UNIT V APPLICATION LAYER
Traditional applications -Electronic Mail (SMTP, POP3, IMAP, MIME) – HTTP – Web Services – DNS- DDNS - TELNET - File transfer Protocol– SNMP-Cryptography - Basic concepts.
5.1 TRADITIONAL APPLICATIONS These applications use the request/Reply paradigm The request paradigm is used by user to send a request to server The response paradigm is used by server to send a reply based on request Applications are Remote Logging(TELNET) E-Mail File Transfer WWW Web Services SNMP HTTP
5.2 ELECTRONIC MAIL (SMTP, POP3, IMAP, MIME) E-mail is one of the most popular internet services than it was when it was envisaged.
Common Architecture
The following shows components of e-mail system involved in Alice sending a message to Bob
Components
1. User Agent 2. Message 3. Message Transfer Agent 4. Message Access Agent
User Agent
A user agent (UA) is software that is either command (eg. pine, elm) or GUI based (eg. Microsoft Outlook, Netscape). It facilitates: o Composing messages UA helps to compose messages by providing a template that comes with a built-in editor.
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o Reading messages UA checks mail in the incoming box and apart from message provides information such as sender, size, subject and flag (read, new). o Replying to messages UA allows user to reply (send message) back to sender o Forwarding messages UA facilitates forwarding message to a third party. o Handling mailboxes UA creates two mailboxes for each user, namely inbox (to store received emails) and outbox (to keep all sent mails).
Message Format
RFC822 defines message to have two parts namely header and a body. The message header is a series of
Multipurpose Internet Mail Extensions (MIME)
MIME is a supplementary protocol that allows non-ASCII data to be sent through e-mail. MIME transforms non-ASCII data to NVT ASCII and delivers to client MTA. The NVT ASCII data is converted back to non-ASCII form at the recipient mail server. MIME defines five headers. They are: specifies the current version 1.1 o MIME-Version o Content-Type specifies message type such as text (plain, html), image (jpeg, gif), audio, video and application (postscript, msword). If more than one type exists, then it is termed as multipart (mixed). o Content-Transfer-Encoding defines how data in the message body is encoded such as binary, base64, 7-bit, etc. o Content-Id unique identifier the whole message in a multiple message type. o Content-Description describes type of the message body.
For example, a message containing plain text and an image file looks like:
MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="------417CA6E2DE4ABCAFBC5" From: Alice Smith
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Content-Transfer-Encoding: 7bit Bob, Here's the jpeg image and draft report I promised. --Alice ------417CA6E2DE4ABCAFBC5 Content-Type: image/jpeg Content- Transfer-Encoding: base64 . . . unreadable encoding of a jpeg figure
Message Transfer Agent (MTA): SMTP
Message Transfer Agent (MTA) is a mail daemon (a version of sendmail program) that helps to transmit/receive message over the network. To send mail a system must have the client MTA, and to receive mail a system must have a server MTA.
Simple Mail Transfer Protocol (SMTP) defines communication between client/server MTA. SMTP defines how commands and responses must be sent back and forth.
Some commands sent from client MTA are:
Command Description HELO Sender's host name MAIL FROM Sender of the message RCPTTO Recipient of the message DATA Body of the mail QUIT Terminate VRFY Name of recipient to be verified before forwarding EXPN Mailing list to be expanded
Common responses sent from server MTA are:
Code Description 220 Service ready 221 Service closing transmission channel 250 Request completed 354 Start mail input 450 Mailbox not available 500 Syntax error; unrecognized command 550 Command not executed, mailbox unavailable 551 User not local
SMTP uses TCP connection on port 25 to forward the entire message and store at intermediate mail servers/mail gateways until it reaches the recipient mail server.
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The following example shows commands and responses using SMTP protocol
HELO cs.princeton.edu 250 Hello [email protected] [128.12.169.24] MAIL FROM:
In each exchange, the client posts a command and the server responds with a code. The server also returns a human-readable explanation for the code. After the commands and responses, client sends the message which is ended by a period (.) and terminates the connection. Message Access Agent (MAA)/Mail Reader: POP and IMAP
MAA or mail reader allows user to retrieve messages from the mailbox, so that user can perform actions such as reply, forwarding, etc. The two message access protocols are: o Post Office Protocol, version 3 (POP3) o Internet Mail Access Protocol, version 4 (IMAP4) SMTP is a push type protocol whereas POP3 and IMAP4 are pop type protocol.
The protocols involved in e-mail are shown below.
POP3
POP3 is simple and limited in functionality POP3 client is installed on the recipient computer and POP3 server on the mail server. The client opens a connection to the server on TCP port 110. The client sends username and password to access the mailbox and retrieve the messages. POP3 works in two modes namely, delete and keep mode. o In delete mode, mail is deleted from the mailbox after retrieval o In keep mode, mail after reading is kept in mailbox for later retrieval.
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