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Versions: 00 01 02 03 04 rfc2919                                        
INTERNET-DRAFT                                      Ravinder Chandhok
draft-chandhok-listid-02.txt                        Geoffrey Wenger
Expires: April 20, 1999                             Within Technology, Inc.
                                                    October 20, 1998

                A Structured Field and Namespace for the
                    Identification of Mailing Lists

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 distribute working documents as

    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.''

        To view the entire list of current Internet-Drafts, please check
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        Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East
        Coast), or ftp.isi.edu (US West Coast).

Copyright Notice

    Copyright (C) The Internet Society 1998.  All Rights Reserved.


    Software that handles electronic mailing list messages (servers
    and user agents) needs a way to reliably identify messages that
    belong to a particular mailing list.  With the advent of list
    management headers [RFC2369] it has become even more important
    to provide an identifier to use to identify a mailing list
    regardless of the particular host that serves as the list
    processor at any given time.

    The List-Id header provides a standard location for such an
    identifier.  In addition, a namespace for list identifiers
    based on fully qualified domain names is described.  This
    namespace is intended to guarantee uniqueness for list owners
    who require it, while allowing for a less rigorous namespace
    for experimental and personal use.

    By including the List-Id field, list servers can make it easier
    for mail clients to provide automated tools for users to
    perform list functions.  The list identifier can serve as a key
    to make many automated processing tasks easier, and hence more
    widely available.

    The key words ''MUST'', ''MUST NOT'', ''REQUIRED'', ''SHALL'',
    ''MAY'', and ''OPTIONAL'' in this document are to be
    interpreted as described in RFC 2119.

1. Introduction

    Internet mailing lists have evolved into fairly sophisticated
    forums for group communication and collaboration; however,
    corresponding changes in the underlying infrastructure have
    lagged behind.  Recent proposals like [RFC2369] have expanded
    the functionality that the MUA can provide by providing more
    information in each message sent by the mailing list
    distribution software.

    In order to further automate (and make more accurate) the
    processing a software agent can do, there needs to be some
    unique identifier to use as an identifier for the mailing list.
    This identifier can be simply used for filter string matching,
    or it can be used in more sophisticated systems to uniquely
    identify messages as belonging to a particular mailing list
    independent of the particular host delivering the actual
    messages.  This identifier can also act as a key into a
    database of mailing lists.

2. The List Identifier Syntax

    The list identifier will, in most cases, appear like a host name
    in the domain of the owner's choosing.  In other words, the
    domain name system is being used to delegate namespace
    authority for list identifiers just as it has been used to
    distribute that authority for other internet resources.  If an
    entity is the owner of a domain or subdomain, then it may
    create list identifiers in the namespace of that domain.  It is
    important to note that it is perfectly acceptable for a list
    identifier to be completely independent of the domain name of
    the host machine servicing the mailing list.

    If the owner of the list does not have access and permission to
    a domain-based namespace, they MAY create unmanaged list
    identifiers in the special unmanaged domain "localhost".  This
    would apply to personal users, or users unable to afford domain
    name registration fees.  The owner of a mailing list MUST NOT
    generate list identifiers in any domain namespace for which
    they do not have administrative control or have prior approval.

    The syntax for a list identifier in ABNF [RFC2234] follows:

    list-id = list-label "." list-id-namespace

    list-label = dot-atom-text

    list-id-namespace = domain-name / unmanaged-list-id-namespace

    unmanaged-list-id-namespace    = "localhost"

    domain-name = dot-atom-text


        dot-atom-text is defined in [DRUMS]

        "localhost" is a reserved domain name is defined in [RTLDN]

    In addition, a list identifier (list-id) MUST NOT be longer than
    255 octets in length, for future compatibility.

3. The List-Id Header Field

    This document presents a header field which will provide an
    identifier for an e-mail distribution list.  This header 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 this particular distinct
    list. There MUST be no more than one of each field present in
    any given message.

    This field MUST only be generated by mailing list software, not
    end users.

    The contents of the List-Id header mostly consist of
    angle-bracket ('<', '>') enclosed identifier, with internal
    whitespace being ignored. MTAs MUST NOT insert whitespace
    within the brackets, but client applications should treat any
    such whitespace, that might be inserted by poorly behaved MTAs,
    as characters to ignore.

    The list header fields are subject to the encoding and
    character restrictions for mail headers as described in

    The List-Id header MAY optionally include a description by
    including it as a "quoted-string" [DRUMS] before the
    angle-bracketed list identifier.  The MUA MAY choose to use
    this description in its user interface.

    The syntax of the List-Id header follows:

        list-id-header = "List-ID:" [quoted-string] "<" list-id ">" CRLF

    where quoted-string and CRLF are as defined in [DRUMS].


    List-Id: "List Header Mailing List" <list-header.nisto.com>
    List-Id: <commonspace-users.list-ids.within.com>
    List-Id: "Lena's Personal Joke List"
    List-Id: "An internal CMU List" <0Jks9449.cmu.edu>
    List-Id: <da39efc25c530ad145d41b86f7420c3b.051998.localhost>

4. Persistence of List Identifiers

    Although the list identifier MAY be changed by the mailing list
    administrator this is not desirable. (Note that there is no
    disadvantage to changing the description portion of the List-Id
    header.) A MUA MAY not recognize the change to the list
    identifier because the MUA SHOULD treat a different list
    identifier as a different list. As such the mailing list
    administrator SHOULD avoid changing the list identifier even
    when the host serving the list changes. On the other hand,
    transitioning from an informal unmanaged-list-id-namespace to a
    domain namespace is an acceptable reason to change the list
    identifier. Also if the focus of the list changes sufficiently
    the administrator may wish to retire the previous list and its
    associated identifier to start a new list reflecting the new

5. Uniqueness of List Identifiers

    This proposal seeks to leverage the existing administrative
    process already in place for domain name allocation.  In
    particular, we exploit the fact that domain name ownership
    creates a namespace that by definition can be used to create
    unique identifiers within the domain.

    In addition, there must be a mechanism for identification of
    mailing lists that are administrated by some entity without
    administrative access to a domain.  In this case, general
    heuristics can be given to reduce the chance of collision, but
    it cannot be guaranteed.  If a list owner requires a guarantee,
    they are free to register a domain name under their control.

    List-IDs not ending with ".localhost" MUST be globally unique
    in reference to all other mailing lists.

    List owners wishing to use the special "localhost" namespace
    for their list identifier SHOULD use the month and year (in the
    form MMYYYY) that they create the list identifier as a
    "subdomain" of the "localhost" namespace.  In addition, some
    portion of the list identifier MUST be a randomly generated
    string.  List owners generating such identifiers should refer
    to [MSGID] for further suggestions on generating a unique
    identifier, and [RFC1750] for suggestions on generating random
    numbers.  In particular, list identifiers that have a random
    component SHOULD contain a hex encoding of 128 bits of
    randomness (resulting in 32 hex characters) as part of the list

    Thus, list identifiers such as
    and <da39efc25c530ad145d41b86f7420c3b.051998.localhost> conform
    to these guidelines, while <lenas-jokes.021999.localhost> and
    <mylist.localhost> do not.   A particular list owner
    with several lists MAY choose to use the same random number
    subdomain when generating list identifiers for each of the

    List-IDs ending with ".localhost" are not guaranteed to be
    globally unique.

6. Operations on List Identifiers

    There is only one operation defined for list identifiers, that
    of case insensitive equality (See Section 3.4.7.  CASE
    INDEPENDENCE [RFC822]).  The sole use of a list identifier is
    to identify a mailing list, and the sole use of the List-Id
    header is to mark a particular message as belonging to that
    list.  The comparison operation MUST ignore any part of the
    List-Id header outside of the angle brackets, the MUA MAY
    choose to inform the user if the descriptive name of a mailing
    list changes.

7. Supporting Nested Lists

    A list that is a sublist for another list in a nested mailing
    list hierarchy MUST NOT modify the List-Id header field.

8. 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 headers
    being  forged, but this problem is inherent in Internet
    e-mail, not specific to the header described in this document.
    Further, the implications are relatively harmless.

    Mail list processors SHOULD NOT allow any user-originated
    List-Id fields to pass through to their lists, lest they
    confuse the user and have the potential to create security

    On the client side, a forged list identifier may break
    automated processing.  The list identifier (in its current
    form) SHOULD NOT be used as an indication of the authenticity
    of the message.

9. Acknowledgements

    The numerous participants of the List-Header [LISTHEADER] and
    ListMom-Talk [LISTMOM] mailing lists contributed much to the
    formation and structure of this document.

    Grant Neufeld <grant@acm.org> focused much of the early
    discussion, and thus was essential in the creation of this


    [DRUMS] P. Resnick, Editor, "Internet Message Format Standard",
        March 13 1998.

    [LISTHEADER] "List-Header" Mail list. list-header@list.nisto.com

    [LISTMOM] "ListMom-Talk" Mail list. listmom-talk@skyweyr.com

    [MSGID] J. Zawinski, M. Curtin, "Recommendations for generating
        Message IDs", July 22 1998.

    [RFC822] David H. Crocker, "Standard for the Format of ARPA
        Internet Text Messages" RFC 822, August 1982.

    [RFC1750] D. Eastlake, 3rd, S. Crocker & J. Schiller. "Randomness
        Recommendations for Security" RFC 1750,  December 1994.

    [RFC2234]   D. Crocker, P. Overell. "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

    [RFC2369] Grant Neufeld and Joshua D. Baer, "The Use of URLs as
        Meta-Syntax for Core Mail List Commands and their Transport
        through Message Header Fields", RFC 2369, July 1998.

    [RTLDN] D. Eastlake, 3rd, S. Panitz. "Reserved Top Level DNS
        Names", July 1998.

Editors' addresses

    Ravinder Chandhok
    Within Technology, Inc.
    RD#1 Box 228
    Waynesburg PA 15370 USA
    Email: chandhok@within.com
    Web: http://www.within.com/~chandhok

    Geoffrey Wenger
    Within Technology, Inc.
    RD#1 Box 228 Waynesburg
    PA 15370 USA
    Email: wenger@within.com
    Web: http://www.within.com/~wenger

Expires: April 20, 1999