polypacket 0.7

A tool for building protocol services

PolyPacket

Poly Packet is backend and code generation tool for creating messaging protocols/Services. Protocols are described in an XML document which can be easily shared with all components of a system.

A python script is used to parse the XML file and generate code as well as documentation. The code generation tool can create the back end service, app layer, and even an entire linux utility app

Protocol Generation

Protocols are generated using XML. The messaging structure is made up 4 entity types:

• Field
• Packet
• Val
• Struct

Fields

A field is a data object within a message

example field:

<Field name="src" type="uint16_t" format="hex" desc="Source address of message" />

name: The name of the field
type: The data type for the field
format: (optional) This sets the display format used for the toString and toJsonString methods [ hex , dec , assci ]
desc: (optional) The description of the field. This is used to create the documentation

Packets

A Packet describes an entire message and is made up of fields

example Packet:

<Packet name="GetData" desc="Message to get data from node" response="RespData">
  <Field name="src" req="true"/>
  <Field name="dst" req="true" desc="address of node to retrieve data from"/>
</Packet>

name: The name of the packet
desc: (optional) description of the packet for documentation
response: (optional) name of the packet type expected in response to this message (if any)

within the packet we reference Fields which have already been declared in the Fields section. these references contain 3 attributes:

name: The name of the field
req: (optional) makes the field a requirement for this packet type
desc: (optional) description of this field for this packet type, will override fields description in the documentation for this packet type only

Val

Val entities are used for defining options in enum and flags fields.

<Field name="cmd" type="enum" format="hex" desc="Command for device">
  <Val name="led_ON" desc="turns on led" />
  <Val name="led_OFF" desc="turns off led" />
  <Val name="reset" desc="resets the device" />
</Field>

In this example an enum is used to set up some predefined options for the cmd field. enums are created with sequential values starting at 0. a flags field is defined in the same way, but instead of sequential numbers, it shifts bits to the left, to create a group of individually set-able flags.

Struct

Structs are essentially the same thing as packets in that they are a collection of fields. The only real difference is the name, and that the code generation tool will create classes for structs.

  <Packets>

    <Packet name="getData" desc="Get values of remote node" response="data"/>

    <Packet name="data" desc="Set values of remote node">
      <Field name="light"/>
    </Packet>

  </Packets>
  <Structs>
    <Struct name="node" desc="Struct for local node properties">
      <Field name="light"/>
    </Struct>
  </Structs>

The idea being that a Struct will hold data locally, and can be easily updated from a message. Any packet can be copied to another packet using the packet copy funtion. this function can be used with any 2 packets/structs but will only copy the fields that have in common:

sp_struct_t thisNode; //must be initialized with sp_struct_build(&thisNode, SP_STRUCT_NODE);

HandlerStatus_e sp_Data_handler(sp_packet_t* sp_data)
{

  sp_packet_copy(&thisNode, sf_data); //update thisNode from incoming data packet

  return PACKET_HANDLED;
}

HandlerStatus_e sp_GetData_handler(sp_packet_t* sp_getData, sp_packet_t* sp_data)
{

  sp_packet_copy( sp_data, &thisNode);  //update data packet with fields from thisNode

  return PACKET_HANDLED;
}

sp is just the prefix for the sample protocol

Example:

The following example show the XML for a simple message protocol.

<?xml version="1.0" encoding="UTF-8"?>
<Protocol name="Sample" prefix="sp"
  desc="This is a sample protocol made up to demonstrate features of the PolyPacket code generation tool. The idea
  is to have a tool that can automatically create parseable/serializable messaging for embedded systems.">
  <!--First we declare all Field descriptors-->
  <Fields>

    <!--Common -->
    <Field name="cmd" type="enum" format="hex" desc="Command for device">
      <Val name="led_ON" desc="turns on led" />
      <Val name="led_OFF" desc="turns off led" />
      <Val name="reset" desc="resets the device" />
    </Field>

    <!-- SensorData -->
    <Field name="sensorA" type="int16" format="dec" desc="Value of Sensor A"/>
    <Field name="sensorB" type="int" format="dec" desc="Value of Sensor B" />
    <Field name="sensorName" type="string[32]" format="ascii" desc="Name of sensor"/>

  </Fields>
  <!--Declare all Packet Types-->
  <Packets>
    <Packet name="SendCmd" desc="Message to set command in node" >
      <Field name="cmd" req="true"/>
    </Packet>

    <Packet name="GetData" desc="Message to get data from node" response="Data">
    </Packet>

    <Packet name="Data" desc="Message containing data from sensor" >
      <Field name="sensorA"/>
      <Field name="sensorB"/>
      <!-- Adding a description here will overwrite the description in documentation for this packet type -->
      <Field name="sensorName" desc="Name of sensor responding to message "/>
    </Packet>

  </Packets>
</Protocol>

The XML sets up 3 Fields:

cmd - Command for device
sensorA - Value of Sensor A
sensorB - Value of Sensor B
sensorName - Name of sensor

It then lists the packets, and which fields are in each packet. fields are considered optional in a packet unless specified with req="true". So in this example you could send a Data packet that only contained sensorB and sensorName.

Using Poly Packet

To use poly packet, write your xml to define the fields and packets in your protocol. Then use poly-packet to generate the source code.

the mako module is required (pip install mako)

poly-packet -i sample_protocol.xml -o . -a

• -i is for input file, this will be the xml file used
• -o is the output directory, this is where the code and documentation will be generated
• -a tells the tool to create an application layer for you
• -u specifies a path to create a standalone serial utility for the service

by default all functions will start with the prefix 'pp'. but the 'prefix' attribute can be used in the xml to set a different prefix. this allows the use of multiple services/protocols in a single project without conflict

This example shows how to use the code to create a service. The service is initialized with 1 interface:

Initializing service

sp_service_init(1); //initialize the service with 1 interface

For devices where multiple hardware ports are being used by the same protocol, you can use more interfaces

---

Register Tx functions

For each interface we need to register a send function. This allows us the service to handle the actual sending so we can automate things like acknowledgements and retries. There are two types of send callbacks that can be registered:

typedef HandlerStatus_e (*poly_tx_byte_callback)(uint8_t* data , int len);
typedef HandlerStatus_e (*poly_tx_packet_callback)(poly_packet_t* packet );

The tx_byte callback will pass the packet as an array of COBS encoded bytes which can be sent directly over a serial connection. The tx_packet will pass a reference to the packet itself which can be converted to JSON, or manipulated before sending.

sp_service_register_tx_bytes(0, &uart_send ); // register sending function for raw bytes
sp_service_register_tx_packet(0, &json_send ); // register sending function for entire packet

once we have registered a callback for an interface, we can send messages to it using the quick send functions generated for the service.

sp_sendGetData();

---

Feed the service

The underlying service is responsible for packing and parsing the data. So wherever you read bytes off of the hardware interface, just feed them to the service.

Encoded Bytes

void uart_rx_handler(uint8_t* data, int len)
{
  sp_service_feed(0,datam len);
}

If you are working with JSON you have two options. you can feed the json message to the service for normal handling or call the json handler to bypass the normal service queue. This option make it easy to use the service in synchronous tasks such as responding to an http request

Async JSON

void app_json_async_handler(char* strJson, int len)
{
  sp_service_feed_json(0,strJson, len);
}

Sync JSON

void app_json_sync_handler(const char* strRequest, int len, char* strResp)
{
  HandlerStatus_e status;
  status = sp_handle_json(strRequest, len, strResp);
}

From here the service will take care of parsing the data and dispatching messages to the proper message handler.

---

Sending messages

The service creates one-liner functions for easily sending simple messages

Using the example protocol we can send a message to get data from a remote device on interface 0 with:

sp_sendGetData(0);

for packet types with data fields, the datafields get turned into the arguments for the function

sp_sendData(0, 97, 98, "My Sensor name");

---

Creating a message (Be sure to clean after)

If for some reason you need to build your own message, you can do that as well, but be sure to clean it when you are done.

sp_packet_t msg;
sp_packet_init(&msg,SP_SETDATA_PACKET);

next we set fields in the message

sp_setSrc(msg,0xABCD );
sp_setDst(msg,0xCDEF);

sp_send(0,&msg);

When creating your own packets, make sure they are cleaned up after

sp_clean(&msg);

---

Receive Handlers

The generated service creates a handler for each packet type, they are created with weak attributes, so they can be overridden by just declaring them again in our code. If you specify a response for a packet in the xml, the generated handler will have a pointer to responding packet. As long as the handler returns 'PACKET_HANDLED', the service will send the reply. If no reponse is specified the service will send an 'Ack' packet

The following is our handler for 'SetData' type packets

/**
  *@brief Handler for receiving GetData packets
  *@param GetData incoming GetData packet
  *@param Data Data packet to respond with
  *@return handling status
  */
HandlerStatus_e sp_GetData_handler(sp_packet_t* sp_GetData, sp_packet_t* sp_Data)
{
  sp_setSensorA(sp_Data, 97);
  sp_setSensorB(sp_Data, 98);
  sp_setSensorName(sp_Data, "My sensor");

  return PACKET_HANDLED;
}

Process

The service is meant to be run on many platforms, so it does not have built in threading/tasking. For it to continue handling messages, we have to call its process function either in a thread/task or in our super-loop

while(1)
{
  sp_service_process();
}