Question

I’m trying to implement a basic MIME parser for the multipart/related in C++/Qt.

So far I’ve been writing some basic parser code for headers, and I’m reading the RFCs to get an idea how to do everything as close to the specification as possible. Unfortunately there is a part in the RFC that confuses me a bit:

From RFC882 Section 3.1.1:

Each header field can be viewed as a single, logical line of
ASCII characters, comprising a field-name and a field-body.
For convenience, the field-body portion of this conceptual
entity can be split into a multiple-line representation; this
is called "folding". The general rule is that wherever there
may be linear-white-space (NOT simply LWSP-chars), a CRLF
immediately followed by AT LEAST one LWSP-char may instead be
inserted. Thus, the single line

Alright, so I simply parse a header field and if a CRLF follows with linear whitespace, I simply concat those in a useful manner to result in a single header line. Let’s proceed…

From RFC2045 Section 5.1:

In the Augmented BNF notation of RFC 822, a Content-Type header field
value is defined as follows:

 content := "Content-Type" ":" type "/" subtype
            *(";" parameter)
            ; Matching of media type and subtype
            ; is ALWAYS case-insensitive.

[…]

 parameter := attribute "=" value

 attribute := token
              ; Matching of attributes
              ; is ALWAYS case-insensitive.

 value := token / quoted-string

 token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
             or tspecials>

Okay. So it seems if you want to specify a Content-Type header with parameters, simply do it like this:

Content-Type: multipart/related; foo=bar; something=else

… and a folded version of the same header would look like this:

Content-Type: multipart/related;
    foo=bar;
    something=else

Correct? Good. As I kept reading the RFCs, I came across the following in RFC2387 Section 5.1 (Examples):

 Content-Type: Multipart/Related; boundary=example-1
         start="<[e-mail]>";
         type="Application/X-FixedRecord"
         start-info="-o ps"

 --example-1
 Content-Type: Application/X-FixedRecord
 Content-ID: <[e-mail]>

 [data]
 --example-1
 Content-Type: Application/octet-stream
 Content-Description: The fixed length records
 Content-Transfer-Encoding: base64
 Content-ID: <[e-mail]>

 [data]

 --example-1--

Hmm, this is odd. Do you see the Content-Type header? It has a number of parameters, but not all have a ";" as parameter delimiter.

Maybe I just didn’t read the RFCs correctly, but if my parser works strictly like the specification defines, the type and start-info parameters would result in a single string or worse, a parser error.

Guys, what’s your thought on this? Just a typo in the RFCs? Or did I miss something?

Thanks!

Answer 1

It is a typo in the examples. Parameters must always be delimited with semicolons correctly, even when folded. The folding is not meant to change the semantics of a header, only to allow for readability and to account for systems that have line length restrictions.