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Re: MIME boundary question recap

1995-02-10 07:25:35
I should point out that Steve Dorner's example is illegal per the
multipart grammar in RFC 1341, it violates the "not occuring"
restrictions in the body-part nonterminal.  

The multipart grammar in 1521 is just plain broken.  The following
multipart is illegal in it, it violates the "not occuring"
restrictions on body-part:

Content-type: multipart/mixed; boundary="foo"


some text--foo


As another example of brokenness, the multipart grammar in 1521 is
ambiguous.  There are two legal parses of the following multipart:

Content-type: multipart/mixed; boundary="bar"




this can parse as an "epilogue" or as part of a "text/plain" inside
of the first "body-part".



Now, if you change the above example such that the line "other-line"
is instead "--bar", then it has an unambiguous legal parse, where the
line starting "this" is inside the first of two body-parts.  One has
to do an arbitrary amount of lookahead in order to determine the
corrct parse for the line starting "this".

Now, I think it's no big deal to expect composers to pick outer
boundary parameters to be long enough to not have 
``CR LF "--" boundary'' be a substring in their enclosed body-parts.
No matter what we do, composers have to pick boundary parameters so
that some sequence is not a substring in their enclosed body-part.

Requiring parsers to look for something like
``CR LF "--" boundary 0*70(LWSP-char) CR LF''
or even worse ``CR LF "--" boundary 0*1000(LWSP-char) CR LF''
like Keith suggests is going to make life incredibly painful.  I
seriously doubt many people would correctly implement such a parser.

_.John G. Myers         Internet: jgm+(_at_)CMU(_dot_)EDU
                        LoseNet:  ...!seismo!ihnp4!!give!up

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