Movatterモバイル変換

Internet Engineering Task Force (IETF)                        J. KlensinRequest for Comments: 6152STD: 71                                                         N. FreedObsoletes:1652                                                   OracleCategory: Standards Track                                        M. RoseISSN: 2070-1721                             Dover Beach Consulting, Inc.                                                         D. Crocker, Ed.                                             Brandenburg InternetWorking                                                              March 2011SMTP Service Extension for 8-bit MIME TransportAbstract   This memo defines an extension to the SMTP service whereby an SMTP   content body consisting of text containing octets outside of the   US-ASCII octet range (hex 00-7F) may be relayed using SMTP.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/rfc6152.Copyright Notice   Copyright (c) 2011 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.Klensin, et al.              Standards Track                    [Page 1]

RFC 6152              SMTP Extension for 8-bit MIME           March 20111.  Introduction   Although SMTP is widely and robustly deployed, various extensions   have been requested by parts of the Internet community.  In   particular, a significant portion of the Internet community wishes to   exchange messages in which the content body consists of a MIME   message [RFC2045][RFC2046][RFC5322] containing arbitrary octet-   aligned material.  This memo uses the mechanism described in the SMTP   specification [RFC5321] to define an extension to the SMTP service   whereby such contents may be exchanged.  Note that this extension   does NOT eliminate the possibility of an SMTP server limiting line   length; servers are free to implement this extension but nevertheless   set a line length limit no lower than 1000 octets.  Given that this   restriction still applies, this extension does NOT provide a means   for transferring unencoded binary via SMTP.2.  Framework for the 8-bit MIME Transport Extension   The 8-bit MIME transport extension is laid out as follows:   1.  the name of the SMTP service extension defined here is       8bit-MIMEtransport;   2.  the EHLO keyword value associated with the extension is 8BITMIME;   3.  no parameter is used with the 8BITMIME EHLO keyword;   4.  one optional parameter using the keyword BODY is added to the       MAIL command.  The value associated with this parameter is a       keyword indicating whether a 7-bit message (in strict compliance       with [RFC5321]) or a MIME message (in strict compliance with       [RFC2046] and [RFC2045]) with arbitrary octet content is being       sent.  The syntax of the value is as follows, using the ABNF       notation of [RFC5234]:       body-value = "7BIT" / "8BITMIME"   5.  no additional SMTP verbs are defined by this extension; and   6.  the next section specifies how support for the extension affects       the behavior of a server and client SMTP.3.  The 8bit-MIMEtransport Service Extension   When a client SMTP wishes to submit (using the MAIL command) a   content body consisting of a MIME message containing arbitrary lines   of octet-aligned material, it first issues the EHLO command to the   server SMTP.  If the server SMTP responds with code 250 to the EHLOKlensin, et al.              Standards Track                    [Page 2]

RFC 6152              SMTP Extension for 8-bit MIME           March 2011   command, and the response includes the EHLO keyword value 8BITMIME,   then the server SMTP is indicating that it supports the extended MAIL   command and will accept MIME messages containing arbitrary octet-   aligned material.   The extended MAIL command is issued by a client SMTP when it wishes   to transmit a content body consisting of a MIME message containing   arbitrary lines of octet-aligned material.  The syntax for this   command is identical to the MAIL command inRFC 5321, except that a   BODY parameter must appear after the address.  Only one BODY   parameter may be used in a single MAIL command.   The complete syntax of this extended command is defined inRFC 5321.   The esmtp-keyword is BODY, and the syntax for esmtp-value is given by   the syntax for body-value shown above.   The value associated with the BODY parameter indicates whether the   content body that will be passed using the DATA command consists of a   MIME message containing some arbitrary octet-aligned material   ("8BITMIME") or is encoded entirely in accordance withRFC 5321   ("7BIT").   A server that supports the 8-bit MIME transport service extension   shall preserve all bits in each octet passed using the DATA command.   Naturally, the usual SMTP data-stuffing algorithm applies, so that a   content that contains the five-character sequence of   <CR> <LF> <DOT> <CR> <LF>   or a content that begins with the three-character sequence of   <DOT> <CR> <LF>   does not prematurely terminate the transfer of the content.  Further,   it should be noted that the CR-LF pair immediately preceding the   final dot is considered part of the content.  Finally, although the   content body contains arbitrary lines of octet-aligned material, the   length of each line (number of octets between two CR-LF pairs) is   still subject to SMTP server line length restrictions (which can   allow as few as 1000 octets, inclusive of the CR-LF pair, on a single   line).  This restriction means that this extension provides the   necessary facilities for transferring a MIME object with the 8BIT   content-transfer-encoding, it DOES NOT provide a means of   transferring an object with the BINARY content-transfer-encoding.   Once a server SMTP supporting the 8bit-MIMEtransport service   extension accepts a content body containing octets with the high-   order (8th) bit set, the server SMTP must deliver or relay the   content in such a way as to preserve all bits in each octet.Klensin, et al.              Standards Track                    [Page 3]

RFC 6152              SMTP Extension for 8-bit MIME           March 2011   If a server SMTP does not support the 8-bit MIME transport extension   (either by not responding with code 250 to the EHLO command, or by   not including the EHLO keyword value 8BITMIME in its response), then   the client SMTP must not, under any circumstances, attempt to   transfer a content that contains characters outside of the US-ASCII   octet range (hex 00-7F).   A client SMTP has two options in this case: first, it may implement a   gateway transformation to convert the message into valid 7-bit MIME,   or second, it may treat the barrier to 8-bit as a permanent error and   handle it in the usual manner for delivery failures.  The specifics   of the transformation from 8-bit MIME to 7-bit MIME are not described   by this RFC; the conversion is nevertheless constrained in the   following ways:   1.  it must cause no loss of information; MIME transport encodings       must be employed as needed to insure this is the case, and   2.  the resulting message must be valid 7-bit MIME.4.  Usage Example   The following dialogue illustrates the use of the 8bit-MIMEtransport   service extension:   S: <wait for connection on TCP port 25>   C: <open connection to server>   S: 220 dbc.mtview.ca.us SMTP service ready   C: EHLO ymir.claremont.edu   S: 250-dbc.mtview.ca.us says hello   S: 250 8BITMIME   C: MAIL FROM:<ned@ymir.claremont.edu> BODY=8BITMIME   S: 250 <ned@ymir.claremont.edu>... Sender and 8BITMIME ok   C: RCPT TO:<mrose@dbc.mtview.ca.us>   S: 250 <mrose@dbc.mtview.ca.us>... Recipient ok   C: DATA   S: 354 Send 8BITMIME message, ending in CRLF.CRLF.    ...   C: .   S: 250 OK   C: QUIT   S: 250 GoodbyeKlensin, et al.              Standards Track                    [Page 4]

RFC 6152              SMTP Extension for 8-bit MIME           March 20115.  Security Considerations   This RFC does not discuss security issues and is not believed to   raise any security issues not already endemic in electronic mail and   present in fully conforming implementations ofRFC 5321, including   attacks facilitated by the presence of an option negotiation   mechanism.  Since MIME semantics are transport-neutral, the 8BITMIME   option provides no more added capability to disseminate malware than   is provided by unextended 7-bit SMTP.6.  IANA Considerations6.1.  SMTP Service Extension Registration   This document defines an SMTP and Submit service extension.  IANA has   updated the 8BITMIME entry in the SMTP Service Extensions registry,   as follows:   Keyword:   8BITMIME   Description:   SMTP and Submit transport of 8-bit MIME content   Reference:   [RFC6152]   Parameters:   SeeSection 2 in this specification.7.  Acknowledgements   E. Stefferud was an original author.  This version of the   specification was produced by the YAM working group.   Original acknowledgements:   This document represents a synthesis of      the ideas of many people and reactions to the ideas and proposals      of others.  Randall Atkinson, Craig Everhart, Risto Kankkunen, and      Greg Vaudreuil contributed ideas and text sufficient to be      considered co-authors.  Other important suggestions, text, or      encouragement came from Harald Alvestrand, Jim Conklin,      Mark Crispin, Frank da Cruz, Olafur Gudmundsson, Per Hedeland,      Christian Huitma, Neil Katin, Eliot Lear, Harold A. Miller,      Keith Moore, Dan Oscarsson, Julian Onions, Neil Rickert,      John Wagner, Rayan Zachariassen, and the contributions of the      entire IETF SMTP Working Group.  Of course, none of the      individuals are necessarily responsible for the combination of      ideas represented here.  Indeed, in some cases, the response to a      particular criticism was to accept the problem identification but      to include an entirely different solution from the one originally      proposed.Klensin, et al.              Standards Track                    [Page 5]

RFC 6152              SMTP Extension for 8-bit MIME           March 20118.  Normative References   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail              Extensions (MIME) Part One: Format of Internet Message              Bodies",RFC 2045, November 1996.   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail              Extensions (MIME) Part Two: Media Types",RFC 2046,              November 1996.   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF", STD 68,RFC 5234, January 2008.   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol",RFC 5321,              October 2008.   [RFC5322]  Resnick, P., Ed., "Internet Message Format",RFC 5322,              October 2008.Klensin, et al.              Standards Track                    [Page 6]

RFC 6152              SMTP Extension for 8-bit MIME           March 2011Authors' Addresses   John C. Klensin   1770 Massachusetts Ave, Ste. 322   Cambridge, MA  02140   USA   Phone: +1 617 245 1457   EMail: john+ietf@jck.com   Ned Freed   Oracle   800 Royal Oaks   Monrovia, CA  91016-6347   USA   EMail: ned.freed@mrochek.com   M. Rose   Dover Beach Consulting, Inc.   POB 255268   Sacramento, CA  95865-5268   USA   Phone: +1 916 538 2535   EMail: mrose17@gmail.com   D. Crocker (editor)   Brandenburg InternetWorking   675 Spruce Dr.   Sunnyvale, CA   USA   Phone: +1 408 246 8253   EMail: dcrocker@bbiw.net   URI:http://bbiw.netKlensin, et al.              Standards Track                    [Page 7]