Web services and electronic mail over the internet

Post Office Protocol version 3 (POP3

• Authorisation: the user agent sends a username and a password.

• •Transaction: the user agent retrieves messages; can mark messages for

deletion, remove deletion marks, and obtain mail statistics.

• • Update: after the client has issued the quit command (ending the POP3 session); the mail server deletes the messages that were marked for deletion.

How does HTTP work?

• The HTTP client first initiates a TCP connection with the server.

• The client sends HTTP request messages and receives HTTP response messages.

• Vice versa for the server

Major components of e-mail

User agents, mail servers, SMTP

three phases of SMTP transfer

handshaking (greeting), transfer of messages, closure

User agents in electronic mail

User agents compose, edit, read and save

(outgoing, incoming messages stored on

server) mail messages

Mail servers

Mail servers have a mailbox a message queue and an SMTP implementation.

Mail access protocols….

transfer messages from the receiver’s mail server to the recipient’s user agent.

Non-persistent HTTP

At most one object is sent over a TCP. Requires multiple connections to download multiple objects.

Persistent HTTP

Multiple objects can be sent over single TCP connection

HTTP Request and Response

The client requests, receives and displays Web objects.

• The web server sends objects in response to

requests. The HTTP server is stateless.

Round trip time (RTT)

The time it takes for a small packet to travel from client to server and then back to the client (including delays)

Persistent HTTP in depth

• The server leaves the connection open after sending the

response message.

• Subsequent HTTP messages between the same client/server

are sent over the open connection.

• The client sends requests as soon as it encounters a

referenced object.

• It can take as little as one RTT for all the referenced objects

Four components of HTTP cookie

• a cookie header line in the HTTP response message;

• a cookie header line in the HTTP request message;

• a cookie file kept on the user's end system and managed by the user's browser.

• back-end database on the Web site.