1. Electronic Mail (SMTP, POP, IMAP, MIME)
Chapter 26

2. Electronic Mail Email used to send memos over an internet Widely used
Popular because it is fast and convenient
Can send small notes or large memos
More users send files with than file transfer
Different concept than other network uses
Must provide for unavailable remote machine
Temporary outage should not cause sender to wait or the transfer to abort

3. Spooling used to handle delayed delivery
When user sends, mail system stores copy
Initiates transfer as a background activity
Sender can proceed with other activities
Background mail process becomes a client
Uses DNS to map destination name to IP address
Attempts to form TCP connection to destination mail server
If successful, passes copy to remote system’s spool area
If not, records attempted time and terminates
Background process periodically sweeps spool area
If finds undelivered mail, tries again
If cannot deliver after an extended time, notifies sender

4. Figure 26.1

5. Three hidden ideas Recipient specified by text string
(ex.
Names independent of other machine names
Mail destination names decoupled from usual domain names
Mail forwarding
Mail that arrives on one machine may be forwarded to another
Includes a mail alias expansion mechanism

6. TCP/IP Standards for Email Service
Two sets of mail standards
Message formats
Exact format for headers
Body of text formatting up to sender
Details of exchanges between computers

7. Simple Mail Transfer Protocol
SMTP
Standard for exchange of mail between machines
Focuses on how underlying mail delivery system passes messages across internet
Not specify how mail system accepts mail from a user
Not specify how user presented with incoming mail
Not specify how mail is stored
Not specify how frequently send attempts made

8. SMTP is straightforward
Client/server communication done in ASCII text
Easily readable by humans
Client establishes reliable stream connection to server; waits for server to send READY FOR MAIL
When get, client sends HELO command
Server responds by identifying itself
Once communication established, sender can:
transmit one or more mail messages
terminate the connection
Request server to reverse roles (mail go other way)
Receiver must acknowledge each message; or can abort connection or abort current message transfer

9. Mail Retrieval & Mailbox Manipulation Protocols
SMTP scheme implies that a server must remain ready to accept at all times
What about user with dialup Internet access?
Only be available when dialed in
Use a two-stage delivery process
User given mailbox on system with permanent connection
That computer runs a conventional SMTP server
User forms dialup connection, runs protocol to get messages
Two protocols allows remote user to get mail from permanent mailbox

10. Post Office Protocol Current version is 3; POP3
User invokes POP3 client
Creates TCP connection to POP3 server
User authenticates by login & password
User client sends commands to retrieve messages
Computer with permanent mailbox must run two servers
SMTP server accepts mail and stores it
POP3 server allows user to get messages

11. Internet Message Access Protocol
Current version is 4: IMAP4
Uses same general paradigm as POP3
But allows user to dynamically create, delete, or rename mailboxes
Also provides more functionality for message retrieval and processing
Examine headers without retrieving
Search for string and retrieve portions of a message

12. MIME Extension for Non-ASCII Data
Multipurpose Internet Mail Extensions
Defined for transfer of Non-ASCII data
Does not replace or change POP3 or SMTP
Allows encoding of arbitrary data in ASCII
Then can be transmitted in a standard message
MIME header lines specify:
Version of MIME used
Type of data being send
Encoding used to covert the data to ASCII

13. Example: From: bill@acollege.edu To: john@example.com
MIME-Version: 1.0
Content-Type: image/gif
Content-Transfer-Encoding: base64
…data for the image…

14. MIME-Version: 1.0
Content-Type: multipart/mixed;
Content-Type: application/msword;
name="assign10.doc"
Content-Transfer-Encoding: base64
Content-Description: assign10.doc
Content-Disposition: attachment;
filename="assign10.doc"
0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAPgADAP7/CQAGAAAAAAAAAAAAAAABAAAAbAAAAAAAAAAA
EAAAbgAAAAEAAAD+////AAAAAGsAAAD/////////////////////////////////////////
EAAAbgAAAAEAAAD+////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15. Content-Type must have two identifiers
A content type and a subtype
Separated by a slash
Figure 26.5

16. MIME Multipart Messages
Multipart content type useful for flexibility
Four subtypes defined
Mixed
Message can have multiple independent submessages
Each can have own type and encoding
Alternative
Message can have multiple representations of the same data
Parallel
Message can have subparts that should be viewed together
Digest
Single message can contain a set of other messages

17. Summary Email is one of most widely available application services
Like other TCP/IP services, uses client-server
Mail system buffers outgoing/incoming msgs
Allows transfer to be done in the background
TCP/IP suite has separate standards for:
Mail message format
Mail transfer

18. SMTP
Defines how mail system on one machine transfers mail to a server on another
POP3
Specifies how a user can retrieve contents of a mailbox
Allows user to have permanent mailbox on a computer with continuous Internet connectivity
User can access from a machine with intermittent connectivity

19. MIME
Provides mechanism allowing arbitrary data to be transferred using SMTP
Lines in header of message define the type of data and the encoding used
Mixed multipart type is especially useful
Single message can contain multiple data types