TCP (Part 1)

TCP (Part 1)

- Reliable Stream Transport Service

D.E. Comer “Internetworking with TCP/IP: Principles, Protocols and Architectures”, Ch. 13, Prentice Hall, 2000

Presented by Ming [email protected]

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Content TCP Introduction Issue in TCP --- Flow Control

Sliding window protocol TCP acknowledgement scheme TCP segment TCP timeout

Summary

TCP Introduction

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Introduction - TCP TCP

Transmission Control Protocol TCP is not a software

Purpose Providing reliable stream delivery Isolating application programs from the details of

networking

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TCP Conceptual Layering

Application

Reliable Stream (TCP)User Datagram (UDP)

Internet (IP), ICMP

Network Interface

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Properties of the reliable delivery service Stream Orientation

data as a stream of bits, divided into 8-bit octets Virtual Circuit Connection Buffered Transfer

transfer more efficiently & minimize network traffic Unstructured Stream Full Duplex Connection

Two-way connection

Issues in TCP

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Two issues in TCP Flow Control

End-to-end flow control problem Congestion control problem

Virtual Circuit Connection

Flow Control

- Sliding window protocol

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End-to-end flow control Problem

Sender can send more traffic that receiver can handle. (Too fast)

Solution variable sliding window protocol each acknowledgement, which specifies how

many octets have been received, contains a window advertisement that specifies how many additional octets receiver are prepared to accept.

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Variable Window Size

… …

Transmitter Receiver

Window Advertisement

Transmitter Window Size Value of

Window Advertisement

Free space in buffer to fill

increase bigger increase

decrease smaller decrease

Stop transmissions 0 full

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Sliding window protocol in TCP TCP allows the window size to vary over

time. Window size changes at the time it slides

forward. Advantage: it provides flow control as well as

reliable transfer.

Flow Control

- TCP acknowledgement scheme

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Acknowledgements and Retransmission

Cumulative acknowledgement scheme is used in TCP.

A TCP ACK specifies “the sequence number of the next octet that the receiver expects to receive”.

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Acknowledgements and Retransmission

Adv. Easy to generate and unambiguous. The lost ACKs don’t force retransmission.

Disadv. No information about all successful transmissions

for the sender, but only a single position in the stream.

Flow Control

- TCP segment

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TCP segment format

Source Port

Sequence Number

Acknowledge Number

HLEN

Checksum

Options (if any)

Data

…

Destination Port

Reserved Code Bits Window

Urgent Pointer

Padding

4 3116 240

Flow Control

- TCP timeout

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Timeout and Retransmission Question

How to determine timeout? Is the timeout always a constant?

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Timeout and Retransmission An adaptive retransmission algorithm is used

in TCP. TCP monitors the performance of each

connection and adjust its timeout parameter accordingly Timeout value may change.

Timeout is adjusted when a new round trip sample ( RTT ) is obtained.

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Timeout and Retransmission

RTT =

(α * Old_RTT) + ((1 – α) * new_RTT_sample ) 0 < α < 1 α close to 1 => no change in a short time α close to 0 => RTT changes too quickly

Timeout = β * RTT β >1 Recommended setting, β= 2

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Accurate Measurement of Round Trip Samples TCP acknowledgements are ambiguous.

It is caused by the cumulative acknowledgement scheme. It happens when retransmission. It causes the question that the first received ACK does corr

espond the original datagram or the retransmitted datagram. RTT couldn’t be measured accurately if the above question

cannot be answered.

How to do?

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Accurate Measurement of Round Trip Samples --- Karn’s Algorithm and Timer Backoff

Karn’s algorithm: when computing the round trip estimate, ignore samples that correspond to retransmitted segments, but use a back-off strategy, and retain the timeout value from a retransmitted packet for subsequent packets until a valid sample is obtained.

Timer back-off strategy: New_timeout = γ * timeout ( typically, γ =2 ) Each time timer expires (retransmit happens), TCP increa

ses timeout value.

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Karn’s Algorithm Use RTT to compute Timeout When retransmission happens, Timeout increases in

γtimes continuously, until transfer successfully. [Backoff strategy]

Use the timeout in the final turn of the last step to send next segment. [Backoff strategy]

When an acknowledgement arrives corresponding to a segment that did not require retransmission, then TCP re-computes the RTT and reset the timeout accordingly

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Responding to High Variance in Delay

Queuing theory: variation in round trip delay is proportional to 1/(1-L), where L is the current network load, 0<L<1

( Timeout = β * RTT ) & (β=2 ) => L < 30% Not efficient

1989 TCP specification requires to use estimated variance in place of β

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New RTT and Timeout Algorithm DIFF = sample – old_RTT Smoothed_RTT = old_RTT + d * DIFF DEV = old_DEV + p (|DIFF| - old_DEV) Timeout = Smoothed_RTT + g * DEV

DEV estimated mean deviation d, a fraction between 0 and 1 to control how quickly the new sample

affects the weighted average p, a fraction between 0 and 1 to control how quickly the new sample

affects mean deviation g, a factor controls how much deviation affects round trip timeout Research suggests: d=1/8, p=1/4 and g=4

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Summary Purpose of TCP End-end Flow Control issue

Variable Sliding window protocol in TCP TCP acknowledgement scheme TCP segment TCP timeout

RTT-Timeout Calculation and Karn’s Algorithm New RTT and Timeout Algorithm

Thanks