How browser maps the web response back to request?

Question

Say i make the web request(www.amazon.com) to amazon web server through browser. Browser makes the connection with Internet through Internet service providers. Request reaches to amazon server which process it and send back the response. Two questions here :-

1. Does Amazon server makes new connection with internet to send the response back or incoming request(initiated by me) waits on socket till amazon process the response ?
2. Once my browser receives the response how does it map the response(sent from amazon) back to particular request . I believe there must be some unique identifier like requestId must be present in response through which browser must be mapping to request. Is that correct ?

Answer 1

HTTP uses Transfer Control Protocol. This is how it happens-

1. Does Amazon server makes new connection with internet to send the response back or incoming request(initiated by me) waits on socket till amazon process the response ?

No. Most browsers use HTTP 1.1 so the connection between client and server is established only once until closed (Persistent connection).

2. Once my browser receives the request how does it map the response(sent from amazon) back to particular request . I believe there must be some unique identifier like requestId must be present in response through which browser must be mapping to request. Is that correct ?

There is a protocol(HTTP) on how the messages are exchanged. HTTP dictates that responses must arrive in the order they were requested. So it goes like-

Request;Response;Request;Response;Request;Response;...

And there is also a specific format of HTTP request (from your browser- HTTP client) and HTTP response message (from amazon HTTP server). There are response status codes that let the browser know if their request has been succeeded, otherwise tell the errors. A few sample codes-

Answer 2

The client opens a TCP-connection to the server, sends an HTTP-request and the server sends the response using the same connection. So, the browser knows from the connection that the response belongs to a specific request. This applies to basic HTTP 1.

This has to be distinguished from the programming model of an AJAX web application which is asynchronous and not synchronous. The application does not actively wait for a response. It is instead triggered later when the response arrives. The connection handling described above is what happens "under the hood".

Back to the connection handling: There are optimizations of HTTP that make things more complicated. HTTP 1.1 has a feature called "keep alive" and HTTP 2 goes further into this direction. The idea is to send more data over a single TCP-connection because establishing a TCP-connection is expensive (-> three way handshake, slow start). So, multiple requests and responses are sent over a single TCP-connection. Your question arises again in case of this optimization. If e. g. there is a sequence of requests A, B and a sequence of corresponding responses B, A within a single HTTP-connection how does the browser know the request a response belongs to? HTTP 2 introduces the concept of streams (RFC 7540, section 5):

A single HTTP/2 connection can contain multiple concurrently open streams, with either endpoint interleaving frames from multiple streams.

The order in which frames are sent on a stream is significant.

Streams are identified by an integer.

So, the stream identifier and the order within a stream can be used by the browser to find out the request a response belongs to.

HTTP 2 introduces another interesting feature which is called "push". The client can proactively send resources to the client that the client has not even requested. So, resources like e. g. images can be already sent when the HTML is requested avoiding another communication roundtrip.

Answer 3

1. Does Amazon server makes new connection with internet to send the response back or incoming request(initiated by me) waits on socket till amazon process the response ?

It uses the same connection. Most of the time it's not even possible to connect back to a web browser due to firewall restrictions or Network Address Translation (NAT).

2. Once my browser receives the request how does it map the response(sent from amazon) back to particular request . I believe there must be some unique identifier like requestId must be present in response through which browser must be mapping to request. Is that correct ?

It receives the response on the same socket. So the socket is the identifier. If HTTP2 multiplexing is used, then each multiplexed stream has a stream identifier, which is used to map the response back to the request.