diff --git a/fastboot/Android.mk b/fastboot/Android.mk
index fcec5b1..65f4e01 100644
--- a/fastboot/Android.mk
+++ b/fastboot/Android.mk
@@ -106,7 +106,11 @@
 LOCAL_MODULE := fastboot_test
 LOCAL_MODULE_HOST_OS := darwin linux windows
 
-LOCAL_SRC_FILES := socket.cpp socket_test.cpp
+LOCAL_SRC_FILES := \
+    socket.cpp \
+    socket_mock.cpp \
+    socket_test.cpp \
+
 LOCAL_STATIC_LIBRARIES := libbase libcutils
 
 LOCAL_CFLAGS += -Wall -Wextra -Werror -Wunreachable-code
diff --git a/fastboot/socket.cpp b/fastboot/socket.cpp
index d41f1fe..0a3ddfa 100644
--- a/fastboot/socket.cpp
+++ b/fastboot/socket.cpp
@@ -28,6 +28,7 @@
 
 #include "socket.h"
 
+#include <android-base/errors.h>
 #include <android-base/stringprintf.h>
 
 Socket::Socket(cutils_socket_t sock) : sock_(sock) {}
@@ -77,6 +78,10 @@
     return total;
 }
 
+int Socket::GetLocalPort() {
+    return socket_get_local_port(sock_);
+}
+
 // Implements the Socket interface for UDP.
 class UdpSocket : public Socket {
   public:
@@ -210,3 +215,12 @@
 
     return nullptr;
 }
+
+std::string Socket::GetErrorMessage() {
+#if defined(_WIN32)
+    DWORD error_code = WSAGetLastError();
+#else
+    int error_code = errno;
+#endif
+    return android::base::SystemErrorCodeToString(error_code);
+}
diff --git a/fastboot/socket.h b/fastboot/socket.h
index 3e66c27..a7481db 100644
--- a/fastboot/socket.h
+++ b/fastboot/socket.h
@@ -44,6 +44,10 @@
   public:
     enum class Protocol { kTcp, kUdp };
 
+    // Returns the socket error message. This must be called immediately after a socket failure
+    // before any other system calls are made.
+    static std::string GetErrorMessage();
+
     // Creates a new client connection. Clients are connected to a specific hostname/port and can
     // only send to that destination.
     // On failure, |error| is filled (if non-null) and nullptr is returned.
@@ -78,6 +82,9 @@
     // connected to the client on success, nullptr on failure.
     virtual std::unique_ptr<Socket> Accept() { return nullptr; }
 
+    // Returns the local port the Socket is bound to or -1 on error.
+    int GetLocalPort();
+
   protected:
     // Protected constructor to force factory function use.
     Socket(cutils_socket_t sock);
diff --git a/fastboot/socket_mock.cpp b/fastboot/socket_mock.cpp
new file mode 100644
index 0000000..8fea554
--- /dev/null
+++ b/fastboot/socket_mock.cpp
@@ -0,0 +1,132 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include "socket_mock.h"
+
+#include <gtest/gtest.h>
+
+SocketMock::SocketMock() : Socket(INVALID_SOCKET) {}
+
+SocketMock::~SocketMock() {
+    if (!events_.empty()) {
+        ADD_FAILURE() << events_.size() << " event(s) were not handled";
+    }
+}
+
+ssize_t SocketMock::Send(const void* data, size_t length) {
+    if (events_.empty()) {
+        ADD_FAILURE() << "Send() was called when no message was expected";
+        return -1;
+    }
+
+    if (events_.front().type != EventType::kSend) {
+        ADD_FAILURE() << "Send() was called out-of-order";
+        return -1;
+    }
+
+    std::string message(reinterpret_cast<const char*>(data), length);
+    if (events_.front().message != message) {
+        ADD_FAILURE() << "Send() expected " << events_.front().message << ", but got " << message;
+        return -1;
+    }
+
+    ssize_t return_value = events_.front().return_value;
+    events_.pop();
+    return return_value;
+}
+
+ssize_t SocketMock::Receive(void* data, size_t length, int /*timeout_ms*/) {
+    if (events_.empty()) {
+        ADD_FAILURE() << "Receive() was called when no message was ready";
+        return -1;
+    }
+
+    if (events_.front().type != EventType::kReceive) {
+        ADD_FAILURE() << "Receive() was called out-of-order";
+        return -1;
+    }
+
+    if (events_.front().return_value > static_cast<ssize_t>(length)) {
+        ADD_FAILURE() << "Receive(): not enough bytes (" << length << ") for "
+                      << events_.front().message;
+        return -1;
+    }
+
+    ssize_t return_value = events_.front().return_value;
+    if (return_value > 0) {
+        memcpy(data, events_.front().message.data(), return_value);
+    }
+    events_.pop();
+    return return_value;
+}
+
+int SocketMock::Close() {
+    return 0;
+}
+
+std::unique_ptr<Socket> SocketMock::Accept() {
+    if (events_.empty()) {
+        ADD_FAILURE() << "Accept() was called when no socket was ready";
+        return nullptr;
+    }
+
+    if (events_.front().type != EventType::kAccept) {
+        ADD_FAILURE() << "Accept() was called out-of-order";
+        return nullptr;
+    }
+
+    std::unique_ptr<Socket> sock = std::move(events_.front().sock);
+    events_.pop();
+    return sock;
+}
+
+void SocketMock::ExpectSend(std::string message) {
+    ssize_t return_value = message.length();
+    events_.push(Event(EventType::kSend, std::move(message), return_value, nullptr));
+}
+
+void SocketMock::ExpectSendFailure(std::string message) {
+    events_.push(Event(EventType::kSend, std::move(message), -1, nullptr));
+}
+
+void SocketMock::AddReceive(std::string message) {
+    ssize_t return_value = message.length();
+    events_.push(Event(EventType::kReceive, std::move(message), return_value, nullptr));
+}
+
+void SocketMock::AddReceiveFailure() {
+    events_.push(Event(EventType::kReceive, "", -1, nullptr));
+}
+
+void SocketMock::AddAccept(std::unique_ptr<Socket> sock) {
+    events_.push(Event(EventType::kAccept, "", 0, std::move(sock)));
+}
+
+SocketMock::Event::Event(EventType _type, std::string _message, ssize_t _return_value,
+                         std::unique_ptr<Socket> _sock)
+        : type(_type), message(_message), return_value(_return_value), sock(std::move(_sock)) {}
diff --git a/fastboot/socket_mock.h b/fastboot/socket_mock.h
new file mode 100644
index 0000000..3e62b33
--- /dev/null
+++ b/fastboot/socket_mock.h
@@ -0,0 +1,97 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef SOCKET_MOCK_H_
+#define SOCKET_MOCK_H_
+
+#include <memory>
+#include <queue>
+#include <string>
+
+#include <android-base/macros.h>
+
+#include "socket.h"
+
+// A mock Socket implementation to be used for testing. Tests can set expectations for messages
+// to be sent and provide messages to be received in order to verify protocol behavior.
+//
+// Example: testing sending "foo" and receiving "bar".
+//   SocketMock mock;
+//   mock.ExpectSend("foo");
+//   mock.AddReceive("bar");
+//   EXPECT_TRUE(DoFooBar(&mock));
+//
+// Example: testing sending "foo" and expecting "bar", but receiving "baz" instead.
+//   SocketMock mock;
+//   mock.ExpectSend("foo");
+//   mock.AddReceive("baz");
+//   EXPECT_FALSE(DoFooBar(&mock));
+class SocketMock : public Socket {
+  public:
+    SocketMock();
+    ~SocketMock() override;
+
+    ssize_t Send(const void* data, size_t length) override;
+    ssize_t Receive(void* data, size_t length, int timeout_ms) override;
+    int Close() override;
+    virtual std::unique_ptr<Socket> Accept();
+
+    // Adds an expectation for Send().
+    void ExpectSend(std::string message);
+
+    // Adds an expectation for Send() that returns -1.
+    void ExpectSendFailure(std::string message);
+
+    // Adds data to provide for Receive().
+    void AddReceive(std::string message);
+
+    // Adds a Receive() failure.
+    void AddReceiveFailure();
+
+    // Adds a Socket to return from Accept().
+    void AddAccept(std::unique_ptr<Socket> sock);
+
+  private:
+    enum class EventType { kSend, kReceive, kAccept };
+
+    struct Event {
+        Event(EventType _type, std::string _message, ssize_t _return_value,
+              std::unique_ptr<Socket> _sock);
+
+        EventType type;
+        std::string message;
+        ssize_t return_value;
+        std::unique_ptr<Socket> sock;
+    };
+
+    std::queue<Event> events_;
+
+    DISALLOW_COPY_AND_ASSIGN(SocketMock);
+};
+
+#endif  // SOCKET_MOCK_H_
diff --git a/fastboot/socket_test.cpp b/fastboot/socket_test.cpp
index 1fd9d7c..7bfe967 100644
--- a/fastboot/socket_test.cpp
+++ b/fastboot/socket_test.cpp
@@ -14,33 +14,31 @@
  * limitations under the License.
  */
 
-// Tests UDP functionality using loopback connections. Requires that kTestPort is available
-// for loopback communication on the host. These tests also assume that no UDP packets are lost,
-// which should be the case for loopback communication, but is not guaranteed.
+// Tests socket functionality using loopback connections. The UDP tests assume that no packets are
+// lost, which should be the case for loopback communication, but is not guaranteed.
+//
+// Also tests our SocketMock class to make sure it works as expected and reports errors properly
+// if the mock expectations aren't met during a test.
 
 #include "socket.h"
+#include "socket_mock.h"
 
 #include <gtest/gtest.h>
+#include <gtest/gtest-spi.h>
 
-enum {
-    // This port must be available for loopback communication.
-    kTestPort = 54321,
-
-    // Don't wait forever in a unit test.
-    kTestTimeoutMs = 3000,
-};
+enum { kTestTimeoutMs = 3000 };
 
 // Creates connected sockets |server| and |client|. Returns true on success.
 bool MakeConnectedSockets(Socket::Protocol protocol, std::unique_ptr<Socket>* server,
-                          std::unique_ptr<Socket>* client, const std::string hostname = "localhost",
-                          int port = kTestPort) {
-    *server = Socket::NewServer(protocol, port);
+                          std::unique_ptr<Socket>* client,
+                          const std::string hostname = "localhost") {
+    *server = Socket::NewServer(protocol, 0);
     if (*server == nullptr) {
         ADD_FAILURE() << "Failed to create server.";
         return false;
     }
 
-    *client = Socket::NewClient(protocol, hostname, port, nullptr);
+    *client = Socket::NewClient(protocol, hostname, (*server)->GetLocalPort(), nullptr);
     if (*client == nullptr) {
         ADD_FAILURE() << "Failed to create client.";
         return false;
@@ -124,3 +122,86 @@
         EXPECT_EQ(-1, bytes);
     }
 }
+
+TEST(SocketMockTest, TestSendSuccess) {
+    SocketMock mock;
+
+    mock.ExpectSend("foo");
+    EXPECT_TRUE(SendString(&mock, "foo"));
+
+    mock.ExpectSend("abc");
+    mock.ExpectSend("123");
+    EXPECT_TRUE(SendString(&mock, "abc"));
+    EXPECT_TRUE(SendString(&mock, "123"));
+}
+
+TEST(SocketMockTest, TestSendFailure) {
+    SocketMock* mock = new SocketMock;
+
+    EXPECT_NONFATAL_FAILURE(SendString(mock, "foo"), "no message was expected");
+
+    mock->ExpectSend("foo");
+    EXPECT_NONFATAL_FAILURE(SendString(mock, "bar"), "expected foo, but got bar");
+    EXPECT_TRUE(SendString(mock, "foo"));
+
+    mock->AddReceive("foo");
+    EXPECT_NONFATAL_FAILURE(SendString(mock, "foo"), "called out-of-order");
+    EXPECT_TRUE(ReceiveString(mock, "foo"));
+
+    mock->ExpectSend("foo");
+    EXPECT_NONFATAL_FAILURE(delete mock, "1 event(s) were not handled");
+}
+
+TEST(SocketMockTest, TestReceiveSuccess) {
+    SocketMock mock;
+
+    mock.AddReceive("foo");
+    EXPECT_TRUE(ReceiveString(&mock, "foo"));
+
+    mock.AddReceive("abc");
+    mock.AddReceive("123");
+    EXPECT_TRUE(ReceiveString(&mock, "abc"));
+    EXPECT_TRUE(ReceiveString(&mock, "123"));
+}
+
+TEST(SocketMockTest, TestReceiveFailure) {
+    SocketMock* mock = new SocketMock;
+
+    EXPECT_NONFATAL_FAILURE(ReceiveString(mock, "foo"), "no message was ready");
+
+    mock->ExpectSend("foo");
+    EXPECT_NONFATAL_FAILURE(ReceiveString(mock, "foo"), "called out-of-order");
+    EXPECT_TRUE(SendString(mock, "foo"));
+
+    char c;
+    mock->AddReceive("foo");
+    EXPECT_NONFATAL_FAILURE(mock->Receive(&c, 1, 0), "not enough bytes (1) for foo");
+    EXPECT_TRUE(ReceiveString(mock, "foo"));
+
+    mock->AddReceive("foo");
+    EXPECT_NONFATAL_FAILURE(delete mock, "1 event(s) were not handled");
+}
+
+TEST(SocketMockTest, TestAcceptSuccess) {
+    SocketMock mock;
+
+    SocketMock* mock_handler = new SocketMock;
+    mock.AddAccept(std::unique_ptr<SocketMock>(mock_handler));
+    EXPECT_EQ(mock_handler, mock.Accept().get());
+
+    mock.AddAccept(nullptr);
+    EXPECT_EQ(nullptr, mock.Accept().get());
+}
+
+TEST(SocketMockTest, TestAcceptFailure) {
+    SocketMock* mock = new SocketMock;
+
+    EXPECT_NONFATAL_FAILURE(mock->Accept(), "no socket was ready");
+
+    mock->ExpectSend("foo");
+    EXPECT_NONFATAL_FAILURE(mock->Accept(), "called out-of-order");
+    EXPECT_TRUE(SendString(mock, "foo"));
+
+    mock->AddAccept(nullptr);
+    EXPECT_NONFATAL_FAILURE(delete mock, "1 event(s) were not handled");
+}