View source for Tutorial:Networking with UDP

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

This tutorial assumes that you are familiar with [[Tutorial:Callback Functions|Callbacks]], and Lua in general. Networking should be considered a moderately advanced topic.

There are 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 it's 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.
<source lang="lua">
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
</source>

===[[love.load]]===
First up, we need a UDP socket, from which we'll do all our networking.
<source lang="lua">
function love.load()
	udp = socket.udp()
</source>
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.
<source lang="lua">
	udp:settimeout(0)
</source>
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''.

{{notice|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.}}

<source lang=lua>
	udp:setpeername(address, port)
</source>
	
Seed the [[wikipedia:PRNG|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.

{{notice|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.)}}

<source lang="lua">
	math.randomseed(os.time()) 
	entity = tostring(math.random(99999))
</source>

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.
<source lang=lua>
	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
</source>

===[[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 [[wikipedia:Massively Multiplayer Online game|MMO]]s), 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.
<source lang="lua">
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
</source>

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.
<source lang=lua>
		local dg = string.format("%s %s %f %f", entity, 'move', x, y)
		udp:send(dg)	
</source>

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

{{notice|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 griefing 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}}

<source lang=lua>
		local dg = string.format("%s %s $", entity, 'update')
		udp:send(dg)

		t=t-updaterate -- set t for the next round
	end
</source>

	
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 <code>udp:send</code>!
<code>udp:receive</code> 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 [http://www.lua.org/manual/5.1/manual.html#5.4 string handling functions] is a must.)
<source lang=lua>
	repeat
		data, msg = udp:receive()

		if data then -- you remember, right? that all values in lua evaluate as true, save nil and false?
</source>
	
<code>string.match</code> 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 [http://www.lua.org/manual/5.1/manual.html#5.4.1 5.4.1:Patterns])
<source lang=lua>
			ent, cmd, parms = data:match("^(%S*) (%S*) (.*)")
			if cmd == 'at' then
				local x, y = parms:match("^(%-?[%d.e]*) (%-?[%d.e]*)$")
</source>
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 <code>tonumber()</code>.
<source lang=lua>
				assert(x and y)
				x, y = tonumber(x), tonumber(y)
				world[ent] = {x=x, y=y}
</source>
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!
<source lang=lua>
			else
				print("unrecognised command:", cmd)
			end
</source>
If ''data'' was <code>nil</code>, then ''msg'' will contain a short description of the problem (which are also double as error IDs...).
The most common will be <code>'timeout'</code>, since we <code>socket:settimeout()</code> to zero, any time there isn't data ''waiting'' for us, it'll <code>'timeout'</code>. 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.

<source lang=lua>
		elseif msg ~= 'timeout' then 
			error("Network error: "..tostring(msg))
		end
	until not data
end
</source>

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

<source lang=lua>
function love.draw()
	-- pretty simple, we 
	for k, v in pairs(world) do
		love.graphics.print(k, v.x, v.y)
	end
end
</source>

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 <code>require</code>ing 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 [https://github.com/rjpcomputing/luaforwindows/releases] installer, I wouldn't know for Mac, and Linux? you guys know what to do :3)''
<source lang=lua>
local socket = require "socket"
local udp = socket.udp()
</source>

And once again, we set the <code>'timeout'</code> 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, <code>'*'</code> 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.

{{notice| 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}}

<source lang=lua>
udp:settimeout(0)
udp:setsockname('*', 12345)
</source>

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.

<source lang=lua>
local world = {} -- the empty world-state
local data, msg_or_ip, port_or_nil
local entity, cmd, parms
</source>
Indefinite loops are probably not something you are 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
<source lang=lua>
local running = true

print "Beginning server loop."
while running do
</source>
This next line looks familiar, I'm sure, but we're using <code>udp:receivefrom()</code> 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 <code>udp:receivefrom()</code> (and its counterpart, <code>udp:sendto()</code>) in the client. there's nothing special about the functions to prevent it, however <code>send</code>/<code>receive</code> are convenient and perform slightly better.)''

<source lang=lua>
	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*) (.*)")
</source>
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.
<source lang=lua>
		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;
</source>
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.
<source lang=lua>
		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."
</source>

==Conclusion==
* [[Tutorial:Networking with UDP-TheClient | The Complete Client Source]]
* [[Tutorial:Networking with UDP-TheServer | The Complete Server Source]]

[[wikipedia:User Datagram Protocol|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. These are 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]]
* [http://w3.impa.br/~diego/software/luasocket/reference.html luaSocket Reference]
* [http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html Beej's Networking Tutorial]
[[Category:Tutorials]]

{{#set:LOVE Version=0.7.1}}
{{#set:Description=Networking with UDP}}

== Other languages ==
{{i18n|Tutorial:Networking with UDP}}