core: implement statesync module

And support GetMPTData and MPTData P2P commands.

internal/fakechain/fakechain.go

@@ -13,6 +13,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/core/blockchainer/services"
 	"github.com/nspcc-dev/neo-go/pkg/core/interop"
 	"github.com/nspcc-dev/neo-go/pkg/core/mempool"
+	"github.com/nspcc-dev/neo-go/pkg/core/mpt"
 	"github.com/nspcc-dev/neo-go/pkg/core/native"
 	"github.com/nspcc-dev/neo-go/pkg/core/state"
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
@@ -42,6 +43,13 @@ type FakeChain struct {
 	UtilityTokenBalance      *big.Int
 }
 
+// FakeStateSync implements StateSync interface.
+type FakeStateSync struct {
+	IsActiveFlag      bool
+	IsInitializedFlag bool
+	InitFunc          func(h uint32) error
+}
+
 // NewFakeChain returns new FakeChain structure.
 func NewFakeChain() *FakeChain {
 	return &FakeChain{
@@ -294,6 +302,16 @@ func (chain *FakeChain) GetStateModule() blockchainer.StateRoot {
 	return nil
 }
 
+// GetStateSyncModule implements Blockchainer interface.
+func (chain *FakeChain) GetStateSyncModule() blockchainer.StateSync {
+	return &FakeStateSync{}
+}
+
+// JumpToState implements Blockchainer interface.
+func (chain *FakeChain) JumpToState(module blockchainer.StateSync) error {
+	panic("TODO")
+}
+
 // GetStorageItem implements Blockchainer interface.
 func (chain *FakeChain) GetStorageItem(id int32, key []byte) state.StorageItem {
 	panic("TODO")
@@ -436,3 +454,57 @@ func (chain *FakeChain) UnsubscribeFromNotifications(ch chan<- *state.Notificati
 func (chain *FakeChain) UnsubscribeFromTransactions(ch chan<- *transaction.Transaction) {
 	panic("TODO")
 }
+
+// AddBlock implements StateSync interface.
+func (s *FakeStateSync) AddBlock(block *block.Block) error {
+	panic("TODO")
+}
+
+// AddHeaders implements StateSync interface.
+func (s *FakeStateSync) AddHeaders(...*block.Header) error {
+	panic("TODO")
+}
+
+// AddMPTNodes implements StateSync interface.
+func (s *FakeStateSync) AddMPTNodes([][]byte) error {
+	panic("TODO")
+}
+
+// BlockHeight implements StateSync interface.
+func (s *FakeStateSync) BlockHeight() uint32 {
+	panic("TODO")
+}
+
+// IsActive implements StateSync interface.
+func (s *FakeStateSync) IsActive() bool { return s.IsActiveFlag }
+
+// IsInitialized implements StateSync interface.
+func (s *FakeStateSync) IsInitialized() bool {
+	return s.IsInitializedFlag
+}
+
+// Init implements StateSync interface.
+func (s *FakeStateSync) Init(currChainHeight uint32) error {
+	if s.InitFunc != nil {
+		return s.InitFunc(currChainHeight)
+	}
+	panic("TODO")
+}
+
+// NeedHeaders implements StateSync interface.
+func (s *FakeStateSync) NeedHeaders() bool { return false }
+
+// NeedMPTNodes implements StateSync interface.
+func (s *FakeStateSync) NeedMPTNodes() bool {
+	panic("TODO")
+}
+
+// Traverse implements StateSync interface.
+func (s *FakeStateSync) Traverse(root util.Uint256, process func(node mpt.Node, nodeBytes []byte) bool) error {
+	panic("TODO")
+}
+
+// GetJumpHeight implements StateSync interface.
+func (s *FakeStateSync) GetJumpHeight() (uint32, error) {
+	panic("TODO")
+}

pkg/core/blockchain.go

@@ -23,6 +23,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/core/native/noderoles"
 	"github.com/nspcc-dev/neo-go/pkg/core/state"
 	"github.com/nspcc-dev/neo-go/pkg/core/stateroot"
+	"github.com/nspcc-dev/neo-go/pkg/core/statesync"
 	"github.com/nspcc-dev/neo-go/pkg/core/storage"
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
 	"github.com/nspcc-dev/neo-go/pkg/crypto/hash"
@@ -409,6 +410,64 @@ func (bc *Blockchain) init() error {
 	return bc.updateExtensibleWhitelist(bHeight)
 }
 
+// JumpToState is an atomic operation that changes Blockchain state to the one
+// specified by the state sync point p. All the data needed for the jump must be
+// collected by the state sync module.
+func (bc *Blockchain) JumpToState(module blockchainer.StateSync) error {
+	bc.lock.Lock()
+	defer bc.lock.Unlock()
+
+	p, err := module.GetJumpHeight()
+	if err != nil {
+		return fmt.Errorf("failed to get jump height: %w", err)
+	}
+	if p+1 >= uint32(len(bc.headerHashes)) {
+		return fmt.Errorf("invalid state sync point")
+	}
+
+	bc.log.Info("jumping to state sync point", zap.Uint32("state sync point", p))
+
+	block, err := bc.dao.GetBlock(bc.headerHashes[p])
+	if err != nil {
+		return fmt.Errorf("failed to get current block: %w", err)
+	}
+	err = bc.dao.StoreAsCurrentBlock(block, nil)
+	if err != nil {
+		return fmt.Errorf("failed to store current block: %w", err)
+	}
+	bc.topBlock.Store(block)
+	atomic.StoreUint32(&bc.blockHeight, p)
+	atomic.StoreUint32(&bc.persistedHeight, p)
+
+	block, err = bc.dao.GetBlock(bc.headerHashes[p+1])
+	if err != nil {
+		return fmt.Errorf("failed to get block to init MPT: %w", err)
+	}
+	if err = bc.stateRoot.JumpToState(&state.MPTRoot{
+		Index: p,
+		Root:  block.PrevStateRoot,
+	}, bc.config.KeepOnlyLatestState); err != nil {
+		return fmt.Errorf("can't perform MPT jump to height %d: %w", p, err)
+	}
+
+	err = bc.contracts.NEO.InitializeCache(bc, bc.dao)
+	if err != nil {
+		return fmt.Errorf("can't init cache for NEO native contract: %w", err)
+	}
+	err = bc.contracts.Management.InitializeCache(bc.dao)
+	if err != nil {
+		return fmt.Errorf("can't init cache for Management native contract: %w", err)
+	}
+	bc.contracts.Designate.InitializeCache()
+
+	if err := bc.updateExtensibleWhitelist(p); err != nil {
+		return fmt.Errorf("failed to update extensible whitelist: %w", err)
+	}
+
+	updateBlockHeightMetric(p)
+	return nil
+}
+
 // Run runs chain loop, it needs to be run as goroutine and executing it is
 // critical for correct Blockchain operation.
 func (bc *Blockchain) Run() {
@@ -696,6 +755,11 @@ func (bc *Blockchain) GetStateModule() blockchainer.StateRoot {
 	return bc.stateRoot
 }
 
+// GetStateSyncModule returns new state sync service instance.
+func (bc *Blockchain) GetStateSyncModule() blockchainer.StateSync {
+	return statesync.NewModule(bc, bc.log, bc.dao)
+}
+
 // storeBlock performs chain update using the block given, it executes all
 // transactions with all appropriate side-effects and updates Blockchain state.
 // This is the only way to change Blockchain state.

pkg/core/blockchainer/blockchainer.go

@@ -21,7 +21,6 @@ import (
 type Blockchainer interface {
 	ApplyPolicyToTxSet([]*transaction.Transaction) []*transaction.Transaction
 	GetConfig() config.ProtocolConfiguration
-	AddHeaders(...*block.Header) error
 	Blockqueuer // Blockqueuer interface
 	CalculateClaimable(h util.Uint160, endHeight uint32) (*big.Int, error)
 	Close()
@@ -56,10 +55,12 @@ type Blockchainer interface {
 	GetStandByCommittee() keys.PublicKeys
 	GetStandByValidators() keys.PublicKeys
 	GetStateModule() StateRoot
+	GetStateSyncModule() StateSync
 	GetStorageItem(id int32, key []byte) state.StorageItem
 	GetStorageItems(id int32) (map[string]state.StorageItem, error)
 	GetTestVM(t trigger.Type, tx *transaction.Transaction, b *block.Block) *vm.VM
 	GetTransaction(util.Uint256) (*transaction.Transaction, uint32, error)
+	JumpToState(module StateSync) error
 	SetOracle(service services.Oracle)
 	mempool.Feer // fee interface
 	ManagementContractHash() util.Uint160

pkg/core/blockchainer/blockqueuer.go

@@ -5,5 +5,6 @@ import "github.com/nspcc-dev/neo-go/pkg/core/block"
 // Blockqueuer is an interface for blockqueue.
 type Blockqueuer interface {
 	AddBlock(block *block.Block) error
+	AddHeaders(...*block.Header) error
 	BlockHeight() uint32
 }

pkg/core/blockchainer/state_root.go

@@ -9,6 +9,7 @@ import (
 // StateRoot represents local state root module.
 type StateRoot interface {
 	AddStateRoot(root *state.MPTRoot) error
+	CurrentLocalHeight() uint32
 	CurrentLocalStateRoot() util.Uint256
 	CurrentValidatedHeight() uint32
 	GetStateProof(root util.Uint256, key []byte) ([][]byte, error)

pkg/core/blockchainer/state_sync.go

@@ -0,0 +1,19 @@
+package blockchainer
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/core/mpt"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+)
+
+// StateSync represents state sync module.
+type StateSync interface {
+	AddMPTNodes([][]byte) error
+	Blockqueuer // Blockqueuer interface
+	Init(currChainHeight uint32) error
+	IsActive() bool
+	IsInitialized() bool
+	GetJumpHeight() (uint32, error)
+	NeedHeaders() bool
+	NeedMPTNodes() bool
+	Traverse(root util.Uint256, process func(node mpt.Node, nodeBytes []byte) bool) error
+}

pkg/core/mpt/helpers.go

@@ -1,5 +1,7 @@
 package mpt
 
+import "github.com/nspcc-dev/neo-go/pkg/util"
+
 // lcp returns longest common prefix of a and b.
 // Note: it does no allocations.
 func lcp(a, b []byte) []byte {
@@ -49,3 +51,36 @@ func fromNibbles(path []byte) []byte {
 	}
 	return result
 }
+
+// GetChildrenPaths returns a set of paths to node's children who are non-empty HashNodes
+// based on the node's path.
+func GetChildrenPaths(path []byte, node Node) map[util.Uint256][][]byte {
+	res := make(map[util.Uint256][][]byte)
+	switch n := node.(type) {
+	case *LeafNode, *HashNode, EmptyNode:
+		return nil
+	case *BranchNode:
+		for i, child := range n.Children {
+			if child.Type() == HashT {
+				cPath := make([]byte, len(path), len(path)+1)
+				copy(cPath, path)
+				if i != lastChild {
+					cPath = append(cPath, byte(i))
+				}
+				paths := res[child.Hash()]
+				paths = append(paths, cPath)
+				res[child.Hash()] = paths
+			}
+		}
+	case *ExtensionNode:
+		if n.next.Type() == HashT {
+			cPath := make([]byte, len(path)+len(n.key))
+			copy(cPath, path)
+			copy(cPath[len(path):], n.key)
+			res[n.next.Hash()] = [][]byte{cPath}
+		}
+	default:
+		panic("unknown Node type")
+	}
+	return res
+}

pkg/core/mpt/helpers_test.go

@@ -3,6 +3,7 @@ package mpt
 import (
 	"testing"
 
+	"github.com/nspcc-dev/neo-go/pkg/util"
 	"github.com/stretchr/testify/require"
 )
 
@@ -18,3 +19,49 @@ func TestToNibblesFromNibbles(t *testing.T) {
 		check(t, []byte{0x01, 0xAC, 0x8d, 0x04, 0xFF})
 	})
 }
+
+func TestGetChildrenPaths(t *testing.T) {
+	h1 := NewHashNode(util.Uint256{1, 2, 3})
+	h2 := NewHashNode(util.Uint256{4, 5, 6})
+	h3 := NewHashNode(util.Uint256{7, 8, 9})
+	l := NewLeafNode([]byte{1, 2, 3})
+	ext1 := NewExtensionNode([]byte{8, 9}, h1)
+	ext2 := NewExtensionNode([]byte{7, 6}, l)
+	branch := NewBranchNode()
+	branch.Children[3] = h1
+	branch.Children[5] = l
+	branch.Children[6] = h1 // 3-th and 6-th children have the same hash
+	branch.Children[7] = h3
+	branch.Children[lastChild] = h2
+	testCases := map[string]struct {
+		node     Node
+		expected map[util.Uint256][][]byte
+	}{
+		"Hash":                         {h1, nil},
+		"Leaf":                         {l, nil},
+		"Extension with next Hash":     {ext1, map[util.Uint256][][]byte{h1.Hash(): {ext1.key}}},
+		"Extension with next non-Hash": {ext2, map[util.Uint256][][]byte{}},
+		"Branch": {branch, map[util.Uint256][][]byte{
+			h1.Hash(): {{0x03}, {0x06}},
+			h2.Hash(): {{}},
+			h3.Hash(): {{0x07}},
+		}},
+	}
+	parentPath := []byte{4, 5, 6}
+	for name, testCase := range testCases {
+		t.Run(name, func(t *testing.T) {
+			require.Equal(t, testCase.expected, GetChildrenPaths([]byte{}, testCase.node))
+			if testCase.expected != nil {
+				expectedWithPrefix := make(map[util.Uint256][][]byte, len(testCase.expected))
+				for h, paths := range testCase.expected {
+					var res [][]byte
+					for _, path := range paths {
+						res = append(res, append(parentPath, path...))
+					}
+					expectedWithPrefix[h] = res
+				}
+				require.Equal(t, expectedWithPrefix, GetChildrenPaths(parentPath, testCase.node))
+			}
+		})
+	}
+}

pkg/core/mpt/proof_test.go

@@ -6,7 +6,7 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func newProofTrie(t *testing.T) *Trie {
+func newProofTrie(t *testing.T, missingHashNode bool) *Trie {
 	l := NewLeafNode([]byte("somevalue"))
 	e := NewExtensionNode([]byte{0x05, 0x06, 0x07}, l)
 	l2 := NewLeafNode([]byte("invalid"))
@@ -20,11 +20,14 @@ func newProofTrie(t *testing.T) *Trie {
 	require.NoError(t, tr.Put([]byte{0x12, 0x32}, []byte("value2")))
 	tr.putToStore(l)
 	tr.putToStore(e)
+	if !missingHashNode {
+		tr.putToStore(l2)
+	}
 	return tr
 }
 
 func TestTrie_GetProof(t *testing.T) {
-	tr := newProofTrie(t)
+	tr := newProofTrie(t, true)
 
 	t.Run("MissingKey", func(t *testing.T) {
 		_, err := tr.GetProof([]byte{0x12})
@@ -43,7 +46,7 @@ func TestTrie_GetProof(t *testing.T) {
 }
 
 func TestVerifyProof(t *testing.T) {
-	tr := newProofTrie(t)
+	tr := newProofTrie(t, true)
 
 	t.Run("Simple", func(t *testing.T) {
 		proof, err := tr.GetProof([]byte{0x12, 0x32})

pkg/core/native/designate.go

@@ -353,3 +353,8 @@ func (s *Designate) getRole(item stackitem.Item) (noderoles.Role, bool) {
 	u := bi.Uint64()
 	return noderoles.Role(u), u <= math.MaxUint8 && s.isValidRole(noderoles.Role(u))
 }
+
+// InitializeCache invalidates native Designate cache.
+func (s *Designate) InitializeCache() {
+	s.rolesChangedFlag.Store(true)
+}

pkg/core/stateroot/module.go

@@ -114,6 +114,25 @@ func (s *Module) Init(height uint32, enableRefCount bool) error {
 	return nil
 }
 
+// JumpToState performs jump to the state specified by given stateroot index.
+func (s *Module) JumpToState(sr *state.MPTRoot, enableRefCount bool) error {
+	if err := s.addLocalStateRoot(s.Store, sr); err != nil {
+		return fmt.Errorf("failed to store local state root: %w", err)
+	}
+
+	data := make([]byte, 4)
+	binary.LittleEndian.PutUint32(data, sr.Index)
+	if err := s.Store.Put([]byte{byte(storage.DataMPT), prefixValidated}, data); err != nil {
+		return fmt.Errorf("failed to store validated height: %w", err)
+	}
+	s.validatedHeight.Store(sr.Index)
+
+	s.currentLocal.Store(sr.Root)
+	s.localHeight.Store(sr.Index)
+	s.mpt = mpt.NewTrie(mpt.NewHashNode(sr.Root), enableRefCount, s.Store)
+	return nil
+}
+
 // AddMPTBatch updates using provided batch.
 func (s *Module) AddMPTBatch(index uint32, b mpt.Batch, cache *storage.MemCachedStore) (*mpt.Trie, *state.MPTRoot, error) {
 	mpt := *s.mpt

pkg/core/statesync/module.go

@@ -0,0 +1,440 @@
+/*
+Package statesync implements module for the P2P state synchronisation process. The
+module manages state synchronisation for non-archival nodes which are joining the
+network and don't have the ability to resync from the genesis block.
+
+Given the currently available state synchronisation point P, sate sync process
+includes the following stages:
+
+1. Fetching headers starting from height 0 up to P+1.
+2. Fetching MPT nodes for height P stating from the corresponding state root.
+3. Fetching blocks starting from height P-MaxTraceableBlocks (or 0) up to P.
+
+Steps 2 and 3 are being performed in parallel. Once all the data are collected
+and stored in the db, an atomic state jump is occurred to the state sync point P.
+Further node operation process is performed using standard sync mechanism until
+the node reaches synchronised state.
+*/
+package statesync
+
+import (
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"sync"
+
+	"github.com/nspcc-dev/neo-go/pkg/core/block"
+	"github.com/nspcc-dev/neo-go/pkg/core/blockchainer"
+	"github.com/nspcc-dev/neo-go/pkg/core/dao"
+	"github.com/nspcc-dev/neo-go/pkg/core/mpt"
+	"github.com/nspcc-dev/neo-go/pkg/core/storage"
+	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+	"go.uber.org/zap"
+)
+
+// stateSyncStage is a type of state synchronisation stage.
+type stateSyncStage uint8
+
+const (
+	// inactive means that state exchange is disabled by the protocol configuration.
+	// Can't be combined with other states.
+	inactive stateSyncStage = 1 << iota
+	// none means that state exchange is enabled in the configuration, but
+	// initialisation of the state sync module wasn't yet performed, i.e.
+	// (*Module).Init wasn't called. Can't be combined with other states.
+	none
+	// initialized means that (*Module).Init was called, but other sync stages
+	// are not yet reached (i.e. that headers are requested, but not yet fetched).
+	// Can't be combined with other states.
+	initialized
+	// headersSynced means that headers for the current state sync point are fetched.
+	// May be combined with mptSynced and/or blocksSynced.
+	headersSynced
+	// mptSynced means that MPT nodes for the current state sync point are fetched.
+	// Always combined with headersSynced; may be combined with blocksSynced.
+	mptSynced
+	// blocksSynced means that blocks up to the current state sync point are stored.
+	// Always combined with headersSynced; may be combined with mptSynced.
+	blocksSynced
+)
+
+// Module represents state sync module and aimed to gather state-related data to
+// perform an atomic state jump.
+type Module struct {
+	lock sync.RWMutex
+	log  *zap.Logger
+
+	// syncPoint is the state synchronisation point P we're currently working against.
+	syncPoint uint32
+	// syncStage is the stage of the sync process.
+	syncStage stateSyncStage
+	// syncInterval is the delta between two adjacent state sync points.
+	syncInterval uint32
+	// blockHeight is the index of the latest stored block.
+	blockHeight uint32
+
+	dao     *dao.Simple
+	bc      blockchainer.Blockchainer
+	mptpool *Pool
+
+	billet *mpt.Billet
+}
+
+// NewModule returns new instance of statesync module.
+func NewModule(bc blockchainer.Blockchainer, log *zap.Logger, s *dao.Simple) *Module {
+	if !(bc.GetConfig().P2PStateExchangeExtensions && bc.GetConfig().RemoveUntraceableBlocks) {
+		return &Module{
+			dao:       s,
+			bc:        bc,
+			syncStage: inactive,
+		}
+	}
+	return &Module{
+		dao:          s,
+		bc:           bc,
+		log:          log,
+		syncInterval: uint32(bc.GetConfig().StateSyncInterval),
+		mptpool:      NewPool(),
+		syncStage:    none,
+	}
+}
+
+// Init initializes state sync module for the current chain's height with given
+// callback for MPT nodes requests.
+func (s *Module) Init(currChainHeight uint32) error {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	if s.syncStage != none {
+		return errors.New("already initialized or inactive")
+	}
+
+	p := (currChainHeight / s.syncInterval) * s.syncInterval
+	if p < 2*s.syncInterval {
+		// chain is too low to start state exchange process, use the standard sync mechanism
+		s.syncStage = inactive
+		return nil
+	}
+	pOld, err := s.dao.GetStateSyncPoint()
+	if err == nil && pOld >= p-s.syncInterval {
+		// old point is still valid, so try to resync states for this point.
+		p = pOld
+	} else if s.bc.BlockHeight() > p-2*s.syncInterval {
+		// chain has already been synchronised up to old state sync point and regular blocks processing was started
+		s.syncStage = inactive
+		return nil
+	}
+
+	s.syncPoint = p
+	err = s.dao.PutStateSyncPoint(p)
+	if err != nil {
+		return fmt.Errorf("failed to store state synchronisation point %d: %w", p, err)
+	}
+	s.syncStage = initialized
+	s.log.Info("try to sync state for the latest state synchronisation point",
+		zap.Uint32("point", p),
+		zap.Uint32("evaluated chain's blockHeight", currChainHeight))
+
+	// check headers sync state first
+	ltstHeaderHeight := s.bc.HeaderHeight()
+	if ltstHeaderHeight > p {
+		s.syncStage = headersSynced
+		s.log.Info("headers are in sync",
+			zap.Uint32("headerHeight", s.bc.HeaderHeight()))
+	}
+
+	// check blocks sync state
+	s.blockHeight = s.getLatestSavedBlock(p)
+	if s.blockHeight >= p {
+		s.syncStage |= blocksSynced
+		s.log.Info("blocks are in sync",
+			zap.Uint32("blockHeight", s.blockHeight))
+	}
+
+	// check MPT sync state
+	if s.blockHeight > p {
+		s.syncStage |= mptSynced
+		s.log.Info("MPT is in sync",
+			zap.Uint32("stateroot height", s.bc.GetStateModule().CurrentLocalHeight()))
+	} else if s.syncStage&headersSynced != 0 {
+		header, err := s.bc.GetHeader(s.bc.GetHeaderHash(int(p + 1)))
+		if err != nil {
+			return fmt.Errorf("failed to get header to initialize MPT billet: %w", err)
+		}
+		s.billet = mpt.NewBillet(header.PrevStateRoot, s.bc.GetConfig().KeepOnlyLatestState, s.dao.Store)
+		s.log.Info("MPT billet initialized",
+			zap.Uint32("height", s.syncPoint),
+			zap.String("state root", header.PrevStateRoot.StringBE()))
+		pool := NewPool()
+		pool.Add(header.PrevStateRoot, []byte{})
+		err = s.billet.Traverse(func(n mpt.Node, _ []byte) bool {
+			nPaths, ok := pool.TryGet(n.Hash())
+			if !ok {
+				// if this situation occurs, then it's a bug in MPT pool or Traverse.
+				panic("failed to get MPT node from the pool")
+			}
+			pool.Remove(n.Hash())
+			childrenPaths := make(map[util.Uint256][][]byte)
+			for _, path := range nPaths {
+				nChildrenPaths := mpt.GetChildrenPaths(path, n)
+				for hash, paths := range nChildrenPaths {
+					childrenPaths[hash] = append(childrenPaths[hash], paths...) // it's OK to have duplicates, they'll be handled by mempool
+				}
+			}
+			pool.Update(nil, childrenPaths)
+			return false
+		}, true)
+		if err != nil {
+			return fmt.Errorf("failed to traverse MPT while initialization: %w", err)
+		}
+		s.mptpool.Update(nil, pool.GetAll())
+		if s.mptpool.Count() == 0 {
+			s.syncStage |= mptSynced
+			s.log.Info("MPT is in sync",
+				zap.Uint32("stateroot height", p))
+		}
+	}
+
+	if s.syncStage == headersSynced|blocksSynced|mptSynced {
+		s.log.Info("state is in sync, starting regular blocks processing")
+		s.syncStage = inactive
+	}
+	return nil
+}
+
+// getLatestSavedBlock returns either current block index (if it's still relevant
+// to continue state sync process) or H-1 where H is the index of the earliest
+// block that should be saved next.
+func (s *Module) getLatestSavedBlock(p uint32) uint32 {
+	var result uint32
+	mtb := s.bc.GetConfig().MaxTraceableBlocks
+	if p > mtb {
+		result = p - mtb
+	}
+	storedH, err := s.dao.GetStateSyncCurrentBlockHeight()
+	if err == nil && storedH > result {
+		result = storedH
+	}
+	actualH := s.bc.BlockHeight()
+	if actualH > result {
+		result = actualH
+	}
+	return result
+}
+
+// AddHeaders validates and adds specified headers to the chain.
+func (s *Module) AddHeaders(hdrs ...*block.Header) error {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	if s.syncStage != initialized {
+		return errors.New("headers were not requested")
+	}
+
+	hdrsErr := s.bc.AddHeaders(hdrs...)
+	if s.bc.HeaderHeight() > s.syncPoint {
+		s.syncStage = headersSynced
+		s.log.Info("headers for state sync are fetched",
+			zap.Uint32("header height", s.bc.HeaderHeight()))
+
+		header, err := s.bc.GetHeader(s.bc.GetHeaderHash(int(s.syncPoint) + 1))
+		if err != nil {
+			s.log.Fatal("failed to get header to initialize MPT billet",
+				zap.Uint32("height", s.syncPoint+1),
+				zap.Error(err))
+		}
+		s.billet = mpt.NewBillet(header.PrevStateRoot, s.bc.GetConfig().KeepOnlyLatestState, s.dao.Store)
+		s.mptpool.Add(header.PrevStateRoot, []byte{})
+		s.log.Info("MPT billet initialized",
+			zap.Uint32("height", s.syncPoint),
+			zap.String("state root", header.PrevStateRoot.StringBE()))
+	}
+	return hdrsErr
+}
+
+// AddBlock verifies and saves block skipping executable scripts.
+func (s *Module) AddBlock(block *block.Block) error {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	if s.syncStage&headersSynced == 0 || s.syncStage&blocksSynced != 0 {
+		return nil
+	}
+
+	if s.blockHeight == s.syncPoint {
+		return nil
+	}
+	expectedHeight := s.blockHeight + 1
+	if expectedHeight != block.Index {
+		return fmt.Errorf("expected %d, got %d: invalid block index", expectedHeight, block.Index)
+	}
+	if s.bc.GetConfig().StateRootInHeader != block.StateRootEnabled {
+		return fmt.Errorf("stateroot setting mismatch: %v != %v", s.bc.GetConfig().StateRootInHeader, block.StateRootEnabled)
+	}
+	if s.bc.GetConfig().VerifyBlocks {
+		merkle := block.ComputeMerkleRoot()
+		if !block.MerkleRoot.Equals(merkle) {
+			return errors.New("invalid block: MerkleRoot mismatch")
+		}
+	}
+	cache := s.dao.GetWrapped()
+	writeBuf := io.NewBufBinWriter()
+	if err := cache.StoreAsBlock(block, writeBuf); err != nil {
+		return err
+	}
+	writeBuf.Reset()
+
+	err := cache.PutStateSyncCurrentBlockHeight(block.Index)
+	if err != nil {
+		return fmt.Errorf("failed to store current block height: %w", err)
+	}
+
+	for _, tx := range block.Transactions {
+		if err := cache.StoreAsTransaction(tx, block.Index, writeBuf); err != nil {
+			return err
+		}
+		writeBuf.Reset()
+	}
+
+	_, err = cache.Persist()
+	if err != nil {
+		return fmt.Errorf("failed to persist results: %w", err)
+	}
+	s.blockHeight = block.Index
+	if s.blockHeight == s.syncPoint {
+		s.syncStage |= blocksSynced
+		s.log.Info("blocks are in sync",
+			zap.Uint32("blockHeight", s.blockHeight))
+		s.checkSyncIsCompleted()
+	}
+	return nil
+}
+
+// AddMPTNodes tries to add provided set of MPT nodes to the MPT billet if they are
+// not yet collected.
+func (s *Module) AddMPTNodes(nodes [][]byte) error {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	if s.syncStage&headersSynced == 0 || s.syncStage&mptSynced != 0 {
+		return errors.New("MPT nodes were not requested")
+	}
+
+	for _, nBytes := range nodes {
+		var n mpt.NodeObject
+		r := io.NewBinReaderFromBuf(nBytes)
+		n.DecodeBinary(r)
+		if r.Err != nil {
+			return fmt.Errorf("failed to decode MPT node: %w", r.Err)
+		}
+		nPaths, ok := s.mptpool.TryGet(n.Hash())
+		if !ok {
+			// it can easily happen after receiving the same data from different peers.
+			return nil
+		}
+
+		var childrenPaths = make(map[util.Uint256][][]byte)
+		for _, path := range nPaths {
+			err := s.billet.RestoreHashNode(path, n.Node)
+			if err != nil {
+				return fmt.Errorf("failed to add MPT node with hash %s and path %s: %w", n.Hash().StringBE(), hex.EncodeToString(path), err)
+			}
+			for h, paths := range mpt.GetChildrenPaths(path, n.Node) {
+				childrenPaths[h] = append(childrenPaths[h], paths...) // it's OK to have duplicates, they'll be handled by mempool
+			}
+		}
+
+		s.mptpool.Update(map[util.Uint256][][]byte{n.Hash(): nPaths}, childrenPaths)
+	}
+	if s.mptpool.Count() == 0 {
+		s.syncStage |= mptSynced
+		s.log.Info("MPT is in sync",
+			zap.Uint32("height", s.syncPoint))
+		s.checkSyncIsCompleted()
+	}
+	return nil
+}
+
+// checkSyncIsCompleted checks whether state sync process is completed, i.e. headers up to P+1
+// height are fetched, blocks up to P height are stored and MPT nodes for P height are stored.
+// If so, then jumping to P state sync point occurs. It is not protected by lock, thus caller
+// should take care of it.
+func (s *Module) checkSyncIsCompleted() {
+	if s.syncStage != headersSynced|mptSynced|blocksSynced {
+		return
+	}
+	s.log.Info("state is in sync",
+		zap.Uint32("state sync point", s.syncPoint))
+	err := s.bc.JumpToState(s)
+	if err != nil {
+		s.log.Fatal("failed to jump to the latest state sync point", zap.Error(err))
+	}
+	s.syncStage = inactive
+	s.dispose()
+}
+
+func (s *Module) dispose() {
+	s.billet = nil
+}
+
+// BlockHeight returns index of the last stored block.
+func (s *Module) BlockHeight() uint32 {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
+	return s.blockHeight
+}
+
+// IsActive tells whether state sync module is on and still gathering state
+// synchronisation data (headers, blocks or MPT nodes).
+func (s *Module) IsActive() bool {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
+	return !(s.syncStage == inactive || (s.syncStage == headersSynced|mptSynced|blocksSynced))
+}
+
+// IsInitialized tells whether state sync module does not require initialization.
+// If `false` is returned then Init can be safely called.
+func (s *Module) IsInitialized() bool {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
+	return s.syncStage != none
+}
+
+// NeedHeaders tells whether the module hasn't completed headers synchronisation.
+func (s *Module) NeedHeaders() bool {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
+	return s.syncStage == initialized
+}
+
+// NeedMPTNodes returns whether the module hasn't completed MPT synchronisation.
+func (s *Module) NeedMPTNodes() bool {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
+	return s.syncStage&headersSynced != 0 && s.syncStage&mptSynced == 0
+}
+
+// Traverse traverses local MPT nodes starting from the specified root down to its
+// children calling `process` for each serialised node until stop condition is satisfied.
+func (s *Module) Traverse(root util.Uint256, process func(node mpt.Node, nodeBytes []byte) bool) error {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
+	b := mpt.NewBillet(root, s.bc.GetConfig().KeepOnlyLatestState, storage.NewMemCachedStore(s.dao.Store))
+	return b.Traverse(process, false)
+}
+
+// GetJumpHeight returns state sync point to jump to. It is not protected by mutex and should be called
+// under the module lock.
+func (s *Module) GetJumpHeight() (uint32, error) {
+	if s.syncStage != headersSynced|mptSynced|blocksSynced {
+		return 0, errors.New("state sync module has wong state to perform state jump")
+	}
+	return s.syncPoint, nil
+}

pkg/core/statesync/mptpool.go

@@ -0,0 +1,119 @@
+package statesync
+
+import (
+	"bytes"
+	"sort"
+	"sync"
+
+	"github.com/nspcc-dev/neo-go/pkg/util"
+)
+
+// Pool stores unknown MPT nodes along with the corresponding paths (single node is
+// allowed to have multiple MPT paths).
+type Pool struct {
+	lock   sync.RWMutex
+	hashes map[util.Uint256][][]byte
+}
+
+// NewPool returns new MPT node hashes pool.
+func NewPool() *Pool {
+	return &Pool{
+		hashes: make(map[util.Uint256][][]byte),
+	}
+}
+
+// ContainsKey checks if MPT node with the specified hash is in the Pool.
+func (mp *Pool) ContainsKey(hash util.Uint256) bool {
+	mp.lock.RLock()
+	defer mp.lock.RUnlock()
+
+	_, ok := mp.hashes[hash]
+	return ok
+}
+
+// TryGet returns a set of MPT paths for the specified HashNode.
+func (mp *Pool) TryGet(hash util.Uint256) ([][]byte, bool) {
+	mp.lock.RLock()
+	defer mp.lock.RUnlock()
+
+	paths, ok := mp.hashes[hash]
+	return paths, ok
+}
+
+// GetAll returns all MPT nodes with the corresponding paths from the pool.
+func (mp *Pool) GetAll() map[util.Uint256][][]byte {
+	mp.lock.RLock()
+	defer mp.lock.RUnlock()
+
+	return mp.hashes
+}
+
+// Remove removes MPT node from the pool by the specified hash.
+func (mp *Pool) Remove(hash util.Uint256) {
+	mp.lock.Lock()
+	defer mp.lock.Unlock()
+
+	delete(mp.hashes, hash)
+}
+
+// Add adds path to the set of paths for the specified node.
+func (mp *Pool) Add(hash util.Uint256, path []byte) {
+	mp.lock.Lock()
+	defer mp.lock.Unlock()
+
+	mp.addPaths(hash, [][]byte{path})
+}
+
+// Update is an atomic operation and removes/adds specified nodes from/to the pool.
+func (mp *Pool) Update(remove map[util.Uint256][][]byte, add map[util.Uint256][][]byte) {
+	mp.lock.Lock()
+	defer mp.lock.Unlock()
+
+	for h, paths := range remove {
+		old := mp.hashes[h]
+		for _, path := range paths {
+			i := sort.Search(len(old), func(i int) bool {
+				return bytes.Compare(old[i], path) >= 0
+			})
+			if i < len(old) && bytes.Equal(old[i], path) {
+				old = append(old[:i], old[i+1:]...)
+			}
+		}
+		if len(old) == 0 {
+			delete(mp.hashes, h)
+		} else {
+			mp.hashes[h] = old
+		}
+	}
+	for h, paths := range add {
+		mp.addPaths(h, paths)
+	}
+}
+
+// addPaths adds set of the specified node paths to the pool.
+func (mp *Pool) addPaths(nodeHash util.Uint256, paths [][]byte) {
+	old := mp.hashes[nodeHash]
+	for _, path := range paths {
+		i := sort.Search(len(old), func(i int) bool {
+			return bytes.Compare(old[i], path) >= 0
+		})
+		if i < len(old) && bytes.Equal(old[i], path) {
+			// then path is already added
+			continue
+		}
+		old = append(old, path)
+		if i != len(old)-1 {
+			copy(old[i+1:], old[i:])
+			old[i] = path
+		}
+	}
+	mp.hashes[nodeHash] = old
+}
+
+// Count returns the number of nodes in the pool.
+func (mp *Pool) Count() int {
+	mp.lock.RLock()
+	defer mp.lock.RUnlock()
+
+	return len(mp.hashes)
+}

pkg/network/blockqueue.go

@@ -4,6 +4,7 @@ import (
 	"github.com/Workiva/go-datastructures/queue"
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
 	"github.com/nspcc-dev/neo-go/pkg/core/blockchainer"
+	"go.uber.org/atomic"
 	"go.uber.org/zap"
 )
 
@@ -13,6 +14,7 @@ type blockQueue struct {
 	checkBlocks chan struct{}
 	chain       blockchainer.Blockqueuer
 	relayF      func(*block.Block)
+	discarded   *atomic.Bool
 }
 
 const (
@@ -32,6 +34,7 @@ func newBlockQueue(capacity int, bc blockchainer.Blockqueuer, log *zap.Logger, r
 		checkBlocks: make(chan struct{}, 1),
 		chain:       bc,
 		relayF:      relayer,
+		discarded:   atomic.NewBool(false),
 	}
 }
 
@@ -91,8 +94,10 @@ func (bq *blockQueue) putBlock(block *block.Block) error {
 }
 
 func (bq *blockQueue) discard() {
-	close(bq.checkBlocks)
-	bq.queue.Dispose()
+	if bq.discarded.CAS(false, true) {
+		close(bq.checkBlocks)
+		bq.queue.Dispose()
+	}
 }
 
 func (bq *blockQueue) length() int {

pkg/network/message.go

@@ -71,6 +71,8 @@ const (
 	CMDBlock                        = CommandType(payload.BlockType)
 	CMDExtensible                   = CommandType(payload.ExtensibleType)
 	CMDP2PNotaryRequest             = CommandType(payload.P2PNotaryRequestType)
+	CMDGetMPTData       CommandType = 0x51 // 0x5.. commands are used for extensions (P2PNotary, state exchange cmds)
+	CMDMPTData          CommandType = 0x52
 	CMDReject           CommandType = 0x2f
 
 	// SPV protocol.
@@ -136,6 +138,10 @@ func (m *Message) decodePayload() error {
 		p = &payload.Version{}
 	case CMDInv, CMDGetData:
 		p = &payload.Inventory{}
+	case CMDGetMPTData:
+		p = &payload.MPTInventory{}
+	case CMDMPTData:
+		p = &payload.MPTData{}
 	case CMDAddr:
 		p = &payload.AddressList{}
 	case CMDBlock:
@@ -221,7 +227,7 @@ func (m *Message) tryCompressPayload() error {
 	if m.Flags&Compressed == 0 {
 		switch m.Payload.(type) {
 		case *payload.Headers, *payload.MerkleBlock, payload.NullPayload,
-			*payload.Inventory:
+			*payload.Inventory, *payload.MPTInventory:
 			break
 		default:
 			size := len(compressedPayload)

pkg/network/message_string.go

@@ -26,6 +26,8 @@ func _() {
 	_ = x[CMDBlock-44]
 	_ = x[CMDExtensible-46]
 	_ = x[CMDP2PNotaryRequest-80]
+	_ = x[CMDGetMPTData-81]
+	_ = x[CMDMPTData-82]
 	_ = x[CMDReject-47]
 	_ = x[CMDFilterLoad-48]
 	_ = x[CMDFilterAdd-49]
@@ -44,7 +46,7 @@ const (
 	_CommandType_name_6 = "CMDExtensibleCMDRejectCMDFilterLoadCMDFilterAddCMDFilterClear"
 	_CommandType_name_7 = "CMDMerkleBlock"
 	_CommandType_name_8 = "CMDAlert"
-	_CommandType_name_9 = "CMDP2PNotaryRequest"
+	_CommandType_name_9 = "CMDP2PNotaryRequestCMDGetMPTDataCMDMPTData"
 )
 
 var (
@@ -55,6 +57,7 @@ var (
 	_CommandType_index_4 = [...]uint8{0, 12, 22}
 	_CommandType_index_5 = [...]uint8{0, 6, 16, 34, 45, 50, 58}
 	_CommandType_index_6 = [...]uint8{0, 13, 22, 35, 47, 61}
+	_CommandType_index_9 = [...]uint8{0, 19, 32, 42}
 )
 
 func (i CommandType) String() string {
@@ -83,8 +86,9 @@ func (i CommandType) String() string {
 		return _CommandType_name_7
 	case i == 64:
 		return _CommandType_name_8
-	case i == 80:
-		return _CommandType_name_9
+	case 80 <= i && i <= 82:
+		i -= 80
+		return _CommandType_name_9[_CommandType_index_9[i]:_CommandType_index_9[i+1]]
 	default:
 		return "CommandType(" + strconv.FormatInt(int64(i), 10) + ")"
 	}

pkg/network/message_test.go

@@ -258,6 +258,21 @@ func TestEncodeDecodeNotFound(t *testing.T) {
 	})
 }
 
+func TestEncodeDecodeGetMPTData(t *testing.T) {
+	testEncodeDecode(t, CMDGetMPTData, &payload.MPTInventory{
+		Hashes: []util.Uint256{
+			{1, 2, 3},
+			{4, 5, 6},
+		},
+	})
+}
+
+func TestEncodeDecodeMPTData(t *testing.T) {
+	testEncodeDecode(t, CMDMPTData, &payload.MPTData{
+		Nodes: [][]byte{{1, 2, 3}, {4, 5, 6}},
+	})
+}
+
 func TestInvalidMessages(t *testing.T) {
 	t.Run("CMDBlock, empty payload", func(t *testing.T) {
 		testEncodeDecodeFail(t, CMDBlock, payload.NullPayload{})

pkg/network/payload/mptdata.go

@@ -0,0 +1,35 @@
+package payload
+
+import (
+	"errors"
+
+	"github.com/nspcc-dev/neo-go/pkg/io"
+)
+
+// MPTData represents the set of serialized MPT nodes.
+type MPTData struct {
+	Nodes [][]byte
+}
+
+// EncodeBinary implements io.Serializable.
+func (d *MPTData) EncodeBinary(w *io.BinWriter) {
+	w.WriteVarUint(uint64(len(d.Nodes)))
+	for _, n := range d.Nodes {
+		w.WriteVarBytes(n)
+	}
+}
+
+// DecodeBinary implements io.Serializable.
+func (d *MPTData) DecodeBinary(r *io.BinReader) {
+	sz := r.ReadVarUint()
+	if sz == 0 {
+		r.Err = errors.New("empty MPT nodes list")
+		return
+	}
+	for i := uint64(0); i < sz; i++ {
+		d.Nodes = append(d.Nodes, r.ReadVarBytes())
+		if r.Err != nil {
+			return
+		}
+	}
+}

pkg/network/payload/mptdata_test.go

@@ -0,0 +1,24 @@
+package payload
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/internal/testserdes"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMPTData_EncodeDecodeBinary(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		d := new(MPTData)
+		bytes, err := testserdes.EncodeBinary(d)
+		require.NoError(t, err)
+		require.Error(t, testserdes.DecodeBinary(bytes, new(MPTData)))
+	})
+
+	t.Run("good", func(t *testing.T) {
+		d := &MPTData{
+			Nodes: [][]byte{{}, {1}, {1, 2, 3}},
+		}
+		testserdes.EncodeDecodeBinary(t, d, new(MPTData))
+	})
+}

pkg/network/payload/mptinventory.go

@@ -0,0 +1,32 @@
+package payload
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+)
+
+// MaxMPTHashesCount is the maximum number of requested MPT nodes hashes.
+const MaxMPTHashesCount = 32
+
+// MPTInventory payload.
+type MPTInventory struct {
+	// A list of requested MPT nodes hashes.
+	Hashes []util.Uint256
+}
+
+// NewMPTInventory return a pointer to an MPTInventory.
+func NewMPTInventory(hashes []util.Uint256) *MPTInventory {
+	return &MPTInventory{
+		Hashes: hashes,
+	}
+}
+
+// DecodeBinary implements Serializable interface.
+func (p *MPTInventory) DecodeBinary(br *io.BinReader) {
+	br.ReadArray(&p.Hashes, MaxMPTHashesCount)
+}
+
+// EncodeBinary implements Serializable interface.
+func (p *MPTInventory) EncodeBinary(bw *io.BinWriter) {
+	bw.WriteArray(p.Hashes)
+}

pkg/network/payload/mptinventory_test.go

@@ -0,0 +1,38 @@
+package payload
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/internal/testserdes"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMPTInventory_EncodeDecodeBinary(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		testserdes.EncodeDecodeBinary(t, NewMPTInventory([]util.Uint256{}), new(MPTInventory))
+	})
+
+	t.Run("good", func(t *testing.T) {
+		inv := NewMPTInventory([]util.Uint256{{1, 2, 3}, {2, 3, 4}})
+		testserdes.EncodeDecodeBinary(t, inv, new(MPTInventory))
+	})
+
+	t.Run("too large", func(t *testing.T) {
+		check := func(t *testing.T, count int, fail bool) {
+			h := make([]util.Uint256, count)
+			for i := range h {
+				h[i] = util.Uint256{1, 2, 3}
+			}
+			if fail {
+				bytes, err := testserdes.EncodeBinary(NewMPTInventory(h))
+				require.NoError(t, err)
+				require.Error(t, testserdes.DecodeBinary(bytes, new(MPTInventory)))
+			} else {
+				testserdes.EncodeDecodeBinary(t, NewMPTInventory(h), new(MPTInventory))
+			}
+		}
+		check(t, MaxMPTHashesCount, false)
+		check(t, MaxMPTHashesCount+1, true)
+	})
+}

pkg/network/server.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	mrand "math/rand"
 	"net"
+	"sort"
 	"strconv"
 	"sync"
 	"time"
@@ -17,7 +18,9 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/core/blockchainer"
 	"github.com/nspcc-dev/neo-go/pkg/core/mempool"
 	"github.com/nspcc-dev/neo-go/pkg/core/mempoolevent"
+	"github.com/nspcc-dev/neo-go/pkg/core/mpt"
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
+	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/network/capability"
 	"github.com/nspcc-dev/neo-go/pkg/network/extpool"
 	"github.com/nspcc-dev/neo-go/pkg/network/payload"
@@ -67,6 +70,7 @@ type (
 		discovery         Discoverer
 		chain             blockchainer.Blockchainer
 		bQueue            *blockQueue
+		bSyncQueue        *blockQueue
 		consensus         consensus.Service
 		mempool           *mempool.Pool
 		notaryRequestPool *mempool.Pool
@@ -93,6 +97,7 @@ type (
 
 		oracle    *oracle.Oracle
 		stateRoot stateroot.Service
+		stateSync blockchainer.StateSync
 
 		log *zap.Logger
 	}
@@ -191,6 +196,10 @@ func newServerFromConstructors(config ServerConfig, chain blockchainer.Blockchai
 	}
 	s.stateRoot = sr
 
+	sSync := chain.GetStateSyncModule()
+	s.stateSync = sSync
+	s.bSyncQueue = newBlockQueue(maxBlockBatch, sSync, log, nil)
+
 	if config.OracleCfg.Enabled {
 		orcCfg := oracle.Config{
 			Log:     log,
@@ -277,6 +286,7 @@ func (s *Server) Start(errChan chan error) {
 	go s.broadcastTxLoop()
 	go s.relayBlocksLoop()
 	go s.bQueue.run()
+	go s.bSyncQueue.run()
 	go s.transport.Accept()
 	setServerAndNodeVersions(s.UserAgent, strconv.FormatUint(uint64(s.id), 10))
 	s.run()
@@ -292,6 +302,7 @@ func (s *Server) Shutdown() {
 		p.Disconnect(errServerShutdown)
 	}
 	s.bQueue.discard()
+	s.bSyncQueue.discard()
 	if s.StateRootCfg.Enabled {
 		s.stateRoot.Shutdown()
 	}
@@ -573,6 +584,10 @@ func (s *Server) IsInSync() bool {
 	var peersNumber int
 	var notHigher int
 
+	if s.stateSync.IsActive() {
+		return false
+	}
+
 	if s.MinPeers == 0 {
 		return true
 	}
@@ -630,6 +645,9 @@ func (s *Server) handleVersionCmd(p Peer, version *payload.Version) error {
 
 // handleBlockCmd processes the received block received from its peer.
 func (s *Server) handleBlockCmd(p Peer, block *block.Block) error {
+	if s.stateSync.IsActive() {
+		return s.bSyncQueue.putBlock(block)
+	}
 	return s.bQueue.putBlock(block)
 }
 
@@ -639,25 +657,46 @@ func (s *Server) handlePing(p Peer, ping *payload.Ping) error {
 	if err != nil {
 		return err
 	}
-	if s.chain.BlockHeight() < ping.LastBlockIndex {
-		err = s.requestBlocks(p)
-		if err != nil {
-			return err
-		}
+	err = s.requestBlocksOrHeaders(p)
+	if err != nil {
+		return err
 	}
 	return p.EnqueueP2PMessage(NewMessage(CMDPong, payload.NewPing(s.chain.BlockHeight(), s.id)))
 }
 
+func (s *Server) requestBlocksOrHeaders(p Peer) error {
+	if s.stateSync.NeedHeaders() {
+		if s.chain.HeaderHeight() < p.LastBlockIndex() {
+			return s.requestHeaders(p)
+		}
+		return nil
+	}
+	var bq blockchainer.Blockqueuer = s.chain
+	if s.stateSync.IsActive() {
+		bq = s.stateSync
+	}
+	if bq.BlockHeight() < p.LastBlockIndex() {
+		return s.requestBlocks(bq, p)
+	}
+	return nil
+}
+
+// requestHeaders sends a CMDGetHeaders message to the peer to sync up in headers.
+func (s *Server) requestHeaders(p Peer) error {
+	// TODO: optimize
+	currHeight := s.chain.HeaderHeight()
+	needHeight := currHeight + 1
+	payload := payload.NewGetBlockByIndex(needHeight, -1)
+	return p.EnqueueP2PMessage(NewMessage(CMDGetHeaders, payload))
+}
+
 // handlePing processes pong request.
 func (s *Server) handlePong(p Peer, pong *payload.Ping) error {
 	err := p.HandlePong(pong)
 	if err != nil {
 		return err
 	}
-	if s.chain.BlockHeight() < pong.LastBlockIndex {
-		return s.requestBlocks(p)
-	}
-	return nil
+	return s.requestBlocksOrHeaders(p)
 }
 
 // handleInvCmd processes the received inventory.
@@ -766,6 +805,50 @@ func (s *Server) handleGetDataCmd(p Peer, inv *payload.Inventory) error {
 	return nil
 }
 
+// handleGetMPTDataCmd processes the received MPT inventory.
+func (s *Server) handleGetMPTDataCmd(p Peer, inv *payload.MPTInventory) error {
+	if !s.chain.GetConfig().P2PStateExchangeExtensions {
+		return errors.New("GetMPTDataCMD was received, but P2PStateExchangeExtensions are disabled")
+	}
+	if s.chain.GetConfig().KeepOnlyLatestState {
+		// TODO: implement keeping MPT states for P1 and P2 height (#2095, #2152 related)
+		return errors.New("GetMPTDataCMD was received, but only latest MPT state is supported")
+	}
+	resp := payload.MPTData{}
+	capLeft := payload.MaxSize - 8 // max(io.GetVarSize(len(resp.Nodes)))
+	for _, h := range inv.Hashes {
+		if capLeft <= 2 { // at least 1 byte for len(nodeBytes) and 1 byte for node type
+			break
+		}
+		err := s.stateSync.Traverse(h,
+			func(_ mpt.Node, node []byte) bool {
+				l := len(node)
+				size := l + io.GetVarSize(l)
+				if size > capLeft {
+					return true
+				}
+				resp.Nodes = append(resp.Nodes, node)
+				capLeft -= size
+				return false
+			})
+		if err != nil {
+			return fmt.Errorf("failed to traverse MPT starting from %s: %w", h.StringBE(), err)
+		}
+	}
+	if len(resp.Nodes) > 0 {
+		msg := NewMessage(CMDMPTData, &resp)
+		return p.EnqueueP2PMessage(msg)
+	}
+	return nil
+}
+
+func (s *Server) handleMPTDataCmd(p Peer, data *payload.MPTData) error {
+	if !s.chain.GetConfig().P2PStateExchangeExtensions {
+		return errors.New("MPTDataCMD was received, but P2PStateExchangeExtensions are disabled")
+	}
+	return s.stateSync.AddMPTNodes(data.Nodes)
+}
+
 // handleGetBlocksCmd processes the getblocks request.
 func (s *Server) handleGetBlocksCmd(p Peer, gb *payload.GetBlocks) error {
 	count := gb.Count
@@ -845,6 +928,11 @@ func (s *Server) handleGetHeadersCmd(p Peer, gh *payload.GetBlockByIndex) error
 	return p.EnqueueP2PMessage(msg)
 }
 
+// handleHeadersCmd processes headers payload.
+func (s *Server) handleHeadersCmd(p Peer, h *payload.Headers) error {
+	return s.stateSync.AddHeaders(h.Hdrs...)
+}
+
 // handleExtensibleCmd processes received extensible payload.
 func (s *Server) handleExtensibleCmd(e *payload.Extensible) error {
 	if !s.syncReached.Load() {
@@ -993,8 +1081,8 @@ func (s *Server) handleGetAddrCmd(p Peer) error {
 // 1. Block range is divided into chunks of payload.MaxHashesCount.
 // 2. Send requests for chunk in increasing order.
 // 3. After all requests were sent, request random height.
-func (s *Server) requestBlocks(p Peer) error {
-	var currHeight = s.chain.BlockHeight()
+func (s *Server) requestBlocks(bq blockchainer.Blockqueuer, p Peer) error {
+	var currHeight = bq.BlockHeight()
 	var peerHeight = p.LastBlockIndex()
 	var needHeight uint32
 	// lastRequestedHeight can only be increased.
@@ -1051,9 +1139,18 @@ func (s *Server) handleMessage(peer Peer, msg *Message) error {
 		case CMDGetData:
 			inv := msg.Payload.(*payload.Inventory)
 			return s.handleGetDataCmd(peer, inv)
+		case CMDGetMPTData:
+			inv := msg.Payload.(*payload.MPTInventory)
+			return s.handleGetMPTDataCmd(peer, inv)
+		case CMDMPTData:
+			inv := msg.Payload.(*payload.MPTData)
+			return s.handleMPTDataCmd(peer, inv)
 		case CMDGetHeaders:
 			gh := msg.Payload.(*payload.GetBlockByIndex)
 			return s.handleGetHeadersCmd(peer, gh)
+		case CMDHeaders:
+			h := msg.Payload.(*payload.Headers)
+			return s.handleHeadersCmd(peer, h)
 		case CMDInv:
 			inventory := msg.Payload.(*payload.Inventory)
 			return s.handleInvCmd(peer, inventory)
@@ -1093,6 +1190,7 @@ func (s *Server) handleMessage(peer Peer, msg *Message) error {
 			}
 			go peer.StartProtocol()
 
+			s.tryInitStateSync()
 			s.tryStartServices()
 		default:
 			return fmt.Errorf("received '%s' during handshake", msg.Command.String())
@@ -1101,6 +1199,52 @@ func (s *Server) handleMessage(peer Peer, msg *Message) error {
 	return nil
 }
 
+func (s *Server) tryInitStateSync() {
+	if !s.stateSync.IsActive() {
+		s.bSyncQueue.discard()
+		return
+	}
+
+	if s.stateSync.IsInitialized() {
+		return
+	}
+
+	var peersNumber int
+	s.lock.RLock()
+	heights := make([]uint32, 0)
+	for p := range s.peers {
+		if p.Handshaked() {
+			peersNumber++
+			peerLastBlock := p.LastBlockIndex()
+			i := sort.Search(len(heights), func(i int) bool {
+				return heights[i] >= peerLastBlock
+			})
+			heights = append(heights, peerLastBlock)
+			if i != len(heights)-1 {
+				copy(heights[i+1:], heights[i:])
+				heights[i] = peerLastBlock
+			}
+		}
+	}
+	s.lock.RUnlock()
+	if peersNumber >= s.MinPeers && len(heights) > 0 {
+		// choose the height of the median peer as current chain's height
+		h := heights[len(heights)/2]
+		err := s.stateSync.Init(h)
+		if err != nil {
+			s.log.Fatal("failed to init state sync module",
+				zap.Uint32("evaluated chain's blockHeight", h),
+				zap.Uint32("blockHeight", s.chain.BlockHeight()),
+				zap.Uint32("headerHeight", s.chain.HeaderHeight()),
+				zap.Error(err))
+		}
+
+		// module can be inactive after init (i.e. full state is collected and ordinary block processing is needed)
+		if !s.stateSync.IsActive() {
+			s.bSyncQueue.discard()
+		}
+	}
+}
 func (s *Server) handleNewPayload(p *payload.Extensible) {
 	_, err := s.extensiblePool.Add(p)
 	if err != nil {

pkg/network/server_test.go

@@ -2,6 +2,7 @@ package network
 
 import (
 	"errors"
+	"fmt"
 	"math/big"
 	"net"
 	"strconv"
@@ -46,7 +47,10 @@ func (f *fakeConsensus) OnTransaction(tx *transaction.Transaction)     { f.txs =
 func (f *fakeConsensus) GetPayload(h util.Uint256) *payload.Extensible { panic("implement me") }
 
 func TestNewServer(t *testing.T) {
-	bc := &fakechain.FakeChain{}
+	bc := &fakechain.FakeChain{ProtocolConfiguration: config.ProtocolConfiguration{
+		P2PStateExchangeExtensions: true,
+		StateRootInHeader:          true,
+	}}
 	s, err := newServerFromConstructors(ServerConfig{}, bc, nil, newFakeTransp, newFakeConsensus, newTestDiscovery)
 	require.Error(t, err)
 
@@ -899,3 +903,39 @@ func TestVerifyNotaryRequest(t *testing.T) {
 		require.NoError(t, verifyNotaryRequest(bc, nil, r))
 	})
 }
+
+func TestTryInitStateSync(t *testing.T) {
+	t.Run("module inactive", func(t *testing.T) {
+		s := startTestServer(t)
+		s.tryInitStateSync()
+	})
+
+	t.Run("module already initialized", func(t *testing.T) {
+		s := startTestServer(t)
+		s.stateSync = &fakechain.FakeStateSync{IsActiveFlag: true, IsInitializedFlag: true}
+		s.tryInitStateSync()
+	})
+
+	t.Run("good", func(t *testing.T) {
+		s := startTestServer(t)
+		for _, h := range []uint32{10, 8, 7, 4, 11, 4} {
+			p := newLocalPeer(t, s)
+			p.handshaked = true
+			p.lastBlockIndex = h
+			s.peers[p] = true
+		}
+		p := newLocalPeer(t, s)
+		p.handshaked = false // one disconnected peer to check it won't be taken into attention
+		p.lastBlockIndex = 5
+		s.peers[p] = true
+		var expectedH uint32 = 8 // median peer
+
+		s.stateSync = &fakechain.FakeStateSync{IsActiveFlag: true, IsInitializedFlag: false, InitFunc: func(h uint32) error {
+			if h != expectedH {
+				return fmt.Errorf("invalid height: expected %d, got %d", expectedH, h)
+			}
+			return nil
+		}}
+		s.tryInitStateSync()
+	})
+}

pkg/network/tcp_peer.go

@@ -267,12 +267,10 @@ func (p *TCPPeer) StartProtocol() {
 		zap.Uint32("id", p.Version().Nonce))
 
 	p.server.discovery.RegisterGoodAddr(p.PeerAddr().String(), p.version.Capabilities)
-	if p.server.chain.BlockHeight() < p.LastBlockIndex() {
-		err = p.server.requestBlocks(p)
-		if err != nil {
-			p.Disconnect(err)
-			return
-		}
+	err = p.server.requestBlocksOrHeaders(p)
+	if err != nil {
+		p.Disconnect(err)
+		return
 	}
 
 	timer := time.NewTimer(p.server.ProtoTickInterval)
@@ -281,10 +279,8 @@ func (p *TCPPeer) StartProtocol() {
 		case <-p.done:
 			return
 		case <-timer.C:
-			// Try to sync in headers and block with the peer if his block height is higher then ours.
-			if p.LastBlockIndex() > p.server.chain.BlockHeight() {
-				err = p.server.requestBlocks(p)
-			}
+			// Try to sync in headers and block with the peer if his block height is higher than ours.
+			err = p.server.requestBlocksOrHeaders(p)
 			if err == nil {
 				timer.Reset(p.server.ProtoTickInterval)
 			}