Solana
PoH Back Reference
Context
This figure appears in the 'Proof of History' section, in the discussion of how the PoH sequence ensures causal consistency and prevents an attacker from silently forking the timeline. A back reference is a specific form of data insertion where a node inserts a hash of a known prior PoH state into the current hash computation, creating an explicit dependency link between two points in the sequence.
What This Figure Shows
The diagram illustrates a back reference: at position n, the generator inserts a reference to an earlier checkpoint (or the output of another PoH generator) by hashing it into the current state. The resulting entry records that the referenced prior state was observed and incorporated before position n was computed. This creates a provable causal ordering: the referenced event must have existed and been known to the generator before the back-reference entry. If a secondary PoH generator inserts a hash of the primary generator's output into its own sequence, both sequences become cross-referenced — each proves awareness of the other at a specific point in time — enabling horizontal scaling and inter-generator consistency.
Significance
Back references are what allow multiple PoH generators to coordinate without a central authority, and what allow validators to prove that they observed specific network events at specific times. This mechanism is essential for the Turbine block propagation protocol, where validators acknowledge receipt of shreds by inserting references into their local PoH streams, creating an unforgeable audit trail of data propagation.
