LIS508 lecture 12: email

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LIS508 lecture 12email

• Thomas Krichel
• 2002-12-08

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Structure

• Email
• Structure of mail
• Transport
• Email lists
• http (only the verry basics)

3
history

• 1982 ARPANET issue email protocols
• RFC821 by a hippy
• RFC822 revised by a grad student
• 1984 CCITT (Comité Consultatif International
  Télégraphique et Téléphonique, now the
  International Telecommunication Union) drafted
  X.400, backed by all major industry players. This
  standard is almost totally uninteroperable with
  the ARPANET standards.

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email

• Nowadays emails runs of top of DNS, all mails are
  sent to
• user_at_hostname where hostname is a host name and
  user name on that host.
• To which machine mail is sent depends on the MX
  record for the name you are sending the mail to.
  See the mx record in the DNS example.

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RFC821

• Defines the original SMTP, the simple mail
  transfer protocol.
• By Jonathan B. Postel, who was the RFC editor.
• He worked at the IANA, the Internet Assigned
  Number Autority.

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smtp

• TCP connection to port 25 by a client, sending
  machine.
• It waits for a response from the receiving
  machine acting as a server to say
• Who it is
• Whether it ready to receive mail
• Senders says who the mail is from and where it
  goes to.
• Receiver checks this and give go-ahead

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Typical dialog

• R 220 liu.edu SMTP service ready
• S HELO openlib.org
• R 250 liu.edu says hello to abc.com
• S MAIL from ltkrichel_at_openlib.orggt
• R 250 sender ok
• S RCPT TO michael.koenig_at_liu.edu
• R 250 recipient ok
• S DATA
• R 354 send mail, end with . on a line by
  itself
• S From
• .
• .
• R 250 message received
• S Quit

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RFC 822

• Is the one document that originally define email.
  It is the most famous RFC known to man.
• Specification is limited to US ASCII, 7 bit
• A mail message is defines as a text file. It has
• Header fields (mandatory)
• Body (optional)

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Email headers

• Email headers are composed of logical lines.
• These a physical lines (terminated by
  carriage-return/line-feed) not starting with a
  blank.
• They are of the form
• header-nameheader-value
• eg.
• To Thomas Krichel ltkrichel_at_openlib.orggt

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Header Fields

• Return-Path Added by the final delivery host to
  give a back path to the originator.
• Received Added each time another machine
  receives the mail. Note that the mail may be
  relayed by a number of machines.
• From Person who wanted to send the mail.
• Reply-to Where to send a reply to

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Email headers II

• To Primary recipient
• CC Secondary (informational) recipient
• BCC Additional (hidden) recipient
• Message-Id An identifier for the message
• Subject has the subject
• Sender whom to report problems to

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example

• Return-path ltadickey_at_liu.edugt
• Envelope-to krichel_at_openlib.org
• Received from phoenix.liunet.edu (148.4.5.3
  helophoenix.liu.edu)
• by lists.repec.org with esmtp (Exim 3.36
  1 (Debian)) id 18GQaW-0005ad-00
• for ltkrichel_at_openlib.orggt Mon, 25 Nov
  2002 151616 -0600
• Received from webmail (webmail.liunet.edu
  148.4.5.10)
• by phoenix.liu.edu (Switch-2.0.3/Switch-2.
  0.3) with ESMTP id gAPLIDU13872
• for ltkrichel_at_openlib.orggt Mon, 25 Nov
  2002 161813 -0500
• X-WebMail-UserID adickey_at_liu.edu
• Date Mon, 25 Nov 2002 162902 -0500
• Sender Alison Dickey ltadickey_at_liu.edugt
• From Alison Dickey ltadickey_at_liu.edugt
• To Thomas Krichel ltkrichel_at_openlib.orggt
• X-EXP32-SerialNo 00002866
• Subject RE course
• Message-ID lt3E120D84_at_webmailgt
• Mime-Version 1.0
• Content-Type text/plain charset"ISO-8859-1"
• Content-Transfer-Encoding 7bit

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Multipurpose Internet Mail Extensions MIME

• MIME allows to attach arbitrary files to emails.
  It is an Internet standard defined in RFC 2045 to
  2049
• MIME provides a way for non-text information to
  be encoded as text. This encoding is known as
  base64
• The MIME format is also very similar to the
  format of information that is exchanged between a
  Web browser and the Web server it connects to.
  This related format is specified as part of the
  Hypertext Transfer Protocol (HTTP).

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Basic idea

• Continue to use RFC822 but give the body of the
  mail a richer structure
• Five new headers
• MIME-version shows the version
• Content-description human readable string
• Content-id Unique identifier
• Content-transfer-encoding how the body is
  wrapped for transmission
• Content-type nature of the message

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Content-type

• The value of this header is known as the MIME
  type of the form type/subtype. Both type and
  subtype have to be specified.
• application/octet-stream is the catchall.
• The Internet Assigned Number Authority act as a
  registrar for these types.
• They provide some controlled vocabulary for file
  types. It is not perfect.

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Email lists

• An email list is a list of email addresses
  managed by a computer program..
• A list has an email address.
• When a person writes to the email address of the
  list, the message may be distributed to all
  addresses on the list.

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Concepts involved with lists

• The list owner is a person or group of person who
  has the power to add and remove addresses from
  the list. The owner may also have the following
  duties/powers
• define charter and policy
• answer technical questions
• A list is closed if a potential subscriber has to
  ask the list owner to be subscribed.
• A list is open if anyone can subscribe to a list.
  

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More concepts involved with lists

• A list is moderated if the moderator(s) are the
  only people allowed to send messages to the list.
  Messages sent to the list are forwarded to the
  moderator(s) by the list processing software.
• The owner of the list is not necessarily the
  moderator.
• Usually, the owner has moderator powers too.

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Hypertext transfer protocol http

• An application-level protocol for distributed,
  collaborative, hypermedia information systems.
• HTTP is also used as a generic protocol for
  communication between user agents and
  proxies/gateways to other Internet systems,
  including those supported by the SMTP, NNTP,
  FTP, Gopher, and WAIS protocols. In this way,
  HTTP allows basic hypermedia access to resources
  available from diverse applications.
• history
• 1990 version 0.9 allows for transfer of raw
  data.
• 1996 rfc1945 defines version 1.0. by adding
  attributevalue headers.
• 1999 rfc 2616 adds support for hierarchical
  proxies, caching, virtual hosts and some support
  for persistent connections, and is more
  stringent.

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request/response

• Server sends response
• required items
• Status line
• Protocol version
• Success or error code
• optional items
• Server information
• body

• Client sends request
• required items
• method
• request URI
• protocol version
• optional items
• request modifiers
• client information
• body

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General format of messages

• Start line
• Headers attributevalue form
• An empty line
• Body
• Just like in an email!

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Start line of the request

• Aka the resquest line, of the form
• method URI protocol-version
• method takes the values OPTIONS'', GET'',
  HEAD'', POST'', PUT'', DELETE'',
  TRACE'', CONNECT''.
• URI is a URL generally, though it can take more
  general form
• protocol-version is the version of the protocol
• Example
• GET http//openlib.org/home/krichel HTTP/1.1

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Start line of the response

• Aka the status line, of the form
• HTTP-Version Status-Code Reason-Phrase
• Only the code is required. Famous codes are
• 404 Not Found
• 403 Forbidden
• 200 OK
• Example
• HTTP/1.1 200 ok

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http//openlib.org/home/krichel

• Thank you for your attention!