As per Relevance of the word locators, we have this rfc below:

---

Network Working Group R.
Request for Comments: 1808 UC
Category: Standards Track June 1995


Relative Uniform Resource

Status of this

This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited



A Uniform Resource Locator (URL) is a compact representation of
location and access method for a resource available via the Internet
When embedded within a base document, a URL in its absolute form
contain a great deal of information which is already known from
context of that base document's retrieval, including the scheme
network location, and parts of the url-path. In situations where
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
context rather than re-specifying it in every instance.
document defines the syntax and semantics for such Relative
Resource Locators

1.

This document describes the syntax and semantics for "relative
Uniform Resource Locators (relative URLs): a compact
of the location of a resource relative to an absolute base URL.
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
document (referred to as the "base" document) for the purpose
identifying other Internet-accessible resources. For example,
hypertext documents, URLs can be used as the identifiers
hypertext link destinations

Absolute URLs contain a great deal of information which may
be known from the context of the base document's retrieval,
the scheme, network location, and parts of the URL path.
situations where the base URL is well-defined and known, it is
to be able to embed a URL reference which inherits that



Fielding Standards Track [Page 1]

RFC 1808 Relative Uniform Resource Locators June 1995


rather than re-specifying it within each instance. Relative URLs
also be used within data-entry dialogs to decrease the number
characters necessary to describe a location

In addition, it is often the case that a group or "tree" of
has been constructed to serve a common purpose; the vast majority
URLs in these documents point to locations within the tree
than outside of it. Similarly, documents located at a
Internet site are much more likely to refer to other resources
that site than to resources at remote sites

Relative addressing of URLs allows document trees to be
independent of their location and access scheme. For instance, it
possible for a single set of hypertext documents to be
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,
changing any of the embedded URLs. Experience within the World-
Web has demonstrated that the ability to perform relative
is necessary for the long-term usability of embedded URLs

2. Relative URL

The syntax for relative URLs is a shortened form of that for
URLs [2], where some prefix of the URL is missing and certain
components ("." and "..") have a special meaning when interpreting
relative path. Because a relative URL may appear in any context
could hold an absolute URL, systems that support relative URLs
be able to recognize them as part of the URL parsing process

Although this document does not seek to define the overall
syntax, some discussion of it is necessary in order to describe
parsing of relative URLs. In particular, base documents can
make use of relative URLs when their base URL fits within
generic-RL syntax described below. Although some URL schemes do
require this generic-RL syntax, it is assumed that any document
contains a relative reference does have a base URL that obeys
syntax. In other words, relative URLs cannot be used
documents that have unsuitable base URLs

2.1. URL Syntactic

The URL syntax is dependent upon the scheme. Some schemes
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
resolve relative URLs based upon a single, generic-RL syntax.
generic-RL syntax consists of six components



Fielding Standards Track [Page 2]

RFC 1808 Relative Uniform Resource Locators June 1995


:///;?#<fragment

each of which, except , may be absent from a particular URL
These components are defined as follows (a complete BNF is
in Section 2.2):

scheme ":" ::= scheme name, as per Section 2.1 of RFC 1738 [2].

"//" net_loc ::= network location and login information, as
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
Section 3.2.2 of RFC 1738 [2]).

"?" query ::= query information, as per Section 3.3
RFC 1738 [2].

"#" fragment ::= fragment identifier

Note that the fragment identifier (and the "#" that precedes it)
not considered part of the URL. However, since it is commonly
within the same string context as a URL, a parser must be able
recognize the fragment when it is present and set it aside as part
the parsing process

The order of the components is important. If both
are present, the information must occur after
.

2.2. BNF for Relative

This is a BNF-like description of the Relative Uniform
Locator syntax, using the conventions of RFC 822 [5], except that "|"
is used to designate alternatives. Briefly, literals are quoted
"", parentheses "(" and ")" are used to group elements,
elements are enclosed in [brackets], and elements may be
with * to designate n or more repetitions of the
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 RFC 1738 [2]
that all schemes are required to use a single set of
characters and use them consistently within the major URL components






Fielding Standards Track [Page 3]

RFC 1808 Relative Uniform Resource Locators June 1995


URL = ( absoluteURL | relativeURL ) [ "#" fragment ]

absoluteURL = generic-RL | ( scheme ":" *( uchar | reserved ) )

generic-RL = scheme ":"

relativeURL = net_path | abs_path | rel_

net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_
rel_path = [ path ] [ ";" params ] [ "?" query ]

path = fsegment *( "/" segment )
fsegment = 1*
segment = *

params = param *( ";" param )
param = *( pchar | "/" )

scheme = 1*( alpha | digit | "+" | "-" | "." )
net_loc = *( pchar | ";" | "?" )
query = *( uchar | reserved )
fragment = *( uchar | reserved )

pchar = uchar | ":" | "@" | "&" | "="
uchar = unreserved |
unreserved = alpha | digit | safe |

escape = "%" hex
hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f

alpha = lowalpha |
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 = "<" | ">" | "#" | "%" | <">



Fielding Standards Track [Page 4]

RFC 1808 Relative Uniform Resource Locators June 1995


2.3. Specific Schemes and their Syntactic

Each URL scheme has its own rules regarding the presence or
of the syntactic components described in Sections 2.1 and 2.2.
addition, some schemes are never appropriate for use with
URLs. However, since relative URLs will only be used within
in which they are useful, these scheme-specific differences can
ignored by the resolution process

Within this section, we include as examples only those schemes
have a defined URL syntax in RFC 1738 [2]. The following schemes
never used with relative URLs

mailto Electronic
news USENET
telnet TELNET Protocol for Interactive

Some URL schemes allow the use of reserved characters for
outside the generic-RL syntax given above. However, such use
rare. Relative URLs can be used with these schemes whenever
applicable base URL follows the generic-RL syntax

gopher Gopher and Gopher+
prospero Prospero Directory
wais Wide Area Information Servers

Users of gopher URLs should note that gopher-type information
almost always included at the beginning of what would be
generic-RL path. If present, this type information
relative-path references to documents with differing gopher-types

Finally, the following schemes can always be parsed using
generic-RL syntax. This does not necessarily imply that
URLs will be useful with these schemes -- that decision is left
the system implementation and the author of the base document

file Host-specific
ftp File Transfer
http Hypertext Transfer
nntp USENET news using NNTP

NOTE: Section 5 of RFC 1738 specifies that the question-
character ("?") is allowed in an ftp or file path segment
However, this is not true in practice and is believed to be
error in the RFC. Similarly, RFC 1738 allows the
character semicolon (";") within an http path segment, but
not define its semantics; the correct semantics are as
by this document for .



Fielding Standards Track [Page 5]

RFC 1808 Relative Uniform Resource Locators June 1995


We recommend that new schemes be designed to be parsable via
generic-RL syntax if they are intended to be used with relative URLs
A description of the allowed relative forms should be included when
new scheme is registered, as per Section 4 of RFC 1738 [2].

2.4. Parsing a

An accepted method for parsing URLs is useful to clarify
generic-RL syntax of Section 2.2 and to describe the algorithm
resolving relative URLs presented in Section 4. This
describes the parsing rules for breaking down a URL (relative
absolute) into the component parts described in Section 2.1.
rules assume that the URL has already been separated from
surrounding text and copied to a "parse string". The rules
listed in the order in which they would be applied by the parser

2.4.1. Parsing the Fragment

If the parse string contains a crosshatch "#" character, then
substring after the first (left-most) crosshatch "#" and up to
end of the parse string is the <fragment> identifier. If
crosshatch is the last character, or no crosshatch is present,
the fragment identifier is empty. The matched substring,
the crosshatch character, is removed from the parse string
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
to recognize and set aside fragment identifiers as part of
process

2.4.2. Parsing the

If the parse string contains a colon ":" after the first
and before any characters not allowed as part of a scheme name (i.e.,
any not an alphanumeric, plus "+", period ".", or hyphen "-"),
of the URL is the substring of characters up to but
including the first colon. These characters and the colon are
removed from the parse string before continuing

2.4.3. Parsing the Network Location/

If the parse string begins with a double-slash "//", then
substring of characters after the double-slash and up to, but
including, the next slash "/" character is the network location/
() of the URL. If no trailing slash "/" is present,
entire remaining parse string is assigned to . The double
slash and are removed from the parse string



Fielding Standards Track [Page 6]

RFC 1808 Relative Uniform Resource Locators June 1995


continuing

2.4.4. Parsing the Query

If the parse string contains a question mark "?" character, then
substring after the first (left-most) question mark "?" and up to
end of the parse string is the information. If the
mark is the last character, or no question mark is present, then
query information is empty. The matched substring, including
question mark character, is removed from the parse string
continuing

2.4.5. Parsing the

If the parse string contains a semicolon ";" character, then
substring after the first (left-most) semicolon ";" and up to the
of the parse string is the parameters (). If the
is the last character, or no semicolon is present, then
empty. The matched substring, including the semicolon character,
removed from the parse string before continuing

2.4.6. Parsing the

After the above steps, all that is left of the parse string is
URL and the slash "/" that may precede it. Even though
initial slash is not part of the URL path, the parser must
whether or not it was present so that later processes
differentiate between relative and absolute paths. Often this
done by simply storing the preceding slash along with the path

3. Establishing a Base

The term "relative URL" implies that there exists some absolute "
URL" against which the relative reference is applied. Indeed,
base URL is necessary to define the semantics of any
relative URLs; without it, a relative reference is meaningless.
order for relative URLs to be usable within a document, the base
of that document must be known to the parser













Fielding Standards Track [Page 7]

RFC 1808 Relative Uniform Resource Locators June 1995


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
thought of in terms of layers, where the innermost defined base
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

Within certain document media types, the base URL of the document
be embedded within the content itself such that it can be
obtained by a parser. This can be useful for descriptive documents
such as tables of content, which may be transmitted to others
protocols other than their usual retrieval context (e.g., E-Mail
USENET news).

It is beyond the scope of this document to specify how, for
media type, the base URL can be embedded. It is assumed that
agents manipulating such media types will be able to obtain
appropriate syntax from that media type's specification. An
of how the base URL can be embedded in the Hypertext Markup
(HTML) [3] is provided in an Appendix (Section 10).

Messages are considered to be composite documents. The base URL of
message can be specified within the message headers (or
tagged metainformation) of the message. For protocols that make
of message headers like those described in RFC 822 [5], we
that the format of this header be

base-header = "Base" ":" ""

where "Base" is case-insensitive and any whitespace (including
used for line folding) inside the angle brackets is ignored.
example, the header




Fielding Standards Track [Page 8]

RFC 1808 Relative Uniform Resource Locators June 1995


Base:
would indicate that the base URL for that message is the
"http://www.ics.uci.edu/Test/a/b/c". The base URL for a
serves as both the base for any relative URLs within the
headers and the default base URL for documents enclosed within
message, as described in the next section

Protocols which do not use the RFC 822 message header syntax,
which do allow some form of tagged metainformation to be
within messages, may define their own syntax for defining the
URL as part of a message

3.2. Base URL from the Encapsulating

If no base URL is embedded, the base URL of a document is defined
the document's retrieval context. For a document that is
within another entity (such as a message or another document),
retrieval context is that entity; thus, the default base URL of
document is the base URL of the entity in which the document
encapsulated

Composite media types, such as the "multipart/*" and "message/*"
media types defined by MIME (RFC 1521, [4]), define a hierarchy
retrieval context for their enclosed documents. In other words,
retrieval context of a component part is the base URL of
composite entity of which it is a part. Thus, a composite entity
redefine the retrieval context of its component parts via
inclusion of a base-header, and this redefinition applies
for a hierarchy of composite parts. Note that this might not
the base URL of the components, since each component may include
embedded base URL or base-header that takes precedence over
retrieval context

3.3. Base URL from the Retrieval

If no base URL is embedded and the document is not
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
be considered the base URL. Note that if the retrieval was
result of a redirected request, the last URL used (i.e., that
resulted in the actual retrieval of the document) is the base URL

3.4. Default Base

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
embedded URLs within that document are assumed to be absolute URLs



Fielding Standards Track [Page 9]

RFC 1808 Relative Uniform Resource Locators June 1995


It is the responsibility of the distributor(s) of a
containing relative URLs to ensure that the base URL for
document can be established. It must be emphasized that
URLs cannot be used reliably in situations where the document's
URL is not well-defined

4. Resolving Relative

This section describes an example algorithm for resolving URLs
a context in which the URLs may be relative, such that the result
always a URL in absolute form. Although this algorithm
guarantee that the resulting URL will equal that intended by
original author, it does guarantee that any valid URL (relative
absolute) can be consistently transformed to an absolute form given
valid base URL

The following steps are performed in order

Step 1: The base URL is established according to the rules
Section 3. If the base URL is the empty string (unknown),
the embedded URL is interpreted as an absolute URL
we are done

Step 2: Both the base and embedded URLs are parsed into
component parts as described in Section 2.4.

a) If the embedded URL is entirely empty, it inherits
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
interpreted as an absolute URL and we are done

c) Otherwise, the embedded URL inherits the scheme
the base URL

Step 3: If the embedded URL's is non-empty, we skip
Step 7. Otherwise, the embedded URL inherits the (if any) of the base URL

Step 4: If the embedded URL path is preceded by a slash "/",
path is not relative and we skip to Step 7.









Fielding Standards Track [Page 10]

RFC 1808 Relative Uniform Resource Locators June 1995


Step 5: If the embedded URL path is empty (and not preceded by
slash), then the embedded URL inherits the base URL path


a) if the embedded URL's is non-empty, we skip
step 7; otherwise, it inherits the of the
URL (if any)

b) if the embedded URL's is non-empty, we skip
step 7; otherwise, it inherits the of the
URL (if any) and we skip to step 7.

Step 6: The last segment of the base URL's path (
following the rightmost slash "/", or the entire path if
slash is present) is removed and the embedded URL's path
appended in its place. The following operations
then applied, in order, to the new path

a) All occurrences of "./", where "." is a complete
segment, are removed

b) If the path ends with "." as a complete path segment
that "." is removed

c) All occurrences of "/../", where is
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 "/..", where is
complete path segment not equal to "..",
"/.." is removed

Step 7: The resulting URL components, including any inherited
the base URL, are recombined to give the absolute form
the embedded URL

Parameters, regardless of their purpose, do not form a part of
URL path and thus do not affect the resolving of relative paths.
particular, the presence or absence of the ";type=d" parameter on
ftp URL does not affect the interpretation of paths relative to
URL. Fragment identifiers are only inherited from the base URL
the entire embedded URL is empty







Fielding Standards Track [Page 11]

RFC 1808 Relative Uniform Resource Locators June 1995


The above algorithm is intended to provide an example by which
output of implementations can be tested -- implementation of
algorithm itself is not required. For example, some systems may
it more efficient to implement Step 6 as a pair of segment
being merged, rather than as a series of string pattern matches

5. Examples and Recommended

Within an object with a well-defined base URL

Base:
the relative URLs would be resolved as follows

5.1. Normal

g:h = g = ./g = g/ =
/g = //g = ?y = g?y = g?y/./x = #s = g#s = g#s/./x = g?y#s = ;x = g;x = g;x?y#s = . =
./ =
.. =
../ =
../g = ../.. =
../../ =
../../g =
5.2. Abnormal

Although the following abnormal examples are unlikely to occur
normal practice, all URL parsers should be capable of resolving
consistently. Each example uses the same base as above





Fielding Standards Track [Page 12]

RFC 1808 Relative Uniform Resource Locators June 1995


An empty reference resolves to the complete base URL

<> =
Parsers must be careful in handling the case where there are
relative path ".." segments than there are hierarchical levels in
base URL's path. Note that the ".." syntax cannot be used to
the of a URL

../../../g = ../../../../g =
Similarly, parsers must avoid treating "." and ".." as special
they are not complete components of a relative path

/./g = /../g = g. =
.g = g.. =
..g =
Less likely are cases where the relative URL uses unnecessary
nonsensical forms of the "." and ".." complete path segments

./../g = ./g/. =
g/./h = g/../h =
Finally, some older parsers allow the scheme name to be present in
relative URL if it is the same as the base URL scheme. This
considered to be a loophole in prior specifications of partial
[1] and should be avoided by future parsers

http:g = http: =

5.3. Recommended

Authors should be aware that path names which contain a colon ":"
character cannot be used as the first component of a relative
path (e.g., "this:that") because they will likely be mistaken for
scheme name. It is therefore necessary to precede such cases
other components (e.g., "./this:that"), or to escape the
character (e.g., "this%3Athat"), in order for them to be
parsed. The former solution is preferred because it does not
the absolute form of the URL



Fielding Standards Track [Page 13]

RFC 1808 Relative Uniform Resource Locators June 1995


There is an ambiguity in the semantics for the ftp URL
regarding the use of a trailing slash ("/") character and/or
parameter ";type=d" to indicate a resource that is an ftp directory
If the result of retrieving that directory includes embedded
URLs, it is necessary that the base URL path for that result
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

There are no security considerations in the use or parsing
relative URLs. However, once a relative URL has been resolved to
absolute form, the same security considerations apply as
described in RFC 1738 [2].

7.

This work is derived from concepts introduced by Tim Berners-Lee
the World-Wide Web global information initiative. Relative URLs
described as "Partial URLs" in RFC 1630 [1]. That description
expanded for inclusion as an appendix for an early draft of RFC 1738,
"Uniform Resource Locators (URL)" [2]. However, after
discussion, the URI-WG decided to specify Relative URLs
from the primary URL draft

This document is intended to fulfill the recommendations for
Resource Locators as stated in [6]. It has benefited greatly
the comments of all those participating in the URI-WG.
thanks go to Larry Masinter, Michael A. Dolan, Guido van Rossum,
Kristol, David Robinson, and Brad Barber for
problems/deficiencies in earlier drafts

8.

[1] Berners-Lee, T., "Universal Resource Identifiers in WWW:
Unifying Syntax for the Expression of Names and Addresses
Objects on the Network as used in the World-Wide Web", RFC 1630,
CERN, June 1994.

[2] Berners-Lee, T., Masinter, L., and M. McCahill, Editors, "
Resource Locators (URL)", RFC 1738, CERN, Xerox Corporation
University of Minnesota, December 1994.

[3] Berners-Lee T., and D. Connolly, "HyperText Markup
Specification -- 2.0", Work in Progress, MIT, HaL
Systems, February 1995.





Fielding Standards Track [Page 14]

RFC 1808 Relative Uniform Resource Locators June 1995


[4] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet
Extensions): Mechanisms for Specifying and Describing the
of Internet Message Bodies", RFC 1521, Bellcore, Innosoft
September 1993.

[5] Crocker, D., "Standard for the Format of ARPA Internet
Messages", STD 11, RFC 822, UDEL, August 1982.

[6] Kunze, J., "Functional Recommendations for Internet
Locators", RFC 1736, IS&T, UC Berkeley, February 1995.

9. Author's

Roy T.
Department of Information and Computer
University of
Irvine, CA 92717-3425
U.S.A

Tel: +1 (714) 824-4049
Fax: +1 (714) 824-4056
EMail: fielding@ics.uci.

10. Appendix - Embedding the Base URL in HTML

It is useful to consider an example of how the base URL of a
can be embedded within the document's content. In this appendix,
describe how documents written in the Hypertext Markup
(HTML) [3] can include an embedded base URL. This appendix does
form a part of the relative URL specification and should not
considered as anything more than a descriptive example

HTML defines a special element "BASE" which, when present in
"HEAD" portion of a document, signals that the parser should use
BASE element's "HREF" attribute as the base URL for resolving
relative URLs. The "HREF" attribute must be an absolute URL.
that, in HTML, element and attribute names are case-insensitive.
example