usrsctp: Support sending and receiving empty messages

Add new PPIDs 56 and 57. When sending an empty message,
we use the corresponding PPID with a single byte data chunk.
On the receiving side, when detecting such a PPID, we just
ignore the payload content.

Bug: webrtc:12697
Change-Id: I6af481e7281db10d9663e1c0aaf97b3e608432a1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215931
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33808}

media/sctp/usrsctp_transport.cc

@@ -74,24 +74,25 @@ static constexpr size_t kSctpMtu = 1191;
 ABSL_CONST_INIT int g_usrsctp_usage_count = 0;
 ABSL_CONST_INIT bool g_usrsctp_initialized_ = false;
 ABSL_CONST_INIT webrtc::GlobalMutex g_usrsctp_lock_(absl::kConstInit);
+ABSL_CONST_INIT char kZero[] = {'\0'};
 
 // DataMessageType is used for the SCTP "Payload Protocol Identifier", as
 // defined in http://tools.ietf.org/html/rfc4960#section-14.4
 //
 // For the list of IANA approved values see:
+// https://tools.ietf.org/html/rfc8831 Sec. 8
 // http://www.iana.org/assignments/sctp-parameters/sctp-parameters.xml
 // The value is not used by SCTP itself. It indicates the protocol running
 // on top of SCTP.
 enum {
   PPID_NONE = 0,  // No protocol is specified.
-  // Matches the PPIDs in mozilla source and
-  // https://datatracker.ietf.org/doc/draft-ietf-rtcweb-data-protocol Sec. 9
-  // They're not yet assigned by IANA.
   PPID_CONTROL = 50,
-  PPID_BINARY_PARTIAL = 52,
+  PPID_TEXT_LAST = 51,
+  PPID_BINARY_PARTIAL = 52,  // Deprecated
   PPID_BINARY_LAST = 53,
-  PPID_TEXT_PARTIAL = 54,
+  PPID_TEXT_PARTIAL = 54,  // Deprecated
-  PPID_TEXT_LAST = 51
+  PPID_TEXT_EMPTY = 56,
+  PPID_BINARY_EMPTY = 57,
 };
 
 // Should only be modified by UsrSctpWrapper.
@@ -128,7 +129,7 @@ void DebugSctpPrintf(const char* format, ...) {
 }
 
 // Get the PPID to use for the terminating fragment of this type.
-uint32_t GetPpid(cricket::DataMessageType type) {
+uint32_t GetPpid(cricket::DataMessageType type, size_t size) {
   switch (type) {
     default:
     case cricket::DMT_NONE:
@@ -136,9 +137,9 @@ uint32_t GetPpid(cricket::DataMessageType type) {
     case cricket::DMT_CONTROL:
       return PPID_CONTROL;
     case cricket::DMT_BINARY:
-      return PPID_BINARY_LAST;
+      return size > 0 ? PPID_BINARY_LAST : PPID_BINARY_EMPTY;
     case cricket::DMT_TEXT:
-      return PPID_TEXT_LAST;
+      return size > 0 ? PPID_TEXT_LAST : PPID_TEXT_EMPTY;
   }
 }
 
@@ -147,11 +148,13 @@ bool GetDataMediaType(uint32_t ppid, cricket::DataMessageType* dest) {
   switch (ppid) {
     case PPID_BINARY_PARTIAL:
     case PPID_BINARY_LAST:
+    case PPID_BINARY_EMPTY:
       *dest = cricket::DMT_BINARY;
       return true;
 
     case PPID_TEXT_PARTIAL:
     case PPID_TEXT_LAST:
+    case PPID_TEXT_EMPTY:
       *dest = cricket::DMT_TEXT;
       return true;
 
@@ -168,6 +171,10 @@ bool GetDataMediaType(uint32_t ppid, cricket::DataMessageType* dest) {
   }
 }
 
+bool IsEmptyPPID(uint32_t ppid) {
+  return ppid == PPID_BINARY_EMPTY || ppid == PPID_TEXT_EMPTY;
+}
+
 // Log the packet in text2pcap format, if log level is at LS_VERBOSE.
 //
 // In order to turn these logs into a pcap file you can use, first filter the
@@ -205,11 +212,12 @@ void VerboseLogPacket(const void* data, size_t length, int direction) {
 
 // Creates the sctp_sendv_spa struct used for setting flags in the
 // sctp_sendv() call.
-sctp_sendv_spa CreateSctpSendParams(const cricket::SendDataParams& params) {
+sctp_sendv_spa CreateSctpSendParams(const cricket::SendDataParams& params,
+                                    size_t size) {
   struct sctp_sendv_spa spa = {0};
   spa.sendv_flags |= SCTP_SEND_SNDINFO_VALID;
   spa.sendv_sndinfo.snd_sid = params.sid;
-  spa.sendv_sndinfo.snd_ppid = rtc::HostToNetwork32(GetPpid(params.type));
+  spa.sendv_sndinfo.snd_ppid = rtc::HostToNetwork32(GetPpid(params.type, size));
   // Explicitly marking the EOR flag turns the usrsctp_sendv call below into a
   // non atomic operation. This means that the sctp lib might only accept the
   // message partially. This is done in order to improve throughput, so that we
@@ -792,13 +800,23 @@ SendDataResult UsrsctpTransport::SendMessageInternal(OutgoingMessage* message) {
   }
 
   // Send data using SCTP.
-  sctp_sendv_spa spa = CreateSctpSendParams(message->send_params());
+  sctp_sendv_spa spa =
+      CreateSctpSendParams(message->send_params(), message->size());
+  const void* data = message->data();
+  size_t data_length = message->size();
+  if (message->size() == 0) {
+    // Empty messages are replaced by a single NUL byte on the wire as SCTP
+    // doesn't support empty messages.
+    // The PPID carries the information that the payload needs to be ignored.
+    data = kZero;
+    data_length = 1;
+  }
   // Note: this send call is not atomic because the EOR bit is set. This means
   // that usrsctp can partially accept this message and it is our duty to buffer
   // the rest.
-  ssize_t send_res = usrsctp_sendv(
+  ssize_t send_res = usrsctp_sendv(sock_, data, data_length, NULL, 0, &spa,
-      sock_, message->data(), message->size(), NULL, 0, &spa,
+                                   rtc::checked_cast<socklen_t>(sizeof(spa)),
-      rtc::checked_cast<socklen_t>(sizeof(spa)), SCTP_SENDV_SPA, 0);
+                                   SCTP_SENDV_SPA, 0);
   if (send_res < 0) {
     if (errno == SCTP_EWOULDBLOCK) {
       ready_to_send_data_ = false;
@@ -814,7 +832,8 @@ SendDataResult UsrsctpTransport::SendMessageInternal(OutgoingMessage* message) {
   }
 
   size_t amount_sent = static_cast<size_t>(send_res);
-  RTC_DCHECK_LE(amount_sent, message->size());
+  RTC_DCHECK_LE(amount_sent, data_length);
+  if (message->size() != 0)
     message->Advance(amount_sent);
   // Only way out now is success.
   return SDR_SUCCESS;
@@ -1319,7 +1338,10 @@ void UsrsctpTransport::OnDataOrNotificationFromSctp(const void* data,
   // association.
   params.seq_num = rcv.rcv_ssn;
 
-  // Append the chunk's data to the message buffer
+  // Append the chunk's data to the message buffer unless we have a chunk with a
+  // PPID marking an empty message.
+  // See: https://tools.ietf.org/html/rfc8831#section-6.6
+  if (!IsEmptyPPID(ppid))
     partial_incoming_message_.AppendData(reinterpret_cast<const uint8_t*>(data),
                                          length);
   partial_params_ = params;

pc/data_channel_integrationtest.cc

@@ -218,6 +218,52 @@ TEST_P(DataChannelIntegrationTest,
   }
 }
 
+// This test sets up a call between two parties with an SCTP
+// data channel only, and sends empty messages
+TEST_P(DataChannelIntegrationTest,
+       EndToEndCallWithSctpDataChannelEmptyMessages) {
+  ASSERT_TRUE(CreatePeerConnectionWrappers());
+  ConnectFakeSignaling();
+  // Expect that data channel created on caller side will show up for callee as
+  // well.
+  caller()->CreateDataChannel();
+  caller()->CreateAndSetAndSignalOffer();
+  ASSERT_TRUE_WAIT(SignalingStateStable(), kDefaultTimeout);
+  // Caller data channel should already exist (it created one). Callee data
+  // channel may not exist yet, since negotiation happens in-band, not in SDP.
+  ASSERT_NE(nullptr, caller()->data_channel());
+  ASSERT_TRUE_WAIT(callee()->data_channel() != nullptr, kDefaultTimeout);
+  EXPECT_TRUE_WAIT(caller()->data_observer()->IsOpen(), kDefaultTimeout);
+  EXPECT_TRUE_WAIT(callee()->data_observer()->IsOpen(), kDefaultTimeout);
+
+  // Ensure data can be sent in both directions.
+  // Sending empty string data
+  std::string data = "";
+  caller()->data_channel()->Send(DataBuffer(data));
+  EXPECT_EQ_WAIT(1u, callee()->data_observer()->received_message_count(),
+                 kDefaultTimeout);
+  EXPECT_TRUE(callee()->data_observer()->last_message().empty());
+  EXPECT_FALSE(callee()->data_observer()->messages().back().binary);
+  callee()->data_channel()->Send(DataBuffer(data));
+  EXPECT_EQ_WAIT(1u, caller()->data_observer()->received_message_count(),
+                 kDefaultTimeout);
+  EXPECT_TRUE(caller()->data_observer()->last_message().empty());
+  EXPECT_FALSE(caller()->data_observer()->messages().back().binary);
+
+  // Sending empty binary data
+  rtc::CopyOnWriteBuffer empty_buffer;
+  caller()->data_channel()->Send(DataBuffer(empty_buffer, true));
+  EXPECT_EQ_WAIT(2u, callee()->data_observer()->received_message_count(),
+                 kDefaultTimeout);
+  EXPECT_TRUE(callee()->data_observer()->last_message().empty());
+  EXPECT_TRUE(callee()->data_observer()->messages().back().binary);
+  callee()->data_channel()->Send(DataBuffer(empty_buffer, true));
+  EXPECT_EQ_WAIT(2u, caller()->data_observer()->received_message_count(),
+                 kDefaultTimeout);
+  EXPECT_TRUE(caller()->data_observer()->last_message().empty());
+  EXPECT_TRUE(caller()->data_observer()->messages().back().binary);
+}
+
 TEST_P(DataChannelIntegrationTest,
        EndToEndCallWithSctpDataChannelLowestSafeMtu) {
   // The lowest payload size limit that's tested and found safe for this
@@ -386,10 +432,16 @@ TEST_P(DataChannelIntegrationTest, StressTestUnorderedSctpDataChannel) {
                  kDefaultTimeout);
 
   // Sort and compare to make sure none of the messages were corrupted.
-  std::vector<std::string> caller_received_messages =
+  std::vector<std::string> caller_received_messages;
-      caller()->data_observer()->messages();
+  absl::c_transform(caller()->data_observer()->messages(),
-  std::vector<std::string> callee_received_messages =
+                    std::back_inserter(caller_received_messages),
-      callee()->data_observer()->messages();
+                    [](const auto& a) { return a.data; });
+
+  std::vector<std::string> callee_received_messages;
+  absl::c_transform(callee()->data_observer()->messages(),
+                    std::back_inserter(callee_received_messages),
+                    [](const auto& a) { return a.data; });
+
   absl::c_sort(sent_messages);
   absl::c_sort(caller_received_messages);
   absl::c_sort(callee_received_messages);

pc/sctp_data_channel.cc

@@ -294,13 +294,6 @@ bool SctpDataChannel::Send(const DataBuffer& buffer) {
     return false;
   }
 
-  // TODO(jiayl): the spec is unclear about if the remote side should get the
-  // onmessage event. We need to figure out the expected behavior and change the
-  // code accordingly.
-  if (buffer.size() == 0) {
-    return true;
-  }
-
   buffered_amount_ += buffer.size();
 
   // If the queue is non-empty, we're waiting for SignalReadyToSend,

pc/test/fake_data_channel_provider.h

@@ -36,7 +36,7 @@ class FakeDataChannelProvider
       return false;
     }
 
-    if (transport_error_ || payload.size() == 0) {
+    if (transport_error_) {
       *result = cricket::SDR_ERROR;
       return false;
     }

pc/test/mock_peer_connection_observers.h

@@ -351,6 +351,11 @@ class FakeSetRemoteDescriptionObserver
 
 class MockDataChannelObserver : public webrtc::DataChannelObserver {
  public:
+  struct Message {
+    std::string data;
+    bool binary;
+  };
+
   explicit MockDataChannelObserver(webrtc::DataChannelInterface* channel)
       : channel_(channel) {
     channel_->RegisterObserver(this);
@@ -363,20 +368,29 @@ class MockDataChannelObserver : public webrtc::DataChannelObserver {
   void OnStateChange() override { state_ = channel_->state(); }
   void OnMessage(const DataBuffer& buffer) override {
     messages_.push_back(
-        std::string(buffer.data.data<char>(), buffer.data.size()));
+        {std::string(buffer.data.data<char>(), buffer.data.size()),
+         buffer.binary});
   }
 
   bool IsOpen() const { return state_ == DataChannelInterface::kOpen; }
-  std::vector<std::string> messages() const { return messages_; }
+  std::vector<Message> messages() const { return messages_; }
   std::string last_message() const {
-    return messages_.empty() ? std::string() : messages_.back();
+    if (messages_.empty())
+      return {};
+
+    return messages_.back().data;
+  }
+  bool last_message_is_binary() const {
+    if (messages_.empty())
+      return false;
+    return messages_.back().binary;
   }
   size_t received_message_count() const { return messages_.size(); }
 
  private:
   rtc::scoped_refptr<webrtc::DataChannelInterface> channel_;
   DataChannelInterface::DataState state_;
-  std::vector<std::string> messages_;
+  std::vector<Message> messages_;
 };
 
 class MockStatsObserver : public webrtc::StatsObserver {