OSI Model

  • Open Systems Interconnection Model


Sr No Layer Data unit Examples
1 Physical bit Ethernet, USB, Wi-Fi, Bluetooth, DSL
2 Data link Frame L2TP, PPP
3 Network Packet IPv4, IPv6
4 Transport Segment (Datagram) TCP, UDP
6 Presentation Data ASCII, JPEG
7 Application Data Chrome,

Data Units

  • Frame
    • Structure: Frame header (e.g. Ethernet) + (Network header + Transport header + data) + Frame footer
    • Source and destination MAC addresses, length, checksum
  • Packet
    • Structure: Network header (e.g. IP) + (Transport header + data)
    • Version (IPv4/IPv6), source and destination IP addresses, length flags, TTL, protocol, checksum
  • Segment
    • Structure: Transport header (e.g. TCP) + data
    • Source and destination ports, sequence number, ack number, data offsets, flags, checksum


  • Internet Protocol
  • No concept of connection.
  • Packets are passed from one computer to the next, until reaching the destination.
  • No delivery guarantee, no receiving ack.
  • Sometimes multiple copies of the same packet are passes, taking different paths, and thus arriving at different times.
  • Designed to be able to route around connectivity problems.


  • Transmission Control Protocol
  • Built on-top of IP.
  • Connection-based.
  • Once a connection has been made between two parties, sending data between the two is much like writing to a file on one side and reading from a file on the other.
  • Reliable and ordered, i.e., arrival and ordering are guaranteed.
  • Takes care of splitting your data info packets and sending those across the network, so you can write bytes as a stream of data.
  • Makes sure it doesn't send data too fast for the Internet connection to handle (flow control).
  • Hides all complexities of packets and unreliability.
  • Sends an ack for every packet received.
  • Queues up data until there's enough to send as a packet.
  • TCP tends to induce packet loss for UDP packets whenever they share a bottleneck node (same LAN/WAN).


  • User Datagram Protocol
  • Built on-top of IP, very thin layer over it.
  • Unreliable protocol, usually around 1-5% packet loss.
  • No guarantee of ordering.
  • Minimizes transmission delay.
  • Send a packet to destination IP address and port; the packet will get passed from computer to computer and arrive at destination or get lost.
  • Receiver simply listens on specific port and gets notified when a packet arrives, with the sender address:port, and packet size.
  • One guarantee over IP – a packet will either arrive as a whole (all of it) at destination or not at all (no partial delivery).
  • You need to manually break your data up into packets and send them.
  • You need to make sure you don't send data too fast for your Internet connection to handle.
  • Good for when you want data to get as quickly as possible from client to server without having to wait for lost data to be resent, usually real-time data.
  • Examples: real-time gaming, metrics reporting, video/audio streaming.

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