In-practice ideal timeout length for idle HTTP connections

In an embedded device, What is a practical amount time to allow idle HTTP connections to stay open?

I know back in the olden days of the net circa 1999 internet chat rooms would sometimes just hold the connection open and send replies as they came in. Idle timeouts and session length of HTTP connections needed to be longer in those days...

How about today with ajax and such?

REASONING: I am writing a transparent proxy for an 开发者_如何学Pythonembedded system with low memory. I am looking for ways to prevent DoS attacks.

My guess would be 3 minutes, or 1 minute. The system has extremely limited RAM and it's okay if it breaks rare and unpopular sites.

In the old days (about 2000), an idle timeout was up to 5 minutes standard. These days it tends to be 5 seconds to 50 seconds. Apache's default is 5 seconds. With some special apps defaulting to 120 seconds.

So my assumption is, that with AJAX, long held-open HTTP connections are no longer needed.

How about allowing idle HTTP connections to remain open unless another communication request comes in? If a connection is open and no one else is trying to communicate, the open connection won't hurt anything. If someone else does try to communicate, send a FIN+ACK to the first connection and open the second. Many http clients will attempt to receive multiple files using the same connection if possible, but can reconnect between files if necessary.