Transcript Networks
Networks Network Protocols Peer-to-peer Client-Server Configurations Trust Nov 27, Fall 2006 IAT 410 1 Networks Required for multiplayer games 3 Standard technologies – Modems – Ethernet – Internet Nov 27, Fall 2006 IAT 410 2 Internet The greatest thing since sliced bread The savior of humanity Will increase freedom and democracy – Around the world – In your neighborhood Nov 27, Fall 2006 IAT 410 3 Internet User Protocols TCP – Connection – Reliable – Bytes arrive in order they were sent – Collects small packets and transmits them together – Stream of bytes Nov 27, Fall 2006 UDP – Connectionless – Unreliable – Arbitrary arrival order IAT 410 4 TCP Reliable stream of bytes – Implies the need for a “connection” Connection sets up data structures – Hold incoming packets – Hold outgoing packets – Handle retransmits Nov 27, Fall 2006 IAT 410 5 TCP Reliability Each packet does Send-ReceiveAcknowledge Sender Send Receiver Receive Acknowledge Sender holds sent packet until ACK is received Sender retransmits if ACK takes too long Nov 27, Fall 2006 IAT 410 6 TCP One Send-Receive-Ack takes time Overlay Sends and Acks Maintain a queue in sender and receiver Sender Send 0 Receiver 1 0 2 1 0 3 2 1 3 2 Ack Sender 3 Nov 27, Fall 2006 IAT 410 7 TCP Circular Queue -- Sender Sends data and Puts it in send queue Sets timer on this queue item If timer expires, and no ACK, re-send data – Set another, longer timer – Exponentially increasing time When ACK received – If this queue slot is the oldest, • Free the slot for new data If no queue space avail, sender app waits! Nov 27, Fall 2006 IAT 410 8 TCP Receive Queue Receiver maintains a queue the same size as the sender’s When a packet arrives, send ACK If the packet is next in sequence – Give it to application Else Keep it in queue – Another, earlier packet is on its way Nov 27, Fall 2006 IAT 410 9 TCP If no ACKS arrive for a long enough time – Temporarily gives up – Sends test packets When test packets get through – Starts slow, builds up Nov 27, Fall 2006 IAT 410 10 TCP Wrap-up Connection sets up sequencing and queues – Reliable arrival: – Reliable order: Retransmit Sequence numbers TCP bunches up data on 200ms intervals – Minimizes overhead for small chunks of data – This option can be turned off TCP Has an “emergency” channel – OOB Out Of Band Nov 27, Fall 2006 IAT 410 11 UDP Connectionless! – No underlying data to maintain Unreliable transmission – If you lose a packet, it’s gone – Network software must handle this Out-of-order arrival – Network software must handle that, too! Fast – When the port gets the data, the app gets it Nov 27, Fall 2006 IAT 410 12 UDP Packets will drop! – 1 in 5 over non-local connection Have to do your own re-send Some packets are time sensitive – Care little about the past ship location No header compression – May end up with greater overhead than TCP with PPP Nov 27, Fall 2006 IAT 410 13 Game Architectures Peer-to-peer Client/Server – One server per game Floating server – One client is also a server Distributed server – Multiple servers for large world Nov 27, Fall 2006 IAT 410 14 Peer-to-Peer Simple version: Lockstep – eg. Doom – Each client transmits to other – Wait for everyone to get data – Proceed to next step Nov 27, Fall 2006 IAT 410 15 Peer-to-Peer Advantages – Simple – Nobody has to provide a server • Including the Game’s authors! – Good for turn-based games with low bandwidth – TCP Nov 27, Fall 2006 IAT 410 Disadvantages – Frame rate is that of • Slowest machine • Worst connection – Hackable – Not good for real-time games 16 Client/Server Server per game – MUDs, Fireteam, NetTrek – Someone must provide server ($$$) • Possibly the game’s authors – Less hackable – Single point of failure – Server must be big & well-connected Nov 27, Fall 2006 IAT 410 17 Floating Server Peer-to-peer Server resolves the action One peer is the server – Unreal • One player elects to be the server – X-Wing vs Tie-Fighter: • First player to enter session – Starcraft • Player with the CD Nov 27, Fall 2006 IAT 410 18 Multiple Server Many machines coordinate service – Ultima Online, Everquest, AOL Used for large virtual worlds Everquest – One server per game-geographic region – Freeze on handoff affects game play Nov 27, Fall 2006 IAT 410 19 What Data to Send? Sending entire world state is usually too much Can send just user actions – Simulation engine does the same thing at each client – Pseudo-random numbers from same seed Nov 27, Fall 2006 IAT 410 20 Sending User Actions-Problems Any error in engine – Divergence in worlds – Small error can lead to big divergence X-Wing vs Tie Fighter – Created a resynchronize protocol – Causes jumps • Wrote smoothing algorithm for resynchs Sim City 2000 Network Edition – Send checksums for world state each turn Nov 27, Fall 2006 IAT 410 21 Prediction Eg. Unreal Waiting for user inputs is too slow Client does prediction – Motion prediction Server Nov 27, Fall 2006 corrects things if client is wrong IAT 410 22 Prediction: Dead Reckoning Eg. SIMNET (US Army Tank Simulator) Each vehicle simulates own tank Sends data every 5 seconds, updating – Position, Speed, Acceleration – Expected path – Prediction violation criteria Receiver simulates own tank – AND simulates local copy of other tanks Nov 27, Fall 2006 IAT 410 23 Dead Recokoning Receiver gets latest 5-second update Updates own copy of other tanks Predicts other tanks – Using prediction data – Until new data arrives Each simulator also sends update – When own prediction violates own criteria Assumes Nov 27, Fall 2006 latencies < 500ms IAT 410 24 Dead Reckoning Sim A A’s Predicted Path Predict B Sim B Sim A A’s Predicted Path B’s Predicted Path Predict A Predict B Transmit new prediction every 5 seconds Nov 27, Fall 2006 IAT 410 Sim B B’s Predicted Path Predict A B Exceeds prediction: predict again and transmit 25 Dead Reckoning: Requirements Data structures for other entities Model of entity behavior – Vehicle speed, acceleration range, turn radius – Responsiveness to commands Situation parameters – Following a road – Precomputed path (NPCs) Nov 27, Fall 2006 IAT 410 26 Multiple Copies Maintain 2 Data sets Now – Accurate self – Predicted others – “Zero” latency for self Ground Truth – Accurate everybody – Large latency for almost everybody – 200-500ms ago Nov 27, Fall 2006 IAT 410 27 Latency Issues When latencies get high – Prediction gets worse and worse Correcting prediction errors may cause visual jumps – Easy to notice! If jumps are large enough – Temporarily interpolate between wrong prediction and the new correction Nov 27, Fall 2006 IAT 410 28 Prediction Interpolation Interpolated Response Real Predicted Nov 27, Fall 2006 IAT 410 29 Token Ownership Some games may allow distributed ownership – Ballistic simulation – Shooter fires bullet – Intended target receives the simulation Sports - eg. Tennis – Player A hits ball – Player B gets simulation token – B simulates ball path from A’s racket Nov 27, Fall 2006 IAT 410 30 Trust “Never trust the client” Data on the user’s hard drive is insecure – Diablo utility to modify character data – Wrote patch to prevent hacking • Throws out your stuff if there’s a time inconsistency – Daylight savings nuked my stuff! Nov 27, Fall 2006 IAT 410 31 Trust Network communications are insecure NetTrek communications are encrypted NetTrek also requires “blessed” client – Servers have different policies on requiring a blessed client – Prevents robot players or assistants Nov 27, Fall 2006 IAT 410 32 Trust -- Checksums First line of defense: – Checksum of all packets – Include header in checksum! – Stops casual tampering Hash function – Hard to compute source value from result – MD5 Nov 27, Fall 2006 IAT 410 33 Checksums Not immune to: – Code disassembly – Packet replay Packet replay attack: – Capture a legal packet, and re-send it more frequently than allowed – Client can restrict send frequency – Server cannot reject high-frequency packets • Internet bunch-ups are source of OK bunch-ups Nov 27, Fall 2006 IAT 410 34 Combating Replay Each new packet client sends is different – Add a pseudo-random number to each packet – Not just sequence number! – Client & Server match pseudo-random numbers Random numbers – Seeds must match! – Dropped packets: include sequence number! Nov 27, Fall 2006 IAT 410 35 Combating Replay XOR each packet with a pseudo-random bit pattern – Make sure the bit patterns are in sync! – Based on previous synchronized pseudorandom numbers Add junk – Confuse length analysis Nov 27, Fall 2006 IAT 410 36 Reverse Engineering Remove symbols Put encryption code in with rest of network stuff Compute magic numbers: – At runtime – In server Encrypt Nov 27, Fall 2006 from the start! IAT 410 37 Lists Of Servers Denial of service: – Send a packet to server-server saying “I’m a server” – Fake the IP return address with a random IP# – Server-server adds “new server” to list – Server may run out of memory storing hundreds of thousands of fake servers Nov 27, Fall 2006 IAT 410 38 List of Servers Require a dialog – Server-list server responds with • Password • Keepalive interval – Password must be given by attacker at the correct time – Works OK if client is not better connected! Nov 27, Fall 2006 IAT 410 39