How does information reach my computer, if I do not have any ports open?

Question

This is a very newb question and probably demonstrates misunderstanding of networking; so please be patient.

I am using a network managed by someone else; this network appears to have all ports blocked by the router. I can understand that I am able to send a request (such as HTTP) to another computer which does have an open port on the Internet (such as example.com:80) but I don't understand how the response reaches my computer.

The server can't reply to me through port 80 or 443 because those aren't open. So I assume the router listens in some way for data, then forwards that to my computer.

Thus I have the following questions:

• How does this mechanism work?
• Can I only receive data once I have requested it? In what way is it tied to the connection I have made?
• Would it not be possible to keep this "path" open until either server is closed? Meaning that a server/client/whoever can send me data constantly?
• Is this mechanism used in online gaming? Or is it a TCP-specific feature?

Thank you.

Answer 1

How does this mechanism work?

The firewall inspects packets and uses the information in their headers (and sometimes their payloads) to keep track of connections.

This information can be used to allow only incoming packets that relate to an existing connection. It can also be used for network address translation to hide a large numper of clients behind a single public IP.

Can I only receive data once I have requested it? In what way is it tied to the connection I have made?

IP addresses and port numbers primerally. Sometimes application payloads may also be inspected/translated (for example this is needed to make active mode FTP work).

Would it not be possible to keep this "path" open until either server is closed? Meaning that a server/client/whoever can send me data constantly?

Yes you can typically keep the path open as long as you want.

Is this mechanism used in online gaming? Or is it a TCP-specific feature?

The mecahnism works equally well for TCP and UDP.

What is tricky though is setting up a connection where both devices are behind such a firewall/nat. There are tricks that can be used but they don't always work depending on the exact behaviour of the translation mechanism.

Answer 2

Stateful firewalls keep state information. So, port(s) are in fact opened for your response to reach you.

Answer 3

Mainly, network services running on devices like servers, pc, voip phones, are associated to ports, eg: 80 for http, 443 for https, 21 for ftp, 22 for ssh. Those are well known ports, ther are lower than 1024. The thing is, by security reason, on edge devices, are places some access list to protect internal users, on that, are blocked traffic destinated to ports (mainly well known ports), that can compromise some system. Then , when you request some web page and made a request to some server on 80 port, the request from your machine is generated on sone port upper 1024, that it's not listening for nothing, but your application, on this case, your iexplorer, or mozilla, or chrome, has the possibility to open and send a receive traffic accross then, and once you get what you want, close it. I guess you need to learn and read about osi model and networking, you can begin with Computer Networks, by Tanembaun, it is a GREAT start point for you, and this stuff is easily explained.