



Internet Engineering Task Force (IETF)                        D. Crocker
Request for Comments: 9078                   Brandenburg InternetWorking
Category: Experimental                                         R. Signes
ISSN: 2070-1721                                                 Fastmail
                                                                N. Freed
                                                                  Oracle
                                                             August 2021


           Reaction: Indicating Summary Reaction to a Message

Abstract

   The popularity of social media has led to user comfort with easily
   signaling basic reactions to an author's posting, such as with a
   'thumbs up' or 'smiley' graphic.  This specification permits a
   similar facility for Internet Mail.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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 candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9078.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Reaction Content-Disposition
   4.  Reaction Message Processing
   5.  Usability Considerations
     5.1.  Example Message
     5.2.  Example Display
   6.  Security Considerations
   7.  IANA Considerations
   8.  Experimental Goals
   9.  Normative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The popularity of social media has led to user comfort with easily
   signaling summary reactions to an author's posting, by using emoji
   graphics, such as with a 'thumbs up', 'heart', or 'smiley'
   indication.  Sometimes the permitted repertoire is constrained to a
   small set, and sometimes a more extensive range of indicators is
   supported.

   This specification extends this existing practice in social media and
   instant messaging into Internet Mail.

   While it is already possible to include symbols and graphics as part
   of an email reply's content, there has not been an established means
   of signaling the semantic substance that such data are to be taken as
   a summary 'reaction' to the original message -- that is, a mechanism
   to identify symbols as specifically providing a summary reaction to
   the cited message rather than merely being part of the free text in
   the body of a response.  Such a structured use of the symbol(s)
   allows recipient Mail User Agents (MUAs) to correlate this reaction
   to the original message and possibly to display the information
   distinctively.

   This facility defines a new MIME Content-Disposition, to be used in
   conjunction with the In-Reply-To header field, to specify that a part
   of a message containing one or more emojis can be treated as a
   summary reaction to a previous message.

2.  Terminology

   Unless provided here, terminology, architecture, and specification
   notation used in this document are incorporated from:

   *  [Mail-Arch]

   *  [Mail-Fmt]

   *  [MIME]

   Syntax is specified with

   *  [ABNF]

   The ABNF rule emoji-sequence is inherited from [Emoji-Seq]; details
   are in Section 3.

   Normative language, per [RFC2119] and [RFC8174]:

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Reaction Content-Disposition

   A message sent as a reply MAY include a part containing:

   Content-Disposition: reaction

   If such a field is specified, the Content-Type of the part MUST be:

   Content-Type: text/plain; charset=utf-8

   The content of this part is restricted to a single line of emoji.
   The [ABNF] is:

   part-content    = emoji *(WSP emoji) CRLF

   emoji           = emoji-sequence
   emoji-sequence  = { defined in [Emoji-Seq] }

   base-emojis     = thumbs-up / thumbs-down / grinning-face /
                     frowning-face / crying-face
                     ; Basic set of emojis, drawn from [Emoji-Seq]

   ; thumbs-up       = {U+1F44D}
   ; thumbs-down     = {U+1F44E}
   ; grinning-face   = {U+1F600}
   ; frowning-face   = {U+2639}
   ; crying-face     = {U+1F622}

   The part-content is either the message's single MIME body or the
   content portion of the first MIME multipart body part.

   The ABNF rule emoji-sequence is inherited from [Emoji-Seq].  It
   defines a set of Unicode code point sequences, which must then be
   encoded as UTF-8.  Each sequence forms a single pictograph.  The BNF
   syntax used in [Emoji-Seq] differs from [ABNF] and MUST be
   interpreted as used in Unicode documentation.  The referenced
   document describes these as sequences of code points.

      |  Note: The part-content can first be parsed into candidate
      |  reactions, separated by WSP.  Each candidate reaction that does
      |  not constitute a single emoji-sequence (as per [Emoji-Seq]) is
      |  invalid.  Invalid candidates can be treated individually,
      |  rather than affecting the remainder of the part-content's
      |  processing.  The remaining candidates form the set of reactions
      |  to be processed.  This approach assumes use of a mechanism for
      |  emoji sequence validation that is not specified here.

   The rule base-emojis is provided as a simple, common list, or
   'vocabulary' of emojis.  It was developed from some existing practice
   in social networking and is intended for similar use.  However,
   support for it as a base vocabulary is not required.  Having
   providers and consumers employ a common set will facilitate user
   interoperability, but different sets of users might want to have
   different, common (shared) sets.

   The reaction emoji or emojis are linked to the current message's In-
   Reply-To field, which references an earlier message and provides a
   summary reaction to that earlier message [Mail-Fmt].  For processing
   details, see Section 4.

   Reference to unallocated code points SHOULD NOT be treated as an
   error; the corresponding UTF-8-encoded code points SHOULD be
   processed using the system default method for denoting an unallocated
   or undisplayable code point.

      |  Note: The "emoji" token looks simple.  It isn't.  Implementers
      |  are well advised not to assume that emoji sequences are trivial
      |  to parse or validate.  Among other concerns, an implementation
      |  of the Unicode Character Database is required.  An emoji is
      |  more than a stand-in for a simple alternation of characters.
      |  Similarly, one emoji sequence is not interchangeable with, or
      |  equivalent to, another one, and comparisons require detailed
      |  understanding of the relevant Unicode mechanisms.  Use of an
      |  existing Unicode implementation will typically prove extremely
      |  helpful, as will an understanding of the error modes that may
      |  arise with a chosen implementation.

4.  Reaction Message Processing

   The presentation aspects of reaction processing are necessarily MUA
   specific and beyond the scope of this specification.  In terms of the
   message itself, a recipient MUA that supports this mechanism operates
   as follows:

   1.  If a received message R's header contains an In-Reply-To field,
       check to see if it references a previous message that the MUA has
       sent or received.

   2.  If R's In-Reply-To: does reference one, then check R's message
       content for a part with a "reaction" Content-Disposition header
       field, at either the outermost level or as part of a multipart at
       the outermost level.

   3.  If such a part is found and the content of the part conforms to
       the restrictions outlined above, remove the part from the message
       and process the part as a reaction.

      |  Note: A message's content might include other, nested messages.
      |  These can be analyzed for reactions, independently of the
      |  containing message, applying the above algorithm for each
      |  contained message, separately.

   Again, the handling of a message that has been successfully processed
   is MUA specific and beyond the scope of this specification.

5.  Usability Considerations

   This specification defines a mechanism for the structuring and
   carriage of information.  It does not define any user-level details
   of use.  However, the design of the user-level mechanisms associated
   with this facility is paramount.  This section discusses some issues
   to consider.

   Creation:  Because an email environment is different from a typical
      social media platform, there are significant -- and potentially
      challenging -- choices in the design of the user interface, to
      support indication of a reaction.  Is the reaction to be sent only
      to the original author, or should it be sent to all recipients?
      Should the reaction always be sent in a discrete message
      containing only the reaction, or should the user also be able to
      include other message content?  (Note that carriage of the
      reaction in a normal email message enables inclusion of this other
      content.)

   Display:  Reaction indications might be more useful when displayed in
      close visual proximity to the original message, rather than merely
      as part of an email response thread.  The handling of multiple
      reactions, from the same person, is also an opportunity for making
      a user experience design choice that could be interesting.

   Culture:  The use of an image, intended to serve as a semantic
      signal, is determined and affected by cultural factors, which
      differ in complexity and nuance.  It is important to remain aware
      that an author's intent when sending a particular emoji might not
      match how the recipient interprets it.  Even simple, commonly used
      emojis can be subject to these cultural differences.

5.1.  Example Message

   A simple message exchange might be:

   To: recipient@example.org
   From: author@example.com
   Date: Today, 29 February 2021 00:00:00 -800
   Message-ID: 12345@example.com
   Subject: Meeting

   Can we chat at 1pm pacific, today?

   with a thumbs-up, affirmative response of:

   To: author@example.com
   From: recipient@example.org
   Date: Today, 29 February 2021 00:00:10 -800
   Message-ID: 56789@example.org
   In-Reply-To: 12345@example.com
   Subject: Meeting
   Mime-Version: 1.0 (1.0)
   Content-Type: text/plain; charset=utf-8
   Content-Disposition: reaction

   {U+1F44D}

   The Unicode character, represented here as "{U+1F44D}" for
   readability, would actually be sent as the UTF-8-encoded character.

   The example could, of course, be more elaborate, such as the first of
   a MIME multipart sequence.

5.2.  Example Display

   Repeating the caution that actual use of this capability requires
   careful usability design and testing, this section describes simple
   examples -- which have not been tested -- of how the reaction
   response might be displayed in a summary list of messages:

   Summary:  Summary listings of messages in a folder include columns
      such as Subject, From, and Date.  Another might be added to show
      common reactions and a count of how many of them have been
      received.

   Message:  A complete message is often displayed with a tailored
      section for header fields, enhancing the format and showing only
      selected header fields.  A pseudo-field might be added for
      reactions, again showing the symbol and a count.

6.  Security Considerations

   This specification employs message content that is a strict subset of
   existing possible content and thus introduces no new content-specific
   security considerations.  The fact that this content is structured
   might seem to make it a new threat surface, but there is no analysis
   demonstrating that it does.

   This specification defines a distinct Content-Disposition value for
   specialized message content.  Processing that handles the content
   differently from other content in the message body might introduce
   vulnerabilities.  Since this capability is likely to produce new user
   interaction features, that might also produce new social engineering
   vulnerabilities.

7.  IANA Considerations

   IANA has registered the Reaction MIME Content-Disposition parameter,
   per [RFC2183].

   Content-Disposition parameter name:  reaction

   Allowable values for this parameter:  (none)

   Description:  Permit a recipient to respond by signaling basic
      reactions to an author's posting, such as with a 'thumbs up' or
      'smiley' graphic

8.  Experimental Goals

   The basic, email-specific mechanics for this capability are well
   established and well understood.  Points of concern, therefore, are:

   *  Technical issues in using emojis within a message body

   *  Market interest

   *  Usability

   So the questions to answer for this Experimental specification are:

   *  Is there demonstrated interest by MUA developers?

   *  If MUA developers add this capability, is it used by authors?

   *  Does the presence of the Reaction capability create any
      operational problems for recipients?

   *  Does the presence of the Reaction capability demonstrate
      additional security issues?

   *  What specific changes to the specification are needed?

   *  What other comments will aid in use of this mechanism?

   Please send comments to ietf-822@ietf.org.

9.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [Emoji-Seq]
              Davis, M., Ed. and P. Edberg, Ed., "Unicode Technical
              Standard #51: Unicode Emoji", September 2020,
              <https://www.unicode.org/reports/
              tr51/#def_emoji_sequence>.

   [Mail-Arch]
              Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <https://www.rfc-editor.org/info/rfc5598>.

   [Mail-Fmt] Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [MIME]     Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183,
              DOI 10.17487/RFC2183, August 1997,
              <https://www.rfc-editor.org/info/rfc2183>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Acknowledgements

   This specification had substantive commentary on three IETF mailing
   lists.

   This work began as a private exercise, in July 2020, with private
   discussion, for draft-crocker-reply-emoji.  It morphed into draft-
   crocker-inreply-react, with significant discussion on the ietf-822
   mailing list <https://www.ietf.org/mailman/listinfo/ietf-822>,
   September through November 2020.  The discussion produced a
   fundamental change from proposing a new header field to instead
   defining a new Content-Disposition type, as well as significantly
   enhancing its text concerning Unicode.  It also produced two
   additional coauthors.

   In November 2020, the Dispatch mailing list
   <https://www.ietf.org/mailman/listinfo/dispatch> was queried about
   the draft, but it produced no discussion, though it did garner one
   statement of interest.

   A 4-week Last Call was issued on this document, January 2021,
   resulting in quite a bit of fresh discussion on the last-call mailing
   list <https://www.ietf.org/mailman/listinfo/last-call> and producing
   further changes to this document.  After Last Call completed,
   additional concerns regarding the Unicode-related details surfaced,
   producing yet more changes to the document.  It also produced a
   challenge that prompted the current version of this Acknowledgements
   section.

   Readers who are interested in the details of the document's history
   are encouraged to peruse the archives for the three lists, searching
   Subject fields for "react".

Authors' Addresses

   Dave Crocker
   Brandenburg InternetWorking

   Email: dcrocker@bbiw.net


   Ricardo Signes
   Fastmail

   Email: rjbs@semiotic.systems


   Ned Freed
   Oracle

   Email: ned.freed@mrochek.com
