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Networking Fundamentals · Foundational

Transport Layer (L4)

OSI Layer 4, which provides host-to-host delivery, segmentation, port addressing and optionally reliability.

Definition

The Transport layer multiplexes application conversations using port numbers and breaks data into segments (TCP) or datagrams (UDP). TCP adds sequencing, acknowledgments, retransmission, flow control and congestion control, while UDP provides minimal, connectionless delivery. It sits at the same position in both the OSI and TCP/IP models.

Example

A web browser uses TCP destination port 443 to reach an HTTPS server.

Exam tip

Port numbers and the TCP versus UDP choice live at Layer 4; firewalls that filter on ports are making Layer 4 decisions.

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