On the Internet Printing Protocol list the following issues were
raised in the context of a discussion of whether or not IPP should
use HTTP/1.1 as a 'transport' protocol, defining IPP operations as
usage conventions and extentions.
I will attempt to respond based on my understanding; if anyone else
on http-wg disagrees, please correct or amplify.
IPP> We discussed how MIME differs from the HTTP MIME-like
IPP> protocol. There was a concern that the HTTP version of MIME
IPP> doesn't support Content-Transfer-Encoding, though we think that
IPP> we probably could add such an entity-header as an extension and
IPP> support it through a CGI script if necessary (though we aren't
IPP> sure about this).
Content-Transfer-Encoding is not supported by HTTP because it isn't
needed; the HTTP transport is 8 bit clean.
IPP> There was also a question about how to send binary data in a
IPP> multipart/mixed, especially in the chunked case because there is
IPP> no way to know if a CRLF in the midst of binary data is really a
IPP> CRLF. Thus it is hard to find the boundary string.
CRLF is what it is - if it is in the binary data and is followed by
the specified boundary string and another CRLF then you are at the
end of the body part. What is the question?
As I understand it, the selection of a boundary string in MIME is
already 'probabilistic'; the sender is responsible for choosing a
string that 'probably' won't appear in the body (I do not claim to
be an authority on MIME).
IPP> We believe that chunked applies to the entire multipart/mixed
IPP> entity and cannot be used for one of the sub-entities alone.
IPP> Thus there is no length to mark the boundary of a sub-entity.
Correct; the 'Transfer-Encoding: chunked' applies to all of the HTTP
message body. For completeness, my companys' server implementation
does support chunked encoding of the entire multipart/* body part,
but we think it doesn't make much sense (because it is redundant) so
that support may be compiled out to save code.
Scott Lawrence EmWeb Embedded Server <lawrence at agranat.com>
Agranat Systems, Inc. Engineering http://www.agranat.com/