GATE CS 2017 Set 2 Q18 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

To understand the behavior of a connected UDP socket in Linux, let us analyze both statements:

• Statement I: "A connected UDP socket can be used to communicate with multiple peers simultaneously."
  Incorrect. When connect() is called on a UDP socket, the operating system kernel binds the socket to a specific remote IP address and port number. Consequently:
  • The socket can only send datagrams to this specific peer (using write() or send() instead of sendto()).
  • The socket will only receive datagrams originating from this specific peer; datagrams from any other source are discarded by the kernel.
  Thus, it cannot be used to communicate with multiple peers simultaneously.


• Statement II: "A process can successfully call connect function again for an already connected UDP socket."
  Correct. Unlike TCP, where a connection is established via a three-way handshake and cannot be re-connected, UDP is connectionless. Calling connect() on a UDP socket simply updates the destination association in the kernel. A process can call connect() again to:
  • Specify a new peer (changing the destination IP/port).
  • Disconnect the socket entirely by setting the address family to AF_UNSPEC.

Therefore, only Statement II is correct.