Segmentation and fragmentation are both processes that involve dividing data for network transmission, but they occur at different layers of the TCP/IP model and serve different purposes. Here's a breakdown:
Understanding the Difference
| Feature | Segmentation | Fragmentation |
|---|---|---|
| Purpose | Divides large data streams into manageable segments for transport. | Breaks down large IP packets into smaller fragments to accommodate network limitations. |
| Layer | Transport Layer (e.g., TCP) | Network Layer (IP) |
| Process | Data is divided before it's encapsulated in a network packet. | Network packets are divided after they are created. |
| Location | Usually performed by the sending host. | Can be performed by the sending host or by intermediate routers. |
| Goal | To prepare data for reliable delivery via TCP. | To allow packets to traverse networks with smaller Maximum Transmission Units (MTU). |
| Reassembly | Reassembled at the receiving transport layer. | Reassembled at the receiving network layer. |
| Headers | Segments often have TCP headers for flow control, sequencing, etc. | Fragments contain IP headers with additional flags for reassembly. |
Segmentation Explained
- Purpose: TCP, a connection-oriented protocol, needs to break down the application data into smaller chunks because it is not always practical to send very large amounts of data at once. These chunks are called segments.
- Process: The sending device divides the data, adds TCP headers that include sequence numbers for the proper order of reassembly, and sends the resulting segments to the IP layer.
- Reassembly: The destination host reassembles the segments back into the original application data based on the sequence numbers, ensuring that the data is received in the correct order, and handles the flow control.
- Example: When you download a file, your browser receives data in segments, which are then reassembled at the transport layer to present the complete file to the application.
Fragmentation Explained
- Purpose: IP packets may be too large to travel through a network that has a smaller Maximum Transmission Unit (MTU). For example, an Ethernet network can have an MTU of around 1500 bytes, while some other networks have different MTUs.
- Process: When an IP packet is too large for a particular network segment, the sending host or an intermediate router divides the IP packet into smaller fragments. These fragments each get an IP header, and the original IP header gets modified to show these packets are fragments.
- Reassembly: The receiving host uses the headers to reassemble the IP fragments back into the original IP packet.
- Example: When a large data stream is sent from a host using an Ethernet network and the packet goes through a smaller MTU network such as a VPN connection, intermediate routers may fragment the packet as it traverses the VPN. The packet is then reassembled at the destination.
- Important consideration: If one fragment is lost or corrupted during transmission, the entire packet will be lost, because fragmented packets are not reassembled until all fragments are received. This could lead to network performance issues.
Key Differences Summarized
In short, segmentation is performed by TCP (at the transport layer) to manage data flow and reliable delivery, while fragmentation is performed by IP (at the network layer) to adapt to different network MTU sizes.
According to the provided reference: "segmentation refers to dividing a large piece of data into smaller segments, while fragmentation refers to breaking up a packet into smaller fragments to enable its transmission over a network with a lower MTU."
