Understanding IP, TCP, and UDP Packet Structures: A Quick Guide

IP Packet Format

Introduction

The IP protocol encapsulates transport‑layer messages in an IP header that contains information such as the originating transport protocol, destination machine, source machine, and other useful data. IP is connection‑less, meaning it does not establish a session before sending data, which reduces overhead.

IP Header Fields

Version (0‑3 bits) : Binary value; 4 for IPv4 (0100) and 6 for IPv6 (0110).

Internet Header Length (IHL) (4‑7 bits) : Length of the header in 4‑byte units; a header without options is 20 bytes, so the value is 5.

TOS (Type of Service) (8‑15 bits) : Complex field updated several times. Sub‑fields include:

0‑2 bits: Priority

3rd bit: Delay

4th bit: Throughput

5th bit: Reliability

Last two bits: Reserved

Total Length (TL) : Total size of the packet in bytes, including header and data, up to 65 535 bytes.

Identification (ID) : 16‑bit unique identifier for each IP packet, used for reassembly of fragments.

Flags (3 bits) :

Bit 0 (Reserved): Usually 0.

DF (Don’t Fragment): 0 = can fragment, 1 = cannot fragment.

MF (More Fragments): 0 = last fragment, 1 = more fragments follow.

Fragment Offset (13 bits) : Position of this fragment relative to the start of the original packet; 0 when no fragmentation occurs.

Time To Live (TTL) (8 bits) : Decremented by one at each router hop; packet is discarded when TTL reaches 0 to prevent endless circulation.

Protocol Number (8 bits) : Indicates the encapsulated transport‑layer protocol (e.g., TCP = 6, UDP = 17, ICMP = 1).

Header Checksum (16 bits) : Used for error detection of the header.

Source Address (SA) : 32‑bit IP address of the sender.

Destination Address (DA) : 32‑bit IP address of the receiver.

Options : Optional fields for security, routing, timestamps, etc.

Padding : Added to ensure the header length is a multiple of 4 bytes.

TCP Packet Format

Purpose of TCP

TCP sits on top of IP and provides a stateful, reliable transmission service. It ensures data is delivered in order, without loss, and acknowledges receipt.

TCP Header

Key Fields

The TCP header includes source/destination ports, sequence number, acknowledgment number, data offset, reserved bits, control flags (SYN, ACK, FIN, etc.), window size, checksum, urgent pointer, and optional options.

UDP Packet Format

Introduction

UDP is a simplified version of TCP, containing only source/destination ports, length, and checksum.

UDP Header

Key Fields

Fields are source port, destination port, length (header + data), and checksum.

Summary of TCP vs UDP

TCP is connection‑oriented; UDP is connection‑less.

TCP provides reliable, ordered delivery; UDP offers faster, best‑effort delivery.

TCP incurs higher latency and resource usage; UDP is lightweight.

TCP guarantees data integrity; UDP may experience packet loss.

TCP requires more system resources; UDP requires fewer.