diff --git a/Cargo.lock b/Cargo.lock
index 1d81b0474..b9cbff049 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -513,7 +513,9 @@ dependencies = [
  "ethcore-ipc-codegen 1.6.0",
  "ethcore-network 1.6.0",
  "ethcore-util 1.6.0",
+ "futures 0.1.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rand 0.3.14 (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)",
diff --git a/ethcore/light/Cargo.toml b/ethcore/light/Cargo.toml
index 9372f522f..534b0e5c1 100644
--- a/ethcore/light/Cargo.toml
+++ b/ethcore/light/Cargo.toml
@@ -20,6 +20,8 @@ ethcore-ipc = { path = "../../ipc/rpc", optional = true }
 rlp = { path = "../../util/rlp" }
 time = "0.1"
 smallvec = "0.3.1"
+futures = "0.1"
+rand = "0.3"
 
 [features]
 default = []
diff --git a/ethcore/light/src/lib.rs b/ethcore/light/src/lib.rs
index 5cdc3addc..0e650f992 100644
--- a/ethcore/light/src/lib.rs
+++ b/ethcore/light/src/lib.rs
@@ -34,6 +34,7 @@
 
 pub mod client;
 pub mod net;
+pub mod on_demand;
 
 #[cfg(not(feature = "ipc"))]
 pub mod provider;
@@ -64,6 +65,8 @@ extern crate ethcore_io as io;
 extern crate rlp;
 extern crate smallvec;
 extern crate time;
+extern crate futures;
+extern crate rand;
 
 #[cfg(feature = "ipc")]
 extern crate ethcore_ipc as ipc;
diff --git a/ethcore/light/src/net/mod.rs b/ethcore/light/src/net/mod.rs
index 03a19e14b..18425152c 100644
--- a/ethcore/light/src/net/mod.rs
+++ b/ethcore/light/src/net/mod.rs
@@ -30,6 +30,7 @@ use util::{Bytes, Mutex, RwLock, U256};
 use time::{Duration, SteadyTime};
 
 use std::collections::HashMap;
+use std::fmt;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
@@ -123,6 +124,12 @@ mod timeout {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct ReqId(usize);
 
+impl fmt::Display for ReqId {
+	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+		write!(f, "Request #{}", self.0)
+	}
+}
+
 // A pending peer: one we've sent our status to but
 // may not have received one for.
 struct PendingPeer {
@@ -186,12 +193,12 @@ pub trait Handler: Send + Sync {
 	fn on_block_headers(&self, _ctx: &EventContext, _req_id: ReqId, _headers: &[Bytes]) { }
 	/// Called when a peer responds with block receipts.
 	fn on_receipts(&self, _ctx: &EventContext, _req_id: ReqId, _receipts: &[Vec<Receipt>]) { }
-	/// Called when a peer responds with state proofs. Each proof is a series of trie
+	/// Called when a peer responds with state proofs. Each proof should be a series of trie
 	/// nodes in ascending order by distance from the root.
 	fn on_state_proofs(&self, _ctx: &EventContext, _req_id: ReqId, _proofs: &[Vec<Bytes>]) { }
 	/// Called when a peer responds with contract code.
 	fn on_code(&self, _ctx: &EventContext, _req_id: ReqId, _codes: &[Bytes]) { }
-	/// Called when a peer responds with header proofs. Each proof is a block header coupled
+	/// Called when a peer responds with header proofs. Each proof should be 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 to "tick" the handler periodically.
diff --git a/ethcore/light/src/net/status.rs b/ethcore/light/src/net/status.rs
index d058bc2f2..a11ed5b11 100644
--- a/ethcore/light/src/net/status.rs
+++ b/ethcore/light/src/net/status.rs
@@ -158,6 +158,16 @@ pub struct Status {
 	pub last_head: Option<(H256, u64)>,
 }
 
+impl Status {
+	/// Update the status from an announcement.
+	pub fn update_from(&mut self, announcement: &Announcement) {
+		self.last_head = Some((self.head_hash, announcement.reorg_depth));
+		self.head_td = announcement.head_td;
+		self.head_hash = announcement.head_hash;
+		self.head_num = announcement.head_num;
+	}
+}
+
 /// Peer capabilities.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Capabilities {
diff --git a/ethcore/light/src/on_demand/mod.rs b/ethcore/light/src/on_demand/mod.rs
new file mode 100644
index 000000000..aadb43c0e
--- /dev/null
+++ b/ethcore/light/src/on_demand/mod.rs
@@ -0,0 +1,520 @@
+// 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/>.
+
+//! On-demand chain requests over LES. This is a major building block for RPCs.
+//! The request service is implemented using Futures. Higher level request handlers
+//! will take the raw data received here and extract meaningful results from it.
+
+use std::collections::HashMap;
+
+use ethcore::basic_account::BasicAccount;
+use ethcore::encoded;
+use ethcore::receipt::Receipt;
+
+use futures::{Async, Poll, Future};
+use futures::sync::oneshot;
+use network::PeerId;
+
+use net::{Handler, Status, Capabilities, Announcement, EventContext, BasicContext, ReqId};
+use util::{Bytes, RwLock};
+use types::les_request::{self as les_request, Request as LesRequest};
+
+pub mod request;
+
+/// Errors which can occur while trying to fulfill a request.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Error {
+	/// Request was canceled.
+	Canceled,
+	/// No suitable peers available to serve the request.
+	NoPeersAvailable,
+	/// Request timed out.
+	TimedOut,
+}
+
+impl From<oneshot::Canceled> for Error {
+	fn from(_: oneshot::Canceled) -> Self {
+		Error::Canceled
+	}
+}
+
+/// Future which awaits a response to an on-demand request.
+pub struct Response<T>(oneshot::Receiver<Result<T, Error>>);
+
+impl<T> Future for Response<T> {
+	type Item = T;
+	type Error = Error;
+
+	fn poll(&mut self) -> Poll<T, Error> {
+		match self.0.poll().map_err(Into::into) {
+			Ok(Async::Ready(val)) => val.map(Async::Ready),
+			Ok(Async::NotReady) => Ok(Async::NotReady),
+			Err(e) => Err(e),
+		}
+	}
+}
+
+type Sender<T> = oneshot::Sender<Result<T, Error>>;
+
+// relevant peer info.
+struct Peer {
+	status: Status,
+	capabilities: Capabilities,
+}
+
+// Attempted request info and sender to put received value.
+enum Pending {
+	HeaderByNumber(request::HeaderByNumber, Sender<encoded::Header>), // num + CHT root
+	HeaderByHash(request::HeaderByHash, Sender<encoded::Header>),
+	Block(request::Body, Sender<encoded::Block>),
+	BlockReceipts(request::BlockReceipts, Sender<Vec<Receipt>>),
+	Account(request::Account, Sender<BasicAccount>),
+}
+
+/// On demand request service. See module docs for more details.
+/// Accumulates info about all peers' capabilities and dispatches
+/// requests to them accordingly.
+pub struct OnDemand {
+	peers: RwLock<HashMap<PeerId, Peer>>,
+	pending_requests: RwLock<HashMap<ReqId, Pending>>,
+}
+
+impl Default for OnDemand {
+	fn default() -> Self {
+		OnDemand {
+			peers: RwLock::new(HashMap::new()),
+			pending_requests: RwLock::new(HashMap::new()),
+		}
+	}
+}
+
+impl OnDemand {
+	/// Request a header by block number and CHT root hash.
+	pub fn header_by_number(&self, ctx: &BasicContext, req: request::HeaderByNumber) -> Response<encoded::Header> {
+		let (sender, receiver) = oneshot::channel();
+		self.dispatch_header_by_number(ctx, req, sender);
+		Response(receiver)
+	}
+
+	// dispatch the request, completing the request if no peers available.
+	fn dispatch_header_by_number(&self, ctx: &BasicContext, req: request::HeaderByNumber, sender: Sender<encoded::Header>) {
+		let num = req.num;
+		let cht_num = ::client::cht::block_to_cht_number(req.num);
+		let les_req = LesRequest::HeaderProofs(les_request::HeaderProofs {
+			requests: vec![les_request::HeaderProof {
+				cht_number: cht_num,
+				block_number: req.num,
+				from_level: 0,
+			}],
+		});
+
+		// we're looking for a peer with serveHeaders who's far enough along in the
+		// chain.
+		for (id, peer) in self.peers.read().iter() {
+			if peer.capabilities.serve_headers && peer.status.head_num >= num {
+				match ctx.request_from(*id, les_req.clone()) {
+					Ok(req_id) => {
+						trace!(target: "on_demand", "Assigning request to peer {}", id);
+						self.pending_requests.write().insert(
+							req_id,
+							Pending::HeaderByNumber(req, sender)
+						);
+						return
+					},
+					Err(e) =>
+						trace!(target: "on_demand", "Failed to make request of peer {}: {:?}", id, e),
+				}
+			}
+		}
+
+		// TODO: retrying
+		trace!(target: "on_demand", "No suitable peer for request");
+		sender.complete(Err(Error::NoPeersAvailable));
+	}
+
+	/// Request a header by hash. This is less accurate than by-number because we don't know
+	/// where in the chain this header lies, and therefore can't find a peer who is supposed to have
+	/// it as easily.
+	pub fn header_by_hash(&self, ctx: &BasicContext, req: request::HeaderByHash) -> Response<encoded::Header> {
+		let (sender, receiver) = oneshot::channel();
+		self.dispatch_header_by_hash(ctx, req, sender);
+		Response(receiver)
+	}
+
+	fn dispatch_header_by_hash(&self, ctx: &BasicContext, req: request::HeaderByHash, sender: Sender<encoded::Header>) {
+		let les_req = LesRequest::Headers(les_request::Headers {
+			start: req.0.into(),
+			max: 1,
+			skip: 0,
+			reverse: false,
+		});
+
+		// all we've got is a hash, so we'll just guess at peers who might have
+		// it randomly.
+		let mut potential_peers = self.peers.read().iter()
+			.filter(|&(_, peer)| peer.capabilities.serve_headers)
+			.map(|(id, _)| *id)
+			.collect::<Vec<_>>();
+
+		let mut rng = ::rand::thread_rng();
+
+		::rand::Rng::shuffle(&mut rng, &mut potential_peers);
+
+		for id in potential_peers {
+			match ctx.request_from(id, les_req.clone()) {
+				Ok(req_id) => {
+					trace!(target: "on_demand", "Assigning request to peer {}", id);
+					self.pending_requests.write().insert(
+						req_id,
+						Pending::HeaderByHash(req, sender),
+					);
+					return
+				}
+				Err(e) =>
+					trace!(target: "on_demand", "Failed to make request of peer {}: {:?}", id, e),
+			}
+		}
+
+		// TODO: retrying
+		trace!(target: "on_demand", "No suitable peer for request");
+		sender.complete(Err(Error::NoPeersAvailable));
+	}
+
+	/// Request a block, given its header. Block bodies are requestable by hash only,
+	/// and the header is required anyway to verify and complete the block body
+	/// -- this just doesn't obscure the network query.
+	pub fn block(&self, ctx: &BasicContext, req: request::Body) -> Response<encoded::Block> {
+		let (sender, receiver) = oneshot::channel();
+		self.dispatch_block(ctx, req, sender);
+		Response(receiver)
+	}
+
+	fn dispatch_block(&self, ctx: &BasicContext, req: request::Body, sender: Sender<encoded::Block>) {
+		let num = req.header.number();
+		let les_req = LesRequest::Bodies(les_request::Bodies {
+			block_hashes: vec![req.hash],
+		});
+
+		// we're looking for a peer with serveChainSince(num)
+		for (id, peer) in self.peers.read().iter() {
+			if peer.capabilities.serve_chain_since.as_ref().map_or(false, |x| *x >= num) {
+				match ctx.request_from(*id, les_req.clone()) {
+					Ok(req_id) => {
+						trace!(target: "on_demand", "Assigning request to peer {}", id);
+						self.pending_requests.write().insert(
+							req_id,
+							Pending::Block(req, sender)
+						);
+						return
+					}
+					Err(e) =>
+						trace!(target: "on_demand", "Failed to make request of peer {}: {:?}", id, e),
+				}
+			}
+		}
+
+		// TODO: retrying
+		trace!(target: "on_demand", "No suitable peer for request");
+		sender.complete(Err(Error::NoPeersAvailable));
+	}
+
+	/// Request the receipts for a block. The header serves two purposes:
+	/// provide the block hash to fetch receipts for, and for verification of the receipts root.
+	pub fn block_receipts(&self, ctx: &BasicContext, req: request::BlockReceipts) -> Response<Vec<Receipt>> {
+		let (sender, receiver) = oneshot::channel();
+		self.dispatch_block_receipts(ctx, req, sender);
+		Response(receiver)
+	}
+
+	fn dispatch_block_receipts(&self, ctx: &BasicContext, req: request::BlockReceipts, sender: Sender<Vec<Receipt>>) {
+		let num = req.0.number();
+		let les_req = LesRequest::Receipts(les_request::Receipts {
+			block_hashes: vec![req.0.hash()],
+		});
+
+		// we're looking for a peer with serveChainSince(num)
+		for (id, peer) in self.peers.read().iter() {
+			if peer.capabilities.serve_chain_since.as_ref().map_or(false, |x| *x >= num) {
+				match ctx.request_from(*id, les_req.clone()) {
+					Ok(req_id) => {
+						trace!(target: "on_demand", "Assigning request to peer {}", id);
+						self.pending_requests.write().insert(
+							req_id,
+							Pending::BlockReceipts(req, sender)
+						);
+						return
+					}
+					Err(e) =>
+						trace!(target: "on_demand", "Failed to make request of peer {}: {:?}", id, e),
+				}
+			}
+		}
+
+		// TODO: retrying
+		trace!(target: "on_demand", "No suitable peer for request");
+		sender.complete(Err(Error::NoPeersAvailable));
+	}
+
+	/// Request an account by address and block header -- which gives a hash to query and a state root
+	/// to verify against.
+	pub fn account(&self, ctx: &BasicContext, req: request::Account) -> Response<BasicAccount> {
+		let (sender, receiver) = oneshot::channel();
+		self.dispatch_account(ctx, req, sender);
+		Response(receiver)
+	}
+
+	fn dispatch_account(&self, ctx: &BasicContext, req: request::Account, sender: Sender<BasicAccount>) {
+		let num = req.header.number();
+		let les_req = LesRequest::StateProofs(les_request::StateProofs {
+			requests: vec![les_request::StateProof {
+				block: req.header.hash(),
+				key1: ::util::Hashable::sha3(&req.address),
+				key2: None,
+				from_level: 0,
+			}],
+		});
+
+		// we're looking for a peer with serveStateSince(num)
+		for (id, peer) in self.peers.read().iter() {
+			if peer.capabilities.serve_state_since.as_ref().map_or(false, |x| *x >= num) {
+				match ctx.request_from(*id, les_req.clone()) {
+					Ok(req_id) => {
+						trace!(target: "on_demand", "Assigning request to peer {}", id);
+						self.pending_requests.write().insert(
+							req_id,
+							Pending::Account(req, sender)
+						);
+						return
+					}
+					Err(e) =>
+						trace!(target: "on_demand", "Failed to make request of peer {}: {:?}", id, e),
+				}
+			}
+		}
+
+		// TODO: retrying
+		trace!(target: "on_demand", "No suitable peer for request");
+		sender.complete(Err(Error::NoPeersAvailable));
+	}
+}
+
+impl Handler for OnDemand {
+	fn on_connect(&self, ctx: &EventContext, status: &Status, capabilities: &Capabilities) {
+		self.peers.write().insert(ctx.peer(), Peer { status: status.clone(), capabilities: capabilities.clone() });
+	}
+
+	fn on_disconnect(&self, ctx: &EventContext, unfulfilled: &[ReqId]) {
+		self.peers.write().remove(&ctx.peer());
+		let ctx = ctx.as_basic();
+
+		for unfulfilled in unfulfilled {
+			if let Some(pending) = self.pending_requests.write().remove(unfulfilled) {
+				trace!(target: "on_demand", "Attempting to reassign dropped request");
+				match pending {
+					Pending::HeaderByNumber(req, sender)
+						=> self.dispatch_header_by_number(ctx, req, sender),
+					Pending::HeaderByHash(req, sender)
+						=> self.dispatch_header_by_hash(ctx, req, sender),
+					Pending::Block(req, sender)
+						=> self.dispatch_block(ctx, req, sender),
+					Pending::BlockReceipts(req, sender)
+						=> self.dispatch_block_receipts(ctx, req, sender),
+					Pending::Account(req, sender)
+						=> self.dispatch_account(ctx, req, sender),
+				}
+			}
+		}
+	}
+
+	fn on_announcement(&self, ctx: &EventContext, announcement: &Announcement) {
+		let mut peers = self.peers.write();
+		if let Some(ref mut peer) = peers.get_mut(&ctx.peer()) {
+			peer.status.update_from(&announcement);
+			peer.capabilities.update_from(&announcement);
+		}
+	}
+
+	fn on_header_proofs(&self, ctx: &EventContext, req_id: ReqId, proofs: &[(Bytes, Vec<Bytes>)]) {
+		let peer = ctx.peer();
+		let req = match self.pending_requests.write().remove(&req_id) {
+			Some(req) => req,
+			None => return,
+		};
+
+		match req {
+			Pending::HeaderByNumber(req, sender) => {
+				if let Some(&(ref header, ref proof)) = proofs.get(0) {
+					match req.check_response(header, proof) {
+						Ok(header) => {
+							sender.complete(Ok(header));
+							return
+						}
+						Err(e) => {
+							warn!("Error handling response for header request: {:?}", e);
+							ctx.disable_peer(peer);
+						}
+					}
+				}
+
+				self.dispatch_header_by_number(ctx.as_basic(), req, sender);
+			}
+			_ => panic!("Only header by number request fetches header proofs; qed"),
+		}
+	}
+
+	fn on_block_headers(&self, ctx: &EventContext, req_id: ReqId, headers: &[Bytes]) {
+		let peer = ctx.peer();
+		let req = match self.pending_requests.write().remove(&req_id) {
+			Some(req) => req,
+			None => return,
+		};
+
+		match req {
+			Pending::HeaderByHash(req, sender) => {
+				if let Some(ref header) = headers.get(0) {
+					match req.check_response(header) {
+						Ok(header) => {
+							sender.complete(Ok(header));
+							return
+						}
+						Err(e) => {
+							warn!("Error handling response for header request: {:?}", e);
+							ctx.disable_peer(peer);
+						}
+					}
+				}
+
+				self.dispatch_header_by_hash(ctx.as_basic(), req, sender);
+			}
+			_ => panic!("Only header by hash request fetches headers; qed"),
+		}
+	}
+
+	fn on_block_bodies(&self, ctx: &EventContext, req_id: ReqId, bodies: &[Bytes]) {
+		let peer = ctx.peer();
+		let req = match self.pending_requests.write().remove(&req_id) {
+			Some(req) => req,
+			None => return,
+		};
+
+		match req {
+			Pending::Block(req, sender) => {
+				if let Some(ref block) = bodies.get(0) {
+					match req.check_response(block) {
+						Ok(block) => {
+							sender.complete(Ok(block));
+							return
+						}
+						Err(e) => {
+							warn!("Error handling response for block request: {:?}", e);
+							ctx.disable_peer(peer);
+						}
+					}
+				}
+
+				self.dispatch_block(ctx.as_basic(), req, sender);
+			}
+			_ => panic!("Only block request fetches bodies; qed"),
+		}
+	}
+
+	fn on_receipts(&self, ctx: &EventContext, req_id: ReqId, receipts: &[Vec<Receipt>]) {
+		let peer = ctx.peer();
+		let req = match self.pending_requests.write().remove(&req_id) {
+			Some(req) => req,
+			None => return,
+		};
+
+		match req {
+			Pending::BlockReceipts(req, sender) => {
+				if let Some(ref receipts) = receipts.get(0) {
+					match req.check_response(receipts) {
+						Ok(receipts) => {
+							sender.complete(Ok(receipts));
+							return
+						}
+						Err(e) => {
+							warn!("Error handling response for receipts request: {:?}", e);
+							ctx.disable_peer(peer);
+						}
+					}
+				}
+
+				self.dispatch_block_receipts(ctx.as_basic(), req, sender);
+			}
+			_ => panic!("Only receipts request fetches receipts; qed"),
+		}
+	}
+
+	fn on_state_proofs(&self, ctx: &EventContext, req_id: ReqId, proofs: &[Vec<Bytes>]) {
+		let peer = ctx.peer();
+		let req = match self.pending_requests.write().remove(&req_id) {
+			Some(req) => req,
+			None => return,
+		};
+
+		match req {
+			Pending::Account(req, sender) => {
+				if let Some(ref proof) = proofs.get(0) {
+					match req.check_response(proof) {
+						Ok(proof) => {
+							sender.complete(Ok(proof));
+							return
+						}
+						Err(e) => {
+							warn!("Error handling response for state request: {:?}", e);
+							ctx.disable_peer(peer);
+						}
+					}
+				}
+
+				self.dispatch_account(ctx.as_basic(), req, sender);
+			}
+			_ => panic!("Only account request fetches state proof; qed"),
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use net::{Announcement, BasicContext, ReqId, Error as LesError};
+	use request::{Request as LesRequest, Kind as LesRequestKind};
+	use network::{PeerId, NodeId};
+	use futures::Future;
+	use util::H256;
+
+	struct FakeContext;
+
+	impl BasicContext for FakeContext {
+		fn persistent_peer_id(&self, _: PeerId) -> Option<NodeId> { None }
+		fn request_from(&self, _: PeerId, _: LesRequest) -> Result<ReqId, LesError> {
+			unimplemented!()
+		}
+		fn make_announcement(&self, _: Announcement) { }
+		fn max_requests(&self, _: PeerId, _: LesRequestKind) -> usize { 0 }
+		fn disconnect_peer(&self, _: PeerId) { }
+		fn disable_peer(&self, _: PeerId) { }
+	}
+
+	#[test]
+	fn no_peers() {
+		let on_demand = OnDemand::default();
+		let result = on_demand.header_by_hash(&FakeContext, request::HeaderByHash(H256::default()));
+
+		assert_eq!(result.wait().unwrap_err(), Error::NoPeersAvailable);
+	}
+}
diff --git a/ethcore/light/src/on_demand/request.rs b/ethcore/light/src/on_demand/request.rs
new file mode 100644
index 000000000..86fcc5d05
--- /dev/null
+++ b/ethcore/light/src/on_demand/request.rs
@@ -0,0 +1,337 @@
+// 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/>.
+
+//! Request types, verification, and verification errors.
+
+use ethcore::basic_account::BasicAccount;
+use ethcore::encoded;
+use ethcore::receipt::Receipt;
+
+use rlp::{RlpStream, Stream, UntrustedRlp, View};
+use util::{Address, Bytes, HashDB, H256};
+use util::memorydb::MemoryDB;
+use util::sha3::Hashable;
+use util::trie::{Trie, TrieDB, TrieError};
+
+/// Errors in verification.
+#[derive(Debug, PartialEq)]
+pub enum Error {
+	/// RLP decoder error.
+	Decoder(::rlp::DecoderError),
+	/// Trie lookup error (result of bad proof)
+	Trie(TrieError),
+	/// Bad inclusion proof
+	BadProof,
+	/// Wrong header number.
+	WrongNumber(u64, u64),
+	/// Wrong header hash.
+	WrongHash(H256, H256),
+	/// Wrong trie root.
+	WrongTrieRoot(H256, H256),
+}
+
+impl From<::rlp::DecoderError> for Error {
+	fn from(err: ::rlp::DecoderError) -> Self {
+		Error::Decoder(err)
+	}
+}
+
+impl From<Box<TrieError>> for Error {
+	fn from(err: Box<TrieError>) -> Self {
+		Error::Trie(*err)
+	}
+}
+
+/// Request for a header by number.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct HeaderByNumber {
+	/// The header's number.
+	pub num: u64,
+	/// The root of the CHT containing this header.
+	pub cht_root: H256,
+}
+
+impl HeaderByNumber {
+	/// Check a response with a header and cht proof.
+	pub fn check_response(&self, header: &[u8], proof: &[Bytes]) -> Result<encoded::Header, Error> {
+		use util::trie::{Trie, TrieDB};
+
+		// check the proof
+		let mut db = MemoryDB::new();
+
+		for node in proof { db.insert(&node[..]); }
+		let key = ::rlp::encode(&self.num);
+
+		let expected_hash: H256 = match TrieDB::new(&db, &self.cht_root).and_then(|t| t.get(&*key))? {
+			Some(val) => ::rlp::decode(&val),
+			None => return Err(Error::BadProof)
+		};
+
+		// and compare the hash to the found header.
+		let found_hash = header.sha3();
+		match expected_hash == found_hash {
+			true => Ok(encoded::Header::new(header.to_vec())),
+			false => Err(Error::WrongHash(expected_hash, found_hash)),
+		}
+	}
+}
+
+/// Request for a header by hash.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct HeaderByHash(pub H256);
+
+impl HeaderByHash {
+	/// Check a response for the header.
+	pub fn check_response(&self, header: &[u8]) -> Result<encoded::Header, Error> {
+		let hash = header.sha3();
+		match hash == self.0 {
+			true => Ok(encoded::Header::new(header.to_vec())),
+			false => Err(Error::WrongHash(self.0, hash)),
+		}
+	}
+}
+
+/// Request for a block, with header and precomputed hash.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Body {
+	/// The block's header.
+	pub header: encoded::Header,
+	/// The block's hash.
+	pub hash: H256,
+}
+
+impl Body {
+	/// Check a response for this block body.
+	pub fn check_response(&self, body: &[u8]) -> Result<encoded::Block, Error> {
+		let body_view = UntrustedRlp::new(&body);
+
+		// check the integrity of the the body against the header
+		let tx_root = ::util::triehash::ordered_trie_root(body_view.at(0)?.iter().map(|r| r.as_raw().to_vec()));
+		if tx_root != self.header.transactions_root() {
+			return Err(Error::WrongTrieRoot(self.header.transactions_root(), tx_root));
+		}
+
+		let uncles_hash = body_view.at(1)?.as_raw().sha3();
+		if uncles_hash != self.header.uncles_hash() {
+			return Err(Error::WrongHash(self.header.uncles_hash(), uncles_hash));
+		}
+
+		// concatenate the header and the body.
+		let mut stream = RlpStream::new_list(3);
+		stream.append_raw(self.header.rlp().as_raw(), 1);
+		stream.append_raw(body, 2);
+
+		Ok(encoded::Block::new(stream.out()))
+	}
+}
+
+/// Request for a block's receipts with header for verification.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct BlockReceipts(pub encoded::Header);
+
+impl BlockReceipts {
+	/// Check a response with receipts against the stored header.
+	pub fn check_response(&self, receipts: &[Receipt]) -> Result<Vec<Receipt>, Error> {
+		let receipts_root = self.0.receipts_root();
+		let found_root = ::util::triehash::ordered_trie_root(receipts.iter().map(|r| ::rlp::encode(r).to_vec()));
+
+		match receipts_root == found_root {
+			true => Ok(receipts.to_vec()),
+			false => Err(Error::WrongTrieRoot(receipts_root, found_root)),
+		}
+	}
+}
+
+/// Request for an account structure.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Account {
+	/// Header for verification.
+	pub header: encoded::Header,
+	/// Address requested.
+	pub address: Address,
+}
+
+impl Account {
+	/// Check a response with an account against the stored header.
+	pub fn check_response(&self, proof: &[Bytes]) -> Result<BasicAccount, Error> {
+		let state_root = self.header.state_root();
+
+		let mut db = MemoryDB::new();
+		for node in proof { db.insert(&node[..]); }
+
+		match TrieDB::new(&db, &state_root).and_then(|t| t.get(&self.address.sha3()))? {
+			Some(val) => {
+				let rlp = UntrustedRlp::new(&val);
+				Ok(BasicAccount {
+					nonce: rlp.val_at(0)?,
+					balance: rlp.val_at(1)?,
+					storage_root: rlp.val_at(2)?,
+					code_hash: rlp.val_at(3)?,
+				})
+			},
+			None => Err(Error::BadProof)
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use util::{MemoryDB, Address, H256, FixedHash};
+	use util::trie::{Trie, TrieMut, TrieDB, SecTrieDB, TrieDBMut, SecTrieDBMut};
+	use util::trie::recorder::Recorder;
+
+	use ethcore::header::Header;
+	use ethcore::encoded;
+	use ethcore::receipt::Receipt;
+
+	#[test]
+	fn check_header_by_number() {
+		let mut root = H256::default();
+		let mut db = MemoryDB::new();
+		let mut header = Header::new();
+		header.set_number(10_000);
+		header.set_extra_data(b"test_header".to_vec());
+
+		{
+			let mut trie = TrieDBMut::new(&mut db, &mut root);
+			for i in (0..2048u64).map(|x| x + 8192) {
+				let hash = if i == 10_000 {
+					header.hash()
+				} else {
+					H256::random()
+				};
+				trie.insert(&*::rlp::encode(&i), &*::rlp::encode(&hash)).unwrap();
+			}
+		}
+
+		let proof = {
+			let trie = TrieDB::new(&db, &root).unwrap();
+			let key = ::rlp::encode(&10_000u64);
+			let mut recorder = Recorder::new();
+
+			trie.get_with(&*key, &mut recorder).unwrap().unwrap();
+
+			recorder.drain().into_iter().map(|r| r.data).collect::<Vec<_>>()
+		};
+
+		let req = HeaderByNumber {
+			num: 10_000,
+			cht_root: root,
+		};
+
+		let raw_header = ::rlp::encode(&header);
+
+		assert!(req.check_response(&*raw_header, &proof[..]).is_ok());
+	}
+
+	#[test]
+	fn check_header_by_hash() {
+		let mut header = Header::new();
+		header.set_number(10_000);
+		header.set_extra_data(b"test_header".to_vec());
+		let hash = header.hash();
+		let raw_header = ::rlp::encode(&header);
+
+		assert!(HeaderByHash(hash).check_response(&*raw_header).is_ok())
+	}
+
+	#[test]
+	fn check_body() {
+		use rlp::{RlpStream, Stream};
+
+		let header = Header::new();
+		let mut body_stream = RlpStream::new_list(2);
+		body_stream.begin_list(0).begin_list(0);
+
+		let req = Body {
+			header: encoded::Header::new(::rlp::encode(&header).to_vec()),
+			hash: header.hash(),
+		};
+
+		assert!(req.check_response(&*body_stream.drain()).is_ok())
+	}
+
+	#[test]
+	fn check_receipts() {
+		let receipts = (0..5).map(|_| Receipt {
+			state_root: H256::random(),
+			gas_used: 21_000u64.into(),
+			log_bloom: Default::default(),
+			logs: Vec::new(),
+		}).collect::<Vec<_>>();
+
+		let mut header = Header::new();
+		let receipts_root = ::util::triehash::ordered_trie_root(
+			receipts.iter().map(|x| ::rlp::encode(x).to_vec())
+		);
+
+		header.set_receipts_root(receipts_root);
+
+		let req = BlockReceipts(encoded::Header::new(::rlp::encode(&header).to_vec()));
+
+		assert!(req.check_response(&receipts).is_ok())
+	}
+
+	#[test]
+	fn check_state_proof() {
+		use rlp::{RlpStream, Stream};
+
+		let mut root = H256::default();
+		let mut db = MemoryDB::new();
+		let mut header = Header::new();
+		header.set_number(123_456);
+		header.set_extra_data(b"test_header".to_vec());
+
+		let addr = Address::random();
+		let rand_acc = || {
+			let mut stream = RlpStream::new_list(4);
+			stream.append(&2u64)
+				.append(&100_000_000u64)
+				.append(&H256::random())
+				.append(&H256::random());
+
+			stream.out()
+		};
+		{
+			let mut trie = SecTrieDBMut::new(&mut db, &mut root);
+			for _ in 0..100 {
+				let address = Address::random();
+				trie.insert(&*address, &rand_acc()).unwrap();
+			}
+
+			trie.insert(&*addr, &rand_acc()).unwrap();
+		}
+
+		let proof = {
+			let trie = SecTrieDB::new(&db, &root).unwrap();
+			let mut recorder = Recorder::new();
+
+			trie.get_with(&*addr, &mut recorder).unwrap().unwrap();
+
+			recorder.drain().into_iter().map(|r| r.data).collect::<Vec<_>>()
+		};
+
+		header.set_state_root(root.clone());
+
+		let req = Account {
+			header: encoded::Header::new(::rlp::encode(&header).to_vec()),
+			address: addr,
+		};
+
+		assert!(req.check_response(&proof[..]).is_ok());
+	}
+}