diff --git a/ethcore/light/src/client/fetch.rs b/ethcore/light/src/client/fetch.rs
new file mode 100644
index 000000000..0f11ef2f4
--- /dev/null
+++ b/ethcore/light/src/client/fetch.rs
@@ -0,0 +1,69 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Trait for fetching chain data.
+
+use ethcore::header::Header;
+use ethcore::receipt::Receipt;
+use futures::future::IntoFuture;
+
+/// Provides full chain data.
+pub trait ChainDataFetcher: Send + Sync + 'static {
+	/// Error type when data unavailable.
+	type Error: ::std::fmt::Debug;
+
+	/// Future for fetching block body.
+	type Body: IntoFuture<Item=Vec<u8>,Error=Self::Error>;
+	/// Future for fetching block receipts.
+	type Receipts: IntoFuture<Item=Vec<Receipt>,Error=Self::Error>;
+	/// Future for fetching epoch transition
+	type Transition: IntoFuture<Item=Vec<u8>,Error=Self::Error>;
+
+	/// Fetch a block body.
+	fn block_body(&self, header: &Header) -> Self::Body;
+
+	/// Fetch block receipts.
+	fn block_receipts(&self, header: &Header) -> Self::Receipts;
+
+	/// Fetch epoch transition proof at given header.
+	fn epoch_transition(&self, header: &Header) -> Self::Transition;
+}
+
+/// Fetcher implementation which cannot fetch anything.
+pub struct Unavailable;
+
+/// Create a fetcher which has all data unavailable.
+pub fn unavailable() -> Unavailable { Unavailable }
+
+impl ChainDataFetcher for Unavailable {
+	type Error = &'static str;
+
+	type Body = Result<Vec<u8>, &'static str>;
+	type Receipts = Result<Vec<Receipt>, &'static str>;
+	type Transition = Result<Vec<u8>, &'static str>;
+
+	fn block_body(&self, _header: &Header) -> Self::Body {
+		Err("fetching block bodies unavailable")
+	}
+
+	fn block_receipts(&self, _header: &Header) -> Self::Receipts {
+		Err("fetching block receipts unavailable")
+	}
+
+	fn epoch_transition(&self, _header: &Header) -> Self::Body {
+		Err("fetching epoch transition proofs unavailable")
+	}
+}
diff --git a/ethcore/light/src/client/mod.rs b/ethcore/light/src/client/mod.rs
index 3f0e50584..b3d2dff4f 100644
--- a/ethcore/light/src/client/mod.rs
+++ b/ethcore/light/src/client/mod.rs
@@ -20,7 +20,7 @@ use std::sync::{Weak, Arc};
 
 use ethcore::block_status::BlockStatus;
 use ethcore::client::{ClientReport, EnvInfo};
-use ethcore::engines::Engine;
+use ethcore::engines::{Engine, EpochChange, Proof, Unsure};
 use ethcore::error::BlockImportError;
 use ethcore::ids::BlockId;
 use ethcore::header::Header;
@@ -31,9 +31,12 @@ use ethcore::service::ClientIoMessage;
 use ethcore::encoded;
 use io::IoChannel;
 
+use futures::{IntoFuture, Future};
+
 use util::{H256, U256, Mutex, RwLock};
 use util::kvdb::{KeyValueDB, CompactionProfile};
 
+use self::fetch::ChainDataFetcher;
 use self::header_chain::{AncestryIter, HeaderChain};
 
 use cache::Cache;
@@ -43,6 +46,8 @@ pub use self::service::Service;
 mod header_chain;
 mod service;
 
+pub mod fetch;
+
 /// Configuration for the light client.
 #[derive(Debug, Clone)]
 pub struct Config {
@@ -154,7 +159,7 @@ impl<T: LightChainClient> AsLightClient for T {
 }
 
 /// Light client implementation.
-pub struct Client {
+pub struct Client<T> {
 	queue: HeaderQueue,
 	engine: Arc<Engine>,
 	chain: HeaderChain,
@@ -162,12 +167,21 @@ pub struct Client {
 	import_lock: Mutex<()>,
 	db: Arc<KeyValueDB>,
 	listeners: RwLock<Vec<Weak<LightChainNotify>>>,
+	fetcher: T,
 	verify_full: bool,
 }
 
-impl Client {
+impl<T: ChainDataFetcher> Client<T> {
 	/// Create a new `Client`.
-	pub fn new(config: Config, db: Arc<KeyValueDB>, chain_col: Option<u32>, spec: &Spec, io_channel: IoChannel<ClientIoMessage>, cache: Arc<Mutex<Cache>>) -> Result<Self, String> {
+	pub fn new(
+		config: Config,
+		db: Arc<KeyValueDB>,
+		chain_col: Option<u32>,
+		spec: &Spec,
+		fetcher: T,
+		io_channel: IoChannel<ClientIoMessage>,
+		cache: Arc<Mutex<Cache>>
+	) -> Result<Self, String> {
 		let gh = ::rlp::encode(&spec.genesis_header());
 
 		Ok(Client {
@@ -178,6 +192,7 @@ impl Client {
 			import_lock: Mutex::new(()),
 			db: db,
 			listeners: RwLock::new(vec![]),
+			fetcher: fetcher,
 			verify_full: config.verify_full,
 		})
 	}
@@ -189,10 +204,24 @@ impl Client {
 
 	/// Create a new `Client` backed purely in-memory.
 	/// This will ignore all database options in the configuration.
-	pub fn in_memory(config: Config, spec: &Spec, io_channel: IoChannel<ClientIoMessage>, cache: Arc<Mutex<Cache>>) -> Self {
+	pub fn in_memory(
+		config: Config,
+		spec: &Spec,
+		fetcher: T,
+		io_channel: IoChannel<ClientIoMessage>,
+		cache: Arc<Mutex<Cache>>
+	) -> Self {
 		let db = ::util::kvdb::in_memory(0);
 
-		Client::new(config, Arc::new(db), None, spec, io_channel, cache).expect("New DB creation infallible; qed")
+		Client::new(
+			config,
+			Arc::new(db),
+			None,
+			spec,
+			fetcher,
+			io_channel,
+			cache
+		).expect("New DB creation infallible; qed")
 	}
 
 	/// Import a header to the queue for additional verification.
@@ -291,9 +320,14 @@ impl Client {
 				continue
 			}
 
-			// TODO: `epoch_end_signal`, `is_epoch_end`.
-			// proofs we get from the network would be _complete_, whereas we need
-			// _incomplete_ signals
+			let _write_proof_result = match self.check_epoch_signal(&verified_header) {
+				Ok(Some(proof)) => self.write_pending_proof(&verified_header, proof),
+				Ok(None) => Ok(()),
+				Err(e) =>
+					panic!("Unable to fetch epoch transition proof: {:?}", e),
+			};
+
+			// TODO: check epoch end.
 
 			let mut tx = self.db.transaction();
 			let pending = match self.chain.insert(&mut tx, verified_header) {
@@ -419,9 +453,71 @@ impl Client {
 
 		true
 	}
+
+	fn check_epoch_signal(&self, verified_header: &Header) -> Result<Option<Proof>, T::Error> {
+		let (mut block, mut receipts) = (None, None);
+
+		// First, check without providing auxiliary data.
+		match self.engine.signals_epoch_end(verified_header, None, None) {
+			EpochChange::No => return Ok(None),
+			EpochChange::Yes(proof) => return Ok(Some(proof)),
+			EpochChange::Unsure(unsure) => {
+				let (b, r) = match unsure {
+					Unsure::NeedsBody =>
+						(Some(self.fetcher.block_body(verified_header)), None),
+					Unsure::NeedsReceipts =>
+						(None, Some(self.fetcher.block_receipts(verified_header))),
+					Unsure::NeedsBoth => (
+						Some(self.fetcher.block_body(verified_header)),
+						Some(self.fetcher.block_receipts(verified_header)),
+					),
+				};
+
+				if let Some(b) = b {
+					block = Some(b.into_future().wait()?);
+				}
+
+				if let Some(r) = r {
+					receipts = Some(r.into_future().wait()?);
+				}
+			}
+		}
+
+		let block = block.as_ref().map(|x| &x[..]);
+		let receipts = receipts.as_ref().map(|x| &x[..]);
+
+		// Check again now that required data has been fetched.
+		match self.engine.signals_epoch_end(verified_header, block, receipts) {
+			EpochChange::No => return Ok(None),
+			EpochChange::Yes(proof) => return Ok(Some(proof)),
+			EpochChange::Unsure(_) =>
+				panic!("Detected faulty engine implementation: requests additional \
+					data to check epoch end signal when everything necessary provided"),
+		}
+	}
+
+	// attempts to fetch the epoch proof from the network until successful.
+	fn write_pending_proof(&self, header: &Header, proof: Proof) -> Result<(), T::Error> {
+		let _proof = match proof {
+			Proof::Known(known) => known,
+			Proof::WithState(state_dependent) => {
+				loop {
+					let proof = self.fetcher.epoch_transition(header).into_future().wait()?;
+					match state_dependent.check_proof(&*self.engine, &proof) {
+						Ok(()) => break proof,
+						Err(e) =>
+							debug!(target: "client", "Fetched bad epoch transition proof from network: {}", e),
+					}
+				}
+			}
+		};
+
+		// TODO: actually write it
+		unimplemented!()
+	}
 }
 
-impl LightChainClient for Client {
+impl<T: ChainDataFetcher> LightChainClient for Client<T> {
 	fn chain_info(&self) -> BlockChainInfo { Client::chain_info(self) }
 
 	fn queue_header(&self, header: Header) -> Result<H256, BlockImportError> {
diff --git a/ethcore/light/src/client/service.rs b/ethcore/light/src/client/service.rs
index 83949a2f1..59a5d28c6 100644
--- a/ethcore/light/src/client/service.rs
+++ b/ethcore/light/src/client/service.rs
@@ -30,7 +30,7 @@ use util::kvdb::{Database, DatabaseConfig};
 use cache::Cache;
 use util::Mutex;
 
-use super::{Client, Config as ClientConfig};
+use super::{ChainDataFetcher, Client, Config as ClientConfig};
 
 /// Errors on service initialization.
 #[derive(Debug)]
@@ -51,14 +51,14 @@ impl fmt::Display for Error {
 }
 
 /// Light client service.
-pub struct Service {
-	client: Arc<Client>,
+pub struct Service<T> {
+	client: Arc<Client<T>>,
 	io_service: IoService<ClientIoMessage>,
 }
 
-impl Service {
+impl<T: ChainDataFetcher> Service<T> {
 	/// Start the service: initialize I/O workers and client itself.
-	pub fn start(config: ClientConfig, spec: &Spec, path: &Path, cache: Arc<Mutex<Cache>>) -> Result<Self, Error> {
+	pub fn start(config: ClientConfig, spec: &Spec, fetcher: T, path: &Path, cache: Arc<Mutex<Cache>>) -> Result<Self, Error> {
 
 		// initialize database.
 		let mut db_config = DatabaseConfig::with_columns(db::NUM_COLUMNS);
@@ -81,6 +81,7 @@ impl Service {
 			db,
 			db::COL_LIGHT_CHAIN,
 			spec,
+			fetcher,
 			io_service.channel(),
 			cache,
 		).map_err(Error::Database)?);
@@ -97,14 +98,14 @@ impl Service {
 	}
 
 	/// Get a handle to the client.
-	pub fn client(&self) -> &Arc<Client> {
+	pub fn client(&self) -> &Arc<Client<T>> {
 		&self.client
 	}
 }
 
-struct ImportBlocks(Arc<Client>);
+struct ImportBlocks<T>(Arc<Client<T>>);
 
-impl IoHandler<ClientIoMessage> for ImportBlocks {
+impl<T: ChainDataFetcher> IoHandler<ClientIoMessage> for ImportBlocks<T> {
 	fn message(&self, _io: &IoContext<ClientIoMessage>, message: &ClientIoMessage) {
 		if let ClientIoMessage::BlockVerified = *message {
 			self.0.import_verified();
@@ -117,9 +118,10 @@ mod tests {
 	use super::Service;
 	use devtools::RandomTempPath;
 	use ethcore::spec::Spec;
-	
+
 	use std::sync::Arc;
 	use cache::Cache;
+	use client::fetch;
 	use time::Duration;
 	use util::Mutex;
 
@@ -129,6 +131,6 @@ mod tests {
 		let temp_path = RandomTempPath::new();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
 
-		Service::start(Default::default(), &spec, temp_path.as_path(), cache).unwrap();
+		Service::start(Default::default(), &spec, fetch::unavailable(), temp_path.as_path(), cache).unwrap();
 	}
 }
diff --git a/ethcore/src/engines/authority_round/mod.rs b/ethcore/src/engines/authority_round/mod.rs
index 3210368db..0dc33d342 100644
--- a/ethcore/src/engines/authority_round/mod.rs
+++ b/ethcore/src/engines/authority_round/mod.rs
@@ -666,6 +666,8 @@ impl Engine for AuthorityRound {
 			(&active_set as &_, epoch_manager.epoch_transition_number)
 		};
 
+		// always report with "self.validators" so that the report actually gets
+		// to the contract.
 		let report = |report| match report {
 			Report::Benign(address, block_number) =>
 				self.validators.report_benign(&address, set_number, block_number),
diff --git a/ethcore/src/engines/mod.rs b/ethcore/src/engines/mod.rs
index e28ae0bc1..3997b66c3 100644
--- a/ethcore/src/engines/mod.rs
+++ b/ethcore/src/engines/mod.rs
@@ -132,7 +132,7 @@ pub trait StateDependentProof {
 
 /// Proof generated on epoch change.
 pub enum Proof {
-	/// Known proof (exctracted from signal)
+	/// Known proof (extracted from signal)
 	Known(Vec<u8>),
 	/// State dependent proof.
 	WithState(Box<StateDependentProof>),