diff --git a/fastboot/Android.mk b/fastboot/Android.mk
index e0f7c73..a326f55 100644
--- a/fastboot/Android.mk
+++ b/fastboot/Android.mk
@@ -32,6 +32,7 @@
     protocol.cpp \
     socket.cpp \
     tcp.cpp \
+    udp.cpp \
     util.cpp \
 
 LOCAL_MODULE := fastboot
@@ -114,6 +115,8 @@
     socket_test.cpp \
     tcp.cpp \
     tcp_test.cpp \
+    udp.cpp \
+    udp_test.cpp \
 
 LOCAL_STATIC_LIBRARIES := libbase libcutils
 
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 7c7d417..636092e 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -59,6 +59,7 @@
 #include "fs.h"
 #include "tcp.h"
 #include "transport.h"
+#include "udp.h"
 #include "usb.h"
 
 #ifndef O_BINARY
@@ -245,22 +246,41 @@
         return transport;
     }
 
+    Socket::Protocol protocol = Socket::Protocol::kTcp;
     std::string host;
-    int port = tcp::kDefaultPort;
-    if (serial != nullptr && android::base::StartsWith(serial, "tcp:")) {
-        std::string error;
-        const char* address = serial + strlen("tcp:");
+    int port = 0;
+    if (serial != nullptr) {
+        const char* net_address = nullptr;
 
-        if (!android::base::ParseNetAddress(address, &host, &port, nullptr, &error)) {
-            fprintf(stderr, "error: Invalid network address '%s': %s\n", address, error.c_str());
-            return nullptr;
+        if (android::base::StartsWith(serial, "tcp:")) {
+            protocol = Socket::Protocol::kTcp;
+            port = tcp::kDefaultPort;
+            net_address = serial + strlen("tcp:");
+        } else if (android::base::StartsWith(serial, "udp:")) {
+            protocol = Socket::Protocol::kUdp;
+            port = udp::kDefaultPort;
+            net_address = serial + strlen("udp:");
+        }
+
+        if (net_address != nullptr) {
+            std::string error;
+            if (!android::base::ParseNetAddress(net_address, &host, &port, nullptr, &error)) {
+                fprintf(stderr, "error: Invalid network address '%s': %s\n", net_address,
+                        error.c_str());
+                return nullptr;
+            }
         }
     }
 
     while (true) {
         if (!host.empty()) {
             std::string error;
-            transport = tcp::Connect(host, port, &error).release();
+            if (protocol == Socket::Protocol::kTcp) {
+                transport = tcp::Connect(host, port, &error).release();
+            } else if (protocol == Socket::Protocol::kUdp) {
+                transport = udp::Connect(host, port, &error).release();
+            }
+
             if (transport == nullptr && announce) {
                 fprintf(stderr, "error: %s\n", error.c_str());
             }
@@ -337,8 +357,9 @@
             "                                           formatting.\n"
             "  -s <specific device>                     Specify a device. For USB, provide either\n"
             "                                           a serial number or path to device port.\n"
-            "                                           For TCP, provide an address in the form\n"
-            "                                           tcp:<hostname>[:port].\n"
+            "                                           For ethernet, provide an address in the"
+            "                                           form <protocol>:<hostname>[:port] where"
+            "                                           <protocol> is either tcp or udp.\n"
             "  -p <product>                             Specify product name.\n"
             "  -c <cmdline>                             Override kernel commandline.\n"
             "  -i <vendor id>                           Specify a custom USB vendor id.\n"
diff --git a/fastboot/fastboot_protocol.txt b/fastboot/fastboot_protocol.txt
index 4aa48b1..2801703 100644
--- a/fastboot/fastboot_protocol.txt
+++ b/fastboot/fastboot_protocol.txt
@@ -17,9 +17,9 @@
   * The protocol is entirely host-driven and synchronous (unlike the
     multi-channel, bi-directional, asynchronous ADB protocol)
 
-* TCP
+* TCP or UDP
   * Device must be reachable via IP.
-  * Device will act as the TCP server, fastboot will be the client.
+  * Device will act as the server, fastboot will be the client.
   * Fastboot data is wrapped in a simple protocol; see below for details.
 
 
@@ -217,3 +217,226 @@
 Host    [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x0B]getvar:none
 Device  [0x00][0x00][0x00][0x00][0x00][0x00][0x00][0x04]OKAY
 Host    <disconnect>
+
+
+UDP Protocol v1
+---------------
+
+The UDP protocol is more complex than TCP since we must implement reliability
+to ensure no packets are lost, but the general concept of wrapping the fastboot
+protocol is the same.
+
+Overview:
+  1. As with TCP, the device will listen on UDP port 5554.
+  2. Maximum UDP packet size is negotiated during initialization.
+  3. The host drives all communication; the device may only send a packet as a
+     response to a host packet.
+  4. If the host does not receive a response in 500ms it will re-transmit.
+
+-- UDP Packet format --
+  +----------+----+-------+-------+--------------------+
+  | Byte #   | 0  |   1   | 2 - 3 |  4+                |
+  +----------+----+-------+-------+--------------------+
+  | Contents | ID | Flags | Seq # | Data               |
+  +----------+----+-------+-------+--------------------+
+
+  ID      Packet ID:
+            0x00: Error.
+            0x01: Query.
+            0x02: Initialization.
+            0x03: Fastboot.
+
+          Packet types are described in more detail below.
+
+  Flags   Packet flags: 0 0 0 0 0 0 0 C
+            C=1 indicates a continuation packet; the data is too large and will
+                continue in the next packet.
+
+            Remaining bits are reserved for future use and must be set to 0.
+
+  Seq #   2-byte packet sequence number (big-endian). The host will increment
+          this by 1 with each new packet, and the device must provide the
+          corresponding sequence number in the response packets.
+
+  Data    Packet data, not present in all packets.
+
+-- Packet Types --
+Query     The host sends a query packet once on startup to sync with the device.
+          The host will not know the current sequence number, so the device must
+          respond to all query packets regardless of sequence number.
+
+          The response data field should contain a 2-byte big-endian value
+          giving the next expected sequence number.
+
+Init      The host sends an init packet once the query response is returned. The
+          device must abort any in-progress operation and prepare for a new
+          fastboot session. This message is meant to allow recovery if a
+          previous session failed, e.g. due to network error or user Ctrl+C.
+
+          The data field contains two big-endian 2-byte values, a protocol
+          version and the max UDP packet size (including the 4-byte header).
+          Both the host and device will send these values, and in each case
+          the minimum of the sent values must be used.
+
+Fastboot  These packets wrap the fastboot protocol. To write, the host will
+          send a packet with fastboot data, and the device will reply with an
+          empty packet as an ACK. To read, the host will send an empty packet,
+          and the device will reply with fastboot data. The device may not give
+          any data in the ACK packet.
+
+Error     The device may respond to any packet with an error packet to indicate
+          a UDP protocol error. The data field should contain an ASCII string
+          describing the error. This is the only case where a device is allowed
+          to return a packet ID other than the one sent by the host.
+
+-- Packet Size --
+The maximum packet size is negotiated by the host and device in the Init packet.
+Devices must support at least 512-byte packets, but packet size has a direct
+correlation with download speed, so devices are strongly suggested to support at
+least 1024-byte packets. On a local network with 0.5ms round-trip time this will
+provide transfer rates of ~2MB/s. Over WiFi it will likely be significantly
+less.
+
+Query and Initialization packets, which are sent before size negotiation is
+complete, must always be 512 bytes or less.
+
+-- Packet Re-Transmission --
+The host will re-transmit any packet that does not receive a response. The
+requirement of exactly one device response packet per host packet is how we
+achieve reliability and in-order delivery of packets.
+
+For simplicity of implementation, there is no windowing of multiple
+unacknowledged packets in this version of the protocol. The host will continue
+to send the same packet until a response is received. Windowing functionality
+may be implemented in future versions if necessary to increase performance.
+
+The first Query packet will only be attempted a small number of times, but
+subsequent packets will attempt to retransmit for at least 1 minute before
+giving up. This means a device may safely ignore host UDP packets for up to 1
+minute during long operations, e.g. writing to flash.
+
+-- Continuation Packets --
+Any packet may set the continuation flag to indicate that the data is
+incomplete. Large data such as downloading an image may require many
+continuation packets. The receiver should respond to a continuation packet with
+an empty packet to acknowledge receipt. See examples below.
+
+-- Summary --
+The host starts with a Query packet, then an Initialization packet, after
+which only Fastboot packets are sent. Fastboot packets may contain data from
+the host for writes, or from the device for reads, but not both.
+
+Given a next expected sequence number S and a received packet P, the device
+behavior should be:
+  if P is a Query packet:
+    * respond with a Query packet with S in the data field
+  else if P has sequence == S:
+    * process P and take any required action
+    * create a response packet R with the same ID and sequence as P, containing
+      any response data required.
+    * transmit R and save it in case of re-transmission
+    * increment S
+  else if P has sequence == S - 1:
+    * re-transmit the saved response packet R from above
+  else:
+    * ignore the packet
+
+-- Examples --
+In the examples below, S indicates the starting client sequence number.
+
+Host                                    Client
+======================================================================
+[Initialization, S = 0x55AA]
+[Host: version 1, 2048-byte packets. Client: version 2, 1024-byte packets.]
+[Resulting values to use: version = 1, max packet size = 1024]
+ID   Flag SeqH SeqL Data                ID   Flag SeqH SeqL Data
+----------------------------------------------------------------------
+0x01 0x00 0x00 0x00
+                                        0x01 0x00 0x00 0x00 0x55 0xAA
+0x02 0x00 0x55 0xAA 0x00 0x01 0x08 0x00
+                                        0x02 0x00 0x55 0xAA 0x00 0x02 0x04 0x00
+
+----------------------------------------------------------------------
+[fastboot "getvar" commands, S = 0x0001]
+ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
+----------------------------------------------------------------------
+0x03  0x00  0x00  0x01  getvar:version
+                                        0x03  0x00  0x00  0x01
+0x03  0x00  0x00  0x02
+                                        0x03  0x00  0x00  0x02  OKAY0.4
+0x03  0x00  0x00  0x03  getvar:foo
+                                        0x03  0x00  0x00  0x03
+0x03  0x00  0x00  0x04
+                                        0x03  0x00  0x00  0x04  OKAY
+
+----------------------------------------------------------------------
+[fastboot "INFO" responses, S = 0x0000]
+ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
+----------------------------------------------------------------------
+0x03  0x00  0x00  0x00  <command>
+                                        0x03  0x00  0x00  0x00
+0x03  0x00  0x00  0x01
+                                        0x03  0x00  0x00  0x01  INFOWait1
+0x03  0x00  0x00  0x02
+                                        0x03  0x00  0x00  0x02  INFOWait2
+0x03  0x00  0x00  0x03
+                                        0x03  0x00  0x00  0x03  OKAY
+
+----------------------------------------------------------------------
+[Chunking 2100 bytes of data, max packet size = 1024, S = 0xFFFF]
+ID   Flag SeqH SeqL Data                ID   Flag SeqH SeqL Data
+----------------------------------------------------------------------
+0x03 0x00 0xFF 0xFF download:0000834
+                                        0x03 0x00 0xFF 0xFF
+0x03 0x00 0x00 0x00
+                                        0x03 0x00 0x00 0x00 DATA0000834
+0x03 0x01 0x00 0x01 <1020 bytes>
+                                        0x03 0x00 0x00 0x01
+0x03 0x01 0x00 0x02 <1020 bytes>
+                                        0x03 0x00 0x00 0x02
+0x03 0x00 0x00 0x03 <60 bytes>
+                                        0x03 0x00 0x00 0x03
+0x03 0x00 0x00 0x04
+                                        0x03 0x00 0x00 0x04 OKAY
+
+----------------------------------------------------------------------
+[Unknown ID error, S = 0x0000]
+ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
+----------------------------------------------------------------------
+0x10  0x00  0x00  0x00
+                                        0x00  0x00  0x00  0x00  <error message>
+
+----------------------------------------------------------------------
+[Host packet loss and retransmission, S = 0x0000]
+ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
+----------------------------------------------------------------------
+0x03  0x00  0x00  0x00  getvar:version [lost]
+0x03  0x00  0x00  0x00  getvar:version [lost]
+0x03  0x00  0x00  0x00  getvar:version
+                                        0x03  0x00  0x00  0x00
+0x03  0x00  0x00  0x01
+                                        0x03  0x00  0x00  0x01  OKAY0.4
+
+----------------------------------------------------------------------
+[Client packet loss and retransmission, S = 0x0000]
+ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
+----------------------------------------------------------------------
+0x03  0x00  0x00  0x00  getvar:version
+                                        0x03  0x00  0x00  0x00 [lost]
+0x03  0x00  0x00  0x00  getvar:version
+                                        0x03  0x00  0x00  0x00 [lost]
+0x03  0x00  0x00  0x00  getvar:version
+                                        0x03  0x00  0x00  0x00
+0x03  0x00  0x00  0x01
+                                        0x03  0x00  0x00  0x01  OKAY0.4
+
+----------------------------------------------------------------------
+[Host packet delayed, S = 0x0000]
+ID    Flags SeqH  SeqL  Data            ID    Flags SeqH  SeqL  Data
+----------------------------------------------------------------------
+0x03  0x00  0x00  0x00  getvar:version [delayed]
+0x03  0x00  0x00  0x00  getvar:version
+                                        0x03  0x00  0x00  0x00
+0x03  0x00  0x00  0x01
+                                        0x03  0x00  0x00  0x01  OKAY0.4
+0x03  0x00  0x00  0x00  getvar:version [arrives late with old seq#, is ignored]
diff --git a/fastboot/socket.cpp b/fastboot/socket.cpp
index d49f47f..14ecd93 100644
--- a/fastboot/socket.cpp
+++ b/fastboot/socket.cpp
@@ -48,18 +48,6 @@
     return ret;
 }
 
-bool Socket::SetReceiveTimeout(int timeout_ms) {
-    if (timeout_ms != receive_timeout_ms_) {
-        if (socket_set_receive_timeout(sock_, timeout_ms) == 0) {
-            receive_timeout_ms_ = timeout_ms;
-            return true;
-        }
-        return false;
-    }
-
-    return true;
-}
-
 ssize_t Socket::ReceiveAll(void* data, size_t length, int timeout_ms) {
     size_t total = 0;
 
@@ -82,6 +70,40 @@
     return socket_get_local_port(sock_);
 }
 
+// According to Windows setsockopt() documentation, if a Windows socket times out during send() or
+// recv() the state is indeterminate and should not be used. Our UDP protocol relies on being able
+// to re-send after a timeout, so we must use select() rather than SO_RCVTIMEO.
+// See https://msdn.microsoft.com/en-us/library/windows/desktop/ms740476(v=vs.85).aspx.
+bool Socket::WaitForRecv(int timeout_ms) {
+    receive_timed_out_ = false;
+
+    // In our usage |timeout_ms| <= 0 means block forever, so just return true immediately and let
+    // the subsequent recv() do the blocking.
+    if (timeout_ms <= 0) {
+        return true;
+    }
+
+    // select() doesn't always check this case and will block for |timeout_ms| if we let it.
+    if (sock_ == INVALID_SOCKET) {
+        return false;
+    }
+
+    fd_set read_set;
+    FD_ZERO(&read_set);
+    FD_SET(sock_, &read_set);
+
+    timeval timeout;
+    timeout.tv_sec = timeout_ms / 1000;
+    timeout.tv_usec = (timeout_ms % 1000) * 1000;
+
+    int result = TEMP_FAILURE_RETRY(select(sock_ + 1, &read_set, nullptr, nullptr, &timeout));
+
+    if (result == 0) {
+        receive_timed_out_ = true;
+    }
+    return result == 1;
+}
+
 // Implements the Socket interface for UDP.
 class UdpSocket : public Socket {
   public:
@@ -127,7 +149,7 @@
 }
 
 ssize_t UdpSocket::Receive(void* data, size_t length, int timeout_ms) {
-    if (!SetReceiveTimeout(timeout_ms)) {
+    if (!WaitForRecv(timeout_ms)) {
         return -1;
     }
 
@@ -206,7 +228,7 @@
 }
 
 ssize_t TcpSocket::Receive(void* data, size_t length, int timeout_ms) {
-    if (!SetReceiveTimeout(timeout_ms)) {
+    if (!WaitForRecv(timeout_ms)) {
         return -1;
     }
 
diff --git a/fastboot/socket.h b/fastboot/socket.h
index c0bd7c9..de543db 100644
--- a/fastboot/socket.h
+++ b/fastboot/socket.h
@@ -81,13 +81,17 @@
     virtual bool Send(std::vector<cutils_socket_buffer_t> buffers) = 0;
 
     // Waits up to |timeout_ms| to receive up to |length| bytes of data. |timout_ms| of 0 will
-    // block forever. Returns the number of bytes received or -1 on error/timeout. On timeout
-    // errno will be set to EAGAIN or EWOULDBLOCK.
+    // block forever. Returns the number of bytes received or -1 on error/timeout; see
+    // ReceiveTimedOut() to distinguish between the two.
     virtual ssize_t Receive(void* data, size_t length, int timeout_ms) = 0;
 
     // Calls Receive() until exactly |length| bytes have been received or an error occurs.
     virtual ssize_t ReceiveAll(void* data, size_t length, int timeout_ms);
 
+    // Returns true if the last Receive() call timed out normally and can be retried; fatal errors
+    // or successful reads will return false.
+    bool ReceiveTimedOut() { return receive_timed_out_; }
+
     // Closes the socket. Returns 0 on success, -1 on error.
     virtual int Close();
 
@@ -102,10 +106,13 @@
     // Protected constructor to force factory function use.
     Socket(cutils_socket_t sock);
 
-    // Update the socket receive timeout if necessary.
-    bool SetReceiveTimeout(int timeout_ms);
+    // Blocks up to |timeout_ms| until a read is possible on |sock_|, and sets |receive_timed_out_|
+    // as appropriate to help distinguish between normal timeouts and fatal errors. Returns true if
+    // a subsequent recv() on |sock_| will complete without blocking or if |timeout_ms| <= 0.
+    bool WaitForRecv(int timeout_ms);
 
     cutils_socket_t sock_ = INVALID_SOCKET;
+    bool receive_timed_out_ = false;
 
     // Non-class functions we want to override during tests to verify functionality. Implementation
     // should call this rather than using socket_send_buffers() directly.
@@ -113,8 +120,6 @@
             socket_send_buffers_function_ = &socket_send_buffers;
 
   private:
-    int receive_timeout_ms_ = 0;
-
     FRIEND_TEST(SocketTest, TestTcpSendBuffers);
     FRIEND_TEST(SocketTest, TestUdpSendBuffers);
 
diff --git a/fastboot/socket_mock.cpp b/fastboot/socket_mock.cpp
index c962f30..2531b53 100644
--- a/fastboot/socket_mock.cpp
+++ b/fastboot/socket_mock.cpp
@@ -55,7 +55,7 @@
         return false;
     }
 
-    bool return_value = events_.front().return_value;
+    bool return_value = events_.front().status;
     events_.pop();
     return return_value;
 }
@@ -76,21 +76,28 @@
         return -1;
     }
 
-    if (events_.front().type != EventType::kReceive) {
+    const Event& event = events_.front();
+    if (event.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;
+    const std::string& message = event.message;
+    if (message.length() > length) {
+        ADD_FAILURE() << "Receive(): not enough bytes (" << length << ") for " << message;
         return -1;
     }
 
-    ssize_t return_value = events_.front().return_value;
-    if (return_value > 0) {
-        memcpy(data, events_.front().message.data(), return_value);
+    receive_timed_out_ = event.status;
+    ssize_t return_value = message.length();
+
+    // Empty message indicates failure.
+    if (message.empty()) {
+        return_value = -1;
+    } else {
+        memcpy(data, message.data(), message.length());
     }
+
     events_.pop();
     return return_value;
 }
@@ -124,18 +131,21 @@
 }
 
 void SocketMock::AddReceive(std::string message) {
-    ssize_t return_value = message.length();
-    events_.push(Event(EventType::kReceive, std::move(message), return_value, nullptr));
+    events_.push(Event(EventType::kReceive, std::move(message), false, nullptr));
+}
+
+void SocketMock::AddReceiveTimeout() {
+    events_.push(Event(EventType::kReceive, "", true, nullptr));
 }
 
 void SocketMock::AddReceiveFailure() {
-    events_.push(Event(EventType::kReceive, "", -1, nullptr));
+    events_.push(Event(EventType::kReceive, "", false, nullptr));
 }
 
 void SocketMock::AddAccept(std::unique_ptr<Socket> sock) {
-    events_.push(Event(EventType::kAccept, "", 0, std::move(sock)));
+    events_.push(Event(EventType::kAccept, "", false, std::move(sock)));
 }
 
-SocketMock::Event::Event(EventType _type, std::string _message, ssize_t _return_value,
+SocketMock::Event::Event(EventType _type, std::string _message, ssize_t _status,
                          std::unique_ptr<Socket> _sock)
-        : type(_type), message(_message), return_value(_return_value), sock(std::move(_sock)) {}
+        : type(_type), message(_message), status(_status), sock(std::move(_sock)) {}
diff --git a/fastboot/socket_mock.h b/fastboot/socket_mock.h
index 41fe06d..eacd6bb 100644
--- a/fastboot/socket_mock.h
+++ b/fastboot/socket_mock.h
@@ -71,7 +71,10 @@
     // Adds data to provide for Receive().
     void AddReceive(std::string message);
 
-    // Adds a Receive() failure.
+    // Adds a Receive() timeout after which ReceiveTimedOut() will return true.
+    void AddReceiveTimeout();
+
+    // Adds a Receive() failure after which ReceiveTimedOut() will return false.
     void AddReceiveFailure();
 
     // Adds a Socket to return from Accept().
@@ -81,12 +84,12 @@
     enum class EventType { kSend, kReceive, kAccept };
 
     struct Event {
-        Event(EventType _type, std::string _message, ssize_t _return_value,
+        Event(EventType _type, std::string _message, ssize_t _status,
               std::unique_ptr<Socket> _sock);
 
         EventType type;
         std::string message;
-        ssize_t return_value;
+        bool status;  // Return value for Send() or timeout status for Receive().
         std::unique_ptr<Socket> sock;
     };
 
diff --git a/fastboot/socket_test.cpp b/fastboot/socket_test.cpp
index cc71075..affbdfd 100644
--- a/fastboot/socket_test.cpp
+++ b/fastboot/socket_test.cpp
@@ -28,7 +28,8 @@
 #include <gtest/gtest-spi.h>
 #include <gtest/gtest.h>
 
-enum { kTestTimeoutMs = 3000 };
+static constexpr int kShortTimeoutMs = 10;
+static constexpr int kTestTimeoutMs = 3000;
 
 // Creates connected sockets |server| and |client|. Returns true on success.
 bool MakeConnectedSockets(Socket::Protocol protocol, std::unique_ptr<Socket>* server,
@@ -87,6 +88,50 @@
     }
 }
 
+TEST(SocketTest, TestReceiveTimeout) {
+    std::unique_ptr<Socket> server, client;
+    char buffer[16];
+
+    for (Socket::Protocol protocol : {Socket::Protocol::kUdp, Socket::Protocol::kTcp}) {
+        ASSERT_TRUE(MakeConnectedSockets(protocol, &server, &client));
+
+        EXPECT_EQ(-1, server->Receive(buffer, sizeof(buffer), kShortTimeoutMs));
+        EXPECT_TRUE(server->ReceiveTimedOut());
+
+        EXPECT_EQ(-1, client->Receive(buffer, sizeof(buffer), kShortTimeoutMs));
+        EXPECT_TRUE(client->ReceiveTimedOut());
+    }
+
+    // UDP will wait for timeout if the other side closes.
+    ASSERT_TRUE(MakeConnectedSockets(Socket::Protocol::kUdp, &server, &client));
+    EXPECT_EQ(0, server->Close());
+    EXPECT_EQ(-1, client->Receive(buffer, sizeof(buffer), kShortTimeoutMs));
+    EXPECT_TRUE(client->ReceiveTimedOut());
+}
+
+TEST(SocketTest, TestReceiveFailure) {
+    std::unique_ptr<Socket> server, client;
+    char buffer[16];
+
+    for (Socket::Protocol protocol : {Socket::Protocol::kUdp, Socket::Protocol::kTcp}) {
+        ASSERT_TRUE(MakeConnectedSockets(protocol, &server, &client));
+
+        EXPECT_EQ(0, server->Close());
+        EXPECT_EQ(-1, server->Receive(buffer, sizeof(buffer), kTestTimeoutMs));
+        EXPECT_FALSE(server->ReceiveTimedOut());
+
+        EXPECT_EQ(0, client->Close());
+        EXPECT_EQ(-1, client->Receive(buffer, sizeof(buffer), kTestTimeoutMs));
+        EXPECT_FALSE(client->ReceiveTimedOut());
+    }
+
+    // TCP knows right away when the other side closes and returns 0 to indicate EOF.
+    ASSERT_TRUE(MakeConnectedSockets(Socket::Protocol::kTcp, &server, &client));
+    EXPECT_EQ(0, server->Close());
+    EXPECT_EQ(0, client->Receive(buffer, sizeof(buffer), kTestTimeoutMs));
+    EXPECT_FALSE(client->ReceiveTimedOut());
+}
+
 // Tests sending and receiving large packets.
 TEST(SocketTest, TestLargePackets) {
     std::string message(1024, '\0');
@@ -290,6 +335,11 @@
 
     mock->AddReceiveFailure();
     EXPECT_FALSE(ReceiveString(mock, "foo"));
+    EXPECT_FALSE(mock->ReceiveTimedOut());
+
+    mock->AddReceiveTimeout();
+    EXPECT_FALSE(ReceiveString(mock, "foo"));
+    EXPECT_TRUE(mock->ReceiveTimedOut());
 
     mock->AddReceive("foo");
     mock->AddReceiveFailure();
diff --git a/fastboot/tcp.cpp b/fastboot/tcp.cpp
index da2880a..e42c4e1 100644
--- a/fastboot/tcp.cpp
+++ b/fastboot/tcp.cpp
@@ -28,6 +28,7 @@
 
 #include "tcp.h"
 
+#include <android-base/parseint.h>
 #include <android-base/stringprintf.h>
 
 namespace tcp {
@@ -98,7 +99,8 @@
         return false;
     }
 
-    char buffer[kHandshakeLength];
+    char buffer[kHandshakeLength + 1];
+    buffer[kHandshakeLength] = '\0';
     if (socket_->ReceiveAll(buffer, kHandshakeLength, kHandshakeTimeoutMs) != kHandshakeLength) {
         *error = android::base::StringPrintf(
                 "No initialization message received (%s). Target may not support TCP fastboot",
@@ -111,9 +113,10 @@
         return false;
     }
 
-    if (memcmp(buffer + 2, "01", 2) != 0) {
+    int version = 0;
+    if (!android::base::ParseInt(buffer + 2, &version) || version < kProtocolVersion) {
         *error = android::base::StringPrintf("Unknown TCP protocol version %s (host version %02d)",
-                                             std::string(buffer + 2, 2).c_str(), kProtocolVersion);
+                                             buffer + 2, kProtocolVersion);
         return false;
     }
 
diff --git a/fastboot/tcp_test.cpp b/fastboot/tcp_test.cpp
index 7d80d76..6e867ae 100644
--- a/fastboot/tcp_test.cpp
+++ b/fastboot/tcp_test.cpp
@@ -42,6 +42,16 @@
     EXPECT_EQ("", error);
 }
 
+TEST(TcpConnectTest, TestNewerVersionSuccess) {
+    std::unique_ptr<SocketMock> mock(new SocketMock);
+    mock->ExpectSend("FB01");
+    mock->AddReceive("FB99");
+
+    std::string error;
+    EXPECT_NE(nullptr, tcp::internal::Connect(std::move(mock), &error));
+    EXPECT_EQ("", error);
+}
+
 TEST(TcpConnectTest, TestSendFailure) {
     std::unique_ptr<SocketMock> mock(new SocketMock);
     mock->ExpectSendFailure("FB01");
@@ -74,11 +84,11 @@
 TEST(TcpConnectTest, TestUnknownVersionFailure) {
     std::unique_ptr<SocketMock> mock(new SocketMock);
     mock->ExpectSend("FB01");
-    mock->AddReceive("FB02");
+    mock->AddReceive("FB00");
 
     std::string error;
     EXPECT_EQ(nullptr, tcp::internal::Connect(std::move(mock), &error));
-    EXPECT_EQ("Unknown TCP protocol version 02 (host version 01)", error);
+    EXPECT_EQ("Unknown TCP protocol version 00 (host version 01)", error);
 }
 
 // Fixture to configure a SocketMock for a successful TCP connection.
diff --git a/fastboot/udp.cpp b/fastboot/udp.cpp
new file mode 100644
index 0000000..b36bd60
--- /dev/null
+++ b/fastboot/udp.cpp
@@ -0,0 +1,391 @@
+/*
+ * Copyright (C) 2015 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.
+ */
+
+// This file implements the fastboot UDP protocol; see fastboot_protocol.txt for documentation.
+
+#include "udp.h"
+
+#include <errno.h>
+#include <stdio.h>
+
+#include <list>
+#include <memory>
+#include <vector>
+
+#include <android-base/macros.h>
+#include <android-base/stringprintf.h>
+
+#include "socket.h"
+
+namespace udp {
+
+using namespace internal;
+
+constexpr size_t kMinPacketSize = 512;
+constexpr size_t kHeaderSize = 4;
+
+enum Index {
+    kIndexId = 0,
+    kIndexFlags = 1,
+    kIndexSeqH = 2,
+    kIndexSeqL = 3,
+};
+
+// Extracts a big-endian uint16_t from a byte array.
+static uint16_t ExtractUint16(const uint8_t* bytes) {
+    return (static_cast<uint16_t>(bytes[0]) << 8) | bytes[1];
+}
+
+// Packet header handling.
+class Header {
+  public:
+    Header();
+    ~Header() = default;
+
+    uint8_t id() const { return bytes_[kIndexId]; }
+    const uint8_t* bytes() const { return bytes_; }
+
+    void Set(uint8_t id, uint16_t sequence, Flag flag);
+
+    // Checks whether |response| is a match for this header.
+    bool Matches(const uint8_t* response);
+
+  private:
+    uint8_t bytes_[kHeaderSize];
+};
+
+Header::Header() {
+    Set(kIdError, 0, kFlagNone);
+}
+
+void Header::Set(uint8_t id, uint16_t sequence, Flag flag) {
+    bytes_[kIndexId] = id;
+    bytes_[kIndexFlags] = flag;
+    bytes_[kIndexSeqH] = sequence >> 8;
+    bytes_[kIndexSeqL] = sequence;
+}
+
+bool Header::Matches(const uint8_t* response) {
+    // Sequence numbers must be the same to match, but the response ID can either be the same
+    // or an error response which is always accepted.
+    return bytes_[kIndexSeqH] == response[kIndexSeqH] &&
+           bytes_[kIndexSeqL] == response[kIndexSeqL] &&
+           (bytes_[kIndexId] == response[kIndexId] || response[kIndexId] == kIdError);
+}
+
+// Implements the Transport interface to work with the fastboot engine.
+class UdpTransport : public Transport {
+  public:
+    // Factory function so we can return nullptr if initialization fails.
+    static std::unique_ptr<UdpTransport> NewTransport(std::unique_ptr<Socket> socket,
+                                                      std::string* error);
+    ~UdpTransport() override = default;
+
+    ssize_t Read(void* data, size_t length) override;
+    ssize_t Write(const void* data, size_t length) override;
+    int Close() override;
+
+  private:
+    UdpTransport(std::unique_ptr<Socket> socket) : socket_(std::move(socket)) {}
+
+    // Performs the UDP initialization procedure. Returns true on success.
+    bool InitializeProtocol(std::string* error);
+
+    // Sends |length| bytes from |data| and waits for the response packet up to |attempts| times.
+    // Continuation packets are handled automatically and any return data is written to |rx_data|.
+    // Excess bytes that cannot fit in |rx_data| are dropped.
+    // On success, returns the number of response data bytes received, which may be greater than
+    // |rx_length|. On failure, returns -1 and fills |error| on failure.
+    ssize_t SendData(Id id, const uint8_t* tx_data, size_t tx_length, uint8_t* rx_data,
+                     size_t rx_length, int attempts, std::string* error);
+
+    // Helper for SendData(); sends a single packet and handles the response. |header| specifies
+    // the initial outgoing packet information but may be modified by this function.
+    ssize_t SendSinglePacketHelper(Header* header, const uint8_t* tx_data, size_t tx_length,
+                                   uint8_t* rx_data, size_t rx_length, int attempts,
+                                   std::string* error);
+
+    std::unique_ptr<Socket> socket_;
+    int sequence_ = -1;
+    size_t max_data_length_ = kMinPacketSize - kHeaderSize;
+    std::vector<uint8_t> rx_packet_;
+
+    DISALLOW_COPY_AND_ASSIGN(UdpTransport);
+};
+
+std::unique_ptr<UdpTransport> UdpTransport::NewTransport(std::unique_ptr<Socket> socket,
+                                                         std::string* error) {
+    std::unique_ptr<UdpTransport> transport(new UdpTransport(std::move(socket)));
+
+    if (!transport->InitializeProtocol(error)) {
+        return nullptr;
+    }
+
+    return transport;
+}
+
+bool UdpTransport::InitializeProtocol(std::string* error) {
+    uint8_t rx_data[4];
+
+    sequence_ = 0;
+    rx_packet_.resize(kMinPacketSize);
+
+    // First send the query packet to sync with the target. Only attempt this a small number of
+    // times so we can fail out quickly if the target isn't available.
+    ssize_t rx_bytes = SendData(kIdDeviceQuery, nullptr, 0, rx_data, sizeof(rx_data),
+                                kMaxConnectAttempts, error);
+    if (rx_bytes == -1) {
+        return false;
+    } else if (rx_bytes < 2) {
+        *error = "invalid query response from target";
+        return false;
+    }
+    // The first two bytes contain the next expected sequence number.
+    sequence_ = ExtractUint16(rx_data);
+
+    // Now send the initialization packet with our version and maximum packet size.
+    uint8_t init_data[] = {kProtocolVersion >> 8, kProtocolVersion & 0xFF,
+                           kHostMaxPacketSize >> 8, kHostMaxPacketSize & 0xFF};
+    rx_bytes = SendData(kIdInitialization, init_data, sizeof(init_data), rx_data, sizeof(rx_data),
+                        kMaxTransmissionAttempts, error);
+    if (rx_bytes == -1) {
+        return false;
+    } else if (rx_bytes < 4) {
+        *error = "invalid initialization response from target";
+        return false;
+    }
+
+    // The first two data bytes contain the version, the second two bytes contain the target max
+    // supported packet size, which must be at least 512 bytes.
+    uint16_t version = ExtractUint16(rx_data);
+    if (version < kProtocolVersion) {
+        *error = android::base::StringPrintf("target reported invalid protocol version %d",
+                                             version);
+        return false;
+    }
+    uint16_t packet_size = ExtractUint16(rx_data + 2);
+    if (packet_size < kMinPacketSize) {
+        *error = android::base::StringPrintf("target reported invalid packet size %d", packet_size);
+        return false;
+    }
+
+    packet_size = std::min(kHostMaxPacketSize, packet_size);
+    max_data_length_ = packet_size - kHeaderSize;
+    rx_packet_.resize(packet_size);
+
+    return true;
+}
+
+// SendData() is just responsible for chunking |data| into packets until it's all been sent.
+// Per-packet timeout/retransmission logic is done in SendSinglePacketHelper().
+ssize_t UdpTransport::SendData(Id id, const uint8_t* tx_data, size_t tx_length, uint8_t* rx_data,
+                               size_t rx_length, int attempts, std::string* error) {
+    if (socket_ == nullptr) {
+        *error = "socket is closed";
+        return -1;
+    }
+
+    Header header;
+    size_t packet_data_length;
+    ssize_t ret = 0;
+    // We often send header-only packets with no data as part of the protocol, so always send at
+    // least once even if |length| == 0, then repeat until we've sent all of |data|.
+    do {
+        // Set the continuation flag and truncate packet data if needed.
+        if (tx_length > max_data_length_) {
+            packet_data_length = max_data_length_;
+            header.Set(id, sequence_, kFlagContinuation);
+        } else {
+            packet_data_length = tx_length;
+            header.Set(id, sequence_, kFlagNone);
+        }
+
+        ssize_t bytes = SendSinglePacketHelper(&header, tx_data, packet_data_length, rx_data,
+                                               rx_length, attempts, error);
+
+        // Advance our read and write buffers for the next packet. Keep going even if we run out
+        // of receive buffer space so we can detect overflows.
+        if (bytes == -1) {
+            return -1;
+        } else if (static_cast<size_t>(bytes) < rx_length) {
+            rx_data += bytes;
+            rx_length -= bytes;
+        } else {
+            rx_data = nullptr;
+            rx_length = 0;
+        }
+
+        tx_length -= packet_data_length;
+        tx_data += packet_data_length;
+
+        ret += bytes;
+    } while (tx_length > 0);
+
+    return ret;
+}
+
+ssize_t UdpTransport::SendSinglePacketHelper(
+        Header* header, const uint8_t* tx_data, size_t tx_length, uint8_t* rx_data,
+        size_t rx_length, const int attempts, std::string* error) {
+    ssize_t total_data_bytes = 0;
+    error->clear();
+
+    int attempts_left = attempts;
+    while (attempts_left > 0) {
+        if (!socket_->Send({{header->bytes(), kHeaderSize}, {tx_data, tx_length}})) {
+            *error = Socket::GetErrorMessage();
+            return -1;
+        }
+
+        // Keep receiving until we get a matching response or we timeout.
+        ssize_t bytes = 0;
+        do {
+            bytes = socket_->Receive(rx_packet_.data(), rx_packet_.size(), kResponseTimeoutMs);
+            if (bytes == -1) {
+                if (socket_->ReceiveTimedOut()) {
+                    break;
+                }
+                *error = Socket::GetErrorMessage();
+                return -1;
+            } else if (bytes < static_cast<ssize_t>(kHeaderSize)) {
+                *error = "protocol error: incomplete header";
+                return -1;
+            }
+        } while (!header->Matches(rx_packet_.data()));
+
+        if (socket_->ReceiveTimedOut()) {
+            --attempts_left;
+            continue;
+        }
+        ++sequence_;
+
+        // Save to |error| or |rx_data| as appropriate.
+        if (rx_packet_[kIndexId] == kIdError) {
+            error->append(rx_packet_.data() + kHeaderSize, rx_packet_.data() + bytes);
+        } else {
+            total_data_bytes += bytes - kHeaderSize;
+            size_t rx_data_bytes = std::min<size_t>(bytes - kHeaderSize, rx_length);
+            if (rx_data_bytes > 0) {
+                memcpy(rx_data, rx_packet_.data() + kHeaderSize, rx_data_bytes);
+                rx_data += rx_data_bytes;
+                rx_length -= rx_data_bytes;
+            }
+        }
+
+        // If the response has a continuation flag we need to prompt for more data by sending
+        // an empty packet.
+        if (rx_packet_[kIndexFlags] & kFlagContinuation) {
+            // We got a valid response so reset our attempt counter.
+            attempts_left = attempts;
+            header->Set(rx_packet_[kIndexId], sequence_, kFlagNone);
+            tx_data = nullptr;
+            tx_length = 0;
+            continue;
+        }
+
+        break;
+    }
+
+    if (attempts_left <= 0) {
+        *error = "no response from target";
+        return -1;
+    }
+
+    if (rx_packet_[kIndexId] == kIdError) {
+        *error = "target reported error: " + *error;
+        return -1;
+    }
+
+    return total_data_bytes;
+}
+
+ssize_t UdpTransport::Read(void* data, size_t length) {
+    // Read from the target by sending an empty packet.
+    std::string error;
+    ssize_t bytes = SendData(kIdFastboot, nullptr, 0, reinterpret_cast<uint8_t*>(data), length,
+                             kMaxTransmissionAttempts, &error);
+
+    if (bytes == -1) {
+        fprintf(stderr, "UDP error: %s\n", error.c_str());
+        return -1;
+    } else if (static_cast<size_t>(bytes) > length) {
+        // Fastboot protocol error: the target sent more data than our fastboot engine was prepared
+        // to receive.
+        fprintf(stderr, "UDP error: receive overflow, target sent too much fastboot data\n");
+        return -1;
+    }
+
+    return bytes;
+}
+
+ssize_t UdpTransport::Write(const void* data, size_t length) {
+    std::string error;
+    ssize_t bytes = SendData(kIdFastboot, reinterpret_cast<const uint8_t*>(data), length, nullptr,
+                             0, kMaxTransmissionAttempts, &error);
+
+    if (bytes == -1) {
+        fprintf(stderr, "UDP error: %s\n", error.c_str());
+        return -1;
+    } else if (bytes > 0) {
+        // UDP protocol error: only empty ACK packets are allowed when writing to a device.
+        fprintf(stderr, "UDP error: target sent fastboot data out-of-turn\n");
+        return -1;
+    }
+
+    return length;
+}
+
+int UdpTransport::Close() {
+    if (socket_ == nullptr) {
+        return 0;
+    }
+
+    int result = socket_->Close();
+    socket_.reset();
+    return result;
+}
+
+std::unique_ptr<Transport> Connect(const std::string& hostname, int port, std::string* error) {
+    return internal::Connect(Socket::NewClient(Socket::Protocol::kUdp, hostname, port, error),
+                             error);
+}
+
+namespace internal {
+
+std::unique_ptr<Transport> Connect(std::unique_ptr<Socket> sock, std::string* error) {
+    if (sock == nullptr) {
+        // If Socket creation failed |error| is already set.
+        return nullptr;
+    }
+
+    return UdpTransport::NewTransport(std::move(sock), error);
+}
+
+}  // namespace internal
+
+}  // namespace udp
diff --git a/fastboot/udp.h b/fastboot/udp.h
new file mode 100644
index 0000000..14f5b35
--- /dev/null
+++ b/fastboot/udp.h
@@ -0,0 +1,81 @@
+/*
+ * Copyright (C) 2015 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 UDP_H_
+#define UDP_H_
+
+#include <memory>
+#include <string>
+
+#include "socket.h"
+#include "transport.h"
+
+namespace udp {
+
+constexpr int kDefaultPort = 5554;
+
+// Returns a newly allocated Transport object connected to |hostname|:|port|. On failure, |error| is
+// filled and nullptr is returned.
+std::unique_ptr<Transport> Connect(const std::string& hostname, int port, std::string* error);
+
+// Internal namespace for test use only.
+namespace internal {
+
+constexpr uint16_t kProtocolVersion = 1;
+
+// This will be negotiated with the device so may end up being smaller.
+constexpr uint16_t kHostMaxPacketSize = 8192;
+
+// Retransmission constants. Retransmission timeout must be at least 500ms, and the host must
+// attempt to send packets for at least 1 minute once the device has connected. See
+// fastboot_protocol.txt for more information.
+constexpr int kResponseTimeoutMs = 500;
+constexpr int kMaxConnectAttempts = 4;
+constexpr int kMaxTransmissionAttempts = 60 * 1000 / kResponseTimeoutMs;
+
+enum Id : uint8_t {
+    kIdError = 0x00,
+    kIdDeviceQuery = 0x01,
+    kIdInitialization = 0x02,
+    kIdFastboot = 0x03
+};
+
+enum Flag : uint8_t {
+    kFlagNone = 0x00,
+    kFlagContinuation = 0x01
+};
+
+// Creates a UDP Transport object using a given Socket. Used for unit tests to create a Transport
+// object that uses a SocketMock.
+std::unique_ptr<Transport> Connect(std::unique_ptr<Socket> sock, std::string* error);
+
+}  // namespace internal
+
+}  // namespace udp
+
+#endif  // UDP_H_
diff --git a/fastboot/udp_test.cpp b/fastboot/udp_test.cpp
new file mode 100644
index 0000000..ff8cf0f
--- /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)));
+}