How does a http client associate an http response with a request (with Netty) or in general?

Question

Is a http end point suppose to respond to requests from a particular client in order that they are received?

What about if it doesn't make sense to in the case of requests handled by cluster behind a proxy or in requests handled with NIO where one request is finished faster than the other?

Is there a standard way of associating a unique id with each http request to associate with the response? How is this handled in clients like http componenets httpclient or curl?

The question comes down to the following case:

Suppose, I am downloading a file from a server and the request is not finished. Is a client capable of completing other requests on the same keep-alive connection?

Answer 1

Whenever a TCP connection is opened, the connection is recognized by the source and destination ports and IP addresses. So if I connect to www.google.com on destination port 80 (default for HTTP), I need a free source port which the OS will generate.

The reply of the web server is then sent to the source port (and IP). This is also how NAT works, remembering which source port belongs to which internal IP address (and vice versa for incoming connections).

As for your edit: no, a single http connection can execute one command (GET/POST/etc) at the same time. If you send another command while you are retreiving data from a previously issued command, the results may vary per client and server implementation. I guess that Apache, for example, will transmit the result of the second request after the data of the first request is sent.