
			Networking Maelstrom

                            Sam Lantinga
June 2, 1996


Design Goals:

	I originally envisioned being able to play Maelstrom
one-on-one against other people over Linux SLIP lines.  We could log
into a central server and play one another, similar to the way Quake (tm)
is planned to be.

Design Process:

	The first idea was to create one central server that did
all calculation of player movement, collision detection, etc, and then
broadcast updates, every frame time, to each player.  The disadvantages
of this approach are that it places a great computation load on a single
machine, and requires high bandwidth to transmit the entire game state
thirty times per second.  I started coding the client to duplicate some
of the state calculation of the game and reduce the bandwidth requirement,
but it was a great deal of work, and subtle differences in the client
and server code could result in synchronization bugs.

	The next design paradigm was to have N independent games
running, and have each independent game control a player.  To reduce 
bandwidth, each game would run its game computations independently, and 
the only traffic would be synchronization packets every frame, and keystrokes
to update player actions.  Actually, the only traffic really needed is
the keystroke information, but in a shoot'em up game such as Maelstrom,
the frame on which keystrokes are delivered is critical.  If a game runs
really fast on one machine, and a keystroke arrives from a slower player,
then a player can die on one machine and still be alive on another machine.
This fragments game consistency and must be avoided.  The way to avoid this
is to have each game instance (player) checkpoint at every frame, and 
deliver keystrokes destined for that frame.

	Creating multiple players had the unexpected side effect
of requiring the game logic to be rewritten using C++ objects.  The
original game logic assumed that all game objects were generic
structures fit into an array, and the object at array position 0 was
the player.  With multiple players there had to be a way to allow
them to shoot at eachother, and for objects to interact in general
ways, and in then sometimes in particular ways depending on the type
of objects interacting.  This lent itself well to C++ object inheritance
design, and took about three days of coding to get mostly working.

	Synchronization between running games was assured by the
use of the original random number generator used in Macintosh Maelstrom,
and the translation of time-based events to frame-based events.
The original random number generator uses a very specific algorithm to
take the random seed, translate it into a pseudo-random number and then
update the random seed.  Event translation works because frames are
supposed to occur exactly 30 times per second and the original event timings
were always some multiple of frame times apart.  The only completely
random event in the game is the completion of sound events, and the only
time they are ever noticed is during thrust, and no calculation affecting
the game is done at that time.  Combined, this allows multiple games,
given the same seed and input, to run exactly the same over multiple 
instances.

	The only problem now was how to guarantee the delivery of the same
input to each instance of the game.  TCP was my first choice, as it has the
advantage of reliable delivery and would guarantee that my frame packets
would arrive in the correct order.  However, TCP has the disadvantages of
high overhead and buffered data.  In the course of ECS152B, I learned about
the slow start algorithm, congestion avoidance, and other features of TCP
that makes it very suited for reliable stream data, but a poor choice for
timely high speed packet transmission.  

	UDP is the perfect choice of the IP protocol suite for high-speed,
low overhead packet based transmissions.  It has zero connection establishment
overhead, packets are sent as soon as they are queued, and if packets are lost,
with careful packet management and caching, your application can recover
almost immediately.  In my final design, I use a modified stop-wait protocol
that uses packet caching to do very little waiting.

	To synchronize the game we need to make sure all keystrokes arrive
on the interval in which they were pressed.  The original game polls for
keystrokes on every frame, and testing shows that while for most purposes
polling every other frame is acceptable, but for pitched battles, the 30'th
of a second resolution is critical.  This means that we need synchronization
every frame.  Keystroke information can ride piggy-back on synchronization
packets, so the only overhead that is important is that of the sync packets
themselves.  If any player gets more than one partial frame ahead of another,
we may lose synchronization, so some sort of stop and wait protocol is
suggested.

	Using stop and wait protocol, the total overhead of synchronization
packets, assuming a two-player game, is:

	30*(sizeof(IP header)+sizeof(UDP header)+sizeof(packet))
	30*(20+20+1) = 1230 bytes per player

If we ack each frame packet, we effectively double the bandwidth to
nearly 2500 bytes per second for each player.  This is far beyond the
current modem IP bandwidth for 14.4Kbps modems.

If we somehow eliminate ack packets, and compress the IP header, we
can reach a low bandwidth requirement of 30*(4+20+1) 750 bytes per
second for each player.  At an average 200 ms round trip time, our game
could reach a maximum of 10-15 frames per second -- much too low for 
high speed interactive games.  Thus, the solution seems to be that 
high speed modem play is infeasible over IP.  Note that without the
overhead of UDP/IP communications, i.e. direct modem or null-modem
connection, our bandwidth requirement goes down to 30*(1+1) bytes
per second, or 60 bytes per second for each player.  You could play
this over a 2400 baud modem!

	In practice, it seems that an ethernet-speed network is
required to run networked Maelstrom over UDP/IP.  So, the challenge
becomes reducing the overhead of the network to levels that allow the 
other parts of the game to proceed at thirty frames per second.


The Algorithm:

	Already mentioned was a modified stop and wait with little
waiting.  Here's how it works.  Each frame, every player sends its
current frame packet to each other player and waits for frame information
from every other player.  Since this happens simultaneously, each player
sends and receives the current frame packet almost instantaneously, and
they continue processing the game.

	* A player will never advance the frame if it doesn't receive
	  frame updates from all other players.
	* If it doesn't receive a frame packet within 1 second it will
	  rebroadcast the current frame packet.
	* If a player receives a frame packet for the last frame, it will
	  send its previous frame packet which it has cached.
	* Packets from frames farther back than the previous frame are
	  ignored.

In this situation, if a packet is lost, then the player waiting for it
will time out, the other players will be one frame ahead, the player
waiting for the packet will resend it's packet, and the player who lost
the packet will resend.  The waiting player will continue, and all will
be well.  It is guaranteed that no player will be more than one frame
ahead in this case.

Unfortunately there is a real possibility of a lock-step problem here:

	Player 1  (frame 10)		Player 2  (frame 10)
		10 -->				<-- 10
		dropped packet
	Player 1  (frame 10)		Player 2  (frame 11)
		wait for 10			<-- 11
		get 11, timeout
	Player 1  (frame 10)		Player 2  (frame 11)
		10 -->				<-- 10 (cached)
Here player 2 has already sent the packet for 11
	Player 1  (frame 11)		Player 2 (frame 11)
		11 -->				wait for 11
	Player 1  (frame 11)		Player 2 (frame 12)
		wait for 11			<-- 12
		get 12, timeout

This goes on, repeating, as the two players advance frames in lock-step,
one frame per timeout.

The solution is for Player 1 to immediately resend its current frame packet
if it receives a packet for the next frame.  At that point it knows that
the other player has already advanced to the next frame and has already
sent the packet for the current frame.  If it caches the packet for the
next frame, the next time around, it just sends the packet for the next
frame, and advances immediately.  The two players are now back in sync.

This assumes that few packets will be dropped or otherwise lost.
This seems to be a valid assumption.  It was tested over a 32.6K link to 
Australia and though the game was unplayably slow because of round trip
times, dropped, lost, and timed out packets were few and far between.

The new senario:

	Player 1  (frame 10)		Player 2  (frame 10)
		10 -->				<-- 10
		dropped packet
	Player 1  (frame 10)		Player 2  (frame 11)
		get 11, cache 11, 10 -->		<-- 11
	Player 1  (frame 10)		Player 2  (frame 11)
		get 10				<-- 10 (cached)
	Player 1  (frame 11)		Player 2  (frame 11)
		11 -->				get 11
	Player 1  (frame 12)		Player 2 (frame 12)
		12 -->				<-- 12

The players are now synchronized again.

	This run-wait scheme works very well when you have a set
of hosts that are broadcasting synchronization packets to each other at
high rates.  It prevents the stop-wait syndrome, and reduces the necessity
of timeouts, the main disadvantages of using UDP for small packet based
communications.  It has been shown, through extensive testing, to be highly
effective for frame-based network games such as Maelstrom.

	IT WORKS!!

		Anybody wanna play? :-)

(slouken@libsdl.org)
