RFC 2369
Network Working Group G. Neufeld
Request for Comments: 2369 Nisto
Category: Standards Track J. Baer
SkyWeyr Technologies
July 1998
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 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 in RFC 2119.
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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
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). See appendix 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.
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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.
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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: (List Instructions)
List-Help:
List-Help: (Info about the list)
List-Help: ,
List-Help: (FTP),
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:
List-Unsubscribe: (Use this command to get off the list)
List-Unsubscribe:
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List-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:
List-Subscribe:
List-Subscribe: (Use this command to join the list)
List-Subscribe:
List-Subscribe: ,
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:
List-Post: (Postings are Moderated)
List-Post:
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: (Contact Person for Help)
List-Owner: (Grant Neufeld)
List-Owner:
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3.6. List-Archive
The List-Archive field describes how to access archives for the list.
Examples:
List-Archive:
List-Archive:
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, it
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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 and Christopher Allen
provided guidance on the standards
process.
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A. 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.
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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.
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A.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.
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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
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.
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B. Client Implementation
B.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 to
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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
[6] "ListMom-Talk" Mail list. listmom-talk@skyweyr.com
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Editors' 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/
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Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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