Email Address Internationalization J. Klensin (EAI) Internet-Draft Y. Ko Obsoletes: RFCs 4952, 5504, 5825 ICU (if approved) September 17, 2010 Intended status: Informational Expires: March 21, 2011 Overview and Framework for Internationalized Email draft-ietf-eai-frmwrk-4952bis-08 Abstract Full use of electronic mail throughout the world requires that (subject to other constraints) people be able to use close variations on their own names (written correctly in their own languages and scripts) as mailbox names in email addresses. This document introduces a series of specifications that define mechanisms and protocol extensions needed to fully support internationalized email addresses. These changes include an SMTP extension and extension of email header syntax to accommodate UTF-8 data. The document set also includes discussion of key assumptions and issues in deploying fully internationalized email. This document is an update of RFC 4952; it reflects additional issues identified since that document was published. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on March 21, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. Klensin & Ko Expires March 21, 2011 [Page 1]
Internet-Draft EAI Framework September 2010 This document is subject to BCP 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. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Klensin & Ko Expires March 21, 2011 [Page 2]
Internet-Draft EAI Framework September 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Role of This Specification . . . . . . . . . . . . . . . . . . 5 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Mail User and Mail Transfer Agents . . . . . . . . . . . . 7 4.2. Address Character Sets . . . . . . . . . . . . . . . . . . 8 4.3. User Types . . . . . . . . . . . . . . . . . . . . . . . . 8 4.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.5. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 9 4.6. Conventional Message and Internationalized Message . . . . 9 4.7. Undeliverable Messages, Notification, and Delivery Receipts . . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Overview of the Approach and Document Plan . . . . . . . . . . 10 6. Review of Experimental Results . . . . . . . . . . . . . . . . 10 7. Overview of Protocol Extensions and Changes . . . . . . . . . 11 7.1. SMTP Extension for Internationalized Email Address . . . . 11 7.2. Transmission of Email Header Fields in UTF-8 Encoding . . 12 7.3. SMTP Service Extension for DSNs . . . . . . . . . . . . . 13 8. Downgrading before and after SMTP Transactions . . . . . . . . 13 8.1. Downgrading before or during Message Submission . . . . . 14 8.2. Downgrading or Other Processing After Final SMTP Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 15 9. Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 16 10. User Interface and Configuration Issues . . . . . . . . . . . 16 10.1. Choices of Mailbox Names and Unicode Normalization . . . . 16 11. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 18 11.1. Impact on URIs and IRIs . . . . . . . . . . . . . . . . . 18 11.2. Use of Email Addresses as Identifiers . . . . . . . . . . 18 11.3. Encoded Words, Signed Messages, and Downgrading . . . . . 18 11.4. Other Uses of Local Parts . . . . . . . . . . . . . . . . 19 11.5. Non-Standard Encapsulation Formats . . . . . . . . . . . . 19 12. Key Changes From the Experimental Protocols and Framework . . 19 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 14. Security Considerations . . . . . . . . . . . . . . . . . . . 20 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 16.1. Normative References . . . . . . . . . . . . . . . . . . . 22 16.2. Informative References . . . . . . . . . . . . . . . . . . 23 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 27 A.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 27 A.2. Changes between -01 and -02 . . . . . . . . . . . . . . . 27 A.3. Changes between -02 and -03 . . . . . . . . . . . . . . . 29 A.4. Changes between -03 and -04 . . . . . . . . . . . . . . . 29 A.5. Changes between -04 and -05 . . . . . . . . . . . . . . . 29 A.6. Changes between -05 and -06 . . . . . . . . . . . . . . . 29 A.7. Changes between -06 and -07 . . . . . . . . . . . . . . . 29 Klensin & Ko Expires March 21, 2011 [Page 3]
Internet-Draft EAI Framework September 2010 A.8. Changes between -07 and -08 (after IETF Last Call) . . . . 30 Klensin & Ko Expires March 21, 2011 [Page 4]
Internet-Draft EAI Framework September 2010 1. Introduction Note in Draft and to RFC Editor: The keyword represented in this document by "UTF8SMTPbis" (and in the XML source by &EAISMTPkeyword;) is a placeholder. The actual keyword will be assigned when the standards track SMTP extension in this series [RFC5336bis-SMTP] is approved for publication and should be substituted here. This paragraph should be treated as normative reference to that SMTP extension draft, creating a reference hold until it is approved by the IESG. The paragraph should be removed before RFC publication. In order to use internationalized email addresses, we need to internationalize both the domain part and the local part of email addresses. The domain part of email addresses is already internationalized [RFC5890], while the local part is not. Without the extensions specified in this document, the mailbox name is restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME [RFC2045] enables the transport of non-ASCII data, it does not provide a mechanism for internationalized email addresses. In RFC 2047 [RFC2047], MIME defines an encoding mechanism for some specific message header fields to accommodate non-ASCII data. However, it does not permit the use of email addresses that include non-ASCII characters. Without the extensions defined here, or some equivalent set, the only way to incorporate non-ASCII characters in any part of email addresses is to use RFC 2047 coding to embed them in what RFC 5322 [RFC5322] calls the "display name" (known as a "name phrase" or by other terms elsewhere) of the relevant header fields. Information coded into the display name is invisible in the message envelope and, for many purposes, is not part of the address at all. This document is an update of RFC 4952 [RFC4952]; it reflects additional issues, shared terminology, and some architectural changes identified since that document was published. The pronouns "he" and "she" are used interchangeably to indicate a human of indeterminate gender. 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 BCP 14, RFC 2119 [RFC2119]. Although this document is Informational, those requirements are consistent with requirements specified in the Standards Track documents in this set as described in Section 5. 2. Role of This Specification This document presents the overview and framework for an approach to the next stage of email internationalization. This new stage Klensin & Ko Expires March 21, 2011 [Page 5]
Internet-Draft EAI Framework September 2010 requires not only internationalization of addresses and header fields, but also associated transport and delivery models. A prior version of this specification, RFC 4952 [RFC4952], also provided an introduction to a series of experimental protocols [RFC5335] [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. This revised form provides overview and conceptual information for the standards-track successors of a subset of those protocols. Details of the documents and the relationships among them appear in Section 5 and a discussion of what was learned from the Experimental protocols and their implementations appears in Section 6. Taken together, these specifications provide the details for a way to implement and support internationalized email. The document itself describes how the various elements of email internationalization fit together and the relationships among the primary specifications associated with message transport, header formats, and handling. This document, and others that comprise the collection described above, assume a reasonable familiarity with the basic Internet electronic mail specifications and terminology [RFC5321][RFC5322] and the MIME [RFC2045] and 8BITMIME [RFC1652] ones as well. While not strictly required to implement this specification, a general familiarity with the terminology and functions of IDNA [RFC5890][RFC5891] [RFC5892][RFC5893] [RFC5894] are also assumed. 3. Problem Statement Internationalizing Domain Names in Applications (IDNA) [RFC5890] permits internationalized domain names, but deployment has not yet reached most users. One of the reasons for this is that we do not yet have fully internationalized naming schemes. Domain names are just one of the various names and identifiers that are required to be internationalized. In many contexts, until more of those identifiers are internationalized, internationalized domain names alone have little value. Email addresses are prime examples of why it is not good enough to just internationalize the domain name. As most observers have learned from experience, users strongly prefer email addresses that resemble names or initials to those involving seemingly meaningless strings of letters or numbers. Unless the entire email address can use familiar characters and formats, users will perceive email as being culturally unfriendly. If the names and initials used in email addresses can be expressed in the native languages and writing systems of the users, the Internet will be perceived as more natural, especially by those whose native language is not written in a subset of a Roman-derived script. Klensin & Ko Expires March 21, 2011 [Page 6]
Internet-Draft EAI Framework September 2010 Internationalization of email addresses is not merely a matter of changing the SMTP envelope; or of modifying the From, To, and Cc header fields; or of permitting upgraded Mail User Agents (MUAs) to decode a special coding and respond by displaying local characters. To be perceived as usable, the addresses must be internationalized and handled consistently in all of the contexts in which they occur. This requirement has far-reaching implications: collections of patches and workarounds are not adequate. Even if they were adequate, a workaround-based approach may result in an assortment of implementations with different sets of patches and workarounds having been applied with consequent user confusion about what is actually usable and supported. Instead, we need to build a fully internationalized email environment, focusing on permitting efficient communication among those who share a language or other community. That, in turn, implies changes to the mail header environment to permit the full range of Unicode characters where that makes sense, an SMTP Extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing and delivery of those extended header fields, support for internationalization of delivery and service notifications [RFC3461] [RFC3464], and (finally) a requirement for support of the 8BITMIME SMTP Extension [RFC1652] so that all of these can be transported through the mail system without having to overcome the limitation that header fields do not have content-transfer-encodings. 4. Terminology This document assumes a reasonable understanding of the protocols and terminology of the core email standards as documented in [RFC5321] and [RFC5322]. 4.1. Mail User and Mail Transfer Agents Much of the description in this document depends on the abstractions of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"). However, it is important to understand that those terms and the underlying concepts postdate the design of the Internet's email architecture and the application of the "protocols on the wire" principle to it. That email architecture, as it has evolved, and that "on the wire" principle have prevented any strong and standardized distinctions about how MTAs and MUAs interact on a given origin or destination host (or even whether they are separate). However, the term "final delivery MTA" is used in this document in a fashion equivalent to the term "delivery system" or "final delivery system" of RFC 5321. This is the SMTP server that controls the format of the local parts of addresses and is permitted to inspect and interpret them. It receives messages from the network for delivery to mailboxes or for other local processing, including any Klensin & Ko Expires March 21, 2011 [Page 7]
Internet-Draft EAI Framework September 2010 forwarding or aliasing that changes envelope addresses, rather than relaying. From the perspective of the network, any local delivery arrangements such as saving to a message store, handoff to specific message delivery programs or agents, and mechanisms for retrieving messages are all "behind" the final delivery MTA and hence are not part of the SMTP transport or delivery process. 4.2. Address Character Sets In this document, an address is "all-ASCII", or just an "ASCII address", if every character in the address is in the ASCII character repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address", if any character is not in the ASCII character repertoire. Such addresses MAY be restricted in other ways, but those restrictions are not relevant to this definition. The term "all-ASCII" is also applied to other protocol elements when the distinction is important, with "non-ASCII" or "internationalized" as its opposite. The umbrella term to describe the email address internationalization specified by this document and its companion documents is "UTF8SMTPbis". [[anchor3: Note in Draft: Keyword to be changed before publication.]] For example, an address permitted by this specification is referred to as a "UTF8SMTPbis (compliant) address". Please note that, according to the definitions given here, the set of all "all-ASCII" addresses and the set of all "non-ASCII" addresses are mutually exclusive. The set of all addresses permitted when UTF8SMTPbis appears is the union of these two sets. 4.3. User Types An "ASCII user" (i) exclusively uses email addresses that contain ASCII characters only, and (ii) cannot generate recipient addresses that contain non-ASCII characters. An "i18mail user" has one or more non-ASCII email addresses, or is able to generate recipient addresses that contain non-ASCII characters. Such a user may have ASCII addresses too; if the user has more than one email account and a corresponding address, or more than one alias for the same address, he or she has some method to choose which address to use on outgoing email. Note that under this definition, it is not possible to tell from an ASCII address if the owner of that address is an i18mail user or not. (A non-ASCII address implies a belief that the owner of that address is an i18mail user.) There is no such thing as an "i18mail message"; the term applies only to users and their agents and capabilities. In particular, the use of non-ASCII message content is an integral part Klensin & Ko Expires March 21, 2011 [Page 8]
Internet-Draft EAI Framework September 2010 of the MIME specifications [RFC2045] and does not require these extensions (although it is compatible with them). 4.4. Messages A "message" is sent from one user (sender) using a particular email address to one or more other recipient email addresses (often referred to just as "users" or "recipient users"). 4.5. Mailing Lists A "mailing list" is a mechanism whereby a message may be distributed to multiple recipients by sending it to one recipient address. An agent (typically not a human being) at that single address then causes the message to be redistributed to the target recipients. This agent sets the envelope return address of the redistributed message to a different address from that of the original single recipient message. Using a different envelope return address (reverse-path) causes error (and other automatically generated) messages to go to an error handling address. Special provisions for managing mailing lists that might contain non- ASCII addresses are discussed in a document that is specific to that topic [EAI-Mailinglist] [RFCNNNNbis-MailingList]. 4.6. Conventional Message and Internationalized Message o A conventional message is one that does not use any extension defined in the SMTP extension document [RFC5336] or in the UTF8header specification [RFC5335], and is strictly conformant to RFC 5322 [RFC5322]. o An internationalized message is a message utilizing one or more of the extensions defined in this set of specifications, so that it is no longer conformant to the traditional specification of an email message or its transport. 4.7. Undeliverable Messages, Notification, and Delivery Receipts As specified in RFC 5321, a message that is undeliverable for some reason is expected to result in notification to the sender. This can occur in either of two ways. One, typically called "Rejection", occurs when an SMTP server returns a reply code indicating a fatal error (a "5yz" code) or persistently returns a temporary failure error (a "4yz" code). The other involves accepting the message during SMTP processing and then generating a message to the sender, typically known as a "Non-delivery Notification" or "NDN". Current practice often favors rejection over NDNs because of the reduced Klensin & Ko Expires March 21, 2011 [Page 9]
Internet-Draft EAI Framework September 2010 likelihood that the generation of NDNs will be used as a spamming technique. The latter, NDN, case is unavoidable if an intermediate MTA accepts a message that is then rejected by the next-hop server. A sender MAY also explicitly request message receipts [RFC3461] that raise the same issues for these internationalization extensions as NDNs. 5. Overview of the Approach and Document Plan This set of specifications changes both SMTP and the character encoding of email message headers to permit non-ASCII characters to be represented directly. Each important component of the work is described in a separate document. The document set, whose members are described below, also contains informational documents whose purpose is to provide implementation suggestions and guidance for the protocols. In addition to this document, the following documents make up this specification and provide advice and context for it. o SMTP extensions. This document [RFC5336bis-SMTP] provides an SMTP extension (as provided for in RFC 5321) for internationalized addresses. o Email message headers in UTF-8. This document [RFC5335bis-Hdrs] essentially updates RFC 5322 to permit some information in email message headers to be expressed directly by Unicode characters encoded in UTF-8 when the SMTP extension described above is used. This document, possibly with one or more supplemental ones, will also need to address the interactions with MIME, including relationships between UTF8SMTPbis and internal MIME headers and content types. o Extensions to delivery status and notification handling to adapt to internationalized addresses [RFC5337bis-DSN]. o Extensions to the IMAP protocol to support internationalized message headers [RFC5738bis-IMAP]. o Parallel extensions to the POP protocol [RFC5721] [RFC5721bis-POP3]. 6. Review of Experimental Results The key difference between this set of protocols and the experimental set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a Klensin & Ko Expires March 21, 2011 [Page 10]
Internet-Draft EAI Framework September 2010 mechanism for in-transit downgrading of messages (described in detail in RFC 5504). That mechanism permitted, and essentially required, that each non-ASCII address be accompanied by an all-ASCII equivalent. That, in turn, raised security concerns associated with pairing of addresses that could not be authenticated. It also introduced the first incompatible change to Internet mail addressing in many years, raising concerns about interoperability issues if the new address forms "leaked" into legacy email implementations. The WG concluded that the advantages of in-transit downgrading, were it feasible operationally, would be significant enough to overcome those concerns. Operationally that turned out to not be the case, with interoperability problems among initial implementations. Prior to starting on the work that led to this set of specifications, the WG concluded that the combination of requirements and long-term implications of that earlier model were too complex to be satisfactory and that work should move ahead without it. 7. Overview of Protocol Extensions and Changes 7.1. SMTP Extension for Internationalized Email Address An SMTP extension, "UTF8SMTPbis" is specified as follows: o Permits the use of UTF-8 strings in email addresses, both local parts and domain names. o Permits the selective use of UTF-8 strings in email message headers (see Section 7.2). o Requires that the server advertise the 8BITMIME extension [RFC1652] and that the client support 8-bit transmission so that header information can be transmitted without using a special content-transfer-encoding. Some general principles affect the development decisions underlying this work. 1. Email addresses enter subsystems (such as a user interface) that may perform charset conversions or other encoding changes. When the left hand side of the address includes characters outside the US-ASCII character repertoire, use of ASCII-compatible encoding (ACE) [RFC3492] [RFC5890] on the right hand side is discouraged to promote consistent processing of characters throughout the address. Klensin & Ko Expires March 21, 2011 [Page 11]
Internet-Draft EAI Framework September 2010 2. An SMTP relay MUST * Either recognize the format explicitly, agreeing to do so via an ESMTP option, or * Reject the message or, if necessary, return a non-delivery notification message, so that the sender can make another plan. 3. If the message cannot be forwarded because the next-hop system cannot accept the extension it MUST be rejected or a non-delivery message MUST be generated and sent. 4. In the interest of interoperability, charsets other than UTF-8 are prohibited in mail addresses and message headers being transmitted over the Internet. There is no practical way to identify multiple charsets properly with an extension similar to this without introducing great complexity. Conformance to the group of standards specified here for email transport and delivery requires implementation of the SMTP Extension specification and the UTF-8 Header specification. If the system implements IMAP or POP, it MUST conform to the i18n IMAP or POP specifications respectively. 7.2. Transmission of Email Header Fields in UTF-8 Encoding There are many places in MUAs or in a user presentation in which email addresses or domain names appear. Examples include the conventional From, To, or Cc header fields; Message-ID and In-Reply-To header fields that normally contain domain names (but that may be a special case); and in message bodies. Each of these must be examined from an internationalization perspective. The user will expect to see mailbox and domain names in local characters, and to see them consistently. If non-obvious encodings, such as protocol-specific ASCII-Compatible Encoding (ACE) variants, are used, the user will inevitably, if only occasionally, see them rather than "native" characters and will find that discomfiting or astonishing. Similarly, if different codings are used for mail transport and message bodies, the user is particularly likely to be surprised, if only as a consequence of the long-established "things leak" principle. The only practical way to avoid these sources of discomfort, in both the medium and the longer term, is to have the encodings used in transport be as similar to the encodings used in message headers and message bodies as possible. When email local parts are internationalized, they SHOULD be accompanied by arrangements for the message headers to be in the Klensin & Ko Expires March 21, 2011 [Page 12]
Internet-Draft EAI Framework September 2010 fully internationalized form. That form SHOULD use UTF-8 rather than ASCII as the base character set for the contents of header fields (protocol elements such as the header field names themselves are unchanged and remain entirely in ASCII). For transition purposes and compatibility with legacy systems, this can done by extending the traditional MIME encoding models for non-ASCII characters in headers [RFC2045] [RFC2231], but even these should be based on UTF-8. rather than other encodings, if at all possible [IAB-idn-encoding]. However, the target is fully internationalized message headers, as discussed in [RFC5335bis-Hdrs] and not an extended and painful transition. 7.3. SMTP Service Extension for DSNs The existing Draft Standard Delivery status notifications (DSNs) specification [RFC3461] is limited to ASCII text in the machine readable portions of the protocol. "International Delivery and Disposition Notifications" [RFC5337bis-DSN] adds a new address type for international email addresses so an original recipient address with non-ASCII characters can be correctly preserved even after downgrading. If an SMTP server advertises both the UTF8SMTPbis and the DSN extension, that server MUST implement internationalized DSNs including support for the ORCPT parameter specified in RFC 3461 [RFC3461]. 8. Downgrading before and after SMTP Transactions An important issue with these extensions is how to handle interactions between systems that support non-ASCII addresses and legacy systems that expect ASCII. There is, of course, no problem with ASCII-only systems sending to those that can handle internationalized forms because the ASCII forms are just a proper subset. But, when systems that support these extensions send mail, they MAY include non-ASCII addresses for senders, receivers, or both and might also provide non-ASCII header information other than addresses. If the extension is not supported by the first-hop system (SMTP server accessed by the Submission server acting as an SMTP client), message originating systems SHOULD be prepared to either send conventional envelopes and message headers or to return the message to the originating user so the message may be manually downgraded to the traditional form, possibly using encoded words [RFC2047] in the message headers. Of course, such transformations imply that the originating user or system must have ASCII-only addresses available for all senders and recipients. Mechanisms by which such addresses may be found or identified are outside the scope of these specifications as are decisions about the design of originating systems such as whether any required transformations are made by the user, the originating MUA, or the Submission server. Klensin & Ko Expires March 21, 2011 [Page 13]
Internet-Draft EAI Framework September 2010 A somewhat more complex situation arises when the first-hop system supports these extensions but some subsequent server in the SMTP transmission chain does not. It is important to note that most cases of that situation with forward-pointing addresses will be the result of configuration errors: especially if it hosts non-ASCII addresses, a final delivery MTA that accepts these extensions SHOULD NOT be configured with lower-preference MX hosts that do not. When the only non-ASCII address being transmitted is backward-pointing (e.g., in an SMTP MAIL command), recipient configuration can not help in general. On the other hand, alternate, all-ASCII, addresses for senders are those most likely to be authoritatively known by the submission environment or the sender herself. Consequently, if an intermediate SMTP relay that requires these extensions then discovers that the next system in the chain does not support them, it will have little choice other than to reject or return the message. As discussed above, downgrading to an ASCII-only form may occur before or during the initial message submission. It might also occur after the delivery to the final delivery MTA in order to accommodate messages stores or IMAP or POP servers or clients that have different capabilities than the delivery MTA. These two cases are discussed in the subsections below. 8.1. Downgrading before or during Message Submission The IETF has traditionally avoided specifying the precise behavior of MUAs to provide maximum flexibility in the associated user interfaces. The SMTP standard [RFC5321], Section 6.4, gives wide latitude to MUAs and Submission servers as to what might be supplied by the user as long as the result conforms with "on the wire" standards once it is injected into the public Internet. In that tradition, the discussion in the remainder of this section is provided as general guidance rather than normative requirements. It is likely that the most common cases in which a message that requires these extensions is sent to a system that does not will involve the combination of ASCII-only forward-pointing addresses with a non-ASCII backward-pointing one. Until the extensions described here have been universally implemented in the Internet email environment, senders who prefer to use non-ASCII addresses (or raw UTF-8 characters in header fields) even when their intended recipients use and expect all-ASCII ones will need to be especially careful about the error conditions that can arise, especially if they are working in an environment in which non-delivery messages (or other indications from submission servers) are routinely dropped or ignored. Perhaps obviously, the most convenient time to find an ASCII address Klensin & Ko Expires March 21, 2011 [Page 14]
Internet-Draft EAI Framework September 2010 corresponding to an internationalized address is at the originating MUA or closely-associated systems. This can occur either before the message is sent or after the internationalized form of the message is rejected. It is also the most convenient time to convert a message from the internationalized form into conventional ASCII form or to generate a non-delivery message to the sender if either is necessary. At that point, the user has a full range of choices available, including changing backward-pointing addresses, contacting the intended recipient out of band for an alternate address, consulting appropriate directories, arranging for translation of both addresses and message content into a different language, and so on. While it is natural to think of message downgrading as optimally being a fully-automated process, we should not underestimate the capabilities of a user of at least moderate intelligence who wishes to communicate with another such user. In this context, one can easily imagine modifications to message submission servers (as described in RFC 4409 [RFC4409]) so that they would perform downgrading operations or perhaps even upgrading ones. Such operations would permit receiving messages with one or more of the internationalization extensions discussed here and adapting the outgoing message, as needed, to respond to the delivery or next-hop environment the submission server encounters. 8.2. Downgrading or Other Processing After Final SMTP Delivery When an email message is received by a final delivery MTA, it is usually stored in some form. Then it is retrieved either by software that reads the stored form directly or by client software via some email retrieval mechanisms such as POP or IMAP. The SMTP extension described in Section 7.1 provides protection only in transport. It does not prevent MUAs and email retrieval mechanisms that have not been upgraded to understand internationalized addresses and UTF-8 message headers from accessing stored internationalized emails. Since the final delivery MTA (or, to be more specific, its corresponding mail storage agent) cannot safely assume that agents accessing email storage will always be capable of handling the extensions proposed here, it MAY downgrade internationalized emails, specially identify messages that utilize these extensions, or both. If this is done, the final delivery MTA SHOULD include a mechanism to preserve or recover the original internationalized forms without information loss to support access by UTF8SMTPbis-aware agents. Klensin & Ko Expires March 21, 2011 [Page 15]
Internet-Draft EAI Framework September 2010 9. Downgrading in Transit The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321]) states that "due to a long history of problems when intermediate hosts have attempted to optimize transport by modifying them, the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address". This is not a new requirement; equivalent statements appeared in specifications in 2001 [RFC2821] and even in 1989 [RFC1123]. Adherence to this rule means that a downgrade mechanism that transforms the local-part of an email address cannot be utilized in transit. It can only be applied at the endpoints, specifically by the MUA or submission server or by the final delivery MTA. One of the reasons for this rule has to do with legacy email systems that embed mail routing information in the local-part of the address field. Transforming the email address destroys such routing information. There is no way a server other than the final delivery server can know, for example, whether the local-part of user%foo@example.com is a route ("user" is reached via "foo") or simply a local address. 10. User Interface and Configuration Issues Internationalization of addresses and message headers, especially in combination with variations on character coding that are inherent to Unicode, may make careful choices of addresses and careful configuration of servers and DNS records even more important than they are for traditional Internet email. It is likely that, as experience develops with the use of these protocols, it will be desirable to produce one or more additional documents that offer guidance for configuration and interfaces. A document that discusses issues with mail user agents (MUAs), especially with regard to downgrading [EAI-MUA-issues], is expected to be developed in the EAI Working Group. The subsections below address some other issues. 10.1. Choices of Mailbox Names and Unicode Normalization It has long been the case that the email syntax permits choices about mailbox names that are unwise in practice if one actually intends the mailboxes to be accessible to a broad range of senders. The most- often-cited examples involve the use of case-sensitivity and tricky quoting of embedded characters in mailbox local parts. While these are permitted by the protocols and servers are expected to support them and there are special cases where they can provide value, taking advantage of those features is almost always bad practice unless the intent is to create some form of security by obscurity. Klensin & Ko Expires March 21, 2011 [Page 16]
Internet-Draft EAI Framework September 2010 In the absence of these extensions, SMTP clients and servers are constrained to using only those addresses permitted by RFC 5321. The local parts of those addresses MAY be made up of any ASCII characters except the control characters that 5321 prohibits, although some of them MUST be quoted as specified there. It is notable in an internationalization context that there is a long history on some systems of using overstruck ASCII characters (a character, a backspace, and another character) within a quoted string to approximate non-ASCII characters. This form of internationalization was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321 because it requires a backspace character (a prohibited C0 control). Because RFC 5321 (and its predecessor, RFC 2821) prohibit the use of this character in ASCII mailbox names and it is even more problematic (for canonicalization and normalization reasons) in non-ASCII strings, backspace MUST NOT appear in UTF8SMTPbis mailbox names. For the particular case of EAI mailbox names, special attention MUST be paid to Unicode normalization [Unicode-UAX15], in part because Unicode strings may be normalized by other processes independent of what a mail protocol specifies (this is exactly analogous to what may happen with quoting and dequoting in traditional addresses). Consequently, the following principles are offered as advice to those who are selecting names for mailboxes: o In general, it is wise to support addresses in Normalized form, using either Normalization Form NFC and, except in unusual circumstances, NFKC. o It may be wise to support other forms of the same local-part string, either as aliases or by normalization of strings reaching the delivery server, in the event that the sender does not send the strings in normalized form. o Stated differently and in more specific terms, the rules of the protocol for local-part strings essentially provide that: * Unnormalized strings are valid, but sufficiently bad practice that they may not work reliably on a global basis. * C0 (and presumably C1) controls (see The Unicode Standard [Unicode52]) are prohibited, the first in RFC 5321 and the second by an obvious extension from it [RFC5198]. * Other kinds of punctuation, spaces, etc., are risky practice. Perhaps they will work, and SMTP receiver code is required to handle them, but creating dependencies on them in mailbox names that are chosen is usually a bad practice and may lead to interoperability problems. Klensin & Ko Expires March 21, 2011 [Page 17]
Internet-Draft EAI Framework September 2010 11. Additional Issues This section identifies issues that are not covered, or not covered comprehensively, as part of this set of specifications, but that will require ongoing review as part of deployment of email address and header internationalization. 11.1. Impact on URIs and IRIs The mailto: schema [RFC2368] and discussed in the Internationalized Resource Identifier (IRI) specification [RFC3987] may need to be modified when this work is completed and standardized. 11.2. Use of Email Addresses as Identifiers There are a number of places in contemporary Internet usage in which email addresses are used as identifiers for individuals, including as identifiers to Web servers supporting some electronic commerce sites and in some X.509 certificates [RFC5280]. These documents do not address those uses, but it is reasonable to expect that some difficulties will be encountered when internationalized addresses are first used in those contexts, many of which cannot even handle the full range of addresses permitted today. 11.3. Encoded Words, Signed Messages, and Downgrading One particular characteristic of the email format is its persistency: MUAs are expected to handle messages that were originally sent decades ago and not just those delivered seconds ago. As such, MUAs and mail filtering software, such as that specified in Sieve [RFC5228], will need to continue to accept and decode header fields that use the "encoded word" mechanism [RFC2047] to accommodate non- ASCII characters in some header fields. While extensions to both POP3 [RFC1939] and IMAP [RFC3501] have been defined that include automatic upgrading of messages that carry non-ASCII information in encoded form -- including RFC 2047 decoding -- of messages by the POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are message structures and MIME content-types for which that cannot be done or where the change would have unacceptable side effects. For example, message parts that are cryptographically signed, using e.g., S/MIME [RFC3851] or Pretty Good Privacy (PGP) [RFC3156], cannot be upgraded from the RFC 2047 form to normal UTF-8 characters without breaking the signature. Similarly, message parts that are encrypted may contain, when decrypted, header fields that use the RFC 2047 encoding; such messages cannot be 'fully' upgraded without access to cryptographic keys. Klensin & Ko Expires March 21, 2011 [Page 18]
Internet-Draft EAI Framework September 2010 Similar issues may arise if messages are signed and then subsequently downgraded, e.g., as discussed in Section 8.1, and then an attempt is made to upgrade them to the original form and then verify the signatures. Even the very subtle changes that may result from algorithms to downgrade and then upgrade again may be sufficient to invalidate the signatures if they impact either the primary or MIME bodypart headers. When signatures are present, downgrading MUST be performed with extreme care if at all. 11.4. Other Uses of Local Parts Local parts are sometimes used to construct domain labels, e.g., the local part "user" in the address user@domain.example could be converted into a vanity host user.domain.example with its Web space at <http://user.domain.example> and the catchall addresses any.thing.goes@user.domain.example. Such schemes are obviously limited by, among other things, the SMTP rules for domain names, and will not work without further restrictions for other local parts such as the <utf8-local-part> specified in [RFC5335bis-Hdrs]. Whether those limitations are relevant to these specifications is an open question. It may be simply another case of the considerable flexibility accorded to delivery MTAs in determining the mailbox names they will accept and how they are interpreted. 11.5. Non-Standard Encapsulation Formats Some applications use formats similar to the application/mbox format defined in [RFC4155] instead of the message/digest form described in RFC 2046, Section 5.1.5 [RFC2046] to transfer multiple messages as single units. Insofar as such applications assume that all stored messages use the message/rfc822 format described in RFC 2046, Section 5.2.1 [RFC2046] with US-ASCII message headers, they are not ready for the extensions specified in this series of documents and special measures may be needed to properly detect and process them. 12. Key Changes From the Experimental Protocols and Framework The original framework for internationalized email addresses and headers was described in RFC 4952 and a subsequent set of experimental protocol documents. Those relationships are described in Section 3. The key architectural difference between the experimental specifications and this newer set is that the earlier specifications supported in-transit downgrading including providing syntax and functions to support passing alternate, all-ASCII, addresses with the non-ASCII ones and special headers to indicate the downgraded status of messages. Those features were eliminated after Klensin & Ko Expires March 21, 2011 [Page 19]
Internet-Draft EAI Framework September 2010 experimentation indicated that they were more complex and less necessary than had been assumed earlier. Those issues are described in more detail in Section 6 and Section 9. 13. IANA Considerations This overview description and framework document does not contemplate any IANA registrations or other actions. Some of the documents in the group have their own IANA considerations sections and requirements. 14. Security Considerations Any expansion of permitted characters and encoding forms in email addresses raises some risks. There have been discussions on so called "IDN-spoofing" or "IDN homograph attacks". These attacks allow an attacker (or "phisher") to spoof the domain or URLs of businesses. The same kind of attack is also possible on the local part of internationalized email addresses. It should be noted that the proposed fix involving forcing all displayed elements into normalized lower-case works for domain names in URLs, but not email local parts since those are case sensitive. Since email addresses are often transcribed from business cards and notes on paper, they are subject to problems arising from confusable characters (see [RFC4690]). These problems are somewhat reduced if the domain associated with the mailbox is unambiguous and supports a relatively small number of mailboxes whose names follow local system conventions. They are increased with very large mail systems in which users can freely select their own addresses. The internationalization of email addresses and message headers must not leave the Internet less secure than it is without the required extensions. The requirements and mechanisms documented in this set of specifications do not, in general, raise any new security issues. They do require a review of issues associated with confusable characters -- a topic that is being explored thoroughly elsewhere (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other transformations. Normalization and other issues associated with transformations and standard forms are also part of the subject of work described elsewhere [RFC5198] [RFC5893] [IAB-idn-encoding]. Some issues specifically related to internationalized addresses and message headers are discussed in more detail in the other documents in this set. However, in particular, caution should be taken that any "downgrading" mechanism, or use of downgraded addresses, does not Klensin & Ko Expires March 21, 2011 [Page 20]
Internet-Draft EAI Framework September 2010 inappropriately assume authenticated bindings between the internationalized and ASCII addresses. Expecting and most or all such transformations prior to final delivery be done by systems that are presumed to be under the administrative control of the sending user ameliorates the potential problem somewhat as compared to what it would be if the relationships were changed in transit. The new UTF-8 header and message formats might also raise, or aggravate, another known issue. If the model creates new forms of an 'invalid' or 'malformed' message, then a new email attack is created: in an effort to be robust, some or most agents will accept such message and interpret them as if they were well-formed. If a filter interprets such a message differently than the MUA used by the recipient, then it may be possible to create a message that appears acceptable under the filter's interpretation but that should be rejected under the interpretation given to it by that MUA. Such attacks already exist for existing messages and encoding layers, e.g., invalid MIME syntax, invalid HTML markup, and invalid coding of particular image types. In addition, email addresses are used in many contexts other than sending mail, such as for identifiers under various circumstances (see Section 11.2). Each of those contexts will need to be evaluated, in turn, to determine whether the use of non-ASCII forms is appropriate and what particular issues they raise. This work will clearly affect any systems or mechanisms that are dependent on digital signatures or similar integrity protection for email message headers (see also the discussion in Section 11.3). Many conventional uses of PGP and S/MIME are not affected since they are used to sign body parts but not message headers. On the other hand, the developing work on domain keys identified mail (DKIM) [RFC5863] will eventually need to consider this work and vice versa: while this specification does not address or solve the issues raised by DKIM and other signed header mechanisms, the issues will have to be coordinated and resolved eventually if the two sets of protocols are to co-exist. In addition, to the degree to which email addresses appear in PKI (Public Key Infrastructure) certificates, standards addressing such certificates will need to be upgraded to address these internationalized addresses. Those upgrades will need to address questions of spoofing by look-alikes of the addresses themselves. 15. Acknowledgments This document is an update to, and derived from, RFC 4952. This document would have been impossible without the work and contributions acknowledged in it. The present document benefited Klensin & Ko Expires March 21, 2011 [Page 21]
Internet-Draft EAI Framework September 2010 significantly from discussions in the EAI WG and elsewhere after RFC 4952 was published, especially discussions about the experimental versions of other documents in the internationalized email collection, and from RFC errata on RFC 4952 itself. Special thanks are due to Ernie Dainow for careful reviews and suggested text in this version. 16. References 16.1. Normative References [ASCII] American National Standards Institute (formerly United States of America Standards Institute), "USA Code for Information Interchange", ANSI X3.4-1968, 1968. ANSI X3.4-1968 has been replaced by newer versions with slight modifications, but the 1968 version remains definitive for the Internet. [RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July 1994. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [RFC5335bis-Hdrs] Yang, A. and S. Steele, "Internationalized Email Headers", July 2010, <https:// datatracker.ietf.org/doc/ draft-ietf-eai-rfc5335bis/>. [RFC5336bis-SMTP] Yao, J. and W. Mao, "SMTP Extension for Klensin & Ko Expires March 21, 2011 [Page 22]
Internet-Draft EAI Framework September 2010 Internationalized Email Address", August 2010, <https://datatracker.ietf.org/ doc/draft-ietf-eai-rfc5336bis/>. [RFC5337bis-DSN] Not yet posted?, "Internationalized Delivery Status and Disposition Notifications", Unwritten waiting for I-D, 2010. [RFC5721bis-POP3] Not yet posted?, "POP3 Support for UTF-8", Unwritten waiting for I-D, 2010. [RFC5738bis-IMAP] Not yet posted?, "IMAP Support for UTF-8", Unwritten waiting for I-D, 2010. [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010. [RFCNNNNbis-MailingList] Not yet posted?, "Mailing Lists and Internationalized Email Addresses", First Version still not in RFC Editor queue https ://datatracker.ietf.org/doc/ draft-ietf-eai-mailinglist/, Unwritten waiting for I-D, 2010. 16.2. Informative References [EAI-MUA-issues] EAI WG, "Still-unnamed proposed document on MUA issues", Not assigned or agreed to yet, 2011. Note to IESG and RFC Editor: While there is provision for a document on this subject in the WG Charter, there is, as yet, no plan for producing it or even for adding it to the WG's task list with benchmarks. If the present document is approved for publication before the is at least a title and author(s) for an I-D, the citation and reference should simply be dropped. [EAI-Mailinglist] Gellens, R., "Mailing Lists and Internationalized Email Addresses", June 2010, <https://datatracker.ietf.org/ doc/draft-ietf-eai-mailinglist/>. Klensin & Ko Expires March 21, 2011 [Page 23]
Internet-Draft EAI Framework September 2010 [IAB-idn-encoding] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on Encodings for Internationalized Domain Names", 2010, <htt ps://datatracker.ietf.org/doc/ draft-iab-idn-encoding/>. [RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821, August 1982. [RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033, October 1996. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Characte r Sets, Languages, and Continuations", RFC 2231, November 1997. [RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, "The mailto URL scheme", RFC 2368, July 1998. [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME Security with OpenPGP", Klensin & Ko Expires March 21, 2011 [Page 24]
Internet-Draft EAI Framework September 2010 RFC 3156, August 2001. [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)", RFC 3461, January 2003. [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications", RFC 3464, January 2003. [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003. [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 3851, July 2004. [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005. [RFC4155] Hall, E., "The application/mbox Media Type", RFC 4155, September 2005. [RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", RFC 4409, April 2006. [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and Recommendations for Internationalized Domain Names (IDNs)", RFC 4690, September 2006. [RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC 4952, July 2007. [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network Interchange", RFC 5198, March 2008. Klensin & Ko Expires March 21, 2011 [Page 25]
Internet-Draft EAI Framework September 2010 [RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering Language", RFC 5228, January 2008. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. [RFC5335] Abel, Y., "Internationalized Email Headers", RFC 5335, September 2008. [RFC5336] Yao, J. and W. Mao, "SMTP Extension for Internationalized Email Addresses", RFC 5336, September 2008. [RFC5337] Newman, C. and A. Melnikov, "Internationalized Delivery Status and Disposition Notifications", RFC 5337, September 2008. [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for Email Address Internationalization", RFC 5504, March 2009. [RFC5721] Gellens, R. and C. Newman, "POP3 Support for UTF-8", RFC 5721, February 2010. [RFC5738] Resnick, P. and C. Newman, "IMAP Support for UTF-8", RFC 5738, March 2010. [RFC5825] Fujiwara, K. and B. Leiba, "Displaying Downgraded Messages for Email Address Internationalization", RFC 5825, April 2010. [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, "DomainKeys Identified Mail (DKIM) Development, Deployment, and Operations", RFC 5863, May 2010. [RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, August 2010. [RFC5892] Faltstrom, P., "The Unicode Code Points and Klensin & Ko Expires March 21, 2011 [Page 26]
Internet-Draft EAI Framework September 2010 Internationalized Domain Names for Applications (IDNA)", RFC 5892, August 2010. [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)", RFC 5893, August 2010. [RFC5894] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale", RFC 5894, August 2010. [Unicode-UAX15] The Unicode Consortium, "Unicode Standard Annex #15: Unicode Normalization Forms", March 2008, <http://www.unicode.org/reports/tr15/>. [Unicode52] The Unicode Consortium. The Unicode Standard, Version 5.2.0, defined by:, "The Unicode Standard, Version 5.2.0", (Mountain View, CA: The Unicode Consortium, 2009. ISBN 978-1-936213-00-9)., <http:// www.unicode.org/versions/Unicode5.2.0/>. Appendix A. Change Log [[RFC Editor: Please remove this section prior to publication.]] A.1. Changes between -00 and -01 o Because there has been no feedback on the mailing list, updated the various questions to refer to this version as well. o Reflected RFC Editor erratum #1507 by correcting terminology for headers and header fields and distinguishing between "message headers" and different sorts of headers (e.g., the MIME ones). A.2. Changes between -01 and -02 Note that section numbers in the list that follows may refer to -01 and not -02. o Discussion of RFC 5825 ("downgraded display") has been removed per the earlier note and on-list discussion. Any needed discussion about reconstructed messages will need to appear in the IMAP and POP documents. However, the introductory material has been Klensin & Ko Expires March 21, 2011 [Page 27]
Internet-Draft EAI Framework September 2010 reworded to permit keeping 5504 and 5825 on the list there, without which the back chain would not be complete. For consistency with this change, 5504 and 5825 have been added to the "Obsoletes" list (as far as I know, an Informational spec can obsolete or update Experimental ones, so no downref problem here --JcK). o Reference to alternate addresses dropped from (former) Section 7.1. o Reference to RFC 5504 added to (former) Section 8 for completeness. o Ernie's draft comments added (with some minor edits) to replace the placeholder in (former) Section 9 ("Downgrading in Transit"). It is intended to capture at least an introduction the earlier discussions of algorithmic downgrading generally and ACE/Punycode transformations in particular. Anyone who is unhappy with it should say so and propose alternate text. RSN. o In the interest of clarity and consistency with the terminology in Section 4.1, all uses of "final delivery SMTP server" and "final delivery server" have been changed to "final delivery MTA". o Placeholder at the end of Section 2 has been removed and the text revised to promise less. The "Document Plan" (Section 5) has been revised accordingly. We need to discuss this at IETF 78 if not sooner. o Sections 5 and 6 have been collapsed into one -- there wasn't enough left in the former Section 5 to justify a separate section. o Former Section 11.1 has been dropped and the DSN document moved up into the "Document Plan" as suggested earlier. o Section 12, "Experimental Targets", has been removed. o Updated references for the new version EAI documents and added placeholders for all of the known remaining drafts that will become part of the core EAI series but that have not been written. o Inserted an additional clarification about the relationship of these extensions to non-ASCII messages. o Changed some normative/informative reference classifications based on review of the new text. Klensin & Ko Expires March 21, 2011 [Page 28]
Internet-Draft EAI Framework September 2010 o Removed references to the pre-EAI documents that were cited for historical context in 4952. o Got rid of a remaining pointer to address downgrading in the discussion of an updated MAILTO URI. o Minor additional editorial cleanups and tuning. A.3. Changes between -02 and -03 o Inserted paragraph clarifying the status of the UTF8SMTPbis keyword as a result of discussion prior to and during IETF 79. o Adjusted some references including adding an explicit citation of RFC 821. o Removed the discussion of the experimental work from an inline aside to a separate section, Section 6. o Rewrote the discussion of configuration errors in MX setups to make it clear that they are an issue with forward-pointing addresses only and improved the discussion of backward-pointing addresses. o Removed some now-obsolete placeholder notes and resolved the remaining one to a dangling reference. A.4. Changes between -03 and -04 o Several minor editorial changes. o Added a discussion of the relationship to the base email, MIME, and IDNA specifications. A.5. Changes between -04 and -05 o Several more minor editorial changes. A.6. Changes between -05 and -06 o Corrections to more precisely reflect RFC 2119 language requirements and closely-related issues.. A.7. Changes between -06 and -07 o Added a new section (now Section 12) to explicitly discuss the changes from the previous version. Klensin & Ko Expires March 21, 2011 [Page 29]
Internet-Draft EAI Framework September 2010 o Removed the discussion of LMTP from Section 11; it is more appropriately placed in the SMTP Extension document (5336bis). A.8. Changes between -07 and -08 (after IETF Last Call) o Modified Section 7.2 to make the last paragraph less tentative and more clear. o Modified Section 8.1 to add an introductory paragraph that clarifies what the IETF does and does not specify about email protocols. Authors' Addresses John C KLENSIN 1770 Massachusetts Ave, #322 Cambridge, MA 02140 USA Phone: +1 617 491 5735 EMail: john-ietf@jck.com YangWoo KO ICU 119 Munjiro Yuseong-gu, Daejeon 305-732 Republic of Korea EMail: yw@mrko.pe.kr Klensin & Ko Expires March 21, 2011 [Page 30]
