1. TCP Sequence Number Plots
Carey Williamson
Department of Computer Science
University of Calgary
Winter 2018

2. Used for email, Web, ftp, telnet, p2p,…
Tutorial: TCP 101
The Transmission Control Protocol (TCP) is the protocol that sends your data reliably
Used for , 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...
“TCP is the four-wheel drive of transport-layer protocols.
It can go anywhere, but sometimes it is a pretty bumpy ride!”

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

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

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

6. TCP slow-start and congestion avoidance
TCP 101 (Cont’d)
TCP slow-start and congestion avoidance
ACK

7. This (exponential growth) “slow start” process continues until either:
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

8. 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: 20 KB Web page (14 packets)…

9. SeqNum Time Key: X Data Packet + Ack Packet X + + X + X X + X + X + X

10. So What?
What can it tell you?
Everything!!!

11. SeqNum Key: X Data Packet + Ack Packet X + + X X + X + X + X + X + X +
RTT + X + X + X X +

12. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X + X +
TCP
Seg.
Size + X + X X +

13. SeqNum Key: X Data Packet + Ack Packet X + + X X + X + X + X + + X X +
TCP Connection Duration

14. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X + X +
Bytes
Sent X + + X + X + X + X X +

15. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X +
Bytes
Avg Throughput (Bytes/Sec) X + X + + X + X + X + X X +
Sec

16. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + Access
Network
Bandwidth
(Bytes/Sec) X + X + X + X + + X + X + X + X X +

17. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X + X +
Sender’s
Flow Control
Window Size + X + X + X + X X +

18. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X + TCP
Slow
Start X + X + + X + X + X + X X +

19. SeqNum Key: X Data Packet + Ack Packet X + + X X X + X X + X X + X
Delayed ACK + X X + X X X +

20. SeqNum Key: X Data Packet + Ack Packet X X + + Packet Loss X + X + X +
Duplicate
ACK + X + X + X + X X +

21. SeqNum Cumulative ACK Key: X Data Packet + Ack Packet + X X X + + X +
Retransmit X + X + + X + X + X + X X +

22. SeqNum Key: X Data Packet + Ack Packet + X X X + + X + X + X + X + X +
RTO X +

23. What happens when a packet loss occurs? Quiz Time...
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

24. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X + X +

25. SeqNum ? Key: X Data Packet + Ack Packet + X + X + X X + + X + X X + +

26. SeqNum X + Key: X Data Packet + Ack Packet + X + X + X X + + X + X X +

27. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X + X +

28. SeqNum ? Key: X Data Packet + Ack Packet X + X X + X + X + X + X + X +

29. SeqNum Key: X Data Packet + Ack Packet X + X + + X X + X + X + X + X +

30. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X + X +

31. SeqNum ? Key: X Data Packet + Ack Packet X X X X + X + X + X + X + + X

32. SeqNum Key: X Data Packet + Ack Packet X + X X X X + + + + X + X + X +

33. SeqNum Key: X Data Packet + Ack Packet X + + X + X X + X + X + X + X +

34. Key: X Data Packet
+ Ack Packet
SeqNum ? + X X +

35. SeqNum Key: X Data Packet + Ack Packet X + X + + X X + X + X X + + + X

36. TCP 201 (Cont’d) Main observation:
“Not all packet losses are created equal” - CLW 2002
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

37. Congratulations!
You are now a TCP expert!