moparisthebest/xeps
1
0
Fork 0
mirror of https://github.com/moparisthebest/xeps synced 2024-11-24 18:22:24 -05:00
Code Issues Releases Wiki Activity

Merge pull request #454 from Flowdalic/isr-sasl2-protoxep

Browse Source 
Add ISR-SASL2 ProtoXEP to inbox/
This commit is contained in:
Florian Schmaus 2017-03-20 18:47:38 +01:00 committed by GitHub
commit 3e5394df98
2 changed files with 587 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

586
inbox/isr-sasl2.xml Normal file
View File

@ -0,0 +1,586 @@
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
<!ENTITY tls13 "<span class='ref'><link url='https://tools.ietf.org/html/draft-ietf-tls-tls13-19'>draft-ietf-tls-tls13-19</link></span> <note>The Transport Layer Security (TLS) Protocol Version 1.3 &lt;<link url='https://tools.ietf.org/html/draft-ietf-tls-tls13-19'>https://tools.ietf.org/html/draft-ietf-tls-tls13-19</link>&gt;.</note>" >
<!ENTITY iana-hash-alg "<span class='ref'><link url='https://www.iana.org/assignments/named-information/named-information.xhtml#hash-alg'>IANA Named Information Hash Algorithm Registry</link></span><note>IANA Named Information Hash Algorithm Registry &lt;<link url='https://www.iana.org/assignments/named-information/named-information.xhtml#hash-alg'>https://www.iana.org/assignments/named-information/named-information.xhtml#hash-alg</link>&gt;.</note>" >
<!ENTITY iana-cbt "<span class='ref'><link url='https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml'>IANA Channel-Binding Types</link></span><note>IANA Channel-Binding Types&lt;<link url='https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml'>https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml</link>&gt;.</note>" >
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>Instant Stream Resumption</title>
<abstract>This specification introduces a mechanism for instant
stream resumption, based on Stream Management (XEP-0198), allowing
XMPP entities to instantaneously resume an XMPP stream.</abstract>
<legal>
<copyright>This XMPP Extension Protocol is copyright (c) 1999 -
2016 by the XMPP Standards Foundation (XSF).</copyright>
<permissions>Permission is hereby granted, free of charge, to any
person obtaining a copy of this specification (the
&quot;Specification&quot;), to make use of the Specification
without restriction, including without limitation the rights to
implement the Specification in a software program, deploy the
Specification in a network service, and copy, modify, merge,
publish, translate, distribute, sublicense, or sell copies of the
Specification, and to permit persons to whom the Specification is
furnished to do so, subject to the condition that the foregoing
copyright notice and this permission notice shall be included in
all copies or substantial portions of the Specification. Unless
separate permission is granted, modified works that are
redistributed shall not contain misleading information regarding
the authors, title, number, or publisher of the Specification, and
shall not claim endorsement of the modified works by the authors,
any organization or project to which the authors belong, or the
XMPP Standards Foundation.</permissions>
<warranty>## NOTE WELL: This Specification is provided on an
&quot;AS IS&quot; BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, express or implied, including, without limitation, any
warranties or conditions of TITLE, NON-INFRINGEMENT,
MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. In no event
shall the XMPP Standards Foundation or the authors of this
Specification be liable for any claim, damages, or other
liability, whether in an action of contract, tort, or otherwise,
arising from, out of, or in connection with the Specification or
the implementation, deployment, or other use of the
Specification. ##</warranty>
<liability>In no event and under no legal theory, whether in tort
(including negligence), contract, or otherwise, unless required by
applicable law (such as deliberate and grossly negligent acts) or
agreed to in writing, shall the XMPP Standards Foundation or any
author of this Specification be liable for damages, including any
direct, indirect, special, incidental, or consequential damages of
any character arising out of the use or inability to use the
Specification (including but not limited to damages for loss of
goodwill, work stoppage, computer failure or malfunction, or any
and all other commercial damages or losses), even if the XMPP
Standards Foundation or such author has been advised of the
possibility of such damages.</liability>
<conformance>This XMPP Extension Protocol has been contributed in
full conformance with the XSF's Intellectual Property Rights
Policy (a copy of which may be found at &lt;<link
url='http://xmpp.org/extensions/ipr-policy.shtml'>http://xmpp.org/extensions/ipr-policy.shtml</link>&gt;
or obtained by writing to XSF, P.O. Box 1641, Denver, CO 80201
USA).</conformance>
</legal>
<number>xxxx</number>
<status>ProtoXEP</status>
<type>Standards Track</type>
<sig>Standards</sig>
<approver>Council</approver>
<dependencies>
<spec>XMPP Core</spec>
<spec>XEP-0198</spec>
<spec>XEP-0388</spec>
</dependencies>
<supersedes/>
<supersededby/>
<shortname>isr</shortname>
<author>
<firstname>Florian</firstname>
<surname>Schmaus</surname>
<email>flo@geekplace.eu</email>
<jid>flo@geekplace.eu</jid>
</author>
<revision>
<version>0.0.3</version>
<date>2017-03-17</date>
<initials>fs</initials>
<remark><p>Based ISR on SASL2.</p></remark>
</revision>
<revision>
<version>0.0.2</version>
<date>2016-03-11</date>
<initials>fs</initials>
<remark><p>Second draft.</p></remark>
</revision>
<revision>
<version>0.0.1</version>
<date>2016-02-12</date>
<initials>fs</initials>
<remark><p>First draft.</p></remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>This XEP specifies an instant stream resumption mechanism based
on &xep0198;, allowing XMPP entities to instantaneously resume an
XMPP stream. This can be seen as the complementary part to &xep0305;
allowing for fast XMPP session (re-)establishment.</p>
<p>Compared to the existing stream resumption mechanism of <link
url='http://xmpp.org/extensions/xep-0198.html#resumption'><cite>XEP-0198</cite>
§ 5</link>, the approach defined herein reduces the round trips
required to resume a stream to exactly <em>one</em>. This is
achieved by using just a secure short-lived key to resume the
stream.</p>
</section1>
<section1 topic='Glossary' anchor='glossary'>
<dl>
<di>
<dt>ISR</dt>
<dd>Instant Stream Resumption.</dd>
</di>
<di>
<dt>Instant Stream Resumption Key (ISR Key)</dt>
<dd>A shared secret that is exclusively ephemeral and represented as string.</dd>
</di>
<di>
<dt>TLS</dt>
<dd>Transport Layer Security (&rfc5246;).</dd>
</di>
</dl>
</section1>
<!--
<section1 topic='Use Cases' anchor='usecases'>
<p>STRONGLY RECOMMENDED.</p>
</section1>
-->
<section1 topic='Stream Feature'>
<p>XMPP entities providing Instant Stream Resumption MUST announce
that functionality as stream feature, but only if an instant stream
resumption is possible at this stage. The ISR stream future consists
of an &lt;isr/&gt; element qualified by the 'urn:xmpp:isr:0'
namespace. And since ISR requires TLS, this means that the
&lt;isr/&gt; stream feature only appears on TLS secured
connections.</p>
<p>The ISR stream feature element MUST contain a &lt;mechanisms/&gt;
element as defined in &rfc6120;. This element contains the SASL
mechanism which are available to be used for instant stream
resumption.</p>
<example caption='Server announces the Instant Stream Resumption Stream Feature'><![CDATA[
<stream:stream
from='example.com'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/stream'
version='1.0'>
<stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
<sm xmlns='urn:xmpp:sm:3'/>
<isr xmlns='urn:xmpp:isr:0'>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>X-HT-SHA-256-ENDP</mechanism>
</mechaisms>
</isr>
</stream:features>
]]></example>
<p>Every ISR enabled entity MUST support the X-HT-SHA-256-ENDP
mechanism, support for X-HT-SHA-256-UNIQ is RECOMMENDED. The family
of SASL <cite>X-HT-*</cite> mechanisms is defined below in <link
url='#ht-sasl'>Section 6</link>.</p>
</section1>
<section1 topic='Obtaining a Instant Stream Resumption Key' anchor='obtain'>
<p>In order to obtain an ISR key, the requesting entity must add a
'mechanism' attribute qualified by the 'urn:xmpp:isr:0' namespace to
the &lt;enable/&gt; element as defined in &xep0198; when attempting
to enable Stream Management. The value of the 'mechanism' attribute
is the name of the SASL mechanism the requesting entity will use
when performing ISR with the returned key. The entities involved in
ISR MUST only use or allow this mechanism when performing ISR with
the according key. This effectively pins the SASL
mechanism <note>Pinning the SASL mechanism is believed to
increase the security</note>.</p>
<example caption='An &lt;enable/gt; Nonza with the ISR &apos;mechanism&apos; attribute'><![CDATA[
<enable
xmlns='urn:xmpp:sm:3'
xmlns:isr='urn:xmpp:isr:0'
isr:mechanism='X-HT-SHA-256-ENDP'/>
]]></example>
<p>Next, the &lt;enabled/&gt; Nonza (see &xep0360;) which is send as
positive reply upon a request to enable Stream Management, MUST
contain an 'key' attribute qualified by the 'urn:xmpp:isr:0'
namespace containing a ISR key. The key MUST be newly generated by a
cryptographically secure random number generator and MUST contain at
lest 128 bit of entropy. The Nonza can optionally also contain a
'location' attribute qualified by the 'urn:xmpp:isr:0' namespace
which specifies the preferred IP address or hostname, and a TCP port
number of the host which should be used for instant stream
resumption.</p>
<example caption='An &lt;enabled/&gt; Nonza with a ISR key'><![CDATA[
<enabled
xmlns='urn:xmpp:sm:3'
xmlns:isr='urn:xmpp:isr:0'
isr:key='a0b9162d-0981-4c7d-9174-1f55aedd1f52'/>]]></example>
<example caption='An &lt;enabled/&gt; Nonza with a ISR key and location'><![CDATA[
<enabled
xmlns='urn:xmpp:sm:3'
xmlns:isr='urn:xmpp:isr:0'
isr:key='a0b9162d-0981-4c7d-9174-1f55aedd1f52'
isr:location='isr.example.org:5222'/>]]></example>
<p>The &lt;enabled/&gt; Nonza containing an ISR key MUST only be
sent over TLS secured connections.</p>
</section1>
<section1 topic='Instant Stream Resumption' anchor='isr'>
<p>In order to instantaneously resume an XMPP stream the initiating
entity, which is either an XMPP client or server, must posses a
valid ISR key. After it has obtained the ISR key, using the process
described in the previous section, it first determines the host for
resumption, and after that, tries to perform the instant stream
resumption.</p>
<section2 topic='Determing the Host for Resumption' anchor='host'>
<p>The lookup mechanism order to determine host candidates for ISR
resumption is as follows:</p>
<ol>
<li>The host provided in the optional 'location' attribute
qualified by the 'urn:xmpp:isr:0' namespace found in the
&lt;enabled/&gt; element of <cite>XEP-0198</cite> (the
"isr:location").
</li>
<li>The hosts determined by means of &xep0368;.</li>
<li>The host announced in the 'location' attribute of the
&lt;enabled/&gt; Nonza defined in <cite>XEP-0198</cite>.</li>
<li>Standard host lookup mechanisms.</li>
</ol>
<p>The host candidates retrieved by those mechanisms SHOULD be
tried by the initiating entity in this order.</p>
<p>Note that the hosts announced by the 'location' attribute
qualified by the 'urn:xmpp:isr:0' namespace MUST be connected to
using TLS from the beginning, i.e. &lt;starttls/&gt; MUST NOT be
used, instead the TLS handshake is performed right after
establishing the connection.</p>
<p>This order prefers hosts which allow connections where TLS is
enabled from the beginning. This is desirable to reduce the
required round trips by skipping the &lt;starttls/&gt; step.</p>
</section2>
<section2 topic='Performing Instant Stream Resumption' anchor='resume'>
<p>After the remote host on which the instant stream resumption
should be performed was determined, the initiating entity connects
to the host, and establishes TLS by either</p>
<ol>
<li>establishing a TLS session right away, or</li>
<li>performing STARTTLS (&rfc6120; § 5).</li>
</ol>
<p>Now the initiating entity sends an XMPP &lt;stream&gt; open
element followed by a &lt;authenticate/&gt; Nonza as specified in
the &xep0388;. The initiating entity must also provide a
&lt;inst-resume/&gt; element qualified by the 'urn:xmpp:isr:0'
namespace, which must contain a &lt;resume/&gt; element as defined
in &xep0198;.</p>
<p>If the 'without-isr-token' attribute is set to true, then the
SASL mechanisms are performed as when traditionally authenticating
the XMPP session. If the value of the attribute is 'false', which is
the default value for this attribute, then the "password" given to
the SASL mechanism is the ISR key. Note that this implies that only
SASL mechanisms which take a password/token can be used this
way.</p>
<p>All ISR implementations MUST support the
X-HT-SHA-256-ENDP mechanism.</p>
<example caption='Initiating entity requests instant stream resumption via the Extensible SASL Profile (XEP-0388)'><![CDATA[
<?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<authenticate xmlns='urn:xmpp:sasl:0' mechanism='X-HT-SHA-256-ENDP'>
<initial-response></initial-response>
<inst-resume xmlns='urn:xmpp:isr:0' without-isr-token='false'/>
<resume xmlns='urn:xmpp:sm:3'
h='some-sequence-number'
previd='some-long-sm-id'/>
</inst-resume>
</authenticate>
]]></example>
<p>Note that the initiating entity SHOULD pipeline the instant
stream resumption request together with then initial
&lt;stream&gt; open element. The initiating entity is able to do
so since it already knows that the service supports ISR because it
announced an ISR key.</p>
<p>Servers MUST destroy the ISR key of a stream after an instant
stream resumption was attempted for that stream with an invalid ISR
key. Server implementations MUST implement the ISR key comparision in
linear runtime.</p>
<section3 topic='Successful Stream Resumption' anchor='isr-success'>
<example caption='Successful Instant Stream Resumption'><![CDATA[
<success xmlns='urn:xmpp:sasl:0'>z
<success-data></success-data>
<inst-resumed xmlns='urn:xmpp:isr:o'
key='006b1a29-c549-41c7-a12c-2a931822f8c0'>
<resumed xmlns='urn:xmpp:sm:3' h='354' previd='123'/>
</inst-resumed>
</success>
]]></example>
<p>On success the server replies with a &lt;success/&gt; nonza as
specified in the &xep0388;, which must include a
&lt;inst-resumed/&gt; element qualified by the 'urn:xmpp:isr:0'
namespace. This element MUST contain a <em>new</em> ISR Key found in
the 'key' attribute. It also MUST include a &lt;resumed/&gt; as
specified in &xep0198; containing the sequence number of the last by
Stream Management handled stanza in the 'h' attribute and the
'previd' attribute.</p>
<p>In case of an successful Instant Stream Resumption authenticated
by an ISR key, the server MUST immediately destroy the ISR key after
authentication, i.e., it MUST no longer be possible to perform an
ISR using that ISR key and Stream Management ID (SM-ID, see
&xep0198;) tuple.</p>
<p>After the &lt;inst-resumed/&gt; was received and has been
verified both entities MUST consider the resumed stream to be
re-established. This includes all previously negotiated stream
features like &xep0138;. It does however not include the specific
state of the features: For example in case of Stream Compression,
the dictionary used by the compression mechanism of the resumed
stream MUST NOT be considered to be restored after instant stream
resumption.</p>
<p class='box'>Note that this behavior is different from &xep0198;
stream resumption, where "outer stream" features like compression
are not restored. Since such a behavior would be counterproductive
towards the goal of this XEP, it specifies that the negotiation
state of such "outer stream" features is also restored (besides the
features which where already negotiated at ISR-time, i.e. TLS).</p>
</section3>
<section3 topic='Successful Authentication but failed Stream Resumption' anchor='isr-auth-success-resumption-failed'>
<p>If the server was able to authenticate the initiating entity
but is unable to resume the stream instantly it MUST reply with a
&lt;success/&gt; Nonza as defined in the &xep0388; containing
a &lt;inst-resume-failed/&gt; element qualified by the
'urn:xmpp:isr:0' namespace. This
&lt;inst-resume-failed/&gt; MUST contain a &lt;failed/&gt;
element as defined in &xep0198;.</p>
<example caption='Server indicates instant stream resumption failure'><![CDATA[
<success xmlns='urn:xmpp:sasl:0'>
<inst-resume-failed xmlns='urn:xmpp:isr:0'>
<failed xmlns='urn:xmpp:sm:3'
h='another-sequence-number'>
<item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</failed>
</inst-resume-failed>
</sucess>
]]></example>
<p>Instant stream resumption errors SHOULD be considered
recoverable, the initiating entity MAY continue with normal
session establishment; however, misuse of stream management MAY
result in termination of the stream. Since the initiating entity is
authenticated, it could continue with resource binding by using
&rfc6120; § 7. or &xep0386;.</p>
</section3>
<section3 topic='Multi SASL Mechanism ISR' anchor='multi-sasl-mech-isr'>
<p>As specified in the &xep0388; § 2.6.4, a single SASL
mechanism may not be sufficient for authentication. In this case,
the remote entity sends a &lt;continue/&gt; element as defined in
&xep0388; to request the local entity to perform another
SASL mechanism. Performing instant stream resumption using
multiple SASL mechanisms MUST only be done if the
'without-isr-token' attribute is set to 'true'.</p>
<example caption='Server requires Multi SASL Mechanism ISR'><![CDATA[
<continue xmlns='urn:xmpp:sasl:0'>
<additional-data>
T3B0aW9uYWwgQmFzZSA2NCBlbmNvZGVkIFNBU0wgc3VjY2VzcyBkYXRh
</additional-data>
<mechanisms>
<mechanism>HOTP-EXAMPLE</mechanism>
<mechanism>TOTP-EXAMPLE</mechanism>
<mechanisms>
</continue>
]]></example>
</section3>
<section3 topic='Failed ISR Authentication' anchor='isr-auth-failed'>
<p>If the server is unable to authenticate the initiating entity it
MUST reply with a &lt;failure/&gt; Nonza as defined in
&xep0388;.</p>
<example caption='Server indicates instant stream resumption failure'><![CDATA[
<failure xmlns='urn:xmpp:sasl:0'>
<not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
</failure>
]]></example>
<p>After the ISR authentication has failed, the initiating entity
could continue with normal authentication (&xep0388;,
…).</p>
</section3>
</section2>
</section1>
<section1 topic='The X-HT-* SASL Mechanism' anchor='ht-sasl'>
<p>This section specifies the Hashed Token (X-HT-*) SASL
mechanism. This mechanism was designed to be used with short-lived
tokens (shared secrets) for authentication. It provides hash
agility, mutual authentication and is secured by channel
binding. Since the token is not salted, and only one iteration is
used, the X-HT mechanism is not suitable to protect long-lived
shared secrets (e.g. "passwords"). You may want to look at &rfc5802;
for that.</p>
<section2 topic='The X-HT-* Family of Mechanisms' anchor='ht-sasl-mechanism-family'>
<p>Each mechanism in this family differs by the choice of the hash
algorithm and the choice of the channel binding type. Each
mechanism has a name of the form X-HT-[HA]-[CBT] where [HA] is the
"Hash Name String" of the &iana-hash-alg; registry in capital
letters, and [CBT] is one of 'ENDP' or 'UNIQ'. In case of 'ENDP',
the tls-server-end-point channel binding type is used. In case of
'UNIQ', the tls-unique channel binding type is used. For more
information about channel binding, see &rfc5929; and the
&iana-cbt; registry.</p>
<table caption='Mapping of CBT to Channel Bindings'>
<tr><th>CBT</th><th>Channel Binding Type</th></tr>
<tr><td>ENDP</td><td>tls-server-end-point</td></tr>
<tr><td>UNIQ</td><td>tls-unique</td></tr>
</table>
<p>The following table lists a few examples of X-HT-* SASL
mechanism names.</p>
<table caption='Example X-HT-* SASL mechanisms'>
<tr><th>Mechanism Name</th><th>Hash Algorithm</th><th>Channel-binding unique prefix</th></tr>
<tr><td>X-HT-SHA-512-ENDP</td><td>SHA-512 (FIPS 180-4)</td><td>tls-server-end-point</td></tr>
<tr><td>X-HT-SHA3-256-ENDP</td><td>SHA3-512 (FIPS 202)</td><td>tls-server-end-point</td></tr>
<tr><td>X-HT-SHA-512-UNIQ</td><td>SHA-512 (FIPS 180-4)</td><td>tls-unique</td></tr>
<tr><td>X-HT-SHA-256-UNIQ</td><td>SHA-256 (&rfc6920;)</td><td>tls-unique</td></tr>
</table>
</section2>
<section2 topic='The X-HT Mechanism' anchor='ht-sasl-mechanism'>
<p>The mechanism consists of a simple exchange of exactly two
messages between the initiator and responder. It starts with the
message from the initiator to the responder. This
'initiator-message' is defined as follows:</p>
<p class='box'>
initiator-message = HMAC(token, "Initiator" || cb-data)
</p>
<p>HMAC() is the function defined in &rfc2104; with H being the
chosen hash algorithm, 'cb-data' represents channel binding type
data, and 'token' are the UTF-8 (see &rfc3629;) encoded bytes of
the token String which acts as shared secret between initiator and
responder. The initiator-message MUST NOT be included in TLS 1.3
0-RTT early data (&tls13;).</p>
<p>This message is followed by a message from the responder to the
initiator. This 'responder-message' is defined as follows:</p>
<p class='box'>
responder-message = HMAC(token, "Responder" || cb-data)
</p>
<p>The initiating entity MUST verify the responder-message to
achieve mutual authentication.</p>
</section2>
<section2 topic='Security Considerations for the X-HT SASL Mechanism' anchor='ht-sasl-security-considerations'>
<p>To be secure, X-HT MUST be used over a TLS channel that has had
the session hash extension &rfc7627; negotiated, or session
resumption MUST NOT have been used.</p>
</section2>
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>Putting ISR data in <cite>TLS 1.3</cite> 0-RTT early data is
forbidden. (TODO: Shall we weaken this requirement to allow early
data?. It would be technically possible if the sender does not add
additional data, for example Stanzas, after the ISR/XEP-0388 data at
the end of the early data. And if the receiver does ensure that the
existence of such additional data is causing an ISR failure.)</p>
<p>It is of vital importance that the Instant Stream Resumption Key
is generated by a cryptographically secure random generator. See
&rfc4086; for more information about Randomness Requirements for
Security</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
<p>This document requires no interaction with &IANA;.</p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<p>The &REGISTRAR; includes 'urn:xmpp:isr:0' in its registry of protocol namespaces (see &NAMESPACES;).</p>
</section1>
<section1 topic='XML Schema' anchor='schema'>
<p>TODO: Add after the XEP leaves the 'experimental' state.</p>
</section1>
<section1 topic='Acknowledgements' anchor='acknowledgements'>
<p>Thanks to Jonas Wielicki, Thijs Alkemade, Dave Cridland,
Maxime Buquet and Alexander Würstlein for their feedback.</p>
</section1>
</xep>

2
xep.dtd
View File

@ -121,7 +121,7 @@ THE SOFTWARE.
alt CDATA ''
height CDATA ''
width CDATA ''>
<!ELEMENT link (#PCDATA)* >
<!ELEMENT link (#PCDATA | cite)* >
<!ATTLIST link url CDATA '' >
<!ELEMENT note (#PCDATA | link | tt | dfn | em | strong | cite | span | note)* >
<!ELEMENT blockquote (#PCDATA)* >