Intended Status: Experimental.
Internet Draft: Mailing Lists and Internationalized R. Gellens
Email Addresses Qualcomm
Document: draft-ietf-eai-mailinglist-05.txt
Expires: June 12, 2010 January 12, 2010
Mailing Lists and Internationalized Email Addresses
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Abstract
This document describes considerations for mailing lists with the
introduction of internationalized email addresses.
This document makes some specific recommendations on how mailing
lists should act in various situations.
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Table of Contents
1. Conventions Used in this Document . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scenarios Involving Mailing Lists . . . . . . . . . . . . . 5
4. Capabilities and Requirements . . . . . . . . . . . . . . . 6
5. List Header Fields . . . . . . . . . . . . . . . . . . . . 7
6. Further Discussion . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . 9
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 9
10. Normative References . . . . . . . . . . . . . . . . . . . 9
11. Informative References . . . . . . . . . . . . . . . . . . . 10
12. Authors' Addresses . . . . . . . . . . . . . . . . . . . . 10
Appendix A: Changes from Previous Version . . . . . . . . . . 10
1. Conventions Used in this Document
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 [KEYWORDS].
2. Introduction
Mailing lists are an important part of email usage and collaborative
communications. The introduction of internationalized email
addresses affects mailing lists in three main areas: (1) transport
(receiving and sending messages); (2) message headers of received
and retransmitted messages; and (3) mailing list operational
policies.
A mailing list is a mechanism whereby a message may be distributed
to multiple recipients by sending to one address. An agent
(typically not a human being) at that single address receives the
message and then causes the message to be redistributed to a list of
recipients. This agent sets the envelope return address of the
redistributed message to a different address from that of the
original message. Using a different envelope return address
(reverse-path) directs error (and other automatically generated)
messages to an error handling address associated with the mailing
list. (This avoids having error and other automatic messages go to
the original sender, who typically doesn't control the list and
hence can't do anything about them.)
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In most cases, the mailing list agent redistributes a received
message to its subscribers as a new message, that is, conceptually
it uses message submission [submission] (as did the sender of the
original message). The exception, where the mailing list is not a
separate agent that receives and redistributes messages in separate
transactions, but is instead an expansion step within an SMTP
transaction where one local address expands to multiple local or
non-local addresses, is out of scope for this document.
Some mailing lists alter the message header, while others do not. A
number of standardized list-related header fields have been defined,
and many lists add one or more of these headers. Separate from
these standardized list-specific header fields, and despite a
history of interoperability problems from doing so, some lists alter
or add header fields in an attempt to control where replies are
sent. Such lists typically add or replace the "Reply-To" field and
some add or replace the "Sender" field. Poorly-behaved lists may
alter or replace other fields, including "From".
Among these list-specific header fields are those specified in RFC
2369 ("The Use of URLs as Meta-Syntax for Core Mail List Commands
and their Transport through Message Header Fields") [List-*] and RFC
2919 ("List-Id: A Structured Field and Namespace for the
Identification of Mailing Lists") [List-ID]. For more information,
see Section 5.
While the mail transport protocol does not differ between regular
email recipients and mailing list recipients, lists have special
considerations with internationalized email addresses because they
retransmit messages composed by other agents to potentially many
recipients.
There are considerations for internationalized email addresses in
the envelope as well as in header fields of redistributed messages.
In particular, an internationalized message cannot be downgraded
unless all envelope addresses are available in ASCII (that is, each
address either is ASCII or has an ALT-ADDRESS).
With mailing lists, there are two different types of considerations:
first, the purely technical ones involving message handling, error
cases, downgrades, and the like, and second, those that arise from
the fact that humans use mailing lists to communicate. As an
example of the first, mailing lists might choose to reject all
messages from internationalized addresses that lack an alt-address,
or even all internationalized messages that can not be downgraded.
As an example of the second, a user who sends a message to a list
often is unaware of the list membership. In particular, the user
often doesn't know if the members are i18mail users or not, and
often neither the original sender nor the recipients personally know
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each other. As a consequence of this, remedies that may be readily
available for one-to-one communication might not be appropriate when
dealing with mailing lists. For example, if a user sends a message
which is undeliverable, normally the telephone, instant messaging,
or other forms of communication are available to obtain a working
address. With mailing lists, the users may not have any recourse.
Of course, with mailing lists, the original sender usually does not
know if the message was successfully received by any list members,
or if it was undeliverable to some.
Conceptually, a mailing list's internationalization can be divided
into three capabilities: First, does it have a UTF-8 submission or
return-path address? Second, does it accept subscriptions to UTF-8
addresses? And third, does it accept [UTF8SMTP] messages? This is
explored in Section 4.
A brief discussion on a few additional considerations for mailing
list operation is in Section 6.
3. Scenarios Involving Mailing Lists
Generally (and exclusively within the scope of this document), an
original message is sent to a mailing list as a completely separate
and independent transaction from the mailing list agent sending the
retransmitted message to one or more list recipients. In both
cases, the message might have only one recipient, or might have
multiple recipients. That is, the original message might be sent to
additional recipients as well as the mailing list agent, and the
mailing list might choose to send the retransmitted message to each
list recipient in a separate message submission [submission]
transaction, or might choose to include multiple recipients per
transaction. (Often, mailing lists are constructed to work in
cooperation with, rather than include the functionality of, a
message submission server [submission], and hence the list transmits
to a single submission server one copy of the retransmitted message,
with all list recipients specified in the SMTP envelope. The
submission server then decides which recipients to include in which
transaction.)
The retransmitted message sent by the mailing list to its
subscribers might need to be downgraded [EAI-Downgrade]. In order
for a downgrade to be possible, the return path set by the mailing
list agent must be an ASCII address or have ALT-ADDRESS specified.
In addition, the recipient addresses need to have ASCII addresses
available. It may be advisable for mailing list operators to
pre-obtain an alt-address for all its internationalized member
addresses.
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In the case where a member or non-member with an internationalized
email address is sending to a mailing list, no alt-address is
specified, and a downgrade is required, the message cannot be
delivered. To protect against this, a UTF8SMTP-aware [UTF8SMTP]
mailing list might prefer to reject submissions from
internationalized email addresses that lack an alt-address.
(Note that the situation is not unique to mailing lists. Mail
relays that are UTF8SMTP-aware will potentially encounter the same
situation.) Further discussions are included in section 6 of this
document.
4. Capabilities and Requirements
There are three primary internationalization capabilities of mailing
lists: First, does it have a UTF-8 submission or return-path
address? Second, does it allow subscriptions from UTF-8 addresses?
And third, does it accept [UTF8SMTP] messages?
In theory, any list can support any combination of these. In
practice, only some offer any benefit. For example, neither
allowing UTF-8 addresses to subscribe, nor accepting UTF8SMTP
messages, makes much sense without the other (an all-ASCII address
might or might not be capable of receiving UTF8SMTP messages, but a
UTF-8 address of necessity needs to accept UTF8SMTP messages).
Likewise, there is no real benefit to a list in using a UTF-8
submission address unless it also accepts UTF8SMTP messages and
permits UTF-8 addresses to subscribe.
However, requirements for lists can be discussed separately for each
of the three capabilities.
1. If the list uses a UTF-8 submission or return-path address, it
SHOULD specify an alt-address for it. Clearly, it needs to sit
behind a UTF8SMTP-enabled final-delivery SMTP server [UTF8SMTP] and
delivery agent. Likewise, if a list uses a UTF-8 return-path
address, then its MSA [submission] needs to support UTF8SMTP.
The list's return-path address is usually separate from its
submission address (so that delivery reports and other
automatically-generated messages are not sent to the submission
address). For reliability in receiving delivery status
notifications, a list MAY choose to use an all-ASCII return-path
even if it uses a UTF-8 submission address. If the list does use a
UTF-8 return path, it MUST specify an alt-address (or else there is
a high risk of being unable to receive non-delivery reports).
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There are also implications for the List-* headers (see below).
2. If it allows UTF-8 addresses to subscribe, it MAY require an
alt-address to be specified for each UTF-8 subscriber.
Naturally, if it permits UTF-8 addresses to subscribe, it needs a
mechanism to accept subscription requests from such addresses
(preferably specified in the form <utf8@utf8 <ascii@ascii>>).
Likewise, its MSA needs to support [UTF8SMTP].
3. If it accepts UTF8SMTP messages, its MSA needs to support
UTF8SMTP.
5. List Header Fields
A number of header fields specifically for mailing lists have been
introduced in RFC 2369 and RFC 2919. These include, for example:
List-Id: List Header Mailing List <list-header.nisto.com>
List-Help: <mailto:list@host.com?subject=help> (List Instructions)
List-Unsubscribe: <mailto:list@host.com?subject=unsubscribe>
List-Subscribe: <mailto:list@host.com?subject=subscribe>
List-Post: <mailto:list@host.com>
List-Owner: <mailto:listmom@host.com> (Contact Person for Help)
List-Archive: <mailto:archive@host.com?subject=index%20list>
As described in RFC 2369, "The contents of the list header fields
mostly consist of angle-bracket ('<', '>') enclosed URLs, with
internal whitespace being ignored." [List-*] Whereas RFC2919
specifies that, "The list identifier will, in most cases, appear
like a host name in a domain of the list owner." [List-ID]
These mailing list header fields contain URLs. The most common
schemes are generally HTTP, HTTPS, mailto, and FTP. The URLs in
these fields can use RFC3987 "Internationalized Resource Identifier
(IRI)" [IRI] encoded as URLs. Future work may extend these header
fields or define replacements to directly support non-encoded UTF-8
in IRIs (for example, [mailto-bis]), but in the absence of such
extension or replacement, non-ASCII characters can only appear
within IRIs when properly encoded. Note that internationalized
domain names could potentially be either percent-encoded or
puny-coded, but punycode is likely to have better results.
Even without these header fields being extended to support UTF-8,
some special provisions may be helpful when downgrading. In
particular, if a List-* header contains a UTF-8 mailto (even encoded
in ASCII) followed by an ASCII mailto, it may be advisable to not
only copy and preserve the original header as usual (ENCAPSULATION
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method of [EAI-Downgrade]), but also to edit the header to remove
the UTF-8 address. Otherwise, a client might run into trouble if
the decoded mailto results in a non-ASCII address. [[[EDITOR'S NOTE:
This needs to be vetted by the eai list.]]]
When mailing lists use a UTF-8 form of a List-* header, an ASCII
form SHOULD also be used. These headers are vital to good
operations and use of mailing lists; caution is called for when
considering how to form and use these headers in a non-ASCII
environment.
The most commonly-used URI schemes in List-* headers tend to be HTTP
and mailto. The current specification for mailto does not permit
unencoded UTF-8 characters, although work has been proposed to
extend or more likely replace mailto in order to permit this. For
mailto URIs, a separate consideration is how to include an alternate
ASCII address (alt-address) for a UTF-8 address. Note that the
existing ability to specify multiple URLs within each List-* header
field provides one solution.
[List-*] says:
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.
When a UTF-8 mailto is used in a List-* header field, an
alt-address, if available, SHOULD immediately follow it.
The List-ID header field uniquely identifies a list. The intent is
that the value of this header remain constant, even if the machine
or system used to operate or host the list changes. This header
field is often used in various filters and tests, such as
client-side filters, Sieve filters, and so forth. Such filters and
tests may not properly compare a non-ASCII value which has been
encoded into ASCII. In addition to these comparison considerations,
it is generally desirable that this header field contain something
meaningful that users can type in. However, ASCII encodings of
non-ASCII characters are unlikely to be meaningful to users or easy
for them to accurately type.
6. Further Discussion
While mailing lists do not create a significant additional burden to
the deployment of internationalized email address functionalities,
there are some specific areas that need to be considered when the
operator of a mailing list or of a final delivery MTA that serves a
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mailing list upgrades to internationalized mail.
Mailing lists face additional complexity since they redistribute
messages composed by other agents. Hence, they may be asked to
accept a message with non-ASCII headers composed by a UTF8SMTP-aware
user agent [UTF8SMTP], and redistribute it to i18mail and
non-i18mail users via systems that are not UTF8SMTP-aware.
1. Obtaining Downgrade Information -- for a mailing list, or mail
relay server for that matter, that is UTF8SMTP-aware, receiving mail
from an internationalized email address, the alt-address is not
required from the sending MTA for the transport to be complete.
When the mailing list then retransmits the message to its
subscribers, it may encounter paths where a downgrade is needed (if
a relay or final MSA does not supports UTF8SMTP). In order to
mitigate this situation, the mailing list might perhaps decide to
reject all incoming mail from an internationalized email address
that lacks an alt-address. However, note that in general,
downgrades are not expected to be the normal case.
2. Downgrading Considerations for mailto URLs -- UTF-8 addresses in
mailto links in List-* headers will be easier to downgrade if they
contain an alt-address.
7. IANA Considerations
None.
8. Security Considerations
Security considerations are discussed in the Framework document
[EAI-Framework]. No further security considerations are raised by
this document.
9. Acknowledgments
Edmon Chung of Afilias wrote the original version of this document.
Thanks to Harald Alvestrand for his comments.
10. Normative References
[EAI-Framework] J. Klensin and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 4952, July 2007.
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[KEYWORDS] "Key words for use in RFCs to Indicate Requirement
Levels", S. Bradner, RFC 2119, BCP 14, March 1997.
11. Informative References
[mailto-bis] M. Duerst and L. Masinter, "The mailto URI scheme",
draft-duerst-mailto-bis-xx (work in progress).
[List-*] G. Neufeld 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
[List-ID] R. Chandhok and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists", RFC 2919,
March 2001
[IRI] M. Duerst and M. Suignard,"Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005
[submission] R. Gellens and J. Klensin, "Message Submission for
Mail", RFC 4409, April 2006.
[UTF8SMTP] J. Yao and W. Mao, "SMTP Extension for Internationalized
Email Addresses", RFC 5336, September 2008.
[EAI-Downgrade] K. Fujiwara and Y. Yoneya, "Downgrading Mechanism
for Email Address Internationalization", RFC 5504, March 2009.
12. Authors' Addresses
Randall Gellens
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego, CA 92121
rg+ietf@qualcomm.com
Appendix A: Changes from Previous Version
THIS SECTION TO BE REMOVED PRIOR TO PUBLICATION.
Changes made from version -04 to -05:
o Added [submission] as informative reference.
o Normalized "UTF-8" for descriptive terms except as part of
hyphenated compounds.
o Reworded Introduction text on envelope addresses and ASCII.
o Added informative reference to RFC 5336.
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o Reworded discussion of List-ID in "List Header Fields" section
to more clearly explain the risks of using ASCII encodings of
non-ASCII characters.
o Added text to "Further Discussion" clarifying that downgrade
might be needed if an MAA between the list and a subscriber does
not support UTF8SMTP.
o Corrected some references.
Changes made from version -03 to -04:
o Rewrote text in Section 5 on List-* headers. Added new text
specifically on List-ID. Noted that currently, IRIs in List-*
headers must be encoded as ASCII.
Changes made from version -02 to -03:
o Deleted Section 6.
o Restored missing text in third paragraph from the end of Section
3.1.
o Deleted broken suggestion in Section 5.
o Additional text fixes.
o Reworked text on List-* header fields.
o Removed most Editor's Notes, including deletion of all text that
had been followed by an Editor's Note asking if it was useful.
o Modified Abstract.
o Edited Sections 3, 4, and 5.
Changes made from version -01 to -02:
o Significant changes throughout the document. Sorry.
Changes made from version -00 to -01:
o Fixed SMTP envelope versus message header confusion.
o Fixed erroneous mailing list operation text.
o Removed references to ATOMIC.
o Removed unneeded scenarios.
o Added discussion of human considerations which arise with lists.
o Fixed some typos.
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