wrote up documentation on how the MIME stuff works now

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@556 7612ce4b-ef26-0410-bec9-ea0150e637f0

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+mime.txt
+by Luke Ehresman
+June 22, 2000 - Last updated: June 22, 2000
+
+Who should read this?
+
+   The intended audience for this document are people who want to understand how
+   the MIME code works.  This is a technical documentation of how mime.php 
+   works and how it parses a MIME encoded message.
+
+
+Object Structure
+
+   There are two objects that are used: "message" and "msg_header".  here is a
+   brief overview of what each object contains.
+
+   msg_header
+      Contains variables for all the necessary parts of the header of a
+      message.  This includes (but is not limited to) the following:  to, from,
+      subject, type (type0), subtype (type1), filename ...
+      
+   message
+      This contains the structure for the message.  It contains two parts:
+      $header and $entities[].  $header is of type msg_header, and $entities[]
+      is an array of type $message.  The $entities[] array is optional.  If
+      it does not exist, then we are at a leaf node, and have an actual
+      attachment (entity) that can be displayed.  Here is a tree view of how
+      this object functions.
+
+      header
+      entities
+         |
+         +--- header
+         |
+         +--- header
+         |    entities
+         |       |
+         |       +--- header
+         |       |
+         |       +--- header
+         |
+         +--- header
+
+
+Getting the Structure
+
+   Previously (version 0.4 and below), SquirrelMail handled all the parsing of
+   the email message.  It would read the entire message in, search for 
+   boundaries, and created an array similar to the $message object discribed
+   above.  This was very inefficient.
+
+   Currently, all the parsing of the body of the message takes place on the
+   IMAP server itself.  According to RFC 2060 section 7.4.2, we can use the
+   BODYSTRUCTURE function which will return the structure of the body (imagine
+   that).  It goes into detail of how the bodystructure should be formatted,
+   and we have based our new MIME support on this specification.  
+
+   A simple text/plain message would have a BODYSTRUCTURE similar to the 
+   following:
+
+      ("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)
+
+   A more complicated multipart message with an attachment would look like:   
+
+      (("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)("TEXT" 
+      "PLAIN" ("CHARSET" "US-ASCII" "NAME" "cc.diff") 
+      "<960723163407.20117h@cac.washington.edu>" "Compiler diff" "BASE64" 
+      4554 73) "MIXED"))
+   
+   Our MIME functionality implements different functions that recursively
+   run through this text and parses out the structure of the message.  If you
+   want to learn more about how the structure of a message is returned with
+   the BODYSTRUCTURE function, please see RFC 2060 section 7.4.2.
+
+   NOTE:  SquirrelMail passes the MIME Torture Test written by Mark
+          Crispin (author of the IMAP protocol).  This message is crazy!  It
+          has about 30 parts nested inside each other.  A very good test, 
+          and SquirrelMail passed it.  It can be found here:
+
+          ftp://ftp.lysator.liu.se/mirror/unix/imapd/mime/torture-test.mbox
+
+Getting the Body
+
+   Once all of the structure of the message has been read into the $message
+   object, we then need to display the body of one entity.  There are a number
+   of ways we decide which entity to display at a certain time, and I won't go
+   into that here.  
+
+   Each entity has its own ID.  Entity IDs look something like "1.2.1", or 
+   "4.1", or just "2".  You can find a detailed description of how entities
+   should be identified by reading RFC 2060 section 6.4.5.  To fetch the body
+   of a particular entity, we use the function "BODY[<section>]".  For 
+   instance, if we were wanting to return entity 1.2.1, we would send the
+   IMAP server the command: "a001 FETCH <msg_id> BODY[1.2.1]".
+
+   This returns a string of the entire body.  Based upon what is in the header,
+   we may need to decode it or do other things to it.
+
+
+Closing Notes  
+
+   That is basically how it works.  There is a variable in mime.php called
+   $debug_mime that is defined at the top of that file.  If you set it to true,
+   it will output all kinds of valuable information while it tries to decode
+   the MIME message.
+
+   The code in mime.php is pretty well documented, so you might want to poke
+   around there as well to find out more details of how this works.
+   
+   If you have questions about this, please direct them to our mailing list:
+   squirrelmail-list@sourceforge.net