diff --git a/transport/internet/internal/pool.go b/transport/internet/internal/pool.go
index 07af634a9..4fa5a2dcb 100644
--- a/transport/internet/internal/pool.go
+++ b/transport/internet/internal/pool.go
@@ -9,50 +9,59 @@ import (
 	"v2ray.com/core/common/signal"
 )
 
+// ConnectionRecyler is the interface for recycling connections.
 type ConnectionRecyler interface {
-	Put(ConnectionId, net.Conn)
+	// Put returns a connection back to a connection pool.
+	Put(ConnectionID, net.Conn)
 }
 
-type ConnectionId struct {
+// ConnectionID is the ID of a connection.
+type ConnectionID struct {
 	Local      v2net.Address
 	Remote     v2net.Address
 	RemotePort v2net.Port
 }
 
-func NewConnectionId(source v2net.Address, dest v2net.Destination) ConnectionId {
-	return ConnectionId{
+// NewConnectionID creates a new ConnectionId.
+func NewConnectionID(source v2net.Address, dest v2net.Destination) ConnectionID {
+	return ConnectionID{
 		Local:      source,
 		Remote:     dest.Address,
 		RemotePort: dest.Port,
 	}
 }
 
+// ExpiringConnection is a connection that will expire in certain time.
 type ExpiringConnection struct {
 	conn   net.Conn
 	expire time.Time
 }
 
-func (o *ExpiringConnection) Expired() bool {
-	return o.expire.Before(time.Now())
+// Expired returns true if the connection has expired.
+func (ec *ExpiringConnection) Expired() bool {
+	return ec.expire.Before(time.Now())
 }
 
+// Pool is a connection pool.
 type Pool struct {
 	sync.Mutex
-	connsByDest map[ConnectionId][]*ExpiringConnection
+	connsByDest map[ConnectionID][]*ExpiringConnection
 	cleanupOnce signal.Once
 }
 
+// NewConnectionPool creates a new Pool.
 func NewConnectionPool() *Pool {
 	return &Pool{
-		connsByDest: make(map[ConnectionId][]*ExpiringConnection),
+		connsByDest: make(map[ConnectionID][]*ExpiringConnection),
 	}
 }
 
-func (o *Pool) Get(id ConnectionId) net.Conn {
-	o.Lock()
-	defer o.Unlock()
+// Get returns a connection with matching connection ID. Nil if not found.
+func (p *Pool) Get(id ConnectionID) net.Conn {
+	p.Lock()
+	defer p.Unlock()
 
-	list, found := o.connsByDest[id]
+	list, found := p.connsByDest[id]
 	if !found {
 		return nil
 	}
@@ -72,18 +81,18 @@ func (o *Pool) Get(id ConnectionId) net.Conn {
 		list[connIdx] = list[listLen-1]
 	}
 	list = list[:listLen-1]
-	o.connsByDest[id] = list
+	p.connsByDest[id] = list
 	return conn.conn
 }
 
-func (o *Pool) Cleanup() {
-	defer o.cleanupOnce.Reset()
+func (p *Pool) cleanup() {
+	defer p.cleanupOnce.Reset()
 
-	for len(o.connsByDest) > 0 {
+	for len(p.connsByDest) > 0 {
 		time.Sleep(time.Second * 5)
 		expiredConns := make([]net.Conn, 0, 16)
-		o.Lock()
-		for dest, list := range o.connsByDest {
+		p.Lock()
+		for dest, list := range p.connsByDest {
 			validConns := make([]*ExpiringConnection, 0, len(list))
 			for _, conn := range list {
 				if conn.Expired() {
@@ -93,34 +102,35 @@ func (o *Pool) Cleanup() {
 				}
 			}
 			if len(validConns) != len(list) {
-				o.connsByDest[dest] = validConns
+				p.connsByDest[dest] = validConns
 			}
 		}
-		o.Unlock()
+		p.Unlock()
 		for _, conn := range expiredConns {
 			conn.Close()
 		}
 	}
 }
 
-func (o *Pool) Put(id ConnectionId, conn net.Conn) {
+// Put implements ConnectionRecyler.Put().
+func (p *Pool) Put(id ConnectionID, conn net.Conn) {
 	expiringConn := &ExpiringConnection{
 		conn:   conn,
 		expire: time.Now().Add(time.Second * 4),
 	}
 
-	o.Lock()
-	defer o.Unlock()
+	p.Lock()
+	defer p.Unlock()
 
-	list, found := o.connsByDest[id]
+	list, found := p.connsByDest[id]
 	if !found {
 		list = []*ExpiringConnection{expiringConn}
 	} else {
 		list = append(list, expiringConn)
 	}
-	o.connsByDest[id] = list
+	p.connsByDest[id] = list
 
-	o.cleanupOnce.Do(func() {
-		go o.Cleanup()
+	p.cleanupOnce.Do(func() {
+		go p.cleanup()
 	})
 }
diff --git a/transport/internet/internal/pool_test.go b/transport/internet/internal/pool_test.go
index 0c4592779..65f5a6d0d 100644
--- a/transport/internet/internal/pool_test.go
+++ b/transport/internet/internal/pool_test.go
@@ -51,11 +51,11 @@ func TestConnectionCache(t *testing.T) {
 	assert := assert.On(t)
 
 	pool := NewConnectionPool()
-	conn := pool.Get(NewConnectionId(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))))
+	conn := pool.Get(NewConnectionID(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))))
 	assert.Pointer(conn).IsNil()
 
-	pool.Put(NewConnectionId(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))), &TestConnection{id: "test"})
-	conn = pool.Get(NewConnectionId(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))))
+	pool.Put(NewConnectionID(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))), &TestConnection{id: "test"})
+	conn = pool.Get(NewConnectionID(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))))
 	assert.String(conn.(*TestConnection).id).Equals("test")
 }
 
@@ -64,9 +64,9 @@ func TestConnectionRecycle(t *testing.T) {
 
 	pool := NewConnectionPool()
 	c := &TestConnection{id: "test"}
-	pool.Put(NewConnectionId(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))), c)
+	pool.Put(NewConnectionID(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))), c)
 	time.Sleep(6 * time.Second)
 	assert.Bool(c.closed).IsTrue()
-	conn := pool.Get(NewConnectionId(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))))
+	conn := pool.Get(NewConnectionID(v2net.LocalHostIP, v2net.TCPDestination(v2net.LocalHostIP, v2net.Port(80))))
 	assert.Pointer(conn).IsNil()
 }
diff --git a/transport/internet/internal/sysfd.go b/transport/internet/internal/sysfd.go
index bf0f6493b..31be9c53b 100644
--- a/transport/internet/internal/sysfd.go
+++ b/transport/internet/internal/sysfd.go
@@ -3,13 +3,15 @@ package internal
 import (
 	"net"
 	"reflect"
+
 	"v2ray.com/core/common/errors"
 )
 
 var (
-	ErrInvalidConn = errors.New("Invalid Connection.")
+	errInvalidConn = errors.New("Invalid Connection.")
 )
 
+// GetSysFd returns the underlying fd of a connection.
 func GetSysFd(conn net.Conn) (int, error) {
 	cv := reflect.ValueOf(conn)
 	switch ce := cv.Elem(); ce.Kind() {
@@ -21,5 +23,5 @@ func GetSysFd(conn net.Conn) (int, error) {
 			return int(fd.Int()), nil
 		}
 	}
-	return 0, ErrInvalidConn
+	return 0, errInvalidConn
 }
diff --git a/transport/internet/kcp/connection.go b/transport/internet/kcp/connection.go
index b18d9c89a..dd2b69c16 100644
--- a/transport/internet/kcp/connection.go
+++ b/transport/internet/kcp/connection.go
@@ -156,7 +156,7 @@ func (v *Updater) Run() {
 
 type SystemConnection interface {
 	net.Conn
-	Id() internal.ConnectionId
+	Id() internal.ConnectionID
 	Reset(func([]Segment))
 	Overhead() int
 }
diff --git a/transport/internet/kcp/connection_test.go b/transport/internet/kcp/connection_test.go
index 5edfa5625..1c47b5acd 100644
--- a/transport/internet/kcp/connection_test.go
+++ b/transport/internet/kcp/connection_test.go
@@ -48,15 +48,15 @@ func (o *NoOpConn) SetWriteDeadline(time.Time) error {
 	return nil
 }
 
-func (o *NoOpConn) Id() internal.ConnectionId {
-	return internal.ConnectionId{}
+func (o *NoOpConn) Id() internal.ConnectionID {
+	return internal.ConnectionID{}
 }
 
 func (o *NoOpConn) Reset(input func([]Segment)) {}
 
 type NoOpRecycler struct{}
 
-func (o *NoOpRecycler) Put(internal.ConnectionId, net.Conn) {}
+func (o *NoOpRecycler) Put(internal.ConnectionID, net.Conn) {}
 
 func TestConnectionReadTimeout(t *testing.T) {
 	assert := assert.On(t)
diff --git a/transport/internet/kcp/dialer.go b/transport/internet/kcp/dialer.go
index d02325038..90cdb5a6e 100644
--- a/transport/internet/kcp/dialer.go
+++ b/transport/internet/kcp/dialer.go
@@ -26,7 +26,7 @@ var (
 type ClientConnection struct {
 	sync.RWMutex
 	net.Conn
-	id     internal.ConnectionId
+	id     internal.ConnectionID
 	input  func([]Segment)
 	reader PacketReader
 	writer PacketWriter
@@ -56,7 +56,7 @@ func (o *ClientConnection) Read([]byte) (int, error) {
 	panic("KCP|ClientConnection: Read should not be called.")
 }
 
-func (o *ClientConnection) Id() internal.ConnectionId {
+func (o *ClientConnection) Id() internal.ConnectionID {
 	return o.id
 }
 
@@ -112,7 +112,7 @@ func DialKCP(src v2net.Address, dest v2net.Destination, options internet.DialerO
 	dest.Network = v2net.Network_UDP
 	log.Info("KCP|Dialer: Dialing KCP to ", dest)
 
-	id := internal.NewConnectionId(src, dest)
+	id := internal.NewConnectionID(src, dest)
 	conn := globalPool.Get(id)
 	if conn == nil {
 		rawConn, err := internet.DialToDest(src, dest)
diff --git a/transport/internet/kcp/listener.go b/transport/internet/kcp/listener.go
index b3db87c06..dae4dc0d2 100644
--- a/transport/internet/kcp/listener.go
+++ b/transport/internet/kcp/listener.go
@@ -27,7 +27,7 @@ type ConnectionID struct {
 }
 
 type ServerConnection struct {
-	id     internal.ConnectionId
+	id     internal.ConnectionID
 	local  net.Addr
 	remote net.Addr
 	writer PacketWriter
@@ -73,7 +73,7 @@ func (o *ServerConnection) SetWriteDeadline(time.Time) error {
 	return nil
 }
 
-func (o *ServerConnection) Id() internal.ConnectionId {
+func (o *ServerConnection) Id() internal.ConnectionID {
 	return o.id
 }
 
@@ -188,7 +188,7 @@ func (v *Listener) OnReceive(payload *buf.Buffer, session *proxy.SessionInfo) {
 		}
 		localAddr := v.hub.Addr()
 		sConn := &ServerConnection{
-			id:     internal.NewConnectionId(v2net.LocalHostIP, src),
+			id:     internal.NewConnectionID(v2net.LocalHostIP, src),
 			local:  localAddr,
 			remote: remoteAddr,
 			writer: &KCPPacketWriter{
@@ -274,7 +274,7 @@ func (v *Listener) Addr() net.Addr {
 	return v.hub.Addr()
 }
 
-func (v *Listener) Put(internal.ConnectionId, net.Conn) {}
+func (v *Listener) Put(internal.ConnectionID, net.Conn) {}
 
 type Writer struct {
 	id       ConnectionID
diff --git a/transport/internet/tcp/connection.go b/transport/internet/tcp/connection.go
index 37e98dd9c..2995db272 100644
--- a/transport/internet/tcp/connection.go
+++ b/transport/internet/tcp/connection.go
@@ -9,7 +9,7 @@ import (
 )
 
 type ConnectionManager interface {
-	Put(internal.ConnectionId, net.Conn)
+	Put(internal.ConnectionID, net.Conn)
 }
 
 type RawConnection struct {
@@ -27,14 +27,14 @@ func (v *RawConnection) SysFd() (int, error) {
 }
 
 type Connection struct {
-	id       internal.ConnectionId
+	id       internal.ConnectionID
 	reusable bool
 	conn     net.Conn
 	listener ConnectionManager
 	config   *Config
 }
 
-func NewConnection(id internal.ConnectionId, conn net.Conn, manager ConnectionManager, config *Config) *Connection {
+func NewConnection(id internal.ConnectionID, conn net.Conn, manager ConnectionManager, config *Config) *Connection {
 	return &Connection{
 		id:       id,
 		conn:     conn,
diff --git a/transport/internet/tcp/dialer.go b/transport/internet/tcp/dialer.go
index 7f4696a53..3cf4df13e 100644
--- a/transport/internet/tcp/dialer.go
+++ b/transport/internet/tcp/dialer.go
@@ -27,7 +27,7 @@ func Dial(src v2net.Address, dest v2net.Destination, options internet.DialerOpti
 	}
 	tcpSettings := networkSettings.(*Config)
 
-	id := internal.NewConnectionId(src, dest)
+	id := internal.NewConnectionID(src, dest)
 	var conn net.Conn
 	if dest.Network == v2net.Network_TCP && tcpSettings.IsConnectionReuse() {
 		conn = globalCache.Get(id)
diff --git a/transport/internet/tcp/hub.go b/transport/internet/tcp/hub.go
index 6ed346f4c..834919b2e 100644
--- a/transport/internet/tcp/hub.go
+++ b/transport/internet/tcp/hub.go
@@ -89,7 +89,7 @@ func (v *TCPListener) Accept() (internet.Connection, error) {
 				return nil, connErr.err
 			}
 			conn := connErr.conn
-			return NewConnection(internal.ConnectionId{}, conn, v, v.config), nil
+			return NewConnection(internal.ConnectionID{}, conn, v, v.config), nil
 		case <-time.After(time.Second * 2):
 		}
 	}
@@ -125,7 +125,7 @@ func (v *TCPListener) KeepAccepting() {
 	}
 }
 
-func (v *TCPListener) Put(id internal.ConnectionId, conn net.Conn) {
+func (v *TCPListener) Put(id internal.ConnectionID, conn net.Conn) {
 	v.Lock()
 	defer v.Unlock()
 	if !v.acccepting {