One document matched: draft-ietf-dkim-ssp-03.xml
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<rfc category="std" docName="draft-ietf-dkim-ssp-03" ipr="full3978">
<front>
<title>DKIM Author Signing Practices (ASP)</title>
<author fullname="Eric Allman" initials="E." surname="Allman">
<organization>Sendmail, Inc.</organization>
<address>
<postal>
<street>6475 Christie Ave, Suite 350</street>
<city>Emeryville</city>
<code>94608</code>
<region>CA</region>
</postal>
<phone>+1 510 594 5501</phone>
<email>eric+dkim@sendmail.org</email>
</address>
</author>
<author fullname="Jim Fenton" initials="J." surname="Fenton">
<organization>Cisco Systems, Inc.</organization>
<address>
<postal>
<street>MS SJ-9/2</street>
<street>170 W. Tasman Drive</street>
<city>San Jose</city>
<code>95134-1706</code>
<region>CA</region>
</postal>
<phone>+1 408 526 5914</phone>
<email>fenton@cisco.com</email>
</address>
</author>
<author fullname="Mark Delany" initials="M." surname="Delany">
<organization>Yahoo! Inc.</organization>
<address>
<postal>
<street>701 First Avenue</street>
<city>Sunnyvale</city>
<code>94089</code>
<region>CA</region>
</postal>
<phone>+1 408 349 6831</phone>
<email>markd+dkim@yahoo-inc.com</email>
</address>
</author>
<author fullname="John Levine" initials="J." surname="Levine">
<organization>Taughannock Networks</organization>
<address>
<postal>
<street>PO Box 727</street>
<city>Trumansburg</city>
<code>14886</code>
<region>NY</region>
</postal>
<phone>+1 831 480 2300</phone>
<email>standards@taugh.com</email>
<uri>http://www.taugh.com</uri>
</address>
</author>
<date day="23" month="February" year="2008"/>
<area>Applications / Email</area>
<!-- <workgroup>DKIM</workgroup> -->
<keyword>email, e-mail, rfc2822, rfc 2822, rfc822, rfc 822, rfc2821, rfc
2821, rfc821, rfc 821, rfc4871, rfc 4871, DKIM, domain keys, ASP,
architecture, mta, user, delivery, smtp,
submission, Internet, mailfrom, author, return address</keyword>
<abstract>
<t>DomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email using public-key cryptography and
key server technology to permit verification of the source and
contents of messages by either Mail Transport Agents (MTAs) or Mail
User Agents (MUAs). The primary DKIM protocol is described in
<xref target="RFC4871"/>.
This document describes the records that authors' domains can use to advertise
their practices for signing their outgoing mail, and how other hosts can access
those records.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>DomainKeys Identified Mail (DKIM) defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain
to claim responsibility for the introduction of a message into the
mail stream. Message recipients can verify the signature by querying
the signer's domain directly to retrieve the appropriate public key,
and thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain.</t>
<t>However, the legacy of the Internet is such that not all messages
will be signed, and the absence of a signature on a message is not an
a priori indication of forgery. In fact, during early phases of
deployment it is very likely that most messages will remain
unsigned. However, some domains might decide to sign all of their
outgoing mail, for example, to protect their brand name. It is
desirable for such domains to be able to advertise that fact
to other hosts. This is the topic of Author Signing Practices (ASP).</t>
<t>Hosts implementing
this specification can inquire what Author Signing Practices
a domain advertises. This inquiry is called an Author
Signing Practices check.</t>
<t>The detailed requirements for Author Signing Practices are given in
<xref target="RFC5016" />. This document refers extensively
to <xref target="RFC4871"/> and assumes the reader
is familiar with it.</t>
<t><list style="hanging">
<t hangText="Requirements Notation:">
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 <xref target="RFC2119"/>
</t>
</list>
</t>
</section>
<section title="Language and Terminology">
<section title="Terms Imported from DKIM Signatures Specification">
<t>Some terminology used herein is derived directly from
<xref target="RFC4871" />.
In several cases, references in that document to Sender have
been changed to Author here, to emphasize the relationship to
the Author address(es) in the From: header field described
in <xref target="RFC2822" />.
Briefly,
<list style="symbols">
<t>A "Signer" is the agent that signs a message, as defined in
section 2.1 of <xref target="RFC4871" />.</t>
<t>A "Selector" specifies which of the keys published by a signing
domain is to be queried, as defined in section 3.1 of <xref target="RFC4871" />.</t>
<t>A "Local-part" is the part of an address preceding the @ sign,
as defined in <xref target="RFC2822" /> and used in
<xref target="RFC4871" />.</t>
</list></t>
</section>
<section title="Valid Signature">
<t>A "Valid Signature" is any signature on a message which correctly
verifies using the procedure described in section 6.1 of <xref target="RFC4871" />.</t>
</section>
<section title="Author Address">
<t>An "Author Address" is an email address in the From header
field of a message <xref target="RFC2822" />.
If the From header field
contains multiple addresses, the message has multiple Author Addresses.</t>
</section>
<section title="Author Domain">
<t>An "Author Domain" is everything to the right of the "@" in an
Author Address (excluding the "@" itself).</t>
</section>
<!-- <section title="Alleged Signer">
<t>An "Alleged Signer" is the identity of the signer claimed in a DKIM-Signature
header field in a message received; it is
"alleged" because it has not yet been verified.</t>
</section> -->
<section title="Alleged Author">
<t>An "Alleged Author" is an Author Address of a message;
it is "alleged" because it has not yet been
verified.</t>
</section>
<section title="Author Signing Practices">
<t>"Author Signing Practices" (or just "practices") consist of a
machine-readable record published by the domain of an Alleged
Author which includes statements about the domain's practices
with respect to mail it sends with its domain in the From: line.</t>
</section>
<section title="Author Signature">
<t>An "Author Signature" is any Valid Signature where the
identity of the user or agent on behalf of which the message is
signed (listed in the "<spanx style="verb">i=</spanx>"
tag or its default value from the "<spanx style="verb">d=</spanx>"
tag) matches an Author Address in the message.
When the identity of the user or agent includes a
Local-part, the identities match if the Local-parts match and
the domains match. Otherwise,
the identities match if the domains match.</t>
<t>For example, if
a message has a Valid Signature, with the DKIM-Signature
field containing <spanx style="verb">i=a@domain.example</spanx>,
then domain.example
is asserting that it takes responsibility for the message.
If the message's From: field contains the address
<spanx style="verb">b@domain.example</spanx>
and an ASP query produces a <spanx style="verb">dkim=all</spanx>
or <spanx style="verb">dkim=discardable</spanx> result,
that would mean that the message does not have a valid Author
Signature. Even though the message is signed by the same domain,
its failure to satisfy ASP could be problematic.</t>
</section>
</section>
<section title="Operation Overview">
<t>Domain owners can publish Author Signing Practices via a query
mechanism such as the Domain Name System; specific details are
given in <xref target="dnsrep" />.</t>
<t>Hosts can look up the Author Signing Practices of
the domain(s) specified by the Author Address(es) as described in
<xref target="checkproc" />.
If a message has multiple Author Addresses
the ASP lookups SHOULD be performed independently on each address.
This standard does not address the process a host might use to
combine the lookup results.</t>
<section title="ASP Usage">
<t>Depending on the Author Domain(s) and the signatures in a
message, a recipient gets varying amounts of useful information
from each ASP lookup.
<list style="symbols">
<t>If a message has no Valid Signature, the ASP result is directly
relevant to the message.</t>
<t>If a message has a Valid Signature from an Author Domain,
ASP provides no benefit relative to that domain
since the message is already known
to be compliant with any possible ASP for that domain.</t>
<t>If a message has a Valid Signature from a domain other than
an Author Domain, the receiver can use both the Signature and
the ASP result in its evaluation of the message.</t>
</list></t>
</section>
<section title="ASP Results">
<t>An Author Signing Practices lookup for an Author Address produces one of four possible
results:
<list style="symbols">
<t>Messages from this domain might or might not have an author signature.
This is the default
if the domain exists in the DNS but no record is found.</t>
<t>All messages from this domain are signed.</t>
<t>All messages from this domain are signed and discardable.</t>
<t>The domain does not exist.</t>
</list></t>
</section>
</section>
<section title="Detailed Description">
<section title="DNS Representation" anchor='dnsrep'>
<t>Author Signing Practices records are published using the DNS TXT
resource record type.</t>
<t>NON-NORMATIVE DISCUSSION [to be removed before publication]: There has been considerable discussion
on the DKIM WG mailing list regarding the relative advantages of
TXT and a new resource record (RR) type. Read the archive for details.</t>
<t>The RDATA for ASP resource records is textual in format, with
specific syntax and semantics relating to their role in describing
Author Signing Practices. The "Tag=Value List" syntax described in
section 3.2 of <xref target="RFC4871"/> is used. Records not in compliance with
that syntax or the syntax of individual tags described in Section 4.3
MUST be ignored (considered equivalent to a NODATA result) for
purposes of ASP, although they MAY cause the logging
of warning messages via an appropriate system logging mechanism.
If the RDATA contains multiple character strings, the strings are
logically concatenated with no delimiters between the strings.
</t>
<t>The ASP record for a domain is published at a location
in the domain's
DNS hierarchy prefixed by _asp._domainkey.; e.g., the ASP record for
example.com would be a TXT record that is published at
<spanx style="verb">_asp._domainkey.example.com</spanx>.
A domain MUST NOT publish more than one ASP record;
the semantics of an ASP lookup that returns multiple ASP records
for a single domain are undefined.
(Note that example.com and mail.example.com are different domains.)
</t>
</section>
<section title="Publication of ASP Records">
<t>Author Signing Practices are intended to apply to all mail sent
from the domain of an Alleged Author. In order to ensure that ASP
applies to any hosts within that domain (e.g., www.example.com,
ftp.example.com.) the ASP lookup algorithm looks up one level in
the domain tree. For example, mail signed by www.example.com could
be covered by the ASP record for example.com. This avoids
the need to include an ASP record for every name
within a given domain.</t>
<t>Normally, a domain expressing Author Signing Practices will want to
do so for both itself and all of its "descendants" (child domains at
all lower levels). Domains wishing to do so MUST publish ASP records
for the domain itself and any subdomains.</t>
<t>Wildcards within a domain publishing ASP records pose a
particular problem. This is discussed in more detail in <xref
target="wildcards" />.</t>
<section title="Record Syntax" anchor='syntax'>
<t>ASP records use the "tag=value" syntax described in section 3.2 of
<xref target="RFC4871"/>.</t>
<t>Tags used in ASP records are described below. Unrecognized tags MUST be
ignored. In the ABNF below, the WSP token is imported from
<xref target="RFC2822"/>. The ALPHA and DIGIT tokens
are imported from <xref target="RFC5234"/>.
<list style='hanging'>
<t hangText="dkim=">Outbound signing practices for the domain (plain-text;
REQUIRED). Possible values are as follows:
<list style='hanging'>
<t hangText="unknown">The domain might sign some or all email.</t>
<t hangText="all">All mail from the domain is signed
with an Author Signature.</t>
<t hangText="discardable">All mail from the domain is signed
with an Author Signature.
Furthermore, if a message arrives without a
valid Author Signature
due to modification in transit, submission via a path
without access to a signing key, or other reason,
the domain encourages the recipient(s) to discard it.</t>
</list>
<figure><preamble>ABNF:</preamble>
<artwork>
asp-dkim-tag = %x64.6b.69.6d *WSP "="
*WSP ("unknown" / "all" / "discardable")
</artwork>
</figure>
</t>
<t hangText="t=">Flags, represented as a colon-separated list of names (plain-text;
OPTIONAL, default is that no flags are set). Flag values are:
<list style='hanging'>
<t hangText="s">The signing practices apply only to the named domain, and not
to subdomains.</t>
</list>
<figure><preamble>ABNF:</preamble>
<artwork>
asp-t-tag = %x74 *WSP "=" *WSP { asp-t-tag-flag
0*( *WSP ":" *WSP asp-t-tag-flag )
asp-t-tag-flag = "s" / hyphenated-word
; for future extension
hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-")
(ALPHA / DIGIT) ]
</artwork>
</figure>
</t>
<t>Unrecognized flags MUST be ignored.</t>
</list></t>
</section>
<section anchor='checkproc' title="Author Signing Practices Lookup Procedure">
<t>Hosts doing an ASP lookup MUST produce a result that is
semantically equivalent to applying the following steps in the order
listed below. In practice, several of these steps can be performed in
parallel in order to improve performance. However, implementations
SHOULD avoid doing unnecessary DNS lookups. For the purposes of this
section a "valid ASP record" is one that is both syntactically and
semantically correct; in particular, it matches the ABNF for a
<spanx style="verb">tag-list</spanx> and includes a defined <spanx
style="verb">dkim=</spanx> tag.<list style="numbers">
<t><spanx>Fetch Named ASP Record.</spanx> The host
MUST query DNS for a TXT record corresponding to the
Author Domain prefixed by <spanx
style="verb">_asp._domainkey.</spanx> (note the trailing
dot). If the result of this query is a <spanx
style="verb">NOERROR</spanx> response with an
answer which is a valid ASP record, use that record;
otherwise, continue to the next step.</t>
<t><spanx>Verify Domain Exists.</spanx> The host MUST
perform a DNS query for a record corresponding to the Author
Domain (with no prefix). The type of the query can be of any type,
since this step is only to determine if the domain itself exists
in DNS. This query MAY be done in parallel with the query made in
step 2. If the result of this query is an
<spanx style="verb">NXDOMAIN</spanx> error, the algorithm MUST terminate
with an appropriate error.
<list style="empty">
<t>NON-NORMATIVE DISCUSSION: Any resource record type
could be used for this query since the existence of a
resource record of any type will prevent an <spanx
style="verb">NXDOMAIN</spanx> error.
MX is a reasonable choice for
this purpose is because this record type is thought to
be the most common for likely domains, and will
therefore result in a result which can be more readily
cached than a negative result.</t>
</list></t>
<t><spanx>Try Parent Domain.</spanx> The host MUST
query DNS for a TXT record for the immediate parent
domain, prefixed with <spanx style="verb">_asp._domainkey.</spanx>
If the result of
this query is anything other than a <spanx
style="verb">NOERROR</spanx> response with a valid ASP
record, the algorithm terminates with a result
indicating that no ASP record was present. If the ASP "t"
tag exists in the response and any of the flags is "s"
(indicating it does not apply to a subdomain), the
algorithm also terminates without finding an ASP record.
Otherwise, use that record.</t>
</list></t>
<t>If any of the queries involved in the Author Signing Practices
Check result in a <spanx style="verb">SERVFAIL</spanx> error response,
the algorithm terminates without returning a result;
possible actions include queuing the message or returning an SMTP
error indicating a temporary failure.</t>
</section>
</section>
</section>
<section title="IANA Considerations">
<t>ASP introduces some new namespaces that have been registered with
IANA. In all cases, new values are assigned only for values that
have been documented in a published RFC that has IETF Consensus
<xref target='RFC2434' />.</t>
<t>INFORMATIVE NOTE [ to be removed before publication ]: RFC 4871 defines
a selector as a sub-domain, importing the term from RFC 2822. A sub-domain
starts with a letter or digit, hence names such as _asp that start with
an underscore cannot collide with valid selectors.</t>
<section title="ASP Specification Tag Registry">
<t>An ASP record provides for a list of specification tags. IANA has
established the ASP Specification Tag Registry for
specification tags that can be used in ASP fields.</t>
<t>The initial entries in the registry comprise:
<figure>
<artwork>
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| dkim | (this document) |
| t | (this document) |
+------+-----------------+
</artwork>
<postamble>ASP Specification Tag Registry Initial Values</postamble>
</figure>
</t>
</section>
<section title="ASP Outbound Signing Practices Registry">
<t>The <spanx style="verb">dkim=</spanx> tag spec, defined in <xref target="syntax" />, provides for
a value specifying Outbound Signing Practices. IANA has
established the ASP Outbound Signing Practices Registry for
Outbound Signing Practices.</t>
<t>The initial entries in the registry comprise:
<figure>
<artwork>
+-------------+-----------------+
| TYPE | REFERENCE |
+-------------+-----------------+
| unknown | (this document) |
| all | (this document) |
| discardable | (this document) |
+-------------+-----------------+
</artwork>
<postamble>ASP Outbound Signing Practices Registry Initial Values</postamble>
</figure>
</t>
</section>
<section title="ASP Flags Registry">
<t>The <spanx style="verb">t=</spanx> tag spec, defined in <xref target="syntax" />, provides for
a value specifying Flags. IANA has
established the ASP Flags Registry for
ASP Flags.</t>
<t>The initial entries in the registry comprise:
<figure>
<artwork>
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| s | (this document) |
+------+-----------------+
</artwork>
<postamble>ASP Flags Registry Initial Values</postamble>
</figure>
</t>
</section>
</section>
<section title="Security Considerations">
<t>Security considerations in the Author Signing Practices are
mostly related to attempts on the part of malicious senders to
represent themselves as authors for whom they are not authorized
to send mail, often in an attempt to defraud either the recipient
or an Alleged Author.</t>
<t>Additional security considerations regarding Author Signing
Practices are found in <xref target="RFC4686">the DKIM threat
analysis</xref>.</t>
<section title="ASP Threat Model">
<t>Email recipients often have a core set of content authors that
they already trust. Common examples include financial
institutions with which they have an existing relationship and
Internet web transaction sites with which they conduct
business.</t>
<t>Email abuse often seeks to exploit the name-recognition that
recipients will have, for a legitimate email author, by
using its domain name in the From: header field. Especially
since many popular MUAs do not display the author's email
address, there is no empirical evidence of the extent
that this particular
unauthorized use of a domain name contributes to recipient
deception or that eliminating it will have significant effect.</t>
<t>However, closing this exploit could facilitate some types of
optimized processing by receive-side message filtering engines,
since it could permit them to maintain higher-confidence
assertions about From: header field uses of a domain, when the
occurrence is authorized.</t>
<t>Unauthorized uses of domain names occur elsewhere in messages,
as do unauthorized uses of organizations' names. These attacks
are outside the scope of this specification.</t>
<t>ASP does not provide any benefit--nor, indeed, have any effect
at all--unless an external system acts upon the verdict, either by
treating the message differently during the delivery process or by
showing some indicator to the end recipient. Such a system is out
of scope for this specification.</t>
<t>ASP Checkers perform up to three DNS lookups per Alleged Author
Domain. Since these lookups are driven by domain names in email
message headers of possibly fraudulent email, legitimate ASP
Checkers can become participants in traffic multiplication
attacks.</t>
</section>
<section title="DNS Attacks">
<t>An attacker might attack the DNS infrastructure in an
attempt to impersonate ASP records, in an attempt to influence
a receiver's decision on how it will handle mail. However,
such an attacker is more likely to attack at a higher level,
e.g., redirecting A or MX record lookups in order to
capture traffic that was legitimately intended for the target
domain. These DNS security issues are addressed by
<xref target="RFC4033">DNSSEC</xref>.</t>
<t>Because ASP operates within the framework of the legacy e-mail
system, the default result in the absence of an ASP record is that
the domain does not sign all of its messages. It is therefore
important that the ASP clients distinguish a DNS failure such as
<spanx style="verb">SERVFAIL</spanx>
from other DNS errors so that appropriate actions can be taken.</t>
</section>
<section title="DNS Wildcards" anchor='wildcards'>
<t>Wildcards within a domain publishing ASP records, including
but not limited to wildcard MX records,
pose a particular
problem. While referencing the immediate parent domain
allows the discovery of an ASP record corresponding to an
unintended immediate-child subdomain, wildcard records apply
at multiple levels. For example, if there is a wildcard
MX record for "example.com", the domain
"foo.bar.example.com" can receive mail through the named
mail exchanger. Conversely, the existence of the record
makes it impossible to tell whether "foo.bar.example.com" is
a legitimate name since a query for that name will not
return an <spanx style="verb">NXDOMAIN</spanx> error.
For that reason, ASP coverage
for subdomains of domains containing a wildcard record is
incomplete.
<vspace blankLines='1' />
NON-NORMATIVE NOTE: Complete ASP coverage of domains containing
(or where any parent contains) wildcards generally cannot be
provided by standard DNS servers.</t>
</section>
</section>
</middle>
<back>
<references title="References - Normative">
&rfc2119;
&rfc2434;
&rfc2822;
&rfc4033;
&rfc5234;
&rfc4686;
&rfc4871;
</references>
<references title="References - Informative">
&rfc5016;
</references>
<section title="Usage Examples">
<t>These examples are intended to illustrate typical uses of ASP.
They are not intended to be exhaustive, nor to apply to every
domain's or mail system's individual situation.</t>
<t>Domain managers are advised to consider the ways that mail
processing can modify messages in ways that will invalidate
an existing DKIM signature, such as mailing lists, courtesy
forwarders, and other paths that could add or modify
headers, or modify the message body.
In that case, if the modifications invalidate the DKIM signature,
recipient hosts will consider the mail not to
have an Author Signature, even though the signature was present
when the mail was originally sent.</t>
<section title="Single Location Domains">
<t>A common mail system configuration handles all of a domain's
users' incoming and outgoing mail through a single MTA or
group of MTAs.
In that case, the MTA(s) can be configured to sign outgoing
mail with an Author Signature.</t>
<t>In this situation it might be appropriate to publish an ASP
record for the domain containing "all", depending on
whether the users also send mail through other paths that do
not apply an Author Signature. Such paths could include
MTAs at hotels or hotspot networks used by travelling
users, or web sites that provide "mail an article"
features.</t>
</section>
<section title="Bulk Mailing Domains">
<t>Another common configuration uses a domain solely for bulk
or broadcast mail, with no individual human users, again typically
sending all the mail through a single MTA or group of MTAs
that can apply an Author Signature.
In this case, the domain's management can be confident that all
of its outgoing mail will be sent through the signing MTA.
Lacking individual users, the domain is unlikely to participate in
mailing lists, but could still send mail through other paths
that might invalidate signatures.</t>
<t>Domain owners often use specialist mailing providers to
send their bulk mail.
In that case, the mailing provider needs access to a
suitable signing key in order to apply an Author Signature.
One possible route would be for the domain owner to generate
the key and give it to the mailing provider.
Another would be for the domain to delegate a subdomain to
the mailing provider, for example, bigbank.example might
delegate email.bigbank.example to such a provider.
In that case, the provider can generate the
keys and DKIM DNS records itself and use the subdomain
in the Author address in the mail.</t>
</section>
<section title="Bulk Mailing Domains with Discardable Mail">
<t>In some cases, a domain might sign all its outgoing mail with
an Author Signature, but prefer that recipient systems
discard mail without a valid Author Signature to avoid confusion
from mail sent from sources that do not apply an Author Signature.
(This latter kind of mail is sometimes loosely called
"forgeries".)
In that case, it might be appropriate to publish an ASP record
containing "discardable".
Note that a domain SHOULD NOT publish a "discardable" record
if it wishes to maximize the likelihood that mail
from the domain is delivered,
since it could cause some fraction of the mail the
domain sends to be discarded.</t>
<t>As a special case, if a domain sends no mail at all, it can safely
publish a "discardable" ASP record, since any mail with an
author address in the domain is a forgery.</t>
</section>
<section title="Third Party Senders">
<t>Another common use case is for a third party to enter into
an agreement whereby that third party will send bulk or
other mail on behalf of a designated author domain, using
that domain in the RFC2822 From: or other headers. Due to
the many and varied complexities of such agreements, third
party signing is not addressed in this specification. <!-- The
authors anticipate that as mail systems gain experience
with DKIM, it will become possible to codify best practices
of this and other usages of DKIM. --></t>
</section>
</section>
<section title="Acknowledgements">
<t>This document greatly benefited from comments by
Steve Atkins, Jon Callas, Dave Crocker,
JD Falk, Arvel Hathcock,
Ellen Siegel, Michael Thomas, and Wietse Venema.</t>
</section>
<section title="Change Log">
<t><spanx style="strong">NOTE TO RFC EDITOR: This section may be removed
upon publication of this document as an RFC.</spanx></t>
<section title="Changes since -ietf-dkim-02">
<t>
<list style="symbols">
<t>Merge in more text from ASP draft.</t>
<t>Phrase actions as host's rather than checker.</t>
<t>Explanatory description of i= matching.</t>
<t>Lookup procedure consistently refers to one ASP record per lookup.</t>
<t>Update security section w/ language from W. Venema</t>
<t>Simplify imports of terms from other RFCs, add Local-part,
4234 -> 5234.</t>
<t>Add usage example appendix.</t>
<t>Add IANA considerations.</t>
<t>Update authors list</t>
</list></t>
</section>
<section title="Changes since -ietf-dkim-ssp-01">
<t><list style="symbols">
<t>Reworded introduction for clarity.</t>
<t>Various definition
clarifications.</t>
<t>Changed names of practices to unknown, all, and discardable.</t>
<t>Removed normative language mandating use of SSP in
particular situations (issue 1538).</t>
<t>Clarified possible confusion over handling of syntax
errors.</t>
<t>Removed normative language from Introduction (issue 1538).</t>
<t>Changed "Originator" to "Author" throughout (issue 1529).</t>
<t>Removed all references to Third-Party Signatures (issues 1512,
1521).</t>
<t>Removed all mention of "Suspicious" (issues 1528, 1530).</t>
<t>Removed "t=y" (testing) flag (issue 1540).</t>
<t>Removed "handling" tag (issue 1513).</t>
<t>Broke up the "Sender Signing Practices Check Procedure" into
two algorithms: fetching the SSP record and interpretation thereof
(issues 1531, 1535; partially addresses issue 1520). Interpretation
is now the responsibility of the Evaluator.</t>
<t>Document restructuring for better flow and remove redundancies
(some may address issue 1523, but I'm not sure I understand that
issue completely; also issues 1532, 1537).</t>
<t>Removed all mention of how this interacts with users, even
though it makes parts of the document harder to understand (issue
1526).</t>
<t>Introduced the concepts of "SSP Checker" and "Evaluator".</t>
<t>Multiple author case now handled my separate invocations
of SSP checker by Evaluator (issue 1525).</t>
<t>Removed check to avoid querying top-level domains.</t>
<t>Changed ABNF use of whitespace from [FWS] to *WSP
(partially addresses issue 1543).</t>
</list></t>
</section>
<section title="Changes since -ietf-dkim-ssp-00">
<t><list style="symbols">
<t>Clarified Operation Overview and eliminated use of Legitimate
as the counterpart of Suspicious since the words have different
meanings.</t>
<t>Improved discussion (courtesy of Arvel Hathcock) of the use of
TXT records in DNS vs. a new RR type.</t>
<t>Clarified publication rules for multilevel names.</t>
<t>Better description of overall record syntax, in particular that
records with unknown tags are considered syntactically
correct.</t>
<t>Clarified Sender Signing Practices Check Procedure, primarily
by use of new term Author Domain.</t>
<t>Eliminated section "Third-Party Signatures and Mailing Lists"
that is better included in the DKIM overview document.</t>
<t>Added "handling" tag to express alleged sending domain's
preference about handling of Suspicious messages.</t>
<t>Clarified handling of SERVFAIL error in SSP check.</t>
<t>Replaced "entity" with "domain", since with the removal of
user-granularity SSP, the only entities having sender signing
policies are domains.</t>
</list></t>
</section>
<section title="Changes since -allman-ssp-02">
<t><list style="symbols">
<t>Removed user-granularity SSP and u= tag.</t>
<t>Replaced DKIMP resource record with a TXT record.</t>
<t>Changed name of the primary tag from "p" to "dkim".</t>
<t>Replaced lookup algorithm with one which traverses upward at
most one level.</t>
<t>Added description of records to be published, and
effect of wildcard records within the domain, on SSP.</t>
</list></t>
</section>
<section title="Changes since -allman-ssp-01">
<t><list style="symbols">
<t>Changed term "Sender Signing Policy" to "Sender Signing
Practices".</t>
<t>Changed query methodology to use a separate DNS resource record
type, DKIMP.</t>
<t>Changed tag values from SPF-like symbols to words.</t>
<t>User level policies now default to that of the domain if not
specified.</t>
<t>Removed the "Compliance" section since we're still not clear on
what goes here.</t>
<t>Changed the "parent domain" policy to only search up one level
(assumes that subdomains will publish SSP records if
appropriate).</t>
<t>Added detailed description of SSP check procedure.</t>
</list></t>
</section>
<section title="Changes since -allman-ssp-00">
<t>From a "diff" perspective, the changes are extensive. Semantically,
the changes are:<list style="symbols">
<t>Added section on "Third-Party Signatures and Mailing Lists"</t>
<t>Added "Compliance" (transferred from -base document). I'm not
clear on what needs to be done here.</t>
<t>Extensive restructuring.</t>
</list></t>
</section>
</section>
</back>
</rfc>
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