RFC4865

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Network Working Group G. White Request for Comments: 4865 Independent Updates: 3463, 3464 G. Vaudreuil Category: Standards Track Alcatel-Lucent 

                                                            May 2007

  SMTP Submission Service Extension for Future Message Release

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The IETF Trust (2007).

Abstract

This memo defines an extension to the SMTP submission protocol for a client to indicate a future time for the message to be released for delivery. This extension permits a client to use server-based storage for a message that should be held in queue until an appointed time in the future. This is useful for clients which do not have local storage or are otherwise unable to release a message for delivery at an appointed time.

Introduction

There is a widely used feature within the voice messaging community to compose and send a message for delivery in the future. This is useful for sending announcements to be heard at the beginning of a work day, to send birthday greetings a day or so ahead, or to use as a lightweight facility to build a personal reminder service.

This extension uses the SMTP submission protocol [n3] to allow a client, when submitting a message, to indicate a future time for the message to be released for delivery.

Terminology

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 [n1].

Framework

The Future Message Release service extension for SMTP submission uses the SMTP service extension mechanism [n4] to extend the SMTP submission protocol [n3]. The following SMTP submission service extension is hereby defined:

The name of the SMTP submission service extension is "Future Message Release".

1) The Extended Hello (EHLO) keyword associated with this service extension is "FUTURERELEASE".

2) Two required parameters, the max-future-release-interval and the max-future-release-date-time, are combined with the EHLO keyword in the manner specified in [n4].

The max-future-release-interval is a positive integer indicating the maximum amount of time for which the message submission server (MSA) will hold messages for future release.

Using ABNF [n2], the syntax of this parameter is as follows: 

     future-release-integer = %x31-39 *8DIGIT
                              ; integer in the range 1-999999999
                              ; measured in seconds

     max-future-release-interval = future-release-integer

  The max-future-release-date-time is a timestamp, normalized to
  Universal Coordinated Time (UTC), indicating the most remote date
  and time in the future until which the MSA will hold messages for
  future release.

  Using ABNF [n2], the syntax of this parameter is as follows:

     max-future-release-date-time = date-time

  where the format of date-time is defined in [n10].

3) When forming the portion of the EHLO reply containing the 

  FUTURERELEASE keyword, the keyword is followed by the max-future-
  release-interval, and then the max-future-release-date-time.  The
  keyword and two values are delimited by spaces.

  For example, the ABNF for a continuation line in the EHLO response
  that contains the FUTURERELEASE keyword is:

     line = "250-FUTURERELEASE" SP max-future-release-interval
                                SP max-future-release-date-time

4) One required parameter, the hold-param, is added to the MAIL 

  command using either the keyword "HOLDFOR" or the keyword
  "HOLDUNTIL".

  The HOLDFOR parameter value is a future-release-interval, which is
  a positive integer indicating the amount of time the message is to
  be held by the MSA before release.

  The HOLDUNTIL parameter value is a future-release-date-time, which
  is a timestamp, normalized to UTC, indicating the future date and
  time until which the message is to be held by the MSA before
  release.

  Using ABNF [n2], the syntax of this parameter is as follows:

     future-release-interval = future-release-integer

     future-release-date-time = Internet-style-date-time-UTC

     hold-for-param = "HOLDFOR=" future-release-interval

     hold-until-param = "HOLDUNTIL=" future-release-date-time

     hold-param = hold-for-param / hold-until-param

  The absence of this parameter on the MAIL command does not imply a
  default value for this parameter.

5) The maximum length of a MAIL command is increased by 34 characters 

  by the possible addition of the hold-param.

6) No additional SMTP verbs are defined by this extension.

7) This service extension is appropriate only for the SMTP submission 

  protocol [n3].  This service extension is not appropriate for
  standard SMTP [n4].

Behavior

It is unfortunate to define two seemingly identical ways to indicate a future message release time. When the client has both accurate time and accurate time zone information, either interval or date-time can be trivially calculated from the other. However, in the current world of clients, there are clients with accurate local time but no indication of their time zone, and clients without a suitably accurate clock. Based on the limited facilities available to these time-challenged clients, it is likely that only one or the other of these mechanisms will be useful.

It is believed that servers will have accurate time, and can trivially convert between these mechanisms. It is also accepted that the protocol and implementation overhead of offering these two mechanisms is low, and that few interoperability challenges are anticipated.

SMTP Client

1) An SMTP client preparing to use Future Message Release MUST first 

  verify that the MSA supports this extension.

2) An SMTP client using Future Message Release MUST include one, and 

  only one, hold-param with the MAIL command.

3) An SMTP client using Future Message Release with the "for" option 

  of the hold-param MUST ensure that the future-release-interval is
  less than or equal to the max-future-release-interval advertised
  by the MSA.

4) An SMTP client using Future Message Release with the "until" 

  option of the hold-param MUST ensure that the future-release-
  date-time is earlier than or equal to the max-future-release-
  date-time advertised by the MSA.

MSA

1) An MSA supporting Future Message Release MUST comply with the SMTP 

  submission protocol as described in [n3].

2) An MSA supporting Future Message Release MUST NOT advertise this 

  support (i.e. include the FUTURERELEASE keyword in its EHLO reply)
  on any port other than the submission port.

3) An MSA supporting Future Message Release MUST include the 

  FUTURERELEASE keyword, and associated max-future-release-interval
  and max-future-release-date-time parameters, in its reply to the
  EHLO command.

4) An MSA supporting Future Message Release MUST accept a MAIL 

  command containing a valid hold-param, given that the MAIL command
  contains no other errors.

5) An MSA that accepts a message with a request for Future Message 

  Release indicating the "for" option MUST NOT release the message
  until the amount of time specified in the future-release-interval
  elapses.

6) An MSA that accepts a message with a request for Future Message 

  Release indicating the "until" option MUST NOT release the message
  until the date and time indicated by the future-release-date-time
  occurs.

7) An MSA supporting Future Message Release MUST reject a MAIL 

  command containing the "for" option specifying a value that is
  greater than the advertised max-future-release-interval, or
  otherwise invalid.

8) An MSA supporting Future Message Release MUST reject a MAIL 

  command containing the "until" option specifying a value that is
  later than the advertised max-future-release-date-time, or
  otherwise invalid.

9) An MSA supporting Future Message Release MUST reject a MAIL 

  command containing more than one hold-param.

10) An MSA supporting Future Message Release, when rejecting a MAIL 

  command per items 7, 8, or 9, above, SHOULD supply the reply code
  501 (syntax error in parameters or arguments [n4]) in the reply.

11) An MSA supporting Future Message Release, when rejecting a MAIL 

  command per items 7, 8, or 9, above, SHOULD supply the Enhanced
  Mail System Status Code 5.5.4 (invalid command arguments [i1]) in
  the reply.

Protocol Interactions

Interaction with the DSN SMTP Service Extensions

The Delivery Status Notification (DSN) service extension is described in [n7], and DSN message format is described in [n8].

SMTP Client Interaction with DSN

1) An SMTP client MUST NOT request Future Message Release when 

  sending a DSN to the MSA.

MSA Interaction with DSN

1) If an MSA generates a DSN for a message that includes a Future 

  Message Release request, the MSA MUST include an Arrival-Date
  field in the machine-readable body part of the DSN.

2) If an MSA generates a DSN for a message that includes a Future 

  Message Release request, the MSA MUST include a Future-Release-
  Request field in the machine-readable body part of the DSN.  The
  value of this field is the value of the HOLD parameter contained
  in the MAIL command of the original message.

  The Future-Release-Request field is an extension to the set of DSN
  per-message fields described in [n8].  Using ABNF [n2], the syntax
  of this new field is as follows:

     orig-hold-param-value = ("for;" future-release-interval) /
                             ("until;" future-release-date-time)
                        ; this is the value of the HOLD param from
                        ; the MAIL command of the original message

     future-release-request-field = "Future-Release-Request:"
                                    orig-hold-param-value

Interaction with the DELIVERBY SMTP Service Extension

If an MSA supports the Future Message release and Deliver By service extensions, it is possible for an SMTP client to make simultaneous requests for future message release and deliver-by times when submitting a message. A problem will occur if the future message release time is farther in the future than the deliver-by time. In order to honor the deliver-by request, the future message release request has to be ignored. In order to honor the future message release request, the deliver-by request has to be ignored. This section addresses that problem. The Deliver By extension is described in [n6].

SMTP Client Interaction with DELIVERBY

1) When an SMTP client wishes to use the Future Message Release and 

  Deliver By extensions with the same message, the client MUST
  ensure that the specified deliver-by time is farther in the future
  than the specified ("until" option) or implied ("for" option)
  future message release time.

MSA Interaction with DELIVERBY

1) If an MSA supports Future Message Release and Deliver By 

  extensions, and receives a message requesting the use of both
  extensions, the MSA MUST reject the MAIL command if it determines
  that the future message release time is farther in the future than
  the deliver-by time.

2) When an MSA is rejecting a MAIL command per item 1, above, it 

  SHOULD supply the reply code 501 (syntax error in parameters or
  arguments [n4]) in the reply.

3) When an MSA is rejecting a MAIL command per item 1, above, it 

  SHOULD supply the Enhanced Mail System Status Code 5.5.4 (invalid
  command arguments [i1]) in the reply.

Interaction with the MDN Function

The Message Disposition Notification (MDN) function is described in [n9].

SMTP Client Interaction with MDN

1) An SMTP client MUST NOT request Future Message Release when 

  sending an MDN to the MSA.

Security Considerations

The Future Message Release service extension presents a number of security considerations:

1) Unauthorized future-release messages provide a means to overwhelm 

  the storage of an MSA.  The authorization mechanisms required for
  the base mail submission protocol [n3] are expected to provide
  appropriate defense against such attacks.

2) Authorized future message release without a per-user quota may 

  also provide a way to overwhelm an MSA's storage.  An MSA's future
  release message storage SHOULD be subject to a per-user quota.

3) If an MSA is imposing a per-user quota on future-release message 

  storage, and detects that an incoming future-release message will
  exceed the user's future-release message storage quota, the MSA
  MUST reject the MAIL command.

4) When an MSA is rejecting a MAIL command per 5.3, it SHOULD supply 

  the reply code 552 (requested mail action aborted: exceeded
  storage allocation [n4]) in the reply.

5) When an MSA is rejecting a MAIL command per 5.3, it SHOULD supply 

  the new Enhanced Mail System Status Code defined for this purpose.
  This new status code updates [i1].

  X.7.16   Future release per-user message quota exceeded

     There is insufficient per-user quota to queue the message for
     future release.  This code suggests the client can submit again
     only after the per-user queue has drained.

  X.7.17   Future release system message quota exceeded

     There is insufficient system quota to queue the message for
     future release.  This code suggests the client can submit again
     after the system queue has drained.

6) Inaccurate time on the MSA may result in premature or delayed 

  release of messages.  Both HOLDUNTIL and HOLDFOR request
  mechanisms are sensitive to inaccurate or changing clocks on the
  MSA.

7) Some element of deception is inherent in the future message 

  release concept.  The message release time is intentionally
  delayed past the time it would otherwise be released; hence, the
  message delivery time is delayed past the time it would otherwise
  be delivered.  This extension provides no mechanism for hiding
  this from the message recipient.  The RFC 2822 [n5] message
  header, and specifically the Date field, remain unchanged after
  submission.  While a sending client MAY elect to place the
  future-message-release-time as the date in the Date field, there
  is no requirement or expectation that the Received fields and
  other trace information be modified by the transport system to
  further this deception.

IANA Considerations

This extension has been added to the list of SMTP Service Extensions on the Mail Parameters Web page.

Acknowledgments

Much of the credit for this document is due to the LEMONADE working group. Through many revisions, the discussion resulted in fundamental new understandings of this protocol and corresponding refinement of the implied requirements and protocol. Special thanks to those who patiently lead the WG to understand that doing both interval and date-time was the pragmatically correct approach to the needs of diverse clients.

Normative References

[n1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 

     Levels", BCP 14, RFC 2119, March 1997.

[n2] Crocker, D. and P. Overell, "Augmented BNF for Syntax 

     Specifications: ABNF", RFC 4234, October 2005.

[n3] Gellens, R. and J. Klensin, "Message Submission for Mail", RFC 

     4409, April 2006.

[n4] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 

     2001.

[n5] Resnick, P., "Internet Message Format", RFC 2822, April 2001.

[n6] Newman, D., "Deliver By SMTP Service Extension", RFC 2852, June 

     2000.

[n7] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 

     Extension for Delivery Status Notifications (DSNs)", RFC 3461,
     January 2003.

[n8] Moore, K. and G. Vaudreuil, "An Extensible Message Format for 

     Delivery Status Notifications", RFC 3464, January 2003.

[n9] Hansen, T. and G. Vaudreuil, "Message Disposition 

     Notification", RFC 3798, May 2004.

[n10] Klyne, G. and C. Newman, "Date and Time on the Internet: 

     Timestamps", RFC 3339, July 2002

10. Informative References

[i1] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463, 

     January 2003.

Authors' Addresses

Gregory A. White 6519 Camille Ave. Dallas, TX 75252 USA EMail: [email protected]

Gregory M. Vaudreuil Alcatel-Lucent 9489 Bartgis Ct Frederick, MD 21702 USA EMail: [email protected]

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