TCP/IP Model

The TCP/IP model organizes the protocols used by Internet hosts into functional layers. A common four-layer form uses Application, Transport, Internet, and Link. Some teaching models split the physical medium from the link layer and therefore show five layers.

Application Layer

Application protocols define messages and behavior for services such as HTTP, DNS, SSH, and SMTP. This layer also includes many representation and session responsibilities that the OSI model discusses separately.

Transport Layer

Transport protocols provide communication between application endpoints. TCP offers a reliable ordered byte stream with congestion and flow control. UDP provides independent datagrams without TCP's connection, ordering, or retransmission guarantees. Port numbers help identify transport endpoints, but a port number alone does not prove which application is listening.

Internet Layer

Internet Protocol carries packets using source and destination IP addresses. Routers examine routing information and decrement hop limits while forwarding packets toward the destination. ICMP communicates control and error information for IP operation. Delivery remains best effort; higher layers or applications handle any required recovery.

Link Layer and Encapsulation

The link layer sends an IP packet across one local link using Ethernet, Wi-Fi, a point-to-point protocol, or another technology. As application data moves downward, each layer adds information needed for its scope. At the receiver, layers validate and remove their own encapsulation before delivering data upward.

Link headers normally change at each routed hop; transport and application conversations are end-to-end unless a middlebox terminates or transforms them.

Summary

You can now place common Internet functions in the TCP/IP model.

1. Associate service protocols with the application layer.
2. Distinguish TCP streams from UDP datagrams.
3. Place IP addressing and routing at the Internet layer.
4. Treat link framing as local-hop encapsulation.