diff --git a/ethcore/light/src/lib.rs b/ethcore/light/src/lib.rs
index 82b6ea12686..5e970b837fd 100644
--- a/ethcore/light/src/lib.rs
+++ b/ethcore/light/src/lib.rs
@@ -44,13 +44,13 @@ pub mod provider;
 
 #[cfg(feature = "ipc")]
 pub mod provider {
-    #![allow(dead_code, unused_assignments, unused_variables, missing_docs)] // codegen issues
+	#![allow(dead_code, unused_assignments, unused_variables, missing_docs)] // codegen issues
 	include!(concat!(env!("OUT_DIR"), "/provider.rs"));
 }
 
 #[cfg(feature = "ipc")]
 pub mod remote {
-    pub use provider::LightProviderClient;
+	pub use provider::LightProviderClient;
 }
 
 mod types;
diff --git a/ethcore/light/src/net/context.rs b/ethcore/light/src/net/context.rs
index 64ddd19a355..33009d7f607 100644
--- a/ethcore/light/src/net/context.rs
+++ b/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::Requests;
+use request::NetworkRequests as Requests;
 
 /// An I/O context which allows sending and receiving packets as well as
 /// disconnecting peers. This is used as a generalization of the portions
diff --git a/ethcore/light/src/net/mod.rs b/ethcore/light/src/net/mod.rs
index 1919724514d..798f68fe525 100644
--- a/ethcore/light/src/net/mod.rs
+++ b/ethcore/light/src/net/mod.rs
@@ -33,7 +33,7 @@ use std::sync::Arc;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 use provider::Provider;
-use request::{Request, Requests, Response};
+use request::{Request, NetworkRequests as Requests, Response};
 
 use self::request_credits::{Credits, FlowParams};
 use self::context::{Ctx, TickCtx};
@@ -108,9 +108,14 @@ mod timeout {
 }
 
 /// A request id.
+#[cfg(not(test))]
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Ord, PartialOrd, Hash)]
 pub struct ReqId(usize);
 
+#[cfg(test)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Ord, PartialOrd, Hash)]
+pub struct ReqId(pub usize);
+
 impl fmt::Display for ReqId {
 	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 		write!(f, "Request #{}", self.0)
diff --git a/ethcore/light/src/net/request_set.rs b/ethcore/light/src/net/request_set.rs
index 094fa189420..c5608050f54 100644
--- a/ethcore/light/src/net/request_set.rs
+++ b/ethcore/light/src/net/request_set.rs
@@ -25,7 +25,7 @@ use std::collections::{BTreeMap, HashMap};
 use std::iter::FromIterator;
 
 use request::Request;
-use request::Requests;
+use request::NetworkRequests as Requests;
 use net::{timeout, ReqId};
 use util::U256;
 
diff --git a/ethcore/light/src/net/tests/mod.rs b/ethcore/light/src/net/tests/mod.rs
index 6dc5fbe7ec5..94788a72707 100644
--- a/ethcore/light/src/net/tests/mod.rs
+++ b/ethcore/light/src/net/tests/mod.rs
@@ -39,14 +39,14 @@ use std::sync::Arc;
 
 // helper for encoding a single request into a packet.
 // panics on bad backreference.
-fn encode_single(request: Request) -> Requests {
+fn encode_single(request: Request) -> NetworkRequests {
 	let mut builder = RequestBuilder::default();
 	builder.push(request).unwrap();
 	builder.build()
 }
 
 // helper for making a packet out of `Requests`.
-fn make_packet(req_id: usize, requests: &Requests) -> Vec<u8> {
+fn make_packet(req_id: usize, requests: &NetworkRequests) -> Vec<u8> {
 	let mut stream = RlpStream::new_list(2);
 	stream.append(&req_id).append_list(&requests.requests());
 	stream.out()
diff --git a/ethcore/light/src/on_demand/mod.rs b/ethcore/light/src/on_demand/mod.rs
index c756844c9a1..e61c126d6f5 100644
--- a/ethcore/light/src/on_demand/mod.rs
+++ b/ethcore/light/src/on_demand/mod.rs
@@ -18,20 +18,17 @@
 //! The request service is implemented using Futures. Higher level request handlers
 //! will take the raw data received here and extract meaningful results from it.
 
-// TODO [ToDr] Suppressing deprecation warnings. Rob will fix the API anyway.
-#![allow(deprecated)]
-
 use std::collections::HashMap;
+use std::marker::PhantomData;
 use std::sync::Arc;
 
 use ethcore::basic_account::BasicAccount;
 use ethcore::encoded;
 use ethcore::receipt::Receipt;
-use ethcore::state::ProvedExecution;
 use ethcore::executed::{Executed, ExecutionError};
 
-use futures::{Async, Poll, Future};
-use futures::sync::oneshot::{self, Sender, Receiver};
+use futures::{future, Async, Poll, Future, BoxFuture};
+use futures::sync::oneshot::{self, Sender, Receiver, Canceled};
 use network::PeerId;
 use rlp::RlpStream;
 use util::{Bytes, RwLock, Mutex, U256, H256};
@@ -39,10 +36,19 @@ use util::sha3::{SHA3_NULL_RLP, SHA3_EMPTY, SHA3_EMPTY_LIST_RLP};
 
 use net::{self, Handler, Status, Capabilities, Announcement, EventContext, BasicContext, ReqId};
 use cache::Cache;
-use request::{self as basic_request, Request as NetworkRequest, Response as NetworkResponse};
+use request::{self as basic_request, Request as NetworkRequest};
+use self::request::CheckedRequest;
+
+pub use self::request::{Request, Response};
+
+#[cfg(test)]
+mod tests;
 
 pub mod request;
 
+/// The result of execution
+pub type ExecutionResult = Result<Executed, ExecutionError>;
+
 // relevant peer info.
 struct Peer {
 	status: Status,
@@ -50,146 +56,154 @@ struct Peer {
 }
 
 impl Peer {
-	// Whether a given peer can handle a specific request.
-	fn can_handle(&self, pending: &Pending) -> bool {
-		match *pending {
-			Pending::HeaderProof(ref req, _) =>
-				self.capabilities.serve_headers && self.status.head_num > req.num(),
-			Pending::HeaderByHash(_, _) => self.capabilities.serve_headers,
-			Pending::Block(ref req, _) =>
-				self.capabilities.serve_chain_since.as_ref().map_or(false, |x| *x <= req.header.number()),
-			Pending::BlockReceipts(ref req, _) =>
-				self.capabilities.serve_chain_since.as_ref().map_or(false, |x| *x <= req.0.number()),
-			Pending::Account(ref req, _) =>
-				self.capabilities.serve_state_since.as_ref().map_or(false, |x| *x <= req.header.number()),
-			Pending::Code(ref req, _) =>
-				self.capabilities.serve_state_since.as_ref().map_or(false, |x| *x <= req.block_id.1),
-			Pending::TxProof(ref req, _) =>
-				self.capabilities.serve_state_since.as_ref().map_or(false, |x| *x <= req.header.number()),
-		}
-	}
-}
+	// whether this peer can fulfill the
+	fn can_fulfill(&self, c: &Capabilities) -> bool {
+		let caps = &self.capabilities;
 
-// Which portions of a CHT proof should be sent.
-enum ChtProofSender {
-	Both(Sender<(H256, U256)>),
-	Hash(Sender<H256>),
-	ChainScore(Sender<U256>),
+		caps.serve_headers == c.serve_headers &&
+			caps.serve_chain_since >= c.serve_chain_since &&
+			caps.serve_state_since >= c.serve_chain_since
+	}
 }
 
 // Attempted request info and sender to put received value.
-enum Pending {
-	HeaderProof(request::HeaderProof, ChtProofSender),
-	HeaderByHash(request::HeaderByHash, Sender<encoded::Header>),
-	Block(request::Body, Sender<encoded::Block>),
-	BlockReceipts(request::BlockReceipts, Sender<Vec<Receipt>>),
-	Account(request::Account, Sender<BasicAccount>),
-	Code(request::Code, Sender<Bytes>),
-	TxProof(request::TransactionProof, Sender<Result<Executed, ExecutionError>>),
+struct Pending {
+	requests: basic_request::Requests<CheckedRequest>,
+	net_requests: basic_request::Requests<NetworkRequest>,
+	required_capabilities: Capabilities,
+	responses: Vec<Response>,
+	sender: oneshot::Sender<Vec<Response>>,
 }
 
-impl Pending {
-	// Create a network request.
-	fn make_request(&self) -> NetworkRequest {
-		match *self {
-			Pending::HeaderByHash(ref req, _) => NetworkRequest::Headers(basic_request::IncompleteHeadersRequest {
-				start: basic_request::HashOrNumber::Hash(req.0).into(),
-				skip: 0,
-				max: 1,
-				reverse: false,
-			}),
-			Pending::HeaderProof(ref req, _) => NetworkRequest::HeaderProof(basic_request::IncompleteHeaderProofRequest {
-				num: req.num().into(),
-			}),
-			Pending::Block(ref req, _) => NetworkRequest::Body(basic_request::IncompleteBodyRequest {
-				hash: req.hash.into(),
-			}),
-			Pending::BlockReceipts(ref req, _) => NetworkRequest::Receipts(basic_request::IncompleteReceiptsRequest {
-				hash: req.0.hash().into(),
-			}),
-			Pending::Account(ref req, _) => NetworkRequest::Account(basic_request::IncompleteAccountRequest {
-				block_hash: req.header.hash().into(),
-				address_hash: ::util::Hashable::sha3(&req.address).into(),
-			}),
-			Pending::Code(ref req, _) => NetworkRequest::Code(basic_request::IncompleteCodeRequest {
-				block_hash: req.block_id.0.into(),
-				code_hash: req.code_hash.into(),
-			}),
-			Pending::TxProof(ref req, _) => NetworkRequest::Execution(basic_request::IncompleteExecutionRequest {
-				block_hash: req.header.hash().into(),
-				from: req.tx.sender(),
-				gas: req.tx.gas,
-				gas_price: req.tx.gas_price,
-				action: req.tx.action.clone(),
-				value: req.tx.value,
-				data: req.tx.data.clone(),
-			}),
+// helper to guess capabilities required for a given batch of network requests.
+fn guess_capabilities(requests: &[CheckedRequest]) -> Capabilities {
+	let mut caps = Capabilities {
+		serve_headers: false,
+		serve_chain_since: None,
+		serve_state_since: None,
+		tx_relay: false,
+	};
+
+	let update_since = |current: &mut Option<u64>, new|
+		*current = match *current {
+			Some(x) => Some(::std::cmp::min(x, new)),
+			None => Some(new),
+		};
+
+	for request in requests {
+		match *request {
+			// TODO: might be worth returning a required block number for this also.
+			CheckedRequest::HeaderProof(_, _) =>
+				caps.serve_headers = true,
+			CheckedRequest::HeaderByHash(_, _) =>
+				caps.serve_headers = true,
+			CheckedRequest::Body(ref req, _) =>
+				update_since(&mut caps.serve_chain_since, req.header.number()),
+			CheckedRequest::Receipts(ref req, _) =>
+				update_since(&mut caps.serve_chain_since, req.0.number()),
+			CheckedRequest::Account(ref req, _) =>
+				update_since(&mut caps.serve_state_since, req.header.number()),
+			CheckedRequest::Code(ref req, _) =>
+				update_since(&mut caps.serve_state_since, req.block_id.1),
+			CheckedRequest::Execution(ref req, _) =>
+				update_since(&mut caps.serve_state_since, req.header.number()),
 		}
 	}
+
+	caps
+}
+
+/// A future extracting the concrete output type of the generic adapter
+/// from a vector of responses.
+pub struct OnResponses<T: request::RequestAdapter> {
+	receiver: Receiver<Vec<Response>>,
+	_marker: PhantomData<T>,
+}
+
+impl<T: request::RequestAdapter> Future for OnResponses<T> {
+	type Item = T::Out;
+	type Error = Canceled;
+
+	fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+		self.receiver.poll().map(|async| async.map(T::extract_from))
+	}
 }
 
 /// On demand request service. See module docs for more details.
 /// Accumulates info about all peers' capabilities and dispatches
 /// requests to them accordingly.
+// lock in declaration order.
 pub struct OnDemand {
+	pending: RwLock<Vec<Pending>>,
 	peers: RwLock<HashMap<PeerId, Peer>>,
-	pending_requests: RwLock<HashMap<ReqId, Pending>>,
+	in_transit: RwLock<HashMap<ReqId, Pending>>,
 	cache: Arc<Mutex<Cache>>,
-	orphaned_requests: RwLock<Vec<Pending>>,
-	start_nonce: U256,
+	no_immediate_dispatch: bool,
 }
 
-const RECEIVER_IN_SCOPE: &'static str = "Receiver is still in scope, so it's not dropped; qed";
-
 impl OnDemand {
 	/// Create a new `OnDemand` service with the given cache.
-	pub fn new(cache: Arc<Mutex<Cache>>, account_start_nonce: U256) -> Self {
+	pub fn new(cache: Arc<Mutex<Cache>>) -> Self {
 		OnDemand {
+			pending: RwLock::new(Vec::new()),
 			peers: RwLock::new(HashMap::new()),
-			pending_requests: RwLock::new(HashMap::new()),
+			in_transit: RwLock::new(HashMap::new()),
 			cache: cache,
-			orphaned_requests: RwLock::new(Vec::new()),
-			start_nonce: account_start_nonce,
+			no_immediate_dispatch: true,
 		}
 	}
 
+	// make a test version: this doesn't dispatch pending requests
+	// until you trigger it manually.
+	#[cfg(test)]
+	fn new_test(cache: Arc<Mutex<Cache>>) -> Self {
+		let mut me = OnDemand::new(cache);
+		me.no_immediate_dispatch = true;
+
+		me
+	}
+
 	/// Request a header's hash by block number and CHT root hash.
 	/// Returns the hash.
-	pub fn hash_by_number(&self, ctx: &BasicContext, req: request::HeaderProof) -> Receiver<H256> {
-		let (sender, receiver) = oneshot::channel();
+	pub fn hash_by_number(&self, ctx: &BasicContext, req: request::HeaderProof) -> BoxFuture<H256, Canceled> {
 		let cached = {
 			let mut cache = self.cache.lock();
 			cache.block_hash(&req.num())
 		};
 
 		match cached {
-			Some(hash) => sender.send(hash).expect(RECEIVER_IN_SCOPE),
-			None => self.dispatch(ctx, Pending::HeaderProof(req, ChtProofSender::Hash(sender))),
+			Some(hash) => future::ok(hash).boxed(),
+			None => {
+				self.request(ctx, req)
+					.expect("request given fully fleshed out; qed")
+					.map(|(h, _)| h)
+					.boxed()
+			},
 		}
-		receiver
 	}
 
 	/// Request a canonical block's chain score.
 	/// Returns the chain score.
-	pub fn chain_score_by_number(&self, ctx: &BasicContext, req: request::HeaderProof) -> Receiver<U256> {
-		let (sender, receiver) = oneshot::channel();
+	pub fn chain_score_by_number(&self, ctx: &BasicContext, req: request::HeaderProof) -> BoxFuture<U256, Canceled> {
 		let cached = {
 			let mut cache = self.cache.lock();
 			cache.block_hash(&req.num()).and_then(|hash| cache.chain_score(&hash))
 		};
 
 		match cached {
-			Some(score) => sender.send(score).expect(RECEIVER_IN_SCOPE),
-			None => self.dispatch(ctx, Pending::HeaderProof(req, ChtProofSender::ChainScore(sender))),
+			Some(score) => future::ok(score).boxed(),
+			None => {
+				self.request(ctx, req)
+					.expect("request given fully fleshed out; qed")
+					.map(|(_, s)| s)
+					.boxed()
+			},
 		}
-
-		receiver
 	}
 
 	/// Request a canonical block's hash and chain score by number.
 	/// Returns the hash and chain score.
-	pub fn hash_and_score_by_number(&self, ctx: &BasicContext, req: request::HeaderProof) -> Receiver<(H256, U256)> {
-		let (sender, receiver) = oneshot::channel();
+	pub fn hash_and_score_by_number(&self, ctx: &BasicContext, req: request::HeaderProof) -> BoxFuture<(H256, U256), Canceled> {
 		let cached = {
 			let mut cache = self.cache.lock();
 			let hash = cache.block_hash(&req.num());
@@ -200,31 +214,33 @@ impl OnDemand {
 		};
 
 		match cached {
-			(Some(hash), Some(score)) => sender.send((hash, score)).expect(RECEIVER_IN_SCOPE),
-			_ => self.dispatch(ctx, Pending::HeaderProof(req, ChtProofSender::Both(sender))),
+			(Some(hash), Some(score)) => future::ok((hash, score)).boxed(),
+			_ => {
+				self.request(ctx, req)
+					.expect("request given fully fleshed out; qed")
+					.boxed()
+			},
 		}
-
-		receiver
 	}
 
 	/// 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) -> Receiver<encoded::Header> {
-		let (sender, receiver) = oneshot::channel();
+	pub fn header_by_hash(&self, ctx: &BasicContext, req: request::HeaderByHash) -> BoxFuture<encoded::Header, Canceled> {
 		match { self.cache.lock().block_header(&req.0) } {
-			Some(hdr) => sender.send(hdr).expect(RECEIVER_IN_SCOPE),
-			None => self.dispatch(ctx, Pending::HeaderByHash(req, sender)),
+			Some(hdr) => future::ok(hdr).boxed(),
+			None => {
+				self.request(ctx, req)
+					.expect("request given fully fleshed out; qed")
+					.boxed()
+			},
 		}
-		receiver
 	}
 
 	/// 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) -> Receiver<encoded::Block> {
-		let (sender, receiver) = oneshot::channel();
-
+	pub fn block(&self, ctx: &BasicContext, req: request::Body) -> BoxFuture<encoded::Block, Canceled> {
 		// fast path for empty body.
 		if req.header.transactions_root() == SHA3_NULL_RLP && req.header.uncles_hash() == SHA3_EMPTY_LIST_RLP {
 			let mut stream = RlpStream::new_list(3);
@@ -232,7 +248,7 @@ impl OnDemand {
 			stream.begin_list(0);
 			stream.begin_list(0);
 
-			sender.send(encoded::Block::new(stream.out())).expect(RECEIVER_IN_SCOPE);
+			future::ok(encoded::Block::new(stream.out())).boxed()
 		} else {
 			match { self.cache.lock().block_body(&req.hash) } {
 				Some(body) => {
@@ -242,98 +258,124 @@ impl OnDemand {
 					stream.append_raw(&body.at(0).as_raw(), 1);
 					stream.append_raw(&body.at(1).as_raw(), 1);
 
-					sender.send(encoded::Block::new(stream.out())).expect(RECEIVER_IN_SCOPE);
+					future::ok(encoded::Block::new(stream.out())).boxed()
+				}
+				None => {
+					self.request(ctx, req)
+						.expect("request given fully fleshed out; qed")
+						.boxed()
 				}
-				None => self.dispatch(ctx, Pending::Block(req, sender)),
 			}
 		}
-		receiver
 	}
 
 	/// 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) -> Receiver<Vec<Receipt>> {
-		let (sender, receiver) = oneshot::channel();
-
+	pub fn block_receipts(&self, ctx: &BasicContext, req: request::BlockReceipts) -> BoxFuture<Vec<Receipt>, Canceled> {
 		// fast path for empty receipts.
 		if req.0.receipts_root() == SHA3_NULL_RLP {
-			sender.send(Vec::new()).expect(RECEIVER_IN_SCOPE);
-		} else {
-			match { self.cache.lock().block_receipts(&req.0.hash()) } {
-				Some(receipts) => sender.send(receipts).expect(RECEIVER_IN_SCOPE),
-				None => self.dispatch(ctx, Pending::BlockReceipts(req, sender)),
-			}
+			return future::ok(Vec::new()).boxed()
 		}
 
-		receiver
+		match { self.cache.lock().block_receipts(&req.0.hash()) } {
+			Some(receipts) => future::ok(receipts).boxed(),
+			None => {
+				self.request(ctx, req)
+					.expect("request given fully fleshed out; qed")
+					.boxed()
+			},
+		}
 	}
 
 	/// 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) -> Receiver<BasicAccount> {
-		let (sender, receiver) = oneshot::channel();
-		self.dispatch(ctx, Pending::Account(req, sender));
-		receiver
+	/// `None` here means that no account by the queried key exists in the queried state.
+	pub fn account(&self, ctx: &BasicContext, req: request::Account) -> BoxFuture<Option<BasicAccount>, Canceled> {
+		self.request(ctx, req)
+			.expect("request given fully fleshed out; qed")
+			.boxed()
 	}
 
 	/// Request code by address, known code hash, and block header.
-	pub fn code(&self, ctx: &BasicContext, req: request::Code) -> Receiver<Bytes> {
-		let (sender, receiver) = oneshot::channel();
-
+	pub fn code(&self, ctx: &BasicContext, req: request::Code) -> BoxFuture<Bytes, Canceled> {
 		// fast path for no code.
 		if req.code_hash == SHA3_EMPTY {
-			sender.send(Vec::new()).expect(RECEIVER_IN_SCOPE)
+			future::ok(Vec::new()).boxed()
 		} else {
-			self.dispatch(ctx, Pending::Code(req, sender));
+			self.request(ctx, req)
+				.expect("request given fully fleshed out; qed")
+				.boxed()
 		}
-
-		receiver
 	}
 
 	/// Request proof-of-execution for a transaction.
-	pub fn transaction_proof(&self, ctx: &BasicContext, req: request::TransactionProof) -> Receiver<Result<Executed, ExecutionError>> {
-		let (sender, receiver) = oneshot::channel();
+	pub fn transaction_proof(&self, ctx: &BasicContext, req: request::TransactionProof) -> BoxFuture<ExecutionResult, Canceled> {
+		self.request(ctx, req)
+			.expect("request given fully fleshed out; qed")
+			.boxed()
+	}
 
-		self.dispatch(ctx, Pending::TxProof(req, sender));
+	/// Submit a vector of requests to be processed together.
+	///
+	/// Fails if back-references are not coherent.
+	/// The returned vector of responses will correspond to the requests exactly.
+	pub fn request_raw(&self, ctx: &BasicContext, requests: Vec<Request>)
+		-> Result<Receiver<Vec<Response>>, basic_request::NoSuchOutput>
+	{
+		let (sender, receiver) = oneshot::channel();
 
-		receiver
-	}
+		if requests.is_empty() {
+			assert!(sender.send(Vec::new()).is_ok(), "receiver still in scope; qed");
+			return Ok(receiver);
+		}
 
-	// dispatch the request, with a "suitability" function to filter acceptable peers.
-	fn dispatch(&self, ctx: &BasicContext, pending: Pending) {
 		let mut builder = basic_request::RequestBuilder::default();
-		builder.push(pending.make_request())
-			.expect("make_request always returns fully complete request; qed");
-
-		let complete = builder.build();
-
-		let kind = complete.requests()[0].kind();
-		for (id, peer) in self.peers.read().iter() {
-			if !peer.can_handle(&pending) { continue }
-			match ctx.request_from(*id, complete.clone()) {
-				Ok(req_id) => {
-					trace!(target: "on_demand", "{}: Assigned {:?} to peer {}",
-						req_id, kind, id);
-
-					self.pending_requests.write().insert(
-						req_id,
-						pending,
-					);
-					return
-				}
-				Err(net::Error::NoCredits) => {}
-				Err(e) =>
-					trace!(target: "on_demand", "Failed to make request of peer {}: {:?}", id, e),
-			}
+
+		let responses = Vec::with_capacity(requests.len());
+		for request in requests {
+			builder.push(CheckedRequest::from(request))?;
 		}
 
-		self.orphaned_requests.write().push(pending);
+		let requests = builder.build();
+		let net_requests = requests.clone().map_requests(|req| req.into_net_request());
+		let capabilities = guess_capabilities(requests.requests());
+
+		self.pending.write().push(Pending {
+			requests: requests,
+			net_requests: net_requests,
+			required_capabilities: capabilities,
+			responses: responses,
+			sender: sender,
+		});
+
+		self.attempt_dispatch(ctx);
+
+		Ok(receiver)
 	}
 
+	/// Submit a strongly-typed batch of requests.
+	///
+	/// Fails if back-reference are not coherent.
+	pub fn request<T>(&self, ctx: &BasicContext, requests: T) -> Result<OnResponses<T>, basic_request::NoSuchOutput>
+		where T: request::RequestAdapter
+	{
+		self.request_raw(ctx, requests.make_requests()).map(|recv| OnResponses {
+			receiver: recv,
+			_marker: PhantomData,
+		})
+	}
 
-	// dispatch orphaned requests, and discard those for which the corresponding
+	// maybe dispatch pending requests.
+	// sometimes
+	fn attempt_dispatch(&self, ctx: &BasicContext) {
+		if !self.no_immediate_dispatch {
+			self.dispatch_pending(ctx)
+		}
+	}
+
+	// dispatch pending requests, and discard those for which the corresponding
 	// receiver has been dropped.
-	fn dispatch_orphaned(&self, ctx: &BasicContext) {
+	fn dispatch_pending(&self, ctx: &BasicContext) {
 		// wrapper future for calling `poll_cancel` on our `Senders` to preserve
 		// the invariant that it's always within a task.
 		struct CheckHangup<'a, T: 'a>(&'a mut Sender<T>);
@@ -356,35 +398,44 @@ impl OnDemand {
 			CheckHangup(send).wait().expect("CheckHangup always returns ok; qed")
 		}
 
-		if self.orphaned_requests.read().is_empty() { return }
-
-		let to_dispatch = ::std::mem::replace(&mut *self.orphaned_requests.write(), Vec::new());
-
-		trace!(target: "on_demand", "Attempting to dispatch {} orphaned requests.", to_dispatch.len());
-		for mut orphaned in to_dispatch {
-			let hung_up = match orphaned {
-				Pending::HeaderProof(_, ref mut sender) => match *sender {
-						ChtProofSender::Both(ref mut s) => check_hangup(s),
-						ChtProofSender::Hash(ref mut s) => check_hangup(s),
-						ChtProofSender::ChainScore(ref mut s) => check_hangup(s),
-				},
-				Pending::HeaderByHash(_, ref mut sender) => check_hangup(sender),
-				Pending::Block(_, ref mut sender) => check_hangup(sender),
-				Pending::BlockReceipts(_, ref mut sender) => check_hangup(sender),
-				Pending::Account(_, ref mut sender) => check_hangup(sender),
-				Pending::Code(_, ref mut sender) => check_hangup(sender),
-				Pending::TxProof(_, ref mut sender) => check_hangup(sender),
-			};
-
-			if !hung_up { self.dispatch(ctx, orphaned) }
-		}
+		if self.pending.read().is_empty() { return }
+		let mut pending = self.pending.write();
+
+		// iterate over all pending requests, and check them for hang-up.
+		// then, try and find a peer who can serve it.
+		let peers = self.peers.read();
+		*pending = ::std::mem::replace(&mut *pending, Vec::new()).into_iter()
+			.filter_map(|mut pending| match check_hangup(&mut pending.sender) {
+				false => Some(pending),
+				true => None,
+			})
+			.filter_map(|pending| {
+				for (peer_id, peer) in peers.iter() { // .shuffle?
+					// TODO: see which requests can be answered by the cache?
+
+					if !peer.can_fulfill(&pending.required_capabilities) {
+						continue
+					}
+
+					match ctx.request_from(*peer_id, pending.net_requests.clone()) {
+						Ok(req_id) => {
+							self.in_transit.write().insert(req_id, pending);
+							return None
+						}
+						Err(net::Error::NoCredits) => {}
+						Err(e) => debug!(target: "on_demand", "Error dispatching request to peer: {}", e),
+					}
+				}
+				Some(pending)
+			})
+			.collect(); // `pending` now contains all requests we couldn't dispatch.
 	}
 }
 
 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() });
-		self.dispatch_orphaned(ctx.as_basic());
+		self.attempt_dispatch(ctx.as_basic());
 	}
 
 	fn on_disconnect(&self, ctx: &EventContext, unfulfilled: &[ReqId]) {
@@ -392,16 +443,16 @@ impl Handler for OnDemand {
 		let ctx = ctx.as_basic();
 
 		{
-			let mut orphaned = self.orphaned_requests.write();
+			let mut pending = self.pending.write();
 			for unfulfilled in unfulfilled {
-				if let Some(pending) = self.pending_requests.write().remove(unfulfilled) {
+				if let Some(unfulfilled) = self.in_transit.write().remove(unfulfilled) {
 					trace!(target: "on_demand", "Attempting to reassign dropped request");
-					orphaned.push(pending);
+					pending.push(unfulfilled);
 				}
 			}
 		}
 
-		self.dispatch_orphaned(ctx);
+		self.attempt_dispatch(ctx);
 	}
 
 	fn on_announcement(&self, ctx: &EventContext, announcement: &Announcement) {
@@ -413,183 +464,70 @@ impl Handler for OnDemand {
 			}
 		}
 
-		self.dispatch_orphaned(ctx.as_basic());
+		self.attempt_dispatch(ctx.as_basic());
 	}
 
 	fn on_responses(&self, ctx: &EventContext, req_id: ReqId, responses: &[basic_request::Response]) {
-		let peer = ctx.peer();
-		let req = match self.pending_requests.write().remove(&req_id) {
+		use request::IncompleteRequest;
+
+		let mut pending = match self.in_transit.write().remove(&req_id) {
 			Some(req) => req,
 			None => return,
 		};
 
-		let response = match responses.get(0) {
-			Some(response) => response,
-			None => {
-				trace!(target: "on_demand", "Ignoring empty response for request {}", req_id);
-				self.dispatch(ctx.as_basic(), req);
-				return;
-			}
-		};
-
-		trace!(target: "on_demand", "Handling response for request {}, kind={:?}", req_id, response.kind());
-
-		// handle the response appropriately for the request.
-		// all branches which do not return early lead to disabling of the peer
-		// due to misbehavior.
-		match req {
-			Pending::HeaderProof(req, sender) => {
-				if let NetworkResponse::HeaderProof(ref response) = *response {
-					match req.check_response(&response.proof) {
-						Ok((hash, score)) => {
-							let mut cache = self.cache.lock();
-							cache.insert_block_hash(req.num(), hash);
-							cache.insert_chain_score(hash, score);
-
-							match sender {
-								ChtProofSender::Both(sender) => { let _ = sender.send((hash, score)); }
-								ChtProofSender::Hash(sender) => { let _ = sender.send(hash); }
-								ChtProofSender::ChainScore(sender) => { let _ = sender.send(score); }
-							}
-							return
-						}
-						Err(e) => warn!(target: "on_demand", "Error handling response for header request: {:?}", e),
-					}
-				}
-			}
-			Pending::HeaderByHash(req, sender) => {
-				if let NetworkResponse::Headers(ref response) = *response {
-					if let Some(header) = response.headers.get(0) {
-						match req.check_response(header) {
-							Ok(header) => {
-								self.cache.lock().insert_block_header(req.0, header.clone());
-								let _ = sender.send(header);
-								return
-							}
-							Err(e) => warn!(target: "on_demand", "Error handling response for header request: {:?}", e),
-						}
-					}
-				}
-			}
-			Pending::Block(req, sender) => {
-				if let NetworkResponse::Body(ref response) = *response {
-					match req.check_response(&response.body) {
-						Ok(block) => {
-							self.cache.lock().insert_block_body(req.hash, response.body.clone());
-							let _ = sender.send(block);
-							return
-						}
-						Err(e) => warn!(target: "on_demand", "Error handling response for block request: {:?}", e),
-					}
-				}
-			}
-			Pending::BlockReceipts(req, sender) => {
-				if let NetworkResponse::Receipts(ref response) = *response {
-					match req.check_response(&response.receipts) {
-						Ok(receipts) => {
-							let hash = req.0.hash();
-							self.cache.lock().insert_block_receipts(hash, receipts.clone());
-							let _ = sender.send(receipts);
-							return
-						}
-						Err(e) => warn!(target: "on_demand", "Error handling response for receipts request: {:?}", e),
-					}
+		// for each incoming response
+		//   1. ensure verification data filled. (still TODO since on_demand doesn't use back-references yet)
+		//   2. pending.requests.supply_response
+		//   3. if extracted on-demand response, keep it for later.
+		for response in responses {
+			match pending.requests.supply_response(&*self.cache, response) {
+				Ok(response) => {
+					pending.responses.push(response)
 				}
-			}
-			Pending::Account(req, sender) => {
-				if let NetworkResponse::Account(ref response) = *response {
-					match req.check_response(&response.proof) {
-						Ok(account) => {
-							let account = account.unwrap_or_else(|| {
-								BasicAccount {
-									balance: 0.into(),
-									nonce: self.start_nonce,
-									code_hash: SHA3_EMPTY,
-									storage_root: SHA3_NULL_RLP
-								}
-							});
-
-							// TODO: validate against request outputs.
-							// needs engine + env info as part of request.
-							let _ = sender.send(account);
-							return
-						}
-						Err(e) => warn!(target: "on_demand", "Error handling response for state request: {:?}", e),
-					}
-				}
-			}
-			Pending::Code(req, sender) => {
-				if let NetworkResponse::Code(ref response) = *response {
-					match req.check_response(response.code.as_slice()) {
-						Ok(()) => {
-							let _ = sender.send(response.code.clone());
-							return
-						}
-						Err(e) => warn!(target: "on_demand", "Error handling response for code request: {:?}", e),
-					}
-				}
-			}
-			Pending::TxProof(req, sender) => {
-				if let NetworkResponse::Execution(ref response) = *response {
-					match req.check_response(&response.items) {
-						ProvedExecution::Complete(executed) => {
-							let _ = sender.send(Ok(executed));
-							return
-						}
-						ProvedExecution::Failed(err) => {
-							let _ = sender.send(Err(err));
-							return
-						}
-						ProvedExecution::BadProof => warn!(target: "on_demand", "Error handling response for transaction proof request"),
-					}
+				Err(e) => {
+					let peer = ctx.peer();
+					debug!(target: "on_demand", "Peer {} gave bad response: {:?}", peer, e);
+					ctx.disable_peer(peer);
+
+					break;
 				}
 			}
 		}
 
-		ctx.disable_peer(peer);
-	}
+		pending.requests.fill_unanswered();
+		if pending.requests.is_complete() {
+			let _ = pending.sender.send(pending.responses);
 
-	fn tick(&self, ctx: &BasicContext) {
-		self.dispatch_orphaned(ctx)
-	}
-}
-
-#[cfg(test)]
-mod tests {
-	use super::*;
-
-	use std::sync::Arc;
-
-	use cache::Cache;
-	use net::{Announcement, BasicContext, ReqId, Error as LesError};
-	use request::Requests;
-
-	use network::{PeerId, NodeId};
-	use time::Duration;
-	use util::{H256, Mutex};
+			return;
+		}
 
-	struct FakeContext;
 
-	impl BasicContext for FakeContext {
-		fn persistent_peer_id(&self, _: PeerId) -> Option<NodeId> { None }
-		fn request_from(&self, _: PeerId, _: Requests) -> Result<ReqId, LesError> {
-			unimplemented!()
+		// update network requests (unless we're done, in which case fulfill the future.)
+		let mut builder = basic_request::RequestBuilder::default();
+		let num_answered = pending.requests.num_answered();
+		let mut mapping = move |idx| idx - num_answered;
+
+		for request in pending.requests.requests().iter().skip(num_answered) {
+			let mut net_req = request.clone().into_net_request();
+
+			// all back-references with request index less than `num_answered` have
+			// been filled by now. all remaining requests point to nothing earlier
+			// than the next unanswered request.
+			net_req.adjust_refs(&mut mapping);
+			builder.push(net_req)
+				.expect("all back-references to answered requests have been filled; qed");
 		}
-		fn make_announcement(&self, _: Announcement) { }
-		fn disconnect_peer(&self, _: PeerId) { }
-		fn disable_peer(&self, _: PeerId) { }
-	}
 
-	#[test]
-	fn detects_hangup() {
-		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
-		let on_demand = OnDemand::new(cache, 0.into());
-		let result = on_demand.header_by_hash(&FakeContext, request::HeaderByHash(H256::default()));
+		// update pending fields and re-queue.
+		let capabilities = guess_capabilities(&pending.requests.requests()[num_answered..]);
+		pending.net_requests = builder.build();
+		pending.required_capabilities = capabilities;
 
-		assert!(on_demand.orphaned_requests.read().len() == 1);
-		drop(result);
+		self.pending.write().push(pending);
+		self.attempt_dispatch(ctx.as_basic());
+	}
 
-		on_demand.dispatch_orphaned(&FakeContext);
-		assert!(on_demand.orphaned_requests.read().is_empty());
+	fn tick(&self, ctx: &BasicContext) {
+		self.attempt_dispatch(ctx)
 	}
 }
diff --git a/ethcore/light/src/on_demand/request.rs b/ethcore/light/src/on_demand/request.rs
index 8a37ddf7b41..c9a5c4d9b26 100644
--- a/ethcore/light/src/on_demand/request.rs
+++ b/ethcore/light/src/on_demand/request.rs
@@ -26,17 +26,374 @@ use ethcore::receipt::Receipt;
 use ethcore::state::{self, ProvedExecution};
 use ethcore::transaction::SignedTransaction;
 
+use request::{self as net_request, IncompleteRequest, Output, OutputKind};
+
 use rlp::{RlpStream, UntrustedRlp};
-use util::{Address, Bytes, DBValue, HashDB, H256, U256};
+use util::{Address, Bytes, DBValue, HashDB, Mutex, H256, U256};
 use util::memorydb::MemoryDB;
 use util::sha3::Hashable;
 use util::trie::{Trie, TrieDB, TrieError};
 
+const SUPPLIED_MATCHES: &'static str = "supplied responses always match produced requests; enforced by `check_response`; qed";
+
+/// Core unit of the API: submit batches of these to be answered with `Response`s.
+#[derive(Clone)]
+pub enum Request {
+	/// A request for a header proof.
+	HeaderProof(HeaderProof),
+	/// A request for a header by hash.
+	HeaderByHash(HeaderByHash),
+	/// A request for block receipts.
+	Receipts(BlockReceipts),
+	/// A request for a block body.
+	Body(Body),
+	/// A request for an account.
+	Account(Account),
+	/// A request for a contract's code.
+	Code(Code),
+	/// A request for proof of execution.
+	Execution(TransactionProof),
+}
+
+/// A request argument.
+pub trait RequestArg {
+	/// the response type.
+	type Out;
+
+	/// Create the request type.
+	/// `extract` must not fail when presented with the corresponding
+	/// `Response`.
+	fn make(self) -> Request;
+
+	/// May not panic if the response corresponds with the request
+	/// from `make`.
+	/// Is free to panic otherwise.
+	fn extract(r: Response) -> Self::Out;
+}
+
+/// An adapter can be thought of as a grouping of request argument types.
+/// This is implemented for various tuples and convenient types.
+pub trait RequestAdapter {
+	/// The output type.
+	type Out;
+
+	/// Infallibly produce requests. When `extract_from` is presented
+	/// with the corresponding response vector, it may not fail.
+	fn make_requests(self) -> Vec<Request>;
+
+	/// Extract the output type from the given responses.
+	/// If they are the corresponding responses to the requests
+	/// made by `make_requests`, do not panic.
+	fn extract_from(Vec<Response>) -> Self::Out;
+}
+
+// helper to implement `RequestArg` and `From` for a single request kind.
+macro_rules! impl_single {
+	($variant: ident, $me: ty, $out: ty) => {
+		impl RequestArg for $me {
+			type Out = $out;
+
+			fn make(self) -> Request {
+				Request::$variant(self)
+			}
+
+			fn extract(r: Response) -> $out {
+				match r {
+					Response::$variant(x) => x,
+					_ => panic!(SUPPLIED_MATCHES),
+				}
+			}
+		}
+
+		impl From<$me> for Request {
+			fn from(me: $me) -> Request {
+				Request::$variant(me)
+			}
+		}
+	}
+}
+
+// implement traits for each kind of request.
+impl_single!(HeaderProof, HeaderProof, (H256, U256));
+impl_single!(HeaderByHash, HeaderByHash, encoded::Header);
+impl_single!(Receipts, BlockReceipts, Vec<Receipt>);
+impl_single!(Body, Body, encoded::Block);
+impl_single!(Account, Account, Option<BasicAccount>);
+impl_single!(Code, Code, Bytes);
+impl_single!(Execution, TransactionProof, super::ExecutionResult);
+
+macro_rules! impl_args {
+	() => {
+		impl<T: RequestArg> RequestAdapter for T {
+			type Out = T::Out;
+
+			fn make_requests(self) -> Vec<Request> {
+				vec![self.make()]
+			}
+
+			fn extract_from(mut responses: Vec<Response>) -> Self::Out {
+				T::extract(responses.pop().expect(SUPPLIED_MATCHES))
+			}
+		}
+	};
+	($first: ident, $($next: ident,)*) => {
+		impl<
+			$first: RequestArg,
+			$($next: RequestArg,)*
+		>
+		RequestAdapter for ($first, $($next,)*) {
+			type Out = ($first::Out, $($next::Out,)*);
+
+			fn make_requests(self) -> Vec<Request> {
+				let ($first, $($next,)*) = self;
+
+				vec![
+					$first.make(),
+					$($next.make(),)*
+				]
+			}
+
+			fn extract_from(responses: Vec<Response>) -> Self::Out {
+				let mut iter = responses.into_iter();
+				(
+					$first::extract(iter.next().expect(SUPPLIED_MATCHES)),
+					$($next::extract(iter.next().expect(SUPPLIED_MATCHES)),)*
+				)
+			}
+		}
+		impl_args!($($next,)*);
+	}
+}
+
+mod impls {
+	#![allow(non_snake_case)]
+
+	use super::{RequestAdapter, RequestArg, Request, Response, SUPPLIED_MATCHES};
+
+	impl_args!(A, B, C, D, E, F, G, H, I, J, K, L,);
+}
+
+/// Requests coupled with their required data for verification.
+/// This is used internally but not part of the public API.
+#[derive(Clone)]
+#[allow(missing_docs)]
+pub enum CheckedRequest {
+	HeaderProof(HeaderProof, net_request::IncompleteHeaderProofRequest),
+	HeaderByHash(HeaderByHash, net_request::IncompleteHeadersRequest),
+	Receipts(BlockReceipts, net_request::IncompleteReceiptsRequest),
+	Body(Body, net_request::IncompleteBodyRequest),
+	Account(Account, net_request::IncompleteAccountRequest),
+	Code(Code, net_request::IncompleteCodeRequest),
+	Execution(TransactionProof, net_request::IncompleteExecutionRequest),
+}
+
+impl From<Request> for CheckedRequest {
+	fn from(req: Request) -> Self {
+		match req {
+			Request::HeaderByHash(req) => {
+				let net_req = net_request::IncompleteHeadersRequest {
+					start: net_request::HashOrNumber::Hash(req.0).into(),
+					skip: 0,
+					max: 1,
+					reverse: false,
+				};
+				CheckedRequest::HeaderByHash(req, net_req)
+			}
+			Request::HeaderProof(req) => {
+				let net_req = net_request::IncompleteHeaderProofRequest {
+					num: req.num().into(),
+				};
+				CheckedRequest::HeaderProof(req, net_req)
+			}
+			Request::Body(req) =>  {
+				let net_req = net_request::IncompleteBodyRequest {
+					hash: req.hash.into(),
+				};
+				CheckedRequest::Body(req, net_req)
+			}
+			Request::Receipts(req) => {
+				let net_req = net_request::IncompleteReceiptsRequest {
+					hash: req.0.hash().into(),
+				};
+				CheckedRequest::Receipts(req, net_req)
+			}
+			Request::Account(req) =>  {
+				let net_req = net_request::IncompleteAccountRequest {
+					block_hash: req.header.hash().into(),
+					address_hash: ::util::Hashable::sha3(&req.address).into(),
+				};
+				CheckedRequest::Account(req, net_req)
+			}
+			Request::Code(req) => {
+				let net_req = net_request::IncompleteCodeRequest {
+					block_hash: req.block_id.0.into(),
+					code_hash: req.code_hash.into(),
+				};
+				CheckedRequest::Code(req, net_req)
+			}
+			Request::Execution(req) => {
+				let net_req = net_request::IncompleteExecutionRequest {
+					block_hash: req.header.hash().into(),
+					from: req.tx.sender(),
+					gas: req.tx.gas,
+					gas_price: req.tx.gas_price,
+					action: req.tx.action.clone(),
+					value: req.tx.value,
+					data: req.tx.data.clone(),
+				};
+				CheckedRequest::Execution(req, net_req)
+			}
+		}
+	}
+}
+
+impl CheckedRequest {
+	/// Convert this into a network request.
+	pub fn into_net_request(self) -> net_request::Request {
+		use ::request::Request as NetRequest;
+
+		match self {
+			CheckedRequest::HeaderProof(_, req) => NetRequest::HeaderProof(req),
+			CheckedRequest::HeaderByHash(_, req) => NetRequest::Headers(req),
+			CheckedRequest::Receipts(_, req) => NetRequest::Receipts(req),
+			CheckedRequest::Body(_, req) => NetRequest::Body(req),
+			CheckedRequest::Account(_, req) => NetRequest::Account(req),
+			CheckedRequest::Code(_, req) => NetRequest::Code(req),
+			CheckedRequest::Execution(_, req) => NetRequest::Execution(req),
+		}
+	}
+}
+
+macro_rules! match_me {
+	($me: expr, ($check: pat, $req: pat) => $e: expr) => {
+		match $me {
+			CheckedRequest::HeaderProof($check, $req) => $e,
+			CheckedRequest::HeaderByHash($check, $req) => $e,
+			CheckedRequest::Receipts($check, $req) => $e,
+			CheckedRequest::Body($check, $req) => $e,
+			CheckedRequest::Account($check, $req) => $e,
+			CheckedRequest::Code($check, $req) => $e,
+			CheckedRequest::Execution($check, $req) => $e,
+		}
+	}
+}
+
+impl IncompleteRequest for CheckedRequest {
+	type Complete = net_request::CompleteRequest;
+	type Response = net_request::Response;
+
+	/// Check prior outputs against the needed inputs.
+	///
+	/// This is called to ensure consistency of this request with
+	/// others in the same packet.
+	fn check_outputs<F>(&self, f: F) -> Result<(), net_request::NoSuchOutput>
+		where F: FnMut(usize, usize, OutputKind) -> Result<(), net_request::NoSuchOutput>
+	{
+		match_me!(*self, (_, ref req) => req.check_outputs(f))
+	}
+
+	/// Note that this request will produce the following outputs.
+	fn note_outputs<F>(&self, f: F) where F: FnMut(usize, OutputKind) {
+		match_me!(*self, (_, ref req) => req.note_outputs(f))
+	}
+
+	/// Fill fields of the request.
+	///
+	/// This function is provided an "output oracle" which allows fetching of
+	/// prior request outputs.
+	/// Only outputs previously checked with `check_outputs` may be available.
+	fn fill<F>(&mut self, f: F) where F: Fn(usize, usize) -> Result<Output, net_request::NoSuchOutput> {
+		match_me!(*self, (_, ref mut req) => req.fill(f))
+	}
+
+	/// Will succeed if all fields have been filled, will fail otherwise.
+	fn complete(self) -> Result<Self::Complete, net_request::NoSuchOutput> {
+		use ::request::CompleteRequest;
+
+		match self {
+			CheckedRequest::HeaderProof(_, req) => req.complete().map(CompleteRequest::HeaderProof),
+			CheckedRequest::HeaderByHash(_, req) => req.complete().map(CompleteRequest::Headers),
+			CheckedRequest::Receipts(_, req) => req.complete().map(CompleteRequest::Receipts),
+			CheckedRequest::Body(_, req) => req.complete().map(CompleteRequest::Body),
+			CheckedRequest::Account(_, req) => req.complete().map(CompleteRequest::Account),
+			CheckedRequest::Code(_, req) => req.complete().map(CompleteRequest::Code),
+			CheckedRequest::Execution(_, req) => req.complete().map(CompleteRequest::Execution),
+		}
+	}
+
+
+	fn adjust_refs<F>(&mut self, mapping: F) where F: FnMut(usize) -> usize {
+		match_me!(*self, (_, ref mut req) => req.adjust_refs(mapping))
+	}
+}
+
+impl net_request::CheckedRequest for CheckedRequest {
+	type Extract = Response;
+	type Error = Error;
+	type Environment = Mutex<::cache::Cache>;
+
+	/// Check whether the response matches (beyond the type).
+	fn check_response(&self, cache: &Mutex<::cache::Cache>, response: &Self::Response) -> Result<Response, Error> {
+		use ::request::Response as NetResponse;
+
+		// helper for expecting a specific response for a given request.
+		macro_rules! expect {
+			($res: pat => $e: expr) => {
+				match *response {
+					$res => $e,
+					_ => Err(Error::WrongKind),
+				}
+			}
+		}
+
+		// check response against contained prover.
+		match *self {
+			CheckedRequest::HeaderProof(ref prover, _) => expect!(NetResponse::HeaderProof(ref res) =>
+				prover.check_response(cache, &res.proof).map(Response::HeaderProof)),
+			CheckedRequest::HeaderByHash(ref prover, _) => expect!(NetResponse::Headers(ref res) =>
+				prover.check_response(cache, &res.headers).map(Response::HeaderByHash)),
+			CheckedRequest::Receipts(ref prover, _) => expect!(NetResponse::Receipts(ref res) =>
+				prover.check_response(cache, &res.receipts).map(Response::Receipts)),
+			CheckedRequest::Body(ref prover, _) => expect!(NetResponse::Body(ref res) =>
+				prover.check_response(cache, &res.body).map(Response::Body)),
+			CheckedRequest::Account(ref prover, _) => expect!(NetResponse::Account(ref res) =>
+				prover.check_response(cache, &res.proof).map(Response::Account)),
+			CheckedRequest::Code(ref prover, _) => expect!(NetResponse::Code(ref res) =>
+				prover.check_response(cache, &res.code).map(Response::Code)),
+			CheckedRequest::Execution(ref prover, _) => expect!(NetResponse::Execution(ref res) =>
+				prover.check_response(cache, &res.items).map(Response::Execution)),
+		}
+	 }
+}
+
+/// Responses to on-demand requests.
+/// All of these are checked.
+pub enum Response {
+	/// Response to a header proof request.
+	/// Returns the hash and chain score.
+	HeaderProof((H256, U256)),
+	/// Response to a header-by-hash request.
+	HeaderByHash(encoded::Header),
+	/// Response to a receipts request.
+	Receipts(Vec<Receipt>),
+	/// Response to a block body request.
+	Body(encoded::Block),
+	/// Response to an Account request.
+	// TODO: `unwrap_or(engine_defaults)`
+	Account(Option<BasicAccount>),
+	/// Response to a request for code.
+	Code(Vec<u8>),
+	/// Response to a request for proved execution.
+	Execution(super::ExecutionResult),
+}
+
 /// Errors in verification.
 #[derive(Debug, PartialEq)]
 pub enum Error {
 	/// RLP decoder error.
 	Decoder(::rlp::DecoderError),
+	/// Empty response.
+	Empty,
 	/// Trie lookup error (result of bad proof)
 	Trie(TrieError),
 	/// Bad inclusion proof
@@ -47,6 +404,8 @@ pub enum Error {
 	WrongHash(H256, H256),
 	/// Wrong trie root.
 	WrongTrieRoot(H256, H256),
+	/// Wrong response kind.
+	WrongKind,
 }
 
 impl From<::rlp::DecoderError> for Error {
@@ -93,9 +452,15 @@ impl HeaderProof {
 	pub fn cht_root(&self) -> H256 { self.cht_root }
 
 	/// Check a response with a CHT proof, get a hash and total difficulty back.
-	pub fn check_response(&self, proof: &[Bytes]) -> Result<(H256, U256), Error> {
+	pub fn check_response(&self, cache: &Mutex<::cache::Cache>, proof: &[Bytes]) -> Result<(H256, U256), Error> {
 		match ::cht::check_proof(proof, self.num, self.cht_root) {
-			Some((expected_hash, td)) => Ok((expected_hash, td)),
+			Some((expected_hash, td)) => {
+				let mut cache = cache.lock();
+				cache.insert_block_hash(self.num, expected_hash);
+				cache.insert_chain_score(expected_hash, td);
+
+				Ok((expected_hash, td))
+			}
 			None => Err(Error::BadProof),
 		}
 	}
@@ -107,10 +472,14 @@ pub struct HeaderByHash(pub H256);
 
 impl HeaderByHash {
 	/// Check a response for the header.
-	pub fn check_response(&self, header: &encoded::Header) -> Result<encoded::Header, Error> {
+	pub fn check_response(&self, cache: &Mutex<::cache::Cache>, headers: &[encoded::Header]) -> Result<encoded::Header, Error> {
+		let header = headers.get(0).ok_or(Error::Empty)?;
 		let hash = header.sha3();
 		match hash == self.0 {
-			true => Ok(header.clone()),
+			true => {
+				cache.lock().insert_block_header(hash, header.clone());
+				Ok(header.clone())
+			}
 			false => Err(Error::WrongHash(self.0, hash)),
 		}
 	}
@@ -136,7 +505,7 @@ impl Body {
 	}
 
 	/// Check a response for this block body.
-	pub fn check_response(&self, body: &encoded::Body) -> Result<encoded::Block, Error> {
+	pub fn check_response(&self, cache: &Mutex<::cache::Cache>, body: &encoded::Body) -> Result<encoded::Block, Error> {
 		// check the integrity of the the body against the header
 		let tx_root = ::util::triehash::ordered_trie_root(body.rlp().at(0).iter().map(|r| r.as_raw().to_vec()));
 		if tx_root != self.header.transactions_root() {
@@ -154,6 +523,8 @@ impl Body {
 		stream.append_raw(body.rlp().at(0).as_raw(), 1);
 		stream.append_raw(body.rlp().at(1).as_raw(), 1);
 
+		cache.lock().insert_block_body(self.hash, body.clone());
+
 		Ok(encoded::Block::new(stream.out()))
 	}
 }
@@ -164,12 +535,15 @@ 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> {
+	pub fn check_response(&self, cache: &Mutex<::cache::Cache>, 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()),
+			true => {
+				cache.lock().insert_block_receipts(receipts_root, receipts.to_vec());
+				Ok(receipts.to_vec())
+			}
 			false => Err(Error::WrongTrieRoot(receipts_root, found_root)),
 		}
 	}
@@ -186,7 +560,7 @@ pub struct Account {
 
 impl Account {
 	/// Check a response with an account against the stored header.
-	pub fn check_response(&self, proof: &[Bytes]) -> Result<Option<BasicAccount>, Error> {
+	pub fn check_response(&self, _: &Mutex<::cache::Cache>, proof: &[Bytes]) -> Result<Option<BasicAccount>, Error> {
 		let state_root = self.header.state_root();
 
 		let mut db = MemoryDB::new();
@@ -208,6 +582,7 @@ impl Account {
 }
 
 /// Request for account code.
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Code {
 	/// Block hash, number pair.
 	pub block_id: (H256, u64),
@@ -217,10 +592,10 @@ pub struct Code {
 
 impl Code {
 	/// Check a response with code against the code hash.
-	pub fn check_response(&self, code: &[u8]) -> Result<(), Error> {
+	pub fn check_response(&self, _: &Mutex<::cache::Cache>, code: &[u8]) -> Result<Vec<u8>, Error> {
 		let found_hash = code.sha3();
 		if found_hash == self.code_hash {
-			Ok(())
+			Ok(code.to_vec())
 		} else {
 			Err(Error::WrongHash(self.code_hash, found_hash))
 		}
@@ -228,6 +603,7 @@ impl Code {
 }
 
 /// Request for transaction execution, along with the parts necessary to verify the proof.
+#[derive(Clone)]
 pub struct TransactionProof {
 	/// The transaction to request proof of.
 	pub tx: SignedTransaction,
@@ -241,25 +617,32 @@ pub struct TransactionProof {
 
 impl TransactionProof {
 	/// Check the proof, returning the proved execution or indicate that the proof was bad.
-	pub fn check_response(&self, state_items: &[DBValue]) -> ProvedExecution {
+	pub fn check_response(&self, _: &Mutex<::cache::Cache>, state_items: &[DBValue]) -> Result<super::ExecutionResult, Error> {
 		let root = self.header.state_root();
 
 		let mut env_info = self.env_info.clone();
 		env_info.gas_limit = self.tx.gas.clone();
-		state::check_proof(
+
+		let proved_execution = state::check_proof(
 			state_items,
 			root,
 			&self.tx,
 			&*self.engine,
-			&env_info,
-		)
+			&self.env_info,
+		);
+
+		match proved_execution {
+			ProvedExecution::BadProof => Err(Error::BadProof),
+			ProvedExecution::Failed(e) => Ok(Err(e)),
+			ProvedExecution::Complete(e) => Ok(Ok(e)),
+		}
 	}
 }
 
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use util::{MemoryDB, Address, H256};
+	use util::{MemoryDB, Address, Mutex, H256};
 	use util::trie::{Trie, TrieMut, SecTrieDB, SecTrieDBMut};
 	use util::trie::recorder::Recorder;
 
@@ -268,6 +651,10 @@ mod tests {
 	use ethcore::encoded;
 	use ethcore::receipt::Receipt;
 
+	fn make_cache() -> ::cache::Cache {
+		::cache::Cache::new(Default::default(), ::time::Duration::seconds(1))
+	}
+
 	#[test]
 	fn no_invalid_header_by_number() {
 		assert!(HeaderProof::new(0, Default::default()).is_none())
@@ -297,7 +684,8 @@ mod tests {
 		let proof = cht.prove(10_000, 0).unwrap().unwrap();
 		let req = HeaderProof::new(10_000, cht.root()).unwrap();
 
-		assert!(req.check_response(&proof[..]).is_ok());
+		let cache = Mutex::new(make_cache());
+		assert!(req.check_response(&cache, &proof[..]).is_ok());
 	}
 
 	#[test]
@@ -308,7 +696,8 @@ mod tests {
 		let hash = header.hash();
 		let raw_header = encoded::Header::new(::rlp::encode(&header).to_vec());
 
-		assert!(HeaderByHash(hash).check_response(&raw_header).is_ok())
+		let cache = Mutex::new(make_cache());
+		assert!(HeaderByHash(hash).check_response(&cache, &[raw_header]).is_ok())
 	}
 
 	#[test]
@@ -324,8 +713,9 @@ mod tests {
 			hash: header.hash(),
 		};
 
+		let cache = Mutex::new(make_cache());
 		let response = encoded::Body::new(body_stream.drain().to_vec());
-		assert!(req.check_response(&response).is_ok())
+		assert!(req.check_response(&cache, &response).is_ok())
 	}
 
 	#[test]
@@ -346,7 +736,8 @@ mod tests {
 
 		let req = BlockReceipts(encoded::Header::new(::rlp::encode(&header).to_vec()));
 
-		assert!(req.check_response(&receipts).is_ok())
+		let cache = Mutex::new(make_cache());
+		assert!(req.check_response(&cache, &receipts).is_ok())
 	}
 
 	#[test]
@@ -395,7 +786,8 @@ mod tests {
 			address: addr,
 		};
 
-		assert!(req.check_response(&proof[..]).is_ok());
+		let cache = Mutex::new(make_cache());
+		assert!(req.check_response(&cache, &proof[..]).is_ok());
 	}
 
 	#[test]
@@ -406,7 +798,8 @@ mod tests {
 			code_hash: ::util::Hashable::sha3(&code),
 		};
 
-		assert!(req.check_response(&code).is_ok());
-		assert!(req.check_response(&[]).is_err());
+		let cache = Mutex::new(make_cache());
+		assert!(req.check_response(&cache, &code).is_ok());
+		assert!(req.check_response(&cache, &[]).is_err());
 	}
 }
diff --git a/ethcore/light/src/on_demand/tests.rs b/ethcore/light/src/on_demand/tests.rs
new file mode 100644
index 00000000000..d5789c5e184
--- /dev/null
+++ b/ethcore/light/src/on_demand/tests.rs
@@ -0,0 +1,397 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Tests for the on-demand service.
+
+use cache::Cache;
+use ethcore::encoded;
+use ethcore::header::{Header, Seal};
+use futures::Future;
+use network::{PeerId, NodeId};
+use net::*;
+use util::{H256, Mutex};
+use time::Duration;
+use ::request::{self as basic_request, Response};
+
+use std::sync::Arc;
+
+use super::{request, OnDemand, Peer};
+
+// useful contexts to give the service.
+enum Context {
+	NoOp,
+	WithPeer(PeerId),
+	RequestFrom(PeerId, ReqId),
+	Punish(PeerId),
+}
+
+impl EventContext for Context {
+	fn peer(&self) -> PeerId {
+		match *self {
+			Context::WithPeer(id)
+			| Context::RequestFrom(id, _)
+			| Context::Punish(id) => id,
+			_ => panic!("didn't expect to have peer queried."),
+		}
+	}
+
+	fn as_basic(&self) -> &BasicContext { self }
+}
+
+impl BasicContext for Context {
+	/// Returns the relevant's peer persistent Id (aka NodeId).
+	fn persistent_peer_id(&self, _: PeerId) -> Option<NodeId> {
+		panic!("didn't expect to provide persistent ID")
+	}
+
+	fn request_from(&self, peer_id: PeerId, _: ::request::NetworkRequests) -> Result<ReqId, Error> {
+		match *self {
+			Context::RequestFrom(id, req_id) => if peer_id == id { Ok(req_id) } else { Err(Error::NoCredits) },
+			_ => panic!("didn't expect to have requests dispatched."),
+		}
+	}
+
+	fn make_announcement(&self, _: Announcement) {
+		panic!("didn't expect to make announcement")
+	}
+
+	fn disconnect_peer(&self, id: PeerId) {
+		self.disable_peer(id)
+	}
+
+	fn disable_peer(&self, peer_id: PeerId) {
+		match *self {
+			Context::Punish(id) if id == peer_id => {},
+			_ => panic!("Unexpectedly punished peer."),
+		}
+	}
+}
+
+// test harness.
+struct Harness {
+	service: OnDemand,
+}
+
+impl Harness {
+	fn create() -> Self {
+		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::minutes(1))));
+		Harness {
+			service: OnDemand::new_test(cache),
+		}
+	}
+
+	fn inject_peer(&self, id: PeerId, peer: Peer) {
+		self.service.peers.write().insert(id, peer);
+	}
+}
+
+fn dummy_status() -> Status {
+	Status {
+		protocol_version: 1,
+		network_id: 999,
+		head_td: 1.into(),
+		head_hash: H256::default(),
+		head_num: 1359,
+		genesis_hash: H256::default(),
+		last_head: None,
+	}
+}
+
+fn dummy_capabilities() -> Capabilities {
+	Capabilities {
+		serve_headers: true,
+		serve_chain_since: Some(1),
+		serve_state_since: Some(1),
+		tx_relay: true,
+	}
+}
+
+#[test]
+fn detects_hangup() {
+	let on_demand = Harness::create().service;
+	let result = on_demand.header_by_hash(&Context::NoOp, request::HeaderByHash(H256::default()));
+
+	assert_eq!(on_demand.pending.read().len(), 1);
+	drop(result);
+
+	on_demand.dispatch_pending(&Context::NoOp);
+	assert!(on_demand.pending.read().is_empty());
+}
+
+#[test]
+fn single_request() {
+	let harness = Harness::create();
+
+	let peer_id = 10101;
+	let req_id = ReqId(14426);
+
+	harness.inject_peer(peer_id, Peer {
+		status: dummy_status(),
+		capabilities: dummy_capabilities(),
+	});
+
+	let header = Header::default();
+	let encoded = encoded::Header::new(header.rlp(Seal::With));
+
+	let recv = harness.service.request_raw(
+		&Context::NoOp,
+		vec![request::HeaderByHash(header.hash()).into()]
+	).unwrap();
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.service.dispatch_pending(&Context::RequestFrom(peer_id, req_id));
+
+	assert_eq!(harness.service.pending.read().len(), 0);
+
+	harness.service.on_responses(
+		&Context::WithPeer(peer_id),
+		req_id,
+		&[Response::Headers(basic_request::HeadersResponse { headers: vec![encoded] })]
+	);
+
+	assert!(recv.wait().is_ok());
+}
+
+#[test]
+fn no_capabilities() {
+	let harness = Harness::create();
+
+	let peer_id = 10101;
+
+	let mut capabilities = dummy_capabilities();
+	capabilities.serve_headers = false;
+
+	harness.inject_peer(peer_id, Peer {
+		status: dummy_status(),
+		capabilities: capabilities,
+	});
+
+	let _recv = harness.service.request_raw(
+		&Context::NoOp,
+		vec![request::HeaderByHash(Default::default()).into()]
+	).unwrap();
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.service.dispatch_pending(&Context::NoOp);
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+}
+
+#[test]
+fn reassign() {
+	let harness = Harness::create();
+
+	let peer_ids = (10101, 12345);
+	let req_ids = (ReqId(14426), ReqId(555));
+
+	harness.inject_peer(peer_ids.0, Peer {
+		status: dummy_status(),
+		capabilities: dummy_capabilities(),
+	});
+
+	let header = Header::default();
+	let encoded = encoded::Header::new(header.rlp(Seal::With));
+
+	let recv = harness.service.request_raw(
+		&Context::NoOp,
+		vec![request::HeaderByHash(header.hash()).into()]
+	).unwrap();
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.service.dispatch_pending(&Context::RequestFrom(peer_ids.0, req_ids.0));
+	assert_eq!(harness.service.pending.read().len(), 0);
+
+	harness.service.on_disconnect(&Context::WithPeer(peer_ids.0), &[req_ids.0]);
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.inject_peer(peer_ids.1, Peer {
+		status: dummy_status(),
+		capabilities: dummy_capabilities(),
+	});
+
+	harness.service.dispatch_pending(&Context::RequestFrom(peer_ids.1, req_ids.1));
+	assert_eq!(harness.service.pending.read().len(), 0);
+
+	harness.service.on_responses(
+		&Context::WithPeer(peer_ids.1),
+		req_ids.1,
+		&[Response::Headers(basic_request::HeadersResponse { headers: vec![encoded] })]
+	);
+
+	assert!(recv.wait().is_ok());
+}
+
+#[test]
+fn partial_response() {
+	let harness = Harness::create();
+
+	let peer_id = 111;
+	let req_ids = (ReqId(14426), ReqId(555));
+
+	harness.inject_peer(peer_id, Peer {
+		status: dummy_status(),
+		capabilities: dummy_capabilities(),
+	});
+
+	let make = |num| {
+		let mut hdr = Header::default();
+		hdr.set_number(num);
+
+		let encoded = encoded::Header::new(hdr.rlp(Seal::With));
+		(hdr, encoded)
+	};
+
+	let (header1, encoded1) = make(5);
+	let (header2, encoded2) = make(23452);
+
+	// request two headers.
+	let recv = harness.service.request_raw(
+		&Context::NoOp,
+		vec![
+			request::HeaderByHash(header1.hash()).into(),
+			request::HeaderByHash(header2.hash()).into(),
+		],
+	).unwrap();
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.service.dispatch_pending(&Context::RequestFrom(peer_id, req_ids.0));
+	assert_eq!(harness.service.pending.read().len(), 0);
+
+	// supply only the first one.
+	harness.service.on_responses(
+		&Context::WithPeer(peer_id),
+		req_ids.0,
+		&[Response::Headers(basic_request::HeadersResponse { headers: vec![encoded1] })]
+	);
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.service.dispatch_pending(&Context::RequestFrom(peer_id, req_ids.1));
+	assert_eq!(harness.service.pending.read().len(), 0);
+
+	// supply the next one.
+	harness.service.on_responses(
+		&Context::WithPeer(peer_id),
+		req_ids.1,
+		&[Response::Headers(basic_request::HeadersResponse { headers: vec![encoded2] })]
+	);
+
+	assert!(recv.wait().is_ok());
+}
+
+#[test]
+fn part_bad_part_good() {
+	let harness = Harness::create();
+
+	let peer_id = 111;
+	let req_ids = (ReqId(14426), ReqId(555));
+
+	harness.inject_peer(peer_id, Peer {
+		status: dummy_status(),
+		capabilities: dummy_capabilities(),
+	});
+
+	let make = |num| {
+		let mut hdr = Header::default();
+		hdr.set_number(num);
+
+		let encoded = encoded::Header::new(hdr.rlp(Seal::With));
+		(hdr, encoded)
+	};
+
+	let (header1, encoded1) = make(5);
+	let (header2, encoded2) = make(23452);
+
+	// request two headers.
+	let recv = harness.service.request_raw(
+		&Context::NoOp,
+		vec![
+			request::HeaderByHash(header1.hash()).into(),
+			request::HeaderByHash(header2.hash()).into(),
+		],
+	).unwrap();
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.service.dispatch_pending(&Context::RequestFrom(peer_id, req_ids.0));
+	assert_eq!(harness.service.pending.read().len(), 0);
+
+	// supply only the first one, but followed by the wrong kind of response.
+	// the first header should be processed.
+	harness.service.on_responses(
+		&Context::Punish(peer_id),
+		req_ids.0,
+		&[
+			Response::Headers(basic_request::HeadersResponse { headers: vec![encoded1] }),
+			Response::Receipts(basic_request::ReceiptsResponse { receipts: vec![] } ),
+		]
+	);
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.inject_peer(peer_id, Peer {
+		status: dummy_status(),
+		capabilities: dummy_capabilities(),
+	});
+
+	harness.service.dispatch_pending(&Context::RequestFrom(peer_id, req_ids.1));
+	assert_eq!(harness.service.pending.read().len(), 0);
+
+	// supply the next one.
+	harness.service.on_responses(
+		&Context::WithPeer(peer_id),
+		req_ids.1,
+		&[Response::Headers(basic_request::HeadersResponse { headers: vec![encoded2] })]
+	);
+
+	assert!(recv.wait().is_ok());
+}
+
+#[test]
+fn wrong_kind() {
+	let harness = Harness::create();
+
+	let peer_id = 10101;
+	let req_id = ReqId(14426);
+
+	harness.inject_peer(peer_id, Peer {
+		status: dummy_status(),
+		capabilities: dummy_capabilities(),
+	});
+
+	let _recv = harness.service.request_raw(
+		&Context::NoOp,
+		vec![request::HeaderByHash(Default::default()).into()]
+	).unwrap();
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+
+	harness.service.dispatch_pending(&Context::RequestFrom(peer_id, req_id));
+
+	assert_eq!(harness.service.pending.read().len(), 0);
+
+	harness.service.on_responses(
+		&Context::Punish(peer_id),
+		req_id,
+		&[Response::Receipts(basic_request::ReceiptsResponse { receipts: vec![] })]
+	);
+
+	assert_eq!(harness.service.pending.read().len(), 1);
+}
diff --git a/ethcore/light/src/transaction_queue.rs b/ethcore/light/src/transaction_queue.rs
index d17a863f511..7ce2bd53dd8 100644
--- a/ethcore/light/src/transaction_queue.rs
+++ b/ethcore/light/src/transaction_queue.rs
@@ -131,7 +131,7 @@ impl TransactionQueue {
 
 		if self.by_hash.contains_key(&hash) { return Err(TransactionError::AlreadyImported) }
 
-	    let res = match self.by_account.entry(sender) {
+		let res = match self.by_account.entry(sender) {
 			Entry::Vacant(entry) => {
 				entry.insert(AccountTransactions {
 					cur_nonce: CurrentNonce::Assumed(nonce),
diff --git a/ethcore/light/src/types/request/builder.rs b/ethcore/light/src/types/request/builder.rs
index 77f1389c2bb..dff33513aa1 100644
--- a/ethcore/light/src/types/request/builder.rs
+++ b/ethcore/light/src/types/request/builder.rs
@@ -20,22 +20,30 @@
 
 use std::collections::HashMap;
 use request::{
-	IncompleteRequest, CompleteRequest, Request,
-	OutputKind, Output, NoSuchOutput, Response, ResponseError,
+	IncompleteRequest, OutputKind, Output, NoSuchOutput, ResponseError, ResponseLike,
 };
 
 /// Build chained requests. Push them onto the series with `push`,
 /// and produce a `Requests` object with `build`. Outputs are checked for consistency.
-#[derive(Debug, Default, Clone, PartialEq, Eq)]
-pub struct RequestBuilder {
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RequestBuilder<T> {
 	output_kinds: HashMap<(usize, usize), OutputKind>,
-	requests: Vec<Request>,
+	requests: Vec<T>,
+}
+
+impl<T> Default for RequestBuilder<T> {
+	fn default() -> Self {
+		RequestBuilder {
+			output_kinds: HashMap::new(),
+			requests: Vec::new(),
+		}
+	}
 }
 
-impl RequestBuilder {
+impl<T: IncompleteRequest> RequestBuilder<T> {
 	/// Attempt to push a request onto the request chain. Fails if the request
 	/// references a non-existent output of a prior request.
-	pub fn push(&mut self, request: Request) -> Result<(), NoSuchOutput> {
+	pub fn push(&mut self, request: T) -> Result<(), NoSuchOutput> {
 		request.check_outputs(|req, idx, kind| {
 			match self.output_kinds.get(&(req, idx)) {
 				Some(k) if k == &kind => Ok(()),
@@ -54,7 +62,7 @@ impl RequestBuilder {
 	}
 
 	/// Convert this into a "requests" object.
-	pub fn build(self) -> Requests {
+	pub fn build(self) -> Requests<T> {
 		Requests {
 			outputs: HashMap::new(),
 			requests: self.requests,
@@ -65,44 +73,41 @@ impl RequestBuilder {
 
 /// Requests pending responses.
 #[derive(Debug, Clone, PartialEq, Eq)]
-pub struct Requests {
+pub struct Requests<T> {
 	outputs: HashMap<(usize, usize), Output>,
-	requests: Vec<Request>,
+	requests: Vec<T>,
 	answered: usize,
 }
 
-impl Requests {
-	/// For each request, produce responses for each.
-	/// The responses vector produced goes up to the point where the responder
-	/// first returns `None`, an invalid response, or until all requests have been responded to.
-	pub fn respond_to_all<F>(mut self, responder: F) -> Vec<Response>
-		where F: Fn(CompleteRequest) -> Option<Response>
-	{
-		let mut responses = Vec::new();
-
-		while let Some(response) = self.next_complete().and_then(&responder) {
-			match self.supply_response(&response) {
-				Ok(()) => responses.push(response),
-				Err(e) => {
-					debug!(target: "pip", "produced bad response to request: {:?}", e);
-					return responses;
-				}
-			}
-		}
-
-		responses
-	}
-
+impl<T> Requests<T> {
 	/// Get access to the underlying slice of requests.
 	// TODO: unimplemented -> Vec<Request>, // do we _have to_ allocate?
-	pub fn requests(&self) -> &[Request] { &self.requests }
+	pub fn requests(&self) -> &[T] { &self.requests }
 
 	/// Get the number of answered requests.
 	pub fn num_answered(&self) -> usize { self.answered }
 
+	/// Whether the batch is complete.
+	pub fn is_complete(&self) -> bool {
+		self.answered == self.requests.len()
+	}
+
+	/// Map requests from one type into another.
+	pub fn map_requests<F, U>(self, f: F) -> Requests<U>
+		where F: FnMut(T) -> U, U: IncompleteRequest
+	{
+		Requests {
+			outputs: self.outputs,
+			requests: self.requests.into_iter().map(f).collect(),
+			answered: self.answered,
+		}
+	}
+}
+
+impl<T: IncompleteRequest + Clone> Requests<T> {
 	/// Get the next request as a filled request. Returns `None` when all requests answered.
-	pub fn next_complete(&self) -> Option<CompleteRequest> {
-		if self.answered == self.requests.len() {
+	pub fn next_complete(&self) -> Option<T::Complete> {
+		if self.is_complete() {
 			None
 		} else {
 			Some(self.requests[self.answered].clone()
@@ -111,14 +116,29 @@ impl Requests {
 		}
 	}
 
+	/// Sweep through all unanswered requests, filling them as necessary.
+	pub fn fill_unanswered(&mut self) {
+		let outputs = &mut self.outputs;
+
+		for req in self.requests.iter_mut().skip(self.answered) {
+			req.fill(|req_idx, out_idx| outputs.get(&(req_idx, out_idx)).cloned().ok_or(NoSuchOutput))
+		}
+	}
+}
+
+impl<T: super::CheckedRequest> Requests<T> {
 	/// Supply a response for the next request.
 	/// Fails on: wrong request kind, all requests answered already.
-	pub fn supply_response(&mut self, response: &Response) -> Result<(), ResponseError> {
+	pub fn supply_response(&mut self, env: &T::Environment, response: &T::Response)
+		-> Result<T::Extract, ResponseError<T::Error>>
+	{
 		let idx = self.answered;
 
 		// check validity.
-		if idx == self.requests.len() { return Err(ResponseError::Unexpected) }
-		if self.requests[idx].kind() != response.kind() { return Err(ResponseError::WrongKind) }
+		if self.is_complete() { return Err(ResponseError::Unexpected) }
+
+		let extracted = self.requests[idx]
+			.check_response(env, response).map_err(ResponseError::Validity)?;
 
 		let outputs = &mut self.outputs;
 		response.fill_outputs(|out_idx, output| {
@@ -135,7 +155,30 @@ impl Requests {
 			req.fill(|req_idx, out_idx| outputs.get(&(req_idx, out_idx)).cloned().ok_or(NoSuchOutput))
 		}
 
-		Ok(())
+		Ok(extracted)
+	}
+}
+
+impl Requests<super::Request> {
+	/// For each request, produce a response.
+	/// The responses vector produced goes up to the point where the responder
+	/// first returns `None`, an invalid response, or until all requests have been responded to.
+	pub fn respond_to_all<F>(mut self, responder: F) -> Vec<super::Response>
+		where F: Fn(super::CompleteRequest) -> Option<super::Response>
+	{
+		let mut responses = Vec::new();
+
+		while let Some(response) = self.next_complete().and_then(&responder) {
+			match self.supply_response(&(), &response) {
+				Ok(()) => responses.push(response),
+				Err(e) => {
+					debug!(target: "pip", "produced bad response to request: {:?}", e);
+					return responses;
+				}
+			}
+		}
+
+		responses
 	}
 }
 
diff --git a/ethcore/light/src/types/request/mod.rs b/ethcore/light/src/types/request/mod.rs
index 3099f8fedcc..3dd2db6290d 100644
--- a/ethcore/light/src/types/request/mod.rs
+++ b/ethcore/light/src/types/request/mod.rs
@@ -69,11 +69,15 @@ pub use self::builder::{RequestBuilder, Requests};
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct NoSuchOutput;
 
+/// Wrong kind of response corresponding to request.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct WrongKind;
+
 /// Error on processing a response.
 #[derive(Debug, Clone, PartialEq, Eq)]
-pub enum ResponseError {
-	/// Wrong kind of response.
-	WrongKind,
+pub enum ResponseError<T> {
+	/// Error in validity.
+	Validity(T),
 	/// No responses expected.
 	Unexpected,
 }
@@ -96,6 +100,12 @@ impl<T> Field<T> {
 			_ => Err(NoSuchOutput),
 		}
 	}
+
+	fn adjust_req<F>(&mut self, mut mapping: F) where F: FnMut(usize) -> usize {
+		if let Field::BackReference(ref mut req_idx, _) = *self {
+			*req_idx = mapping(*req_idx)
+		}
+	}
 }
 
 impl<T> From<T> for Field<T> {
@@ -197,6 +207,9 @@ impl Encodable for HashOrNumber {
 	}
 }
 
+/// Type alias for "network requests".
+pub type NetworkRequests = Requests<Request>;
+
 /// All request types, as they're sent over the network.
 /// They may be incomplete, with back-references to outputs
 /// of prior requests.
@@ -296,6 +309,7 @@ impl Encodable for Request {
 
 impl IncompleteRequest for Request {
 	type Complete = CompleteRequest;
+	type Response = Response;
 
 	fn check_outputs<F>(&self, f: F) -> Result<(), NoSuchOutput>
 		where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -350,6 +364,33 @@ impl IncompleteRequest for Request {
 			Request::Execution(req) => req.complete().map(CompleteRequest::Execution),
 		}
 	}
+
+	fn adjust_refs<F>(&mut self, mapping: F) where F: FnMut(usize) -> usize {
+		match *self {
+			Request::Headers(ref mut req) => req.adjust_refs(mapping),
+			Request::HeaderProof(ref mut req) => req.adjust_refs(mapping),
+			Request::Receipts(ref mut req) => req.adjust_refs(mapping),
+			Request::Body(ref mut req) => req.adjust_refs(mapping),
+			Request::Account(ref mut req) => req.adjust_refs(mapping),
+			Request::Storage(ref mut req) => req.adjust_refs(mapping),
+			Request::Code(ref mut req) => req.adjust_refs(mapping),
+			Request::Execution(ref mut req) => req.adjust_refs(mapping),
+		}
+	}
+}
+
+impl CheckedRequest for Request {
+	type Extract = ();
+	type Error = WrongKind;
+	type Environment = ();
+
+	fn check_response(&self, _: &(), response: &Response) -> Result<(), WrongKind> {
+		if self.kind() == response.kind() {
+			Ok(())
+		} else {
+			Err(WrongKind)
+		}
+	}
 }
 
 /// Kinds of requests.
@@ -421,9 +462,9 @@ pub enum Response {
 	Execution(ExecutionResponse),
 }
 
-impl Response {
+impl ResponseLike for Response {
 	/// Fill reusable outputs by writing them into the function.
-	pub fn fill_outputs<F>(&self, f: F) where F: FnMut(usize, Output) {
+	fn fill_outputs<F>(&self, f: F) where F: FnMut(usize, Output) {
 		match *self {
 			Response::Headers(ref res) => res.fill_outputs(f),
 			Response::HeaderProof(ref res) => res.fill_outputs(f),
@@ -435,7 +476,9 @@ impl Response {
 			Response::Execution(ref res) => res.fill_outputs(f),
 		}
 	}
+}
 
+impl Response {
 	/// Inspect the kind of this response.
 	pub fn kind(&self) -> Kind {
 		match *self {
@@ -490,6 +533,8 @@ impl Encodable for Response {
 pub trait IncompleteRequest: Sized {
 	/// The complete variant of this request.
 	type Complete;
+	/// The response to this request.
+	type Response: ResponseLike;
 
 	/// Check prior outputs against the needed inputs.
 	///
@@ -511,6 +556,30 @@ pub trait IncompleteRequest: Sized {
 	/// Attempt to convert this request into its complete variant.
 	/// Will succeed if all fields have been filled, will fail otherwise.
 	fn complete(self) -> Result<Self::Complete, NoSuchOutput>;
+
+	/// Adjust back-reference request indices.
+	fn adjust_refs<F>(&mut self, mapping: F) where F: FnMut(usize) -> usize;
+}
+
+/// A request which can be checked against its response for more validity.
+pub trait CheckedRequest: IncompleteRequest {
+	/// Data extracted during the check.
+	type Extract;
+	/// Error encountered during the check.
+	type Error;
+	/// Environment passed to response check.
+	type Environment;
+
+	/// Check whether the response matches (beyond the type).
+	fn check_response(&self, &Self::Environment, &Self::Response) -> Result<Self::Extract, Self::Error>;
+}
+
+/// A response-like object.
+///
+/// These contain re-usable outputs.
+pub trait ResponseLike {
+	/// Write all re-usable outputs into the provided function.
+	fn fill_outputs<F>(&self, output_store: F) where F: FnMut(usize, Output);
 }
 
 /// Header request.
@@ -555,6 +624,7 @@ pub mod header {
 
 	impl super::IncompleteRequest for Incomplete {
 		type Complete = Complete;
+		type Response = Response;
 
 		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
 			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -586,6 +656,10 @@ pub mod header {
 				reverse: self.reverse,
 			})
 		}
+
+		fn adjust_refs<F>(&mut self, mapping: F) where F: FnMut(usize) -> usize {
+			self.start.adjust_req(mapping)
+		}
 	}
 
 	/// A complete header request.
@@ -608,9 +682,9 @@ pub mod header {
 		pub headers: Vec<encoded::Header>,
 	}
 
-	impl Response {
+	impl super::ResponseLike for Response {
 		/// Fill reusable outputs by writing them into the function.
-		pub fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) { }
+		fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) { }
 	}
 
 	impl Decodable for Response {
@@ -671,6 +745,7 @@ pub mod header_proof {
 
 	impl super::IncompleteRequest for Incomplete {
 		type Complete = Complete;
+		type Response = Response;
 
 		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
 			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -699,6 +774,10 @@ pub mod header_proof {
 				num: self.num.into_scalar()?,
 			})
 		}
+
+		fn adjust_refs<F>(&mut self, mapping: F) where F: FnMut(usize) -> usize {
+			self.num.adjust_req(mapping)
+		}
 	}
 
 	/// A complete header proof request.
@@ -719,9 +798,9 @@ pub mod header_proof {
 		pub td: U256,
 	}
 
-	impl Response {
+	impl super::ResponseLike for Response {
 		/// Fill reusable outputs by providing them to the function.
-		pub fn fill_outputs<F>(&self, mut f: F) where F: FnMut(usize, Output) {
+		fn fill_outputs<F>(&self, mut f: F) where F: FnMut(usize, Output) {
 			f(0, Output::Hash(self.hash));
 		}
 	}
@@ -776,6 +855,7 @@ pub mod block_receipts {
 
 	impl super::IncompleteRequest for Incomplete {
 		type Complete = Complete;
+		type Response = Response;
 
 		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
 			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -802,6 +882,10 @@ pub mod block_receipts {
 				hash: self.hash.into_scalar()?,
 			})
 		}
+
+		fn adjust_refs<F>(&mut self, mapping: F) where F: FnMut(usize) -> usize {
+			self.hash.adjust_req(mapping)
+		}
 	}
 
 	/// A complete block receipts request.
@@ -818,9 +902,9 @@ pub mod block_receipts {
 		pub receipts: Vec<Receipt>
 	}
 
-	impl Response {
+	impl super::ResponseLike for Response {
 		/// Fill reusable outputs by providing them to the function.
-		pub fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
+		fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
 	}
 
 	impl Decodable for Response {
@@ -868,6 +952,7 @@ pub mod block_body {
 
 	impl super::IncompleteRequest for Incomplete {
 		type Complete = Complete;
+		type Response = Response;
 
 		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
 			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -894,6 +979,10 @@ pub mod block_body {
 				hash: self.hash.into_scalar()?,
 			})
 		}
+
+		fn adjust_refs<F>(&mut self, mapping: F) where F: FnMut(usize) -> usize {
+			self.hash.adjust_req(mapping)
+		}
 	}
 
 	/// A complete block body request.
@@ -910,9 +999,9 @@ pub mod block_body {
 		pub body: encoded::Body,
 	}
 
-	impl Response {
+	impl super::ResponseLike for Response {
 		/// Fill reusable outputs by providing them to the function.
-		pub fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
+		fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
 	}
 
 	impl Decodable for Response {
@@ -971,6 +1060,7 @@ pub mod account {
 
 	impl super::IncompleteRequest for Incomplete {
 		type Complete = Complete;
+		type Response = Response;
 
 		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
 			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -1013,6 +1103,11 @@ pub mod account {
 				address_hash: self.address_hash.into_scalar()?,
 			})
 		}
+
+		fn adjust_refs<F>(&mut self, mut mapping: F) where F: FnMut(usize) -> usize {
+			self.block_hash.adjust_req(&mut mapping);
+			self.address_hash.adjust_req(&mut mapping);
+		}
 	}
 
 	/// A complete request for an account.
@@ -1039,9 +1134,9 @@ pub mod account {
 		pub storage_root: H256,
 	}
 
-	impl Response {
+	impl super::ResponseLike for Response {
 		/// Fill reusable outputs by providing them to the function.
-		pub fn fill_outputs<F>(&self, mut f: F) where F: FnMut(usize, Output) {
+		fn fill_outputs<F>(&self, mut f: F) where F: FnMut(usize, Output) {
 			f(0, Output::Hash(self.code_hash));
 			f(1, Output::Hash(self.storage_root));
 		}
@@ -1109,6 +1204,7 @@ pub mod storage {
 
 	impl super::IncompleteRequest for Incomplete {
 		type Complete = Complete;
+		type Response = Response;
 
 		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
 			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -1162,6 +1258,12 @@ pub mod storage {
 				key_hash: self.key_hash.into_scalar()?,
 			})
 		}
+
+		fn adjust_refs<F>(&mut self, mut mapping: F) where F: FnMut(usize) -> usize {
+			self.block_hash.adjust_req(&mut mapping);
+			self.address_hash.adjust_req(&mut mapping);
+			self.key_hash.adjust_req(&mut mapping);
+		}
 	}
 
 	/// A complete request for a storage proof.
@@ -1184,9 +1286,9 @@ pub mod storage {
 		pub value: H256,
 	}
 
-	impl Response {
+	impl super::ResponseLike for Response {
 		/// Fill reusable outputs by providing them to the function.
-		pub fn fill_outputs<F>(&self, mut f: F) where F: FnMut(usize, Output) {
+		fn fill_outputs<F>(&self, mut f: F) where F: FnMut(usize, Output) {
 			f(0, Output::Hash(self.value));
 		}
 	}
@@ -1243,6 +1345,7 @@ pub mod contract_code {
 
 	impl super::IncompleteRequest for Incomplete {
 		type Complete = Complete;
+		type Response = Response;
 
 		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
 			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -1281,6 +1384,11 @@ pub mod contract_code {
 				code_hash: self.code_hash.into_scalar()?,
 			})
 		}
+
+		fn adjust_refs<F>(&mut self, mut mapping: F) where F: FnMut(usize) -> usize {
+			self.block_hash.adjust_req(&mut mapping);
+			self.code_hash.adjust_req(&mut mapping);
+		}
 	}
 
 	/// A complete request.
@@ -1299,9 +1407,9 @@ pub mod contract_code {
 		pub code: Bytes,
 	}
 
-	impl Response {
+	impl super::ResponseLike for Response {
 		/// Fill reusable outputs by providing them to the function.
-		pub fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
+		fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
 	}
 
 	impl Decodable for Response {
@@ -1380,6 +1488,7 @@ pub mod execution {
 
 	impl super::IncompleteRequest for Incomplete {
 		type Complete = Complete;
+		type Response = Response;
 
 		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
 			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
@@ -1412,6 +1521,10 @@ pub mod execution {
 				data: self.data,
 			})
 		}
+
+		fn adjust_refs<F>(&mut self, mapping: F) where F: FnMut(usize) -> usize {
+			self.block_hash.adjust_req(mapping);
+		}
 	}
 
 	/// A complete request.
@@ -1440,9 +1553,9 @@ pub mod execution {
 		pub items: Vec<DBValue>,
 	}
 
-	impl Response {
+	impl super::ResponseLike for Response {
 		/// Fill reusable outputs by providing them to the function.
-		pub fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
+		fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
 	}
 
 	impl Decodable for Response {
diff --git a/ethcore/src/header.rs b/ethcore/src/header.rs
index 4517c576437..e8b1fc43090 100644
--- a/ethcore/src/header.rs
+++ b/ethcore/src/header.rs
@@ -17,12 +17,14 @@
 //! Block header.
 
 use util::*;
-use basic_types::{LogBloom, Seal, ZERO_LOGBLOOM};
+use basic_types::{LogBloom, ZERO_LOGBLOOM};
 use time::get_time;
 use rlp::*;
 
 use std::cell::RefCell;
 
+pub use basic_types::Seal;
+
 /// Type for Block number
 pub type BlockNumber = u64;
 
diff --git a/parity/light_helpers/queue_cull.rs b/parity/light_helpers/queue_cull.rs
index 548ee33cdf0..c643daa2aa2 100644
--- a/parity/light_helpers/queue_cull.rs
+++ b/parity/light_helpers/queue_cull.rs
@@ -67,6 +67,7 @@ impl IoHandler<ClientIoMessage> for QueueCull {
 
 		let (sync, on_demand, txq) = (self.sync.clone(), self.on_demand.clone(), self.txq.clone());
 		let best_header = self.client.best_block_header();
+		let start_nonce = self.client.engine().account_start_nonce();
 
 		info!(target: "cull", "Attempting to cull queued transactions from {} senders.", senders.len());
 		self.remote.spawn_with_timeout(move || {
@@ -74,7 +75,10 @@ impl IoHandler<ClientIoMessage> for QueueCull {
 				// fetch the nonce of each sender in the queue.
 				let nonce_futures = senders.iter()
 					.map(|&address| request::Account { header: best_header.clone(), address: address })
-					.map(|request| on_demand.account(ctx, request).map(|acc| acc.nonce))
+					.map(move |request| {
+						on_demand.account(ctx, request)
+							.map(move |maybe_acc| maybe_acc.map_or(start_nonce, |acc| acc.nonce))
+					})
 					.zip(senders.iter())
 					.map(|(fut, &addr)| fut.map(move |nonce| (addr, nonce)));
 
diff --git a/parity/run.rs b/parity/run.rs
index 018360f1b92..6f0987a7e14 100644
--- a/parity/run.rs
+++ b/parity/run.rs
@@ -228,8 +228,7 @@ fn execute_light(cmd: RunCmd, can_restart: bool, logger: Arc<RotatingLogger>) ->
 	}
 
 	// start on_demand service.
-	let account_start_nonce = service.client().engine().account_start_nonce();
-	let on_demand = Arc::new(::light::on_demand::OnDemand::new(cache.clone(), account_start_nonce));
+	let on_demand = Arc::new(::light::on_demand::OnDemand::new(cache.clone()));
 
 	// set network path.
 	net_conf.net_config_path = Some(db_dirs.network_path().to_string_lossy().into_owned());
diff --git a/rpc/src/v1/helpers/dispatch.rs b/rpc/src/v1/helpers/dispatch.rs
index 877e4c24b11..68fff78b731 100644
--- a/rpc/src/v1/helpers/dispatch.rs
+++ b/rpc/src/v1/helpers/dispatch.rs
@@ -281,6 +281,7 @@ impl LightDispatcher {
 		}
 
 		let best_header = self.client.best_block_header();
+		let account_start_nonce = self.client.engine().account_start_nonce();
 		let nonce_future = self.sync.with_context(|ctx| self.on_demand.account(ctx, request::Account {
 			header: best_header,
 			address: addr,
@@ -288,7 +289,7 @@ impl LightDispatcher {
 
 		match nonce_future {
 			Some(x) =>
-				x.map(|acc| acc.nonce)
+				x.map(move |acc| acc.map_or(account_start_nonce, |acc| acc.nonce))
 					.map_err(|_| errors::no_light_peers())
 					.boxed(),
 			None =>  future::err(errors::network_disabled()).boxed()
diff --git a/rpc/src/v1/helpers/light_fetch.rs b/rpc/src/v1/helpers/light_fetch.rs
index 7cf5c3dc959..7132106cbcc 100644
--- a/rpc/src/v1/helpers/light_fetch.rs
+++ b/rpc/src/v1/helpers/light_fetch.rs
@@ -57,16 +57,16 @@ pub type ExecutionResult = Result<Executed, ExecutionError>;
 
 impl LightFetch {
 	/// Get a block header from the on demand service or client, or error.
-	pub fn header(&self, id: BlockId) -> BoxFuture<Option<encoded::Header>, Error> {
+	pub fn header(&self, id: BlockId) -> BoxFuture<encoded::Header, Error> {
 		if let Some(h) = self.client.block_header(id) {
-			return future::ok(Some(h)).boxed()
+			return future::ok(h).boxed()
 		}
 
 		let maybe_future = match id {
 			BlockId::Number(n) => {
 				let cht_root = cht::block_to_cht_number(n).and_then(|cn| self.client.cht_root(cn as usize));
 				match cht_root {
-					None => return future::ok(None).boxed(),
+					None => return future::err(errors::unknown_block()).boxed(),
 					Some(root) => {
 						let req = request::HeaderProof::new(n, root)
 							.expect("only fails for 0; client always stores genesis; client already queried; qed");
@@ -82,7 +82,7 @@ impl LightFetch {
 									Some(fut) => fut.map_err(errors::on_demand_cancel).boxed(),
 									None => future::err(errors::network_disabled()).boxed(),
 								}
-							}).map(Some).boxed()
+							}).boxed()
 						})
 					}
 				}
@@ -91,7 +91,7 @@ impl LightFetch {
 				self.sync.with_context(|ctx|
 					self.on_demand.header_by_hash(ctx, request::HeaderByHash(h))
 						.then(|res| future::done(match res {
-							Ok(h) => Ok(Some(h)),
+							Ok(h) => Ok(h),
 							Err(e) => Err(errors::on_demand_cancel(e)),
 						}))
 						.boxed()
@@ -106,22 +106,21 @@ impl LightFetch {
 		}
 	}
 
-	// Get account info at a given block. `None` signifies no such account existing.
+	/// helper for getting account info at a given block.
+	/// `None` indicates the account doesn't exist at the given block.
 	pub fn account(&self, address: Address, id: BlockId) -> BoxFuture<Option<BasicAccount>, Error> {
 		let (sync, on_demand) = (self.sync.clone(), self.on_demand.clone());
 
 		self.header(id).and_then(move |header| {
-			let header = match header {
-				None => return future::ok(None).boxed(),
-				Some(hdr) => hdr,
-			};
-
-			sync.with_context(|ctx| on_demand.account(ctx, request::Account {
+			let maybe_fut = sync.with_context(|ctx| on_demand.account(ctx, request::Account {
 				header: header,
 				address: address,
-			}).map(Some))
-				.map(|x| x.map_err(errors::on_demand_cancel).boxed())
-				.unwrap_or_else(|| future::err(errors::network_disabled()).boxed())
+			}));
+
+			match maybe_fut {
+				Some(fut) => fut.map_err(errors::on_demand_cancel).boxed(),
+				None => future::err(errors::network_disabled()).boxed(),
+			}
 		}).boxed()
 	}
 
@@ -176,10 +175,11 @@ impl LightFetch {
 		}).join(header_fut).and_then(move |(tx, hdr)| {
 			// then request proved execution.
 			// TODO: get last-hashes from network.
-			let (env_info, hdr) = match (client.env_info(id), hdr) {
-				(Some(env_info), Some(hdr)) => (env_info, hdr),
+			let env_info = match client.env_info(id) {
+				Some(env_info) => env_info,
 				_ => return future::err(errors::unknown_block()).boxed(),
 			};
+
 			let request = request::TransactionProof {
 				tx: tx,
 				header: hdr,
@@ -198,18 +198,13 @@ impl LightFetch {
 		}).boxed()
 	}
 
-	/// Get a block.
-	pub fn block(&self, id: BlockId) -> BoxFuture<Option<encoded::Block>, Error> {
+	/// get a block itself. fails on unknown block ID.
+	pub fn block(&self, id: BlockId) -> BoxFuture<encoded::Block, Error> {
 		let (on_demand, sync) = (self.on_demand.clone(), self.sync.clone());
 
-		self.header(id).and_then(move |hdr| {
-			let req = match hdr {
-				Some(hdr) => request::Body::new(hdr),
-				None => return future::ok(None).boxed(),
-			};
-
+		self.header(id).map(request::Body::new).and_then(move |req| {
 			match sync.with_context(move |ctx| on_demand.block(ctx, req)) {
-				Some(fut) => fut.map_err(errors::on_demand_cancel).map(Some).boxed(),
+				Some(fut) => fut.map_err(errors::on_demand_cancel).boxed(),
 				None => future::err(errors::network_disabled()).boxed(),
 			}
 		}).boxed()
diff --git a/rpc/src/v1/impls/light/eth.rs b/rpc/src/v1/impls/light/eth.rs
index e45397f7db0..e02ccc987e9 100644
--- a/rpc/src/v1/impls/light/eth.rs
+++ b/rpc/src/v1/impls/light/eth.rs
@@ -115,12 +115,11 @@ impl EthClient {
 			on_demand: self.on_demand.clone(),
 			sync: self.sync.clone(),
 			cache: self.cache.clone(),
-
 		}
 	}
 
-	// get a "rich" block structure
-	fn rich_block(&self, id: BlockId, include_txs: bool) -> BoxFuture<Option<RichBlock>, Error> {
+	// get a "rich" block structure. Fails on unknown block.
+	fn rich_block(&self, id: BlockId, include_txs: bool) -> BoxFuture<RichBlock, Error> {
 		let (on_demand, sync) = (self.on_demand.clone(), self.sync.clone());
 		let (client, engine) = (self.client.clone(), self.client.engine().clone());
 		let eip86_transition = self.client.eip86_transition();
@@ -160,49 +159,45 @@ impl EthClient {
 		};
 
 		// get the block itself.
-		self.fetcher().block(id).and_then(move |block| match block {
-			None => return future::ok(None).boxed(),
-			Some(block) => {
-				// then fetch the total difficulty (this is much easier after getting the block).
-				match client.score(id) {
-					Some(score) => future::ok(fill_rich(block, Some(score))).map(Some).boxed(),
-					None => {
-						// make a CHT request to fetch the chain score.
-						let req = cht::block_to_cht_number(block.number())
-							.and_then(|num| client.cht_root(num as usize))
-							.and_then(|root| request::HeaderProof::new(block.number(), root));
-
-
-						let req = match req {
-							Some(req) => req,
-							None => {
-								// somehow the genesis block slipped past other checks.
-								// return it now.
-								let score = client.block_header(BlockId::Number(0))
-									.expect("genesis always stored; qed")
-									.difficulty();
-
-								return future::ok(fill_rich(block, Some(score))).map(Some).boxed()
-							}
-						};
-
-						// three possible outcomes:
-						//   - network is down.
-						//   - we get a score, but our hash is non-canonical.
-						//   - we get ascore, and our hash is canonical.
-						let maybe_fut = sync.with_context(move |ctx| on_demand.hash_and_score_by_number(ctx, req));
-						match maybe_fut {
-							Some(fut) => fut.map(move |(hash, score)| {
-									let score = if hash == block.hash() {
-										Some(score)
-									} else {
-										None
-									};
-
-									Some(fill_rich(block, score))
-								}).map_err(errors::on_demand_cancel).boxed(),
-							None => return future::err(errors::network_disabled()).boxed(),
+		self.fetcher().block(id).and_then(move |block| {
+			// then fetch the total difficulty (this is much easier after getting the block).
+			match client.score(id) {
+				Some(score) => future::ok(fill_rich(block, Some(score))).boxed(),
+				None => {
+					// make a CHT request to fetch the chain score.
+					let req = cht::block_to_cht_number(block.number())
+						.and_then(|num| client.cht_root(num as usize))
+						.and_then(|root| request::HeaderProof::new(block.number(), root));
+
+					let req = match req {
+						Some(req) => req,
+						None => {
+							// somehow the genesis block slipped past other checks.
+							// return it now.
+							let score = client.block_header(BlockId::Number(0))
+								.expect("genesis always stored; qed")
+								.difficulty();
+
+							return future::ok(fill_rich(block, Some(score))).boxed()
 						}
+					};
+
+					// three possible outcomes:
+					//   - network is down.
+					//   - we get a score, but our hash is non-canonical.
+					//   - we get a score, and our hash is canonical.
+					let maybe_fut = sync.with_context(move |ctx| on_demand.hash_and_score_by_number(ctx, req));
+					match maybe_fut {
+						Some(fut) => fut.map(move |(hash, score)| {
+								let score = if hash == block.hash() {
+									Some(score)
+								} else {
+									None
+								};
+
+								fill_rich(block, score)
+							}).map_err(errors::on_demand_cancel).boxed(),
+						None => return future::err(errors::network_disabled()).boxed(),
 					}
 				}
 			}
@@ -281,11 +276,11 @@ impl Eth for EthClient {
 	}
 
 	fn block_by_hash(&self, hash: RpcH256, include_txs: bool) -> BoxFuture<Option<RichBlock>, Error> {
-		self.rich_block(BlockId::Hash(hash.into()), include_txs)
+		self.rich_block(BlockId::Hash(hash.into()), include_txs).map(Some).boxed()
 	}
 
 	fn block_by_number(&self, num: BlockNumber, include_txs: bool) -> BoxFuture<Option<RichBlock>, Error> {
-		self.rich_block(num.into(), include_txs)
+		self.rich_block(num.into(), include_txs).map(Some).boxed()
 	}
 
 	fn transaction_count(&self, address: RpcH160, num: Trailing<BlockNumber>) -> BoxFuture<RpcU256, Error> {
@@ -297,11 +292,6 @@ impl Eth for EthClient {
 		let (sync, on_demand) = (self.sync.clone(), self.on_demand.clone());
 
 		self.fetcher().header(BlockId::Hash(hash.into())).and_then(move |hdr| {
-			let hdr = match hdr {
-				None => return future::ok(None).boxed(),
-				Some(hdr) => hdr,
-			};
-
 			if hdr.transactions_root() == SHA3_NULL_RLP {
 				future::ok(Some(U256::from(0).into())).boxed()
 			} else {
@@ -317,11 +307,6 @@ impl Eth for EthClient {
 		let (sync, on_demand) = (self.sync.clone(), self.on_demand.clone());
 
 		self.fetcher().header(num.into()).and_then(move |hdr| {
-			let hdr = match hdr {
-				None => return future::ok(None).boxed(),
-				Some(hdr) => hdr,
-			};
-
 			if hdr.transactions_root() == SHA3_NULL_RLP {
 				future::ok(Some(U256::from(0).into())).boxed()
 			} else {
@@ -337,11 +322,6 @@ impl Eth for EthClient {
 		let (sync, on_demand) = (self.sync.clone(), self.on_demand.clone());
 
 		self.fetcher().header(BlockId::Hash(hash.into())).and_then(move |hdr| {
-			let hdr = match hdr {
-				None => return future::ok(None).boxed(),
-				Some(hdr) => hdr,
-			};
-
 			if hdr.uncles_hash() == SHA3_EMPTY_LIST_RLP {
 				future::ok(Some(U256::from(0).into())).boxed()
 			} else {
@@ -357,11 +337,6 @@ impl Eth for EthClient {
 		let (sync, on_demand) = (self.sync.clone(), self.on_demand.clone());
 
 		self.fetcher().header(num.into()).and_then(move |hdr| {
-			let hdr = match hdr {
-				None => return future::ok(None).boxed(),
-				Some(hdr) => hdr,
-			};
-
 			if hdr.uncles_hash() == SHA3_EMPTY_LIST_RLP {
 				future::ok(Some(U256::from(0).into())).boxed()
 			} else {
diff --git a/rpc/src/v1/impls/light/parity.rs b/rpc/src/v1/impls/light/parity.rs
index 63e1c64b6bd..015d4fef75f 100644
--- a/rpc/src/v1/impls/light/parity.rs
+++ b/rpc/src/v1/impls/light/parity.rs
@@ -360,7 +360,7 @@ impl Parity for ParityClient {
 		})
 	}
 
-	fn block_header(&self, number: Trailing<BlockNumber>) -> BoxFuture<Option<RichHeader>, Error> {
+	fn block_header(&self, number: Trailing<BlockNumber>) -> BoxFuture<RichHeader, Error> {
 		use ethcore::encoded;
 
 		let engine = self.light_dispatch.client.engine().clone();
@@ -391,7 +391,7 @@ impl Parity for ParityClient {
 			}
 		};
 
-		self.fetcher().header(number.0.into()).map(move |encoded| encoded.map(from_encoded)).boxed()
+		self.fetcher().header(number.0.into()).map(from_encoded).boxed()
 	}
 
 	fn ipfs_cid(&self, content: Bytes) -> Result<String, Error> {
diff --git a/rpc/src/v1/impls/parity.rs b/rpc/src/v1/impls/parity.rs
index 689ba3e708a..9c3673ee462 100644
--- a/rpc/src/v1/impls/parity.rs
+++ b/rpc/src/v1/impls/parity.rs
@@ -400,17 +400,17 @@ impl<C, M, S: ?Sized, U> Parity for ParityClient<C, M, S, U> where
 		})
 	}
 
-	fn block_header(&self, number: Trailing<BlockNumber>) -> BoxFuture<Option<RichHeader>, Error> {
+	fn block_header(&self, number: Trailing<BlockNumber>) -> BoxFuture<RichHeader, Error> {
 		const EXTRA_INFO_PROOF: &'static str = "Object exists in in blockchain (fetched earlier), extra_info is always available if object exists; qed";
 
 		let client = take_weakf!(self.client);
 		let id: BlockId = number.0.into();
 		let encoded = match client.block_header(id.clone()) {
 			Some(encoded) => encoded,
-			None => return future::ok(None).boxed(),
+			None => return future::err(errors::unknown_block()).boxed(),
 		};
 
-		future::ok(Some(RichHeader {
+		future::ok(RichHeader {
 			inner: Header {
 				hash: Some(encoded.hash().into()),
 				size: Some(encoded.rlp().as_raw().len().into()),
@@ -431,7 +431,7 @@ impl<C, M, S: ?Sized, U> Parity for ParityClient<C, M, S, U> where
 				extra_data: Bytes::new(encoded.extra_data()),
 			},
 			extra_info: client.block_extra_info(id).expect(EXTRA_INFO_PROOF),
-		})).boxed()
+		}).boxed()
 	}
 
 	fn ipfs_cid(&self, content: Bytes) -> Result<String, Error> {
diff --git a/rpc/src/v1/traits/parity.rs b/rpc/src/v1/traits/parity.rs
index 272d874955e..498d1b82dea 100644
--- a/rpc/src/v1/traits/parity.rs
+++ b/rpc/src/v1/traits/parity.rs
@@ -202,7 +202,7 @@ build_rpc_trait! {
 		/// Get block header.
 		/// Same as `eth_getBlockByNumber` but without uncles and transactions.
 		#[rpc(async, name = "parity_getBlockHeaderByNumber")]
-		fn block_header(&self, Trailing<BlockNumber>) -> BoxFuture<Option<RichHeader>, Error>;
+		fn block_header(&self, Trailing<BlockNumber>) -> BoxFuture<RichHeader, Error>;
 
 		/// Get IPFS CIDv0 given protobuf encoded bytes.
 		#[rpc(name = "parity_cidV0")]