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Application Protocols
Application Protocols
SMTP
Problems• Transferring mail across different networks• Use different transport services• Different domains• Appropriate messages and regular updating
mechanism • Security
• Allows you to send messages via computer networks to other computer users.– The message may include a file (attachments)– You must know the e-mail address of the
recipient, just as you need to know the phone number to use a phone.
Introduction
• What is SMTP? The Internet’s standard protocol used in sending and receiving e-mail in order to transfer mail reliably and efficiently
SMTP
• Simple Mail Transfer Protocol.
• The SMTP service is a modification of the FTP– SMTP is an electronic mail routing protocol
that uses TCP/IP to route mail messages between network hosts.
– SMTP does not deal with local mail user issues. This is handled by an e-mail tool (Outlook Express, Netscape Messenger)
Solutions
• 2-way transmission channel• Server confirmation• Host and the server identify each other• Commands and Replies• Extension services• Mail could be directly delivered or could be relayed
across networks • Server takes full responsibility
Solutions
• Mail object• Envelope and Content • Envelope-Originator address• - Recipient address or addresses• - Optional protocol extension
material
SMTP Mail Message Format
– Messages are a Human Readable Text File– Two Parts: Envelope and Contents
• Envelope:– Keyword/Value Pairs – One Per Line– Common Keywords
» Subject:» Date:» From:» Reply-To:
– Common Mailer Generated Keywords:» Received:» Message-Id:
• Message Contents– Separated From Envelope By a Blank Line
Definition :
Internet method of Transferring Electronic
Mails (Allows Message to be packed and
Delivered to remote systems
Transferring Mail Between Two Hosts
E-mail standards• Simple Mail Transfer Protocol (SMTP):
part of the TCP/IP protocol suite.• SMTP is the main e-mail standard for
mail transfer agents in use today, but was written to only handle text files.
• SMTP is usually implemented using a two-tier client-server architecture.
• Two other competing standards for e-mail are Common Messaging Calls (CMC) and X.400.
Simple Mail Transfer Protocol (SMTP)
• Sends and receive e-mail messages from one e-mail server to another.
• Defines a dialog between a sending and receiving system.
Two-Tier E-mail Architecture• SMTP’s uses two types of programs: • User agents, aka e-mail clients, are
programs running on client computers that send e-mail to e-mail servers and download e-mail from mailboxes on those servers.– Examples: Eudora, Outlook.
• Mail transfer agents, also called mail server software, are used by e-mail servers, sends e-mail through the Internet between e-mail servers and maintains individual mailboxes. – Post Office Protocol (POP) & Internet Mail Access
Protocol (IMAP) are the main protocols for user agent to mail server communications.
Complementary Protocols
• SMTP is limited in its ability to queue messages at the receiving end. One of two others are used in conjunction:– POP3– IMAP4 (Internet Message Access Protocol)
Client computer withe-mail client software(“user agent”)
Server computer withe-mail server
software
(“message transfer agent”)
SMTP packet
SMTP packet
LAN
Client computer withe-mail client software(“user agent”)
Server computer withe-mail server
software
(“message transfer agent”)
SMTP packet
LAN
SMTP packet
IMAP or SMTP packet
Internet
Simple Mail Transfer Protocol
• SMTP E-mail messages have the following structure:– Header: lists source and destination
addresses, date, subject and other information about the e-mail message.
– Body: the message itself– Attachments: additional files that have been
included along with the e-mail message.
Figure 2-13 Sample E-mail Message using SMTP
From: “Alan Dennis;”<[email protected]>TO: “Pat Someone” <[email protected]>Date: Mon 06 Aug 2001 19:03:02 GMTSubject: Sample NoteMessage-Id:
<[email protected]>DATA: This is an example of an e-mail message
Messageheader
Messagebody
How it works
• Sender SMTP establishes a two-way transmission channel to a receiver SMTP.
• Once the channel is established, there are three basic steps to SMTP mail transactions:– MAIL FROM:– RCPT TO:– DATA
• At each step, the procedure can be accepted (OK) or rejected (Failure reply)
How Mail Gets Transfer From One Hosts to Another
SMTP defines a set of commands with strict
Syntax that are used to move mail
Messages from one hosts to another. These
Functions are not user-visible.
SMTP delivers mail by establishing a
Transmission path between an SMTP
Client (“Sender-SMTP”)
SMTP Commands
• HELO - identifies sender• MAIL FROM: - starts a mail transaction and
identifies the mail originator• RCPT TO: - identifies individual recipient. There
may be multiple RCPT TO: commands.• DATA - sender ready to transmit a series of lines of
text, each ends with \r\n. A line containing only a period ‘.’ indicates the end of the data.
SMPT example
>MAIL FROM: <[email protected]>
>250 Mail.mydomain.com MYPC>HELO MYPC
>250 OK
>RCPT TO: <[email protected]>
>250 OK
>DATA >354 Start mail input, end with <CRLF>.<CRLF>.
>Hello Efrim <CRLF>.<CRLF>.>250 OK
>QUIT>221 Goodbye MYPC
Simple Mail Transfer Protocol (SMTP)
Attachments in MIME• Because SMTP was developed for transferring text files, a
special techniques needed to be developed to send graphical information.
• Graphics capable mail transfer agent protocols include MIME (Multipurpose Internet Mail Extension), uuencode and binhex.
• Included as part of an e-mail client, MIME translates graphical information into text allowing the graphic to be sent as part of an SMTP message.
• The receiver’s e-mail client then translates the MIME attachment from text back into graphical format.
Encoding: Header Information– MIME – Multipurpose Internet Mail Extensions
• Mime Format in Mail Header Identifies the Contents • MIME Body Types
– Text» Plain – No Encoding Needed» RTF – Allows Some Formatting
– Image– Audio– Application
» A Specific Application Format (e.g. Microsoft Word)– Structured
» Allows Multiple Types in One Document– Message
» Used To Identify Partial Messages
Encoding Techniques (2)
• Non-Portable Formats– Will Cause Corruption if an Intermediate Server Has
Limitations
– Eight-Bit• No Encoding• No Line Exceeds 1000 Characters
– Binary• No Encoding• Lines May Be Too Long
Retrieving Mail• Offline Model
– User Doesn’t Have to Be Connected In Order to Receive Messages
– Mail Server Stores Messages For a User Locally• User Then Downloads Them To a Local File
• POP – Post Office Protocol• Uses Port 110• Current Version: POP3 (Version 3)
– Allows Selective Downloading• Session Based:
– Authorization» User Name, Password
– Transaction» STAT – Request Statistics (Number of Messages, Size)» LIST – List of Messages With Size» RETR – Download a Message» DELE – Delete Message From Server» QUIT – End Session
SENDMAIL Client
Transport ControlProtocol
Internet Protocol
Network InterfaceProtocol
SENDMAIL Client
Transport ControlProtocol
Internet Protocol
Network InterfaceProtocol
Host A Host B
Messages
Physical Network
File Transfer Protocol (FTP) and Telnet
• FTP enables sending and receiving files over the Internet.
• Telnet allows remote logins over the Internet.
• Both were much more commonly used during the pre-WWW days of the Internet.
• FTP still commonly used today for uploading web pages.
E-mail Models
• Online– Email resides on the server, access to email
(including reading etc.) requires connection to server.
• Offline– Mail is retrieved from server during connection, and
stored on client.
• Disconnected– Mail is retrieved and manipulated on-line when
connected, cached on the local machine when off-line, and syncronized when connected again.
File Transfer Protocol (FTP)• Protocol that allows transfer of files across
Internet
• Both computers need to run FTP - one acts as a server (remote), the other as a client (local)
• Many shareware files available
• Often stored in ZIP format or self-extracting EXE
• Disadvantage - must know name of file
FTP
• The File Transfer Protocol allows files to be moved from one machine to another.
• FTP is client-Server based where the server acts as a store for files. Clients may connect and request files on the server
• FTP is becoming less popular with changes that have been made in HTTP.
How to FTP
• Steps to FTP:1. Log on to machine e.g. to csusap
2. Connect to a another remote machine by either:
i. anonymous FTP - very common - but can only access files in the pub directory
Login: anonymousPassword: <your email address>
ii. log in as yourself - this requires you to have an account on that machine i.e. username and password Gives you access to files that you own on that machine.
Pictorial representation of FTP
ComputerA
Computer B
file1.txt
file2.txtput file2.txt
get file1.txt
Computer Bestablishes an FTPsession with Computer A and issues the get and put statement
Both Computer A and Computer B have FTP running
Transferring Files Between Hosts
The File Transfer (ftp) commands is used to
Transfer files between hosts of possibly
Dissimilar file system.
FTP establishes two connection between
The client and the server
One for control information such as
Commands and responses,
And a second for transferring data
Which FTP Program• A number available:
WinQVT - traditionally used on Windows 3.1• Unix ftp - activated through Start, Run, ftp• CuteFTP - excellent, windows based, easy to use• WS_FTP - on most CSU machines (CSU
Applications)• Tucows - excellent repository for FTP programs:
Web browser - e.g. Netscape/Explorer can 'get' files.
How to Uses the ftp commands
• Transferring and managing
• Changing and listing contents of Directories
• Managing which files will be transferred
• Managing connections with remote hosts
Basic FTP commands (Unix)• ftp> open csusap.csu.edu.au - establishes FTP session ftp>
user ausernam – login as "ausernam" • ftp> ls -l - equivalent to dir on remote machine• ftp> cd ... - changes to new directory on remote machine • ftp> lcd ... - changes to new directory on local machine• ftp> lcd ... - changes to new directory on local machine • ftp> get filename - gets filename from remote machine • ftp> put filename - puts filename on remote machine • ftp> quit - terminates FTP session.
FTP Objectives• promote sharing of files• encourage indirect use of remote computers• shield user from variations in file storage• transfer data reliably and efficiently• “FTP, although usable directly by a user at a
terminal, is designed mainly for use by programs”
Control and Data Connections
• Control functions (commands) and reply codes are transferred over the control connection.
• All data transfer takes place over the data connection.
• The control connection must be “up” while data transfer takes place.
How the ftp server function
• Ftp commands and replies are exchanged
Through one connection, while
• Data is transferred over a separate
Connection
• The FTP client communicates with the
FTP server on the remote host using TCP
Standard Connection Model
ControlControl
DataDataAAAA BBBB
FTP Client/Server interaction
FTPControl
FTPTransfer
FTPControl
FTPTransfer
FTP Client User
FTP Client User
Commands
RepliesData
Data
FTP
Transport ControlProtocol
Internet Protocol
Network InterfaceProtocol
FTPServer
Transport ControlProtocol
Internet Protocol
Network InterfaceProtocol
Host A Host B
Messages
Physical Network
The TELNET Protocol
Telnet Protocol
• Telnet - complex protocol that allows one machine to log into another on the Internet
• Stilled based on TCP/IP• Why have a telnet session?• Local machine can access remote computer
hardware• Reduces organisations hardware costs.
TELNET
• This protocol enables users to log on to other machines remotely.– Essentially their terminal becomes the
equivalent of the dumb terminal of mainframe systems, allowing the user to run programs as if they were physically sitting at the machine.
– Normally requires a username and password
• Telnet Uses the client-server model
TELNET vs. telnet• TELNET is a protocol that provides a
general bi-directional, eight-bit byte
oriented communications facility.
• telnet is a program that supports the
TELNET protocol over TCP.
• Many application protocols are built
upon the TELNET protocol.
The TELNET Protocol
• TCP connection
• data and control over the same connection.
• Network Virtual Terminal
• negotiated options
Network Virtual Terminal (NVT)
The Network virtual Terminal (NVT) is an
Imaginary device that is used as a model for
Real terminals.
The NVT is a bidirectional, character-based
Device with a printer and a keyboard
Pictorial representation of Telnet
ComputerA
Computer B
SoftwareData
Computer Bestablishes a Telnetsession with Computer A and can now access programs and resources on Computer A
Both Computer A and Computer B have Telnet running
Network Virtual Terminal
• intermediate representation of a generic terminal.
• provides a standard language for communication of terminal control functions.
Network Virtual Terminal
NVTNVT NVTNVT
ServerServerProcessProcess
TCPTCP TCPTCP
Negotiated Options
• All NVTs support a minimal set of capabilities.
• Some terminals have more capabilities than the minimal set.
• The 2 endpoints negotiate a set of mutually acceptable options (character set, echo mode, etc).
Connecting to Remote Host Telnet
The telnet command provides remote login
Services so that an interactive user on a
Client system can connect with a server on
A remote system
The telnet command uses TCP as the
Protocol to ensure error checking.
The telnet command establishes a
Connection with a remote host, it maps the
Local terminal into a model of a terminal
Called a “ Network Virtual Terminal “ (NVT)
Negotiated Options
• The protocol for requesting optional features is well defined and includes rules for eliminating possible negotiation “loops”.
• The set of options is not part of the TELNET protocol, so that new terminal features can be incorporated without changing the TELNET protocol.
How to use the telnet command
Entering the telnet command in its simplest
Form
telnet
Will start the telnet process, and the prompt
telnet>
To start a login session with a host, enter
The telnet command in the form
telnet Host
Option examples
• Line mode vs. character mode
• echo modes
• character set (EBCDIC vs. ASCII)
Telnet send command argumentsService Required Send Argument
Break or kill brk
Erase character ec
Erase line el
Interrupt process ip
Go Ahead ga
No operation nop
How the telnet server Functions
The telnet commands uses the TCP and IP
Protocol to establish communication with
Remote server
TELNET
Transport ControlProtocol
Internet Protocol
Network InterfaceProtocol
TELNETServer
Transport ControlProtocol
Internet Protocol
Network InterfaceProtocol
Host A Host B
Messages
Physical Network
World Wide Web (WWW)
• A protocol that uses hypertext links• Accessed by using a browser• Uniform Resource Locator (URL) - a specific
address to a particular resource protocol://host/directory/resource
• Examples• http://csu.edu.au/images/csu.gif• ftp://ftp.monash.edu.au/pub/win95/update.txt• telnet://lorenz.mur.csu.edu.au• news:aus:computing
Thank You
