Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Read offline with no ads or diagram watermarks!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
                     9  TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME
                          9  TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME)

Previous Topic/Section
MIME Message Format Overview, Motivation, History and Standards
Previous Page
Pages in Current Topic/Section
1
234
Next Page
MIME Content-Type Header and Discrete Media: Types, Subtypes and Parameters
Next Topic/Section

MIME Basic Structures and Headers
(Page 1 of 4)

The creators of the Multipurpose Internet Mail Extensions (MIME) standard had a difficult challenge on their hands: how to bring flexibility in the types of data contained in e-mail messages, when RFC 822 said that they had to be comprised of only ASCII text. To accomplish this, they had to exploit the areas of flexibility that had already been put into the existing RFC 822.

There were two such “opportunities” available. The first was the fact that RFC 822 message bodies are allowed to contain any type of ASCII text, as long as lines don't exceed 998 text characters and each line ends with a “CRLF” control code combination. Even though the creators of RFC 822 naturally assumed this ASCII text would be human-readable, there was nothing stopping it from being machine-readable code. The second was the facility built into RFC 822 (and the protocols that use it, such as SMTP) to allow custom “user-defined” header fields to be added to any e-mail message.

The non-specific nature of RFC 822 message bodies forms the basis for how MIME itself works. An e-mail client that supports the MIME standard uses special encoding algorithms that transform non-ASCII information into ASCII form. It then places this set of encoded ASCII characters into the body of the message, as if it had been typed by a user, using one of two special structures.

The ability to add new headers to RFC 822 is used to communicate information about the use of MIME from the sender to the recipient. The devices transporting a MIME message don't care that MIME was used, because they don't pay attention to the contents of the message body. However, when the message reaches its destination, the recipient’s e-mail client program must have some way of knowing that MIME was used, and must also be told how the information in the message was encoded. Otherwise, it might just present the encoded non-ASCII data to the user as ASCII text (which would look like random gibberish!)


Previous Topic/Section
MIME Message Format Overview, Motivation, History and Standards
Previous Page
Pages in Current Topic/Section
1
234
Next Page
MIME Content-Type Header and Discrete Media: Types, Subtypes and Parameters
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.