How are different TCP connections in HTTP requests identified?

Question

From what I understand, each HTTP request uses its own TCP connection (please correct me if i'm wrong). So, let's say that there are two current connections to the same server. For example, client side javascript code triggering a couple of AJAX POST requests using the XMLHttpRequest object, one right after the other, before getting the response to the first one. So we're talking about two connections to the same server, each waiting for a response in order to route it to each separate callback function.

Now here's the thing that I don't understand: The TCP packet includes source and destination ip and port, but won't both of these connections have the same src and dest ip addresses, and port 80? How can the packets be differentiated and routed to appropriately? Does it have anything to do with the packet sequence number which is different for each connection?

Answer 1

When your browser creates a new connection to the HTTP server, it uses a different source port.

For example, say your browser creates two connections to a server and that your IP address is 60.12.34.56. The first connection might originate from source port 60123 and the second from 60127. This is embedded in the TCP header of each packet sent to the server. When the server replies to each connection, it uses the appropriate port (e.g. 60123 or 60127) so that the packet makes it back to the right spot.

One of the best ways to learn about this is to download Wireshark and just observe traffic on your own network. It will show you this and much more.

Additionally, this gives insight into how Network Address Translation (NAT) works on a router. You can have many computers share the same IP address and the router will rewrite the request to use a different port so that two computers can simultaneously connect to places like AOL Instant Messenger.

Answer 2

They're differentiated by the source port.

The main reason for each HTTP request to not generate a separate TCP connection is called keepalives, incidentally.

Answer 3

A socket, in packet network communications, is considered to be the combination of 4 elements: server IP, server port, client IP, client port. The second one is usually fixed in a protocol, e.g. http usually listen in port 80, but the client port is a random number usually in the range 1024-65535. This is because the operating system could use those ports for known server protocols (e.g. 21 for FTP, 22 for SSH, etc.). The same network device can not use the same client port to open two different connections even to different servers and if two different clients use the same port, the server can tell them apart by their IP addresses. If a port is being used in a system either to listen for connection or to establish a connection, it can not be used for anything else. That's how the operating system can dispatch packets to the correct process once received by the network card.