Internet Draft                                Paul Hoffman
draft-hoffman-rescap-mua-02.txt               Internet Mail Consortium
November 20, 1999
Expires in six months

                Rescap Profile for Mail User Agents

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other
groups may also distribute working documents as Internet-Drafts.

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. Introduction

This document defines a profile of the rescap protocol for mail user
agents (MUAs) and mail recipients. It describes the attributes that a
mail sender might want or need to know about a particular mail
recipient before sending a message.

The attributes are divided into four general categories:
- MIME handling
- S/MIME
- OpenPGP
- General

Note: this list is very preliminary. The process of defining the
requirements for rescap has just begun. Because the rescap protocol has
not even had a first draft, it is likely that there will be many
significant changes to this draft in the future as rescap gets worked
on.

In this document, "recipient" is used to indicate the user who can
accept mail at the URL provided in the rescap request and "sender" is
the person or process who requested the rescap information. Note that
some of the attributes in this document apply to the MUA a recipient is
using, while others apply directly to the mail recipient (which might
be a human or a mail-processing program).

The attributes described in this document are those that a mail sender
would want to know about a recipient or the recipient's MUA. Attributes
about the mail recipient that have no relevance to a mail sender (such
if the MUA uses IMAP to access its message store) are not included.

. MIME Handling

The attributes in this section describe general MIME handling. They
include some specific MIME profiles as well as more general MIME
characteristics.

Identifier:   PlainTextOnly
Value type:   Boolean
Description:  Can only read single-part text/plain messages. Put
another way, cannot parse a MIME message.

Identifier:   MIMEIntlHeaders
Value type:   Boolean
Description:  Conforms to [MIME-HEADER-EXTENSIONS], which describes
many extensions for MIME headers, such as for non-ASCII characters.

Identifier:   MIMEParamExtensions
Value type:   Boolean
Description:  Conforms to [MIME-PARAM], which describes many extensions
for MIME parameter values and encoded words.

Identifier:   DisplayableMedia
Value type:   Conneg string
Description:  A list of MIME types and subtypes that are natively
displayed by the receiving MUA without falling back to a default media
type. The string is in the format of [CONNEG], as extended by
[CONNEG-MEDIA]. This string should contain only MIME types and
subtypes, not additional media features.

Identifier:   MediaFeatures
Value type:   Conneg string
Description:  A list of media features of the MUA. The string is in the
format of [CONNEG].

Identifier:   CharsetsDisplayed
Value type:   Conneg string
Description:  The list of charset labels that describe the charsets
[CHARSET] that can be displayed. Note that US-ASCII, and support for at
least the US-ASCII subset of ISO-8859-*, is assumed regardless of the
value of this attribute. The string is in the format of [CONNEG], using
the tags defined in [CONNEG-CHARLANG].

Identifier:   PreferredLanguages
Value type:   List of strings
Description:  The lists of languages understandable to the recipient,
as described in [LANG]. The string is in the format of [CONNEG], using
the tags defined in [CONNEG-CHARLANG].

Identifier:   LineLength
Value type:   Integer
Description:  The width, in characters, of a line in the display of the
MUA. For variable-width displays, this should be an estimate of the
number of characters per line from a typical mail message. For systems
that have no line limitations, this value should be set to 0.

Identifier:   HandlesMHTML
Value type:   Boolean
Description:  Handles MHTML content natively, as described in [MHTML].

Identifier:   HandlesContentMD5
Value type:   Boolean
Description:  Handles Content-MD5 headers, as described in
[CONTENT-MD5]. If the recipient does not handle Content-MD5 headers, as
many don't, this the sender can save time by not constructing one.

Identifier:   HandlesMailingListURLs
Value type:   Boolean
Description:  Handles mailing list URL headers, as described in
[LIST-URLS].

Identifier:   RepliesToMDNs
Value type:   Boolean
Description:  Is able to reply to message disposition notification
requests, as described in [MDN]. Note that this does not mean that the
client will necessarily send an MDN back to a particular request, just
that it is able to reply to such requests. This field helps a sending
MUA decide whether or not to keep track of the MDNs sent to the
recipient; if the recipient is known not to reply to MDNs, then the
sender doesn't need to track them. This can also reduce the size of
messages sent over bandwidth-restricted lines.

Identifier:   CalendarClient
Value type:   Boolean
Description:  Can act as an iCalendar iMIP agent [IMIP].


. S/MIME

The attributes in this section indicate the S/MIME capabilities of the
recipient as described in [SMIME-MSG], [SMIME-CERT], and associated
documents.

Note that some S/MIME public keys are used for both encrypting and
signing. This means that there may be duplicated certificates in the
SMIMESigningCertsBasic and SMIMEEncryptingCerts lists.

Identifier:   SMIMEVerifiesSigned
Value type:   List of strings
Description:  Indicates that the recipient can verify the signatures on
S/MIME signed messages. The strings in the list indicate the type of
signatures accepted. The values currently are limited to "id-dsa" and
"rsaEncryption". The list is in decreasing order of preference.

Identifier:   SMIMESigningCertsBasic
Value type:   List of binary
Description:  Provides the S/MIME certificates for public signing keys
of the recipient. The list is in decreasing order of preference.

Identifier:   SMIMESigningCertsExtended
Value type:   List of binary
Description:  Provides the S/MIME certificates for public signing keys
of the recipient, including additional signed attributes, as described
in [SMIME-CERTDIST]. The list is in decreasing order of preference.

Identifier:   SMIMEEncryptingCerts
Value type:   List of binary
Description:  Provides the S/MIME certificates for public encrypting
keys of the recipient. The list is in decreasing order of preference.

Identifier:   SMIMEHigherCerts
Value type:   List of binary
Description:  Provides the S/MIME certificates for certificate
authorities that have signed the recipient's signing and encrypting
certificates. These higher-level certificates can be used by the sender
to validate the recipient's certificates. The list is in no particular
order.

Identifier:   SMIMESignedReceipts
Value type:   Boolean
Description:  Responds to requests for S/MIME signed receipts described
in [SMIME-ESS].

Identifier:   SMIMESecurityLabels
Value type:   Boolean
Description:  Acts on S/MIME security labels, or is behind a gateway
that does security label handling, as described in [SMIME-ESS].

Identifier:   SMIMESecureMailingList
Value type:   Boolean
Description:  Is a mailing list that uses secure mailing list
handling described in [SMIME-ESS].

Identifier:   SMIMEHandlesSigningCert
Value type:   Boolean
Description:  Handles the signed SigningCertificate attribute
described in [SMIME-ESS].


. OpenPGP

The attributes in this section indicate the OpenPGP capabilities of the
recipient as described in [OPEN-PGP] and associated documents.

Identifier:   OpenPGPVerifiesSigned
Value type:   List of strings
Description:  Indicates that the recipient can verify the signatures on
OpenPGP signed messages. The strings in the list indicate the type of
signatures accepted. The values currently are limited to "DSA" and
"RSA". The list is in decreasing order of preference.

Identifier:   OpenPGPSigningCertsBasic
Value type:   List of binary
Description:  Provides the OpenPGP certificates for public signing keys
of the recipient. The list is in decreasing order of preference.

Identifier:   OpenPGPEncryptingCerts
Value type:   List of binary
Description:  Provides the OpenPGP certificates for public encrypting
keys of the recipient. The list is in decreasing order of preference.

Identifier:   OpenPGPHigherCerts
Value type:   List of binary
Description:  Provides the OpenPGP certificates for users and
certificate authorities that have signed the recipient's signing and
encrypting certificates. These higher-level certificates can be used by
the sender to validate the recipient's certificates. The list is in no
particular order.


. General

User agent and recipient attributes that don't fit into the other
categories appear in this section.

Identifier:   UBEPrefernces
Value type:   List of pairs of strings
Description:  Specifies the preferences of the recipient for receiving
unsolicited bulk email (UBE). Each entry in the list is a pair of
strings. The first entry in the pair is a tag indicating the law or
policy being referred to, and the second entry is the value specified
for that law or policy. The identities of the laws and policies must be
registered with IANA.

Identifier:   MailingListInfo
Value type:   String
Description:  Gives information about a mailing list. The format of the
information is single string consisting of RFC 822 headers, as
described in [MAILLIST]. If the recipient is not a mailing list and
this attribute is included in the rescap response, the string should be
empty.

Identifier:   GeneralInfo
Value type:   vCard string
Description:  Gives information about the person or system that is
associated with the recipient. The information is returned in the vCard
format described in [VCARD]. Note that any information in this
attribute that can also be represented in other attributes in this
profile should also be delivered in the other attributes. No client
should have to retrieve the value for this attribute in order to get
information that is also available in other attributes.

Identifier:   AssociatedEmailAddresses
Value type:   List of lists
Description:  Lists the email addresses used by this recipient. The
list contains items that contain a pair of string items. The pairs
consist of an email address and a description. The description should
be the strings "home", "work", "all", "unused". The "unused" term
indicates an email address that is no longer valid for the recipient.


. Security Considerations

The rescap protocol will control the security of the passing the values
for the attributes described here. If digital signatures are not used,
an attacker can alter the values that the client receives from the
server, thereby causing false values or no values to be received. For
example, an attacker can change the legal notices sent, which can cause
damage to the named recipient. If encryption is not used, an attacker
can watch the values of the attributes as they are transmitted over the
Internet.


. References

[CHARSET] "IANA Charset Registration Procedures", RFC 2278

[CONNEG] "A Syntax for Describing Media Feature Sets", RFC 2533.

[CONNEG-CHARLANG] "Registration of Charset and Languages Media Features
Tags", draft-hoffman-char-lang-media.

[CONNEG-MEDIA] "MIME content types in media feature expressions",
draft-ietf-conneg-feature-type.

[CONTENT-MD5] "The Content-MD5 Header Field", RFC 1864.

[IMIP] "iCalendar Message-Based Interoperability Protocol (iMIP)", RFC
2447.

[LANG] "Tags for the Identification of Languages", RFC 1766.

[LIST-URLS] "The Use of URLs as Meta-Syntax for Core Mail List Commands
and their Transport through Message Header Fields", RFC 2369.

[MAILLIST] "The Use of URLs as Meta-Syntax for Core Mail List Commands
and their Transport through Message Header Fields", RFC 2369.

[MDN] "An Extensible Message Format for Message Disposition
Notifications", RFC 2298.

[MHTML] "MIME E-mail Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2110.

[MIME-HEADER-EXTENSIONS] "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047.

[MIME-PARAM] "MIME Parameter Value and Encoded Word Extensions:
Character Sets, Languages, and Continuations", RFC 2231.

[OPEN-PGP] "OpenPGP Message Format", RFC 2440.

[SMIME-CERT] "S/MIME Version 3 Certificate Handling",
RFC 2632.

[SMIME-CERTDIST] "Certificate Distribution Specification",
draft-ietf-smime-certdist.

[SMIME-ESS] "Enhanced Security Services for S/MIME",
RFC 2634.

[SMIME-MSG] "S/MIME Version 3 Message Specification",
RFC 2633.

[VCARD] "vCard MIME Directory Profile", RFC 2426.


A. IANA Registrations

A.1 Attribute Identifier Registrations

[[It is likely that all the attribute identifiers in this document will
need to be registered.]]

A.2 Additional Registrations

[[Registration of UCE law and policy identifiers]]


B. Acknowledgments

The following people have contributed changes and additions to this
document:

Chris Newman
Graham Klyne
Larry Masinter
Tony Hansen


C. Changes between versions of the draft

C.1 Changes between -01 and -02

Corrected RFC number for [CONNEG].

Changed CharsetsDisplayed and PreferredLanguages to conneg strings.

Added LineLegth.

Removed multipart-onepass because the feature is dead.

Added reference to [CONNEG-CHARLANG].

Updated the S/MIME references to the RFCs.


D. Author's Address

Paul Hoffman
Internet Mail Consortium
 Segre Place
Santa Cruz, CA  95060
phoffman@imc.org