Transmission Control Protocol (TCP) communication of an HTTPS request through a proxy

The communication during the process of an HTTPS request made to a web server through a proxy involves the use of the Transmission Control Protocol (TCP), which provides reliable, ordered, and error-checked delivery of data. Here's how TCP communication is involved:

1. Three-Way Handshake:

?? When the browser initiates a connection to the proxy server, a three-way handshake is performed to establish a TCP connection. The steps are:

?? - The browser sends a "SYN" (synchronize) packet to the proxy to initiate the connection.

?? - The proxy responds with a "SYN-ACK" (synchronize-acknowledge) packet to acknowledge the connection request.

?? - The browser sends an "ACK" (acknowledge) packet back to the proxy to confirm the connection.

2. Proxy-Browser Communication:

?? Once the TCP connection is established, the browser can send the HTTPS request data to the proxy server. This data is broken into packets, each containing a portion of the request. The proxy server acknowledges the receipt of each packet.

3. Proxy-Web Server Communication:

?? The proxy server, acting on behalf of the browser, establishes a separate TCP connection with the web server. A similar three-way handshake process occurs to establish this connection.

4. Web Server-Proxy Communication:

?? The web server sends the HTTPS response data back to the proxy server over the established TCP connection. The proxy server acknowledges the receipt of each packet.

5. Proxy-Browser Communication (Response):

?? The proxy server sends the received HTTPS response data to the browser over the initial TCP connection. The browser acknowledges the receipt of each packet.

6. Connection Termination:

?? Once the data transmission is complete, the connections need to be gracefully terminated. This involves a four-way handshake:

?? - The browser sends a "FIN" (finish) packet to indicate it has finished sending data.

?? - The proxy server acknowledges the FIN packet.

?? - The proxy server sends its own FIN packet to indicate it has finished sending data.

?? - The browser acknowledges the proxy's FIN packet.

Throughout this process, TCP ensures that data is reliably transmitted between the different parties while managing issues like packet loss, out-of-order packets, and retransmissions. It provides a reliable transport layer for the HTTPS communication, allowing both the browser-proxy and proxy-web server interactions to happen seamlessly and securely.