CSCI 6433CLASS 10Dave Roberts

2PRINCIPLES

Good Citizen Principle It’s about giving up resources when they are scarce It’s not about normal operation

Layering Isolates operations from each other Flexibility in configuration

End to End Operation High level functions performed by endpoints Superior reliability

Conserving router time No Internet if router job is too difficult Drives design of all Internet protocols

ELECTRONIC MAILSimple Mail Transport Protocol

SMTPPOPIMAPMIME 3

ELECTRONIC MAIL

Email allows users to send memos across the Internet.

Notes can be short or quite large Notes can have multiple attachments Must work when remote machine is

unreachable

4

ACTORS IN THE EMAIL SYSTEM

Post Office SMTP servers move email between each other SMTP servers store email for delivery to end

users Users

POP, IMAP clients pick up email from SMTP servers

POP, IMAP clients hand outgoing mail to SMTP servers

5

6COMPONENTS

DELAYED DELIVERY7Sender and receiver do not need to be

connected to the server at the same time

8AN SMTP TRANSACTION

MAIL command: establishes return address and bounce address

RCPT command: establishes recipient of this message

DATA signals beginning of the message text

9QUESTIONS

What protocol do SMTP servers use to deliver messages?

How does SMTP differ between v4 and v6?

SMTP TRANSACTION SEQUENCE

10

11EXAMPLE

S: 220 smtp.example.com ESMTP Postfix C: HELO relay.example.org S: 250 Hello relay.example.org, I am glad to meet you C: MAIL FROM:<bob@example.org> S: 250 Ok C: RCPT TO:<alice@example.com> S: 250 Ok C: RCPT TO:<theboss@example.com> S: 250 Ok C: DATA S: 354 End data with <CR><LF>.<CR><LF> C: From: "Bob Example" <bob@example.org> C: To: "Alice Example" <alice@example.com> C: Cc: theboss@example.com C: Date: Tue, 15 January 2008 16:02:43 -0500 C: Subject: Test message C: C: Hello Alice. C: This is a test message with 5 header fields and 4 lines in

the message body. C: Your friend, C: Bob C: . S: 250 Ok: queued as 12345 C: QUIT S: 221 Bye {The server closes the connection}

ALIAS EXPANSION, MAIL FORWARDING

12

INTERNETWORKING AND EMAIL

TCP/IP internet makes universal delivery service possible

Mail systems built on TCP/IP are inherently reliable because of end-to-end delivery

Alternatively, mail gateways are used Allow mail transfer between different systems When gateway has a message, sender

discards it

13Question: Is email through a gateway better or worse than a direct

SMTP transfer?

TCP/IP EMAIL STANDARDS

Format and computer interaction are specified separately

Format: header, blank line, body Body unspecified Header is key word, colon, value Some keywords required, others optional Header is readable

14

EMAIL ADDRESSES

local-part @ domain-name

domain-name: name of mail destination local-part: address of a mailbox at destinationNote: when gateways are employed, mail

addresses are site-dependent

15

SMTP

Simple Mail Transport Protocol Focuses on how mail delivery system passes

messages from one machine to a server on another machine

Does not specify anything about user interface Does not specify how mail is stored

16

ASSURED DELIVERY

SMTP server forms TCP/IP connection with receiving server

Once receiving server has put message into safe store, then it acknowledges and sender discards message

If SMTP can’t transfer message on the first try, it keeps trying

After several days of failure, SMTP reports failure to deliver.

17

SMTP HIGHLIGHTS

All communications is readable ASCII text Transcript of interactions is readable Each message is acknowledged separately Addresses of the form local-part@domain-

name

18

MAIL RETRIEVAL AND MAILBOX MANIPULATION

POP3—Post Office Protocol POP3 client creates TCP connection to POP3

server on mailbox computer Mailbox computer runs two servers:

SMTP to place mail into user mailboxes POP3 server to allow user to extract messages

from user mailbox POP3 retrieves messages, deletes from user

mailbox Two servers coordinate use of the user

mailbox

19

INTERNET MESSAGE ACCESS PROTOCOL

POP stores messages offline; not compatible with the use of multiple computers

IMAP allows message access, manipulation from multiple computers

Platform-independent access to mail

20

Question: How does server resource usage of IMAP compare with POP?

MIME EXTENSION FOR NON-ASCII DATA

MIME—Multipurpose Internet Mail Extensions Defined to allow transmission of non-ASCII data

through mail MIME allows arbitrary data to be encoded in

ASCII, transmitted as standard email message MIME message tells recipient type of data, type

of encoding used Data type and subtype is specified MIME information is in 822 mail header

21

MIME CONTENT TYPES22

MIME EXAMPLE

From: bill@acollege.eduTo: john@example.comMIME-Version: 1/0Content-Type: image/gifContent-Transfer Encoding: base64

….data for the image….

23

MAIL RETRIEVAL, MAILBOX PROTOCOLS

Post Office Protocol—client login with userid/password

Client can then retrieve, delete messages Server computer must run two servers: POP

and SMTP POP and SMTP must coordinate use of mailbox

24

SUMMARY

Email is a very important Internet service Separate standards are used for message

format and transfer SMTP—how a mail system on one machine

transfers to a server on another POP3—how a user can retrieve contents of a

mailbox IMAP—user protocol for use from multiple

computers MIME allows arbitrary data to be exchanged

using SMTP

25