- more minor UI tweaks
[squirrelmail.git] / doc / mime.txt
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cd3dc100 1mime.txt
2by Luke Ehresman
3June 22, 2000 - Last updated: June 22, 2000
4
5Who should read this?
c4809aca 6---------------------
cd3dc100 7 The intended audience for this document are people who want to understand how
8 the MIME code works. This is a technical documentation of how mime.php
9 works and how it parses a MIME encoded message.
10
11
12Object Structure
c4809aca 13----------------
cd3dc100 14 There are two objects that are used: "message" and "msg_header". here is a
15 brief overview of what each object contains.
16
17 msg_header
18 Contains variables for all the necessary parts of the header of a
19 message. This includes (but is not limited to) the following: to, from,
20 subject, type (type0), subtype (type1), filename ...
21
22 message
23 This contains the structure for the message. It contains two parts:
24 $header and $entities[]. $header is of type msg_header, and $entities[]
25 is an array of type $message. The $entities[] array is optional. If
26 it does not exist, then we are at a leaf node, and have an actual
27 attachment (entity) that can be displayed. Here is a tree view of how
28 this object functions.
29
30 header
31 entities
32 |
33 +--- header
34 |
35 +--- header
36 | entities
37 | |
38 | +--- header
39 | |
40 | +--- header
41 |
42 +--- header
43
44
45Getting the Structure
c4809aca 46---------------------
cd3dc100 47 Previously (version 0.4 and below), SquirrelMail handled all the parsing of
48 the email message. It would read the entire message in, search for
49 boundaries, and created an array similar to the $message object discribed
50 above. This was very inefficient.
51
52 Currently, all the parsing of the body of the message takes place on the
53 IMAP server itself. According to RFC 2060 section 7.4.2, we can use the
54 BODYSTRUCTURE function which will return the structure of the body (imagine
55 that). It goes into detail of how the bodystructure should be formatted,
56 and we have based our new MIME support on this specification.
57
58 A simple text/plain message would have a BODYSTRUCTURE similar to the
59 following:
60
61 ("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)
62
63 A more complicated multipart message with an attachment would look like:
64
65 (("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)("TEXT"
66 "PLAIN" ("CHARSET" "US-ASCII" "NAME" "cc.diff")
67 "<960723163407.20117h@cac.washington.edu>" "Compiler diff" "BASE64"
68 4554 73) "MIXED"))
69
70 Our MIME functionality implements different functions that recursively
71 run through this text and parses out the structure of the message. If you
72 want to learn more about how the structure of a message is returned with
73 the BODYSTRUCTURE function, please see RFC 2060 section 7.4.2.
74
75 NOTE: SquirrelMail passes the MIME Torture Test written by Mark
76 Crispin (author of the IMAP protocol). This message is crazy! It
77 has about 30 parts nested inside each other. A very good test,
78 and SquirrelMail passed it. It can be found here:
79
80 ftp://ftp.lysator.liu.se/mirror/unix/imapd/mime/torture-test.mbox
81
82Getting the Body
c4809aca 83----------------
cd3dc100 84 Once all of the structure of the message has been read into the $message
85 object, we then need to display the body of one entity. There are a number
86 of ways we decide which entity to display at a certain time, and I won't go
87 into that here.
88
89 Each entity has its own ID. Entity IDs look something like "1.2.1", or
90 "4.1", or just "2". You can find a detailed description of how entities
91 should be identified by reading RFC 2060 section 6.4.5. To fetch the body
92 of a particular entity, we use the function "BODY[<section>]". For
93 instance, if we were wanting to return entity 1.2.1, we would send the
94 IMAP server the command: "a001 FETCH <msg_id> BODY[1.2.1]".
95
96 This returns a string of the entire body. Based upon what is in the header,
97 we may need to decode it or do other things to it.
98
99
100Closing Notes
c4809aca 101-------------
cd3dc100 102 That is basically how it works. There is a variable in mime.php called
103 $debug_mime that is defined at the top of that file. If you set it to true,
104 it will output all kinds of valuable information while it tries to decode
105 the MIME message.
106
107 The code in mime.php is pretty well documented, so you might want to poke
108 around there as well to find out more details of how this works.
109
110 If you have questions about this, please direct them to our mailing list:
111 squirrelmail-list@sourceforge.net