Email

Question 1

Application architecture (Three major components) of Email

Answer 1

• Mail servers
• User Agents
• “Simple Mail Transfer Protocol” (SMTP)

Question 2

Mail Servers

Answer 2

• Composition of a mail server
  
  • Mailbox contains incoming messages for users
  • Message queue of outgoing (to be sent) mail messages
  • Messages are sent between mail servers using SMTP
• Mail servers play both a “client” role and a “server” role
  
  • “Server” role: receive emails from an upstream server (or UA)
  • “Client” role: sending emails to a downstream server

Question 3

User Agent

Answer 3

• “Mail reader”
• Outlook, Thunderbird, iPhone mail client, etc.
• Used to compose, edit, send, read… mail messages
• Both outgoing and incoming messages are stored on the associated server, not on the UA itself

Question 4

Simple Mail Transfer Protocol [RFC 5321]

Answer 4

• Principal application-layer protocol for Internet electronic mail
• Older than HTTP!
• RFC 5321 published in 1982, but SMTP was used before that too
• Still in widespread use, despite its age

Question 5

SMTP archaic characteristics

Answer 5

• Body must be simple 7-bit ASCII
• This made sense in the 1980s when throughput was scarce
• However, with the advent of the multimedia era, sending images, audio, etc. became more common
• Binary multimedia data has to be encoded to ASCII before sending
  
  • Decoded on the receiver side too

Question 6

SMTP (Transport, Three phases of transfer,Command/response style)

Answer 6

• Uses TCP to reliably transfer email message from mail server to mail server
  
  • Runs over TCP port 25
  • Direct transfer between sending server and receiving server
• Three phases of transfer:
  
  • Handshake (greeting)
  • Transfer of messages
  • Close
• Command/response style interaction (like HTTP)
  
  • Commands: ASCII text
  • Response: status code + text

Question 7

Scenario:
Alice wants to send a message to Bob

Answer 7

1) Alice uses UA to compose message “to” bob@someschool.edu
2) Alice’s UA sends message to her mail server; message placed in message queue
3) Client side of SMTP opens TCP connection with Bob’s mail server
4) SMTP client sends Alice’s message over the TCP connection; let’s assume that Bob’s mail server queue is full and rejects the message
5) Alice’s mail server retries every 30 minutes until it’s successful; the message is eventually delivered
6) Bob’s mail server places the message in Bob’s mailbox
7) Bob invokes his user agent to read message

Question 8

Intermediate Servers

Answer 8

• Sending does not necessarily use intermediate mail servers
  
  • Even if the mail servers are on the other side of world
• Example: Alice’s server is in Lancaster, Bob’s server is in New York: direct TCP connection between the two servers
• If Bob’s mail server is not receiving additional messages, the message will remain on Alice’s mail server in Lancaster
  
  • It does not get placed in an intermediate server

Question 9

Sample SMTP Interaction
Much like face-to-face communication!

Answer 9

Question 10

Comparison to HTTP

Answer 10

• HTTP: pull
• SMTP: push
• Both have ASCII command/response interaction, status codes
• HTTP: no format restrictions
• SMTP: enforces 7-bit ASCII format
• HTTP: each “object” is encapsulated in its own response message
• SMTP: multiple objects sent in multipart message

Question 11

Mail Message Format

Answer 11

Question 12

Mail Access Protocols

Answer 12

SMTP: delivery to receiver’s server
* Mail access protocol: subsequent retrieval from server
* POP: Post Office Protocol [RFC 1939]: authorisation, download
* IMAP: Internet Mail Access Protocol [RFC 1730]: more sophisticated, e.g. allows manipulation of stored messages on server
* HTTP: GMail, Hotmail, Yahoo! Mail, etc.

Question 13

POP3 example

Question 14

POP3 contd.

Answer 14

• Previous example used “download and delete” mode
  
  • Bob cannot re-read e-mail if he changes client, as it has been deleted
• “download-and-keep” mode allows copies of messages to be kept on different clients
• POP3 is stateless across sessions
  
  • Simple protocol, but limited functionality
  • Still in widespread use

Question 15

IMAP

Answer 15

• Can keep and manipulate all messages at the server
• Allows users to organise messages in folders
  
  • With POP3 this would be achieved locally, on the client-side
  • However, IMAP allows this to be done remotely
• Keeps user state across sessions:
  
  • Names of folders and mappings between message IDs and folder names
• IMAP also allows parts of a message to be retrieved
  
  • Useful with low bandwidth connections
  • Avoid fetching attachments, for example

Question 16

Web-based Email

Answer 16

• Increasingly popular…
  
  • As provided by Google (and others - e.g. Lancaster University)
• Web browser acts as User Agent
  
  • When an email is accessed from a mailbox, it is retrieved using HTTP
• Importantly, the mail servers still send and receive messages using SMTP

Question 17

SMTP: Final Words

Answer 17

• SMTP uses persistent connections
• SMTP requires message (header & body) to be in 7-bit ASCII
• SMTP server uses CRLF.CRLF to determine end of message