Transport Layer: Port Numbers, Segmentation, and Reliable Data Delivery

Transport Layer

Also called the End-to-End or Host-to-Host layer.

Reliable Delivery

Ensures all data arrives without loss or errors—retransmits lost or corrupted data if required (TCP is an example of a protocol offering this).

Segmentation

Breaks down large messages into segments for easier transmission, each segment is tracked for reassembly.

Port Numbers

Numerical identifiers for different applications (e.g., 80 for HTTP, 25 for email). Allows multiple apps to use the network simultaneously.

Multiplexing/Demultiplexing

Multiple streams of data (from/to various apps or devices) are combined onto a single network link (multiplexed) and then separated back into streams at the receiving host (demultiplexed).

Connection-Oriented (TCP) vs. Connectionless (UDP)

TCP provides reliability, UDP is faster and used for time-sensitive traffic.

Layer 4 Segment

The data (now called a Layer 4 segment) is wrapped with the correct port number, then handed to the Network layer as part of the payload.

Network Transmission

Traffic is sent across the network as Layer 3 packets, further encapsulated in Layer 2 frames.

De-Multiplexing on Arrival

Host 2.1 receives the segment, checks the port number, and delivers it to the correct application (e.g., web browser for port 80, email client for port 25).

Multilayer switches (load balancers)

Direct traffic based on port/application; can balance load across servers.

Advanced firewalls

Filter/block traffic based on port/application/protocol.

Intrusion Detection Systems (IDS)

Analyze segments for malicious or abnormal behavior.

Port Number 80

HTTP (web traffic)

Port Number 25

SMTP (email)

Port Number 445

Windows SMB

Port Number 5061

SIP/TLS (VoIP).

Traffic from Hosts 2.2 and 2.3

Send different types of traffic to host 2.1 (e.g., port 25 for email, 80 for HTTP, etc.).

Layer 4 Devices

Devices that operate at the Transport Layer, managing traffic based on port numbers.

What does Layer 4 (Transport) use to deliver data to the correct application on the receiving device?

port numbers

Why is TCP reliable?

The Transport Layer will check for successful delivery, retransmitting segments as needed for reliable protocols (e.g., TCP).