Difference Between Segments, Packets and Frames

When data is sent over the internet or any network, it isn't transmitted as one large block. Instead, it's divided into smaller units known as segments, packets, and frames. These units are different layers of packaging that help the data travel efficiently through the network.

Each one plays a specific role in ensuring that information reaches its destination accurately and quickly. Understanding these terms is key to grasping how data flows through computer networks.

Segment

When data is sent from the application layer, it is broken down into smaller parts according to the Maximum Segment Size (MSS) defined by the network. Each of these smaller data parts is then encapsulated with a TCP header and form a segment.

Key Parts of a TCP Header:

• Source Port: Shows which port the data is coming from.
• Destination Port: Tells where the data is going (which app/service on the receiving device).
• Flag Bits: Special signals that help control how data is sent—like whether it can be broken up or if more data is coming.
• Sequence Number: Helps make sure the pieces are reassembled in the right order on the receiving side.
• Checksum: A kind of error check that makes sure the data wasn’t damaged along the way.
• Options: Extra settings, like how big the data chunks can be or how fast they can be sent.

Source and Destination port are required because it tells in which PDU is to be delivered in the receiver host. The checksum field of the TCP is calculated by taking into account the TCP header, data and IP pseudo-header. The Checksum ensures that correct data is sent and received. Thus, after all these processing the broken data packets are called Segments. 

Packets

Once the Transport Layer generates segments, the Network Layer (usually using the IP ) processes these segments further to create packets. These packets are crucial for routing data across different networks.

Structure of an IP Packet : An IP packet consists of :-

• Header : typically 20 bytes, but can go up to 60 bytes if options are used
• Body : which contains the segment from the transport layer

Fields in the IP Header

• Source IP Address : Specifies the origin of the packet.
• Destination IP Address : Specifies the final destination.
• TTL (Time To Live) : Defines the packet's lifespan. It decreases by 1 with each hop (router). When TTL reaches 0, the packet is discarded.
• Identification : Aids in reassembling fragmented packets.
• Protocol Type : Indicates the transport layer protocol being used (e.g., TCP, UDP).
• Version : Specifies the IP protocol version (IPv4 or IPv6).

Thus the IP header along with the body (which contains the segment from the Transport layer) makes the IP Packet or popularly only Packet. This layer is also responsible for fragmentation if required , when MTU of the network is less . This fragmentation is done at the Routers. 

Frames

When a packet arrives, it gets wrapped with another header (sometimes a trailer), turning it into a frame. This wrapping helps the data move from one physical device (like your computer) to another (like a router or switch) on the same local network.

Frame Header Contents

The frame header contains key information used for local delivery between directly connected devices:

• MAC (Media Access Control) Address Source: Identifies the device sending the frame.
• MAC Address – Destination: Identifies the device meant to receive the frame.
• Type/Length Field: Tells what kind of data is being carried (like an IPv4 packet).
• Error Checking (usually in trailer): Often a CRC (Cyclic Redundancy Check) is added to help detect any errors that happened while the data was moving across the wire.

Difference Between Segments, Packets and Frames