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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
    • What is congestion?
    • 3. Myths about congestion
    • 4. Congestion control possibilities
    • 5. ATM – Vocabulary
    • 6. ATM – Congestion Control
    • ATM – CC evaluation
    • 7. TCP
    • 8. TCP – Congestion Control
    • 9. TCP – CC evaluation
    • 10. Bibliography
  • 11. What is congestion?
    • ”network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates” - Wikipedia
    • 12. QoS
  • 16. Congestion control possibilities
    • Admission control
    • 17. Traffic access control
    • 18. Packet scheduling (prioritization)
    • 19. Buffer management
    • 20. Flow control (speed matching)
    • 21. QoS routing
  • 22. Congestion control possibilities
    • Admission control
    • 23. Traffic access control
    • 24. Packet scheduling (prioritization)
    • 25. Buffer management
    • 26. Flow control (speed matching)
    • 27. QoS routing
    Congestion
  • 30. CC Objectives
    • Scalability
    • 31. Fairness
      • Max-Min
      • 32. Min + Equal share
      • 33. Maximum of the previous too
      • 34. Allocation proportional with Min
      • 35. Weighted allocation
    • Robustness
    • 36. Implementability
  • 37. ATM
    • Protocol implemented over different media
    • 38. Fixed sized cells (53 bytes – 48 byte of data)
    • 39. Vocabulary:
      • VCI – Virtual Circuit Identifier
      • 40. PCR – Peak Cell Rate
      • 41. SCR – Sustained Cell Rate
      • 42. MBR – Maximum Burst Size
      • 43. MCR – Minimum Cell Rate
  • 44. ATM
    • Vocabulary
      • CTD – Cell Transfer Delay (maximum / mean)
      • 45. CDV – Cell Delay Variation (peak-to-peak / instant.)
      • 46. CLR – Cell Loss Ratio
    • Service categories
      • 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
  • 51. ATM – Congestion Control
    • Admission control – CAC (Call Access Control)
      • Hard or soft (statistical) limits
    • Traffic access control – GCRA (Generic Cell Rate Algorith)
      • TAT – Theoretical Arrival Time
      • 52. I – Increment
      • 53. L - Limit
    Cell arrival at t t <= TAT TAT = t t+L < TAT Conforming cell TAT = TAT + I Non-conforming cell
  • 54. ATM – Congestion Recovery
    • Mainly for ABR traffic
    • 55. Closed loop, Rate based, Binary & Explicit feedback and Queue growth rate based CC
    • 56. Resource Management (RM) Cells
      • DIR – Direction (forward / backward)
      • 57. BN – Backward Notification
      • 58. CI – Congestion Indication
      • 59. NI – No Increase
      • 60. ER – Explicit Rate
    • CLP – Cell Loss Priority bit
  • 61. ATM – Congestion Recovery
    • 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
  • 65. ATM – CC evaluation
    • Explicit rather than implicit
    • 66. Each switch can control the flow
    • 67. Can provide guarantess (QoS)
    • Complex system with many parameters
    • 68. Needs ”smart-network”
  • 69. TCP
    • Possibly the most well-known communication protocol
    • 70. Three-way handshake
    • 71. Flow multiplexing with source/destination ports
    • 72. Positive acknowledgement
    • 73. Windowing
  • 74. TCP – Congestion Control
    • 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)
  • 79. TCP – Congestion Control
    • Additive increase (+1) / multiplicative decrease (*0.5)
    • 80. Slow start
    • 81. Fast retransmit and recovery
  • 82. TCP – CC evaluation
    • Simple (no parameters to tweak)
    • 83. Doesn't need cooperation from intermediate routers
    • No guarantees by itself
    • 84. It'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
  • 87. Bibliography
    • 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
  • 91. Thank you

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