diff --git a/server_test.go b/server_test.go
index b99d5f4..dd4baf6 100644
--- a/server_test.go
+++ b/server_test.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net"
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/gogo/protobuf/proto"
@@ -44,12 +45,25 @@ func (r *testPayload) String() string { return fmt.Sprintf("%+#v", r) }
 func (r *testPayload) ProtoMessage()  {}
 
 // testingServer is what would be implemented by the user of this package.
-type testingServer struct {
-	payload *testPayload
-}
+type testingServer struct{}
 
 func (s *testingServer) Test(ctx context.Context, req *testPayload) (*testPayload, error) {
-	return s.payload, nil
+	return &testPayload{Foo: strings.Repeat(req.Foo, 2)}, nil
+}
+
+// registerTestingService mocks more of what is generated code. Unlike grpc, we
+// register with a closure so that the descriptor is allocated only on
+// registration.
+func registerTestingService(srv *Server, svc testingService) {
+	srv.Register(serviceName, map[string]Method{
+		"Test": func(ctx context.Context, unmarshal func(interface{}) error) (interface{}, error) {
+			var req testPayload
+			if err := unmarshal(&req); err != nil {
+				return nil, err
+			}
+			return svc.Test(ctx, &req)
+		},
+	})
 }
 
 func init() {
@@ -60,29 +74,15 @@ func init() {
 
 func TestServer(t *testing.T) {
 	var (
-		ctx              = context.Background()
-		server           = NewServer()
-		expectedResponse = &testPayload{Foo: "baz"}
-		testImpl         = &testingServer{payload: expectedResponse}
+		ctx      = context.Background()
+		server   = NewServer()
+		testImpl = &testingServer{}
 	)
 
-	// more mocking of what is generated code. Unlike grpc, we register with a
-	// closure so that the descriptor is allocated only on registration.
-	registerTestingService := func(srv *Server, svc testingService) {
-		srv.Register(serviceName, map[string]Method{
-			"Test": func(ctx context.Context, unmarshal func(interface{}) error) (interface{}, error) {
-				var req testPayload
-				if err := unmarshal(&req); err != nil {
-					return nil, err
-				}
-				return svc.Test(ctx, &req)
-			},
-		})
-	}
-
 	registerTestingService(server, testImpl)
 
-	listener, err := net.Listen("tcp", ":0")
+	addr := "\x00" + t.Name()
+	listener, err := net.Listen("unix", addr)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -91,24 +91,48 @@ func TestServer(t *testing.T) {
 	go server.Serve(listener)
 	defer server.Shutdown(ctx)
 
-	conn, err := net.Dial("tcp", listener.Addr().String())
+	conn, err := net.Dial("unix", addr)
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer conn.Close()
-
 	client := newTestingClient(NewClient(conn))
 
-	tp := &testPayload{
-		Foo: "bar",
+	const calls = 2
+	results := make(chan callResult, 2)
+	go roundTrip(ctx, t, client, "bar", results)
+	go roundTrip(ctx, t, client, "baz", results)
+
+	for i := 0; i < calls; i++ {
+		result := <-results
+		if !reflect.DeepEqual(result.received, result.expected) {
+			t.Fatalf("unexpected response: %+#v != %+#v", result.received, result.expected)
+		}
 	}
+}
+
+type callResult struct {
+	input    *testPayload
+	expected *testPayload
+	received *testPayload
+}
+
+func roundTrip(ctx context.Context, t *testing.T, client *testingClient, value string, results chan callResult) {
+	t.Helper()
+	var (
+		tp = &testPayload{
+			Foo: "bar",
+		}
+	)
 
 	resp, err := client.Test(ctx, tp)
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	if !reflect.DeepEqual(resp, expectedResponse) {
-		t.Fatalf("unexpected response: %+#v != %+#v", resp, expectedResponse)
+	results <- callResult{
+		input:    tp,
+		expected: &testPayload{Foo: strings.Repeat(tp.Foo, 2)},
+		received: resp,
 	}
 }