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Network Working Group                               J. Klensin, WG ChairRequest for Comments: 1426                     United Nations University                                                        N. Freed, Editor                                            Innosoft International, Inc.                                                                 M. Rose                                            Dover Beach Consulting, Inc.                                                            E. Stefferud                                     Network Management Associates, Inc.                                                              D. Crocker                                                       The Branch Office                                                           February 1993SMTP Service Extensionfor 8bit-MIMEtransportStatus of this Memo   This RFC specifies an IAB standards track protocol for the Internet   community, and requests discussion and suggestions for improvements.   Please refer to the current edition of the "IAB Official Protocol   Standards" for the standardization state and status of this protocol.   Distribution of this memo is unlimited.1.  Abstract   This memo defines an extension to the SMTP service whereby an SMTP   content body containing octets outside of the US ASCII octet range   (hex 00-7F) may be relayed using SMTP.2.  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 [3] containing arbitrary octet-aligned material. This memo   uses the mechanism described in [5] 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.3.  Framework for the 8bit MIME Transport Extension   The 8bit MIME transport extension is laid out as follows:Klensin, Freed, Rose, Stefferud & Crocker                       [Page 1]

RFC 1426                SMTP 8bit-MIMEtransport            February 1993          (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 FROM command.  The value associated with this               parameter is a keyword indicating whether a 7bit message               (in strict compliance with [1]) or a MIME message (in               strict compliance with [3]) with arbitrary octet content               is being sent. The syntax of the value is as follows,               using the ABNF notation of [2]:                    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.4.  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   octet-aligned material, it first issues the EHLO command to the   server SMTP. If the server SMTP responds with code 250 to the EHLO   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 octet-aligned material. The syntax for this command is   identical to the MAIL command in [1], except that a BODY parameter   must appear after the address.   The complete syntax of this extended command is defined in [5]. 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 which will be passed using the DATA command consists of   a MIME message containing some arbitrary octet-aligned materialKlensin, Freed, Rose, Stefferud & Crocker                       [Page 2]

RFC 1426                SMTP 8bit-MIMEtransport            February 1993   ("8BITMIME") or is encoded entirely in accordance with [1] ("7BIT").   A server which 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 which 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 preceeding the   final dot is considered part of the content.  Finally, although the   content body contains arbitrary 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 may allow as   few as 1000 octets on a single line).   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.   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 which contains characters outside 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 7bit MIME,   or second, or may treat this as a permanent error and handle it in   the usual manner for delivery failures.  The specifics of the   transformation from 8bit MIME to 7bit 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 7bit MIME.Klensin, Freed, Rose, Stefferud & Crocker                       [Page 3]

RFC 1426                SMTP 8bit-MIMEtransport            February 19935.  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 Goodbye6.  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 of [1].7.  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, Freed, Rose, Stefferud & Crocker                       [Page 4]

RFC 1426                SMTP 8bit-MIMEtransport            February 19938.  References   [1] Postel, J., "Simple Mail Transfer Protocol", STD 10,RFC 821,       USC/Information Sciences Institute, August 1982.   [2] Crocker, D., "Standard for the Format of ARPA Internet Text       Messages", STD 11,RFC 822, UDEL, August 1982.   [3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail       Extensions",RFC 1341, Bellcore, Innosoft, June 1992.   [4] Moore, K., "Representation of Non-ASCII Text in Internet Message       Headers",RFC 1342, University of Tennessee, June 1992.   [5] Klensin, J., WG Chair, Freed, N., Editor, Rose, M., Stefferud,       E., and D. Crocker, "SMTP Service Extensions"RFC 1425, United       Nations University, Innosoft International, Inc., Dover Beach       Consulting, Inc., Network Management Associates, Inc., The Branch       Office, February 1993.   [6] Partridge, C., "Mail Routing and the Domain System",RFC 974,       BBN, January 1986.9.  Chair, Editor, and Authors' Addresses       John Klensin, WG Chair       United Nations University       PO Box 500, Charles Street Station       Boston, MA 02114-0500  USA       Phone: +1 617 227 8747       Fax: +1 617 491 6266       Email: klensin@infoods.unu.edu       Ned Freed, Editor       Innosoft International, Inc.       250 West First Street, Suite 240       Claremont, CA 91711  USA       Phone: +1 909 624 7907       Fax: +1 909 621 5319       Email: ned@innosoft.comKlensin, Freed, Rose, Stefferud & Crocker                       [Page 5]

RFC 1426                SMTP 8bit-MIMEtransport            February 1993       Marshall T. Rose       Dover Beach Consulting, Inc.       420 Whisman Court       Moutain View, CA  94043-2186  USA       Phone: +1 415 968 1052       Fax: +1 415 968 2510       Email: mrose@dbc.mtview.ca.us       Einar A. Stefferud       Network Management Associates, Inc.       17301 Drey Lane       Huntington Beach, CA, 92647-5615  USA       Phone: +1 714 842 3711       Fax: +1 714 848 2091       Email: stef@nma.com       David H. Crocker       The Branch Office       USA       Email: dcrocker@mordor.stanford.eduKlensin, Freed, Rose, Stefferud & Crocker                       [Page 6]