The TCP/IP Model

Definition: The TCP/IP model is the four-layer reference model used by the Internet protocol suite.

Layered structure

Functions of the layers

1. Application layer: Provides network services to applications, such as web browsing, e-mail, file transfer and name resolution. TCP/IP does not define separate session and presentation layers; those functions are handled by applications or their protocols.
2. Transport layer (Host-to-Host): Provides process-to-process communication using port numbers. TCP offers reliable, ordered delivery and flow/congestion control; UDP offers connectionless, low-overhead delivery without a delivery guarantee.
3. Internet layer: Provides logical IP addressing and routes packets across interconnected networks. IP is connectionless and best-effort; ICMP reports network-related errors and diagnostic information.
4. Network Access layer: Covers link-level and physical transmission functions, including framing, MAC addressing, media access and signal transmission. It maps broadly to OSI Data Link and Physical layers.

Correspondence with OSI

TCP/IP layer	Corresponding OSI layer(s)	Examples
Application	Application, Presentation, Session	HTTP, DNS, SMTP
Transport	Transport	TCP, UDP
Internet	Network	IP, ICMP
Network Access	Data Link, Physical	Ethernet, Wi-Fi

Critiques of the TCP/IP model

1. The Network Access layer combines the OSI Physical and Data Link layers, so it does not distinguish their functions clearly.
2. It has no separate Session and Presentation layers; functions such as dialogue control, data-format conversion and encryption must be handled elsewhere.
3. It was developed around existing protocols, rather than first being designed as a general reference model, so it is less suitable for describing every networking architecture.
4. Layer boundaries are less strict than in the OSI model, and some protocols do not fit neatly into a single layer.
5. The model is commonly described through the Internet protocol suite and its RFCs rather than one universally definitive model specification.

Conclusion: TCP/IP is the practical foundation of Internet communication, while the OSI model is often more useful for learning and separating networking functions.