Optimistic transaction propagation signal doc

docs/src/proposals/optimistic-transaction-propagation-signal.md

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+---
+title: Optimistic Transaction Propagation Signal
+---
+
+## Current Retransmit behavior
+
+The retransmission tree currently considers:
+1. epoch staked nodes
+2. tvu peers (filtered by contact info and shred version)
+3. current validator
+
+concatenating (1), (2), and (3)
+deduplicating this list of entries by pubkey favoring entries with contact info
+filtering this list by entries with contact info
+
+This list is then is randomly shuffled by stake weight.
+
+Shreds are then retransmitted to up to FANOUT neighbors and up to FANOUT
+children.
+
+## Deterministic retransmission tree
+
+`weighted_shuffle` will use a deterministic seed when
+`enable_deterministic_seed` has been enabled based on the triple (shred slot,
+shred index, leader pubkey):
+
+```
+if enable_deterministic_seed(self.slot(), root_bank) {
+    hashv(&[
+        &self.slot().to_le_bytes(),
+        &self.index().to_le_bytes(),
+        &leader_pubkey.to_bytes(),
+    ])
+```
+
+First, only epoch staked nodes will be considered regardless of presence of
+contact info (and possibly including the validator node itself).
+
+A deterministic ordering of the epoch staked nodes will be created based on the
+derministic shred seed using weighted_shuffle.
+
+Let `neighbor_set` be selected from up to FANOUT neighbors of the current node.
+Let `child_set` be selected from up to FANOUT children of the current node.
+
+Filter `neighbor_set` by contact info.
+Filter `child_set` by contact info.
+
+Let `epoch_set` be the union of `neighbor_set` and `child_set`.
+
+Let `remaining_set` be all other nodes with contact info not contained in
+`epoch_set`.
+
+If `epoch_set.len < 2*FANOUT` then we may randomly select up to
+`2*FANOUT - epoch_set.len` nodes to to retransmit to from `remaining_set`.
+
+## Receiving retransmitted shred
+
+If the current validator node is not in the set of epoch staked nodes for the
+shred epoch then no early retransmission information can be obtained.
+
+Compute the deterministic shred seed.
+
+Run the deterministic epoch_stakes shuffle.
+
+Find position of self in the neighbor or child sets.
+
+Calculate the sum of the stakes of all nodes in the current and prior
+distribution levels.
+
+### Stake summation considerations:
+
+- Stake sum could include stakes of nodes which had been skipped in prior
+distribution levels because of lack of contact info.
+- Current node was part of original epoch staked shuffle from retransmitter
+but was filtered out because of missing contact info. Current node subsequently
+receives retransmisison of shred and assumes that the retransmit was a result
+of the deterministic tree calculation and not from subsequent random selection.
+This should be benign because the current node will underestimate prior stake
+weight in the retransmission tree.
+
+### General considerations:
+
+attack by leader (level 0):
+- transmits shred for distribution through the tree as normal
+- additionally transmits shred (or fake shred) directly to node(s) at level >=2
+leading the node(s) to believe a greater percentage of the tree retransmission
+tree had been processed
+
+attack by node at level n:
+- retransmits shred to node(s) at level >=n+2 leading the node(s) to believe a
+greater percentage of the tree retransmission tree had been processed
+
+### Questions
+
+- Should receiving nodes attempt to verify that the origin of the shred was
+retransmitted from the expected node? If so, consideration of spoofing?
+- How is this information consumed?
+
+## Notes
+
+Practically, signals should fall into the following buckets:
+1. current leader (can signal layer 1 when broadcast is sent)
+2. layer 1
+1.1. can signal layer 1 when shred is received
+1.2. can signal layer 1 + subset of layer 2 when retransmit is sent
+3. layer 2
+3.1. can signal layer 2 when shred is received
+3.2. can signal layer 2 + subset of layer 3 when retrnasmit is sent
+4. current node not a member of epoch staked nodes, no signal can be sent