diff --git a/internal/httptransport/httptransport.go b/internal/httptransport/httptransport.go
index 1f5bc34..27e2568 100644
--- a/internal/httptransport/httptransport.go
+++ b/internal/httptransport/httptransport.go
@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/ooni/netx/internal/httptransport/toptripper"
+	"github.com/ooni/netx/internal/httptransport/transactioner"
 	"github.com/ooni/netx/model"
 	"golang.org/x/net/http2"
 )
@@ -35,9 +36,9 @@ func NewTransport(beginning time.Time, handler model.Handler) *Transport {
 			TLSHandshakeTimeout:   10 * time.Second,
 		},
 	}
-	transport.roundTripper = toptripper.New(
+	transport.roundTripper = transactioner.New(toptripper.New(
 		beginning, handler, transport.Transport,
-	)
+	))
 	// Configure h2 and make sure that the custom TLSConfig we use for dialing
 	// is actually compatible with upgrading to h2. (This mainly means we
 	// need to make sure we include "h2" in the NextProtos array.) Because
diff --git a/internal/httptransport/transactioner/transactioner.go b/internal/httptransport/transactioner/transactioner.go
new file mode 100644
index 0000000..0b6f210
--- /dev/null
+++ b/internal/httptransport/transactioner/transactioner.go
@@ -0,0 +1,55 @@
+// Package transactioner contains the transaction assigning round tripper
+package transactioner
+
+import (
+	"context"
+	"net/http"
+	"sync/atomic"
+)
+
+type contextkey struct{}
+
+var id int64
+
+// WithTransactionID returns a copy of ctx with TransactionID
+func WithTransactionID(ctx context.Context) context.Context {
+	return context.WithValue(
+		ctx, contextkey{}, atomic.AddInt64(&id, 1),
+	)
+}
+
+// ContextTransactionID returns the TransactionID of the context, or zero
+func ContextTransactionID(ctx context.Context) int64 {
+	id, _ := ctx.Value(contextkey{}).(int64)
+	return id
+}
+
+// Transport performs single HTTP transactions.
+type Transport struct {
+	roundTripper http.RoundTripper
+}
+
+// New creates a new Transport.
+func New(roundTripper http.RoundTripper) *Transport {
+	return &Transport{
+		roundTripper: roundTripper,
+	}
+}
+
+// RoundTrip executes a single HTTP transaction, returning
+// a Response for the provided Request.
+func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
+	ctx := WithTransactionID(req.Context())
+	return t.roundTripper.RoundTrip(req.WithContext(ctx))
+}
+
+// CloseIdleConnections closes the idle connections.
+func (t *Transport) CloseIdleConnections() {
+	// Adapted from net/http code
+	type closeIdler interface {
+		CloseIdleConnections()
+	}
+	if tr, ok := t.roundTripper.(closeIdler); ok {
+		tr.CloseIdleConnections()
+	}
+}
diff --git a/internal/httptransport/transactioner/transactioner_test.go b/internal/httptransport/transactioner/transactioner_test.go
new file mode 100644
index 0000000..0a5498e
--- /dev/null
+++ b/internal/httptransport/transactioner/transactioner_test.go
@@ -0,0 +1,55 @@
+package transactioner
+
+import (
+	"io/ioutil"
+	"net/http"
+	"testing"
+)
+
+type transport struct {
+	roundTripper http.RoundTripper
+	t            *testing.T
+}
+
+func (t *transport) RoundTrip(req *http.Request) (*http.Response, error) {
+	ctx := req.Context()
+	if id := ContextTransactionID(ctx); id == 0 {
+		t.t.Fatal("transaction ID not set")
+	}
+	return t.roundTripper.RoundTrip(req)
+}
+
+func TestIntegration(t *testing.T) {
+	client := &http.Client{
+		Transport: New(&transport{
+			roundTripper: http.DefaultTransport,
+			t:            t,
+		}),
+	}
+	resp, err := client.Get("https://www.google.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+	_, err = ioutil.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	client.CloseIdleConnections()
+}
+
+func TestIntegrationFailure(t *testing.T) {
+	client := &http.Client{
+		Transport: New(http.DefaultTransport),
+	}
+	// This fails the request because we attempt to speak cleartext HTTP with
+	// a server that instead is expecting TLS.
+	resp, err := client.Get("http://www.google.com:443")
+	if err == nil {
+		t.Fatal("expected an error here")
+	}
+	if resp != nil {
+		t.Fatal("expected a nil response here")
+	}
+	client.CloseIdleConnections()
+}