One document matched: draft-fielding-url-syntax-00.txt
Network Working Group T. Berners-Lee
INTERNET-DRAFT MIT/LCS
<draft-fielding-url-syntax-00> R. Fielding
Expires six months after publication date. U.C. Irvine
L. Masinter
Xerox Corporation
13 November 1996
Uniform Resource Locators (URL)
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as
``work in progress.''
To learn the current status of any Internet-Draft, please check
the ``1id-abstracts.txt'' listing contained in the Internet-Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast),
or ftp.isi.edu (US West Coast).
Issues:
1. The descriptions of the components in Section 4 are not yet
complete.
2. We need to define a mechanism for using IPv6 addresses in the
URL hostname which will not break existing systems too badly.
3. Section 7 (New URL Schemes) needs input from the Applications
Area A.D.'s.
Abstract
A Uniform Resource Locator (URL) is a compact string representation
of the location for a resource that is available via the Internet.
This document defines the general syntax and semantics of URLs,
including both absolute and relative locators, and guidelines for
their use and for the definition of new URL schemes. It revises and
replaces the generic definitions in RFC 1738 and RFC 1808.
1. Introduction
Uniform Resource Locators (URLs) provide a simple and extensible
means for identifying a resource by its location. This specification
of URL syntax and semantics 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]. The specification of URLs is
designed to meet the recommendations laid out in "Functional
Recommendations for Internet Resource Locators", RFC 1736 [8].
This document updates and merges RFC 1738 "Uniform Resource Locators"
[2] and RFC 1808 "Relative Uniform Resource Locators" [7] in order to
define a single, general syntax for all URLs. It excludes those
portions of RFC 1738 that defined the specific syntax of individual
URL schemes; those portions will be updated as separate documents.
URLs are characterized by the following definitions:
Uniform
Uniformity of syntax and semantics allows the mechanism for
referencing resources to be independent of the mechanism used
to locate those resources and the operations applied to those
resources once they have been located. New types of resources,
access mechanisms, and operations can be introduced without
changing the protocols and data formats that use URLs.
Resource
A resource can be anything that has identity. Familiar
examples include an electronic document, an image, a service
(e.g., "today's weather report for Los Angeles"), and a
collection of other resources. Not all resources are network
"retrievable", such as human beings, corporations, and actual
books in a library.
The resource is the conceptual entity defined by the identity,
not necessarily the thing which holds that identity at one
particular instance in time. Thus, a resource can remain
constant even when its content---the thing(s) to which it
currently corresponds---changes over time, provided that the
identity is not changed in the process.
Locator
A locator is an object that identifies a resource by its
location. In the case of URLs, the object is a sequence of
characters with a restricted syntax. An absolute locator
identifies a location independent of any context, whereas a
relative locator identifies a location relative to the
context in which it is found.
URLs are used to `locate' resources by providing an abstract
identification of the resource location. Having located a resource,
a system may perform a variety of operations on the resource, as
might be characterized by such words as `access', `update',
`replace', or `find attributes'. This specification is only
concerned with the issue of identifying a resource by its location.
1.1. Example URLs
The following examples illustrate URLs which are in common use.
ftp://ds.internic.net/rfc/rfc1808.txt
-- ftp scheme for File Transfer Protocol services
gopher://spinaltap.micro.umn.edu/00/Weather/California/Los%20Angeles
-- gopher scheme for Gopher and Gopher+ Protocol services
http://www.ics.uci.edu/pub/ietf/uri/
-- http scheme for Hypertext Transfer Protocol services
mailto:masinter@parc.xerox.com
-- mailto scheme for electronic mail addresses
news:comp.infosystems.www.servers.unix
-- news scheme for USENET news groups and articles
telnet://melvyl.ucop.edu/
-- telnet scheme for interactive services via the TELNET Protocol
Many other URL schemes have been defined. Section 7 describes how
new schemes are defined and registered. Although many URL schemes
are named after protocols, this does not imply that the only way to
access the URL's resource is via the named protocol. Gateways,
proxies, caches, and name resolution services might be used to access
some resources, independent of the protocol of their origin.
1.2. URL Transcribability
The URL syntax has been designed to promote transcribability over all
other concerns. A URL is a sequence of characters, i.e., letters,
digits, and special characters. A URL may be represented in a
variety of ways: e.g., ink on paper, pixels on a screen, or a
sequence of octets in a coded character set. The interpretation of a
URL depends only on the identity of the characters used.
The goal of transcribability can be described by a simple scenario.
Imagine two colleagues, Sam and Kim, sitting in a pub at an
international conference and exchanging research ideas. Sam asks Kim
for a location to get more information, so Kim writes the URL for the
research site on a napkin. Upon returning home, Sam takes out the
napkin and types the URL into a computer, which then retrieves the
information to which Kim referred.
There are several design concerns revealed by the scenario:
o A URL is a sequence of characters, which is not always
represented as a sequence of octets.
o A URL may be transcribed from a non-network source, and thus
should consist of characters which are most likely to be able
to be typed into a computer, within the constraints imposed by
keyboards (and related input devices) across nationalities and
languages.
o A URL often needs to be remembered by people, and it is easier
for people to remember a URL when it consists of meaningful
components.
These design concerns are not always in alignment. For example, it
is often the case that the most meaningful name for a URL component
would require characters which cannot be typed on most keyboards.
In such cases, the ability to access a resource is considered more
important than having its URL consist of the most meaningful of
components.
1.3. Syntax Notation and Common Elements
This document uses two conventions to describe and define the syntax
for Uniform Resource Locators. The first, called the layout form, is
a general description of the order of components and component
separators, as in
<first>/<second>;<third>?<fourth>
The component names are enclosed in angle-brackets and any characters
outside angle-brackets are literal separators. Whitespace should be
ignored. These descriptions are used informally and do not define
the syntax requirements.
The second convention is a BNF-like grammar, used to define the
formal URL syntax. The grammar is that of RFC 822 [6], except that
"|" is used to designate alternatives. Briefly, rules are separated
from definitions by an equal "=", indentation is used to continue a
rule definition over more than one line, literals are quoted with "",
parentheses "(" and ")" are used to group elements, optional elements
are enclosed in "[" and "]" brackets, and elements may be preceded
with <n>* to designate n or more repetitions of the following
element; n defaults to 0.
Unlike many specifications which use a BNF-like grammar to define the
bytes (octets) allowed by a protocol, the URL grammar is defined in
terms of characters. Each literal in the grammar corresponds to the
character it represents, rather than to the octet encoding of that
character in any particular coded character set. How a URL is
represented in terms of bits and bytes on the wire is dependent upon
the character encoding of the protocol used to transport it, or the
charset of the document which contains it.
The following definitions are common to many elements:
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"
alphanum = alpha | digit
The complete URL syntax is collected in Appendix A.
2. URL Characters and Character Escaping
All URLs consist of a restricted set of characters, chosen to
maximize their transcribability and usability across varying computer
systems, natural languages, and nationalities. This restricted set
corresponds to a subset of the graphic printable characters of the
US-ASCII coded character set [10].
The set of characters allowed for use within URLs can be described in
three categories: reserved, unreserved, and escaped.
urlchar = reserved | unreserved | escaped
2.1. Reserved Characters
Many URLs include components consisting of, or delimited by, certain
special characters. These characters are called "reserved", since
their usage within the URL component is limited to their reserved
purpose. If the data characters for a URL component would conflict
with the reserved purpose, then the conflicting characters must be
escaped before forming the URL.
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
This specification uses the "reserved" set to refer to those
characters which are allowed within a URL, but which may not be
allowed within a particular component of the generic URL syntax; they
are used as delimiters of the components described in Section 4.
Characters in the "reserved" set are not always reserved. The set of
characters actually reserved within any given URL component is
defined by that component. In general, a character is reserved if
escaping that character would change the semantics of the URL.
2.2. Unreserved Characters
Data characters which are allowed in a URL but do not have a reserved
purpose are called unreserved. These include upper and lower case
letters, decimal digits, and a subset of the punctuation marks and
symbols found in US-ASCII.
unreserved = alpha | digit | mark
mark = "$" | "-" | "_" | "." | "!" | "~" |
"*" | "'" | "(" | ")" | ","
Unreserved characters can be escaped without changing the semantics
of the URL, but this should not be done unless the URL is being used
in a context which does not allow the unescaped character to appear.
2.3. Escaped Characters
A character needs to be escaped if it is non-printable, if it is
often used to delimit a URL from its context, if it is not found in
the US-ASCII coded character set, if it is known to cause problems
when passed through some e-mail gateways, or if it is being used as
normal data within a component in which it is reserved.
2.3.1. Escaped Encoding
An escaped character is encoded as a character triplet, consisting of
the percent character "%" followed by the two hexadecimal digits
representing the character's octet code in an 8-bit coded character
set. For example, "%20" is the escaped encoding for the space
character.
escaped = "%" hex hex
hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f"
The 8-bit coded character set of the octet must be a superset of the
US-ASCII coded character set, such that the US-ASCII characters have
the same escaped encoding regardless of the larger octet character
set. The coded character set chosen must correspond to the character
set of the mechanism that will interpret the URL component in which
the escaped character is used.
2.3.2. When to Escape and Unescape
A URL is always in an escaped form, since escaping or unescaping a
completed URL might change its semantics. The only time that
characters within a URL can be safely escaped is when the URL is
being created from its component parts. Each component may have its
own set of characters which are reserved, so only the mechanism
responsible for generating or interpreting that component can
determine whether or not escaping a character will change its
semantics. Likewise, a URL must be separated into its components
before the escaped characters within those components can be
safely unescaped.
Because the percent "%" character always has the reserved purpose of
being the escape indicator, it must be escaped as "%25" in order to
be used as data within a URL. Implementers should be careful not to
escape or unescape the same string more than once, since unescaping
an already unescaped string might lead to misinterpreting a percent
data character as another escaped character, or vice versa in the
case of escaping an already escaped string.
2.3.3. Excluded Characters
Although they are not used within the URL syntax, we include here a
description of those characters which have been excluded and the
reasons for their exclusion.
excluded = control | space | delims | unwise | national
All characters corresponding to the control characters in the
US-ASCII coded character set are unsafe to use within a URL, both
because they are non-printable and because they are likely to be
misinterpreted by some control mechanisms.
control = <US-ASCII coded characters 00-1F and 7F hexadecimal>
The space character is excluded because significant spaces may
disappear and insignificant spaces may be introduced when URLs are
transcribed or typeset or subjected to the treatment of
word-processing programs. Whitespace is also used to delimit URLs in
many contexts.
space = <US-ASCII coded character 20 hexadecimal>
The angle-bracket "<" and ">" and double-quote (`"') characters are
excluded because they are often used as the delimiters around URLs in
text documents and protocol fields. The character "#" is excluded
because it is used to delimit a URL from a fragment identifier in URL
references. The percent character "%" is excluded because it is used
for the encoding of escaped characters.
delims = "<" | ">" | "#" | "%" | <">
Other characters are excluded because gateways and other transport
agents are known to sometimes modify such characters.
unwise = "{" | "}" | "|" | "\" | "^" | "[" | "]" | "`"
Finally, all other characters besides those mentioned in the above
sections are excluded because they are often difficult or impossible
to transcribe using traditional computer keyboards and software.
national = <Any character not in the reserved, unreserved,
control, space, delims, or unwise sets>
Excluded characters must be escaped in order to be properly
represented within a URL. However, there do exist some systems that
allow characters from the "unwise" and "national" sets to be used in
URL references; a robust implementation should be prepared to handle
those characters when it is possible to do so.
3. URL References
A common source of confusion in the use and interpretation of Uniform
Resource Locators is the distinction between a reference to a URL and
the URL itself. A URL reference may be absolute or relative, and may
be attached to additional information in the form of a fragment
identifier. However, "the URL" which results from such a reference
includes only the absolute URL after the fragment identifier (if any)
is removed and after any relative URL is resolved to its absolute
form. Although it is possible to limit the discussion of URL syntax
and semantics to that of the absolute result, most usage of URLs
is within general URL references, and it is impossible to obtain the
URL from such a reference without also parsing the fragment and
resolving the relative form.
URL-reference = [ absoluteURL | relativeURL ] [ "#" fragment ]
The syntax for relative URLs is a shortened form of that for absolute
URLs, where some prefix of the URL is missing and certain path
components ("." and "..") have a special meaning when interpreting a
relative path.
When a URL reference is used to perform a retrieval action on the
identified resource, the optional fragment identifier, separated from
the URL by a crosshatch ("#") character, consists of additional
reference information to be interpreted by the user agent after the
retrieval action has been successfully completed. As such, it is not
part of a URL, but is often used in conjunction with a URL. The
format and interpretation of fragment identifiers is dependent on the
media type of the retrieved resource.
fragment = *urlchar
A URL reference which does not contain a URL is a reference to the
current document. In other words, an empty URL reference within a
document is interpreted as a reference to the top of that document,
and a reference containing only a fragment identifier is a reference
to the identified fragment of that document. Traversal of such a
reference should not result in an additional retrieval action.
When parsing a URL reference, the fragment identifier (if any) is
extracted first. If the reference 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 crosshatch separator is
discarded.
4. Generic URL Syntax
4.1. Scheme
Just as there are many different methods of access to resources,
there are a variety of schemes for describing the location of such
resources. The URL syntax consists of a sequence of components
separated by reserved characters, with the first component defining
the semantics for the remainder of the URL string.
In general, absolute URLs are written as follows:
<scheme>:<scheme-specific-part>
An absolute URL contains the name of the scheme being used (<scheme>)
followed by a colon (":") and then a string (the <scheme-specific-
part>) whose interpretation depends on the scheme.
Scheme names consist of a sequence of characters. The lower case
letters "a"--"z", digits, and the characters plus ("+"), period
("."), and hyphen ("-") are allowed. For resiliency, programs
interpreting URLs should treat upper case letters as equivalent to
lower case in scheme names (e.g., allow "HTTP" as well as "http").
scheme = 1*( alpha | digit | "+" | "-" | "." )
When parsing a URL reference, the scheme (if any) is extracted after
the fragment. If the reference contains a colon ":" after the first
character and before any characters not allowed as part of a scheme
name, the scheme of the URL is the substring of characters up to, but
not including, the first colon. The colon separator is discarded.
Relative URL references are distinguished from absolute URLs in that
they do not begin with a scheme name. Instead, the scheme is
inherited from the base URL, as described in Section 7.
4.2. Opaque and Hierarchical URLs
The URL syntax does not require that the scheme-specific-part have
any general structure or set of semantics which is common among all
URLs. However, a subset of URLs do share a common syntax for
representing hierarchical relationships within the locator namespace.
This generic-URL syntax is used in interpreting relative URLs.
absoluteURL = generic-URL | opaque-URL
opaque-URL = scheme ":" *urlchar
generic-URL = scheme ":" relativeURL
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 server
are much more likely to refer to other resources on that server 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, such document trees can be moved, as a whole, without
changing any of the relative references. Experience within the WWW
has demonstrated that the ability to perform relative referencing
is necessary for the long-term usability of embedded URLs.
relativeURL = net_path | abs_path | rel_path
net_path = "//" server [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" parameters ] [ "?" query ]
It is not necessary for all URLs within a given scheme to be
restricted to the generic-URL syntax, since the hierarchical
properties of that syntax are only necessary when relative URLs are
used within a particular document. Documents can only make use of
relative URLs when their base URL fits within the generic-URL syntax.
It is assumed that any document which contains a relative reference
will also have a base URL that obeys the syntax. In other words,
relative URLs cannot be used within documents that have unsuitable
base URLs.
URLs which are hierarchical in nature use the slash "/" character for
separating hierarchical components. For some file systems, the "/"
used to denote the hierarchical structure of a URL corresponds to the
delimiter used to construct a file name hierarchy, and thus the URL
path will look similar to a file pathname. This does NOT imply that
the URL is a Unix pathname.
4.3. 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,
most URL schemes use a common sequence of five main components to
define the location of a resource
<scheme>://<server>/<path>;<parameters>?<query>
each of which, except <scheme>, may be absent from a particular URL.
The order of the components is important. If both <parameters> and
<query> are present, the <query> information must occur after the
<parameters>. A URL reference is parsed into its components from the
outside-in: fragment, scheme, server, query, parameters, and then
path.
4.3.1. Server Component
URL schemes that involve the direct use of an IP-based protocol to a
specified host on the Internet use a common syntax for the server
component of the URL's scheme-specific data:
<user>:<password>@<host>:<port>
Some or all of the parts "<user>:<password>@", ":<password>", and
":<port>" may be excluded. The server component is preceded by a
double slash "//" and is terminated by the next slash "/" or by the
end of the URL. The different components obey the following rules:
server = [ [ user [ ":" password ] "@" ] hostport ]
user = *[ unreserved | escaped |
";" | "?" | "&" | "=" | "+" ]
password = *[ unreserved | escaped |
";" | "?" | "&" | "=" | "+" ]
The user name (and password), if present, are followed by a
commercial at-sign "@". Within the user and password fields, the
characters ":", "@", and "/" are reserved.
Note that an empty user name or password is different than no user
name or password; there is no way to specify a password without
specifying a user name. E.g., <ftp://@host.com/> has an empty
user name and no password, <ftp://host.com/> has no user name,
while <ftp://foo:@host.com/> has a user name of "foo" and an
empty password.
hostport = host [ ":" port ]
host = hostname | hostnumber
hostname = *( domainlabel "." ) toplabel
domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum
toplabel = alpha | alpha *( alphanum | "-" ) alphanum
hostnumber = 1*digit "." 1*digit "." 1*digit "." 1*digit
port = *digit
The host is a domain name of a network host, or its IP address as a
set of four decimal digit groups separated by ".". Fully qualified
domain names take the form as described in Section 3.5 of RFC 1034
[9] and Section 2.1 of RFC 1123 [5]: a sequence of domain labels
separated by ".", each domain label starting and ending with an
alphanumerical character and possibly also containing "-" characters.
The rightmost domain label will never start with a digit, though,
which syntactically distinguishes all domain names from the IP
addresses.
The port is the network port number for the server. Most schemes
designate protocols that have a default port number. Another port
number may optionally be supplied, in decimal, separated from the
host by a colon. If the port is omitted, the default port number is
assumed.
When parsing a URL reference, the server component (if any) is
extracted after the scheme. If the remaining reference 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 server component of the URL. If no trailing slash
is present, the entire remaining reference is the server component.
The double-slash separator is discarded.
4.3.2. Path Component
The path component contains data specific to the scheme or server.
It typically specifies the details of how the resource can be
accessed. Note that the "/" between the host (or port) and the
url-path is NOT part of the url-path.
path = fsegment *( "/" segment )
fsegment = 1*pchar
segment = *pchar
pchar = unreserved | escaped | ":" | "@" | "&" | "=" | "+"
When parsing a URL reference, the path is extracted after all other
components. The remaining reference is the URL path and the slash
"/" that might precede it. Although 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, as described in Section 7.
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") in order for them to be
referenced as a relative path.
4.3.3. Parameters Component
The parameters component is a sequence of semi-colon-separated
attributes.
parameters = param *( ";" param )
param = *( pchar | "/" )
When parsing a URL reference, the parameters component (if any) is
extracted after the query component. If the remaining reference
contains a semicolon ";" character, then the substring after the
first (left-most) semicolon and up to the end of the reference is the
parameters component. If the semicolon is the last character, or no
semicolon is present, then the parameters component is empty. The
semicolon separator is discarded.
4.3.4. Query Component
The query component is a string of information to be interpreted by
the resource.
query = *urlchar
When parsing a URL reference, the query component (if any) is
extracted after the server component. If the remaining reference
contains a question mark "?" character, then the substring after the
first (left-most) question mark and up to the end of the reference is
the query component. If the question mark is the last character, or
no question mark is present, then the query component is empty. The
question mark separator is discarded.
5. 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 relative URL
reference; 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:
.----------------------------------------------------------.
| .----------------------------------------------------. |
| | .----------------------------------------------. | |
| | | .----------------------------------------. | | |
| | | | (5.1) Base URL embedded in the | | | |
| | | | document's content | | | |
| | | `----------------------------------------' | | |
| | | (5.2) Base URL of the encapsulating entity | | |
| | | (message, document, or none). | | |
| | `----------------------------------------------' | |
| | (5.3) URL used to retrieve the entity | |
| `----------------------------------------------------' |
| (5.4) Base URL = "" (undefined) |
`----------------------------------------------------------'
5.1. 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 Appendix B.
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 MIME [4], the base URL
can be specified by the Content-Base or Content-Location header
fields.
Content-Base = "Content-Base" ":" absoluteURL
Content-Location = "Content-Location" ":"
( absoluteURL | relativeURL )
The field names are case-insensitive and any whitespace inside
the field value (including that used for line folding) is ignored.
Content-Base takes precedence over any Content-Location. If the
latter is relative, it must be resolved to its absolute form (like
any relative URL) before it can be used as the base URL for other
references.
For example, the header field
Content-Base: http://www.ics.uci.edu/Test/a/b/c
would indicate that the base URL for that message is the string
"http://www.ics.uci.edu/Test/a/b/c". The base URL for a message
serves as both the base for any relative URLs within the message
headers and the default base URL for documents enclosed within the
message, as described in the next section.
Protocols which do not use the RFC 822 message header syntax, but
which do allow some form of tagged metainformation to be included
within messages, may define their own syntax for defining the base
URL as part of a message.
5.2. 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 (RFC 1521, [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 Content-Base or Content-Location 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.
5.3. 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.
5.4. Default Base URL
If none of the conditions described in Sections 5.1 -- 5.3 apply,
then the base URL is considered to be the empty string and all
URL references 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.
6. Resolving Relative URLs
This section describes an example algorithm for resolving URL
references 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 5. If the base URL is the empty string (unknown),
the URL reference is interpreted as an absolute URL and
we are done.
Step 2: Both the base and URL reference are parsed into their
component parts as described in Section 4.
a) If the URL reference is entirely empty, or consists only
of a fragment identifier, it is interpreted as a reference
to the current document and we are done.
b) If the URL reference starts with a scheme name, it is
interpreted as an absolute URL and we are done.
c) Otherwise, the URL reference inherits the scheme of
the base URL.
Step 3: If the URL reference's <server> is non-empty, we skip to
Step 7. Otherwise, the URL reference inherits the <server>
(if any) of the base URL.
Step 4: If the URL reference path is preceded by a slash "/", the
path is not relative and we skip to Step 7.
Step 5: If the URL reference path is empty (and not preceded by a
slash), then the URL reference inherits the base URL path,
and
a) if the URL reference's <parameters> is non-empty, we skip
to step 7; otherwise, it inherits the <parameters> of the
base URL (if any) and
b) if the URL reference'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 URL reference'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 URL reference.
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.
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.
Further examples are provided in Appendix C.
7. Adding New Schemes
The Internet Assigned Numbers Authority (IANA) maintains a registry
of URL schemes.
The current process for defining URL schemes is via the Internet
standards process: new URL schemes should be described in
standards-track RFCs. Over time, other methods of registering URL
schemes may be added.
URL schemes must have demonstrable utility and operability. One way
to provide such a demonstration is via a gateway which provides
objects in the new scheme for clients using an existing protocol. If
the new scheme does not locate resources that are data objects, the
properties of names in the new space must be clearly defined.
URL schemes should follow the same syntactic conventions of existing
schemes when appropriate. URL schemes should use the generic-URL
syntax if they are intended to be used with relative URLs. A
description of the allowed relative forms should be included in the
scheme's definition.
8. Security Considerations
The URL scheme does not in itself pose a security threat. Users
should beware that there is no general guarantee that a URL which at
one time points to a given object continues to do so, and does not
even at some later time point to a different object due to the
movement of objects on servers.
A URL-related security threat is that it is sometimes possible to
construct a URL such that an attempt to perform a harmless idempotent
operation such as the retrieval of the object will in fact cause a
possibly damaging remote operation to occur. The unsafe URL is
typically constructed by specifying a port number other than that
reserved for the network protocol in question. The client
unwittingly contacts a server which is in fact running a different
protocol. The content of the URL contains instructions which, when
interpreted according to this other protocol, cause an unexpected
operation. An example has been the use of gopher URLs to cause a rude
message to be sent via a SMTP server. Caution should be used when
using any URL which specifies a port number other than the default
for the protocol, especially when it is a number within the reserved
space.
Care should be taken when URLs contain escaped delimiters for a given
protocol (for example, CR and LF characters for telnet protocols)
that these are not unencoded before transmission. This would violate
the protocol but could be used to simulate an extra operation or
parameter, again causing an unexpected and possible harmful remote
operation to be performed.
The use of URLs containing passwords that should be secret is clearly
unwise.
9. Acknowledgements
Most of this document was derived from RFC 1738 [2] and RFC 1808 [7];
the acknowledgements in those specifications still apply.
10. 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,
CERN, June 1994.
[2] Berners-Lee, T., Masinter, L., and M. McCahill, Editors, "Uniform
Resource Locators (URL)", RFC 1738, CERN, Xerox Corporation,
University of Minnesota, December 1994.
[3] Berners-Lee T., and D. Connolly, "HyperText Markup Language
Specification -- 2.0", RFC 1866, MIT/W3C, November 1995.
[4] Borenstein, N., 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.
[5] Braden, R., Editor, "Requirements for Internet Hosts --
Application and Support", STD 3, RFC 1123, IETF, October 1989.
[6] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.
[7] Fielding, R., "Relative Uniform Resource Locators", RFC 1808,
UC Irvine, June 1995.
[8] Kunze, J., "Functional Recommendations for Internet Resource
Locators", RFC 1736, IS&T, UC Berkeley, February 1995.
[9] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, USC/Information Sciences Institute,
November 1987.
[10] US-ASCII. "Coded Character Set -- 7-bit American Standard Code
for Information Interchange", ANSI X3.4-1986.
11. Authors' Addresses
Tim Berners-Lee
World Wide Web Consortium
MIT Laboratory for Computer Science, NE43-356
545 Technology Square
Cambridge, MA 02139
Fax: +1(617)258-8682
EMail: timbl@w3.org
Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425
U.S.A.
Fax: +1(714)824-4056
EMail: fielding@ics.uci.edu
Larry Masinter
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA 94034
Fax: +1(415)812-4333
EMail: masinter@parc.xerox.com
Appendices
A. Collected BNF for URLs
# To be inserted when it is completed.
B. Recommendations for Delimiting URLs in Context
URIs, including URLs, are intended to be transmitted through
protocols which provide a context for their interpretation.
In some cases, it will be necessary to distinguish URLs from other
possible data structures in a syntactic structure. In this case, is
recommended that URLs be preceded with a prefix consisting of the
characters "URL:". For example, this prefix may be used to
distinguish URLs from other kinds of URIs.
In addition, there are many occasions when URLs are included in other
kinds of text; examples include simple plain text sent in electronic
mail, USENET news messages, or, most importantly, printed on paper.
In such cases, it is important to be able to delimit the URL from the
rest of the text, and in particular from punctuation marks that might
be mistaken for part of the URL.
In practice, URLs are delimited in a variety of ways, using brackets
[http://test.com/] or angle brackets <http://www.w3.org> or even
paired braces {http://test.com}, with or (usually) without the "URL:"
prefix. This wrapper does not form part of the URL. In the case
where a fragment/anchor identifier is associated with a URL
(following a "#"), the identifier would be placed within the brackets
as well.
In some cases, extra whitespace (spaces, linebreaks, tabs, etc.) may
need to be added to break long URLs across lines. The whitespace
should be ignored when extracting the URL.
No whitespace should be introduced after a hyphen ("-") character.
Because some typesetters and printers may (erroneously) introduce a
hyphen at the end of line when breaking a line, the interpreter of a
URL containing a line break immediately after a hyphen should ignore
all unencoded whitespace around the line break, and should be aware
that the hyphen may or may not actually be part of the URL.
Examples:
Yes, Jim, I found it under <ftp://info.cern.ch/pub/www/doc;
type=d> but you can probably pick it up from <ftp://ds.in
ternic.net/rfc>. Note the warning in <http://ds.internic.
net/instructions/overview.html#WARNING>.
C. Examples of Resolving Relative URLs
Within an object with a well-defined base URL of
Content-Base: http://a/b/c/d;p?q
the relative URLs would be resolved as follows:
C.1. Normal Examples
g:h = g:h
g = http://a/b/c/g
./g = http://a/b/c/g
g/ = http://a/b/c/g/
/g = http://a/g
//g = http://g
?y = http://a/b/c/d;p?y
g?y = http://a/b/c/g?y
g?y/./x = http://a/b/c/g?y/./x
#s = http://a/b/c/d;p?q#s
g#s = http://a/b/c/g#s
g#s/./x = http://a/b/c/g#s/./x
g?y#s = http://a/b/c/g?y#s
;x = http://a/b/c/d;x
g;x = http://a/b/c/g;x
g;x?y#s = http://a/b/c/g;x?y#s
. = http://a/b/c/
./ = http://a/b/c/
.. = http://a/b/
../ = http://a/b/
../g = http://a/b/g
../.. = http://a/
../../ = http://a/
../../g = http://a/g
C.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 refers to the top of the current document.
<> = http://a/b/c/d;p?q
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 <server> of a URL.
../../../g = http://a/../g
../../../../g = http://a/../../g
Similarly, parsers must avoid treating "." and ".." as special when
they are not complete components of a relative path.
/./g = http://a/./g
/../g = http://a/../g
g. = http://a/b/c/g.
.g = http://a/b/c/.g
g.. = http://a/b/c/g..
..g = 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 = http://a/b/g
./g/. = http://a/b/c/g/
g/./h = http://a/b/c/g/h
g/../h = 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 = http:g
http: = http:
D. 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
<http://www.ics.uci.edu/Test/a/x>
regardless of the context in which the example document was obtained.
E. Summary of Non-editorial Changes
E.1. Additions
Section 1 (Introduction) is entirely new. Design rationale for the
scope of URLs and the chosen URL character set has been added in
order to address common misconceptions about what would and would not
be appropriate for additional URL schemes, and why the allowed
character set is limited to US-ASCII characters.
Section 3 (URL References) was added to stem the confusion regarding
"what is a URL" and how to describe fragment identifiers given that
they are not part of the URL, but are part of the URL syntax and
parsing concerns. In addition, it provides a reference definition
for use by other IETF specifications (HTML, HTTP, etc.) which have
previously attempted to redefine the URL syntax in order to account
for the presence of fragment identifiers in URL references.
Section 2.3.2 (When to Escape and Unescape) was added in response to
many (mis)implementation questions on the subject.
E.2. Modifications from both RFC 1738 and RFC 1808
Confusion regarding the terms "character encoding", the URL
"character set", and the escaping of characters with %<hex><hex>
equivalents has (hopefully) been reduced. Many of the BNF rule names
regarding the character sets have been changed to more accurately
describe their purpose and to encompass all "characters" rather than
just US-ASCII octets. Unless otherwise noted here, these
modifications do not affect the URL syntax.
Both RFC 1738 and RFC 1808 refer to the "reserved" set of characters
as if URL-interpreting servers were limited to a single set of
characters with a reserved purpose (i.e., as meaning something other
than the data to which the characters correspond), and that this set
was fixed by the URL scheme. However, this has not been true in
practice; any character which is interpreted differently when it is
escaped is, in effect, reserved. Furthermore, the interpreting
engine on a server is often dependent on the resource, not just the
URL scheme. The description of reserved characters has been changed
accordingly.
The plus "+" character was added to those in the "reserved" set,
since it is treated as reserved within some URL components.
The tilde "~" character was added to those in the "unreserved" set,
since it is extensively used on the Internet in spite of the
difficulty to transcribe it with some keyboards.
E.3. Modifications from RFC 1738
The definition of specific URL schemes and their scheme-specific
syntax and semantics has been moved to separate documents.
The URL host was defined as a fully-qualified domain name. However,
many URLs are used without fully-qualified domain names (in contexts
for which the full qualification is not necessary), without any host
(as in some file URLs), or with a host of "localhost".
The URL port is now *digit instead of 1*digit, since systems are
expected to handle the case where the ":" separator between host and
port is supplied without a port.
E.4. Modifications from RFC 1808
RFC 1808 (Section 4) defined an empty URL reference (a reference
containing nothing aside from the fragment identifier) as being a
reference to the base URL. Unfortunately, that definition could be
interpreted, upon selection of such a reference, as a new retrieval
action on that resource. Since the normal intent of such references
is for the user agent to change its view of the current document to
the beginning of the specified fragment within that document, not to
make an additional request of the resource, a description of how to
correctly interpret an empty reference has been added in Section 3.
The description of the mythical Base header field has been replaced
with the Content-Base and Content-Location header fields defined by
HTTP/1.1 and MHTML.
RFC 1808 described various schemes as either having or not having the
properties of the generic-URL syntax. However, the only requirement
is that the particular document containing the relative references
have a base URL which abides by the generic-URL syntax, regardless of
the URL scheme, so the associated description has been updated to
reflect that.
The BNF term <net_loc> has been replaced with <server>, since the
latter more accurately describes its use and purpose.
| PAFTECH AB 2003-2026 | 2026-04-24 05:25:16 |