The Transport layer can provide two types of service depending on the protocol used:

2.1 Connection-Oriented Service

This service establishes a logical connection before data transmission begins. It guarantees:

• Reliable delivery: Data arrives without errors
• Ordered delivery: Data arrives in the exact sequence sent
• Error correction: Lost or corrupted data is retransmitted

Example: TCP (Transmission Control Protocol) provides connection-oriented service.

2.2 Connectionless Service

This service sends data without establishing a connection first. There is no guarantee of delivery, ordering, or error correction.

Example: UDP (User Datagram Protocol) provides connectionless service.

To achieve reliable data delivery, transport protocols implement several mechanisms:

3.1 Virtual Circuit Setup

In connection-oriented transport, a virtual circuit must be established before data transfer. This involves a process called the three-way handshake:

1. SYN: The sender sends a synchronize segment to request connection
2. SYN-ACK: The receiver acknowledges and synchronizes back
3. ACK: The sender confirms the connection

This exchange ensures both parties are ready to communicate.

3.2 Sequencing

Each data segment is assigned a sequence number. This allows the receiver to:

• Reassemble segments in the correct order
• Identify missing segments for retransmission
• Prevent duplicate segments from being processed

The receiving host examines sequence numbers to reorder segments correctly before passing data to the application.

3.3 Acknowledgments

When the receiver successfully obtains a data segment, it sends an acknowledgment (ACK) back to the sender. If the sender does not receive an ACK within a timeout period, it retransmits the segment.

This mechanism is called Positive Acknowledgment with Retransmission (PAR).

3.4 Flow Control

Flow control prevents the sender from overwhelming the receiver with too much data. The receiver has a limited buffer size, and if the sender transmits too fast, data may be lost.

Techniques include:

• Buffering: Excess data is temporarily stored in memory
• Windowing: The sender transmits multiple segments before waiting for acknowledgments
• Congestion avoidance: Regulates data flow to prevent network overload

The Transmission Control Protocol (TCP) is the primary connection-oriented transport protocol. It provides reliable, ordered, error-checked delivery of data between applications.

4.1 TCP Header Structure

A TCP segment includes:

• Source Port (16 bits)
• Destination Port (16 bits)
• Sequence Number (32 bits)
• Acknowledgment Number (32 bits)
• Flags (SYN, ACK, FIN, RST, PSH, URG)
• Window Size (16 bits)
• Checksum (16 bits)
• Urgent Pointer (16 bits)

4.2 How TCP Works

Step 1: Connection Establishment (Three-way Handshake)

Sender → SYN (seq=x) → Receiver
Sender ← SYN-ACK (seq=y, ack=x+1) ← Receiver
Sender → ACK (ack=y+1) → Receiver

Step 2: Data Transfer

• Sender transmits segments with sequence numbers
• Receiver sends ACKs confirming receipt
• If ACK is not received, sender retransmits

Step 3: Connection Termination

Sender → FIN → Receiver
Sender ← ACK ← Receiver
Sender ← FIN ← Receiver
Sender → ACK → Receiver

4.3 Flow Control in TCP

TCP uses a sliding window mechanism:

• The window size determines how many bytes can be sent before requiring an ACK
• The receiver advertises its receive window (rwnd) to the sender
• If the window reaches zero, the sender stops transmitting until buffer space opens

4.4 Congestion Control in TCP

TCP implements several algorithms to prevent network congestion:

• Slow Start: Begins with a small window, exponentially increases
• Congestion Avoidance: Additive increase of window size
• Fast Retransmit: Retransmits after three duplicate ACKs
• Fast Recovery: Continues after fast retransmit

The User Datagram Protocol (UDP) is a simpler protocol that does not establish connections before sending data.

5.1 Characteristics of UDP

• No connection setup: Data is sent immediately
• No acknowledgments: No confirmation of delivery
• No sequencing: Packets may arrive out of order
• No flow control: Sender can overwhelm receiver
• No congestion control: May contribute to network congestion

5.2 UDP Header Structure

UDP has a much simpler header than TCP:

• Source Port (16 bits)
• Destination Port (16 bits)
• Length (16 bits)
• Checksum (16 bits)

5.3 Why Use UDP?

UDP is faster and has less overhead because it does not perform error recovery, sequencing, or flow control. It is suitable for applications where:

• Speed is critical (real-time audio, video streaming)
• Occasional data loss is acceptable
• Error handling is done at the application layer

Scenario: A student in Dar es Salaam opens a web browser to visit a website hosted on a server in Mwanza.

Step-by-step:

1. Client (browser) sends SYN:

   • Sets SYN flag = 1
   • Generates Initial Sequence Number (ISN), for example, 1000
   • Segment: SYN, seq=1000

2. Server responds with SYN-ACK:

   • Sets SYN = 1, ACK = 1
   • Acknowledges client's ISN: ack=1001
   • Generates its own ISN, for example, 2500
   • Segment: SYN-ACK, seq=2500, ack=1001

3. Client sends ACK:

   • Sets ACK = 1
   • Acknowledges server's ISN: ack=2501
   • Segment: ACK, seq=1001, ack=2501

Now the TCP connection is established, and data transfer can begin.

When you use mobile money (M-Pesa) to send money from your phone, both TCP and UDP play roles behind the scenes. The initial login and authentication use TCP to ensure your credentials are reliably transmitted and acknowledged. However, for real-time transaction notifications that must arrive instantly, UDP is often used to reduce delay—ensuring you see the message that money was sent immediately, even if the occasional notification is lost. This combination of reliable connection-oriented communication for critical data and fast connectionless communication for timely updates is how the Transport layer enables Tanzania's mobile money systems to function effectively.