Electronic Mail (SMTP, POP, IMAP, MIME)

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

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 email 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

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

Figure 26.1

Three hidden ideas • Recipient specified by text string local-part@domain-part (ex. rhammell@towson.edu) • 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

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 email exchanges between computers

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

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

Mail Retrieval & Mailbox Manipulation Protocols • SMTP scheme implies that a server must remain ready to accept email 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

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

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

MIME Extension for Non-ASCII Data • Multipurpose Internet Mail Extensions • Defined for email 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 email message • MIME header lines specify: • Version of MIME used • Type of data being send • Encoding used to covert the data to ASCII

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…

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+//// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////s pcEAA2AJBAAA8BK/AAAAAAAAEAAAAAAABgAAXA0AAA4AYmpiastzy3MAAAAAAAAAAAAAAAAAAAAA AAAJBBYANBoAAKkZAQCpGQEAXAUAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD//w8AAAAA AAAAAAD//w8AAAAAAAAAAAD//w8AAAAAAAAAAAAAAAAAAAAAAKQAAAAAAHAEAAAAAAAAcAQAAHAE AAAAAAAAcAQAAAAAAABwBAAAAAAAAHAEAAAAAAAAcAQAABQAAAAAAAAAAAAAAIQEAAAAAAAADAoA AAAAAAAMCgAAAAAAAAwKAAAAAAAADAoAABwAAAAoCgAAFAAAAIQEAAAAAAAAhpMAAGgBAABICgAA AAAAAEgKAAAAAAAASAoAAAAAAABICgAAAAAAAEgKAAAAAAAASAoAAAAAAABICgAAAAAAAEgKAAAA AAAAKZMAAAIAAAArkwAAAAAAACuTAAAAAAAAK5MAAAAAAAArkwAAAAAAACuTAAAAAAAAK5MAAAAA AADulAAAaAIAAFaXAACAAAAAK5MAABUAAAAAAAAAAAAAAAAAAAAAAAAAcAQAAAAAAABICgAAAAAA AAAAAAAAAAAAAAAAAAAAAABICgAAAAAAAEgKAAAAAAAASAoAAAAAAABICgAAAAAAACuTAAAAAAAA AAAAAAAAAABwBAAAAAAAAHAEAAAAAAAASAoAAAAAAAAAAAAAAAAAAEgKAAAAAAAAQJMAABYAAABm CwAAAAAAAGYLAAAAAAAAZgsAAAAAAABICgAACgAAAHAEAAAAAAAASAoAAAAAAABwBAAAAAAAAEgK AAAAAAAAKZMAAAAAAAAAAAAAAAAAAGYLAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Content-Type must have two identifiers • A content type and a subtype • Separated by a slash Figure 26.5

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

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

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

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

Electronic Mail (SMTP, POP, IMAP, MIME)