INTERNET-DRAFT Eric A. Hall Document: draft-hall-mime-app-mbox-03.doc January 2005 Expires: July, 2005 Category: Standards-Track The APPLICATION/MBOX Media-Type Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This memo requests that the application/mbox media-type be authorized for allocation by the IESG, according to the terms specified in RFC 2048 [RFC2048]. This memo also defines a default format for the mbox database, which must be supported by all conformant implementations. Internet Draft draft-hall-mime-app-mbox-03.doc January 2005 1. Background and Overview UNIX-like operating systems have historically made widespread use of "mbox" database files for a variety of local email purposes. In the common case, mbox files store linear sequences of one or more electronic mail messages, with local email clients treating the database as a kind of "folder" of email messages. mbox databases are also commonly used by a variety of other messaging tools, such as mailing list programs, archival and filtering utilities, email servers, and other applications. In recent years, mbox databases have also become common on a large number of non-UNIX computing platforms, for similar kinds of purposes. The increased pervasiveness of these files has led to an increased demand for a standardized, network-wide interchange of these files as discrete database objects. In turn, this dictates a need for a media-type definition for mbox files in general, which is the subject and purpose of this memo. 2. About the mbox Database The mbox database format is not documented in an authoritative specification, but instead exists as a well-known output format, or is only anecdotally documented, or is only authoritatively documented for a specific platform or tool. As a result, the mbox database has been adapted and mutated by various systems and utilities over the years, and does not exist in a singular form across all messaging platforms. In general, mbox files typically contain a linear sequence of electronic mail messages. Each message begins with a separator line that identifies the message sender, and also identifies the date and time at which the message was received by the final recipient (either the last-hop system in the transfer path, or the system which serves as the recipient's mailstore). The end of the database is usually recognizable by the absence of any additional data, or by the presence of an end-of-file marker. The structure of the separator lines vary somewhat across implementations, but almost always contain the exact character sequence of "From", followed by a single space character, an email address of some kind, another space character, a timestamp sequence of some kind, and an end-of-line marker. The exact structure of the email address, the timestamp, the end-of-line marker, and even the whole separator line, are all known to differ Hall I-D Expires: July 2005 [page 2]
Internet Draft draft-hall-mime-app-mbox-03.doc January 2005 across implementations. For example, the email address can reflect any addressing syntax which has ever been used on any system in all of history (specifically including addressing forms which are not compatible with Internet mail), and the timestamp sequences can also vary according to system output, while the end-of-line sequences will often reflects platform-specific requirements. Some messaging systems have also been known to append additional information after the timestamp, while the presence of such data is known to break other messaging systems. The exact format of the separator line in use with a particular mbox database can often be determined by examining the first line of the file itself, although implementers are cautioned that multiple mbox files may have been concatenated together, or a single file may have been accessed by multiple messaging clients (each of which uses its own syntax), resulting in different separator line formats being used within a single file. Many implementations are also known to escape message body lines that begin with the character sequence of "From", so as to prevent confusion with overly-liberal parsers that do not search for full separator lines. In the common case, a leading Greater-Than symbol (0x3E) is used for this purpose, with "From" becoming ">From". However, other implementations are known not to escape such lines unless they are immediately preceded by a blank line or if they also appear to contain an email address and a timestamp, while others are known to perform secondary escapes against such lines which are already escaped or quoted. Message data within mbox databases is often undefined, or will reflect on site-specific peculiarities. For example, it is entirely possible for the message body or headers in an mbox database to contain untagged eight-bit character data that implicitly reflects a site-specific default language or locale. Similarly, message data can also contain unencoded eight-bit binary data, or to use encoding formats which represent a specific platform (E.G., BINHEX or UUENCODE sequences). Along these same lines, email addresses can also reflect site-specific messaging platforms or make default assumptions about domain names, or for header fields to reflect a particular platform, and so forth. A comprehensive description of mbox database files on UNIX-like systems can be found at http://qmail.org./man/man5/mbox.html, which should be treated as mostly authoritative for those variations which are otherwise only documented in anecdotal form. However, readers are advised that many other platforms and tools Hall I-D Expires: July 2005 [page 3]
Internet Draft draft-hall-mime-app-mbox-03.doc January 2005 make use of mbox databases, and that there are many more potential variations that can be encountered in the wild. In order to mitigate errors that may arise from such vagaries, this specification defines a "format" parameter to the APPLICATION/MBOX media-type declaration, which can be used to identify the specific kind of mbox database that is being transferred. Furthermore, this specification defines a "default" database format which MUST be supported by implementations that claim to be compliant with this specification, and which is to be used as the implicit format for undeclared APPLICATION/MBOX data objects. Additional format types are to be defined in subsequent specifications. Messaging systems which receive an unknown format SHOULD treat the data as an opaque binary object, as if the data had been declared as APPLICATION/OCTET-STREAM. Refer to Appendix A for a description of the default mbox format. Note that RFC 2046 [RFC2046] defines the multipart/digest media- type for transferring platform-independent message files. Since that specification defines a set of neutral and strict formatting rules, the multipart/digest media-type already facilitates highly- predictable transfer and conversion operations, and as such implementers are strongly encouraged to support and use that media-type where possible. 3. Prerequisites and Terminology Readers of this document are expected to be familiar with the specification for MIME [RFC2045] and MIME-type registrations [RFC2048]. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 4. The APPLICATION/MBOX Media-Type Registration This section provides the media-type registration application (as per [RFC2048]), which will be submitted to IANA after IESG approval of this specification. MIME media type name: application MIME subtype name: mbox Hall I-D Expires: July 2005 [page 4]
Internet Draft draft-hall-mime-app-mbox-03.doc January 2005 Required parameters: none Optional parameters: The "format" parameter identifies the format of the mbox database and the messages contained therein. The default value for the "format" parameter is "default", and refers to the formatting rules defined in Appendix A. mbox databases that do not have a "format" parameter SHOULD be interpreted as having the implicit "format" value of "default". mbox databases that have an unknown value for the "format" parameter SHOULD be treated as opaque data objects, as if the media-type had been specified as APPLICATION/OCTET-STREAM. Encoding considerations: Due to a lack of tagging mechanisms in the traditional mbox format, it will often be necessary to encode the contents of an mbox database prior to transmission. In particular, if an email client receives an mbox database as a message attachment and writes that message (and its attachment) to a local mbox database, the contents of the two database files may become irreversibly intermingled, such that neither database is independently recognizable. In order to avoid these collisions, messaging clients which support this specification MUST encode an mbox database (or at a minimum, the message-separator lines within the database) with a non-transparent encoding (such as BASE64 or Quoted-Printable) prior to transfer. mbox databases transferred over other media SHOULD also be similarly encoded to allow for any subsequent retransmission which may occur. Implementers should also be prepared to encode mbox data locally if non-compliant data is received. Security considerations: mbox data is passive, and does not generally represent a unique or new security threat. However, there is some risk in sharing any kind of data, in that unintentional information may be exposed, and this risk certainly applies to mbox data as well. Interoperability considerations: Due to the lack of any single formal specification for mbox databases, there are a large number of variations between database formats, and it is expected that non-conformant data will be exchanged. As such, it will be necessary for implementations to examine the mbox database in order to successfully import or otherwise utilize the data. Furthermore, different organizations will often have locally- relevant assumptions about message data (such as the character set in use, or the default domain name for unqualified addresses, among others), with no way to carry these local assumptions across Hall I-D Expires: July 2005 [page 5]
Internet Draft draft-hall-mime-app-mbox-03.doc January 2005 the broader Internet. Although the "default" format specified in this memo does not allow for these kinds of vagaries, prior negotiation or agreement between humans will often be needed when non-compliant database formats are used. Published specification: see Appendix A. Applications which use this media type: hundreds of messaging products make use of the mbox database format, in one form or another. Magic number(s): no standard File extension(s): mbox database files sometimes have an ".mbox" extension, but this is not required nor expected. Macintosh File Type Code(s): no standard Person & email address to contact for further information: Eric A. Hall (ehall@ntrg.com) Intended usage: COMMON 5. Security Considerations See the discussion in section 4. 6. IANA Considerations After any IESG approval which may be forthcoming, IANA would be expected to register the application/mbox media-type, using the application provided in section 4 above. 7. Normative References [RFC2046] Freed, N., Borenstein, N., "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [RFC2048] Freed, N., Klensin, J., Postel, J., "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", BCP 13, RFC 2048, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Hall I-D Expires: July 2005 [page 6]
Internet Draft draft-hall-mime-app-mbox-03.doc January 2005 Appendix A. The "default" mbox Database Format In order to improve interoperability among messaging systems, this memo defines a "default" mbox database format, which MUST be supported by all implementations claiming to be compliant with this specification. The "default" mbox database format uses a linear sequence of Internet messages, with each message being immediately prefaced by a separator line, and being terminated by an empty line. More specifically: o Each message within the database MUST follow the syntax and formatting rules defined in RFC 2822 [RFC2822] and its related specifications, with the exception that the canonical mbox database MUST use a single Line-Feed character (0x0A) as the end-of-line sequence, and MUST NOT use a Carriage- Return/Line-Feed pair (NB: this requirement only applies to the canonical mbox database, and is not to be interpreted to override any other specifications). This usage represents the most common historical representation of the mbox database format, and allows for the least amount of conversion. o Messages within the default mbox database MUST consist of seven-bit characters within an eight-bit stream. Eight-bit data within the stream MUST be converted to a seven-bit form (using an appropriate, standardized encoding) and appropriately tagged (with the correct header fields) before the database is transferred. o Message headers and data in the default mbox database MUST be fully-qualified, as per the relevant specification[s]. For example, email addresses in the various header fields MUST have legitimate domain names (as per RFC 2822), while extended characters and encodings MUST be specified in the appropriate location (as per the appropriate MIME specifications), and so forth. o Each message in the mbox database MUST be immediately preceded by a single separator line, which MUST conform to the following syntax: The exact character sequence of "From"; Hall I-D Expires: July 2005 [page 7]
Internet Draft draft-hall-mime-app-mbox-03.doc January 2005 a single Space character (0x20); the email address of the message sender (as obtained from the message envelope or other authoritative source), conformant with the "addr-spec" syntax from RFC 2822; a single Space character; a timestamp indicating the UTC date and time when the message was originally received, conformant with the syntax of the traditional UNIX 'ctime' output sans timezone (note that the use of UTC precludes the need for a timezone indicator); an end-of-line marker. o Each message in the database MUST be terminated by an empty line, containing a single end-of-line marker. o Message body lines that begin with the character sequence of "From" MUST be escaped with a leading Greater-Than symbol (0x3E) prior to transfer. Recipient systems MUST remove these escape sequences upon receipt (NB: this rule does not prohibit such systems from escaping the lines again, as may be needed for storage or subsequent processing). Note that the first message in an mbox database will only be prefaced by a separator line, while every other message will begin with two end-of-line sequences (one at the end of the message itself, and another to mark the end of the message within the mbox database file stream) and a separator line (marking the new message). The end of the database is reached when no more separator lines exist. Acknowledgments Funding for the RFC editor function is currently provided by the Internet Society. Authors' Addresses Eric A. Hall ehall@ntrg.com Hall I-D Expires: December 2004 [page 8]
Internet Draft draft-hall-mime-app-mbox-03.doc January 2005 Full Copyright Statement Copyright (C) The Internet Society 2004. This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Hall I-D Expires: December 2004 [page 9]
