Transport layer
Also known as: layer 4, osi layer 4
The transport layer is Layer 4 of the OSI model, responsible for end-to-end communication between applications on different hosts. It uses port numbers to direct traffic to the right application and provides either reliable delivery with TCP or low-overhead delivery with UDP.
The layers below the transport layer solve different problems: the network layer gets packets from one host to another across networks, and the data link layer moves frames across a single link. Neither knows which program on the destination host should receive the data. The transport layer supplies that missing piece by identifying communicating processes rather than hosts. It accepts data from the application layer, breaks it into segments, hands them down for delivery, and reassembles them on the far side.
Port numbers are the mechanism. Each segment carries a source and destination port, and the combination of IP address and port identifies a specific conversation. Well-known ports below 1024 are reserved for common services — 80 for HTTP, 443 for HTTPS, 22 for SSH, 53 for DNS — while clients draw from a range of ephemeral ports for their side of the connection. This is what allows one host to run a browser, an email client, and a file transfer simultaneously over a single IP address, a capability called multiplexing.
Two protocols dominate the layer. TCP is connection-oriented: it establishes a session with a three-way handshake, numbers each byte with sequence numbers, requires acknowledgments, retransmits anything that goes unacknowledged, reorders segments that arrive out of order, and adjusts its sending rate through windowing and congestion control. That reliability costs header size and round trips. UDP is connectionless and does none of it — no handshake, no acknowledgments, no reordering — which makes it fast and lightweight. Applications that tolerate occasional loss but not delay, such as voice, video streaming, and DNS queries, generally choose UDP, while file transfers, web traffic, and email use TCP.
The transport layer is foundational material on the Cisco CCNA exam. Candidates are expected to know where Layer 4 sits in both the OSI and TCP/IP models, how port numbers enable multiplexing, the steps of the TCP three-way handshake, and which common applications rely on TCP versus UDP.
Key takeaways
- The transport layer is OSI Layer 4 and provides end-to-end communication between applications, not just between hosts.
- Port numbers identify the specific application, allowing multiplexing of many conversations over one IP address.
- TCP is connection-oriented and reliable, using a three-way handshake, sequence numbers, acknowledgments, and windowing.
- UDP is connectionless and adds minimal overhead, trading reliability for speed.
- Latency-sensitive traffic such as voice and video typically uses UDP, while web, email, and file transfer use TCP.