Socket connection timeouts: where is the specification?

Question

The context of my work is my local network.

The code examples below are written in Java, but my question is about TCP, not programming.

I experienced the following connection timeout:

• 2 ms when establishing a connection
• 1 005 ms when the host is alive but not listening on the specified socket port
• 21,000 ms with no host

These values ​​come from monitoring my network, but I believe there is an RFC.

The following is information about the timeout:

• RFC 1122
• RFC 793
• Nagle's_algorithm and TCP_NO_DELAY

Can you give me more directions?

@Override public void run() { for( int port = _portFirst; port < _portLast; ++port ) { String host = "192.168.1." + _address; boolean success = false; long before = System.currentTimeMillis(); try { Socket socket = new Socket(); SocketAddress endpoint = new InetSocketAddress( host, port ); socket.connect( endpoint, 0 ); success = true; socket.close(); }// try catch( ConnectException c ){/**/} catch( Throwable t ){ t.printStackTrace(); } long duration = System.currentTimeMillis() - before; System.err.println( host + ":" + port + " = " + duration ); _listener.hostPinged( host, port, success, duration ); } } 

Answer 1

No RFC for connection timeouts. For any RFC or other document, it is not possible to know in advance the conditions that exist on any network.

In general, you can expect a successful connection to be very fast; a ECONNREFUSED ( ConnectException: connection refused ) is about as fast; and the connection timeout ( ConnectException: connect timeout ) until it accepts, depending on the reason, the platform at both ends and the nature of the intermediate network. On Windows, I believe that the connection timeout consists of the total time of three connection attempts with timeouts of 6s, 12s and 24s, a total of 42s; on different Unixes, I think the total is more like the 70s, which may be the result of 3 attempts with timeouts of 10, 20 and 40. As you can see, it depends on the platform. There is also the problem that populating the backup queue on a Windows server will result in an RST for inbound SYNs, where on a Unix / Linux server this will not cause any response to inbound SYNs. A.

It should also be noted that in Java and contrary to many years of Javadoc:

• A timeout with a zero connection does not imply an infinite timeout; it implies a default timeout for the platform, which, as shown above, does not exceed about 70 seconds,

• You cannot specify a connection timeout that extends the platform by default; you can use it only to reduce the default platform.

Answer 2

As you found out, you can specify a timeout in the call to the connect(...) method as such:

 connect( SocketAddress endpoint , int timeout ) 

And remember that ... The zero timeout is interpreted as an infinite timeout „ (where“ infinite ”often actually means“ use the default OS ").

As for the default values, they depend on the OS, but, as a rule, are quite consistent. If you are in the linux window, look at /proc/sys/net/ipv4/tcp_keepalive* (but do not change them if you do not know what you are doing :-)

Greetings