Within certain document media types, the base URL of the document can be embedded within the content itself such that it can be readily obtained by a parser. This can be useful for descriptive documents, such as tables of content, which may be transmitted to others through protocols other than their usual retrieval context (e.g., E-Mail or USENET news).
It is beyond the scope of this document to specify how, for each media type, the base URL can be embedded. It is assumed that user agents manipulating such media types will be able to obtain the appropriate syntax from that media type's specification. An example of how the base URL can be embedded in the Hypertext Markup Language (HTML) [3] is provided in an Appendix (Section 10).
Messages are considered to be composite documents. The base URL of a message can be specified within the message headers (or equivalent tagged metainformation) of the message. For protocols that make use of message headers like those described in RFC 822 [5], we recommend that the format of this header be:
base-header = "Base" ":" "<URL:" absoluteURL ">"
where "Base" is case-insensitive and any whitespace (including that used for line folding) inside the angle brackets is ignored. For example, the header field
Base: <URL:http://www.ics.uci.edu/Test/a/b/c>
would indicate that the base URL for that message is the string "http://www.ics.uci.edu/Test/a/b/c". The base URL for a message serves as both the base for any relative URLs within the message headers and the default base URL for documents enclosed within the message, as described in the next section.
Protocols which do not use the RFC 822 message header syntax, but which do allow some form of tagged metainformation to be included within messages, may define their own syntax for defining the base URL as part of a message.