CPET 355
Data Communications & Networking
7. The Application Layer:
Email
Paul I-Hai Lin, Professor
Electrical and Computer Engineering
Technology
Purdue University, Fort Wayne Campus
April 5, 2004
Prof. Paul Lin
1
Electronic Mail

ARPANET e-mail proposals (1982)
• RFC 821 – transmission protocol
• RFC 822 – (message format)

RFC 2821 and RFC 2822
• Minor revisions of RFC 821 and 822
• Internet e-mail standards
April 5, 2004
Prof. Paul Lin
2
E-Mail Architecture and
Services

Two Subsystems
• User agents – local programs
• Message transfer agents

Five Basic Functions
•
•
•
•
•
Email composition
Transfer
Reporting
Displaying
Disposition
April 5, 2004
Prof. Paul Lin
3
E-Mail Architecture and
Services

Email Envelope
• Destination address
• Priority level
• Security level

Message (inside the envelope)
• Header (control info for user agent programs)
• Body – the message
April 5, 2004
Prof. Paul Lin
4
RFC 2822

Header (related to message transport)
•
•
•
•
•
•
•
TO: Primary recipients
Cc:
Secondary recipients
Bcc: Blind carbon copy
From: Person who created email
Sender: E-mail address of the actual sender
Received: Transfer agent added info
Return-Path: A path back to the sender
April 5, 2004
Prof. Paul Lin
5
RFC 2822

Message Header
• Date: the date and time the message was
sent
• Reply-To: the email address to be replied
• Message-Id: unique message reference ID
• In-Reply-To: message ID
• References: other relevant message IDs
• Keywords: user-chosen keywords
• Subject: short message for one-line display
April 5, 2004
Prof. Paul Lin
6
MIME- Multipurpose Internet
Mail Extensions

Five Message Headers
• MIME-Version: 1.0
• Content-Description: Human-readable string
telling what is in the message
• Content-Id: Unique Identifier
• Content-Transfer-Encoding: 5 schemes
• Content-Type: 7-types defined in RFC 2045
April 5, 2004
Prof. Paul Lin
7
MIME- Multipurpose Internet
Mail Extensions

Content-Type: 7-types defined in RFC
2045
•
•
•
•
•
•
•
Text: plain, enriched
Image: gif, jpeg
Audio: basic
Video: Mpeg
Application: Octet-stream, Postscript
Message: Rfc822, Partial, External-body
Multipart: Mixed, Alternative, Parallel, Digest
April 5, 2004
Prof. Paul Lin
8
MIME- Multipurpose Internet
Mail Extensions

Content-Type:text/subtype Examples
•
•
•
•
Content-Type:text/plain
Content-Type:text/enriched
Content-Type:text/html - RFC 2854
Content-Type:text/xml - RFC 3023
April 5, 2004
Prof. Paul Lin
9
MIME- Multipurpose Internet
Mail Extensions

Content-Type:image/subtype Examples
• Content-Type:image/gif
• Content-Type:text/jpg

Content-Type:audio/subtype Examples
• Content-Type:audio/basic
• Content-Type:audio/mpeg
April 5, 2004
Prof. Paul Lin
- RFC 3003
10
Message Transfer

SMTP – Simple Mail Transfer Protocol
•
•
•
•

TCP Port 25
< 64 KB message length
Time-out issue
Assume that the hosts are on-line all the time
ESMTP – Extended SMTP (RFC 2821)
• To get around these SMTP related problems
April 5, 2004
Prof. Paul Lin
11
Final Message Delivery

POP3 – Post Office Protocol Version 3
• RFC 1939
• ISP’s POP server port 110
• Download emails to the user machine

IMAP – Internet Message Access Protocol
• RFC 2060
• Port 143
• For creating, destroying, and manipulating multiple
mailboxes on the server

Webmail
April 5, 2004
Prof. Paul Lin
12
Delivery Features

Filters (rules – conditions, actions)
• Spam (junk-email)
• Forwarding emails (vacation daemon)
April 5, 2004
Prof. Paul Lin
13