Internet Engineering Course Electronic Mail. Most heavily used application on any network Uses...
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Internet Engineering Course Electronic Mail
Internet Engineering Course Electronic Mail. Most heavily used application on any network Uses client-server architecture ◦ Electronic mail client accepts
Most heavily used application on any network Uses client-server
architecture Electronic mail client accepts mail from user and
delivers to server on destination computer Many variations and
styles of delivery Simple Mail Transfer Protocol (SMTP) TCP/IP
Delivery of simple text messages Multi-purpose Internet Mail
Extension (MIME) Delivery of other types of data Voice, images,
video clips
Slide 3
SMTP Concept
Slide 4
User Agents and Mail Transfer Agents
Slide 5
Relay MTAs
Slide 6
Mail Gateways
Slide 7
The Entire E-mail System
Slide 8
SMTP Simple Mail Transfer Protocol is standard application
protocol for delivery of mail from source to destination Built on
top of TCP: provides reliable delivery of data SMTP does not
normally use intermediate mail servers for sending mail SMTP uses
persistent connections Other functions: E-mail address lookup
E-mail address verification Example protocol exchange
SMTP RFC 821 Not concerned with format of messages or data
Covered in RFC 822 (see later) SMTP uses info written on envelope
of mail Message header Does not look at contents Message body
Except: Standardize message character set to 7 bit ASCII
Slide 12
Basic Operation Mail created by user agent program (mail
client) Message consists of: Header containing recipients address
and other info Body containing user data Messages queued and sent
as input to SMTP sender program Typically a server process (daemon
on UNIX)
Slide 13
Mail Message Contents Each queued message has: Message text RFC
822 header with message envelope and list of recipients Message
body, composed by user A list of mail destinations Derived by user
agent from header May be listed in header May require expansion of
mailing lists May need replacement of mnemonic names with mailbox
names If BCCs indicated, user agent needs to prepare correct
message format
Slide 14
SMTP Sender Takes message from queue Transmits to proper
destination host Via SMTP transaction Over one or more TCP
connections to port 25 Host may have multiple senders active When
delivery complete, sender deletes destination from list for that
message When all destinations processed, message is deleted
Slide 15
Optimization If message destined for multiple users on a given
host, it is sent only once Delivery to users handled at destination
host If multiple messages ready for given host, a single TCP
connection can be used Saves overhead of setting up and dropping
connection
Slide 16
Possible Errors Host unreachable Host out of operation TCP
connection fail during transfer Sender can re-queue mail Give up
after a period Faulty destination address User error Target user
changed address Redirect if possible Inform user if not
Slide 17
SMTP Protocol - Reliability Used to transfer messages from
sender to receiver over TCP connection Attempts to provide reliable
service No guarantee to recover lost messages No end to end
acknowledgement to originator Error indication delivery not
guaranteed Generally considered reliable
Slide 18
SMTP Receiver Accepts arriving message Places in user mailbox
or copies to outgoing queue for forwarding Receiver must: Verify
local mail destinations Deal with errors Transmission Lack of disk
space Sender responsible for message until receiver confirm
complete transfer Indicates mail has arrived at host, not user
Slide 19
SMTP Forwarding Mostly direct transfer from sender host to
receiver host May go through intermediate machine via forwarding
capability Sender can specify route Target user may have moved
Slide 20
Conversation SMTP limited to conversation between sender and
receiver Main function is to transfer messages Rest of mail
handling beyond scope of SMTP May differ between systems
Slide 21
SMTP Mail Flow
Slide 22
SMTP System Overview Commands and responses between sender and
receiver Initiative with sender Establishes TCP connection Sender
sends commands to receiver e.g. HELO Each command generates exactly
one reply e.g. 250 requested mail action ok; completed
Operation Phases Connection setup Exchange of command-response
pairs Connection termination
Slide 25
Connection Setup Sender opens TCP connection with receiver Once
connected, receiver identifies itself 220 service ready Sender
identifies itself HELO Receiver accepts senders identification 250
OK If mail service not available, step 2 above becomes: 421 service
not available
Slide 26
Mail Transfer Sender may send one or more messages to receiver
MAIL command identifies originator Gives reverse path to used for
error reporting Receiver returns 250 OK or appropriate fail/error
message One or more RCPT commands identifies recipients for the
message Separate reply for each recipient DATA command transfers
message text End of message indicated by line containing just
period (.)
Slide 27
Closing Connection Two steps Sender sends QUIT and waits for
reply Then initiate TCP close operation Receiver initiates TCP
close after sending reply to QUIT
Slide 28
Electronic Mailboxes and Addresses E-mail users have an
electronic mailbox into which mail is deposited Usually associated
with computer account; one user may have different electronic
mailboxes User accesses mail with a mail reader program Electronic
mailbox is identified by an e- mail address mailbox@computer
Slide 29
E-mail message format Simple two-part format Header includes
delivery information Body carries text of message Header and body
is separated by blank line
Slide 30
E-mail Headers Lines of text in format keyword: information
keyword identifies information; information can appear in any order
Essential information: To: list of recipients From: sender Cc: list
of copy recipients Useful information Reply-to: different address
than From: Received-by: for debugging Frivolous information:
Favorite drink: lemonade Phase-of-the-moon: gibbous
Slide 31
Data in E-mail Original Internet mail carried only 7-bit ASCII
data Couldnt contain arbitrary binary values; e.g., executable
program Techniques for encoding binary data allowed transport of
binary data Uuencode: three 8-bit binary values as four ASCII
characters (6 bits each) Also carries file name and protection
information Incurs 33% overhead
Slide 32
Multipurpose Internet Mail Extension (MIME) Extension to RFC822
SMTP can not transmit executables Uuencode and other schemes are
available Not standardized Can not transmit text including
international characters (e.g. , , , , , , ) Need 8 bit ASCII
Servers may reject mail over certain size Translation between ASCII
and EBCDIC not standard Some SMTP implementations do not adhere to
standard CRLF, truncate or wrap long lines, removal of white space,
etc.
Slide 33
MIME Multipart Internet Mail Extensions - Extends and automates
encoding mechanisms Allows inclusion of separate components
programs, pictures, audio clips in a single mail message Sending
program identifies the components so receiving program can
automatically extract and inform mail recipient Header includes
MIME-Version: 1.0 Content-Type: Multipart/Mixed;
Boundary=Mime_separator Separator line gives information about
specific encoding Content-Type: text/plain
Slide 34
MIME (cont.) MIME is extensible sender and receiver agree on
encoding scheme MIME is compatible with existing mail systems
Everything encoded as ASCII Headers and separators ignored by
non-MIME mail systems MIME encapsulates binary data in ASCII mail
envelope
Slide 35
MIME
Slide 36
Overview of MIME Five new message header fields MIME version
Content type Content transfer encoding Content Id Content
Description
Slide 37
Content Types Text body Multipart Mixed, Parallel, Alternative,
Digest Message RFC 822, Partial, External-body Image jpeg, gif
Video mpeg Audio Basic Application Postscript octet stream
Slide 38
MIME Transfer Encodings Reliable delivery across wide largest
range of environments Content transfer encoding field Six values
Three (7bit, 8bit, binary) no encoding done Provide info about
nature of data Quoted-printable Data largely printable ASCII
characters Non-printing characters represented by hex code Base64
Maps arbitrary binary input onto printable output X-token Named
nonstandard encoding
Slide 39
39 Message format: multimedia extensions MIME: multimedia mail
extension, RFC 2045, 2056 additional lines in msg header declare
MIME content type From: [email protected] To: [email protected]
Subject: Picture of yummy crepe. MIME-Version: 1.0
Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64
encoded data....................................base64 encoded data
multimedia data type, subtype, parameter declaration method used to
encode data MIME version encoded data
Slide 40
2: Application Layer40 Multipart Type From: [email protected] To:
[email protected] Subject: Picture of yummy crepe. MIME-Version:
1.0 Content-Type: multipart/mixed; boundary=98766789 --98766789
Content-Transfer-Encoding: quoted-printable Content-Type:
text/plain Dear Bob, Please find a picture of a crepe. --98766789
Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64
encoded data....................................base64 encoded data
--98766789-- Compare with HTTP? Break into parts?
Slide 41
Printable Encoding of Binary Data into Radix-64 Format
Slide 42
Programs as mail recipients Can arrange for e-mailbox to be
associated with a program rather than a users mail reader Incoming
mail automatically processed as input to program Example mailing
list subscription administration Can be used to implement
client-server processing Client request in incoming mail message
Server response in returned mail reply
Slide 43
Mailing Lists and Forwarders E-mail addresses can be attached
to programs as well as electronic mailboxes Mail exploder or mail
forwarder resends copies of message to e-mail addresses in mailing
list UNIX mail program sendmail provides mail aliases Mailing list
processor, e.g. listserv, can also interpret subscription
management commands
Slide 44
Mail Gateways Mailing list processing may take significant
resources in large organizations May be segregated to a dedicated
server computer: mail gateway Provides single mail destination
point for all incoming mail Can use MX records in DNS to cause all
mail to be delivered to gateway
Slide 45
Mail gateways and forwarding Users within an organization may
want to read mail on local or departmental computer Can arrange to
have mail forwarded from mail gateway Message now makes multiple
hops for delivery Hops may be recorded in header Forwarded mail may
use proprietary (non-SMTP) mail system
Slide 46
Mailbox Access Where should mailbox be located? Users want to
access mail from most commonly used computer Cannot always use
desktop computer as mail server Not always running Requires
multitasking OS Requires local disk storage Can TELNET to remote
computer with mail server Can use WEB based interfaces to access
mailbox
Slide 47
Mail Access Protocols Instead of TELNET, use protocol that
accesses mail on remote computer directly Mail access protocol
transfers mail from server to local PC 2 popular protocols: POP3
(Post Office Protocol v3) IMAP (Internet Mail Access Protocol) Cant
use SMTP because getting messages is a pull operation & SMTP is
a push protocol.
Slide 48
Post Office Protocol (POP) TCP/IP suite includes Post Office
Protocol (POP) for remote mailbox access Computer with mailboxes
runs POP server User runs POP client on local computer POP client
can access and retrieve messages from mailbox Requires
authentication (password) Local computer uses SMTP for outgoing
mail
Slide 49
POP3 and SMTP
Slide 50
POP3 Stands for Post Office Protocol Version 3 RFC in 1993 POP3
is intended to permit a workstation to dynamically access a
maildrop on a server host in a useful fashion. Usually, this means
that the POP3 protocol is used to allow a workstation to retrieve
mail that the server is holding for it.
Slide 51
POP3 Doesnt provide extensive mail operations on the server
Normally mail is downloaded, then deleted Deletion doesnt really
occur until QUIT Uses port 110
Slide 52
POP3 Three phases Authorization Transaction Update During a
POP3 session, the server maintains state information POP3 downloads
mail messages to the local machine not good for nomadic user
Slide 53
POP3 protocol authorization phase client commands: user:
declare username pass: password server responses +OK -ERR
transaction phase, client: list: list message numbers retr:
retrieve message by number dele: delete quit C: list S: 1 498 S: 2
912 S:. C: retr 1 S: S:. C: dele 1 C: retr 2 S: S:. C: dele 2 C:
quit S: +OK POP3 server signing off S: +OK POP3 server ready C:
user bob S: +OK C: pass hungry S: +OK user successfully logged
on
Slide 54
Dialup Access and POP POP is useful for dialup connection Users
computer not always connected Can download all mail at once and
read off- line Can compose mail off-line and mail in one
connection
Slide 55
Minimal POP3 Commands USER name users name PASS string users
password QUIT quits, and updates the mailbox STAT status LIST [msg]
lists the mail messages (#s only) RETR msg retrieves a mail message
DELE msg deletes a message NOOP No operation RSET resets the mail
to its original state
Slide 56
IMAP Internet Mail Access Protocol Allows users to manipulate
remote mailboxes Must maintain info about user (e.g., folder
hierarchy) even after the user quits IMAP server is always in 1 of
4 states: Nonauthenticated Authenticated Selected Logout
Slide 57
POP3 (more) and IMAP More about POP3 Previous example uses
download and delete mode. Bob cannot re-read e-mail if he changes
client Download-and-keep: copies of messages on different clients
POP3 is stateless across sessions IMAP Keep all messages in one
place: the server Allows user to organize messages in folders IMAP
keeps user state across sessions: names of folders and mappings
between message IDs and folder name
Slide 58
Security SMTP, POP3 and IMAP are plain text protocols Secured
version available to protect the user SMTPS POP3s Need to be
configured both on server and client
Slide 59
Stunnel Name of an application/service Even if the mail server
application does not implement secure version What does it do?
gathers the encrypted message from client Decrypts the data Sends
plain text data using a local connection to insecure port Server
application considers it a normal client
Slide 60
STunneling Server Client Stunnel (on server) Listening to port
995 Send decrypted data to port 25 of the server Encrypted data
(Command/mail body)
Slide 61
Web interface Nowadays there are many web interface user agents
They allow accessing messages from any client with only a web
browser However the interface is not as sophisticated as special
mail agents But it is very useful if you need to check your emails
from different computers
Slide 62
Summary for Email Electronic mail allows quick, asynchronous
communication across entire Internet Can attach e-mail addresses to
programs for processing Mailing lists Other client-server
applications Simple Mail Transfer Protocol (SMTP) is Internet
standard for mail delivery Mail gateways Provide uniform user
addressing outside organizations Translate from Internet mail (e.g.
SMTP) to proprietary systems Post Office Protocol (POP) and
Internet Mail Access Protocol (IMAP) allow remote access to
electronic mailboxes.
