Communications and Network Security Flashcards
What does OSI stand for?
Open Systems Interconnection - created by ISO (International standard for organisation)
What is the most widely used protocol?
TCP/IP protocol (based on TCP/IP model)
Explain a protocol?
A protocol is the way in which two computers are able to communication over a network (think of it like a path between houses)
What is the main goal of the OSI?
To support data exchange and network communications. Also to ensure different products can communicate across vendors - i.e CISCO switch can communicate with netgear router.
Name the Seven layers of the OSI?
All People Seem To Need Domino’s Pizza
What moves across the physical wire?
Electrons over copper
Photons over fiber
Radio signals through the air (wifi)
What part of the OSI model does each layer communicate with?
Up, down and to it’s peer layer (for authentication)
Explain encapsulation/descaps
Employed by protocols
The addition of a header or footer to direct the data
They combine to become the payload
They contain instructions so the peer layers can communicate
At what point is the message converted into electrical impulses that represent bits?
In the physical layer so that it can be transferred over the wire
What does the physical layer do when it receives the message?
converts the message into bits. Before converting it to frames at the data link layer
Name the different types of message at each layer?
Data (session, presentation, application) Segement/datagram (transport layer) packet (network layer) Frame (data link layer) Bits (physical)
The message in a network is a segment when?
It uses TCP protocols
The message in a network is a datagram when?
It uses UDP protocols
What are the two networking model and which one do all networks use?
OSI and TCP/IP - all networks support TCP/IP stack but OSI is used as a benchmark.
They are standards on how systems are built and networks communicate. It means we can all talk together and use the same protocols etc.
Name the layers of the TCP/IP model?
All twats need domino’s pizza
What layer does a hub operate at?
Hubs operate at layer 1 because they are simple and stupid - they only repeat electric signals. Similar to a repeater on layer 1 too.
What is the first thing that Ron must do before he tried to buy coffee off Starbucks?
- Send out ARP request for DNS server and then send out a DNS request to get the IP address of website
- Do the same for the router so that he can go outside
- then send the message to the switch > router > switch > Web server
What happens at the application level?
You attempt to access a website for example, or attempt to send an email. They are protocols used by web browsers for example. Includes HTTPs/
What happens at layer 4 (transport layer)?
This is where you outline and add headers that describe how you are going to send the data and which ports you are going to use - i.e HTTPS and port 443 - so that the other layers know
When your browser tries to access a website and it adds a HTTP header to the data - what is this called?
Encapsulation
Each layer adds a header to the data/segment/frame/packets/bits - what is this called?
Encapsulation
What is the benefit of adding layer 3 headings?
It add’s destination and source IP addresses and tells the router where to direct the message (this message is a packet)
What is the benefit of adding layer 2 headings?
It gives instructions to our switch (this message is a frame) - where to send the message
Once you encapsulate the data and send your message - what type of message does the switch (on your side) receive at layer 2?
It receives a frame, which only contains the MAC address data (the rest is encapsulated and can’t be read). The Switch see’s this information by decapsulating the message and uses the MAC address to forward on the message.
This will typically be the MAC address of the router, as it needs to be send outside the network.
How does the router learn the IP address of a website?
It does this through ARP - and then encapsulates this information into a new mac address header and footer. The original footer would have contained IP/MAC adderesses of the sender and router.
What happens when the server receives the frame from a switch?
It can read all the layers and it checks each one to make sure it contains his/her MAC address, IP address, and see’s that it’s using TCP on port 443 (for example). It then passes it up to the application level and it can see that it’s HTTP and for the browser.
What are the two protocols that you can choose when using the transport layer?
TCP, UDP
What parts of the OSI model are employed more when watching youtube, rather then sending emails?
Application, presentation and session layers
What type of request would be coming from the application layer when trying to stream something like youtube?
It would be a data request from your browser, in the form of a ‘get’ request - i.e get me that data
What are the two main things that the presentation layer is concerned with?
Data format - this will be file types such as .PDF. they are common formats that your computer knows how to open.
Data encryption - SSL would sit here.
What is the data format (presentation layer) for our web browser?
HTML! Your presentation layer puts the data in a .HTML file.
For example, if youtube send you the homepage of youtube - it will send you a homepage.html file
What does the session layer do
It opens up the session and starts the conversation between source and destination. Ensures authentication and a smooth communication. It happens whilst everything else is going through the other layers. Session layer has one job - keep the conversation going throughout (makes it quicker and easier)
give a working example of a session layer communication?
Between your application and the web server of a website
Examples of session layer protocols?
L2TP - layer 2 tunnelling protocl (VPN)
RTCP - remote transport control (phone calls)
H.2ys - for video calls
SOCKS - this sets up proxies
If we refer to the TCP/IP model - what does the application layer do?
Gets the data ready, encrypts it, formats it, opens up a session.
application layer is quite relevant for programmers (not networkers)
What is the transport layer concerned with?
How it is going to get the data to the other person!
It’s like sending a package - we’ve boxed it up and secured it, but how will it get there through the lower levels?
To ‘transport’ it - you can choose out of the two main protocols; TCP/UDP
TCP |
Transmission control protocol
More reliable
has a verification process to make sure your data has been received
Also requires a three way handshake
what is the TCP three way handshake?
It sets up the connection before you start exchanging the data.
Youtube trying to send data - Send a synchronisation message - Syn
You send a synchronisation acknowledgement message - SyncAck
Youtube send an acknowledgement message - Ack
Once this has been done you can exchange data
UDP
Doesn’t wait for verification - it just sends the message back to you
Why would your communication with youtube switch from TCP to UDP
Youtube will send a .HTML file through TCP as it will send the actual home page. But videos will come through as UDP.
How do you communication with youtube?
you need to access their server (which is obviously publicly available)
What are ports?
They are used by protocols.
How do you tell a server that you want to access SSH rather than watch a video? You do this via port numbers!
You know HTTPS uses port 443. So when you type in https//:youtube ….. you are saying that you want to access the youtube server via port 443. It could also look like this 174.37.192: 443
The ports allow us to run multiple services on one server.
What is wireshark?
It captures your network traffic
Should one server host one service?
No - it can be a web server, RDP server, SSH, FTP
You just need to open up other ports on your server!
Are ports both TCP and UDP?
Some use both, but others only use one or the other.
How many ports in general and how many ‘well known’ ports?
0 - 6k
0 - 1k ish
Name some well known ports?
22 - SSH 23 - Tel net 25 - SMTP 80 - HTTP 443 - HTTPS (accessing web server for youtube)
Which port will the application send you data back on?
You can tell the application which port it should send you application data on - i.e port 5709
you will be receiving lots of data all at once
What;s an ephemeral port?
It’s a temporary port for youtube or other applications to send you data back on
What does your transport layer do?
It determines which ports and protocols you will be using and then adds this on as a header before sending it to the network layer (which adds on the IP address header) - it also adds in your source IP address so it knows where to send the data back to
What types of devices might be connected to you switch (working environment?)
Computer, phones, WAP (for wifi), web server, router
Why should you never connect a switch to another switch within your network?
It’s a single point of failure!
How do you become redundant in your network design?
a) connect all of your switches to your router (but that isn’t good for devices communicating on your own network)
b) connect all fo your switches to a multi-layer swtich.
What does a multi-layer switch do?
It sits at layer 3 (instead of layer 2). It can handle IP addresses and MAC addresses!! Also ensures redundancy if any of your switches go down.
It will connect directly to your router.
Why don’t businesses design their network without single points of failure?
It can be really expensive! The more devices you have, the more expensive it is. You need to take baby steps.
When you add in a multi-layer switch - what is this network architecture called?
2-tier architecture. You switches and devices sit at access layer/tier one and your multi-layer switch/router sit at distribution layer/tier 2.
It’s a distribution layer because everything goes through the ‘distribution switch’ - it needs to be able to handle all of the traffic.
What devices sit at layer 1?
Hub, repeaters, extenders, buffers
What devices sit at layer 2?
Switch and bridges
What does the network layer packet header include?
Source and IP address
Which layer manages traffic control?
The network layer - it looks at error detection and node traffic data
What are TCP/IP protocols?
Protocols that are widely used within the TCP/IP model
What devices sit at layer 3?
Routers and bridge routers
Can you use non IP protocols (at the network layer)?
Yes you can, but they are for special purpose networks. Much more bespoke - IPX, Apple Talk, NetBEUI
What is a Brouter?
It sits at layer 3 and attempts to route first, but if that fails it defaults to bridging at layer 2 (non-routable protocols)
At the session layer - how do communication sessions operate?
Simplex - One-way communication
Half Duplex - Two way comms - but can only send data one way at a time
Full Duplex - two way comms - data can be sent both ways
Name some of the format standards that operate within the application layer?
File formats:
ASCII - represent text in computers (binary) 10100110 TIFF - for raster graphics images JPEG - contains high quality images MPEG - digital audio / movie format MIDI - shares musical information
At what level does your web browser or other application directly communicate with protocols?
Application layer
Does the application itself sit in layer 7?
No - just the protocols and services used to transmit files, messages etc.
What other devices sit at layer 7?
Application layer gateway (protocol translation tool)
Application layer firewall
