fastboot/udp.cpp - android-core - Git at Google

 /*
  * 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
	/*
	* 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