fastboot: add UDP protocol.

Implements a UDP protocol for fastboot, documented in
fastboot_protocol.txt.

This version of the protocol does not include multi-packet windowing,
which will likely be needed to achieve reasonable speeds over WiFi.
Since we don't have any WiFi use cases yet, we'd like to get this in
now and update the protocol later if it becomes necessary.

Usage:
  fastboot -s udp:<hostname>[:port] <command>

Bug: http://b/26154914
Change-Id: Ia5bbae6bcd6405671f594d7120b3994746d234d3

1 files changed, 531 insertions, 0 deletions

diff --git a/fastboot/udp_test.cpp b/fastboot/udp_test.cpp
new file mode 100644
index 000000000..ff8cf0fc8
--- /dev/null
+++ b/fastboot/udp_test.cpp
@@ -0,0 +1,531 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "udp.h"
+
+#include <gtest/gtest.h>
+
+#include "socket.h"
+#include "socket_mock.h"
+
+using namespace udp;
+using namespace udp::internal;
+
+// Some possible corner case sequence numbers we want to check.
+static const uint16_t kTestSequenceNumbers[] = {0x0000, 0x0001, 0x00FF, 0x0100,
+                                                0x7FFF, 0x8000, 0xFFFF};
+
+// Converts |value| to a binary big-endian string.
+static std::string PacketValue(uint16_t value) {
+    return std::string{static_cast<char>(value >> 8), static_cast<char>(value)};
+}
+
+// Returns an Error packet.
+static std::string ErrorPacket(uint16_t sequence, const std::string& message = "",
+                               char flags = kFlagNone) {
+    return std::string{kIdError, flags} + PacketValue(sequence) + message;
+}
+
+// Returns a Query packet with no data.
+static std::string QueryPacket(uint16_t sequence) {
+    return std::string{kIdDeviceQuery, kFlagNone} + PacketValue(sequence);
+}
+
+// Returns a Query packet with a 2-byte |new_sequence|.
+static std::string QueryPacket(uint16_t sequence, uint16_t new_sequence) {
+    return std::string{kIdDeviceQuery, kFlagNone} + PacketValue(sequence) +
+           PacketValue(new_sequence);
+}
+
+// Returns an Init packet with a 2-byte |version| and |max_packet_size|.
+static std::string InitPacket(uint16_t sequence, uint16_t version, uint16_t max_packet_size) {
+    return std::string{kIdInitialization, kFlagNone} + PacketValue(sequence) +
+           PacketValue(version) + PacketValue(max_packet_size);
+}
+
+// Returns a Fastboot packet with |data|.
+static std::string FastbootPacket(uint16_t sequence, const std::string& data = "",
+                                  char flags = kFlagNone) {
+    return std::string{kIdFastboot, flags} + PacketValue(sequence) + data;
+}
+
+// Fixture class to test protocol initialization. Usage is to set up the expected calls to the
+// SocketMock object then call UdpConnect() and check the result.
+class UdpConnectTest : public ::testing::Test {
+  public:
+    UdpConnectTest() : mock_socket_(new SocketMock) {}
+
+    // Run the initialization, return whether it was successful or not. This passes ownership of
+    // the current |mock_socket_| but allocates a new one for re-use.
+    bool UdpConnect(std::string* error = nullptr) {
+        std::string local_error;
+        if (error == nullptr) {
+            error = &local_error;
+        }
+        std::unique_ptr<Transport> transport(Connect(std::move(mock_socket_), error));
+        mock_socket_.reset(new SocketMock);
+        return transport != nullptr && error->empty();
+    }
+
+  protected:
+    std::unique_ptr<SocketMock> mock_socket_;
+};
+
+// Tests a successful protocol initialization with various starting sequence numbers.
+TEST_F(UdpConnectTest, InitializationSuccess) {
+    for (uint16_t seq : kTestSequenceNumbers) {
+        mock_socket_->ExpectSend(QueryPacket(0));
+        mock_socket_->AddReceive(QueryPacket(0, seq));
+        mock_socket_->ExpectSend(InitPacket(seq, kProtocolVersion, kHostMaxPacketSize));
+        mock_socket_->AddReceive(InitPacket(seq, kProtocolVersion, 1024));
+
+        EXPECT_TRUE(UdpConnect());
+    }
+}
+
+// Tests continuation packets during initialization.
+TEST_F(UdpConnectTest, InitializationContinuationSuccess) {
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(std::string{kIdDeviceQuery, kFlagContinuation, 0, 0, 0x44});
+    mock_socket_->ExpectSend(std::string{kIdDeviceQuery, kFlagNone, 0, 1});
+    mock_socket_->AddReceive(std::string{kIdDeviceQuery, kFlagNone, 0, 1, 0x55});
+
+    mock_socket_->ExpectSend(InitPacket(0x4455, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(std::string{kIdInitialization, kFlagContinuation, 0x44, 0x55, 0});
+    mock_socket_->ExpectSend(std::string{kIdInitialization, kFlagNone, 0x44, 0x56});
+    mock_socket_->AddReceive(std::string{kIdInitialization, kFlagContinuation, 0x44, 0x56, 1});
+    mock_socket_->ExpectSend(std::string{kIdInitialization, kFlagNone, 0x44, 0x57});
+    mock_socket_->AddReceive(std::string{kIdInitialization, kFlagContinuation, 0x44, 0x57, 2});
+    mock_socket_->ExpectSend(std::string{kIdInitialization, kFlagNone, 0x44, 0x58});
+    mock_socket_->AddReceive(std::string{kIdInitialization, kFlagNone, 0x44, 0x58, 0});
+
+    EXPECT_TRUE(UdpConnect());
+}
+
+
+// Tests a mismatched version number; as long as the minimum of the two versions is supported
+// we should allow the connection.
+TEST_F(UdpConnectTest, InitializationVersionMismatch) {
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(InitPacket(0, 2, 1024));
+
+    EXPECT_TRUE(UdpConnect());
+
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(InitPacket(0, 0, 1024));
+
+    EXPECT_FALSE(UdpConnect());
+}
+
+TEST_F(UdpConnectTest, QueryResponseTimeoutFailure) {
+    for (int i = 0; i < kMaxConnectAttempts; ++i) {
+        mock_socket_->ExpectSend(QueryPacket(0));
+        mock_socket_->AddReceiveTimeout();
+    }
+
+    EXPECT_FALSE(UdpConnect());
+}
+
+TEST_F(UdpConnectTest, QueryResponseReceiveFailure) {
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceiveFailure();
+
+    EXPECT_FALSE(UdpConnect());
+}
+
+TEST_F(UdpConnectTest, InitResponseTimeoutFailure) {
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+    for (int i = 0; i < kMaxTransmissionAttempts; ++i) {
+        mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+        mock_socket_->AddReceiveTimeout();
+    }
+
+    EXPECT_FALSE(UdpConnect());
+}
+
+TEST_F(UdpConnectTest, InitResponseReceiveFailure) {
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceiveFailure();
+
+    EXPECT_FALSE(UdpConnect());
+}
+
+// Tests that we can recover up to the maximum number of allowed retries.
+TEST_F(UdpConnectTest, ResponseRecovery) {
+    // The device query packet can recover from up to (kMaxConnectAttempts - 1) timeouts.
+    for (int i = 0; i < kMaxConnectAttempts - 1; ++i) {
+        mock_socket_->ExpectSend(QueryPacket(0));
+        mock_socket_->AddReceiveTimeout();
+    }
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+
+    // Subsequent packets try up to (kMaxTransmissionAttempts - 1) times.
+    for (int i = 0; i < kMaxTransmissionAttempts - 1; ++i) {
+        mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+        mock_socket_->AddReceiveTimeout();
+    }
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(InitPacket(0, kProtocolVersion, 1024));
+
+    EXPECT_TRUE(UdpConnect());
+}
+
+// Tests that the host can handle receiving additional bytes for forward compatibility.
+TEST_F(UdpConnectTest, ExtraResponseDataSuccess) {
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0) + "foo");
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(InitPacket(0, kProtocolVersion, 1024) + "bar");
+
+    EXPECT_TRUE(UdpConnect());
+}
+
+// Tests mismatched response sequence numbers. A wrong sequence number is interpreted as a previous
+// retransmission and just ignored so we should be able to recover.
+TEST_F(UdpConnectTest, WrongSequenceRecovery) {
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(1, 0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(InitPacket(1, kProtocolVersion, 1024));
+    mock_socket_->AddReceive(InitPacket(0, kProtocolVersion, 1024));
+
+    EXPECT_TRUE(UdpConnect());
+}
+
+// Tests mismatched response IDs. This should also be interpreted as a retransmission and ignored.
+TEST_F(UdpConnectTest, WrongIdRecovery) {
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(FastbootPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(FastbootPacket(0));
+    mock_socket_->AddReceive(InitPacket(0, kProtocolVersion, 1024));
+
+    EXPECT_TRUE(UdpConnect());
+}
+
+// Tests an invalid query response. Query responses must have at least 2 bytes of data.
+TEST_F(UdpConnectTest, InvalidQueryResponseFailure) {
+    std::string error;
+
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0));
+
+    EXPECT_FALSE(UdpConnect(&error));
+    EXPECT_EQ("invalid query response from target", error);
+
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0) + std::string{0x00});
+
+    EXPECT_FALSE(UdpConnect(&error));
+    EXPECT_EQ("invalid query response from target", error);
+}
+
+// Tests an invalid initialization response. Max packet size must be at least 512 bytes.
+TEST_F(UdpConnectTest, InvalidInitResponseFailure) {
+    std::string error;
+
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(InitPacket(0, kProtocolVersion, 511));
+
+    EXPECT_FALSE(UdpConnect(&error));
+    EXPECT_EQ("target reported invalid packet size 511", error);
+
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(InitPacket(0, 0, 1024));
+
+    EXPECT_FALSE(UdpConnect(&error));
+    EXPECT_EQ("target reported invalid protocol version 0", error);
+}
+
+TEST_F(UdpConnectTest, ErrorResponseFailure) {
+    std::string error;
+
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(ErrorPacket(0, "error1"));
+
+    EXPECT_FALSE(UdpConnect(&error));
+    EXPECT_NE(std::string::npos, error.find("error1"));
+
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(QueryPacket(0, 0));
+    mock_socket_->ExpectSend(InitPacket(0, kProtocolVersion, kHostMaxPacketSize));
+    mock_socket_->AddReceive(ErrorPacket(0, "error2"));
+
+    EXPECT_FALSE(UdpConnect(&error));
+    EXPECT_NE(std::string::npos, error.find("error2"));
+}
+
+// Tests an error response with continuation flag.
+TEST_F(UdpConnectTest, ErrorContinuationFailure) {
+    std::string error;
+
+    mock_socket_->ExpectSend(QueryPacket(0));
+    mock_socket_->AddReceive(ErrorPacket(0, "error1", kFlagContinuation));
+    mock_socket_->ExpectSend(ErrorPacket(1));
+    mock_socket_->AddReceive(ErrorPacket(1, " ", kFlagContinuation));
+    mock_socket_->ExpectSend(ErrorPacket(2));
+    mock_socket_->AddReceive(ErrorPacket(2, "error2"));
+
+    EXPECT_FALSE(UdpConnect(&error));
+    EXPECT_NE(std::string::npos, error.find("error1 error2"));
+}
+
+// Fixture class to test UDP Transport read/write functionality.
+class UdpTest : public ::testing::Test {
+  public:
+    void SetUp() override {
+        // Create |transport_| starting at sequence 0 with 512 byte max packet size. Tests can call
+        // InitializeTransport() again to change settings.
+        ASSERT_TRUE(InitializeTransport(0, 512));
+    }
+
+    // Sets up |mock_socket_| to correctly initialize the protocol and creates |transport_|. This
+    // can be called multiple times in a test if needed.
+    bool InitializeTransport(uint16_t starting_sequence, int device_max_packet_size = 512) {
+        mock_socket_ = new SocketMock;
+        mock_socket_->ExpectSend(QueryPacket(0));
+        mock_socket_->AddReceive(QueryPacket(0, starting_sequence));
+        mock_socket_->ExpectSend(
+                InitPacket(starting_sequence, kProtocolVersion, kHostMaxPacketSize));
+        mock_socket_->AddReceive(
+                InitPacket(starting_sequence, kProtocolVersion, device_max_packet_size));
+
+        std::string error;
+        transport_ = Connect(std::unique_ptr<Socket>(mock_socket_), &error);
+        return transport_ != nullptr && error.empty();
+    }
+
+    // Writes |message| to |transport_|, returns true on success.
+    bool Write(const std::string& message) {
+        return transport_->Write(message.data(), message.length()) ==
+                static_cast<ssize_t>(message.length());
+    }
+
+    // Reads from |transport_|, returns true if it matches |message|.
+    bool Read(const std::string& message) {
+        std::string buffer(message.length(), '\0');
+        return transport_->Read(&buffer[0], buffer.length()) ==
+                static_cast<ssize_t>(message.length()) && buffer == message;
+    }
+
+  protected:
+    // |mock_socket_| is a raw pointer here because we transfer ownership to |transport_| but we
+    // need to retain a pointer to set send and receive expectations.
+    SocketMock* mock_socket_ = nullptr;
+    std::unique_ptr<Transport> transport_;
+};
+
+// Tests sequence behavior with various starting sequence numbers.
+TEST_F(UdpTest, SequenceIncrementCheck) {
+    for (uint16_t seq : kTestSequenceNumbers) {
+        ASSERT_TRUE(InitializeTransport(seq));
+
+        for (int i = 0; i < 10; ++i) {
+            mock_socket_->ExpectSend(FastbootPacket(++seq, "foo"));
+            mock_socket_->AddReceive(FastbootPacket(seq, ""));
+            mock_socket_->ExpectSend(FastbootPacket(++seq, ""));
+            mock_socket_->AddReceive(FastbootPacket(seq, "bar"));
+
+            EXPECT_TRUE(Write("foo"));
+            EXPECT_TRUE(Read("bar"));
+        }
+    }
+}
+
+// Tests sending and receiving a few small packets.
+TEST_F(UdpTest, ReadAndWriteSmallPackets) {
+    mock_socket_->ExpectSend(FastbootPacket(1, "foo"));
+    mock_socket_->AddReceive(FastbootPacket(1, ""));
+    mock_socket_->ExpectSend(FastbootPacket(2, ""));
+    mock_socket_->AddReceive(FastbootPacket(2, "bar"));
+
+    EXPECT_TRUE(Write("foo"));
+    EXPECT_TRUE(Read("bar"));
+
+    mock_socket_->ExpectSend(FastbootPacket(3, "12345 67890"));
+    mock_socket_->AddReceive(FastbootPacket(3));
+    mock_socket_->ExpectSend(FastbootPacket(4, "\x01\x02\x03\x04\x05"));
+    mock_socket_->AddReceive(FastbootPacket(4));
+
+    EXPECT_TRUE(Write("12345 67890"));
+    EXPECT_TRUE(Write("\x01\x02\x03\x04\x05"));
+
+    // Reads are done by sending empty packets.
+    mock_socket_->ExpectSend(FastbootPacket(5));
+    mock_socket_->AddReceive(FastbootPacket(5, "foo bar baz"));
+    mock_socket_->ExpectSend(FastbootPacket(6));
+    mock_socket_->AddReceive(FastbootPacket(6, "\x01\x02\x03\x04\x05"));
+
+    EXPECT_TRUE(Read("foo bar baz"));
+    EXPECT_TRUE(Read("\x01\x02\x03\x04\x05"));
+}
+
+TEST_F(UdpTest, ResponseTimeoutFailure) {
+    for (int i = 0; i < kMaxTransmissionAttempts; ++i) {
+        mock_socket_->ExpectSend(FastbootPacket(1, "foo"));
+        mock_socket_->AddReceiveTimeout();
+    }
+
+    EXPECT_FALSE(Write("foo"));
+}
+
+TEST_F(UdpTest, ResponseReceiveFailure) {
+    mock_socket_->ExpectSend(FastbootPacket(1, "foo"));
+    mock_socket_->AddReceiveFailure();
+
+    EXPECT_FALSE(Write("foo"));
+}
+
+TEST_F(UdpTest, ResponseTimeoutRecovery) {
+    for (int i = 0; i < kMaxTransmissionAttempts - 1; ++i) {
+        mock_socket_->ExpectSend(FastbootPacket(1, "foo"));
+        mock_socket_->AddReceiveTimeout();
+    }
+    mock_socket_->ExpectSend(FastbootPacket(1, "foo"));
+    mock_socket_->AddReceive(FastbootPacket(1, ""));
+
+    EXPECT_TRUE(Write("foo"));
+}
+
+// Tests continuation packets for various max packet sizes.
+// The important part of this test is that regardless of what kind of packet fragmentation happens
+// at the socket layer, a single call to Transport::Read() and Transport::Write() is all the
+// fastboot code needs to do.
+TEST_F(UdpTest, ContinuationPackets) {
+    for (uint16_t max_packet_size : {512, 1024, 1200}) {
+        ASSERT_TRUE(InitializeTransport(0, max_packet_size));
+
+        // Initialize the data we want to send. Use (size - 4) to leave room for the header.
+        size_t max_data_size = max_packet_size - 4;
+        std::string data(max_data_size * 3, '\0');
+        for (size_t i = 0; i < data.length(); ++i) {
+            data[i] = i;
+        }
+        std::string chunks[] = {data.substr(0, max_data_size),
+                                data.substr(max_data_size, max_data_size),
+                                data.substr(max_data_size * 2, max_data_size)};
+
+        // Write data: split into 3 UDP packets, each of which will be ACKed.
+        mock_socket_->ExpectSend(FastbootPacket(1, chunks[0], kFlagContinuation));
+        mock_socket_->AddReceive(FastbootPacket(1));
+        mock_socket_->ExpectSend(FastbootPacket(2, chunks[1], kFlagContinuation));
+        mock_socket_->AddReceive(FastbootPacket(2));
+        mock_socket_->ExpectSend(FastbootPacket(3, chunks[2]));
+        mock_socket_->AddReceive(FastbootPacket(3));
+        EXPECT_TRUE(Write(data));
+
+        // Same thing for reading the data.
+        mock_socket_->ExpectSend(FastbootPacket(4));
+        mock_socket_->AddReceive(FastbootPacket(4, chunks[0], kFlagContinuation));
+        mock_socket_->ExpectSend(FastbootPacket(5));
+        mock_socket_->AddReceive(FastbootPacket(5, chunks[1], kFlagContinuation));
+        mock_socket_->ExpectSend(FastbootPacket(6));
+        mock_socket_->AddReceive(FastbootPacket(6, chunks[2]));
+        EXPECT_TRUE(Read(data));
+    }
+}
+
+// Tests that the continuation bit is respected even if the packet isn't max size.
+TEST_F(UdpTest, SmallContinuationPackets) {
+    mock_socket_->ExpectSend(FastbootPacket(1));
+    mock_socket_->AddReceive(FastbootPacket(1, "foo", kFlagContinuation));
+    mock_socket_->ExpectSend(FastbootPacket(2));
+    mock_socket_->AddReceive(FastbootPacket(2, "bar"));
+
+    EXPECT_TRUE(Read("foobar"));
+}
+
+// Tests receiving an error packet mid-continuation.
+TEST_F(UdpTest, ContinuationPacketError) {
+    mock_socket_->ExpectSend(FastbootPacket(1));
+    mock_socket_->AddReceive(FastbootPacket(1, "foo", kFlagContinuation));
+    mock_socket_->ExpectSend(FastbootPacket(2));
+    mock_socket_->AddReceive(ErrorPacket(2, "test error"));
+
+    EXPECT_FALSE(Read("foo"));
+}
+
+// Tests timeout during a continuation sequence.
+TEST_F(UdpTest, ContinuationTimeoutRecovery) {
+    mock_socket_->ExpectSend(FastbootPacket(1));
+    mock_socket_->AddReceive(FastbootPacket(1, "foo", kFlagContinuation));
+    mock_socket_->ExpectSend(FastbootPacket(2));
+    mock_socket_->AddReceiveTimeout();
+    mock_socket_->ExpectSend(FastbootPacket(2));
+    mock_socket_->AddReceive(FastbootPacket(2, "bar"));
+
+    EXPECT_TRUE(Read("foobar"));
+}
+
+// Tests read overflow returns -1 to indicate the failure.
+TEST_F(UdpTest, MultipleReadPacket) {
+    mock_socket_->ExpectSend(FastbootPacket(1));
+    mock_socket_->AddReceive(FastbootPacket(1, "foobarbaz"));
+
+    char buffer[3];
+    EXPECT_EQ(-1, transport_->Read(buffer, 3));
+}
+
+// Tests that packets arriving out-of-order are ignored.
+TEST_F(UdpTest, IgnoreOutOfOrderPackets) {
+    mock_socket_->ExpectSend(FastbootPacket(1));
+    mock_socket_->AddReceive(FastbootPacket(0, "sequence too low"));
+    mock_socket_->AddReceive(FastbootPacket(2, "sequence too high"));
+    mock_socket_->AddReceive(QueryPacket(1));
+    mock_socket_->AddReceive(FastbootPacket(1, "correct"));
+
+    EXPECT_TRUE(Read("correct"));
+}
+
+// Tests that an error response with the correct sequence number causes immediate failure.
+TEST_F(UdpTest, ErrorResponse) {
+    // Error packets with the wrong sequence number should be ignored like any other packet.
+    mock_socket_->ExpectSend(FastbootPacket(1, "foo"));
+    mock_socket_->AddReceive(ErrorPacket(0, "ignored error"));
+    mock_socket_->AddReceive(FastbootPacket(1));
+
+    EXPECT_TRUE(Write("foo"));
+
+    // Error packets with the correct sequence should abort immediately without retransmission.
+    mock_socket_->ExpectSend(FastbootPacket(2, "foo"));
+    mock_socket_->AddReceive(ErrorPacket(2, "test error"));
+
+    EXPECT_FALSE(Write("foo"));
+}
+
+// Tests that attempting to use a closed transport returns -1 without making any socket calls.
+TEST_F(UdpTest, CloseTransport) {
+    char buffer[32];
+    EXPECT_EQ(0, transport_->Close());
+    EXPECT_EQ(-1, transport_->Write("foo", 3));
+    EXPECT_EQ(-1, transport_->Read(buffer, sizeof(buffer)));
+}