Weave Packet Generator

components/proximity_auth/ble/bluetooth_low_energy_weave_packet_generator.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/proximity_auth/ble/bluetooth_low_energy_weave_packet_generator.h"
+
+#include <string.h>
+
+#include <algorithm>
+
+#include "base/logging.h"
+
+namespace {
+
+typedef proximity_auth::BluetoothLowEnergyWeavePacketGenerator::Packet Packet;
+
+const uint16_t kMinSupportedWeaveVersion = 1;
+const uint16_t kMaxSupportedWeaveVersion = 1;
+const uint16_t kServerWeaveVersion = 1;
+const uint16_t kMinConnectionRequestSize = 7;
+const uint16_t kMinConnectionResponseSize = 5;
+const uint16_t kMinConnectionCloseSize = 3;
+const uint32_t kDefaultDataPacketSize = 20;
+// Max packet size is 0, which means defer the decision to the server.
+const uint16_t kMaxSupportedPacketSize = 0;
+const uint8_t kMaxPacketCounter = 8;
+
+}  // namespace
+
+namespace proximity_auth {
+
+// static.
+BluetoothLowEnergyWeavePacketGenerator::Factory*
+    BluetoothLowEnergyWeavePacketGenerator::Factory::factory_instance_ =
+        nullptr;
+
+// static.
+std::unique_ptr<BluetoothLowEnergyWeavePacketGenerator>
+BluetoothLowEnergyWeavePacketGenerator::Factory::NewInstance() {
+  if (factory_instance_ == nullptr) {
+    factory_instance_ = new Factory();
+  }
+  return factory_instance_->BuildInstance();
+}
+
+// static.
+void BluetoothLowEnergyWeavePacketGenerator::Factory::SetInstanceForTesting(
+    Factory* factory) {
+  factory_instance_ = factory;
+}
+
+std::unique_ptr<BluetoothLowEnergyWeavePacketGenerator>
+BluetoothLowEnergyWeavePacketGenerator::Factory::BuildInstance() {
+  return std::unique_ptr<BluetoothLowEnergyWeavePacketGenerator>(
+      new BluetoothLowEnergyWeavePacketGenerator());
+}
+
+BluetoothLowEnergyWeavePacketGenerator::BluetoothLowEnergyWeavePacketGenerator()
+    : packet_size_(kDefaultDataPacketSize), packet_number_(0) {}
+
+Packet BluetoothLowEnergyWeavePacketGenerator::CreateConnectionRequest() {
+  Packet packet = CreateControlPacket(kMinConnectionRequestSize);
+
+  SetControlCommand(&packet, ControlCommand::CONNECTION_REQUEST);

[Kyle Horimoto, 2016/06/17 04:04:12]

Instead of passing |&packet| everywhere, just make the Set*() functions take a
|Packet&| as a parameter and pass |packet|.

[Kyle Horimoto, 2016/06/20 18:01:23]

Ping.

[jingxuy, 2016/06/20 21:29:32]

Is it the convention that if the caller expects a parameter to be modified, it
should be passed around as a pointer.
Maybe instead I could do Packet* packet =
&(CreaterControlPacket(kMinConnectionRequestSize);

[Kyle Horimoto, 2016/06/20 21:52:17]

Ah, you're right. Nope, please leave it as-is. However, the convention is
actually:

FunctionName(<value parameters>, <input parameters>, <output parameters)

So, you should always be passing the packet as the last parameter to these
functions since it is an output parameter.

[Kyle Horimoto, 2016/06/21 02:15:29]

Ping.

[jingxuy, 2016/06/21 06:03:10]

Done.

+  SetShortField(&packet, 1, kMinSupportedWeaveVersion);
+  SetShortField(&packet, 3, kMaxSupportedWeaveVersion);
+  SetShortField(&packet, 5, kMaxSupportedPacketSize);
+
+  return packet;
+}
+
+Packet BluetoothLowEnergyWeavePacketGenerator::CreateConnectionResponse() {
+  Packet packet = CreateControlPacket(kMinConnectionResponseSize);
+
+  SetControlCommand(&packet, ControlCommand::CONNECTION_RESPONSE);
+  SetShortField(&packet, 1, kServerWeaveVersion);
+  SetShortField(&packet, 3, packet_size_);
+
+  return packet;
+}
+
+Packet BluetoothLowEnergyWeavePacketGenerator::CreateConnectionClose(
+    ReasonForClose reason_for_close) {
+  Packet packet = CreateControlPacket(kMinConnectionCloseSize);
+
+  SetControlCommand(&packet, ControlCommand::CONNECTION_CLOSE);
+  SetShortField(&packet, 1, reason_for_close);
+
+  return packet;
+}
+
+Packet BluetoothLowEnergyWeavePacketGenerator::CreateControlPacket(
+    uint16_t size) {
+  Packet packet(size, 0);
+
+  // Packet is a control packet.
+  SetPacketTypeBit(&packet, PacketType::CONTROL);
+  SetPacketCounter(&packet);
+
+  return packet;
+}
+
+void BluetoothLowEnergyWeavePacketGenerator::SetDataPacketSize(uint32_t size) {
+  DCHECK(size >= 2);

[sacomoto, 2016/06/17 15:38:04]

s/2/20.

The BLE specs guarantee at least 20 for the MTU.

[jingxuy, 2016/06/17 18:59:00]

I think 20 is the guaranteed max size, the 2 is just a sanity check for the min
size.

[Kyle Horimoto, 2016/06/20 18:01:23]

Nope - 20 should be the minimum, not the maximum.

[jingxuy, 2016/06/20 21:29:32]

I think 20 is the minimum maximum. As the receiver must be able to receive
packets up to 20 bytes. But I think creating packets of less than 20 bytes is
legal. Though I'm not sure if it's necessary to allow that implementation in the
generator. I left it there for the flexibility

[Kyle Horimoto, 2016/06/20 21:52:17]

I think there's a misunderstanding. We aren't saying that sending a data packet
<20 bytes is illegal - it certainly is legal to do so. This SetDataPacketSize()
function is setting the maximum packet size allowed. By default, this is set to
20 bytes, but this can be increased based on the "connection request" packet.

It might be a good idea to rename this function so that it is clear that you are
setting the size of a "full" packet and that it is still valid to send data
packets which are shorter than that value.

[Kyle Horimoto, 2016/06/21 02:15:29]

Ping.

[jingxuy, 2016/06/21 06:03:10]

I knew what you guys meant. I just think we should restrict the max packet size
sent. But now I think of it, the packet_size_ parameter is also used in control
packet. So I suppose sending something less than 20 doesn't make any sense. So I
will change the DCHECK to be 20

+  packet_size_ = size;
+}
+
+std::vector<Packet> BluetoothLowEnergyWeavePacketGenerator::EncodeDataMessage(
+    std::string message) {

[Kyle Horimoto, 2016/06/17 04:04:12]

Should we throw an error here if message is empty? I don't think it makes sense
to return a packet if there was no message.

[sacomoto, 2016/06/17 15:38:04]

+1

I suggested a DCHECK in an earlier comment, but an error is fine too.

[jingxuy, 2016/06/17 18:59:00]

Done.

+  // The first byte of a packet is used by the uWeave protocol,
+  // hence the payload is 1 byte smaller than the packet size.
+  uint32_t packet_payload_size = packet_size_ - 1;
+
+  uint32_t message_length = message.length();
+  // (packet_payload_size - 1) is used to enforce rounding up.
+  uint32_t num_packets =
+      (message_length + (packet_payload_size - 1)) / packet_payload_size;
+
+  // Empty message resulted in an empty but existent packet.
+  num_packets = std::max(num_packets, static_cast<uint32_t>(1));
+
+  std::vector<Packet> weave_message(num_packets);
+
+  const char* byte_message = message.c_str();
+
+  for (uint32_t i = 0; i < num_packets; ++i) {
+    Packet* packet = &weave_message[i];
+    uint32_t begin = packet_payload_size * i;
+    uint32_t end = std::min(begin + packet_payload_size, message_length);
+
+    packet->push_back(0);
+
+    SetPacketTypeBit(packet, PacketType::DATA);
+    SetPacketCounter(packet);
+
+    for (uint32_t i = begin; i < end; ++i) {
+      packet->push_back(byte_message[i]);
+    }
+  }
+
+  // Guaranteed to have at least one packet.
+  SetDataFirstBit(&weave_message[0]);
+  SetDataLastBit(&weave_message[num_packets - 1]);
+
+  return weave_message;
+}
+
+void BluetoothLowEnergyWeavePacketGenerator::SetShortField(Packet* packet,
+                                                           uint32_t byte_offset,
+                                                           uint16_t val) {
+  DCHECK(packet);
+  DCHECK_LT(byte_offset, packet->size());
+  DCHECK_LT(byte_offset + 1, packet->size());
+
+  uint8_t upper = (val >> 8) & 0xFF;
+  uint8_t lower = val & 0xFF;
+
+  packet->at(byte_offset) = lower;
+  packet->at(byte_offset + 1) = upper;
+}
+
+void BluetoothLowEnergyWeavePacketGenerator::SetPacketTypeBit(Packet* packet,
+                                                              PacketType type) {
+  DCHECK(packet);
+  DCHECK(!packet->empty());
+
+  // Type bit is the highest bit of the first byte of the packet.
+  // So clear the highest bit and set it according to val.
+  packet->at(0) = (packet->at(0) & 0x7F) | (type << 7);
+}
+
+void BluetoothLowEnergyWeavePacketGenerator::SetControlCommand(
+    Packet* packet,
+    ControlCommand command) {
+  DCHECK(packet);
+  DCHECK(!packet->empty());
+
+  // Control Command is the lower 4 bits of the packet's first byte.
+  // So clear the lower 4 bites and set it according to val.
+  packet->at(0) = (packet->at(0) & 0xF0) | command;
+}
+
+void BluetoothLowEnergyWeavePacketGenerator::SetPacketCounter(Packet* packet) {
+  DCHECK(packet);
+  DCHECK(!packet->empty());
+  uint8_t counter = packet_number_ % kMaxPacketCounter;
+
+  // Packet counter is the bits 4, 5, and 6 of the packet's first byte.
+  // So clear those bits and set them according to current packet counter
+  // modular max packet counter.
+  packet->at(0) = (packet->at(0) & 0x8F) | (counter << 4);
+  packet_number_++;
+}
+
+void BluetoothLowEnergyWeavePacketGenerator::SetDataFirstBit(Packet* packet) {
+  DCHECK(packet);
+  DCHECK(!packet->empty());
+
+  // First bit is bit 3 of the packet's first byte and set it to 1.
+  packet->at(0) = packet->at(0) | (1 << 3);
+}
+
+void BluetoothLowEnergyWeavePacketGenerator::SetDataLastBit(Packet* packet) {
+  DCHECK(packet);
+  DCHECK(!packet->empty());
+
+  // Last bit is the bit 2 of the packet's first byte and set it to 1.
+  packet->at(0) = packet->at(0) | (1 << 2);
+}
+
+}  // namespace proximity_auth