Ravindra BagaleCourses & study guides

Chapter 2: The OSI Model and TCP/IP Model

2.2 The seven layers

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   Sender                                                    Receiver
 +------------------+        (logical communication)      +------------------+
 | 7 Application    | <---------------------------------> | 7 Application    |
 | 6 Presentation   | <---------------------------------> | 6 Presentation   |
 | 5 Session        | <---------------------------------> | 5 Session        |
 | 4 Transport      | <------------ segments -----------> | 4 Transport      |
 | 3 Network        | <------------ packets ------------> | 3 Network        |
 | 2 Data Link      | <------------ frames -------------> | 2 Data Link      |
 | 1 Physical       | ============== bits ==============  | 1 Physical       |
 +------------------+     cable / fibre / radio           +------------------+
   Data goes DOWN the stack (encapsulation) and UP at the receiver (decapsulation)
# Layer Main function Protocols / standards Devices Data unit (PDU)
7 Application Interface for user applications; network services HTTP, HTTPS, FTP, SMTP, DNS, SSH, DHCP, SNMP Gateways, L7 load balancers (AWS ALB), WAF Data / message
6 Presentation Translation, encryption, compression, data format TLS/SSL, JPEG, PNG, ASCII, UTF-8, MPEG — Data
5 Session Establish, maintain, terminate sessions; checkpoints NetBIOS, RPC, SQL session, PPTP — Data
4 Transport End-to-end delivery, ports, segmentation, flow & error control TCP, UDP L4 load balancers (AWS NLB), firewalls Segment (TCP) / Datagram (UDP)
3 Network Logical addressing (IP), routing between networks IPv4, IPv6, ICMP, ARP*, OSPF, BGP Router, L3 switch Packet
2 Data Link Physical (MAC) addressing, framing, error detection on a link Ethernet (802.3), Wi-Fi (802.11), PPP, VLAN (802.1Q) Switch, bridge, NIC Frame
1 Physical Transmission of raw bits over the medium; voltages, connectors RJ-45, fibre, DSL, Bluetooth radio, 802.11 PHY Hub, repeater, cables, modem Bit

* ARP works between Layer 2 and 3 (it maps IP → MAC), so books place it in either.

Layer-by-layer in simple words

  • Layer 7 – Application: What the user's program speaks. The browser speaks HTTP; ssh speaks the SSH protocol. Not the application itself (Chrome), but the protocol it uses.
  • Layer 6 – Presentation: Makes data understandable to both sides — character encoding (UTF-8), compression (gzip) and encryption (TLS). HTTPS = HTTP + TLS.
  • Layer 5 – Session: Opens, manages and closes a conversation (session). E.g. keeping you logged in, resuming a large download.
  • Layer 4 – Transport: Adds source and destination port numbers. TCP is reliable (3-way handshake, acknowledgements, retransmission, ordering) — used by HTTP, SSH, MySQL. UDP is fast and connectionless — used by DNS, video calls, online games.
  • Layer 3 – Network: Adds source and destination IP addresses and chooses the path (routing). Routers work here. ping (ICMP) tests this layer.
  • Layer 2 – Data Link: Adds MAC addresses (48-bit hardware address like 0a:1b:2c:3d:4e:5f) to move frames within one local network. Switches work here.
  • Layer 1 – Physical: Converts frames into electrical signals, light pulses or radio waves.

TCP vs UDP

In my sessions, students often ask: "If TCP is reliable, why not use it everywhere?" Bagha — reliability has a cost (handshakes, acknowledgements, retransmissions). For a quick DNS query or a live video call, speed matters more than perfection.

Feature TCP UDP
Connection Connection-oriented (3-way handshake SYN, SYN-ACK, ACK) Connectionless
Reliability Guaranteed delivery, ordering, retransmission Best effort; may lose packets
Speed / overhead Slower, 20-byte header Faster, 8-byte header
Uses Web (HTTP/1.1, HTTP/2), SSH, e-mail, databases DNS, DHCP, VoIP, streaming, gaming, HTTP/3 (QUIC)

Encapsulation

As data moves down, each layer adds its own header (Layer 2 also adds a trailer):

 L7-5   [                 HTTP data                   ]
 L4     [TCP hdr: ports ][        HTTP data           ]            = Segment
 L3     [IP hdr: IPs][TCP hdr][      HTTP data        ]            = Packet
 L2     [Eth hdr: MACs][IP][TCP][    HTTP data   ][FCS]            = Frame
 L1     0101100101110100101010010101001010101010101...            = Bits