Remote transaction execution (#4684)

* return errors on database corruption

* fix tests, json tests

* fix remainder of build

* buffer flow -> request credits

* proving state backend

* generate transaction proofs from provider

* network messages for transaction proof

* transaction proof test

* test for transaction proof message

* fix call bug

* request transaction proofs from on_demand

* most of proved_execution rpc

* proved execution future

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()
 	}

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"),

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()
 		}
 	}

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());
 	}
 }

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

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};
 

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,

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)
 	}

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::*;

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)

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,
+			}
 		}
 	}
 }