For example, this means that you can get the most recent value of a
name from the name-registry without worrying about fork censorship
attacks, without posting a commit and waiting for confirmations. It's
fast, secure, and free!

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Merkle
======
For an overview of Merkle trees, see
`wikipedia <https://en.wikipedia.org/wiki/Merkle_tree>`__.
There are two types of Merkle trees used in Tendermint.
- ```IAVL+ Tree`` <#iavl-tree>`__: An immutable self-balancing binary
tree for persistent application state
- ```Simple Tree`` <#simple-tree>`__: A simple compact binary tree for
a static list of items
IAVL+ Tree
----------
The purpose of this data structure is to provide persistent storage for
key-value pairs (e.g. account state, name-registrar data, and
per-contract data) such that a deterministic merkle root hash can be
computed. The tree is balanced using a variant of the `AVL
algorithm <http://en.wikipedia.org/wiki/AVL_tree>`__ so all operations
are O(log(n)).
Nodes of this tree are immutable and indexed by its hash. Thus any node
serves as an immutable snapshot which lets us stage uncommitted
transactions from the mempool cheaply, and we can instantly roll back to
the last committed state to process transactions of a newly committed
block (which may not be the same set of transactions as those from the
mempool).
In an AVL tree, the heights of the two child subtrees of any node differ
by at most one. Whenever this condition is violated upon an update, the
tree is rebalanced by creating O(log(n)) new nodes that point to
unmodified nodes of the old tree. In the original AVL algorithm, inner
nodes can also hold key-value pairs. The AVL+ algorithm (note the plus)
modifies the AVL algorithm to keep all values on leaf nodes, while only
using branch-nodes to store keys. This simplifies the algorithm while
minimizing the size of merkle proofs
In Ethereum, the analog is the `Patricia
trie <http://en.wikipedia.org/wiki/Radix_tree>`__. There are tradeoffs.
Keys do not need to be hashed prior to insertion in IAVL+ trees, so this
provides faster iteration in the key space which may benefit some
applications. The logic is simpler to implement, requiring only two
types of nodes -- inner nodes and leaf nodes. The IAVL+ tree is a binary
tree, so merkle proofs are much shorter than the base 16 Patricia trie.
On the other hand, while IAVL+ trees provide a deterministic merkle root
hash, it depends on the order of updates. In practice this shouldn't be
a problem, since you can efficiently encode the tree structure when
serializing the tree contents.
Simple Tree
-----------
For merkelizing smaller static lists, use the Simple Tree. The
transactions and validation signatures of a block are hashed using this
simple merkle tree logic.
If the number of items is not a power of two, the tree will not be full
and some leaf nodes will be at different levels. Simple Tree tries to
keep both sides of the tree the same size, but the left side may be one
greater.
::
Simple Tree with 6 items Simple Tree with 7 items
* *
/ \ / \
/ \ / \
/ \ / \
/ \ / \
* * * *
/ \ / \ / \ / \
/ \ / \ / \ / \
/ \ / \ / \ / \
* h2 * h5 * * * h6
/ \ / \ / \ / \ / \
h0 h1 h3 h4 h0 h1 h2 h3 h4 h5
Simple Tree with Dictionaries
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Simple Tree is used to merkelize a list of items, so to merkelize a
(short) dictionary of key-value pairs, encode the dictionary as an
ordered list of ``KVPair`` structs. The block hash is such a hash
derived from all the fields of the block ``Header``. The state hash is
similarly derived.

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RPC
===
Coming soon: RPC docs powered by `slate <https://github.com/lord/slate>`__. Until then, read on.
Tendermint supports the following RPC protocols:
- URI over HTTP
- JSONRPC over HTTP
- JSONRPC over websockets
Tendermint RPC is build using `our own RPC
library <https://github.com/tendermint/tendermint/tree/master/rpc/lib>`__.
Documentation and tests for that library could be found at
``tendermint/rpc/lib`` directory.
Configuration
~~~~~~~~~~~~~
Set the ``laddr`` config parameter under ``[rpc]`` table in the
$TMHOME/config.toml file or the ``--rpc.laddr`` command-line flag to the
desired protocol://host:port setting. Default: ``tcp://0.0.0.0:46657``.
Arguments
~~~~~~~~~
Arguments which expect strings or byte arrays may be passed as quoted
strings, like ``"abc"`` or as ``0x``-prefixed strings, like
``0x616263``.
URI/HTTP
~~~~~~~~
Example request:
.. code:: bash
curl -s 'http://localhost:46657/broadcast_tx_sync?tx="abc"' | jq .
Response:
.. code:: json
{
"error": "",
"result": {
"hash": "2B8EC32BA2579B3B8606E42C06DE2F7AFA2556EF",
"log": "",
"data": "",
"code": 0
},
"id": "",
"jsonrpc": "2.0"
}
The first entry in the result-array (``96``) is the method this response
correlates with. ``96`` refers to "ResultTypeBroadcastTx", see
`responses.go <https://github.com/tendermint/tendermint/blob/master/rpc/core/types/responses.go>`__
for a complete overview.
JSONRPC/HTTP
~~~~~~~~~~~~
JSONRPC requests can be POST'd to the root RPC endpoint via HTTP (e.g.
``http://localhost:46657/``).
Example request:
.. code:: json
{
"method": "broadcast_tx_sync",
"jsonrpc": "2.0",
"params": [ "abc" ],
"id": "dontcare"
}
JSONRPC/websockets
~~~~~~~~~~~~~~~~~~
JSONRPC requests can be made via websocket. The websocket endpoint is at
``/websocket``, e.g. ``http://localhost:46657/websocket``. Asynchronous
RPC functions like event ``subscribe`` and ``unsubscribe`` are only
available via websockets.
Endpoints
~~~~~~~~~
An HTTP Get request to the root RPC endpoint (e.g.
``http://localhost:46657``) shows a list of available endpoints.
::
Available endpoints:
http://localhost:46657/abci_info
http://localhost:46657/dump_consensus_state
http://localhost:46657/genesis
http://localhost:46657/net_info
http://localhost:46657/num_unconfirmed_txs
http://localhost:46657/status
http://localhost:46657/unconfirmed_txs
http://localhost:46657/unsafe_flush_mempool
http://localhost:46657/unsafe_stop_cpu_profiler
http://localhost:46657/validators
Endpoints that require arguments:
http://localhost:46657/abci_query?path=_&data=_&prove=_
http://localhost:46657/block?height=_
http://localhost:46657/blockchain?minHeight=_&maxHeight=_
http://localhost:46657/broadcast_tx_async?tx=_
http://localhost:46657/broadcast_tx_commit?tx=_
http://localhost:46657/broadcast_tx_sync?tx=_
http://localhost:46657/commit?height=_
http://localhost:46657/dial_seeds?seeds=_
http://localhost:46657/subscribe?event=_
http://localhost:46657/tx?hash=_&prove=_
http://localhost:46657/unsafe_start_cpu_profiler?filename=_
http://localhost:46657/unsafe_write_heap_profile?filename=_
http://localhost:46657/unsubscribe?event=_
tx
~~
Returns a transaction matching the given transaction hash.
**Parameters**
1. hash - the transaction hash
2. prove - include a proof of the transaction inclusion in the block in
the result (optional, default: false)
**Returns**
- ``proof``: the ``types.TxProof`` object
- ``tx``: ``[]byte`` - the transaction
- ``tx_result``: the ``abci.Result`` object
- ``index``: ``int`` - index of the transaction
- ``height``: ``int`` - height of the block where this transaction was
in
**Example**
.. code:: bash
curl -s 'http://localhost:46657/broadcast_tx_commit?tx="abc"' | jq .
# {
# "error": "",
# "result": {
# "hash": "2B8EC32BA2579B3B8606E42C06DE2F7AFA2556EF",
# "log": "",
# "data": "",
# "code": 0
# },
# "id": "",
# "jsonrpc": "2.0"
# }
curl -s 'http://localhost:46657/tx?hash=0x2B8EC32BA2579B3B8606E42C06DE2F7AFA2556EF' | jq .
# {
# "error": "",
# "result": {
# "proof": {
# "Proof": {
# "aunts": []
# },
# "Data": "YWJjZA==",
# "RootHash": "2B8EC32BA2579B3B8606E42C06DE2F7AFA2556EF",
# "Total": 1,
# "Index": 0
# },
# "tx": "YWJjZA==",
# "tx_result": {
# "log": "",
# "data": "",
# "code": 0
# },
# "index": 0,
# "height": 52
# },
# "id": "",
# "jsonrpc": "2.0"
# }
More Examples
~~~~~~~~~~~~~
See the various bash tests using curl in ``test/``, and examples using
the ``Go`` API in ``rpc/client/``.

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Secure P2P
==========
The Tendermint p2p protocol uses an authenticated encryption scheme
based on the `Station-to-Station
Protocol <https://en.wikipedia.org/wiki/Station-to-Station_protocol>`__.
The implementation uses
`golang's <https://godoc.org/golang.org/x/crypto/nacl/box>`__ `nacl
box <http://nacl.cr.yp.to/box.html>`__ for the actual authenticated
encryption algorithm.
Each peer generates an ED25519 key-pair to use as a persistent
(long-term) id.
When two peers establish a TCP connection, they first each generate an
ephemeral ED25519 key-pair to use for this session, and send each other
their respective ephemeral public keys. This happens in the clear.
They then each compute the shared secret. The shared secret is the
multiplication of the peer's ephemeral private key by the other peer's
ephemeral public key. The result is the same for both peers by the magic
of `elliptic
curves <https://en.wikipedia.org/wiki/Elliptic_curve_cryptography>`__.
The shared secret is used as the symmetric key for the encryption
algorithm.
The two ephemeral public keys are sorted to establish a canonical order.
Then a 24-byte nonce is generated by concatenating the public keys and
hashing them with Ripemd160. Note Ripemd160 produces 20byte hashes, so
the nonce ends with four 0s.
The nonce is used to seed the encryption - it is critical that the same
nonce never be used twice with the same private key. For convenience,
the last bit of the nonce is flipped, giving us two nonces: one for
encrypting our own messages, one for decrypting our peer's. Which ever
peer has the higher public key uses the "bit-flipped" nonce for
encryption.
Now, a challenge is generated by concatenating the ephemeral public keys
and taking the SHA256 hash.
Each peer signs the challenge with their persistent private key, and
sends the other peer an AuthSigMsg, containing their persistent public
key and the signature. On receiving an AuthSigMsg, the peer verifies the
signature.
The peers are now authenticated.
All future communications can now be encrypted using the shared secret
and the generated nonces, where each nonce is incremented by one each
time it is used. The communications maintain Perfect Forward Secrecy, as
the persistent key pair was not used for generating secrets - only for
authenticating.
Caveat
------
This system is still vulnerable to a Man-In-The-Middle attack if the
persistent public key of the remote node is not known in advance. The
only way to mitigate this is with a public key authentication system,
such as the Web-of-Trust or Certificate Authorities. In our case, we can
use the blockchain itself as a certificate authority to ensure that we
are connected to at least one validator.
Additional Reading
------------------
- `Implementation <https://github.com/tendermint/go-p2p/blob/master/secret_connection.go#L49>`__
- `Original STS paper by Whitfield Diffie, Paul C. van Oorschot and
Michael J.
Wiener <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.216.6107&rep=rep1&type=pdf>`__
- `Further work on secret
handshakes <https://dominictarr.github.io/secret-handshake-paper/shs.pdf>`__

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Validators
==========
Validators are responsible for committing new blocks in the blockchain.
These validators participate in the consensus protocol by broadcasting
*votes* which contain cryptographic signatures signed by each
validator's public key.
Some Proof-of-Stake consensus algorithms aim to create a "completely"
decentralized system where all stakeholders (even those who are not
always available online) participate in the committing of blocks.
Tendermint has a different approach to block creation. Validators are
expected to be online, and the set of validators is permissioned/curated
by some external process. Proof-of-stake is not required, but can be
implemented on top of Tendermint consensus. That is, validators may be
required to post collateral on-chain, off-chain, or may not be required
to post any collateral at all.
Validators have a cryptographic key-pair and an associated amount of
"voting power". Voting power need not be the same.
Becoming a Validator
--------------------
There are two ways to become validator.
1. They can be pre-established in the `genesis
state <./genesis.html>`__
2. The `ABCI app responds to the EndBlock
message <https://github.com/tendermint/abci>`__ with changes to the
existing validator set.
Committing a Block
------------------
*+2/3 is short for "more than 2/3"*
A block is committed when +2/3 of the validator set sign `precommit
votes <./block-structure.html#vote>`__ for that block at the same
``round``. The +2/3 set of precommit votes is
called a `*commit* <./block-structure.html#commit>`__. While any
+2/3 set of precommits for the same block at the same height&round can
serve as validation, the canonical commit is included in the next block
(see `LastCommit <./block-structure.html>`__).

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Wire Protocol
=============
The `Tendermint wire protocol <https://github.com/tendermint/go-wire>`__
encodes data in `c-style binary <#binary>`__ and `JSON <#json>`__ form.
Supported types
---------------
- Primitive types
- ``uint8`` (aka ``byte``), ``uint16``, ``uint32``, ``uint64``
- ``int8``, ``int16``, ``int32``, ``int64``
- ``uint``, ``int``: variable length (un)signed integers
- ``string``, ``[]byte``
- ``time``
- Derived types
- structs
- var-length arrays of a particular type
- fixed-length arrays of a particular type
- interfaces: registered union types preceded by a ``type byte``
- pointers
Binary
------
**Fixed-length primitive types** are encoded with 1,2,3, or 4 big-endian
bytes. - ``uint8`` (aka ``byte``), ``uint16``, ``uint32``, ``uint64``:
takes 1,2,3, and 4 bytes respectively - ``int8``, ``int16``, ``int32``,
``int64``: takes 1,2,3, and 4 bytes respectively - ``time``: ``int64``
representation of nanoseconds since epoch
**Variable-length integers** are encoded with a single leading byte
representing the length of the following big-endian bytes. For signed
negative integers, the most significant bit of the leading byte is a 1.
- ``uint``: 1-byte length prefixed variable-size (0 ~ 255 bytes)
unsigned integers
- ``int``: 1-byte length prefixed variable-size (0 ~ 127 bytes) signed
integers
NOTE: While the number 0 (zero) is encoded with a single byte ``x00``,
the number 1 (one) takes two bytes to represent: ``x0101``. This isn't
the most efficient representation, but the rules are easier to remember.
+---------------+----------------+----------------+
| number | binary | binary ``int`` |
| | ``uint`` | |
+===============+================+================+
| 0 | ``x00`` | ``x00`` |
+---------------+----------------+----------------+
| 1 | ``x0101`` | ``x0101`` |
+---------------+----------------+----------------+
| 2 | ``x0102`` | ``x0102`` |
+---------------+----------------+----------------+
| 256 | ``x020100`` | ``x020100`` |
+---------------+----------------+----------------+
| 2^(127\ *8)-1 | ``x800100...`` | overflow |
| \| | | |
| ``x7FFFFF...` | | |
| ` | | |
| \| | | |
| ``x7FFFFF...` | | |
| ` | | |
| \| \| | | |
| 2^(127*\ 8) | | |
+---------------+----------------+----------------+
| 2^(255\*8)-1 |
| \| |
| ``xFFFFFF...` |
| ` |
| \| overflow |
| \| \| -1 \| |
| n/a \| |
| ``x8101`` \| |
| \| -2 \| n/a |
| \| ``x8102`` |
| \| \| -256 \| |
| n/a \| |
| ``x820100`` |
| \| |
+---------------+----------------+----------------+
**Structures** are encoded by encoding the field values in order of
declaration.
.. code:: go
type Foo struct {
MyString string
MyUint32 uint32
}
var foo = Foo{"626172", math.MaxUint32}
/* The binary representation of foo:
0103626172FFFFFFFF
0103: `int` encoded length of string, here 3
626172: 3 bytes of string "bar"
FFFFFFFF: 4 bytes of uint32 MaxUint32
*/
**Variable-length arrays** are encoded with a leading ``int`` denoting
the length of the array followed by the binary representation of the
items. **Fixed-length arrays** are similar but aren't preceded by the
leading ``int``.
.. code:: go
foos := []Foo{foo, foo}
/* The binary representation of foos:
01020103626172FFFFFFFF0103626172FFFFFFFF
0102: `int` encoded length of array, here 2
0103626172FFFFFFFF: the first `foo`
0103626172FFFFFFFF: the second `foo`
*/
foos := [2]Foo{foo, foo} // fixed-length array
/* The binary representation of foos:
0103626172FFFFFFFF0103626172FFFFFFFF
0103626172FFFFFFFF: the first `foo`
0103626172FFFFFFFF: the second `foo`
*/
**Interfaces** can represent one of any number of concrete types. The
concrete types of an interface must first be declared with their
corresponding ``type byte``. An interface is then encoded with the
leading ``type byte``, then the binary encoding of the underlying
concrete type.
NOTE: The byte ``x00`` is reserved for the ``nil`` interface value and
``nil`` pointer values.
.. code:: go
type Animal interface{}
type Dog uint32
type Cat string
RegisterInterface(
struct{ Animal }{}, // Convenience for referencing the 'Animal' interface
ConcreteType{Dog(0), 0x01}, // Register the byte 0x01 to denote a Dog
ConcreteType{Cat(""), 0x02}, // Register the byte 0x02 to denote a Cat
)
var animal Animal = Dog(02)
/* The binary representation of animal:
010102
01: the type byte for a `Dog`
0102: the bytes of Dog(02)
*/
**Pointers** are encoded with a single leading byte ``x00`` for ``nil``
pointers, otherwise encoded with a leading byte ``x01`` followed by the
binary encoding of the value pointed to.
NOTE: It's easy to convert pointer types into interface types, since the
``type byte`` ``x00`` is always ``nil``.
JSON
----
The JSON codec is compatible with the ```binary`` <#binary>`__ codec,
and is fairly intuitive if you're already familiar with golang's JSON
encoding. Some quirks are noted below:
- variable-length and fixed-length bytes are encoded as uppercase
hexadecimal strings
- interface values are encoded as an array of two items:
``[type_byte, concrete_value]``
- times are encoded as rfc2822 strings