Merge pull request #3413 from zsfelfoldi/light-topic4

les, p2p/discv5: implement server pool, improve peer selection, light fetcher and topic searching

21 files changed, 1913 insertions, 546 deletions

diff --git a/eth/backend.go b/eth/backend.go
index d5b767b12..f98c9b724 100644
--- a/eth/backend.go
+++ b/eth/backend.go
@@ -104,6 +104,7 @@ type Config struct {
 
 type LesServer interface {
 	Start(srvr *p2p.Server)
+	Synced()
 	Stop()
 	Protocols() []p2p.Protocol
 }
@@ -145,6 +146,7 @@ type Ethereum struct {
 
 func (s *Ethereum) AddLesServer(ls LesServer) {
 	s.lesServer = ls
+	s.protocolManager.lesServer = ls
 }
 
 // New creates a new Ethereum object (including the
diff --git a/eth/handler.go b/eth/handler.go
index 8f05cc5b1..771e69b8d 100644
--- a/eth/handler.go
+++ b/eth/handler.go
@@ -87,6 +87,8 @@ type ProtocolManager struct {
 	quitSync    chan struct{}
 	noMorePeers chan struct{}
 
+	lesServer LesServer
+
 	// wait group is used for graceful shutdowns during downloading
 	// and processing
 	wg sync.WaitGroup
@@ -171,7 +173,7 @@ func NewProtocolManager(config *params.ChainConfig, fastSync bool, networkId int
 		return blockchain.CurrentBlock().NumberU64()
 	}
 	inserter := func(blocks types.Blocks) (int, error) {
-		atomic.StoreUint32(&manager.synced, 1) // Mark initial sync done on any fetcher import
+		manager.setSynced() // Mark initial sync done on any fetcher import
 		return manager.insertChain(blocks)
 	}
 	manager.fetcher = fetcher.New(blockchain.GetBlockByHash, validator, manager.BroadcastBlock, heighter, inserter, manager.removePeer)
diff --git a/eth/sync.go b/eth/sync.go
index 373cc2054..234534b4f 100644
--- a/eth/sync.go
+++ b/eth/sync.go
@@ -181,7 +181,7 @@ func (pm *ProtocolManager) synchronise(peer *peer) {
 	if err := pm.downloader.Synchronise(peer.id, pHead, pTd, mode); err != nil {
 		return
 	}
-	atomic.StoreUint32(&pm.synced, 1) // Mark initial sync done
+	pm.setSynced() // Mark initial sync done
 
 	// If fast sync was enabled, and we synced up, disable it
 	if atomic.LoadUint32(&pm.fastSync) == 1 {
@@ -192,3 +192,10 @@ func (pm *ProtocolManager) synchronise(peer *peer) {
 		}
 	}
 }
+
+// setSynced sets the synced flag and notifies the light server if present
+func (pm *ProtocolManager) setSynced() {
+	if atomic.SwapUint32(&pm.synced, 1) == 0 && pm.lesServer != nil {
+		pm.lesServer.Synced()
+	}
+}
diff --git a/les/fetcher.go b/les/fetcher.go
index ae9bf8474..c23af8da3 100644
--- a/les/fetcher.go
+++ b/les/fetcher.go
@@ -23,172 +23,426 @@ import (
 	"time"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/mclock"
 	"github.com/ethereum/go-ethereum/core"
 	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/light"
+	"github.com/ethereum/go-ethereum/logger"
+	"github.com/ethereum/go-ethereum/logger/glog"
 )
 
+const (
+	blockDelayTimeout = time.Second * 10 // timeout for a peer to announce a head that has already been confirmed by others
+	maxNodeCount      = 20               // maximum number of fetcherTreeNode entries remembered for each peer
+)
+
+// lightFetcher
 type lightFetcher struct {
 	pm    *ProtocolManager
 	odr   *LesOdr
-	chain BlockChain
-
-	headAnnouncedMu sync.Mutex
-	headAnnouncedBy map[common.Hash][]*peer
-	currentTd       *big.Int
-	deliverChn      chan fetchResponse
-	reqMu           sync.RWMutex
-	requested       map[uint64]fetchRequest
-	timeoutChn      chan uint64
-	notifyChn       chan bool // true if initiated from outside
-	syncing         bool
-	syncDone        chan struct{}
+	chain *light.LightChain
+
+	maxConfirmedTd  *big.Int
+	peers           map[*peer]*fetcherPeerInfo
+	lastUpdateStats *updateStatsEntry
+
+	lock       sync.Mutex // qwerqwerqwe
+	deliverChn chan fetchResponse
+	reqMu      sync.RWMutex
+	requested  map[uint64]fetchRequest
+	timeoutChn chan uint64
+	requestChn chan bool // true if initiated from outside
+	syncing    bool
+	syncDone   chan *peer
+}
+
+// fetcherPeerInfo holds fetcher-specific information about each active peer
+type fetcherPeerInfo struct {
+	root, lastAnnounced *fetcherTreeNode
+	nodeCnt             int
+	confirmedTd         *big.Int
+	bestConfirmed       *fetcherTreeNode
+	nodeByHash          map[common.Hash]*fetcherTreeNode
+	firstUpdateStats    *updateStatsEntry
 }
 
+// fetcherTreeNode is a node of a tree that holds information about blocks recently
+// announced and confirmed by a certain peer. Each new announce message from a peer
+// adds nodes to the tree, based on the previous announced head and the reorg depth.
+// There are three possible states for a tree node:
+// - announced: not downloaded (known) yet, but we know its head, number and td
+// - intermediate: not known, hash and td are empty, they are filled out when it becomes known
+// - known: both announced by this peer and downloaded (from any peer).
+// This structure makes it possible to always know which peer has a certain block,
+// which is necessary for selecting a suitable peer for ODR requests and also for
+// canonizing new heads. It also helps to always download the minimum necessary
+// amount of headers with a single request.
+type fetcherTreeNode struct {
+	hash             common.Hash
+	number           uint64
+	td               *big.Int
+	known, requested bool
+	parent           *fetcherTreeNode
+	children         []*fetcherTreeNode
+}
+
+// fetchRequest represents a header download request
 type fetchRequest struct {
-	hash   common.Hash
-	amount uint64
-	peer   *peer
+	hash    common.Hash
+	amount  uint64
+	peer    *peer
+	sent    mclock.AbsTime
+	timeout bool
 }
 
+// fetchResponse represents a header download response
 type fetchResponse struct {
 	reqID   uint64
 	headers []*types.Header
+	peer    *peer
 }
 
+// newLightFetcher creates a new light fetcher
 func newLightFetcher(pm *ProtocolManager) *lightFetcher {
 	f := &lightFetcher{
-		pm:              pm,
-		chain:           pm.blockchain,
-		odr:             pm.odr,
-		headAnnouncedBy: make(map[common.Hash][]*peer),
-		deliverChn:      make(chan fetchResponse, 100),
-		requested:       make(map[uint64]fetchRequest),
-		timeoutChn:      make(chan uint64),
-		notifyChn:       make(chan bool, 100),
-		syncDone:        make(chan struct{}),
-		currentTd:       big.NewInt(0),
+		pm:             pm,
+		chain:          pm.blockchain.(*light.LightChain),
+		odr:            pm.odr,
+		peers:          make(map[*peer]*fetcherPeerInfo),
+		deliverChn:     make(chan fetchResponse, 100),
+		requested:      make(map[uint64]fetchRequest),
+		timeoutChn:     make(chan uint64),
+		requestChn:     make(chan bool, 100),
+		syncDone:       make(chan *peer),
+		maxConfirmedTd: big.NewInt(0),
 	}
 	go f.syncLoop()
 	return f
 }
 
-func (f *lightFetcher) notify(p *peer, head *announceData) {
-	var headHash common.Hash
-	if head == nil {
-		// initial notify
-		headHash = p.Head()
-	} else {
-		if core.GetTd(f.pm.chainDb, head.Hash, head.Number) != nil {
-			head.haveHeaders = head.Number
-		}
-		//fmt.Println("notify", p.id, head.Number, head.ReorgDepth, head.haveHeaders)
-		if !p.addNotify(head) {
-			//fmt.Println("addNotify fail")
-			f.pm.removePeer(p.id)
+// syncLoop is the main event loop of the light fetcher
+func (f *lightFetcher) syncLoop() {
+	f.pm.wg.Add(1)
+	defer f.pm.wg.Done()
+
+	requestStarted := false
+	for {
+		select {
+		case <-f.pm.quitSync:
+			return
+		// when a new announce is received, request loop keeps running until
+		// no further requests are necessary or possible
+		case newAnnounce := <-f.requestChn:
+			f.lock.Lock()
+			s := requestStarted
+			requestStarted = false
+			if !f.syncing && !(newAnnounce && s) {
+				if peer, node, amount := f.nextRequest(); node != nil {
+					requestStarted = true
+					reqID, started := f.request(peer, node, amount)
+					if started {
+						go func() {
+							time.Sleep(softRequestTimeout)
+							f.reqMu.Lock()
+							req, ok := f.requested[reqID]
+							if ok {
+								req.timeout = true
+								f.requested[reqID] = req
+							}
+							f.reqMu.Unlock()
+							// keep starting new requests while possible
+							f.requestChn <- false
+						}()
+					}
+				}
+			}
+			f.lock.Unlock()
+		case reqID := <-f.timeoutChn:
+			f.reqMu.Lock()
+			req, ok := f.requested[reqID]
+			if ok {
+				delete(f.requested, reqID)
+			}
+			f.reqMu.Unlock()
+			if ok {
+				f.pm.serverPool.adjustResponseTime(req.peer.poolEntry, time.Duration(mclock.Now()-req.sent), true)
+				glog.V(logger.Debug).Infof("hard timeout by peer %v", req.peer.id)
+				go f.pm.removePeer(req.peer.id)
+			}
+		case resp := <-f.deliverChn:
+			f.reqMu.Lock()
+			req, ok := f.requested[resp.reqID]
+			if ok && req.peer != resp.peer {
+				ok = false
+			}
+			if ok {
+				delete(f.requested, resp.reqID)
+			}
+			f.reqMu.Unlock()
+			if ok {
+				f.pm.serverPool.adjustResponseTime(req.peer.poolEntry, time.Duration(mclock.Now()-req.sent), req.timeout)
+			}
+			f.lock.Lock()
+			if !ok || !(f.syncing || f.processResponse(req, resp)) {
+				glog.V(logger.Debug).Infof("failed processing response by peer %v", resp.peer.id)
+				go f.pm.removePeer(resp.peer.id)
+			}
+			f.lock.Unlock()
+		case p := <-f.syncDone:
+			f.lock.Lock()
+			glog.V(logger.Debug).Infof("done synchronising with peer %v", p.id)
+			f.checkSyncedHeaders(p)
+			f.syncing = false
+			f.lock.Unlock()
 		}
-		headHash = head.Hash
 	}
-	f.headAnnouncedMu.Lock()
-	f.headAnnouncedBy[headHash] = append(f.headAnnouncedBy[headHash], p)
-	f.headAnnouncedMu.Unlock()
-	f.notifyChn <- true
 }
 
-func (f *lightFetcher) gotHeader(header *types.Header) {
-	f.headAnnouncedMu.Lock()
-	defer f.headAnnouncedMu.Unlock()
+// addPeer adds a new peer to the fetcher's peer set
+func (f *lightFetcher) addPeer(p *peer) {
+	p.lock.Lock()
+	p.hasBlock = func(hash common.Hash, number uint64) bool {
+		return f.peerHasBlock(p, hash, number)
+	}
+	p.lock.Unlock()
+
+	f.lock.Lock()
+	defer f.lock.Unlock()
+
+	f.peers[p] = &fetcherPeerInfo{nodeByHash: make(map[common.Hash]*fetcherTreeNode)}
+}
+
+// removePeer removes a new peer from the fetcher's peer set
+func (f *lightFetcher) removePeer(p *peer) {
+	p.lock.Lock()
+	p.hasBlock = nil
+	p.lock.Unlock()
+
+	f.lock.Lock()
+	defer f.lock.Unlock()
+
+	// check for potential timed out block delay statistics
+	f.checkUpdateStats(p, nil)
+	delete(f.peers, p)
+}
+
+// announce processes a new announcement message received from a peer, adding new
+// nodes to the peer's block tree and removing old nodes if necessary
+func (f *lightFetcher) announce(p *peer, head *announceData) {
+	f.lock.Lock()
+	defer f.lock.Unlock()
+	glog.V(logger.Debug).Infof("received announce from peer %v  #%d  %016x  reorg: %d", p.id, head.Number, head.Hash[:8], head.ReorgDepth)
+
+	fp := f.peers[p]
+	if fp == nil {
+		glog.V(logger.Debug).Infof("announce: unknown peer")
+		return
+	}
 
-	hash := header.Hash()
-	peerList := f.headAnnouncedBy[hash]
-	if peerList == nil {
+	if fp.lastAnnounced != nil && head.Td.Cmp(fp.lastAnnounced.td) <= 0 {
+		// announced tds should be strictly monotonic
+		glog.V(logger.Debug).Infof("non-monotonic Td from peer %v", p.id)
+		go f.pm.removePeer(p.id)
 		return
 	}
-	number := header.Number.Uint64()
-	td := core.GetTd(f.pm.chainDb, hash, number)
-	for _, peer := range peerList {
-		peer.lock.Lock()
-		ok := peer.gotHeader(hash, number, td)
-		peer.lock.Unlock()
-		if !ok {
-			//fmt.Println("gotHeader fail")
-			f.pm.removePeer(peer.id)
+
+	n := fp.lastAnnounced
+	for i := uint64(0); i < head.ReorgDepth; i++ {
+		if n == nil {
+			break
 		}
+		n = n.parent
 	}
-	delete(f.headAnnouncedBy, hash)
-}
+	if n != nil {
+		// n is now the reorg common ancestor, add a new branch of nodes
+		// check if the node count is too high to add new nodes
+		locked := false
+		for uint64(fp.nodeCnt)+head.Number-n.number > maxNodeCount && fp.root != nil {
+			if !locked {
+				f.chain.LockChain()
+				defer f.chain.UnlockChain()
+				locked = true
+			}
+			// if one of root's children is canonical, keep it, delete other branches and root itself
+			var newRoot *fetcherTreeNode
+			for i, nn := range fp.root.children {
+				if core.GetCanonicalHash(f.pm.chainDb, nn.number) == nn.hash {
+					fp.root.children = append(fp.root.children[:i], fp.root.children[i+1:]...)
+					nn.parent = nil
+					newRoot = nn
+					break
+				}
+			}
+			fp.deleteNode(fp.root)
+			if n == fp.root {
+				n = newRoot
+			}
+			fp.root = newRoot
+			if newRoot == nil || !f.checkKnownNode(p, newRoot) {
+				fp.bestConfirmed = nil
+				fp.confirmedTd = nil
+			}
 
-func (f *lightFetcher) nextRequest() (*peer, *announceData) {
-	var bestPeer *peer
-	bestTd := f.currentTd
-	for _, peer := range f.pm.peers.AllPeers() {
-		peer.lock.RLock()
-		if !peer.headInfo.requested && (peer.headInfo.Td.Cmp(bestTd) > 0 ||
-			(bestPeer != nil && peer.headInfo.Td.Cmp(bestTd) == 0 && peer.headInfo.haveHeaders > bestPeer.headInfo.haveHeaders)) {
-			bestPeer = peer
-			bestTd = peer.headInfo.Td
+			if n == nil {
+				break
+			}
 		}
-		peer.lock.RUnlock()
-	}
-	if bestPeer == nil {
-		return nil, nil
-	}
-	bestPeer.lock.Lock()
-	res := bestPeer.headInfo
-	res.requested = true
-	bestPeer.lock.Unlock()
-	for _, peer := range f.pm.peers.AllPeers() {
-		if peer != bestPeer {
-			peer.lock.Lock()
-			if peer.headInfo.Hash == bestPeer.headInfo.Hash && peer.headInfo.haveHeaders == bestPeer.headInfo.haveHeaders {
-				peer.headInfo.requested = true
+		if n != nil {
+			for n.number < head.Number {
+				nn := &fetcherTreeNode{number: n.number + 1, parent: n}
+				n.children = append(n.children, nn)
+				n = nn
+				fp.nodeCnt++
 			}
-			peer.lock.Unlock()
+			n.hash = head.Hash
+			n.td = head.Td
+			fp.nodeByHash[n.hash] = n
 		}
 	}
-	return bestPeer, res
-}
+	if n == nil {
+		// could not find reorg common ancestor or had to delete entire tree, a new root and a resync is needed
+		if fp.root != nil {
+			fp.deleteNode(fp.root)
+		}
+		n = &fetcherTreeNode{hash: head.Hash, number: head.Number, td: head.Td}
+		fp.root = n
+		fp.nodeCnt++
+		fp.nodeByHash[n.hash] = n
+		fp.bestConfirmed = nil
+		fp.confirmedTd = nil
+	}
 
-func (f *lightFetcher) deliverHeaders(reqID uint64, headers []*types.Header) {
-	f.deliverChn <- fetchResponse{reqID: reqID, headers: headers}
+	f.checkKnownNode(p, n)
+	p.lock.Lock()
+	p.headInfo = head
+	fp.lastAnnounced = n
+	p.lock.Unlock()
+	f.checkUpdateStats(p, nil)
+	f.requestChn <- true
 }
 
-func (f *lightFetcher) requestedID(reqID uint64) bool {
-	f.reqMu.RLock()
-	_, ok := f.requested[reqID]
-	f.reqMu.RUnlock()
-	return ok
+// peerHasBlock returns true if we can assume the peer knows the given block
+// based on its announcements
+func (f *lightFetcher) peerHasBlock(p *peer, hash common.Hash, number uint64) bool {
+	f.lock.Lock()
+	defer f.lock.Unlock()
+
+	fp := f.peers[p]
+	if fp == nil || fp.root == nil {
+		return false
+	}
+
+	if number >= fp.root.number {
+		// it is recent enough that if it is known, is should be in the peer's block tree
+		return fp.nodeByHash[hash] != nil
+	}
+	f.chain.LockChain()
+	defer f.chain.UnlockChain()
+	// if it's older than the peer's block tree root but it's in the same canonical chain
+	// than the root, we can still be sure the peer knows it
+	return core.GetCanonicalHash(f.pm.chainDb, fp.root.number) == fp.root.hash && core.GetCanonicalHash(f.pm.chainDb, number) == hash
 }
 
-func (f *lightFetcher) request(p *peer, block *announceData) {
-	//fmt.Println("request", p.id, block.Number, block.haveHeaders)
-	amount := block.Number - block.haveHeaders
-	if amount == 0 {
-		return
+// request initiates a header download request from a certain peer
+func (f *lightFetcher) request(p *peer, n *fetcherTreeNode, amount uint64) (uint64, bool) {
+	fp := f.peers[p]
+	if fp == nil {
+		glog.V(logger.Debug).Infof("request: unknown peer")
+		return 0, false
 	}
-	if amount > 100 {
+	if fp.bestConfirmed == nil || fp.root == nil || !f.checkKnownNode(p, fp.root) {
 		f.syncing = true
 		go func() {
-			//fmt.Println("f.pm.synchronise(p)")
+			glog.V(logger.Debug).Infof("synchronising with peer %v", p.id)
 			f.pm.synchronise(p)
-			//fmt.Println("sync done")
-			f.syncDone <- struct{}{}
+			f.syncDone <- p
 		}()
-		return
+		return 0, false
 	}
 
-	reqID := f.odr.getNextReqID()
-	f.reqMu.Lock()
-	f.requested[reqID] = fetchRequest{hash: block.Hash, amount: amount, peer: p}
-	f.reqMu.Unlock()
+	reqID := getNextReqID()
+	n.requested = true
 	cost := p.GetRequestCost(GetBlockHeadersMsg, int(amount))
 	p.fcServer.SendRequest(reqID, cost)
-	go p.RequestHeadersByHash(reqID, cost, block.Hash, int(amount), 0, true)
+	f.reqMu.Lock()
+	f.requested[reqID] = fetchRequest{hash: n.hash, amount: amount, peer: p, sent: mclock.Now()}
+	f.reqMu.Unlock()
+	go p.RequestHeadersByHash(reqID, cost, n.hash, int(amount), 0, true)
 	go func() {
 		time.Sleep(hardRequestTimeout)
 		f.timeoutChn <- reqID
 	}()
+	return reqID, true
+}
+
+// requestAmount calculates the amount of headers to be downloaded starting
+// from a certain head backwards
+func (f *lightFetcher) requestAmount(p *peer, n *fetcherTreeNode) uint64 {
+	amount := uint64(0)
+	nn := n
+	for nn != nil && !f.checkKnownNode(p, nn) {
+		nn = nn.parent
+		amount++
+	}
+	if nn == nil {
+		amount = n.number
+	}
+	return amount
 }
 
+// requestedID tells if a certain reqID has been requested by the fetcher
+func (f *lightFetcher) requestedID(reqID uint64) bool {
+	f.reqMu.RLock()
+	_, ok := f.requested[reqID]
+	f.reqMu.RUnlock()
+	return ok
+}
+
+// nextRequest selects the peer and announced head to be requested next, amount
+// to be downloaded starting from the head backwards is also returned
+func (f *lightFetcher) nextRequest() (*peer, *fetcherTreeNode, uint64) {
+	var (
+		bestHash   common.Hash
+		bestAmount uint64
+	)
+	bestTd := f.maxConfirmedTd
+
+	for p, fp := range f.peers {
+		for hash, n := range fp.nodeByHash {
+			if !f.checkKnownNode(p, n) && !n.requested && (bestTd == nil || n.td.Cmp(bestTd) >= 0) {
+				amount := f.requestAmount(p, n)
+				if bestTd == nil || n.td.Cmp(bestTd) > 0 || amount < bestAmount {
+					bestHash = hash
+					bestAmount = amount
+					bestTd = n.td
+				}
+			}
+		}
+	}
+	if bestTd == f.maxConfirmedTd {
+		return nil, nil, 0
+	}
+
+	peer := f.pm.serverPool.selectPeer(func(p *peer) (bool, uint64) {
+		fp := f.peers[p]
+		if fp == nil || fp.nodeByHash[bestHash] == nil {
+			return false, 0
+		}
+		return true, p.fcServer.CanSend(p.GetRequestCost(GetBlockHeadersMsg, int(bestAmount)))
+	})
+	var node *fetcherTreeNode
+	if peer != nil {
+		node = f.peers[peer].nodeByHash[bestHash]
+	}
+	return peer, node, bestAmount
+}
+
+// deliverHeaders delivers header download request responses for processing
+func (f *lightFetcher) deliverHeaders(peer *peer, reqID uint64, headers []*types.Header) {
+	f.deliverChn <- fetchResponse{reqID: reqID, headers: headers, peer: peer}
+}
+
+// processResponse processes header download request responses
 func (f *lightFetcher) processResponse(req fetchRequest, resp fetchResponse) bool {
 	if uint64(len(resp.headers)) != req.amount || resp.headers[0].Hash() != req.hash {
 		return false
@@ -200,96 +454,248 @@ func (f *lightFetcher) processResponse(req fetchRequest, resp fetchResponse) boo
 	if _, err := f.chain.InsertHeaderChain(headers, 1); err != nil {
 		return false
 	}
-	for _, header := range headers {
-		td := core.GetTd(f.pm.chainDb, header.Hash(), header.Number.Uint64())
+	tds := make([]*big.Int, len(headers))
+	for i, header := range headers {
+		td := f.chain.GetTd(header.Hash(), header.Number.Uint64())
 		if td == nil {
 			return false
 		}
-		if td.Cmp(f.currentTd) > 0 {
-			f.currentTd = td
-		}
-		f.gotHeader(header)
+		tds[i] = td
 	}
+	f.newHeaders(headers, tds)
 	return true
 }
 
-func (f *lightFetcher) checkSyncedHeaders() {
-	//fmt.Println("checkSyncedHeaders()")
-	for _, peer := range f.pm.peers.AllPeers() {
-		peer.lock.Lock()
-		h := peer.firstHeadInfo
-		remove := false
-	loop:
-		for h != nil {
-			if td := core.GetTd(f.pm.chainDb, h.Hash, h.Number); td != nil {
-				//fmt.Println(" found", h.Number)
-				ok := peer.gotHeader(h.Hash, h.Number, td)
-				if !ok {
-					remove = true
-					break loop
-				}
-				if td.Cmp(f.currentTd) > 0 {
-					f.currentTd = td
-				}
-			}
-			h = h.next
+// newHeaders updates the block trees of all active peers according to a newly
+// downloaded and validated batch or headers
+func (f *lightFetcher) newHeaders(headers []*types.Header, tds []*big.Int) {
+	var maxTd *big.Int
+	for p, fp := range f.peers {
+		if !f.checkAnnouncedHeaders(fp, headers, tds) {
+			glog.V(logger.Debug).Infof("announce inconsistency by peer %v", p.id)
+			go f.pm.removePeer(p.id)
 		}
-		peer.lock.Unlock()
-		if remove {
-			//fmt.Println("checkSync fail")
-			f.pm.removePeer(peer.id)
+		if fp.confirmedTd != nil && (maxTd == nil || maxTd.Cmp(fp.confirmedTd) > 0) {
+			maxTd = fp.confirmedTd
 		}
 	}
+	if maxTd != nil {
+		f.updateMaxConfirmedTd(maxTd)
+	}
 }
 
-func (f *lightFetcher) syncLoop() {
-	f.pm.wg.Add(1)
-	defer f.pm.wg.Done()
+// checkAnnouncedHeaders updates peer's block tree if necessary after validating
+// a batch of headers. It searches for the latest header in the batch that has a
+// matching tree node (if any), and if it has not been marked as known already,
+// sets it and its parents to known (even those which are older than the currently
+// validated ones). Return value shows if all hashes, numbers and Tds matched
+// correctly to the announced values (otherwise the peer should be dropped).
+func (f *lightFetcher) checkAnnouncedHeaders(fp *fetcherPeerInfo, headers []*types.Header, tds []*big.Int) bool {
+	var (
+		n      *fetcherTreeNode
+		header *types.Header
+		td     *big.Int
+	)
 
-	srtoNotify := false
-	for {
-		select {
-		case <-f.pm.quitSync:
-			return
-		case ext := <-f.notifyChn:
-			//fmt.Println("<-f.notifyChn", f.syncing, ext, srtoNotify)
-			s := srtoNotify
-			srtoNotify = false
-			if !f.syncing && !(ext && s) {
-				if p, r := f.nextRequest(); r != nil {
-					srtoNotify = true
-					go func() {
-						time.Sleep(softRequestTimeout)
-						f.notifyChn <- false
-					}()
-					f.request(p, r)
+	for i := len(headers) - 1; ; i-- {
+		if i < 0 {
+			if n == nil {
+				// no more headers and nothing to match
+				return true
+			}
+			// we ran out of recently delivered headers but have not reached a node known by this peer yet, continue matching
+			td = f.chain.GetTd(header.ParentHash, header.Number.Uint64()-1)
+			header = f.chain.GetHeader(header.ParentHash, header.Number.Uint64()-1)
+		} else {
+			header = headers[i]
+			td = tds[i]
+		}
+		hash := header.Hash()
+		number := header.Number.Uint64()
+		if n == nil {
+			n = fp.nodeByHash[hash]
+		}
+		if n != nil {
+			if n.td == nil {
+				// node was unannounced
+				if nn := fp.nodeByHash[hash]; nn != nil {
+					// if there was already a node with the same hash, continue there and drop this one
+					nn.children = append(nn.children, n.children...)
+					n.children = nil
+					fp.deleteNode(n)
+					n = nn
+				} else {
+					n.hash = hash
+					n.td = td
+					fp.nodeByHash[hash] = n
 				}
 			}
-		case reqID := <-f.timeoutChn:
-			f.reqMu.Lock()
-			req, ok := f.requested[reqID]
-			if ok {
-				delete(f.requested, reqID)
+			// check if it matches the header
+			if n.hash != hash || n.number != number || n.td.Cmp(td) != 0 {
+				// peer has previously made an invalid announcement
+				return false
 			}
-			f.reqMu.Unlock()
-			if ok {
-				//fmt.Println("hard timeout")
-				f.pm.removePeer(req.peer.id)
+			if n.known {
+				// we reached a known node that matched our expectations, return with success
+				return true
 			}
-		case resp := <-f.deliverChn:
-			//fmt.Println("<-f.deliverChn", f.syncing)
-			f.reqMu.Lock()
-			req, ok := f.requested[resp.reqID]
-			delete(f.requested, resp.reqID)
-			f.reqMu.Unlock()
-			if !ok || !(f.syncing || f.processResponse(req, resp)) {
-				//fmt.Println("processResponse fail")
-				f.pm.removePeer(req.peer.id)
+			n.known = true
+			if fp.confirmedTd == nil || td.Cmp(fp.confirmedTd) > 0 {
+				fp.confirmedTd = td
+				fp.bestConfirmed = n
 			}
-		case <-f.syncDone:
-			//fmt.Println("<-f.syncDone", f.syncing)
-			f.checkSyncedHeaders()
-			f.syncing = false
+			n = n.parent
+			if n == nil {
+				return true
+			}
+		}
+	}
+}
+
+// checkSyncedHeaders updates peer's block tree after synchronisation by marking
+// downloaded headers as known. If none of the announced headers are found after
+// syncing, the peer is dropped.
+func (f *lightFetcher) checkSyncedHeaders(p *peer) {
+	fp := f.peers[p]
+	if fp == nil {
+		glog.V(logger.Debug).Infof("checkSyncedHeaders: unknown peer")
+		return
+	}
+	n := fp.lastAnnounced
+	var td *big.Int
+	for n != nil {
+		if td = f.chain.GetTd(n.hash, n.number); td != nil {
+			break
+		}
+		n = n.parent
+	}
+	// now n is the latest downloaded header after syncing
+	if n == nil {
+		glog.V(logger.Debug).Infof("synchronisation failed with peer %v", p.id)
+		go f.pm.removePeer(p.id)
+	} else {
+		header := f.chain.GetHeader(n.hash, n.number)
+		f.newHeaders([]*types.Header{header}, []*big.Int{td})
+	}
+}
+
+// checkKnownNode checks if a block tree node is known (downloaded and validated)
+// If it was not known previously but found in the database, sets its known flag
+func (f *lightFetcher) checkKnownNode(p *peer, n *fetcherTreeNode) bool {
+	if n.known {
+		return true
+	}
+	td := f.chain.GetTd(n.hash, n.number)
+	if td == nil {
+		return false
+	}
+
+	fp := f.peers[p]
+	if fp == nil {
+		glog.V(logger.Debug).Infof("checkKnownNode: unknown peer")
+		return false
+	}
+	header := f.chain.GetHeader(n.hash, n.number)
+	if !f.checkAnnouncedHeaders(fp, []*types.Header{header}, []*big.Int{td}) {
+		glog.V(logger.Debug).Infof("announce inconsistency by peer %v", p.id)
+		go f.pm.removePeer(p.id)
+	}
+	if fp.confirmedTd != nil {
+		f.updateMaxConfirmedTd(fp.confirmedTd)
+	}
+	return n.known
+}
+
+// deleteNode deletes a node and its child subtrees from a peer's block tree
+func (fp *fetcherPeerInfo) deleteNode(n *fetcherTreeNode) {
+	if n.parent != nil {
+		for i, nn := range n.parent.children {
+			if nn == n {
+				n.parent.children = append(n.parent.children[:i], n.parent.children[i+1:]...)
+				break
+			}
+		}
+	}
+	for {
+		if n.td != nil {
+			delete(fp.nodeByHash, n.hash)
+		}
+		fp.nodeCnt--
+		if len(n.children) == 0 {
+			return
+		}
+		for i, nn := range n.children {
+			if i == 0 {
+				n = nn
+			} else {
+				fp.deleteNode(nn)
+			}
+		}
+	}
+}
+
+// updateStatsEntry items form a linked list that is expanded with a new item every time a new head with a higher Td
+// than the previous one has been downloaded and validated. The list contains a series of maximum confirmed Td values
+// and the time these values have been confirmed, both increasing monotonically. A maximum confirmed Td is calculated
+// both globally for all peers and also for each individual peer (meaning that the given peer has announced the head
+// and it has also been downloaded from any peer, either before or after the given announcement).
+// The linked list has a global tail where new confirmed Td entries are added and a separate head for each peer,
+// pointing to the next Td entry that is higher than the peer's max confirmed Td (nil if it has already confirmed
+// the current global head).
+type updateStatsEntry struct {
+	time mclock.AbsTime
+	td   *big.Int
+	next *updateStatsEntry
+}
+
+// updateMaxConfirmedTd updates the block delay statistics of active peers. Whenever a new highest Td is confirmed,
+// adds it to the end of a linked list together with the time it has been confirmed. Then checks which peers have
+// already confirmed a head with the same or higher Td (which counts as zero block delay) and updates their statistics.
+// Those who have not confirmed such a head by now will be updated by a subsequent checkUpdateStats call with a
+// positive block delay value.
+func (f *lightFetcher) updateMaxConfirmedTd(td *big.Int) {
+	if f.maxConfirmedTd == nil || td.Cmp(f.maxConfirmedTd) > 0 {
+		f.maxConfirmedTd = td
+		newEntry := &updateStatsEntry{
+			time: mclock.Now(),
+			td:   td,
+		}
+		if f.lastUpdateStats != nil {
+			f.lastUpdateStats.next = newEntry
+		}
+		f.lastUpdateStats = newEntry
+		for p, _ := range f.peers {
+			f.checkUpdateStats(p, newEntry)
+		}
+	}
+}
+
+// checkUpdateStats checks those peers who have not confirmed a certain highest Td (or a larger one) by the time it
+// has been confirmed by another peer. If they have confirmed such a head by now, their stats are updated with the
+// block delay which is (this peer's confirmation time)-(first confirmation time). After blockDelayTimeout has passed,
+// the stats are updated with blockDelayTimeout value. In either case, the confirmed or timed out updateStatsEntry
+// items are removed from the head of the linked list.
+// If a new entry has been added to the global tail, it is passed as a parameter here even though this function
+// assumes that it has already been added, so that if the peer's list is empty (all heads confirmed, head is nil),
+// it can set the new head to newEntry.
+func (f *lightFetcher) checkUpdateStats(p *peer, newEntry *updateStatsEntry) {
+	now := mclock.Now()
+	fp := f.peers[p]
+	if fp == nil {
+		glog.V(logger.Debug).Infof("checkUpdateStats: unknown peer")
+		return
+	}
+	if newEntry != nil && fp.firstUpdateStats == nil {
+		fp.firstUpdateStats = newEntry
+	}
+	for fp.firstUpdateStats != nil && fp.firstUpdateStats.time <= now-mclock.AbsTime(blockDelayTimeout) {
+		f.pm.serverPool.adjustBlockDelay(p.poolEntry, blockDelayTimeout)
+		fp.firstUpdateStats = fp.firstUpdateStats.next
+	}
+	if fp.confirmedTd != nil {
+		for fp.firstUpdateStats != nil && fp.firstUpdateStats.td.Cmp(fp.confirmedTd) <= 0 {
+			f.pm.serverPool.adjustBlockDelay(p.poolEntry, time.Duration(now-fp.firstUpdateStats.time))
+			fp.firstUpdateStats = fp.firstUpdateStats.next
 		}
 	}
 }
diff --git a/les/handler.go b/les/handler.go
index fdf4e6e8a..b024841f2 100644
--- a/les/handler.go
+++ b/les/handler.go
@@ -22,8 +22,8 @@ import (
 	"errors"
 	"fmt"
 	"math/big"
+	"net"
 	"sync"
-	"time"
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core"
@@ -58,7 +58,7 @@ const (
 	MaxHeaderProofsFetch = 64  // Amount of merkle proofs to be fetched per retrieval request
 	MaxTxSend            = 64  // Amount of transactions to be send per request
 
-	disableClientRemovePeer = true
+	disableClientRemovePeer = false
 )
 
 // errIncompatibleConfig is returned if the requested protocols and configs are
@@ -101,10 +101,7 @@ type ProtocolManager struct {
 	chainDb     ethdb.Database
 	odr         *LesOdr
 	server      *LesServer
-
-	topicDisc *discv5.Network
-	lesTopic  discv5.Topic
-	p2pServer *p2p.Server
+	serverPool  *serverPool
 
 	downloader *downloader.Downloader
 	fetcher    *lightFetcher
@@ -157,13 +154,29 @@ func NewProtocolManager(chainConfig *params.ChainConfig, lightSync bool, network
 			Version: version,
 			Length:  ProtocolLengths[i],
 			Run: func(p *p2p.Peer, rw p2p.MsgReadWriter) error {
+				var entry *poolEntry
 				peer := manager.newPeer(int(version), networkId, p, rw)
+				if manager.serverPool != nil {
+					addr := p.RemoteAddr().(*net.TCPAddr)
+					entry = manager.serverPool.connect(peer, addr.IP, uint16(addr.Port))
+					if entry == nil {
+						return fmt.Errorf("unwanted connection")
+					}
+				}
+				peer.poolEntry = entry
 				select {
 				case manager.newPeerCh <- peer:
 					manager.wg.Add(1)
 					defer manager.wg.Done()
-					return manager.handle(peer)
+					err := manager.handle(peer)
+					if entry != nil {
+						manager.serverPool.disconnect(entry)
+					}
+					return err
 				case <-manager.quitSync:
+					if entry != nil {
+						manager.serverPool.disconnect(entry)
+					}
 					return p2p.DiscQuitting
 				}
 			},
@@ -192,7 +205,6 @@ func NewProtocolManager(chainConfig *params.ChainConfig, lightSync bool, network
 		manager.downloader = downloader.New(downloader.LightSync, chainDb, manager.eventMux, blockchain.HasHeader, nil, blockchain.GetHeaderByHash,
 			nil, blockchain.CurrentHeader, nil, nil, nil, blockchain.GetTdByHash,
 			blockchain.InsertHeaderChain, nil, nil, blockchain.Rollback, removePeer)
-		manager.fetcher = newLightFetcher(manager)
 	}
 
 	if odr != nil {
@@ -222,10 +234,12 @@ func (pm *ProtocolManager) removePeer(id string) {
 	glog.V(logger.Debug).Infof("LES: unregister peer %v", id)
 	if pm.lightSync {
 		pm.downloader.UnregisterPeer(id)
-		pm.odr.UnregisterPeer(peer)
 		if pm.txrelay != nil {
 			pm.txrelay.removePeer(id)
 		}
+		if pm.fetcher != nil {
+			pm.fetcher.removePeer(peer)
+		}
 	}
 	if err := pm.peers.Unregister(id); err != nil {
 		glog.V(logger.Error).Infoln("Removal failed:", err)
@@ -236,54 +250,26 @@ func (pm *ProtocolManager) removePeer(id string) {
 	}
 }
 
-func (pm *ProtocolManager) findServers() {
-	if pm.p2pServer == nil || pm.topicDisc == nil {
-		return
-	}
-	glog.V(logger.Debug).Infoln("Looking for topic", string(pm.lesTopic))
-	enodes := make(chan string, 100)
-	stop := make(chan struct{})
-	go pm.topicDisc.SearchTopic(pm.lesTopic, stop, enodes)
-	go func() {
-		added := make(map[string]bool)
-		for {
-			select {
-			case enode := <-enodes:
-				if !added[enode] {
-					glog.V(logger.Info).Infoln("Found LES server:", enode)
-					added[enode] = true
-					if node, err := discover.ParseNode(enode); err == nil {
-						pm.p2pServer.AddPeer(node)
-					}
-				}
-			case <-stop:
-				return
-			}
-		}
-	}()
-	select {
-	case <-time.After(time.Second * 20):
-	case <-pm.quitSync:
-	}
-	close(stop)
-}
-
 func (pm *ProtocolManager) Start(srvr *p2p.Server) {
-	pm.p2pServer = srvr
+	var topicDisc *discv5.Network
 	if srvr != nil {
-		pm.topicDisc = srvr.DiscV5
+		topicDisc = srvr.DiscV5
 	}
-	pm.lesTopic = discv5.Topic("LES@" + common.Bytes2Hex(pm.blockchain.Genesis().Hash().Bytes()[0:8]))
+	lesTopic := discv5.Topic("LES@" + common.Bytes2Hex(pm.blockchain.Genesis().Hash().Bytes()[0:8]))
 	if pm.lightSync {
 		// start sync handler
-		go pm.findServers()
+		if srvr != nil { // srvr is nil during testing
+			pm.serverPool = newServerPool(pm.chainDb, []byte("serverPool/"), srvr, lesTopic, pm.quitSync, &pm.wg)
+			pm.odr.serverPool = pm.serverPool
+			pm.fetcher = newLightFetcher(pm)
+		}
 		go pm.syncer()
 	} else {
-		if pm.topicDisc != nil {
+		if topicDisc != nil {
 			go func() {
-				glog.V(logger.Debug).Infoln("Starting registering topic", string(pm.lesTopic))
-				pm.topicDisc.RegisterTopic(pm.lesTopic, pm.quitSync)
-				glog.V(logger.Debug).Infoln("Stopped registering topic", string(pm.lesTopic))
+				glog.V(logger.Info).Infoln("Starting registering topic", string(lesTopic))
+				topicDisc.RegisterTopic(lesTopic, pm.quitSync)
+				glog.V(logger.Info).Infoln("Stopped registering topic", string(lesTopic))
 			}()
 		}
 		go func() {
@@ -352,13 +338,13 @@ func (pm *ProtocolManager) handle(p *peer) error {
 	glog.V(logger.Debug).Infof("LES: register peer %v", p.id)
 	if pm.lightSync {
 		requestHeadersByHash := func(origin common.Hash, amount int, skip int, reverse bool) error {
-			reqID := pm.odr.getNextReqID()
+			reqID := getNextReqID()
 			cost := p.GetRequestCost(GetBlockHeadersMsg, amount)
 			p.fcServer.SendRequest(reqID, cost)
 			return p.RequestHeadersByHash(reqID, cost, origin, amount, skip, reverse)
 		}
 		requestHeadersByNumber := func(origin uint64, amount int, skip int, reverse bool) error {
-			reqID := pm.odr.getNextReqID()
+			reqID := getNextReqID()
 			cost := p.GetRequestCost(GetBlockHeadersMsg, amount)
 			p.fcServer.SendRequest(reqID, cost)
 			return p.RequestHeadersByNumber(reqID, cost, origin, amount, skip, reverse)
@@ -367,12 +353,21 @@ func (pm *ProtocolManager) handle(p *peer) error {
 			requestHeadersByHash, requestHeadersByNumber, nil, nil, nil); err != nil {
 			return err
 		}
-		pm.odr.RegisterPeer(p)
 		if pm.txrelay != nil {
 			pm.txrelay.addPeer(p)
 		}
 
-		pm.fetcher.notify(p, nil)
+		p.lock.Lock()
+		head := p.headInfo
+		p.lock.Unlock()
+		if pm.fetcher != nil {
+			pm.fetcher.addPeer(p)
+			pm.fetcher.announce(p, head)
+		}
+
+		if p.poolEntry != nil {
+			pm.serverPool.registered(p.poolEntry)
+		}
 	}
 
 	stop := make(chan struct{})
@@ -454,7 +449,9 @@ func (pm *ProtocolManager) handleMsg(p *peer) error {
 			return errResp(ErrDecode, "%v: %v", msg, err)
 		}
 		glog.V(logger.Detail).Infoln("AnnounceMsg:", req.Number, req.Hash, req.Td, req.ReorgDepth)
-		pm.fetcher.notify(p, &req)
+		if pm.fetcher != nil {
+			go pm.fetcher.announce(p, &req)
+		}
 
 	case GetBlockHeadersMsg:
 		glog.V(logger.Debug).Infof("<=== GetBlockHeadersMsg from peer %v", p.id)
@@ -552,8 +549,8 @@ func (pm *ProtocolManager) handleMsg(p *peer) error {
 			return errResp(ErrDecode, "msg %v: %v", msg, err)
 		}
 		p.fcServer.GotReply(resp.ReqID, resp.BV)
-		if pm.fetcher.requestedID(resp.ReqID) {
-			pm.fetcher.deliverHeaders(resp.ReqID, resp.Headers)
+		if pm.fetcher != nil && pm.fetcher.requestedID(resp.ReqID) {
+			pm.fetcher.deliverHeaders(p, resp.ReqID, resp.Headers)
 		} else {
 			err := pm.downloader.DeliverHeaders(p.id, resp.Headers)
 			if err != nil {
diff --git a/les/helper_test.go b/les/helper_test.go
index a5428e954..2df3faab2 100644
--- a/les/helper_test.go
+++ b/les/helper_test.go
@@ -25,6 +25,7 @@ import (
 	"math/big"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core"
@@ -334,3 +335,13 @@ func (p *testPeer) handshake(t *testing.T, td *big.Int, head common.Hash, headNu
 func (p *testPeer) close() {
 	p.app.Close()
 }
+
+type testServerPool peer
+
+func (p *testServerPool) selectPeer(func(*peer) (bool, uint64)) *peer {
+	return (*peer)(p)
+}
+
+func (p *testServerPool) adjustResponseTime(*poolEntry, time.Duration, bool) {
+
+}
diff --git a/les/odr.go b/les/odr.go
index 444b1da2a..8878508c4 100644
--- a/les/odr.go
+++ b/les/odr.go
@@ -17,6 +17,8 @@
 package les
 
 import (
+	"crypto/rand"
+	"encoding/binary"
 	"sync"
 	"time"
 
@@ -37,6 +39,11 @@ var (
 // peerDropFn is a callback type for dropping a peer detected as malicious.
 type peerDropFn func(id string)
 
+type odrPeerSelector interface {
+	selectPeer(func(*peer) (bool, uint64)) *peer
+	adjustResponseTime(*poolEntry, time.Duration, bool)
+}
+
 type LesOdr struct {
 	light.OdrBackend
 	db           ethdb.Database
@@ -44,15 +51,13 @@ type LesOdr struct {
 	removePeer   peerDropFn
 	mlock, clock sync.Mutex
 	sentReqs     map[uint64]*sentReq
-	peers        *odrPeerSet
-	lastReqID    uint64
+	serverPool   odrPeerSelector
 }
 
 func NewLesOdr(db ethdb.Database) *LesOdr {
 	return &LesOdr{
 		db:       db,
 		stop:     make(chan struct{}),
-		peers:    newOdrPeerSet(),
 		sentReqs: make(map[uint64]*sentReq),
 	}
 }
@@ -77,16 +82,6 @@ type sentReq struct {
 	answered chan struct{} // closed and set to nil when any peer answers it
 }
 
-// RegisterPeer registers a new LES peer to the ODR capable peer set
-func (self *LesOdr) RegisterPeer(p *peer) error {
-	return self.peers.register(p)
-}
-
-// UnregisterPeer removes a peer from the ODR capable peer set
-func (self *LesOdr) UnregisterPeer(p *peer) {
-	self.peers.unregister(p)
-}
-
 const (
 	MsgBlockBodies = iota
 	MsgCode
@@ -142,29 +137,26 @@ func (self *LesOdr) requestPeer(req *sentReq, peer *peer, delivered, timeout cha
 
 	select {
 	case <-delivered:
-		servTime := uint64(mclock.Now() - stime)
-		self.peers.updateTimeout(peer, false)
-		self.peers.updateServTime(peer, servTime)
+		if self.serverPool != nil {
+			self.serverPool.adjustResponseTime(peer.poolEntry, time.Duration(mclock.Now()-stime), false)
+		}
 		return
 	case <-time.After(softRequestTimeout):
 		close(timeout)
-		if self.peers.updateTimeout(peer, true) {
-			self.removePeer(peer.id)
-		}
 	case <-self.stop:
 		return
 	}
 
 	select {
 	case <-delivered:
-		servTime := uint64(mclock.Now() - stime)
-		self.peers.updateServTime(peer, servTime)
-		return
 	case <-time.After(hardRequestTimeout):
-		self.removePeer(peer.id)
+		go self.removePeer(peer.id)
 	case <-self.stop:
 		return
 	}
+	if self.serverPool != nil {
+		self.serverPool.adjustResponseTime(peer.poolEntry, time.Duration(mclock.Now()-stime), true)
+	}
 }
 
 // networkRequest sends a request to known peers until an answer is received
@@ -176,7 +168,7 @@ func (self *LesOdr) networkRequest(ctx context.Context, lreq LesOdrRequest) erro
 		sentTo:   make(map[*peer]chan struct{}),
 		answered: answered, // reply delivered by any peer
 	}
-	reqID := self.getNextReqID()
+	reqID := getNextReqID()
 	self.mlock.Lock()
 	self.sentReqs[reqID] = req
 	self.mlock.Unlock()
@@ -193,7 +185,16 @@ func (self *LesOdr) networkRequest(ctx context.Context, lreq LesOdrRequest) erro
 
 	exclude := make(map[*peer]struct{})
 	for {
-		if peer := self.peers.bestPeer(lreq, exclude); peer == nil {
+		var p *peer
+		if self.serverPool != nil {
+			p = self.serverPool.selectPeer(func(p *peer) (bool, uint64) {
+				if !lreq.CanSend(p) {
+					return false, 0
+				}
+				return true, p.fcServer.CanSend(lreq.GetCost(p))
+			})
+		}
+		if p == nil {
 			select {
 			case <-ctx.Done():
 				return ctx.Err()
@@ -202,17 +203,17 @@ func (self *LesOdr) networkRequest(ctx context.Context, lreq LesOdrRequest) erro
 			case <-time.After(retryPeers):
 			}
 		} else {
-			exclude[peer] = struct{}{}
+			exclude[p] = struct{}{}
 			delivered := make(chan struct{})
 			timeout := make(chan struct{})
 			req.lock.Lock()
-			req.sentTo[peer] = delivered
+			req.sentTo[p] = delivered
 			req.lock.Unlock()
 			reqWg.Add(1)
-			cost := lreq.GetCost(peer)
-			peer.fcServer.SendRequest(reqID, cost)
-			go self.requestPeer(req, peer, delivered, timeout, reqWg)
-			lreq.Request(reqID, peer)
+			cost := lreq.GetCost(p)
+			p.fcServer.SendRequest(reqID, cost)
+			go self.requestPeer(req, p, delivered, timeout, reqWg)
+			lreq.Request(reqID, p)
 
 			select {
 			case <-ctx.Done():
@@ -239,10 +240,8 @@ func (self *LesOdr) Retrieve(ctx context.Context, req light.OdrRequest) (err err
 	return
 }
 
-func (self *LesOdr) getNextReqID() uint64 {
-	self.clock.Lock()
-	defer self.clock.Unlock()
-
-	self.lastReqID++
-	return self.lastReqID
+func getNextReqID() uint64 {
+	var rnd [8]byte
+	rand.Read(rnd[:])
+	return binary.BigEndian.Uint64(rnd[:])
 }
diff --git a/les/odr_peerset.go b/les/odr_peerset.go
deleted file mode 100644
index e9b7eec7f..000000000
--- a/les/odr_peerset.go
+++ /dev/null
@@ -1,120 +0,0 @@
-// Copyright 2016 The go-ethereum Authors
-// This file is part of the go-ethereum library.
-//
-// The go-ethereum library is free software: you can redistribute it and/or modify
-// it under the terms of the GNU Lesser General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// The go-ethereum library 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 Lesser General Public License for more details.
-//
-// You should have received a copy of the GNU Lesser General Public License
-// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
-
-package les
-
-import (
-	"sync"
-)
-
-const dropTimeoutRatio = 20
-
-type odrPeerInfo struct {
-	reqTimeSum, reqTimeCnt, reqCnt, timeoutCnt uint64
-}
-
-// odrPeerSet represents the collection of active peer participating in the block
-// download procedure.
-type odrPeerSet struct {
-	peers map[*peer]*odrPeerInfo
-	lock  sync.RWMutex
-}
-
-// newPeerSet creates a new peer set top track the active download sources.
-func newOdrPeerSet() *odrPeerSet {
-	return &odrPeerSet{
-		peers: make(map[*peer]*odrPeerInfo),
-	}
-}
-
-// Register injects a new peer into the working set, or returns an error if the
-// peer is already known.
-func (ps *odrPeerSet) register(p *peer) error {
-	ps.lock.Lock()
-	defer ps.lock.Unlock()
-
-	if _, ok := ps.peers[p]; ok {
-		return errAlreadyRegistered
-	}
-	ps.peers[p] = &odrPeerInfo{}
-	return nil
-}
-
-// Unregister removes a remote peer from the active set, disabling any further
-// actions to/from that particular entity.
-func (ps *odrPeerSet) unregister(p *peer) error {
-	ps.lock.Lock()
-	defer ps.lock.Unlock()
-
-	if _, ok := ps.peers[p]; !ok {
-		return errNotRegistered
-	}
-	delete(ps.peers, p)
-	return nil
-}
-
-func (ps *odrPeerSet) peerPriority(p *peer, info *odrPeerInfo, req LesOdrRequest) uint64 {
-	tm := p.fcServer.CanSend(req.GetCost(p))
-	if info.reqTimeCnt > 0 {
-		tm += info.reqTimeSum / info.reqTimeCnt
-	}
-	return tm
-}
-
-func (ps *odrPeerSet) bestPeer(req LesOdrRequest, exclude map[*peer]struct{}) *peer {
-	var best *peer
-	var bpv uint64
-	ps.lock.Lock()
-	defer ps.lock.Unlock()
-
-	for p, info := range ps.peers {
-		if _, ok := exclude[p]; !ok {
-			pv := ps.peerPriority(p, info, req)
-			if best == nil || pv < bpv {
-				best = p
-				bpv = pv
-			}
-		}
-	}
-	return best
-}
-
-func (ps *odrPeerSet) updateTimeout(p *peer, timeout bool) (drop bool) {
-	ps.lock.Lock()
-	defer ps.lock.Unlock()
-
-	if info, ok := ps.peers[p]; ok {
-		info.reqCnt++
-		if timeout {
-			// check ratio before increase to allow an extra timeout
-			if info.timeoutCnt*dropTimeoutRatio >= info.reqCnt {
-				return true
-			}
-			info.timeoutCnt++
-		}
-	}
-	return false
-}
-
-func (ps *odrPeerSet) updateServTime(p *peer, servTime uint64) {
-	ps.lock.Lock()
-	defer ps.lock.Unlock()
-
-	if info, ok := ps.peers[p]; ok {
-		info.reqTimeSum += servTime
-		info.reqTimeCnt++
-	}
-}
diff --git a/les/odr_requests.go b/les/odr_requests.go
index f4bd51888..a4fbd79f6 100644
--- a/les/odr_requests.go
+++ b/les/odr_requests.go
@@ -36,6 +36,7 @@ import (
 
 type LesOdrRequest interface {
 	GetCost(*peer) uint64
+	CanSend(*peer) bool
 	Request(uint64, *peer) error
 	Valid(ethdb.Database, *Msg) bool // if true, keeps the retrieved object
 }
@@ -66,6 +67,11 @@ func (self *BlockRequest) GetCost(peer *peer) uint64 {
 	return peer.GetRequestCost(GetBlockBodiesMsg, 1)
 }
 
+// CanSend tells if a certain peer is suitable for serving the given request
+func (self *BlockRequest) CanSend(peer *peer) bool {
+	return peer.HasBlock(self.Hash, self.Number)
+}
+
 // Request sends an ODR request to the LES network (implementation of LesOdrRequest)
 func (self *BlockRequest) Request(reqID uint64, peer *peer) error {
 	glog.V(logger.Debug).Infof("ODR: requesting body of block %08x from peer %v", self.Hash[:4], peer.id)
@@ -121,6 +127,11 @@ func (self *ReceiptsRequest) GetCost(peer *peer) uint64 {
 	return peer.GetRequestCost(GetReceiptsMsg, 1)
 }
 
+// CanSend tells if a certain peer is suitable for serving the given request
+func (self *ReceiptsRequest) CanSend(peer *peer) bool {
+	return peer.HasBlock(self.Hash, self.Number)
+}
+
 // Request sends an ODR request to the LES network (implementation of LesOdrRequest)
 func (self *ReceiptsRequest) Request(reqID uint64, peer *peer) error {
 	glog.V(logger.Debug).Infof("ODR: requesting receipts for block %08x from peer %v", self.Hash[:4], peer.id)
@@ -171,6 +182,11 @@ func (self *TrieRequest) GetCost(peer *peer) uint64 {
 	return peer.GetRequestCost(GetProofsMsg, 1)
 }
 
+// CanSend tells if a certain peer is suitable for serving the given request
+func (self *TrieRequest) CanSend(peer *peer) bool {
+	return peer.HasBlock(self.Id.BlockHash, self.Id.BlockNumber)
+}
+
 // Request sends an ODR request to the LES network (implementation of LesOdrRequest)
 func (self *TrieRequest) Request(reqID uint64, peer *peer) error {
 	glog.V(logger.Debug).Infof("ODR: requesting trie root %08x key %08x from peer %v", self.Id.Root[:4], self.Key[:4], peer.id)
@@ -221,6 +237,11 @@ func (self *CodeRequest) GetCost(peer *peer) uint64 {
 	return peer.GetRequestCost(GetCodeMsg, 1)
 }
 
+// CanSend tells if a certain peer is suitable for serving the given request
+func (self *CodeRequest) CanSend(peer *peer) bool {
+	return peer.HasBlock(self.Id.BlockHash, self.Id.BlockNumber)
+}
+
 // Request sends an ODR request to the LES network (implementation of LesOdrRequest)
 func (self *CodeRequest) Request(reqID uint64, peer *peer) error {
 	glog.V(logger.Debug).Infof("ODR: requesting node data for hash %08x from peer %v", self.Hash[:4], peer.id)
@@ -274,6 +295,14 @@ func (self *ChtRequest) GetCost(peer *peer) uint64 {
 	return peer.GetRequestCost(GetHeaderProofsMsg, 1)
 }
 
+// CanSend tells if a certain peer is suitable for serving the given request
+func (self *ChtRequest) CanSend(peer *peer) bool {
+	peer.lock.RLock()
+	defer peer.lock.RUnlock()
+
+	return self.ChtNum <= (peer.headInfo.Number-light.ChtConfirmations)/light.ChtFrequency
+}
+
 // Request sends an ODR request to the LES network (implementation of LesOdrRequest)
 func (self *ChtRequest) Request(reqID uint64, peer *peer) error {
 	glog.V(logger.Debug).Infof("ODR: requesting CHT #%d block #%d from peer %v", self.ChtNum, self.BlockNum, peer.id)
diff --git a/les/odr_test.go b/les/odr_test.go
index 27e3b283d..685435996 100644
--- a/les/odr_test.go
+++ b/les/odr_test.go
@@ -160,6 +160,8 @@ func testOdr(t *testing.T, protocol int, expFail uint64, fn odrTestFn) {
 	pm, db, odr := newTestProtocolManagerMust(t, false, 4, testChainGen)
 	lpm, ldb, odr := newTestProtocolManagerMust(t, true, 0, nil)
 	_, err1, lpeer, err2 := newTestPeerPair("peer", protocol, pm, lpm)
+	pool := (*testServerPool)(lpeer)
+	odr.serverPool = pool
 	select {
 	case <-time.After(time.Millisecond * 100):
 	case err := <-err1:
@@ -188,13 +190,13 @@ func testOdr(t *testing.T, protocol int, expFail uint64, fn odrTestFn) {
 	}
 
 	// temporarily remove peer to test odr fails
-	odr.UnregisterPeer(lpeer)
+	odr.serverPool = nil
 	// expect retrievals to fail (except genesis block) without a les peer
 	test(expFail)
-	odr.RegisterPeer(lpeer)
+	odr.serverPool = pool
 	// expect all retrievals to pass
 	test(5)
-	odr.UnregisterPeer(lpeer)
+	odr.serverPool = nil
 	// still expect all retrievals to pass, now data should be cached locally
 	test(5)
 }
diff --git a/les/peer.go b/les/peer.go
index 5d566d899..0a8db4975 100644
--- a/les/peer.go
+++ b/les/peer.go
@@ -51,12 +51,14 @@ type peer struct {
 
 	id string
 
-	firstHeadInfo, headInfo *announceData
-	headInfoLen             int
-	lock                    sync.RWMutex
+	headInfo *announceData
+	lock     sync.RWMutex
 
 	announceChn chan announceData
 
+	poolEntry *poolEntry
+	hasBlock  func(common.Hash, uint64) bool
+
 	fcClient       *flowcontrol.ClientNode // nil if the peer is server only
 	fcServer       *flowcontrol.ServerNode // nil if the peer is client only
 	fcServerParams *flowcontrol.ServerParams
@@ -109,67 +111,6 @@ func (p *peer) headBlockInfo() blockInfo {
 	return blockInfo{Hash: p.headInfo.Hash, Number: p.headInfo.Number, Td: p.headInfo.Td}
 }
 
-func (p *peer) addNotify(announce *announceData) bool {
-	p.lock.Lock()
-	defer p.lock.Unlock()
-
-	if announce.Td.Cmp(p.headInfo.Td) < 1 {
-		return false
-	}
-	if p.headInfoLen >= maxHeadInfoLen {
-		//return false
-		p.firstHeadInfo = p.firstHeadInfo.next
-		p.headInfoLen--
-	}
-	if announce.haveHeaders == 0 {
-		hh := p.headInfo.Number - announce.ReorgDepth
-		if p.headInfo.haveHeaders < hh {
-			hh = p.headInfo.haveHeaders
-		}
-		announce.haveHeaders = hh
-	}
-	p.headInfo.next = announce
-	p.headInfo = announce
-	p.headInfoLen++
-	return true
-}
-
-func (p *peer) gotHeader(hash common.Hash, number uint64, td *big.Int) bool {
-	h := p.firstHeadInfo
-	ptr := 0
-	for h != nil {
-		if h.Hash == hash {
-			if h.Number != number || h.Td.Cmp(td) != 0 {
-				return false
-			}
-			h.headKnown = true
-			h.haveHeaders = h.Number
-			p.firstHeadInfo = h
-			p.headInfoLen -= ptr
-			last := h
-			h = h.next
-			// propagate haveHeaders through the chain
-			for h != nil {
-				hh := last.Number - h.ReorgDepth
-				if last.haveHeaders < hh {
-					hh = last.haveHeaders
-				}
-				if hh > h.haveHeaders {
-					h.haveHeaders = hh
-				} else {
-					return true
-				}
-				last = h
-				h = h.next
-			}
-			return true
-		}
-		h = h.next
-		ptr++
-	}
-	return true
-}
-
 // Td retrieves the current total difficulty of a peer.
 func (p *peer) Td() *big.Int {
 	p.lock.RLock()
@@ -195,6 +136,9 @@ func sendResponse(w p2p.MsgWriter, msgcode, reqID, bv uint64, data interface{})
 }
 
 func (p *peer) GetRequestCost(msgcode uint64, amount int) uint64 {
+	p.lock.RLock()
+	defer p.lock.RUnlock()
+
 	cost := p.fcCosts[msgcode].baseCost + p.fcCosts[msgcode].reqCost*uint64(amount)
 	if cost > p.fcServerParams.BufLimit {
 		cost = p.fcServerParams.BufLimit
@@ -202,6 +146,14 @@ func (p *peer) GetRequestCost(msgcode uint64, amount int) uint64 {
 	return cost
 }
 
+// HasBlock checks if the peer has a given block
+func (p *peer) HasBlock(hash common.Hash, number uint64) bool {
+	p.lock.RLock()
+	hashBlock := p.hasBlock
+	p.lock.RUnlock()
+	return hashBlock != nil && hashBlock(hash, number)
+}
+
 // SendAnnounce announces the availability of a number of blocks through
 // a hash notification.
 func (p *peer) SendAnnounce(request announceData) error {
@@ -453,9 +405,7 @@ func (p *peer) Handshake(td *big.Int, head common.Hash, headNum uint64, genesis
 		p.fcCosts = MRC.decode()
 	}
 
-	p.firstHeadInfo = &announceData{Td: rTd, Hash: rHash, Number: rNum}
-	p.headInfo = p.firstHeadInfo
-	p.headInfoLen = 1
+	p.headInfo = &announceData{Td: rTd, Hash: rHash, Number: rNum}
 	return nil
 }
 
diff --git a/les/randselect.go b/les/randselect.go
new file mode 100644
index 000000000..1a9d0695b
--- /dev/null
+++ b/les/randselect.go
@@ -0,0 +1,173 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library 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 Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// Package les implements the Light Ethereum Subprotocol.
+package les
+
+import (
+	"math/rand"
+)
+
+// wrsItem interface should be implemented by any entries that are to be selected from
+// a weightedRandomSelect set. Note that recalculating monotonously decreasing item
+// weights on-demand (without constantly calling update) is allowed
+type wrsItem interface {
+	Weight() int64
+}
+
+// weightedRandomSelect is capable of weighted random selection from a set of items
+type weightedRandomSelect struct {
+	root *wrsNode
+	idx  map[wrsItem]int
+}
+
+// newWeightedRandomSelect returns a new weightedRandomSelect structure
+func newWeightedRandomSelect() *weightedRandomSelect {
+	return &weightedRandomSelect{root: &wrsNode{maxItems: wrsBranches}, idx: make(map[wrsItem]int)}
+}
+
+// update updates an item's weight, adds it if it was non-existent or removes it if
+// the new weight is zero. Note that explicitly updating decreasing weights is not necessary.
+func (w *weightedRandomSelect) update(item wrsItem) {
+	w.setWeight(item, item.Weight())
+}
+
+// remove removes an item from the set
+func (w *weightedRandomSelect) remove(item wrsItem) {
+	w.setWeight(item, 0)
+}
+
+// setWeight sets an item's weight to a specific value (removes it if zero)
+func (w *weightedRandomSelect) setWeight(item wrsItem, weight int64) {
+	idx, ok := w.idx[item]
+	if ok {
+		w.root.setWeight(idx, weight)
+		if weight == 0 {
+			delete(w.idx, item)
+		}
+	} else {
+		if weight != 0 {
+			if w.root.itemCnt == w.root.maxItems {
+				// add a new level
+				newRoot := &wrsNode{sumWeight: w.root.sumWeight, itemCnt: w.root.itemCnt, level: w.root.level + 1, maxItems: w.root.maxItems * wrsBranches}
+				newRoot.items[0] = w.root
+				newRoot.weights[0] = w.root.sumWeight
+				w.root = newRoot
+			}
+			w.idx[item] = w.root.insert(item, weight)
+		}
+	}
+}
+
+// choose randomly selects an item from the set, with a chance proportional to its
+// current weight. If the weight of the chosen element has been decreased since the
+// last stored value, returns it with a newWeight/oldWeight chance, otherwise just
+// updates its weight and selects another one
+func (w *weightedRandomSelect) choose() wrsItem {
+	for {
+		if w.root.sumWeight == 0 {
+			return nil
+		}
+		val := rand.Int63n(w.root.sumWeight)
+		choice, lastWeight := w.root.choose(val)
+		weight := choice.Weight()
+		if weight != lastWeight {
+			w.setWeight(choice, weight)
+		}
+		if weight >= lastWeight || rand.Int63n(lastWeight) < weight {
+			return choice
+		}
+	}
+}
+
+const wrsBranches = 8 // max number of branches in the wrsNode tree
+
+// wrsNode is a node of a tree structure that can store wrsItems or further wrsNodes.
+type wrsNode struct {
+	items                    [wrsBranches]interface{}
+	weights                  [wrsBranches]int64
+	sumWeight                int64
+	level, itemCnt, maxItems int
+}
+
+// insert recursively inserts a new item to the tree and returns the item index
+func (n *wrsNode) insert(item wrsItem, weight int64) int {
+	branch := 0
+	for n.items[branch] != nil && (n.level == 0 || n.items[branch].(*wrsNode).itemCnt == n.items[branch].(*wrsNode).maxItems) {
+		branch++
+		if branch == wrsBranches {
+			panic(nil)
+		}
+	}
+	n.itemCnt++
+	n.sumWeight += weight
+	n.weights[branch] += weight
+	if n.level == 0 {
+		n.items[branch] = item
+		return branch
+	} else {
+		var subNode *wrsNode
+		if n.items[branch] == nil {
+			subNode = &wrsNode{maxItems: n.maxItems / wrsBranches, level: n.level - 1}
+			n.items[branch] = subNode
+		} else {
+			subNode = n.items[branch].(*wrsNode)
+		}
+		subIdx := subNode.insert(item, weight)
+		return subNode.maxItems*branch + subIdx
+	}
+}
+
+// setWeight updates the weight of a certain item (which should exist) and returns
+// the change of the last weight value stored in the tree
+func (n *wrsNode) setWeight(idx int, weight int64) int64 {
+	if n.level == 0 {
+		oldWeight := n.weights[idx]
+		n.weights[idx] = weight
+		diff := weight - oldWeight
+		n.sumWeight += diff
+		if weight == 0 {
+			n.items[idx] = nil
+			n.itemCnt--
+		}
+		return diff
+	}
+	branchItems := n.maxItems / wrsBranches
+	branch := idx / branchItems
+	diff := n.items[branch].(*wrsNode).setWeight(idx-branch*branchItems, weight)
+	n.weights[branch] += diff
+	n.sumWeight += diff
+	if weight == 0 {
+		n.itemCnt--
+	}
+	return diff
+}
+
+// choose recursively selects an item from the tree and returns it along with its weight
+func (n *wrsNode) choose(val int64) (wrsItem, int64) {
+	for i, w := range n.weights {
+		if val < w {
+			if n.level == 0 {
+				return n.items[i].(wrsItem), n.weights[i]
+			} else {
+				return n.items[i].(*wrsNode).choose(val)
+			}
+		} else {
+			val -= w
+		}
+	}
+	panic(nil)
+}
diff --git a/les/randselect_test.go b/les/randselect_test.go
new file mode 100644
index 000000000..f3c34305e
--- /dev/null
+++ b/les/randselect_test.go
@@ -0,0 +1,67 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library 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 Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package les
+
+import (
+	"math/rand"
+	"testing"
+)
+
+type testWrsItem struct {
+	idx  int
+	widx *int
+}
+
+func (t *testWrsItem) Weight() int64 {
+	w := *t.widx
+	if w == -1 || w == t.idx {
+		return int64(t.idx + 1)
+	}
+	return 0
+}
+
+func TestWeightedRandomSelect(t *testing.T) {
+	testFn := func(cnt int) {
+		s := newWeightedRandomSelect()
+		w := -1
+		list := make([]testWrsItem, cnt)
+		for i, _ := range list {
+			list[i] = testWrsItem{idx: i, widx: &w}
+			s.update(&list[i])
+		}
+		w = rand.Intn(cnt)
+		c := s.choose()
+		if c == nil {
+			t.Errorf("expected item, got nil")
+		} else {
+			if c.(*testWrsItem).idx != w {
+				t.Errorf("expected another item")
+			}
+		}
+		w = -2
+		if s.choose() != nil {
+			t.Errorf("expected nil, got item")
+		}
+	}
+	testFn(1)
+	testFn(10)
+	testFn(100)
+	testFn(1000)
+	testFn(10000)
+	testFn(100000)
+	testFn(1000000)
+}
diff --git a/les/request_test.go b/les/request_test.go
index a6fbb06ce..03b946771 100644
--- a/les/request_test.go
+++ b/les/request_test.go
@@ -71,6 +71,8 @@ func testAccess(t *testing.T, protocol int, fn accessTestFn) {
 	pm, db, _ := newTestProtocolManagerMust(t, false, 4, testChainGen)
 	lpm, ldb, odr := newTestProtocolManagerMust(t, true, 0, nil)
 	_, err1, lpeer, err2 := newTestPeerPair("peer", protocol, pm, lpm)
+	pool := (*testServerPool)(lpeer)
+	odr.serverPool = pool
 	select {
 	case <-time.After(time.Millisecond * 100):
 	case err := <-err1:
@@ -100,11 +102,10 @@ func testAccess(t *testing.T, protocol int, fn accessTestFn) {
 	}
 
 	// temporarily remove peer to test odr fails
-	odr.UnregisterPeer(lpeer)
+	odr.serverPool = nil
 	// expect retrievals to fail (except genesis block) without a les peer
 	test(0)
-	odr.RegisterPeer(lpeer)
+	odr.serverPool = pool
 	// expect all retrievals to pass
 	test(5)
-	odr.UnregisterPeer(lpeer)
 }
diff --git a/les/server.go b/les/server.go
index c763e8c63..e55616a44 100644
--- a/les/server.go
+++ b/les/server.go
@@ -42,6 +42,9 @@ type LesServer struct {
 	fcManager       *flowcontrol.ClientManager // nil if our node is client only
 	fcCostStats     *requestCostStats
 	defParams       *flowcontrol.ServerParams
+	srvr            *p2p.Server
+	synced, stopped bool
+	lock            sync.Mutex
 }
 
 func NewLesServer(eth *eth.Ethereum, config *eth.Config) (*LesServer, error) {
@@ -67,12 +70,35 @@ func (s *LesServer) Protocols() []p2p.Protocol {
 	return s.protocolManager.SubProtocols
 }
 
+// Start only starts the actual service if the ETH protocol has already been synced,
+// otherwise it will be started by Synced()
 func (s *LesServer) Start(srvr *p2p.Server) {
-	s.protocolManager.Start(srvr)
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	s.srvr = srvr
+	if s.synced {
+		s.protocolManager.Start(s.srvr)
+	}
+}
+
+// Synced notifies the server that the ETH protocol has been synced and LES service can be started
+func (s *LesServer) Synced() {
+	s.lock.Lock()
+	defer s.lock.Unlock()
 
+	s.synced = true
+	if s.srvr != nil && !s.stopped {
+		s.protocolManager.Start(s.srvr)
+	}
 }
 
+// Stop stops the LES service
 func (s *LesServer) Stop() {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	s.stopped = true
 	s.fcCostStats.store()
 	s.fcManager.Stop()
 	go func() {
@@ -323,9 +349,8 @@ func (pm *ProtocolManager) blockLoop() {
 }
 
 var (
-	lastChtKey       = []byte("LastChtNumber") // chtNum (uint64 big endian)
-	chtPrefix        = []byte("cht")           // chtPrefix + chtNum (uint64 big endian) -> trie root hash
-	chtConfirmations = light.ChtFrequency / 2
+	lastChtKey = []byte("LastChtNumber") // chtNum (uint64 big endian)
+	chtPrefix  = []byte("cht")           // chtPrefix + chtNum (uint64 big endian) -> trie root hash
 )
 
 func getChtRoot(db ethdb.Database, num uint64) common.Hash {
@@ -346,8 +371,8 @@ func makeCht(db ethdb.Database) bool {
 	headNum := core.GetBlockNumber(db, headHash)
 
 	var newChtNum uint64
-	if headNum > chtConfirmations {
-		newChtNum = (headNum - chtConfirmations) / light.ChtFrequency
+	if headNum > light.ChtConfirmations {
+		newChtNum = (headNum - light.ChtConfirmations) / light.ChtFrequency
 	}
 
 	var lastChtNum uint64
diff --git a/les/serverpool.go b/les/serverpool.go
new file mode 100644
index 000000000..f5e880460
--- /dev/null
+++ b/les/serverpool.go
@@ -0,0 +1,766 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library 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 Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// Package les implements the Light Ethereum Subprotocol.
+package les
+
+import (
+	"io"
+	"math"
+	"math/rand"
+	"net"
+	"strconv"
+	"sync"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common/mclock"
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/logger"
+	"github.com/ethereum/go-ethereum/logger/glog"
+	"github.com/ethereum/go-ethereum/p2p"
+	"github.com/ethereum/go-ethereum/p2p/discover"
+	"github.com/ethereum/go-ethereum/p2p/discv5"
+	"github.com/ethereum/go-ethereum/rlp"
+)
+
+const (
+	// After a connection has been ended or timed out, there is a waiting period
+	// before it can be selected for connection again.
+	// waiting period = base delay * (1 + random(1))
+	// base delay = shortRetryDelay for the first shortRetryCnt times after a
+	// successful connection, after that longRetryDelay is applied
+	shortRetryCnt   = 5
+	shortRetryDelay = time.Second * 5
+	longRetryDelay  = time.Minute * 10
+	// maxNewEntries is the maximum number of newly discovered (never connected) nodes.
+	// If the limit is reached, the least recently discovered one is thrown out.
+	maxNewEntries = 1000
+	// maxKnownEntries is the maximum number of known (already connected) nodes.
+	// If the limit is reached, the least recently connected one is thrown out.
+	// (not that unlike new entries, known entries are persistent)
+	maxKnownEntries = 1000
+	// target for simultaneously connected servers
+	targetServerCount = 5
+	// target for servers selected from the known table
+	// (we leave room for trying new ones if there is any)
+	targetKnownSelect = 3
+	// after dialTimeout, consider the server unavailable and adjust statistics
+	dialTimeout = time.Second * 30
+	// targetConnTime is the minimum expected connection duration before a server
+	// drops a client without any specific reason
+	targetConnTime = time.Minute * 10
+	// new entry selection weight calculation based on most recent discovery time:
+	// unity until discoverExpireStart, then exponential decay with discoverExpireConst
+	discoverExpireStart = time.Minute * 20
+	discoverExpireConst = time.Minute * 20
+	// known entry selection weight is dropped by a factor of exp(-failDropLn) after
+	// each unsuccessful connection (restored after a successful one)
+	failDropLn = 0.1
+	// known node connection success and quality statistics have a long term average
+	// and a short term value which is adjusted exponentially with a factor of
+	// pstatRecentAdjust with each dial/connection and also returned exponentially
+	// to the average with the time constant pstatReturnToMeanTC
+	pstatRecentAdjust   = 0.1
+	pstatReturnToMeanTC = time.Hour
+	// node address selection weight is dropped by a factor of exp(-addrFailDropLn) after
+	// each unsuccessful connection (restored after a successful one)
+	addrFailDropLn = math.Ln2
+	// responseScoreTC and delayScoreTC are exponential decay time constants for
+	// calculating selection chances from response times and block delay times
+	responseScoreTC = time.Millisecond * 100
+	delayScoreTC    = time.Second * 5
+	timeoutPow      = 10
+	// peerSelectMinWeight is added to calculated weights at request peer selection
+	// to give poorly performing peers a little chance of coming back
+	peerSelectMinWeight = 0.005
+	// initStatsWeight is used to initialize previously unknown peers with good
+	// statistics to give a chance to prove themselves
+	initStatsWeight = 1
+)
+
+// serverPool implements a pool for storing and selecting newly discovered and already
+// known light server nodes. It received discovered nodes, stores statistics about
+// known nodes and takes care of always having enough good quality servers connected.
+type serverPool struct {
+	db     ethdb.Database
+	dbKey  []byte
+	server *p2p.Server
+	quit   chan struct{}
+	wg     *sync.WaitGroup
+	connWg sync.WaitGroup
+
+	discSetPeriod chan time.Duration
+	discNodes     chan *discv5.Node
+	discLookups   chan bool
+
+	entries              map[discover.NodeID]*poolEntry
+	lock                 sync.Mutex
+	timeout, enableRetry chan *poolEntry
+	adjustStats          chan poolStatAdjust
+
+	knownQueue, newQueue       poolEntryQueue
+	knownSelect, newSelect     *weightedRandomSelect
+	knownSelected, newSelected int
+	fastDiscover               bool
+}
+
+// newServerPool creates a new serverPool instance
+func newServerPool(db ethdb.Database, dbPrefix []byte, server *p2p.Server, topic discv5.Topic, quit chan struct{}, wg *sync.WaitGroup) *serverPool {
+	pool := &serverPool{
+		db:           db,
+		dbKey:        append(dbPrefix, []byte(topic)...),
+		server:       server,
+		quit:         quit,
+		wg:           wg,
+		entries:      make(map[discover.NodeID]*poolEntry),
+		timeout:      make(chan *poolEntry, 1),
+		adjustStats:  make(chan poolStatAdjust, 100),
+		enableRetry:  make(chan *poolEntry, 1),
+		knownSelect:  newWeightedRandomSelect(),
+		newSelect:    newWeightedRandomSelect(),
+		fastDiscover: true,
+	}
+	pool.knownQueue = newPoolEntryQueue(maxKnownEntries, pool.removeEntry)
+	pool.newQueue = newPoolEntryQueue(maxNewEntries, pool.removeEntry)
+	wg.Add(1)
+	pool.loadNodes()
+	pool.checkDial()
+
+	if pool.server.DiscV5 != nil {
+		pool.discSetPeriod = make(chan time.Duration, 1)
+		pool.discNodes = make(chan *discv5.Node, 100)
+		pool.discLookups = make(chan bool, 100)
+		go pool.server.DiscV5.SearchTopic(topic, pool.discSetPeriod, pool.discNodes, pool.discLookups)
+	}
+
+	go pool.eventLoop()
+	return pool
+}
+
+// connect should be called upon any incoming connection. If the connection has been
+// dialed by the server pool recently, the appropriate pool entry is returned.
+// Otherwise, the connection should be rejected.
+// Note that whenever a connection has been accepted and a pool entry has been returned,
+// disconnect should also always be called.
+func (pool *serverPool) connect(p *peer, ip net.IP, port uint16) *poolEntry {
+	pool.lock.Lock()
+	defer pool.lock.Unlock()
+	entry := pool.entries[p.ID()]
+	if entry == nil {
+		return nil
+	}
+	glog.V(logger.Debug).Infof("connecting to %v, state: %v", p.id, entry.state)
+	if entry.state != psDialed {
+		return nil
+	}
+	pool.connWg.Add(1)
+	entry.peer = p
+	entry.state = psConnected
+	addr := &poolEntryAddress{
+		ip:       ip,
+		port:     port,
+		lastSeen: mclock.Now(),
+	}
+	entry.lastConnected = addr
+	entry.addr = make(map[string]*poolEntryAddress)
+	entry.addr[addr.strKey()] = addr
+	entry.addrSelect = *newWeightedRandomSelect()
+	entry.addrSelect.update(addr)
+	return entry
+}
+
+// registered should be called after a successful handshake
+func (pool *serverPool) registered(entry *poolEntry) {
+	glog.V(logger.Debug).Infof("registered %v", entry.id.String())
+	pool.lock.Lock()
+	defer pool.lock.Unlock()
+
+	entry.state = psRegistered
+	entry.regTime = mclock.Now()
+	if !entry.known {
+		pool.newQueue.remove(entry)
+		entry.known = true
+	}
+	pool.knownQueue.setLatest(entry)
+	entry.shortRetry = shortRetryCnt
+}
+
+// disconnect should be called when ending a connection. Service quality statistics
+// can be updated optionally (not updated if no registration happened, in this case
+// only connection statistics are updated, just like in case of timeout)
+func (pool *serverPool) disconnect(entry *poolEntry) {
+	glog.V(logger.Debug).Infof("disconnected %v", entry.id.String())
+	pool.lock.Lock()
+	defer pool.lock.Unlock()
+
+	if entry.state == psRegistered {
+		connTime := mclock.Now() - entry.regTime
+		connAdjust := float64(connTime) / float64(targetConnTime)
+		if connAdjust > 1 {
+			connAdjust = 1
+		}
+		stopped := false
+		select {
+		case <-pool.quit:
+			stopped = true
+		default:
+		}
+		if stopped {
+			entry.connectStats.add(1, connAdjust)
+		} else {
+			entry.connectStats.add(connAdjust, 1)
+		}
+	}
+
+	entry.state = psNotConnected
+	if entry.knownSelected {
+		pool.knownSelected--
+	} else {
+		pool.newSelected--
+	}
+	pool.setRetryDial(entry)
+	pool.connWg.Done()
+}
+
+const (
+	pseBlockDelay = iota
+	pseResponseTime
+	pseResponseTimeout
+)
+
+// poolStatAdjust records are sent to adjust peer block delay/response time statistics
+type poolStatAdjust struct {
+	adjustType int
+	entry      *poolEntry
+	time       time.Duration
+}
+
+// adjustBlockDelay adjusts the block announce delay statistics of a node
+func (pool *serverPool) adjustBlockDelay(entry *poolEntry, time time.Duration) {
+	pool.adjustStats <- poolStatAdjust{pseBlockDelay, entry, time}
+}
+
+// adjustResponseTime adjusts the request response time statistics of a node
+func (pool *serverPool) adjustResponseTime(entry *poolEntry, time time.Duration, timeout bool) {
+	if timeout {
+		pool.adjustStats <- poolStatAdjust{pseResponseTimeout, entry, time}
+	} else {
+		pool.adjustStats <- poolStatAdjust{pseResponseTime, entry, time}
+	}
+}
+
+type selectPeerItem struct {
+	peer   *peer
+	weight int64
+}
+
+func (sp selectPeerItem) Weight() int64 {
+	return sp.weight
+}
+
+// selectPeer selects a suitable peer for a request
+func (pool *serverPool) selectPeer(canSend func(*peer) (bool, uint64)) *peer {
+	pool.lock.Lock()
+	defer pool.lock.Unlock()
+
+	sel := newWeightedRandomSelect()
+	for _, entry := range pool.entries {
+		if entry.state == psRegistered {
+			p := entry.peer
+			ok, cost := canSend(p)
+			if ok {
+				w := int64(1000000000 * (peerSelectMinWeight + math.Exp(-(entry.responseStats.recentAvg()+float64(cost))/float64(responseScoreTC))*math.Pow((1-entry.timeoutStats.recentAvg()), timeoutPow)))
+				sel.update(selectPeerItem{peer: p, weight: w})
+			}
+		}
+	}
+	choice := sel.choose()
+	if choice == nil {
+		return nil
+	}
+	return choice.(selectPeerItem).peer
+}
+
+// eventLoop handles pool events and mutex locking for all internal functions
+func (pool *serverPool) eventLoop() {
+	lookupCnt := 0
+	var convTime mclock.AbsTime
+	pool.discSetPeriod <- time.Millisecond * 100
+	for {
+		select {
+		case entry := <-pool.timeout:
+			pool.lock.Lock()
+			if !entry.removed {
+				pool.checkDialTimeout(entry)
+			}
+			pool.lock.Unlock()
+
+		case entry := <-pool.enableRetry:
+			pool.lock.Lock()
+			if !entry.removed {
+				entry.delayedRetry = false
+				pool.updateCheckDial(entry)
+			}
+			pool.lock.Unlock()
+
+		case adj := <-pool.adjustStats:
+			pool.lock.Lock()
+			switch adj.adjustType {
+			case pseBlockDelay:
+				adj.entry.delayStats.add(float64(adj.time), 1)
+			case pseResponseTime:
+				adj.entry.responseStats.add(float64(adj.time), 1)
+				adj.entry.timeoutStats.add(0, 1)
+			case pseResponseTimeout:
+				adj.entry.timeoutStats.add(1, 1)
+			}
+			pool.lock.Unlock()
+
+		case node := <-pool.discNodes:
+			pool.lock.Lock()
+			now := mclock.Now()
+			id := discover.NodeID(node.ID)
+			entry := pool.entries[id]
+			if entry == nil {
+				glog.V(logger.Debug).Infof("discovered %v", node.String())
+				entry = &poolEntry{
+					id:         id,
+					addr:       make(map[string]*poolEntryAddress),
+					addrSelect: *newWeightedRandomSelect(),
+					shortRetry: shortRetryCnt,
+				}
+				pool.entries[id] = entry
+				// initialize previously unknown peers with good statistics to give a chance to prove themselves
+				entry.connectStats.add(1, initStatsWeight)
+				entry.delayStats.add(0, initStatsWeight)
+				entry.responseStats.add(0, initStatsWeight)
+				entry.timeoutStats.add(0, initStatsWeight)
+			}
+			entry.lastDiscovered = now
+			addr := &poolEntryAddress{
+				ip:   node.IP,
+				port: node.TCP,
+			}
+			if a, ok := entry.addr[addr.strKey()]; ok {
+				addr = a
+			} else {
+				entry.addr[addr.strKey()] = addr
+			}
+			addr.lastSeen = now
+			entry.addrSelect.update(addr)
+			if !entry.known {
+				pool.newQueue.setLatest(entry)
+			}
+			pool.updateCheckDial(entry)
+			pool.lock.Unlock()
+
+		case conv := <-pool.discLookups:
+			if conv {
+				if lookupCnt == 0 {
+					convTime = mclock.Now()
+				}
+				lookupCnt++
+				if pool.fastDiscover && (lookupCnt == 50 || time.Duration(mclock.Now()-convTime) > time.Minute) {
+					pool.fastDiscover = false
+					pool.discSetPeriod <- time.Minute
+				}
+			}
+
+		case <-pool.quit:
+			close(pool.discSetPeriod)
+			pool.connWg.Wait()
+			pool.saveNodes()
+			pool.wg.Done()
+			return
+
+		}
+	}
+}
+
+// loadNodes loads known nodes and their statistics from the database
+func (pool *serverPool) loadNodes() {
+	enc, err := pool.db.Get(pool.dbKey)
+	if err != nil {
+		return
+	}
+	var list []*poolEntry
+	err = rlp.DecodeBytes(enc, &list)
+	if err != nil {
+		glog.V(logger.Debug).Infof("node list decode error: %v", err)
+		return
+	}
+	for _, e := range list {
+		glog.V(logger.Debug).Infof("loaded server stats %016x  fails: %v  connStats: %v / %v  delayStats: %v / %v  responseStats: %v / %v  timeoutStats: %v / %v", e.id[0:8], e.lastConnected.fails, e.connectStats.avg, e.connectStats.weight, time.Duration(e.delayStats.avg), e.delayStats.weight, time.Duration(e.responseStats.avg), e.responseStats.weight, e.timeoutStats.avg, e.timeoutStats.weight)
+		pool.entries[e.id] = e
+		pool.knownQueue.setLatest(e)
+		pool.knownSelect.update((*knownEntry)(e))
+	}
+}
+
+// saveNodes saves known nodes and their statistics into the database. Nodes are
+// ordered from least to most recently connected.
+func (pool *serverPool) saveNodes() {
+	list := make([]*poolEntry, len(pool.knownQueue.queue))
+	for i, _ := range list {
+		list[i] = pool.knownQueue.fetchOldest()
+	}
+	enc, err := rlp.EncodeToBytes(list)
+	if err == nil {
+		pool.db.Put(pool.dbKey, enc)
+	}
+}
+
+// removeEntry removes a pool entry when the entry count limit is reached.
+// Note that it is called by the new/known queues from which the entry has already
+// been removed so removing it from the queues is not necessary.
+func (pool *serverPool) removeEntry(entry *poolEntry) {
+	pool.newSelect.remove((*discoveredEntry)(entry))
+	pool.knownSelect.remove((*knownEntry)(entry))
+	entry.removed = true
+	delete(pool.entries, entry.id)
+}
+
+// setRetryDial starts the timer which will enable dialing a certain node again
+func (pool *serverPool) setRetryDial(entry *poolEntry) {
+	delay := longRetryDelay
+	if entry.shortRetry > 0 {
+		entry.shortRetry--
+		delay = shortRetryDelay
+	}
+	delay += time.Duration(rand.Int63n(int64(delay) + 1))
+	entry.delayedRetry = true
+	go func() {
+		select {
+		case <-pool.quit:
+		case <-time.After(delay):
+			select {
+			case <-pool.quit:
+			case pool.enableRetry <- entry:
+			}
+		}
+	}()
+}
+
+// updateCheckDial is called when an entry can potentially be dialed again. It updates
+// its selection weights and checks if new dials can/should be made.
+func (pool *serverPool) updateCheckDial(entry *poolEntry) {
+	pool.newSelect.update((*discoveredEntry)(entry))
+	pool.knownSelect.update((*knownEntry)(entry))
+	pool.checkDial()
+}
+
+// checkDial checks if new dials can/should be made. It tries to select servers both
+// based on good statistics and recent discovery.
+func (pool *serverPool) checkDial() {
+	fillWithKnownSelects := !pool.fastDiscover
+	for pool.knownSelected < targetKnownSelect {
+		entry := pool.knownSelect.choose()
+		if entry == nil {
+			fillWithKnownSelects = false
+			break
+		}
+		pool.dial((*poolEntry)(entry.(*knownEntry)), true)
+	}
+	for pool.knownSelected+pool.newSelected < targetServerCount {
+		entry := pool.newSelect.choose()
+		if entry == nil {
+			break
+		}
+		pool.dial((*poolEntry)(entry.(*discoveredEntry)), false)
+	}
+	if fillWithKnownSelects {
+		// no more newly discovered nodes to select and since fast discover period
+		// is over, we probably won't find more in the near future so select more
+		// known entries if possible
+		for pool.knownSelected < targetServerCount {
+			entry := pool.knownSelect.choose()
+			if entry == nil {
+				break
+			}
+			pool.dial((*poolEntry)(entry.(*knownEntry)), true)
+		}
+	}
+}
+
+// dial initiates a new connection
+func (pool *serverPool) dial(entry *poolEntry, knownSelected bool) {
+	if entry.state != psNotConnected {
+		return
+	}
+	entry.state = psDialed
+	entry.knownSelected = knownSelected
+	if knownSelected {
+		pool.knownSelected++
+	} else {
+		pool.newSelected++
+	}
+	addr := entry.addrSelect.choose().(*poolEntryAddress)
+	glog.V(logger.Debug).Infof("dialing %v out of %v, known: %v", entry.id.String()+"@"+addr.strKey(), len(entry.addr), knownSelected)
+	entry.dialed = addr
+	go func() {
+		pool.server.AddPeer(discover.NewNode(entry.id, addr.ip, addr.port, addr.port))
+		select {
+		case <-pool.quit:
+		case <-time.After(dialTimeout):
+			select {
+			case <-pool.quit:
+			case pool.timeout <- entry:
+			}
+		}
+	}()
+}
+
+// checkDialTimeout checks if the node is still in dialed state and if so, resets it
+// and adjusts connection statistics accordingly.
+func (pool *serverPool) checkDialTimeout(entry *poolEntry) {
+	if entry.state != psDialed {
+		return
+	}
+	glog.V(logger.Debug).Infof("timeout %v", entry.id.String()+"@"+entry.dialed.strKey())
+	entry.state = psNotConnected
+	if entry.knownSelected {
+		pool.knownSelected--
+	} else {
+		pool.newSelected--
+	}
+	entry.connectStats.add(0, 1)
+	entry.dialed.fails++
+	pool.setRetryDial(entry)
+}
+
+const (
+	psNotConnected = iota
+	psDialed
+	psConnected
+	psRegistered
+)
+
+// poolEntry represents a server node and stores its current state and statistics.
+type poolEntry struct {
+	peer                  *peer
+	id                    discover.NodeID
+	addr                  map[string]*poolEntryAddress
+	lastConnected, dialed *poolEntryAddress
+	addrSelect            weightedRandomSelect
+
+	lastDiscovered              mclock.AbsTime
+	known, knownSelected        bool
+	connectStats, delayStats    poolStats
+	responseStats, timeoutStats poolStats
+	state                       int
+	regTime                     mclock.AbsTime
+	queueIdx                    int
+	removed                     bool
+
+	delayedRetry bool
+	shortRetry   int
+}
+
+func (e *poolEntry) EncodeRLP(w io.Writer) error {
+	return rlp.Encode(w, []interface{}{e.id, e.lastConnected.ip, e.lastConnected.port, e.lastConnected.fails, &e.connectStats, &e.delayStats, &e.responseStats, &e.timeoutStats})
+}
+
+func (e *poolEntry) DecodeRLP(s *rlp.Stream) error {
+	var entry struct {
+		ID                         discover.NodeID
+		IP                         net.IP
+		Port                       uint16
+		Fails                      uint
+		CStat, DStat, RStat, TStat poolStats
+	}
+	if err := s.Decode(&entry); err != nil {
+		return err
+	}
+	addr := &poolEntryAddress{ip: entry.IP, port: entry.Port, fails: entry.Fails, lastSeen: mclock.Now()}
+	e.id = entry.ID
+	e.addr = make(map[string]*poolEntryAddress)
+	e.addr[addr.strKey()] = addr
+	e.addrSelect = *newWeightedRandomSelect()
+	e.addrSelect.update(addr)
+	e.lastConnected = addr
+	e.connectStats = entry.CStat
+	e.delayStats = entry.DStat
+	e.responseStats = entry.RStat
+	e.timeoutStats = entry.TStat
+	e.shortRetry = shortRetryCnt
+	e.known = true
+	return nil
+}
+
+// discoveredEntry implements wrsItem
+type discoveredEntry poolEntry
+
+// Weight calculates random selection weight for newly discovered entries
+func (e *discoveredEntry) Weight() int64 {
+	if e.state != psNotConnected || e.delayedRetry {
+		return 0
+	}
+	t := time.Duration(mclock.Now() - e.lastDiscovered)
+	if t <= discoverExpireStart {
+		return 1000000000
+	} else {
+		return int64(1000000000 * math.Exp(-float64(t-discoverExpireStart)/float64(discoverExpireConst)))
+	}
+}
+
+// knownEntry implements wrsItem
+type knownEntry poolEntry
+
+// Weight calculates random selection weight for known entries
+func (e *knownEntry) Weight() int64 {
+	if e.state != psNotConnected || !e.known || e.delayedRetry {
+		return 0
+	}
+	return int64(1000000000 * e.connectStats.recentAvg() * math.Exp(-float64(e.lastConnected.fails)*failDropLn-e.responseStats.recentAvg()/float64(responseScoreTC)-e.delayStats.recentAvg()/float64(delayScoreTC)) * math.Pow((1-e.timeoutStats.recentAvg()), timeoutPow))
+}
+
+// poolEntryAddress is a separate object because currently it is necessary to remember
+// multiple potential network addresses for a pool entry. This will be removed after
+// the final implementation of v5 discovery which will retrieve signed and serial
+// numbered advertisements, making it clear which IP/port is the latest one.
+type poolEntryAddress struct {
+	ip       net.IP
+	port     uint16
+	lastSeen mclock.AbsTime // last time it was discovered, connected or loaded from db
+	fails    uint           // connection failures since last successful connection (persistent)
+}
+
+func (a *poolEntryAddress) Weight() int64 {
+	t := time.Duration(mclock.Now() - a.lastSeen)
+	return int64(1000000*math.Exp(-float64(t)/float64(discoverExpireConst)-float64(a.fails)*addrFailDropLn)) + 1
+}
+
+func (a *poolEntryAddress) strKey() string {
+	return a.ip.String() + ":" + strconv.Itoa(int(a.port))
+}
+
+// poolStats implement statistics for a certain quantity with a long term average
+// and a short term value which is adjusted exponentially with a factor of
+// pstatRecentAdjust with each update and also returned exponentially to the
+// average with the time constant pstatReturnToMeanTC
+type poolStats struct {
+	sum, weight, avg, recent float64
+	lastRecalc               mclock.AbsTime
+}
+
+// init initializes stats with a long term sum/update count pair retrieved from the database
+func (s *poolStats) init(sum, weight float64) {
+	s.sum = sum
+	s.weight = weight
+	var avg float64
+	if weight > 0 {
+		avg = s.sum / weight
+	}
+	s.avg = avg
+	s.recent = avg
+	s.lastRecalc = mclock.Now()
+}
+
+// recalc recalculates recent value return-to-mean and long term average
+func (s *poolStats) recalc() {
+	now := mclock.Now()
+	s.recent = s.avg + (s.recent-s.avg)*math.Exp(-float64(now-s.lastRecalc)/float64(pstatReturnToMeanTC))
+	if s.sum == 0 {
+		s.avg = 0
+	} else {
+		if s.sum > s.weight*1e30 {
+			s.avg = 1e30
+		} else {
+			s.avg = s.sum / s.weight
+		}
+	}
+	s.lastRecalc = now
+}
+
+// add updates the stats with a new value
+func (s *poolStats) add(value, weight float64) {
+	s.weight += weight
+	s.sum += value * weight
+	s.recalc()
+}
+
+// recentAvg returns the short-term adjusted average
+func (s *poolStats) recentAvg() float64 {
+	s.recalc()
+	return s.recent
+}
+
+func (s *poolStats) EncodeRLP(w io.Writer) error {
+	return rlp.Encode(w, []interface{}{math.Float64bits(s.sum), math.Float64bits(s.weight)})
+}
+
+func (s *poolStats) DecodeRLP(st *rlp.Stream) error {
+	var stats struct {
+		SumUint, WeightUint uint64
+	}
+	if err := st.Decode(&stats); err != nil {
+		return err
+	}
+	s.init(math.Float64frombits(stats.SumUint), math.Float64frombits(stats.WeightUint))
+	return nil
+}
+
+// poolEntryQueue keeps track of its least recently accessed entries and removes
+// them when the number of entries reaches the limit
+type poolEntryQueue struct {
+	queue                  map[int]*poolEntry // known nodes indexed by their latest lastConnCnt value
+	newPtr, oldPtr, maxCnt int
+	removeFromPool         func(*poolEntry)
+}
+
+// newPoolEntryQueue returns a new poolEntryQueue
+func newPoolEntryQueue(maxCnt int, removeFromPool func(*poolEntry)) poolEntryQueue {
+	return poolEntryQueue{queue: make(map[int]*poolEntry), maxCnt: maxCnt, removeFromPool: removeFromPool}
+}
+
+// fetchOldest returns and removes the least recently accessed entry
+func (q *poolEntryQueue) fetchOldest() *poolEntry {
+	if len(q.queue) == 0 {
+		return nil
+	}
+	for {
+		if e := q.queue[q.oldPtr]; e != nil {
+			delete(q.queue, q.oldPtr)
+			q.oldPtr++
+			return e
+		}
+		q.oldPtr++
+	}
+}
+
+// remove removes an entry from the queue
+func (q *poolEntryQueue) remove(entry *poolEntry) {
+	if q.queue[entry.queueIdx] == entry {
+		delete(q.queue, entry.queueIdx)
+	}
+}
+
+// setLatest adds or updates a recently accessed entry. It also checks if an old entry
+// needs to be removed and removes it from the parent pool too with a callback function.
+func (q *poolEntryQueue) setLatest(entry *poolEntry) {
+	if q.queue[entry.queueIdx] == entry {
+		delete(q.queue, entry.queueIdx)
+	} else {
+		if len(q.queue) == q.maxCnt {
+			e := q.fetchOldest()
+			q.remove(e)
+			q.removeFromPool(e)
+		}
+	}
+	entry.queueIdx = q.newPtr
+	q.queue[entry.queueIdx] = entry
+	q.newPtr++
+}
diff --git a/light/lightchain.go b/light/lightchain.go
index 1cea7a892..d397f5006 100644
--- a/light/lightchain.go
+++ b/light/lightchain.go
@@ -505,3 +505,14 @@ func (self *LightChain) SyncCht(ctx context.Context) bool {
 	}
 	return false
 }
+
+// LockChain locks the chain mutex for reading so that multiple canonical hashes can be
+// retrieved while it is guaranteed that they belong to the same version of the chain
+func (self *LightChain) LockChain() {
+	self.chainmu.RLock()
+}
+
+// UnlockChain unlocks the chain mutex
+func (self *LightChain) UnlockChain() {
+	self.chainmu.RUnlock()
+}
diff --git a/light/odr.go b/light/odr.go
index 679569bf9..4f6ef6b9e 100644
--- a/light/odr.go
+++ b/light/odr.go
@@ -48,6 +48,7 @@ type OdrRequest interface {
 // TrieID identifies a state or account storage trie
 type TrieID struct {
 	BlockHash, Root common.Hash
+	BlockNumber     uint64
 	AccKey          []byte
 }
 
@@ -55,9 +56,10 @@ type TrieID struct {
 // header.
 func StateTrieID(header *types.Header) *TrieID {
 	return &TrieID{
-		BlockHash: header.Hash(),
-		AccKey:    nil,
-		Root:      header.Root,
+		BlockHash:   header.Hash(),
+		BlockNumber: header.Number.Uint64(),
+		AccKey:      nil,
+		Root:        header.Root,
 	}
 }
 
@@ -66,9 +68,10 @@ func StateTrieID(header *types.Header) *TrieID {
 // checking Merkle proofs.
 func StorageTrieID(state *TrieID, addr common.Address, root common.Hash) *TrieID {
 	return &TrieID{
-		BlockHash: state.BlockHash,
-		AccKey:    crypto.Keccak256(addr[:]),
-		Root:      root,
+		BlockHash:   state.BlockHash,
+		BlockNumber: state.BlockNumber,
+		AccKey:      crypto.Keccak256(addr[:]),
+		Root:        root,
 	}
 }
 
diff --git a/light/odr_util.go b/light/odr_util.go
index 5c72f90e9..761711621 100644
--- a/light/odr_util.go
+++ b/light/odr_util.go
@@ -38,8 +38,9 @@ var (
 	ErrNoTrustedCht = errors.New("No trusted canonical hash trie")
 	ErrNoHeader     = errors.New("Header not found")
 
-	ChtFrequency  = uint64(4096)
-	trustedChtKey = []byte("TrustedCHT")
+	ChtFrequency     = uint64(4096)
+	ChtConfirmations = uint64(2048)
+	trustedChtKey    = []byte("TrustedCHT")
 )
 
 type ChtNode struct {
diff --git a/p2p/discv5/net.go b/p2p/discv5/net.go
index d1c48904e..1348c6774 100644
--- a/p2p/discv5/net.go
+++ b/p2p/discv5/net.go
@@ -126,8 +126,15 @@ type topicRegisterReq struct {
 }
 
 type topicSearchReq struct {
-	topic Topic
-	found chan<- string
+	topic  Topic
+	found  chan<- *Node
+	lookup chan<- bool
+	delay  time.Duration
+}
+
+type topicSearchResult struct {
+	target lookupInfo
+	nodes  []*Node
 }
 
 type timeoutEvent struct {
@@ -263,16 +270,23 @@ func (net *Network) lookup(target common.Hash, stopOnMatch bool) []*Node {
 			break
 		}
 		// Wait for the next reply.
-		for _, n := range <-reply {
-			if n != nil && !seen[n.ID] {
-				seen[n.ID] = true
-				result.push(n, bucketSize)
-				if stopOnMatch && n.sha == target {
-					return result.entries
+		select {
+		case nodes := <-reply:
+			for _, n := range nodes {
+				if n != nil && !seen[n.ID] {
+					seen[n.ID] = true
+					result.push(n, bucketSize)
+					if stopOnMatch && n.sha == target {
+						return result.entries
+					}
 				}
 			}
+			pendingQueries--
+		case <-time.After(respTimeout):
+			// forget all pending requests, start new ones
+			pendingQueries = 0
+			reply = make(chan []*Node, alpha)
 		}
-		pendingQueries--
 	}
 	return result.entries
 }
@@ -293,18 +307,20 @@ func (net *Network) RegisterTopic(topic Topic, stop <-chan struct{}) {
 	}
 }
 
-func (net *Network) SearchTopic(topic Topic, stop <-chan struct{}, found chan<- string) {
-	select {
-	case net.topicSearchReq <- topicSearchReq{topic, found}:
-	case <-net.closed:
-		return
-	}
-	select {
-	case <-net.closed:
-	case <-stop:
+func (net *Network) SearchTopic(topic Topic, setPeriod <-chan time.Duration, found chan<- *Node, lookup chan<- bool) {
+	for {
 		select {
-		case net.topicSearchReq <- topicSearchReq{topic, nil}:
 		case <-net.closed:
+			return
+		case delay, ok := <-setPeriod:
+			select {
+			case net.topicSearchReq <- topicSearchReq{topic: topic, found: found, lookup: lookup, delay: delay}:
+			case <-net.closed:
+				return
+			}
+			if !ok {
+				return
+			}
 		}
 	}
 }
@@ -347,6 +363,13 @@ func (net *Network) reqTableOp(f func()) (called bool) {
 
 // TODO: external address handling.
 
+type topicSearchInfo struct {
+	lookupChn chan<- bool
+	period    time.Duration
+}
+
+const maxSearchCount = 5
+
 func (net *Network) loop() {
 	var (
 		refreshTimer       = time.NewTicker(autoRefreshInterval)
@@ -385,10 +408,12 @@ func (net *Network) loop() {
 		topicRegisterLookupTarget lookupInfo
 		topicRegisterLookupDone   chan []*Node
 		topicRegisterLookupTick   = time.NewTimer(0)
-		topicSearchLookupTarget   lookupInfo
 		searchReqWhenRefreshDone  []topicSearchReq
+		searchInfo                = make(map[Topic]topicSearchInfo)
+		activeSearchCount         int
 	)
-	topicSearchLookupDone := make(chan []*Node, 1)
+	topicSearchLookupDone := make(chan topicSearchResult, 100)
+	topicSearch := make(chan Topic, 100)
 	<-topicRegisterLookupTick.C
 
 	statsDump := time.NewTicker(10 * time.Second)
@@ -504,21 +529,52 @@ loop:
 		case req := <-net.topicSearchReq:
 			if refreshDone == nil {
 				debugLog("<-net.topicSearchReq")
-				if req.found == nil {
-					net.ticketStore.removeSearchTopic(req.topic)
+				info, ok := searchInfo[req.topic]
+				if ok {
+					if req.delay == time.Duration(0) {
+						delete(searchInfo, req.topic)
+						net.ticketStore.removeSearchTopic(req.topic)
+					} else {
+						info.period = req.delay
+						searchInfo[req.topic] = info
+					}
 					continue
 				}
-				net.ticketStore.addSearchTopic(req.topic, req.found)
-				if (topicSearchLookupTarget.target == common.Hash{}) {
-					topicSearchLookupDone <- nil
+				if req.delay != time.Duration(0) {
+					var info topicSearchInfo
+					info.period = req.delay
+					info.lookupChn = req.lookup
+					searchInfo[req.topic] = info
+					net.ticketStore.addSearchTopic(req.topic, req.found)
+					topicSearch <- req.topic
 				}
 			} else {
 				searchReqWhenRefreshDone = append(searchReqWhenRefreshDone, req)
 			}
 
-		case nodes := <-topicSearchLookupDone:
-			debugLog("<-topicSearchLookupDone")
-			net.ticketStore.searchLookupDone(topicSearchLookupTarget, nodes, func(n *Node) []byte {
+		case topic := <-topicSearch:
+			if activeSearchCount < maxSearchCount {
+				activeSearchCount++
+				target := net.ticketStore.nextSearchLookup(topic)
+				go func() {
+					nodes := net.lookup(target.target, false)
+					topicSearchLookupDone <- topicSearchResult{target: target, nodes: nodes}
+				}()
+			}
+			period := searchInfo[topic].period
+			if period != time.Duration(0) {
+				go func() {
+					time.Sleep(period)
+					topicSearch <- topic
+				}()
+			}
+
+		case res := <-topicSearchLookupDone:
+			activeSearchCount--
+			if lookupChn := searchInfo[res.target.topic].lookupChn; lookupChn != nil {
+				lookupChn <- net.ticketStore.radius[res.target.topic].converged
+			}
+			net.ticketStore.searchLookupDone(res.target, res.nodes, func(n *Node) []byte {
 				net.ping(n, n.addr())
 				return n.pingEcho
 			}, func(n *Node, topic Topic) []byte {
@@ -531,11 +587,6 @@ loop:
 					return nil
 				}
 			})
-			topicSearchLookupTarget = net.ticketStore.nextSearchLookup()
-			target := topicSearchLookupTarget.target
-			if (target != common.Hash{}) {
-				go func() { topicSearchLookupDone <- net.lookup(target, false) }()
-			}
 
 		case <-statsDump.C:
 			debugLog("<-statsDump.C")
diff --git a/p2p/discv5/ticket.go b/p2p/discv5/ticket.go
index 202504314..752fdc9b4 100644
--- a/p2p/discv5/ticket.go
+++ b/p2p/discv5/ticket.go
@@ -138,16 +138,12 @@ type ticketStore struct {
 	nextTicketReg     mclock.AbsTime
 
 	searchTopicMap        map[Topic]searchTopic
-	searchTopicList       []Topic
-	searchTopicPtr        int
 	nextTopicQueryCleanup mclock.AbsTime
 	queriesSent           map[*Node]map[common.Hash]sentQuery
-	radiusLookupCnt       int
 }
 
 type searchTopic struct {
-	foundChn chan<- string
-	listIdx  int
+	foundChn chan<- *Node
 }
 
 type sentQuery struct {
@@ -183,23 +179,15 @@ func (s *ticketStore) addTopic(t Topic, register bool) {
 	}
 }
 
-func (s *ticketStore) addSearchTopic(t Topic, foundChn chan<- string) {
+func (s *ticketStore) addSearchTopic(t Topic, foundChn chan<- *Node) {
 	s.addTopic(t, false)
 	if s.searchTopicMap[t].foundChn == nil {
-		s.searchTopicList = append(s.searchTopicList, t)
-		s.searchTopicMap[t] = searchTopic{foundChn: foundChn, listIdx: len(s.searchTopicList) - 1}
+		s.searchTopicMap[t] = searchTopic{foundChn: foundChn}
 	}
 }
 
 func (s *ticketStore) removeSearchTopic(t Topic) {
 	if st := s.searchTopicMap[t]; st.foundChn != nil {
-		lastIdx := len(s.searchTopicList) - 1
-		lastTopic := s.searchTopicList[lastIdx]
-		s.searchTopicList[st.listIdx] = lastTopic
-		sl := s.searchTopicMap[lastTopic]
-		sl.listIdx = st.listIdx
-		s.searchTopicMap[lastTopic] = sl
-		s.searchTopicList = s.searchTopicList[:lastIdx]
 		delete(s.searchTopicMap, t)
 	}
 }
@@ -247,20 +235,13 @@ func (s *ticketStore) nextRegisterLookup() (lookup lookupInfo, delay time.Durati
 	return lookupInfo{}, 40 * time.Second
 }
 
-func (s *ticketStore) nextSearchLookup() lookupInfo {
-	if len(s.searchTopicList) == 0 {
-		return lookupInfo{}
-	}
-	if s.searchTopicPtr >= len(s.searchTopicList) {
-		s.searchTopicPtr = 0
-	}
-	topic := s.searchTopicList[s.searchTopicPtr]
-	s.searchTopicPtr++
-	target := s.radius[topic].nextTarget(s.radiusLookupCnt >= searchForceQuery)
+func (s *ticketStore) nextSearchLookup(topic Topic) lookupInfo {
+	tr := s.radius[topic]
+	target := tr.nextTarget(tr.radiusLookupCnt >= searchForceQuery)
 	if target.radiusLookup {
-		s.radiusLookupCnt++
+		tr.radiusLookupCnt++
 	} else {
-		s.radiusLookupCnt = 0
+		tr.radiusLookupCnt = 0
 	}
 	return target
 }
@@ -662,9 +643,9 @@ func (s *ticketStore) gotTopicNodes(from *Node, hash common.Hash, nodes []rpcNod
 		if ip.IsUnspecified() || ip.IsLoopback() {
 			ip = from.IP
 		}
-		enode := NewNode(node.ID, ip, node.UDP-1, node.TCP-1).String() // subtract one from port while discv5 is running in test mode on UDPport+1
+		n := NewNode(node.ID, ip, node.UDP-1, node.TCP-1) // subtract one from port while discv5 is running in test mode on UDPport+1
 		select {
-		case chn <- enode:
+		case chn <- n:
 		default:
 			return false
 		}
@@ -677,6 +658,8 @@ type topicRadius struct {
 	topicHashPrefix   uint64
 	radius, minRadius uint64
 	buckets           []topicRadiusBucket
+	converged         bool
+	radiusLookupCnt   int
 }
 
 type topicRadiusEvent int
@@ -706,7 +689,7 @@ func (b *topicRadiusBucket) update(now mclock.AbsTime) {
 	b.lastTime = now
 
 	for target, tm := range b.lookupSent {
-		if now-tm > mclock.AbsTime(pingTimeout) {
+		if now-tm > mclock.AbsTime(respTimeout) {
 			b.weights[trNoAdjust] += 1
 			delete(b.lookupSent, target)
 		}
@@ -906,6 +889,7 @@ func (r *topicRadius) recalcRadius() (radius uint64, radiusLookup int) {
 
 	if radiusLookup == -1 {
 		// no more radius lookups needed at the moment, return a radius
+		r.converged = true
 		rad := maxBucket
 		if minRadBucket < rad {
 			rad = minRadBucket