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    <p>EDIT: As noted in the comments below, my original assertion that there is a 64K limit based on the number of ports is incorrect, however there <em>is</em> a 32K limit on the number of <em>socket handles</em>, so my suggested design is valid.</p> <p>With a typical TCP/IP server design, you're limited in the number of simultaneous open connections you can have. The server has one listening port, and when a client connects to it the server makes an accept call, and that creates a new socket on a random port for the rest of the connection. </p> <p>To handle more than 64K simultaneous connections I think you need to use UDP instead. You only need one port for the server to listen on, and you need to manage the connections using a 32-bit client ID in the packet data instead of having a separate port for each client. The 32-bit client ID could be the client's IP address, and the client can listen on a known UDP port for messages coming back from the server. That port would be the only one that needs to be open on the firewall.</p> <p>With this approach, your only limitation is how quickly you can handle and respond to UDP messages. With millions of clients, even sparse traffic could give you large spikes, and if you don't read the packets fast enough your input queue will fill up and you'll start dropping packets. The C10K page Greg points to will give you strategies for that.</p>
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    1. POHow to retain one million simultaneous TCP connections?
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    1. COMultiple client connections to a server do not use additional *server* ports. There will still be technical limitations, but it's not the number of ports. Connections are identified on the server side using the unique 4-tuple (server_ip, server_port, client_ip, client_port). You may be thinking of the *socket handles*, where a server socket appears to spawn more socket handles through the `accept()` call.
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      1. USGreg Hewgill
    2. COHmm. Yeah, I guess you're right. http://linux.die.net/man/2/accept says accept will fail if you run out of file descriptors, not ports. I took a look in the linux kernel source code, and from what I can tell the maximum number of files is passed around as an int. That probably limits you to 32K file descriptors, though it'll vary by platform. I'm wrong on the details of why TCP is limited, but I think it's still limited and UDP is a workable alternative.
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    3. COthanks for comments from you both. i saw the c10k page weeks ago. i remember it suggest epoll(). i wish i could use it with UDP. howver, as i know, epoll is irrelevant to a UDP server that has only one socket to receive, am i correct? the urgency to read packets fast enough is valid. however, i cannot think of a better way to read faster than using a dedicate thread to read packets off the UDP socket and put them in a queue for worker threads to consume. could you advise better methods? many thanks in advance.
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