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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP Application Layer Addressing: Uniform Resource Identifiers, Locators and Names (URIs, URLs and URNs)                 9  Uniform Resource Locators (URLs)

URL Schemes (Applications / Access Methods) and Scheme-Specific Syntaxes 1 2 3 4 URL Length and Complexity Issues

Creating and Interpreting Relative URLs

It is for these reasons that URL syntax was extended to include a relative form. In simplest terms, a relative URL is the same as an absolute URL but with pieces of information omitted that are implied by context. Like our “go downstairs” instruction, a relative URL does not by itself contain enough information to specify a resource. A relative URL must be interpreted within a context that provides the missing information.

The context needed to find a resource from a relative URL is provided in the form of a base URL that provides the missing information. A base URL must be either a specific absolute URL, or itself a relative URL that refers to some other absolute base. The base URL may be either explicitly stated or may be inferred from use. The RFCs dealing with URLs define three methods for determining the base URL, which are arranged into the following precedence:

1. Base URL Within Document: Some documents allow the base URL to be explicitly stated. If present, this specification is used for any relative URLs in the document.
   
   
2. Base URL From Encapsulating Entity: In cases where no explicit base URL is specified in a document, but the document is part of a higher-level entity enclosing it, the base URL is the URL of the “parent” document. For example, a document within a body part of a MIME multipart message can use the URL of the message as a whole as the base URL for relative references.
   
   
3. Base URL From Retrieval URL: If neither of those two methods are feasible, the base URL is inferred from the URL used to retrieve the document containing the relative URL.

Of these three methods, #1 and #3 are the most common. HTML, the language used for the Web, allows a base URL to be explicitly stated which removes any doubt about how relative URLs are to be interpreted. Failing this, method #3 is commonly used for images and other links in HTML documents that are specified in relative terms.

For example, let's go back to the poor slob maintaining “http://www.longdomainnamesareirritating.com/index.htm”. By default, any images referenced from that “index.htm” HTML document can use relative URLs—the base URL will be assumed from the name of the document itself. So he can just say “companylogo.gif” instead of “http://www.longdomainnamesareirritating.com/companylogo.gif”, as long as that file is in the same directory on the same server as “index.htm”.

If all three of these methods fail for whatever reason, then no base URL can be determined. Relative URLs in such a document will be interpreted as absolute URLs, and since they do not contain complete information, they will not work properly.

Also, relative URLs only have meaning for certain URL schemes. For others, they make no sense and cannot be used. In particular, relative URLs are never used for the “telnet”, “mailto” and “news” schemes. They are very commonly used for HTTP documents, and may also be used for FTP and file URLs.

URL Schemes (Applications / Access Methods) and Scheme-Specific Syntaxes 1 2 3 4 URL Length and Complexity Issues