Advanced Notes - 9.4 The Transmission Control Protocol - Internet Protocol (TCP IP)

AQA Computer Science A-Level

4.9.4 The Transmission Control Protocol /

Internet Protocol (TCP/IP)

Advanced Notes

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Specification:

4.9.4.1 TCP/IP:

Describe the role of the four layers of the TCP/IP stack (application,

transport, network, link)

Describe the role of sockets in the TCP/IP stack

Be familiar with the role of MAC (Media Access Control) addresses

Explain what the well-known ports and client ports are used for and the

differences between them.

4.9.4.2 Standard application layer protocols:

Be familiar with the following protocols:

● FTP (File Transfer Protocol)

● HTTP (Hypertext Transfer Protocol)

● HTTPS (Hypertext Transfer Protocol Secure)

● POP3 (Post Office Protocol (v3))

● SMTP (Simple Mail Transfer Protocol)

● SSH (Secure Shell)

Be familiar with FTP client software and an FTP server, with regard to

transferring files using anonymous and non-anonymous access

Be familiar with how SSH is used for remote management

Know how an SSH client is used to make a TCP connection to a remote

port for the purpose of sending commands to this port using application level

protocols such as GET for HTTP, SMTP commands for sending email and

POP3 for retrieving email

Be familiar with using SSH to log in securely to a remote computer and

execute commands

Explain the role of an email server in retrieving and sending email

Explain the role of a web server in serving up web pages in text form

Understand the role of a web browser in retrieving web pages and web

page resources and rendering these accordingly

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4.9.4.3 IP address structure:

Know that an IP address is split into a network identifier part and a host

identifier part

4.9.4.4 Subnet masking:

Know that networks can be divided into subnets and know how a

subnet mask is used to identify the network identifier part of the IP address

4.9.4.5 IP standards:

Know that there are currently two standards of IP address, v4 and v6

Know why v6 was introduced

4.9.4.6 Public and private IP addresses:

Distinguish between routable and non-routable IP addresses

4.9.4.7 Dynamic Host Configuration Protocol (DHCP):

Understand the purpose and function of the DHCP system

4.9.4.8 Network Address Translation (NAT):

Explain the basic concept of NAT and why it is used

4.9.4.9 Port forwarding:

Explain the basic concept of port forwarding and why it is used

4.9.4.10 Client server model:

Be familiar with the client server model

Be familiar with the Websocket protocol and know why it is used and

where it is used

Be familiar with the principles of Web CRUD Applications and REST:

● CRUD is an acronym for create, retrieve, update, delete

● REST enables CRUD to be mapped to database functions (SQL)

Compare JSON (Javascript object notation) with XML

4.9.4.11 Thin- versus thick-client computing

Compare and contrast thin-client computing with thick-client computing

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TCP / IP

TCP / IP stands for transmission control protocol / internet protocol. The protocol is used in

all parts of the Internet to enable different devices to communicate.

The protocol is formed from four distinct layers that form the TCP / IP stack. These layers

are application, transport, network, link and each is responsible for a seperate part of

communication over the Internet.

Layer

Role

Application

Selects and uses the correct protocol to transmit data. The layer

interacts with the user with application software like a web browser.

Let’s suppose we’re sending the following message over the Internet:

Would you like to go bowling tonight?

Transport

Establishes what’s called a virtual path: an end to end connection

between the sender and the receiver. The layer then splits the

transmission into packets.

Each packet has a sequence number which identifies a packet’s

position in relation to other packets that form part of the same message.

Also contained in each packet is the port number to be used which

identifies the protocol in use. In this case, port 443 is HTTPS.

Network

Provides the correct IP addresses for each packet’s source and

destination.

Routers work within the network layer, using the IP addresses on a

packet to send it to its destination.

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Link

Controls physical connections between pieces of hardware in a network.

Adds MAC addresses to packets which it receives from the network

layer. MAC (which stands for media access control) addresses are

assigned to every device that can connect to a network by their

manufacturer and are unique to that device.

The MAC addresses identify the hardware to which a packet should be

sent. These MAC addresses change with every hop through a network.

At the receiving end

Once a packet has been received by its intended recipient, it is stripped of its extra

information by reversing the TCP / IP stack.

Firstly, the link layer removes MAC addresses from the packet. Next, the network layer

removes IP addresses before the transport layer uses the packet’s port number to

determine the correct application to send the packet to. The transport layer also uses the

packet’s sequence number to ensure that it is in the correct position relative to other

packets in the same transmission.

Finally, the application layer receives the packets and displays the information to the user

accordingly.

Socket addresses

When an IP address is combined with a port number, a socket address is formed. These

are formed from an IP address, followed by a colon, followed by the port number in use.



A socket address identifies which of the applications on the recipient device a packet

should be sent to.

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Well-known ports

The table below lists some well-known ports, their port number and their purpose.

Protocol

Port(s)

Purpose

FTP

(File transfer protocol)

20 & 21

Used for sending files between devices. FTP

client software can run on devices, allowing

them to connect to FTP servers which send

files requested by the client.

Clients can access FTP servers anonymously

or non-anonymously by logging in with a

username and password.

SSH

(Secure shell)

Used for remote management of computers,

allowing computers to be accessed and

controlled remotely.

In order to access a remote computer, SSH

requires a username / password combination

and encrypts information during transmission.

SSH clients are pieces of software which can

be used to make a TCP connection to a

remote port. Once a connection is established,

commands for application-level protocols can

be sent to the remote computer.

HTTP

(Hypertext transfer protocol)

Web servers hold web pages in text form,

which they can deliver to clients using HTTP.

Application software on the client receives

these text files and renders them as web

pages.

HTTPS

(Hypertext transfer protocol

secure)

443

HTTPS performs the same function as HTTP,

but encrypts information during transmission.

This doesn’t just keep information sent by

clients secure, but helps to prevent information

from being tampered with or modified during

transmission.

POP3

(Post office protocol 3)

110 & 995

Used for retrieving emails from an email

server. Email servers are responsible for

receiving and sending emails. POP3

communicates with email servers to check for

and download any new messages.

SMTP

(Simple mail transfer protocol)

25, 587

& 465

Used for sending emails. SMTP is used

between a client and an email server.

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The structure of IP addresses

An IP address is split into two parts: a network identifier and a host identifier. Each of the

computers in a network shares the same network identifier but has its own host identifier.

Networks can be divided into smaller

networks, called subnets. Each subnet has

a different network identifier.

The network identifier part of an IP address

can be determined with a subnet mask.

Let’s say that a device has an IP address of

192.168.3.24 and a subnet mask of

255.255.255.0.

In order to work out which subnet the

device belongs to, we have to apply the

subnet mask as follows.

192.168.3.24

IP Address of device

11000000.10101000.00000011.00011000

IP Address of device (binary)

255.255.255.0

Subnet mask

11111111.11111111.11111111.00000000

Subnet mask (binary)

11000000.10101000.00000011.00000000

IP address AND subnet mask

192.168.3.0

Network identifier (IP address AND subnet mask, in decimal)

As it happens, 255.255.255.0 is a fairly common subnet mask that is easy to use in

examples. However, the same procedure can be applied to any subnet mask.

The more bits that are assigned to the network identifier of an IP address, the more

different subnets a network can have. In the same way, the more bits that are assigned to

the host identifier, the more different devices can be connected to each subnet

simultaneously.

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IP address standards

There are two types of IP address in common use: versions four and six (IPv4 and IPv6).

IPv4

IPv4 addresses are dotted quad numbers, meaning that they consist of four parts that are

separated by dots. Each of the four parts of an IPv4 address is assigned one byte (eight

bits) allowing for numbers from 0 to 255 to be represented.

192.168.34.7

This allows for a total of slightly over 4 billion (256

4

) unique IPv4 addresses. That may

sound like a lot, but IPv4 addresses are in short supply. The number of devices on the

Internet that require a routable IP address is increasing so rapidly that a new version of IP

address had to be created. This was IPv6.

IPv6

IPv6 addresses are formed of eight blocks separated by colons. Each block contains four

hexadecimal characters (a-z and 0-9).

2071:0eb8:85a3:8a2f:0000:0000:0370:7264

IPv6 addresses use 128 bits which, compared to IPv4’s use of 32, allows for far more (in

the order of 10

37

) unique permutations.

Public and private IP addresses

An IP address is said to be either routable or non-routable (public or private). If every

device that is connected to a network had its own public IP address, there wouldn’t be

enough IP addresses to go around.

Instead, each home or business that requires internet access has a small number of public

IP addresses. Most homes have just one public IP address while some businesses will

have multiple addresses.

Routable IP addresses are globally unique whereas millions of devices can have the same

non-routable IP address, provided they are not on the same network. Global authorities

are responsible for assigning routable IP addresses which ensures that the same address

is never issued twice.

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Dynamic host configuration protocol (DHCP)

The number of available private IP addresses within a private network is limited. Assigning

each device on a network its own private IP address would not be sensible, as that device

may leave and never join again, resulting in a wasted IP address.

Instead, DHCP is used to assign IP addresses to devices as they join a network. DHCP

uses a pool of available IP addresses to allocate IP addresses to new devices for the

duration of their session. Once a device leaves the network, the IP address that the device

was using is returned to the pool for allocation to a new device.

Network address translation (NAT)

The diagram shows a network

consisting of three computers, each

of which is allocated a private

(non-routable) IP address. The

network’s router has two IP

addresses, one private address on

the private network’s side and a

public (routable) address on the

Internet’s side. There is a server

connected to the Internet with a

routable IP address.

If a device on the network sent a packet to the server, the server couldn’t respond to the

computer directly because the computer’s IP address is non-routable - not globally unique.

NAT gets around this problem.

When a device

on the private network (let’s use 192.168.1.2)

needs to communicate with a device on the

Internet (let’s use the server), it sends packets

through the router, which makes a record of

the packet before replacing the private IP

address of the computer with its own routable

IP address.

When a response is received, it is sent to the

router’s public IP address, which then

forwards the response to the correct private IP

address by using the record it made when

sending the packet.

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Port forwarding

Port forwarding is used when a client needs to communicate with a server that is

connected to a private network.

The client sends packets to the public IP

address of the router belonging to the

server’s private network.

The packets sent by the client contain the

port number of the application running on

the server that the client wishes to access.

The private network’s router then forwards the packets to the server using NAT.

The client server model

In a network that uses the client server model, clients send request messages to servers,

which reply to the clients with response messages. These messages may contain

requested information, a confirmation that a requested action has been completed, or a

message explaining why the requested action hasn’t been completed.

There are a number of different types of servers, each of which specialise in a certain task.

For example: file servers, database servers and email servers.

APIs

An API (application programming interface) is a name given to a set of protocols relating to

how different applications communicate with each other. They define how interaction

between the applications should be carried out, allowing applications to make use of other

applications.

The websocket protocol

The websocket protocol is an example of an API which operates in the application layer of

the TCP / IP stack. The protocol can be used to provide a constant stream of information

between two devices, usually a client’s web browser and a server.

The connection created by the websocket protocol is full-duplex, meaning that data can be

transmitted in both directions at the same time.

The websocket protocol, which allows for fast transmission of data by reducing the size of

packet headers, is used in video streaming, online games and instant messaging.

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Web CRUD Applications and REST

CRUD

CRUD is an acronym for c

reate, retrieve, update, delete; four commands that can be used

to query online databases. Each of the four CRUD commands has a SQL equivalent.

CRUD

SQL

Create

INSERT

Retrieve

SELECT

Update

UPDATE

Delete

DELETE

REST

An acronym for re

presentational state transfer, REST is a design methodology for online

database applications that are queried with a web browser.

REST uses the four HTTP request methods POST, GET, PUT and DELETE to query

databases.

HTTP

SQL

POST

INSERT

GET

SELECT

PUT

UPDATE

DELETE

When a client needs to connect to a database, it follows these steps:

1. Client-server request made by the client to the web browser

2. Web browser responds with the requested web page (which is delivered as a text

file)

3. This text file contains JavaScript which loads an API. The API uses REST to enable

the database server to be queried by the client with the use of HTTP request

methods.

4. The client sends HTTP requests to the database server.

5. The database server responds to the client’s requests using either JSON or XML.

6. The client’s browser processes the JSON or XML and displays the response to the

user.

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XML and JSON

Database servers deliver responses to queries using either XML or JSON. XML stands for

ex

tensible markup language and does the same job as JavaScript object notation

(JSON).

XML

JSON

 

<Department>

<Subject>

 <Name>Physics</Name>

 <Board>Edexcel</Board>

</Subject>

<Subject>

<Name>Maths</Name>

<Board>AQA</Board>

</Subject>

</Department>

{

"Department": {

"Subject": [

{

"Name": "Physics",

"Board": "Edexcel"

},

{

"Name": "Maths",

"Board": "AQA"

}

]

}

The table above shows the same information represented as both XML and JSON. As the

table shows, JSON is more compact, easier to read, easier to create and faster for

computers to process than XML. However, XML is sometimes seen to be more flexible

than JSON.

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Thin- and thick-client computing

Networks can be configured as either thin-client or thick-client networks.

Thin-client networks

In thin-client networks, the majority of the network’s processing power belongs to servers

which provide services and resources including storage and processing.

It’s easy to add new clients to thin-client networks and the clients themselves are

inexpensive machines. Thin-client networks also allow for greater centralised control of the

network as software updates and security can be managed from the server.

However, thin-client networks require a powerful server which is expensive and requires

expertise to set up and maintain.

Thick-client networks

In a thick-client network, the clients are powerful enough to provide their own processing

power and storage. This independence eliminates the requirement for a server, although it

is possible for thick-client networks can make use of a server.

Thick-client networks require more powerful clients than their thin-client counterparts,

making the network expensive to set up. However, the cost and expertise required in

setting up and maintaining an expensive server is done away with.

Thick-client networks are harder to maintain because there is no facility to issue updates

and manage security from a central server.

Compared to thin-client networks which suffer from high volumes of traffic communicating

between clients and the server, thick-client networks boast much quieter communication

channels which reduces the likelihood of data collisions.