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Network Layer: Delivery, Forwarding, and Routing

* UDP and TCP are transport-layer protocols that create a process-to-process communication.
* UDP is an unreliable and connectionless protocol that requires little overhead and offers fast delivery.
* In the client-server paradigm, an application program on the local host, called the client, needs services from an application program on the remote host, called a server.
* Each application program has a unique port number that distinguishes it from other programs running at the same time on the same machine.
* The client program is assigned a random port number called the ephemeral port number.
* The server program is assigned a universal port number called a well-known port number.
* The combination of the IP address and the port number, called the socket address, uniquely defines a process and a host.
* The UDP packet is called a user datagram.
* UDP has no flow control mechanism.
* Transmission Control Protocol (TCP) is a connection-oriented, reliable, stream transport-layer protocol in the Internet model.
* The unit of data transfer between two devices using TCP software is called a segment; it has 20 to 60 bytes of header, followed by data from the application program.
* TCP uses a sliding window mechanism for flow control.
* Error detection is handled in TCP by the checksum, acknowledgment, and time-out.
* Corrupted and lost segments are retransmitted, and duplicate segments are discarded.
* TCP uses four timers—retransmission, persistence, keep-alive, and time-waited—in its operation.
* Connection establishment requires three steps; connection termination normally requires four steps.
* TCP software is implemented as a finite state machine.
* The TCP window size is determined by the receiver.