rfc4096
Network Working Group C. Malamud
Request for Comments: 4096 Memory Palace Press
Category: Informational May 2005
Policy-Mandated Labels Such as "Adv:" in Email Subject Headers
Considered Ineffective At Best
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo discusses policies that require certain labels to be
inserted in the "Subject:" header of a mail message. Such policies
are difficult to specify accurately while remaining compliant with
key RFCs and are likely to be ineffective at best. This memo
discusses an alternate, standards-compliant approach that is
significantly simpler to specify and is somewhat less likely to be
ineffective.
Table of Contents
1. Labeling Requirements ...........................................2
1.1. Terminology ................................................3
2. Subject Line Encoding ...........................................3
3. Implementing a Labeling Requirement .............................5
4. Subjects are For Humans, Not Labels .............................6
5. Solicitation Class Keywords .....................................8
6. Security Considerations ........................................10
7. Recommendations ................................................10
8. Acknowledgements ...............................................10
9. References .....................................................11
9.1. Normative References ......................................11
9.2. Informative References ....................................11
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1. Labeling Requirements
The U.S. Congress and President have enacted the Controlling the
Assault of Non-Solicited Pornography and Marketing Act of 2003
(CAN-SPAM Act of 2003) [US], which requires in Section 11(2) that the
Federal Trade Commission:
"[transmit to the Congress] a report, within 18 months after the
date of enactment of this Act, that sets forth a plan for
requiring commercial electronic mail to be identifiable from its
subject line, by means of compliance with Internet Engineering
Task Force Standards, the use of the characters "ADV" in the
subject line, or other comparable identifier, or an explanation of
any concerns the Commission has that cause the Commission to
recommend against this plan."
The Korean Government has enacted the Act on Promotion of Information
and Communication and Communications Network Utilization and
Information Protection of 2001 [Korea]. As explained by the Korea
Information Security Agency, the government body with enforcement
authority under the act, Korean law makes it mandatory as of June,
2003 to:
"include the '@' (at) symbol in the title portion (right-side) of
any commercial e-mail address, in addition to the words
'(Advertisement)' or '(Adult Advertisement)' as applicable. The
inclusion of the '@' symbol, as proposed by the Korean government,
is intended to indicate an e-mail advertisement. Because e-mails
easily cross international borders, the '@' symbol may be used as
a symbol for filtering advertisement mails." [KISA]
The State of Colorado has enacted the Colorado Junk Email Law, which
states:
"It shall be a violation of this article for any person that sends
an unsolicited commercial electronic mail message to fail to use
the exact characters "ADV:" (the capital letters "A", "D", and
"V", in that order, followed immediately by a colon) as the first
four characters in the subject line of an unsolicited commercial
electronic mail message." [Colorado]
The Rules of Professional Conduct of the Florida Bar require, in Rule
4-7.6(c)(3) states:
"A lawyer shall not send, or knowingly permit to be sent, on the
lawyer's behalf or on behalf of the lawyer's firm or partner, an
associate, or any other lawyer affiliated with the lawyer or the
lawyer's firm, an unsolicited electronic mail communication
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directly or indirectly to a prospective client for the purpose of
obtaining professional employment unless ... the subject line of
the communication states 'legal advertisement.'" [Florida]
A subject line that complies with the above requirements might read
as follows:
Subject: ADV: @ (Advertisement) legal advertisement
A more comprehensive survey of applicable laws would, no doubt,
lengthen the above example considerably.
1.1. Terminology
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 BCP 14, [RFC2119].
2. Subject Line Encoding
The basic definition of the "Subject:" of an electronic mail message
is contained in [RFC2822]. The normative requirements that apply to
all headers are:
o The maximum length of the header field is 998 characters.
o Each line must be no longer than 78 characters.
A multi-line subject field is indicated by the presence of a carriage
return and white space, as follows:
Subject: This
is a test
On the subject of the three unstructured fields ( "Subject:",
"Comments:", and "Keywords:"), the standard indicates that these are
"intended to have only human-readable content with information about
the message." In addition, on the specific subject of the "Subject:"
field, the standard states:
The "Subject:" field is the most common and contains a short
string identifying the topic of the message. When used in a
reply, the field body MAY start with the string "Re: " (from the
Latin "res", in the matter of) followed by the contents of the
"Subject:" field body of the original message. If this is done,
only one instance of the literal string "Re: " ought to be used
since use of other strings or more than one instance can lead to
undesirable consequences.
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Further guidance on the structure of the "Subject:" field is
contained in [RFC2047], which species the mechanisms for character
set encoding in mail headers. [RFC2978] specifies a mechanism for
registering different character sets with the [IANA].
In addition to choosing a character set, [RFC2047] uses two
algorithms, known as "Base64 Encoding" and "Quoted Printable", which
are two different methods for encoding characters that fall outside
the basic 7-bit ASCII requirements that are specified in the core
electronic mail standards.
Thus, an encoded piece of text consists of the following components:
o The string "=?", which signifies the beginning of encoded text.
o A valid character set indicator.
o The string "?", which is a delimiter.
o The string "b" if "Base64 Encoding" is used or the string "q" if
"Quoted Printable" encoding is used.
o The string "?", which is a delimiter.
o The text, which has been properly encoded.
o The string "?=", which signifies the ending of the encoded text.
A simple example would be to use the popular [8859-1] character set,
which has accents and other characters not found in the ASCII
character set:
o "Subject: This is an ADV:" is an unencoded header.
o "Subject: =?iso-8859-1?b?VGhpcyBpcyBhbiBBRFY6?=" is encoded using
Base64.
o "Subject: =?iso-8859-1?q?This=20is=20an=20ADV:?=" is encoded using
Quoted Printable.
o "Subject: =?iso-8859-1?q?This=20is=20an=20=41=44=56=3A?=" is also
encoded using Quoted Printable, but instead the last four
characters are encoded with their hexadecimal representations.
Note that both character set and encoding indicators are case
insensitive. Additional complexity can be introduced by appending a
language specification to the character set indication, as specified
in [RFC2231] and [RFC3066]. This language specification consists of
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the string "*", followed by a valid language indicator. For example,
"US-ASCII*EN" indicates the "US-ASCII" character set and the English
language.
When a message is read, the "Subject:" field is decoded, with
appropriate characters from the character set displayed to the user.
Section 7 (Conformance) of [RFC2047] specifies that a conforming mail
reading program must perform the following tasks:
"The program must be able to display the unencoded text if the
character set is "US-ASCII". For the ISO-8859-* character sets,
the mail reading program must at least be able to display the
characters which are also in the ASCII set."
However, there is no requirement for every system to have every
character set. Mail readers that are unable to display a particular
set of characters resort to a variety of strategies, including
silently ignoring the unknown text, or generating an error or warning
message.
Two characteristics of many common Message User Agents (MUAs) (e.g.,
mail readers) are worth noting:
o Although the subject line is, in theory, of unlimited length, many
mail readers only show the reader the first few dozen characters.
o Electronic mail is often transmitted through gateways, reaching
pagers or cell phones with SMS capability. Those systems
typically require short subject lines.
3. Implementing a Labeling Requirement
In this section, we posit a hypothetical situation with two key
players:
o John Doe [Doe] is an attorney at the firm of Dewey, Cheatem &
Howe, LLC [Stooges].
o The Federal Trust Commission (FTC) has been entrusted with
implementing a recent labeling requirement, promulgated by the
Sovereign Government of Freedonia [Duck]. Specifically, President
Firefly directed the FTC to "make sure that anybody spamming folks
get the symbol 'spam:' in the subject line and or shoot them, if
you can find them."
Based on this directive, the FTC promulgated a very simple regulation
which read: "Please obey the law." John Doe, being a lawyer, read
the law, and promptly proceeded to spam everybody using a fairly
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obvious loophole: he made sure his subject line was really long, and
he shoved all the stuff like "spam:" and the "@" symbol and other
verbiage near the end of the 998 allowed characters. He was
complying with the law, but of course, nobody saw the labels in their
reader.
Based on a periodic review, the FTC decided to be more specific, and
re-promulgated their regulation as follows: "If you send spam, put
'spam:' at the _beginning_ of the subject line." The Freedonian FTC
promptly received a visit from the Sylvanian Ambassador, who
complained that this conflicted with his country's requirements under
the Marx Doctrine to place the string "WATCH OUT! THE CONTENTS OF
THIS MESSAGE ARE SUSPECT!" at the beginning of the subject line.
The re-promulgation of the regulation was rescinded, more experts
were called in, and a new regulation was issued: "Put it as close to
the beginning of the subject line as you can, modulo any requirements
by other governments." John Doe looked at this, scratched his head,
and applied a clever little hack, picking the ISO [8859-8] character
set for Hebrew, and duly spelling out the letters ":" Mem Alef Pe
Samech.
Subject: =?iso-8859-8?q?=f1=f4=e0=ee=3a?=
Some receivers of this message get an error message because they
don't have Hebrew installed on their systems. Others get some
cryptic indicator of a missing character set, such as
"[?iso-8859-8?]".
The FTC called a summit of leading thinkers, and the regulation was
amended to read "but don't use languages that go from right to left
or up and down instead of plain old left to right." Needless to say,
the reaction from the Freedonian League for the Defense of Linguistic
Diversity killed that proposed regulation really quickly.
The commission continued the cycle of re-promulgation and refinement,
but ultimately, the regulations continued to contain either a
loophole, objectionable requirements, or violations of the relevant
RFCs.
4. Subjects are For Humans, Not Labels
The use of an unknown character set, or of a very, very long subject
line are just two examples of how people can try to get around
labeling requirements. In order to specify a regulation without
ambiguity, it would need to be extremely complex in order to avoid
loopholes such as these.
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Drafting of regulations is one issue, but there is another. Subject
lines are used to specify, as [RFC2822] says, a "short string
identifying the topic of the message."
Any regulation has to compete with the other words in the subject,
and this mixing of purposes makes it very difficult for a machine to
filter out messages at the direction of the user. For example, if
one looks for the "@" symbol, per the Korean law, checks have to be
made that this symbol is not a legitimate part of a legitimate
message.
Not only do multiple labeling requirements compete with legitimate
subject lines, but also there is no easy way for the sender of a
legitimate message to effectively insert other labels that indicate
to the recipient that-- although the message may have a required
label-- it is actually a message the user might want to see, based
on, for example, a prior relationship.
Even if one considers only the sender of the message, it is very
difficult to specify a loophole-free way of putting a specific label
in a specific place. And, even if we could control what the sender
does, it is an unfortunate fact of life that other agents may also
alter the subject line. For example, mailing list management
software and even personal email filtering systems will often "munge"
the subject line to add information such as the name of a mailing
list, or the fact that a message comes from a certain group of
people. Such transformations have long been generally accepted as
being potentially harmful [RFC0886], and are the subject of continued
discussions, which are outside the scope of the present document (see
[Koch] and [RFC3834]).
The "Subject:" field is currently overloaded; it has become a handy
place for a variety of agents to attempt to insert information.
Because of that overloading, it is a poor location for specifying
mandatory use of a label, because it is unlikely that label will
"rise to the top" and become apparent to the reader of a message or
even to the mail-filtering software that examines the mail before the
user. The difficulty of implementing subject line labeling, without
taking additional steps, has been noted by several other
commentators, including [Moore-1], [Lessig], and [Levine]. Indeed,
the problem is a general one. Keith Moore has pointed out seven good
reasons why tags of any sort in the "Subject:" field have potential
problems:
1. The "Subject:" field space is not strictly limited and long
fields can be folded.
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2. PDAs, phones, and other devices and software have only a limited
space to display the "Subject:" field.
3. A variety of different kinds of labels such as "ADV:" and
"[Mailing List Name]" compete for scarce display space.
4. There are conflicting legal requirements from different
jurisdictions.
5. There is a conflict between human use of the "Subject:" field and
use of that field for filtering and filing:
* Machine-readable tokens interfere with human readability.
* Representation of human-readable text was not designed with
machine interpretation in mind and, thus, does not have a
canonical form.
6. Lack of support in a few existing mail readers for displaying
information from elsewhere in the message header (e.g., from
newly-defined fields), along with familiarity, motivates
additional uses of the "Subject:", further compounding the
problem.
7. Any text-based tags added or imposed by outside parties (i.e.,
those that are not the sender or recipient of the message) will
not be reliably meaningful in the recipient's language.
Source: [Moore-2].
5. Solicitation Class Keywords
[RFC3865] defines the "solicitation class keyword", an arbitrary
label that can be associated with an electronic mail message and
transported by the ESMTP mail service, as defined in [RFC2821] and
related documents. Solicitation class keywords are formatted like
domain names, but reversed. For example, the registrant of
"example.com" might specify a particular solicitation class keyword
such as "com.example.adv" that could be inserted in a "No-Solicit:"
header or in a trace field. Anybody with a domain name can specify a
solicitation class keyword, and anybody sending a message can use any
solicitation class keyword that has been defined by themselves or by
others.
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This memo argues that the "No-Solicit:" approach is either a superior
alternative or a necessary complement to "Subject:" field labeling
requirements because:
o One can specify very precisely what a label should be and where it
should go using the "No-Solicit:" header, which is designed
specifically for this purpose.
o The sender of a message can add additional solicitation class
keywords to help distinguish the message. For example, if the
"Freedonian Direct Marketing Council" wished to form a voluntary
consortium of direct marketers who subscribe to certain practices,
they could specify a keyword (e.g.,
"org.example.freedonia.good.spam") and educate the public to set
their filters to receive these types of messages.
o Message Transfer Agents (MTAs) may insert solicitation class
keywords in the "received:" trace fields, thus providing
additional tools for recipients to use for filtering messages.
o A recipient can also define a solicitation class keyword, a tool
that allows them to give friends and correspondents a "pass key"
so the recipient's mail filtering software always passes through
messages containing that keyword.
As can be seen, the solicitation class keyword approach is flexible
enough to serve as a tool for government-mandated labeling and for
other purposes as well.
Most modern email software gives users a variety of filtering tools.
For example, the popular Eudora program allows a user to specify the
name of a message header, the desired match (e.g., a wild card or
regular expression, or simply a phrase to match), and an action to
take (e.g., moving the message to a particular folder, sounding an
alarm, or even automatically deleting messages with harmful content
such as viruses). There is one popular email reader that only allows
filtering on selected fields, such as "To:", "From:", or "Subject:",
but that program is the exception to the rule.
In summary, for senders and receivers of email, use of the
"No-Solicit:" mechanism would be simple to understand and use. For
policy makers, it would be extremely simple to specify the format and
placement of the solicitation class keyword. Needless to say, the
issue of how to define what classes of messages are subject to such a
requirement, and how to enforce it, are beyond the scope of this
discussion.
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6. Security Considerations
The use of labels in the "Subject:" field gives users and policy
makers an unwarranted illusion that certain classes of messages will
be "flagged" correctly by the MUA of the recipient. The difficulty
in specifying requirements for labels in the "Subject:" field in a
precise, unambiguous manner makes it difficult for the MUA to
systematically identify messages that are labeled; this leads to both
false positive and false negative indications.
In addition, conflicting labeling requirements by policy makers, as
well as other current practices that use the "Subject:" for a variety
of purposes, make that field "overloaded." In order to meet these
conflicting requirements, software designers and bulk mail senders
resort to a variety of tactics, some of which may violate fundamental
requirements of the mail standards, such as the practice of an
intermediate MTA inserting various labels into the "Subject:" field.
Such practices increase the likelihood of non-compliant mail messages
and, thus, threaten interoperability between implementations.
7. Recommendations
This document makes three recommendations:
1. There is widespread skepticism in the technical community that
labels of any sort will be effective. Such labels should
probably be avoided as ineffective at best.
2. Despite the widespread skepticism expressed in point 1, over 36
states in the U.S. and 27 countries have passed anti-spam
measures, many of which require labels [Sorkin]. If such labels
are to be used, despite the widespread skepticism expressed in
point 1, there is a fairly broad consensus in the technical
community that such labels should not be put in the "Subject:"
field and should go in a designated header field.
3. If, despite points 1 and 2, a policy of mandating labels in the
"Subject:" field is adopted, a complementary requirement to use
the "No-Solicit:" should also be added.
8. Acknowledgements
The author would like to thank the following for their helpful
suggestions and reviews of this document: Joe Abley, Harald
Alvestrand, Elwyn Davies, Alain Durand, Frank Ellermann, Ted Hardie,
Tony Hansen, Scott Hollenbeck, Peter Koch, Bruce Lilly, Keith Moore,
Pekka Savola, and Mark Townsley.
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9. References
9.1. Normative References
[IANA] IANA, "Registry of Official Names for Character Sets That
May Be Used on the Internet", February 2004,
<http://www.iana.org/assignments/character-sets>.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC3865] Malamud, C., "A No Soliciting Simple Mail Transfer
Protocol (SMTP) Service Extension", RFC 3865,
September 2004.
9.2. Informative References
[8859-1] International Organization for Standardization,
"Information technology - 8-bit single byte coded graphic
- character sets - Part 1: Latin alphabet No. 1, JTC1/
SC2", ISO Standard 8859-1, 1987.
[8859-8] International Organization for Standardization,
"Information Processing - 8-bit Single-Byte Coded Graphic
Character Sets, Part 8: Latin/Hebrew alphabet",
ISO Standard 8859-8, 1988.
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[Colorado] Sixty-Second General Assembly of the State of Colorado,
"Colorado Junk Email Law", House Bill 1309, June 2000,
<http://www.spamlaws.com/state/co.html>.
[Doe] Frank Capra (Director), "Meet John Doe", IMDB Movie
No. 0033891, 1941, <http://us.imdb.com/title/tt0033891/>.
[Duck] The Mark Brothers, "Duck Soup", IMDB Movie No. 0023969,
1933, <http://us.imdb.com/title/tt0023969/>.
[Florida] The Florida Bar, "Rules of Professional Conduct", 2005,
<http://www.flabar.org/divexe/rrtfb.nsf/
WContents?OpenView&Start=1&Count=30&Expand=4.8#4.8>.
[KISA] Korea Information Security Agency, "Korea Spam Response
Center -- Legislation for Anti-Spam Regulations: Mandatory
Indication of Advertisement", 2003,
<http://www.spamcop.or.kr/eng/m_2.html>.
[Koch] Koch, P., "Subject: [tags] Considered Harmful", Work in
Progress, November 2004.
[Korea] National Assembly of the Republic of Korea, "Act on
Promotion of Information and Communication and
Communications Network Utilization and Information
Protection of 2001", 2001, <http://www.mic.go.kr/eng/res/
res_pub_db/res_pub_mic_wp/2003/whitepaper2003/in3_7.htm>.
[Lessig] Lessig, L., "How to unspam the Internet", The
Philadelphia Inquirer, May 2003, <http://www.philly.com/
mld/inquirer/news/editorial/5778539.htm?1c>.
[Levine] Levine, J., "Comments In the Matter of: REPORT TO CONGRESS
PURSUANT TO CAN-SPAM ACT", Federal Trade Commission,
Matter No. PO44405, February 2004, <http://www.ftc.gov/
reports/dneregistry/xscripts/dne040226pm.pdf>.
[Moore-1] Moore, K., "Individual Comment of Mr. Keith Moore Re:
Label for E-mail Messages", Federal Trade Commission of
the U.S., NPRM Comment RIN 3084-AA96, February 2004, <http
://www.ftc.gov/os/comments/adultemaillabeling/
040216moore.pdf>.
[Moore-2] Moore, K., "E-mail Message to the Author and the IESG",
March 2005.
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[RFC0886] Rose, M., "Proposed standard for message header munging",
RFC 886, December 1983.
[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, August 2004.
[Sorkin] Sorkin, D., "http://www.spamlaws.com/", 2005,
<http://www.spamlaws.com/>.
[Stooges] The Three Stooges, "Heavenly Daze", IMDB Movie
No. 0040429, 1948, <http://us.imdb.com/title/tt0040429/>.
[US] United States Congress, "The Controlling the Assault of
Non-Solicited Pornography and Marketing Act of 2003 (CAN-
SPAM Act of 2003)", Public Law 108-187, 117 STAT. 2699, 15
USC 7701, December 2003, <http://frwebgate.access.gpo.gov/
cgi-bin/getdoc.cgi?dbname=108_cong_public_laws
&docid=f:publ187.108.pdf>.
Author's Address
Carl Malamud
Memory Palace Press
PO Box 300
Sixes, OR 97476
US
EMail: carl@media.org
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ERRATA
