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).
AbstractThis 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. n3] to allow a client, when submitting a message, to indicate a future time for the message to be released for delivery.
n1]. 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].
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.
n7], and DSN message format is described in [n8]. 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 n6].
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. n9]. 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.
[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
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