What is Segment?

Understanding Segment

Data representation at Layer 4 of the Open Systems Interconnection (OSI) model. A segment is a unit of data at the transport layer (Layer 4) of the OSI model, typically encapsulating application data with information like source and destination ports, sequence numbers, and control flags. TCP segments and UDP datagrams are the primary types of transport layer protocol data units. Transport layer specifications are defined in RFCs for TCP (RFC 793) and UDP (RFC 768). Organizations secure segments through stateful firewalls, transport layer security protocols, deep packet inspection, and monitoring for abnormal segment patterns. For example, a network security team might implement stateful inspection firewalls that examine TCP segment flags and sequence numbers to detect attempts to bypass access controls through abnormal connection establishment, while using TLS to encrypt segment contents during transmission. Related terms: Transport layer, TCP, UDP, Port number, Sequence number, Stateful inspection, Flags, OSI model, Encapsulation.