CPSC 601.381 Tutorial: TCP 101 r The Transmission Control Protocol (TCP) is the protocol that sends...

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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...

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TCP 101 (Cont’d)

TCP is a connection-oriented protocol

SYNSYN/ACK

ACKGET URL

YOUR DATA HERE

FIN FIN/ACKACK

Web Client Web Server

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TCP 101 (Cont’d)

TCP slow-start and congestion avoidance

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TCP 101 (Cont’d)

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TCP 101 (Cont’d)

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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: Consider a 14-packet transfer

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Key: X Data Packet + Ack Packet X

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So What?

What can it tell you?

Everything!!!

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TCPSeg.Size

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TCP Connection Duration

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NumBytesSent

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t (Byte

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++Access

Network Bandwidth(Bytes/Sec)

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Sender’sFlow ControlWindow Size

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TCP SlowStart

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Delayed ACK

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Key: X Data Packet + Ack Packet

PacketLoss

DuplicateACK

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Retransmit

Cumulative ACK

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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

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X + ++X

X +X +X +

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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

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Congratulations!

You are now a TCP expert!

CPSC 601.381 Tutorial: TCP 101 r The Transmission Control Protocol (TCP) is the protocol that sends...

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