OSI model & TCP-IP suite

Networking model

• categorizes and provides a structure for networking protocols/standards (set of rules, logical rules, defining how network devices and software should work together)

• if Dell and Apple made their own networking model with their own protocols, they would be able to communicate within their own ecosystem but not with one another

OSI model

• Open Systems Interconnection

• conceptual model that categorizes and standardizes the different functions in a network

• an attempt at standardizing network communications (not in use nowadays)

• functions are divided into 7 layers

• top 3 :

  • Application, Presentation, Session

  • application developers work with these top layers to connect their applications over networks

• bottom 4 :

  • Transport, Network, Data Link, Physical

  • network engineers work with the bottom layers

7. Application layer

• closest to end user

• interacts with software applications like web browsers

• eg: HTTP, HTTPS, [[DHCP]]

• identifies communication partners and synchronizes communication

• Note: Layer 7 doesn't include the application itself, but rather the protocols that interact with the application like HTTPS

• Functions:

  • Identifying communication partners

  • Synchronizing communication

Adjacent layer interaction

• interaction between different layers of the OSI model

• eg: encapsulation and de-encapsulation

Same layer interaction

• eg: communication between the application layers of 2 different systems

• this same layer interaction between application layers is what allows the application layer to perform its functions of identifying communication partners, syncing communications etc

6. Presentation layer

• data in application layer is in 'application format'

• translates data to the appropriate format (between application and network formats)

• also translates between different application layer formats

• eg: encryption of data when sent to this layer and decryption when received

5. Session layer

• controls dialogues (sessions) between communicating hosts

• establishes, manages and terminates connections between local application (web browser) and remote application (youtube)

• eg: youtube servers are used by countless people at every moment, so there has to be a way to manage all of these sessions

4. Transport layer

• adds a header in front of the data

• Segment: data + L4 header

• eg: UDP

• segments and reassembles data for communication between end hosts

• breaks large pieces of data into smaller segments which can be easily sent over the network and are less likely to cause transmission problems, if error occurs

• provides host-to-host communication (end-to-end communication), process-to-process communication for applications

• if data being sent is large enough, it will be segmented into smaller parts, and a Layer 4 header will be added on to each segment

3. Network layer

• provides connectivity between end hosts on different networks (outside of the LAN)

• provides logical addressing ( source and destination IP addresses)

• Packet: data + L4 header + L3 header

• eg: OSPF

• provides path selection between source and destination (in complex networks like internet, there can be many different possible paths to a destination)

• routers operate at layer 3

2. Data Link layer

• provides node-to-node connectivity and data transfer (eg: PC to switch, switch to router, router to router)

• defines how data is formatted for transmission over a physical medium (eg: copper UTP cables)

• detects and possibly corrects Physical layer errors

• Layer 2 addressing (separate from L3)

• switches operate at Layer 2 (switches look at destination L2 address to determine where to send the data)

• Frame : L2 trailer + data + L4 header + L3 header + L2 header

• eg: STP

• This frame is sent over the connection, whether it is electrical signals over a wire or wireless signals in case of wifi.

1. Physical layer

• defines physical characteristics (voltage levels, max transmission distances, physical connectors, cable specs) of the medium used to transfer data between devices

• Digital bits are converted into electrical (for wired connections) or radio (for wireless connections) signals.

• Everything in [[Interfaces and cables]] is related to this layer

Protocol Data Units (PDU)

• Segment is Layer 4 PDU, Frame is a Layer 2 PDU

• at Layer 1, the PDU is bit

Data flow

• A software application interacts with application layer and wants to send some data to the system on the right.

• This data is processed through the OSI stack (Layer 7 to Layer 1), each layer adding something to the original data. This is called encapsulation, as the original data is encapsulated inside this additional information which is added on. ^encapsulation

• By the time it reaches the physical layer, it is electrical signals on wire and is sent to the neighboring system.

• The neighboring system performs the opposite process - the additions of each layer are stripped off until the data reaches the application layer of the neighboring system. This process is called de-encapsulation ^de-encapsulation

TCP / IP suite

• similar structure to OSI model, but with fewer layers

• used in internet and other modern networks

• developed by US Department of Defense

• 4 layers:

  • application, presentation, session layer of OSI model are equivalent to the Application layer in TCP/IP model. (actually this is how network engineers perceive, as they won't work above Transport layer)

  • but the numbering is used from OSI model. eg: Layer 4 problem means OSI's layer 4 (transport layer) and not TCP/IP's layer 4 (application layer)

  • Transport layer is shared between the both models

  • Network layer of OSI is equivalent to Internet layer in TCP/IP model

  • DataLink and Physical layer of OSI is mapped to Link layer in TCP/IP