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Network Working Group R. Chandhok Request for Comments: 2919 G. Wenger Category: Standards Track QUALCOMM, Inc.

                                                              March 2001
              A Structured Field and Namespace for the
                  Identification of Mailing Lists

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 (2001).  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, it has become even more important to provide a unique
 identifier for 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.

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

Chandhok & Wenger Standards Track [Page 1] RFC 2919 List-Id March 2001

 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.
 Actually implementing such functionality in the MUA depends on the
 ability to accurately identify messages as belonging to a particular
 mailing list.  The problem then becomes what attribute or property to
 use to identify a mailing list.  The most likely candidate is the
 submission address of the mailing list itself.  Unfortunately, when
 the list server host, the list processing software, or the submission
 policy of the list changes the submission address itself can change.
 This causes great difficulty for automated processing and filtering.
 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 string matching in a filter, 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.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC 2119.

2. The List Identifier Syntax

 The list identifier will, in most cases, appear like a host name in a
 domain of the list owner.  In other words, the domain name system is
 used to delegate namespace authority for list identifiers just as it
 has been used to distribute that authority for other internet
 Using the domain name system as a basis for the list identifier
 namespace is intended to leverage an existing authority structure
 into a new area of application.  By using the domain name system to
 delegate list identifier namespace authority, it becomes instantly
 clear who has the right to create a particular list identifier, and
 separates the list identifier from any particular delivery host or
 mechanism.  Only the rights-holder of a domain or subdomain has the
 authority to create list identifiers in the namespace of that domain.
 For example, only the rights-holder to the "" domain has the
 authority to create list identifiers in "" domain.

Chandhok & Wenger Standards Track [Page 2] RFC 2919 List-Id March 2001

 While it is perfectly acceptable for a list identifier to be
 completely independent of the domain name of the host machine
 servicing the mailing list, the owner of a mailing list MUST NOT
 generate list identifiers in any domain namespace for which they do
 not have authority.  For example, a mailing list hosting service may
 choose to assign list identifiers in their own domain based
 namespace, or they may allow their clients (the list owners) to
 provide list identifiers in a namespace for which the owner has
 If the owner of the list does not have the authority to create a list
 identifier in 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 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 [RFC2606]
 In addition, a list identifier (list-id) MUST NOT be longer than 255
 octets in length, for future compatibility.  It should be noted that
 "localhost" is not valid for the domain-name rule.

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.

Chandhok & Wenger Standards Track [Page 3] RFC 2919 List-Id March 2001

 This field MUST only be generated by mailing list software, not end
 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 [RFC822].
 The List-Id header MAY optionally include a description by including
 it as a "phrase" [DRUMS] before the angle-bracketed list identifier.
 The MUA MAY choose to use this description in its user interface;
 however, any MUA that intends to make use of the description should
 be prepared to properly parse and decode any encoded strings or other
 legal phrase components.  For many MUAs the parsing of the List-Id
 header will simply consist of extracting the list identifier from
 between the delimiting angle brackets.
 The syntax of the List-Id header follows:
 list-id-header = "List-ID:" [phrase] "<" list-id ">" CRLF
 where phrase and CRLF are as defined in [DRUMS].  Unlike most headers
 in [RFC822], the List-Id header does not allow free insertion of
 whitespace and comments around tokens.  Any descriptive text must be
 presented in the optional phrase component of the header.

List-Id: List Header Mailing List <> List-Id: <> List-Id: "Lena's Personal Joke List"


List-Id: "An internal CMU List" <> List-Id: <da39efc25c530ad145d41b86f7420c3b.052000.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

Chandhok & Wenger Standards Track [Page 4] RFC 2919 List-Id March 2001

 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 focus.

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
 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.
 It is suggested, but not required, that list identifiers be created
 under a subdomain of "list-id" within any given domain.  This can
 help to reduce internal conflicts between the administrators of the
 subdomains of large organizations.  For example, list identifiers at
 "" are generated in the subdomain of "".
 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 identifier
 Thus, list identifiers such as
 <lenas-jokes.da39efc25c530ad145d41b86f7420c3b.021999.localhost> and
 <da39efc25c530ad145d41b86f7420c3b.051998.localhost> conform to these
 guidelines, while <lenas-jokes.021999.localhost> and

Chandhok & Wenger Standards Track [Page 5] RFC 2919 List-Id March 2001

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

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; however, this
 will only be possible when the nested mailing list is aware of the
 relationship between it and its "parent" mailing lists.  If a mailing
 list processor encounters a List-Id header field from any unexpected
 source it SHOULD NOT pass it through to the list.  This implies that
 mailing list processors may have to be updated to properly support
 List-Ids for nested lists.

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.
 As mentioned above, 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 problems.
 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.

Chandhok & Wenger Standards Track [Page 6] RFC 2919 List-Id March 2001

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 <> focused much of the early discussion,
 and thus was essential in the creation of this document.


 [LISTHEADER] "List-Header" Mail list.
 [LISTMOM]    "ListMom-Talk" Mail list.
 [MSGID]      J. Zawinski, M. Curtin, "Recommendations for generating
              Message IDs", Work in Progress.
 [RFC822]     Crocker, D., "Standard for the Format of ARPA Internet
              Text Messages", RFC 822, August 1982.
 [RFC1750]    Eastlake, D., Crocker S. and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.
 [RFC2234]    Crocker, D. and P. Overell. "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.
 [RFC2369]    Neufeld G. and J. Baer, "The Use of URLs as Meta-Syntax
              for Core Mail List Commands and their Transport through
              Message Header Fields", RFC 2369, July 1998.
 [RFC2606]    Eastlake, 3rd, D., and S. Panitz. "Reserved Top Level
              DNS Names", BCP 32, RFC 2606, June 1999.
 [RFC2822]    Resnick, P., Editor, "Internet Message Format Standard",
              STD 11, RFC 2822, March 2001.

Chandhok & Wenger Standards Track [Page 7] RFC 2919 List-Id March 2001

Authors' Addresses

 Ravinder Chandhok
 5775 Morehouse Drive
 San Diego, CA 92121 USA
 Geoffrey Wenger
 5775 Morehouse Drive
 San Diego, CA 92121 USA

Chandhok & Wenger Standards Track [Page 8] RFC 2919 List-Id March 2001

Full Copyright Statement

 Copyright (C) The Internet Society (2001).  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
 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


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Chandhok & Wenger Standards Track [Page 9]

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