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TCP/IP Protocol Suite 1

Chapter

Upon completion you wil l be able to:

TransmissionControl Protocol

Be able to name and understand the services offered by TCP

Understand TCPs flow and error control and congestion control

Be famil iar wi th the fields in a TCP segmentUnderstand the phases in a connection-oriented connection

Understand the TCP transition state diagram

Be able to name and understand the timers used in TCP

Be famil iar wi th the TCP options

Objectives

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Figure 1.1 TCP/I P protocol sui te

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Tcp Session 1 cont..

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1.1 TCP SERVICES

We explain the services offered by TCP to the processes at the appli cationlayer.

The topics discussed in this section include:

Process-to-Process Communication

Stream Delivery Service

Ful l-Duplex Communication

Connection-Oriented Service

Reliable Service

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Table 1.1 Well-known ports used by TCP

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Figure 1.3 Sending and receiving buff ers

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Figure 1.4 TCP segments

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1.2 TCP FEATURES

To provide the services mentioned in the previous section, TCP hasseveral f eatures that are brief ly summarized in this section.


Numbering System

F low Control

Er ror Control

Congestion Control

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The bytes of data being transferred in

each connection are numbered by TCP.The numbering starts with a randomly

generated number.

Note:

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Suppose a TCP connection is transferr ing a fi le of 5000 bytes.The first byte is numbered 10001. What are the sequence

numbers for each segment i f data is sent in f ive segments, each

carrying 1000 bytes?

Example

SolutionThe following shows the sequence number for each segment:

Segment 1 Sequence Number: 10,001 (range: 10,001 to 11,000)





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The value in the sequence number

f ield of a segment defines the numberof the first data byte contained

in that segment.

Note:

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The value of the acknowledgment f ield

in a segment defines the number of thenext byte a party expects to receive.

The acknowledgment number is

cumulative.

Note:

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1.3 SEGMENT

A packet in TCP is called a segment


Format

Encapsulation

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Figure 1.5 TCP segment format

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Figure 1.6 Control fi eld

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I

Table 1.2 Description of f lags in the control f ield

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Figure 1.7 Pseudoheader added to the TCP datagram

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The inclusion of the checksum in TCP

is mandatory.

Note:

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Figure 1.8 Encapsulation and decapsulation

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1.4 A TCP CONNECTION

TCP is connection-oriented. A connection-oriented transport protocolestabli shes a vir tual path between the source and destination. All of the

segments belonging to a message are then sent over this vir tual path. A

connection-oriented transmission requires three phases: connection

establishment, data transfer, and connection termination.


Connection Establishment

Data Transfer

Connection Termination

Connection Reset

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A SYN segment cannot carry data, buti t consumes one sequence number.

Note:

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A SYN + ACK segment cannot carry

data, but does consume one

sequence number.

Note:

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An ACK segment, if carrying no data,

consumes no sequence number.

Note:

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Figure 1.10 Data transfer

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The F IN segment consumes one

sequence number if it does not carry

data.

Note:

Fi 1 11 C ti t i ti i th h d h ki

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Figure 1.11 Connection termination using three-way handshaking

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The F IN + ACK segment consumes

one sequence number if it does not

carry data.

Note:

Fi 1 12 H lf l

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Figure 1.12 Half-close

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1.5 STATE TRANSITION DIAGRAM

To keep track of al l the dif ferent events happening dur ing connectionestablishment, connection termination, and data transfer, the TCP

software is implemented as a f ini te state machine. .


Scenarios

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Table 1.3 States for TCP

Figure 1 13 State transition diagram

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Figure 1.13 State transition diagram

Figure 1 14 Common scenar io

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Figure 1.14 Common scenar io

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The common value forMSL(maximum segment l i fe) is

between 30 seconds and 1 minute.

Note:

Figure 1 15 Three way handshake

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Figure 1.15 Three-way handshake

Figure 1 16 Simultaneous open

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Figure 1.16 Simultaneous open

Figure 1 17 Simultaneous close

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Figure 1.17 Simultaneous close

Figure 1 18 Denying a connection

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Figure 1.18 Denying a connection

Figure 1 19 Aborting a connection

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Figure 1.19 Aborting a connection

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1.6 FLOW CONTROL

Flow control r egulates the amount of data a source can send beforereceiving an acknowledgment from the destination. TCP defines a

window that is imposed on the buffer of data delivered from the

application program.


Sliding Window Protocol

Sil ly Window Syndrome

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Figure 1.20 Sliding window

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Figure 1.20 Sliding window

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1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1

2

3

ACK 4

Window size = 6Octets/Bytes received

Host A - Sender Host B - Receiver

Example:-

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1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

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1

2

3

ACK 4

45

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

ACK 6

Host B - ReceiverHost A - Sender

Window size = 6

Window Size increase by 3because received ACK for 3

octets/bytes

Host B - ReceiverHost A - Sender

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1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1

2

3

ACK 4

4

5

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

ACK 6

Window size = 6

1 2 3 4 5 6 7 8 9 10 11 12 13

7

6

9

8

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

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A sliding window is used to make

transmission more eff icient as well as

to control the flow of data so that thedestination does not become

overwhelmed with data.

TCPs sliding windows are byteoriented.

Note:

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Figure 1.21 Example 5

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g


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SI LLYWINDOW SYNDROME by sender

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If the data is send byte by byte

For sending 1 byte of data 20 bytes of ipheader and 20 bytes of tcp header is usedtotal 41bytes for sending one byte of data

This reduces the efficiency of theoperation

Which indicates that we are using capacityof the network very inefficiently.


SI LLY WINDOW SYNDROME by sender

SI LLY WINDOW SYNDROME

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To overcome nagles algorithm wasintroduced

Send piece of data even of 1 byte

Now sender accumulates enough data to fill

maximum-size segment

Transmit full segment at end



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If the data is received byte by byte whenbuffer is full

Once data is filled in buffer and the bufferis full

Receiver sends zero window size

As soon as one byte is empty the it sendcurrent window 1

This leads to slow data consuming


SI LLY WINDOW SYNDROME by receiver


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To overcome clarks solution wasintroduced

When buffer is full

Send ack as soon as data arrives but anounce

window size zero

Once the buffer has a space to accommodatea maximum size segment

Send the max window size




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To overcome delayed acknowledgementwas introduced

Delay the ack even if receiver receives data

But acknowledgement should not delay more

than 500 ms



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To avoid shrinking the sender window,

the receiver must wait unti l more

space is available in its buffer.

Note:

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Some points about TCPs sliding windows:

The size of the window is the lesser of rwndand cwnd.

The source does not have to send a full windows

worth of data.The window can be opened or closed by the receiver,

but should not be shrunk.

The destination can send an acknowledgment at any

time as long as it does not result in a shrinking window.

The receiver can temporari ly shut down the window;

the sender, however, can always send a segment of one

byte after the window is shut down.

Note:

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1.7 ERROR CONTROL

TCP provides rel iabil i ty using error control, which detects corrupted,

lost, out-of-order, and duplicated segments. Error control in TCP is

achieved through the use of the checksum, acknowledgment, and time-

out.


Checksum

Acknowledgment

Acknowledgment Type

Retransmission

Out-of -Order SegmentsSome Scenar ios

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ACK segments do not consume

sequence numbers and are not

acknowledged.

Note:

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No retransmission timer is set for an

ACK segment.

Note:

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Data may arr ive out of order and be

temporari ly stored by the receiving TCP,

but TCP guarantees that no out-of-order

segment is delivered to the process.

Note:

Figure 1.25 Normal operation

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Figure 1.26 Lost segment

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The receiver TCP delivers onlyordered data to the process.

Note:

Figure 1.27 Fast retransmission

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Figure 1.28 Lost acknowledgment

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Figure 1.29 Lost acknowledgment corrected by resending a segment

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Lost acknowledgments may create

deadlock if they are not proper ly

handled.

Note:

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Figure 1.30 Router queues

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Figure 1.31 Packet delay and network load

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Figure 1.32 Throughput versus network load

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Figure 1.33 Slow start, exponential i ncrease

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Figure 1.34 Congestion avoidance, additive increase

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I n the congestion avoidance algor ithm

the size of the congestion window

increases additively unti l

congestion is detected.

Note:

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Most implementations react differently to

congestion detection:

I f detection is by time-out, a new slow start phase

starts.

I f detection is by three ACKs, a new congestionavoidance phase star ts.

Note:

Figure 1.36 Congestion example

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Tcp Session 2 cont..

1 9 TCP TIMERS

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1.9 TCP TIMERS

To perform i ts operation smoothly, most TCP implementations use at

least four timers.


Retransmission Timer

Persistence TimerKeepali ve Timer

TIME-WAI T Timer

Figure 1.37 TCP timers

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Retransmission timer

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Retransmission timer

If the ack is received before timer goes offtimer is destroyed

If the ack is not received and timer goes

off the segment is resent

Calculation of retransmission time Retransmission time = 2 X RTT

RTT = 90% X previous RTT + 10% X current RTT Eg RTT = 0.9 X 100 + 0.1 X 80 = 98microsec


Persistence timer

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Persistence timer

It deals with zero window size When an sender tcp receives zero window

size its starts persistence timer

When persistence timer goes off Sender sends special segment called

probewhich contains 1 byte of data

This probe alerts the sender and senderresends the ack


Keep alive timer

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Keep alive timer

Keep alive timer is used to prevent long idleconnection

Widen communication client suddenly stayssilent(clients system shutdown)

Time out is usually of 2 hours If client doesnt respond witen 2 hours

Server sends probe segment to client

If there is no response after 10 probes Server terminates the connection


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I n TCP, there can be only be one RTT

measurement in progress at any time.

Note:

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TCP does not consider the RTT of a

retransmitted segment in its

calculation of a new RTO.

Note:

1 10 OPTIONS

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1.10 OPTIONS

The TCP header can have up to 40 bytes of optional information.

Options convey additional information to the destination or align other

options.

Figure 1.40 Options

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Figure 1.41 End-of-option option

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EOP can be used only once.

Note:

Figure 1.42 No-operation option

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NOP can be used more than once.

Note:

Figure 1.43 Maximum-segment-size option

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The value of MSS is determined

dur ing connection establ ishment and

does not change dur ing the

connection.

Note:

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The value of the window scale factor

can be determined only dur ing

connection establishment; it does not

change dur ing the connection.

Note:

Figure 1.45 Timestamp option

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One application of the timestamp

option is the calculation of round tr ip

time (RTT).

Note:

1.11 TCP PACKAGE

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We present a simpl if ied, bare-bones TCP package to simulate the heart of

TCP. The package involves tables call ed transmission control blocks, a

set of timers, and three software modules.


Transmission Control Blocks (TCBs)

TimersMain M odule

I nput Processing Module

Output Processing Module

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Figure 1.52 TCBs

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chap-01 tcp

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