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Tutorial: TCP 101

Tutorial: TCP 101. The Transmission Control Protocol (TCP) is the protocol that sends your data reliably Used for email, Web, ftp, telnet, p2p,… Makes sure that data is received correctly: right data, right order, exactly once

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Tutorial: TCP 101

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  1. Tutorial: TCP 101 • The Transmission Control Protocol (TCP) is the protocol that sends your data reliably • Used for email, Web, ftp, telnet, p2p,… • Makes sure that data is received correctly: right data, right order, exactly once • Detects and recovers from any problems that occur at the IP network layer • Mechanisms for reliable data transfer: sequence numbers, acknowledgements, timers, retransmissions, flow control... CPSC 601.38

  2. SYN SYN/ACK ACK GET URL FIN FIN/ACK ACK TCP 101 (Cont’d) • TCP is a connection-oriented protocol YOUR DATA HERE Web Client Web Server CPSC 601.38

  3. ACK TCP 101 (Cont’d) • TCP slow-start and congestion avoidance CPSC 601.38

  4. ACK TCP 101 (Cont’d) • TCP slow-start and congestion avoidance CPSC 601.38

  5. ACK TCP 101 (Cont’d) • TCP slow-start and congestion avoidance CPSC 601.38

  6. TCP 101 (Cont’d) • This (exponential growth) “slow start” process continues until either: • packet loss: after a brief recovery phase, you enter a (linear growth) “congestion avoidance” phase based on slow-start threshold found • limit reached: slow-start threshold, or maximum advertised receive window size • all done: terminate connection and go home CPSC 601.38

  7. Tutorial: TCP 201 • There is a beautiful way to plot and visualize the dynamics of TCP behaviour • Called a “TCP Sequence Number Plot” • Plot packet events (data and acks) as points in 2-D space, with time on the horizontal axis, and sequence number on the vertical axis • Example: Consider a 14-packet transfer CPSC 601.38

  8. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  9. So What? • What can it tell you? • Everything!!! CPSC 601.38

  10. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time RTT CPSC 601.38

  11. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X + X + X TCP Seg. Size + X + X + X Time CPSC 601.38

  12. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time TCP Connection Duration CPSC 601.38

  13. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + Num Bytes Sent + X + X + X + X + X + X Time CPSC 601.38

  14. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X Bytes Avg Throughput (Bytes/Sec) X + + X + X + X + X + X + X Sec Time CPSC 601.38

  15. + Key: X Data Packet + Ack Packet X + X + X + X + X Access Network Bandwidth (Bytes/Sec) SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  16. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X Sender’s Flow Control Window Size + X + X + X + X + X Time CPSC 601.38

  17. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X TCP Slow Start X + + X + X + X + X + X + X Time CPSC 601.38

  18. + Key: X Data Packet + Ack Packet X + X X + X X SeqNum + X X X + X Delayed ACK + X X + X X + X Time CPSC 601.38

  19. Key: X Data Packet + Ack Packet X + + X + Packet Loss X SeqNum + X + X X + + Duplicate ACK X + X + X + X + X + X Time CPSC 601.38

  20. Cumulative ACK Key: X Data Packet + Ack Packet + X X + + X + X SeqNum + X + Retransmit X X + + X + X + X + X + X + X Time CPSC 601.38

  21. Key: X Data Packet + Ack Packet + X X + + X + X SeqNum + X + X X + + X + X + X + X + X + X Time RTO CPSC 601.38

  22. TCP 201 (Cont’d) • What happens when a packet loss occurs? • Quiz Time... • Consider a 14-packet Web document • For simplicity, consider only a single packet loss CPSC 601.38

  23. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  24. ? Key: X Data Packet + Ack Packet + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  25. + X Key: X Data Packet + Ack Packet + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  26. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  27. Key: X Data Packet + Ack Packet X ? + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  28. + Key: X Data Packet + Ack Packet X X + + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  29. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  30. Key: X Data Packet + Ack Packet X X X ? + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  31. + Key: X Data Packet + Ack Packet X X X X + + + + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  32. + Key: X Data Packet + Ack Packet X + X + X + X + X SeqNum + X + X X + + X + X + X + X + X + X Time CPSC 601.38

  33. Key: X Data Packet + Ack Packet SeqNum ? + X + X Time CPSC 601.38

  34. X X + + X + + X Key: X Data Packet + Ack Packet SeqNum + X + X X X + + + X + X Time CPSC 601.38

  35. TCP 201 (Cont’d) • Main observation: • “Not all packet losses are created equal” • Losses early in the transfer have a huge adverse impact on the transfer latency • Losses near the end of the transfer always cost at least a retransmit timeout • Losses in the middle may or may not hurt, depending on congestion window size at the time of the loss CPSC 601.38

  36. Congratulations! • You are now a TCP expert! CPSC 601.38

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