1. CPSC 601.381 Tutorial: TCP 101 r The Transmission Control Protocol (TCP) is the protocol that sends your data reliably r Used for email, Web, ftp, telnet, p2p,… r Makes sure that data is received correctly: right data, right order, exactly once r Detects and recovers from any problems that occur at the IP network layer r Mechanisms for reliable data transfer: sequence numbers, acknowledgements, timers, retransmissions, flow control...

2. CPSC 601.382 TCP 101 (Cont’d) r TCP is a connection-oriented protocol SYN SYN/ACK ACK GET URL YOUR DATA HERE FIN FIN/ACK ACK

3. CPSC 601.383 TCP 101 (Cont’d) r TCP slow-start and congestion avoidance ACK

4. CPSC 601.384 TCP 101 (Cont’d) r TCP slow-start and congestion avoidance ACK

5. CPSC 601.385 TCP 101 (Cont’d) r TCP slow-start and congestion avoidance ACK

6. CPSC 601.386 TCP 101 (Cont’d) r This (exponential growth) “slow start” process continues until either: m packet loss: after a brief recovery phase, you enter a (linear growth) “congestion avoidance” phase based on slow-start threshold found m limit reached: slow-start threshold, or maximum advertised receive window size m all done: terminate connection and go home

7. CPSC 601.387 Tutorial: TCP 201 r There is a beautiful way to plot and visualize the dynamics of TCP behaviour r Called a “TCP Sequence Number Plot” r 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

8. CPSC 601.388 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

9. CPSC 601.389 TCP 201 (Cont’d) r What happens when a packet loss occurs? r Quiz Time... m Consider a 14-packet Web document m For simplicity, consider only a single packet loss

10. CPSC 601.3810 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

11. CPSC 601.3811 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + + + + + + ?

12. CPSC 601.3812 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + + + + + + X +

13. CPSC 601.3813 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

14. CPSC 601.3814 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + + + + + ?

15. CPSC 601.3815 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + ++ + + +

16. CPSC 601.3816 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

17. CPSC 601.3817 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + + + ?

18. CPSC 601.3818 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + ++++ + X +

19. CPSC 601.3819 Time SeqNum X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

20. CPSC 601.3820 Time SeqNum X + Key: X Data Packet + Ack Packet X + ?

21. CPSC 601.3821 Time SeqNum X + Key: X Data Packet + Ack Packet X X X + + + X X X + + + X + X + X +

22. CPSC 601.3822 TCP 201 (Cont’d) r Main observation: m “Not all packet losses are created equal” r Losses early in the transfer have a huge adverse impact on the transfer latency r Losses near the end of the transfer always cost at least a retransmit timeout r Losses in the middle may or may not hurt, depending on congestion window size at the time of the loss