Question

I am using Java (although I think the socket options is implement in most languages) to implement a client and server. The server sends data to the client for processing which the client acknowledges. On another port the client then sends the results of the processing back to the server. When it comes to options such as

• SO_LINGER
• SO_KEEPALIVE
• SO_NODELAY
• SO_REUSEADDRESS
• SO_SENDBUFFER
• SO_RECBUFFER
• TCP_NODELAY

We have noticed that the connection between the client and server occasionally breaks. There will be a timeout on the send or the receive. When this happens will kill the socket and open a new one to continue.

What would be the best options to set in terms of the above scenario and is there anything that we could do from our side (programmatically or options-wise) to try minimize the amount of times the connection is dropped. We are using normal TCP/IP.

UPDATE:
The bounty on this ends soon. I haven’t had a satisfactory answer yet so it is still open. I think everyone is missing the point of the quest. What is the best practice with regards to the options above for sockets that continuously chat. I have already got a ping packet in that if there is no work to be done (hardly ever the scenario) the normal message is sent with no inner elements so there is always processing.

Answer 1

Strictly speaking, you don’t need any of these socket options:

* SO_LINGER

You need to set SO_LINGER only if your application still has outstanding packets to send when close(2) or shutdown(2) has been called. Not really applicable for your application.

* SO_KEEPALIVE

Sending keepalive-pings every two hours would really only help very long-lived but -very- quiet connections going through stateful firewalls with very long session timeouts. (Two hours between pings is entirely too long to be practical in today’s Internet.)

* SO_NODELAY

This (presumably an alias for TCP_NODELAY) disables Nagle’s algorithm, which is just a small-packet-avoidance problem. Perhaps Nagle is getting in the way in your application, but it takes special sequences of packets to introduce 500ms delays into processing; it never just hangs connections.

* SO_REUSEADDRESS

Useful for all ‘servers’ that listen on well-known port numbers; use on ‘clients’ is almost always covering up some bug or other, but it is sometimes necessary if requests must come from a well-known port number.

* SO_SENDBUFFER
* SO_RECBUFFER

These buffer sizes influence the kernel-side buffer sizes maintained for receiving or sending data while your program (receive buffer) or the socket (send buffer) isn’t yet ready to accept more data. If these are set too small, your application might not transfer data as smoothly as possible, reducing throughput, but it should not lead to any stalls if these are set smaller than optimal. Of course, too large may put unreasonable demands on kernel memory, but there should be a reasonable system-wide maximum allowed size.

* TCP_NODELAY

Disables Nagle. Not likely to do more than introduce 500ms delays if your application sends multiple small packets before attempting a blocking read.

Really, you shouldn’t need to set any socket options.

Can you distill your code into something that could be pasted here and tested or inspected? I’m used to TCP sessions surviving for days or weeks without trouble, so this is pretty surprising.