Understanding TCP vs UDP: Reliable and Unreliable Transport Explained

Transport Layer Overview

The transport layer provides end‑to‑end communication between applications. It offers two core protocols: TCP (connection‑oriented, reliable) and UDP (connection‑less, unreliable).

IP Addresses and Port Numbers

An IP address identifies a host. The IP header contains a protocol field (6 for TCP, 17 for UDP) that indicates which transport protocol carries the payload. To differentiate multiple applications on the same host, the transport layer uses 16‑bit port numbers , often called program addresses .

Port Number Ranges

Well‑known ports (0‑1023) – reserved for standard services (e.g., HTTP 80, SSH 22).

Registered ports (1024‑49151) – assigned to specific applications.

Dynamic (private) ports (49152‑65535) – allocated by the operating system for client connections.

TCP vs UDP

TCP delivers a reliable byte‑stream. It provides sequence control, acknowledgments, retransmission, flow control, congestion control, and optional extensions. UDP delivers datagrams without guaranteeing delivery, order, or congestion control; applications must implement any required reliability.

Key Differences

TCP establishes a connection with a three‑way handshake, uses ACKs, and retransmits lost segments.

UDP has lower overhead and is suited for DNS, video/audio streaming, LAN‑only services, and broadcast/multicast.

TCP Connection Management

TCP creates a connection using a three‑way handshake:

SYN: client sends a SYN segment with initial sequence number a.

SYN‑ACK: server replies with SYN and ACK, sequence b, acknowledgment a+1.

ACK: client acknowledges with ACK, sequence a+1, acknowledgment b+1. Data transfer can then begin.

Connection termination uses a four‑step handshake (FIN/ACK exchange) to close both directions.

Sequence Numbers and Acknowledgments

Each TCP segment carries a 32‑bit sequence number. The receiver returns an ACK indicating the next expected sequence number (N+1 means all bytes up to N were received). Duplicate ACKs trigger fast retransmission.

Timeout Retransmission

If an ACK is not received within the measured round‑trip time (RTT), the sender retransmits the segment. Retransmission intervals double exponentially until a limit is reached, after which the connection is aborted.

Window and Flow Control

TCP uses a sliding window to allow multiple unacknowledged segments. The 16‑bit window field specifies how many bytes the receiver can accept. A zero window signals that the receiver’s buffer is full, prompting the sender to pause. The receiver advertises its available buffer size; the sender must not exceed this advertised window, preventing buffer overflow.

Congestion Control

TCP employs slow start: the congestion window (cwnd) starts at one maximum‑segment‑size (MSS) and grows by one MSS for each ACK received until loss is detected. This limits the amount of data injected into the network, avoiding congestion collapse.

UDP Header Format

UDP has a fixed 8‑byte header followed by payload data. The header fields are:

Source port (16 bits)

Destination port (16 bits)

Length (16 bits) – total length of header and data

Checksum (16 bits) – optional error‑checking field

TCP Header Format

TCP’s header consists of a mandatory 20‑byte portion plus optional fields. Key fields include:

Source port (16 bits)

Destination port (16 bits)

Sequence number (32 bits)

Acknowledgment number (32 bits)

Data offset (4 bits) – header length

Reserved (6 bits)

Control flags (6 bits): URG, ACK, PSH, RST, SYN, FIN

Window size (16 bits) – receiver’s advertised buffer

Checksum (16 bits)

Urgent pointer (16 bits) – valid if URG is set

Options (variable length) – e.g., MSS, window scaling

Additional TCP Mechanisms

Maximum Segment Size (MSS)

During the three‑way handshake each side advertises its MSS. The smaller value is used for segment sizing. Typical defaults are 536 bytes (minimum) and 1460 bytes (optimal for Ethernet with a 1500‑byte MTU).

Fast Retransmit and Fast Recovery

When the sender receives three duplicate ACKs, it performs a fast retransmit of the missing segment without waiting for the retransmission timer. The congestion window is then reduced (typically to half) and grows again using additive increase.

Window Probing

If the advertised window is zero, the sender may send a one‑byte “window probe” to elicit an updated window size from the receiver.

Summary of Differences

TCP provides reliable, ordered delivery with flow and congestion control; suitable for applications that require data integrity (e.g., web, file transfer).

UDP provides low‑latency, connection‑less delivery without reliability guarantees; suitable for latency‑sensitive or broadcast applications (e.g., DNS, VoIP, streaming).