Uniform Resource Identifiers Working Group R. T. Fielding
INTERNET-DRAFT UC Irvine
Expires July 30, 1995 January 30, 1995
Relative Uniform Resource Locators
<draft-ietf-uri-relative-url-05.txt>
Status of this Memo
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Abstract
A Uniform Resource Locator (URL) is 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 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 document describes the syntax and semantics for "relative"
Uniform Resource Locators (relative URLs): a compact representation
of the location of a resource relative to an absolute base URL.
It is a companion to RFC 1738, "Uniform Resource Locators (URL)" [2],
which specifies the syntax and semantics of absolute URLs.
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 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.
Similarly, relative URLs can be used within data-entry dialogs to
decrease the number of characters necessary to describe a location.
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 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"
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 a shortened form of that for absolute
URLs [2], where some prefix of the URL is missing and certain path
components ("." and "..") have a special meaning when interpreting a
relative path. Because a relative URL may appear in any context that
could hold an absolute URL, systems that support relative URLs must
be able to recognize them as part of the URL parsing process.
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. In particular, base documents can only
make use of relative URLs when their base URL fits within the
generic-RL syntax described below. Although some URL schemes do not
require this generic-RL syntax, it is assumed that any document which
contains a relative reference does have a base URL that obeys the
syntax. In other words, relative URLs cannot be used within
documents that have unsuitable base URLs.
2.1. URL Syntactic Components
The URL syntax is dependent upon the scheme. Some schemes use
reserved characters like "?" and ";" to indicate special components,
while others just consider them to be part of the path. However,
there is enough uniformity in the use of URLs to allow a parser
to resolve relative URLs based upon a single, generic-RL syntax.
This generic-RL syntax consists of six components:
<scheme>://<net_loc>/<path>;<params>?<query>#<fragment>
each of which, except <scheme>, may be absent from a particular URL.
These components are defined as follows (a complete BNF is provided
in Section 2.2):
scheme ":" ::= scheme name, as per Section 2.1 of RFC 1738 [2].
"//" net_loc ::= network location and login information, as per
Section 3.1 of RFC 1738 [2].
"/" path ::= URL path, as per Section 3.1 of RFC 1738 [2].
";" params ::= object parameters (e.g. ";type=a" as in
Section 3.2.2 of RFC 1738 [2]).
"?" query ::= query information, as per Section 3.3 of
RFC 1738 [2].
"#" fragment ::= fragment identifier.
Note that the fragment identifier (and the "#" that precedes it) is
not considered part of the URL. However, since it is commonly used
within the same string context as a URL, a parser must be able to
recognize the fragment when it is present and set it aside as part
of the parsing process.
The order of the components is important. If both <params> and
<query> are present, the <query> information must occur after the
<params>.
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 [5], except that
"|" is used to designate alternatives. Briefly, literals are quoted
with "", parentheses "(" and ")" are used to group elements, 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.
URL = ( absoluteURL | relativeURL ) [ "#" fragment ]
absoluteURL = generic-RL | ( scheme ":" *( uchar | reserved ) )
generic-RL = scheme ":" relativeURL
relativeURL = net_path | abs_path | rel_path
net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]
path = fsegment *( "/" segment )
fsegment = 1*pchar
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 scheme has its own rules regarding the presence or absence
of the syntactic components described in Sections 2.1 and 2.2.
In addition, some schemes are never appropriate for use with relative
URLs. However, since relative URLs will only be used within contexts
in which they are useful, these scheme-specific differences can be
ignored by the resolution process.
Within this section, we include as examples only those schemes that
have a defined URL syntax in RFC 1738 [2]. The following schemes are
never used with relative URLs:
mailto Electronic Mail
news USENET news
telnet TELNET Protocol for Interactive Sessions
Some URL schemes allow the use of reserved characters for purposes
outside the generic-RL syntax given above. However, such use is
rare. Relative URLs can be used with these schemes whenever the
applicable base URL follows the generic-RL syntax.
gopher Gopher and Gopher+ Protocols
prospero Prospero Directory Service
wais Wide Area Information Servers Protocol
Users of gopher URLs should note that gopher-type information is
often included at the beginning of what would be the generic-RL path.
If present, this type information prevents relative-path references
to documents with differing gopher-types.
Finally, the following schemes can always be parsed using the
generic-RL syntax.
file Host-specific Files
ftp File Transfer Protocol
http Hypertext Transfer Protocol
nntp USENET news using NNTP access
It is recommended that new schemes be designed to be parsable via
the generic-RL syntax if they are intended to be used with relative
URLs. A description of the allowed relative forms should be included
when a new scheme is registered, as per Section 4 of RFC 1738 [2].
2.4. Parsing a URL
An accepted method for parsing URLs is useful to clarify the
generic-RL 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 would be applied by the parser.
2.4.1. Parsing the Fragment Identifier
If the parse string contains a crosshatch "#" character, then the
substring after the first (left-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.
2.4.2. 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.
2.4.3. Parsing the Network Location/Login
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 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 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
The term "relative URL" implies that there exists some absolute
"base URL" against which the relative reference is applied. Indeed,
the base URL is necessary to define the semantics of any embedded
relative URLs; without it, a relative reference is meaningless.
In order for relative URLs to be usable within a document, the base
URL of that document must be known to the parser.
The base URL of a document can be established in one of four ways,
listed below in order of precedence. The order of precedence can be
thought of in terms of layers, where the innermost defined base URL
has the highest precedence. This can be visualized graphically as:
.---------------------------------------------------------.
| .---------------------------------------------------. |
| | .---------------------------------------------. | |
| | | .---------------------------------------. | | |
| | | | (3.1) Base URL embedded in the | | | |
| | | | document's content | | | |
| | | `---------------------------------------' | | |
| | | (3.2) URL defined by a "Base" message | | |
| | | header (or equivalent) | | |
| | `---------------------------------------------' | |
| | (3.3) URL of the document's retrieval context | |
| `---------------------------------------------------' |
| (3.4) Base URL = "" (undefined) |
`---------------------------------------------------------'
3.1. Base URL within Document Content
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. However, an example of
how this is done for the Hypertext Markup Language (HTML) [3] is
provided in an Appendix (Section 10).
3.2. Base URL within Message Headers
A second method for identifying the base URL of a document is to
specify it within the message headers (or equivalent tagged
metainformation) of the message enclosing the document. For
protocols that make use of message headers like those described in
RFC 822 [5], it is recommended that the format of this header be:
base-header = "Base" ":" "<URL:" absoluteURL ">"
where "Base" is case-insensitive. For example, the header
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>.
Any whitespace (including that used for line folding) inside the
angle brackets should be ignored.
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 passing the base URL
as part of a message. Describing the syntax for all possible
protocols is beyond the scope of this document. It is assumed that
user agents using such a protocol will be able to obtain the
appropriate syntax from that protocol's specification.
In situations where both an embedded base URL (as described in
Section 3.1) and a base-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-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.
Composite media types, such as the "multipart/*" and "message/*"
media types defined by MIME (RFC 1521, [4]), require special
processing in order to determine the retrieval context of an enclosed
document. For these types, the base URL of the composite entity
must be determined first; this base is then considered the retrieval
context for its component parts, and thus the base URL for any part
that does not define its own base via one of the methods described
in Sections 3.1 and 3.2. This logic is applied recursively for
component parts that are themselves composite entities.
In other words, the retrieval context (Section 3.3) of a component
part is the base URL of the composite entity of which it is a part.
Thus, a composite entity can redefine the retrieval context of its
component parts via inclusion of a base-header, and this redefinition
applies recursively for a hierarchy of composite parts. Note that
this is not necessarily the same as defining the base URL of the
components, since each component may include an embedded base URL
or base-header that takes precedence over the retrieval context.
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 are assumed to be absolute URLs.
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. If the base URL is the empty string (unknown),
the embedded URL is interpreted as an absolute URL and
we are done.
Step 2: Both the base and embedded URLs are parsed into their
component parts as described in Section 2.4.
a) If the embedded URL is entirely empty, it inherits the
entire base URL (i.e. is set equal to the base URL)
and we are done.
b) If the embedded URL starts with a scheme name, it is
interpreted as an absolute URL and we are done.
c) Otherwise, the embedded URL inherits the scheme of
the base URL.
Step 3: If the embedded URL's <net_loc> is non-empty, we skip to
Step 7. Otherwise, the embedded URL inherits the <net_loc>
(if any) of the base URL.
Step 4: If the embedded URL path is preceded by a slash "/", the
path is not relative and we skip to Step 7.
Step 5: If the embedded URL path is empty (and not preceded by a
slash), then the embedded URL inherits the base URL path,
and
a) if the embedded URL's <params> is non-empty, we skip to
step 7; otherwise, it inherits the <params> of the base
URL (if any) and
b) if the embedded URL's <query> is non-empty, we skip to
step 7; otherwise, it inherits the <query> of the base
URL (if any) and we skip to step 7.
Step 6: The last 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 path:
a) All occurrences of "./", where "." is a complete path
segment, are removed.
b) If the 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 path ends with "<segment>/..", that "<segment>/.."
is removed.
Step 7: 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 only inherited from the base
URL when the entire embedded URL is empty.
5. Examples and Recommended Practice
Within an object with a well-defined base URL of
Base: <URL:http://a/b/c/d;p?q#f>
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;p?y>
g?y = <URL:http://a/b/c/g?y>
g?y/./x = <URL:http://a/b/c/g?y/./x>
#s = <URL:http://a/b/c/d;p?q#s>
g#s = <URL:http://a/b/c/g#s>
g#s/./x = <URL:http://a/b/c/g#s/./x>
g?y#s = <URL:http://a/b/c/g?y#s>
;x = <URL:http://a/b/c/d;x>
g;x = <URL:http://a/b/c/g;x>
g;x?y#s = <URL:http://a/b/c/g;x?y#s>
. = <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/>
../../ = <URL:http://a/>
../../g = <URL:http://a/g>
5.2. Abnormal Examples
Although the following abnormal examples are unlikely to occur
in normal practice, all URL parsers should be capable of resolving
them consistently. Each example uses the same base as above.
An empty reference resolves to the complete base URL:
<> = <URL:http://a/b/c/d;p?q#f>
Parsers must be careful in handling the case where there are more
relative path ".." segments than there are hierarchical levels in
the base URL's path. Note that the ".." syntax cannot be used to
change the <net_loc> of a URL.
../../../g = <URL:http://a/../g>
Similarly, parsers must avoid treating "." and ".." as special when
they are not complete components of a relative path.
/./g = <URL:http://a/./g>
/../g = <URL:http://a/../g>
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/b/c/..g>
Less likely are cases where the relative URL uses unnecessary or
nonsensical forms of the "." and ".." complete path segments.
./../g = <URL:http://a/b/g>
./g/. = <URL:http://a/b/c/g/>
g/./h = <URL:http://a/b/c/g/h>
g/../h = <URL:http://a/b/c/h>
Finally, some older parsers allow the scheme name to be present in
a relative URL if it is the same as the base URL scheme. This is
considered to be a loophole in prior specifications of partial
URLs [1] and should be avoided by future parsers.
http:g = <URL:http:g>
http: = <URL:http:>
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.
There is an ambiguity in the semantics for the ftp URL scheme
regarding the use of a trailing slash ("/") character and/or a
parameter ";type=d" to indicate a resource that is an ftp directory.
If the result of retrieving that directory includes embedded
relative URLs, it is necessary that the base URL path for that result
include a trailing slash. For this reason, it is recommended that
the ";type=d" parameter value not be used within contexts that allow
relative URLs.
6. Security Considerations
There are no security considerations in the use or parsing of relative
URLs. However, once a relative URL has been resolved to its absolute
form, the same security considerations apply as those described in
RFC 1738 [2].
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 an early draft of RFC 1738,
"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 [6]. 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] T. Berners-Lee, "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,
CERN, June 1994. <URL:ftp://ds.internic.net/rfc/rfc1630.txt>
[2] T. Berners-Lee, L. Masinter, and M. McCahill, Editors,
"Uniform Resource Locators (URL)", RFC 1738, CERN,
Xerox Corporation, University of Minnesota, December 1994.
<URL:ftp://ds.internic.net/rfc/rfc1738.txt>
[3] T. Berners-Lee and D. Connolly, "HyperText Markup Language
Specification -- 2.0", Work in Progress, MIT, HaL Computer
Systems, November 1994.
<URL:http://www.ics.uci.edu/pub/ietf/html/>
[4] N. Borenstein and N. Freed, "MIME (Multipurpose Internet Mail
Extensions): Mechanisms for Specifying and Describing the Format
of Internet Message Bodies", RFC 1521, Bellcore, Innosoft,
September 1993. <URL:ftp://ds.internic.net/rfc/rfc1521.txt>
[5] D. H. Crocker, "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC 822, UDEL, August 1982.
<URL:ftp://ds.internic.net/rfc/rfc822.txt>
[6] J. Kunze, "Functional Requirements for Internet Resource
Locators", Work in Progress, IS&T, UC Berkeley, January 1995.
<URL:ftp://ds.internic.net/internet-drafts/
draft-ietf-uri-irl-fun-req-03.txt>
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 July 30, 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) [3] 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.