Movatterモバイル変換

Internet Engineering Task Force (IETF)                         J. LevineRequest for Comments: 6783                          Taughannock NetworksObsoletes:5983                                               R. GellensCategory: Informational                            Qualcomm IncorporatedISSN: 2070-1721                                            November 2012Mailing Lists and Non-ASCII AddressesAbstract   This document describes considerations for mailing lists with the   introduction of non-ASCII UTF-8 email addresses.  It outlines some   possible scenarios for handling lists with mixtures of non-ASCII and   traditional addresses but does not specify protocol changes or offer   implementation or deployment advice.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6783.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Levine & Gellens              Informational                     [Page 1]

RFC 6783          Mailing Lists and Non-ASCII Addresses    November 2012Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Mailing List Header Additions and Modifications . . . . . .31.2.  Non-ASCII Email Addresses . . . . . . . . . . . . . . . . .32.  Scenarios Involving Mailing Lists . . . . . . . . . . . . . . .42.1.  Fully SMTPUTF8 Lists  . . . . . . . . . . . . . . . . . . .42.2.  Mixed SMTPUTF8 and ASCII Lists  . . . . . . . . . . . . . .52.3.  SMTP Issues . . . . . . . . . . . . . . . . . . . . . . . .53.  List Headers  . . . . . . . . . . . . . . . . . . . . . . . . .63.1.  SMTPUTF8 List Headers . . . . . . . . . . . . . . . . . . .63.2.  Downgrading List Headers  . . . . . . . . . . . . . . . . .73.3.  Subscribers' Addresses in Downgraded Headers  . . . . . . .84.  Security Considerations . . . . . . . . . . . . . . . . . . . .85.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .85.1.  Normative References  . . . . . . . . . . . . . . . . . . .85.2.  Informative References  . . . . . . . . . . . . . . . . . .91.  Introduction   This document describes considerations for mailing lists with the   introduction of non-ASCII UTF-8 email addresses.  The usage of such   addresses is described in [RFC6530].   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, usually software rather than a person, at that single address   receives the message and then causes the message to be redistributed   to a list of recipients.  This agent usually sets the envelope return   address (henceforth called the "bounce address") of the redistributed   message to a different address from that of the original message.   Using a different bounce address 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, rather 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 managed by aLevine & Gellens              Informational                     [Page 2]

RFC 6783          Mailing Lists and Non-ASCII Addresses    November 2012   separate agent that receives and redistributes messages in separate   transactions but is implemented by an expansion step within an SMTP   transaction where one local address expands to multiple local or non-   local addresses, is not addressed by this document.1.1.  Mailing List Header Additions and Modifications   Some list agents 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 RFCs   2369 [RFC2369] and 2919 [RFC2919].  For more information, seeSection 3.1.2.  Non-ASCII Email Addresses   While the mail transport protocol is the same for regular email   recipients and mailing list recipients, list agents have special   considerations with non-ASCII email addresses because they retransmit   messages composed by other agents to potentially many recipients.   There are considerations for non-ASCII email addresses in the   envelope as well as in header fields of redistributed messages.  In   particular, a message with non-ASCII addresses in the headers or   envelope cannot be sent to non-SMTPUTF8 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,   list agents might choose to reject all messages from non-ASCII   addresses if they are unprepared to handle SMTPUTF8 mail.  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 SMTPUTF8 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   that is undeliverable, normally the telephone, instant messaging, or   other forms of communication are available to obtain a workingLevine & Gellens              Informational                     [Page 3]

RFC 6783          Mailing Lists and Non-ASCII Addresses    November 2012   address.  With mailing lists, the users may not have any recourse.   Of course, with mailing lists, the original sender usually does not   know which list members successfully received a message or if it was   undeliverable to some.   Conceptually, a mailing list's internationalization can be divided   into three capabilities.  First, does the list have a non-ASCII   submission address?  Second, does the list agent accept subscriptions   for addresses containing non-ASCII characters?  And third, does the   list agent accept messages that require SMTPUTF8 capabilities?   If a list has subscribers with ASCII addresses, those subscribers   might or might not be able to accept SMTPUTF8 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 list agent sending the   retransmitted message to one or more list recipients.  In both cases,   the message might be addressed only to the list address or might have   recipients in addition to the list.  Furthermore, the list agent   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, list agents are   constructed to work in cooperation with, rather than include the   functionality of, a message submission server; hence, the list   transmits to a single submission server one copy of the retransmitted   message.  The submission server then decides which recipients to   include in which transaction.2.1.  Fully SMTPUTF8 Lists   Some lists may wish to be fully SMTPUTF8.  That is, all subscribers   are expected to be able to receive SMTPUTF8 mail.  For list hygiene   reasons, such a list would probably want to prevent subscriptions   from addresses that are unable to receive SMTPUTF8 mail.  If a   putative subscriber has a non-ASCII address, it must be able to   receive SMTPUTF8 mail, but there is no way to tell whether a   subscriber with an ASCII address can receive SMTPUTF8 mail short of   sending an SMTPUTF8 probe or confirmation message and somehow finding   out whether it was delivered, e.g., if the user clicked a link in the   confirmation message.Levine & Gellens              Informational                     [Page 4]

RFC 6783          Mailing Lists and Non-ASCII Addresses    November 20122.2.  Mixed SMTPUTF8 and ASCII Lists   Other lists may wish to handle a mixture of SMTPUTF8 and ASCII   subscribers, either as a transitional measure as subscribers upgrade   to SMTPUTF8-capable mail software or as an ongoing feature.  While it   is not possible in general to downgrade SMTPUTF8 mail to ASCII mail,   list software might divide the recipients into two sets, SMTPUTF8 and   ASCII recipients, and create a downgraded version of SMTPUTF8 list   messages to send to ASCII recipients.  See Sections3.2 and3.3.   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 SMTPUTF8 test   message, 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 SMTPUTF8 SMTP session is an SMTPUTF8 message or might examine the   headers and body of the message to see whether it needs SMTPUTF8   treatment.  Depending on the interface between the list software and   the Mail Transfer Agent (MTA) and Mail Delivery Agent (MDA) that   handle incoming messages, it may not be able to tell the type of   session for incoming messages.2.3.  SMTP Issues   Mailing list software usually changes the envelope addresses on each   message.  The bounce address is set to an address that will return   bounces to the list agent, 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 Variable   Envelope Return Paths or VERP [VERP].   The bounce address for a list typically includes the name of the   list, so a list with a non-ASCII name will have a non-ASCII bounce   address.  Given the unknown paths that bounce messages might take,   list software might instead use an ASCII bounce address to make it   more likely that bounces can be delivered back to the list agent.   Similarly, a VERP address for each subscriber typically embeds a   version of the subscriber's address so the VERP bounce address for a   non-ASCII subscriber address will be a non-ASCII address.  For the   same reason, the list software might use ASCII bounce addresses that   encode the recipient's identity in some other way.Levine & Gellens              Informational                     [Page 5]

RFC 6783          Mailing Lists and Non-ASCII Addresses    November 20123.  List Headers   List agents typically add list-specific headers to each message   before resending the message to list recipients.3.1.  SMTPUTF8 List Headers   The list headers in RFCs 2369 [RFC2369] and 2919 [RFC2919] were all   specified before SMTPUTF8 mail existed, and their definitions do not   address where non-ASCII characters might appear.  These include, for   example:   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], "[t]he contents of the list header fields   mostly consist of angle-bracket ('<', '>') enclosed URLs, with   internal whitespace being ignored".  [RFC2919] specifies that "[t]he   list identifier will, in most cases, appear like a host name in a   domain of the list owner".  Since these headers were defined in the   context of ASCII mail, these headers permit only ASCII text,   including in the URLs.   The most commonly used URI schemes in List-* headers tend to be http   and mailto [RFC6068], although they sometimes include https and ftp   and, in principle, can contain any valid URI.   Even if a scheme permits an internationalized form, it should use a   pure ASCII form of the URI described in [RFC3986].  Future work may   extend these header fields or define replacements to directly support   unencoded non-ASCII outside the ASCII repertoire in these and other   header fields, but in the absence of such extension or replacement,   non-ASCII characters can only be included by encoding them as ASCII.Levine & Gellens              Informational                     [Page 6]

RFC 6783          Mailing Lists and Non-ASCII Addresses    November 2012   The encoding technique specified in [RFC3986] is to use a pair of hex   digits preceded by a percent sign, but percent signs have been used   informally in mail addresses to do source routing.  Although few mail   systems still permit source routing, a lot of mail software still   forbids or escapes characters formerly used for source routing, which   can lead to unfortunate interactions with percent-encoded URIs or any   URI that includes one of those characters.  If a program interpreting   a mailto: URI knew that the Mail User Agent (MUA) in use were able to   handle non-ASCII data, the program could pass the URI in unencoded   non-ASCII, avoiding problems with misinterpreted percent signs, but   at this point, there is no standard or even informal way for MUAs to   signal SMTPUTF8 capabilities.  Also, note that whether   internationalized domain names should be percent-encoded or appear in   A-label form [RFC5890] depends on the context in which they occur.   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 [RFC5228], and so forth.  If the definition of   a List-ID header field were to be extended to allow non-ASCII text,   filters and tests might not properly compare encoded and unencoded   versions of a non-ASCII value.  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   If list software prepares a downgraded version of an SMTPUTF8   message, all the List-* headers must be downgraded.  In particular,   if a List-* header contains a non-ASCII mailto (even encoded in   ASCII), it may be advisable to edit the header to remove the non-   ASCII 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.  If a   header that contains a mailto URL is downgraded by percent encoding,   some mail software may misinterpret the percent signs as attempted   source routing.   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 non-ASCII 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.Levine & Gellens              Informational                     [Page 7]

RFC 6783          Mailing Lists and Non-ASCII Addresses    November 2012   If lists permit mail with multiple MIME parts, some MIME headers in   SMTPUTF8 messages may include non-ASCII characters in file names 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 references 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 recipients 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 non-ASCII address,   there is no way to downgrade the From: header and preserve the   address so that ASCII recipients can respond to it, since non-   SMTPUTF8 mail systems can't send mail to non-ASCII addresses.   Possible work-arounds (none implemented that we know of) might   include allowing subscribers with non-ASCII 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 directly to the submitter.4.  Security Considerations   None beyond what mailing list agents do now.5.  References5.1.  Normative References   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform              Resource Identifier (URI): Generic Syntax", STD 66,RFC 3986, January 2005.   [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'              URI Scheme",RFC 6068, October 2010.   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",              STD 72,RFC 6409, November 2011.   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for              Internationalized Email",RFC 6530, February 2012.Levine & Gellens              Informational                     [Page 8]

RFC 6783          Mailing Lists and Non-ASCII Addresses    November 20125.2.  Informative References   [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.   [RFC2919]  Chandhok, R. and G. Wenger, "List-Id: A Structured Field              and Namespace for the Identification of Mailing Lists",RFC 2919, March 2001.   [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering              Language",RFC 5228, January 2008.   [RFC5890]  Klensin, J., "Internationalized Domain Names for              Applications (IDNA): Definitions and Document Framework",RFC 5890, August 2010.   [VERP]     Bernstein, D., "Variable Envelope Return Paths",              February 1997, <http://cr.yp.to/proto/verp.txt>.Authors' Addresses   John Levine   Taughannock Networks   PO Box 727   Trumansburg, NY  14886   US   Phone: +1 831 480 2300   EMail: standards@taugh.com   URI:http://jl.ly   Randall Gellens   Qualcomm Incorporated   5775 Morehouse Drive   San Diego, CA  92121   US   EMail: rg+ietf@qti.qualcomm.comLevine & Gellens              Informational                     [Page 9]