Tutorial:Networking with UDP

This is an introduction to networking, using Luasocket. Don't run away! Luasocket is compiled into LÖVE, and are really not that bad once you get used to it.

This tutorial assumes that you are familiar with Callbacks, and Lua in general. Networking should be considered a moderately advanced topic.

There two basic kinds of sockets, and we'll be covering UDP in this tutorial. UDP networking is message-oriented (as opposed to TCP being stream-oriented), meaning that its oriented around distinct (and otherwise independent) messages called datagrams.

In the long run it pays to have a solid understanding on how networking works, but for now lets just get cracking. :3 We'll start with the LÖVE client, then follow up with a stand-alone server written in Lua.

The Client

To start, we need to require the "socket" lib. socket provides low-level networking features.

local socket = require "socket"

-- the address and port of the server
local address, port = "localhost", 12345

local entity -- entity is what we'll be controlling
local updaterate = 0.1 -- how long to wait, in seconds, before requesting an update

local world = {} -- the empty world-state
local t

love.load

First up, we need a UDP socket, from which we'll do all our networking.

function love.load()
	udp = socket.udp()

Normally socket reads block (cause your game to stop and wait) until they have data. That doesn't suit us, so we tell it not to do that by setting the timeout to zero.

	udp:settimeout(0)

Unlike the server, we'll just be talking to the one machine, so we'll "connect" this socket to the server's address and port using udp:setpeername.

UDP is actually connectionless, this is purely a convenience provided by the socket library. It doesn't actually change the 'bits on the wire', and in fact we can change/remove this at any time.

	udp:setpeername(address, port)

Seed the PRNG, so we don't just get the same numbers each time. entity will be what we'll be controlling, for the sake of this tutorial. It's just a number, but it'll do. We'll just use math.random to give us a reasonably unique identity for little effort.

A random number to identify yourself isn't actually a very good way of doing this, but the "correct" ways are beyond the scope of this article.)

	math.randomseed(os.time()) 
	entity = tostring(math.random(99999))

Here we do our first bit of actual networking: we set up a string containing the data we want to send (using string.format) and then send it using udp.send. Since we used udp:setpeername earlier we don't even have to specify where to send it.

Thats... it, really. the rest of this is just putting this context and practical use.

	local dg = string.format("%s %s %d %d", entity, 'at', 320, 240)
	udp:send(dg) -- the magic line in question.
	
	-- t is just a variable we use to help us with the update rate in love.update.
	t = 0 -- (re)set t to 0
end

love.update

We start with a little bit of nonsense involving t we declared earlier; It's very easy to completely saturate a network connection if you aren't careful with the packets we send (or request!), so we hedge our chances by limiting how often we send (and request) updates.

(For the record, ten times a second is considered good for most normal games (including many MMOs), and you shouldn't ever really need more than 30 updates a second, even for fast-paced games.)

We could send updates for every little move, but we'll consolidate the last update-worth here into a single packet, drastically reducing our bandwidth use.

function love.update(deltatime)
	t = t + deltatime -- increase t by the deltatime
	
	if t > updaterate then
		local x, y = 0, 0
		if love.keyboard.isDown('up') then 	y=y-(20*t) end
		if love.keyboard.isDown('down') then 	y=y+(20*t) end
		if love.keyboard.isDown('left') then 	x=x-(20*t) end
		if love.keyboard.isDown('right') then 	x=x+(20*t) end

Again, we prepare a packet payload using string.format, then send it on its way with udp:send. This is the move update mentioned above.

		local dg = string.format("%s %s %f %f", entity, 'move', x, y)
		udp:send(dg)

And again! This is a request that the server send us an update for the world state.

In most designs you don't request world-state updates, you just get them sent to you periodically. There's various reasons for this, but there's one BIG one you will have to solemnly take note of: "anti-griefing". World-updates are probably one of biggest things the average game-server will pump out on a regular basis, and greifing with forged update requests would be simple effective. So they just don't support update requests, instead giving them out when they feel its appropriate

		local dg = string.format("%s %s $", entity, 'update')
		udp:send(dg)

		t=t-updaterate -- set t for the next round
	end

There could well be more than one message waiting for us, so we'll loop until we run out!

And here is something new, the much anticipated other end of udp:send! udp:receive will return a waiting packet (or nil, and an error message). data is a string, the payload of the far-end's udp:send. we can deal with it the same ways we could deal with any other string in Lua (needless to say, getting familiar with Lua's string handling functions is a must.)

	repeat
		data, msg = udp:receive()

		if data then -- you remember, right? that all values in lua evaluate as true, save nil and false?

string.match is our friend here, its part of string.*, and data is (or should be!) a string. That funky set of characters bares some explanation, though. (Which I haven't gotten to, but I'll leave you with a link to 5.4.1:Patterns)

			entity, cmd, parms = data:match("^(%S*) (%S*) (.*)")
			if cmd == 'at' then
				local x, y = parms:match("^(%-?[%d.e]*) (%-?[%d.e]*)$")

Confirming that the values you received are what you expect is important, since you never known who or what is on the other end (or in between...). Since this is just an example, we'll just use asserts.

And don't forget, even if you matched a "number", the result is still a string! Thankfully conversion is easy in Lua using tonumber().

				assert(x and y)
				x, y = tonumber(x), tonumber(y)
				world[entity] = {x=x, y=y}

This case shouldn't trigger often, but its always a good idea to check (and log!) any unexpected messages and events. It can help you find bugs in your code...or people trying to hack the server. Never forget, you can not trust the client!

			else
				print("unrecognised command:", cmd)
			end

If data was nil, then msg will contain a short description of the problem (which are also double as error IDs...). The most common will be 'timeout', since we socket:settimeout() to zero, any time there isn't data waiting for us, it'll 'timeout'. But we should check to see if its a different error, and act accordingly. In this case we don't even try to save ourselves, we just error out.

		elseif msg ~= 'timeout' then 
			error("Network error: "..tostring(msg))
		end
	until not data
end

love.draw

Draw is stunningly simple, since its not really the meat of this example. it just just loops over the world table, and print the name (key) of everything in their, at its own stored co-ords.

function love.draw()
	-- pretty simple, we 
	for k, v in pairs(world) do
		love.graphics.print(k, v.x, v.y)
	end
end

And that's the end of the Client code.

The Server

The server is a little different, for starters its a stand-alone Lua program: it doesn't run in LÖVE.

Once again we begin by requireing socket, and creating a UDP socket.

(LuaSocket isn't compiled into Lua by default. If you are on Windows just get the all-in-one Lua for Windows installer, I wouldn't know for Mac, and Linux? you guys know what to do :3)

local socket = require "socket"
local udp = socket.udp()

And once again, we set the 'timeout' to zero.

But next we do something a little different; unlike the client, the server has to be specific about where its 'bound', or the poor clients will never find it. Thus while we can happily let the client auto-bind to whatever it likes, we have to tell the server to bind to something known.

The first part is which interface we should bind to, '*' basically means "all of them". port is simple, the system maintains a list of up to 65535 (!) "ports" ... really just numbers.

Point is that if you send to a particular port, then only things "listening" to that port will be able to receive it, and likewise you can only read data sent to ports you are listening too.

Generally speaking, if an address is which machine you want to talk to, then a port is what program on that machine you want to talk to.

On some operating systems, ports between 0 and 1024 are "reserved for privileged processes". Its a security precaution for those system. Generally speaking, just not using ports in that range avoids a lot of problems

udp:settimeout(0)
udp:setsockname('*', 12345)

We declare a whole bunch of local variables that we'll be using the in main server loop below. you probably recognise some of them from the client example, but you are also probably wondering what's with the fruity names, msg_or_ip? port_or_nil?

Well, we're using a slightly different function this time, you'll see when we get there.

local world = {} -- the empty world-state
local data, msg_or_ip, port_or_nil
local entity, cmd, parms

Indefinite loops are probably not something you used to if you only know love, but they are quite common. and in fact love has one at its heart, you just don't see it. Regardless, we'll be needing one for our server. and this little variable lets us stop it :3

local running = true

print "Beginning server loop."
while running do

This next line looks familiar, I'm sure, but we're using udp:receivefrom() this time. its similar to receive, but returns the data, sender's ip address, and the sender's port (which you'll hopefully recognise as the two things we need to send messages to someone). We didn't have to do this in the client example because we just bound the socket to the server, but that also ignores messages from sources other than what we've bound to, which obviously won't do at all as a server.

(Strictly, we could have just used udp:receivefrom() (and its counterpart, udp:sendto()) in the client. there's nothing special about the functions to prevent it, indeed. send/receive are just convenience functions, sendto/receivefom from are the real workers.)

	data, msg_or_ip, port_or_nil = udp:receivefrom()
	if data then
		-- more of these funky match paterns!
		entity, cmd, parms = data:match("^(%S*) (%S*) (.*)")

The server implements a few more commands than the client does, the 'move' command updates the position of an entity relative to its current position, 'at' simply sets an entity's location (which we saw in the client), then there's update, which loops through the server's world-state and sends 'at' commands back to the client. and finally there's 'quit', which kills the server.

		if cmd == 'move' then
			local x, y = parms:match("^(%-?[%d.e]*) (%-?[%d.e]*)$")
			assert(x and y) -- validation is better, but asserts will serve.
			-- don't forget, even if you matched a "number", the result is still a string!
			-- thankfully conversion is easy in lua.
			x, y = tonumber(x), tonumber(y)
			-- and finally we stash it away
			local ent = world[entity] or {x=0, y=0}
			world[entity] = {x=ent.x+x, y=ent.y+y}
		elseif cmd == 'at' then
			local x, y = parms:match("^(%-?[%d.e]*) (%-?[%d.e]*)$")
			assert(x and y) -- validation is better, but asserts will serve.
			x, y = tonumber(x), tonumber(y)
			world[entity] = {x=x, y=y}
		elseif cmd == 'update' then
			for k, v in pairs(world) do
				udp:sendto(string.format("%s %s %d %d", k, 'at', v.x, v.y), msg_or_ip,  port_or_nil)
			end
		elseif cmd == 'quit' then
			running = false;

There's nothing much left to see, other than the socket.sleep' call, which helps reduce the CPU load of the server. Generally, people seem to prefer to just rely on blocking behaviour (which we turned off) to keep CPU usage down, but its nice to know how do this without blocking.

		else
			print("unrecognised command:", cmd)
		end
	elseif msg_or_ip ~= 'timeout' then
		error("Unknown network error: "..tostring(msg))
	end
	
	socket.sleep(0.01)
end

print "Thank you."

Conclusion

• The Complete Client Source
• The Complete Server Source

UDP is simple in its usage, but also relies on the developer to get a lot more right. since UDP doesn't make any assurances about the order that datagrams arrive, or even that they arrive at all. Things that you obviously have to take into account when designing your protocol.

Additionally UDP datagrams have limited sizes, the luasocket Documentation specifically notes that it doesn't support anything over 8k, and frankly you should assume much less.

See also

• Networking with TCP
• luaSocket Reference
• Beej's Networking Tutorial

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