CS4344 09/10 Lecture 10: Transport Protocol for Networked Games
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The document discusses transport protocols for networked games and compares TCP and UDP. While TCP provides reliable delivery, it has higher latency than UDP. UDP has lower overhead but is unreliable. The document examines why certain popular games use TCP or UDP and outlines strategies to make TCP perform better for games, such as reducing delays, retransmitting bundles of data, and combining thin streams. It suggests the Stream Control Transmission Protocol (SCTP) as a potentially ideal transport for games since it allows flexibility in reliability and ordering of messages.
CS4344 09/10 Lecture 10: Transport Protocol for Networked Games
- 1. Lecture 10 Transport Protocol for Networked Games 29 March 2010 1
- 2. TCP or UDP ? 2
- 3. Why use TCP? • TCP provides reliable, in-order delivery • TCP goes through most firewalls, UDP does not • TCP manages connection for us 3
- 4. Why not to use TCP? • TCP incurs higher latency • Don’t always need reliability and in-order delivery • High header overhead 4
- 5. position = 10 position = 13 X position = 15 Updated position not delivered to application until (outdated) lost packet is received 5
- 6. A’s position = 10 B’s position = 13 X C’s position = 15 Some messages need not be delivered in sequence. 6
- 7. Gestures from someone far away need not be received reliably. 7
- 8. 46% of ShenZhou Online bandwidth is occupied by TCP header 8
- 9. enet.cubik.org A library that provides reliability, sequencing, connection managements over UDP 9
- 10. Delivery can be stream-oriented (like TCP) or message-oriented (like UDP) 10
- 11. Supports partial reliability enet_packet_create (“abc”, 4, ENET_PACKET_FLAG_RELIABLE) 11
- 12. Retransmission triggered by timeout-based on RTT 12
- 13. Data in queue are bundled into one packet if there is space 13
- 14. enet.cubik.org Portable, easy to use, but still, most firewalls block UDP traffic 14
- 15. • Uses TCP: WoW, Lineage I/II, Guild Wars, Ragnarok Online, Anarchy Online, Mabinogi • Uses UDP: EverQuest, SW Galaxies, City of Heroes, Ultima Online, Asherons Call, FFXI 15
- 16. Need to study the use of TCP for networked games 16
- 17. How to provide reliability over UDP? 17
- 18. How slow is TCP, really? Which part of TCP is the root of slowness? Can we fix TCP? 18
- 19. A Quick Review of TCP 19
- 20. s=100 s=200 s=300 s=400 a=300 a=500 TCP Delayed ACK 20
- 21. TCP Spec: max 500ms delay Most implementation: 200ms 21
- 22. Why delay ACK? • reduce num of ACKs • in case receiver wants to send data within 200ms (in which case it can piggyback the ACK with data) • give sender time to buffer more data for sending (avoid silly window syndrome) 22
- 23. s=100 s=200 s=300 a=200 X s=400 s=500 a=200 a=200 a=200 s=200 TCP Fast Retransmission 23
- 24. Definition of Dup ACKs in 4.4BSD and Stevens: “pure ACK with no data” 24
- 25. RTO X X 2xRTO TCP Timeout + Exponential Back-off 25
- 26. RTO Spurious Retransmission 26
- 27. RTO estimation Ei = 7Ei-1/8 + RTT/8 Vi = 3Vi-1/4 + |RTT-Ei-1|/4 RTO = max(Ei + 4Vi, 1s) 27
- 28. Linux’s RTO estimation Ei = 7Ei-1/8 + RTT/8 Vi = 3Vi-1/4 + |RTT-Ei-1|/4 Wi = min(Vi, 50ms) RTO = max(200ms, Ei+Wi) 28
- 29. delayed ACK increase RTT increase RTO 29
- 30. Congestion Control 30
- 31. Window Size Packet Loss Time TCP Congestion Control 31
- 32. Congestion window resets to 2 after an idle period (> RTO) 32
- 33. What does real game traffic look like? 33
- 34. low packet rate small packet size 34
- 35. “Thin Streams” 35
- 36. 36
- 37. Max Application Delay Max RTT 250ms Avg RTT 37
- 38. 38
- 39. About 4 packets / sec 39
- 40. Average Payload: 100 Bytes 40
- 41. Loss Rate 1% 41
- 42. But some experience 6 retransmissions 42
- 43. ShenZhou Online 43
- 44. 44
- 45. 45
- 46. 46
- 47. “Thin Streams” 47
- 48. Findings 1: Fast retransmission rarely triggered 48
- 49. In ShenZhou Online traces, fail to trigger fast retransmission because insufficient dup ACK (50%) interrupted by data (50%) 49
- 50. 50
- 51. 51
- 52. Findings 2: Delay due mostly to timeout 52
- 53. 53
- 54. Findings 3: Congestion window reset is frequent 54
- 55. 55
- 56. 12% - 18% of packets faces window reset 56
- 57. think.. think.. think.. click (tank attack here) click (missile launch there) click (charge soldiers) The last command is delayed as congestion window = 2 57
- 58. How to make TCP (or, transport protocol) go faster in these games? 58
- 59. 1. Remove exponential backoff 59
- 60. TO X X TO TCP Timeout 60
- 61. 2. Make RTO Smaller 61
- 62. make sure minimum RTO is not 1s 62
- 63. spurious retransmission is not disastrous 63
- 64. 3. Make Fast Retransmit Faster 64
- 65. s=100 s=200 s=300 a=200 X s=400 s=500 a=200 a=200 a=200 s=200 Retransmit after one duplicate ACK 65
- 66. 4. Retransmission Bundling 66
- 67. X Retransmit all unacknowledge data in queue 67
- 68. 5. Redundant Data Bundling 68
- 69. X a=300 a=500 Send any unacknowledged segment in queue as long as there is space. Lost data gets recovered in the next transmission before retransmission. 69
- 70. 6. Turn off or reduce Delayed ACKs 70
- 71. Packet interarrival time on average > 200ms (can’t combine two ACKs into one) 71
- 72. 7. Combine Thin Streams into Thicker Stream 72
- 73. Server Proxy 73
- 74. TCP for Games • remove exponential backoff • reduce RTO • make fast retransmit faster • retransmit agressively • don’t delay ACK • combine into thick streams 74
- 75. Beyond TCP and UDP? 75
- 76. Ideal Transport Protocol for Games • optional reliability • optional order-of-delivery • flexibility in organizing messages into different classes with different requirements 76
- 77. SCTP Stream Control Transport Protocol 77
- 78. Multi-streaming: multiple independent streams 78
- 79. A stream can be either reliable or non-reliable 79
- 80. Data from multiple streams can be bundled into one packets 80
- 81. Message-oriented (like UDP) 81
- 82. Message can be flagged for unordered delivery 82
- 83. Ideal Transport Protocol for Games • optional reliability • optional order-of-delivery • flexibility in organizing messages into different classes with different requirements 83
- 84. SCTP for Games? 84
- 85. States of SCTP • rumored to be in Vista • available in FreeBSD 7 and Solaris 10 • lksctp project for Linux 85