# Leader to Leader Transition

This design describes how leaders transition production of the PoH ledger
between each other as each leader generates its own slot.

## Challenges

Current leader and the next leader are both racing to generate the final tick
for the current slot.  The next leader may arrive at that slot while still
processing the current leader's entries.

The ideal scenario would be that the next leader generated its own slot right
after it was able to vote for the current leader.  It is very likely that the
next leader will arrive at their PoH slot height before the current leader
finishes broadcasting the entire block.

The next leader has to make the decision of attaching its own block to the last
completed block, or wait to finalize the pending block.  It is possible that the
next leader will produce a block that proposes that the current leader failed,
even though the rest of the network observes that block succeeding.

The current leader has incentives to start its slot as early as possible to
capture economic rewards.  Those incentives need to be balanced by the leader's
need to attach its block to a block that has the most commitment from the rest
of the network.

## Leader timeout

While a leader is actively receiving entries for the previous slot, the leader
can delay broadcasting the start of its block in real time.  The delay is
locally configurable by each leader, and can be dynamically based on the
previous leader's behavior.  If the previous leader's block is confirmed by the
leader's TVU before the timeout, the PoH is reset to the start of the slot and
this leader produces its block immediately.

The downsides:

* Leader delays its own slot, potentially allowing the next leader more time to
catch up.

The upsides compared to guards:

* All the space in a block is used for entries.

* The timeout is not fixed.

* The timeout is local to the leader, and therefore can be clever.  The leader's
heuristic can take into account avalanche performance.

* This design doesn't require a ledger hard fork to update.

* The previous leader can redundantly transmit the last entry in the block to
the next leader, and the next leader can speculatively decide to trust it to
generate its block without verification of the previous block.

* The leader can speculatively generate the last tick from the last received
entry.

* The leader can speculatively process transactions and guess which ones are not
going to be encoded by the previous leader.  This is also a censorship attack
vector.  The current leader may withhold transactions that it receives from the
clients so it can encode them into its own slot. Once processed, entries can be
replayed into PoH quickly.

## Alternative design options

### Guard tick at the end of the slot

A leader does not produce entries in its block after the *penultimate tick*,
which is the last tick before the first tick of the next slot.  The network
votes on the *last tick*, so the time difference between the *penultimate tick*
and the *last tick* is the forced delay for the entire network, as well as the
next leader before a new slot can be generated.  The network can produce the
*last tick* from the *penultimate tick*.

If the next leader receives the *penultimate tick* before it produces its own
*first tick*, it will reset its PoH and produce the *first tick* from the
previous leader's *penultimate tick*.  The rest of the network will also reset
its PoH to produce the *last tick* as the id to vote on.

The downsides:

* Every vote, and therefore confirmation, is delayed by a fixed timeout. 1 tick,
or around 100ms.

* Average case confirmation time for a transaction would be at least 50ms worse.

* It is part of the ledger definition, so to change this behavior would require
a hard fork.

* Not all the available space is used for entries.

The upsides compared to leader timeout:

* The next leader has received all the previous entries, so it can start
processing transactions without recording them into PoH.

* The previous leader can redundantly transmit the last entry containing the
*penultimate tick* to the next leader.  The next leader can speculatively
generate the *last tick* as soon as it receives the *penultimate tick*, even
before verifying it.
