Headers-first synchronization

Many changes:
* Do not use 'getblocks', but 'getheaders', and use it to build a headers tree.
* Blocks are fetched in parallel from all available outbound peers, using a
  limited moving window. When one peer stalls the movement of the window, it is
  disconnected.
* No more orphan blocks. At all. We only ever request a block for which we have
  verified the headers, and store it to disk immediately. This means that a
  disk-fill attack would require PoW.
* Require protocol version 31800 for every peer (released in december 2010).
* No more syncnode (we sync from everyone we can, though limited to 1 during
  initial *headers* sync).
* Introduce some extra named constants, comments and asserts.

9 files changed, 375 insertions, 370 deletions

diff --git a/src/chain.h b/src/chain.h
index 0aafb40b98..4e6a466c6a 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -49,12 +49,29 @@ struct CDiskBlockPos
 };
 
 enum BlockStatus {
+    // Unused.
     BLOCK_VALID_UNKNOWN      =    0,
-    BLOCK_VALID_HEADER       =    1, // parsed, version ok, hash satisfies claimed PoW, 1 <= vtx count <= max, timestamp not in future
-    BLOCK_VALID_TREE         =    2, // parent found, difficulty matches, timestamp >= median previous, checkpoint
-    BLOCK_VALID_TRANSACTIONS =    3, // only first tx is coinbase, 2 <= coinbase input script length <= 100, transactions valid, no duplicate txids, sigops, size, merkle root
-    BLOCK_VALID_CHAIN        =    4, // outputs do not overspend inputs, no double spends, coinbase output ok, immature coinbase spends, BIP30
-    BLOCK_VALID_SCRIPTS      =    5, // scripts/signatures ok
+
+    // Parsed, version ok, hash satisfies claimed PoW, 1 <= vtx count <= max, timestamp not in future
+    BLOCK_VALID_HEADER       =    1,
+
+    // All parent headers found, difficulty matches, timestamp >= median previous, checkpoint. Implies all parents
+    // are also at least TREE.
+    BLOCK_VALID_TREE         =    2,
+
+    // Only first tx is coinbase, 2 <= coinbase input script length <= 100, transactions valid, no duplicate txids,
+    // sigops, size, merkle root. Implies all parents are at least TREE but not necessarily TRANSACTIONS. When all
+    // parent blocks also have TRANSACTIONS, CBlockIndex::nChainTx will be set.
+    BLOCK_VALID_TRANSACTIONS =    3,
+
+    // Outputs do not overspend inputs, no double spends, coinbase output ok, immature coinbase spends, BIP30.
+    // Implies all parents are also at least CHAIN.
+    BLOCK_VALID_CHAIN        =    4,
+
+    // Scripts & signatures ok. Implies all parents are also at least SCRIPTS.
+    BLOCK_VALID_SCRIPTS      =    5,
+
+    // All validity bits.
     BLOCK_VALID_MASK         =   BLOCK_VALID_HEADER | BLOCK_VALID_TREE | BLOCK_VALID_TRANSACTIONS |
                                  BLOCK_VALID_CHAIN | BLOCK_VALID_SCRIPTS,
 
@@ -103,7 +120,8 @@ public:
     // Note: in a potential headers-first mode, this number cannot be relied upon
     unsigned int nTx;
 
-    // (memory only) Number of transactions in the chain up to and including this block
+    // (memory only) Number of transactions in the chain up to and including this block.
+    // This value will be non-zero only if and only if transactions for this block and all its parents are available.
     unsigned int nChainTx; // change to 64-bit type when necessary; won't happen before 2030
 
     // Verification status of this block. See enum BlockStatus
@@ -146,7 +164,7 @@ public:
         SetNull();
     }
 
-    CBlockIndex(CBlockHeader& block)
+    CBlockIndex(const CBlockHeader& block)
     {
         SetNull();
 
diff --git a/src/main.cpp b/src/main.cpp
index fc8167e40e..816cab7e1f 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -56,14 +56,6 @@ CFeeRate minRelayTxFee = CFeeRate(1000);
 
 CTxMemPool mempool(::minRelayTxFee);
 
-struct COrphanBlock {
-    uint256 hashBlock;
-    uint256 hashPrev;
-    vector<unsigned char> vchBlock;
-};
-map<uint256, COrphanBlock*> mapOrphanBlocks;
-multimap<uint256, COrphanBlock*> mapOrphanBlocksByPrev;
-
 struct COrphanTx {
     CTransaction tx;
     NodeId fromPeer;
@@ -106,6 +98,12 @@ namespace {
     // The set of all CBlockIndex entries with BLOCK_VALID_TRANSACTIONS or better that are at least
     // as good as our current tip. Entries may be failed, though.
     set<CBlockIndex*, CBlockIndexWorkComparator> setBlockIndexValid;
+    // Best header we've seen so far (used for getheaders queries' starting points).
+    CBlockIndex *pindexBestHeader = NULL;
+    // Number of nodes with fSyncStarted.
+    int nSyncStarted = 0;
+    // All pairs A->B, where A (or one if its ancestors) misses transactions, but B has transactions.
+    multimap<CBlockIndex*, CBlockIndex*> mapBlocksUnlinked;
 
     CCriticalSection cs_LastBlockFile;
     CBlockFileInfo infoLastBlockFile;
@@ -125,11 +123,10 @@ namespace {
     // Protected by cs_main.
     struct QueuedBlock {
         uint256 hash;
+        CBlockIndex *pindex;  // Optional.
         int64_t nTime;  // Time of "getdata" request in microseconds.
-        int nQueuedBefore;  // Number of blocks in flight at the time of request.
     };
     map<uint256, pair<NodeId, list<QueuedBlock>::iterator> > mapBlocksInFlight;
-    map<uint256, pair<NodeId, list<uint256>::iterator> > mapBlocksToDownload;
 
 } // anon namespace
 
@@ -220,22 +217,24 @@ struct CNodeState {
     CBlockIndex *pindexBestKnownBlock;
     // The hash of the last unknown block this peer has announced.
     uint256 hashLastUnknownBlock;
+    // The last full block we both have.
+    CBlockIndex *pindexLastCommonBlock;
+    // Whether we've started headers synchronization with this peer.
+    bool fSyncStarted;
+    // Since when we're stalling block download progress (in microseconds), or 0.
+    int64_t nStallingSince;
     list<QueuedBlock> vBlocksInFlight;
     int nBlocksInFlight;
-    list<uint256> vBlocksToDownload;
-    int nBlocksToDownload;
-    int64_t nLastBlockReceive;
-    int64_t nLastBlockProcess;
 
     CNodeState() {
         nMisbehavior = 0;
         fShouldBan = false;
         pindexBestKnownBlock = NULL;
         hashLastUnknownBlock = uint256(0);
-        nBlocksToDownload = 0;
+        pindexLastCommonBlock = NULL;
+        fSyncStarted = false;
+        nStallingSince = 0;
         nBlocksInFlight = 0;
-        nLastBlockReceive = 0;
-        nLastBlockProcess = 0;
     }
 };
 
@@ -266,64 +265,37 @@ void FinalizeNode(NodeId nodeid) {
     LOCK(cs_main);
     CNodeState *state = State(nodeid);
 
+    if (state->fSyncStarted)
+        nSyncStarted--;
+
     BOOST_FOREACH(const QueuedBlock& entry, state->vBlocksInFlight)
         mapBlocksInFlight.erase(entry.hash);
-    BOOST_FOREACH(const uint256& hash, state->vBlocksToDownload)
-        mapBlocksToDownload.erase(hash);
     EraseOrphansFor(nodeid);
 
     mapNodeState.erase(nodeid);
 }
 
 // Requires cs_main.
-void MarkBlockAsReceived(const uint256 &hash, NodeId nodeFrom = -1) {
-    map<uint256, pair<NodeId, list<uint256>::iterator> >::iterator itToDownload = mapBlocksToDownload.find(hash);
-    if (itToDownload != mapBlocksToDownload.end()) {
-        CNodeState *state = State(itToDownload->second.first);
-        state->vBlocksToDownload.erase(itToDownload->second.second);
-        state->nBlocksToDownload--;
-        mapBlocksToDownload.erase(itToDownload);
-    }
-
+void MarkBlockAsReceived(const uint256& hash) {
     map<uint256, pair<NodeId, list<QueuedBlock>::iterator> >::iterator itInFlight = mapBlocksInFlight.find(hash);
     if (itInFlight != mapBlocksInFlight.end()) {
         CNodeState *state = State(itInFlight->second.first);
         state->vBlocksInFlight.erase(itInFlight->second.second);
         state->nBlocksInFlight--;
-        if (itInFlight->second.first == nodeFrom)
-            state->nLastBlockReceive = GetTimeMicros();
+        state->nStallingSince = 0;
         mapBlocksInFlight.erase(itInFlight);
     }
 }
 
 // Requires cs_main.
-bool AddBlockToQueue(NodeId nodeid, const uint256 &hash) {
-    if (mapBlocksToDownload.count(hash) || mapBlocksInFlight.count(hash))
-        return false;
-
-    CNodeState *state = State(nodeid);
-    if (state == NULL)
-        return false;
-
-    list<uint256>::iterator it = state->vBlocksToDownload.insert(state->vBlocksToDownload.end(), hash);
-    state->nBlocksToDownload++;
-    if (state->nBlocksToDownload > 5000)
-        Misbehaving(nodeid, 10);
-    mapBlocksToDownload[hash] = std::make_pair(nodeid, it);
-    return true;
-}
-
-// Requires cs_main.
-void MarkBlockAsInFlight(NodeId nodeid, const uint256 &hash) {
+void MarkBlockAsInFlight(NodeId nodeid, const uint256& hash, CBlockIndex *pindex = NULL) {
     CNodeState *state = State(nodeid);
     assert(state != NULL);
 
     // Make sure it's not listed somewhere already.
     MarkBlockAsReceived(hash);
 
-    QueuedBlock newentry = {hash, GetTimeMicros(), state->nBlocksInFlight};
-    if (state->nBlocksInFlight == 0)
-        state->nLastBlockReceive = newentry.nTime; // Reset when a first request is sent.
+    QueuedBlock newentry = {hash, pindex, GetTimeMicros()};
     list<QueuedBlock>::iterator it = state->vBlocksInFlight.insert(state->vBlocksInFlight.end(), newentry);
     state->nBlocksInFlight++;
     mapBlocksInFlight[hash] = std::make_pair(nodeid, it);
@@ -362,6 +334,103 @@ void UpdateBlockAvailability(NodeId nodeid, const uint256 &hash) {
     }
 }
 
+/** Find the last common ancestor two blocks have.
+ *  Both pa and pb must be non-NULL. */
+CBlockIndex* LastCommonAncestor(CBlockIndex* pa, CBlockIndex* pb) {
+    if (pa->nHeight > pb->nHeight) {
+        pa = pa->GetAncestor(pb->nHeight);
+    } else if (pb->nHeight > pa->nHeight) {
+        pb = pb->GetAncestor(pa->nHeight);
+    }
+
+    while (pa != pb && pa && pb) {
+        pa = pa->pprev;
+        pb = pb->pprev;
+    }
+
+    // Eventually all chain branches meet at the genesis block.
+    assert(pa == pb);
+    return pa;
+}
+
+/** Update pindexLastCommonBlock and add not-in-flight missing successors to vBlocks, until it has
+ *  at most count entries. */
+void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<CBlockIndex*>& vBlocks, NodeId& nodeStaller) {
+    if (count == 0)
+        return;
+
+    vBlocks.reserve(vBlocks.size() + count);
+    CNodeState *state = State(nodeid);
+    assert(state != NULL);
+
+    // Make sure pindexBestKnownBlock is up to date, we'll need it.
+    ProcessBlockAvailability(nodeid);
+
+    if (state->pindexBestKnownBlock == NULL || state->pindexBestKnownBlock->nChainWork < chainActive.Tip()->nChainWork) {
+        // This peer has nothing interesting.
+        return;
+    }
+
+    if (state->pindexLastCommonBlock == NULL) {
+        // Bootstrap quickly by guessing a parent of our best tip is the forking point.
+        // Guessing wrong in either direction is not a problem.
+        state->pindexLastCommonBlock = chainActive[std::min(state->pindexBestKnownBlock->nHeight, chainActive.Height())];
+    }
+
+    // If the peer reorganized, our previous pindexLastCommonBlock may not be an ancestor
+    // of their current tip anymore. Go back enough to fix that.
+    state->pindexLastCommonBlock = LastCommonAncestor(state->pindexLastCommonBlock, state->pindexBestKnownBlock);
+    if (state->pindexLastCommonBlock == state->pindexBestKnownBlock)
+        return;
+
+    std::vector<CBlockIndex*> vToFetch;
+    CBlockIndex *pindexWalk = state->pindexLastCommonBlock;
+    // Never fetch further than the best block we know the peer has, or more than BLOCK_DOWNLOAD_WINDOW + 1 beyond our
+    // current tip. The +1 is so we can detect stalling, namely if we would be able to download that next block if the
+    // window were 1 larger.
+    int nMaxHeight = std::min<int>(state->pindexBestKnownBlock->nHeight, chainActive.Height() + BLOCK_DOWNLOAD_WINDOW + 1);
+    NodeId waitingfor = -1;
+    while (pindexWalk->nHeight < nMaxHeight) {
+        // Read up to 128 (or more, if more blocks than that are needed) successors of pindexWalk (towards
+        // pindexBestKnownBlock) into vToFetch. We fetch 128, because CBlockIndex::GetAncestor may be as expensive
+        // as iterating over ~100 CBlockIndex* entries anyway.
+        int nToFetch = std::min(nMaxHeight - pindexWalk->nHeight, std::max<int>(count - vBlocks.size(), 128));
+        vToFetch.resize(nToFetch);
+        pindexWalk = state->pindexBestKnownBlock->GetAncestor(pindexWalk->nHeight + nToFetch);
+        vToFetch[nToFetch - 1] = pindexWalk;
+        for (unsigned int i = nToFetch - 1; i > 0; i--) {
+            vToFetch[i - 1] = vToFetch[i]->pprev;
+        }
+
+        // Iterate over those blocks in vToFetch (in forward direction), adding the ones that
+        // are not yet downloaded and not in flight to vBlocks. In the mean time, update
+        // pindexLastCommonBlock as long as all ancestors are already downloaded.
+        BOOST_FOREACH(CBlockIndex* pindex, vToFetch) {
+            if (pindex->nStatus & BLOCK_HAVE_DATA) {
+                if (pindex->nChainTx)
+                    state->pindexLastCommonBlock = pindex;
+            } else if (mapBlocksInFlight.count(pindex->GetBlockHash()) == 0) {
+                // The block is not already downloaded, and not yet in flight.
+                if (pindex->nHeight > chainActive.Height() + (int)BLOCK_DOWNLOAD_WINDOW) {
+                    // We reached the end of the window.
+                    if (vBlocks.size() == 0 && waitingfor != nodeid) {
+                        // We aren't able to fetch anything, but we would be if the download window was one larger.
+                        nodeStaller = waitingfor;
+                    }
+                    return;
+                }
+                vBlocks.push_back(pindex);
+                if (vBlocks.size() == count) {
+                    return;
+                }
+            } else if (waitingfor == -1) {
+                // This is the first already-in-flight block.
+                waitingfor = mapBlocksInFlight[pindex->GetBlockHash()].first;
+            }
+        }
+    }
+}
+
 } // anon namespace
 
 bool GetNodeStateStats(NodeId nodeid, CNodeStateStats &stats) {
@@ -1086,46 +1155,6 @@ bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex)
     return true;
 }
 
-uint256 static GetOrphanRoot(const uint256& hash)
-{
-    map<uint256, COrphanBlock*>::iterator it = mapOrphanBlocks.find(hash);
-    if (it == mapOrphanBlocks.end())
-        return hash;
-
-    // Work back to the first block in the orphan chain
-    do {
-        map<uint256, COrphanBlock*>::iterator it2 = mapOrphanBlocks.find(it->second->hashPrev);
-        if (it2 == mapOrphanBlocks.end())
-            return it->first;
-        it = it2;
-    } while(true);
-}
-
-// Remove a random orphan block (which does not have any dependent orphans).
-void static PruneOrphanBlocks()
-{
-    if (mapOrphanBlocksByPrev.size() <= (size_t)std::max((int64_t)0, GetArg("-maxorphanblocks", DEFAULT_MAX_ORPHAN_BLOCKS)))
-        return;
-
-    // Pick a random orphan block.
-    int pos = insecure_rand() % mapOrphanBlocksByPrev.size();
-    std::multimap<uint256, COrphanBlock*>::iterator it = mapOrphanBlocksByPrev.begin();
-    while (pos--) it++;
-
-    // As long as this block has other orphans depending on it, move to one of those successors.
-    do {
-        std::multimap<uint256, COrphanBlock*>::iterator it2 = mapOrphanBlocksByPrev.find(it->second->hashBlock);
-        if (it2 == mapOrphanBlocksByPrev.end())
-            break;
-        it = it2;
-    } while(1);
-
-    uint256 hash = it->second->hashBlock;
-    delete it->second;
-    mapOrphanBlocksByPrev.erase(it);
-    mapOrphanBlocks.erase(hash);
-}
-
 CAmount GetBlockValue(int nHeight, const CAmount& nFees)
 {
     int64_t nSubsidy = 50 * COIN;
@@ -1664,11 +1693,6 @@ bool ConnectBlock(CBlock& block, CValidationState& state, CBlockIndex* pindex, C
     if (fJustCheck)
         return true;
 
-    // Correct transaction counts.
-    pindex->nTx = block.vtx.size();
-    if (pindex->pprev)
-        pindex->nChainTx = pindex->pprev->nChainTx + block.vtx.size();
-
     // Write undo information to disk
     if (pindex->GetUndoPos().IsNull() || !pindex->IsValid(BLOCK_VALID_SCRIPTS))
     {
@@ -1900,6 +1924,8 @@ static CBlockIndex* FindMostWorkChain() {
         CBlockIndex *pindexTest = pindexNew;
         bool fInvalidAncestor = false;
         while (pindexTest && !chainActive.Contains(pindexTest)) {
+            assert(pindexTest->nStatus & BLOCK_HAVE_DATA);
+            assert(pindexTest->nChainTx || pindexTest->nHeight == 0);
             if (pindexTest->nStatus & BLOCK_FAILED_MASK) {
                 // Candidate has an invalid ancestor, remove entire chain from the set.
                 if (pindexBestInvalid == NULL || pindexNew->nChainWork > pindexBestInvalid->nChainWork)
@@ -2032,7 +2058,7 @@ bool ActivateBestChain(CValidationState &state, CBlock *pblock) {
     return true;
 }
 
-CBlockIndex* AddToBlockIndex(CBlockHeader& block)
+CBlockIndex* AddToBlockIndex(const CBlockHeader& block)
 {
     // Check for duplicate
     uint256 hash = block.GetHash();
@@ -2043,10 +2069,10 @@ CBlockIndex* AddToBlockIndex(CBlockHeader& block)
     // Construct new block index object
     CBlockIndex* pindexNew = new CBlockIndex(block);
     assert(pindexNew);
-    {
-         LOCK(cs_nBlockSequenceId);
-         pindexNew->nSequenceId = nBlockSequenceId++;
-    }
+    // We assign the sequence id to blocks only when the full data is available,
+    // to avoid miners withholding blocks but broadcasting headers, to get a
+    // competitive advantage.
+    pindexNew->nSequenceId = 0;
     BlockMap::iterator mi = mapBlockIndex.insert(make_pair(hash, pindexNew)).first;
     pindexNew->phashBlock = &((*mi).first);
     BlockMap::iterator miPrev = mapBlockIndex.find(block.hashPrevBlock);
@@ -2058,6 +2084,11 @@ CBlockIndex* AddToBlockIndex(CBlockHeader& block)
     }
     pindexNew->nChainWork = (pindexNew->pprev ? pindexNew->pprev->nChainWork : 0) + pindexNew->GetBlockWork();
     pindexNew->RaiseValidity(BLOCK_VALID_TREE);
+    if (pindexBestHeader == NULL || pindexBestHeader->nChainWork < pindexNew->nChainWork)
+        pindexBestHeader = pindexNew;
+
+    // Ok if it fails, we'll download the header again next time.
+    pblocktree->WriteBlockIndex(CDiskBlockIndex(pindexNew));
 
     return pindexNew;
 }
@@ -2066,30 +2097,45 @@ CBlockIndex* AddToBlockIndex(CBlockHeader& block)
 bool ReceivedBlockTransactions(const CBlock &block, CValidationState& state, CBlockIndex *pindexNew, const CDiskBlockPos& pos)
 {
     pindexNew->nTx = block.vtx.size();
-    if (pindexNew->pprev) {
-        // Not the genesis block.
-        if (pindexNew->pprev->nChainTx) {
-            // This parent's block's total number transactions is known, so compute outs.
-            pindexNew->nChainTx = pindexNew->pprev->nChainTx + pindexNew->nTx;
-        } else {
-            // The total number of transactions isn't known yet.
-            // We will compute it when the block is connected.
-            pindexNew->nChainTx = 0;
-        }
-    } else {
-        // Genesis block.
-        pindexNew->nChainTx = pindexNew->nTx;
-    }
+    pindexNew->nChainTx = 0;
     pindexNew->nFile = pos.nFile;
     pindexNew->nDataPos = pos.nPos;
     pindexNew->nUndoPos = 0;
     pindexNew->nStatus |= BLOCK_HAVE_DATA;
+    pindexNew->RaiseValidity(BLOCK_VALID_TRANSACTIONS);
+    {
+         LOCK(cs_nBlockSequenceId);
+         pindexNew->nSequenceId = nBlockSequenceId++;
+    }
 
-    if (pindexNew->RaiseValidity(BLOCK_VALID_TRANSACTIONS))
-        setBlockIndexValid.insert(pindexNew);
+    if (pindexNew->pprev == NULL || pindexNew->pprev->nChainTx) {
+        // If pindexNew is the genesis block or all parents are BLOCK_VALID_TRANSACTIONS.
+        deque<CBlockIndex*> queue;
+        queue.push_back(pindexNew);
 
-    if (!pblocktree->WriteBlockIndex(CDiskBlockIndex(pindexNew)))
-        return state.Abort("Failed to write block index");
+        // Recursively process any descendant blocks that now may be eligible to be connected.
+        while (!queue.empty()) {
+            CBlockIndex *pindex = queue.front();
+            queue.pop_front();
+            pindex->nChainTx = (pindex->pprev ? pindex->pprev->nChainTx : 0) + pindex->nTx;
+            setBlockIndexValid.insert(pindex);
+            std::pair<std::multimap<CBlockIndex*, CBlockIndex*>::iterator, std::multimap<CBlockIndex*, CBlockIndex*>::iterator> range = mapBlocksUnlinked.equal_range(pindex);
+            while (range.first != range.second) {
+                std::multimap<CBlockIndex*, CBlockIndex*>::iterator it = range.first;
+                queue.push_back(it->second);
+                range.first++;
+                mapBlocksUnlinked.erase(it);
+            }
+            if (!pblocktree->WriteBlockIndex(CDiskBlockIndex(pindex)))
+                return state.Abort("Failed to write block index");
+        }
+    } else {
+        if (pindexNew->pprev && pindexNew->pprev->IsValid(BLOCK_VALID_TREE)) {
+            mapBlocksUnlinked.insert(std::make_pair(pindexNew->pprev, pindexNew));
+        }
+        if (!pblocktree->WriteBlockIndex(CDiskBlockIndex(pindexNew)))
+            return state.Abort("Failed to write block index");
+    }
 
     return true;
 }
@@ -2205,12 +2251,31 @@ bool CheckBlockHeader(const CBlockHeader& block, CValidationState& state, bool f
 
 bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bool fCheckMerkleRoot)
 {
-    // These are checks that are independent of context
-    // that can be verified before saving an orphan block.
+    // These are checks that are independent of context.
 
     if (!CheckBlockHeader(block, state, fCheckPOW))
         return false;
 
+    // Check the merkle root.
+    if (fCheckMerkleRoot) {
+        bool mutated;
+        uint256 hashMerkleRoot2 = block.BuildMerkleTree(&mutated);
+        if (block.hashMerkleRoot != hashMerkleRoot2)
+            return state.DoS(100, error("CheckBlock() : hashMerkleRoot mismatch"),
+                             REJECT_INVALID, "bad-txnmrklroot", true);
+
+        // Check for merkle tree malleability (CVE-2012-2459): repeating sequences
+        // of transactions in a block without affecting the merkle root of a block,
+        // while still invalidating it.
+        if (mutated)
+            return state.DoS(100, error("CheckBlock() : duplicate transaction"),
+                             REJECT_INVALID, "bad-txns-duplicate", true);
+    }
+
+    // All potential-corruption validation must be done before we do any
+    // transaction validation, as otherwise we may mark the header as invalid
+    // because we receive the wrong transactions for it.
+
     // Size limits
     if (block.vtx.empty() || block.vtx.size() > MAX_BLOCK_SIZE || ::GetSerializeSize(block, SER_NETWORK, PROTOCOL_VERSION) > MAX_BLOCK_SIZE)
         return state.DoS(100, error("CheckBlock() : size limits failed"),
@@ -2230,15 +2295,6 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
         if (!CheckTransaction(tx, state))
             return error("CheckBlock() : CheckTransaction failed");
 
-    // Check for merkle tree malleability (CVE-2012-2459): repeating sequences
-    // of transactions in a block without affecting the merkle root of a block,
-    // while still invalidating it.
-    bool mutated;
-    uint256 hashMerkleRoot2 = block.BuildMerkleTree(&mutated);
-    if (mutated)
-        return state.DoS(100, error("CheckBlock() : duplicate transaction"),
-                         REJECT_INVALID, "bad-txns-duplicate", true);
-
     unsigned int nSigOps = 0;
     BOOST_FOREACH(const CTransaction& tx, block.vtx)
     {
@@ -2248,15 +2304,10 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
         return state.DoS(100, error("CheckBlock() : out-of-bounds SigOpCount"),
                          REJECT_INVALID, "bad-blk-sigops", true);
 
-    // Check merkle root
-    if (fCheckMerkleRoot && block.hashMerkleRoot != hashMerkleRoot2)
-        return state.DoS(100, error("CheckBlock() : hashMerkleRoot mismatch"),
-                         REJECT_INVALID, "bad-txnmrklroot", true);
-
     return true;
 }
 
-bool AcceptBlockHeader(CBlockHeader& block, CValidationState& state, CBlockIndex** ppindex)
+bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, CBlockIndex** ppindex)
 {
     AssertLockHeld(cs_main);
     // Check for duplicate
@@ -2264,9 +2315,13 @@ bool AcceptBlockHeader(CBlockHeader& block, CValidationState& state, CBlockIndex
     BlockMap::iterator miSelf = mapBlockIndex.find(hash);
     CBlockIndex *pindex = NULL;
     if (miSelf != mapBlockIndex.end()) {
+        // Block header is already known.
         pindex = miSelf->second;
+        if (ppindex)
+            *ppindex = pindex;
         if (pindex->nStatus & BLOCK_FAILED_MASK)
             return state.Invalid(error("%s : block is marked invalid", __func__), 0, "duplicate");
+        return true;
     }
 
     CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint();
@@ -2344,6 +2399,12 @@ bool AcceptBlock(CBlock& block, CValidationState& state, CBlockIndex** ppindex,
     if (!AcceptBlockHeader(block, state, &pindex))
         return false;
 
+    if (pindex->nStatus & BLOCK_HAVE_DATA) {
+        // TODO: deal better with duplicate blocks.
+        // return state.DoS(20, error("AcceptBlock() : already have block %d %s", pindex->nHeight, pindex->GetBlockHash().ToString()), REJECT_DUPLICATE, "duplicate");
+        return true;
+    }
+
     if (!CheckBlock(block, state)) {
         if (state.IsInvalid() && !state.CorruptionPossible()) {
             pindex->nStatus |= BLOCK_FAILED_VALID;
@@ -2456,93 +2517,26 @@ void CBlockIndex::BuildSkip()
         pskip = pprev->GetAncestor(GetSkipHeight(nHeight));
 }
 
-void PushGetBlocks(CNode* pnode, CBlockIndex* pindexBegin, uint256 hashEnd)
-{
-    AssertLockHeld(cs_main);
-    // Filter out duplicate requests
-    if (pindexBegin == pnode->pindexLastGetBlocksBegin && hashEnd == pnode->hashLastGetBlocksEnd)
-        return;
-    pnode->pindexLastGetBlocksBegin = pindexBegin;
-    pnode->hashLastGetBlocksEnd = hashEnd;
-
-    pnode->PushMessage("getblocks", chainActive.GetLocator(pindexBegin), hashEnd);
-}
-
 bool ProcessBlock(CValidationState &state, CNode* pfrom, CBlock* pblock, CDiskBlockPos *dbp)
 {
-    // Check for duplicate
-    uint256 hash = pblock->GetHash();
-
-    {
-    LOCK(cs_main);
-    if (mapBlockIndex.count(hash))
-        return state.Invalid(error("ProcessBlock() : already have block %d %s", mapBlockIndex[hash]->nHeight, hash.ToString()), 0, "duplicate");
-    if (mapOrphanBlocks.count(hash))
-        return state.Invalid(error("ProcessBlock() : already have block (orphan) %s", hash.ToString()), 0, "duplicate");
-
     // Preliminary checks
-    if (!CheckBlock(*pblock, state))
-        return error("ProcessBlock() : CheckBlock FAILED");
+    bool checked = CheckBlock(*pblock, state);
 
-    // If we don't already have its previous block (with full data), shunt it off to holding area until we get it
-    BlockMap::iterator it = mapBlockIndex.find(pblock->hashPrevBlock);
-    if (pblock->hashPrevBlock != 0 && (it == mapBlockIndex.end() || !(it->second->nStatus & BLOCK_HAVE_DATA)))
     {
-        LogPrintf("ProcessBlock: ORPHAN BLOCK %lu, prev=%s\n", (unsigned long)mapOrphanBlocks.size(), pblock->hashPrevBlock.ToString());
-
-        // Accept orphans as long as there is a node to request its parents from
-        if (pfrom) {
-            PruneOrphanBlocks();
-            COrphanBlock* pblock2 = new COrphanBlock();
-            {
-                CDataStream ss(SER_DISK, CLIENT_VERSION);
-                ss << *pblock;
-                pblock2->vchBlock = std::vector<unsigned char>(ss.begin(), ss.end());
-            }
-            pblock2->hashBlock = hash;
-            pblock2->hashPrev = pblock->hashPrevBlock;
-            mapOrphanBlocks.insert(make_pair(hash, pblock2));
-            mapOrphanBlocksByPrev.insert(make_pair(pblock2->hashPrev, pblock2));
-
-            // Ask this guy to fill in what we're missing
-            PushGetBlocks(pfrom, chainActive.Tip(), GetOrphanRoot(hash));
+        LOCK(cs_main);
+        MarkBlockAsReceived(pblock->GetHash());
+        if (!checked) {
+            return error("ProcessBlock() : CheckBlock FAILED");
         }
-        return true;
-    }
 
-    // Store to disk
-    CBlockIndex *pindex = NULL;
-    bool ret = AcceptBlock(*pblock, state, &pindex, dbp);
-    if (!ret)
-        return error("ProcessBlock() : AcceptBlock FAILED");
-
-    // Recursively process any orphan blocks that depended on this one
-    vector<uint256> vWorkQueue;
-    vWorkQueue.push_back(hash);
-    for (unsigned int i = 0; i < vWorkQueue.size(); i++)
-    {
-        uint256 hashPrev = vWorkQueue[i];
-        for (multimap<uint256, COrphanBlock*>::iterator mi = mapOrphanBlocksByPrev.lower_bound(hashPrev);
-             mi != mapOrphanBlocksByPrev.upper_bound(hashPrev);
-             ++mi)
-        {
-            CBlock block;
-            {
-                CDataStream ss(mi->second->vchBlock, SER_DISK, CLIENT_VERSION);
-                ss >> block;
-            }
-            block.BuildMerkleTree();
-            // Use a dummy CValidationState so someone can't setup nodes to counter-DoS based on orphan resolution (that is, feeding people an invalid block based on LegitBlockX in order to get anyone relaying LegitBlockX banned)
-            CValidationState stateDummy;
-            CBlockIndex *pindexChild = NULL;
-            if (AcceptBlock(block, stateDummy, &pindexChild))
-                vWorkQueue.push_back(mi->second->hashBlock);
-            mapOrphanBlocks.erase(mi->second->hashBlock);
-            delete mi->second;
+        // Store to disk
+        CBlockIndex *pindex = NULL;
+        bool ret = AcceptBlock(*pblock, state, &pindex, dbp);
+        if (pindex && pfrom) {
+            mapBlockSource[pindex->GetBlockHash()] = pfrom->GetId();
         }
-        mapOrphanBlocksByPrev.erase(hashPrev);
-    }
-
+        if (!ret)
+            return error("ProcessBlock() : AcceptBlock FAILED");
     }
 
     if (!ActivateBestChain(state, pblock))
@@ -2808,13 +2802,26 @@ bool static LoadBlockIndexDB()
     {
         CBlockIndex* pindex = item.second;
         pindex->nChainWork = (pindex->pprev ? pindex->pprev->nChainWork : 0) + pindex->GetBlockWork();
-        pindex->nChainTx = (pindex->pprev ? pindex->pprev->nChainTx : 0) + pindex->nTx;
-        if (pindex->IsValid(BLOCK_VALID_TRANSACTIONS))
+        if (pindex->nStatus & BLOCK_HAVE_DATA) {
+            if (pindex->pprev) {
+                if (pindex->pprev->nChainTx) {
+                    pindex->nChainTx = pindex->pprev->nChainTx + pindex->nTx;
+                } else {
+                    pindex->nChainTx = 0;
+                    mapBlocksUnlinked.insert(std::make_pair(pindex->pprev, pindex));
+                }
+            } else {
+                pindex->nChainTx = pindex->nTx;
+            }
+        }
+        if (pindex->IsValid(BLOCK_VALID_TRANSACTIONS) && (pindex->nChainTx || pindex->pprev == NULL))
             setBlockIndexValid.insert(pindex);
         if (pindex->nStatus & BLOCK_FAILED_MASK && (!pindexBestInvalid || pindex->nChainWork > pindexBestInvalid->nChainWork))
             pindexBestInvalid = pindex;
         if (pindex->pprev)
             pindex->BuildSkip();
+        if (pindex->IsValid(BLOCK_VALID_TREE) && (pindexBestHeader == NULL || CBlockIndexWorkComparator()(pindexBestHeader, pindex)))
+            pindexBestHeader = pindex;
     }
 
     // Load block file info
@@ -3226,8 +3233,7 @@ bool static AlreadyHave(const CInv& inv)
                 pcoinsTip->HaveCoins(inv.hash);
         }
     case MSG_BLOCK:
-        return mapBlockIndex.count(inv.hash) ||
-               mapOrphanBlocks.count(inv.hash);
+        return mapBlockIndex.count(inv.hash);
     }
     // Don't know what it is, just say we already got one
     return true;
@@ -3375,10 +3381,6 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         return true;
     }
 
-    {
-        LOCK(cs_main);
-        State(pfrom->GetId())->nLastBlockProcess = GetTimeMicros();
-    }
 
 
 
@@ -3587,6 +3589,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
         LOCK(cs_main);
 
+        std::vector<CInv> vToFetch;
+
         for (unsigned int nInv = 0; nInv < vInv.size(); nInv++)
         {
             const CInv &inv = vInv[nInv];
@@ -3597,19 +3601,29 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             bool fAlreadyHave = AlreadyHave(inv);
             LogPrint("net", "got inv: %s  %s peer=%d\n", inv.ToString(), fAlreadyHave ? "have" : "new", pfrom->id);
 
-            if (!fAlreadyHave) {
-                if (!fImporting && !fReindex) {
-                    if (inv.type == MSG_BLOCK)
-                        AddBlockToQueue(pfrom->GetId(), inv.hash);
-                    else
-                        pfrom->AskFor(inv);
-                }
-            } else if (inv.type == MSG_BLOCK && mapOrphanBlocks.count(inv.hash)) {
-                PushGetBlocks(pfrom, chainActive.Tip(), GetOrphanRoot(inv.hash));
-            }
+            if (!fAlreadyHave && !fImporting && !fReindex && inv.type != MSG_BLOCK)
+                pfrom->AskFor(inv);
 
-            if (inv.type == MSG_BLOCK)
+            if (inv.type == MSG_BLOCK) {
                 UpdateBlockAvailability(pfrom->GetId(), inv.hash);
+                if (!fAlreadyHave && !fImporting && !fReindex && !mapBlocksInFlight.count(inv.hash)) {
+                    // First request the headers preceeding the announced block. In the normal fully-synced
+                    // case where a new block is announced that succeeds the current tip (no reorganization),
+                    // there are no such headers.
+                    // Secondly, and only when we are close to being synced, we request the announced block directly,
+                    // to avoid an extra round-trip. Note that we must *first* ask for the headers, so by the
+                    // time the block arrives, the header chain leading up to it is already validated. Not
+                    // doing this will result in the received block being rejected as an orphan in case it is
+                    // not a direct successor.
+                    pfrom->PushMessage("getheaders", chainActive.GetLocator(pindexBestHeader), inv.hash);
+                    if (chainActive.Tip()->GetBlockTime() > GetAdjustedTime() - Params().TargetSpacing() * 20) {
+                        vToFetch.push_back(inv);
+                        // Mark block as in flight already, even though the actual "getdata" message only goes out
+                        // later (within the same cs_main lock, though).
+                        MarkBlockAsInFlight(pfrom->GetId(), inv.hash);
+                    }
+                }
+            }
 
             // Track requests for our stuff
             g_signals.Inventory(inv.hash);
@@ -3619,6 +3633,9 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                 return error("send buffer size() = %u", pfrom->nSendSize);
             }
         }
+
+        if (!vToFetch.empty())
+            pfrom->PushMessage("getdata", vToFetch);
     }
 
 
@@ -3706,7 +3723,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
         // we must use CBlocks, as CBlockHeaders won't include the 0x00 nTx count at the end
         vector<CBlock> vHeaders;
-        int nLimit = 2000;
+        int nLimit = MAX_HEADERS_RESULTS;
         LogPrint("net", "getheaders %d to %s\n", (pindex ? pindex->nHeight : -1), hashStop.ToString());
         for (; pindex; pindex = chainActive.Next(pindex))
         {
@@ -3826,22 +3843,66 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
     }
 
 
+    else if (strCommand == "headers" && !fImporting && !fReindex) // Ignore headers received while importing
+    {
+        std::vector<CBlockHeader> headers;
+
+        // Bypass the normal CBlock deserialization, as we don't want to risk deserializing 2000 full blocks.
+        unsigned int nCount = ReadCompactSize(vRecv);
+        if (nCount > MAX_HEADERS_RESULTS) {
+            Misbehaving(pfrom->GetId(), 20);
+            return error("headers message size = %u", nCount);
+        }
+        headers.resize(nCount);
+        for (unsigned int n = 0; n < nCount; n++) {
+            vRecv >> headers[n];
+            ReadCompactSize(vRecv); // ignore tx count; assume it is 0.
+        }
+
+        LOCK(cs_main);
+
+        if (nCount == 0) {
+            // Nothing interesting. Stop asking this peers for more headers.
+            return true;
+        }
+
+        CBlockIndex *pindexLast = NULL;
+        BOOST_FOREACH(const CBlockHeader& header, headers) {
+            CValidationState state;
+            if (pindexLast != NULL && header.hashPrevBlock != pindexLast->GetBlockHash()) {
+                Misbehaving(pfrom->GetId(), 20);
+                return error("non-continuous headers sequence");
+            }
+            if (!AcceptBlockHeader(header, state, &pindexLast)) {
+                int nDoS;
+                if (state.IsInvalid(nDoS)) {
+                    if (nDoS > 0)
+                        Misbehaving(pfrom->GetId(), nDoS);
+                    return error("invalid header received");
+                }
+            }
+        }
+
+        if (pindexLast)
+            UpdateBlockAvailability(pfrom->GetId(), pindexLast->GetBlockHash());
+
+        if (nCount == MAX_HEADERS_RESULTS && pindexLast) {
+            // Headers message had its maximum size; the peer may have more headers.
+            // TODO: optimize: if pindexLast is an ancestor of chainActive.Tip or pindexBestHeader, continue
+            // from there instead.
+            pfrom->PushMessage("getheaders", chainActive.GetLocator(pindexLast), uint256(0));
+        }
+    }
+
     else if (strCommand == "block" && !fImporting && !fReindex) // Ignore blocks received while importing
     {
         CBlock block;
         vRecv >> block;
 
-        LogPrint("net", "received block %s peer=%d\n", block.GetHash().ToString(), pfrom->id);
-
         CInv inv(MSG_BLOCK, block.GetHash());
-        pfrom->AddInventoryKnown(inv);
+        LogPrint("net", "received block %s peer=%d\n", inv.hash.ToString(), pfrom->id);
 
-        {
-            LOCK(cs_main);
-            // Remember who we got this block from.
-            mapBlockSource[inv.hash] = pfrom->GetId();
-            MarkBlockAsReceived(inv.hash, pfrom->GetId());
-        }
+        pfrom->AddInventoryKnown(inv);
 
         CValidationState state;
         ProcessBlock(state, pfrom, &block);
@@ -4323,9 +4384,17 @@ bool SendMessages(CNode* pto, bool fSendTrickle)
         state.rejects.clear();
 
         // Start block sync
-        if (pto->fStartSync && !fImporting && !fReindex) {
-            pto->fStartSync = false;
-            PushGetBlocks(pto, chainActive.Tip(), uint256(0));
+        if (pindexBestHeader == NULL)
+            pindexBestHeader = chainActive.Tip();
+        bool fFetch = !pto->fInbound || (pindexBestHeader && (state.pindexLastCommonBlock ? state.pindexLastCommonBlock->nHeight : 0) + 144 > pindexBestHeader->nHeight);
+        if (!state.fSyncStarted && !pto->fClient && fFetch && !fImporting && !fReindex) {
+            // Only actively request headers from a single peer, unless we're close to today.
+            if (nSyncStarted == 0 || pindexBestHeader->GetBlockTime() > GetAdjustedTime() - 24 * 60 * 60) {
+                state.fSyncStarted = true;
+                nSyncStarted++;
+                CBlockIndex *pindexStart = pindexBestHeader->pprev ? pindexBestHeader->pprev : pindexBestHeader;
+                pto->PushMessage("getheaders", chainActive.GetLocator(pindexStart), uint256(0));
+            }
         }
 
         // Resend wallet transactions that haven't gotten in a block yet
@@ -4384,35 +4453,32 @@ bool SendMessages(CNode* pto, bool fSendTrickle)
         if (!vInv.empty())
             pto->PushMessage("inv", vInv);
 
-
-        // Detect stalled peers. Require that blocks are in flight, we haven't
-        // received a (requested) block in one minute, and that all blocks are
-        // in flight for over two minutes, since we first had a chance to
-        // process an incoming block.
+        // Detect whether we're stalling
         int64_t nNow = GetTimeMicros();
-        if (!pto->fDisconnect && state.nBlocksInFlight &&
-            state.nLastBlockReceive < state.nLastBlockProcess - BLOCK_DOWNLOAD_TIMEOUT*1000000 &&
-            state.vBlocksInFlight.front().nTime < state.nLastBlockProcess - 2*BLOCK_DOWNLOAD_TIMEOUT*1000000) {
-            LogPrintf("Peer %s is stalling block download, disconnecting\n", state.name);
+        if (!pto->fDisconnect && state.nStallingSince && state.nStallingSince < nNow - 1000000 * BLOCK_STALLING_TIMEOUT) {
+            // Stalling only triggers when the block download window cannot move. During normal steady state,
+            // the download window should be much larger than the to-be-downloaded set of blocks, so disconnection
+            // should only happen during initial block download.
+            LogPrintf("Peer=%d is stalling block download, disconnecting\n", pto->id);
             pto->fDisconnect = true;
         }
 
-        // Update knowledge of peer's block availability.
-        ProcessBlockAvailability(pto->GetId());
-
         //
         // Message: getdata (blocks)
         //
         vector<CInv> vGetData;
-        while (!pto->fDisconnect && state.nBlocksToDownload && state.nBlocksInFlight < MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
-            uint256 hash = state.vBlocksToDownload.front();
-            vGetData.push_back(CInv(MSG_BLOCK, hash));
-            MarkBlockAsInFlight(pto->GetId(), hash);
-            LogPrint("net", "Requesting block %s peer=%d\n", hash.ToString(), pto->id);
-            if (vGetData.size() >= 1000)
-            {
-                pto->PushMessage("getdata", vGetData);
-                vGetData.clear();
+        if (!pto->fDisconnect && !pto->fClient && fFetch && state.nBlocksInFlight < MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
+            vector<CBlockIndex*> vToDownload;
+            NodeId staller = -1;
+            FindNextBlocksToDownload(pto->GetId(), MAX_BLOCKS_IN_TRANSIT_PER_PEER - state.nBlocksInFlight, vToDownload, staller);
+            BOOST_FOREACH(CBlockIndex *pindex, vToDownload) {
+                vGetData.push_back(CInv(MSG_BLOCK, pindex->GetBlockHash()));
+                MarkBlockAsInFlight(pto->GetId(), pindex->GetBlockHash(), pindex);
+                LogPrint("net", "Requesting block %s peer=%d\n", pindex->GetBlockHash().ToString(), pto->id);
+            }
+            if (state.nBlocksInFlight == 0 && staller != -1) {
+                if (State(staller)->nStallingSince == 0)
+                    State(staller)->nStallingSince = nNow;
             }
         }
 
@@ -4519,12 +4585,6 @@ public:
             delete (*it1).second;
         mapBlockIndex.clear();
 
-        // orphan blocks
-        std::map<uint256, COrphanBlock*>::iterator it2 = mapOrphanBlocks.begin();
-        for (; it2 != mapOrphanBlocks.end(); it2++)
-            delete (*it2).second;
-        mapOrphanBlocks.clear();
-
         // orphan transactions
         mapOrphanTransactions.clear();
         mapOrphanTransactionsByPrev.clear();
diff --git a/src/main.h b/src/main.h
index cad7eebfb7..674f2fe4a4 100644
--- a/src/main.h
+++ b/src/main.h
@@ -72,9 +72,17 @@ static const int MAX_SCRIPTCHECK_THREADS = 16;
 /** -par default (number of script-checking threads, 0 = auto) */
 static const int DEFAULT_SCRIPTCHECK_THREADS = 0;
 /** Number of blocks that can be requested at any given time from a single peer. */
-static const int MAX_BLOCKS_IN_TRANSIT_PER_PEER = 128;
-/** Timeout in seconds before considering a block download peer unresponsive. */
-static const unsigned int BLOCK_DOWNLOAD_TIMEOUT = 60;
+static const int MAX_BLOCKS_IN_TRANSIT_PER_PEER = 16;
+/** Timeout in seconds during which a peer must stall block download progress before being disconnected. */
+static const unsigned int BLOCK_STALLING_TIMEOUT = 2;
+/** Number of headers sent in one getheaders result. We rely on the assumption that if a peer sends
+ *  less than this number, we reached their tip. Changing this value is a protocol upgrade. */
+static const unsigned int MAX_HEADERS_RESULTS = 2000;
+/** Size of the "block download window": how far ahead of our current height do we fetch?
+ *  Larger windows tolerate larger download speed differences between peer, but increase the potential
+ *  degree of disordering of blocks on disk (which make reindexing and in the future perhaps pruning
+ *  harder). We'll probably want to make this a per-peer adaptive value at some point. */
+static const unsigned int BLOCK_DOWNLOAD_WINDOW = 1024;
 
 /** "reject" message codes **/
 static const unsigned char REJECT_MALFORMED = 0x01;
@@ -137,8 +145,6 @@ void RegisterNodeSignals(CNodeSignals& nodeSignals);
 /** Unregister a network node */
 void UnregisterNodeSignals(CNodeSignals& nodeSignals);
 
-void PushGetBlocks(CNode* pnode, CBlockIndex* pindexBegin, uint256 hashEnd);
-
 /** Process an incoming block */
 bool ProcessBlock(CValidationState &state, CNode* pfrom, CBlock* pblock, CDiskBlockPos *dbp = NULL);
 /** Check whether enough disk space is available for an incoming block */
@@ -439,9 +445,6 @@ bool DisconnectBlock(CBlock& block, CValidationState& state, CBlockIndex* pindex
 // Apply the effects of this block (with given index) on the UTXO set represented by coins
 bool ConnectBlock(CBlock& block, CValidationState& state, CBlockIndex* pindex, CCoinsViewCache& coins, bool fJustCheck = false);
 
-// Add this block to the block index, and if necessary, switch the active block chain to this
-bool AddToBlockIndex(CBlock& block, CValidationState& state, const CDiskBlockPos& pos);
-
 // Context-independent validity checks
 bool CheckBlockHeader(const CBlockHeader& block, CValidationState& state, bool fCheckPOW = true);
 bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW = true, bool fCheckMerkleRoot = true);
@@ -449,7 +452,7 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW = t
 // Store block on disk
 // if dbp is provided, the file is known to already reside on disk
 bool AcceptBlock(CBlock& block, CValidationState& state, CBlockIndex **pindex, CDiskBlockPos* dbp = NULL);
-bool AcceptBlockHeader(CBlockHeader& block, CValidationState& state, CBlockIndex **ppindex= NULL);
+bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, CBlockIndex **ppindex= NULL);
 
 
 
diff --git a/src/net.cpp b/src/net.cpp
index dd5cb480cc..50b435cf14 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -73,7 +73,6 @@ map<CNetAddr, LocalServiceInfo> mapLocalHost;
 static bool vfReachable[NET_MAX] = {};
 static bool vfLimited[NET_MAX] = {};
 static CNode* pnodeLocalHost = NULL;
-static CNode* pnodeSync = NULL;
 uint64_t nLocalHostNonce = 0;
 static std::vector<ListenSocket> vhListenSocket;
 CAddrMan addrman;
@@ -519,10 +518,6 @@ void CNode::CloseSocketDisconnect()
     TRY_LOCK(cs_vRecvMsg, lockRecv);
     if (lockRecv)
         vRecvMsg.clear();
-
-    // if this was the sync node, we'll need a new one
-    if (this == pnodeSync)
-        pnodeSync = NULL;
 }
 
 void CNode::PushVersion()
@@ -615,7 +610,6 @@ void CNode::copyStats(CNodeStats &stats)
     X(nSendBytes);
     X(nRecvBytes);
     X(fWhitelisted);
-    stats.fSyncNode = (this == pnodeSync);
 
     // It is common for nodes with good ping times to suddenly become lagged,
     // due to a new block arriving or other large transfer.
@@ -1487,61 +1481,20 @@ bool OpenNetworkConnection(const CAddress& addrConnect, CSemaphoreGrant *grantOu
 }
 
 
-// for now, use a very simple selection metric: the node from which we received
-// most recently
-static int64_t NodeSyncScore(const CNode *pnode) {
-    return pnode->nLastRecv;
-}
-
-void static StartSync(const vector<CNode*> &vNodes) {
-    CNode *pnodeNewSync = NULL;
-    int64_t nBestScore = 0;
-
-    int nBestHeight = g_signals.GetHeight().get_value_or(0);
-
-    // Iterate over all nodes
-    BOOST_FOREACH(CNode* pnode, vNodes) {
-        // check preconditions for allowing a sync
-        if (!pnode->fClient && !pnode->fOneShot &&
-            !pnode->fDisconnect && pnode->fSuccessfullyConnected &&
-            (pnode->nStartingHeight > (nBestHeight - 144)) &&
-            (pnode->nVersion < NOBLKS_VERSION_START || pnode->nVersion >= NOBLKS_VERSION_END)) {
-            // if ok, compare node's score with the best so far
-            int64_t nScore = NodeSyncScore(pnode);
-            if (pnodeNewSync == NULL || nScore > nBestScore) {
-                pnodeNewSync = pnode;
-                nBestScore = nScore;
-            }
-        }
-    }
-    // if a new sync candidate was found, start sync!
-    if (pnodeNewSync) {
-        pnodeNewSync->fStartSync = true;
-        pnodeSync = pnodeNewSync;
-    }
-}
-
 void ThreadMessageHandler()
 {
     SetThreadPriority(THREAD_PRIORITY_BELOW_NORMAL);
     while (true)
     {
-        bool fHaveSyncNode = false;
-
         vector<CNode*> vNodesCopy;
         {
             LOCK(cs_vNodes);
             vNodesCopy = vNodes;
             BOOST_FOREACH(CNode* pnode, vNodesCopy) {
                 pnode->AddRef();
-                if (pnode == pnodeSync)
-                    fHaveSyncNode = true;
             }
         }
 
-        if (!fHaveSyncNode)
-            StartSync(vNodesCopy);
-
         // Poll the connected nodes for messages
         CNode* pnodeTrickle = NULL;
         if (!vNodesCopy.empty())
@@ -2078,10 +2031,7 @@ CNode::CNode(SOCKET hSocketIn, CAddress addrIn, std::string addrNameIn, bool fIn
     nSendSize = 0;
     nSendOffset = 0;
     hashContinue = 0;
-    pindexLastGetBlocksBegin = 0;
-    hashLastGetBlocksEnd = 0;
     nStartingHeight = -1;
-    fStartSync = false;
     fGetAddr = false;
     fRelayTxes = false;
     setInventoryKnown.max_size(SendBufferSize() / 1000);
diff --git a/src/net.h b/src/net.h
index ad0a1df7e2..18da24183f 100644
--- a/src/net.h
+++ b/src/net.h
@@ -158,7 +158,6 @@ public:
     int nStartingHeight;
     uint64_t nSendBytes;
     uint64_t nRecvBytes;
-    bool fSyncNode;
     bool fWhitelisted;
     double dPingTime;
     double dPingWait;
@@ -276,10 +275,7 @@ protected:
 
 public:
     uint256 hashContinue;
-    CBlockIndex* pindexLastGetBlocksBegin;
-    uint256 hashLastGetBlocksEnd;
     int nStartingHeight;
-    bool fStartSync;
 
     // flood relay
     std::vector<CAddress> vAddrToSend;
diff --git a/src/qt/forms/rpcconsole.ui b/src/qt/forms/rpcconsole.ui
index 7f28209c9a..898df2b080 100644
--- a/src/qt/forms/rpcconsole.ui
+++ b/src/qt/forms/rpcconsole.ui
@@ -836,29 +836,6 @@
             </property>
            </widget>
           </item>
-          <item row="4" column="0">
-           <widget class="QLabel" name="label_25">
-            <property name="text">
-             <string>Sync Node</string>
-            </property>
-           </widget>
-          </item>
-          <item row="4" column="2">
-           <widget class="QLabel" name="peerSyncNode">
-            <property name="cursor">
-             <cursorShape>IBeamCursor</cursorShape>
-            </property>
-            <property name="text">
-             <string>N/A</string>
-            </property>
-            <property name="textFormat">
-             <enum>Qt::PlainText</enum>
-            </property>
-            <property name="textInteractionFlags">
-             <set>Qt::LinksAccessibleByMouse|Qt::TextSelectableByKeyboard|Qt::TextSelectableByMouse</set>
-            </property>
-           </widget>
-          </item>
           <item row="5" column="0">
            <widget class="QLabel" name="label_29">
             <property name="text">
diff --git a/src/qt/rpcconsole.cpp b/src/qt/rpcconsole.cpp
index 8129353d4b..2d2d448b49 100644
--- a/src/qt/rpcconsole.cpp
+++ b/src/qt/rpcconsole.cpp
@@ -611,7 +611,6 @@ void RPCConsole::updateNodeDetail(const CNodeCombinedStats *stats)
     ui->peerSubversion->setText(QString::fromStdString(stats->nodeStats.cleanSubVer));
     ui->peerDirection->setText(stats->nodeStats.fInbound ? tr("Inbound") : tr("Outbound"));
     ui->peerHeight->setText(QString("%1").arg(stats->nodeStats.nStartingHeight));
-    ui->peerSyncNode->setText(stats->nodeStats.fSyncNode ? tr("Yes") : tr("No"));
 
     // This check fails for example if the lock was busy and
     // nodeStateStats couldn't be fetched.
diff --git a/src/rpcnet.cpp b/src/rpcnet.cpp
index bc19d1372a..3bc4bb30c3 100644
--- a/src/rpcnet.cpp
+++ b/src/rpcnet.cpp
@@ -139,7 +139,6 @@ Value getpeerinfo(const Array& params, bool fHelp)
             obj.push_back(Pair("banscore", statestats.nMisbehavior));
             obj.push_back(Pair("syncheight", statestats.nSyncHeight));
         }
-        obj.push_back(Pair("syncnode", stats.fSyncNode));
         obj.push_back(Pair("whitelisted", stats.fWhitelisted));
 
         ret.push_back(obj);
diff --git a/src/version.h b/src/version.h
index 75cbec39b7..a1e440de24 100644
--- a/src/version.h
+++ b/src/version.h
@@ -33,8 +33,11 @@ static const int PROTOCOL_VERSION = 70002;
 // initial proto version, to be increased after version/verack negotiation
 static const int INIT_PROTO_VERSION = 209;
 
+// In this version, 'getheaders' was introduced.
+static const int GETHEADERS_VERSION = 31800;
+
 // disconnect from peers older than this proto version
-static const int MIN_PEER_PROTO_VERSION = 209;
+static const int MIN_PEER_PROTO_VERSION = GETHEADERS_VERSION;
 
 // nTime field added to CAddress, starting with this version;
 // if possible, avoid requesting addresses nodes older than this