Client/server architecture and FTP operation

Client/server architecture

In client/server architecture the work is divided between two kinds of programs. A server provides a service (files, web pages, database, mail) and waits for requests. A client starts the communication, sends a request and displays the result. They communicate over a network by a protocol; one server serves many clients.

 Client 1 --\
 Client 2 ---+--request--> [ Server ] <--> data
 Client 3 --/   <-response-

Benefits

• Central control of data, security and backup.
• Easy to manage, update and upgrade the server.
• Resources and expensive hardware are shared by many clients.
• Scalable: servers can be added or upgraded.
• Clients and servers can be of different platforms; roles are clear.

Drawbacks

• The server is a single point of failure; if it is down, clients cannot work.
• Heavy load on the server and network can cause congestion.
• Servers and administration are costly.
• Needs skilled staff and strong security for the server.
• Clients depend on the network.

FTP client/server operation

File Transfer Protocol (FTP) (RFC 959) transfers files between a client and a server over TCP. It uses two connections:

• Control connection: TCP port 21; stays open for the whole session and carries commands (USER, PASS, LIST, RETR, STOR, QUIT) and replies (220 ready, 331 password needed, 230 logged in, 226 transfer complete).
• Data connection: opened for each file transfer or directory listing (TCP port 20 in active mode) and closed after the transfer.

 FTP Client                        FTP Server
 [User interface]---control (21)---[Server PI]
 [Data transfer ]---data (20/n)----[Data transfer]

Steps: (1) client connects to port 21; (2) client logs in; (3) client sends a command such as RETR file; (4) a data connection is opened and the file is sent; (5) the data connection closes; (6) QUIT closes the session.

Types of FTP

1. Active mode: client sends PORT with its address and port; the server opens the data connection from port 20 to the client. Firewalls and NAT on the client side often block it.
2. Passive mode: client sends PASV; the server opens a random port and the client makes both connections. This works through firewalls and is the default today.
3. Anonymous FTP: public access with user "anonymous" and an e-mail as password; for public downloads.
4. Authenticated FTP: needs a user name and password.
5. Secure variants: FTPS (FTP over TLS) and SFTP (file transfer over SSH).
6. Transfer modes: ASCII (text) and binary (images, programs).