How TCP Guarantees Reliable Data Transfer: Packets, Sequencing, and Flow Control Explained

1. Role of TCP in the Internet

The Internet is built on layered protocols. Ethernet provides point‑to‑point communication within a LAN, IP supplies addressing and routing across networks, and TCP sits above IP to guarantee reliable end‑to‑end data transfer.

Ethernet defines how electrical signals form packets but cannot interconnect multiple LANs. IP solves this by defining IP addresses and routing tables, enabling hosts in different LANs to communicate.

Routers forward packets based on the destination IP address using their routing tables.

2. TCP Packet Size

Ethernet frames are 1518 bytes (later 1522 bytes). The payload is 1500 bytes and the Ethernet header is 22 bytes.

IP packets reside inside the Ethernet payload and require a minimum 20‑byte IP header, leaving a maximum IP payload of 1480 bytes.

TCP adds its own header (minimum 20 bytes), so the maximum TCP payload is roughly 1460 bytes. In practice, optional headers reduce the usable payload to about 1400 bytes.

A 1500‑byte HTTP request often spans two TCP packets. HTTP/2 compresses header fields, allowing a single request to fit into one TCP packet and thus improving latency.

3. TCP Sequence Numbers (SEQ)

Each TCP segment carries a sequence number (SEQ) so the receiver can reorder packets and detect loss. The first segment starts with a random number; for illustration we treat it as 1.

If a segment contains 100 bytes of data, the next segment’s SEQ will be 101, allowing the receiver to know both the current segment’s position and the start of the following segment.

4. Reassembly of TCP Segments

The operating system reassembles received TCP segments; applications never handle raw TCP packets directly. Once reassembled, data is delivered to the appropriate application based on the destination port (e.g., 21 for FTP, 25 for SMTP, 80 for HTTP).

5. Slow Start and ACK Mechanism

To avoid overwhelming the network, TCP begins with a small congestion window (typically 10 segments, defined by the constant TCP_INIT_CWND in the Linux kernel). The window size grows if no packet loss is detected and shrinks when loss occurs.

The receiver acknowledges received data with ACK packets. By default, an ACK is sent for every two received TCP segments. An ACK carries two pieces of information: the next expected sequence number and the remaining space in the receiver’s window.

Expected next sequence number Remaining receive‑window capacity

6. Handling Lost Packets

If a segment is lost, the receiver continues to send duplicate ACKs indicating the same next‑expected sequence number. When the sender receives three duplicate ACKs (or a timeout occurs), it infers packet loss and retransmits the missing segment.