# Tower BFT

This design describes Solana's *Tower BFT* algorithm. It addresses the
following problems:

* Some forks may not end up accepted by the super-majority of the cluster, and
voters need to recover from voting on such forks.

* Many forks may be votable by different voters, and each voter may see a
different set of votable forks.  The selected forks should eventually converge
for the cluster.

* Reward based votes have an associated risk.  Voters should have the ability to
configure how much risk they take on.

* The [cost of rollback](#cost-of-rollback) needs to be computable.  It is
important to clients that rely on some measurable form of Consistency.  The
costs to break consistency need to be computable, and increase super-linearly
for older votes.

* ASIC speeds are different between nodes, and attackers could employ Proof of
History ASICS that are much faster than the rest of the cluster.  Consensus
needs to be resistant to attacks that exploit the variability in Proof of
History ASIC speed.

For brevity this design assumes that a single voter with a stake is deployed as
an individual validator in the cluster.

## Time

The Solana cluster generates a source of time via a Verifiable Delay Function we
are calling [Proof of History](book/src/synchronization.md).

Proof of History is used to create a deterministic round robin schedule for all
the active leaders.  At any given time only 1 leader, which can be computed from
the ledger itself, can propose a fork.  For more details, see [fork
generation](fork-generation.md) and [leader rotation](leader-rotation.md).

## Lockouts

The purpose of the lockout is to force a validator to commit opportunity cost to
a specific fork.  Lockouts are measured in slots, and therefor represent a
real-time forced delay that a validator needs to wait before breaking the
commitment to a fork.

Validators that violate the lockouts and vote for a diverging fork within the
lockout should be punished. The proposed punishment is to slash the validator
stake if a concurrent vote within a lockout for a non-descendant fork can be
proven to the cluster.

## Algorithm

The basic idea to this approach is to stack consensus votes and double lockouts.
Each vote in the stack is a confirmation of a fork.  Each confirmed fork is an
ancestor of the fork above it.  Each vote has a `lockout` in units of slots
before the validator can submit a vote that does not contain the confirmed fork
as an ancestor.

When a vote is added to the stack, the lockouts of all the previous votes in the
stack are doubled (more on this in [Rollback](#Rollback)).  With each new vote,
a validator commits the previous votes to an ever-increasing lockout.  At 32
votes we can consider the vote to be at `max lockout` any votes with a lockout
equal to or above `1<<32` are dequeued (FIFO).  Dequeuing a vote is the trigger
for a reward.  If a vote expires before it is dequeued, it and all the votes
above it are popped (LIFO) from the vote stack.  The validator needs to start
rebuilding the stack from that point.

### Rollback

Before a vote is pushed to the stack, all the votes leading up to vote with a
lower lock time than the new vote are popped.  After rollback lockouts are not
doubled until the validator catches up to the rollback height of votes.

For example, a vote stack with the following state:

| vote | vote time | lockout | lock expiration time |
|-----:|----------:|--------:|---------------------:|
|    4 |         4 |      2  |                    6 |
|    3 |         3 |      4  |                    7 |
|    2 |         2 |      8  |                   10 |
|    1 |         1 |      16 |                   17 |

*Vote 5* is at time 9, and the resulting state is

| vote | vote time | lockout | lock expiration time |
|-----:|----------:|--------:|---------------------:|
|    5 |         9 |      2  |                   11 |
|    2 |         2 |      8  |                   10 |
|    1 |         1 |      16 |                   17 |

*Vote 6* is at time 10

| vote | vote time | lockout | lock expiration time |
|-----:|----------:|--------:|---------------------:|
|    6 |        10 |       2 |                   12 |
|    5 |         9 |       4 |                   13 |
|    2 |         2 |       8 |                   10 |
|    1 |         1 |      16 |                   17 |

At time 10 the new votes caught up to the previous votes.  But *vote 2* expires
at 10, so the when *vote 7* at time 11 is applied the votes including and above
*vote 2* will be popped.

| vote | vote time | lockout | lock expiration time |
|-----:|----------:|--------:|---------------------:|
|    7 |        11 |       2 |                   13 |
|    1 |         1 |      16 |                   17 |

The lockout for vote 1 will not increase from 16 until the stack contains 5
votes.

### Slashing and Rewards

Validators should be rewarded for selecting the fork that the rest of the
cluster selected as often as possible.  This is well-aligned with generating a
reward when the vote stack is full and the oldest vote needs to be dequeued.
Thus a reward should be generated for each successful dequeue.

### Cost of Rollback

Cost of rollback of *fork A* is defined as the cost in terms of lockout time to
the validator to confirm any other fork that does not include *fork A* as an
ancestor.

The **Economic Finality** of *fork A* can be calculated as the loss of all the
rewards from rollback of *fork A* and its descendants, plus the opportunity cost
of reward due to the exponentially growing lockout of the votes that have
confirmed *fork A*.

### Thresholds

Each validator can independently set a threshold of cluster commitment to a fork
before that validator commits to a fork.  For example, at vote stack index 7,
the lockout is 256 time units.  A validator may withhold votes and let votes 0-7
expire unless the vote at index 7 has at greater than 50% commitment in the
cluster.  This allows each validator to independently control how much risk to
commit to a fork.  Committing to forks at a higher frequency would allow the
validator to earn more rewards.

### Algorithm parameters

The following parameters need to be tuned:

* Number of votes in the stack before dequeue occurs (32).

* Rate of growth for lockouts in the stack (2x).

* Starting default lockout (2).

* Threshold depth for minimum cluster commitment before committing to the fork
(8).

* Minimum cluster commitment size at threshold depth (50%+).

### Free Choice

A "Free Choice" is an unenforcible validator action.  There is no way for the
protocol to encode and enforce these actions since each validator can modify the
code and adjust the algorithm.  A validator that maximizes self-reward over all
possible futures should behave in such a way that the system is stable, and the
local greedy choice should result in a greedy choice over all possible futures.
A set of validator that are engaging in choices to disrupt the protocol should
be bound by their stake weight to the denial of service.  Two options exits for
validator:

* a validator can outrun previous validator in virtual generation and submit a
  concurrent fork

* a validator can withhold a vote to observe multiple forks before voting

In both cases, the validator in the cluster have several forks to pick from
concurrently, even though each fork represents a different height.  In both
cases it is impossible for the protocol to detect if the validator behavior is
intentional or not.

### Greedy Choice for Concurrent Forks

When evaluating multiple forks, each validator should use the following rules:

1. Forks must satisfy the *Threshold* rule.

2. Pick the fork that maximizes the total cluster lockout time for all the
ancestor forks.

3. Pick the fork that has the greatest amount of cluster transaction fees.

4. Pick the latest fork in terms of PoH.

Cluster transaction fees are fees that are deposited to the mining pool as
described in the [Staking Rewards](book/src/staking-rewards.md) section.

## PoH ASIC Resistance

Votes and lockouts grow exponentially while ASIC speed up is linear.  There are
two possible attack vectors involving a faster ASIC.

### ASIC censorship

An attacker generates a concurrent fork that outruns previous leaders in an
effort to censor them. A fork proposed by this attacker will be available
concurrently with the next available leader.  For nodes to pick this fork it
must satisfy the *Greedy Choice* rule.

1. Fork must have equal number of votes for the ancestor fork.

2. Fork cannot be so far a head as to cause expired votes.

3. Fork must have a greater amount of cluster transaction fees.

This attack is then limited to censoring the previous leaders fees, and
individual transactions.  But it cannot halt the cluster, or reduce the
validator set compared to the concurrent fork.  Fee censorship is limited to
access fees going to the leaders but not the validators.

### ASIC Rollback

An attacker generates a concurrent fork from an older block to try to rollback
the cluster.  In this attack the concurrent fork is competing with forks that
have already been voted on.  This attack is limited by the exponential growth of
the lockouts.

* 1 vote has a lockout of 2 slots.  Concurrent fork must be at least 2 slots
ahead, and be produced in 1 slot. Therefore requires an ASIC 2x faster.

* 2 votes have a lockout of 4 slots.  Concurrent fork must be at least 4 slots
ahead and produced in 2 slots. Therefore requires an ASIC 2x faster.

* 3 votes have a lockout of 8 slots.  Concurrent fork must be at least 8 slots
ahead and produced in 3 slots. Therefore requires an ASIC 2.6x faster.

* 10 votes have a lockout of 1024 slots.  1024/10, or 102.4x faster ASIC.

* 20 votes have a lockout of 2^20 slots.  2^20/20, or 52,428.8x faster ASIC.
