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+# ADR 005: Consensus Params
+
+## Context
+
+Consensus critical parameters controlling blockchain capacity have until now been hard coded, loaded from a local config, or neglected.
+Since they may be need to be different in different networks, and potentially to evolve over time within
+networks, we seek to initialize them in a genesis file, and expose them through the ABCI.
+
+While we have some specific parameters now, like maximum block and transaction size, we expect to have more in the future,
+such as a period over which evidence is valid, or the frequency of checkpoints. 
+
+## Decision
+
+### ConsensusParams
+
+A new `ConsensusParams` is optionally included in the `genesis.json` file,
+and loaded into the `State`. Any items not included are set to their default value.
+A value of 0 is undefined (see ABCI, below). A value of -1 is used to indicate the parameter does not apply.
+No consensus critical parameters should ever be found in the `config.toml`.
+
+```
+type ConsensusParams struct {
+    BlockSizeParams
+    TxSizeParams
+    BlockGossipParams
+}
+
+type BlockSizeParams struct {
+    BlockSizeBytes int
+    BlockSizeTxs int
+    BlockSizeGas int
+}
+
+type TxSizeParams struct {
+    TxSizeBytes int
+    TxSizeGas int
+}
+
+type BlockGossipParams struct {
+    BlockPartSizeBytes int
+}
+```
+
+The `ConsensusParams` can evolve over time by adding new structs that cover different aspects of the consensus rules.
+
+### ABCI
+
+#### InitChain
+
+InitChain currently takes the initial validator set. It should be extended to also take the ConsensusParams.
+In fact, it might as well just consume the whole Genesis.
+
+#### EndBlock
+
+The EndBlock response includes a `ConsensusParams`, which includes BlockSizeParams and TxSizeParams, but not BlockGossipParams.
+Other param struct can be added to `ConsensusParams` in the future.
+The `0` value is used to denote no change. 
+Any other value will update that parameter in the `State.ConsensusParams`, to be applied for the next block.
+Tendermint should have hard-coded upper limits as sanity checks.
+
+## Status
+
+Proposed.
+
+## Consequences
+
+### Positive
+
+- Alternative capacity limits and consensus parameters can be specified without re-compiling the software.
+- They can also change over time under the control of the application
+
+### Negative
+
+- More exposed parameters is more complexity
+- Different rules at different heights in the blockchain complicates fast sync
+
+### Neutral