- From: Larry Masinter <masinter@parc.xerox.com>
- Date: Sun, 29 Dec 1996 01:33:13 PST
- To: internet-drafts@ietf.org
- Cc: uri@bunyip.com
This replaces any previous draft-fielding-url-syntax. (It seems that
draft-fielding-url-syntax-01 and draft-fielding-url-syntax-02 were
submitted but have not appeared? In any case, they were already
circulated on the mailing list.)
================================================================
Network Working Group T. Berners-Lee
INTERNET-DRAFT MIT/LCS
<draft-ietf-url-syntax-00> R. Fielding
Expires six months after publication date. U.C. Irvine
L. Masinter
Xerox Corporation
29 December 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. We need to define a mechanism for using IPv6 addresses in the
URL hostname which will not break existing systems too badly.
2. Need a specific reference to the documents
defining Content-Base and Content-Language.
3. Examples should include one with multiple parameters and
one with multiple queries.
4. Suggestion to include a 'normalization' algorithm. Should we?
5. Is there semantics to empty fragment identifiers?
6. clarify issue with http://4kids/blah, where non FQDN is used.
7. Add [MHTML] reference
8. URN/URI/URL issue
Abstract
A Uniform Resource Locator (URL) is a compact string representation
of a location for use in identifying an abstract or physical
resource. This document defines the general syntax and semantics of
URLs, including both absolute and relative locators, and guidelines
for their use. 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 [9].
This document updates and merges RFC 1738 "Uniform Resource Locators"
[2] and RFC 1808 "Relative Uniform Resource Locators" [6] 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,
as will the process for registration of new URL schemes.
All significant changes from the prior RFCs are noted in Appendix F.
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"; e.g., human beings, corporations, and bound
books in a library can also be considered resources.
The resource is the conceptual mapping to an entity or set of
entities, not necessarily the entity which corresponds to that
mapping at any particular instance in time. Thus, a resource
can remain constant even when its content---the entities to
which it currently corresponds---changes over time, provided
that the conceptual mapping 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. URL, URN, and URI
URLs are a subset of Uniform Resource Identifiers (URI), which also
includes the notion of Uniform Resource Names (URN). A URN differs
from a URL in that it identifies a resource in a location-independent
fashion (see RFC 1737, [11]). URNs are defined by a separate set of
specifications.
Although this specification restricts its discussion to URLs, the
syntax defined is that of URI in general. Any requirements placed on
the URL syntax also apply to the URI syntax. This uniform syntax for
all resource identifiers allows a URN to be used in any data field
that might otherwise hold a URL.
1.2. Example URLs
The following examples illustrate URLs which are in common use.
ftp://ftp.is.co.za/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.math.uio.no/faq/compression-faq/part1.html
-- http scheme for Hypertext Transfer Protocol services
mailto:mduerst@ifi.unizh.ch
-- 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.
The scheme defines the namespace of the URL. 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, and the resolution of some URLs may require the use of more
than one protocol (e.g., both DNS and HTTP are typically used to
access an "http" URL's resource when it can't be found in a local
cache).
1.3. URL Transcribability
The URL syntax has been designed to promote transcribability as one
of its main 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 characters used and not how those characters
are represented on the wire.
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 languages and
locales.
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.
The ability to transcribe the resource
location from one medium to another was considered more
important than having its URL consist of the most meaningful of
components.
In a few cases, exceptions were made for characters already in
widespread use within URLs: the "~", "$" and "#" characters might
have otherwise been excluded from URLs.
1.4. 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 [5], 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 | upalpha
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"
upalpha = "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, primarily chosen
to aid transcribability and usability both in computer
systems and in non-computer communications. In addition, characters
used conventionally as delimiters around URLs were excluded. The
restricted set of characters consists of digits, letters, and a few
graphic symbols corresponding to a subset of the graphic printable
characters of the US-ASCII coded character set [12]; they are
common to most of the character encodings and input facilities
available to Internet users.
Within a URL, characters are either used as delimiters, or to
represent strings of data (octets) within delimited portions. When
used to represent data directly, the character denotes the octet
corresponding to the US-ASCII code for that character. In
addition, an octet may be represented by an escaped encoding.
Thus, the set of "characters" allowed within URLs can be described in
three categories: reserved, unreserved, and escaped.
urlc = reserved | unreserved | escaped
1.5. Characters, octets, and encodings
URLs are sequences of characters. Parts of those sequences of
characters are then used to represent sequences of octets. In turn,
sequences of octets are (frequently) used (with a character
encoding scheme) to represent characters. This means that when
dealing with URLs it's necessary to work at three levels:
represented characters
^
|
v
octets
^
|
v
URL characters
This looks more complicated than necessary if all one is dealing
with is file names in ASCII, but is necessary when dealing with the
wide variety of systems in use. URL characters may represent octets
directly or with escape sequences (Section 2.3). Octets may
sometimes represent characters in ASCII, in other character
encodings, or sometimes be used to represent data that does not
correspond to characters at all.
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 for a URL component would conflict
with the reserved purpose, then the conflicting data must be
escaped before forming the URL.
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
The "reserved" syntax class above refers 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 4.3.
Characters in the "reserved" set are not reserved in all contexts.
The set of characters actually reserved within any given URL
component is defined by that component. In general, a character is
reserved if the semantics of the URL changes if the character is
replaced with its escaped ASCII encoding.
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 limited set of punctuation marks and
symbols.
unreserved = alphanum | 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. Escape Sequences
Data must be escaped if it does not have a representation using an
unreserved character; this includes data that does not correspond
to a printable character of the US-ASCII coded character set, and
also data that corresponds to characters used to delimit a URL from
its context.
2.3.1. Escaped Encoding
An escaped octet is encoded as a character triplet, consisting
of the percent character "%" followed by the two hexadecimal digits
representing the octet code. For example, "%20" is the escaped
encoding for the US-ASCII space character.
escaped = "%" hex hex
hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f"
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. Normally, the only time
escape encodings can safely be made 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 decoded.
In some cases, data that could be represented by an unreserved
character may appear escaped; for example, some of the unreserved
mark characters are automatically escaped by some systems. It
is safe to unescape these within the body of a URL.
For example, "%7e" is sometimes used instead of "~" in http URL
path, but the two can be used interchangably.
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 US-ASCII 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 (Section 3). 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, or they are
used as delimiters.
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>
Data corresponding to 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 (section 3); 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
have additional information attached 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 resource referenced by the URL.
fragment = *urlc
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 start 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.
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 | "+" | "-" | "." )
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 5.2.
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 ":" *urlc
generic-URL = scheme ":" relativeURL
URLs which are hierarchical in nature use the slash "/" character for
separating hierarchical components. For some file systems, a "/"
character (used to denote the hierarchical structure of a URL) is 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 four main components to
define the location of a resource
<scheme>://<server><path>?<query>
each of which, except <scheme>, may be absent from a particular URL.
For example, some URL schemes do not allow a server component, and
others do not use a query component.
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. Within the server component, the characters ":",
"@", "?", and "/" are reserved.
server = [ [ user [ ":" password ] "@" ] hostport ]
The user name and password, if present, are followed by a commercial
at-sign "@".
user = *( unreserved | escaped | ";" | "&" | "=" | "+" )
password = *( unreserved | escaped | ";" | "&" | "=" | "+" )
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.
The host is a domain name of a network host, or its IPv4 address as a
set of four decimal digit groups separated by ".". A suitable
representation for IPv6 addresses has not yet been determined.
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
Domain names take the form as described in Section 3.5 of RFC 1034
[10] and Section 2.1 of RFC 1123 [4]: 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.
A server component is not required for a URL scheme to make use of
relative references. A base URL without a server component implies
that any relative reference will also be without a server component.
4.3.2. Path Component
The path component contains data, specific to the scheme or server,
regarding the details of how the resource can be accessed.
path = [ "/" ] path_segments
path_segments = segment *( "/" segment )
segment = *pchar *( ";" param )
param = *pchar
pchar = unreserved | escaped | ":" | "@" | "&" | "=" | "+"
The path may consist of a sequence of path segments separated by a
single slash "/" character. Within a path segment, the characters
"/", ";", "=", and "?" are reserved. Each path segment may include a
sequence of parameters, indicated by the semicolon ";" character.
The parameters are not significant to the parsing of relative
references.
4.3.3. Query Component
The query component is a string of information to be interpreted by
the resource.
query = *urlc
Within a query component, the characters "/", "&", "=", and "+" are
reserved.
4.4. Parsing a URL Reference
A URL reference is typically parsed according to the four main
components in order to determine what components are present and
whether or not the reference is relative or absolute. The individual
components are then parsed for their subparts and to verify their
validity. A reference is parsed as if it is a generic-URL, even
though it might be considered opaque by later processes.
Although the BNF defines what is allowed in each component, it is
ambiguous in terms of differentiating between a server component and
a path component that begins with two slash characters. The greedy
algorithm is used for disambiguation: the left-most matching rule
soaks up as much of the URL reference string as it is capable of
matching. In other words, the server component wins.
Readers familiar with regular expressions should see Appendix B for a
concrete parsing example and test oracle.
5. Relative URL References
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
A relative reference beginning with two slash characters is termed a
network-path reference. Such references are rarely used.
net_path = "//" server [ abs_path ]
A relative reference beginning with a single slash character is
termed an absolute-path reference.
abs_path = "/" rel_path
A relative reference which does not begin with a scheme name or a
slash character is termed a relative-path reference.
rel_path = [ path_segments ] [ "?" query ]
Within a relative-path reference, the complete path segments "." and
".." have special meanings: "the current hierarchy level" and "the
level above this hierarchy level", respectively. Although this is
very similar to their use within Unix-based filesystems to indicate
directory levels, these path components are only considered special
when resolving a relative-path reference to its absolute form
(Section 5.2).
Authors should be aware that a path segment which contains a colon
character cannot be used as the first segment of a relative URL path
(e.g., "this:that"), because it would be mistaken for a scheme name.
It is therefore necessary to precede such segments with other
segments (e.g., "./this:that") in order for them to be referenced as
a relative path.
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 a document that has an unsuitable
base URL.
5.1. 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:
.----------------------------------------------------------.
| .----------------------------------------------------. |
| | .----------------------------------------------. | |
| | | .----------------------------------------. | | |
| | | | .----------------------------------. | | | |
| | | | | <relative_reference> | | | | |
| | | | `----------------------------------' | | | |
| | | | (5.1.1) Base URL embedded in the | | | |
| | | | document's content | | | |
| | | `----------------------------------------' | | |
| | | (5.1.2) Base URL of the encapsulating entity | | |
| | | (message, document, or none). | | |
| | `----------------------------------------------' | |
| | (5.1.3) URL used to retrieve the entity | |
| `----------------------------------------------------' |
| (5.1.4) Base URL = "" (undefined) |
`----------------------------------------------------------'
5.1.1. 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 D.
MIME messages [7] 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 [7], 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.
5.1.2. 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[8], 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.
5.1.3. 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.
5.1.4. Default Base URL
If none of the conditions described in Sections 5.1.1--5.1.3 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.
5.2. Resolving Relative References to Absolute Form
This section describes an example algorithm for resolving URL
references which might be relative to a given base URL.
The base URL is established according to the rules of Section 5.1 and
parsed into the four main components as described in Section 4.4.
Note that only the scheme component is required to be present in the
base URL; the other components may be empty or undefined. A
component is undefined if its preceding separator does not appear in
the URL reference; the path component is never undefined, though it
may be empty. The base URL's query component is not used by the
resolution algorithm and may be discarded.
For each URL reference, the following steps are performed in order:
1) The URL reference is parsed into the potential four components and
fragment identifier, as described in Section 4.4.
2) If the path component is empty and the scheme, server, and query
components are undefined, then it is a reference to the current
document and we are done.
3) If the scheme component is defined, indicating that the reference
starts with a scheme name, then the reference is interpreted as an
absolute URL and we are done. Otherwise, the reference URL's
scheme is inherited from the base URL's scheme component.
4) If the server component is defined, then the reference is a
network-path and we skip to step 7. Otherwise, the reference
URL's server is inherited from the base URL's server component,
which will also be undefined if the URL scheme does not use a
server component.
5) If the path component begins with a slash character ("/"), then
the reference is an absolute-path and we skip to step 7.
6) If this step is reached, then we are resolving a relative-path
reference. The relative path needs to be merged with the base
URL's path. Although there are many ways to do this, we will
describe a simple method using a separate string buffer.
a) All but the last segment of the base URL's path component is
copied to the buffer. In other words, any characters after the
last (right-most) slash character, if any, are excluded.
b) The reference's path component is appended to the buffer
string.
c) If the reference's query component is defined, then a "?"
character is appended to the buffer string, followed by the
query component.
d) All occurrences of "./", where "." is a complete path segment,
are removed from the buffer string.
e) If the buffer string ends with "." as a complete path segment,
that "." is removed.
f) All occurrences of "<segment>/../", where <segment> is a
complete path segment not equal to "..", are removed from the
buffer string. Removal of these path segments is performed
iteratively, removing the leftmost matching pattern on each
iteration, until no matching pattern remains.
g) If the buffer string ends with "<segment>/..", where <segment>
is a complete path segment not equal to "..", that
"<segment>/.." is removed.
h) If the buffer string contains a question-mark "?" character,
then the reference URL's query component is the substring after
the first (left-most) question-mark. Otherwise, the reference
URL's query component is set undefined.
i) The reference URL's new path component is the buffer string up
to, but not including, the first question-mark character or the
end of the buffer string.
7) The resulting URL components, including any inherited from the
base URL, are recombined to give the absolute form of the URL
reference. Using pseudocode, this would be
result = ""
if scheme is defined then
append scheme to result
append ":" to result
if server is defined then
append "//" to result
append server to result
append path to result
if query is defined then
append "?" to result
append query to result
if fragment is defined then
append "#" to result
append fragment to result
return result
Note that we must be careful to preserve the distinction between a
component that is undefined, meaning that its separator was not
present in the reference, and a component that is empty, meaning
that the separator was present and was immediately followed by the
next component separator or the end of the reference.
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 replacements.
Resolution examples are provided in Appendix C.
6. Security Considerations
A URL does not in itself pose a security threat. Users should beware
that there is no general guarantee that a URL, which at one time
located a given resource, will continue to do so. Nor is there any
guarantee that a URL will not locate a different resource at some
later point in time, due to the lack of any constraint on how a given
server apportions its namespace. Such a guarantee can only be
obtained from the person(s) controlling that namespace and the
resource in question.
It is sometimes possible to construct a URL such that an attempt to
perform a seemingly harmless, idempotent operation, such as the
retrieval of an entity associated with the resource, 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 an
unintended or impersonating 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 unescaped before transmission. This might violate
the protocol, but avoids the potential for such characters to be used
to simulate an extra operation or parameter in that protocol, which
might lead to an unexpected and possibly harmful remote operation to
be performed.
It is clearly unwise to use a URL that contains a password which is
intended to be secret.
7. Acknowledgements
This document was derived from RFC 1738 [2] and RFC 1808 [6]; the
acknowledgements in those specifications still apply. In addition,
contributions by Lauren Wood, Martin Duerst, Gisle Aas, Martijn
Koster, Ryan Moats and Foteos Macrides are gratefully acknowledged.
8. References
[1] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
Unifying Syntax for the Expression of Names and Addresses of
Objects on the Network as used in the World-Wide Web", RFC 1630,
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] Braden, R., Editor, "Requirements for Internet Hosts --
Application and Support", STD 3, RFC 1123, IETF, October 1989.
[5] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.
[6] Fielding, R., "Relative Uniform Resource Locators", RFC 1808,
UC Irvine, June 1995.
[7] N. Freed & N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies,"
RFC 2045, November 1996.
[8] Freed, N., and N. Freed, "Multipurpose Internet Mail
Extensions (MIME): Part Two: Media Types", RFC 2046, Innosoft, Bellcore,
November 1996.
[9] Kunze, J., "Functional Recommendations for Internet Resource
Locators", RFC 1736, IS&T, UC Berkeley, February 1995.
[10] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, USC/Information Sciences Institute,
November 1987.
[11] Sollins, K., and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,
December 1994.
[12] US-ASCII. "Coded Character Set -- 7-bit American Standard Code
for Information Interchange", ANSI X3.4-1986.
9. 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
Irvine, CA 92697-3425
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
URL-reference = [ absoluteURL | relativeURL ] [ "#" fragment ]
absoluteURL = generic-URL | opaque-URL
opaque-URL = scheme ":" *urlc
generic-URL = scheme ":" relativeURL
relativeURL = net_path | abs_path | rel_path
net_path = "//" server [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path_segments ] [ "?" query ]
scheme = 1*( alpha | digit | "+" | "-" | "." )
server = [ [ user [ ":" password ] "@" ] hostport ]
user = *( unreserved | escaped | ";" | "&" | "=" | "+" )
password = *( unreserved | escaped | ";" | "&" | "=" | "+" )
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
path = [ "/" ] path_segments
path_segments = segment *( "/" segment )
segment = *pchar *( ";" param )
param = *pchar
pchar = unreserved | escaped | ":" | "@" | "&" | "=" | "+"
query = *urlc
fragment = *urlc
urlc = reserved | unreserved | escaped
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
unreserved = alpha | digit | mark
mark = "$" | "-" | "_" | "." | "!" | "~" |
"*" | "'" | "(" | ")" | ","
escaped = "%" hex hex
hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f"
alphanum = alpha | digit
alpha = lowalpha | upalpha
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"
upalpha = "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"
B. Parsing a URL Reference with a Regular Expression
As described in Section 4.4, the generic-URL syntax is not sufficient
to disambiguate the components of some forms of URL. Since the
"greedy algorithm" described in that section is identical to the
disambiguation method used by POSIX regular expressions, it is
natural and commonplace to use a regular expression for parsing the
potential four components and fragment identifier of a URL reference.
The following line is the regular expression for breaking-down a URL
reference into its components.
^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
12 3 4 5 6 7 8 9
The numbers in the second line above are only to assist readability;
they indicate the reference points for each subexpression (i.e., each
paired parenthesis). We refer to the value matched for subexpression
<n> as $<n>. For example, matching the above expression to
http://www.ics.uci.edu/pub/ietf/uri/#Related
results in the following subexpression matches:
$1 = http:
$2 = http
$3 = //www.ics.uci.edu
$4 = www.ics.uci.edu
$5 = /pub/ietf/uri/
$6 = <undefined>
$7 = <undefined>
$8 = #Related
$9 = Related
where <undefined> indicates that the component is not present, as is
the case for the query component in the above example. Therefore, we
can determine the value of the four components and fragment as
scheme = $2
server = $4
path = $5
query = $7
fragment = $9
and, going in the opposite direction, we can recreate a URL reference
from its components using the algorithm in step 7 of Section 5.2.
C. Examples of Resolving Relative URL References
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/?y
g?y = http://a/b/c/g?y
#s = (current document)#s
g#s = http://a/b/c/g#s
g?y#s = http://a/b/c/g?y#s
;x = http://a/b/c/;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 start of the current document.
<> = (current document)
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 component 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
g;x=1/./y = http://a/b/c/g;x=1/y
g;x=1/../y = http://a/b/c/y
g?y/./x = http://a/b/c/g?y/x
g?y/../x = http://a/b/c/x
g#s/./x = http://a/b/c/g#s/./x
g#s/../x = http://a/b/c/g#s/../x
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. Recommendations for Delimiting URLs in Context
URLs are often transmitted through formats which do not provide a
clear context for their interpretation. For example, there are many
occasions when URLs are included in plain text; examples include text
sent in electronic mail, USENET news messages, and, 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, but usually
within double-quotes "http://test.com/", angle brackets
<http://test.com/>, or just using whitespace
http://test.com/
These wrappers do not form part of the URL.
In the case where a fragment identifier is associated with a URL
reference, the fragment would be placed within the brackets as well
(separated from the URL with a "#" character).
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 unescaped whitespace around the line break, and should be aware
that the hyphen may or may not actually be part of the URL.
Using <> angle brackets around each URL is especially recommended
as a delimiting style for URLs that contain whitespace.
The prefix "URL:" (with or without a trailing space) was
recommended as a way to used to help distinguish a URL from other
bracketed designators, although this is not common in pratice.
For robustness, software that accepts user-typed URLs should
attempt to recognize and strip both delimiters and embedded
whitespace..
Examples:
Yes, Jim, I found it under "http://www.w3.org/pub/WWW/",
but you can probably pick it up from <ftp://ds.internic.
net/rfc/>. Note the warning in <http://ds.internic.net/
instructions/overview.html#WARNING>.
F. Summary of Non-editorial Changes
F.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. A definition of URI
is also given, and how the URI syntax equates to the URL syntax, so
that other IETF specifications (e.g., HTTP, HTML, etc.) can refer to
a single definition of URI.
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.
F.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.
The question-mark "?" character was removed from the set of allowed
characters for the user and password in the server component, since
testing showed that many applications treat it as reserved for
separating the query component from the rest of the URL.
RFC 1738 specified that the path was separated from the server
portion of a URL by a slash. RFC 1808 followed suit, but with a
fudge of carrying around the separator as a "prefix" in order to
describe the parsing algorithm. RFC 1630 never had this problem,
since it considered the slash to be part of the path. In writing
this specification, it was found to be impossible to accurately
describe and retain the difference between the two URLs
<foo:/bar> and <foo:bar>
without either considering the slash to be part of the path (as
corresponds to actual practice) or creating a separate component just
to hold that slash. We chose the former.
F.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.
The recommendations for delimiting URLs in context (Appendix E) have
been adjusted to reflect current practice.
F.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.[palme]
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.
Extensive testing of current client applications demonstrated that
the majority of deployed systems do not use the ";" character to
indicate trailing parameter information, and that the presence of a
semicolon in a path segment does not affect the relative parsing of
that segment. Therefore, parameters have been removed as a separate
component and may now appear in any path segment. Their influence
has been removed from the algorithm for resolving a relative URL
reference. The resolution examples in Appendix C have been modified
to reflect this change.
Testing has also revealed that most client applications remove the
query component from the base URL before resolving relative URLs, and
append the reference's query component to a relative path before
merging it with the base path. The resolution algorithm has been
changed accordingly.
Received on Monday, 30 December 1996 12:13:48 UTC