eth, eth/downloader: transition to eth 61

4 files changed, 606 insertions, 81 deletions

diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 1b31f5dbd..b4154c166 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -19,6 +19,11 @@ import (
 	"gopkg.in/fatih/set.v0"
 )
 
+const (
+	eth60 = 60 // Constant to check for old protocol support
+	eth61 = 61 // Constant to check for new protocol support
+)
+
 var (
 	MinHashFetch  = 512  // Minimum amount of hashes to not consider a peer stalling
 	MaxHashFetch  = 2048 // Amount of hashes to be fetched per retrieval request
@@ -58,6 +63,9 @@ type hashCheckFn func(common.Hash) bool
 // blockRetrievalFn is a callback type for retrieving a block from the local chain.
 type blockRetrievalFn func(common.Hash) *types.Block
 
+// headRetrievalFn is a callback type for retrieving the head block from the local chain.
+type headRetrievalFn func() *types.Block
+
 // chainInsertFn is a callback type to insert a batch of blocks into the local chain.
 type chainInsertFn func(types.Blocks) (int, error)
 
@@ -98,6 +106,7 @@ type Downloader struct {
 	// Callbacks
 	hasBlock    hashCheckFn      // Checks if a block is present in the chain
 	getBlock    blockRetrievalFn // Retrieves a block from the chain
+	headBlock   headRetrievalFn  // Retrieves the head block from the chain
 	insertChain chainInsertFn    // Injects a batch of blocks into the chain
 	dropPeer    peerDropFn       // Drops a peer for misbehaving
 
@@ -109,8 +118,9 @@ type Downloader struct {
 
 	// Channels
 	newPeerCh chan *peer
-	hashCh    chan hashPack
-	blockCh   chan blockPack
+	hashCh    chan hashPack  // Channel receiving inbound hashes
+	blockCh   chan blockPack // Channel receiving inbound blocks
+	processCh chan bool      // Channel to signal the block fetcher of new or finished work
 
 	cancelCh   chan struct{} // Channel to cancel mid-flight syncs
 	cancelLock sync.RWMutex  // Lock to protect the cancel channel in delivers
@@ -123,7 +133,7 @@ type Block struct {
 }
 
 // New creates a new downloader to fetch hashes and blocks from remote peers.
-func New(mux *event.TypeMux, hasBlock hashCheckFn, getBlock blockRetrievalFn, insertChain chainInsertFn, dropPeer peerDropFn) *Downloader {
+func New(mux *event.TypeMux, hasBlock hashCheckFn, getBlock blockRetrievalFn, headBlock headRetrievalFn, insertChain chainInsertFn, dropPeer peerDropFn) *Downloader {
 	// Create the base downloader
 	downloader := &Downloader{
 		mux:         mux,
@@ -131,11 +141,13 @@ func New(mux *event.TypeMux, hasBlock hashCheckFn, getBlock blockRetrievalFn, in
 		peers:       newPeerSet(),
 		hasBlock:    hasBlock,
 		getBlock:    getBlock,
+		headBlock:   headBlock,
 		insertChain: insertChain,
 		dropPeer:    dropPeer,
 		newPeerCh:   make(chan *peer, 1),
 		hashCh:      make(chan hashPack, 1),
 		blockCh:     make(chan blockPack, 1),
+		processCh:   make(chan bool, 1),
 	}
 	// Inject all the known bad hashes
 	downloader.banned = set.New()
@@ -175,7 +187,7 @@ func (d *Downloader) Synchronising() bool {
 
 // RegisterPeer injects a new download peer into the set of block source to be
 // used for fetching hashes and blocks from.
-func (d *Downloader) RegisterPeer(id string, version int, head common.Hash, getHashes hashFetcherFn, getBlocks blockFetcherFn) error {
+func (d *Downloader) RegisterPeer(id string, version int, head common.Hash, getRelHashes relativeHashFetcherFn, getAbsHashes absoluteHashFetcherFn, getBlocks blockFetcherFn) error {
 	// If the peer wants to send a banned hash, reject
 	if d.banned.Has(head) {
 		glog.V(logger.Debug).Infoln("Register rejected, head hash banned:", id)
@@ -183,7 +195,7 @@ func (d *Downloader) RegisterPeer(id string, version int, head common.Hash, getH
 	}
 	// Otherwise try to construct and register the peer
 	glog.V(logger.Detail).Infoln("Registering peer", id)
-	if err := d.peers.Register(newPeer(id, version, head, getHashes, getBlocks)); err != nil {
+	if err := d.peers.Register(newPeer(id, version, head, getRelHashes, getAbsHashes, getBlocks)); err != nil {
 		glog.V(logger.Error).Infoln("Register failed:", err)
 		return err
 	}
@@ -289,12 +301,38 @@ func (d *Downloader) syncWithPeer(p *peer, hash common.Hash) (err error) {
 		}
 	}()
 
-	glog.V(logger.Debug).Infoln("Synchronizing with the network using:", p.id)
-	if err = d.fetchHashes(p, hash); err != nil {
-		return err
-	}
-	if err = d.fetchBlocks(); err != nil {
-		return err
+	glog.V(logger.Debug).Infof("Synchronizing with the network using: %s, eth/%d", p.id, p.version)
+	switch p.version {
+	case eth60:
+		// Old eth/60 version, use reverse hash retrieval algorithm
+		if err = d.fetchHashes60(p, hash); err != nil {
+			return err
+		}
+		if err = d.fetchBlocks60(); err != nil {
+			return err
+		}
+	case eth61:
+		// New eth/61, use forward, concurrent hash and block retrieval algorithm
+		number, err := d.findAncestor(p)
+		if err != nil {
+			return err
+		}
+		errc := make(chan error, 2)
+		go func() { errc <- d.fetchHashes(p, number+1) }()
+		go func() { errc <- d.fetchBlocks(number + 1) }()
+
+		// If any fetcher fails, cancel the other
+		if err := <-errc; err != nil {
+			d.cancel()
+			<-errc
+			return err
+		}
+		return <-errc
+
+	default:
+		// Something very wrong, stop right here
+		glog.V(logger.Error).Infof("Unsupported eth protocol: %d", p.version)
+		return errBadPeer
 	}
 	glog.V(logger.Debug).Infoln("Synchronization completed")
 
@@ -326,10 +364,10 @@ func (d *Downloader) Terminate() {
 	d.cancel()
 }
 
-// fetchHahes starts retrieving hashes backwards from a specific peer and hash,
+// fetchHashes60 starts retrieving hashes backwards from a specific peer and hash,
 // up until it finds a common ancestor. If the source peer times out, alternative
 // ones are tried for continuation.
-func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
+func (d *Downloader) fetchHashes60(p *peer, h common.Hash) error {
 	var (
 		start  = time.Now()
 		active = p             // active peer will help determine the current active peer
@@ -346,12 +384,12 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 	<-timeout.C // timeout channel should be initially empty.
 
 	getHashes := func(from common.Hash) {
-		go active.getHashes(from)
+		go active.getRelHashes(from)
 		timeout.Reset(hashTTL)
 	}
 
 	// Add the hash to the queue, and start hash retrieval.
-	d.queue.Insert([]common.Hash{h})
+	d.queue.Insert([]common.Hash{h}, false)
 	getHashes(h)
 
 	attempted[p.id] = true
@@ -377,7 +415,7 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 				if d.banned.Has(hash) {
 					glog.V(logger.Debug).Infof("Peer (%s) sent a known invalid chain", active.id)
 
-					d.queue.Insert(hashPack.hashes[:index+1])
+					d.queue.Insert(hashPack.hashes[:index+1], false)
 					if err := d.banBlocks(active.id, hash); err != nil {
 						glog.V(logger.Debug).Infof("Failed to ban batch of blocks: %v", err)
 					}
@@ -395,7 +433,7 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 				}
 			}
 			// Insert all the new hashes, but only continue if got something useful
-			inserts := d.queue.Insert(hashPack.hashes)
+			inserts := d.queue.Insert(hashPack.hashes, false)
 			if len(inserts) == 0 && !done {
 				glog.V(logger.Debug).Infof("Peer (%s) responded with stale hashes", active.id)
 				return errBadPeer
@@ -422,9 +460,9 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 				continue
 			}
 			// We're done, prepare the download cache and proceed pulling the blocks
-			offset := 0
+			offset := uint64(0)
 			if block := d.getBlock(head); block != nil {
-				offset = int(block.NumberU64() + 1)
+				offset = block.NumberU64() + 1
 			}
 			d.queue.Prepare(offset)
 			finished = true
@@ -481,10 +519,10 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 	return nil
 }
 
-// fetchBlocks iteratively downloads the entire schedules block-chain, taking
+// fetchBlocks60 iteratively downloads the entire schedules block-chain, taking
 // any available peers, reserving a chunk of blocks for each, wait for delivery
 // and periodically checking for timeouts.
-func (d *Downloader) fetchBlocks() error {
+func (d *Downloader) fetchBlocks60() error {
 	glog.V(logger.Debug).Infoln("Downloading", d.queue.Pending(), "block(s)")
 	start := time.Now()
 
@@ -619,6 +657,323 @@ out:
 	return nil
 }
 
+// findAncestor tries to locate the common ancestor block of the local chain and
+// a remote peers blockchain. In the general case when our node was in sync and
+// on the correct chain, checking the top N blocks should already get us a match.
+// In the rare scenario when we ended up on a long soft fork (i.e. none of the
+// head blocks match), we do a binary search to find the common ancestor.
+func (d *Downloader) findAncestor(p *peer) (uint64, error) {
+	glog.V(logger.Debug).Infof("%v: looking for common ancestor", p)
+
+	// Request out head blocks to short circuit ancestor location
+	head := d.headBlock().NumberU64()
+	from := int64(head) - int64(MaxHashFetch)
+	if from < 0 {
+		from = 0
+	}
+	go p.getAbsHashes(uint64(from), MaxHashFetch)
+
+	// Wait for the remote response to the head fetch
+	number, hash := uint64(0), common.Hash{}
+	timeout := time.After(hashTTL)
+
+	for finished := false; !finished; {
+		select {
+		case <-d.cancelCh:
+			return 0, errCancelHashFetch
+
+		case hashPack := <-d.hashCh:
+			// Discard anything not from the origin peer
+			if hashPack.peerId != p.id {
+				glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", hashPack.peerId)
+				break
+			}
+			// Make sure the peer actually gave something valid
+			hashes := hashPack.hashes
+			if len(hashes) == 0 {
+				glog.V(logger.Debug).Infof("%v: empty head hash set", p)
+				return 0, errEmptyHashSet
+			}
+			// Check if a common ancestor was found
+			finished = true
+			for i := len(hashes) - 1; i >= 0; i-- {
+				if d.hasBlock(hashes[i]) {
+					number, hash = uint64(from)+uint64(i), hashes[i]
+					break
+				}
+			}
+
+		case <-d.blockCh:
+			// Out of bounds blocks received, ignore them
+
+		case <-timeout:
+			glog.V(logger.Debug).Infof("%v: head hash timeout", p)
+			return 0, errTimeout
+		}
+	}
+	// If the head fetch already found an ancestor, return
+	if !common.EmptyHash(hash) {
+		glog.V(logger.Debug).Infof("%v: common ancestor: #%d [%x]", p, number, hash[:4])
+		return number, nil
+	}
+	// Ancestor not found, we need to binary search over our chain
+	start, end := uint64(0), head
+	for start+1 < end {
+		// Split our chain interval in two, and request the hash to cross check
+		check := (start + end) / 2
+
+		timeout := time.After(hashTTL)
+		go p.getAbsHashes(uint64(check), 1)
+
+		// Wait until a reply arrives to this request
+		for arrived := false; !arrived; {
+			select {
+			case <-d.cancelCh:
+				return 0, errCancelHashFetch
+
+			case hashPack := <-d.hashCh:
+				// Discard anything not from the origin peer
+				if hashPack.peerId != p.id {
+					glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", hashPack.peerId)
+					break
+				}
+				// Make sure the peer actually gave something valid
+				hashes := hashPack.hashes
+				if len(hashes) != 1 {
+					glog.V(logger.Debug).Infof("%v: invalid search hash set (%d)", p, len(hashes))
+					return 0, errBadPeer
+				}
+				arrived = true
+
+				// Modify the search interval based on the response
+				block := d.getBlock(hashes[0])
+				if block == nil {
+					end = check
+					break
+				}
+				if block.NumberU64() != check {
+					glog.V(logger.Debug).Infof("%v: non requested hash #%d [%x], instead of #%d", p, block.NumberU64(), block.Hash().Bytes()[:4], check)
+					return 0, errBadPeer
+				}
+				start = check
+
+			case <-d.blockCh:
+				// Out of bounds blocks received, ignore them
+
+			case <-timeout:
+				glog.V(logger.Debug).Infof("%v: search hash timeout", p)
+				return 0, errTimeout
+			}
+		}
+	}
+	return start, nil
+}
+
+// fetchHashes keeps retrieving hashes from the requested number, until no more
+// are returned, potentially throttling on the way.
+func (d *Downloader) fetchHashes(p *peer, from uint64) error {
+	glog.V(logger.Debug).Infof("%v: downloading hashes from #%d", p, from)
+
+	// Create a timeout timer, and the associated hash fetcher
+	timeout := time.NewTimer(0) // timer to dump a non-responsive active peer
+	<-timeout.C                 // timeout channel should be initially empty
+	defer timeout.Stop()
+
+	getHashes := func(from uint64) {
+		go p.getAbsHashes(from, MaxHashFetch)
+		timeout.Reset(hashTTL)
+	}
+	// Start pulling hashes, until all are exhausted
+	getHashes(from)
+	for {
+		select {
+		case <-d.cancelCh:
+			return errCancelHashFetch
+
+		case hashPack := <-d.hashCh:
+			// Make sure the active peer is giving us the hashes
+			if hashPack.peerId != p.id {
+				glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", hashPack.peerId)
+				break
+			}
+			timeout.Stop()
+
+			// If no more hashes are inbound, notify the block fetcher and return
+			if len(hashPack.hashes) == 0 {
+				glog.V(logger.Debug).Infof("%v: no available hashes", p)
+
+				select {
+				case d.processCh <- false:
+				case <-d.cancelCh:
+				}
+				return nil
+			}
+			// Otherwise insert all the new hashes, aborting in case of junk
+			inserts := d.queue.Insert(hashPack.hashes, true)
+			if len(inserts) != len(hashPack.hashes) {
+				glog.V(logger.Debug).Infof("%v: stale hashes", p)
+				return errBadPeer
+			}
+			// Notify the block fetcher of new hashes, and continue fetching
+			select {
+			case d.processCh <- true:
+			default:
+			}
+			from += uint64(len(hashPack.hashes))
+			getHashes(from)
+
+		case <-timeout.C:
+			glog.V(logger.Debug).Infof("%v: hash request timed out", p)
+			return errTimeout
+		}
+	}
+}
+
+// fetchBlocks iteratively downloads the scheduled hashes, taking any available
+// peers, reserving a chunk of blocks for each, waiting for delivery and also
+// periodically checking for timeouts.
+func (d *Downloader) fetchBlocks(from uint64) error {
+	glog.V(logger.Debug).Infof("Downloading blocks from #%d", from)
+	defer glog.V(logger.Debug).Infof("Block download terminated")
+
+	// Create a timeout timer for scheduling expiration tasks
+	ticker := time.NewTicker(100 * time.Millisecond)
+	defer ticker.Stop()
+
+	update := make(chan struct{}, 1)
+
+	// Prepare the queue and fetch blocks until the hash fetcher's done
+	d.queue.Prepare(from)
+	finished := false
+
+	for {
+		select {
+		case <-d.cancelCh:
+			return errCancelBlockFetch
+
+		case blockPack := <-d.blockCh:
+			// If the peer was previously banned and failed to deliver it's pack
+			// in a reasonable time frame, ignore it's message.
+			if peer := d.peers.Peer(blockPack.peerId); peer != nil {
+				// Deliver the received chunk of blocks, and demote in case of errors
+				err := d.queue.Deliver(blockPack.peerId, blockPack.blocks)
+				switch err {
+				case nil:
+					// If no blocks were delivered, demote the peer (need the delivery above)
+					if len(blockPack.blocks) == 0 {
+						peer.Demote()
+						peer.SetIdle()
+						glog.V(logger.Detail).Infof("%s: no blocks delivered", peer)
+						break
+					}
+					// All was successful, promote the peer and potentially start processing
+					peer.Promote()
+					peer.SetIdle()
+					glog.V(logger.Detail).Infof("%s: delivered %d blocks", peer, len(blockPack.blocks))
+					go d.process()
+
+				case errInvalidChain:
+					// The hash chain is invalid (blocks are not ordered properly), abort
+					return err
+
+				case errNoFetchesPending:
+					// Peer probably timed out with its delivery but came through
+					// in the end, demote, but allow to to pull from this peer.
+					peer.Demote()
+					peer.SetIdle()
+					glog.V(logger.Detail).Infof("%s: out of bound delivery", peer)
+
+				case errStaleDelivery:
+					// Delivered something completely else than requested, usually
+					// caused by a timeout and delivery during a new sync cycle.
+					// Don't set it to idle as the original request should still be
+					// in flight.
+					peer.Demote()
+					glog.V(logger.Detail).Infof("%s: stale delivery", peer)
+
+				default:
+					// Peer did something semi-useful, demote but keep it around
+					peer.Demote()
+					peer.SetIdle()
+					glog.V(logger.Detail).Infof("%s: delivery partially failed: %v", peer, err)
+					go d.process()
+				}
+			}
+			// Blocks arrived, try to update the progress
+			select {
+			case update <- struct{}{}:
+			default:
+			}
+
+		case cont := <-d.processCh:
+			// The hash fetcher sent a continuation flag, check if it's done
+			if !cont {
+				finished = true
+			}
+			// Hashes arrive, try to update the progress
+			select {
+			case update <- struct{}{}:
+			default:
+			}
+
+		case <-ticker.C:
+			// Sanity check update the progress
+			select {
+			case update <- struct{}{}:
+			default:
+			}
+
+		case <-update:
+			// Short circuit if we lost all our peers
+			if d.peers.Len() == 0 {
+				return errNoPeers
+			}
+			// Check for block request timeouts and demote the responsible peers
+			for _, pid := range d.queue.Expire(blockHardTTL) {
+				if peer := d.peers.Peer(pid); peer != nil {
+					peer.Demote()
+					glog.V(logger.Detail).Infof("%s: block delivery timeout", peer)
+				}
+			}
+			// If there's noting more to fetch, wait or terminate
+			if d.queue.Pending() == 0 {
+				if d.queue.InFlight() == 0 && finished {
+					glog.V(logger.Debug).Infof("Block fetching completed")
+					return nil
+				}
+				break
+			}
+			// Send a download request to all idle peers, until throttled
+			for _, peer := range d.peers.IdlePeers() {
+				// Short circuit if throttling activated
+				if d.queue.Throttle() {
+					break
+				}
+				// Reserve a chunk of hashes for a peer. A nil can mean either that
+				// no more hashes are available, or that the peer is known not to
+				// have them.
+				request := d.queue.Reserve(peer, peer.Capacity())
+				if request == nil {
+					continue
+				}
+				if glog.V(logger.Detail) {
+					glog.Infof("%s: requesting %d blocks", peer, len(request.Hashes))
+				}
+				// Fetch the chunk and make sure any errors return the hashes to the queue
+				if err := peer.Fetch(request); err != nil {
+					glog.V(logger.Error).Infof("%v: fetch failed, rescheduling", peer)
+					d.queue.Cancel(request)
+				}
+			}
+			// Make sure that we have peers available for fetching. If all peers have been tried
+			// and all failed throw an error
+			if !d.queue.Throttle() && d.queue.InFlight() == 0 {
+				return errPeersUnavailable
+			}
+		}
+	}
+}
+
 // banBlocks retrieves a batch of blocks from a peer feeding us invalid hashes,
 // and bans the head of the retrieved batch.
 //
diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index 0e9b58005..c5fb00289 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -16,17 +16,12 @@ import (
 	"github.com/ethereum/go-ethereum/event"
 )
 
-const (
-	eth60 = 60
-	eth61 = 61
-)
-
 var (
 	testdb, _ = ethdb.NewMemDatabase()
 	genesis   = core.GenesisBlockForTesting(testdb, common.Address{}, big.NewInt(0))
 )
 
-// makeChain creates a chain of n blocks starting at and including
+// makeChain creates a chain of n blocks starting at but not including
 // parent. the returned hash chain is ordered head->parent.
 func makeChain(n int, seed byte, parent *types.Block) ([]common.Hash, map[common.Hash]*types.Block) {
 	blocks := core.GenerateChain(parent, testdb, n, func(i int, gen *core.BlockGen) {
@@ -47,7 +42,7 @@ func makeChain(n int, seed byte, parent *types.Block) ([]common.Hash, map[common
 // h2[:f] are different but have a common suffix of length n-f.
 func makeChainFork(n, f int, parent *types.Block) (h1, h2 []common.Hash, b1, b2 map[common.Hash]*types.Block) {
 	// Create the common suffix.
-	h, b := makeChain(n-f-1, 0, parent)
+	h, b := makeChain(n-f, 0, parent)
 	// Create the forks.
 	h1, b1 = makeChain(f, 1, b[h[0]])
 	h1 = append(h1, h[1:]...)
@@ -80,7 +75,7 @@ func newTester() *downloadTester {
 		peerHashes: make(map[string][]common.Hash),
 		peerBlocks: make(map[string]map[common.Hash]*types.Block),
 	}
-	tester.downloader = New(new(event.TypeMux), tester.hasBlock, tester.getBlock, tester.insertChain, tester.dropPeer)
+	tester.downloader = New(new(event.TypeMux), tester.hasBlock, tester.getBlock, tester.headBlock, tester.insertChain, tester.dropPeer)
 
 	return tester
 }
@@ -104,6 +99,11 @@ func (dl *downloadTester) getBlock(hash common.Hash) *types.Block {
 	return dl.ownBlocks[hash]
 }
 
+// headBlock retrieves the current head block from the canonical chain.
+func (dl *downloadTester) headBlock() *types.Block {
+	return dl.getBlock(dl.ownHashes[len(dl.ownHashes)-1])
+}
+
 // insertChain injects a new batch of blocks into the simulated chain.
 func (dl *downloadTester) insertChain(blocks types.Blocks) (int, error) {
 	for i, block := range blocks {
@@ -125,7 +125,7 @@ func (dl *downloadTester) newPeer(id string, version int, hashes []common.Hash,
 // specific delay time on processing the network packets sent to it, simulating
 // potentially slow network IO.
 func (dl *downloadTester) newSlowPeer(id string, version int, hashes []common.Hash, blocks map[common.Hash]*types.Block, delay time.Duration) error {
-	err := dl.downloader.RegisterPeer(id, version, hashes[0], dl.peerGetHashesFn(id, delay), dl.peerGetBlocksFn(id, delay))
+	err := dl.downloader.RegisterPeer(id, version, hashes[0], dl.peerGetRelHashesFn(id, delay), dl.peerGetAbsHashesFn(id, version, delay), dl.peerGetBlocksFn(id, delay))
 	if err == nil {
 		// Assign the owned hashes and blocks to the peer (deep copy)
 		dl.peerHashes[id] = make([]common.Hash, len(hashes))
@@ -146,10 +146,10 @@ func (dl *downloadTester) dropPeer(id string) {
 	dl.downloader.UnregisterPeer(id)
 }
 
-// peerGetBlocksFn constructs a getHashes function associated with a particular
+// peerGetRelHashesFn constructs a GetHashes function associated with a specific
 // peer in the download tester. The returned function can be used to retrieve
 // batches of hashes from the particularly requested peer.
-func (dl *downloadTester) peerGetHashesFn(id string, delay time.Duration) func(head common.Hash) error {
+func (dl *downloadTester) peerGetRelHashesFn(id string, delay time.Duration) func(head common.Hash) error {
 	return func(head common.Hash) error {
 		time.Sleep(delay)
 
@@ -179,13 +179,43 @@ func (dl *downloadTester) peerGetHashesFn(id string, delay time.Duration) func(h
 	}
 }
 
+// peerGetAbsHashesFn constructs a GetHashesFromNumber function associated with
+// a particular peer in the download tester. The returned function can be used to
+// retrieve batches of hashes from the particularly requested peer.
+func (dl *downloadTester) peerGetAbsHashesFn(id string, version int, delay time.Duration) func(uint64, int) error {
+	// If the simulated peer runs eth/60, this message is not supported
+	if version == eth60 {
+		return func(uint64, int) error { return nil }
+	}
+	// Otherwise create a method to request the blocks by number
+	return func(head uint64, count int) error {
+		time.Sleep(delay)
+
+		limit := count
+		if dl.maxHashFetch > 0 {
+			limit = dl.maxHashFetch
+		}
+		// Gather the next batch of hashes
+		hashes := dl.peerHashes[id]
+		result := make([]common.Hash, 0, limit)
+		for i := 0; i < limit && len(hashes)-int(head)-1-i >= 0; i++ {
+			result = append(result, hashes[len(hashes)-int(head)-1-i])
+		}
+		// Delay delivery a bit to allow attacks to unfold
+		go func() {
+			time.Sleep(time.Millisecond)
+			dl.downloader.DeliverHashes(id, result)
+		}()
+		return nil
+	}
+}
+
 // peerGetBlocksFn constructs a getBlocks function associated with a particular
 // peer in the download tester. The returned function can be used to retrieve
 // batches of blocks from the particularly requested peer.
 func (dl *downloadTester) peerGetBlocksFn(id string, delay time.Duration) func([]common.Hash) error {
 	return func(hashes []common.Hash) error {
 		time.Sleep(delay)
-
 		blocks := dl.peerBlocks[id]
 		result := make([]*types.Block, 0, len(hashes))
 		for _, hash := range hashes {
@@ -200,7 +230,7 @@ func (dl *downloadTester) peerGetBlocksFn(id string, delay time.Duration) func([
 }
 
 // Tests that simple synchronization, without throttling from a good peer works.
-func TestSynchronisation(t *testing.T) {
+func TestSynchronisation60(t *testing.T) {
 	// Create a small enough block chain to download and the tester
 	targetBlocks := blockCacheLimit - 15
 	hashes, blocks := makeChain(targetBlocks, 0, genesis)
@@ -217,42 +247,79 @@ func TestSynchronisation(t *testing.T) {
 	}
 }
 
-// Tests that an inactive downloader will not accept incoming hashes and blocks.
-func TestInactiveDownloader(t *testing.T) {
+// Tests that simple synchronization against a canonical chain works correctly.
+// In this test common ancestor lookup should be short circuited and not require
+// binary searching.
+func TestCanonicalSynchronisation(t *testing.T) {
+	// Create a small enough block chain to download
+	targetBlocks := blockCacheLimit - 15
+	hashes, blocks := makeChain(targetBlocks, 0, genesis)
+
 	tester := newTester()
+	tester.newPeer("peer", eth61, hashes, blocks)
 
-	// Check that neither hashes nor blocks are accepted
-	if err := tester.downloader.DeliverHashes("bad peer", []common.Hash{}); err != errNoSyncActive {
-		t.Errorf("error mismatch: have %v, want %v", err, errNoSyncActive)
+	// Synchronise with the peer and make sure all blocks were retrieved
+	if err := tester.sync("peer"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
-	if err := tester.downloader.DeliverBlocks("bad peer", []*types.Block{}); err != errNoSyncActive {
-		t.Errorf("error mismatch: have %v, want %v", err, errNoSyncActive)
+	if imported := len(tester.ownBlocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
 	}
 }
 
-// Tests that a canceled download wipes all previously accumulated state.
-func TestCancel(t *testing.T) {
-	// Create a small enough block chain to download and the tester
-	targetBlocks := blockCacheLimit - 15
+// Tests that if a large batch of blocks are being downloaded, it is throttled
+// until the cached blocks are retrieved.
+func TestThrottling60(t *testing.T) {
+	// Create a long block chain to download and the tester
+	targetBlocks := 8 * blockCacheLimit
 	hashes, blocks := makeChain(targetBlocks, 0, genesis)
 
 	tester := newTester()
 	tester.newPeer("peer", eth60, hashes, blocks)
 
-	// Make sure canceling works with a pristine downloader
-	tester.downloader.cancel()
-	hashCount, blockCount := tester.downloader.queue.Size()
-	if hashCount > 0 || blockCount > 0 {
-		t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
+	// Wrap the importer to allow stepping
+	done := make(chan int)
+	tester.downloader.insertChain = func(blocks types.Blocks) (int, error) {
+		n, err := tester.insertChain(blocks)
+		done <- n
+		return n, err
 	}
-	// Synchronise with the peer, but cancel afterwards
-	if err := tester.sync("peer"); err != nil {
-		t.Fatalf("failed to synchronise blocks: %v", err)
+	// Start a synchronisation concurrently
+	errc := make(chan error)
+	go func() {
+		errc <- tester.sync("peer")
+	}()
+	// Iteratively take some blocks, always checking the retrieval count
+	for len(tester.ownBlocks) < targetBlocks+1 {
+		// Wait a bit for sync to throttle itself
+		var cached int
+		for start := time.Now(); time.Since(start) < 3*time.Second; {
+			time.Sleep(25 * time.Millisecond)
+
+			cached = len(tester.downloader.queue.blockPool)
+			if cached == blockCacheLimit || len(tester.ownBlocks)+cached == targetBlocks+1 {
+				break
+			}
+		}
+		// Make sure we filled up the cache, then exhaust it
+		time.Sleep(25 * time.Millisecond) // give it a chance to screw up
+		if cached != blockCacheLimit && len(tester.ownBlocks)+cached < targetBlocks+1 {
+			t.Fatalf("block count mismatch: have %v, want %v", cached, blockCacheLimit)
+		}
+		<-done // finish previous blocking import
+		for cached > maxBlockProcess {
+			cached -= <-done
+		}
+		time.Sleep(25 * time.Millisecond) // yield to the insertion
 	}
-	tester.downloader.cancel()
-	hashCount, blockCount = tester.downloader.queue.Size()
-	if hashCount > 0 || blockCount > 0 {
-		t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
+	<-done // finish the last blocking import
+
+	// Check that we haven't pulled more blocks than available
+	if len(tester.ownBlocks) > targetBlocks+1 {
+		t.Fatalf("target block count mismatch: have %v, want %v", len(tester.ownBlocks), targetBlocks+1)
+	}
+	if err := <-errc; err != nil {
+		t.Fatalf("block synchronization failed: %v", err)
 	}
 }
 
@@ -264,7 +331,7 @@ func TestThrottling(t *testing.T) {
 	hashes, blocks := makeChain(targetBlocks, 0, genesis)
 
 	tester := newTester()
-	tester.newPeer("peer", eth60, hashes, blocks)
+	tester.newPeer("peer", eth61, hashes, blocks)
 
 	// Wrap the importer to allow stepping
 	done := make(chan int)
@@ -312,6 +379,102 @@ func TestThrottling(t *testing.T) {
 	}
 }
 
+// Tests that simple synchronization against a forked chain works correctly. In
+// this test common ancestor lookup should *not* be short circuited, and a full
+// binary search should be executed.
+func TestForkedSynchronisation(t *testing.T) {
+	// Create a long enough forked chain
+	common, fork := MaxHashFetch, 2*MaxHashFetch
+	hashesA, hashesB, blocksA, blocksB := makeChainFork(common+fork, fork, genesis)
+
+	tester := newTester()
+	tester.newPeer("fork A", eth61, hashesA, blocksA)
+	tester.newPeer("fork B", eth61, hashesB, blocksB)
+
+	// Synchronise with the peer and make sure all blocks were retrieved
+	if err := tester.sync("fork A"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	if imported := len(tester.ownBlocks); imported != common+fork+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, common+fork+1)
+	}
+	// Synchronise with the second peer and make sure that fork is pulled too
+	if err := tester.sync("fork B"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	if imported := len(tester.ownBlocks); imported != common+2*fork+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, common+2*fork+1)
+	}
+}
+
+// Tests that an inactive downloader will not accept incoming hashes and blocks.
+func TestInactiveDownloader(t *testing.T) {
+	tester := newTester()
+
+	// Check that neither hashes nor blocks are accepted
+	if err := tester.downloader.DeliverHashes("bad peer", []common.Hash{}); err != errNoSyncActive {
+		t.Errorf("error mismatch: have %v, want %v", err, errNoSyncActive)
+	}
+	if err := tester.downloader.DeliverBlocks("bad peer", []*types.Block{}); err != errNoSyncActive {
+		t.Errorf("error mismatch: have %v, want %v", err, errNoSyncActive)
+	}
+}
+
+// Tests that a canceled download wipes all previously accumulated state.
+func TestCancel60(t *testing.T) {
+	// Create a small enough block chain to download and the tester
+	targetBlocks := blockCacheLimit - 15
+	hashes, blocks := makeChain(targetBlocks, 0, genesis)
+
+	tester := newTester()
+	tester.newPeer("peer", eth60, hashes, blocks)
+
+	// Make sure canceling works with a pristine downloader
+	tester.downloader.cancel()
+	hashCount, blockCount := tester.downloader.queue.Size()
+	if hashCount > 0 || blockCount > 0 {
+		t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
+	}
+	// Synchronise with the peer, but cancel afterwards
+	if err := tester.sync("peer"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	tester.downloader.cancel()
+	hashCount, blockCount = tester.downloader.queue.Size()
+	if hashCount > 0 || blockCount > 0 {
+		t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
+	}
+}
+
+// Tests that a canceled download wipes all previously accumulated state.
+func TestCancel(t *testing.T) {
+	// Create a small enough block chain to download and the tester
+	targetBlocks := blockCacheLimit - 15
+	if targetBlocks >= MaxHashFetch {
+		targetBlocks = MaxHashFetch - 15
+	}
+	hashes, blocks := makeChain(targetBlocks, 0, genesis)
+
+	tester := newTester()
+	tester.newPeer("peer", eth61, hashes, blocks)
+
+	// Make sure canceling works with a pristine downloader
+	tester.downloader.cancel()
+	hashCount, blockCount := tester.downloader.queue.Size()
+	if hashCount > 0 || blockCount > 0 {
+		t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
+	}
+	// Synchronise with the peer, but cancel afterwards
+	if err := tester.sync("peer"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	tester.downloader.cancel()
+	hashCount, blockCount = tester.downloader.queue.Size()
+	if hashCount > 0 || blockCount > 0 {
+		t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
+	}
+}
+
 // Tests that synchronisation from multiple peers works as intended (multi thread sanity test).
 func TestMultiSynchronisation(t *testing.T) {
 	// Create various peers with various parts of the chain
diff --git a/eth/downloader/peer.go b/eth/downloader/peer.go
index 7176cc06b..bd58b4dc8 100644
--- a/eth/downloader/peer.go
+++ b/eth/downloader/peer.go
@@ -15,7 +15,8 @@ import (
 	"gopkg.in/fatih/set.v0"
 )
 
-type hashFetcherFn func(common.Hash) error
+type relativeHashFetcherFn func(common.Hash) error
+type absoluteHashFetcherFn func(uint64, int) error
 type blockFetcherFn func([]common.Hash) error
 
 var (
@@ -37,23 +38,25 @@ type peer struct {
 
 	ignored *set.Set // Set of hashes not to request (didn't have previously)
 
-	getHashes hashFetcherFn  // Method to retrieve a batch of hashes (mockable for testing)
-	getBlocks blockFetcherFn // Method to retrieve a batch of blocks (mockable for testing)
+	getRelHashes relativeHashFetcherFn // Method to retrieve a batch of hashes from an origin hash
+	getAbsHashes absoluteHashFetcherFn // Method to retrieve a batch of hashes from an absolute position
+	getBlocks    blockFetcherFn        // Method to retrieve a batch of blocks
 
 	version int // Eth protocol version number to switch strategies
 }
 
 // newPeer create a new downloader peer, with specific hash and block retrieval
 // mechanisms.
-func newPeer(id string, version int, head common.Hash, getHashes hashFetcherFn, getBlocks blockFetcherFn) *peer {
+func newPeer(id string, version int, head common.Hash, getRelHashes relativeHashFetcherFn, getAbsHashes absoluteHashFetcherFn, getBlocks blockFetcherFn) *peer {
 	return &peer{
-		id:        id,
-		head:      head,
-		capacity:  1,
-		getHashes: getHashes,
-		getBlocks: getBlocks,
-		ignored:   set.New(),
-		version:   version,
+		id:           id,
+		head:         head,
+		capacity:     1,
+		getRelHashes: getRelHashes,
+		getAbsHashes: getAbsHashes,
+		getBlocks:    getBlocks,
+		ignored:      set.New(),
+		version:      version,
 	}
 }
 
diff --git a/eth/downloader/queue.go b/eth/downloader/queue.go
index 903f043eb..b24ce42e8 100644
--- a/eth/downloader/queue.go
+++ b/eth/downloader/queue.go
@@ -40,9 +40,9 @@ type queue struct {
 
 	pendPool map[string]*fetchRequest // Currently pending block retrieval operations
 
-	blockPool   map[common.Hash]int // Hash-set of the downloaded data blocks, mapping to cache indexes
-	blockCache  []*Block            // Downloaded but not yet delivered blocks
-	blockOffset int                 // Offset of the first cached block in the block-chain
+	blockPool   map[common.Hash]uint64 // Hash-set of the downloaded data blocks, mapping to cache indexes
+	blockCache  []*Block               // Downloaded but not yet delivered blocks
+	blockOffset uint64                 // Offset of the first cached block in the block-chain
 
 	lock sync.RWMutex
 }
@@ -53,7 +53,7 @@ func newQueue() *queue {
 		hashPool:   make(map[common.Hash]int),
 		hashQueue:  prque.New(),
 		pendPool:   make(map[string]*fetchRequest),
-		blockPool:  make(map[common.Hash]int),
+		blockPool:  make(map[common.Hash]uint64),
 		blockCache: make([]*Block, blockCacheLimit),
 	}
 }
@@ -69,7 +69,7 @@ func (q *queue) Reset() {
 
 	q.pendPool = make(map[string]*fetchRequest)
 
-	q.blockPool = make(map[common.Hash]int)
+	q.blockPool = make(map[common.Hash]uint64)
 	q.blockOffset = 0
 	q.blockCache = make([]*Block, blockCacheLimit)
 }
@@ -130,7 +130,7 @@ func (q *queue) Has(hash common.Hash) bool {
 
 // Insert adds a set of hashes for the download queue for scheduling, returning
 // the new hashes encountered.
-func (q *queue) Insert(hashes []common.Hash) []common.Hash {
+func (q *queue) Insert(hashes []common.Hash, fifo bool) []common.Hash {
 	q.lock.Lock()
 	defer q.lock.Unlock()
 
@@ -147,7 +147,11 @@ func (q *queue) Insert(hashes []common.Hash) []common.Hash {
 		inserts = append(inserts, hash)
 
 		q.hashPool[hash] = q.hashCounter
-		q.hashQueue.Push(hash, float32(q.hashCounter)) // Highest gets schedules first
+		if fifo {
+			q.hashQueue.Push(hash, -float32(q.hashCounter)) // Lowest gets schedules first
+		} else {
+			q.hashQueue.Push(hash, float32(q.hashCounter)) // Highest gets schedules first
+		}
 	}
 	return inserts
 }
@@ -175,7 +179,7 @@ func (q *queue) GetBlock(hash common.Hash) *Block {
 		return nil
 	}
 	// Return the block if it's still available in the cache
-	if q.blockOffset <= index && index < q.blockOffset+len(q.blockCache) {
+	if q.blockOffset <= index && index < q.blockOffset+uint64(len(q.blockCache)) {
 		return q.blockCache[index-q.blockOffset]
 	}
 	return nil
@@ -202,7 +206,7 @@ func (q *queue) TakeBlocks() []*Block {
 	for k, n := len(q.blockCache)-len(blocks), len(q.blockCache); k < n; k++ {
 		q.blockCache[k] = nil
 	}
-	q.blockOffset += len(blocks)
+	q.blockOffset += uint64(len(blocks))
 
 	return blocks
 }
@@ -318,7 +322,7 @@ func (q *queue) Deliver(id string, blocks []*types.Block) (err error) {
 			continue
 		}
 		// If a requested block falls out of the range, the hash chain is invalid
-		index := int(block.NumberU64()) - q.blockOffset
+		index := int(int64(block.NumberU64()) - int64(q.blockOffset))
 		if index >= len(q.blockCache) || index < 0 {
 			return errInvalidChain
 		}
@@ -329,7 +333,7 @@ func (q *queue) Deliver(id string, blocks []*types.Block) (err error) {
 		}
 		delete(request.Hashes, hash)
 		delete(q.hashPool, hash)
-		q.blockPool[hash] = int(block.NumberU64())
+		q.blockPool[hash] = block.NumberU64()
 	}
 	// Return all failed or missing fetches to the queue
 	for hash, index := range request.Hashes {
@@ -346,7 +350,7 @@ func (q *queue) Deliver(id string, blocks []*types.Block) (err error) {
 }
 
 // Prepare configures the block cache offset to allow accepting inbound blocks.
-func (q *queue) Prepare(offset int) {
+func (q *queue) Prepare(offset uint64) {
 	q.lock.Lock()
 	defer q.lock.Unlock()