draft-ietf-appsawg-rrvs-header-field-00.txt   draft-ietf-appsawg-rrvs-header-field-01.txt 
Network Working Group W. Mills Network Working Group W. Mills
Internet-Draft Yahoo! Inc. Internet-Draft Yahoo! Inc.
Intended status: Standards Track M. Kucherawy Intended status: Standards Track M. Kucherawy
Expires: February 19, 2014 Facebook, Inc. Expires: April 24, 2014 Facebook, Inc.
August 18, 2013 October 21, 2013
The Require-Recipient-Valid-Since Header Field The Require-Recipient-Valid-Since Header Field and SMTP Service
draft-ietf-appsawg-rrvs-header-field-00 Extension
draft-ietf-appsawg-rrvs-header-field-01
Abstract Abstract
This document defines an email header field, Require-Recipient-Valid- This document defines an extension for the Simple Mail Transfer
Since, to provide a method for senders to indicate to receivers the Protocol called RRVS, and a header field called Require-Recipient-
time when the sender last confirmed the ownership of the target Valid-Since, to provide a method for senders to indicate to receivers
the time when the sender last confirmed the ownership of the target
mailbox. This can be used to detect changes of mailbox ownership, mailbox. This can be used to detect changes of mailbox ownership,
and thus prevent mail from being delivered to the wrong party. and thus prevent mail from being delivered to the wrong party.
The intended use of this header field is on automatically generated The intended use of these facilities is on automatically generated
messages that might contain sensitive information. messages that might contain sensitive information, though it may also
be useful in other applications.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 19, 2014. This Internet-Draft will expire on April 24, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 14 skipping to change at page 2, line 17
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Description . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Description . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Use with Mailing Lists . . . . . . . . . . . . . . . . . . . . 5 3.1. The 'RRVS' SMTP Extension . . . . . . . . . . . . . . . . 4
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. The 'Require-Recipient-Valid-Since' Header Field . . . . . 4
6. Continuous Ownership . . . . . . . . . . . . . . . . . . . . . 6 4. Handling By Receivers . . . . . . . . . . . . . . . . . . . . 5
7. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. SMTP Extension Used . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 4.2. Header Field Used . . . . . . . . . . . . . . . . . . . . 5
8.1. Abuse Countermeasures . . . . . . . . . . . . . . . . . . 7 5. Role Accounts . . . . . . . . . . . . . . . . . . . . . . . . 7
8.2. Suggested Use Restrictions . . . . . . . . . . . . . . . . 8 6. Method Conversion . . . . . . . . . . . . . . . . . . . . . . 7
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 8 7. Use with Mailing Lists . . . . . . . . . . . . . . . . . . . . 7
9.1. Probing Attacks . . . . . . . . . . . . . . . . . . . . . 8 8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.2. Envelope Recipients . . . . . . . . . . . . . . . . . . . 9 9. Continuous Ownership . . . . . . . . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Header Field Registration . . . . . . . . . . . . . . . . 9 10.1. SMTP Extension Example . . . . . . . . . . . . . . . . . . 9
10.2. Enhanced Status Code Registration . . . . . . . . . . . . 9 10.2. Header Field Example . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 11. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10 11.1. Abuse Countermeasures . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 10 11.2. Suggested Use Restrictions . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 10 12. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 11
12.1. Probing Attacks . . . . . . . . . . . . . . . . . . . . . 11
12.2. Envelope Recipients . . . . . . . . . . . . . . . . . . . 12
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
13.1. SMTP Extension Registration . . . . . . . . . . . . . . . 12
13.2. Header Field Registration . . . . . . . . . . . . . . . . 12
13.3. Enhanced Status Code Registration . . . . . . . . . . . . 12
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
14.1. Normative References . . . . . . . . . . . . . . . . . . . 13
14.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
Email addresses sometimes get reassigned to a different person. For Email addresses sometimes get reassigned to a different person. For
example, employment changes at a company can cause an address used example, employment changes at a company can cause an address used
for an ex-employee to be assigned to a new employee, or a mail for an ex-employee to be assigned to a new employee, or a mail
service provider (MSP) might expire an account and then let someone service provider (MSP) might expire an account and then let someone
else register for the local-part that was previously used. Those who else register for the local-part that was previously used. Those who
sent mail to the previous owner of an address might not know that it sent mail to the previous owner of an address might not know that it
has been reassigned. This can lead to the sending of email to the has been reassigned. This can lead to the sending of email to the
correct address, but the wrong recipient. correct address, but the wrong recipient.
What is needed is a way to indicate an attribute of the recipient What is needed is a way to indicate an attribute of the recipient
that will distinguish between the previous owner of an address and that will distinguish between the previous owner of an address and
its current owner. if they are different. Further, this needs to be its current owner, if they are different. Further, this needs to be
done in a way that respects privacy. done in a way that respects privacy.
The mechanism specified here allows the sender of the mail to The mechanisms specified here allow the sender of the mail to
indicate how "old" the address assignment is expected to be. In indicate how "old" the address assignment is expected to be. In
effect, the sender is saying, "The person to whom I am sending to had effect, the sender is saying, "The person to whom I am sending to had
this address assigned to as far back as this date-time." A receiving this address assigned to as far back as this date-time." A receiving
system can then compare this information against the date and time system can then compare this information against the date and time
the address was assigned to its current user. If the assignment was the address was assigned to its current user. If the assignment was
made later than the date-time indicated in the message, there is a made later than the date-time indicated in the message, there is a
good chance the current user of the address is not the correct good chance the current user of the address is not the correct
recipient. The receiving system can then choose to prevent delivery recipient. The receiving system can then choose to prevent delivery
and, possibly, to notify the original sender of the problem. and, possibly, to notify the original sender of the problem.
The primary application is automatically generated messages rather The primary application is automatically generated messages rather
than user-authored content. than user-authored content, though it may be useful in other
contexts.
2. Definitions 2. Definitions
For a description of the email architecture, consult [EMAIL-ARCH]. For a description of the email architecture, consult [EMAIL-ARCH].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS]. document are to be interpreted as described in [KEYWORDS].
3. Description 3. Description
The Require-Recipient-Valid-Since header field includes an intended To address the problem described above, a mail sending client needs
recipient coupled with a timestamp indicating the most recent date to indicate to the server to which it is connecting that there is an
and time when the message author believed the destination mailbox to expectation that the destination of the message has been under
be under the continuous ownership (see Section 6) of a specific continuous ownership since some date-time, presumably the most recent
party. Presumably there has been some confirmation process applied time the message author had confirmed its understanding of who owned
to establish this ownership; however, the method of making such that mailbox. Two mechanisms are defined here: an extension to the
determinations is a local matter and outside the scope of this Simple Mail Transfer Protocol [SMTP], for use between a client and
document. server that both implement the extension, and a header field that can
be used when passing a message to a server that appears not to
implement this extension.
The SMTP extenion is called "RRVS" (Require Recipient Valid Since),
and adds a parameter to the SMTP "RCPT" command that indicates the
most recent date and time when the message author believed the
destination mailbox to be under the continuous ownership (see
Section 9) of a specific party. Similarly, the Require-Recipient-
Valid-Since header field includes an intended recipient coupled with
a timestamp indicating the same thing. Presumably there has been
some confirmation process applied to establish this ownership;
however, the method of making such determinations is a local matter
and outside the scope of this document.
3.1. The 'RRVS' SMTP Extension
Extensions to SMTP are described in Section 2.2 of [SMTP].
The name of the extension is "RRVS", an abbreviation of "Require
Recipient Valid Since". Servers implementing the SMTP extension
advertise an additional EHLO keyword of "RRVS", which has no
associated parameters, introduces no new SMTP verbs, and does not
alter the MAIL verb.
An MTA implementing RRVS can transmit or accept a new parameter to
the RCPT command. The new parameter is "RRVS", which takes a value
that is an integer timestamp expressed as an "epoch" time, namely the
number of seconds since midnight on January 1, 1970. Accordingly,
this extension increases the maximum command length for the RCPT verb
by 16 characters.
The meaning of this extension, when used, is described in
Section 4.1.
3.2. The 'Require-Recipient-Valid-Since' Header Field
The general constraints on syntax and placement of header fields in a The general constraints on syntax and placement of header fields in a
message are defined in Internet Message Format [MAIL]. message are defined in Internet Message Format [MAIL].
Using Augmented Backus-Naur Form [ABNF], the syntax for the field is: Using Augmented Backus-Naur Form [ABNF], the syntax for the field is:
rrvs = "Require-Recipient-Valid-Since:" addr-spec; date-time CRLF rrvs = "Require-Recipient-Valid-Since:" addr-spec; date-time CRLF
"CFWS" is defined in Section 3.2.2, "date-time" is defined in Section "CFWS" is defined in Section 3.2.2, "date-time" is defined in Section
3.3, and "addr-spec" is defined in Section 3.4.1, of [MAIL]. 3.3, and "addr-spec" is defined in Section 3.4.1, of [MAIL].
A receiving system that implements this specification checks whether 4. Handling By Receivers
the current mailbox owner has held it continuously, far enough back
If a receiver implements the RRVS SMTP extension, then there are two
possible evaluation paths:
1. The sending client implements the extension, and so there was an
RRVS parameter on a RCPT TO command in the SMTP session; or
2. The sending client does not (or elected not to) implement the
extension, so the RRVS parameter was not present on the RCPT TO
commands in the SMTP session.
4.1. SMTP Extension Used
A receiving system that implements the SMTP extension declared above
and observes an RRVS parameter on a RCPT TO command checks whether
the current owner of the destination mailbox has held it
continuously, far enough back to inclue the given date-time, and
delivers it unless that check returns in the negative. Expressed as
a sequence of steps:
1. Ignore the parameter if the named mailbox is a role account as
listed in Mailbox Names For Common Services, Roles And Functions
[ROLES]. (See Section 5.)
2. Determine if the named address is serviced for local delivery.
If so, and if that address, has not been under continuous
ownership since the specified timestamp, return a 550 error to
the RCPT command. (See also Section 13.3.)
3. RECOMMENDED: If any Require-Recipient-Valid-Since header fields
are present and refer to the named address, remove them prior to
delivery or relaying. (See Section 4.2 for discussion.)
4.2. Header Field Used
A receiving system that implements this specification, upon receiving
a message bearing a Require-Recipient-Valid-Since header field when
no corresponding RRVS SMTP extension was used, checks whether the
destination mailbox owner has held it continuously, far enough back
to include the given date-time, and delivers it unless that check to include the given date-time, and delivers it unless that check
returns in the negative. Expressed as a sequence of steps: returns in the negative. Expressed as a sequence of steps:
1. Extract the set of Require-Recipient-Valid-Since fields from the 1. Extract the set of Require-Recipient-Valid-Since fields from the
message. message for which no corresponding RRVS SMTP extension was used.
2. Discard any such fields that are syntactically invalid. 2. Discard any such fields that are syntactically invalid.
3. Discard any such fields that name a role account as listed in 3. Discard any such fields that name a role account as listed in
Mailbox Names For Common Services, Roles And Functions [ROLES]. [ROLES] (see Section 5).
4. Discard any such fields for which the "addr-spec" portion does 4. Discard any such fields for which the "addr-spec" portion does
not match a current recipient, as listed in the RCPT TO commands not match a current recipient, as listed in the RCPT TO commands
in the corresponding Simple Mail Transfer Protocol [SMTP] in the SMTP session.
session.
5. For each field remaining, determine if the named address has been 5. Discard any such fields for which the "addr-spec" portion does
not refer to a mailbox handled for local delivery by this MTA.
6. For each field remaining, determine if the named address has been
under continuous ownership since the corresponding timestamp. If under continuous ownership since the corresponding timestamp. If
it has not, reject the message. it has not, reject the message.
6. RECOMMENDED: If local delivery is being performed, remove all 7. RECOMMENDED: If local delivery is being performed, remove all
instances of this field prior to delivery to a mailbox; if the instances of this field prior to delivery to a mailbox; if the
message is being forwarded, remove those instances of this header message is being forwarded, remove those instances of this header
field that were not discarded by steps 1-4 above. field that were not discarded by steps 1-4 above.
Handling proceeds normally upon completion of the above steps if
rejection has not been performed.
The final step is not mandatory as not all mail handling agents are The final step is not mandatory as not all mail handling agents are
capable of stripping away header fields. capable of stripping away header fields, and there are sometimes
reasons to keep the field intact such as debugging or presence of
digital signatures that might be invalidated by such a change.
If a message is to be rejected within the SMTP protocol itself If a message is to be rejected within the SMTP protocol itself
(versus generating a rejection message separately), servers (versus generating a rejection message separately), servers
implementing this protocol and the SMTP extensions described in implementing this protocol and the SMTP extensions described in
Enhanced Mail System Status Codes [ESC] SHOULD use the enhanced Enhanced Mail System Status Codes [ESC] SHOULD use the enhanced
status code described in Section 10.2. status code described in Section 13.3.
Implementation is expected to be transparent to non participants, Implementation by this method is expected to be transparent to non-
since they would typically ignore this header field. participants, since they would typically ignore this header field.
This header field is not typically added to a message that is This header field is not normally added to a message that is
addressed to multiple recipients. The intended use of this field addressed to multiple recipients. The intended use of this field
involves an author seeking to protect transactional or otherwise involves an author seeking to protect transactional or otherwise
sensitive data intended for a single recipient, and thus generating sensitive data intended for a single recipient, and thus generating
independent messages for each individual recipient is normal independent messages for each individual recipient is normal
practice. Because of the nature of SMTP, a message bearing this practice. Because of the nature of SMTP, a message bearing this
header field for multiple addressees could result in a single header field for multiple addressees could result in a single
delivery attempt for multiple recipients (in particular, if two of delivery attempt for multiple recipients (in particular, if two of
the recipients are handled by the same server), and if any one of the recipients are handled by the same server), and if any one of
them fails the test, the delivery fails to all of them; it then them fails the test, the delivery fails to all of them; it then
becomes necessary to generate a Delivery Status Notification [DSN] becomes necessary to generate a Delivery Status Notification [DSN]
message for each of the failed recipients indicating the specific message for each of the failed recipients indicating the specific
failure cause for each. failure cause for each.
To further obscure account details on the receiving system, the 5. Role Accounts
receiver SHOULD ignore the header field if the address within it has
had one continuous owner since it was created, regardless of the
purported confirmation date of the address. This is further
discussed in Section 8.
4. Use with Mailing Lists It is necessary not to interfere with delivery of messages to role
mailboxes (see [ROLES]), but it could be useful to indicate to users
handling those mailboxes that a change of ownership might have taken
place where doing so is possible.
6. Method Conversion
Use of the SMTP extension provided here is preferable over the header
field method, since the additional detail about the relationship
between the message author and its intended recipient is at best a
property of the message transaction and not part of the message
itself. The header field mechanism is defined only to enable passage
of the request between and through systems that that do not implement
the SMTP extension.
If an SMTP server receives a message from a client and both of them
use the SMTP extension described here, the server thus has "valid-
since" timestamps associated with one or more of the destination
mailboxes. If that server needs to relay the message on to another
server (thereby becoming a client), but this new server does not
advertise the SMTP extension, the client SHOULD add Require-
Recipient-Valid-Since header fields matching each mailbox to which
relaying is being done, and the corresponding valid-since timestamp
for each.
Similarly, if an SMTP server receives a message bearing one or more
Require-Recipient-Valid-Since header fields for which it must now
relay the message (thereby becoming a client) and the new server
advertises support for the SMTP extension, the client SHOULD delete
the header field(s) and instead relay this information by making use
of the SMTP extension.
7. Use with Mailing Lists
Mailing list services can store the timestamp at which a subscriber Mailing list services can store the timestamp at which a subscriber
was added to a mailing list. This specification can be used in was added to a mailing list. This specification can be used in
conjunction with that information in order to restrict traffic to the conjunction with that information in order to restrict traffic to the
original subscriber, rather than a different person now in possession original subscriber, rather than a different person now in possession
of an address under which the original subscriber registered. Upon of an address under which the original subscriber registered. Upon
receiving a rejection caused by this specification, the list service receiving a rejection caused by this specification, the list service
can remove that address from further distribution. can remove that address from further distribution.
A mailing list service that receives a message containing this field A mailing list service that receives a message containing this field
removes it from the message prior to redistributing it, limiting removes it from the message prior to redistributing it, limiting
exposure of information regarding the relationship between the exposure of information regarding the relationship between the
message's author and mailing list. message's author and mailing list.
5. Discussion 8. Discussion
It can be argued that placing this information in a message header To further obscure account details on the receiving system, the
field is not the ideal architectural choice and that it would have receiver SHOULD ignore the SMTP extension or the header field if the
been better to introduce this capability as an extension to SMTP. address specified has had one continuous owner since it was created,
The exchange of meta data about the target address is not part of the regardless of the purported confirmation date of the address. This
actual message content, nor is it meta data about the content. is further discussed in Section 11.
However, if the author and the target address are separated by an
SMTP server that does not implement the SMTP extension, the check
will not be able to propagate to the intended receiving system.
Implementing this service as a header field allows the check to occur
even across non-participating systems, effectively tunneling the
request.
The presence of the intended address in the field content supports The presence of the intended address in the field content supports
the case where a message bearing this header field is forwarded. The the case where a message bearing this header field is forwarded. The
specific use case is as follows: specific use case is as follows:
1. A user subscribes to a service "S" on date "D" and confirms an 1. A user subscribes to a service "S" on date "D" and confirms an
email address at the user's current location, "A"; email address at the user's current location, "A";
2. At some later date, the user intends to leave the current 2. At some later date, the user intends to leave the current
location, and thus creates a new mailbox elsewhere, at "B"; location, and thus creates a new mailbox elsewhere, at "B";
skipping to change at page 6, line 36 skipping to change at page 8, line 46
7. On receipt at "B", either the header field has been removed, or 7. On receipt at "B", either the header field has been removed, or
the header field does not refer to a current envelope recipient, the header field does not refer to a current envelope recipient,
and in either case delivers the message. and in either case delivers the message.
Some services generate messages with an RFC5322.To field that does Some services generate messages with an RFC5322.To field that does
not contain a valid address, in order to obscure the intended not contain a valid address, in order to obscure the intended
recipient. For this reason, the original intended recipient is recipient. For this reason, the original intended recipient is
included in this header field. included in this header field.
6. Continuous Ownership 9. Continuous Ownership
Determining continuous ownership of a mailbox is a local matter at Determining continuous ownership of a mailbox is a local matter at
the receiving site. In particular, the only possible answers to the the receiving site. In particular, the only possible answers to the
continuous-ownership-since question are "yes", "no", and "unknown"; continuous-ownership-since question are "yes", "no", and "unknown";
the action to be taken in the "unknown" case is a matter of local the action to be taken in the "unknown" case is a matter of local
policy. policy.
For example, when control of a domain name is transferred, the new For example, when control of a domain name is transferred, the new
domain owner might be unable to determine whether the owner of the domain owner might be unable to determine whether the owner of the
subject address has been under continuous ownership since the stated subject address has been under continuous ownership since the stated
date if the mailbox history is not also transferred (or was not date if the mailbox history is not also transferred (or was not
previously maintained). previously maintained).
It will also be "unknown" if whatever database contains mailbox It will also be "unknown" if whatever database contains mailbox
ownership data is temporarily unavailable at the time a message ownership data is temporarily unavailable at the time a message
arrives for delivery. In this case, typical SMTP temporary failure arrives for delivery. In this case, typical SMTP temporary failure
handling is appropriate. handling is appropriate.
7. Example 10. Examples
In the following example, "C:" indicates data sent by an SMTP client, In the following examples, "C:" indicates data sent by an SMTP
and "S:" indicates responses by the SMTP server. Message content is client, and "S:" indicates responses by the SMTP server. Message
CRLF terminated, though these are omitted here for ease of reading. content is CRLF terminated, though these are omitted here for ease of
reading.
10.1. SMTP Extension Example
C: [connection established]
S: 220 server.example.com ESMTP ready
C: EHLO client.example.net
S: 250-server.example.com
S: 250 RRVS
C: MAIL FROM:<sender@example.net>
S: 250 OK
C: RCPT TO:<receiver@example.com> RRVS=1381993177
S: 550 5.7.15 receiver@example.com is no longer valid
C: QUIT
S: 221 So long!
10.2. Header Field Example
C: [connection established] C: [connection established]
S: 220 server.example.com ESMTP ready S: 220 server.example.com ESMTP ready
C: HELO client.example.net C: HELO client.example.net
S: 250 server.example.com S: 250 server.example.com
C: MAIL FROM:<sender@example.net> C: MAIL FROM:<sender@example.net>
S: 250 OK S: 250 OK
C: RCPT TO:<receiver@example.com> C: RCPT TO:<receiver@example.com>
S: 250 OK S: 250 OK
C: DATA C: DATA
S: 354 Ready for message content S: 354 Ready for message content
skipping to change at page 7, line 42 skipping to change at page 10, line 34
C: QUIT C: QUIT
S: 221 So long! S: 221 So long!
If an authentication scheme is applied to claim the added header If an authentication scheme is applied to claim the added header
field is valid, but the scheme fails, a receiver might reject the field is valid, but the scheme fails, a receiver might reject the
message with an SMTP reply such as: message with an SMTP reply such as:
S: 550-5.7.7 Use of Require-Recipient-Valid-Since header S: 550-5.7.7 Use of Require-Recipient-Valid-Since header
S: 550 field requires a valid signature S: 550 field requires a valid signature
8. Security Considerations 11. Security Considerations
8.1. Abuse Countermeasures 11.1. Abuse Countermeasures
The response of a server implementing this protocol can disclose The response of a server implementing this protocol can disclose
information about the age of existing email mailbox. Implementation information about the age of existing email mailbox. Implementation
of countermeasures against probing attacks is advised. For example, of countermeasures against probing attacks is advised. For example,
an operator could track appearance of this field with respect to a an operator could track appearance of this field with respect to a
particular mailbox and observe the timestamps being submitted for particular mailbox and observe the timestamps being submitted for
testing; if it appears a variety of timestamps is being tried against testing; if it appears a variety of timestamps is being tried against
a single mailbox in short order, the field could be ignored and the a single mailbox in short order, the field could be ignored and the
message silently discarded. This concern is discussed further in message silently discarded. This concern is discussed further in
Section 9. Section 12.
8.2. Suggested Use Restrictions 11.2. Suggested Use Restrictions
If the mailbox named in the field is known to have had only a single If the mailbox named in the field is known to have had only a single
continuous owner since creation, or not to have existed at all (under continuous owner since creation, or not to have existed at all (under
any owner) prior to the date specified in the field, then the field any owner) prior to the date specified in the field, then the field
can be silently ignored and normal message handling applied so that can be silently ignored and normal message handling applied so that
this information is not disclosed. Such fields are likely the this information is not disclosed. Such fields are likely the
product of either gross error or an attack. product of either gross error or an attack.
A message author using this specification might restrict inclusion of A message author using this specification might restrict inclusion of
the header field such that it is only done for recipients known also the header field such that it is only done for recipients known also
to implement this specification, in order to reduce the possibility to implement this specification, in order to reduce the possibility
of revealing information about the relationship between the author of revealing information about the relationship between the author
and the mailbox. and the mailbox.
9. Privacy Considerations If ownership of an entire domain is transferred, the new owner may
not know what addresses were assigned in the past by the prior owner.
Hence, no address can be known not to have had a single owner, or to
have existed (or not) at all.
9.1. Probing Attacks 12. Privacy Considerations
12.1. Probing Attacks
As described above, use of this header field in probing attacks can As described above, use of this header field in probing attacks can
disclose information about the history of the mailbox. The harm that disclose information about the history of the mailbox. The harm that
can be done by leaking any kind of private information is difficult can be done by leaking any kind of private information is difficult
to predict, so it is prudent to be sensitive to this sort of to predict, so it is prudent to be sensitive to this sort of
disclosure, either inadvertently or in response to probing by an disclosure, either inadvertently or in response to probing by an
attacker. It bears restating, then, that implementing attacker. It bears restating, then, that implementing
countermeasures to abuse of this capability needs strong countermeasures to abuse of this capability needs strong
consideration. consideration.
skipping to change at page 9, line 5 skipping to change at page 12, line 5
information. Supplying such credentials to the wrong party after a information. Supplying such credentials to the wrong party after a
mailbox ownership change could allow the previous owner's data to be mailbox ownership change could allow the previous owner's data to be
exposed without his or her authorization or knowledge. In contrast, exposed without his or her authorization or knowledge. In contrast,
the information that may be exposed to a third party via the proposal the information that may be exposed to a third party via the proposal
in this document is limited to information about the mailbox history. in this document is limited to information about the mailbox history.
Given that MSPs have chosen to allow transfers of mailbox ownership Given that MSPs have chosen to allow transfers of mailbox ownership
without the prior owner's involvement, the information leakage from without the prior owner's involvement, the information leakage from
the header field specified here creates far fewer risks than the the header field specified here creates far fewer risks than the
potential for delivering mail to the wrong party. potential for delivering mail to the wrong party.
9.2. Envelope Recipients 12.2. Envelope Recipients
The email To and Cc header fields are not required to be populated The email To and Cc header fields are not required to be populated
with addresses that match the envelope recipient set, and Cc may even with addresses that match the envelope recipient set, and Cc may even
be absent. However, the algorithm in Section 3 requires that this be absent. However, the algorithm in Section 3 requires that this
header field contain a match for an envelope recipient in order to be header field contain a match for an envelope recipient in order to be
actionable. As such, use of this specification can reveal some or actionable. As such, use of this specification can reveal some or
all of the original intended recipient set to any party that can see all of the original intended recipient set to any party that can see
the message in transit or upon delivery. the message in transit or upon delivery.
For a message destined to a single recipient, this is unlikely to be For a message destined to a single recipient, this is unlikely to be
a concern, which is one of the reasons use of this specification on a concern, which is one of the reasons use of this specification on
multi-recipient messages is discouraged. multi-recipient messages is discouraged.
10. IANA Considerations 13. IANA Considerations
10.1. Header Field Registration 13.1. SMTP Extension Registration
IANA is requested to register the SMTP extension described in
Section 3.1.
13.2. Header Field Registration
IANA is requested to add the following entry to the Permanent Message IANA is requested to add the following entry to the Permanent Message
Header Field registry, as per the procedure found in [IANA-HEADERS]: Header Field registry, as per the procedure found in [IANA-HEADERS]:
Header field name: Require-Recipient-Valid-Since Header field name: Require-Recipient-Valid-Since
Applicable protocol: mail ([MAIL]) Applicable protocol: mail ([MAIL])
Status: Standard Status: Standard
Author/Change controller: IETF Author/Change controller: IETF
Specification document(s): [this document] Specification document(s): [this document]
Related information: Related information:
Requesting review of any proposed changes and additions to Requesting review of any proposed changes and additions to
this field is recommended. this field is recommended.
10.2. Enhanced Status Code Registration 13.3. Enhanced Status Code Registration
IANA is requested to register the following in the SMTP Enhanced IANA is requested to register the following in the SMTP Enhanced
Status Codes registry: Status Codes registry:
Code: X.7.15 Code: X.7.15
Sample Text: Mailbox owner has changed Sample Text: Mailbox owner has changed
Associated basic status code: 5 Associated basic status code: 5
Description: This status code is returned when a message is Description: This status code is returned when a message is
received with a Require-Recipient-Valid-Since received with a Require-Recipient-Valid-Since
field and the receiving system is able to field and the receiving system is able to
determine that the intended recipient mailbox determine that the intended recipient mailbox
has not been under continuous ownership since has not been under continuous ownership since
the specified date. the specified date.
Reference: [this document] Reference: [this document]
Submitter: M. Kucherawy Submitter: M. Kucherawy
Change controller: IESG Change controller: IESG
11. References 14. References
11.1. Normative References 14.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 5234, January 2008. Syntax Specifications: ABNF", RFC 5234, January 2008.
[IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul, [IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul,
"Registration Procedures for Message Header Fields", "Registration Procedures for Message Header Fields",
BCP 90, RFC 3864, September 2004. BCP 90, RFC 3864, September 2004.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322, [MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008. October 2008.
[ROLES] Crocker, D., "Mailbox Names For Common Services, [ROLES] Crocker, D., "Mailbox Names For Common Services,
Roles And Functions", RFC 2142, May 1997. Roles And Functions", RFC 2142, May 1997.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", [SMTP] Klensin, J., "Simple Mail Transfer Protocol",
RFC 5321, October 2008. RFC 5321, October 2008.
11.2. Informative References 14.2. Informative References
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message
Format for Delivery Status Notifications", RFC 3464, Format for Delivery Status Notifications", RFC 3464,
January 2003. January 2003.
[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009. July 2009.
[ESC] Vaudreuil, G., "Enhanced Mail System Status Codes", [ESC] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003. RFC 3463, January 2003.
Appendix A. Acknowledgments Appendix A. Acknowledgments
Erling Ellingsen proposed the idea. Erling Ellingsen proposed the idea.
Reviews and comments were provided by Michael Adkins, Kurt Andersen, Reviews and comments were provided by Michael Adkins, Kurt Andersen,
Alissa Cooper, Dave Crocker, Ned Freed, John Levine, Gregg Stefancik, Alissa Cooper, Dave Cridland, Dave Crocker, Ned Freed, John Levine,
Ed Zayas, (others) Hector Santos, Gregg Stefancik, Ed Zayas, (others)
Authors' Addresses Authors' Addresses
William J. Mills William J. Mills
Yahoo! Inc. Yahoo! Inc.
EMail: wmills_92105@yahoo.com EMail: wmills_92105@yahoo.com
Murray S. Kucherawy Murray S. Kucherawy
Facebook, Inc. Facebook, Inc.
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