Uniform Resource Identifiers Working Group R. T. Fielding
INTERNET-DRAFT UC Irvine
Expires May 27, 1995 November 27, 1994
Relative Uniform Resource Locators
<draft-ietf-uri-relative-url-02.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this document is unlimited. Please send comments
to the author, Roy T. Fielding <fielding@ics.uci.edu>, or to the
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Abstract
Uniform Resource Locators (URLs) are a compact representation of the
location and access method for a resource available via the Internet.
When embedded within a base document, a URL in its absolute form may
contain a great deal of information which is already known from the
context of that base document's retrieval, including the access
scheme, network location, and parts of the url-path. In situations
where the base URL is well-defined and known to the parser (human or
machine), it is useful to be able to embed URL references which
inherit that context rather than re-specifying it in every instance.
This document defines the syntax and semantics for such Relative
Uniform Resource Locators.
1. Introduction
This work is derived from concepts introduced by the World-Wide Web
global information initiative, whose use of such objects dates from
1990 and is described in "Universal Resource Identifiers in WWW",
RFC 1630 [1]. This document is a companion to the Internet-Draft
"Uniform Resource Locators (URL)" [2], which specifies the
syntax and semantics of absolute URLs. A URL is "absolute" if it
can be interpreted consistently and unambiguously, with global scope,
independent of any other URL.
This document describes the syntax and semantics for "relative"
Uniform Resource Locators (relative URLs): a compact representation
of the location and access method for a resource available via the
Internet relative to an absolute base URL. The name space of
relative URLs is a superset of that defined in [2] for Uniform
Resource Locators, in that all absolute URLs can be interpreted
consistently relative to any Internet-accessible resource. For the
sake of clarity, however, this document will only term "relative"
those URLs which obtain global scope only when interpreted relative
to a separate base URL.
A common use for Uniform Resource Locators is to embed them within
a document (referred to as the "base" document) for the purpose of
identifying other Internet-accessible resources. For example, in
hypertext documents, URLs can be used as the identifiers for
hypertext link destinations.
Absolute URLs contain a great deal of information which may already
be known from the context of the base document's retrieval,
including the access scheme, network location, and parts of the
URL path. In situations where the base URL is well-defined and
known, it is useful to be able to embed a URL reference which
inherits that context rather than re-specifying it within each
instance.
It is often the case that a group or "tree" of documents has been
constructed to serve a common purpose; the vast majority of URLs
within these documents point to locations within the tree rather
than outside of it. Similarly, documents located at a particular
Internet site are much more likely to refer to other resources at
that site than to resources at remote sites.
Relative addressing of URLs allows document trees to be partially
independent of their location and/or access scheme. For instance,
if they refer to each other using relative URLs, it is possible for
a single set of documents to be simultaneously accessible and, if
hypertext, traversable via each of the "file", "http", and "ftp"
access schemes. Furthermore, document trees can be moved, as a whole,
without changing any of the embedded URLs. Experience within the
World-Wide Web has demonstrated that the ability to perform relative
referencing is necessary for the long-term usability of embedded
URLs.
2. Relative URL Syntax
The syntax for relative URLs is the same as that for absolute URLs
[2], with the exception that portions of the URL may be missing, and
certain path components ("." and "..") have a special meaning when
interpreting a relative URL path. Although this document does not
seek to define the overall URL syntax, some discussion of it is
necessary in order to describe the parsing of relative URLs.
2.1. URL Syntactic Components
Like absolute URLs, relative URL syntax is dependent upon the access
scheme. Some schemes use "?" and ";" to indicate special reserved
components, while others just consider them to be part of the path.
However, there is enough uniformity in the syntax to allow a parser
to resolve relative URLs based upon a few syntactic categories.
These categories are described in Section 2.3.
In general, the relative URL syntax consists of six components:
<scheme>://<net_loc>/<path>;<params>?<query>#<fragment>
each of which may be absent or may be disallowed by a particular
scheme. They are defined as follows (a complete BNF is provided in
Section 2.2):
scheme ":" ::= access scheme name, as per Section 2.1 of [2].
"//" net_loc ::= network location and login information, as per
Section 3.1 of [2].
"/" path ::= URL path, as per Section 3.1 of [2].
";" params ::= object parameters (e.g. ";type=a" as in
Section 3.2.2 of [2]).
"?" query ::= query information, as per Section 3.3 of [2].
"#" fragment ::= fragment identifier (currently only used within
the World-Wide Web initiative).
The order of the components is important. If both <params> and
<query> are present, the <query> information must occur after the
<params>. Relative components are resolved from left-to-right,
according to the rules given in Section 4.
2.2. BNF for Relative URLs
This is a BNF-like description of the Relative Uniform Resource
Locator syntax, using the conventions of RFC 822 [7], except that
"|" is used to designate alternatives, and brackets "[]" are used
around optional or repeated elements. Briefly, literals are quoted
with "", optional elements are enclosed in [brackets], and elements
may be preceded with <n>* to designate n or more repetitions of the
following element; n defaults to 0.
Because relative URLs are parsed within the context of the base URL,
this BNF is not sufficient to completely specify the allowed syntax
within any given context. Section 2.4 describes a context-sensitive
parsing algorithm which disambiguates the grammar.
relativeURL = [ absoluteURL | location | abs_path | rel_path ]
[ "#" fragment ]
absoluteURL = scheme ":" *[ uchar | reserved ]
location = "//" net_loc [ "/" rel_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]
path = segment *[ "/" segment ]
segment = *[ pchar | ";" ]
params = param *[ ";" param ]
param = *[ pchar | "/" ]
scheme = 1*[ alpha | digit | "+" | "-" | "." ]
net_loc = *[ pchar | ";" ]
query = *[ uchar | reserved ]
fragment = *[ uchar | reserved ]
pchar = [ uchar | "?" | ":" | "@" | "&" | "=" ]
uchar = unreserved | escape
unreserved = alpha | digit | safe | extra | national
escape = "%" hex hex
hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f"
alpha = lowalpha | hialpha
lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
"j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
"s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
hialpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
"J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
"S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
"8" | "9"
safe = "$" | "-" | "_" | "." | "+"
extra = "!" | "*" | "'" | "(" | ")" | ","
national = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`"
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "="
punctuation = "<" | ">" | "#" | "%" | <">
2.3. Specific Schemes and their Syntactic Categories
Each URL access scheme has its own rules regarding the presence or
absence of the syntactic components described in Section 2.1 and 2.2.
However, there is enough commonality among the schemes to be able
to group them into just a few categories. These categories are
sufficiently general to allow new schemes to be added without
substantial changes to the algorithm for resolving relative URLs.
Within this section, we include as examples only those schemes
which have a defined URL syntax in [2]. This includes:
ftp File Transfer Protocol [3]
http Hypertext Transfer Protocol [4]
gopher Gopher and Gopher+ Protocols [5, 6]
mailto Electronic Mail [7]
news USENET news [8]
nntp USENET news using NNTP access [9]
telnet TELNET Protocol for Interactive Sessions [10]
wais Wide Area Information Servers Protocol [11,12]
file Host-specific Files
prospero Prospero Directory Service [13]
It is recommended that new schemes include a description of their
membership in the following categories when they are registered,
as per Section 4 of [2]. Membership in the five categories is
described in terms of named sets: Uses-Netloc, Non-Hierarchical,
Uses-Params, Uses-Query, and Uses-Fragment.
2.3.1 The Uses-Netloc Set
The Uses-Netloc set includes those access schemes which use the
Common Internet Scheme Syntax described in Section 3.1 of [2], where
the network location and/or login information starts with a
double-slash "//" to indicate its presence, and continues until the
following slash "/", if any.
Uses-Netloc = {ftp, http, gopher, nntp, telnet, wais,
file, prospero}
2.3.2 The Non-Hierarchical Set
The Non-Hierarchical set includes those access schemes which do not
use "/" to indicate hierarchical segments in the URL path.
Non-Hierarchical = {gopher, wais, mailto, news, telnet, prospero}
When resolving relative paths for schemes not in the Non-Hierarchical
set, the complete path segments ".." and "." have a significance
reserved for representing the path hierarchy, indicating up-one-level
and current-level, respectively.
2.3.3 The Uses-Params Set
The Uses-Params set includes those access schemes which allow the
semicolon ";" character to separate object parameters from the
URL path. There may be more than one parameter, each being
separated by a semicolon ";".
Uses-Params = {ftp, http, prospero}
2.3.4 The Uses-Query Set
The Uses-Query set includes those access schemes which allow the
question mark "?" character to separate query information from the
URL path.
Uses-Query = {http, wais}
2.3.5 The Uses-Fragment Set
The Uses-Fragment set includes those access schemes which allow the
crosshatch "#" character to separate a fragment identifier from
the rest of the URL. Within systems that use fragment identifiers,
Uses-Fragment = {ftp, http, gopher, news, nntp, wais,
file, prospero}
Unlike the other sets, however, the fragment identifier is only
reserved within systems which use it. Outside of those systems,
Uses-Fragment is equal to the empty set.
2.3.6. Summary of Categories by Scheme
Uses- Non-Hier Uses- Uses- Uses-
Netloc archical Params Query Fragment
.--------------------------------------------.
ftp | XXXX | | XXXX | | XXXX |
http | XXXX | | XXXX | XXXX | XXXX |
gopher | XXXX | XXXX | | | XXXX |
mailto | | XXXX | | | |
news | | XXXX | | | XXXX |
nntp | XXXX | | | | XXXX |
telnet | XXXX | XXXX | | | |
wais | XXXX | XXXX | | XXXX | XXXX |
file | XXXX | | | | XXXX |
prospero | XXXX | XXXX | XXXX | | XXXX |
`--------------------------------------------'
2.4. Parsing a URL
An accepted method for parsing URLs is necessary to disambiguate the
relative URL syntax of Section 2.2 and to describe the algorithm for
resolving relative URLs presented in Section 4. This section
describes the parsing rules for breaking down a URL (relative or
absolute) into the component parts described in Section 2.1. The
rules assume that the URL has already been separated from any
surrounding text and copied to a "parse string". The rules are
listed in the order in which they must be applied by the parser.
2.4.1. Parsing the Scheme
If the parse string contains a colon ":" after the first character
and before any characters not allowed as part of a scheme name
(i.e. any not an alphanumeric, plus "+", period ".", or hyphen "-"),
the scheme of the URL is the substring of characters up to but not
including the first colon. These characters and the colon are then
removed from the parse string before continuing.
2.4.2. Parsing the Fragment Identifier
If the scheme is not a member of the Uses-Fragment set, this section
is skipped.
If the parse string contains a crosshatch "#" character, then the
substring after the last (right-most) crosshatch "#" and up to the
end of the parse string is the fragment identifier. If the
crosshatch is the last character, or no crosshatch is present, then
the fragment identifier is empty. The matched substring, including
the crosshatch character, is removed from the parse string before
continuing.
Note that the fragment identifier is not considered part of the URL.
However, since it is often attached to the URL, parsers must be able
to recognize and set aside fragment identifiers as part of the
process.
2.4.3. Parsing the Network Location/Login
If the scheme is not a member of the Uses-Netloc set, this section
is skipped.
If the parse string begins with a double-slash "//", then the
substring of characters after the double-slash and up to, but not
including, the next slash "/" character is the network location/login
(<net_loc>) of the URL. If no trailing slash "/" is present, the
entire remaining parse string is assigned to <net_loc>. The
double-slash and <net_loc> are removed from the parse string before
continuing.
2.4.4. Parsing the Query Information
If the scheme is not a member of the Uses-Query set, this section
is skipped.
If the parse string contains a question mark "?" character, then the
substring after the first (left-most) question mark "?" and up to the
end of the parse string is the query information. If the question
mark is the last character, or no question mark is present, then the
query information is empty. The matched substring, including the
question mark character, is removed from the parse string before
continuing.
2.4.5. Parsing the Parameters
If the scheme is not a member of the Uses-Params set, this section
is skipped.
If the parse string contains a semicolon ";" character, then the
substring after the first (left-most) semicolon ";" and up to the
end of the parse string is the parameters (<params>). If the
semicolon is the last character, or no semicolon is present, then
<params> is empty. The matched substring, including the semicolon
character, is removed from the parse string before continuing.
2.4.6. Parsing the Path
After the above steps, all that is left of the parse string is
the URL path and the slash "/" that may precede it. Even though
the initial slash is not part of the URL path, the parser must
remember whether or not it was present so that later processes
can differentiate between relative and absolute paths. Often this
is done by simply storing the preceding slash along with the path.
3. Establishing a Base URL
In order for relative URLs to be usable within a base document,
the absolute "base URL" of that document must be known to the
parser. There are three methods for obtaining the base URL of
a document, listed here in order of precedence.
3.1. Base URL within Document Content
Within certain document content-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 schemes which do not support relative addressing
(e.g. E-Mail or USENET news).
It is beyond the scope of this document to specify how, for each
content-type, the base URL can be embedded. However, an example of
how this is done for the Hypertext Markup Language (HTML) [14] is
provided in an Appendix (Section 10).
3.2. Base URL within Message Headers
For access schemes which make use of message headers like those
described in RFC 822 [7], a second method for identifying the base
URL of a document is to include that URL in the message headers.
It is recommended that the format of this header be:
Base-URL: absoluteURL
where "Base-URL" is case-insensitive. For example,
Base-URL: http://www.ics.uci.edu/Test/a/b/c
would indicate that any relative URLs found within the document
should be parsed relative to <URL:http://www.ics.uci.edu/Test/a/b/c>.
In situations where both an embedded base URL (as described in
Section 3.1) and a "Base-URL" message header are present, the
embedded base URL takes precedence.
3.3. Base URL from the Retrieval Context
If neither an embedded base URL nor a "Base-URL" message header
is present, then, if a URL was used to retrieve the base document,
that URL shall be considered the base URL. Note that if the
retrieval was the result of a redirected request, the last URL used
(i.e., that which resulted in the actual retrieval of the document)
is the base URL.
3.4. Default Base URL
If none of the conditions described in Sections 3.1 -- 3.3 apply,
then the base URL is considered to be the empty string and all
embedded URLs within that document shall be interpreted as absolute.
It is the responsibility of the distributor(s) of a document
containing relative URLs to ensure that the base URL for that
document can be established. It must be emphasized that relative
URLs cannot be used reliably in situations where the object's base
URL is not well-defined.
4. Resolving Relative URLs
This section describes an example algorithm for resolving URLs
within a context in which the URLs may be relative, such that the
result is always a URL in absolute form. Although this algorithm
cannot guarantee that the resulting URL will equal that intended
by the original author, it does guarantee that any valid URL
(relative or absolute) can be consistently transformed to an
absolute form given a valid base URL.
The following steps are performed in order:
Step 1: The base URL is established according to the rules of
Section 3.
Step 2: If the base URL is the empty string (unknown), the embedded
URL is interpreted as an absolute URL and we are done.
Step 3: Both the base and embedded URLs are parsed into their
component parts as described in Section 2.4.
a) If the embedded URL starts with a scheme name, it is
interpreted as an absolute URL and we are done.
b) Otherwise, the embedded URL inherits the scheme of
the base URL.
Step 4: If the scheme is a member of the Uses-Netloc set
(Section 2.3.1), then
a) If the embedded URL's <net_loc> is non-empty, we skip to
Step 8.
b) Otherwise, the embedded URL inherits the <net_loc> of the
base URL.
Step 5: If the embedded URL path is preceded by a slash "/", the
path is not relative and we skip to Step 8.
Step 6: If the embedded URL path is empty (and not preceded by a
slash), then
a) The embedded URL inherits the base URL path; and,
b) If the embedded URL's <params> is empty, it
inherits the <params> of the base URL (if any); and,
c) If the embedded URL's <query> is empty, it inherits
the <query> of the base URL (if any); and,
d) We skip to Step 8.
Step 7: The last path segment of the base URL's path (anything
following the rightmost slash "/", or the entire path if no
slash is present) is removed and the embedded URL's path is
appended in its place. The following operations are
then applied, in order, to the new URL path:
a) All occurrences of "./", where "." is a complete path
segment, are removed.
b) If the URL path ends with "." as a complete path segment,
that "." is removed.
c) All occurrences of "<segment>/../", where <segment> and
".." are complete path segments, are removed. Removal of
these path segments is performed iteratively, removing the
leftmost matching pattern on each iteration, until no
matching pattern remains.
d) If the URL path ends with "<segment>/..", that
"<segment>/.." is removed.
Step 8: The resulting URL components, including any inherited from
the base URL, are recombined to give the absolute form of
the embedded URL.
Parameters, regardless of their purpose, do not form a part of the
URL path and thus have no effect on the resolving of relative paths.
In particular, the presence or absence of the ";type=d" parameter
on an ftp URL has no effect on the interpretation of paths relative
to that URL. Fragment identifiers are never inherited from the
base URL.
5. Examples and Recommended Practice
Within an object with a well-defined base URL of
<URL:http://a/b/c/d>
the relative URLs would be resolved as follows:
5.1. Normal Examples
g:h = <URL:g:h>
g = <URL:http://a/b/c/g>
./g = <URL:http://a/b/c/g>
g/ = <URL:http://a/b/c/g/>
/g = <URL:http://a/g>
//g = <URL:http://g>
?y = <URL:http://a/b/c/d?y>
g?y = <URL:http://a/b/c/g?y>
g?y/./x = <URL:http://a/b/c/g?y/./x>
. = <URL:http://a/b/c/>
./ = <URL:http://a/b/c/>
.. = <URL:http://a/b/>
../ = <URL:http://a/b/>
../g = <URL:http://a/b/g>
../.. = <URL:http://a/>
../../g = <URL:http://a/g>
5.2. Abnormal Examples
../../../g = <URL:http://a/../g>
./../g = <URL:http://a/b/g>
./g/. = <URL:http://a/b/c/g/>
/./g = <URL:http://a/./g>
g/./h = <URL:http://a/b/c/g/h>
g/../h = <URL:http://a/b/c/h>
http:g = <URL:http:g>
http: = <URL:http:>
Note that, although the abnormal examples are not likely to occur
for a normal relative URL, all URL parsers should be capable of
resolving them consistently.
5.3. Recommended Practice
Authors should be aware that path names which contain a colon
":" character cannot be used as the first component of a relative
URL path (e.g. "this:that") because they will likely be mistaken for
a scheme name. It is therefore necessary to precede such cases with
other components (e.g., "./this:that"), or to escape the colon
character (e.g., "this%3Athat"), in order for them to be correctly
parsed. The former solution is preferred because it has no effect
on the absolute form of the URL.
6. Security Considerations
None.
7. Acknowledgements
This work is derived from concepts introduced by Tim Berners-Lee and
the World-Wide Web global information initiative. Relative URLs are
described as "Partial URLs" in RFC 1630 [1]. That description was
expanded for inclusion as an appendix for the Internet-Draft
"Uniform Resource Locators (URL)" [2]. However, after further
discussion, the URI-WG decided to specify Relative URLs separately
from the primary URL draft.
This document is intended to fulfill the requirements for Internet
Resource Locators as stated in [15]. It has benefited greatly from
the comments of all those participating in the URI-WG. Particular
thanks go to Larry Masinter, Michael A. Dolan, Guido van Rossum, and
Dave Kristol for identifying problems/deficiencies in earlier drafts.
8. References
[1] Berners-Lee, T., "Universal Resource Identifiers in WWW:
A Unifying Syntax for the Expression of Names and Addresses of
Objects on the Network as used in the World-Wide Web", RFC 1630,
<URL:ftp://ds.internic.net/rfc/rfc1630.txt>, June 1994.
[2] Berners-Lee, T., Masinter, L., and McCahill, M., Editors,
"Uniform Resource Locators (URL)", Internet-Draft (work in
progress), <URL:ftp://ds.internic.net/internet-drafts/
draft-ietf-uri-url-08.txt>, October 1994.
[3] Postel, J. and Reynolds, J.K., "File Transfer Protocol (FTP)",
STD 9, RFC 959, <URL:ftp://ds.internic.net/rfc/rfc959.txt>,
October 1985.
[4] Berners-Lee, T ., "Hypertext Transfer Protocol (HTTP)" ,
CERN, <URL:ftp://info.cern.ch/pub/www/doc/http-spec.txt.Z>,
November 1993.
[5] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
Torrey, D., and Alberti, B., "The Internet Gopher Protocol:
A distributed document search and retrieval protocol",
RFC 1436, <URL:ftp://ds.internic.net/rfc/rfc1436.txt>,
March 1993.
[6] Anklesaria, F., Lindner, P., McCahill, M., Torrey, D.,
Johnson, D., and Alberti, B., "Gopher+: Upward compatible
enhancements to the Internet Gopher protocol",
University of Minnesota, <URL:ftp://boombox.micro.umn.edu
/pub/gopher/gopher_protocol/Gopher+/Gopher+.txt>, July 1993.
[7] Crocker, D. H., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822,
<URL:ftp://ds.internic.net/rfc/rfc822.txt>, August 1982.
[8] Horton, M. and Adams, R., "Standard For Interchange of USENET
messages", RFC 1036, <URL:ftp://ds.internic.net/rfc/rfc1036.txt>,
December 1987.
[9] Kantor, B. and Lapsley, P., "Network News Transfer Protocol:
A Proposed Standard for the Stream-Based Transmission of News",
RFC977, <URL:ftp://ds.internic.net/rfc/rfc977.txt>,
February 1986.
[10] Postel, J. and Reynolds, J.K., "TELNET Protocol Specification",
RFC 854, <URL:ftp://ds.internic.net/rfc/rfc854.txt>, May 1983.
[11] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
Sui, J., and Grinbaum, M., "WAIS Interface Protocol Prototype
Functional Specification", (v1.5), Thinking Machines Corporation,
<URL:ftp://quake.think.com/pub/wais/doc/protspec.txt>,
April 1990.
[12] St. Pierre, M, Fullton, J., Gamiel, K., Goldman, J., Kahle, B.,
Kunze, J., Morris, H., and Schiettecatte, F.,
"WAIS over Z39.50-1988", RFC 1625,
<URL:ftp://ds.internic.net/rfc/rfc1625.txt>, June 1994.
[13] Neuman, B.C., and Augart, S. "The Prospero Protocol",
USC Information Sciences Institute, <URL:
ftp://prospero.isi.edu/pub/prospero/doc/prospero-protocol.PS.Z>,
June 1993.
[14] Berners-Lee, T., Connolly, D., et al. "HyperText Markup Language
Specification -- 2.0", Internet-Draft (work in progress),
<URL:ftp://www.ics.uci.edu/pub/ietf/html/>, November 1994.
[15] Kunze, J., "Functional Requirements for Internet Resource
Locators", Internet-Draft (work in progress),
<URL:ftp://ds.internic.net/internet-drafts/
draft-ietf-uri-irl-fun-req-01.txt>, July 1994.
9. Author's Address
Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425
U.S.A.
Tel: +1 (714) 824-4049
Fax: +1 (714) 824-4056
Email: fielding@ics.uci.edu
This Internet-Draft expires May 27, 1995.
10. Appendix - Embedding the Base URL in HTML documents.
It is useful to consider an example of how the base URL of a
document can be embedded within the document's content. In this
appendix, we describe how documents written in the Hypertext Markup
Language (HTML) [14] can include an embedded base URL. This appendix
does not form a part of the relative URL specification and should not
be considered as anything more than a descriptive example.
HTML defines a special element "BASE" which, when present in the
"HEAD" portion of a document, signals that the parser should use
the BASE element's "HREF" attribute as the base URL for resolving
any relative URLs. The "HREF" attribute must be an absolute URL.
Note that, in HTML, element and attribute names are case-insensitive.
For example:
<!doctype html public "-//IETF//DTD HTML//EN">
<HTML><HEAD>
<TITLE>An example HTML document</TITLE>
<BASE href="http://www.ics.uci.edu/Test/a/b/c">
</HEAD><BODY>
... <A href="../x">a hypertext anchor</A> ...
</BODY></HTML>
A parser reading the example document should interpret the given
relative URL "../x" as representing the absolute URL
<URL:http://www.ics.uci.edu/Test/a/x>
regardless of the context in which the example document was obtained.