Replication De-Obfuscation

Replication De-Obfuscation Introduction

Ask any halfway decent UnrealScript programmer what the most confusing aspect of Unreal coding, and he'll be sure to tell you replication. (or perhaps PlayerMovement or pawn/bot AI :) I've been there myself, and I still have trouble with it sometimes, but I think I know enough to write some documentation. Lots of trial and error helped me get far, along with various insights from people that had the source code to the engine. I've tried to piece it all together to create this document. I have however, recently gotten a lot more information about exactly how replication works, which has helped me immensely in understanding replication. Since I can't post that information directly because of NDA, I'll try my best to explain it to you through this networking documentation. Hopefully I'll be able to expand upon what was described in Epic's documents at: http://unreal.epicgames.com/Network.htm and http://unreal.epicgames.com/UTMods.html. Please read it first, as it will help a lot with background and context. In writing this document, I'm going to assume you've programmed in UnrealScript before, you have at the very least considered the problems of replication, you are reasonably intelligent, you know how to read, and you think my jokes are funny.

First, Unreal Tournament uses a client-server architecture. The client is basically a dumb terminal, that sends data to the server about where the player moved, if they're jumping, if they're shooting, etc. It displays to the user what is given to it by the server. The client's information is necessarily out of date with what exists on the server, because of lag, ping time, and other factors. So in order to present a believable world, the client has logic to simulate what it thinks the server will be sending it. It knows the laws of physics, and that if something is travelling in a straight line, then it will contine to travel with that same velocity. Likewise, if it's falling through the air, it's likely to continue to be accelerated under the force of gravity. The dumb terminal predicts where how the game state will be progressing, and so can provide the user with a more believable game. On the other end of the connection is the server, which holds the authoritative state of what's going on in the game. If the server thinks you are dead, then you are dead. Like your teachers, they're never wrong, no matter how much evidence you've seen to the contrary (like how you *thought* you dodged that rocket). The client gets information sent to it by the server (like where other people are, what things have been shot, etc) , and sends other information back to the server (like where the client wants to move, shoot, etc). In the client-server architecture, the client only communicates with the server, and the server communicates with all of the clients (people connected to the server).

Server as Authority

With the recent brew-ha-ha regarding the ZeroPing mod, which aims to give the client more control, there have been two sides to the issue of why the server as authority is the best model.

A good description of the history of the networking architectures in games can be found at the top of the tech page's networking document. Go read the "History" section now, and then come back. The current model is that of a client-server architecture, where the client has prediction to minimize the effects of lag. In current versions of UT, the client tells the server all about it's input. Where is the user looking, in which direction are they moving, and are they firing primary or secondary fires.. The server then uses that data to calculate information about the player, and to tell other clients about that player. For the most part, that is all there is to it. ZeroPing pioneered an idea of giving the client more power. It allowed the client to decide whether they hit the other player. In theory, this sounds great. The user gets to decide what's happening, and if they hit the player, then they hit the player. Unfortunately, there are problems with this idea.

In a large-scale game like UT or Q3A, many people try to cheat. The people that make the game have to prevent that from happening. With these games, the client does so little logic, that there is little room to cheat. The client sends its data to the server, and the trusted server decides whether they hit the player or not. Cheats are extremely limited in what they can do, as they can only modify the direction the player is going, and whether to cheat or not. It's not something that is really worthwhile. However, when you have something like ZeroPing, you are letting the client decide whether or not you hit the player. A cheat can pretend that it hit every player, and can wreck havoc in such a game, where it is killing everyone the moment they enter view, and sometimes even getting repeated kills ten times in a row on a single player as they respawn and then are 'hit' again. Mods like ZeroPing are mods, and so do not ahve the same public appeal that a full game does. As such, mods are less likely to be hacked for cheats. But it does not make them immune. If game makers were to use the approach that ZeroPing uses, there would be a flood of cheats within weeks of the game's release. You can understand the resistance that the game companies have for such an approach. It is not because it's the 'not-invented-here' syndrome as many have suggested, but rather that it really is much more open to cheating.

As a side note, I guess the ZeroPing author does realize this. He has hidden his code to make it impossible to cheat with. He took the security through obfuscation approach. This will work for awhile, until code decompilers become available, or the code gets out. While I am not working on any of these myself, I do know of some UnrealScript code decompilers in the works. So this approach really will not work in the long run.

Replication Overview

Replication deals with how information is sent from the server to the client and vice versa. Because of bandwidth limitations for modems, not everything can be sent, and because of the unpredictability of the Internet, not everything can be ensured of arriving on time. Only a subset of all the information is sent to the client, and replication deals with that subset that travels from the server to the client, and deals with how the client sends its relevant information back to the server.

While the definition of replication may sound simple, there is a lot of underlying assumptions and quirks that you must be aware of, which UT uses in the interest of optimum bandwidth and speed. It's an unmapped jungle out there, and many an explorer has been lost to the dangerous native inhabitants. This guide you are reading is your map and survival guide.

First, the Unreal server does some logic like the following for each client connected:

Run all events (like tick, timer, hitwall, etc)
Go through each client connected to the server
Get the list of relevant actors for this client.
For each actor, replicate variables that need to be replicated.
Replicate any function calls that should be replicated to that particular client, if they were called during the previous server-side events run above. 

The client does the following logic:

Receive data from the server, and update its local copies of the actors
Run any replicated calls that were called on the server
Run any simulated events (like tick, timer, hitwall, etc), and do the local playerpawn handling
Find out what replicated were called on the client and need to be replicated to the server, and send them 

If that didn't make any sense, don't worry about it. They just outline the main parts of replication, and they will all be explained later.

If you did understand that, there's still plenty to learn. UT has a lot of logic going on under the hood in regards to replication, most of which is not immediately apparent. That was the purpose of this document, to explain these mysteries to you, the reader, so that your networking development will go that much more smoothly. Again, all this will be explained in due time, assuming you read the whole document. :)

Relevant Actors

First, we need to define what a relevant actor is. A client, as the dumb terminal, needs to know about everything that it will need to display on screen. It needs to know about the other players, and weapons in the level, if any projectiles are visible, if the flag is visible, health packs, etc. All of these actors constitute the relevant set of actors. how is the relevant set of actors determined? If an actor is not relevant, no amount of replication statements or voodoo magic will cause its information to be sent to the client. It must be relevant to that particular client in order for data to be sent to that client. For Unreal / Unreal Tournament, the method of determining relevant actors is as follows (ripped from the tech page's networking document):

The following logic is performed on each actor, and you should be able to tell whether it is relevant. The actor must pass two discrete sets of steps in order to be considered relevant. One it passes the first test, it must then pass the second. The rules higher in the list have precedence over rules that follow them:

If it is bNetTemporary, then it does not pass the first stage.
If it is not bStatic and not bNoDelete, then it passes the first stage.
If it is a LevelInfo, it passes the first stage.
If it is a temporary network actor, (as defined by bNetTemporary), and it's been sent to this client already, then it does not pass the first stage. These types of actors will be decribed in more detail later.
If its Role is ROLE_None, then does not pass the first stage.
If it is bAlwaysRelevant, and time since the last relevancy is less than 1 / NetUpdateFrequency (described farther below) seconds, then it does not pass the first stage. This basically means an actor will only update it's variables to the client NetUpdateFrequency times per second.(NetUpdateFrequency values: Actor: 100, ZoneInfo: 4, GameReplicationInfo: 4, PlayerReplicationInfo: 2, or Inventory: 8).

If it is bNetOptional, and it has a non-zero LifeSpan (meaning it does not exist forever), and it is not in the first 150 ms of its lifetime, then it passes the first stage. Any attempt at replicating a bNetOptional actor after the first 150 ms, (because it was pushed off the replication list due to bandwidth contraints for that client, etc,) will fail.

If the time since the last relevancy is greater than 1 / ~NetUpdateFrequency seconds, it passes the first stage. (This is a more finely tuned optimization of NetUpdateFrequency, tuned to the type of actor. It will only perform the calculations needed if it has passed through the weeding techniques above.)

It does not pass the first stage. 

If it passed all of those steps, it's not through and clear to be relevant yet. It has another step to go through. If it is deemed relevant during the following steps, then it is relevant to this client.

If it is bAlwaysRelevant, then it is relevant.
If it is owned by the client or owned by the actor we are viewing through (as returned by PlayerCalcView), then it is relevant.
If it is the client or the actor we are viewing through (as returned by PlayerCalcView), then it is relevant.
If its Instigator is the client, then it is relevant.
If it has an ambient sound and is within 'audible distance', then it is relevant.
If it is a weapon that is owned by a visible Pawn owner, then it is relevant.
If it is bHidden or and not bBlockPlayers and the AmbientSound is none, then the actor is not relevant.
If it is not relevant (per the above tests), but if any of the above conditions made it relevant within the last RelevantTimeout seconds, (by default, five seconds, but is defined in UnrealTournament.ini:[IpDrv.TcpNetDriver],) then it is relevant.
It is not relevant. 

Those rules, while seeming complicated, are exactly how UT determines what is relevant to each client. Many times such checks are not related to what you are doing, but they are always good to know. Thinking about it, it makes sense. If something is not visible, then the client probably does not need to know about it. If it's a player on the other side of CTF-LavaGiant, what use does this client have for it? Or if it's an invisible pathnode used for Bot AI, the client also has no use for it. Sending data and variables for such actors would be wasteful on a tiny little 28.8 kbps modem.

Working this discussion into a guideline for your own code, you should set your actors' bAlwaysRelevant to true if they are data storage units that the client needs to know about, like PlayerReplicationInfo or GameReplicationInfo. Things like PlayerReplicationInfo, which store information about the player, are always relevant. This allows a client to view the Scoreboard and see the information (name, score, ping, packetloss, etc) for everyone in the level, regardless of whether that player actor is relevant to the client. PlayerReplicationInfo's allow precious information that's small, bandwidth wise, to be relevant and replicated to each client, yet have information like the location, velocity, weapon, animations, etc of the player to not be replicated when it's not necessary (because it's in the Pawn/PlayerPawn, which is not always relevant.) Hopefully I've not lost you here. It's an important thing to know and remember about Unreal replication. Where to put your variables, and how to make them relevant to the client. If you needed some important game information to be seen by all clients, your best bet would be to subclass ReplicationInfo and utilize that for your data variables. Information that is only needed when it's seen is best put in regular actors, that will go in and out of relevancy as time passes.

In a nutshell, if the client needs to know about it, they will. All inventory is relevant to the client, as is all nearby players. All projectiles and weapon effects that are visible are relelvant. Anything that affects the player's world on the client, will be replicated. This includes things that block the player's movement, or actors that play ambient music. There is an overhead to spawning a new actor in netplay, and so UT does not immediately remove an actor from the relevancy list when it goes out of view. It gives it a few seconds before it is really considered not relevant. Oh, and UT takes into account the fact that a player may be viewing through another actor's eyes, as in the case of the GuidedWarhead, or the teammate views when you hit the number keys on the right side of your keyboard.

NetPriorities

While that's the "simple" description, figuring out why things happen can get a bit more complicated. Unreal Tournament gives different priorities for different types of actors. NetPriority, which should typically be between 1 and 3, determines how to prioritize a given actor's bandwidth "allocation" in the stream being sent to each client. Actors are first sorted by a calculation based upon their priority (adjusted to take into account how close the client is looking at them, etc.) Actors marked bNetOptional are sent to the bottom of the list. So....actors with a higher priority are sent first in the bandwidth stream, and when the connection becomes saturated, any remaining actors are starved of bandwidth until later. (UT remembers which actors have not been replicated, and so it can give bNetOptional actors 150 ms to get replicated before they are discarded. See the above relevancy list for more information.)

Here's a wide and almost complete sampling of UT's NetPriorities:

Bots:             3.0
PlayerPawns:      3.0
GuidedRedeemer:   3.0
Movers:           2.7
Projectiles:      2.5
Carcasses:        2.5
Other Pawns:      2.0 (Spectators, Titans, Bunny Rabbits, etc)
Effects:          2.0 (sparks, blood, smoke)
Simpler effects:  1.4 (shell casings)
Inventory:        1.4 or 2.5
Everything else:  1.0 

This shows that the server considers a Player's (be it bot or playerpawn) information to be much more important than something like bullet casings from a gun. And it makes sense. A guided redeemer also has unpredictability to it, since it is controlled by a player, and thus needs a higher NetPriority. A projectile follows the simple Physics given to it, eg: PHYS_Falling, PHYS_Pojectile, and so they are not very unpredictable. Thus, its NetPriority is not as high as a player's. However, projectiles are still quite important to gameplay (doesn't it suck to get hit by that rocket you never saw? :), and they get a high NetPriority. It's the same with the carcasses, since their animations as they fly through the air or fall to the ground are quite important (not to gameplay, but to that touchy-feel-good feeling when you kill someone.) You should use these examples as a guide when creating your own actors that need a different priority.

One additional thing to note is that you can change the NetPriority on the fly. This is done with Inventory, for example. It exists as 1.4 for most of its life, but when it is thrown from a player's inventory, it now is a moving actor with more importance attached to it. It is then given a higher NetPriority. Once it hits the ground, it gets a NetPriority of 1.4 again, since it does not need to be updated as often when it is sitting stationary on the ground.

In general, all of the relevant actors for a particular client are put in an array each tick, and then sorted by their priorities. The server then starts replication the actors, starting with the highest NetPrioritized actor, working it's way down. UT then stops replicating actors when the bandwidth becomes saturated. That's why it's important to prioritize your actors correctly, so that when a lot of actors are on screen, or on the lowly 28.8K connections, UT will know how to perform triage, and who to starve of bandwidth first. If an actor is not replicated, the variables it was supposed to tell the client about are not forgotten. They will be put on the list during the next tick, to have another go at getting replicated. Note that doubling the size of the actor's NetPriority does not actually change anything in regards to giving it more bandwidth, since a NetPriority only has meaning in relation to other NetPriorities. Since 3.0 is the highest on the list, there is no real difference between giving your actor a NetPriority of 4.0 and 4000.0, since they will both be placed at the top of the list.

Roles and RemoteRoles Introduction

Now is a good a time as any to explain the purposes of the various roles in Unreal. Roles simply guide UT in determining what role this particular actor plays in the game of replication. For example, a player needs to be replicated quite differently from an ammo box sitting on the ground, or a projectile moving according to the rules of physics. On a server, the Role will be ROLE_Authority, which makes sense since that actor is currently being evaluated on the authoritative server. The RemoteRole of that actor will be set to any of a variety of roles, defined below. On the client, the Role and RemoteRole will be reversed. On the client, the RemoteRole will be ROLE_Authority, and the Role will be one of the other role types. Let's go through the various roles (which will be set in the RemoteRole default properties), in order of increasing 'control.'

ROLE_None

This role is probably the simplest. It simply tells the server not to make this actor relevant to the client. No information will be sent about it, and it will exist where it was created only. If it was created on both the client and the server (via a simulated function, described later), and given a Role of ROLE_None, then it would exist both on the server and on the client, and they would both operate independently of each other. 

ROLE_DumbProxy

This role is used for objects that don't move much, yet still need information about them updated to the client. The server does this by sending periodic updates about this actor's location and velocity to the client. If the actor has any sort of logic to it, with Physics, for example, it's much better to use the following role. 

ROLE_SimulatedProxy

This is used for anything that should be simulated over the network, via prediction. Take a rocket, for example. It always travels in a straight line, and is an ideal candidate for SimulatedProxy. By the same token, UT can predict the falling physics clientside, and so grenades, and falling people are also good candidates for this role. Anything that can be predicted clientside because of Physics or clientside physics should use this Role. All Bots and PlayerPawns use this role as well, since when a player is running, he's more than likely to continue to run. It's the best prediction method that can be used for unpredictable players. The only exception to this is the playerpawn you yourself are playing as. 

ROLE_AutonomousProxy

This is used for you. When you play online, you yourself should be treated differently from all other playerpawns. You don't want your own self being predicted based upon the server's expectations. Rather, you want to control yourself and tell the server where you are moving. With this Role, that's easily possible. Many things that should only be done for your own playerpawn, like server-corrections of your location when you get lagged out, are only done with this role. 

ROLE_Authority

This role is different from the others in that it is always the Role on the server. On the server, all Roles will be ROLE_Authority, and the RemoteRole will be one of the above. On the client, the reverse will be true. This role doesn't have any real significance, beyond it's use in replication statements, described below. 

General Replication

For each actor that is deemed to be relevant to a particular client, the server needs to determine what subset of its variables will be sent to the client. The most obvious rule is to only send variables if they change. Unfortunately, that rule alone is not enough to fit data over a modem. Instead, the server has to further whittle down its list of variables, and it does this through replication statements. These replication statements are executed for each client, for each relevant actor to that client. When a subset of the variables of a subset of the actors are sent over the network, only then can it be fit on a regular modem. But how can a client operate without all of the information in the game? Once you stop and think about it, it's not quite that unthinkable. Do all of a Bot's AI variables need to be replicated to the client? Only the result of the AI needs to be sent, which basically consists of the same thing for any other Pawn in the game. Also, depending upon the RemoteRole, different things are replicated. Some cause the Location to be updated periodically as a poor man's simulation of its motion, while another role causes the physics to be replicated, and the location not to be, creating true client-side simulation of the actor's motion.

No Recursive Relevancy

Please note one important point that can easily be overlooked. Relevancy is not recursive with replicated references to variables. Let's say that Actor A contains a reference to Actor B in Property C, and Actor A replicates Property C (currently set to Actor B). Let's also say that Actor A is relevant, and Actor B is not. (Don't worry, there's only three letters in this example.) In this example, Actor A is relevant, and so it's Property C variable will be replicated to the client. Now what does the client know? The client knows about Actor B through Actor A's reference, but Actor B is not relevant. All of Actor B's variables are not checked for replication. The client only knows about Actor B, which really is of no use whatsoever, since the variables are not replicated. I initially thought that replicated variables to actors caused those actors to be relevant and parsed themselves for replicated variables, but that view is false. An actor can only be relevant according to the rules stated above, which make no provision for a reliable reference to them from a relevant actor.

Replication Checks

When you think about the number of clients in a server, and the number of relevant actors for each client, and the number of replication checks on variables for each actor, there's a large number of such checks. Special care should be taken when implementing replication statements. As each conditional is evaluated, you should be considerate of the server's limited processing ability. Don't call functions from within replication statements, as function calls are much too slow for replication statements. Along the same lines, you should try to keep your checks as simple as possible, only evaluating a condition if its necessary. Finally, note that Epic had a problem with replication checks taking too long for some important actors like Actor or Inventory, both of which replicate many variables. To alleviate the situation on servers, Epic moved the checks to native code. That's why you will see the 'nativereplication' on a few of the Engine classes. Any variables being evaluated in those classes will be covered by the nativereplication code, making any UnrealScript replication statements useless in that class, (except as a form of documentation for UnrealScript programmers.) Any variables that are not defined in a nativereplication class, (defined in a non-nativereplication subclass or superclass,) will be replicated via the unrealscript definitions. Despite the fact that the nativereplication checks are in C++, the UnrealScript checks that remain in those classes are still accurate, and a good guide to go by in determining problems with replication, and ways around it.

Special Replication Variables

When writing replication conditionals, there are a few things you should be aware of. During replication, a few helpful variables are set for your use in evaluating whether it should be replicated. These variables are defined in actor, and are:

bNetInitial

true if this is the first time this actor is being replicated across the network. Useful for variables that differ from the defaultproperties, yet will not change over the life of the actor. 

bNetOwner

true if the player we are replicating to owns this actor directly. 

bSimulatedPawn

true if the actor is a pawn (or a pawn subclass) with Role == ROLE_SimulatedProxy. In other words, true if it's a pawn that's not the client. 

bDemoRecording bClientDemoRecording bClientDemoNetFunc

used for demo recording purposes. 

bNetRelevant bNetSee bNetHear bNetFeel

Not used anymore, never even set.

NetUpdateFrequency

NetUpdateFrequency is another variable useful in replication. It effectively causes an otherwise relevant (not useful when the actor is not relevant) actor to be relevant NetUpdateFrequency times per second. This is useful when the data changes frequently but you only want periodic updates. For example, PlayerReplicationInfo contains a lot of data about the client. Stuff like the various player's pings may change quite frequently, and having the client keep track of such information would be quite a network hog. But since PlayerReplicationInfo has a NetUpdateFrequency of 2, it is only updated twice a second, which is much nicer for that player's bandwidth. The list of the current NetUpdateFrequency's for all UT classes are listed below:

Actor:                  100 Hz (times per second)
ZoneInfo:               4 Hz
GameReplicationInfo:    4 Hz
PlayerReplicationInfo:  2 Hz
Inventory:              8 Hz Reliable versus Unreliable 

Another optimization used in networking is the difference between Reliable and Unreliable data. In the case of variables, all data is reliable. It is all guaranteed to reach the client, even if it arrives in the wrong order. (Pawns however, have code that prevent them from appearing jumpy, although the effects of this can still be seen at times when the player bounces back and forth in a somewhat laggy game.....location updates are being received out of order.) Functions however, exist in both reliable and unreliable forms. A reliable function, like reliable data, is guaranteed to reach the client. An unreliable function however, is sent to the client, but it is not guaranteed of reaching the client. Something that sends messages to the client, like BroadcastMessage or BroadcastLocalizedMessage, are reliable to ensure that the client gets the message. Something like a client's update to the server about its position need not be reliable, however. Since the client sends these update requests to the server every single tick (in ServerMove, if you're wondering), it does not matter if they all reach the server. And since each update is timestamped, the server knows which ones to regard because they are out of order or lagged. And so, the server can be assured of getting updates from the client, while not requiring that they ALL be sent. A reliable function is repeatedly sent over the network until the client acknowledges it. This can lead to a one second delay or more if it has to try to send the message once or twice. Since you don't want to bog down the network connection with reliable updates about where the client is looking or moving, unreliable is best in this situation since the server only needs to get "most" of them to work great.

Writing Replication Statements

There are a few useful things to know about when writing your own replication statements. First, you cannot override replication statements. If a replication statement exists in a parent in regards to Location, Velocity, some function, or another variable you want to change the conditions for...you can't. Sorry. Instead, you have to mold your actor into fitting the requirements so that it's property or function will be replicated. Usually you can accomplish this by setting a variable or two (you can't set the "const" ones which are set natively of course), or by fiddling with the RemoteRole until it matches up. However, don't go around doing that blindly, or you're sure never to succeed. Instead, keep reading, and you will discover (hopefully, if I do it right :), exactly what each of the Roles are for, and what side effects they cause. Next, keep them simple. We already discussed the importance of not doing anything overly complicated, and that rule should be adhered to. Second, use the bNet* variables if possible. These variables, like bNetInitial, bNetOwner, bSimulatedPawn are quite useful, and are gratis. There is no additional CPU requirement to use these variables, as they are set for you automatically. Another very important thing to hinge your variables on is the Role and RemoteRole. "if (Role == ROLE_Authority)" would cause that variable/function to be replicated from the server (where the Role equals ROLE_Authority) to the client. Something like: "if (Role < ROLE_Authority)" would cause the variable to be replicated from the client (where the Role is less powerful than ROLE_Authority) to the server. Checks can also be performed to do various things depending upon if the Role/RemoteRole is a SimulatedProxy or a DumbProxy. The best place to find examples of these more intricate checks is in the Engine classes, particularly Actor, Pawn, and PlayerPawn. It should also go without saying that you should not change anything in a replication statement. Doing i++ in a replication statement causes i to change unpredictably as it is being replicated, which is not something you really want to do. And finally, as the last word advice, the server checks all incoming data to be sure it is legitimate. If the client replicates something (a variable or a function) to the server, the server will exchange the Role and RemoteRole variables temporarily, and evaluate the replication condition to see if the other end had a legitimate reason for sending that data. If the check fails, then the data (variable or function call) is discarded. That means that when you are writing replication statements, make sure that the server also has all the data needed to perform the replication check. Otherwise, it will never be processed or accepted by the server.

Variable Replication

Variables are one type of data that can be sent through replication statements. The other is function calls (RPC, if that helps). Here, we'll be discussing how data is replicated from the server to the client, and vice versa. There's a lot of little things that server to complicate it more than the replication described above. As I already stated above, all variables are considered reliable, so the unreliable versus reliable discussion is irrelevant here.

Newly Relevant Actors (Spawning)

When an actor becomes relevant to a client, it is spawned on the client, with all the defaultproperties of the actor. If the server set variables immediately after the spawning of the actor, (before the relevancy checks), then those variables will be sent to the client as an 'addendum' of sorts, assuming those variables meet the replication statements. The server will send any variables that differ from the defaultproperties. Things can get hairy though, when the server has a different idea of an actor's defaultproperties than the client does. Initially, the defaultproperties on the server should match up with what the client has. If a defaultproperty is changed on the server, that change is not replicated to the client. You must change it in a simulated function in order for that to occur. However, what happens if the actor becomes irrelevant? In that case, bad things happen. When the actor becomes relevant again, the default spawned actor on the client will have different defaultproperties from the server. When the server then proceeds to perform the checks on what to replicate, it sees that the default var is the same as that var, and so that var is not replicated. The client then never gets data, and so has the REAL default variable, of a fresh actor. That variable can be incorrect, and cause strange behavior to occur.

As an example, for a particular class-based mod, a programmer (not me, this time ;-) had set both the Mesh and Default.Mesh to the playerclass that the player had chosen. Players however, go in and out of scope, to save on network bandwidth. Normally they are only relevant when you can see them, and for about three seconds once them disappear from view. Let's say the player changed their playerclass at that point. Or say they changed their class at the start of the game while the round has not started and they are invisible, and thus not relevant. When they did become relevant to the client, because they turned a corner, or because the round just started and they are spawned and made visible, the Mesh == Default.Mesh, and so the Mesh is never replicated. The skin however, was still replicated normally. To clients in the game, they saw the player as the original mesh, with a inappropriate skin that was intended for another model. Like I said, strange behavior, that can be hard to trace down. The way to handle this, is to not set the Default variables unless you are really know what you are doing. Just setting the Default.Mesh because it feels right will only cause problems. In this case, the Default.Mesh was not needed for everything, and removing that assignment caused everything to work fine.

A Time for Replication

Replicated variables are only replicated at the end of each tick. That means that if you change the variable repeatedly during a loop, or some other block of code, only the final value will be replicated. If you change a variable many times during a tick, and change it back to its original value when the tick completes, then no variable change will have been seen, and no network bandwidth will have been spent.

Client-Server Replication

Another quirk to be aware of is that client-server replication exists ONLY for your own playerpawn. That means that if you want a variable replicated from the client to the server, it needs to be done in your current PlayerPawn (or one of its parent classes). (You can use function replication to get around this limitation however.) Other playerpawns in the level are not the one that you currently 'possess', so their variables will not be replicated to the server. Just be aware of that when trying to replicate your variables and they aren't working. :) Thinking about it, it makes sense. What right does the client have to send any information to the server about another playerpawn, or some inventory item? The client is only concerned with variable changes that the player himself (or herself) makes, and thus those are the only ones that are sent to the server.

Replication Only When Necessary

Further optimizations exist for variable replications as well. A variable will only be replicated if the server thinks the client has the incorrect version compared to the server's latest version. This usually happens when the server changes a variable, and it knows the clients do not know about such a change. (They would if the function that changed them was simulated and was called clientside, meaning that you technically would not need to replicate that variable. This is explained later.. :) This means your code can change stuff clientside, and it will not be written over by server-side data until the server's data changes. As an example, I used this technique to implement a simple little static LOD back in the Unreal days before Epic introduced their realtime LOD. The client determined how far away the player was, and switched in variously detailed models depending on the distance. This allowed more models onscreen at once. And in multiplayer games, the client could change the mesh to the various LOD meshes, and it would work fine. Since the server never saw a change in the server's mesh (assuming it was a dedicated server), the client could change the LOD mesh without any problems. When the server did change the mesh however, the client would get a new mesh variable sent to it, and so it would have to then change it again. As a bragging aside, it even modified the intensity at which is applied LOD depending upon the framerate. ;)

Replication Data Approximation

Another optimization is that of how Unreal sends vectors, rotators, and arrays across the internet. The rules for this can best be described from the Unreal Tech page's Networking document, and I've copied it here to keep it in context:

Vectors and Rotators: To improve bandwidth efficiency, Unreal reduces the accuracy of vectors and rotators slightly, a process known as quantizing. The X, Y, Z components of vectors are converted to 16-bit signed integers before being sent, thus any fractional values or values out of the range -32768..32767 are lost. The Pitch, Yaw, Roll components of vectors are converted to bytes, of the form (Pitch8)&255. So, you need to be careful with vectors and rotators. If you absolutely must have full precision, then use int or float variables for the individual components; all other data types are sent with their complete precision.

General structs are replicated by sending all of their components. A struct is sent as an "all or nothing" type of thing.
Arrays of variables can be replicated, but only of the size of the array (in bytes) is less than 448 bytes.
Arrays are replicated efficiently; if a single element of a large array changes, only that element is sent. 

Variable Replication Examples

Now would probably be a good time to go through some important examples of replication and how it is used with variables, to give you a better idea of how things work, and to help give examples for your own adventures in coding.

Let's take a few examples, and describe exactly what they do. We'll start off with the easy ones, and gradually work our way up in complexity. The following examples are all taken from Actor, unless specified otherwise.

In PlayerPawn:

reliable if ( Role < ROLE_Authority ) Password, bReadyToPlay;

This here shows where your Password is sent to the server, when trying to join passworded servers, or to log in as an admin to that server. The server needs your password in order to validate it, and this code sends it to the server. Let's evaluate this check both on the server and on the client. Server: On the server, Role == ROLE_Authority. Looking at the conditional, it can easily be shown to be false, and so the Password is not replicated server-side. This means that the server does not send the Password to the other end of the connection. Client: Since on the client, the Role is not ROLE_Authority (remember it's only Authority on the server itself), this replication check evaluations to true on the client. And since variable replication only works in your current PlayerPawn, it will be sent to the server for yourself alone. (It makes sense not to send other's passwords, doesn't it? ;) 

This also shows how bReadyToPlay, which is used in Tournament style games, where everyone has to click to start the game. Note that this is only sent for yourself, since the client can only replicate variables to the server if they are part of it's own playerpawn actor. 

unreliable if( Role == ROLE_Authority ) Owner, Role, RemoteRole;

This ensures that the owner of an object is always replicated to the client. Remember that the owner's variables themselves are not replicated, (because there is no recursive replication,) but the actor itself is. This means that simple comparisons "if (PlayerPawn == Owner)" will work fine because the reference to the Owner is replicated to the client. Remember that the unreliable versus reliable makes no difference in UT, as they are both treated equivilently. They may have had an effect at sone point during the Unreal 1 days, but they no longer play a factor in development. Don't let that confuse you.

Replicating Role and RemoteRole may seem strange at first, since they should be reversed on the client. However, there's native code that causes that switch to be made. On the client, it will ensure that those two variables are reversed, so that RemoteRole is ROLE_Authority. You may wonder why they are even replicated in the first place. While the client never has any direct use for these variables in the code, there is one easy-to-overlook place where they are vitally important: Replication Statements. When the client is evaluating whether it needs to replicate a certain function call to the server, or a playerpawn's variable to the server, the client needs accurate copies of what Role and RemoteRole are. Without those, it would be unable to make the necessary decisions on what to send to the server. And since the server checks the validity of the replication data sent to the server with Role/RemoteRole, the client needs the latest copy in order to successfully replicate that data. 

unreliable if( bNetOwner && Role == ROLE_Authority ) bNetOwner, Inventory;

Inventory is the head of the linked list that lists the entire Inventory for the Actor (usually only utilized with Pawn and PlayerPawn). We want the inventory to be replicated to the client, so that he knows what Inventory he has, to display on his HUD, and stuff (makes sense, right?). Simply replicating the head of a linked list (where each object points to the next in the list) is not enough however. Each actor must itself be relevant (satisfied by the criterion that it must be owned by the current playerpawn), and each link itself must be replicated. And since Inventory subclass Actor somewhere in the chain, Inventory also has a replicated Inventory variable. And that's how you are able to see your Inventory client-side. And since I don't need to know the full Inventory list of what every other player in the game has (the current weapon and the shieldbelt effect are transferred via other means), the Inventory will only be replicated if the client is the owner.

The other variable in the above replication statement is bNetOwner. Since this is set natively on both the client and server, this variable does not need to be replicated. It could be argued that it adds a tiny amount to bandwidth, but its effect is probably negligible. Perhaps it will be removed in a future patch. 

unreliable if( DrawType == DT_Mesh && Role == ROLE_Authority ) Mesh, PrePivot, bMeshEnviroMap, Skin, MultiSkins, Fatness, AmbientGlow, ScaleGlow, bUnlit;

Here we see that all these mesh-specific variables are ONLY replicated if this actor is currently being displayed as a mesh. If it's a sprite, or a brush, or even no drawtype at all, there is no reason to send these variables. 
Now let's get into some of the more complex variable replication statements... 

unreliable if( RemoteRole == ROLE_SimulatedProxy ) Base;

Here we see a slightly more complex replication statement, that does not involve Role == ROLE_Authority. Here we see that the current Base (set via SetBase) is only replicated if the actor is set to be a simulated proxy. If you set the actor to a DumbProxy, you'll see no Base change clientside, and instead will get the jerky Location updates that the server sends to you, (more on this later). So if you are using SetBase, make sure it's a SimulatedProxy, or if it's not a SimulatedProxy, use something other than SetBase. :) Or if you need both SetBase and a SimulatedProxy, the Base will not be replicated for you. You'll have to work out some other mechanism to do that, probably a simulated function that sets the base, so it is run on the client, (more on this later, too.) You might still have troubles when the actor gets its simulated function called while the actor was not relevant. This results in the simulated function never being called clientside, and then when the actor does finally become relevant, it's as if that function was never called (and the base isn't set), which will result in those things affectionately referred to as 'bugs.' You can make the actor being attached (and what it's being attached to) bAlwaysRelevant so that the simulated functions will always be called, but that might be a bit too harsh on network bandwidth. :) 

unreliable if( RemoteRole == ROLE_SimulatedProxy && Physics == PHYS_Rotating && bNetInitial ) bFixedRotationDir, bRotateToDesired, RotationRate, DesiredRotation;

This one is still relatively easy, but it's getting more complex. Here we have similar logic to what was seen above, with the server only sending these variables to the client if the client is a SimulatedProxy. However, since all these variables apply ONLY to PHYS_Rotating, there is no need to replicate these variables to the client if the client is not using PHYS_Rotating. And finally, a very important clause at the end is the bNetInitial one. This states that these variables will only be replicated to the client when it is being replicated for the first time. 

unreliable if( bSimFall || (RemoteRole == ROLE_SimulatedProxy && bNetInitial && !bSimulatedPawn) ) Physics, Acceleration, bBounce;

Here we have some of the interesting variables replicated. We see that if the bSimFall is set to true, it will replicate the Physics to the client. This is used when tossing weapons from your inventory. When they are tossed, they require a Physics change from PHYS_None while sitting in your Inventory to PHYS_Falling as they fly through the air. The then get set back to PHYS_None when they land on the ground. All these Physics changes are accomplished by setting bSimFall at the key points during the TournamentWeapon's life. It is set to true when it is thrown from the inventory, to capture the change, and left on until it hits the ground, where it changes Physics again. After it finally hits and has its physics reset, bSimFall is set to false so no further Physics changes occur. Looking at the second half of the statement, we see that the Physics is replicated ONLY if it's a SimulatedProxy, it's the first time this actor is being replicated over the internet, AND it's not a simulated pawn. The bSimulatedPawn variable is set to true if it's a Pawn with a RemoteRole of ROLE_SimulatedProxy (who would have thought? :) This means any changes to the Physics of a SimulatedProxy non-pawn actor before it is replicated will be replicated to the client. The Physics are never replicated for pawns. Rather, the code for their physics is hard-wired into the code to cause them to be pushed to the ground. Basically, when a pawn jumps, the server sets his vertical velocity so that he travels upwards. That Velocity is then replicated to the client (described below). The client then checks if the pawn is a player (eg: a bot or a playerpawn), that they are currently unable to fly (set via the Pawn's bCanFly variable), and that they are not in a water zone (since that involves different gravities and physics). So in fact, the Physics for a pawn is never replicated to the client, it only appears to do so. If you are attempting to create alternate physics with the playerpawn, you will need to ensure that you set bCanFly to true so that the native code does not enforce the falling Physics upon the player when he's on the wall or in the air. You must then implement the alternate means of transportation yourself. 

unreliable if( !bCarriedItem && (bNetInitial || bSimulatedPawn || RemoteRole < ROLE_SimulatedProxy) && Role == ROLE_Authority ) Location;

Here we see another important variable, Location, and it's replication. We see that it is not replicated for carried items. This is useful in the case of Inventory. When a player is carrying it, there's no reason its location should be replicated, since it isn't used for anything. replicating a player's entire inventory would be quite the strain on network bandwidth. Let's look at the next section of the statement. The location is replicated if this is the first time the actor is being replicated (assuming the other parts of the conditional are true). This is very useful, since all pawns will be spawned in different locations when you enter the game, and rockets will need their start locations set when they spawn out of a rocket launcher, etc. The location is also sent if this is a bSimulatedPawn. This helps correct any errors that may occur in replicating pawns. Since a pawn can change direction, and a client may not always know about it (due to lag, etc), this location resetting is the only way in which the locational updates can be 'corrected'. Note that this is not used to correct your own location when you experience lag. When that happens, you are an AutonomousProxy, and functions specific to PlayerPawn handle that (namely ClientAdjustPosition). For example, PHYS_Walking is not like PHYS_Projectile, where the position can be predicted with great accuracy. PHYS_Walking just means "keep them attached to the ground", basically, The client's only updates (without location) would have been their velocity, which can easily lead to errors in the player's movements over any significant amount of time. This location is used as a correcting factor, making sure the client's view does not straw too far from where he expects the player to be, while at the same time keeping the velocity's replication so he can be predicted inbetween server updates. The other option for allowing the Location to replicate is that of where the RemoteRole is less than a ROLE_SimulatedProxy, namely, a ROLE_DumbProxy, since the ROLE_None prevents relevancy in the first place. DumbProxies get periodic updates from the server every few ticks, and would create a jumpy effect in netplay. It was the cause of a jerky puck in the hockey mod I attempted awhile ago, although I knew nowhere near enough to fix it at the time. :) A SimulatedProxy would not get sent Location updates, since it would be predicted through the use of the Velocity and the current Physics. Since DumbProxies aren't simulated client-side to achieve smoothness, they only get the Location updates. I'm going to clear this all up after the examples of the various replicated variables. 

unreliable if( !bCarriedItem && (DrawType == DT_Mesh || DrawType == DT_Brush) && (bNetInitial || bSimulatedPawn || RemoteRole < ROLE_SimulatedProxy) && Role == ROLE_Authority ) Rotation;

Here we see another important variable, Rotation. This also has the same !bCarriedItem clause, for the same reasons as described above. The rotation is also only replicated if this actor is a mesh or a brush. Since sprites always face the player, replicating their rotation is useless. The bNetInitial also causes the rotation to be replicated the first time through, (again, assuming if the other conditions are true,) since rockets need to be facing in the right direction when it has them flying through the air. Their velocity determines their direction of movement, but the direction they are facing which is equally important, is determined by their rotation. The rotation is replicated if they are a simulated pawn (every playerpawn and bot in the level except your own self). This is so you can see what direction the other people are facing. The ViewRotation (which determines where the client is looking), is sent to the server via ServerMove, where it is translated into a rotation. it is this rotation that is then replicated to the clients. And finally, if it is a DumbProxy (the only valid one less than SimulatedProxy), it also gets the rotation updates. Rotation updates do not need any interpolating between updates like location does. Location lag is much more noticeable when the player is moving quickly, but you never really notice Rotation lag. Besides, there is no real way to predict or forecast rotation. It depends entirely upon the other user's mouse. 

unreliable if( bSimFall || ((RemoteRole == ROLE_SimulatedProxy && (bNetInitial || bSimulatedPawn)) || bIsMover) ) Velocity;

Here's one of the last important variables with a complex replication statement. The velocity is replicated if it has bSimFall set, (used when throwing weapons from your inventory). The PHYS_Falling alone isn't enough. It needs to know exactly what it's initial velocity is when it is launched from the pawn. And no, bNetInitial will not work here, since it's not the first time it's being replicated. It's existed as a weapon for quite some time. It's just for the initial velocity of when it's being thrown from the player that we want updates. Moving along, we see that SimulatedProxies get their Velocities replicated if it the first time on the replication channel, for newly spawned rockets, or for grenades, or shock projectiles, etc. SimulatedProxies also get their velocities replicated if they are a simulated pawn, as all the various pawns need their velocities replicated so that they can be predicated locally. And finally, movers get their velocities replicated, so that the client can accurately predict the mover's movement locally. In the early Unreal 1 days, mover's velocity was not replicated, and so the client got periodic update locations because it was a DumbProxy. This resulted in very jumpy movers in netplay, something that was fixed for Unreal 224+. 

unreliable if( DrawType == DT_Mesh && ((RemoteRole <= ROLE_SimulatedProxy && (!bNetOwner || !bClientAnim)) || bDemoRecording) ) AnimSequence, SimAnim, AnimMinRate, bAnimNotify;

There are a few more variables here, all of which relate to animation. These variables are only replicated if it is currently being drawn as a Mesh. If we are recording a demo, then the animations are always sent. Otherwise, it checks the somewhat confusing conditional. If the actor is a DumbProxy, and the player is not the owner of this object, and bClientAnim is not set, then it replicates the animation variables. In the case of weapons, you see those animations clientside, and so the animation variables do not need to be replicated from the server. That is because all TournamentWeapons have the bClientAnim set to true, indicating that their animations are handled clientside. If the animations are not handled clientside, and the actor is a dumb proxy, then it will send them from the server. SimulatedProxies, which include pawns and rockets and other projectiles, are all simulated proxies, and so they do not receive animations from the server. Instead, the client animates them with its client-side prediction. In the case of pawns, this is handled internally in the engine (for another tutorial ;), and you need not worry about it.