Search code examples
httplong-pollingkeep-alive

HTTP-Long Polling keep-alive and handshakes

I'm doing a test where I examine how much HTTP-long polling compared to Websockets is affecting the battery performance on my iPhone. Basically what I have is a Node.js with express server that sends out a random string every 0.5 or 10th second to the iPhone. I've inspected the messages in Chrome and I can see the keep-alive header is present. I know keep-alive is a default feature since HTTP/1.1. From what I've understood the TCP-connection will be held open and can be used for pipelining, and this is certainly the case when I'm sending out pings from the server every 0.5 seconds. But when I send out every 10 seconds, will the connection be closed during that time?

  1. How do I know how long the connection is open? This seems to be a crucial part to have in mind when doing the tests.

  2. Will the HTTP-handshake still be made when the TCP-connection is open?

Solution

  • AFAIK, in HTTP 1, the server cannot send a response back to the client if that client didn't send a request first. That might sound irrelevant to your question but bear with me.

    The Connection: keep-alive header tells the client that it can reuse the connection if he want to, not that it must. The client can decide to close it any time, it all depends on the client library implementation and you don't have any guarantee.

    The only way to force the client to not close the connection is to not finish the response. The only way to do that is to send a response with a Transfer-Encoding: chunked, and never send the final chunk (this has some serious caveats, like a buffer overrun on the client...).

    So to answer your 2 points:

    1. You can't, this low-level detail is totally hidden (for good reasons) from the client.
    2. There is no HTTP handshake, there is a TCP handshake which is made when the client socket connects to the server socket. There is the TLS handshake which is made after the TCP connection and before any request is made. Once the connection is open, http requests are sent by the client and the server responds with resources.