Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose...

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Electronic Mail

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Transcript of Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose...

Page 1: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Electronic Mail

Page 2: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Familiar to most of us What are the functions we expect?

– Outgoing mail• Compose• send

– compress– encrypt

– Incoming Mail• read• forward• repl• store (retain) (and organize into folders, search, …)• discard• others????

Page 3: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

The X.400 Functional Model

UA UA

AUMTA

MS

PDAU

MTA

User UserP2

P3

P3

P1MTS

MHSPostal

Services

Other Services

(ex: SMTP)

(ex: SMTP)

(ex: SMTP)

(ex: POP or IMAP)

Page 4: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Protocols User to the User Agent

– local issue, no need to standardize P1: Message Transfer Agent interaction

– concerned with routing and protection– not interested in message format or content

P2: User Agent to User Agent– common packaging and interpretation of labels

P3: Between the UA or MS and the MTA– hand over the message for safe delivery

AU = Access Unit; PDAU = Physical Delivery Access Unit– non standard delivery, such as e-mail to fax, e-mail to postal

mail, etc.

Page 5: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Sending a message

Use DNS to get IP address of the destination mail server

Establish a TCP/IP connection to the server (Optional) establish a SSL connection to the

server Send header and message

Ref: http://docs.iplanet.com/docs/manuals/messaging/nms41/ag/routing.htm#1017549

Page 6: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Routing a message

1. Queries the Directory Server to determine whether the recipient is local or remote.

2. If the recipient is local, delivers the message, typically placing it in the message store. Otherwise, proceeds to step 3.

3. If the recipient is remote:

a. Queries DNS to find the MX (mail exchange) servers for the domain.

b. Queries DNS to find the IP address of the remote messaging server.

c. Establishes a TCP/IP connection to the remote messaging server.

d. (Optional) Establishes an SSL connection to the remote messaging server.

e. Sends the message to the remote messaging server (SMTP-Deliver).

Ref: http://docs.iplanet.com/docs/manuals/messaging/nms41/ag/routing.htm#1017549

Page 7: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Retrieving a messageTo retrieve a message, the client must know the IP address of the messaging server, establish a connection to the server, then retrieve the message using one of the retrieval protocols: POP, IMAP, or HTTP. The following summarizes the steps the client takes to retrieve a message.

1.Queries DNS to find the IP address of the server.

2.Establishes a TCP/IP connection to the server.

3.(Optional) Establishes an SSL connection to the server.

4.Establishes a POP3, IMAP4, or HTTP connection to the server to retrieve the message.

Ref: http://docs.iplanet.com/docs/manuals/messaging/nms41/ag/routing.htm#1017549

Page 8: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Characteristics User Interface

– mail program environment• hints and clues about what to do next• reminders of folder names, aliases

– set of individual commands to execute as needed• easy to incorporate mail functions into other programs

Knowing about success or failure• failure can occur at any of several places in the mail

delivery system

Page 9: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Mail Interface

Netscape Outlook unix mail Webmail Mail interface programs often provide other

functions, such as calendars and to-do lists in the same system

Page 10: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Mail system failuresSubject: Re: SIGCSE 2000 sites

Date: 10 Sep 98 09:57:08 -0400

From: [email protected] (Nell Dale)

To: <beck>

CC: Boots Cassel <cassel>

My problem with the Arbaretum is its distance from the airport and

town--and there is no public transportation to speak of. We will have to

arrange buses to take people to and from the airport and to and from town.

Taxis are not expensive, but are not used very much here.

NellThis was actually a failure on our receiver mail

system. It decided that the recipient of the message did not need to know the e-mail address of anyone

except the sender of the message. It stripped off everything from the @ on. ( It is fixed now.)

Page 11: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Problem with aliases

.Subject: Re:VU-UD tickets Date: Wed, 22 Sep 1999 11:57:22 -0400 From: [email protected] To: [email protected] CC: [email protected], [email protected], [email protected], [email protected], [email protected]

<message>

Here the problem is that the alias used locally is shown to the recipients. Those

aliases are of no use in sending replies to this message

Page 12: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Result Reporting Success reporting is usually optional Failure reports are more consistent

– Unknown user at the destination site– Inability to find the destination site– Can find, but cannot connect to the destination site– Badly formed message not processed locally

Status reporting– Verbose mode often available to show all the steps

in mail delivery

Page 13: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

SMTP

Workhorse of mail interaction Defines message format and transfer

methods Uses TCP on port 25 Send mail to a MTA Send mail between MTAs

Page 14: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

The List of Reply Codes

211 System status, or system help reply 214 Help message [Information on how to use the receiver or the meaning

of a particular non-standard command; t his reply is useful only to the human user] 220 <domain> Service ready 221 <domain> Service closing transmission channel 250 Requested mail action okay, completed 251 User not local; will forward to <forward-path> 354 Start mail input; end with <CRLF>.<CRLF> 421 <domain> Service not available, closing transmission

channel [This may be a reply to any command if the service knows it

must shut down] 450 Requested mail action not taken: mailbox unavailable [E.g., mailbox busy] 451 Requested action aborted: local error in processing

452 Requested action not taken: insufficient system storage

500 Syntax error, command unrecognized [This may include errors such as command

line too long] 501 Syntax error in parameters or argument 502 Command not implemented 503 Bad sequence of commands 504 Command parameter not implemented 550 Requested action not taken: mailbox

unavailable [E.g., mailbox not found, no access]

551 User not local; please try <forward-path> 552 Requested mail action aborted: exceeded

storage allocation 553 Requested action not taken: mailbox name not

allowed [E.g., mailbox syntax incorrect]

554 Transaction failed

Ref: http://raddist.rad.com/networks/1998/smtp/smtp.htm#commands2

Page 15: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

11 monet:/mnt/a/cassel> mail -v [email protected]: test messageChecking mail steps.EOT

monet:/mnt/a/cassel> [email protected]... Connecting to mail.acm.org. via esmtp...220 mail.acm.org ESMTP Sendmail 8.9.3/8.9.3; Wed, 4 Oct 2000 14:47:43 -0400>>> EHLO monet.vill.edu250-mail.acm.org Hello monet.csc.vill.edu [153.104.202.173], pleased to meet you250-8BITMIME250-SIZE 2000000250-DSN250-ONEX250-XUSR250 HELP>>> MAIL From:<[email protected]> SIZE=81250 <[email protected]>... Sender ok>>> RCPT To:<[email protected]>250 <[email protected]>... Recipient ok>>> DATA354 Enter mail, end with "." on a line by itself>>> .250 OAA201890 Message accepted for [email protected]... Sent (OAA201890 Message accepted for delivery)Closing connection to mail.acm.org.>>> QUIT221 mail.acm.org closing connection

From acm.org serverFrom monet

Page 16: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Return-Path: <[email protected]> Received: from mail.acm.org ([199.222.69.4]) by skinner.villanova.edu (Netscape Messaging Server 4.15) with ESMTP id G1X5J300.I88 for <[email protected]>; Wed, 4 Oct 2000 14:47:27 -0400 Received: from monet.vill.edu (monet.csc.vill.edu [153.104.202.173]) by mail.acm.org (8.9.3/8.9.3) with ESMTP id OAA201890 for <[email protected]>; Wed, 4 Oct 2000 14:47:43 -0400 Received: (from cassel@localhost) by monet.vill.edu (8.9.0/8.9.0) id OAA15022 for [email protected]; Wed, 4 Oct 2000 14:47:44 -0400 (EDT) Date: Wed, 4 Oct 2000 14:47:44 -0400 (EDT) From: boots cassel <[email protected]> Message-ID: <[email protected]> To: [email protected] Subject: test message

Expanded Message Header

Page 17: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Addresses

Local– same machine– same local network environment

remote– same mail system on a remote host– different type of mail system

Page 18: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Message Store Protocols Client interacting with a mail server

(see RFC 1733)

– offline• move mail from server to client machine

– online• keep mail on server and stay connected

– disconnected• keep mail on server• download a cache copy and disconnect• reconnect to synchronize client and server

Page 19: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

POP and IMAP

Post Office Protocolsee RFC 1725 and later revision

– works on local copy on the client machine (offline access)

– may leave copy on the server, but does not resynchronize

Internet Message Access Protocolsee RFC 1730

– allows all three interaction modes

Page 20: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

POP commands

USER Client identifies mailbox to be downloaded PASS Password STAT Get number of messages and size of mailbox LIST N Return size of message N LAST Get highest message number accessed RETR N Retrieve a full message TOP N M Retrieve only headers and the first N lines DELE N Delete message QUIT Release service NOOP See if POP server is functioning RPOP Insecure authentication

Page 21: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Offline, online, disconnected Offline is usual for ISPs.

– They store your messages until you collect them, but don’t provide long term disk space to keep all your mail for you.

Online works when the mail is delivered, read, and stored on the same system– You login, get your mail, read it, store it, go back to it when you want to.

Disconnected– One copy on your local machine; master copy on a server– Access available while traveling, from home and office.– Long distance phone connections are minimized

Page 22: Electronic Mail. Familiar to most of us 4 What are the functions we expect? –Outgoing mail Compose send –compress –encrypt –Incoming Mail read forward.

Mail Summary

Early in the ARPAnet days, e-mail came to dominate usage

With Web use, it remains a major contributor to traffic on the Internet

The user interface to the mail system comes in many varieties

Behind the scenes, a few message transfer systems deliver the mail


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