Unraveling TCP/IP: From Its Origins to the Full Packet Journey

Historical Background

In the 1960s the U.S. Department of Defense funded research on a communication method that could survive line failures. This led to packet switching and the creation of ARPANET, the first distributed wide‑area packet network and the direct predecessor of the TCP/IP suite.

Standardization

The suite is defined by open, implementation‑oriented specifications published as RFCs by the IETF. RFCs contain both protocol syntax and guidance for developers.

TCP/IP Layer Model

TCP/IP condenses the seven‑layer OSI model into four layers: Link, Internet, Transport, and Application.

Link Layer

The link layer encompasses the physical medium (e.g., Ethernet cables, telephone lines) and the data‑link functions that frame and transmit bits over a single link. It defines MAC addressing and EtherType fields.

Internet Layer

The Internet layer uses the IP protocol to route packets based on IPv4/IPv6 addresses. Supporting protocols include ICMP for error reporting and diagnostics, and ARP for resolving IPv4 addresses to MAC addresses on the local link.

Transport Layer

Two transport protocols dominate:

TCP – connection‑oriented, reliable delivery with sequence numbers, checksums, flow control, and congestion control.

UDP – connection‑less, best‑effort delivery, low latency, suitable for streaming, multicast, and DNS.

Application Layer

The application layer merges OSI’s session and presentation functions. Applications typically follow a client‑server model; servers listen on well‑known ports, clients initiate connections.

Packet Construction and Transmission

Header Stacking

Each layer prepends its own header to the payload, forming a stacked packet that travels from the application down to the physical medium.

Sending Process

Application – Data is encoded and handed to the transport layer; a TCP connection is established if required.

Transport (TCP) – A TCP header containing source/destination ports, sequence number, acknowledgment number, flags, window size, and checksum is added.

Internet (IP) – An IP header with source/destination IP addresses, TTL, protocol identifier, and checksum is wrapped around the TCP segment.

Link (Ethernet) – An Ethernet frame adds source/destination MAC addresses, EtherType, and a frame check sequence (FCS) before the bits are placed on the wire.

Receiving Process

The NIC discards frames whose destination MAC does not match.

The Ethernet frame is stripped, and the IP packet is passed to the IP layer.

The IP layer verifies the destination IP, removes its header, and forwards the payload to the indicated transport protocol.

The transport layer validates the checksum, reassembles segments (TCP) or delivers the datagram (UDP), and finally passes the data to the application.

Key Protocol Details

ARP

– Resolves an IPv4 address to a MAC address on the local network when the destination MAC is unknown. ICMP – Provides network diagnostics (e.g., ping, destination unreachable) and error messages for IP.

TCP header fields: source port, destination port, sequence number, acknowledgment number, data offset, reserved bits, flags (SYN, ACK, FIN, etc.), window size, checksum, urgent pointer, optional options.

UDP header fields: source port, destination port, length, checksum.

IP header fields (IPv4): version, header length, DSCP, total length, identification, flags, fragment offset, TTL, protocol, header checksum, source address, destination address, optional options.