[squirrelmail.git] / doc / mime.txt
1 SquirrelMail MIME Support Introduction
2 ======================================
4 The intended audience for this document are people who want to understand how
5 the MIME code works. This is a technical documentation of how mime.php
6 works and how it parses a MIME encoded message.
9 Object Structure
10 ----------------
11 There are two objects that are used: "message" and "msg_header". here is a
12 brief overview of what each object contains.
14 msg_header
15 Contains variables for all the necessary parts of the header of a
16 message. This includes (but is not limited to) the following: to, from,
17 subject, type (type0), subtype (type1), filename ...
19 message
20 This contains the structure for the message. It contains two parts:
21 $header and $entities[]. $header is of type msg_header, and $entities[]
22 is an array of type $message. The $entities[] array is optional. If
23 it does not exist, then we are at a leaf node, and have an actual
24 attachment (entity) that can be displayed. Here is a tree view of how
25 this object functions.
27 header
28 entities
29 |
30 +--- header
31 |
32 +--- header
33 | entities
34 | |
35 | +--- header
36 | |
37 | +--- header
38 |
39 +--- header
42 Getting the Structure
43 ---------------------
44 Previously (version 0.4 and below), SquirrelMail handled all the parsing of
45 the email message. It would read the entire message in, search for
46 boundaries, and created an array similar to the $message object discribed
47 above. This was very inefficient.
49 Currently, all the parsing of the body of the message takes place on the
50 IMAP server itself. According to RFC 2060 section 7.4.2, we can use the
51 BODYSTRUCTURE function which will return the structure of the body (imagine
52 that). It goes into detail of how the bodystructure should be formatted,
53 and we have based our new MIME support on this specification.
55 A simple text/plain message would have a BODYSTRUCTURE similar to the
56 following:
58 ("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)
60 A more complicated multipart message with an attachment would look like:
62 (("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)("TEXT"
63 "PLAIN" ("CHARSET" "US-ASCII" "NAME" "cc.diff")
64 "<960723163407.20117h@cac.washington.edu>" "Compiler diff" "BASE64"
65 4554 73) "MIXED"))
67 Our MIME functionality implements different functions that recursively
68 run through this text and parses out the structure of the message. If you
69 want to learn more about how the structure of a message is returned with
70 the BODYSTRUCTURE function, please see RFC 2060 section 7.4.2.
72 NOTE: SquirrelMail passes the MIME Torture Test written by Mark
73 Crispin (author of the IMAP protocol). This message is crazy! It
74 has about 30 parts nested inside each other. A very good test,
75 and SquirrelMail passed it. It can be found here:
77 ftp://ftp.lysator.liu.se/mirror/unix/imapd/mime/torture-test.mbox
79 Getting the Body
80 ----------------
81 Once all of the structure of the message has been read into the $message
82 object, we then need to display the body of one entity. There are a number
83 of ways we decide which entity to display at a certain time, and I won't go
84 into that here.
86 Each entity has its own ID. Entity IDs look something like "1.2.1", or
87 "4.1", or just "2". You can find a detailed description of how entities
88 should be identified by reading RFC 2060 section 6.4.5. To fetch the body
89 of a particular entity, we use the function "BODY[<section>]". For
90 instance, if we were wanting to return entity 1.2.1, we would send the
91 IMAP server the command: "a001 FETCH <msg_id> BODY[1.2.1]".
93 This returns a string of the entire body. Based upon what is in the header,
94 we may need to decode it or do other things to it.
97 Closing Notes
98 -------------
99 That is basically how it works. There is a variable in mime.php called
100 $debug_mime that is defined at the top of that file. If you set it to true,
101 it will output all kinds of valuable information while it tries to decode
102 the MIME message.
104 The code in mime.php is pretty well documented, so you might want to poke
105 around there as well to find out more details of how this works.
107 If you have questions about this, please direct them to our mailing list:
108 squirrelmail-list@sourceforge.net