Whats the difference between a socket which is open and a socket which is connected?

Question

The Java Socket class has two methods isClosed and isConnected to check whether the socket is closed or connected respectively. I wanted to know what's the difference between a TCP socket which is only open and a TCP socket which is open and connected, and how is this different from UDP.

Poco

Answer 1

To put things simply, a Socket that is open is a socket that is either waiting for connection or has successfully connected with another Socket. When a socket has been closed, it means that this socket is no longer available for connection, and that it's resources has already been released. A Socket that is connected, well, it means that the socket is connected to another Socket.

So a Socket can..

• be open and connected at the same time.
• be open and not connected at the same time.
• not be connected when closed.

UPDATE

from @Bryan

Apparently, there are half-closed or half-open states for TCP Sockets; which usage (today) is different from its original meaning. More on this link.

Crambo

Not to confuse things, but TCP also has a half-closed state where there is still a connection but it's now unidirectional. – Isoleucine

Answer 2

This page gives a fairly good overview on socket states: http://diranieh.com/SOCKETS/SocketStates.htm and the difference between TCP and UDP sockets. Particularly:

• State "Open" (TCP and UDP): An unnamed socket has been created. An unnamed socket is one that is not bound to a local address and port
• State "Connected" (TCP only): An association (virtual circuit) has been established between a local and remote host. Sending and receiving data is now possible.

Note that newer implementations of java.net.DatagramSocket support an extension to the TCP/IP network states: A DatagramSocket can also be in the state "connected", so that isConnected() does not necessarily return false even though a datagram socket is never "connected" at the network layer. In particular: "When a DatagramSocket is connected to a remote address, packets may only be sent to or received from that address. By default a datagram socket is not connected."

For more information, see the sources and the JavaDoc.

Gotama