diff --git a/sync/src/light_sync/downloader.rs b/sync/src/light_sync/downloader.rs
new file mode 100644
index 000000000..dd78a3144
--- /dev/null
+++ b/sync/src/light_sync/downloader.rs
@@ -0,0 +1,173 @@
+// Copyright 2015, 2016 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Header download state machine.
+
+use std::collections::{HashMap, VecDeque};
+use std::mem;
+
+use ethcore::header::Header;
+
+use light::net::{EventContext, ReqId};
+use light::request::Headers as HeadersRequest;
+
+use network::PeerId;
+use rlp::{UntrustedRlp, View};
+use util::{Bytes, H256, Mutex};
+
+use super::{Error, Peer};
+use super::response::{self, Constraint};
+
+// amount of blocks between each scaffold entry.
+// TODO: move these into paraeters for `RoundStart::new`?
+const ROUND_SKIP: usize = 255;
+
+// amount of scaffold frames: these are the blank spaces in "X___X___X"
+const ROUND_FRAMES: u64 = 255;
+
+// number of attempts to make to get a full scaffold for a sync round.
+const SCAFFOLD_ATTEMPTS: usize = 3;
+
+// A request for headers with a known starting header
+// and a known parent hash for the last block.
+struct Request {
+	headers: HeadersRequest,
+	end_parent: H256,
+}
+
+pub struct Fetcher {
+	sparse: Vec<Header>, // sparse header chain.
+	requests: VecDeque<Request>,
+	pending: HashMap<ReqId, Request>,
+}
+
+impl Fetcher {
+	// Produce a new fetcher given a sparse headerchain, in ascending order.
+	// The headers must be valid RLP at this point.
+	fn new(sparse_headers: Vec<Header>) -> Self {
+		let mut requests = VecDeque::with_capacity(sparse_headers.len() - 1);
+		for pair in sparse_headers.windows(2) {
+			let low_rung = &pair[0];
+			let high_rung = &pair[1];
+
+			let diff = high_rung.number() - low_rung.number();
+			if diff < 2 { continue } // these headers are already adjacent.
+
+			let needed_headers = HeadersRequest {
+				start: high_rung.parent_hash().clone().into(),
+				max: diff as usize - 1,
+				skip: 0,
+				reverse: true,
+			};
+
+			requests.push_back(Request {
+				headers: needed_headers,
+				end_parent: low_rung.hash(),
+			});
+		}
+
+		Fetcher {
+			sparse: sparse_headers,
+			requests: requests,
+			pending: HashMap::new(),
+		}
+	}
+
+	fn process_response(self, req_id: ReqId, headers: &[Bytes]) -> (Downloader, Result<(), Error>) {
+		unimplemented!()
+	}
+}
+
+// Round started: get stepped header chain.
+// from a start block with number X we request 256 headers stepped by 256 from
+// block X + 1.
+struct RoundStart {
+	start_block: (u64, H256),
+	pending_req: Option<(ReqId, HeadersRequest)>,
+	sparse_headers: Vec<Header>,
+	attempt: 0,
+}
+
+impl RoundStart {
+	fn new(start: (u64, H256)) -> Self {
+		RoundStart {
+			start_block: start.clone(),
+			pending_req: None,
+			sparse_headers: Vec::new(),
+		}
+	}
+
+	fn process_response(mut self, req_id: ReqId, headers: &[Bytes]) -> (Downloader, Result<(), Error>) {
+		let req = match self.pending_req.take() {
+			Some((id, req)) if req_id == id { req.clone() }
+			other => {
+				self.pending_req = other;
+				return (self, Ok(()))
+			}
+		};
+
+		self.attempt += 1;
+		let headers = match response::decode_and_verify(headers, &req) {
+			Ok(headers) => {
+				self.sparse_headers.extend(headers);
+
+				if self.sparse_headers.len() == ROUND_FRAMES + 1
+					|| self.attempt >= SCAFFOLD_ATTEMPTS
+				{
+					let fetcher = Fetcher::new(self.sparse_headers);
+					(Downloader::Fetch(fetcher), Ok(()))
+				}
+			}
+		}
+	}
+}
+
+/// Downloader state machine.
+pub enum Downloader {
+	/// Waiting for peers.
+	Nothing,
+	/// Searching for common block with best chain.
+	SearchCommon,
+	/// Beginning a sync round.
+	RoundStart(RoundStart),
+	/// Fetching intermediate blocks during a sync round.
+	Fetch(Fetcher),
+}
+
+impl Downloader {
+	// Process an answer to a request. Unknown requests will be ignored.
+	fn process_response(self, req_id: ReqId, headers: &[Bytes]) -> (Self, Result<(), Error>) {
+		match self {
+			Downloader::RoundStart(round_start) => round_start.process_response(req_id, headers),
+			Downloader::Fetch(fetcher) => fetcher.process_response(req_id, headers),
+			other => (other, Ok(())),
+		}
+	}
+
+	// Return unfulfilled requests from disconnected peer. Unknown requests will be ignored.
+	fn requests_abandoned(self, abandoned: &[ReqId]) -> (Self, Result<(), Error>) {
+
+	}
+
+	// Dispatch pending requests. The dispatcher provided will attempt to
+	// find a suitable peer to serve the request.
+	// TODO: have dispatcher take capabilities argument?
+	fn dispatch_requests<D>(self, dispatcher: D) -> (Self, Result<(), Error>)
+		where D: Fn(HeadersRequest) -> Option<ReqId>
+	{
+		unimplemented!()
+	}
+}