sess.go

// Package kcp-go is a Reliable-UDP library for golang.
//
// This library intents to provide a smooth, resilient, ordered,
// error-checked and anonymous delivery of streams over UDP packets.
//
// The interfaces of this package aims to be compatible with
// net.Conn in standard library, but offers powerful features for advanced users.
package kcp

import (
	"crypto/rand"
	"encoding/binary"
	"hash/crc32"
	"io"
	"net"
	"sync"
	"sync/atomic"
	"time"

	"github.com/pkg/errors"
	"golang.org/x/net/ipv4"
	"golang.org/x/net/ipv6"
)

const (
	// 16-bytes nonce for each packet
	nonceSize = 16

	// 4-bytes packet checksum
	crcSize = 4

	// overall crypto header size
	cryptHeaderSize = nonceSize + crcSize

	// maximum packet size
	mtuLimit = 1500

	// accept backlog
	acceptBacklog = 128
)

var (
	errInvalidOperation = errors.New("invalid operation")
	errTimeout          = errors.New("timeout")
)

var (
	// a system-wide packet buffer shared among sending, receiving and FEC
	// to mitigate high-frequency memory allocation for packets, bytes from xmitBuf
	// is aligned to 64bit
	xmitBuf sync.Pool
)

func init() {
	xmitBuf.New = func() interface{} {
		return make([]byte, mtuLimit)
	}
}

type (
	// UDPSession defines a KCP session implemented by UDP
	UDPSession struct {
		conn    net.PacketConn // the underlying packet connection
		ownConn bool           // true if we created conn internally, false if provided by caller
		kcp     *KCP           // KCP ARQ protocol
		l       *Listener      // pointing to the Listener object if it's been accepted by a Listener
		block   BlockCrypt     // block encryption object

		// kcp receiving is based on packets
		// recvbuf turns packets into stream
		recvbuf []byte
		bufptr  []byte

		// FEC codec
		fecDecoder *fecDecoder
		fecEncoder *fecEncoder

		// settings
		remote     net.Addr  // remote peer address
		rd         time.Time // read deadline
		wd         time.Time // write deadline
		headerSize int       // the header size additional to a KCP frame
		ackNoDelay bool      // send ack immediately for each incoming packet(testing purpose)
		writeDelay bool      // delay kcp.flush() for Write() for bulk transfer
		dup        int       // duplicate udp packets(testing purpose)

		// notifications
		die          chan struct{} // notify current session has Closed
		dieOnce      sync.Once
		chReadEvent  chan struct{} // notify Read() can be called without blocking
		chWriteEvent chan struct{} // notify Write() can be called without blocking

		// socket error handling
		socketReadError      atomic.Value
		socketWriteError     atomic.Value
		chSocketReadError    chan struct{}
		chSocketWriteError   chan struct{}
		socketReadErrorOnce  sync.Once
		socketWriteErrorOnce sync.Once

		// nonce generator
		nonce Entropy

		// packets waiting to be sent on wire
		txqueue         []ipv4.Message
		xconn           batchConn // for x/net
		xconnWriteError error

		mu sync.Mutex
	}

	setReadBuffer interface {
		SetReadBuffer(bytes int) error
	}

	setWriteBuffer interface {
		SetWriteBuffer(bytes int) error
	}

	setDSCP interface {
		SetDSCP(int) error
	}
)

// newUDPSession create a new udp session for client or server
func newUDPSession(conv uint32, dataShards, parityShards int, l *Listener, conn net.PacketConn, ownConn bool, remote net.Addr, block BlockCrypt) *UDPSession {
	sess := new(UDPSession)
	sess.die = make(chan struct{})
	sess.nonce = new(nonceAES128)
	sess.nonce.Init()
	sess.chReadEvent = make(chan struct{}, 1)
	sess.chWriteEvent = make(chan struct{}, 1)
	sess.chSocketReadError = make(chan struct{})
	sess.chSocketWriteError = make(chan struct{})
	sess.remote = remote
	sess.conn = conn
	sess.ownConn = ownConn
	sess.l = l
	sess.block = block
	sess.recvbuf = make([]byte, mtuLimit)

	// cast to writebatch conn
	if _, ok := conn.(*net.UDPConn); ok {
		addr, err := net.ResolveUDPAddr("udp", conn.LocalAddr().String())
		if err == nil {
			if addr.IP.To4() != nil {
				sess.xconn = ipv4.NewPacketConn(conn)
			} else {
				sess.xconn = ipv6.NewPacketConn(conn)
			}
		}
	}

	// FEC codec initialization
	sess.fecDecoder = newFECDecoder(dataShards, parityShards)
	if sess.block != nil {
		sess.fecEncoder = newFECEncoder(dataShards, parityShards, cryptHeaderSize)
	} else {
		sess.fecEncoder = newFECEncoder(dataShards, parityShards, 0)
	}

	// calculate additional header size introduced by FEC and encryption
	if sess.block != nil {
		sess.headerSize += cryptHeaderSize
	}
	if sess.fecEncoder != nil {
		sess.headerSize += fecHeaderSizePlus2
	}

	sess.kcp = NewKCP(conv, func(buf []byte, size int) {
		if size >= IKCP_OVERHEAD+sess.headerSize {
			sess.output(buf[:size])
		}
	})
	sess.kcp.ReserveBytes(sess.headerSize)

	if sess.l == nil { // it's a client connection
		go sess.readLoop()
		atomic.AddUint64(&DefaultSnmp.ActiveOpens, 1)
	} else {
		atomic.AddUint64(&DefaultSnmp.PassiveOpens, 1)
	}

	// start per-session updater
	SystemTimedSched.Put(sess.update, time.Now())

	currestab := atomic.AddUint64(&DefaultSnmp.CurrEstab, 1)
	maxconn := atomic.LoadUint64(&DefaultSnmp.MaxConn)
	if currestab > maxconn {
		atomic.CompareAndSwapUint64(&DefaultSnmp.MaxConn, maxconn, currestab)
	}

	return sess
}

// Read implements net.Conn
func (s *UDPSession) Read(b []byte) (n int, err error) {
	var timeout *time.Timer
	// deadline for current reading operation
	var c <-chan time.Time
	if !s.rd.IsZero() {
		delay := time.Until(s.rd)
		timeout = time.NewTimer(delay)
		c = timeout.C
		defer timeout.Stop()
	}

	for {
		s.mu.Lock()
		if len(s.bufptr) > 0 { // copy from buffer into b
			n = copy(b, s.bufptr)
			s.bufptr = s.bufptr[n:]
			s.mu.Unlock()
			atomic.AddUint64(&DefaultSnmp.BytesReceived, uint64(n))
			return n, nil
		}

		if size := s.kcp.PeekSize(); size > 0 { // peek data size from kcp
			if len(b) >= size { // receive data into 'b' directly
				s.kcp.Recv(b)
				s.mu.Unlock()
				atomic.AddUint64(&DefaultSnmp.BytesReceived, uint64(size))
				return size, nil
			}

			// if necessary resize the stream buffer to guarantee a sufficient buffer space
			if cap(s.recvbuf) < size {
				s.recvbuf = make([]byte, size)
			}

			// resize the length of recvbuf to correspond to data size
			s.recvbuf = s.recvbuf[:size]
			s.kcp.Recv(s.recvbuf)
			n = copy(b, s.recvbuf)   // copy to 'b'
			s.bufptr = s.recvbuf[n:] // pointer update
			s.mu.Unlock()
			atomic.AddUint64(&DefaultSnmp.BytesReceived, uint64(n))
			return n, nil
		}

		s.mu.Unlock()

		// wait for read event or timeout or error
		select {
		case <-s.chReadEvent:
		case <-c:
			return 0, errors.WithStack(errTimeout)
		case <-s.chSocketReadError:
			return 0, s.socketReadError.Load().(error)
		case <-s.die:
			return 0, errors.WithStack(io.ErrClosedPipe)
		}
	}
}

// Write implements net.Conn
func (s *UDPSession) Write(b []byte) (n int, err error) { return s.WriteBuffers([][]byte{b}) }

// WriteBuffers write a vector of byte slices to the underlying connection
func (s *UDPSession) WriteBuffers(v [][]byte) (n int, err error) {
	var timeout *time.Timer
	var c <-chan time.Time
	if !s.wd.IsZero() {
		delay := time.Until(s.wd)
		timeout = time.NewTimer(delay)
		c = timeout.C
		defer timeout.Stop()
	}

	for {
		select {
		case <-s.chSocketWriteError:
			return 0, s.socketWriteError.Load().(error)
		case <-s.die:
			return 0, errors.WithStack(io.ErrClosedPipe)
		default:
		}

		s.mu.Lock()

		// make sure write do not overflow the max sliding window on both side
		waitsnd := s.kcp.WaitSnd()
		if waitsnd < int(s.kcp.snd_wnd) && waitsnd < int(s.kcp.rmt_wnd) {
			for _, b := range v {
				n += len(b)
				for {
					if len(b) <= int(s.kcp.mss) {
						s.kcp.Send(b)
						break
					} else {
						s.kcp.Send(b[:s.kcp.mss])
						b = b[s.kcp.mss:]
					}
				}
			}

			waitsnd = s.kcp.WaitSnd()
			if waitsnd >= int(s.kcp.snd_wnd) || waitsnd >= int(s.kcp.rmt_wnd) || !s.writeDelay {
				s.kcp.flush(false)
				s.uncork()
			}
			s.mu.Unlock()
			atomic.AddUint64(&DefaultSnmp.BytesSent, uint64(n))
			return n, nil
		}

		s.mu.Unlock()

		select {
		case <-s.chWriteEvent:
		case <-c:
			return 0, errors.WithStack(errTimeout)
		case <-s.chSocketWriteError:
			return 0, s.socketWriteError.Load().(error)
		case <-s.die:
			return 0, errors.WithStack(io.ErrClosedPipe)
		}
	}
}

// uncork sends data in txqueue if there is any
func (s *UDPSession) uncork() {
	if len(s.txqueue) > 0 {
		s.tx(s.txqueue)
		// recycle
		for k := range s.txqueue {
			xmitBuf.Put(s.txqueue[k].Buffers[0])
			s.txqueue[k].Buffers = nil
		}
		s.txqueue = s.txqueue[:0]
	}
}

// Close closes the connection.
func (s *UDPSession) Close() error {
	var once bool
	s.dieOnce.Do(func() {
		close(s.die)
		once = true
	})

	if once {
		atomic.AddUint64(&DefaultSnmp.CurrEstab, ^uint64(0))

		// try best to send all queued messages
		s.mu.Lock()
		s.kcp.flush(false)
		s.uncork()
		// release pending segments
		s.kcp.ReleaseTX()
		if s.fecDecoder != nil {
			s.fecDecoder.release()
		}
		s.mu.Unlock()

		if s.l != nil { // belongs to listener
			s.l.closeSession(s.remote)
			return nil
		} else if s.ownConn { // client socket close
			return s.conn.Close()
		} else {
			return nil
		}
	} else {
		return errors.WithStack(io.ErrClosedPipe)
	}
}

// LocalAddr returns the local network address. The Addr returned is shared by all invocations of LocalAddr, so do not modify it.
func (s *UDPSession) LocalAddr() net.Addr { return s.conn.LocalAddr() }

// RemoteAddr returns the remote network address. The Addr returned is shared by all invocations of RemoteAddr, so do not modify it.
func (s *UDPSession) RemoteAddr() net.Addr { return s.remote }

// SetDeadline sets the deadline associated with the listener. A zero time value disables the deadline.
func (s *UDPSession) SetDeadline(t time.Time) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.rd = t
	s.wd = t
	s.notifyReadEvent()
	s.notifyWriteEvent()
	return nil
}

// SetReadDeadline implements the Conn SetReadDeadline method.
func (s *UDPSession) SetReadDeadline(t time.Time) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.rd = t
	s.notifyReadEvent()
	return nil
}

// SetWriteDeadline implements the Conn SetWriteDeadline method.
func (s *UDPSession) SetWriteDeadline(t time.Time) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.wd = t
	s.notifyWriteEvent()
	return nil
}

// SetWriteDelay delays write for bulk transfer until the next update interval
func (s *UDPSession) SetWriteDelay(delay bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.writeDelay = delay
}

// SetWindowSize set maximum window size
func (s *UDPSession) SetWindowSize(sndwnd, rcvwnd int) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.kcp.WndSize(sndwnd, rcvwnd)
}

// SetMtu sets the maximum transmission unit(not including UDP header)
func (s *UDPSession) SetMtu(mtu int) bool {
	if mtu > mtuLimit {
		return false
	}

	s.mu.Lock()
	defer s.mu.Unlock()
	s.kcp.SetMtu(mtu)
	return true
}

// SetStreamMode toggles the stream mode on/off
func (s *UDPSession) SetStreamMode(enable bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if enable {
		s.kcp.stream = 1
	} else {
		s.kcp.stream = 0
	}
}

// SetACKNoDelay changes ack flush option, set true to flush ack immediately,
func (s *UDPSession) SetACKNoDelay(nodelay bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.ackNoDelay = nodelay
}

// (deprecated)
//
// SetDUP duplicates udp packets for kcp output.
func (s *UDPSession) SetDUP(dup int) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.dup = dup
}

// SetNoDelay calls nodelay() of kcp
// https://github.com/skywind3000/kcp/blob/master/README.en.md#protocol-configuration
func (s *UDPSession) SetNoDelay(nodelay, interval, resend, nc int) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.kcp.NoDelay(nodelay, interval, resend, nc)
}

// SetDSCP sets the 6bit DSCP field in IPv4 header, or 8bit Traffic Class in IPv6 header.
//
// if the underlying connection has implemented `func SetDSCP(int) error`, SetDSCP() will invoke
// this function instead.
//
// It has no effect if it's accepted from Listener.
func (s *UDPSession) SetDSCP(dscp int) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.l != nil {
		return errInvalidOperation
	}

	// interface enabled
	if ts, ok := s.conn.(setDSCP); ok {
		return ts.SetDSCP(dscp)
	}

	if nc, ok := s.conn.(net.Conn); ok {
		var succeed bool
		if err := ipv4.NewConn(nc).SetTOS(dscp << 2); err == nil {
			succeed = true
		}
		if err := ipv6.NewConn(nc).SetTrafficClass(dscp); err == nil {
			succeed = true
		}

		if succeed {
			return nil
		}
	}
	return errInvalidOperation
}

// SetReadBuffer sets the socket read buffer, no effect if it's accepted from Listener
func (s *UDPSession) SetReadBuffer(bytes int) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.l == nil {
		if nc, ok := s.conn.(setReadBuffer); ok {
			return nc.SetReadBuffer(bytes)
		}
	}
	return errInvalidOperation
}

// SetWriteBuffer sets the socket write buffer, no effect if it's accepted from Listener
func (s *UDPSession) SetWriteBuffer(bytes int) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.l == nil {
		if nc, ok := s.conn.(setWriteBuffer); ok {
			return nc.SetWriteBuffer(bytes)
		}
	}
	return errInvalidOperation
}

// post-processing for sending a packet from kcp core
// steps:
// 1. FEC packet generation
// 2. CRC32 integrity
// 3. Encryption
// 4. TxQueue
func (s *UDPSession) output(buf []byte) {
	var ecc [][]byte

	// 1. FEC encoding
	if s.fecEncoder != nil {
		ecc = s.fecEncoder.encode(buf)
	}

	// 2&3. crc32 & encryption
	if s.block != nil {
		s.nonce.Fill(buf[:nonceSize])
		checksum := crc32.ChecksumIEEE(buf[cryptHeaderSize:])
		binary.LittleEndian.PutUint32(buf[nonceSize:], checksum)
		s.block.Encrypt(buf, buf)

		for k := range ecc {
			s.nonce.Fill(ecc[k][:nonceSize])
			checksum := crc32.ChecksumIEEE(ecc[k][cryptHeaderSize:])
			binary.LittleEndian.PutUint32(ecc[k][nonceSize:], checksum)
			s.block.Encrypt(ecc[k], ecc[k])
		}
	}

	// 4. TxQueue
	var msg ipv4.Message
	for i := 0; i < s.dup+1; i++ {
		bts := xmitBuf.Get().([]byte)[:len(buf)]
		copy(bts, buf)
		msg.Buffers = [][]byte{bts}
		msg.Addr = s.remote
		s.txqueue = append(s.txqueue, msg)
	}

	for k := range ecc {
		bts := xmitBuf.Get().([]byte)[:len(ecc[k])]
		copy(bts, ecc[k])
		msg.Buffers = [][]byte{bts}
		msg.Addr = s.remote
		s.txqueue = append(s.txqueue, msg)
	}
}

// sess update to trigger protocol
func (s *UDPSession) update() {
	select {
	case <-s.die:
	default:
		s.mu.Lock()
		interval := s.kcp.flush(false)
		waitsnd := s.kcp.WaitSnd()
		if waitsnd < int(s.kcp.snd_wnd) && waitsnd < int(s.kcp.rmt_wnd) {
			s.notifyWriteEvent()
		}
		s.uncork()
		s.mu.Unlock()
		// self-synchronized timed scheduling
		SystemTimedSched.Put(s.update, time.Now().Add(time.Duration(interval)*time.Millisecond))
	}
}

// GetConv gets conversation id of a session
func (s *UDPSession) GetConv() uint32 { return s.kcp.conv }

// GetRTO gets current rto of the session
func (s *UDPSession) GetRTO() uint32 {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.kcp.rx_rto
}

// GetSRTT gets current srtt of the session
func (s *UDPSession) GetSRTT() int32 {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.kcp.rx_srtt
}

// GetRTTVar gets current rtt variance of the session
func (s *UDPSession) GetSRTTVar() int32 {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.kcp.rx_rttvar
}

func (s *UDPSession) notifyReadEvent() {
	select {
	case s.chReadEvent <- struct{}{}:
	default:
	}
}

func (s *UDPSession) notifyWriteEvent() {
	select {
	case s.chWriteEvent <- struct{}{}:
	default:
	}
}

func (s *UDPSession) notifyReadError(err error) {
	s.socketReadErrorOnce.Do(func() {
		s.socketReadError.Store(err)
		close(s.chSocketReadError)
	})
}

func (s *UDPSession) notifyWriteError(err error) {
	s.socketWriteErrorOnce.Do(func() {
		s.socketWriteError.Store(err)
		close(s.chSocketWriteError)
	})
}

// packet input stage
func (s *UDPSession) packetInput(data []byte) {
	decrypted := false
	if s.block != nil && len(data) >= cryptHeaderSize {
		s.block.Decrypt(data, data)
		data = data[nonceSize:]
		checksum := crc32.ChecksumIEEE(data[crcSize:])
		if checksum == binary.LittleEndian.Uint32(data) {
			data = data[crcSize:]
			decrypted = true
		} else {
			atomic.AddUint64(&DefaultSnmp.InCsumErrors, 1)
		}
	} else if s.block == nil {
		decrypted = true
	}

	if decrypted && len(data) >= IKCP_OVERHEAD {
		s.kcpInput(data)
	}
}

func (s *UDPSession) kcpInput(data []byte) {
	var kcpInErrors, fecErrs, fecRecovered, fecParityShards uint64

	fecFlag := binary.LittleEndian.Uint16(data[4:])
	if fecFlag == typeData || fecFlag == typeParity { // 16bit kcp cmd [81-84] and frg [0-255] will not overlap with FEC type 0x00f1 0x00f2
		if len(data) >= fecHeaderSizePlus2 {
			f := fecPacket(data)
			if f.flag() == typeParity {
				fecParityShards++
			}

			// lock
			s.mu.Lock()
			// if fecDecoder is not initialized, create one with default parameter
			if s.fecDecoder == nil {
				s.fecDecoder = newFECDecoder(1, 1)
			}
			recovers := s.fecDecoder.decode(f)
			if f.flag() == typeData {
				if ret := s.kcp.Input(data[fecHeaderSizePlus2:], true, s.ackNoDelay); ret != 0 {
					kcpInErrors++
				}
			}

			for _, r := range recovers {
				if len(r) >= 2 { // must be larger than 2bytes
					sz := binary.LittleEndian.Uint16(r)
					if int(sz) <= len(r) && sz >= 2 {
						if ret := s.kcp.Input(r[2:sz], false, s.ackNoDelay); ret == 0 {
							fecRecovered++
						} else {
							kcpInErrors++
						}
					} else {
						fecErrs++
					}
				} else {
					fecErrs++
				}
				// recycle the recovers
				xmitBuf.Put(r)
			}

			// to notify the readers to receive the data
			if n := s.kcp.PeekSize(); n > 0 {
				s.notifyReadEvent()
			}
			// to notify the writers
			waitsnd := s.kcp.WaitSnd()
			if waitsnd < int(s.kcp.snd_wnd) && waitsnd < int(s.kcp.rmt_wnd) {
				s.notifyWriteEvent()
			}

			s.uncork()
			s.mu.Unlock()
		} else {
			atomic.AddUint64(&DefaultSnmp.InErrs, 1)
		}
	} else {
		s.mu.Lock()
		if ret := s.kcp.Input(data, true, s.ackNoDelay); ret != 0 {
			kcpInErrors++
		}
		if n := s.kcp.PeekSize(); n > 0 {
			s.notifyReadEvent()
		}
		waitsnd := s.kcp.WaitSnd()
		if waitsnd < int(s.kcp.snd_wnd) && waitsnd < int(s.kcp.rmt_wnd) {
			s.notifyWriteEvent()
		}
		s.uncork()
		s.mu.Unlock()
	}

	atomic.AddUint64(&DefaultSnmp.InPkts, 1)
	atomic.AddUint64(&DefaultSnmp.InBytes, uint64(len(data)))
	if fecParityShards > 0 {
		atomic.AddUint64(&DefaultSnmp.FECParityShards, fecParityShards)
	}
	if kcpInErrors > 0 {
		atomic.AddUint64(&DefaultSnmp.KCPInErrors, kcpInErrors)
	}
	if fecErrs > 0 {
		atomic.AddUint64(&DefaultSnmp.FECErrs, fecErrs)
	}
	if fecRecovered > 0 {
		atomic.AddUint64(&DefaultSnmp.FECRecovered, fecRecovered)
	}

}

type (
	// Listener defines a server which will be waiting to accept incoming connections
	Listener struct {
		block        BlockCrypt     // block encryption
		dataShards   int            // FEC data shard
		parityShards int            // FEC parity shard
		conn         net.PacketConn // the underlying packet connection
		ownConn      bool           // true if we created conn internally, false if provided by caller

		sessions        map[string]*UDPSession // all sessions accepted by this Listener
		sessionLock     sync.RWMutex
		chAccepts       chan *UDPSession // Listen() backlog
		chSessionClosed chan net.Addr    // session close queue

		die     chan struct{} // notify the listener has closed
		dieOnce sync.Once

		// socket error handling
		socketReadError     atomic.Value
		chSocketReadError   chan struct{}
		socketReadErrorOnce sync.Once

		rd atomic.Value // read deadline for Accept()
	}
)

// packet input stage
func (l *Listener) packetInput(data []byte, addr net.Addr) {
	decrypted := false
	if l.block != nil && len(data) >= cryptHeaderSize {
		l.block.Decrypt(data, data)
		data = data[nonceSize:]
		checksum := crc32.ChecksumIEEE(data[crcSize:])
		if checksum == binary.LittleEndian.Uint32(data) {
			data = data[crcSize:]
			decrypted = true
		} else {
			atomic.AddUint64(&DefaultSnmp.InCsumErrors, 1)
		}
	} else if l.block == nil {
		decrypted = true
	}

	if decrypted && len(data) >= IKCP_OVERHEAD {
		l.sessionLock.RLock()
		s, ok := l.sessions[addr.String()]
		l.sessionLock.RUnlock()

		var conv, sn uint32
		convRecovered := false
		fecFlag := binary.LittleEndian.Uint16(data[4:])
		if fecFlag == typeData || fecFlag == typeParity { // 16bit kcp cmd [81-84] and frg [0-255] will not overlap with FEC type 0x00f1 0x00f2
			// packet with FEC
			if fecFlag == typeData && len(data) >= fecHeaderSizePlus2+IKCP_OVERHEAD {
				conv = binary.LittleEndian.Uint32(data[fecHeaderSizePlus2:])
				sn = binary.LittleEndian.Uint32(data[fecHeaderSizePlus2+IKCP_SN_OFFSET:])
				convRecovered = true
			}
		} else {
			// packet without FEC
			conv = binary.LittleEndian.Uint32(data)
			sn = binary.LittleEndian.Uint32(data[IKCP_SN_OFFSET:])
			convRecovered = true
		}

		if ok { // existing connection
			if !convRecovered || conv == s.kcp.conv { // parity data or valid conversation
				s.kcpInput(data)
			} else if sn == 0 { // should replace current connection
				s.Close()
				s = nil
			}
		}

		if s == nil && convRecovered { // new session
			if len(l.chAccepts) < cap(l.chAccepts) { // do not let the new sessions overwhelm accept queue
				s := newUDPSession(conv, l.dataShards, l.parityShards, l, l.conn, false, addr, l.block)
				s.kcpInput(data)
				l.sessionLock.Lock()
				l.sessions[addr.String()] = s
				l.sessionLock.Unlock()
				l.chAccepts <- s
			}
		}
	}
}

func (l *Listener) notifyReadError(err error) {
	l.socketReadErrorOnce.Do(func() {
		l.socketReadError.Store(err)
		close(l.chSocketReadError)

		// propagate read error to all sessions
		l.sessionLock.RLock()
		for _, s := range l.sessions {
			s.notifyReadError(err)
		}
		l.sessionLock.RUnlock()
	})
}

// SetReadBuffer sets the socket read buffer for the Listener
func (l *Listener) SetReadBuffer(bytes int) error {
	if nc, ok := l.conn.(setReadBuffer); ok {
		return nc.SetReadBuffer(bytes)
	}
	return errInvalidOperation
}

// SetWriteBuffer sets the socket write buffer for the Listener
func (l *Listener) SetWriteBuffer(bytes int) error {
	if nc, ok := l.conn.(setWriteBuffer); ok {
		return nc.SetWriteBuffer(bytes)
	}
	return errInvalidOperation
}

// SetDSCP sets the 6bit DSCP field in IPv4 header, or 8bit Traffic Class in IPv6 header.
//
// if the underlying connection has implemented `func SetDSCP(int) error`, SetDSCP() will invoke
// this function instead.
func (l *Listener) SetDSCP(dscp int) error {
	// interface enabled
	if ts, ok := l.conn.(setDSCP); ok {
		return ts.SetDSCP(dscp)
	}

	if nc, ok := l.conn.(net.Conn); ok {
		var succeed bool
		if err := ipv4.NewConn(nc).SetTOS(dscp << 2); err == nil {
			succeed = true
		}
		if err := ipv6.NewConn(nc).SetTrafficClass(dscp); err == nil {
			succeed = true
		}

		if succeed {
			return nil
		}
	}
	return errInvalidOperation
}

// Accept implements the Accept method in the Listener interface; it waits for the next call and returns a generic Conn.
func (l *Listener) Accept() (net.Conn, error) {
	return l.AcceptKCP()
}

// AcceptKCP accepts a KCP connection
func (l *Listener) AcceptKCP() (*UDPSession, error) {
	var timeout <-chan time.Time
	if tdeadline, ok := l.rd.Load().(time.Time); ok && !tdeadline.IsZero() {
		timeout = time.After(time.Until(tdeadline))
	}

	select {
	case <-timeout:
		return nil, errors.WithStack(errTimeout)
	case c := <-l.chAccepts:
		return c, nil
	case <-l.chSocketReadError:
		return nil, l.socketReadError.Load().(error)
	case <-l.die:
		return nil, errors.WithStack(io.ErrClosedPipe)
	}
}

// SetDeadline sets the deadline associated with the listener. A zero time value disables the deadline.
func (l *Listener) SetDeadline(t time.Time) error {
	l.SetReadDeadline(t)
	l.SetWriteDeadline(t)
	return nil
}

// SetReadDeadline implements the Conn SetReadDeadline method.
func (l *Listener) SetReadDeadline(t time.Time) error {
	l.rd.Store(t)
	return nil
}

// SetWriteDeadline implements the Conn SetWriteDeadline method.
func (l *Listener) SetWriteDeadline(t time.Time) error { return errInvalidOperation }

// Close stops listening on the UDP address, and closes the socket
func (l *Listener) Close() error {
	var once bool
	l.dieOnce.Do(func() {
		close(l.die)
		once = true
	})

	var err error
	if once {
		if l.ownConn {
			err = l.conn.Close()
		}
	} else {
		err = errors.WithStack(io.ErrClosedPipe)
	}
	return err
}

// closeSession notify the listener that a session has closed
func (l *Listener) closeSession(remote net.Addr) (ret bool) {
	l.sessionLock.Lock()
	defer l.sessionLock.Unlock()
	if _, ok := l.sessions[remote.String()]; ok {
		delete(l.sessions, remote.String())
		return true
	}
	return false
}

// Addr returns the listener's network address, The Addr returned is shared by all invocations of Addr, so do not modify it.
func (l *Listener) Addr() net.Addr { return l.conn.LocalAddr() }

// Listen listens for incoming KCP packets addressed to the local address laddr on the network "udp",
func Listen(laddr string) (net.Listener, error) { return ListenWithOptions(laddr, nil, 0, 0) }

// ListenWithOptions listens for incoming KCP packets addressed to the local address laddr on the network "udp" with packet encryption.
//
// 'block' is the block encryption algorithm to encrypt packets.
//
// 'dataShards', 'parityShards' specify how many parity packets will be generated following the data packets.
//
// Check https://github.com/klauspost/reedsolomon for details
func ListenWithOptions(laddr string, block BlockCrypt, dataShards, parityShards int) (*Listener, error) {
	udpaddr, err := net.ResolveUDPAddr("udp", laddr)
	if err != nil {
		return nil, errors.WithStack(err)
	}
	conn, err := net.ListenUDP("udp", udpaddr)
	if err != nil {
		return nil, errors.WithStack(err)
	}

	return serveConn(block, dataShards, parityShards, conn, true)
}

// ServeConn serves KCP protocol for a single packet connection.
func ServeConn(block BlockCrypt, dataShards, parityShards int, conn net.PacketConn) (*Listener, error) {
	return serveConn(block, dataShards, parityShards, conn, false)
}

func serveConn(block BlockCrypt, dataShards, parityShards int, conn net.PacketConn, ownConn bool) (*Listener, error) {
	l := new(Listener)
	l.conn = conn
	l.ownConn = ownConn
	l.sessions = make(map[string]*UDPSession)
	l.chAccepts = make(chan *UDPSession, acceptBacklog)
	l.chSessionClosed = make(chan net.Addr)
	l.die = make(chan struct{})
	l.dataShards = dataShards
	l.parityShards = parityShards
	l.block = block
	l.chSocketReadError = make(chan struct{})
	go l.monitor()
	return l, nil
}

// Dial connects to the remote address "raddr" on the network "udp" without encryption and FEC
func Dial(raddr string) (net.Conn, error) { return DialWithOptions(raddr, nil, 0, 0) }

// DialWithOptions connects to the remote address "raddr" on the network "udp" with packet encryption
//
// 'block' is the block encryption algorithm to encrypt packets.
//
// 'dataShards', 'parityShards' specify how many parity packets will be generated following the data packets.
//
// Check https://github.com/klauspost/reedsolomon for details
func DialWithOptions(raddr string, block BlockCrypt, dataShards, parityShards int) (*UDPSession, error) {
	// network type detection
	udpaddr, err := net.ResolveUDPAddr("udp", raddr)
	if err != nil {
		return nil, errors.WithStack(err)
	}
	network := "udp4"
	if udpaddr.IP.To4() == nil {
		network = "udp"
	}

	conn, err := net.ListenUDP(network, nil)
	if err != nil {
		return nil, errors.WithStack(err)
	}

	var convid uint32
	binary.Read(rand.Reader, binary.LittleEndian, &convid)
	return newUDPSession(convid, dataShards, parityShards, nil, conn, true, udpaddr, block), nil
}

// NewConn3 establishes a session and talks KCP protocol over a packet connection.
func NewConn3(convid uint32, raddr net.Addr, block BlockCrypt, dataShards, parityShards int, conn net.PacketConn) (*UDPSession, error) {
	return newUDPSession(convid, dataShards, parityShards, nil, conn, false, raddr, block), nil
}

// NewConn2 establishes a session and talks KCP protocol over a packet connection.
func NewConn2(raddr net.Addr, block BlockCrypt, dataShards, parityShards int, conn net.PacketConn) (*UDPSession, error) {
	var convid uint32
	binary.Read(rand.Reader, binary.LittleEndian, &convid)
	return NewConn3(convid, raddr, block, dataShards, parityShards, conn)
}

// NewConn establishes a session and talks KCP protocol over a packet connection.
func NewConn(raddr string, block BlockCrypt, dataShards, parityShards int, conn net.PacketConn) (*UDPSession, error) {
	udpaddr, err := net.ResolveUDPAddr("udp", raddr)
	if err != nil {
		return nil, errors.WithStack(err)
	}
	return NewConn2(udpaddr, block, dataShards, parityShards, conn)
}