CPET 355 Data Communications

1
CPET 355Data Communications Networking

• 7. The Application Layer
• Email
• Paul I-Hai Lin, Professor
• Electrical and Computer Engineering Technology
• Purdue University, Fort Wayne Campus

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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

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E-Mail Architecture and Services

• Two Subsystems
• User agents local programs
• Message transfer agents
• Five Basic Functions
• Email composition
• Transfer
• Reporting
• Displaying
• Disposition

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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

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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

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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

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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

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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

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MIME- Multipurpose Internet Mail Extensions

• Content-Typetext/subtype Examples
• Content-Typetext/plain
• Content-Typetext/enriched
• Content-Typetext/html - RFC 2854
• Content-Typetext/xml - RFC 3023

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MIME- Multipurpose Internet Mail Extensions

• Content-Typeimage/subtype Examples
• Content-Typeimage/gif
• Content-Typetext/jpg
• Content-Typeaudio/subtype Examples
• Content-Typeaudio/basic
• Content-Typeaudio/mpeg - RFC 3003

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Message Transfer

• SMTP Simple Mail Transfer Protocol
• TCP Port 25
• lt 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

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Final Message Delivery

• POP3 Post Office Protocol Version 3
• RFC 1939
• ISPs 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

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Delivery Features

• Filters (rules conditions, actions)
• Spam (junk-email)
• Forwarding emails (vacation daemon)