Use multiple parallel KCP state machines in the server.

To distribute CPU load.

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40200

server/lib/http.go

@@ -3,6 +3,10 @@ package snowflake_server
 import (
 	"bufio"
 	"bytes"
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/binary"
 	"fmt"
 	"io"
 	"log"
@@ -49,9 +53,45 @@ var upgrader = websocket.Upgrader{
 var clientIDAddrMap = newClientIDMap(clientIDAddrMapCapacity)
 
 type httpHandler struct {
-	// pconn is the adapter layer between stream-oriented WebSocket
-	// connections and the packet-oriented KCP layer.
-	pconn *turbotunnel.QueuePacketConn
+	// pconns is the adapter layer between stream-oriented WebSocket
+	// connections and the packet-oriented KCP layer. There are multiple of
+	// these, corresponding to the multiple kcp.ServeConn in
+	// Transport.Listen. Clients are assigned to a particular instance by a
+	// hash of ClientID, indexed by a hash of the ClientID, in order to
+	// distribute KCP processing load across CPU cores.
+	pconns []*turbotunnel.QueuePacketConn
+
+	// clientIDLookupKey is a secret key used to tweak the hash-based
+	// assignement of ClientID to pconn, in order to avoid manipulation of
+	// hash assignments.
+	clientIDLookupKey []byte
+}
+
+// newHTTPHandler creates a new http.Handler that exchanges encapsulated packets
+// over incoming WebSocket connections.
+func newHTTPHandler(localAddr net.Addr, numInstances int) *httpHandler {
+	pconns := make([]*turbotunnel.QueuePacketConn, 0, numInstances)
+	for i := 0; i < numInstances; i++ {
+		pconns = append(pconns, turbotunnel.NewQueuePacketConn(localAddr, clientMapTimeout))
+	}
+
+	clientIDLookupKey := make([]byte, 16)
+	_, err := rand.Read(clientIDLookupKey)
+	if err != nil {
+		panic(err)
+	}
+
+	return &httpHandler{
+		pconns:            pconns,
+		clientIDLookupKey: clientIDLookupKey,
+	}
+}
+
+// lookupPacketConn returns the element of pconns that corresponds to client ID,
+// according to the hash-based mapping.
+func (handler *httpHandler) lookupPacketConn(clientID turbotunnel.ClientID) *turbotunnel.QueuePacketConn {
+	s := hmac.New(sha256.New, handler.clientIDLookupKey).Sum(clientID[:])
+	return handler.pconns[binary.LittleEndian.Uint64(s)%uint64(len(handler.pconns))]
 }
 
 func (handler *httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
@@ -82,7 +122,7 @@ func (handler *httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	switch {
 	case bytes.Equal(token[:], turbotunnel.Token[:]):
-		err = turbotunnelMode(conn, addr, handler.pconn)
+		err = handler.turbotunnelMode(conn, addr)
 	default:
 		// We didn't find a matching token, which means that we are
 		// dealing with a client that doesn't know about such things.
@@ -100,7 +140,7 @@ func (handler *httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 // turbotunnelMode handles clients that sent turbotunnel.Token at the start of
 // their stream. These clients expect to send and receive encapsulated packets,
 // with a long-lived session identified by ClientID.
-func turbotunnelMode(conn net.Conn, addr net.Addr, pconn *turbotunnel.QueuePacketConn) error {
+func (handler *httpHandler) turbotunnelMode(conn net.Conn, addr net.Addr) error {
 	// Read the ClientID prefix. Every packet encapsulated in this WebSocket
 	// connection pertains to the same ClientID.
 	var clientID turbotunnel.ClientID
@@ -120,6 +160,8 @@ func turbotunnelMode(conn net.Conn, addr net.Addr, pconn *turbotunnel.QueuePacke
 	// credited for the entire KCP session.
 	clientIDAddrMap.Set(clientID, addr)
 
+	pconn := handler.lookupPacketConn(clientID)
+
 	var wg sync.WaitGroup
 	wg.Add(2)
 	done := make(chan struct{})

server/lib/snowflake.go

@@ -55,6 +55,11 @@ const (
 	WindowSize = 65535
 	// StreamSize controls the maximum amount of in flight data between a client and server.
 	StreamSize = 1048576 //1MB
+
+	// numKCPInstances is the number of parallel KCP state machines to run.
+	// Clients are assigned to a particular KCP instance by a hash of their
+	// ClientID.
+	numKCPInstances = 2
 )
 
 // Transport is a structure with methods that conform to the Go PT v2.1 API
@@ -76,17 +81,13 @@ func (t *Transport) Listen(addr net.Addr) (*SnowflakeListener, error) {
 		addr:   addr,
 		queue:  make(chan net.Conn, 65534),
 		closed: make(chan struct{}),
+		ln:     make([]*kcp.Listener, 0, numKCPInstances),
 	}
 
-	handler := httpHandler{
-		// pconn is shared among all connections to this server. It
-		// overlays packet-based client sessions on top of ephemeral
-		// WebSocket connections.
-		pconn: turbotunnel.NewQueuePacketConn(addr, clientMapTimeout),
-	}
+	handler := newHTTPHandler(addr, numKCPInstances)
 	server := &http.Server{
 		Addr:        addr.String(),
-		Handler:     &handler,
+		Handler:     handler,
 		ReadTimeout: requestTimeout,
 	}
 	// We need to override server.TLSConfig.GetCertificate--but first
@@ -139,12 +140,13 @@ func (t *Transport) Listen(addr net.Addr) (*SnowflakeListener, error) {
 
 	listener.server = server
 
-	// Start a KCP engine, set up to read and write its packets over the
+	// Start the KCP engines, set up to read and write its packets over the
 	// WebSocket connections that arrive at the web server.
 	// handler.ServeHTTP is responsible for encapsulation/decapsulation of
 	// packets on behalf of KCP. KCP takes those packets and turns them into
 	// sessions which appear in the acceptSessions function.
-	ln, err := kcp.ServeConn(nil, 0, 0, handler.pconn)
+	for i, pconn := range handler.pconns {
+		ln, err := kcp.ServeConn(nil, 0, 0, pconn)
 		if err != nil {
 			server.Close()
 			return nil, err
@@ -153,11 +155,11 @@ func (t *Transport) Listen(addr net.Addr) (*SnowflakeListener, error) {
 			defer ln.Close()
 			err := listener.acceptSessions(ln)
 			if err != nil {
-			log.Printf("acceptSessions: %v", err)
+				log.Printf("acceptSessions %d: %v", i, err)
 			}
 		}()
-
-	listener.ln = ln
+		listener.ln = append(listener.ln, ln)
+	}
 
 	return listener, nil
 
@@ -167,7 +169,7 @@ type SnowflakeListener struct {
 	addr      net.Addr
 	queue     chan net.Conn
 	server    *http.Server
-	ln        *kcp.Listener
+	ln        []*kcp.Listener
 	closed    chan struct{}
 	closeOnce sync.Once
 }
@@ -196,7 +198,9 @@ func (l *SnowflakeListener) Close() error {
 	l.closeOnce.Do(func() {
 		close(l.closed)
 		l.server.Close()
-		l.ln.Close()
+		for _, ln := range l.ln {
+			ln.Close()
+		}
 	})
 	return nil
 }