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Network Working Group                                      G. NeufeldRequest for Comments: 2369                                      NistoCategory: Standards Track                                     J. Baer                                                 SkyWeyr Technologies                                                            July 1998The Use of URLs as Meta-Syntax for Core Mail List Commandsand their Transport through Message Header FieldsStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1998).  All Rights Reserved.Abstract   The mailing list command specification header fields are a set of   structured fields to be added to email messages sent by email   distribution lists. Each field typically contains a URL (usually   mailto [RFC2368]) locating the relevant information or performing the   command directly. The three core header fields described in this   document are List-Help, List-Subscribe, and List-Unsubscribe.   There are three other header fields described here which, although   not as widely applicable, will have utility for a sufficient number   of mailing lists to justify their formalization here. These are   List-Post, List-Owner and List-Archive.   By including these header fields, list servers can make it possible   for mail clients to provide automated tools for users to perform list   functions. This could take the form of a menu item, push button, or   other user interface element. The intent is to simplify the user   experience, providing a common interface to the often cryptic and   varied mailing list manager commands.   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 inRFC 2119.Neufeld & Baer              Standards Track                     [Page 1]

RFC 2369                  URLs as Meta-Syntax                  July 19981. Introduction   This is a proposal for additional header fields to be added to email   messages sent by email distribution lists. The content of each new   field is typically a URL - usually mailto [RFC2368] - which locates   the relevant information or performs the command directly. MTAs   generating the header fields SHOULD usually include a mailto based   command, in addition to any other protocols used, in order to support   users who do not have access to non-mail-based protocols.   Implementing these fields will be optional. Significant functionality   and convenience can be gained by including them, however. Many list   managers, especially as the proposal first gains acceptance, MAY   choose to implement only one or two of the fields.  The List-Help   field is the most useful individual field since it provides an access   point to detailed user support information, and accommodates almost   all existing list managers command sets. The List-Subscribe and   List-Unsubscribe fields are also very useful, but cannot describe   some list manager syntaxes at this time (those which require variable   substitution). Seeappendix A.5 for an explanation.   The description of command syntax provided by the fields can be used   by mail client applications to provide simplified and consistent user   access to email distribution list functions. This could take the form   of menu items, push buttons, or other user interface elements.  The   intent is to simplify the user experience, providing a common   interface to the often cryptic and varied mailing list manager   commands.   Consideration has been given to avoiding the creation of too many   fields, while at the same time avoiding the overloading of individual   fields and keeping the syntax clear and simple.   The use of these fields does not remove the requirement to support   the -Request command address for mailing lists [RFC2142].2. The Command Syntax   The list header fields are subject to the encoding and character   restrictions for mail headers as described in [RFC822]. Additionally,   the URL content is further restricted to the set of URL safe   characters [RFC1738].   The contents of the list header fields mostly consist of angle-   bracket ('<', '>') enclosed URLs, with internal whitespace being   ignored. MTAs MUST NOT insert whitespace within the brackets, but   client applications should treat any whitespace, that might be   inserted by poorly behaved MTAs, as characters to ignore.Neufeld & Baer              Standards Track                     [Page 2]

RFC 2369                  URLs as Meta-Syntax                  July 1998   A list of multiple, alternate, URLs MAY be specified by a comma-   separated list of angle-bracket enclosed URLs. The URLs have order of   preference from left to right. The client application should use the   left most protocol that it supports, or knows how to access by a   separate application. By this mechanism, protocols like http may be   specified while still providing the basic mailto support for those   clients who do not have access to non-mail protocols. The client   should only use one of the available URLs for a command, using   another only if the first one used failed.   The use of URLs allows for the use of the syntax with existing URL   supporting applications. As the standard for URLs is extended, the   list header fields will gain the benefit of those extensions.   Additionally, the use of URLs provides access to multiple transport   protocols (such as ftp and http) although it is expected that the   "mailto" protocol [RFC2368] will be the focus of most use of the list   header fields. Use of non-mailto protocols should be considered in   light of those users who do not have access to the specified   mechanism (those who only have email - with no web access).   Command syntaxes requiring variable fields to be set by the client   (such as including the user's email address within a command) are not   supported by this implementation. However, systems using such   syntaxes SHOULD still take advantage of the List-Help field to   provide the user with detailed instructions as needed or - perhaps   more usefully - provide access to some form of structured command   interface such as an HTML-based form.   The additional complications of supporting variable fields within the   command syntax was determined to be too difficult to support by this   protocol and would compromise the likelihood of implementation by   software authors.   To allow for future extension, client applications MUST follow the   following guidelines for handling the contents of the header fields   described in this document:   1) Except where noted for specific fields, if the content of the      field (following any leading whitespace, including comments)      begins with any character other than the opening angle bracket      '<', the field SHOULD be ignored.   2) Any characters following an angle bracket enclosed URL SHOULD be      ignored, unless a comma is the first non-whitespace/comment      character after the closing angle bracket.Neufeld & Baer              Standards Track                     [Page 3]

RFC 2369                  URLs as Meta-Syntax                  July 1998   3) If a sub-item (comma-separated item) within the field is not an      angle-bracket enclosed URL, the remainder of the field (the      current, and all subsequent, sub-items) SHOULD be ignored.3. The List Header Fields      This document presents header fields which will provide the      command syntax description for the 'core' and key secondary      functions of most email distribution lists. The fields implemented      on a given list SHOULD be included on all messages distributed by      the list (including command responses to individual users), and on      other messages where the message clearly applies to one distinct      list. There MUST be no more than one of each field present in any      given message.      These fields MUST only be generated by mailing lists, not end      users.3.1. List-Help      The List-Help field is the most important of the header fields      described in this document. It would be acceptable for a list      manager to include only this field, since by definition it SHOULD      direct the user to complete instructions for all other commands.      Typically, the URL specified would request the help file, perhaps      incorporating an HTML form for list commands, for the list, and      alternatively provide access to an instructive website.      Examples:     List-Help: <mailto:list@host.com?subject=help> (List Instructions)     List-Help: <mailto:list-manager@host.com?body=info>     List-Help: <mailto:list-info@host.com> (Info about the list)     List-Help: <http://www.host.com/list/>, <mailto:list-info@host.com>     List-Help: <ftp://ftp.host.com/list.txt> (FTP),         <mailto:list@host.com?subject=help>3.2. List-Unsubscribe   The List-Unsubscribe field describes the command (preferably using   mail) to directly unsubscribe the user (removing them from the list).   Examples:     List-Unsubscribe: <mailto:list@host.com?subject=unsubscribe>     List-Unsubscribe: (Use this command to get off the list)         <mailto:list-manager@host.com?body=unsubscribe%20list>     List-Unsubscribe: <mailto:list-off@host.com>Neufeld & Baer              Standards Track                     [Page 4]

RFC 2369                  URLs as Meta-Syntax                  July 1998     List-Unsubscribe: <http://www.host.com/list.cgi?cmd=unsub&lst=list>,         <mailto:list-request@host.com?subject=unsubscribe>3.3. List-Subscribe   The List-Subscribe field describes the command (preferably using   mail) to directly subscribe the user (request addition to the list).   Examples:     List-Subscribe: <mailto:list@host.com?subject=subscribe>     List-Subscribe: <mailto:list-request@host.com?subject=subscribe>     List-Subscribe: (Use this command to join the list)         <mailto:list-manager@host.com?body=subscribe%20list>     List-Subscribe: <mailto:list-on@host.com>     List-Subscribe: <http://www.host.com/list.cgi?cmd=sub&lst=list>,         <mailto:list-manager@host.com?body=subscribe%20list>3.4. List-Post   The List-Post field describes the method for posting to the list.   This is typically the address of the list, but MAY be a moderator, or   potentially some other form of submission. For the special case of a   list that does not allow posting (e.g., an announcements list), the   List-Post field may contain the special value "NO".   Examples:     List-Post: <mailto:list@host.com>     List-Post: <mailto:moderator@host.com> (Postings are Moderated)     List-Post: <mailto:moderator@host.com?subject=list%20posting>     List-Post: NO (posting not allowed on this list)3.5. List-Owner   The List-Owner field identifies the path to contact a human   administrator for the list. The URL MAY contain the address of a   administrator for the list, the mail system administrator, or any   other person who can handle user contact for the list. There is no   need to specify List-Owner if it is the same person as the mail   system administrator (postmaster).   Examples:     List-Owner: <mailto:listmom@host.com> (Contact Person for Help)     List-Owner: <mailto:grant@foo.bar> (Grant Neufeld)     List-Owner: <mailto:josh@foo.bar?Subject=list>Neufeld & Baer              Standards Track                     [Page 5]

RFC 2369                  URLs as Meta-Syntax                  July 19983.6. List-Archive   The List-Archive field describes how to access archives for the list.   Examples:     List-Archive: <mailto:archive@host.com?subject=index%20list>     List-Archive: <ftp://ftp.host.com/pub/list/archive/>     List-Archive: <http://www.host.com/list/archive/> (Web Archive)4. Supporting Nested Lists   A list that is a sublist for another list in a nested mailing list   hierarchy will need to modify some of the List- header fields, while   leaving others as the parent list set them.   Sublists SHOULD remove the parent list's List-Help, List-Subscribe,   List-Unsubscribe and List-Owner fields, and SHOULD insert their own   versions of those fields.   If the sublist provides its own archive, it SHOULD replace the List-   Archive with its own. Otherwise, it MUST leave the List-Archive field   untouched.   Dependant on how postings to the list are handled, the sublist MAY   replace the List-Post field. The appropriateness of whether to   replace List-Post is left to the determination of the individual list   managers. If the intention is that postings should be distributed to   all members of the primary list, List-Post should not be changed by a   sublist in such a way that postings will be distributed only to   members of the sublist.5. Security Considerations   There are very few new security concerns generated with this   proposal. Message headers are an existing standard, designed to   easily accommodate new types. There may be concern with multiple   fields being inserted or headers being forged, but these are problems   inherent in Internet email, not specific to the protocol described in   this document. Further, the implications are relatively harmless.   Mail list processors should not allow any user-originated list header   fields to pass through to their lists, lest they confuse the user and   have the potential to create security problems.   On the client side, there may be some concern with posts or commands   being sent in error. It is required that the user have a chance to   confirm any action before it is executed. In the case of mailto, itNeufeld & Baer              Standards Track                     [Page 6]

RFC 2369                  URLs as Meta-Syntax                  July 1998   may be appropriate to create the correctly formatted message without   sending it, allowing the user to see exactly what is happening and   giving the user the opportunity to approve or discard the message   before it is sent.   All security considerations for the use of URLs [RFC1738] apply   equally to this protocol. Mail client applications should not support   list header field URLs which could compromise the security of the   user's system. This includes the "file://" URL type which could   potentially be used to trigger the execution of a local application   on some user systems.6. Acknowledgements   The numerous participants of the List-Header [5], ListMom-Talk [6],   List-Managers and MIDA-Mail mailing lists contributed much to the   formation and structure of this document.   Keith Moore <moore@cs.utk.edu> and Christopher Allen   <ChristopherA@consensus.com> provided guidance on the standards   process.Neufeld & Baer              Standards Track                     [Page 7]

RFC 2369                  URLs as Meta-Syntax                  July 1998A. Background Discussion   This proposal arose from discussions started on the ListMom-Talk   Discussion List [6]. When the discussion reached a sufficient level,   a separate list was formed for discussing this proposal, the List   Headers Mail List [5] for deeper discussion. We have included   summaries of key issues raised, in order to show some of the   alternatives examined and reasons for our decisions.A.1. Multiple header fields vs. a single header field   Use of a single header field for transporting command meta-syntax was   rejected for a number of reasons.   Such a field would require the creation of a new meta-syntax in order   to describe the list commands (as opposed to the use of the widely   deployed URL syntax which was chosen for this implementation).  Every   additional layer of complexity and newness reduces the likelihood of   actual implementation because it will require additional work to   support. Also, by using the existing URL syntax, we can profit from   the end users' knowledge of that syntax and ability to use it even if   their client applications do not support the list header fields.   Restricting the transport of meta-syntax to the use of a single   header field also introduces complications with header field size   limitations. Most individual commands can easily be described in a   single line, but describing a multitude of commands can take up many   lines in the field and runs a greater risk of being modified by an   existing server on route.   The client implementation is also easier with multiple fields, since   each command can be supported and implemented individually,   completely independent of the others. Thus, some list managers or   mail clients can choose to implement a subset of the fields based on   the specific needs of their individual lists.   Finally, the format described in this document is simple and well   recognized, which reduces the chances of errors in implementation and   parsing.A.2. URLs vs. parameter lists   URLs are already an established syntax which is flexible, well-   defined, and in wide spread use. As its definition matures and   expands, the abilities of the list fields will grow as well, without   requiring modification of this proposal. URLs are well prepared to   handle future protocols and developments, and can easily describe the   different existing access protocols such as mailto, http and ftp.Neufeld & Baer              Standards Track                     [Page 8]

RFC 2369                  URLs as Meta-Syntax                  July 1998   Many clients already have functionality for recognizing, parsing, and   evaluating URLs, either internally or by passing the request to a   helper application. This makes implementation easier and more   realistic. As an example, this existing support for URL parsing   allowed us to add prototype list header functionality to existing   mail clients (Eudora and Emailer for the Macintosh) without modifying   their source code.A.3. Why not just create a standard command language?   A standard command language, supported by all email list services,   would go a long way to reducing the problems of list access that   currently plague existing services. It would reduce the amount of   learning required by end users and allow for a number of common   support tools to be developed.   However, such standardization does pose problems in the areas of   multi-lingual support and the custom needs of individual mailing   lists. The development of such a standard is also expected to be met   with a slow adoption rate by software developers and list service   providers.   These points do not preclude the development of such a standard (in   fact, it would suggest that we should start sooner rather than   later), but we do need a solution that can be widely supported by the   current list services.   We can support most existing list manager command syntaxes without a   standard command language. By using URLs, we allow alternate access   methods a standard command language probably wouldn't enable, such as   web based control.   Finally, client support for a standard command language is not at all   clear or necessarily simple to implement. The variety and large   number of commands existing today would require complicated user   interfaces which could be confusing and difficult to implement. By   restricting this proposal to the core functions, the client   implementation is much simpler, which significantly increases the   likelihood of implementation (as evidenced by the support already   announced by a number of client and server application authors).A.4. Internationalization   Multilingual support is up to the URL standard. If URLs support it,   then the List- header fields support it. This is another advantage of   using URLs as the building blocks for the list header fields.Neufeld & Baer              Standards Track                     [Page 9]

RFC 2369                  URLs as Meta-Syntax                  July 1998A.5. Variable Substitution   Variables would allow the List- header fields to accommodate nearly   every existing list manager. However, it would immeasurably increase   the complexity of the entire proposal, and possibly involve   redefining the URL standard, or force us to use something more   complicated (and hence more difficult to implement) than URLs to   describe the command syntax.   Parameters would either have to be mandatory (i.e. the user agent   doesn't submit the message if it doesn't know what text to   substitute) or you need a way to say "if you know this parameter, add   its text here; otherwise, do this" where "this" is either: (a)   substitute a constant string, or (b) fail.   The reason you would want a facility like this is because some list   server applications insist on having certain parameters like users'   names, which the user agent might or might not know. e.g. listserv   insists on having a first name and a last name if you supply either   one.   Which could lead to something like the UNIX shell syntax, where   ${foo-bar} means substitute the value of parameter "foo" if "foo" is   defined, else substitute the string "bar". Perhaps $foo would mean   "substitute the value of parameter foo if it is defined, else   substitute the empty string"   This all seems far too complicated for the gains involved, especially   since the use of variables can often be avoided.   The use of variables in the command syntaxes of list services appears   to be lessening and does not, in any case, apply to all commands.   While the unsubscribe and subscribe command header fields may not be   usable by those systems which require the use of variables, the help   field will still provide end users with a consistent point of access   through which they can get support for their use of the list.A.6. Why not use a specialized MIME part instead of header fields?   MIME parts were considered, but because most mail clients currently   either don't support MIME or are not equipped to handle such   specialized parts - such an implementation would result in problems   for end users. It is also not as easy for many list servers to   implement MIME as it is to implement new header fields.   However, we are looking at the design of a MIME part to more fully   describe list command syntax, as well as trying to find ways to get   it supported by the applicable software.Neufeld & Baer              Standards Track                    [Page 10]

RFC 2369                  URLs as Meta-Syntax                  July 1998A.7. Why include a Subscribe command?   Subscribe and Unsubscribe are the key commands needed by almost every   list. Other commands, such as digest mode, are not as widely   supported.   Additionally, users who have unsubscribed (before going on vacation,   or for whatever other reason) may want to resubscribe to a list. Or,   a message may be forwarded/bounced from a subscriber to a non-   subscriber. Or, the user may change addresses and want to subscribe   from their new address. Having the List-Subscribe field available   could certainly help in all these cases.A.8. The Dangers of Header Bloat   At what point are there just too many header fields?  It really   varies on a list by list basis. On some lists, the majority of users   will never be aware of a field unless the client software provides   some alternative user interface to it (akin to the Reply-To field).   On others, the users will often see the header fields of messages and   would be able to recognize the function of the URLs contained within.   The flexibility afforded by the protocol described in this document   (in that the header fields may be individually implemented as deemed   appropriate) provides list administrators with sufficient 'room to   maneuver' to meet their individual needs.Neufeld & Baer              Standards Track                    [Page 11]

RFC 2369                  URLs as Meta-Syntax                  July 1998B. Client ImplementationB.1. Guidelines   For 'mailto' URL based commands, mail client applications may choose   to provide specialized feedback (such as presenting a dialog or   alert), instead of the actual command email message, asking for   command confirmation from the user. The feedback should identify the   message destination and command within a more descriptive   explanation. For example:     "Do you want to send the unsubscription command 'unsubscribe     somelist' to 'somelist-request@some.host.com'?  Sending the command     will result in your removal from the associated list."   If the user has multiple email addresses supported by the mail   client, the client application should prompt the user for which   address to use when subscribing or performing some other action where   the address to use cannot be specifically determined. When   unsubscribing or such, the address that is subscribed should be used,   unless that is not known by the application and cannot be determined   from the message headers.B.2. Implementation Options   The following implementation possibilities are suggested here to give   some idea as to why these new header fields will be useful, and how   they could be supported.   In most cases, it may be helpful to disable the interface for the   commands when not applicable to the currently selected message.B.2.1. Key combinations and command lines   On text based systems which utilize command lines or key   combinations, each field could be implemented as a separate command.   Thus one combination would subscribe the user, another would   unsubscribe, a third request help, etc. The commands would only be   available on messages containing the list header fields.B.2.2. Menu items   On graphical systems which have menus, these commands could take the   form of a menu or sub-menu of items. For example, a "Lists" menu   might appear when viewing messages containing the header fields, with   items named "Subscribe", "Unsubscribe", "Get Help", "Post Message toNeufeld & Baer              Standards Track                    [Page 12]

RFC 2369                  URLs as Meta-Syntax                  July 1998   List", "Contact List Owner" and "Access List Archive". This menu   could be disabled when not applicable to the current message or   disappear entirely.B.2.3. Push Buttons and Pallettes   On graphical window systems, buttons could be placed in the window of   the message, a toolbar, or in a floating pallette of their own. Each   button could correspond to a command, with names "Subscribe",   "Unsubscribe", "Get Help", "Post to List", "List Owner" and   "Archive". These buttons or pallettes could be disabled when not   applicable to the current message or disappear entirely.B.2.4 Feedback to the User   If using a dialog interface (or other feedback element) the client   application MUST include an option for the user to review (and   possibly modify) the message before it is sent. The application may   also find it useful to provide a link to more detailed context-   sensitive assistance about mail list access in general.References   [RFC822] Crocker, D., "Standard for the Format of ARPA            Internet Text Messages", STD 11,RFC 822, August 1982.   [RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill,             "Uniform Resource Locators (URL)"RFC 1738, December 1994.   [RFC2142] Crocker, D., "Mailbox Names for Common Services, Roles and             Functions",RFC 2142, May 1997.   [RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, "The mailto URL             scheme",RFC 2368, July 1998.   [5] "List-Header" Mail list. list-header@list.nisto.com       <URL:http://www.nisto.com/listspec/mail/>       <URL:http://www.nisto.com/listspec/>   [6] "ListMom-Talk" Mail list. listmom-talk@skyweyr.com       <URL:http://cgi.skyweyr.com/ListMom.Home>Neufeld & Baer              Standards Track                    [Page 13]

RFC 2369                  URLs as Meta-Syntax                  July 1998Editors' Addresses   Joshua D. Baer   Box 273   4902 Forbes Avenue   Pittsburgh, PA 15213-3799   USA   EMail: josh@skyweyr.com   Grant Neufeld   Calgary, Alberta   Canada   EMail: grant@acm.org   Web:http://www.nisto.com/Neufeld & Baer              Standards Track                    [Page 14]

RFC 2369                  URLs as Meta-Syntax                  July 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS 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.Neufeld & Baer              Standards Track                    [Page 15]