WebSub provides a common mechanism for communication between publishers of any kind of Web content and their subscribers, based on HTTP web hooks. Subscription requests are relayed through hubs, which validate and verify the request. Hubs then distribute new and updated content to subscribers when it becomes available. WebSub was previously known as PubSubHubbub.
This document was published by theSocial Web Working Group as a Recommendation. Comments regarding this document are welcome. All interested parties are invited to provide implementation and bug reports and other comments through the Working Group'sIssue tracker. These will be discussed by theSocial Web Community Group and considered in any future versions of this specification.
An HTTP [RFC7230] (or HTTPS [RFC2818]) resource URL. The unit to which one can subscribe to changes.
Hub ("the hub")
The server (URL [URL]) which implements both sides of this protocol. Any hubMAY implement its own policies on who can use it.
Publisher
An owner of a topic. Notifies the hub when the topic feed has been updated. As in almost all pubsub systems, the publisher is unaware of the subscribers, if any. Other pubsub systems might call the publisher the "source".
Subscriber
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.
Subscription
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.
Subscriber Callback URL
The URL [URL] at which a subscriber wishes to receive content distribution requests.
Event
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, delivering the content to subscribers.
Content Distribution Notification /(Content Distribution Request)
A payload describing how a topic's contents have changed, or the full updated content. Depending on the topic's content type, the difference (or "delta") may be computed by the hub and sent to all subscribers.
2.High-level protocol flow
(This section is non-normative.)
Subscribers discover the hub of a topic URL, and makes a POST to one or more of the advertised hubs in order to receive updates when the topic changes.
Publishers notify their hub(s) URLs when their topic(s) change.
When the hub identifies a change in the topic, it sends a content distribution notification to all registered subscribers.
Earlier versions of this protocol were called PubSubHubbub:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
3.1Conformance Classes
WebSub describes three roles: publishers, subscribers and hubs. This section describes the conformance criteria for each role.
Publishers
A conforming publisherMUST advertise topic and hub URLs for a given resource URL as described inDiscovery.
Subscribers
A conforming subscriber:
MUST support each discovery mechanism in the specified order to discover the topic and hub URLs as described inDiscovery.
This specification exited the CR stage with at least two independent, interoperable implementations of each feature. Each feature may have been implemented by a different set of products. There was no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:
3.2.1Publisher
A WebSub Publisher is an implementation that advertises a topic and hub URL on one or more resource URLs. The conformance criteria are described inConformance Classes above.
3.2.2Subscriber
A WebSub Subscriber is an implementation that discovers the hub and topic URL given a resource URL, subscribes to updates at the hub, and accepts content distribution requests from the hub. The subscriberMAY supportauthenticated content distribution. The conformance criteria are described inConformance Classes above.
3.2.3Hub
A WebSub Hub is an implementation that handles subscription requests and distributes the content to subscribers when the corresponding topic URL has been updated. HubsMUST support subscription requests with a secret and deliverauthenticated requests when requested. HubsMUST deliver the full contents of the topic URL in the request, andMAY reduce the payload to a diff if the content type supports it. The conformance criteria are described inConformance Classes above.
3.2.4Independent
Each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.
3.2.5Interoperable
A Subscriber and Hub implementation are considered interoperable for a specific feature when the Hub takes the defined action that the Subscriber requests, the Subscriber gets the expected response from a Hub according to the feature, and the Hub sends the expected response to the Subscriber.
3.2.6Feature
For the purposes of evaluating exit criteria, each of the following is considered a feature:
Discovering the hub and topic URLs by looking at the HTTP headers of the resource URL.
Discovering the hub and topic URLs by looking at the contents of the resource URL as an XML document.
Discovering the hub and topic URLs by looking at the contents of the resource URL as an HTML document.
Subscribing to the hub with a callback URL.
Subscribing to the hub and requesting a specific lease duration.
Subscribing to the hub with a secret and handling authenticated content distribution.
Requesting that a subscription is deactivated by sending an unsubscribe request.
The Subscriber acknowledges a pending subscription on a validation request.
The Subscriber rejects a subscription validation request for an invalid topic URL.
The Subscriber returns an HTTP 2xx response when the payload is delivered.
The Subscriber verifies the signature for authenticated content distribution requests.
The Subscriber rejects the distribution request if the signature does not validate.
The Subscriber rejects the distribution request when no signature is present if the subscription was made with a secret.
The Hub respects the requested lease duration during a subscription request.
The Hub allows Subscribers to re-request already active subscriptions, extending the lease duration.
The Hub sends the full contents of the topic URL in the distribution request.
The Hub sends a diff of the topic URL for the formats that support it.
The Hub sends a valid signature for subscriptions that were made with a secret.
4.Discovery
The discovery mechanism aims at identifying at least 2 URLs.
The URL of one or more hubs designated by the publisher. If more than one hub URL is specified, it is expected that the publisher notifies each hub, so the subscriber may subscribe to one or more of them.
The canonical URL for the topic to which subscribers are expected to use for subscriptions.
Note
Publishers may wish to advertise and publish to more than one hub for fault tolerance and redundancy. If one hub fails to propagate an update to the document, then using multiple independent hub is a way to increase the liklihood of delivery to subscribers. As such, subscribers may subscribe to one or more of the advertised hubs.
The protocol currently supports the following discovery mechanisms. PublishersMUST implement at least one of them:
Link Headers [RFC5988]: the publisherSHOULD include at least one Link Header [RFC5988] withrel=hub (a hub link header) as well as exactly one Link Header [RFC5988] withrel=self (the self link header)
If the topic is an XML based feed, publishersSHOULD use embedded link elements as described in Appendix B of Web Linking [RFC5988]. Similarly, for HTML pages, publishersSHOULD use embedded link elements as described in Appendix A of Web Linking [RFC5988].
Note
Since<link> has been limited to being placed in the<head> for many years, some consuming code might only check the<head>. Therefore it is more robust to place the<link> tags only in the HTML<head> rather than in the<body>.
When perfoming discovery, subscribersMUST implement all three discovery mechanisms in the following order, stopping at the first match:
Issue a GET or HEAD request to retrieve the topic URL. SubscribersMUST check for HTTP Link headers first.
In the absence of HTTP Link headers, and if the topic is an XML based feed or an HTML page, subscribersMUST check for embedded link elements.
4.1Content Negotiation
For practical purposes, it is important that therel=self URL only offers a single representation. As the hub has no way of knowing what Media Type ([RFC6838]) or language may have been requested by the subscriber upon discovery, it would not be able to deliver the content using the appropriate representation of the document.
It is, however, possible to perform content negotiation by returning an appropriaterel=self URL according to the HTTP headers used in the initial discovery request. For example, a request to/feed with anAccept header containingapplication/json could return arel=self value of/feed.json.
The example below illustrates how a topic URL can return differentLink headers depending on theAccept header that was sent.
Subscribing to a topic URL consists of four parts that may occur immediately in sequence or have a delay.
Subscriber requests a subscription at the hub
The hub validates the subscription with the publisher (OPTIONAL)
The hub confirms the subscription was actually requested by the subscriber
The hub periodically reconfirms the subscription is still active (OPTIONAL)
Unsubscribing works in the same way, except with a single parameter changed to indicate the desire to unsubscribe. Also, the Hub will not validate unsubscription requests with the publisher.
5.1Subscriber Sends Subscription Request
Subscription is initiated by the subscriber making an HTTPS or HTTP POST [RFC7231] request to the hub URL. This requestMUST have a Content-Type header ofapplication/x-www-form-urlencoded (described in Section 4.10.22.6 [HTML5]),MUST use UTF-8 [Encoding] as the document character encoding, andMUST use the following parameters in its body, formatted accordingly:
hub.callback
REQUIRED. The subscriber's callback URL where content distribution notifications should be delivered. The callback URLSHOULD be an unguessable URL that is unique per subscription. ([capability-urls])
hub.mode
REQUIRED. The literal string "subscribe" or "unsubscribe", depending on the goal of the request.
hub.topic
REQUIRED. The topic URL that the subscriber wishes to subscribe to or unsubscribe from. Note that thisMUST be the "self" URL found during the discovery step, which may be different from the URL that was used to make the discovery request.
hub.lease_seconds
OPTIONAL. Number of seconds for which the subscriber would like to have the subscription active, given as a positive decimal integer. HubsMAY choose to respect this value or not, depending on their own policies, andMAY set a default value if the subscriber omits the parameter. This parameterMAY be present for unsubscription requests andMUST be ignored by the hub in that case.
hub.secret
OPTIONAL. A subscriber-provided cryptographically random unique secret string that will be used to compute an HMAC digest forauthorized content distribution. If not supplied, the HMAC digest will not be present for content distribution requests. This parameterSHOULD only be specified when the request was made over HTTPS [RFC2818]. This parameterMUST be less than 200 bytes in length.
SubscribersMAY also include additional HTTP [RFC7230] request parameters, as well as HTTP [RFC7230] Headers if they are required by the hub.
HubsMUST ignore additional request parameters they do not understand.
HubsMUST allow subscribers to re-request subscriptions that are already activated. Each subsequent request to a hub to subscribe or unsubscribeMUST override the previous subscription state for a specific topic URL and callback URL combination, but only once the action is verified (Section 4.3). If verification fails, the subscription stateMUST be left unchanged. This is required so subscribers can renew their subscriptions before the lease seconds period is over without any interruption. The subscriberMAY use a new hub.secret value in a future subscription, andMAY make a new subscription without a hub.secret.
5.1.1Subscription Parameter Details
The topic and callback URLsMAY use HTTP [RFC7230] or HTTPS [RFC2818] schemes. The topic URLMUST be the one advertised by the publisher in a Self Link Header during the discovery phase. (SeeSection 3). HubsMAY refuse subscriptions if the topic URL does not correspond to the one advertised by the publisher. The topic URL can otherwise be free-form following the URL spec [URL]. HubsMUST always decode non-reserved characters for these URL parameters; see section 1.2 on"Percent-encoded bytes" in [URL].
The callback URLSHOULD be an unguessable unique URL ([capability-urls]) andSHOULD use HTTPS [RFC7230]. The callback URL acts as authentication from the hub to the subscriber when confirming subscriptions and delivering the content. Additionally, the callbackSHOULD be unique (not re-used for multiple hubs) and changed when subscriptions are renewed.
The callback URLMAY contain arbitrary query string parameters (e.g.,?foo=bar&red=fish). HubsMUST preserve the query string during subscription verification by appending new parameters to the end of the list using the& (ampersand) character to join. Existing parameters with names that overlap with those used by verification requests will not be overwritten. When sending the content distribution request, the hub will make a POST request to the callback URL including any query string parameters in the URL portion of the request, not as POST body parameters.
5.1.2Subscription Response Details
If the hub URL supports WebSub and is able to handle the subscription or unsubscription request, itMUST respond to a subscription request with an HTTP [RFC7231] 202 "Accepted" response to indicate that the request was received and will now be verified (Section 4.3) and validated (Section 4.2) by the hub. The hubSHOULD perform the verification and validation of intent as soon as possible.
If a hub finds any errors in the subscription request, an appropriate HTTP [RFC7231] error response code (4xx or 5xx)MUST be returned. In the event of an error, hubsSHOULD return a description of the error in the response body as plain text, used to assist the client developer in understanding the error. This is not meant to be shown to the end user. HubsMAY decide to reject some callback URLs or topic URLs based on their own policies (e.g., domain authorization, topic URL port numbers). However, since verification and validation of intent are asynchronous steps that logically begin after the HTTP response has been returned, the HTTP responseMUST NOT depend on the process or outcome of verification or validation.
If the hub URL is not able to handle subscription or unsubscription requests, itMAY redirect to another hub which supports WebSub. It does so by yielding an HTTP [RFC7231] 307 (temporary redirect) or 308 (permanent redirect) response. ItMUST also include at least a HTTP [RFC7230] Location Header containing a preferred URL reference for the hub to use by the subscriber. The subscriber is expected to retry the subscription or unsubscription at the new hub URL.
5.2Subscription Validation
SubscriptionsMAY be validated by the Hubs who may require more details to accept or refuse a subscription. The HubMAY also check with the publisher whether the subscription should be accepted.
If (and when) the subscription is accepted, the hubMUST perform theverification of intent of the subscriber.
If (and when) the subscription is denied, the hubMUST inform the subscriber by sending an HTTP [RFC7231] (or HTTPS [RFC2818]) 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 4 of [URL]):
hub.mode
REQUIRED. The literal string "denied".
hub.topic
REQUIRED. The topic URL given in the corresponding subscription request.
hub.reason
OPTIONAL. The hub may include a reason for which the subscription has been denied.
The subscriptionMAY be denied by the hub at any point (even if it was previously accepted). The SubscriberSHOULD then consider that the subscription is not possible anymore.
5.3Hub Verifies Intent of the Subscriber
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.
The hub verifies a subscription request by sending an HTTP [RFC7231] (or HTTPS [RFC2818]) 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 4 of [URL]):
hub.mode
REQUIRED. The literal string "subscribe" or "unsubscribe", which matches the original request to the hub from the subscriber.
hub.topic
REQUIRED. The topic URL given in the corresponding subscription request.
hub.challenge
REQUIRED. A hub-generated, random string thatMUST be echoed by the subscriber to verify the subscription.
hub.lease_seconds
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. HubsMUST supply this parameter whenhub.mode is set to "subscribe". This parameterMAY be present whenhub.mode is "unsubscribe" andMUST be ignored by subscribers in that case.
5.3.1Verification Details
The subscriberMUST confirm that thehub.topic corresponds to a pending subscription or unsubscription that it wishes to carry out. If so, the subscriberMUST respond with an HTTP success (2xx) code with a response body equal to thehub.challenge parameter. If the subscriber does not agree with the action, the subscriberMUST respond with a 404 "Not Found" response.
The hubMUST consider other server response codes (3xx, 4xx, 5xx) to mean that the verification request has failed. If the subscriber returns an HTTP [RFC7231] success (2xx) but the content body does not match thehub.challenge parameter, the hubMUST also consider verification to have failed.
HubsMAY make thehub.lease_seconds equal to the value the subscriber passed in their subscription request butMAY change the value depending on the hub's policies. To sustain a subscription, the subscriberMUST re-request the subscription on the hub beforehub.lease_seconds seconds has elapsed.
HubsMUST enforce lease expirations, andMUST NOT issue perpetual lease durations.
Note
The spec uses GET vs POST to differentiate between the confirmation/denial of the subscription request and delivering the content. While this is not considered "best practice" from a web architecture perspective, it does make implementation of the callback URL simpler. Since the POST body of the content distribution request may be any arbitrary content type and only includes the actual content of the document, using the GET vs POST distinction to switch between handling these two modes makes implementations simpler.
6.Publishing
The publisherMUST inform the hubs it previously designated when a topic has been updated. The hub and the publisher can agree on any mechanism, as long as the hub is eventually able send the updated payload to the subscribers.
Note
The specific mechanism for the publisher to inform the hub is left unspecified. For example, some existing public hubs[1][2][3] ask publishers to send a POST request with the keyshub.mode="publish" andhub.url=(the URL of the resource that was updated).
6.1Subscription Migration
If the publisher wishes to migrate existing subscriptions to a new topic URL, it can do so using HTTP redirects.
The previous topic URL should send a redirect to the new topic URL. This will provide a seamless transition for any HTTP client that did not use WebSub but instead was polling the topic URL.
When existing WebSub subscriptions expire, subscribers will attempt to renew the subscription. The first step of renewing a subscription is to fetch the topic URL, which means the subscriber will encounter the redirect and end up at the new topic URL.
At the new topic URL, the subscriber will see the newrel=self URL and the new hub, and will subscribe to the new topic URL at the new hub.
This does not require any participation on the part of the previous hub, and works whether or not the publisher changes hubs as well.
7.Content Distribution
A content distribution request is sent from the Hub to the Subscriber when new content is available for a topic URL. The request is an HTTP [RFC7231] (or HTTPS [RFC2818]) POST request from the hub to the subscriber's callback URL. The HTTP body of the POST requestMUST include the payload of the content distribution notification. The content distribution requestMUST have aContent-Type Header corresponding to theContent-Type of the topic, andMUST contain the full contents of the topic URL, with an exception allowed as described below.
For Atom ([RFC4287]) and RSS ([RSS-2.0]) feeds, the hubMAY remove already-deliveredatom:entry orrss:item elements from the feed.
The requestMUST include at least one Link Header [RFC5988] withrel=hub pointing to a Hub associated with the topic being updated. ItMUST also include one Link Header [RFC5988] withrel=self set to the canonical URL of the topic being updated. The HubSHOULD combine these headers into a single Link Header [RFC5988]. All these URLs are those resulting from the discovery process (Section 3). The subscriberMUST NOT use these Link headers to identify the subscription corresponding to the content distribution request, because the Link headers are metadata associated with the topic content, not with any particular subscription. For example, the topic URL in the content distribution request may be different from the topic URL that was originally subscribed to.
The subscriber's callback URLMUST return an HTTP [RFC7231] 2xx response code to indicate a success. The subscriber's callback URLMAY return an HTTP 410 code to indicate that the subscription has been deleted, and the hubMAY terminate the subscription if it receives that code as a response. The hubMUST consider all other subscriber response codes as failures; that means subscribersMUST NOT use HTTP redirects for moving subscriptions. SubscribersSHOULD respond to content distribution requests as quickly as possible; their success response codeSHOULD only indicate receipt of the message, not acknowledgment that it was successfully processed by the subscriber. The response body from the subscriberMUST be ignored by the hub. HubsSHOULD retry content distribution requests up to self-imposed limits on the number of times and the overall time period to retry. When the failing delivery exceeds the hub's limits, the hub stops attempting to deliver that nofication. The hubMUST keep the subscription active until the end of the lease duration, and if a new update is published to the topic,MUST continue to retry delivery to the previously-failing subscriber.
7.1Authenticated Content Distribution
If the subscriber supplied a value forhub.secret in their subscription request, the hubMUST generate an HMAC signature of the payload and include that signature in the request headers of the content distribution request. TheX-Hub-Signature header's valueMUST be in the formmethod=signature wheremethod is one of the recognized algorithm names andsignature is the hexadecimal representation of the signature. The signatureMUST be computed using the HMAC algorithm [RFC6151] with the request body as the data and thehub.secret as the key.
7.1.1Recognized algorithm names
The following algorithms are the initially registered algorithm names, based on the contents of the [FIPS-PUB-180-4] registry at the time of publishing.
sha1
The SHA-1 algorithm as specified in Section 6.1 of [FIPS-PUB-180-4]
sha256
The SHA-256 algorithm as specified in Section 6.2 of [FIPS-PUB-180-4]
sha384
The SHA-384 algorithm as specified in Section 6.5 of [FIPS-PUB-180-4]
sha512
The SHA-512 algorithm as specified in Section 6.4 of [FIPS-PUB-180-4]
In the future, an extension may be specified allowing subscribers to indicate which algorithms they can use for validation. As of this writing, most hubs sign with SHA-1, despite its known cryptographic weakness, in order to be interoperable with older subscribers.
7.1.2Signature validation
When subscribers receive a content distribution request with theX-Hub-Signature header specified, theySHOULD recompute the signature with the shared secret using the same method (provided in theX-Hub-Signature header) as the hub. If the signature does not match, subscribersMUST locally ignore the message as invalid. SubscribersMAY still acknowledge this request with a 2xx response code in order to be able to process the message asynchronously and/or prevent brute-force attempts of the signature. Using this technique along with HTTPS [RFC2818] for subscription requests enables simple subscribers to receive authenticated content distribution requests from hubs without the need for subscribers to run an HTTPS [RFC2818] server.
Please note however that this signature only ensures that the payload was not forged. Since the request also includes headers, these should not be considered as safe by the subscriber, unless of course the subscriber uses HTTPS [RFC2818] callbacks.
8.Security Considerations
Here is a summary of security considerations. It is important to note that WebSub is a server-to-server protocol which relies only on HTTP. It is strongly recommended to use HTTPS for all requests.
8.1Discovery
The decision about whether a subscriber should look for<link> elements inside a page's<body> (as well as the<head>) is not straightforward, and there is currently no clear consensus. One reason to ignore the<body> during discovery is that some web sites might (perhaps accidentally) allow users to post content containing<link> elements, though the working group does not know of any specific examples of such sites. If WebSub discovery uses such<link> elements, a user contributing to such sites could potentially maliciously cause all subscribers to use an alternate hub which later delivers malicious content. Given this potential attack, it may be prudent to do discovery only in the<head> of HTML documents.
8.2Subscriptions
First, subscribersSHOULD always favor the HTTPS URL for hubs (even if the URL is advertised as HTTP). Second, subscribersSHOULD use unique unguessable capability URLs for the callbacks, as well as make them available via HTTPS. Finally, subscribersSHOULD use ahub.secret when subscribing to allow signature of the content distribution.
HubsSHOULD enforce short livedhub.lease_seconds (10 days is a good default). When performing intent verification, the hubSHOULD use a random, single-usehub.challenge.
8.3Distribution
The HubMUST use the exact callback used by the subscriber (including the use of HTTPS). HubsMUST sign their requests using thehub.secret supplied by subscribers if requested.
If the subscriber included a hub.secret in the subscription request, the subscriberSHOULD validate the hub's provided signature, and if they do so, theyMUST use the server's stated signature mechanism, and discard requests which fail the test.
If a subscriber does not use a secure callback URL (HTTPS), or if it is suspected that the TLS transport between the hub and subscriber may be compromised, then the integrity of the content delivery notification is only protected by thehub.secret and the hashing algorithm used. In this case, an appropriate hashing algorithm should be used based on the security requirements of the application. As SHA-1 has been demonstrated to be compromised as of the date of this publication, a minimum of SHA-256 should be used.
8.4Security and Privacy Review
These questions provide an overview of security and privacy considerations for this specification as guided by Self-Review Questionnaire: Security and Privacy ([security-privacy-questionnaire]).
Does this specification deal with personally-identifiable information?
The only potentially personally-identifiable information involved are topic and callback URLs.
Does this specification deal with high-value data?
No, there is no authentication or other credentials involved.
Does this specification introduce new state for an origin that persists across browsing sessions?
No.
Does this specification expose persistent, cross-origin state to the web?
The WebSub subscriber should create a resource with information about the topic to which it subscribes.
Does this specification expose any other data to an origin that it doesn't currently have access to?
No.
Does this specification enable new script execution/loading mechanisms?
No.
Does this specification allow an origin access to a user's location?
No.
Does this specification allow an origin access to sensors on a user's device?
No.
Does this specification allow an origin access to aspects of a user's local computing environment?
No.
Does this specification allow an origin access to other devices?
No.
Does this specification allow an origin some measure of control over a user agent's native UI?
No.
Does this specification expose temporary identifiers to the web?
No.
Does this specification distinguish between behavior in first-party and third-party contexts?
No.
How should this specification work in the context of a user agent's "incognito" mode?
WebSub is a server to server protocol, in which "incognito" mode does not have a meaning.
Does this specification persist data to a user's local device?
No.
Does this specification allow downgrading default security characteristics?
B.1Changes from 03 October 2017 PR to this version
Added link to content negotiation section from corresponding item in conformance criteria
Updated reference name from "WHATWG-URL" to "[URL]", but does not change the actual reference.
Rephrase sentence on hub URL discovery to better clarify which URLs are being talked about, and use "notify" instead of "ping"
Correct "mime-type" to "Media Type" and add informative reference to RFC.
Rephrase summary of "subscribing and unsubscribing" to explicitly mention the actor of each step
Clarify sending the POST request to the subscriber's callback URL
Add reference to HTTPS when previously only HTTP was mentioned in cases where an HTTPS URL may be used
Clarify when hub.lease_seconds parameter is required or optional
Reword section on subscription migration to make it more clear
Move note about publisher->hub notification outside of the subscription migration section
Remove references to "notification" as a standalone term, replaced by "content distribution request" or "content distribution notification" as appropriate
Update CR exit criteria text to past tense
Added explanatory text about why a publisher may advertise multiple hubs
Replaced sentence in section 8 to better describe the situation "the topic URL in the content distribution request may be different from the topic URL that was originally subscribed to"
Dropped at-risk limitation of the link tag in the head
Added security consideration for discovery related to link rel discovery in body vs head
Dropped unused feature of the hub rejecting the subscription and providing an alternate topic URL
Added sequence diagram in high-level overview section, and minor updates to phrasing of that section
Added acknowledgements
Note that hubs may set a default value for lease_seconds
B.2Changes from 11 April 2017 CR to 03 October 2017 PR
Adds informative guidelines for how publishers can migrate subscribers to a new topic URL
Allow subscribers to reject invalid signatures with response codes other than 2xx
Suggest subscribers return HTTP 410 if a subscription has been deleted
Add informative note about the lack of specification of publisher-hub relationship
Drop at-risk.host-meta discovery feature due to lack of implementations (Issue #97)
Add text to note the subscriber'shub.secret should be cryptographically random and unique
Clarify that failed delivery of notifications should not delete the subscription before the lease duration ends (Issue #119)
Replaced note about referring to FIPS document for algorithm extensions with a note that says a proper WebSub extension should be defined in the future that enables negotiation of the hashing algorithm between subscribers and hubs
Add example of returning a different rel=self URL depending on the HTTPAccept-Language header
Add a note in Security Considerations about the hashing algorithm protecting against a compromised TLS channel
Moved "at risk" section out of the "sotd" section which was causing a respec error
Replaced hyperlink to FIPS PUB 180-4 with an in-page reference
B.3Changes from 24 November WD to 11 April 2017 CR
Clarified wording on supported algorithms for authenticated distribution
Only allow<link> tags in the HTML <head> element
Added conformance criteria and CR exit criteria
Added examples of discovery request and response
Added example of using differentrel=self URLs to support content negotiation
Added a security considerations section
Updated references to WHATWG-URL instead of HTML 4
Replaced abstract with updated description
B.4Changes from 20 October FPWD to 24 November 2016
Added informative reference to previous versions of the spec, PubSubHubbub 0.3 and 0.4
Split discovery section into separate publisher and subscriber sections
Clarify that publishers can use any available discovery method, and subscribers must support all
Marked host-meta discovery method At Risk due to no known implementations, and fixed reference to Host Meta spec instead of the previous reference to Well-Known
Recommend using Capability URLs as the subscriber's callback URLs for security and authenticating the notification delivery
Recommend not reusing callback URLs on subscription renewals
Clarify that thehub.topic must be theself URL that was discovered
Dropped the recommendation of including theFrom header on subscription requests
Clarify that the hub response to subscription requests must not depend on the verification or validation
Hubs must enforce lease expirations
Clarify that the notification payload should contain the full contents of the topic URL
Recommend that hubs should retry failed notification delivery up to self-imposed limits
Clarify that future defined signature methods in FIPS PUB 180-4 are allowed
Added informative note about the use of GET vs POST at the callback URL
