TCP Plots

1
TCP Plots

• Slides originally from Williamson at Calgary
• Minor modifications are made

2
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...

3
TCP 101 (Contd)

• TCP is a connection-oriented protocol

YOUR DATA HERE
Web Client
Web Server
4
TCP 101 (Contd)

• TCP slow-start and congestion avoidance

5
TCP 101 (Contd)

• TCP slow-start and congestion avoidance

6
TCP 101 (Contd)

• TCP slow-start and congestion avoidance

7
TCP 101 (Contd)

• 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 Consider a 14-packet transfer

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Key X Data Packet Ack Packet
X

X

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
10
So What?

• What can it tell you?
• Everything!!!

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Key X Data Packet Ack Packet
X

X

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
RTT
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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
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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
14

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
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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
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Key X Data Packet Ack Packet
X

X

X

X

X
SeqNum
Access Network Bandwidth (Bytes/Sec)

X

X
X


X

X

X

X

X

X
Time
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Key X Data Packet Ack Packet
X

X

X

X

X
SeqNum

X

X
X


X
Senders Flow Control Window Size

X

X

X

X

X
Time
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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
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Key X Data Packet Ack Packet
X

X
X

X
X
SeqNum

X
X
X

X
Delayed ACK

X
X

X
X

X
Time
20
Key X Data Packet Ack Packet
X


X

X
Packet Loss
SeqNum

X

X
X


Duplicate ACK
X

X

X

X

X

X
Time
21
Cumulative ACK
Key X Data Packet Ack Packet

X
X


X

X
SeqNum

X

Retransmit
X
X


X

X

X

X

X

X
Time
22
Key X Data Packet Ack Packet

X
X


X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
RTO (Retranmission Timeout)
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TCP 201 (Contd)

• What happens when a packet loss occurs?
• Quiz Time...
• Consider a 14-packet Web document
• For simplicity, consider only a single packet
  loss

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Key X Data Packet Ack Packet
X

X

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
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?
Key X Data Packet Ack Packet

X

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
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Key X Data Packet Ack Packet

X

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
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Key X Data Packet Ack Packet
X

X

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
28
Key X Data Packet Ack Packet
X
?

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
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Key X Data Packet Ack Packet
X
X


X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
30

Key X Data Packet Ack Packet
X

X

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
31
Key X Data Packet Ack Packet
X
X
X
?

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
32

Key X Data Packet Ack Packet
X
X
X
X




X
SeqNum

X

X
X


X

X

X

X

X

X
Time
33

Key X Data Packet Ack Packet
X

X

X

X

X
SeqNum

X

X
X


X

X

X

X

X

X
Time
34
Key X Data Packet Ack Packet
SeqNum
?

X

X
Time
35
Key X Data Packet Ack Packet
SeqNum

X

X
X
X



X

X
Time
36
TCP 201 (Contd)

• 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

37
Congratulations!

• You are now a TCP expert!
• Or you are so confused now that you will finally
  take my advice and buy the TCP book and read
  through some chapters