Technical definition
The OSI Transport layer is layer 4 of the OSI reference model. It provides end-to-end data transfer between transport-service users. The selected transport protocol determines the connection mode, multiplexing, segmentation and delivery-control mechanisms.
In plain English: Delivers data between the right applications on two hosts, using port numbers. TCP adds reliability and ordering.
What this layer does
An IP address gets traffic to a host interface; a port helps the host give it to the right application endpoint. TCP establishes a connection, numbers the bytes, acknowledges received data and retransmits missing data. Its sliding window lets it send multiple segments before an acknowledgement arrives.
Receive flow control limits data to what the receiver can accept; congestion control responds to network conditions. UDP sends separate datagrams without built-in acknowledgements, ordering or retransmission.
TCP provides a reliable, ordered byte stream while the connection works; UDP itself does not guarantee delivery or order.
Follow one request from PC-1
PC-1 is a user's computer opening a page on the lab web server. Its IPv4 address is 192.168.1.20/24 and its gateway is R1 at 192.168.1.1. The server is on another subnet, at 203.0.113.20/24, with gateway 203.0.113.1. The /24 means that the first 24 bits identify each subnet. R1 joins the two subnets.
On small screens, swipe sideways or focus the diagram and use the arrow keys.
In this request: For this TCP-based HTTPS example, your PC uses source port 51524 and destination port 443. TCP recovers missing bytes, but a long outage can still make the connection fail.
The interface labels identify the ends of the cables: Eth0 is the example Ethernet interface on each host, while Gi0/0 and Gi0/1 are R1's two Gigabit Ethernet interfaces. Actual device names vary. An IP address identifies an interface; TCP port 443 identifies the HTTPS service endpoint. It is not a physical socket on the router.
Where it fits in the other model
TCP/IP also calls this the Transport layer. The comparison concerns the job, not a requirement for the same implementation in every protocol suite.
The trade-off is between a more detailed vocabulary and a simpler description of deployed protocols. OSI names seven responsibilities; the four-layer TCP/IP model groups some of them together. Neither view requires a separate header for every named responsibility.
When it does not work
If ping succeeds but the TCP connection fails, avoid concluding that the cable is broken. ICMP and TCP may be treated differently. Check whether the server is listening on the requested port and whether host or network firewalls allow the connection. TCP can recover some loss, but repeated retransmissions and a connection timeout do not identify the cause on their own.
Try a practical check
Use PowerShell on Windows for this observation exercise. These commands inspect your own connection or request a public example service; they do not configure R1 or contact the documentation server in the figure.
Test-NetConnection example.com -Port 443In Windows PowerShell, inspect RemoteAddress and TcpTestSucceeded. True means this TCP connection test to port 443 succeeded at that time. It does not prove that TLS certificate validation or an HTTP request succeeds. False needs investigation: a service may be stopped, a policy may block the connection, or a path may be unavailable. This test does not test UDP.
Your task: identify one useful field in the result, state what it shows, then name something it cannot prove. If the command is unavailable or a request is blocked, record that limitation rather than treating it as proof of a failed layer.
Check your understanding
Ping works, but a TCP connection to port 443 fails. What is the useful next check?