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		<title>Format-specific URI limitations &lt;https://y.st./en/URI_research/limitations.xhtml&gt;</title>
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			<h1>Format-specific <abbr title="Uniform Resource Identifier">URI</abbr> limitations</h1>
			<p>Some valid <abbr title="Uniform Resource Identifier">URI</abbr>s aren&apos;t available in every context.</p>
		</header>
<p>This page is still in its infancy, as I haven&apos;t done much research on standard limitations on <abbr title="Uniform Resource Identifier">URI</abbr>s just yet.</p>
<h2>Sitemaps</h2>
<p>The <a href="http://www.sitemaps.org/protocol.html#xmlTagDefinitions">Sitemap</a> format defines the <code>&lt;loc/&gt;</code> tag as containing a <abbr title="Uniform Resource Identifier">URI</abbr> that is not more than 2047 characters in length. Read carefully; it says <q>less than 2,048 characters</q>, not <q>less than or equal to 2,048 characters</q>. Furthermore, this limitation is only placed on <code>&lt;loc/&gt;</code> tags within a Sitemap file, not within a Sitemap index file.</p>
<h2>Onion addresses</h2>
<p>While <abbr title="Internet Assigned Numbers Authority">IANA</abbr> did <a href="https://www.iana.org./assignments/special-use-domain-names/special-use-domain-names.xhtml">reserve the <q>onion.</q> <abbr title="Top Level Domain">TLD</abbr> for use with <abbr title="The Onion Router">Tor</abbr></a>, they don&apos;t require that <abbr title="The Onion Router">Tor</abbr> used <abbr title="Domain Name System">DNS</abbr> to implement these names. In fact, the <abbr title="Request for Comments">RFC</abbr> says that <a href="https://tools.ietf.org./html/rfc7686#section-1">these names are to be resolved outside of <abbr title="Domain Name System">DNS</abbr></a>. Essentially, the <abbr title="Request for Comments">RFC</abbr> just validates <abbr title="The Onion Router">Tor</abbr>&apos;s use of onion addresses and specified that applications that are not <abbr title="The Onion Router">Tor</abbr>-aware should reject these names without without attempting to look them up, instead returning an error. It also says that <abbr title="Domain Name System">DNS</abbr> servers should return NXDOMAIN responses instead of attempting to resolve them. <abbr title="The Onion Router">Tor</abbr>-aware applications are required to either access the resource directly without a lookup or use a proxy to do so, using the tor-rendezvous protocol, which is defined by <abbr title="The Onion Router">Tor</abbr> itself. <abbr title="The Onion Router">Tor</abbr> doesn&apos;t handle the trailing dot on onion addresses, unlike <abbr title="Domain Name System">DNS</abbr> names. However, because <abbr title="The Onion Router">Tor</abbr> doesn&apos;t allow resolution of onion addresses with trailing dots and the proper handling of onion addresses is defined as whatever <abbr title="The Onion Router">Tor</abbr> does, rejecting onion addresses with a trailing dot is actually the correct behavior. If the <code>//onion.</code> <abbr title="Top Level Domain">TLD</abbr> is used, the trailing dot should be omitted every time. It&apos;s worth mentioning the one mention of an onion address&apos;s <a href="https://gitweb.torproject.org./torspec.git/tree/rend-spec.txt#n1017">format</a> though:</p>

<blockquote><pre>Valid onion addresses contain 16 characters in a-z2-7 plus
&quot;.onion&quot;, and valid auth cookies contain 22 characters in
A-Za-z0-9+/. The service name is only used for internal purposes,
e.g., for Tor controllers; nothing in Tor itself requires or uses
it.</pre></blockquote>

<p>It makes no mention of trailing dot being valid or invalid. Furthermore, it doesn&apos;t mention use of subdomains being valid, though the <abbr title="Request for Comments">RFC</abbr> explicitly give permission to use subdomains.</p>
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