diff --git a/core/client/src/client.rs b/core/client/src/client.rs
index d0a53802fad7a..8ae21ca6ab4e0 100644
--- a/core/client/src/client.rs
+++ b/core/client/src/client.rs
@@ -47,7 +47,7 @@ use state_machine::{
 };
 use executor::{RuntimeVersion, RuntimeInfo};
 use consensus::{
-	Error as ConsensusError, BlockImportParams,
+	Error as ConsensusError, BlockStatus, BlockImportParams,
 	ImportResult, BlockOrigin, ForkChoiceStrategy,
 	well_known_cache_keys::Id as CacheKeyId,
 	SelectChain, self,
@@ -171,21 +171,6 @@ pub struct ClientInfo<Block: BlockT> {
 	pub used_state_cache_size: Option<usize>,
 }
 
-/// Block status.
-#[derive(Debug, PartialEq, Eq)]
-pub enum BlockStatus {
-	/// Added to the import queue.
-	Queued,
-	/// Already in the blockchain and the state is available.
-	InChainWithState,
-	/// In the blockchain, but the state is not available.
-	InChainPruned,
-	/// Block or parent is known to be bad.
-	KnownBad,
-	/// Not in the queue or the blockchain.
-	Unknown,
-}
-
 /// Summary of an imported block
 #[derive(Clone, Debug)]
 pub struct BlockImportNotification<Block: BlockT> {
@@ -1187,10 +1172,7 @@ impl<B, E, Block, RA> Client<B, E, Block, RA> where
 		Ok(())
 	}
 
-	fn notify_imported(
-		&self,
-		notify_import: ImportSummary<Block>,
-	) -> error::Result<()> {
+	fn notify_imported(&self, notify_import: ImportSummary<Block>) -> error::Result<()> {
 		if let Some(storage_changes) = notify_import.storage_changes {
 			// TODO [ToDr] How to handle re-orgs? Should we re-emit all storage changes?
 			self.storage_notifications.lock()
@@ -1941,6 +1923,16 @@ pub mod utils {
 	}
 }
 
+impl<BE, E, B, RA> consensus::block_validation::Chain<B> for Client<BE, E, B, RA>
+	where BE: backend::Backend<B, Blake2Hasher>,
+		  E: CallExecutor<B, Blake2Hasher>,
+		  B: BlockT<Hash = H256>
+{
+	fn block_status(&self, id: &BlockId<B>) -> Result<BlockStatus, Box<dyn std::error::Error + Send>> {
+		Client::block_status(self, id).map_err(|e| Box::new(e) as Box<_>)
+	}
+}
+
 #[cfg(test)]
 pub(crate) mod tests {
 	use std::collections::HashMap;
diff --git a/core/client/src/lib.rs b/core/client/src/lib.rs
index 3919b3970f889..5a6d65f7d0824 100644
--- a/core/client/src/lib.rs
+++ b/core/client/src/lib.rs
@@ -113,7 +113,7 @@ pub use crate::call_executor::{CallExecutor, LocalCallExecutor};
 pub use crate::client::{
 	new_with_backend,
 	new_in_mem,
-	BlockBody, BlockStatus, ImportNotifications, FinalityNotifications, BlockchainEvents,
+	BlockBody, ImportNotifications, FinalityNotifications, BlockchainEvents,
 	BlockImportNotification, Client, ClientInfo, ExecutionStrategies, FinalityNotification,
 	LongestChain, BlockOf, ProvideUncles,
 	utils, apply_aux,
diff --git a/core/consensus/common/src/block_validation.rs b/core/consensus/common/src/block_validation.rs
new file mode 100644
index 0000000000000..be181b05c6f4d
--- /dev/null
+++ b/core/consensus/common/src/block_validation.rs
@@ -0,0 +1,66 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+//
+// Substrate 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.
+//
+// Substrate 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Block announcement validation.
+
+use crate::BlockStatus;
+use sr_primitives::{generic::BlockId, traits::Block};
+use std::{error::Error, sync::Arc};
+
+/// A type which provides access to chain information.
+pub trait Chain<B: Block> {
+	/// Retrieve the status of the block denoted by the given [`BlockId`].
+	fn block_status(&self, id: &BlockId<B>) -> Result<BlockStatus, Box<dyn Error + Send>>;
+}
+
+impl<T: Chain<B>, B: Block> Chain<B> for Arc<T> {
+	fn block_status(&self, id: &BlockId<B>) -> Result<BlockStatus, Box<dyn Error + Send>> {
+		(&**self).block_status(id)
+	}
+}
+
+/// Result of `BlockAnnounceValidator::validate`.
+#[derive(Debug, PartialEq, Eq)]
+pub enum Validation {
+	/// Valid block announcement.
+	Success,
+	/// Invalid block announcement.
+	Failure,
+}
+
+/// Type which checks incoming block announcements.
+pub trait BlockAnnounceValidator<B: Block> {
+	/// Validate the announced header and its associated data.
+	fn validate(&mut self, header: &B::Header, data: &[u8]) -> Result<Validation, Box<dyn Error + Send>>;
+}
+
+/// Default implementation of `BlockAnnounceValidator`.
+#[derive(Debug)]
+pub struct DefaultBlockAnnounceValidator<C> {
+	chain: C
+}
+
+impl<C> DefaultBlockAnnounceValidator<C> {
+	pub fn new(chain: C) -> Self {
+		Self { chain }
+	}
+}
+
+impl<B: Block, C: Chain<B>> BlockAnnounceValidator<B> for DefaultBlockAnnounceValidator<C> {
+	fn validate(&mut self, _h: &B::Header, _d: &[u8]) -> Result<Validation, Box<dyn Error + Send>> {
+		Ok(Validation::Success)
+	}
+}
diff --git a/core/consensus/common/src/lib.rs b/core/consensus/common/src/lib.rs
index 3fd0a0c694c7e..4c5797219d2a5 100644
--- a/core/consensus/common/src/lib.rs
+++ b/core/consensus/common/src/lib.rs
@@ -35,6 +35,7 @@ use sr_primitives::traits::{Block as BlockT, DigestFor};
 use futures::prelude::*;
 pub use inherents::InherentData;
 
+pub mod block_validation;
 pub mod offline_tracker;
 pub mod error;
 pub mod block_import;
@@ -52,6 +53,21 @@ pub use block_import::{
 };
 pub use select_chain::SelectChain;
 
+/// Block status.
+#[derive(Debug, PartialEq, Eq)]
+pub enum BlockStatus {
+	/// Added to the import queue.
+	Queued,
+	/// Already in the blockchain and the state is available.
+	InChainWithState,
+	/// In the blockchain, but the state is not available.
+	InChainPruned,
+	/// Block or parent is known to be bad.
+	KnownBad,
+	/// Not in the queue or the blockchain.
+	Unknown,
+}
+
 /// Environment producer for a Consensus instance. Creates proposer instance and communication streams.
 pub trait Environment<B: BlockT> {
 	/// The proposer type this creates.
diff --git a/core/finality-grandpa/src/communication/mod.rs b/core/finality-grandpa/src/communication/mod.rs
index 7185e93208730..652c33c0262a5 100644
--- a/core/finality-grandpa/src/communication/mod.rs
+++ b/core/finality-grandpa/src/communication/mod.rs
@@ -127,7 +127,7 @@ pub trait Network<Block: BlockT>: Clone + Send + 'static {
 	fn report(&self, who: network::PeerId, cost_benefit: i32);
 
 	/// Inform peers that a block with given hash should be downloaded.
-	fn announce(&self, block: Block::Hash);
+	fn announce(&self, block: Block::Hash, associated_data: Vec<u8>);
 }
 
 /// Create a unique topic for a round and set-id combo.
@@ -197,8 +197,8 @@ impl<B, S, H> Network<B> for Arc<NetworkService<B, S, H>> where
 		self.report_peer(who, cost_benefit)
 	}
 
-	fn announce(&self, block: B::Hash) {
-		self.announce_block(block)
+	fn announce(&self, block: B::Hash, associated_data: Vec<u8>) {
+		self.announce_block(block, associated_data)
 	}
 }
 
@@ -727,7 +727,7 @@ impl<Block: BlockT, N: Network<Block>> Sink for OutgoingMessages<Block, N>
 			);
 
 			// send the target block hash to the background block announcer
-			self.announce_sender.send(target_hash);
+			self.announce_sender.send(target_hash, Vec::new());
 
 			// propagate the message to peers
 			let topic = round_topic::<Block>(self.round, self.set_id);
diff --git a/core/finality-grandpa/src/communication/periodic.rs b/core/finality-grandpa/src/communication/periodic.rs
index 333178dec2343..81c18891d03b5 100644
--- a/core/finality-grandpa/src/communication/periodic.rs
+++ b/core/finality-grandpa/src/communication/periodic.rs
@@ -118,7 +118,7 @@ pub(super) fn neighbor_packet_worker<B, N>(net: N) -> (
 /// A background worker for performing block announcements.
 struct BlockAnnouncer<B: BlockT, N> {
 	net: N,
-	block_rx: mpsc::UnboundedReceiver<B::Hash>,
+	block_rx: mpsc::UnboundedReceiver<(B::Hash, Vec<u8>)>,
 	latest_voted_blocks: VecDeque<B::Hash>,
 	reannounce_after: Duration,
 	delay: Delay,
@@ -199,8 +199,8 @@ impl<B: BlockT, N: Network<B>> Future for BlockAnnouncer<B, N> {
 			match self.block_rx.poll().expect("unbounded receivers do not error; qed") {
 				Async::Ready(None) => return Ok(Async::Ready(())),
 				Async::Ready(Some(block)) => {
-					if self.note_block(block) {
-						self.net.announce(block);
+					if self.note_block(block.0) {
+						self.net.announce(block.0, block.1);
 						self.reset_delay();
 					}
 				},
@@ -229,7 +229,7 @@ impl<B: BlockT, N: Network<B>> Future for BlockAnnouncer<B, N> {
 					);
 
 					for block in self.latest_voted_blocks.iter() {
-						self.net.announce(*block);
+						self.net.announce(*block, Vec::new());
 					}
 				},
 				Ok(Async::NotReady) => return Ok(Async::NotReady),
@@ -240,15 +240,12 @@ impl<B: BlockT, N: Network<B>> Future for BlockAnnouncer<B, N> {
 
 /// A sender used to send block hashes to announce to a background job.
 #[derive(Clone)]
-pub(super) struct BlockAnnounceSender<B: BlockT>(mpsc::UnboundedSender<B::Hash>);
+pub(super) struct BlockAnnounceSender<B: BlockT>(mpsc::UnboundedSender<(B::Hash, Vec<u8>)>);
 
 impl<B: BlockT> BlockAnnounceSender<B> {
 	/// Send a block hash for the background worker to announce.
-	pub fn send(
-		&self,
-		block: B::Hash,
-	) {
-		if let Err(err) = self.0.unbounded_send(block) {
+	pub fn send(&self, block: B::Hash, associated_data: Vec<u8>) {
+		if let Err(err) = self.0.unbounded_send((block, associated_data)) {
 			debug!(target: "afg", "Failed to send block to background announcer: {:?}", err);
 		}
 	}
diff --git a/core/finality-grandpa/src/communication/tests.rs b/core/finality-grandpa/src/communication/tests.rs
index 28de0e538c84a..6215e30b80966 100644
--- a/core/finality-grandpa/src/communication/tests.rs
+++ b/core/finality-grandpa/src/communication/tests.rs
@@ -88,7 +88,7 @@ impl super::Network<Block> for TestNetwork {
 	}
 
 	/// Inform peers that a block with given hash should be downloaded.
-	fn announce(&self, block: Hash) {
+	fn announce(&self, block: Hash, _associated_data: Vec<u8>) {
 		let _ = self.sender.unbounded_send(Event::Announce(block));
 	}
 }
@@ -559,7 +559,7 @@ fn periodically_reannounce_voted_blocks_on_stall() {
 			for _ in 0..=12 {
 				let hash = Hash::random();
 				hashes.lock().push(hash);
-				announce_sender.send(hash);
+				announce_sender.send(hash, Vec::new());
 			}
 
 			// we should see an event for each of those announcements
diff --git a/core/network/src/chain.rs b/core/network/src/chain.rs
index b7465685ae8c1..d23068179ff55 100644
--- a/core/network/src/chain.rs
+++ b/core/network/src/chain.rs
@@ -16,10 +16,10 @@
 
 //! Blockchain access trait
 
-use client::{self, Client as SubstrateClient, ClientInfo, BlockStatus, CallExecutor};
+use client::{self, Client as SubstrateClient, ClientInfo, CallExecutor};
 use client::error::Error;
 use client::light::fetcher::ChangesProof;
-use consensus::{BlockImport, Error as ConsensusError};
+use consensus::{BlockImport, BlockStatus, Error as ConsensusError};
 use sr_primitives::traits::{Block as BlockT, Header as HeaderT};
 use sr_primitives::generic::{BlockId};
 use sr_primitives::Justification;
diff --git a/core/network/src/config.rs b/core/network/src/config.rs
index 7392a4aaf7b4e..0582fcb300f1e 100644
--- a/core/network/src/config.rs
+++ b/core/network/src/config.rs
@@ -26,7 +26,7 @@ use crate::chain::{Client, FinalityProofProvider};
 use crate::on_demand_layer::OnDemand;
 use crate::service::{ExHashT, TransactionPool};
 use bitflags::bitflags;
-use consensus::import_queue::ImportQueue;
+use consensus::{block_validation::BlockAnnounceValidator, import_queue::ImportQueue};
 use sr_primitives::traits::{Block as BlockT};
 use std::sync::Arc;
 use libp2p::identity::{Keypair, secp256k1, ed25519};
@@ -80,6 +80,9 @@ pub struct Params<B: BlockT, S, H: ExHashT> {
 
 	/// Customization of the network. Use this to plug additional networking capabilities.
 	pub specialization: S,
+
+	/// Type to check incoming block announcements.
+	pub block_announce_validator: Box<dyn BlockAnnounceValidator<B> + Send>
 }
 
 bitflags! {
diff --git a/core/network/src/protocol.rs b/core/network/src/protocol.rs
index ea993bdce0b38..b2cea2cd5c82b 100644
--- a/core/network/src/protocol.rs
+++ b/core/network/src/protocol.rs
@@ -23,14 +23,17 @@ use libp2p::core::{ConnectedPoint, nodes::Substream, muxing::StreamMuxerBox};
 use libp2p::swarm::{ProtocolsHandler, IntoProtocolsHandler};
 use libp2p::swarm::{NetworkBehaviour, NetworkBehaviourAction, PollParameters};
 use primitives::storage::StorageKey;
-use consensus::{import_queue::IncomingBlock, import_queue::Origin, BlockOrigin};
+use consensus::{
+	BlockOrigin,
+	block_validation::BlockAnnounceValidator,
+	import_queue::{BlockImportResult, BlockImportError, IncomingBlock, Origin}
+};
 use sr_primitives::{generic::BlockId, ConsensusEngineId, Justification};
 use sr_primitives::traits::{
 	Block as BlockT, Header as HeaderT, NumberFor, One, Zero,
 	CheckedSub, SaturatedConversion
 };
-use consensus::import_queue::{BlockImportResult, BlockImportError};
-use message::{BlockAttributes, Direction, FromBlock, Message, RequestId};
+use message::{BlockAnnounce, BlockAttributes, Direction, FromBlock, Message, RequestId};
 use message::generic::{Message as GenericMessage, ConsensusMessage};
 use event::Event;
 use consensus_gossip::{ConsensusGossip, MessageRecipient as GossipMessageRecipient};
@@ -366,9 +369,16 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 		finality_proof_request_builder: Option<BoxFinalityProofRequestBuilder<B>>,
 		protocol_id: ProtocolId,
 		peerset_config: peerset::PeersetConfig,
+		block_announce_validator: Box<dyn BlockAnnounceValidator<B> + Send>
 	) -> error::Result<(Protocol<B, S, H>, peerset::PeersetHandle)> {
 		let info = chain.info();
-		let sync = ChainSync::new(config.roles, chain.clone(), &info, finality_proof_request_builder);
+		let sync = ChainSync::new(
+			config.roles,
+			chain.clone(),
+			&info,
+			finality_proof_request_builder,
+			block_announce_validator,
+		);
 		let (peerset, peerset_handle) = peerset::Peerset::from_config(peerset_config);
 		let versions = &((MIN_VERSION as u8)..=(CURRENT_VERSION as u8)).collect::<Vec<u8>>();
 		let behaviour = LegacyProto::new(protocol_id, versions, peerset);
@@ -376,7 +386,7 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 		let protocol = Protocol {
 			tick_timeout: Box::new(futures_timer::Interval::new(TICK_TIMEOUT).map(|v| Ok::<_, ()>(v)).compat()),
 			propagate_timeout: Box::new(futures_timer::Interval::new(PROPAGATE_TIMEOUT).map(|v| Ok::<_, ()>(v)).compat()),
-			config: config,
+			config,
 			context_data: ContextData {
 				peers: HashMap::new(),
 				chain,
@@ -384,7 +394,7 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 			light_dispatch: LightDispatch::new(checker),
 			genesis_hash: info.chain.genesis_hash,
 			sync,
-			specialization: specialization,
+			specialization,
 			consensus_gossip: ConsensusGossip::new(),
 			handshaking_peers: HashMap::new(),
 			transaction_pool,
@@ -1010,7 +1020,7 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 	///
 	/// In chain-based consensus, we often need to make sure non-best forks are
 	/// at least temporarily synced.
-	pub fn announce_block(&mut self, hash: B::Hash) {
+	pub fn announce_block(&mut self, hash: B::Hash, data: Vec<u8>) {
 		let header = match self.context_data.chain.header(&BlockId::Hash(hash)) {
 			Ok(Some(header)) => header,
 			Ok(None) => {
@@ -1030,10 +1040,10 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 
 		let is_best = self.context_data.chain.info().chain.best_hash == hash;
 		debug!(target: "sync", "Reannouncing block {:?}", hash);
-		self.send_announcement(&header, is_best, true)
+		self.send_announcement(&header, data, is_best, true)
 	}
 
-	fn send_announcement(&mut self, header: &B::Header, is_best: bool, force: bool) {
+	fn send_announcement(&mut self, header: &B::Header, data: Vec<u8>, is_best: bool, force: bool) {
 		let hash = header.hash();
 
 		for (who, ref mut peer) in self.context_data.peers.iter_mut() {
@@ -1050,7 +1060,12 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 						}
 					} else  {
 						None
-					}
+					},
+					data: if peer.info.protocol_version >= 4 {
+						Some(data.clone())
+					} else {
+						None
+					},
 				});
 
 				self.behaviour.send_packet(who, message)
@@ -1074,29 +1089,22 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 		self.send_message(who, GenericMessage::Status(status))
 	}
 
-	fn on_block_announce(
-		&mut self,
-		who: PeerId,
-		announce: message::BlockAnnounce<B::Header>
-	) -> CustomMessageOutcome<B>  {
-		let header = announce.header;
-		let hash = header.hash();
-		{
-			if let Some(ref mut peer) = self.context_data.peers.get_mut(&who) {
-				peer.known_blocks.insert(hash.clone());
-			}
+	fn on_block_announce(&mut self, who: PeerId, announce: BlockAnnounce<B::Header>) -> CustomMessageOutcome<B> {
+		let hash = announce.header.hash();
+		if let Some(ref mut peer) = self.context_data.peers.get_mut(&who) {
+			peer.known_blocks.insert(hash.clone());
 		}
 		self.light_dispatch.update_best_number(LightDispatchIn {
 			behaviour: &mut self.behaviour,
 			peerset: self.peerset_handle.clone(),
-		}, who.clone(), *header.number());
+		}, who.clone(), *announce.header.number());
 
 		let is_their_best = match announce.state.unwrap_or(message::BlockState::Best) {
 			message::BlockState::Best => true,
 			message::BlockState::Normal => false,
 		};
 
-		match self.sync.on_block_announce(who.clone(), hash, &header, is_their_best) {
+		match self.sync.on_block_announce(who.clone(), hash, &announce, is_their_best) {
 			sync::OnBlockAnnounce::Request(peer, req) => {
 				self.send_message(peer, GenericMessage::BlockRequest(req));
 				return CustomMessageOutcome::None
@@ -1131,7 +1139,7 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 				blocks: vec![
 					message::generic::BlockData {
 						hash: hash,
-						header: Some(header),
+						header: Some(announce.header),
 						body: None,
 						receipt: None,
 						message_queue: None,
@@ -1156,7 +1164,7 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 
 	/// Call this when a block has been imported in the import queue and we should announce it on
 	/// the network.
-	pub fn on_block_imported(&mut self, hash: B::Hash, header: &B::Header, is_best: bool) {
+	pub fn on_block_imported(&mut self, hash: B::Hash, header: &B::Header, data: Vec<u8>, is_best: bool) {
 		if is_best {
 			self.sync.update_chain_info(header);
 		}
@@ -1172,7 +1180,7 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 		}
 
 		// send out block announcements
-		self.send_announcement(&header, is_best, false);
+		self.send_announcement(header, data, is_best, false);
 	}
 
 	/// Call this when a block has been finalized. The sync layer may have some additional
diff --git a/core/network/src/protocol/message.rs b/core/network/src/protocol/message.rs
index 2faa68a4e2002..6560ed0f13a52 100644
--- a/core/network/src/protocol/message.rs
+++ b/core/network/src/protocol/message.rs
@@ -273,6 +273,8 @@ pub mod generic {
 		pub header: H,
 		/// Block state. TODO: Remove `Option` and custom encoding when v4 becomes common.
 		pub state: Option<BlockState>,
+		/// Data associated with this block announcement, e.g. a candidate message.
+		pub data: Option<Vec<u8>>,
 	}
 
 	// Custom Encode/Decode impl to maintain backwards compatibility with v3.
@@ -284,6 +286,9 @@ pub mod generic {
 			if let Some(state) = &self.state {
 				state.encode_to(dest);
 			}
+			if let Some(data) = &self.data {
+				data.encode_to(dest)
+			}
 		}
 	}
 
@@ -291,9 +296,11 @@ pub mod generic {
 		fn decode<I: Input>(input: &mut I) -> Result<Self, codec::Error> {
 			let header = H::decode(input)?;
 			let state = BlockState::decode(input).ok();
+			let data = Vec::decode(input).ok();
 			Ok(BlockAnnounce {
 				header,
 				state,
+				data,
 			})
 		}
 	}
diff --git a/core/network/src/protocol/sync.rs b/core/network/src/protocol/sync.rs
index 52e88034a385d..cf7d899d6dc7b 100644
--- a/core/network/src/protocol/sync.rs
+++ b/core/network/src/protocol/sync.rs
@@ -28,11 +28,15 @@
 //!
 
 use blocks::BlockCollection;
-use client::{BlockStatus, ClientInfo, error::Error as ClientError};
-use consensus::{BlockOrigin, import_queue::{IncomingBlock, BlockImportResult, BlockImportError}};
+use client::{ClientInfo, error::Error as ClientError};
+use consensus::{BlockOrigin, BlockStatus,
+	block_validation::{BlockAnnounceValidator, Validation},
+	import_queue::{IncomingBlock, BlockImportResult, BlockImportError}
+};
 use crate::{
 	config::{Roles, BoxFinalityProofRequestBuilder},
-	message::{self, generic::FinalityProofRequest, BlockAttributes, BlockRequest, BlockResponse, FinalityProofResponse},
+	message::{self, generic::FinalityProofRequest, BlockAnnounce, BlockAttributes, BlockRequest, BlockResponse,
+	FinalityProofResponse},
 	protocol
 };
 use either::Either;
@@ -122,6 +126,8 @@ pub struct ChainSync<B: BlockT> {
 	request_builder: Option<BoxFinalityProofRequestBuilder<B>>,
 	/// A flag that caches idle state with no pending requests.
 	is_idle: bool,
+	/// A type to check incoming block announcements.
+	block_announce_validator: Box<dyn BlockAnnounceValidator<B> + Send>
 }
 
 /// All the data we have about a Peer that we are trying to sync with
@@ -271,7 +277,8 @@ impl<B: BlockT> ChainSync<B> {
 		role: Roles,
 		client: Arc<dyn crate::chain::Client<B>>,
 		info: &ClientInfo<B>,
-		request_builder: Option<BoxFinalityProofRequestBuilder<B>>
+		request_builder: Option<BoxFinalityProofRequestBuilder<B>>,
+		block_announce_validator: Box<dyn BlockAnnounceValidator<B> + Send>
 	) -> Self {
 		let mut required_block_attributes = BlockAttributes::HEADER | BlockAttributes::JUSTIFICATION;
 
@@ -293,6 +300,7 @@ impl<B: BlockT> ChainSync<B> {
 			best_importing_number: Zero::zero(),
 			request_builder,
 			is_idle: false,
+			block_announce_validator,
 		}
 	}
 
@@ -885,9 +893,10 @@ impl<B: BlockT> ChainSync<B> {
 	/// header (call `on_block_data`). The network request isn't sent
 	/// in this case. Both hash and header is passed as an optimization
 	/// to avoid rehashing the header.
-	pub fn on_block_announce(&mut self, who: PeerId, hash: B::Hash, header: &B::Header, is_best: bool)
+	pub fn on_block_announce(&mut self, who: PeerId, hash: B::Hash, announce: &BlockAnnounce<B::Header>, is_best: bool)
 		-> OnBlockAnnounce<B>
 	{
+		let header = &announce.header;
 		let number = *header.number();
 		debug!(target: "sync", "Received block announcement with number {:?}", number);
 		if number.is_zero() {
@@ -932,6 +941,20 @@ impl<B: BlockT> ChainSync<B> {
 			return OnBlockAnnounce::Nothing
 		}
 
+		// Let external validator check the block announcement.
+		let assoc_data = announce.data.as_ref().map_or(&[][..], |v| v.as_slice());
+		match self.block_announce_validator.validate(&header, assoc_data) {
+			Ok(Validation::Success) => (),
+			Ok(Validation::Failure) => {
+				debug!(target: "sync", "block announcement validation of block {} from {} failed", hash, who);
+				return OnBlockAnnounce::Nothing
+			}
+			Err(e) => {
+				error!(target: "sync", "block announcement validation errored: {}", e);
+				return OnBlockAnnounce::Nothing
+			}
+		}
+
 		// stale block case
 		let requires_additional_data = !self.role.is_light();
 		if number <= self.best_queued_number {
diff --git a/core/network/src/service.rs b/core/network/src/service.rs
index a1cba0395e360..3ca7bffdb45c5 100644
--- a/core/network/src/service.rs
+++ b/core/network/src/service.rs
@@ -113,9 +113,7 @@ impl<B: BlockT + 'static, S: NetworkSpecialization<B>, H: ExHashT> NetworkWorker
 	/// Returns a `NetworkWorker` that implements `Future` and must be regularly polled in order
 	/// for the network processing to advance. From it, you can extract a `NetworkService` using
 	/// `worker.service()`. The `NetworkService` can be shared through the codebase.
-	pub fn new(
-		params: Params<B, S, H>,
-	) -> Result<NetworkWorker<B, S, H>, Error> {
+	pub fn new(params: Params<B, S, H>) -> Result<NetworkWorker<B, S, H>, Error> {
 		let (to_worker, from_worker) = mpsc::unbounded();
 
 		if let Some(ref path) = params.network_config.net_config_path {
@@ -178,6 +176,7 @@ impl<B: BlockT + 'static, S: NetworkSpecialization<B>, H: ExHashT> NetworkWorker
 			params.finality_proof_request_builder,
 			params.protocol_id,
 			peerset_config,
+			params.block_announce_validator
 		)?;
 
 		// Build the swarm.
@@ -297,8 +296,8 @@ impl<B: BlockT + 'static, S: NetworkSpecialization<B>, H: ExHashT> NetworkWorker
 	}
 
 	/// You must call this when a new block is imported by the client.
-	pub fn on_block_imported(&mut self, hash: B::Hash, header: B::Header, is_best: bool) {
-		self.network_service.user_protocol_mut().on_block_imported(hash, &header, is_best);
+	pub fn on_block_imported(&mut self, hash: B::Hash, header: B::Header, data: Vec<u8>, is_best: bool) {
+		self.network_service.user_protocol_mut().on_block_imported(hash, &header, data, is_best);
 	}
 
 	/// You must call this when a new block is finalized by the client.
@@ -394,8 +393,8 @@ impl<B: BlockT + 'static, S: NetworkSpecialization<B>, H: ExHashT> NetworkServic
 	///
 	/// In chain-based consensus, we often need to make sure non-best forks are
 	/// at least temporarily synced. This function forces such an announcement.
-	pub fn announce_block(&self, hash: B::Hash) {
-		let _ = self.to_worker.unbounded_send(ServerToWorkerMsg::AnnounceBlock(hash));
+	pub fn announce_block(&self, hash: B::Hash, data: Vec<u8>) {
+		let _ = self.to_worker.unbounded_send(ServerToWorkerMsg::AnnounceBlock(hash, data));
 	}
 
 	/// Send a consensus message through the gossip
@@ -580,7 +579,7 @@ impl<B, S, H> NetworkStateInfo for NetworkService<B, S, H>
 enum ServerToWorkerMsg<B: BlockT, S: NetworkSpecialization<B>> {
 	PropagateExtrinsics,
 	RequestJustification(B::Hash, NumberFor<B>),
-	AnnounceBlock(B::Hash),
+	AnnounceBlock(B::Hash, Vec<u8>),
 	ExecuteWithSpec(Box<dyn FnOnce(&mut S, &mut dyn Context<B>) + Send>),
 	ExecuteWithGossip(Box<dyn FnOnce(&mut ConsensusGossip<B>, &mut dyn Context<B>) + Send>),
 	GossipConsensusMessage(B::Hash, ConsensusEngineId, Vec<u8>, GossipMessageRecipient),
@@ -653,8 +652,8 @@ impl<B: BlockT + 'static, S: NetworkSpecialization<B>, H: ExHashT> Stream for Ne
 				}
 				ServerToWorkerMsg::GossipConsensusMessage(topic, engine_id, message, recipient) =>
 					self.network_service.user_protocol_mut().gossip_consensus_message(topic, engine_id, message, recipient),
-				ServerToWorkerMsg::AnnounceBlock(hash) =>
-					self.network_service.user_protocol_mut().announce_block(hash),
+				ServerToWorkerMsg::AnnounceBlock(hash, data) =>
+					self.network_service.user_protocol_mut().announce_block(hash, data),
 				ServerToWorkerMsg::RequestJustification(hash, number) =>
 					self.network_service.user_protocol_mut().request_justification(&hash, number),
 				ServerToWorkerMsg::PropagateExtrinsics =>
diff --git a/core/network/src/test/mod.rs b/core/network/src/test/mod.rs
index 5fd67acd1b625..ae3163380a0f0 100644
--- a/core/network/src/test/mod.rs
+++ b/core/network/src/test/mod.rs
@@ -35,6 +35,7 @@ use client::error::Result as ClientResult;
 use client::block_builder::BlockBuilder;
 use client::backend::{AuxStore, Backend, Finalizer};
 use crate::config::Roles;
+use consensus::block_validation::DefaultBlockAnnounceValidator;
 use consensus::import_queue::BasicQueue;
 use consensus::import_queue::{
 	BoxBlockImport, BoxJustificationImport, Verifier, BoxFinalityProofImport,
@@ -252,8 +253,8 @@ impl<D, S: NetworkSpecialization<Block>> Peer<D, S> {
 	}
 
 	/// Announces an important block on the network.
-	pub fn announce_block(&self, hash: <Block as BlockT>::Hash) {
-		self.network.service().announce_block(hash);
+	pub fn announce_block(&self, hash: <Block as BlockT>::Hash, data: Vec<u8>) {
+		self.network.service().announce_block(hash, data);
 	}
 
 	/// Add blocks to the peer -- edit the block before adding
@@ -300,11 +301,11 @@ impl<D, S: NetworkSpecialization<Block>> Peer<D, S> {
 				Default::default()
 			};
 			self.block_import.import_block(import_block, cache).expect("block_import failed");
-			self.network.on_block_imported(hash, header, true);
+			self.network.on_block_imported(hash, header, Vec::new(), true);
 			at = hash;
 		}
 
-		self.network.service().announce_block(at.clone());
+		self.network.service().announce_block(at.clone(), Vec::new());
 		at
 	}
 
@@ -537,6 +538,7 @@ pub trait TestNetFactory: Sized {
 			protocol_id: ProtocolId::from(&b"test-protocol-name"[..]),
 			import_queue,
 			specialization: self::SpecializationFactory::create(),
+			block_announce_validator: Box::new(DefaultBlockAnnounceValidator::new(client.clone()))
 		}).unwrap();
 
 		self.mut_peers(|peers| {
@@ -600,6 +602,7 @@ pub trait TestNetFactory: Sized {
 			protocol_id: ProtocolId::from(&b"test-protocol-name"[..]),
 			import_queue,
 			specialization: self::SpecializationFactory::create(),
+			block_announce_validator: Box::new(DefaultBlockAnnounceValidator::new(client.clone()))
 		}).unwrap();
 
 		self.mut_peers(|peers| {
@@ -662,7 +665,7 @@ pub trait TestNetFactory: Sized {
 
 				// We poll `imported_blocks_stream`.
 				while let Ok(Async::Ready(Some(notification))) = peer.imported_blocks_stream.poll() {
-					peer.network.on_block_imported(notification.hash, notification.header, true);
+					peer.network.on_block_imported(notification.hash, notification.header, Vec::new(), true);
 				}
 
 				// We poll `finality_notification_stream`, but we only take the last event.
diff --git a/core/network/src/test/sync.rs b/core/network/src/test/sync.rs
index 6cba21c1719bd..d50190f6573ff 100644
--- a/core/network/src/test/sync.rs
+++ b/core/network/src/test/sync.rs
@@ -444,7 +444,7 @@ fn can_sync_small_non_best_forks() {
 	assert!(net.peer(0).client().header(&BlockId::Hash(small_hash)).unwrap().is_some());
 	assert!(!net.peer(1).client().header(&BlockId::Hash(small_hash)).unwrap().is_some());
 
-	net.peer(0).announce_block(small_hash);
+	net.peer(0).announce_block(small_hash, Vec::new());
 
 	// after announcing, peer 1 downloads the block.
 
@@ -499,7 +499,7 @@ fn light_peer_imports_header_from_announce() {
 	let mut runtime = current_thread::Runtime::new().unwrap();
 
 	fn import_with_announce(net: &mut TestNet, runtime: &mut current_thread::Runtime, hash: H256) {
-		net.peer(0).announce_block(hash);
+		net.peer(0).announce_block(hash, Vec::new());
 
 		runtime.block_on(futures::future::poll_fn::<(), (), _>(|| {
 			net.poll();
diff --git a/core/service/src/lib.rs b/core/service/src/lib.rs
index 431776b31e9b4..cf84df3ef6b1d 100644
--- a/core/service/src/lib.rs
+++ b/core/service/src/lib.rs
@@ -190,6 +190,9 @@ macro_rules! new_impl {
 			network::config::ProtocolId::from(protocol_id_full)
 		};
 
+		let block_announce_validator =
+			Box::new(consensus_common::block_validation::DefaultBlockAnnounceValidator::new(client.clone()));
+
 		let network_params = network::config::Params {
 			roles: $config.roles,
 			network_config: $config.network.clone(),
@@ -201,6 +204,7 @@ macro_rules! new_impl {
 			import_queue,
 			protocol_id,
 			specialization: network_protocol,
+			block_announce_validator,
 		};
 
 		let has_bootnodes = !network_params.network_config.boot_nodes.is_empty();
@@ -682,7 +686,7 @@ fn build_network_future<
 
 		// We poll `imported_blocks_stream`.
 		while let Ok(Async::Ready(Some(notification))) = imported_blocks_stream.poll() {
-			network.on_block_imported(notification.hash, notification.header, notification.is_new_best);
+			network.on_block_imported(notification.hash, notification.header, Vec::new(), notification.is_new_best);
 		}
 
 		// We poll `finality_notification_stream`, but we only take the last event.