In TCP/IP networking, what is meant by a TCP session or TCP connection between two endpoints?

Introduction / Context:
The term "TCP session" or "TCP connection" appears frequently in discussions about web browsing, file transfer, and other client-server applications. It refers to the logical relationship established between two endpoints over which reliable data exchange occurs. Interviewers may ask what a TCP session is to test understanding of connection-oriented transport protocols and state management in TCP/IP.

Given Data / Assumptions:

• We are using the Transmission Control Protocol (TCP) on top of IP.
• Two endpoints are typically identified by IP addresses and port numbers.
• TCP uses a three-way handshake to set up the connection.
• Once established, the connection tracks sequence numbers, acknowledgements, and window sizes.

Concept / Approach:
A TCP session is defined by a four-tuple: source IP, source port, destination IP, and destination port. When a client wants to talk to a server, it initiates a three-way handshake: sending a SYN segment, receiving a SYN-ACK, and replying with an ACK. This handshake establishes shared state, including initial sequence numbers. Throughout the session, TCP keeps track of bytes sent and acknowledged, retransmits lost data, and applies flow and congestion control. The session ends with a connection termination sequence using FIN and ACK segments or may be aborted with a reset (RST).

Step-by-Step Solution:
Step 1: Describe a TCP session as a logical connection identified by IP addresses and port numbers at both ends. Step 2: Explain that it is established using a three-way handshake (SYN, SYN-ACK, ACK). Step 3: Note that once established, TCP maintains state, including sequence numbers, acknowledgement numbers, and window sizes for each direction. Step 4: Mention that data is transmitted in segments over this connection, with TCP ensuring reliability and ordering. Step 5: Conclude that the session is gracefully closed with a four-step FIN/ACK exchange or forcibly reset if needed.

Verification / Alternative check:
Monitoring network traffic with a packet analyzer shows distinct TCP connections, each beginning with a SYN from the client and a SYN-ACK from the server. Subsequent packets in the connection carry the same source and destination IP and port numbers, with sequence and acknowledgement numbers evolving over time. When the communication ends, FIN and ACK flags appear, clearly marking the termination of the TCP session. This observed pattern confirms the definition of a TCP session as a stateful logical connection.

Why Other Options Are Wrong:
Option B confuses a session with a single broadcast packet, which is unrelated to TCP's connection oriented nature. Option C describes a physical cable between switches, which is a link layer concept, not a transport layer session. Option D refers to user authentication on an email server, which is an application layer issue, not a TCP level connection definition.

Common Pitfalls:
One pitfall is to equate a TCP session with a user session at the application layer; multiple TCP connections can exist within one logical user session, particularly in modern web applications. Another error is to forget that firewalls and NAT devices track TCP sessions using the four-tuple and timeout idle connections accordingly. Understanding TCP sessions helps when analyzing connection failures, timeouts, and resource usage on servers and network devices.

Final Answer:
A TCP session is a logical, connection-oriented channel established between two endpoints using a three-way handshake, maintaining state such as ports and sequence numbers to provide reliable data transfer.