RFC1808 - Relative Uniform Resource Locators

Network Working Group R. Fielding

Request for Comments: 1808 UC Irvine

Category: Standards Track June 1995

Relative Uniform Resource Locators

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

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 RFC1738, "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. Relative URLs can

also be used within data-entry dialogs to decrease the number of

characters necessary to describe a location.

In addition, 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 in 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, it is

possible for a single set of hypertext documents to be simultaneously

accessible and traversable via each of the "file", "http", and "FTP"

schemes if the documents refer to each other using relative URLs.

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:

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 RFC1738 [2].

"//" net_loc ::= network location and login information, as per

Section 3.1 of RFC1738 [2].

"/" path ::= URL path, as per Section 3.1 of RFC1738 [2].

";" params ::= object parameters (e.g., ";type=a" as in

Section 3.2.2 of RFC1738 [2]).

"?" query ::= query information, as per Section 3.3 of

RFC1738 [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 RFC822 [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.

This BNF also describes the generic-RL syntax for valid base URLs.

Note that this differs from the URL syntax defined in RFC1738 [2] in

that all schemes are required to use a single set of reserved

characters and use them consistently within the major URL components.

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

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 RFC1738 [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

almost always 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. This does not necessarily imply that relative

URLs will be useful with these schemes -- that decision is left to

the system implementation and the author of the base document.

file Host-specific Files

ftp File Transfer Protocol

http Hypertext Transfer Protocol

nntp USENET news using NNTP access

NOTE: Section 5 of RFC1738 specifies that the question-mark

character ("?") is allowed in an ftp or file path segment.

However, this is not true in practice and is believed to be an

error in the RFC. Similarly, RFC1738 allows the reserved

character semicolon (";") within an http path segment, but does

not define its semantics; the correct semantics are as defined

by this document for <params>.

We recommend 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 RFC1738 [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) Base URL of the encapsulating entity

(message, document, or none).

`----------------------------------------------'

(3.3) URL used to retrieve the entity

`----------------------------------------------------'

(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. 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 RFC822 [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 RFC822 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.

3.2. Base URL from the Encapsulating Entity

If no base URL is embedded, the base URL of a document is defined by

the document's retrieval context. For a document that is enclosed

within another entity (such as a message or another document), the

retrieval context is that entity; thus, the default base URL of the

document is the base URL of the entity in which the document is

encapsulated.

Composite media types, such as the "multipart/*" and "message/*"

media types defined by MIME (RFC1521, [4]), define a hierarchy of

retrieval context for their enclosed documents. In other words, the

retrieval context 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 the

inclusion of a base-header, and this redefinition applies recursively

for a hierarchy of composite parts. Note that this might not change

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.

3.3. Base URL from the Retrieval URL

If no base URL is embedded and the document is not encapsulated

within some other entity (e.g., the top level of a composite entity),

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 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 document'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> is a

complete path segment not equal to "..", 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>/..", where <segment> is a

complete path segment not equal to "..", 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 do not affect the resolving of relative paths. In

particular, the presence or absence of the ";type=d" parameter on an

ftp URL does not affect the interpretation of paths relative to that

URL. Fragment identifiers are only inherited from the base URL when

the entire embedded URL is empty.

The above algorithm is intended to provide an example by which the

output of implementations can be tested -- implementation of the

algorithm itself is not required. For example, some systems may find

it more efficient to implement Step 6 as a pair of segment stacks

being merged, rather than as a series of string pattern matches.

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>

../../../../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 does not affect

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, we recommend 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 RFC1738 [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 RFC1630 [1]. That description was

expanded for inclusion as an appendix for an early draft of RFC1738,

"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 recommendations 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, Dave

Kristol, David Robinson, and Brad Barber 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", RFC1630,

CERN, June 1994.

[2] Berners-Lee, T., Masinter, L., and M. McCahill, Editors, "Uniform

Resource Locators (URL)", RFC1738, CERN, Xerox Corporation,

University of Minnesota, December 1994.

[3] Berners-Lee T., and D. Connolly, "HyperText Markup Language

Specification -- 2.0", Work in Progress, MIT, HaL Computer

Systems, February 1995.

<URL:http://www.ics.uci.edu/pub/ietf/html/>

[4] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail

Extensions): Mechanisms for Specifying and Describing the Format

of Internet Message Bodies", RFC1521, Bellcore, Innosoft,

September 1993.

[5] Crocker, D., "Standard for the Format of ARPA Internet Text

Messages", STD 11, RFC822, UDEL, August 1982.

[6] Kunze, J., "Functional Recommendations for Internet Resource

Locators", RFC1736, IS&T, UC Berkeley, February 1995.

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

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">

<TITLE>An example HTML document</TITLE>

</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.