Much of this binary posting specification deals with the MIME Content-Type header. The goal is for clients to be able to obtain all necessary metadata by examining just one header in the message.
Current practice is to use the Subject header to convey metadata about the file contained in the message. Various formats are used, and user-agents are left to parse them with no real specification, resulting in ugly and imprecise code. yEnc attempts to address this problem with a more detailed specification of the format.
However, the use of the Subject line is essentially a hack, used because it's there, and because clients generally download the overview information upon entering a newsgroup, and the overview includes the Subject line. The Subject line is not well-suited for this purpose. It is intended as free-form text for human consumption, not as a structured and machine-parsed field. This specification eventually will free up the Subject line to be a Subject line by placing the metadata elsewhere.
When a newsreading client enters a group, it typically downloads the overview for that group. In a binary group, especially a multipart binary group, much or most of this data is redundant or otherwise unnecessary. For example, if someone has posted a file in 100 segments, you are downloading redundant information such as the From and Date headers 100 times. The References are generally not used at all, and the Subject (if it didn't contain the necessary metadata) is redundant as well. Xref need not be used unless and until the user marks the message as read, and only if he wants his client to chase crossposts. Since many binary groups have enormous overview files, it is desirable to free the client from needing to do this, resulting in a savings in both time and bandwidth. But even with single-part binaries, such as images, a well-defined method of specifying metadata is desirable.
This specification is fairly basic, and much of it is restating things from the MIME specification. It is intended that it can serve as a starting point for programmers to use for implementing MIME binary posting and downloading in a newsreader (along with reading the appropriate RFCs, of course). In addition, it serves as a base specification which can be extended for other purposes, for example to support multipart binaries.
As an implementation guideline, the headers which should be used in binary posts are specified below, with references to the applicable RFCs where appropriate. In addition, we add a couple of things to Content-Type, which is almost certain to be the most controversial part of this proposal.
The basic MIME Content-Type header, as could be found in use right now, looks like these examples:
Content-Type: text/plain; charset=iso-8859-1
This defines the type of file contained within the MIME entity, with optional additional metadata given as parameters (the "charset" parameter in the above example). The goal of this specification is to add more metadata to this header with additional parameters. For basic single-part binaries, the number of new parameters will be small, but Content-Type should still be used to maintain consistency for clients with the specification for multipart binaries, where we will be defining more metadata parameters.
[Because this document adds additional parameters to existing content types, and the procedure for doing this is not well-documented in the MIME RFCs (that I can find), I would appreciate feedback on how best to do this. Ie, should they begin with an x- prefix until some sort of standardization is achieved? And, because we are basically overloading Content-Type by adding non-type-specific parameters, will it break anything? See below.]
The three additional parameters are defined below).
The first two above parameters are duplicates of the filename parameter of the Content-Disposition header, and of the Content-MD5 header, respectively. The purpose for the duplication is so that clients don't need to retrieve three headers in order to figure out what the user actually wants to download, and so that server implementations need not optimize for retrieval of many extra headers. The information is in fact redundant, but the advantage of having it all in the single header outweighs that, in my opinion.
The name parameter is already in widespread use, and thus should not cause any problems with existing software. However, the other two are new. This is the most dubious part of this proposal, and I am in need of serious feedback on whether this will break anything. I feel that it is important for all the necessary metadata to be in one header, which is why I want to go forward with this in spite of the somewhat less-than-ideal method used.
The Content-MD5 header is described in RFC 1864. Although this header has been defined for some time, its use has not been widespread. However, it can be used to detect corrupt messages, and to detect duplicate files even when the name has been changed. Because identical files will have identical checksums, it can be used to detect two posts containing the same file under different names.
Note that the checksum applies to the original file, before any decoding, and thus should clients attempting to verify using it should decode the file first, calculate the MD5 of the decoded data, and compare that result with the value of this header. This also has the result that the checksum will remain useful even if the file has been re-encoded into base64 or another transfer encoding by a relaying agent.
When a client is attempting to verify the integrity of a decoded file, if the checksum is present in both the Content-MD5 header and in the md5 parameter of the Content-Type header, the Content-MD5 header should take priority.
The Content-Description header is described in RFC 2045, section 8. It is a free-form text field intended to describe the content of the entity. It can be used to provide a description of the file being posted, and can be displayed as such by the client. In particular, a user might examine a list of files, and click on one to obtain more information prior to deciding whether to actually download the full post. Content-Description can be used in that case to provide the user with the information he needs to make the decision.
This header specifies the encoding method used on the file. It is described in RFC 2045, section 6. Currently, when posting binary files, the content of this field will most often be base64. However, if one of the new, smaller encoding schemes described on my transfer-encoding page is used, this is where that will be specified.
The Content-Disposition header is described in RFC 2183. In general, there are two values which commonly appear for this header: inline and attachment. A value of inline means that the file is intended to be viewed directly within the message, while a value of attachment means it is intended to be offered for the user to save to disk (though clients do often also display attachments inline where they feel it is appropriate).
The filename parameter specifies the filename of the attachment. If this is present, an implementation can suggest it as a filename under which to save the attachment. However, software should not blindly use the value of this parameter when saving to disk! It should be examined first to ensure that it is suitable for the target system, and that it presents no security risks. In particular, any path information should be removed and ignored.
With the suggestion that the filename be present in the Content-Type header, if it is specified in both places with different values, the value in Content-Disposition should take priority.
This header simply contains the value 1.0, and must be present.
At this point you may be wondering what this gains us. Well, in addition to the ability to use new transfer-encodings, we get machine-readable metadata for binary posts.
As mentioned previously, when a client enters a newsgroup, it typically downloads the full overview for the group. This can be a very large amount of data and can take a long time. In a binary group, a user may only be interested in seeing what is available in the group, and may not care about anything else unless something looks interesting.
With posts in this format, a client could alternatively scan a newsgroup as follows. First, issue the GROUP command as normal. But, instead of doing an XOVER on all the messages (or all the unread messages, if the client caches overviews), it could do an XHDR to grab only the Content-Type headers for the new messages. At this point, in a perfect Usenet where everyone is posting in MIME according to this specification, the client how has all the information it needs to present the user with a list of files available in the group.
Now, the user could, for example, tell the client he wants more information about a certain file. At that point, the client could use XOVER or even HEAD to grab all the available information about the file, including the identity of the poster (the From header), the Content-Description, etc. In the case of a multipart post, the client need only do this on one message in the series.
This can drastically reduce the amount of data that must be downloaded, and can make the process of scanning newsgroups for items of interest much faster. As with much of this specification, this becomes far more significant when we extend it to support multipart binaries.
In this imperfect Usenet, in particular during the transition period, clients could still reduce the amount of data downloaded by also using XHDR to grab the Subject lines to be parsed the old-fashioned way. In the beginning, when few posts are being made according to this specification, clients might choose to continue to simply use the old method until the advantage becomes greater.
Files may sometimes be posted as a MIME multipart message (here, I'm not talking about multipart posts consisiting of several articles, but rather a single post with a root Content-Type of multipart/mixed. Such a post might contain two leaf entities, the first being a text/plain part containing a description or other information, and the second being the actual file which was the purpose of the post. In this case, the metadata about the file is currently "hidden" within the body of the message, and using the current methods, cannot be obtained by the client from just the headers.
In a case where a single attached file is the main purpose of the post, and the other parts are incidental (a description, a signature, etc), it is desirable to have a way for a client to obtain the metadata for this file using only the message headers. When posting such a message, we can use multipart/related (defined in RFC 2387) rather than multipart/mixed.
In a post of this type, the "root" object (as defined for multipart/related) is the entity which is the main purpose of the post. In the case of a post containing an image part, and a text part with a short description of the image, the image part would be the root entity.
A posting client would create the message as multipart/related (rather than multipart/mixed). Each entity within the message would have a Content-ID. The parameters of the Content-Type header would be created as follows.
No means of including the filename in the header is specified. This is unfortunate, and could be changed in a future revision.
Unfortunately, this method of centering on the Content-Type header does create one unfortunate drawback: a character-set problem. With the filename specified in the header, we are functionally limited to us-ascii characters in the filenames in order to ensure consistent operation. There is no way to specify the character set of message headers, and the encoding described in RFC 2047 cannot be used in this context.
This problem will solve itself before very long. The next version of the Usenet article format standard will specify that headers use UTF-8. Until that happens, it looks like we will be stuck with the same character-set problems that we have had all along.
The only current way to specify filenames with non-ascii characters in a reliable manner would be to have them in a text/plain entity within the message body, using MIME to define the character set of the entity. This is clearly even worse than the present character set problem. However, this is hardly a new problem, so I don't consider the failure to solve it a show-stopper. (In particular, even if the current Subject header hacks were to be continued, my reading of the spec is that 2047 encoding would not be appropriate in that context either, so I see no loss in even potential functionality.)
RFC 2231 specifies a means of encoding non-ascii characters into structured MIME headers. While this would appear to be the right solution to the problem, I am only aware of a few newsreaders which support it.
What the above leaves us with is a simple specification which can be extended for a variety of purposes, including posting multipart binaries and media-specific enhancements.