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The Webmention FAQ is for documenting frequently asked questions about Webmention and their answers. Infrequently asked questions may be collected in resolved Issues.


Are Webmentions like Pingbacks

Short answer: No, Webmentions are not like Pingbacks. Webmentions are like comments and notifications.

Longer: No, Webmentions power existing user-centric features like comments, likes, notifications etc. across websites, while Pingback (perhaps unintentionally) created a new (and ugly) user-feature "Pingback" which was more confusing than helpful. Under the covers, Webmention is a much simpler, cleaner, and more secure protocol than Pingback despite having some structural similarities.

Developer Questions

What is the canonical URL for the Webmention spec

Webmention is a W3C spec and lives at https://www.w3.org/TR/webmention/

Why webmention instead of pingback

Webmention is simpler & more reliable than Pingback, more thoroughly specified (e.g. explicit support for updates & deletes), and designed specifically to support rich social web interactions such as cross-site comments, likes, reposts, RSVPs, and more.

Webmentions are:

  • Simpler. By dropping XML-RPC, webmention is simpler than Pingback. This means:
    • Less work to implement
    • Easier to test
    • Both of which combine to enable more reliable, interoperable implementations, sooner.
  • Re-using only HTTP enables easier testing and UI. By using only HTTP, it's possible to construct simple HTML forms that exercise the protocol, which is a good design principle for web protocols in general[1]. This enables simpler/easier testing (with just a static HTML file), and the ability to provide simple (no JS needed) webmention submit forms on blog posts for others to paste URLs to their replies (some IndieWeb participants are already doing this, e.g. http://adactio.com/ and http://aaronparecki.com/ posts).

Why webmention instead of Trackback

Main article: Trackback

Which links should receive webmentions

Which links in a post should receive webmentions?

Webmentions should be sent to all URLs a post can be considered to mention — this includes

  • in-reply-to URL(s)
  • rsvp URLs
  • URLs linked to in the content, including URLs of people mentioned

What do existing implementations do?

  • I send pingbacks (webmentions coming soon) to the in-reply-to URL if set, and all URLs in the post content field. --Waterpigs.co.uk 09:14, 10 April 2013 (PDT)

Should webmentions be sent for links to static assets

Should webmentions be sent for links to static asset URLs e.g. images, audio?

What do existing implementations do?

Should webmentions be sent for embeds

Should webmentions be sent for resources embedded in a post like the URLs in "src" and "data" attributes?

  • <audio src>
  • <embed src>
  • <iframe src>
  • <img src>
  • <object data>
  • <source src>
  • <video src>
  • ... others?

And what about "srcset"? Is anyone on the indieweb even bothering to publish with srcset?

What would the receiver of a webmention for an embed do with that notification?

Should webmentions be sent for removals

Main article: deleted
  • If the source post is edited to remove a link to the target, should a webmention be sent to the target to inform them of the removal of the mention?
    • [kartikprabhu.com] sends webmentions to links that are removed while editing an article.
  • Do any implementations for receiving webmentions do anything with such a webmention POST?
    • [kartikprabhu.com] deletes the existing mention-response from database, if it receives a mention where the source no longer links to target.

How does this solve the spam problem

How does this solve the spam problem that plagued Trackback and Pingback?

Trackback was particularly susceptible to spam since the URL and content of the comment was sent in the payload.

Pingback improved on this by making the "ping" only contain the URL of the comment, so that the link could be verified.

Webmention is a simplification of Pingback, so by itself does not provide any new anti-spam measures.

Instead, spam is treated as its own issue, with several possible solutions being documented on the Spam Prevention page, including most recently, the Vouch protocol.

In practice, Webmention receivers look for specific h-entry markup to render comments, likes, reposts, and other responses, which all add a minor barrier (for now) for spammers above and beyond Pingback.

We expect that as Webmention grows in popularity, implementers will need to implement:

  • The Vouch extension to Webmention

POSSE or Send Webmentions First

Should an indieweb publisher POSSE first, or send webmentions first?

It is better to POSSE a post first, and then send webmentions for all links in a post, for two reasons:

  1. Easier de-duplication at the receiver. By sending webmentions after POSSEing and adding u-syndication links on your post to the POSSE copies, the webmention receiver(s) will see those u-syndication links and be able to use them immediately to de-duplicate any proxy webmentions it may have received due to your POSSE copies. The opposite order is harder to de-dupe since it requires original-post-discovery which is more work than simply getting u-syndication links from parsing the h-entry.
  2. POSSE reply threading. When indie A replies to indie B and sends B a webmention, when B goes to reply back to A, if B's copy of A's post already knows the Ap POSSE copy, then B can immediately reply back both to A, and POSSE reply thread their Bp POSSE copy to Ap.

Reference: 2015-05-28 IRC discussion/background

Why is the target URL a required parameter

Technically, the "target" parameter does not add any semantics to the webmention request. Without the target parameter, the webmention request means essentially the same thing, which is indicating the source URL may be of interest to the webmention endpoint.

By including the "target" parameter in the webmention request, it makes verifying the webmention easier for the recipient, since they have an exact string to look for on the source page. With the target parameter, the receiver searches for an exact match. Without the target parameter, the receiver doesn't know exactly what to look for, and would have to look for any link matching the receiver's domain, adding complexity to the verification code.

Additionally, since the receiver also must follow redirects on the target URL, expecting the receiver to follow redirects for all URLs on a page is unreasonable. Having a single URL to follow is much simpler.

Since it's no extra burden on the sender to include the target parameter, and makes verification easier for the receiver, including the target parameter is required in the spec.

Also, a webmention endpoint may handle multiple possible target sites, as webmention.herokuapp.com does, for example, so can't make an a priori choice about which domain to check the source for links to.

Issue: "Possible" handling of multiple target sites is at best a feature of a Webmention endpoint. It is not documented or even expected in the current Webmention recommendation. -- 07:42, 26 July 2015 (PDT) Sarven Capadisli

  • Similar to the reason we have a webmention endpoint in the first place (rather than having the URL handle webmentions itself), we prefer to design protocols that don't limit the implementation details. Having the ability for an endpoint to handle multiple domains is a good thing, and we should not design the protocol where that is impossible. Aaron Parecki 08:01, 26 July 2015 (PDT)

How does Webmention scale

Because it is peer-to-peer, Webmention scales naturally with the web, in contrast to centralized update methods such as pinging a central ping server which quickly gets overloaded, or having to consume a firehose of all updates (e.g. the twitter firehose) which becomes too big a burden.

What if multiple webmention endpoints are discovered

(or, what if I want people to send me webmentions to multiple endpoints)

Senders are expected to only send the webmention to the first endpoint they discover. This is to avoid a situation where a web page can advertise a large number of endpoints and require the sender make an unreasonable number of requests to deliver the webmentions.

If a receiver wants to use multiple webmention endpoints (for example, if one endpoint exists to send notifications, and another exists to archive the source documents, and another exists to collect a backup copy of comments), it should set up a single endpoint and forward a copy of incoming mentions to the other endpoints. In the real world, the only way a receiver can guarantee its multiple endpoints will receive mentions is if it sends them itself, since even if the spec says senders should send to all advertised endpoints, there is no way to guarantee the sender will actually do so.

See Also