OSI model & TCP-IP suite
Last updated
Last updated
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
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
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
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
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
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
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
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.
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
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