Sieve Notification Mechanism: mailto
draft-ietf-sieve-notify-mailto-10

Status of this Memo

By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.

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.

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.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on June 7, 2009.

Abstract

This document describes a profile of the Sieve extension for notifications, to allow notifications to be sent by electronic mail.

Table of Contents

1.  Introduction
1.1.  Overview
1.2.  Conventions used in this document

2.  Definition
2.1.  Notify parameter "method"
2.2.  Test notify_method_capability
2.3.  Notify tag ":from"
2.4.  Notify tag ":importance"
2.5.  Notify tag ":options"
2.6.  Notify tag ":message"
2.7.  Other Definitions
2.7.1.  The Auto-Submitted header field

3.  Examples

4.  Internationalization Considerations

5.  Security Considerations

6.  IANA Considerations
6.1.  Registration of notification mechanism
6.2.  New registry for Auto-Submitted header field keywords
6.3.  Initial registration of Auto-Submitted header field keywords

7.  References
7.1.  Normative References
7.2.  Non-Normative References

§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

1.1.  Overview

The [Notify] (Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T. Martin, “Sieve Extension: Notifications,” December 2007.) extension to the [Sieve] (Guenther, P., Ed. and T. Showalter, Ed., “Sieve: An Email Filtering Language,” January 2008.) mail filtering language is a framework for providing notifications by employing URIs to specify the notification mechanism. This document defines how [mailto] (Hoffman, P., Masinter, L., and J. Zawinski, “The mailto URL scheme,” July 1998.) URIs are used to generate notifications by e-mail.

1.2.  Conventions used in this document

Conventions for notations are as in [Sieve] (Guenther, P., Ed. and T. Showalter, Ed., “Sieve: An Email Filtering Language,” January 2008.) section 1.1, including the use of [Kwds] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [Kwds] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Definition

The mailto mechanism results in the sending of a new email message (a "notification message") to notify a recipient about a "triggering message".

2.1.  Notify parameter "method"

The mailto notification mechanism uses standard mailto URIs as specified in [mailto] (Hoffman, P., Masinter, L., and J. Zawinski, “The mailto URL scheme,” July 1998.). mailto URIs may contain header fields consisting of a header name and value. These header fields are called "URI headers" to distinguish them from "message headers".

2.2.  Test notify_method_capability

The notify_method_capability test for "online" may return "yes" or "no" only if the Sieve processor can determine with certainty whether or not the recipients of the notification message are online and logged in. Otherwise, the test returns "maybe" for this notification method.

2.3.  Notify tag ":from"

The :from tag overrides the default sender of the notification message. "Sender", here, refers to the value used in the [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) "From" header. Implementations MAY also use this value in the [RFC5321] (Klensin, J., Ed., “Simple Mail Transfer Protocol,” October 2008.) "MAIL FROM" command (the "envelope sender"), or they may prefer to establish a mailbox that receives bounces from notification messages.

2.4.  Notify tag ":importance"

The :importance tag has no special meaning for this notification mechanism, and this specification puts no restriction on its use. Implementations MAY use the value of :importance to set a priority or importance indication on the notification message (perhaps a visual indication, or perhaps making use of one of the non-standard but commonly used message headers).

2.5.  Notify tag ":options"

This tag is not used by the mailto method.

2.6.  Notify tag ":message"

The value of this tag, if it is present, is used as the subject of the notification message, and overrides all other mechanisms for determining the subject (as described below). Its value SHOULD NOT normally be truncated, though it may be sensible to truncate an excessively long value.

2.7.  Other Definitions

Because the receipt of an email message is generating another email message, implementations MUST take steps to avoid mail loops. The REQUIRED inclusion of an "Auto-Submitted:" field, as described in the message composition guidelines, will also help in loop detection and avoidance.

Implementations SHOULD NOT trigger notifications for messages containing "Auto-Submitted:" header fields with any value other than "No".

Implementations MUST allow messages with empty envelope senders to trigger notifications.

Because this notification method uses a store-and-forward system for delivery of the notification message, the Sieve processor should not have a need to retry notifications. Therefore, implementations of this method SHOULD use normal mechanisms for submitting SMTP messages and for retrying the initial submission. Once the notification message is submitted, implementations MUST NOT resubmit it, as this is likely to result in multiple notifications, and increases the danger of message loops.

The overall notification message is composed using the following guidelines (see [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) for references to message header fields):

• If the envelope sender of the triggering message is empty, the envelope sender of the notification message MUST be empty as well, to avoid message loops. Otherwise, the envelope sender of the notification message SHOULD be set to the value of the ":from" parameter to the notify action, if one is specified, has email address syntax and is valid according to the implementation specific security checks (see Section 3.3 of [Notify] (Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T. Martin, “Sieve Extension: Notifications,” December 2007.)). If ":from" is not specified or is not valid, the envelope sender of the notification message SHOULD be set either to the envelope "to" field from the triggering message, as used by Sieve, or to an email address associated with the notification system, at the discretion of the implementation. This MUST NOT be overridden by a "from" URI header, and any such URI header MUST be ignored.
• The envelope recipient(s) of the notification message SHOULD be set to the address(es) specified in the URI (including any URI headers where the hname is "to" or "cc").
• The header field "Auto-Submitted: auto-notified" MUST be included in the notification message (see Section 2.7.1 (The Auto-Submitted header field)). This is to reduce the likelihood of message loops, by tagging this as an automatically generated message. Among other results, it will inform other notification systems not to generate further notifications. mailto URI headers with hname "auto-submitted" are considered unsafe and MUST be ignored.
• The "From:" header field of the notification message SHOULD be set to the value of the ":from" parameter to the notify action, if one is specified, has email address syntax and is valid according to the implementation specific security checks (see Section 3.3 of [Notify] (Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T. Martin, “Sieve Extension: Notifications,” December 2007.)). If ":from" is not specified or is not valid, the "From:" header field of the notification message SHOULD be set either to the envelope "to" field from the triggering message, as used by Sieve, or to an email address associated with the notification system, at the discretion of the implementation. This MUST NOT be overridden by a "from" URI header, and any such URI header MUST be ignored.
• The "To:" header field of the notification message SHOULD be set to the address(es) specified in the URI (including any URI headers where the hname is "to").
• The "Subject:" field of the notification message SHOULD contain the value defined by the :message notify tag, as described in [Notify] (Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T. Martin, “Sieve Extension: Notifications,” December 2007.). If there is no :message tag and there is a "subject" header on the URI, then that value SHOULD be used. If that is also absent, the subject SHOULD be retained from the triggering message. Note that Sieve [Variables] (Homme, K., “Sieve Extension: Variables,” January 2008.) can be used to advantage here, as shown in the example in Section 3 (Examples).
• The "References:" field of the notification message MAY be set to refer to the triggering message, and MAY include references from the triggering message.
• If the mailto URI contains a "body" header, the value of that header SHOULD be used as the body of the notification message. If there is no "body" header, it is up to the implementation whether to leave the body empty or to use an excerpt of the original message.
• The "Received:" fields from the triggering message MAY be retained in the notification message, as these could provide useful trace/history/diagnostic information. The "Auto-Submitted" header field MUST be placed above these (see Section 2.7.1 (The Auto-Submitted header field)). URI headers with hname "received" are considered unsafe, and MUST be ignored.
• Other header fields of the notification message that are normally related to an individual new message (such as "Message-ID" and "Date") are generated for the notification message in the normal manner, and MUST NOT be copied from the triggering message. Any URI headers with those names MUST be ignored. Further, the "Date" header serves as the notification timestamp defined in [Notify] (Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T. Martin, “Sieve Extension: Notifications,” December 2007.).
• All other header fields of the notification message either are as specified by URI headers, or have implementation-specific values; their values are not defined here. It is suggested that the implementation capitalize the first letter of URI headers and add a space character after the colon between the mail header name and value when adding URI headers to the message, to be consistent with common practice in email headers.

2.7.1.  The Auto-Submitted header field

The header field "Auto-Submitted: auto-notified" MUST be included in the notification message (see [RFC3834] (Moore, K., “Recommendations for Automatic Responses to Electronic Mail,” August 2004.)). The "Auto-Submitted" header field is considered a "trace field", similar to "Received" header fields (see [RFC5321] (Klensin, J., Ed., “Simple Mail Transfer Protocol,” October 2008.)). If the implementation retains the "Received" fields from the triggering message (see above), the "Auto-Submitted" field MUST be placed above those "Received" fields, serving as a boundary between the ones from the triggering message and those that will be part of the notification message.

The auto-notified Auto-Submitted field MUST include one or both of the following parameters:

• owner-email - specifies an email address of the owner of the Sieve script that generated this notification. If specified, it might be used to identify or contact the script's owner. The parameter attribute is "owner-email", and the parameter value is a quoted string containing an email address, as defined by "addr-spec" in [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.). Example:
  
    Auto-Submitted: auto-notified; owner-email="me@example.com"
• owner-token - specifies an opaque token that the administrative domain of the owner of the Sieve script that generated this notification can identify the owner with. This might be used to allow identification of the owner while protecting the owner's privacy. The parameter attribute is "owner-token", and the parameter value is as defined by "token" in [RFC3834] (Moore, K., “Recommendations for Automatic Responses to Electronic Mail,” August 2004.). Example:
  
    Auto-Submitted: auto-notified; owner-token=af3NN2pK5dDXI0W

See Section 5 (Security Considerations) for discussion of possible uses of these parameters.

3.  Examples

Triggering message (received by recipient@example.org):

   Return-Path: <knitting-bounces@example.com>
   Received: from mail.example.com by mail.example.org
     for <recipient@example.org>; Wed, 7 Dec 2005 05:08:02 -0500
   Received: from hobbies.example.com by mail.example.com
     for <knitting@example.com>; Wed, 7 Dec 2005 02:00:26 -0800
   Message-ID: <1234567.89ABCDEF@example.com>
   Date: Wed, 07 Dec 2005 10:59:19 +0100
   Precedence: list
   List-Id: Knitting Mailing List <knitting.example.com>
   Sender: knitting-bounces@example.com
   Errors-To: knitting-bounces@example.com
   From: "Jeff Smith" <jeff@hobbies.example.com>
   To: "Knitting Mailing List" <knitting@example.com>
   Subject: [Knitting] A new sweater

   I just finished a great new sweater!


Sieve script (run on behalf of recipient@example.org):

   require ["notify", "variables"];

   if header :contains "list-id" "knitting.example.com" {
     if header :matches "Subject" "[*] *" {
       notify :message "From ${1} list: ${2}"
           :importance "3"
           "mailto:0123456789@sms.example.net?to=backup@example.com";
     }
   }


Notification message:

   Auto-Submitted: auto-notified; owner-email="recipient@example.org"
   Received: from mail.example.com by mail.example.org
     for <recipient@example.org>; Wed, 7 Dec 2005 05:08:02 -0500
   Received: from hobbies.example.com by mail.example.com
     for <knitting@example.com>; Wed, 7 Dec 2005 02:00:26 -0800
   Date: Wed, 7 Dec 2005 05:08:55 -0500
   Message-ID: <A2299BB.FF7788@example.org>
   From: recipient@example.org
   To: 0123456789@sms.example.net, backup@example.com
   Subject: From Knitting list: A new sweater

Note that:

• Fields such as "Message-ID:" and "Date:" were generated afresh for the notification message, and do not relate to the triggering message.
• Additional "Received:" fields will be added to the notification message in transit; the ones shown were copied from the triggering message. New ones will be added above the "Auto-Submitted:" field.
• If this message should appear at the mail.example.org server again, the server can use the presence of a "mail.example.org" received line to recognize that. The Auto-Submitted header field is also present to tell the server to avoid sending another notification, and it includes an optional owner-email parameter for identification.

4.  Internationalization Considerations

This specification introduces no specific internationalization issues that are not already addressed in [Sieve] (Guenther, P., Ed. and T. Showalter, Ed., “Sieve: An Email Filtering Language,” January 2008.) and in [Notify] (Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T. Martin, “Sieve Extension: Notifications,” December 2007.).

5.  Security Considerations

Sending a notification is comparable with forwarding mail to the notification recipient. Care must be taken when forwarding mail automatically, to ensure that confidential information is not sent into an insecure environment.

The automated sending of email messages exposes the system to mail loops, which can cause operational problems. Implementations of this specification MUST protect themselves against mail loops; see Section 2.7 (Other Definitions) for discussion of this and some suggestions. Other possible mitigations for mail loops involve types of service limitations. For example, the number of notifications generated for a single user might be limited to no more than, say, 30 in a 60-minute period. Of course, this technique presents its own problems, in that the actual rate limit must be selected carefully, to allow most legitimate situations in the given environment, and even with careful selection it's inevitable that there will be false positives -- and false negatives.

Ultimately, human intervention may be necessary to re-enable notifications that have been disabled because a loop was detected, or to terminate a very slow loop that's under the automatic-detection radar. Administrative mechanisms MUST be available to handle these sorts of situations.

Email addresses specified as recipients of notifications might not be owned by the entity that owns the Sieve script. As a result, a notification recipient could wind up as the target of unwanted notifications, either through intent (using scripts to mount a mail-bomb attack) or by accident (an address was mistyped or has been reassigned). The situation is arguably no worse than any other in which a recipient gets unwanted email, and some of the same mechanisms can be used in this case. But those deploying this extension have to be aware of the potential extra problems here, where scripts might be created through means that do not adequately validate email addresses, and such scripts might then be forgotten and left to run indefinitely.

In particular, note that the Auto-Submitted header field is required to include a value that a recipient can use when contacting the source domain of the notification message (see Section 2.7.1 (The Auto-Submitted header field)). That value will allow the domain to track down the script's owner and have the script corrected or disabled. Domains that enable this extension MUST be prepared to respond to such complaints, in order to limit the damage caused by a faulty script.

Problems can also show up if notification messages are sent to a gateway into another service, such as SMS. Information from the email message is often lost in the gateway translation, and in this case critical information needed to avoid loops, to contact the script owner, and to resolve other problems might be lost. Developers of email gateways should consider these issues, and try to preseve as much information as possible, including what appears in email trace headers and Auto-Submitted.

Additional security considerations are discussed in [Sieve] (Guenther, P., Ed. and T. Showalter, Ed., “Sieve: An Email Filtering Language,” January 2008.) and in [Notify] (Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T. Martin, “Sieve Extension: Notifications,” December 2007.).

6.  IANA Considerations

6.1.  Registration of notification mechanism

The following template specifies the IANA registration of the Sieve notification mechanism specified in this document:

To: iana@iana.org
Subject: Registration of new Sieve notification mechanism
Mechanism name: mailto
Mechanism URI: RFC2368
Mechanism-specific tags: none
Standards Track/IESG-approved experimental RFC number: this RFC
Person and email address to contact for further information:
    Michael Haardt <michael.haardt@freenet.ag>

This information should be added to the list of sieve notification mechanisms given on http://www.iana.org/assignments/sieve-notification.

6.2.  New registry for Auto-Submitted header field keywords

Because [RFC3834] (Moore, K., “Recommendations for Automatic Responses to Electronic Mail,” August 2004.) does not define a registry for new keywords used in the Auto-Submitted header field, we define one here, to be created as http://www.iana.org/assignments/auto-submitted-keywords. Keywords are registered using the "Specification Required" policy [IANA] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

This defines the template to be used to register new keywords. Initial entries to this registry follow in Section 6.3 (Initial registration of Auto-Submitted header field keywords).

To: iana@iana.org
Subject: Registration of new auto-submitted header field keyword
Keyword value: [the text value of the field]
Description: [a brief explanation of the purpose of this value]
Parameters: [list any keyword-specific parameters, specify their meanings, specify whether they are required or optional; use "none" if there are none]
Standards Track/IESG-approved experimental RFC number: [identifies the specification that defines the value being registered]
Contact: [name and email address to contact for further information]

6.3.  Initial registration of Auto-Submitted header field keywords

The following are the initial keywords to be registered for the Auto-Submitted header field, to be entered in http://www.iana.org/assignments/auto-submitted-keywords.

Keyword value: no
Description: Indicates that a message was NOT automatically generated, but was created by a human. It is the equivalent to the absence of an Auto-Submitted header altogether.
Parameters: none
Standards Track/IESG-approved experimental RFC number: RFC3834
Contact: Keith Moore <moore@network-heretics.com>

Keyword value: auto-generated
Description: Indicates that a message was generated by an automatic process, and is not a direct response to another message.
Parameters: none
Standards Track/IESG-approved experimental RFC number: RFC3834
Contact: Keith Moore <moore@network-heretics.com>

Keyword value: auto-replied
Description: Indicates that a message was automatically generated as a direct response to another message.
Parameters: none
Standards Track/IESG-approved experimental RFC number: RFC3834
Contact: Keith Moore <moore@network-heretics.com>

Keyword value: auto-notified
Description: Indicates that a message was generated by a Sieve notification system.
Parameters: owner-email, owner-token. Both optional, both refer to the owner of the Sieve script that generated this message. See the relevant RFC for details.
Standards Track/IESG-approved experimental RFC number: this RFC
Contact: Michael Haardt <michael.haardt@freenet.ag>

7.  References

7.1. Normative References

7.2. Non-Normative References

Authors' Addresses

Full Copyright Statement

Copyright © The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.