OSI Network Model on tplat

Physical Layer

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Physical Layer

The physical layer defines the electrical, mechanical, and procedural interface to the transmission medium. It covers connectors, signalling, line coding, frequencies, and other low-level transmission details.

Data Link Layer

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Data Link Layer

The data link layer transfers frames between devices on the same local network segment. It also defines how devices identify one another on the local link and how frames move across the physical medium.

The data link layer handles: The data link layer transfers frames between nodes on the same local network segment. It can also detect and sometimes correct errors from the physical layer.

The data link layer handles:

Network Layer

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Network Layer

The network layer is responsible for packet forwarding between hosts, including routing through intermediate routers. It provides:

Connectionless communication.
Host addressing.
Message forwarding.

Internet Protocol

The Internet Protocol (IP), introduced in RFC 791, exists at the network layer. IP routes packets, encapsulates data, and fragments and reassembles datagrams when they reach the destination host. By design, IP is connectionless and provides no guarantee that data will reach its intended recipient.

Transport Layer

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Transport Layer

The transport layer provides end-to-end communication services for applications. It handles connection-oriented communication, reliability, flow control, congestion control, and multiplexing.

Connection-oriented communication.
In-order delivery.
Reliability.
Flow control.
Congestion control.
Multiplexing.

TCP vs. UDP

The Transmission Control Protocol (TCP) (RFC 793) and User Datagram Protocol (UDP) (RFC 768) reside at the transport layer.

Characteristic	TCP	UDP
Transmission	Connection-oriented	Connectionless.
Connection establishment	Uses a three-way handshake.	Does not verify that the destination is listening.
Data delivery	Stream-based.	Packet-based.
Receipt of data	Uses sequence and acknowledgment numbers.	Does not track receipt.
Speed	More overhead, but reliable.	Faster, but unreliable.

TCP

TCP uses flags in the header. The three flags worth remembering first are SYN, ACK, and FIN.

Session Layer

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Session Layer

The session layer provides the mechanism for opening, closing, and managing a session between end-user application processes. It supports a semi-permanent dialogue between endpoints.

Presentation Layer

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Presentation Layer

The presentation layer serves as the data translator for the network. It provides:

Data conversion.
Character code translation.
Compression.
Encryption and decryption.
Serialisation.

Application Layer

Mon, 01 Jan 0001 00:00:00 +0000

Application Layer

The application layer defines the communication protocols and interface methods used by hosts in a network.

HTTP

Hypertext Transfer Protocol (HTTP) lives at the application layer and enables cleartext communication between a client and server over TCP. HTTP commonly uses ports 80 and 8000.

Method	Description
HEAD	Requests the same resource as `GET` without the response body. Useful for metadata and health checks.
GET	Reads data from the server.
POST	Submits data for processing.
PUT	Creates or replaces a resource at the target URI.
DELETE	Removes the resource at the target URI.
TRACE	Echoes the request for diagnostics.
OPTIONS	Reports the methods supported by the target resource.
CONNECT	Establishes a tunnel, usually through a proxy.

HTTPS

HTTP Secure (HTTPS) wraps HTTP in TLS so the conversation can be encrypted end to end. It commonly uses ports 443 and 8443 instead of port 80.