From c07daa3eca376935200ec43cb20052da66f7f521 Mon Sep 17 00:00:00 2001
From: Bora Alper <boramalper@gmail.com>
Date: Tue, 24 Jul 2018 15:41:13 +0300
Subject: [PATCH] magneticod: metadata leech refactored heavily, now much more
 readable <3

+ persistence: we now make sure that rows are always closed using `defer`
---
 Gopkg.toml                                    |   4 +
 cmd/magneticod/bittorrent/metadata/leech.go   | 439 ++++++++++++++++
 .../leech_test.go}                            |   2 +-
 .../{sinkMetadata.go => metadata/sink.go}     |  55 +-
 cmd/magneticod/bittorrent/operations.go       | 480 ------------------
 cmd/magneticod/dht/mainline/protocol.go       |   3 +-
 cmd/magneticod/dht/mainline/service.go        |  41 +-
 cmd/magneticod/dht/mainline/transport.go      |  86 +++-
 cmd/magneticod/dht/managers.go                |   1 +
 cmd/magneticod/main.go                        |  20 +-
 pkg/persistence/sqlite3.go                    |  37 +-
 11 files changed, 607 insertions(+), 561 deletions(-)
 create mode 100644 cmd/magneticod/bittorrent/metadata/leech.go
 rename cmd/magneticod/bittorrent/{operations_test.go => metadata/leech_test.go} (98%)
 rename cmd/magneticod/bittorrent/{sinkMetadata.go => metadata/sink.go} (63%)
 delete mode 100644 cmd/magneticod/bittorrent/operations.go

diff --git a/Gopkg.toml b/Gopkg.toml
index 980964b..2a98fca 100644
--- a/Gopkg.toml
+++ b/Gopkg.toml
@@ -56,3 +56,7 @@
 [[constraint]]
   name = "go.uber.org/zap"
   version = "1.7.1"
+
+[[constraint]]
+  name = "github.com/libp2p/go-sockaddr"
+  version = "1.0.3"
diff --git a/cmd/magneticod/bittorrent/metadata/leech.go b/cmd/magneticod/bittorrent/metadata/leech.go
new file mode 100644
index 0000000..ff1c18c
--- /dev/null
+++ b/cmd/magneticod/bittorrent/metadata/leech.go
@@ -0,0 +1,439 @@
+package metadata
+
+import (
+	"bytes"
+	"crypto/rand"
+	"crypto/sha1"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"math"
+	"net"
+	"time"
+
+	"github.com/anacrolix/torrent/bencode"
+	"github.com/anacrolix/torrent/metainfo"
+	"github.com/pkg/errors"
+
+	"go.uber.org/zap"
+
+	"github.com/boramalper/magnetico/pkg/persistence"
+)
+
+const MAX_METADATA_SIZE = 10 * 1024 * 1024
+
+type rootDict struct {
+	M            mDict `bencode:"m"`
+	MetadataSize int   `bencode:"metadata_size"`
+}
+
+type mDict struct {
+	UTMetadata int `bencode:"ut_metadata"`
+}
+
+type extDict struct {
+	MsgType int `bencode:"msg_type"`
+	Piece   int `bencode:"piece"`
+}
+
+type Leech struct {
+	infoHash                       [20]byte
+	peerAddr                       *net.TCPAddr
+	ev                             LeechEventHandlers
+
+	conn                           *net.TCPConn
+	clientID                       [20]byte
+
+	ut_metadata                    uint8
+	metadataReceived, metadataSize uint
+	metadata                       []byte
+}
+
+type LeechEventHandlers struct {
+	OnSuccess func(Metadata)         // must be supplied. args: metadata
+	OnError   func([20]byte, error)  // must be supplied. args: infohash, error
+}
+
+func NewLeech(infoHash [20]byte, peerAddr *net.TCPAddr, ev LeechEventHandlers) *Leech {
+	l := new(Leech)
+	l.infoHash = infoHash
+	l.peerAddr = peerAddr
+	l.ev = ev
+
+	if _, err := rand.Read(l.clientID[:]); err != nil {
+		panic(err.Error())
+	}
+
+	return l
+}
+
+func (l *Leech) writeAll(b []byte) error {
+	for len(b) != 0 {
+		n, err := l.conn.Write(b)
+		if err != nil {
+			return err
+		}
+		b = b[n:]
+	}
+	return nil
+}
+
+func (l *Leech) doBtHandshake() error {
+	lHandshake := []byte(fmt.Sprintf(
+		"\x13BitTorrent protocol\x00\x00\x00\x00\x00\x10\x00\x01%s%s",
+		l.infoHash[:],
+		l.clientID,
+	))
+
+	// ASSERTION
+	if len(lHandshake) != 68 { panic(fmt.Sprintf("len(lHandshake) == %d", len(lHandshake))) }
+
+
+	err := l.writeAll(lHandshake)
+	if err != nil {
+		return errors.Wrap(err, "writeAll lHandshake")
+	}
+
+	zap.L().Debug("BitTorrent handshake sent, waiting for the remote's...")
+
+	rHandshake, err := l.readExactly(68)
+	if err != nil {
+		return errors.Wrap(err, "readExactly rHandshake")
+	}
+	if !bytes.HasPrefix(rHandshake, []byte("\x13BitTorrent protocol")) {
+		return fmt.Errorf("corrupt BitTorrent handshake received")
+	}
+
+	// TODO: maybe check for the infohash sent by the remote peer to double check?
+
+	if (rHandshake[25] & 0x10) == 0 {
+		return fmt.Errorf("peer does not support the extension protocol")
+	}
+
+	return nil
+}
+
+func (l *Leech) doExHandshake() error {
+	err := l.writeAll([]byte("\x00\x00\x00\x1a\x14\x00d1:md11:ut_metadatai1eee"))
+	if err != nil {
+		return errors.Wrap(err, "writeAll lHandshake")
+	}
+
+	rExMessage, err := l.readExMessage()
+	if err != nil {
+		return errors.Wrap(err, "readExMessage")
+	}
+
+	// Extension Handshake has the Extension Message ID = 0x00
+	if rExMessage[1] != 0 {
+		return errors.Wrap(err, "first extension message is not an extension handshake")
+	}
+
+	rRootDict := new(rootDict)
+	err = bencode.Unmarshal(rExMessage[2:], rRootDict)
+	if err != nil {
+		return errors.Wrap(err, "unmarshal rExMessage")
+	}
+
+	if !(0 < rRootDict.MetadataSize && rRootDict.MetadataSize < MAX_METADATA_SIZE) {
+		return fmt.Errorf("metadata too big or its size is less than or equal zero")
+	}
+
+	if !(0 < rRootDict.M.UTMetadata && rRootDict.M.UTMetadata < 255) {
+		return fmt.Errorf("ut_metadata is not an uint8")
+	}
+
+	l.ut_metadata = uint8(rRootDict.M.UTMetadata) // Save the ut_metadata code the remote peer uses
+	l.metadataSize = uint(rRootDict.MetadataSize)
+	l.metadata = make([]byte, l.metadataSize)
+
+	return nil
+}
+
+func (l *Leech) requestAllPieces() error {
+	// Request all the pieces of metadata
+	nPieces := int(math.Ceil(float64(l.metadataSize) / math.Pow(2, 14)))
+	for piece := 0; piece < nPieces; piece++ {
+		// __request_metadata_piece(piece)
+		// ...............................
+		extDictDump, err := bencode.Marshal(extDict{
+			MsgType: 0,
+			Piece:   piece,
+		})
+		if err != nil {  // ASSERT
+			panic(errors.Wrap(err, "marshal extDict"))
+		}
+
+		err = l.writeAll([]byte(fmt.Sprintf(
+			"%s\x14%s%s",
+			toBigEndian(uint(2 + len(extDictDump)), 4),
+			toBigEndian(uint(l.ut_metadata), 1),
+			extDictDump,
+		)))
+		if err != nil {
+			return errors.Wrap(err, "writeAll piece request")
+		}
+	}
+
+	return nil
+}
+
+// readMessage returns a BitTorrent message, sans the first 4 bytes indicating its length.
+func (l *Leech) readMessage() ([]byte, error) {
+	rLengthB, err := l.readExactly(4)
+	if err != nil {
+		return nil, errors.Wrap(err, "readExactly rLengthB")
+	}
+
+	rLength := uint(binary.BigEndian.Uint32(rLengthB))
+
+	rMessage, err := l.readExactly(rLength)
+	if err != nil {
+		return nil, errors.Wrap(err, "readExactly rMessage")
+	}
+
+	return rMessage, nil
+}
+
+// readExMessage returns an *extension* message, sans the first 4 bytes indicating its length.
+//
+// It will IGNORE all non-extension messages!
+func (l *Leech) readExMessage() ([]byte, error) {
+	for {
+		rMessage, err := l.readMessage()
+		if err != nil {
+			return nil, errors.Wrap(err, "readMessage")
+		}
+
+		// We are interested only in extension messages, whose first byte is always 20
+		if rMessage[0] == 20 {
+			return rMessage, nil
+		}
+	}
+}
+
+// readUmMessage returns an ut_metadata extension message, sans the first 4 bytes indicating its
+// length.
+//
+// It will IGNORE all non-"ut_metadata extension" messages!
+func (l *Leech) readUmMessage() ([]byte, error) {
+	for {
+		rExMessage, err := l.readExMessage()
+		if err != nil {
+			return nil, errors.Wrap(err, "readExMessage")
+		}
+
+		if rExMessage[1] == 0x01 {
+			return rExMessage, nil
+		}
+	}
+}
+
+func (l *Leech) connect(deadline time.Time) error {
+	var err error
+
+	l.conn, err = net.DialTCP("tcp", nil, l.peerAddr)
+	if err != nil {
+		return errors.Wrap(err, "dial")
+	}
+	defer l.conn.Close()
+
+	err = l.conn.SetNoDelay(true)
+	if err != nil {
+		return errors.Wrap(err, "NODELAY")
+	}
+
+	err = l.conn.SetDeadline(deadline)
+	if err != nil {
+		return errors.Wrap(err, "SetDeadline")
+	}
+
+	return nil
+}
+
+func (l *Leech) Do(deadline time.Time) {
+	err := l.connect(deadline)
+	if err != nil {
+		l.OnError(errors.Wrap(err, "connect"))
+		return
+	}
+
+	err = l.doBtHandshake()
+	if err != nil {
+		l.OnError(errors.Wrap(err, "doBtHandshake"))
+		return
+	}
+
+	err = l.doExHandshake()
+	if err != nil {
+		l.OnError(errors.Wrap(err, "doExHandshake"))
+		return
+	}
+
+	err = l.requestAllPieces()
+	if err != nil {
+		l.OnError(errors.Wrap(err, "requestAllPieces"))
+		return
+	}
+
+	for l.metadataReceived < l.metadataSize {
+		rUmMessage, err := l.readUmMessage()
+		if err != nil {
+			l.OnError(errors.Wrap(err, "readUmMessage"))
+			return
+		}
+
+		// Run TestDecoder() function in leech_test.go in case you have any doubts.
+		rMessageBuf := bytes.NewBuffer(rUmMessage[2:])
+		rExtDict := new(extDict)
+		err = bencode.NewDecoder(rMessageBuf).Decode(rExtDict)
+		if err != nil {
+			zap.L().Warn("Couldn't decode extension message in the loop!", zap.Error(err))
+			return
+		}
+
+		if rExtDict.MsgType == 2 { // reject
+			l.OnError(fmt.Errorf("remote peer rejected sending metadata"))
+			return
+		}
+		
+		if rExtDict.MsgType == 1 { // data
+			// Get the unread bytes!
+			metadataPiece := rMessageBuf.Bytes()
+
+			// BEP 9 explicitly states:
+			//   > If the piece is the last piece of the metadata, it may be less than 16kiB. If
+			//   > it is not the last piece of the metadata, it MUST be 16kiB.
+			//
+			// Hence...
+			//   ... if the length of @metadataPiece is more than 16kiB, we err.
+			if len(metadataPiece) > 16*1024 {
+				l.OnError(fmt.Errorf("metadataPiece > 16kiB"))
+				return
+			}
+			
+			piece := rExtDict.Piece
+			// metadata[piece * 2**14: piece * 2**14 + len(metadataPiece)] = metadataPiece is how it'd be done in Python
+			copy(l.metadata[piece*int(math.Pow(2, 14)):piece*int(math.Pow(2, 14))+len(metadataPiece)], metadataPiece)
+			l.metadataReceived += uint(len(metadataPiece))
+			
+			// ... if the length of @metadataPiece is less than 16kiB AND metadata is NOT
+			// complete then we err.
+			if len(metadataPiece) < 16*1024 && l.metadataReceived != l.metadataSize {
+				l.OnError(fmt.Errorf("metadataPiece < 16 kiB but incomplete"))
+				return
+			}
+
+			if l.metadataReceived > l.metadataSize {
+				l.OnError(fmt.Errorf("metadataReceived > metadataSize"))
+				return
+			}
+		}
+	}
+	
+	// Verify the checksum
+	sha1Sum := sha1.Sum(l.metadata)
+	if !bytes.Equal(sha1Sum[:], l.infoHash[:]) {
+		l.OnError(fmt.Errorf("infohash mismatch"))
+		return
+	}
+	
+	// Check the info dictionary
+	info := new(metainfo.Info)
+	err = bencode.Unmarshal(l.metadata, info)
+	if err != nil {
+		l.OnError(errors.Wrap(err, "unmarshal info"))
+		return
+	}
+	err = validateInfo(info)
+	if err != nil {
+		l.OnError(errors.Wrap(err, "validateInfo"))
+		return
+	}
+
+	var files []persistence.File
+	// If there is only one file, there won't be a Files slice. That's why we need to add it here
+	if len(info.Files) == 0 {
+		files = append(files, persistence.File{
+			Size: info.Length,
+			Path: info.Name,
+		})
+	} else {
+		for _, file := range info.Files {
+			files = append(files, persistence.File{
+				Size: file.Length,
+				Path: file.DisplayPath(info),
+			})
+		}
+	}
+
+	var totalSize uint64
+	for _, file := range files {
+		if file.Size < 0 {
+			l.OnError(fmt.Errorf("file size less than zero"))
+			return
+		}
+		
+		totalSize += uint64(file.Size)
+	}
+
+	l.ev.OnSuccess(Metadata{
+		InfoHash:     l.infoHash[:],
+		Name:         info.Name,
+		TotalSize:    totalSize,
+		DiscoveredOn: time.Now().Unix(),
+		Files:        files,
+	})
+}
+
+// COPIED FROM anacrolix/torrent
+func validateInfo(info *metainfo.Info) error {
+	if len(info.Pieces)%20 != 0 {
+		return errors.New("pieces has invalid length")
+	}
+	if info.PieceLength == 0 {
+		if info.TotalLength() != 0 {
+			return errors.New("zero piece length")
+		}
+	} else {
+		if int((info.TotalLength()+info.PieceLength-1)/info.PieceLength) != info.NumPieces() {
+			return errors.New("piece count and file lengths are at odds")
+		}
+	}
+	return nil
+}
+
+func (l *Leech) readExactly(n uint) ([]byte, error) {
+	b := make([]byte, n)
+	_, err := io.ReadFull(l.conn, b)
+	return b, err
+}
+
+func (l *Leech) OnError(err error) {
+	l.ev.OnError(l.infoHash, err)
+}
+
+// TODO: add bounds checking!
+func toBigEndian(i uint, n int) []byte {
+	b := make([]byte, n)
+	switch n {
+	case 1:
+		b = []byte{byte(i)}
+
+	case 2:
+		binary.BigEndian.PutUint16(b, uint16(i))
+
+	case 4:
+		binary.BigEndian.PutUint32(b, uint32(i))
+
+	default:
+		panic(fmt.Sprintf("n must be 1, 2, or 4!"))
+	}
+
+	if len(b) != n {
+		panic(fmt.Sprintf("postcondition failed: len(b) != n in intToBigEndian (i %d, n %d, len b %d, b %s)", i, n, len(b), b))
+	}
+
+	return b
+}
+
diff --git a/cmd/magneticod/bittorrent/operations_test.go b/cmd/magneticod/bittorrent/metadata/leech_test.go
similarity index 98%
rename from cmd/magneticod/bittorrent/operations_test.go
rename to cmd/magneticod/bittorrent/metadata/leech_test.go
index 8c24fa6..9e35686 100644
--- a/cmd/magneticod/bittorrent/operations_test.go
+++ b/cmd/magneticod/bittorrent/metadata/leech_test.go
@@ -1,4 +1,4 @@
-package bittorrent
+package metadata
 
 import (
 	"bytes"
diff --git a/cmd/magneticod/bittorrent/sinkMetadata.go b/cmd/magneticod/bittorrent/metadata/sink.go
similarity index 63%
rename from cmd/magneticod/bittorrent/sinkMetadata.go
rename to cmd/magneticod/bittorrent/metadata/sink.go
index 9e5049e..fef4572 100644
--- a/cmd/magneticod/bittorrent/sinkMetadata.go
+++ b/cmd/magneticod/bittorrent/metadata/sink.go
@@ -1,4 +1,4 @@
-package bittorrent
+package metadata
 
 import (
 	"crypto/rand"
@@ -24,21 +24,17 @@ type Metadata struct {
 	Files []persistence.File
 }
 
-type Peer struct {
-	Addr *net.TCPAddr
-}
-
-type MetadataSink struct {
-	clientID    []byte
-	deadline    time.Duration
-	drain       chan Metadata
+type Sink struct {
+	clientID           []byte
+	deadline           time.Duration
+	drain              chan Metadata
 	incomingInfoHashes map[[20]byte]struct{}
-	terminated  bool
-	termination chan interface{}
+	terminated         bool
+	termination        chan interface{}
 }
 
-func NewMetadataSink(deadline time.Duration) *MetadataSink {
-	ms := new(MetadataSink)
+func NewSink(deadline time.Duration) *Sink {
+	ms := new(Sink)
 
 	ms.clientID = make([]byte, 20)
 	_, err := rand.Read(ms.clientID)
@@ -52,27 +48,29 @@ func NewMetadataSink(deadline time.Duration) *MetadataSink {
 	return ms
 }
 
-func (ms *MetadataSink) Sink(res mainline.TrawlingResult) {
+func (ms *Sink) Sink(res mainline.TrawlingResult) {
 	if ms.terminated {
-		zap.L().Panic("Trying to Sink() an already closed MetadataSink!")
+		zap.L().Panic("Trying to Sink() an already closed Sink!")
 	}
 
 	if _, exists := ms.incomingInfoHashes[res.InfoHash]; exists {
 		return
 	}
 	// BEWARE!
-	// Although not crucial, the assumption is that MetadataSink.Sink() will be called by only one
+	// Although not crucial, the assumption is that Sink.Sink() will be called by only one
 	// goroutine (i.e. it's not thread-safe), lest there might be a race condition between where we
 	// check whether res.infoHash exists in the ms.incomingInfoHashes, and where we add the infoHash
 	// to the incomingInfoHashes at the end of this function.
 
+	zap.L().Info("Sunk!", zap.String("infoHash", res.InfoHash.String()))
+
 	IPs := res.PeerIP.String()
 	var rhostport string
 	if IPs == "<nil>" {
-		zap.L().Debug("MetadataSink.Sink: Peer IP is nil!")
+		zap.L().Debug("Sink.Sink: Peer IP is nil!")
 		return
 	} else if IPs[0] == '?' {
-		zap.L().Debug("MetadataSink.Sink: Peer IP is invalid!")
+		zap.L().Debug("Sink.Sink: Peer IP is invalid!")
 		return
 	} else if strings.ContainsRune(IPs, ':') { // IPv6
 		rhostport = fmt.Sprintf("[%s]:%d", IPs, res.PeerPort)
@@ -82,29 +80,33 @@ func (ms *MetadataSink) Sink(res mainline.TrawlingResult) {
 
 	raddr, err := net.ResolveTCPAddr("tcp", rhostport)
 	if err != nil {
-		zap.L().Debug("MetadataSink.Sink: Couldn't resolve peer address!", zap.Error(err))
+		zap.L().Debug("Sink.Sink: Couldn't resolve peer address!", zap.Error(err))
 		return
 	}
 
-	go ms.awaitMetadata(res.InfoHash, Peer{Addr: raddr})
+	leech := NewLeech(res.InfoHash, raddr, LeechEventHandlers{
+		OnSuccess: ms.flush,
+		OnError: ms.onLeechError,
+	})
+	go leech.Do(time.Now().Add(ms.deadline))
 
 	ms.incomingInfoHashes[res.InfoHash] = struct{}{}
 }
 
-func (ms *MetadataSink) Drain() <-chan Metadata {
+func (ms *Sink) Drain() <-chan Metadata {
 	if ms.terminated {
-		zap.L().Panic("Trying to Drain() an already closed MetadataSink!")
+		zap.L().Panic("Trying to Drain() an already closed Sink!")
 	}
 	return ms.drain
 }
 
-func (ms *MetadataSink) Terminate() {
+func (ms *Sink) Terminate() {
 	ms.terminated = true
 	close(ms.termination)
 	close(ms.drain)
 }
 
-func (ms *MetadataSink) flush(result Metadata) {
+func (ms *Sink) flush(result Metadata) {
 	if !ms.terminated {
 		ms.drain <- result
 		// Delete the infoHash from ms.incomingInfoHashes ONLY AFTER once we've flushed the
@@ -114,3 +116,8 @@ func (ms *MetadataSink) flush(result Metadata) {
 		delete(ms.incomingInfoHashes, infoHash)
 	}
 }
+
+func (ms *Sink) onLeechError(infoHash [20]byte, err error) {
+	zap.L().Debug("leech error", zap.ByteString("infoHash", infoHash[:]), zap.Error(err))
+	delete(ms.incomingInfoHashes, infoHash)
+}
diff --git a/cmd/magneticod/bittorrent/operations.go b/cmd/magneticod/bittorrent/operations.go
deleted file mode 100644
index d0e65df..0000000
--- a/cmd/magneticod/bittorrent/operations.go
+++ /dev/null
@@ -1,480 +0,0 @@
-package bittorrent
-
-import (
-	"bytes"
-	"crypto/sha1"
-	"encoding/binary"
-	"errors"
-	"fmt"
-	"io"
-	"math"
-	"net"
-	"time"
-
-	"github.com/anacrolix/torrent/bencode"
-	"github.com/anacrolix/torrent/metainfo"
-
-	"go.uber.org/zap"
-
-	"github.com/boramalper/magnetico/pkg/persistence"
-)
-
-const MAX_METADATA_SIZE = 10 * 1024 * 1024
-
-type rootDict struct {
-	M            mDict `bencode:"m"`
-	MetadataSize int   `bencode:"metadata_size"`
-}
-
-type mDict struct {
-	UTMetadata int `bencode:"ut_metadata"`
-}
-
-type extDict struct {
-	MsgType int `bencode:"msg_type"`
-	Piece   int `bencode:"piece"`
-}
-
-func (ms *MetadataSink) awaitMetadata(infoHash metainfo.Hash, peer Peer) {
-	conn, err := net.DialTCP("tcp", nil, peer.Addr)
-	if err != nil {
-		zap.L().Debug(
-			"awaitMetadata couldn't connect to the peer!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.String("remotePeerAddr", peer.Addr.String()),
-			zap.Error(err),
-		)
-		return
-	}
-	defer conn.Close()
-
-	err = conn.SetNoDelay(true)
-	if err != nil {
-		zap.L().Panic(
-			"Couldn't set NODELAY!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.String("remotePeerAddr", peer.Addr.String()),
-			zap.Error(err),
-		)
-		return
-	}
-	err = conn.SetDeadline(time.Now().Add(ms.deadline))
-	if err != nil {
-		zap.L().Panic(
-			"Couldn't set the deadline!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.String("remotePeerAddr", peer.Addr.String()),
-			zap.Error(err),
-		)
-		return
-	}
-
-	// State Variables
-	var isExtHandshakeDone, done bool
-	var ut_metadata, metadataReceived, metadataSize int
-	var metadata []byte
-
-	lHandshake := []byte(fmt.Sprintf(
-		"\x13BitTorrent protocol\x00\x00\x00\x00\x00\x10\x00\x01%s%s",
-		infoHash[:],
-		ms.clientID,
-	))
-	if len(lHandshake) != 68 {
-		zap.L().Panic(
-			"Generated BitTorrent handshake is not of length 68!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.Int("len_lHandshake", len(lHandshake)),
-		)
-	}
-	err = writeAll(conn, lHandshake)
-	if err != nil {
-		zap.L().Debug(
-			"Couldn't write BitTorrent handshake!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.String("remotePeerAddr", peer.Addr.String()),
-			zap.Error(err),
-		)
-		return
-	}
-
-	zap.L().Debug("BitTorrent handshake sent, waiting for the remote's...")
-
-	rHandshake, err := readExactly(conn, 68)
-	if err != nil {
-		zap.L().Debug(
-			"Couldn't read remote BitTorrent handshake!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.String("remotePeerAddr", peer.Addr.String()),
-			zap.Error(err),
-		)
-		return
-	}
-	if !bytes.HasPrefix(rHandshake, []byte("\x13BitTorrent protocol")) {
-		zap.L().Debug(
-			"Remote BitTorrent handshake is not what it is supposed to be!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.String("remotePeerAddr", peer.Addr.String()),
-			zap.ByteString("rHandshake[:20]", rHandshake[:20]),
-		)
-		return
-	}
-
-	// __on_bt_handshake
-	// ================
-	if rHandshake[25] != 16 { // TODO (later): do *not* compare the whole byte, check the bit instead! (0x10)
-		zap.L().Debug(
-			"Peer does not support the extension protocol!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.String("remotePeerAddr", peer.Addr.String()),
-		)
-		return
-	}
-
-	writeAll(conn, []byte("\x00\x00\x00\x1a\x14\x00d1:md11:ut_metadatai1eee"))
-	zap.L().Debug(
-		"Extension handshake sent, waiting for the remote's...",
-		zap.ByteString("infoHash", infoHash[:]),
-		zap.String("remotePeerAddr", peer.Addr.String()),
-	)
-
-	// the loop!
-	// =========
-	for !done {
-		rLengthB, err := readExactly(conn, 4)
-		if err != nil {
-			zap.L().Debug(
-				"Couldn't read the first 4 bytes from the remote peer in the loop!",
-				zap.ByteString("infoHash", infoHash[:]),
-				zap.String("remotePeerAddr", peer.Addr.String()),
-				zap.Error(err),
-			)
-			return
-		}
-
-		// The messages we are interested in have the length of AT LEAST two bytes
-		// (TODO: actually a bit more than that but SURELY when it's less than two bytes, the
-		//        program panics)
-		rLength := bigEndianToInt(rLengthB)
-		if rLength < 2 {
-			continue
-		}
-
-		rMessage, err := readExactly(conn, rLength)
-		if err != nil {
-			zap.L().Debug(
-				"Couldn't read the rest of the message from the remote peer in the loop!",
-				zap.ByteString("infoHash", infoHash[:]),
-				zap.String("remotePeerAddr", peer.Addr.String()),
-				zap.Error(err),
-			)
-			return
-		}
-
-		// __on_message
-		// ------------
-		if rMessage[0] != 0x14 { // We are interested only in extension messages, whose first byte is always 0x14
-			zap.L().Debug(
-				"Ignoring the non-extension message.",
-				zap.ByteString("infoHash", infoHash[:]),
-			)
-			continue
-		}
-
-		if rMessage[1] == 0x00 { // Extension Handshake has the Extension Message ID = 0x00
-			// __on_ext_handshake_message(message[2:])
-			// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-			// TODO: continue editing log messages from here
-
-			if isExtHandshakeDone {
-				return
-			}
-
-			rRootDict := new(rootDict)
-			err := bencode.Unmarshal(rMessage[2:], rRootDict)
-			if err != nil {
-				zap.L().Debug("Couldn't unmarshal extension handshake!", zap.Error(err))
-				return
-			}
-
-			if rRootDict.MetadataSize <= 0 || rRootDict.MetadataSize > MAX_METADATA_SIZE {
-				zap.L().Debug("Unacceptable metadata size!", zap.Int("metadata_size", rRootDict.MetadataSize))
-				return
-			}
-
-			ut_metadata = rRootDict.M.UTMetadata // Save the ut_metadata code the remote peer uses
-			metadataSize = rRootDict.MetadataSize
-			metadata = make([]byte, metadataSize)
-			isExtHandshakeDone = true
-
-			zap.L().Debug("GOT EXTENSION HANDSHAKE!", zap.Int("ut_metadata", ut_metadata), zap.Int("metadata_size", metadataSize))
-
-			// Request all the pieces of metadata
-			n_pieces := int(math.Ceil(float64(metadataSize) / math.Pow(2, 14)))
-			for piece := 0; piece < n_pieces; piece++ {
-				// __request_metadata_piece(piece)
-				// ...............................
-				extDictDump, err := bencode.Marshal(extDict{
-					MsgType: 0,
-					Piece:   piece,
-				})
-				if err != nil {
-					zap.L().Warn("Couldn't marshal extDictDump!", zap.Error(err))
-					return
-				}
-				writeAll(conn, []byte(fmt.Sprintf(
-					"%s\x14%s%s",
-					intToBigEndian(2+len(extDictDump), 4),
-					intToBigEndian(ut_metadata, 1),
-					extDictDump,
-				)))
-			}
-
-			zap.L().Warn("requested all metadata pieces!")
-
-		} else if rMessage[1] == 0x01 {
-			// __on_ext_message(message[2:])
-			// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-			// Run TestDecoder() function in operations_test.go in case you have any doubts.
-			rMessageBuf := bytes.NewBuffer(rMessage[2:])
-			rExtDict := new(extDict)
-			err := bencode.NewDecoder(rMessageBuf).Decode(rExtDict)
-			if err != nil {
-				zap.L().Warn("Couldn't decode extension message in the loop!", zap.Error(err))
-				return
-			}
-
-			if rExtDict.MsgType == 1 { // data
-				// Get the unread bytes!
-				metadataPiece := rMessageBuf.Bytes()
-				piece := rExtDict.Piece
-				// metadata[piece * 2**14: piece * 2**14 + len(metadataPiece)] = metadataPiece is how it'd be done in Python
-				copy(metadata[piece*int(math.Pow(2, 14)):piece*int(math.Pow(2, 14))+len(metadataPiece)], metadataPiece)
-				metadataReceived += len(metadataPiece)
-				done = metadataReceived == metadataSize
-
-				// BEP 9 explicitly states:
-				//   > If the piece is the last piece of the metadata, it may be less than 16kiB. If
-				//   > it is not the last piece of the metadata, it MUST be 16kiB.
-				//
-				// Hence...
-				//   ... if the length of @metadataPiece is more than 16kiB, we err.
-				if len(metadataPiece) > 16*1024 {
-					zap.L().Debug(
-						"metadataPiece is bigger than 16kiB!",
-						zap.Int("len_metadataPiece", len(metadataPiece)),
-						zap.Int("metadataReceived", metadataReceived),
-						zap.Int("metadataSize", metadataSize),
-						zap.Int("metadataPieceIndex", bytes.Index(rMessage, metadataPiece)),
-					)
-					return
-				}
-
-				// ... if the length of @metadataPiece is less than 16kiB AND metadata is NOT
-				// complete (!done) then we err.
-				if len(metadataPiece) < 16*1024 && !done {
-					zap.L().Debug(
-						"metadataPiece is less than 16kiB and metadata is incomplete!",
-						zap.Int("len_metadataPiece", len(metadataPiece)),
-						zap.Int("metadataReceived", metadataReceived),
-						zap.Int("metadataSize", metadataSize),
-						zap.Int("metadataPieceIndex", bytes.Index(rMessage, metadataPiece)),
-					)
-					return
-				}
-
-				if metadataReceived > metadataSize {
-					zap.L().Debug(
-						"metadataReceived is greater than metadataSize!",
-						zap.Int("len_metadataPiece", len(metadataPiece)),
-						zap.Int("metadataReceived", metadataReceived),
-						zap.Int("metadataSize", metadataSize),
-						zap.Int("metadataPieceIndex", bytes.Index(rMessage, metadataPiece)),
-					)
-					return
-				}
-
-				zap.L().Debug(
-					"Fetching...",
-					zap.ByteString("infoHash", infoHash[:]),
-					zap.String("remotePeerAddr", conn.RemoteAddr().String()),
-					zap.Int("metadataReceived", metadataReceived),
-					zap.Int("metadataSize", metadataSize),
-				)
-			} else if rExtDict.MsgType == 2 { // reject
-				zap.L().Debug(
-					"Remote peer rejected sending metadata!",
-					zap.ByteString("infoHash", infoHash[:]),
-					zap.String("remotePeerAddr", conn.RemoteAddr().String()),
-				)
-				return
-			}
-
-		} else {
-			zap.L().Debug(
-				"Message is not an ut_metadata message! (ignoring)",
-				zap.ByteString("msg", rMessage[:100]),
-			)
-			// no return!
-		}
-	}
-
-	zap.L().Debug(
-		"Metadata is complete, verifying the checksum...",
-		zap.ByteString("infoHash", infoHash[:]),
-	)
-
-	sha1Sum := sha1.Sum(metadata)
-	if !bytes.Equal(sha1Sum[:], infoHash[:]) {
-		zap.L().Debug(
-			"Info-hash mismatch!",
-			zap.ByteString("expectedInfoHash", infoHash[:]),
-			zap.ByteString("actualInfoHash", sha1Sum[:]),
-		)
-		return
-	}
-
-	zap.L().Debug(
-		"Checksum verified, checking the info dictionary...",
-		zap.ByteString("infoHash", infoHash[:]),
-	)
-
-	info := new(metainfo.Info)
-	err = bencode.Unmarshal(metadata, info)
-	if err != nil {
-		zap.L().Debug(
-			"Couldn't unmarshal info bytes!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.Error(err),
-		)
-		return
-	}
-	err = validateInfo(info)
-	if err != nil {
-		zap.L().Debug(
-			"Bad info dictionary!",
-			zap.ByteString("infoHash", infoHash[:]),
-			zap.Error(err),
-		)
-		return
-	}
-
-	var files []persistence.File
-	// If there is only one file, there won't be a Files slice. That's why we need to add it here
-	if len(info.Files) == 0 {
-		files = append(files, persistence.File{
-			Size: info.Length,
-			Path: info.Name,
-		})
-	}
-
-	for _, file := range info.Files {
-		if file.Length < 0 {
-			zap.L().Debug(
-				"File size is less than zero!",
-				zap.ByteString("infoHash", infoHash[:]),
-				zap.String("filePath", file.DisplayPath(info)),
-				zap.Int64("fileSize", file.Length),
-			)
-			return
-		}
-
-		files = append(files, persistence.File{
-			Size: file.Length,
-			Path: file.DisplayPath(info),
-		})
-	}
-
-	var totalSize uint64
-	for _, file := range files {
-		totalSize += uint64(file.Size)
-	}
-
-	zap.L().Debug(
-		"Flushing metadata...",
-		zap.ByteString("infoHash", infoHash[:]),
-	)
-
-	ms.flush(Metadata{
-		InfoHash:     infoHash[:],
-		Name:         info.Name,
-		TotalSize:    totalSize,
-		DiscoveredOn: time.Now().Unix(),
-		Files:        files,
-	})
-}
-
-// COPIED FROM anacrolix/torrent
-func validateInfo(info *metainfo.Info) error {
-	if len(info.Pieces)%20 != 0 {
-		return errors.New("pieces has invalid length")
-	}
-	if info.PieceLength == 0 {
-		if info.TotalLength() != 0 {
-			return errors.New("zero piece length")
-		}
-	} else {
-		if int((info.TotalLength()+info.PieceLength-1)/info.PieceLength) != info.NumPieces() {
-			return errors.New("piece count and file lengths are at odds")
-		}
-	}
-	return nil
-}
-
-func writeAll(c *net.TCPConn, b []byte) error {
-	for len(b) != 0 {
-		n, err := c.Write(b)
-		if err != nil {
-			return err
-		}
-		b = b[n:]
-	}
-	return nil
-}
-
-func readExactly(c *net.TCPConn, n int) ([]byte, error) {
-	b := make([]byte, n)
-	_, err := io.ReadFull(c, b)
-	return b, err
-}
-
-// TODO: add bounds checking!
-func intToBigEndian(i int, n int) []byte {
-	b := make([]byte, n)
-	switch n {
-	case 1:
-		b = []byte{byte(i)}
-
-	case 2:
-		binary.BigEndian.PutUint16(b, uint16(i))
-
-	case 4:
-		binary.BigEndian.PutUint32(b, uint32(i))
-
-	default:
-		panic(fmt.Sprintf("n must be 1, 2, or 4!"))
-	}
-
-	if len(b) != n {
-		panic(fmt.Sprintf("postcondition failed: len(b) != n in intToBigEndian (i %d, n %d, len b %d, b %s)", i, n, len(b), b))
-	}
-
-	return b
-}
-
-func bigEndianToInt(b []byte) int {
-	switch len(b) {
-	case 1:
-		return int(b[0])
-
-	case 2:
-		return int(binary.BigEndian.Uint16(b))
-
-	case 4:
-		return int(binary.BigEndian.Uint32(b))
-
-	default:
-		panic(fmt.Sprintf("bigEndianToInt: b is too long! (%d bytes)", len(b)))
-	}
-}
diff --git a/cmd/magneticod/dht/mainline/protocol.go b/cmd/magneticod/dht/mainline/protocol.go
index 53459fe..9ec2849 100644
--- a/cmd/magneticod/dht/mainline/protocol.go
+++ b/cmd/magneticod/dht/mainline/protocol.go
@@ -26,11 +26,12 @@ type ProtocolEventHandlers struct {
 	OnGetPeersResponse           func(*Message, net.Addr)
 	OnFindNodeResponse           func(*Message, net.Addr)
 	OnPingORAnnouncePeerResponse func(*Message, net.Addr)
+	OnCongestion                 func()
 }
 
 func NewProtocol(laddr string, eventHandlers ProtocolEventHandlers) (p *Protocol) {
 	p = new(Protocol)
-	p.transport = NewTransport(laddr, p.onMessage)
+	p.transport = NewTransport(laddr, p.onMessage, p.eventHandlers.OnCongestion)
 	p.eventHandlers = eventHandlers
 
 	p.currentTokenSecret, p.previousTokenSecret = make([]byte, 20), make([]byte, 20)
diff --git a/cmd/magneticod/dht/mainline/service.go b/cmd/magneticod/dht/mainline/service.go
index c5c8c0a..e0e754e 100644
--- a/cmd/magneticod/dht/mainline/service.go
+++ b/cmd/magneticod/dht/mainline/service.go
@@ -31,13 +31,14 @@ type TrawlingService struct {
 	//                                                                  ^~~~~~
 	routingTable      map[string]net.Addr
 	routingTableMutex *sync.Mutex
+	maxNeighbors      uint
 }
 
 type TrawlingServiceEventHandlers struct {
 	OnResult func(TrawlingResult)
 }
 
-func NewTrawlingService(laddr string, eventHandlers TrawlingServiceEventHandlers) *TrawlingService {
+func NewTrawlingService(laddr string, initialMaxNeighbors uint, eventHandlers TrawlingServiceEventHandlers) *TrawlingService {
 	service := new(TrawlingService)
 	service.protocol = NewProtocol(
 		laddr,
@@ -45,12 +46,14 @@ func NewTrawlingService(laddr string, eventHandlers TrawlingServiceEventHandlers
 			OnGetPeersQuery:     service.onGetPeersQuery,
 			OnAnnouncePeerQuery: service.onAnnouncePeerQuery,
 			OnFindNodeResponse:  service.onFindNodeResponse,
+			OnCongestion:        service.onCongestion,
 		},
 	)
 	service.trueNodeID = make([]byte, 20)
 	service.routingTable = make(map[string]net.Addr)
 	service.routingTableMutex = new(sync.Mutex)
 	service.eventHandlers = eventHandlers
+	service.maxNeighbors = initialMaxNeighbors
 
 	_, err := rand.Read(service.trueNodeID)
 	if err != nil {
@@ -78,11 +81,14 @@ func (s *TrawlingService) Terminate() {
 
 func (s *TrawlingService) trawl() {
 	for range time.Tick(3 * time.Second) {
+		s.maxNeighbors = uint(float32(s.maxNeighbors) * 1.01)
+
 		s.routingTableMutex.Lock()
 		if len(s.routingTable) == 0 {
 			s.bootstrap()
 		} else {
-			zap.L().Debug("Latest status:", zap.Int("n", len(s.routingTable)))
+			zap.L().Warn("Latest status:", zap.Int("n", len(s.routingTable)),
+				zap.Uint("maxNeighbors", s.maxNeighbors))
 			s.findNeighbors()
 			s.routingTable = make(map[string]net.Addr)
 		}
@@ -185,11 +191,34 @@ func (s *TrawlingService) onFindNodeResponse(response *Message, addr net.Addr) {
 	s.routingTableMutex.Lock()
 	defer s.routingTableMutex.Unlock()
 
+	zap.L().Debug("find node response!!", zap.Uint("maxNeighbors", s.maxNeighbors),
+		zap.Int("response.R.Nodes length", len(response.R.Nodes)))
+
 	for _, node := range response.R.Nodes {
-		if node.Addr.Port != 0 { // Ignore nodes who "use" port 0.
-			if len(s.routingTable) < 8000 {
-				s.routingTable[string(node.ID)] = &node.Addr
-			}
+		if uint(len(s.routingTable)) >= s.maxNeighbors {
+			break
 		}
+		if node.Addr.Port == 0 { // Ignore nodes who "use" port 0.
+			zap.L().Debug("ignoring 0 port!!!")
+			continue
+		}
+
+		s.routingTable[string(node.ID)] = &node.Addr
 	}
 }
+
+func (s *TrawlingService) onCongestion() {
+	/* The Congestion Prevention Strategy:
+	 *
+	 * In case of congestion, decrease the maximum number of nodes to the 90% of the current value.
+	 */
+	 if s.maxNeighbors < 200 {
+		 zap.L().Warn("Max. number of neighbours are < 200 and there is still congestion!" +
+		 	"(check your network connection if this message recurs)")
+		 return
+	 }
+
+	 s.maxNeighbors = uint(float32(s.maxNeighbors) * 0.9)
+	 zap.L().Debug("Max. number of neighbours updated!",
+	 	zap.Uint("s.maxNeighbors", s.maxNeighbors))
+}
diff --git a/cmd/magneticod/dht/mainline/transport.go b/cmd/magneticod/dht/mainline/transport.go
index 2d97187..60f37e7 100644
--- a/cmd/magneticod/dht/mainline/transport.go
+++ b/cmd/magneticod/dht/mainline/transport.go
@@ -4,6 +4,7 @@ import (
 	"net"
 
 	"github.com/anacrolix/torrent/bencode"
+	sockaddr "github.com/libp2p/go-sockaddr/net"
 	"go.uber.org/zap"
 	"golang.org/x/sys/unix"
 )
@@ -12,23 +13,43 @@ type Transport struct {
 	fd      int
 	laddr   *net.UDPAddr
 	started bool
+	buffer  []byte
 
 	// OnMessage is the function that will be called when Transport receives a packet that is
 	// successfully unmarshalled as a syntactically correct Message (but -of course- the checking
 	// the semantic correctness of the Message is left to Protocol).
 	onMessage func(*Message, net.Addr)
+	// OnCongestion
+	onCongestion func()
 }
 
-func NewTransport(laddr string, onMessage func(*Message, net.Addr)) *Transport {
-	transport := new(Transport)
-	transport.onMessage = onMessage
+func NewTransport(laddr string, onMessage func(*Message, net.Addr), onCongestion func()) *Transport {
+	t := new(Transport)
+	/*   The field size sets a theoretical limit of 65,535 bytes (8 byte header + 65,527 bytes of
+	 * data) for a UDP datagram. However the actual limit for the data length, which is imposed by
+	 * the underlying IPv4 protocol, is 65,507 bytes (65,535 − 8 byte UDP header − 20 byte IP
+	 * header).
+	 *
+	 *   In IPv6 jumbograms it is possible to have UDP packets of size greater than 65,535 bytes.
+	 * RFC 2675 specifies that the length field is set to zero if the length of the UDP header plus
+	 * UDP data is greater than 65,535.
+	 *
+	 * https://en.wikipedia.org/wiki/User_Datagram_Protocol
+	 */
+	t.buffer       = make([]byte, 65507)
+	t.onMessage    = onMessage
+	t.onCongestion = onCongestion
+
 	var err error
-	transport.laddr, err = net.ResolveUDPAddr("udp", laddr)
+	t.laddr, err = net.ResolveUDPAddr("udp", laddr)
 	if err != nil {
 		zap.L().Panic("Could not resolve the UDP address for the trawler!", zap.Error(err))
 	}
+	if t.laddr.IP.To4() == nil {
+		zap.L().Panic("IP address is not IPv4!")
+	}
 
-	return transport
+	return t
 }
 
 func (t *Transport) Start() {
@@ -51,7 +72,12 @@ func (t *Transport) Start() {
 		zap.L().Fatal("Could NOT create a UDP socket!", zap.Error(err))
 	}
 
-	unix.Bind(t.fd, unix.SockaddrInet4{Addr: t.laddr.IP, Port: t.laddr.Port})
+	var ip [4]byte
+	copy(ip[:], t.laddr.IP.To4())
+	err = unix.Bind(t.fd, &unix.SockaddrInet4{Addr: ip, Port: t.laddr.Port})
+	if err != nil {
+		zap.L().Fatal("Could NOT bind the socket!", zap.Error(err))
+	}
 
 	go t.readMessages()
 }
@@ -62,21 +88,33 @@ func (t *Transport) Terminate() {
 
 // readMessages is a goroutine!
 func (t *Transport) readMessages() {
-	buffer := make([]byte, 65536)
-
 	for {
-		n, from, err := unix.Recvfrom(t.fd, buffer, 0)
-		if err != nil {
+		n, fromSA, err := unix.Recvfrom(t.fd, t.buffer, 0)
+		if err == unix.EPERM || err == unix.ENOBUFS { // todo: are these errors possible for recvfrom?
+			zap.L().Warn("READ CONGESTION!", zap.Error(err))
+			t.onCongestion()
+		} else if err != nil {
 			// TODO: isn't there a more reliable way to detect if UDPConn is closed?
 			zap.L().Debug("Could NOT read an UDP packet!", zap.Error(err))
 		}
 
+		if n == 0 {
+			/*   Datagram sockets in various domains  (e.g., the UNIX and Internet domains) permit
+			 * zero-length datagrams. When such a datagram is received, the return value (n) is 0.
+			 */
+			zap.L().Debug("zero-length received!!")
+			continue
+		}
+
+		from := sockaddr.SockaddrToUDPAddr(fromSA)
+
 		var msg Message
-		err = bencode.Unmarshal(buffer[:n], &msg)
+		err = bencode.Unmarshal(t.buffer[:n], &msg)
 		if err != nil {
 			zap.L().Debug("Could NOT unmarshal packet data!", zap.Error(err))
 		}
 
+		zap.L().Debug("message read! (first 20...)", zap.ByteString("msg", t.buffer[:20]))
 		t.onMessage(&msg, from)
 	}
 }
@@ -87,9 +125,29 @@ func (t *Transport) WriteMessages(msg *Message, addr net.Addr) {
 		zap.L().Panic("Could NOT marshal an outgoing message! (Programmer error.)")
 	}
 
-	err = unix.Sendto(t.fd, data, 0, addr)
-	// TODO: isn't there a more reliable way to detect if UDPConn is closed?
-	if err != nil {
+	addrSA := sockaddr.NetAddrToSockaddr(addr)
+
+	zap.L().Debug("sent message!!!")
+
+	err = unix.Sendto(t.fd, data, 0, addrSA)
+	if err == unix.EPERM || err == unix.ENOBUFS {
+		/*   EPERM (errno: 1) is kernel's way of saying that "you are far too fast, chill". It is
+		 * also likely that we have received a ICMP source quench packet (meaning, that we *really*
+		 * need to slow down.
+		 *
+		 * Read more here: http://www.archivum.info/comp.protocols.tcp-ip/2009-05/00088/UDP-socket-amp-amp-sendto-amp-amp-EPERM.html
+		 *
+		 * >   Note On BSD systems (OS X, FreeBSD, etc.) flow control is not supported for
+		 * > DatagramProtocol, because send failures caused by writing too many packets cannot be
+		 * > detected easily. The socket always appears ‘ready’ and excess packets are dropped; an
+		 * > OSError with errno set to errno.ENOBUFS may or may not be raised; if it is raised, it
+		 * > will be reported to DatagramProtocol.error_received() but otherwise ignored.
+		 *
+		 * Source: https://docs.python.org/3/library/asyncio-protocol.html#flow-control-callbacks
+		 */
+		zap.L().Warn("WRITE CONGESTION!", zap.Error(err))
+		t.onCongestion()
+	} else if err != nil {
 		zap.L().Debug("Could NOT write an UDP packet!", zap.Error(err))
 	}
 }
diff --git a/cmd/magneticod/dht/managers.go b/cmd/magneticod/dht/managers.go
index c26ccaf..95990d0 100644
--- a/cmd/magneticod/dht/managers.go
+++ b/cmd/magneticod/dht/managers.go
@@ -18,6 +18,7 @@ func NewTrawlingManager(mlAddrs []string) *TrawlingManager {
 	for _, addr := range mlAddrs {
 		manager.services = append(manager.services, mainline.NewTrawlingService(
 			addr,
+			2000,
 			mainline.TrawlingServiceEventHandlers{
 				OnResult: manager.onResult,
 			},
diff --git a/cmd/magneticod/main.go b/cmd/magneticod/main.go
index 0a35e4f..9d92ff1 100644
--- a/cmd/magneticod/main.go
+++ b/cmd/magneticod/main.go
@@ -6,7 +6,6 @@ import (
 	"net"
 	"os"
 	"os/signal"
-	"path"
 	"runtime/pprof"
 	"time"
 
@@ -14,7 +13,7 @@ import (
 	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
 
-	"github.com/boramalper/magnetico/cmd/magneticod/bittorrent"
+	"github.com/boramalper/magnetico/cmd/magneticod/bittorrent/metadata"
 	"github.com/boramalper/magnetico/cmd/magneticod/dht"
 
 	"github.com/Wessie/appdirs"
@@ -75,6 +74,8 @@ func main() {
 
 	zap.ReplaceGlobals(logger)
 
+	zap.L().Debug("debug message!")
+
 	switch opFlags.Profile {
 	case "cpu":
 		file, err := os.OpenFile("magneticod_cpu.prof", os.O_CREATE | os.O_WRONLY, 0755)
@@ -96,19 +97,19 @@ func main() {
 	interruptChan := make(chan os.Signal)
 	signal.Notify(interruptChan, os.Interrupt)
 
-	database, err := persistence.MakeDatabase(opFlags.DatabaseURL, false, logger)
+	database, err := persistence.MakeDatabase(opFlags.DatabaseURL, logger)
 	if err != nil {
 		logger.Sugar().Fatalf("Could not open the database at `%s`: %s", opFlags.DatabaseURL, err.Error())
 	}
 
 	trawlingManager := dht.NewTrawlingManager(opFlags.TrawlerMlAddrs)
-	metadataSink := bittorrent.NewMetadataSink(2 * time.Minute)
+	metadataSink := metadata.NewSink(2 * time.Minute)
 
 	// The Event Loop
 	for stopped := false; !stopped; {
 		select {
 		case result := <-trawlingManager.Output():
-			zap.L().Info("Trawled!", zap.String("infoHash", result.InfoHash.String()))
+			zap.L().Debug("Trawled!", zap.String("infoHash", result.InfoHash.String()))
 			exists, err := database.DoesTorrentExist(result.InfoHash[:])
 			if err != nil {
 				zap.L().Fatal("Could not check whether torrent exists!", zap.Error(err))
@@ -144,10 +145,11 @@ func parseFlags() (*opFlags, error) {
 	}
 
 	if cmdF.DatabaseURL == "" {
-		opF.DatabaseURL = "sqlite3://" + path.Join(
-				appdirs.UserDataDir("magneticod", "", "", false),
-				"database.sqlite3",
-			)
+		opF.DatabaseURL =
+			"sqlite3://" +
+			appdirs.UserDataDir("magneticod", "", "", false) +
+			"/database.sqlite3" +
+			"?_journal_mode=WAL"  // https://github.com/mattn/go-sqlite3#connection-string
 	} else {
 		opF.DatabaseURL = cmdF.DatabaseURL
 	}
diff --git a/pkg/persistence/sqlite3.go b/pkg/persistence/sqlite3.go
index 1579eda..131d89a 100644
--- a/pkg/persistence/sqlite3.go
+++ b/pkg/persistence/sqlite3.go
@@ -14,6 +14,9 @@ import (
 	"go.uber.org/zap"
 )
 
+// Close your rows lest you get "database table is locked" error(s)!
+// See https://github.com/mattn/go-sqlite3/issues/2741
+
 type sqlite3Database struct {
 	conn *sql.DB
 }
@@ -55,15 +58,12 @@ func (db *sqlite3Database) DoesTorrentExist(infoHash []byte) (bool, error) {
 	if err != nil {
 		return false, err
 	}
+	defer rows.Close()
 
 	// If rows.Next() returns true, meaning that the torrent is in the database, return true; else
 	// return false.
 	exists := rows.Next()
-	if !exists && rows.Err() != nil {
-		return false, err
-	}
-
-	if err = rows.Close(); err != nil {
+	if rows.Err() != nil {
 		return false, err
 	}
 
@@ -143,6 +143,7 @@ func (db *sqlite3Database) GetNumberOfTorrents() (uint, error) {
 	if err != nil {
 		return 0, err
 	}
+	defer rows.Close()
 
 	if rows.Next() != true {
 		fmt.Errorf("No rows returned from `SELECT MAX(ROWID)`")
@@ -153,10 +154,6 @@ func (db *sqlite3Database) GetNumberOfTorrents() (uint, error) {
 		return 0, err
 	}
 
-	if err = rows.Close(); err != nil {
-		return 0, err
-	}
-
 	return n, nil
 }
 
@@ -247,6 +244,7 @@ func (db *sqlite3Database) QueryTorrents(
 	queryArgs = append(queryArgs, limit)
 
 	rows, err := db.conn.Query(sqlQuery, queryArgs...)
+	defer rows.Close()
 	if err != nil {
 		return nil, fmt.Errorf("error while querying torrents: %s", err.Error())
 	}
@@ -269,10 +267,6 @@ func (db *sqlite3Database) QueryTorrents(
 		torrents = append(torrents, torrent)
 	}
 
-	if err := rows.Close(); err != nil {
-		return nil, err
-	}
-
 	return torrents, nil
 }
 
@@ -307,6 +301,7 @@ func (db *sqlite3Database) GetTorrent(infoHash []byte) (*TorrentMetadata, error)
 		WHERE info_hash = ?`,
 		infoHash,
 	)
+	defer rows.Close()
 	if err != nil {
 		return nil, err
 	}
@@ -320,10 +315,6 @@ func (db *sqlite3Database) GetTorrent(infoHash []byte) (*TorrentMetadata, error)
 		return nil, err
 	}
 
-	if err = rows.Close(); err != nil {
-		return nil, err
-	}
-
 	return &tm, nil
 }
 
@@ -331,6 +322,7 @@ func (db *sqlite3Database) GetFiles(infoHash []byte) ([]File, error) {
 	rows, err := db.conn.Query(
 		"SELECT size, path FROM files, torrents WHERE files.torrent_id = torrents.id AND torrents.info_hash = ?;",
 		infoHash)
+	defer rows.Close()
 	if err != nil {
 		return nil, err
 	}
@@ -344,10 +336,6 @@ func (db *sqlite3Database) GetFiles(infoHash []byte) ([]File, error) {
 		files = append(files, file)
 	}
 
-	if err := rows.Close(); err != nil {
-		return nil, err
-	}
-
 	return files, nil
 }
 
@@ -391,6 +379,7 @@ func (db *sqlite3Database) GetStatistics(from string, n uint) (*Statistics, erro
 			GROUP BY dt;`,
 			timef),
 		fromTime.Unix(), toTime.Unix())
+	defer rows.Close()
 	if err != nil {
 		return nil, err
 	}
@@ -478,6 +467,7 @@ func (db *sqlite3Database) setupDatabase() error {
 	if err != nil {
 		return fmt.Errorf("sql.Tx.Query (user_version): %s", err.Error())
 	}
+	defer rows.Close()
 	var userVersion int
 	if rows.Next() != true {
 		return fmt.Errorf("sql.Rows.Next (user_version): PRAGMA user_version did not return any rows!")
@@ -485,11 +475,6 @@ func (db *sqlite3Database) setupDatabase() error {
 	if err = rows.Scan(&userVersion); err != nil {
 		return fmt.Errorf("sql.Rows.Scan (user_version): %s", err.Error())
 	}
-	// Close your rows lest you get "database table is locked" error(s)!
-	// See https://github.com/mattn/go-sqlite3/issues/2741
-	if err = rows.Close(); err != nil {
-		return fmt.Errorf("sql.Rows.Close (user_version): %s", err.Error())
-	}
 
 	switch userVersion {
 	case 0:  // FROZEN.