Moved sync to a separate crate

2016-01-29 15:56:06 +01:00 · 2016-01-29 15:56:06 +01:00 · 1e91b6f5db
commit 1e91b6f5db
parent 53f9bff95c
16 changed files with 1880 additions and 16 deletions
--- a/bin/Cargo.toml
+++ b/bin/Cargo.toml
@ -15,6 +15,7 @@ ctrlc = "1.0"
 ethcore-util = { path = "../util" }
 ethcore-rpc = { path = "../rpc", optional = true }
 ethcore = { path = ".." }
+ethsync = { path = "../sync" }
 clippy = "0.0.37"

 [features]
--- a/bin/src/main.rs
+++ b/bin/src/main.rs
@ -8,6 +8,7 @@ extern crate docopt;
 extern crate rustc_serialize;
 extern crate ethcore_util as util;
 extern crate ethcore;
+extern crate ethsync;
 extern crate log;
 extern crate env_logger;
 extern crate ctrlc;
@ -24,7 +25,7 @@ use ethcore::client::*;
 use ethcore::service::{ClientService, NetSyncMessage};
 use ethcore::ethereum;
 use ethcore::blockchain::CacheSize;
-use ethcore::sync::EthSync;
+use ethsync::EthSync;

 docopt!(Args derive Debug, "
 Parity. Ethereum Client.
@ -81,8 +82,10 @@ fn main() {
 	let mut net_settings = NetworkConfiguration::new();
 	net_settings.boot_nodes = init_nodes;
 	let mut service = ClientService::start(spec, net_settings).unwrap();
-	setup_rpc_server(service.client(), service.sync());
-	let io_handler  = Arc::new(ClientIoHandler { client: service.client(), info: Default::default(), sync: service.sync() });
+	let client = service.client().clone();
+	let sync = EthSync::register(service.network(), client);
+	setup_rpc_server(service.client(), sync.clone());
+	let io_handler  = Arc::new(ClientIoHandler { client: service.client(), info: Default::default(), sync: sync });
 	service.io().register_handler(io_handler).expect("Error registering IO handler");

 	let exit = Arc::new(Condvar::new());
--- a/src/lib.rs
+++ b/src/lib.rs
@ -151,8 +151,6 @@ mod tests;
 /// TODO [arkpar] Please document me
 pub mod client;
 /// TODO [arkpar] Please document me
-pub mod sync;
-/// TODO [arkpar] Please document me
 pub mod block;
 /// TODO [arkpar] Please document me
 pub mod verification;
--- a/src/service.rs
+++ b/src/service.rs
@ -1,5 +1,4 @@
 use util::*;
-use sync::*;
 use spec::Spec;
 use error::*;
 use std::env;
@ -21,7 +20,6 @@ pub type NetSyncMessage = NetworkIoMessage<SyncMessage>;
 pub struct ClientService {
 	net_service: NetworkService<SyncMessage>,
 	client: Arc<Client>,
-	sync: Arc<EthSync>,
 }

 impl ClientService {
@ -34,7 +32,6 @@ impl ClientService {
 		dir.push(".parity");
 		dir.push(H64::from(spec.genesis_header().hash()).hex());
 		let client = try!(Client::new(spec, &dir, net_service.io().channel()));
-		let sync = EthSync::register(&mut net_service, client.clone());
 		let client_io = Arc::new(ClientIoHandler {
 			client: client.clone()
 		});
@ -43,29 +40,28 @@ impl ClientService {
 		Ok(ClientService {
 			net_service: net_service,
 			client: client,
-			sync: sync,
 		})
 	}

-	/// Get the network service.
+	/// Add a node to network
 	pub fn add_node(&mut self, _enode: &str) {
 		unimplemented!();
 	}

-	/// TODO [arkpar] Please document me
+	/// Get general IO interface
 	pub fn io(&mut self) -> &mut IoService<NetSyncMessage> {
 		self.net_service.io()
 	}

-	/// TODO [arkpar] Please document me
+	/// Get client interface
 	pub fn client(&self) -> Arc<Client> {
 		self.client.clone()
 	
 	}
-	
-	/// Get shared sync handler
-	pub fn sync(&self) -> Arc<EthSync> {
-		self.sync.clone()
+
+	/// Get network service component
+	pub fn network(&mut self) -> &mut NetworkService<SyncMessage> {
+		&mut self.net_service
 	}
 }

--- a/sync/Cargo.toml
+++ b/sync/Cargo.toml
@ -0,0 +1,16 @@
+[package]
+description = "Ethcore blockchain sync"
+name = "ethsync"
+version = "0.1.0"
+license = "GPL-3.0"
+authors = ["Ethcore <admin@ethcore.io"]
+
+[lib]
+
+[dependencies]
+ethcore-util = { path = "../util" }
+ethcore = { path = ".." }
+clippy = "0.0.37"
+log = "0.3"
+env_logger = "0.3"
+
--- a/src/sync/chain.rs
+++ b/src/sync/chain.rs
--- a/src/sync/io.rs
+++ b/src/sync/io.rs
--- a/src/sync/mod.rs
+++ b/src/sync/mod.rs
--- a/src/sync/range_collection.rs
+++ b/src/sync/range_collection.rs
--- a/sync/src/chain.rs
+++ b/sync/src/chain.rs
@ -0,0 +1,969 @@
+/// 
+/// BlockChain synchronization strategy.
+/// Syncs to peers and keeps up to date. 
+/// This implementation uses ethereum protocol v63
+///
+/// Syncing strategy.
+///
+/// 1. A peer arrives with a total difficulty better than ours
+/// 2. Find a common best block between our an peer chain. 
+/// Start with out best block and request headers from peer backwards until a common block is found
+/// 3. Download headers and block bodies from peers in parallel. 
+/// As soon as a set of the blocks is fully downloaded at the head of the queue it is fed to the blockchain
+/// 4. Maintain sync by handling NewBlocks/NewHashes messages
+///
+
+use util::*;
+use std::mem::{replace};
+use ethcore::views::{HeaderView};
+use ethcore::header::{BlockNumber, Header as BlockHeader};
+use ethcore::client::{BlockChainClient, BlockStatus};
+use range_collection::{RangeCollection, ToUsize, FromUsize};
+use ethcore::error::*;
+use io::SyncIo;
+
+impl ToUsize for BlockNumber {
+	fn to_usize(&self) -> usize {
+		*self as usize
+	}
+}
+
+impl FromUsize for BlockNumber {
+	fn from_usize(s: usize) -> BlockNumber {
+		s as BlockNumber
+	}
+}
+
+type PacketDecodeError = DecoderError;
+
+const PROTOCOL_VERSION: u8 = 63u8;
+const MAX_BODIES_TO_SEND: usize = 256;
+const MAX_HEADERS_TO_SEND: usize = 512;
+const MAX_NODE_DATA_TO_SEND: usize = 1024;
+const MAX_RECEIPTS_TO_SEND: usize = 1024;
+const MAX_HEADERS_TO_REQUEST: usize = 512;
+const MAX_BODIES_TO_REQUEST: usize = 256;
+
+const STATUS_PACKET: u8 = 0x00;
+const NEW_BLOCK_HASHES_PACKET: u8 = 0x01;
+const TRANSACTIONS_PACKET: u8 = 0x02;
+const GET_BLOCK_HEADERS_PACKET: u8 = 0x03;
+const BLOCK_HEADERS_PACKET: u8 = 0x04;
+const GET_BLOCK_BODIES_PACKET: u8 = 0x05;
+const BLOCK_BODIES_PACKET: u8 = 0x06;
+const NEW_BLOCK_PACKET: u8 = 0x07;
+
+const GET_NODE_DATA_PACKET: u8 = 0x0d;
+const NODE_DATA_PACKET: u8 = 0x0e;
+const GET_RECEIPTS_PACKET: u8 = 0x0f;
+const RECEIPTS_PACKET: u8 = 0x10;
+
+const NETWORK_ID: U256 = ONE_U256; //TODO: get this from parent
+
+struct Header {
+	/// Header data
+	data: Bytes,
+	/// Block hash
+	hash: H256,
+	/// Parent hash
+	parent: H256,
+}
+
+/// Used to identify header by transactions and uncles hashes
+#[derive(Eq, PartialEq, Hash)]
+struct HeaderId {
+	transactions_root: H256,
+	uncles: H256
+}
+
+#[derive(Copy, Clone, Eq, PartialEq, Debug)]
+/// Sync state 
+pub enum SyncState {
+	/// Initial chain sync has not started yet
+	NotSynced,
+	/// Initial chain sync complete. Waiting for new packets
+	Idle,
+	/// Block downloading paused. Waiting for block queue to process blocks and free some space
+	Waiting,
+	/// Downloading blocks
+	Blocks,
+	/// Downloading blocks learned from NewHashes packet
+	NewBlocks,
+}
+
+/// Syncing status and statistics
+pub struct SyncStatus {
+	/// State
+	pub state: SyncState,
+	/// Syncing protocol version. That's the maximum protocol version we connect to.
+	pub protocol_version: u8,
+	/// BlockChain height for the moment the sync started.
+	pub start_block_number: BlockNumber,
+	/// Last fully downloaded and imported block number.
+	pub last_imported_block_number: BlockNumber,
+	/// Highest block number in the download queue.
+	pub highest_block_number: BlockNumber,
+	/// Total number of blocks for the sync process.
+	pub blocks_total: usize,
+	/// Number of blocks downloaded so far.
+	pub blocks_received: usize,
+	/// Total number of connected peers
+	pub num_peers: usize,
+	/// Total number of active peers
+	pub num_active_peers: usize,
+}
+
+#[derive(PartialEq, Eq, Debug)]
+/// Peer data type requested
+enum PeerAsking {
+	Nothing,
+	BlockHeaders,
+	BlockBodies,
+}
+
+/// Syncing peer information
+struct PeerInfo {
+	/// eth protocol version
+	protocol_version: u32,
+	/// Peer chain genesis hash
+	genesis: H256,
+	/// Peer network id 
+	network_id: U256,
+	/// Peer best block hash
+	latest: H256,
+	/// Peer total difficulty
+	difficulty: U256,
+	/// Type of data currenty being requested from peer.
+	asking: PeerAsking,
+	/// A set of block numbers being requested
+	asking_blocks: Vec<BlockNumber>,
+}
+
+/// Blockchain sync handler.
+/// See module documentation for more details.
+pub struct ChainSync {
+	/// Sync state
+	state: SyncState,
+	/// Last block number for the start of sync
+	starting_block: BlockNumber,
+	/// Highest block number seen
+	highest_block: BlockNumber,
+	/// Set of block header numbers being downloaded
+	downloading_headers: HashSet<BlockNumber>,
+	/// Set of block body numbers being downloaded
+	downloading_bodies: HashSet<BlockNumber>,
+	/// Downloaded headers.
+	headers: Vec<(BlockNumber, Vec<Header>)>, //TODO: use BTreeMap once range API is sable. For now it is a vector sorted in descending order
+	/// Downloaded bodies
+	bodies: Vec<(BlockNumber, Vec<Bytes>)>, //TODO: use BTreeMap once range API is sable. For now it is a vector sorted in descending order
+	/// Peer info
+	peers: HashMap<PeerId, PeerInfo>,
+	/// Used to map body to header
+	header_ids: HashMap<HeaderId, BlockNumber>,
+	/// Last impoted block number
+	last_imported_block: BlockNumber,
+	/// Last impoted block hash
+	last_imported_hash: H256,
+	/// Syncing total  difficulty
+	syncing_difficulty: U256,
+	/// True if common block for our and remote chain has been found
+	have_common_block: bool,
+}
+
+
+impl ChainSync {
+	/// Create a new instance of syncing strategy.
+	pub fn new() -> ChainSync {
+		ChainSync {
+			state: SyncState::NotSynced,
+			starting_block: 0,
+			highest_block: 0,
+			downloading_headers: HashSet::new(),
+			downloading_bodies: HashSet::new(),
+			headers: Vec::new(),
+			bodies: Vec::new(),
+			peers: HashMap::new(),
+			header_ids: HashMap::new(),
+			last_imported_block: 0,
+			last_imported_hash: H256::new(),
+			syncing_difficulty: U256::from(0u64),
+			have_common_block: false,
+		}
+	}
+
+	/// @returns Synchonization status
+	pub fn status(&self) -> SyncStatus {
+		SyncStatus {
+			state: self.state.clone(),
+			protocol_version: 63,
+			start_block_number: self.starting_block,
+			last_imported_block_number: self.last_imported_block,
+			highest_block_number: self.highest_block,
+			blocks_received: (self.last_imported_block - self.starting_block) as usize,
+			blocks_total: (self.highest_block - self.starting_block) as usize,
+			num_peers: self.peers.len(),
+			num_active_peers: self.peers.values().filter(|p| p.asking != PeerAsking::Nothing).count(),
+		}
+	}
+
+	/// Abort all sync activity
+	pub fn abort(&mut self, io: &mut SyncIo) {
+		self.restart(io);
+		self.peers.clear();
+	}
+
+	/// Rest sync. Clear all downloaded data but keep the queue
+	fn reset(&mut self) {
+		self.downloading_headers.clear();
+		self.downloading_bodies.clear();
+		self.headers.clear();
+		self.bodies.clear();
+		for (_, ref mut p) in &mut self.peers {
+			p.asking_blocks.clear();
+		}
+		self.header_ids.clear();
+		self.syncing_difficulty = From::from(0u64);
+		self.state = SyncState::Idle;
+	}
+
+	/// Restart sync
+	pub fn restart(&mut self, io: &mut SyncIo) {
+		self.reset();
+		self.last_imported_block = 0;
+		self.last_imported_hash = H256::new();
+		self.starting_block = 0;
+		self.highest_block = 0;
+		self.have_common_block = false;
+		io.chain().clear_queue();
+		self.starting_block = io.chain().chain_info().best_block_number;
+		self.state = SyncState::NotSynced;
+	}
+
+	/// Called by peer to report status
+	fn on_peer_status(&mut self, io: &mut SyncIo, peer_id: PeerId, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		let peer = PeerInfo {
+			protocol_version: try!(r.val_at(0)),
+			network_id: try!(r.val_at(1)),
+			difficulty: try!(r.val_at(2)),
+			latest: try!(r.val_at(3)),
+			genesis: try!(r.val_at(4)),
+			asking: PeerAsking::Nothing,
+			asking_blocks: Vec::new(),
+		};
+
+		trace!(target: "sync", "New peer {} (protocol: {}, network: {:?}, difficulty: {:?}, latest:{}, genesis:{})", peer_id, peer.protocol_version, peer.network_id, peer.difficulty, peer.latest, peer.genesis);
+		
+		let chain_info = io.chain().chain_info();
+		if peer.genesis != chain_info.genesis_hash {
+			io.disable_peer(peer_id);
+			trace!(target: "sync", "Peer {} genesis hash not matched", peer_id);
+			return Ok(());
+		}
+		if peer.network_id != NETWORK_ID {
+			io.disable_peer(peer_id);
+			trace!(target: "sync", "Peer {} network id not matched", peer_id);
+			return Ok(());
+		}
+
+		let old = self.peers.insert(peer_id.clone(), peer);
+		if old.is_some() {
+			panic!("ChainSync: new peer already exists");
+		}
+		info!(target: "sync", "Connected {}:{}", peer_id, io.peer_info(peer_id));
+		self.sync_peer(io, peer_id, false);
+		Ok(())
+	}
+
+	#[allow(cyclomatic_complexity)]
+	/// Called by peer once it has new block headers during sync
+	fn on_peer_block_headers(&mut self, io: &mut SyncIo, peer_id: PeerId, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		self.reset_peer_asking(peer_id, PeerAsking::BlockHeaders);
+		let item_count = r.item_count();
+		trace!(target: "sync", "{} -> BlockHeaders ({} entries)", peer_id, item_count);
+		self.clear_peer_download(peer_id);
+		if self.state != SyncState::Blocks && self.state != SyncState::NewBlocks && self.state != SyncState::Waiting {
+			trace!(target: "sync", "Ignored unexpected block headers");
+			return Ok(());
+		}
+		if self.state == SyncState::Waiting {
+			trace!(target: "sync", "Ignored block headers while waiting");
+			return Ok(());
+		}
+
+		for i in 0..item_count {
+			let info: BlockHeader = try!(r.val_at(i));
+			let number = BlockNumber::from(info.number);
+			if number <= self.last_imported_block || self.headers.have_item(&number) {
+				trace!(target: "sync", "Skipping existing block header");
+				continue;
+			}
+			if number > self.highest_block {
+				self.highest_block = number;
+			}
+			let hash = info.hash();
+			match io.chain().block_status(&hash) {
+				BlockStatus::InChain => {
+					self.have_common_block = true;
+					self.last_imported_block = number;
+					self.last_imported_hash = hash.clone();
+					trace!(target: "sync", "Found common header {} ({})", number, hash);
+				},
+				_ => {
+					if self.have_common_block {
+						//validate chain
+						if self.have_common_block && number == self.last_imported_block + 1 && info.parent_hash != self.last_imported_hash {
+							// TODO: lower peer rating
+							debug!(target: "sync", "Mismatched block header {} {}", number, hash);
+							continue;
+						}
+						if self.headers.find_item(&(number - 1)).map_or(false, |p| p.hash != info.parent_hash) {
+							// mismatching parent id, delete the previous block and don't add this one
+							// TODO: lower peer rating
+							debug!(target: "sync", "Mismatched block header {} {}", number, hash);
+							self.remove_downloaded_blocks(number - 1);
+							continue;
+						}
+						if self.headers.find_item(&(number + 1)).map_or(false, |p| p.parent != hash) {
+							// mismatching parent id for the next block, clear following headers
+							debug!(target: "sync", "Mismatched block header {}", number + 1);
+							self.remove_downloaded_blocks(number + 1);
+						}
+					}
+					let hdr = Header {
+						data: try!(r.at(i)).as_raw().to_vec(),
+						hash: hash.clone(),
+						parent: info.parent_hash,
+					};
+					self.headers.insert_item(number, hdr);
+					let header_id = HeaderId {
+						transactions_root: info.transactions_root,
+						uncles: info.uncles_hash
+					};
+					trace!(target: "sync", "Got header {} ({})", number, hash);
+					if header_id.transactions_root == rlp::SHA3_NULL_RLP && header_id.uncles == rlp::SHA3_EMPTY_LIST_RLP {
+						//empty body, just mark as downloaded
+						let mut body_stream = RlpStream::new_list(2);
+						body_stream.append_raw(&rlp::NULL_RLP, 1);
+						body_stream.append_raw(&rlp::EMPTY_LIST_RLP, 1);
+						self.bodies.insert_item(number, body_stream.out());
+					}
+					else {
+						self.header_ids.insert(header_id, number);
+					}
+				}
+			}
+		}
+		self.collect_blocks(io);
+		self.continue_sync(io);
+		Ok(())
+	}
+
+	/// Called by peer once it has new block bodies
+	fn on_peer_block_bodies(&mut self, io: &mut SyncIo, peer_id: PeerId, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		use util::triehash::ordered_trie_root;
+		self.reset_peer_asking(peer_id, PeerAsking::BlockBodies);
+		let item_count = r.item_count();
+		trace!(target: "sync", "{} -> BlockBodies ({} entries)", peer_id, item_count);
+		self.clear_peer_download(peer_id);
+		if self.state != SyncState::Blocks && self.state != SyncState::NewBlocks && self.state != SyncState::Waiting {
+			trace!(target: "sync", "Ignored unexpected block bodies");
+			return Ok(());
+		}
+		if self.state  == SyncState::Waiting {
+			trace!(target: "sync", "Ignored block bodies while waiting");
+			return Ok(());
+		}
+		for i in 0..item_count {
+			let body = try!(r.at(i));
+			let tx = try!(body.at(0));
+			let tx_root = ordered_trie_root(tx.iter().map(|r| r.as_raw().to_vec()).collect()); //TODO: get rid of vectors here
+			let uncles = try!(body.at(1)).as_raw().sha3();
+			let header_id = HeaderId {
+				transactions_root: tx_root,
+				uncles: uncles
+			};
+			match self.header_ids.get(&header_id).cloned() {
+				Some(n) => {
+					self.header_ids.remove(&header_id);
+					self.bodies.insert_item(n, body.as_raw().to_vec());
+					trace!(target: "sync", "Got body {}", n);
+				}
+				None =>  {
+					debug!(target: "sync", "Ignored unknown block body");
+				}
+			}
+		}
+		self.collect_blocks(io);
+		self.continue_sync(io);
+		Ok(())
+	}
+
+	/// Called by peer once it has new block bodies
+	fn on_peer_new_block(&mut self, io: &mut SyncIo, peer_id: PeerId, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		let block_rlp = try!(r.at(0));
+		let header_rlp = try!(block_rlp.at(0));
+		let h = header_rlp.as_raw().sha3();
+
+		trace!(target: "sync", "{} -> NewBlock ({})", peer_id, h);
+		let header_view = HeaderView::new(header_rlp.as_raw());
+		// TODO: Decompose block and add to self.headers and self.bodies instead
+		if header_view.number() == From::from(self.last_imported_block + 1) {
+			match io.chain().import_block(block_rlp.as_raw().to_vec()) {
+				Err(ImportError::AlreadyInChain) => {
+					trace!(target: "sync", "New block already in chain {:?}", h);
+				},
+				Err(ImportError::AlreadyQueued) => {
+					trace!(target: "sync", "New block already queued {:?}", h);
+				},
+				Ok(_) => {
+					trace!(target: "sync", "New block queued {:?}", h);
+				},
+				Err(e) => {
+					debug!(target: "sync", "Bad new block {:?} : {:?}", h, e);
+					io.disable_peer(peer_id);
+				}
+			};
+		} 
+		else {
+			trace!(target: "sync", "New block unknown {:?}", h);
+			//TODO: handle too many unknown blocks
+			let difficulty: U256 = try!(r.val_at(1));
+			let peer_difficulty = self.peers.get_mut(&peer_id).expect("ChainSync: unknown peer").difficulty;
+			if difficulty > peer_difficulty {
+				trace!(target: "sync", "Received block {:?}  with no known parent. Peer needs syncing...", h);
+				{
+					let peer = self.peers.get_mut(&peer_id).expect("ChainSync: unknown peer");
+					peer.latest = header_view.sha3();
+				}
+				self.sync_peer(io, peer_id, true);
+			}
+		}
+		Ok(())
+	}
+
+	/// Handles NewHashes packet. Initiates headers download for any unknown hashes. 
+	fn on_peer_new_hashes(&mut self, io: &mut SyncIo, peer_id: PeerId, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		if self.peers.get_mut(&peer_id).expect("ChainSync: unknown peer").asking != PeerAsking::Nothing {
+			trace!(target: "sync", "Ignoring new hashes since we're already downloading.");
+			return Ok(());
+		}
+		trace!(target: "sync", "{} -> NewHashes ({} entries)", peer_id, r.item_count());
+		let hashes = r.iter().map(|item| (item.val_at::<H256>(0), item.val_at::<U256>(1)));
+		let mut max_height: U256 = From::from(0);
+		for (rh, rd) in hashes {
+			let h = try!(rh);
+			let d = try!(rd);
+			match io.chain().block_status(&h) {
+				BlockStatus::InChain  => {
+					trace!(target: "sync", "New block hash already in chain {:?}", h);
+				},
+				BlockStatus::Queued => {
+					trace!(target: "sync", "New hash block already queued {:?}", h);
+				},
+				BlockStatus::Unknown => {
+					trace!(target: "sync", "New unknown block hash {:?}", h);
+					if d > max_height {
+						let peer = self.peers.get_mut(&peer_id).expect("ChainSync: unknown peer");
+						peer.latest = h.clone();
+						max_height = d;
+					}
+				},
+				BlockStatus::Bad =>{
+					debug!(target: "sync", "Bad new block hash {:?}", h);
+					io.disable_peer(peer_id);
+					return Ok(());
+				}
+			}
+		};
+		if max_height != x!(0) {
+			self.sync_peer(io, peer_id, true);
+		}
+		Ok(())
+	}
+
+	/// Called by peer when it is disconnecting
+	pub fn on_peer_aborting(&mut self, io: &mut SyncIo, peer: PeerId) {
+		trace!(target: "sync", "== Disconnecting {}", peer);
+		if self.peers.contains_key(&peer) {
+			info!(target: "sync", "Disconnected {}:{}", peer, io.peer_info(peer));
+			self.clear_peer_download(peer);
+			self.peers.remove(&peer);
+			self.continue_sync(io);
+		}
+	}
+
+	/// Called when a new peer is connected
+	pub fn on_peer_connected(&mut self, io: &mut SyncIo, peer: PeerId) {
+		trace!(target: "sync", "== Connected {}", peer);
+		self.send_status(io, peer);
+	}
+
+	/// Resume downloading
+	fn continue_sync(&mut self, io: &mut SyncIo) {
+		let mut peers: Vec<(PeerId, U256)> = self.peers.iter().map(|(k, p)| (*k, p.difficulty)).collect();
+		peers.sort_by(|&(_, d1), &(_, d2)| d1.cmp(&d2).reverse()); //TODO: sort by rating
+		for (p, _) in peers {
+			self.sync_peer(io, p, false);
+		}
+	}
+
+	/// Called after all blocks have been donloaded
+	fn complete_sync(&mut self) {
+		trace!(target: "sync", "Sync complete");
+		self.reset();
+		self.state = SyncState::Idle;
+	}
+
+	/// Enter waiting state
+	fn pause_sync(&mut self) {
+		trace!(target: "sync", "Block queue full, pausing sync");
+		self.state = SyncState::Waiting;
+	}
+
+	/// Find something to do for a peer. Called for a new peer or when a peer is done with it's task.
+	fn sync_peer(&mut self, io: &mut SyncIo,  peer_id: PeerId, force: bool) {
+		let (peer_latest, peer_difficulty) = {
+			let peer = self.peers.get_mut(&peer_id).expect("ChainSync: unknown peer");
+			if peer.asking != PeerAsking::Nothing {
+				return;
+			}
+			if self.state == SyncState::Waiting {
+				trace!(target: "sync", "Waiting for block queue");
+				return;
+			}
+			(peer.latest.clone(), peer.difficulty.clone())
+		};
+
+		let td = io.chain().chain_info().pending_total_difficulty;
+		let syncing_difficulty = max(self.syncing_difficulty, td);
+		if force || peer_difficulty > syncing_difficulty {
+			// start sync
+			self.syncing_difficulty = peer_difficulty;
+			if self.state == SyncState::Idle || self.state == SyncState::NotSynced {
+				self.state = SyncState::Blocks;
+			}
+			trace!(target: "sync", "Starting sync with better chain");
+			self.request_headers_by_hash(io, peer_id, &peer_latest, 1, 0, false);
+		}
+		else if self.state == SyncState::Blocks {
+			self.request_blocks(io, peer_id);
+		}
+	}
+
+	/// Find some headers or blocks to download for a peer.
+	fn request_blocks(&mut self, io: &mut SyncIo, peer_id: PeerId) {
+		self.clear_peer_download(peer_id);
+
+		if io.chain().queue_info().full {
+			self.pause_sync();
+			return;
+		}
+
+		// check to see if we need to download any block bodies first
+		let mut needed_bodies: Vec<H256> = Vec::new();
+		let mut needed_numbers: Vec<BlockNumber> = Vec::new();
+
+		if self.have_common_block && !self.headers.is_empty() && self.headers.range_iter().next().unwrap().0 == self.last_imported_block + 1 {
+			for (start, ref items) in self.headers.range_iter() {
+				if needed_bodies.len() > MAX_BODIES_TO_REQUEST {
+					break;
+				}
+				let mut index: BlockNumber = 0;
+				while index != items.len() as BlockNumber && needed_bodies.len() < MAX_BODIES_TO_REQUEST {
+					let block = start + index;
+					if !self.downloading_bodies.contains(&block) && !self.bodies.have_item(&block) {
+						needed_bodies.push(items[index as usize].hash.clone());
+						needed_numbers.push(block);
+						self.downloading_bodies.insert(block);
+					}
+					index += 1;
+				}
+			}
+		}
+		if !needed_bodies.is_empty() {
+			replace(&mut self.peers.get_mut(&peer_id).unwrap().asking_blocks, needed_numbers);
+			self.request_bodies(io, peer_id, needed_bodies);
+		}
+		else {
+			// check if need to download headers
+			let mut start = 0usize;
+			if !self.have_common_block {
+				// download backwards until common block is found 1 header at a time
+				let chain_info = io.chain().chain_info();
+				start = chain_info.best_block_number as usize;
+				if !self.headers.is_empty() {
+					start = min(start, self.headers.range_iter().next().unwrap().0 as usize - 1);
+				}
+				if start == 0 {
+					self.have_common_block = true; //reached genesis
+					self.last_imported_hash = chain_info.genesis_hash;
+				}
+			}
+			if self.have_common_block {
+				let mut headers: Vec<BlockNumber> = Vec::new();
+				let mut prev = self.last_imported_block + 1;
+				for (next, ref items) in self.headers.range_iter() {
+					if !headers.is_empty() {
+						break;
+					}
+					if next <= prev {
+						prev = next + items.len() as BlockNumber;
+						continue;
+					}
+					let mut block = prev;
+					while block < next && headers.len() <= MAX_HEADERS_TO_REQUEST {
+						if !self.downloading_headers.contains(&(block as BlockNumber)) {
+							headers.push(block as BlockNumber);
+							self.downloading_headers.insert(block as BlockNumber);
+						}
+						block += 1;
+					}
+					prev = next + items.len() as BlockNumber;
+				}
+
+				if !headers.is_empty() {
+					start = headers[0] as usize;
+					let count = headers.len();
+					replace(&mut self.peers.get_mut(&peer_id).unwrap().asking_blocks, headers);
+					assert!(!self.headers.have_item(&(start as BlockNumber)));
+					self.request_headers_by_number(io, peer_id, start as BlockNumber, count, 0, false);
+				}
+			}
+			else {
+				self.request_headers_by_number(io, peer_id, start as BlockNumber, 1, 0, false);
+			}
+		}
+	}
+
+	/// Clear all blocks/headers marked as being downloaded by a peer.
+	fn clear_peer_download(&mut self, peer_id: PeerId) {
+		let peer = self.peers.get_mut(&peer_id).expect("ChainSync: unknown peer");
+		for b in &peer.asking_blocks {
+			self.downloading_headers.remove(&b);
+			self.downloading_bodies.remove(&b);
+		}
+		peer.asking_blocks.clear();
+	}
+
+	/// Checks if there are blocks fully downloaded that can be imported into the blockchain and does the import.
+	fn collect_blocks(&mut self, io: &mut SyncIo) {
+		if !self.have_common_block || self.headers.is_empty() || self.bodies.is_empty() {
+			return;
+		}
+
+		let mut restart = false;
+		// merge headers and bodies
+		{
+			let headers = self.headers.range_iter().next().unwrap();
+			let bodies = self.bodies.range_iter().next().unwrap();
+			if headers.0 != bodies.0 || headers.0 != self.last_imported_block + 1 {
+				return;
+			}
+
+			let count = min(headers.1.len(), bodies.1.len());
+			let mut imported = 0;
+			for i in 0..count {
+				let mut block_rlp = RlpStream::new_list(3);
+				block_rlp.append_raw(&headers.1[i].data, 1);
+				let body = Rlp::new(&bodies.1[i]);
+				block_rlp.append_raw(body.at(0).as_raw(), 1);
+				block_rlp.append_raw(body.at(1).as_raw(), 1);
+				let h = &headers.1[i].hash;
+				match io.chain().import_block(block_rlp.out()) {
+					Err(ImportError::AlreadyInChain) => {
+						trace!(target: "sync", "Block already in chain {:?}", h);
+						self.last_imported_block = headers.0 + i as BlockNumber;
+						self.last_imported_hash = h.clone();
+					},
+					Err(ImportError::AlreadyQueued) => {
+						trace!(target: "sync", "Block already queued {:?}", h);
+						self.last_imported_block = headers.0 + i as BlockNumber;
+						self.last_imported_hash = h.clone();
+					},
+					Ok(_) => {
+						trace!(target: "sync", "Block queued {:?}", h);
+						self.last_imported_block = headers.0 + i as BlockNumber;
+						self.last_imported_hash = h.clone();
+						imported += 1;
+					},
+					Err(e) => {
+						debug!(target: "sync", "Bad block {:?} : {:?}", h, e);
+						restart = true;
+					}
+				}
+			}
+			trace!(target: "sync", "Imported {} of {}", imported, count);
+		}
+
+		if restart {
+			self.restart(io);
+			return;
+		}
+
+		self.headers.remove_head(&(self.last_imported_block + 1));
+		self.bodies.remove_head(&(self.last_imported_block + 1));
+
+		if self.headers.is_empty() {
+			assert!(self.bodies.is_empty());
+			self.complete_sync();
+		}
+	}
+
+	/// Remove downloaded bocks/headers starting from specified number. 
+	/// Used to recover from an error and re-download parts of the chain detected as bad.
+	fn remove_downloaded_blocks(&mut self, start: BlockNumber) {
+		for n in self.headers.get_tail(&start) {
+			if let Some(ref header_data) = self.headers.find_item(&n) {
+				let header_to_delete = HeaderView::new(&header_data.data);
+				let header_id = HeaderId {
+					transactions_root: header_to_delete.transactions_root(),
+					uncles: header_to_delete.uncles_hash()
+				};
+				self.header_ids.remove(&header_id);
+			}
+			self.downloading_bodies.remove(&n);
+			self.downloading_headers.remove(&n);
+		}
+		self.headers.remove_tail(&start);
+		self.bodies.remove_tail(&start);
+	}
+
+	/// Request headers from a peer by block hash
+	fn request_headers_by_hash(&mut self, sync: &mut SyncIo, peer_id: PeerId, h: &H256, count: usize, skip: usize, reverse: bool) {
+		trace!(target: "sync", "{} <- GetBlockHeaders: {} entries starting from {}", peer_id, count, h);
+		let mut rlp = RlpStream::new_list(4);
+		rlp.append(h);
+		rlp.append(&count);
+		rlp.append(&skip);
+		rlp.append(&if reverse {1u32} else {0u32});
+		self.send_request(sync, peer_id, PeerAsking::BlockHeaders, GET_BLOCK_HEADERS_PACKET, rlp.out());
+	}
+
+	/// Request headers from a peer by block number
+	fn request_headers_by_number(&mut self, sync: &mut SyncIo, peer_id: PeerId, n: BlockNumber, count: usize, skip: usize, reverse: bool) {
+		let mut rlp = RlpStream::new_list(4);
+		trace!(target: "sync", "{} <- GetBlockHeaders: {} entries starting from {}", peer_id, count, n);
+		rlp.append(&n);
+		rlp.append(&count);
+		rlp.append(&skip);
+		rlp.append(&if reverse {1u32} else {0u32});
+		self.send_request(sync, peer_id, PeerAsking::BlockHeaders, GET_BLOCK_HEADERS_PACKET, rlp.out());
+	}
+
+	/// Request block bodies from a peer
+	fn request_bodies(&mut self, sync: &mut SyncIo, peer_id: PeerId, hashes: Vec<H256>) {
+		let mut rlp = RlpStream::new_list(hashes.len());
+		trace!(target: "sync", "{} <- GetBlockBodies: {} entries", peer_id, hashes.len());
+		for h in hashes {
+			rlp.append(&h);
+		}
+		self.send_request(sync, peer_id, PeerAsking::BlockBodies, GET_BLOCK_BODIES_PACKET, rlp.out());
+	}
+
+	/// Reset peer status after request is complete.
+	fn reset_peer_asking(&mut self, peer_id: PeerId, asking: PeerAsking) {
+		let peer = self.peers.get_mut(&peer_id).expect("ChainSync: unknown peer");
+		if peer.asking != asking {
+			warn!(target:"sync", "Asking {:?} while expected {:?}", peer.asking, asking);
+		}
+		else {
+			peer.asking = PeerAsking::Nothing;
+		}
+	}
+
+	/// Generic request sender
+	fn send_request(&mut self, sync: &mut SyncIo, peer_id: PeerId, asking: PeerAsking,  packet_id: PacketId, packet: Bytes) {
+		{
+			let peer = self.peers.get_mut(&peer_id).expect("ChainSync: unknown peer");
+			if peer.asking != PeerAsking::Nothing {
+				warn!(target:"sync", "Asking {:?} while requesting {:?}", asking, peer.asking);
+			}
+		}
+		match sync.send(peer_id, packet_id, packet) {
+			Err(e) => {
+				warn!(target:"sync", "Error sending request: {:?}", e);
+				sync.disable_peer(peer_id);
+				self.on_peer_aborting(sync, peer_id);
+			}
+			Ok(_) => {
+				let mut peer = self.peers.get_mut(&peer_id).unwrap();
+				peer.asking = asking;
+			}
+		}
+	}
+
+	/// Called when peer sends us new transactions
+	fn on_peer_transactions(&mut self, _io: &mut SyncIo, _peer_id: PeerId, _r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		Ok(())
+	}
+
+	/// Send Status message
+	fn send_status(&mut self, io: &mut SyncIo, peer_id: PeerId) {
+		let mut packet = RlpStream::new_list(5);
+		let chain = io.chain().chain_info();
+		packet.append(&(PROTOCOL_VERSION as u32));
+		packet.append(&NETWORK_ID); //TODO: network id
+		packet.append(&chain.total_difficulty);
+		packet.append(&chain.best_block_hash);
+		packet.append(&chain.genesis_hash);
+		//TODO: handle timeout for status request
+		if let Err(e) = io.send(peer_id, STATUS_PACKET, packet.out()) {
+			warn!(target:"sync", "Error sending status request: {:?}", e);
+			io.disable_peer(peer_id);
+		}
+	}
+
+	/// Respond to GetBlockHeaders request
+	fn return_block_headers(&self, io: &mut SyncIo, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		// Packet layout:
+		// [ block: { P , B_32 }, maxHeaders: P, skip: P, reverse: P in { 0 , 1 } ]
+		let max_headers: usize = try!(r.val_at(1));
+		let skip: usize = try!(r.val_at(2));
+		let reverse: bool = try!(r.val_at(3));
+		let last = io.chain().chain_info().best_block_number;
+		let mut number = if try!(r.at(0)).size() == 32 {
+			// id is a hash
+			let hash: H256 = try!(r.val_at(0));
+			trace!(target: "sync", "-> GetBlockHeaders (hash: {}, max: {}, skip: {}, reverse:{})", hash, max_headers, skip, reverse);
+			match io.chain().block_header(&hash) {
+				Some(hdr) => From::from(HeaderView::new(&hdr).number()),
+				None => last
+			}
+		}
+		else {
+			trace!(target: "sync", "-> GetBlockHeaders (number: {}, max: {}, skip: {}, reverse:{})", try!(r.val_at::<BlockNumber>(0)), max_headers, skip, reverse);
+			try!(r.val_at(0))
+		};
+
+		if reverse {
+			number = min(last, number);
+		} else {
+			number = max(1, number);
+		}
+		let max_count = min(MAX_HEADERS_TO_SEND, max_headers);
+		let mut count = 0;
+		let mut data = Bytes::new();
+		let inc = (skip + 1) as BlockNumber;
+		while number <= last && number > 0 && count < max_count {
+			if let Some(mut hdr) = io.chain().block_header_at(number) {
+				data.append(&mut hdr);
+				count += 1;
+			}
+			if reverse {
+				if number <= inc {
+					break;
+				}
+				number -= inc;
+			}
+			else {
+				number += inc;
+			}
+		}
+		let mut rlp = RlpStream::new_list(count as usize);
+		rlp.append_raw(&data, count as usize);
+		io.respond(BLOCK_HEADERS_PACKET, rlp.out()).unwrap_or_else(|e|
+			debug!(target: "sync", "Error sending headers: {:?}", e));
+		trace!(target: "sync", "-> GetBlockHeaders: returned {} entries", count);
+		Ok(())
+	}
+
+	/// Respond to GetBlockBodies request
+	fn return_block_bodies(&self, io: &mut SyncIo, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		let mut count = r.item_count();
+		if count == 0 {
+			debug!(target: "sync", "Empty GetBlockBodies request, ignoring.");
+			return Ok(());
+		}
+		trace!(target: "sync", "-> GetBlockBodies: {} entries", count);
+		count = min(count, MAX_BODIES_TO_SEND);
+		let mut added = 0usize;
+		let mut data = Bytes::new();
+		for i in 0..count {
+			if let Some(mut hdr) = io.chain().block_body(&try!(r.val_at::<H256>(i))) {
+				data.append(&mut hdr);
+				added += 1;
+			}
+		}
+		let mut rlp = RlpStream::new_list(added);
+		rlp.append_raw(&data, added);
+		io.respond(BLOCK_BODIES_PACKET, rlp.out()).unwrap_or_else(|e|
+			debug!(target: "sync", "Error sending headers: {:?}", e));
+		trace!(target: "sync", "-> GetBlockBodies: returned {} entries", added);
+		Ok(())
+	}
+
+	/// Respond to GetNodeData request
+	fn return_node_data(&self, io: &mut SyncIo, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		let mut count = r.item_count();
+		if count == 0 {
+			debug!(target: "sync", "Empty GetNodeData request, ignoring.");
+			return Ok(());
+		}
+		count = min(count, MAX_NODE_DATA_TO_SEND);
+		let mut added = 0usize;
+		let mut data = Bytes::new();
+		for i in 0..count {
+			if let Some(mut hdr) = io.chain().state_data(&try!(r.val_at::<H256>(i))) {
+				data.append(&mut hdr);
+				added += 1;
+			}
+		}
+		let mut rlp = RlpStream::new_list(added);
+		rlp.append_raw(&data, added);
+		io.respond(NODE_DATA_PACKET, rlp.out()).unwrap_or_else(|e|
+			debug!(target: "sync", "Error sending headers: {:?}", e));
+		Ok(())
+	}
+
+	/// Respond to GetReceipts request
+	fn return_receipts(&self, io: &mut SyncIo, r: &UntrustedRlp) -> Result<(), PacketDecodeError> {
+		let mut count = r.item_count();
+		if count == 0 {
+			debug!(target: "sync", "Empty GetReceipts request, ignoring.");
+			return Ok(());
+		}
+		count = min(count, MAX_RECEIPTS_TO_SEND);
+		let mut added = 0usize;
+		let mut data = Bytes::new();
+		for i in 0..count {
+			if let Some(mut hdr) = io.chain().block_receipts(&try!(r.val_at::<H256>(i))) {
+				data.append(&mut hdr);
+				added += 1;
+			}
+		}
+		let mut rlp = RlpStream::new_list(added);
+		rlp.append_raw(&data, added);
+		io.respond(RECEIPTS_PACKET, rlp.out()).unwrap_or_else(|e|
+			debug!(target: "sync", "Error sending headers: {:?}", e));
+		Ok(())
+	}
+
+	/// Dispatch incoming requests and responses
+	pub fn on_packet(&mut self, io: &mut SyncIo, peer: PeerId, packet_id: u8, data: &[u8]) {
+		let rlp = UntrustedRlp::new(data);
+		let result = match packet_id {
+			STATUS_PACKET => self.on_peer_status(io, peer, &rlp),
+			TRANSACTIONS_PACKET => self.on_peer_transactions(io, peer, &rlp),
+			GET_BLOCK_HEADERS_PACKET => self.return_block_headers(io, &rlp),
+			BLOCK_HEADERS_PACKET => self.on_peer_block_headers(io, peer, &rlp),
+			GET_BLOCK_BODIES_PACKET => self.return_block_bodies(io, &rlp),
+			BLOCK_BODIES_PACKET => self.on_peer_block_bodies(io, peer, &rlp),
+			NEW_BLOCK_PACKET => self.on_peer_new_block(io, peer, &rlp),
+			NEW_BLOCK_HASHES_PACKET => self.on_peer_new_hashes(io, peer, &rlp),
+			GET_NODE_DATA_PACKET => self.return_node_data(io, &rlp),
+			GET_RECEIPTS_PACKET => self.return_receipts(io, &rlp),
+			_ => { 
+				debug!(target: "sync", "Unknown packet {}", packet_id);
+				Ok(())
+			}
+		};
+		result.unwrap_or_else(|e| {
+			debug!(target:"sync", "{} -> Malformed packet {} : {}", peer, packet_id, e);
+		})
+	}
+
+	/// Maintain other peers. Send out any new blocks and transactions
+	pub fn _maintain_sync(&mut self, _io: &mut SyncIo) {
+	}
+}
+
--- a/sync/src/io.rs
+++ b/sync/src/io.rs
@ -0,0 +1,62 @@
+use ethcore::client::BlockChainClient;
+use util::{NetworkContext, PeerId, PacketId,};
+use util::error::UtilError;
+use ethcore::service::SyncMessage;
+
+/// IO interface for the syning handler.
+/// Provides peer connection management and an interface to the blockchain client.
+// TODO: ratings
+pub trait SyncIo {
+	/// Disable a peer
+	fn disable_peer(&mut self, peer_id: PeerId);
+	/// Respond to current request with a packet. Can be called from an IO handler for incoming packet.
+	fn respond(&mut self, packet_id: PacketId, data: Vec<u8>) -> Result<(), UtilError>;
+	/// Send a packet to a peer.
+	fn send(&mut self, peer_id: PeerId, packet_id: PacketId, data: Vec<u8>) -> Result<(), UtilError>;
+	/// Get the blockchain
+	fn chain(&self) -> &BlockChainClient;
+	/// Returns peer client identifier string
+	fn peer_info(&self, peer_id: PeerId) -> String {
+		peer_id.to_string()
+	}
+}
+
+/// Wraps `NetworkContext` and the blockchain client
+pub struct NetSyncIo<'s, 'h> where 'h: 's {
+	network: &'s NetworkContext<'h, SyncMessage>,
+	chain: &'s BlockChainClient
+}
+
+impl<'s, 'h> NetSyncIo<'s, 'h> {
+	/// Creates a new instance from the `NetworkContext` and the blockchain client reference.
+	pub fn new(network: &'s NetworkContext<'h, SyncMessage>, chain: &'s BlockChainClient) -> NetSyncIo<'s, 'h> {
+		NetSyncIo {
+			network: network,
+			chain: chain,
+		}
+	}
+}
+
+impl<'s, 'h> SyncIo for NetSyncIo<'s, 'h> {
+	fn disable_peer(&mut self, peer_id: PeerId) {
+		self.network.disable_peer(peer_id);
+	}
+
+	fn respond(&mut self, packet_id: PacketId, data: Vec<u8>) -> Result<(), UtilError>{
+		self.network.respond(packet_id, data)
+	}
+
+	fn send(&mut self, peer_id: PeerId, packet_id: PacketId, data: Vec<u8>) -> Result<(), UtilError>{
+		self.network.send(peer_id, packet_id, data)
+	}
+
+	fn chain(&self) -> &BlockChainClient {
+		self.chain
+	}
+
+	fn peer_info(&self, peer_id: PeerId) -> String {
+		self.network.peer_info(peer_id)
+	}
+}
+
+
--- a/sync/src/lib.rs
+++ b/sync/src/lib.rs
@ -0,0 +1,106 @@
+#![warn(missing_docs)]
+#![feature(plugin)]
+#![plugin(clippy)]
+#![feature(augmented_assignments)]
+//! Blockchain sync module
+//! Implements ethereum protocol version 63 as specified here:
+//! https://github.com/ethereum/wiki/wiki/Ethereum-Wire-Protocol
+//!
+//! Usage example:
+//!
+//! ```rust
+//! extern crate ethcore_util as util;
+//! extern crate ethcore;
+//! extern crate ethsync;
+//! use std::env;
+//! use std::sync::Arc;
+//! use util::network::{NetworkService, NetworkConfiguration};
+//! use ethcore::client::Client;
+//! use ethsync::EthSync;
+//! use ethcore::ethereum;
+//!
+//! fn main() {
+//! 	let mut service = NetworkService::start(NetworkConfiguration::new()).unwrap();
+//! 	let dir = env::temp_dir();
+//! 	let client = Client::new(ethereum::new_frontier(), &dir, service.io().channel()).unwrap();
+//! 	EthSync::register(&mut service, client);
+//! }
+//! ```
+
+#[macro_use]
+extern crate log;
+#[macro_use]
+extern crate ethcore_util as util;
+extern crate ethcore;
+extern crate env_logger;
+
+use std::ops::*;
+use std::sync::*;
+use ethcore::client::Client;
+use util::network::{NetworkProtocolHandler, NetworkService, NetworkContext, PeerId};
+use chain::ChainSync;
+use ethcore::service::SyncMessage;
+use io::NetSyncIo;
+
+mod chain;
+mod io;
+mod range_collection;
+
+#[cfg(test)]
+mod tests;
+
+/// Ethereum network protocol handler
+pub struct EthSync {
+	/// Shared blockchain client. TODO: this should evetually become an IPC endpoint
+	chain: Arc<Client>,
+	/// Sync strategy
+	sync: RwLock<ChainSync>
+}
+
+pub use self::chain::SyncStatus;
+
+impl EthSync {
+	/// Creates and register protocol with the network service
+	pub fn register(service: &mut NetworkService<SyncMessage>, chain: Arc<Client>) -> Arc<EthSync> {
+		let sync = Arc::new(EthSync {
+			chain: chain,
+			sync: RwLock::new(ChainSync::new()),
+		});
+		service.register_protocol(sync.clone(), "eth", &[62u8, 63u8]).expect("Error registering eth protocol handler");
+		sync
+	}
+
+	/// Get sync status
+	pub fn status(&self) -> SyncStatus {
+		self.sync.read().unwrap().status()
+	}
+
+	/// Stop sync
+	pub fn stop(&mut self, io: &mut NetworkContext<SyncMessage>) {
+		self.sync.write().unwrap().abort(&mut NetSyncIo::new(io, self.chain.deref()));
+	}
+
+	/// Restart sync
+	pub fn restart(&mut self, io: &mut NetworkContext<SyncMessage>) {
+		self.sync.write().unwrap().restart(&mut NetSyncIo::new(io, self.chain.deref()));
+	}
+}
+
+impl NetworkProtocolHandler<SyncMessage> for EthSync {
+	fn initialize(&self, _io: &NetworkContext<SyncMessage>) {
+	}
+
+	fn read(&self, io: &NetworkContext<SyncMessage>, peer: &PeerId, packet_id: u8, data: &[u8]) {
+		self.sync.write().unwrap().on_packet(&mut NetSyncIo::new(io, self.chain.deref()) , *peer, packet_id, data);
+	}
+
+	fn connected(&self, io: &NetworkContext<SyncMessage>, peer: &PeerId) {
+		self.sync.write().unwrap().on_peer_connected(&mut NetSyncIo::new(io, self.chain.deref()), *peer);
+	}
+
+	fn disconnected(&self, io: &NetworkContext<SyncMessage>, peer: &PeerId) {
+		self.sync.write().unwrap().on_peer_aborting(&mut NetSyncIo::new(io, self.chain.deref()), *peer);
+	}
+}
+
+
--- a/sync/src/range_collection.rs
+++ b/sync/src/range_collection.rs
@ -0,0 +1,260 @@
+/// This module defines a trait for a collection of ranged values and an implementation
+/// for this trait over sorted vector.
+
+use std::ops::{Add, Sub, Range};
+
+pub trait ToUsize {
+	fn to_usize(&self) -> usize;
+}
+
+pub trait FromUsize {
+	fn from_usize(s: usize) -> Self;
+}
+
+/// A key-value collection orderd by key with sequential key-value pairs grouped together.
+/// Such group is called a range.
+/// E.g. a set of collection of 5 pairs {1, a}, {2, b}, {10, x}, {11, y}, {12, z} will be grouped into two ranges: {1, [a,b]}, {10, [x,y,z]}
+pub trait RangeCollection<K, V> {
+	/// Check if the given key is present in the collection.
+	fn have_item(&self, key: &K) -> bool;
+	/// Get value by key.
+	fn find_item(&self, key: &K) -> Option<&V>;
+	/// Get a range of keys from `key` till the end of the range that has `key`
+	/// Returns an empty range is key does not exist.
+	fn get_tail(&mut self, key: &K) -> Range<K>;
+	/// Remove all elements < `start` in the range that contains `start` - 1
+	fn remove_head(&mut self, start: &K);
+	/// Remove all elements >= `start` in the range that contains `start` 
+	fn remove_tail(&mut self, start: &K);
+	/// Remove all elements >= `tail`
+	fn insert_item(&mut self, key: K, value: V);
+	/// Get an iterator over ranges
+	fn range_iter(& self) -> RangeIterator<K, V>;
+}
+
+/// Range iterator. For each range yelds a key for the first element of the range and a vector of values.
+pub struct RangeIterator<'c, K:'c, V:'c> {
+	range: usize,
+	collection: &'c Vec<(K, Vec<V>)>
+}
+
+impl<'c, K:'c, V:'c> Iterator for RangeIterator<'c, K, V> where K: Add<Output = K> + FromUsize + ToUsize + Copy {
+    type Item = (K, &'c [V]);
+    // The 'Iterator' trait only requires the 'next' method to be defined. The
+    // return type is 'Option<T>', 'None' is returned when the 'Iterator' is
+    // over, otherwise the next value is returned wrapped in 'Some'
+    fn next(&mut self) -> Option<(K, &'c [V])> {
+		if self.range > 0 {
+			self.range -= 1;
+		}
+		else {
+			return None;
+		}
+		match self.collection.get(self.range) {
+			Some(&(ref k, ref vec)) => {
+				Some((*k, &vec))
+			},
+			None => None
+		}
+    }
+}
+
+impl<K, V> RangeCollection<K, V> for Vec<(K, Vec<V>)> where K: Ord + PartialEq + Add<Output = K> + Sub<Output = K> + Copy + FromUsize + ToUsize {
+	fn range_iter(&self) -> RangeIterator<K, V> {
+		RangeIterator {
+			range: self.len(),
+			collection: self
+		}
+	}
+
+	fn have_item(&self, key: &K) -> bool {
+		match self.binary_search_by(|&(k, _)| k.cmp(key).reverse()) {
+			Ok(_) => true,
+			Err(index) => match self.get(index) {
+				Some(&(ref k, ref v)) => k <= key && (*k + FromUsize::from_usize(v.len())) > *key,
+				_ => false
+			},
+		}
+	}
+
+	fn find_item(&self, key: &K) -> Option<&V> {
+		match self.binary_search_by(|&(k, _)| k.cmp(key).reverse()) {
+			Ok(index) => self.get(index).unwrap().1.get(0),
+			Err(index) => match self.get(index) {
+				Some(&(ref k, ref v)) if k <= key && (*k + FromUsize::from_usize(v.len())) > *key => v.get((*key - *k).to_usize()),
+				_ => None
+			},
+		}
+	}
+
+	fn get_tail(&mut self, key: &K) -> Range<K> {
+		let kv = *key;
+		match self.binary_search_by(|&(k, _)| k.cmp(key).reverse()) {
+			Ok(index) => kv..(kv + FromUsize::from_usize(self[index].1.len())),
+			Err(index) => {
+				match self.get_mut(index) {
+					Some(&mut (ref k, ref mut v)) if k <= key && (*k + FromUsize::from_usize(v.len())) > *key => {
+						kv..(*k + FromUsize::from_usize(v.len()))
+					}
+					_ => kv..kv
+				}
+			},
+		}
+	}
+	/// Remove element key and following elements in the same range
+	fn remove_tail(&mut self, key: &K) {
+		match self.binary_search_by(|&(k, _)| k.cmp(key).reverse()) {
+			Ok(index) => { self.remove(index); },
+			Err(index) =>{
+				let mut empty = false;
+				match self.get_mut(index) {
+					Some(&mut (ref k, ref mut v)) if k <= key && (*k + FromUsize::from_usize(v.len())) > *key => {
+						v.truncate((*key - *k).to_usize());
+						empty = v.is_empty();
+					}
+					_ => {}
+				}
+				if empty {
+					self.remove(index);
+				}
+			},
+		}
+	}
+
+	/// Remove range elements up to key
+	fn remove_head(&mut self, key: &K) {
+		if *key == FromUsize::from_usize(0) {
+			return
+		}
+
+		let prev = *key - FromUsize::from_usize(1);
+		match self.binary_search_by(|&(k, _)| k.cmp(key).reverse()) {
+			Ok(_) => { }, //start of range, do nothing.
+			Err(index) => {
+				let mut empty = false;
+				match self.get_mut(index) {
+					Some(&mut (ref mut k, ref mut v)) if *k <= prev && (*k + FromUsize::from_usize(v.len())) > prev => {
+						let tail = v.split_off((*key - *k).to_usize());
+						empty = tail.is_empty();
+						let removed = ::std::mem::replace(v, tail);
+						let new_k = *k + FromUsize::from_usize(removed.len());
+						::std::mem::replace(k, new_k);
+					}
+					_ => {}
+				}
+				if empty {
+					self.remove(index);
+				}
+			},
+		}
+	}
+
+	fn insert_item(&mut self, key: K, value: V) {
+		assert!(!self.have_item(&key));
+
+		let lower = match self.binary_search_by(|&(k, _)| k.cmp(&key).reverse()) {
+			Ok(index) => index,
+			Err(index) => index,
+		};
+
+		let mut to_remove: Option<usize> = None;
+		if lower < self.len() && self[lower].0 + FromUsize::from_usize(self[lower].1.len()) == key {
+				// extend into existing chunk
+				self[lower].1.push(value);
+		}
+		else {
+			// insert a new chunk
+			let range: Vec<V> = vec![value];
+			self.insert(lower, (key, range));
+		};
+		if lower > 0 {
+			let next = lower - 1;
+			if next < self.len()
+			{
+				{
+					let (mut next, mut inserted) = self.split_at_mut(lower);
+					let mut next = next.last_mut().unwrap();
+					let mut inserted = inserted.first_mut().unwrap();
+					if next.0 == key + FromUsize::from_usize(1)
+					{
+						inserted.1.append(&mut next.1);
+						to_remove = Some(lower - 1);
+					}
+				}
+
+				if let Some(r) = to_remove {
+					self.remove(r);
+				}
+			}
+		}
+	}
+}
+
+#[test]
+#[allow(cyclomatic_complexity)]
+fn test_range() {
+	use std::cmp::{Ordering};
+
+	let mut ranges: Vec<(u64, Vec<char>)> = Vec::new();
+	assert_eq!(ranges.range_iter().next(), None);
+	assert_eq!(ranges.find_item(&1), None);
+	assert!(!ranges.have_item(&1));
+	assert_eq!(ranges.get_tail(&0), 0..0);
+
+	ranges.insert_item(17, 'q');
+	assert_eq!(ranges.range_iter().cmp(vec![(17, &['q'][..])]),  Ordering::Equal);
+	assert_eq!(ranges.find_item(&17), Some(&'q'));
+	assert!(ranges.have_item(&17));
+	assert_eq!(ranges.get_tail(&17), 17..18);
+
+	ranges.insert_item(18, 'r');
+	assert_eq!(ranges.range_iter().cmp(vec![(17, &['q', 'r'][..])]),  Ordering::Equal);
+	assert_eq!(ranges.find_item(&18), Some(&'r'));
+	assert!(ranges.have_item(&18));
+	assert_eq!(ranges.get_tail(&17), 17..19);
+
+	ranges.insert_item(16, 'p');
+	assert_eq!(ranges.range_iter().cmp(vec![(16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+	assert_eq!(ranges.find_item(&16), Some(&'p'));
+	assert_eq!(ranges.find_item(&17), Some(&'q'));
+	assert_eq!(ranges.find_item(&18), Some(&'r'));
+	assert!(ranges.have_item(&16));
+	assert_eq!(ranges.get_tail(&17), 17..19);
+	assert_eq!(ranges.get_tail(&16), 16..19);
+
+	ranges.insert_item(2, 'b');
+	assert_eq!(ranges.range_iter().cmp(vec![(2, &['b'][..]),  (16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+	assert_eq!(ranges.find_item(&2), Some(&'b'));
+
+	ranges.insert_item(3, 'c');
+	ranges.insert_item(4, 'd');
+	assert_eq!(ranges.get_tail(&3), 3..5);
+	assert_eq!(ranges.range_iter().cmp(vec![(2, &['b', 'c', 'd'][..]),  (16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+
+	let mut r = ranges.clone();
+	r.remove_head(&1);
+	assert_eq!(r.range_iter().cmp(vec![(2, &['b', 'c', 'd'][..]),  (16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+	r.remove_head(&2);
+	assert_eq!(r.range_iter().cmp(vec![(2, &['b', 'c', 'd'][..]),  (16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+	r.remove_head(&3);
+	assert_eq!(r.range_iter().cmp(vec![(3, &['c', 'd'][..]),  (16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+	r.remove_head(&10);
+	assert_eq!(r.range_iter().cmp(vec![(3, &['c', 'd'][..]),  (16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+	r.remove_head(&5);
+	assert_eq!(r.range_iter().cmp(vec![(16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+	r.remove_head(&19);
+	assert_eq!(r.range_iter().next(), None);
+
+	let mut r = ranges.clone();
+	r.remove_tail(&20);
+	assert_eq!(r.range_iter().cmp(vec![(2, &['b', 'c', 'd'][..]),  (16, &['p', 'q', 'r'][..])]),  Ordering::Equal);
+	r.remove_tail(&17);
+	assert_eq!(r.range_iter().cmp(vec![(2, &['b', 'c', 'd'][..]),  (16, &['p'][..])]),  Ordering::Equal);
+	r.remove_tail(&16);
+	assert_eq!(r.range_iter().cmp(vec![(2, &['b', 'c', 'd'][..])]),  Ordering::Equal);
+	r.remove_tail(&3);
+	assert_eq!(r.range_iter().cmp(vec![(2, &['b'][..])]),  Ordering::Equal);
+	r.remove_tail(&2);
+	assert_eq!(r.range_iter().next(), None);
+}
+
--- a/sync/src/service.rs
+++ b/sync/src/service.rs
@ -0,0 +1,104 @@
+use util::*;
+use sync::*;
+use spec::Spec;
+use error::*;
+use std::env;
+use client::Client;
+
+/// Message type for external and internal events
+#[derive(Clone)]
+pub enum SyncMessage {
+	/// New block has been imported into the blockchain
+	NewChainBlock(Bytes), //TODO: use Cow
+	/// A block is ready 
+	BlockVerified,
+}
+
+/// TODO [arkpar] Please document me
+pub type NetSyncMessage = NetworkIoMessage<SyncMessage>;
+
+/// Client service setup. Creates and registers client and network services with the IO subsystem.
+pub struct ClientService {
+	net_service: NetworkService<SyncMessage>,
+	client: Arc<Client>,
+	sync: Arc<EthSync>,
+}
+
+impl ClientService {
+	/// Start the service in a separate thread.
+	pub fn start(spec: Spec, net_config: NetworkConfiguration) -> Result<ClientService, Error> {
+		let mut net_service = try!(NetworkService::start(net_config));
+		info!("Starting {}", net_service.host_info());
+		info!("Configured for {} using {} engine", spec.name, spec.engine_name);
+		let mut dir = env::home_dir().unwrap();
+		dir.push(".parity");
+		dir.push(H64::from(spec.genesis_header().hash()).hex());
+		let client = try!(Client::new(spec, &dir, net_service.io().channel()));
+		let sync = EthSync::register(&mut net_service, client.clone());
+		let client_io = Arc::new(ClientIoHandler {
+			client: client.clone()
+		});
+		try!(net_service.io().register_handler(client_io));
+
+		Ok(ClientService {
+			net_service: net_service,
+			client: client,
+			sync: sync,
+		})
+	}
+
+	/// Get the network service.
+	pub fn add_node(&mut self, _enode: &str) {
+		unimplemented!();
+	}
+
+	/// TODO [arkpar] Please document me
+	pub fn io(&mut self) -> &mut IoService<NetSyncMessage> {
+		self.net_service.io()
+	}
+
+	/// TODO [arkpar] Please document me
+	pub fn client(&self) -> Arc<Client> {
+		self.client.clone()
+	
+	}
+	
+	/// Get shared sync handler
+	pub fn sync(&self) -> Arc<EthSync> {
+		self.sync.clone()
+	}
+}
+
+/// IO interface for the Client handler
+struct ClientIoHandler {
+	client: Arc<Client>
+}
+
+const CLIENT_TICK_TIMER: TimerToken = 0;
+const CLIENT_TICK_MS: u64 = 5000;
+
+impl IoHandler<NetSyncMessage> for ClientIoHandler {
+	fn initialize(&self, io: &IoContext<NetSyncMessage>) {
+		io.register_timer(CLIENT_TICK_TIMER, CLIENT_TICK_MS).expect("Error registering client timer");
+	}
+
+	fn timeout(&self, _io: &IoContext<NetSyncMessage>, timer: TimerToken) {
+		if timer == CLIENT_TICK_TIMER {
+			self.client.tick();
+		}
+	}
+
+	#[allow(match_ref_pats)]
+	#[allow(single_match)]
+	fn message(&self, io: &IoContext<NetSyncMessage>, net_message: &NetSyncMessage) {
+		if let &UserMessage(ref message) = net_message {
+			match message {
+				&SyncMessage::BlockVerified => {
+					self.client.import_verified_blocks(&io.channel());
+				},
+				_ => {}, // ignore other messages
+			}
+		}
+	}
+}
+
--- a/sync/src/tests.rs
+++ b/sync/src/tests.rs
@ -0,0 +1,349 @@
+use util::*;
+use ethcore::client::{BlockChainClient, BlockStatus, TreeRoute, BlockChainInfo};
+use ethcore::block_queue::BlockQueueInfo;
+use ethcore::header::{Header as BlockHeader, BlockNumber};
+use ethcore::error::*;
+use io::SyncIo;
+use chain::ChainSync;
+
+struct TestBlockChainClient {
+	blocks: RwLock<HashMap<H256, Bytes>>,
+ 	numbers: RwLock<HashMap<usize, H256>>,
+	genesis_hash: H256,
+	last_hash: RwLock<H256>,
+	difficulty: RwLock<U256>,
+}
+
+impl TestBlockChainClient {
+	fn new() -> TestBlockChainClient {
+
+		let mut client = TestBlockChainClient {
+			blocks: RwLock::new(HashMap::new()),
+			numbers: RwLock::new(HashMap::new()),
+			genesis_hash: H256::new(),
+			last_hash: RwLock::new(H256::new()),
+			difficulty: RwLock::new(From::from(0)),
+		};
+		client.add_blocks(1, true); // add genesis block
+		client.genesis_hash = client.last_hash.read().unwrap().clone();
+		client
+	}
+
+	pub fn add_blocks(&mut self, count: usize, empty: bool) {
+		let len = self.numbers.read().unwrap().len();
+		for n in len..(len + count) {
+			let mut header = BlockHeader::new();
+			header.difficulty = From::from(n);
+			header.parent_hash = self.last_hash.read().unwrap().clone();
+			header.number = n as BlockNumber;
+			let mut uncles = RlpStream::new_list(if empty {0} else {1});
+			if !empty {
+				uncles.append(&H256::from(&U256::from(n)));
+				header.uncles_hash = uncles.as_raw().sha3();
+			}
+			let mut rlp = RlpStream::new_list(3);
+			rlp.append(&header);
+			rlp.append_raw(&rlp::NULL_RLP, 1);
+			rlp.append_raw(uncles.as_raw(), 1);
+			self.import_block(rlp.as_raw().to_vec()).unwrap();
+		}
+	}
+}
+
+impl BlockChainClient for TestBlockChainClient {
+	fn block_total_difficulty(&self, _h: &H256) -> Option<U256> {
+		unimplemented!();
+	}
+
+	fn block_header(&self, h: &H256) -> Option<Bytes> {
+		self.blocks.read().unwrap().get(h).map(|r| Rlp::new(r).at(0).as_raw().to_vec())
+
+	}
+
+	fn block_body(&self, h: &H256) -> Option<Bytes> {
+		self.blocks.read().unwrap().get(h).map(|r| {
+			let mut stream = RlpStream::new_list(2);
+			stream.append_raw(Rlp::new(&r).at(1).as_raw(), 1);
+			stream.append_raw(Rlp::new(&r).at(2).as_raw(), 1);
+			stream.out()
+		})
+	}
+
+	fn block(&self, h: &H256) -> Option<Bytes> {
+		self.blocks.read().unwrap().get(h).cloned()
+	}
+
+	fn block_status(&self, h: &H256) -> BlockStatus {
+		match self.blocks.read().unwrap().get(h) {
+			Some(_) => BlockStatus::InChain,
+			None => BlockStatus::Unknown
+		}
+	}
+
+	fn block_total_difficulty_at(&self, _number: BlockNumber) -> Option<U256> {
+		unimplemented!();
+	}
+
+	fn block_header_at(&self, n: BlockNumber) -> Option<Bytes> {
+		self.numbers.read().unwrap().get(&(n as usize)).and_then(|h| self.block_header(h))
+	}
+
+	fn block_body_at(&self, n: BlockNumber) -> Option<Bytes> {
+		self.numbers.read().unwrap().get(&(n as usize)).and_then(|h| self.block_body(h))
+	}
+
+	fn block_at(&self, n: BlockNumber) -> Option<Bytes> {
+		self.numbers.read().unwrap().get(&(n as usize)).map(|h| self.blocks.read().unwrap().get(h).unwrap().clone())
+	}
+
+	fn block_status_at(&self, n: BlockNumber) -> BlockStatus {
+		if (n as usize) < self.blocks.read().unwrap().len() {
+			BlockStatus::InChain
+		} else {
+			BlockStatus::Unknown
+		}
+	}
+
+	fn tree_route(&self, _from: &H256, _to: &H256) -> Option<TreeRoute> {
+		Some(TreeRoute {
+			blocks: Vec::new(),
+			ancestor: H256::new(),
+			index: 0
+		})
+	}
+
+	fn state_data(&self, _h: &H256) -> Option<Bytes> {
+		None
+	}
+
+	fn block_receipts(&self, _h: &H256) -> Option<Bytes> {
+		None
+	}
+
+	fn import_block(&self, b: Bytes) -> ImportResult {
+		let header = Rlp::new(&b).val_at::<BlockHeader>(0);
+		let h = header.hash();
+		let number: usize = header.number as usize;
+		if number > self.blocks.read().unwrap().len() {
+			panic!("Unexpected block number. Expected {}, got {}", self.blocks.read().unwrap().len(), number);
+		}
+		if number > 0 {
+			match self.blocks.read().unwrap().get(&header.parent_hash) {
+				Some(parent) => {
+					let parent = Rlp::new(parent).val_at::<BlockHeader>(0);
+					if parent.number != (header.number - 1) {
+						panic!("Unexpected block parent");
+					}
+				},
+				None => {
+					panic!("Unknown block parent {:?} for block {}", header.parent_hash, number);
+				}
+			}
+		}
+		let len = self.numbers.read().unwrap().len();
+		if number == len {
+			*self.difficulty.write().unwrap().deref_mut() += header.difficulty;
+			mem::replace(self.last_hash.write().unwrap().deref_mut(), h.clone());
+			self.blocks.write().unwrap().insert(h.clone(), b);
+			self.numbers.write().unwrap().insert(number, h.clone());
+			let mut parent_hash = header.parent_hash;
+			if number > 0 {
+				let mut n = number - 1;
+				while n > 0 && self.numbers.read().unwrap()[&n] != parent_hash {
+					*self.numbers.write().unwrap().get_mut(&n).unwrap() = parent_hash.clone();
+					n -= 1;
+					parent_hash = Rlp::new(&self.blocks.read().unwrap()[&parent_hash]).val_at::<BlockHeader>(0).parent_hash;
+				}
+			}
+		}
+		else {
+			self.blocks.write().unwrap().insert(h.clone(), b.to_vec());
+		}
+		Ok(h)
+	}
+
+	fn queue_info(&self) -> BlockQueueInfo {
+		BlockQueueInfo {
+			full: false,
+			verified_queue_size: 0,
+			unverified_queue_size: 0,
+			verifying_queue_size: 0,
+		}
+	}
+
+	fn clear_queue(&self) {
+	}
+
+	fn chain_info(&self) -> BlockChainInfo {
+		BlockChainInfo {
+			total_difficulty: *self.difficulty.read().unwrap(),
+			pending_total_difficulty: *self.difficulty.read().unwrap(),
+			genesis_hash: self.genesis_hash.clone(),
+			best_block_hash: self.last_hash.read().unwrap().clone(),
+			best_block_number: self.blocks.read().unwrap().len() as BlockNumber - 1,
+		}
+	}
+}
+
+struct TestIo<'p> {
+	chain: &'p mut TestBlockChainClient,
+	queue: &'p mut VecDeque<TestPacket>,
+	sender: Option<PeerId>,
+}
+
+impl<'p> TestIo<'p> {
+	fn new(chain: &'p mut TestBlockChainClient, queue: &'p mut VecDeque<TestPacket>, sender: Option<PeerId>) -> TestIo<'p> {
+		TestIo {
+			chain: chain,
+			queue: queue,
+			sender: sender
+		}
+	}
+}
+
+impl<'p> SyncIo for TestIo<'p> {
+	fn disable_peer(&mut self, _peer_id: PeerId) {
+	}
+
+	fn respond(&mut self, packet_id: PacketId, data: Vec<u8>) -> Result<(), UtilError> {
+		self.queue.push_back(TestPacket {
+			data: data,
+			packet_id: packet_id,
+			recipient: self.sender.unwrap()
+		});
+		Ok(())
+	}
+
+	fn send(&mut self, peer_id: PeerId, packet_id: PacketId, data: Vec<u8>) -> Result<(), UtilError> {
+		self.queue.push_back(TestPacket {
+			data: data,
+			packet_id: packet_id,
+			recipient: peer_id,
+		});
+		Ok(())
+	}
+
+	fn chain(&self) -> &BlockChainClient {
+		self.chain
+	}
+}
+
+struct TestPacket {
+	data: Bytes,
+	packet_id: PacketId,
+	recipient: PeerId,
+}
+
+struct TestPeer {
+	chain: TestBlockChainClient,
+	sync: ChainSync,
+	queue: VecDeque<TestPacket>,
+}
+
+struct TestNet {
+	peers: Vec<TestPeer>
+}
+
+impl TestNet {
+	pub fn new(n: usize) -> TestNet {
+		let mut net = TestNet {
+			peers: Vec::new(),
+		};
+		for _ in 0..n {
+			net.peers.push(TestPeer {
+				chain: TestBlockChainClient::new(),
+				sync: ChainSync::new(),
+				queue: VecDeque::new(),
+			});
+		}
+		net
+	}
+
+	pub fn peer(&self, i: usize) -> &TestPeer {
+		self.peers.get(i).unwrap()
+	}
+
+	pub fn peer_mut(&mut self, i: usize) -> &mut TestPeer {
+		self.peers.get_mut(i).unwrap()
+	}
+
+	pub fn start(&mut self) {
+		for peer in 0..self.peers.len() {
+			for client in 0..self.peers.len() {
+				if peer != client {
+					let mut p = self.peers.get_mut(peer).unwrap();
+					p.sync.on_peer_connected(&mut TestIo::new(&mut p.chain, &mut p.queue, Some(client as PeerId)), client as PeerId);
+				}
+			}
+		}
+	}
+
+	pub fn sync_step(&mut self) {
+		for peer in 0..self.peers.len() {
+			if let Some(packet) = self.peers[peer].queue.pop_front() {
+				let mut p = self.peers.get_mut(packet.recipient).unwrap();
+				trace!("--- {} -> {} ---", peer, packet.recipient);
+				p.sync.on_packet(&mut TestIo::new(&mut p.chain, &mut p.queue, Some(peer as PeerId)), peer as PeerId, packet.packet_id, &packet.data);
+				trace!("----------------");
+			}
+			let mut p = self.peers.get_mut(peer).unwrap();
+			p.sync._maintain_sync(&mut TestIo::new(&mut p.chain, &mut p.queue, None));
+		}
+	}
+
+	pub fn sync(&mut self) {
+		self.start();
+		while !self.done() {
+			self.sync_step()
+		}
+	}
+
+	pub fn done(&self) -> bool {
+		self.peers.iter().all(|p| p.queue.is_empty())
+	}
+}
+
+
+#[test]
+fn full_sync_two_peers() {
+	::env_logger::init().ok();
+	let mut net = TestNet::new(3);
+	net.peer_mut(1).chain.add_blocks(1000, false);
+	net.peer_mut(2).chain.add_blocks(1000, false);
+	net.sync();
+	assert!(net.peer(0).chain.block_at(1000).is_some());
+	assert_eq!(net.peer(0).chain.blocks.read().unwrap().deref(), net.peer(1).chain.blocks.read().unwrap().deref());
+}
+
+#[test]
+fn full_sync_empty_blocks() {
+	::env_logger::init().ok();
+	let mut net = TestNet::new(3);
+	for n in 0..200 {
+		net.peer_mut(1).chain.add_blocks(5, n % 2 == 0);
+		net.peer_mut(2).chain.add_blocks(5, n % 2 == 0);
+	}
+	net.sync();
+	assert!(net.peer(0).chain.block_at(1000).is_some());
+	assert_eq!(net.peer(0).chain.blocks.read().unwrap().deref(), net.peer(1).chain.blocks.read().unwrap().deref());
+}
+
+#[test]
+fn forked_sync() {
+	::env_logger::init().ok();
+	let mut net = TestNet::new(3);
+	net.peer_mut(0).chain.add_blocks(300, false);
+	net.peer_mut(1).chain.add_blocks(300, false);
+	net.peer_mut(2).chain.add_blocks(300, false);
+	net.peer_mut(0).chain.add_blocks(100, true); //fork
+	net.peer_mut(1).chain.add_blocks(200, false);
+	net.peer_mut(2).chain.add_blocks(200, false);
+	net.peer_mut(1).chain.add_blocks(100, false); //fork between 1 and 2
+	net.peer_mut(2).chain.add_blocks(10, true);
+	// peer 1 has the best chain of 601 blocks
+	let peer1_chain = net.peer(1).chain.numbers.read().unwrap().clone();
+	net.sync();
+	assert_eq!(net.peer(0).chain.numbers.read().unwrap().deref(), &peer1_chain);
+	assert_eq!(net.peer(1).chain.numbers.read().unwrap().deref(), &peer1_chain);
+	assert_eq!(net.peer(2).chain.numbers.read().unwrap().deref(), &peer1_chain);
+}
--- a/src/sync/tests.rs
+++ b/src/sync/tests.rs