diff --git a/internal/engine/experiment/urlgetter/getter.go b/internal/engine/experiment/urlgetter/getter.go
index 240b6ad..e29ce8a 100644
--- a/internal/engine/experiment/urlgetter/getter.go
+++ b/internal/engine/experiment/urlgetter/getter.go
@@ -94,7 +94,7 @@ func (g Getter) get(ctx context.Context, saver *trace.Saver) (TestKeys, error) {
 	// start tunnel
 	var proxyURL *url.URL
 	if g.Config.Tunnel != "" {
-		tun, err := tunnel.Start(ctx, tunnel.Config{
+		tun, err := tunnel.Start(ctx, &tunnel.Config{
 			Name:    g.Config.Tunnel,
 			Session: g.Session,
 			WorkDir: filepath.Join(g.Session.TempDir(), "urlgetter-tunnel"),
diff --git a/internal/engine/session.go b/internal/engine/session.go
index e688c35..d6a5869 100644
--- a/internal/engine/session.go
+++ b/internal/engine/session.go
@@ -350,7 +350,7 @@ func (s *Session) MaybeStartTunnel(ctx context.Context, name string) error {
 		// sets a proxy, the second check for s.tunnel is for robustness.
 		return ErrAlreadyUsingProxy
 	}
-	tunnel, err := tunnel.Start(ctx, tunnel.Config{
+	tunnel, err := tunnel.Start(ctx, &tunnel.Config{
 		Name:    name,
 		Session: s,
 	})
diff --git a/internal/engine/tunnel/integration_test.go b/internal/engine/tunnel/integration_test.go
index 7f3998d..61e54b3 100644
--- a/internal/engine/tunnel/integration_test.go
+++ b/internal/engine/tunnel/integration_test.go
@@ -21,7 +21,7 @@ func TestPsiphonStartWithCancelledContext(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	tunnel, err := tunnel.Start(ctx, tunnel.Config{
+	tunnel, err := tunnel.Start(ctx, &tunnel.Config{
 		Name:    "psiphon",
 		Session: sess,
 	})
@@ -45,7 +45,7 @@ func TestPsiphonStartStop(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	tunnel, err := tunnel.Start(context.Background(), tunnel.Config{
+	tunnel, err := tunnel.Start(context.Background(), &tunnel.Config{
 		Name:    "psiphon",
 		Session: sess,
 	})
diff --git a/internal/engine/tunnel/tunnel.go b/internal/engine/tunnel/tunnel.go
index 95a6654..a714e94 100644
--- a/internal/engine/tunnel/tunnel.go
+++ b/internal/engine/tunnel/tunnel.go
@@ -27,16 +27,23 @@ type Tunnel interface {
 	Stop()
 }
 
-// Config contains config for the session tunnel.
+// Config contains the configuration for creating a Tunnel instance.
 type Config struct {
-	Name    string
+	// Name is the mandatory name of the tunnel. We support
+	// "tor" and "psiphon" tunnels.
+	Name string
+
+	// Session is the current measurement session.
 	Session Session
+
+	// WorkDir is the directory in which the tunnel SHOULD
+	// store its state, if any.
 	WorkDir string
 }
 
 // Start starts a new tunnel by name or returns an error. Note that if you
 // pass to this function the "" tunnel, you get back nil, nil.
-func Start(ctx context.Context, config Config) (Tunnel, error) {
+func Start(ctx context.Context, config *Config) (Tunnel, error) {
 	logger := config.Session.Logger()
 	switch config.Name {
 	case "":
diff --git a/internal/engine/tunnel/tunnel_test.go b/internal/engine/tunnel/tunnel_test.go
index f87e700..b46d9c9 100644
--- a/internal/engine/tunnel/tunnel_test.go
+++ b/internal/engine/tunnel/tunnel_test.go
@@ -12,7 +12,7 @@ import (
 func TestStartNoTunnel(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
-	tunnel, err := Start(ctx, Config{
+	tunnel, err := Start(ctx, &Config{
 		Name: "",
 		Session: &mockable.Session{
 			MockableLogger: log.Log,
@@ -29,7 +29,7 @@ func TestStartNoTunnel(t *testing.T) {
 func TestStartPsiphonTunnel(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
-	tunnel, err := Start(ctx, Config{
+	tunnel, err := Start(ctx, &Config{
 		Name: "psiphon",
 		Session: &mockable.Session{
 			MockableLogger: log.Log,
@@ -46,7 +46,7 @@ func TestStartPsiphonTunnel(t *testing.T) {
 func TestStartTorTunnel(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
-	tunnel, err := Start(ctx, Config{
+	tunnel, err := Start(ctx, &Config{
 		Name: "tor",
 		Session: &mockable.Session{
 			MockableLogger: log.Log,
@@ -63,7 +63,7 @@ func TestStartTorTunnel(t *testing.T) {
 func TestStartInvalidTunnel(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
-	tunnel, err := Start(ctx, Config{
+	tunnel, err := Start(ctx, &Config{
 		Name: "antani",
 		Session: &mockable.Session{
 			MockableLogger: log.Log,