Indicating Supported Media Features Using Extensions to DSN and MDN
A way for mail and fax receivers to report which media features they support inside delivery and read receipts.
Wing, D., "Indicating Supported Media Features Using Extensions to DSN and MDN", RFC 2530, DOI 10.17487/RFC2530, March 1999, <https://www.rfc-editor.org/info/rfc2530>.
@misc{rfc2530,
series = {Request for Comments},
number = 2530,
howpublished = {RFC 2530},
publisher = {RFC Editor},
doi = {10.17487/RFC2530},
url = {https://www.rfc-editor.org/info/rfc2530},
author = {D. Wing},
title = {{Indicating Supported Media Features Using Extensions to DSN and MDN}},
pagetotal = 5,
year = 1999,
month = mar,
} RFC text
Contents
Network Working Group D. Wing
Request for Comments: 2530 Cisco Systems
Category: Standards Track March 1999
Indicating Supported Media Features Using
Extensions to DSN and MDN
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 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].
3. 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 returned
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.
8. 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.com
9. 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.
AI ELI5
On this page
§1Reporting Media Features
In Internet mail and Internet fax, a recipient needs to indicate the media features it supports so that senders do not exceed its abilities. This memo defines a format for Message Disposition Notifications (MDNs, RFC 2298) and Delivery Status Notifications (DSNs, RFC 1894) that carry this information.
Both have drawbacks. DSNs are not available between all senders and receivers. MDNs require the receiver to disclose disposition information, or, with the denied disposition-type, the time the notification was generated.
A receipt for a letter can also say what kinds of paper and pictures the receiver can handle.
§2The Media-Accept-Features Field
For a DSN, the field is a per-recipient field. For an MDN, it is an extension field. In ABNF it is media-features = "Media-Accept-Features" ":" media-feature-tags.
The tags are defined in separate schema documents that MUST use the language of the media feature set syntax document and MUST be registered under RFC 2506. A valid example, assuming a schema defines the tags, is Media-Accept-Features: (& (paper-size=a4) (color=binary) ).
The receipt gets one extra line listing what the receiver can show, like "A4 paper, color".
§3MTA Advice and Security
If the recipient's MTA decides a message cannot be processed, it is strongly encouraged to reject it with status code 5.6.1. This may be returned at the end-of-mail-data indicator if the MTA supports enhanced error codes, or later in a delivery failure DSN.
Inaccurate media feature information could cause a denial of service, since later messages would be sent in forms the recipient cannot process. It may come from a malicious attack, such as a spoofed DSN or MDN, or from misconfiguration.
If a mail server cannot handle a message it should say so clearly. A fake or wrong receipt could trick senders into sending things the receiver cannot read.