TCP/IP Stack

TCP/IP¶

To transmit a packet of information (say, a picture) from, for example, Boston in North America to Voronezh in Russia over common data transmission networks through layers of heterogeneous equipment, a universally understood protocol stack is necessary. This stack encompasses the entire transport chain of digital data transfer, formalizing the rules for packet formation and transport. The protocol stack must cover all levels of the data transmission model, starting from the user-visible level (e.g., HTTP requests in a browser) to the level directly responsible for interacting with the physical transmission medium (e.g., the backbone fiber optic cable laid at the bottom of the Atlantic Ocean).

The protocol stack that ensures end-to-end data transmission over computer networks in general and through the Internet in particular is called the TCP/IP. It consists of four levels: application, transport, network, and link.

The topmost, application layer, provides the interaction between the network and the user. Here, the user refers not to a human but rather to various software and hardware devices (e.g., IoT sensors) that interact with the network. The application layer is the entry point of information into the network. The most well-known application layer protocol is HTTP.

The transport layer is a set of mechanisms that ensure the correct sequencing of data transmission and reception, elimination of duplication, and, if necessary, the organization of retransmissions in case of data loss.

The following protocols operate at the transport layer:
TCP — a protocol that organizes retransmissions in case of information loss and eliminates duplication when two copies of the same packet are received;
UDP — sends packets without ensuring data integrity; used for streaming, online gaming, internet telephony, and other applications where the speed of data transmission and immediacy of information are more critical than transmission accuracy, and retransmissions of lost packets often do not make sense;
DCCP — a newer protocol that, like UDP, does not guarantee data delivery but provides access to congestion control mechanisms, offering information about lost packets; it is not very widespread since the mechanisms that control data loss in streaming networks are often implemented at the higher application level;
SCTP — a relatively new protocol that provides enhanced security features and multi-threaded transmission.

The internet layer is the level at which routers operate, sequentially transmitting packets from one network to another. At the network layer, the specific content of information packets is irrelevant; only addressing and routing matter. The most important protocol at this level is IP, which unites network segments into a whole and ensures data delivery between any network nodes through any number of intermediate routers.

The link layer (or data link layer) describes network operation at the physical level and deals with low-level concepts such as bit sequences or noise immunity. For example, the most widespread network level protocol, Ethernet, specifies special bit sequences that define the beginning and end of a data packet.

The physical layer, which covers electrical specifications, is not officially part of the TCP/IP stack but is often mentioned for educational purposes, such as in Andrew Tanenbaum's "Computer Networks."

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