Internet DRAFT - draft-crocker-email-deliveredto
draft-crocker-email-deliveredto
Network Working Group D. Crocker, Ed.
Internet-Draft Brandenburg InternetWorking
Intended status: Experimental 9 February 2022
Expires: 13 August 2022
Delivered-To Email Header Field
draft-crocker-email-deliveredto-10
Abstract
The address to which email is delivered might be different than any
of the addresses shown in any of the content header fields that were
created by the email's author. For example, the address used by the
email transport service is provided separately, such as through
SMTP's "RCPT TO" command, and might not match any address in the To:
or cc: fields. In addition before final delivery, handling can
entail a sequence of submission/delivery events, using a sequence of
different destination addresses that (eventually) lead to the
recipient. As well, a receiving system's delivery process can
produce local address transformations.
It can be helpful for a message to have a common way to record each
delivery in such a sequence, and to note each address used in the
sequence to that recipient, such as for analyzing the path a message
has taken, or for loop detection, or for formulating the author's
address in a reply message. This document defines a header field for
this information.
Email handling information discloses details about the email
infrastructure, as well as about a particular recipient; this can
raise privacy concerns.
A header field such as this is not automatically assured of
widespread use. Therefore this is being published as an Experiment,
looking for constituency and for operational utility. The document
is produced through the Independent RFC stream and was not subject to
the IETF's approval process.
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 https://datatracker.ietf.org/drafts/current/.
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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 13 August 2022.
Copyright Notice
Copyright (c) 2022 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
Provisions Relating to IETF Documents (https://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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Framework & Terminology . . . . . . . . . . . . . . . . . . . 5
4. Delivered-To . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Multi-delivery Example . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Experimental Goals . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The address to which email is delivered might be different than any
of the addresses shown in any of the content header fields
[Mail-Fmt], such as the To: and cc: fields that were created by the
author's Mail User Agent (MUA) [Mail-Arch]. The address used by the
Message Handling Service (MHS) is provided separately, in envelope
information, such as through an "RCPT TO" command in [SMTP].
Delivery is a transition of responsibility for a message, from the
MHS, over to an agent of the destination, as represented by the
envelope address (Section 4.3.3 [Mail-Arch]). That is, when the
destination address is fully and successfully processed, and any
additional processing is by an agent working on behalf of that
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address, the message has been delivered. Rather than placing the
message into a recipient inbox, or otherwise completing the handling
of the message, that agent might create additional processing,
including to one or more, different addresses. Each transition of
responsibility, from the MHS to an agent of a current addressee,
constitutes a distinct delivery. Given handling sequences that can
include aliasing, mailing lists, and the like, the transit of a
message from its author to a final recipient might include a series
of submission/delivery events. Also, the delivery process at a
receiving system can produce local (internal) address
transformations.
Header fields that provide information about handling can be used
when assessing email traffic issues and when diagnosing specific
handling problems. To this end, it can be helpful for a message to
have a common way of indicating each delivery in the handling
sequence, and to include each address that led to the final delivery.
This can aid in the analysis of a message's transit handling.
An additional use can as be an aid in detecting a delivery sequence
loop, based on a specific address. With a problematic loop, the same
copy of a message is delivered to the same email address more than
once. This is different from having different copies delivered to
the same address, such as happens when a message is sent directly to
an address, as well as via a mailing list. It is also different from
having two copies of the same message arrive at the same, ultimate
destination address, having been originally posted to two different
addresses. Further, this is different from noting when a message
simply transits the same MTA more than once, which might be
necessary, such as when it is processed through a mailing list; an
MTA services many addresses.
Delivering the same copy of a message more than once, to the same
address, is almost certainly not an intended activity. An example of
a problematic arrangement would be to send a message to mailing list
List-A, where List-A contains an entry for List-B, and List-B
contains an entry for List-A. The message will enter an infinite
loop. Loop detection for email can be a complicated affair. The
Delivered-To: header field provides helpful information, with a
definitive indication that this copy of a message has (already) been
delivered to a specific address.
When specifying new activity that is related to existing activity,
there is a choice of design approach:
* Seeking to change (some) of the existing behavior
* Adding to the activity without changing what is already being done
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* Calling for separate, new activity
On the average, attempting to change existing activities is the least
likely to obtain adoption; it can create operational confusion
between old and new activities, which in turn creates resistance to
adoption. Seeking new activity can make sense when that activity is
sufficiently different and deemed sufficiently beneficial. Adding to
existing activity has the selling point of building upon an installed
base. The current specification builds upon an existing installed
base of Delivered-To: activity. It calls for little technical
enhancement, but rather, it simply provides for wider range of
application.
Considerations:
* Email handling information, such as this, provides information
about the email infrastructure, as well as about the recipient.
Disclosure of this information might engender privacy concerns.
* A specification, is not automatically assured of adoption or use.
Therefore it is being published as an Experiment, looking for
extended constituency and for general operational utility.
* This document was produced through the Independent RFC stream and
was not subject to the IETF's approval process.
2. Background
Ad hoc use of a "Delivered-To:" email header field appears to date
back to the 1990s, primarily for loop detection, although
documentation is spotty and system-specific. A listing of some
implementations is offered in [Prior].
It appears that all uses include a string in the form of an email
address, although at least one example has leading text that is a
comment about the address. In some cases, the string appears to be
the email transport destination address, such as used in SMTP's "RCPT
TO" command. In other cases, it appears to be the result of some
internal mapping at the receiving system, although tending to be a
variant of the transport address.
Email loop detection tends to be accomplished through a variety of
different methods, such as counting Received: header fields. These
methods are often combined to greater effect.
The Received: header field's 'for' clause is sometimes useful for
disclosing the recipient's address. However the clause is not used
reliably and its semantics are not thoroughly defined. Also it
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references an addressing value that is received, but might be
different from the value that is ultimately used (as the result of a
transformation.) That is, the value in a 'for' clause might be a
sufficient indicator of delivery addressing, but it might not.
3. Framework & Terminology
Unless otherwise indicated, basic architecture and terminology used
in this document are taken from:
* [Mail-Arch]
* [SMTP]
* [Mail-Fmt]
and syntax is specified with:
* [ABNF]
Normative language, is per [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.
4. Delivered-To
The "Delivered-To:" header field annotates an email delivery event.
The header field contains information about the individual address
used to effect that transition.
* When a message is delivered, as a transition from control by the
MHS to the recipient's store or their agent, a Delivered-To:
header field SHOULD be added, with the _addr-spec_ value
containing the address that was used by the service to reach the
recipient.
* If a receiving system's delivery process applies mappings or
transformations, from the address used by the MHS, to a local
value, this new value SHOULD also be recorded into a separate
Delivered-To: field, when transit and processing using that
addresses successfully completes. This ensures a detailed record
of the sequence of handling addresses used for the message.
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* As with some other information, each additional Delivered-To:
header field MUST be placed at the current 'top' of the message's
set of header fields -- that is, as the first header field, in a
fashion similar to the trace fields specified in [SMTP], such as
in its Section 4.1.1.4. This produces a sequence of Delivered-To:
header fields that represent the sequence of deliveries, with the
first being at the 'bottom' of the sequence and the final one
being at the top.
* As with other fields placed incrementally in this way, with each
added at the current top, the Delivered-To: header field MUST NOT
be reordered with respect to other Delivered-To: fields and those
other fields. This is intended to preserve the fields as
representing the message handling sequence.
The Delivered-To: header field is added at the time of delivery, when
responsibility for a message transitions from the Message Handling
(Transport) Service (MHS) to an agent of the specified individual
recipient address. The field can also be added as a result of
internal system processing, to note address transformations.
Note: The presence of an existing Delivered-To: header field, for
the same address, typically indicates a handling loop for this
instance of the message.
The syntax of the header field is:
"Delivered-To:" FWS addr-spec FWS CRLF ; addr-spec is from [Mail-Fmt]
The field records information about a single address, for one
recipient. See [Section 6] for the privacy-related concerns about
divulging addresses.
5. Multi-delivery Example
The Delivered-To: header field can be used to document a sequence of
deliveries of a message. Each time an address is fully processed, a
Delivered-To: header field is added, recording a handling sequence,
with the most recent one being towards the 'top' of the sequence of
header fields.
This example demonstrates a message traveling from its original
posting, through a remote group mailing list, on through an
independent personal aliasing mechanism, and then reaching final
delivery at yet another independent email provider.
1. Origination @ com.example
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The message, as submitted. The destination address is the
same as the value in the message's To: header field.
From: Ann Author <aauthor@com.example>
Date: Mon, 25 Jan 2021 18:29:00 -0500
To: list@org.example
Subject: [list] Sending through a list and alias
Sender: Ann Author <aauthor@com.example>
2. List @ org.example
As delivered, with one Delivered-To: header field, to the list
processing module, which will then re-submit the message for
further transport to the list member "Recipient-
alumn@edu.example".
Delivered-To: list@org.example
Received: by submit.org.example with SMTP id i17so17480689ljn.1
for <list@org.example> from mail.com.example;
Mon, 25 Jan 2021 15:29:19 -0800 (PST)
Received: by mail.com.example; Mon, 25 Jan 2021 15:29:00 -0800 (PST)
From: Ann Author <aauthor@com.example>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@org.example
Subject: [list] Sending through a list and alias
Sender: Ann Author <aauthor@com.example>
3. Alias @ edu.example
The message, as delivered with two Delivered-To: header
fields, to the alias processing module, which sends the
message on to "theRecipient@example.net".
Delivered-To: Recipient-alumn@edu.example
Received: from mail.org.example
by relay.edu.example; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Received: by submit.org.example;
Mon, 25 Jan 2021 23:29:21 +0000 (UTC)
Delivered-To: list@org.example
Received: by submit.org.example with SMTP id i17so17480689ljn.1
for <list@org.example> from mail.com.example;
Mon, 25 Jan 2021 15:29:19 -0800 (PST)
Received: by mail.com.example; Mon, 25 Jan 2021 15:29:00 -0800 (PST)
From: Ann Author <aauthor@com.example>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@org.example
Subject: [list] Sending through a list and alias
Sender: list-bounces@org.example
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4. Delivery @ example.net
The message, as finally delivered with three Delivered-To:
header fields, to the recipient at "theRecipient@example.net".
Delivered-To: theRecipient@example.net
Received: from mail.edu.example (mail.edu.example [4.31.198.45])
by relay.example.net; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Delivered-To: Recipient-alumn@edu.example
Received: from mail.org.example
by relay.edu.example; Mon, 25 Jan 2021 23:29:24 +0000 (UTC)
Received: by submit.org.example;
Mon, 25 Jan 2021 23:29:21 +0000 (UTC)
Delivered-To: list@org.example
Received: by submit.org.example with SMTP id i17so17480689ljn.1
for <list@org.example> from mail.com.example;
Mon, 25 Jan 2021 15:29:19 -0800 (PST)
Received: by mail.com.example; Mon, 25 Jan 2021 15:29:00 -0800 (PST)
From: Ann Author <aauthor@com.example>
Date: Mon, 25 Jan 2021 18:29:06 -0500
To: list@org.example
Subject: [list] Sending through a list and alias
Sender: list-bounces@org.example
6. Security Considerations
As with Received: header fields, the presence of a Delivered-To:
header field discloses handling information and, possibly, personal
information.
Security and privacy are essential, if challenging, topics for email
in general and for the handling of metadata in particular. The
purpose of this section is to note points of potential concern,
rather than to provide details for mitigation. The basic mechanism
described here has a long history of use, with no history of being
problematic. However the expanded use described here might create
new scenarios that are problematic.
An issue specific to this mechanism is disclosure of a sequence of
addresses, applied to the same recipient, if a message goes through a
series of recipient address replacements. The document calls for
each of these addresses to be recorded in a separate Delivered-To:
field. This does not disclose addresses of other recipients, but it
does disclose a address-transformation handling path for the
recipient.
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Where this disclosure is most likely to be a concern is when a
recipient manually forwards a message and includes all of the
original header fields. This will expose, to a later recipient, any
intermediate addresses used for getting the original message to the
original recipient. Such a disclosure is likely to be unintended and
might be (highly) problematic. Note that a basic version of this
unintended disclosure has long existed, by virtue of a later
recipient's seeing Received: header fields, but especially any with a
'for' clause. However a Delivered-To: header field sequence can
disclose significantly more recipient-specific handling detail.
An issue that is entirely implementation specific -- and therefore
out of scope to this document -- is that in some systems, a message
that is for multiple (local) recipients is stored as a single, shared
version. Supporting Delivered-To:, while maintaining recipient
privacy, creates a challenge in this case, since exposing different
recipient addresses to other recipients can be problematic.
7. IANA Considerations
Registration of the "Delivered-To:" header field is requested, per
[RFC3864]:
Header field name: Delivered-To
Applicable protocol: mail
Status: Provisional
Author/Change controller: Dave Crocker
Specification document(s): *** This document ***
Related information: None.
8. Experimental Goals
Specific feedback is sought concerning:
* Technical issues in recording the Delivered-To: field into a
message, through its entire submission/delivery sequence
* Market interest in the uses described here
* Utility for the purposes described here, or for other uses
So the questions to answer for this Experimental document are:
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* Is there demonstrated interest by MSA/MTA/MDA developers?
* If the capability is implemented and the header field generated,
is it used by operators or MUAs?
* Does the presence of the header field create any operational
problems?
* Does the presence of the header field demonstrate additional
security issues?
* What specific changes to the document are needed?
* What other comments will aid in use of this mechanism?
Please send comments to ietf-smtp@ietf.org.
9. References
9.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[Mail-Arch]
Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009, <https://www.rfc-editor.org/rfc/rfc5598>.
[Mail-Fmt] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008, <https://www.rfc-editor.org/rfc/rfc5322>.
[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>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[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>.
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[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
9.2. Informative References
[Prior] Dukhovni, V. and J. J. Levine, "The Delivered-To Message
Header Field", I-D draft-duklev-deliveredto-00, 16 August
2021.
Appendix A. Acknowledgements
Even a simple, narrow specification can elicit a remarkable range and
intensity of debate. In spite of the current document's being a case
of that challenge, useful discussion has taken place, first in the
IETF's emailcore working group mailing list, and then on the long-
standing ietf-smtp mailing list.
Helpful information and suggestions were provided by: Anonymous,
Richard Clayton, Viktor Dukhovni, Adrian Farrel, Ned Freed, John
Klensin, Barry Leiba, Brandon Long, George Michaelson, Michael
Peddemors, Phil Pennock, Pete Resnick, Sam Varshavchik, Alessandro
Vesely, Tim Wicinski.
Author's Address
Dave Crocker (editor)
Brandenburg InternetWorking
Email: dcrocker@bbiw.net
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