dcsctp: Negotiate zero checksum

If configured, attempt to negotiate "zero checksum
acceptable" capability, which will make the outgoing
packets have a fixed checksum of zero. Received
packets will not be verified for a correct checksum
if it's zero.

Also includes some boilerplate:
 - Setting capability in state cookie
 - Adding capability to handover state
 - Adding metric to know if the feature is used

This feature is not enabled by default, as it will be
first evaluated with an A/B experiment before making
it the default.

When the feature is enabled, lower CPU consumption for
both receiving and sending packets is expected. How
much depends on the architecture, as some architectures
have better support for generating CRC32 in hardware
than others.

Bug: webrtc:14997
Change-Id: If23f73e87220c7d42bd4f9a92772cda16bc18fcb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/299076
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39920}

net/dcsctp/public/dcsctp_handover_state.h

@@ -40,6 +40,7 @@ struct DcSctpSocketHandoverState {
     bool partial_reliability = false;
     bool message_interleaving = false;
     bool reconfig = false;
+    bool zero_checksum = false;
     uint16_t negotiated_maximum_incoming_streams = 0;
     uint16_t negotiated_maximum_outgoing_streams = 0;
   };

net/dcsctp/public/dcsctp_socket.h

@@ -245,6 +245,10 @@ struct Metrics {
   // peers.
   bool uses_message_interleaving = false;
 
+  // Indicates if draft-tuexen-tsvwg-sctp-zero-checksum-00 has been negotiated
+  // by both peers.
+  bool uses_zero_checksum = false;
+
   // The number of negotiated incoming and outgoing streams, which is configured
   // locally as `DcSctpOptions::announced_maximum_incoming_streams` and
   // `DcSctpOptions::announced_maximum_outgoing_streams`, and which will be

net/dcsctp/socket/capabilities.h

@@ -21,6 +21,8 @@ struct Capabilities {
   bool message_interleaving = false;
   // RFC6525 Stream Reconfiguration
   bool reconfig = false;
+  // https://datatracker.ietf.org/doc/draft-tuexen-tsvwg-sctp-zero-checksum/
+  bool zero_checksum = false;
   // Negotiated maximum incoming and outgoing stream count.
   uint16_t negotiated_maximum_incoming_streams = 0;
   uint16_t negotiated_maximum_outgoing_streams = 0;

net/dcsctp/socket/dcsctp_socket.cc

@@ -56,6 +56,7 @@
 #include "net/dcsctp/packet/parameter/parameter.h"
 #include "net/dcsctp/packet/parameter/state_cookie_parameter.h"
 #include "net/dcsctp/packet/parameter/supported_extensions_parameter.h"
+#include "net/dcsctp/packet/parameter/zero_checksum_acceptable_chunk_parameter.h"
 #include "net/dcsctp/packet/sctp_packet.h"
 #include "net/dcsctp/packet/tlv_trait.h"
 #include "net/dcsctp/public/dcsctp_message.h"
@@ -117,6 +118,11 @@ Capabilities ComputeCapabilities(const DcSctpOptions& options,
     capabilities.reconfig = true;
   }
 
+  if (options.enable_zero_checksum &&
+      parameters.get<ZeroChecksumAcceptableChunkParameter>().has_value()) {
+    capabilities.zero_checksum = true;
+  }
+
   capabilities.negotiated_maximum_incoming_streams = std::min(
       options.announced_maximum_incoming_streams, peer_nbr_outbound_streams);
   capabilities.negotiated_maximum_outgoing_streams = std::min(
@@ -137,6 +143,9 @@ void AddCapabilityParameters(const DcSctpOptions& options,
     chunk_types.push_back(IDataChunk::kType);
     chunk_types.push_back(IForwardTsnChunk::kType);
   }
+  if (options.enable_zero_checksum) {
+    builder.Add(ZeroChecksumAcceptableChunkParameter());
+  }
   builder.Add(SupportedExtensionsParameter(std::move(chunk_types)));
 }
 
@@ -279,7 +288,10 @@ void DcSctpSocket::SendInit() {
                  connect_params_.initial_tsn, params_builder.Build());
   SctpPacket::Builder b(VerificationTag(0), options_);
   b.Add(init);
-  packet_sender_.Send(b);
+  // https://www.ietf.org/archive/id/draft-tuexen-tsvwg-sctp-zero-checksum-01.html#section-4.2
+  // "When an end point sends a packet containing an INIT chunk, it MUST include
+  // a correct CRC32c checksum in the packet containing the INIT chunk."
+  packet_sender_.Send(b, /*write_checksum=*/true);
 }
 
 void DcSctpSocket::MakeConnectionParameters() {
@@ -320,6 +332,7 @@ void DcSctpSocket::CreateTransmissionControlBlock(
     size_t a_rwnd,
     TieTag tie_tag) {
   metrics_.uses_message_interleaving = capabilities.message_interleaving;
+  metrics_.uses_zero_checksum = capabilities.zero_checksum;
   metrics_.negotiated_maximum_incoming_streams =
       capabilities.negotiated_maximum_incoming_streams;
   metrics_.negotiated_maximum_outgoing_streams =
@@ -351,6 +364,7 @@ void DcSctpSocket::RestoreFromState(const DcSctpSocketHandoverState& state) {
       capabilities.message_interleaving =
           state.capabilities.message_interleaving;
       capabilities.reconfig = state.capabilities.reconfig;
+      capabilities.zero_checksum = state.capabilities.zero_checksum;
       capabilities.negotiated_maximum_incoming_streams =
           state.capabilities.negotiated_maximum_incoming_streams;
       capabilities.negotiated_maximum_outgoing_streams =
@@ -1209,7 +1223,9 @@ void DcSctpSocket::HandleInit(const CommonHeader& header,
                         options_.announced_maximum_incoming_streams,
                         connect_params_.initial_tsn, params_builder.Build());
   b.Add(init_ack);
-  packet_sender_.Send(b);
+  // If the peer has signaled that it supports zero checksum, INIT-ACK can then
+  // have its checksum as zero.
+  packet_sender_.Send(b, /*write_checksum=*/!capabilities.zero_checksum);
 }
 
 void DcSctpSocket::HandleInitAck(

net/dcsctp/socket/dcsctp_socket_test.cc

@@ -23,6 +23,7 @@
 #include "api/array_view.h"
 #include "net/dcsctp/common/handover_testing.h"
 #include "net/dcsctp/packet/chunk/chunk.h"
+#include "net/dcsctp/packet/chunk/cookie_ack_chunk.h"
 #include "net/dcsctp/packet/chunk/cookie_echo_chunk.h"
 #include "net/dcsctp/packet/chunk/data_chunk.h"
 #include "net/dcsctp/packet/chunk/data_common.h"
@@ -30,6 +31,7 @@
 #include "net/dcsctp/packet/chunk/heartbeat_ack_chunk.h"
 #include "net/dcsctp/packet/chunk/heartbeat_request_chunk.h"
 #include "net/dcsctp/packet/chunk/idata_chunk.h"
+#include "net/dcsctp/packet/chunk/init_ack_chunk.h"
 #include "net/dcsctp/packet/chunk/init_chunk.h"
 #include "net/dcsctp/packet/chunk/sack_chunk.h"
 #include "net/dcsctp/packet/chunk/shutdown_chunk.h"
@@ -58,8 +60,10 @@ namespace {
 using ::testing::_;
 using ::testing::AllOf;
 using ::testing::ElementsAre;
+using ::testing::Eq;
 using ::testing::HasSubstr;
 using ::testing::IsEmpty;
+using ::testing::Not;
 using ::testing::SizeIs;
 using ::testing::UnorderedElementsAre;
 
@@ -2864,5 +2868,156 @@ TEST(DcSctpSocketTest, ResetStreamsWithPausedSenderResumesWhenPerformed) {
   EXPECT_EQ(msg2->payload().size(), kSmallMessageSize);
 }
 
+TEST_P(DcSctpSocketParametrizedTest, ZeroChecksumMetricsAreSet) {
+  std::vector<std::pair<bool, bool>> combinations = {
+      {false, false}, {false, true}, {true, false}, {true, true}};
+  for (const auto& [a_enable, z_enable] : combinations) {
+    DcSctpOptions a_options = {.enable_zero_checksum = a_enable};
+    DcSctpOptions z_options = {.enable_zero_checksum = z_enable};
+
+    SocketUnderTest a("A", a_options);
+    auto z = std::make_unique<SocketUnderTest>("Z", z_options);
+
+    ConnectSockets(a, *z);
+    z = MaybeHandoverSocket(std::move(z));
+
+    EXPECT_EQ(a.socket.GetMetrics()->uses_zero_checksum, a_enable && z_enable);
+    EXPECT_EQ(z->socket.GetMetrics()->uses_zero_checksum, a_enable && z_enable);
+  }
+}
+
+TEST(DcSctpSocketTest, AlwaysSendsInitWithNonZeroChecksum) {
+  DcSctpOptions options = {.enable_zero_checksum = true};
+  SocketUnderTest a("A", options);
+
+  a.socket.Connect();
+  std::vector<uint8_t> data = a.cb.ConsumeSentPacket();
+  ASSERT_HAS_VALUE_AND_ASSIGN(SctpPacket packet,
+                              SctpPacket::Parse(data, options));
+  EXPECT_THAT(packet.descriptors(),
+              ElementsAre(testing::Field(&SctpPacket::ChunkDescriptor::type,
+                                         InitChunk::kType)));
+  EXPECT_THAT(packet.common_header().checksum, Not(Eq(0u)));
+}
+
+TEST(DcSctpSocketTest, MaySendInitAckWithZeroChecksum) {
+  DcSctpOptions options = {.enable_zero_checksum = true};
+  SocketUnderTest a("A", options);
+  SocketUnderTest z("Z", options);
+
+  a.socket.Connect();
+  z.socket.ReceivePacket(a.cb.ConsumeSentPacket());  // INIT
+
+  std::vector<uint8_t> data = z.cb.ConsumeSentPacket();
+  ASSERT_HAS_VALUE_AND_ASSIGN(SctpPacket packet,
+                              SctpPacket::Parse(data, options));
+  EXPECT_THAT(packet.descriptors(),
+              ElementsAre(testing::Field(&SctpPacket::ChunkDescriptor::type,
+                                         InitAckChunk::kType)));
+  EXPECT_THAT(packet.common_header().checksum, 0u);
+}
+
+TEST(DcSctpSocketTest, AlwaysSendsCookieEchoWithNonZeroChecksum) {
+  DcSctpOptions options = {.enable_zero_checksum = true};
+  SocketUnderTest a("A", options);
+  SocketUnderTest z("Z", options);
+
+  a.socket.Connect();
+  z.socket.ReceivePacket(a.cb.ConsumeSentPacket());  // INIT
+  a.socket.ReceivePacket(z.cb.ConsumeSentPacket());  // INIT-ACK
+
+  std::vector<uint8_t> data = a.cb.ConsumeSentPacket();
+  ASSERT_HAS_VALUE_AND_ASSIGN(SctpPacket packet,
+                              SctpPacket::Parse(data, options));
+  EXPECT_THAT(packet.descriptors(),
+              ElementsAre(testing::Field(&SctpPacket::ChunkDescriptor::type,
+                                         CookieEchoChunk::kType)));
+  EXPECT_THAT(packet.common_header().checksum, Not(Eq(0u)));
+}
+
+TEST(DcSctpSocketTest, SendsCookieAckWithZeroChecksum) {
+  DcSctpOptions options = {.enable_zero_checksum = true};
+  SocketUnderTest a("A", options);
+  SocketUnderTest z("Z", options);
+
+  a.socket.Connect();
+  z.socket.ReceivePacket(a.cb.ConsumeSentPacket());  // INIT
+  a.socket.ReceivePacket(z.cb.ConsumeSentPacket());  // INIT-ACK
+  z.socket.ReceivePacket(a.cb.ConsumeSentPacket());  // COOKIE-ECHO
+
+  std::vector<uint8_t> data = z.cb.ConsumeSentPacket();
+  ASSERT_HAS_VALUE_AND_ASSIGN(SctpPacket packet,
+                              SctpPacket::Parse(data, options));
+  EXPECT_THAT(packet.descriptors(),
+              ElementsAre(testing::Field(&SctpPacket::ChunkDescriptor::type,
+                                         CookieAckChunk::kType)));
+  EXPECT_THAT(packet.common_header().checksum, 0u);
+}
+
+TEST_P(DcSctpSocketParametrizedTest, SendsDataWithZeroChecksum) {
+  DcSctpOptions options = {.enable_zero_checksum = true};
+  SocketUnderTest a("A", options);
+  auto z = std::make_unique<SocketUnderTest>("Z", options);
+
+  ConnectSockets(a, *z);
+  z = MaybeHandoverSocket(std::move(z));
+
+  std::vector<uint8_t> payload(a.options.mtu - 100);
+  a.socket.Send(DcSctpMessage(StreamID(1), PPID(53), payload), {});
+
+  std::vector<uint8_t> data = a.cb.ConsumeSentPacket();
+  z->socket.ReceivePacket(data);
+  ASSERT_HAS_VALUE_AND_ASSIGN(SctpPacket packet,
+                              SctpPacket::Parse(data, options));
+  EXPECT_THAT(packet.descriptors(),
+              ElementsAre(testing::Field(&SctpPacket::ChunkDescriptor::type,
+                                         DataChunk::kType)));
+  EXPECT_THAT(packet.common_header().checksum, 0u);
+
+  MaybeHandoverSocketAndSendMessage(a, std::move(z));
+}
+
+TEST_P(DcSctpSocketParametrizedTest, AllPacketsAfterConnectHaveZeroChecksum) {
+  DcSctpOptions options = {.enable_zero_checksum = true};
+  SocketUnderTest a("A", options);
+  auto z = std::make_unique<SocketUnderTest>("Z", options);
+
+  ConnectSockets(a, *z);
+  z = MaybeHandoverSocket(std::move(z));
+
+  // Send large messages in both directions, and verify that they arrive and
+  // that every packet has zero checksum.
+  std::vector<uint8_t> payload(kLargeMessageSize);
+  a.socket.Send(DcSctpMessage(StreamID(1), PPID(53), payload), kSendOptions);
+  z->socket.Send(DcSctpMessage(StreamID(1), PPID(53), payload), kSendOptions);
+
+  for (;;) {
+    if (auto data = a.cb.ConsumeSentPacket(); !data.empty()) {
+      ASSERT_HAS_VALUE_AND_ASSIGN(SctpPacket packet,
+                                  SctpPacket::Parse(data, options));
+      EXPECT_THAT(packet.common_header().checksum, 0u);
+      z->socket.ReceivePacket(std::move(data));
+
+    } else if (auto data = z->cb.ConsumeSentPacket(); !data.empty()) {
+      ASSERT_HAS_VALUE_AND_ASSIGN(SctpPacket packet,
+                                  SctpPacket::Parse(data, options));
+      EXPECT_THAT(packet.common_header().checksum, 0u);
+      a.socket.ReceivePacket(std::move(data));
+
+    } else {
+      break;
+    }
+  }
+
+  absl::optional<DcSctpMessage> msg1 = z->cb.ConsumeReceivedMessage();
+  ASSERT_TRUE(msg1.has_value());
+  EXPECT_THAT(msg1->payload(), SizeIs(kLargeMessageSize));
+
+  absl::optional<DcSctpMessage> msg2 = a.cb.ConsumeReceivedMessage();
+  ASSERT_TRUE(msg2.has_value());
+  EXPECT_THAT(msg2->payload(), SizeIs(kLargeMessageSize));
+
+  MaybeHandoverSocketAndSendMessage(a, std::move(z));
+}
 }  // namespace
 }  // namespace dcsctp

net/dcsctp/socket/packet_sender.cc

@@ -21,12 +21,12 @@ PacketSender::PacketSender(DcSctpSocketCallbacks& callbacks,
                                               SendPacketStatus)> on_sent_packet)
     : callbacks_(callbacks), on_sent_packet_(std::move(on_sent_packet)) {}
 
-bool PacketSender::Send(SctpPacket::Builder& builder) {
+bool PacketSender::Send(SctpPacket::Builder& builder, bool write_checksum) {
   if (builder.empty()) {
     return false;
   }
 
-  std::vector<uint8_t> payload = builder.Build();
+  std::vector<uint8_t> payload = builder.Build(write_checksum);
 
   SendPacketStatus status = callbacks_.SendPacketWithStatus(payload);
   on_sent_packet_(payload, status);

net/dcsctp/socket/packet_sender.h

@@ -25,7 +25,7 @@ class PacketSender {
                                   SendPacketStatus)> on_sent_packet);
 
   // Sends the packet, and returns true if it was sent successfully.
-  bool Send(SctpPacket::Builder& builder);
+  bool Send(SctpPacket::Builder& builder, bool write_checksum = true);
 
  private:
   DcSctpSocketCallbacks& callbacks_;

net/dcsctp/socket/state_cookie.cc

@@ -42,6 +42,7 @@ std::vector<uint8_t> StateCookie::Serialize() {
   buffer.Store8<30>(capabilities_.reconfig);
   buffer.Store16<32>(capabilities_.negotiated_maximum_incoming_streams);
   buffer.Store16<34>(capabilities_.negotiated_maximum_outgoing_streams);
+  buffer.Store8<36>(capabilities_.zero_checksum);
   return cookie;
 }
 
@@ -74,6 +75,7 @@ absl::optional<StateCookie> StateCookie::Deserialize(
   capabilities.reconfig = buffer.Load8<30>() != 0;
   capabilities.negotiated_maximum_incoming_streams = buffer.Load16<32>();
   capabilities.negotiated_maximum_outgoing_streams = buffer.Load16<34>();
+  capabilities.zero_checksum = buffer.Load8<36>() != 0;
 
   return StateCookie(verification_tag, initial_tsn, a_rwnd, tie_tag,
                      capabilities);

net/dcsctp/socket/state_cookie.h

@@ -27,7 +27,7 @@ namespace dcsctp {
 // Do not trust anything in it; no pointers or anything like that.
 class StateCookie {
  public:
-  static constexpr size_t kCookieSize = 36;
+  static constexpr size_t kCookieSize = 37;
 
   StateCookie(VerificationTag initiate_tag,
               TSN initial_tsn,

net/dcsctp/socket/state_cookie_test.cc

@@ -21,6 +21,7 @@ TEST(StateCookieTest, SerializeAndDeserialize) {
   Capabilities capabilities = {.partial_reliability = true,
                                .message_interleaving = false,
                                .reconfig = true,
+                               .zero_checksum = true,
                                .negotiated_maximum_incoming_streams = 123,
                                .negotiated_maximum_outgoing_streams = 234};
   StateCookie cookie(VerificationTag(123), TSN(456),
@@ -36,6 +37,7 @@ TEST(StateCookieTest, SerializeAndDeserialize) {
   EXPECT_TRUE(deserialized.capabilities().partial_reliability);
   EXPECT_FALSE(deserialized.capabilities().message_interleaving);
   EXPECT_TRUE(deserialized.capabilities().reconfig);
+  EXPECT_TRUE(deserialized.capabilities().zero_checksum);
   EXPECT_EQ(deserialized.capabilities().negotiated_maximum_incoming_streams,
             123);
   EXPECT_EQ(deserialized.capabilities().negotiated_maximum_outgoing_streams,

net/dcsctp/socket/transmission_control_block.cc

@@ -163,7 +163,7 @@ void TransmissionControlBlock::MaybeSendForwardTsn(SctpPacket::Builder& builder,
     } else {
       builder.Add(retransmission_queue_.CreateForwardTsn());
     }
-    packet_sender_.Send(builder);
+    Send(builder);
     // https://datatracker.ietf.org/doc/html/rfc3758
     // "IMPLEMENTATION NOTE: An implementation may wish to limit the number of
     // duplicate FORWARD TSN chunks it sends by ... waiting a full RTT before
@@ -198,7 +198,7 @@ void TransmissionControlBlock::MaybeSendFastRetransmit() {
       builder.Add(DataChunk(tsn, std::move(data), false));
     }
   }
-  packet_sender_.Send(builder);
+  Send(builder);
 }
 
 void TransmissionControlBlock::SendBufferedPackets(SctpPacket::Builder& builder,
@@ -245,7 +245,13 @@ void TransmissionControlBlock::SendBufferedPackets(SctpPacket::Builder& builder,
       }
     }
 
-    if (!packet_sender_.Send(builder)) {
+    // https://www.ietf.org/archive/id/draft-tuexen-tsvwg-sctp-zero-checksum-02.html#section-4.2
+    // "When an end point sends a packet containing a COOKIE ECHO chunk, it MUST
+    // include a correct CRC32c checksum in the packet containing the COOKIE
+    // ECHO chunk."
+    bool write_checksum =
+        !capabilities_.zero_checksum || cookie_echo_chunk_.has_value();
+    if (!packet_sender_.Send(builder, write_checksum)) {
       break;
     }
 
@@ -274,6 +280,9 @@ std::string TransmissionControlBlock::ToString() const {
   if (capabilities_.reconfig) {
     sb << "Reconfig,";
   }
+  if (capabilities_.zero_checksum) {
+    sb << "ZeroChecksum,";
+  }
   sb << " max_in=" << capabilities_.negotiated_maximum_incoming_streams;
   sb << " max_out=" << capabilities_.negotiated_maximum_outgoing_streams;
 
@@ -294,6 +303,7 @@ void TransmissionControlBlock::AddHandoverState(
   state.capabilities.partial_reliability = capabilities_.partial_reliability;
   state.capabilities.message_interleaving = capabilities_.message_interleaving;
   state.capabilities.reconfig = capabilities_.reconfig;
+  state.capabilities.zero_checksum = capabilities_.zero_checksum;
   state.capabilities.negotiated_maximum_incoming_streams =
       capabilities_.negotiated_maximum_incoming_streams;
   state.capabilities.negotiated_maximum_outgoing_streams =

net/dcsctp/socket/transmission_control_block.h

@@ -80,7 +80,8 @@ class TransmissionControlBlock : public Context {
     return tx_error_counter_.IsExhausted();
   }
   void Send(SctpPacket::Builder& builder) override {
-    packet_sender_.Send(builder);
+    packet_sender_.Send(builder,
+                        /*write_checksum=*/!capabilities_.zero_checksum);
   }
 
   // Other accessors

net/dcsctp/socket/transmission_control_block_test.cc

@@ -92,6 +92,7 @@ TEST_F(TransmissionControlBlockTest, LogsAllCapabilitiesInToSring) {
   capabilities_.negotiated_maximum_outgoing_streams = 2000;
   capabilities_.message_interleaving = true;
   capabilities_.partial_reliability = true;
+  capabilities_.zero_checksum = true;
   capabilities_.reconfig = true;
 
   TransmissionControlBlock tcb(
@@ -99,9 +100,10 @@ TEST_F(TransmissionControlBlockTest, LogsAllCapabilitiesInToSring) {
       kMyVerificationTag, kMyInitialTsn, kPeerVerificationTag, kPeerInitialTsn,
       kArwnd, kTieTag, sender_, on_connection_established.AsStdFunction());
 
-  EXPECT_EQ(tcb.ToString(),
-            "verification_tag=000001c8, last_cumulative_ack=999, "
-            "capabilities=PR,IL,Reconfig, max_in=1000 max_out=2000");
+  EXPECT_EQ(
+      tcb.ToString(),
+      "verification_tag=000001c8, last_cumulative_ack=999, "
+      "capabilities=PR,IL,Reconfig,ZeroChecksum, max_in=1000 max_out=2000");
 }
 
 TEST_F(TransmissionControlBlockTest, IsInitiallyHandoverReady) {