The internet

Structure of the internet, DNS, TCP/IP stack and Application layer protocols

(Structure of) The Internet

A global network of interconnected networks that communicate using a common set of standards and protocols

Individuals and organisations connect to the internet through an …

… internet service provider (ISP)

Edge router

Router that links one network to another

Subscriber router

• Small scale, low-cost devices with two interfaces (one for each network).

• Often provided to homes and small businesses.

• Type of edge router

Enterprise router

• Used to connect large businesses and ISP networks to the internet.

• Very powerful and expensive as they are a type of edge router

Router

A networking device that forwards data packets between computer networks

Core router

• Routers that are part of the internet backbone

• These routers have multiple interfaces, all of which can work at the highest speed simultaneously

Components of an IP packet

header, payload and trailer

Packet switching

A method of communicating packets of data across a network

A hop

A router-to-router link

How do routers determine the best 'next hop'

They use a routing table (set of rules) to decide what to do with incoming packets. Each packet is treated individually and a separate routing decision is made for each one.

Time to live (TTL) counter in a packet header

• To prevent packets being infinitely passed in a loop by routers

• Initially set when the packet is created, and reduced by one every time it goes through a router.

• If the counter reaches zero, the packet is discarded.

Type of Service (ToS) field in the packet header

• Makes it possible to mark packets with a priority level

• Allows request of special treatment i.e. to be placed at the front of the queue of packets to be routed

• Routers may choose to implement or ignore these requests.

Circuit switching

• Method of communication over a network.

• A direct link is created between two devices (and is maintained for the entire duration of the conversation). Data can then be transferred for of duration of the link.

• A fixed bandwith is reserved for the duration of the link.

• Sometimes used for phone calls

What is the key requirement of circuit switching?

The transfer and download rates must be identical on both devices.

What is the domain name system (DNS)?

A hierarchical system used to name internet resources and find the IP address associated with a given domain name

DNS is the abreviation of …

Abbreviation of domain name system

DNS lookup

The process of getting the IP address of a domain

Sequence of events in a DNS lookup for a URL (e.g. https://www.raspberrypi.org/learn/index.html)

• Browser sends the hostname part of the URL (e.g. www.raspberrypi.org) to a domain name server.

• The DNS server checks for the hostname in its lookup table

• The DNS server sends the corresponding IP address back to the browser

Sequence of events to load a URL (e.g https://www.raspberrypi.org/learn/index.html)

• Browser sends the hostname part of the URL (e.g. www.raspberrypi.org) to a domain name server. This is usually provided by your ISP.

• The DNS server checks for the hostname in its lookup table

• The DNS server sends the corresponding IP address back to the browser

• The browser sends a request for the resource identified in the path (/learn/index.html) to the web server located at the IP address

• The server then returns the resource to the browser

TLD is the abreviation of …

Abreviation of top level domain

2LD is the abreviation of …

Abreviation of second level domain

Benefit for humans of using domain names/ URLs instead of IPs

Beacause domain names/URLs are much easier to remember

Recursive name server

• DNS server that handles a high volumes of DNS queries and will store a lot of IP addresses in cache (but not all).

• Returns the corresponding address if it has it stored. If not, it performs a full DNS lookup

DNS root servers

• DNS servers at the root of the internet controlled by ICANN.

• Stores the domain of all TLD servers

• Returns IP address of the TLD server needed to the recursive name server

Top level domain (TLD) server

• Stores details of authoritative name servers for each of its registered domains

• Returns IP address of the authoritative name server associated with the domain to the recursive name server

Authoratative name sever

• Returns the IP address of the webserver to the recursive name server

URL is the abreviation of …

… abreviation of uniform resource locator

Uniform resource locator(URL)

Unique address of a webpage or other internet resource

Techniques networks use to exchange data

circuit switching and packet switching

Advantages of packet switching

• Multiple methods to ensure data arrives intact eg. checksums and cyclic redundancy checks

• Multiple routes can be used between devices, so if one path breaks, another can be used

• Can transfer data over very large networks to allow communication globally

Disadvantages of packet switching

• Time is spent deconstructing and reconstructing the data packets

• Must wait for all packets to arrive before data can be received.

Advantages of circuit switching

• Data arrives in a logical order which results in a quicker reconstruction of the data.

• Enables two users to hold a call without delay in speech.

Disadvantages of circuit switching

• Bandwidth is wasted during periods of time in which no data is being sent.

• Devices must transfer and receive data at the same rate.

• Using switches means electrical interference may be produced which can corrupt or destroy data.

• Ties up sections of the network which cannot be used by others data until transmission has been completed

4 layers of the TCP/IP protocol stack (top to bottom)

• Application layer

• Transport layer

• Internet layer (aka network layer)

• Link layer (aka data link layer)

Name of data block produced by the application layer

Application data

What is the name of data block produced by the transport layer?

Segments (e.g. TCP segment) or datagrams (UDP datagram)

Name of data block produced by the internet layer.

Packets

Name of data block produced by the link layer

Frames (e.g. Ethernet frames)

Trailer

Contains a checksum or cyclic redundancy check. These codes are used to detect whether any errors have occurred during transmission.

Application layer

• Based at the top of the stack

• Specifies what protocols need to be used to relate the application to what it’s being used for

• For example, if the application is a browser then it would select a protocol such as HTTP, POP3, FTP

What happens at the application layer?

• The protocol used is decided based on the application

• E.g. HTTPS for browser based service, SMTP / IMAP for messaging service

• Passes data on to transport layer to send

• Gets data from transport layer when receiving

• Unpacks message ready for display / removes headers or other non-viewable data

What does the application layer do when it receives data?

Presents data in the form it was sent

Transport layer

• Establishes an end-to-end connection between the source and recipient computers.

• Splits data into segments

• Can provide either a reliable or unreliable delivery service using TCP or UDP respectively

• Adds the the source and destination port numbers to the segment headers

TCP is the abreviation of …

abreviation of transmission control protocol

Transmission control protocol

• ‘Reliable’ protocol - ensures none of the data is lost

• Splits data into TCP segments which are often reffered to as TCP packets

• Labels TCP segments with their sequence number, the total number of segments the original data was split up into and the port number being used for communication.

• The recieving end acknowledges each segment it recieves.

• If the sender does not receive an acknowledgement, the segment has been lost, and so is resent

• In short: If any packets get lost, the transport layer requests retransmissions of these lost packets

UDP is the abreviation of …

abreviation of user datagram protocol

What does the transport layer do when it receives the data?

Removes the port number and reassembles the segments

User datagram protocol

• ‘Unreliable’ protocol - doesn’t ensures no of the data is lost

• Used when speed of data transfer is the most important criteria, and the application can tolerate some data loss

• Applications such as ‘Skype’ and ‘Facetime’ use UDP as well as other media streaming services

Port number

Addressable location on a network that links to an application (or process). Used to return data to the corret place (correct application)

Socket (address)

• Combination of an IP address and a port number.

• Facilitate asynchronous communication - a socket is designated the source or the destination, depending on which side (client or server) is transmitting the data

• Sockets are used to specify which device packets must be sent to and the application being used on that device

Well-known port numbers

• Servers use _________ so they do not need to be looked up by the client that is making a request

• These numbers lie in the range 0-1024

Network/Internet layer

• IP is the standard protocol used

• Segments from the transport layer are encapsulated by the IP protocol to create IP packets

• Each packet is given a header to assist with the delivery of data. The most important header data is the scource and destination IP addresses.

• Routers operate at this layer

Link layer

• Bottom of the TCP/IP stack

• Receives data from internet layer to send

• Adds the source and destination MAC addresses to the packet

• Passes and receives data across wireless network (to WAN / other machine)

• Passes data back up to internet layer when receiving

What does the network layer do when it receives the data?

It removes the sender and recipient IP addresses

What does the link layer do when it receives the data?

Removes the (source and destination) MAC addresses

HTTP(s) is the abreviation of …

abreviation of hypertext transfer protocol (secure)

Hypertext transfer protocol (secure)

• Encrypted version of HTTP

IP packet header

Contains the following fields:

• The sender and recipient IP addresses

• Protocols being used

• Order of the packets

• The time to live / hop limit

• (Type of service is a bonus)

Payload

the raw data being transmitted