TCP Sequence Number Plots Carey Williamson Department of Computer - - PowerPoint PPT Presentation

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TCP Sequence Number Plots Carey Williamson Department of Computer - - PowerPoint PPT Presentation

Aug 29, 2023 238 likes •614 views

TCP Sequence Number Plots Carey Williamson Department of Computer Science University of Calgary Tutorial: TCP 101 The Transmission Control Protocol (TCP) is the protocol that sends your data reliably Used for email, Web, ftp, telnet,

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TCP Sequence Number Plots

Carey Williamson Department of Computer Science University of Calgary

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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...

“TCP is the four-wheel drive of transport-layer protocols. It can go anywhere, but sometimes it is a pretty bumpy ride!”

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TCP 101 (Cont’d) ▪ TCP is a connection-oriented protocol

SYN SYN/ACK ACK GET URL YOUR DATA HERE FIN FIN/ACK ACK Web Client Web Server

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TCP 101 (Cont’d) ▪ TCP slow-start and congestion avoidance

ACK

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TCP 101 (Cont’d) ▪ TCP slow-start and congestion avoidance

ACK

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TCP 101 (Cont’d) ▪ TCP slow-start and congestion avoidance

ACK

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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

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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)…

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Time SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

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So What? ▪ What can it tell you? ▪ Everything!!!

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + + RTT

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SeqNum

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

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + + TCP Connection Duration

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + + Num Bytes Sent

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + + Bytes Sec

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SeqNum

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

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SeqNum

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

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + + TCP Slow Start

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + Delayed ACK

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X + + + + + + + + + + + Packet Loss Duplicate ACK

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X + + + + + + + + + + X + Retransmit Cumulative ACK +

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X + + + + + + + + + + X + RTO +

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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

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + + + + + +

?

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + + + + + + X +

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + + + + +

?

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + + +

?

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X + + + + + + + ++ + + + X +

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SeqNum

X + Key: X Data Packet + Ack Packet X X X X X X X X X X X X X + + + + + + + + + + + + +

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SeqNum

X + Key: X Data Packet + Ack Packet X +

?

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SeqNum

X + Key: X Data Packet + Ack Packet X X X + ++ X X X + + + X + X + X +

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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

  • n congestion window size at the time of the loss
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Congratulations! ▪ You are now a TCP expert!