# Linkback

A linkback is a method for Web authors to obtain notifications when other authors link to one of their documents. This enables authors to keep track of who is linking to, or referring to, their articles. The four methods (refback, trackback, pingback and webmention) differ in how they accomplish this task.

## Overview

"Linkback" is the generalized term used to reference four methods of communication between websites. While sometimes confused with one another, linkbacks and backlinks are not the same type of entity. A backlink is what the person referring to a page creates while a linkback is what the publisher of the page being referred to receives.
Any of the four terms—linkback, trackback, pingback, or (rarely) refback—might also refer colloquially to items within a section upon the linked page that display the received notifications, usually along with a reciprocal link; trackback is used most often for this purpose. Also, the word trackback is often used colloquially to mean any kind of linkback.
| | Refback | Trackback | Pingback | Webmention |
| Trigger mechanism | Visitor to linking site clicks on the link, and their browser takes them to the linked site | Code on linking server examines added or updated documents, extracts links, and sends notification to linked server for each link found | Code on linking server examines added or updated documents, extracts links, and sends notification to linked server for each link found | Code on linking server examines added or updated documents, extracts links, and sends notification to linked server for each link found |
| Notification medium | HTTP referrer value | HTTP POST | XML-RPC call | HTTP POST with source and target parameters |
| Capture mechanism | Examination of incoming HTTP referrer values | Trackback capture script | XML-RPC function | Webmention capture script |
| Information sent by linking server | None | Linking site name (Optional) Linking post title (Optional) Linking post excerpt (Optional) Linking post URL | Linked post URL Linking post URL | Linked post URL (target) Linking post URL (source) |
| Additional information presented to linked server | HTTP referrer sent by a visitor's browser upon clicking the link | IP address of linking server | IP address of linking server | IP address of linking server |
| Autodiscovery mechanism (how the linking server finds out how and where to send the notification) | None | LINK tag in the header of the linked page or trackback RDF documents | Special HTTP header or LINK tag on the linked page | HTTP Link header or link element on the linked page |
| Action required when notification is received | Extract referrer value from incoming HTTP headers Retrieve referring page Parse retrieved page for desired information | Gather desired information from Given parameters or retrieving and parsing the given URL | Retrieve page at "linking post URL" Parse retrieved page for desired information | Verifying that linking page does indeed link to linked page is recommended, not explicitly required |
| Advantages | Requires no special code on linking server (the link itself becomes the notification when someone clicks on it) | All the information desired by the linked server (Linking site name, post title, excerpt) is present in the notification itself | Notification mechanism has a complete technical specification Less susceptible to spamming | Uses well-known parts of HTTP wherever possible (autodiscovery, encoding of data, response status) Reuses pingback's existing semantics Minimum amount of data transferred on-the-wire |
| Disadvantages | No notification unless someone actually clicks on the link Relies upon visitors' browsers sending proper HTTP referrer information Linked site must retrieve and parse linking site's page to extract the information it wants | Notification requires positive action by linking server Notification mechanism has only a partial technical specification Autodiscovery information may prevent XHTML validation | Notification requires positive action by linking server Linked site must retrieve and parse linking site's page to extract the information it wants Can be abused for DDOS attacks | Notification requires positive action by linking server Linked site must retrieve and parse linking site's page to extract the information it wants Relatively new, so less widely implemented |