Does UDP allow two clients to connect at the same time to a server port?
DatagramSocket udp1 = new DatagramSocket(8000); // = localhost: 8000 ?
DatagramSocket udp2 = new DatagramSocket(8000);
What happens if udp1 and udp2 are created from two different IPs and send data at the same time?
Will it cause any issue?
UDP doesn’t really have a concept of “connect”, just sending and receiving packets. (e.g. if making a TCP connection is analogous to making telephone call, then sending a UDP packet is more like mailing a letter).
Regarding two sockets arriving at the same UDP port on a server at the same time: the TCP/IP stack keeps a fixed-size receive-buffer for each socket that the server creates, and whenever a packet arrives at the port that socket is bound to, the packet is placed into that buffer. Then the server program is woken up and can recv() the data whenever it cares to do so.
So in most cases, both packets will be placed into the buffer and then recv()’d and processed by the server program. The exception would be if there is not enough room left in the buffer for one or both of the packets to fit into it (remember it’s a fixed-size buffer); in that case, the packet(s) that wouldn’t fit into the buffer will simply be dropped and never seen again.