OSI Model

To standardize the process of networking, what was created?

The Open Systems Interconnection (OSI) model

What are the seven layers of the OSI?

1. Physical Layer

2. Data Link Layer

3. Network Layer

4. Transport Layer

5. Session Layer

6. Presentation Layer

7. Application Layer

Application Layer (Layer 7)

• The Topmost Layer

• Involves communication between end-user applications (the program you interact with on your computer) and the network

  • This involves a user interface and protocols to allow communication

• Services at this layer include web browsing, email, file transfer, and remote access

Hypertext Transfer Protocol (HTTP)

Used to retrieve content from web servers to enable web browsing

File Transfer Protocol (FTP)

Transfer files from client to server

Simple Mail Transfer Protocol (SMTP)

Sends electronic mail from clients to mail server, or between mail servers.

Secure Shell (SSH)

Remote access and command over another machine, with traffic encrypted

Presentation Layer (Layer 6)

• Involves the from in which data is presented and is responsible for:

  • Data Translation: Ensuring the data sent by one system can be understood by the other.

  • Data Compression: Decreasing the size of data to increase efficiency

  • Data Encryption: Enhancing security by converting data into an unreadable format, reversible by the intended recipient

Data Translation

• Necessary because not all systems represent data in the same way

• Ex. Some systems use Little Endian and others use Big Endian, so we need to ensure data is translatable across systems

Data Compression

• Allows us to optimize communication by reducing the number of bits that need to be transmitted across the network

• Compression methods vary, but they tend to leverage patterns to reduce the amount of data needed to express the same concept.

Data Compression can be ___ or ___

lossless or lossy

Lossless Compression

The original data can be perfectly reconstructed (ex. ZIP files). This is essential for text files, executables, etc.

Lossy Compression

Less important or redundant data is removed entirely, resulting in loss. This is permissible when reducing size is more important than retaining quality

Session Layer (Layer 5)

• Involves managing the session between two communicating devices - i.e., a single, continuous exchange of data.

• Negotiating a session may involve:

  • Authentication: Verifying the identity of a user/device

  • Authorization: Checking permissions of a user/device

  • Agreeing on the structure of the conversation (e.g. who gets to talk and when)

For two (or more) applications to communicate across a network, a ___ must be established

Session

Each participant in a session receives a unique ___ to distinguish it from other traffic

identifier (a string, a UUID*, etc.)

Full Duplex

Allows both endpoints to communicate simultaneously (ex. video call). Most modern devices permit concurrent communication.

Half Duplex

Only allows one endpoint to communicate at a time (i.e. they must take turns).

Terms of a conversation must be ___. This decision is based on ___, not on physical medium

negotiated, protocol

Transport Layer (Layer 4)

• Involves managing the traffic between hosts and destinations. Responsibilities include:

  • Segmentation: Breaking data into small chunks (i.e. segments) and reassembling them at the destination.

  • Flow Control: Adjusting the speed/quantity of data transmissions based on the receiver’s capacity

  • Error Control: Ensuring that data is delivered correctly, detecting corruption, and resubmitting lost data (if desired).

Flow Control

Ensures the host sends data at a rate that the destination can receive. One device may be able to transmit data more quickly than the other can accept it, which will lead to buffer overflow. The same can be true if the hose is sending data too slowly, leading to buffer underflow.

If this is a problem, the receiver can send data to the host explaining how much data it can accept, and the host can adjust its transmission speed.

What are the two most common Transport Layer protocols?

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)

Transmission Control Protocol (TCP)

Allows for reliable, well-ordered transmission with error checking. It is used for web browsing, email, file transfers, etc., where correctness is more important than performance.

User Datagram Protocol (UDP)

Allows for unreliable, unordered transmission of data without error checking or flow control. It is used for voice calls, online video games, streaming services, etc. where performance is more important than reliability

Network Layer (Layer 3)

• Involves routing packets across networks to ensure they arrive at the appropriate destinations (this is the layer in which routers are involved).

• Elements of the Layer include:

  • Logical Addressing: Assigning and using logical addresses to identify devices across the network

  • Path Determination/Routing: Determining the optimal course through the network and moving packets along it.

The most prominent Network Layer protocol is called ___ ___, which is responsible for most logical addressing

Internet Protocol (IP)

Each device on a network has a unique ___ ___ that is used for routing purposes

IP Address

IP Addresses allow the network to __

send data to the appropriate destination

What are the two types of IP addresses?

IPv4 and IPv6

IPv4 Address

They are 32 bits total and are represented as four 8-bit decimal values divided by periods

ex. 192.168.5.12

The IP Address identifies both the ___ and the ___

network, host

The ___ ___ of the address is always at the beginning, and the ___ ___ of the address is always at the end

network portion, host portion

Originally, IPv4 addresses were divided into ___

classes

What does the class of an address determine?

How many bits were dedicated to the network and how many were dedicated to the host

Class A address

allows for a small number of networks with a large number of devices

These were intended for use by governments and very large organizations

Start: 0.0.0.0

End: 127.255.255.255

Class B address

Allows for more networks than class A, but fewer devices per network

These were intended for medium-sized organizations and internet service providers (ISPs)

Start: 128.0.0.0

End: 191.255.255.255

Class C address

Allows for a large number of networks with a smaller number of devices

These were intended for small businesses and personal home networks

Start: 192.0.0.0

End: 223.255.255.255

Class D address

Not used for standard networking. Instead, used for multicast communication

Multicasting means sending data to multiple (not all) hosts at once. Video calls, online games, etc.

Start: 224.0.0.0

End: 239.255.255.255

Class E address

Not used. These addresses are reserved for experimental purposes

Not used on the public Internet and are ostensibly for research and development purposes

Start: 240.0.0.0

End: 255.255.255.255

Classful Networking

Dividing IP address ranges

What are the issues with classful networking?

• Fixed size: address space is assigned in fixed chunks; each network in a class will have the same number of host addresses assigned to it

• Inefficiency: Assigned address space is too large; class A networks, for example, can have up to 16 million hosts each.

• Space exhaustion: the two issues above causes us to run out of addresses more quickly than we otherwise would

What was introduced to solve the issues of classful networking?

Classless Inter-Domain Routing (CIDR)

Classless Inter-Domain Routing (CIDR)

Replaces classful networking with a more refined system that allows us to create networks of virtually any size though applying a network mask

Network Mask

A number of bits that specifies the size of the network portion for each individual network

Where does the Network Mask appear in an IP address?

at the end of the address, preceded by a forward slash

ex. 192.168.5.12/24

the number represents the first # of bits that are dedicated to the network portion, whereas the remainder of the bits are dedicated to the host portion

What do Network Masks look like in traditional classful addressing?

Class A: 255.0.0.0 (/8)

Class B: 255.255.0.0 (/16)

Class C: 255.255.255.0 (/24)

By taking a network mask and applying a ___ ___ operation with a given IP address, you produce the ___ ___

bitwise AND, network address

Bitwise AND opreationg compares two bits. If both are 1, corresponding bit is 1. Otherwise, it is 0.

IPv6 Addresses

• Intended to solve the problem of exhausting address space

• 128 bits and are represented using eight groups of four hexits separated by colons.

• Ex. 2001:0DB8:85A3:0000:0000:8A2E:0370:7334

• Highly unlikely to exhaust this address space

Because IPv6 addresses are so long, we often abbreviate them by doing what?

Eliminating leading 0’s and (once per address) replacing and entire block of 0’s with a double colon (::)

ex. 2001:DB8:85A3::8A2E:370:7334

For what reasons is IPv6 still not widely used?

• Address exhaustion was not as significant an issue with IPv4 as was expected

• Changing systems is costly and effort-intesnvie

• IPv6 is not backwards compatible with IPv4, making integration difficult

Data Link Layer (Layer 2)

• Involves organizing raw bits from the Physical Layer (Layer 1) into frames containing information about the source and destination, the payload, and error checking mechanisms

• Includes Media Access Control (MAC) addresses, which facilitate access to physical devices on the same network

Media Access Control (MAC) Address

Address used for communication in the Data Link Layer

MAC addresses are assigned to the __ __ __ by the manufacturer

Network Interface Card (NIC) - the hardware that handles Layer 2 data transmission and reception

Each MAC Address is ___ and is ___ into the device itself, meaning it cannot be changed by conventional means

unique, hardcoded

MAC Addresses are typically ___ bit addresses displayed as ___ hexits

48, 12

Windows: 00-1A-2B-3C-4D-5E

MAC Addresses are used to ___ individual devices at Layer 2

Identify

IP addresses can ___; ISPs may provide new ___, or the device may ___ between networks

change, addresses, move

True or False: MAC Addresses will change when going to new networks or changing networks

False

Physical Layer (Layer 1)

• Involves the raw, physical transmission of signals across media

• Digital data is communicated through hardware in the form of electric signals, light pulses, radio waves, etc.

• The Physical Layer exists independently of meaning; the data is just a physical signal - raw bits. Assigning significance is the responsibility of higher layers.