The OSI Model

Each layer provides services to the layer above it and consumes services from the layer below. Layers communicate with their peer layer on the remote host via protocols, but the actual data flows down the stack on the sender, across the physical medium, and up the stack on the receiver.

Note: The OSI model never "won" as an implementation. TCP/IP did. But OSI remains the dominant mental model for reasoning about networking. When someone says "that's a Layer 2 problem" or "this operates at Layer 7," they're referencing OSI.

Defines the electrical, optical, or radio specifications for transmitting raw bits over a physical medium. This layer is concerned with voltages, pin layouts, cable specs, modulation schemes, and bit timing.

• Converts bits into signals (electrical pulses, light pulses, or radio waves)
• Defines the physical connectors (RJ-45, SFP+, LC fiber connector)
• Specifies data rates (1 Gbps, 10 Gbps, 25 Gbps)
• Manages bit synchronization and encoding (e.g., Manchester encoding, 8b/10b)

PDU: Bits. At this layer, there's no concept of addresses, packets, or meaning — just a stream of 1s and 0s transmitted over a medium.

Provides node-to-node (hop-by-hop) data transfer between directly connected devices on the same network segment. This layer frames raw bits into structured frames, handles MAC addressing, error detection, and media access control.

• Framing: Encapsulates packets into frames with headers and trailers
• MAC Addressing: Uses 48-bit hardware addresses (e.g., aa:bb:cc:dd:ee:ff) to identify NICs on the local segment
• Error Detection: Appends a CRC (FCS field) to detect corrupted frames
• Media Access Control: Manages access to the shared medium (CSMA/CD for wired Ethernet, CSMA/CA for Wi-Fi)
• Flow Control: Prevents a fast sender from overwhelming a slow receiver (e.g., Ethernet PAUSE frames)

PDU: Frame. An Ethernet frame includes a destination MAC, source MAC, EtherType, payload, and a 4-byte FCS (Frame Check Sequence).

Provides end-to-end logical addressing and routing across multiple networks. This is the layer that enables internetworking — connecting different LANs into a single routed network.

• Logical Addressing: IP addresses (IPv4: 32-bit, IPv6: 128-bit) that are hierarchical and routable
• Routing: Determines the best path for packets across multiple hops (routers)
• Fragmentation/Reassembly: Splits packets that exceed the MTU of a link (though modern practice avoids this via Path MTU Discovery)
• TTL/Hop Limit: Prevents packets from looping forever

PDU: Packet. An IP packet includes source IP, destination IP, TTL, protocol field, and the payload (typically a transport segment).

Provides end-to-end communication between processes on different hosts. This layer introduces port numbers to multiplex multiple applications over a single IP address, and optionally provides reliability, ordering, and flow control.

• Segmentation: Breaks application data into segments/datagrams
• Port Multiplexing: 16-bit port numbers (0-65535) identify specific processes
• Reliability (TCP): Acknowledgments, retransmission, in-order delivery
• Flow Control (TCP): Sliding window mechanism to prevent receiver overload
• Congestion Control (TCP): Algorithms (Reno, CUBIC, BBR) to avoid overwhelming the network

PDU: Segment (TCP) or Datagram (UDP). Includes source port, destination port, and protocol-specific fields (sequence numbers for TCP, length for UDP).

Manages sessions (dialogs) between applications. Responsible for establishing, maintaining, synchronizing, and tearing down communication sessions. This includes checkpointing, so a long transfer can resume after interruption.

• Session establishment, maintenance, and termination
• Synchronization and recovery (checkpoints in data streams)
• Full-duplex, half-duplex, or simplex mode management

In practice: Layer 5 is one of the "blurriest" layers. In the TCP/IP world, session management is typically handled by application protocols or by TLS session resumption. Distinct L5 protocols are rare in modern networking.

Handles data translation, encryption, and compression between the application and the network. Ensures that data from the application layer of one system can be read by the application layer of another, regardless of internal representation.

• Data Translation: Character encoding (ASCII, UTF-8, EBCDIC), byte order
• Encryption/Decryption: TLS/SSL encryption technically operates here
• Compression: gzip, data serialization formats (ASN.1, Protocol Buffers, JSON)
• Serialization: Converting data structures to wire format and back

In practice: Like Layer 5, this layer is largely absorbed into the application layer in real-world TCP/IP. TLS, data serialization (JSON, protobuf), and compression (gzip) are all handled at the application level or by libraries.

The layer closest to the end user. Provides network services directly to applications. This is where protocols define the semantics of the communication — what the data means and how it's structured.

• Defines application-specific protocols and data formats
• Provides interfaces for user applications to access network services
• Handles authentication, authorization at the application level

Note: Layers 5-7 all share the same PDU name ("Data") in the OSI model because there's no standard framing between them — they all operate on the application data before it reaches the transport layer.

• Shared vocabulary: "L4 load balancer" vs "L7 load balancer" is universally understood because of OSI numbering
• Troubleshooting framework: Work bottom-up — "Is the cable plugged in? (L1) → Can I see the MAC? (L2) → Can I ping the IP? (L3) → Is the port open? (L4) → Is the app responding? (L7)"
• Vendor-neutral: Every networking vendor, cloud provider, and certification uses OSI layer numbering
• Separation of concerns: Forces you to think about which layer a problem or technology belongs to