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Versions: 00 01 rfc2369                                                 
INTERNET-DRAFT                                          Grant Neufeld
draft-baer-listspec-00.txt                      independent developer
Expires August 27, 1997                                Joshua D. Baer
                                                 SkyWeyr Technologies
                                                       March 22, 1997

       The Use of URLs as Meta-Syntax for Core Mail List Commands
           and their Transport through Message Header Fields

Status of this Memo

      This document is an Internet-Draft.  Internet-Drafts are working
      documents of the Internet Engineering Task Force (IETF), its
      areas, and its working groups.  Note that other groups may also
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      Internet-Drafts are draft documents valid for a maximum of six
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   The mailing list command specification header fields are a simple set
   of fields to be added to email messages sent by email distribution
   lists. Each field contains a URL (usually mailto or http) 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.

   By including these header fields, mail clients can provide automated
   tools for performing these 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.

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1. 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
   header is a URL (usually mailto or http) locating the relevant
   information or performing the command directly.  The three core
   headers are List-Help, List-Subscribe, and List-Unsubscribe.

   Implementing these headers 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 only choose to implement one or two of the headers.
   The List-Help header is the most useful individual header 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 headers are also very useful, but
   cannot describe some list manager syntaxes at this time (those which
   require variable substitution).  See the appendix for an explanation.

   The description of command syntax provided by the headers 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

   Consideration has been given to avoiding the creation of too many
   headers, while at the same time avoiding the overloading of
   individual headers and keeping the syntax clear and simple.

2. The Command Syntax

   The contents of the list headers consist of angle-bracket ('<', '>')
   enclosed URLs, with internal whitespace being ignored.

   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 headers 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 will be the focus of most use of the list headers.

   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 at this time.  However, systems
   using such syntaxes may still take advantage of the List-Help header
   to provide the user with detailed instructions as needed or -

Baer & Neufeld                                                  [Page 2]

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   perhaps more usefully - provide access to a web based command

   The additional complications of supporting variable fields within the
   command syntax was determined to be too difficult to support at this
   stage 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 headers
   described in this document:

   1) If the content of the header (following any leading whitespace)
      begins with any character other than the opening angle bracket
      '<', the header should be ignored.
   2) Any characters in the header after the first closing angle
      bracket '>' are to be ignored.

3. The Core Headers

   This document presents three headers which will provide the command
   syntax description for the 'core' functions of most email
   distribution lists.  The headers implemented on a given list should
   be included on all posts to the list, and on other messages where the
   message clearly applies to one distinct list.  Only one header of
   each type should be present in any given message, to avoid any
   confusion on the part of the mail client.

   The headers are presented in order of suggested priority.

3.1. List-Help

   The List-Help header is the most important of these header fields.
   It would be perfectly acceptable for a list manager to include only
   this header, since by definition it should direct the user to
   complete instructions for all other commands.  Typically, this would
   return the help file for the list or a web page with list
   instructions. Of all four headers, this one is the most likely
   candidate for an http URL rather than a mailto, since a web page can
   be used to provide a lot more information about the list, as well as
   a form interface for command access.


    List-Help: <mailto:list@host.com?subject=help>
    List-Help: <mailto:list-manager@host.com?body=info>
    List-Help: <http://www.host.com/list/>
    List-Help: <mailto:list-info@host.com>
    List-Help: <ftp://ftp.host.com/list.txt>

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3.2. List-Unsubscribe

   The URL of the List-Unsubscribe header should generally describe the
   mail message (mailto) to directly unsubscribe the user (removing them
   from the list).  Alternately, the URL may connect the user to some
   other mechanism (such as a web page) which performs this function.


    List-Unsubscribe: <mailto:list@host.com?subject=unsubscribe>
    List-Unsubscribe: <http://www.host.com/list.cgi?cmd=unsub&lst=list>
    List-Unsubscribe: <mailto:list-off@host.com>

3.3. List-Subscribe

   The URL of the List-Subscribe header should generally describe the
   mail message (mailto) to directly subscribe the user (requesting
   addition to the list).  Alternately, the URL may connect the user to
   some other mechanism (such as a web page ) which performs this


    List-Subscribe: <mailto:list@host.com?subject=subscribe>
    List-Subscribe: <mailto:list-request@host.com?subject=subscribe>
    List-Subscribe: <http://www.host.com/list.cgi>
    List-Subscribe: <mailto:list-on@host.com>

4. Security Considerations

   There are very few new security concerns generated with this
   proposal.  Headers are an existing standard, designed to easily
   accommodate new types. There may be concern with multiple headers
   being inserted or headers being forged, but these are problems
   inherent in Internet email, not specific to this proposal.  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 suggested that the user have a chance to

Baer & Neufeld                                                  [Page 4]

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   confirm any action before it is executed.  In the case of mailto, it
   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 ability to stop the message before it is sent.

   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.

5. Acknowledgements

   The participants of the ListMom-Talk, List-Managers, MIDA-Mail and
   List-Header 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


A. Background Discussion

   This proposal arose from discussions started on the ListMom-Talk
   Discussion List [2].  When the discussion reached a sufficient level,
   a separate list was formed for discussing this proposal, the List
   Headers Mail List [3] for deeper discussion.  We have included edited
   excerpts from that discussion here, 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 header 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 size

Baer & Neufeld                                                  [Page 5]

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   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 header and runs a greater risk of being modified by an
   existing server on route.

   The client implementation is also easier with multiple headers, 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 headers based on
   the specific needs of their individual lists.

   Finally, this format 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 headers 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.

   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
   the 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

   These 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

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   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 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 some
   client and server application authors) and makes the entire proposal
   more realistic.

A.4. Internationalization

   Multilingual support is up to the URL standard.  If URLs support it,
   this proposal supports.  This is another advantage of using URLs as
   the building blocks of the headers.

A.5. Variable Substitution

   Variables would allow this proposal to accommodate pretty much every
   existing list manager.  However, it would immeasurably increase the
   complexity of the entire proposal, and possibly involve redefining
   the URL standard.

   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

   Which would 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.

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   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 in problems for end
   users.  It is also not as easy for many list servers to implement
   MIME as it is to implement new headers.

   However, we are looking at how to implement MIME support in the
   future when the software is able to handle it.

A.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

   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 header
   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 header).
   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.

   We feel the flexibility afforded by this proposal (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.

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A.9. Additional header fields for future consideration

   Some of the other headers fields which have been considered, but
   which have been set aside for further study, are as follows:

     List-Digest: a command URL to switch to digest mode.
     List-Post: email address to post to as a mailto URL.
     List-Admin: contact URL for list administrator (usually a human).
     List-Archive: access URL for list archives (old messages).
     List-Software: list processing software name and version

B. Client Implementation

B.1. Guidelines

   For 'mailto' URL based commands, mail client applications should
   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

     "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 why these new headers will be useful, and how they could be
   supported.  Prototype menu items and floating pallettes have already
   been implemented in more than one mail client.

   In most cases, it may be helpful to disable the commands when not
   applicable to the currently selected message.

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B.2.1. Key combinations and command lines

   On text based systems which utilize command lines or key
   combinations, each header 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 headers.

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 headers, with items
   named "Subscribe", "Unsubscribe" and "Get Help".  This menu could be
   disabled when not applicable to the current message or disappear

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 header, with names "Subscribe",
   "Unsubscribe" and "Get Help".  These buttons or pallettes could be
   disabled when not applicable to the current message or disappear

B.2.4 Feedback to the User

   When putting up a dialog (or other feedback element) the client
   application may find it useful to 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


  [1] David H. Crocker, "Standard for the Format of ARPA
      Internet Text Messages" RFC 822, August 1982.

  [2] P. Hoffman and L. Masinter, "The mailto URL scheme"
      'work in progress' January 1997. <URL:ftp://ftp.internic.net/

  [3] T. Berners-Lee, L. Masinter and M. McCahill,
      "Uniform Resource Locators (URL)" RFC 1738, December 1994.

Baer & Neufeld                                                 [Page 10]

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Editors' addresses

Joshua D. Baer
Box 273
4902 Forbes Avenue
Pittsburgh, PA 15213-3799

Email: josh@skyweyr.com

Grant Neufeld
Ottawa, Ontario

Email: grant@acm.org

This document expires August 27, 1997

Baer & Neufeld                                                 [Page 11]