portability/udp_sockets.hpp
Platform-Independent UDP Internet Classes

Introduction

UDP is an internet protocol that is faster and simpler than the more common TCP. It achieves this by being unreliable. Packets may be lost or arrive in a different order to that sent. Why would you use this? There are some applications where performance matters, but reliability doesn't. For example, a live video feed - if a frame gets lost, so what? The most important thing is that the picture stays as up-to-date as possible. It also allows you to design reliability features into your protocol that are faster then TCP's. For example, you could implement a backup system that fires packets to the backup server in a long burst, then gets back a list of all packets received, then resends any that got lost. This might be more efficient than a TCP-based system which has to stop every time a packet is lost whilst the missing packet is re-requested.

This package provides simple, clean and platform-independent interfaces to the Internet protocols provided by UDP. The motivation for writing it was to make these protocols more accessible to the developer. The underlying mechanisms for UDP are very low level and unnecessarily complicated. These classes encapsulate the essence of UDP in two very simple to use classes.

There are two 'ends' to an internet connection, known as the client and the server. A client is a program that requests a connection whilst a server provides that connection. In most cases, developers will only be interested in the client class, since that is all that is needed to build an internet application. However, this package also provides the server class that would be required to build an internet server.

Note: some people might object to this description on the grounds that UDP is often described as "connectionless". This is a misunderstanding since all internet protocols are connectionless; the connection is an abstract concept, but communication is always in packet form. UDP is more accurately described as "stateless" in that the protocol does not assume any relationships between packets and delivers each as a separate entity, rather than the ordered stream of TCP. In fact TCP makes the unordered Internet protocol look ordered by numbering the packets on send, sorting the packets into order on receipt and re-requesting lost packets after a "reasonable" wait. However, despite this, I find it is still helpful in understanding UDP (and TCP) to think of there being a connection from client to server.

The UDP classes only provide the underlying transport mechanism of UDP. It does not provide the information protocols layered onto it. These information protocols could be provided as additional classes layered onto the UDP classes.

The UDP classes are derived from the IP_socket class. This class is not documented separately because it is not intended to be used directly. IP_socket implements low-level socket operations in a platform-independent, object-orientated form. It hides the operating-system differences and the sheer ugliness of the socket interfaces that implement the internet protocol (IP). The UDP classes then inherit from that layer, implementing the UDP functionality using this platform-independent interface to simplify the task. The intention is that you only use the UDP classes and not the base class.

UDP Client

The UDP client is used to create a connection to a UDP server anywhere on the internet. The client is responsible for initiating an internet connection which the server then responds to. In fact, in the UDP protocol, there is no initial communication from client to server, the remote host information is simply stored ready for use. Only when packets are actually sent is a connection made and then only per-packet.

The interface to the UDP_client class is shown below.

class UDP_client
{
public:
  UDP_client(void);
  UDP_client(const std::string& remote_address, unsigned short remote_port, unsigned short local_port = 0);
  UDP_client(unsigned long remote_address, unsigned short remote_port, unsigned short local_port = 0);
  ~UDP_client(void);
  UDP_client(const UDP_client&);
  UDP_client& operator=(const UDP_client&);

  unsigned long ip_lookup(const std::string& remote_address);
  bool initialise(const std::string& remote_address, unsigned short remote_port, unsigned short local_port = 0);
  bool initialise(unsigned long remote_address, unsigned short remote_port, unsigned short local_port = 0);
  bool initialised(void) const;
  bool close(void);

  bool send_ready(unsigned wait = 0);
  bool send (std::string& packet);
  bool receive_ready(unsigned wait = 0);
  bool receive (std::string& packet);

  unsigned short local_port(void) const;
  unsigned long remote_address(void) const;
  unsigned short remote_port(void) const;

  void clear_error (void) const;
  int error(void) const;
  std::string message(void) const;
};

There are two ways of creating an initialised UDP client. You can either create an uninitialised client (calling the void constructor) and then initialise it later, or you can initialise the connection at construction time and then later test to see if it initialised correctly. In the former case, the initialise method will return true if the initialisation succeeded and false otherwise. In the latter case, constructors cannot return a result so you must call the initialised test to see if the connection initialised correctly. If the connection fails, the error method will give an error code corresponding to the cause for the failure and the message method will give a textual representation of the error.

The first initialise method (and the second constructor) takes three arguments. The first is a string containing the internet address to connect to, either as a name (e.g. stlplus.sourceforge.net) or as a number (e.g. 195.0.0.1) and the second is the port to connect to (e.g. 80 for the HTTP service). The third argument is the local port to connect from; if set to zero, the operating system will choose an 'ephemeral' port.

The second initialise method (and the third constructor) also takes three arguments. The first is the internet address to connect to represented as an integer (actually an unsigned long). This integer is the result from the ip_lookup method. It enables the name lookup to be separated out from the actual connection process. The second and third argument, and indeed the operation of the method, is the same as the previous initialisation method.

The UDP client class supports assignment. This is done using a smart pointer to the underlying data structures. Thus, when you assign UDP clients, you simply create multiple pointers to the same data structure. This support for assignment makes it easy to create data structures of UDP clients. For example, to support multiple simultaneous connections, you could have an STL vector of UDP_client objects. Each object could be created outside the vector, initialised and then only copied into to the vector if initialisation worked.

Once a client connection is finished with, it should be closed by calling close. The destructor also calls close, so allowing the client object to be destroyed will close the connection automatically.

The next block of methods actually perform the communication phase of the connection. All communications are done in non-blocking mode, so these methods will not block and therefore will not lock up the program. However, this means that you have to be prepared to retry.

To send data down the connection to the server, use the send method. The string passed to send is consumed by the send method. In other words, if the send succeeds, the string will be empty afterwards. The send method returns false only if an error occurred - failure to send because the server is not ready does not class as an error. All that will happen there is that the method will return true but the string will still contain the same data. You should retry, possibly after a short delay.

You can use the send_ready method to determine whether the server is ready to receive what you send and then call send to actually send the data. The send_ready method allows you to specify a time delay to wait for the server to become ready if it is not at the time of the call. This delay is specified in micro-seconds (so giving the delay the value 1000000 will cause a delay of 1s). Setting the delay to zero will simply test whether the server is ready now. This is a non-blocking test which will be the best choice if you are using the UDP classes as part of a bigger program such as a GUI where there are other things to deal with, or if you are trying to poll multiple connections. If the server is not ready after this time delay has expired, the send_ready method will return false to indicate not ready. If the server becomes available at any time during the time delay, the timer will be stopped and the method will return true immediately, it will not wait for the full delay before returning.

A similar argument applies to the receive and receive_ready methods. Calling receive_ready checks whether there is any server data ready to be received by the client. If it is, then receive will collect it. Bear in mind that network delays mean that, even if receive_ready returns false, that doesn't mean all the data has been received, simply that no data has arrived within the timeout period. Again, receive_ready can be used with little or no timeout to prevent blocking of the program.

The receive method appends data received to the string passed as its argument, so you could in principle allow the data to accumulate over many calls to receive. However, this may not be wise, since the receive order is not guaranteed, so it is recommended that each packet is read, then cleared from the string before the next one.

The remote_address and remote_port methods allow access to the connection details once the connection has been made. The local_port method allows the local port being used for the connection to be retrieved. If the local port was set to zero on initialisation, this will be a port picked from a pool of port numbers set aside by the operating system for use by clients, and will not be one of the service numbers such as 80 for HTTP. Each open connection uses a different local port number.

Finally, there are three error handling methods. The error method returns the latest error code, zero if there hasn't been an error. This is the test for no error. Then, if there is an error, the message method returns a string description of the error. Finally, if the error is non-fatal, it can be cleared by clear_error and communications can continue.

In typical use, bear in mind that packet delivery is not guaranteed, so do not invent protocols that rely on responses - they will lock up. Instead, just poll the received_ready method periodically to see if there is data ready to read. Send whenever send_ready says it is possible to do so. At the end of the communication, it is usually the client's job to close the connection, although some protocols allow the server to close the connection as soon as a request has been fulfilled.

Since there is no actual connection, there is not really a concept of closing a connection and therefore no sure way of knowing that the client has finished, or that the server has been closed down - unlike TCP where the conceptual connection can be closed by either end of the connection. If the server closes down for any reason, the client will not know this unless a send fails repeatedly to initiate a response. Even one fail doesn't indicate a closed server because delivery isn't guaranteed. The close method in the client can be thought of as a de-initialise method which clears the socket and prevents its further use.

UDP Server

A UDP Server is a program that spends most of its time doing nothing very much. It listens to a port, waiting for any client to send a packet. When a packet is sent, the server can either just receive the data if the protocol doesn't require a response or respond to it by sending back another packet or packets to the client. There is no concept of a client connection and each packet is handled as a separate communication. The server does not know about thge client (e.g. its address or port) until the packet is received, containing that data. Unlike TCP, a UDP server handles the send/receive itself, it does not delegate communications to another port.

The server interface is shown below:

class UDP_server
{
public:
  UDP_server(void);
  UDP_server(unsigned short port);
  ~UDP_server(void);
  UDP_server(const UDP_server&);
  UDP_server& operator=(const UDP_server&);

  unsigned long ip_lookup(const std::string& remote_address);
  bool initialise(unsigned short port);
  bool initialised(void) const;
  bool close(void);

  bool send_ready(unsigned timeout = 0);
  bool send(std::string& packet, const std::string& remote_address, unsigned short remote_port);
  bool send(std::string& packet, unsigned long remote_address, unsigned short remote_port);
  bool receive_ready(unsigned timeout = 0);
  bool receive(std::string& packet, unsigned long& remote_address, unsigned short& remote_port);

  unsigned short local_port(void) const;

  void clear_error (void) const;
  int error(void) const;
  std::string message(void) const;
};

There are two ways of creating an initialised UDP server. You can either create an uninitialised server (calling the void constructor) and then initialise it later, or you can initialise the server at construction time and then later test to see if the server initialised correctly. In the former case, the initialise method will return true if the initialisation succeeded and false otherwise. In the latter case, constructors cannot return a result so you must call the initialised test to see if the server initialised correctly. If the initialisation fails, the error method will give an error code corresponding to the cause for the failure and the message method will give a textual representation of the error.

The initialise method (and the second constructor) takes one argument. This is the port number to listen to for requests.

The UDP server class supports assignment. This is done using a smart pointer to the underlying data structures. Thus, when you assign UDP servers, you simply create multiple pointers to the same data structure as with the UDP client. This support for assignment makes it easy to create data structures of UDP servers. For example, to listen to many different ports, you could have a vector of UDP_server objects, one for each port being listened to. Each object could be created outside the vector, initialised and then only copied into to the vector if initialisation worked.

Once a server is finished with, it should be closed by calling close. The destructor also calls close, so allowing the server object to be destroyed will close the listening port automatically.

The communication methods mirror the client data exchange methods. The send method sends data to the client. Because UDP is stateless, the remote address and port need to be specified every time. The address of the first send method is a string containing the remote address as either a name (e.g. stlplus.sourceforge.net) or as a dotted number (e.g. 127.0.0.1). The second send method takes this argument as an unsigned long encoded form of the internet address as returned by the ip_lookup method. The final parameter is the remote port to send to. If the send succeeds, then the sent data is removed from the string passed as the argument. You can optionally call send_ready to check whether calling send would result in something actually being sent. The send_receive method also gives a mechanism for allowing the program to wait a time delay before sending, since network traffic is typically slow compared with the CPU. This would be appropriate if you are writing a program that has nothing else to do but process the connection. However, if you are in a GUI or in a program with many connections to serve, either no wait or a very small wait should be used. If no wait is required there is really no point in calling send_ready since send does not fail if the client is not ready.

A similar argument applies to the receive and receive_ready methods. Calling receive_ready checks whether there is any client data ready to be received by the server. If it is, then receive will collect it. At the same time, the receive method retrieves the remote address and port of the client sending the request, which can then be used as arguments to the second send method when responding to the request.

This means that the code for responding to a request can be tightly coded:

if (server.receive_ready())
{
  unsigned long remote_address = 0;
  unsigned short remote_port = 0;
  std::string packet;
  server.receive(packet, remote_address, remote_port);
  server.send(response_to(packet), remote_address, remote_port);
}

The receive method appends data received to the string passed as its argument. On reading the contents of the string, you should then clear the string ready to receive the next batch of data.

There is no concept of closing a connection to a server, packets just come in at arbitrary times and the task of the server is to respond to them. Typically the server will only close when the program exits. However, there is a close method which effectively de-initialises the server, disconnecting it from the local port. Clients trying to send to that service will thereafter get no response because the packets will be rejected by the operating system due to there being no listener on that port.

Example

The example is taken from a test program used to test the UDP sockets code. It sets up a local server and then runs a second program which then acts as the client.

The server code is:

int server(std::vector<std::string> values, int port, const std::string& command)
{
  int errors = 0;
  std::cerr << "server: " << stlplus::build() << std::endl;
  // this is the first-run instance and acts as the server
  // setup a server on the shared port
  std::cerr << "server: started" << std::endl;
  stlplus::UDP_server connection(port);
  std::cerr << "server: set up UDP server on port " << port << std::endl;
  // now spawn a second copy of this program
  stlplus::async_subprocess client;
  client.spawn(command);
  std::cerr << "server: spawned client with command: " << command << std::endl;
  // UDP servers just respond immediately to incoming datagrams
  std::cerr << "server: waiting for incoming connection" << std::endl;
  for (unsigned request = 0; request < values.size(); request++)
  {
    std::cerr << "server: checking for receive ready" << std::endl;
    // check connection for incoming request
    if (!connection.receive_ready(timeout))
    {
      std::cerr << "server: ERROR: wait for receive timed out" << std::endl;
      ++errors;
    }
    else
    {
      // receive the request
      std::string data;
      unsigned long remote_address = 0;
      unsigned short remote_port = 0;
      if (!connection.receive(data, remote_address, remote_port))
      {
        std::cerr << "server: ERROR: connection receive failed after receive_ready was true" << std::endl;
        ++errors;
      }
      else
      {
        // report the value and check it's correct
        std::cerr << "server: received \"" << data << "\" from " << remote_address << ":" << remote_port << std::endl;
        std::string value = std::string("error");
        if (request < values.size())
          value = values[request];
        if (data != value)
        {
          std::cerr << "server: ERROR: received data \"" << data << "\" != \"" << value << "\"" << std::endl;
          ++errors;
        }
        // now throw the same data back
        std::cerr << "server: testing for send ready" << std::endl;
        if (!connection.send_ready(timeout))
        {
          std::cerr << "server: ERROR: wait for send timed out" << std::endl;
          ++errors;
        }
        else
        {
          std::cerr << "server: sending \"" << data << "\"" << std::endl;
          if (!connection.send(data, remote_address, remote_port))
          {
            std::cerr << "server: ERROR: connection send failed after send_ready was true" << std::endl;
            ++errors;
          }
        }
      }            
    }
  }
  std::cerr << "server: exited server loop" << std::endl;
  // wait for client to exit
  while (client.tick())
  {
    std::cerr << "server: client has not exited - waiting" << std::endl;
    sleep(1);
  }
  if (client.exit_status() != 0)
  {
    std::cerr << "server: ERROR: client has exited with status " << client.exit_status() << std::endl;
    errors += client.exit_status();
  }
  std::cerr << "server: " << (errors ? "ERROR:" : "SUCCESS:") << " exiting with " << errors << " errors" << std::endl;
  return errors;
}

The client is run in the second instance of this program, which is spawned using the async_subprocess class (see the Subprocesses package). The client function is:

int client(std::vector<std::string> values, int port)
{
  int errors = 0;
  // this is the second-run instance and acts as the client
  std::cerr << "client: " << stlplus::build() << std::endl;
  std::cerr << "client: creating connection" << std::endl;
  stlplus::UDP_client connection("localhost", port, timeout);
  for(unsigned request = 0; request < values.size(); request++)
  {
    std::string value = values[request];
    // send this value
    std::cerr << "client: testing for send ready" << std::endl;
    if (!connection.send_ready(timeout))
    {
      std::cerr << "client: ERROR: wait for send timed out" << std::endl;
      ++errors;
    }
    else
    {
      // avoid the data being consumed by the send
      std::string data = value;
      std::cerr << "client: sending \"" << data << "\"" << std::endl;
      if (!connection.send(data))
      {
        std::cerr << "client: ERROR: connection send failed after send_ready was true" << std::endl;
        ++errors;
      }
    }
    // check connection for incoming request
    std::cerr << "client: testing for receive ready" << std::endl;
    if (!connection.receive_ready(timeout))
    {
      std::cerr << "client: ERROR: wait for receive timed out" << std::endl;
      ++errors;
    }
    else
    {
      // receive the request
      std::string data;
      if (!connection.receive(data))
      {
        std::cerr << "client: ERROR: connection receive failed after receive_ready was true" << std::endl;
        ++errors;
      }
      else
      {
        // report the value and check it's correct
        std::cerr << "client: received \"" << data << "\"" << std::endl;
        if (data != value)
        {
          std::cerr << "client: ERROR: received data \"" << data << "\" != \"" << value << "\"" << std::endl;
          ++errors;
        }
      }            
    }
  }
  std::cerr << "client: exited communication loop" << std::endl;
  std::cerr << "client: " << (errors ? "ERROR:" : "SUCCESS:") << " exiting with " << errors << " errors" << std::endl;
  return errors;
}

Finally here is the overall program that puts all this together:

#include "build.hpp"
#include "udp_sockets.hpp"
#include "string_utilities.hpp"
#include "subprocesses.hpp"
#include <vector>
#include <string>
#include <iostream>

////////////////////////////////////////////////////////////////////////////////
// UDP test: this program is designed to be run twice - once without an
// argument as the server, which itself runs a second copy with an argument as
// the client. The two instances then swap information using UDP
////////////////////////////////////////////////////////////////////////////////

unsigned timeout = 10000000;

////////////////////////////////////////////////////////////////////////////////
// Server code
// sets up a UDP server then spawns this program again in client mode

<server function>

////////////////////////////////////////////////////////////////////////////////
// client

<client function>

////////////////////////////////////////////////////////////////////////////////
// main program

int main (int argc, char* argv[])
{
  // code common to both instances to ensure both have the same information
  std::vector<std::string> values = stlplus::split("one:two:three", ":");
  int port = 3000;

  // now branch for the two instances
  int errors = 0;
  if (argc == 1)
  {
    std::string command = std::string(argv[0]) + std::string(" client");
    errors = server(values, port, command);
  }
  else
  {
    errors = client(values, port);
  }
  return errors;
}