Merge pull request #3892 from ethcore/lightsync

Naive light client synchronization

Cargo.lock

@@ -522,6 +522,7 @@ dependencies = [
  "ethcore-util 1.5.0",
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "rlp 0.1.0",
+ "smallvec 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "time 0.1.35 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
@@ -2000,6 +2001,11 @@ name = "smallvec"
 version = "0.1.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "smallvec"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "solicit"
 version = "0.4.4"
@@ -2514,6 +2520,7 @@ dependencies = [
 "checksum slab 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "6dbdd334bd28d328dad1c41b0ea662517883d8880d8533895ef96c8003dec9c4"
 "checksum slab 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "17b4fcaed89ab08ef143da37bc52adbcc04d4a69014f4c1208d6b51f0c47bc23"
 "checksum smallvec 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)" = "fcc8d19212aacecf95e4a7a2179b26f7aeb9732a915cf01f05b0d3e044865410"
+"checksum smallvec 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)" = "3a3c84984c278afe61a46e19868e8b23e2ee3be5b3cc6dea6edad4893bc6c841"
 "checksum solicit 0.4.4 (registry+https://github.com/rust-lang/crates.io-index)" = "172382bac9424588d7840732b250faeeef88942e37b6e35317dce98cafdd75b2"
 "checksum spmc 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "93bdab61c1a413e591c4d17388ffa859eaff2df27f1e13a5ec8b716700605adf"
 "checksum stable-heap 0.1.0 (git+https://github.com/carllerche/stable-heap?rev=3c5cd1ca47)" = "<none>"

ethcore/light/Cargo.toml

@@ -19,6 +19,7 @@ ethcore-io = { path = "../../util/io" }
 ethcore-ipc = { path = "../../ipc/rpc", optional = true }
 rlp = { path = "../../util/rlp" }
 time = "0.1"
+smallvec = "0.3.1"
 
 [features]
 default = []

ethcore/light/src/client.rs

@@ -1,121 +0,0 @@
-// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
-// This file is part of Parity.
-
-// Parity is free software: you can redistribute it and/or modify
-// it under the terms of the GNU General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-
-// Parity is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU General Public License for more details.
-
-// You should have received a copy of the GNU General Public License
-// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
-
-//! Light client implementation. Stores data from light sync
-
-use std::sync::Arc;
-
-use ethcore::engines::Engine;
-use ethcore::ids::BlockId;
-use ethcore::service::ClientIoMessage;
-use ethcore::block_import_error::BlockImportError;
-use ethcore::block_status::BlockStatus;
-use ethcore::verification::queue::{HeaderQueue, QueueInfo};
-use ethcore::transaction::{SignedTransaction, PendingTransaction};
-use ethcore::blockchain_info::BlockChainInfo;
-use ethcore::encoded;
-
-use io::IoChannel;
-use util::hash::{H256, H256FastMap};
-use util::{Bytes, Mutex};
-
-use provider::Provider;
-use request;
-
-/// Light client implementation.
-pub struct Client {
-	_engine: Arc<Engine>,
-	header_queue: HeaderQueue,
-	_message_channel: Mutex<IoChannel<ClientIoMessage>>,
-	tx_pool: Mutex<H256FastMap<SignedTransaction>>,
-}
-
-impl Client {
-	/// Import a header as rlp-encoded bytes.
-	pub fn import_header(&self, bytes: Bytes) -> Result<H256, BlockImportError> {
-		let header = ::rlp::decode(&bytes);
-
-		self.header_queue.import(header).map_err(Into::into)
-	}
-
-	/// Whether the block is already known (but not necessarily part of the canonical chain)
-	pub fn is_known(&self, _id: BlockId) -> bool {
-		false
-	}
-
-	/// Import a local transaction.
-	pub fn import_own_transaction(&self, tx: SignedTransaction) {
-		self.tx_pool.lock().insert(tx.hash(), tx);
-	}
-
-	/// Fetch a vector of all pending transactions.
-	pub fn pending_transactions(&self) -> Vec<SignedTransaction> {
-		self.tx_pool.lock().values().cloned().collect()
-	}
-
-	/// Inquire about the status of a given block (or header).
-	pub fn status(&self, _id: BlockId) -> BlockStatus {
-		BlockStatus::Unknown
-	}
-
-	/// Get the header queue info.
-	pub fn queue_info(&self) -> QueueInfo {
-		self.header_queue.queue_info()
-	}
-}
-
-// dummy implementation -- may draw from canonical cache further on.
-impl Provider for Client {
-	fn chain_info(&self) -> BlockChainInfo {
-		unimplemented!()
-	}
-
-	fn reorg_depth(&self, _a: &H256, _b: &H256) -> Option<u64> {
-		None
-	}
-
-	fn earliest_state(&self) -> Option<u64> {
-		None
-	}
-
-	fn block_header(&self, _id: BlockId) -> Option<encoded::Header> {
-		None
-	}
-
-	fn block_body(&self, _id: BlockId) -> Option<encoded::Body> {
-		None
-	}
-
-	fn block_receipts(&self, _hash: &H256) -> Option<Bytes> {
-		None
-	}
-
-	fn state_proof(&self, _req: request::StateProof) -> Vec<Bytes> {
-		Vec::new()
-	}
-
-	fn contract_code(&self, _req: request::ContractCode) -> Bytes {
-		Vec::new()
-	}
-
-	fn header_proof(&self, _req: request::HeaderProof) -> Option<(encoded::Header, Vec<Bytes>)> {
-		None
-	}
-
-	fn ready_transactions(&self) -> Vec<PendingTransaction> {
-		Vec::new()
-	}
-}

ethcore/light/src/client/cht.rs

@@ -0,0 +1,22 @@
+// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+//! Canonical hash trie definitions and helper functions.
+
+/// The size of each CHT.
+pub const SIZE: u64 = 2048;
+
+/// Convert a block number to a CHT number.
+pub fn block_to_cht_number(block_num: u64) -> u64 {
+	(block_num + 1) / SIZE
+}

ethcore/light/src/client/header_chain.rs

@@ -0,0 +1,365 @@
+// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Light client header chain.
+//!
+//! Unlike a full node's `BlockChain` this doesn't store much in the database.
+//! It stores candidates for the last 2048-4096 blocks as well as CHT roots for
+//! historical blocks all the way to the genesis.
+//!
+//! This is separate from the `BlockChain` for two reasons:
+//!   - It stores only headers (and a pruned subset of them)
+//!   - To allow for flexibility in the database layout once that's incorporated.
+// TODO: use DB instead of memory. DB Layout: just the contents of `candidates`/`headers`
+//
+
+use std::collections::{BTreeMap, HashMap};
+
+use client::cht;
+
+use ethcore::block_status::BlockStatus;
+use ethcore::error::BlockError;
+use ethcore::ids::BlockId;
+use ethcore::views::HeaderView;
+use util::{Bytes, H256, U256, HeapSizeOf, Mutex, RwLock};
+
+use smallvec::SmallVec;
+
+/// Store at least this many candidate headers at all times.
+/// Also functions as the delay for computing CHTs as they aren't
+/// relevant to any blocks we've got in memory.
+const HISTORY: u64 = 2048;
+
+/// Information about a block.
+#[derive(Debug, Clone)]
+pub struct BlockDescriptor {
+	/// The block's hash
+	pub hash: H256,
+	/// The block's number
+	pub number: u64,
+	/// The block's total difficulty.
+	pub total_difficulty: U256,
+}
+
+// candidate block description.
+struct Candidate {
+	hash: H256,
+	parent_hash: H256,
+	total_difficulty: U256,
+}
+
+struct Entry {
+	candidates: SmallVec<[Candidate; 3]>, // 3 arbitrarily chosen
+	canonical_hash: H256,
+}
+
+impl HeapSizeOf for Entry {
+	fn heap_size_of_children(&self) -> usize {
+		match self.candidates.spilled() {
+			false => 0,
+			true => self.candidates.capacity() * ::std::mem::size_of::<Candidate>(),
+		}
+	}
+}
+
+/// Header chain. See module docs for more details.
+pub struct HeaderChain {
+	genesis_header: Bytes, // special-case the genesis.
+	candidates: RwLock<BTreeMap<u64, Entry>>,
+	headers: RwLock<HashMap<H256, Bytes>>,
+	best_block: RwLock<BlockDescriptor>,
+	cht_roots: Mutex<Vec<H256>>,
+}
+
+impl HeaderChain {
+	/// Create a new header chain given this genesis block.
+	pub fn new(genesis: &[u8]) -> Self {
+		let g_view = HeaderView::new(genesis);
+
+		HeaderChain {
+			genesis_header: genesis.to_owned(),
+			best_block: RwLock::new(BlockDescriptor {
+				hash: g_view.hash(),
+				number: 0,
+				total_difficulty: g_view.difficulty(),
+			}),
+			candidates: RwLock::new(BTreeMap::new()),
+			headers: RwLock::new(HashMap::new()),
+			cht_roots: Mutex::new(Vec::new()),
+		}
+	}
+
+	/// Insert a pre-verified header.
+	///
+	/// This blindly trusts that the data given to it is
+	/// a) valid RLP encoding of a header and
+	/// b) has sensible data contained within it.
+	pub fn insert(&self, header: Bytes) -> Result<(), BlockError> {
+		let view = HeaderView::new(&header);
+		let hash = view.hash();
+		let number = view.number();
+		let parent_hash = view.parent_hash();
+
+		// hold candidates the whole time to guard import order.
+		let mut candidates = self.candidates.write();
+
+		// find parent details.
+		let parent_td =
+			if number == 1 {
+				let g_view = HeaderView::new(&self.genesis_header);
+				g_view.difficulty()
+			} else {
+				candidates.get(&(number - 1))
+					.and_then(|entry| entry.candidates.iter().find(|c| c.hash == parent_hash))
+					.map(|c| c.total_difficulty)
+					.ok_or_else(|| BlockError::UnknownParent(parent_hash))?
+			};
+
+		let total_difficulty = parent_td + view.difficulty();
+
+		// insert headers and candidates entries.
+		candidates.entry(number).or_insert_with(|| Entry { candidates: SmallVec::new(), canonical_hash: hash})
+			.candidates.push(Candidate {
+				hash: hash,
+				parent_hash: parent_hash,
+				total_difficulty: total_difficulty,
+		});
+
+		self.headers.write().insert(hash, header.clone());
+
+		// reorganize ancestors so canonical entries are first in their
+		// respective candidates vectors.
+		if self.best_block.read().total_difficulty < total_difficulty {
+			let mut canon_hash = hash;
+			for (_, entry) in candidates.iter_mut().rev().skip_while(|&(height, _)| *height > number) {
+				if entry.canonical_hash == canon_hash { break; }
+
+				let canon = entry.candidates.iter().find(|x| x.hash == canon_hash)
+					.expect("blocks are only inserted if parent is present; or this is the block we just added; qed");
+
+				// what about reorgs > cht::SIZE + HISTORY?
+				// resetting to the last block of a given CHT should be possible.
+				canon_hash = canon.parent_hash;
+			}
+
+			*self.best_block.write() = BlockDescriptor {
+				hash: hash,
+				number: number,
+				total_difficulty: total_difficulty,
+			};
+
+			// produce next CHT root if it's time.
+			let earliest_era = *candidates.keys().next().expect("at least one era just created; qed");
+			if earliest_era + HISTORY + cht::SIZE <= number {
+				let mut values = Vec::with_capacity(cht::SIZE as usize);
+				{
+					let mut headers = self.headers.write();
+					for i in (0..cht::SIZE).map(|x| x + earliest_era) {
+						let era_entry = candidates.remove(&i)
+							.expect("all eras are sequential with no gaps; qed");
+
+						for ancient in &era_entry.candidates {
+							headers.remove(&ancient.hash);
+						}
+
+						values.push((
+							::rlp::encode(&i).to_vec(),
+							::rlp::encode(&era_entry.canonical_hash).to_vec(),
+						));
+					}
+				}
+
+				let cht_root = ::util::triehash::trie_root(values);
+				debug!(target: "chain", "Produced CHT {} root: {:?}", (earliest_era - 1) % cht::SIZE, cht_root);
+
+				self.cht_roots.lock().push(cht_root);
+			}
+		}
+
+		Ok(())
+	}
+
+	/// Get a block header. In the case of query by number, only canonical blocks
+	/// will be returned.
+	pub fn get_header(&self, id: BlockId) -> Option<Bytes> {
+		match id {
+			BlockId::Earliest | BlockId::Number(0) => Some(self.genesis_header.clone()),
+			BlockId::Hash(hash) => self.headers.read().get(&hash).map(|x| x.to_vec()),
+			BlockId::Number(num) => {
+				if self.best_block.read().number < num { return None }
+
+				self.candidates.read().get(&num).map(|entry| entry.canonical_hash)
+					.and_then(|hash| self.headers.read().get(&hash).map(|x| x.to_vec()))
+			}
+			BlockId::Latest | BlockId::Pending => {
+				let hash = self.best_block.read().hash;
+				self.headers.read().get(&hash).map(|x| x.to_vec())
+			}
+		}
+	}
+
+	/// Get the nth CHT root, if it's been computed.
+	///
+	/// CHT root 0 is from block `1..2048`.
+	/// CHT root 1 is from block `2049..4096`
+	/// and so on.
+	///
+	/// This is because it's assumed that the genesis hash is known,
+	/// so including it within a CHT would be redundant.
+	pub fn cht_root(&self, n: usize) -> Option<H256> {
+		self.cht_roots.lock().get(n).map(|h| h.clone())
+	}
+
+	/// Get the genesis hash.
+	pub fn genesis_hash(&self) -> H256 {
+		::util::Hashable::sha3(&self.genesis_header)
+	}
+
+	/// Get the best block's data.
+	pub fn best_block(&self) -> BlockDescriptor {
+		self.best_block.read().clone()
+	}
+
+	/// If there is a gap between the genesis and the rest
+	/// of the stored blocks, return the first post-gap block.
+	pub fn first_block(&self) -> Option<BlockDescriptor> {
+		let candidates = self.candidates.read();
+		match candidates.iter().next() {
+			None | Some((&1, _)) => None,
+			Some((&height, entry)) => Some(BlockDescriptor {
+				number: height,
+				hash: entry.canonical_hash,
+				total_difficulty: entry.candidates.iter().find(|x| x.hash == entry.canonical_hash)
+					.expect("entry always stores canonical candidate; qed").total_difficulty,
+			})
+		}
+	}
+
+	/// Get block status.
+	pub fn status(&self, hash: &H256) -> BlockStatus {
+		match self.headers.read().contains_key(hash) {
+			true => BlockStatus::InChain,
+			false => BlockStatus::Unknown,
+		}
+	}
+}
+
+impl HeapSizeOf for HeaderChain {
+	fn heap_size_of_children(&self) -> usize {
+		self.candidates.read().heap_size_of_children() +
+			self.headers.read().heap_size_of_children() +
+			self.cht_roots.lock().heap_size_of_children()
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::HeaderChain;
+	use ethcore::ids::BlockId;
+	use ethcore::header::Header;
+	use ethcore::spec::Spec;
+
+	#[test]
+	fn basic_chain() {
+		let spec = Spec::new_test();
+		let genesis_header = spec.genesis_header();
+
+		let chain = HeaderChain::new(&::rlp::encode(&genesis_header));
+
+		let mut parent_hash = genesis_header.hash();
+		let mut rolling_timestamp = genesis_header.timestamp();
+		for i in 1..10000 {
+			let mut header = Header::new();
+			header.set_parent_hash(parent_hash);
+			header.set_number(i);
+			header.set_timestamp(rolling_timestamp);
+			header.set_difficulty(*genesis_header.difficulty() * i.into());
+
+			chain.insert(::rlp::encode(&header).to_vec()).unwrap();
+
+			parent_hash = header.hash();
+			rolling_timestamp += 10;
+		}
+
+		assert!(chain.get_header(BlockId::Number(10)).is_none());
+		assert!(chain.get_header(BlockId::Number(9000)).is_some());
+		assert!(chain.cht_root(2).is_some());
+		assert!(chain.cht_root(3).is_none());
+	}
+
+	#[test]
+	fn reorganize() {
+		let spec = Spec::new_test();
+		let genesis_header = spec.genesis_header();
+
+		let chain = HeaderChain::new(&::rlp::encode(&genesis_header));
+
+		let mut parent_hash = genesis_header.hash();
+		let mut rolling_timestamp = genesis_header.timestamp();
+		for i in 1..6 {
+			let mut header = Header::new();
+			header.set_parent_hash(parent_hash);
+			header.set_number(i);
+			header.set_timestamp(rolling_timestamp);
+			header.set_difficulty(*genesis_header.difficulty() * i.into());
+
+			chain.insert(::rlp::encode(&header).to_vec()).unwrap();
+
+			parent_hash = header.hash();
+			rolling_timestamp += 10;
+		}
+
+		{
+			let mut rolling_timestamp = rolling_timestamp;
+			let mut parent_hash = parent_hash;
+			for i in 6..16 {
+				let mut header = Header::new();
+				header.set_parent_hash(parent_hash);
+				header.set_number(i);
+				header.set_timestamp(rolling_timestamp);
+				header.set_difficulty(*genesis_header.difficulty() * i.into());
+
+				chain.insert(::rlp::encode(&header).to_vec()).unwrap();
+
+				parent_hash = header.hash();
+				rolling_timestamp += 10;
+			}
+		}
+
+		assert_eq!(chain.best_block().number, 15);
+
+		{
+			let mut rolling_timestamp = rolling_timestamp;
+			let mut parent_hash = parent_hash;
+
+			// import a shorter chain which has better TD.
+			for i in 6..13 {
+				let mut header = Header::new();
+				header.set_parent_hash(parent_hash);
+				header.set_number(i);
+				header.set_timestamp(rolling_timestamp);
+				header.set_difficulty(*genesis_header.difficulty() * (i * i).into());
+
+				chain.insert(::rlp::encode(&header).to_vec()).unwrap();
+
+				parent_hash = header.hash();
+				rolling_timestamp += 11;
+			}
+		}
+
+		assert_eq!(chain.best_block().number, 12);
+	}
+}

ethcore/light/src/client/mod.rs

@@ -0,0 +1,264 @@
+// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Light client implementation. Stores data from light sync
+
+use ethcore::block_import_error::BlockImportError;
+use ethcore::block_status::BlockStatus;
+use ethcore::client::ClientReport;
+use ethcore::ids::BlockId;
+use ethcore::header::Header;
+use ethcore::verification::queue::{self, HeaderQueue};
+use ethcore::transaction::PendingTransaction;
+use ethcore::blockchain_info::BlockChainInfo;
+use ethcore::spec::Spec;
+use ethcore::service::ClientIoMessage;
+use ethcore::encoded;
+use io::IoChannel;
+
+use util::hash::{H256, H256FastMap};
+use util::{Bytes, Mutex, RwLock};
+
+use provider::Provider;
+use request;
+
+use self::header_chain::HeaderChain;
+
+pub use self::service::Service;
+
+pub mod cht;
+
+mod header_chain;
+mod service;
+
+/// Configuration for the light client.
+#[derive(Debug, Default, Clone)]
+pub struct Config {
+	/// Verification queue config.
+	pub queue: queue::Config,
+}
+
+/// Trait for interacting with the header chain abstractly.
+pub trait LightChainClient: Send + Sync {
+	/// Get chain info.
+	fn chain_info(&self) -> BlockChainInfo;
+
+	/// Queue header to be verified. Required that all headers queued have their
+	/// parent queued prior.
+	fn queue_header(&self, header: Header) -> Result<H256, BlockImportError>;
+
+	/// Query whether a block is known.
+	fn is_known(&self, hash: &H256) -> bool;
+
+	/// Clear the queue.
+	fn clear_queue(&self);
+
+	/// Get queue info.
+	fn queue_info(&self) -> queue::QueueInfo;
+
+	/// Get the `i`th CHT root.
+	fn cht_root(&self, i: usize) -> Option<H256>;
+}
+
+/// Light client implementation.
+pub struct Client {
+	queue: HeaderQueue,
+	chain: HeaderChain,
+	tx_pool: Mutex<H256FastMap<PendingTransaction>>,
+	report: RwLock<ClientReport>,
+	import_lock: Mutex<()>,
+}
+
+impl Client {
+	/// Create a new `Client`.
+	pub fn new(config: Config, spec: &Spec, io_channel: IoChannel<ClientIoMessage>) -> Self {
+		Client {
+			queue: HeaderQueue::new(config.queue, spec.engine.clone(), io_channel, true),
+			chain: HeaderChain::new(&::rlp::encode(&spec.genesis_header())),
+			tx_pool: Mutex::new(Default::default()),
+			report: RwLock::new(ClientReport::default()),
+			import_lock: Mutex::new(()),
+		}
+	}
+
+	/// Import a header to the queue for additional verification.
+	pub fn import_header(&self, header: Header) -> Result<H256, BlockImportError> {
+		self.queue.import(header).map_err(Into::into)
+	}
+
+	/// Import a local transaction.
+	pub fn import_own_transaction(&self, tx: PendingTransaction) {
+		self.tx_pool.lock().insert(tx.transaction.hash(), tx);
+	}
+
+	/// Fetch a vector of all pending transactions.
+	pub fn ready_transactions(&self) -> Vec<PendingTransaction> {
+		let best_num = self.chain.best_block().number;
+		self.tx_pool.lock()
+			.values()
+			.filter(|t| t.min_block.as_ref().map_or(true, |x| x <= &best_num))
+			.cloned()
+			.collect()
+	}
+
+	/// Inquire about the status of a given header.
+	pub fn status(&self, hash: &H256) -> BlockStatus {
+		match self.queue.status(hash) {
+			queue::Status::Unknown => self.chain.status(hash),
+			other => other.into(),
+		}
+	}
+
+	/// Get the chain info.
+	pub fn chain_info(&self) -> BlockChainInfo {
+		let best_block = self.chain.best_block();
+		let first_block = self.chain.first_block();
+		let genesis_hash = self.chain.genesis_hash();
+
+		BlockChainInfo {
+			total_difficulty: best_block.total_difficulty,
+			pending_total_difficulty: best_block.total_difficulty,
+			genesis_hash: genesis_hash,
+			best_block_hash: best_block.hash,
+			best_block_number: best_block.number,
+			ancient_block_hash: if first_block.is_some() { Some(genesis_hash) } else { None },
+			ancient_block_number: if first_block.is_some() { Some(0) } else { None },
+			first_block_hash: first_block.as_ref().map(|first| first.hash),
+			first_block_number: first_block.as_ref().map(|first| first.number),
+		}
+	}
+
+	/// Get the header queue info.
+	pub fn queue_info(&self) -> queue::QueueInfo {
+		self.queue.queue_info()
+	}
+
+	/// Get a block header by Id.
+	pub fn get_header(&self, id: BlockId) -> Option<Bytes> {
+		self.chain.get_header(id)
+	}
+
+	/// Get the `i`th CHT root.
+	pub fn cht_root(&self, i: usize) -> Option<H256> {
+		self.chain.cht_root(i)
+	}
+
+	/// Import a set of pre-verified headers from the queue.
+	pub fn import_verified(&self) {
+		const MAX: usize = 256;
+
+		let _lock = self.import_lock.lock();
+
+		let mut bad = Vec::new();
+		let mut good = Vec::new();
+		for verified_header in self.queue.drain(MAX) {
+			let (num, hash) = (verified_header.number(), verified_header.hash());
+
+			match self.chain.insert(::rlp::encode(&verified_header).to_vec()) {
+				Ok(()) => {
+					good.push(hash);
+					self.report.write().blocks_imported += 1;
+				}
+				Err(e) => {
+					debug!(target: "client", "Error importing header {:?}: {}", (num, hash), e);
+					bad.push(hash);
+				}
+			}
+		}
+
+		self.queue.mark_as_bad(&bad);
+		self.queue.mark_as_good(&good);
+	}
+
+	/// Get a report about blocks imported.
+	pub fn report(&self) -> ClientReport {
+		::std::mem::replace(&mut *self.report.write(), ClientReport::default())
+	}
+
+	/// Get blockchain mem usage in bytes.
+	pub fn chain_mem_used(&self) -> usize {
+		use util::HeapSizeOf;
+
+		self.chain.heap_size_of_children()
+	}
+}
+
+impl LightChainClient for Client {
+	fn chain_info(&self) -> BlockChainInfo { Client::chain_info(self) }
+
+	fn queue_header(&self, header: Header) -> Result<H256, BlockImportError> {
+		self.import_header(header)
+	}
+
+	fn is_known(&self, hash: &H256) -> bool {
+		self.status(hash) == BlockStatus::InChain
+	}
+
+	fn clear_queue(&self) {
+		self.queue.clear()
+	}
+
+	fn queue_info(&self) -> queue::QueueInfo {
+		self.queue.queue_info()
+	}
+
+	fn cht_root(&self, i: usize) -> Option<H256> {
+		Client::cht_root(self, i)
+	}
+}
+
+// dummy implementation -- may draw from canonical cache further on.
+impl Provider for Client {
+	fn chain_info(&self) -> BlockChainInfo {
+		Client::chain_info(self)
+	}
+
+	fn reorg_depth(&self, _a: &H256, _b: &H256) -> Option<u64> {
+		None
+	}
+
+	fn earliest_state(&self) -> Option<u64> {
+		None
+	}
+
+	fn block_header(&self, id: BlockId) -> Option<encoded::Header> {
+		self.chain.get_header(id).map(encoded::Header::new)
+	}
+
+	fn block_body(&self, _id: BlockId) -> Option<encoded::Body> {
+		None
+	}
+
+	fn block_receipts(&self, _hash: &H256) -> Option<Bytes> {
+		None
+	}
+
+	fn state_proof(&self, _req: request::StateProof) -> Vec<Bytes> {
+		Vec::new()
+	}
+
+	fn contract_code(&self, _req: request::ContractCode) -> Bytes {
+		Vec::new()
+	}
+
+	fn header_proof(&self, _req: request::HeaderProof) -> Option<(encoded::Header, Vec<Bytes>)> {
+		None
+	}
+
+	fn ready_transactions(&self) -> Vec<PendingTransaction> {
+		Vec::new()
+	}
+}

ethcore/light/src/client/service.rs

@@ -0,0 +1,73 @@
+// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Minimal IO service for light client.
+//! Just handles block import messages and passes them to the client.
+
+use std::sync::Arc;
+
+use ethcore::service::ClientIoMessage;
+use ethcore::spec::Spec;
+use io::{IoContext, IoError, IoHandler, IoService};
+
+use super::{Client, Config as ClientConfig};
+
+/// Light client service.
+pub struct Service {
+	client: Arc<Client>,
+	_io_service: IoService<ClientIoMessage>,
+}
+
+impl Service {
+	/// Start the service: initialize I/O workers and client itself.
+	pub fn start(config: ClientConfig, spec: &Spec) -> Result<Self, IoError> {
+		let io_service = try!(IoService::<ClientIoMessage>::start());
+		let client = Arc::new(Client::new(config, spec, io_service.channel()));
+		try!(io_service.register_handler(Arc::new(ImportBlocks(client.clone()))));
+
+		Ok(Service {
+			client: client,
+			_io_service: io_service,
+		})
+	}
+
+	/// Get a handle to the client.
+	pub fn client(&self) -> &Arc<Client> {
+		&self.client
+	}
+}
+
+struct ImportBlocks(Arc<Client>);
+
+impl IoHandler<ClientIoMessage> for ImportBlocks {
+	fn message(&self, _io: &IoContext<ClientIoMessage>, message: &ClientIoMessage) {
+		if let ClientIoMessage::BlockVerified = *message {
+			self.0.import_verified();
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::Service;
+	use ethcore::spec::Spec;
+
+	#[test]
+	fn it_works() {
+		let spec = Spec::new_test();
+		Service::start(Default::default(), &spec).unwrap();
+	}
+}

ethcore/light/src/lib.rs

@@ -25,8 +25,10 @@
 //! low-latency applications, but perfectly suitable for simple everyday
 //! use-cases like sending transactions from a personal account.
 //!
-//! It starts by performing a header-only sync, verifying random samples
+//! The light client performs a header-only sync, doing verification and pruning
-//! of members of the chain to varying degrees.
+//! historical blocks. Upon pruning, batches of 2048 blocks have a number => hash
+//! mapping sealed into "canonical hash tries" which can later be used to verify
+//! historical block queries from peers.
 
 #![deny(missing_docs)]
 
@@ -60,6 +62,7 @@ extern crate ethcore_util as util;
 extern crate ethcore_network as network;
 extern crate ethcore_io as io;
 extern crate rlp;
+extern crate smallvec;
 extern crate time;
 
 #[cfg(feature = "ipc")]

ethcore/light/src/net/context.rs

@@ -20,7 +20,7 @@ use network::{NetworkContext, PeerId, NodeId};
 
 use super::{Announcement, LightProtocol, ReqId};
 use super::error::Error;
-use request::Request;
+use request::{self, Request};
 
 /// An I/O context which allows sending and receiving packets as well as
 /// disconnecting peers. This is used as a generalization of the portions
@@ -77,12 +77,8 @@ impl<'a> IoContext for NetworkContext<'a> {
 	}
 }
 
-/// Context for a protocol event.
+/// Basic context for a the protocol.
-pub trait EventContext {
+pub trait BasicContext {
-	/// Get the peer relevant to the event e.g. message sender,
-	/// disconnected/connected peer.
-	fn peer(&self) -> PeerId;
-
 	/// Returns the relevant's peer persistent Id (aka NodeId).
 	fn persistent_peer_id(&self, peer: PeerId) -> Option<NodeId>;
 
@@ -93,6 +89,10 @@ pub trait EventContext {
 	// TODO: maybe just put this on a timer in LightProtocol?
 	fn make_announcement(&self, announcement: Announcement);
 
+	/// Find the maximum number of requests of a specific type which can be made from
+	/// supplied peer.
+	fn max_requests(&self, peer: PeerId, kind: request::Kind) -> usize;
+
 	/// Disconnect a peer.
 	fn disconnect_peer(&self, peer: PeerId);
 
@@ -100,6 +100,50 @@ pub trait EventContext {
 	fn disable_peer(&self, peer: PeerId);
 }
 
+/// Context for a protocol event which has a peer ID attached.
+pub trait EventContext: BasicContext {
+	/// Get the peer relevant to the event e.g. message sender,
+	/// disconnected/connected peer.
+	fn peer(&self) -> PeerId;
+
+	/// Treat the event context as a basic context.
+	fn as_basic(&self) -> &BasicContext;
+}
+
+/// Basic context.
+pub struct TickCtx<'a> {
+	/// Io context to enable dispatch.
+	pub io: &'a IoContext,
+	/// Protocol implementation.
+	pub proto: &'a LightProtocol,
+}
+
+impl<'a> BasicContext for TickCtx<'a> {
+	fn persistent_peer_id(&self, id: PeerId) -> Option<NodeId> {
+		self.io.persistent_peer_id(id)
+	}
+
+	fn request_from(&self, peer: PeerId, request: Request) -> Result<ReqId, Error> {
+		self.proto.request_from(self.io, &peer, request)
+	}
+
+	fn make_announcement(&self, announcement: Announcement) {
+		self.proto.make_announcement(self.io, announcement);
+	}
+
+	fn max_requests(&self, peer: PeerId, kind: request::Kind) -> usize {
+		self.proto.max_requests(peer, kind)
+	}
+
+	fn disconnect_peer(&self, peer: PeerId) {
+		self.io.disconnect_peer(peer);
+	}
+
+	fn disable_peer(&self, peer: PeerId) {
+		self.io.disable_peer(peer);
+	}
+}
+
 /// Concrete implementation of `EventContext` over the light protocol struct and
 /// an io context.
 pub struct Ctx<'a> {
@@ -111,11 +155,7 @@ pub struct Ctx<'a> {
 	pub peer: PeerId,
 }
 
-impl<'a> EventContext for Ctx<'a> {
+impl<'a> BasicContext for Ctx<'a> {
-	fn peer(&self) -> PeerId {
-		self.peer
-	}
-
 	fn persistent_peer_id(&self, id: PeerId) -> Option<NodeId> {
 		self.io.persistent_peer_id(id)
 	}
@@ -128,6 +168,10 @@ impl<'a> EventContext for Ctx<'a> {
 		self.proto.make_announcement(self.io, announcement);
 	}
 
+	fn max_requests(&self, peer: PeerId, kind: request::Kind) -> usize {
+		self.proto.max_requests(peer, kind)
+	}
+
 	fn disconnect_peer(&self, peer: PeerId) {
 		self.io.disconnect_peer(peer);
 	}
@@ -136,3 +180,13 @@ impl<'a> EventContext for Ctx<'a> {
 		self.io.disable_peer(peer);
 	}
 }
+
+impl<'a> EventContext for Ctx<'a> {
+	fn peer(&self) -> PeerId {
+		self.peer
+	}
+
+	fn as_basic(&self) -> &BasicContext {
+		&*self
+	}
+}

ethcore/light/src/net/error.rs

@@ -62,6 +62,8 @@ pub enum Error {
 	UnsupportedProtocolVersion(u8),
 	/// Bad protocol version.
 	BadProtocolVersion,
+	/// Peer is overburdened.
+	Overburdened,
 }
 
 impl Error {
@@ -79,6 +81,7 @@ impl Error {
 			Error::NotServer => Punishment::Disable,
 			Error::UnsupportedProtocolVersion(_) => Punishment::Disable,
 			Error::BadProtocolVersion => Punishment::Disable,
+			Error::Overburdened => Punishment::None,
 		}
 	}
 }
@@ -109,6 +112,7 @@ impl fmt::Display for Error {
 			Error::NotServer => write!(f, "Peer not a server."),
 			Error::UnsupportedProtocolVersion(pv) => write!(f, "Unsupported protocol version: {}", pv),
 			Error::BadProtocolVersion => write!(f, "Bad protocol version in handshake"),
+			Error::Overburdened => write!(f, "Peer overburdened"),
 		}
 	}
 }

ethcore/light/src/net/mod.rs

@@ -37,8 +37,8 @@ use provider::Provider;
 use request::{self, HashOrNumber, Request};
 
 use self::buffer_flow::{Buffer, FlowParams};
-use self::context::Ctx;
+use self::context::{Ctx, TickCtx};
-use self::error::{Error, Punishment};
+use self::error::Punishment;
 
 mod buffer_flow;
 mod context;
@@ -48,12 +48,16 @@ mod status;
 #[cfg(test)]
 mod tests;
 
-pub use self::context::{EventContext, IoContext};
+pub use self::error::Error;
+pub use self::context::{BasicContext, EventContext, IoContext};
 pub use self::status::{Status, Capabilities, Announcement};
 
 const TIMEOUT: TimerToken = 0;
 const TIMEOUT_INTERVAL_MS: u64 = 1000;
 
+const TICK_TIMEOUT: TimerToken = 1;
+const TICK_TIMEOUT_INTERVAL_MS: u64 = 5000;
+
 // minimum interval between updates.
 const UPDATE_INTERVAL_MS: i64 = 5000;
 
@@ -131,8 +135,9 @@ struct Peer {
 	status: Status,
 	capabilities: Capabilities,
 	remote_flow: Option<(Buffer, FlowParams)>,
-	sent_head: H256, // last head we've given them.
+	sent_head: H256, // last chain head we've given them.
 	last_update: SteadyTime,
+	idle: bool, // make into a current percentage of max buffer being requested?
 }
 
 impl Peer {
@@ -188,7 +193,11 @@ pub trait Handler: Send + Sync {
 	/// Called when a peer responds with header proofs. Each proof is a block header coupled
 	/// with a series of trie nodes is ascending order by distance from the root.
 	fn on_header_proofs(&self, _ctx: &EventContext, _req_id: ReqId, _proofs: &[(Bytes, Vec<Bytes>)]) { }
-	/// Called on abort.
+	/// Called to "tick" the handler periodically.
+	fn tick(&self, _ctx: &BasicContext) { }
+	/// Called on abort. This signals to handlers that they should clean up
+	/// and ignore peers.
+	// TODO: coreresponding `on_activate`?
 	fn on_abort(&self) { }
 }
 
@@ -253,18 +262,25 @@ impl LightProtocol {
 	}
 
 	/// Check the maximum amount of requests of a specific type
-	/// which a peer would be able to serve.
+	/// which a peer would be able to serve. Returns zero if the
-	pub fn max_requests(&self, peer: PeerId, kind: request::Kind) -> Option<usize> {
+	/// peer is unknown or has no buffer flow parameters.
+	fn max_requests(&self, peer: PeerId, kind: request::Kind) -> usize {
 		self.peers.read().get(&peer).and_then(|peer| {
 			let mut peer = peer.lock();
-			match peer.remote_flow.as_mut() {
+			let idle = peer.idle;
-				Some(&mut (ref mut buf, ref flow)) => {
+			match peer.remote_flow {
+				Some((ref mut buf, ref flow)) => {
 					flow.recharge(buf);
-					Some(flow.max_amount(&*buf, kind))
+
+					if !idle {
+						Some(0)
+					} else {
+						Some(flow.max_amount(&*buf, kind))
+					}
 				}
 				None => None,
 			}
-		})
+		}).unwrap_or(0)
 	}
 
 	/// Make a request to a peer.
@@ -278,8 +294,10 @@ impl LightProtocol {
 		let peer = peers.get(peer_id).ok_or_else(|| Error::UnknownPeer)?;
 		let mut peer = peer.lock();
 
-		match peer.remote_flow.as_mut() {
+		if !peer.idle { return Err(Error::Overburdened) }
-			Some(&mut (ref mut buf, ref flow)) => {
+
+		match peer.remote_flow {
+			Some((ref mut buf, ref flow)) => {
 				flow.recharge(buf);
 				let max = flow.compute_cost(request.kind(), request.amount());
 				buf.deduct_cost(max)?;
@@ -290,6 +308,8 @@ impl LightProtocol {
 		let req_id = self.req_id.fetch_add(1, Ordering::SeqCst);
 		let packet_data = encode_request(&request, req_id);
 
+		trace!(target: "les", "Dispatching request {} to peer {}", req_id, peer_id);
+
 		let packet_id = match request.kind() {
 			request::Kind::Headers => packet::GET_BLOCK_HEADERS,
 			request::Kind::Bodies => packet::GET_BLOCK_BODIES,
@@ -301,6 +321,7 @@ impl LightProtocol {
 
 		io.send(*peer_id, packet_id, packet_data);
 
+		peer.idle = false;
 		self.pending_requests.write().insert(req_id, Requested {
 			request: request,
 			timestamp: SteadyTime::now(),
@@ -404,6 +425,8 @@ impl LightProtocol {
 		match peers.get(peer) {
 			Some(peer_info) => {
 				let mut peer_info = peer_info.lock();
+				peer_info.idle = true;
+
 				match peer_info.remote_flow.as_mut() {
 					Some(&mut (ref mut buf, ref mut flow)) => {
 						let actual_buffer = ::std::cmp::min(cur_buffer, *flow.limit());
@@ -505,6 +528,15 @@ impl LightProtocol {
 			}
 		}
 	}
+
+	fn tick_handlers(&self, io: &IoContext) {
+		for handler in &self.handlers {
+			handler.tick(&TickCtx {
+				io: io,
+				proto: self,
+			})
+		}
+	}
 }
 
 impl LightProtocol {
@@ -603,6 +635,7 @@ impl LightProtocol {
 			remote_flow: remote_flow,
 			sent_head: pending.sent_head,
 			last_update: pending.last_update,
+			idle: true,
 		}));
 
 		for handler in &self.handlers {
@@ -1123,7 +1156,10 @@ fn punish(peer: PeerId, io: &IoContext, e: Error) {
 
 impl NetworkProtocolHandler for LightProtocol {
 	fn initialize(&self, io: &NetworkContext) {
-		io.register_timer(TIMEOUT, TIMEOUT_INTERVAL_MS).expect("Error registering sync timer.");
+		io.register_timer(TIMEOUT, TIMEOUT_INTERVAL_MS)
+			.expect("Error registering sync timer.");
+		io.register_timer(TICK_TIMEOUT, TICK_TIMEOUT_INTERVAL_MS)
+			.expect("Error registering sync timer.");
 	}
 
 	fn read(&self, io: &NetworkContext, peer: &PeerId, packet_id: u8, data: &[u8]) {
@@ -1141,6 +1177,7 @@ impl NetworkProtocolHandler for LightProtocol {
 	fn timeout(&self, io: &NetworkContext, timer: TimerToken) {
 		match timer {
 			TIMEOUT => self.timeout_check(io),
+			TICK_TIMEOUT => self.tick_handlers(io),
 			_ => warn!(target: "les", "received timeout on unknown token {}", timer),
 		}
 	}

parity/cli/config.full.toml

@@ -12,6 +12,7 @@ base_path = "$HOME/.parity"
 db_path = "$HOME/.parity/chains"
 keys_path = "$HOME/.parity/keys"
 identity = ""
+light = false
 
 [account]
 unlock = ["0xdeadbeefcafe0000000000000000000000000000"]

parity/run.rs

@@ -32,12 +32,10 @@ use ethcore::verification::queue::VerifierSettings;
 use ethsync::SyncConfig;
 use informant::Informant;
 use updater::{UpdatePolicy, Updater};
-use parity_reactor::{EventLoop, EventLoopHandle};
+use parity_reactor::EventLoop;
 use hash_fetch::fetch::{Fetch, Client as FetchClient};
 
-use rpc::{HttpServer, IpcServer, HttpConfiguration, IpcConfiguration};
+use rpc::{HttpConfiguration, IpcConfiguration};
-use signer::SignerServer;
-use dapps::WebappServer;
 use params::{
 	SpecType, Pruning, AccountsConfig, GasPricerConfig, MinerExtras, Switch,
 	tracing_switch_to_bool, fatdb_switch_to_bool, mode_switch_to_bool
@@ -444,16 +442,10 @@ pub fn execute(cmd: RunCmd, can_restart: bool, logger: Arc<RotatingLogger>) -> R
 	}
 
 	// Handle exit
-	let restart = wait_for_exit(
+	let restart = wait_for_exit(panic_handler, Some(updater), can_restart);
-		panic_handler,
+
-		http_server,
+	// drop this stuff as soon as exit detected.
-		ipc_server,
+	drop((http_server, ipc_server, dapps_server, signer_server, event_loop));
-		dapps_server,
-		signer_server,
-		event_loop.into(),
-		updater,
-		can_restart,
-	);
 
 	info!("Finishing work, please wait...");
 
@@ -523,12 +515,7 @@ fn build_create_account_hint(spec: &SpecType, keys: &str) -> String {
 
 fn wait_for_exit(
 	panic_handler: Arc<PanicHandler>,
-	_http_server: Option<HttpServer>,
+	updater: Option<Arc<Updater>>,
-	_ipc_server: Option<IpcServer>,
-	_dapps_server: Option<WebappServer>,
-	_signer_server: Option<SignerServer>,
-	_event_loop: EventLoopHandle,
-	updater: Arc<Updater>,
 	can_restart: bool
 ) -> bool {
 	let exit = Arc::new((Mutex::new(false), Condvar::new()));
@@ -541,12 +528,14 @@ fn wait_for_exit(
 	let e = exit.clone();
 	panic_handler.on_panic(move |_reason| { e.1.notify_all(); });
 
-	// Handle updater wanting to restart us
+	if let Some(updater) = updater {
-	if can_restart {
+		// Handle updater wanting to restart us
-		let e = exit.clone();
+		if can_restart {
-		updater.set_exit_handler(move || { *e.0.lock() = true; e.1.notify_all(); });
+			let e = exit.clone();
-	} else {
+			updater.set_exit_handler(move || { *e.0.lock() = true; e.1.notify_all(); });
-		updater.set_exit_handler(|| info!("Update installed; ready for restart."));
+		} else {
+			updater.set_exit_handler(|| info!("Update installed; ready for restart."));
+		}
 	}
 
 	// Wait for signal

sync/src/api.rs

@@ -33,6 +33,8 @@ use ipc::{BinaryConvertable, BinaryConvertError, IpcConfig};
 use std::str::FromStr;
 use parking_lot::RwLock;
 use chain::{ETH_PACKET_COUNT, SNAPSHOT_SYNC_PACKET_COUNT};
+use light::client::LightChainClient;
+use light::Provider;
 use light::net::{LightProtocol, Params as LightParams, Capabilities, Handler as LightHandler, EventContext};
 
 /// Parity sync protocol
@@ -567,3 +569,101 @@ pub struct ServiceConfiguration {
 	/// IPC path.
 	pub io_path: String,
 }
+
+/// Configuration for the light sync.
+pub struct LightSyncParams<L> {
+	/// Network configuration.
+	pub network_config: BasicNetworkConfiguration,
+	/// Light client to sync to.
+	pub client: Arc<L>,
+	/// Network ID.
+	pub network_id: u64,
+	/// Subprotocol name.
+	pub subprotocol_name: [u8; 3],
+}
+
+/// Service for light synchronization.
+pub struct LightSync {
+	proto: Arc<LightProtocol>,
+	network: NetworkService,
+	subprotocol_name: [u8; 3],
+}
+
+impl LightSync {
+	/// Create a new light sync service.
+	pub fn new<L>(params: LightSyncParams<L>) -> Result<Self, NetworkError>
+		where L: LightChainClient + Provider + 'static
+	{
+		use light_sync::LightSync as SyncHandler;
+
+		// initialize light protocol handler and attach sync module.
+		let light_proto = {
+			let light_params = LightParams {
+				network_id: params.network_id,
+				flow_params: Default::default(), // or `None`?
+				capabilities: Capabilities {
+					serve_headers: false,
+					serve_chain_since: None,
+					serve_state_since: None,
+					tx_relay: false,
+				},
+			};
+
+			let mut light_proto = LightProtocol::new(params.client.clone(), light_params);
+			let sync_handler = try!(SyncHandler::new(params.client.clone()));
+			light_proto.add_handler(Box::new(sync_handler));
+
+			Arc::new(light_proto)
+		};
+
+		let service = try!(NetworkService::new(params.network_config));
+
+		Ok(LightSync {
+			proto: light_proto,
+			network: service,
+			subprotocol_name: params.subprotocol_name,
+		})
+	}
+}
+
+impl ManageNetwork for LightSync {
+	fn accept_unreserved_peers(&self) {
+		self.network.set_non_reserved_mode(NonReservedPeerMode::Accept);
+	}
+
+	fn deny_unreserved_peers(&self) {
+		self.network.set_non_reserved_mode(NonReservedPeerMode::Deny);
+	}
+
+	fn remove_reserved_peer(&self, peer: String) -> Result<(), String> {
+		self.network.remove_reserved_peer(&peer).map_err(|e| format!("{:?}", e))
+	}
+
+	fn add_reserved_peer(&self, peer: String) -> Result<(), String> {
+		self.network.add_reserved_peer(&peer).map_err(|e| format!("{:?}", e))
+	}
+
+	fn start_network(&self) {
+		match self.network.start() {
+			Err(NetworkError::StdIo(ref e)) if  e.kind() == io::ErrorKind::AddrInUse => warn!("Network port {:?} is already in use, make sure that another instance of an Ethereum client is not running or change the port using the --port option.", self.network.config().listen_address.expect("Listen address is not set.")),
+			Err(err) => warn!("Error starting network: {}", err),
+			_ => {},
+		}
+
+		let light_proto = self.proto.clone();
+
+		self.network.register_protocol(light_proto, self.subprotocol_name, ::light::net::PACKET_COUNT, ::light::net::PROTOCOL_VERSIONS)
+			.unwrap_or_else(|e| warn!("Error registering light client protocol: {:?}", e));
+	}
+
+	fn stop_network(&self) {
+		self.proto.abort();
+		if let Err(e) = self.network.stop() {
+			warn!("Error stopping network: {}", e);
+		}
+	}
+
+	fn network_config(&self) -> NetworkConfiguration {
+		NetworkConfiguration::from(self.network.config().clone())
+	}
+}

sync/src/lib.rs

@@ -58,6 +58,8 @@ mod sync_io;
 mod snapshot;
 mod transactions_stats;
 
+pub mod light_sync;
+
 #[cfg(test)]
 mod tests;
 
@@ -70,8 +72,11 @@ mod api {
 #[cfg(not(feature = "ipc"))]
 mod api;
 
-pub use api::{EthSync, Params, SyncProvider, ManageNetwork, SyncConfig,
+pub use api::{
-	ServiceConfiguration, NetworkConfiguration, PeerInfo, AllowIP, TransactionStats};
+	EthSync, Params, SyncProvider, ManageNetwork, SyncConfig,
+	ServiceConfiguration, NetworkConfiguration, PeerInfo, AllowIP, TransactionStats,
+	LightSync, LightSyncParams,
+};
 pub use chain::{SyncStatus, SyncState};
 pub use network::{is_valid_node_url, NonReservedPeerMode, NetworkError};
 

sync/src/light_sync/mod.rs

@@ -0,0 +1,459 @@
+// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Light client synchronization.
+//!
+//! This will synchronize the header chain using LES messages.
+//! Dataflow is largely one-directional as headers are pushed into
+//! the light client queue for import. Where possible, they are batched
+//! in groups.
+//!
+//! This is written assuming that the client and sync service are running
+//! in the same binary; unlike a full node which might communicate via IPC.
+
+use std::collections::HashMap;
+use std::mem;
+use std::sync::Arc;
+
+use light::client::LightChainClient;
+use light::net::{
+	Announcement, Handler, BasicContext, EventContext,
+	Capabilities, ReqId, Status,
+};
+use light::request;
+use network::PeerId;
+use util::{Bytes, U256, H256, Mutex, RwLock};
+use rand::{Rng, OsRng};
+
+use self::sync_round::{AbortReason, SyncRound, ResponseContext};
+
+mod response;
+mod sync_round;
+
+/// Peer chain info.
+#[derive(Clone)]
+struct ChainInfo {
+	head_td: U256,
+	head_hash: H256,
+	head_num: u64,
+}
+
+struct Peer {
+	status: ChainInfo,
+}
+
+impl Peer {
+	// Create a new peer.
+	fn new(chain_info: ChainInfo) -> Self {
+		Peer {
+			status: chain_info,
+		}
+	}
+}
+// search for a common ancestor with the best chain.
+enum AncestorSearch {
+	Queued(u64), // queued to search for blocks starting from here.
+	Awaiting(ReqId, u64, request::Headers), // awaiting response for this request.
+	Prehistoric, // prehistoric block found. TODO: start to roll back CHTs.
+	FoundCommon(u64, H256), // common block found.
+	Genesis, // common ancestor is the genesis.
+}
+
+impl AncestorSearch {
+	fn begin(best_num: u64) -> Self {
+		match best_num {
+			0 => AncestorSearch::Genesis,
+			_ => AncestorSearch::Queued(best_num),
+		}
+	}
+
+	fn process_response<L>(self, ctx: &ResponseContext, client: &L) -> AncestorSearch
+		where L: LightChainClient
+	{
+		let first_num = client.chain_info().first_block_number.unwrap_or(0);
+		match self {
+			AncestorSearch::Awaiting(id, start, req) => {
+				if &id == ctx.req_id() {
+					match response::decode_and_verify(ctx.data(), &req) {
+						Ok(headers) => {
+							for header in &headers {
+								if client.is_known(&header.hash()) {
+									debug!(target: "sync", "Found common ancestor with best chain");
+									return AncestorSearch::FoundCommon(header.number(), header.hash());
+								}
+
+								if header.number() <= first_num {
+									debug!(target: "sync", "Prehistoric common ancestor with best chain.");
+									return AncestorSearch::Prehistoric;
+								}
+							}
+
+							AncestorSearch::Queued(start - headers.len() as u64)
+						}
+						Err(e) => {
+							trace!(target: "sync", "Bad headers response from {}: {}", ctx.responder(), e);
+
+							ctx.punish_responder();
+							AncestorSearch::Queued(start)
+						}
+					}
+				} else {
+					AncestorSearch::Awaiting(id, start, req)
+				}
+			}
+			other => other,
+		}
+	}
+
+	fn dispatch_request<F>(self, mut dispatcher: F) -> AncestorSearch
+		where F: FnMut(request::Headers) -> Option<ReqId>
+	{
+		const BATCH_SIZE: usize = 64;
+
+		match self {
+			AncestorSearch::Queued(start) => {
+				let req = request::Headers {
+					start: start.into(),
+					max: ::std::cmp::min(start as usize, BATCH_SIZE),
+					skip: 0,
+					reverse: true,
+				};
+
+				match dispatcher(req.clone()) {
+					Some(req_id) => AncestorSearch::Awaiting(req_id, start, req),
+					None => AncestorSearch::Queued(start),
+				}
+			}
+			other => other,
+		}
+	}
+}
+
+// synchronization state machine.
+enum SyncState {
+	// Idle (waiting for peers)
+	Idle,
+	// searching for common ancestor with best chain.
+	// queue should be cleared at this phase.
+	AncestorSearch(AncestorSearch),
+	// Doing sync rounds.
+	Rounds(SyncRound),
+}
+
+struct ResponseCtx<'a> {
+	peer: PeerId,
+	req_id: ReqId,
+	ctx: &'a BasicContext,
+	data: &'a [Bytes],
+}
+
+impl<'a> ResponseContext for ResponseCtx<'a> {
+	fn responder(&self) -> PeerId { self.peer }
+	fn req_id(&self) -> &ReqId { &self.req_id }
+	fn data(&self) -> &[Bytes] { self.data }
+	fn punish_responder(&self) { self.ctx.disable_peer(self.peer) }
+}
+
+/// Light client synchronization manager. See module docs for more details.
+pub struct LightSync<L: LightChainClient> {
+	best_seen: Mutex<Option<(H256, U256)>>, // best seen block on the network.
+	peers: RwLock<HashMap<PeerId, Mutex<Peer>>>, // peers which are relevant to synchronization.
+	client: Arc<L>,
+	rng: Mutex<OsRng>,
+	state: Mutex<SyncState>,
+}
+
+impl<L: LightChainClient> Handler for LightSync<L> {
+	fn on_connect(&self, ctx: &EventContext, status: &Status, capabilities: &Capabilities) {
+		let our_best = self.client.chain_info().best_block_number;
+
+		if !capabilities.serve_headers || status.head_num <= our_best {
+			trace!(target: "sync", "Disconnecting irrelevant peer: {}", ctx.peer());
+			ctx.disconnect_peer(ctx.peer());
+			return;
+		}
+
+		let chain_info = ChainInfo {
+			head_td: status.head_td,
+			head_hash: status.head_hash,
+			head_num: status.head_num,
+		};
+
+		{
+			let mut best = self.best_seen.lock();
+			if best.as_ref().map_or(true, |b| status.head_td > b.1) {
+				*best = Some((status.head_hash, status.head_td));
+			}
+		}
+
+		self.peers.write().insert(ctx.peer(), Mutex::new(Peer::new(chain_info)));
+		self.maintain_sync(ctx.as_basic());
+	}
+
+	fn on_disconnect(&self, ctx: &EventContext, unfulfilled: &[ReqId]) {
+		let peer_id = ctx.peer();
+
+		let peer = match self.peers.write().remove(&peer_id).map(|p| p.into_inner()) {
+			Some(peer) => peer,
+			None => return,
+		};
+
+		trace!(target: "sync", "peer {} disconnecting", peer_id);
+
+		let new_best = {
+			let mut best = self.best_seen.lock();
+			let peer_best = (peer.status.head_hash, peer.status.head_td);
+
+			if best.as_ref().map_or(false, |b| b == &peer_best) {
+				// search for next-best block.
+				let next_best: Option<(H256, U256)> = self.peers.read().values()
+					.map(|p| p.lock())
+					.map(|p| (p.status.head_hash, p.status.head_td))
+					.fold(None, |acc, x| match acc {
+						Some(acc) => if x.1 > acc.1 { Some(x) } else { Some(acc) },
+						None => Some(x),
+					});
+
+				*best = next_best;
+			}
+
+			best.clone()
+		};
+
+		if new_best.is_none() {
+			debug!(target: "sync", "No peers remain. Reverting to idle");
+			*self.state.lock() = SyncState::Idle;
+		} else {
+			let mut state = self.state.lock();
+
+			*state = match mem::replace(&mut *state, SyncState::Idle) {
+				SyncState::Idle => SyncState::Idle,
+				SyncState::AncestorSearch(search) => SyncState::AncestorSearch(search),
+				SyncState::Rounds(round) => SyncState::Rounds(round.requests_abandoned(unfulfilled)),
+			};
+		}
+
+		self.maintain_sync(ctx.as_basic());
+	}
+
+	fn on_announcement(&self, ctx: &EventContext, announcement: &Announcement) {
+		let last_td = {
+			let peers = self.peers.read();
+			match peers.get(&ctx.peer()) {
+				None => return,
+				Some(peer) => {
+					let mut peer = peer.lock();
+					let last_td = peer.status.head_td;
+					peer.status = ChainInfo {
+						head_td: announcement.head_td,
+						head_hash: announcement.head_hash,
+						head_num: announcement.head_num,
+					};
+					last_td
+				}
+			}
+		};
+
+		trace!(target: "sync", "Announcement from peer {}: new chain head {:?}, reorg depth {}",
+			ctx.peer(), (announcement.head_hash, announcement.head_num), announcement.reorg_depth);
+
+		if last_td > announcement.head_td {
+			trace!(target: "sync", "Peer {} moved backwards.", ctx.peer());
+			self.peers.write().remove(&ctx.peer());
+			ctx.disconnect_peer(ctx.peer());
+		}
+
+		{
+			let mut best = self.best_seen.lock();
+			if best.as_ref().map_or(true, |b| announcement.head_td > b.1) {
+				*best = Some((announcement.head_hash, announcement.head_td));
+			}
+		}
+
+		self.maintain_sync(ctx.as_basic());
+	}
+
+	fn on_block_headers(&self, ctx: &EventContext, req_id: ReqId, headers: &[Bytes]) {
+		if !self.peers.read().contains_key(&ctx.peer()) {
+			return
+		}
+
+		{
+			let mut state = self.state.lock();
+
+			let ctx = ResponseCtx {
+				peer: ctx.peer(),
+				req_id: req_id,
+				ctx: ctx.as_basic(),
+				data: headers,
+			};
+
+			*state = match mem::replace(&mut *state, SyncState::Idle) {
+				SyncState::Idle => SyncState::Idle,
+				SyncState::AncestorSearch(search) =>
+					SyncState::AncestorSearch(search.process_response(&ctx, &*self.client)),
+				SyncState::Rounds(round) => SyncState::Rounds(round.process_response(&ctx)),
+			};
+		}
+
+		self.maintain_sync(ctx.as_basic());
+	}
+
+	fn tick(&self, ctx: &BasicContext) {
+		self.maintain_sync(ctx);
+	}
+}
+
+// private helpers
+impl<L: LightChainClient> LightSync<L> {
+	// Begins a search for the common ancestor and our best block.
+	// does not lock state, instead has a mutable reference to it passed.
+	fn begin_search(&self, state: &mut SyncState) {
+		if let None =  *self.best_seen.lock() {
+			// no peers.
+			*state = SyncState::Idle;
+			return;
+		}
+
+		trace!(target: "sync", "Beginning search for common ancestor");
+		self.client.clear_queue();
+		let chain_info = self.client.chain_info();
+
+		*state = SyncState::AncestorSearch(AncestorSearch::begin(chain_info.best_block_number));
+	}
+
+	fn maintain_sync(&self, ctx: &BasicContext) {
+		const DRAIN_AMOUNT: usize = 128;
+
+		debug!(target: "sync", "Maintaining sync.");
+
+		let mut state = self.state.lock();
+
+		// drain any pending blocks into the queue.
+		{
+			let mut sink = Vec::with_capacity(DRAIN_AMOUNT);
+
+			'a:
+			loop {
+				let queue_info = self.client.queue_info();
+				if queue_info.is_full() { break }
+
+				*state = match mem::replace(&mut *state, SyncState::Idle) {
+					SyncState::Rounds(round)
+						=> SyncState::Rounds(round.drain(&mut sink, Some(DRAIN_AMOUNT))),
+					other => other,
+				};
+
+				if sink.is_empty() { break }
+
+				for header in sink.drain(..) {
+					if let Err(e) = self.client.queue_header(header) {
+						debug!(target: "sync", "Found bad header ({:?}). Reset to search state.", e);
+
+						self.begin_search(&mut state);
+						break 'a;
+					}
+				}
+			}
+		}
+
+		// handle state transitions.
+		{
+			match mem::replace(&mut *state, SyncState::Idle) {
+				SyncState::Rounds(SyncRound::Abort(reason)) => {
+					match reason {
+						AbortReason::BadScaffold(bad_peers) => {
+							debug!(target: "sync", "Disabling peers responsible for bad scaffold");
+							for peer in bad_peers {
+								ctx.disable_peer(peer);
+							}
+						}
+						AbortReason::NoResponses => {}
+					}
+
+					debug!(target: "sync", "Beginning search after aborted sync round");
+					self.begin_search(&mut state);
+				}
+				SyncState::AncestorSearch(AncestorSearch::FoundCommon(num, hash)) => {
+					// TODO: compare to best block and switch to another downloading
+					// method when close.
+					*state = SyncState::Rounds(SyncRound::begin(num, hash));
+				}
+				SyncState::AncestorSearch(AncestorSearch::Genesis) => {
+					// Same here.
+					let g_hash = self.client.chain_info().genesis_hash;
+					*state = SyncState::Rounds(SyncRound::begin(0, g_hash));
+				}
+				SyncState::Idle => self.begin_search(&mut state),
+				other => *state = other, // restore displaced state.
+			}
+		}
+
+		// allow dispatching of requests.
+		// TODO: maybe wait until the amount of cumulative requests remaining is high enough
+		// to avoid pumping the failure rate.
+		{
+			let peers = self.peers.read();
+			let mut peer_ids: Vec<_> = peers.keys().cloned().collect();
+			let mut rng = self.rng.lock();
+
+			// naive request dispatcher: just give to any peer which says it will
+			// give us responses.
+			let dispatcher = move |req: request::Headers| {
+				rng.shuffle(&mut peer_ids);
+
+				for peer in &peer_ids {
+					if ctx.max_requests(*peer, request::Kind::Headers) >= req.max {
+						match ctx.request_from(*peer, request::Request::Headers(req.clone())) {
+							Ok(id) => {
+								return Some(id)
+							}
+							Err(e) =>
+								trace!(target: "sync", "Error requesting headers from viable peer: {}", e),
+						}
+					}
+				}
+
+				None
+			};
+
+			*state = match mem::replace(&mut *state, SyncState::Idle) {
+				SyncState::Rounds(round) =>
+					SyncState::Rounds(round.dispatch_requests(dispatcher)),
+				SyncState::AncestorSearch(search) =>
+					SyncState::AncestorSearch(search.dispatch_request(dispatcher)),
+				other => other,
+			};
+		}
+	}
+}
+
+// public API
+impl<L: LightChainClient> LightSync<L> {
+	/// Create a new instance of `LightSync`.
+	///
+	/// This won't do anything until registered as a handler
+	/// so it can act on events.
+	pub fn new(client: Arc<L>) -> Result<Self, ::std::io::Error> {
+		Ok(LightSync {
+			best_seen: Mutex::new(None),
+			peers: RwLock::new(HashMap::new()),
+			client: client,
+			rng: Mutex::new(try!(OsRng::new())),
+			state: Mutex::new(SyncState::Idle),
+		})
+	}
+}

sync/src/light_sync/response.rs

@@ -0,0 +1,254 @@
+// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Helpers for decoding and verifying responses for headers.
+
+use std::fmt;
+
+use ethcore::header::Header;
+use light::request::{HashOrNumber, Headers as HeadersRequest};
+use rlp::{DecoderError, UntrustedRlp, View};
+use util::{Bytes, H256};
+
+/// Errors found when decoding headers and verifying with basic constraints.
+#[derive(Debug, PartialEq)]
+pub enum BasicError {
+	/// Wrong skip value: expected, found (if any).
+	WrongSkip(u64, Option<u64>),
+	/// Wrong start number.
+	WrongStartNumber(u64, u64),
+	/// Wrong start hash.
+	WrongStartHash(H256, H256),
+	/// Too many headers.
+	TooManyHeaders(usize, usize),
+	/// Decoder error.
+	Decoder(DecoderError),
+}
+
+impl From<DecoderError> for BasicError {
+	fn from(err: DecoderError) -> Self {
+		BasicError::Decoder(err)
+	}
+}
+
+impl fmt::Display for BasicError {
+	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+		try!(write!(f, "Header response verification error: "));
+
+		match *self {
+			BasicError::WrongSkip(ref exp, ref got)
+				=> write!(f, "wrong skip (expected {}, got {:?})", exp, got),
+			BasicError::WrongStartNumber(ref exp, ref got)
+				=> write!(f, "wrong start number (expected {}, got {})", exp, got),
+			BasicError::WrongStartHash(ref exp, ref got)
+				=> write!(f, "wrong start hash (expected {}, got {})", exp, got),
+			BasicError::TooManyHeaders(ref max, ref got)
+				=> write!(f, "too many headers (max {}, got {})", max, got),
+			BasicError::Decoder(ref err)
+				=> write!(f, "{}", err),
+		}
+	}
+}
+
+/// Request verification constraint.
+pub trait Constraint {
+	type Error;
+
+	/// Verify headers against this.
+	fn verify(&self, headers: &[Header], reverse: bool) -> Result<(), Self::Error>;
+}
+
+/// Decode a response and do basic verification against a request.
+pub fn decode_and_verify(headers: &[Bytes], request: &HeadersRequest) -> Result<Vec<Header>, BasicError> {
+	let headers: Vec<_> = try!(headers.iter().map(|x| UntrustedRlp::new(&x).as_val()).collect());
+
+	let reverse = request.reverse;
+
+	try!(Max(request.max).verify(&headers, reverse));
+	match request.start {
+		HashOrNumber::Number(ref num) => try!(StartsAtNumber(*num).verify(&headers, reverse)),
+		HashOrNumber::Hash(ref hash) => try!(StartsAtHash(*hash).verify(&headers, reverse)),
+	}
+
+	try!(SkipsBetween(request.skip).verify(&headers, reverse));
+
+	Ok(headers)
+}
+
+struct StartsAtNumber(u64);
+struct StartsAtHash(H256);
+struct SkipsBetween(u64);
+struct Max(usize);
+
+impl Constraint for StartsAtNumber {
+	type Error = BasicError;
+
+	fn verify(&self, headers: &[Header], _reverse: bool) -> Result<(), BasicError> {
+		headers.first().map_or(Ok(()), |h| {
+			if h.number() == self.0 {
+				Ok(())
+			} else {
+				Err(BasicError::WrongStartNumber(self.0, h.number()))
+			}
+		})
+	}
+}
+
+impl Constraint for StartsAtHash {
+	type Error = BasicError;
+
+	fn verify(&self, headers: &[Header], _reverse: bool) -> Result<(), BasicError> {
+		headers.first().map_or(Ok(()), |h| {
+			if h.hash() == self.0 {
+				Ok(())
+			} else {
+				Err(BasicError::WrongStartHash(self.0, h.hash()))
+			}
+		})
+	}
+}
+
+impl Constraint for SkipsBetween {
+	type Error = BasicError;
+
+	fn verify(&self, headers: &[Header], reverse: bool) -> Result<(), BasicError> {
+		for pair in headers.windows(2) {
+			let (low, high) = if reverse { (&pair[1], &pair[0]) } else { (&pair[0], &pair[1]) };
+			if low.number() >= high.number() { return Err(BasicError::WrongSkip(self.0, None)) }
+
+			let skip = (high.number() - low.number()) - 1;
+			if skip != self.0 { return Err(BasicError::WrongSkip(self.0, Some(skip))) }
+		}
+
+		Ok(())
+	}
+}
+
+impl Constraint for Max {
+	type Error = BasicError;
+
+	fn verify(&self, headers: &[Header], _reverse: bool) -> Result<(), BasicError> {
+		match headers.len() > self.0 {
+			true => Err(BasicError::TooManyHeaders(self.0, headers.len())),
+			false => Ok(())
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use ethcore::header::Header;
+	use light::request::Headers as HeadersRequest;
+
+	use super::*;
+
+	#[test]
+	fn sequential_forward() {
+		let request = HeadersRequest {
+			start: 10.into(),
+			max: 30,
+			skip: 0,
+			reverse: false,
+		};
+
+		let mut parent_hash = None;
+		let headers: Vec<_> = (0..25).map(|x| x + 10).map(|x| {
+			let mut header = Header::default();
+			header.set_number(x);
+
+			if let Some(parent_hash) = parent_hash {
+				header.set_parent_hash(parent_hash);
+			}
+
+			parent_hash = Some(header.hash());
+
+			::rlp::encode(&header).to_vec()
+		}).collect();
+
+		assert!(decode_and_verify(&headers, &request).is_ok());
+	}
+
+	#[test]
+	fn sequential_backward() {
+		let request = HeadersRequest {
+			start: 34.into(),
+			max: 30,
+			skip: 0,
+			reverse: true,
+		};
+
+		let mut parent_hash = None;
+		let headers: Vec<_> = (0..25).map(|x| x + 10).rev().map(|x| {
+			let mut header = Header::default();
+			header.set_number(x);
+
+			if let Some(parent_hash) = parent_hash {
+				header.set_parent_hash(parent_hash);
+			}
+
+			parent_hash = Some(header.hash());
+
+			::rlp::encode(&header).to_vec()
+		}).collect();
+
+		assert!(decode_and_verify(&headers, &request).is_ok());
+	}
+
+	#[test]
+	fn too_many() {
+		let request = HeadersRequest {
+			start: 10.into(),
+			max: 20,
+			skip: 0,
+			reverse: false,
+		};
+
+		let mut parent_hash = None;
+		let headers: Vec<_> = (0..25).map(|x| x + 10).map(|x| {
+			let mut header = Header::default();
+			header.set_number(x);
+
+			if let Some(parent_hash) = parent_hash {
+				header.set_parent_hash(parent_hash);
+			}
+
+			parent_hash = Some(header.hash());
+
+			::rlp::encode(&header).to_vec()
+		}).collect();
+
+		assert_eq!(decode_and_verify(&headers, &request), Err(BasicError::TooManyHeaders(20, 25)));
+	}
+
+	#[test]
+	fn wrong_skip() {
+		let request = HeadersRequest {
+			start: 10.into(),
+			max: 30,
+			skip: 5,
+			reverse: false,
+		};
+
+		let headers: Vec<_> = (0..25).map(|x| x * 3).map(|x| x + 10).map(|x| {
+			let mut header = Header::default();
+			header.set_number(x);
+
+			::rlp::encode(&header).to_vec()
+		}).collect();
+
+		assert_eq!(decode_and_verify(&headers, &request), Err(BasicError::WrongSkip(5, Some(2))));
+	}
+}

sync/src/light_sync/sync_round.rs

@@ -0,0 +1,456 @@
+// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Header download state machine.
+
+use std::cmp::Ordering;
+use std::collections::{BinaryHeap, HashMap, HashSet, VecDeque};
+
+use ethcore::header::Header;
+
+use light::net::ReqId;
+use light::request::Headers as HeadersRequest;
+
+use network::PeerId;
+use util::{Bytes, H256};
+
+use super::response;
+
+// amount of blocks between each scaffold entry.
+// TODO: move these into parameters for `RoundStart::new`?
+const ROUND_SKIP: u64 = 255;
+
+// amount of scaffold frames: these are the blank spaces in "X___X___X"
+const ROUND_FRAMES: usize = 255;
+
+// number of attempts to make to get a full scaffold for a sync round.
+const SCAFFOLD_ATTEMPTS: usize = 3;
+
+/// Context for a headers response.
+pub trait ResponseContext {
+	/// Get the peer who sent this response.
+	fn responder(&self) ->	PeerId;
+	/// Get the request ID this response corresponds to.
+	fn req_id(&self) -> &ReqId;
+	/// Get the (unverified) response data.
+	fn data(&self) -> &[Bytes];
+	/// Punish the responder.
+	fn punish_responder(&self);
+}
+
+/// Reasons for sync round abort.
+#[derive(Debug, Clone)]
+pub enum AbortReason {
+	/// Bad sparse header chain along with a list of peers who contributed to it.
+	BadScaffold(Vec<PeerId>),
+	/// No incoming data.
+	NoResponses,
+}
+
+// A request for headers with a known starting header hash.
+// and a known parent hash for the last block.
+#[derive(PartialEq, Eq)]
+struct SubchainRequest {
+	subchain_parent: (u64, H256),
+	headers_request: HeadersRequest,
+	subchain_end: (u64, H256),
+	downloaded: VecDeque<Header>,
+}
+
+// ordered by subchain parent number so pending requests towards the
+// front of the round are dispatched first.
+impl PartialOrd for SubchainRequest {
+	fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+		self.subchain_parent.0
+			.partial_cmp(&other.subchain_parent.0)
+			.map(Ordering::reverse)
+	}
+}
+
+impl Ord for SubchainRequest {
+	fn cmp(&self, other: &Self) -> Ordering {
+		self.subchain_parent.0.cmp(&other.subchain_parent.0).reverse()
+	}
+}
+
+/// Manages downloading of interior blocks of a sparse header chain.
+pub struct Fetcher {
+	sparse: VecDeque<Header>, // sparse header chain.
+	requests: BinaryHeap<SubchainRequest>,
+	complete_requests: HashMap<H256, SubchainRequest>,
+	pending: HashMap<ReqId, SubchainRequest>,
+	scaffold_contributors: Vec<PeerId>,
+	ready: VecDeque<Header>,
+	end: (u64, H256),
+}
+
+impl Fetcher {
+	// Produce a new fetcher given a sparse headerchain, in ascending order along
+	// with a list of peers who helped produce the chain.
+	// The headers must be valid RLP at this point and must have a consistent
+	// non-zero gap between them. Will abort the round if found wrong.
+	fn new(sparse_headers: Vec<Header>, contributors: Vec<PeerId>) -> SyncRound {
+		let mut requests = BinaryHeap::with_capacity(sparse_headers.len() - 1);
+
+		for pair in sparse_headers.windows(2) {
+			let low_rung = &pair[0];
+			let high_rung = &pair[1];
+
+			let diff = high_rung.number() - low_rung.number();
+
+			// should never happen as long as we verify the gaps
+			// gotten from SyncRound::Start
+			if diff < 2 { continue }
+
+			let needed_headers = HeadersRequest {
+				start: high_rung.parent_hash().clone().into(),
+				max: diff as usize - 1,
+				skip: 0,
+				reverse: true,
+			};
+
+			requests.push(SubchainRequest {
+				headers_request: needed_headers,
+				subchain_end: (high_rung.number() - 1, *high_rung.parent_hash()),
+				downloaded: VecDeque::new(),
+				subchain_parent: (low_rung.number(), low_rung.hash()),
+			});
+		}
+
+		let end = match sparse_headers.last().map(|h| (h.number(), h.hash())) {
+			Some(end) => end,
+			None => return SyncRound::abort(AbortReason::BadScaffold(contributors)),
+		};
+
+		SyncRound::Fetch(Fetcher {
+			sparse: sparse_headers.into(),
+			requests: requests,
+			complete_requests: HashMap::new(),
+			pending: HashMap::new(),
+			scaffold_contributors: contributors,
+			ready: VecDeque::new(),
+			end: end,
+		})
+	}
+
+	// collect complete requests and their subchain from the sparse header chain
+	// into the ready set in order.
+	fn collect_ready(&mut self) {
+		loop {
+			let start_hash = match self.sparse.front() {
+				Some(first) => first.hash(),
+				None => break,
+			};
+
+			match self.complete_requests.remove(&start_hash) {
+				None => break,
+				Some(complete_req) => {
+					self.ready.push_back(self.sparse.pop_front().expect("first known to exist; qed"));
+					self.ready.extend(complete_req.downloaded);
+				}
+			}
+		}
+
+		// frames are between two sparse headers and keyed by subchain parent, so the last
+		// remaining will be the last header.
+		if self.sparse.len() == 1 {
+			self.ready.push_back(self.sparse.pop_back().expect("sparse known to have one entry; qed"))
+		}
+	}
+
+	fn process_response<R: ResponseContext>(mut self, ctx: &R) -> SyncRound {
+		let mut request = match self.pending.remove(ctx.req_id()) {
+			Some(request) => request,
+			None => return SyncRound::Fetch(self),
+		};
+
+		let headers = ctx.data();
+
+		if headers.len() == 0 {
+			trace!(target: "sync", "Punishing peer {} for empty response", ctx.responder());
+			ctx.punish_responder();
+			return SyncRound::Fetch(self);
+		}
+
+		match response::decode_and_verify(headers, &request.headers_request) {
+			Err(e) => {
+				trace!(target: "sync", "Punishing peer {} for invalid response ({})", ctx.responder(), e);
+				ctx.punish_responder();
+
+				// TODO: track number of attempts per request,
+				// abort if failure rate too high.
+				self.requests.push(request);
+				SyncRound::Fetch(self)
+			}
+			Ok(headers) => {
+				let mut parent_hash = None;
+				for header in headers {
+					if parent_hash.as_ref().map_or(false, |h| h != &header.hash()) {
+						trace!(target: "sync", "Punishing peer {} for parent mismatch", ctx.responder());
+						ctx.punish_responder();
+
+						self.requests.push(request);
+						return SyncRound::Fetch(self);
+					}
+
+					// incrementally update the frame request as we go so we can
+					// return at any time in the loop.
+					parent_hash = Some(header.parent_hash().clone());
+					request.headers_request.start = header.parent_hash().clone().into();
+					request.headers_request.max -= 1;
+
+					request.downloaded.push_front(header);
+				}
+
+				let subchain_parent = request.subchain_parent.1;
+
+				if request.headers_request.max == 0 {
+					if parent_hash.map_or(true, |hash| hash != subchain_parent) {
+						let abort = AbortReason::BadScaffold(self.scaffold_contributors);
+						return SyncRound::Abort(abort);
+					}
+
+					self.complete_requests.insert(subchain_parent, request);
+					self.collect_ready();
+				}
+
+				// state transition not triggered until drain is finished.
+				(SyncRound::Fetch(self))
+			}
+		}
+	}
+
+	fn requests_abandoned(mut self, abandoned: &[ReqId]) -> SyncRound {
+		for abandoned in abandoned {
+			match self.pending.remove(abandoned) {
+				None => {},
+				Some(req) => self.requests.push(req),
+			}
+		}
+
+		// TODO: track failure rate and potentially abort.
+		SyncRound::Fetch(self)
+	}
+
+	fn dispatch_requests<D>(mut self, mut dispatcher: D) -> SyncRound
+		where D: FnMut(HeadersRequest) -> Option<ReqId>
+	{
+		while let Some(pending_req) = self.requests.pop() {
+			match dispatcher(pending_req.headers_request.clone()) {
+				Some(req_id) => {
+					trace!(target: "sync", "Assigned request for subchain ({} -> {})",
+						pending_req.subchain_parent.0 + 1, pending_req.subchain_end.0);
+
+					self.pending.insert(req_id, pending_req);
+				}
+				None => {
+					self.requests.push(pending_req);
+					break;
+				}
+			}
+		}
+
+		SyncRound::Fetch(self)
+	}
+
+	fn drain(mut self, headers: &mut Vec<Header>, max: Option<usize>) -> SyncRound {
+		let max = ::std::cmp::min(max.unwrap_or(usize::max_value()), self.ready.len());
+		headers.extend(self.ready.drain(0..max));
+
+		if self.sparse.is_empty() && self.ready.is_empty() {
+			SyncRound::Start(RoundStart::new(self.end))
+		} else {
+			SyncRound::Fetch(self)
+		}
+	}
+}
+
+/// Round started: get stepped header chain.
+/// from a start block with number X we request 256 headers stepped by 256 from
+/// block X + 1.
+pub struct RoundStart {
+	start_block: (u64, H256),
+	pending_req: Option<(ReqId, HeadersRequest)>,
+	sparse_headers: Vec<Header>,
+	contributors: HashSet<PeerId>,
+	attempt: usize,
+}
+
+impl RoundStart {
+	fn new(start: (u64, H256)) -> Self {
+		RoundStart {
+			start_block: start.clone(),
+			pending_req: None,
+			sparse_headers: Vec::new(),
+			contributors: HashSet::new(),
+			attempt: 0,
+		}
+	}
+
+	// called on failed attempt. may trigger a transition after a number of attempts.
+	// a failed attempt is defined as any time a peer returns invalid or incomplete response
+	fn failed_attempt(mut self) -> SyncRound {
+		self.attempt += 1;
+
+		if self.attempt >= SCAFFOLD_ATTEMPTS {
+			if self.sparse_headers.len() > 1 {
+				Fetcher::new(self.sparse_headers, self.contributors.into_iter().collect())
+			} else {
+				SyncRound::Abort(AbortReason::NoResponses)
+			}
+		} else {
+			SyncRound::Start(self)
+		}
+	}
+
+	fn process_response<R: ResponseContext>(mut self, ctx: &R) -> SyncRound {
+		let req = match self.pending_req.take() {
+			Some((id, ref req)) if ctx.req_id() == &id => { req.clone() }
+			other => {
+				self.pending_req = other;
+				return SyncRound::Start(self);
+			}
+		};
+
+		match response::decode_and_verify(ctx.data(), &req) {
+			Ok(headers) => {
+				if self.sparse_headers.len() == 0
+					&& headers.get(0).map_or(false, |x| x.parent_hash() != &self.start_block.1) {
+					trace!(target: "sync", "Wrong parent for first header in round");
+					ctx.punish_responder(); // or should we reset?
+				}
+
+				self.contributors.insert(ctx.responder());
+				self.sparse_headers.extend(headers);
+
+				if self.sparse_headers.len() == ROUND_FRAMES + 1 {
+					trace!(target: "sync", "Beginning fetch of blocks between {} sparse headers",
+						self.sparse_headers.len());
+
+					return Fetcher::new(self.sparse_headers, self.contributors.into_iter().collect());
+				}
+			}
+			Err(e) => {
+				trace!(target: "sync", "Punishing peer {} for malformed response ({})", ctx.responder(), e);
+				ctx.punish_responder();
+			}
+		};
+
+		self.failed_attempt()
+	}
+
+	fn requests_abandoned(mut self, abandoned: &[ReqId]) -> SyncRound {
+		match self.pending_req.take() {
+			Some((id, req)) => {
+				if abandoned.iter().any(|r| r == &id) {
+					self.pending_req = None;
+					self.failed_attempt()
+				} else {
+					self.pending_req = Some((id, req));
+					SyncRound::Start(self)
+				}
+			}
+			None => SyncRound::Start(self),
+		}
+	}
+
+	fn dispatch_requests<D>(mut self, mut dispatcher: D) -> SyncRound
+		where D: FnMut(HeadersRequest) -> Option<ReqId>
+	{
+		if self.pending_req.is_none() {
+			// beginning offset + first block expected after last header we have.
+			let start = (self.start_block.0 + 1)
+				+ self.sparse_headers.len() as u64 * (ROUND_SKIP + 1);
+
+			let headers_request = HeadersRequest {
+				start: start.into(),
+				max: (ROUND_FRAMES - 1) - self.sparse_headers.len(),
+				skip: ROUND_SKIP,
+				reverse: false,
+			};
+
+			if let Some(req_id) = dispatcher(headers_request.clone()) {
+				self.pending_req = Some((req_id, headers_request));
+			}
+		}
+
+		SyncRound::Start(self)
+	}
+}
+
+/// Sync round state machine.
+pub enum SyncRound {
+	/// Beginning a sync round.
+	Start(RoundStart),
+	/// Fetching intermediate blocks during a sync round.
+	Fetch(Fetcher),
+	/// Aborted.
+	Abort(AbortReason),
+}
+
+impl SyncRound {
+	fn abort(reason: AbortReason) -> Self {
+		trace!(target: "sync", "Aborting sync round: {:?}", reason);
+
+		SyncRound::Abort(reason)
+	}
+
+	/// Begin sync rounds from a starting block.
+	pub fn begin(num: u64, hash: H256) -> Self {
+		SyncRound::Start(RoundStart::new((num, hash)))
+	}
+
+	/// Process an answer to a request. Unknown requests will be ignored.
+	pub fn process_response<R: ResponseContext>(self, ctx: &R) -> Self {
+		match self {
+			SyncRound::Start(round_start) => round_start.process_response(ctx),
+			SyncRound::Fetch(fetcher) => fetcher.process_response(ctx),
+			other => other,
+		}
+	}
+
+	/// Return unfulfilled requests from disconnected peer. Unknown requests will be ignored.
+	pub fn requests_abandoned(self, abandoned: &[ReqId]) -> Self {
+		match self {
+			SyncRound::Start(round_start) => round_start.requests_abandoned(abandoned),
+			SyncRound::Fetch(fetcher) => fetcher.requests_abandoned(abandoned),
+			other => other,
+		}
+	}
+
+	/// Dispatch pending requests. The dispatcher provided will attempt to
+	/// find a suitable peer to serve the request.
+	// TODO: have dispatcher take capabilities argument? and return an error as
+	// to why no suitable peer can be found? (no buffer, no chain heads that high, etc)
+	pub fn dispatch_requests<D>(self, dispatcher: D) -> Self
+		where D: FnMut(HeadersRequest) -> Option<ReqId>
+	{
+		match self {
+			SyncRound::Start(round_start) => round_start.dispatch_requests(dispatcher),
+			SyncRound::Fetch(fetcher) => fetcher.dispatch_requests(dispatcher),
+			other => other,
+		}
+	}
+
+	/// Drain up to a maximum number (None -> all) of headers (continuous, starting with a child of
+	/// the round start block) from the round, starting a new one once finished.
+	pub fn drain(self, v: &mut Vec<Header>, max: Option<usize>) -> Self {
+		match self {
+			SyncRound::Fetch(fetcher) => fetcher.drain(v, max),
+			other => other,
+		}
+	}
+}