FTP is a TCP service that uses a data port and a control or command port. Commonly those ports are 20 and 21, respectively. But it’s not always that way…specially if you are behind a firewall.

Active FTP

Let’s see how ftp connections works normally:

• The client connects from a random unprivileged port (N >1024) to ftp server’s command port (21)
• The client starts listening on port N+1 and sends this ftp command port to the server
• The server will connect back to port N+1 from its data port (20)

Active FTP

Here is a connection example:

1. Client’ s command port (1026) connects to server’s command port (21) and sends data port 1027
2. Server sends back an ACK to the client’s command port
3. Server starts a connection between its data port (20) and client’s data port (1027)
4. Client sends an ACK to server

Passive FTP

The active mode has a problem, if the client is behind a firewall the server won’t be able to establish a connection to the client’s random data port (the one which is above 1024). In order to fix this problem, there’s another connection method called passive FTP:

• The client opens two random unprivileged ports (N > 1024 and N+1)
• The first port (command port) connects to port 21 on server side (command port) and sends a PASV command
• Then, the server opens a random unprivileged port (P > 1024) and sends the PORT P command back to the client
• Finally, the client starts a connection from port N+1 to port P on the server to transfer data

Passive FTP

Here’ s an example.

1. Client’ s command port (1026) connects to server’s command port (21) and sends PASV command
2. Server replies to the client with port 2024, the random data port opened for data connections
3. The client starts a connection from its data port (1027) to server’s data port (2024)
4. Finally, the server sends back an ACK  to the client’s data port

Reference

• Active FTP vs. Passive FTP, a Definitive Explanation

According to Wikipedia: “RAID is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. The different schemes/architectures are named by the word RAID followed by a number, as in RAID 0, RAID 1, etc. RAID’s various designs all involve two key design goals: increased data reliability or increased input/output performance. When multiple physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across multiple disks, but the array is seen by the computer user and operating system as one single disk. RAID can be set up to serve several different purposes”.

The distribution of data across multiple drives can be managed either by dedicated hardware or by software, or even by hybrid RAID based on software ad specific hardware.

According to the RAID you choose, these are some RAID’ s features:

• Data reliability
• Better fault tolerance
• Better performance
• More space
• Integration of low cost disks

Original RAID’s  specification suggested some numbers called “RAID levels”, each of them with theoretical pros and cons. Over the years, different RAID concepts have appeared, but most of them differs substantially from the original RAID levels, but it’s still common to use a number (e.g, RAID7)
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