Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Informational R. Gellens
Expires: May 14, 2012 Qualcomm Incorporated
November 2011
Mailing Lists and UTF-8 Addresses
draft-ietf-eai-mailinglistbis-00
Abstract
This document describes considerations for mailing lists with the
introduction of internationalized email addresses. It outlines some
possible scenarios for handling lists with mixtures of
internationalized and traditional addresses, but does not offer
implementation or deployment advice.
Status of this Memo
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This Internet-Draft will expire on May 14, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Scenarios Involving Mailing Lists . . . . . . . . . . . . . . 4
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2.1. Fully EAI lists . . . . . . . . . . . . . . . . . . . . . 4
2.2. Mixed EAI and ASCII lists . . . . . . . . . . . . . . . . 4
2.3. SMTP issues . . . . . . . . . . . . . . . . . . . . . . . 5
3. List headers . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. EAI list headers . . . . . . . . . . . . . . . . . . . . . 5
3.2. Downgrading list headers . . . . . . . . . . . . . . . . . 6
3.3. Subscribers' addresses in downgraded headers . . . . . . . 7
4. Security considerations . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 8
Appendix A.1. Changes up to -00 . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document describes considerations for mailing lists with the
introduction of internationalized email addresses.
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 that distributes a message to multiple
recipients when the originator sends it to a single 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 sends error and other automatic messages to the list
agent, which can often do something useful with them, rathern than to
the original sender, who typically doesn't control the list and hence
can't do anything about them.
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 [RFC6409] (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-
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local addresses, is not addressed by this document.
Some mailing lists alter message header fields, 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. Some lists alter or replace other
fields, including "From".
Among these list-specific header fields are those specified in
[RFC2369] and [RFC2919]. For more information, see [listheaders].
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, a message with UTF-8 addresses in the headers or envelope
cannot be sent to non-UTF8 recipients.
With mailing lists, there are two different types of considerations:
first, the purely technical ones involving message handling, error
cases, 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. 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 UTF-8
mail users or not, and often neither the original sender nor the
recipients personally know 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
delivered to 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 EAI messages?
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If a list has subscribers with ASCII addresses, those subscribers
might or might not be able to accept EAI messages.
2. 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 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, 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.
2.1. Fully EAI lists
Some lists may wish to be fully EAI. That is, all subscribers are
expected to be able to receive EAI mail. For list hygeine reasons,
such a list would probably want to prevent subscriptions from
addresses that are unable to receive EAI mail. If a putative
subscriber has a UTF-8 address, it must be able to receive EAI mail,
but there is no way to tell whether a subscriber with an ASCII
address can receive EAI mail short of sending an EAI probe or
confirmation message and somehow finding out whether it was
delivered.
2.2. Mixed EAI and ASCII lists
Other lists may wish to handle a mixture of EAI and ASCII
subscribers, either as a transitional measure as subscribers upgrade
to EAI-capable mail software, or as an ongoing feature. While it is
not possible in general to downgrade EAI mail to ASCII mail, list
software might divide the recipients into two sets, EAI and ASCII
recipients, and create a downgraded version of EAI list messages to
send to ASCII recipients.
To determine which set an address belongs in, list software might
make the conservative assumption that ASCII addresses get ASCII
messages, it might try to probe the address with an EAI test message,
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or it might let the subscriber set the message format manually,
similar to the way that some lists now let subscribers choose between
plain text and HTML mail, or individual messages and a daily digest.
To determine whether a message needs to be downgraded for ASCII
recipients, list software might assume that any message received via
an EAI SMTP session is an EAI message, or might examine the headers
and body of the message to see whether it needs EAI treatment.
Depending on the interface between the list software and the MTA and
MDA that handle incoming messages, it may not be able to tell the
type of of session for incoming messages.
2.3. SMTP issues
Mailing list software invariably changes the envelope addresses on
each message. The return path is set to an address that will return
bounces to the list software, and the recipient addresses are set to
the subscribers of the list. For some lists, all messages to a list
get the same bounce address. For others, list software may create a
bounce address per recipient, or a unique bounce address per message
per recipient, bounce management techniques known as VERP.
The bounce address for a list typically includes the name of the
list, so a list with an EAI name will have an EAI bounce address.
Given the unknown paths that bounce messages might take, list
software might modify the bounce address to be ASCII to make it more
likely that bounces can be delivered back to the list. Similarly, a
VERP address for each subscriber typically embeds a version of the
subscriber's address so the VERP bounce address for an EAI subscriber
address will be an EAI address. For the same reason, the list
software might want to modify the VERP bounce addresses to be ASCII.
3. List headers
List software typically adds list-specific headers to each message
before resending the message to list recipients.
3.1. EAI list headers
The list headers in [RFC2369] and [RFC2919] were all specified before
EAI mail existed and their definitions do not address where UTF-8
characters might appear. These include, for example:
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List-Id: List Header Mailing List
<list-header.example.com>
List-Help:
<mailto:list@example.com?subject=help>
List-Unsubscribe:
<mailto:list@example.com?subject=unsubscribe>
List-Subscribe:
<mailto:list@example.com?subject=subscribe>
List-Post:
<mailto:list@example.com>
List-Owner:
<mailto:listmom@example.com> (Contact Person for Help)
List-Archive: <mailto:archive@example.com?subject=index%20list>
As described in [RFC2369], "The contents of the list header fields
mostly consist of angle-bracket ('<', '>') enclosed URLs, with
internal whitespace being ignored." [RFC2919] specifies that, "The
list identifier will, in most cases, appear like a host name in a
domain of the list owner."
These mailing list header fields contain URLs. The most common
schemes are generally HTTP, HTTPS, mailto, and FTP. Schemes that
permit both URI and IRI forms should use the URI-encoded form
described in [RFC3987]. Future work may extend these header fields
or define replacements to directly support non-encoded UTF-8 in IRIs
(for example, [I-D.duerst-mailto-bis]), but in the absence of such
extension or replacement, non-ASCII characters can only appear within
when encoded as ASCII. Note that discussion on whether
internationalized domain names should be percent-encoded or puny-
coded, is ongoing; see [I-D.duerst-iri-bis].
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.
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.
3.2. Downgrading list headers
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If list software prepares a downgraded version of an EAI message, all
the List-* headers must be downgraded. In particular, if a List-*
header contains a UTF-8 mailto (even encoded in ASCII), it may be
advisable to edit the header to remove the UTF-8 address, or replace
it with an equivalent ASCII address if one is known to the list
software. Otherwise, a client might run into trouble if the decoded
mailto results in a non-ASCII address.
When downgrading list headers, it may not be possible to produce a
downgraded version that is satisfactorily equivalent to the original
header. In particular, if a UTF-8 List-ID is downgraded to an ASCII
version, software and humans at recipient systems will typically not
be able to tell that both refer to the same list.
If lists permit mail with multiple MIME parts, some MIME headers in
EAI messages may include UTF-8 characters in filenames and other
descriptive text strings. Downgrading these strings may lose the
sense of the names, break references from other MIME parts (such as
HTML IMG refereneces to embedded images) and otherwise damage the
mail.
3.3. Subscribers' addresses in downgraded headers
List software typically leaves the original submitter's address in
the From: line, both so that receipients can tell who wrote the
message, and so that they have a choice of responding to the list or
directly to the submitter. If a submitter has a UTF-8 address, there
is no way to downgrade the From: header and preserve the address so
that ASCII recipients can respond to it, since non-EAI mail systems
can't send mail to UTF-8 addresses.
Possible workarounds (none implemented that we know of) might include
allowing subscribers with UTF-8 addresses to register an alternate
ASCII address with the list software, having the list software itself
create ASCII forwarding addresses, or just putting the list's address
in the From: line and losing the ability to respond to the submitter.
4. Security considerations
None beyond what mailing lists do now.
5. References
[I-D.duerst-iri-bis]
Duerst, M, Suignard, M and L Masinter, "Internationalized
Resource Identifiers (IRIs)", Internet-Draft draft-duerst-
iri-bis-07, October 2009.
[I-D.duerst-mailto-bis]
Duerst, M, Masinter, L and J Zawinski, "The 'mailto' URI
Scheme", Internet-Draft draft-duerst-mailto-bis-10, May
2010.
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[RFC2369] Neufeld, G. and J.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.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
Appendix A. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.*
Appendix A.1. Changes up to -00
Rewrite completely.
Authors' Addresses
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Phone: +1 831 480 2300
Email: standards@taugh.com
URI: http://jl.ly
Randall Gellens
Qualcomm Incorporated
5775 Morehouse Drive
San Diego, CA 92121
Email: rg+ietf@qualcomm.com
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