diff --git a/errorcodes/error_codes.go b/errorcodes/error_codes.go
index 30a8b2e68..5f2244dd0 100644
--- a/errorcodes/error_codes.go
+++ b/errorcodes/error_codes.go
@@ -19,10 +19,10 @@ const (
 	QUIC_ENCRYPTION_FAILURE            = protocol.ErrorCode(13)
 	QUIC_PACKET_TOO_LARGE              = protocol.ErrorCode(14)
 	// QUIC_PACKET_FOR_NONEXISTENT_STREAM= Data was sent for a stream which did not exist.
-	QUIC_PEER_GOING_AWAY       = protocol.ErrorCode(16)
-	QUIC_INVALID_STREAM_ID     = protocol.ErrorCode(17)
-	QUIC_TOO_MANY_OPEN_STREAMS = protocol.ErrorCode(18)
-	// QUIC_CONNECTION_TIMED_OUT= We hit our pre-negotiated (or default) timeout
+	QUIC_PEER_GOING_AWAY                         = protocol.ErrorCode(16)
+	QUIC_INVALID_STREAM_ID                       = protocol.ErrorCode(17)
+	QUIC_TOO_MANY_OPEN_STREAMS                   = protocol.ErrorCode(18)
+	QUIC_NETWORK_IDLE_TIMEOUT                    = protocol.ErrorCode(25)
 	QUIC_CRYPTO_TAGS_OUT_OF_ORDER                = protocol.ErrorCode(29)
 	QUIC_CRYPTO_TOO_MANY_ENTRIES                 = protocol.ErrorCode(30)
 	QUIC_CRYPTO_INVALID_VALUE_LENGTH             = protocol.ErrorCode(31)
diff --git a/handshake/connection_parameters_manager.go b/handshake/connection_parameters_manager.go
index 697670f62..d25479ef7 100644
--- a/handshake/connection_parameters_manager.go
+++ b/handshake/connection_parameters_manager.go
@@ -79,14 +79,13 @@ func (h *ConnectionParametersManager) GetStreamFlowControlWindow() (protocol.Byt
 }
 
 // GetIdleConnectionStateLifetime gets the idle timeout
-func (h *ConnectionParametersManager) GetIdleConnectionStateLifetime() (time.Duration, error) {
+func (h *ConnectionParametersManager) GetIdleConnectionStateLifetime() time.Duration {
 	rawValue, err := h.GetRawValue(TagICSL)
 	if err != nil {
-		return 0, err
+		panic("ConnectionParameters: Could not find ICSL")
 	}
-
 	if len(rawValue) != 4 {
-		return 0, errors.New("expected uint32 for ICSL")
+		panic("ConnectionParameters: ICSL has invalid value")
 	}
-	return time.Duration(binary.LittleEndian.Uint32(rawValue)) * time.Second, nil
+	return time.Duration(binary.LittleEndian.Uint32(rawValue)) * time.Second
 }
diff --git a/handshake/connection_parameters_manager_test.go b/handshake/connection_parameters_manager_test.go
index a1c7c6750..acb062f48 100644
--- a/handshake/connection_parameters_manager_test.go
+++ b/handshake/connection_parameters_manager_test.go
@@ -62,8 +62,7 @@ var _ = Describe("ConnectionsParameterManager", func() {
 
 	It("gets idle connection state lifetime", func() {
 		cpm.params[TagICSL] = []byte{0xad, 0xfb, 0xca, 0xde}
-		val, err := cpm.GetIdleConnectionStateLifetime()
-		Expect(err).ToNot(HaveOccurred())
+		val := cpm.GetIdleConnectionStateLifetime()
 		Expect(val).To(Equal(0xdecafbad * time.Second))
 	})
 })
diff --git a/session.go b/session.go
index 2d34c5f84..93def6a5d 100644
--- a/session.go
+++ b/session.go
@@ -119,6 +119,8 @@ func (s *Session) Run() {
 			s.scheduleSending()
 		case <-s.sendingScheduled:
 			err = s.sendPacket()
+		case <-time.After(s.connectionParametersManager.GetIdleConnectionStateLifetime()):
+			s.Close(protocol.NewQuicError(errorcodes.QUIC_NETWORK_IDLE_TIMEOUT, "No recent network activity."), true)
 		}
 
 		if err != nil {
diff --git a/session_test.go b/session_test.go
index 7a4f90eb7..2a831ebe7 100644
--- a/session_test.go
+++ b/session_test.go
@@ -45,6 +45,9 @@ var _ = Describe("Session", func() {
 			streams:                     make(map[protocol.StreamID]*stream),
 			streamCallback:              func(*Session, utils.Stream) { callbackCalled = true },
 			connectionParametersManager: handshake.NewConnectionParamatersManager(),
+			closeChan:                   make(chan struct{}, 1),
+			closeCallback:               func(*Session) {},
+			packer:                      &packetPacker{aead: &crypto.NullAEAD{}},
 		}
 	})
 
@@ -344,4 +347,13 @@ var _ = Describe("Session", func() {
 		Expect(conn.written).To(HaveLen(1))
 		Expect(conn.written[0]).To(ContainSubstring(string([]byte("PRST"))))
 	})
+
+	It("times out", func(done Done) {
+		session.connectionParametersManager.SetFromMap(map[handshake.Tag][]byte{
+			handshake.TagICSL: {0, 0, 0, 0},
+		})
+		session.Run() // Would normally not return
+		Expect(conn.written[0]).To(ContainSubstring("No recent network activity."))
+		close(done)
+	}, 0.5)
 })