TCP (The Three-Way Handshake)

TCP is a connection‑oriented, reliable transport‑layer protocol. Before any data is exchanged, TCP establishes a session using a three‑way handshake, then guarantees in‑order, error‑free delivery via sequencing, acknowledgments, flow control, and retransmissions. Finally, it cleanly tears down the connection with a four‑step FIN sequence.

Key Characteristics

Connection‑oriented: Requires setup (three‑way handshake) before sending data.
Reliable delivery: Uses sequence numbers, ACKs, checksums, and retransmissions.
Ordered data stream: Segments arrive in order—or are reordered by the receiver.
Flow control: Receiver advertises a window size to throttle sender speed.
Congestion control: Algorithms (e.g., Slow Start, Congestion Avoidance) adapt to network conditions.

TCP Segment Structure

Each TCP segment consists of a header (20–60 bytes) and payload.

Field	Size	Description
Source Port	16 bits	Port on the sender’s host (chosen from ephemeral range).
Destination Port	16 bits	Port where the receiving service is listening (e.g., 80 for HTTP).
Sequence Number	32 bits	Byte index of the first data byte in this segment.
Acknowledgment Number	32 bits	Next byte the sender expects to receive (cumulative ACK).
Data Offset (HLEN)	4 bits	Header length in 4‑byte words (min 5 → 20 bytes; max 15 → 60 bytes).
Reserved	3 bits	Reserved for future use; set to zero.
Control Flags	9 bits	URG, ACK, PSH, RST, SYN, FIN, plus ECE, CWR, NS for congestion control.
Window Size	16 bits	Bytes the receiver can accept (flow‑control window).
Checksum	16 bits	Covers header + data for error detection.
Urgent Pointer	16 bits	If URG flag set, points to last urgent data byte.
Options & Padding	0–40 bytes	Extra features (MSS, window scaling, timestamps).
Payload (Data)	Variable	The actual application data (up to MSS).

Three‑Way Handshake

Establishes a TCP connection and synchronizes initial sequence numbers (ISNs):

Step	Packet	Description
1	`SYN`	Client → Server: “SYNchronize. My ISN = A.”
2	`SYN + ACK`	Server → Client: “ACK your A. My ISN = B, and I SYNc with you.”
3	`ACK`	Client → Server: “ACK your B. Let’s begin data transfer at Seq = A+1, Ack = B+1.”

Once Step 3 completes, the TCP connection is established.