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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en"><head><title>Draft: PubSubHubbub Core 0.3 -- Working Draft</title>
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<body>
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<table summary="layout" width="66%" border="0" cellpadding="0" cellspacing="0"><tr><td><table summary="layout" width="100%" border="0" cellpadding="2" cellspacing="1">
<tr><td class="header">Draft</td><td class="header">B. Fitzpatrick</td></tr>
<tr><td class="header"> </td><td class="header">B. Slatkin</td></tr>
<tr><td class="header"> </td><td class="header">Google, Inc</td></tr>
<tr><td class="header"> </td><td class="header">M. Atkins</td></tr>
<tr><td class="header"> </td><td class="header">Six Apart Ltd.</td></tr>
<tr><td class="header"> </td><td class="header">February 8, 2010</td></tr>
</table></td></tr></table>
<h1><br />PubSubHubbub Core 0.3 -- Working Draft</h1>
<h3>Abstract</h3>
<p>An open, simple, web-scale pubsub protocol, along with an open source
reference implentation targeting Google App Engine. Notably, however,
nothing in the protocol is centralized, or Google- or App
Engine-specific. Anybody can play.
</p>
<p>As opposed to more developed (and more complex) pubsub specs like <a class='info' href='#XEP-0060'>Jabber Publish-Subscribe<span> (</span><span class='info'>Millard, P., Saint-Andre, P., and R. Meijer, “Publish-Subscribe,” .</span><span>)</span></a> [XEP‑0060] this spec's base profile
(the barrier-to-entry to speak it) is dead simple. The fancy bits required
for high-volume publishers and subscribers are optional. The base profile
is HTTP-based, as opposed to XMPP (see more on this below).
</p>
<p>To dramatically simplify this spec in several places where we had to
choose between supporting A or B, we took it upon ourselves to say "only
A", rather than making it an implementation decision.
</p>
<p>We offer this spec in hopes that it fills a need or at least advances
the state of the discussion in the pubsub space. Polling sucks. We think
a decentralized pubsub layer is a fundamental, missing layer in the
Internet architecture today and its existence, more than just enabling
the obvious lower latency feed readers, would enable many cool
applications, most of which we can't even imagine. But we're looking
forward to decentralized social networking.
</p><a name="toc"></a><br /><hr />
<h3>Table of Contents</h3>
<p class="toc">
<a href="#anchor1">1.</a>
Notation and Conventions<br />
<a href="#anchor2">2.</a>
Definitions<br />
<a href="#anchor3">3.</a>
High-level protocol flow<br />
<a href="#anchor4">4.</a>
Atom Details<br />
<a href="#discovery">5.</a>
Discovery<br />
<a href="#subscribing">6.</a>
Subscribing and Unsubscribing<br />
<a href="#anchor5">6.1.</a>
Subscriber Sends Subscription Request<br />
<a href="#verifysub">6.2.</a>
Hub Verifies Intent of the Subscriber<br />
<a href="#autorefresh">6.3.</a>
Automatic Subscription Refreshing<br />
<a href="#publishing">7.</a>
Publishing<br />
<a href="#anchor9">7.1.</a>
New Content Notification<br />
<a href="#contentfetch">7.2.</a>
Content Fetch<br />
<a href="#contentdistribution">7.3.</a>
Content Distribution<br />
<a href="#authednotify">7.4.</a>
Authenticated Content Distribution<br />
<a href="#aggregatedistribution">7.5.</a>
Aggregated Content Distribution<br />
<a href="#bestpractices">8.</a>
Best Practices<br />
<a href="#hubbestpractices">8.1.</a>
For Hubs<br />
<a href="#subbestpractices">8.2.</a>
For Subscribers<br />
<a href="#rfc.references1">9.</a>
References<br />
<a href="#anchor11">Appendix A.</a>
Specification Feedback<br />
<a href="#rfc.authors">§</a>
Authors' Addresses<br />
</p>
<br clear="all" />
<a name="anchor1"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1"></a><h3>1.
Notation and Conventions</h3>
<p>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 <a class='info' href='#RFC2119'>[RFC2119]<span> (</span><span class='info'>Bradner, B., “Key words for use in RFCs to Indicate Requirement Levels,” .</span><span>)</span></a>. Domain name examples use <a class='info' href='#RFC2606'>[RFC2606]<span> (</span><span class='info'>Eastlake, D. and A. Panitz, “Reserved Top Level DNS Names,” .</span><span>)</span></a>.
</p>
<a name="anchor2"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.2"></a><h3>2.
Definitions</h3>
<p></p>
<blockquote class="text"><dl>
<dt>Topic:</dt>
<dd>An <a class='info' href='#RFC4287'>Atom<span> (</span><span class='info'>Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” .</span><span>)</span></a> [RFC4287] or <a class='info' href='#RSS20'>RSS<span> (</span><span class='info'>Winer, D., “RSS 2.0,” .</span><span>)</span></a> [RSS20] feed <a class='info' href='#RFC3986'>URL<span> (</span><span class='info'>Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> [RFC3986]. The
unit to which one can subscribe to changes. This spec currently only
addresses feed URLs that require no additional authorization
headers.
</dd>
<dt>Hub ("the hub"):</dt>
<dd>The server (<a class='info' href='#RFC3986'>URL<span> (</span><span class='info'>Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> [RFC3986]) which implements both sides of this
protocol. We have implemented this and are running a server at <a href='http://pubsubhubbub.appspot.com/'>http://pubsubhubbub.appspot.com</a> that is, at least for now, open
to anybody for use, as either a publisher or subscriber. Any hub MAY
implement its own policies on who can use it.
</dd>
<dt>Publisher:</dt>
<dd>An owner of a topic. Notifies the hub when
the topic feed has been updated. It just notifies that it <em>has</em> been updated, but not how. As in almost all
pubsub systems, the publisher is unaware of the subscribers, if any.
Other pubsub systems might call the publisher the "source".
</dd>
<dt>Subscriber:</dt>
<dd>An entity (person or program) that wants
to be notified of changes on a topic. The subscriber must be
directly network-accessible and is identified by its Subscriber
Callback URL.
</dd>
<dt>Subscription:</dt>
<dd>A unique relation to a topic by a
subscriber that indicates it should receive updates for that topic. A
subscription's unique key is the tuple (Topic URL, Subscriber Callback
URL). Subscriptions may (at the hub's decision) have expiration times
akin to DHCP leases which must be periodically renewed.
</dd>
<dt>Subscriber Callback URL:</dt>
<dd>The <a class='info' href='#RFC3986'>URL<span> (</span><span class='info'>Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> [RFC3986] at which a subscriber wishes to receive
notifications.
</dd>
<dt>Event:</dt>
<dd>An event that causes updates to multiple topics.
For each event that happens (e.g. "Brad posted to the Linux
Community."), multiple topics could be affected (e.g. "Brad posted."
and "Linux community has new post"). Publisher events cause topics to
be updated and the hub looks up all subscriptions for affected topics,
sending out notifications to subscribers.
</dd>
<dt>Notification:</dt>
<dd>A payload describing how a topic's
contents have changed. This difference (or "delta") is computed by the
hub and sent to all subscribers. The format of the notification will
be an Atom or RSS feed served by the publisher with only those entries
present which are new or have changed. The notification can be the
result of a publisher telling the hub of an update, or the hub
proactively polling a topic feed, perhaps for a subscriber subscribing
to a topic that's not pubsub-aware. Note also that a notification to a
subscriber can be a payload consisting of updates for multiple topics.
Hubs MAY choose to send multi-topic notifications as an optimization
for heavy subscribers, but subscribers MUST understand them. See <a class='info' href='#contentdistribution'>Section 7.3<span> (</span><span class='info'>Content Distribution</span><span>)</span></a> for format details.
</dd>
</dl></blockquote>
<a name="anchor3"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.3"></a><h3>3.
High-level protocol flow</h3>
<p>(This section is non-normative.)
</p>
<p></p>
<ul class="text">
<li>Publishers POST a ping to their hub(s) URLs when their topic(s)
change.
</li>
<li>Subscribers POST to one or more of the advertised hubs for a
topic they're interested in. Alternatively, some hubs may offer
auto-polling capability, to let {their,any} subscribers subscribe to
topics which don't advertise a hub.
</li>
<li>The hub caches minimal metadata (id, data, entry digest) about
each topic's previous state. When the hub re-fetches a topic feed (on
its own initiative or as a result of a publisher's ping) and finds a
delta, it enqueues a notification to all registered subscribers.
</li>
</ul>
<a name="anchor4"></a><br /><hr />
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<a name="rfc.section.4"></a><h3>4.
Atom Details</h3>
<p>Notification and source formats will be <a class='info' href='#RFC4287'>Atom<span> (</span><span class='info'>Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” .</span><span>)</span></a> [RFC4287] or <a class='info' href='#RSS20'>RSS<span> (</span><span class='info'>Winer, D., “RSS 2.0,” .</span><span>)</span></a> [RSS20]. The
Publisher makes the decision as to include full body, truncated body, or
meta data of most recent event(s). One of:
</p>
<p></p>
<ul class="text">
<li>URL + metadata
</li>
<li>URL + metadata + truncated
</li>
<li>URL + metadata + full
</li>
</ul>
<p>The trade-off between including all content in outgoing notifications
or having the thundering herd (by clients who fetch the <tt>//atom:feed/entry/link</tt> in response to a notification)
is up to the publisher. Entries of the most recent events (for recipient
to know whether or not they'd missed any recent items-- like TCP SACK) MAY
be provided as context. Some examples of Atom feed entries follow.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre><?xml version="1.0"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<!-- Normally here would be source, title, author, id, etc ... -->
<link rel="hub" href="http://myhub.example.com/endpoint" />
<link rel="self" href="http://publisher.example.com/happycats.xml" />
<updated>2008-08-11T02:15:01Z</updated>
<!-- Example of a full entry. -->
<entry>
<title>Heathcliff</title>
<link href="http://publisher.example.com/happycat25.xml" />
<id>http://publisher.example.com/happycat25.xml</id>
<updated>2008-08-11T02:15:01Z</updated>
<content>
What a happy cat. Full content goes here.
</content>
</entry>
<!-- Example of an entity that isn't full/is truncated. This is implied
by the lack of a <content> element and a <summary> element instead. -->
<entry >
<title>Heathcliff</title>
<link href="http://publisher.example.com/happycat25.xml" />
<id>http://publisher.example.com/happycat25.xml</id>
<updated>2008-08-11T02:15:01Z</updated>
<summary>
What a happy cat!
</summary>
</entry>
<!-- Meta-data only; implied by the lack of <content> and
<summary> elements. -->
<entry>
<title>Garfield</title>
<link rel="alternate" href="http://publisher.example.com/happycat24.xml" />
<id>http://publisher.example.com/happycat25.xml</id>
<updated>2008-08-11T02:15:01Z</updated>
</entry>
<!-- Context entry that's meta-data only and not new. -->
<entry>
<title>Nermal</title>
<link rel="alternate" href="http://publisher.example.com/happycat23s.xml" />
<id>http://publisher.example.com/happycat25.xml</id>
<updated>2008-07-10T12:28:13Z</updated>
</entry>
</feed>
</pre></div>
<a name="discovery"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5"></a><h3>5.
Discovery</h3>
<p>A potential subscriber initiates discovery by retrieving the feed to
which it wants to subscribe. A feed that acts as a topic as per this
specification MUST publish, as a child of <tt>//atom:feed</tt> or <tt>//rss:rss/channel</tt> , an <tt>atom:link</tt> element whose <tt>rel</tt> attribute has the value <tt>hub</tt> and whose <tt>href</tt>
attribute contains the hub's endpoint URL. Feeds MAY contain multiple
<tt>atom:link[@rel="hub"]</tt> elements if the
publisher wishes to notify multiple hubs. When a potential subscriber
encounters one or more such links, that subscriber MAY subscribe to the
feed using one or more hubs URLs as described in <a class='info' href='#subscribing'>Section 6<span> (</span><span class='info'>Subscribing and Unsubscribing</span><span>)</span></a>.
</p>
<p>Example:
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre><?xml version="1.0"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<!-- Normally here would be source, title, author, id, etc ... -->
<link rel="hub" href="https://myhub.example.com/endpoint" />
<link rel="self" href="http://publisher.example.com/topic.xml" />
....
<entry>
....
</entry>
<entry>
....
</entry>
</feed>
</pre></div>
<p>Hubs MUST use the same URL for both the publishing and subscribing
interfaces, which is why only a single <tt>atom:link</tt>
element is required to declare a hub. Publishers SHOULD use <a class='info' href='#RFC2818'>HTTPS<span> (</span><span class='info'>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a> [RFC2616] in their hubs' discovery URLs. However,
subscribers that do not support <a class='info' href='#RFC2818'>HTTPS<span> (</span><span class='info'>Rescorla, E., “HTTP Over TLS,” May 2000.</span><span>)</span></a> [RFC2818] MAY
try to fallback to <a class='info' href='#RFC2616'>HTTP<span> (</span><span class='info'>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a> [RFC2616], which MAY work
depending on the hub's policy.
</p>
<a name="subscribing"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.6"></a><h3>6.
Subscribing and Unsubscribing</h3>
<p>Subscribing to a topic URL consists of three parts that may occur
immediately in sequence or have a delay.
</p>
<p></p>
<ul class="text">
<li>Requesting a subscription using the hub
</li>
<li>Confirming the subscription was actually desired
</li>
<li>Periodically reconfirming the subscription is still active
</li>
</ul>
<p>Unsubscribing works in the same way, except with a single parameter
changed to indicate the desire to unsubscribe.
</p>
<a name="anchor5"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.6.1"></a><h3>6.1.
Subscriber Sends Subscription Request</h3>
<p>Subscription is initiated by the subscriber making an <a class='info' href='#RFC2616'>HTTP<span> (</span><span class='info'>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a> [RFC2616] POST request to the hub URL. This request
has a Content-Type of <tt>application/x-www-form-urlencoded</tt> (described in
Section 17.13.4 of <a class='info' href='#W3C.REC-html401-19991224'>[W3C.REC‑html401‑19991224]<span> (</span><span class='info'>Raggett, D., Hors, A., and I. Jacobs, “HTML 4.01 Specification,” December 1999.</span><span>)</span></a>) and the
following parameters in its body:
</p>
<p></p>
<blockquote class="text"><dl>
<dt>hub.callback</dt>
<dd>REQUIRED. The subscriber's callback URL where notifications should be delivered.
</dd>
<dt>hub.mode</dt>
<dd>REQUIRED. The literal string "subscribe" or
"unsubscribe", depending on the goal of the request.
</dd>
<dt>hub.topic</dt>
<dd>REQUIRED. The topic URL that the subscriber
wishes to subscribe to.
</dd>
<dt>hub.verify</dt>
<dd>REQUIRED. Keyword describing verification
modes supported by this subscriber, as described below. This
parameter may be repeated to indicate multiple supported modes.
</dd>
<dt>hub.lease_seconds</dt>
<dd>OPTIONAL. Number of seconds for
which the subscriber would like to have the subscription active. If
not present or an empty value, the subscription will be permanent
(or active until <a class='info' href='#autorefresh'>automatic
refreshing<span> (</span><span class='info'>Automatic Subscription Refreshing</span><span>)</span></a> removes the subscription). Hubs MAY choose to
respect this value or not, depending on their own policies. This
parameter MAY be present for unsubscription requests and MUST be
ignored by the hub in that case.
</dd>
<dt>hub.secret</dt>
<dd>OPTIONAL. A subscriber-provided secret
string that will be used to compute an HMAC digest for <a class='info' href='#authednotify'>authorized content distribution<span> (</span><span class='info'>Authenticated Content Distribution</span><span>)</span></a>. If not
supplied, the HMAC digest will not be present for content
distribution requests. This parameter SHOULD only be specified when
the request was made over <a class='info' href='#RFC2818'>HTTPS<span> (</span><span class='info'>Rescorla, E., “HTTP Over TLS,” May 2000.</span><span>)</span></a> [RFC2818]. This
parameter MUST be less than 200 bytes in length.
</dd>
<dt>hub.verify_token</dt>
<dd>OPTIONAL. A subscriber-provided
opaque token that will be echoed back in the verification request to
assist the subscriber in identifying which subscription request is
being verified. If this is not included, no token will be included
in the verification request.
</dd>
</dl></blockquote>
<p>The following keywords are supported for hub.verify:
</p>
<p></p>
<blockquote class="text"><dl>
<dt>sync</dt>
<dd>The subscriber supports synchronous
verification, where the verification request must occur before the
subscription request's HTTP response is returned.
</dd>
<dt>async</dt>
<dd>The subscriber supports asynchronous
verification, where the verification request may occur at a later
point after the subscription request has returned.
</dd>
</dl></blockquote>
<p>Where repeated keywords are used, their order indicates the
subscriber's order of preference. Subscribers MUST use at least one of
the modes indicated in the list above, but MAY include additional
keywords defined by extension specifications. Hubs MUST ignore verify
mode keywords that they do not understand.
</p>
<p>Hubs MUST ignore additional request parameters they do not
understand.
</p>
<p>Hubs MUST allow subscribers to re-request subscriptions that are
already activate. Each subsequent request to a hub to subscribe or
unsubscribe MUST override the previous subscription state for a
specific topic URL and callback URL combination once the action is
verified. Any failures to confirm the subscription action MUST leave
the subscription state unchanged. This is required so subscribers can
renew their subscriptions before the lease seconds period is over
without any interruption.
</p>
<a name="anchor6"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.6.1.1"></a><h3>6.1.1.
Subscription Parameter Details</h3>
<p>The topic and callback URLs MUST NOT contain an anchor fragment.
These URLs MAY use <a class='info' href='#RFC2616'>HTTP<span> (</span><span class='info'>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a> [RFC2616] or <a class='info' href='#RFC2818'>HTTPS<span> (</span><span class='info'>Rescorla, E., “HTTP Over TLS,” May 2000.</span><span>)</span></a> [RFC2818] schemes. These URLs MAY have port
numbers specified; however, hubs MAY choose to disallow certain ports
based on their own policies (e.g., security) and return errors for
these requests. The topic URL can otherwise be free-form following
<a class='info' href='#RFC3986'>the URI spec<span> (</span><span class='info'>Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> [RFC3986]. Hubs MUST always decode
non-reserved characters for these URL parameters; see section 2.4 on
<em>"When to Encode or Decode"</em> in <a class='info' href='#RFC3986'>the URI spec<span> (</span><span class='info'>Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> [RFC3986].
</p>
<p>The callback URL MAY contain arbitrary query string parameters
(e.g., <tt>?foo=bar&red=fish</tt>). Hubs MUST
preserve the query string during subscription verification by
appending new parameters to the end of the list using the <tt>&</tt> (ampersand) character to join. Existing
parameters with names that overlap with those used by verification
requests will not be overwritten; Hubs MUST only append verification
parameters to the existing list, if any. For event notification, the
callback URL will be POSTed to including any query-string parameters
in the URL portion of the request, not as POST body parameters.
</p>
<p>Subscribers MAY choose to use <a class='info' href='#RFC2818'>HTTPS<span> (</span><span class='info'>Rescorla, E., “HTTP Over TLS,” May 2000.</span><span>)</span></a> [RFC2818]
for their callback URLs if they care about the privacy of
notifications as they come over the wire from the Hub. The use of
mechanisms (such as XML signatures) to verify the integrity of
notifications coming from the original publisher is out of the scope
of this specification.
</p>
<a name="anchor7"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.6.1.2"></a><h3>6.1.2.
Subscription Response Details</h3>
<p>The hub MUST respond to a subscription request with an HTTP 204 "No
Content" response to indicate that the request was verified and that
the subscription is active. If the subscription has yet to be verified
(i.e., the hub is using asynchronous verification), the hub MUST
respond with a 202 "Accepted" code.
</p>
<p>If a hub finds any errors in the subscription request, an
appropriate HTTP error response code (4xx or 5xx) MUST be returned. In
the event of an error, hubs SHOULD return a description of the error
in the response body as plain text. Hubs MAY decide to reject some
callback URLs or topic URLs based on their own policies (e.g., domain
authorization, topic URL port numbers).
</p>
<p>In synchronous mode, the verification (<a class='info' href='#verifysub'>Section 6.2<span> (</span><span class='info'>Hub Verifies Intent of the Subscriber</span><span>)</span></a>) MUST be completed before the hub returns a
response. In asynchronous mode, the verification MAY be deferred until
a later time. This is useful to enable hubs to defer work; this could
allow them to alleviate servers under heavy load or do verification
work in batches.
</p>
<a name="verifysub"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.6.2"></a><h3>6.2.
Hub Verifies Intent of the Subscriber</h3>
<p>In order to prevent an attacker from creating unwanted subscriptions
on behalf of a subscriber (or unsubscribing desired ones), a hub must
ensure that the subscriber did indeed send the subscription request.
</p>
<p>The hub verifies a subscription request by sending an <a class='info' href='#RFC2616'>HTTP<span> (</span><span class='info'>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a> [RFC2616] GET request to the subscriber's callback
URL as given in the subscription request. This request has the following
query string arguments appended (format described in Section 17.13.4 of
<a class='info' href='#W3C.REC-html401-19991224'>[W3C.REC‑html401‑19991224]<span> (</span><span class='info'>Raggett, D., Hors, A., and I. Jacobs, “HTML 4.01 Specification,” December 1999.</span><span>)</span></a>):
</p>
<p></p>
<blockquote class="text"><dl>
<dt>hub.mode</dt>
<dd>REQUIRED. The literal string "subscribe" or
"unsubscribe", which matches the original request to the hub from
the subscriber.
</dd>
<dt>hub.topic</dt>
<dd>REQUIRED. The topic URL given in the
corresponding subscription request.
</dd>
<dt>hub.challenge</dt>
<dd>REQUIRED. A hub-generated, random string
that MUST be echoed by the subscriber to verify the
subscription.
</dd>
<dt>hub.lease_seconds</dt>
<dd>REQUIRED/OPTIONAL. The
hub-determined number of seconds that the subscription will stay
active before expiring, measured from the time the verification
request was made from the hub to the subscriber. Hubs MUST supply
this parameter for subscription requests. This parameter MAY be
present for unsubscribe requests and MUST be ignored by subscribers
during unsubscription.
</dd>
<dt>hub.verify_token</dt>
<dd>OPTIONAL. The subscriber-provided
opaque token from the corresponding subscription request, if one was
provided.
</dd>
</dl></blockquote>
<a name="anchor8"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.6.2.1"></a><h3>6.2.1.
Verification Details</h3>
<p>The subscriber MUST confirm that the <tt>hub.topic
</tt> and <tt>hub.verify_token</tt> correspond
to a pending subscription or unsubscription that it wishes to carry
out. If so, the subscriber MUST respond with an HTTP success (2xx)
code with a response body equal to the <tt>hub.challenge</tt> parameter. If the subscriber does
not agree with the action, the subscriber MUST respond with a 404 "Not
Found" response.
</p>
<p>For synchronous verification, the hub MUST consider other server
response codes (3xx, 4xx, 5xx) to mean that the verification request
has immediately failed and no retries should occur. If the subscriber
returns an HTTP success (2xx) but the content body does not match the
<tt>hub.challenge</tt> parameter, the hub MUST also
consider verification to have failed.
</p>
<p>For asynchronous verification, the hub MUST consider other server
response codes (3xx, 4xx, and 5xx) to mean that the subscription
action was <em>temporarily</em> not verified. If
the subscriber returns an HTTP success (2xx) but the content body does
not match the <tt>hub.challenge</tt> parameter, the
hub MUST consider this to be a <em>temporary</em>
failure and retry. The hub SHOULD retry verification a reasonable
number of times over the course of a longer time period (e.g., 6
hours) until a definite acknowledgement (positive or negative) is
received. If a definite response still cannot be determined after this
retry period, the subscription action verification MUST be abandoned,
leaving the previous subscription state.
</p>
<p>Hubs MAY make the <tt>hub.lease_seconds</tt>
equal to the period the subscriber passed in their subscription
request but MAY change the value depending on the hub's policies. To
sustain a temporary subscription, the subscriber MUST re-request the
subscription on the hub before <tt>hub.lease_seconds</tt> seconds has elapsed. For
permanent subscriptions with no <tt>hub.lease_seconds</tt> value specified, the behavior
is different as described in the section on <a class='info' href='#autorefresh'>automatic subscription refreshing<span> (</span><span class='info'>Automatic Subscription Refreshing</span><span>)</span></a>.
</p>
<a name="autorefresh"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.6.3"></a><h3>6.3.
Automatic Subscription Refreshing</h3>
<p>Before a subscription expires (i.e., before <tt>hub.lease_seconds</tt> elapses), Hubs MUST recheck with
subscribers to see if a continued subscription is desired. Hubs do this
by sending the subscriber a <a class='info' href='#verifysub'>verification
request<span> (</span><span class='info'>Hub Verifies Intent of the Subscriber</span><span>)</span></a> with <tt>hub.mode</tt> equal to
subscribe. This request MUST match the original verification request
sent to the subscriber (but with a new <tt>hub.challenge</tt>).
</p>
<p>The response codes returned by the subscriber MUST be interpreted the
same way as during a subscriber-initiated verification flow. However,
this refresh request MUST behave like an initial subscription request;
this means that if an auto-refresh response from the subscriber
constantly returns an error, the hub MUST give up on the subscription
verification action altogether and remove the subscription.
</p>
<p>In the case of permanent subscriptions (with no <tt>hub.lease_seconds</tt> specified in the original
request), the <tt>hub.lease_seconds</tt> value
supplied by the hub in the verification request to the subscriber SHOULD
represent how many seconds until the hub expects it will next initiate
automatic subscription refreshing to ensure that the subscriber is still
interested in the topic. This behavior provides the best of both worlds:
maximum simplicity of the subscriber through infinitely-long
subscriptions, but still garbage collectable subscriptions for hub
hygiene.
</p>
<a name="publishing"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.7"></a><h3>7.
Publishing</h3>
<p>A publisher pings the hub with the topic URL(s) which have been updated
and the hub schedules those topics to be fetched and delivered. Because
it's just a ping to notify the hub of the topic URL (without a payload),
no authentication from the publisher is required.
</p>
<a name="anchor9"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.7.1"></a><h3>7.1.
New Content Notification</h3>
<p>When new content is added to a feed, a notification is sent to
the hub by the publisher. The hub MUST accept a POST request to
the hub URL containing the notification. This request MUST have a
Content-Type of <tt>application/x-www-form-urlencoded
</tt> (described in Section 17.13.4 of <a class='info' href='#W3C.REC-html401-19991224'>[W3C.REC‑html401‑19991224]<span> (</span><span class='info'>Raggett, D., Hors, A., and I. Jacobs, “HTML 4.01 Specification,” December 1999.</span><span>)</span></a>) and the following parameters in
its body:
</p>
<p></p>
<blockquote class="text"><dl>
<dt>hub.mode</dt>
<dd>REQUIRED. The literal string "publish".
</dd>
<dt>hub.url</dt>
<dd>REQUIRED. The topic URL of the topic that
has been updated. This field may be repeated to indicate multiple
topics that have been updated.
</dd>
</dl></blockquote>
<p>The new content notification is a signal to the hub that there is new
content available. The hub SHOULD arrange for a content fetch request
(<a class='info' href='#contentfetch'>Section 7.2<span> (</span><span class='info'>Content Fetch</span><span>)</span></a>) to be performed in the near future
to retrieve the new content. If the notification was acceptable, the hub
MUST return a 204 No Content response. If the notification is not
acceptable for some reason, the hub MUST return an appropriate HTTP
error response code (4xx and 5xx). Hubs MUST return a 204 No Content
response even when they do not have any subscribers for all of the
specified topic URLs.
</p>
<a name="contentfetch"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.7.2"></a><h3>7.2.
Content Fetch</h3>
<p>When the hub wishes to retrieve new content for a topic, the hub
sends an <a class='info' href='#RFC2616'>HTTP<span> (</span><span class='info'>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a> [RFC2616] GET request to the topic
URL. The hub MUST follow HTTP redirects. The Hub SHOULD use best
practices for caching headers in its requests (e.g., <tt>If-None-Match</tt>, <tt>If-Modified-Since</tt>).
</p>
<p>The request SHOULD include the header field <tt>User-Agent</tt> in the form expected by <a class='info' href='#RFC2616'>HTTP<span> (</span><span class='info'>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a> [RFC2616]. The header MAY have one or more additional
suffixes like <tt>(example.com; 20 subscribers)</tt>
to indicate the number of subscriptions the hub has active for this
topic.
</p>
<p>If present, the first suffix MUST indicate the total number of
subscriptions the hub has aggregated across all subscriber domains by
means of the <tt>X-Hub-On-Behalf-Of</tt> header
(<a class='info' href='#contentdistribution'>Section 7.3<span> (</span><span class='info'>Content Distribution</span><span>)</span></a>). Any additional suffixes
indicate a breakdown of subscriber counts across subscriber domains.
This allows content publishers to distinguish the source of their
subscriber counts and mitigate subscriber count spam. An example header
could be (ignoring line-wrapping):
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
User-Agent: MyHub (+http://hub.example.com; 26 subscribers)
(sub.example.com; 4 subscribers)
(other-sub.example.com; 22 subscribers)
</pre></div>
<p>If, after a content fetch, the hub determines that the topic feed
content has changed, the hub MUST send information about the changes to
each of the subscribers to the topic (<a class='info' href='#contentdistribution'>Section 7.3<span> (</span><span class='info'>Content Distribution</span><span>)</span></a>). Hubs MUST consider new feed
entries, updated entries, or changes to the surrounding feed document as
significant content changes that require content distribution.
</p>
<a name="contentdistribution"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.7.3"></a><h3>7.3.
Content Distribution</h3>
<p>A content distribution request is an <a class='info' href='#RFC2616'>HTTP<span> (</span><span class='info'>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a> [RFC2616] POST request from hub to the subscriber's
callback URL with the list of new and changed entries. This request MUST
have a <tt>Content-Type</tt> of <tt>application/atom+xml</tt> when the request body is an
<a class='info' href='#RFC4287'>Atom<span> (</span><span class='info'>Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” .</span><span>)</span></a> [RFC4287] feed document, or a <tt>Content-Type</tt> of <tt>application/rss+xml</tt> when the request body is an
<a class='info' href='#RSS20'>RSS<span> (</span><span class='info'>Winer, D., “RSS 2.0,” .</span><span>)</span></a> [RSS20] feed document. The behavior for other
content types is not yet defined. Hubs MAY transform the content type
and request body as desired (e.g., for language translation).
</p>
<p>If the document represents a single feed being replicated for the
subscriber, then the feed-level elements SHOULD be preserved aside from
the <tt>atom:entry</tt> or <tt>rss:item</tt> elements. However, the <tt>atom:id</tt> element MUST be reproduced exactly. The
other <tt>atom:updated</tt> and <tt>atom:title</tt> elements required by the Atom
specification SHOULD be present. Each <tt>atom:entry</tt> or <tt>rss:item</tt>
element in the feed contains the content from an entry in the single
topic that the subscriber has an active subscription for. Essentially,
in the single feed case the subscriber will receive a feed document
that looks like the original but with old content removed.
</p>
<p>The successful response from the subscriber's callback URL MUST be an
HTTP success (2xx) code. The hub MUST consider all other subscriber
response codes as failures; that means subscribers MUST not use HTTP
redirects for moving subscriptions. The response body from the
subscriber MUST be ignored by the hub. Hubs SHOULD retry notifications
repeatedly until successful (up to some reasonable maximum over a
reasonable time period). Subscribers SHOULD respond to notifications as
quickly as possible; their success response code SHOULD only indicate
receipt of the message, not acknowledgment that it was successfully
processed by the subscriber.
</p>
<p>The subscriber's callback response MAY include the header field
<tt>X-Hub-On-Behalf-Of</tt> with an integer value,
possibly approximate, representing the number of subscribers on behalf
of which this feed notification was delivered. This value SHOULD be
aggregated by the hub across all subscribers and used to provide the
subscriber counts in the <tt>User-Agent</tt> header
field sent to publishers (<a class='info' href='#contentfetch'>Section 7.2<span> (</span><span class='info'>Content Fetch</span><span>)</span></a>). Hubs MAY
ignore or respect <tt>X-Hub-On-Behalf-Of</tt> values
from subscribers depending on their own policies (i.e., to prevent
spam).
</p>
<a name="authednotify"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.7.4"></a><h3>7.4.
Authenticated Content Distribution</h3>
<p>If the subscriber supplied a value for <tt>hub.secret</tt> in their subscription request, the hub
MUST generate an HMAC signature of the payload and include that
signature in the request headers of the content distribution request.
The <tt>X-Hub-Signature</tt> header's value MUST be
in the form <tt>sha1=signature</tt> where <tt>signature</tt> is a 40-byte, hexadecimal representation
of a <a class='info' href='#RFC3174'>SHA1 signature<span> (</span><span class='info'>Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” September 2001.</span><span>)</span></a> [RFC3174]. The signature MUST be
computed using the <a class='info' href='#RFC2104'>HMAC algorithm<span> (</span><span class='info'>Krawczyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” .</span><span>)</span></a> [RFC2104] with the
request body as the data and the <tt>hub.secret</tt>
as the key.
</p>
<p>When subscribers receive a content distribution request with the
<tt>X-Hub-Signature</tt> header specified, they
SHOULD recompute the SHA1 signature with the shared secret using the
same method as the hub. If the signature does not match, subscribers
MUST still return a 2xx success response to acknowledge receipt, but
locally ignore the message as invalid. Using this technique along with
HTTPS for subscription requests enables simple subscribers to receive
authenticated content delivery from hubs without the need for
subscribers to run an HTTPS server.
</p>
<a name="aggregatedistribution"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.7.5"></a><h3>7.5.
Aggregated Content Distribution</h3>
<p>For Atom feeds only. Pending further review.
</p>
<p>When a subscriber indicates the same callback URL is used for
multiple subscriptions, hubs MAY choose to combine content delivery
requests into a single payload containing an aggregated set of feeds.
This bulk delivery results in fewer requests and more efficient
distribution. If the subscriber indicated a <tt>hub.secret</tt> value for these overlapping
subscriptions, the secret MUST also be the same for all subscriptions.
This allows the hub to generate a single <tt>X-Hub-Signature</tt> header to sign the entire payload.
Hubs MUST return an error response (4xx, 5xx) for subscription
requests with overlapping callback URLs and different secret values.
</p>
<p>With an aggregated set of feeds, the hub SHOULD reproduce all of the
elements from the source feed <em>inside</em> the
corresponding <tt>atom:entry</tt> in the content
distribution request by using an <tt>atom:source</tt>
element. However, the <tt>atom:id</tt> value MUST be
reproduced exactly within the source element. If the source entry does
not have an <tt>atom:source</tt> element, the hub
MUST create an <tt>atom:source</tt> element
containing the <tt>atom:id</tt> element. The hub
SHOULD also include the <tt>atom:title</tt> element
and an <tt>atom:link</tt> element with <tt>rel="self"</tt> values that are functionally
equivalent to the corresponding elements in the original topic feed.
</p>
<p>Example aggregated feed:
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre><?xml version="1.0"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>Aggregated feed</title>
<updated>2008-08-11T02:17:44Z</updated>
<id>http://myhub.example.com/aggregated?1232427842-39823</id>
<entry>
<source>
<id>http://www.example.com/foo</id>
<link rel="self" href="http://publisher.example.com/foo.xml" />
<author>
<name>Mr. Bar</name>
</author>
</source>
<title>Testing Foo</title>
<link href="http://publisher.example.com/foo24.xml" />
<id>http://publisher.example.com/foo24.xml</id>
<updated>2008-08-11T02:15:01Z</updated>
<content>
This is some content from the user named foo.
</content>
</entry>
<entry>
<source>
<id>http://www.example.com/bar</id>
<link rel="self" href="http://publisher.example.com/bar.xml" />
<author>
<name>Mr. Bar</name>
</author>
</source>
<title>Testing Bar</title>
<link href="http://publisher.example.com/bar18.xml" />
<id>http://publisher.example.com/bar18.xml</id>
<updated>2008-08-11T02:17:44Z</updated>
<content>
Some data from the user named bar.
</content>
</entry>
<feed>
</pre></div>
<a name="bestpractices"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.8"></a><h3>8.
Best Practices</h3>
<p>(This section is non-normative.)
</p>
<a name="hubbestpractices"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.8.1"></a><h3>8.1.
For Hubs</h3>
<p>The vast majority of feeds on the web have multiple <em>variants</em> that contain essentially the same content
but appear on different URLs. A common set of variants is <tt>http://example.com/feed?format=atom</tt> and <tt>http://example.com/feed?format=rss</tt>. In practice,
these URLs also fail to set their <tt>//atom:feed/link[@rel="self"]</tt> values properly,
meaning it's difficult for subscribers to discover which feed URL they
truly wanted. Making matters worse, feeds often have redirects
(temporary or permanent) to new hosting locations, analytics providers,
or other feed-processors. To solve this, it's important for Hub
implementations to determine feed URL equivalences using heuristics.
Examples: follow all feed URLs redirects and see if they end up at the
same location; use overlaps in feed HTML alternate links; use the <tt>atom:id</tt> value across all domains. This specific
problem is considered out of scope of this specification but this
section is meant as a reminder to implementors that feed URL aliasing is
an important issue that hubs should address instead of putting
the burden on publishers and subscribers.
</p>
<a name="subbestpractices"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.8.2"></a><h3>8.2.
For Subscribers</h3>
<p>The <tt>hub.verify_token</tt> parameter in
subscription requests enables subscribers to verify the identity and
intent of the hub making the verification request. Subscribers should
use the token to retrieve internal state to ensure the subscription
request outcome is what they intended.
</p>
<a name="rfc.references1"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<h3>9. References</h3>
<table width="99%" border="0">
<tr><td class="author-text" valign="top"><a name="RFC2104">[RFC2104]</a></td>
<td class="author-text">Krawczyk, H., Bellare, M., and R. Canetti, “<a href="http://tools.ietf.org/html/rfc2104">HMAC: Keyed-Hashing for Message Authentication</a>,” RFC 2104.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC2119">[RFC2119]</a></td>
<td class="author-text">Bradner, B., “<a href="http://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement
Levels</a>,” RFC 2119.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC2606">[RFC2606]</a></td>
<td class="author-text">Eastlake, D. and A. Panitz, “<a href="http://tools.ietf.org/html/rfc2606">Reserved Top Level DNS Names</a>,” RFC 2606.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC2616">[RFC2616]</a></td>
<td class="author-text">Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “<a href="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>,” RFC 2616.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC2818">[RFC2818]</a></td>
<td class="author-text">Rescorla, E., “<a href="http://tools.ietf.org/html/rfc2818">HTTP Over TLS</a>,” RFC 2818, May 2000 (<a href="ftp://ftp.isi.edu/in-notes/rfc2818.txt">TXT</a>).</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC3174">[RFC3174]</a></td>
<td class="author-text">Eastlake, D. and P. Jones, “<a href="http://tools.ietf.org/html/rfc3174">US Secure Hash Algorithm 1 (SHA1)</a>,” RFC 3174, September 2001 (<a href="ftp://ftp.isi.edu/in-notes/rfc3174.txt">TXT</a>).</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC3986">[RFC3986]</a></td>
<td class="author-text">Berners-Lee, T., “<a href="http://tools.ietf.org/html/rfc3986">Uniform Resource Identifiers (URI): Generic Syntax</a>,” RFC 3986.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC4287">[RFC4287]</a></td>
<td class="author-text">Nottingham, M., Ed. and R. Sayre, Ed., “<a href="http://tools.ietf.org/html/rfc4287">The Atom Syndication Format</a>,” RFC 4287 (<a href="http://xml.resource.org/public/rfc/html/rfc4287.html">HTML</a>).</td></tr>
<tr><td class="author-text" valign="top"><a name="RSS20">[RSS20]</a></td>
<td class="author-text">Winer, D., “<a href="http://www.rssboard.org/rss-specification">RSS 2.0</a>.”</td></tr>
<tr><td class="author-text" valign="top"><a name="W3C.REC-html401-19991224">[W3C.REC-html401-19991224]</a></td>
<td class="author-text">Raggett, D., Hors, A., and I. Jacobs, “<a href="http://www.w3.org/TR/1999/REC-html401-19991224">HTML 4.01 Specification</a>,” World Wide Web Consortium Recommendation REC-html401-19991224, December 1999 (<a href="http://www.w3.org/TR/1999/REC-html401-19991224">HTML</a>).</td></tr>
<tr><td class="author-text" valign="top"><a name="XEP-0060">[XEP-0060]</a></td>
<td class="author-text">Millard, P., Saint-Andre, P., and R. Meijer, “<a href="http://www.xmpp.org/extensions/xep-0060.html">Publish-Subscribe</a>,” XSF XEP 0060.</td></tr>