Encapsulation and Decapsulation in the Transport Layer

 

Encapsulation and Decapsulation in the Transport Layer

Introduction

For communication between two application processes, the Transport Layer must prepare the data before transmission and recover it at the destination. This is accomplished through two important operations:

  • Encapsulation – Performed at the sender.
  • Decapsulation – Performed at the receiver.

These operations enable the transport layer to provide process-to-process communication by adding and removing transport-layer information such as port numbers and other control fields.

What is Encapsulation?

Encapsulation is the process of adding protocol control information (header) to the application message before sending it over the network.

The transport layer receives the message from the application layer, adds a transport-layer header, and passes the resulting packet to the network layer.

The transport-layer packet is called:

  • Segment (TCP)
  • User Datagram (UDP)
  • Packet (general term)

Encapsulation Process

The following steps occur at the sender:

Step 1: Application Creates the Message

The application (process) generates the message to be transmitted.

Example:

Hello Bob

Step 2: Message is Passed to the Transport Layer

The application passes the following information to the transport layer:

  • Application message
  • Source socket address
  • Destination socket address
  • Other protocol-specific information

Step 3: Transport Layer Adds Header

The transport layer creates a header containing information such as:

  • Source Port Number
  • Destination Port Number
  • Sequence Number (TCP)
  • Acknowledgment Number (TCP)
  • Checksum
  • Control Flags
  • Window Size (TCP)

The message becomes a transport-layer packet.

+-----------------------+----------------------+
| Transport Header      | Application Data     |
+-----------------------+----------------------+

Step 4: Packet is Sent to the Network Layer

The completed transport packet is handed to the network layer, which further encapsulates it inside an IP packet.

Encapsulation Diagram

Application Layer

        Message
           │
           ▼

+----------------------+
|   Transport Layer    |
| Adds Header          |
+----------------------+
           │
           ▼

+----------------------+----------------------+
| Transport Header     | Application Message  |
+----------------------+----------------------+
           │
           ▼
      Network Layer

Example of Encapsulation

Suppose Alice opens a web browser and sends an HTTP request.

Application Message

GET /index.html

Transport Layer Header

Source Port      = 52000
Destination Port = 80

Transport Packet

+-------------------------------------------+
| Src Port | Dest Port | Application Data   |
| 52000    | 80        | GET /index.html    |
+-------------------------------------------+

The packet is then forwarded to the network layer.

What is Decapsulation?

Decapsulation is the reverse process of encapsulation.

At the destination, the transport layer:

  • Receives the transport packet.
  • Removes the transport-layer header.
  • Delivers the original application message to the appropriate process.

Decapsulation Process

The following steps occur at the receiver.

Step 1: Packet Arrives

The network layer delivers the transport packet.

+----------------------+----------------------+
| Transport Header     | Application Message  |
+----------------------+----------------------+

Step 2: Header is Examined

The transport layer checks

  • Destination Port Number
  • Error Detection Information (Checksum)
  • Sequence Number (TCP)
  • Acknowledgment Number (TCP)

Step 3: Header is Removed

After processing, the transport header is discarded.

Only the application message remains.

Application Message

GET /index.html

Step 4: Deliver to Correct Process

Using the destination port number, the transport layer delivers the message to the correct application.

Example

Destination Port = 80

↓

HTTP Server

If the destination port is

25

The message is delivered to the

SMTP Server

Decapsulation Diagram

Network Layer
        │
        ▼

+----------------------+----------------------+
| Transport Header     | Application Data     |
+----------------------+----------------------+
        │
 Remove Header
        ▼

Application Layer

Application Message

Complete Communication Process

Information Added During Encapsulation

The transport-layer header may contain:

FieldPurpose
Source Port Number    Identifies sending process
Destination Port Number    Identifies receiving process
Sequence Number (TCP)    Orders segments
Acknowledgment Number (TCP)    Confirms receipt
Checksum    Error detection
Control Flags    Connection management
Window Size (TCP)    Flow control

Information Removed During Decapsulation

At the receiver, the transport layer:

  • Reads the destination port number.
  • Verifies checksum.
  • Reorders packets (TCP).
  • Removes the transport header.
  • Delivers the original application message.
  • Passes the sender's socket address to the application if a reply is needed.

Why is Encapsulation Necessary?

Encapsulation allows the transport layer to:

  • Identify the source and destination processes.
  • Detect transmission errors.
  • Provide reliable communication (TCP).
  • Perform flow control.
  • Support multiplexing and demultiplexing.
  • Enable end-to-end communication.

Why is Decapsulation Necessary?

Decapsulation enables the receiver to:

  • Extract the original application message.
  • Identify the intended application using the destination port number.
  • Verify data integrity.
  • Deliver the message to the correct process.

Encapsulation vs Decapsulation

FeatureEncapsulationDecapsulation
Performed At    SenderReceiver
Purpose    Add transport headerRemove transport header
Input    Application messageTransport packet
Output    Transport packet (Segment/User Datagram)Original application message
Next Layer    Network LayerApplication Layer

Summary

  • Encapsulation is the process of adding a transport-layer header to the application message before transmission.
  • During encapsulation, the transport layer adds information such as source and destination port numbers, checksum, and other protocol-specific fields, creating a segment (TCP) or user datagram (UDP).
  • Decapsulation is the reverse process in which the transport layer receives the packet, verifies the header, removes it, and delivers the original message to the correct application process.
  • These operations ensure process-to-process communication, allowing multiple applications on different hosts to communicate reliably and efficiently.

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