check datagram size in PMTUD integration test

integrationtests/self/dplpmtud_test.go

@@ -17,10 +17,14 @@ import (
 
 var _ = Describe("DPLPMTUD", func() {
 	It("discovers the MTU", func() {
-		const rtt = 100 * time.Millisecond
+		rtt := scaleDuration(10 * time.Millisecond)
 		const mtu = 1400
 
-		ln, err := quic.ListenAddr("localhost:0", getTLSConfig(), getQuicConfig(&quic.Config{DisablePathMTUDiscovery: true}))
+		ln, err := quic.ListenAddr(
+			"localhost:0",
+			getTLSConfig(),
+			getQuicConfig(&quic.Config{DisablePathMTUDiscovery: true, EnableDatagrams: true}),
+		)
 		Expect(err).ToNot(HaveOccurred())
 		defer ln.Close()
 		go func() {
@@ -73,7 +77,7 @@ var _ = Describe("DPLPMTUD", func() {
 			context.Background(),
 			proxy.LocalAddr(),
 			getTLSClientConfig(),
-			getQuicConfig(nil),
+			getQuicConfig(&quic.Config{EnableDatagrams: true}),
 		)
 		Expect(err).ToNot(HaveOccurred())
 		defer conn.CloseWithError(0, "")
@@ -87,15 +91,27 @@ var _ = Describe("DPLPMTUD", func() {
 			Expect(err).ToNot(HaveOccurred())
 			Expect(data).To(Equal(PRDataLong))
 		}()
+		err = conn.SendDatagram(make([]byte, 2000))
+		Expect(err).To(BeAssignableToTypeOf(&quic.DatagramTooLargeError{}))
+		initialMaxDatagramSize := err.(*quic.DatagramTooLargeError).MaxDatagramPayloadSize
 		_, err = str.Write(PRDataLong)
 		Expect(err).ToNot(HaveOccurred())
 		str.Close()
 		Eventually(done, 20*time.Second).Should(BeClosed())
+		err = conn.SendDatagram(make([]byte, 2000))
+		Expect(err).To(BeAssignableToTypeOf(&quic.DatagramTooLargeError{}))
+		finalMaxDatagramSize := err.(*quic.DatagramTooLargeError).MaxDatagramPayloadSize
 
 		mx.Lock()
 		defer mx.Unlock()
 		fmt.Fprintf(GinkgoWriter, "max client packet size: %d, MTU: %d\n", maxPacketSizeClient, mtu)
+		fmt.Fprintf(GinkgoWriter, "max datagram size: initial: %d, final: %d\n", initialMaxDatagramSize, finalMaxDatagramSize)
 		fmt.Fprintf(GinkgoWriter, "max server packet size: %d, MTU: %d\n", maxPacketSizeServer, mtu)
 		Expect(maxPacketSizeClient).To(BeNumerically(">=", mtu-25))
+		const maxDiff = 40 // this includes the 21 bytes for the short header, 16 bytes for the encryption tag, and framing overhead
+		Expect(initialMaxDatagramSize).To(BeNumerically(">=", 1252-maxDiff))
+		Expect(finalMaxDatagramSize).To(BeNumerically(">=", maxPacketSizeClient-maxDiff))
+		// MTU discovery was disabled on the server side
+		Expect(maxPacketSizeServer).To(Equal(1252))
 	})
 })