The Application Layer

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The Application Layer

• Chapter 7-1

Institute of Information Science and Technology .
Chengdu University YiYong 2008?2?25?
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Teaching object

• Through this section of the study , student
  should be understand the basic concepts of the
  data link layer, on top of stop waiting for the
  agreement

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Focus and Difficulties

• 1.DNSThe Domain Name System
• 2.EMAIL

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• 7.1 DNSThe Domain Name System
• 7.2 Electronic Mail
• 7.3 The World Wide Web
• 7.4 Multimedia

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7.1 DNS The Domain Name System

• The DNS Name Space
• Resource Records
• Name Servers

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7.1.1 The DNS Name Space

• A portion of the Internet domain name space.

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7.1.2 Resource Records

• The principal DNS resource records types.

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Resource Records (2)

• A portion of a possible DNS database for cs.vu.nl.

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7.1.3 Name Servers

• Part of the DNS name space showing the division
  into zones.

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Name Servers (2)

• How a resolver looks up a remote name in eight
  steps.

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7.2 Electronic Mail

• Architecture and Services
• The User Agent
• Message Formats
• Message Transfer
• Final Delivery

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Electronic Mail (2)

• Some smileys. They will not be on the final exam
  -).

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7.2.1 Architecture and Services

• Basic functions
• Composition
• Transfer
• Reporting
• Displaying
• Disposition

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7.2.2 The User Agent

• Envelopes and messages. (a) Paper mail. (b)
  Electronic mail.

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Reading E-mail

• An example display of the contents of a mailbox.

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7.2.3 Message Formats RFC 822

• RFC 822 header fields related to message
  transport.

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Message Formats RFC 822 (2)

• Some fields used in the RFC 822 message header.

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MIME Multipurpose Internet Mail Extensions

• Problems with international languages
• Languages with accents (French, German).
• Languages in non-Latin alphabets (Hebrew,
  Russian).
• Languages without alphabets (Chinese, Japanese).
• Messages not containing text at all (audio or
  images).

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MIME (2)

• RFC 822 headers added by MIME.

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MIME (3)

• The MIME types and subtypes defined in RFC 2045.

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MIME (4)

• A multipart message containing enriched and audio
  alternatives.

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7.2.4 Message Transfer

• Transferring a message from elinore_at_abc.com to
  carolyn_at_xyz.com.

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Final Delivery
(a) Sending and reading mail when the receiver
has a permanent Internet connection and the user
agent runs on the same machine as the message
transfer agent. (b) Reading e-mail when the
receiver has a dial-up connection to an ISP.
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POP3

• Using POP3 to fetch three messages.

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IMAP

• A comparison of POP3 and IMAP.