Turbo Tunnel client and server.

The client opts into turbotunnel mode by sending a magic token at the
beginning of each WebSocket connection (before sending even the
ClientID). The token is just a random byte string I generated. The
server peeks at the token and, if it matches, uses turbotunnel mode.
Otherwise, it unreads the token and continues in the old
one-session-per-WebSocket mode.

client/lib/snowflake.go

@@ -1,11 +1,16 @@
 package lib
 
 import (
+	"context"
 	"errors"
 	"io"
 	"log"
 	"net"
 	"time"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/turbotunnel"
+	"github.com/xtaci/kcp-go/v5"
+	"github.com/xtaci/smux"
 )
 
 const (
@@ -13,43 +18,98 @@ const (
 	SnowflakeTimeout = 30 * time.Second
 )
 
+type dummyAddr struct{}
+
+func (addr dummyAddr) Network() string { return "dummy" }
+func (addr dummyAddr) String() string  { return "dummy" }
+
 // Given an accepted SOCKS connection, establish a WebRTC connection to the
 // remote peer and exchange traffic.
 func Handler(socks net.Conn, snowflakes SnowflakeCollector) error {
-	// Obtain an available WebRTC remote. May block.
-	snowflake := snowflakes.Pop()
-	if nil == snowflake {
-		return errors.New("handler: Received invalid Snowflake")
+	clientID := turbotunnel.NewClientID()
+
+	// We build a persistent KCP session on a sequence of ephemeral WebRTC
+	// connections. This dialContext tells RedialPacketConn how to get a new
+	// WebRTC connection when the previous one dies. Inside each WebRTC
+	// connection, we use EncapsulationPacketConn to encode packets into a
+	// stream.
+	dialContext := func(ctx context.Context) (net.PacketConn, error) {
+		log.Printf("redialing on same connection")
+		// Obtain an available WebRTC remote. May block.
+		conn := snowflakes.Pop()
+		if conn == nil {
+			return nil, errors.New("handler: Received invalid Snowflake")
+		}
+		log.Println("---- Handler: snowflake assigned ----")
+		// Send the magic Turbo Tunnel token.
+		_, err := conn.Write(turbotunnel.Token[:])
+		if err != nil {
+			return nil, err
+		}
+		// Send ClientID prefix.
+		_, err = conn.Write(clientID[:])
+		if err != nil {
+			return nil, err
+		}
+		return NewEncapsulationPacketConn(dummyAddr{}, dummyAddr{}, conn), nil
 	}
-	defer snowflake.Close()
-	log.Println("---- Handler: snowflake assigned ----")
+	pconn := turbotunnel.NewRedialPacketConn(dummyAddr{}, dummyAddr{}, dialContext)
+	defer pconn.Close()
 
-	go func() {
-		// When WebRTC resets, close the SOCKS connection too.
-		snowflake.WaitForReset()
-		socks.Close()
-	}()
+	// conn is built on the underlying RedialPacketConn—when one WebRTC
+	// connection dies, another one will be found to take its place. The
+	// sequence of packets across multiple WebRTC connections drives the KCP
+	// engine.
+	conn, err := kcp.NewConn2(dummyAddr{}, nil, 0, 0, pconn)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+	// Permit coalescing the payloads of consecutive sends.
+	conn.SetStreamMode(true)
+	// Disable the dynamic congestion window (limit only by the
+	// maximum of local and remote static windows).
+	conn.SetNoDelay(
+		0, // default nodelay
+		0, // default interval
+		0, // default resend
+		1, // nc=1 => congestion window off
+	)
+	// On the KCP connection we overlay an smux session and stream.
+	smuxConfig := smux.DefaultConfig()
+	smuxConfig.Version = 2
+	smuxConfig.KeepAliveTimeout = 10 * time.Minute
+	sess, err := smux.Client(conn, smuxConfig)
+	if err != nil {
+		return err
+	}
+	defer sess.Close()
+	stream, err := sess.OpenStream()
+	if err != nil {
+		return err
+	}
+	defer stream.Close()
 
-	// Begin exchanging data. Either WebRTC or localhost SOCKS will close first.
-	// In eithercase, this closes the handler and induces a new handler.
-	copyLoop(socks, snowflake)
-	log.Println("---- Handler: closed ---")
+	// Begin exchanging data.
+	log.Printf("---- Handler: begin stream %v ---", stream.ID())
+	copyLoop(socks, stream)
+	log.Printf("---- Handler: closed stream %v ---", stream.ID())
 	return nil
 }
 
 // Exchanges bytes between two ReadWriters.
-// (In this case, between a SOCKS and WebRTC connection.)
-func copyLoop(socks, webRTC io.ReadWriter) {
+// (In this case, between a SOCKS connection and smux stream.)
+func copyLoop(socks, stream io.ReadWriter) {
 	done := make(chan struct{}, 2)
 	go func() {
-		if _, err := io.Copy(socks, webRTC); err != nil {
+		if _, err := io.Copy(socks, stream); err != nil {
 			log.Printf("copying WebRTC to SOCKS resulted in error: %v", err)
 		}
 		done <- struct{}{}
 	}()
 	go func() {
-		if _, err := io.Copy(webRTC, socks); err != nil {
-			log.Printf("copying SOCKS to WebRTC resulted in error: %v", err)
+		if _, err := io.Copy(stream, socks); err != nil {
+			log.Printf("copying SOCKS to stream resulted in error: %v", err)
 		}
 		done <- struct{}{}
 	}()

client/lib/turbotunnel.go

@@ -0,0 +1,68 @@
+package lib
+
+import (
+	"bufio"
+	"errors"
+	"io"
+	"net"
+	"time"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/encapsulation"
+)
+
+var errNotImplemented = errors.New("not implemented")
+
+// EncapsulationPacketConn implements the net.PacketConn interface over an
+// io.ReadWriteCloser stream, using the encapsulation package to represent
+// packets in a stream.
+type EncapsulationPacketConn struct {
+	io.ReadWriteCloser
+	localAddr  net.Addr
+	remoteAddr net.Addr
+	bw         *bufio.Writer
+}
+
+// NewEncapsulationPacketConn makes
+func NewEncapsulationPacketConn(
+	localAddr, remoteAddr net.Addr,
+	conn io.ReadWriteCloser,
+) *EncapsulationPacketConn {
+	return &EncapsulationPacketConn{
+		ReadWriteCloser: conn,
+		localAddr:       localAddr,
+		remoteAddr:      remoteAddr,
+		bw:              bufio.NewWriter(conn),
+	}
+}
+
+// ReadFrom reads an encapsulated packet from the stream.
+func (c *EncapsulationPacketConn) ReadFrom(p []byte) (int, net.Addr, error) {
+	data, err := encapsulation.ReadData(c.ReadWriteCloser)
+	if err != nil {
+		return 0, c.remoteAddr, err
+	}
+	return copy(p, data), c.remoteAddr, nil
+}
+
+// WriteTo writes an encapsulated packet to the stream.
+func (c *EncapsulationPacketConn) WriteTo(p []byte, addr net.Addr) (int, error) {
+	// addr is ignored.
+	_, err := encapsulation.WriteData(c.bw, p)
+	if err == nil {
+		err = c.bw.Flush()
+	}
+	if err != nil {
+		return 0, err
+	}
+	return len(p), nil
+}
+
+// LocalAddr returns the localAddr value that was passed to
+// NewEncapsulationPacketConn.
+func (c *EncapsulationPacketConn) LocalAddr() net.Addr {
+	return c.localAddr
+}
+
+func (c *EncapsulationPacketConn) SetDeadline(t time.Time) error      { return errNotImplemented }
+func (c *EncapsulationPacketConn) SetReadDeadline(t time.Time) error  { return errNotImplemented }
+func (c *EncapsulationPacketConn) SetWriteDeadline(t time.Time) error { return errNotImplemented }