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Network Working Group                                            D. WingRequest for Comments: 2530                                 Cisco SystemsCategory: Standards Track                                     March 1999Indicating Supported Media Features UsingExtensions to DSN and MDNStatus 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 Internet Society (1999).  All Rights Reserved.1.  Abstract   There is a need in Internet mail and Internet fax for a recipient to   indicate the media features it supports so that messages can be   generated by senders without exceeding the recipient's abilities.   This memo describes a format for generating Message Disposition   Notifications [RFC2298] and Delivery Status Notifications [RFC1894]   which contain such information.  This information can be used by   senders to avoid exceeding the recipient's capabilities when sending   subsequent messages.2. Introduction   The extensions described in this document can be used in Message   Disposition Notifications [RFC2298] or Delivery Status Notifications   [RFC1894], as appropriate for the implementation.   Note that both DSNs and MDNs have drawbacks: DSNs are not available   between all senders and receivers, and MDNs require the receiver to   disclose message disposition information (or, if using the "denied"   disposition-type, the time the disposition notification was   generated).   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 [RFC2119].Wing                        Standards Track                     [Page 1]

RFC 2530            Media Features using DSN and MDN          March 19993.  Extensions for use by DSN and MDN   The following extension is available to both DSN [RFC1894] and MDN   [RFC2298] messages.   For a DSN message, the following per-recipient fields are defined   (section 2.3 of [RFC1894]).  For an MDN message, the following   extension fields are defined (section 3.1 of [RFC2298]).  Using the   language of [RFC2234]:      extension-field    = media-features CRLF      media-features     = "Media-Accept-Features" ":"                            media-feature-tags      media-feature-tags = <*text as defined below,                            with LWSP wrapping>   The <media-feature-tags> are defined in separate schema documents   which MUST utilize the language described in [SYNTAX].  The schema   MUST be registered following the registration requirements of   [RFC2506].3.1.  Examples   The following examples assume there is a schema document which   defines the tags shown.3.1.1.  Paper-size and Color   Assuming there is a schema document which describes the tags paper-   size and color, the following example is valid:      Media-Accept-Features: (& (paper-size=a4) (color=binary) )3.1.2.  UA-Media, Paper-size, and Color   Assuming there is a schema document which describes the tags paper-   size, color, and grey:      Media-Accept-Features: (& (| (paper-size=a4) (paper-size=letter) )        (| (& (color=grey) (dpi=200) (dpi-xyratio=200/100) )        (& (color=limited) (dpi=200) (dpi-xy=200/100) ) )4.  MTA Implmentation Recommendation   If the recipient's MTA determines that a message cannot be processed,   the recipient's MTA is strongly encouraged to reject the message with   a status code of 5.6.1 [RFC1893].  This status code may be returnedWing                        Standards Track                     [Page 2]

RFC 2530            Media Features using DSN and MDN          March 1999   in response to the end-of-mail-data indicator if the MTA supports   reporting of enhanced error codes [RFC2034], or after message   reception by generating a delivery failure DSN ("bounce").5.  Security Considerations   Inaccurate media feature information could cause a denial of service,   causing subsequent messages to be sent which the recipient is unable   to process.   The media feature information could be inaccurate due to a malicious   attack (spoofed DSN or MDN) or misconfiguration.6.  Acknowledgments   The author thanks the members of the Internet Fax working group for   assistance with this document, and especially Larry Masinter, Graham   Klyne, and Ned Freed.7.  References   [RFC2506] Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag             Registration Procedure",BCP 31,RFC 2506, March 1999.   [RFC1894] Moore, K. and G. Vaudreuil, "An Extensible Message Format             for Delivery Status Notifications",RFC 1894, January 1996.   [RFC2034] Freed, N., "SMTP Service Extension for Returning Enhanced             Error Codes",RFC 2034, October 1996.   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax             Specifications:  ABNF",RFC 2234, November 1997.   [RFC2298] Fajman, R., "An Extensible Message Format for Message             Disposition Notifications",RFC 2298, March 1998.   [SYNTAX]  Klyne, G., "A Syntax for Describing Media Feature Sets",RFC 2533, March 1999.Wing                        Standards Track                     [Page 3]

RFC 2530            Media Features using DSN and MDN          March 19998.  Author's Address   Dan Wing   Cisco Systems, Inc.   101 Cooper Street   Santa Cruz, CA 95060  USA   Phone: +1 831 457 5200   Fax:   +1 831 457 5208   EMail: dwing@cisco.comWing                        Standards Track                     [Page 4]

RFC 2530            Media Features using DSN and MDN          March 19999.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Wing                        Standards Track                     [Page 5]