1. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Last Time More collision detection packages Time critical collision detection Introduction to networking for games

2. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Today Networking protocols Cheating and Security –Great article: Evaluations

3. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Internet Protocols There are only two internet protocols that are widely deployed and useful for games: UDP and TCP/IP –TCP/IP (Transmission Control Protocol/Internet Protocol) is most commonly used –UDP (User Datagram Protocol) is also widely deployed and used Other protocols exist: –Proprietary standards –Broadcast and Multicast are standard protocols with some useful properties, but they are not widely deployed –If the ISPs don’t provide it, you can’t use it

4. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin TCP/IP Overview Advantages: –Guaranteed packet delivery –Ordered packet delivery –Packet checksum checking (some error checking) –Transmission flow control Disadvantages: –Point-to-point transport –Bandwidth and latency overhead –Packets may be delayed to preserve order Uses: –Data that must be reliably sent, or requires one of the other properties –Games that can tolerate latency

5. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin UDP Overview Advantages: –Low overhead in bandwidth and latency –Immediate delivery - as soon as it arrives it goes to the client Disadvantages: –Point to point connectivity –No reliability guarantees –No ordering guarantees –Packets can be corrupted –Can cause problems with some firewalls Uses: –Data that is sent frequently and goes out of date quickly

6. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Choosing a Protocol The best way to do it is decide on the requirements and find the protocol to match –In most cases, that means TCP/IP You can also design your own “protocol” by designing the contents of packets –A protocol is simply a set of rules for putting data in the packets and acting upon it –Add cheat detection or error correction, for instance –You then wrap you protocol inside TCP/IP or UDP – just send your “packet” as a regular chunk of data

7. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Reducing Bandwidth Demands Bandwidth is plentiful on the internet today, so it only becomes an issue with large environments –Even “slow” modems have more impact through high latency than low bandwidth (due to compression, error checking and analogue/digital conversion) Regardless, smaller packets reduce both bandwidth and latency –Latency is measured from the time the first bit leaves to the time the last bit arrives - so fewer bits have lower latency There are two primary ways to reduce bandwidth demands: –Dead reckoning allows you to send state less frequently –Area of interest management avoids sending irrelevant data

8. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Area of Interest Management Area of interest management is the networking equivalent of visibility - only send data to the people who need it There is a catch, however: In a network you may not know where everyone is, so you don’t know what they can see –A chicken-and-egg problem Hence, area-of-interest schemes are typically employed in client-server environments: –The server has complete information –It decides who needs to receive what information, and only sends information to those who need it Two approaches: grid methods and aura methods –Sound familiar? (replace aura with bounding box)

9. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Grid and Aura Methods Grid methods break the world into a grid –Associate information with cells –Associate players with cells –Only send information to players in the same, or neighboring, cells –This has all the same issues as grid based visibility and collision detection Aura methods associate an aura with each piece of information –Only send information to players that intersect the aura –Just like broad-phase collision detection with bounding volumes Players need to find out all the information about a space when they enter it, regardless how long ago that information last changed –In other words, have to be careful with “state” information that is infrequently sent

10. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Building for Networking It is generally agreed that networking must be a consideration from the start of development There are many good practices – see game programming web sites for tips (and descriptions of past mistakes) One good practice: Access all data through a consistent interface, which both the networking and local code uses –Avoids different side effects from different access points –Can check for validity of data in one place, which makes checking for network errors easier –Can decrypt/encrypt in one place –Can automatically propagate data onto network

11. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Cheating It’s an arms race… –http://www.gamasutra.com/features/20000724/pritchard_01.htmhttp://www.gamasutra.com/features/20000724/pritchard_01.htm

12. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Why Care About Cheats? Online gaming is big business Cheats can achieve financial advantage: –Competitive games with prizes are the obvious example (casinos) –Also consider EverQuest: People play the game, build good characters, and then auction them on ebay. If they can cheat to obtain good characters, they are achieving unfair financial advantage Cheats can ruin the game for everyone: –Players tend to have a strong sense of fairness –If they believe they are being cheated, they will not play, and you will not make any money Single player cheaters typically only affect themselves, so you don’t care

13. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Sources of Cheats Reflex augmentation: Use a cheat to improve some aspect of physical performance, such as the firing rate or aiming Authoritative clients: Clients issue commands that are inconsistent with the game-play, or mimic the server Information Exposure: Clients obtain information that should be hidden Compromised servers: A hacked server biases game-play toward the group that knows of the hacks Bugs and Design Loopholes: Bugs are found and exploited, or parts of the program intended for one purpose are used for another Environmental Weakness: Differences or problems with the OS or network environment are exploited

14. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Observations About Cheating The only way to make a system 100% secure is to completely isolate it (Eric Brewer, 1996) Pritchard’s Rules (Gamasutra article): –If you build it, they will come - to hack and cheat –Hacking attempts increase as a game becomes more successful –Cheaters actively try to control knowledge of their cheats –Your game, along with everything on the cheater’s computer, in not secure - not memory, not files, not devices and networks –Obscurity is not security –Any communication over an open line is subject to interception, analysis and modification –There is no such thing as a harmless cheat –Trust in the server is everything in client-server games –Honest player would like the game to tip them off to cheater, hackers hate it

15. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Reflex Augmentation Aiming proxies intercept communications, build a map of where people are, and automatically shoot them Rapid-fire proxies take each “shoot” packet and replicate it Fix #1: The server validates player actions - if they are too good the player is considered a cheat and kicked out –What’s hard about this? Fix #2: Make it difficult to insert non-valid packets –Encrypt the packets, but your encryption must be cheap, and cheap encryption can be broken –Make the encryption depend on the game state or other time-dependent “random” value. Hard to do with UDP. Why? –If using guaranteed delivery (TCP) serialize packets with a unique sequence of numbers - cannot then replicate or insert extra packets

16. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin A Word on Encryption Typically: A key – known only to intended users – is used to convert regular data into something that looks random –Cannot go from random data back to key, or to the real data Many ways to come up with the key –Agree on it ahead of time –Transmit it – key exchange algorithms –Derive it from somewhere else in such a way that all parties derive the same key (e.g. from game state) Most encryption algorithms work on blocks of a fixed size –Split large amounts of data into smaller blocks –Pad blocks that are too small

17. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Authoritative Clients Occurs when one player’s game informs everyone else that a definitive event has occurred: e.g. I just got a power-up Hacked clients can be created in many ways: change the executable, change game data in other files, hack packets Fix is to insert command request steps: –Player request an action, its validity is checked, it is sent out on the network, and added to the player’s pending event queue –Incoming actions also go on the pending queue –Actions come off the pending queue, are validated again, and then are implemented Sometimes validation is hard to get right, so try synchronization –Occasionally send complete game state around, and compare it –Actually, send something derived from complete game state

18. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Information Exposure Some classics: Modify the renderer to make walls transparent, modify maps to remove the fog of war Basically, display variables must be modified in memory, or read out and displayed elsewhere –Hackers are very good at finding the locations of key data in memory, and modifying them transparently Fixes: –Check that players agree on the value of certain variables, and the validity of actions - synchronization again Note that you can look for actions that cannot be valid with the correct display –Compile statistics on drawing, and see of they look off (eg # polygons drawn) –Encrypt data in memory to avoid passive attacks

19. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Compromised Servers Many servers have customization options, and the community is encouraged to modify the server –This is completely legal However, as a game becomes popular, naïve people start to play the game –They do not have the skills or knowledge to check that the server they are playing on is “pure” –They will grow frustrated, blame the developer, and complain to their friends Some modifications can be very insidious, and may not be legal. For example, hack the server to do different damage for opponents Solution is to warn people when they connect to the server, and about any other non standard properties (found through validation)

20. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Bugs and Design Issues Some bugs enable cheating, such as a bug that enables fast reloading, or one that incorrectly validates commands Some design decisions make cheating easier: –Embedding cheats codes in single player mode makes it easy for a hacker to track down the variables that control cheats –Poor networking or event handling can allow repeat commands or other exploitations Age of Empires and Starcraft example: all resource management is done after all events for a turn are processed. Poor networking allowed multiple cancel events on the queue, which restored multiple resources Solution is to avoid bugs and think carefully about the implications of design decisions on hacking

21. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Environmental Weaknesses Facilities to deal with the OS or network may leave you vulnerable to some forms of attack –Interaction with the clipboard can introduce non-printable characters –Interaction with almost any scripting language may leave you open to hacks not related to the game itself (ie your game could be a way in) –Network connection drops or overloading can cause problems Some cheats destroy the game for every player (tip the board) which can be useful if you are losing Others knock off a specific player (your worst enemy, one assumes)

22. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin The Moral of the Story You can’t win, you just try to make cheating as hard as possible and contain the damage

23. 12/09/03CS679 - Fall 2003 - Copyright Univ. of Wisconsin Todo By Thursday, Dec 11, Midday, Final Demo Monday, Dec 15, Final Exam, 2:45 P.M, RM 1221 CS