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Internet Engineering Task Force (IETF)                       A. MelnikovRequest for Comments: 6558                                 Isode LimitedCategory: Standards Track                                       B. LeibaISSN: 2070-1721                                                    K. Li                                                     Huawei Technologies                                                              March 2012Sieve Extension for Converting Messages before DeliveryAbstract   This document describes how the "CONVERT" IMAP extension can be used   within the Sieve mail filtering language to transform messages before   final delivery.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6558.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://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.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Melnikov, et al.             Standards Track                    [Page 1]

RFC 6558                      Sieve CONVERT                   March 2012Table of Contents1. Introduction ....................................................21.1. Conventions Used in This Document ..........................22. "convert" Action ................................................22.1. Interaction with Other Tests and Actions ...................32.2. "convert" as a Test ........................................43. Examples ........................................................53.1. Example 1 ..................................................53.2. Example 2 ..................................................53.3. Example 3 ..................................................53.4. Example 4 ..................................................64. Security Considerations .........................................75. IANA Considerations .............................................76. Acknowledgements ................................................77. Normative References ............................................71.  Introduction   The IMAP "CONVERT" extension [RFC5259] adds an IMAP command for   performing client-controlled conversions on whole messages or their   body parts.  This document defines a similar extension to the Sieve   mail filtering language [RFC5228], which reuses the conversion   parameters and framework established by IMAP CONVERT.1.1.  Conventions Used in This Document   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 inRFC 2119 [RFC2119].   Conventions for notations are as in Sieve[RFC5228], Section 1.1.2.  "convert" Action   Usage: convert  <quoted-from-media-type: string>                   <quoted-to-media-type: string>                   <transcoding-params: string-list>   The "convert" action specifies that all body parts with a media type   [RFC2046] (sometimes called "MIME type") equal to <quoted-from-media-   type> be converted to the media type in <quoted-to-media-type> using   conversion parameters specified in <transcoding-params>.  Each   conversion parameter value has the following syntax: "<transcoding-   param-name>=<transcoding-param-value>", where <transcoding-param-   name> and <transcoding-param-value> are defined in CONVERT [RFC5259].   Messages that don't have any body parts with the <quoted-from-media-   type> media type are not affected by the conversion.Melnikov, et al.             Standards Track                    [Page 2]

RFC 6558                      Sieve CONVERT                   March 2012   The "convert" action can be used with Sieve MIME Part Tests   [RFC5703], in the case that some, but not all of the body parts need   to be converted, or where different body parts might require   different conversions.  When the "convert" action appears in a   "foreverypart" loop, it applies only to the body part being   processed, and not to any other body parts (seeSection 3.2 for an   example).   When the "convert" action appears outside a "foreverypart" loop, the   conversion applies equally to all body parts -- that is, all body   parts that have the "quoted-from-media-type" are converted, using the   same transcoding parameters.   A single "convert" action will only apply once to any body part.  If,   for example, << convert "image/jpeg" "image/jpeg" ["pix-x=100","pix-   y=120"] >> converts a larger JPEG image to the smaller 100 x 120   size, that will be the end of that "convert" action on that body   part.  The action will not see a "new" JPEG body part to process,   resulting from the conversion.   If a "convert" action cannot be completed -- perhaps because the   conversion failed, or because the requested conversion is not   available -- that "convert" action MUST terminate and leave the   message unchanged, rolling back any other conversions done by that   action.  The script processing continues, and prior or subsequent   "convert" actions are not affected.  No error condition is raised,   and no partial conversions from a single "convert" action are   allowed.   Implementations might defer any actual conversion until the results   of the conversion are needed for script processing, to avoid doing   conversions unnecessarily.  Consider the case wherein a "convert"   action is processed but a "discard" action results without the need   to actually perform the conversion.   When conversions actually need to be done, they can put a significant   load on the server.  Computationally expensive conversions of a lot   of body parts can constitute an attack vector; even if done   legitimately, they can create an unacceptable load.  Servers MAY   refuse conversions, or do them at lower priority, effectively slowing   the requesting process in order to avoid negative effects on service   to other processes.2.1.  Interaction with Other Tests and Actions   Whether or not the actual conversion has been done yet, a successful   "convert" action effectively changes the message, and all subsequent   actions, including any other "convert" actions, apply to the changedMelnikov, et al.             Standards Track                    [Page 3]

RFC 6558                      Sieve CONVERT                   March 2012   message.  The "convert" action does not affect the applicability of   other actions; any action that was applicable before the "convert" is   equally applicable to the changed message afterward.   When a disposition-type action, such as "fileinto" or "redirect", is   encountered, the state of the message with respect to conversions is   "locked in" for that disposition-type action.  Whether the   implementation performs the action at that point or batches it for   later, it MUST perform the action on the message as it stood at the   time, and MUST NOT include subsequent conversions encountered later   in the script processing.  Therefore, the sequence "convert,   fileinto, convert, fileinto" will store two different versions of the   message: the first "fileinto" uses only the first conversion, while   the second uses both.  SeeSection 3.4 for an example of how this can   be used.   In addition, any tests done on the message and its parts will test   the message after prior conversions have been done.  The fourth block   ofSection 3.4 shows an example of this situation.   Convert actions are cumulative, and each conversion operates on the   message as it stands after all prior conversions.  See the fourth   block ofSection 3.4 for an example of how this might be tricky.   Because the implicit keep (seeSection 2.10.2 of [RFC5228]), if it is   in effect, acts on the final state of the message, all conversions   are performed before any implicit keep.2.2.  "convert" as a Test   To simplify testing for supported and successful conversions, the   "convert" action can also be used as a test.  As such, it will   attempt to perform the requested conversion(s) and will evaluate to   "false" if and only if at least one conversion failed.  The failure   can be because a conversion was unsupported or because the data could   not be converted (perhaps it had been corrupted in transit or   mislabeled at its origin).   This creates a new type of Sieve action, a "testable action".  The   usage as a test is exactly the same as for an action, and it doubles   as an action and a test of the action's result at the same time.  SeeSection 3.2 for an example of how this test can be used.   Note that defining this new testable action does not change the   definitions of any other actions -- it does not imply that other   actions can be used as tests.  Future extensions might define other   testable actions, but those specifications would be responsible for   clearly specifying that.Melnikov, et al.             Standards Track                    [Page 4]

RFC 6558                      Sieve CONVERT                   March 20123.  Examples3.1.  Example 1   In the following example, all "image/tiff" body parts of the message   are converted to "image/jpeg" with image resolution of 320x240   pixels.  The converted message is then subject to the implicit keep.       require ["convert"];       convert "image/tiff" "image/jpeg" ["pix-x=320","pix-y=240"];3.2.  Example 2   In the following example, all "image/tiff" body parts of the message   are converted to "image/jpeg", as in Example 1.  If the conversions   were successful, those messages are then filed into a mailbox called   "INBOX.pics".  Other messages (those with no image/tiff body parts)   are subject to the implicit keep, and have not been converted.       require ["mime", "fileinto", "convert"];       if header :mime :anychild :contenttype                 "Content-Type" "image/tiff"       {        if (convert "image/tiff" "image/jpeg" ["pix-x=320","pix-y=240"])        {         fileinto "INBOX.pics";        }       }3.3.  Example 3   In the following example, only "image/tiff" body parts with a   Content-Disposition of "inline" are converted.  Matching parts that   are larger than 500 kilobytes are converted using an image resolution   of 640x480 pixels, and those smaller are converted to 320x240 pixels.   The message disposition is not changed, so the implicit keep will be   in effect unless something else in the script changes that.       require ["mime", "foreverypart", "fileinto", "convert"];       foreverypart       {         if header :mime :param "filename" :contains                   "Content-Disposition" "inline"         {           if size :over "500K"           {            convert "image/tiff" "image/jpeg" ["pix-x=640","pix-y=480"];           } else {Melnikov, et al.             Standards Track                    [Page 5]

RFC 6558                      Sieve CONVERT                   March 2012            convert "image/tiff" "image/jpeg" ["pix-x=320","pix-y=240"];           }         }       }       [... script continues ...]3.4.  Example 4   The following example shows some tricky interactions between multiple   "convert" actions and other disposition-type actions.       require ["mime", "foreverypart",                "fileinto", "redirect", "convert"];       # The first "if" block will convert all image/tiff body parts       # to 640x480 jpegs and will file the message       # into the "INBOX.pics" mailbox as converted at this point.       if header :mime :anychild :contenttype                 "Content-Type" "image/tiff"       {         convert "image/tiff" "image/jpeg" ["pix-x=640","pix-y=480"];         fileinto "INBOX.pics";       }       # The second block, the "foreverypart" loop, will convert all       # inline jpegs to 320x240 resolution... including any tiff body       # parts that had been converted in the first block, above.       # Therefore, any tiff that had been converted to a 640x480 jpeg       # will be re-converted to a 320x240 jpeg here if its       # Content-Disposition is specified as "inline".       foreverypart       {         if header :mime :param "filename" :contains                   "Content-Disposition" "inline"         {           convert "image/jpeg" "image/jpeg" ["pix-x=320","pix-y=240"];         }       }       # The third block will take any message that contains a header       # field called "Mobile-Link" and redirect it to the user's       # mobile address.  The redirected message will include both       # conversions above, from block one and block two.       if exists "Mobile-Link"       {         redirect "joe@mobile.example.com";       }Melnikov, et al.             Standards Track                    [Page 6]

RFC 6558                      Sieve CONVERT                   March 2012       # The fourth block will file the message into "Tiff" if it       # contains any tiff body parts.  But because of the earlier       # conversion (in the first block), there will never be any       # tiff body parts, so this "fileinto" will never happen.       if header :mime :anychild :contenttype                 "Content-Type" "image/tiff"       {         fileinto "Tiff";       }       # Now, at the end of the script processing, the Sieve       # processor will perform an implicit keep if none of       # the "fileinto" and "redirect" actions were taken.       # The kept message will include any conversions that       # were done (that is, any from the second block).4.  Security Considerations   Security considerations given in IMAP CONVERT [RFC5259] and Sieve   [RFC5228] are relevant to this document.  There are no additional   security considerations resulting from combining the two.5.  IANA Considerations   IANA has added the following registration to the "Sieve Extensions"   registry, as defined inRFC 5228:   Capability name:  convert   Description:  adds a new Sieve test and action that enable Sieve      scripts to perform data conversions on the message being      delivered.   RFC number:RFC 6558   Contact address:  The Sieve discussion list <sieve@ietf.org>6.  Acknowledgements   The authors also want to thank all who have contributed key insight   and extensively reviewed and discussed the concepts of CONVERT.   Qian Sun contributed text to this document.7.  Normative References   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail              Extensions (MIME) Part Two: Media Types",RFC 2046,              November 1996.Melnikov, et al.             Standards Track                    [Page 7]

RFC 6558                      Sieve CONVERT                   March 2012   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering              Language",RFC 5228, January 2008.   [RFC5259]  Melnikov, A. and P. Coates, "Internet Message Access              Protocol - CONVERT Extension",RFC 5259, July 2008.   [RFC5703]  Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part              Tests, Iteration, Extraction, Replacement, and Enclosure",RFC 5703, October 2009.Authors' Addresses   Alexey Melnikov   Isode Limited   5 Castle Business Village   36 Station Road   Hampton, Middlesex  TW12 2BX   UK   EMail: Alexey.Melnikov@isode.com   URI:http://www.melnikov.ca/   Barry Leiba   Huawei Technologies   Phone: +1 646 827 0648   EMail: barryleiba@computer.org   URI:http://internetmessagingtechnology.org/   Kepeng Li   Huawei Technologies   Huawei Base, Bantian, Longgang District   Shenzhen, Guangdong  518129   P. R. China   Phone: +86-755-28974289   EMail: likepeng@huawei.comMelnikov, et al.             Standards Track                    [Page 8]