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Part 2 : Actual Internet Performance Requirements and Constraints

Part 2 : Actual Internet Performance Requirements and Constraints. Anthony Steed. Actual Internet Performance - What bandwidth can we expect? - Sources of latency - What other issues must we consider? Requirements and Constraints - Requirements on consistency

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Part 2 : Actual Internet Performance Requirements and Constraints

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  1. Part 2:Actual Internet PerformanceRequirements and Constraints Anthony Steed

  2. Actual Internet Performance • - What bandwidth can we expect? • - Sources of latency • - What other issues must we consider? • Requirements and Constraints • - Requirements on consistency • - Requirements on latency • - User response to inconsistency and latency

  3. Actual Internet Performance

  4. Actual Internet Performance • - What bandwidth can we expect? • - Sources of latency • - What other issues must we consider?

  5. Network properties • Latency (Round Trip Time) • Devicestake time to send data (e.g. Modems) • Data takes time to transmit (speedoflight) • Jitter • Routersinsertbandwidth • Bandwidth (Capacity) • Bandwidthcostsmoney • In UK: 8Mbps isfairly standard @£10($15)/month • Loss (Congestion, Reliability) • Routersdroppackets, links do godown, routes do fluctuate

  6. Latency & Jitter

  7. It is not possible for EVERY user to share the EXACT same state at EVERY instance Reality Check GOLDEN RULE Information propagation IS NOT instantaneous

  8. Impact on the Shared Experience Host C Host A Host B

  9. Overview of the Challenge Senses Local Host Devices Mental Model Human Brain Access Network Muscles Human System Network Internal Processing Local Processing Network Processing The total processing time must not exceed the interactive threshold which is determined by Gameplay

  10. Latency and Jitter : Single Host Application System Model Input Simulation Rendering 1 2 3

  11. Latency and Jitter : Networked Host Application System Model Input Simulation Rendering 1 2 3 5 Network 4 Link Physical Internet

  12. Latency and Jitter : Client and Server Application System Model Input Simulation Rendering 1 2 3 6 Network 4 5 Link Physical Internet Server Application Simulation

  13. Latency : Network Perspective Input Queues Output Queues Routing Table Handler

  14. Latency : Network Perspective Input Queues Output Queues Latency Latency Routing Table Latency Handler

  15. Jitter • Jitter is change in latency • Jitter is caused by the technology of the Internet • Wired routers • Wireless access • Two problems: • Routers are almost certainly capacity bound and demand on routers changes rapidly • Some link layers (notably wireless) are shared medium so transmitters will conflict

  16. Latency and Jitter : Network Perspective Jittered Timing Regular Timing Internet Receiver Sender Network Latency Transmission Delay : time it takes to put a packet on the outgoing link Propagation Delay : time it takes for the packet to arrive at destination

  17. Variance of inter-packet arrival times Correct spacing Gaussian distribution Observed distribution Frequency of occurrence Interpacket arrival time

  18. Bandwidth & Loss

  19. Bandwidth • Bandwidth is a shared resource • At local level we shared the wireless or share a home or office router • Can be much more outbound or requested inbound traffic that the local network can access • However probably, the bottleneck is likely to be upstream to our ISP • ISP have intra-ISP (and “senior” ISP) bottlenecks • The destination site (BBC, Facebook) might have inbound capacity limits

  20. It is the Internet’s defence against failure Dropping packets (hopefully) causes senders (processes or users) to rate-limit Loss Another GOLDEN RULE Packet Loss is a Good Thing

  21. Loss : Network Perspective Input Queues Output Queues Loss Routing Table Handler

  22. Throughput : Network Perspective Throughput : number of bits per time of unit

  23. Throughput : Network Perspective Throughput : number of bits per time of unit Potential Loss and Increased Delay

  24. STATE OF THE INTERNET

  25. Bandwidth and Latency: Wired • Much literature in the area is based on 56kbps modems … • Broadband is now common in homes • 500Kbps – 1Gbps • Depends on technology (twisted-pair v. optical) • Offices have always been different • 1Gbps Ethernet, switched (not shared) is common • Outbound varies enormously • Latency is good

  26. Bandwidth and Latency: Wireless • 2G • Don’t try, run web or sms-based applications! • 3G / 4G • 3G: ~2.4Mbps • 4G: 100Mbps – 1Gbps • 802.11a-n • b: 11 Mbps • n: 54 Mbps • Be skeptical: its shared bandwidth • Latency is moderate-poor: its shared bandwidth

  27. Bandwidth Availability Average connection speed by country, Q4 2008. Based on (Akamai, 2009)

  28. Effect of distance on throughput and download times Based on (Leighton, 2009)

  29. Requirements and Constraints

  30. Requirements and Constraints • - Requirements on consistency • - Requirements on latency • - User response to inconsistency and latency

  31. Consistency : System Perspective • C1 : Local changes replicated at each site • C2 : Simulation should not diverge over time • C3 : Casual order of events should be preserved • C4 : Temporal and motion characteristics of events should be preserved

  32. Consistency : User Perspective • C5 : The joint perception of events should be plausible • C6 : The outcome of the events should be fair • C7 : The system should preserve the users’ intentions

  33. Latency Impact

  34. Impact: Timings Activity Onset CarA CarB CarA CarB CarA A=1, V=0 CarB A=1, V=0 CarA A=1, V=1 CarB A=1, V=1 CarA A=1, V=2 CarB A=1, V=2 CarA A=1, V=3 CarB A=1, V=3 ClientA ClientB

  35. Impact: Inconsistent State Changes Door is Closed & Unlocked Door is Closed & Unlocked Door is Closed & Locked Door is Open & Unlocked Lock Door Open Door Can’t apply open state Door is Open & Locked ClientA ClientB

  36. Impact: Fireproof Players Shooter (PlayerA) Target (PlayerB) ClientA Server ClientB

  37. Impact: Shooting Around Corners Shooter (PlayerA) Target (PlayerB) ClientA Server ClientB

  38. Latency Acceptability Lower Precision Aiming Weapon / Shooting Sniper Aiming & Shooting Machine Gun Drinking Health Potion Casting Area Spell Vehicle Racing Moving (God Game) Building (God Game) Run command Combat Shooting Rockets Fighting (God Game) Exploring (God Game) Mouse Control Highest Avatar Control Camera Control Several tasks plotted on the Precision/Deadline axes. Based on Claypool and Claypool (2006). Tightest Deadline Lowest Immediate Control Tasks

  39. BANDWIDTH

  40. Bandwidth Requirements • Obviously depends on activity • Downloading models • Sending small, game specific commands • Rate of command sending (very sensitive to type of game) • Typically: • FPS & real-time send commands at fixed rate (e.g. 20 Hz) • RTS and other send commands at issue rate (e.g. up to 5Hz with StarCraft)

  41. Packet Rates Server packet rates and sizes for three FPS games, from Feng et al. (2005)

  42. Packet Rates Client packet rates and sizes for four MMORPG games, from Molnár & Szabó (2008)

  43. Packet Rates Bandwidth of Second Life for different region types and different modes of travel. From Kinicki & Claypool (2008)

  44. CONNECTIVITY

  45. Network Address Translation • The biggest hiccup for any peer to peer networking • Many (most?) computers on the Internet are behind a NAT • We are behind a NAT • 192.168.14.32 is in a reserved IP address domain • Your home probably runs a NAT • You have one address from your ISP • You PAY to have this be a static IP address • You pay more to have more than one

  46. Reserved Addresses IP address. (2011, March 18). In Wikipedia, The Free Encyclopedia. Retrieved 07:24, March 19, 2011, from http://en.wikipedia.org/w/index.php?title=IP_address&oldid=419473743

  47. What Does NAT Do? • Network Address Translation is a function of your router (gateway) • You have any number of devices on your LAN • All appear to have the same IP to the outside world • The NAT replaces the source address and source port of the IP packets

  48. What a NAT Does • Store a table • Not trivial to do this, some systems use lots of connections and ports • Overload is a common cause of WLAN falling over

  49. Implication of NATs • There is no problem calling out • Calling in you can’t know automatically which machine behind a NAT uses what ports • The NAT needs to discover or be told that port 80 (web service) packets need to be routed to a specific machine • Most home gateways have functionality for this specifically for running game services! • This is a problem for any peer to peer system. Your likely experience with it is using Skype

  50. NAT Traversal • NAT breaks one of the fundamental assumption of Internet: that all machines are peers and are routable by IP number • NAT traversal is supported by some protocols • E.G. callback: protocol can work in either way, TCP is also bidirectional • Otherwise need to do it yourself

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