Layer 4 — Transport

What This Layer Does

A packet arriving at your laptop has reached the right device. It has not reached the right program. At any moment your machine may be running a browser, a music app, a video call, and a background update, all using the network at once. Something has to sort the arriving data between them.

That is the first job of the Transport layer, and it is done with port numbers. A port number identifies one specific network conversation on a device. Data leaves with a source port and a destination port, and the receiving system uses the destination port to decide which program gets it. Web servers listen on port 443 for encrypted traffic and port 80 for unencrypted traffic; a browser opens a temporary high-numbered port for its own side.

The second job is deciding how carefully to deliver. This is where the layer offers a real choice, and the choice is between two protocols.

TCP — the Transmission Control Protocol — is the careful one. Before sending anything it establishes a connection with the other side. It numbers the data it sends, waits for the receiver to acknowledge what arrived, resends anything that went missing, and restores the original order before handing the data up. If delivery fails, TCP reports it rather than pretending. Its unit is a segment.

UDP — the User Datagram Protocol — is the fast one. It sends data without setting up a connection first, and it does not number, acknowledge, resend, or reorder anything. If something is lost, UDP neither knows nor cares. Its unit is a datagram.

UDP is not a worse TCP. It is a different trade. TCP's checking costs time, and in a live call a chunk of audio that arrives late is worthless anyway — better a momentary glitch than a conversation falling further behind every second. Applications that value timing over completeness choose UDP and handle the gaps themselves.

Common Examples

• TCP, used by web browsing, email, and file transfer
• UDP, used by many live video and voice calls and by most DNS lookups
• Port 443, used by HTTPS
• Port 53, used by DNS
• Temporary high-numbered ports, opened by client programs for their side of a conversation

Port numbers run from 0 to 65535. The lowest range is reserved for well-known services, the middle range is registered for particular applications, and the highest is left free for temporary use. IANA maintains the registry, and it is not worth memorizing.

Simple Real-World Example

You open two browser tabs: one loading a news site, one playing a video from a different site. Both arrive at your laptop over the same connection, with the same IP address on the outside.

Your laptop keeps them apart because each conversation uses a different source port number. Data for port 51314 goes to the first tab's connection; data for port 51315 goes to the second. Without port numbers, your machine could not tell the two streams apart.

  MAC address   →  which device on the local network
  IP address    →  which network, and which device on it
  Port number   →  which conversation on that device

  Building analogy:
     street address  = IP address
     name on the door = MAC address
     room number      = port number

What Can Go Wrong?

• The port is blocked. A firewall between the two devices allows the packets through at Layer 3 and drops them at Layer 4, so the connection simply times out.
• Nothing is listening on the port. The data arrives at the right device and no program is waiting for it, so the connection is refused.
• The wrong port is configured. The application is running and reachable and still fails, because the client is knocking on a different door.
• UDP data is lost silently. Nothing reports an error, because reporting errors was never UDP's job. The symptom is a stuttering call, not a message.

Remember This

Layer 4 uses TCP or UDP and port numbers to deliver data to the correct application.