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RFC 4865 - SMTP Submission Service Extension for Future Message


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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.

1.  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.

2.  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].

3.  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].

4.  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.

4.1.  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.

4.2.  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.

5.  Protocol Interactions

5.1.  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].

5.1.1.  SMTP Client Interaction with DSN

   1) An SMTP client MUST NOT request Future Message Release when
      sending a DSN to the MSA.

5.1.2.  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

5.2.  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].

5.2.1.  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.

5.2.2.  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.

5.3.  Interaction with the MDN Function

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

5.3.1.  SMTP Client Interaction with MDN

   1) An SMTP client MUST NOT request Future Message Release when
      sending an MDN to the MSA.

6.  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.

7.  IANA Considerations

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

8.  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.

9.  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: g.a.white@tx.rr.com

   Gregory M. Vaudreuil
   Alcatel-Lucent
   9489 Bartgis Ct
   Frederick, MD 21702
   USA
   EMail: GregV@ieee.org

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