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Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
Congestion Control in Computer Networks - ATM and TCP
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Congestion Control in Computer Networks - ATM and TCP

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  • 1. Congestion Control in Networks ATM and TCP Balazs Attila-Mihaly
  • 2. Outline <ul><li>What is congestion?
  • 3. Myths about congestion
  • 4. Congestion control possibilities
  • 5. ATM – Vocabulary
  • 6. ATM – Congestion Control </li></ul><ul><li>ATM – CC evaluation
  • 7. TCP
  • 8. TCP – Congestion Control
  • 9. TCP – CC evaluation
  • 10. Bibliography </li></ul>
  • 11. What is congestion? <ul><li>”network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates” - Wikipedia
  • 12. QoS </li><ul><li>Packet loss
  • 13. Queuing delay
  • 14. Jitter
  • 15. Low troughput </li></ul></ul>
  • 16. Congestion control possibilities <ul><li>Admission control
  • 17. Traffic access control
  • 18. Packet scheduling (prioritization)
  • 19. Buffer management
  • 20. Flow control (speed matching)
  • 21. QoS routing </li></ul>
  • 22. Congestion control possibilities <ul><li>Admission control
  • 23. Traffic access control
  • 24. Packet scheduling (prioritization)
  • 25. Buffer management
  • 26. Flow control (speed matching)
  • 27. QoS routing </li></ul>Congestion <ul><li>prevention
  • 28. avoidance
  • 29. recovery </li></ul>
  • 30. CC Objectives <ul><li>Scalability
  • 31. Fairness </li><ul><li>Max-Min
  • 32. Min + Equal share
  • 33. Maximum of the previous too
  • 34. Allocation proportional with Min
  • 35. Weighted allocation </li></ul><li>Robustness
  • 36. Implementability </li></ul>
  • 37. ATM <ul><li>Protocol implemented over different media
  • 38. Fixed sized cells (53 bytes – 48 byte of data)
  • 39. Vocabulary: </li><ul><li>VCI – Virtual Circuit Identifier
  • 40. PCR – Peak Cell Rate
  • 41. SCR – Sustained Cell Rate
  • 42. MBR – Maximum Burst Size
  • 43. MCR – Minimum Cell Rate </li></ul></ul>
  • 44. ATM <ul><li>Vocabulary </li><ul><li>CTD – Cell Transfer Delay (maximum / mean)
  • 45. CDV – Cell Delay Variation (peak-to-peak / instant.)
  • 46. CLR – Cell Loss Ratio </li></ul><li>Service categories </li><ul><li>CBR – Constant Bit Rate
  • 47. rt-VBR – Real-Time Variable Bit Rate
  • 48. nrt-VBR – Non-Real-Time Variable Bit Rate
  • 49. UBR – Unspecified Bit Rate
  • 50. ABR – Available Bit Rate </li></ul></ul>
  • 51. ATM – Congestion Control <ul><li>Admission control – CAC (Call Access Control) </li><ul><li>Hard or soft (statistical) limits </li></ul><li>Traffic access control – GCRA (Generic Cell Rate Algorith) </li><ul><li>TAT – Theoretical Arrival Time
  • 52. I – Increment
  • 53. L - Limit </li></ul></ul>Cell arrival at t t &lt;= TAT TAT = t t+L &lt; TAT Conforming cell TAT = TAT + I Non-conforming cell
  • 54. ATM – Congestion Recovery <ul><li>Mainly for ABR traffic
  • 55. Closed loop, Rate based, Binary &amp; Explicit feedback and Queue growth rate based CC
  • 56. Resource Management (RM) Cells </li><ul><li>DIR – Direction (forward / backward)
  • 57. BN – Backward Notification
  • 58. CI – Congestion Indication
  • 59. NI – No Increase
  • 60. ER – Explicit Rate </li></ul><li>CLP – Cell Loss Priority bit </li></ul>
  • 61. ATM – Congestion Recovery <ul><li>RM Cells sent every N cells (for example 32)
  • 62. Considered out of band cells
  • 63. Can be sent by the intermediate switches
  • 64. Tagged packets (CLP) can be dropped at intermediate switches if a treshold is hit </li></ul>
  • 65. ATM – CC evaluation <ul><li>Explicit rather than implicit
  • 66. Each switch can control the flow
  • 67. Can provide guarantess (QoS) </li></ul><ul><li>Complex system with many parameters
  • 68. Needs ”smart-network” </li></ul>
  • 69. TCP <ul><li>Possibly the most well-known communication protocol
  • 70. Three-way handshake
  • 71. Flow multiplexing with source/destination ports
  • 72. Positive acknowledgement
  • 73. Windowing </li></ul>
  • 74. TCP – Congestion Control <ul><li>Not well known, but fascinating
  • 75. Not part of the original spec
  • 76. As a result in 1986 NSFnet capacity dropped more than 800 (!!!) times
  • 77. Only reactive – needs to respect the ”smart endpoints – dumb network” principle
  • 78. There are alternatives (SACK, RVSP, etc) </li></ul>
  • 79. TCP – Congestion Control <ul><li>Additive increase (+1) / multiplicative decrease (*0.5)
  • 80. Slow start
  • 81. Fast retransmit and recovery </li></ul>
  • 82. TCP – CC evaluation <ul><li>Simple (no parameters to tweak)
  • 83. Doesn&apos;t need cooperation from intermediate routers </li></ul><ul><li>No guarantees by itself
  • 84. It&apos;s reactive
  • 85. It actually relies on congestion happening to detect the link speed (for slow start)
  • 86. It assumes that all packed loss is due to congestion </li></ul>
  • 87. Bibliography <ul><li>William Stallings – Data and Computer Communications – Fifth edition
  • 88. Jagannathan Sarangapani – Wireless, Ad Hoc and Senso Networks – Protocol, Performance and Control
  • 89. Steve Sjoquist, Andrew Tucker – Comparison of ATM and TCP Congestion Control
  • 90. http://www.cis.ohio-state.edu/~jain/cis788-95/atm_cong/index.html </li></ul>
  • 91. Thank you

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