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Network Working Group                                          R. ChandhokRequest for Comments: 2919                                       G. WengerCategory: Standards Track                                   QUALCOMM, Inc.                                                                March 2001List-Id:A Structured Field and Namespace for theIdentification of Mailing ListsStatus 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.Abstract   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.  RecentChandhok & 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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 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   resources.   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 "acm.org" domain has the   authority to create list identifiers in "acm.org" 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   authority.   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   Where:       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   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 [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.   Examples:List-Id: List Header Mailing List <list-header.nisto.com>List-Id: <commonspace-users.list-id.within.com>List-Id: "Lena's Personal Joke List"         <lenas-jokes.da39efc25c530ad145d41b86f7420c3b.021999.localhost>List-Id: "An internal CMU List" <0Jks9449.list-id.cmu.edu>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 listChandhok & 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   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.   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   "within.com" are generated in the subdomain of "list-id.within.com".   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> andChandhok & 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   unique.6. Operations on List Identifiers   There is only one operation defined for list identifiers, that of   case insensitive equality (SeeSection 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 20019. 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 document.References   [LISTHEADER] "List-Header" Mail list. list-header@list.nisto.com                <http://www.nisto.com/listspec/mail/>                <http://www.nisto.com/listspec/>   [LISTMOM]    "ListMom-Talk" Mail list. listmom-talk@skyweyr.com                <http://cgi.skyweyr.com/ListMom.Home>   [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 2001Authors' Addresses   Ravinder Chandhok   QUALCOMM, Inc.   5775 Morehouse Drive   San Diego, CA 92121 USA   EMail: chandhok@qualcomm.com   Geoffrey Wenger   QUALCOMM, Inc.   5775 Morehouse Drive   San Diego, CA 92121 USA   EMail: gwenger@qualcomm.comChandhok & Wenger           Standards Track                     [Page 8]

RFC 2919                        List-Id                       March 2001Full 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   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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Chandhok & Wenger           Standards Track                     [Page 9]