HTTP Keep Alive and TCP keep alive

Question

How is HTTP Keep Alive implemented? Does it internally use TCP Keep Alive? If not, how does the server detect if the client is dead or alive?

Answer 1

I know this is an old question, but still:

• HTTP Keep-Alive is a feature that allows HTTP client (usually browser) and server (webserver) to send multiple request/response pairs over the same TCP connection. This decreases latency for 2nd, 3rd,... HTTP request, decreases network traffic and similar.

• TCP keepalive is a totally different beast. It keeps TCP connection opened by sending small packets. Additionally, when the packet is sent this serves as a check so the sender is notified as soon as connection drops (note that this is NOT the case otherwise - until we try to communicate through TCP connection we have no idea if it is ok or not).

To answer your questions about HTTP Keep-Alive:

How is HTTP Keep Alive implemented?  

To put it simply, the HTTP server doesn't close the TCP connection after each response but waits some time if some other HTTP request will come over it too. After some timeout it closes it anyway.

Does it internally use TCP Keep Alive?  

No, at least I see no point in it.

If not, how does the server detect if the client is dead or alive? 

It doesn't - it doesn't need to. If a client sends a request, it will get the response. If the client doesn't send anything over TCP connection (maybe because the connection is dead) then a timeout will close the connection; client will of course notice this and will send request through another TCP connection if needed.

Answer 2

HTTP Keep-Alive is a feature of HTTP protocol. The web-server, implementing Keep-Alive Feature, has to check the connection/socket periodically (for incoming HTTP request) for the time span since it sent the last HTTP response (in case there was corresponding HTTP Request). If no HTTP request is received by the time of the configured keep-alive time (seconds) the web server closes the connection. No further HTTP request will be possible after the 'close' done by Web Server. On the other hand, TCP Keep-Alive is managed by OS in the TCP layer. HTTP Keep-Alive and TCP Keep-Alive is totally unrelated things.