diff --git a/ethcore/light/src/client/mod.rs b/ethcore/light/src/client/mod.rs
index 5701fc606..2872e0eec 100644
--- a/ethcore/light/src/client/mod.rs
+++ b/ethcore/light/src/client/mod.rs
@@ -31,7 +31,7 @@ use ethcore::service::ClientIoMessage;
 use ethcore::encoded;
 use io::IoChannel;
 
-use util::{Bytes, H256, Mutex, RwLock};
+use util::{Bytes, DBValue, H256, Mutex, RwLock};
 
 use self::header_chain::{AncestryIter, HeaderChain};
 
@@ -230,22 +230,32 @@ impl Client {
 	}
 
 	/// Get a handle to the verification engine.
-	pub fn engine(&self) -> &Engine {
-		&*self.engine
+	pub fn engine(&self) -> &Arc<Engine> {
+		&self.engine
 	}
 
-	fn latest_env_info(&self) -> EnvInfo {
-		let header = self.best_block_header();
+	/// Get the latest environment info.
+	pub fn latest_env_info(&self) -> EnvInfo {
+		self.env_info(BlockId::Latest)
+			.expect("Best block header and recent hashes always stored; qed")
+	}
 
-		EnvInfo {
+	/// Get environment info for a given block.
+	pub fn env_info(&self, id: BlockId) -> Option<EnvInfo> {
+		let header = match self.block_header(id) {
+			Some(hdr) => hdr,
+			None => return None,
+		};
+
+		Some(EnvInfo {
 			number: header.number(),
 			author: header.author(),
 			timestamp: header.timestamp(),
 			difficulty: header.difficulty(),
-			last_hashes: self.build_last_hashes(header.hash()),
+			last_hashes: self.build_last_hashes(header.parent_hash()),
 			gas_used: Default::default(),
 			gas_limit: header.gas_limit(),
-		}
+		})
 	}
 
 	fn build_last_hashes(&self, mut parent_hash: H256) -> Arc<Vec<H256>> {
@@ -344,6 +354,10 @@ impl ::provider::Provider for Client {
 		None
 	}
 
+	fn transaction_proof(&self, _req: ::request::TransactionProof) -> Option<Vec<DBValue>> {
+		None
+	}
+
 	fn ready_transactions(&self) -> Vec<::ethcore::transaction::PendingTransaction> {
 		Vec::new()
 	}
diff --git a/ethcore/light/src/net/error.rs b/ethcore/light/src/net/error.rs
index 627a7ef0f..dda78e0b6 100644
--- a/ethcore/light/src/net/error.rs
+++ b/ethcore/light/src/net/error.rs
@@ -44,8 +44,8 @@ pub enum Error {
 	Rlp(DecoderError),
 	/// A network error.
 	Network(NetworkError),
-	/// Out of buffer.
-	BufferEmpty,
+	/// Out of credits.
+	NoCredits,
 	/// Unrecognized packet code.
 	UnrecognizedPacket(u8),
 	/// Unexpected handshake.
@@ -72,7 +72,7 @@ impl Error {
 		match *self {
 			Error::Rlp(_) => Punishment::Disable,
 			Error::Network(_) => Punishment::None,
-			Error::BufferEmpty => Punishment::Disable,
+			Error::NoCredits => Punishment::Disable,
 			Error::UnrecognizedPacket(_) => Punishment::Disconnect,
 			Error::UnexpectedHandshake => Punishment::Disconnect,
 			Error::WrongNetwork => Punishment::Disable,
@@ -103,7 +103,7 @@ impl fmt::Display for Error {
 		match *self {
 			Error::Rlp(ref err) => err.fmt(f),
 			Error::Network(ref err) => err.fmt(f),
-			Error::BufferEmpty => write!(f, "Out of buffer"),
+			Error::NoCredits => write!(f, "Out of request credits"),
 			Error::UnrecognizedPacket(code) => write!(f, "Unrecognized packet: 0x{:x}", code),
 			Error::UnexpectedHandshake => write!(f, "Unexpected handshake"),
 			Error::WrongNetwork => write!(f, "Wrong network"),
diff --git a/ethcore/light/src/net/mod.rs b/ethcore/light/src/net/mod.rs
index eb5677cfa..181f95e95 100644
--- a/ethcore/light/src/net/mod.rs
+++ b/ethcore/light/src/net/mod.rs
@@ -19,14 +19,14 @@
 //! This uses a "Provider" to answer requests.
 //! See https://github.com/ethcore/parity/wiki/Light-Ethereum-Subprotocol-(LES)
 
-use ethcore::transaction::UnverifiedTransaction;
+use ethcore::transaction::{Action, UnverifiedTransaction};
 use ethcore::receipt::Receipt;
 
 use io::TimerToken;
 use network::{NetworkProtocolHandler, NetworkContext, PeerId};
 use rlp::{RlpStream, Stream, UntrustedRlp, View};
 use util::hash::H256;
-use util::{Bytes, Mutex, RwLock, U256};
+use util::{Bytes, DBValue, Mutex, RwLock, U256};
 use time::{Duration, SteadyTime};
 
 use std::collections::HashMap;
@@ -37,7 +37,7 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use provider::Provider;
 use request::{self, HashOrNumber, Request};
 
-use self::buffer_flow::{Buffer, FlowParams};
+use self::request_credits::{Credits, FlowParams};
 use self::context::{Ctx, TickCtx};
 use self::error::Punishment;
 use self::request_set::RequestSet;
@@ -51,7 +51,7 @@ mod request_set;
 #[cfg(test)]
 mod tests;
 
-pub mod buffer_flow;
+pub mod request_credits;
 
 pub use self::error::Error;
 pub use self::context::{BasicContext, EventContext, IoContext};
@@ -73,7 +73,7 @@ pub const PROTOCOL_VERSIONS: &'static [u8] = &[1];
 pub const MAX_PROTOCOL_VERSION: u8 = 1;
 
 /// Packet count for LES.
-pub const PACKET_COUNT: u8 = 15;
+pub const PACKET_COUNT: u8 = 17;
 
 // packet ID definitions.
 mod packet {
@@ -109,6 +109,10 @@ mod packet {
 	// request and response for header proofs in a CHT.
 	pub const GET_HEADER_PROOFS: u8 = 0x0d;
 	pub const HEADER_PROOFS: u8 = 0x0e;
+
+	// request and response for transaction proof.
+	pub const GET_TRANSACTION_PROOF: u8 = 0x0f;
+	pub const TRANSACTION_PROOF: u8 = 0x10;
 }
 
 // timeouts for different kinds of requests. all values are in milliseconds.
@@ -121,6 +125,7 @@ mod timeout {
 	pub const PROOFS: i64 = 4000;
 	pub const CONTRACT_CODES: i64 = 5000;
 	pub const HEADER_PROOFS: i64 = 3500;
+	pub const TRANSACTION_PROOF: i64 = 5000;
 }
 
 /// A request id.
@@ -143,10 +148,10 @@ struct PendingPeer {
 /// Relevant data to each peer. Not accessible publicly, only `pub` due to
 /// limitations of the privacy system.
 pub struct Peer {
-	local_buffer: Buffer, // their buffer relative to us
+	local_credits: Credits, // their credits relative to us
 	status: Status,
 	capabilities: Capabilities,
-	remote_flow: Option<(Buffer, FlowParams)>,
+	remote_flow: Option<(Credits, FlowParams)>,
 	sent_head: H256, // last chain head we've given them.
 	last_update: SteadyTime,
 	pending_requests: RequestSet,
@@ -155,21 +160,21 @@ pub struct Peer {
 
 impl Peer {
 	// check the maximum cost of a request, returning an error if there's
-	// not enough buffer left.
+	// not enough credits left.
 	// returns the calculated maximum cost.
 	fn deduct_max(&mut self, flow_params: &FlowParams, kind: request::Kind, max: usize) -> Result<U256, Error> {
-		flow_params.recharge(&mut self.local_buffer);
+		flow_params.recharge(&mut self.local_credits);
 
 		let max_cost = flow_params.compute_cost(kind, max);
-		self.local_buffer.deduct_cost(max_cost)?;
+		self.local_credits.deduct_cost(max_cost)?;
 		Ok(max_cost)
 	}
 
-	// refund buffer for a request. returns new buffer amount.
+	// refund credits for a request. returns new amount of credits.
 	fn refund(&mut self, flow_params: &FlowParams, amount: U256) -> U256 {
-		flow_params.refund(&mut self.local_buffer, amount);
+		flow_params.refund(&mut self.local_credits, amount);
 
-		self.local_buffer.current()
+		self.local_credits.current()
 	}
 }
 
@@ -206,6 +211,8 @@ pub trait Handler: Send + Sync {
 	/// Called when a peer responds with header proofs. Each proof should be a block header coupled
 	/// with a series of trie nodes is ascending order by distance from the root.
 	fn on_header_proofs(&self, _ctx: &EventContext, _req_id: ReqId, _proofs: &[(Bytes, Vec<Bytes>)]) { }
+	/// Called when a peer responds with a transaction proof. Each proof is a vector of state items.
+	fn on_transaction_proof(&self, _ctx: &EventContext, _req_id: ReqId, _state_items: &[DBValue]) { }
 	/// Called to "tick" the handler periodically.
 	fn tick(&self, _ctx: &BasicContext) { }
 	/// Called on abort. This signals to handlers that they should clean up
@@ -218,7 +225,7 @@ pub trait Handler: Send + Sync {
 pub struct Params {
 	/// Network id.
 	pub network_id: u64,
-	/// Buffer flow parameters.
+	/// Request credits parameters.
 	pub flow_params: FlowParams,
 	/// Initial capabilities.
 	pub capabilities: Capabilities,
@@ -334,14 +341,14 @@ impl LightProtocol {
 
 	/// Check the maximum amount of requests of a specific type
 	/// which a peer would be able to serve. Returns zero if the
-	/// peer is unknown or has no buffer flow parameters.
+	/// peer is unknown or has no credit parameters.
 	fn max_requests(&self, peer: PeerId, kind: request::Kind) -> usize {
 		self.peers.read().get(&peer).and_then(|peer| {
 			let mut peer = peer.lock();
 			match peer.remote_flow {
-				Some((ref mut buf, ref flow)) => {
-					flow.recharge(buf);
-					Some(flow.max_amount(&*buf, kind))
+				Some((ref mut c, ref flow)) => {
+					flow.recharge(c);
+					Some(flow.max_amount(&*c, kind))
 				}
 				None => None,
 			}
@@ -351,7 +358,7 @@ impl LightProtocol {
 	/// Make a request to a peer.
 	///
 	/// Fails on: nonexistent peer, network error, peer not server,
-	/// insufficient buffer. Does not check capabilities before sending.
+	/// insufficient credits. Does not check capabilities before sending.
 	/// On success, returns a request id which can later be coordinated
 	/// with an event.
 	pub fn request_from(&self, io: &IoContext, peer_id: &PeerId, request: Request) -> Result<ReqId, Error> {
@@ -360,10 +367,10 @@ impl LightProtocol {
 		let mut peer = peer.lock();
 
 		match peer.remote_flow {
-			Some((ref mut buf, ref flow)) => {
-				flow.recharge(buf);
+			Some((ref mut c, ref flow)) => {
+				flow.recharge(c);
 				let max = flow.compute_cost(request.kind(), request.amount());
-				buf.deduct_cost(max)?;
+				c.deduct_cost(max)?;
 			}
 			None => return Err(Error::NotServer),
 		}
@@ -380,6 +387,7 @@ impl LightProtocol {
 			request::Kind::StateProofs => packet::GET_PROOFS,
 			request::Kind::Codes => packet::GET_CONTRACT_CODES,
 			request::Kind::HeaderProofs => packet::GET_HEADER_PROOFS,
+			request::Kind::TransactionProof => packet::GET_TRANSACTION_PROOF,
 		};
 
 		io.send(*peer_id, packet_id, packet_data);
@@ -464,7 +472,7 @@ impl LightProtocol {
 	//   - check whether request kinds match
 	fn pre_verify_response(&self, peer: &PeerId, kind: request::Kind, raw: &UntrustedRlp) -> Result<IdGuard, Error> {
 		let req_id = ReqId(raw.val_at(0)?);
-		let cur_buffer: U256 = raw.val_at(1)?;
+		let cur_credits: U256 = raw.val_at(1)?;
 
 		trace!(target: "les", "pre-verifying response from peer {}, kind={:?}", peer, kind);
 
@@ -480,9 +488,9 @@ impl LightProtocol {
 					(Some(request), Some(flow_info)) => {
 						had_req = true;
 
-						let &mut (ref mut buf, ref mut flow) = flow_info;
-						let actual_buffer = ::std::cmp::min(cur_buffer, *flow.limit());
-						buf.update_to(actual_buffer);
+						let &mut (ref mut c, ref mut flow) = flow_info;
+						let actual_credits = ::std::cmp::min(cur_credits, *flow.limit());
+						c.update_to(actual_credits);
 
 						if request.kind() != kind {
 							Some(Error::UnsolicitedResponse)
@@ -539,6 +547,9 @@ impl LightProtocol {
 			packet::GET_HEADER_PROOFS => self.get_header_proofs(peer, io, rlp),
 			packet::HEADER_PROOFS => self.header_proofs(peer, io, rlp),
 
+			packet::GET_TRANSACTION_PROOF => self.get_transaction_proof(peer, io, rlp),
+			packet::TRANSACTION_PROOF => self.transaction_proof(peer, io, rlp),
+
 			packet::SEND_TRANSACTIONS => self.relay_transactions(peer, io, rlp),
 
 			other => {
@@ -685,10 +696,10 @@ impl LightProtocol {
 			return Err(Error::BadProtocolVersion);
 		}
 
-		let remote_flow = flow_params.map(|params| (params.create_buffer(), params));
+		let remote_flow = flow_params.map(|params| (params.create_credits(), params));
 
 		self.peers.write().insert(*peer, Mutex::new(Peer {
-			local_buffer: self.flow_params.create_buffer(),
+			local_credits: self.flow_params.create_credits(),
 			status: status.clone(),
 			capabilities: capabilities.clone(),
 			remote_flow: remote_flow,
@@ -793,10 +804,10 @@ impl LightProtocol {
 		let actual_cost = self.flow_params.compute_cost(request::Kind::Headers, response.len());
 		assert!(max_cost >= actual_cost, "Actual cost exceeded maximum computed cost.");
 
-		let cur_buffer = peer.refund(&self.flow_params, max_cost - actual_cost);
+		let cur_credits = peer.refund(&self.flow_params, max_cost - actual_cost);
 		io.respond(packet::BLOCK_HEADERS, {
 			let mut stream = RlpStream::new_list(3);
-			stream.append(&req_id).append(&cur_buffer).begin_list(response.len());
+			stream.append(&req_id).append(&cur_credits).begin_list(response.len());
 
 			for header in response {
 				stream.append_raw(&header.into_inner(), 1);
@@ -855,11 +866,11 @@ impl LightProtocol {
 		let actual_cost = self.flow_params.compute_cost(request::Kind::Bodies, response_len);
 		assert!(max_cost >= actual_cost, "Actual cost exceeded maximum computed cost.");
 
-		let cur_buffer = peer.refund(&self.flow_params, max_cost - actual_cost);
+		let cur_credits = peer.refund(&self.flow_params, max_cost - actual_cost);
 
 		io.respond(packet::BLOCK_BODIES, {
 			let mut stream = RlpStream::new_list(3);
-			stream.append(&req_id).append(&cur_buffer).begin_list(response.len());
+			stream.append(&req_id).append(&cur_credits).begin_list(response.len());
 
 			for body in response {
 				match body {
@@ -921,11 +932,11 @@ impl LightProtocol {
 		let actual_cost = self.flow_params.compute_cost(request::Kind::Receipts, response_len);
 		assert!(max_cost >= actual_cost, "Actual cost exceeded maximum computed cost.");
 
-		let cur_buffer = peer.refund(&self.flow_params, max_cost - actual_cost);
+		let cur_credits = peer.refund(&self.flow_params, max_cost - actual_cost);
 
 		io.respond(packet::RECEIPTS, {
 			let mut stream = RlpStream::new_list(3);
-			stream.append(&req_id).append(&cur_buffer).begin_list(response.len());
+			stream.append(&req_id).append(&cur_credits).begin_list(response.len());
 
 			for receipts in response {
 				stream.append_raw(&receipts, 1);
@@ -995,11 +1006,11 @@ impl LightProtocol {
 		let actual_cost = self.flow_params.compute_cost(request::Kind::StateProofs, response_len);
 		assert!(max_cost >= actual_cost, "Actual cost exceeded maximum computed cost.");
 
-		let cur_buffer = peer.refund(&self.flow_params, max_cost - actual_cost);
+		let cur_credits = peer.refund(&self.flow_params, max_cost - actual_cost);
 
 		io.respond(packet::PROOFS, {
 			let mut stream = RlpStream::new_list(3);
-			stream.append(&req_id).append(&cur_buffer).begin_list(response.len());
+			stream.append(&req_id).append(&cur_credits).begin_list(response.len());
 
 			for proof in response {
 				stream.append_raw(&proof, 1);
@@ -1067,11 +1078,11 @@ impl LightProtocol {
 		let actual_cost = self.flow_params.compute_cost(request::Kind::Codes, response_len);
 		assert!(max_cost >= actual_cost, "Actual cost exceeded maximum computed cost.");
 
-		let cur_buffer = peer.refund(&self.flow_params, max_cost - actual_cost);
+		let cur_credits = peer.refund(&self.flow_params, max_cost - actual_cost);
 
 		io.respond(packet::CONTRACT_CODES, {
 			let mut stream = RlpStream::new_list(3);
-			stream.append(&req_id).append(&cur_buffer).begin_list(response.len());
+			stream.append(&req_id).append(&cur_credits).begin_list(response.len());
 
 			for code in response {
 				stream.append(&code);
@@ -1140,11 +1151,11 @@ impl LightProtocol {
 		let actual_cost = self.flow_params.compute_cost(request::Kind::HeaderProofs, response_len);
 		assert!(max_cost >= actual_cost, "Actual cost exceeded maximum computed cost.");
 
-		let cur_buffer = peer.refund(&self.flow_params, max_cost - actual_cost);
+		let cur_credits = peer.refund(&self.flow_params, max_cost - actual_cost);
 
 		io.respond(packet::HEADER_PROOFS, {
 			let mut stream = RlpStream::new_list(3);
-			stream.append(&req_id).append(&cur_buffer).begin_list(response.len());
+			stream.append(&req_id).append(&cur_credits).begin_list(response.len());
 
 			for proof in response {
 				stream.append_raw(&proof, 1);
@@ -1182,6 +1193,90 @@ impl LightProtocol {
 		Ok(())
 	}
 
+	// Receive a request for proof-of-execution.
+	fn get_transaction_proof(&self, peer: &PeerId, io: &IoContext, raw: UntrustedRlp) -> Result<(), Error> {
+		// refuse to execute more than this amount of gas at once.
+		// this is appx. the point at which the proof of execution would no longer fit in
+		// a single Devp2p packet.
+		const MAX_GAS: usize = 50_000_000;
+		use util::Uint;
+
+		let peers = self.peers.read();
+		let peer = match peers.get(peer) {
+			Some(peer) => peer,
+			None => {
+				debug!(target: "les", "Ignoring request from unknown peer");
+				return Ok(())
+			}
+		};
+		let mut peer = peer.lock();
+
+		let req_id: u64 = raw.val_at(0)?;
+
+		let req = {
+			let req_rlp = raw.at(1)?;
+			request::TransactionProof {
+				at: req_rlp.val_at(0)?,
+				from: req_rlp.val_at(1)?,
+				action: if req_rlp.at(2)?.is_empty() {
+					Action::Create
+				} else {
+					Action::Call(req_rlp.val_at(2)?)
+				},
+				gas: ::std::cmp::min(req_rlp.val_at(3)?, MAX_GAS.into()),
+				gas_price: req_rlp.val_at(4)?,
+				value: req_rlp.val_at(5)?,
+				data: req_rlp.val_at(6)?,
+			}
+		};
+
+		// always charge the peer for all the gas.
+		peer.deduct_max(&self.flow_params, request::Kind::TransactionProof, req.gas.low_u64() as usize)?;
+
+		let response = match self.provider.transaction_proof(req) {
+			Some(res) => res,
+			None => vec![],
+		};
+
+		let cur_credits = peer.local_credits.current();
+
+		io.respond(packet::TRANSACTION_PROOF, {
+			let mut stream = RlpStream::new_list(3);
+			stream.append(&req_id).append(&cur_credits).begin_list(response.len());
+
+			for state_item in response {
+				stream.append(&&state_item[..]);
+			}
+
+			stream.out()
+		});
+
+		Ok(())
+	}
+
+	// Receive a response for proof-of-execution.
+	fn transaction_proof(&self, peer: &PeerId, io: &IoContext, raw: UntrustedRlp) -> Result<(), Error> {
+		let id_guard = self.pre_verify_response(peer, request::Kind::HeaderProofs, &raw)?;
+		let raw_proof: Vec<DBValue> = raw.at(2)?.iter()
+			.map(|rlp| {
+				let mut db_val = DBValue::new();
+				db_val.append_slice(rlp.data()?);
+				Ok(db_val)
+			})
+			.collect::<Result<Vec<_>, ::rlp::DecoderError>>()?;
+
+		let req_id = id_guard.defuse();
+		for handler in &self.handlers {
+			handler.on_transaction_proof(&Ctx {
+				peer: *peer,
+				io: io,
+				proto: self,
+			}, req_id, &raw_proof);
+		}
+
+		Ok(())
+	}
+
 	// Receive a set of transactions to relay.
 	fn relay_transactions(&self, peer: &PeerId, io: &IoContext, data: UntrustedRlp) -> Result<(), Error> {
 		const MAX_TRANSACTIONS: usize = 256;
@@ -1330,6 +1425,25 @@ fn encode_request(req: &Request, req_id: usize) -> Vec<u8> {
 					.append(&proof_req.from_level);
 			}
 
+			stream.out()
+		}
+		Request::TransactionProof(ref request) => {
+			let mut stream = RlpStream::new_list(2);
+			stream.append(&req_id).begin_list(7)
+				.append(&request.at)
+				.append(&request.from);
+
+			match request.action {
+				Action::Create => stream.append_empty_data(),
+				Action::Call(ref to) => stream.append(to),
+			};
+
+			stream
+				.append(&request.gas)
+				.append(&request.gas_price)
+				.append(&request.value)
+				.append(&request.data);
+
 			stream.out()
 		}
 	}
diff --git a/ethcore/light/src/net/buffer_flow.rs b/ethcore/light/src/net/request_credits.rs
similarity index 77%
rename from ethcore/light/src/net/buffer_flow.rs
rename to ethcore/light/src/net/request_credits.rs
index cce54da59..97aa9b431 100644
--- a/ethcore/light/src/net/buffer_flow.rs
+++ b/ethcore/light/src/net/request_credits.rs
@@ -14,14 +14,14 @@
 // You should have received a copy of the GNU General Public License
 // along with Parity.  If not, see <http://www.gnu.org/licenses/>.
 
-//! LES buffer flow management.
+//! Request credit management.
 //!
-//! Every request in the LES protocol leads to a reduction
-//! of the requester's buffer value as a rate-limiting mechanism.
-//! This buffer value will recharge at a set rate.
+//! Every request in the light protocol leads to a reduction
+//! of the requester's amount of credits as a rate-limiting mechanism.
+//! The amount of credits will recharge at a set rate.
 //!
-//! This module provides an interface for configuration of buffer
-//! flow costs and recharge rates.
+//! This module provides an interface for configuration of
+//! costs and recharge rates of request credits.
 //!
 //! Current default costs are picked completely arbitrarily, not based
 //! on any empirical timings or mathematical models.
@@ -38,19 +38,19 @@ use time::{Duration, SteadyTime};
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Cost(pub U256, pub U256);
 
-/// Buffer value.
+/// Credits value.
 ///
 /// Produced and recharged using `FlowParams`.
 /// Definitive updates can be made as well -- these will reset the recharge
 /// point to the time of the update.
 #[derive(Debug, Clone, PartialEq, Eq)]
-pub struct Buffer {
+pub struct Credits {
 	estimate: U256,
 	recharge_point: SteadyTime,
 }
 
-impl Buffer {
-	/// Get the current buffer value.
+impl Credits {
+	/// Get the current amount of credits..
 	pub fn current(&self) -> U256 { self.estimate.clone() }
 
 	/// Make a definitive update.
@@ -61,7 +61,7 @@ impl Buffer {
 		self.recharge_point = SteadyTime::now();
 	}
 
-	/// Attempt to apply the given cost to the buffer.
+	/// Attempt to apply the given cost to the amount of credits.
 	///
 	/// If successful, the cost will be deducted successfully.
 	///
@@ -69,7 +69,7 @@ impl Buffer {
 	/// error will be produced.
 	pub fn deduct_cost(&mut self, cost: U256) -> Result<(), Error> {
 		match cost > self.estimate {
-			true => Err(Error::BufferEmpty),
+			true => Err(Error::NoCredits),
 			false => {
 				self.estimate = self.estimate - cost;
 				Ok(())
@@ -81,12 +81,13 @@ impl Buffer {
 /// A cost table, mapping requests to base and per-request costs.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct CostTable {
-	headers: Cost,
+	headers: Cost, // cost per header
 	bodies: Cost,
 	receipts: Cost,
 	state_proofs: Cost,
 	contract_codes: Cost,
 	header_proofs: Cost,
+	transaction_proof: Cost, // cost per gas.
 }
 
 impl Default for CostTable {
@@ -99,6 +100,7 @@ impl Default for CostTable {
 			state_proofs: Cost(250000.into(), 25000.into()),
 			contract_codes: Cost(200000.into(), 20000.into()),
 			header_proofs: Cost(150000.into(), 15000.into()),
+			transaction_proof: Cost(100000.into(), 2.into()),
 		}
 	}
 }
@@ -112,7 +114,7 @@ impl RlpEncodable for CostTable {
 				.append(&cost.1);
 		}
 
-		s.begin_list(6);
+		s.begin_list(7);
 
 		append_cost(s, packet::GET_BLOCK_HEADERS, &self.headers);
 		append_cost(s, packet::GET_BLOCK_BODIES, &self.bodies);
@@ -120,6 +122,7 @@ impl RlpEncodable for CostTable {
 		append_cost(s, packet::GET_PROOFS, &self.state_proofs);
 		append_cost(s, packet::GET_CONTRACT_CODES, &self.contract_codes);
 		append_cost(s, packet::GET_HEADER_PROOFS, &self.header_proofs);
+		append_cost(s, packet::GET_TRANSACTION_PROOF, &self.transaction_proof);
 	}
 }
 
@@ -133,6 +136,7 @@ impl RlpDecodable for CostTable {
 		let mut state_proofs = None;
 		let mut contract_codes = None;
 		let mut header_proofs = None;
+		let mut transaction_proof = None;
 
 		for row in rlp.iter() {
 			let msg_id: u8 = row.val_at(0)?;
@@ -150,6 +154,7 @@ impl RlpDecodable for CostTable {
 				packet::GET_PROOFS => state_proofs = Some(cost),
 				packet::GET_CONTRACT_CODES => contract_codes = Some(cost),
 				packet::GET_HEADER_PROOFS => header_proofs = Some(cost),
+				packet::GET_TRANSACTION_PROOF => transaction_proof = Some(cost),
 				_ => return Err(DecoderError::Custom("Unrecognized message in cost table")),
 			}
 		}
@@ -161,11 +166,12 @@ impl RlpDecodable for CostTable {
 			state_proofs: state_proofs.ok_or(DecoderError::Custom("No proofs cost specified"))?,
 			contract_codes: contract_codes.ok_or(DecoderError::Custom("No contract codes specified"))?,
 			header_proofs: header_proofs.ok_or(DecoderError::Custom("No header proofs cost specified"))?,
+			transaction_proof: transaction_proof.ok_or(DecoderError::Custom("No transaction proof gas cost specified"))?,
 		})
 	}
 }
 
-/// A buffer-flow manager handles costs, recharge, limits
+/// Handles costs, recharge, limits of request credits.
 #[derive(Debug, Clone, PartialEq)]
 pub struct FlowParams {
 	costs: CostTable,
@@ -175,7 +181,7 @@ pub struct FlowParams {
 
 impl FlowParams {
 	/// Create new flow parameters from a request cost table,
-	/// buffer limit, and (minimum) rate of recharge.
+	/// credit limit, and (minimum) rate of recharge.
 	pub fn new(limit: U256, costs: CostTable, recharge: U256) -> Self {
 		FlowParams {
 			costs: costs,
@@ -197,11 +203,12 @@ impl FlowParams {
 				state_proofs: free_cost.clone(),
 				contract_codes: free_cost.clone(),
 				header_proofs: free_cost.clone(),
+				transaction_proof: free_cost,
 			}
 		}
 	}
 
-	/// Get a reference to the buffer limit.
+	/// Get a reference to the credit limit.
 	pub fn limit(&self) -> &U256 { &self.limit }
 
 	/// Get a reference to the cost table.
@@ -220,6 +227,7 @@ impl FlowParams {
 			request::Kind::StateProofs => &self.costs.state_proofs,
 			request::Kind::Codes => &self.costs.contract_codes,
 			request::Kind::HeaderProofs => &self.costs.header_proofs,
+			request::Kind::TransactionProof => &self.costs.transaction_proof,
 		};
 
 		let amount: U256 = amount.into();
@@ -227,10 +235,10 @@ impl FlowParams {
 	}
 
 	/// Compute the maximum number of costs of a specific kind which can be made
-	/// with the given buffer.
+	/// with the given amount of credits
 	/// Saturates at `usize::max()`. This is not a problem in practice because
 	/// this amount of requests is already prohibitively large.
-	pub fn max_amount(&self, buffer: &Buffer, kind: request::Kind) -> usize {
+	pub fn max_amount(&self, credits: &Credits, kind: request::Kind) -> usize {
 		use util::Uint;
 		use std::usize;
 
@@ -241,9 +249,10 @@ impl FlowParams {
 			request::Kind::StateProofs => &self.costs.state_proofs,
 			request::Kind::Codes => &self.costs.contract_codes,
 			request::Kind::HeaderProofs => &self.costs.header_proofs,
+			request::Kind::TransactionProof => &self.costs.transaction_proof,
 		};
 
-		let start = buffer.current();
+		let start = credits.current();
 
 		if start <= cost.0 {
 			return 0;
@@ -259,36 +268,36 @@ impl FlowParams {
 		}
 	}
 
-	/// Create initial buffer parameter.
-	pub fn create_buffer(&self) -> Buffer {
-		Buffer {
+	/// Create initial credits..
+	pub fn create_credits(&self) -> Credits {
+		Credits {
 			estimate: self.limit,
 			recharge_point: SteadyTime::now(),
 		}
 	}
 
-	/// Recharge the buffer based on time passed since last
+	/// Recharge the given credits based on time passed since last
 	/// update.
-	pub fn recharge(&self, buf: &mut Buffer) {
+	pub fn recharge(&self, credits: &mut Credits) {
 		let now = SteadyTime::now();
 
 		// recompute and update only in terms of full seconds elapsed
 		// in order to keep the estimate as an underestimate.
-		let elapsed = (now - buf.recharge_point).num_seconds();
-		buf.recharge_point = buf.recharge_point + Duration::seconds(elapsed);
+		let elapsed = (now - credits.recharge_point).num_seconds();
+		credits.recharge_point = credits.recharge_point + Duration::seconds(elapsed);
 
 		let elapsed: U256 = elapsed.into();
 
-		buf.estimate = ::std::cmp::min(self.limit, buf.estimate + (elapsed * self.recharge));
+		credits.estimate = ::std::cmp::min(self.limit, credits.estimate + (elapsed * self.recharge));
 	}
 
-	/// Refund some buffer which was previously deducted.
+	/// Refund some credits which were previously deducted.
 	/// Does not update the recharge timestamp.
-	pub fn refund(&self, buf: &mut Buffer, refund_amount: U256) {
-		buf.estimate = buf.estimate + refund_amount;
+	pub fn refund(&self, credits: &mut Credits, refund_amount: U256) {
+		credits.estimate = credits.estimate + refund_amount;
 
-		if buf.estimate > self.limit {
-			buf.estimate = self.limit
+		if credits.estimate > self.limit {
+			credits.estimate = self.limit
 		}
 	}
 }
@@ -318,20 +327,20 @@ mod tests {
 	}
 
 	#[test]
-	fn buffer_mechanism() {
+	fn credits_mechanism() {
 		use std::thread;
 		use std::time::Duration;
 
 		let flow_params = FlowParams::new(100.into(), Default::default(), 20.into());
-		let mut buffer =  flow_params.create_buffer();
+		let mut credits =  flow_params.create_credits();
 
-		assert!(buffer.deduct_cost(101.into()).is_err());
-		assert!(buffer.deduct_cost(10.into()).is_ok());
+		assert!(credits.deduct_cost(101.into()).is_err());
+		assert!(credits.deduct_cost(10.into()).is_ok());
 
 		thread::sleep(Duration::from_secs(1));
 
-		flow_params.recharge(&mut buffer);
+		flow_params.recharge(&mut credits);
 
-		assert_eq!(buffer.estimate, 100.into());
+		assert_eq!(credits.estimate, 100.into());
 	}
 }
diff --git a/ethcore/light/src/net/request_set.rs b/ethcore/light/src/net/request_set.rs
index 9a26b24b1..e6d4068da 100644
--- a/ethcore/light/src/net/request_set.rs
+++ b/ethcore/light/src/net/request_set.rs
@@ -101,6 +101,7 @@ impl RequestSet {
 			request::Kind::StateProofs => timeout::PROOFS,
 			request::Kind::Codes => timeout::CONTRACT_CODES,
 			request::Kind::HeaderProofs => timeout::HEADER_PROOFS,
+			request::Kind::TransactionProof => timeout::TRANSACTION_PROOF,
 		};
 
 		base + Duration::milliseconds(kind_timeout) <= now
diff --git a/ethcore/light/src/net/status.rs b/ethcore/light/src/net/status.rs
index 655dc404f..3e32f6609 100644
--- a/ethcore/light/src/net/status.rs
+++ b/ethcore/light/src/net/status.rs
@@ -19,7 +19,7 @@
 use rlp::{DecoderError, RlpDecodable, RlpEncodable, RlpStream, Stream, UntrustedRlp, View};
 use util::{H256, U256};
 
-use super::buffer_flow::FlowParams;
+use super::request_credits::FlowParams;
 
 // recognized handshake/announcement keys.
 // unknown keys are to be skipped, known keys have a defined order.
@@ -207,7 +207,7 @@ impl Capabilities {
 /// Attempt to parse a handshake message into its three parts:
 ///   - chain status
 ///   - serving capabilities
-///   - buffer flow parameters
+///   - request credit parameters
 pub fn parse_handshake(rlp: UntrustedRlp) -> Result<(Status, Capabilities, Option<FlowParams>), DecoderError> {
 	let mut parser = Parser {
 		pos: 0,
@@ -300,7 +300,7 @@ pub struct Announcement {
 	pub serve_chain_since: Option<u64>,
 	/// optional new transaction-relay capability. false means "no change"
 	pub tx_relay: bool,
-	// TODO: changes in buffer flow?
+	// TODO: changes in request credits.
 }
 
 /// Parse an announcement.
@@ -372,7 +372,7 @@ pub fn write_announcement(announcement: &Announcement) -> Vec<u8> {
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use super::super::buffer_flow::FlowParams;
+	use super::super::request_credits::FlowParams;
 	use util::{U256, H256, FixedHash};
 	use rlp::{RlpStream, Stream ,UntrustedRlp, View};
 
diff --git a/ethcore/light/src/net/tests/mod.rs b/ethcore/light/src/net/tests/mod.rs
index 47d73aef2..6a9de1467 100644
--- a/ethcore/light/src/net/tests/mod.rs
+++ b/ethcore/light/src/net/tests/mod.rs
@@ -20,11 +20,11 @@
 use ethcore::blockchain_info::BlockChainInfo;
 use ethcore::client::{EachBlockWith, TestBlockChainClient};
 use ethcore::ids::BlockId;
-use ethcore::transaction::PendingTransaction;
+use ethcore::transaction::{Action, PendingTransaction};
 use ethcore::encoded;
 use network::{PeerId, NodeId};
 
-use net::buffer_flow::FlowParams;
+use net::request_credits::FlowParams;
 use net::context::IoContext;
 use net::status::{Capabilities, Status, write_handshake};
 use net::{encode_request, LightProtocol, Params, packet, Peer};
@@ -32,7 +32,7 @@ use provider::Provider;
 use request::{self, Request, Headers};
 
 use rlp::*;
-use util::{Bytes, H256, U256};
+use util::{Address, Bytes, DBValue, H256, U256};
 
 use std::sync::Arc;
 
@@ -127,6 +127,10 @@ impl Provider for TestProvider {
 		None
 	}
 
+	fn transaction_proof(&self, _req: request::TransactionProof) -> Option<Vec<DBValue>> {
+		None
+	}
+
 	fn ready_transactions(&self) -> Vec<PendingTransaction> {
 		self.0.client.ready_transactions()
 	}
@@ -203,7 +207,7 @@ fn genesis_mismatch() {
 }
 
 #[test]
-fn buffer_overflow() {
+fn credit_overflow() {
 	let flow_params = make_flow_params();
 	let capabilities = capabilities();
 
@@ -268,11 +272,11 @@ fn get_block_headers() {
 		let headers: Vec<_> = (0..10).map(|i| provider.client.block_header(BlockId::Number(i + 1)).unwrap()).collect();
 		assert_eq!(headers.len(), 10);
 
-		let new_buf = *flow_params.limit() - flow_params.compute_cost(request::Kind::Headers, 10);
+		let new_creds = *flow_params.limit() - flow_params.compute_cost(request::Kind::Headers, 10);
 
 		let mut response_stream = RlpStream::new_list(3);
 
-		response_stream.append(&req_id).append(&new_buf).begin_list(10);
+		response_stream.append(&req_id).append(&new_creds).begin_list(10);
 		for header in headers {
 			response_stream.append_raw(&header.into_inner(), 1);
 		}
@@ -317,11 +321,11 @@ fn get_block_bodies() {
 		let bodies: Vec<_> = (0..10).map(|i| provider.client.block_body(BlockId::Number(i + 1)).unwrap()).collect();
 		assert_eq!(bodies.len(), 10);
 
-		let new_buf = *flow_params.limit() - flow_params.compute_cost(request::Kind::Bodies, 10);
+		let new_creds = *flow_params.limit() - flow_params.compute_cost(request::Kind::Bodies, 10);
 
 		let mut response_stream = RlpStream::new_list(3);
 
-		response_stream.append(&req_id).append(&new_buf).begin_list(10);
+		response_stream.append(&req_id).append(&new_creds).begin_list(10);
 		for body in bodies {
 			response_stream.append_raw(&body.into_inner(), 1);
 		}
@@ -371,11 +375,11 @@ fn get_block_receipts() {
 			.map(|hash| provider.client.block_receipts(hash).unwrap())
 			.collect();
 
-		let new_buf = *flow_params.limit() - flow_params.compute_cost(request::Kind::Receipts, receipts.len());
+		let new_creds = *flow_params.limit() - flow_params.compute_cost(request::Kind::Receipts, receipts.len());
 
 		let mut response_stream = RlpStream::new_list(3);
 
-		response_stream.append(&req_id).append(&new_buf).begin_list(receipts.len());
+		response_stream.append(&req_id).append(&new_creds).begin_list(receipts.len());
 		for block_receipts in receipts {
 			response_stream.append_raw(&block_receipts, 1);
 		}
@@ -420,11 +424,11 @@ fn get_state_proofs() {
 			vec![::util::sha3::SHA3_NULL_RLP.to_vec()],
 		];
 
-		let new_buf = *flow_params.limit() - flow_params.compute_cost(request::Kind::StateProofs, 2);
+		let new_creds = *flow_params.limit() - flow_params.compute_cost(request::Kind::StateProofs, 2);
 
 		let mut response_stream = RlpStream::new_list(3);
 
-		response_stream.append(&req_id).append(&new_buf).begin_list(2);
+		response_stream.append(&req_id).append(&new_creds).begin_list(2);
 		for proof in proofs {
 			response_stream.begin_list(proof.len());
 			for node in proof {
@@ -472,11 +476,11 @@ fn get_contract_code() {
             key2.iter().chain(key2.iter()).cloned().collect(),
 		];
 
-		let new_buf = *flow_params.limit() - flow_params.compute_cost(request::Kind::Codes, 2);
+		let new_creds = *flow_params.limit() - flow_params.compute_cost(request::Kind::Codes, 2);
 
 		let mut response_stream = RlpStream::new_list(3);
 
-		response_stream.append(&req_id).append(&new_buf).begin_list(2);
+		response_stream.append(&req_id).append(&new_creds).begin_list(2);
 		for code in codes {
 			response_stream.append(&code);
 		}
@@ -488,6 +492,56 @@ fn get_contract_code() {
 	proto.handle_packet(&expected, &1, packet::GET_CONTRACT_CODES, &request_body);
 }
 
+#[test]
+fn proof_of_execution() {
+	let flow_params = FlowParams::new(5_000_000.into(), Default::default(), 0.into());
+	let capabilities = capabilities();
+
+	let (provider, proto) = setup(flow_params.clone(), capabilities.clone());
+
+	let cur_status = status(provider.client.chain_info());
+
+	{
+		let packet_body = write_handshake(&cur_status, &capabilities, Some(&flow_params));
+		proto.on_connect(&1, &Expect::Send(1, packet::STATUS, packet_body.clone()));
+		proto.handle_packet(&Expect::Nothing, &1, packet::STATUS, &packet_body);
+	}
+
+	let req_id = 112;
+	let mut request = Request::TransactionProof (request::TransactionProof {
+		at: H256::default(),
+		from: Address::default(),
+		action: Action::Call(Address::default()),
+		gas: 100.into(),
+		gas_price: 0.into(),
+		value: 0.into(),
+		data: Vec::new(),
+	});
+
+	// first: a valid amount to request execution of.
+	let request_body = encode_request(&request, req_id);
+	let response = {
+		let new_creds = *flow_params.limit() - flow_params.compute_cost(request::Kind::TransactionProof, 100);
+
+		let mut response_stream = RlpStream::new_list(3);
+		response_stream.append(&req_id).append(&new_creds).begin_list(0);
+
+		response_stream.out()
+	};
+
+	let expected = Expect::Respond(packet::TRANSACTION_PROOF, response);
+	proto.handle_packet(&expected, &1, packet::GET_TRANSACTION_PROOF, &request_body);
+
+	// next: way too much requested gas.
+	if let Request::TransactionProof(ref mut req) = request {
+		req.gas = 100_000_000.into();
+	}
+	let req_id = 113;
+	let request_body = encode_request(&request, req_id);
+	let expected = Expect::Punish(1);
+	proto.handle_packet(&expected, &1, packet::GET_TRANSACTION_PROOF, &request_body);
+}
+
 #[test]
 fn id_guard() {
 	use super::request_set::RequestSet;
@@ -515,10 +569,10 @@ fn id_guard() {
 	pending_requests.insert(req_id_2, req, ::time::SteadyTime::now());
 
 	proto.peers.write().insert(peer_id, ::util::Mutex::new(Peer {
-		local_buffer: flow_params.create_buffer(),
+		local_credits: flow_params.create_credits(),
 		status: status(provider.client.chain_info()),
 		capabilities: capabilities.clone(),
-		remote_flow: Some((flow_params.create_buffer(), flow_params)),
+		remote_flow: Some((flow_params.create_credits(), flow_params)),
 		sent_head: provider.client.chain_info().best_block_hash,
 		last_update: ::time::SteadyTime::now(),
 		pending_requests: pending_requests,
diff --git a/ethcore/light/src/on_demand/mod.rs b/ethcore/light/src/on_demand/mod.rs
index ec3b758ce..25cde402b 100644
--- a/ethcore/light/src/on_demand/mod.rs
+++ b/ethcore/light/src/on_demand/mod.rs
@@ -24,12 +24,14 @@ 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 network::PeerId;
 use rlp::{RlpStream, Stream};
-use util::{Bytes, RwLock, Mutex, U256};
+use util::{Bytes, DBValue, RwLock, Mutex, U256};
 use util::sha3::{SHA3_NULL_RLP, SHA3_EMPTY_LIST_RLP};
 
 use net::{Handler, Status, Capabilities, Announcement, EventContext, BasicContext, ReqId};
@@ -59,6 +61,7 @@ enum Pending {
 	BlockReceipts(request::BlockReceipts, Sender<Vec<Receipt>>),
 	Account(request::Account, Sender<BasicAccount>),
 	Code(request::Code, Sender<Bytes>),
+	TxProof(request::TransactionProof, Sender<Result<Executed, ExecutionError>>),
 }
 
 /// On demand request service. See module docs for more details.
@@ -418,6 +421,50 @@ impl OnDemand {
 		self.orphaned_requests.write().push(pending)
 	}
 
+	/// 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();
+
+		self.dispatch_transaction_proof(ctx, req, sender);
+
+		receiver
+	}
+
+	fn dispatch_transaction_proof(&self, ctx: &BasicContext, req: request::TransactionProof, sender: Sender<Result<Executed, ExecutionError>>) {
+		let num = req.header.number();
+		let les_req = LesRequest::TransactionProof(les_request::TransactionProof {
+			at: req.header.hash(),
+			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(),
+		});
+		let pending = Pending::TxProof(req, sender);
+
+		// we're looking for a peer with serveStateSince(num)
+		for (id, peer) in self.peers.read().iter() {
+			if peer.capabilities.serve_state_since.as_ref().map_or(false, |x| *x >= num) {
+				match ctx.request_from(*id, les_req.clone()) {
+					Ok(req_id) => {
+						trace!(target: "on_demand", "Assigning request to peer {}", id);
+						self.pending_requests.write().insert(
+							req_id,
+							pending
+						);
+						return
+					}
+					Err(e) =>
+						trace!(target: "on_demand", "Failed to make request of peer {}: {:?}", id, e),
+				}
+			}
+		}
+
+		trace!(target: "on_demand", "No suitable peer for request");
+		self.orphaned_requests.write().push(pending)
+	}
+
 	// dispatch orphaned requests, and discard those for which the corresponding
 	// receiver has been dropped.
 	fn dispatch_orphaned(&self, ctx: &BasicContext) {
@@ -468,6 +515,8 @@ impl OnDemand {
 					if !check_hangup(&mut sender) { self.dispatch_account(ctx, req, sender) },
 				Pending::Code(req, mut sender) =>
 					if !check_hangup(&mut sender) { self.dispatch_code(ctx, req, sender) },
+				Pending::TxProof(req, mut sender) =>
+					if !check_hangup(&mut sender) { self.dispatch_transaction_proof(ctx, req, sender) }
 			}
 		}
 	}
@@ -690,6 +739,36 @@ impl Handler for OnDemand {
 		}
 	}
 
+	fn on_transaction_proof(&self, ctx: &EventContext, req_id: ReqId, items: &[DBValue]) {
+		let peer = ctx.peer();
+		let req = match self.pending_requests.write().remove(&req_id) {
+			Some(req) => req,
+			None => return,
+		};
+
+		match req {
+			Pending::TxProof(req, sender) => {
+				match req.check_response(items) {
+					ProvedExecution::Complete(executed) => {
+						sender.complete(Ok(executed));
+						return
+					}
+					ProvedExecution::Failed(err) => {
+						sender.complete(Err(err));
+						return
+					}
+					ProvedExecution::BadProof => {
+						warn!("Error handling response for transaction proof request");
+						ctx.disable_peer(peer);
+					}
+				}
+
+				self.dispatch_transaction_proof(ctx.as_basic(), req, sender);
+			}
+			_ => panic!("Only transaction proof request dispatches transaction proof requests; qed"),
+		}
+	}
+
 	fn tick(&self, ctx: &BasicContext) {
 		self.dispatch_orphaned(ctx)
 	}
diff --git a/ethcore/light/src/on_demand/request.rs b/ethcore/light/src/on_demand/request.rs
index 3964137d9..3a72db51d 100644
--- a/ethcore/light/src/on_demand/request.rs
+++ b/ethcore/light/src/on_demand/request.rs
@@ -16,12 +16,18 @@
 
 //! Request types, verification, and verification errors.
 
+use std::sync::Arc;
+
 use ethcore::basic_account::BasicAccount;
 use ethcore::encoded;
+use ethcore::engines::Engine;
+use ethcore::env_info::EnvInfo;
 use ethcore::receipt::Receipt;
+use ethcore::state::{self, ProvedExecution};
+use ethcore::transaction::SignedTransaction;
 
 use rlp::{RlpStream, Stream, UntrustedRlp, View};
-use util::{Address, Bytes, HashDB, H256, U256};
+use util::{Address, Bytes, DBValue, HashDB, H256, U256};
 use util::memorydb::MemoryDB;
 use util::sha3::Hashable;
 use util::trie::{Trie, TrieDB, TrieError};
@@ -231,6 +237,33 @@ impl Code {
 	}
 }
 
+/// Request for transaction execution, along with the parts necessary to verify the proof.
+pub struct TransactionProof {
+	/// The transaction to request proof of.
+	pub tx: SignedTransaction,
+	/// Block header.
+	pub header: encoded::Header,
+	/// Transaction environment info.
+	pub env_info: EnvInfo,
+	/// Consensus engine.
+	pub engine: Arc<Engine>,
+}
+
+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 {
+		let root = self.header.state_root();
+
+		state::check_proof(
+			state_items,
+			root,
+			&self.tx,
+			&*self.engine,
+			&self.env_info,
+		)
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;
diff --git a/ethcore/light/src/provider.rs b/ethcore/light/src/provider.rs
index caade3857..3f55a6b99 100644
--- a/ethcore/light/src/provider.rs
+++ b/ethcore/light/src/provider.rs
@@ -24,7 +24,7 @@ use ethcore::client::{BlockChainClient, ProvingBlockChainClient};
 use ethcore::transaction::PendingTransaction;
 use ethcore::ids::BlockId;
 use ethcore::encoded;
-use util::{Bytes, RwLock, H256};
+use util::{Bytes, DBValue, RwLock, H256};
 
 use cht::{self, BlockInfo};
 use client::{LightChainClient, AsLightClient};
@@ -193,6 +193,10 @@ pub trait Provider: Send + Sync {
 
 	/// Provide pending transactions.
 	fn ready_transactions(&self) -> Vec<PendingTransaction>;
+
+	/// Provide a proof-of-execution for the given transaction proof request.
+	/// Returns a vector of all state items necessary to execute the transaction.
+	fn transaction_proof(&self, req: request::TransactionProof) -> Option<Vec<DBValue>>;
 }
 
 // Implementation of a light client data provider for a client.
@@ -283,6 +287,26 @@ impl<T: ProvingBlockChainClient + ?Sized> Provider for T {
 		}
 	}
 
+	fn transaction_proof(&self, req: request::TransactionProof) -> Option<Vec<DBValue>> {
+		use ethcore::transaction::Transaction;
+
+		let id = BlockId::Hash(req.at);
+		let nonce = match self.nonce(&req.from, id.clone()) {
+			Some(nonce) => nonce,
+			None => return None,
+		};
+		let transaction = Transaction {
+			nonce: nonce,
+			gas: req.gas,
+			gas_price: req.gas_price,
+			action: req.action,
+			value: req.value,
+			data: req.data,
+		}.fake_sign(req.from);
+
+		self.prove_transaction(transaction, id)
+	}
+
 	fn ready_transactions(&self) -> Vec<PendingTransaction> {
 		BlockChainClient::ready_transactions(self)
 	}
@@ -343,6 +367,10 @@ impl<L: AsLightClient + Send + Sync> Provider for LightProvider<L> {
 		None
 	}
 
+	fn transaction_proof(&self, _req: request::TransactionProof) -> Option<Vec<DBValue>> {
+		None
+	}
+
 	fn ready_transactions(&self) -> Vec<PendingTransaction> {
 		let chain_info = self.chain_info();
 		self.txqueue.read().ready_transactions(chain_info.best_block_number, chain_info.best_block_timestamp)
diff --git a/ethcore/light/src/types/les_request.rs b/ethcore/light/src/types/les_request.rs
index b4940980e..dbff19eb5 100644
--- a/ethcore/light/src/types/les_request.rs
+++ b/ethcore/light/src/types/les_request.rs
@@ -16,7 +16,8 @@
 
 //! LES request types.
 
-use util::H256;
+use ethcore::transaction::Action;
+use util::{Address, H256, U256, Uint};
 
 /// Either a hash or a number.
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -134,6 +135,26 @@ pub struct HeaderProofs {
 	pub requests: Vec<HeaderProof>,
 }
 
+/// A request for proof of (simulated) transaction execution.
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "ipc", binary)]
+pub struct TransactionProof {
+	/// Block hash to request for.
+	pub at: H256,
+	/// Address to treat as the caller.
+	pub from: Address,
+	/// Action to take: either a call or a create.
+	pub action: Action,
+	/// Amount of gas to request proof-of-execution for.
+	pub gas: U256,
+	/// Price for each gas.
+	pub gas_price: U256,
+	/// Value to simulate sending.
+	pub value: U256,
+	/// Transaction data.
+	pub data: Vec<u8>,
+}
+
 /// Kinds of requests.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 #[cfg_attr(feature = "ipc", binary)]
@@ -150,6 +171,8 @@ pub enum Kind {
 	Codes,
 	/// Requesting header proofs (from the CHT).
 	HeaderProofs,
+	/// Requesting proof of transaction execution.
+	TransactionProof,
 }
 
 /// Encompasses all possible types of requests in a single structure.
@@ -168,6 +191,8 @@ pub enum Request {
 	Codes(ContractCodes),
 	/// Requesting header proofs.
 	HeaderProofs(HeaderProofs),
+	/// Requesting proof of transaction execution.
+	TransactionProof(TransactionProof),
 }
 
 impl Request {
@@ -180,10 +205,12 @@ impl Request {
 			Request::StateProofs(_) => Kind::StateProofs,
 			Request::Codes(_) => Kind::Codes,
 			Request::HeaderProofs(_) => Kind::HeaderProofs,
+			Request::TransactionProof(_) => Kind::TransactionProof,
 		}
 	}
 
 	/// Get the amount of requests being made.
+	/// In the case of `TransactionProof`, this is the amount of gas being requested.
 	pub fn amount(&self) -> usize {
 		match *self {
 			Request::Headers(ref req) => req.max,
@@ -192,6 +219,10 @@ impl Request {
 			Request::StateProofs(ref req) => req.requests.len(),
 			Request::Codes(ref req) => req.code_requests.len(),
 			Request::HeaderProofs(ref req) => req.requests.len(),
+			Request::TransactionProof(ref req) => match req.gas > usize::max_value().into() {
+				true => usize::max_value(),
+				false => req.gas.low_u64() as usize,
+			}
 		}
 	}
 }
diff --git a/ethcore/src/client/client.rs b/ethcore/src/client/client.rs
index 7f209bad1..63be1da07 100644
--- a/ethcore/src/client/client.rs
+++ b/ethcore/src/client/client.rs
@@ -24,7 +24,7 @@ use time::precise_time_ns;
 
 // util
 use util::{Bytes, PerfTimer, Itertools, Mutex, RwLock, MutexGuard, Hashable};
-use util::{journaldb, TrieFactory, Trie};
+use util::{journaldb, DBValue, TrieFactory, Trie};
 use util::{U256, H256, Address, H2048, Uint, FixedHash};
 use util::trie::TrieSpec;
 use util::kvdb::*;
@@ -34,7 +34,7 @@ use io::*;
 use views::BlockView;
 use error::{ImportError, ExecutionError, CallError, BlockError, ImportResult, Error as EthcoreError};
 use header::BlockNumber;
-use state::{State, CleanupMode};
+use state::{self, State, CleanupMode};
 use spec::Spec;
 use basic_types::Seal;
 use engines::Engine;
@@ -308,18 +308,24 @@ impl Client {
 	}
 
 	/// The env info as of the best block.
-	fn latest_env_info(&self) -> EnvInfo {
-		let header = self.best_block_header();
+	pub fn latest_env_info(&self) -> EnvInfo {
+		self.env_info(BlockId::Latest).expect("Best block header always stored; qed")
+	}
 
-		EnvInfo {
-			number: header.number(),
-			author: header.author(),
-			timestamp: header.timestamp(),
-			difficulty: header.difficulty(),
-			last_hashes: self.build_last_hashes(header.hash()),
-			gas_used: U256::default(),
-			gas_limit: header.gas_limit(),
-		}
+	/// The env info as of a given block.
+	/// returns `None` if the block unknown.
+	pub fn env_info(&self, id: BlockId) -> Option<EnvInfo> {
+		self.block_header(id).map(|header| {
+			EnvInfo {
+				number: header.number(),
+				author: header.author(),
+				timestamp: header.timestamp(),
+				difficulty: header.difficulty(),
+				last_hashes: self.build_last_hashes(header.parent_hash()),
+				gas_used: U256::default(),
+				gas_limit: header.gas_limit(),
+			}
+		})
 	}
 
 	fn build_last_hashes(&self, parent_hash: H256) -> Arc<LastHashes> {
@@ -874,17 +880,9 @@ impl snapshot::DatabaseRestore for Client {
 
 impl BlockChainClient for Client {
 	fn call(&self, t: &SignedTransaction, block: BlockId, analytics: CallAnalytics) -> Result<Executed, CallError> {
-		let header = self.block_header(block).ok_or(CallError::StatePruned)?;
-		let last_hashes = self.build_last_hashes(header.parent_hash());
-		let env_info = EnvInfo {
-			number: header.number(),
-			author: header.author(),
-			timestamp: header.timestamp(),
-			difficulty: header.difficulty(),
-			last_hashes: last_hashes,
-			gas_used: U256::zero(),
-			gas_limit: U256::max_value(),
-		};
+		let mut env_info = self.env_info(block).ok_or(CallError::StatePruned)?;
+		env_info.gas_limit = U256::max_value();
+
 		// that's just a copy of the state.
 		let mut state = self.state_at(block).ok_or(CallError::StatePruned)?;
 		let original_state = if analytics.state_diffing { Some(state.clone()) } else { None };
@@ -910,17 +908,13 @@ impl BlockChainClient for Client {
 
 	fn estimate_gas(&self, t: &SignedTransaction, block: BlockId) -> Result<U256, CallError> {
 		const UPPER_CEILING: u64 = 1_000_000_000_000u64;
-		let header = self.block_header(block).ok_or(CallError::StatePruned)?;
-		let last_hashes = self.build_last_hashes(header.parent_hash());
-		let env_info = EnvInfo {
-			number: header.number(),
-			author: header.author(),
-			timestamp: header.timestamp(),
-			difficulty: header.difficulty(),
-			last_hashes: last_hashes,
-			gas_used: U256::zero(),
-			gas_limit: UPPER_CEILING.into(),
+		let (mut upper, env_info)  = {
+			let mut env_info = self.env_info(block).ok_or(CallError::StatePruned)?;
+			let initial_upper = env_info.gas_limit;
+			env_info.gas_limit = UPPER_CEILING.into();
+			(initial_upper, env_info)
 		};
+
 		// that's just a copy of the state.
 		let original_state = self.state_at(block).ok_or(CallError::StatePruned)?;
 		let sender = t.sender();
@@ -946,7 +940,6 @@ impl BlockChainClient for Client {
 				.unwrap_or(false))
 		};
 
-		let mut upper = header.gas_limit();
 		if !cond(upper)? {
 			// impossible at block gas limit - try `UPPER_CEILING` instead.
 			// TODO: consider raising limit by powers of two.
@@ -989,7 +982,7 @@ impl BlockChainClient for Client {
 
 	fn replay(&self, id: TransactionId, analytics: CallAnalytics) -> Result<Executed, CallError> {
 		let address = self.transaction_address(id).ok_or(CallError::TransactionNotFound)?;
-		let header = self.block_header(BlockId::Hash(address.block_hash)).ok_or(CallError::StatePruned)?;
+		let mut env_info = self.env_info(BlockId::Hash(address.block_hash)).ok_or(CallError::StatePruned)?;
 		let body = self.block_body(BlockId::Hash(address.block_hash)).ok_or(CallError::StatePruned)?;
 		let mut state = self.state_at_beginning(BlockId::Hash(address.block_hash)).ok_or(CallError::StatePruned)?;
 		let mut txs = body.transactions();
@@ -999,16 +992,6 @@ impl BlockChainClient for Client {
 		}
 
 		let options = TransactOptions { tracing: analytics.transaction_tracing, vm_tracing: analytics.vm_tracing, check_nonce: false };
-		let last_hashes = self.build_last_hashes(header.hash());
-		let mut env_info = EnvInfo {
-			number: header.number(),
-			author: header.author(),
-			timestamp: header.timestamp(),
-			difficulty: header.difficulty(),
-			last_hashes: last_hashes,
-			gas_used: U256::default(),
-			gas_limit: header.gas_limit(),
-		};
 		const PROOF: &'static str = "Transactions fetched from blockchain; blockchain transactions are valid; qed";
 		let rest = txs.split_off(address.index);
 		for t in txs {
@@ -1620,6 +1603,25 @@ impl ::client::ProvingBlockChainClient for Client {
 			.and_then(|x| x)
 			.unwrap_or_else(Vec::new)
 	}
+
+	fn prove_transaction(&self, transaction: SignedTransaction, id: BlockId) -> Option<Vec<DBValue>> {
+		let (state, env_info) = match (self.state_at(id), self.env_info(id)) {
+			(Some(s), Some(e)) => (s, e),
+			_ => return None,
+		};
+		let mut jdb = self.state_db.lock().journal_db().boxed_clone();
+		let backend = state::backend::Proving::new(jdb.as_hashdb_mut());
+
+		let mut state = state.replace_backend(backend);
+		let options = TransactOptions { tracing: false, vm_tracing: false, check_nonce: false };
+		let res = Executive::new(&mut state, &env_info, &*self.engine, &self.factories.vm).transact(&transaction, options);
+
+		match res {
+			Err(ExecutionError::Internal(_)) => return None,
+			_ => return Some(state.drop().1.extract_proof()),
+		}
+	}
+
 }
 
 impl Drop for Client {
diff --git a/ethcore/src/client/test_client.rs b/ethcore/src/client/test_client.rs
index dc9cb5944..5d436f4c5 100644
--- a/ethcore/src/client/test_client.rs
+++ b/ethcore/src/client/test_client.rs
@@ -765,6 +765,10 @@ impl ProvingBlockChainClient for TestBlockChainClient {
 	fn code_by_hash(&self, _: H256, _: BlockId) -> Bytes {
 		Vec::new()
 	}
+
+	fn prove_transaction(&self, _: SignedTransaction, _: BlockId) -> Option<Vec<DBValue>> {
+		None
+	}
 }
 
 impl EngineClient for TestBlockChainClient {
diff --git a/ethcore/src/client/traits.rs b/ethcore/src/client/traits.rs
index 6e1ea9d31..4af20de0f 100644
--- a/ethcore/src/client/traits.rs
+++ b/ethcore/src/client/traits.rs
@@ -16,6 +16,7 @@
 
 use std::collections::BTreeMap;
 use util::{U256, Address, H256, H2048, Bytes, Itertools};
+use util::hashdb::DBValue;
 use blockchain::TreeRoute;
 use verification::queue::QueueInfo as BlockQueueInfo;
 use block::{OpenBlock, SealedBlock};
@@ -321,4 +322,7 @@ pub trait ProvingBlockChainClient: BlockChainClient {
 
 	/// Get code by address hash.
 	fn code_by_hash(&self, account_key: H256, id: BlockId) -> Bytes;
+
+	/// Prove execution of a transaction at the given block.
+	fn prove_transaction(&self, transaction: SignedTransaction, id: BlockId) -> Option<Vec<DBValue>>;
 }
diff --git a/ethcore/src/env_info.rs b/ethcore/src/env_info.rs
index 9e1bb6a40..cc42008d5 100644
--- a/ethcore/src/env_info.rs
+++ b/ethcore/src/env_info.rs
@@ -14,6 +14,8 @@
 // You should have received a copy of the GNU General Public License
 // along with Parity.  If not, see <http://www.gnu.org/licenses/>.
 
+//! Environment information for transaction execution.
+
 use std::cmp;
 use std::sync::Arc;
 use util::{U256, Address, H256, Hashable};
@@ -25,7 +27,7 @@ use ethjson;
 pub type LastHashes = Vec<H256>;
 
 /// Information concerning the execution environment for a message-call/contract-creation.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct EnvInfo {
 	/// The block number.
 	pub number: BlockNumber,
diff --git a/ethcore/src/lib.rs b/ethcore/src/lib.rs
index 2d3307b66..a78e2120f 100644
--- a/ethcore/src/lib.rs
+++ b/ethcore/src/lib.rs
@@ -79,7 +79,6 @@
 //!   cargo build --release
 //!   ```
 
-
 extern crate ethcore_io as io;
 extern crate rustc_serialize;
 extern crate crypto;
@@ -141,12 +140,12 @@ pub mod action_params;
 pub mod db;
 pub mod verification;
 pub mod state;
+pub mod env_info;
 #[macro_use] pub mod evm;
 
 mod cache_manager;
 mod blooms;
 mod basic_types;
-mod env_info;
 mod pod_account;
 mod state_db;
 mod account_db;
diff --git a/ethcore/src/state/backend.rs b/ethcore/src/state/backend.rs
index 81a770fe7..5ab620b0e 100644
--- a/ethcore/src/state/backend.rs
+++ b/ethcore/src/state/backend.rs
@@ -21,10 +21,12 @@
 //! should become general over time to the point where not even a
 //! merkle trie is strictly necessary.
 
+use std::collections::{HashSet, HashMap};
 use std::sync::Arc;
 
 use state::Account;
-use util::{Address, AsHashDB, HashDB, H256};
+use util::{Address, MemoryDB, Mutex, H256};
+use util::hashdb::{AsHashDB, HashDB, DBValue};
 
 /// State backend. See module docs for more details.
 pub trait Backend: Send {
@@ -64,21 +66,48 @@ pub trait Backend: Send {
 	fn is_known_null(&self, address: &Address) -> bool;
 }
 
-/// A raw backend which simply wraps a hashdb and does no caching.
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub struct NoCache<T>(T);
+/// A raw backend used to check proofs of execution.
+///
+/// This doesn't delete anything since execution proofs won't have mangled keys
+/// and we want to avoid collisions.
+// TODO: when account lookup moved into backends, this won't rely as tenuously on intended
+// usage.
+#[derive(Clone, PartialEq)]
+pub struct ProofCheck(MemoryDB);
 
-impl<T> NoCache<T> {
-	/// Create a new `NoCache` backend.
-	pub fn new(inner: T) -> Self { NoCache(inner) }
-
-	/// Consume the backend, yielding the inner database.
-	pub fn into_inner(self) -> T { self.0 }
+impl ProofCheck {
+	/// Create a new `ProofCheck` backend from the given state items.
+	pub fn new(proof: &[DBValue]) -> Self {
+		let mut db = MemoryDB::new();
+		for item in proof { db.insert(item); }
+		ProofCheck(db)
+	}
 }
 
-impl<T: AsHashDB + Send> Backend for NoCache<T> {
-	fn as_hashdb(&self) -> &HashDB { self.0.as_hashdb() }
-	fn as_hashdb_mut(&mut self) -> &mut HashDB { self.0.as_hashdb_mut() }
+impl HashDB for ProofCheck {
+	fn keys(&self) -> HashMap<H256, i32> { self.0.keys() }
+	fn get(&self, key: &H256) -> Option<DBValue> {
+		self.0.get(key)
+	}
+
+	fn contains(&self, key: &H256) -> bool {
+		self.0.contains(key)
+	}
+
+	fn insert(&mut self, value: &[u8]) -> H256 {
+		self.0.insert(value)
+	}
+
+	fn emplace(&mut self, key: H256, value: DBValue) {
+		self.0.emplace(key, value)
+	}
+
+	fn remove(&mut self, _key: &H256) { }
+}
+
+impl Backend for ProofCheck {
+	fn as_hashdb(&self) -> &HashDB { self }
+	fn as_hashdb_mut(&mut self) -> &mut HashDB { self }
 	fn add_to_account_cache(&mut self, _addr: Address, _data: Option<Account>, _modified: bool) {}
 	fn cache_code(&self, _hash: H256, _code: Arc<Vec<u8>>) {}
 	fn get_cached_account(&self, _addr: &Address) -> Option<Option<Account>> { None }
@@ -91,3 +120,104 @@ impl<T: AsHashDB + Send> Backend for NoCache<T> {
 	fn note_non_null_account(&self, _address: &Address) {}
 	fn is_known_null(&self, _address: &Address) -> bool { false }
 }
+
+/// Proving state backend.
+/// This keeps track of all state values loaded during usage of this backend.
+/// The proof-of-execution can be extracted with `extract_proof`.
+///
+/// This doesn't cache anything or rely on the canonical state caches.
+pub struct Proving<H: AsHashDB> {
+	base: H, // state we're proving values from.
+	changed: MemoryDB, // changed state via insertions.
+	proof: Mutex<HashSet<DBValue>>,
+}
+
+impl<H: AsHashDB + Send + Sync> HashDB for Proving<H> {
+	fn keys(&self) -> HashMap<H256, i32> {
+		let mut keys = self.base.as_hashdb().keys();
+		keys.extend(self.changed.keys());
+		keys
+	}
+
+	fn get(&self, key: &H256) -> Option<DBValue> {
+		match self.base.as_hashdb().get(key) {
+			Some(val) => {
+				self.proof.lock().insert(val.clone());
+				Some(val)
+			}
+			None => self.changed.get(key)
+		}
+	}
+
+	fn contains(&self, key: &H256) -> bool {
+		self.get(key).is_some()
+	}
+
+	fn insert(&mut self, value: &[u8]) -> H256 {
+		self.changed.insert(value)
+	}
+
+	fn emplace(&mut self, key: H256, value: DBValue) {
+		self.changed.emplace(key, value)
+	}
+
+	fn remove(&mut self, key: &H256) {
+		// only remove from `changed`
+		if self.changed.contains(key) {
+			self.changed.remove(key)
+		}
+	}
+}
+
+impl<H: AsHashDB + Send + Sync> Backend for Proving<H> {
+	fn as_hashdb(&self) -> &HashDB {
+		self
+	}
+
+	fn as_hashdb_mut(&mut self) -> &mut HashDB {
+		self
+	}
+
+	fn add_to_account_cache(&mut self, _: Address, _: Option<Account>, _: bool) { }
+
+	fn cache_code(&self, _: H256, _: Arc<Vec<u8>>) { }
+
+	fn get_cached_account(&self, _: &Address) -> Option<Option<Account>> { None }
+
+	fn get_cached<F, U>(&self, _: &Address, _: F) -> Option<U>
+		where F: FnOnce(Option<&mut Account>) -> U
+	{
+		None
+	}
+
+	fn get_cached_code(&self, _: &H256) -> Option<Arc<Vec<u8>>> { None }
+	fn note_non_null_account(&self, _: &Address) { }
+	fn is_known_null(&self, _: &Address) -> bool { false }
+}
+
+impl<H: AsHashDB> Proving<H> {
+	/// Create a new `Proving` over a base database.
+	/// This will store all values ever fetched from that base.
+	pub fn new(base: H) -> Self {
+		Proving {
+			base: base,
+			changed: MemoryDB::new(),
+			proof: Mutex::new(HashSet::new()),
+		}
+	}
+
+	/// Consume the backend, extracting the gathered proof.
+	pub fn extract_proof(self) -> Vec<DBValue> {
+		self.proof.into_inner().into_iter().collect()
+	}
+}
+
+impl<H: AsHashDB + Clone> Clone for Proving<H> {
+	fn clone(&self) -> Self {
+		Proving {
+			base: self.base.clone(),
+			changed: self.changed.clone(),
+			proof: Mutex::new(self.proof.lock().clone()),
+		}
+	}
+}
diff --git a/ethcore/src/state/mod.rs b/ethcore/src/state/mod.rs
index ebac907aa..3c5a3bc09 100644
--- a/ethcore/src/state/mod.rs
+++ b/ethcore/src/state/mod.rs
@@ -31,6 +31,7 @@ use factory::Factories;
 use trace::FlatTrace;
 use pod_account::*;
 use pod_state::{self, PodState};
+use types::executed::{Executed, ExecutionError};
 use types::state_diff::StateDiff;
 use transaction::SignedTransaction;
 use state_db::StateDB;
@@ -60,6 +61,17 @@ pub struct ApplyOutcome {
 /// Result type for the execution ("application") of a transaction.
 pub type ApplyResult = Result<ApplyOutcome, Error>;
 
+/// Return type of proof validity check.
+#[derive(Debug, Clone)]
+pub enum ProvedExecution {
+	/// Proof wasn't enough to complete execution.
+	BadProof,
+	/// The transaction failed, but not due to a bad proof.
+	Failed(ExecutionError),
+	/// The transaction successfully completd with the given proof.
+	Complete(Executed),
+}
+
 #[derive(Eq, PartialEq, Clone, Copy, Debug)]
 /// Account modification state. Used to check if the account was
 /// Modified in between commits and overall.
@@ -150,6 +162,39 @@ impl AccountEntry {
 	}
 }
 
+/// Check the given proof of execution.
+/// `Err(ExecutionError::Internal)` indicates failure, everything else indicates
+/// a successful proof (as the transaction itself may be poorly chosen).
+pub fn check_proof(
+	proof: &[::util::DBValue],
+	root: H256,
+	transaction: &SignedTransaction,
+	engine: &Engine,
+	env_info: &EnvInfo,
+) -> ProvedExecution {
+	let backend = self::backend::ProofCheck::new(proof);
+	let mut factories = Factories::default();
+	factories.accountdb = ::account_db::Factory::Plain;
+
+	let res = State::from_existing(
+		backend,
+		root,
+		engine.account_start_nonce(),
+		factories
+	);
+
+	let mut state = match res {
+		Ok(state) => state,
+		Err(_) => return ProvedExecution::BadProof,
+	};
+
+	match state.execute(env_info, engine, transaction, false) {
+		Ok(executed) => ProvedExecution::Complete(executed),
+		Err(ExecutionError::Internal(_)) => ProvedExecution::BadProof,
+		Err(e) => ProvedExecution::Failed(e),
+	}
+}
+
 /// Representation of the entire state of all accounts in the system.
 ///
 /// `State` can work together with `StateDB` to share account cache.
@@ -264,6 +309,19 @@ impl<B: Backend> State<B> {
 		Ok(state)
 	}
 
+	/// Swap the current backend for another.
+	// TODO: [rob] find a less hacky way to avoid duplication of `Client::state_at`.
+	pub fn replace_backend<T: Backend>(self, backend: T) -> State<T> {
+		State {
+			db: backend,
+			root: self.root,
+			cache: self.cache,
+			checkpoints: self.checkpoints,
+			account_start_nonce: self.account_start_nonce,
+			factories: self.factories,
+		}
+	}
+
 	/// Create a recoverable checkpoint of this state.
 	pub fn checkpoint(&mut self) {
 		self.checkpoints.get_mut().push(HashMap::new());
@@ -535,16 +593,12 @@ impl<B: Backend> State<B> {
 		Ok(())
 	}
 
-	/// Execute a given transaction.
+	/// Execute a given transaction, producing a receipt and an optional trace.
 	/// This will change the state accordingly.
 	pub fn apply(&mut self, env_info: &EnvInfo, engine: &Engine, t: &SignedTransaction, tracing: bool) -> ApplyResult {
 //		let old = self.to_pod();
 
-		let options = TransactOptions { tracing: tracing, vm_tracing: false, check_nonce: true };
-		let vm_factory = self.factories.vm.clone();
-		let e = Executive::new(self, env_info, engine, &vm_factory).transact(t, options)?;
-
-		// TODO uncomment once to_pod() works correctly.
+		let e = self.execute(env_info, engine, t, tracing)?;
 //		trace!("Applied transaction. Diff:\n{}\n", state_diff::diff_pod(&old, &self.to_pod()));
 		let state_root = if env_info.number < engine.params().eip98_transition {
 			self.commit()?;
@@ -557,6 +611,15 @@ impl<B: Backend> State<B> {
 		Ok(ApplyOutcome{receipt: receipt, trace: e.trace})
 	}
 
+	// Execute a given transaction.
+	fn execute(&mut self, env_info: &EnvInfo, engine: &Engine, t: &SignedTransaction, tracing: bool) -> Result<Executed, ExecutionError> {
+		let options = TransactOptions { tracing: tracing, vm_tracing: false, check_nonce: true };
+		let vm_factory = self.factories.vm.clone();
+
+		Executive::new(self, env_info, engine, &vm_factory).transact(t, options)
+	}
+
+
 	/// Commit accounts to SecTrieDBMut. This is similar to cpp-ethereum's dev::eth::commit.
 	/// `accounts` is mutable because we may need to commit the code or storage and record that.
 	#[cfg_attr(feature="dev", allow(match_ref_pats))]
diff --git a/ethcore/src/tests/client.rs b/ethcore/src/tests/client.rs
index 526738586..3734c5520 100644
--- a/ethcore/src/tests/client.rs
+++ b/ethcore/src/tests/client.rs
@@ -16,7 +16,8 @@
 
 use io::IoChannel;
 use client::{BlockChainClient, MiningBlockChainClient, Client, ClientConfig, BlockId};
-use state::CleanupMode;
+use state::{self, State, CleanupMode};
+use executive::Executive;
 use ethereum;
 use block::IsBlock;
 use tests::helpers::*;
@@ -341,3 +342,43 @@ fn does_not_propagate_delayed_transactions() {
 	assert_eq!(2, client.ready_transactions().len());
 	assert_eq!(2, client.miner().pending_transactions().len());
 }
+
+#[test]
+fn transaction_proof() {
+	use ::client::ProvingBlockChainClient;
+
+	let client_result = generate_dummy_client(0);
+	let client = client_result.reference();
+	let address = Address::random();
+	let test_spec = Spec::new_test();
+	for _ in 0..20 {
+		let mut b = client.prepare_open_block(Address::default(), (3141562.into(), 31415620.into()), vec![]);
+		b.block_mut().fields_mut().state.add_balance(&address, &5.into(), CleanupMode::NoEmpty).unwrap();
+		b.block_mut().fields_mut().state.commit().unwrap();
+		let b = b.close_and_lock().seal(&*test_spec.engine, vec![]).unwrap();
+		client.import_sealed_block(b).unwrap(); // account change is in the journal overlay
+	}
+
+	let transaction = Transaction {
+		nonce: 0.into(),
+		gas_price: 0.into(),
+		gas: 21000.into(),
+		action: Action::Call(Address::default()),
+		value: 5.into(),
+		data: Vec::new(),
+	}.fake_sign(address);
+
+	let proof = client.prove_transaction(transaction.clone(), BlockId::Latest).unwrap();
+	let backend = state::backend::ProofCheck::new(&proof);
+
+	let mut factories = ::factory::Factories::default();
+	factories.accountdb = ::account_db::Factory::Plain; // raw state values, no mangled keys.
+	let root = client.best_block_header().state_root();
+
+	let mut state = State::from_existing(backend, root, 0.into(), factories.clone()).unwrap();
+	Executive::new(&mut state, &client.latest_env_info(), &*test_spec.engine, &factories.vm)
+		.transact(&transaction, Default::default()).unwrap();
+
+	assert_eq!(state.balance(&Address::default()).unwrap(), 5.into());
+	assert_eq!(state.balance(&address).unwrap(), 95.into());
+}
diff --git a/rpc/src/v1/helpers/dispatch.rs b/rpc/src/v1/helpers/dispatch.rs
index 0bea7f9a1..b11ada048 100644
--- a/rpc/src/v1/helpers/dispatch.rs
+++ b/rpc/src/v1/helpers/dispatch.rs
@@ -158,6 +158,54 @@ impl<C: MiningBlockChainClient, M: MinerService> Dispatcher for FullDispatcher<C
 	}
 }
 
+/// Get a recent gas price corpus.
+// TODO: this could be `impl Trait`.
+pub fn fetch_gas_price_corpus(
+	sync: Arc<LightSync>,
+	client: Arc<LightChainClient>,
+	on_demand: Arc<OnDemand>,
+	cache: Arc<Mutex<LightDataCache>>,
+) -> BoxFuture<Corpus<U256>, Error> {
+	const GAS_PRICE_SAMPLE_SIZE: usize = 100;
+
+	if let Some(cached) = cache.lock().gas_price_corpus() {
+		return future::ok(cached).boxed()
+	}
+
+	let cache = cache.clone();
+	let eventual_corpus = sync.with_context(|ctx| {
+		// get some recent headers with gas used,
+		// and request each of the blocks from the network.
+		let block_futures = client.ancestry_iter(BlockId::Latest)
+			.filter(|hdr| hdr.gas_used() != U256::default())
+			.take(GAS_PRICE_SAMPLE_SIZE)
+			.map(request::Body::new)
+			.map(|req| on_demand.block(ctx, req));
+
+		// as the blocks come in, collect gas prices into a vector
+		stream::futures_unordered(block_futures)
+			.fold(Vec::new(), |mut v, block| {
+				for t in block.transaction_views().iter() {
+					v.push(t.gas_price())
+				}
+
+				future::ok(v)
+			})
+			.map(move |v| {
+				// produce a corpus from the vector, cache it, and return
+				// the median as the intended gas price.
+				let corpus: ::stats::Corpus<_> = v.into();
+				cache.lock().set_gas_price_corpus(corpus.clone());
+				corpus
+			})
+	});
+
+	match eventual_corpus {
+		Some(corp) => corp.map_err(|_| errors::no_light_peers()).boxed(),
+		None => future::err(errors::network_disabled()).boxed(),
+	}
+}
+
 /// Dispatcher for light clients -- fetches default gas price, next nonce, etc. from network.
 /// Light client `ETH` RPC.
 #[derive(Clone)]
@@ -197,44 +245,12 @@ impl LightDispatcher {
 	/// Get a recent gas price corpus.
 	// TODO: this could be `impl Trait`.
 	pub fn gas_price_corpus(&self) -> BoxFuture<Corpus<U256>, Error> {
-		const GAS_PRICE_SAMPLE_SIZE: usize = 100;
-
-		if let Some(cached) = self.cache.lock().gas_price_corpus() {
-			return future::ok(cached).boxed()
-		}
-
-		let cache = self.cache.clone();
-		let eventual_corpus = self.sync.with_context(|ctx| {
-			// get some recent headers with gas used,
-			// and request each of the blocks from the network.
-			let block_futures = self.client.ancestry_iter(BlockId::Latest)
-				.filter(|hdr| hdr.gas_used() != U256::default())
-				.take(GAS_PRICE_SAMPLE_SIZE)
-				.map(request::Body::new)
-				.map(|req| self.on_demand.block(ctx, req));
-
-			// as the blocks come in, collect gas prices into a vector
-			stream::futures_unordered(block_futures)
-				.fold(Vec::new(), |mut v, block| {
-					for t in block.transaction_views().iter() {
-						v.push(t.gas_price())
-					}
-
-					future::ok(v)
-				})
-				.map(move |v| {
-					// produce a corpus from the vector, cache it, and return
-					// the median as the intended gas price.
-					let corpus: ::stats::Corpus<_> = v.into();
-					cache.lock().set_gas_price_corpus(corpus.clone());
-					corpus
-				})
-		});
-
-		match eventual_corpus {
-			Some(corp) => corp.map_err(|_| errors::no_light_peers()).boxed(),
-			None => future::err(errors::network_disabled()).boxed(),
-		}
+		fetch_gas_price_corpus(
+			self.sync.clone(),
+			self.client.clone(),
+			self.on_demand.clone(),
+			self.cache.clone(),
+		)
 	}
 
 	/// Get an account's next nonce.
@@ -285,7 +301,12 @@ impl Dispatcher for LightDispatcher {
 		// fast path for known gas price.
 		match request_gas_price {
 			Some(gas_price) => future::ok(with_gas_price(gas_price)).boxed(),
-			None => self.gas_price_corpus().and_then(|corp| match corp.median() {
+			None => fetch_gas_price_corpus(
+				self.sync.clone(),
+				self.client.clone(),
+				self.on_demand.clone(),
+				self.cache.clone()
+			).and_then(|corp| match corp.median() {
 				Some(median) => future::ok(*median),
 				None => future::ok(DEFAULT_GAS_PRICE), // fall back to default on error.
 			}).map(with_gas_price).boxed()
diff --git a/rpc/src/v1/impls/eth.rs b/rpc/src/v1/impls/eth.rs
index 6a1702a44..6df8f5278 100644
--- a/rpc/src/v1/impls/eth.rs
+++ b/rpc/src/v1/impls/eth.rs
@@ -632,26 +632,34 @@ impl<C, SN: ?Sized, S: ?Sized, M, EM> Eth for EthClient<C, SN, S, M, EM> where
 		self.send_raw_transaction(raw)
 	}
 
-	fn call(&self, request: CallRequest, num: Trailing<BlockNumber>) -> Result<Bytes, Error> {
+	fn call(&self, request: CallRequest, num: Trailing<BlockNumber>) -> BoxFuture<Bytes, Error> {
 		let request = CallRequest::into(request);
-		let signed = self.sign_call(request)?;
-
-		let result = match num.0 {
-			BlockNumber::Pending => take_weak!(self.miner).call(&*take_weak!(self.client), &signed, Default::default()),
-			num => take_weak!(self.client).call(&signed, num.into(), Default::default()),
+		let signed = match self.sign_call(request) {
+			Ok(signed) => signed,
+			Err(e) => return future::err(e).boxed(),
 		};
 
-		result
+		let result = match num.0 {
+			BlockNumber::Pending => take_weakf!(self.miner).call(&*take_weakf!(self.client), &signed, Default::default()),
+			num => take_weakf!(self.client).call(&signed, num.into(), Default::default()),
+		};
+
+		future::done(result
 			.map(|b| b.output.into())
 			.map_err(errors::from_call_error)
+		).boxed()
 	}
 
-	fn estimate_gas(&self, request: CallRequest, num: Trailing<BlockNumber>) -> Result<RpcU256, Error> {
+	fn estimate_gas(&self, request: CallRequest, num: Trailing<BlockNumber>) -> BoxFuture<RpcU256, Error> {
 		let request = CallRequest::into(request);
-		let signed = self.sign_call(request)?;
-		take_weak!(self.client).estimate_gas(&signed, num.0.into())
+		let signed = match self.sign_call(request) {
+			Ok(signed) => signed,
+			Err(e) => return future::err(e).boxed(),
+		};
+		future::done(take_weakf!(self.client).estimate_gas(&signed, num.0.into())
 			.map(Into::into)
 			.map_err(errors::from_call_error)
+		).boxed()
 	}
 
 	fn compile_lll(&self, _: String) -> Result<Bytes, Error> {
diff --git a/rpc/src/v1/impls/light/eth.rs b/rpc/src/v1/impls/light/eth.rs
index 6251b67fc..f889faf00 100644
--- a/rpc/src/v1/impls/light/eth.rs
+++ b/rpc/src/v1/impls/light/eth.rs
@@ -24,6 +24,7 @@ use std::sync::Arc;
 use jsonrpc_core::Error;
 use jsonrpc_macros::Trailing;
 
+use light::cache::Cache as LightDataCache;
 use light::client::Client as LightClient;
 use light::{cht, TransactionQueue};
 use light::on_demand::{request, OnDemand};
@@ -31,17 +32,18 @@ use light::on_demand::{request, OnDemand};
 use ethcore::account_provider::{AccountProvider, DappId};
 use ethcore::basic_account::BasicAccount;
 use ethcore::encoded;
+use ethcore::executed::{Executed, ExecutionError};
 use ethcore::ids::BlockId;
-use ethcore::transaction::SignedTransaction;
+use ethcore::transaction::{Action, SignedTransaction, Transaction as EthTransaction};
 use ethsync::LightSync;
 use rlp::{UntrustedRlp, View};
 use util::sha3::{SHA3_NULL_RLP, SHA3_EMPTY_LIST_RLP};
-use util::{RwLock, U256};
+use util::{RwLock, Mutex, FixedHash, Uint, U256};
 
-use futures::{future, Future, BoxFuture};
+use futures::{future, Future, BoxFuture, IntoFuture};
 use futures::sync::oneshot;
 
-use v1::helpers::{CallRequest as CRequest, errors, limit_logs};
+use v1::helpers::{CallRequest as CRequest, errors, limit_logs, dispatch};
 use v1::helpers::block_import::is_major_importing;
 use v1::traits::Eth;
 use v1::types::{
@@ -60,6 +62,7 @@ pub struct EthClient {
 	on_demand: Arc<OnDemand>,
 	transaction_queue: Arc<RwLock<TransactionQueue>>,
 	accounts: Arc<AccountProvider>,
+	cache: Arc<Mutex<LightDataCache>>,
 }
 
 // helper for internal error: on demand sender cancelled.
@@ -67,6 +70,8 @@ fn err_premature_cancel(_cancel: oneshot::Canceled) -> Error {
 	errors::internal("on-demand sender prematurely cancelled", "")
 }
 
+type ExecutionResult = Result<Executed, ExecutionError>;
+
 impl EthClient {
 	/// Create a new `EthClient` with a handle to the light sync instance, client,
 	/// and on-demand request service, which is assumed to be attached as a handler.
@@ -76,6 +81,7 @@ impl EthClient {
 		on_demand: Arc<OnDemand>,
 		transaction_queue: Arc<RwLock<TransactionQueue>>,
 		accounts: Arc<AccountProvider>,
+		cache: Arc<Mutex<LightDataCache>>,
 	) -> Self {
 		EthClient {
 			sync: sync,
@@ -83,6 +89,7 @@ impl EthClient {
 			on_demand: on_demand,
 			transaction_queue: transaction_queue,
 			accounts: accounts,
+			cache: cache,
 		}
 	}
 
@@ -147,6 +154,80 @@ impl EthClient {
 				.unwrap_or_else(|| future::err(errors::network_disabled()).boxed())
 		}).boxed()
 	}
+
+	// helper for getting proved execution.
+	fn proved_execution(&self, req: CallRequest, num: Trailing<BlockNumber>) -> BoxFuture<ExecutionResult, Error> {
+		const DEFAULT_GAS_PRICE: U256 = U256([0, 0, 0, 21_000_000]);
+
+
+		let (sync, on_demand, client) = (self.sync.clone(), self.on_demand.clone(), self.client.clone());
+		let req: CRequest = req.into();
+		let id = num.0.into();
+
+		let from = req.from.unwrap_or(Address::zero());
+		let nonce_fut = match req.nonce {
+			Some(nonce) => future::ok(Some(nonce)).boxed(),
+			None => self.account(from, id).map(|acc| acc.map(|a| a.nonce)).boxed(),
+		};
+
+		let gas_price_fut = match req.gas_price {
+			Some(price) => future::ok(price).boxed(),
+			None => dispatch::fetch_gas_price_corpus(
+				self.sync.clone(),
+				self.client.clone(),
+				self.on_demand.clone(),
+				self.cache.clone(),
+			).map(|corp| match corp.median() {
+				Some(median) => *median,
+				None => DEFAULT_GAS_PRICE,
+			}).boxed()
+		};
+
+		// if nonce resolves, this should too since it'll be in the LRU-cache.
+		let header_fut = self.header(id);
+
+		// fetch missing transaction fields from the network.
+		nonce_fut.join(gas_price_fut).and_then(move |(nonce, gas_price)| {
+			let action = req.to.map_or(Action::Create, Action::Call);
+			let gas = req.gas.unwrap_or(U256::from(10_000_000)); // better gas amount?
+			let value = req.value.unwrap_or_else(U256::zero);
+			let data = req.data.map_or_else(Vec::new, |d| d.to_vec());
+
+			future::done(match nonce {
+				Some(n) => Ok(EthTransaction {
+					nonce: n,
+					action: action,
+					gas: gas,
+					gas_price: gas_price,
+					value: value,
+					data: data,
+				}.fake_sign(from)),
+				None => Err(errors::unknown_block()),
+			})
+		}).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),
+				_ => return future::err(errors::unknown_block()).boxed(),
+			};
+			let request = request::TransactionProof {
+				tx: tx,
+				header: hdr,
+				env_info: env_info,
+				engine: client.engine().clone(),
+			};
+
+			let proved_future = sync.with_context(move |ctx| {
+				on_demand.transaction_proof(ctx, request).map_err(err_premature_cancel).boxed()
+			});
+
+			match proved_future {
+				Some(fut) => fut.boxed(),
+				None => future::err(errors::network_disabled()).boxed(),
+			}
+		}).boxed()
+	}
 }
 
 impl Eth for EthClient {
@@ -322,12 +403,23 @@ impl Eth for EthClient {
 		self.send_raw_transaction(raw)
 	}
 
-	fn call(&self, req: CallRequest, num: Trailing<BlockNumber>) -> Result<Bytes, Error> {
-		Err(errors::unimplemented(None))
+	fn call(&self, req: CallRequest, num: Trailing<BlockNumber>) -> BoxFuture<Bytes, Error> {
+		self.proved_execution(req, num).and_then(|res| {
+			match res {
+				Ok(exec) => Ok(exec.output.into()),
+				Err(e) => Err(errors::execution(e)),
+			}
+		}).boxed()
 	}
 
-	fn estimate_gas(&self, req: CallRequest, num: Trailing<BlockNumber>) -> Result<RpcU256, Error> {
-		Err(errors::unimplemented(None))
+	fn estimate_gas(&self, req: CallRequest, num: Trailing<BlockNumber>) -> BoxFuture<RpcU256, Error> {
+		// TODO: binary chop for more accurate estimates.
+		self.proved_execution(req, num).and_then(|res| {
+			match res {
+				Ok(exec) => Ok((exec.refunded + exec.gas_used).into()),
+				Err(e) => Err(errors::execution(e)),
+			}
+		}).boxed()
 	}
 
 	fn transaction_by_hash(&self, hash: RpcH256) -> Result<Option<Transaction>, Error> {
@@ -355,19 +447,20 @@ impl Eth for EthClient {
 	}
 
 	fn compilers(&self) -> Result<Vec<String>, Error> {
-		Err(errors::unimplemented(None))
+		Err(errors::deprecated("Compilation functionality is deprecated.".to_string()))
+
 	}
 
-	fn compile_lll(&self, _code: String) -> Result<Bytes, Error> {
-		Err(errors::unimplemented(None))
+	fn compile_lll(&self, _: String) -> Result<Bytes, Error> {
+		Err(errors::deprecated("Compilation of LLL via RPC is deprecated".to_string()))
 	}
 
-	fn compile_solidity(&self, _code: String) -> Result<Bytes, Error> {
-		Err(errors::unimplemented(None))
+	fn compile_serpent(&self, _: String) -> Result<Bytes, Error> {
+		Err(errors::deprecated("Compilation of Serpent via RPC is deprecated".to_string()))
 	}
 
-	fn compile_serpent(&self, _code: String) -> Result<Bytes, Error> {
-		Err(errors::unimplemented(None))
+	fn compile_solidity(&self, _: String) -> Result<Bytes, Error> {
+		Err(errors::deprecated("Compilation of Solidity via RPC is deprecated".to_string()))
 	}
 
 	fn logs(&self, _filter: Filter) -> Result<Vec<Log>, Error> {
diff --git a/rpc/src/v1/traits/eth.rs b/rpc/src/v1/traits/eth.rs
index eaf608c60..365ad9320 100644
--- a/rpc/src/v1/traits/eth.rs
+++ b/rpc/src/v1/traits/eth.rs
@@ -110,12 +110,12 @@ build_rpc_trait! {
 		fn submit_transaction(&self, Bytes) -> Result<H256, Error>;
 
 		/// Call contract, returning the output data.
-		#[rpc(name = "eth_call")]
-		fn call(&self, CallRequest, Trailing<BlockNumber>) -> Result<Bytes, Error>;
+		#[rpc(async, name = "eth_call")]
+		fn call(&self, CallRequest, Trailing<BlockNumber>) -> BoxFuture<Bytes, Error>;
 
 		/// Estimate gas needed for execution of given contract.
-		#[rpc(name = "eth_estimateGas")]
-		fn estimate_gas(&self, CallRequest, Trailing<BlockNumber>) -> Result<U256, Error>;
+		#[rpc(async, name = "eth_estimateGas")]
+		fn estimate_gas(&self, CallRequest, Trailing<BlockNumber>) -> BoxFuture<U256, Error>;
 
 		/// Get transaction by its hash.
 		#[rpc(name = "eth_getTransactionByHash")]
diff --git a/sync/src/light_sync/tests/test_net.rs b/sync/src/light_sync/tests/test_net.rs
index b73da48bb..d0e472374 100644
--- a/sync/src/light_sync/tests/test_net.rs
+++ b/sync/src/light_sync/tests/test_net.rs
@@ -27,7 +27,7 @@ use ethcore::spec::Spec;
 use io::IoChannel;
 use light::client::Client as LightClient;
 use light::net::{LightProtocol, IoContext, Capabilities, Params as LightParams};
-use light::net::buffer_flow::FlowParams;
+use light::net::request_credits::FlowParams;
 use network::{NodeId, PeerId};
 use util::RwLock;
 
diff --git a/util/src/hashdb.rs b/util/src/hashdb.rs
index 3b1939cae..8217413ef 100644
--- a/util/src/hashdb.rs
+++ b/util/src/hashdb.rs
@@ -125,3 +125,13 @@ impl<T: HashDB> AsHashDB for T {
 		self
 	}
 }
+
+impl<'a> AsHashDB for &'a mut HashDB {
+	fn as_hashdb(&self) -> &HashDB {
+		&**self
+	}
+
+	fn as_hashdb_mut(&mut self) -> &mut HashDB {
+		&mut **self
+	}
+}