ACTIVE - cna there be collisions on port 20?

Question

Hello,
I understand that all ACTIVE transfers occur on port 20.

If multiple ACTIVE transfers occur simultaniously, that is perhaps there are multiple versions of code running, what happens when they all try to use port 20?

Assuming the same problem occurs in PASV but because transfer ports are random, the problem is less an issue?

But for both situations, port 21 I assume could be overly taxed?

Answer 1

Most FTP servers will have no trouble supporting many multiple connections from/to clients on the same port.

Multiple active transfers implies multiple clients, all connecting to the one FTP server (listening on port 21).

I doubt the ports would be overly taxed - most FTP servers would set a maximum number of clients to prevent this from happening.

Depending on how scalable the server is, this maximum could be quite high, e.g. hundreds of clients. Of course, it depends what the clients are actually doing.

Hello,
I understand that all ACTIVE transfers occur on port 20.

If multiple ACTIVE transfers occur simultaniously, that is perhaps there are multiple versions of code running, what happens when they all try to use port 20?

Assuming the same problem occurs in PASV but because transfer ports are random, the problem is less an issue?

But for both situations, port 21 I assume could be overly taxed?