Acceptable Link Format
=========================
The Acceptable Link Format (ALF) is an older HTTP Link Format that was widely used until 2019. It has been largely replaced by the more Modern and Secure HTTP Link Type: <a href="/Rel" class="missing-article">Rel</a>=<a href="/Canonical" class="missing-article">Canonical</a> Link Type.
History
ALF was introduced in RFC 5988, a standard for representing links in HTML, which was published in 1998. The Format is based on the idea of allowing users to specify alternative sources for web pages that contain information about an original resource.
Syntax
The ALF Link Format consists of three parts:
<a href="/Rel" class="missing-article">Rel</a>: one or more words specifying the Type of Link.href: the target URL of the Link, which may be empty if no specific target is specified.- Optional parameters for additional context or attributes.
Example
<a href="[HTTP](/HTTP)://example.com" <a href="/Rel" class="missing-article">Rel</a>="alternate">
<strong>Read this page on [Wikipedia](/Wikipedia)</strong>
<span class="external">https://en.[Wikipedia](/Wikipedia).org/wiki/This_page</span>
</a>
In this example, the <a href="/Rel" class="missing-article">Rel</a> attribute specifies that the linked resource is an alternate version of the original source (in this case, Wikipedia), while the href attribute points to a specific URL on Wikipedia. The optional parameter class="external" is applied to the external Link.
Usage
ALF was commonly used in web applications and documentation to provide alternative links or resources for users who want to access information about an original resource.
Example Use Case
Suppose a website has multiple versions of its main page, each with some differences. To allow users to view the latest version, the site could use ALF:
<a href="[HTTP](/HTTP)://example.com/old-version" <a href="/Rel" class="missing-article">Rel</a>="alternate">
<strong>Read this old version</strong>
</a>
<a href="[HTTP](/HTTP)://example.com/latest" <a href="/Rel" class="missing-article">Rel</a>="<a href="/Canonical" class="missing-article">Canonical</a>">View the latest version</a>
In this example, the <a href="/Rel" class="missing-article">Rel</a> attribute specifies that the linked resource is an alternate version of the original source (the “old version”), while the href attribute points to a specific URL on the website’s main page. The <a href="/Canonical" class="missing-article">Canonical</a> attribute specifies that the most up-to-date version should be displayed by default.
Security Considerations
ALF was vulnerable to attacks such as HTTP Injection and man-in-the-middle (MITM), where an attacker could manipulate the href attribute to redirect users to malicious URLs.
The HTTP Link Type: <a href="/Rel" class="missing-article">Rel</a>=<a href="/Canonical" class="missing-article">Canonical</a> Link Type was introduced in RFC 8284 to provide a more Secure and robust alternative for specifying links that require clients to always display the most up-to-date version of an original resource.
Conclusion
The Acceptable Link Format (ALF) is an older HTTP Link Format that has been largely replaced by more Modern Standards such as the <a href="/Rel" class="missing-article">Rel</a>=<a href="/Canonical" class="missing-article">Canonical</a> Link Type. While ALF was useful in its time, it lacks the security features and robustness of Modern Link types, making it vulnerable to attacks such as HTTP Injection and MITM.
As a result, the <a href="/Rel" class="missing-article">Rel</a>=<a href="/Canonical" class="missing-article">Canonical</a> Link Type has become the preferred choice for specifying links that require clients to always display the most up-to-date version of an original resource.