Internet Engineering Task Force (IETF) D. Crocker

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
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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: [email protected]
From: [email protected]
Date: Today, 29 February 2021 00:00:00 -800
Message-ID: [email protected]
Subject: Meeting

Can we chat at 1pm pacific, today?

with a thumbs-up, affirmative response of:

To: [email protected]
From: [email protected]
Date: Today, 29 February 2021 00:00:10 -800
Message-ID: [email protected]
In-Reply-To: [email protected]
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 [email protected].

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: [email protected]

Ricardo Signes
Fastmail

Email: [email protected]

Ned Freed
Oracle

Email: [email protected]