Httpd – How to test keep-alive is working on client end

The benefit from having keep-alive on is that a client will be able to request more than one entity from your server without having to create another TCP connection (3-way handshake with its round-trips included). The problem with this is that if, say, you have a connection limit in apache set to 300, if there's 300 active connections all the others will have to wait until the first 300 clients are done and/or the timeout expires.

Disabling keep-alive will force the clients to create 1 connection per request. When the socket is properly closed on both ends, it goes into the TIME_WAIT status, as you've noticed. This happens to ensure that the port used in that connection does not receive data from a previous connection for a while before being available (see this, but there's much more out there). In my linux system, /proc/sys/net/ipv4/tcp_fin_timeout is set to 60 seconds. You can try reducing that, but don't go too far down. How far? It depends on how many connections per second you are getting. For 100-200req/s, don't bother changing the default.

Keep alive is meant to reduce the number of tcp-setups, which are chatty and are heavily influenced by latency. It's main goal is to improve client performance - as for a modern server-os there is a low cost in setting up tcp connections.

Some proxies/loadbalancers lean disable keep-alive to the webservers because they assume the actual servers and are on a LAN with neglible latency, and therefore neglible tcp setup penalty. It also lessens the complexity of the balancing software, as there is not necessarily a one-to-one relationship between incomming and outgoing (from the LB's perspective) connections.

There are few reasons why a server would disable keep-alive, so I suspect it's probably admins at fault. One exception are the a-child-per-connection servers (apache), where releasing that child/thread as soon as possible offers advantages, even if the total browser experience (and bandwidth usage) becomes poorer.

Note that afaik keep-alive is an optional feature and no service is required to implement it.