TCP/IP Distilled
The TCP/IP model is the ubiquitous standard in computer networks. It came out as a set of protocols developed at DARPA(Defence Advanced Research Projects Agency). Like the OSI reference model, TCP/IP is a layered model consisting of four layers; Application, Transport, Internet, and the Link layer.
The TCP/IP model combines the application, presentation, and session layer in the OSI reference model into a single application layer. Likewise, the OSI reference model's data link and physical layer are combined into a single link layer. The transport and network layer have a one-to-one correspondence in both models.
Application Layer
The TCP/IP application layer interacts directly with software applications. As a result, most of the applications we write use protocols defined in the application layer. Common TCP/IP application-level protocols include HTTP, FTP, and SMTP. In addition, DHCP(Dynamic Host Configuration Protocol) that provides addressing, and DNS(Domain Name System), which provides name resolution, also functions at the application layer.
Transport Layer
Transport Layer protocols handle the transfer of data between two nodes. The transport layer ensures that the data received at the destination is in the correct order without duplicates or missing datagrams. The primary transport-level protocols are TCP and UDP. Applications that rely on the transport layer to handle error correction, flow control, retransmission, and transport acknowledgment use TCP. Applications that need maximal throughput or do not require session management or error checking leverage UDP.
Network Layer
The network layer is responsible for transmitting data between nodes. It allows an application to send data to a network address without having a direct connection to the remote node. The network layer abstracts away the complexity of routing packets, often over multiple networks and heterogenous physical media devices. Common protocols that provide host identification and routing include the Internet Protocol(V4 and V6), Border Gateway Protocol(BGP), Internet Control Message Protocol (ICMP), Internet Group Management Protocol(IGMP), and Internet Protocol Security(IPSec).
Link Layer
The link layer is the interface between the core TCP/IP protocols and the physical media. The Address Resolution Protocol(ARP) in the link layer translates the node's IP address to the MAC address of its network interface. The link layer adds the MAC address in each frame header before passing the frame to the physical network. Common link layer protocols include Serial Line Internet Protocol(SLIP) and Point to Point Protocol(PPP).
However, not all TCP/IP implementations include link layer protocols. For example, TCP/IP implementations over Ethernet, wireless, and fiber optics do not include link layer protocols. Instead, they depend on device drivers and network hardware to fulfill the capabilities of the link layer of their TCP/IP stack.
