Network Working Group R. Chandhok
Request for Comments: 2919 G. Wenger
Category: Standards Track QUALCOMM, Inc.
March 2001 List-Id:
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 (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
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.
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.
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 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 "acm.org" domain has the
authority to create list identifiers in "acm.org" domain.
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
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.
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-header.nisto.com>
List-Id: "Lena's Personal Joke List"
List-Id: "An internal CMU List" <0Jks9449.list-id.cmu.edu>
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 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
"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
<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 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.
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