Which action is performed by a client when establishing communication with a server via the use of UDP at the transport layer?

When a client establishes communication with a server using UDP (User Datagram Protocol) at the transport layer, the following steps are typically involved:

1. The client application creates a UDP socket: The client application initiates the process by creating a UDP socket. A socket is an endpoint for sending or receiving data across a computer network. In this case, the socket is specifically designed for sending and receiving datagrams using the UDP protocol.

2. The client sends a UDP datagram to the server: Once the socket is created, the client can send a UDP datagram to the server. This datagram includes the data to be sent as well as the destination IP address and port number. Unlike TCP (Transmission Control Protocol), UDP does not establish a connection with the server before sending the datagram. This is why UDP is sometimes referred to as a "connectionless" protocol.

3. The server receives the UDP datagram: The server, which also has a UDP socket set up, receives the datagram sent by the client. The server's socket "listens" for incoming datagrams on the specified port number.

4. The server processes the UDP datagram: Once the server receives the datagram, it processes the data contained in the datagram. This could involve a variety of tasks, depending on the specific application.

5. The server sends a response (if necessary): Depending on the nature of the communication, the server may send a response back to the client. This response would also be sent as a UDP datagram.

It's important to note that UDP does not guarantee delivery of datagrams, nor does it require an acknowledgement that a datagram has been received. This makes UDP faster and more efficient than TCP, but also less reliable.